Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands

Abstract

We describe herein the synthesis and characterization of ruthenium complexes with multifunctional bipyridyl diphosphonate ligands as well as initial water oxidation studies. In these complexes, the phosphonate groups provide redox-potential leveling through charge compensation and σ donation to allow facile access to high oxidation states. These complexes display unique pH-dependent electrochemistry associated with deprotonation of the phosphonic acid groups. The position of these groups allows them to shuttle protons in and out of the catalytic site and reduce activation barriers. A mechanism for water oxidation by these catalysts is proposed on the basis of experimental results and DFT calculations. The unprecedented attack of water at a neutral six-coordinate [Ru(IV) ] center to yield an anionic seven-coordinate [Ru(IV) -OH](-) intermediate is one of the key steps of a single-site mechanism in which all species are anionic or neutral. These complexes are among the fastest single-site catalysts reported to date.

Supramolecular Approaches to Improve the Performance of Ruthenium‐Based Water Oxidation Catalysts

“…[21] A substantial increase in stability was indeed observed for 17, which might be explained by the aforementioned reasons. As expected, the macrocyclic catalyst 17 operates via the WNA mechanism, as it has been proven by 18 O labeling experiments, kinetic studies regarding the catalyst and oxidant concentrations, and H/D kinetic isotope effects. Interestingly, 17 exhibits a remarkably high catalytic activity under CAN driven reaction conditions for the usually slow WNA mechanism (TON = 7400; TOF = 150 s −1 ).…”

“…[18] This novel ligand system is a strong electron donor and capable of charge compensation upon oxidation of the ruthenium center by deprotonation of the phosphonic acid groups. Consequently, higher ruthenium oxidation states are more easily accessible since this complex is either neutrally or even negatively charged.…”

Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2Mn Complex

“…The lower oxidation state transitions from A to B and B to C are not observed experimentally probably owing to the low quality of the electrochemical response for this complex. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations...…”

“…[41] It is worth mentioning here that the active species, derived from A,t hat are responsible for the OÀOb ond formation, reach formal the oxidation state Ru1 IV Mn V Ru2 IV .T his is in good agreement with the OEC-PSII,w here af ormal Mn V is also neededi nt he S 4 state. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein.…”

Impact of the Ligand Flexibility and Solvent on the O–O Bond Formation Step in a Highly Active Ruthenium Water Oxidation Catalyst

“…Such an enhancement of the catalytic activity by the presence of a pendant base to abstract the proton during WNA has also been observed in a number of previous studies. 11,12 …”