Concerted O atom–proton transfer in the O—O bond forming step in water oxidation

Abstract

As the terminal step in photosystem II, and a potential half-reaction for artificial photosynthesis, water oxidation (2H 2 O → O 2 þ 4e − þ 4H þ ) is key, but it imposes a significant mechanistic challenge with requirements for both 4e − ∕4H þ loss and O-O bond formation. Significant progress in water oxidation catalysis has been achieved recently by use of single-site Ru metal complex catalysts such as ½RuðMebimpyÞðbpyÞðOH 2 Þ 2þ [Mebimpy ¼ 2,6-bisð1-methylbenzimidazol-2-ylÞpyridine; bpy ¼ 2,2 0 -bipyridine]. When oxidized from Ru II -OH 2 2þ to Ru V ¼ O 3þ , these complexes undergo O-O bond formation by O-atom attack on a H 2 O molecule, which is often the rate-limiting step. Microscopic details of O-O bond formation have been explored by quantum mechanical/molecular mechanical (QM/MM) simulations the results of which provide detailed insight into mechanism and a strategy for enhancing catalytic rates. It utilizes added bases as proton acceptors and concerted atom-proton transfer (APT) with O-atom transfer to the O atom of a water molecule in concert with proton transfer to the base (B). Base catalyzed APT reactivity in water oxidation is observed both in solution and on the surfaces of oxide electrodes derivatized by attached phosphonated metal complex catalysts. These results have important implications for catalytic, electrocatalytic, and photoelectrocatalytic water oxidation.water split | O-O coupling | base effect | isotope effect I n natural photosynthesis, and in many schemes for artificial photosynthesis, water oxidation (is a key reaction with requirements for both 4e − ∕4H þ loss and O -O bond formation. Significant progress in water oxidation catalysis has been achieved recently by using single-site molecular catalysts (1-8). This includes elucidation of a mechanism in Ce (IV) catalyzed water oxidation by ½RuðtpyÞðbpmÞðOH 2 Þ 2þ and ½RuðtpyÞðbpzÞðOH 2 Þ 2þ (tpy ¼ 2; 2 0 ∶6 0 ; 2 00 -terpyridine; bpm ¼ 2; 2 0 -bipyrimidine; bpz ¼ 2; 2 0 -bipyrazine), Scheme 1 (1). Although undergoing multiple turnovers unchanged, these catalysts are relatively slow with O-O bond formation, often the rate-limiting step. In Scheme 1, O-atom transfer from ½Ru V ðtpyÞðbpmÞðOÞ 3þ to H 2 O is rate limiting and occurs with kð0.1 M HNO 3 ; 25°CÞ ¼ 8.9 × 10 −3 s −1 (5). To put rate into perspective, in a practical solar energy conversion scheme, a turnover rate on the millisecond or submillisecond time scale is required to match or exceed the rate of solar insolation. Achieving rates of this magnitude poses a considerable challenge (9).A useful strategy for achieving faster rates in metal complex catalysts exists based on an interplay between mechanism and systematic synthetic modifications. Ligand variations can be used to modify redox potentials, increase driving force, and decrease barriers (6). Mechanistic insight can uncover previously undescribed reaction pathways. We report here the use of a previously unidentified pathway to achieve greatly enhanced rates of electrocatalytic water oxidation for the catalyst, ½RuðMebim...

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

“…This suggests that in this concentration regime the catalyst operates via the I2M reaction mechanism with the O−O coupling as the RDS. In line with this, a low secondary isotope effect (KIE<1.5) is observed (Figure 2 b) 21. When the local catalyst concentration is higher than 0.27  m , the reaction becomes first order with respect to local catalyst concentration (Figure 2 a), while the KIE increases in this window (1.5–2.2; Figure 2 b).…”

Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

“…Retention of electrocatalytic reactivity on the surface is demonstrated and water oxidation catalysis investigated over a wide pH range. Clear evidence is found in these studies that added proton acceptor bases enhance the kinetic pathways in the key, rate-limiting step (O-O bond formation) via an atom-proton transfer (APT) mechanism (22,35). In addition, a facile pathway has been identified with direct attack by OH − on an activated oxo form of the catalyst with rate enhancements of up to 10 6 for water oxidation.…”

“…Based on kinetic isotope effects (35) and quantum mechanics/molecular mechanics (QM/MM)-minimal free-energy path method calculations (42), O-O bond formation occurs in concert with solvation of the released proton by a neighboring water molecule or water cluster (35). However, water is a poor proton acceptor base (pK a = −1.74 for H 3 O + ) and catalysis is enhanced with the added proton acceptor bases HPO 2− 4 and CH 3 COO − .…”

“…Surface-bound carboxylates are typically unstable to hydrolysis in water, whereas phosphonates are unstable in neutral or basic solutions (27,34). For water oxidation catalysis this is particularly detrimental given the accelerated rates that are accessible for catalytic water oxidation as the pH is increased due to the intervention of base-catalyzed pathways with concerted atom-proton transfer accompanying O-O bond formation (35).…”

Revisit of Unusual Ruthenium(III) Dichloro Complex

“…1 Especially oxo-metal reagents in high oxidation state have been found to be versatile stoichiometric and/or catalytic oxidants toward a variety of organic and inorganic substrates via electron transfer, oxygen atom transfer, hydride transfer, and proton-coupled electron transfer pathways. 2 Among multiple ruthenium species appeared in the syntheses and mechanistic studies, Ru(III) complex is quite interesting material to investigate. However, there is a complication since Ru(III), once formed, undergoes disproportionation reaction to give Ru(II) and Ru(IV).…”