Hydrothermal Syntheses, Crystal Structures And Properties Of Metal Coordination Complexes Of 4-hydroxy-6-methylnicotinic Acid

In recent years, considerable attention has been devoted to the design and synthesis ofmetal coordination complexes due to their interesting photophysical properties which havepotential application in sensors, liquid crystalline materials, optical fiber lasers and amplifiers,luminescent label design for specific biomolecule interactions, magnetic molecular materialsand electroluminescent material. The various ligands, which contain N, O-donor atoms, suchas pyrazinecarboxylic acid, pyridinecarboxylic acid, imidazolecarboxylic acid, are particularlyvaluable in investigation.4-Hydroxy-6-methylnicotinic acid （HL） with carboxyl and hydroxy functional groups hasseveral interesting characteristics. First, the carboxylic and hydroxylic hydrogen atoms can bepartially or completely removed, so the ligand can not only act as a potential proton acceptor,but also a proton donor; second, it possesses multidentate coordination sites, the hydroxyloxygen, the pyridyl nitrogen and carboxyl oxygen atoms are all capable of coordinating tometal ions. But, up to now, only several coordination complexes with hydroxynicotinic ligandhave been reported. Herein, 8 metal coordination complexes have been synthesized andcharacterized. The main points are summarized as follows:1. A novel 1D coordination polymer, [La（L）₂（2H₂O） 2H₂O]_n（1）, has been synthesizedby hydrothermal reaction. Single-crystal X-ray analysis reveals the asymmetric unit of 1contains one lanthanide ion, two L ligands anions, two-coordinated water molecules and fourfree water molecules. Each lanthanide ion is in ten-coordination environment with a distorteddi-capped square-antiprismatic geometry. Furthermore 1D chains are connected by hydrogenbonds giving a 3D network.2. Six isostructural lanthanide complexes, [Ln₂（L）₆（H₂O）₂] 2H₂O （Ln= La （2）, Ce （3）,Nd （4）, Sm （5）, Pr （6）, Gd （7））, have been synthesized and characterized. Single-crystal X-rayanalyses suggest that six complexes are dimer in which each lanthanide ion is coordinated bynine oxygen atoms with a distorted tri-capped trigonal prismatic geometry. The binonuclearunits are further linked via hydrogen bonds giving a 3D supramolecular network. Thephotoluminescence properties of compounds 5 and 6 in the solid state at room temperaturehave been investigated. The complex 5 is demonstrated orange-red luminescence, while theluminescence spectrum of complex 6 is typical for the praseodymium complexes.3. A novel iron mononuclear coordination polymer, Fe[（L）₂（H₂O）₂] 2H₂O （8）, has beensynthesized and and characterized. The mononuclear units are further linked via hydrogenbonds giving a 3D supramolecular network.