Guest metal complexes

In this chapter we discuss host-guest complexes of arenediazonium salts with crown ethers and related compounds. Transition metal complexes of arenediazonium ions are treated together with those of dinitrogen and of diazoalkanes in our second book (Zollinger, 1995, Sec. 10.1). 

A number of capped cyclodextrins which are able to bind a metal ion at one end of their cavity, together with an organic guest in the cavity, have been synthesized. Such species parallel in several respects the family of completely synthetic vaulted transition-metal complexes prepared by Busch and coworkers and already discussed in Chapter 3 (section 3.5). 

In the field of porous supramolecular metal complexes, both molecular and extended-solid materials have been extensively studied in recent years. A particularly well-studied class of compounds is the metal-containing molecular squares, that is, square-shaped porous tetrameric structures (30,108). These have been prepared by several approaches, the most common being the reaction of an organic bridging ligand with a metal complex that has available cis-coordination sites (109-113) (Fig. 13). However, the resulting metal centers are usually coordinatively saturated, which makes it difficult for guest molecules to interact directly with the metal atoms. 

The only example of P.E. spectra of a mixed arene-carbonyl metal complex is that of Cr(bz)(CO)3 reported by Guest et al. (76). In the latter compound, the two d-bands observed in the spectrum of Cr(bz)2 merge into a single band, with maximum at 7.42 eV, containing the ionizations of all six -electrons. A second band, at 10.70 eV, is related to a benzene orbital, while a third band, at 12.70 eV, has both carbonyl and benzene character. The assignment is supported by theoretical calculations and by comparison between He(I) and He(II) spectra (76). 

FABMS has been used as a semiquantitative indication of the selectivity of receptors for particular guest metal cations (Johnstone and Rose, 1983). The FABMS competition experiment on [7] with equimolar amounts of the nitrates of sodium, potassium, rubidium and caesium gave gas-phase complex ions of ([7] + K)+ ion (m/z 809) and a minor peak ([7] + Rb)+ ion (m/z 855) exclusively. The relative peak intensities therefore suggested a selectivity order of K+ Rb+ Na+, Cs+, indicative of the bis-crown effect, the ability of bis-crown ether ligands to complex a metal cation of size larger than the cavity of a single crown ether unit, forming a sandwich structure. 

A study similar to that of Hersey and Robinson has been reported by Seiyama and coworkers.From a stopped-flow, kinetic study of the interaction of various azo dyes and some azo dye-metal complexes with alpha cyclodextrin, they observed two kinetic processes. The dependence of the observed rate-constants for these two processes on the alpha cyclodextrin concentration was found to be explainable in terms of a mechanism identical to that proposed earlier by Hersey and Robinson. In the case of the guests used by Seiyama and coworkers, however, values for the rate constants of the binding step could be determined from the concentration dependence of kobs for the faster process thus,... 

Based on host-guest interaction, microporous zeolites have been used as heterogeneous host for encapsulation of metal complexes and organometallic fragments. For zeolite-encapsulated photosensitizer, the steric and electrostatic constraint imposed on the complexes within the channels or cages of zeolites can alter the photochemical and photophysical properties of the guest complexes and diminish the photodegradation and undesirable electron transfer reactions [6]. But, the pore sizes (-13 A) of microporous zeolites are too small for... 

Gyclodextrin cavities form the early models of host molecules involved in supramolecular assemblies. There are many other molecules known as cryptands which can be designed to offer a cavity of fairly precise dimensions to accommodate various ions or metal complexes. It may be possible to locate not just one, but two, guest molecules inside a cryptand cavity, and this may lead to new electron transfer reactions in restricted environments another step towards synthetic photoinduced biochemical reactions. 

---