Modeling Reactive Metal Oxides. Kinetics, Thermodynamics, and Mechanism of M3 Cap Isomerization in Polyoxometalates
journal contributionposted on 10.07.2002, 00:00 by Travis M. Anderson, Craig L. Hill
An investigation of M3O13 unit (“M3 cap”) isomerization in the classical polytungstodiphosphates α- and β-P2W18O626- has been undertaken because cap isomerism is an important and structurally well-studied phenomenon in many polyoxometalate families. The relative thermodynamic stabilities of the α (more stable) versus β isomers were established both in the solid state by differential scanning calorimetry (4.36 ± 0.64 kcal/mol) and in solution by 31P NMR (3.80 ± 0.57 kcal/mol). The isomerization of β-P2W18O626- to α-P2W18O626-, followed by 31P NMR, has a bimolecular rate constant k2 of 9.3 × 10-1 M-1 s-1 at 343 K in pH 4.24 acetate buffer. Several lines of evidence establish the validity of suggestions in the literature that isomerization goes through a lacunary (defect) intermediate. First, the rate is proportional to [OH-]. Second, isomerization increases at higher ionic strengths, and a Debye−Hückel plot is consistent with a rate-limiting reaction between β-P2W18O626- and OH- (two species with a charge product of 6). Third, alkali-metal cations stabilize the bimolecular transition state (K+ > Na+ > Li+), consistent with recent ion-pairing studies in polyoxometalate systems. Fourth, the monovanadium-substituted products α1- and α2-P2VW17O627- (51V NMR δ −554 ppm) form during isomerization in the presence of VO2+. The known lacunary compounds (α1- and α2-P2W17O6110-) also react rapidly with the same vanadium precursor. Fifth, solvent studies establish that isomerization does not occur when OH- is absent. A mechanism is proposed involving attack of OH- on β-P2W18O626-, loss of monomeric W(VI) from the M3 (M3O13) terminal cap, isomerization of the resulting lacunary compound to α-P2W17O6110-, and finally reaction of this species with monomeric W(VI) to form the thermodynamic and observed product, α-P2W18O626-.