Hydrogen Release Studies of Alkali Metal Amidoboranes

A series of metal amidoboranes LiNH2BH3 (LAB), NaNH2BH3 (SAB), LiNH(Me)BH3 (LMAB), NaNH(Me)BH3 (SMAB), KNH(Me)BH3 (PMAB), and KNH(Bu-t)BH3 (PBAB) were synthesized by solution phase methods, and the thermal release of H-2 in the solid state was studied. The mechanism of hydrogen release from metal amidoboranes, a metal ion assisted hydride transfer, is very different than the mechanism of hydrogen release from the parent compound ammonia borane (AB). On the basis of the observed trends in reaction rates of H > Me > Bu-t, K > Na > Li, and the kinetic isotope effect, the mechanism of hydrogen release from MAB compounds was found to proceed through a bimolecular mechanism involving the intermediacy of a MH (M = Li, Na, or K). The non-volatile products formed from MABs are significantly different than the products formed after hydrogen release from AB. The boron containing products resulting from the release of 1 equiv of hydrogen from the metal amidoboranes were characterized by MAS B-11 NMR spectroscopy and found to contain both BH3 and sp(2) hybridized BH groups, consistent with the general structure MN(R)=BHN(R)MBH3.