Coordination chemistry of the larger calixarenes
Introduction
The coordination chemistry of the simplest of the calix[n]arenes, the calix[4]arene, is now relatively well developed as highlighted by a number of recent reviews [1], [1](a), [1](b), [1](c), [1](d), [1](e), [1](f), [1](g), [1](h), [1](i). The vast majority of these metallocalix[4]arene derivatives exist as either mono or binuclear complexes, retaining a cone-like conformation for the parent ligand. By contrast, metal compounds containing the larger ring systems (n≥5) are still quite rare despite the conformational variations offered by the increased flexibility of the larger number of polyphenolic rings. An additional attractive feature of the latter is their ability to simultaneously coordinate more than one metal centre. In this context, we herein review the coordination chemistry for calix[n]arene complexes, where n≥5, paying particular attention to both the degree of ‘metallation’ of the ring and the conformations adopted.
Section snippets
Alkali metal and alkali earth metals
Interest in calixarenes of Group I stems from the observation that they effect the transport of ions of this group through water/organic solvent/water membranes [2]. Further to this, it has been postulated that the larger calix[n]arenes, by virtue of their increased flexibility, may transport more than one metal at a time [3]. Surprisingly though, structural information on such systems is scant. Harrowfield et al. have isolated the calcium complex [Ca(p-tert-butylcalix[8]areneH6)(dmf)4] (1)
Titanium
One of the earliest reports of a transition metal calixarene complex was that of Atwood in 1986 who reacted hexamethoxycalix[6]arene with 4 equivalents of TiCl4 isolating a centrosymmetric complex 30 which contained two bimetallic units of the form [Cl3TiOTiCl2], the calix[6]arene ligand adopting an eliptical cone conformation [19]. The reaction is thought to proceed via cleavage of an OMe bond and ultimately the loss of CH3Cl molecules.
The tetrametallic complex 31, obtained from the reaction
Iron
A violet, water-soluble complex of iron(III) is formed on interaction of a solution generated from iron(III) perchlorate with calix[6]arene-p-sulfonic acid [H12CS] over the pH range 2.5–5.5 [40]. Studies of the acid–base properties of such ligands have found that at pH<11 only two of the hydroxyl protons are readily disociated, whilst the six sulfonic groups possess high acidity. In the present reaction, potentiometric and spectrophotometric studies are suggestive of the formula {[Fe(OH)2]2H6CS}
Aluminium
Atwood and co-workers reacted the methyl-ethers of calix[8]arene and p-tert-butylcalix[8]arene with trimethylaluminium (8 equivalents) obtaining in each case colourless crystals of the hexaaluminium species 99 and 100, respectively [57]. The absence of bulky tert-butyl groups in 99 allows all six Me3Al groups to point outward away from the centre of the calixarene ring; in 100 (not shown) two of these Me3Al groups are more inwardly directed. The crystal packing of each complex highlights
Lathanides and actinides
Calixarenes complexes of both the actinides and Lanthanides have been comprehensively reviewed very recently [1g]. We will therefore only discuss here one or two highlights.
The p-sulfonatocalix[5]arene 13 reacts with La(NO3)3·H2O in water in the presence of pyridine N-oxide to afford Na2[La(H2O)9][p-sulfonatocalix[5]arene]·pyridine N-oxide·10H2O (129) [9]. The La(III) centre is not directly bound to the calix[5]arene ligand, preferring instead to hydrogen bond via its aqua ligands. A disordered
Concluding remarks
This review highlights the significant progress that has been made in the development of the coordination chemistry of the larger calixarene ring systems. In a short period of time since the discovery of the first metallocalixarene in the mid 1980s', this exciting area has revealed a variety of structural motifs and there is doubtless much rich chemistry yet to be discovered.
Acknowledgments
The author wishes to acknowledge The Royal Society, The Leverhulme Trust, The EPSRC and Professor V.C. Gibson (Imperial College) for past financial support and the continuing superb crystallographic efforts of Dr Mark R.J. Elsegood (University of Loughborough).
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