J. Eric D. Davies University of Lancaster Lancaster, Lancashire LAI 4YA. Great Britain
Species in Layers, Cavities and Channels (or Trapped Species)
The host lattice must be capable of crystallizing in a very open structure containing species in which the guest molecule can be trapped.
In addition to the usual states of two component systems such as miscihle and immiscible liquids, an insoluhlesolid in contact with a liquid, and a solute in a solvent, we also have the so-called inclusion compounds. Here the two components are usually referred t o as the host lattice and the guest molecule, a requirement of the host lattice being that i t must be capable of crystallizing in a very open type of structure containing spaces in which the guest molecule can be trapped. A large number of materials can act as host lattices giving voids of varying shapes and dimensions. This article will deal with selected examples of host lattices which illustrate the three main types of voids encountered: interlayer spaces ( I 1, enclosed cavities, and long channels. The article will attempt to show how the reactivity of a species can he modified by its inclusion in a host lattice, how the use of inclusion compounds enables one to study both host lattices and guest molecules in forms otherwise unobtainable and even to study single molecules a t temperatures where they would not normally exist! Graphite Intercalates One of the simplest compounds which has a layered structure is one of the allotropes of carbon, viz. graphite, whose structure is shown in Figure 1. Each laver -~ ~,consists of hexaeonal carbon units with the layers stacked one on top of anuther such that ewry alternate laver is sunerimoosahle. The intralaver carbon-carbon dist a k e is 14i pm ihereas the interlayerseparation is very large, (335 om). means that the interlaver . . This laree - seoaration . forces are \ w y weak Van der Wads forces and thls allows ;he layers to slide over each other. It is this movement which gives graphite its luhrlcatlng properties. The laree interlaver . s.~ a c i -n aalso allows atoms and molecules to penetrate between the layers giving graphite intercalates (2). Potassium, for example, will form a series of intercalates of limiting compositions CsK, C&, C2&, C32K etc., and the structure of C8K is shown in Figure 2. ~
Table 1. Reduction of Ketones with C,K. Ketone
Benmphenone Acetophenone
% Yield
Product
98
Cycloheptanone Cyclohexan~ne Camphor
Alcohol ~lcohol ~inacol Al~ohol Al~ohol Alcohol
4-phenylbutenone
Saturated ketone
45 45 92 75 60 ( e m )
4 0 (endo) 75
In CsK there are potassium atoms between each plane, in C,.K there are ootassium atoms between evew alternate laver, atoms are between &ery third laier, in&^ the etc. The intercalate also has a greater interlayer spacing than graphite, 540 pm in CsK compared with 335 pm in graphite itself. A variety of species, such as K, CrOs, SbFs, and AIC13, form graphite intercalates. Organic chemists will recognize these bulk reagents as reducing, oxidizing, and fluorinating agents, and as a Friedel-Crafts catalyst. The graphite intercalates of these snecies can also be used in svntbetic oreanic reactions. and it wo~lldhr interesting to compare the reactivity of the molecule inside the eranhite lattice with the reartivitv of the bulk reagent. ~ o m e k f f e r e n c emight s be expected in;for examnle. reaction rates. since the reactine molecule might have w diffuse into the graphik lattice in ord& to reach thereagent molecule. The intercalates might also show some selectivity depending on the size of the mkecule or the stereochemistry around the reaction center because of the limitation imposed by the interlayer spacing. ~
~
-
.
The hexagona grapnrle an cc lF8g~re1 from Aenn g. G R Prog lnarg Chsm. 1. 125 (19591 Reproa~ccdoy permossom of Jonn WI ey 8 Sons. 1°C.)
Fogure 1
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Journal of Chemical Education
Figure 2.
0K
The srmchlre of C.K. (Figure 2 from Lalancene.J. M., Rollin, G.. and Dumas, P., Canad. J. Chem., SO, 3058 (1972). Reproduced by permission of the National Research Council of Canada.1
tercalate is the monoethyl derivative, whereas bulk AIC13 gives the triethyl derivative as the predominant product. Another advantage of the intercalate is that it is much more resistant to hydrolysis than bulk AlCh and this simplifies the experimental procedure. Hofmann Compounds
1 from Lalancene. Fioure 3. Stereochemistw of reductionwith CaK. . IFiWre . . J. M., Rollin, G.. andDumas. P.. Canad. J. Chem.. 50,3058 (1972). Repmduced by permission of the National Research Council of Canada.) Table 2.
