Molecular Zinc Hydride Cations [ZnH]⁺ : Synthesis, Structure, and CO₂ Hydrosilylation Catalysis

Protonolysis of [ZnH₂ ]_n with the conjugated Brønsted acid of the bidentate diamine TMEDA (N,N,N',N'-tetramethylethane-1,2-diamine) and TEEDA (N,N,N',N'-tetraethylethane-1,2-diamine) gave the zinc hydride cation [(L₂ )ZnH]⁺ , isolable either as the mononuclear THF adduct [(L₂ )ZnH(thf)]⁺ [BAr^F ₄ ]^- (L₂ =TMEDA; BAr^F ₄ ^- =[B(3,5-(CF₃ )₂ -C₆ H₃ )₄ ]^- ) or as the dimer [{(L₂ )Zn)}₂ (μ-H)₂ ]²⁺ [BAr^F ₄ ]^- ₂ (L₂ =TEEDA). In contrast to [ZnH₂ ]_n , the cationic zinc hydrides are thermally stable and soluble in THF. [(L₂ )ZnH]⁺ was also shown to form di- and trinuclear adducts of the elusive neutral [(L₂ )ZnH₂ ]. All hydride-containing cations readily inserted CO₂ to give the corresponding formate complexes. [(TMEDA)ZnH]⁺ [BAr^F ₄ ]^- catalyzed the hydrosilylation of CO₂ with tertiary hydrosilanes to give stepwise formoxy silane, methyl formate, and methoxy silane. The unexpected formation of methyl formate was shown to result from the zinc-catalyzed transesterification of methoxy silane with formoxy silane, which was eventually converted into methoxy silane as well.