Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Abstract

Reactions of β-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-ⁱPr₂C₆H₃)}₂M(THF)_n{N(SiMe₃)₂}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH₂)₄NH·BH₃, were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-ⁱPr₂C₆H₃)}₂MN(CH₂)₄·BH₃]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH₂)₄NH·BH₃ to the Mg derivative induced the formation of a species, [HC{(Me)CN(2,6-ⁱPr₂C₆H₃)}₂Mg{N(CH₂)₄ BH₂NMe₂BH₃}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H₂. 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(SiMe₃)₂}₂] (M = Mg, Ca, Sr, Ba), with (CH₃)₂NH·BH₃ and ⁱPr₂NH·BH₃ reactivity consistent with successive amidoborane β-hydride elimination and [R₂NBH₂] insertion is described as a means to induce the B–N dehydrocoupling between amine borane substrates.