Altering the Site of Electron Abstraction in Cobalt Corroles via meso-Trifluoromethyl Substituents

Abstract

A mono-DMSO cobalt meso-CF3 corrole, formulated as (CF3)3CorCo­(DMSO), where (CF3)3Cor is the trianion of 5,10,15-tris­(trifluoromethyl)­corrole, was synthesized and characterized as to its spectral and electrochemical properties in nonaqueous media with a focus on its coordination chemistry and electronic structure. Cyclic voltammetric measurements showed more facile reductions and difficult oxidations compared to the cobalt triarylcorrole possessing p-CF3Ph units at the meso-positions, a result consistent with the enhanced inductive effect of the electron-withdrawing trifluoromethyl substituents linked directly at the meso-carbon atoms of the macrocycle. The effects of DMSO, pyridine, and cyanide anions (CN–) on the electrochemistry and spectral properties of the compound were investigated, and it was found that only two molar equivalents are needed to form the bis-CN adduct, which exhibited two 1-electron oxidations at 0.27 and 0.95 V vs saturated calomel electrode (SCE) in CH2Cl2/0.1 M TBAP. The sites of electron transfer in the first oxidation and reduction were investigated by spectroelectrochemistry and confirmed that the first electron addition affords a Cor3–CoII complex under all solution conditions independent of the initial coordination and/or electronic configuration (i.e., innocent Cor3–CoIII or noninnocent Cor•2–CoII). In contrast, data for the first oxidation suggests that the site of electron abstraction (ligand or metal) depended upon coordination of the neutral and in situ generated complexes under the various solution conditions, leading to a Co­(IV)-corrole3– product for both the bis-pyridine and bis-cyanide adducts.

Beneficial Effects on the Cobalt-Catalyzed Hydrogen Evolution Reaction Induced by Corrole Chelation

“…A brief examination of the formal cobalt­(IV) hydride intermediate was performed, even though such a species may well remain elusive for at least two different reasons: (i) the difficulties in the undoubtful characterization of even cobalt­(III) hydrides, and (b) the fact that, in all of the proposed mechanisms, it either will not be formed (the E­(CE)­E process in Figure b) or will not accumulate. Our computations revealed that most of the spin density was spread over the macrocycle (Figure S27), similar to what has been reported for alkyl- and aryl-cobalt­(IV) corroles. − The distinction between the heterolytic and homolytic pathways leading from the cobalt­(IV) hydride intermediates to the products (the lower and upper parts in Figure b, respectively) was not addressed by the computations, as both pathways are very much dependent on the acidity of the proton source, the catalyst concentrations, and whether the reactions are performed under homogeneous or heterogeneous conditions.…”

Effect of Apical Ligands, Substituents and Oxidation States on the Electronic Structure of Co(III) Corrolates

“…CASSCF Calculations. It has been reported that in cobalt corroles with one Py [17] or with DMSO [8,18,21,29] ligands, the corrole ligands act as non-innocent, resulting in an antiferromagnetlically coupled Co(II)Cor *2À ground state. Our DFT calculations support this analysis in the case of DMSO complexes, but are less straightforward in the case of mono-Py ligated cobalt corroles.…”

Size and Electronic Effects on the Performance of (Corrolato)cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis

“…The less obvious (and more important for ORR catalysis) deduction comes from analysis of the first reduction process. This Co II /Co III transformation frequently proceeds by the well-known electrochemical “box mechanism” that includes ligand association/disassociation. , The 5-coordinate low-spin d 6 cobalt­(III) complexes bind their PPh 3 ligand tightly, but the affinity is strongly reduced upon reduction to much more labile d 7 cobalt­(II). A reversible Co II /Co III redox process is obtained only for 2t , thus indicating that this complex is reoxidized without losing its axial ligand.…”

“…The situation is reversed for the bis­(pyridine) complexes: the most electron-rich 3b is now the only one to display a reversible Co II /Co III couple. The reason is that, although all derivatives are present in the solid state (X-ray determination) as 6-coordinate complexes, they easily lose either one or even both axial ligands in dilute solutions . The electron-richness of corroles is a major factor in that regard, coming into effect by 3 orders of magnitude differences in the bis­(pyridine)­cobalt­(III) complex dissociation constants. , A plausible explanation is that the original pyridine ligands in the least Lewis acidic complex 3b are fully dissociated (presumably replaced by the coordinating acetonitrile solvent) even prior to reduction.…”

“…The reason is that, although all derivatives are present in the solid state (X-ray determination) as 6-coordinate complexes, they easily lose either one or even both axial ligands in dilute solutions . The electron-richness of corroles is a major factor in that regard, coming into effect by 3 orders of magnitude differences in the bis­(pyridine)­cobalt­(III) complex dissociation constants. , A plausible explanation is that the original pyridine ligands in the least Lewis acidic complex 3b are fully dissociated (presumably replaced by the coordinating acetonitrile solvent) even prior to reduction. Supporting evidence for this conclusion is based on two other observations: (a) the reoxidation process (i.e., the reverse scan) of [ 3b ] − is maximal at −0.5 V, identical with the one assigned for PPh 3 -free [ 3t ] − ; (b) while the second reduction of 2b occurs at a much less negative potential than that of 3b , consistent with the difference between them regarding the electron-withdrawing effect of CF 3 versus H, the opposite is true for their first reduction process.…”