Herein, we report the synthesis and species distribution of copper(ii) complexes based on two different ligand scaffolds and the application of the two complexes in the electrochemical proton reduction catalysis. The ligands bind to one or two copper(II) ions and the pH-dependent mono/dinuclear equilibrium depends on the steric bulk of the ligands. The two water soluble copper(II) complexes were investigated for their activities in the electrochemical hydrogen evolution reaction (HER). In both complexes the copper(ii) ions have a N4-coordination environment composed of N-heterocycles, although in different coordination geometries (SPY-5 and TBPY-5). The solutions of the complexes were highly active catalysts in water at acidic pH but the complexes decompose under catalytic conditions. They act as precursors for highly active copper(0) and Cu2O deposits at the electrode surface, which are in turn the active catalysts. The absence or presence of the ligands has neither an influence on the catalytic activity of the solutions nor an influence on the activity of the deposit formed during controlled potential electrolysis. Finally, we can draw some conclusions on the stability of copper catalysts in the aqueous electrochemical HER.
The synthesis of three new ligands and their coordination behavior towards zinc ions with strongly coordinating anions and cobalt ions with weakly coordinating anions are reported. The ligands have two adjacent imidazolyl‐pyridinyl and pyrazolyl‐pyridinyl binding pockets, respectively, which are linked by a phenol unit. We also investigated the dynamic behavior of the ligand having the imidazolyl‐pyridiyl sidearm in solution. The reaction of the ligands and ZnCl2 yielded complexes of the type [LZn2Cl3]. When we used CoII salts with weakly coordinating anions, complexes of the general formula [L2Co2]2+ were formed.
Herein, we report the synthesis of a new ligand, which is preorganised for the complexation of two metal ions and has two 1H‐imidazolyl units in the outer ligand sphere. The zinc and cobalt complexes with the ligand were synthesised and fully characterised, and their solid‐state structures were determined. The complexes show a highly symmetric [M2L2]2+ structural motif. The thermodynamic data, that is, the overall stability constants and redox potential of the Co2+/Co3+ couple, were determined. Potentiometric studies revealed the presence of mononuclear (below pH 7) and differently protonated dimeric species (above pH 7). The formation of distinct species was corroborated by ESI‐MS measurements. Similar species were obtained for the corresponding zinc complexes. Additionally, we determined the dependence of the Co2+/Co3+ redox couple on the protonation state of the complex. The studies showed that there is a considerably strong thermodynamic coupling between the NH protons of the imidazolyl units and the oxidation state of the metal ion. Upon deprotonation of all four NH units, the potential of the redox couple shifted by ca. 1.1 V. We also synthesised the analogous ligand with N‐methylimidazolyl units to investigate the influence of the NH protons on the properties of the complexes. The methylation of the NH unit has virtually no influence on the redox, magnetic and structural properties of the zinc and cobalt complexes.
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