smart n heterocyclic carbene ligands in catalysis - uji2018).pdf · smart n‑heterocyclic carbene...

Smart N‑Heterocyclic Carbene Ligands in CatalysisEduardo Peris

Institute of Advanced Materials Universitat Jaume I Avenida Vicente Sos Baynat sn Castellon E-12071 Spain

ABSTRACT It is well-recognized that N-heterocyclic carbene (NHC) ligands have provideda new dimension to the design of homogeneous catalysts Part of the success of this type ofligands resides in the limitless access to a variety of topologies with tuned electronic propertiesbut also in the ability of a family of NHCs that are able to adapt their properties to the specificrequirements of individual catalytic transformations The term ldquosmartrdquo is used here to refer toswitchable multifunctional adaptable or tunable ligands and in general to all those ligandsthat are able to modify their steric or electronic properties to fulfill the requirements of adefined catalytic reaction The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusingattention on the catalytically relevant ligand-based reactivity

CONTENTS

1 Introduction A2 Switchable N-Heterocyclic Carbene Ligands B

21 Photoswitchable N-Heterocyclic CarbeneLigands B

22 Redox-Switchable N-Heterocyclic CarbeneLigands C

23 Chemoactive N-Heterocyclic Carbene Li-gands H

231 Postfunctionalization of N-HeterocyclicCarbenes with Reactive Backbones H

232 Postfunctionalization by Coordinationof Metal Fragments M

233 Proton-Responsive N-Heterocyclic Car-bene Ligands P

3 Multifunctional N-Heterocyclic Carbene Ligands Q31 Hemilabile and Conformationally Labile N-

Heterocyclic Carbene Ligands Q311 N-Heterocyclic Carbenes with N-Donors R312 N-Heterocyclic Carbenes with S-Donors T313 N-Heterocyclic Carbenes with O-Donors U314 N-Heterocyclic Carbenes with Alkenyl

and Other C-Donors W315 Conformationally Labile N-Heterocyclic

Carbenes Y32 Bifunctional N-Heterocyclic Carbene Ligands Z

321 Bifunctional Processes Involving theParticipation of CminusH OminusH or NminusHBonds of the Ligand Z

322 Bifunctional Processes through Aroma-tizationDearomatization AC

323 Bifunctional N-Heterocyclic Carbene Li-gands with Rigid Polyaromatic Func-tionalities Homogeneous Catalysis andπ-Stacking AE

4 Conclusions and Outlook AGAuthor Information AI

Corresponding Author AIORCID AI

Notes AIBiography AI

Acknowledgments AIReferences AI

1 INTRODUCTIONNoninnocent12 cooperative2minus5 switchable67 and multifunc-tional89 are different forms for referring to ligands that do morethan providing an electronic environment and a stericallydefined pocket to the metal fragment in a catalyst Such ligandsare able to adapt their properties to the specific requirementsfor individual organic transformations and therefore the termldquosmartrdquo may seem rather appropriate In general all ligands areimportant in the sense that they determine the reactivity of acomplex in solution stabilize specific oxidation states or definethe size and shape of the catalytic site Once a ligand is chosenthe coordination environment of the catalyst is fixed and thisdetermines the rate and selectivity for a given reaction ldquoSmartrdquoligands are Switchable Multifunctional Adaptable oR Tunableand therefore they are able to directly interact with thesubstrate or modify their steric or electronic properties to fulfillthe requirements of a defined catalytic transformation In manycases the adaptability of a smart ligand is recognizedpostfactum when the outcome of a specific catalytic reactionis shown to be different than the one that should have beenexpected if the role of the ligand was assumed to be only asspectator However once the cooperative effect has beenidentified the new pattern of ligand-based reactivity can beused for the design of more effective multifunctional ligandsWhy smart N-heterocyclic carbene (NHC) ligands It is well

recognized that the impact of NHC ligands in homogeneouscatalysis can only be compared with the impact that phosphineshad during the 1970minus1990 period The success of NHC ligandsis arguably attributed to their relatively easy preparation10

Special Issue Carbene Chemistry

Received October 10 2016

Review

pubsacsorgCR

copy XXXX American Chemical Society A DOI 101021acschemrev6b00695Chem Rev XXXX XXX XXXminusXXX

which gives access to a large variety of structures with diversetopologies and electron-donating properties11minus17 Also NHCsmay incorporate a limitless library of additional functions andthis gives this type of unique ligands clear advantages for theon-demand (or tailor-made) synthesis of homogeneouscatalystsThe purpose of this review is to describe all types of smart

NHC ligands by considering the classification given in Figure1 We are aware that some other classifications may be equallyvalid but we thought that this one gives a clear overview ofwhat has been done in the field and allows a clear distinction ofall types of different ligands The review will place moreemphasis on those examples for which the role of the ligand hasa clear influence on the catalytic properties of the complex butattention will also be given to those cases in which relevantstudies on modification of the steric and electronic propertiesof NHC-based metal complexes is produced by external stimuliAlthough several review articles have appeared regarding theproperties of the different types of smart ligands as describedhere this review is the first one to globally consider all differenttypes of smart NHCsAs described here smart ligands have been classified into

switchable NHC ligands (section 2) and multifunctional NHCligands (section 3) The term switchable catalysis is used torefer to all those catalysts that are able to switch their activitiesby virtue of a range of stimuli such as light pH coordinationevents redox processes and changes of reaction conditions6

Although switchable catalysis is often understood as all thecatalytic processes that can be reversibly switched on and off7

we will here consider also those examples in which the activityof the catalyst can be turned on or off by external stimulidisregarding whether this change is reversible or not Cases ofreversible switchable catalysis are easily found for redox-1819

pH-9 or light-driven20minus24 processes Irreversible switchablecatalysis is often found when the catalyst is turned on or off byvirtue of an irreversible chemical modification involving thecoordination sphere of the metal complexMultifunctional ligands are those that have been designed to

do more than one thing8 This means that multifunctionalligands have additional functions that that can be useful incatalysis9 In our review this group will include all existingexamples regarding hemilabile ligands (section 31) andbifunctional ligands (section 32) Both hemilabile andbifunctional ligands refer to ligands whose reactivity dependson direct interaction with the substrates and therefore their

presence has a direct influence on the catalytic properties of thecatalyst

2 SWITCHABLE N-HETEROCYCLIC CARBENELIGANDS

The following subsections will be devoted to the photochemical(section 21) redox (section 22) and chemical (section 23)switching of NHC ligands and their applications in specifichomogeneously catalyzed processes Strategies aiming toappend functional groups by modifying coordinated NHCligands have been explored by a number of research groups fora handful of years A review article regarding the post-modification of transition metalminusNHC complexes as a toolboxfor bioorganometallic chemistry was recently published byCisnetti et al25 In that article the authors wisely suggested thatin the near future the postmodification of NHC ligands mayhelp increase the number of biocompatible and bioconjugatedNHC-based metal complexes In the following subsectionsexisting examples of postfunctionalization of NHC ligands forhomogeneous catalysis will be described

21 Photoswitchable N-Heterocyclic Carbene Ligands

Photoswitchable systems allow for external (remote) modu-lation of chemical reactions by light22 A photoswitchablecatalyst involves a catalytically active species that is susceptibleto undergoing a reversible photochemical transformation andas a result modifying its intrinsic catalytic properties2023 Thefirst example of photoswitchable postmodification of a NHCligand was described in 2009 by Yam et al26 who obtained aseries of di(thiophenyl)ethene-containing NHC complexes ofPd(II) Ag(I) and Au(I) whose steric and electronic propertiescould be modified by the photochemical reversible skeletalreorganization of the ligand via a CminusC bond activation(Scheme 1) Shortly after the work was extended to thepreparation of half-sandwich-type ruthenium(II) complexeswhich showed photochromic properties upon UV irradiation to

Figure 1 Presently known types of smart ligands

Scheme 1 First Photoswitchable NHC-based Ligand26

Chemical Reviews Review

DOI 101021acschemrev6b00695Chem Rev XXXX XXX XXXminusXXX

B

generate the annulated form of the dithienylethene moiety27

Interestingly Bielawski and co-workers21 studied the mod-ification of the electron-donating character of the NHC ligandupon cyclization of the dithienylethene fragment by estimatingthe shift in the Tolman electronic parameter (TEP) fromevaluation of the CO stretching frequencies of the related[IrCl(NHC)(CO)2] complexes and by using the well-acceptedcorrelations122829 From their studies they concluded that thecyclization of the ligand produced a positive shift of 6 cmminus1thus significantly reducing the electron-donating character ofthe NHC21

The same type of ligand was later used by Neilson andBielawski30 to obtain a Rh(I) compound whose catalyticproperties in the hydroboration of alkynes and alkenes werestudied Upon photocyclization of the ligand the activity of therhodium complex was reduced by an order of magnitude thusaffording one of the first examples clearly showing modulationof the activity of a catalyst by phototuning its electronicproperties (Scheme 2) The attenuated activity of thecycloannulated complex 4 with respect to the open form 3was attributed to the decrease in electron-donor power of theligand upon photoannulation which in turn inhibits the rate-determining reductive elimination step of the hydroborationBecause the ring-closure reaction is reversible the authors wereable to perform an experiment in which activity of the catalystwas turned on and off by successively irradiating the reactionvessel with UV light (λ = 313 nm) and visible light (λ gt 500nm)

The catalytic photoswitchable properties of this dithienyli-midazolylidene were observed to be associated with its role notonly as a ligand (coordinated to a metal fragment) but also asan organocatalyst where it showed interesting properties intransesterifications and transamidations31 and in the ring-opening polymerization of ε-caprolactone and δ-valerolactone(Scheme 3)32 In both cases the activity of the NHC catalystwas significantly attenuated upon exposure to UV light as aconsequence of ring closure of the imidazolylidene This wasexplained by the authors as a consequence of decreasedelectron density at the carbene of the ring-closed species (6)which forms an NHCminusalcohol adduct32 The photoswitchableproperties of the isolated free carbenes (5 and 6) were alsostudied33 Furthermore the ring-closed isomer (6) was foundto capture ammonia by NminusH activation which could then bereleased by the open isomer 5 therefore affording an elegantexample of a photoswitchable transformation

Another interesting example of a photoswitchable NHCligand was described by Monkowius and co-workers in 201334

This group obtained an azobenzene-functionalized NHC thatwas coordinated to silver and gold Scheme 4 shows oneexample of this type of reaction for a selected compound Allgold complexes featured E rarr Z isomerization of theazobenzene tag upon irradiation with UV light The thermalreverse reaction was relatively slow Unfortunately the catalyticproperties of these complexes were not evaluated

22 Redox-Switchable N-Heterocyclic Carbene Ligands

In a complex with a redox-active ligand the ligand is consideredto be the dominant source of electrons with the metal retainingits original oxidation state18 In principle the participation of aredox-active ligand in the catalytic cycle may be by accepting ordonating electrons or by actively cleaving or forming covalentbonds35 In general redox-switchable catalysis seeks to regulatechemical reactions by simple reductionminusoxidation manipula-tion Redox-active ligands can be used as a reversible trigger tocontrol catalytic reactivity In the case of existing examples ofredox-active NHC ligands for catalysis most cases refer tosituations in which oxidationreduction of the ligand is used fortuning the electronic properties of the metal19 In this regardredox-active NHCs are not ascribed to the classical concept ofnoninnocent redox ligands for which the noninnocent behaviorrefers to the ambiguity or uncertainty of the oxidation state ofthe metal that is generated upon coordination to such a ligand2

Rather than approaching this section in a chronologicalmanner we will first approach the studies dealing withelectronic modification of redox-active NHC ligands uponintroduction of a redox stimulus and then we will describe theexisting examples in which these ligands were used in redox-switchable catalysis

Scheme 2 Photoswitchable Catalytic Hydroboration ofAlkenes and Alkynes30

Scheme 3 Photoswitchable Organocatalytic Reactions withDithienylimidazolylidene

Scheme 4 Photoswitchable Behavior of Azobenzene-TaggedNHC Coordinated to Au(I)



C

In 2008 Bielawski and co-workers36 described a series of six-membered N-heterocyclic carbenes with a 11prime-ferrocenediylbackbone (diaminocarbene[3]ferrocenophanes) which werecoordinated to Rh(I) to afford the related [RhCl(NHC)-(COD)] and [RhCl(NHC)(CO)2] complexes [Scheme 5

shows a selected Rh(I) complex] The oxidation of theferrocene moiety here is irreversible and leads to fastdecomposition Bulk electrolysis combined with time-resolvedIR spectroscopy allowed examination of the carbonyl stretchingfrequency as a function of the oxidation state of the Fe centerThe authors concluded that oxidation of the ferrocenylfragment in 8 (Scheme 5) produced a positive shift of 205cmminus1 which is consistent with an important reduction ofelectron density at the rhodium center as a consequence ofintroducing a positive charge at the iron atom Neverthelesstwo later works by the same group reported lower shifts ofνav(CO) for related [IrCl(NHC)(CO)2] complexes (with theNHC ligands being the same as in 8 but with n-Bu and CH3

groups instead of phenyl at the nitrogen atoms of theheterocycle) upon oxidation of the iron center These shifts

Scheme 5 CminusO Stretching Frequency Shift upon Oxidationof a Diaminocarbeneferrocenophane

