Insight into Varied Reaction Pathways for O-H and N-H Bond Activation by Bis(imino)pyridine Complexes of Al(III)

The activation of O-H bonds in alcohol substrates is the initial step in acceptor-less catalytic dehydrogenation of alcohols. At room temperature, the bis(imino)pyridine-ligated aluminum hydride compound, denoted as ((I2P2-)-I-Ph)AlH (1), activates O-H bonds in alcohols through a metal-ligand cooperative pathway to afford the phenmdde and benzyloxide complexes with protonated ligand: ((HI2P1-)-H-Ph)Al(OR)H, where R = Ph, Bn. Thermochemical measurements indicate that the amido nitrogen of the protonated ligand in ((HI2P1-)-H-Ph)Al(OPh)H is far more basic (pK(a) = 36-45) than the analogous proton for the previously reported ((HI2P1-)-H-Ph)Al(NHPh)H (pK(a) = 10-14), and this is consistent with reactivity we observe, where ((HI2P1-)-H-Ph)Al(OPh)H complexes do not intramolecularly liberate H-2. The inability of ((HI2P1-)-H-Ph)Al(OR)H to release H-2 upon heating precludes access to a four-coordinate Al center and results in an inability of 1 to dehydrogenate benzyl alcohol to benzaldehyde. These observations also lend further support to the mechanism for benzylamine dehydrogenation that we have previously proposed and provide insights for future catalyst design using metalligand cooperative pathways with Al.