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 = CuII, MnII, CoII, NiII, ZnII, CdII),
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) Å3, 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.