A clathrate may be described as a single-phased solid consisting of two distinct compo- nents: the host and the guest. The guest is retained in closed cavities or cages provided by the crystalline structure of the host. Generally, a cage and its en- closed molecule or molecules are taken as a unit cell.

In 1897, Hofmann and Kiispert (1) published a paper in which they stated that when benzene is added to a solution of nickel cyanide in aqueous ammonia and containing acetic acid, a precipitate of the composition [Ni(CN)z.NH3,C6H6] is formed. Other examples of these molecular compounds are those formed between nickel cyanide-ammonia complex, Ni(CN)%.NH3, and molecules of aniline, benzene, phenol (8), and pyridine, furan, pyrrole, and thiophene (3). The maximum composition formula for this series of monoammine- nickel(I1) cyanide clathrates is [Ni(CN)z.NH,,M], where M is the organic molecules previously men- tioned. Cambi, et al. (4) made careful magnetochemi- cal investigations on nickel cyanide and complex derivatives. They measured the magnetic sus- ceptibility of [Ni(CN)2.NH3.C6H6] and gave the value of 11.04. The magnetic susceptibility of this clathrate was also measured by Janes (6) over a temperature range from 82.5 to 296'K. Although he took into account the effect of a crystal field, he found that there was no possibility of the theoretical formula for molar susceptibilities agreeing with the experimental data,

Vijay Mohan Bhatnagarl Punjab University

Chandigarh, Punjab, India

I~emrusr tlw mngt~rtie moment of the uickrl ntom equal to 2.27 Hohr nucnrtons at 3OO.G0K 1ws subnormal.

Benzene

i.e., smaller than the spin-only value, 2.83 ~ o h r magnetons. He tentatively suggested an explanation in terms of an assumption that there is a predominantly rhombic or perhaps axial crystal field, which quenches not only the orbital but also a part of the spin con- tribution to the magnetic moment. Janes tacitly assumed that all the nickel atoms in the compound are equivalent to one another.

A single crystal X-ray investigation was undertaken on the benzene clathrate (6, 7), and the large distance between the carbon atoms of benzene and the atoms of the caging material was interpreted to mean that the organic molecules are not bonded but are simply trapped in the crystal lattice. The crystal structure of this clathrate is an extended two dimensional flat network of alternating nickel and cyanide groups. In the presence of the guest component, the complex crystallizes in planar arrays of nickel cyanide with the ammonia molecules, bonded to nickel atoms, projected above and below each other. Each guest molecule is located in a cavity bound by two planes of nickel cyanide and by vertically aligned pairs of ammonia molecules (Fig. 1).

The merely space-filling part played by benzene is evidenced by the fact that other molecules of com-

Present address: Instituto di Chimica, Generale ed In- organics, Universita di Roma, Rome, Italy.

parable volume can replace it; these are as follows (molecular volumes a t 15'C in parentheses): aniline (90.5), phenol (88.8), pyridine (80.0), pyrrole (69.3), thiophene (78.5), and furan (72.0). All these com- pounds are included in the ammino-nickel cyanide structure to yield crystalline products of compositions corresponding to and properties almost identical with1 that formed by benzene [molecular volume (88.6)]. The nickel complex is, however, quite indifferent to the following larger molecules: toluene (105.6), nitro- benzene (103.6), fluoro-, chloro-, bromo-, and iodo-1 benzene (93.8-110), toluidine (107.0), cresol (102.0),8 and naphthalene (111.0).

Figure 1. Cage lawice structure of benzene, ammonia, and nickel eyonidd [Ni ICNh . NHa . CaHsl.

It is interesting to compare (8) the packing of benzenl when caged in the nickel complex with that found in t h ~ solid pure hydrocarbon, which crystallizes in t h ~ orthorhombic system, with a unit cell of $imension# a = 7.44 A, b = 9.65 A, and e = 6.81 A (9). T ~ I flat rings are arranged in parallel layers, with the plant of the ring perpendicular to the layer planes; succeed ing layers are identical but each is translated by distance a relative to the neighboring plane. Th closest approach of carbon atoms is between 3.8 an1 3.9 A. The similar plan for the caged structure, i~ which the cage imposes the higher tetragonal sym metry on the benzene, indicates that benzene molp le are closely packed, the closest approach, 3.4-3.5 A.

