Fe2 and Fe4 Clusters Encapsulated in Vacant Polyoxotungstates: Hydrothermal Synthesis, Magnetic, Electrochemical Properties, and DFT calculations

摘要

While the reaction of [PW11O39]7- towards first row transition metal ions Mn+ under usual bench conditions only leads to monosubstituted {PW11O39M(H2O)} anions, we show that the use of this precursor under hydrothermal conditions allows to isolate a family of novel polynuclear molecular magnetic polyoxometalates (POMs). The hybrid asymmetric [FeII(bpy)3][PW11O39Fe2III(OH)(bpy)2]·12H2O complex (1) contains the dinuclear {Fe(μ-O(W))(μ-OH)Fe} core where one iron atom is coordinated to a monovacant POM while the other is coordinated to two bipyridine ligands. Magnetic measurements indicate that in 1 the FeIII centers are weakly antiferromagnetically coupled (J = -11.2 cm-1, Ĥ = -JŜ1Ŝ2) compared to {Fe(μ-O)(μ-OH)Fe} systems. This is due to the long distance 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. (Hdmbpy)2[FeII(dmbpy)3]2[(PW11O39)2Fe4IIIO2(dmbpy)4]·14H2O (2) (dmby = 5,5’-dimethyl-2,2’-bpy) and H2[FeII(dmbpy)3]2[(PW11O39)2Fe4IIIO2(dmbpy)4]·10H2O (3) represent the first butterfly-like POM complexes. In these species, a tetranuclear FeIII complex is sandwiched between two lacunary polyoxotungstates which are pentacoordinated to two FeIII cations, the remaining paramagnetic centers being each coordinated to two dmbpy ligands. The best fit of the χMT = f(T) curve leads to Jwb = -59.6 cm-1 and Jbb = -10.2 cm-1 (Ĥ = -Jwb(Ŝ1Ŝ2 + Ŝ1Ŝ2* + Ŝ1*Ŝ2 + Ŝ1*Ŝ2*) – Jbb(Ŝ2Ŝ2*)). While the Jbb value is within the range of related exchange parameter previously reported for non-POM butterfly systems, the Jwb constant is significantly lower. As for complex 1, this can be justified considering Few-O distances. Finally, in absence of coordinating ligand, the dimeric complex [N(CH3)4]10[(PW11O39FeIII)2O]·12H2O (4) has been isolated. In this complex, the two single oxo-bridged FeIII centers are very