The Synthesis and Electronic Structure of a Novel [Ni‘S4’Fe2(CO)6] Radical Cluster: Implications for the Active Site of the [NiFe] Hydrogenases

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Abstract
A novel [Ni‘S4’Fe2(CO)6] cluster (1: ‘S4’=(CH3C6H3S2)2(CH2)3) has been synthesised, structurally characterised and has been shown to undergo a chemically reversible reduction process at −1.31 V versus Fc+/Fc to generate the EPR‐active monoanion 1−. Multifrequency Q‐, X‐ and S‐band EPR spectra of 61Ni‐enriched 1− show a well‐resolved quartet hyperfine splitting in the low‐field region due to the interaction with a single 61Ni (I=3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non‐coincidence between g1 and A1, and g3 and A3 to reproduce all of the features in the S‐ and X‐band spectra. This behaviour provides a rare example of the detection and measurement of non‐coincidence effects from frozen‐solution EPR spectra without the need for single‐crystal measurements, and in which the S‐band experiment is sensitive to the non‐coincidence. An analysis of the EPR spectra of 1− reveals a 24 % Ni contribution to the SOMO in 1−, supporting a delocalisation of the spin‐density across the NiFe2 cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, ν(CO), shift to lower frequency by about 70 cm−1 when 1 is reduced to 1−. Density functional calculations provide a framework for the interpretation of the spectroscopic properties of 1− and suggest that the SOMO is delocalised over the whole cluster, but with little S‐centre participation. This electronic structure contrasts with that of the Ni‐A, ‐B, ‐C and ‐L forms of [NiFe] hydrogenase in which there is considerable S participation in the SOMO.

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