The synthesis and electronic structure of a novel [NiS4Fe2(CO)(6)] radical cluster: Implications for the active site of the [NiFe] hydrogenases

A novel [Ni'S-4'Fe-2(CO)(6)] cluster (1: 'S-4'=(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 Ni-61-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single Ni-61 (I = 3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g, and A(1), and g(3) and A(3) 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, v(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.