Kenji Nomiya

Catalysis by heteropolyacids—VIII. Immobilization of keggin-type heteropolyacids on poly(4-vinylpyridine)

Abstract Anchoring of a Keggin-type heteropolyacid (HPA) to a polymeric support and the effect of heterogenization on the HPA catalysis of the oxidation of a primary diol have been studied. A coloured HPA such as H6[Co(II)W12O40] (blue green), H4[SiMo12O40] (yellow) or H3[PMo12O40] (yellow), as the Keggin-type HPA and poly(4-vinylpyridine) as a polymeric support have been used. The anchoring of H6[Co(II)W12O40] has led to a markedly increased stability of the oxidation catalyst.

Anderson-type heteropolyanions of molybdenum(VI) and tungsten(VI)

Abstract The previously reported preparation of some Anderson-type molybdopolyanions containing divalent metal ions (Zn, Cu, Co or Mn) as a heteroatom has been reinvestigated. The molybdopolyanions of Zn(II) and Cu(II) were confirmed, although the Cu(II) polyanion was not stable and could not be recrystallized. On the other hand, the polyanions of Co(II) and Mn(II) could not be reproduced. Another type of heteropoly compound, [X(H2O)6-x(Mo7O24)]4− [X = Cu(II), Co(II) or Mn(II)], was isolated as solids, which are not stable thermally. The mixed-type Anderson polyanions, [Ni(II)Mo6-xWx,O24H6]4−, which have been questioned as mixtures of species with different x values, were also reinvestigated using IR, UV absorption and MCD spectra. They are single species, but not mixtures, although some positional isomers may be present for the compounds where x = 2-4. The possibility of oxidation of the heteroatom with the Anderson structure maintained was examined. The oxidation of [Ni(II)Mo6O24H6]4− by the S2O2−8 ion in aqueous solution gave the Waugh-type [Ni(IV)Mo9O32]6− polyanion, whereas the oxidation of [Ni(II)W6O24H6]4− gave no heteropoly compound.

Charge-transfer absorption spectra of some tungsten(VI) and molybdenum(VI) polyoxoanions

Abstract The charge-transfer (CT) absorption spectra in the UV region (30,000–50,000 cm−1) of most W(VI) and Mo(VI) polyoxoanions, attributed to the oxygen-to-metal transitions, are interpreted on the basis of structural considerations through edge- and/or corner-sharings of some octahedral units. Polyoxoanions studied here are classified into two groups according to a common fragment constituting their structures; one group contains an ‘M5O18’ fragment [M = W(VI) and Mo(VI)] such as Weakley-type [Ce(IV)(W5O18)2]8− anion, [W10O32]4− and [M′W5O19]n− [M′ = V(V), Nb(V), W(VI)] polyanions and the other contains an A-α-‘XM9O34’ fragment (X = heteroatom) such as Keggin-type [α-XM12O40]n− and Dawson-type [α-P2M18O62]6− heteropolyanions. The first- and second-lowest energy CT (LECT) bands of the respective group are qualitatively discussed from the CT interactions among some fragments.

Catalysis by heteropolyacid. VII: Catalytic oxidation of cyclohexanol by dodecamolybdate

Abstract In the oxidation of cyclohexanol to cyclohexanone and the further oxidation to adipic acid, a dodecamolybdo-heteropolyanion (HPA) functions catalytically in the presence of aqueous hydrogen peroxide, whereas the HPA without hydrogen peroxide reacts only stoichiometrically. Air- or oxygen-passing through the refluxing system of cyclohexanone and the HPA in the presence of active charcoal, without hydrogen peroxide, has also promoted the oxidation catalytically. Since both hydrogen peroxide and aeration in the presence of active charcoal convert the reduced-form (blue) of the HPA produced by the oxidation of substrate to the oxidized-form (yellow), they play a role as a promoter or a reoxidizing reagent for the oxidation by the HPA. The reoxidizing effect of the aqueous hydrogen peroxide is much the more effective.

