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 mole-cules 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 Xray 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.

H8[PV5Mo7O40] – A Unique Polyoxometalate for Acid and RedOx Catalysis: Synthesis, Characterization, and Modern Applications in Green Chemical Processes

“…[78] Recently, we were able to show that these RedOx potential can be tuned by the additional incorporation of another transition metal, such as Mn II . [79] This opens up possibilities for future development of tailor-made POM catalysts. [79]…”

“…This phenomenon is well known and observed for all higher substituted compounds of this family. [28,60,61] The 51 V NMR data of an aqueous HPA-5 solution at pH 1 are presented in Figure 7. The broad peak at À 544 ppm (literature -545 ppm) [41] belongs to the hydrated VO 2 + cation.…”

“…It is derived from the phosphomolybdic acid H 3 [PMo 12 O 40 ], in which Mo is partly replaced by V. This replacement leads to a series of compounds HPA-x with the general formula H 3 + x [PV x Mo 12À x O 40 ] with x ranging from 1 to 6. [25][26][27][28] Out of this series however, HPA-5 stands out for its exceptional catalytic capabilities and resulting applications. On the one hand side, it can act as a strong Brønsted acid, catalysing esterification, dehydration, or delignification reactions in solution.…”

Study on the incorporation of various elements into the Keggin lacunary-type phosphomolybdate [PMo9O34]9− and subsequent purification of the polyoxometalates by nanofiltration

“…6 The incorporation of different elements, especially transition metals, into the Keggin-type phosphomolybdate POMs (also known as heteropolyanions/heteropolyacids (HPA)) [11][12][13] is a currently very active area of research. 14,15 It is an interesting field not only from a synthetic point of view, but also from a catalytic perspective; in fact a major goal of this field is the synthesis of tailor-made catalysts. [15][16][17] By substituting the framework metal with redox active transition metals, the redox potentials and thus the redox activity of the POM can be tuned.…”

Recent Advances in Confining Polyoxometalates and the Applications

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In short, the unique structure and properties of POMs make them attractive in catalysis, [15,16] biomedicine, [17][18][19] nanotechnology, [20,21] crystallography, [22][23][24] and material science. [25,26] Nevertheless, the practical application of POMs is greatly limited by their instability and easy-agglomeration characteristics.Specifically, the structure of POM is labile in aqueous/organic solvents because the polyhedral subunits of POM anion may detach from the skeletons and multiple POM species may form when the solution is in equilibrium.

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