Slow dynamics of aluminium-citrate complexes studied by ~1H- and ~(13)C-NMR spectroscopy

摘要

Inter- and intra-molecular exchange reactions of the major Al(Ⅲ)-citrate (Cit) complexes have been studied under equilibrium conditions in aqueous solution by ~1H- and ~(13)C-NMR using band shape analysis and magnetization transfer methods. Rate equations and activation parameters have been evaluated from concentration, pH and temperature dependent studies. The lability with respect to ligand exchange was: Al(Cit)_2~(3-) (k_(298) = 1.1 +- 0.1 s~(-1)) > Al_3(H_(-1)Cit)_3(OH)_4~(7-) (Sy; k_(298) = 0.08 +- 0.01 s~(-1))Al_3(H_(-1)Cit)_3(OH)~(4-) (As). The variation in lability appears to be related to the different coordination modes of the citrate ligands in the complexes. The ligand exchange reactions show an I_a mechanism for Al(Cit)_2~(3-) and Sy. These two complexes are fluxional (k_(298)~(Sy) = 230 s~(-1)), the intra-molecular rearrangement of Sy goes through a bond rupture mechanism. As is neither fluxional nor labile for ligand exchange, k_(As) ≤ 0.03 s~(-1) at 353 K.