Nickel Pyridinethiolate Complexes as Catalysts for
the Light-Driven Production of Hydrogen from Aqueous Solutions in
Noble-Metal-Free Systems
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Abstract
A series
of mononuclear nickel(II) thiolate complexes (Et<sub>4</sub>N)Ni(X-pyS)<sub>3</sub> (Et<sub>4</sub>N = tetraethylammonium; X
= 5-H (<b>1a</b>), 5-Cl (<b>1b</b>), 5-CF<sub>3</sub> (<b>1c</b>), 6-CH<sub>3</sub> (<b>1d</b>); pyS = pyridine-2-thiolate),
Ni(pySH)<sub>4</sub>(NO<sub>3</sub>)<sub>2</sub> (<b>2</b>),
(Et<sub>4</sub>N)Ni(4,6-Y<sub>2</sub>-pymS)<sub>3</sub> (Y = H (<b>3a</b>), CH<sub>3</sub> (<b>3b</b>); pymS = pyrimidine-2-thiolate),
and Ni(4,4′-Z-2,2′-bpy)(pyS)<sub>2</sub> (Z = H (<b>4a</b>), CH<sub>3</sub> (<b>4b</b>), OCH<sub>3</sub> (<b>4c</b>); bpy = bipyridine) have been synthesized in high yield
and characterized. X-ray diffraction studies show that <b>2</b> is square planar, while the other complexes possess tris-chelated
distorted-octahedral geometries. All of the complexes are active catalysts
for both the photocatalytic and electrocatalytic production of hydrogen
in 1/1 EtOH/H<sub>2</sub>O. When coupled with fluorescein (Fl) as
the photosensitizer (PS) and triethylamine (TEA) as the sacrificial
electron donor, these complexes exhibit activity for light-driven
hydrogen generation that correlates with ligand electron donor ability.
Complex <b>4c</b> achieves over 7300 turnovers of H<sub>2</sub> in 30 h, which is among the highest reported for a molecular noble
metal-free system. The initial photochemical step is reductive quenching
of Fl* by TEA because of the latter’s greater concentration.
When system concentrations are modified so that oxidative quenching
of Fl* by catalyst becomes more dominant, system durability increases,
with a system lifetime of over 60 h. System variations and cyclic
voltammetry experiments are consistent with a CECE mechanism that
is common to electrocatalytic and photocatalytic hydrogen production.
This mechanism involves initial protonation of the catalyst followed
by reduction and then additional protonation and reduction steps to
give a key Ni–H<sup>–</sup>/N–H<sup>+</sup> intermediate
that forms the H–H bond in the turnover-limiting step of the
catalytic cycle. A key to the activity of these catalysts is the reversible
dechelation and protonation of the pyridine N atoms, which enable
an internal heterocoupling of a metal hydride and an N-bound proton
to produce H<sub>2</sub>
