Electrochemical Generation of Rhodium Porphyrin Hydrides. Catalysis of Hydrogen Evolution

Chemical properties of the Pd4(dppm)4(H)2(2+) cluster and the homogeneous electrocatalytical behavior of hydrogen evolution and formate decomposition

Two new reductive electrochemical (CO2 + H2O + 2e-; HCO2H + 2e-) and two new chemical methods (Al(CH3)3 + proton donor; NaO2CH) to prepare the title compound from Pd2(dppm)2Cl2 are reported. For the latter method, an intermediate species formulated as Pd2(dppm)4(O2CH)2(2+) is identified spectroscopically (1H NMR, 31P NMR, IR, and FAB-MS). Limited stability of the title compound in the presence of Cl- and Br- as counteranions is noticed and is due to sensitivity of the cluster toward nucleophilic attack of the halide ions. This result is corroborated by the rapid decomposition of these clusters in the presence of CN- to form the binuclear species Pd2(dppm)2(CN)4 and by the preparation of the stable salts [Pd4(dppm)4(H)2](X)2(X- = BF4-, PF6-, BPh4-). Upon a two-electron electrochemical reduction of this cluster to the neutral species (E1/2 = -1.42 V vs SCE in DMF) in the presence of 1 equiv of HCO2H, a highly reactive species formulated as [Pd4(dppm)4(H)3]+ is generated and characterized by 1H NMR, 31P NMR, and cyclic voltammetry. Subsequent addition of H+ (via RCO2H; R = H, CH3, CF3, C6H5) under the same reducing conditions, induces the homogeneous catalysis of H2 evolution. The turnover number is found to be 134 in 2 h, with no evidence for catalyst decomposition. This same species also exhibits a one-electron oxidation process (E1/2 = -0.61 V vs SCE in DMF) that induces the catalytical decomposition of formate (HCO2- --> CO2 + 1/2H2 + 1e-). This double catalysis from the same cluster intermediate is unprecedented.