Catalytic photooxidation of some secondary alcohols by decatungstate isopolyanion and Keggin-type dodecatungstophosphate heteropolyanion in homogeneous system under excess of oxygen and the effect of counterions on redox cycle of polyanions

Abstract Under excess of oxygen, catalytic photooxidation of secondary alcohols to ketones, initiated by irradiation of charge-transfer bands of polyanions such as decatungstate isopolyanion and Keggin-type heteropolytungstophosphate, has been investigated. Photooxidation of organic substrates proceeds in conjunction with photoreduction of polyanions and the catalytic cycle is attained by reoxidation of reduced polyanions by the reaction with O 2 , where consumed O 2 is converted to water. Polyanions studied are H 3 [PW 12 O 40 ] and K 4 [W 10 O 32 ] which are soluble in water, and (TBA) 3 [PW 12 O 40 ] and (TBA) 4− [W 10 O 32 ] (TBA=tetrabutylammonium) which are soluble in acetonitrile. Organic substrates studied are secondary alcohols such as isopropyl alcohol, cyclohexanol and ethyl lactate. Catalysis by the Keggin polyanion is based on the redox cycle between the one-electron (1e) reduced species and the oxidized form, whereas that by the decatungstate is based on the redox cycle between the 2e-species and the oxidized form. These two cycles are strongly influenced by the counterion. This is interpreted in that some of the TBA cations are binding with the polyanions in acetonitrile solution.

Catalysis by heteropolyacids—ix. Photocatalytic oxidation of isopropyl alcohol to acetone under oxygen using tetrabutylammonium decatungstate

Abstract Homogeneous photocatalytic oxidation of isopropyl alcohol by tetrabutylammonium decatungstate (Bu4N)4[W10O32] in acetonitrile using O2 as a reoxidizing reagent has been studied. The photocatalysis is interpreted on the basis of a rapid redox cycle between 2e-reduced species and the oxidized species of decatungstate; O2 as a reoxidizing reagent is converted to water. The catalytic activity and the mechanism of the redox cycle of decatungstate are in contrast to those of Keggin-type heteropolytungstic acids. When the concentration of isopropyl alcohol is lowered by prolonged irradiation, pale yellow precipitates are produced by side reactions due to electron transfer from the counter-ion Bu+4 to the decatungstate.

Tetrahedral metal complexes of [MW12O40]-type (MAlIII, ZnII) with dodecatungstate as tetrahedral ligand

Abstract Tetrabutylammonium salts of 12-heteropolytungstates with central metal ions in a tetrahedral oxygen environment, MW12O40 (MAl, Zn), have been prepared and their spectroscopic data (IR, UV and MCD) presented. IR studies in acetonitrile solution suggest that the tetrahedral structure around AlO4 moiety is largely distorted, whereas the ZnO4 moiety retains excellent tetrahedral symmetry.

Structural stability index of heteropoly- and isopoly-anions—III. Application to site-vacant fragments of keggin and dawson polyanions α-[PW12O

Abstract For some site-vacant species of Keggin and Dawson polyanions α-[PW12O40]3− and α-[P2W18O62]6− containing hypothetic

Induced cotton effect of keggin-type and mixed-coordination-type heteropolyanions in nonaqueous media containing l-brucine sulphate

The reactivities, or the interactions in nonaqueous media of the Keggin-type heteropolyanions [XM12O40]n- (X = CoII, ZnII, AlIII, Si, P, 2H+, M = W; X = P, Si, M = Mo) and the Fe(III)-substituted mixed-type polyanions [PFeM11O40H2]4- (M= Mo, W) with a chiral organic compound such as l-brucine sulphate have been studied using the electronic absorption, differences absorption and CD spectra. Two types of the interaction of l-brucine with the polyanions have been indicated from the observed Cotton effect; one is an outer-sphere association of the brucinium ion with the surface bridged oxygens of dodecametallate and the other a coordination of brucine to the Fe(III) ion in the mixed-type polyanions. In the latter, an outer-sphere association between the PM11O397- moiety and brucinium ion has been also suggested.

Preparation and spectrochemical properties of soluble vanadophosphate polyanions with bicapped-Keggin structure

Abstract Some soluble forms in aqueous and nonaqueous media of vanadophosphate with a bicapped-Keggin structure have been prepared. Some isopoly decavanadates derived during such preparations have also been reported. Their spectrochemical data (IR, UV-vis and MCD) are presented.