Pyridyl ‘ring-flipping’ in the dimers [Me2E(2-py)]2 (E = B, Al, Ga; 2-py = 2-pyridyl)

The boron and aluminium dimers [Me2E(µ-py)]2 [E = B (1); Al (2)] are formed as mixtures of two isomers in which the group 13 centres are coordinated by the bridging 2-py ligands in a cis or trans manner, however, in contrast to previous studies, we find that simply heating the mixtures of these isomers of 1 and 2 gives the more thermodynamically stable (synthetically useful) trans isomers exclusively (the trans isomer being the only product in the case of the gallium analogue 3).