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 Mn II or Co II . 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.