Coordination self-assemblies from pyridine bis-tridentate N,N,O-tridentate (1,2,3-triazol-4-yl)-picolinamide (tzpa) ligands: example of Cu(II) disphenoid (M4L4) shaped macrocycle formations.

Three bis N,N,O-tridentate (1,2,3-triazol-4-yl)-picolinamide (tzpa) ligands H2L1-H2L3 were synthesised and characterised and used in conjunction with Cu(I) and Cu(II) to develop supramolecular self-assemblies which were studied in methanol solution. The three bis tzpa ligands all have terminal pyridyl units (as 2- or 3-pyridyl) but differ in the nature of the flexible spacer between the two triazole units, which in the case of H2L1 was aliphatic, while H2L2 and H2L3 are based on p-xylyl and m-xylyl units, respectively. While both H2L1 and H2L3 were found to be readily soluble in range of organic solvents, H2L2 was not, but found to be soluble in aqueous solution, which upon acidification with HCl gave rise to the formation of a hydrogel. The coordination properties of H2L1 and H2L2 with Cu(I) and Cu(II) salts in range of solvents were probed. While these did not result in the formation of crystalline material, the interaction with Cu(I) and H2L3 in methanol resulted in the formation of crystalline material of X-ray quality. The resulting assembly is a tetranuclear supramolecular M4HL34 species, that resembled a disphenoid (rhombic disphenoid) shaped macrocyle, where the Cu(I) had oxidised to Cu(II) and the amides of H2L3 were deprotonated to give L3 ligand. The changes in the ground and the excited state properties of the tzpa ligand H2L3 were used to monitor the formation of the supramolecular structures in situ using Cu(I) in methanol, which also confirmed the formation of the M4L34 assembly in solution. Likewise we were able to examine H2L1 with Cu(I) and Cu(II) in solution, but only the former resulted in the generation of the M4L14 assembly, as the use of Cu(II) favoured the M2L1 structure in solution.