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Keywords:

  • boranes;
  • calcium;
  • dehydrocoupling;
  • magnesium;
  • structure elucidation

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 Me2NHBH3. Reactions of either magnesium dialkyls or the β-diketiminate complex [HC{(Me)CN(Dipp)}2MgnBu] with four or two equivalents of Me2NHBH3, respectively, produce compounds containing the [H3BNMe2BH2Me2N] ion, which coordinates to the magnesium centers through Mg[BOND]N and Mg⋅⋅⋅HB interactions in both the solution and solid states. Thermolysis of these compounds at 60 °C produces the cyclic product [(H2BNMe2)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 H2B[DOUBLE BOND]NMe2 into polarized M[BOND]N bonds as the major B[BOND]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.