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

Abstract

The synthesis and characterisation of the trinuclear Ni–Fe complex [Ni(L²)SFe₂(CO)₆] (1) formed from the reaction of [Ni(L¹)] with Fe₃(CO)₁₂ is described. The single-crystal X-ray structure of 1 shows Ni(II) bound to three thioether R₂-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) Å] is identical to that for the bridging sulfide found in the oxidised inactive form of the [NiFe] hydrogenase from D. vulgaris (2.16 Å). Thus, 1 provides a useful comparison for biological [NiFe] centres bridged by sulfide donors.