Supramolecular Architecture - American Chemical Society

Chapter 19

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Crystal Engineering of Novel Materials Composed of Infinite Two- and ThreeDimensional Frameworks Richard Robson, Brendan F. Abrahams, Stuart R. Batten, Robert W. Gable, Bernard F. Hoskins, and Jianping Liu Inorganic Section, School of Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia A general approach to the construction of new types of infinite frameworks based on a number of simple structural prototypes is described. Whereas Cu(4,4',4'',4'''tetracyanotetraphenylmethane)BF .xC H NO contains a single diamond-like framework which generates huge intra-framework spaces filled with essentially fluid nitrobenzene, Cu(1,4dicyanobenzene) BF contains five independent diamond-like frameworks which interpenetrate leaving no space for solvent. Simple mixing of the components NMe4 , Zn , Cu andCN leads to the spontaneous assembly of the intended diamond-related array of composition [NMe4][ZnCu(CN)4]. A new simple structural prototype is provided by Cd(CN) .2/3H O.tBuOH which contains an infinite honeycomb-like framework consisting of interconnected square planar and tetrahedral centres in 1:2 proportions; Cd(CN) .1/3hexamethylenetetramine has a geometrically very different framework structure which nevertheless has an identical connectivity or topology. A number of metal-4,4'-bipyridine derivatives consist of two perpendicular stacks of 2D square grid sheets which interpenetrate to give an unprecedented 3D concatenation. M[C(CN) ] crystals (M=Zn, Cu, Cd, Ni, Co, Mn) consist of two independent, interpenetrating rutile-like frameworks. A new type of 3D net is produced by interconnecting 5,10,15,20tetra(4-pyridyl)-21H,23H-porphine palladium building blocks via Cd centres. A PtS-relatedframeworkcan be deliberately constructed by interconnecting square planar Pt(CN)4 building blocks by tetrahedralCu centres. 4

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It is possible that unusual materials could be made by building infinite frameworks based on a number of structural prototypes in which each atom of the parent net has been replaced by a stereochemically appropriate molecular building block and each bond of the parent has been replaced by an appropriate molecular connection. It is probably wise in early attempts at this framework construction to use the simplest available structural prototypes such as diamond and Lonsdalite (tetrahedral centres), α-polonium (octahedral centres), NbO (square planar centres), PtS (equal numbers

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of tetrahedral and square planar centres) and rutile (trigonal and octahedral centres in 2:1 proportions) but others that exist or that can be envisaged could be put to similar use. Given the wide range of molecular building blocks and connectors that can be conceived for these purposes each prototype net in principle affords a whole family of related frameworks. Such solids would be of fundamental structural interest and may have useful properties. The working hypothesis we have adopted is that self-assembly to yield ordered infinite frameworks may occur spontaneously in reaction mixtures containing appropriately functionalised building blocks and connecting units. In ideal cases one could imagine devising reaction systems having no option but to condense into the intended infinite array. Channels, Cavities and Interpénétration Examination of models and consideration of the sorts of molecular building blocks that might realistically be used for framework construction leads to the realisation that, for many simple nets, if the connecting units have an element of rod-like rigidity they need have only modest length (by normal molecular standards) to provide structures with relatively large cavities, windows and channels. As a consequence, these materials may have interesting and useful properties with applications in areas such as ion exchange, molecular sieves and zeolite-like catalysis. Appropriate functionalisation of the components, either before framework construction or afterwards may make possible the synthesis of a range of permeable solid catalysts each tailor-made for a particular chemical transformation. Compared with zeolites these frameworks in principle offer bigger channels, more facile and more widely variable functionalisation for the introduction of catalytic sites and better substrate access to those sites. An aspect of these frameworks which is becoming more apparent the more of them we study, is their tendency to yield remarkable structures in which two or more entirely independent giant molecules are intimately entangled, but in an ordered fashion. In such cases the channels and cavities generated by one framework are very neatly filled by the other(s). A few interpenetrating networks have been recognised for many years. The discovery of the first example, CU2O, consisting of two independent, interpenetrating diamond-like nets dates back to the very early days of structural analysis (1). Most known cases of interpénétration involve varying numbers of diamond-related nets eg. two independent nets (2), three nets (3), five nets (4) and six nets (5). Rare examples of interpenetrating 3D nets not derived from diamond are provided by the silicate mineral neptunite, in which two essentially 3connected nets interpenetrate (6) and β-quinol (7). Rare examples of interpenetrating sheet structures are provided by Ag[C(CN)3] (8), benzene- 1,3,5-tricarboxylic acid (9) and certain of its inclusion compounds (10) all of which involve sheets resembling hexagonal mesh chicken wire and Hittorfs violet phosphorus (11). Several of these examples involve rather weakly Η-bonded networks where the distinction from collections of discrete molecules is not clear cut and examples of mtzTczntXrzurig frameworks that are strongly bonded internally remain relatively rare. It does seem likely at this stage that work along the Unes described below will greatly increase the range of known types of interpénétration. Concatenated arrays are of interest not only at a fundamental structural level as new and special geometrical and topological types but also because they may show unusual properties stemming from their unusual structures. For example, the ordered entanglement may lead to unusual mechanical properties. It may also provide a means of positioning and orienting sub-units of separate frameworks into unusually close contact; if the sub-units have appropriate delocalised electronic π-systems this oriented close contact may afford an approach to the development of organic superconductors with higher T 's than than those currently known. c


