Charge-Transfer Spectroscopy of Bisaxially Coordinated Iron(II) Phthalocyanines through the Prism of the Lever's E L Parameters Scale, MCD Spectroscopy, and TDDFT Calculations.

The position of the experimentally observed (in the UV-vis and magnetic circular dichroism (MCD) spectra) low-energy metal-to-ligand charge-transfer (MLCT) band in low-spin iron(II) phthalocyanine complexes of general formula PcFeL 2 , PcFeL'L″, and [PcFeX 2 ] 2- (L, L', or L″ are neutral and X - is an anionic axial ligand) was correlated with the Lever's electrochemical E L scale values for the axial ligands. The time-dependent density functional theory (TDDFT)-predicted UV-vis spectra are in very good agreement with the experimental data for all complexes. In the majority of compounds, TDDFT predicts that the first degenerate MLCT band that correlates with the MCD A -term observed between 360 and 480 nm is dominated by an e g (Fe, d π ) → b 1 u (Pc, π*) single-electron excitation (in traditional D 4h point group notation) and agrees well with the previous assignment discussed by Stillman and co-workers[ Inorg. Chem. 1994, 33, 573-583]. The TDDFT calculations also suggest a small energy gap for b 1u / b 2u (Pc, π*) orbital splitting and closeness of the MLCT 1 e g (Fe, d π ) → b 1u (Pc, π*) and MLCT 2 e g (Fe, d π ) → b 2u (Pc, π*) transitions. In the case of the PcFeL 2 complexes with phosphines as the axial ligands, additional degenerate charge-transfer transitions were observed between 450 and 500 nm. These transitions are dominated by a 2u (Pc + L, π) → e g (Pc, π*) single-electron excitations and are unique for the PcFe(PR 3 ) 2 complexes. The energy of the phthalocyanine-based a 2u orbital has large axial ligand dependency and is the reason for a large energy deviation for B1 a 2u (Pc + L, π) → e g (Pc, π*) transition. The energies of the axial ligand-to-iron, axial ligand-to-phthalocyanine, iron-to-axial ligand, and phthalocyanine-to-axial ligand charge-transfer transitions were discussed on the basis of TDDFT calculations.