Temperature-/solvent-dependent low-dimensional compounds based on quinoline-2,3-dicarboxylic acid: Structures and fluorescent properties

Abstract

A series of 0-D, 1-D, and 2-D metal–organic compounds through reactions of quinoline-2,3-dicarboxylic acid (2,3-H₂qldc) with transition metal salts MCl₂, namely, M(2,3-Hqldc)₂(H₂O)₂ (M = Co(1), Zn(4) and Cd(7)), [M(3-qlc)₂(H₂O)₂]_n (M = Co(2), Zn(5) and Cd(8)), M(2-qldc-3-OCH₃)₂(CH₃OH)₂ (M = Co(3) and Zn(6)) and [Cd(2,3-qldc-OCH₃)(μ₂-Cl)]_2n (9) (where, 3-Hqlc = quinoline-3-carboxylic acid and 2-qldc-3-OCH₃ = 3-(methoxycarbonyl)quinoline-2-carboxylic acid), were synthesized and characterized by elemental analysis, IR, thermogravimetric analysis (TG), and single-crystal X-ray diffraction. When the temperature ranged from room temperature to 70 °C, three isomorphous mononuclear complexes 1, 4 and 7 were obtained in H₂O/H₂O + CH₃OH. As the temperature rose further to above 90 °C, due to the decomposition of 2-position carboxyl group in ligand 2,3-H₂qldc, the same reactions, respectively, produced three isomorphous 2-D layer-like structures 2, 5 and 8 with 4⁴ topology in water. By contrast, when the mixed solvent of H₂O + CH₃OH at a 1 : 1 ratio (v/v) was applied, the three above-mentioned reactions respectively gave compounds 3, 6 and 9 with the 3-position esterification of 2,3-H₂qldc. Compounds 3 and 6 are mononuclear and isomorphous, while complex 9 has a 1-D double-stranded chain-like structure connected by two μ₂-Cl bridges. Obviously, these results reveal that the reaction temperature and solvent play a critical role in structural direction of these low-dimensional compounds. Meanwhile, the photoluminescent property of the selected compounds is also investigated.