MOLECULAR MODELING 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-1, C-13 and P-31) and electronic spectroscopies have been u sed to study the tautomeric forms of 3-phenyl-4-benzoyl-isoxazol-5-one , ''HPBI'', and its interaction with tri-n-octylphosphine oxide, ''TOP O'', in wet toluene or benzene, and in chloroform. These metal extract ing media have been compared with analogous solutions containing 1-phe nyl-3-methyl-4-acyl-pyrazol-5-ols. The spectroscopic results are compa red with those of molecular modelling studies. Spectroscopic criteria that allow a choice between the various tautomeric forms of pyrazolone s or isoxazolones are briefly reviewed. In the solid, in wet toluene, benzene or chloroform solutions, HPBI mainly exists as a B-keto-enol c helate, quite different from its tautomeric form in methanol, which is most probably a diketone-enamine. Molecular modelling studies show th at the energy difference between the chelated 5-hydroxy- and 5-one- ta utomeric forms of the free molecule is rather low. They cannot be disc riminated by spectroscopy. As shown by UV and PMR spectroscopies, the interaction of TOPO wi th HPBI is stronger than with 4-acyl-5-hydroxyp yrazoles (acyl: lauroyl, benzoyl, thenoyl). The most likely tautomeric form for HPBI in the H-bonded [TOPO.HPBI] moiety is the diketone-enam ine (b'). Accordingly, molecular modelling studies show that the R(3)P O ... H-N-HPBI hydrogen bond is 0.15 Angstrom shorter than the R(3)PO ... H-O-HPBI one, when comparing two isomeric forms of the 1:1 [TOPO.H PBI]complex.