A nonanuclear copper(II) polyoxometalate assembled around a μ-1,1,1,3,3,3-azido ligand and its parent tetranuclear complex

Reaction of Cu-II, [gamma-SiW10O36](8-), and N-3(-) affords three azido polyoxotungstate complexes. Two of them have been characterized by single-crystal X-ray diffraction. Complex KNaCs10[{gamma-SiW10O36Cu2(H2O)(N-3)(2)}(2)](.)26H(2)O (1) is obtained as crystals in few hours after addition of CsCl. This linear tetranuclear Cu-II complex consists in two [gamma-SiW10O36Cu2(H2O)(N-3)(2)](6-) units connected through two W=O bridges. When the filtrate is left to stand for one night, a new complex is obtained. From both elemental analysis and IR spectroscopy, it has been postulated that this compound could be formulated K15Cs5.5[SiW10O37Cu2(H2O)(2)(N-3)](.)14H(2)O (1a), showing the loss of one azido ligand per polyoxometalate unit. Finally, when no cesium salt is added to the reaction medium, the nonanuclear complex K12Na7[{SiW8O31Cu3(OH)(H2O)(2)(N-3)}(3)(N-3)](.)24H(2)O (2) is obtained after three days. Compound 2 crystallizes in the R3c space group and consists in three {Cu-3} units related by a C-3 axis passing through the exceptional mu-1,1,1,3,3,3-azido bridging ligand. Each trinuclear Cu-II unit is embedded in the [gamma-SiW8O31](10-) ligand, an unprecedented tetravacant polyoxometalate, showing that partial decomposition of the [gamma-SiW10O36](8-) precursor occurs with time in such experimental conditions. Magnetically, complex 1 behaves as two isolated {Cu-2(mu(1.1)-N-3)(2)} pairs in which the metal centers are strongly ferromagnetically coupled (J=+224 cm(-1), g=2.20), the coupling through the W=O bridges being negligible. The magnetic behavior of complex 2 has also been studied. Relatively weak ferromagnetic couplings (J(1) = +1.0 cm(-1), J(2) = + 20.0 cm(-1), g = 2.17) have been found inside the {Cu-3} units, while the intertrimeric magnetic interactions occurring through the hexadentate azido ligand have been found to be antiferromagnetic (J(3) = -5.4 cm(-1)) and ferromagnetic (J(4) = + 1.3 cm(-1)) with respect to the end-to-end and end-on azido-bridged Cu-II pairs, respectively.