Association of Keggin-type anions with cationic meso-substituted porphyrins: synthesis, characterization and oxidative catalytic studies

Abstract: New compounds based on the association of polyoxometalates and cationic porphyrins or metalloporphyrins were prepared with the aim of obtaining new bi-functional catalysts. A combined study on the syntheses, characterization and catalytic activity of these compounds was performed. The new compounds have the general formula (porphyrin) x H y [XM12O40]·z solv, x =0.75 or 1, X=P or Si and M=W or Mo. The porphyrins used were 5,10,15,20-tetrakis(4-pyridyl)porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin, 5,10,15-tris(2,6-dichorophenyl)-20-(4-pyridyl)porphyrin, and 5,10,15-tris(2,6-dichlorophenyl)-20-(1-methyl-4-pyridinio)porphyrin, either as free bases or complexed with MnIII (one case with ZnII). A few compounds with metal substituted Keggin anions ([PW11Y(H2O)O39] n−, Y=Mn, Fe, Ni, Zn) were also prepared. All compounds were characterized by spectroscopic and analytical techniques. The oxidation of cis-cyclooctene, geraniol and (+)-3-carene by hydrogen peroxide catalysed by the polyoxometalate/MnIIIporphyrin associations and co-catalysed by ammonium acetate was examined. Some of the associations significantly increased the percentage of conversion of the substrates relatively to the respective metalloporphyrin alone. However, the regio-, chemo- and stereoselectivity of the oxidation reactions catalysed by metalloporphyrins were globally preserved: cis-cyclooctene was selectively epoxidised to epoxycyclooctane; geraniol gave 6,7-epoxygeraniol as the major product and (+)-3-carene was preferably oxidised to the α-3,4-epoxycarane. In the conditions studied, the polyoxoanions seem to contribute mainly to stabilise the metalloporphyrin against deactivation during catalytic cycles. [Copyright &y& Elsevier]