Synthesis and Physical-Chemical Study of Two Sandwich-Type Heteropolyoxometalates With Dinuclear Vanadium Clusters

Nicoleta Joo^a, Mihaela Hossu^b, Dan Rusu^c, Anca Marcu^b, Mariana Rusu^a, Cristina Pasca^d and Leontin David^b*
^a Deptment of Chemistry, “Babes-Bolyai” University, 11 Arany Janos Str., 400028, Cluj-Napoca, Romania
^b Deptment of Physics, “Babes-Bolyai” University, 1 Kogalniceanu Str., 400084, Cluj-Napoca, Romania
^c Deptment of Chemistry – Physics, “Iuliu Hatieganu” Medical and Pharmaceutical University, 6 Pasteur Str.,
400349, Cluj-Napoca, Romania
^d Departmant of Biology, “Babes-Bolyai” University, 5–7 Clinicilor Str., 400006, Cluj-Napoca, Romania
Tel.: + 004 0264 405300 5185; E-mail: leodavid@ohys.ubbcluj.ro

Abstract
The sandwich-type K₁₂[(VO)₂Sb₂W₂₀O₇₀] · 31H₂O (1) and K₁₁[(VO)₂Bi₂W₂₀O₇₀] · 21H₂O (2) heteropolyoxotungstates were investigated by means of elemental analyses, thermogravimetric and spectroscopic (FT-IR, UV–VIS and EPR) methods.
The analysis of vanadium ions coordination mode was made comparing the FT-IR spectra of the complexes 1 and 2 with the K₁₂[Sb₂W₂₂O₇₄(OH)₂] · 38H₂O (L₁) and K₁₂[Bi₂W₂₂O₇₄(OH)₂] · 40H₂O (L₂) ligands. FT-IR spectra of the complexes show the presence of the V=O bonds characterized by ν_as(V=O) vibrations. In both complexes, the coordination of the vanadium shifts the ν_s(W–O_c,e–W) vibration bands. In UV spectra, the charge transfer p_π(Oc,e) → d_π*(W) band is shifted in complexes compared to the ligands spectra with ≈ 245 nm for 1 towards higher wavelength and with ≈ 250 nm for 2 towards lower wave numbers.
Visible spectra contain ²B₂(d_xy) → ²E(d_xz,yz) and ²B₂(d_xy) → ²B₁(d_x2 _–y2) transition bands for vanadyl ions in C_4v local symmetry, at 12020 cm^–1 respectively 14710 cm^–1 for the complex 1, and at 12405 cm^–1 and 15905 cm^–1 for the complex 2. The powder EPR spectra at room temperature exhibit eight components both in the perpendicular and in the parallel bands and are typical for mononuclear oxovanadium species in axial environment.

Keywords: Heteropolyoxotungstates, VO complexes, FT-IR spectroscopy, UV-VIS spectroscopy, EPR spectroscopy