Distinguishing Novel Sandwich-Type Tungsten Oxide Cluster Compounds Using Two-Dimensional Correlation Infrared Spectroscopy

Background Introduction

Polyoxometalates (POMs) are metal-oxygen cluster compounds linked through oxygen coordination with transition metal ions. They play significant roles in catalysis, optics, medicine, and magnetism. However, due to their structural diversity and complexity, the spectroscopic properties of POMs are often similar, making their characterization and differentiation highly challenging. Particularly, for single-crystal X-ray diffraction (SC-XRD), obtaining high-quality crystals is a prerequisite, which can be difficult for certain POMs.

In prior studies, two-dimensional correlation spectroscopy (2D-COS) has been proposed as a technique capable of detecting subtle differences in complex spectra via induced changes in variables such as temperature and magnetic fields. This method has seen extensive applications in fields like chemistry, environmental science, and pharmaceuticals. However, applying it to distinguish tungsten oxide cluster compounds remains in the exploratory phase.

In this context, a research team from the College of Chemistry at Fuzhou University and the Fujian Institute of Research on the Structure of Matter at the Chinese Academy of Sciences investigated the molecular structures and vibrational characteristics of two novel sandwich-type tungsten oxide cluster compounds containing manganese (Mn) and cobalt (Co). Using two-dimensional correlation infrared spectroscopy (2D-COS-IR) combined with single-crystal X-ray diffraction, the team aimed to reveal the spectral response differences from magnetic and thermal perturbations.

Source of the Study

This study, titled “A spectroscopic method for distinguishing two novel sandwich-type tungsten oxide cluster compounds,” was authored by Wen-Jun Mi, Wen-Chao Bi, Ming-Ze Meng, Yi-Ping Chen, and Yan-Qiong Sun. Published in the journal Applied Spectroscopy, Volume 79, 2025, it explores the application of 2D-COS-IR in distinguishing structurally similar tungsten oxide cluster compounds. The work was financially supported by the National Natural Science Foundation of China and the Natural Science Foundation of Fujian Province.

Research Workflow and Experimental Details

Synthesis and Sample Preparation

The research team synthesized two novel sandwich-type tungsten oxide cluster compounds via hydrothermal methods, designated as Compound 1 (H4(C6H12N2H2)3{Na(H2O)2[Mn2(H2O)(GeW9O34)]}2) and Compound 2 (H2(C6H12N2H2)3.5{Na3(H2O)4[Co2(H2O)(GeW9O34)]2}·17H2O). Both compounds contained the cluster anion [GeW9O34]10−, coordinated with transition metals (Mn or Co), and were stabilized by triethylenediamine as an organic ligand.

Single-Crystal X-ray Diffraction (SC-XRD)

Low-temperature structural analyses of the single crystals were performed using Rigaku Saturn 724 and Bruker APEX3 CCD diffractometers. The results revealed that both compounds crystallized in the hexagonal crystal system with the P63/m space group, exhibiting typical sandwich-type structures.

In Compound 1, the [GeW9O34]10− cluster structure featured a triple-deficient Keggin framework, bridged by Mn2+ ions to form [Mn2(H2O)(GeW9O34)]212−. Further analysis showed that Na+ ions stabilized the cluster externally through a seven-coordination mode. Compound 2 exhibited a similar structure, except that Mn2+ was replaced by Co2+, with a distinct hydrogen bonding network forming a three-dimensional framework.

Infrared Spectroscopy (IR)

Fourier transform infrared spectroscopy (FT-IR) was employed for initial characterization of the molecular vibrations of the two compounds. Characteristic absorption peaks of the Keggin cluster framework were observed in the range of 500–1000 cm⁻¹ (corresponding to W–O stretching vibrations), while the N–H and C–N vibrational peaks of the ligands appeared in the ranges of 1000–2000 cm⁻¹ and 3000–3500 cm⁻¹, respectively.

While one-dimensional IR spectra demonstrated similarities between the two compounds, two-dimensional correlation spectroscopy revealed significant differences in their responses to magnetic and thermal perturbations.

Two-Dimensional Correlation Infrared Spectroscopy Analysis (2D-COS-IR)

Magnetic Field Perturbation Study

When exposed to magnetic field variations ranging from 5 to 50 milliteslas (mT), Compound 1 exhibited a prominent response peak at 870 cm⁻¹ (νas(W–Ob–W), asymmetric stretching vibration of W–O bonds), while Compound 2 displayed a distinct response at 920 cm⁻¹ (νas(W=Od), terminal W=O bonds). This difference was attributed to the distinct transition metal ions (Mn2+ and Co2+) in the compounds, resulting in varied dipole moment responses of functional groups to the magnetic field.

Thermal Perturbation Study

Within the temperature range of 50–120°C, Compound 1 demonstrated prominent response peaks for W–O stretching vibrations at 760 cm⁻¹ and 865 cm⁻¹. These changes were likely related to the hydrogen bonding interactions between the cluster anion and triethylenediamine. In contrast, Compound 2 exhibited strong response peaks at 900 cm⁻¹ and 950 cm⁻¹ for νas(W=Od). The study indicated that differing hydrogen bonding networks played a significant role in influencing the vibrational modes of the tungsten-oxygen bonds.

Research Results and Significance

Key Conclusions

This study demonstrated that 2D-COS-IR could distinguish tungsten oxide cluster compounds based on their responses to magnetic and thermal perturbations. Despite sharing a similar cluster framework, [GeW9O34]10−, Compounds 1 and 2 exhibited notable spectral differences attributable to the varied transition metal ions (Mn or Co) and hydrogen bonding interactions. Additionally, this research highlighted the utility of 2D-COS-IR in analyzing and identifying compounds with closely related structures.

Importance and Value

1. Scientific Value: This study provided both experimental and theoretical foundations for the application of 2D-COS technology to POMs, enriching the literature on 2D-COS-IR.
2. Practical Value: The findings offer a more precise characterization method for the development of novel functional materials, such as catalysts and pharmaceuticals.
3. Research Highlights: By designing compound systems with explicit magnetic or thermodynamic differences, the study showcased the sensitivity of 2D-COS-IR to subtle molecular structure variations.

Unique Features

Key features of this research include: (1) advancing the boundaries of 2D-COS applications in distinguishing complex molecular systems, and (2) employing a dual approach of SC-XRD and 2D-COS-IR, ensuring consistency between molecular structural data and spectroscopic results.

Conclusion

Through meticulous experimental design and the application of cutting-edge spectroscopy, the research team successfully unveiled critical vibrational differences between two tungsten oxide cluster compounds. This work provides new pathways for researchers in related fields and contributes to the ongoing development of two-dimensional correlation spectroscopy.