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Diamond-Related Frameworks M,

4,4',4",4 -tetrasubstituted tetraphenylmethanes and 1,3,5,7-tetrasubstituted adamantanes are natural choices as building blocks for the construction of diamondrelated frameworks, a number of general approaches to which have been considered elsewhere (12). Replacement of the acetonitrile ligands in C u ( C H 3 C N ) 4 by 4,4',4 ,4 "-tetracyanotetraphenylmethane led to the assembly of a diamond-like array (12). Part of the structure of the positively charged framework is represented in Figure 1, which highlights an adamantane-like unit, a fundamental structural component (see Figure 2) of the diamond net. The structure consists of alternating tetrahedral C and Cu centres, inter-connected by C-C6H4-CN-CU rods 8.856(2) Â in length. The figure gives a visual impression of the large relative size of the intraframework space which accounts for approximately two thirds of the volume of the crystal and which is occupied by large amounts of fluid nitrobenzene (at least 7.7 C 6 H 5 N O 2 per Cu) together with mobile BF4- ions. A crystal two thirds of which is fluid is quite extraordinary. When the acetonitrile ligands of 0 ^ ( Ο ί 3 θ Ν ) 4 Β Ρ 4 are substituted by 1,4dicyanobenzene under conditions identical to those used above for the tetraphenylmethane system, the outcome is very different. The crystals formed contain no solvent and have composition Cu (l,4-dicyanobenzene)2BF4. Diamond like frameworks are indeed formed (Cu-NC-Qify-CN-Cu rods 11.76Â in length) but the structure contains five independent frameworks which interpenetrate as represented in Figures 3a and 3b. A n adamantane unit has three 2-fold axes of symmetry one of which is indicated in Figure 2a; the view looking almost directly down a 2-fold axis is shown in Figure 2b. The framework structure of Cu(l,4dicyanobenzene)2BF4, showing only the metal centres and their connectivity is represented in Figure 3a, as seen from a viewpoint slightly off the 2-fold axis analogous to that in Figure 2b. As can be seen, the adamantane units are distorted. The heavy connections represent one framework. The interpenetrating arrangement is such that the adamantane units of the 2nd, 3rd, 4th and 5th frameworks can be imagined generated from the first by a translation along the 2-fold axis common to all of them by l/5s, 2/5s, 3/5s and 4/5s respectively (see Figure 2a for s). Channels of rhombic cross-section are generated in which the BF4- ions are located as shown in Figure 3b. A study of the way the structure varies, in particular whether or not interpénétration occurs, as counter ion and solvent are varied, would clearly be valuable with these Cu(I) derivatives of 1,4-dicyanobenzene and 4,4',4",4 tetracyanotetraphenylmethane. Zn(CN)2 and Cd(CN)2 each contain two independent diamond-related frameworks with tetrahedral metal centres and M C N M rods, one neatly filling the spaces generated by the other (12). As an exercise in the deliberate manipulation of framework assembly we attempted to devise ways of creating a single, noninterpenetrating, diamond-like structure consisting of tetrahedral metal centres interconnected by linear cyanide bridges. One strategy explored was to attempt the construction of a negatively charged single framework by substituting every other Z n in a Zn(CN)2 diamond net with C u , the required counter cations thus making interpénétration impossible. Models suggested N M e 4 would fit snugly into the adamantane cavities of a single framework and the overall framework charge required that only half the cavities would need to be occupied by cations. Bringing together in aqueous solution the components Z n , C u , CN" and N M e 4 under extremely simple conditions led to the spontaneous assembly of the intended array (12). Iwamoto has shown independently that in the presence of CCI4, Cd(CN)2 can be I

