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PERSPECTIVE: Synthesis and coordination chemistry of macrocyclic ligands featuring NHC donor groups Peter G. Edwards and F. Ekkehardt Hahn Dalton Trans., 2011, 10.1039/C1DT10864F

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Switching on oxygen activation by cobalt complexes of pentadentate ligands†

Mads S. Vad,a Anne Nielsen,a Anders Lennartson,a Andrew D. Bond,a John E. McGradyb andChristine J. McKenzie*a

Received 7th April 2011, Accepted 26th July 2011DOI: 10.1039/c1dt10594a

The monoanionic N4O ligand N-methyl-N,N¢-bis(2-pyridylmethyl)ethylenediamine-N¢-acetate(mebpena-) undergoes oxidative C–N bond cleavage in the presence of Co(II) and O2. The two resultantfragments are coordinated to the metal ion in the product [CoIII(2-pyridylformate)(mepena)]ClO4

(mepena- = N-methyl-N¢-(2-pyridylmethyl)ethylenediamine-N¢-acetato). Bond cleavage does not occurin the presence of chloride ions and [CoIII(mebpena)Cl]+, containing intact mebpena-, can be isolated.The oxidative instability of the mebpena- in the presence of Co(II) and air stands in contrast to theoxidative stability of the family of very closely related penta- and hexa-dentate ligands in their cobaltcomplexes. Cyclic voltammetry on the matched pair [CoIIICl(mebpena)]+ and [CoIICl(bztpen)]+,bztpen = N-benzyl-N,N¢,N¢-tris(2-pyridylmethyl)ethylenediamine, shows that substitution of a pyridinedonor for a carboxylato donor results in a relatively small cathodic shift of 150 mV in theE◦(Co(II)/Co(III)) oxidation potential, presumably this is enough to determine the contrasting metaloxidation state in the complexes isolated under ambient conditions. DFT calculations support aproposal that [CoII(mebpena)]+ reacts with O2 to form a Co(III)-superoxide complex which can abstractan H atom from a ligand methylene C atom as the initial step towards the observed oxidative C–Nbond cleavage.

Introduction

Cobalt–O2 complexes of a range of ligands includingdien,1 salen,2 porphyrin,3 pentacyano,4 trispyrazoleborato5 andtetraazamacrocyclic,6 ligands have served as important bench-mark structural and spectroscopic models for elucidating dioxy-gen binding in heme enzymes.7 Aware of this well-knownbackground in the utility of cobalt for trapping dioxygen-derived adducts, we explored the cobalt chemistry of thematched pair of pentadentate monoanionic N4O and neutralN5 ligands, N-alkyl-N,N¢-bis(2-pyridylmethyl)ethylenediamine-N¢-acetate (Rbpena-), Scheme 1(a), and N-alkyl-N,N¢,N¢-tris(2-pyridylmethyl)ethylenediamine (Rtpen), Scheme 1(b). These lig-ands were first prepared in our laboratory,8,9,10 and both theiron and manganese complexes of Rtpen and Rbpena- ex-hibit biomimetic reactivity pertinent to O2 catabolism and an-abolism. This reactivity implies the formation of as yet un-detected dioxygen adducts. The parent hexadentate monoan-ionic N5O ligand N,N,N¢-tris(2-pyridylmethyl)ethylenediamine-N¢-acetate (tpena-)11 and neutral N6 ligand, N,N,N¢,N¢-tetrakis(2-

aDepartment of Physics and Chemistry, University of Southern Denmark,Campusvej 55, 5230, Odense M, Denmark. E-mail: chk@ifk.sdu.dk;Fax: (+45) 6615 8760; Tel: (+45) 6550 2518bInorganic Chemistry Laboratory, Department of Chemistry, University ofOxford, South Parks Road, Oxford, OX1 3QR, United Kingdom† Electronic supplementary information (ESI) available. CCDC referencenumbers 821028–821032. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/c1dt10594a

Scheme 1 Ligands in this study. The monoanionic N4O andneutral N5 pentadentate ligands (a) N-R-N,N¢-bis(2-pyridylmethyl)-ethylenediamine-N¢-acetate (Rbpena-), R = methyl, benzyl and(b) N-R¢-N,N¢,N¢-tris(2-pyridylmethyl)ethylenediamine (Rtpen), R =methyl, ethyl, propyl, isopropyl, 2-hydroxyethyl, phenyl, benzyl.The monoanionic N5O and neutral N6 hexadentate ligands (c)N,N,N¢-tris(2-pyridylmethyl)ethylenediamine-N¢-acetate (tpena-) and (d)N,N,N¢N¢-tetrakis(2-pyridylmethyl)ethylenediamine (tpen).

pyridylmethyl)ethylenediamine (tpen)12 used for comparisons inour work are depicted also in Scheme 1. Notably, all of the

10698 | Dalton Trans., 2011, 40, 10698–10707 This journal is © The Royal Society of Chemistry 2011

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ethylendiamine-based ligands in Scheme 1 are close relatives ofone of the mainstays in coordination chemistry, edta4-.13

Predominantly air-stable mononuclear Fe(II) complexes[Fe(X)(Rtpen]n+, n = 1 or 2, X = Cl-,9 Br-,14 I-,14b CN-,14b

NCS-,14b NCO-,14b CF3SO3-,15 CH3CO2

-,16 or NO,16 form in thereactions of Rtpen (Scheme 1(b)) with Fe(II) salts. By contrast,analogous reactions with Fe(II) salts and Rbpena- (Scheme 1(a))give spontaneously Fe(III) complexes, for example the oxo-bridgeddinuclear complex [Fe2(O)(bzbpena)2]2+.17 The contrasting aerobicchemistry of the Fe complexes of Rtpen and Rbpena- is a clearindication of the expected redox, and consequent reactivity, tuningimposed by alternating a pyridyl donor arm for a carboxylatodonor arm, in otherwise geometrically analogous ligands. Thereactivity is reminiscent of biological O2 reactivity seen for non-heme enzymes: introduction of terminally coordinating aspartateor glutamate, which ostensibly replaces histidine donors in thecoordination spheres of Fe, can effect a switching between oxygenactivation vs. reversible O2 binding, viz. the activity of hemerythrinvs. methane monoxygenase.

In parallel with the aerobic Fe(II) and Fe(III) chemistry of themonoanionic N4O and neutral N5 ligand systems respectively,a difference of one oxidation state is emerging for the high-valent iron-oxide chemistry of these ligands: Reactions withperoxides of starting Fe(II) and Fe(III) complexes of Rtpen andRbpena- respectively, give contrasting results. [FeII(Cl)(Rtpen]+

reacts with dihydrogenperoxide to form unstable but spectroscop-ically identifiable peroxo adducts, namely the purple low-spin[FeIIIOOH(Rtpen)]2+ and blue high-spin [FeIIIOO(Rtpen)]+,9,14a,18

Scheme 2(a) and (b).19 Over the course of a few minutes atroom temperature these peroxide complexes decompose and thestarting Fe(II) complexes can be identified in condensed phases.Homolytic cleavage of the O–O bond in [FeIIIOOH(Rtpen)]2+

Scheme 2 (a) Fe(III) hydroperoxides and (b) Fe(III) peroxides derivedfrom [FeII(Rtpen)]2+ R = methyl, ethyl, propyl, isopropyl, 2-hydroxyethyl,phenyl, benzyl. (c) [FeIII(bzObpena)]+: oxygen atom inserted productderived from [FeII(bzbpena)]+. (d) [FeIII(Cl)2(HmebpenaO)]+: oxygen atominserted product derived from [FeII(mebpena)]+.

was observed in the gas phase and an ion assigned to theferryl species [FeIVO(Rtpen)]2+ is generated. This ion undergoessubsequent loss of the mass equivalent to the aldehydic derivativeof the dangling alkyl group R, suggesting that a C–H activationand O atom transfer can occur in the gas phase. Since thisreport, related ferryl(IV) complexes based on neutral N5 donorsets have been structurally characterised.20,21 By contrast, oxygeninserted products are isolated from the reaction of the Fe(III)–Rbpena- complexes with dihydrogen peroxide, alkyl peroxidesor O2 and ascorbic acid.17 When the R group in Rbpena- is abenzyl group, its oxygenation results in the formation of an Fe(III)complex of the previously unknown hexadentate dianionic ligandN -(2-oxidobenzyl)-N,N¢-bis(2-pyridylmethyl)ethylenediamine-N¢-acetate, (bzObpena)]+, Scheme 2(c). When the dangling benzylgroup is replaced by a methyl group, regiospecific insertion ofan O atom into an Namine–Fe bond to give a coordinated N-oxide (Fe–O–NR3), Scheme 2(d) occurs. Both the products inScheme 2(c) and (d) provided strong circumstantial evidencefor the generation of an undetected Fe(V) perferryl O atomtransfer reagent. In summary, it seems that the Fe species andligand oxidation chemistry we have observed indicates that themonoanionic N4O systems and their neutral N5 counterparts canaccess the FeVO and FeIVO species, respectively, and that boththese species are capable of C–H activation. By contrast, air-stable Mn(II) complexes of both Rtpen22 and Rbpena- 8 have beenisolated.

