A New Fluorescent Calix Crown Ether: Synthesis and Complex Formation with Alkali Metal Ions

Abstract

We synthesised a new N-benzylaza-21-crown-7 ether 5 with a dihydroxy coumarin as a fluorescence sensor and investigated the binding behaviour towards alkali metal cations in methanol by fluorescence titrations. The association constants are within one order of magnitude, with the exception of sodium. Potassium is the preferred binding partner (K(Na)=330 M(-1); K(K)=8600 M(-1); K(Rb)=8200 M(-1); K(Cs)=4400 M(-1)). The corresponding aza-21-crown-7 ether (6) was attached by a methylene unit to a resorcarene to give fluorescent calix crown ether 12. The binding abilities of the calix crown ether towards alkali metal ions in methanol have also been investigated, and an increasing complex stability, distinct for potassium and rubidium in comparison with 5, was found: K(Na)=440 M(-1); K(K)=110,000 M(-1); K(Rb)=63,000 M(-1); K(Cs)=20,000 M(-1). Like bis(crown ether)s, a cooperative complexation of the crown ether and the cavitand scaffold can be assumed. The proposed complex geometry is supported by Kohn-Sham DFT calculations for the potassium and caesium complexes.

A perspective to resorcinarene crowns

“…A similar procedure was applied to the synthesis of aza-n-crown-m ethers 14b and 14c (n=21 or 15, m=7 or 5; R=C 11 H 23 , Figure 5) with a fused fluorescent methyl coumarine moiety. 27,28 The fluorescent podand derivatives 14b and 14c…”

“…26 The association constants for mono-substituted cavitand crowns 14b and 14c were determined by an emission titration. 27,28 The derivative 14b with a slightly smaller crown ring showed affinity toward Na + (log K 11 =3.08 M -1 ) and K + (log K 11 =2.70 M -1 ) whereas 14c bound Na + (log K 11 =2.46 M -1 ) but its affinity towards larger cations K + (log K 11 =5.03 M -1 ), Rb + (log K 11 =4.80 M -1 ) and Cs + (log K 11 =4.29 M -1 ) was clearly higher. Job plots assured a formation of 1:1 lithium complex for 14b but the association constants could not be determined.…”

“…Stoll et al 27,28 exploited Kohn-Sham DFT calculations to study the structures of alkali metal complexes formed by the mono-substituted resorcinarene crowns 14b and 14c. The sodium and potassium complexes of 14b with smaller crown moiety had two similar local minima: one with the crown moiety rotated away from the cavitand (sidewise-like complex, Figure 15a) and one with the crown turned over the cavitand (sandwich-like complex, Figure 15b).…”

“…For the mono-substituted cavitand 14c with the bigger azacrown the cesium complex in addition to the potassium complex was calculated. 28 In contrast to the calculation made with 14b only sandwich-like conformation was used for the structure optimization and two coordination isomers were preselected as starting configurations: one in which the metal is closer to the azacrown and one in which the metal is closer to the resorcinarene cavity (Figure 15c and d). In the first structure potassium forms seven coordination bonds to oxygens of which six is between the crown moiety and one with the cavitand, and in the latter structure the total number of interactions is six of which one is to nitrogen, three to crown oxygens and two to the cavity.…”

New Fluorescent Calix Crown Ethers, Part II: Synthesis and Complex Formation in Solution and the Solid State

“…[9] In our previous work, we presented the combination of the cavitand with a fluorescent 1-aza-21-crown-7 ether. [10] The binding strength of selected alkali-metal-ion complexes was increased by at least one order of magnitude by the attached cavitand over that of the crown ether alone. Herein, we report that DFT calculations of a container-like supramolecule containing the cavitand and crown ether indicated cooperative binding interactions.…”