Anion-Induced Structural Diversity and Optical Chromism in a Series of Cyano-Bridged Heterometallic 3d-4f Coordination Polymers

The self-assembly reaction of the neutral dicyano-bis(1,10-phenanthroline) iron(II) complex with lanthanide ions (Ln = Eu(III), Gd(III), Er(III)) provided two different classes of heterometallic cyano-bridged 3d-4f architectures depending on the nature of the counteranion, irrespective of the size of the 4f metal ion. Tetranuclear oligomers with a squared Fe2Ln2 core were isolated when using nitrate salts, whereas unusual 1D polymeric chains were obtained when resorting to triflate salts under the same synthetic conditions. It is shown that the different structural motifs have a remarkable impact on the thermal stability and the optical properties of the compounds, which display a notable optical ipsochromism of the parent Fe(II) complex upon coordination with the Ln ion. This effect is significantly more pronounced in the polymeric chain than in the Fe2Ln2 oligomer both in solution and in the solid state. Structural evidence suggests that this behavior is likely related to the geometry of the CN-Ln bridge. On the other hand, more extended π-stacking interactions in the oligomer give rise to a broad charge-transfer absorption (600-1500 nm), making this compound promising as NIR absorber. Density Functional Theory calculations and electrochemical studies demonstrate that the observed negative chromism originates from the stabilization of a mixed metal/cyanide character HOMO with respect to a phenanthroline-centered LUMO.