By analogy to the use of Co–dioxygen adducts in the elucidationof heme-Fe chemistry, it is clear that structural characterisationof a Co–O2 adduct of Rbpena- and Rtpen would shed light onthe possible mechanisms involved in the formation of the Fe(III)species in Scheme 2, and the above mentioned O2 evolutionreaction with Mn complexes of Rbpena- and this was ourmotivation for the work here; an investigation of the Co chemistryof the series of related penta- and hexa-dentate N4O, N5, N5O andN6 ligands depicted in Scheme 1. Although we did not succeedin isolating Co–O2 adducts, we have unearthed yet another typeof C–H activation: the N4O ligands mebpena- and bzbpena-,Scheme 1(a) R = Me, Bz, are susceptible to oxidation in their Co(II)complexes in the presence of O2. By contrast, ligand oxidations arenot observed under equivalent conditions for the Co complexes ofthe analogous neutral N5 ligand or their hexadentate N5O and N6

counterparts (Scheme 1(b), (c) and (d)).

Results and discussion

Synthesis and characterisation

The reaction between CoCl2 and mebpenaH, Scheme 1(a) R =CH3 in methanol gives the red Co(III) complex [CoCl(mebpena)]+

(1) in Scheme 3. Elemental analysis of the solid compoundisolated after the addition of sodium perchlorate to solutionsof [CoCl(mebpena)]+ fits best with a mixture containing both[CoCl(mebpena)]ClO4 (1·ClO4) and the sodium perchlorate dou-ble salt 12[Na(ClO4)2(OH2)2]ClO4. The crystal selected for X-rayanalysis was found to be the double salt and no attempt wasmade to obtain the simple 1 : 1 salt or the double salt of 1 inpure bulk samples. Analogously, reaction between CoCl2 and theneutral pentadentate analogue bztpen, Scheme 1(b) R = CH2C6H5,in methanol, followed by addition of perchlorate gave the purple

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 10698–10707 | 10699

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Scheme 3 Chemical structures of cations derived from single crystalX-ray studies.

air-stable [CoCl(bztpen)]ClO4 (2·ClO4) depicted in Scheme 3. Thechlorido complexes 1 and 2 are a matched pair of monocationiccomplexes [CoCl(L)]+, L = mebpena- and bztpen, with the Co ionsin the +2 and +3 oxidation states, respectively. The formulationsreflect the expected tendency towards stabilisation of the harderCo(III) by a negatively charged hard carboxylato ligand. Thepresence of the methyl and benzyl R groups in mebpena- andbztpen, respectively, is not expected to contribute significantly tothe redox chemistry of the metal ions. The cyclic voltammogramsmeasured for [CoCl(L)]ClO4, L = mebpena- or bztpen, 1 and 2,show quasi-reversible redox waves for the Co(II)/Co(III) processescentered around -307 mV for 1 and -159 mV for 2 vs. ferrocene,Fig. 1. The separation of 211 mV for the Epa and Epc process for 1 issignificantly larger than the 143 mV for 2. It is somewhat surprisingthat the difference in redox potentials for the Co(II)/Co(III)complexes with the monoanionic N5Cl and dianionic N4OClcoordination spheres was not greater, given that aerobic oxidationof the solution mebpena- and bztpen Co(II) systems, vide infra,gives very different outcomes in the absence of chloride: the formershows metal and ligand oxidation, while the latter shows metaloxidation only.

In contrast to the reactions producing the matched pair 1 and2 (apart from the respective Co(III) and Co(II) oxidation states),the reactions of Co(II) with mebpena- and bztpen do not resultin analogous outcomes (apart from a common Co(III) oxidationstates when chloride is absent from the reaction mixtures). Reac-tion of bztpen and Co(II) perchlorate in methanol–water (1 : 1) inair gave the Co(III) hydroxo compound [Co(OH)(bztpen)](ClO4)2

(3·(ClO4)2·H2O), Scheme 3. This contrasts to the Co(II) oxidationstate seen for the chlorido complex, 2, despite the fact that chlorido

Fig. 1 The cyclic voltammograms of 1 (red line) and 2 (blue line)were recorded in 0.1 M tetrabutylammoniumperchlorate in a solution ofacetonitrile against a Ag/AgCl reference electrode and calibrated againstthe ferrocene/ferrocenium redox couple.

and hydroxido are both monodentate, monoanionic auxillary lig-ands. The hydroxide/Co(III) combination rather than alternativewater/Co(II) combination is consistent with the M–L bond lengthsand metal ion geometry seen in the single-crystal X-ray structuredescribed below. A comparison of the optimised structure for 3and its reduced and protonated counterpart [CoII(OH2)(bztpen)]2+

calculated using DFT supports the assignment of Co(III)–OH inthe crystal structure (Co–O = 1.8709(12) A): the Co–O distancein the optimised structure of 3 is 1.89 A, compared to 2.28 Aand 3.01 A in the high-spin and low-spin optimised structuresof [CoII(OH2)(bztpen)]2+, respectively. The trend is repeated inthe remaining five metal–ligand bond lengths where the averageabsolute deviation compared to the crystal structure is larger forthe two spin states of [CoII(OH2)(bztpen)]2+: 9 pm and 25 pm forthe doublet and the quartet respectively compared to 7 pm for[CoIII(OH)(bztpen)]2+. Overlays of the DFT-optimized structureswith the crystal structure of 3 are included as ESI.†

Analogous complexation reactions using mebpenaH andcobalt(II) perchlorate were less straightforward. The ESI massspectrum (ESI, Fig. S1†) of a red solid isolated from the reactionshows two dominant ions, one assignable to 1 (m/z 407.3) and oneat m/z 403.3 (mass equivalent to {Co(mebpena) + 2O–H}+). Apossible source of chloride ions in the reaction needed for the for-mation of 1 is from the ligand synthesis. Despite considerable effort(e.g. attempted syntheses using the ester protection/deprotectionof the carboxylate group coupled with chromatography or treat-ment with silver nitrate), we were not able to rectify this without anunacceptable loss of ligand. Thus as an alternative synthetic routewe performed a metathesis reaction using the Mn(II) complex[Mn2(mebpena)2(H2O)2](ClO4)2.8 Reaction of cobalt perchloratewith [Mn2(mebpena)2(H2O)2](ClO4)2 in MeOH–H2O solution inair, or under a stream of O2, at room temperature results, afterseveral days, in the formation of red crystals of the heterolepticCo(III) complex of the derivatives of mebpena- resulting fromoxidative C–N cleavage adjacent to the N–CH3 group. The redCo(III) complex [Co(2-pyridylformate)(mepena)]+ (4), Scheme 3,where mepena- is the unknown tetradentate ligand N-methyl-N¢-(2-pyridylmethyl)ethylenediamine-N¢-acetate (Scheme 4), was

10700 | Dalton Trans., 2011, 40, 10698–10707 This journal is © The Royal Society of Chemistry 2011

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Scheme 4 C–N cleavage of mebpena- to give 2-pyridylformate (pfa-) andN-methyl-N¢-(2-pyridylmethyl)ethylenediamine-N¢-acetate (mepena-).

crystallographically characterised in 4·ClO4. This result indicatedthat m/z 403.3 ion with a mass equivalent to {Co(mebpena) +2O–H}+ seen in initial attempts was in fact complex 4 and thiswas confirmed by the ESI mass spectra of the crystals (ESI,Fig. S2†).