Scheme 6 Variations in ν(CO) Frequencies of a Series of CarbonylminusMetal Complexes with Redox-Active NHC Ligandsa

aData were taken from refs 37 41 and 42



D

were 12 and 13 cmminus1 for ligands containing n-Bu37 or CH338

groups respectively as will be described in more detail later inthis sectionIn 2009 Siemeling et al3940 described a series of complexes

of rhodium and molybdenum with the same type ofdiaminocarbene[3]ferrocenophanes These authors showedthat the carbenes could be easily oxidized to the correspondingradical cations By computational calculations it was establishedthat the spin density of the radical oxidized diaminocarbene[3]-ferrocenophane is located at the iron atom and at the carbon ofthe carbene39

Bielawski and co-workers37 made a strong contribution tothe study of variation of the donating abilities of a variety offerrocenyl-functionalized NHCs upon oxidation of theferrocenyl units In order to perform a detailed analysis ofthe problem a wide set of ferrocenyl-NHC ligands were tested(Scheme 6) These authors sought to evaluate how severalstructural characteristics influence the electronic consequencesof incorporating redox-active fragments in the ligand Suchcharacteristics were (i) use of an imidazolylidene versus abenzoimidazolylidene (ii) incorporation of one versus twoferrocenyl fragments and (iii) the presence or absence of anaromatic ring fused to the backbone of the NHC They alsostudied the influence of these characteristics on the electronicproperties of the NHC ligand by introducing a reducible redox-active functionality such as a naphthoquinoneFrom the data obtained (shown in Scheme 6) it could be

concluded that oxidation of the ferrocenyl-containing Ir(I)complexes resulted in nearly identical shifts in νav(CO)(typically between 13 and 15 cmminus1) An interesting exampleis the case of the iridium complex with a ferrocenyl groupbound to the NHC nitrogen by a methylenic group for whichthe electronic communication of the ferrocenyl fragment and

the iridium center is disrupted yet the νav(CO) shift observedlies in a similar range42 For the reduction of thedimesitylnaphthoquinone-based NHC the νav(CO) shift wasminus125 cmminus1 (similar value but negative) in accordance with theenhancement of donor properties of the NHC ligand41 Shiftswere also observed for the A1 carbonyl frequencies of thecarbonyl ligands in the [M(NHC)(CO)5] complexes (M = MoW frequency shifts ranged between 91 and 93 cmminus1 forcomplexes with ferrocenyl-containing ligands and between minus93and minus102 cmminus1 for complexes with dimesitylnaphthoquinone-based ligand)41 Analysis of these results indicated that thescaffold structure or the number of ferrocenyl units bound tothe ligand does not produce an appreciable change in thetunability properties of the ligand upon oxidation or in otherwords that the redox tunability of the ligands does not stronglydepend on the chemical identity of the redox-active functionalgroup Rather than that it seems that the change in ligand-donating ability is due to the overall charge imparted to themolecule upon redox change37 This important workconstituted the first detailed study on quantification of theelectron-donating tunability of redox active-NHC ligandsMore recently the same authors obtained a series of iridium

cationic complexes bearing two bulky ferrocenylated NHCligands Oxidation of the ferrocenylated ligands on these[Ir(NHC)2(CO)2]

+ cations increased the measured νav(CO) by10 cmminus1 and this value is independent of the number of redox-active groups at the compex43 in agreement with resultspublished before37

The first redox-switchable NHC-based complex was reportedby Sussner and Plenio in 200544 It was also the first example inwhich this type of redox-active ligand was used to pursue acatalytic application The complex was a ruthenium-basedGrubbsminusHoveyda-type olefin metathesis catalyst with two

Scheme 7 Soluble Switchable Catalysts Used for Ring-Closing Metathesis of N-Tosyldiallylamide and for Ring-OpeningMetathesisminusPolymerization of Norbornene



E

ferrocenyl fragments as redox-switchable tags (9 in Scheme 7)Oxidation of the ferrocenyl fragments with acetylferroceniumtriflate produced a highly insoluble bisferrocenium complexand this was elegantly used by the authors to control thesolubility of the catalyst by reversibly oxidizing or reducing thecomplex The catalyst was tested in the ring-closing metathesis(RCM) of N-tosyldiallylamide and the authors showed thatthey were able to switch on and off the catalyst activity severaltimes by adding acetylferrocenium triflate (off) or octamethyl-ferrocene (on) More interestingly they were able to separatethe catalyst from the products of the catalytic reaction at anytime during the reaction and after its completion In 2010 thesame group studied the behavior of complex 10 as a redox-switchable catalyst for ring-opening metathesisminuspolymerization(ROMP) of norbornene45 Both neutral and dicationiccomplexes resulting from the oxidation of 10 showed similaractivities in the reaction and rendered polymers with similar EZ ratio The authors rationalized these results on the basis ofthe small difference in electron donation of the NHC ligand in10 and 102+ which in this case did not provide a significantchange in the stereochemistry of the polymer obtainedRing-closing metathesis of diethylmalonate was tested with

the ferrocene-containing NHC complex of Ru(II) 11 shown inScheme 842 The related compound with a 1prime2prime3prime4prime5prime-pentamethylferrocenyl moiety 12 was also obtained and theactivities of these two complexes indicated a higher activity of11 compared to that shown by 12 When complex 11 was usedin RCM of diethyl malonate conversion reached 80 in 1 hAddition of an oxidant reduced the activity of the catalyst andthe conversion decreased to 7 under the same conditionsRegeneration of neutral 11 by addition of a reductant did notrestore the full activity of the catalyst (only 13 activity wasrestored) The same type of experiment was performed forcatalyst 12 for which the subsequent addition of an oxidant(acetylferrocenium tetrafluoroborate) and a reductant (deca-methylferrocene) produced deactivation of the catalyst andreactivation of its catalytic activity For this complex the activitycould be restored to 9442

Bielawski and co-workers38 also studied the ROMP of ciscis-cyclooctadiene with 13 as catalyst (Scheme 9) They observedthat addition of an oxidant (23-dichloro-56-dicyanoquinone)

reduced the rate constant of the reaction by over an order ofmagnitude Subsequent reduction of the oxidized catalyst (byaddition of decamethylferrocene) restored the catalytic activitySarkar and co-workers46 were the first to use a mesoionic

carbene (MIC) complex as a redox-switchable catalyst Theyobtained a ferrocenyl-based mesoionic Au(I) complex (14Scheme 10) which was used for cyclization of N-(2-propyn-1-yl)benzamide to 5-methylene-2-phenyl-45-dihydrooxazole Itwas observed that the neutral complex was inactive in thereaction but the oxidized cationic radical delivered 40conversion after 5 h and 88 after 24 h46 The higher activityof the oxidized complex (obtained by in situ oxidation of 14with acetylferrocenium) was attributed to its higher Lewisacidity compared to the unoxidized form These studies werecarried out in more detail in a more recent work where a largerset of redox-active MIC ligands was used and in some cases theoxidized MIC-based gold complexes outperformed their neutralcounterparts by almost a factor of 1047 With thesecontributions the authors illustrated how oxidized MIC-basedcatalysts may offer excellent performance when electron-poorligands are needed This is an interesting finding because MICligands are mostly used when strong electron-donating ligandsare neededPoyatos Peris and co-workers48 also contributed to the field

by describing a benzo-fused ferrocenyl-imidazolylidene ligandwhich was coordinated to iridium(I) and gold(I) (Scheme 11)After a detailed experimental and computational analysis theauthors concluded that oxidation of the metal complexes withacetylferrocenium tetrafluoroborate produced mixtures of therelated oxidized complexes containing a ferrocenium-imidazo-

Scheme 8 Ferrocenyl-based NHC Complexes of Ru(II) Used in Ring-Closing Metathesis of Diethylmalonate

Scheme 9 Redox-Switchable Catalyst for ROMP ofCyclooctadiene



F

lylidene ligand (Fe3+) together with a protonated ferrocenyl-imidazolylidene (Fe2+) compound Protonation of the NHCligand is produced at one unsubstituted nitrogen of theheterocycle closest to the ferrocenyl group and is presumablydue to hydrogen abstraction from solvent by the cationicoxidized radical (redox-induced hydrogen abstraction) Oxida-tion of the ligand (regardless of whether it produced oxidationof the ferrocenyl fragment or protonation by hydrogenabstraction) produced a subtle increase in νav(CO) of thecorresponding [IrCl(NHC)(CO)2] complex [see Scheme 11for differences between 15 and 1616-H νav(CO) shift =29cmminus1] which is lower than the value expected according topreviously published results by other authors37 Coordination

of the ligand to gold(I) produced a redox-switchable catalyst(17) that was used in hydroamination of terminal alkynes andin cyclization of alkynes with furans48 For both reactions it wasassumed that the higher Lewis acidity of the gold complex uponoxidation (18 or 18-H) should provide a more active catalystThis assumption was proven right for both reactions In thehydroamination of terminal alkynes the results indicated thatoxidation of the ligand produced a moderate increase in activityof the gold catalyst In the cyclization of alkynes with furans theneutral complex was inactive while the oxidized catalystproduced moderate to good yields of the final products48

The same benzo-fused ferrocenyl-imidazolylidene ligand waslater coordinated to ruthenium(II) (19 Scheme 12) and the

Scheme 10 First MIC-based Redox-Active Catalyst

Scheme 11 Benzo-fused Ferrocenyl-imidazolylidene Complexes with Ir(I) and Au(I)

Scheme 12 Redox-Switchable NHCminusRuthenium Catalyst in Transfer Hydrogenation



G

resulting complex was used as catalyst in the reduction ofketones and imines by transfer hydrogenation with 2-propanolas the hydrogen source49 In this case the neutral complex wasfar more active than the oxidized form in the reduction ofketones but afforded similar activity in the reduction of iminesThis allowed modulation of the activity of the catalyst bysuccessively adding acetylferrocenium tetrafluoroborate andcobaltocene for the specific case of reduction of hexanophe-none Addition of acetylferrocenium tetrafluoroborate inter-rupted reduction of the substrate while addition of cobaltocenerestored the catalytic activity These results indicated that theoxidized complex could potentially be used in the selectivereduction of imines containing carbonyl groups49

Some other ferrocenyl-containing NHC complexes showingredox-active properties were also reported50minus57 but theirproperties were not exploited in homogeneous catalysisFerrocenyl groups were not the only redox-active fragments

introduced into NHC ligands in order to design redox-switchable NHC ligands As already mentioned someresearchers introduced quinone groups aiming that thereduction of quinone fragment should increase the electron-donating character of the NHC ligands For example Bielawskiand co-workers58 obtained a series of naphthoquinimidazoly-lidene complexes with Ni(II) Pd(II) and Pt(II) to study theredox-switchable Kumada cross-coupling reaction Theyobserved that the Ni(II) complex (20 Scheme 13) showedhigh activity in the process and they studied the effect ofcontrolling the activity of the complex by altering the redoxstate of the supporting NHC ligand Addition of a reducingagent (2 equiv of cobaltocene) significantly reduced thecatalytic activity of the complex while subsequent addition ofan oxidant (2 equiv of ferrocenium tetrafluoroborate) restored

the activity The authors hypothesized that the diminishedactivity of the reduced catalyst reflected greater electron densityat the nickel center causing the transmetalation and reductiveelimination steps of the catalytic cycle to occur at a much lowerrate58

23 Chemoactive N-Heterocyclic Carbene Ligands

In 2010 Cesar and Lavigne and co-workers59 used the termchemoactive NHC ligands to refer to N-heterocyclic carbeneswith a functional group at the backbone susceptible to allowingfurther chemical modification once the ligand is bound to themetal fragment This strategy allows real-time tuning of theelectronic properties and therefore can be used to adapt theligand to the requirements for a specific catalytic trans-formation Therefore chemoactive NHCs refer to all types ofmethods allowing the chemical postmodification of a NHCligand once this is already bound to a metal This strategy tomodulate the chemical properties of the NHC ligand differsfrom the premodification strategy which implies that thefunctionality is added to the NHC in the multistep syntheticprocedure of the carbene precursor often an azolium salt Thetypes of chemical postmodification that will be treated in thissection are addition of a functionality to the NHC backbonecoordination to a metal and proton-responsive systems

231 Postfunctionalization of N-Heterocyclic Car-benes with Reactive Backbones In 2009 Cesar andLavigne and co-workers60 obtained an imidazolylidene-olatewhich can be considered as the first preprogrammed NHC forfurther derivatization In fact these authors were probably thefirst ones to study in detail the rich chemistry introduced byanionic NHC ligands61 Coordination of the in situ preparedanionic NHC (21) with [RhCl(COD)]2 in the presence ofhydrochloric acid afforded the keto-imidazolylidene rhodium(I)

Scheme 13 DimesitylnaphthoquinimidazolylideneminusNi(II) Complex in Redox-Switchable Kumada Coupling

