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.