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

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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 Fe III (TIM) (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene). The reaction of trans -[Fe(TIM)Cl 2 ]PF 6 with either lithiated aryl or arylmagnesium Grignard reagents affords the trans -[Fe(TIM)(Ar) 2 ]PF 6 complexes [Ar = Ph ( 1 ), p -C 6 H 4 CF 3 ( 2 ), p -C 6 H 4 F ( 3 ), p -C 6 H 4 OCH 3 ( 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) Fe III center and a ( d xy ) 2 ( d xz, yz ) 3 ground-state configuration that is supported by DFT studies.