Scheme 14 Postfunctionalization of Imidazolylidene-olate



H

complex 2260 This compound is assumed to show a tautomericequilibrium with the enol-imidazolylidene so reaction withLiN(SiMe3)2 followed by addition of an electrophile[terbutyldimethylsilyl chloride (TBDMSCl) or Ph2P(O)Cl]produced functionalization at the oxygen atom (complex 23Scheme 14) C-functionalization of the carbene ligand can beperformed with paraformaldehyde as electrophile via analdolizationminuscrotonization sequence affording the olefiniccarbene complex 24Hashmi et al62 extended the coordination of this type of

chemoswitchable imidazolylidene-olate ligands to gold palla-dium and platinum The ligands contained a wide variety ofaromatic and aliphatic wingtips (substituents at the nitrogenatoms of the N-heterocycle) Interestingly the crystal structureof one of the gold complexes indicated the presence of twoindependent mesomeric molecules (25 and 25prime) Switching ofthe electronic properties of the ligand was achieved bydeprotonation of the carbonyl-imidazolylidene complex ofgold and subsequent addition of an electrophile (Scheme 15)The resulting complexes were active in the cycloisomerizationof propargyl amides to alkylideneoxazolines and oxazines

although the enolate-carbene complexes (27) did not showmeasurable different activities compared to their mothercarbonyl-NHC gold complex 2562 The same keto-imidazoly-lidene was recently used by Buchowicz and co-workers63 for thepreparation of the corresponding [NiCpCl(NHC)] complexThis complex together with other half-sandwich nickelcomplexes with backbone-functionalized NHCs were testedin the polymerization of styrene and methyl methacrylate andin the SuzukiminusMiyaura cross-couplingAnother interesting example of a chemoactive NHC ligand

reported by Cesar and Lavigne and co-workers64 consisted of aimidazolylidene with a free NH function at the backbone Thiscarbene is in a tautomeric equilibrium with its mesoionicimidazolium isomer although the carbene form is trapped byreaction with metal fragments (Scheme 16) Reaction of thisspecies with [RhCl(COD)]2 in the presence of potassium tert-butoxide produced a complex with the heterocyclic ligand inthe form of the imidazolylidene (28) which does not show anytendency to tautomerize This complex reacted in the presenceof SiO2 in air by incorporating an oxygen atom to thebackbone generating an amido-amidino-carbene complex (29)

Scheme 15 Gold Complexes with Chemoswitchable Carbenes

Scheme 16 Postfunctionalization of Amino-imidazolylidene



I

Calculation of the Tolman electronic parameters (TEP) of theligands (by obtaining the related [RhCl(NHC)(CO)2]complexes and comparing their IR CminusO stretching frequen-cies) in 28 and 29 indicated that the four-electron oxidation ofthe carbene ligand was accompanied by an increase in the TEPvalue of 78 cmminus1 as a consequence of the lower electron-donating character of the ligand in 2964

The same researchers described a six-membered NHC with aremote anionic functional group within the heterocyclicbackbone which afforded zwitterionic metal complexes suchas 30 (Scheme 17)65 This complex underwent reaction withvarious electrophiles to afford cationic complexes 3166

Addition of the electrophile produced an important decreasein the ligand donicity (Δνav(CO) in the range 13minus16 cmminus1according to variation of CminusO stretching frequencies of therelated [RhCl(NHC)(CO)2] complexes) The complexes weretested in the catalytic polymerization of phenylacetylene and inthe hydroboration of styrene (Scheme 17) and for both casesthe zwitterionic complex was the best catalyst thus revealing anapparent correlation between the donor character of the ligandand the activity of the complexSix-membered ring N-heterocyclic carbenes with malonate or

imidate backbones were incorporated to the coordinationsphere of cyclopentadienyl iron carbonyl complexes affording aseries of zwitterionic complexes (32 and 33 Scheme 18)67 Thedonor properties of the ligands could be switched (reduced) byaddition of methyl triflate which reacts with one oxygen of themalonate group (34) or the nitrogen of the imidate backbone(35) The complexes were tested in hydrosilylation ofbenzaldehyde where the zwitterionic complex with themalonate-NHC ligand was the one displaying higher activityThe authors rationalized these results as a consequence of thestronger electron-donating character of the malonate-NHCligand in 32 Complex 32 was further tested in thehydrosilylation of a variety of aldehydes showing goodefficiency and excellent chemoselectivity67 The same catalyst

showed moderate activity in hydrosilylation of ketones andgood activity in hydrosilylation of iminesSeveral examples have been reported in which postfunction-

alization of the NHC ligand is performed through anldquoautoclickrdquo copper- or ruthenium-catalyzed alkyneminusazide cyclo-addition (CuAAC or RuAAC)68minus70 Although some exampleshave been described in which this type of strategy is used tomodify the catalytic properties of NHC-based metal complexesthe method has not yet been used for the preparation of

Scheme 17 Effects of Chemical Modification of Anionic NHC Ligand on Catalyst Activity in Polymerization of Phenylacetyleneand Hydroboration of Styrene

Scheme 18 Malonate- and Imidate-NHC Complexes withFeCp(CO)2



J

switchable catalysts7 in the sense that modifications of theproperties are not reversible and also the ldquoswitchingrdquo is neverproduced in situ during the course of the catalytic reactionThis type of postmodification strategy was first used by Elsevierand co-workers in 201071 who described the modular approachto bidentate palladium complexes with triazolyl-NHC chelateligands The monodentate palladium alkenyl-imidazolylidenecomplex 36 underwent a copper-catalyzed click reaction withazidoadamantane to afford the CminusN ligated complex 37 asdescribed in Scheme 19 Complex 37 and other CminusN similarchelate palladium complexes were tested in the reduction of 1-phenyl-1-propyne by transfer hydrogenation with formic acid ashydrogen source71

Only one year later Guichard and Bellemin-Laponnaz andco-workers72 used the RuAAC click process for functionaliza-tion of a series of NHC complexes of palladium and platinumThese authors used a wide set of azides and alkynes aiming togenerate an extensive library of NHC-based complexes withcytotoxic activity An example of functionalization of a platinumcomplex with estrogen is shown in Scheme 20While these two previous examples were based on the

introduction of alkyne functionalities onto the backbone of theNHC to facilitate the click process by reaction with afunctionalized azide Gautier and Cisnetti and co-workers73

were the first ones to invert the process by using azide-functionalized NHC complexes to facilitate postfunctionaliza-tion of the ligand They first prepared a series of silver(I) and

gold(I) complexes functionalized with diverse 123-triazolesInterestingly the reaction with an activated alkyne could beproduced by thermally-promoted and strain-promoted 13-dipolar cycloaddition without the need to add any coppercatalyst73 By following a similar methodology the sameauthors prepared a series of azido-NHCminusCu(I) complexeswhich underwent a CuAAC reaction in the presence of alkynesin a very interesting self-catalyzed (or autoclick) processwithout the need for any additional copper catalysts (Scheme21)74 The reaction worked well for saturated and unsaturatedNHCs More recently by using this autoclick process theseauthors were able to prepare a wide variety of functionalizedgold and copper complexes including a gold compound with afluorescent biomarker75

By using a bromopropyl-substituted benzoimidazolylideneminuspalladium complex (42 in Scheme 22) Huynh and Teng76

were able to easily postmodify their complexes by adding anumber of functionalities including iodide esters azidothiocyanato and thioester groups (43minus47) Although thisstrategy was not tested in homogeneous catalysis the ease ofpostfunctionalization of these palladium complexes envisages apromising future in the field also as a simple way for graftingthe metal complexes onto solid materials for heterogenizationof the catalysts In a more recent work using the samebromopropyl-substituted benzoimidazolylidene ligands theauthors prepared a bis(benzoimidazolylidene) complex 48which was postfunctionalized with a series of groups includinghydroxyl azido thioether thioester amino and thiocyanate77

Some of the resulting complexes were tested in the MizorokiminusHeck coupling reaction between aryl halides and tert-butylacrylate (a selected example is shown in Scheme 22) where thepresence of the ancillary ammonium group in 50 showed asignificant positive effect on the activity of the catalyst possiblydue to the stabilization of the catalyst via formation of ionpairs77 Although the presence of the ancillary ammoniumgroups are clearly beneficial for the activity of 50 the catalystperforms better in the presence of an extra amount ofammonium saltsThe same strategy was used for the postmodification of

indazolinylidene complexes of palladium(II) In this case aseries of trans-[PdBr2(amine) (indy)] (indy = indazolinylidene)

Scheme 19 CuAAC Click Reaction for Preparation of CminusNChelate Pd(II) Complex

Scheme 20 Functionalization of Platinum(II) Complex with Estrogen



K

with pendant amines (52 in Scheme 23) were obtained byreaction of a single precursor with an indazolinylidene ligandhaving a bromopropyl substituent at the nitrogen (51)78 Allthese palladium complexes were tested in the arylation of 1-methylpyrrole with 4-bromoacetophenone where the amine-functionalized catalysts showed a slightly higher activity

compared to 51 The best activity was achieved for thereaction carried out with catalyst 52 bearing a diethylaminogroupHuynh and co-workers also developed an interesting example

in which a series of thiolato-bridged dinuclear complexes ofpalladium (53) are oxidized by oxone to yield the relatedsulfonate-functionalized monometallic complexes (54)79 Thesecomplexes are interesting because their solubility in water isvery high and for this reason they were used for the MizorokiminusHeck coupling of aryl halides with tert-butyl acrylate in aqueousmedium (Scheme 24) The activity of complex 54 with a o-xylyl-sulfonate group was very high for the coupling ofbromobenzaldehyde even at low catalyst loadings (005 mol) but it failed to catalyze the coupling of aryl chlorides orunactivated aryl bromides79

In a recent work Bellemin-Laponnaz and co-workers81 usedthe oxime ligation80 as a click reaction methodology forpostfunctionalization of a NHCminusPt(II) complex bearing abenzaldehyde moiety (55 in Scheme 25) This process wascombined with metalminusligand exchange thus offering a verypractical approach to the multifunctionalization of NHCminusplatinum complexes The work constituted a new advance in

Scheme 21 CuAAC Click Reaction of Azide-Tagged Cu(I)minusNHC Complex

Scheme 22 Postfunctionalization of Palladium(II)minusNHC Complexes by Nucleophilic Substitution

Scheme 23 Postmodification of Palladium(II) Complex withIndazolinylidene Ligand



L

the search for simple methods to fine-tune NHCminusmetalcomplexes with biological activity81 Scheme 25 shows thegeneral procedure together with one example of the manypossible multifunctionalized Pt(II) complexes described (57prime)An intriguing way of postfunctionalizing a chelate cyclo-

metalated NHC ligand was described by Deng and co-workersin 201382 The authors obtained a cobalt(II) bis(carbene)complex with cyclometalated mesityl groups (58 Scheme 26)The reaction of this complex with an arylhydrosilane afforded asilyl-donor-functionalized NHC complex (59) through acobalt-mediated CminusH activation and silylation protocol Theresulting complexes with strong electron-donating ligands dueto the presence of the silyl-metal-bound moieties afforded veryhigh activity in the hydrosilylation of 1-octene with PhSiH3also providing high selectivity toward the linear isomer product

232 Postfunctionalization by Coordination of MetalFragments The coordination of a metal fragment to a distantcoordination site attached to an NHC ligand may also be usedfor modification of the electronic and steric properties of the

Scheme 24 Preparation of Sulfonate-Functionalized NHCminusPd(II) Complexes and Catalytic Activity in MizorokiminusHeckCoupling in Water

Scheme 25 NHC Functionalization by Oxime Ligation Followed by Ligand Exchange

Scheme 26 Cobalt-Mediated Silylation of NHC Ligand



M

NHC ligand Some early examples of this strategy were used byPeris and co-workers83minus90 who used a simple 124-triazoldiylidene ligand to prepare a series of homo- andheterodimetallic complexes which were basically used for thedesign of tandem processes by the combination of orthogonalcatalytic cycles facilitated by each of the metal fragments boundto the di-NHC ligand (Figure 2) These results were reviewedin a recent article91 and for this reason we will not go into deepdetails about this research

An interesting finding derived from this research arose bycomputational study of variation of the Tolman electronicparameter (TEP) of the triazoldiylidene ligand as aconsequence of the modification of the metal fragmentbound to the other edge of the carbene ligand92 This variationis shown in Figure 3 which reflects how switching from thecoordination of [IrCp(CO)] to a [RhCl(CO)2] fragment isaccompanied by a positive shift of the TEP value of 45 cmminus1thus indicating an important decrease in electron-donatingpower of the ligand By choosing other metal fragments a fine-tuning of the electronic properties of the ligand can beobtained92

Cesar and Lavigne and co-workers93 used anotherambidentate Janus-type ligand which combined an N-heterocyclic carbene and an anionic imidate (60 in Chart 1)This bifunctional ligand could be bound to two different metalfragments and therefore as for all the examples described inthis section the electronic structure is assumed not to be an

intrinsic property of the ligand Other selected examples ofpolyfunctional carbene ligands that can be modified bycoordination of metal fragments are the ones described byWright and Gade and co-workers94 (61 and 62) Gates andStreubel and co-workers95 (63) Ruiz and Mesa96 (64)Bertrand and co-workers97 and Bellemin-Lapponaz and Cesarand co-workers98 However none of these species were testedin catalysis Some other examples are known but theirchemistry and catalytic properties were described in a previousreview article91

Ganter and co-workers also contributed to the developmentof NHC-based metal complexes that could be modified bycoordination of a second metal fragment by attaching acationic pentamethylcyclopentadienylruthenium fragment([RuCp]+) to a metal-coordinated benzoimidazolylideneligand99minus101 and then to a dimethylperimidinylidene complex(Figure 4)102 The TEP of the cationic species is shifted byapproximately 5 cmminus1 for both types of ligands as aconsequence of reduction of the electron-donating characterof the carbene upon attachment of the cationic metal fragmentIn line with these findings Poyatos and co-workers103 obtaineda pyrene-imidazolylidene ligand whose electronic propertieswere modified by 6 cmminus1 (based on TEP units) byintroduction of the same cationic ruthenium moietyA very interesting example in which the electronic properties

of a NHC ligand can be switched by metal coordination was

Figure 2 Triazoldiylidene complexes used for homogeneouslycatalyzed tandem processes

Figure 3 Modification of the TEP value of a triazoldiylidene ligand by coordination of different metal fragments

Chart 1 Selected Examples of Janus-type NHC ComplexesPostfunctionalized by Coordination