I t has been shown (5)that some intercalatedspecies have considerable translational mobility within the graphite lattice. If blocking groups were present between the layers then this mobility would he reduced and the intercalated species contained. Such blocking groups cannot of course be incorporated in the graphite structure, hut they are found in the metal1 ammonialcyanide complexes of the general formula M(NH&Ni(CN)a whose structure is shown in Figure 4. The s t ~ c t u r is e similar to that of graphite in that it contains layers conmining the rned atoms and the cyanide groups, but between the layers Iirr NH., groups which form a cavity in the
Oxidation of ~ I c o h o l w s ith CrOJGra~hite
Alcohol
Confact time ihr)
1-Hexadecanol Benlyl alcohol Cinnamyl alcohol Furfury1 alcohol citronello1 1.6-Hexanediol Phenylethanediol CvclohexvlmethanOl ~~&hex&l t-Amy1 alcohol laopuiegol
24 24 24
Table 3.
% Yield o f aldehyde or ketone
2 0 0
The Use of the AICI, Intercalate ProdUCfS "ring
Bulk System ~ e n z e n eand ethyl bromide in a sealed tube at 25- for 48 hr. Toluene and ethyl bromide i n a sealed tube at 2s0 for 24 hr.
AICI, Benzene
MonoethylDiethyiTriethylTetraethylToluene MonoethylDiethylTvietnyl
2 13 27 54 4 0 Tracer 40 60
AICI, infercalafe 29 40 23 8
aN
C
Traces
17 52 31 Traces
Table 1contains the results obtained (3) when reducing a variety of ketones with CsK. One interesting result is the reduction of camphor to give the exo alcohol as the predominant product. This ratio is quite different from that obtained by reduction with potassium in tetrahydrofuran when the endo alcohol is the predominant product. The reason for this difference is shown in Figure 3, where the proposed mechanism involves adsorption at the surface of a CsK crystallite. Since this adsorption is easier in a position more favorable for the formation of the exo alcohol, this isomer becomes the dominant product able 2 contains the results obtained (3) during the oxidation of alcohols by the chromic oxide intercalate. The number of secondarjand tertiary alcohols in the list is very limited but in general, the chromic oxide intercalate converts primary alcohols to aldehydes, secondary alcohols to ketones, and it does not affect tertiary alcohols. The variation in yield can he related to the stereochemistry around the hydroxyl group which controls the diffusion of the molecule between the graphite layers. A final example of the use of a graphite intercalate is illustrated in Table 3 which contains a comparison (4) of the reactivities of bulk AIC13 and the AlC13 intercalate when used as catalvsts for Friedel Crafts reactions. In these alkylation reactiuk the intercalate gives a slwver reaction and here is also less . ~olvsubstitution. In the toluene/ethyl hrwnide reaction, for example, the predominant product with the in-
Floure 4 me structure of Hofmann-woe . " .. clathrate MiNKlrM'iCN)r.2CeHe M = M' = Ni in Hofmann's dakrate. Pmtons of the benzene and ammonia are not shown. (Figure 1 from lwamoto. T.. etal.. lnorg. Chim. Acta. 2,31311968). Reproduced by permission of Elseviw Sequ6ia S. A.)
i
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(6)
Figure 5. Manner of hydrogenbonding of hydroquinone molecules. (a) In plan. Each regular hexagon denotes six hydrogen bonds between oxygen atoms. Hexagons at different levels are repre sented by different line thickness. me tapered lines, representing the 0-0 axis of a hydroquinone molecule, show the method of linkingtoforman infinite Uvee-dimensional cagework. Each taper points downward fmm the observer. (b) Perspective drawing corresponding to the above. The hexagons denote the hydrogen bonds: Me longer lines cannecting different hexagons denote the 0-0 axis of the hydroquinone m o l e cule. The m p l e t e sbuctve as faund in the molecular compounds consists of the cagework shown, together with a second identical interpenetrating cagewak, which is displaced vertically half-way between the top and bottom hexagons. (From Palin. D. E., and Powell. H. M., J. Chem. Soc.. 208 (1947). Reprcduced by permission of The Chemical Society.)
