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

Functional Organic Zeolite Analogues 147
2.5
Modes of Guest Binding
The percent volume occupied by host or guest provides a measure of the porosity
of a network. In this respect, many of the networks such as 11 (Fig. 1), 23
(Fig. 7), 30 (Fig. 9), 38 (Fig. 10), 42 (Fig. 11), and 45 (Fig. 12) are highly porous,
i.e. ≥ 50% of the total volume of the adduct is occupied by the guest or only
≥ 50% of that is occupied by the host framework. Some of the guest-binding
channels have a cross-section as wide as 10 Å. For comparison, typical pore sizes
of zeolites (molecular sieves 4 A, 5 A, and 13X) fall in the range 4–13 Å and
void volumes are generally < 50% [63].
For the stoichiometry, selectivity, mobility and catalysis it is of crucial importance
if there is host-guest interaction. In many cases of multiple hydrogen
bonds such as those between carboxyl groups (1 and 2 in Fig 1), pyridones (3 in
Fig. 1),or 2,4-diaminotriazines (38 in Fig. 10),all or most of the hydrogen-bonding
capacity is consumed to build a robust network. In most metal-organic
solids whose crystal structures have been determined, the metal centres are
coordinatively saturated [58, 64]; there is no guest-metal interaction expected
unless guest/ligand exchange occurs. In the absence of strong host-guest (metalguest)
interaction, guest (solvent) molecules included in a large cavity or channel
are, not surprisingly, highly disordered. Thus, it is remarkable that two-thirds by
volume of what is definitely a crystal (adduct 11 ◊ x(C 6H 5NO 2) (x ≥ 7.7) is essentially
a liquid [35]. It is also likely that the host-guest stoichiometry is primarily
controlled on a size basis. Host 35 (Fig. 10), for example, forms such adducts as
35 ◊ 5(dioxane), 35 ◊ 10(CH 3CN), 35 ◊ 10(HCO 2H) and 35 ◊ 21(H 2O) [54].
The simple hydrogen bond between OH groups is coordinatively unsaturated.
Two or one such free protons (O-H◊◊◊O-H) capable of hydrogen-bonding
fixation of a polar guest are available for the diresorcinol or monoresorcinol
host 29 or 32 (Fig. 9), respectively. This gives a 1:2 29:guest or 1:1 32:guest
stoichiometry for various polar guests having different sizes [52, 53]. In fact,
when crystallized from ethyl acetete, host 29 only affords a 1:2 adduct. On the
other hand, when crystallized from a mixture of ethyl acetate and benzene, the
host gives rise to a ternary adduct 29 ◊ 2(ester) ◊ 2(benzene) [50]. Thus the
stoichiometry of the polar host-guest interaction is very strict and the void
space still available is better filled with a hydrocarbon guest. The formation of
such a ternary adduct containing polar and nonpolar guests simultaneously
bound in a cavity provides a basis of its application as a solid catalyst to the
bimolecular Diels-Alder reactions [65].
An analogous diresorcinol derivative of Zn II -porphyrin (39) generates a similar
2D net 40 (Fig. 11), having cavities much wider than those (30 in Fig. 9) of the
anthracene host 29. The porphyrin host forms adduct 39 ◊ 4THF [55]. Two molecules
of the guest are hydrogen-bonded to the host as above,while the remaining
two are coordinated to the central Zn II ions. Cooperation of coordination and
hydrogen bonding would be a potential tool for assembling different types of
guests in a cavity.
The host-guest hydrogen bonding can also be applied to achieve a particular
alignment of host and guest components. Quinones as strong electron-acceptors