N

reported by Choudhury and co-workers in 2014104 Their ideawas to use a NHC-pyridyl ligand and use the labilecoordination abilities of the pyridyl group for the reversiblecoordination of different metal fragments They were successfulin reversibly coordinating [IrCpCl2] and [RuCl2(p-cymene)]fragments to the pyridyl moiety which could then bedecoordinated by adding triphenylphosphine (Scheme 27)104

The overall process constitutes a neat example of a pronouncedon-demand reversible modulation of the electronic propertiesof the main (NHC-coordinated) metal center

One of the very few examples in which the effects of remotecoordination of a metal to a preformed NHCminusmetal complexwere tested in catalysis was reported by Richeter and co-workers in 2013105 These authors prepared a NHC ligandfused to a porphyrin so that the coordination abilities of the

porphyrin could be used for the incorporation of a secondmetal Modification of the electronic properties of the carbeneligand was investigated by studying the modification of COstretching frequencies of the related [RhCl(porphyrin-NHC)-(CO)2] complexes (Chart 2) It was established that porphyrin-NHC ligands with Mn(III) and Al(III) were weaker electrondonors than the porphyrin-NHC ligand without any metalcation or with Ni(II) or Zn(II) (2minus3 cmminus1 shift of average CminusO stretching frequency of the related [RhCl(NHC)(CO)2]complexes) The authors studied the organocatalyzed ring-opening polymerization of L-lactide with the porphyrin-NHCand they observed that the properties of the catalyst wereaffected by the presence and nature of the metal cations boundto the porphyrin fragment105 It was assumed that thedifferences were due to subtle modifications of the electronicproperties of the NHC organocatalysts although some otherphenomena (such as aggregation of the porphyrin catalyst orcoordination of ligands to the metal in the inner part of theporphyrin) could also be at play By using the same porphyrin-NHC ligand the same authors obtained mono-NHC and bis-NHC complexes of Au(I) with and without a nickel atomcoordinated to the inner part of the porphyrin106 Themonoporphyrin-NHCminusAu(I)minusCl complex was tested in thephotooxidation of cholesterol where it showed higher activitythan the free ligand without the AuminusCl fragment This resultwas attributed to the ldquoheavy atomrdquo effect of Au(I) whichpromotes the intercrossing system S1 rarr T1 allowing formation

Figure 4 TEP variation by introduction of a [RuCp]+ fragment on a series of arene-functionalized NHCminusmetal complexes

Scheme 27 Reversible Coordination (Coordination Switch)of Metal Fragment to NHC-PyridylminusRu(II) Complex

Chart 2 Porphyrin-NHCminusRhodium Carbonyl Complexes and Average CminusO Stretching Frequencies



O

of 1O2 Other porphyrin-functionalized NHCs were alsoreported by Furuta and co-workers107 Wang et al108 andRuppert and co-workers109110

233 Proton-Responsive N-Heterocyclic CarbeneLigands Although natural chemical processes driven by pH-responsive systems are very common the number of examplesdescribing NHC-based catalysts using this type of strategy isvery scarce and they mostly refer to Ru(II)-catalyzed olefinmetathesis processes In this regard the first pH-responsiveRuminusNHC complexes used for olefin metathesis were describedby Schanz and co-workers in 2008111 These researchersprepared two Ru(II) complexes (73 and 74) each bearing amono-NHC ligand with two anchored NMe2 groups Theinitial idea is that these species may react with HCl to afford themono- and diprotonated species (73prime and 74prime) which showdifferent reactivity patterns related to the mother unprotonatedcomplexes and also are more soluble in aqueous media The

two catalysts were tested in the ROMP reaction of exo-7-oxanorbornene where they showed negligible activity Thesame two catalysts were tested in the RCM of diallylmalonicacid where they both produced moderate conversions (Scheme28) These results were unexpected as it was assumed thatacidic conditions should have accelerated the initiation rates ofthe metathesis reaction The authors interpreted thedeactivation of catalytic activity as a consequence of thelower electron-donating character of the NHC ligand uponprotonation111 The authors also performed a detailed study inwhich they proved that the activity in olefin metathesis can beexternally controlled with acid addition due to gradualprotonation of NMe2 groups of the NHC ligand and theconcomitant reduction of electron-donating power of the NHCligand which was also verified by density functional theory(DFT) calculations112

Scheme 28 Proton-Responsive RuminusNHC Complexes Used in Olefin Metathesis

Scheme 29 A GrubbsminusHoveyda Metathesis Catalyst Combining Two Proton-Responsive Ligands and Its Activity in TwoStandard Processes



P

Plenio and co-workers113 used a related Ru(II) GrubbsminusHoveyda catalyst with a NEt2-substituted NHC and comparedthe activity of neutral and protonated catalysts in the ROMP ofnorbornene Upon protonation the double-bond geometrychanged (from EZ = 078 to EZ = 104) and for this reasonthe authors concluded that addition of acid during thepolymerization reaction allows for fine-tuning the EZ ratioof the resulting norborneneMore recently Schanz and co-workers114 in line with their

previous works prepared two new pH-responsive Ru catalystseach containing a dimethylaminopyridine ligand (75 with twodifferent R groups as shown in Scheme 29) In organic solventsthese two catalysts gave good activities in the ROMP and RCMprocesses with standard substrates The ROMP reaction inbenzene was accelerated when 2 equiv (referred to the catalyst)of an acid (H3PO4) were added but the activity decreased assoon as the amount of acid increased probably as aconsequence of significant reduction of the electron-donatingcharacter of the carbene ligand upon protonation of the aminemoieties attached to the NHC The RCM reaction in acidicaqueous medium did not work probably due to catalystdegradation114

Some other proton-responsive NHC-based complexes weredescribed although their catalytic properties were notevaluated For example in 2009 Glorius and co-workers115

performed one of the very first studies on switching theelectron-donor power of a NHC ligand by reversibleprotonationdeprotonation of an enolate-imidazolylideneligand (Scheme 30) which was previously described by Cesar

and Lavigne and co-workers60 Upon coordination this ligandis known to stabilize the keto-imidazolylidene form asmentioned in section 231 The coordination of this NHC toform the corresponding [IrCl(NHC)(CO)2] complex (76)allowed for study of the variation of CminusO stretching frequencyand then to estimate the TEP shift produced by reversibledeprotonation of the ligand115 This deprotonation wasaccompanied by a TEP decrease of 14 cmminus1 thereforeindicating a very significant increase in electron-donatingpower of the NHC ligand The reversibility of the processadds significance to the potential of this class of NHC ligandsRicheter and co-workers116 also estimated the change in

electron-donating power of their porphyrin-NHC ligand uponprotonation by studying the shift in CminusO stretching frequencyof their related Rh(I) complex 67 (Chart 2) This protonationwas produced by use of trifluoroacetic acid and afforded therelated dicationic species in which all four nitrogens of theporphyrin are bound to a proton In this case the protonationwas accompanied by an increase in average CminusO stretchingfrequency of 7 cmminus1 as a consequence of the lower σ-donating

character of the complex resulting from the diprotonation of67116

Choudhury and co-workers104 also used their ruthenium(II)complex with a NHC-pyridyl ligand (65) as a proton-responsive system in which the electronic properties of theligand were reversibly modulated by addition of an acid or abase (Scheme 31)

3 MULTIFUNCTIONAL N-HETEROCYCLIC CARBENELIGANDS

As defined by Burrows in 20028 ldquoa multifunctional ligand issimply a ligand that has been designed to do more than onethingrdquo Crabtree9 made a more specific definition of theseligands by suggesting that multifunctional ligands are thosehaving additional functions that can be useful in catalysisAlthough these two definitions would include all the NHCligands described in section 2 we leave to this section only theexamples regarding the use of hemilabile ligands (section 31)and ligands that have a relevant participation in the catalyticcycle as the so-called bifunctional or cooperative ligands(section 32)117 Related to hemilabile NHCs ligands withflexible steric bulk (conformationally labile NHCs) will also beconsidered here Therefore while section 2 refers to ligandsthat adapt (or switch) by external stimuli their steric orelectronic properties to the requirements of the catalytictransformations multifunctional ligands as described in thissection will deal with all ligands whose reactivity depends ondirect interaction with the substrates and therefore are directlyinvolved in the catalytic cycle31 Hemilabile and Conformationally LabileN-Heterocyclic Carbene Ligands

The term hemilability in coordination ligands was introducedby Jeffrey and Rauchfuss in 1979118 A hemilabile ligand refersto a hybrid polydentate ligand where one of the coordinatinggroups can be displaced from the coordination sphere of themetal while at least one group remains firmly bound to themetal center119minus121 This means that one of the donor groupsneeds to not be strongly bonded so that it can reversiblydissociate in solution to open a coordination site Thereforehemilabile ligands yield the stability of a coordinative saturatedmetal complex while providing open coordination sites duringthe catalytic cycle In NHC-based hemilabile ligands the NHCpart of the ligand is assumed to be the edge that is stronglybound to the metal and therefore these types of ligands areoften heteroleptic (or hybrid) ligands with a weakly boundfunctionality which is often (but not exclusively) a N O or Sdonor group (Scheme 32) Although many different types ofdonor-functionalized NHC ligands have been described122minus124

in this section we will only discuss those examples in which thestudy of hemilability of the ligand is explicitly described and

Scheme 30 Deprotonation of Keto-imidazolylidene Ligandand TEP Change

Scheme 31 Reversible Protonation of Ruthenium Complexwith Pyridyl-NHC Ligand



Q

related to homogeneously catalyzed processes These exampleswill mostly refer to NHC hybrid ligands with N and O since Pis often considered to bind strongly to the metal center and thechemistry of NHC-S ligands has been thoroughly reviewedrecently125minus127 A detailed review on anionic tethered NHCchemistry was also published by Arnold and co-workers in2007128 and this included NHC-O hybrid ligands An excellentreview article regarding palladium complexes with NHC-basedhemilabile ligands129 has also been publishedConformationally labile ligands may also be considered as

multifunctional ligands in many ways and they can also beconsidered related to hemilabile ligands For this type ofsystem the steric hindrance imposed by the ligand can betuned depending on the requirements of a specific catalyticreaction Some key examples of this type of NHC ligand will beintroduced in section 315311 N-Heterocyclic Carbenes with N-Donors One of

the first NHC-based hemilabile ligands used in catalysis wasdescribed by McGuinness and Cavell in 2000130 These authorsprepared a series of NHC ligands containing pyridine andcarbonyl donors which were coordinated to palladiumaffording neutral and cationic complexes Depending on themetal precursors used and on the type of ligand the resultingPdminusNHC complexes displayed chelating or dangling donorsWhile NHCs with carbonyl donors afforded the monodentatecoordination of the ligands with dangling carbonyl function-alities the NHC-pyridine ligands exhibited monodentate andchelating behavior thus reflecting the hemilabile behavior ofthe NHC-pyridine ligand The complexes were tested in theHeck and SuzukiminusMiyaura couplings where the two mostactive catalysts were 78 and 79 (Scheme 33) Complex 78

achieved 947 000 turnovers in the coupling of butyl acrylatewith 4-bromoacetophenone (71 conversion) while in thesame reaction conditions 79 gave a turnover number (TON) of610 000 Both catalysts showed exceptional long-term stabilityIn the Suzuki coupling of 4-bromoacetophenone with phenyl-boronic acid TONs over 100 000 were achieved130

Chen and Lin131 studied the hemilability of a series ofpalladium(II) complexes with a bis(pyridyl)-NHC ligand Twoligands can bind to the metal in a bidentate chelating modeyielding complex 80 (Scheme 34) This complex is susceptibleto substitution by halides and phosphines yielding complexesthat contain both CN-bidentate and monodentate NHCligands Complex 80 shows fluxional behavior in coordinatingsolvents as a consequence of solvent-assisted exchangeinvolving unbound and coordinated pyridine Complex 80was active in the catalytic polymerization of COnorbor-nene131

Since the publication of these two early examples of pyridine-NHC hemilabile ligands in 2000130131 the number ofcomplexes containing NHC ligands with nitrogen donorgroups has increased enormously132 and these includeexamples of coordination to palladium133minus158 platinum154

rhodium122150minus152159minus169 iridium122161minus163165166170minus183

ruthenium179181182184minus193 nickel146192194minus197 silver198minus203

gold203 copper201202204 and iron152205 A selected group ofthese complexes is shown in Chart 3Despite the large number of complexes with NHC ligands

with nitrogen donors the number of examples in which thehemilability of these complexes has been related to theircatalytic properties is relatively scarce An interesting study wasdescribed by Jimenez Oro and co-workers in 2008167 Theauthors obtained a series of [RhCl(NHC)(COD)] complexesin which the NHCs contained aminoalkyl functionalities (99Scheme 35) Upon chloride abstraction with silver salts theaminopropyl-substituted NHC complex (n = 3 Scheme 35)undergoes CN-chelation by binding the NMe2 group to themetal This chelate coordination could not be achieved forother aminoalkyl-NHC ligands (ie n = 2) thus suggesting thatthe chelate coordination is only possible for an appropriate

Scheme 32 NHC-based Hemilabile Ligands

Scheme 33 Palladium(II) Complexes with NHC-Pyridyl Ligands in CminusC Coupling Reactions



R

linker length (n = 3) The complexes were tested inhydrosilylation of terminal alkynes where the neutralcomplexes were found to be the most active and selectivecatalysts167

Also in 2008 a series of rhodium and iridium complexes withquinoline-tethered hemilabile NHCs were obtained by Websterand Li and co-workers163 Interestingly the coordination to[IrCpCl2]2 rendered an equilibrium mixture of neutral and

cationic species (100 and 101 Scheme 36) The reaction of theligand (or its azolium precursor) with [MCl(CO)2]2 (M = Rhor Ir) afforded a mixture of chelating and monodentatecomplexes for the case of rhodium and only the chelatingcomplex for the case of iridium (Scheme 36)163

