KINETICS OF BROMIDE OXIDATION BY PEROXO COMPLEXES OF MOLYBDENUM(VI) AND TUNGSTEN(VI)

The kinetics of bromide oxidation by several peroxo complexes of molyb denum(VI) and tungsten(VI) were studied. All systems obey the rate law d[Br+]/dt = k[metal complex][Br-]. At pH 1.0, we quantify the initial rate of bromine production by the spectrophotometric monitoring of th e disappearance of fluorescein. The complexes and their rate constants at pH 1.0 and 25 degrees C are MoO(O-2)(nta)(-) (nta = nitrilotriacet ate) (k = (1.64 +/- 0.02) x 10(-4) M-1 s(-1)), MoO(O-2)(dipic)(H2O) (d ipic = pyridine-2,6-dicarboxylate) (k = (5.1 +/- 0.2) x 10(-4) M-1 s(- 1)), WO(O-2)(dipic)(H2O) (k = (2.79 +/- 0.02) x 10(-2) M-1 s(-1)), MoO (O-2)(2)(pic)(-) (pic = pyridine-2-carboxylate) (k = (1.633 +/- 0.001) x 10(-1) M-1 s(-1)) and MoO(O-2)(2)(H2O)(2) (k = 6.12 +/- 0.03) x 10( -1) M-1 s(-1)). At pH 5.0, the initial rates were measured spectrophot ometrically by the production of bromophenol blue from phenol red. The complexes and their rate constants at pH 5.0 and 25 degrees C are MoO (O-2)(2)(OH)(H2O)(-) (k = (8 +/- 1) x 10(-4) M-1 s(-1)), WO(O-2)(2)(OH )(H2O)(-) (k = (8.3 +/- 0.7) x 10(-3) M-1 s(-1)) and WO(O-2)(2)(C2O4)( 2-) (k = (1.3 +/- 0.2) x 10(-2) M-1 s(-1)). The O-17 chemical shifts f or the peroxo groups of MoO(O-2)(nta)(-) (501 ppm), MoO(O-2)(dipic)(H2 O) (537 ppm), WO(O-2)(dipic)(H2O) (364 ppm), MoO(O-2)(2)(pic)(-) (434 ppm) and WO(O-2)(2)(C2O4)(2-) (368 ppm) were measured after enrichment with (H2O2)-O-17. As expected, these values correlate with the recipr ocal of the transition energies of the electronic spectra. The values nu(Mo = O) = 930 cm(-1) and nu(O-O) = 900 cm(-1) for MoO(O-2)(nta)(-) are consistent with the vibrational features of similar compounds. The relationship between the spectroscopic properties and reactivity towa rd bromide oxidation follows previously observed patterns and may allo w the reactivity of peroxometal complexes to be predicted from spectro scopic information. (C) 1997 Elsevier Science S.A.