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