Metal complexes with N-(2-pyridylmethyl)iminodiacetate: from discrete polynuclear compounds to 1D coordination polymers

The reactions of M(ClO4)(2)center dot 6H(2)O (M = Mn2+, Co2+ and Ni2+) with N-(2-pyridylmethyl)-iminodiacetic acid (H(2)pmida) gave three discrete polynuclear complexes {Na[Mn-3(pmida)(3)(H2O)(3)]}ClO4 (1), {Na[Co-3(pmida)(3)(H2O)(3)](H2O)}ClO4 (2) and [Co(H2O)(6)][Co-3(pmida)(3)(H2O)(3)](2)(ClO4)(2)center dot 4H(2)O (3) and two 1D polymers {[Co(pmida)(H2O)]center dot H2O}(n) (4) and {[Ni(pmida)(H2O)]center dot H2O}(n) (5). All complexes were characterized by elemental analysis, infrared (IR) spectroscopy and single-crystal X-ray diffraction analysis. Complexes 1 and 2 show similar structures with an unprecedented trinuclear unit [M-3(pmida)(3)(H2O)(3)] in which the three six-coordinated divalent metal ions are joined by three coplanar syn-anti bridging carboxylate groups of three ligands into a closed planar twelve-membered ring with one Na+ ion located on the C-3 axis. Complex 3 possesses two kinds of coordination units: mononuclear cation [Co(H2O)(6)](2+) and trinuclear unit [Co-3(pmida)(3)(H2O)(3)]. The Co2+ and Ni2+ ions in complexes 4 and 5, respectively, present distorted octahedral geometries and are bridged by anti-anti bridging carboxylate groups of the ligands to form 1D chains. Compounds 1-5 display a structure variation from discrete polynuclear to 1D coordination polymer along Mn(II), Co(II) to Ni(II), and a dependence of the formation of discrete compound or coordination polymer on the identity of the metal ion. The magnetic investigations of 1-3 were also carried out. The negative Weiss constants and coupling constants obtained from two kinds of fitting models indicate the presence of dominant antiferromagnetic exchanges mediated by the syn-anti bridging carboxylate groups between the metal centers of three compounds.