Unusual formation of a [NiSFe2(CO)(6)] cluster: a structural model for the inactive form of [NiFe] hydrogenase

The synthesis and characterisation of the trinuclear Ni-Fe complex [Ni(L-2) SFe2(CO)(6)] (1) formed from the reaction of [Ni(L-1)] with Fe-3(CO)(12) is described. The single-crystal X-ray structure of 1 shows Ni(II) bound to three thioether R-2-S donors and bridged by a sulfide (S2-) group to two Fe(CO)(3) units. 1 undergoes a reversible one-electron reduction process at E-1/2 = -1.62 V vs. Fc(+)/Fc to generate 1(-), which has been characterised by UV-vis and IR spectroelectrochemistry and by EPR spectroscopy. DFT calculations on 1 and 1(-) reveal electronic structures that are delocalised across the NiFe2 core. The SOMO in 1(-) possesses Ni-Fe and Fe-Fe anti-bonding character and lies approximately in the plane defined by the equilateral triangle of Ni and Fe atoms. It possesses d-orbital contributions of 18.5, 15.0 and 19.8% for the Ni(1), Fe(1) and Fe(2) atoms, respectively. The Ni-S(sulfide) bond length in 1 [2.1654(7)angstrom] is identical to that for the bridging sulfide found in the oxidised inactive form of the [NiFe] hydrogenase from D. vulgaris (2.16 angstrom). Thus, 1 provides a useful comparison for biological [NiFe] centres bridged by sulfide donors.