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Unraveling and optimizing oxygen-oxygen bond formation pathways promoted by highly preoganized binuclear transition metal complexes

Subject Area Inorganic Molecular Chemistry - Synthesis and Characterisation
Term from 2010 to 2014
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 164130766
 
Final Report Year 2018

Final Report Abstract

Water splitting is one of the key steps for converting sunlight into useful chemical fuels via artificial photosynthesis schemes. However, the development of rugged and highly efficient catalysts for the oxidative part of water splitting, viz. for the water oxidation reaction generating molecular dioxygen, represents a severe bottleneck in the field. At the outset of this project, a limited number of well-defined molecular transition metal complexes capable of catalyzing the thermodynamically and kinetically demanding water oxidation reaction had been described in the literature, and few were mechanistically well understood. Within this project, a new family of dinuclear ruthenium complexes called Ru2-bbp has been developed, and these complexes were shown to serve as rugged and efficient water oxidation catalysts (WOCs) when oxidizing equivalents are supplied either chemically, electrochemically, or photochemically. They are based on a compartmental pyrazole-bridged ligand (bbp) that provides two meridionally tridentate binding sites, placing two ruthenium ions at well-defined spatial distance. A versatile synthetic access towards these Ru2-bbp complexes was established, which allowed for deriving useful structure-activity correlations. Detailed mechanistic investigations were carried out for a Ru2-bbp derivative bearing peripheral sulfonate groups to impart water solubility over a wide pH range. Specifically, a Pourbaix analysis revealed the sequence of coupled electron and proton transfer steps upon oxidation of the diruthenium core to its active RuIVRuV state, and 18O labeling experiments evidenced that the key O-O bond formation occurs via the so-called water nucleophilic attack (WNA) pathway, in stark contrast the intramolecular coupling of two Ru=O units (I2M) reported for a closely related Ru 2 catalyst. These findings showed that subtle geometrical variations of the diruthenium core can switch the O–O bond formation pathway from I2M toward WNA, thus demonstrating that the different pathways may have relatively close activation barriers. In combination with a suitable photosensitizer and a sacrificial electron acceptor, the Ru2-bbp complex showed excellent photocatalytic performance with high turnover numbers and turnover frequencies, representing a new benchmark for the homogeneous light-induced water oxidation. During the course of this research project it was recognized that a reliable technique for benchmarking different molecular WOCs is needed, to be able to identify the best catalyst for a given application. For this purpose, the so-called foot of the wave analysis (FOWA) has been adapted for the electrocatalytic water oxidation reaction, and FOWA results were used to construct catalytic Tafel plots that assessed the performance of several ruthenium-based molecular WOCs as a function of overpotential under analogous conditions. In pursuing further elaborations of the preorganizing ligand scaffold, a series of new pyrazolatebased ligands featuring modified compartments for hosting two metal ions have been synthesized, and their diruthenium complexes have been investigated. Particularly promising is the Ru2-bcpp system, which is based on a new pyrazolate ligand (bcpp) equipped with peripheral carboxylate groups that provide additional anionic charges and thus lower the sequential oxidation potentials. This complex exhibits very high activity in catalytic water oxidation, which demonstrates how rational ligand design, founded on the knowledge gained in previous WOC research, leads to significantly improved catalysts. Advancement of the project benefitted greatly from the close cooperation between the two involved research groups at the University of Göttingen (Germany) and the ICIQ Tarragona (Spain), which also included mutual student exchange for research stays at the partner institute.

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