RESUMO
Based on studies with microcrystals of [Ru(bpy)(3)](2)[alpha-W(18)O(54)(SO(3))(2)] adhered to an electrode surface in contact with the ionic liquids [BMIM][PF(6)] and [BMIM][BF(4)], it has been found that a series of well defined monoelectronic reduction processes associated with the tungsten-oxo cage of the [alpha-W(18)O(54)(SO(3))(2)](4-) species are observed at much less negative potentials than found in conventional organic solvents. Furthermore, since the potentials of the [Ru(bpy)(3)](2+) 2,2'-bipyridyl ligand based reduction processes are not strongly medium dependent, ionic liquid investigations allowed the characterisation of the [Ru(bpy)(3)](2)[alpha-W(18)O(54)(SO(3))(2)] complex without extensive overlap of the ligand based reductions from the [Ru(bpy)(3)](2+) cation and the [W(18)O(54)(SO(3))(2)](4-) anions. In the case of the Mo analogues, [alpha and beta-Mo(18)O(54)(SO(3))(2)](4-), the much higher reactivity of the reduced forms precluded access to well defined monoelectronic reversible steps at negative potentials in all media examined, even though reduction is far easier and hence availability of extensively reduced species should be simpler, in a thermodynamic sense.
RESUMO
While the reaction of [PW(11)O(39)](7-) with first row transition-metal ions M(n+) under usual bench conditions only leads to monosubstituted {PW(11)O(39)M(H(2)O)} anions, we have shown that the use of this precursor under hydrothermal conditions allows the isolation of a family of novel polynuclear discrete magnetic polyoxometalates (POMs). The hybrid asymmetric [Fe(II)(bpy)(3)][PW(11)O(39)Fe(2) (III)(OH)(bpy)(2)]12 H(2)O (bpy=bipyridine) complex (1) contains the dinuclear {Fe(micro-O(W))(micro-OH)Fe} core in which one iron atom is coordinated to a monovacant POM, while the other is coordinated to two bipyridine ligands. Magnetic measurements indicate that the Fe(III) centers in complex 1 are weakly antiferromagnetically coupled (J=-11.2 cm(-1), H=-JS(1)S(2)) compared to other {Fe(micro-O)(micro-OH)Fe} systems. This is due to the long distances between the iron center embedded in the POM and the oxygen atom of the POM bridging the two magnetic centers, but also, as shown by DFT calculations, to the important mixing of bridging oxygen orbitals with orbitals of the POM tungsten atoms. The complexes [Hdmbpy](2)[Fe(II)(dmbpy)(3)](2)[(PW(11)O(39))(2)Fe(4) (III)O(2)(dmbpy)(4)]14 H(2)O (2) (dmbpy=5,5'-dimethyl-2,2'-bipyridine) and H(2)[Fe(II)(dmbpy)(3)](2)[(PW(11)O(39))(2)Fe(4) (III)O(2)(dmbpy)(4)]10 H(2)O (3) represent the first butterfly-like POM complexes. In these species, a tetranuclear Fe(III) complex is sandwiched between two lacunary polyoxotungstates that are pentacoordinated to two Fe(III) cations, the remaining paramagnetic centers each being coordinated to two dmbpy ligands. The best fit of the chi(M)T=f(T) curve leads to J(wb)=-59.6 cm(-1) and J(bb)=-10.2 cm(-1) (H=-J(wb)(S(1)S(2)+S(1)S(2*)+S(1*)S(2)+S(1*)S(2*))-J(bb)(S(2)S(2*))). While the J(bb) value is within the range of related exchange parameters previously reported for non-POM butterfly systems, the J(wb) constant is significantly lower. As for complex 1, this can be justified considering Fe(w)--O distances. Finally, in the absence of a coordinating ligand, the dimeric complex [N(CH(3))(4)](10)[(PW(11)O(39)Fe(III))(2)O]12 H(2)O (4) has been isolated. In this complex, the two single oxo-bridged Fe(III) centers are very strongly antiferromagnetically coupled (J=-211.7 cm(-1), H=-JS(1)S(2)). The electrochemical behavior of compound 1 both in dimethyl sulfoxide (DMSO) and in the solid state is also presented, while the electrochemical properties of complex 2, which is insoluble in common solvents, have been studied in the solid state.