In the case of complex 91 (Chart 3) described by Crabtreeand co-workers179 the authors did not find evidence ofhemilability of the chelate NHC-pyrimidine ligand but the free

Scheme 34 Hemilability Shown by a Bis(pyridyl)-NHC Ligand

Chart 3 Some Examples of NHC Hemilabile Ligands with N-Donor Groups



S

pyrimidyl group acted as an internal base in a series ofborrowing-hydrogen processes (transfer hydrogenation β-alkylation of secondary alcohols with primary alcohols andN-alkylation of amines with primary alcohols) so that a mildbase such as NaHCO3 was sufficient to promote the process179

Messerle and co-workers206 also developed two NCN pincerligands that displayed diverse coordination abilities withrhodium(I) and iridium(I) (Chart 4 shows examples of therhodium complexes) It was found that increasing the length ofthe alkyl chain between the NHC and the pyrazole ringsincreased the lability of the pyrazole donors For example thecoordination of the ethyl-connected ligand is tridentate insolution but in the solid state it was found to be bidentate(106 Chart 4) The new complexes were tested in theintramolecular addition of NH and OH bonds where therhodium complex with the ethylene-connected ligand (106)was more active than the related one with a connectingmethylene (105) therefore indicating that the hemilabilebehavior of the ligand in 106 has a positive impact on itscatalytic activity206

In 2014 Pothig and Kuhn and co-workers172 published avery detailed study on the hemilabile behavior of a series ofiridium(I) complexes with N-functionalized NHC ligands of

the type NCN (Scheme 37) The ligand shows a dynamicfluxional behavior between the κ1C and κ2CN coordinationand this situation is not stopped even at temperatures as low asminus90 degC The authors related this behavior to the high catalyticactivity of these complexes in the reduction of acetophenone bytransfer hydrogenation Further analysis of the reactivity ofthese types of complexes demonstrated that addition of a ligandlike a phosphine produces substitution of the halide rather thandisplacement of the pyridine (109) which remains showingfluxionality On the other hand CO bubbling producesreplacement of the COD ligand and stabilization of the specieswith the chelating κ2CN coordination (110 in Scheme 37)172

A very recent work was published by Bertrand and co-workers207 describing the preparation of a series of hemilabilecyclic alkyl(amino)carbenes (CAACs) featuring alkene etheramine imine and phosphine groups These CAACs werecoordinated to Cu(I) and Au(I) The authors showed how thecarbene with a pendant imine could stabilize Au(III) andtherefore facilitate the CminusC oxidative addition of biphenyleneto the Au(I) complex with an imine-functionalized carbene(Scheme 38) The same gold(I) complex showed high activityin the hydroarylation of α-methylstyrene with NN-dimethylani-line The authors reasoned that the presence of the imine groupattached to the CAAC ligand should assist this catalytic reactionthat involves a proton transfer and for that reason the activityof complex 111 in the hydroarylation is higher than the activityshown by other related Au(I) complexes without the iminegroup207

312 N-Heterocyclic Carbenes with S-Donors Sincethe recent review published by Fliedel and Braunstein in2014127 very few S-functionalized NHC ligands have beenreported208209 An interesting example was described by Liuand Chen and co-workers209 who prepared a series ofpalladium(II) complexes with bidentate sulfoxide-NHC ligands

Scheme 35 Hemilabile NHC-Aminoalkyl Ligand

Scheme 36 Hemilabile Behavior of NHC-Quinoline Ligand



T

(113minus118 Scheme 39) These authors also studied thebehavior of an analogous thioether-functionalized NHCcomplex for comparison Although both types of ligandsshowed hemilability the sulfoxides were more labile than thethioethers The sulfoxide-NHC ligands exhibited variouscoordination forms including mono- and bidentate chelationsulfur- and oxygen-bound sulfoxide coordination and also the

presence of one or two ligands bound to the metal Thehemilability of the sulfoxide-NHC ligand is explained as acombination of intramolecular competition and intermolecularcompetition with solvent molecules as illustrated by theexample shown in Scheme 39

313 N-Heterocyclic Carbenes with O-Donors A largenumber of complexes with NHC ligands with oxygen donorgroups have also been extensively studied These may containalcohol210minus225 alkoxide193222minus227 phenoxide228minus235

ether166236minus238 sulfonate239minus243 or carboxylate244 groupsSome selected recent examples of this type of complexes aredepicted in Chart 5 With regard to their catalytic activity mostof the studies have provided evidence of the bifunctional (orcooperative) behavior of these complexes245 rather than theirhemilabile behavior Some selected examples related to thehemilabile behavior of these types of ligands are discussedbelow

Chart 4 Rhodium(I) Complexes with Bis(pyrazolyl)imidadolylidenes

Scheme 37 Hemilabile Behavior and Reactivity of Iridium(I) Complex with NHC-based NCN Ligands

Scheme 38 Oxidative Addition of Phenylene to Au(I)Complex with Imine-CAAC Ligand

Scheme 39 Hemilability Shown by Palladium Complexes with Sulfoxide-NHC Ligands



U

Albrecht and co-workers153 described in 2014 the prepara-tion of the pincer type NHC-carboxylate complex 121 Thecoordination and decoordination of the carboxylate arm of theligand is triggered by addition of a base or acid respectivelyThe bis-chelate or tris-chelate (pincer) palladium(II) com-plexes displayed distinct catalytic activities in SuzukiminusMiyauraand HeckminusMizoroki cross-coupling reactions for which thecoordinating carboxylate in 121 seemed to be beneficialcompared to the uncoordinated form153 Wang and co-workers235 used palladium complex 122 for the polymerizationof norbornene with methylaluminoxane (MAO) as cocatalystachieving excellent catalytic activities Braunstein and co-workers246 obtained a series of nickel complexes with ether-functionalized NHC ligands such as 123 which were active inthe polymerization of ethylene The activity of the relatedNi(II) complexes with unfunctionalized NHC ligands showedsimilar activity therefore indicating a reduced role of the etherside arms246 Palladium complex 127221 featuring an alcoholfunctional group was tested in the arylation of furan andthiophene where it showed moderate activity but excellentregioselectivity although the authors did not find clear evidenceof the benefits provided by the O-donor functionalityIglesias and Oro and co-workers247 prepared the 14-electron

Ir(III) complex 125 in 2012 The complex shows two latentcoordination sites which are accessible under catalyticconditions The complex was tested in hydrosilylation ofterminal alkynes affording very good selectivity in theformation of the β-(Z)-vinylsilanes This selectivity was firstexplained by a modified ChalkminusHarrod mechanism taking

place248 in which a metal-assisted isomerization of the metal-alkenylsilane should be produced in the final steps of thecatalytic cycle (see Scheme 40) and all this can be explained bythe generation of a cavity by stepwise dissociation of the sidearms of the ligand247 However the possibility of this classicalmechanism was discarded soon after basically because DFTcalculations suggested that oxidative addition of the SiminusH bondto the rhodium or iridium center to afford a M(V) species wasunfeasible and also because alkynes did not react with 125 orwith its rhodium analogue249 On this basis an outer-spheremechanism was proposed where again the cavity generated bydecoordination of the side arms favors formation of the β-(Z)-vinylsilanes Further modification of these type of catalysts bythe same authors provided a more detailed insight into the roleof hemilabile ligands in this catalytic reaction250 The influenceof these hemilabile ligands was also examined by the sameauthors by studying the behavior of catalysts in transferhydrogenation of ketones and imines251 and in hydrolysis andmethanolysis of silanes252

Braunstein and co-workers225 described the hemilabilebehavior of alkoxide-functionalized NHC ligands coordinatedto nickel and palladium (Chart 6) The authors were able toselectively deprotonate and coordinate or reprotonate anddecoordinate the alkoxide side arm Dinickel complex 132 wastested in oligomerization of ethylene where it showed higheractivity than other Ni(II) complexes with unfunctionalizedNHC ligands The authors interpreted this result as aconsequence of the increased lifetime of the catalyst probably

Chart 5 Some Recent Examples of Complexes Bearing NHC Ligands with Oxygen Donor Groups



V

as a consequence of the presence of the CO chelate or bridgingligandZhou and Jordan239 prepared a palladium complex with a

sulfonate-NHC ligand whose reactivity is dominated by thedissociation or displacement of the sulfonate functionality(Scheme 41) They studied the isomerization betweencomplexes 133 and 134 and concluded that the reactionshould proceed by dissociation of the sulfonate unit to form athree-coordinate intermediate (Scheme 40) The reaction isaccelerated in solvents with hydrogen-bond donors (CD3OD)and by the addition of Lewis acids [B(C6F5)3] that can labilizethe sulfonate groupIn a very recent work Ruddlesden and Ducket253 described

an iridium(I) complex bearing a phenolate-NHC ligand (135)which was a very effective precatalyst in the SABRE (signal

amplification by reversible exchange) process (Scheme 42)The complex contains an IrminusO bond which is sensitive to thepolarity of the solvent and for this reason the catalytic activityof the complex is solvent-dependent In nonprotic solvents thisbond is strong and reacts with pyridine and H2 to form theIr(III) complex 136 In protic solvents such as methanol theIrminusO bond is labile and the reaction with pyridine andmolecular hydrogen yields complex 137 Both species haveinequivalent hydrides (a requisite for acting as SABREcatalysts) and are active in the catalytic transfer of polarizationfrom para-hydrogen to loosely bound ligands such as pyridinenicotinaldehyde and nicotinePapish and co-workers254 published a study aiming to

determine which is the role of hemilability of the etherfunctionality in the transfer hydrogenation of ketones using(arene)Ru(II) complexes with an ether-functionalized NHCligand In their study the authors did not find any evidence ofO binding to the metal and they concluded that for this type ofcomplex [(η6-arene)Ru(II)] arene lability has a very importantimpact on the activity This conclusion was supported by thefact that the trend of activity of the complexes was related tolability of the arene ligands in the order benzene gt p-cymene gthexamethylbenzene254

314 N-Heterocyclic Carbenes with Alkenyl andOther C-Donors Alkenyl-NHCs provide an interesting typeof hemilabile ligand with potential applications in homoge-neous catalysis The first example of coordination of this type ofligand was described by Oro and Hahn and co-workers255256

who described a series of iridium(I) complexes with N-allyl-substituted benzimidazolylidenes256 and imidazolylidenes(Scheme 43)255 By reaction of an alkenyl-imidazolium saltwith [Ir(μ-OMe)(COD)]2 the authors obtained the five-coordinate complex 138 in which one of the coordination sitesis occupied by the alkenyl side arm Halide abstraction withAgBF4 yields the cationic complex 139 with the two alkenylarms bound to the metal256 Reaction of 138 with NaOEt inethanol affords the tetracoordinated complex 140 in which thetwo allyl fragments were hydrogenated to generate two n-propyl chains The activity of 138 and 140 was tested in the

Scheme 40 Proposed Mechanism for Hydrosilylation ofTerminal Alkynes with Complex 125

Chart 6 Complexes of Nickel and Palladium with Alkoxide-Functionalized NHCs

Scheme 41 Isomerization of Palladium Complex withTethered Sulfonate-NHC Ligand



W

reduction of cyclohexanone by transfer hydrogenation iniPrOH (KOH was used as base) where 140 showed muchhigher activity This was interpreted by the authors as a factorindicating that the allyl substituents remain unaffected along thecatalytic reaction and therefore the availability of the vacantcoordination site is reduced in 138 thus providing a less activecatalystInspired by these results Peris and co-workers257 reported a

series of CpIr(III) with N-alkenyl-imidazolylidene ligandsRather than aiming to obtain a hemilabile system in which theolefin arm of the NHC ligand could reversibly decoordinatecoordinate to the metal the authors aimed to irreversibly cleave

the olefinminusmetal bond under conditions used in a catalyticreaction (reduction of ketones and imines by transferhydrogenation) and for that reason this type of ligand wasreferred to as hemicleavable It was found that the chelatingcoordination of the alkenyl-imidazolylidene could only beachieved when short linkers between the NHC and the olefinwere present (as in 141 in Scheme 44) while longer linkersproduced monocoordination of the ligand via the NHC partThe chelate carbene-alkenyl complexes showed very goodactivity in the reduction of ketones and imines by transferhydrogenation It was proposed that under the reactionconditions used for the catalytic reaction (iPrOH 80 degC andKOH) the catalyst was activated by irreversible hydrogenationof the coordinated alkene generating a N-alkyl fragment(142)257

In 2008 Peris and co-workers258 described a series of Ir(I)complexes with alkenyl-functionalized ligands These com-plexes were similar to those previously reported by Hahn andOro and co-workers255 (in their case the complexes containedbromide ligands instead of chlorides) Complexes 143 (Scheme45) showed fluxional behavior due to the coordinationdecoordination of the alkene with the iridium center258 Bystudying this fluxional behavior by variable-temperature (VT)NMR spectroscopy the authors were able to experimentallyestimate that the Irminusolefin bond energy was ΔH = 209 kcalmol for 143 with X = H Complexes 143 were active in thehydrosilylation of terminal alkynes where they proved to bevery selective in the production of Z-vinylsilanes258

Alkenyl-imidazolylidene ligands were also coordinated in thechelate fashion to CpNi(II) complexes259 but the catalyticactivities of the resulting complexes were not reported Hornand Albrecht260 described a (p-cymene)Ru(II) complex with ahemilabile alkenyl-NHC ligand (145) which showed high

Scheme 42 Solvent-Dependent Behavior of Iridium Complex with NHC-Phenolate Ligand

