The “Picrate Effect” on Extraction Selectivities of Aromatic Group-Containing Crown Ethers for Alkali Metal Cations¹

Abstract

As evaluated with 15 crown ethers which contain varying numbers of benzo substituents, the magnitude of the extraction selectivities (and in one case, the selectivity order) of aromatic group-containing ionophores for alkali metal picrates may vary significantly from those for alkali metal salts with inorganic anions as a result of π−π interactions between picrate ion and an aromatic unit of the ionophore. The importance of the “picrate effect” increases as the number of benzo groups in the crown ether is enhanced and varies with their location in the macrocycle. To verify the involvement of picrate−crown ether π-stacking in complexation, crown ether−alkali metal picrate complexes were examined in solution by 1H NMR spectroscopy and solid-state structures for nine complexes were determined by X-ray diffraction. Dependence of the chemical shift for the picrate proton singlet in the NMR spectrum on the metal cation and/or macrocycle identity in the metal picrate−crown ether complex was found to be a convenient tool for studying anion−ligand π−π interactions in solution.

Fine Tuning of the Cation Affinity of Artificial Receptors Based on Cyclic Peptides by Intramolecular Conformational Control

“…In addition, the high stability of the cyclopeptide complexes with BTMA ϩ picrate may be a consequence of the so-called ''picrate effect''. [24] This effect is due to πϪπ interactions between the picrate anions and aromatic subunits of an artificial receptor, which occur when cations are incorporated into the receptor cavity. In agreement with these observations, we found a small but characteristic upfield shift of the picrate proton signals in the 1 H NMR spectra of the BTMA ϩ picrate complexes of our cyclopeptides.…”

Selective Transport of Silver(I) Cation Across a Bulk Liquid Membrane Containing Bis-β-enamino Ester as Ion Carrier

“…It is well known that the soft, polarizable and weakly hydrated picrate ion is a convenient counter ion that provides greater distribution ratios and membrane transport rates for metal ion-neutral ligand complexes than do inorganic ions such as NO 3 − and ClO 4 − . 33 For this reason, picrate ion was selected as a suitable counter ion. Then, the influence of the picric acid concentration in the feed phase on the silver ion transport was studied.…”

Polybenzocrown ethers: synthesis by cesium-assisted cyclization and solid-state structures

“…From our previous work, we are able to compare the structure of 8 with the structure of the same crown as a ligand in Rb[tribenzo-21-crown-7]picrate (Rb7picrate, Figure 1k) 13 . As was the case in the comparison of 6 with K6picrate, the complexed metal cation significantly reduces the folding of two of the benzo substituents; although for 8, this folding was already minimal.…”