The Photon Driven Reduction of Zn(II) to Zinc Metal by Transition Metal Complexes

The photon driven reduction of Zn(II) to zinc metal is a novel field of study with many potential applications including the use of zinc metal as a solar fuel. Although it has not been widely considered for such a role in the past, zinc is an appealing means to store solar energy as a stable fuel without the safety risks of hydrogen gas. As a solar fuel, zinc metal would be used in efficient zinc air batteries and then regenerated by the photocatalytic reduction of Zn(II) and a concurrent oxidation reaction. In this vein, this work reports the photocatalytic reduction of Zn(II) to zinc metal using visible light. Heteroleptic bis-cyclometallated iridium complexes and zinc quinolates that form in situ are found to be effective photocatalysts. Zinc generating photoreactions are conducted on a custom-designed home-built photoreactor, with triethylamine (TEA) employed as the sacrificial reductant. Photocatalyst structure–activity relationships, ideal reaction conditions, and reaction kinetics are described for the iridium catalyzed reaction. Cyclic voltammetry of zinc salts is used to supplement the investigation of the Zn(II) reduction mechanism. A maximum of 430 catalyst turnovers is reported. Similar experiments are performed in order to characterize the zinc quinolate catalyzed reduction of Zn(II) and confirm aspects of the reaction mechanism. Additionally, fluorescence quenching and ¹H NMR titration experiments are employed to identify the zinc quinolate that forms in situ from 5,7-dichloro-8-hydroxyquinoline. Subsequently, ligands with extended conjugated pi systems relative to 8-hydroxyquinoline are synthesized and their photocatalytic activity is assessed. Second generation iridium photocatalysts with bridging nitrile functionalities are also synthesized and evaluated in an attempt to improve catalytic activity.