Since the formation of 4 is an oxidative process, it is pertinentto consider the metal-based oxidants which might be gener-ated from reaction of a [CoII(mebpena)]+ species and O2. TheESI mass spectra of a reaction mixture containing equimolar(in terms of mebpenaH : Co2+) [Mn2(mebpena)2(H2O)2](ClO4)2

and cobalt(II) perchlorate, recorded after purging for 10 minwith O2 (ESI, Fig. S3†) shows no sign of 4: however thereis a major ion at m/z 372.2 corresponding to the proposedintermediate [CoII(mebpena)]+ and some less abundant ionsfor solvated derivates, [CoII(mebpena)(H2O)]+ m/z 390.2 and[CoII(mebpena)(CH3OH)]+ m/z 404.2. Over the course of days,while the peak for 4 grows in, no ions assignable to viableintermediates other than [CoII(mebpena)]+ and its solvates aredetected. The fact that e.g., a Co-dioxygen adduct is not observedcannot be used to conclude its absence in solution phases. Forexample, gas phase dioxygen adducts are expected to be veryunstable due to facile reductive dissociation of neutral O2.23,24

This is the first documentation of the occurrence of oxidative C–N cleavage with the series of ligands in Scheme 1. The formationof 4 is noteworthy in the sense that the product derived from thealkyl group being cleaved also remains bound to the metal ion,albeit in a modified form. In other words, all of the daughterproducts of the precursor ligand remain bound to cobalt. Colbranand co-workers,25 have most likely observed a similar reaction: C–N cleavage of 2,6-bis(bis(2-pyridylmethyl)amino)methypyridine(bpa–tpa) to give the Co(III) complex of carboxylated tpa,[Co(tpaCO2)Cl]+. Likewise, Comba and co-workers26 observeddemethylation of one of the two tertiary amine donors of atetradentate bispidine N4 ligand. In both of these reactions, the fateof the leaving alkyl group is assumed, e.g., this would be undetectedformaldehyde in the demethylation reaction. These examples showclose resemblance to the system reported here, however the litera-ture also contains examples of ligand degradation via oxidativeC–N cleavage by metal-based oxidants generated through O2

activation by vanadium, cobalt, copper and iron.27,28,29,30 We havealso carried out parallel studies using the benzylated N4O ligand,bzbpena-, Scheme 1(a) R = CH2C6H5. The results are morecomplicated; aside from C–N cleavage, oxygenation of the benzenering, analogously to the reaction seen for the corresponding ironcomplex17 occurs (see ESI†).

Given the differences in the reactivity of the Co complexesof mebpena-, bzbpena- compared with bztpen, it was perti-nent also to compare the aerobic Co chemistry of the com-plexes of their parent hexadentate ligands. A Co(III) com-plex, [Co(tpen)](ClO4)3 of the neutral N6 ligand N,N,N¢,N¢-tetrakis(2-pyridylmethyl)ethylenediamine (tpen),12 Scheme 1(d),

was reported 20 years ago from an aerobic synthesis startingwith a Co(II) source.31 We have not repeated this synthesis,however a conventional outer-sphere oxidation of [CoII(tpen)]2+

in solution seems likely. The N5O ligand N,N,N¢-tris(2-pyridylmethyl)ethylenediamine-N¢-acetate (tpena-), Scheme 1(c),has only very recently been reported.11 As an aside, until thisreport, tpena- was the missing link in the series of hexadenateethylenediamine based ligands containing four glycinato and/or 2-pyridylmethyl arms spanning from edta4- to tpen. For the purposeof this study we had independently prepared this new ligand usinga different synthesis (described in the experimental section) andcan report here the first complex to be isolated in the solid stateand its X-ray crystal structure. Reaction of tpenaH with Co(ClO4)2

produces the simple Co(III) complex, [Co(tpena)](ClO4)2·1/2H2O(5·ClO4·1/2H2O), Scheme 3. The yield of [Co(tpena)]2+ was notaffected by variation of O2 pressure. This suggests that a freecoordination site/labile exchangeable auxiliary solvent ligand isnecessary for the mebpena- oxidation to occur and this corrob-orates the observations made with the coordinatively saturatedN4OCl coordination sphere.

X-Ray crystal structures

Single-crystal X-ray structures have been obtained for compoundscontaining all of the key cations 1–5 shown in Scheme 3. Thecations are shown in Fig. 2(a)–(e) and selected bond distances andangles are listed in Table 1. Except for 2, all of the complexescontain Co(III), and the bond distances and angles are consistentwith the Co(III) and Co(II) oxidation states. The tetrafluoroboratesalt of 2 was reported recently16 and the conformation of theligand in the new perchlorate salt is identical. For the matchedpair of chloride complexes 1 and 2, the average Co–N/O bonddistance is 1.930 A for 1 and 2.185 A for 2, and the mean of thedeviations from 90◦ for cis donor angles is 4.1◦ for 1 and 9.9◦ for2. The carboxylate group of 1 is trans to a pyridine donor. Asmentioned above, 1·[Na(ClO4)2(OH2)2]·ClO4 is a double salt. TheNa+ cations are octahedrally coordinated by the O atoms of twowater molecules, two perchlorate anions and two carbonyl groupsof adjacent molecules of 1. The chloride ligands in complexes 1and 2 are both located trans to the aliphatic amine donors, thatbearing the methyl group in 1 and that bearing the two pyridylgroups in 2. The carboxylato and pyridine arms attached to thesame tertiary N atom in 1 are bound meridionally. The carboxylategroups of 2-pyridylformate and mepena- in 4 are located trans toeach other and the remaining N donors of mepena- are arrangedmeridionally. The conformation of the bztpen in complex 3 is thesame as in complex 2, i.e., the hydroxido and chlorido ligandoccupy equivalent sites in these Co(III) and Co(II) complexesrespectively. In 5·ClO4·1/2H2O, the Co-donor bond distances andthe slightly distorted octahedral coordination geometries aroundCo1 are comparable in two crystallographically distinct cations. Incontrast to the structure of 1, the carboxylato and pyridine armsattached to the same tertiary N atom are bound facially.

Mechanistic considerations for aerobic ligand oxidations in theCo(II) complexes of mebpena-

The experimental work summarised in the previous paragraphsclearly indicates that mebpena-, in its solution-state Co(II)

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Table 1 Details of the Co coordination environment in 1–5 (A, ◦)

