Structure and UV-Vis spectroscopy of the iron-sulfur dinuclear nitrosyl complexes [Fe2S2(NO)4]2− and [Fe2(SR)2(NO)4]

Abstract

Calculations of the electronic structure, geometry and electronic spectra of Roussin’s red salt dianion (RRS) and Roussin’s red diester (RRE) were carried out with the RB3LYP and UB3LYP methods. The electronic structure emerging from these calculations may be described as composed of two {Fe(NO)₂}⁹ units, in which ferric ion (S = 5/2) is antiferromagnetically coupled to two NO⁻ ligands (each with S = 1), giving S = 1/2; the units are antiferromagnetically coupled to each other yielding a total S = 0. The S²⁻ bridges (in RRS) or SR⁻ bridges (RRE) mediate the antiferromagnetic coupling. The character of the frontier orbitals controls the dinuclear species’ reactivity, which is initiated by electrophilic attack on S-localized HOMO orbitals (RRS) or nucleophilic attack on the Fe–S antibonding LUMO orbital (RRE). The contrasting susceptibility to electrophilic/nucleophilic attack is also assisted by the sulfur charge, which is negative in RRS and positive in RRE. The calculated spectra of RRS and RRE show substantial resemblance to the experimental spectra. The calculated transitions are mainly of charge transfer character: At long wavelengths they are described as π*_NO → d (LMCT), at short wavelengths (below 250 nm) the most intense transitions are d → π*_NO (MLCT). In the middle part of the spectra both types of transitions are present. Some contribution of sulfur to the transitions throughout the whole spectrum is observed. The π*_NO → d transitions are assumed to be responsible for the photochemical reactivity of both compounds, which is initiated by photodissociation of the NO group.