Tuning the Redox Potentials and Ligand Field Strength of Fe(II) Polypyridines: The Dual π‑Donor and π‑Acceptor Character of Bipyridine

The quintet–singlet energy difference (ΔEQ/S) in Fe­(II) polypyridine complexes is often interpreted in terms of metal–ligand π interactions. DFT calculations on a series of substituted [Fe­(bpy)3]2+ (bpy = 2,2′-bipyridine) complexes show the disparate magnitudes of substituent effects on tuning ΔEQ/S and reduction potentials (E°). In this series, E° spans a much larger range than ΔEQ/S (2.07 vs 0.29 eV). While small changes in ΔEQ/S are controlled by metal–ligand π interactions, large changes in E° arise from modification of the electrostatic environment around the Fe center. Molecular orbital analysis reveals that, contrary to the typical description of bpy as a π-acceptor, bpy is better described as acting as both a π-donor and π-acceptor in [Fe­(bpy)3]2+ complexes, even when it is substituted with highly electron withdrawing substituents. Overall, substituent modification is a useful strategy for fine-tuning the ligand field strength but not for significant reordering of the spin-state manifold, despite the large effect on metal–ligand electrostatic interactions.