Electronic Structure, Spectroscopy, and Redox Properties of Highly Soluble (μ-oxo)Bis[Tetra(tert-butyl)(phthalocyaninato)Iron(III)] Complex and Its Pyridine Adduct Under Anaerobic Conditions in Non-Coordinating Solvents

Abstract

The solid-state Mössbauer spectra of a highly soluble (μ-oxo)bis[tetra(tert-butyl)(phthalocyaninato)iron(III)] complex 1 ((PctBuFe)2O) consist of two doublets that represent bent geometry μ-oxo(1) (1a, ΔEQ = 0.43 mm/s, T = 10K) and linear geometry μ-oxo(2) (1b) isomers (ΔEQ = 1.40 mm/s, T = 10K) with the ratio between two isomers depending on the purification method. Both isomers were found to be diamagnetic and transform entirely in 1a isomer in solution. The room- and low-temperature MCD spectra of 1a show one Faraday A- and one B-terms 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 independent of the temperature. Both 1a and 1b are diamagnetic between room temperature and 4 K. Electrochemical experiments show up to three oxidation and up to four reduction processes in 1a. Its oxidation under spectroelectrochemical or chemical (in the absence of oxygen-containing oxidants) oxidation conditions in non-coordinating solvents results in the formation of a broad NIR band 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 monomeric PctBu(1-)FeIIIX2 complexes 5X (X = Cl- or CF3CO2 -). Pyridine adduct of 1a ((PyPctBuFe)2O; 2Py) is paramagnetic (μB = 2.19, g = 2.11, and J = -6.1cm-1) and has a major peak at 627 nm in UV-Vis spectrum, which is associated with MCD pseudo A-term. Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations, along with Kasha’s and Spano’s exciton coupling theories, were used to explain unusually red-shifted intense transitions in 1a as well as H-aggregate like spectra of the pyridine adduct 2Py.