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.

Three nanoscale supramolecules were synthesized via Pt(II) mediated self-assembly. The first [3+3] self-assembled triangle 5 resulted from the reaction of 1 equivalent of cis-Pt(Et3)2(OTf)2 and 1 equivalent of flexible pyridine-4-carbaldehyde azine. The second analogous structure [3+3] molecular triangle 6 was obtained by the self-assembly of cis-Pt(Et3)2(OTf)2 and 4,4′azobispyridine, while the third [2+2] molecular rectangle 7 was synthesized via reaction between 1 equivalent of cis-Pt(Et3)2(OTf)2 and 1 equivalent of pyridine-3-carbaldehyde azine. All three structures were studied by multinuclear NMR (1H and 31P) and have been established by X-ray crystallography.Figure optionsDownload as PowerPoint slideHighlights
► Three new Pt metallacycles were self-assembled.
► All metalla-cycles were well characterized by various techniques.
► The ligand flexibility effect on formation of self-assembly was studied.