Spectral study of some polyoxovanadium borates

Four novel polyoxovanadium borates with [(VO)_ 12O_6B_ 18O_ 42], namely (enH_2)_6H_3[(VO)_ 12O_6B_ 18O_ 42]·13H_2O(1), [Ni(en)_2]_6H_3[(VO)_ 12O_6B_ 18O_ 42]·8H_2O(2), [Cu(en)_2]_5H_3[(VO)_ 12O_6B_ 18O_ 42][B(OH)_3]_2·16H_2O(3) and(enH_2)_4Na_4H_3[(VO)_ 12O_6B_ 18O_ 42]·8H_2O(4), were synthesized. The relationship between their properties and structures were studied by using FTIR, UV-Vis DRS and fluorescence etc. Among these compounds, compound (1) possesses isolated [(VO)_ 12O_6B_ 18O_ 42] cage, there is a ring of B_ 18O_ 42 sandwiched between two vanadium-oxygen clusters V_6O_ 18 by eighteen B—(μ_3—O)—V bonds, while compound (2) is rather six [Ni(en)_2], each connected with the B_ 18O_ 42 ring by two Ni—(μ_3—O)—B. In compounds (3) and (4), the anion clusters [(VO)_ 12O_6B_ 18O_ 42] 13- are connected with [Cu(en)_2] 2+ and Na+, respectively. Thus, the compounds (3) and (4) are extended to an infinite two-dimensional network structure, respectively. These characteristic vibrational frequencies of polyoxovanadium borates are related to the structures of these compounds. In UV/Vis DRS spectra, there are two characteristic peaks of polyoxometalates at 205 and 260 nm, respectively. The fluorescence spectra of these four compounds have been studied, and they have emission peaks at 415 nm excited by 310 nm which are caused by O_μ→Mo. The fluorescence lifetimes of these compounds have been studied.