Functional tetranuclear square grids synthesized by in situ oxidation of ligand

Abstract

Tetranuclear square grids, namely, [Cd4(L–O)4]·1.5DMF (1), [Ni4(L–O)4]·H2O (2), [Zn4(L–O)4]·3H2O (3), [Zn4(L–O)4] (4), [Cu4(L–O)4]·H2O (5), and {[Cu4(L–O)4][Cu4(L–O)4]}·2.5DMF·1.5H2O (6) (DMF=N,N-dimethylformamide, H2L–O=2,6-bis(pyrazine-2-carboxamido)pyridine-N-oxide), formed by the reactions of M ions (M=CdII, NiII, ZnII and CuII) with pyridyldicarboxamide ligands 2,6-bis(pyrazine-2-carboxamido)pyridine (H2L), was fully characterized. Notably, the central pyridines of ligands H2L were oxidized into pyridine-N-oxides under air conditions, producing the new anionic ligands (L–O)2− to fabricate those grids. In those square grids every octahedrally coordinated metal node is chelated by two doubly deprotonated ligands (L–O)2− in the mer configuration, and each ligand (L–O)2− acts as a rigid bis(tridentate) linker with the Ncarboxamide and one Npyrazine in each arm chelating metal ions plus the O atom in the pyridine-N-oxide group bridging the two bound M ions. The CdII and ZnII grids show uncommonly-excellent thermal stabilities and interesting green emissions. Magnetic studies upon grid [Ni4(L–O)4] suggest an antiferromagnetic coupling between the adjacent NiII mainly exchanged through μ–O with a coupling constant of J=−15.06cm−1.