Synthesis, characterization and solvent/structural effects on spectral and redox properties of cobalt triphenylcorroles in nonaqueous media

Five cobalt(III) triphenylcorroles with different electron-withdrawing or electron-donating substituents and an axially bound triphenylphosphine ligand were synthesized and characterized by spectroscopic and electrochemical techniques. The investigated compounds are represented as (4-XPh)(3)CorCo(PPh3), where Ph(3)Cor is the trianion of a triphenylcorrole and X is a OMe, Me, H, F or Cl substituent on the meso-phenyl rings. Each corrole was examined by UV-vis, H-1 NMR and IR spectroscopy, mass spectrometry, electrochemistry and thin-layer spectroelectrochemistry. Redox potentials and spectra of each oxidized and reduced species were examined in dichloromethane and N,N'-dimethylformamide containing 0.1 M tetra-n-butylammonium perchlorate. Each Co(III) corrole undergoes up to five one-electron transfer reactions, some of which are reversible and others which are not. The Co-III/Co-II process is irreversible in both solvents due to the loss of the triphenylphosphine axial ligand following electron transfer. The Co-II/Co-I process is reversible in DMF but irreversible in CH2 Cl-2 due to a homogenous chemical reaction between the electrogenerated Co(I) corrole and the chlorinated solvent. The potential for the first oxidation of the investigated corroles varies little with change of solvent, consistent with the lack of solvent binding to the neutral and singly oxidized forms of (4-XPh)(3)CorCo(PPh3). However, a single DMF molecule strongly binds to the doubly oxidized corrole in DMF or DMF/CH2Cl2 mixtures. This results in an easier oxidation and a negative shift of similar to 200 mV in E-1/2 upon going from CH2Cl2 to DMF as solvent. The effect of substitutents and solvent on redox potentials is discussed and an overall electroreduction/electrooxidation mechanism is proposed.