Reactivity of a Bulky Bora-Amidine Ligand with Trimethylaluminum

Reaction of Me3Al with HB[N(H)DIPP](2) (DIPP = 2,6-iPr(2)-C6H3) in hexane gave a complex mixture of compounds instead of HB(N-DIPP)(2)AlMe. After addition of THF, one of the products could be trapped in trace amounts and was characterized by crystal structure determination: DIPP-N(H)B(H)(N-DIPP)AlMe2 center dot THF (1). Heating a hexane or fluorobenzene solution of HB[N(H)DIPP](2) and Me3Al in the absence of THF gave a relatively clean conversion and allowed for the isolation of [(DIPPNH)AlMe2(mu-H)AlMe2](2) (2), a product that was formed through B/Al ligand exchange. Complex 2 contains protic and hydridic hydrogen atoms in close vicinity. The compound remains intact upon heating, however, it readily releases H-2 when dissolved in THF forming DIPPN(AlMe2 center dot THF)(2) (3). The latter bimetallic complex contains two THF stabilized Lewis acidic central aluminum atoms in close proximity.