Volume 54, Number 9. September 1977 1 537
structure wherein a molecule such as henzene can he trapped. The general definition of an inclusion compound assumes that there is very little interaction between the host lattice and the guest molecule. However spectroscopic work carried out jointly a t Lancaster and a t Hacettepe University, Ankara, has shown ( 6 )that in these Hofmann compounds there is in fact appreciable interaction between the benzene guest molecule and the NH3 groups. In addition to reducing the mobility of the guest molecule by the presence of the blocking NH3 groups some constraints have also been placed on the size of the molecule which can he included, e.g., while the ahove lattice will accommodate a benzene guest molecule, the toluene molecule is too large to fit into the cavity (7). Ouinol Clathrates
In the ahove lattice the henzene molecule is not rompletely enclosed hut can slowly diffuse out. Compounds in which the guest molecule is cornpletelv enclosed in a cage formed hy the host lsttire are called clathrates ( b ) and quinol, C,jH4(0H)z, is an example of a host lattice which formsspherical cavities in the hydroxen bonded lattice of the &polymorph (Fig. 5). Thp dimensions of this rnvitv are now essentidlv fixed with a diameter of about 800 pm. There is consequentiy an upper limit on the size of guest molecule which can he accommodated. This is illustrated by the fact that while methanol can be accommodated in the quiuol cavity, the ethanol molecule is too large. The cavity in these quinol clathrates is thus quite small and it can be used to accommodate a single diatomic molecule as illustrated in Figure 6. These clathrates can thus he used to study the rotational, vibrational, and nmr spectra of isolated molecules over very wide temperature ranges. They can even be used to study single moiecules a t teiperaturks where they would not normally exist! ~ i t r i coxide, NO, is an interesting molecule since i t is paramagnetic with one unpaired electron, and it is interesting in determine the temperal&e dependence of its effective oh; magneton number since theory predirm that this should iall to &at low temperature. The cnn\,entional way of studying isdated molerules is U Duse thegasat low pressure, hut nitric nxfde liquificsat 121 K, and the liquid nolnnger has a monomeric structure but consists of diamagnetic dimers. If, however, the quinol clathrate is prepared then the nitric
oxide molecule is isolated in a cavity from which it cannot escape. Using the clathrate (9) it is thus possihle to obtain measurements on the isolated nitric oxide molecule down to 10 K, i.e., at temperatures where the molecule would normally exist as the dimeric species. Quinol will not form a clathrate with ethanol and it is not possihle to change the quinol structure so that the cavity would he large enough to accept an ethanol guest molecule. Thus if i t is required to study ethanol as a guest molecule a host lattice with a sufficiently large cavity, must he used and such a compound is Dianin's compound (Fig. 7, Structure I). The cavities formed by this compound are large enough to accommodate ethanol and even larger molecules such as nhexanol. The cavity is found to have an unusual shape (Fig. Lla), and the waist is thouaht to he d w to one of the reminal methyl groups in structure I, Figure 7. With this par%cular host 1 2 tice it might thus he possihle to modify the dimensions of the cavity by removing one of these methyl groups. Structure 11, Figure 7, which contains one methyl group less than Dianin's compound has recently been synthesized (10)and has been found capable of acting as a host lattice. Furthermore a crystal structure determination shows (Fig. 8b) that the waist present in Dianin's compound has been removed. Such crystal engineering techniques make slight modifications of cavity dimensions possihle, hut they are unlikely to lead t o any significant modifications.
Fig~le8 The cavities of Dim n's compo~ndand its 2-nor anslag. lF,gae 2 irom hardy, A. 0. U . McKendrlCk. J J . and MacNieol. D 0.. J.C.S. Chem Comm. 355 11976).Reprodxed qr permissionot Thechemical Socoely.)
Ftgure 6. Doatomic guest molecule in 3-9"no1 cavity. (Figure 1 fromBurgiel. J C . Meyer. h . andRchards. P L.. J. Chem Phys.. 43.4291 (19651.Reprod u c e ~by PBrmiSsion 01 the Amellcan lnst rule of Phys c6.I
1111
01
Floure 7. A D lJ
Shlcture of
Dianin's ComDound and its 2-nor analoo. iFrom Hardv.
. McKendrlck. J J . and MacNml. D D . J C S Chem Comm. 355
(19761 Reprodww by
permlsolon ot Tne Chemical Soc ery 1
538 1 Journal of Chemical Education
Figure 9. Cross Jsetianal view of urea-rrparaffin complex. (Figure 11 from Smith, A. E., Acta Crysr, 5, 224 (1952).Reprcdvced by permission of A m Crystallogrsphlca.)
Urea and Thiourea lncluslon Compounds Urea Another examole of a molecule which can act as a host lattice is urea (NH&CO. When it is recrystallized from methanol containine some hvdrocarbon i t c ~ s t a l l i z e sin a hexaeonal modifica
