Liquid clathrates crystallizations

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

The model of the solution behaviour of liquid clathrates that appears to best fit the observations is depicted in Figure 13.26. Large regions of liquid order exist in which cations and anions interact strongly with one another. This prevents simple dissolution, but association into aggregates such as dimers and trimers (possibly leading to crystal nucleation) is also prevented by the steric properties of the anions and/or cations. The role of the aromatic solvent is to support the structure and stabilise... 

Figure 13.28 X-ray crystal structure of the liquid-clathrate-derived inclusion compound N,N -dimethylimidazolium+(PF6 )-0.5C6H6 showing the stacking of the benzene guest with the imidazolium...

Jerry L. Atwood is Curator s Professor of Chemistry at the University of Missouri-Columbia. His research has focused on supramolecular chemistry. His research group has synthesized and examined a broad array of host-guest chemical systems (e.g., liquid clathrates, macromolecular hosts). A principal method for characterization of these systems has been single crystal X-ray structure determination [41-43],... 

The experiment began by charging the equilibrium cell with about 30 cm3 of either phenoPp-cresol or phenol-water solution mixture. The cell was then pressurized with either methane or carbon dioxide until the phenol clathrate formed under sufficient pressure. The systems were cooled to about 5 K below the anticipated clathrate-forming temperature. Clathrate nucleation was then induced by agitating the magnetic spin bar. After the clathrates formed, the cell temperature was slowly increased until the clathrate phase coexisted with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to diminish hysteresis phenomenon. The clathrates, however, exhibited minimal hysteresis and the excellent reproducibility of dissociation pressures was attained for all the temperatures and found to be within 0.1 K and 1.0 bar at each time. When a minute amount of phenol or p-cresol clathrate crystals remains and the system temperature was kept constant for at least 8 hours after attaining pressure stabilization, the pressure was considered as an equilibrium dissociation pressure at that specified temperature. 

In an extension to this work it was found that simple metal halides (and some carbonyls) could also form liquid clathrates with [18] crown-6 in toluene or benzene when HC1 was bubbled through the mixture, but only in the presence of trace water (or fortuitous water , as it became known within the group) [4]. These systems also generated hydronium ions that were stabilized by the crown ether and crystallized from solution. 

Anionic coordination complexes of Mo and W which crystallize from liquid clathrate media with oxonium ion-crown ether cations, J. L. Atwood, S. G. Bott, P. C. Junk and M. T. May, J. Coord. Chem., 1996, 37, 89. 

Temperature is all important to the existence of liquid clathrates. K[Al2(CH3)6N3] benzene is reasonably stable (Reaction 4) at 25°C and completely stable at 40°C. Cs[A1o(CH3)gN03] benzene is exceedingly unstable (Reaction 4) at 25°C but completely stable at 80°C. K[AL(CH3)gN3] p-xylene does not exist at 25°C, but it is stable at 130 °C. It does not seem unreasonable that the proper combination of cation, anion, and guest might form a liquid clathrate at slightly elevated temperatures, but at room temperature it would form a solid with a structure similar to that of the liquid clathrate. We now have what may be an example of this in the crystal structure of K[CH3Se Al(CH3)3 3] 2CgHg. 

Table 3 Oxonium ion-crown ether complexes crystallized from liquid clathrates...

Liquid clathrates as crystallization media have resulted in many new structures. Crystals of crown ether, cryptand, and tetramethylethylenediammonium and triphenylphosphonium complexes were prepared. Presented in Table 3 is a short list of some oxonium ion crown-ether crystal complexes which deposited from the liquid clathrate mixture. Oxonium ion complexes with crown ethers from a liquid clathrate media were formed of a suitable size and shape to crystallize lanthanides and actinides. 

K. D. Atwood, J.L. Synthesis and x-ray structure of [H30 - 18-crown-6][l7 ]. A new infinite sawhorse geometry for l7 crystallized from a liquid clathrate medium. Inorg. Chem. 1995. 34. 5395-5396. 

Barbour, L.J. MacGillivray, L.R. Atwood, J.L. Crystal and molecular structure of [H30-18-crown-6]2[ReCl6] isolated from a liquid clathrate medium. J. Chern. Crystallogr. 1996, 26. 59-61. 

MacGillivray, L.R. Atwood. J.L. Insight into the mechanism of the protonation of cry ptand 222 within a liquid clathrate medium Synthesis and x-ray crystal structure of [H30][222-2H][(CoCl3)2( x-Cl)]. Chem. Commun. 1996. 

Equation 13 has an important implication a clathrate behaves as an ideally dilute solution insofar as the chemical potential of the solvent is independent of the nature of the solutes and is uniquely determined by the total solute concentrations 2K yK1.. . 2x yKn in the different types of cavities. For a clathrate with one type of cavity the reverse is also true for a given value of fjiq (e.g. given concentration of Q in a liquid solution from which the clathrate is being crystallized) the fraction of cavities occupied 2kVk s uniquely determined by Eq. 13. When there are several types of cavities, however, this is no longer so since the individual occupation numbers 2k2/ki . ..,2k yKn, and hence the total solute concentration... 

The points R have to be on a straight line terminating in the composition of the methanol hydroquinone clathrate (A) in equilibrium with a-hydroquinone at 25°C. The point B roughly corresponds to the composition of the clathrate obtained by Palin and Powell24 when crystallizing hydroquinone from methanol points between A and B form a continuous range of solid solutions in equilibrium with liquid phases whose compositions lie on the curve CE. It is found that the equilibrium clathrate has a composition corresponding to y — 0.474 at 25°C. 

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