| Summary: | Aqua/hydroxo mononuclear Al3+ species coordinated by F in aqueous solution are investigated using density functional theory (DFT B3LYP/6-311++G(d,p)) and the polarized continuum model (PCM). Optimized gas-phase geometries have been obtained for the species AlF(OH)(n)(H2O)(m)((2-n)+) in which n = 0, 1, 2, or 3 while (n + m) = 3, 4, or 5. Analysis of the AlF, Al-O, and 0 H bond lengths and the Al, F, 0, and H natural charges of these complexes reveals clear trends that suggest increased acidity with decreasing coordination number (CN) and decreased water stability with increased hydrolysis. These observations are supported by the calculation and analysis of the dehydration and hydrolysis reaction Gibbs free energies Delta G(aqueous)(dehydration) and Delta G(aqueous)(dehydration) of the AlF(OH)(n)(H2O)(m)((2-n)+) complexes, which clearly show a strong correlation between increased hydrolysis. and a preference to coordinate fewer water molecules. The combination of the appropriate Delta G(aqueous)(dehydration) and Delta G(aqueous)(dehydration) values generate the aqueous Gibbs free energies relative to AlF (H2O)(5)(2+) and demonstrate the clear transition from a 6 to 5 to 4 coordinate species as a function of ligand hydrolysis. Calculation of the equilibrium mole fraction of each species as a function of pH shows that this system is largely dominated by the AlF(OH)(1)(H2O)(4)(1+) and AlF(OH)(3)(1-) species. A comparison of structural and electronic data with the aqueous Al3+ complexes shows a remarkable similarity when plotted against the number negative ligands (F- or OH-), suggesting that the F- anion coordinates the Al3+ cation in a similar way to the remaining OH- anions. The comparison of the calculated equilibrium mole fractions of each species displays important changes in the composition of our model system upon Al3+ coordination by F- in the direction of increased acidity of these complexes. Our predicted decreased stability of the Al water bond is in complete agreement with experimental NMR observations of an increased water exchange rate upon F coordination of aqueous aluminum complexes. Our prediction of stable hydroxide ternary complexes is not in agreement with recent NMR data, which indicate that these complexes do not readily form. An explanation for this may lie in the increased lability of these complexes, which may lead to difficulties in NMR detection.
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