Electrocatalytic Water Oxidation by a Water-Soluble Copper(II) Complex with a Copper-Bound Carbonate Group Acting as a Potential Proton Shuttle

Abstract

Water-soluble copper(II) complexes of the dianionic tridentate pincer ligand N,N'-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate (L) are catalysts for water oxidation. In [L-Cu-DMF] (1, DMF = dimethylformamide) and [L-Cu-OAc] (2, OAc = acetate), ligand L binds Cu through three N atoms, which define an equatorial plane. The fourth coordination site of the equatorial plane is occupied by DMF in 1 and by OAc in 2. These two complexes can electrocatalyze water oxidation to evolve O in 0.1 M pH 10 carbonate buffer. Spectroscopic, titration, and crystallographic studies show that both 1 and 2 undergo ligand exchange when they are dissolved in carbonate buffer to give [L-Cu-COH] (3). Complex 3 has a similar structure as those of 1 and 2 except for having a carbonate group at the fourth equatorial position. A catalytic cycle for water oxidation by 3 is proposed based on experimental and theoretical results. The two-electron oxidized form of 3 is the catalytically active species for water oxidation. Importantly, for these two oxidation events, the calculated potential values of E = 1.01 and 1.59 V vs normal hydrogen electrode (NHE) agree well with the experimental values of E = 0.93 and 1.51 V vs NHE in pH 10 carbonate buffer. The potential difference between the two oxidation events is 0.58 V for both experimental and calculated results. With computational evidence, this Cu-bound carbonate group may act as a proton shuttle to remove protons for water activation, a key role resembling intramolecular bases as reported previously.

Electrochemical Water Oxidation Catalyzed by N4‐Coordinate Copper Complexes with Different Backbones: Insight into the Structure‐Activity Relationship of Copper Catalysts

“…[31] The plausible mechanisms for the OER catalyzed by 1-3 are proposed on the basis of the available experimental evidence and related literatures (Scheme 1). [18,20,23] In the light of the results of kinetic studies, the OER catalyzed by 1 and 3 is proposed to proceed via a unimolecular mechanism of water nucleophilic attack (WNA), as reported for most mononuclear copper catalysts, [8,[18][19][20]23] while the second-order kinetic property of the OER catalyzed by 2 suggests a bimolecular catalytic mechanism, either via the dimerization of two metal-hydroxyl units (D2MH), [4,6,9,13] or via the coupling of two metal-oxo units (C2MO). [11,32] According to the estimated pK a value (10.5) of similar N 4coordinate copper complexes reported previously, [15] the coordinated water molecule in 1À 3 could not be deprotonated in neutral PBSs.…”

“…[2] In recent years, molecular copper catalysts have drawn special attention for water oxidation due to their diverse redox properties and well-defined coordination chemistry, as well as their low cost and earth abundancy. [3] Most reported molecular copper catalysts are the complexes of bi-, [4][5][6][7] tri-, [8,9] tetra-, [10][11][12][13][14][15][16][17][18][19] and pentadentate nitrogen ligands. [20,21] In addition, some NObidentate [22] and N 4 O-pentadentate copper complexes, [23,24] as well as Cu 4 clusters, [25,26] have also been reported to be catalytically active molecular catalysts for OER.…”

Catalysis of a Single Transition Metal Site for Water Oxidation: From Mononuclear Molecules to Single Atoms

“…These ligands are directly attached to the high‐valent TM ions, forming stable four‐coordinated TMN 4 units ( Figure a) as water oxidation centers . Besides, additional oxygen‐containing groups (e.g., carbonate, acetate) may occupy partially the coordination sites in the equatorial or axial position of the ligands, so TM ions can bond with N and O atoms simultaneously, which are also the active sites for water oxidation by SMMCs . In the meanwhile, introducing additional substituents (R) into the second coordination sphere helps decrease the electron density of the macrocycle, and the catalytic property of SMMCs can be thus improved .…”

Ultra‐thin Co−Fe Layered Double Hydroxide Hollow Nanocubes for Efficient Electrocatalytic Water Oxidation

“…As the increasing fossil fuel consumption, electrochemical water splitting is a promising solution for the problem . Because of the sluggish kinetics and large overpotential resulting from the complex oxidation pathway, oxygen evolution reaction (OER) is regarded as the main bottleneck for water splitting . Along with the fact, many endeavours have been developed to invented efficient electrocatalysts to reduce over‐potential and speed up the OER kinetics.…”