Host-guest complexes of mixed glycol-bipyridine cryptands: prediction of ion selectivity by quantum chemical calculations, part V.

Svetlana Begel,Rudi van Eldik,Ralph Puchta

doi:10.3762/bjoc.9.142

Svetlana Begel, Rudi van Eldik + Show 1 more

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https://doi.org/10.3762/bjoc.9.142

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Abstract

The selectivity of the cryptands [2.2.bpy] and [2.bpy.bpy] for the endohedral complexation of alkali, alkaline-earth and earth metal ions was predicted on the basis of the DFT (B3LYP/LANL2DZp) calculated structures and complex-formation energies. The cavity size in both cryptands lay between that for [2.2.2] and [bpy.bpy.bpy], such that the complexation of K+, Sr2+ and Tl3+ is most favorable. While the [2.2.bpy] is moderately larger, preferring Rb+ complexation and demonstrating equal priority for Sr2+ and Ba2+, the slightly smaller [2.bpy.bpy] yields more stable cryptates with Na+ and Ca2+. Although the CH2-units containing molecular bars fixed at the bridgehead nitrogen atoms determine the flexibility of the cryptands, the twist angles associated with the bipyridine and glycol building blocks also contribute considerably.

Highlights

The present report continues a series of contributions from our group dealing with quantum chemical investigations of the selective complexation of alkali and alkaline-earth metal cations by supramolecular species, predominantly cryptands and their derivatives [1,2,3,4]
The CH2 and (CH2)-units containing molecular bars fixed at the bridgehead nitrogen atoms determine the flexibility of the cryptands, the twist angles associated with the bipyridine and glycol building blocks contribute considerably
According to our DFT-calculations, [2.2.bpy] and [2.bpy.bpy] have somewhat smaller cavities than [2.2.2]. Both cryptands prefer to bind Tl3+ as earth metal and K+ as alkali metal ions, [2.2.bpy] favors Rb+ as the best and [2.bpy.bpy] favors Na+. It is the selectivity of the alkaline-earth metal ions that is more significant if one wants to ascertain the cavity size of a cryptand

Summary

Introduction

The present report continues a series of contributions from our group dealing with quantum chemical investigations of the selective complexation of alkali and alkaline-earth metal cations by supramolecular species, predominantly cryptands and their derivatives [1,2,3,4]. The high value for the movement of Ga3+ inside the [2.2.bpy] results from the very effective complexation of earth metal ions by cryptands compared with single solvent molecules, e.g., H2O or NH3, and a large amount of released free energy.

Results

Conclusion