Characterization of manganese(V)-oxo polyoxometalate intermediates and their properties in oxygen-transfer reactions

A manganese(III)-substituted polyoxometalate, [α₂-P ₂Mn^III(L)W₁₇O₆₁]^7- (P₂W₁₇M^III), was studied as an oxidation catalyst using iodopentafluorobenzene bis(tifluoroacetate) (F ₅Phl(TFAc)₂) as a monooxygen donor. Pink P ₂W₁₇Mn^III turns green upon addition of F ₅Phl(TFAc)₂. The ¹⁹F NMR spectrum of F ₅Phl(TFAc)₂ with excess P₂W₁₇Mn ^III at -50°C showed the formation of an intermediate attributed to P₂W₁₇Mn^III-F₅-Phl(TFAc) ₂ that disappeared upon warming. The ³¹P NMR spectra of P₂W₁₇Mn^III with excess F₅Phl(TFAc) ₂ at -50 and -20°C showed a pair of narrow peaks attributed to a diamagnetic, singlet manganese(V)-oxo species, P₂W ₁₇Mn^V=O. An additional broad peak at -10.6 ppm was attributed to both the P₂W₁₇-Mn^III-F ₅Phl(TFAc)₂ complex and a paramagnetic, triplet manganese(V)-oxo species. The electronic structure and reactivity of P ₂W₁₇Mn^V=O were modeled by DFT calculations using the analogous Keggin compound, [PMn^V=OW₁₁O ₃₉]^4-. Calculations with a pure functional, UBLYP, showed singlet and triplet ground states of similar energy. Further calculations using both the UBLYP and UB3LYP functionals for epoxidation and hydroxylation of propene showed lowest lying triplet transition states for both transformations, while singlet and quintet transition states were of higher energy. The calculations especially after corrections for the solvent effect indicate that [PMn^V=OW₁₁O₃₉]^4- should be highly reactive, even more reactive than analogous Mn^V=O porphyrin species. Kinetic measurements of the reaction of P₂W₁₇Mn ^V=O with 1-octene indicated, however, that P₂W ₁₇Mn^V=O was less reactive than a Mn^V=O porphyrin. The experimental enthalpy of activation confirmed that the energy barrier for epoxidation is low, but the highly negative entropy of activation leads to a high free energy of activation. This result originates in our view from the strong solvation of the highly charged polyoxometalate by the polar solvent used and adventitious water. The higher negative charge of the polyoxometalate in the transition versus ground state leads to electrostriction of the solvent molecules and to a loss of degrees of freedom, resulting in a highly negative entropy of activation and slower reactions.