Visible-Light Induced Fixation of SO2 into Organic Molecules with Polypyridine Chromium(III) Complexes

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.

Automated summary

In this study, a photocatalytic approach using sensitizers made from earth-abundant chromium(III) ions and visible light was employed to incorporate sulfur dioxide into organic compounds, leading to the formation of sulfones and sulfonamides. Three different chromium(III) sensitizers were used, and their reactivity and stability under the reaction conditions were investigated. Through various experiments, the key reaction steps and challenges in these transformations were identified, and the reactivity of the reduced chromium complex was found to be a crucial factor determining the reaction outcomes.