Equilibria and dynamics of some aqueous peroxomolybdate catalysts: a O-17 NMR spectroscopic study

The equilibrium speciation at 278 K and the dynamics in the systems pH(+) + qMoO(4)(2-) + rH(2)O(2) + sL reversible arrow (H+)(p-)(MoO42-)(q) (H2O2)(r) L-s (L = SO42- or Cl-) in 0.3 M Na-2(SO4) and 0.6 M Na(Cl) medium has been determined from O-17 NMR integral and shift data in the range 0.7 less than or equal to pH less than or equal to 4.1 and at excess of peroxide ([H2O2](tot)/[Mo](tot) = 3). In 0.3 M Na-2(SO4) medium species with the following compositions were found: MoX2- (1,1,2,0), MoX2 (2,1,2,0), MoX2S2- (2,1,2,1), MoX2S- (3,1,2,1), Mo2X42- (2,2,4,0), Mo2X4- (3,2,4,0) and Mo2X62- (2,2,6,0), where (Mo) corresponds to MoO42-, (X) to O-2(2-) or HOO- and (S) to SO42-. In 0.6 M Na(Cl) medium the diperoxomolybdosulfate complex MoX2S- is replaced with a novel diperoxomolybdochloride complex MoX2Cl- (2,1,2,1), where (Cl) corresponds to Cl-. The numbers in parentheses refer to the values of pq,r and s in the formula above. The stoichiometries of each complex are given by the abbreviated formula MoqXrSs(2q + 2s - p)- and MoqXrCls(2q + s - p)-. The following formation constants with 3sigma were obtained with values for 0.6 M Na(Cl) in parenthesis: log beta(1,1,2,0) = 11.61 (11.61); log beta(2,1,2,0) = 13.70 +/- 0.10, pK(a) = 2.09 (13.86 +/- 0.10, pK(a) = 2.25); log beta(2,1,2,1) = 14.57 +/- 0.07 (13.87 +/- 0.15); log beta(3,1,2,1) = 15.27 +/- 0.17, pK(a) = 0.70, log beta(2,2,4,0) = 24.06 +/- 0.05 (24.08 +/- 0.04); log beta(3,2,4,0) = 25.99 +/- 0.18, pK(a) = 1.93 (26.23 +/- 0.17, pK(a) = 2.15); log beta(2,2,6,0) = 24.02 +/- 0.15 (23.9 +/- 0.3). The pK(a) value for HSO4- was determined to be 1.06 +/- 0.11. At 278 K the MOX2 and Mo2X4 complexes are in chemical exchange and the rate of exchange increases upon protonation. At 312 K there is also measurable exchange between MoX2S and the MoX2 and Mo2X4 complexes. The dynamics in 0.6 M Na(Cl) medium seem to be very similar to those in the sulfate medium