Hydrogen Evolution from Water under Visible-Light Irradiation Using Keggin-Type Platinum(II)-Coordinated Phospho-, Silico-, and Germanotungstates as Co-Catalysts

The tetramethylammonium salts of diplatinum(II)complexes composed of mono-lacunary Keggin-type silico and germanotungstates, [(CH3)4N]4[α-SiW11O39{cis-Pt(NH3)2}2]·13H2O (TMA-Si-Pt)and [(CH3)4N]4[α-GeW11O39{cis-Pt(NH3)2}2]·11H2O (TMA-Ge-Pt),were synthesized and crystallized. Single crystals of a tetramethylammoniumsalt of Keggin-type diplatinum(II)-coordinatedphosphotungstate [(CH3)4N]3[α-PW11O39{cis-Pt(NH3)2}2]×10H2O (TMA-P-Pt) were also obtained. The X-ray structural analyses revealed that the two cis-platinum(II) moieties, [cis-Pt(NH3)2]2+,were each coordinated to two oxygen atoms in a mono-vacant site of [XW11O39](12-n)- (Xn+ = Si4+, Ge4+, P5+). FTIRspectra of TMA-Si-Pt and TMA-Ge-Pt also suggested that the two platinum(II)moieties were coordinated to the vacant siteof [SiW11O39]8- and [GeW11O39]8-.The 1H NMR spectra in DMSO-d6 of TMA-Si-Pt and TMA-Ge-Ptshowed signals assigned to the two sets of NH3 ligands coordinated to the platinum sites. These threeplatinum compounds showed hydrogenevolution from aqueous triethanolamine solution under visible lightirradiation (λ ≥ 400 nm) in the presence of eosin Y, α-Keggin mono-aluminum-substitutedsilicotungstate, and titanium dioxide. The catalytic activities were influenced by thecentral atoms, and TMA-P-Pt showed the highest activities among the threeplatinum compounds.