Inclusion complexes formed by versatile hosts

Abstract

Versatility in a host can be achieved in different ways, all leading to the capability of a single host species to form a variety of inclusion complexes. A small number of examples has been chosen here. Tri-o-thymotide (TOT) interacts with its neighbours (hosts and guests) only via van der Waals forces, forming clathrates with smaller guests and tunnel inclusion complexes of a number of types with larger or elongated guests; some 70 complexes of various types have been recorded. Hydrogen-bonded trimesic acid (TMA) gives tunnel inclusion complexes, with guests ranging from long-chain hydrocarbons to polyiodides, interstitial complexes with halogens and various small molecules as guests; it also forms hydrogen-bonded compounds with suitable acceptors and salts with components having some basic functionality. The Heilbron host E,E-1-[p-dimethylaminophenyl]-5-[o-hydroxyphenyl]-penta-1,4-dien-3-one has two possible conformations and can act both as hydrogen-bond donor and acceptor. These potentialities are all exploited in the different complexes studied, which have CHCl3, m-dinitrobenzene, and p-dimethylaminobenzaldehyde as second component. Over 100 complexes of racemic gossypol have been prepared and most can be grouped into eight different types of hydrogen-bonded arrangement, with more undoubtedly awaiting discovery. The tripod molecule tris(5-acetyl-3-thienyl)methane (TATM) is a rare example of a flexile molecule which forms host-guest inclusion complexes with a large variety of guests. Some forty types of guest have been reported to form inclusion complexes. Five types of crystal structure, with nine different guests, have been reported. Analysis of this substantial but incomplete data base shows that each group of crystallographically isomorphous structures contains a particular TATM conformer with characteristic torsion angles. Over 200 complexes of a wide variety of guests have been prepared with unsubstituted tetraphenylmetalloporphyrins (containing metals, e.g., Zn, Mn, Fe, 2H) as hosts. Many structural resemblances suggest that there is a common interaction in all these complexes, possibly based on charge transfer guest-core and guest-metal interactions.