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

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 L n = 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 [(L n )­Zn­(OCHO)2] were isolated and structurally characterized, either as a molecule (L n = TMEDA, TMPDA, PMDTA) or a charge-separated ion pair [(L n )­Zn­(OCHO)]­[OCHO] (L n = Me4TACD). The reaction of [ZnH2] n with the mild Lewis acid BPh3 in the presence of chelating N-donor ligands L n gave a series of hydridotriphenylborates, either as a contact ion pair [(L2)­Zn­(H)­(HBPh3)] (L2 = TMEDA, TMPDA) or a separated ion pair [(L n )­Zn­(H)]­[HBPh3] (L n = 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 [(L n )­Zn­(H)­(HBPh3)] underwent CO2 insertion to give (formato)zinc (formoxy)­triphenylborate complexes [(L n )­Zn­(OCHO)]­[(OCHO)­BPh3] (L n = TMPDA, PMDTA, Me4TACD). For L n = TMEDA, a dinuclear complex [(L n )2Zn2(μ-OCHO)3]­[(OCHO)­BPh3] was isolated. Hydridotriphenylborates [(L n )­Zn­(H)­(HBPh3)] catalyzed the hydrosilylation of CO2 (1 bar) by n BuMe2SiH in THF at 70 °C to give formoxysilane and (methoxy)­silane.