Group 2 Promoted Hydrogen Release from NMe2H⋅BH3: Intermediates and Catalysis

Abstract

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 Me 2 NH⋅BH 3. Reactions of either magnesium dialkyls or the β‐diketiminate complex [HC{(Me)CN(Dipp)}2 Mg n Bu] with four or two equivalents of Me 2 NHBH 3, respectively, produce compounds containing the [H 3 BNMe 2 BH 2 Me 2 N]− ion, which coordinates to the magnesium centers through MgN and Mg⋅⋅⋅HB interactions in both the solution and solid states. Thermolysis of these compounds at 60 °C produces the cyclic product [(H 2 BNMe 2)2] and, it is proposed, magnesium hydrido species by an unprecedented δ‐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 H 2 BNMe 2 into polarized MN bonds as the major BN bond‐forming step. The efficacy of this insertion and subsequent β‐ or δ‐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.