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(^MePDP)₂ and Zr­(^MePDP)₂ (^MePDP = 2,6-bis­(5-methyl-3-phenyl-1H-pyrrol-2-yl)­pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr­(^MePDP)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(^MePDP)₂ acts as a substitute for the precious metal photosensitizer [Ru­(bpy)₃]<sup>2+