DOI: 10.1002/chem.201402875

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& Homogeneous Catalysis

Zincocene and Dizincocene N-Heterocyclic Carbene Complexes and Catalytic Hydrogenation of Imines and Ketones Phillip Jochmann[a] and Douglas W. Stephan*[a, b]

Abstract: The N-heterocyclic carbene (NHC) adducts Zn(CpR)2(NHC)] (CpR = C5HMe4, C5H4SiMe3 ; NHC = ItBu, IDipp (Dipp = 2,6-diisopropylphenyl), IMes (Mes = mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*2] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh2)2(aItBu)]2 was formed from spon-

taneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated ZnI adducts [Zn2Cp*2(NHC)] (NHC = ItBu, SIMes) are presented and characterized as weak adducts on the basis of 13C NMR spectroscopic and X-ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp*2] with H2 to give [Zn2Cp*2] is discussed.

Introduction

engo et al., reporting the first zincocene NHC adduct,[14] only a limited number of related complexes has been isolated. NHC-supported zinc alkoxides have been employed for the polymerization of d,l-lactide,[7a, b] and rac-lactide.[7c] NHC adducts of zinc bromide, chlorides, and acetates have also been used to effect cycloaddition of CO2 to epoxides and the copolymerization of CO2 with cyclohexene oxide.[15, 7d] Rapid functionalization of CO2 by insertion into Zn C bonds and ringopening polymerization of epoxides are additional examples of applications of zinc–NHC complexes.[16, 17] Even though decamethylzincocene [ZnCp*2] (1; Cp* = C5Me5) and related zincocenes have been known for decades,[18] recent attention[19] has focused on the ZnI compound [Zn2Cp*2] (2), which contains a Zn Zn bond.[20] In extending our earlier communication, we report a series of zincocene- and dizincocene–NHC adducts. The former complexes were evaluated in the catalytic hydrogenation of imines and ketones. The reaction of 1 with H2 to form the latter ZnI compounds is discussed from a mechanistic point of view.

The activation of dihydrogen and the catalytic reduction of unsaturated organic molecules is a fundamentally important process.[1] Indeed, the hydrogenation of imines and ketones constitutes an atom economic way for the production of amines and alcohols.[2] Platinum-group metals for catalytic hydrogenation are increasingly undesirable due to their rising cost and toxicity. As a result, there has been increasing interest in earthabundant element-based catalyst systems in recent years.[3] On the main group side, we and others have described efficient metal-free hydrogenation utilizing frustrated Lewis pairs (FLPs).[4] In terms of earth-abundant metals, a number of groups have focused on s-block metals,[5] as well as Fe-, Ni-, and Co-based catalysts.[6] In this vein, zinc-based catalysts have been developed for important transformations including (de)polymerization,[7] hydrosilylation,[8] dehydrocoupling,[8b, 9] and hydroamination reactions.[10, 11] In the case of hydrogenation, while Beller and co-workers had previously exploited Zn(OTf)2 to generate an in situ catalyst for the hydrogenation of imines, we have reported the first example of a well-defined organozinc(hydride) precatalyst, [Zn(Cp*)(H)(SIMes)] (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene).[12] This catalyst, derived from the combination of an organozinc reagent with N-heterocyclic carbenes (NHCs) is a rare example of a (Cp*)Zn–NHC complex.[13] After pioneering work by Ardu-

Results and Discussion Zincocene complexes Because the above-described previous results demonstrated that NHCs can stabilize cyclopentadienyl–zinc hydrides, we sought to probe related systems. To that end, the parent octamethyl- and bis(trimethylsilyl)zincocenes, [Zn(CpMe4)2] and [Zn(CpTMS)2], which are known to be coordination polymers in the solid state, were prepared and converted with NHCs (Scheme 1).[21] Although the reported syntheses used [LiC5H4SiMe3] and [Mg(C5Me4H)2] as metathesis reagents, the present approach is similar to that reported for analogous ZnCp*2 derivatives.[18] Thus, the pro-ligands CpRH were treated with 1.2 equivalent of NaH and subsequently reacted with

[a] Dr. P. Jochmann, Prof. Dr. D. W. Stephan Department of Chemistry, University of Toronto 80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada) E-mail: [email protected] [b] Prof. Dr. D. W. Stephan Chemistry Department, Faculty of Science King Abdulaziz University (KAU), Jeddah 21589 (Saudi Arabia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402875. Chem. Eur. J. 2014, 20, 1 – 10

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Scheme 1. Synthesis of compounds 3–10.

