MOLECULAR MODELING STUDIES ON THE STRUCTURE AND ELECTRONIC-PROPERTIESOF BIS(THIOPHOSPHORYLAMINES) AND THEIR ZINC-COMPLEXES

The structures and charge distributions of a number of bidentate bis(d isubstituted-thiophosphorylamine) ligands used to extract zinc have be en assessed by both semiempirical and ab initio molecular orbital meth ods and the results compared with crystallographic data where availabl e. At the semiempirical level, the MNDO method gives superior results to either the AM1 or PM3 methods, which give poor charge distributions at the N, P, and S atoms. Five possible conformers of two tautomeric types were calculated for both alkyl and aryl derivatives. The results suggest that ligands containing an alpha-branched alkyl chain at phos phorus, which are good extractants for zinc, exist predominantly as ci s SH tautomers with the acidic hydrogen covalently bonded to one sulfu r and hydrogen bonded to the other. In contrast, those ligands contain ing a primary carbon or oxygen at phosphorus, which are poor extractan ts for zinc, are predicted to exist as NH tautomers with both sulfur a toms forming a trans arrangement to one another. These structural diff erences appear to explain the marked difference in the P-31 chemical s hifts between the alpha-branched compounds and the other types.