Diborane heterolysis: breaking and making B–B bonds at magnesium†
Abstract
Reaction of the dimeric β-diketiminato magnesium hydride [(BDI)MgH]2 (BDI = HC{(Me)CN-2,6-i-Pr2C6H3}2) with bis-pinacolatodiborane (B2pin2) 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) (B2hex2) provided a B(sp2)–B(sp3) 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 (B2pin2). 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(sp2)–B(sp3) 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.