Ethynyl group as a supramolecular halogen and C≡C-H⋯C≡C trimer synthon in 2,4,6-Tris(4-ethynylphenoxy)-1,3,5-triazine

Abstract

Halogen-ethynyl interaction mimicry and the role of halogen trimer synthon (X⋯X)3 in crystal packing have been discussed in several papers. However, the ethynyl trimer motif (C≡C-H⋯C≡C) 3 has not been systematically studied in crystal engineering. The (C≡C-H⋯C≡C)3 synthon is reproduced in 10 isostructural host-guest crystalline adducts in the R3̄ space group. Not only are the ethynyl structures (4-EPOT) identical to the iodo series (4-IPOT) because of deliberate design but they also show tighter guest inclusion due to the stronger C≡C-H⋯π hydrogen bond compared to the I⋯I interaction. 4-EPOT-guest inclusion adducts are more stable than 4-IPOT inclusion compounds on the basis of Cerius2 lattice energy computations (ΔE = ∼8 kcal mol-1). The temperature for guest release in thermal gravimetric analysis from the 4-EPOT-host framework is higher than the solvent boiling point (Tonset - Tbp) by as much as 123°C for CH2Cl2, 103°C for C 6F6, and 83°C for Et2O. The guest molecules are included in the host cage by a good size fit and van der Waals interactions. Benzene is a guest in the slightly smaller hexagonal cavity of 4-EPOT, although its inclusion was not possible in 4-XPOT hosts (X = Cl, Br, I). Stretching frequency Δv of 14-25 cm-1 in the infrared spectrum and hydrogen-bond shortening in the distance-angle scatter plot indicate cooperativity in extended (C≡C-H⋯C≡C)n arrays. The "missing" (C≡C-H⋯C≡C)3 synthon in isostructural halogen/ ethynyl motifs is now reported. © 2007 American Chemical Society.