A combined theoretical-experimental study of the inclusion of niobocene dichloride in native and permethylated beta-cyclodextrins

摘要

Inclusion complexes comprising niobocene dichloride, CP2NbCl2 and either native beta-cyclodextrin (beta-CD) or permethylated beta-CD (TRIMEB) were prepared with a host:guest molar ratio of 1: 1. The adducts were characterized in the solid state by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), C-13{H-1} CP/MAS NMR, FTIR, and FT Raman spectroscopies. Ab initio calculations were performed in order to elucidate the possible inclusion geometries and calculate vibrational frequencies for Cp2NbCl2 in the region 100- 1800 cm(-1). The vibrational spectra indicated that neither the hosts nor the guest were chemically modified upon inclusion complexation. Small wavenumber shifts in C-H, C-C, and metal-ligand vibrational modes were attributed to the effect of the host-guest interaction. Powder XRD showed that the P-CD adduct was of low crystallinity, while the TRIMEB adduct was quite crystalline. The powder XRD pattern of the TRIMEB adduct could be satisfactorily indexed in the monoclinic system with the space group P2(1). A hypothetical structural model for the crystal packing was obtained through Monte Carlo optimizations using a reported TRIMEB geometry comprising a distorted elliptical cavity. An unrestrained Rietveld refinement of the structural model could not be carried out due to the relatively low quality of the experimental powder XRD pattern. However, the plausibility of the host-guest interaction geometry, involving inclusion of one of the guest Cp ligands inside the CD cavity, was fully supported by the ab initio calculations.