PRASEODYMIUM NMR-SHIFT REAGENTS FOR CARBOXYLIC-ACIDS AND THEIR CARBOXYLATES - SYNTHESIS, X-RAY STRUCTURE AND APPLICATIONS

The compound (Pr(tpip)(3) 1, tris(tetraphenylimidodiphosphinato) prase odymium, was obtained from PrCl3 . 6H(2)O and tpipK in aqueous medium free from any external ligand, and characterized by X-ray crystallogra phy. Recrystallization of 1 from acetone solution gave Pr(tpip)(3)(CH3 COCH3) 3. Under the same conditions DyCl3 . 6H(2)O gave Dy(tpip)(3) 2. Pr(tpip)(3) induces important upheld shifts in the H-1 NMR spectra of carboxylic acids, as a result of the formation of labile Pr(tpip)(3)( RCOOH) adducts. It can thus be used for the straightforward simplifica tion and analysis of the H-1 NMR spectra of carboxylic acids. Attempts to isolate Pr(tpip)(3)(AcOH) only gave small amounts of Pr(tpip)(3)(H 2O) 4. However, in the case of chloropropionic acid, the adduct Pr(tpi p)(3)(CH3CHClCOOH)(2) 5 could be isolated and fully characterized by X -ray crystallography. Pr(tpip)(3) reacts instantaneously with potassiu m or ammonium carboxylates to give, upon exchange of one tpip Ligand f ollowed by dimerization, the dinuclear complexes Pr-2(tpip)(4)(RCOO)(2 ) 6 in which tpip acts both as chelating and bridging ligands, whereas the carboxylate groups act as tridentate bridging-chelating ligands. These new complexes have been fully characterized by X-ray analysis in the case of citronelic and 3-hydroxybutyric acids. The H-1 NMR spectr a of such complexes disclose very important high-field shifts for the protons associated with the carboxylate groups. Moreover, in the case of chiral acids, the achiral Pr(tpip)(3) reagent can be used, in assoc iation with carboxylates to determine their optical purity by means of the formation of diastereoisomeric dinuclear complexes which can be d ifferentiated by H-1 NMR spectroscopy. Applications of these new reage nts will be provided.