Modulation in the reactivity of imine group by aryl and alkyl thiolato sulfur on ligands coordinated to Co(III) center: relevance to the Co-containing enzyme nitrile hydratase

In order to determine the effect of ligand backbone on imine-to-carboxamide (ITC) conversion on Co(III) metal centers, we have synthesized and characterized the Co(III) complex of the designed ligand PyMSH. The ligand contains an alkyl thiol group and is similar to the previously reported ligand PyASH that contains an aryl thiol group. The Co(III) complex of PyAS− namely, [Co(PyAS)2]Cl (1), readily undergoes ITC conversion with OH− in air. In contrast, [Co(PyMS)2]BF4 (3) does not afford the corresponding carboxamido complex ((Et4N)[Co(PyPepRS)2] (4)) under similar reaction conditions. The difference in reactivity is attributed to (a) loss of conjugation around the imine moiety and (b) greater stabilization of the Co(III) metal center by the alkyl thiolato sulfur donors. Electrochemical data indicate that replacement of aryl thiolates with alkyl thiolates (1 and 3) shifts the E1/2 value of Co(III)–Co(II) couple cathodically by 0.16 V. Replacement of imine nitrogen with carboxamido nitrogen, however, imparts much greater stability to the Co(III) metal center; the E1/2 value of 3 in DMF is −0.82 V while complex 4 shows no reduction upto −1.8 V (vs. SCE) in DMF. These results suggest that good models of the non-redox enzyme nitrile hydratase must comprise carboxamido nitrogens in the coordination sphere of the M(III) (M=Fe, Co) center.