Copper bis(thiosemicarbazone) Complexes with Pendent Polyamines: Effects of Proton Relays and Charged Moieties on Electrocatalytic HER

A series of new bis(thiosemicarbazonato) Cu(II) complexes with pendent polyamines, diacetyl‐(N, ‐dimethylethylenediaminothiosemicarbazonato)‐(N’‐methyl‐3‐thio‐semicarbazonato)butane‐2,3‐diimine)‐copper(II) (Cu‐1), diacetyl‐bis(N‐dimethylethylenediamino‐3‐thiosemicarbazonato)butane‐2,3‐diimine)‐copper(II) (Cu‐3), and their cationic derivatives Cu‐2 and Cu‐4, have been synthesized and fully characterized by spectroscopic, electrochemical, and X‐ray diffraction methods. Complexes Cu‐1–Cu‐4 are analogues of Cu(ATSM), which contains a similar N2S2 donor core with terminal non‐coordinating amines. Substitution of the methyl group(s) of the terminal amines of H2ATSM with N,N‐dimethylethylenediamine followed by alkylation generates a charged quaternary amine in the ligand framework. The charged site tunes the redox potentials of the complexes with minimal changes in their physical and electronic properties. The HER activity of all four copper complexes were evaluated in acetonitrile with glacial acetic acid. All of the complexes have lower HER overpotentials than Cu(ATSM), which is attributed to charge effects. The pendent amines of Cu‐1 and Cu‐3 have the lowest HER overpotential as the pendent tertiary amine also serves as a proton relay to enhance proton rearrangement under catalytic conditions. Complex Cu‐3 showed the highest activity with a TOF of 12 × 103 s−1, an overpotential of 0.65 V, and faradaic efficiency of 100 %. The introduction of pendent amines to copper bis(thiosemicarbazone) complexes enhances electrocatalytic HER activity through second coordination sphere effects. Optimal activity is observed with basic, tertiary pendent amines that serve as proton relays and introduction point charges upon protonation under acid saturated catalytic conditions.