Seven-coordinate Mn(II) and Co(II) complexes of the pentadentate ligand 2,6-diacetyl-4-carboxymethyl-pyridine bis(benzoylhydrazone): Synthesis, crystal structure and magnetic properties

The metal complexation properties of a functionalized N3O2 donor ligand H2L2, where H2L2 stands for 2,6-diacetyl-4-carboxymethyl-pyridine bis(benzoylhydrazone), are investigated by structural and spectroscopic (IR, ESI-MS and EPR) characterization of its Mn(II) and Co(II) complexes. The ligand H2L2 is observed to react essentially in the same fashion as its unmodified parent H2L1 producing mixed-ligand [M(H2L2)(Cl2)] complexes (M = MnII (1), CoII (3)) upon treatment with MCl2. Complexes [M(HL2)(H2O)(EtOH)]BPh4 (M = Mn 2, M = Co 4), incorporating the supporting ligand in the partially deprotonated form (HL2)−, are formed by salt elimination of the [M(H2L2)(Cl2)] compounds with NaBPh4. Compounds 2 and 4 are isostructural featuring distorted pentagonal–bipyramidal coordinated MnII and CoII ions, with the H2O and EtOH ligands bound in axial positions. Intermolecular hydrogen bonding interactions of the type M–OH2⋯O–M involving the H2O ligands and the carbonyl functions of the supporting ligand assembles the complexes into dimers. Temperature-dependent magnetic susceptibility measurements (2–300 K) show a substantially paramagnetic Curie behavior for the Mn2+ compound (2) influenced by zero-field splitting and significant orbital angular momentum contribution for 4 (high-spin CoII). The exchange coupling across the MnII–OH2⋯O–MnII bridges in 2 was found to be less than 0.1 cm−1, suggesting that no significant intradimer exchange coupling occurs via this path.