Electronic Structure and Hypercorrole Features of Deprotonated Free-Base and Reduced Copper Corroles

We report the synthesis, crystal structure, and spectroscopic and computational electronic-structural characterization of two copper corroles, [Cu(5,15-bis(4-methylcarboxyphenyl)-10-(2-methylcarboxyphenyl)corrole)] (1Cu) and [Cu(5,15-bis(4-nitrophenyl)-10-(2-methylcarboxyphenyl)corrole)] (2Cu), as well as spectroscopic and computational studies on the corrole ligands (H₃L1 and H₃L2) in their neutral and anionic forms. We have found that the anionic corroles containing the 4-nitrophenyl substituents in positions 5 and 15 of the corrole ring show hypercorrole behavior, where the Q-bands are considerably more intense and red-shifted compared to those in the 4-methylcarboxyphenyl-substituted corroles. Electronic structure calculations using wave function methods (CASSCF/NEVPT2) reveal that the intense Q-bands, which extend slightly into the NIR region, are charge-transfer bands from the anionic corrole core to the strongly electron-withdrawing 4-nitrophenyl substituents. We further show that 2Cu can be reduced indirectly in the presence of excess F^- or OH^-, and the Q-band shifts toward the red in the polar environment containing the corresponding salts. Our study provides examples of easy-to-prepare anionic corroles, metalated and unmetalated, that show hypercorrole behavior and NIR absorption, and thus could find use in hyperthermal processes.