Lewis acid hosts

Hawthorne and co-workers have also produced a series of macrocyclic Lewis acid hosts called mercuracarborands (156, 157, and 158) (Fig. 84) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. They were synthesized by a kinetic halide ion template effect that afforded tetrameric cycles or cyclic trimers in the presence or absence of halide ion templates, respectively.163 These complexes, which can bind a variety of electron-rich guests, are ideal for catalytic and ion-sensing applications, as well as for the assembly of supramolecular architectures. 

Even before the discovery of the katapinands vide supra), work upon bidentate Lewis acid hosts suggested the possibility of chelation of anionic sjjecies by acyclic-boron-containing ligands such as BF2CH2CH2BF2. In many ways mul-... 

This compound represents an example of an inverse crown ether stmcture (Lewis acidic host-Lewis basic guest macrocyclic heterometallic alkoxides or amides/ . 

Blanda, M. T., Horner, J. H., Newcomb, M., Macrocycles containing tin - preparation of macrobicyclic Lewis acidic hosts containing 2 tin atoms and Sn-119 Nmr-studies of their chloride and bromide binding-properties in solution. J. Org. Chem. 1989,54,4626-4636. 

In addition to the areas already discussed in this article and, in particular, development of applications in biological, medicinal, and materials chemistry, several aspects are of current research interest. Section 4.10 introduced the steadily expanding area of single-cage metallacarbaborane clusters with more than 12 vertices. The role of carbaboranes and metallacarbaboranes within supramolecular chemistry has been actively investigated in the last decade. One of the best-explored areas is that of multicluster mercuracarbaboranes, which act as macrocyclic Lewis acidic hosts for electron-rich (anionic or neutral) guests. ... 

J. H. Horner, P. J. Squatritto, N. McGuire, J. P. Riebenspies, and M. Newcomb, Organometallics, 10, 1741 (1991). Macrocycles Containing Tin. Crystal Structures and Molecular Mechanics Calculated Structures of Macrocyclic Lewis Acidic Hosts. 

Lewis acidic hosts (Section V.A) illustrated important theoretical concepts such as the chelate effect and binding cooperativity, which have now been shown to exist for anion as well as cation binding. This work has also resulted in the crystallographic determination of eye-catching solid state receptor-anion complexes, while heteroelement NMR has allowed an accurate means of probing the solution phase structure of these complexes. Already, multinuclear tin systems are being built into functioning anion selective electrodes. 

For a summary of published reports on anion complexation by multidentate Lewis acid hosts, see [3a]. 

The interaction of halide anions with a series of macrobicyclic compounds (1—4) containing two tin atoms in bridgehead positions was also studied by Sn-NMR. It turned out that bicyclic host 1 is able to bind fluoride anion, but it does not react with bulkier chloride, bromide, and iodide ions. By contrasL compounds 2-4 readily coordinate chloride ion but they are unable to bind fluoride anion. According to x-ray diffraction data, the fluoride ion in the complex with 1 is bonded to both tin centers of the Lewis acidic host, whereas the chloride ion in its adduct with 2 is coordinated only to a single tin atom. 

Newcomb, M. Horner, J.H. Blanda. M.T. Squattrito. P.J. Macrocycles containing tin. Solid complexes of anions encrypted in macrobicyclic Lewis acidic hosts. J. Am. Chem. Soc. 1989. 111.6294-6301. 

Hawthorne. M.F. Zheng, Z. Recognition of electron-donating guests by carborane-supported multidentate macrocyclic Lewis acid hosts Mercuracarborand chemistry. Acc. Chem. Res. 1997. 30. 267-276. 

Zinn, A.A. Knobler, C.B. Karwell. D.E. Hawthorne, M.F. Molecular aggregates of nitrate ion with the tetravalent Lewis acid host 12-mercuracarborand-4 Novel trihapto coordination of NO. Inorg. Chem. 1999. 38. 2227-2230. 

An easily visualised example is the multidentate Lewis-acid host 2.77, a colourimetric fluoride sensor. The receptor contains a non-irmocent ferrocenyl linker between the boronate groups. Reaction of 2.77 with n-Bu4N+F in CHCI3... 

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