Modification of MOF catalysts by manipulation of counter-ions: experimental and theoretical studies of photochemical hydrogen production from water over microporous diruthenium (II, III) coordination polymers

In this study we investigated the photochemical production of hydrogen from water using three heterogeneous microporous ruthenium coordination polymers [Ru2(p-BDC)2X]n (p-BDC=1,4-benzenedicarboxylate, X=Cl, Br and BF4) in the presence of multi-component systems. The order of catalytic performances is [Ru2(p-BDC)2Br]n[Ru2(p-BDC)2BF4]n[Ru2(p-BDC)2Cl]n. The most active catalyst, [Ru2(p-BDC)2Br]n, caused the evolution of 46.7mol hydrogen molecules with a turn-over number of 18.7 based on [Ru2(p-BDC)2Br]n under visible light irradiation for 4h. We ascertained that the differences in catalytic activities originated from (1) the efficiency of the quenching of methyl-viologen radicals by [Ru2(p-BDC)2X]n and (2) the durability of the structure in the reaction. In order to examine the catalytic reaction mechanism, we performed theoretical calculations for neutral model structures [Ru2(HCOO)4X(H2O)] (X=Cl, Br), one-electron reduction model complexes [Ru2(HCOO)4X(H2O)]- and deduced intermediate model structures [H-Ru2(HCOO)4X] using broken-symmetry hybrid density functional theory methods.