ZnCl2. The isolated yields were 65 % for [Zn(CpMe4)2] and 30 % for [Zn(CpTMS)2]. The particularly low latter yield was attributed to contamination of the crude product with residual chloride species. This dictated a minimum of three extractions with pentane and subsequent drying in vacuo to give [Zn(CpTMS)2] free of chloride impurities. Adduct formations of [Zn(CpMe4)2] and [Zn(CpTMS)2] were achieved by simply mixing equimolar amounts of zinc species and free carbene in benzene or toluene for one hour. In this fashion, the compounds [Zn(CpMe4)2(NHC)] (Scheme 1, 3–6,) and [Zn(CpTMS)2(NHC)] (7–10) (NHC = 1,3-di-tert-butyl-imidazol-2-ylidene : tBu, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene: IDipp, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene: IMes, 1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene: SIMes) were prepared, isolated, and characterized by 1H and 13C NMR analysis. Solutions of adducts 3–10 in aromatic solvents showed 1H NMR coupling patterns indicative of h5-coordinated CpR ligands. 1 H NMR spectra of [Zn(CpMe4)2(NHC)] (NHC = ItBu, IDipp, IMes, SIMes) (3–6) showed a strong dependence of the chemical shift of the CH proton of the CpMe4 ligand on the nature of the coordinated NHC (3: d = 4.30, 4: 3.32, 5: 3.68 and 6: 3.63 ppm). Due to the very low solubility of 5 and 6 in aliphatic and aromatic solvents (< 2 mg mL 1), no reliable 13C NMR data could be obtained. The more soluble adducts 3 and 4 showed 13 C NMR resonances for the metal bound CNHC atoms at d = 173.5 and 189.4 ppm, respectively. Single crystals of [Zn(CpMe4)2(ItBu)] (3) and [Zn(CpMe4)2(SIMes)] (6) were obtained by cooling concentrated solutions in benzene and THF, respectively. Even though this method allowed recrystallization of the other adducts (i.e., 4 and 5), the obtained materials were frequently found to be microcrystalline or of low quality and thus not suitable for X-ray structure determination. The molecular structures of 3 and 6 in the solid state feature a trigonal planar coordination around the metal center with the sum of the angles about the metal totaling 3608 (Figure 1). The Zn C distances for the carbene ligands were found to be 2.062(2) and 2.074(3) . A ring slip of the CpMe4 rings leads to comparatively &

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Figure 1. Pov-ray depiction of [Zn(CpMe4)2(ItBu)](C6H6) (3; top) and [Zn(CpMe4)2(SIMes)](THF)2 (6; bottom). Co-crystallized solvent molecules are omitted for clarity. Only C5Me4H protons are shown for clarity.

short Zn C bond lengths of 2.099(2) and 2.111(2)  for 3 and 2.037(3) and 2.126(3)  for 6. The cyclopentadienyl hydrogen atoms of the CpMe4 ligands gave rise to Zn H distances of 1.886 and 1.833  in 3 and 2.567 and 2.745  in 6. For compound 3, these values suggest the possibility of an agostic interaction. Although the C5HMe4 ring in 3 is coplanar with the remaining proton in the C5-plane, this proton is markedly outof-plane (above) in 6. In the former case, the coordination can be described as bis(h1-p)-coordination, reflecting sp2 hybridization of the zinc-bound carbon. In contrast, in compound 6, the zinc-bound carbon atom is sp3 hybridized, and thus the coordination is best described as bis(h1-s). Steric interactions of the CpMe4 fragment with the carbene substituents in 6 may account for the twisting of the CpMe4 ligand away from the carbene. The orientation of the CpMe4 ligand towards the carbene in 3 may be stabilized by this agostic interaction; however, the dynamic nature of the bis(h5)-Cp* binding in solution preclud2

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Full Paper ed confirmation of such interactions in 3 by NMR spectroscopy. The 1H NMR shifts of [Zn(CpTMS)2(NHC)] (NHC = ItBu, IDipp, IMes, SIMes) (7–10) appeared to be insensitive to the nature of the NHC. Only [Zn(CpTMS)2(ItBu)] (7) displayed slightly upfield CpTMS resonances (d = 6.90 and 6.11 ppm), whereas the other three adducts 8–10 showed resonances in the narrow range of 6.19 to 5.66 ppm, and the TMS groups gave resonances between 0.39 and 0.37 ppm for 7–10. The zinc-bound CNHC carbon atoms showed 13C resonances at d = 186.6 (7), 178.0 (8), 176.3 (9), and 201.3 ppm (10). Single crystals of [Zn(CpTMS)2(ItBu)] (7) were obtained by cooling a concentrated solution in toluene and featured slipped CpTMS rings, a three-coordinated metal center with an angle sum about Zn of 3608 and a Zn CNHC distance of 2.048(4)  (Figure 2). The zinc–carbon bond

Table 1. Catalytic hydrogenation of imines.[a,c] No.

Cat.

Substrate[b]

Yield [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

3 3 3 3 3 4[b] 4[b] 4[b] 4[b] 4[b] 7 7 7 7 7 8 8 8 8 8 9 9 9 9 9 10 10 10 10 10

tBuN=CHPh PhCH2N=CHPh tBuN=CH-p-tBu-C6H4 tBuN=CH-p-Br-C6H4 tBuN=CH-m-Br-C6H4 tBuN=CHPh PhCH2N=CHPh tBuN=CH-p-tBu-C6H4 tBuN=CH-p-Br-C6H4 tBuN=CH-m-Br-C6H4 tBuN=CHPh PhCH2N=CHPh tBuN=CH-p-tBu-C6H4 tBuN=CH-p-Br-C6H4 tBuN=CH-m-Br-C6H4 tBuN=CHPh PhCH2N=CHPh tBuN=CH-p-tBu-C6H4 tBuN=CH-p-Br-C6H4 tBuN=CH-m-Br-C6H4 tBuN=CHPh PhCH2N=CHPh tBuN=CH-p-tBu-C6H4 tBuN=CH-p-Br-C6H4 tBuN = CH-m-Br-C6H4 tBuN=CHPh PhCH2N=CHPh tBuN=CH-p-tBu-C6H4 tBuN=CH-p-Br-C6H4 tBuN=CH-m-Br-C6H4

0 99 0 0 0 3 100 trace 4 9 0 25 0 trace 0 11 100 19 25 22 7 100 9 10 11 trace 15 0 0 0

[a] H2 (100 bar), 72 h, 50 8C, [Zn(CpR)2(NHC)] (10 mol %, conc. ca. 4 mm in C6D6). [b] This experiment was performed at a catalyst loading of 7 mol %, due to limited solubility. [c] Yields are based on 1H NMR data.

