Rates of solvent exchange in aqueous aluminium(III)-maltolate complexes

Results from single-crystal X-ray diffraction and ²⁷Al- and ¹⁷O-NMR spectroscopy are reported for a series of aluminium-maltolate complexes. Maltolate bonds to Al(III) via 3-oxy and 4-pyronate functional groups and in acidic solutions forms bidentate complexes with Al(III) that have the stoichiometry: Al(ma)_n(H₂O)_6-2n^3-n (ma = maltolate and n = 0, 1, 2, 3), which was confirmed by the structure refinement of the n = 3 compound. The relative concentrations of these complexes determined by ²⁷Al-NMR compare well with those predicted from thermodynamic data derived from potentiometry at 298 K and I = 0.6. The rate parameters for exchange of inner-sphere water molecules with bulk solution were determined by ¹⁷O-NMR for the Al(ma)(H₂O)₄²⁺ complex: k_ex²⁹⁸ = 304 (±26) s^-1, ΔH‡ = 63 (±2) kJ mol^-1, and ΔS‡ = 14 (±7) J mol^-1 K^-1 and for Al(ma)₂(H₂O)₂⁺: k_ex²⁹⁸ = 1950 (±91) s^-1, ΔH‡ = 49 (±2) kJ mol^-1, and ΔS‡ = -319 (±6) J mol^-1 K^-1. Surprisingly, maltolate labilizes inner-sphere water molecules to an extent that is similar to bidentate dicarboxylate and carboxylate plus phenolic ligands studied previously. Substitution of a single maltolate into the inner-coordination sphere of Al(III) increases the exchange rate of the remaining bound water molecules with the bulk solution by a factor of ≈10². Substitution of a second maltolate increases the rates by another factor of 6-7, which is similar to simple aliphatic organic acids, such as methylmalonate.