Fe(III) Bis-Alkynyls Supported by the TIM Macrocycle: Molecular and Electronic Structures and Altering the Nature of Charge Transfer Transitions through Reduction

The bis-alkynyl complexes based on FeIII(TIM) (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) are reported herein. A modified synthesis of trans-[Fe­(TIM)­Cl2]­PF6 ([1]­PF6) in a 90% yield was developed. The reaction between [1]­PF6 and LiC2R (R = Ph, SiEt3, or SiiPr3) resulted in the bis-alkynyl complexes, trans-[Fe­(TIM)­(C2R)2]­PF6, in good yields, 43% for 2 (R = Ph), 29% for 3 (R = SiEt3), and 18% for 4 (R = SiiPr3). X-ray diffraction studies yielded molecular structures of complexes 1–3, which feature a tightened Fe–N4 coordination in comparison with analogous Fe­(cyclam) complexes. Electron paramagnetic resonance studies confirmed that the complex ions [Fe­(TIM)­Cl2]+ and [Fe­(TIM)­(C2R)2]+ are low-spin d5 species with S = 1/2 ground states. Electronic absorption spectra of bis-alkynyl complexes 2–4 all feature intense LMCT (π­(C≡C) →dπ­(Fe)) bands in the visible region, and the assignment was supported by the time-dependent density functional theory analysis. These complexes undergo multiple one-electron redox processes, and the first one-electron reduction is reversible in all four complexes and assigned as an Fe­(+3/+2) couple. Spectroelectrochemical studies of 1 and 2 reveal that the first one-electron reduction leads to the emergence of intense metal-to-ligand charge transfer (dπ­(Fe) → π*­(α-diimine)) bands and the concurrent disappearance of the ligand-to-metal charge transfer bands.