Synthesis and Reactivity Studies of Benzo-Substituted Bis(indenyl)Iron and Zirconium Complexes: The Difference a Methyl Group CanMakeGregory P. McGovern,† Fernando Hung-Low,† Jesse W. Tye,‡ and Christopher A. Bradley*,†

†Department of Chemistry and Biochemistry, Texas Tech University, Box 41061 Lubbock, Texas 79409-1061, United States‡Department of Chemistry, Ball State University, Muncie, Indiana 47306, United States

*S Supporting Information

ABSTRACT: The synthesis of the sterically hindered 1,3,4,7-tetrasubstituted indenyl ligand 1,3-(CHMe2)2-4,7-Me2-C9H3 isaccomplished via initial preparation of 4,7-dimethylindene and subsequent installation of isopropyl groups on the five-memberedring. Synthesis of the corresponding bis(indenyl) iron complex (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2Fe (3) and comparison to abis(indenyl) iron analogue devoid of benzo substituents, (η5-C9H5-1,3-(CHMe2)2)2Fe (4), through variable-temperature NMRstudies and electrochemistry, establishes the new ligand as both more sterically demanding and slightly more electron rich. Alkali-metal reduction of (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2ZrCl2 (5) yields an equilibrium mixture of the η5,η9 sandwich complex 7and the cyclometalated hydride 8, indicating both that benzo binding is still possible when the six-membered ring is substitutedand that ligand activation can be modulated by the choice of substituents, as the η5,η9 zirconium sandwich 9, which lacks methylgroups on the benzo ring, does not cyclometalate under ambient conditions. The reactivity of 8 was explored, demonstrating thatthe cyclometalated species can act as a source of both Zr(II), via ligand-induced reductive elimination, and Zr(IV), throughinsertion or σ bond metathesis, depending on the added reagent. Addition of H2 to 8 gives (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2ZrH2 (17), which upon prolonged thermolysis results in benzo CC bond insertion into the Zr hydride. The reaction ratein comparison to that of the bis(indenyl) zirconium dihydride analogue 19, without benzo substituents, suggests that the methylgroups on the six-membered ring significantly reduce the rate of intramolecular insertion. These studies show that benzosubstitution accomplishes both major intended goals: destabilizing the interaction of the benzo ring with low-valent metals whilereducing the rate of insertion of a benzo CC bond into a metal hydride in high-oxidation-state complexes.

■ INTRODUCTIONCyclopentadienyl (Cp) ancillary ligands and related benzo-substituted derivatives (i.e., indenyl or fluorenyl) have played acritical role in the development of organometallic chemistry,serving as supports for transition metals that (1) mediate avariety of stoichiometric and catalytic small-molecule trans-formations,1−4 (2) act as materials with attractive physical andelectronic properties,5,6 and (3) serve as molecular precursorsfor nanomaterials,7 among other applications.8,9 Though Cpand related ligands are often considered robust spectators,modification or unusual interactions of these supports are notuncommon. For example, the scission and rearrangement of theC−C bond skeleton of Cp and indenyl ligands by presumedlow-valent Ti centers have been studied extensively and utilized

in the construction of more elaborate carbocyclic ligands.10−13

Cyclopentadienyl ligands have also been shown to bridge twometal centers, forming triple-decker complexes,14 specificallywith group 9 metals.15 Indenyl and fluorenyl ligands havedemonstrated the ability for the benzo ring of the ligand toserve as a stabilizing fragment for low-valent metals, both in anintramolecular fashion16,17 and in the synthesis of multimetallicframeworks,18−21 and have also displayed the ability toparticipate in insertion of a benzo CC bond into a Zrhydride.22,23 The flexibility of indenyl/fluorenyl hapticity is alsowell documented.24,25

Received: November 18, 2011

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In the context of our research, we hoped to further extendthe use of indenyl ligands as intramolecular dialkene donors tometals, specifically using Co, to mediate C−H activation forultimate application to hydrocarbon activation. Specifically, wetargeted alkane dehydrogenation, a reaction mediated by pincerIr complexes through presumed unsaturated, low-valent metalintermediates.26 Initially, we postulated that intramolecularbenzo coordination could give rise to an 18-electron complex,which due to ring strain could dissociate the L2 donor, a processobserved in related η5, η9 Zr sandwich complexes,16 and provideaccess to a reactive 14-electron Co(I) species which mightfacilitate C−H activation in a manner similar to that in thepincer Ir systems (Scheme 1).However, when the bis(indenyl) cobalt complex (η5-C9H5-

1,3-(SiMe3)2)2Co (1) was reduced in the presence ofvinyltrimethylsilane, synthesis of dimer 2 was observed, whereeach metal binds in an intermolecular fashion to anotherindenyl Co(I) unit (Scheme 2).27 Though 2 serves as a ready

source of Co(I) in the presence of carbon monoxide,27 to datethe complex has shown no propensity to activate C−H bonds.This lack of reactivity likely involves the inability to access bonafide 14-electron indenyl Co(I) fragments, as the formation of 2is thought to proceed through 16-electron (η5-C9H5-1,3-(SiMe3)2)Co(L) intermediates, akin to related Cp*CoL (Cp*= η5-C5Me5) complexes which are unreactive with C−Hbonds,28 presumably due to their spin state.29−31

Reduction of 1 in the absence of strong donor ligands givesrise to the unusual 20-electron Co(I) anion [Na(THF)n][(η

5-C9H5-1,3-(SiMe3)2)2Co].

32 As with 2, this anion can act as asource of an indenyl Co(I) equivalent in the presence of avariety of substrates but does not react with alkanes. Forexample, even in the presence of activated substrates, such assilyl-protected amines popularized by Brookhart and co-workers for transfer dehydrogenation of sp3 C−H bondsusing Cp*Co(η2-H2C=CHSiMe3)2,

33 in situ generation of theanionic Co(I) complex results only in the synthesis of 2(Scheme 2).32

Though dimer formation prevents access to the desiredtransient 14-electron Co(I) species, one simple strategy toeither establish a monomer−dimer equilibrium or destabilizedimer formation altogether involves appending substituents onthe benzo ring of the indenyl ligand (eq 1). Given the dramatic

ancillary ligand effects observed by subtle alterations in relatedCp ligands, with regard to dinitrogen binding34 and partialreduction using group 4 metals,2 it seemed worthwhile toexplore simple ligand modifications of the indenyl scaffold topromote generation of more reactive indenyl Co(I) equivalents.The steric and electronic effects of benzo ligand substitution

on metal reactivity have been examined to some extent.35,36

With group 4 metal catalysts active for olefin polymerization,electron-withdrawing groups tend to decrease catalyst activityand polymer molecular weight, while electron-rich groups havelittle effect on catalyst or subsequent polymer production.37,38

Structural studies of the effect of permethylation of indenyl39,40

and fluorenyl41 complexes of both early and late transitionmetals suggest the groups impart an increase of electrondensity, as expected. Reports of selective methyl groupappendage on indenyl ligands have also appeared involvingboth Cr42 and Fe,43 demonstrating that the spin state andligand electronics can be modulated considerably on the basisof the position of the substituents. Recently, we have foundmethyl substitutions of the Cp or benzo rings of bis(indenyl)Co or Zr complexes suggest that steric effects are most

Scheme 1. (a) Ideal Co-Catalyzed Alkane Dehydrogenation and (b) Proposed Mechanism

Scheme 2. Synthesis and Proposed Mechanism of Formationof Dimer 2 from Bis(indenyl) Co(II) Complex 1

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pronounced when groups are placed on the five-memberedring, while benzo substitution can attenuate the electron-donating ability of the methyl group.44 Despite these studies,little research has focused on examining the effects on thechemistry of low-valent metals ligated by benzo-substitutedligands.45

Here, we report studies of the synthesis and reactivity of ironand zirconium complexes supported by the 1,3,4,7-tetrasub-stituted indenide ligand (1,3-(CHMe2)2-4,7-Me2-C9H3). Coor-dination to Fe and Zr indicates that the ligand is both moresterically encumbering and electron rich relative to the related1,3-diisopropylindenide. This work establishes that low-valentmetals, specifically Zr(II), can still bind despite benzo ringsubstitution, but the interaction is metastable, resulting inpromotion of cyclometalation of ligand substituents. Further-more, the reaction of high-oxidation-state Zr complexessupported by the new indenyl ligand demonstrates the rate ofintramolecular benzo insertion into the metal hydride has beensignificantly reduced. These studies provide promise thatsecond-generation indenyl ligands can potentially permit accessto the desired monomeric 14-electron Co(I) equivalents andthat two key steps required to close a catalytic cycle of alkanedehydrogenation, (1) promoting C−H activation and (2)limiting catalyst deactivation by intramolecular insertion(Scheme 1, steps i and iii), can perhaps both be modulatedby careful ligand choice.

