Structures and magnetism of azide- and carboxylate-bridged metal(ii) systems derived from 1,2-bis(N-carboxymethyl-4-pyridinio)ethane

Four coordination polymers of different transition metal ions with azide and the zwitterionic dicarboxylate ligand 1,2-bis(N-carboxymethyl-4-pyridinio) ethane (bcpe) were synthesized, and structurally and magnetically characterized. They are formulated as [M₂(bcpe)(N₃)₄] ·H₂O (M = Mn, 1; Co, 2; and Ni, 3) and [Cu₃(bcpe) (N₃)₆]·(H₂O)₂ (4). In the isomorphous compounds 1-3, octahedral metal ions are linked by the mixed ( ₂-syn,syn-COO)(₂-EO-N₃)₂ triple bridges (EO = end-on) to give anionic uniform chains, which are cross-linked by the cationic bis(pyridinium) spacers to produce two-dimensional coordination layers. Magnetic studies demonstrated that the magnetic coupling through the mixed triple bridge is antiferromagnetic in the Mn(ii) compound (1) but ferromagnetic in the Co(ii) and Ni(ii) species (2 and 3). Compound 4 also consists of two-dimensional coordination layers in which Cu(ii) chains are interlinked by the organic spacers, but the chain has mixed ₂-EO- azide, ₃-EO-azide and ₂-O_carboxylate bridges and features tetranuclear dicubane-based units sharing Cu ions or alternatively linear trinuclear units connected by long axial Cu-N/O bonds. Magnetic studies suggested that the Cu(ii) ions linked by the double azide bridges in the equatorial-equatorial fashion are strongly coupled to give a ferromagnetic S = 3/2 ground state for the trinuclear units, with very weak antiferromagnetic interactions through the equatorial-axial bridges between the units.