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 toπ→π^* and n→π^* 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)→Ag(II) process, and the reduction wave may be attributable to the Ag(II)→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.