The rates of water exchange in AI(III)-salicylate and AI(III)-sulfosalicylate complexes

Rate parameters are reported for exchange of hydration waters from the inner coordination sphere of Al(III)-sulfosalicylate [Al(sSal)⁺] and Al(III)-salicylate [Al(Sal)⁺] complexes to bulk solution as determined with ¹⁷O-NMR. The rate parameters for the Al(sSal)⁺ complex are: k(ex)²⁹⁸ = (3.0 ± 0.2) · 10³ s^-1, ΔH(++) = 37(±3) kJ mol^-1, ΔS(++) = -54(±9) J mol^-1 K^-1; and for the Al(Sal)⁺ complex are: k(ex)²⁹⁸ = 4.9(±0.3) · 10³ s^-1, ΔH(++) = 35(±3) kJ mol^-1, ΔS(++) = -57(±11) J mol^-1 K^-1. These results, along with previous work, suggest that the lability of water molecules in bidentate carboxylate-phenolic complexes scales with the electron-donating properties of the ligand oxygens. Replacement of a coordinated carboxyl with a phenolic group in the ligand increases both the Lewis basicity and the value of k(ex)²⁹⁸. A correlation between these parameters is proposed that can be used to predict rate coefficients for other bidentate aluminum complexes.