■ EXPERIMENTAL SECTION46

Preparation of 1,3-Diisopropyl-4,7-dimethyl-1H-indene. In aflask containing 14.51 g (0.10 mol) of 4,7-dimethylindene47 underdynamic nitrogen, 150 mL of dry THF was added via cannula transferand the reaction mixture was then cooled in a dry ice/acetone bath.Butyllithium in hexanes (1.6 M, 69.5 mL, 0.11 mol) was addeddropwise via syringe, and the mixture was warmed to ambienttemperature. After it was stirred for 3 h, the solution was returned to adry ice/acetone bath, and 18.80 g (0.11 mol) of 2-iodopropane wasadded dropwise via syringe over 10 min. The solution was againwarmed to ambient temperature and stirred overnight. Butyllithiumand 2-iodopropane were then added during a second sequence asdescribed above and the mixture was stirred overnight again. Thesolvent was then removed in vacuo. Water (250 mL) was added, andthe organic compounds were extracted with three 300 mL portions ofether. The combined ether layers were dried over MgSO4 and filtered,and the solvent was removed in vacuo to afford 19.13 g (83.3%) ofdark yellow product. 1H NMR (CDCl3): δ 0.26 (d, 7 Hz, 3H,CHMe2), 1.21 (m, 6H, CHMe2), 1.25 (d, 7 Hz, 3H, CHMe2), 2.34 (s,3H, Benzo-Me), 2.53 (s, 3H, Benzo-Me), 2.61 (m, 1H, CHMe2), 3.11(m, 1H, CHMe2), 3.34 (br, 1H, CH), 6.18 (s, 1H, Cp), 6.92 (m, 2H,Benzo). 13C NMR (CDCl3): δ 15.05, 18.97, 19.96, 23.01, 23.14, 23.70,27.46, 27.86 (CHMe2/Benzo-Me), 53.70 (Cp CH), 126.52, 127.11,127.59, 129.79, 130.40, 142.57, 146.97, 153.52 (Cp/Benzo). Massspectrum (EI) for C17H24: calcd, m/z 228.2; found, m/z 228.3.Preparation of Li[C9H3-1,3-(CHMe2)2-4,7-Me2]. In a glovebox, a

1 L round-bottom flask was charged with 8.68 g (38 mmol) of 1,3-diisopropyl-4,7-dimethyl-1H-indene and 450 mL of pentane. The flaskwas placed in a liquid-nitrogen-cooled cold well for 20 min. Uponremoval from the cold well, 26.1 mL (41.8 mmol) of 1.6 Mbutyllithium in hexanes was added. The reaction mixture was warmedto ambient temperature and stirred for 16 h. The product was thencollected by filtration to yield 6.00 g (67%) of a pale white solid.Preparation of (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2ZrCl2 (5). In a

flask containing 100 mL of ether, 1.53 g (13.2 mmol) of ZrCl4 in aglovebox was added, and the flask was then placed in a liquid-nitrogen-cooled cold well for 20 min. Li[(C9H3-(CHMe2)2-4,7-(Me2)2] (3.09 g,6.60 mmol) was then added, and the reaction mixture was warmed toambient temperature and stirred for an additional 16 h. Solvent wasthen removed in vacuo, the resulting yellow solid was suspended in

pentane, and the suspension was then filtered. The solid was dried invacuo to yield orange-yellow 5. The solid was then dissolved inCH2Cl2 and filtered through Celite to remove residual LiCl. Theyellow solution was dried in vacuo to yield 2.73 g (67%) of analyticallypure 5. Anal. Calcd for C34H46Cl2Zr: C, 66.20; H, 7.52. Found: C,65.93; H, 7.33. 1H NMR (C6D6): δ 0.94 (d, 7 Hz, 12H, CHMe2), 1.34(d, 7 Hz, 12H, CHMe2), 2.55 (s, 12H, Benzo-Me), 3.24 (m, 4H,CHMe2), 6.62 (s, 4H, Benzo), 7.03 (s, 2H, Cp). 13C NMR (C6D6): δ21.16, 22.47, 27.41, 28.21 (CHMe2/Benzo-Me), 127.48, 134.13 (Cp/Benzo). Three Cp/Benzo resonances were not located.

Observation of (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)(η9-C9H3-

(CHMe2)2-4,7-Me2)Zr (7). A flask with 20 mL of pentane wascharged with 41.56 g (207 mmol) of mercury in a glovebox. Smallpieces of sodium (0.21 g, 9.03 mmol) were added slowly and allowedto amalgamate with the mercury. The resulting amalgam was stirredfor 15 min. Then, 1.01 g (1.81 mmol) of 5 was added as a pentaneslurry and stirred under ambient conditions for 48 h. The resultingdark solution was decanted onto a Celite pad and filtered. The solventwas removed in vacuo to yield 0.481 g (49%) of a mixture of 7 and 8as a dark brown solid. Note that 7 cannot be isolated in the absence of8 and equilibrates with the cyclometalated hydride over the course ofhours. Combustion analysis, though satisfactory, provides noindication of purity, since 7 and 8 have the same empirical formula.1H NMR (toluene-d8, −20 °C): δ 0.90 (m, 9H, CHMe2), 1.09 (d, 6Hz, 3H, CHMe2), 1.15 (d, 6 Hz, 3H, CHMe2), 1.20 (d, 6 Hz, 3H,CHMe2), 1.26 (d, 6 Hz, 6H, CHMe2) 2.25 (s, 6H, Benzo-Me), 2.59 (s,6H, Benzo-Me), 3.01 (sept, 6 Hz, 2H, CHMe2), 3.37 (s, 2H, η9-Benzo), 3.49 (sept, 6 Hz, 2H, CHMe2), 6.14 (s, 1H, Cp), 6.36 (s, 2H,Benzo), 6.39 (s, 1H, Cp).

