Structural Differences and Redox Properties of Unsymmetric Diiron PDIxCy Complexes

We present two bimetallic iron complexes, [Fe₂(PDIeCy)(OTf)₄] (1) and [Fe₂(PDIpCy)(THF)(OTf)₄] (2) coordinated by an unsymmetric ligand. The new ligand, PDIeCy (PDI = pyridyldiimine; e = ethyl; Cy = cyclam), is a variant of the previously reported PDIpCy (p = propyl) ligand, featuring a shorter linker between the two metal coordination sites. The structural and electronic properties of 1 and 2, both in the solid and solution state, were analyzed by means of X-ray crystallography, and spectroscopic methods, including ¹⁹F-NMR. The two ligand platforms yield markedly different diiron structures: the PDIeCy ligand permits formation of a bridged, µ-OTf complex, while the two iron centers of the PDIpCy-based 2 remain unconnected, directly, under all conditions examined. Both compounds contain electronically non-coupled high-spin (S = 2) ferrous centers, as established by Mössbauer spectroscopy and magnetic susceptibility studies. Cyclic voltammetry demonstrates the rich redox chemistry of the compounds, involving both ligand and metal-centered redox processes. Moreover, we synthesized the two-electron reduced [Fe₂(PDIeCy)]²⁺ form of 1, which contains the dianionic PDI^2– ligand, and represents a two-electron charge localized complex.