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

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