三例同构稀土/钨氧簇化合物二维红外相关光谱研究

doi:10.3964/j.issn.1000-0593(2020)07-2104-06

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Abstract Polyoxometalates (POMs) are an important class of metal-oxygen clusters, which are composed of cluster anions bridged by pre-transition metals (V^Ⅴ, Nb^Ⅴ, Ta^Ⅴ, Mo^Ⅵ, Mo^Ⅴ and W^Ⅵ) and oxygen. On the basis of previous studies (H₂en)₂{SiW₁₁O₃₉Sm(H₂O)₂}·(H₃O)·6H₂O, we changed rare earth salts and successfully synthesized three isomorphic crystals (H₂en)₂{SiW₁₁O₃₉Ln(H₂O)₂}·(H₃O)·6H₂O[Ln=Ce(1), Pr(2), Nd(3)], X-ray single crystal diffraction experiment measured that the four belong to the triclinic system, the P1 space group, and the unit cell parameters are consistent, indicating that they have the same crystal structure. XRD experiments show that they have the same peaks, indicating that the substances are identical. Due to the same cluster anions and only the substituted rare earth ions are different, these isomorphic crystals exhibit similar phenomena in many characterization methods, for example the similar absorption curves in 1D infrared spectroscopy: the vibrational absorption of anion skeleton belonging to Keggin cluster appears at 1 039, 949, 889 and 787 cm^-1, and the absorption peaks of ν_as(O—H) and δ(O—H) occur in the vicinity of 3 600～3 300 and 1 600～1 630 cm^-1. The stretching vibration peaks of N—H and C—H ligands of ethylenediamine were observed in 3 277, 2 927 and 2 855 cm^-1. However, the two-dimensional infrared correlation spectra under magnetic perturbation are sensitive to the magnetic field response. The two-dimensional infrared correlation spectra under thermal perturbation are easy to capture the subtle changes of hydrogen bond vibration modes. Therefore, two-dimensional infrared spectroscopy can be used for fine determination of molecular structure, and the comparative analysis of two-dimensional infrared spectroscopy of such isomorphic polyoxotungstate has not been reported. Two-dimensional infrared correlation spectra under magnetic perturbation show that compound 1 has response peaks at 468, 560 and 810 cm^-1, which belong to ν_as(Ln—O), skeleton ν(W—O) and ν_as(W—O_b), respectively. Compound 2 exhibits as (Ln—O) at 450, 464, 475 cm^-1, and the response peak at 570, 675 cm^-1 belongs to skeleton ν(W—O). The response peaks of compound 3 at 452, 468, 472 cm^-1 belong to ν_as (Ln—O), and 518, 533, 545, 565, 695 cm^-1 belong to skeleton ν(W—O). The number of response peaks of compound 1, 2, 3 belongs to skeleton ν(W—O) increases. This is due to the valence electron configurations of Ce³⁺, Pr³⁺, Nd³⁺ are 4f¹, 4f², 4f³. The valence electron number increases, so the influence of magnetic particle Ln³⁺ on adjacent W—O bonds increases. Two-dimensional infrared correlation spectra under thermal perturbation show that compounds 1, 2 and 3 have as (Ln—O) response peaks at about 400 cm^-1, and the response peaks of ν_as (W—O_b) and ν_as (W—O_d) appear at 810, 860 and 940 cm^-1, which are due to the same cluster skeleton and the same hydrogen bond. However, the most strong peak positions of ν_as (W—O) belong to compounds 1, 2, 3 appear at 810, 850, 855 cm^-1, which may because the different polarities of Ln ions substituted on cluster skeletons have different effects on the dipole moments of adjacent W—O bonds. The similarities and differences of these isomorphic rare earth substituted polyoxotungstate can be well analyzed by two-dimensional infrared spectroscopy.

Key words：Polyoxotungstates; Keggin structure; Two dimensional infrared correlation spectral

Received: 2019-06-11 Accepted: 2019-10-30

ZTFLH:

O657.3

Corresponding Authors: CHEN Yi-ping E-mail: ypchen@fzu.edu.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)07-2104-06 OR https://www.gpxygpfx.com/EN/Y2020/V40/I07/2104