Lowering water oxidation overpotentials using the ionisable imidazole of copper(2-(2′-pyridyl)imidazole)

Abstract

Rapid and low overpotential oxidation of water to dioxygen remains a key hurdle for storage of solar energy. Here, we address this issue by demonstrating that deprotonation of 2-(2'-pyridyl)-imidazole (pimH)-ligated copper complexes promotes water oxidation at low overpotential and low catalyst loading. This improves upon other work on homogeneous copper-based water oxidation catalysts, which are highly active, but limited by high overpotentials. EPR and UV-vis spectroscopic evaluation of catalyst speciation shows that at pH ≥ 12 coordinated pimH is deprotonated and a bis(hydroxide) Cu active catalyst forms. Rapid electrochemical water oxidation (35 s, 0.85 V onset potential) was observed with 150 μM catalyst. These results demonstrate that catalytic water oxidation potentials can be shifted by hundreds of mV in homogeneous metal catalysts bearing an ionisable imidazole ligand.

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

“…The plausible mechanisms for the OER catalyzed by 1 – 3 are proposed on the basis of the available experimental evidence and related literatures (Scheme ) . 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, 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), or via the coupling of two metal‐oxo units (C2MO) …”

“…In addition, some NO‐bidentate and N 4 O‐pentadentate copper complexes, as well as Cu 4 clusters, have also been reported to be catalytically active molecular catalysts for OER. Several examples have been presented in the literatures for improvement of catalytic property of molecular copper catalysts through ligand modification . Recently, we explored the impact of the backbone of diamine‐dipyridine ligands on the catalytic performance of copper complexes, to have an understanding of the intrinsic correlation between the backbone structure and catalytic behavior of molecular catalysts.…”

“…We presumed that the wide open N py −Cu−N py bond angle could allow the dimerization of the two Cu III species upon oxidation of 2 . The plausible pathway is the dimerization of [(L2)Cu III (OH)] 2+ to [(L2)Cu III (μ‐OH) 2 Cu III (L2)] 4+ by the formation of two μ‐OH bridges (D2MH), followed by the PCET oxidation to build the O−O bond . This pathway avoids the formation of the high valence copper (Cu IV ) intermediate in the rate‐limiting step of OER.…”

“…[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.…”

“…Several examples have been presented in the literatures for improvement of catalytic property of molecular copper catalysts through ligand modification. [5,6,[12][13][14][15] Recently, we explored the impact of the backbone of diamine-dipyridine ligands on the catalytic performance of copper complexes, to have an understanding of the intrinsic correlation between the backbone structure and catalytic behavior of molecular catalysts. Herein, we report the electrochemical OER catalytic properties of three N 4 -coordinate copper complexes, [(L)Cu(OH 2 )](BF 4 ) 2 (1, L1 = N,N'-dimethyl-N,N'bis(pyridin-2-ylmethyl)-1,2-diaminoethane; 2, L2 = N,N'-bis(pyridin-2-ylmethyl)piperazine); 3, L3 = N,N'-bis(pyridin-2-ylmethyl) diazepane), which have the same first coordination environment but different backbones of N 4 -ligands.…”