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

Photoinduced Oxygen Evolution Catalysis Promoted by Polyoxometalate Salts of Cationic Photosensitizers

The insoluble salt Cs15K[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (CsCo9) is tested as heterogeneous oxygen evolution catalyst in light-induced experiments, when combined with the homogeneous photosensitizer [Ru(bpy)3]2+ and the oxidant Na2S2O8 in neutral pH. Oxygen evolution occurs in parallel to a solid transformation. Post-catalytic essays indicate that the CsCo9 salt is transformed into the corresponding [Ru(bpy)3]2+ salt, upon cesium loss. Remarkably, analogous photoactivated oxygen evolution experiments starting with the [Ru(bpy)3](5+x)K(6−2x)[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]·(39+x)H2O (RuCo9) salt demonstrate much higher efficiency and kinetics. The origin of this improved performance is at the cation-anion, photosensitizer-catalyst pairing in the solid state. This is beneficial for the electron transfer event, and for the long-term stability of the photosensitizer. The latter was confirmed as the limiting process during these oxygen evolution reactions, with the polyoxometalate catalyst exhibiting robust performance in multiple cycles, upon addition of photosensitizer, and/or oxidant to the reaction mixture