The catalytic activity of the bimolecular reaction was affected by many parameters. Although many efforts have been dedicated to investigate the influence of secondary interactions in pre-organizing catalysts, the hydrophobic effect on Ru-bda-type water oxidation catalysts remains unclear as a result of the lack of an ideal catalytic model. In this work, four catalysts 1–4 with variable hydrophobicity have been synthesized, and cerium(IV)-driven water oxidation results showed that the hydrophobic complexes 3 and 4 outperformed the hydrophilic complex 2. Steric mapping, nuclear magnetic resonance, and differential pulse voltammogram studies indicated that the increase in activity has no correlation with electronic and steric effects but has correlation with hydrophobicity. Molecular dynamics have shown that the modifications of the hydrophobicity on the axial pyridine ligands of the Ru-bda type of catalysts can improve the water oxidation catalytic activity by stabilizing the pre-reactive catalyst dimer.

Water provides an ideal source for the production of protons and electrons required for generation of renewable fuels. Among the most-prominent electrocatalysts capable of water oxidation at low overpotentials are Ru(bda)L-2-type catalysts. Although many studies were dedicated to the investigation of the influence of structural variations, the true implication of the bda backbone on catalysis remains mostly unclarified. In this work, we further investigated if electronic effects are contributing to catalysis by Ru(bda)(pic)(2) or if the intrinsic catalytic activity mainly originates from the structural features of the ligand. Through introduction of pyrazines in the bda backbone, forming Ru(N-1-bda)(pic)(2) and Ru(N-2-bda)(pic)(2), electronic differences were maximized while minimizing changes in the geometry and other intermolecular interactions. Through a combination of electrochemical analysis, chemical oxygen evolution, and density functional theory calculations, we reveal that the catalytic activity is unaffected by the electronic features of the backbone and that the unique bimolecular reactivity of the Ru(bda)L-2 family of catalysts thus purely depends on the spatial geometry of the ligand.

Interaction of two metal–oxyl radicals (I2M) mechanism can theoretically provide water oxidation catalysts with lower overpotentials because they avoid forming the high-energy metal-OOH intermediate. However, only two Ru-based catalytic systems have been reported involving intermolecular I2M pathway. Herein, a Ru-pda-type (pda is 1,10-phenanthroline-2,9-dicarboxylate acid) water oxidation catalyst was designed and synthesized. Through synergistic modulation of oxo spin-density and organizational entropy, mechanism switching from the water nucleophilic attack (WNA) to I2M was realized, accompanied by a rate increase of around two orders of magnitude.

Isolation of RuIII-bda (17-electron specie) complex with an aqua ligand (2-electron donor) is challenging due to violation of the 18-electron rule. Although considerable efforts have been dedicated to mechanistic studies of water oxidation by the Ru-bda family, the structure and initial formation of the RuIII-bda aqua complex are still controversial. Herein, we challenge this often overlooked step by designing a pocket-shape Ru-based complex 1. The computational studies showed that 1 possesses the crucial hydrophobicity at the RuV(O) state as well as similar probability of access of terminal O to solvent water molecules when compared with classic Ru-bda catalysts. Through characterization of single-crystal structures at the RuII and RuIII states, a pseudo seven-coordinate “ready-to-go” aqua ligand with RuIII...O distance of 3.62 Å was observed. This aqua ligand was also found to be part of a formed hydrogen-bonding network, providing a good indication of how the RuIII-OH2 complex is formed.

The outer coordination sphere of metalloenzyme often plays an important role in its high catalytic activity, however, this principle is rarely considered in the design of man-made molecular catalysts. Herein, four Ru-bda based molecular water oxidation catalysts with well-defined outer spheres are designed and synthesized. Experimental and theoretical studies showed that the hydrophobic environment around the Ru center could lead to thermodynamic stabilization of the high-valent intermediates and kinetic acceleration of the proton transfer process during catalytic water oxidation. By this outer sphere stabilization, a 6-fold rate increase for water oxidation catalysis has been achieved.