Versatile photocatalytic systems for H₂ generation in water based on an efficient DuBois-type nickel catalyst

The generation of renewable H₂ through an efficient photochemical route requires photoinduced electron transfer (ET) from a light harvester to an efficient electrocatalyst in water. Here, we report on a molecular H₂ evolution catalyst (NiP) with a DuBois-type [Ni(P ₂^R′N₂^R″) ₂]²⁺ core (P₂^R′N ₂^R″ = bis(1,5-R′-diphospha-3,7-R″- diazacyclooctane), which contains an outer coordination sphere with phosphonic acid groups. The latter functionality allows for good solubility in water and immobilization on metal oxide semiconductors. Electrochemical studies confirm that NiP is a highly active electrocatalyst in aqueous electrolyte solution (overpotential of approximately 200 mV at pH 4.5 with a Faradaic yield of 85 ± 4%). Photocatalytic experiments and investigations on the ET kinetics were carried out in combination with a phosphonated Ru(II) tris(bipyridine) dye (RuP) in homogeneous and heterogeneous environments. Time-resolved luminescence and transient absorption spectroscopy studies confirmed that directed ET from RuP to NiP occurs efficiently in all systems on the nano- to microsecond time scale, through three distinct routes: reductive quenching of RuP in solution or on the surface of ZrO₂ ("on particle" system) or oxidative quenching of RuP when the compounds were immobilized on TiO₂ ("through particle" system). Our studies show that NiP can be used in a purely aqueous solution and on a semiconductor surface with a high degree of versatility. A high TOF of 460 ± 60 h^-1 with a TON of 723 ± 171 for photocatalytic H₂ generation with a molecular Ni catalyst in water and a photon-to-H₂ quantum yield of approximately 10% were achieved for the homogeneous system.