Diorganotin(IV) complexes of D-galacturonic acid: solid-state and solution-phase structural study

Three diorganotin(IV) complexes of D-galacturonic acid (H2GalA; R = Me, n-Bu, Ph), two of which are new derivatives (R = Me, Ph), have been synthesized and their solid-state and solution-phase investigated by IR, Mossbauer, 1H, 13C and 119Sn NMR spectroscopy. The FTIR data suggest that H2GalA, in the dialkyltin derivatives, behaves as a dianionic ligand, coordinating the tin(IV) atom through an ester-type carboxylate and deprotonated alcoholic hydroxo groups, whereas a bridging carboxylate occurs in the diphenyltin(IV) complex. Octahedral and trigonal bipyramidal local geometries on tin(IV) atoms are proposed for dialkyltin(IV)GalA and diphenyltin(IV)GalA complexes respectively on the basis of Mossbauer spectroscopy. In D2O solution, R2Sn(IV) moieties seem to be mainly coordinated by the GalA2− in the β-furanosidic (β-GalfA2−) form, involving the β-furanosidic ring in rotational equilibrium. However, the α-furanosidic (α-GalfA2−), α-pyranosidic (α-GalpA2−) and the β-pyranosidic (β-GalpA2−) isomers are also involved in the coordination upon the tin atom to a minor extent. In DMSO-d6 solution, R2Sn(IV) moieties are coordinated almost entirely by the GalA2− in the β-furanosidic form.