ligand and Zn0 or a colorless precipitate after two weeks. Even though not investigated in detail, the latter precipitate is believed to be ZnH2 or a derivative thereof. In addition, 3–4 and 7–9 were evaluated as catalysts for the hydrogenation of C=N double bonds (Table 1). In general, these species were not highly effective, giving only moderate reduction at best. The one exception was the substrate PhCH2N=CHPh, which was effectively reduced by using 3–4 and 7–9 as the catalyst precursors. In a similar fashion, catalytic hydrogenation of a series of ketones by 1/NHC (NHC = ItBu, IDipp, SIMes) was assessed using 10 mol % catalyst, and 100 bar H2 at 25 8C typically for 72 h (Table 2). No significant reduction was observed for 2-methylpentan-3-one, benzophenone, and diisopropylphenyl methyl ketone (Table 2, entries 5–7, 12–14, 19–21). In contrast, moderate to quantitative yields of the corresponding alcohols were obtained for the hydrogenation of iPr2CO (entries 1, 8, 15) and tBu2CO (entries 2, 9, 16). In these cases, 1/SIMes is the most efficient catalyst. It is noteworthy that SIMes did not form an adduct with 1 due to steric crowding, and thus this situation resembles frustrated Lewis pair (FLP) behavior. The poorer catalytic ability of 1/NHC to reduce ketone versus imine substrates is consistent with the greater bond strength of Zn O over Zn N.[12] This is thought to slow product release and thus

Figure 2. Pov-ray depiction of [Zn(CpTMS)2(ItBu)] (7). Hydrogen atoms are omitted for clarity.

lengths for the CpTMS ligands of 2.126(4) and 2.140(4)  are indicative of two h1-CpTMS ligands. However, the zinc-bound CH moieties are not fully pyramidalized, what speaks for a certain degree of charge delocalization in the slipped CpTMS ligands. Notably, the zinc center binds to the carbon atoms in g position of the CpTMS ligands. This is likely a consequence of steric effects, and a similar binding was observed for the parent compound [Zn(CpTMS)2]n.[21] The overall observed complex geometry is reminiscent of the paddle-wheel nature of 6. Catalytic hydrogenations Efforts to react 3, 4, and 7–10 with H2 (4 bar) were monitored in situ by NMR spectroscopy. Compounds 5 and 6 were found to be unsuitable, due to their low solubility. Although 3 and 8 showed no apparent reaction, 4, 7, 9, and 10 reacted slowly with H2 leading to hydrogenolysis of the cyclopentadienyl Chem. Eur. J. 2014, 20, 1 – 10

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Full Paper Table 2. Catalytic hydrogenation of ketones with 1/NHC. No.[a]

Cat.

Substrate

Yield [%][b]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

3 3 3 3 3 3 3 4 4 4 4 4 4 4 6 6 6 6 6 6 6

iPr2C=O tBu2C=O iPr(Me)C=O Cy(Me)C=O iPr(Et)C=O Ph2C=O Dipp(Me)C=O iPr2C=O tBu2C=O iPr(Me)C=O Cy(Me)C=O iPr(Et)C=O Ph2C=O Dipp(Me)C=O iPr2C=O tBu2C=O iPr(Me)C=O Cy(Me)C=O iPr(Et)C=O Ph2C=O Dipp(Me)C=O

90 57 22 23 trace 0 0 54 32 40 26 trace 0 0 100 62 26 14 trace 0 0

Figure 3. Pov-ray depiction of 11. Solvent molecules and hydrogen atoms are omitted for clarity.

[a] H2 (100 bar), 72 h, 25 8C, [ZnCp*2]/NHC (10 mol %, conc. ca. 4 mm in C6D6). [b] Yields are based on 1H NMR data. Cy = cyclohexyl.

tures an oxygen-bridged dimer. The bridging ligands exhibited slightly elongated Zn O bond lengths of 2.016(1) , whereas the nonbridging alkoxy ligands displayed Zn O distances of 1.927(1) . The terminal aItBu ligands showed longer bond lengths of Zn C 2.027(2) , which is slightly shorter compared to previously reported NHC adducts of zinc.[23a] The corresponding reactions with IDipp and Ph2CO or tBu2CO gave crystals of [Zn(OCHPh2)2(IDipp)] (12) and [Zn(OCHtBu2)2(IDipp)] (13). The latter species was isolated in 50 % yield from the hydrogenation of the ketone, whereas the former species 12 was more conveniently prepared from the reaction of [ZnCp*2] with benzhydrol in the presence of IDipp (Scheme 3). The nature of 12 and 13 was confirmed crystallographically (Figure 4). These species exhibit trigonal planar ge-

slow hydrogenation. In agreement with this hypothesis, single crystals of zinc alkoxide products were repeatedly obtained in hydrogenation reactions (see below). Crystals of a product 11 were obtained from the attempted hydrogenation of benzophenone with [ZnCp*2] (1) in the presence of ItBu (Scheme 2). This product was independently synthesized by initial reaction of 1 with Ph2CHOH and subsequent

Scheme 2. Reaction of 1 to the abnormal adduct 11.

