Redox flexibility of iron complexes supported by sulfur-based tris(o-methylenethiophenolato)amine relative to its tripodal oxygen-based congener

Abstract

Tripodal ligands designed to generate a local C₃ symmetry have resulted in novel types of metal complexes that feature unusual bonding and electronic properties. However, most complexes reported to date are characterised by strong field ligands that enforce low or intermediate-spin states for the metal centres. Moreover, anionic sulfur-based tripodal ligands are particularly scarce due to their challenging synthesis. In this context, we herein report the synthesis, spectral characterization, structural, and electronic properties of an iron complex supported by the tripodal, trianionic ligand [N(CH₂ArS)₃]³⁻ as the trigonal-bipyramidal complexes [Fe{N(CH₂ArS)₃}(X)] (1^X, X = DMSO, THF). The solid-state structures reveal local C_3v symmetry around the Fe³⁺ ions, while electron spin resonance measurements established a high-spin state (S = 5/2). Electrochemical studies demonstrate the redox flexibility of the FeS₃ fragment by direct comparison with the oxygen-based analogue N(CH₂ArOH)₃, which displays an irreversible reduction; in contrast, 1^THF has a reversible Fe³⁺/Fe²⁺ redox process at −0.83 V (relative to the ferrocenium/ferrocene redox couple). The high spin and redox properties of 1^THF are attributable to the weak ligand field provided by the NS₃ fragment, as confirmed by the electronic structure calculated by density functional theory, which reveals substantial electronic delocalisation and covalency of the Fe–S bonds in 1^X.