Backbone Immobilization of the Bis(bipyridyl)pyrazolate Diruthenium Catalyst for Electrochemical Water Oxidation

Efficient
catalysts for water oxidation are key to all scenarios
for artificial solar water splitting, and, in the case of molecular
catalysts, their immobilization on conductive solid supports is considered
essential for the construction of a photoelectrochemical cell. In
this work, derivatives of the rugged 3,5-bis­(bipyridyl)­pyrazolate
(bbp)-based diruthenium water oxidation precatalyst [(μ-bbp)­{Ru­(py)2}2(μ-X)]2+ (X = OAc, CO3H) have been synthesized that are equipped with a carboxylate anchor
at the ligand backbone. The para-C6H4COOR (R = Et, H) substituent at the pyrazolate-C4 is shown to not have any significant effect on the electronic properties
of the bbp-based diruthenium core, or on the catalytic performance
when using CeIV as a chemical oxidant. The bbp-based complex
with labile exogenous bicarbonate bridge (4) has been
successfully anchored on mesoporous ITO (mesoITO),
and the hybrid ITO|mesoITO|4 shows electrochemical
signatures similar to those of the homogeneous system. Its electrocatalytic
water oxidation performance at pH 1 has been investigated by rotating
ring disk electrode (RRDE) measurements, and high faradaic efficiency
is evidenced by the agreement of Tafel slopes for the disk and ring
currents. X-ray photoelectron spectroscopy (XPS) data before and after
electrochemical measurements were compared to evaluate the chemical
stability of the immobilized complex 4 during catalysis,
showing that the surface-bound complex species remains intact without
any decomposition to RuO2. However, gradual leaching of
the complex leads to decreasing catalyst concentration and decreasing
activity during electrocatalysis, reflecting weak attachment of the
carboxylate anchor to the oxide substrate and emphasizing the need
for improved anchoring strategies.