From Ru-bda to Ru-bds: a step forward to highly efficient molecular water oxidation electrocatalysts under acidic and neutral conditions

Significant advances during the past decades in the design and studies of Ru complexes with polypyridine ligands have led to the great development of molecular water oxidation catalysts and understanding on the O-O bond formation mechanisms. Here we report a Ru-based molecular water oxidation catalyst [Ru(bds)(pic)(2)] (Ru-bds; bds(2-) = 2,2-bipyridine-6,6 ' -disulfonate) containing a tetradentate, dianionic sulfonate ligand at the equatorial position and two 4-picoline ligands at the axial positions. This Ru-bds catalyst electrochemically catalyzes water oxidation with turnover frequencies (TOF) of 160 and 12,900s(-1) under acidic and neutral conditions respectively, showing much better performance than the state-of-art Ru-bda catalyst. Density functional theory calculations reveal that (i) under acidic conditions, the high valent Ru intermediate Ru-V=O featuring the 7-coordination configuration is involved in the O-O bond formation step; (ii) under neutral conditions, the seven-coordinate Ru-IV=O triggers the O-O bond formation; (iii) in both cases, the I2M (interaction of two M-O units) pathway is dominant over the WNA (water nucleophilic attack) pathway. Developing efficient molecular water oxidation catalysts for artificial photosynthesis is a challenging task. Here the authors introduce a ruthenium based complex with negatively charged sulfonate groups to effectively drive water oxidation under both acidic and neutral conditions.

Automated summary

In this study, a ruthenium-based molecular water oxidation catalyst with negatively charged sulfonate groups was introduced, showing efficient performance under both acidic and neutral conditions. The high valent Ru intermediate Ru-V=O and seven-coordinate Ru-IV=O were found to be involved in the O-O bond formation step under acidic and neutral conditions, respectively. In both cases, the I2M pathway was dominant over the WNA pathway for O-O bond formation.