Abstract
Three mono-CN ligated anionic cobalt A3-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)3CorCoIII(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 β-pyrrole substituents. Formation constants (log K) for conversion of the five-coordinate mono-CN complex to its six-coordinate bis-CN form ranged from 102.8 for Ar = p-(OMe)Ph to 104.7 for Ar = p-(CN)Ph in DMSO as determined by spectroscopic methodologies. The in situ generated bis-CN complexes, represented as [(Ar)3CorCoIII(CN)2]2-(TBA+)2, and the mixed ligand complexes, represented as [(Ar)3CorCoIII(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 CH2Cl2, pyridine, or DMSO. Linear free energy relationships were elucidated between the meso-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.
Highlights
The propensity of the corrole macrocycle to behave as a noninnocent ligand under specific conditions has often hindered the ability to definitively assign an exact metal and ligand oxidation state, but published reports from our laboratory[2,18,42−50] and others[13,21,26,37,38,51−55] have clearly demonstrated that noninnocent corrole macrocycles are significantly easier to reduce than innocent macrocyclic systems having the same formal oxidation state of the central metal ion, providing insights into the electronic configuration of the molecule
We established an electrochemical litmus test, where the existence of extremely facile reductions (∼0.00 to −0.20 V vs saturated calomel electrode (SCE)) for transition-metal corroles can be used as one criterion to establish the presence of a noninnocent ligand in the case of metallocorroles with iron, cobalt, nickel, or copper.[2,43,45,47−49] For example, in the case of cobalt triarylcorroles, the four-coordinate complexes are reduced at the macrocycle, while the six-coordinate species are reduced at the central metal ion, giving, in both cases, the same cobalt(II) reduction product
In search of nonlabile coordinating ligands, we recently investigated the possible binding of anionic ligands to neutral cobalt triarylcorroles and found that 10 of the 11 investigated anionic ligands (PF6−, BF4−, HSO4−, ClO4−, Br−, I−, Cl−, OAc−, F−, OTs−) were unable to bind to the cobalt center of the neutral complex, with the only exception being complexes are the mixed-ligand (CN)−, where a stepwise formation of both mono- and bis-CN adducts was observed in solutions of CH2Cl2.47 (Ar)3CorCoII(DMSO) + CN−
Summary
The electrochemical and spectroscopic properties of numerous corroles possessing a wide variety of meso- and β-pyrrole substituents have been reported over the last three decades.[1−8] Many of these reports have focused on the use of these compounds for applications as redox and bioinspired catalysts[9−25] or sensors,[26−36] with the focal point of cobalt corrolate application involving oxygen reduction[12,23−25] and carbon monoxide detection.[31−36] a particular emphasis of corrole research has been placed on unveiling the electronic configuration of the molecule[4,37−40] to aid in the design of new catalysts.[41].
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