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. 1 H 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 1 H 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-)Fe II ] - , [Pc(3-)Fe II L] - , and [Pc(3-)Fe II X] 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.