Organotin(IV) complexes of polyhydroxyalkyl carboxylic acids and some related ligands

A number of dibutyltin(IV) complexes of polyhydroxyalkyl carboxylic acids (O donor atoms) and amino acids (O,N donor atoms) were prepared in the solid state. The binding sites of the ligands were determined by means of FT-IR, Raman and ¹³C NMR spectroscopy. Partial quadrupole splitting calculations were utilized to determine the coordination geometry around the Sn(IV) centre by means of Mössbauer measurements. The results showed that in the solid state oligomeric complexes are formed, with the -COO^- groups as bridges between the organometallic cations. The {Sn} atoms are mostly in trigonal bipyramidal surroundings. The Sn-O and Sn-C bond distances were determined by EXAFS measurements to be 207-234 and 295 pm, respectively. Evaluation of the pH-metric and NMR titration curves in Me₂Sn(IV)-D-gluconic acid system revealed that the equilibria in aqueous solution are fairly complicated. In acidic solution, the formation of 1 : 1 and 1 : 2 -COO^- coordinated species predominate, but deprotonation of the alcoholic -OH groups also starts at very low pH. In the pH range 5-9, NMR provides experimental evidence of ligand-exchange reactions without pH-metrically detectable proton release. In alkaline solution, further deprotonation processes occur, resulting in either alkoxo or mixed hydroxo complexes. The carboxylate coordination is expected for the amino acid ligands but the shift of the νN-H stretching vibrations in the FT-IR spectra demonstrated that the ammine group also binds to the metal ion in the solid Bu₂Sn(IV)complexes.