The preparation of benzene clathrate has bee] described by Hofmann, et al. (1, Z), Palmer ( lo) , an1 Bhatnagar and Fujiwara (11).

In 1955, Leicester and Bradley (18) reported tha certain biphenyls (biphenyl, 4-amino biphenyl, an1 4hydroxy biphenyl) co-crystallize with monoammino nickel (11) cyanide as solids with a characteristi1

646 / Journol of Chemical Education

formula of 2[Ni(NH3) I2+.2CN .M. However, 2-am- mino diphenyl and 2-2'-dihydroxy diphenyl do not form complexes under similar conditions and neither does stilhene. Baur and Schwarzenbach (13) synthesized a few new clathrates from double cyanides, M Ni(CN)a, where M stands for Cu2+, Cda+, or ZnZ+. Examples of synthesized and examined clathrates are [Cu(NHa)r Ni(CN),.C6H,], [Cd(NH3),Ni(CN),.3/2C~H~], and [Zn Ni(CN)4.'/2C6H6]. They are suggested to be similar to [T\'i(CN)2.NHa .C6H6].

A process for the recovery of benzene by selective clathration was patented by Jones and Fay (14). The invention related to a process and apparatus for the recovery of benzene from hydrocarbon stocks and, more particularly, to a cyclic process adapted for con- tinuous operation for the recovery of benzene of high purity from low benzene hydrocarbon stocks, such as refinery naphthas. The process comprised continu- ously blending a stream of the stock with a stream of nickel cyanide-ammonia; conducting the blend to a homogenizer and homogenizing the blend to form a solid clathrate compound with benzene conducting the resultant slurry to a centrifugal separator, and centrifugally separating the solid clathrate compound from the stock; conducting the clathrate compound to a stripper and countercurrently flowing the slurry against a stream of steam and ammonia to decompose he clathrate compound into benzene and nickel cyanide- mmonia; recovering henzene, steam, and ammonia t the top of the stripper; drawing off recovered nickel yanide-ammonia a t the bottom of the stripper; 1 nd recycling the nickel cyanide-ammonia.

Aynsley, et at. (15) examined the physical properties 3f [Ni(CN)2. NH3. C6HB], and contrary to previous zssertions (7, 8), they found that the benzene could be removed in vaeuo (without removal of ammonia), 3lowly a t room temperature and rapidly (50% in 1-3 hours) a t 40-60°C. Up to a t least SOYo removal of benzene the decomposition was of zero order, varying In rate from sample to sample. They presumed that liberation of benzene was accompanied by a partial :ollapse of the lattice, which is then not wholly available or reabsorption of henzene. Discussing the two Ienzene frequencies a t 1573 and 1166 ern-', they ltated that due to relaxation of the selection rules, .hese frequencies appeared in liquid benzene a t 1,587 md 1179 cm-', but not in the benzene crystals. They :xpected the same restriction to be applied to benzene n the clathrate compound, and yet the band a t 1166 :m-' was the strongest hand in the region 40W650 :m-l. Spectroscopic studies on benzene clathrate have Ieen completed by Bhatnagar and Cole (16).

Kondo and Kubo (17) measured the magnetic :usceptibilities of dicyanoamminenickel (II) , and the :lathrate compounds thereof with benzene and pyridine tt room temperature and the effective magnetic noments per one nickel atom were calculated. These noments, which were approximately equal to 2.2 Bohr nagnetons, were explained by the presence of equal lumbers of nickel atoms having no unpaired electrons. he former have four ligands in square coordination

hybridization, while the latter are with six ligands in octahedral coordination

volving dZsp3 hybridization of the central nickel et al. (18) explained that since planar

nickel (11) atoms are usually diimagnetic, it is pos- sible that the benzene, instead of being trapped in the crystal lattice, was interacting with the nickel to produce a paramagnetic tetragonal atom. They re- determined the magnetic data and examined the infrared spectra to provide additional structural in- formation. A structure containing trapped benzene, compatible both with the X-ray work and with.the magnetic moment of 2.1 Bohr magnetons obtained in their study, was proposed. The X-ray powder diffraction pattern of decomposed benzene clathrate (19), electron paramagnetic resonance (If) , and proton magnetic resonance spectra (30) have been described.

