Why Is There a Barrier in the Coupling of Two Radicals in the Water Oxidation Reaction?

Two
radicals can form a bond without an energetic barrier. However,
the radical coupling mechanism in ruthenium-catalyzed water oxidation
has been found to be associated with substantial activation energies.
Here we have investigated the coupling reaction of [RuO­(bda)­L2]+ catalysts with different axial L ligands. The
interaction between the two oxo radical moieties at the Ru­(V) state
was found to have a favorable interaction in the transition state
in comparison to the prereactive complex. To further understand the
existence of the activation energy, the activation energy has been
decomposed into distortion energy and interaction energy. No correlation
between the experimental rates and the calculated coupling barriers
of different axial L was found, showing that more aspects such as
solvation, supramolecular properties, and solvent dynamics likely
play important roles in the equilibrium between the free RuVO monomer and the [RuVO···ORuV] dimer. On the basis of our findings, we give general guidelines
for the design of catalysts that operate by the radical coupling mechanism.