Phosphorylated cavitand

Keywords Phosphorylated hosts Cavitands Hemicryptophanes Complexation ... 

Scheme 1 Typical examples of phosphorylated cryptands A [12, 13], B [14, 15], C [16], macrocycle [17], hemispherand [18], and cavitand [19]...

The cavitands are essentially synthesized from their resorc[4]arene precursors which are readily obtained by resorcinol condensation with aldehydes. The main feature comes from the different configurations that are expected for this tetrameric species and the relative thermodynamical stability of each isomer, which has been widely investigated by several authors. In addition, the conformational mobility of the resorc[4]arene molecules will depend on substitution at the upper and lower rims [28, 36, 40, 41]. The first attempt to synthesize a phosphorus bridged cavitand was to treat resorc[4]arene la (1, R=CH3) with phenylphosphonic dichloride or phenylphosphonothioic dichloride. Only inseparable isomer mixtures were obtained and isolation of the desired cavitands was not possible [42]. The first isolated phosphorylated resorcinol-based cavitand was described in 1992 by Markovsky et al., who prepared compound D from la and four equivalents of o-phenylenechlorophos-phate in the presence of triethylamine [43, 44]. For this compound, a tautomeric temperature and solvent dependent equilibrium exists between the spirophosphorane structure and the cyclic phosphate form (Scheme 4). 

We will focus now on two series of tetra-bridged phosphorylated cavitands, which are of importance in regards to their potential host-guest properties and therefore as elements for the design of supramolecular systems. These are the phosphate derivatives, and the P-phenyl phosphonate or thio-phosphonate compounds, which have been particularly investigated in our group. 

The complexation of neutral guests by tetra-bridged phosphorylated cavitands has been quite seldom investigated, although some specific host-guest interactions should favor the encapsulation of neutral species (H-bonding, van der Waals forces, hydrophobic effects or specific r-interaction). With the tetra-nuclear complexes of 2 described in the previous section, evidence for the encapsulation of alkyl-amine was reported. Only amine guest can in-... 

The complexation of anionic species by tetra-bridged phosphorylated cavitands concerns mainly the work of Puddephatt et al. who described the selective complexation of halides by the tetra-copper and tetra-silver complexes of 2 (see Scheme 17). The complexes are size selective hosts for halide anions and it was demonstrated that in the copper complex, iodide is preferred over chloride. Iodide is large enough to bridge the four copper atoms but chloride is too small and can coordinate only to three of them to form the [2-Cu4(yU-Cl)4(yU3-Cl)] complex so that in a mixed iodide-chloride complex, iodide is preferentially encapsulated inside the cavity. In the [2-Ag4(//-Cl)4(yU4-Cl)] silver complex, the larger size of the Ag(I) atom allowed the inner chloride atom to bind with the four silver atoms. The X-ray crystal structure of the complexes revealed that one Y halide ion is encapsulated in the center of the cavity and bound to 3 copper atoms in [2-Cu4(//-Cl)4(//3-Cl)] (Y=C1) [45] or to 4 copper atoms in [2-Cu4(/U-Cl)4(/U4-I)] (Y=I) and to 4 silver atoms in [2-Ag4(/i-Cl)4(/i4-Cl)] [47]. NMR studies in solution of the inclusion process showed that multiple coordination types take place in the supramolecular complexes. 

The main feature for cation recognition by tetra-bridged phosphorylated cavitands arises from the cooperative effect of the four phosphorus groups and the aromatic molecular cavity. In the phosphorus(IV) cavitands guest binding will be achieved through O (P=0) or S (P=S) coordination with different affinity for hard or soft metal ions. On the other hand, transition metal rim complexes described above can act as host for metal cation. 

The general trend is similar for the four hosts, although some discrepancy appears along these data. In spite of some systematic errors arising from the extraction method, it must be underlined for example the discrepancy of data for Ag. The silver(I) cation is much better extracted by the thioether-substituted host 12g probably because Ag can interact not only with the phosphorylated binding sites of the cavitand, but also with the thioether functionality of the lower rim. Furthermore, it must be pointed out that the Hpophihcity of the host can interfere in the extraction process. For both al-kahne and alkaline-earth picrate salts, the extractability increases with the... 

Fig. 4 % Extraction of metal picrates by phosphorylated cavitands 12b, 12c, 12d, and 12g (extraction of Ag(I) with 12b was not measured)...

This new type of supramolecular assembly opens the route to the design of more elaborated systems and for instance the propensity of the phosphorylated cavitand 12b to bind to poly-methylammonium guests, was further illustrated by the formation of the supramolecular complex depicted in Fig. 12. The fourfold symmetrical tetrapyridinium porphyrin [porphyry ll insoluble in chloroform was used as guest and was readily dissolved in chloroform solution in the presence of four equivalents of 12b [93]. 

Fig. 13 Schematic representation of supramolecular assemblies of phosphorylated cavitand tectons...

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