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

Briana R. Schrage, Wen Zhou, Laurel A. Harrison, Dustin E. Nevonen, John R. Thompson, Kathleen E. Prosser, Charles J. Walsby, Christopher J. Ziegler, Daniel B. Leznoff, Victor N. Nemykin

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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 [ 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(dxzyz)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. copy; 2022 American Chemical Society.

Original languageEnglish
Pages (from-to)20177-20199
Number of pages23
JournalInorganic Chemistry
Volume61
Issue number50
DOIs
StatePublished - Dec 19 2022
Externally publishedYes

Funding

Generous support from the NSF (CHE-2153081), Minnesota Supercomputing Institute, WestGrid Canada, and the University of Tennessee to V.N.N. is greatly appreciated. K.E.P., C.W., and D.B.L. are grateful to NSERC of Canada for generous support of this research via a Vanier Postgraduate Fellowship (K.E.P.) and the Discovery Grants program (C.W., D.B.L.). We want to acknowledge Prof. Karl Kadish for some electrochemical data.

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