Water exchange in fluoroaluminate complexes in aqueous solution: A variable temperature multinuclear NMR study

An ¹⁷O, ¹⁹F, and ²⁷Al NMR study of fluoroaluminate complexes (AlF_n(H₂O)_6-n^(3-n)+, n = 0, 1, and 2) in aqueous solution supports the idea that for each substitution of a bound water molecule by a fluoride anion, the exchange rate of bound water with free water increases by about 2 orders of magnitude. New rate coefficients for exchange of inner-sphere water molecules in AlF(H₂O)₅²⁺ are k_ex²⁹⁸ = 230(±20) s^-1, ΔH^≠ = 65(±3) kJ mol^-1, and ΔS^≠ = 19(±10) J mol^-1 K^-1. The corresponding new values for the AlF₂(H₂O)₄⁺ complex are: k_ex²⁹⁸ = 17 100(±500) s^-1, ΔH^≠ = 66(±2) kJ mol^-1, and ΔS^≠ = 57(±8) J mol^-1 K^-1. When these new results are combined with those of our previous study, we find no dependence of the solvent exchange rate, in either AlF(H₂O)₅²⁺ or AlF₂(H₂O)₄⁺, on the concentration of fluoride or protons over the range of ∑F = 0.06-0.50 M and [H⁺] = 0.01-0.44 M. A paramagnetic shift of ²⁷Al resonances results from addition of Mn(II) to the aqueous solution as a relaxation agent for bulk waters. This shift allows resolution of the AlF_n(H₂O)_6-n^(3-n)+ species in ₂₇Al NMR spectra and comparison of the speciation determined via thermodynamic calculations with that determined by ²⁷Al, ¹⁹F, and ¹⁷O NMR.