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

The reaction of cis-[FeIII(cyclam)Cl2]Cl with 1 equiv of sodium N-diethyldithiocarbamate, toluene-3,4-dithiolate, and maleonitriledithiolate in methanol in the presence of triethylamine afforded the cations [FeIII(cyclam)(Et2dtc)]2+ (1), [FeIII(cyclam)(tdt)]+ (2), and [FeIII(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 d5). These salts were characterized by X-ray crystallography, UV−vis, Mössbauer, and electron paramagnetic resonance spectroscopies. Cyclic voltammetry revealed that 2 and 3 are reversibly one-electron-reduced, generating neutral 2red and 3red, respectively, and one-electron-oxidized, generating dicationic 2ox and 3ox, respectively. Fe and S K-edge X-ray absorption spectroscopy (XAS) revealed that 2 (S = 1/2) and 2ox (S = 0) possess a low-spin ferric ion. Complexes 2 and 3 are S,S′-coordinated to a closed-shell dithiolate(2−) ligand, whereas 2ox and 3ox consist of a low-spin ferric ion antiferromagnetically coupled to a dithiolate(1−) π radical ligand. They are singlet diradicals [FeIII(cyclam)(dithiolate•)]2+. The analysis of the sulfur K pre-edge transitions reveals significant multiplet effects in the spectra of 2 and 2ox, which provide rare experimental evidence for a singlet diradical description for 2ox. Mössbauer spectroscopy on frozen solutions of 2red 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 Mössbauer spectra.