A Nonanuclear Copper(II) Polyoxometalate Assembled Around a μ-1,1,1,3,3,3-Azido Ligand and Its Parent Tetranuclear Complex

Reaction of CuII, [γ‐SiW10O36]8−, and N3− affords three azido polyoxotungstate complexes. Two of them have been characterized by single‐crystal X‐ray diffraction. Complex KNaCs10[{γ‐SiW10O36Cu2(H2O)(N3)2}2]⋅26 H2O (1) is obtained as crystals in few hours after addition of CsCl. This linear tetranuclear CuII complex consists in two [γ‐SiW10O36Cu2(H2O)(N3)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 K1.5Cs5.5[SiW10O37Cu2(H2O)2(N3)]⋅14 H2O (1 a), 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(N3)}3(N3)]⋅24 H2O (2) is obtained after three days. Compound 2 crystallizes in the R3c space group and consists in three {Cu3} units related by a C3 axis passing through the exceptional μ‐1,1,1,3,3,3‐azido bridging ligand. Each trinuclear CuII unit is embedded in the [γ‐SiW8O31]10− ligand, an unprecedented tetravacant polyoxometalate, showing that partial decomposition of the [γ‐SiW10O36]8− precursor occurs with time in such experimental conditions. Magnetically, complex 1 behaves as two isolated {Cu2(μ1,1‐N3)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 (J1=+1.0 cm−1, J2= +20.0 cm−1, g=2.17) have been found inside the {Cu3} units, while the intertrimeric magnetic interactions occurring through the hexadentate azido ligand have been found to be antiferromagnetic (J3=−5.4 cm−1) and ferromagnetic (J4=+1.3 cm−1) with respect to the end‐to‐end and end‐on azido‐bridged CuII pairs, respectively. Unusual bridging mode: Reaction of CuII, [γ‐SiW10O36]8−, and N3− affords three azido polyoxotungstate complexes that can be selectively isolated at different reaction steps. While a tetranuclear CuII complex based on the [γ‐SiW10O36]8− unit is isolated after a few hours, the nonanuclear complex formed of three [γ‐SiW8O31Cu2(OH)(H2O)2]7− units connected through the exceptional μ‐1,1,1,3,3,3‐azido bridging ligand (illustrated here) is obtained after three days.