X-ray structures, thermal stabilities and kinetics of guest desolvation of complexes of three fluorenone-derived host compounds with the polar aprotic guest solvent, tetramethylurea

The wheel-and-axle host compounds 9,9′-(1,4-phenylene)bis(fluoren-9-ol) (H1), 9,9′-(ethyne-1,2-diyl)bis(fluoren-9-ol) (H2) and 9,9′-(biphenyl-4,4′-diyl)bis(fluoren-9-ol) (H3) each formed complexes with tetramethylurea (TMU), a polar aprotic organic solvent, with host: guest ratios of 1:2. Single crystal X-ray diffraction revealed that these complexes crystallized in the monoclinic space group P2₁/c, their analyses being performed in P2₁/c for H1⋅2(TMU) and in the alternative setting P2₁/n for both H2·2(TMU) and H3·2(TMU). Furthermore, these inclusion compounds are stabilized by both classical and non-classical hydrogen bonds between the host and guest molecules. Hirshfeld surface analyses demonstrated that the percentage of interatomic (host)H···O(guest) interactions ranged between 7.8 and 10.3%, while thermal analyses showed that the relative thermal stabilities of these complexes were high, with the onset temperatures for the guest release event, T_on, being 83.1 (H1·2(TMU)), 81.1 (H2·2(TMU)) and 90.3 °C (H3·2(TMU)). Moreover, the calculated mass loss percentages, after heating each complex in a controlled manner to release the guest species, correlated closely with those expected for these 1:2 host: guest inclusion complexes. Finally, determination of the activation energies for complex desolvation yielded 148.7 ± 5.4, 128.6 ± 10.8 and 149.4 ± 0.8 kJ·mol^‒1 for H1·2(TMU), H2·2(TMU) and H3·2(TMU) respectively. A single guest desolvation mechanism was at work in the first and last of these complexes, while this mechanism in H2·2(TMU) changed during this process. The H1·2(TMU) inclusion complex has been reported previously, and the results obtained in that work are also compared with those from the present investigation.