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