Abstract
We report the formation of heavy alkali metal complexes of bicyclic host molecules including the caesium(I) complex that catches the central metal ion with the deep pocket of the host similar to a baseball glove. For this, three regioisomers of singly bridged calix[6]crown-6 [1,2-bridged (H4 L 1,2), 1,3-bridged (H4 L 1,3) and 1,4-bridged (H4 L 1,4)] have been synthesized by alkyl-ation of calix[6]arene with penta-ethyl-ene glycol di-tosyl-ate in the presence of M 2CO3 (M = Na, K, Rb and Cs). The larger the cation size of the metal carbonate, the higher the yield of the H4 L 1,4 isomer, indicating the size-based template effect. A combination of H4 L 1,2 and RbOH allowed isolation of the mononuclear rubidium(I) complex (1) in which the metal center is six-coordinated in a loose fashion, the remaining two oxygen donors in the crown loop and two phenols in the calix rim are uncoordinated. Notably, the complexation of H4 L 1,2 with CsOH yielded the mononuclear caesium(I) complex (2), in which all possible ten binding sites on the deep and good-fit pocket participate in coordination via high cooperativity between the crown loop and calix rim, similar to a baseball glove. In dipolar organic solution, the caesium(I) complex 2 remains intact. H4 L 1,4 afforded a dicesium(I) complex (3) and adjacent complexes are linked by intermolecular cation-π interactions, giving rise to a pseudo one-dimensional coordination polymer. These results provide insight into the metal carbonate-dependent synthesis of calix[6]crowns and the influence of regioisomers on caesium(I) complexation.
Highlights
Since the pioneering works of Pedersen, Lehn and Cram, supramolecular chemistry of macrocycles is still significant in chemistry and biology because many of the roots and concepts came from the phenomena found in biological process such as molecular recognition, self-assembly, enzyme catalysis and ion transport (Pedersen, 1967; Izatt et al, 1995; Lindoy, 1990; Lehn, 1995)
As shown in Scheme 1, regioisomers of the singly bridged calix[6]crown-6 were synthesized via direct alkylation of calix[6]arene with pentaethylene glycol ditosylate in the presence of M2CO3 (M = Na, K, Rb and Cs)
Compared with the spectrum of free H4L1,2, its caesium(I) complex 2 shows remarkable peak broadening, with the signal of OCH2 of the crown loop being shifted downfield because of the presence of multiple Cs—O bonds restricting the conformational mobility of the ligand (Fig. 3)
Summary
Since the pioneering works of Pedersen, Lehn and Cram, supramolecular chemistry of macrocycles is still significant in chemistry and biology because many of the roots and concepts came from the phenomena found in biological process such as molecular recognition, self-assembly, enzyme catalysis and ion transport (Pedersen, 1967; Izatt et al, 1995; Lindoy, 1990; Lehn, 1995). For the complexation of heavy alkali metal ions, hetero-di and polytopic receptors incorporating two different macrocycle units could be more advantageous due to the deep pocket being formed rather than simple flat macrocycles (Lehn, 1978) In this sense, calix[n]crowns are one of the most important host molecules (Asfari et al, 2001; Harrowfield & Koutsantonis, 2007; Ikeda & Shinkai, 1997; Kim, Lee et al, 2012; Nimse & Kim, 2013). The phenol groups under the basic reaction conditions could be advantageous for a few reasons: (i) charge neutrality of the complexes can be achieved by phenolate groups, (ii) the phenolate group can acts as a strong cation-ligation site In this regard, such modification of calix[6]arene entity by bridging with proper crown loops could be a potential design tool for engineering new coordination products in terms of stoichiometries, topologies and coordination modes.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
