SOLUTION THERMODYNAMICS OF GEOMETRICAL-ISOMERS OF PYRIDINO CALIX(4)ARENES AND THEIR INTERACTION WITH THE SILVER CATION - THE X-RAY STRUCTURE OF A 1 1 COMPLEX OF SILVER PERCHLORATE AND ACETONITRILE WITH ,27,28-TETRAKIS(2-PYRIDYLMETHYL)OXY]CALIX(4)ARENE/

The solubility and derived standard Gibbs energies of solution of thre e geometrical isomers of pyridinocalix(4)arenes tert-butyl-[25,26,27,2 8-tetrakis-(n-pyridylmethyl) oxy]-p-tert-butylcalix(4) arene, with n = 2, 3, 4} in a variety of solvents at 298.15 K are reported. Solvation effects are assessed from the standard transfer Gibbs energies of the se ligands, using acetonitrile as the reference solvent. For the 2-pyr idyl derivative in alcohols as the aliphatic chain length of the alcoh ol increases, the extent of solvent-ligand interaction increases, wher eas for the 3- and the 4-pyridyl derivatives the opposite is observed. The protonation constants for the 4-pyridyl derivative in methanol at 298.15 K are reported. The results, compared with corresponding value s for the 3-pyridyl derivative show that the basic character of these ligands follows the sequence; 4-pyridyl > 3-pyridyl > 2-pyridyl calix( 4)arene. Thermodynamic parameters for the complexation of these macroc ycles (2 and 4-pyridyl derivatives) and the silver cation in acetonitr ile show that, as the distance between the phenolic oxygens and the py ridyl nitrogens increases, the strength of complexation decreases. The rmodynamic data for the complexation of silver and the 2-pyridylcalix( 4)arene derivative are compared with corresponding values for sodium a nd this ligand in the same solvent. The results suggest that the 2-pyr idyl derivative provides phenolic oxygens and pyridyl nitrogens as the active sites for complexation with silver. This is corroborated by H- 1 NMR measurements of this ligand and silver in CD3CN at 298 K. The cr ystal, structure of a 1 : 1 monoacetonitrile and silver complex of the 2-pyridyl derivative with perchlorate as the counter-ion has been det ermined from XRD data. The substance crystallises in the tetragonal sp ace group P4/n with a = 15.580(2) Angstrom, c = 13.082(2) Angstrom, an d z = 2. The macrocycle is sited on a fourfold symmetry axis and consi sts of two conical parts, one hydrophobic, filled with an acetonitrile molecule, the other hydrophilic, encapsulating the Ag+ ion through th e ethereal oxygen [d(Ag ... O) = 2.923(3) Angstrom] and pyridine nitro gen atoms [d(Ag ... N) = 2.483(5) Angstrom]. The coordination around t he Ag+ ion has the form of a distorted Archimedean square antiprism. T he ability of the macrocycle to complex the silver cation in its hydro philic cavity is demonstrated.