Assembling Organic Receptors around Transition Metal Templates: Functionalized Catechols and Dioxomolybdenum(VI) for...

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.