Inclusion compounds guest exchange

Muller and coworkers have recently reported the molecular behavior of 1,3,5-trioxane in a cyclophosphazene inclusion compound using H-2 NMR [68]. The experimental data were obtained by variable-temperature line shape analysis, spin-spin and spin-lattice relaxation time measurements and by 2D exchange between 30 and 370 K. At room temperature, highly mobile trioxane guests were observed. They undergo various overall and conformational motions which give rise to substantial orientational disorder within the hexagonal cyclophosphazene channels. 

The question of guest exchange in inclusion compounds received little attention, despite that fact that such processes are important for sensing and catalysis based on inclusion. However, a number of host-guest systems with organic and metal-containing hosts that entrap volatile guests were recently studied.The host l,l,6,6-tetraphenyl-2,4-diyne-1.6-diol H) forms inclusion compounds with tetrahydrofuran (THF), and thiophene (THIO), and we analyzed the dynamics of the reaction ... 

Caira, M.R. Nassimbeni, L.R. Toda, F. Vujovic D. Inclusion by a diol host compound Structure and dynamics of volatile guest exchange. J. Chem. Soc., Perkin Trans., 2 2001, 2119-2124. 

Crystal isostructurality refers to identical or nearly identical packing arrays of chemically distinct compounds and is inversely related to the phenomenon of polymorphism, which, instead, refers to the ability of a single compound to crystallize in different packing arrays. A series of isostructural inclusion compounds is typically based on a common host framework with voids that may be occupied by different guest molecules. The constant guest environment provided by such a series is a factor that facilitates interpretation of the mechanisms of processes such as thermal decomposition and guest exchange. 

Guest exchange processes in urea inclusion compounds... 

To obtain deeper insights into such guest exchange processes in urea inclusion compounds, confocal Raman microspectrometry has been exploited as an in situ probe (Figure 29) to yield information on the spatial distribution of the original and new guest molecules within the crystal, and to establish details of how the spatial distribution of... 

Figure 29 Schematic representation of the experimental assembly for in situ Raman microspectrometry studies of guest exchange in a urea inclusion compound, comprising the single crystal of the urea inclusion compound (green), initially containing 1,8-dibromooctane guest molecules, attached to a reservoir containing liquid pentadecane (blue).

Figure 28 Schematic illustration of a mechanism for guest exchange in a single crystal of a urea inclusion compound. In the single crystal shown, the tnnnels in the urea host structure are vertical.

These examples represent rare cases of guest exchange processes occurring with retention of the crystal integrity of the inclusion compound. The fact that structural changes of the magnitude implied in this system can occur without loss of crystal integrity may be attributed to the relative structural flexibility of CA inclusion compounds. 

Similar single-crystal to single-crystal guest exchange processes have been observed for inclusion compounds formed between host molecule S (Figure 31) and cationic guest molecules. ... 

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