If you want to produce hydrogen electrochemically, you will probably have to use the water splitting reaction. But without catalysts based on expensive and scarce metals, the half reaction of oxidising water remains a challenge. If you want to use the cheaper metals from the fourth row of the periodic table (Ni, Fe, Cu), then choosing the right ligand is essential. Daan den Boer, Dennis Hetterscheid and colleagues at Leiden University have used a number of variants of a ligand to see how the changes affect the water oxidation reaction.

The basis for the ligands is 6,6’-bis(2-aminopyridyl)-2,2’-bipyridine, which is a redox-active ligand and therefore contributes to catalysis. When it is coordinated to copper, water can be oxidised by single electron transfer hydroxide attack (SET-HA), as the researchers have previously shown. The secondary amine of the ligand seemed to play an important role in this process. To confirm this, they synthesised ligands with secondary amines that they either methylated or did not methylate.

After extensive analysis of the electrochemical properties of the ligands, the researchers arrived at three guidelines for copper-catalysed water oxidation via the SET-HA mechanism: ‘1) The redox-active ligand should contain a negative charge of at least −1 or have the ability to oxidize the ligand via PCET whereby a proton is removed from the ligand. 2) The oxidation potential of the redox-active ligands can be controlled by introduction of substituents. To decrease this potential, the electron density should be directed at the position of the ligand where oxidation occurs. 3) Stabilization of the oxidized ligand by electronic or delocalization effects decreases the oxidation potential.’

Den Boer, D. et al. (2023) EurJIC e202300475, DOI: 10.1002/ejic.202300475 (open access)