Recognition of Achiral and Chiral Ammonium Salts by Neutral Ditopic Receptors Based on Chiral Salen-UO₂ Macrocycles

Abstract

A mononuclear (M20) and a dinuclear (M40) uranyl chiral macrocyclic complex, incorporating both a salen unit containing two phenyl rings linked to a chiral diimine bridge and the (R)-BINOL unit, behaves as an efficient ditopic receptor for achiral and chiral quaternary ammonium salts. Binding affinities in chloroform solution have been measured for 1:1 complexes of many quaternary salts encompassing tetramethylammonium (TMA), tetraethylammonium (TEA), tetrabutylammonium (TBA), and acetylcholine (ACh), as well as trimethylanilinium (TriMAn), benzyltrimethylammonium (BnTriMA), (alpha-methylbenzyl)trimethylammonium and pyrrolidinium cations. The anion of the salt is bound by the hard Lewis acidic uranyl site, with an increasing binding efficiency on increasing the anion hardness (I(-) < Br(-) < Cl(-)), whereas CH-pi or pi-pi attractions by binapthyl moiety, or the salicylaldehyde unit, or the phenyl rings of diimine bridge ensure the recognition of the cation partner. Optimized structures of receptor-anion-cation ternary complexes obtained by MM calculations are supported by 2D-ROESY NMR measurements.

Heteroditopic Chiral Uranyl–Salen Receptor for Molecular Recognition of Amino Acid Ammonium Salts

“…These findings seem to indicate that the carboxylate anion of the amino acid behaves as the hard chloride in binding the uranyl metal center driving the molecular recognition event. [10,17] To the best of our knowledge, this uranyl complex represents the first hitherto reported chiral receptor able to recognize and strongly bind carboxylate anions of amino acids. NMR spectroscopic data support this conclusion.…”

“…[10] However, so far only few examples of chiral uranyl-salen complexes have been described in the literature. [11] Here we report the synthesis of a new heteroditopic chiral uranyl-salen derivative bearing two pyrenyl side arms and discuss its ability in the enantioselective molecular recognition of ammonium salts also containing α-amino acid moieties.…”

NMR Studies of Optically Active Schiff Bases

“…The ability of di-and tetra-Schiff base uranyl macrocyclic complexes containing the (R)-BINOL unit M40 [11] and M20 [12] to recognise a series ammonium salts has been verified. 115 The 2D T-ROESY experiments of M40-tetrabutylammonium chloride complex have shown the dipolar correlation cross-peaks between OCH 2 protons of the BINOL unit and Nmethylene and N-methyl protons of the salt. Such ROE contacts have indicated the proximity of the receptor and ammonium group and can evidence cation location inside the receptor and nearby the uranyl site.…”

A DFT study on the recognition of α-amino acid derivatives by chiral uranyl–salen

“…Chiral uranyl salen complexes have proved to be efficient hosts for amino acid salts [24][25][26][27][28], due to the ability of the uranyl metal center to coordinate one molecule of carboxylate anion in its equatorial fifth position [29,30]. Very recently, we reported on two uranyl-salen receptors 1 and 2 that differ for the presence of two tert-butyl groups covalently linked to the salen scaffold [31].…”