Electrocatalytic hydrogen evolution mediated by an organotelluroxane macrocycle stabilized through secondary interactions†‡

A discrete liphophilic organotelluroxane macrocycle has been found to catalyse the hydrogen evolution reaction (HER) by proton reduction efficiently. The macrocycle is synthesized via chloride abstraction from bis(p-methoxyphenyl) tellurium dichloride (p-MeOC₆H₅)₂TeCl₂ (1) by silver salts AgMX₄ (MX₄ = BF₄⁻, and ClO₄⁻) resulting in in situ generated di-cationic tetraorganoditelluroxane units; two such units are held together by two weak anions μ₂-MX₄, bridging to form 12-membered di-cationic macrocycles [((p-MeO-C₆H₄)₂Te)₂(μ-O)(μ₂-F₂BF₂)₂]²⁺ (2) and [((p-MeO-C₆H₄)₂Te)₂(μ-O)(μ₂-O₂ClO₂)₂]²⁺ (3) stabilized via Te–(μ₂-BF₄/ClO₄), with secondary interactions. The charge is balanced by the presence of two more anions, one above and another below the plane of the macrocycle. Similar reaction at higher temperatures leads to the formation of telluronium salts R₃TeX [X = BF₄⁻ (4), ClO₄⁻ (5)] as a major product. The BF₄⁻ anion containing macrocycle and telluronium salt were monitored using ¹⁹F NMR. HRMS confirmed the structural stability of all the compounds in the solution state. The organotelluroxane macrocycle 2 has been found to act as an efficient electrocatalyst for proton reduction in an organic medium in the presence of p-toluene sulfonic acid as a protic source.