Synthesis and characterization of four novel manganese (II) chains formed by 4,4′-azobis (pyridine) and benzoate or nitrobenzoates: stabilization of unusual ladder structures

Abstract

Four new manganese (II) coordination polymers: [Mn(4,4′-azpy)(C₆H₅COO)₂](4,4′-azpy)_0.5 (1), [Mn(4,4′-azpy)(p-(NO₂)C₆H₄COO)₂] (2), [Mn(4,4′-azpy)(m-(NO₂)C₆H₄COO)₂] (3) and [Mn(4,4′-azpy)(o-(NO₂)C₆H₄COO)₂(H₂O)₂] (4), where 4,4′-azpy = 4,4′-azobis(pyridine), have been synthesized by self-assembly of MnX₂(X = benzoate, p-, m- or o-nitrobenzoates) together with 4,4′-azpy. All four complexes were characterized by elemental analyses, IR spectroscopy, thermal analyses, single-crystal X-ray diffraction analyses and variable-temperature magnetic measurements. The structural analyses reveal that complexes 1, 2 and 3 feature a 1D molecular ladder formed by syn–syn (complex 1) or syn–anti (complexes 2 and 3) carboxylate-bridged dimeric Mn (II) units which are joined together by 4,4′-azpy ligands. In complex 1, these ladders assemble with the help of π–π and C–H⋯π interactions to form a nanoporous framework that incorporates non coordinated 4,4′-azpy molecules by exploiting host–guest C–H⋯π and hydrogen bonding interactions. Complex 2 presents a 3D supramolecular framework by π–π and CH⋯π interactions, whereas, complex 3 having a similar ladder structure to 1 and 2, forms a 2D grid through π–π interactions. On the other hand, complex 4 is a 4,4′-azpy bridged fish-bone chain of carboxylate-coordinated mononuclear manganese (II) units, which are linked together by strong hydrogen bonds to form a 2D structure. Variable-temperature (2–300 K) magnetic susceptibility measurements show the presence of weak antiferromagnetic interactions within the discrete Mn–(OCO)₂–Mn dimers for complexes 1, 2 and 3 that have been fitted with a S = 5/2 dimer model (J = −0.8, −0.5 and −0.4 cm⁻¹ respectively). The magnetic data of complex 4 can be reproduced with a S = 5/2 monomer model including a Zero Field Splitting (|D| = 1.7 cm⁻¹).