AB-INITIO MO-LCAO INVESTIGATION OF THE STRUCTURE AND REACTIVITY TOWARDS ALKENES OF MODEL TUNGSTEN(VI) PEROXO COMPLEXES DERIVED FROM THE TETRAKIS (OXODIPEROXOTUNGSTO) PHOSPHATE(3-) COMPLEX, (PO4[W(O)(O-2)(2)](4))(3-)

An ab initio MO-LCAO investigation was undertaken concerning the struc ture and reactivity towards alkenes of model tungsten(VI) peroxo compl exes derived from the tetrakis (oxodiperoxotungsto) phosphate(3-) comp lex, {PO4[W(O)(O-2)(2)](4)}(3-), an efficient reagent (as ''onium'' sa lt) for olefin epoxidation in nonprotic solvents, as well as a highly effective catalyst for the same reaction when used in combination with hydrogen peroxide in a two-phase system. The complex of formula [H2PO 4(W2O10)](-) (A), chosen as a simplified model of the (PO4[W(O)(O-2)(2 )](4)}(3-) anion, and the related ''partially reduced'' complexes of g eneral formula [H2PO4(W2O9)](-)(B-1,B-2) and [H2PO4(W2O8)](-) (C1-3) w ere considered. The best molecular geometries for all the investigated complexes were obtained at Hartree-Fock level. using a relativistic e ffective potential for core electrons of the W atoms. The Hartree-Fock energies corrected for electron correlation by means of Lee-Yang-Parr density functional were used to evaluate the reaction energies regard ing the peroxidic oxygen transfer from complexes A and B-1,B-2 to an a lkene (ethylene). The energetics of these complexes is discussed in th e framework of a global process leading to the formation of epoxide an d restoring the starting complexes by reaction of the resulting ''redu ced'' species with hydrogen peroxide. All the reaction steps, identifi ed on the basis of a purely theoretical investigation, were found to b e, to a different extent, exothermic. The analysis of the net atomic c harges carried by the peroxidic oxygens in complexes A and B-1,B-2 rev eals that the oxygen expected to be the ''most electrophilic'' is the one adjacent to the triply shared oxygen. Its transfer to ethylene is associated with the highest energy gain when complex A is involved, bu t such a behaviour is no longer regularly observed when a complex of t ype B is considered, (C) 1997 Academic Press.