Association Constants Of Receptors Research Articles

Anion recognition by 2,2′-binaphthalene derivatives bearing urea and thiourea groups at 8- and 8′-positions by UV–vis and fluorescence spectroscopies

Synthesis and anion recognition properties of 2,2′-binaphthalene derivatives bearing two thiourea ( 1) and urea ( 2) groups at 8- and 8′-positions were studied. The structure of receptor 1 was determined by X-ray crystallography. UV–vis spectra of the receptors showed characteristic changes around 300–400 nm through isosbestic points upon the addition of biologically relevant anions such as acetate, dihydrogenphosphate, and chloride in MeCN and DMSO due to restriction of the rotation around the single bond connecting two naphthyl moieties by cooperative guest binding of two recognition sites. Job’s plots showed 1:1 complexation for guest anions. The fluorescence quantum yields of free form of 1 and 2 in MeCN were determined to be 0.021 and 0.57, respectively. The fluorescence intensities of the receptors diminished upon the addition of anions in MeCN. The association constants of receptors 1 and 2 were one or two orders of magnitude greater than the corresponding monothiourea and urea receptors 3 and 4 indicating cooperative hydrogen bonding with guest anions. The selectivity trends of association of anions were F −>AcO −>H 2PO 4 −>Cl −>>HSO 4 −≈NO 3 −≈Br −≈I − for 1, and F −>AcO −≈Cl −>H 2PO 4 −>Br −>HSO 4 −>I −≈NO 3 − for 2. Receptor 2 showed remarkable Cl − selectivity presumably owing to suitable orientation for effective hydrogen bond formation with Cl −.

Synthesis and Binding Properties of 1,3,5-Tris(2-arylthiomethyl)mesitylene: A Selective Ag (I) Ionophore

The efficient synthesis of four mesitylene-based receptors 1-4 and their potentiometric response characteristics to alkali metal, alkaline earth metal, and transition metal ions, under various pH conditions are outlined. Receptor 1-based electrode exhibited more sensitive response to Ag+ ion (49 mV/decade of range from 10-6 to 10-2 M) than the 2-based electrode (47 mV/decade of range from 3 ´ 10-5 to 10-2 M), while the 3- and 4-based ones revealed sub-Nernstian below 40 mV/pAg+. All electrodes showed substantial responses to Ag+ ion under acidic condition, but there was almost nil-response to other transition metal ions (Fe2+, Co2+, Zn2+, Ni2+, Pb2+, Cd2+, Cu2+ and Hg2+). The association constant of receptor 1 toward Ag+ ion, measured by 1H NMR titration, showed the largest value (200 M-1) among the tested receptors. The results were interpreted with semi empirically-modeled structures.

Stereoselective Synthesis and Binding Properties of Novel Concave-Shaped Indolizino[3,4-b]quinolines

Novel all-cis-configurated indolizino[3,4-b]quinoline receptors 3, 4were prepared via diastereoselective Lewis acid-catalyzed cyclization of N-arylimines 6, 7 as the key step. In order to obtain the indolizino[3,4-b]quinoline derivative 21 without a gem-dimethyl group at C-7, an N-arylimine precursor 18 bearing a vinyldisilane terminus was prepared in 8 steps from L-prolinol 15. In contrast to the known β-effect of silyl groups cyclization of 18 proceeded via an α-carbenium ion species to give the diastereomeric products 19, 20, which were desilylated to 21, 22. The association constants for receptors 2—4 and 21 decreased in the order 21 > 2 > 4 > 3 for both acetic acid and N-Z-phenylalanine as substrates.

Assembling Organic Receptors around Transition Metal Templates: Functionalized Catechols and Dioxomolybdenum(VI) for the Recognition of Dicarboxylic Acids

The synthesis of two receptors for dicarboxylic acids [12](2)(-) and [13](2)(-), based on the self arrangement of two functionalized catechols 1 and 2 around a cis-[MoO(2)](2)(+) core, is described. Among the three pairs of enantiomers which may be produced during the complexation of two unsymmetrical catecholates around one molybdenum(VI) ion, only one is observed for each catechol derivative 1 or 2. Depending on the base used during the complexation of catechols to the molybdenum atom, the dianionic receptors obtained display different solubility properties. These molybdenum-based receptors are chromogenic and, in methylene chloride, the affinities of the assembled receptors for dicarboxylic acids ranging from C(4) to C(8) have been assessed by UV-visible titrations after determining the stoichiometry of the complex formation using Job's method. While receptor [12](2)(-) displays selectivity for C(4) and C(5) acids, the more flexible receptor [13](2)(-) exhibits selectivity for C(7) and C(8). The binding mode of the diacids to the molybdenum receptor has been determined based on (1)H NMR titration. Due to the intrinsic chirality of the receptors, their binding properties versus chiral dicarboxylic acid have been examined. The enantioselective binding of N-carbobenzyloxy protected L and D-glutamic acid due to additional pi-pi interactions of the protecting group with the receptor's framework is reported for [12](2)(-) in methylene chloride. For comparison, the association constants of receptor [12](2)(-) with a Boc protected L-glutamic acid and the racemic mixture of N-carbobenzyloxy protected glutamic acid have been determined.