1 2 3 4 5a

Co1–Cl1 2.2346(8) Co1–Cl1 2.3500(7) Co1–O1 1.8709(12) Co1–O1 1.8803(14) Co1–O1 1.885(3) 1.890(3)Co1–O1 1.8939(18) Co1–N1 2.1933(19) Co1–N1 1.9630(15) Co1–O3 1.9041(14) Co1–N1 1.935(4) 1.942(4)Co1–N1 1.975(2) Co1–N2 2.2083(19) Co1–N2 1.9975(15) Co1–N1 1.9391(17) Co1–N2 1.940(4) 1.949(4)Co1–N2 1.928(2) Co1–N3 2.116(2) Co1–N3 1.9306(16) Co1–N2 1.9693(18) Co1–N3 1.950(4) 1.953(4)Co1–N3 1.935(2) Co1–N4 2.254(2) Co1–N4 1.9376(15) Co1–N3 1.9466(17) Co1–N4 1.941(4) 1.929(4)Co1–N4 1.918(2) Co1–N5 2.151(2) Co1–N5 1.9338(15) Co1–N4 1.9198(17) Co1–N5 1.931(4) 1.944(4)O1–Co1–N1 87.39(9) N1–Co1–N2 80.41(7) O1–Co1–N1 176.27(6) O1–Co1–O3 177.94(6) O1–Co1–N1 91.53(15) 92.79(14)O1–Co1–N2 86.17(9) N1–Co1–N3 76.24(7) O1–Co1–N2 93.05(6) O1–Co1–N1 87.49(7) O1–Co1–N2 86.52(14) 86.60(14)O1–Co1–N3 88.80(9) N1–Co1–N4 99.82(7) O1–Co1–N3 95.55(6) O1–Co1–N2 91.61(7) O1–Co1–N3 175.67(15) 176.48(14)O1–Co1–N4 170.21(9) N1–Co1–N5 77.50(7) O1–Co1–N4 88.54(6) O1–Co1–N3 92.75(7) O1–Co1–N4 88.91(14) 88.79(14)O1–Co1–Cl1 89.10(6) N1–Co1–Cl1 171.92(5) O1–Co1–N5 90.90(6) O1–Co1–N4 93.46(7) O1–Co1–N5 87.19(15) 86.71(14)N1–Co1–Cl1 175.24(7) N2–Co1–N3 145.34(7) N1–Co1–N2 86.94(6) O3–Co1–N1 94.53(7) N1–Co1–N2 89.48(16) 89.12(15)N1–Co1–N2 88.76(10) N2–Co1–N4 74.85(7) N1–Co1–N3 84.63(6) O3–Co1–N2 88.08(7) N1–Co1–N3 85.07(16) 85.51(16)N1–Co1–N3 83.15(10) N2–Co1–N5 96.18(7) N1–Co1–N4 95.16(6) O3–Co1–N3 87.89(7) N1–Co1–N4 171.36(16) 170.91(16)N1–Co1–N4 96.37(10) N2–Co1–Cl1 102.39(5) N1–Co1–N5 85.41(6) O3–Co1–N4 84.53(7) N1–Co1–N5 82.69(17) 82.43(15)N2–Co1–Cl1 94.22(8) N3–Co1–N4 84.24(7) N2–Co1–N3 171.11(6) N1–Co1–N2 87.58(7) N2–Co1–N3 96.12(15) 96.44(15)N2–Co1–N3 170.66(10) N3–Co1–N5 103.26(7) N2–Co1–N4 84.33(6) N1–Co1–N3 83.47(7) N2–Co1–N4 81.94(16) 82.04(15)N2–Co1–N4 84.88(10) N3–Co1–Cl1 104.30(6) N2–Co1–N5 98.46(6) N1–Co1–N4 178.27(7) N2–Co1–N5 169.81(16) 168.95(15)N3–Co1–Cl1 93.56(8) N4–Co1–N5 170.99(7) N3–Co1–N4 93.62(6) N2–Co1–N3 169.87(7) N3–Co1–N4 94.86(16) 93.37(15)N3–Co1–N4 100.60(9) N4–Co1–Cl1 88.24(5) N3–Co1–N5 83.68(6) N2–Co1–N4 93.83(7) N3–Co1–N5 89.72(15) 90.01(15)N4–Co1–Cl1 87.61(7) N5–Co1–Cl1 94.61(5) N4–Co1–N5 177.18(6) N3–Co1–N4 95.04(7) N4–Co1–N5 105.95(16) 106.60(16)a Two complexes in the asymmetric unit, labelled with suffixes A and B in the crystal structure.

Fig. 2 The structures of (a) 1 in the crystal structure of 12[Na(ClO4)2(OH2)2]ClO4 (b) 2 in the crystal structure 2·ClO4, (c) 3 in the structure of3(ClO4)2·1.15H2O, (d) 4 in the structure of 4(ClO4) and (e) 5 in the structure of 5(ClO4)2·0.5H2O.

10702 | Dalton Trans., 2011, 40, 10698–10707 This journal is © The Royal Society of Chemistry 2011

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Scheme 5 Proposed mechanism for C–N cleavage in mebpena- in the presence of Co(II), dioxygen and water.

complex, is unstable towards aerobic oxidation, resultingin C–N cleavage. The reaction apparently depends on thepresence of a free coordination site/labile sixth ligand. Asmentioned in the Introduction, there is ample precedence forthe formation of mononuclear Co(III) superoxide complexesby direct reaction of an appropriate Co(II) precursor and O2.This seemed an obvious starting point when considering themechanism. A minimum corresponding to a superoxido complex[CoIII(mebpena)OO∑]+ (Scheme 5, 6), the putative product of thereaction of [CoII(mebpena)(OH2)]+ with O2, was located usingDFT. This H2O/O2 exchange reaction is marginally exergonic(DrG = -3.7 kcal mol-1). From this intermediate, the C–N bondcleavage seen in mebpena- could be initialised by an intramolecularhydrogen atom abstraction from the methylene group adjacent tothe methyl-bearing amine nitrogen by the superoxido ligand in 6.It is important to emphasise that pseudo-octahedral complexesof mebpena- such as [CoIII(mebpena)OO∑]+ have a number ofcoordination isomers, and indeed we have located seven distinctisomers of 6 that lie within 5 kcal mol-1 of each other. The freeenergy surface for C–H activation starting from the most stableof these is shown in Fig. S19,† and leads to a radical intermediate[CoIII(mebpena-H∑)OOH]+ with a barrier of DG‡ = 23.7 kcalmol-1. The free energy surfaces for C–H activation starting fromalternative coordination isomers of 6 lie approximately parallelto that shown in Fig. S19,† and lead to a variety of coordinationisomers of [CoIII(mebpena-H∑)OOH]+, including the one thatwould lead directly to the crystallised product. Barriers (measuredrelative to the most stable coordination isomer of 6) lie between23.7 and 33.7 kcal mol-1.

After H atom abstraction to give [CoIII(mebpena–H∑)OOH]+

a subsequent rapid cascade of reactions could start with Co(III)oxidation of the radical arm to generate an iminium ion, whichis readily hydrolysed to a secondary amine and an aldehyde. Thisoxidation step could either be intramolecular (i.e. the Co(III) isfrom the same complex) or intermolecular, involving a secondCo(III) centre. The products of the iminium ion hydrolysis are

an aldehyde and a secondary amine. Hydrogen peroxide andperoxyacids32,33 are known to oxidise aldehydes to carboxylicacids, thus the aldehyde could be further oxidized to a car-boxylate by the hydroperoxide ligand in a Bayer–Villiger34 typereaction, leaving a final rapid outer-sphere oxidation of themetal center to give the isolated product complex 4. Carbon-based radicals have been suggested previously as intermediatesin oxidative N-dealkylations.25a,35,36 Comba and co-workers26 pro-posed that a carbon-based radical can be generated by H atomtransfer from a methyl group to a weakly Co(II)-coordinateddihydrogenperoxide, to form ultimately a Co(III)–hydroxide andwater.

A comparison with the complexes of the corresponding neutralmetpen ligand, where we have found no evidence for ligandoxidation, are also informative here. The O2 binding reaction,[CoII(metpen)(OH2)]2+ + O2 → [CoIII(metpen)(O2)]2+ + H2O isslightly less exergonic (DrG = -1.8 kcal mol-1), as might be expectedfor a neutral ligand, and the barrier to the subsequent C–Hactivation event is marginally higher (DG‡ = 25.8 kcal mol-1). Thesmall differences are consistent with the fact that the Co(III)/Co(II)redox potentials for 1 and 2 were only 150 mV apart. Thus bothtrends are consistent with lower reactivity for the complexes ofmetpen, although the differences in the calculated DrG are smalland within the confidence limits of DFT.

An alternative explanation may relate to the potential de-protonation of the aquo precursors [CoII(mebpena)(OH2)]+ and[CoII(metpen)(OH2)]2+, which would be expected to be morefavourable in the latter, dicationic, case. Formation of a hydroxidoligand would certainly reduce the rate of exchange with O2,allowing an irreversible outer-sphere oxidation to dominate,producing the structurally characterised complex 3. This impliesthat it may be possible to access a similar metal-based oxidantderived from [CoII(metpen)]2+ and O2 in the strict absence of asixth auxiliary ligand (solvent or anion), and form a complexanalogous to that in the structurally characterised 2 and 3 (O2

∑-

replaces Cl-/OH-).

