Halide binding, size selecting

The complexation of anionic species by tetra-bridged phosphorylated cavitands concerns mainly the work of Puddephatt et al. who described the selective complexation of halides by the tetra-copper and tetra-silver complexes of 2 (see Scheme 17). The complexes are size selective hosts for halide anions and it was demonstrated that in the copper complex, iodide is preferred over chloride. Iodide is large enough to bridge the four copper atoms but chloride is too small and can coordinate only to three of them to form the [2-Cu4(yU-Cl)4(yU3-Cl)] complex so that in a mixed iodide-chloride complex, iodide is preferentially encapsulated inside the cavity. In the [2-Ag4(//-Cl)4(yU4-Cl)] silver complex, the larger size of the Ag(I) atom allowed the inner chloride atom to bind with the four silver atoms. The X-ray crystal structure of the complexes revealed that one Y halide ion is encapsulated in the center of the cavity and bound to 3 copper atoms in [2-Cu4(//-Cl)4(//3-Cl)] (Y=C1) [45] or to 4 copper atoms in [2-Cu4(/U-Cl)4(/U4-I)] (Y=I) and to 4 silver atoms in [2-Ag4(/i-Cl)4(/i4-Cl)] [47]. NMR studies in solution of the inclusion process showed that multiple coordination types take place in the supramolecular complexes. 

A theoretical (AMI molecular orbital) treatment of boron analogues of the katapinand series as well as macrotricyclic compounds related to 16 has also recently appeared. As with the ammonium-based host molecules the macrotricyclic hosts containing four boron atoms exhibited a greater degree of anion specificity as a consequence of the rigidity of their binding sites. In all cases, size selective complexation of halide anions was observed with accompanying decreases in B - B distances and partial sp -> sp rehybridization. ... 

Extending this line of thought, Puddephatt s group reported in 1993 cavitands bridged with P(III) moieties. They demonstrated the formation of several tetra-gold (I) and tetra-copper(I) complexes and showed that they were capable of the size-selective binding of halide ions [197]. Molecular mechanics and NMR data were consistent with the stereochemistry of the free ligand in which the lone pairs on the phosphorus point towards the center of the pocket (60). 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

Iron(II) has been used as a supramolecular template for the formation of a tris-imidazoUum receptor from Ugand 117 [79]. NMR studies and X-ray crystal structure determination were used to demonstrate the encapsulation of bromide in the cavity of the receptor, with the anion coordinated by six C-H fragments (Fig. 2). Spectrophotometric titrations in acetonitrile solution revealed that this receptor binds halides with selectivity for chloride > bromide > iodide, but has no affinity for dihydrogenphosphate or hydrogensulfate. Presumably the restricted size of the receptor cavity excludes the binding of these larger tetrahedral anions. The Hnear anions azide, cyanate and thiocyanate also produced a response in the UV/vis spectrum, and azide was found to bind preferentially to 117 in comparison to the non-symmetrical linear anions. 

Sessler and co-workers further synthesized strapped calixpyrroles, including isophthalamide strapped system 74 [136, 137]. The aliphatic bridges were found to increase the binding affinity for halide anions in acetonitrile, but failed to provide size-based selectivity amongst anions (Br, CF and F) due to the tilt of the strap to one side of the receptor, which allowed the formation of 2 1 (receptor anion) complexes. 

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