Visible-light induced fixation of SO2 into organic molecules with polypyridine chromium(III) complexes

Abstract

Incorporation of sulfur dioxide into organic compounds is achieved by a photocatalytic approach using sensitizers made from earth-abundant chromium(III) ions and visible light leading to sulfones and sulfonamides. We employed three different chromium(III) sensitizers [Cr(ddpd)2]3+, [Cr(bpmp)2]3+ and [Cr(tpe)2]3+ with long excited state lifetimes and different ground and excited state redox potentials as well as varying stability under the reaction conditions (ddpd=N,N’-dimethyl-N,N’-dipyridin-2-yl-pyridine-2,6-diamine; bpmp=2,6-bis(2-pyridylmethyl)pyridine; tpe=1,1,1-tris(pyrid-2-yl)ethane). Key reaction steps of the catalytic cycles are identified by electrochemical, luminescence quenching, photolysis, laser flash photolysis and catalytic experiments delivering a detailed picture of the challenges in these transformations. The reactivity of the reduced chromium complex was identified as a key property to explain the reaction outcomes. Initial cage escape yield determinations with [Cr(tpe)2]3+ revealed that desired photoreactions occur with unusually high quantum efficiencies, whereas side reactions are almost unproductive.