A Neutral Aluminocene Sandwich Complex: η1- versus η5-Coordination Modes of a Pentaarylborole with ECp* (E=Al, Ga; Cp*=C5Me5)

The pentaaryl borole (Ph*C)(4)BXyl(F) [Ph*=3,5-tBu(2)(C6H3); Xyl(F)=3,5-(CF3)(2)(C6H3)] reacts with low-valent Group 13 precursors AlCp* and GaCp* by two divergent routes. In the case of [AlCp*](4), the borole reacts as an oxidising agent and accepts two electrons. Structural, spectroscopic, and computational analysis of the resulting unprecedented neutral eta(5)-Cp*,eta(5)-[(Ph*C)(4)BXyl(F)] complex of Al-III revealed a strong, ionic bonding interaction. The formation of the heteroleptic borole-cyclopentadienyl aluminocene leads to significant changes in the C-13 NMR chemical shifts within the borole unit. In the case of the less-reductive GaCp*, borole (Ph*C)(4)BXyl(F) reacts as a Lewis acid to form a dynamic adduct with a dative 2-center-2-electron Ga-B bond. The Lewis adduct was also studied structurally, spectroscopically, and computationally.