Scheme 3. Formation of alkoxides 12 and 13.

treatment of the intermediate alkoxide product with ItBu.[22] This product proved to be insoluble in aromatic solvents and unstable in CD2Cl2. However, when a freshly prepared solution of 11 in CD2Cl2 was kept in liquid N2 and warmed prior to acquisition of NMR data, 1H resonances at d = 7.56 and 6.86 ppm and a 13C resonance at 155.3 ppm were consistent with the presence of an abnormal carbene.[23] Two sets of resonances inferred two alkoxide (OCHPh2) fragments leading to speculation of a dimeric structure in solution. The corresponding reaction of 1 with benzophenone under D2 effected the analogous conversion, prompting the formulation of the product as [Zn(OCDPh2)2(aItBu)] ([D1]11). Crystallographic data confirmed this formulation of 11 (Figure 3). The molecular structure of 11 fea&

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ometries at the Zn center with angle sums of > 3598. The Zn O bond lengths are 1.835(4) to 1.846(4)  for 12 and 1.859(1) and 1.866(1)  for 13. The IDipp ligands gave rise to Zn C distances of 1.981(6) and 1.994(6)  in 12 and 2.052(2)  in 13. The closely related [Zn(OCH2Ph)2(IMes)]2 and [ZnCl(OCH2Ph)(IDipp)]2 were structurally characterized by Tolman and coworkers and shown to act as catalysts for the polymerization of lactide.[7a, b] It is noteworthy that these species were observed to exist as dimers in the solid state, but appear to be monomers in solution. 4

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Full Paper resonances at 205.8 and 208.4 ppm for the coordinating carbon atoms of the NHCs, respectively.[12b] These values indicate a low sensitivity of the Cp* resonances to carbene coordination. The carbene carbon 13C resonances are significantly shifted downfield, compared to NHC adducts of ZnII cyclopentadienyls.[12b, 14, 16] The molecular solid-state structures of 14 and 15 were established and revealed the Cp* rings on the Zn atom to which the carbene binds to adopt slipped binding modes (Figure 5). The Zn Zn bond lengths are 2.3392(4) and 2.3682(6)  for 14 and 15, respectively, and are between the corresponding distances observed for the parent compound 2 (2.305(3) )[20a] and [Zn2Cp*2(dmap)2] (dmap = 4-dimethylaminopyridine 2.418(1) )[24] and within the general range for

Figure 4. Pov-ray depiction of (12; top) and (13; bottom); one iPr group per IDipp ligand was disordered. In the case of 12, there are two independent molecules per unit cell, one is shown. All hydrogen atoms are omitted for clarity.

Dizincocenes We reported earlier the in situ observation of dizincocene adducts [Zn2Cp*2(ItBu)] (14) and [Zn2Cp*2(SIMes)] (15) during the conversion of 1 to 2 in the presence of H2 and NHCs.[12b] These adducts were independently synthesized by the combination of equimolar amounts of 2 and NHCs in toluene (Scheme 4). Compounds 14 and 15 showed characteristic 1H NMR resonances at d = 2.07 and 2.11 ppm for the h5-Cp* ligands and 13C

Figure 5. Pov-ray depiction of (14; top) and (15; bottom). For compound 14, one of two disordered slipped Cp* positions is shown. All hydrogen atoms are omitted for clarity.

Scheme 4. Synthesis of compounds 14 and 15. Chem. Eur. J. 2014, 20, 1 – 10

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Full Paper Zn2R2 compounds.[19c] Although one ring remains coordinated in a slightly slipped h5-fashion (Zn C bond lengths: 2.269(4) to 2.384(4)  for 14 and 2.323(5)–2.414(4)  for 15), the other ring is best described as h2 (Zn C 2.209(15)–2.472(18) ) for 14 and h3 (Zn C 2.126(4) to 2.548(4) ) for 15. This slippage is consistent with a minimal pyramidalization around the zinc-bound carbon atoms, because the methyl groups are slightly above the C5 plane (0.41 and 0.47  for 14 and 15, respectively). The Zn NHC bond distances are 2.101(3) (14) and 2.147(4)  (15), slightly elongated compared to known ItBu and SIMes adducts of [Cp*ZnR] (R = C6F5, H). This is in agreement with a diminished Lewis acidity of the Zn22 + unit. The original in situ observation of dizincocene adducts 14 and 15 was made during the conversion of 1 and the corresponding NHC with H2 to cleanly give 2.[12a, b] This facile and high-yielding route gave 2 in the presence or absence of NHCs. However, in the presence of NHCs, the formation of [Cp*ZnH(NHC)] and NHC-derived aminals were observed as intermediates and/or byproducts. The formation of 2 is commonly believed to proceed by dimerization of [ZnCp*]· radicals. This view was supported by the detailed work of Carmona and co-workers,[20d] reactivity studies of the hydride complexes [ArMH]2 (M = Zn, Cd; Ar = C6H3-2,6-(C6H2-2,4,6-iPr3)2, C6H2-2,6(C6H2-2,4,6-iPr3)2-4-SiMe3) reported by Power and co-workers[25] and theoretical calculations.[26] In the case of the reaction of 1 with H2, the stability of 1 precludes a pathway involving the homolytic dissociation into [ZnCp*]· and Cp*· radicals in a pre-equilibrium. This view was further supported by calculations by Hepperle and Wang that showed this dissociation is uphill by 32.3 kcal mol 1,[26] although 6.7 kcal mol 1 less costly than the analogous dissociation for [ZnCp2]. A more likely pathway involves heterolytic cleavage of H2 to eliminate Cp*H and generate [Cp*ZnH]. Dimerization and reductive elimination of H2 would lead to the formation of 2 (Scheme 5). This mechanistic scenario was intimated by Power and co-workers[25a] and was demonstrated for

intermediacy of [Cp*ZnH]1 2 was further supported by our recent report of [Cp*ZnH(NHC)] (NHC = SIMes), which can be regarded as the NHC-stabilized intermediate. Similarly, Rit et al. have described [ZnH2(NHC)]2 (NHC = IDipp, IMes).[9b] Interestingly, reduction of [ZnCp*2] or [ZnH2(NHC)]2 with H2 at elevated temperatures gave metallic Zn0.[9b, 12] This implies that soluble zinc hydrides can undergo reductive elimination under reducing conditions. Because dimeric zinc hydrides are key intermediates en route to 2, NHCs react with the transient Zn hydride accounting for the observed formation of NHC-derived aminals. Transfer of a hydride to the NHC and subsequent hydrogenolysis is thought to release the aminal and regenerate the zinc hydride. Indeed, independent reactions showed that this oxidative addition of H2 to the NHC is catalytic in zinc. Such hydride-transfer reactions were previously observed for beryllium, silicon and boron reagents and calculated to constitute the first step in the ring expansion of NHCs.[28]

