A Noble-Metal-Free, Tetra-nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution

A tetra-nickel-containing polyoxotungstate,
Na6K4[Ni4(H2O)2(PW9O34)2]·32H2O
(Na6K4-Ni4P2), has been synthesized in
high yield and
systematically characterized. The X-ray crystal structure confirms
that a tetra-nickel cluster core [Ni4O14] is
sandwiched by two trivacant, heptadentate [PW9O34]9– POM ligands. When coupled with (4,4′-di-tert-butyl-2,2′-dipyridyl)-bis­(2-phenylpyridine­(1H))-iridium­(III) hexafluorophosphate [Ir­(ppy)2(dtbbpy)]­[PF6] as photosensitizer and triethanolamine
(TEOA) as sacrificial electron donor, the noble-metal-free complex Ni4P2 works as an efficient and robust molecular catalyst for H2 production upon visible light irradiation. Under minimally
optimized conditions, Ni4P2 catalyzes H2 production
over 1 week and achieves a turnover number (TON) of as high as 6500
with almost no loss in activity. Mechanistic studies (emission quenching,
time-resolved fluorescence decay, and transient absorption spectroscopy)
confirm that, under visible light irradiation, the excited state [Ir­(ppy)­2­(dtbbpy)]+* can be both oxidatively and reductively quenched by Ni4P2 and TEOA, respectively. Extensive stability studies (e.g.,
UV–vis absorption, FT-IR, mercury-poison test, dynamic light
scattering (DLS) and transmission electron microscopy (TEM)) provide
very strong evidence that Ni4P2 catalyst remains homogeneous
and intact under turnover conditions.