Second Coordination Sphere Effects in an Evolved Ru Complex Based on Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis

A new
Ru complex containing the deprotonated 2,2′:6′,2′′-terpyridine-6,6′′-diphosphonic
acid (H4tPa) and pyridine (py) of general formula [RuII(H3tPa-κ-N3O)­(py)2]+, 2+, has been prepared and
thoroughly characterized by means of spectroscopic and electrochemical
techniques, X-ray diffraction analysis, and density functional theory
(DFT) calculations. Complex 2+ presents a
dynamic behavior in the solution that involves the synchronous coordination
and the decoordination of the dangling phosphonic groups of the tPa4– ligand. However, at oxidation state IV, complex 2+ becomes seven coordinated with the two phosphonic
groups now bonded to the metal center. Further, at this
oxidation state at neutral and basic pH, the Ru complex undergoes
the coordination of an exogenous OH– group from
the solvent that leads to an intramolecular aromatic O atom insertion
into the CH bond of one of the pyridyl groups, forming the corresponding
phenoxo-phosphonate Ru complex [RuIII(tPaO-κ-N2OPOC)­(py)2]2–, 42–, where tPaO5– is the 3-(hydroxo-[2,2′:6′,2′′-terpyridine]-6,6′′-diyl)­bis­(phosphonate)
ligand. This new in situ generated Ru complex, 42–, has been isolated and spectroscopically and electrochemically
characterized. In addition, a crystal structure has been also obtained
using single-crystal X-ray diffraction techniques. Complex 42– turns out to be an exceptional water oxidation
catalyst achieving record maximum turnover
frequencies (TOFmax) on the order of 16 000 s–1. A mechanistic analysis complemented with DFT calculations
has also been carried out, showing the critical role
of intramolecular second coordination sphere effects exerted by the
phosphonate groups in lowering the activation energy at the rate-determining
step.