Self-assembly and host-guest chemistry of big metallosupramolecular M4L4 tetrahedra

Metallosupramolecular tetrahedra M-8[L4Ti4] are easily obtained by self-assembly from the triangular ligands L-H-6 and titanoyl bis(acetylacetonate) in the presence of alkali metal carbonates as base. All the complexes can be well characterized by H-1 NMR in combination with ESI FT-ICR MS. Force field calculations reveal that the tetrahedra show Ti - Ti separations of 17 angstrom ([L-1 Ti-4(4)](8-)) and 23.5 angstrom ([L-2 Ti-4(4)](8-)), respectively, leading to huge internal cavities. The cavity is readily shielded in the case of L-1 but possesses big pores with the bigger ligand L-2. [L-1 Ti-4(4)](8-) was used to investigate the host - guest chemistry of these container molecules and it was found that cationic organic guest species like anilinium can be introduced in the interior of the complex. Inclusion is nicely followed by NMR spectroscopy. Upon addition of one equivalent of guest the symmetry of the tetrahedron is lost but is regained after addition of significantly more than four equivalents.