Isolation of Bis(indenyl) Zr Cyclometalated Hydride (8). Thereaction was performed in a glovebox. A 20 mL scintillation vial wascharged with 0.481 g (0.077 mmol) of a mixture of 7 and 8 dissolvedin 10 mL of pentane, and the solution was stirred under ambientconditions for 1 week. The resulting suspension was filtered and rinsedwith cold pentane to remove any 7, and 0.187 g (38.9%) of yellow 8was collected from the filter. Anal. Calcd for C34H46Zr: C, 74.80; H,8.49. Found: C, 74.53; H, 8.69. 1H NMR (C6D6): δ −1.88 (br m, 1H,Zr-CH2), −1.13 (br m, 1H, Zr-CH2), 0.91 (d, 7 Hz, 3H, CHMe2), 1.01(d, 7 Hz, 3H, CHMe2), 1.10 (d, 7 Hz, 3H, CHMe2), 1.13 (d, 7 Hz, 3H,CHMe2), 1.25 (d, 7 Hz, 3H, CHMe2), 1.45 (d, 7 Hz, 3H, CHMe2),1.55 (d, 7 Hz, 3H, CHMe2), 2.16 (s, 3H, Benzo-Me), 2.18 (s, 3H,Benzo-Me), 2.28 (s, 3H, Benzo-Me), 2.42 (sept, 7 Hz, 1H, CHMe2),2.49 (s, 3H, Benzo-Me), 2.58 (sept, 7 Hz, 1H, CHMe2), 4.09 (sept, 7Hz, 1H, CHMe2), 4.48 (m, 1H, CHMe2), 6.00 (s, 1H, Zr-H), 6.35−6.46 (br, 3H, Benzo), 6.51 (d, 7 Hz, 1H, Benzo), 6.99 (s, 1H, Cp),7.75 (s, 1H, Cp). 13C NMR (C6D6): δ 21.47, 21.65, 21.77, 21.88,22.44, 22.63, 25.85, 26.17, 28.50, 28.82, 29.19, 29.88, 30.10 (CHMe2/Benzo-Me), 43.64 (Zr-CH2), 102.37, 107.23, 111.57, 119.11, 121.33,122.19, 122.48, 123.12, 123.47, 123.82, 124.56, 124.85, 125.73, 127.22,131.37, 131.70, 131.80, 132.71 (Cp/Benzo). Two CHMe2/Benzo-Meresonances were not found.

Preparation of the Bis(indenyl) Zr DMAP Hydride (10). A 75mL thick-walled reaction vessel was charged with 0.158 g (0.282mmol) of 8, 0.034 g (0.282 mmol) of dimethylaminopyridine(DMAP), and 15 mL of toluene. The reaction mixture was stirredat 70 °C for 5 days. The solvent was removed in vacuo, and theproduct was rinsed with pentane to yield 0.135 g (69.8%) of whitesolid. Analytically pure material could be obtained by washing thecrude product with cold pentane. Anal. Calcd for C41H56N2Zr: C,73.71; H, 8.45; N, 4.19. Found: C, 73.49; H, 8.22; N, 4.06. 1H NMR(C6D6): δ 1.14 (d, 7 Hz, 6H, CHMe2), 1.19 (d, 7 Hz, 6H, CHMe2),1.69 (d, 7 Hz, 6H, CHMe2), 1.79 (d, 6 Hz, 6H, CHMe2), 2.11 (s, 6H,NMe2 or Benzo-Me), 2.38 (s, 6H, NMe2 or Benzo-Me), 2.73 (s, 6H,NMe2 or Benzo-Me), 3.75 (sept, 7 Hz, 2H, CHMe2), 4.05 (s, 1H, Zr-H), 4.08 (sept, 7 Hz, 2H, CHMe2), 5.76 (d, 6 Hz, 1H, DMAP CH),6.03 (d, 8 Hz, 2H, Benzo), 6.01 (s, 2H, Cp), 6.11 (d, 8 Hz, 2H,Benzo), 6.90 (s, 1H, DMAP CH), 7.09 (d, 6 Hz, 1H, DMAP CH). 13CNMR (C6D6): δ 21.42, 21.85, 22.11, 23.12, 28.42, 28.69, 29.76(CHMe2/Benzo-Me), 38.97 (NMe2), 100.39, 105.89, 108.77, 116.87,120.75, 121.00, 123.67, 124.92, 130.06, 132.06, 132.84, 141.28, 154.52

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(Cp/Benzo/DMAP). One CHMe2/Benzo-Me and one Cp/Benzo/DMAP resonance were not located.Formation of the Bis(indenyl) Zr Acyl Carbonyl Complex

(13). A 75 mL thick-walled reaction vessel was charged with 0.273 g(0.500 mmol) of 8 and 15 mL of toluene. The vessel was sealed,removed from the glovebox, and frozen in liquid nitrogen. Fouratmospheres of carbon monoxide was then added to the reaction. Thevessel was warmed to ambient temperature and the mixture stirredvigorously for 15 min. The volatiles were removed in vacuo, and theresulting product was rinsed with cold pentane to yield 0.224 g(74.4%) of analytically pure 13 as a red-orange solid. Anal. Calcd forC36H46O2Zr: C, 71.83; H, 7.70. Found: C, 71.50; H, 7.37.

1H NMR(C6D6): δ 0.71 (d, 7 Hz, 3H, CHMe2), 0.85 (d, 7 Hz, 3H, CHMe2),0.93 (d, 7 Hz, 3H, CHMe2), 1.04 (m, 6H, CHMe2), 1.16 (d, 7 Hz, 3H,CHMe2), 1.53 (d, 7 Hz, 3H, CHMe2), 2.24 (m, 1H, CHMe2), 2.33 (m,1H, CH2), 2.37 (s, 3H, Benzo-Me), 2.54 (s, 3H, Benzo-Me), 2.60 (s,3H, Benzo-Me), 2.83 (s, 3H, Benzo-Me), 3.08 (m, 1H, CH2), 3.30 (m,2H, CHMe2), 3.84 (br, 1H, CHO), 5.43 (s, 1H, Cp), 5.77 (s, 1H, Cp),6.55 (d, 8 Hz, 1H, Benzo), 6.60 (d, 8 Hz, 1H, Benzo), 6.66 (d, 8 Hz,1H, Benzo), 6.71 (d, 8 Hz, 1H, Benzo). The CHMeCH2 resonancewas not located. 13C NMR (C6D6): δ 20.30, 21.32, 21.76, 22.70, 22.80,23.40, 25.50, 26.18, 27.18, 28.46, 28.49, 28.85, 29.73 (CHMe2/Benzo-Me), 47.48 (CH2), 89.82 (CHO), 103.69, 113.49, 114.15, 114.92,118.19, 118.85, 119.28, 122.55, 124.73, 125.44, 125.78, 125.96, 126.24,130.28, 130.88, 131.10, 133.09 (Cp/Benzo), 258.31 (CO). TwoCHMe2/Benzo-Me and one Cp/Benzo resonance were not located. IR(pentane): 1935 cm−1 (CO).Observation of the Insertion Product from Reaction of 8

with tert-Butyl Isocyanide (16). A 20 mL scintillation vial wascharged with 0.168 g (0.030 mmol) of 8 and excess tert-butylisocyanide (0.036 g, 0.0433 mmol) along with 10 mL of toluene. Theresulting solution was stirred for 30 min and filtered through a glassfrit. The volatiles were removed in vacuo to yield 0.102 g (52.6%) of16 as a black solid. Compound 16 decomposes in the solid state or insolution over a few hours at ambient temperature to give an intractablemixture of products. 1H NMR (C6D6, 7 °C): δ 0.09 (m, 1H, ZrCH2),0.20 (m, 1H, ZrCH2), 0.94 (m, 9H, CHMe2), 1.00 (s, 9H, CMe3), 1.21(d, 7 Hz, 3H, CHMe2), 1.44. (d, 7 Hz, 3H, CHMe2), 1.68 (m, 6H,CHMe), 2.48 (s, 3H, Benzo-Me), 2.49 (s, 3H, Benzo-Me), 2.68 (s, 3H,Benzo-Me), 2.70 (s, 3H, Benzo-Me), 3.15 (sept, 7 Hz, 1H, CHMe2),3.42 (sept, 7 Hz, 1H, CHMe2), 3.63 (sept, 7 Hz, 1H, CHMe2), 3.97 (s,1H, Cp), 4.71 (m, 1H, CHMeCH2), 4.92 (s, 1H, Cp), 6.18 (d, 7 Hz,1H, Benzo), 6.34 (d, 7 Hz, 1H, Benzo), 6.52 (d, 7 Hz, 1H, Benzo),6.63 (d, 7 Hz, 1H, Benzo), 9.97 (s, 1H, Zr−CH). 13C NMR (C6D6, 7°C): δ 1.63 (ZrCH2), 0.95, 21.30, 21.84, 22.06, 23.57, 24.65, 26.46,27.55, 28.33, 28.49, 28.63, 28.77, 28.82, 28.88 (CHMe2/Benzo-Me),30.10 (CMe3), 61.52 (CMe3), 98.50, 104.87, 111.01, 116.16, 118.34,119.90, 123.10, 124.10, 124.23, 124.39, 125.82, 125.94, 126.89, 129.14,129.56, 132.59, 133.52 (Cp/Benzo), 221.71 (CHNCMe3). OneCHMe2/Benzo Me and one Cp/Benzo resonance were not located.Synthesis of (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2Zr(H2) (17). A 75

