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 <i>monoaryls</i>. Reported herein are a series of unique <i>bis-aryl</i> 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 <i>trans</i>-[Fe(TIM)Cl₂]PF₆ with either lithiated aryl or arylmagnesium Grignard reagents affords the <i>trans</i>-[Fe(TIM)(Ar)₂]PF₆ complexes [Ar = Ph (<b>1</b>), <i>p</i>-C₆H₄CF₃ (<b>2</b>), <i>p</i>-C₆H₄F (<b>3</b>), <i>p</i>-C₆H₄OCH₃ (<b>4</b>), and 2-naphthyl (<b>5</b>)]. The molecular structures of <b>1</b>, <b>3</b>, <b>4,</b> and <b>5</b> 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 <b>1-</b><b>4</b> 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 <b>1</b>-<b>5</b> provides evidence for a low-spin (<i>S</i> = 1/2) Fe^III center and a (<i>d</i>_<i>xy</i>)²(<i>d</i>_<i>xz</i>,_<i>yz</i>)³ ground-state configuration that is supported by DFT studies.