Modeling reactive metal oxides. Kinetics,thermodynamics, and mechanism of M(3) cap isomerization in polyoxometalates

An investigation of M(3)O(13) unit ("M(3) cap") isomerization in the classical polytungstodiphosphates alpha- and beta-P(2)W(18)O(62)(6)(-) has been undertaken because cap isomerism is an important and structurally well-studied phenomenon in many polyoxometalate families. The relative thermodynamic stabilities of the alpha (more stable) versus beta isomers were established both in the solid state by differential scanning calorimetry (4.36 +/- 0.64 kcal/mol) and in solution by (31)P NMR (3.80 +/- 0.57 kcal/mol). The isomerization of beta-P(2)W(18)O(62)(6)(-) to alpha-P(2)W(18)O(62)(6)(-), followed by (31)P NMR, has a bimolecular rate constant k(2) of 9.3 x 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 beta-P(2)W(18)O(62)(6)(-) 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 alpha(1)- and alpha(2)-P(2)VW(17)O(62)(7)(-) ((51)V NMR delta -554 ppm) form during isomerization in the presence of VO(2+). The known lacunary compounds (alpha(1)- and alpha(2)-P(2)W(17)O(61)(10)(-)) 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 beta-P(2)W(18)O(62)(6)(-), loss of monomeric W(VI) from the M(3) (M(3)O(13)) terminal cap, isomerization of the resulting lacunary compound to alpha-P(2)W(17)O(61)(10)(-), and finally reaction of this species with monomeric W(VI) to form the thermodynamic and observed product, alpha-P(2)W(18)O(62)(6)(-).