Rational syntheses, characterization, crystal structure, and replacement reactions of coordinated water molecules of [As2W18M4(H2O)2O68]10− (M = Cd, Co, Cu, Fe, Mn, Ni or Zn)

Abstract

Rational, high-yield, and isomerically pure syntheses of [As₂W₁₈M₄(H₂O)₂O₆₈]¹⁰⁻ (M = Cd, Co, Cu, Fe, Mn, Ni or Zn) have been achieved. The products were well characterized by means of IR, UV-vis, CV, ¹⁸³W NMR, TG-DSC, and elemental analyses. The properties have been studied. The results allow a number of conclusions to be drawn. (i) The B-type tri(tungsten)vacant form of B-[AsW₉O₃₄]⁹⁻ is a key structural requirement for formation of the dimetal(2+)-substituted dimers [As₂W₁₈M₄(H₂O)₂O₆₈]¹⁰⁻. (ii) The structures of the compounds K₁₀[Mn₄(H₂O)₂(AsW₉O₃₄)₂]·18H₂O and K₈Na₂[Cu₄(H₂O)₂(AsW₉O₃₄)₂]·32H₂O have been solved, and are similar to that of the zinc derivative, comprising a rhomb-like M₄O₁₆ group encapsulated between two fragments of the trivacant Keggin polyanion [AsW₉O₃₄]⁹⁻. (iii) The crystal structures of the arsenic series have been compared with those of the phosphorus-series. Jahn–Teller distortions of the CuO₆ groups and the absence of Jahn–Teller distortions in the MO₆ octahedra for M = Mn and Zn are predicted. (iv) Two water molecules are coordinated to two metal atoms. [Fe(CN)₆]⁴⁻, [Fe(CN)₆]³⁻, SO₃²⁻, SCN⁻, etc. can replace the coordinated water giving characteristic colors in aqueous solutions, while in organic solvents the coordinated water molecules are lost, leaving unshared coordination positions that can be occupied by organic ligands such as pyridine, lactic acid, and acetone to restore the octahedral coordination of M^II. (v) The crystal morphologies of [Co₄(H₂O)₂(AsW₉O₃₄)₂]¹⁰⁻ anion are dependent on different organic ligands, which substitute the two coordinated waters after phase transfer.

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