What is keggin heteropoly acid, what is Dawson heteropoly acid, and what is the difference between them?

In 1934, J. F. Keggin, a young physicist from the Bragg Research Group in Manchester, England, synthesized

H3 PW12O40 ·5H2O in the laboratory. He compared the results of the X-ray diffraction experiment of the substance powder with the calculated values. Proposed an epoch-making Keggin structural model (1: 12 series A type). Forty years later, in 1974, another measurement proved that Keggin

's structure was correct. In 1953, Dawson used X-ray diffraction for the first time to determine the structure of K6 [P2W18O60] ·14H2O.

The results showed that it was a triclinic crystal system. After determining the structure of Na6 [P2Mo18O60] ·24H2O, Strandbery pointed out:

Na6 [P2Mo18O60] ·24H2O and K6 [P2W18O60] ·14H2O have the same structural configuration. Since then, the structures of some related 2: 18

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series of heteropoly compounds have been determined one after another, and they all have similar skeletons to K6 [P2W18O60] ·14H2O.

In order to commemorate Dawson, later generations called the 2:18 series of heteropoly compounds Dawson structured heteropoly compounds. As early as 1937, J. A.

Anderson had already speculated the structure of 1:6 type heteropoly compounds, such as: [IMo6O24]6 -, where I(Ⅶ):Mo = 1:6, but < /p>

It was not finally determined until 1974, so the 1:6 series of heteropoly compounds are called Anderson structure heteropoly compounds, but the first real Anderson structure compound is considered to be 1948 Evans reported [FeMo6O24]6-. In 1953,

Wangh synthesized (NH4 ) 6 [XMo9O12 ] (X =Ni4 + ,Mn4 + ) for the first time; in 1960, Brown. D. H reported 1: 9BeW9

Synthesis of molybdenum; since the 1970s, heteropoly compounds of molybdenum with P, Si, and As as heteroatoms and molybdenum heteropoly compounds with P, Si,

As, Ge, and Sb as heteroatoms have been synthesized. Tungsten heteropoly compounds, later known as Wangh structure (1: 9 series) heteropoly compounds. In addition, there is the Silverton (1: 12 series B type) structure, which together with Keggin, Dawson, Anderson and the Lindqvist structure (M6O19 structure) of polyacid

are called the 6 basic structures of polyacid [ 2 ] .

Due to the large number of atoms and complex structures in polyacid compounds, the traditional description method is to regard their structures as MOn polyhedrons centered on metal

** *A combination of corner oxygen and edge oxygen. Due to limitations of testing methods, by 1971, there were only 14 types of polyacid crystals (12 of which were single crystals) that could be structurally analyzed. Since the 1980s, with the popularization of

four-circle X-ray diffractometer, more than 100 polyacid structures have been identified so far, among which Keggin structure and Dawson structure

are Two common basic structures [4].

(1) Keggin structure (1: 12 series A type)

The general structural formula of heteropolyanions with Keggin structure is [XM12 O40]n - (X = P, Si , Ge, As, etc., M =Mo,

W). The tetrahedral XO4 is located in the center of the molecular structure, and is surrounded by 12 octahedral MO6 with corner oxygen and edge oxygen.

XO4. Keggin structure heteropolyanion*** has 5 isomers: α, β, γ, δ and ε

(2) Dawson structure (2: 18 series)

This It is a class of heteropoly compounds as important as Keggin structure.