Old Dog, New Tricks: Innocent, Five-Coordinate Cyanocobalt Corroles

Three mono-CN ligated anionic cobalt A(3)-triarylcorroles were synthesized and investigated as to their spectroscopic and electrochemical properties in CH2Cl2, pyridine (Py), and dimethyl sulfoxide (DMSO). The newly synthesized corroles provide the first examples of air-stable cobalt corroles with an anionic axial ligand and are represented as [(Ar)(3)CorCo(III)(CN)](-)TBA(+), where Cor is the trivalent corrole macrocycle, Ar is p-(CN)Ph, p-(CF3)Ph, or p-(OMe)Ph, and TBA(+) is the tetra-n-butylammonium (TBA) cation. Multiple redox reactions are observed for each mono-CN derivative with a key feature being a more facile first oxidation and a more difficult first reduction in all three solvents as compared to all previously examined corroles with similar meso- and beta-pyrrole substituents. Formation constants (log K) for conversion of the five-coordinate mono-CN complex to its six-coordinate bis-CN form ranged from 10(2.8) for Ar = p-(OMe)Ph to 10(4.7) for Ar = p-(CN)Ph in DMSO as determined by spectroscopic methodologies. The in situ generated bis-CN complexes, represented as [(Ar)(3)CorCo(III)(CN)(2)](2-)(TBA(+))(2), and the mixed ligand complexes, represented as [(Ar)(3)CorCo(III)(CN)(Py)](-)TBA(+), were also investigated as to their electrochemical and spectroscopic properties. UV-visible spectra and electrode reactions of the synthesized mono-CN- derivatives are compared with the neutral mono-DMSO cobalt corrole complexes and the in situ generated bis-CN and bis-Py complexes, and the noninnocent (or innocent) nature of each cobalt corrole system is addressed. The data demonstrate the ability of the CINI - axial ligand(s) to stabilize the high-valent forms of the metallocorrole, leading to systems with innocent macrocyclic ligands. Although a number of six-coordinate cobalt(III) corroles with N-donor ligands were characterized in the solid state, a dissociation of one axial ligand readily occurs in nonaqueous solvents, and this behavior contrasts with the high stability of the currently studied bis-CN adducts in CH2Cl2, pyridine, or DMSO. Linear free energy relationships were elucidated between the meso-phenyl Hammett substituent constants (Sigma sigma) and the measured binding constants, the redox potentials, and the energy of the band positions in the mono-CN and bis-CN complexes in their neutral or singly oxidized forms, revealing highly predictable trends in the physicochemical properties of the anionic corroles.