Heterobimetallic Calix[4]arene Complexes: Interconversion of Dimeric (Ca, Sr or Ba)/Ti^IV Complexes with a Monomeric K/Ti^IV Complex

Reaction of calix[4]arene with alkaline earth metals (Ca/Sr/Ba) in methanol followed by the addition of various titanium(IV) sources afforded novel dimeric 1:1 alkaline earth/titanium(IV) calix[4]arene complexes. In each complex the central core of the crystallographically imposed centrosymmetric dimer consists of a rhombus of alternating alkaline earth metals and μ ₃-oxo-centres. Five terminal methanol ligands form seven-coordinate calcium and strontium centres, while the barium centres are bridged by two additional methanol ligands, making them nine-coordinate. Each oxo centre is the apical ligand of a square-pyramidal titanium(IV) centre, whose basal plane ligands are the four phenolate oxygens from a calix[4]arene in the symmetrical cone-conformation. Deprotonation of calix[4]arene with potassium metal, followed by the addition of a titanium(IV) source containing at least one acetylacetonate ligand, formed a mixed potassium/titanium complex based on one calix[4]arene. The potassium cation in this complex binds within the π-basic calix[4]arene cavity, whilst titanium is octahedrally complexed in the exo-position, by four phenolates and one acetylacetonate. The alkaline earth titanium dimers and potassium/titanium monomers could be interconverted merely by changing the solvent, with the appropriate cations and ligands present in solution.