Estrutura de Dawson

A estrutura de Dawson ou estrutura de Wells-Dawson é um motivo estrutural bem conhecido para heteropoliácidos. A estrutura de Dawson pode ser vista como a fusão de duas estruturas de Keggin defeituosas, fragmentos com três octaedros faltantes. Como nas estruturas de Keggin, a estrutura de Dawson tem um oxiânion em seu núcleo. Ao contrário das estruturas de Keggin, existem dois desses ânions, um em cada lado do ânion elipsoidal. Um exemplo é [S
2W
18O
62]4– , que também pode ser descrito como [(SO
4)
2(WO
3)
18]4– .[1]

Comumente, as estruturas de Dawson apresentam fosfato como os oxiânions centrais, mas também podem apresentar outros grupos como o sulfato e o ortossilicato. Quando o ânion Keggin [PW
12O
40]3– é deixado por muito tempo em solução aquosa com pH próximo da neutralidade, ele se converte reversivelmente em [P
2W
18O
62]6– .[2]

Estrutura

Estrutura de Dawson.

A estrutura de Dawson pode ser encarada como fruto da união entre duas porções da estrutura de Keggin, na qual um dos conjuntos de três octaedros adjacentes M
3O
12 em cada uma é removido, e as duas estruturas lacunares HnXM
9O
28(OH)
6 resultantes (em que M é o metal adendo e X é o heteroátomo) são unidas com perda de moléculas de água e formação de seis pontes M–O–M no ponto onde os grupos octaédricos foram removidos. A estrutura resultante possui uma forma “oval” alongada, ao contrário da estrutura Keggin que era aproximadamente “esférica”, e contém dois heteroátomos em seu centro ao invés de um. Em torno dos dois grupos do oxoânion XO
4 n- centrais (que não estão diretamente ligados entre si), são ligados dezoito grupos octaédricos MO
6 , que formam uma gaiola em torno dos oxoânions centrais. Cada heteroátomo se liga a quatro oxigênios; um oxigênio de cada um desses grupos, próximo às extremidades do cluster, também se liga a três átomos de adendo além do heteroátomo, enquanto os outros três oxigênios de cada grupo central se liga ao elemento X e a dois átomos do metal adendo. Há 36 átomos de oxigênio que atuam como pontes entre dois octaedros de adendo adjacentes, bem como 18 oxigênios terminais ligados a cada átomo do metal (formando um número variável de ligações M=O e M–OH). Os octaedros encontram-se um pouco distorcidos e há forte deslocalização da carga negativa do ânion resultante da desprotonação do ácido está deslocalizada ao longo da estrutura.

Referências

  1. ↑ Richardt, Peter J. S.; Gable, Robert W.; Bond, Alan M.; Wedd, Anthony G. (2001). «Synthesis and Redox Characterization of the Polyoxo Anion, γ*-[S2W18O62]4-: A Unique Fast Oxidation Pathway Determines the Characteristic Reversible Electrochemical Behavior of Polyoxometalate Anions in Acidic Media». Inorganic Chemistry. 40 (4): 703–709. PMID 11225112. doi:10.1021/ic000793q 
  2. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (em inglês) 2ª ed. [S.l.]: Butterworth-Heinemann. ISBN 978-0-08-037941-8 

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