Molecular Self Assembly: Solvent Guests Tune the Conformation of a Series of 2,6-Bis(2-anilinoethynyl)pyridine-Based Ureas

The conformations of 2,6-bis(2-anilinoethynyl)pyridine-based urea receptors were studied by single crystal X-ray diffraction methods and revealed a rich conformational flexibility influenced by solvents. Whereas receptor L(1) in DMSO prefers an "S" conformation, receptor L(1) crystallizes in an "O" conformation from DMSO/CH(3)OH binary solvent system, and a "W" conformation in the ternary solvent mixture DMSO/toluene/1,4-dioxane. In the case of L(2), the molecule adopts an "S" conformation where water molecules are sandwiched between two molecules of L(2) to form a dimer. Similar to L(2), L(3) also forms a dimer where water molecules are sandwiched between L(3) molecules, which are capped with two molecules of DMSO. Such a capping DMSO solvate is lacking in the case of L(2). Taken together, these results demonstrate that the conformation of 2,6-bis(2-anilinoethynyl) pyridine-based urea receptors can be dramatically manipulated and tuned by the choice of crystallization solvents.