The highly selective catalytic epoxidation of terminal alkenes by the complex WVI/PV/H202/CHC13/PTC (PTC = phase transfer catalyst) system (Ishii-Venturello chemistry) has been extensively investigated by groups in several countries and recently commercialized, yet little is known with certainty about the mechanism. The substrate conversions and epoxide selectivities observed under biphasic conditions, aqueous H202/alkene in CHCI3, with 21 polyoxometalates (cetylpyridinium chloride as the phase-transfer catalyst, PTC) including the Ishii precursor complex, [PW12O40]3~, clearly indicate that only [PW^Oito]3-456and [PWnC>3C)]7-89, which both rapidly form {P04[W0(C>2)2]4}3-, 1, are effective. Simultaneous monitoring of organic oxygenated products and gaseous products (nearly all O2) with several of these polyoxometalates confirm that H2O2 disproportionation is by far the dominant side reaction with several d-electron transition metal-substituted polyoxometalate catalyst precursors. Analysis of the 2/w-p coupling satellites in the 31P NMR spectra of the polytungstophosphate products from the stoichiometric reaction of 1 with alkene substrates as a function of cation, solvent, field strength, and time indicates that both a PW4 and a PW3 specie are formed initially and one PW2 specie subsequently. Several lines of kinetic and spectroscopic evidence indicate that two processes dominate over all others during Ishii-Venturello epoxidation: a slow epoxidation, 1 (PW4) + alkene -PW4, PW3, and PW2 (henceforth called "subsequent peroxo species" or SPS) + epoxide, followed by a rapid regeneration of 1 with H2O2. First, little epoxidation is observed until 1 is in appreciable concentration. Second, the rate law for epoxidation of 1-octene by the Arquad salt of 1, Arql (Arquad = [(CigH37)75% +Third, 1 is the dominant polytungstophosphate present under steady state turnover conditions. Fourth, the ratio of the initial rates of epoxidation is vo(Aiqsps/vo(Arqi) = 0.13 ± 0.01. Fifth, the dominant inorganic product in the formation of 1, {[WCX* 1C^hdUOjhO}2-, is two orders of magnitude slower in alkene epoxidation than Arql under identical conditions at both 23 and 60 °C. Additional 31P NMR studies address both ion pairing effects and dynamic exchange in 1 and the SPS PW4. A linear correlation was found between the change in both chemical shift and 27w-p coupling constant for the SPS PW4 specie but not the SPS PW2 or the SPS PW3 species as a function of reaction time. This is consistent with the SPS PW4 specie undergoing rapid dynamic exchange on the 31P NMR time scale. Addition of 1 equiv of 1,2-epoxybutane to tetran-hexylammonium SPS (THASPS) does shift the SPS PW4 resonance to high field with a larger 2Jw-p coupling constant in accord with the correlation. Consequently, the dynamics of SPS PW4 may reflect exchange of epoxide product. Rapid catalyst inactivation despite being one of two success limiting features of Ishii-Venturello epoxidation was not addressed in any previous work. Under the typical biphasic reaction conditions, ...

We report here picosecond flash excitation results on [W10O32]4-, 1, which demonstrate that the initially prepared ligand-to-metal charge-transfer (LMCT) excited state decays within ~30 ps to a single intermediate, 2, that persists for > 15 ns. Little or no substrate reaction is derived from the short-lived LMCT excited state. Furthermore, the long-lived intermediate, 2, is not the 1-electron-reduced species [W10O32]5-, 3, or one of its protonated derivatives. This long-lived intermediate, 2, is the primary photoreactant and has substantial charge-transfer character itself. Additionally, 2 and 3 are likely to have similar W-orbital electron density; the principal differences in electronic structure derive from the presence of an oxidized oxygen site in 2 which is lacking in 3.

The reaction of [P 2 W 15 O 56 ] 12-and aqueous Fe 3+ forms two ferric polytungstophosphate derivatives that have been isolated, purified, and characterized by X-ray crystallography and several physical methods. At pH ∼3 and [Na + ] ) 2 M, needles of Na 12 [Fe III 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ]‚58H 2 O, Na-1, form, in which the conventional ion packing arrangement in solid heteropoly acids is observed: each polyanion, [Fe III 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 12-(1), is well separated by the cations (Na + ). At pH ∼1 and [Na + ] ) 1 M, however, elongated prisms of H 3 Na 9 [Fe III 4 (H 2 O) 2 -(P 2 W 15 O 56 ) 2 ]‚62H 2 O, (Na-1) n , form. (Na-1) n is a novel one-dimensional inorganic polymer composed of 1 linked via ion pairing with strongly associated hydrated sodium cations. Both Na-1 and (Na-1) n display identical characteristics in H 2 O solution. Seven lines of evidence are consistent with the Fe III 4 oxidation state in 1: (1) the synthesis itself (only Fe 3+ used); (2) bond-length-based valence sum calculations based on the two X-ray structures;(3) the electrochemical behavior; (4) the charge requirements and elemental analyses; (5) no reaction between 1 and Ce 4+ ; (6) the lack of a Jahn-Teller distortion; and (7) the infrared spectrum relative to those of the literature [M 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16-sandwich compounds, where M ) a divalent first-row transition metal ion in all cases. The tetra-n-butylammonium salt of 1, TBA-1, exhibits catalytic activity for oxidation of alkenes by H 2 O 2 . The catalytic effect is substantial. For example, in the reaction of cyclohexene + H 2 O 2 , rate(with TBA-1)/rate(without TBA-1) (-d[cyclohexene]/dt) is ∼25. Alkene oxidation is characterized predominantly by allylic attack on aliphatic substrates and oxidative cleavage of stilbenes. The polyanion, 1, is quite stable under catalytic conditions (no decomposition of 1 was detected by 31 P NMR after a solution of 1, alkene, and 0.25 M H 2 O 2 was incubated for 48 h).

Reported here are both STM images and spatially resolved tunneling spectra of four different polyoxometalate (POM) structural class members: Keggin structure, H(3)[PW(12)O(40)] (spherical); Finke-Droege (FD) structure, Na(16)[Cu(4)(H(2)O)(2)(P(2)W(15)O(56))(2)] (prolate spheroidal); Wells-Dawson (WD) structure, H(7)[P(2)Mo(17)VO(62)] (prolate spheroidal); and Pope-Jeannin-Preyssler (PJP) structure, K(12.5)Na(1.5)[NaP(5)W(30)O(110)] and (NH(4))(14) [NaP(5)W(30)O(110)] (oblate spheroidal). In all four cases, the results demonstrate the formation of well-ordered 2-D inorganic POM anion arrays (composed of catalytically active molecular constituents) on graphite. Importantly, the image shapes and lattice spacings accurately reflect the POM anisotropies, permitting the determination of anion orientation with respect to the surface plane.