mL thick-walled reaction vessel was charged with 0.242 g (0.443mmol) of 8 and 15 mL of toluene. The vessel was then sealed,removed from the glovebox, and frozen in liquid nitrogen. Thereaction mixture was then charged with 4 atm of hydrogen on aSchlenk line. The vessel was then warmed to ambient temperature,and the resulting mixture was stirred for 30 min. Volatiles wereremoved in vacuo, and the crude yellow solid was recrystallized inpentane to give 0.213 g (87.7%) of 17. Anal. Calcd for C34H48Zr: C,74.52; H, 8.83. Found: C, 74.36; H, 8.56. 1H NMR (C6D6): δ 1.11 (d,7 Hz, 12H, CHMe2), 1.21 (d, 7 Hz, 12H, CHMe2), 2.34 (s, 12H,Benzo-Me), 3.13 (sept, 7 Hz, 4H, CHMe2), 6.43 (s, 2H, ZrH), 6.44 (s,4H, Benzo), 7.82 (s, 2H, Cp). 13C NMR (C6D6): δ 21.87, 22.20,27.81, 29.02 (CHMe2 and Benzo-Me), 109.91, 122.03, 122.42, 124.81,132.41 (Cp/Benzo).Preparation of syn-/anti-(η5-C9H3-1,3-(CHMe2)2-4,7-Me2)-

(η5:η3-C9H4-1,3-(CHMe2)2-4,7-Me2)ZrH (syn-/anti-18). In a drybox,a 225 mL thick-walled glass vessel was charged with 0.242 g (0.441mmol) of 17 and 20 mL of toluene. The vessel was sealed, removedfrom the glovebox, and heated to 45 °C for 7 days. The volatiles were

removed in vacuo, and the crude product was washed with coldpentane to afford 0.213 g (88%) of syn-/anti-18 as a mixture of twoisomers as a yellow solid. Anal. Calcd for C34H48Zr: C, 74.52; H, 8.83.Found: C, 74.27; H, 8.81. 1H NMR syn (major isomer) (C6D6): δ 0.65(d, 7 Hz, 3H, inserted Me), 1.00 (d, 7 Hz, 3H, CHMe2), 1.02 (d, 7 Hz,3H, CHMe2), 1.10 (m, 6H, CHMe2), 1.24 (s, 1H, Zr-H), 1.29 (d, 7Hz, 3H, CHMe2), 1.43 (d, 7 Hz, 3H, CHMe2), 1.46, (d, 7 Hz, 3H,CHMe2), 1.52 (d, 7 Hz, 3H, CHMe2), 2.11 (s, 3H, Benzo-Me), 2.34 (s,3H, Benzo-Me), 2.35 (s, 3H, Benzo-Me), 2.45 (sept, 7 Hz, 1H,CHMe2), 2.56 (m, 1H, CHMe), 3.04 (sept, 6 Hz, 1H, CHMe2), 3.19(sept, 6 Hz, 1H, CHMe2), 3.30 (sept, 6 Hz, 1H, CHMe2), 3.39 (m,1H, allyl CH), 4.08 (d, 7 Hz, 1H, alkene CH), 5.54 (s, 1H, Cp), 6.48(d, 7 Hz, 1H, Benzo), 6.54 (d, 7 Hz, 1H, Benzo), 6.64 (s, 1H, Cp). 1HNMR anti (minor isomer) (C6D6): δ 0.33 (s, 1H, Zr-H), 0.87 (d, 7Hz, 3H, inserted Me), 1.36 (d, 7 Hz, 3H, CHMe2), 1.38 (d, 7 Hz, 3H,CHMe2), 2.18 (m, 1H, allyl CH), 2.37 (s, 3H, Benzo-Me), 2.62 (s, 3H,Benzo-Me), 2.66 (s, 3H, Benzo-Me), 2.96 (m, 1H, inserted CH), 3.50(sept, 7 Hz, 1H, CHMe2), 3.94 (sept, 7 Hz, 1H, CHMe2), 4.26 (d, 4Hz, 1H, alkene CH), 5.23 (s, 1H, Cp), 6.22 (s, 1H, Cp), 6.45 (d, 7 Hz,1H, Benzo). Isopropyl methyl (18H), methine (2H), and benzo (1H)resonances were not located for the minor isomer. 13C NMR syn(major isomer) (C6D6): δ 18.19, 21.69, 21.95, 22.17, 22.30, 22.46,22.78, 24.67, 25.84, 27.00, 27.30, 27.82, 28.10, 28.19, 28.84, 29.29,30.02 (CHMe2/Benzo-Me, and inserted CH), 50.40 (allyl C), 98.53,100.19, 104.97, 106.03, 107.99, 111.50, 116.45, 118.03, 119.49, 120.27,123.07, 123.48, 123.92, 129.33, 130.81, 133.19 (Cp/Benzo/alkene).13C NMR anti (minor isomer) (C6D6): δ 21.52, 21.63, 21.72, 21.83,22.52, 24.36, 25.18, 25.61, 28.27, 28.47, 28.65, 28.73, 31.28, 39.33(CHMe2/Benzo-Me, and inserted CH), 54.00 (allyl C), 96.18, 97.31,103.24, 109.74, 110.57, 116.25, 116.58, 118.52, 120.24, 120.92, 122.47,127.12, 128.29 (Cp/Benzo/alkene). Three CHMe2/Benzo-Me andthree Cp/Benzo/alkene resonances were not located for the minorisomer.

■ RESULTS AND DISCUSSIONLigand Synthesis. To avoid completely hindering the

ability of a metal to bind to the benzo ring of the indenyl ligand,a methyl for hydrogen substitution was pursued to ensure thesmallest steric perturbation possible. Substitution proximal tothe five-membered (Cp) ring was targeted, primarily due toease of synthetic access of 4,7-dimethylindene through a base-mediated cyclization of cyclopentadiene and acetonylacetone.47

Incorporation of isopropyl groups on the Cp ring was thenaccomplished via successive in situ deprotonations using nBuLiin THF followed by addition of 2-iodopropane as theelectrophile (eq 2).48 The installation of both isopropyl

substituents can be accomplished in a one-pot procedure inhigh yield without the need to isolate the presumedmono(isopropyl)indene ligand intermediate. This in situdeprotonation/electrophile addition sequence was also usedto prepare 1,3-diisopropylindene in high yield,49 a ligand whichpreviously required a four-step reaction sequence for syn-thesis.50

Gauging Steric and Electronic Effects of the 1,3,4,7-Substituted Indenyl Ligand Using Bis(indenyl) IronComplexes. To detect steric or electronic differences imparted

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by the 4,7-dimethyl substitution, two classes of metalcompounds were targeted: bis(indenyl) iron and zirconiumcomplexes. The iron derivatives could indicate steric effects ofthe new ligand, on the basis of X-ray crystallographic data orlow-temperature NMR studies, on comparison to the ironanalogues lacking benzo substitution,50 while the electronicscould be assayed electrochemically by the reversible Fe(II)/Fe(III) redox couple.50 Bis(indenyl)Zr complexes weretargeted due to their known intramolecular binding of thebenzo fragment of an indenyl ligand16 and because both desiredoxidation states, Zr(II) and Zr(IV), can provide diamagneticproducts amenable to characterization by NMR spectroscopy.Furthermore, we reasoned if benzo coordination was notobserved with zirconium using the more sterically hinderedindenyl ligands, the likelihood of binding to cobalt would besimilarly diminished.Preparation of the bis(indenyl) iron complex (η5-C9H3-1,3-

