Factors affecting the conformational modulation of flexible ligands in the self-assembly process of coordination polymers: synthesis, structural characterization, magnetic properties and theoretical studies of [Co(pda)(bix)]_n, [Ni(pda)(bix)(H₂O)]_n, [Cu(pda)(bix)₂(H₂O)₂]_n•8_nH₂O, [Co₂(μ-OH)(pda)(ptz)]_n •_nH₂O, [Co(hfipbb)(bix)_0.5]_n, and [Co(2,6-pydc)(bix)_1.5]_n •4_nH₂O

Abstract

Six new metal complexes with the formulae [Co(pda)(bix)]_n (1), [Ni(pda)(bix)(H₂O)]_n (2), [Cu(pda)(bix)₂(H₂O)₂]_n·8_nH₂O (3), [Co₂(μ-OH)(pda)(ptz)]_n·nH₂O (4), [Co(hfipbb)(bix)_0.5]_n (5), and [Co(2,6-pydc)(bix)_1.5]_n·4_nH₂O (6) have been synthesized by the reactions of Co(II), Ni(II), and Cu(II) salts with two flexible ligands 1,4-phenylenediaceticacid (H₂pda) and 1,4-bis (imidazole-1-ylmethyl)-benzene (bix) in the presence of coligands 5-(4-pyridyl) tetrazole (4-ptz), 4,4'(hexa-fluoroisopropylidene)bis(benzoicacid) (H₂hfipbb), and 2,6-pyridine dicarboxylic acid (2,6-H₂pydc) and characterized by single crystal X-ray diffraction analysis, IR spectroscopy, and thermogravimetric (TG) analysis. Because of the coordination geometry around the metal ions and the diverse coordination modes of the flexible ligands in combination with the rigid and flexible coligands, the obtained complexes show diverse structures from a one-dimensional (1D) chain to three-dimensional (3D) coordination polymers. 1, 4, 5, and 6 are Co(II) complexes in which Co(II) ions show tetrahedral coordination in 1, trigonal bipyrimidal coordination in 4, and octahedral coordination in 5 and 6. Complexes 2 and 3 are respectively Ni(II) and Cu(II) complexes in which the metal ions are present in octahedral coordination geometries. Factors affecting the conformational change of the flexible ligands in the self-assembly process of coordination polymers, such as, coordination geometry around the metal ions and geometry of the coligands have been systematically studied. The rotation of the bonds C(sp³)-C(sp²) and C(sp³)-N(sp³) in H₂pda and bix ligands causes different conformations (cis, trans, gauche); these conformations have been studied by measuring the torsion angle. A comparative study between the torsion angle of the particular conformation of the ligands and coordination geometry of metal ion/geometry of the coligand has been undertaken. All the possible cis, trans, and gauche conformations of the flexible ligands have been obtained/observed in our complexes. Theoretical calculations were performed to determine the energies of the different conformations of the flexible ligands. The electronic properties of these complexes have also been investigated in the solid state at room temperature. Finally, the temperature-dependent magnetic studies for compounds 4 and 5 are described.