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]2+.