Complexing and Ion-Selective Properties of Some Phosphoryl Podands towards Cadmium Cation

The structures of complexes [CdL1(NCS)(2)(H2O)(2)] (I), [Cd(L-1)(2)(H2O)(2)][CdI4].0.5H(2)O (II), where L-1 is 1,8-bis[(diphenylphosphoryl)methoxy]-3,6-dioxaoctane, and [Cd(mu-L-2)I-2](n) (III), where L-2 is 1,3-bis[oxymethyl(diphenylphosphoryl)]propane have been determined by single-crystal X-ray diffraction. The structures of I and II are the first examples of mononuclear cadmium complexes with phosphoryl podand. The structure of III is formed by 1D chains. The synthesis of 2,6-bis[2-(diphenylphosphoryl)-4-ethylphenoxymethyl]pyridine (L-3) is described. The stability constants of complexes of cadmium nitrate with podands L-2 and L-3 have been determined by spectrophotometric titration in acetonitrile. It has been found that ligand L-3 forms in solution not only a 1 : 1 (M : L) complex (log beta(1) = 4.92 +/- 0.11) but also a 2 : 1 binuclear complex (log beta(2) = 9.42 +/- 0.11), which results in binding of 85.1% of Cd2+ ions at initial reagent concentrations of 0.1 mM. Under the same conditions, podand L-2 binds only 37.4% of Cd2+ ions into a 1 : 1 complex (log beta(1) = 3.98 +/- 0.21). The ion-selective properties of L-1-L-3 as active components of polymeric plasticized membranes of ion-selective electrodes have been tested. A high cadmium selectivity of podand L-3 has been found, which, apparently, is due to the formation of complexes with high stability constants in solution.