Structural Modifications of Mononuclear Ruthenium Complexes:  A Combined Experimental and Theoretical Study on the Kinetics of Ruthenium‐Catalyzed Water Oxidation

Abstract

Practical-efficiency catalysis of the water oxidation process (2 H 2 O!O 2 + 4 e À + 4 H + ) is the highly sought element of emerging artificial photosynthetic energy-conversion technology. [1,2] While oxygen evolution in naturally occurring photosynthesis, which supports nearly all existing life forms, relies on a sophisticated Mn-based complex, [1a, 3] the majority of artificial molecular water-oxidation catalysts (WOCs) are Ru-based complexes with relatively simple polypyridyl ligands. [4, 5] Recently emerged evidence in favor of alternative O 2 -evolving mechanisms for Ru-catalyzed water oxidation, [5] such as solvent water nucleophilic attack (WNA) and direct O À O coupling via interaction of two M-O units (I2M), [5a, 6-8] demonstrated dramatic mechanistic consequences of different ligand designs, which are yet to be fully rationalized. Understanding of intricate ligand-dependent preferences for one mechanism over the other is necessary for further progress to be made. [5, 7] Unfortunately, vast structural differences between Ru-bound ligands of WOCs which operate by the WNA or I2M mechanism hampers determinations of ligand influence on catalytic pathways.

Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

“…18,19,20,21,22 When combined with the theoretical analysis, via density functional theory (DFT) calculations, spectroscopic and electrochemical measurements provide detailed information on the nature of reaction intermediates and activation free energies along the catalytic cycle of water oxidation. 23,24,25,26,27,28,29,30 Strong sigma donation groups like carboxylate ligands in 2,2'-bipyridine-6,6'-dicarboxylic acid (H 2 bda; see Chart 1 for a drawing), together with seven coordination have allowed easy access to reactive species in high oxidation states such as [Ru V (O)(bda)(pic) 2 ] + (pic is 4-picoline) where the metal center is at formal oxidation state of V. 31 Additional tuning of the activation energy barriers can result from supramolecular interactions, based on π-π stacking of ligands with π -extended conjugation such as isoquinoline and its derivatives favoring formation of dinuclear peroxo intermediates. 32,33 Furthermore, hydrogen bonding interactions can play a significant role in the kinetics as demonstrated with strategically substituted fluro-2,2'-bpy ligands.…”

Introducing a closed system approach for the investigation of chemical steps involving proton and electron transfer; as illustrated by a copper-based water oxidation catalyst

“…[1][2][3][4] A key component of the artificial leaf is the water oxidation catalyst, the optimisation of which is an active field of research especially using abundant transition metals such as copper and iron. [5][6][7][8][9][10][11][12][13][14][15][16][17] One way of optimising a proposed water oxidation catalyst is to consider its catalytic cycle and ensuring that it is as favourable as possible with low overpotential. 18,19 Traditionally, catalytic cycles are examined computationally by comparing the free energies of the proposed catalytic intermediates and then, usually, assigning the thermodynamically most favourable cycle as the most likely catalytic cycle.…”

Control over Electrochemical Water Oxidation Catalysis by Preorganization of Molecular Ruthenium Catalysts in Self‐Assembled Nanospheres

“…[6,10] Also,a nchoring of Ru(bda)Het 2 type catalysts on glassy carbon or indium tin oxide surfaces leads to site isolation of the complexes,prohibiting catalysis via the I2M mechanism, and the same complex catalyzes water oxidation via WNAwith lower reaction rates.…”

“…[8b,c, 9] The ligand effect is subtle,a na nalogous complex Ru(phenda)-(pic) 2 (phenda = [1,10] phenanthroline-2,9-dicarboxylic acid, pic = 4-picoline) based on the rigid phenathroline ligand catalyzes water oxidation via the WNAm echanism (Scheme 1), and higher overpotentials are required to drive the reaction. [6,10] Also,a nchoring of Ru(bda)Het 2 type catalysts on glassy carbon or indium tin oxide surfaces leads to site isolation of the complexes,prohibiting catalysis via the I2M mechanism, and the same complex catalyzes water oxidation via WNAwith lower reaction rates.…”

“…Mechanistic pathwaysf or water oxidation with the final OÀ Oformation via nucleophilic attack of water to the ruthenium-oxo species (WNA, blue;that is, Ru(phenda)(pic) 2 [10] )o rv ia interaction of two metal-oxo moieties (I2M, red;t hat is, Ru(bda)(pic) 2 [8b,c] ).…”