(CHMe2)2-4,7-Me2)2Fe (3) was accomplished by the reactionof 2 equiv of the lithium indenide salt with iron(II) chloride inether (eq 3). X-ray-quality crystals of 3 were grown from

pentane at −30 °C, and an ORTEP representation of thestructure is shown in Figure 1 along with metric data in Table1. For comparison, structural data for the related bis(indenyl)iron complex (η5-C9H5-1,3-(CHMe2)2)2Fe (4), without benzosubstitution, were also obtained under identical crystallizationconditions and included in Figure 1 and Table 1. Note that therotational angles for the compounds differ significantly (42.5(4)and 52.5(4)° for the two molecules in the asymmetric unit for 3and 68.5(4)° for 4), defined as the angle between the twoplanes generated from the metal, C(2), and the C(4)−C(9)midpoint for each of the indenyl ligands. This suggests the

methyl groups impart inter-ring repulsions, forcing the ligandsto adopt a conformation which permits optimal gearing of bothbenzo and Cp substituents to relieve unfavorable stericinteractions. Slip-fold parameters, commonly used to evaluateη5 versus η3 hapticity,51 indicate minimal distortion toward η3

coordination in both complexes.To further probe the steric effects of benzo substitution,

variable-temperature NMR studies of 3 and 4 were undertakenin THF-d8. Cooling solutions of 4 to −78 °C produced nosignificant changes in the broadness or number of resonances inthe 1H NMR spectrum. However, below −65 °C, the 1H NMRspectrum of 3 broadens considerably and eventually certainresonances, specifically the benzo methyl groups, begin toseparate into two peaks, indicative of restricted rotation(Supporting Information). Though the low-temperature limitof the resonances could not be accessed in THF-d8, therebypreventing quantitation of activation parameters for the processusing an Eyring plot, qualitatively the observed restrictedrotation suggests that benzo ring substitution can have a similareffect on inter-ring dynamics as installing larger groups on thefive-membered ring, as in the 1,3-substituted bis(indenyl) ironcomplex (η5-C9H5-1,3-(SiMe3)2)2Fe, which also exhibitsrestricted rotation at low temperatures.50

Electrochemical studies of 3 were of interest to determinethe electronic effect of benzo substitution. A cyclic voltammo-

Figure 1. Molecular structures (and overhead views) of (a) 3 (one molecule from the asymmetric unit) and (b) 4 with 30% probability ellipsoids.Hydrogen atoms are omitted for clarity.

Table 1. Structural Parameters for the Bis(indenyl) Iron(II)Complexes

compdslip paramΔ (in Å)

hingeangle(deg)

foldangle(deg)

rotationalangle(deg)

Cpcentroid−Fe−Cpcentroid Angle

(deg)

3a 0.034(4);0.035(4)

1.3(3);3.1(3)

1.1(5);2.0(4)

42.5(4) 178.4(1)

0.040(4);0.043(4)

3.2(3);3.2(3)

2.7(5);3.8(4)

52.5(4) 179.1(1)

4 0.041(4);0.054(4)

0.8(3);1.8(3)

2.5(6);2.9(5)

68.5(4) 178.3(1)

aValues are reported for both molecules in the asymmetric unit.

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gram of 3 was recorded in THF using tetrabutylammoniumhexafluorophosphate (TBAF) as the supporting electrolyte andshows the expected reversible Fe(II)/Fe(III) oxidation couple(Supporting Information).52 The E1/2 value for oxidation of 3under our experimental conditions, at −385 mV relative toferrocene/ferrocenium, indicates that 3 is more easily oxidizedthan 4 (E1/2 = −380 mV) by 5 mV, suggesting indenyl ligandswith benzo methyl groups create a slightly more electron-richenvironment. The lack of a more pronounced effect likely stemsfrom the attenuation of the CH3 group’s electron-donatingability upon benzo substitution, as observed in studies ofrelated bis(indenyl) iron complexes43 and by recent inves-tigations of 1,3-dimethyl- and 4,7-dimethyl-substituted bis-(indenyl) cobalt and zirconium complexes in our group.44

Synthesis of Low- and High-Oxidation-State Bis-(indenyl) Zr Complexes Containing Benzo-SubstitutedIndenyl Ancillary Ligands. Entry into the chemistry of low-valent Zr, to examine if binding of a substituted benzo ring ofthe indenyl ligand is possible, requires access through reductionof appropriate Zr(IV) precursors, typically halide derivatives.53

Accordingly, the zirconium dichloride complex (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2ZrCl2 (5) was prepared by standard saltmetathesis of the lithium indenide with ZrCl4 in ether (eq 4).1H and 13C NMR spectra of 5 are consistent with the expectedC2v-symmetric bent metallocene.

As a secondary probe of ligand electronics to corroborate theelectrochemical studies on 3 (vide supra) and to explore thesynthesis of Zr(II) derivatives in the presence of strong donorligands, the dicarbonyl complex (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2Zr(CO)2 (6) was targeted, as the carbonyl ligandsprovide a convenient handle to judge the electron-donatingability of Cp or indenyl ligands.54 Complex 6 was prepared bymagnesium reduction of 5 in THF under 4 atm of carbonmonoxide (eq 5). X-ray-quality crystals of 6 were grown from

pentane at −30 °C, and an ORTEP representataion of thestructure is shown in Figure 2 with pertinent structuralparameters, accumulated with the other crystallographicallycharacterized Zr derivatives in this study, in Table 2. Salientfeatures of the structure include the rotational angles (86.6(2)and 87.4(1)° for both molecules in the asymmetric unit),suggesting the isopropyl groups on opposite indenyl ligandsdictate the observed gearing to minimize overlap of these ligandsubstituents, much like the case for bulkier 1,3-disilyl-substituted bis(indenyl) zirconium complexes.50 Slip-foldparameters indicate minimal distortion toward η3 coordinationin 6. An IR spectrum of 6, recorded in pentane, displays anaverage CO stretching frequency of 1892 cm−1 (SupportingInformation), suggesting a more electron-rich ligand incomparison to (η5-C9H5-1,3-(CHMe2)2)2Zr(CO)2, with

ν(CO)av 1906 cm−1,50 in accord with the electrochemicaldata obtained for 3 (vide supra).Reduction of 5 using excess sodium amalgam in pentane over

the course of 2 days in the absence of donor ligands results inthe observation of two diamagnetic products (eq 6). The first is

the η5,η9 zirconium sandwich complex (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)(η

9-C9H3-1,3-(CHMe2)2-4,7-Me2)Zr (7).The 1H NMR spectrum of 7 clearly indicates the boundbenzo ring of one of the indenyl ligands, on the basis of thesignificant upfield shifting of the η9 benzo resonance to 3.37ppm (Supporting Information). A low-quality X-ray structureof the complex (Supporting Information) was obtained fromcrystals grown in pentane at −30 °C, further supporting benzobinding, on the basis of the significant distortion from planarityof the η9 indenyl ligand. Though the rate of η5 to η9 indenylligand self-exchange, a process previously documented in thesecompounds,16 would serve as another measure of the effect ofthe bulkier ligand on benzo binding, only weak EXSY exchangepeaks could be discerned in a NOESY spectrum of 7 at a varietyof temperatures, in large part due to conversion of 7 to anothercompound over the time scale of the experiment. The inabilityto separate 7 from the second complex also makes it difficult toobtain better quality crystals of the compound for X-rayanalysis.Though 7 is inseparable from the other reduction product

due to the partial solubility of both compounds in pentane,rinsing the crude reaction mixture with cold pentane allows thesecond complex to be isolated analytically pure. This productforms as the result of intramolecular C−H activation of one ofthe isopropyl substituents, giving the cyclometalated zirconiumhydride 8. The 1H and 13C NMR spectra of 8 in benzene-d6 arediagnostic (Figure 3), on the basis of the distinct C1 symmetryof the compound. Upfield-shifted protons, at −1.88 and −1.13ppm, are further indicative of methylene hydrogens on a carbonbound to zirconium. X-ray-quality crystals of 8 were grownfrom pentane at −30 °C, and an ORTEP representation of themolecule is shown in Figure 4, further establishing the identityof 8. The rotational angles of 84.7(2) and 86.6(2)° for the twomolecules in the asymmetric unit of 8 may result from theconstrained geometry enforced by cyclometalation or are aproduct of gearing due to the bulky isopropyl substituents,much like the case for 6. Slip parameters are within valuestypical for η5-coordinated indenyl ligands (Table 2). Thezirconium−hydride and −carbon bond distances are withinstandard ranges for structurally characterized compoundscontaining these bonding motifs.22,55

