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

154 Y. Aoyama
The desorption of the included guest occurs readily and there is no significant
hysteresis. In marked contrast to the case of hydrogen-bonded host 29 (Fig. 16),
there is no significant structural/conformational change in the host network
upon sorption/desorption of the guest. This is taken as another sort of evidence
for the robustness of metal-coordination networks.
Water is a potential guest of zeolites (molecular sieves). Many hosts [77],
including those described above, bind water as a guest. Apohost 29 (Fig. 9), for
example, incorporates ~16 molecules of water. When immersed in an aqueous
solution of such guests as ethanol, alkyl acetate and cyclohexanol (-diol), the
host preferentially picks up two molecules of the guest together with 3–5 molecules
of water [78]. Thermogravimetry for adduct 29 ◊ 2(CH 3CH 2OH) ◊ 5(H 2O)
indicates that water escapes the cavity even at room temperature and completely
at < 50 °C, while ethanol is desorbed only at > 100 °C. These results, coupled
with IR evidence, suggest that it is the guest (ethanol) molecules that are hydrogen-bonded
to the host and vapour-like water molecules fill the void space left.
On the other hand, treatment of the apohost with an aqueous salt (e.g. NaCl)
solution results in exclusive incorporation of the water.
4
Catalysis
4.1
Criteria of Zeolitic Catalysis
The great advantage of solid catalysts is that they are readily recovered and, in
principle, used repeatedly. For the catalysis to be zeolitic, there seem to be a couple
of criteria. (1) The catalysis should not be a simple surface phenomenon; in
other words, internal cavities should make an essential contribution to the catalysis.
(2) The catalysis should be specific in some way to the solid state of the
catalyst; at least the heterogeneous catalysis should be superior to the corresponding
homogeneous one, when available, in terms of either rate or selectivity.
The facile guest exchange and the availability of protic (in hydrogen-bonded
networks) or metal sites (in coordination networks) suggest their potential
application as zeolite-like acid catalysts. However, there is only scattered information
at best [44]. An apparent general problem is that the stable singlecrystalline
materials suited for structural studies do not usually have vacant
coordination sites required for the catalysis.
4.2
Catalysis by a Hydrogen-Bonded Organic Solid
A prototype of catalysis that meets the above criteria is that of host 29 (Fig. 9) in
the Diels-Alder reaction between acrolein and 1,3-cyclohexadiene (Eq. 2) [65].
68 endo 68 exo
(2)