Deaggregation of Zinc Dihydride by Lewis Acids Including Carbon Dioxide in the Presence of Nitrogen Donors.

Abstract

Thermally sensitive polymeric zinc dihydride [ZnH2]n can conveniently be prepared by the reaction of ZnEt2 with [AlH3(NEt3)]. When reacted with CO2 (1 bar) in the presence of chelating N-donor ligands Ln = N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetramethyl-1,3-propanediamine (TMPDA), N,N,N',N″,N''-pentamethyldiethylenetriamine (PMDTA), and 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (Me4TACD), insertion into the Zn-H bond readily occurred. Depending on the denticity n, formates [(Ln)Zn(OCHO)2] were isolated and structurally characterized, either as a molecule (Ln = TMEDA, TMPDA, PMDTA) or a charge-separated ion pair [(Ln)Zn(OCHO)][OCHO] (Ln = Me4TACD). The reaction of [ZnH2]n with the mild Lewis acid BPh3 in the presence of chelating N-donor ligands Ln gave a series of hydridotriphenylborates, either as a contact ion pair [(L2)Zn(H)(HBPh3)] (L2 = TMEDA, TMPDA) or a separated ion pair [(Ln)Zn(H)][HBPh3] (Ln = PMDTA, Me4TACD). In the crystal, the contact ion pair [(TMEDA)Zn(H)(HBPh3)] showed a bent Zn-H-B bridge indicative of a delocalized Zn-H-B interaction. In contrast, a linear Zn-H-B bridge for [(TMPDA)Zn(H)(HBPh3)] was observed, suggesting a contact ion pair. In THF solution, both complexes show an exchange with free BPh3 as well as [HBPh3]-. DFT calculations suggest the presence of [HBPh3]- anion with a highly polarized B-H bond that interacts with the Lewis acidic zinc hydride cation [(L2)Zn(H)]+. The hydridotriphenylborates [(Ln)Zn(H)(HBPh3)] underwent CO2 insertion to give (formato)zinc (formoxy)triphenylborate complexes [(Ln)Zn(OCHO)][(OCHO)BPh3] (Ln = TMPDA, PMDTA, Me4TACD). For Ln = TMEDA, a dinuclear complex [(Ln)2Zn2(μ-OCHO)3][(OCHO)BPh3] was isolated. Hydridotriphenylborates [(Ln)Zn(H)(HBPh3)] catalyzed the hydrosilylation of CO2 (1 bar) by nBuMe2SiH in THF at 70 °C to give formoxysilane and (methoxy)silane.