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

Abstract

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−)­FeIIIX2 complexes 5X (X = Cl– or CF3CO2 –). The pyridine adduct of 1a ((PyPc tBuFe)2O; 2Py) is paramagnetic (μB = 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.

Spectroscopic techniques to probe magnetic anisotropy and spin–phonon coupling in metal complexes

“…It should be pointed out that magnetic anisotropy of metal complexes has also been determined by cantilever torque magnetometry (CTM) 74,[80][81][82][83][84][85][86] which was recently reviewed by Perfetti, 87 angle-resolved magnetometry, [88][89][90][91] and μ-SQUID magnetometry used by, e.g., Wernsdorfer and coworkers. [92][93][94][95] In addition, electronic structures in metal complexes have been determined by spectroscopic techniques such as magnetic circular dichroism (MCD), 30,48,[96][97][98][99] luminescence, [100][101][102][103][104] and FIRMS. For us, finding spectroscopic methods to directly determine energies of magnetic excited states that are >33 cm −1 , i.e., large magnetic anisotropy, has been one goal of our recent research.…”

“…, Wernsdorfer and coworkers. 92–95 In addition, electronic structures in metal complexes have been determined by spectroscopic techniques such as magnetic circular dichroism (MCD), 30,48,96–99 luminescence, 100–104 and FIRMS. For us, finding spectroscopic methods to directly determine energies of magnetic excited states that are >33 cm −1 , i.e.…”