A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis

Titanium and zirconium complexes carrying two 2,6-bis(pyrrolyl)pyridine ligands have been synthesized and characterized. The neutral complexes Ti((PDP)-P-Me)(2) and Zr((PDP)-P-Me)(2) ((PDP)-P-Me = 2,6-bis(5-methyl-phenyl-1H-pyrrol-2-Apyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr((PDP)-P-Me)(2) exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of -2.16 V vs Fc(+/0). This reactivity was utilized to facilitate dehalogenation reactions, the reduction of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr((PDP)-P-Me)(2) acts as a substitute for the precious metal photosensitizer [Ru(bpy)(3)](2+).