The host behaviour of two compounds, bearing tricyclic fused ring systems, in mixtures of cyclohexanone and its three isomeric methylcyclohexanones

Abstract

Compounds N,N’-bis(9-phenyl-9-xanthenyl)propane-1,3-diamine (H1) and N,N’-bis(9-phenyl-9-xanthenyl)butane-1,4-diamine (H2) were assessed for their host behaviour in cyclohexanone (CYC) and 2-, 3- and 4-methylcyclohexanone (2MeCYC, 3MeCYC and 4MeCYC) through crystallization experiments from these potential guest solvents. It was thus shown that H1 possessed the ability to enclathrate CYC (only), while all four cyclohexanones formed complexes with H2. Thermal analyses and SCXRD revealed that those guest species residing in discrete cavities required more heat in order to be released from their crystals compared with those guests in channels. Furthermore, the thermally most stable complex of H2 was H2·4MeCYC, whilst the least stable one was H2·3MeCYC, as observed from the guest release onset temperatures through thermal analyses. The conformations of the host molecules were also investigated and compared. In H1·CYC, the nitrogen atoms of the diamino linker were involved in an intramolecular (host)N‒H···N(host) hydrogen bond which was absent in the complexes with H2. However, H2·2(CYC), H2·2(3MeCYC) and H2·4MeCYC all experienced strong intermolecular (host)N‒H···O(guest) hydrogen bonds which assisted in retention of the guests in the complexes; this interaction type was absent in both H1·CYC and H2·2(2MeCYC). Guest competition experiments showed that H2 possessed an affinity for, more usually, 3MeCYC, despite the complex of this guest with H2 being the least thermally stable one. Hirshfeld surface analyses, however, were not useful in understanding the host selectivity behaviour in these mixed guest solvents. Finally, it was concluded that, unfortunately, H2 would not be an appropriate host compound for these mixed guest separations through supramolecular chemistry strategies (the more usual fractional distillations are extremely challenging owing to similar boiling points) due to lower than required selectivities in these guest mixtures. (Note that H1 was not employed in analogous guest competition experiments due to its extremely slow crystal growth from these mixtures.)