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.