Mixed azide-carboxylate bridged tri- and tetranuclear Mn^II clusters in coordination polymers derived from a zwitterionic dicarboxylate ligand: Structures and magnetism

Two Mn^II coordination polymers with azide and the zwitterionic dicarboxylate ligand bis(N-carboxymethyl-4-pyridinium) (bcp) were synthesized, and structurally and magnetically characterized. They are formulated as [Mn ₃(bcp)₂(N₃)₂(SO₄) ₂(H₂O)₄]·6H₂O (1) and [Mn₄(bcp)₂(N₃)₈(H₂O) ₂]·4H₂O (2). Compound 1 contains anionic linear [Mn₃(N₃)₂(COO)₄(SO₄) ₂(H₂O)₄]^4- units with simultaneous μ₂-EO (end-on) azide, sulfate and carboxylate bridges, while compound 2 contains [Mn₄(COO)₄(N₃) ₈(H₂O)₂]^4- clusters with mixed μ₂-EO azide, μ₃-EO azide and carboxylate bridges. In these compounds, the anionic tri- or tetranuclear units are linked into coordination chains by the cationic bipyridinium spacers, and are also hydrogen bonded into chains by double O-H⋯O bridges. Magnetic analyses were carried out on temperature-variable susceptibility data for both compounds, and also on isothermal magnetization data for 1. It is revealed that all the mixed double and triple bridges, [(EO-N₃)(COO)(SO₄)] in 1, [(COO)(EO-N₃)₂] and [(COO)(EO-N₃)] in 2, transmit antiferromagnetic coupling between Mn^II ions. The [(EO-N₃)₂] bridge in 2, with Mn-N-Mn = 96.6°, also transmits antiferromagnetic coupling, providing the first example in the antiferromagnetic regime predicted theoretically for double EO-azide bridges between Mn^II ions. The double hydrogen bonding [(O-H⋯O) ₂] bridges in both compounds induce weak antiferromagnetic interactions.