Electrocatalytic Water Oxidation by a Water-Soluble Nickel Porphyrin Complex at Neutral pH with Low Overpotential

The
water-soluble cationic nickel­(II) complex of meso-tetrakis­(4-N-methylpyridyl)­porphyrin (1) can electrocatalyze water oxidation to O2 in neutral
aqueous solution (pH 7.0) with the onset of the catalytic wave appearing
at ∼1.0 V (vs NHE). The homogeneous catalysis with 1 was verified. Catalyst 1 exhibited water oxidation
activity in a pH range 2.0–8.0 and had a strict linear dependence
of catalytic current on its concentration. After 10 h of constant
potential electrolysis at 1.32 V (vs NHE), a negligible difference
of the solution was observed by UV–vis. In addition, inspection
of the working electrode by electrochemistry, scanning electron microscope
(SEM), and energy dispersive X-ray spectroscopy (EDX) showed no sign
of deposition of NiOx films. These results
strongly argued that 1 is a real molecular electrocatalyst
for water oxidation. The turnover frequency (TOF) for this process
was 0.67 s–1 at 20 °C. On the basis of results
from the kinetic isotope effect (KIE) and inhibition experiments,
electrochemical studies in various buffer solutions with different
anions and pHs, and DFT calculations, a catalytic cycle of 1 for water oxidation via a formally Ni­(IV) species was proposed.