Crystal structure of 2-[6-(2-formylphenoxy)-2,4-hexadiynyloxy]- benzaldehyde, C20H14O4

C20H14O4, monoclinic, P121/c1 (no. 14), a = 12.5632(6) A, b = 4.3769(2) A, c = 14.8059(8) A, = 104.213(2)°, V = 789.2 A, Z = 2, Rgt(F) = 0.053, wRref(F) = 0.145, T = 200 K. Source of material 2-[6-(2-Formylphenoxy)-2,4-hexadiynyloxy]benzaldehyde was synthesized according to two different methods: (a) Homocoupling of the acetylenic compound 2-(2-propynyloxy)benzaldehyde [1] with Cu(OAc)2 with or without pyridine (Eglinton reaction), (b) Reaction of salicylaldehyde and 1,6-ditosyl-2,4hexadiyne in the presence of potassium carbonate as a base in THF over 24 h. After filtration and removal of the solvent under reduced pressure, the residue was purified by silica gel column chromatography with a mixture of petroleum ether/ethyl acetate (1:1, v/v) as eluent. The products were crystallized from petroleum ether/dichloromethane (3:1, v/v; m.p. 146-147 °C). Analytical data are available in the CIF. Experimental details All hydrogen atoms were located from difference Fourier maps. In the refinement theywere treated using appropriate ridingmodels. Discussion Alkyne units can function as rigid spacers betweenmolecular fragments, whereas conjugation between the fragments is enabled by the two perpendicular bonds. The conjugation has attracted synthetic chemists for the construction of conjugated systems by coupling of , -polyacetylenes to annulenes [2]. Due to their structural simplicity linear conjugated polyynes have been envisioned as molecular wires [3-6]. Meanwhile, diacetylene compounds are starting material for the synthesis of heterocyclic compounds. Themolecular structure of 2-[6-(2-formylphenoxy)2,4-hexadiynyloxy]benzaldehyde consists of a diacetylene moiety with two additional formyl groups and also two oxygens as heteroatoms.The condensation, namely the formationof theSchiff base, of salicylaldehyde derivatives with a diamine results in the well-known ligand salen. The coordination chemistry of this class of tetradentate Schiff base ligands with transition metal atoms is well investigated. The formation of the Schiff base, however, is not restricted to salicylaldehyde derivatives. It proceeds as well with other aromatic aldehydes [7,8]. Thus we envisioned to use 2-[6-(2-formylphenoxy)-2,4-hexadiynyloxy]benzaldehyde as starting material for the synthesis of some new Schiff bases. The analysis of the structure reveals that the distances in the aromatic rings are not completely uniform. The longest bond (140 pm) was found between the carbon atoms carrying the heterosubstituents. The shortest bonds are the next but one bonds on both sides. This pattern was found agreeing also in the structure of the corresponding monomeric alkyne 2-(2-propynyloxy)benzaldehyde [1]. The molecules exhibit a crystallographically imposed inversion center. As expected the conjugated triple bond (120 pm) is found to be clearly longer than the isolated triple bond in the corresponding monomeric alkyne 2-(2-propynyloxy)benzaldehyde (117 pm, [1]). The bond length of the central sp-sp single bond amounts to 138 pm. The bond angles at these central sp centers exclusively bound to other sp centers arewith 179° significantly closer to 180° than those additionally bound to a methylene group (175° resp. 177° in the corresponding monomeric alkyne [1]). Z. Kristallogr. NCS 221 (2006) 339-340 / DOI 10.1524/ncrs.2006.0100 339 © by Oldenbourg Wissenschaftsverlag, Munchen