FeIII Bipyrrolidine Phenoxide Complexes and Their Oxidized Analogues

FeIII complexes of the symmetric (2S,2′S)-[N,N′-bis­(1-(2-hydroxy-3,5-di-tert-butylphenylmethyl))]-2,2′-bipyrrolidine (H2L1) and dissymmetric (2S,2′S)-[N,N′-(1-(2-hydroxy-3,5-di-tert-butylphenylmethyl))-2-(pyridylmethyl)]-2,2′-bipyrrolidine (HL2) ligands incorporating the bipyrrolidine backbone were prepared, and the electronic structure of the neutral and one-electron oxidized species was investigated. Cyclic voltammograms (CV) of FeL1Cl and FeL2Cl2 showed expected redox waves corresponding to the oxidation of phenoxide moieties to phenoxyl radicals, which was achieved by treating the complexes with 1 equiv of a suitable chemical oxidant. The clean conversion of the neutral complexes to their oxidized forms was monitored by UV–vis-NIR spectroscopy, where an intense π–π* transition characteristic of a phenoxyl radical emerged ([FeL1Cl]+•: 25 500 cm–1 (9000 M–1 cm–1); [FeL2Cl2]+•: 24 100 cm–1 (8300 M–1 cm–1). The resonance Raman (rR) spectra of [FeL1Cl]+• and [FeL2Cl2]+• displayed the characteristic phenoxyl radical ν7a band at 1501 and 1504 cm–1, respectively, confirming ligand-based oxidation. Electron paramagnetic resonance (EPR) spectroscopy exhibited a typical high spin FeIII (S = 5/2) signal for the neutral complexes in perpendicular mode. Upon oxidation, a signal at g ≈ 9 was observed in parallel mode, suggesting the formation of a spin integer system arising from magnetic interactions between the high spin FeIII center and the phenoxyl radical. Density functional theory (DFT) calculations further supports this formulation, where weak antiferromagnetic coupling was predicted for both [FeL1Cl]+• and [FeL2Cl2]+•.