Host-guest interactions between niobocene dichloride and α-, β-, and γ-cyclodextrins:: preparation and characterization

The possible inclusion complexes of Cp2NbCl2 into alpha-, beta-, and gamma-CD hosts have been investigated. The existence of a true inclusion complex in the solid state was confirmed by a combination of thermogravimetric analysis, FTIR, PXRD, and C-13 CP-MAS NMR spectroscopies. The solid-state results demonstrated that alpha-cyclodextrin does not form inclusion complexes with Cp2NbCl2 whereas beta- and gamma-cyclodextrins do form such complexes. PXRD, NMR, and thermal analysis showed that the organometallic molecules of Cp2NbCl2OH are included in the cavities of beta- and gamma-cyclodextrins, possibly adopting a symmetrical conformation in the complex, with each glucose unit in a similar environment. In solution, H-1 NMR experiments suggest that niobocene has a shallow penetration on the beta-CD leading to upfield shift on H-3 signal with a minor perturbation on the H-5 proton while for gamma-CD, both H-3 and H-5 are shifted upfield substantially. This suggests that niobocene penetrates deeper into the gamma-CD cavity than in the beta-CD cavity, as a result of the cavity size.