Scheme 43 Iridium Complexes with Allyl-SubstitutedBenzimidazolylidenes

Scheme 44 Hemicleavable NHC-Alkenyl Ligands bound to CpIr(III)



X

activity in reduction of unfunctionalized olefins by transferhydrogenation In their work the authors compared the activityof several catalyst precursors displaying NHC-based hemilabileligands The catalysts showed strongly diverging behavior in thehydrogenation of dodecene to dodecane with 146 beingcompletely inactive and 145 being the most active one (Scheme46) Catalyst 145 was tested in the hydrogenation of a wide setof olefins where it also showed activity in the olefinisomerization as a competitive process260 In a subsequentstudy the same set of complexes shown in Scheme 46 weretested in the transfer hydrogenation of ketones and activatedolefins261 Again the complex with the tethered olefin was themost active among the four different catalysts Depending onreaction conditions 145 was also able to convert nitriles andαβ-unsaturated ketonesA very interesting example of hemilability was described by

Bullock and co-workers in 2007262 who obtained hydrides ofthe type [CpM(CO)2(IMes)H] [M = W Mo IMes =13-bis(246-trimethylphenyl)imidazol-2-ylidene] Hydride abstrac-tion from these complexes leads to [CpM(CO)2(IMes)]+ inwhich one of the CC double bonds of one of the mesitylgroups binds the metal Ketones and alcohols can displace thisweak coordinated CC group The tungsten complex showedmoderate activity in hydrogenation of acetone with H2

262

N-Arylated NHCs have also shown hemilabile behavior inCpIr(NHC) complexes as in the example reported by Perisand co-workers in 2006263 The reaction of 149 in refluxing

CD3OD afforded deuteration at the ortho position of themetalated phenyl ring in 24 h thus strongly suggesting that adynamic equilibrium with the ldquodemetalatedrdquo complex 150occurs (Scheme 47)

315 Conformationally Labile N-Heterocyclic Car-benes Conformationally labile NHCs are ligands with flexiblesteric bulk so that the steric hindrance imposed by the ligandcan be tuned depending on the requirements of a specificcatalytic reaction These types of ligands may be considered asmultifunctional as they may look somehow related tohemilabile ligands The firsts to apply this concept in catalysiswere Glorius and co-workers264 who developed an imidazo-lylidene with flexible cyclohexyl groups which is expected toexist in the form of three different conformers (Chart 7) Uponcoordination to Pd conformation a is expected to favoroxidative additions on Pd(0) while the more stericallydemanding conformers (b and c) should enhance reductiveelimination and facilitate the formation of Pd monocarbenespecies These ligands were used for the preparation ofpalladium catalysts [in situ from Pd(OAc)2] which wereshown to be efficient for room-temperature SuzukiminusMiyauracoupling of hindered and unhindered activated and deactivatedaryl chlorides and arylboronic acids Soon after the same groupdeveloped a series of NHCs derived from bioxazolines withlarger flexible cycloalkyl groups (structure d) which were alsoapplied for the preparation of effective palladium catalysts forthe SuzukiminusMiyaura coupling Remarkably the catalystsderived from these ligands were able to catalyze the productionof tetra-ortho-substituted biaryls from aryl chlorides265

Scheme 45 Alkenyl-NHC Complexes of Ir(I) inHydrosilylation of Terminal Alkynes

Scheme 46 (p-Cymene)Ru(II) Complexes with Hemilabile NHC-based Ligands Used in Reduction of Dodecene

Scheme 47 Hemilability Shown by Cyclometalated NHC-Phenyl Ligand Coordinated to CpIr(III)



Y

In the same line Dorta and co-workers266 described a familyof imidazolin-2-ylidenes functionalized with naphthyl units(structure e Chart 7) Interconversion of the two possibleatropisomers [C2-symmetric (anti) and Cs-symmetric (syn)]was studied in detail by 1H NMR The palladium-derivedcomplexes showed superior activity compared to otheranalogous palladium complexes without the bulky substituentsSome other recent relevant examples of large yet flexible

NHC ligands were reported by Nolan and co-workers267268

(Pd for SuzukiminusMiyaura and BuchwaldminusHartwig reactionsAu for alkyne hydration nitrile hydration and synthesis ofhomoallylic ketones) and Mauduit and co-workers269 (Ru forolefin metathesis)

32 Bifunctional N-Heterocyclic Carbene Ligands

Bifunctional catalysts operate by establishing control of acatalytic process through multiple weak catalystminussubstrateinteractions117270minus272 and by enabling a cooperative effect thatis translated into a superior ability for substrate activation andselectivity273274 These weak interactions are of the non-covalent type (mostly due to hydrogen bonding or π-stacking)thus bifunctional catalysis can also be regarded within the fieldof supramolecular catalysis275276 A recent review was fullydevoted to bifunctional catalysis with transition metal-basedNHC complexes245 More specific review articles werepublished dealing with the bifunctional behavior of proticNHC complexes277 and NHC complexes bearing NH units278

so the field has been covered in detail until 2016 With thesetaken into account the present review will cover only selectedexamples Section 321 deals with bifunctional processesinvolving the participation of NH OH or CH bonds of theligand and section 322 deals with bifunctional processesthrough aromatizationdearomatization Finally Section 323will describe the most recent advances on the use of NHCligands with rigid polyaromatic fragments and how π-stackinginteractions influence the catalytic properties of the relatedcatalysts321 Bifunctional Processes Involving the Participa-

tion of CminusH OminusH or NminusH Bonds of the Ligand3211 Ligand CminusH Bonds A recent example of metalminusligandcooperativity was described by Mashima and co-workers279

who showed how the hemilability of a N-xylyl-Nprime-methylper-imidine carbene bound to an iridium catalyst was crucial in thedehydrogenative silylation of arylpyridines with triethylsilane(Scheme 48) The precatalyst used was the Ir(I) complex 151The authors observed that intramolecular CminusH activation ofthe N-xylyl group of the perimidine-based NHC efficientlyimproved the catalytic reaction On the basis of their

experimental findings the authors suggested that during thecatalytic cycle the carbene ligand changed its steric andelectronic properties The cyclometalated NHC acted ashydride acceptor returning to the monodentate NHC thusshowing the utility of the metalated xylyl group as a labilecoordination site of hemilabile ligation (see the equilibriumbetween 153 and 154) In their work Mashima and co-workers279 proposed a catalytic cycle from which they wereable to isolate and characterize a number of intermediatesScheme 48 shows the proposed catalytic cycle displaying onlysome selected reaction intermediates

3212 Ligand OminusH Bonds Iridium complexes withtethered O-donor functionalities have been used as bifunctionalcatalysts for the condensation of amines with alcohols withvery relevant works published by Martin-Matute and co-workers222minus224 In their most recent work these authorsperformed a detailed study on mechanistic aspects of the

Chart 7

Scheme 48 Proposed Mechanism for Iridium-CatalyzedSilylation of Arylpyridines



Z

reaction by combining experimental and computationalmethods222 From their study the authors concluded that thealcohol-functionalized ligand participates in almost all steps ofthe catalytic cycle by accepting or releasing protons tosubstrates and intermediates as shown in Scheme 49 The

alcohol functionality also facilitates the reaction by hydrogen-bonding to substrates The bifunctional behavior of this iridiumcatalyst allows the reaction to be performed under base-freeconditions with a 11 ratio of the substrates (no excess of anysubstrate is needed) and under very mild reaction conditions

(T = 50 degC) By starting from the catalyst precursor 119 (Chart5) the catalyst resting state was formed (156 Scheme 49) andcharacterized in situAn interesting example of the acceptorless dehydrogenation

of alcohols to aldehydes and dehydrogenative coupling ofalcohols with amines was described by Bera and co-workers in2014227 using the diruthenium complex 126 (Chart 5) ascatalyst The authors performed a detailed study by usingexperimental and computational methods concluding thatmetalminusmetal and metalminusligand cooperation are crucial in thedehydrogenative coupling of amines and alcohols to afford thecorresponding imines They also found that the presence of theacetate bridging ligand plays an important role by facilitatingthe occurrence of β-elimination227 Scheme 50 shows theproposed mechanism for the process

3213 Ligand NminusH Bonds Morris and co-work-ers189minus192280minus282 have been very active in the research ofNHC-based bifunctional catalysts They prepared series ofruthenium and iridium complexes with NHC ligands tetheredwith amine donors The activity of the cationic [Ru(p-cymene)(NHC-NH2)] complexes 97 (Chart 3) and 162 was studied intransfer hydrogenation of ketones to afford the correspondingalcohols190191 Both experimental and theoretical (DFT)studies concluded that reduction of acetophenone occurredthrough an inner-sphere mechanism involving decoordinationof the amine group of the NHC-NH2 ligand therefore thisligand would only be behaving as hemilabile not exchangingthe hydrogen of the amine with either of the substrates282 Thismechanism was proved to be more feasible than the outer-sphere bifunctional mechanism which should involve thetransfer of a H+Hminus couple to acetophenone from the hydrideamine complex The authors explained that the cationichydride-amine complex 163 (Scheme 51) reacted very slowlywith acetophenone in the absence of base due to the decreasedhydricity of the hydride ligand thus making the outer-spheremechanism unfavored190191

The results are different when the [RuCp(NHC-NH2)-(pyr)](PF6) complex 164 is used This complex can beactivated in the presence of an alkoxide base to form a neutralhydride-amine complex (165) In this case the hydridiccharacter of the neutral metal hydride is expected to favorthe outer-sphere bifunctional mechanism for reduction ofketones On the basis of experimental and theoretical (DFT)

Scheme 49 Proposed Mechanism for Coupling of Aminesand Alcohols with Iridium Catalyst Containing NHC-Alcohol Ligand

Scheme 50 Proposed Mechanism for Dehydrogenative Coupling of Alcohols and Amines by Diruthenium Complex 126



AA

studies Morris and co-workers189 proposed the mechanismshown in Scheme 52 for the reduction of ketones with H2 Onthe other hand theoretical calculations predicted that the outer-sphere mechanism is not favored when the related cationiciridium(III) hydride amine complex [IrCp(NHC-NH2)H]

+ isused due to the poor nucleophilicity of this cationic hydride189

Nevertheless the authors showed evidence that this complex inthe presence of excess alkoxide base hydrogenates ketones viaan outer-sphere mechanism involving a neutral hydrideintermediate [Ir(η4-CpH)(NHC-NH2)H] which was formedby migration of a hydride ligand to the Cp ligand a processthat seems to be favored by the presence of the N-heterocycliccarbene ligand190

In a more recent report the [RuCp(NHC-NH2)(pyr)]-(PF6) complex 164 was used for hydrogenation of esters withH2

281 The authors proved that the reaction occurred through aconcerted bifunctional outer-sphere mechanism via formationof a six-membered ring transition state involving hydrogen-bonding interaction of the carbonyl oxygen of the ester and theNminusH group of the NHC-NH2 ligand The same NHC-NH2ligand was used to prepare the iridium(I) complex [Ir(NHC-NH2)(COD)](PF6) (166) (Scheme 53) Upon oxidativeaddition with HCl the iridium(III) hydride 167 was formedThis complex proved to be very active in the H2 hydrogenationof molecules with unsaturated bonds (TOF gt 800 hminus1 forhydrogenation of acetophenone) although in this case theauthors did not perform mechanistic studies in order toelucidate if the process proceeded via a bifunctional outer-sphere mechanism280

Hahn and co-workers283minus285 developed a series of metalcomplexes with protic NHC ligands (pNHCs) It was proposedthat the NH function of the pNHC ligands may act as adirecting group for formation of hydrogen bonds to selectedsubstrates286287 The NH group of pNHC ligands is known todirectly participate in some catalytic events For example

Grotjahn and co-workers288 showed that the Ru(II) complex168 displaying a pNHC-PPh2 ligand was able to quantitatively

Scheme 51 Inner-Sphere versus Outer-Sphere Mechanisms for Ru(II) Precatalysts with NHC-Amine Ligands

Scheme 52 Alcohol-Assisted Outer-Sphere BifunctionalMechanism with [RuCp(NHC-NH2)H] Catalyst



AB

hydrogenate acetophenone in the presence of iPrOH underbase-free conditions (Scheme 54) By carefully studying thechemical reactivity of the pNHC ligand the authors concludedthat the complex may reduce ketones via a bifunctional outer-sphere mechanism involving the participation of the NH group

Grotjahn and co-workers289 described 169 (Chart 8) aCpIr(III) hydride with a pNHC-PPh2 chelate ligand Theauthors carefully studied the reactivity of the NH bond andsuggested that secondary interactions involving this groupshould provide benefits in homogeneously catalyzed processesby means of the bifunctional character of the ligand Similarstudies were also performed by Hahn and co-workers290 withcomplex 170 bearing a protic benzimidazolylidene ligand

Ghosh and co-workers210291minus293 developed a series of Ni(II)complexes bearing amido-functionalized NHCs (171) for base-free Michael addition of β-dicarbonyls β-keto esters and α-cyano esters with activated ethylenes The fact that the authorswere able to perform the reaction in the absence of base isremarkable Supported by DFT calculations it was proposedthat the rate-limiting-step of the reaction was the nucleophilicattack of a metal-bound enolate of a 13-dicarbonyl adduct tothe approaching activated ethylene291 The reaction pathwayinvolves first the protonation and decoordination of the basicamide function of the NHC-amido ligand by the substrate (2-acetylcyclopentanone in Scheme 55) thus indicating abifunctional process with active participation of the amido

group in the process Then nucleophilic attack of the boundenolate on activated olefin and release of the final productwould complete the cycle The authors were also able to studythe asymmetric version of the reaction by using NHC-amidoligands with chiral ancillary substituents With these catalystschiral inductions ranging between 2 and 75 enantiomericexcess (ee) values were obtained294

