Aryldiazenido derivatives: A new entry to the functionalization of Keggin polyoxometalates

A series of aryldiazenido polyoxomolybdates of the type (nBu₄N)₂Mo₅O₁₃(OMe)₄(NNAr){Na(MeOH)}] (Ar = C₆F₅, 1; Ar = O₂N-o-C₆H₄, 2; Ar = O₂N-m-C₆H₄, 3; Ar = O₂N-p-C₆H₄, 4a; Ar = (O₂N)₂-o,p-C₆H₃, 5) have been obtained by controlled degradation of the parent compounds (nBu₄N)₃Mo₆O₁₈(NNAr)] with NaOH in methanol. They have been characterized by elemental analysis and UV-Vis and IR spectroscopy. In addition, 4a has been characterized by ⁹⁵Mo NMR spectroscopy and the crystal structure of (nBu₄N)₂Mo₅O₁₃(OMe)₄(NNC₆H₄-p-NO₂){Na(H₂O))]·H₂O (4b) has been determined by X-ray diffraction. The molecular structure of the anion of 4b features a lacunary Lindqvist-type anion Mo₅O₁₃(OMe)₄(NNC₆H₄-p-NO₂)]³⁻ interacting with a sodium cation through the four terminal axial oxygen atoms. The 1:1 sodium complexes react with BaCl₂ and BiCl₃ to yield 2:1 complexes which have been isolated as (nBu₄N)₄Ba{Mo₅O₁₃(OMe)₄(NNAr)}₂] (Ar = C₆F₅, 6; Ar = O₂N-p-C₆H₄, 7) and (nBu₄N)₃Bi{Mo₅O₁₃(OMe)₄(NNAr)}₂] (Ar = C₆F₅, 8; Ar = O₂N-p-C₆H₄, 9). X-ray crystallography analysis of 9·Me₂CO has shown that the tetradentate Mo₅O₁₃(OMe)₄(N₂C₆H₄-p-NO₂)]³⁻ anions provide a square-antiprismatic environment for Bi. In contrast, IR spectroscopy provides evidence for a square-prismatic environment of Ba in 6 and 7. In acetonitrile-methanol mixed solvent, Mo₅O₁₃(OMe)₄(NNAr)]³⁻ and PW₁₁O₃₉]⁷⁻, generated in situ by alkaline degradation of their respective parents, Mo₆O₁₈(NNAr)]³⁻ and PW₁₂O₄₀]³⁻, react together to give the Keggin-type diazenido compounds (nBu₄N)₄PW₁₁O₃₉(MoNNAr)] (Ar = O₂N-o-C₆H₄, 10; Ar = O₂N-m-C₆H₄, 11; Ar = O₂N-p-C₆H₄, 12), which have been characterized by ³¹P and ¹⁸³W NMR spectroscopy.