HYDROLYTICALLY STABLE ORGANIC TRIESTER CAPPED POLYOXOMETALATES WITH CATALYTIC OXYGENATION ACTIVITY OF FORMULA [RC(CH2O)3V3P2W15O59]6- (R=CH3, NO2, CH2OH)

The apparent instability of fully oxidized (d0) redox active alkoxypolyoxometalates to hydrolysis inferred from literature studies would appear to obviate complexes involving covalent bonds between polyols (e.g. carbohydrates) and polyoxometalate complexes for applications in aqueous phase catalysis and medicine. The title capped complexes [RC(CH2O)3VV3P2WVI15O59]6- ([1-R]: R = CH3, NO2, and CH2OH), prepared in very high yield (>90%) by the stoichiometric condensation of 1,1,1-tris(hydroxymethyl)ethane derivatives with Q5H4[V3P2W15O62], where Q = (n-C4H9)4N, in CH3CN, however, have substantial kinetic stability with respect to hydrolysis (e.g. tau1/2 approximately 912 h for 40 mM [1-CH3] in D2O with pD = 0 at 100-degrees-C). H-1, P-31, and V-51 NMR spectra during and after the condensation synthesis of the triester complexes [1-R] indicate the absence of polyoxometalate intermediates in significant concentration during the reactions. These techniques and W-183 NMR and IR confirm the only detectable product in each case is the C3v regioisomer involving replacement of the three mu2-OXO edge-bridging oxygens of the V3 cap in the parent polyoxometalate, the most basic oxide ions in the complex, with the three mu2-alkoxy groups of the triols. Two types of experiments, complementary to those addressing hydrolytic removal of the 1,1,1-tris(alkoxymethyl) ''caps'' of the title complexes, indicate both a kinetic and a thermodynamic preference for reaction of the more electron rich alcohols with the parent heteropolyanion, Q5H4[V3P2W15O62], to form the title complexes. First, competitive kinetics methods indicate the relative reactivities of CH3OH, CH3C(CH2OH)3, (HOCH2)C(CH2OH)3, and (O2N)C(CH2OH)3, respectively, with this polyanion were 6:6:5:1. Second, thermodynamic equilibration in cap-cap' exchange (or triple transesterification) reactions, RC(CH2OH)3 + [R'C(CH2O)3HV3P2W15O59]5- --> [RC(CH2O)3HV3P2W15O59]5- + R'C(CH2OH)3, where R, R' = NO2 or CH2OH (DMF solution, 50-degrees-C, 3 days), indicated a 35-fold preference for binding of the HOCH2C(CH2-)3 cap relative to the O2NC(CH2-)3 cap. While the title capped complexes showed stability upon reduction on the cyclic voltammetry time scale (nearly reversible voltammograms), preparative electrochemical or chemical reduction in slightly wet CH3CN resulted in immediate and complete hydrolysis to form the free 1,1,1-tris(alkoxymethyl) caps and [V3P2W15O62]9-. Potentiometric titration, elemental analysis, and chemical reactivity confirmed that the [1-CH3] complex could be prepared in two different protonation states, Q5H[1-CH3] and Q2H4[1-CH3]. Several experiments with the thioether mustard gas analog tetrahydrothiophene (THT) demonstrated that the [1-R] complexes are capable of catalyzing oxygenation. Comparative kinetics methods gave the following order of reactivity for oxidation of THT by tert-butyl hydroperoxide (TBHP): Q2H4[1-CH3] (fastest catalyst) > OV(O-iPr)3 (a monomeric Vv model for breakdown of [1-CH3] during turnover) > (n-Bu4N)2WO4 (a monomeric W(VI) model) > Q5H4[V3P2W15O62] (the parent complex) > Q5H[1-CH3] > Q6[P2W18O62] > no catalyst (slowest). These relative rates coupled with other kinetics measurements and the complete lack of degradation of [1-CH3] examined after 30 turnovers (30 equiv of THTO from THT and TBHP) collectively provide strong evidence that kinetically significant fragmentation during catalysis by [1-CH3] is minimal and that [1-CH3] in varying protonation states is probably the active catalyst. One possible activated complex for the rate-determining transition state is given for THT oxidation by TBHP catalyzed by [1-CH3] consistent with the unusual rate law: v0 = k[THT] [TBHP][[1-CH3]]2. The Arrhenius plot is quite linear (R = 0.988) yielding E(a) approximately 10.1 kcal mol-1.