Acta Physico-Chimica Sinica

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Crystal Structure, Thermal Decomposition Mechanism and Properties of Lanthanide Supramolecular Complexes Based on 2,4,6-Trimethylbenzoic Acid and 5,5’-Dimethyl-2,2’-bipyridine

Mengxue Zhou1,2, Ning Ren3, Jianjun Zhang1,2   

  1. 1 Testing and Analysis Center, Hebei Normal University, Shijiazhuang 050024, P. R. China;
    2 College of Chemistry & Material Science, Hebei Normal University, Shijiazhuang 050024, P. R. China;
    3 College of Chemical Engineering & Material, Hebei Key Laboratory of Heterocyclic Compounds, Handan University, Handan 056005, Hebei Province, P. R. China
  • Received:2020-04-27 Revised:2020-05-15 Accepted:2020-05-15 Published:2020-05-20
  • Supported by:
    The project was supported by the National Natural Science Foundation of China (21803016).

Abstract: Six ternary lanthanide complexes formulated as [Ln(2,4,6-TMBA)3(5,5’-DM-2,2’-bipy)]2 (Ln = Pr 1, Nd 2, Sm 3, Eu 4, Gd 5, Dy 6; 2,4,6-TMBA = 2,4,6-trimethylbenzoate; 5,5’-DM-2,2'-bipy = 5,5'-dimethyl-2,2'-bipyridine) have been synthesized under solvothermal conditions and characterized by single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, etc. The results of crystal diffraction analysis show that complexes 1–6 are binuclear units, crystallizing in the triclinic space group. Complexes 1–5 are isostructural, and each of the central metal ions has a coordination number of 9. The asymmetric unit of complexes 1–5 consists of one Ln3+, one 5,5’-DM-2,2’-bipy ligand, and three 2,4,6-TMBA- moieties with three coordination modes: chelation bidentate, bridging bidentate, and bridging tridentate. The coordination geometry of Ln3+ is distorted monocapped square antiprismatic. The binuclear units of complexes 1–5 form a one-dimensional (1D) supramolecular chain along the c-axis via ππ stacking interactions between the 2,4,6-trimethylbenzoic acid rings. The 1D chains are linked to form a supramolecular two-dimensional (2D) sheet in the bc plane via ππ stacking interactions between the pyridine rings. Although the molecular formulae of complex 6 and complexes 1–5 are similar, the coordination environment of the lanthanide ions is different in the two cases. The asymmetric unit of complex 6 contains a Dy3+ ion coordinated by a bidentate 5,5’-DM-2,2’-bipy and three 2,4,6-TMBA- ligands adopting bidentate and bridging bidentate coordination modes. The Dy3+ metal center has a coordination number of 8, with distorted square antiprismatic molecular geometry. The binuclear molecule of 6 is assembled into a six-nuclear unit by ππ weak staking interactions between two 5,5’-DM-2,2’-bipy ligands; then, adjacent six-nuclear units form a 1D chain via offset ππ interactions between 5,5’-DM-2,2’-bipy ligands on different adjacent units. The adjacent 1D chains are linked by C―H…O hydrogen bonding interactions to form a 2D supramolecular structure. The thermal stability and thermal decomposition mechanism of all the complexes are investigated by the combination of thermogravimetry and infrared spectroscopy (TG/FTIR) techniques under a simulated air atmosphere in the temperature range of 298–973 K at a heating rate of 10 K·min-1. Thermogravimetric studies show that this series of complexes have excellent thermal stability. During the thermal decomposition of the complex, the neutral ligand is lost first, followed by the acid ligand, and finally, the complex is decomposed into rare earth oxides. The three-dimensional infrared results are consistent with the thermogravimetric results. The photoluminescence spectra of complex 4 show the strong characteristic luminescence of Eu3+. The five typical emission peaks at 581, 591, 621, 651, and 701 nm correspond to the 5D07F0, 5D07F1, 5D07F2, 5D07F3, and 5D07F4 electronic transitions of Eu3+, respectively. The emission at 621 nm is due to the electric dipole transition 5D07F2, while that at 591 nm is assigned to the 5D07F1 the magnetic dipole transition. The lifetime (τ) of complex 4 is calculated as 1.15 ms based on the equation τ = (B1τ12 + B2τ22))/(B1τ1 + B2τ2), and the intrinsic quantum yield is calculated to be 45.1%. Further, the magnetic properties of complex 6 in the temperature range of 2–300 K are studied under an applied magnetic field of 1000 Oe.

Key words: Lanthanide supramolecular complexes, Crystal structure,Thermal behavior, Luminescence spectrum, 2,4,6-Trimethylbenzoic acid

MSC2000: 

  • O642