Heteropolyoxotungstates as catalysts for the epoxidation of allylic alcohols

Sandwich-type heteropolyoxotungstate anions of composition [WZn{M(OH2)}2(B-ZnW9O34)2]12-, where M = Mn2+, Co2+, Ni2+, Cu2+ and Zn2+, have been studied as oxygen transfer catalysts for the epoxidation of a range of primary allylic alcohols using aqueous H2O2 under biphasic conditions (1,2-dichloroethane/H2O). These studies have been performed under reaction conditions that allow the effects of the transition metals substituted into the waists of these sandwich-type anions to be differentiated. The catalytic activity of the anions gave the order Zn2+ > Mn2+ ~ Cu2+ > Co2+ > Ni2+, which correlates with the rate of water exchange around the [M(OH2)6]2+ species for species that exchange by a dissociative mechanism (Id or D), and with the theoretically calculated activation energies for a D mechanism. The anomalous position of Cu2+ can be accounted for by the orientation of the (static) Jahn-Teller distorted environment around this ion within the structure. The activity order is consistent with a pre-equilibrium involving exchange of water and allylic alcohol, which holds the latter in place prior to epoxidation of the allyl alcohol by adjacent W(O2) sites. The formation of W(O2) sites from W=O sites in the polyoxotungstate framework has also been studied. Kinetic traces have shown the slow formation of such sites by “induction” periods. Analogous studies have been performed on the tetra- and tri-metallic anions [M4(OH2)2(B-PW9O34)2]10-, where M = Mn2+, Co2+, Ni2+, Cu2+ and Zn2+, [Ni3(OH2)3(B-PW9O34){WO5(H2O)}]7- and [Co3(OH2)3(A-PW9O34)2]12-. Differences in catalytic activity to the [WZn{M(OH2)}2(B-ZnW9O34)2]12- series can be related to the replacement of Zn(II) by P(V) in the (XW9O34)n- anions. These studies have been complemented by investigations on [NaP5W30O110]14- (Preyssler’s anion), and on the iso-and hetero-polyperoxoanions [{W(=O)(O2)2}2O]2- and [PO4{W(=O)(O2)2}4]3-, in order to examine structural features that are important to the reaction mechanism of the epoxidation process. Full kinetic studies, including the evaluation of Arrhenius and Eyring parameters have been performed for epoxidation of 3-methyl-2-buten-1-ol as the allylic alcohol under biphasic conditions with [WZn{Mn(OH2)}2(B-ZnW9O34)2]12-, [Ni3(OH2)3(B-PW9O34){WO5(H2O)}]7-, [Co3(OH2)3(A-PW9O34)2]12-, [{W(=O)(O2)2}2O]2- and for trans-2-hexen-1-ol with the Venturello-Ishii catalyst [PO4{W(=O)(O2)2}4]3-. All hetero- and iso-polyoxotungstate compounds used in these studies were thoroughly characterized by chemical analysis (where appropriate) and by infrared and UV-visible spectroscopy, thermogravimetric and differential thermal analysis, and 31P nmr spectroscopy.