198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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Functional Organic Zeolite Analogues 151 The hydrogen bond is generally much weaker than the metal-ligand interaction. The structural integrity of multiply hydrogen bonded 3D nets may be shown by host (tecton) 38 (Fig. 10) [54]. Up to 63% of the guest can be removed from adduct 38 ◊ 5(dioxane). The resulting material remains optically transparent and undergoes a small but systematic contraction in the cell parameters, indicating that the network remains essentially the same after most of the guest having escaped the cavity. The remarkable robustness of the present network is due to the huge number (16) of hydrogen bonds which participate to maintain a single cavity. This number is equivalent to the number of hydrogen bonds supplied by a host; 16, 8, 4, and 2 for tetra(diaminotriazine) 38, tetracarboxylic acids 7 and 8 (Fig. 1), tetraol 29, and diol 32 (Fig. 9), respectively. In this respect, it may be surprising that a bicyclic but simple diol 47 (Fig. 13) affords a porous guest-free host with empty canals [61]. The unique spiral spine motif 48 may play an important role. This is, however, not the sole origin of the robustness of the helical network, since a similar network but with a larger internal void (51) provided by an analogous host 50 collapses into a layer structure in the absence of guest. It is suggested that the methyl groups and the methylene moieties pointing into the canal sterically interfere with each other (as shown in 49) to protect the structure against collapse in a similar manner as the stone blocks support an arched bridge. When immersed in a guest liquid or kept in contact with a guest vapour, the guest-free apohost readily readsorbs the guest. Guest exchange also occurs. The exchange of nonfunctionalised aliphatic and aromatic molecules makes no change greater than 0.4 Å for any axis in the original cell parameters for adduct 15 ◊ AgOTF ◊ 2(benzene) (Fig. 3) [41]. When single crystals of adduct 43 ◊ n(HCO 2H) ◊ m(dioxane) (cf. Fig. 12) are suspended in dioxane or acetonitrile, complete guest exchange occurs rapidly to produce single crystals of 43◊ 5(dioxane) or 43◊ 10(CH 3CN) [54]. In a similar manner, single crystals of the dioxane adduct, when suspended in water, give rise to single crystals of water adduct 43◊ 21(H 2O). In all the cases, the network remains the same and the unit cell parameters show reasonably small and systematic changes. For metal coordination networks, exchange of counter anions occur readily [35, 73, 74]. Although the detailed mechanism of guest exchange is not clear, especially with respect to the timing of sorption/desorption and the modes of lattice diffusion, there is little doubt that the exchange is indeed a solid-state intracavity event [43, 52, 54, 58]. Recrystallization mechanisms are ruled out, since the hosts are essentially insoluble in the media used, the morphology of the crystals is essentially unaffected, less than one-hundredth at best of the total volume of the crystal, and the guest exchange is much faster (minutes) than the recrystallization which may take days. 3.2 Flexible Networks and Induced-Fit Adjustment Simple hydrogen-bonded networks, especially lower-dimensional ones, do not seem to be robust enough to sustain guest-free cavities. Volatile guests such as ethyl acetate can be readily removed from adduct 29◊2(guest) (cf. Fig. 9) to give
152 Y. Aoyama Fig. 16. Binding isotherm (guest/host ratios plotted against pressure of the guest) for gaseous ethyl acetate with host 29 at 25 °C. The pressure is increased (Æ) from 0 up to the saturation vapour pressure of the guest (P s) for the sorption (open circles) and then decreased (¨) to 0 for the desorption (filled circles) polycrystalline guest-free apohost [52]. There is at least no evidence in support of the presence of a significant void there; the cavity must have collapsed somehow, due to a conformational (anthracene-resorcinol dihedral angle) change in the molecule, a closer approach of the sheets, and possibly partial breakdown of the hydrogen bonds. Nevertheless, apohost 29 in the solid state readily binds various guests including hydrocarbons and haloalkanes (-arenes) not only as liquids and gases but also as solids in some cases. The complexable solid guests include benzophenone, benzoquinone and cyclohexanediol [52]. The solid-liquid complexation occurs instantaneously, while showing no apparent change in the size and shape of the host. There are two remarkable aspects in the present solid-state complexation. One is stoichiometry which is strictly the same as that in recrystallization, i.e. 1:2 (host to guest) in most cases. This is also true for the solid-solid complexation. If a 1:3 (host to guest) mixture is used, for example, the 1:2 adduct results and one equivalent of excess guest remains simply as such. The other is guest-induced structure change; the hostguest adducts resulting from solid-liquid, solid-gas or solid-solid complexations exhibit almost the same X-ray powder diffractions (XRPD) as those of their single-crystalline specimen obtained by recrystallization. The binding isotherm for gaseous ethyl acetate with apohost 29 is shown in Fig. 16, where guest/host molar ratios are plotted against pressure of the guest in the vapour phase up to its saturation vapour pressure at 25 0 °C. The sorption curve is remarkably sigmoidal [75]. In conjunction with XRPD evidence above, this indicates that the transition from the tense guest-free apohost with collapsed cavities to the relaxed host-guest complex with a single-crystal structure occurs in a highly cooperative or allosteric manner (Eq. 1).
