English

Controlled Synthesis of Lanthanide-titanium Oxo Clusters EuTi₆, EuTi₇ and La₂Ti₁₄

• Yang Yamei ,
• Lun Huijie ,
• Long Lasheng ,
• Kong Xiangjian ^, ,
• Zheng Lansun

• Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, P. R. China

Corresponding author: Kong Xiangjian, xjkong@xmu.edu.cn

• Received Date: 02 December 2019
  Accepted Date: 08 January 2020
  Revised Date: 31 December 2019
  Available Online: 15 September 2020
• Fund Project:

  The project was supported by the National Natural Science Foundation of China (21871224, 21673184, 21431005, 21721001)

Abstract: As opposed to nanoparticles, atomically precise metal clusters possess a well-defined surface and crystal structure, which aids in understanding the relationship between the structure and chemical reactivity at the atomic level. As an interesting subgroup of metal cluster compounds, heterometallic lanthanide-titanium oxo clusters (LnTOCs) have attracted extensive attention due to their interesting chemical properties. However, the controlled precise synthesis of LnTOCs remains a great challenge because of the intense hydrolysis of Ti⁴⁺ ions and the competitive coordination of Ln³⁺ ions. Owing to this synthetic difficulty, high-nuclearity LnTOCs are very rare, which obstructs further studies on their properties. Choosing the appropriate chelating ligands should be an effective strategy to synthesize LnTOCs because chelating ligands can reduce the degree of hydrolysis of Ti⁴⁺ ions. Herein, four new LnTOCs, formulated as [EuTi₆(μ₃-O)₃(OC₂H₅)₈(dtbsa)₆(Hdtbsa)]·(C₂H₅OH) (1), [EuTi₇(μ₃-O)₃(μ₂-OH)₂(OⁱPr)₉(dtbsa)₆(Hdtbsa)Cl]·(HOⁱPr)₃ (2), [EuTi₇(μ₃-O)₃(μ₂-OH)₂(OⁱPr)₈(dtbsa)₇(Hdtbsa)]·(HOⁱPr)₃ (3), and [LaTi₇(μ₃-O)₃(μ₂-OH)₂(OC₂H₅)₈(dtbsa)₇(Hdtbsa)]₂·(C₂H₅OH)₄ (4), were prepared by a solvothermal method via the reaction of 3, 5-di-tert-butylsalicylic acid (H₂dtbsa), rare-earth salts, and Ti(OⁱPr)₄. Single-crystal analysis showed that the heptanuclear compound 1 contains a EuTi₆ metal core featuring a trigonal prismatic structure, wherein Eu³⁺ is located at the center of the prism formed by six Ti⁴⁺ ions. The metal core structure of octanuclear compounds 2 and 3 can be viewed as the EuTi₆ unit in 1 connected to another Ti⁴⁺ on one side of the triangular prism. The metal framework of Ln₂Ti₁₄ in 4 can be regarded as a dimer of EuTi₇ in 2. UV-Vis diffuse reflectance spectra revealed that the band gaps of 1, 2, and 3 (2.35, 2.07, and 2.16 eV, respectively) are significantly smaller than that of anatase (3.2 eV). The results of photoelectric tests indicated that the three clusters show an obvious photoelectric response, and the charge separation efficiency of 1 and 2 was better than that of 3. In order to explore the applications of these compounds to photocatalysis, H₂ production by light-driven water splitting under irradiation by a 300 W Xe lamp (300–800 nm) in an aqueous methanol solution (20 mL, 10%) was attempted. The H₂ production rates for 1, 2, and 3 were 112, 106, and 87 μmol∙h^-1∙g^-1, respectively, which were higher than that obtained with the commercial P25. Powder X-ray diffraction (PXRD) spectra and thermogravimetry (TGA) profiles confirmed the optical and thermal stability of the three clusters. This work not only provides a chelating ligand strategy for the synthesis of LnTOCs but also reveals their light-driven photocatalytic activity stemming from the small band-gap.

Key words:
• Lanthanide-titanium oxo cluster
•  /  Well-defined structure
•  /  Solvothermal method
•  /  H₂ production
•  /  Water splitting

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