NICKEL SHIFT REAGENTS IN NMR SPECTROSCOPY
Abstract
The use of (beta)-diketonate nickel (II) complexes as shift reagents in ('1)H and ('13)C NMR spectroscopy was investigated. The reactions of one, bis(2,4-pentanedionato)nickel (II) (Ni(AcAc)(,2)), in chloroform were studied by infrared, visible, ultraviolet and NMR spectroscopy. The following equilibrium constants were measured. 3Ni(AcAc)(,2) (DBLARR) Ni(AcAc)(,2))(,3) K = 7x10('4) l('2)/mol('2) Ni(AcAc)(,2) + 2H(,2)O (DBLARR) Ni(AcAc)(,2)(H(,2)O)(,2) K = 2x10('5) l('2)/mol('2) Ni(AcAc)(,2) + 2n-Butylamine (DBLARR) Ni(AcAc)(,2)(n-Butylamine)(,2) K = 3 x 10('9) l('2)/mol The identity of the intermediate in the final reaction was established to be Ni(AcAc)(,2)(n-Butylamine)(H(,2)O). NMR shift experiments with amines revealed the presence of line broadening in addition to peak shifting. The line broadening was attributed to exchange broadening on the NMR timescale between substrate molecules which are complexed to the shift reagent and those which are uncomplexed. The rate of this reaction was measured by ('13)C NMR lineshape analysis, and the activation energy for the reaction was found to be 13.1 kcal/mol. In the absence of exchange, line broadening was shown to originate predominately from the magnetic dipolar relaxation mechanism. Optimal conditions for performing these shift experiments were defined. These conditions include high temperatures, high concentrations of the shift reagent and ('13)C NMR. A chiral nickel shift reagent, bis(3-trifluoroacetyl-d-camphorateo) nickel (II), was prepared for use in distinguishing chiral substrate molecules. In preliminary experiments, no chiral resolution was detected.
Degree
Ph.D.
Subject Area
Organic chemistry
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