THE COORDINATION CHEMISTRY OF TRIVALENT RARE EARTH METALS WITH FLUORO-SUBSTITUTED BETA-DIKETONE LIGANDS
Optically active ligands 3-trifluoroacetyl-d-camphorate (Hfacam), 3-heptafluorobutyryl-d-camphorate (Hhfbc), 3-pentafluoropropionyl-d-camphorate (Hpfpc), and 3-pentadecafluorooctanoyl-d-camphorate (Hpdfoc) are synthesized in nearly quantitative yield using a magnesium reaction in which d-3-bromocamphor and ethyl perfluoro ester are starting materials. Hfacam is isolated from the reaction mixture by etherate solvent extraction with aqueous KOH, whereas ethanolic anionic ion-exchange chromatography purifies the other four. Transition metal impurities such as iron are eliminated from the final product by fractionally distilling each ligand.^ Hpfpc and Hpdfoc are characterized by synthesizing 28 different rare earth chelates of the general type Ln(pfpc)(,3) and Ln(pdfoc)(,3). The preparation methods also work for anhydrous chelates currently used as optically active n.m.r. shift reagents, Ln(facam)(,3) and Ln(hfbc)(,3). Anhydrous rare earth chelates are made using a barium ligand exchange reaction for all ligands except Hpdfoc which must be reacted directly with the rare earth metal and dehydrated separately because Ba(pdfoc)(,2) is not easily prepared. Even when the exchange reaction is done in anhydrous acetone, partially hydrated chelates form that must be dehydrated in hexane with anhydrous MgSO(,4).^ The coordination reactions are evaluated further in a study in which the stepwise formation constants for the reaction of 13 rare earth metals are studied. Ligands Hfacam, Hpfpc, and Hhfbc are compared with 6,6,7,7,8,8,8-heptafluoro-3,5-octanedione or Hfod, and 2-thenoyltrifluoroacetone or Htta. Hfod is studied because it is similar to Hfacam and Hhfbc in that it forms anhydrous rare earth chelates extensively used as n.m.r. shift reagents; Htta is studied because it is so widely used as a solvent extraction analytical reagent. Coordination trends for 65 different rare earth chelates are compared as stabilities are measured in 70 v/v% acetone-water at 20.0(DEGREES)C using 0.1000 M NaCl. Formation constants are calculated using the potentiometric titration method equivalent to that used by Van Uitert and Haas based on the method of Carlson, et al. and Bjerrum. Stabilities for four rare earth metals with Htta are measured in 75 v/v% dioxane-water at 30.0(DEGREES)C in 0.1000 M KCl for comparison purposes so they may be correlated with stabilities measured in acetone-water. They also correlate linearly with stabilities measured by others using benzene-water solvent extraction at 25(DEGREES)C. The pK(,D) of Hfacam, Hpfpc, Hhfbc, Htta, and Hfod is measured separately to determine how the fluoro-substitution influences the ligand acidity and chelate stability.^ To use acetone-water as a solvent, it is necessary to: (1) measure the hydrogen ion concentration, and (2) know the mean molar activity coefficient, y(+OR-), for the conditions of the study. Activities are needed to: (1) calibrate the electrode so pH can be measured, (2) to calculate pK(,D), and (3) to calculate thermodynamic formation constants for the chelating reactions. Activity coefficients reported in the literature are fitted to a form of the Huckel equation. The glass indicating and calomel reference electrodes are then calibrated so acidity can be measured at any temperature, ionic strength, and for any reasonable acetone-water composition. The claim of Jervis and Neelakantan that acidity in 60 v/v% acetone-water or greater cannot be accurately measured is disproved by showing that the Nernst equation accounts for all electrode responses. To prove the pH meter correction term, log U(,H), is not a function of acidity at high concentrations of acetone, an acid-base titration shows that as acidity is varied in 70 v/v% acetone-water at 20.0(DEGREES)C, log U(,H) remains constant for a wide range of acidity. ^
ROBERTS, JOHN STUART, "THE COORDINATION CHEMISTRY OF TRIVALENT RARE EARTH METALS WITH FLUORO-SUBSTITUTED BETA-DIKETONE LIGANDS" (1981). ETD Collection for Fordham University. AAI8123467.