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Figure 1. Framework structure of Cu(4,4^4'\4 -tetracyanotetraphenylmethane)BF4.xC6H5N02 highlighting an adamantane-like unit ,M

Figure 2. (a) An adamantane-like component of the diamond net showing one of the 2-fold axes, (b) View of adamantane unit looking slightly offset from the 2fold axis.


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Figure 3. (a) Arrangement of copper centres and their connectivity in Cu(l,4dicyanobenzene) BF from a viewpoint similar to that in Figure 2b. Heavy connections indicate one particular framework, (b) Perspective view down one of the rhombic channels of Cu(l,4-dicyanobenzene) BF . 2

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induced to crystallise as a single framework with CCI4 molecules in the adamantane cavities (13). A New Prototype Honeycomb Net Involving Square Planar and Tetrahedral Centres in 1:2 Proportions Cd(CN)2 crystallises from aqueous tBuOH as Cd(CN)2.2/3H20.tBuOH with the novel honeycomb stucture shown in Figure 4 (14). Disordered tBuOH occupies the channels. The infinite [Cd(CN)2] framework consists essentially of square planar and tetrahedral centres in 1:2 proportions. Upon exposure to the atmosphere tBuOH and H 2 O are lost and the crystals collapse to a collection of microcrystals of the normal interpenetrating form of Cd(CN)2 as revealed by powder X-ray diffraction. When Cd(CN)2 is crystallised in the presence of hexamethylenetetramine (HMTA) crystals of Cd(CN)2.1/3(HMTA) are obtained with an infinite 3D [Cd(CN)2] framework which at first sight appears very different from that in Cd(CN)2.2/3H20.tBuOH but which in fact is topologically identical (15). The geometrical relationship between the two is shown in Figures 5a and b. The linear hexagonal channels present in Cd(CN)2.2/3H20.tBuOH (Figure 5a) are deformed, the original connectivity remaining unbroken, to produce in Cd(CN)2- 1/3HMTA pronounced zig-zags (Figure 5b) which allows the four Ν donors of each H M T A to become coordinated. This new, simple 3D net, seen in its geometrically most regular form in Cd(CN)2.2/3H20.tBuOH may provide the prototype (analogous to the diamond prototype in the systems above) for a whole family of potentially interesting solids; e.g. it may be possible to construct frameworks in which the planar ribbons of edgesharing square units apparent in Figures 4 and 5a have been replaced by banks of coplanar porphyrins or similar plate-like building blocks. The hexagonal channels, which with such large building blocks would be of correspondingly large dimensions, would then provide access for substrate molecules to the catalytic metalporphyrin sites lining the channels.


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Square Grid 2D Sheets and their Interpénétration to form a Novel 3D Network Mixtures of Cd(PF6)2 and 4,4'-bipyridine (bipy) in 1:4 ethanol - water yield an initial product in the form of thin plates which is then gradually replaced by a second product consisting of more chunky crystals. The structure of the second type of crystal is presently under investigation. The initial crystals of formulation [Cd(H20)2(bipy)2](PF6)2.2bipy.4H20 consist of stacks of parallel infinite square grid cationic sheets of composition [Cd(H20)2(bipy)2]n with the structure shown in Figure 6. Each Cd is essentially octahedral with two trans water ligands and four bipy Ν donors. Uncoordinated bipy molecules project through the square holes in the sheets. Only minor modification of the reaction conditions, namely, increasing the E t O H : H 0 ratio to 5:2, yields crystals of [Cd(H 0)(OH)(bipy)2]PF6. We have not yet completed a full structural analysis of this material but it has the same space group (P4/ncc) and almost the same unit cell dimensions (a,10.992(5); c,17.608(8) A) as those of the compound [Cd(H20)2(bipy)2]SiF6 (P4/ncc; a,l 1.016(2); c,17.586(3) Â) for which a full analysis has been carried out. The latter compound is obtained from aqueous methanolic solutions containing Cd(C104)2, Na2SiF6 and bipy and it is isostructural with the corresponding Zn (16) and Cu derivatives, consisting of two perpendicular and equivalent stacks of square grid [Cd(H20)2(bipy)2] sheets which interpenetrate to give the remarkable 3D arrangement shown in Figure 7. Any particular sheet has an infinite number of perpendicular ones enmeshed in it. On the evidence above it is almost certain that 2n+