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 10698–10707 | 10703

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Conclusions

The replacement of a pyridine donor by a carboxylate donorin the pentadentate Rtpen and Rbpena- ligands respectivelyenables O2 activation in both their Fe and Co complexes, butwith different outcomes. In the case of Fe, the oxygenationsdepicted in Scheme 2(c) and (d) were observed. For Co oxidativeC–N cleavage reaction occurs. N-dealkylation in complexes ofmultidentate ligands containing aminomethylpyridine moietieshas been sporadically observed, our example is the first whereall products of the C–N cleavage remain coordinated in theproduct. The difference in reactivity for Fe and Co complexesof Rbpena- might be traced back to the different dioxygen-derived metal-based oxidants that the two metals are capable ofstabilising: FeVO and Co(III)–OO(superoxide). It is noteworthythat with its one carboxylate group, the N4O ligand system,Rbpena-, mimics the endogenous monoanionic 3His-1carboxylateor 2His-1carboxylate donor sets found in many non-heme O2

activating enzymes. The one or two extra neutral N donorsin the Rbpena- ligands relative to their biological counterpartsserve to prevent hydrolysis and oligomerisation, in analogy to theprotective function of proteins. A C–N cleavage reaction, namelythe demethylation of nucleobases, is catalyzed by non-heme2His-1Asp Fe(II) a-ketoglutarate dependent enzymes ABH2,ABH337,38,39 and AlkB.40,41,42,43,44 Our results point to dioxygenactivation to give a metal-based oxidant capable of C–H activationby the Co(II) complexes of Rbpena- but not of Rtpen, when thesixth coordination site is not blocked by a ligand that O2 cannotsubstitute, e.g. Cl- in 1 or pyridine in 5. In terms of O2 activation viametal coordination, the comparison of the E 1

2Co(III)/(II) values

for the best matched pair that we have, [CoCl(L)]+, L = mebpena-

or bztpen, 1 and 2, is interesting in the sense that the difference inthese potentials was (at least to us) surprisingly small (150 mV).

Our combined results with the Mn, Fe and Co complexesof the related penta and hexadentate ethylenediamine-basedligands Rtpen, Rbpena-, tpen and tpena-, define some of theoxidative degradation pathways by which transition metal cata-lysts based on multidentate ligands containing carboxylato andaminomethylpyridine groups45 are potentially unstable underoxidising conditions. Such instability may limit the lifetimes ofsuch catalysts. In this respect, it is interesting that we have notyet seen evidence for oxidative ligand degradation reactions in theMn complexes of mebpena-, which apart from being used as thestarting material for the synthesis of the Co complex reportedhere, have been shown to catalyse the tert-butylhydrogenperoxide(TBHP) oxidation of water where turnovers in excess of 104

(mol[O2] per mole of Mn) have been measured.46,47

Experimental

Physical measurements

Elemental analyses were performed at the Chemistry Depart-ment at Copenhagen University, Denmark. IR spectra of thecomplexes in KBr discs were measured using a Hitachi 270-30 IR spectrometer. Electrospray ionisation mass spectra (ESIMS) were obtained using a Finnigan Mat TSQ700-Triple StageQuadrupole mass spectrometer (MDS Proteomics A S-1) and a Q-Star Pulsar quadrupole time-of-flight mass spectrometer (Applied

Biosystems/MDS SCIEX), both equipped with a nanospray ionsource. The isotope patterns of all m/z assignments were checkedby comparison to the calculated theoretical patterns. CyclicVoltammetry (CV) was recorded in acetonitrile solution underdry anaerobic conditions using an Autolab system (Eco Chemie,The Netherlands), controlled by GPES software. The workingelectrode was a Pt disk, the auxiliary electrode was a platinum wireand the reference electrode was Ag/AgCl. 0.1 M TBAClO4 (TBA =tetrabutylammonium) was used as electrolyte. All measurementswere calibrated versus the ferrocene/ferrocenium (Fc0/+) redoxcouple E 1

2= 0.44 V. CV spectra were recorded at a scan rate

of 50 mV s-1.CAUTION! Although we encountered no problems during prepa-

ration of the perchlorate salts, Perchlorate salts of metal complexesare potentially explosive and should be handled with caution in smallquantities.

Synthesis

N -Methyl-N,N¢-bis(2-pyridylmethyl)ethylenediamine-N¢-aceticacid (mebpenaH), [Mn2(mebpena)2(H2O)2](ClO4)2 and[Mn2(bzbpena)2(H2O)2](ClO4)2 were prepared as describedpreviously.8 N,N¢-Bis-2-picolylethylenediamine (bispicene) wasprepared as described previously.48

1,2,3-Tris-2-picolylimidazolidine. Bispicene (3.5317 g,14.6 mmol) and 2-pyridinal (1.393 ml, 14.6 mmol) in drydiethylether (5 ml) were stirred overnight under CaCl2 protection.The volume of the solvent was reduced in vacuo to precipitate theproduct as pale green microcrystals. Recrystallisation from hotdiethylether afforded colorless crystals of X-ray quality (2.1513 g,44%). dH (CDCl3): 2.7, 3.3 (4 H, m, NCH2CH2N, AA¢BB¢), 3.95,3.91, 3.67, 3.62 (4 H, dd, 2 ¥ NCH2C5H4N, AB), 4.26 (1 H, s,NCH(C5H4N)N), 7.06 (2 H, t, 2 ¥ C5H py), 7.17 (1 H, td, C5¢Hpy), 7.32 (2 H, d, 2 ¥ C3H py), 7.55 (2 H, td, 2 ¥ C4H py), 7.68(1 H, td, C4¢H py), 7.87 (1 H, dd, C3¢H py), 8.44 (2 H, dd, 2¥ C6H py), 8.49 (1 H, dd, C6¢H py). dC (CDCl3): 51.504 (2 ¥NCH2C5H4N), 59.024 (NCH2CH2N), 89.322 (NC(C5H4N)N),121.985 (2 ¥ C5 py), 122.957 (2 ¥ C3 py), 123.306 (C5¢ py), 123.347(C3¢ py), 136.395 (2 ¥ C4 py), 136.839 (C4¢ py), 148.509 (C6¢ py),148.988 (2 ¥ C6 py), 159.282 (2 ¥ C2 py), 161.076 (C2¢ py).

N ,N ,N ¢-Tris-2-picolylethylenediamine. 1,2,3-Tris - 2 - picolyli -midazolidine (1.2307 g, 3.71 mmol) was dissolved in dry methanol(40 ml). Solid NaBH3CN (0.2339 g, 3.72 mmol) and trifluoroaceticacid (569 mL, 7.43 mmol) were added carefully (Caution! For-mation of HCN possible) and the reaction mixture was stirredovernight under CaCl2 protection. NaOH (38 ml of a 4 M aqueoussolution) was added. The reaction mixture was stirred for 5–6 h andextracted with CHCl3 (3 ¥ 20 ml). The organic phase was dried onNa2SO4 and filtered. The filtrate was evaporated in vacuo leavingthe product as a thin yellow oil (1.2268 g, 99%). dH (CDCl3):2.755 (4 H, s, NCH2CH2N), 3.799 (6 H, s, NCH2C5H4N), 7.11 (3H, td, 2 ¥ C5H py, C5¢H py), 7.260 (1 H, d, C3¢H py), 7.515(2 H, d, 2 ¥ C3H py), 7.60 (3 H, m, 2 ¥ C4H py, C4¢H py),8.482 (3 H, m, 2 ¥ C6 py, C6H py). dC (CDCl3): 46.780, 54.208(NCH2CH2N), 55.054 (NCH2C5H4N), 60.741 ((NCH2C5H4N)2),121.966, 122.089, 122.290, 122.373, 123.161, 123.404 (2 ¥ C3 py,2 ¥ C5 py, C3¢ py, C5¢ py), 136.478, 136.546, 136.647 (2 ¥ C4 py,

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C4¢ py), 149.104, 149.322 (2 ¥ C6 py, C6¢ py), 159.687, 159.937(2 ¥ C2 py, C2¢ py).

N ,N ,N ¢-Tris-2-picolylethylenediamine-N ¢-acetic acid (tpenaH).A mixture of N,N,N¢-tris-2-picolylethylenediamine (0.1212 g,0.36 mmol), bromoacetic acid (0.0513 g, 0.37 mmol) and Cs2CO3

(0.2208 g, 1.14 mmol) in absolute ethanol (4 ml) was heatedovernight under reflux and N2. The solvent was removed underreduced pressure and the residue was redissolved in chloroformto precipitate CsBr which was filtered off. The filtrate wasevaporated in vacuo leaving a brown oil (0.15 g). dC (CDCl3):51.744, 51.795 (NCH2CH2N), 60.056, 60.401, 60.723, 60.820(2 ¥ NCH2C5H4N, NC¢H2C5H4N, NCH2COOH), 121.940,121.999, 122.077, 122.240, 123.130, 123.666, 124.137 (2 ¥ C3 py, 2¥ C5 py, C3¢ py, C5¢ py), 136.379, 136.409, 136.532 (2 ¥ C4 py, C4¢py), 149.134, 149.187, 149.420 (2 ¥ C6 py, C6¢ py), 158.899, 159.380(2 ¥ C2 py, C2¢ py), 176.042 (C O).

