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 (PhI2P2–)AlH (1), activates O–H bonds in alcohols through a metal–ligand cooperative pathway to afford the phenoxide and benzyloxide complexes with protonated ligand: (PhHI2P1–)Al(OR)H, where R = Ph, Bn. Thermochemical measurements indicate that the amido nitrogen of the protonated ligand in (PhHI2P1–)Al(OPh)H is far more basic (pKa = 36–45) than the analogous proton for the previously reported (PhHI2P1–)Al(NHPh)H (pKa = 10–14), and this is consistent with reactivity we observe, where (PhHI2P1–)Al(OPh)H complexes do not intramolecularly liberate H2. The inability of (PhHI2P1–)Al(OR)H to release H2 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 mech...