By using a N-heterocyclic carbene functionalized with anaphthyridyl group Bera and co-workers295 prepared a Rh(I)complex (172) that showed excellent catalytic activity in thehydration of a wide variety of organonitriles to form thecorresponding organoamides at room temperature A maximumturnover frequency of 20 000 hminus1 was achieved for hydration ofacrylonitrile The presence of the 18-naphthyridyl group iscrucial for the process because an analogous Rh(I) complexwithout the naphthyridyl group [RhBr(COD)(Me2-NHC)]Me2-NHC = 13-dimethylimidazol-2-ylidene turned out to beinactive The proposed mechanism of the catalytic reaction(Scheme 56) involves the participation of the pendantnaphthyridine by forming a double hydrogen bond with amolecule of H2O which provides significant entropic advantageto the hydration process By DFT calculations the authorscalculated the transition state of the process (173 TS)revealing proton movement from water to the naphthyridinenitrogen with a complementary interaction between theoxygen and the nitrile carbon295 The work constitutes a clearexample of bifunctional water activation and cooperative protonmigration as the key steps of the catalytic cycleIn a very recent work Hahn and co-workers296 described

iridium(III) complexes bearing a chelating bis(NHC) com-posed of a classic NHC and an anionic imidazolylidato donorwhich were moderately active in reduction of various imineswith hydrogen The authors hypothesized that the complexesshould act as bifunctional catalysts by initially forming a chelateCNHCminusCpNHC ligand

322 Bifunctional Processes through AromatizationDearomatization Several pincer-type complexes based onpyridine are known to display a mode of metalminusligand

Scheme 53 Hydrogenation of Acetophenone with[IrCl(H)(NHC-NH2)(COD)](PF6)

Scheme 54 Base-Free Hydrogenation of Acetophenone withRu(II) Complex with pNHC-PPh2 Ligand

Chart 8 Two CpIr(III) Complexes with pNHC-PPh2Chelate Ligands

Scheme 55 Bifunctional Pathway Proposed for MichaelAddition Facilitated by Ni(II) Complex with NHC-AmidoLigand



AC

cooperation involving aromatizationdearomatization of thecentral pyridine ring The concept was pioneered and

developed by Milstein and co-workers297minus299 This bifunctionalbehavior occurs without formal change of the metal oxidation

Scheme 56 Bifunctional Mechanism for Hydration of Organonitriles by Rhodium(I) Complex with NHC-Naphthyridyl Ligand

Scheme 57 Activation of Nitriles by Lutidine-Derived Bis(NHC) Ruthenium(II) CNC Pincer Complexes



AD

state and has been widely used for reactions involvingtransformations of amines and alcoholsIn a recent contribution Pidko and co-workers300 described

the preparation of several lutidine-derived bis(NHC)ruthenium(II) CNC pincer complexes (174) In the presenceof phosphazene base these complexes are capable of activatingnitriles to give ketimino compounds (175 Scheme 57)Compound 175 reacts with strong bases to yield adearomatized complex (176) which reacts with H2 to formthe dihydride 177300

Complex 176 was found to be very reactive in the activationof CO2 and H2 although it leads to a stable compound (178)that is not catalytically competent (Scheme 58)301 On the

other hand the bis(hydrido) complex 177 is very active inhydrogenation of CO2 to formate although DFT calculationssuggest that the mechanism does not involve metalminusligandcooperative transformations (Scheme 59)301 The reactionneeds to use H2CO2 ratio of 391 to avoid deactivation of theprocess by formation of 178The bipyridine-NHCminusRu(II) complex 179 was described by

Milstein and co-workers in 2011302 In the presence ofpotassium bis(trimethylsilyl)amide (KHMDS) this complexforms the dearomatized complex 180 (Scheme 60) The in situprepared 180 was tested in the hydrogenation of nonactivatedesters under mild reaction conditions where it proved to be

one of the most active catalysts reported to date302 Forexample under 50 atm pressure of H2 hydrogenation of ethylbenzoate resulted in a high TON of 2840 with almostquantitative formation of benzyl alcohol and ethanolA similar dearomatization process was observed by Song and

co-workers303 when they reacted their Ru-CNN pincercomplex 181 with KHMDS The resulting dearomatizedproduct 182 reacts with H2 to form the ruthenium(II)dihydride 183 Interestingly the experiments carried out withD2 demonstrated that both pincer arms participate in the H2activation thus producing the incorporation of six deuteriumatoms into the molecule as shown in Scheme 61 Complex 181catalyzes the hydrogenation of unactivated esters under mildreaction conditions303

323 Bifunctional N-Heterocyclic Carbene Ligandswith Rigid Polyaromatic Functionalities HomogeneousCatalysis and π-Stacking Noncovalent interactions such asπ-stacking may play an important role in homogeneouslycatalyzed reactions This type of effect which may have beenunrecognized for other systems was systematically studied forthe case of NHC-based metal catalystsClear evidence of the influences of π-stacking and catalysis

was observed by Grela and co-workers back in 2008304

(Scheme 62) These authors observed that the use offluorinated aromatic solvents was beneficial in olefin metathesisreactions catalyzed by Ru Grubbs-type catalysts with a SIMesligand [SIMes = 13-bis(246-trimethylphenyl)-45-dihydroimi-dazol-2-ylidene] On the basis of their studies theyhypothesized that πminusπ stacking interactions between thefluoroaromatic solvent and the N-aromatic substituent of theNHC ligand may enhance the stability of the 14e rutheniumactive species as shown by DFT calculations305 Several authorshad previously suggested that πminusπ stacking interactions shouldplay a role in Ru-catalyzed metathesis reactions306minus309

The π-stacking interaction between aromatic substrates andthe ligand of a catalyst may be boosted if the ligand isfunctionalized with a rigid polyaromatic functionality Theimportant catalytic consequences of using NHC ligandsdecorated with rigid polyaromatic fragments in homogeneouscatalysis were recently pointed out by Peris310 according toresearch mostly developed in his group The first clear evidenceof the participation of π-stacking interactions between aromaticsubstrates and a polyaromatic-based NHC ligand wereobserved for the triphenylene-tris(imidazolylidene)-basedpalladium and gold catalysts 184 and 185 (Scheme 63)311

The studies were performed by comparing the catalyticefficiency of these catalysts in three different reactions(palladium-catalyzed α-arylation of propiophenone with arylbromides SuzukiminusMiyaura coupling between aryl bromidesand arylboronic acids and gold-catalyzed hydroamination ofphenylacetylene) For all three reactions tested the catalytic

Scheme 58 Activation of CO2 by Lutidine-DerivedBis(NHC) Ruthenium(II) CNC Pincer Complex

Scheme 59 Reduction of CO2 to Formate by Lutidine-Derived Bis(NHC) Ruthenium(II) CNC Pincer Complex

Scheme 60 Dearomatization of Bipyridine-NHCminusRu(II)Complex and Activity in Hydrogenation of Esters



AE

activity of 184 and 185 was higher than the activity shown bymodel trimetallic and monometallic complexes formally havingthe same stereoelectronic properties but without polyaromatic

functionalities The higher activity shown by these twocomplexes was hypothesized to be a consequence of the π-stacking interaction between the triphenylene core of thecatalysts and the aromatic substrates which should have aneffect on the catalytic performance of the catalysts The fact thataddition of catalytic amounts of π-stacking additives such aspyrene or hexafluorobenzene produced partial inhibition of theactivity of 184 and 185 was one of the main supports of thishypothesisIn line with these findings several other complexes displaying

polyaromatic NHC-based complexes were obtained and theircatalytic activities were evaluated312313 An interesting studyresulted from comparing the activity of a series of dipalladiumcomplexes bridged by bis(imidazolylidenes) with phenylene orbiphenylene spacers (186minus188 Scheme 64)313 The N-substituents were methyl or methylpyrene groups The resultsin SuzukiminusMiyaura coupling between aryl halides andarylboronic acids revealed that all complexes with pyrene tagsdisplayed higher activity than those with methyl groups Theaddition of a catalytic amount of pyrene (or anthracene) to thereactions resulted in partial inhibition of the activities of the

Scheme 61 Dearomatization and Reactivity of Ru(II) CNN Pincer Complex

Scheme 62 Beneficial Effect of Fluorinated Solvents inActivity of Catalysts in Olefin Metathesis

Scheme 63 Triphenylene-based NHC Complexes of Palladium and Gold and Catalytic Reactions Tested



AF

pyrene-containing complexes while the activity of the complexwith the N-methyl groups was unaffected The same effect wasobserved when the activities of monometallic complexes 189and 190 were studied Again the addition of pyrene produced apartial inhibition of activity of the pyrene-containing complex(189) while not affecting the activity of the methyl-substitutedcatalyst (190)313

Preparation of a series of mono-NHC ligands decorated withpolyaromatic functionalities allowed the study of the influenceof π-stacking additives on electronic properties of NHC ligandsThe electrochemical studies on a series of [NiClCp(NHC)]complexes (NHC = imidazolylidene benzimidazolylidene andpyrene-imidazolylidene for 191minus193 respectively) revealedinteresting results when the three complexes were titrated witha π-stacking additive such as pyrene314 While the imidazoly-lidene and benzimidazolylidene nickel complexes wererelatively insensitive to the addition of pyrene (maximumpositive shift of the half-wave potential observed ΔE12 = 20minus30 mV) the addition of pyrene to the pyrene-imidazolylidenecomplex of nickel (193) produced a maximum positive shift inpotential of 75 mV (Scheme 65) This finding demonstrated

that while all three complexes were sensitive to the addition ofpyrene the complex with the pyrene-imidazolylidene ligandwas the one to show a more pronounced effect most likely dueto π-stacking between added pyrene and the pyrene fragment ofthe ligand314

In order to experimentally quantify the modification ofelectron-donating character of NHC ligands with extendedpolyaromatic fragments a series of [IrCl(NHC)(CO)2]complexes were obtained (Scheme 66)315 The study wascarried out by evaluating the variation of CminusO stretching

frequencies in IR spectra of the complexes upon addition ofpyrene or hexafluorobenzene While the variations werenegligible for the imidazolylidene and benzimidazolylidenecomplexes (194 and 195 respectively) the rest of thecompounds with extended polyaromatic fragments revealedsignificant variations of the CminusO stretching frequencies In allcases the addition of hexafluorobenzene produced a decrease infrequency while the addition of pyrene produced the oppositeeffect The maximum variation was 29 cmminus1 for the case of theiridium complex with the pyrene-imidazolylidene (198) Theseresults revealed that the electronic character of the ligand maybe postmodified by adding a suitable π-stacking additive315

The iridium complexes displayed in Scheme 67 were testedin the reduction of ketones by transfer hydrogenation and inthe β-alkylation of secondary alcohols with primary alcoholsThe study of the reactions revealed the important role ofpyrene tags in the catalysts316 In the reduction of ketoneswhen pyrene-tagged catalysts (199 or 200) and aromaticsubstrates were used the catalytic activity is inhibited byaddition of an external catalytic amount of pyrene Othercombinations of catalysts and substrates do not produce anyvariation in the catalytic outcome of the reaction Detailedstudy of the coupling of secondary alcohols with primaryalcohols revealed important findings about the mechanism ofthe reaction When pyrene-tagged catalysts and aromaticsubstrates were used the reaction followed a zeroth-orderdependence relative to concentration of the substrates Allother combinations afforded a second-order rate Study of thereaction orders with respect to concentration of the catalystsrevealed that the catalysts with a pyrene tag (199 and 200)showed a fractional (lt1) reaction order with respect to thecatalyst while the catalyst without the pyrene functionality(201) showed a reaction order of 1 This result is a clearconsequence of the self-association of pyrene-containingcatalysts All these observations clearly revealed that pyrene-containing catalysts are able to engage in noncovalentinteractions with aromatic molecules which can either besubstrates in a homogeneous catalyzed reaction or the samemolecule of catalyst to afford self-assembled systems316

4 CONCLUSIONS AND OUTLOOKThis review article exemplifies how the role of NHC ligandsgoes far beyond the traditional role of a common spectatorligand The variety of structures awaiting design is limitless andthis places NHC ligands in an advantageous position comparedto other traditional ligands used in homogeneous catalysisNow a number of examples of photoswitchable redox-switchable chemoswitchable hemilabile and bifunctionalNHC ligands are known Most of the examples describedhere refer to types of ligand adaptability that have beenpreviously described for other types of ligands For these casesNHCs have provided new perspectives and new reactivitypatterns that allowed discovery of novel catalytic singularitiesInterestingly NHCs have also provided new types ofadaptability in the case of bifunctional ligands with poly-aromatic functionalities It is probably the special topologicalfeatures of NHCs (mostly their fan-shaped structure) and alsothe easy access to structures with rigid polyaromatic systemsthat have allowed the discovery that π-stacking interactions mayhave important consequences in future catalyst design Thiseffect may also operate for other types of ligands although thelack of a systematic approach probably due to syntheticlimitations has left it unrecognized

Scheme 64 Comparison of Activity of Dimetallic andMonometallic Complexes of Palladium with and withoutPyrene Tags

Scheme 65 Effect of Pyrene Addition on Redox Potential ofThree NHC Complexes of Nickel(II)



