Preparation and Study of a Family of Dinuclear Ru(II) Complexes That Catalyze the Decomposition of Water

Abstract

An approach is developed for the four-electron oxidation of water to provide dioxygen that involves the juxtaposition of two Ru(II) centers such that a metal-bound water molecule might interact with one or both of the metals. The key element in this approach is an appropriate bridging ligand that will hold the metal assembly intact through the full redox cycle. Various synthetic approaches to such ligands are described with the ultimate preparation of four closely related bis-tridentate polypyridine-type systems in which the bridging and distal portions of the ligand are varied. All of these ligands self-assemble with two Ru(II) centers bridged by a Cl ion in the equatorial plane and four axial monodentate substituted pyridines or N-methylimidazoles to form the well-organized catalyst complexes. These complexes are characterized by their distinctive (1)H NMR spectra as well as an X-ray structure of one representative species. The photophysical and electrochemical features of these complexes are consistent with electronegativity and delocalization effects in the equatorial and axial ligands. Of the 14 complexes studied, all but 2, which each contain four axial N-methylimidazole ligands, catalyze the decomposition of water in the presence of excess Ce(IV) as a sacrificial oxidant at pH = 1. Both the rates of oxygen evolution and the catalyst turnover numbers (TNs) are measured. For the active catalysts, the relative rates vary from 1 to 51 and the TNs measure from 80 to 689. Various analytical methods for making these measurements are discussed, and it is found that there is an approximately linear relationship between the rate and TN. Future work will involve optimization of these systems and studies aimed at a better understanding of the mechanism.

Molecular Catalysts that Oxidize Water to Dioxygen

“…Addition of Ce IV to the triflic acid solution generates a spectacular amount of dioxygen, giving turnover number of 538 with an efficiency of 23.6 % with regard to the Ce IV oxidant (The values presented are measured by GC with a thermal conductivity detector (TCD) and are much more reliable than those obtained with the electrochemically based method previously published. [33] ) In this case all ligands bonding to the metal center of the aqua complex are neutral and therefore should generate a rather different thermodynamic scenario than that obtained with 1 and/or 2. It is thus very important to study the electrochemical properties of this series of complexes so as to try to understand the pathways by which they perform the oxidation of water.…”

Mechanistic Analysis of Water Oxidation Catalyst cis-[Ru(bpy)2(H2O)2]2+: Effect of Dimerization

“…Ru-based WOCs, in particular, have been investigated for decades, and today they are still the most extensively studied group of WOCs. Currently there are many mono-Ru [4][5][6][7] and di-Ru [8][9][10][11] complexes capable of oxidizing water; however, the stability of molecular catalysts in highly oxidizing conditions is still a major issue, with most known catalysts deactivating after some time [12]. Under strongly oxidizing conditions one of the reaction pathways for single-site complexes involves the formation of dinuclear complexes [13][14][15].…”

Oxygen Production from Water: Molecular Catalysts

“…A series of related dinuclear complexes have also been prepared by the same author by slightly modifying the bridging ligand and the monodentate ligands that allow the performance of the complexes to be fine tuned with regard to their capacity to oxidize water to dioxygen. 43 In cases where the aquo ligands are involved, all ligands bonded to Ru are neutral and therefore should generate a rather different thermodynamic scenario compared to that of 1 and/or 2. It is thus very important to study the electrochemical properties of this series of complexes, in order to understand the potential pathways through which they bring about the oxidation of water.…”