Inclusion complexes of the antitumour metallocenes Cp₂MCl₂ (M = Mo, Ti) with cucurbit[n]urils MCl₂ (M = Mo, Ti) with cucurbit[n]urils

The encapsulation of the aquated forms of molybdocene dichloride and titanocene dichloride by cucurbit[n]uril (Q[n], where n = 7 and 8) at different pD values has been studied by¹H NMR spectroscopy and molecular modelling.¹H NMR titration experiments indicate that both metallocenes form 1 : 1 host–guest complexes with both Q[7] and Q[8]. In these complexes, both the cyclopentadienyl ligands and metal centre are positioned deep within the cucurbituril cavity. In vitro cell proliferation studies using the cancer cell lines MCF-7 and 2008 showed that the encapsulated molybdocene complex was more active than the corresponding free metallocene, with GI₅₀ values of 210 and 400 μM respectively. However, unexpectedly the encapsulation of Cp₂MoCl_2(aq)at pD 7 catalysed significant degradation of the cucurbituril framework in the presence of oxygen. Encapsulation of Cp₂TiCl_2(aq) by Q[7] greatly slowed the protonolysis of the cyclopentadienyl ligands in aqueous phosphate buffer (pD 7), while encapsulation in Q[8] only slightly retarded the hydrolytic degradation of the metallocene.