Deaggregation of Zinc Dihydride by Lewis Acids Including Carbon Dioxide in the Presence of Nitrogen Donors
Résumé
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 1,4,7,10tetramethyl-1,4,7,10-tetraazacyclododecane (Me(4)TACD), 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 = Me(4)TACD). 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 [(L-2)Zn(H)(HBPh3)] (L-2 = TMEDA, TMPDA) or a separated ion pair [(L-n)Zn(H)][HBPh3] (L-n = PMDTA, Me(4)TACD). 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 [(L-2)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, Me(4)TACD). For L-n = TMEDA, a dinuclear complex [(L-n)(2)Zn-2(mu-OCHO)(3)][(OCHO)BPh3] was isolated. Hydridotriphenylborates [(L-n)Zn(H)(HBPh3)] catalyzed the hydrosilylation of CO2 (1 bar) by (BuMe2SiH)-Bu-n in THF at 70 degrees C to give formoxysilane and (methoxy)silane.