Spectroscopic studies of polyoxometalates: Multinuclear NMR and picosecond laser flash photolysis investigations

The investigations reported here illuminate several general features of polyoxometalate chemistry. First, the experimental procedures necessary for satisfactory acquisition of high-quality natural abundance {dollar}sp{lcub}17{rcub}{dollar}O NMR spectra for polyoxometalates are outlined. {dollar}sp{lcub}17{rcub}{dollar}O NMR evidence is presented which demonstrates an outer-sphere electron transfer mechanism for polyoxometalate catalyst reoxidations by {dollar}rmsp{lcub}17{rcub}Osb2{dollar}. Additionally, a method for extracting electric field gradient information at oxygen sites in polyoxometalates is outlined and the need for accurate electronic structure calculations is noted. Future investigations along this line could lead to a better understanding of metal-oxygen bonding in polyoxometalates. Second, the electrochemical synthesis of an unprotonated reduced complex, (W{dollar}rmsb{lcub}10{rcub}Osb{lcub}32{rcub}{dollar}) {dollar}sp{lcub}6-{rcub}{dollar}, is reported which is characterized by {dollar}sp{lcub}183{rcub}{dollar}W and {dollar}sp{lcub}17{rcub}{dollar}O NMR. Additionally, an analysis of the contributions responsible for {dollar}sp{lcub}183{rcub}{dollar}W chemical shifts in diamagnetic mixed-valence polyoxometalates is presented and previous misconceptions are revealed. Third, the first thorough study of the photophysical processes involved in the remarkable photochemistry of decatungstate on the picosecond timescale is reported. Clear evidence is presented for the preparation of a long-lived non-emissive reactive intermediate. Specifically, this investigation corrects previous misconceptions concerning decatungstate photochemistry, and generally, it suggests that present models of polyoxometalate photochemistry may require revision. Fourth, reported here are new results on the {dollar}sp{lcub}31{rcub}{dollar}P NMR characterization of key peroxophosphotungstate intermediates generated in the recently commercialized catalytic epoxidation of alkenes by the complex Ishii-Venturello system, {dollar}rm Hsb2Osb2/WOsb4sp{lcub}2-{rcub}/POsb4sp{lcub}3-{rcub}/Hsp+.{dollar} Evidence is presented which documents the stereochemical nonrigidity of the active catalyst in the above catalytic epoxidation system, {dollar}rm {lcub}POsb4lbrack WO(Osb2)sb2rbracksb4{rcub}sp{lcub}3-{rcub}.{dollar}...