Solution and Solid State Characterization of a Cadmium Octaazacryptand Complex

The constant for cadmium binding by octaazacryptand L = N(CH2CH2NHCH2CH2NHCH2CH2)3N was determined by potentiometry; log βML =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 cryptate complexes, [CdL](BF4)2 (1a) and [CdL](OAc)2·2H2O (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, P31c, Z = 2, a = 9.581(2) A, c = 17.003(4) A, V = 1351.2(5) A3. Crystallographic data for 1b:  monoclinic, C2/c, Z = 4, a = 10.500(2) A, b = 14.782(3) A, c = 19.395(4) A, β = 99.30(2)°, V = 2971(1) A3. 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 exp...