Role of MCD and Mössbauer Spectroscopy in the Explanation of the Properties of a Highly Soluble (μ-Oxo)bis[tetra(tert-butyl)(phthalocyaninato)iron(III)] Complex, Its Pyridine Adduct, and Redox Forms Oxidized under Anaerobic Conditions in Non-Coordinating Solvents

Solid-state Mössbauer spectra of a highly soluble (μ-oxo)bis[tetra(tert-butyl)(phthalocyaninato)iron(III)] complex 1 ((Pc^tBuFe)₂O) consist of two doublets that represent bent geometry in μ-oxo(1) (1a, ΔE_Q = 0.43 mm/s, T = 10 K) and linear geometry in μ-oxo(2) (1b, ΔE_Q = 1.40 mm/s, T = 10 K) isomers with the ratio between two isomers depending on the purification method. Both isomers were found to be diamagnetic and transform entirely to the 1a isomer in solution. The room- and low-temperature magnetic circular dichroism (MCD) spectra of 1a μ-oxo(1) show one Faraday A- and one B-term between 670 and 720 nm, which correlate with the 690 nm band and 709 nm shoulder observed in the UV-vis spectrum of this compound. UV-vis and MCD spectra of 1a are almost independent of the temperature. Both 1a and 1b are diamagnetic between room temperature and 4 K. Electrochemical experiments show up to three oxidations and up to four reduction processes in 1a. Its oxidation under spectroelectrochemical or chemical (in the absence of oxygen-containing oxidants) conditions in non-coordinating solvents results in the formation of broad NIR bands around 1195 nm (first oxidation) and 1264 nm (second oxidation). The MCD spectra of the redox-active species show a Faraday B-term signal with negative amplitude in this region and are very different from those in the monomeric Pc^tBu(1−)Fe^IIIX₂ complexes 5X (X = Cl^- or CF₃CO₂^-). The pyridine adduct of 1a ((PyPc^tBuFe)₂O; 2Py) is paramagnetic (μ_B = 2.19, g = 2.11, and J = −6.1 cm^-1) and has a major peak at 627 nm of its UV-vis spectrum, which is associated with a MCD pseudo A-term. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, along with the exciton coupling theory, were used to explain the unusually red-shifted intense transitions in 1a as well as the H-aggregate-like spectra of the pyridine adduct 2Py.