JO URNA L O F MO LEC ULA R C A TA LYSIS Journal of MolecularCatalysis94 ( 1994) 323-333 ‘lV-NMR investigation on the formation of peroxo vanadium complexes in aqueous solution: some novel observations Valeria Conte, Fulvio Di Furia *, Stefano Moro Vniversit6 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA di Padova, Centro CNR di Studio sui Meccanismi di Reazioni Organiche, Dipartimento di Chimica Organica, Via Mar2010 I, 35131 Padova. Italy Received6 May 1994;accepted7 June 1994 Abstract Depending on the experimental conditions either monoperoxo or diperoxo vanadium complexes are formed in acid aqueous solutions by addition of H,O, to N&V03. The 51V-NMR spectra of both species have been registered at different pH values. While the signal of the monoperoxo derivative is unchanged in the pH range o-4, the chemical shift of the diperoxo species shows a continuous variation in the same interval thus suggesting the occurrence of a protonation equilibrium leading to a neutral diperoxo vanadium complex. In the presence of bidentate ligands L, e.g., picolinic or pyrazinic acid, under otherwise identical experimental conditions, complexed peroxo vanadium species are formed. The association equilibria can be conveniently studied by ‘IV-NMR spectroscopy. Thus, different species, e.g., VO( O*)L, [ VO( O,)h] -, [ VO( 0,) 2L]z- as a function of the pH has been detected and the corresponding association constants have been measured. Keywords: Aqueoussolutions; NMR spectroscopy; Peroxo complexes; Vanadium zyxwvutsrqponmlkjihgfedcbaZYXWV 1. Introduction The formation of peroxo vanadium derivatives in aqueous acid (pH < 2) solu- tions by addition of hydrogen peroxide to vanadium(V) precursors has been exten- sively studied [l-4]. As a result, the association processes shown in Scheme 1 have been established. Thus, together with the oxocation VO,‘, a red monoperoxo derivative (&I,= 455 nm, l =278) and a yellow diperoxo one (A,,= 350 nm, l =610) may occur depending on the excess of hydrogen peroxide [ 141. * Corresponding author. 0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO304-5102(94)00149-9