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-)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.