Thioether ligation in Co(III) complexes with carboxamido nitrogens as donors: implications on the coordination structure of the cobalt site in nitrile hydratase

In order to determine the role of the sulfur donors around the Co(III) site of the enzyme nitrile hydratase, we have synthesized a designed ligand PyPSMeH2 (Hs are dissociable carboxamide hydrogens) with carboxamido nitrogens and thioether sulfur donors. Although Co(III) complexes with bound thioether groups have previously been synthesized with other ligands, attempts to synthesize such a species with PyPSMe2− have failed. The only complex isolated is the bis complex (Et4N)[Co(PyPSMe)2] (1) in which the potentially pentadentate PyPSMe2− ligand acts as tridentate and all four thioether sulfurs remain as pendant groups. In 1, one of the four carboxamido groups is O-bonded. Methylation of the analogous (Et4N)[Co2(PyPS)2] with iodomethane affords (Et4N)[Co2(PyPS(SMe))2] (2), the only Co(III) complex with weakly bound thioether groups. The dimeric complex 2 contains two bridging thiolato sulfurs and each Co(III) center has one bound thioether sulfur in its coordination sphere. These two terminal thioether groups of 2 are replaced easily by other ligands like pyridine. Reaction of CN− with 2 affords the dicyano complex (Et4N)2[Co(PyPS(SMe)(CN)2] (3) with pendant thioether group. Collectively, these results indicate that Co(III) centers with coordinated carboxamido nitrogen(s) exhibit very low affinity toward thioether sulfur.