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)(Cl-2)] complexes (M = Mn-II (1), Co-II (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)(Cl-2)] compounds with NaBPh4. Compounds 2 and 4 are isostructural featuring distorted pentagonal-bipyramidal coordinated Mn-II and Co-II ions, with the H2O and EtOH ligands bound in axial positions. Intermolecular hydrogen bonding interactions of the type M-OH2 center dot center dot center dot 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 Co-II). The exchange coupling across the Mn-II-OH2 center dot center dot center dot O-Mn-II bridges in 2 was found to be less than 0.1 cm(-1), suggesting that no significant intradimer exchange coupling occurs via this path. (C) 2011 Elsevier B. V. All rights reserved.