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Figure 4. Honeycomb framework in Cd(CN)2.2/3H20.tBuOH. The centres represented are all Cd's and connections are all of the type CdCNCd. C and Ν atoms are not shown, nor are the water molecules coordinated above and below the apparently square planar Cd's. Reproduced with permisson from ref. 14. Copyright 1990 Royal Society of Chemistry.



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b Figure 5. (a) Side-view of one of the hexagonal channels of Cd(CN) .2/3H 0.tBuOH. Heavy connections represent the cyanide linkages between pairs of Cd atoms. Lighter lines indicate only geometrical relationships. The smaller circles represent oxygen atoms of coordinated water, (b) Side-view of zig-zag hexagonal channel in Cd(CN)2.1/3(C H N ) analogous to the linear hexagonal channel in Cd(CN) .2/3H 0.tBuOH shown in (a). Heavy and light lines have the same significance as in (a). Reproduced from ref. 15. Copyright 1991 American Chemical Society. 2

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Figure 6. Structure of [Cd(H20)2(bipy)2](PF6)2.2bipy.4H20 where bipy = 4,4'-bipyridine. Heavy lines represent the Cd.bipy.Cd rod-like components of the square grid sheets.



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Figure 7. Framework structure consisting of interpenetrating square grid sheets common to [Cd(H20)(OH)(bipy)2]PF6 and[M(H20)2(bipy)2]SiF6 ( M = Zn, Cu, Cd). Only metal centres and bipy connections are shown. Reproduced with permisson from ref. 16. Copyright 1990 Royal Society of Chemistry.


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[Cd(H20)(OH)(bipy)2]PF6 has a nearly identical interpenetrating sheet structure in which one of the coordinated water molecules has lost a proton. These Cd-bipy compounds, then, provide the unusual situation where almost identical square grid sheets in one case are stacked one on top of the other to give a non-interpenetrating structure and in another case interpenetrate to yield an unprecedented type of concatenated 3D network. 4,4'-Bipyridine coordinated through both nitrogens to metal cations may have an electronic configuration resembling that in the methyl viologen dication (the l,r-dimethyl-4,4'-bipyridinium or paraquat dication) a species undergoing ready reduction to the stable mono-cation-radical and consequently much used in electron transfer studies. Moreover, bipy is one of a number of bridging species incorporated into mixed valence RuH/Ru * complexes of the famous Creutz-Taube type which mediate in facile electron transfer (17). Infinité framework solids constructed from mixed valence metal centres inter-connected by bipy may therefore show unusual electrical properties.


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Rutile-Related Frameworks The rutile prototype consists of trigonal and octahedral centres in 2:1 proportions. One of the simplest imaginable potential trigonal connectors is the tricyanomethanide ion, C(CN)3-. X-ray structural analyses of Zn[C(CN)3]2 (18) and Cu[C(CN)3h reveal that both consist of two independent rutile-related frameworks which interpenetrate in the manner shown, for the Zn compound, in Figure 8. Cell dimensions and space group determination indicate that the corresponding Cd , N i , Co and M n compounds have the same structure. Tetragonal elongation of two CuN bonds is observed in the Cu compound but otherwise the structure is very similar to that of the Zn compound. As can be seen by inspection of Figure 8, each framework in Zn[C(CN)3]2 contains 4-membered"[Zn2C(3)2] and "6-membered" [Zn3C(3)3] rings. The nature of the interpénétration is such that every 6-membered ring of one framework has a ZnNCC rod of the other framework passing through it, but projection of rods through 4-membered rings does not occur. Parts of the independent frameworks are forced by the interpénétration into unusually close contact eg. C(1)...C(1) = 3.135(4) and 3.138(4) Â; C(2)...N(1) = 3.193(3) A . This highlights the general point made above that interpénétration may lead to enforced proximity and thence to unusual properties. By linking together C(CN)3" units with cations having preferences for geometries other than octahedral, it should be possible to generate a number of new geometrical and topological types of 3D structure; the highly unusual interpenetrating sheet structure in Ag[C(CN)3] (8) in which A g acts as a 3-connecting centre illustrates this potential for the generation of new structural types. Trigonal building blocks larger and more elaborate than C(CN)3 can readily be envisaged and may provide whole families of new materials. Wells (19) has surveyed 3-connected and (3,n)-connected nets, many of which remain purely hypothetical. These trigonal building blocks may yield frameworks providing real examples of hitherto hypothetical nets. 11

