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−)­Fe^II) 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−)­Co^I]⁻ 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_z²)¹ (s = 1) electronic configuration that is antiferromagnetically coupled with the one-electron-reduced Pc(3−) ligand (i.e., [Pc(3−)­Fe^II]⁻, [Pc(3−)­Fe^IIL]⁻, and [Pc(3−)­Fe^IIX]^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.