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

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)(.)10H(2)O (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 [{Cu-5(2,2'-bpy)(5)(H2O)(HPO4)(PO4)}PW11CuO39](.)6H(2)O (2) with 2,2'-bpy and [4,4'-Hbpy][{Cu-2(2,2'-bpy)(2)(4,4'-bpy)(2.5)}PW11CuO39](.)16H(2)O (3) with a mixture of 2,2'- and 4,4'-bpy have been characterized, and a coordination polymer with polyoxometalate guests Na-3[4,4'-Hbpy]{Cu-4(4,4'-bpy)(8)(H2O)(8)}[PW-(H2O)(8)}[PW11Cu2O38(H2O)(2)].38H20 (4) with 4,4'-bpy has been obtained. Finally, in basic medium (pH = 10) the unprecedented molecular cluster Nat[{Cu8(2,2'-bpy)8}(PW803,)2]' 15H20 (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(n) centers. The xN,T =f(T) curve can be fitted using the dinuclear expression appropriate to the HDVV isotropic exchange Hamiltonian H = -JS, x S2, with S, = Sz = Z and J = -105.4 cm-', showing strong antiferromagnetic interactions within the two Cu(ii) pairs.