Synthetic and catalytic studies of polyoxometalates with theoretical and practical value

The photochemistry of polyoxometalates using various organic compounds as substrates is examined. The way the excited polyoxometalate molecules attack organic substrates is similar to that exhibited by the organic radicals. Several sets of experimental results (rate {dollar}sim{dollar} C-H bond energy relationships and product distributions, for instance) confirm that hydrogen abstraction from substrates by polyoxometalate excited states is the rate determining step for this type of reaction. Among the polyoxometalates examined, ({dollar}alpha{dollar}-Mo{dollar}sb8{dollar}O{dollar}sb{lcub}26{rcub}{dollar}) {dollar}sp{lcub}4-{rcub}{dollar} is the most interesting one because of the relationship between its structure and photochemical activity. In contrast to predictions of instability from molecular orbital arguments and the documented redox chemistry of a range of polyoxometalates, ({dollar}alpha{dollar}-Mo{dollar}sb8{dollar}O{dollar}sb{lcub}26{rcub}{dollar}) {dollar}sp{lcub}4-{rcub},{dollar} which contains no Type I MO{dollar}sb6{dollar} octahedra in its structure, undergoes photoreduction without structural decomposition.; The triester from condensation of the organic triol (CH{dollar}sb3{dollar}C(CH{dollar}sb2{dollar}OH){dollar}sb3){dollar} and the trivanadium substituted Wells-Dawson polytungstate, ({dollar}rm Hsb3Psb2Vsb3Wsb{lcub}15{rcub}Osb{lcub}62{rcub}{dollar}) {dollar}sp{lcub}6-{rcub},{dollar} (CH {dollar}rm sb3C(CHsb2O)sb3Psb2Vsb3Wsb{lcub}15{rcub}Osb{lcub}59{rcub}{dollar}) {dollar}sp{lcub}6-{rcub},{dollar} has been synthesized and characterized. The reaction proceeds at room temperature in high selectivity and conversion. Attempted hydrolysis of the lithium salt of this triester complex in aqueous solution shows it to be very stable under harsh conditions. The triester complex is capable of catalyzing the oxidation of tetrahydrothiophene (THT) by t-butylhydroperoxide (TBHP). No decomposition of the catalyst was found based on {dollar}sp{lcub}31{rcub}{dollar}P NMR after 30 turnovers. Control experiments demonstrate that the catalytic activity of the triester complex is not from its decomposition fragments. Kinetic measurements show that the catalytic reaction is first order in both THT and TBHP and second order in catalyst. A model for the rate determining transition state for THTO production via TBHP oxidation of THT catalyzed by Q{dollar}sb2{dollar}H{dollar}sb4{dollar} (CH{dollar}rmsb3C(CHsb2O)sb3Psb2Vsb3Wsb{lcub}15{rcub}Osb{lcub}59{rcub}{dollar}) (Q = (n-(C{dollar}rmsb4Hsb9){dollar}) {dollar}sb4{dollar}N{dollar}sp+){dollar} that is consistent with the rate law, other kinetic and spectroscopic data and literature investigations is proposed.