Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Reactions of beta-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)M(THF)(n){N(SiMe(3))(2)}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH(2))(4)NH center dot BH(3), were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)MN(CH(2))(4)center dot BH(3)]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH(2))(4)NH center dot BH(3) to the Mg derivative induced the formation of a species, [HC{(Me) CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg{N(CH(2))(4)BH(2)NMe(2)BH(3)}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H(2). Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, [M{N(SiMe(3))(2)}(2)] (M= Mg, Ca, Sr, Ba), with (CH(3))(2)NH center dot BH(3) and (i)Pr(2)NH center dot BH(3) reactivity consistent with successive amidoborane beta-hydride elimination and [R(2)N=BH(2)] insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.