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 (Delta E-Q/S) in Fe(II) polypyridine complexes is often interpreted in terms of metal ligand pi 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 Delta E-Q/S and reduction potentials (E degrees). In this series, E spans a much larger range than Delta E-Q/S (2.07 vs 0.29 eV). 'While small changes in Delta E-Q/S are controlled by metal ligand Jr 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 Jr-acceptor, bpy is better described as acting as both a pi-donor and a-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.