Coordination chemistry of N-picolyl-1,8-naphthalimides: colourful low molecular weight metallo-gelators and unique chelation behaviours

We have prepared a series of N-picolyl-1,8-naphthalimide derivatives and explored their coordination chemistry with d-block metal ions in the solid and solution states, and exploited the structural and chemical properties of these ligands to generate a family of robust metallogels. The 2-picolyl substituted ligands N-(2-picolyl)-1,8-naphthalimide L1 and N-(2-picolyl)-4-nitro-1,8-naphthalimide L2 showed interesting coordination behaviour in Mn, Co and Zn complexes 1 – 5, chelating through the pyridyl nitrogen atom and naphthalimide oxygen atom with subsequent bond localisation in the imide ring, the first example of such coordination behaviour to be structurally characterised. The 3-picolyl substituted ligands N-(3-picolyl)-1,8-naphthalimide L3 and N-(3-picolyl)-4-nitro-1,8-naphthalimide L4 vary in their coordination behaviour; the 4-unsubstituted ligand L3 acts as a simple monodentate donor giving crystalline assemblies, while the 4-nitro analogue L4 instead forms robust metallogels on reaction with MnII or CoII. The gels were found to be thermally and chemically reversible under various stimuli, and similar materials could be generated by installing a piperidinyl substituent at the naphthalimide 4-position, giving intensely coloured gels. These results describe previously unknown coordination chemistry and physical properties of the widely-used 1,8-naphthalimide backbone, and outline the important role played by subtle geometric variations in the physical characteristics of supramolecular metallogels