The electronic structures of a series of the low-spin PcFeL2, PcFeL′L″, and [PcFeX2]2- iron(II) phthalocyanine complexes and their reduced [PcFeL]- and [PcFeX]2- anions were probed by electrochemical, spectroelectrochemical and chemical oxidation approaches and complimented by the NMR, UV-Vis, EPR, and MCD spectroscopy, magnetochemistry, as well as theoretical (DFT and TDDFT) studies. In general, energies of the metal-centered occupied orbitals in the various six-coordinate iron phthalocyanine complexes correlate well with Lever’s electrochemical parameter, EL, and intercross the phthalocyanine-centered a1u orbital in several compounds with moderate-to-strong p-accepting axial ligands. In these cases, oxidation of the phthalocyanine macrocycle (Pc(2-)/Pc(1-)) rather than the central metal ion (Fe(II)/Fe(III)) was theoretically predicted and experimentally confirmed. The experimentally derived using MCD spectroscopy or theoretically predicted using TDDFT calculations energy of the MLCT1 transition (dπ→Pc(π*)) on iron(II) phthalocyanines also follows the trend expected for the EL properties of the axial ligands. Based on the experimental and theoretical data, the electronic structure of the reduced anions can be described as [Pc(3-)FeIIL]- and [Pc(3-)FeIIX]2- in which reduced phthalocyanine ligand is antiferromagnetically coupled to the intermediate-spin (s = 1) iron(II) center.

New insights into an old chemistry: Electronic structure and spectroscopy of the low-spin iron(II) phthalocyanines and their reduced forms

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