Spectroscopic Analysis of Catalytic Water Oxidation by [Ru^II(bpy)(tpy)H₂O]²⁺ Suggests That Ru^V═O Is Not a Rate-Limiting Intermediate

Abstract

Modern chemistry's grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru(II)(bpy)(tpy)H2O](2+) (where bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine). Our results reveal that the [Ru(V)(bpy)(tpy)═O](3+) intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru(IV)(bpy)(tpy)═O](2+). [Ru(V)(bpy)(tpy)═O](3+) was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru(III)-peroxo species with (-OOH or -OO- ligands). We also analyzed the [Ru(II)(bpy)(tpy)Cl](+) catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru(III) state is impeded by a lack of proton-coupled electron transfer. Ru-Cl exchange with water is required to form active catalysts with the Ru-H2O fragment. [Ru(II)(bpy)(tpy)H2O](2+) is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru(IV)(bpy)(tpy)═O](2+) intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru(V) intermediate.

Unexpected ligand lability in condition of water oxidation catalysis

“…All solutions of [Ru(bpy)(tpy)I]I were used immediately to avoid exchange of RuAI with water. The spectroscopic behavior of [Ru II (bpy)(tpy)I] + is found to be critically different from the one reported for the similar [Ru II (bpy)(tpy)Cl] + complex [8]. Fig.…”

“…with one equiv of Ce IV results in full conversion to corresponding Ru III complexes with distinct EPR signals [8], the addition of 1 equiv of Ce IV to [Ru(bpy)(tpy)I]I results in an EPR silent sample. The addition of 2-7 equiv to solutions of [Ru(bpy)(tpy)I]I results in an EPR signal with a g-tensor identical to [Ru III (bpy)(tpy)H 2 O] 3+ , with a maximal intensity detected with 5 equiv of Ce IV ( Fig.…”

“…However, the process requires an efficient and durable water oxidation catalyst. Mononuclear Ru-based water oxidation catalysts are currently under active investigation in an effort to uncover the fundamental principles controlling their catalytic activity [3][4][5][6][7][8][9][10][11][12][13]. A mononuclear Ru-based catalyst usually contains polypyridine ligands and one water molecule coordinated with Ru.…”

“…This process produces the oxidized Ru IV @O and Ru V @O species [8,[15][16][17][18]. However, some water oxidation catalysts have been reported which are seemingly incapable of PCET as they lack a ligand suitable for deprotonation, such as water [3,19].…”

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

“…Highly oxidized intermediates (containing Ru IV =O fragment) are produced via proton-coupled electron transfer (PCET) which requires the presence of a ligand that can be deprotonated (usually water). [25,27]. This implies at least one Ru-H 2 O fragment is required in order for the complex to be an active catalyst.…”