Combined MCD/DFT/TDDFT Study of the Electronic Structure of Axially Pyridine Coordinated Metallocorroles

A series of metallocorroles were investigated by UV-vis and magnetic circular dichroism spectroscopies. The diamagnetic distorted square-pyramidal main-group corrole Ga(tpfc)py (2), the diamagnetic distorted octahedral transition-metal adduct Co(tpfc) (py)(2) (3), and paramagnetic distorted octahedral transition-metal Complex Fe(tpfc)(py)(2) (4) [H(3)tpfc = tris(perfluorophenyl)corrole] were studied to investigate similarities and differences in the electronic structure and spectroscopy of the closed- and open-shell metallocorroles. Similar to the free-base H(3)tpfc (1), inspection of the MCD Faraday B-terms for all of the macro cycles presented in this report revealed that a < Delta HOMO < Delta LUMO [Delta HOMO is the energy difference between two highest energy corrole-centered pi-orbitals and Delta LUMO is the energy difference between two lowest energy corrole-centered pi*-orbitals originating from M-L +/- 4 and M-L +/- 5 pairs of perimeter] condition is present for each complex, which results in an unusual sign:reversed sequence for pi-pi* transitions in their MCD spectra. In addition) the MCD spectra of the cobalt and the iron complexes were also complicated by a number of charge-transfer states in the visible region. Iron complex 4 also exhibits a low-energy absorption in the NIR region (1023 nm). DFT and TDDFT calculations were used to elaborate the electronic structures and provide band assignments in UV-vis and MCD spectra of the inetallocorroles. DFT and TDDFT calculations predict that the orientation of the axial pyridine ligand(s) has a very minor influence on the calculated electronic structures and absorption spectra in the target systems.