Clathrate formation of Diels–Alder adduct of phencyclone and acenaphthylene. Key role of CH/π and bidentate CH/O interactions of the phenanthrene ring in construction of host framework

Two clathrate hosts (3a and 3b) were synthesized via the Diels–Alder reaction of phencyclone (1a) and tetracyclone (1b) with acenaphthylene (2), and the clathrate formation properties of these hosts towards a variety of organic guests were investigated. In the presence of aprotic solvents (i.e., aromatic, ketonic and etheric solvents), host 3a formed inclusion complexes with a 2:1 stoichiometric host/guest ratio, whereas 3b primarily formed 1:1 complexes. The desolvation temperatures of the 3a·guest complexes were extremely high in comparison to the boiling points of the pure guest liquids and were also much higher than those of the corresponding 3b·guest complexes, which contain the conformationally flexible stilbene moiety. Structural analyses of the 3a·guest complexes (i.e., 3a·benzene, 3a·toluene, 3a·1,4-dioxane, 3a·acetone and 3a·pentan-3-one) show that the aromatic CH/π (edge-to-face) interactions between phenanthrene and the acenaphthene ring as well as the interaction of the ‘bidentate’ CH/O hydrogen bond between the phenanthrene-ring hydrogen and the bridged carbonyl oxygen play a key role in the construction of the characteristic host ‘column’ structures. The guest molecule of the 3a·benzene complex is held between the stacking columns by aromatic CH/π interactions of the acenaphthene rings of adjacent host molecules. The stable clathrate formation of 3a is discussed based on X-ray structural analyses of six clathrates and PM6 molecular orbital calculations for the clathration model of 3a·benzene.