[Co(mebpena)Cl]ClO4 (1·ClO4). CoCl2·6H2O (640 mg,2.7 mmol) in a mixture of methanol (2 ml) and water (2 ml)and NaClO4·H2O (4.5 g, 32 mmol) in water (2 ml) wereadded to mebpenaH (843 mg, 2.7 mmol) in methanol (3 ml).The product deposited as red crystals and powder (410 mg,30%). It appears to be a mixture of [Co(mebpena)Cl]ClO4

and the double salt [CoCl(mebpena)]2[Na(ClO4)2(OH2)2]ClO4

for which the crystal structure was solved. ESI MS (MeCN)m/z: 407.3 ([Co(mebpena)Cl]+, 100%). IR (KBr) n (cm-1):1667 (C O, vs), 1094 (ClO4, vs). Anal. calcd (%) for([Co(mebpena)Cl]ClO4)x([CoCl(mebpena)]2 [Na(ClO4)2(OH2)2 ]-ClO4)1-x, x = 0.8813, C: 39.61, H: 4.15, N: 10.87. Found C: 39.62,H: 4.13, N: 10.87.

[Co(bztpen)Cl]ClO4 (2·ClO4). CoCl2·6H2O (49 mg,0.21 mmol) in water (2.5 ml) and NaClO4·H2O (250 mg,1.78 mmol) in water (5 ml) were added to bztpen (88 mg,0.21 mmol) in methanol (10 ml). After 2 h, purple crystalsdeposited (94 mg, 73%). ESI MS (CH3CN), m/z: 241.61([Co(bztpen)]2+, 55%), 256.61 ([Co(bztpen)Cl]2+, 40%), 518.17([Co(bztpen)Cl]+, 100%). Anal. calcd (%) for C27H29N5O4Cl2Co([Co(bztpen)Cl]ClO4) C: 52.53, H: 4.73 N: 11.34, found C: 51.74,H: 4.70, N: 11.03.

[Co(OH)(bztpen)](ClO4)2 (3·(ClO4)2·H2O). Co(ClO4)2·6H2O(147 mg, 0.40 mmol) in water (6 ml) was added to bztpen(171 mg, 0.40 mmol) in methanol (10 ml). The reaction mixturewas purged with oxygen for 5 min. After six days, a reddish–brown powder was isolated and recrystallized from methanol–water to give the product as small aggregated crystals (80 mg).ESI MS (MeOH) m/z: 598.1 ([Co(OH2)(bztpen)]ClO4}+, 89%),580.8 ([Co(bztpen)]ClO4}+, 25%), 389.1 ([Co(bztpen - C6H6N)]+,40%), 249.6 ([Co(OH)(bztpen)]2+, 100%), 240.6 ([Co(bztpen -H)]2+, 98%). dH (D2O): 2.618, 3.714, 3.960, 4.382, 4.963 and5.190 (6 ¥ 1 H, d, NCH2C5H4N/NCH2C5H5), 3.003 (1 H, td,NCH2CH2N), 3.258 (2 H, m, NCH2CH2N), 3.810 (1 H, td,NCH2CH2N), 4.264 (d, NCH2C5H4N/NCH2C5H5), 4.802 (d,NCH2C5H4N/NCH2C5H5), 7.022 (1 H, d, C6H py), 7.299–8.046(14 H, aromatic), 8.662 (1 H, d, C6H py), 9.076 (1 H, d, C6

py). IR (KBr) n (cm-1): 1610 (C O, m), 1088 (ClO4, vs). Anal.calcd C27H32N5O10Cl2Co ([Co(bztpen)OH](ClO4)2·H2O) (%) for C:45.27 H:4.50 N: 9.78. Found C: 45.23, H: 4.15 N: 9.58.

[Co(2-pyridylformate)(mepena)]ClO4 (4·ClO4). [Mn2(meb-pena)2(H2O)2](ClO4)2 (203 mg, 0.21 mmol) and Co(ClO4)2·6H2O(154 mg, 0.42 mmol) were mixed in methanol (2 ml) and water(2 ml). Dioxygen was bubbled through the reaction mixture untilthe colour changed from pink to red–brown (5–10 min). Darkred crystals of [Co(2-pyridylformate)(mepena)]ClO4 (84 mg,40%) were deposited after one week at low temperature (4 ◦C).ESI MS (MeOH) m/z: 403.2 ([Co(2-pyridylformate)(mepena)]+,100%), 419.3 ({[Co(2-pyridylformate)(mepena)] + [O]}+,29%). IR (KBr) n (cm-1): 1666 (C O, vs), 1098 (ClO4

-,vs). Anal. calcd (%) for C17H22.6N4O9.3ClCo ([Co(2-pyridylformate)(mepena)]ClO4·1.3H2O) C: 38.81, H: 4.33,N:10.65. Found C: 38.85, H: 3.89, N: 10.46.

[Co(tpena)](ClO4)2·1/2H2O. (5·ClO4). Co(ClO4)2·6H2O(73.7 mg, 0.20 mmol) in water (1 ml) was added to tpenaH(79.1 mg, 0.20 mmol) in methanol (2 ml). After three days,orange crystals of [Co(tpena)](ClO4)2·1/2H2O (80 mg, 60%) weredeposited. ESI MS (MeOH) m/z: 356.1 ([Co(tpena - C6H6N)]+,32%), 388.1 ([Co(tpena - CO2 - 2H)]+, 18%), 389.1 ([Co(tpena -CH3COOH)]+, 22%), 405.1 ([Co(tpena - CO2)]+, 8%), 427.4([Co(tpena - CO2 + Na+)]+, 18%), 448.1 ([Co(tpena - H)]+, 100%),548.1 ({[Co(tpena)]ClO4}+, 27%). IR (KBr) n (cm-1): 1685 (C O,s), 1094 (ClO4

-, vs). Anal. calcd (%) for C22H25N5O10.5Cl2Co([Co(tpena)](ClO4)2·1/2H2O) C: 42.57 H: 5.59 N: 10.79. FoundC: 42.49 H: 4.49 N: 10.85.

Computational details

The quantum chemical calculations were performed with theGaussian03 software package. The functional BLYP49,50,51 withthe basis set TZVP52 were applied on all element except Co. ForCo the SDD53 Stuttgart/Dresden effective core potential basis setwere utilised. Frequency calculations were performed to confirmthe nature of all stationary points and to calculate the free energiesof the molecules in the gas phase.

Single-crystal X-ray diffraction

Selected crystallographic data are presented in Table 2. Diffractiondata were collected using a Bruker-Nonius X8 APEX-II instru-ment (Mo-Ka radiation). Structure solution and refinement wascarried out using SHELXTL.54 H atoms on C atoms were placedat calculated positions and allowed to ride during subsequentrefinement. For 1, the Na+ cation lies on a crystallographicinversion centre, and the perchlorate anion bound to it wasmodelled as disordered in two orientations, each with 50% siteoccupancy. The unbound perchlorate anion was also modelled asdisordered with the Cl atom lying on a crystallographic 2-foldaxis. The H atoms of the water molecule were visible in differenceFourier maps and their positions were refined with restrained O–Hand H ◊ ◊ ◊ H distances. Refinement of 2 was routine. In 3, one of theperchlorate anions was modelled as disordered in two orientationswith 85 : 15% site occupancy. The H atom associated with the OHgroup was clearly visible in the difference Fourier map, and it wasincluded in the as-found position then allowed to ride on atomO1 during subsequent refinement. The structure contains regionsoccupied by H2O molecules: one of them (O1W) was refined withsite occupancy 85%, and its associated H atoms were clearly visibleand included in the model in the as-found positions. These H

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 10698–10707 | 10705

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Table 2 Crystallographic data for 1–5

