Co(III) complexes with coordinated carboxamido nitrogens and thiolato sulfurs as models for co-containing nitrile hydratase and their conversion to the corresponding sulfinato species

Recent spectroscopic data suggest that the Co(III) site in Co-containing ni trile hydratase is ligated to carboxamido nitrogens and thiolato sulfurs an d most possibly one or more of the bound thiolates exist as sulfenato and/o r sulfinato groups. The absence of any Co(III) complex with such coordinati on makes it quite difficult to predict the reactivity of this kind of Co(II I) site. In this paper, the Co(III) complexes of two designed ligands PyPep SH(2) (1) and PyPepRSH(2) (2) have been reported. The two complexes, namely , (Et4N)[Co(PyPepS)(2)] (3) and Na[Co(PyPepRS)2] (4) are the first examples of Co(III) complexes with carboxamido nitrogens and thiolato sulfurs as do nors. The average Co(III)-N-amido and Co(III)-S distances in these complexe s lie in the range 1.90-1.92 and 2.22-2.24 Angstrom, respectively. Reaction of H2O2 With both complexes readily affords Na[Co(PyPepSO(2))(2)] (5) and Na[Co(PyPepRSO(2))(2)] (6), species in which the thiolato sulfurs are conve rted to sulfinato (SO2) groups. Such conversion also occurs when solutions of 3 and 4 are exposed to dioxygen in the presence of activated charcoal, T hese reactions are clean and the S -> SO2 transformation does not introduce significant changes in the metric parameters of these complexes. The react ivity of 3 and 4 indicates that the bound Cys-sulfurs around the biological Co(III) site could be oxidized to sulfinato groups.