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Issue 11, 2015
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Electrocatalytic proton reduction catalysed by the low-valent tetrairon-oxo cluster [Fe4(CO)102-dppn)(μ4-O)]2− [dppn = 1,1′-bis(diphenylphosphino)naphthalene]

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Abstract

The 62-electron oxo-capped tetrairon butterfly cluster, Fe4(CO)102-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)102-dppn)(μ4-O)]+ (1+) and [Fe4(CO)102-dppn)(μ4-O)] (1) respectively. Addition of a second electron affords the 64-electron cluster [Fe4(CO)102-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)102-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)102-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.

Graphical abstract: 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]

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Publication details

The article was received on 28 Oct 2014, accepted on 04 Feb 2015 and first published on 05 Feb 2015


Article type: Paper
DOI: 10.1039/C4DT03323J
Author version available: Download Author version (PDF)
Citation: Dalton Trans., 2015,44, 5160-5169
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    Electrocatalytic proton reduction catalysed by the low-valent tetrairon-oxo cluster [Fe4(CO)102-dppn)(μ4-O)]2− [dppn = 1,1′-bis(diphenylphosphino)naphthalene]

    S. Ghosh, K. B. Holt, S. E. Kabir, M. G. Richmond and G. Hogarth, Dalton Trans., 2015, 44, 5160
    DOI: 10.1039/C4DT03323J

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