Anion Selectivity of Tetravalent Tin Compounds in Membranes Studied by ¹¹⁹Sn‐, ¹³C‐ and ¹H‐NMR

Abstract

The fundamental molecular aspects of trialkyltin compounds of the type R,SnY have been investigated in view of their applicability as ion-selective components in solvent polymeric membranes. The interaction between these compounds and anions has been studied using Il9Sn-and ',C-NMR. Neutral tetracoordinated trialkyltin compounds form a negatively charged pentacoordinated complex upon interaction with C1-ions in homogeneous organic phases as well as in membranes in contact with aqueous solutions. Although in a homogeneous phase, the electronegative substituent Y determines the complex-formation constant, it has no influence on the potentiomctric anion selectivity in liquid membranes containing trialkyltin carriers R3SnY with different Y. The observed selectivity pattern is not given by the magnitude of the stability constants in a homogeneous phase but is dictated by the prevailing association-dissociation process leading to tetracoordinated compounds which change in constitution due to varying sample composition. The results obtained from equilibrium studies of tetravalent monotin compounds with anions in both homogeneous phase and in two-phase systems confirm the earlier hypothesis that trialkyltin compounds incorporated in solvent polymeric membranes act as electrically neutral carriers for anions.

Anion Selectivities of Dibutyltin Dihalides as Neutral Carriers for Ion-Selective Electrodes

“…4,18 However, the selectivity pattern of the electrode containing Bu2SnCl2 heavily depends on the concentration of the ionophore. This tendency has been also indicated by R3SnCl (R=Bu, Oc).…”

“…Therefore, one may conclude that the steric hindrance interferes with the formation of the pentacoordination of the ionophores with sample anions X -. 18 On the other hand, the absence of any marked influence of the electronegative substituents Y on the anion selectivities of the electrodes containing trialkyltin halides R3SnY has been found. 18 For example, the observed selectivity coefficients of the electrodes containing Bu3SnCl, Bu3SnOAc, and Bu3SnCN are almost identical.…”

“…18 On the other hand, the absence of any marked influence of the electronegative substituents Y on the anion selectivities of the electrodes containing trialkyltin halides R3SnY has been found. 18 For example, the observed selectivity coefficients of the electrodes containing Bu3SnCl, Bu3SnOAc, and Bu3SnCN are almost identical. On the basis of NMR study, researchers have concluded that sufficiently fast exchange of the electronegative substituents Y (Y=Cl, OAc, CN) on the Sn atom for X derived from sample anions occurs via the formation of a penta-coordinated Sn atom.…”

“…[14][15][16][17][18] A variety of anion selectivities of trialkyltin compounds has been found by using ionophores which differ in the organic substituents R on the Sn atom. [16][17][18] With more space-consuming substituents R, the observed selectivity patterns have been more similar to that of a blank membrane. Therefore, one may conclude that the steric hindrance interferes with the formation of the pentacoordination of the ionophores with sample anions X -.…”

“…4,18 Therefore, it seems that the electronegative substituent Y on the Sn atom significantly influences the anion selectivity if a sufficiently fast exchange of the electronegative substituent Y does not occur or if another electronegative substituent Y remains on the Sn atom evenafter an exchange of one electronegative substituent Y occurs. Dialkyltin dihalide compounds form complexes more readily than trialkyltin halide compounds because the Lewis acidity of the tin center in dialkyltin dihalides R2SnY2 is increased by two highly electronegative substituents Y.…”