Effect of ligand flexibility on coordination-driven self-assembly of Pt(II) metallacycles

The effect of ligand flexibility on the self-assembly of Pt(II) metallacycles was investigated. Three diazopyridyl ligands with different flexibilities were used to synthesize three nanoscale supramolecular complexes via Pt(II) mediated self-assembly. With comparatively less flexible ligands, 4,4′azobispyridine (2) and pyridine-4-carbaldehyde azine (3), formation of [3+3] molecular triangles was observed; in contrast, the most flexible of the three ligands, pyridine-3-carbaldehyde azine (4), yielded the [2+2] molecular rhomboid. All three complexes were characterized by multinuclear NMR (1H and 31P) and HR-ESI-MS, and the structure of complex 7 was further established by X-ray crystallography. These studies clearly showed that increased ligand flexibility entropically favors the formation of smaller metallacycles as the ligand can reduce angular strain through bending or conformational change.