Water Oxidation Initiated by In Situ Dimerization of the Molecular Ru(pdc) Catalyst

The mononuclear ruthenium
complex [Ru­(pdc)­L3] (H2pdc = 2,6-pyridinedicarboxylic
acid, L = N-heterocycles such
as 4-picoline) has previously shown promising catalytic efficiency
toward water oxidation, both in homogeneous solutions and anchored
on electrode surfaces. However, the detailed water oxidation mechanism
catalyzed by this type of complex has remained unclear. In order to
deepen understanding of this type of catalyst, in the present study,
[Ru­(pdc)­(py)3] (py = pyridine) has been synthesized, and
the detailed catalytic mechanism has been studied by electrochemistry,
UV–vis, NMR, MS, and X-ray crystallography. Interestingly,
it was found that once having reached the RuIV state, this
complex promptly formed a stable ruthenium dimer [RuIII(pdc)­(py)2-O-RuIV(pdc)­(py)2]+. Further investigations suggested that the present dimer,
after one pyridine ligand exchange with water to form [RuIII(pdc)­(py)2-O-RuIV(pdc)­(py)­(H2O)]+, was the true active species to catalyze water oxidation
in homogeneous solutions.

单核钌配合物（mononuclear ruthenium complex）[Ru(pdc)L3]（其中H₂pdc=2,6-吡啶二羧酸（2,6-pyridinedicarboxylic acid），L为诸如4-皮考林的氮杂环（N-heterocycles））此前在均相溶液及锚定电极表面均展现出优异的水氧化催化效能。然而，该类配合物催化的水氧化详细反应机理仍不明确。为深化对该类催化剂的认知，本研究合成了[Ru(pdc)(py)3]（py=吡啶（pyridine）），并通过电化学、紫外-可见光谱（UV–vis）、核磁共振波谱（NMR）、质谱（MS）及X射线晶体学（X-ray crystallography）对其催化机理开展了详细研究。值得注意的是，研究发现该配合物达到四价钌（RuIV）价态后，会迅速形成稳定的钌二聚体[RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+。进一步研究表明，该二聚体经一次吡啶配体与水的配体交换，生成[RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+后，即为催化均相溶液中水氧化反应的真正活性物种。