Na(I) binding by tartaric acid derivatives

A study of the binding of Na⁺ by two readily available tartaric acid derivatives, dimethyl tartrate and dipyrrolidine tartramide, has been carried out. These binding studies reveal binding stoichiometries of 1:1 for the dimethyl tartrate and 2:1 for the dipyrrolidine tartramide, and binding affinities (association constants) of 6.61 M⁻¹ for the dimethyl tartrate and 70.4 M⁻² for the dipyrrolidine tartramide indicating weak binding of Na⁺ in both cases. An X-ray crystal structure of a NaI complex of dipyrrolidine tartramide has also been determined. The binding stoichiometry is 1:1 in the solid state as opposed to the 2:1 binding stoichiometry that is observed in solution. The 1:1 binding in the solid state results in a coordination polymer in which half of the carbonyl oxygens and half of the hydroxy oxygens of the dipyrrolidine tartramide ligand bridge between adjacent Na⁺ cations. This allows each Na⁺ cation to achieve an octahedral coordination geometry. The iodides are ordered in a linear fashion, and each column of iodides is separated from the other columns by the coordination polymer and by linear columns of water molecules.