Computational and Experimental Study of the Mechanism of Hydrogen Generation from Water by a Molecular Molybdenum-Oxo Electrocatalyst

We investigate the mechanism for the electrocatalytic
generation
of hydrogen from water by the molecular molybdenum-oxo complex, [(PY5Me2)­MoO]2+ (PY5Me2 = 2,6-bis­(1,1-bis­(2-pyridyl)­ethyl)­pyridine).
Computational and experimental evidence suggests that the electrocatalysis
consists of three distinct electrochemical reductions, which precede
the onset of catalysis. Cyclic voltammetry studies indicate that the
first two reductions are accompanied by protonations to afford the
Mo-aqua complex, [(PY5Me2)­Mo­(OH2)]+. Calculations support hydrogen evolution from this complex upon
the third reduction, via the oxidative addition of a proton from the
bound water to the metal center and finally an α-H abstraction
to release hydrogen. Calculations further suggest that introducing
electron-withdrawing substituents such as fluorides in the para positions of the pyridine rings can reduce the potential
associated with the reductive steps, without substantially affecting
the kinetics. After the third reduction, there are kinetic bottlenecks
to the formation of the Mo-hydride and subsequent hydrogen release.
Computational evidence also suggests an alternative to direct α-H
abstraction as a mechanism for H2 release which exhibits
a lower barrier. The new mechanism is one in which a water acts as
an intramolecular proton relay between the protons of the hydroxide
and the hydride ligands. The calculated kinetics are in reasonable
agreement with experimental measurements. Additionally, we propose
a mechanism for the stoichiometric reaction of [(PY5Me2)­Mo­(CF3SO3)]+ with water to yield
hydrogen and [(PY5Me2)­MoO]2+ along
with the implications for the viability of an alternate catalytic
cycle involving just two reductions to generate the active catalyst.