TY - JOUR
T1 - Noble Metal (RuIII, PdII, PtII) Substituted “Sandwich” Type Polyoxometalates
T2 - Preparation, Characterization, and Catalytic Activity in Oxidations of Alkanes and Alkenes by Peroxides
AU - Neumann, Ronny
AU - Khenkin, Alexander M.
PY - 1995/10/1
Y1 - 1995/10/1
N2 - The polyoxometalates substituted with noble metals, Pd(II), Pt(II) and Ru(III), K12{[WZnPdII2(H2O)2](ZnW9O34)2}· 38H2O, K12{[WZnPtII2(H2O)2](ZnW9O34)2}·36H2O, and Na11{[WZnRuIII2(OH)(H2O)](ZnW9O34)2}·42H2O, were prepared by exchange of labile zinc atoms with noble metal atoms from the isostructural starting material, Na12-{[WZn3(H2O)2](ZnW9O34)2]}·46H2O. The X-ray crystal structure of the ruthenium compound shows a structure compatible with a sandwich-type structure type with a WRuZnRu (Ru and W, Zn at opposing sides) ring between two B-XW9O34 units. Magnetic susceptibility studies as a function of temperature provide convincing evidence of two ruthenium (III) centers with no magnetic interaction between them. The EPR spectrum is supportive of this formulation showing an anisotropic spectrum of a ruthenium (III) atom (S = 1/2) in an octahedral field. The IR and UV-vis spectra of the ruthenium compound as well as of the diamagnetic palladium and platinum compounds are consistent with an isostructural series of compounds. The water soluble polyoxometalates may be extracted into an organic phase e.g. 1,2-dichloroethane by the addition of methyltricaprylammonium chloride to form their quaternary ammonium salts. The catalytic activity of these compounds was tested for the oxidation of alkenes and alkanes using aqueous 30% hydrogen peroxide and 70% tert-butyl hydroperoxide as oxidants. The alkene oxidation proceeded in high reactivity and moderate selectivity to the epoxide product using 30% H2O2. Kinetic profiles as well as UV-vis and IR spectra before, during and after the reaction indicate that the catalysts are stable throughout the reaction. Formation of epoxides rather than ketonization in the reaction of terminal alkenes as well as low reactivity with iodosobenzene indicates that the reaction is tungsten centered and not noble metal centered. Oxidation of alkenes with tert-butyl hydroperoxide gave mostly allylic oxidation and/ or addition of tert-butyl alcohol to the double bond. Oxidation of cyclic alkanes such as cyclohexane and adamantane was successful with tert-butyl hydroperoxide with catalytic activity 10 times higher than previously found for transition metal substituted Keggin compounds. Ratios of hydroxylation of adamantane at tertiary vs secondary positions indicates different active species in the palladium-, platinum-, and ruthenium substituted-polyoxometalates.
AB - The polyoxometalates substituted with noble metals, Pd(II), Pt(II) and Ru(III), K12{[WZnPdII2(H2O)2](ZnW9O34)2}· 38H2O, K12{[WZnPtII2(H2O)2](ZnW9O34)2}·36H2O, and Na11{[WZnRuIII2(OH)(H2O)](ZnW9O34)2}·42H2O, were prepared by exchange of labile zinc atoms with noble metal atoms from the isostructural starting material, Na12-{[WZn3(H2O)2](ZnW9O34)2]}·46H2O. The X-ray crystal structure of the ruthenium compound shows a structure compatible with a sandwich-type structure type with a WRuZnRu (Ru and W, Zn at opposing sides) ring between two B-XW9O34 units. Magnetic susceptibility studies as a function of temperature provide convincing evidence of two ruthenium (III) centers with no magnetic interaction between them. The EPR spectrum is supportive of this formulation showing an anisotropic spectrum of a ruthenium (III) atom (S = 1/2) in an octahedral field. The IR and UV-vis spectra of the ruthenium compound as well as of the diamagnetic palladium and platinum compounds are consistent with an isostructural series of compounds. The water soluble polyoxometalates may be extracted into an organic phase e.g. 1,2-dichloroethane by the addition of methyltricaprylammonium chloride to form their quaternary ammonium salts. The catalytic activity of these compounds was tested for the oxidation of alkenes and alkanes using aqueous 30% hydrogen peroxide and 70% tert-butyl hydroperoxide as oxidants. The alkene oxidation proceeded in high reactivity and moderate selectivity to the epoxide product using 30% H2O2. Kinetic profiles as well as UV-vis and IR spectra before, during and after the reaction indicate that the catalysts are stable throughout the reaction. Formation of epoxides rather than ketonization in the reaction of terminal alkenes as well as low reactivity with iodosobenzene indicates that the reaction is tungsten centered and not noble metal centered. Oxidation of alkenes with tert-butyl hydroperoxide gave mostly allylic oxidation and/ or addition of tert-butyl alcohol to the double bond. Oxidation of cyclic alkanes such as cyclohexane and adamantane was successful with tert-butyl hydroperoxide with catalytic activity 10 times higher than previously found for transition metal substituted Keggin compounds. Ratios of hydroxylation of adamantane at tertiary vs secondary positions indicates different active species in the palladium-, platinum-, and ruthenium substituted-polyoxometalates.
UR - http://www.scopus.com/inward/record.url?scp=0000656880&partnerID=8YFLogxK
U2 - 10.1021/ic00127a012
DO - 10.1021/ic00127a012
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AN - SCOPUS:0000656880
SN - 0020-1669
VL - 34
SP - 5753
EP - 5760
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 23
ER -