Conclusion The present results expand and document straightforward synthetic routes to NHC adducts of zincocenes and dizincocenes. Although the Zn22 + unit was characterized as a weak Lewis acid, our findings are in line with H2 elimination from a hydride-bridged dimer en route to dizincocenes. The NHC–zincocenes showed moderate to high activity in catalytic hydrogenation, and the first example of organozinc-catalyzed hydrogenation of ketones to give alcohols is reported. In several cases, the incomplete hydrogenation of ketones also allowed the isolation of monomeric NHC coordinated zinc alkoxides. The unprecedented spontaneous rearrangement of an NHC in the coordination sphere of zinc led to the formation of an abnormal alkoxy complex. The utility and reactivity of these Zn–NHC complexes continues to be the subject of study in our laboratories.

Experimental Section General considerations All manipulations were performed under an atmosphere of dry, oxygen-free N2 by means of standard Schlenk or glovebox techniques (MBraun glovebox equipped with a 40 8C freezer), unless otherwise noted. C6D6 and [D8]toluene were dried over Na/benzophenone ketyl, CD2Cl2 was dried over CaH2 and vacuum transferred before use. Other solvents were purified by using an Innovative Technologies solvent-purification system. [ZnCp*2],[18] [Zn2Cp*2],[20c, d, 12] and SIMes[29] were prepared according to published protocols. [Zn(CpMe4)2] and [Zn(CpTMS)2] were prepared according to the protocol for [ZnCp*2].[18] Carbenes ItBu (TCI), IDipp (Aldrich), and IMes (Aldrich) were used as received. NMR spectra were recorded on Bruker Avance 400 MHz, an Agilent DD2 500, or an Agilent DD2 600 Hz spectrometers. CNHC shifts were determined by heteronuclear multiple-bond coherence (HMBC) experiments, if not observed by 1D 13C NMR spectroscopy. Chemical shifts were referenced internally by using the residual solvent resonances and reported relative to tetramethylsilane. X-ray crystallography (Bruker Kappa Apex II) and elemental analysis (PerkinElmer CHN Analyzer)

Scheme 5. Proposed reaction pathway for 1 with H2 to yield 2 and NHC-derived aminal.

the cadmium homologue.[25d] In support of this view, a sample of 1 pressurized with HD gave 2, H2, D2, Cp*H, and Cp*D. A similar mechanistic proposal was recently reported by Fischer and co-workers, in which the formation of 2 from 1 and ZnH2 was explained by in situ formation of [Cp*ZnH] and reductive elimination upon reaction with another equivalent of 1.[27] The &

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Full Paper Synthesis of [Zn(CpTMS)2(NHC)] (NHC = ItBu) 7, IDipp 8, IMes 9, SIMes 10)

were performed in house. We note that satisfactory carbon contents were obtained only for few of the new compounds, even after repeated elemental analysis. These species are highly air and oxygen sensitive and appear to be pure by all spectroscopic methods. A similar phenomenon has been described earlier.[22a]

These compounds were prepared in a similar fashion, and thus only one preparation is detailed. A vial was charged with [Zn(CpTMS)2] (12 mg, 0.035 mmol) and ItBu (6 mg, 0.033 mmol). The solids were dissolved in toluene (1.5 mL) and stirred overnight. All volatiles were removed under reduced pressure, and the resulting solid was washed with n-pentane. After drying in vacuo, the product was obtained.

Synthesis of [Zn(CpMe4)2(NHC)] (NHC = ItBu 3, IDipp 4, IMes 5, SIMes 6) These compounds were prepared in a similar fashion, and thus only one preparation is detailed. A solution of ItBu (12 mg, 0.067 mmol) in toluene (2 mL) was added to a solution of [Zn(CpMe4)2] (20 mg, 0.065 mmol) in toluene (2 mL). All volatiles of the colorless solution were removed, and the resulting powder was recrystallized from toluene at 35 8C to give colorless crystals that were dried in vacuo (31 mg, 0.064 mmol, 96 %).

Data for 7 Pale orange solid (14 mg, 0.027 mmol, 82 %). 1H NMR (400 MHz, C6D6): d = 6.90 (m, 4 H, CHCp), 6.17 (s, 2 H, CHItBu), 6.11 (m, 4 H, CHCp), 1.12 (s, 18 H, MetBu), 0.39 ppm (s, 18 H, SiMe3); 13C NMR (151 MHz, C6D6): d = 186.6 (CItBu), 127.1 (CCp), 123.3 (CHCp), 119.2 (CHItBu), 101.8 (CHCp), 58.3 (CtBu), 31.2 (MetBu), 1.3 ppm (SiMe3); elemental analysis calcd (%) for C27H46N2Si2Zn (520.23): C 62.34, H 8.91, N 5.38; found: C 61.80, H 9.71, N 5.55.

