The rates of exchange of water molecules from Al(III)-methylmalonate complexes: The effect of chelate ring size

Rate coefficients are reported for exchange of hydration waters in the inner-coordination-sphere of Al(III)-methylmalonate complexes with bulk solution as determined via ¹⁷O-NMR. Surprisingly, water molecules in the thermodynamically less-stable complexes containing six-membered chelates are much more labile than those in five-membered oxalate-Al(III) complexes. The rate parameters for the Al(mMal)⁺ complex are: k(ex)²⁹⁸ = 6.6 · 10² s^-1, ΔH = 66(±1) kJ mol^-1; ΔS = 30.6(±2) J mol^-1 K^-1 and for the Al(mMal)₂^- complex are k(ex)²⁹⁸ = 6.9 · 10³ s^-1, ΔH = 55(±3) kJ mol^-1; ΔS = 12.8(±11) J mol^-1 K^-1 The surprising trend in reactivity is attributable either to differences in the Lewis basicities of oxygens in bidentate oxalate and methylmalonate ligands, or to rapid dissociation/reassociation of one of the acetate groups to the metal center. These results identify a useful case where trends in the apparent labilities of dissolved and presumed surface complexes deviate sharply. This deviation could be usefully exploited to probe surfaces if ligand-promoted dissolution rates could be compared at conditions where inner-sphere and outer-sphere chelate complexes could be distinguished spectroscopically. We expect inner-sphere oxalate to have a smaller labilizing effect than malonate or methylmalonate. A contrary result would indicate structural dissimilarity between complexes on the surface and in solution, or perhaps steric hindrance.