Fe(III)(TIM) Bis-Aryl Complexes: Molecular and Electronic Structures and Metal–Ligand/Ligand–Metal Charge-Transfer Spectra

Literature precedents of Fe-aryl species supported by tetra-aza macrocycles such as porphyrins and corroles are dominated by monoaryls. Reported herein are a series of unique bis-aryl complexes based on FeIII(TIM) (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene). The reaction of trans-[Fe(TIM)Cl2]PF6 with either lithiated aryl or arylmagnesium Grignard reagents affords the trans-[Fe(TIM)(Ar)2]PF6 complexes [Ar = Ph (1), p-C6H4CF3 (2), p-C6H4F (3), p-C6H4OCH3 (4), and 2-naphthyl (5)]. The molecular structures of 1, 3, 4, and 5 were elucidated using single-crystal X-ray diffraction. The voltammetry of the series features an irreversible Fe oxidation and a reversible Fe reduction. The electrode potentials of both couples in complexes 1-4 trend with the electron-donating/withdrawing ability of the phenyl substituents. The vis-absorption spectra of all [Fe(TIM)(Ar)2]+ species feature an intense metal-to-ligand charge transfer (MLCT) peak around 580 nm and less intense LMCT peaks between 370 and 470 nm, with the assignment verified by TD-DFT analysis. Upon one electron reduction, the MLCT peak in [Fe(TIM)(Ar)2]0 is shifted to around 740 nm and further intensified. EPR spectra of complexes 1-5 provides evidence for a low-spin (S = 1/2) FeIII center and a (dxy)2(dxz,yz)3 ground-state configuration that is supported by DFT studies.