Toward Controlling Water Oxidation Catalysis: Tunable Activity of Ruthenium Complexes with Axial Imidazole/DMSO Ligands

Using the combinations of imidazole and dimethyl sulfoxide
(DMSO)
as axial ligands and 2,2′-bipyridine-6,6′-dicarboxylate
(bda) as the equatorial ligand, we have synthesized six novel ruthenium
complexes with noticeably different activity as water oxidation catalysts
(WOCs). In four Cs symmetric
RuII(κ3-bda)­(DMSO)­L2 complexes
L = imidazole (1), N-methylimidazole
(2), 5-methylimidazole (3), and 5-bromo-N-methylimidazole (4). Additionally, in two C2v symmetric RuII(κ4-bda)­L2 complexes L = 5-nitroimidazole
(5) and 5-bromo-N-methylimidazole (6), that is, fully equivalent axial imidazoles. A detailed
characterization of all complexes and the mechanistic investigation
of the catalytic water oxidation have been carried out with a number
of experimental techniques, that is, kinetics, electrochemistry and
high resolution mass spectrometry (HR-MS), and density functional
theory (DFT) calculations. We have observed the in situ formation
of a RuII-complex with the accessible seventh coordination
position. The measured catalytic activities and kinetics of complex 1–6 revealed details about an important
structure–activity relation: the connection between the nature
of axial ligands in the combination and either the increase or decrease
of the catalytic activity. In particular, an axial DMSO group substantially
increases the turnover frequency of WOCs reported in the article,
with the ruthenium-complex having one axial 5-bromo-N-methyl-imidazole
and one axial DMSO (4), we have obtained a high initial
turnover frequency of ∼180 s–1. DFT modeling
of the binuclear reaction pathway of the O–O bond formation
in catalytic water oxidation further corroborated the concept of the
mechanistic significance of the axial ligands and rationalized the
experimentally observed difference in the activity of complexes with
imidazole/DMSO and imidazole/imidazole combinations of axial ligands.