Characterization of Complex-B ([Fe(κ3-cys)(CN)(CO)2]−), Biosynthetic Precursor to the [FeFe]-Hydrogenase Active Site, and Related Complexes

The preparation, spectroscopy, and structure are described for [Fe(κ³-cys)(CN)(CO)₂]⁻ ([2]⁻). Otherwise known as complex-B, this coordination complex is an early intermediate in the biosynthesis of the active site of the [FeFe]-hydrogenases. As confirmed by X-ray crystallography, the complex is octahedral with a tridentate cysteinate ligand. The octahedron is completed with two CO ligands and CN^–, which is trans to thiolate. A second isomer is observed in solution. Complex [2]⁻ was prepared by treating [Fe(succinimido)(Br)(CN)(CO)₃]⁻ ([1]⁻) with cysteinate. From [1]⁻ the related penicillamine and homocysteine complexes were also prepared. A practical reagent for probing the biosynthetic role of complex-B, called Syn–B–Br, can be produced in good yield from [1]⁻. Complex [1]⁻ is prepared by oxidative addition of N-bromosuccinimide to [Fe(CN)(CO)₄]⁻. The work is complemented by ¹³CO exchange experiments and DFT calculations.