Bent into shape

The composition and structure of self-assembled coordination cages are very sensitive to subtle changes in the geometry of the bent organic ligands that are used to stitch together the metal ions.

Credit: © 2010 AAAS

Metal ions and organic ligands can be combined in a vast number of different ways to produce self-assembled complexes with a diverse range of structures, including relatively simple shapes such as triangles, squares and cubes, and more complicated interwoven topologies, such as catenanes and knots. In such multicomponent systems, both the coordination geometry of the metal ion and the shape of the organic ligand play a crucial role in determining the final structure of the assembly.

Now, a team of researchers led by Makoto Fujita at the University of Tokyo has investigated¹ how subtle changes in the structure of a dipyridyl ligand affects the self-assembly of well-defined nanoscale coordination complexes. Using a ligand (L) in which two pyridyl rings are linked through a central furan unit, Fujita and co-workers were able to assemble Pd(II) ions (M) into an M₁₂L₂₄ coordination sphere 3.5 nm in diameter. By simply changing the central unit of the ligand to a thiophene ring, however, a larger 5-nm-diameter M₂₄L₄₈ structure was formed.

The difference in self-assembly behaviour is thought to arise from the different 'bite angle' of the two ligands — 149° for the thiophene ligand and 127° for the furan one. Fujita and co-workers also considered how the self-assembly process was affected by using mixtures of the two ligands in different ratios. Only a single product was formed in each case — the M₂₄L₄₈ structure for mixtures containing 30% or more of the thiophene ligand and the smaller M₁₂L₂₄ cage when the quantity dropped to 20% or lower.