Although benzene and perfluorobenzene adopt crystal structures with ªherringboneº (T-shaped) pairwise orientations, combination of the two liquids produces an intimate 1:1 solid complex in which the basic short range order is a face-to-face stack. The driving force for this specific mixing stems from the quadrupole moment and van der Waals surface of benzene and perfluorobenzene. [5±8] Recent dynamic studies have provided quantitative evidence for a general, polar-p phenyl/perfluorophenyl attraction. These studies of ours and related work have developed a design principle for ordered mixed-crystals of aromatic/perfluoroaromatic compounds. [9±14] Studies on tolan/perfluorotolan and related compounds support these ideas. [15±17] A powerful extension of the principle is to use the attractive phenyl/perfluorophenyl interactions to anchor other groups in a specific order within a crystal, that is, to accomplish crystal engineering. We provide a striking demonstration of such control through the crystal structures and mixing behavior of sym-triphenethynylbenzene (1) and sym-tris(perfluorophenethynyl)benzene (1 ± F 15 ); the pure compounds pack in slipped stacks and the 1:1 complex displays face-to-face stacks alternating 1 and 1 ± F 15 .
Reaction of 1,3,5-tribromobenzene with phenylacetylene under Sonagoshira conditions (Pd IV 5 %, dimethylformamide, base) resulted in the formation of 1 in 58 % yield. Under similar conditions 1,3,5-triethynylbenzene and pentafluoroiodobenzene were coupled to form 1 ± F 15 in 30 % yield. Pure 1 was crystallized from hexanes/dichloromethane to produce needles that melted at 142 ± 146 8C (visual inspection), 144 8C (differential scanning calorimetry (DSC)). Pure 1 ± F 15 was recrystallized from hexanes/dichloromethane to produce needles that melted at 199 ± 201 8C (visual inspection), 201 8C (DSC). Equimolar amounts of 1 and 1 ± F 15 were ground in an agate mortar to produce a fine powder (Figure , top). Unlike most solid mixtures, the melting point of the 1:1 mixture was higher than either of the two pure compounds (235 ± 239 8C, visual inspection; 239 8C, DSC). Recrystallization of this powder from hexanes/dichloromethane by evap-