Host–guest interactions between calixarenes and Cp 2NbCl 2

Abstract

The possible inclusion complexes of Cp 2NbCl 2 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 Cp 2NbCl 2. Raman spectroscopy showed, for the inclusion compound of p-sulfonic calix[6]arene–Cp 2NbCl 2, a band between 500 and 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 (Cp 2NbCl 2) dissolves in water and undergoes oxidation and hydrolysis processes to yield Cp 2NbCl 2(OH) species. For that reason this band does not exclude that the Nb–O band belongs to Cp 2NbCl 2(OH). Solid State 13C CP-MAS NMR and solution 1H NMR spectroscopies together with ab-initio results showed that Cp 2NbCl 2 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 Cp 2NbCl 2 in CDCl 3 solution. Cp 2NbCl 2 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 Cp 2NbCl 2 into the cavity.