ReviewCalixarenes as platforms for the construction of multimetallic complexes
Graphical abstract
The use of calix[n]arenes (n = 4−6) in building up multi-metal species based on group 1 or 2 ions and early transition metal ions, through synergistic metal–O and metal⋯π interactions, is reviewed, along with recent advances on metal alkyl/cyclopentadienyl complexes and future prospects on the use of calixarenes in general in building up multi-metal complexes in a controlled way.
Introduction
The assembly of highly organised multimetallic architectures features in catalysis, magnetism, new materials, medicine, molecular electronic devices and associated nano-technologies. Developing methods for tailoring properties and tuning the stoichiometries of metals in multimetallic complexes is a considerable challenge, and for this purpose various synthetic methodologies have been established. The initial limited scope of unidentate ligands in mediating the formation of mixed-metal species through bridging led to the use of multidentate ligands including bi/tri-dentate functionalised moieties [1], [2], [3], [4]. Another approach, which is receiving increasing attention, is the use macrocyclic ligands. Here the organic backbone of the macrocycle provides a platform for the assembly of several metal centres in relatively close proximity. Moreover, the preparation of targeted multimetallic species may be greatly facilitated using macrocyclic ligands that are heterotopic in having two distinct types of coordination sites to bind different types of metal ions.
Although macrocyclic ligands capable of binding different metals have been prepared [5], [6], their syntheses are generally complicated and low yielding, and accordingly can be inherently expensive. For major applications it is essential that the macrocyclic ligands are readily available, cheap and easy to prepare. Calix[n]arenes are potential candidates. They are macrocyclic ligands consisting of phenolic units linked by methylene bridges at their ortho-positions [7], and the hydroxy functionalities are poised to coordinate several metals simultaneously. In addition, the calixarenes can be organised through their conformational flexibility with shape-specific π-rich cavities capable of metal⋯π-arene interactions as well as the metal to oxygen centre interplay. In this article, we focus on multi-organo-metallic calixarene complexes formed purely through O-phenolate complexation, and also those formed through a combination of O-phenolate and metal⋯π-arene interaction, with a view of illustrating how these macrcocyles can be used strategically to prepare multimetallic complexes with control over the metal stoichiometries. This is paramount in preparing complexes of even higher complexity. The discussion focuses on the parent calixarenes and p-alkyl substituted analogues. Metal complexes of highly specialised and functionalised calixarenes including p-sulfonated and O-derivatives have been extensively studied and are more akin to classical coordination chemistry, and are covered elsewhere [7], [8].
Section snippets
Calix[4]arenes
Calix[4]arenes are the lowest oligomers in the series and are readily available, and numerous metal complexes have been prepared and structurally authenticated. It is also the species for which M⋯π complexation is both most predictable and most commonly observed, which is related to its small ring size with well-defined cavities for metal ion inclusion. In the unsubstituted form, calix[4]arenes display four distinct structural conformations at room temperature, these are identified as cone,
Calix[6]arenes
Whereas the relatively rigid calix[4]arene forms complexes predominately in the cone conformation, the larger calix[6]arene is more flexible, offering a greater variety of conformations and coordination interactions, and cavities with different shapes and sizes, with the potential to include larger guest molecules. From a coordination chemistry perspective, calix[6]arene macrocycles have great potential in the design of new complexes containing several metal centres. Nevertheless, there is a
Calix[5]arenes
Like calix[4]arenes, calix[5]arenes are usually predisposed to the cone conformation both in solution and in the solid state, through the formation of strong intramolecular hydrogen bonded torus. However unlike the synthesis of calix[4]arenes, the synthesis of calix[5]arenes low yielding and difficult to prepare [7], which may account for the paucity of structurally chartacterised complexes [31], [32], [33].
An unsymmetrical mixed potassium–titanium calix[5]arene dimer represents the only
Organometallic complexes constructed from O-phenolate coordination
As constrained multidentate hydroxy functionalised ligands, calixarenes have a unique capacity to coordinate several metals centres via O-phenolate complexation. For example, in the partially flattened cone arrangement, identified in aluminium [34], gallium [35] and zinc [36] complexes, Fig. 14, calix[4]arene is able to accommodate from two to four metal atoms simultaneously.
The reaction of two equivalents TiCp2Me2 with calix[4]arene in refluxing hexane, Eq. (2), affords a unique complex
Future prospects
Calixarenes are versatile macrocycles for building up hetero-multi-metallated species based on two different metals, taking advantage of metal–O and metal⋯π interactions, depending on how the conformation of the calixarenes in forming complexes with one metal type effects the proximity of oxygen centres (phenolic/phenolate moieties) and arene rings for binding of metal centres of the same or different type. Hetero-multi-metallated organometallic species are also possible by direct reaction of
Acknowledgement
We thank the ARC for support of this work.
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