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198 Topics in Current Chemistry Edi
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Design of Organic Solids Volume Edi
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Volume Editors Prof. Dr. Edwin Webe
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VIII preface tion at an amazing lev
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Contents of Volume 196 Carbon Rich
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2 J.P. Glusker 5 Interactions of Pl
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4 J.P. Glusker Perturbations to the
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6 J.P. Glusker 2.1 Sources of Cryst
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8 J.P. Glusker other induced, will
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10 J.P. Glusker The forces here hav
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12 J.P. Glusker lographic structure
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14 J.P. Glusker a c Fig. 6 a - c. D
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16 J.P. Glusker hydrogen bond H◊
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18 J.P. Glusker Fig. 9. Citric acid
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20 J.P. Glusker peptide plane and a
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22 J.P. Glusker 4.5 F◊◊◊H Int
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24 J.P. Glusker rine-containing com
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26 J.P. Glusker Fig. 17. The packin
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28 J.P. Glusker bind to a metal ion
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30 J.P. Glusker As for carboxylate
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32 J.P. Glusker Fig. 24. The magnes
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34 J.P. Glusker structures of magne
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36 J.P. Glusker b a Fig. 28 a, b. A
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38 J.P. Glusker Fig. 30. Complexati
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40 J.P. Glusker 7.1 Descriptions of
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42 J.P. Glusker Fig. 34. Glycine cr
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44 J.P. Glusker bonding capabilitie
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46 J.P. Glusker 2. Non-intercalativ
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48 J.P. Glusker d Fig. 37 d. The ch
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50 J.P. Glusker Fig. 39. Hydrogen-b
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52 J.P. Glusker 72, 104], but other
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54 J.P. Glusker 23. Cody V, Murray-
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56 J.P. Glusker: Directional Aspect
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58 A. Nangia · G.R. Desiraju 7 Sup
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60 A. Nangia · G.R. Desiraju elect
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62 A. Nangia · G.R. Desiraju 14 15
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64 A. Nangia · G.R. Desiraju [23]
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66 A. Nangia · G.R. Desiraju struc
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68 A. Nangia · G.R. Desiraju Fig.
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70 A. Nangia · G.R. Desiraju -NH 2
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74 A. Nangia · G.R. Desiraju and r
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76 A. Nangia · G.R. Desiraju bonds
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78 A. Nangia · G.R. Desiraju 2.86-
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80 A. Nangia · G.R. Desiraju from
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82 A. Nangia · G.R. Desiraju ted a
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84 A. Nangia · G.R. Desiraju envir
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86 A. Nangia · G.R. Desiraju PYRAZ
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88 A. Nangia · G.R. Desiraju napht
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90 A. Nangia · G.R. Desiraju b c F
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92 A. Nangia · G.R. Desiraju all a
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94 A. Nangia · G.R. Desiraju 42. J
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Hydrogen-Bonded Ribbons, Tapes and
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Page 165 and 166: 158 Y. Aoyama 5 Concluding Remarks
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Crystalline Polymorphism of Organic
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Author Index Volumes 151-198 Author
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Author Index Volumes 151 - 198 2 1
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Author Index Volumes 151 - 198 213
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Author Index Volumes 151 - 198 2 ]
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Springer and the environment At Spr
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