Liquid clathrates preparation

One of the more important recent developments in organometallic aluminum chemistry has been the formation and isolation of low-coordinate compounds, and, in particular, cations. These were first prepared in reactions of various aluminum reagents with crown ethers to form the inclusion compounds known as liquid clathrates. 71,72 Most of the evidence supports the presence of ion pairs as the basis of the solvent inclusion effect. Indeed, the compound [AlMe2-18-crown-6]+[AlMe2Cl2] was isolated from one such system (the cation is shown in Figure 6(a)).73 This was the first time the Me2Al+ unit had been structurally characterized. 

Preparations. The two different ways by which the liquid clathrates may be prepared are best illustrated by reference to the K[Al2(CH3)6N3] complex. By the method described previously (JO), 0.010 mole Al( CH3 )s was added to 0.005 mole KN3 in N2 atmosphere dry box. The mixture was then sealed in a bomb tube, heated to 80°C, returned to the dry box, and opened another 0.005 mole Al(CH3)3 was added to the powdered contents. After three cycles of grinding, adding Al(CH3)3, and heating. 

A decidedly improved method for producing the compounds involves simply adding 0.005 mole KN3 and 0.010 mole A1(CH3)3 to /- O.IO mole CgHg in the dry box. A liquid clathrate identical in composition to the one prepared by the previous method was obtained in 1 hour. All liquid clathrates reported here were synthesized this way. 

Analysis. The liquid clathrates were analyzed by the integration of NMR spectra recorded on a Perkin-Elmer R20-B instrument. The aromatic stoichiometries in Table II are in most cases the average of three preparations and integrations a realistic standard deviation would be 0.2 molecules. The chemical shifts are accurate to better than 0.02 ppm. 

A second method for the production of liquid clathrates involves the addition of 0.005 mol KN3 and 0.010 mol A1(CH3)3 to 0.10 mol toluene in an N2 atmosphere dry box. This procedure yields a liquid clathrate identical in composition to the one prepared in the previous method. Note that liquid clathrate formation is a visually dramatic event. As the reaction proceeds, separation of two distinct liquid layers becomes obvious. 

Molar miscibility and saturation point measurements describing liquid clathrate behavior are commonly analyzed by the integration of NMR spectra. In most cases, the aromatic stoichiometries are the average of three preparations and integrations with an acceptable standard deviation result of 0.2 molecules. A common feature of all liquid clathrate NMR spectra is the shifting of the entire spectrum 0.2-0.5 ppm downfield relative to the pure aromatic substance. 

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. 

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,... 

The clathrates of quinol were prepared by recrystallisation from the liquid guest or by bubbling the gas through a saturated ethanol solution [3]. Samples of the a-zirconium phosphate intercalates were kindly donated by Prof. U. Costantino, University of Perugia, Italy. 

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