Resolving a Half-Century-Long Controversy between (Magneto)optical and EPR Spectra of Single-Electron-Reduced [PcFe]⁻, [PcFeL]⁻, and [PcFeX]^2– Complexes: Story of a Double Flip

Abstract

The reduction of iron(II) phthalocyanine (Pc(2−)FeII) or its bisaxially coordinated complexes results in the formation of the purple/red [PcFe]−, [PcFeL]−, and [PcFeX]2– (L is neutral and X is anionic ligand) species. The X-ray structure of the [K(DME)4][PcFe] complex exhibits a square-planar [PcFe]− anion. 1H NMR spectra of the reduced species have one or two phthalocyanine broad peaks between 15 and 17 ppm. Solution magnetic moments are consistent with the presence of a single unpaired electron. A solid-state Mössbauer spectrum of [K(DME)4][PcFe] is consistent with an early report [Pure Appl. Chem.197438427438]. The solid-state EPR spectrum of the [PcFe]− anion is close to that recorded by Konarev et al. [Dalton Trans.2012411384113847]. Solution EPR spectra of reduced species have axial symmetry (g ⊥ ∼ 2.08–2.17 and g || ∼ 1.95–1.96) and correlate well with spectra reported by Lever and Wilshire in 1978 [Inorg. Chem.19781711451151]. The UV–vis spectra of pentacoordinated [PcFeL]− and [PcFeX]2– anions consist of the characteristic bands around 810, 690, and 515 nm. These bands correlate well with the set of MCD pseudo A-terms and resemble transitions in the [Pc(3−)M]− and [Pc(3−)ML]− compounds. The UV–vis and MCD spectra of [PcFeL]− and [PcFeX]2– complexes are in stark contrast to the crystallographically characterized reference [Pc(2−)CoI]− anion, which is EPR silent, has a regular diamagnetic 1H NMR spectrum, and has an intense Q-band at 699 nm, which correlates well with the strong MCD A-term. The DFT and TDDFT calculations are suggestive of the iron(II) center in a (d xy )2(d xz , yz )3(d z 2 )1 (s = 1) electronic configuration that is antiferromagnetically coupled with the one-electron-reduced Pc(3−) ligand (i.e., [Pc(3−)FeII]−, [Pc(3−)FeIIL]−, and [Pc(3−)FeIIX]2–). The calculated EPR, Mössbauer, and UV–vis spectra of [PcFe]−, [PcFeL]−, and [PcFeX]2– complexes are in excellent agreement with the experimental data, thus resolving the controversy between axial s = 1/2 like EPR and Pc(3−)-like UV–vis spectra of these compounds.

The fascinating story of axial ligand dependent spectroscopy and redox-properties in iron(ii) phthalocyanines

“…To re-examine this fundamental question of the metal or ring-based reduction in Pc(2-)Fe II L 2 , Pc(2-)Fe II L′L″, and [Pc(2-) Fe II X 2 ] 2− complexes we looked at structural, spectroscopic, and theoretical data for evidence of a [Pc(2-)Fe I L] − versus a [Pc(3-) Fe II L] − assignment. 17 Investigations into the electrochemistry and spectroscopy of the reduction of Pc(2-)Fe II L 2 and [Pc(2-) Fe II X 2 ] 2− can be highly complex, as the reduction can change the affinities for axial ligands. Notably, we observed the formation of five coordinate rather than six coordinate species upon reduction.…”

“…The Mössbauer spectra of the reduced complexes reveal larger quadrupolar splittings than seen in the Pc(2-)Fe II compounds which is at first glance more consistent with a Fe(I) assignment, but we later determined that increasing electron density at the metal center (vide infra) could also explain these splitting without reducing the metal. 17 The contradictory spectroscopic observations on iron phthalocyanine reduction can be resolved by using DFT and TDDFT calculations, which are indicative of the following steps during reduction process (Fig. 7): (i) initial reduction of the phthalocyanine core; (ii) dissociation of one of the axial ligands and stabilization of the iron-centered d z 2 orbital; (iii) transformation of the iron(II) center from low-spin to intermediate-spin (s = 1); (iv) antiferromagnetic coupling between (d π ) 3 and (e g *) 1 orbitals.…”

“…Thus, we decided to probe the reduction chemistry of iron phthalocyanines. 17 The single electron reduction of PcFe II compounds has not been well explored. We would expect, however, to observe similarly complex ligand-dependent behavior in the single electron reduction of Pc(2-)Fe II L 2 , Pc(2-)Fe II L′L′′, and [Pc(2-)Fe II X 2 ] 2− complexes.…”

“…We used MCD to investigate the spectra of these reduced PcFe compounds, and once again the reduction spectra are very similar with some small dependence on the identity of the axial ligand. 17 The UV-visible and MCD spectra resemble those of [Pc(3-)M II ] − complexes such as those seen for the closed-shell metals Mg 2+ and Zn 2+ and are indicative of a single electron reduction of the phthalocyanine ring, and so we can characterize these derivatives as [Pc(3-)Fe II ] − , [Pc(3-)Fe II L] − , or [Pc(3-)Fe II X] 2− species. This electronic ring-based reduction assignments, however, seemingly contradicts the solution EPR data collected by Lever and Wilshire in 1978 20 as well as later work by Dzilinski in 2007.…”

“…The Mössbauer spectra of the reduced complexes reveal larger quadrupolar splittings than seen in the Pc(2-)Fe II compounds which is at first glance more consistent with a Fe( i ) assignment, but we later determined that increasing electron density at the metal center ( vide infra ) could also explain these splitting without reducing the metal. 17…”