Single‐Site Copper(II) Water Oxidation Electrocatalysis: Rate Enhancements with HPO₄²⁻ as a Proton Acceptor at pH 8

Abstract

The complex Cu II (Py 3 P) (1) is an electrocatalyst for water oxidation to dioxygen in H 2 PO 4 À /HPO 4 2À buffered aqueous solutions. Controlled potential electrolysis experiments with 1 at pH 8.0 at an applied potential of 1.40 V versus the normal hydrogen electrode resulted in the formation of dioxygen (84 % Faradaic yield) through multiple catalyst turnovers with minimal catalyst deactivation. The results of an electrochemical kinetics study point to a single-site mechanism for water oxidation catalysis with involvement of phosphate buffer anions either through atom-proton transfer in a rate-limiting OÀO bond-forming step with HPO 4 2À as the acceptor base or by concerted electron-proton transfer with electron transfer to the electrode and proton transfer to the HPO 4 2À base.The oxidation of water to dioxygen is a key half-reaction in natural photosynthesis and a considerable challenge for artificial photosynthetic schemes aimed at converting solar energy into fuels for energy conversion and storage. Microscopically, water oxidation is necessarily complex since it involves the loss of four electrons and four protons with formation of an OÀO bond (2 H 2 O!O 2 + 4 H + + 4 e À , E 0 = 1.23 V vs. NHE, E pH 7 = 0.82 V vs. normal hydrogen electrode, NHE). Considerable progress has been made in catalyzing this reaction by using transition-metal complexes. Examples of Ru, [1] Ir, [2] Fe, [3] Co, [4] and Mn [5] complexes have been identified as single-site or multi-site catalysts in solution or immobilized on metal-oxide surfaces, or as precursors to catalytically active metal oxides or films. Practical strategies for light-driven water oxidation in solar fuel applications require robust and selective catalysts that react at appreciable rates at low overpotentials and do not compete for light absorption with an integrated chromophore or chromophore antenna. Molecular catalysts are particularly attractive in these applications since their catalytic properties can be relatively fine-tuned by systematic structural and electronic modifications, and strategies are available for incorporating them into chromophore-catalyst assemblies.A few recent reports have documented the first examples of Cu II -mediated water oxidation electrocatalysis with simple Cu II complexes or salts. [6] Cu II water oxidation catalysis is appealing since Cu is biologically relevant and abundant with a well-defined coordination chemistry.Here we report that the monomeric Cu II complex Cu II -(Py 3 P) (1, Py 3 P is N,N-bis(2-(2-pyridyl)ethyl)pyridine-2,6dicarboxamidate, Figure 1) is a stable water oxidation electrocatalyst in H 2 PO 4 À /HPO 4 2À buffered solutions. A combination of cyclic voltammetry and controlled-potential electrolysis (CPE) measurements provide experimental evidence for a single-site water oxidation mechanism involving either rate-determining atom-proton transfer or concerted electronproton transfer leading to rapid water oxidation through multiple catalytic turnovers.The electrochemical properties of 1 were explored by CV m...

Effects of Charged Ligand Substituents on the Properties of the Formally Copper(III)-Hydroxide ([CuOH]2+) Unit

“…This proved difficult in the case of the 8a/4a couple, owing to the significant overlap between the oxidative feature and the apparent onset of catalytic water oxidation under basic conditions. 28 Nevertheless, an approximate E 1/2 = 0.75 V (vs normal hydrogen electrode (NHE)) was determined (Figure S29, left) at pH = 11. In the case of the 9b/5b redox couple, an E 1/2 = 1.03 V (vs NHE) was measured at pH = 14 (Figure S29, right).…”

Redox Metal–Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes

“…The storage of oxidative equivalents (holes) by both the metal center and the ligand helps the four-electron removal needed for the water oxidation reaction and thus widens the scope of useful molecular water oxidation catalysts (WOCs). 20 Today the use of well-defined and rugged Cu WOCs is still limited to basic pH, 31,32,37,38,39,40,41,42 with very few examples reported at neutral pH featuring high overpotentials, slow kinetics and/or limited information about the long-term stability. 43,44,45 Further, a detailed analysis at a molecular level is frequently lacking, which is important to understand structure-activity relationships and the possible formation of copper oxides materials as the active catalytic species.…”

Molecular complexes in water oxidation: Pre-catalysts or real catalysts

“…CV of 36 showed two irreversible oxidation waves at 0.87 V and 1.41 V in NaPi buffer at pH 7.0, which were assigned to the oxidation of Ni(II) to Ni(III) and the further oxidation of Ni(III) to Ni(IV). Compared with other homogeneous transition metal-based WOCs (500-800 mV)[27,112,126,127], a relatively low overpotential was exhibited by 36. To verify that the electrocatalysis by 36 is a homogeneous reaction, a series of control experiments were performed: (1) Catalytic current decreased as a result of multiple CV scans and became constant finally, this is in contrast to the observed feature for heterogeneous Ni-based catalysts[122,124].…”