1 2 3 4 5

Formula [C17H21ClCoN4O2+]2

Na+[ClO4-]3·2H2O

[C27H29ClCoN5+] [ClO4

-] [C27H30CoN5O2+][ClO4

-]2·1.15H2O[C17H20CoN4O4

+] [ClO4-] [C22H24CoN5O2

2+][ClO4

-]2· 0.5H2OM/g mol-1 1172.89 617.38 718.50 502.75 657.30Crystal system Monoclinic Orthorhombic Triclinic Monoclinic MonoclinicSpace group C2/c Pbca P1 P21/n P21/ca/A 37.978(5) 16.3600(12) 8.6824(5) 10.1794(3) 17.5150(15)b/A 8.0037(10) 17.8210(10) 11.6313(7) 17.2327(6) 22.3432(18)c/A 14.7034(18) 18.0306(13) 15.8043(9) 11.5761(4) 13.4944(8)a (◦) 90 90 76.450(3) 90 90b (◦) 101.149(2) 90 87.333(3) 105.851(1) 101.220(3)g (◦) 90 90 69.100(2) 90 90V/A3 4385.0(10) 5256.8(6) 1448.32(15) 1953.45(11) 5180.0(7)Z 4 8 2 4 8Dc/g cm-3 1.777 1.560 1.648 1.709 1.686m(Mo-Ka)/mm-1 1.157 0.901 0.846 1.072 0.938T/K 120(2) 120(2) 120(2) 180(2) 180(2)Crystal size/mm 0.18 ¥ 0.14 ¥ 0.13 0.20 ¥ 0.12 ¥ 0.04 0.28 ¥ 0.20 ¥ 0.20 0.23 ¥ 0.10 ¥ 0.08 0.12 ¥ 0.12 ¥ 0.03Crystal colour purple purple purple red orangeq Range/◦ 3.74–25.02 3.55–25.88 3.58–26.00 3.66–27.98 3.56–25.03Reflections collected 20404 77571 72725 19471 40797Unique reflections 3855 4971 5661 4699 9036Rint 0.051 0.079 0.036 0.026 0.096Observed reflections[I > 2s(I)]

3058 3534 5087 4310 5363

No. of parameters 373 352 431 281 730No. of restraints 15 0 20 0 0R1 [I > 2s(I)] 0.033 0.033 0.028 0.039 0.054wR2 (all data) 0.079 0.077 0.070 0.098 0.141GOF on F 2 1.02 1.02 1.04 1.09 1.03

atoms form hydrogen bonds to the OH group bound to Co(III)and also to one perchlorate anion. Two further atom sites werealso clearly identifiable, which were included in the model as O2Wand O3W, both refined with 15% site occupancy. Atom O2W lies ina suitable position to accept a hydrogen bond from the OH groupbound to Co(III), but it was difficult to place H atoms on O2Wand O3W in such a way as to form a coherent hydrogen-bondnetwork. Thus, the H atoms associated with O2W and O3W arenot included in the model, so that the atom sites sum to 1.2 H atomsper unit cell less than the stated empirical formula. Although thisdisordered arrangement of H2O molecules around the OH group isclearly an approximation, the assignment of the OH group can bemade confidently on the basis of the Co(III)–OH bond length (asdescribed in the Discussion). Refinement of 4 and 5 was routine.For 4, the H atom on N2 was visible in the Fourier map, butultimately included in a geometrical position and allowed to rideon N2 for the final stages of refinement. For 5, which contains twocrystallographically distinct complexes, the H atoms of the watermolecule were not apparent from the X-ray data, and these wereincluded so as to make reasonable intermolecular contacts (oneto a perchlorate anion, and one directed towards a neighbouringpyridyl ring).

Acknowledgements

We thank the Danish Councils for Independent Research |Natural Sciences and Technology and Production for financialsupport and Sanne Kjæregaard Knudsen for assistance with someof the synthetic work.

Notes and references

1 F. Miller, J. Simplicio and R. G. Wilkins, J. Am. Chem. Soc., 1969, 91,1962–1967.

2 (a) E. Bolzacchini, C. Canevali, F. Morazzoni, M. Orlandi, B. Rindoneand R. Scotti, J. Chem. Soc., Dalton Trans., 1997, 4695–4699; (b) K.S. Murray, A. van den Bergen, B. J. Kennedy and B. O. West, Aust. J.Chem., 1986, 39, 1479–1493; (c) S. A. Cockle, H. A. O. Hill and R. J. P.Williams, Inorg. Nucl. Chem. Lett., 1970, 6, 131–134.

3 (a) M. Baumgarten, C. J. Winscom and W. Lubitz, Appl. Magn. Reson.,2001, 20, 35–70; (b) J. P. Collman, J. E. Hutchison, M. A. Lopez, A.Tabard, R. Guilard, W. K. Seok, J. A. Ibers and M. L’Her, J. Am.Chem. Soc., 1992, 114, 9869–9877; (c) C. K. Chang, J. Chem. Soc.,Chem. Commun., 1977, 800–801.

4 (a) R. Boca, J. Mol. Catal., 1982, 14, 313–322; (b) R. E. Hester and E.M. Nour, J. Raman Spectrosc., 1981, 11, 43–48.

5 S. Thyagarajan, C. D. Incarvito, A. L. Rheingold and K. H. Theopold,Chem. Commun., 2001, 2198–2199.

6 (a) J. H. Cameron and S. Graham, J. Chem. Soc., Dalton Trans., 1992,385–391; (b) C. Granier and R. Guilard, Microchem. J., 1996, 53, 109–121.

7 (a) E. Hohenester, C. Kratky and B. Krautler, J. Am. Chem. Soc.,1991, 113, 4523–4530; (b) H. Dube, B. Kasumaj, C. Calle, M. Saito,G. Jeschke and F. Diederich, Angew. Chem., Int. Ed., 2008, 47, 2600–2603; (c) S. Nakayama, F. Tani, M. Matsu-Ura and Y. Naruta, Chem.Lett., 2002, 496–497; P. G. Jene and J. A. Ibers, Inorg. Chem., 2000, 39,3823–3827; (d) W. Kanda, H. Okawa and S. Kida, J. Chem. Soc., Chem.Commun., 1983, 973–974; (e) D. H. Busch, P. J. Jackson, M. Kojima,P. Chmielewski, N. Matsumoto, J. C. Stevens, W. Wu, D. Nosco, N.Herron, N. Ye, P. R. Warburton, M. Masarwa, N. A. Stephenson, G.Christoph and N. W. Alcock, Inorg. Chem., 1994, 33, 910–923; (f) I. A.Guzei and A. Bakac, Inorg. Chem., 2001, 40, 2390–2393; (g) P. Doppelt,J. Fischer, L. Ricard and R. Weiss, New J. Chem., 1987, 11, 357–364.

8 C. Baffert, M.-N. Collomb, A. Deronzier, S. Kjærgaard-Knudsen, J.-M. Latour, K. H. Lund, C. J. McKenzie, M. Mortensen, L. P. Nielsenand N. Thorup, Dalton Trans., 2003, 1765–1772.

9 I. Bernal, I.-M. Jensen, K. B. Jensen, C. J. McKenzie, H. Toftlund andJ.-P. Tuchagues, J. Chem. Soc., Dalton Trans., 1995, 3667–3675.

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10 We have previously referred to the Rbpena- ligands, Scheme1(a) R = CH3 and CH2C6H5 as N,N¢-bis(2-pyridylmethyl)ethane-1,2-diamine (mcbpen-) and N-benzyl-N¢-carboxymethyl-N,N¢-bis(2-pyridylmethyl)ethane-1,2-diamine (bcbpen-) respectively, however forease comparison to the tpen, Rtpen, and tpena- ligand systemscontained within this manuscript we have chosen to use the moregeneric Rbpena- acronym.

11 M. Heitzmann, C. Gateau, L. Chareyre, M. Miguirditchian, M.-C.Charbonnel and P. Delangle, New J. Chem., 2010, 34, 108–116.

12 G. Anderegg and F. Wenk, Helv. Chim. Acta, 1967, 50, 2330–2332.13 (a) F. Munz (General Aniline Works), Polyamino carboxylic acids, US

patent 2130505 (1938); (b) E. Anderson and J. Gaunt, Ind. Eng. Chem.,1960, 52, 190–196.

14 (a) A. Hazell, C. J. McKenzie, L. P. Nielsen, S. Schindler and M.Weitzer, J. Chem. Soc., Dalton Trans., 2002, 310–317; (b) N. Ortega-Villar, V. M. Ugalde-Saldivar, M. Carmen Munoz, L. A. Ortiz-Frade,J. G. Alvarado-Rodriguez, J. A. Real and R. Moreno-Esparza, Inorg.Chem., 2007, 46, 7285–7293.

