Molecular Complexes with Calixarenes

This entry deals with various facets of molecular complexation by calixarenes. Most studies published on this subject are connected with cone conformation of calixar-ene skeleton. 

Crown ethers continue to be one of the most useful parts of supramolecular chemistry/91 From the beginning computations of metal ions complexes with synthetic ionophores/101 which have been aptly reviewed/111 emphasized the importance of including explicitly solvation in free energy calculations, also with ab initio calculations on calixarene complexes/121 Molecular dynamics simulations of 18-crown-6 ether complexes in aqueous solutions predict too low affinities, but at least correctly reproduce the sequence trend K+ > Rb+ > Cs+ > Na+. However, only the selection of K+ over Rb+ and Cs+ is ascribed to the cation size relative to that of the crown cavity, whereas K+ appears in these calculations to be selected over Na+ as consequence of the greater free energy penalty involved in displacing water molecules ftomNa/1131... 

G. D. Andreetti, A. Pochini, R. Ungaro, Molecular Inclusion in Functionalized Macrocycles. 6. The Crystal and Molecular-Structures of the Calix[4]arene from Para-(l,l,3,3-Tetramethylbutyl)Phenol and Its 1-1 Complex with Toluene. J. Chem. Soc. Perkin Trans II 1983, 1773-1779 G. D. Andreetti, F. Ugazzoli, in Calixarenes A Versatile Class ofMacrocyclic Compounds, J. Vicens and V. Bohmer, eds. Kluwer, Dordrecht, 1991. 

Capsule-shaped Ir(l) and Rh(l) cationic complexes with a triphosphinocaltx[6]arene as a multidentate ligand were recently synthesized (Figure 33). These organometallic bis-caltxarene complexes showed dynamic behavior with size-selective molecular encapsulation, which was confirmed by variable-temperature P H NMR measurements in the presence of various molecules. X-ray crystal analysis showed that the calix[6]arene moiety adopted the same pinched-cone conformation as the non-coordinated caltxarene. Molecules such as CFlj,Cly or CICH2CH2CI are too small to fit the cavity of the iridium and rhodium bis-calixarene complexes and cannot restrict the dynamic behavior at 25 °C. On the contrary, molecules such as X2CHCHX2 (X = C1 or Br), benzene, toluene, o- or w-xylene just fit in the cavity and show the dynamic behavior. Finally, large molecules (p-xylene, cumene, mesitylene, etc.) could not enter the cavity. 

Fig. 1 Molecular structures of the calixarenes, where, commonly, n = 4. 6. and 8 and the calixresorcinarene (A). The strategy for cation complexation by the introduction of ligand groups at either the lower or upper rims of calixarene derivatives (B). Metal complex with...

Gutsche, C.D. Filling the Baskets Complex Formation with Calixarenes. In Calixarenes Monograph in Supra-molecular Chemistry Royal Society of Chemistry Cambridge, 1989 Vol. 1. 149-185. 

The cavity of these calixarenes is usually too small to accommodate fullerenes, and is confirmed by solution, unless they have extended cavity walls as in calix[4]-naphthalenes. Nevertheless, some calix[4]arenes were shown to form stable crystalline complexes with C60 with the fullerene eno- to the calixarene cavity, and they include p-Ph-calix[4]arene, which has a toluene molecule in the cavity, the overall structure dominated by fullerene-fullerene and eso-calixarene fullerene interactions p-Br-calix[4]arene propyl ether, the structure showing very close interfullerene contacts in a columnar structure (Fig. 2d), which most likely results in opposing induced dipoles from the unidirectionally aligned calix-arenes " and /7-I-calix[4]arene benzyl ether, where the fullerenes are ordered without appreciable interfullerene interactions. There is also a calix[4]resorcinarene, R = CH2CH2Ph, 3, which has a molecular capsule derived from head-to-head hydrogen bonding of two resorcinar-enes and propan-2-01 molecules, with the fullerenes also... 

The number of papers reporting the role of CH/ti interaction is rapidly increasing. Only recent key papers are cited here. Chloroform interacts favorably with an arene n-oloud of fullerene complexes of calixarenes " and cyclotriveratrylene. Evidence for the role of CH/ti interaction in self-assembly was presented.The significance in molecular capsules " " and lattice- and cavity-inclusion type clathrates is well documented. 

The calixarenes are phenol-formaldehyde cyclic oligomers which possess hydro-phobic cavities capable of forming inclusion complexes with aromatic guest molecules in the solid state [1,2]. They also have the ability to function as ion and molecular carriers as well as enzyme mimics [1, 3,4]. 

Very stable complexes between calix[4]arenes and NO catimi have been described in the literature, both in solution and in the solid state, under conditions of host-to-guest excess [35-39]. Under ccmditions of guest-to-host excess, both complexes with stoichiometry greater than 1 1 and covalent nitration products have been obtained [40]. Such calixarene-based systems are now considered as supra-molecular materials for generation, detection, storage, and release of NO [41—43]. 

It took 8 years to publish another structure. In 1976 K. J. Palmer et al. reported two conformers of another resorcin[4] arene derivative, i.e. cis- and trans-C-para-bromophenyl-4,6,10,12,16,18,22,24-0-octaacetylcalix[4]resorcinarene [3]. Than in next 3 years G. D. Andreetti et al. published the first crystal structure of inclusion compound formed by para-ferf-butylcalix[4] arene with toluene [4]. The calixarene molecule was found in cone conformation and the guest molecule could not be located within the host cavity due to its disorder (the molecular complex was located on a crystallographic fourfold axis). The Authors also noticed that the structure undergoes the order-disorder phase transition at 248 K. In 1996 the disorder high temperature phase was resolved with the aid of solid-state NMR data by E. B. Brouwer et al. [5] (unfortunately, the atoms coordinates are not deposited in CSD) and in 1998 the low temperature ordered phase was reported by A. Arduini et al. [6] (Fig. 38.2). The inclusion complex is stabilized by week interaction. The crystal and molecular structure of this... 

---