title: The synthesis and electronic structure of a novel [NiS4Fe2(CO)6] radical cluster: implications for the active site of the [NiFe] hydrogenases.
abstract: A novel [NiS4Fe2(CO)6]cluster (1: 'S(4)'=(CH(3)C(6)H(3)S(2))(2)(CH(2))(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 (61)Ni-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single (61)Ni (I=3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g(1) 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 NiFe(2) cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, nu(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.
authors.affiliation: School of Chemistry, The University of Nottingham, Nottingham, NG7 2RD, UK.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Wang
authors.forename: J Elaine
authors.initials: JE
authors.lastname: Barclay
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Blake
authors.forename: E Stephen
authors.initials: ES
authors.lastname: Davies
authors.forename: David J
authors.initials: DJ
authors.lastname: Evans
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Marr
authors.forename: Eric J L
authors.initials: EJ
authors.lastname: McInnes
authors.forename: Jonathan
authors.initials: J
authors.lastname: McMaster
authors.forename: Claire
authors.initials: C
authors.lastname: Wilson
authors.forename: Martin
authors.initials: M
authors.lastname: Schröder
mesh_terms: D001665:Binding Sites; D002248:Carbon Monoxide / Q000138:chemical synthesis* / Q000737:chemistry; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004581:Electronics; D006864:Hydrogenase / Q000737:chemistry* / Q000378:metabolism*; D015394:Molecular Structure; D009942:Organometallic Compounds; D013057:Spectrum Analysis
chemical_list: C513818:NiS4Fe2(CO)6; D009942:Organometallic Compounds; D002248:Carbon Monoxide; C043296:nickel-iron hydrogenase; D006864:Hydrogenase