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.

Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

“…33). 19 F and 31 P NMR spectroscopy before and after photolysis confirm that the [PF6]counter ions are not photooxidized as the NMR spectra remain unchanged after photolysis (Supplementary Figs. 34,35).…”

“…1,11 With respect to photooxidizing properties, many strongly photooxidizing complexes operate as inner-sphere oxidants in hydrogen atom transfer and M-X bond homolysis reactions, such as high valent oxido and halido complexes of manganese(IV), tungsten(VI), cerium(IV), uranium(VI) and copper(II), [12][13][14][15][16][17][18][19] while strong genuine single-electron outer-sphere photooxidants, which fully retain their coordination sphere, are rare. Recent advances in the field of earth-abundant photocatalysts operating as strong single-electron oxidants include zirconium(IV), 20,21 cobalt(III), 22 iron(III) [23][24][25][26][27] and chromium(III) complexes [28][29][30][31] in their respective excited states, which are of ligand-to-metal charge transfer character (LMCT) 32 for the former and of spin-flip character for chromium(III) complexes. 10,33 Photosensitizers possessing excited states with potentials more positive than +0.80 V vs. SCE (+0.42 V vs. ferrocene) 34 are considered as photosensitizers with extreme redox potentials.…”

“…35,36 The hexacarbene iron(III) complex [Fe(phtmeimb)2] + and various polypyridine chromium(III) complexes realize potentials of Ered* = 1.25 -1.84 V vs. SCE (0.87-1.46 V vs. ferrocene) (phtmeimb = [phenyl(tris(3-methylimidazolin-2ylidene))borate] − ) with blue-green light excitation. [23][24][25][26][27][28][29][30][31] The formally strongest reported photooxidant based on a first row transition metal is [Co(dgpz)2] 3+ with Ered* = 2.75 V vs. SCE (2.37 V vs. ferrocene), yet its strong oxidizing power towards challenging substrates has not yet been exploited (dgpz = 2,6diguanidylpyrazine). 22 Strong organic photooxidants encompass substituted acridinium salts, 37 2,3dichloro-5,6-dicyano-1,4-benzoquinone 38,39 and the electrochemically generated trisaminocyclopropenium radical dication with Ered* = 2.06-3.33 V vs. SCE (1.68-2.95 V vs. ferrocene).…”

Trendbericht: Photochemie

“…46) Auch hier sind die ersten chemischen Schritte die Reduktion des elektro-nisch angeregten *[Cr(tpe) 2 ] 3+ und die Oxidation des Cyclohexylborat-Substrats. 49) Der Photokatalysator [Cr(ddpd) 2 ] 3+ oxidiert bei Bestrahlung mit einer 430-nm-LED 4-Aminopyridin zum Radikalkation, das SO 2 addiert und ein Proton verliert. Das dabei gebildete [Cr(ddpd) 2 ] 2+ -Di kat ion wird durch ein Diphenyliodoniumion regeneriert.…”

“…Das hieraus entstehende Phenylradikal addiert schließlich an das radikalische SO 2 -Addukt zum Sulfonamidprodukt in bis zu 56 Prozent Ausbeute für den Gesamtprozess (Abbildung 6). 49) Molekulare Rubine bieten nicht nur Anwendungen in der Photo(redox)katalyse und Aufwärtskonversion, sondern auch in der Sensorik und der zirkular polarisierten Emission. 50) W…”