Mono-copper far more active than analogous di-copper complex for electrocatalytic hydrogen evolution

Area of Science:

• Inorganic Chemistry
• Electrochemistry
• Catalysis

Background:

• The mono-copper(II) complex [CuIILEt]BF4 (1) exhibits long-lived electrocatalytic hydrogen evolution reaction (HER) activity.
• A di-copper(II) analogue, [CuII2(bis-LEt)](BF4)2 (2), was synthesized to investigate the effect of multiple copper centers on HER.

Purpose of the Study:

• To synthesize and characterize the di-copper(II) complex [CuII2(bis-LEt)](BF4)2 (2).
• To evaluate the electrocatalytic hydrogen evolution reaction (HER) activity of the di-copper(II) complex (2) and compare it with the mono-copper(II) complex (1).

Main Methods:

• Synthesis of the bis-macrocycle ligand bis-HLEt via Schiff base condensation and alkylation.
• Preparation and characterization of the di-copper(II) complex (2) using techniques including single-crystal X-ray diffraction (SCXRD).
• Electrochemical studies including cyclic voltammetry to determine reduction potentials.
• Electrocatalytic HER testing in acetonitrile with acetic acid as a co-catalyst.

Main Results:

• The di-copper(II) complex [CuII2(bis-LEt)](BF4)2·4H2O (2·4H2O) was synthesized in 82% yield.
• SCXRD revealed square planar coordination environments for both copper(II) centers in complex 2, similar to complex 1.
• Complex 2 was easier to reduce than complex 1 (E1/2: -1.20 V vs. -1.39 V).
• Complex 2 was inactive for electrocatalytic HER under the tested conditions, unlike the highly active complex 1.

Conclusions:

• The presence of two copper(II) ions in complex 2 does not enhance, but rather abolishes, the electrocatalytic HER activity observed in the mono-copper(II) complex 1.
• The absence of a secondary amine (NH) group in the bis-macrocycle ligand of complex 2, due to its involvement in linking the two macrocycles, is proposed as the reason for the loss of HER activity.