When pure 8 is left in benzene-d6 over the course of days atambient temperature, appearance of 7 is observed, suggestingan equilibrium between 7 and 8 exists. The Keq value measuredat 25 °C in benzene-d6 is 3.0(2), indicating thermodynamic

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preference for the Zr(IV) cyclometalated species. Theformation of 8 and the strong driving force for intramolecularC−H bond activation relative to 7 is noteworthy, consideringthe related η5,η9 bis(indenyl) Zr sandwich 9 is stable for weeksin benzene-d6 at ambient temperature (eq 7). Thus, thetendency to undergo cyclometalation appears to be modulatedby benzo substitution of the indenyl framework. It is possiblethe methyl substitution creates a more electron-rich ligand andmetal which can then subsequently oxidatively add the sp3 C−H bond. However, on the basis of the relatively minimal

electronic change suggested by the electrochemistry of 3 and 4upon methyl substitution on the benzo ring (vide supra), wefavor a perturbation in the conformational preference of thesandwich 8, imparted by new inter-ring repulsions, resulting inthe increased reactivity toward cyclometalation. Regardless,intramolecular oxidative addition has been enhanced using themore sterically encumbering indenyl ligand, which canhopefully translate to intermolecular reactivity with latermetal systems employing this ligand set.

8 as a Source of Zr(II) or Zr(IV). The frontier orbitals of 8were of interest, to determine the ability of the formally 16-electron complex to react with additional substrates. The X-raycoordinates for one molecule in the asymmetric unit of 8 wereused as the starting point for a geometry optimization by DFTmethods. The computed HOMO and LUMO for 8 are shownin Figure 5. In corroboration of extended Huckel calculations ofrelated Cp2M fragments,56 the LUMO of 8 has significantorbital character directed toward the center of the metallocenewedge,57 indicating the potential for 8 to react with electron-rich molecules.

Figure 2. Molecular structure of one molecule in the asymmetric unit of 6 (including an overhead view) with 30% probability ellipsoids. Hydrogenatoms are omitted for clarity. Pertinent bond ranges (Å): Zr(1)−C(indenyls), 2.526−2.548; Zr−C(carbonyl), 2.149−2.161. Slip parameter (Δ, Å):0.14(1) and 0.00(1). Hinge angle (deg): 4.9(2) and 5.2(2). Fold angle (deg): 2.1(5) and 3.0(2).

Table 2. Structural Parameters for the Bis(indenyl) Zirconium Complexes

compd rotational angle (deg) α (deg) β (deg) γ (deg) τ (deg)

6a 87.4(1); 88.8(2) 28.4(1); 28.6(1) 151.1(1); 151.4(1) 151.5(1); 151.4(1) 0.1(1); 0.2(1)8a 84.7(2); 86.6(2) 22.8(2); 24.1(2) 155.9(2); 157.2(2) 150.3(2); 152.0(2) −2.6(2); −2.8(2)12a 44.8(4); 45.2(4) 47.2(1); 48.3(1) 131.7(1); 132.8(1) 138.2(2); 138.8(1) 3.0(1); 3.2(1)13 84.6(2) 30.8(2) 149.2(2) 144.3(1) −2.4(2)17 88.3(2) 25.9(1) 154.1(1) 150.7(1) −1.7(1)anti-18 44.8(1) 24.5(1) 155.5(1) 149.5(1) −3.0(1)

aTwo entries indicate that multiple molecules are present in the asymmetric unit.

Figure 3. 1H NMR of spectrum of 8 in benzene-d6.

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The reactivity of complex 8 was then pursued, as the strainednature of the cyclometalated compound coupled with the opencoordination site still available in the metallocene wedge mightpermit 8 to serve either as a Zr(II) equivalent, via ligand-induced reductive elimination of the C−H bond,58 or as asource of a zirconium hydride which could participate in typicalZr(IV) (i.e., insertion or σ bond metathesis) chemistry(Scheme 3).59

Addition of a typical σ donor, trimethylphosphine, to 8 inbenzene-d6 results in no observable change in the 1H or 31Pspectrum of the reaction mixture at ambient temperature. Thelack of reactivity with phosphines is perhaps unsurprising, giventhe inability of such ligands to bind with 9 or other known η5,η9

zirconium complexes,60 which is likely a result of the stericallydemanding indenyl ligands. However, 4-(dimethylamino)-

pyridine (DMAP) addition to 8 results in reaction at elevatedtemperatures (70 °C over 5 days in benzene-d6).

61 On the basisof the Cs symmetry of the new product and the disappearanceof upfield-shifted resonances characteristic of 8, along withspectroscopic and elemental analysis data, we assign theproduct as the DMAP hydride complex 10 (Scheme 4). The

Figure 4. Molecular structure of one molecule in the asymmetric unit of 8 (including an overhead view) with 30% probability ellipsoids. Hydrogenatoms are omitted for clarity. Pertinent bond ranges (Å): Zr(1)−C(indenyls), 2.43−2.63; Zr(1)−C(12), 2.30; Zr−H, 1.80. Slip parameter (Δ, Å):0.04(1) and 0.09(1). Hinge angle (deg): 2.9(2) and 2.9(2). Fold angle (deg): 3.0(5) and 3.0(2).

Figure 5. Selected Kohn−Sham orbitals of 8. The orientation of the metallocene wedge is depicted by the Chemdraw representation.

Scheme 3. Potential Zr(II)/Zr(IV) Reactivity Dichotomy for 8

Scheme 4. Reactivity of 8 with DMAP and DMAP-d2

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mechanism of the reaction likely proceeds through reductiveelimination, either induced by the ligand or due to thermalactivation, and subsequent oxidative addition of the accessibleand weaker sp2 C−H α to the nitrogen of DMAP upon bindingto give 10. The proposed C−H activation by 8 serving as aZr(II) equivalent has precedent in related indenyl andcyclopentadienyl zirconium chemistry.61,62 Further evidencefor the formation of 10 via C−H bond oxidative addition isprovided by addition of DMAP-d2