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A New Type of 3D Net Involving 4-Connected Porphyrin Building Blocks Solutions of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine palladium (Pd.py.porph) with excess Cd(N03)2 in water-methanol-ethanol gave crystals of (Pd.py.porph).2Cd(N03)2.hydrate whose structure (20) is represented in Figures 9a and b. A l l porphyrin units are equivalent and are attached via their pyridyl nitrogens to four Cd's (Figure 9a). Two diametrically opposed Cd(2)'s are coordinated by two



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Crystal Engineering of Novel Materiah

Figure 8. Structure of Zn[C(CN)3]2. The left half of the figure represents the actual unit cell. One framework is omitted from the right half so that the relationship to rutile of the framework extending into both halves is easily recognised. Reproduced with permisson from ref. 18. Copyright 1991 Royal Society of Chemistry.


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Figure 9. (a) Environment of the porphyrin unit in (Pd.py.porph).2Cd(N0 ) .hydrate. (b) Extended 3D framework in (Pd.py.porph).2Cd(N0 ) .hydrate. Only the cadmiums and the porphyrin meso(5,10,15,20) carbons are indicated, the latter occupying the corners of the squares seen here obliquely. Reproduced from ref. 20. Copyright 1991 American Chemical Society. 3

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trans pyridines, two trans F^O's and two trans monodentate N03~'s. With regard to the framework structure these Cd(2)'s can be regarded as "linear"connectors. The two Cd(l)'s associated with a particular porphyrin are "bent" connectors because the two coordinated pyridines in these cases are cis. Linear polymeric strips consisting of Pd.py.porph units connected together by linear Cd(2)'s can be seen in Figure 9b, half of them being parallel with each other, the others being parallel to a line at 59° to the first set. Attached to every linear strip via its pendent bent Cd(l)'s is an infinite number of other strips all at 59° to the first; half of these pass over and half pass under the first strip. At these points of overlap essentially parallel Pd.py.porph units make face to face contact (Pd...Pd, 4.68Â) the PdPd vector making an angle of 30° with the normal to the porphyrin planes; in this way stacks of porphyrins are generated with all Pd's co-linear. To the best of our knowledge this is a new geometrical and new topological type of infinite 3D network. On the basis of this preliminary study the prospects look good for constructing many new 2D and 3D nets using porphyrin and phthalocyanin building blocks. Pt S-Related Frameworks The PtS prototype consists of equal numbers of inter-connected square planar and tetrahedral centres (Figures 10a and b). In an experiment similar to the generation of the diamond-related [CuZn(CN>4][NMe4] described above, an attempt was made to link together stable, square planar Pt(CN)4 ~ building blocks via tetrahedral Cul centres. Bringing together C u , Pt(CN)4 ~ and N M e 4 led to the assembly of the [CuPt(CN)4] " framework as the NMe4 derivative which did have the PtS-related structure (21) (Figure 11). Three sorts of channels can be seen. Hexagonal channels of large dimensions run in both the a and b directions, the largest Pt...Pt separation across the channel being 13.50 Â. Also running parallel to a and b are smaller channels of roughly square cross-section. The cations which are disordered over four equivalent orientations are located in the hexagonal channels. A second type of square channel (edge 7.61 Â) runs parallel to c; in crystals sealed together with the aqueous mother liquor in Lindemann tubes these channels are occupied by very disordered water molecules. The framework in this case, in contrast to that in Cd(CN)2-2/3H20.tBuOH, survives loss of solvent, the channels parallel to c being vacant after exposure of the crystals to the atmosphere. The PtS prototype provides a very attractive model for the construction of permeable solids with catalytic potential. If one imagines replacing the planar PtS4 units apparent in Figures 10a and b with flat, rigid, 4-connecting building blocks larger than the Pt(CN)4 " used above, such as porphyrins and phthalocyanins, the potential offered by this net for structures with large channels giving access to banks of catalytic sites can be appreciated. 2

