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 (<b>NiP</b>) with a DuBois-type [Ni­(P₂^R′N₂^R″)₂]²⁺ core (P₂^R′N₂^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 <b>NiP</b> 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 (<b>RuP</b>) in homogeneous and heterogeneous environments. Time-resolved luminescence and transient absorption spectroscopy studies confirmed that directed ET from <b>RuP</b> to <b>NiP</b> occurs efficiently in all systems on the nano- to microsecond time scale, through three distinct routes: reductive quenching of <b>RuP</b> in solution or on the surface of ZrO₂ (“on particle” system) or oxidative quenching of <b>RuP</b> when the compounds were immobilized on TiO₂ (“through particle” system). Our studies show that <b>NiP</b> 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