63 to 8, resulting in theformation of 10-d2 (Scheme 4, Supporting Information). Fromthe 2H NMR spectrum recorded in benzene, the hydride andaryl-deuteride resonances can clearly be discerned, at 4.05 and7.10 ppm, respectively. Importantly, the 2H NMR spectrumalso indicates no detectable amounts of deuterium incorpo-ration into the isopropyl methyl groups. This result suggestsonly a reductive elimination/C−H bond activation sequence isoperative. Hence, addition of DMAP demonstrates 8 exhibitsreactivity consistent with a Zr(II) source.π type ligand reactivity with 8 was also explored, given the

wealth of both Zr(II)64,65 and Zr(IV)66 reactivity with thesetypes of reagents. Addition of 2 equiv of 2-butyne to 8 on apreparatory scale results in sole formation of the metallocycle11 on the basis of NMR spectroscopy (Scheme 5). Monitoring

the reaction by 1H NMR spectroscopy in benzene-d6 at varioustimes indicates only starting material and formation of 11,which is discernible on the basis of loss of upfield CH2resonances from 8 and appearance of a C2v-symmetric productcontaining two new methyl resonances in a 1:1 ratio as a resultof alkyne coupling. Any potential intermediates, resulting frominsertion into the metal hydride, were not observed.Ethylene addition to 8 results in C−C bond coupling to form

the saturated metallacycle 12. Addition of only 1 equiv ofethylene, through use of a calibrated gas bulb, to 8 andmonitoring by 1H NMR spectroscopy in benzene-d6 results inonly a mixture of 12 and 8, suggesting rapid trapping of ligand

by the presumed bis(indenyl) zirconium ethylene adduct. Nointermediates involving ethylene insertion into the Zr−H weredetectable. X-ray-quality crystals of 12 were grown fromtoluene at −30 °C, and an ORTEP representation of thestructure is shown in Figure 6. The rotational angle and othermetric parameters (Table 2) differ significantly from those of 6and 8, likely due to unfavorable steric interactions from thecarbons in the metallacycle and the indenyl substituents whichare not present in the other molecules. For both 2-butyne andethylene, 8 then behaves much like an equivalent of a Zr(II)Negishi type reagent64 in mediating the coupling of C−Cbonds.π acids were also explored in reactions with 8. Addition of 4

atm of carbon monoxide to 8 results in immediate reaction andformation of two products in an 8:1 ratio (eq 8). The minor

product is identified as (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2Zr-(CO)2 (6) based on comparison to the independently preparedcomplex (vide supra). The major product, which can beisolated from 6 by washing with cold pentane, was identified asthe acyl carbonyl compound 13, on the basis of 1H and 13CNMR spectra of 13 recorded in benzene-d6, which indicate a C1-symmetric product (Figure 7). The acyl hydrogen is observedat 3.84 ppm, on the basis of HSQC and COSY spectra, whilethe new methylene hydrogens are easily discerned by an HSQCexperiment at 2.33 and 3.09 ppm, respectively. The IRspectrum of 13 recorded in pentane indicates the presence ofthe bound carbonyl, with an intense stretch at 1935 cm−1. X-ray-quality crystals of 13 were grown from pentane at −30 °C andfurther establish the formation of a new C−C bond (Figure 8).The acyl hydrogen was located. Much like in 8, the rotationalangle (84.6(2)°) may largely be dictated by the secondary

Scheme 5. Addtion of π Type Ligands to 8

Figure 6. Molecular structure of one molecule in the asymmetric unit of 12 (including an overhead view) with 30% probability ellipsoids. Hydrogenatoms are omitted for clarity. Pertinent bond ranges (Å): Zr(1)−C(indenyls), 2.54−2.70; Zr−C(metallacycle), 2.30−2.31. Slip parameter (Δ, Å):0.09(1) and 0.12(1) Å. Hinge angle (deg): 5.4(2) and 7.3(2). Fold angle (deg): 2.2(4) and 3.6(4).

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attachment of one indenyl ligand to the metal. Other metricsare typical of η5 indenyl metal complexes. The C−O bond iselongated from that in typical aldehydes but is in line with otherstructurally characterized dinuclear Zr acyl complexes.67

One potential mechanism for formation of both productsfrom 8 is shown in Scheme 6. Initially, carbon monoxidebinding followed by insertion would give the cyclometalatedacyl intermediate 14.68 This complex could undergo C−Creductive elimination69 to form a transient Zr(II) intermediate,

which upon rebinding the extruded carbonyl would formunsaturated acyl intermediate 15. This species could then giverise to 13 by binding of an additional carbonyl ligand. Theformation of dicarbonyl 6 could result from initial binding ofcarbon monoxide and ligand-induced reductive elimination of aC−H bond, followed by fast trapping with an additional 1 equivof carbon monoxide to give 6.Attempts to monitor the reaction between 8 and CO at

ambient and lower temperatures in toluene-d8, even using

Figure 7. 1H NMR spectrum of 13 in benzene-d6.

Figure 8. Molecular structure of 13 (including an overhead view) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Pertinentbond ranges (Å): Zr(1)−C(indenyls), 2.52−2.64; Zr−C(carbonyl), 2.16; Zr−C(acyl), 2.30; Zr−O(acyl), 2.12. Slip parameter (Δ, Å): 0.04(1) and0.08(1) Å. Hinge angle (deg): 5.2(2) and 5.9(2). Fold angle (deg): 2.3(6) and 4.4(4).

Scheme 6. Proposed Mechanism for Formation of Both 6 and 13

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substoichiometric amounts of CO, through calibrated gas bulbaddition, resulted in only observation of 6 and 13 over time. Asneither 6 nor 13 converted to the other complex in benzene-d6over the course of 1 week at ambient temperature, asmonitored by 1H NMR spectroscopy, the product ratiosindicate an irreversible insertion event is favored over reductiveelimination. This reaction sequence ultimately suggests thecyclometalated complex can serve as both a Zr(IV) and Zr(II)source in succession.tert-Butyl isocyanide was also added to 8, in an effort to

observe or isolate analogues of intermediates from Scheme 6.Addition of either 1 equiv or an excess of tert-butyl isocyanideto 8 results in initial formation of a product identified as thedesired insertion product 16 (eq 9).70 Though 16 is unstable,

converting to a complex mixture of products over minutes, thecompound can be characterized by 1H and 13C NMRspectroscopy when it is cooled to 7 °C in benzene-d6 (Figure9). Clear indication of retention of the cyclometalated isopropylmoiety is supported by the upfield peaks at 0.09 and 0.20 ppmin the 1H NMR spectrum, which are identified as methyleneresonances by an HSQC spectrum. The peak at 9.97 ppm isalso indicative of an imine C−H bond, as a diagnosticcorrelation is observed in the HSQC spectrum with the carbonat 221.71 ppm. Attempts to trap 16 with PMe3 resulted in amixture of intractable products. The formation of 16 providesat least some evidence for the potential of a relatedintermediate in the reaction of 8 with carbon monoxide,though it does not rule out initial insertion into the Zr−Cbond, given the facile reaction of CO with Cp*2ZrMe2.

71 Thelack of an isolable bis(isocyanide) adduct or inability to stabilize16 with strong donor ligands in analogy to 6 likely arises fromthe greater reagent steric bulk of the isocyanide, preventing twomolecules from binding to zirconium.Reactivity of 8 with H2 and Subsequent Chemistry

Associated with the Bis(indenyl) Zirconium Dihydride.Addition of excess hydrogen to 8 provides an immediatereaction to give (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2ZrH2 (17)(eq 10). The dihydride has been characterized by 1H and 13CNMR spectroscopy. The expected C2v symmetry of the

complex is supported by the NMR spectroscopic data(Supporting Information). The Zr−H resonance is located at6.43 ppm and partially overlaps with the Cp hydrogen of theindenyl ligands. This is corroborated by integrations of the 1HNMR spectrum and by preparation of the dideuteride 17-d2(vide infra).X-ray-quality crystals of 17 were grown from pentane at −30

°C, and an ORTEP representation of the structure is shown inFigure 10. Pertinent bond angles and distances are in rangesassociated with typical bis(indenyl) zirconium(IV) complexes,including the metal−hydride bond lengths.22,60 The ligandrotational angle of 88.3(2)° is reminiscent of that seen for thedicarbonyl complex 6 and previously characterized 1,3-disubstituted bis(indenyl) zirconium dihydrides.22

Addition of deuterium gas to 8 generates dideuteride 17-d2(eq 11). Monitoring the reaction by 2H NMR spectroscopy in

benzene indicates that immediately after addition the ratio ofincorporation of deuterium into the hydride and isopropylmethyl positions is 5:1, favoring incorporation of deuteriuminto the hydride position (Supporting Information). Thisexperiment suggests at least three potential pathways areoperative for formation of the dihydride: (1) ligand-inducedreductive elimination upon dihydrogen (or deuterium) binding,which would result in no deuterium incorporation into theisopropyl substituent, (2) σ bond metathesis with dihydrogen(or deuterium), which would result in direct deuteriumincorporation into the isopropyl methyl substituent, and (3)hydride (or deuteride) redistribution when hydrogen (ordeuterium) binds prior to the reductive elimination event.Though differentiating these pathways is difficult, the distinctdeviation from 1:1 incorporation indicates some combinationof the three pathways is operative and is not solely σ bond

Figure 9. 1H NMR spectrum of 16 in benzene-d6, generated in the presence of excess isocyanide.

