Solution and solid state characterization of a cadmium octaazacryptand complex

The constant for cadmium binding by octaazacryptand L = N(CH2CH2NHCH2CH2NHCH2CH2)(3)N was determined by potentiometry; log beta(ML) = 18.3(1). Calculation of pM values (pM = -log [M]) show that L has the highest reported binding affinity for cadmium relative to other [2.2.2] polyoxa-polyaza cryptates. Two cadmium cryplate complexes, [CdL](BF4)(2) (1a) and [CdL](OAc)(2) . 2H(2)O (1b), were synthesized by metalation of the free cryptand. The molecular structures of compounds 1a and 1b were determined by X-ray diffraction. Crystallographic data for 1a: trigonal, <P(3)over bar 1c>, Z = 2, a = 9.581(2) Angstrom, c = 17.003(4) Angstrom, V = 1351.2(5) Angstrom(3). Crystallographic data for 1b: monoclinic, C2/c, Z = 4, a = 10.500(2) Angstrom, b = 14.782(3) Angstrom, c = 19.395(4) Angstrom, beta = 99.30(2)degrees, V = 2971(1) Angstrom(3). The cadmium ions in both complexes are eight-coordinate and display similar coordination geometries but different ligand conformations, designated as parallel and oblique. The cadmium coordination sphere is best described as a bicapped octahedron. Through 2D NMR experiments we have found that the Cd is in a symmetric environment and the conformation of the ligand in solution is consistent with the parallel conformation observed in one of the solid state structures. The Cd-113 NMR shift of this eight-coordinate amine complex is 75 ppm relative to 0.1 M aqueous Cd(ClO4)(2), which correlates with shifts observed for other [2.2.2] cadmium polyoxa-polyaza cryptates.