19F NMR study of the equilibria and dynamics of the Al3+/F- system

Article abstract of DOI:10.1021/ic991248w

A careful reinvestigation by high-field ¹⁹F NMR (470 MHz) spectroscopy has been made of the Al³⁺/F^- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The ¹⁹F NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlF(i)((3-i)+)(aq) with i ≤ 5. There was no need to invoke AlF(i)(OH)(j)((3-i-j)+) mixed complexes in the model under our experimental conditions (pH ≤ 6.5), nor was any evidence obtained for the formation of AlF₆^3-(aq) at very high ratios of F^-/Al³⁺. The stepwise equilibrium constants obtained for the complexes by integration of the ¹⁹F signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log K(i) values are 6.42, 5.41, 3.99, 2.50, and 0.84 (for species i = 1 - 5) and 6.35, 5.25, and 4.11 (for species i = 1 - 3), respectively. Disappearance of the ¹⁹F NMR signals under certain conditions was shown to be due to precipitation. Certain ¹⁹F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F^- exchange rather than complex formation. The detected reactions and their rate constants are AlF₂²⁺ + *F^- ? AlF*F²⁺ + F^- (k₀₂ = (1.8 ± 0.3) x 10⁶ M^-1 s^-1), AlF₃⁰ + *F^- ? AlF₂*F⁰ + F^- (k₀₃ = (3.9 ± 0.9) x 10⁶ M^-1 s^-1), and AlF₃⁰ + H*F ? AlF₂*F⁰ + HF (k(H)₀₃ = (6.6 ± 0.5) x 10⁴ M^-1 s^-1). The rates of these exchange reactions increase markedly with increasing F^- substitution. Thus, the reactions of AlF²⁺(aq) were too inert to be detected even on the T₁ NMR time scale, while some of the reactions of AlF₃⁰(aq) were fast, causing large line broadening. The ligand exchange appears to follow an associative interchange mechanism. The cis-trans isomerization of AlF₂⁺(aq), consistent with octahedral geometry for that complex, is slowed sufficiently to be observed at temperatures around 270 K. Difference between the Al³⁺/F^- system and the much studied Al³⁺/OH^- system are briefly commented on.