Unusual Water Oxidation Mechanism via a Redox-Active Copper Polypyridyl Complex

Abstract

To improve Cu-based water oxidation (WO) catalysts, a proper mechanistic understanding of these systems is required. In contrast to other metals, high-oxidation-state metal-oxo species are unlikely intermediates in Cu-catalyzed WO because π donation from the oxo ligand to the Cu center is difficult due to the high number of d electrons of Cu^II and Cu^III. As a consequence, an alternative WO mechanism must take place instead of the typical water nucleophilic attack and the inter- or intramolecular radical-oxo coupling pathways, which were previously proposed for Ru-based catalysts. [Cu^II(HL)(OTf)₂] [HL = Hbbpya = N,N-bis(2,2'-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active HL ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [Cu^II(L^-)(H₂O)]⁺ is the predominant species in solution. The overall WO mechanism was found to be initiated by two proton-coupled electron-transfer steps. Kinetically, a first-order dependence in the catalyst, a zeroth-order dependence in the phosphate buffer, a kinetic isotope effect of 1.0, a ΔH^⧧ value of 4.49 kcal·mol^-1, a ΔS^⧧ value of -42.6 cal·mol^-1·K^-1, and a ΔG^⧧ value of 17.2 kcal·mol^-1 were found. A computational study supported the formation of a Cu-oxyl intermediate, [Cu^II(L^•)(O^•)(H₂O)]⁺. From this intermediate onward, formation of the O-O bond proceeds via a single-electron transfer from an approaching hydroxide ion to the ligand. Throughout the mechanism, the Cu^II center is proposed to be redox-inactive.

Figure 1

Two types of O–O bond formation mechanisms.

Figure 2

Four types of SET-HA O–O bond formation mechanisms.

Figure 3

Displacement ellipsoid plot (50% probability level) of [Cu(HL)(OTf)₂] (left) and [Cu(L)(MeOH)](OTf) (right) at 110(2) K. Disorder has been removed for clarity.

Figure 4

Three CVs of Cu(HL) in 0.1 M NBu₄PF₆ in MeCN at a scan rate of 100 mV·s^–1 in varying potential windows. Cycle 1 (gray) and 3 (red) overlap exactly and were recorded between −1.0 and +0.35 V vs Fc/Fc⁺. Cycle 2 (black) was recorded between −1.0 and +1.2 V vs Fc/Fc⁺. Boron-doped diamond (BDD, 0.07 cm²), Au, and Ag/AgCl were used as the working electrode (WE), counter electrode (CE), and reference electrode (RE), respectively. Reference potentials were converted to Fc/Fc⁺.

Figure 5

(a) CV of 0.3 mM Cu(L) in a 100 mM pH 11.5 phosphate buffer at a scan rate of 100 mV·s^–1. BDD (0.07 cm²), Au, and reversible hydrogen electrode (RHE) were used as the WE, CE, and RE, respectively. Reference potentials were converted to NHE. (b) Differential-pulse voltammogram of the catalytic wave, visualizing two oxidative waves [recorded on a glassy carbon (GC) WE under the same conditions as the CV].

Figure 6

Pourbaix diagram of Cu(HL)/Cu(L) including assignments of intermediates. Data points were obtained from CVs and differential pulse voltammograms recorded in a 100 mM phosphate electrolyte solution, except for data recorded at pH 1 and 13, for which 0.1 M solutions of H₂SO₄ and NaOH were used, respectively.

Figure 7

Chronoamperometry in combination with EQCM without (a) and in the presence of 0.3 mM Cu(L) (b). Top: Chronoamperogram at a potential of 1.22 V vs NHE in a 100 mM pH 11.5 phosphate buffer. Bottom: Δ frequency response. Au, Au, and RHE were used as the WE, CE, and RE, respectively. Reference potentials were converted to NHE.

Figure 8

CVs of 0.3 mM Cu(L) in H₂O (black) and D₂O (red) in a 100 mM pH/pD 11.6 phosphate buffer at a scan rate of 100 mV·s^–1. BDD (0.07 cm²), Au, and RHE were used as the WE, CE, and RE, respectively. Reference potentials were converted to NHE.

Figure 9

Spin-density distribution in [Cu^II(L^•)(O^•)]⁺·4H₂O (surface isovalue = 0.01 au) with three unpaired spins located at the Cu^II ion, the N bridge of the ligand, and the oxyl ligand.

Figure 10

Free energy scheme of the O–O bond formation via a SET-HA mechanism starting from [Cu(L^•)(O^•)]⁺ and OH^–. The black, blue, and red lines correspond to the quartet (Q₀ and Q₁) and doublet (D₀) spin states of the intermediates given in the graph from left to right.

Figure 11

Proposed mechanistic cycle for the Cu(L)-catalyzed WO.