https://nova.newcastle.edu.au/vital/access/ /manager/Index en-au 5 https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:48280 3(OH₂)₃(B-PW₉O₃₄){WO₅(H₂O)}]⁷⁻ and [Co₃(OH₂)₆(A-PW₉O₃₄)₂]¹²⁻ have been studied as epoxidation catalysts for oxygen transfer from 30% H₂O₂ to a range of allylic alcohols under biphasic conditions (1,2-dichloroethane/H₂O) at 15 °C. The reaction mechanism involves coordination of an allylic alcohol at an M(II) site in each case, prior to transfer of a peroxy oxygen from an adjacent W(O₂) site. The latter is formed from a terminal W = O unit by reaction with H₂O₂. Evidence of W(O₂) formation was obtained through IR studies. The W(O₂) group forms the epoxide by transfer of an oxygen atom to the C=C bond of the coordinated allylic alcohol. Kinetic studies using 3-methyl-2-buten-1-ol as the allylic alcohol substrate have been modelled with all three metal sites catalytically active. The reaction involves an autocatalysis mechanism involving an induction period, which can be rationalised by proposing not only coordination of the allylic alcohol to M(II), but also the product hydroxy epoxide, both through their –OH groups. The autocatalysis is generated by formation of the W(O₂) group adjacent to a coordinated hydroxy epoxide, which competes with coordination of allylic alcohol. The mechanism requires some twenty-one steps involving just the generic steps listed above, with all three metal sites catalytically active. Temperature-dependent kinetic studies and subsequent Eyring analyses have shown that the Co(II)-containing catalyst is the most active of the two. Analogous studies of the epoxidation of 3-methyl-2-buten-1-ol by the two-site [M₄(OH₂)₂(B-PW₉O₃₄)₂]¹⁰⁻ ions as catalysts, where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II), at 15 °C gave an order of reactivity of Cu(II) > Ni(II) > Zn(II), Co(II), Mn(II), which mostly mimics the natural order of stability constants (the Irving-Williams series), suggesting that the formation of the allylic alcohol complexes play a dominant role in this series of related complex anions, with greater replacement of water by allylic alcohol leading to greater reactivity.]]> Tue 14 Mar 2023 08:56:56 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:8178 Zn(II)>Co(II)>Ni(II),and for M(III) and M(IV) substitution is Mn(III)~Ir(IV)>Fe(III)>Cr(III). The observed orders are consistent with the formation of metal(n+)-alcohol species as part of the reaction mechanism. For the more polarizing Ir(IV), however, Ir(IV)-alcoholate species are likely involved in the mechanism. Formation constants for the Mn(III)and Co(II)-phosphopolyoxotungstate-alcohol species with all of the above alcohols have been evaluated in 1,2-dichloroethane at 25 °C and range from 19.0-3.5 M⁻¹. The most likely transition state involves coordination of the alcohol to the transition metal substituted at the lacunary site, or alkoxide in the case of Ir(IV), along with interaction of the double bond of the alcohol with a peroxogroup located at a W(VI) site adjacent to the substituted transition metal.]]> Sat 24 Mar 2018 08:36:16 AEDT ]]>