MOLECULAR MODELLING AND SPECTROSCOPIC STUDIES OF THE INTERACTIONS BETWEEN CHELATING AND ELECTRON-DONOR METAL EXTRACTANTS: THE SYSTEM TRI-N-OCTYLPHOSPHINE OXIDE - 3-PHENYL-4-BENZOYL-ISOXAZOL-5-ONE ( 'HPBI' ) IN WET TOLUENE AND CHLOROFORM

IR, NMR ( H , 13 C and 31 P) and electronic spectroscopies have been used to study the tautomeric forms of 3-phenyl-4-benzoyl-isoxazol-5-one, "HPBI", and its interaction with tri-rc-octylphosphine oxide, 'TOPO', in wet toluene or benzene, and in chloroform. These metal extracting media have been compared with analogous solutions containing l-phenyl-3-methyI-4-acyI-pyrazoI-5-oIs. The spectroscopic results are compared with those of molecular modelling studies. Spectroscopic criteria that allow a choice between the various tautomeric forms of pyrazolones or isoxazolones are briefly reviewed. In the solid, in wet toluene, benzene or chloroform solutions, HPBI mainly exists as a fi-keto-enol chelate, quite different from its tautomeric form in methanol, which is most probably a diketone-enamine. Molecular modelling studies show that the energy difference between the chelated 5-hydroxy- and 5-one- tautomeric forms of the free molecule is rather low. They cannot be discriminated by spectroscopy As shown by UV and PMR spectroscopies, the interaction of TOPO with HPBI is stronger than with 4-acyl-5-hydroxypyrazoles (acyl: lauroyl, benzoyl, thenoyl). The most likely tautomeric form for HPBI in the H-bonded [TOPO.HPBI] moiety is the diketone-enamine (b'). Accordingly, molecular modelling studies show that the R3PO-H-NHPBI hydrogen bond is 0.15A shorter than the R3PO-H-OHPBI one, when comparing two isomeric forms of the 1:1 [TOPO.HPBI]complex.