Conducting Anilate-Based Mixed-Valence Fe(II)Fe(III) Coordination Polymer: Small-Polaron Hopping Model for Oxalate-Type Fe(II)Fe(III) 2D Networks

Abstract

The mixed-valence Fe^IIFe^III 2D coordination polymer formulated as [TAG][Fe^IIFe^III(ClCNAn)₃]·(solvate) 1 (TAG = tris(amino)-guanidinium, ClCNAn^2- = chlorocyanoanilate dianionic ligand) crystallized in the polar trigonal space group P3. In the solid-state structure, determined both at 150 and at 10 K, anionic 2D honeycomb layers [Fe^IIFe^III(ClCNAn)₃]^- establish in the ab plane, with an intralayer metal-metal distance of 7.860 Å, alternating with cationic layers of TAG. The similar Fe-O distances suggest electron delocalization and an average oxidation state of +2.5 for each Fe center. The cation imposes its C₃ symmetry to the structure and engages in intermolecular N-H···Cl hydrogen bonding with the ligand. Magnetic susceptibility characterization indicates magnetic ordering below 4 K and the presence of a hysteresis loop at 2 K with a coercive field of 60 Oe. Mössbauer measurements are in agreement with the existence of Fe(+2.5) ions at RT and statistic charge localization at 10 K. The compound shows semiconducting behavior with the in-plane conductivity of 2 × 10^-3 S/cm, 3 orders of magnitude higher than the perpendicular one. A small-polaron hopping model has been applied to a series of oxalate-type Fe^IIFe^III 2D coordination polymers, providing a clear explanation on the much higher conductivity of the anilate-based systems than the oxalate ones.