Old Dog, New Tricks: Innocent, Five-coordinate Cyanocobalt Corroles.

Three mono-CN ligated anionic cobalt A-triarylcorroles were synthesized and investigated as to their spectroscopic and electrochemical properties in CHCl, 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)CorCo(CN)]TBA, where Cor is the trivalent corrole macrocycle, Ar is -(CN)Ph, -(CF)Ph, or -(OMe)Ph, and TBA is the tetra--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 and β-pyrrole substituents. Formation constants (log ) for conversion of the five-coordinate mono-CN complex to its six-coordinate bis-CN form ranged from 10 for Ar = -(OMe)Ph to 10 for Ar = -(CN)Ph in DMSO as determined by spectroscopic methodologies. The in situ generated bis-CN complexes, represented as [(Ar)CorCo(CN)](TBA), and the mixed ligand complexes, represented as [(Ar)CorCo(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 CN 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 CHCl, pyridine, or DMSO. Linear free energy relationships were elucidated between the -phenyl Hammett substituent constants (Σσ) 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.