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Conclusions and Future Prospects The exploratory work reported here provides considerable encouragement that the idea of using simple 3D nets as prototypes for the construction of new infinite frameworks is not only feasible but potentially extremely fruitful. Deliberate crystal engineering along these lines to produce 2D and 3D frameworks with planned structural features begins to appear a realistic objective. This approach has already uncovered new examples of the little studied phenomenon of interpénétration and the discovery of further types can be anticipated; increasing knowledge in this area may allow, in future materials, the juxtapositioning of structural components for specific applications eg. in the area of electrical properties. The use of appropriately functionalised, large, rigid building blocks such as porphyrins and phthalocyanins


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Figure 10a. axis.

Perspective view of the PtS structure down the tetragonal




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Figure 10b. Perspective view of the PtS structure perpendicular to the tetragonal axis.


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Figure 11. Tetragonal unit cell of [NMe4][CuPt(CN)4]. Reproduced with permisson from ref. 21. Copyright 1990 Royal Society of Chemistry.


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may afford materials with correspondingly large channels and cavities with potentially useful applications in areas such as catalysis. Acknowledgement

We are very grateful for support from the Selby Scientific Foundation. Literature Cited


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3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17 18. 19. 20. 21.

Niggli, P. Z. Krist., 1922, 57, 253. Zhdanov, H.S. C.R.Acad. Sci., 1941, 31, 352. Shugam, E.; Zhdanov, H.S. Acta Physiochim. URSS, 1945, 20, 247. Kamb, B.; Davis, B.L. Proc. Natl. Sci. U. S. Α., 1964, 52, 1433. Kamb, B. Science (Washington. D.C.), 1965, 150, 205. Kamb, B. J. Chem. Phys., 1965, 43, 3917. Brown, A.J.; Whalley, E. J. Chem.Phys.,1966, 45, 4360. Ermer, O.; Eling, A. Angew. Chem.(Int. Edn.), 1988, 27, 829. Ermer, O.; Lindenberg, L. Helv. Chim. Acta, 1988, 71, 1084. Ermer, O. J. Amer. Chem. Soc., 1988, 110, 3747. Kinoshita, Y.; Matsubara,I.;Higuchi, T.; Saito, Y. Bull. Chem. Soc. Japan. 1959c, 32, 1221. Cannillo, E.; Mazzi, F.; Rossi, G. Acta Cryst., 1966, 21, 200. Powell, H.M. J. Chem. Soc., 1950, 300. Konnert, J.; Britton, D. Inorg. Chem., 1966, 5, 1193. Duchamp, D.J.;Marsh, R.E. Acta Cryst., 1969, B25, 5. Herbstein, F.H.; Kapon, M.; Reisner, G.M. Proc. Roy. Soc., London, Ser. A, 1981, 376, 301. Davis, J.E.D.; Finocchiaro, P.; Herbstein, F.H. in "Inclusion Compounds": Atwood, J.L.; Davis, J.E.D.; MacNicol, D.D., Eds.; Academic Press : New York, 1984; Chapter 11, P. 407. Thurn, H.; Krebs, H. ActaCryst.1969, B25, 125. Hoskins, B.F.; Robson, R. J. Amer. Chem. Soc. 1990, 112, 1546. Kitazawa, T.; Nishikiori, R.; Kuroda, R.; Iwamoto, T. Chem. Lett., 1988, 1729. Abrahams, B.F.; Hoskins, B.F.; Robson, R. J. Chem. Soc. Chem. Commun., 1990, 60. Abrahams, B.F.; Hoskins, B.F.; Liu, J.; Robson, R. J. Amer. Chem. Soc., 1991, 113, 3045. Gable, R.W.; Hoskins, B.F.; Robson, R. J. Chem. Soc., Chem. Commun., 1990, 1677. Creutz, C. Progress in Inorg. Chem., 1983, Vol. 30, p.1. Batten, S.R.; Hoskins, B.F.; Robson, R. J. Chem. Soc., Chem. Commun., 1991, in press. Wells, A.F. "Three-Dimensional Nets and Polyhedra", (Wiley, 1977), Chs. 58. Abrahams, B.F.; Hoskins, B.F.; Robson, R. J. Amer. Chem. Soc., 1991, 113, 3606. Gable, R.W.; Hoskins, B.F.; Robson, R. J. Chem. Soc., Chem. Commun., 1990, 762.

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