Data for 3 Data for 8

1

H NMR (400 MHz, C6D6): d = 6.23 (s, 2 H, CHItBu), 4.30 (br hept, J(H,H) = 1 Hz, 2 H, CHCp), 2.24 (s, 12 H, MeCp), 2.15 (s, 12 H, MeCp), 0.93 ppm (s, 18 H, tBu); 13C NMR (125 MHz, C6D6): d = 173.5 (CItBu), 131.5 (CCp), 125.5 (CCp), 119.0 (CHItBu), 65.3 (CHCp), 57.6 (CtBu), 29.4 (MetBu), 15.6 (MeCp), 12.4 ppm (MeCp); elemental analysis calcd (%) for C29H46N2Zn (488.08): C 71.36, H 9.50, N 5.74; found: C 70.46, H 9.46, N 5.52.

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Off-white powder (18 mg, 0.025 mmol, 71 %). 1H NMR (400 MHz, C6D6): d = 7.27 (t, 3J(H,H) = 7.8 Hz, 2 H, p-CHIDipp), 7.12 (d, 3J(H,H) = 7.7 Hz, 4 H, m-CHIDipp), 6.34 (s, 2 H, CHIDipp), 6.19 (br, 4 H, CHCp), 5.66 (br, 4 H, CHCp), 2.68 (pent, 3J(H,H) = 6.8 Hz, 4 H, CHiPr), 1.38 (d, 3 J(H,H) = 6.9 Hz, 12 H, MeiPr), 0.89 (d, 3J(H,H) = 6.9 Hz, 12 H, MeiPr), 0.37 ppm (s, 18 H, SiMe3); 13C NMR (151 MHz, C6D6): d = 178 (CIDipp), 145.9 (o-CIDipp), 135.3 (i-CIDipp), 131.7 (p-CHIDipp), 125.2 (CHIDipp), 124.9 (m-CHIDipp), 119.0 (CHCp), 103.8 (CHCp), 29.4 (CHiPr), 26.4 (MeiPr), 23.3 (MeiPr), 1.6 ppm (SiMe3); elemental analysis calcd (%) for C43H62N2Si2Zn (728.52): C 70.89, H 8.58, N 3.85; found: C 70.34, H 8.63, N 4.10.

Data for 4 Recrystallized from toluene at 35 8C gave yellow crystals that were dried in vacuo (36 mg, 0.052 mmol, 81 %). 1H NMR (400 MHz, C6D6): d = 7.22 (t, 3J(H,H) = 7.8 Hz, 2 H, p-CHIDipp), 7.10 (d, 3J(H,H) = 7.8 Hz, 4 H, m-CHIDipp), 6.48 (s, 2 H, CHIDipp), 3.32 (br s, 2 H, CHCp), 2.85 (hept, 3J(H,H) = 6.8 Hz, 4 H, CHiPr), 2.07 (s, 12 H, MeCp), 1.93 (s, 12 H, MeCp), 1.29 (d, 3J(H,H) = 6.8 Hz, 12 H, MeiPr), 0.98 ppm (d, 3J(H,H) = 6.8 Hz, 12 H, MeiPr); 13C NMR (151 MHz, C6D6): d = 189.4 (CIDipp), 146.2 (o-CIDipp), 137.7 (i-CIDipp), 130.3 (CCp), 128.7 (p-CHIDipp), 126.7 (CCp), 124.6 (m-CHIDipp), 123.5 (CHIDipp), 90.2 (CHCp), 29.3 (CHiPr), 25.4 (MeiPr), 24.1 (MeiPr), 15.2 (MeCp), 12.5 ppm (MeCp); elemental analysis calcd (%) for C45H62N2Zn (696.37): C 77.61, H 8.97, N 4.02; found: C 75.18, H 8.78, N 4.35.

Data for 9 Orange powder (12 mg, 0.019 mmol, 66 %). 1H NMR (400 MHz, C6D6): d = 6.78 (s, 4 H, m-CHIMes), 6.16 (t, 3J(H,H) = 1.9 Hz, 4 H, CHCp), 5.82 (t, 3J(H,H) = 1.9 Hz, 4 H, CHCp), 5.78 (s, 2 H, CHIMes), 2.17 (s, 6 H, MeIMes), 1.92 (s, 12 H, MeIMes), 0.37 ppm (s, 18 H, SiMe3); 13C NMR (125 MHz, C6D6): d = 176.3 (CIMes), 140.6 (p-CIMes), 135.4 (i-CIMes), 135.1 (o-CIMes), 130.3 (m-CHIMes), 128.3 (CCp), 123.3 (CHIMes), 116.4 (CHCp), 105.6 (CHCp), 21.4 (MeIMes), 18.6 (MeIMes), 1.5 ppm (SiMe3); elemental analysis calcd (%) for C37H50N2Si2Zn (644.37): C 68.97, H 7.82, N 4.35; found: C 67.71, H 8.50, N 5.23.

Data for 5

Data for 10 1

Colorless powder (10 mg, 0.016 mmol, 70 %). H NMR (400 MHz, C6D6): d = 6.79 (s, 4 H, m-CHIMes), 5.79 (s, 2 H, CHIMes), 3.68 (br, 2 H, CHCp), 2.21 (s, 12 H, MeCp), 2.11 (s, 6 H, MeIMes), 1.90 (s, 12 H, MeIMes), 1.86 ppm (s, 12 H, MeCp); no 13C NMR data were available due to low solubility; elemental analysis calcd (%) for C39H50N2Zn (612.22): C 76.51, H 8.23, N 4.58; found: C 75.81, H 8.87, N 4.01.

