Host mercuracarborand

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. 

These host mercuracarborand molecules were applied in Ion Selective Electrodes (ISE) and membrane formulations, as selective optical chloride sensors, as sensitive liquid/polymeric membrane electrodes for anions and as catalysts. ... 

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. 

Synthesis of unsubstituted mercuracarborands, as illustrated in Scheme 1, is a two-step process involving deprotonation of the acidic CH vertices of 1,2-carborane 7 followed by treatment with the appropriate mercury salt <1994JA7142, 1997ACR267>. Thus, -butyllithium efficiently deprotonates 1,2-carborane 7 giving doso-X.Z-VXz-1,2-C2BioHio 8, the pivotal intermediate. If treated with mercury acetate, neutral trimer 5 is formed. Using mercury chloride or bromide, however, leads to the formation of tetramer 9 and 10, respectively, as 1 1 anion-host complexes. 

Although H, and nuclear magnetic resonance (NMR) spectroscopy is routinely used to characterize mercuracarborands and their complexes, it has been proved to be of little value in studying their guest-host... 

A tetraphenyl substituted derivative of [12]mercuracarborand-4 (40), which binds one iodide ion in its cavity due to steric hinderance, has been reported. The stereochemistry of the phenyl groups was found to depend on the mercury counteranion used during the synthesis (106). This observation provided yet further evidence for a direct anion templating effect in mercuracarborane syntheses. Most recently, the C—Hg—C link in this type of host has been replaced with a B—Hg—B link, which alters the electron demands of the mercury centers (reducing their electron deficiency) and apparently switches off anion complexation (107). 

All of the mercuracarborand halide complexes react with silver acetate to form the silver halide and liberate the free macrocycle. X-ray crystallography on the THF complex of the host tetramercuracarborand [3d] and on an octa-B-ethyl derivative (prepared to facilitate solubility in hydrocarbon solvents) has revealed that both species have saddle-shaped 54 symmetry with the centers of the icosahedra well outside the plane of the four mercury atoms. [3a, d] The square planar conformation that is found in the chloride com-... 

Figure 3. (a) Structure of the host tetramer (l,2-C2BioHio)4Hg4 in its planar conformation. [3a] (b) Structure of the tetramer with a bound chloride ion. [3a] (c) 3-Center, 2-electron Hg-Cl-Hg bonds in the mercuracarborand-4 chloride ion complex. [3a] (d) Structure of [12]-crown-4. 

The template effect observed in the synthesis of carborane-supported macrocycles, mercuracarborands, as well as their characterization and host-guest... 

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. 

Probably the best known of the anticrowns are the mercuracarborands, e.g. 2.82 and 2.83 (Figure 2.17). As with the fluorinated complexes, the carbo-rane substituents act as elaborate electron-withdrawing groups and increase the Lewis acidity of the mercury(ii) ions. Most crown ethers exhibit one-to-one host guest stoichiometry, but most anticrowns form 2 1 complexes on size-fit grounds. One exception to this rule is the chloride complex of 2.83 in which the chloride ion binds within the centre of the host cavity with a long Hg-Cl distance of 2.94 A. 

The halide anion can be extracted from the complex by Ag+ ions without causing the decomposition of the host. The synthesis of the plain tetramer mercuracarborand, (HgC2B,oHjo)4 (Figure 27.4a) can only be achieved by the extraction of the halide ions with AgOAc from Li2[(HgC2B,oH,o)4 I2] or Li[(HgC2BioHio)4 X] (X= Cl, Br, 1). =... 

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