Intermolecular O-O Bond Formation between High-Valent Ru-oxo Species

Abstract

Despite extensive research on water oxidation catalysts over the past few decades, the relationship between high-valent metal-oxo intermediates and the O-O bond formation pathway has not been well clarified. Our previous study showed that the high spin density on O in Ru^V=O is pivotal for the interaction of two metal-oxyl radical (I2M) pathways. In this study, we found that introducing an axially coordinating ligand, which is favorable for bimolecular coupling, into the Ru-pda catalyst can rearrange its geometry. The shifts in geometric orientation altered its O-O bond formation pathway from water nucleophilic attack (WNA) to I2M, resulting in a 70-fold increase in water oxidation activity. This implies that the I2M pathway is concurrently influenced by the spin density on oxo and the geometry organization of the catalysts. The observed mechanistic switch and theoretical studies provide insights into controlling reaction pathways for homogeneous water oxidation catalysis.

Figure 1

(a) Structures of water oxidation catalysts discussed in this paper; L is the axial ligand. (b) Schematic diagram of spin density and preferable geometry effects for the O–O bond formation mechanism.

Scheme 1. Synthetic Scheme of Complexes 1 and 2 and Crystal Structure of 1 with Thermal Ellipsoids at 50% Probability

Hydrogen atoms are omitted for clarity.

Figure 2

(a) Oxygen evolution vs time at various concentrations of catalyst 1 (30–200 μM) in pH 1 CF₃COOH solution containing [Ce^IV] = 0.17 M and (b) corresponding reaction order determinations.

Figure 3

Free-energy profile for the formation of two [Ru(pda)isoq₂] catalysts via the I2M reaction pathway in the water phase. The snapshots presented in the profile are a configuration of two catalysts positioned relatively far apart (1), a front-to-back configuration (2), a prereactive dimer (3), the transition state (TS), and a product (4), which were obtained from umbrella sampling and EVB-MD simulations. The TIP3P water molecules have been omitted to enhance clarity.