Electrocatalytic proton reduction catalysed by the low-valent tetrairon-oxo cluster [Fe4(CO)10(κ2-dppn)(μ4-O)]2− [dppn = 1,1′-bis(diphenylphosphino)naphthalene]

Abstract

The 62-electron oxo-capped tetrairon butterfly cluster, Fe₄(CO)₁₀(κ²-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(κ²-dppn)(μ₄-O)]⁺ (1⁺) and [Fe₄(CO)₁₀(κ²-dppn)(μ₄-O)]⁻ (1⁻) respectively. Addition of a second electron affords the 64-electron cluster [Fe₄(CO)₁₀(κ²-dppn)(μ₄-O)]²⁻ (1²⁻) which has more limited stability but is stable within the time frame of the electrochemical experiment. While 1 and 1⁻ are inactive as proton reduction catalysts, dianionic 1²⁻ 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(κ²-dppn)(μ₄-O)H]⁻ (1H⁻) resulting from the rapid protonation of 1²⁻. 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(κ²-dppn)(μ₄-O)]²⁻ (1²⁻) and its protonated products density functional theory (DFT) calculations have been employed. Theoretical calculations reveal that the cluster core remains intact in 1²⁻, 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 1²⁻ protonates at dppn-bound iron, accompanied by a wingtip-hinge iron–iron bond scission, and then reacts with a second proton to evolve hydrogen.