Ligand-Directed Assembly of Asymmetric 1,3,5-Triazine Ligands Containing Pyridyl and Hydroxyl Groups

Seven complexes based on the ligands 4,6-bis(4-pyridyl)-1,3,5-triazin-2-ol (4-HOBPT) and 4,6-bis(3-pyridyl)-1,3,5-triazin-2-ol (3-HOBPT), namely, [Fe(4-OBPT)₂(H₂O)₂]·6H₂O (2), M(4-OBPT)(INA)(H₂O) (M = Co(II) (3a), Fe(II) (3b), Zn(II) (3c), HINA = isonicotinic acid), [Co(3-OBPT)₂(H₂O)₂]·H₂O (4), and [M(3-OBPT)(NA)(H₂O)]·0.5CH₃OH (M = Co(II) (5a), Ni(II) (5b), HNA = nicotinic acid), have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction. The thermal stability and coordination model of the ligands are extremely dependent on the reaction temperature and the presence of metal ions. In the range of 90−120 °C, unique complexes 2 and 4 were obtained and the ligands are a N,O-bimonodentate coordination model. At 130 °C, complexes 3 and 5 were afforded in yields of 12−48% depending on the metal ions, in which both of the ligands act as a N,N,O-trimonodentate triangular ligand and the INA and NA anions are generated in situ by hydrolysis of the corresponding parents. The hydrolysis reaction is significantly promoted by Cu(II) ion even at a relative low temperature. Both 4-OBPT and 3-OBPT ligands have different geometries and exhibit ligand-directed supramolecular assembly. In 4-OBPT, the coordination sites locate at the vertexes of the triangular pane; it connects two or three metal ions generating a (4,4) or (6,3) two-dimensional (2D) sheet. Differently, the 3-OBPT ligand chooses to form a M₂L₂ metallamacrocycle as a building unit generating a one-dimensional chain via sharing the metal ions or a (4,4) 2D net due to the coordination nitrogen atoms positioned on the edges of the triangular pane.