Off-white powder (10 mg, 0.015 mmol, 52 %). 1H NMR (400 MHz, C6D6): d = 6.80 (s, 4 H, CHSIMes), 6.18 (t, 3J(H,H) = 2.1 Hz, 4 H, CHCp), 5.76 (t, 3J(H,H) = 2.1 Hz, 4 H, CHCp), 2.81 (s, 4 H, CH2SIMes), 2.16 (s, 6 H, MeSIMes), 2.11 (s, 12 H, MeSIMes), 0.37 ppm (s, 18 H, SiMe3); 13C NMR (125 MHz, C6D6): d = 201.3 (CSIMes), 139.9 (i-CSIMes), 136.3 (o-CSIMes), 134.8 (p-CSIMes), 130.1 (m-CHSIMes), 128.6 (CCp), 117.6 (CHCp), 104.6 (CHCp), 51.11 (CH2SIMes), 21.4 (MeSIMes), 18.8 (MeSIMes), 1.4 (SiMe3); elemental analysis calcd (%) for C37H52N2Si2Zn (646.38): C 68.75, H 8.11, N 4.33; found: C 65.15, H 8.10, N 4.09.

Data for 6 Colorless powder (9 mg, 0.015 mmol, 65 %). 1H NMR (400 MHz, C6D6): d = 6.80 (s, 4 H, m-CHSIMes), 3.63 (br, 2 H, CHCp), 2.87 (s, 4 H, CH2SIMes), 2.21 (s, 12 H, MeCp), 2.11 (s, 6 H, MeSIMes), 2.09 (s, 12 H, MeSIMes), 1.81 ppm (s, 12 H, MeCp); no 13C NMR data were available due to low solubility, elemental analysis calcd (%) for C39H52N2Zn (614.23): C 76.26, H 8.53, N 4.56; found: C 77.57, H 9.04, N 4.14. Chem. Eur. J. 2014, 20, 1 – 10

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Synthesis of [Zn(OCHPh2)2(NHC)]2 (NHC = aItBu 11. IDipp 12) These compounds were prepared in a similar fashion, and thus only one preparation is detailed. A solution of in ItBu (6 mg, 0.033 mmol) in benzene (0.2 mL) was added to a solution of [Zn(OCHPh2)2] (15 mg, 0.031 mmol) in benzene (0.4 mL). After stirring

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Full Paper overnight, the colorless precipitate was separated by decantation and washed with toluene and n-pentane. After drying in vacuo, the product was obtained.

Data for 14 Colorless crystals (9 mg, 0.015 mmol, 60 %). 1H NMR (500 MHz, C6D6): d = 6.67 (s, 2 H, CHItBu), 2.07 (s, 30 H, MeCp*), 1.43 ppm (s, 18 H, tBu); 13C NMR (125 MHz, C6D6): d = 205.8 (CItBu), 116.0 (CHItBu), 110.1 (CCp*), 56.6 (CtBu), 31.6 (MetBu), 11.2 ppm (MeCp*); elemental analysis calcd (%) for C31H50N2Zn2 (581.52): C 64.03, H 8.67, N 4.82; found: C 62.02, H 9.13, N 4.66.

Data for 11 Colorless solid (16 mg, 0.014 mmol, 90 %). 1H NMR (400 MHz, CD2Cl2): d = 7.74 (d, 3J(H,H) = 7.1 Hz, 4 H, o-CHPh1), 7.56 (d, 4J(H,H) = 1.6 Hz, 2 H, 2-CHaItBu), 7.26 (t, 3J(H,H) = 7.2 Hz, 4 H, m-CHPh1), 7.19 (m, 4 H, o-CHPh2), 7.10 (t, 3J(H,H) = 7.3 Hz, 2 H, p-CHPh1), 6.86 (d, 4J(H,H) = 1.7 Hz, 2 H, 5-CHaItBu), 6.76 (m, 4 H, m-CHPh2), 6.65 (m, 2 H, p-CHPh2), 6.37 (s, 2 H, OCH1), 6.02 (s, 2 H, OCH), 1.40 (s, 18 H, tBu), 1.24 ppm (s, 18 H, tBu); 13C NMR (125 MHz, CD2Cl2): d = 155.3 (CaItBu), 154.1 (iC), 151.4 (i-C), 128.0 (CHPh), 127.9 (CHPh), 127.7 (CHPh), 127.7 (CHPh), 127.3 (5-CHaItBu), 126.2 (2-CHaItBu), 125.3 (CHPh), 125.2 (CHPh), 80.6 (OCH), 80.2 (OCH), 58.8 (CtBu), 56.8 (CtBu), 30.6 (MetBu), 30.5 ppm (MetBu); elemental analysis calcd (%) for C74H84N4O4Zn2 (1224.26): C 72.60, H 6.92, N 4.58; found: C 72.63, H 6.68, N 3.58.

