Neutral and reduced Roussin's red salt ester [Fe2(μ-RS)2(NO)4] (R = n-Pr, t-Bu, 6-methyl-2-pyridyl and 4,6-dimethyl-2-pyrimidyl): synthesis, X-ray crystal structures, spectroscopic, electrochemical and density functional theoretical investigationsElectronic supplementary information (ESI) available: Absorption spectra and 1H-NMR spectra for [Fe2(μ-RS)2(NO)4] (R = n-Pr (1), 6-methyl-2-pyridyl (3) and 4,6-dimethyl-2-pyrimidyl (4)), the Cartesian coordinates for all the optimized structures of 1–4, 1−–4−, and Fe(NO)2(CO)2+, and the SOMO of Fe(NO)2(CO)2+. CCDC reference numbers 691873–691875. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b810230a

A series of Roussin''s red salt esters [Fe2(μ-RS)2(NO)4] (R = n-Pr (1), t-Bu (2), 6-methyl-2-pyridyl (3) and 4,6-dimethyl-2-pyrimidyl (4)) were synthesized by the reaction of Fe(NO)2(CO)2with thiols or thiolates. Complexes 1–4were characterized by IR, UV-vis, 1H-NMR, electrochemistry, and single-crystal X-ray diffraction analysis. The IR spectra of complexes 1–4display one weak and two strong NO stretching frequencies (νNO) in solution, but only two strong νNOin the solid. Density functional theoretical (DFT) calculations using complex 1as model suggest that two spatial isomers of these complexes bear a 3 kcal energy difference in solution. Frequency calculations of the two isomers provide insight on the origin of the vibrational bands and explain the IR observation of complexes 1–4in the solid state and in solution. Cyclic voltammetry shows two quasi-reversible, one-electron reductions for complexes 1–2and one quasi-reversible, one-electron reduction for complexes 3–4. The paramagnetic complexes [Fe2(μ-RS)2(NO)4]−(1−–4−), which are prepared by the chemical reduction of neutral complexes [Fe2(μ-RS)2(NO)4] (1–4), have also been investigated by EPR spectroscopy. Interestingly, the EPR spectra of complexes [Fe2(μ-RS)2(NO)4]−(1−–4−) exhibit an isotropic signal of g= 1.998–2.004 without hyperfine splitting in the temperature range 180–298 K. The observations are consistent with the results of the calculations, which reveal that the unpaired electron is dominantly delocalized over the two sulfur and two iron atoms. The difference of the gvalues between the reduced form of Roussin''s red ester and the typical dinitrosyl iron complexes is explained, for the first time, by the difference in unpaired electron distributions between the two types of complexes, which provides the theoretical bases for the use of gvalues as a spectroscopic tool to differentiate these biologically active complexes. [ABSTRACT FROM AUTHOR]