Electrocatalytic hydrogen evolution with copper(II) pincer complexes bearing functionalized pyridyl benzothiazoles and carbo(thio)amides of a κ3-SNS, ONS, SNN or ONN coordination mode

Abstract

Six copper(II) pincer complexes of κ³-SNS, ONS, SNN or ONN coordination modes were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in dimethylformamide using acetic acid and trifluoroacetic acid as proton sources. The copper(II) complexes of general formula [Cu(Lⁿ)(OAc)]•zH₂O (z = 0–3) and Lⁿ = bis-N-(4-chlorophenyl)pyridine-2,6-dicarbothioamide (L¹), bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (L²), N-(2,5-dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (L³), 6-(6‑chloro-1,3-benzothiazol-2-yl)-N-(4-chlorophenyl)pyridine-2-carbothioamide (L⁴), 6-(4,7-dimethoxy-2-benzothiazoyl)-N-(2,5-dimethoxyphenyl)-pyridinecarboxamide (L⁵) and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (L⁶), were prepared by refluxing Cu(OAc)₂•H₂O with Lⁿ in ethanol. The copper complexes with the κ³-SNS coordination mode showed the highest catalytic enhancement and Faradaic efficiencies. The results also suggested that the methoxy substituents were slightly advantageous to the chloro substituent. Moderate overpotentials between 0.69 and 0.83 V at Faradaic yields between 77 and 97 % were obtained for the Cu(II) complexes in acetic acid. Lower overpotentials within the range of 0.57 and 0.73 V were obtained at Faradaic yields between 86 and 98 % when trifluoroacetic acid was used as the proton source. Rate constants were extracted from foot-of-the-wave analysis (FOWA) plots where an EECC mechanism is proposed for the catalytic formation of H₂.