Guest complexes carboxylic acids

Another related host-guest complex has been constructed from citric acid, boric acid, and a strontium salt in a 2 1 1 stoichiometry. In this complex the strontium cation is surrounded by four water molecules, two monodentate carboxyl groups and one oxygen atom of the BO4 unit [161]. 

A carboxylate derivative of a fully aromatic, water-soluble, hyperbranched polyphenylene is considered as a unimolecular micelle due to its ability to complex and solubilize non-polar guest molecules [23]. The carboxylic acid derivative of hyperbranched polyphenylene polymer (HBP) (My,=5750-7077, Mn=3810-3910) consists of 40-60 phenyl units that branch outward from a central point forming a roughly spherical molecule with carboxylates on the outer surface. The free acid form of HBP was suspended in distilled water and dissolved by adding a minimum quantity of NaOH. The solution was adjusted to pH 6.2 with aqueous HCl. Calcium carbonate crystals were growth from supersaturated calcium hydrogencarbonate solution at room temperature. HBP gave... 

Unlike the crystalline cycloamylose complexes, combining ratios of host to guest in solution are usually 1 1. A notable exception is the interaction of the cycloamyloses with long chain aliphatic carboxylic acids. Solubility plots suggest that as many as four cycloheptaamylose molecules may interact with a single molecule of dodecanoic acid (Schlenk and Sand, 1961). In analogy to the crystalline state, cycloamyloses may form channels in solution in order to accomodate extended chains. 

Some of the complexities mentioned above have been encountered by Hui et al. who examined the Norrish II reactivity of two benzoyl carboxylic acids 104a and 104b in aqueous solutions of carboxymethylamylose (CMA) [305]. Like a CD torus, CMA can form pockets where guest molecules like 104 may reside. The CMA can adopt helical conformations whose cross-sectional... 

As discussed in Sect. 2.5, the ability of the host and guest to self-associate must be established prior to any complexation studies. Carboxylic acids 33, 34, 47, and 49 showed no tendency to aggregate in chloroform [59]. In contrast, the lH NMR of carboxylic acid 31 contained several broad resonances that were consistent with a zwitterionic structure and/or an aggregate [56]. For this reason, complexation studies have not been carried out with the highly twisted molecular tweezers 31 or 32. For the guests used in these studies, dimerization constants were known and indicated that self-association would be negligible under the concentrations used. 

A unique dendrimer series possessing an all-hydrocarbon interior and up to 36 surface carboxylic acid groups has exhibited very interesting unimolecular micelle properties. These products have been shown to form host-guest complexes with various hydrophobic guest molecules. Newkome refers to these products as micellanes [137, 138]. 

When the redox active guest anthraquinone-2-carboxylic acid is added to compound 47, a partial quenching of about 20 % of the MLCT emission of the ruthenium center of the complex is observed, while no quenching is observed when the same substrate is added to the metal complex in absence of the cyclodextrin receptor. The partial quenching can be then ascribed to the occurrence of an electron transfer process between the guest complexed into the CD cavity and the appended metal center [88]. 

The boronic acid functionality is being increasingly used in many ways to complex guests containing diols and carboxylic acid units. A few examples are described below. 

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