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.