Crystal Structure and Replacement Reaction of Coordinated Water Molecules of the Heteropoly Compounds of Sandwich-Type Tungstoarsenates

Six new heteropoly compounds in the [M₄(H₂O)₂(As₂W₁₅O₅₆)₂]^16- series (M = Cu^II, Mn^II, Co^II, Ni^II, Zn^II, Cd^II), previously unknown, were synthesized and characterized by means of IR, UV−vis, CV, ¹⁸³W NMR, TG-DSC, and elemental analyses. The synthetic method used in preparing this type of heteropoly compounds was different from that in preparing the corresponding tungstophosphates in that the starting materials were transition metal chlorides in 1.5 times the stoichiometric amount and the required pH value is lower than 2. The crystal structure of Na₁₆[Cu₄(H₂O)₂(As₂W₁₅O₅₆)₂]·47H₂O was solved in triclinic, P1̄ symmetry, with a = 12.721(3) Å, b = 24.516(5) Å, c = 26.450(5) Å, α = 89.90(3)°, β = 77.32(3)°, γ = 89.96(3)°, V = 8048(3) ų, Z = 2, and R = 0.0966. This anion is isostructural with the previously reported [Cu₄(H₂O)₂(P₂W₁₅O₅₆)₂]^16-, having a rhombic tetrameric cluster Cu₄O₁₆ sandwiched by two trivacant Dawson−Wells anions [As₂W₁₅O₅₆]^12-. The range of the bond lengths of the equatorial Cu−O bonds is 1.83−2.05 Å, while that of the axial Cu−O bonds is 2.30−2.39 Å. The distortion of the Cu₄O₁₆ cluster is smaller in the As species than in the P species. Two copper atoms in the Cu₄O₁₆ cluster are coordinated by water molecules. The replacement reactions of the coordinated water molecules of this series of heteropoly compounds in aqueous solutions and in selected organic solvents are also reported here for the first time. The results show that [Fe (CN)₆]^4-, [Fe(CN)₆]^3-, H₂NCH₂CH₂NH₂, etc., can replace the coordinated water to form its characteristic color in aqueous solutions, while in organic solvents the coordinated water molecules are lost, leaving unshared coordination positions that can be occupied by some organic ligands such as pyridine, lactic acid, and acetone to restore the octahedral coordination of M²⁺. The crystallographic morphologies of this series of heteropolyanions after phase transfer are dependent on different transition metal ions present in the central M₄O₁₆ clusters although the anions are isostructural with each other.