Hypercorroles Formed via the Tail that Wagged the Dog: Charge Transfer Interactions from Innocent Corroles to Meso-Nitrophenyl Substituents.

Abstract

A series of cobalt nitrophenylcorroles were spectrally characterized in CH2Cl2, and under certain solution conditions, several compounds were shown to exhibit hypercorrole spectra resulting from charge transfer interactions from the corrole π-system to the redox-active meso-NO2Ph substituents. The resulting spectral pattern has not previously been reported for metallocorroles and in the case of the cobalt derivatives was shown to depend upon the number and position of the meso-nitrophenyl groups on the macrocycle, the position of the NO2 substituent on the meso-phenyl ring(s) (para or meta), and the electronic structure of the corrole, which can exist in its innocent or noninnocent form depending in large part upon the type and number of axial ligands. Cobalt corroles bearing p-nitrophenyl groups at the 5,15- or 5,10,15-positions of the macrocycle exhibited the most marked hypercorrole spectra under solution conditions where the complex was innocent (i.e., Cor3-CoIII), and a systematic analysis of the spectral data suggests the root of this perturbation to be a corrole-to-aryl interaction (i.e., ligand-to-ligand charge transfer or LLCT). The largest interaction between the π-system and the NO2Ph substituents was seen upon coordination of anionic cyanide (CN-) axial ligands to the Co(III) center of the bis-(CN-)-5,15-dinitrophenyl derivative, resulting in a cobalt hypercorrole spectrum where the broad Q-band was red-shifted even further into the NIR region and located at 795 nm in CH2Cl2 and 827 nm in pyridine. Cyclic voltammetry of the bis-CN- adducts showed that the first electrons are added to the LUMOs of the p-NO2Ph substituents rather than the corrole, while the same orbitals for the mono-CN- adducts are nearly degenerate. This redox behavior contrasts with what is seen for the noninnocent nitrophenyl corroles having "normal" unperturbed UV-vis spectra where the first reduction involves the π-system of the macrocycle, followed by reduction of the p-NO2Ph groups at more negative potentials.