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 mechanism for benzylamine dehydrogenation
that we have previously proposed and provide insights for future catalyst
design using metal–ligand cooperative pathways with Al.