Data for 15 Yellow crystals (12 mg, 0.017 mmol, 68 %). 1H NMR (500 MHz, C6D6): d = 6.79 (s, 4 H, m-CHSIMes), 2.95 (s, 4 H, CH2SIMes), 2.25 (s, 12 H, MeSIMes), 2.11 (s, 30 H, MeCp*), 2.07 ppm (s, 6 H, MeSIMes); 13C NMR (125 MHz, C6D6): d = 208.4 (CSIMes), 139.1 (i-CSIMes), 136.1 (o-CSIMes) 135.3 (p-CSIMes), 130.2 (m-CHSIMes), 110.8 (CCp*), 51.2 (CH2SIMes), 21.3 (MeSIMes), 19.3 (MeSIMes), 12.0 ppm (MeCp*); elemental analysis calcd (%) for C41H56N2Zn2 (707.67): C 69.59, H 7.98, N 3.96; found: C 68.82, H 9.02, N 4.07. CCDC-994020 (3), CCDC-994021 (6), CCDC-994022 (7), CCDC994023 (11), CCDC-994024 (12), CCDC-994025 (13), CCDC-994026 (14), and CCDC-994027 (15) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Data for 12 Colorless crystalline solid (12 mg, 0.015 mmol, 83 %). 1H NMR (400 MHz, C6D6): d = 7.35 (t, 3J(H,H) = 7.8 Hz, 2 H, p-CHIDipp), 7.20– 7.07 (br m, 14 H, m-CHIDipp and o-, m-, p-CHPh), 6.36 (s, 2 H, CHIDipp), 5.65 (s, 2 H, OCH), 2.61 (hept, 3J(H,H) = 6.9 Hz, 4 H, CHiPr), 1.31 (d, 3 J(H,H) = 6.9 Hz, 12 H, MeiPr), 1.03 ppm (d, 3J(H,H) = 6.9 Hz, 12 H, MeiPr); 13C NMR (125 MHz, C6D6): d = 178.6 (CIDipp), 152.5 (i-CPh), 146.4 (o-CIDipp), 134.7 (i-CIDipp), 131.3 (p-CHIDipp), 128.0, 127.5 and 125.8 (o-, m-, p-CHPh), 125.0 (m-CHIDipp), 124.1 (CHIDipp), 80.1 (OCH), 29.4 (CHiPr), 25.1 (MeiPr), 24.4 ppm (MeiPr); elemental analysis calcd (%) for C53H58N2O2Zn (820.43): C 77.59, H 7.13, N 3.41; found: C 75.86, H 7.01, N 3.39.

Acknowledgements P.J. is grateful to the Alexander von Humboldt Foundation for a Feodor Lynen Research Fellowship and wants to thank Conor Pranckevicius for his help with X-ray structure determination. D.W.S. gratefully acknowledges the financial support of the NSERC of Canada and the award of a Canada Research Chair. Keywords: carbene ligands · homogeneous hydrogenation · reaction mechanisms · zinc

Synthesis of [Zn(OCHtBu)2(IDipp)] (13) tBu2CO (9 mg, 0.063 mmol) was added to a solution of 1 (10 mg, 0.030 mmol) and IDipp (12 mg, 0.031 mmol) in toluene (0.5 mL). The reaction mixture was exposed to H2 (4 bar) for 4 d, after which all volatiles were removed. The crude product was washed with npentane and dried in vacuo to give an off-white powder (11 mg, 0.015 mmol, 50 %). 1H NMR (400 MHz, C6D6): d = 7.22 (t, 3J(H,H) = 7.7 Hz, 2 H, p-CHIDipp), 7.11 (d, 3J(H,H) = 7.7 Hz, 2 H, m-CHIDipp), 6.33 (s, 2 H, CHIDipp), 3.58 (s, 2 H, OCH), 2.72 (hept, 3J(H,H) = 6.9 Hz, 4 H, CHiPr), 1.51 (d, 3J(H,H) = 6.9 Hz, 4 H, MeiPr), 1.10 (s, 36 H, MetBu), 1.01 ppm (d, 3J(H,H) = 6.9 Hz, 4 H, MeiPr); 13C NMR (125 MHz, C6D6): d = 179.4 (CIDipp), 145.9 (o-CIDipp), 135.7 (o-CIDipp), 131.3 (p-CHIDipp), 125.1 (m-CHIDipp), 124.6 (CHIDipp), 90.0 (OCH), 38.9 (CtBu), 30.6 (MetBu), 29.5 (CHiPr), 24.9 (MeiPr), 24.5 ppm (CHiPr); elemental analysis calcd (%) for C45H74N2O2Zn (740.47): C 72.99, H 10.07, N 3.78; found: C 72.82, H 9,73, N 3.89.

These compounds were prepared in a similar fashion, and thus only one preparation is detailed. A solution of ItBu (5 mg, 0.028 mmol) in toluene (0.5 mL) was added to a solution of 2 (10 mg, 0.025 mmol) in toluene (0.5 mL). All volatiles of the colorless solution were removed, and the resulting powder was recrystallized from toluene at 35 8C to give colorless crystals that were dried in vacuo. &

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Synthesis of [Zn2Cp*2(NHC)] (NHC = ItBu 14, SIMes 15)

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Received: March 31, 2014 Published online on && &&, 0000

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Full Paper

FULL PAPER & Homogeneous Catalysis

Reaction mechanisms: A series of Nheterocyclic carbene (NHC) adducts of zincocenes [Zn(CpR)2(NHC)] (CpR = C5HMe4, C5H4SiMe3 ; NHC = ItBu, IDipp, IMes, SIMes) was isolated and tested in the catalytic hydrogenation of imines. Catalytic hydrogenation of ketones was achieved with [ZnCp*2] (Cp* = C5Me5)/NHC. Adducts of zinc alkoxides and [Zn2Cp*2] were isolated, and their mechanistic implications are discussed (see scheme).

P. Jochmann, D. W. Stephan* && – && Zincocene and Dizincocene NHeterocyclic Carbene Complexes and Catalytic Hydrogenation of Imines and Ketones

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Chem. Eur. J. 2014, 20, 1 – 10

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 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Zincocene and dizincocene N-heterocyclic carbene complexes and catalytic hydrogenation of imines and ketones.

The N-heterocyclic carbene (NHC) adducts Zn(Cp(R))(2) (NHC)] (Cp(R) =C(5)HMe(4), C(5)H(4) SiMe(3); NHC=ItBu, IDipp (Dipp=2,6-diisopropylphenyl), IMes ...
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