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metathesis, again supporting the reactivity dichotomy of 8 toserve as both a Zr(II) and Zr(IV) source.The reactivity of 17 was explored further, in part as related

indenyl cobalt hydrides could be generated in catalytic cyclesinvolving alkane dehydrogenation (vide supra). Addition of aprototypical σ donor ligand, excess PMe3, to 17 results in nonoticeable change at ambient temperature in benzene-d6, byboth 1H and 31P NMR spectroscopy. Cooling a similar reactionmixture in toluene-d8 to −70 °C again indicates no change inthe 1H and 31P NMR spectra. Since the bis(indenyl) Zrdihydride analogue lacking benzo substitution (19, Scheme 7)binds PMe3 readily at ambient temperature,60 the slightmanipulation of the steric environment creates dramaticchanges in small-molecule reactivity of the Zr(IV) complexes.As benzo C−C bond insertions into metal hydrides have

been documented in early-metal systems,22,23 we also examinedthermolysis of 17 to determine if such insertion, which wouldlikely be unproductive in late-metal catalytic cycles, washindered by benzo substitution. When 17 is thermolyzed at45 °C for 7 days, complete conversion to the benzo insertionproducts, syn- and anti-18 is observed, in a 4:1 ratio (eq 12). 1H

and 13C NMR spectroscopy provide evidence for the C1

symmetry of the two isomers, and the splittings of the methylprotons now adjacent to the inserted hydride provide furthersupport for the proposed products (Supporting Information). ANOESY spectrum of the mixture of isomers of 18 indicates themajor product is actually the syn isomer, on the basis of theNOE cross peak present with the inserted hydrogen when themetal hydride is irradiated (Supporting Information). Thehydrides in both isomers were identified by thermolysis of 17-d2 and monitoring by 2H NMR spectroscopy (eq 13 andSupporting Information).X-ray-quality crystals of the mixture were grown from

pentane at −30 °C and reveal the minor isomer anti-18 wascrystallized, as the new zirconium hydride is positioned on theopposite side of the molecule in relation to the endo-insertedhydrogen, both of which were located (Figure 11). Interactionof the former benzo ring in an allyl type fashion with zirconiumis indicated by the short Zr−carbon distances, ranging from2.49 to 2.51 Å.72 Note that the rotational angle for anti-18(44.8(1)°) is much smaller in comparison to that for the non-benzo-substituted analogue syn-20 (Scheme 8), which has RA =82.5(5)°.22 The significant difference likely stems from gearingdue to transannular interactions between the benzo methylgroups in anti-18. Formation of syn- and anti-18 providesupport for intramolecular insertion, as the methyl group is exoto the metal, while endo selectivity would be expected for anintermolecular reaction. Similar intramolecular insertion hasbeen observed in related Zr chemistry with permethylatedfluorenyl derivatives.23

The significant favoring of the syn isomer is comparable tothe regioselectivity observed for the substituted dihydride 19 inthe absence of donor ligands.22 One explanation for similarselectivities of insertion likely deals with the favored rotationalconformation of 17 and 19. In both, based on the rotationalangles in the solid state structures, preference for gauche

Figure 10.Molecular structure of 17 (including an overhead view) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Pertinentbond ranges (Å): Zr(1)−C(indenyls), 2.49−2.58; Zr−hydrides, 1.76−1.92. Slip parameter (Δ, Å): 0.01(1) and 0.02(1) Å. Hinge angle (deg): 3.7(2)and 4.4(1). Fold angle (deg): 4.1(3) and 4.3(2).

Scheme 7. Comparison of Intramolecular Insertion using 17 and 19

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rotamers is observed. From this conformation, insertion wouldgive rise to the syn isomer (Scheme 8).On comparison of the rate of intramolecular insertion for 17

to that for the complex lacking benzo substituents, 19, in aninternal competition experiment, insertion occurs over 3 h at 45°C with 19, while heating for 7 days at 45 °C is required forcomplete conversion of 17 to syn-/anti-18 (Scheme 7). Hence,methyl substitution appears to markedly decrease the rate ofC−C bond insertion in 17 and may provide the opportunity fordesired intermolecular hydride transfer, either via reductiveelimination or a sacrificial alkene, in catalytic cycles involvinglater transition metals such as cobalt.

■ CONCLUSIONS

Appendage of methyl groups on the benzo ring of the indenylligand accomplishes the desired goals. Benzo binding is stillpossible but is hindered relative to unsubstituted indenylanalogues. Ligand modification has also increased thepropensity for cyclometalation, likely through changes inconformational preferences of the bis(indenyl) Zr(II) inter-mediates. Moreover, the rate of intramolecular insertion of thebenzo fragment into metal hydrides has been decreasedmarkedly by placing methyl groups on the six-memberedring. On the basis of these encouraging studies of the effect onreactivity upon benzo substitution, we are now pursuing thesynthesis of related bis(indenyl) Co(II) complexes andsubsequent reduction of these compounds and investigationof their potential as reservoirs for reactive, monomeric Co(I)fragments.

■ ASSOCIATED CONTENT*S Supporting InformationText, figures, tables, and CIF files giving additionalexperimental and computational details, NMR spectra of newcompounds, cyclic voltammograms, selected IR spectra, fullylabeled views, and X-ray data for 3, 4, 6, 8, 12, 13, 17, and anti-18. This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: 806 742 0022. Fax: 806 742 1289. E-mail: chris.bradley@ttu.edu.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank the Robert A. Welch Foundation (Grant D1707) forfinancial support of this research. Texas Tech University isacknowledged for startup funding. The X-ray data werecollected on a Bruker Apex II instrument purchased throughan internal Texas Tech Office of Research Services (ORS)equipment grant. J.W.T. also thanks the Center for Computa-tional Nanoscience at Ball State University for computationaltime and software.

■ REFERENCES(1) For a general treatment of metallocene complexes and theirapplications, see: Metallocenes; Halterman, R. L., Togni, A., Eds.;Wiley-VCH: Weinheim, Germany, 1998.(2) For use of Cp ligands as supports of stoichiometric dinitrogenreduction with Zr, see: (a) Pool, J. A.; Lobkovsky, E.; Chirik, P. J.Nature 2004, 427, 527. (b) Knobloch, D. J.; Lobkovsky, E.; Chirik, P.J. Nature Chem. 2010, 2, 30.(3) For general treatment of group 4 metallocene catalyzed olefinpolymerization, see: (a) Hartwig, J. F. Organotransition MetalChemistry: From Bonding to Catalysis; University Science: Sausalito,CA, 2010. (b) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998,98, 2587.(4) For use of indenyl ligands as supports for early- and late-transition-metal catalysis, see: (a) Sui-Seng, C.; Castonguay, A.; Chen,Y.; Gareau, D.; Grouz, L. F.; Zargarian, D. Top. Catal. 2006, 37, 81.(b) Lin, S.; Waymouth, R. M. Acc. Chem. Res. 2002, 35, 765.

Figure 11. Molecular structure of anti-18 (including an overhead view) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.Pertinent bond ranges (Å): Zr(1)−C(indenyls), 2.30−2.56; Zr−C(benzos), 2.49−2.51; Zr−hydride, 1.73. Slip parameter (Δ, Å): 0.00(1) and0.07(1) Å. Hinge angle (deg): 3.3(4) and 0.1(4). Fold angle (deg): 0.8(2) and 3.6(1).

Scheme 8. Origin of syn Isomer Preference from gaucheRotamersa

aIsopropyl groups are truncated for clarity.

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