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

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 [Taube, R. Pure Appl. Chem.1974, 38, 427−438]. The solid-state EPR spectrum of the [PcFe]− anion is close to that recorded by Konarev et al. [ Dalton Trans.2012, 41, 13841−13847]. 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.1978, 17, 1145−1151]. 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 (dxy)2(dxz,yz)3(dz2)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.

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