Highly efficient and robust molecular water oxidation catalysts based on ruthenium complexes

Abstract

Two monomeric ruthenium molecular catalysts for water oxidation have been prepared, and both of them show high activities in pH 1.0 aqueous solutions, with an initial rate of over 1000 turnover s(-1) by complex 1, and a turnover number of more than 100,000 by complex 2.

Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

“…It is actually one order of magnitude higher than the best one reported so far at pH 1.0, based on Ru-bda type of complexes (see Chart 1). 32,33 Further, at pH 7.0, the complex [Ru(bda)(pic) 2 ] under exactly the same conditions as 3 + is about 3-4 orders of magnitude slower (see SI), assuming a first order behavior of the catalyst at pH = 7.0 as has been recently proposed. 38 In addition, the performance of 4 was evaluated at pH = 8.0 and 10.0, giving impressive TOFs of 25,000 s -1 and 50,000 s -1 respectively, indicating a significant rate enhancement as the pH increases.…”

“…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. 34,35,36 Finally, the presence of an external base can also strongly influence the kinetics of water oxidation reaction by facilitating proton coupled electron transfer (PCET) and deprotonation of the incoming water molecule at the O-O bond formation step as has been recently proposed using phosphate, borate or carboxylate as a base.…”

Behavior of the Ru-bda Water Oxidation Catalyst Covalently Anchored on Glassy Carbon Electrodes

“…This is in sharp contrast with the spectacular performance and stability of the catalyst in the homogeneous phase, where a TOF i close to 1000 cycles per second with an oxidative efficiency close to 100 % is observed under optimized conditions using Ce(IV) as a primary oxidant. 15,16 The radically different behavior of the supported catalyst, when compared to the complex in solution, might be due to dimerization of the complex in the homogeneous phase upon reaching the high oxidation state Ru(V) to generate the RuOORu species via an I2M mechanism and subsequently dioxygen evolution. 10 The low translational mobility of the anchored Ru complex, due to the covalent C-C bond with the graphitic surface, precludes the dimer formation and favors the water nucleophilic attack type of mechanism.…”

“…5,6,7 Water oxidation catalysts (WOCs) benefit from molecular toolkit that exploit electronic and steric effects,and can be efficiently combined to generate extremely fast, oxidatively rugged catalysts. 8,9,10,11,12,13,14,15,16 For such purpose, the effects of ligand perturbations on catalyst performance need to be fully understood, including for example changes in ligand coordination modes, hydrogen-bonding, coordination numbers, inductive effects and site isolation. Finally, molecular WOCs also benefit from an arsenal of spectroscopic techniques that can be applied to molecules and allow to derive detailed information on molecular and electronic structures.…”

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

“…4 Molecular transition metal complexes constitute an excellent platform to examine these factors since significant information based on an arsenal of spectroscopic, electrochemical and analytical techniques can be used together with the valuable complementary information provided by computational studies. 5,6,7,8,9,10,11,12,13,14 The best water oxidation catalysts reported today are based on seven coordinated Ru complexes containing dianionic ligands such as [2,2'-bipyridine]-6,6'-dicarboxylato (bda 2-) 15,16,17 and [2,2':6',2''-terpyridine]-6,6''-dicarboxylato (tda 2-) (see Figure 1 for drawn structures of these ligands). Particularly impressive is the seven coordinate complex [Ru IV (tda--N 3 O)(py)2(O) eq ], 4 IV (O), (the superscript in roman numbers indicates the formal oxidation state of Ru; py is pyridine; the "eq" superscript means equatorial) that is capable of oxidizing water to dioxygen at maximum turnover frequencies (TOFMAX) of 7,700 s -1 and 50,000 s -1 at pH = 7.0 and pH = 10.0 respectively.…”