Spectroscopic, Crystallographic, and Electrochemical Study of Different Manganese(II)-Substituted Keggin-Type Phosphomolybdates

Abstract

Adjusting the RedOx activity of polyoxometalate catalysts is a key challenge for the catalysis of selective oxidation reactions. For this purpose, the possibility of influencing the RedOx potential by the introduction of an additional RedOx-active element was investigated. Thereby, Keggin-type polyoxometalates (POMs) with up to three different elements in the metal framework were created. An advanced and reproducible synthetic procedure to incorporate Mn^II and additionally V^V into Keggin-type heteropolyacids alongside comprehensive characterization of the new molecules is presented. The success of our syntheses was confirmed by vibrational spectroscopy (IR and Raman) and elemental analysis. Furthermore, the new compounds were analyzed by NMR spectroscopy to investigate the characteristics of the POMs in solution. The structures of successfully crystalized compounds were determined by single-crystal X-ray diffraction. Moreover, all synthesized compounds were characterized using UV/Vis spectroscopy and electrochemical analysis to get further insights into the electronic transfer processes and redox potentials.

Keywords: NMR spectroscopy; crystallography; electrochemistry; polyoxometalates; redox active elements.

Figure 1

FTIR (ATR) spectra of the Mn^II‐substituted HPA‐X‐Y POMs compared to H₃[PMo₁₂O₄₀].

Figure 2

Superimposed IR spectra of the two‐fold metal‐substituted POMs HPA‐1‐1, HPA‐0‐2 and HPA‐2‐0, compared with the IR spectrum of HPA‐0‐0.

Figure 3

Raman spectra of the Mn^II‐substituted HPA‐X‐Y POMs compared to H₃[PMo₁₂O₄₀].

Figure 4

Superimposed Raman spectra of the only Mn^II‐substituted POMs HPA‐0‐2, HPA‐0‐1 and in comparison with HPA‐0‐0.

Figure 5

Asymmetric unit of a crystal of HPA‐1‐1 with the corresponding numbering of the atoms, hydrogen atoms have not been modeled. Purple: phosphorous, red: oxygen, and blue: metals (Mo, V, Mn).

Figure 6

Structure of HPA‐1‐1 in the solid state as determined by x‐ray diffraction, hydrogen atoms have not been modeled. The compound crystallized in space group Fd‐3 m (227). There are six atoms in the asymmetric unit and eight formula units per elementary cell. Residual electron density attributed to hydration water has been refined with a solvent mask (aka SQUEEZE). R₁: 2.65, wR₂: 6.03 %, R_int: 2.78 %, GooF: 1.213. Purple: phosphorous, red: oxygen, and blue: metals (Mo, V, Mn).

Figure 7

³¹P NMR spectra of the HPA‐X‐Y POMs in a mixture of 90 % H₂O (pH 1) and 10 % acetone‐d6. The spectra were measured at 242.9 MHz. 85 % H₃PO₄ was used as external standard.

Figure 8

⁵¹V NMR spectra of the HPA−X‐Y POMs in a mixture of 90 % H₂O (pH 1) and 10 % acetone‐d6. The spectra were measured at 157.8 MHz. NaVO₃ was used as external standard.

Figure 9

Comparison of the ³¹P NMR spectra of the two metal substituted POMs HPA‐0‐2 and HPA‐1‐1 in comparison with HPA‐0‐0. The spectra indicate paramagnetism due to the broadening of the peaks in comparison to HPA‐0‐0.

Figure 10

UV/Vis spectra of all HPA−X‐Y compounds in water.

Figure 11

HPA‐X‐Y POMs synthesized in this work with their different colors. From left to the right: HPA‐0‐1, HPA‐1‐1, HPA‐0‐2, HPA‐1‐2, HPA‐3‐2, HPA‐5‐1.

Figure 12

Comparison between the CV measurements of the two‐fold metal substituted POMs HPA‐2‐0, HPA‐0‐2, HPA‐1‐1 and HPA‐0‐0 (concentration 1 mmol/L, scan rate 100 mV/s (CV)/5 mV/s (SWV) and pH 1).

Figure 13

Comparison between the SWV measurements of the two‐fold metal substituted POMs HPA‐2‐0, HPA‐0‐2, HPA‐1‐1 and HPA‐0‐0 (concentration 1 mmol/L, scan rate 100 mV/s (CV)/5 mV/s (SWV) and pH 1).