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

158 Y. Aoyama
5
Concluding Remarks and Future Prospects
5.1
Manipulation of Pores
Zeolites, in a sense, are 3D-networked covalent polymers. The attempts to mimic
their structural integrity and unique properties led to a number of important
discoveries which are summarised as follows. (1) Significant permanent voids
can be sustained by using multiply-bonded coordination and organic networks.
Rigid pores may be used directly as low-dielectric materials and also as the sites
of selective guest binding. Functionalised pores may also be an important future
concern. The construction of robust networks has something to do with the
essence of crystal engineering. It provides a powerful strategy for precisely designing
ordered materials on the basis of predictable intermolecular interactions
which compete favourably with unpredictable van der Waals packing
forces. (2) Simple hydrogen-bonded networks are not so robust as to maintain
guest-free cavities. They are, however, remarkably flexible, dynamic and adjustable.
Guest molecules readily diffuse in to open the channels. The high cooperativity
of organic network materials may find their unique applications
as on-off switchable devices. (3) For the catalysis to occur, porous materials
should have vacant coordination sites. Such sites also bring about selectivities
in the guest binding. Simple hydrogen-bonded systems have this capacity, i.e.
capability of forming additional hydrogen bonds. Vacant metal coordination
sites can be generated by the loss of labile ligands from preformed saturated
adducts [58, 64] or by immobilising soluble metal complexes with a preformed
hydrogen-bonded network [80]. In this way, the catalytic zeolite analogues are
becoming a reality. Two areas of particular interest are shown below. It should
also be noted that the crystal integrity becomes less pronounced upon introduction
of vacant coordination sites.
5.2
Solid Catalysts in Organic Transformations
The catalytic activity of the immobilized Ti complex suggests a bright future in
the use of solid metal-organic catalysts in fine organic synthesis, where soluble
metal complexes and organometallic derivatives have been extensively used. In
principle, there can be various combinations of organic networks as microporous
polymeric ligands and metal complexes as catalytic sites. An especially
interesting area is the use of chiral networks. In view of the importance of asymmetric
transformations, the abundance of chiral soluble catalysts, and the nonexistence
of chiral zeolites, catalytic zeolite analogues may claim their maximal
significance in the chirality control.
Catalysis by metal-organic solids may also be applied to redox reactions. An
especially intriguing target would be manipulation of hydrocarbon transformations.
Many metal-organic networks so far reported in fact contain redox-active
transition metals (Cu II ,Pd II ,Co II and so on). Metalloporphyrins are potential