Molecular and multidimensional polyoxotungstates functionalized by {Cu(bpy)}2+ groups

Abstract

Five new materials built from polyoxotungstates and Cu(II) ions as linkers have been synthesized by hydrothermal reactions from a mixture of sodium tungstate, copper chloride and bipyridine. The value of the initial pH, the nature of the heteroelement (P or Si) and of the ligand (2,2′- and/or 4,4′-bipyridine) permit the control of the nature of the polyoxotungstate clusters and their connectivity via the copper ions, and hence the dimensionality of the framework. A single phase has been obtained with silicon as heteroelement at an initial pH of 5, namely the 2D material [SiW₁₂O₄₀][Cu(2,2′-bpy)₂]₂·10H₂O (1) with saturated Keggin polyoxotungstates linked by {Cu(2,2′-bpy)₂}²⁺ groups. With phosphorous as heteroelement and at the same initial pH, three different structures have been isolated according to the nature of the ligand. Indeed, the two 1D materials [{Cu₅(2,2′-bpy)₅(H₂O)(HPO₄)(PO₄)}PW₁₁CuO₃₉]·6H₂O (2) with 2,2′-bpy and [4,4′-Hbpy][{Cu₂(2,2′-bpy)₂(4,4′-bpy)_2.5}PW₁₁CuO₃₉]·16H₂O (3) with a mixture of 2,2′- and 4,4′-bpy have been characterized, and a coordination polymer with polyoxometalate guests Na₃[4,4′-Hbpy]{Cu₄(4,4′-bpy)₈(H₂O)₈}[PW₁₁CuO₃₉(H₂O)][PW₁₀Cu₂O₃₈(H₂O)₂]·38H₂O (4) with 4,4′-bpy has been obtained. Finally, in basic medium (pH = 10) the unprecedented molecular cluster Na₂[{Cu₈(2,2′-bpy)₈}(PW₈O₃₁)₂]·15H₂O (5) has been evidenced. Magnetic studies of compound 2 revealed that the predominant interactions involve only 4 paramagnetic centers, which are interacting within pairs, among the 6 Cu(II) centers. The χ_MT = f(T) curve can be fitted using the dinuclear expression appropriate to the HDVV isotropic exchange Hamiltonian H = −JS₁×S₂, with S₁ = S₂ = ½ and J = −105.4 cm⁻¹, showing strong antiferromagnetic interactions within the two Cu(II) pairs.