Tuning of molecular topology of polypyridyl spacers via the balance of steric and inductive effects is an important field in supramolecular chemistry. Such polypyridyl ligands that can bridge two or more remote metal centers have been utilized in the synthesis of abiotic functional supramolecules that exhibit mixed valence, anion exchange, photoinduced electron or energy transfer, sensing, polymorphism, and magnetic exchange between paramagnetic centers. Thus, delicate design and synthesis of new tectonics is one of the most important tasks in the field of construction of functional supramolecular materials. In particular, isotropic and anisotropic weak hydrogen bonds as well as electronic and steric effects of substituted moieties play significant role in the synthesis of new spacer ligands. Recently, among various polypyridyl tectonics, some (iso)nicotinoyl-containing spacers have served as appropriate building blocks for nanoscale supramolecular materials. The (iso)nicotinoylcontaining spacers possess characteristic properties such as a potential multidentate, a sp angle around C=O (~120), a malleable length, conformational nonrigidity, and manageable solubility. In this context, we now describe a facile stepwise reaction of 2,5-dimethyl-2,5-heaxanediol with (iso)nicotinoyl chloride via an anisotropic OH...π interaction. The reaction of 2,5-dimethyl-2,5-heaxanediol with (iso)nicotinoyl chloride progresses stepwise as shown in Scheme 1. Formations of the products are strongly dependent on the reaction temperature: the reaction in chloroform-reflux gives mono-substituted product whereas the reaction in toluene-reflux produces di-substituted products. Their compositions and structures were confirmed by chemical analysis, IR, H NMR, and C NMR. Disubstituted products, 1b and 2b, were recrystallized in a mixture of chloroform and n-hexane (1 : 1) to obtain crystalline solids. Mono-isonicotinoyl product 2a is obtained as crystalline solids, but mono-nicotinoyl analogue 1a is obtained as viscous liquids. The products are soluble in common organic solvents such as acetone, chloroform, methanol, ethanol, dimethyl formamide, and dimethyl sulfoxide, but are insoluble in water. All products are stable even in solutions. In order to examine the reason of the prominent stepwise reaction, energy minimizations of the mono-substituted compounds, 1a and 2a, were carried out by the empirical method (MM2) using the Chem3D Ultra 8.0 (Figure 1). As shown in the figure, the remaining OH group is covered with a perpendicular pyridyl group: perpendicular distance of OH...π is 1.98 A for 1a and 1.82 A for 2a. This is, the OH group is locked intramolecularly by a perpendicular pyridyl group, and thus further reaction of the mono-substitutes does not occur at the reaction condition. The actual monosubstituted compounds should be pyridinium salts, 1a·HCl and 2a·HCl during the reaction, and thus, the stable conformations of the salt forms were calculated, resulting in the similar results as shown in Figure 1. However, the OH...π interaction of 2a·HCl (1.97 A) is slightly longer than that of 2a, presumably owing to the deficient π-electron in the ring. If so, the further reaction should need the more vigorous condition to overcome 2-20 kJ/mol of such an OH...π interaction. The similar reactions in toluene give
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Aug 28, 2007
M Bensafi + 3
Open Access
