Fine-Tuned Magnetic and Electrochemical Properties of a Series of Fe(II) Complexes with Asymmetric Jäger-Type (N₂O₂)²⁻ Ligands

A series of seven new asymmetrically substituted Jäger-type (N2O2)2− ligands and their iron(II) complexes is reported, designed to probe how mixed substitution influences molecular and material properties. Comprehensive characterization—including magnetic measurements, X-ray diffraction, spectroscopy, electrochemistry, and density functional theory (DFT) calculations—shows that many optical and redox properties of the asymmetric ligands and complexes follow additive trends from their symmetric counterparts. Cyclic voltammetry reveals predictable fine-tuning of the Fe3+/2+ redox potential, while DFT calculations confirm the decisive role of carbonyl coordination: ketone donors enable spin-crossover (SCO), whereas ester donors stabilize the high-spin state. However, the SCO behavior of the bulk iron(II) materials often deviates; in some cases, SCO is suppressed, while in others, entirely new transition profiles appear. These findings highlight the decisive role of molecular symmetry and intermolecular packing in governing cooperativity in SCO materials.