New Series of Dinuclear Ruthenium(II) Complexes Synthesized Using Photoisomerization for Efficient Water Oxidation Catalysis

A new series of proximal,proximal-[Ru2(tpy)2(L)­XY]n+ (p,p-Ru2XY, tpy = 2,2′:6′,2″-terpyridine,
L = 5-phenyl-2,8-di­(2-pyridyl)-1,9,10-anthyridine, X and Y = other
coordination sites) were synthesized using photoisomerization of a
mononuclear complex. The p,p-Ru2XY complexes undergo
unusual reversible bridge-exchange reactions to generate p,p-Ru2(μ-Cl), p,p-Ru2(μ-OH), and p,p-Ru2(OH)­(OH2) with
μ-Cl, μ-OH, as well as hydroxo and aquo ligands at X and
Y sites of p,p-Ru2XY, respectively. The geometric
and electronic structures of these complexes were characterized based
on UV–vis and 1H NMR spectra, X-ray crystallography,
and density functional theory (DFT) calculations. 1H NMR
data showed C2 symmetry of p,p-Ru2(OH)­(OH2) with the distorted L chelate
and nonequivalence of two tpy ligands, in contrast to the C2v symmetry of p,p-Ru2(μ-Cl) and p,p-Ru2(μ-OH). However,
irrespective of the lower symmetry, p,p-Ru2(OH)­(OH2) is predominantly formed in neutral
and weakly basic conditions due to the specially stabilized core structure
by multiple hydrogen-bond interactions among aquo, hydroxo, and backbone
L ligands. The electrochemical data suggested that p,p-Ru2(OH)­(OH2) (RuII–OH:RuII–OH2) is oxidized to
the RuIII–OH:RuIII–OH state at
0.64 V vs saturated calomel electrode (SCE) and further to RuIVO:RuIVOH at 0.79 V by successive
1-proton-coupled 2-electron processes at pH 7.0. The cyclic voltammogram
data exhibited that the p,p-Ru2(OH)­(OH2) complex works more efficiently for electrocatalytic
water oxidation, compared with a similar mononuclear complex distal-[Ru­(tpy)­(L)­OH2]2+ (d-RuOH2) and p,p-Ru2(μ-Cl) and p,p-Ru2(μ-OH), showing that the p,p-Ru2 core structure with aquo and hydroxo ligands
is important for efficient electrocatalytic water oxidation. Bulk
electrolysis of the p,p-Ru2(OH)­(OH2) solution corroborated the electrocatalytic cycle involving
the RuIII–OH:RuIII–OH state species
as a resting state. The mechanistic insight into O–O bond formation
for O2 production was provided by the isotope effect on
electrocatalytic water oxidation by p,p-Ru2(OH)­(OH2) and d-RuOH2 in H2O and D2O media.