Electrocatalytic proton reduction catalysed by the low-valent tetrairon-oxo cluster [Fe4(CO)10(κ(2)-dppn)(μ4-O)](2-) [dppn = 1,1'-bis(diphenylphosphino)naphthalene

Abstract

The 62-electron oxo-capped tetrairon butterfly cluster, Fe4(CO)10(κ(2)-dppn)(μ4-O) (1) {dppn = 1,8-bis(diphenylphosphino)naphthalene}, undergoes reversible one-electron oxidation and reduction events to generate the 61- and 63-electron radicals [Fe4(CO)10(κ(2)-dppn)(μ4-O)](+) (1+) and [Fe4(CO)10(κ(2)-dppn)(μ4-O)](-) (1-) respectively. Addition of a second electron affords the 64-electron cluster [Fe4(CO)10(κ(2)-dppn)(μ4-O)](2-) (1(2-)) which has more limited stability but is stable within the time frame of the electrochemical experiment. While 1 and 1(-1) are inactive as proton reduction catalysts, dianionic 1(2-) is active for the formation of hydrogen from both CHCl2CO2H and CF3CO2H. This occurs via two separate mechanistic cycles branching at the mono-protonated species [Fe4(CO)10(κ(2)-dppn)(μ4-O)H](-) (1H-) resulting from the rapid protonation of 1(2-). This intermediate then undergoes competing protonation and reduction events leading to EECC and ECEC catalytic cycles respectively with 1- being pivotal to both. In order to understand the nature of [Fe4(CO)10(κ(2)-dppn)(μ4-O)](2-) (1(2-)) and its protonated products density functional theory (DFT) calculations have been employed. Theoretical calculations reveal that the cluster core remains intact in 1(2-), but the two consecutive one-electron reductions lead to an expansion of one of the trigonal-pyramids of this trigonal-bipyramidal cluster. The two-electron reduced cluster 1(2-) protonates at dppn-bound iron, accompanied by a wingtip-hinge iron-iron bond scission, and then reacts with a second proton to evolve hydrogen.