15 J. Kaizer, E. J. Klinker, N. Y. Oh, J.-U. Rohde, W. J. Song, A. Stubna,J. Kim, E. Munck, W. Nam and L. Que Jr., J. Am. Chem. Soc., 2004,126, 472–473.

16 Nebe, A. Beitat, C. Wurtele, C. Ducker-Benfer, R. Van Eldik, C. J.McKenzie and S. Schindler, Dalton Trans., 2010, 39, 7768–7773.

17 A. Nielsen, F. B. Larsen, A. D. Bond and C. J. McKenzie, Angew.Chem., Int. Ed., 2006, 45, 1602–1606.

18 K. B. Jensen, C. J. McKenzie, L. P. Nielsen, J. Z. Pedersen and H. M.Svendsen, Chem. Commun., 1999, 1313–1314.

19 (a) Y. Zang, T. E. Elgren, Y. Dong and L. Que Jr., J. Am. Chem. Soc.,1993, 115, 811–813; (b) M. Lubben, A. Meetsma, E. C. Wilkinson, B.Feringa and L. Que Jr., Angew. Chem., Int. Ed. Engl., 1995, 34, 2048;(c) C. Nguyen, R. J. Guajardo and P. K. Mascharak, Inorg. Chem.,1996, 35, 6273–6281; (d) C. Kim, K. Chen, J. Kim and L. Que Jr., J.Am. Chem. Soc., 1997, 119, 5964–5965.

20 E. J. Klinker, J. Kaizer, W. W. Brennessel, N. L. Woodrum, C. J. Cramerand L. Que Jr., Angew. Chem., Int. Ed., 2005, 44, 3690–3694.

21 A. Thibon, J. England, M. Martinho, V. G. Young Jr., J. R. Frisch, R.Guillot, J.-J. Girerd, F. Banse, L. Que Jr. and E. Munck, Angew. Chem.,Int. Ed., 2008, 47, 7064–7067.

22 S. Groni, G. Blain, R. Guillot, C. Policar and E. Anxolabehere-Mallart,Inorg. Chem., 2007, 46, 1951–1953.

23 F. B. Johansson, A. Bond and C. J. McKenzie, Inorg. Chem., 2007, 46,2224–2236.

24 H. Molina-Svendsen, G. Bojesen and C. J. McKenzie, Inorg. Chem.,1998, 37, 1981–1983.

25 D. G. Lonnon, D. C. Craig and S. B. Colbran, Inorg. Chem. Commun.,2003, 6, 1351–1353.

26 P. Comba, S. Kuwata, G. Linti, H. Pritzkow, M. Tarnai and H.Wadepohl, Chem. Commun., 2006, 2074–2076.

27 (a) A. M. Calafat and L. G. Marzilli, Inorg. Chem., 1993, 32, 2906–2911;(b) M. Yashiro, T. Mori, M. Sekigushi, S. Yoshikawa and S. Shiraishi,J. Chem. Soc., Chem. Commun., 1992, 1167–1168; (c) B. Sonnberger,P. Huhn, P. A. Wasserburger and A. F. Wasgestian, Inorg. Chim. Acta,1992, 196, 65–71; (d) R. K. Egdal, A. D. Bond, F. B. Larsen and C. J.McKenzie, Inorg. Chim. Acta, 2005, 358, 376–382.

28 D. Maiti, A. A. Narducci Sarjeant and K. D. Karlin, Inorg. Chem.,2008, 47, 8736–8747.

29 S. Mahapatra, V. G. Young Jr., S. Kaderli, A. D. Zuberhuhlerand W. B. Tolman, Angew. Chem., Int. Ed. Engl., 1997, 36, 130–133.

30 D. Lee and S. J. Lippard, J. Am. Chem. Soc., 2001, 123, 4611–4612.31 (a) H. Toftlund and S. Yde-Andersen, Acta Chem. Scand., Ser. A, 1981,

35a, 575–585; (b) J. B. Mandel, C. Maricondi and B. E. Douglas, Inorg.Chem., 1988, 27, 2990–2996; (c) J. B. Mandel and B. E. Douglas, Inorg.Chim. Acta, 1989, 155, 55–69.

32 D. Chakraborty, R. R. Gowda and P. Malik, Tetrahedron Lett., 2009,50, 6553–6556.

33 (a) M. Renz and B. Meunier, Eur. J. Org. Chem., 1999, (4), 737–750;(b) P. Y. Bruice, Organic Chemistry Int. Ed., Pearson Prentice Hall,2004, 4th edn, p. 853.

34 A. Bayer and V. Villiger, Ber., 1899, 32, 3625–3633.35 F. Benedini, G. Galliani, M. Nali, B. Rindone and S. Tollari, J. Chem.

Soc., Perkin Trans. 2, 1985, (12), 1963–1967.36 R. M. Hartshorn, J. Chem. Soc., Dalton Trans., 2002, 3214–3218.37 B. Yu, W. C. Edstrom, J. Benach, Y. Hamuro, P. C. Weber, B. R. Gibney

and J. F. Hunt, Nature, 2006, 439, 879–884.38 P. A. Aas, M. Otterlei, P. Ø. Falnes, C. B. Vagbø, F. Skorpen, M. Akbari,

O. Sundheim, M. Bjøras, G. Slupphang, E. Seeberg and H. E. Krokan,Nature, 2003, 421, 859–863.

39 P. Ø. Falnes, M. Bjøras, P. A. Aas, O. Sundheim and E. Seeberg, NucleicAcids Res., 2004, 32, 3456–3461.

40 T. W. Roy and A. S. Bhagwat, Nucl. Acids Res., 2007, 35, e147/1–e147/7.

41 B. Sedgwick, Nat. Rev. Mol. Cell Biol., 2004, 5, 148–157.42 P. Ø. Falnes, A. Klungland and I. Alseth, Neuroscience, 2007, 145,

1222–1232.43 B. Sedgwick and T. Lindahl, Oncogene, 2002, 21, 8886–8894.44 P. Ø. Falnes, Nucleic Acids Res., 2004, 32, 6260–6267.45 (a) M. C. White, A. G. Doyle and E. N. Jacobsen, J. Am. Chem. Soc.,

2001, 123, 7194–7195; (b) M. Kahnes, H. Gorls, L. Gonzalez andMatthias Westerhausen, Organometallics, 2010, 29, 3098–3108; (c) J.England, C. R. Davies, M. Banaru, A. J. P. White and G. J. P. Britovsek,Adv. Synth. Catal., 2008, 350, 883–897; (d) R. Mas-Balleste and L. QueJr., J. Am. Chem. Soc., 2007, 129, 15964–15972.

46 A. K. Poulsen, A. Rompel and C. J. McKenzie, Angew. Chem., Int. Ed.,2005, 44, 6916–6920.

47 R. K. Egdal, A. Nielsen, A. D. Bond, M. J. Bjerrum and C. J. McKenzie,Dalton Trans., 2011, 40, 3849–3858.

48 H. A. Goodwin and F. Lions, J. Am. Chem. Soc., 1960, 82, 5013–5023.49 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785–789.50 A. D. Becke, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38, 3098–3100.51 B. Miehlich, A. Savin, H. Stoll and H. Preuss, Chem. Phys. Lett., 1989,

157, 200–206.52 (a) A. Schafer, H. Horn and R. Ahlrichs, J. Chem. Phys., 1992, 97,

2571–2577; (b) A. Schafer, C. Huber and R. Ahlrichs, J. Chem. Phys.,1994, 100, 5829–5835.

53 (a) M. Dolg, H. Stoll and H. Preuss, Theor. Chim. Acta, 1993,85, 1431–1441; (b) M. Dolg, U. Wedig, H. Stoll and H. Preuss, J.Chem. Phys., 1987, 86, 866–72; (c) T. Leininger, A. Nicklass, H. Stoll,M. Dolg and P. J. Schwerdtfeger, Chem. Phys., 1996, 105, 1052–1059.

54 G. M. Sheldrick, SHELXTL Ver. 6.10, Bruker AXS, Madison,Wisconsin, USA, 2000.

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