Application of Lever’s E L Parameter Scale toward Fe(II)/Fe(III) versus Pc(2-)/Pc(1‑) Oxidation Process Crossover Point in Axially Coordinated Iron(II) Phthalocyanine Complexes

The electronic structures and, particularly, the nature of the HOMO in a series of PcFeL2, PcFeL′L″, and [PcFeX2]2– complexes (Pc = phthalocyaninato­(2-) ligand; L = NH3, n-BuNH2, imidazole (Im), pyridine (Py), PMe3, PBu3, t-BuNC, P­(OBu)3, and DMSO; L′ = CO; L″ = NH3 or n-BuNH2; X = NCO–, NCS–, CN–, imidazolate (Im–), or 1,2,4-triazolate­(Tz–)) were probed by electrochemical, spectroelectrochemical, and chemical oxidation as well as theoretical (density functional theory, DFT) studies. In general, energies of the metal-centered occupied orbitals in various six-coordinate iron phthalocyanine complexes correlate well with Lever Electrochemical Parameter E L and intercross the phthalocyanine-centered a 1u orbital in several compounds with moderate-to-strong π-accepting axial ligands. In these cases, an 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.