A manganese(III)-substituted polyoxometalate, [α2-P 2MnIII(L)W17O61]7- (P2W17MIII), was studied as an oxidation catalyst using iodopentafluorobenzene bis(tifluoroacetate) (F 5Phl(TFAc)2) as a monooxygen donor. Pink P 2W17MnIII 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 P2W17MnIII-F5-Phl(TFAc) 2 that disappeared upon warming. The 31P NMR spectra of P2W17MnIII 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 17MnV=O. An additional broad peak at -10.6 ppm was attributed to both the P2W17-MnIII-F 5Phl(TFAc)2 complex and a paramagnetic, triplet manganese(V)-oxo species. The electronic structure and reactivity of P 2W17MnV=O were modeled by DFT calculations using the analogous Keggin compound, [PMnV=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 [PMnV=OW11O39]4- should be highly reactive, even more reactive than analogous MnV=O porphyrin species. Kinetic measurements of the reaction of P2W17Mn V=O with 1-octene indicated, however, that P2W 17MnV=O was less reactive than a MnV=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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry