Diborane Heterolysis: Breaking and Making B-B bonds at Magnesium

Reaction of the dimeric β-diketiminato magnesium hydride [(BDI)MgH] ₂ (BDI = HC{(Me)CN-2,6-i-Pr ₂C ₆H ₃} ₂) with bis-pinacolatodiborane (B ₂pin ₂) resulted in B-O bond activation and formation of a magnesium complex of an unusual borylborohydride anion. In contrast, similar treatment of the mononuclear organomagnesium [{BDI}Mg(n-Bu)] with 4,4,4′,4′,6,6′-hexamethyl-2,2′-bi(1,3,2-dioxaborinane) (B ₂hex ₂) provided a B(sp ²)-B(sp ³) diborane anion, [(hex)BB(n-Bu)(hex)] ^-, with a constitution which is analogous to that formed in the previously reported reaction with bis(pinacolato)diboron (B ₂pin ₂). Subsequent addition of 4-dimethylaminopyridine to a solution of this compound induced alkylborane displacement and provided a magnesium boryl derivative containing a terminal Mg-B(hex) interaction (Mg-B 2.319(3) Å), a result which reinforces the generality of this approach for the synthesis of boryl anions by B-B bond heterolysis. Further studies of the reactivity of the initially formed B(sp ²)-B(sp ³) anions with diborane small molecules also resulted in alkylborane displacement and the production of triboron anions, which are propagated by contiguous and electron precise (2c-2e) B-B-B interactions.