title: 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 beta-diketiminate complex [HC{(Me)CN(Dipp)}(2)MgnBu] 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 degrees C produces the cyclic product [(H(2)BNMe(2))(2)] and, it is proposed, magnesium hydrido species by an unprecedented delta-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 beta- or delta-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.
authors.affiliation: Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Liptrot
authors.forename: Michael S
authors.initials: MS
authors.lastname: Hill
authors.forename: Mary F
authors.initials: MF
authors.lastname: Mahon
authors.forename: Dugald J
authors.initials: DJ
authors.lastname: MacDougall