Synthesis and Characterization of Novel Wells–Dawson-Type Mono Vanadium(V)-Substituted Tungsto-polyoxometalate Isomers: 1- and 4-[S₂VW₁₇O₆₂]^5–

Abstract

Two vanadium(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S2VW17O62](5-), were prepared as their tetra-alkyl ammonium salts from a W(VI)-H2SO4-V(V) reaction mixture in aqueous CH3CN solution. X-ray crystallographic structural analysis revealed that both isomers have a Wells-Dawson-type structure with a higher occupancy of vanadium at polar sites and belt sites for 1- and 4-[S2VW17O62](5-), respectively. The isomers were also characterized by elemental analysis, infrared, Raman, UV-vis, and (51)V NMR spectroscopies as well as voltammetry, and the data obtained were compared with that derived from [S2W18O62](4-). Significantly, the reversible potentials for the vanadium(V/IV) couple for both 1- and 4-[S2VW17O62](5-) in CH3CN (0.1 M n-Bu4NPF6) are considerably more positive than the tungstate reduction process exhibited by the [S2W18O62](4-) framework, implying that the presence of vanadium should be useful in catalytic reactions. The one-electron-reduced [S2V(IV)W17O62](6-) forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy.

Voltammetric behavior of 1- and 4-[S2VVW17O62]5−in acidified acetonitrile

“…As discussed in ref. 6, the spectra are consistent with the electron being located on one of the cap sites for 1-S 2 V IV W 17 and in a belt site for 4-S 2 V IV W 17 .…”

“…The EPR spectra at 120 K of 1-S 2 V IV W 17 and 4-S 2 V IV W 17 in acetonitrile, frozen after one-electron reduction by bulk electrolysis in the absence of an acid, have been reported previously 6 and are shown in Fig. 5a and 6a respectively.…”

“…Cyclic voltammograms for 4-S 2 V V W 17 in 5 mM CF 3 SO 3 H acidic CH 3 CN are similar to those for 1-S 2 V V W 17 , but with the first process being 80 mV more positive than for 1-S 2 V V W 17 , and the 2 nd and 3 rd processes being slightly distorted because of overlap with the process from a small amount of the 1-isomer as shown in ref. 6 and in the 51 V NMR spectra (see below).…”

“…The precipitate was filtered off, washed thoroughly with water, dried, and recrystallized from CH 3 CN. α 2 -K 10 6 ] was recrystallized at least twice from ethanol before being used as the electrolyte in electrochemical experiments. Acetonitrile of LC-MS grade and acetone, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and nitromethane of spectroscopic) grade were purchased from WAKO and used as provided by the manufacturer.…”

“…1), and their electrochemistry described in neutral CH 3 CN (0.1 M [n-Bu 4 N] [PF 6 ]). 6 In addition, new procedures for extraction of the protonated S 2 V V W 17 and other POMs have been developed recently and their catalytic ability has been demonstrated in several areas of organic syntheses. As a result of this study it is known that under acidic conditions, S 2 V V W 17 is an excellent catalyst for some reactions.…”