Group 2 promoted hydrogen release from NMe₂h·BH₃: intermediates and catalysis

Both homo- and heteroleptic alkyl and amide complexes of the Group 2 elements Mg and Ca are shown to be active for the catalytic dehydrocoupling of Me2NH&middotBH3. Reactions of either magnesium dialkyls or the -diketiminate complex [HC((Me)CN(Dipp)}2MgnBu] with four or two equivalents of Me2NHBH3, respectively, produce compounds containing the [H3BNMe2BH2Me 2N]- ion, which coordinates to the magnesium centers through Mg-N and MgHB interactions in both the solution and solid states. Thermolysis of these compounds at 60C produces the cyclic product [(H2BNMe2)2] and, it is proposed, magnesium hydrido species by an unprecedented -hydride elimination process. Calcium-based species, although less reactive than their magnesium-based counterparts, are found to engage in similar dehydrocoupling reactivity and to produce a similar distribution of products under thermally promoted catalytic conditions. A mechanism for these observations is presented that involves initial production and insertion of H2B=NMe2 into polarized M-N bonds as the major B-N bondforming step. The efficacy of this insertion and subsequent - or -hydride elimination steps is proposed to be dependent upon the charge density and polarizing capability of the participating Group 2 center, providing a rationale for the observed differences in reactivity between magnesium and calcium.