Structural, Electronic, and Theoretical Description of a Series of Cobalt Clathrochelate Complexes in the Co(III), Co(II) and Co(I) Oxidation States

Abstract

Encaged hexacoordinated metal complexes have long been a fascinating family of complexes, as they confer the same ligand environment to metal ions in different oxidation states. We recently reported that cobalt clathrochelate complexes behave as hydrogen-producing catalysts at quite modest overpotential ( Pantani et al. Angew. Chem., Int. Ed. 2008 , 47 , 9948 ). The electrochemical properties evidenced two quasireversible one-electron reduction waves starting from the cobalt(III) derivative, indicating that there are no dramatic changes in the coordination sphere. The intriguing question is the mechanistic pathways for this observed reactivity. In this work, we compare our observed electrochemical and spectroscopic data (UV-visible and EPR spectroscopies) with our theoretical findings based on DFT, TD-DFT, and CASSCF calculations. The properties of the Co(III) and Co(II) species can be explained as low-spin complexes. In contrast, the doubly reduced species, the "Co(I)" form, is a high-spin complex and its electronic description involves partial reduction of the ligand cage. This point is of major importance to understand the catalytic activity.

Approach to the Mechanism of Hydrogen Evolution Electrocatalyzed by a Model Co Clathrochelate: A Theoretical Study by Density Functional Theory

“…Such a one‐electron reduction provoked a movement of 0.7 Å of Co within the organic cage, so that the central atom is nearly in a square‐planar coordination (structure of [ 1 Co] 1− shown in the left of Figure ). The structures of the [Co] 1− species is different to that of the previous work, where low‐spin reduced complex had this quasi‐square planar structure, with two N considered as decoordinated, but the high‐spin counterpart presented a highly symmetric structure. In the context of HER electrocatalysis, the structure of the [Co] 1− obtained in this work is interpreted as favorable because the exposition of the Co should make the coordination of H + more sterically favorable, while the decoordination of two N leaves vacant coordination sites for H + binding to form a reactive intermediate.…”

“…A comparison of the results of this work and previously published data, indicate that the B3LYP functional slightly overestimates bond distances. Even though it should be kept in mind that geometry in the solid state is not necessarily the same as in an organic solution.…”

“…Current literature regarding DFT calculations on clathrochelates, reveals that most calculations have been performed with the B3LYP exchange correlation functional ,,. Though, it has been recognized that transition metal complexes can be described by several other functionals .…”

“…A previous theoretical study on clathrochelates was devoted to the detailed description of a molecule having F and methyl as the apical and ribbed substituents, respectively . The molecule studied here (Figure a) possesses a methyl and Cl atoms as the apical and ribbed substituents, respectively.…”

“…Even though DFT studies of the geometric and electronic structures of these molecules have already been reported in the literature,, the exploration of the HER mechanism has received scarce attention from a theoretical perspective. To the best of our knowledge, the only attempt to rationalize the HER catalyzed by cobalt clathrochelates was presented few years ago .…”