Articles 183 W NMR Study of Peroxotungstates Involved in the Disproportionation of Hydrogen Peroxide into Singlet Oxygen ( 1 O 2 , 1 Δ g ) Catalyzed by Sodium Tungstate in Neutral and Alkaline Water V. Nardello, ² J. Marko, ² G. Vermeersch, ‡ and J. M. Aubry* ,² Equipe de Recherches “Oxydation et Formulation”, URA CNRS 351, Cite´ Scientifique, ENSCL, BP 108, F-59652 Villeneuve d’Ascq Cedex, France, and Laboratoire d’Application RMN, Laboratoire de Physique, URA CNRS 351 Faculte´ de Pharmacie, BP 83, F-59006 Lille Cedex, France ReceiVed December 29, 1997 The disproportionation of aqueous hydrogen peroxide catalyzed by sodium tungstate has been investigated with regard to the multiplicity of the oxygen molecules released. Trapping experiments and detection of the IR luminescence of 1 O 2 have shown that the yield of 1 O 2 is virtually quantitative. The mono-, di-, and tetraperoxotungstate intermediates W(O 2 ) n O 4-n 2- (n ) 1, 2, 4) have been characterized by UV and 183 W NMR spectroscopies. The diperoxo species is proposed as the precursor of 1 O 2 . Introduction In 1985, Aubry reported that about thirty inorganic oxides, hydroxides or oxo-anions induce the decomposition of hydrogen peroxide into excited singlet oxygen ( 1 O 2 , 1 Δ g ) in aqueous alkaline medium. 1 Among these new chemical sources of 1 O 2 , the system hydrogen peroxide/sodium molybdate appeared particularly attractive on account of its efficiency to generate this excited species in homogeneous phase at room tempera- ture. 2,3 Since then, much work has been devoted to this system and both the mechanism of the reaction 4 and its ability to oxidize various organic substrates, in water 5,6 or in microemulsions, 7 have been investigated in details. On the other hand, the very similar system hydrogen peroxide/sodium tungstate has been much less studied in terms of singlet oxygen generation 1,8 whereas it is a well-known epoxidation agent in moderately acidic medium. 9-11 Niu and Foote 8 have recently confirmed the generation of 1 O 2 from the system H 2 O 2 /WO 4 2- by both chemical trapping and detection of the IR luminescence of the excited species. Their study was based on the thermal decomposition of the sodium tetraperoxotungstate, Na 2 W(O 2 ) 4 ‚4H 2 O, which was found to release 1 O 2 at 40 °C in water with a yield of 80%. Nevertheless, the authors specified that this value could be underestimated on account of H 2 O 2 decomposition by side reactions. Moreover, as shown recently by Nardello et al. 4 for the system H 2 O 2 /MoO 4 2- , the mono-, di-, tri-, and tetraperoxo- molybdates formed in water are in equilibrium and it is likely that a similar behavior also takes place with peroxotungstates. Thus, 1 O 2 detected by warming of a tetraperoxotungstate solution, Na 2 W(O 2 ) 4 ‚4H 2 O might be generated by the decom- position of another peroxotungstate. The aim of the present work was to reinvestigate and to study further the disproportionation of hydrogen peroxide by tungstate ions in neutral and alkaline media. The yield of 1 O 2 formation was reconsidered, the peroxotungstate intermediates were characterized by UV and 183 W NMR spectroscopies and the precursor of 1 O 2 was identified. Experimental Section (1) Reagents. Sodium tungstate dihydrate (99%) was from Aldrich Chemie. Hydrogen peroxide (50% Rectapur) was from Prolabo, Paris. Deuterium oxide (99.8% D) was from CEA (Commissariat a` l’Energie Atomique, Saclay, France). Sodium tetraperoxotungstate Na2W(O2)4‚ 4H2O, 12 potassium tetraperoxoditungstate K2[W2O3(O2)4]‚4H2O, 13 and tetrapotassium rubrene-2,3,8,9-tetracarboxylate (RTC) 14 were prepared according to known procedures. (2) Instrumentation. UV/Visible Spectrophotometry. Data were obtained with a Milton Roy Spectronic 3000 spectrophotometer equipped with a diode array photodetector. Full-wavelength scanning measurements can be obtained almost instantaneously, and the detection wavelengths are more reliable than with a scanning spectrophotometer. 183 W NMR Spectroscopy. The natural abundance 183 W NMR spectra were recorded on a Bruker AC 300P FT-spectrometer with a 10-mm VSP 300 broad-band probehead. The spectrometer was ² Equipe de Recherches “Oxydation et Formulation.” ‡ Laboratoire d’Application RMN. (1) Aubry, J. M. J. Am. Chem. Soc. 1985, 107, 5844-5849. (2) Aubry, J. M.; Cazin, B. Inorg. Chem. 1988, 27, 2013-2014. (3) Bo¨hme, K.; Brauer, H.-D. Inorg. Chem. 1992, 31, 3468-3471. (4) Nardello, V.; Marko, J.; Vermeersch, G.; Aubry, J. M. Inorg. Chem. 1995, 34, 4950-4957. (5) Nardello, V.; Bouttemy, S.; Aubry, J. M. J. Mol. Catal. A: Chem. 1997, 117, 439-447. (6) Aubry, J. M.; Cazin, B.; Duprat, F. J. Org. Chem. 1989, 54, 726- 728. (7) Aubry, J. M.; Bouttemy, S. J. Am. Chem. Soc. 1997, 119, 5286- 5294. (8) Niu, Q. J.; Foote, C. S. Inorg. Chem. 1992, 31, 3472-3476. (9) Fort, Y.; Olzewski-Ortar, A.; Caubere, P. Tetrahedron 1992, 48, 5099- 5110. (10) Jorgensen, K. A. Chem. ReV. 1989, 89, 431. (11) Payne, G. B.; Williams, P. H. J. Org. Chem. 1959, 24, 54-55. (12) Jahr, K. F.; Lother, E. Ber. Dtsch. Chem. Ges. 1938, 71, 894-903. (13) Stomberg, R. Acta Chem. Scand. 1968, 22, 1076-1090. (14) Aubry, J. M.; Rigaudy, J.; Cuong, N. K. Photochem. Photobiol. 1981, 33, 149-153. 5418 Inorg. Chem. 1998, 37, 5418-5423 10.1021/ic971607s CCC: $15.00 © 1998 American Chemical Society Published on Web 09/24/1998