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

Abstract

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