AG

If we wanted to schematically illustrate the maturity of thefield of smart NHCs it could be useful to use the known hypecycle which was developed by Gartner Inc in 1995317318 Thehype cycle is a time-dependent graphic that represents aconceptual presentation of the maturity of a technologythrough five different phases (Figure 5) and gives a view ofhow a technology will evolve over time It is considered as thegraphic representation of Amararsquos law319 which states that ldquowetend to overestimate the effect of a technology in the short runand underestimate the effect in the long runrdquo It can also beapplied to specific areas of research On average we canconsider that smart NHCs are at their early adolescence at anage where the first generation of compounds is being developedand enthusiastic expectations are on the rise The individuallocation of each smart NHC type may need a more personalanalysis since the position of each researcher may well differaccording to their research interests The authorrsquos personalopinion was triggered by the suggestion of one of the reviewerswho wisely suggested that a review of this type should clearlyspecify where the field is headed and what areas need attentionand hold opportunity On this basis the positions of each type

of smart NHC depicted in Figure 5 may be rationalized asfollows (1) Switchable NHC-based ligands are located in theplace where the peak of expectations has been surpassed Thehigh expectations created by the potential of this type of ligand

Scheme 66 Qualitative Variation of IR CminusO Stretching Frequencies of a Series of [IrCl(NHC)(CO)2] upon Addition of Pyreneor Hexafluorobenzene

Scheme 67 Comparison of Activity of IridiumminusNHC-based Catalysts with and without Pyrene Tags

Figure 5 Hype cycle with the location of each type of smart NHC(authorrsquos personal estimation)



AH

is now facing the reality that these ligands may have limitedapplicability due to the relative low number of photoswitchablesystems available (more research is needed here) and to thefact that redox-switchable NHCs seem to be sensitive to thecharge generated on the complex rather than to the type ofredox moiety used (2) The number of NHC-based hybridligands continues to grow as their potential use as hemilabileligands increases However the number of hybrid NHC ligandsthat showed clear hemilability in homogeneous catalysis is notso high and the relatively low number of examples provingclear catalytic benefits may indicate that this family of NHCs isclose to reaching the peak of expectations (3) Despite the largenumber of examples regarding bifunctional NHCs we nowseem to be facing a period where limitless applications of thisfamily of NHCs are continuously appearing Within this groupof ligands NHCs with π-stacking abilities are still in theirinfancy and probably some time needs to pass before thenumber of potential practical applications can be envisagedHopefully this classification is useful for encouraging futureresearch in the fieldIn summary smart NHC ligands are capable of adapting the

reactivity of a metal catalyst to the specific requirements of adefined catalytic transformation NHCs themselves are smartin the sense that NHC chemistry has rapidly adapted to newtimes and needs for novel reactivities These advances and thecontinuing development of new applications will certainly helpto ensure an important place for NHC-based catalysts for manyyears to come

AUTHOR INFORMATION

Corresponding Author

E-mail eperisujies

ORCID

Eduardo Peris 0000-0001-9022-2392Notes

The author declares no competing financial interest

Biography

Eduardo Peris graduated in chemistry in 1988 and received his PhDin chemistry (1991) from the Universidad de Valencia under thesupervision of Professor Pascual Lahuerta In 1994 he joined ProfessorRobert Crabtreersquos group at Yale University where he stayed for twoyears working on a research project regarding the determination ofhydrogen bonding to metal hydrides (dihydrogen bond) In October1995 he moved to the Universitat Jaume I (Castellon Spain) asassistant professor (1995minus1997) then associate professor (1997minus2007) and finally full professor of inorganic chemistry At theUniversitat Jaume I he started a research project related to the use oforganometallic pushminuspull compounds with nonlinear optical proper-ties During the past decade he has developed an intense researchactivity on the chemistry and catalytic applications of N-heterocyliccarbene-based organotransition metal catalysts Most of his activity wasdevoted to the preparation of new catalysts for the activation of smallmolecules with special attention to processes implying the activationof CminusH bonds carbon dioxide and reactions involving borrowing-hydrogen methodologies During the last five years the group alsodevoted great attention to the design of catalysts for tandem catalyticreactions The use of heterodimetallic complexes of IrRh and IrPdallowed for studying their activity in catalytic tandem processes inwhich each metal mediated a mechanistically distinct reaction Thecurrent interest of his group is the design of new polytopic rigid N-

heterocyclic carbene ligands (NHCs) for the preparation of improvedcatalysts and the design of supramolecular organometallic structuresfor molecular recognition He has authored or coauthored about 170articles in international chemistry journals and more than 10 bookchapters or monographs In 2012 he was awarded the Spanish RoyalSociety of Chemistry award in the field of inorganic chemistryresearch In the period 2007minus2010 he was a member of the AdvisoryBoard of Organometallics and he is currently the president of theSpanish Organometallic Chemistry Group (GEQO) of the SpanishRoyal Society of Chemistry (RSEQ)

ACKNOWLEDGMENTS

I thank the QOMCAT group from the Universitat Jaume I fortheir continuous support I am very thankful to ProfessorDmitry G Gusev (Wilfrid Laurier University) and to MacarenaPoyatos (Universitat Jaume I) for their valuable help and forrevising the content of this review article I gratefullyacknowledge financial support from the Ministerio deEconomia y Competitividad (MINECO) of Spain(CTQ2014-51999-P) and Universitat Jaume I (P11B2014-02)

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AJ

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(217) Ray L Katiyar V Raihan M J Nanavati H Shaikh M MGhosh P First Example of a Gold(I) N-Heterocyclic-Carbene-BasedInitiator for the Bulk Ring-Opening Polymerization of L-Lactide EurJ Inorg Chem 2006 2006 3724minus3730(218) Benitez M Mas-Marza E Mata J A Peris E Intra-molecular Oxidation of the Alcohol Functionalities in Hydroxyalkyl-N-Heterocyclic Carbene Complexes of Iridium and Rhodium Chem -Eur J 2011 17 10453minus10461(219) Hameury S de Fremont P Breuil P-A R Olivier-Bourbigou H Braunstein P Synthesis and Characterization ofOxygen-Functionalised-NHC Silver(I) Complexes and NHC Trans-metallation to Nickel(II) Dalton Trans 2014 43 4700minus4710(220) Jacques B Hueber D Hameury S Braunstein P Pale PBlanc A de Fremont P Synthesis Characterization and CatalyticActivity of Alcohol-Functionalized NHC Gold(IIII) ComplexesOrganometallics 2014 33 2326minus2335(221) Mariconda A Grisi F Costabile C Falcone S BertolasiV Longo P Synthesis Characterization and Catalytic Behaviour of aPalladium Complex Bearing a Hydroxy-Functionalized N-HeterocyclicCarbene Ligand New J Chem 2014 38 762minus769(222) Bartoszewicz A Miera G G Marcos R Norrby P-OMartin-Matute B Mechanistic Studies on the Alkylation of Amineswith Alcohols Catalyzed by a Bifunctional Iridium Complex ACSCatal 2015 5 3704minus3716(223) Bartoszewicz A Marcos R Sahoo S Inge A K Zou XMartin-Matute B A Highly Active Bifunctional Iridium Complex withan AlcoholAlkoxide-Tethered N-Heterocyclic Carbene for Alkylationof Amines with Alcohols Chem - Eur J 2012 18 14510minus14519(224) Cumpstey I Agrawal S Borbas K E Martin-Matute BIridium-Catalysed Condensation of Alcohols and Amines as a Methodfor Aminosugar Synthesis Chem Commun 2011 47 7827minus7829(225) Hameury S de Fremont P Breuil P-A R Olivier-Bourbigou H Braunstein P Synthesis and Characterization ofPalladium(II) and Nickel(II) Alcoholate-Functionalized NHC Com-plexes and of Mixed Nickel(II)-Lithium(I) Complexes Inorg Chem2014 53 5189minus5200(226) Denizalti S Turkmen H Cetinkaya B Chelating AlkoxyNHC-Rh(I) Complexes and Their Applications in the Arylation ofAldehydes Tetrahedron Lett 2014 55 4129minus4132(227) Saha B Rahaman S M W Daw P Sengupta G Bera J KMetal-Ligand Cooperation on a Diruthenium Platform SelectiveImine Formation through Acceptorless Dehydrogenative Coupling ofAlcohols with Amines Chem - Eur J 2014 20 6542minus6551(228) Lee K S Brown M K Hird A W Hoveyda A H APractical Method for Enantioselective Synthesis of All-CarbonQuaternary Stereogenic Centers through NHC-Cu-Catalyzed Con-jugate Additions of Alkyl- and Arylzinc Reagents to Beta-SubstitutedCyclic Enones J Am Chem Soc 2006 128 7182minus7184(229) Haneda S Sudo K Hayashi M Ni- and Cu-CatalyzedCoupling Reactions Using 2-(45-Dihydro-1H-Imidazo-2-yl)Phenol asa Versatile Phosphine-Free Ligand Heterocycles 2012 84 569minus575(230) Uchida T Katsuki T Construction of a New Type of ChiralBidentate NHC Ligands Copper-Catalyzed Asymmetric ConjugateAlkylation Tetrahedron Lett 2009 50 4741minus4743(231) May T L Brown M K Hoveyda A H EnantioselectiveSynthesis of All-Carbon Quaternary Stereogenic Centers by CatalyticAsymmetric Conjugate Additions of Alkyl and Aryl AluminumReagents to Five- Six- and Seven-Membered-Ring Beta-SubstitutedCyclic Enones Angew Chem Int Ed 2008 47 7358minus7362(232) Ren H Yao P Xu S Song H Wang B A New Class of O-Hydroxyaryl-Substituted N-Heterocyclic Carbene Ligands and TheirComplexes with Palladium J Organomet Chem 2007 692 2092minus2098(233) Mas-Marza E Poyatos M Sanau M Peris E A NewRhodium(III) Complex with a Tripodal Bis(Imidazolylidene) LigandSynthesis and Catalytic Properties Organometallics 2004 23 323minus325(234) Gulcemal D Gulcemal S Robertson C M Xiao J A NewPhenoxide Chelated Ir-III N-Heterocyclic Carbene Complex and its



AO

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AQ

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AR

which gives access to a large variety of structures with diversetopologies and electron-donating properties11minus17 Also NHCsmay incorporate a limitless library of additional functions andthis gives this type of unique ligands clear advantages for theon-demand (or tailor-made) synthesis of homogeneouscatalystsThe purpose of this review is to describe all types of smart

NHC ligands by considering the classification given in Figure1 We are aware that some other classifications may be equallyvalid but we thought that this one gives a clear overview ofwhat has been done in the field and allows a clear distinction ofall types of different ligands The review will place moreemphasis on those examples for which the role of the ligand hasa clear influence on the catalytic properties of the complex butattention will also be given to those cases in which relevantstudies on modification of the steric and electronic propertiesof NHC-based metal complexes is produced by external stimuliAlthough several review articles have appeared regarding theproperties of the different types of smart ligands as describedhere this review is the first one to globally consider all differenttypes of smart NHCsAs described here smart ligands have been classified into

switchable NHC ligands (section 2) and multifunctional NHCligands (section 3) The term switchable catalysis is used torefer to all those catalysts that are able to switch their activitiesby virtue of a range of stimuli such as light pH coordinationevents redox processes and changes of reaction conditions6

Although switchable catalysis is often understood as all thecatalytic processes that can be reversibly switched on and off7

we will here consider also those examples in which the activityof the catalyst can be turned on or off by external stimulidisregarding whether this change is reversible or not Cases ofreversible switchable catalysis are easily found for redox-1819

pH-9 or light-driven20minus24 processes Irreversible switchablecatalysis is often found when the catalyst is turned on or off byvirtue of an irreversible chemical modification involving thecoordination sphere of the metal complexMultifunctional ligands are those that have been designed to

do more than one thing8 This means that multifunctionalligands have additional functions that that can be useful incatalysis9 In our review this group will include all existingexamples regarding hemilabile ligands (section 31) andbifunctional ligands (section 32) Both hemilabile andbifunctional ligands refer to ligands whose reactivity dependson direct interaction with the substrates and therefore their

presence has a direct influence on the catalytic properties of thecatalyst

2 SWITCHABLE N-HETEROCYCLIC CARBENELIGANDS

The following subsections will be devoted to the photochemical(section 21) redox (section 22) and chemical (section 23)switching of NHC ligands and their applications in specifichomogeneously catalyzed processes Strategies aiming toappend functional groups by modifying coordinated NHCligands have been explored by a number of research groups fora handful of years A review article regarding the post-modification of transition metalminusNHC complexes as a toolboxfor bioorganometallic chemistry was recently published byCisnetti et al25 In that article the authors wisely suggested thatin the near future the postmodification of NHC ligands mayhelp increase the number of biocompatible and bioconjugatedNHC-based metal complexes In the following subsectionsexisting examples of postfunctionalization of NHC ligands forhomogeneous catalysis will be described

21 Photoswitchable N-Heterocyclic Carbene Ligands

Photoswitchable systems allow for external (remote) modu-lation of chemical reactions by light22 A photoswitchablecatalyst involves a catalytically active species that is susceptibleto undergoing a reversible photochemical transformation andas a result modifying its intrinsic catalytic properties2023 Thefirst example of photoswitchable postmodification of a NHCligand was described in 2009 by Yam et al26 who obtained aseries of di(thiophenyl)ethene-containing NHC complexes ofPd(II) Ag(I) and Au(I) whose steric and electronic propertiescould be modified by the photochemical reversible skeletalreorganization of the ligand via a CminusC bond activation(Scheme 1) Shortly after the work was extended to thepreparation of half-sandwich-type ruthenium(II) complexeswhich showed photochromic properties upon UV irradiation to

Figure 1 Presently known types of smart ligands

Scheme 1 First Photoswitchable NHC-based Ligand26



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