Of Heterocyclic Carboxylic Complexes, Structure And Properties

In this paper, by treating [(4,6-dimethyl-2-pyrimidinyl)thio] acetic acid (Hdpmta) (L1) or (1,3,4-thiadiazole-2,5-diyldithio)diacetic acid (H2tzda)(L2) with some transition metal ions directly or under the help of some subsidiary ligands, five new metal-organic complexes have been successfully obtained at room temperature conditions. Furthermore, their fluorescence properties and electric properties were also investigated. The results are outlined as follows:(1) Five transition metal(II) complexes have been synthesized. Their formulas are as follows: [Zn(dpmta)₂(1,2-DAP)]·2H₂O (1), [Ag(dpmta)]_n (2), [Cu₂(μ₂-dpmta)₄(CH₃OH)₂]·3CH₃OH (3), {[Cu(tzda)(H₂O)₃]·4H₂O}_n (4) and [Cu(tzda)(phen)(H₂O)]_n (5).(2) The structures of the above complexes have been determined by X-ray single crystal diffraction. Complex 1 shows mononuclear structure. The molecules are linked by hydrogen bonds andÏ€-Ï€packing, forming the final three-dimensional supramolecular network structure. Complex 2 has a 2D-layered network. Each Ag(I) ion is in a distorted tetrahedral coordination environment, surrounded by one carboxyl-oxygen atom from one ligand, one pyridine-nitrogen atom from another ligand, and one carboxyl-oxygen atom and a sulfur atom from the third ligand, respectively. Each ligand acts as a tetradentate ligand and coordinates to three different Ag atoms, forming a 2D brick-wall network. In complex 3, two dinuclear copper(II) clusters with paddle-wheel cage structure based on M₂(CO₄) are formed. Complexes 4 and 5 display similar one-dimensional chains. Every Cu atom is five-coordinated. The tzda ligand biridges two metal ions in bis-monodentate fashion in 4 and 5, generating an infinite 1D chain.(3) The UV-vis absorption spectra and luminescent properties of ligand Hdpmta, 1 and 2 are investigated. The peak positions and spectral shape of 1 and 2 in UV-vis spectra are similar to those of Hdpmta. These absorption peaks for the ground state could be assigned toÏ€â†'Ï€^* and nâ†'Ï€^* transitions of intraligand. Under the same excitation of ultraviolet light, Hdpmta displays one intense emission peak and two weak peaks while 1 and 2 exhibit one intense emission peak and two strong peaks, respectively. Since the emission peak positions of 1 and 2 are similar to those of free ligand Hdpmta, these emission bands in the spectra of 1 and 2 may also attribute to the intraligand transitions.(4) The cyclic voltammogram behaviors of Hdpmta and complexes 1～5 were investigated. In DMF or water, the ligand L1 displays a well-defined cathodic peak and 1 has the similar electrochemical behavior as that of L1. The cyclic voltammety of 2 gives one-pair quasi-reversible redox peaks with oxidation potential at -0.766V as well as reduction potential at -0.869V, whilst the reduction current and oxidation current are -1.6316×10^-6A and -9.513×10^-7A, respectively. The oxidation wave is caused by the Ag(I)â†'Ag(II) process, and the reduction wave may be attributable to the Ag(II)â†'Ag(I) process accompained by the redox process of ligand since the reduction current value is much larger than that of oxidation current. For 3, it shows one-pair quasi-reversible redox peaks ([Cu(dpmta)₂CH₃OH]₂(?) [Cu(dpmta)₂CH₃OH]₂^2-) and a couple of peaks caused by adsorption at the electrode surface between the potentials of-1.6 and +0.4V. In the potential range of-0.4 ⁺0.4V, the redox behavior of the complex 3 shows one pair quasi-reversible redox peaks. The quasi-reversible redox reaction is diffusion-controlled process and the electron transfer number n is 2. The electrochemical studies on 4 and 5 in water show that their redox processes are quasi-reversible (2Cu²⁺+2e(?) 2Cu⁺) and controlled by the diffusion in the potential range of -0.6-+0.4V.