TETRANUCLEAR Cu(II) CLUSTER ENCAPSULATED IN ONE ARSENIC(V) HETEROPOLYOXOTUNGSTATE. SPECTROSCOPIC AND MAGNETIC INVESTIGATION
Leontin David,b* Cora
Crãciun,b Laura Pop-Fanea,b Mihaela Hossu,b
and Mariana Rusuc
a Department of Chemical-Physics, Medical and Pharmaceutical University, 3400 Cluj-Napoca, Romania
b Department of Physics, Babeº-Bolyai University, 3400 Cluj-Napoca, Romania
c Department of Chemistry, Babeº-Bolyai University, 3400 Cluj-Napoca, Romania
Na10[Cu4(H2O)2(AsW9O34)2]×23H2O heteropolyoxometalate was synthesized and investigated by elemental analyses, thermogravimetry, FT-IR, VIS and EPR spectroscopy and magnetic susceptibility techniques. Measurements performed in the present study were used to investigate changes of Keggin units, to evaluate the local symmetry around the Cu ions and to identify the nature of Cu-Cu interactions.
Results of FT-IR investigations showed that asymmetric stretching frequencies (nasym) corresponding to O connecting As and W atoms nasym(W-Oa) and to O connecting corner and edge-sharing octahedra nasym(W-Oc,e-W) were shifted. B1(d x2-y2)® E(dxz,yz) and B1(d x2-y2)® B2(dxy) transitions in the VIS spectrum of the complex were broad. EPR parameters (g, D and E) were estimated using the Kambe model for a tetranuclear cluster of Cu(II) ions. These measurements were used to investigate the ground state (1,1,0) and the excited states (1,1,1) and (1,0,1) of the complex. Magnetic susceptibility data for temperatures ranging from 77 to 273 K were used to calculate the effective magnetic moment.
Spectroscopic investigations demonstrated that Cu(II) ions coordinate to AsW9O34 units, W-Oc-W tricentric bonds in the complex are not equivalent and Cu(II) ions form distorted Oh symmetry with CuO6 and CuO5O* local units. Antiferromagnetic coupling between Cu(II) ions was also identified by magnetic susceptibility measurements.
Key words: polyoxometalate, clusters, Cu, spectroscopy, magnetic susceptibility