A New Series of Cobalt and Iron Clathrochelates with Perfluorinated Ribbed Substituents.

Genrikh E Zelinskii,Yan Z Voloshin,Alexander S Belov,Irina G Belaya,Yurii N Bubnov,Yulia V Nelyubina,Valentin V Novikov,Yan V Zubavichus,Alexander A Pavlov,Anna V Vologzhanina,Nikolay N Efimov

doi:10.1021/acsomega.7b01088

Abstract

The study tackles one of the challenges in developing platinum-free molecular electrocatalysts for hydrogen evolution, which is to seek for new possibilities to ensure large turnover numbers by stabilizing electrocatalytic intermediates. These species are often much more reactive than the initial electrocatalysts, and if not properly stabilized by a suitable choice of functionalizing substituents, they have a limited long-time activity. Here, we describe new iron and cobalt(II) cage complexes (clathrochelates) that in contrast to many previously reported complexes of this type do not act as electrocatalysts for hydrogen evolution. We argue that the most probable reason for this behavior is an excessive stabilization of the metal(I) species by perfluoroaryl ribbed groups, resulting in an unprecedented long-term stability of the metal(I) complexes even in acidic solutions.

Highlights

Cobalt clathrochelates,[1] which are polyazomethine-based cage complexes, have been recently recognized as efficient electrocatalysts for the hydrogen evolution reaction (HER) at low overpotentials;[2] the mechanism of their electrocatalytic activity is still not fully understood
Cobalt(II) clathrochelates were shown to function only as precatalysts that produced electrocatalytically active nanoparticles[3] under acidic conditions, whereas in others, they themselves were homogenous electrocatalysts that required a very small overpotential.2c In contrast to the former, the latter have halogen atoms as their ribbed substituents; they keep the potential of the Co2+/+ reduction close to the thermodynamic potential for the HER2c and increase the chemical stability of Co+ intermediate species,[4] which is a prerequisite for a large catalytic turnover number
The presence of bridging sulfur atoms does not allow thefluoroaryl groups to stabilize the reduced metal(I)containing intermediate.[7]. Removing these atoms may potentially open a new pathway to better clathrochelate-based electrocatalysts for the HER, which are functional analogs of macrocyclic BF2-cross-linked iron(II) complexes of perfluoroaryl-substituted α-dioximates earlier used for this purpose.[8]

Summary

■ INTRODUCTION

Cobalt clathrochelates,[1] which are polyazomethine-based cage complexes, have been recently recognized as efficient electrocatalysts for the hydrogen evolution reaction (HER) at low overpotentials;[2] the mechanism of their electrocatalytic activity is still not fully understood. (C6F5)2GmH2 (0.196 g, 0.47 mmol) and n-butylboronic acid (0.032 g, 0.32 mmol) were dissolved/suspended in nitromethane (20 mL), and FeCl2· 4H2O (0.03 g, 0.15 mmol) was added to the stirring reaction mixture under argon. Phenylboronic acid (0.06 g, 0.50 mmol) and (C6F5)2GmH2 (0.31 g, 0.73 mmol) were dissolved/suspended in nitromethane (10 mL), and CoCl2 (0.03 g, 0.24 mmol) was added to the reaction mixture under stirring in argon. (C6F5)2GmH2 (0.37 g, 0.88 mmol) and n-butylboronic acid (0.075 g, 0.73 mmol) were dissolved/suspended in nitromethane (10 mL), and CoCl2 (0.031 g, 0.24 mmol) was added to the reaction mixture under stirring in argon. The extract was flashchromatographically separated on silica gel (30 mm layer; eluent: dichloromethane), the second orange-red elute was evaporated to dryness, and the solid residue was dried in vacuo.

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES