Fully Collapsed Imploded Cryptophanes in Solution and in the Solid State

Abstract

Cryptophanes with flexible linkers derived from (±)‐tris‐(4‐formyl‐phenyl)‐cyclotriguaiacylene with either bisoxydi(ethylamine) or bis(aminopropyl)ether were isolated as single crystals, the crystal structures of which showed the proposed, but previously uncharacterised, out‐in conformation, in which both cyclotriguaiacylene fragments adopt a crown conformation with one crown sitting inside the other. The usual cage‐like out‐out conformation of the cryptophanes was observed when crystals were dissolved upon heating, and the molecules collapsed back to the out‐in isomers over time. In contrast, a cryptophane also derived from (±)‐tris‐(4‐formyl‐phenyl)‐cyclotriguaiacylene but with rigid dibenzalhydrazine linkers was isolated as the more usual out‐out isomer.

Desolvation Induced Conformational Transition in a Cryptophane Host: Crystal To Crystal Transformation

“…The crystal structure revealed that the out‐out CC conformation is crystalized with guests inside the cavity (Figure 2). It is in agreement with the fact that solvent filled cryptophanes are more stable in out‐out CC conformation [14,21] . Four acetone molecules are present in the unit cell.…”

“…Cryptophanes, are one such class of molecules, [12] contain two bowl shaped cyclic molecular systems (cyclotribenzylenes or its derivatives), joined together with three linkers. Changing linker size changes the free space of the cavity considerably [13,14] . Cryptophanes are well studied for entrapment of small neutral guest molecules CHCl 3 , CH 2 Cl 2 , CHFClBr, [13a] cationic organic molecules, [15,16] metal ions, [17] and gases like Xe, CFCs, CH 4 in solution phase [13,18] .…”

“…Other conformations are expected to form by partial or total collapse of out‐out CC cage structure on removal of included guest molecule(s). These structures have been experimentally obtained by high temperature evacuation or heating the cage in a solvent that is too large to occupy the cavity [14,21,22] . There are three reports on solid‐state characterization of saddle conformers exist in literature.…”

“…Changing linker size changes the free space of the cavity considerably. [13,14] Cryptophanes are well studied for entrapment of small neutral guest molecules CHCl 3 , CH 2 Cl 2 , CHFClBr, [13a] cationic organic molecules, [15,16] metal ions, [17] and gases like Xe, CFCs, CH 4 in solution phase. [13,18] These molecules are shown to bind biologically important molecules such as choline and avidin.…”

“…obtained by high temperature evacuation or heating the cage in a solvent that is too large to occupy the cavity. [14,21,22] There are three reports on solid-state characterization of saddle conformers exist in literature. Holman et al reported the structure of an exo-ester-functionalized m-xylyl-bridged cryptophane, [23] Brotin and co-workers reported propylene bridged cryptophane, [22] and Dmochowski et al reported cryptophane-A, in saddle conformation.…”