Previously, our group discovered a new class of molecular photosensitizers based on the earth abundant metal zirconium. The complexes are highly emissive upon visible light irradiation with exceptionally long-lived 3LMCT/3LLCT excited states of several hundred microseconds. Combined with fully reversible redox events at highly reducing potentials, we were able to carry out photoredox reactions that proceed via a reductive quenching mechanism. However, the air- and water-sensitivity of these complexes made them impossible to handle under “glove-box free” conditions. Also, these zirconium compounds are unstable upon oxidation which significantly limited their applications in photoredox catalysis. Here we report our successful efforts towards synthesizing an air- and water-stable zirconium photosensitizer. A new zirconium photosensitizer, Zr(MesPDPPh)2, equipped with two 2,6-bis(5-mesityl-3-phenyl-1H-pyrrol-2-yl)-pyridine, MesPDPPh, ligands was synthesized. The sterically demanding ligand framework protects the metal center from small molecule penetration (i.e. H2O, HCl, O2), and the complex can easily be manipulated under regular benchtop conditions. Additionally, the reactive 2,5-positions of the pyrrole heterocycles are now protected by the methyl groups of the mesityl moiety, which results in reversible oxidative chemistry. After establishing the optical and electrochemical properties of Zr(MesPDPPh)2, we successfully conducted photocatalytic C-H trifluoromethylation, C-H arylation and atom transfer radical addition reactions that are commonly considered to undergo oxidative quenching pathways. We believe that this robust zirconium photosensitizer can potentially serve as a fully functional replacement for phosphorescent precious metal chromophores.

Second-generation zirconium photosensitizers: oxidative photoredox reactions with an air- and moisture-stable photocatalyst

2019 Spring National Meeting

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