ChemInform Abstract: Nickel‐Containing Keggin‐Type Polyoxometalates as Hydrogen Evolution Catalysts: Photochemical Structure—Activity Relationships.

Abstract

Nickel-Containing Keggin-Type Polyoxometalates as Hydrogen EvolutionCatalysts: Photochemical Structure-Activity Relationships. -The Ni-monosubstituted Keggin-type polyoxometalates (POMs) K 5[Ni(H2O)PW11O39]·12H2O (I), K6[Ni(H2O)GeW11O39]·14H2O (II), and K6[Ni(H2O)SiW11O39]·14H2O (III) are prepared according to a modified published method. As an example, compound (I) is prepared from an acidified (HCl) aqueous 1:3 molar mixture of Na 2WO4 and Na2HPO4 (100 C) which is subjected to solvent extraction by Et 2O. The organic fraction then is heated to 95 C, and a solution of Ni(NO3)3 in acetate buffer (pH 7) is added (2 min stirring) followed by cooling to 65 C and further addition of MeOH. A green precipitate is isolated after keeping the mixture at 8 C overnight (52% yield based on Na 2WO4·2H2O). The compounds crystallize in the tetragonal space group P4n2 (Z = 2, single crystal XRD) and show Ni/W disorder at one of the three symmetry-unique W sites. Nickel substitution is identified as a crucial feature for catalytic activity through comparison with nickel-free reference POMs. Furthermore, turnover numbers (TON) and turnover frequencies strongly depended on the heteroatom (P, Ge, Si) with the highest TON recorded for [Ni(H 2O)GeW11 O39] 6-. Photochemical H2 evolution activity is compared with redox and onset potentials obtained from electrochemical analyses. Furthermore, activity trends are correlated with electronic structure properties derived from DFT calculations. -(VON ALLMEN, K.; MORE, R.; MUELLER, R.; SORIANO-LOPEZ, J.; LINDEN, A.; PATZKE*, G. R.; ChemPlusChem 80 (2015) 9, 1389-1398, http://dx.doi.org/10.1002/cplu.201500074 ; Inst. Chem., Univ. Zuerich, CH-8057 Zuerich, Switz.; Eng.) -J. Schramke 46-010