Studies of the Pathways Open to Copper Water Oxidation Catalysts Containing Proximal Hydroxy Groups During Basic Electrocatalysis

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.