Evans, et al. (31) stated that if the benzene clathrate were formed from thiophene-free but otherwise impure benzene, the benzene recovered on the destruction of the compound should he quite free from the saturated hydrocarbons which are otherwise very difficult to remove. They found that hy this method benzene can be obtained from impure material in good yield and of a purity as good as, if not superior to, that of saniples produced by prolonged fractionation in a very efficient still, or by the tedious and wasteful process of frac- tional crystallization and fusion. For example, in one simple operation benzene was separated from other contaminating hydrocarbons and produced in 99.992% purity by forming the clathrate compound with nickel cyanide-ammonia and then releasing the caged benzene by dissolving the crystals.

I wish to thank Prof. N. S. Bayliss and Dr. A. R. H. Cole, University of Western Australia; Prof. R. C. Paul, Punjab University; and Council of Scientific and Industrial Research, India.

Literature Cited

(1) HOFMANN, K. A,, AND K~~SPERT, F. Z. anorg. Chem., 15, 204 (1897).

(2) HOFMANN. K. A.. AND H~CHTLEN, F., Chem. Rer., 36, 1149 (1903).

(3) HOFWANN. K. A,, AND ARNOLDI, H., Chem. Ber., 39, 339 (1906).

(4) CAMBI, L., CAGNASSO, A., AND TREMOLADA, E., Gaza. Chim. Ital., 64,758 (1934).

(5) .Tams, R. B., Phys. Rev., 48,78 (1935). (6) POWEI.L, H. M., AND RAYNER, J. H., Nahre, 163, 566

(1949). (7) RAYNER, J.H.,ANDPowELL,H.M., J . Chem Soe., 1952,319. (8) PALMER, W. G., "Experimental Inorganic Chemistry,"

Cambridge University Press, London, 1954, p. 518. (9) Cox, E. G. , Proe. Rov. Soe., A135,491(1932).

110) PALMER. W. G.. on. cif . . D. 561. . . .. i l l j B~TNAGAR, V. M., AND FUJIWARA, S., Chem. Ind. (Lon-

don), 1962, 1471. 112) LEICESTER. J.. AND BRADLEY. J. K.. Chem. Ind., 1955, 1449. , . , . (13) BAUR, R., AND SCHWARZENRACH, G., Helv. Chim. .4cla,

43,842 (1960). (14) JONES, A. L., AND FAY, P. S., (to Standard Oil Co., Ohia),

T i . S. 2.732.413, Januarv24. 195fi. C . A , . 50.103'32' (1956). , , , " , . . (15) AYNSLEY, E. E., CAMPBELL, W. A., AND DODD, R. E.,

Proe. Chem. Soc., 1957,210. (16) BHATNAGAR, TT. M., A U D COLS, A. R. H., Unpuhlished work. (17) KONDO, M. AND Kaeo, I f . , J . A m . Chem. Soc., 61, 1648

(1957). (18) DRAGO, R. S., Kwox, J. T., A N D ARCHER, R. D., J . A m .

Chem. Soc. 80,2667 (1958). (19) BHATNAQAR, V. M., J . Indian Chem. SOL, 39,667 (1962). (20) NAKAJIMA, H., BHATNAGAR, Ti. M., AND COLE, A. R.H. ,

J. Phys. Soe. Japan, 17,1194 (1962). (21) EVANS, R. F., ORMROD, O., GOALBY, B. B., AND STAVELEY,

I,. A. K., J . Chem. Soc., 1950,3316.

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