title: Studies of the pathways open to copper water oxidation catalysts containing proximal hydroxy groups during basic electrocatalysis.
abstract: Water oxidation can lead to a sustainable source of energy, but for water oxidation catalysts to be economical they must use earth abundant metals. We report here 2:1 6,6'-dihydroxybipyridine (6,6'-dhbp)/copper complexes that are capable of electrocatalytic water oxidation in aqueous base (pH = 10-14). Two crystal structures of the complex that contains 6,6'-dhbp and copper(II) in a ratio of 2:1 (complex 1) are presented at different protonation states. The thermodynamic acid dissociation constants were measured for complex 1, and these show that the complex is fully deprotonated above pH = 8.3 (i.e., under water oxidation conditions). CW-EPR, ENDOR, and HYSCORE spectroscopy confirmed that the 6,6'-dhbp ligand is bound to the copper ion over a wide pH range which shows how pH influences precatalyst structure. Additional copper(II) complexes were synthesized from the ligands 4,4'-dhbp (complex 2) and 6,6'-dimethoxybipyridine (complexes 3 and 4). A zinc complex of 6,6'-dhbp was also synthesized (complex 5). Crystal structures are reported for 1 (in two protonation states), 3, 4, and 5. Water oxidation studies using several of the above compounds (1, 2, 4, and 5) at pH = 12.6 have illustrated that both copper and proximal OH groups are necessary for water oxidation at a low overpotential. Our most active catalyst 1 was found to have an overpotential of 477 mV for water oxidation at a moderate rate of kcat = 0.356 s(-1) with a competing irreversible oxidation event at a rate of 1.082 s(-1). Furthermore, our combined work supports previous observations in which OH/O(-) groups on the bipyridine rings can hydrogen bond with metal bound substrate, support unusual binding modes, and potentially facilitate proton coupled electron transfer.
authors.affiliation: Department of Chemistry, The University of Alabama , Box 870336, Tuscaloosa, Alabama 35487-0336, United States.
authors.forename: Deidra L
authors.initials: DL
authors.lastname: Gerlach
authors.forename: Salome
authors.initials: S
authors.lastname: Bhagan
authors.forename: Alex A
authors.initials: AA
authors.lastname: Cruce
authors.forename: Dalton B
authors.initials: DB
authors.lastname: Burks
authors.forename: Ismael
authors.initials: I
authors.lastname: Nieto
authors.forename: Hai T
authors.initials: HT
authors.lastname: Truong
authors.forename: Steven P
authors.initials: SP
authors.lastname: Kelley
authors.forename: Corey J
authors.initials: CJ
authors.lastname: Herbst-Gervasoni
authors.forename: Katherine L
authors.initials: KL
authors.lastname: Jernigan
authors.forename: Michael K
authors.initials: MK
authors.lastname: Bowman
authors.forename: Shanlin
authors.initials: S
authors.lastname: Pan
authors.forename: Matthias
authors.initials: M
authors.lastname: Zeller
authors.forename: Elizabeth T
authors.initials: ET
authors.lastname: Papish
mesh_terms: D015082:2,2'-Dipyridyl / Q000737:chemistry*; D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D055664:Electrochemical Techniques; D008958:Models, Molecular; D010084:Oxidation-Reduction; D014867:Water / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D014867:Water; D015082:2,2'-Dipyridyl; D003300:Copper