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

The
mixed-valence FeIIFeIII 2D coordination
polymer formulated as [TAG]­[FeIIFeIII(ClCNAn)3]·(solvate) 1 (TAG = tris­(amino)-guanidinium,
ClCNAn2– = 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 [FeIIFeIII(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 FeIIFeIII 2D coordination polymers, providing a clear
explanation on the much higher conductivity of the anilate-based systems
than the oxalate ones.