TETRANUCLEAR Cu(II) CLUSTER ENCAPSULATED IN ONE ARSENIC(V) HETEROPOLYOXOTUNGSTATE. SPECTROSCOPIC AND MAGNETIC INVESTIGATION
Dan Rusu,a
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
Abstract
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