Autocatalytic photodegradation of [Ru(ii)(2,2′-bipyridine)2DAD]+ (DADH=1,2-dihydroxyanthracene-9,10-dione) by hydrogen peroxide under acidic aqueous conditions

As part of a continuing effort to identify ruthenium agents capable of the photorelease of anthraquinone-based ligands the complexes Delta/?-[Ru(bpy)(2)DAD](+) (bpy = 2,2 '-bipyridine) were produced by the reaction of 1,2-dihydroxyanthracene-9,10-dione (DADH) with chirally pure Delta/?-[Ru(bpy)(2)(py)(2)][(+)-O,O '-dibenzoyl-d-tartrate]center dot 12H(2)O (py = pyridine). A very subtle difference in the chemical shift of the hydroxyl proton in their high-field H-1 NMR spectra was observed, supporting that the OH proton is susceptible to a small change in environment at the metal centre. The excited state lifetime of the complexes, as measured by femtosecond pump-probe spectroscopy, was 7.1 (+/- 0.8) ps in water (pH 2) and 13 (+/- 1) ps in MeCN. Illumination of a sample of ?-[Ru(bpy)(2)DAD](+) in water (pH 2) in the presence of hydrogen peroxide resulted in decomposition of the complex. The decay profile, as monitored at several wavelengths, was sigmoidal indicating the reaction was autocatalytic, in which the product formed catalysed decomposition of the starting complex. A mechanism is proposed that relies on participation of the uncoordinated hydroxyl group on the anthraquinone ligand in promoting water loss and radical formation in the excited state. The radical is oxidised by peroxide to generate the ruthenium(iii) complex, which behaves as an oxidant in the autocatalytic process.

Automated summary

The study focused on identifying ruthenium agents capable of photorelease of anthraquinone-based ligands, with the produced complexes showing a subtle difference in chemical shift in high-field H-1 NMR spectra. The excited state lifetime of the complexes varied in water and MeCN, and the decomposition of the complex in presence of hydrogen peroxide showed an autocatalytic reaction mechanism involving radical formation and ruthenium(iii) complex oxidation.