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

The synthesis of
four new FeII(N4S­(thiolate))
complexes as models of the thiol dioxygenases are described. They
are composed of derivatives of the neutral, tridentate ligand triazacyclononane
(R3TACN; R = Me, iPr) and 2-amino­benzene­thiolate
(abtx; 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(Me3TACN)­(abtx)­(OTf) (1: X = H; 2: X = CF3) and 5-coordinate (5-coord) complexes for [FeII(iPr3TACN)­(abtx)]­(OTf) (3: X = H; 4: X = CF3). Complexes 1–4 were examined by UV–vis, 1H/19F NMR, and Mössbauer spectroscopies, and density functional
theory (DFT) calculations were employed to support the data. Mössbauer
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
FeII active site. Reaction of 1 or 2 with O2 at −95 °C leads to S-oxygenation
of the abt ligand, and in the case of 2, a rare di­(sulfinato)-bridged
complex, [Fe2III(μ-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 O2 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 FeII 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.