Second-sphere coordination guests

First examples of second-sphere coordination adduct between a CD and such organometallic complexes were observed with the ferrocene and its derivatives. Thus, Breslow was the first to report in 1975 that ferrocene forms a 1 1 adduct with the /3-CD in both A/T-dimethylformamide and dimethyl sulfoxide. Nevertheless, the first crystalline inclusion complexes of ferrocene and its derivatives were prepared by Harada and Takahashi in 1984. The inclusion complexes were obtained by direct addition of crystals of ferrocene or its derivatives into aqueous solutions of CD. The stoichiometry of the inclusion complexes was found to be dependent on the sizes of the CD. The /3-CD and y-CD formed 1 1 stoichiometric inclusion complexes whereas a-CD formed a 2 1 (CD guest) complex with ferrocene. From induced circular dichroism experiments, three structures were proposed for these adducts as shown in Figure... 

Cameron, B.R. Corrent, S.S. Loeb, S.J. Transition metal complexes as both host and guest Second-sphere coordination between a Pt-azaerown ether host and a Pt-NH3 guest. Angew. Chem.. Int. Ed. Engl. 1995.34, 2689-2691. 

Figure 3 Ligands 1-9 used to form metal complex receptors that bind guests through the first- and second-sphere coordination.

These effects are well evidenced with the family of caUx[6] complexes (Figure 7). The first coordination sphere is defined by the capping ligand, the second sphere by the crown of phenolic oxygen atoms present at the small rim, and the third by the cavity, which involves CH- 7r interactions with guests and shape selectivity. 

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