Influence of the redox active ligand on the reactivity and electronic structure of a series of Fe(TIM) complexes

The redox properties of Fe and Zn complexes coordinated by an α-diimine based N₄-macrocyclic ligand (TIM) have been examined using spectroscopic methods and density functional theory (DFT) computational analysis. DFT results on the redox series of [Zn(TIM*)]ⁿ and [Fe(TIM*)]ⁿ molecules indicate the preferential reduction of the α-diimine ligand moiety. In addition to the previously reported [Fe(TIM*)]₂ dimer, we have now synthesized and characterized a further series of monomeric and dimeric complexes coordinated by the TIM ligand. This includes the five-coordinate monomeric [Fe(TIM*)I], the neutral and cationic forms of a monomeric phosphite adduct, [Fe(TIM*)(P(OPh) ₃)] and [Fe(TIM*)(P(OPh)₃)](PF₆), as well as a binuclear hydroxy-bridged complex, [{Fe(TIM*)}₂(μ-OH)] (PF₆). Experimental and computational data for these synthetic compounds denote the presence of ferrous and ferric species, suggesting that the α-diimine based macrocycles do not readily support the formation of formally low-valent (M⁰ or M^I) metal complexes as previously speculated. Magnetochemical, Mössbauer, electron paramagnetic resonance (EPR), and electronic spectral data have been employed to experimentally determine the oxidation state of the central metal ion and of the macrocyclic ligand (TIM*) in each compound. The series of compounds is described as follows: [Fe^II(TIM⁰)(CH₃CN ₂)]²⁺, S_Fe = S_T = 0; [Fe ^2.5(TIM^2.5-)]₂, S_T = 1; [{Fe ^III(TIM^2-)}₂(μ-OH)]⁺, S _Fe = 3/2, S_T = 0; [Fe^III(TIM^2-)I], S_Fe = 3/2, S_T = 1/2; [Fe^II(TIM ^2-)(P(OPh₃))], S_Fe = S_T = 0; and [Fe^II(TIM^1-)(P(OPh₃))]¹⁺/[Fe ^I(TIM⁰)(P(OPh₃))]¹⁺, S_T = 1/2. The results have been corroborated by DFT calculations.