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

Abstract

The hydrogen evolution reaction (HER) has attracted much attention within the scientific community because of increasing demands of modern society for clean and renewable energy sources. Molecular complexes of 3d-transition metals, such as cobalt, hold potential to replace platinum for the HER in acidic media. Among these, cage complexes such as tris-glyoximate metal clathrochelates, have demonstrated promising catalytic properties towards the HER. However, it is not clear whether the catalytic activity of this molecule stems from metal-centered activation of H , due to a low oxidation state of the metal stabilized by the surrounding organic cage, or if it is the organic cage playing a further cooperative role in bringing protons together. Herein, we report on a density functional theory study of two possible mechanisms for the HER catalyzed by a model Co clathrochelate. To assess the putative ligand involvement in the mechanism, several combinations of single and double protonation sites were investigated. The structural and energetic analysis of relevant intermediates suggests that the electrocatalytic mechanism is not based on the cooperation between the ligand and the metal. Instead, it is mainly due to the activation of H by the Co metallocenter. Our calculations further suggest that the last step in the mechanism is a proton coupled electron transfer step.

The Use of Density Functional Theory to Decipher the Electrochemical Activity of Metal Clathrochelates with Regard to the Hydrogen Evolution Reaction in the Homogeneous Phase

“…In the case of Co clathrochelates, the literature is dominated by the use of the B3LYP exchange-correlation functional. We have successfully used the B3LYP functional for geometry optimizations, with excellent agreement to experiment [2]; however, all attempts to use other functionals (M06-L, TPSS, BP86, PBE, or PBE0) yielded unreasonable geometries [7]. Regarding the selection of the basis set, the LANL2DZ with effective core potentials for heavy atoms predicts well the geometry of Co clathrochelates; on the other hand, the use of a more complete and more expensive all-electron basis set cc-pVTZ for Co and cc-pVDZ for all other atoms did not improve significantly the results [7].…”

“…This procedure is relatively lengthy and therefore shall not receive further attention in this chapter, but the interested reader is referred to the specialized literature [11][12][13][14]. Conversely, the full calculation of the Born-Haber process may be bypassed, and the optimization and Hessian calculation of the relevant redox species, i.e., O and R, in the desired solvent may be undertaken directly [7,15].…”

“…Until now, the most complete theoretical study of hydrogen evolution involving clathrochelates is found in Ref. [7]. The work was performed on a clathrochelate having chlorine and methyl moieties as the ribbed and apical substituents, respectively.…”

Ligand Effects in Low‐Valent Co(I) Clathrochelate Complexes

“…Following the discovery that Co clathrochelate complexes can be employed as catalyst precursors for the hydrogen evolution reaction, low-valent Co(I) clathrochelates have been studied extensively, both from an experimental as well as theoretical point of view. [7][8][9][10][11][12] Despite these efforts, there was only limited knowledge about the solid state structure of these highly reactive complexes. We have performed crystallographic analyses of three low-valent clathrochelate complexes (3, 4, and 6), along with analyses of the corresponding Co(II) precursors.…”

“…[11] However, the possibility that Co(I) clathrochelate complexes could adopt a singlet spin state (S = 0) was also considered in theoretical studies. [11,12] The calculations indicate that a diamagnetic Co(I) complex would show a very distorted ligand environment, with two long CoÀ N bonds. Below, we report the first structural characterization of Co(I) clathrochelate complexes with alkyl groups in lateral position.…”