Alkaline-Earth Derivatives of the Reactive [HB(C₆F₅)₃]^- Anion

The β-diketiminato magnesium amidoboranes [HC{(Me)CNDipp}₂Mg(NMe₂BH₂NMe₂·BH₃)] and [HC{(t-Bu)CNDipp}₂Mg(NMe₂·BH₃)] are readily converted to the corresponding derivatives of the [HB(C₆F₅)₃]^- anion by treatment with B(C₆F₅)₃. The bis(borohydride) derivatives of the heaviest alkaline-earth elements, strontium and barium, may be similarly synthesized by reaction of strontium or barium dimethylamidoboranes and B(C₆F₅)₃ and by metathesis reactions of either SrI₂ or BaI₂ and 2 molar equiv of K(HB(C₆F₅)₃). The strontium and barium compounds have been fully characterized in solution and in the solid state as the respective tris(diethyl ether) and tetrakis(tetrahydrofuran) adducts. The magnesium compound [HC{(Me)CNDipp}₂Mg(HB(C₆F₅)₃)] has been applied to the catalytic hydroboration of i-PrN=C=N-i-Pr with HBpin. In contrast to carbodiimide hydroboration catalyzed by the corresponding β-diketiminato magnesium hydride, which results in the exclusive production of the monoborylated amidine, use of the [HB(C₆F₅)₃]^- derivative provides the product of bis-borylation, the aminal H₂C(N{Bpin}i-Pr)₂, under mild conditions. A series of stoichiometric reactions highlight that, while this reactivity is likely to be primarily magnesium mediated, B(C₆F₅)₃ plays a vital role both in the delivery of reactive hydride and through the Lewis acid activation of the heteroallene substrate and various reactive intermediates.