P4VP-Ru II (bda) polyelectrolyte-metal complex as water oxidation catalyst: on the unique slow-diffusion and multi-charge effects of the polyelectrolyte ligand.

In this work, we analyze the catalytic mechanism of P4VP-Ru II (bda) polyelectrolyte-metal complex (PMC) as a water oxidation catalyst and elucidate how the unique slow diffusion and multi-charge properties of the polyelectrolyte ligand dominate the catalytic process. Four poly(4-vinyl pyridine)-Ru(bda) (P4VP-Ru) PMCs with different chain lengths and controlled Ru loading amounts were prepared and used as catalysts for catalytic water oxidation. These catalysts present excellent catalytic performance with turnover numbers (TON) from ∼1200 to ∼1700 because of the good hydration properties. Surprisingly, the combined catalysis kinetics and kinetic isotope effect (KIE) studies for P4VP-Ru PMCs confirm the single-site water nucleophilic attack (WNA) mechanism in catalysis, rather than the interaction between two metal oxide units (I2M). A combination of dynamic light scattering characterization, zeta-potential measurement and molecular dynamics simulation reveals that the slow diffusion and multi-charge properties of the polyelectrolyte ligand are responsible for the observed mechanism difference between the P4VP-Ru PMC system and small-molecule multi-nuclear system, though the two systems actually own a similar structural feature (flexible linkages between Ru centers). Our experimental and simulation results highlight the fact that though the existence of flexible linkages between Ru centers could provide large conformation entropy for the occurrence of Ru-dimerization in small-molecule and neutral polymer systems, the entropy elasticity could not overcome the electrostatic interaction energy in the PMC system. Clearly, this work unambiguously clarified why both intra-chain and inter-chain Ru-dimerization (I2M) are prohibited for the PMC system from a perspective of macromolecular chemistry and physics.