Ambivalent binding between a radical-based pincer ligand and iron

Abstract

A complex exhibiting valence delocalization was prepared from 3,5-bis(2-pyridyl)-1,2,4,6-thiatriazinyl (Py₂TTA˙), an inherently redox active pincer-type ligand, coordinated to iron (Fe(Py₂TTA)Cl₂ (1)). Complex 1 can be prepared via two routes, either from the reaction of the neutral radical with FeCl₂ or by treatment of the anionic ligand (Py₂TTA⁻) with FeCl₃, demonstrating its unique redox behaviour. Electrochemical studies, solution absorption and solid-state diffuse reflectance measurements along with X-ray crystallography were carried out to elucidate the molecular and solid-state properties. Temperature- and field-dependent Mössbauer spectroscopy coupled with magnetic measurements revealed that 1 exhibits an isolated S = 5/2 ground spin state for which the low-temperature magnetic behaviour is dominated by exchange interactions between neighbouring molecules. This ground state is rationalized on the basis of DFT calculations that predict the presence of strong electronic interactions between the redox active ligand and metal. This interaction leads to the delocalization of β electron density over the two redox active centres and highlights the difficulty in assigning formal charges to 1.