Solid-state study of the structure and host-guest chemistry of cucurbituril-ferrocene inclusion complexes

Inclusion complexes of ferrocene (Fc) with cucurbit[n]urils (n = 7, 8) have been prepared via a rapid microwave-assisted hydrothermal approach. Solids were isolated and characterised by elemental analysis, powder X-ray diffraction (PXRD), spectroscopic, and thermoanalytical methods. The UV-Vis spectra support the presence of Fc in Fc@CB7 and a mixture of Fc and ferrocenium ions in Fc@CB8. Partial oxidation of Fc to Fc⁺ takes place in situ mainly due to the presence of acid of crystallisation in CB8. On the basis of PXRD, the complex Fc@CB8 is classified into an isostructural series that is formed by several CB8-containing compounds that crystallise in the space group I4₁/a and have similar unit cell dimensions and CB8 packing motifs. The FT-IR and Raman spectra of Fc@CB7 are compared with those of the CB7 host and the Fc guest starting materials, revealing significant frequency shifts of some Fc-centered vibrational modes upon complexation. Blueshifts of the Fe-Cp stretching and ring tilt bands are attributed to encapsulation of Fc monomers in a constrained environment, leading to restricted motion effects and/or a change in the Fc conformation from staggered to eclipsed. The absence of comparable shifts for Fc@CB8 point to a weaker host-guest interaction as a consequence of the larger cavity size. The different host-guest interactions are also evident through a comparison of the ¹³C{¹H} CP MAS NMR spectra. Thermogravimetric analysis for the inclusion compounds reveals that sublimation of Fc is inhibited by molecular encapsulation to the extent that oxidative decomposition of the organoiron species takes place concurrently with cucurbituril decomposition, leading to the formation of hematite, α-Fe₂O₃.