Molecular Recognition in Anion Coordination Chemistry. Structure, Binding Constants and Receptor‐Substrate Complementarity of a Series of Anion Cryptates of a Macrobicyclic Receptor Molecule

Abstract

SummaryThe crystal structures of four anion cryptates [X-c BT-6H'I formed by the protonated macrobicyclic receptor BT-6H+ with F-, C1-, Br-and N; have been determined. They provide a homogeneous series of anion coordination patterns with the same ligand. The small F--ion is tetra-coordinated, while C1-and Br-are bound in an octahedron of H-bonds. The non-complementarity between these spherical anions and the ellipsoidal cavity of BT-6H+ is reflected in ligand distortions. Structural complementarity is achieved for the linear triatomic substrate N;, which is bound by two pyramidal arrays of three H-bonds, each interacting with a terminal N-atom of N;. The formation constants of the complexes formed by BT-6H' with a variety of anions (halides, N;, NO;, carboxylates, SO:-, HPO:-, AMP2-, ADP3-, ATP4-, P,O:-) have been determined. Very strong complexations are found, as well as marked electrostatic and structural effects on stability and selectivity; in particular the binding of F-, C1-, Br-and N; may be analyzed in terms of the crystal structure data. The cryptand BT-6H+ is a molecular receptor containing an ellipsoidal recognition site for linear triatomic substrates of size compatible with the size of the molecular cavity. Further developments of various aspects of anion coordination chemistry are considered.

Anion‐Free Bambus[6]uril and Its Supramolecular Properties

“…Anion-free BU6 was found to be insoluble in any of the investigated solvents; however, it dissolves in methanol/chloroform (1:1) or acetonitrile/water (1:1) mixtures in the presence of the tetrabutylammonium salt of a suitable anion. The association constants with halide ions, BF 4 À , NO 3 À , and CN ) was found for the 1:1 complex of BU6 with I À in acetonitrile/water mixture. A number of crystal structures of BU6 complexes with various anions were obtained.…”

“…In the late 1960s, macrocyclic katapinands were recognized as the first artificial compounds exhibiting complexation properties towards halide ions. [3] Since then, a number of macrocycles with structurally different binding motifs, including ammonium, [4,5] guanidinium, [6,7] urea, [8,9] amide, [10,11] and pyrrole [12][13][14] derivatives, have been developed. As early studies were dedicated especially to the preparation of these structurally different receptors, the current interest aims at the modification of already known structures to obtain receptors with chemically tuned composition showing both high affinity and selectivity towards the corresponding anion.…”

A Dimetallic Cage with a Long Ellipsoidal Cavity for the Fluorescent Detection of Dicarboxylate Anions in Water

“…[3] The hexammonium cage 2 offers an ellipsoidal cavity suitable for the inclusion of the rodlike ion N 3 À . [4] However, other positively charged species, namely transition metal ions, can be placed within a bistren cage for interaction with an included anion. In particular, the dicopper(ii) complex with the all-amine cryptand 2 incorporates ambidentate anions such as N 3 À , [5] NCO À , [5] and HCO 3 À [6] to give stable inclusion complexes: Each metal center is coordinated by a tren subunit in a trigonal bipyramidal arrangement, and an axial position is available for interaction with a further ligand.…”

A Metal‐Based Trisimidazolium Cage That Provides Six CH Hydrogen‐Bond‐Donor Fragments and Includes Anions

“…[10] Positively charged cages capable of establishing hydrogen bonds with both N À H and C À H fragments have also been described. [11] On the other hand, an imidazolium-functionalized acyclic [Ru II (bpy) 3 ] 2+ receptor has recently been investigated for anion sensing in aqueous MeCN.…”