Structures, Spectroscopic Properties, and Dioxygen Reactivity of 5-and 6-Coordinate Nonheme Iron(11) Complexes: A Combined Enzyme/Model Study of Thiol Dioxygenases

The synthesis of four new Fe-11(N4S(thiolate)) complexes as models of the thiol dioxygenases are described. They are composed of derivatives of the neutral, tridentate ligand triazacyclononane (R(3)TACN; R = Me, iPr) and 2-aminobenzenethiolate (abt(x); X = H, CF3), a non-native substrate for thiol dioxygenases. The coordination number of these complexes depends on the identity of the TACN derivative, giving 6-coordinate (6-coord) complexes for FeII(Me(3)TACN)(abt(x))(OTf) (1: X = H; 2: X = CF3) and 5-coordinate (5-coord) complexes for [FeII(iPr(3)TACN)-(abt(x))(OTf) (3: X = H; 4: X = CF3). Complexes 1-4 were examined by UV-vis, H-1/F-19 NMR, and Mossbauer spectroscopies, and density functional theory (DFT) calculations were employed to support the data. Mossbauer spectroscopy reveals that the 6-coord 1-2 and 5-coord 3 4 exhibit distinct spectra, and these data are compared with that for cysteine-bound CDO, helping to clarify the coordination environment of the cys-bound Fe-II active site. Reaction of 1 or 2 with O-2 at -95 degrees C leads to S-oxygenation of the abt ligand, and in the case of 2, a rare di(sulfinato)-bridged complex, [Fe-2(III)(mu-O)((2-NH2)p-CF3C6H3SO2)(2)](OTf)(2) (5), was obtained. Parallel enzymatic studies on the CDO variant C93G were carried out with the abt substrate and show that reaction with O-2 leads to disulfide formation, as opposed to S-oxygenation. The combined model and enzyme studies show that the thiol dioxygenases can operate via a 6-coord Fe-II center, in contrast to the accepted mechanism for nonheme iron dioxygenases, and that proper substrate chelation to Fe appears to be critical for S-oxygenation.