Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

We introduce a new family of complexes
with the general formula
[Run(tda)­(py)2]m+ (n = 2, m = 0, 1; n = 3, m = 1, 2+; n = 4, m = 2, 32+), with tda2– being [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylate,
including complex [RuIV(OH)­(tda-κ-N3O)­(py)2]+, 4H+, which we find
to be an impressive water oxidation catalyst, formed by hydroxo coordination
to 32+ under basic conditions. The complexes
are synthesized, isolated, and thoroughly characterized by analytical,
spectroscopic (UV–vis, nuclear magnetic resonance, electron
paramagnetic resonance), computational, and electrochemical techniques
(cyclic voltammetry, differential pulse voltammetry, coulometry),
including solid-state monocrystal X-ray diffraction analysis. In oxidation
state IV, the Ru center is seven-coordinated and diamagnetic, whereas
in oxidation state II, the complex has an unbonded dangling carboxylate
and is six-coordinated while still diamagnetic. With oxidation state
III, the coordination number is halfway between the coordination of
oxidation states II and IV. Species generated in situ have also been
characterized by spectroscopic, computational, and electrochemical
techniques, together with the related species derived from a different
degree of protonation and oxidation states. 4H+ can be generated potentiometrically, or voltammetrically, from 32+, and both coexist in solution. While complex 32+ is not catalytically active, the catalytic
performance of complex 4H+ is characterized
by the foot of the wave analysis, giving an impressive turnover frequency
record of 8000 s–1 at pH 7.0 and 50 000 s–1 at pH 10.0. Density functional theory calculations
provide a complete description of the water oxidation catalytic cycle
of 4H+, manifesting the key functional role
of the dangling carboxylate in lowering the activation free energies
that lead to O–O bond formation.