Tunnel inclusion complexes formed by hosts of lesser versatility

Abstract

Crystalline tunnel inclusion complexes have one-dimensional tunnels in their structures. The host molecules, which constitute the matrix, may be bonded together by directional bonds or van der Waals forces. The guest molecules in the tunnels generally interact with the host molecules by van der Waals forces, although some examples of host-guest hydrogen bonding are known. The nature of the guest appears to be limited only by the size and shape of the tunnel available, although there are some examples of specific host-guest interactions. The arrangements of the host molecules in the matrix are generally different from those in the neat hosts, and in this situation, the tunnel inclusion complexes are separate phases. The Bishop-Dance complexes, which are primary solid solution of guest in host, are exceptions. Most of the examples quoted show the effects of interaction between host and guest, which manifest themselves as minor changes in cell dimensions of isomorphous crystals or as distortions of a basic crystal structure type. Guest molecules are generally disordered in the tunnels at room temperature but order on cooling, often in a number of stages accompanied by interaction with and distortion of the framework, usually manifested as phase transformations. Diffraction patterns from many complexes are composed of separate contributions from the host framework and the guest arrangement, both being modified by mutual interaction of host and guest. The complicated effects that can ensue are illustrated.