Octahedral Monodithiolene Complexes of Iron: Characterization of S,S′-Coordinated Dithiolate(1-) π Radical Monoanions: A Spectroscopic and Density Functional Theoretical Investigation

The reaction of cis-[Fe-III(cyclam)Cl-2]Cl with 1 equiv of sodium N-diethyldithiocarbamate, toluene-3,4-dithiolate, and maleonitriledithiolate in methanol in the presence of triethylamine afforded the cations [Fe-III(cyclam)(Et(2)dtc)](2+) (1), [Fe-III(cyclam)(tdt)](+) (2), and [Fe-III(cyclam)(mnt)](+) (3), which were isolated as triflate, hexafluorophosphate, and tetrafluoroborate salt, respectively, using sodium triflate, potassium hexafluorophosphate, or sodium tetrafluoroborate as the source for the counteranion. Complexes 1, 2, and 3 possess an S = 1/2 ground state (low-spin ferric d(5)). These salts were characterized by X-ray crystallography, UV-vis, Mossbauer, and electron paramagnetic resonance spectroscopies. Cyclic voltammetry revealed that 2 and 3 are reversibly one-electron-reduced, generating neutral 2(red) and 3(red) respectively, and one-electron-oxidized, generating dicationic 2(ox) and 3(ox), respectively. Fe and S K-edge X-ray absorption spectroscopy (XAS) revealed that 2 (S = 1/2) and 2(ox) (S = 0) possess a low-spin ferric ion. Complexes 2 and 3 are S,S'-coordinated to a closed-shell dithiolate(2-) ligand, whereas 2(ox) and 3(ox) consist of a low-spin ferric ion antiferromagnetically coupled to a dithiolate(1-) pi radical ligand. They are singlet diradicals [Fe-III(cyclam)(dithiolate(center dot))](2+). The analysis of the sulfur K pre-edge transitions reveals significant multiplet effects in the spectra of 2 and 2(ox), which provide rare experimental evidence for a singlet diradical description for 2(ox). Mossbauer spectroscopy on frozen solutions of 2(red) clearly show the presence of a high-spin ferrous ion (S = 2). The experimentally established electronic structures of the three members of the electron transfer series [Fe(cyclam) (dithiolate)](2+,+,0) have been verified by broken symmetry density functional theoretical calculations, which have been calibrated against the experiment by calculating XAS and Mossbauer spectra.