Structure of the stoichiometric complex 18-crown-6•H2O•succinic acid

Abstract

Attempts to prepare stoichiometric complexes of the type dicarboxylic acid•18C6•H2O succeeded only with oxalic, succinic, and glutaric acids. The crystal structure of the succinic acid complex was established by X-ray diffraction at 173 K, and was refined to Rw = 0.038 for 3204 observed reflections. The stoichiometry of the complex is 1:1:1 (succinic acid•H2O•18C6). In the solid state there is a dimeric H-bonded association, through a center of symmetry, of two succinic acid molecules. Each diacid is also hydrogen bonded to a water molecule. The water molecules, in turn, form two hydrogen bonds each with the crown ether. The diacid–diacid, diacid–water, and the two water–18C6 hydrogen bonds are 2.657(2), 2.636(2), 2.819(2), and 2.910(2) Å long respectively. They are all nearly linear. The structural units are associated and form the centrosymmetric dimer (18C6–H2O–succinic acid)2. The conformation of the macrocyclic polyether differs significantly from that normally found. Instead of the usual (ttg)6 conformation, that adopted in this complex is (ttgttgstg)2. The two s torsion angles have values of 90.1(3) and 86.9(3)°. In this complex the succinic acid itself is in the gauche conformation. Keywords: macrocycle – succinic acid complex.