Cyclotriphosphazene clathrates

Inclusion compounds of the Cg aromatic compounds with tris((9-phenylenedioxy)cyclotriphosphazene have been used to separate the individual isomers (43—47). The Schardinger dextrins, such as alpha-cyclodextrin, beta-dextrin, and gamma-dextrin are used for clathration alpha-dextrin is particularly useful for recovering PX from a Cg aromatic mixture (48,49). PyromeUitic dianhydride (50) and beryllium oxybenzoate (51) also form complexes, and procedures for separations were developed. 

Aromatic derivatives of cyclotriphosphazenes, rigid six-membered ring systems built on a framework of alternating P and N atoms, provide one of the more beautiful early examples of hosts that form channel-type clathrates and can be useful for molecular separations 32,42>, Although these clathrate systems were discovered by accident, the conclusions that emerged from their investigation have been extremely helpful for the molecular design of other potential host molecules. 

The range of inclusion adducts formed by the organophosphazenes is very broad, the guest species varying from aliphatic and aromatic hydrocarbons to ethers, ketones and alcohols42. Some of the hosts [e.g., tris(o-phenylenedioxy)cyclotriphosphazene 12)] form clathrates not only when recrystallized from organic solvents but also... 

Not all clathrates are hydrates. Other well-known examples have host lattices formed from hydrogen bonded aggregates of hydroquinone, phenol, and similar organic compounds. Non-hydrogen bonded host structures are also known. One example is a cyclotriphosphazene. (C6H402PN). that traps molecules such as benzene in tunnels in the crystal.2 In addition, coordination polymers are formed by ambidentate ligands, such as CN and SCN, which coordinate to metal ions at both ends (Chapter 12). Perhaps the best known of this type of compound is the series of Ni(CN)2NHj M compounds, where M may be benzene, thiophene, furon. pyrrole, aniline, or phenol. 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

Crystals of tris(o-phenylenedioxyde)cyclotriphosphazene (97) can act as hosts for the inclusion of a number of organic polymers, e.g. cis-1,4-poly butadiene, 1,4-polyisoprene, polyethylene (PE), poly(ethylene oxide) (PEO) and polytetrahydrofuran. X-ray studies of the PE and PEO inclusion compounds show that the polymer chains are extended along the tunnel-like voids of the host lattice. The formation of clathrates appears to be limited by the tunnel dimension of the host crystal lattice. The melting points of the inclusion adducts appear to be higher than those of either the pure host or the pure... 

The most common hosts for inclusion polymerization are urea, thiourea, perhydrotriphenylene (PHTP), deoxycholic acid (DCA), apocholic acid (ACA) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP)(Fig. 2). They have the common feature of forming channel-like clathrates, but differ in many specific properties. For instance, urea and thiourea have a rigid structure in which the host molecules are connected by hydrogen bonds and possess a high selectivity towards the guests. In urea channels are rather narrow whereas in thiourea they are wider as a consequence, linear molecules include only in urea and branched or cyclic molecules in thiourea. On the contrary, chainnels existing in PHTP clathrates are very flexible and can accomodate linear, branched and cyclic molecules. 

Similar phenomena characterize the clathration behavior of the larger host tris(2,3-naphthalenedioxy)cyclotriphosphazene 13). Due to the boIki tack-bone, this host forms, in relation to the former structure type, a markedly expanded (withchaimelsof diameter 9-10 A at the narrowest point) and thus less stable structure. Evidently, the driving force for clathration of the substituted cyclotriphosphazene species is associated with their unusual paddle-wheel like shape. Van der Waals packing of these molecules in a high symmetry arrangement leaves substantial voids in the lattice which are fillta by guest components to lower the enthalpy of the total system. The potential use of this clathration behaviour for separation of hydrocarbons was envisioned already about fifteen years ago... 

Figure 20 The topology of the cyclotriphosphazene lattice present in the clathrate compound formed by host 74 and carbon dioxide, and highlighting the channels formed by the host stacking. Color code C, green N, blue O, red and P, purple. Hydrogen atoms and the disordered guest molecules are omit-ted.134...

C2 nHi8N3O6P3, Tris-(o-phenylenedioxy)cyclotriphosphazene benzene clathrate, 42B, 501... 

Recently, Allcock et al. reported the inclusion polymerization of vinylic, acrylic [89], and diene monomers [90] within the clathrate tunnels formed by tris(o-phenylenedioxy)cyclotriphosphazene. Varying degrees of stereoregularity were obtained from the inclusion polymerization. For example, poly(methacrylonitrile) synthesized within the host adduct showed an enhanced isotactic microstructure. No cross-linked material was obtained from the polymers derived from the acrylates and methacrylonitrile. The copolymer poly(vinylacetate-co-methylacrylate) was also prepared by the inclusion polymerization method. 

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