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 5,11,17,23-tetra-tert-butyl-[25,26,27,28-tetrakis(2- pyridylmethyl)oxy]calix(4)arene

Abstract

The solubility and derived standard Gibbs energies of solution of three geometrical isomers of pyridinocalix(4)arenes {5,11,17,23-tert-butyl-[25,26,27,28-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 these ligands, using acetonitrile as the reference solvent. For the 2-pyridyl derivative in alcohols as the aliphatic chain length of the alcohol increases, the extent of solvent–ligand interaction increases, whereas 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 values 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 macrocycles (2 and 4-pyridyl derivatives) and the silver cation in acetonitrile show that, as the distance between the phenolic oxygens and the pyridyl nitrogens increases, the strength of complexation decreases. Thermodynamic data for the complexation of silver and the 2-pyridylcalix(4)arene derivative are compared with corresponding values for sodium and this ligand in the same solvent. The results suggest that the 2-pyridyl derivative provides phenolic oxygens and pyridyl nitrogens as the active sites for complexation with silver. This is corroborated by ¹H NMR measurements of this ligand and silver in CD₃CN at 298 K.

The crystal structure of a 1:1 monoacetonitrile and silver complex of the 2-pyridyl derivative with perchlorate as the counter-ion has been determined from XRD data. The substance crystallises in the tetragonal space group P4/n with a=15.580(2) Å, c=13.082(2) Å, and z=2. The macrocycle is sited on a fourfold symmetry axis and consists of two conical parts, one hydrophobic, filled with an acetonitrile molecule, the other hydrophilic, encapsulating the Ag⁺ ion through the ethereal oxygen [d(Ag···O)=2.923(3) Å] and pyridine nitrogen atoms [d(Ag···N)=2.483(5) Å]. The coordination around the Ag⁺ ion has the form of a distorted Archimedean square antiprism. The ability of the macrocycle to complex the silver cation in its hydrophilic cavity is demonstrated.

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