CoIII Complexes with Square‐Planar N2S2‐ and N2(SO2)2‐Type Ligands as An Active Site Structural Model for Nitrile Hydratase – Biological Implications of an Amidate Coordination

Abstract

AbstractIn an attempt to understand the unique active site structure of nitrile hydratase, four CoIII complexes with square‐planar N2S2‐ or N2(SO2)2‐type donor sets, Na[CoIII(L:N2S2)] (1‐Na), PPh4[CoIII(L:N2S2)] (1‐PPh4), PPh4[CoIII(L:N2S2)(tBuNC)2] (2), and PPh4[CoIII{L:N2(SO2)2}(tBuNC)2] (3) were synthesized and characterized on the basis of electronic absorption spectroscopy, IR spectroscopy, cyclic voltammetry, and X‐ray structural analysis. Both of the crystal structures of complexes 1‐Na and 1‐PPh4 revealed a square planar structure with N2S2 donating atoms, and 2 exhibited an octahedral structure coordinated with two tert‐butylisocyanide (tBuNC) molecules at the axial sites of complex 1‐PPh4. Complex 3, which showed an octahedral structure with sulfinate sulfur atoms equatorially coordinated to the center, was synthesized by the treatment of 2 with a suitable oxidant. The reduction potential values from CoIII to CoII for complex 3 in solution demonstrated a larger positive shift when compared with those of complexes 1‐PPh4 and 2, which indicates that the oxygenation of the sulfur atoms increased the Lewis acidity of the CoIII center. Interestingly, the coordination equilibrium, the C=O stretching frequency, and the redox potential for 1‐PPh4 were all closely related to the acceptor number (AN) of the solvents. Furthermore, the coordination of monodentate tBuNC to the axial position of 1‐PPh4 was dependent on the solvents used. These findings indicate that an electrophilic interaction between the carbonyl oxygen atoms and the solvent molecules control the Lewis acidity of the metal ion. On the other hand, such a solvent dependence was not detected in the S=O stretching frequency of sulfinates 3. We have concluded that the increase in the redox potential/Lewis acidity of the metal center is a result of the oxygenation of sulfur, and that this increase is controlled by the interaction of the amidate carbonyl oxygen with the secondary coordination sphere. As demonstrated in previous mutation studies, this study suggests that the interaction of the nitrile hydratase active site with the functional groups from the peptide backbone is essential for the catalytic activity of the complex. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)