The possible inclusion complexes of Cp2NbCl2 into calixarenes hosts have been investigated. The existence of a true inclusion complex in the solid state was confirmed by a combination of NMR, ab-initio calculations, thermogravimetric analysis, FTIR, Raman and PXRD. Ab-initio calculations, 1H NMR solution and solid state 13C CP MAS NMR results demonstrated that p-sulfonic calix[6]arene does form an inclusion complex with Cp2NbCl2. Raman spectroscopy showed, for the inclusion compound of p-sulfonic calix[6]arene-Cp2NbCl2, a band between 500–850 cm−1 characteristic of Nb-O vibration. This result suggests that Nb(V) may engage in coordination with the oxygen of the sulfonate group, as part of the host-guest interaction. However, it is important to mention that the niobocene dichloride (Cp2NbCl2) dissolves in water and undergoes oxidation and hydrolysis processes to yield Cp2NbCl2(OH) species. For that reason this band does not exclude that the Nb-O band belongs to Cp2NbCl2(OH). Solid State 13C CP MAS NMR and solution 1H NMR spectroscopies together with ab-initio results showed that Cp2NbCl2 is included in the p-sulfonic calix[6]arene cavity, with both Cp rings inside the cavity. In contrast, the solution 1H NMR results demonstrated that calix[6]arene does not form inclusion complex with Cp2NbCl2 in CDCl3 solution. Cp2NbCl2 is not included in the calix[6]arene cavity, possibly due to the lack of sulfonate heads which promote Nb-O interactions and assist the inclusion of Cp2NbCl2 into the cavity.
Host-guest interactions between α-, β-, and γ-cyclodextrins and vanadocene dichloride (Cp(2)VCl(2)) have been investigated by a combination of thermogravimetric analysis, differential scanning calorimeters, PXRD and solid state and solution EPR spectroscopy. The solid state results demonstrated that only β- and γ-cyclodextrins form 1:1 inclusion complexes, while α-cyclodextrin does not form an inclusion complex with Cp(2)VCl(2). The β- and γ-CD-Cp(2)VCl(2) inclusion complexes exhibited anisotropic electron-(51)V (I = 7/2) hyperfine coupling constants whereas the α-CD- Cp(2)VCl(2) system showed only an asymmetric peak with no anisotropic hyperfine constant. On the other hand, solution EPR spectroscopy showed that α-CD may be involved in weak host-guest interactions in equilibrium with free vanadocene species.
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