Synthetic, X-ray Crystallographic Structural, Non-isothermal Kinetics And Spectral Studies On Rare Earth Complexes Of Homoveratrumic Acid

The great interests in the design and synthesis of metal-organic coordination polymers in supramolecular and materials chemistry, is based on their traits of functional easily and structural levity. By choice of appropriate synthesis methods and versatile organic ligands, numerous coordination polymers have been synthesized as molecular sieves. It also has the promising applications in fields such as catalysis,, molecular magnets, non-linear optical materials, and so on. The rare earth metal ions usually have larger radii and higher coordination numbers, comparing with the transitional metal ions. So the formation of rare earth complex structures is hard to control and forecast. However, the rare earth complexes still attract the researchers' interests for the sake of their intriguing applications in fields of optical, electronic and magnetic characteristics.In this regard, fifteen rare earth complexes were synthesized with 3, 4-dimethoxyphenylacetic acid (HDMPA, homoveratrumic acid), as the first ligand 1,10-phenanthroline(phen), as the ancillary ligand, and trivalent rare earth compounds, by the hydrothermal method and solution method. Eleven single crystals and four powders were obtained.(1) Rare earth complexes 1-15 ([RE(DMPA)₃phen]₂ RE = La, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb, Lu, Y, Ce, Pr, Eu, Tb) were synthesized with 3, 4-dimethoxyphenylacetic acid (HDMPA, homoveratrumic acid), 1,10-phenanthroline (phen), and trivalent rare earth compounds, by the hydrothermal method and solution method. The complexes were characterized by the element analysis, ¹HNMR spectra, IR spectroscopy thermal analysis and X-ray single crystal diffraction. All of the fifteen complexes have the same constitute: [RE(DMPA)3phen]2.(2) Similarity between the powder diffraction patterns of complex 1-11 indicated that these complexes are isostructural. So only the structure of complex 1 was described in detail. There are three coordination modes of the anion of HDMPA ligand in complexes 1. It is a binuclear complex with two coordination environments of centre metal, which can be described as a distorted monocapped square antiprism and a distorted tricapped trigonal prism. The intermolecular hydrogen bond and weakπ…πaromatic interactions contribute to a 3D supramolecular structure and stabilize it.(3) The thermal decomposition kinetic mechanisms of these ternary complexes were studied by non-isothermally TG method, using complex 14 as an example. The data from non-isothermally experiments were analyzed by ACHAR and COATS-REDFERN method. And the activation energy, pre-exponential factor and kinetic equations of the three stages were obtained. The kinetic mechanism of first step can be described as Three-dimensional diffusion (spherical symmetry, 2D3). The kinetic equations are ln{(dα/dt/[3/2(1-α)^2/3[1-(1-α)^1/3]^-1])=lnA-E/RT and ln{[1-(1-α)^1/3]²/T²)=ln(AR/βE)-E/RT. The last two processes are governed by three-dimensional diffusion and the kinetic equations are ln{(dα/dt)/[3/2(1-α)^4/3[1/(1-α)^1/3-1]^-1])=lnA-E/RTå’Œln([1/(1-α)^1/3-1]^1/2/T²)=ln(AR/βE)-E/RT。(4) The UV absorption and emission spectrum of the two ligands, complex 14 and complex 15 were studied. In the UV absorption spectrum, it was found that all of the four compounds exhibit similar spectral profiles, and the rare earth complexes are attributable to metal to ligand charge transfer (MLCT) transitions. The complex 14 has a strong fluorescence, with ⁵D₀→⁷F₀,⁵D₀→⁷F₁,⁵D₀→⁷F₂,⁵D₀→⁷F₃ and ⁵D₀→⁷F₄transitions in 580, 593, 620, 688 and 697 nm, respectively. The intensity of ⁵D₀→⁷F₂is much stronger than of ⁵D₀→⁷F₁, that is the electric dipole transition probality is larger than the magnetic dipole transition probility, which indicate that the Eu(â…¢) ions are not in an inversion center. And in the emission spectrum of complex 15, there are four main peaks, 489, 545, 584 and 622 nm, respectively, corresponding to ⁵D₄→⁷F₆,⁵D₄→⁷F₅,⁵D₄→⁷F₄ and ⁵D₄→⁷F₃ transitions of Tb(â…¢). The intensity of ⁵D₄→⁷F₅ is much stronger than others. All of the results viewed that the two ligands can transfer the energy to the central metal efficiently, and the two ligands can sensitize the central metal.