Metal-Catalyzed Photooxidation of Flavones in Aqueous Media

Abstract

Soluble model compounds, such as flavones, are frequently employed in initial and mechanistic studies under homogeneous conditions in the search for effective bleaching catalysts for raw cotton. The relevance of model substrates, such as morin and chrysin, and especially their reactivity with manganese catalysts [i.e. in combination with 1,4,7-triazacyclononane (tacn) based ligands] applied in raw cotton bleaching with H₂O₂ in alkaline solutions is examined. We show that morin, used frequently as a model, is highly sensitive to oxidation with O₂, by processes catalyzed by trace metal ions, that can be accelerated photochemically, although not involve generation of ¹O₂. The structurally related chrysin is not susceptible to such photo-accelerated oxidation with O₂. Furthermore, chrysin is oxidized by H₂O₂ only in the presence of a Mn-tacn based catalyst, and does not undergo oxidation with O₂ as terminal oxidant. Chrysin mimics the behavior of raw cotton's chromophores in their catalyzed oxidation with H₂O₂, and is likely a mechanistically relevant model compound for the study of transition metal catalysts for dye bleaching catalysts under homogeneous conditions.

Keywords: Bleaching; Chrysin; Manganese; Morin; Oxidation; Photochemistry.

Figure 1

Structure of 1, 2, the ligands Me₄dtne (L) and tmtacn (L′), and Na₅DTPA.

Figure 2

General structure and numbering scheme of flavonoids under investigation.

Figure 3

UV/Vis absorption spectra of morin (40 µm, black) and chrysin (40 µm, red) in water (with 10 mm NaHCO₃) at pH 11.

Figure 4

Raman spectrum of chrysin in the solid state (black, λ _exc 785 nm), and resonance Raman spectrum in water (1 mm, pH 10, red, at λ _exc 266 nm).

Figure 5

Absorbance at 412 nm of Morin (40 µm) in water (10 mm NaHCO₃), pH 10.2, over time during repeated spectral acquisition (at 5 s intervals with 0.5 s exposure time, blue) and after 1 h for a sample held in dark (shown as a red square). The dashed line indicates the initial absorbance.

Figure 6

Absorbance at 412 nm of morin [40 µm] in water (10 mm NaHCO₃) at pH 10.2, without (red), and with (black) Na₅DTPA (5 µm). Spectra recorded at 30 s intervals (1 s exposure time) over 5 h. See also Figure S4.

Figure 7

(Top) UV/Vis absorption spectra of morin (40 µm) with 1 (1 µm), over time (spectra recorded at 50 s intervals for 30 min, for clarity only the spectra at 100 s intervals are shown) at pH 11.4, (10 mm NaHCO₃) at 23 °C. (Bottom) Raman spectra (λ _exc 355 nm) of morin (40 µm) with 1 (1 µm) after 0 min (blue) and 100 min (red), at pH 10.

Figure 8

Absorbance at 405 nm (band c in Figure 7), at pH 8.2 (blue), 9.4 (red), 10.2 (green) and 11.4 (purple) over time of morin (40 µm) with 1 (1 µm), NaHCO₃ (10 mm), 23 °C, and with O₂ (200–350 µm) as terminal oxidant. For changes at ca. 275 nm, and 316 nm (band a and b, respectively, in Figure 7) see Figure S6.

Figure 9

Absorbance at 412 nm (blue) and 417 nm (red) of morin (40 µm) with 1 (1 µm), NaHCO₃(aq) (10 mm) at pH 10 and at pH 11, respectively, over time after addition of H₂O₂ (200 µm) at 23 °C.

Figure 10

Absorbance at 359 nm of chrysin (40 µm, red at pH 10.2 and blue at pH 11.0) and at 412 and 417 nm of morin (40 µm, purple at pH 10.2 and green at pH 11.0, respectively) over time in air equilibrated solution with 1 (1 µm), in aqueous NaHCO₃ (10 mm) at 23 °C.

Scheme 1

Equilibrium between 1 and the µ‐acetate dissociated forms (1a and 1b) formed at high pH.15 and the carbonate bound complex 1c.

Figure 11

Absorbance at 359 nm vs. time showing the extent of oxidation of chrysin with O₂ or H₂O₂ at pH = 10 (left), at pH = 11 (right), (I) 1 under air (red), (II) 1 with H₂O₂ (blue). Conditions: chrysin (40 µm), catalyst (1 µm), H₂O₂ (200 µm), in aqueous NaHCO₃ (10 mm), at 23 °C.

Figure 12

Change in absorption at 358 nm for chrysin [40 µm] with H₂O₂ [10 mm] in H₂O at pH 10, (left) without catalyst and (right) with 1 [1 µm], at 23 °C (black), 40 °C (red), 60 °C (blue). For data at pH 11 see Figure S13. See also Figure S14.

Scheme 2

Degradation pathways for morin with O₂ proposed by Colombini et al.35

Scheme 3

Degradation pathways for morin with O₂ and H₂O₂.