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···CC)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 R₃ 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 (T_onset - T_bp) by as much as 123°C for CH₂Cl₂, 103°C for C₆F₆, 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 Δν 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.