Characterization and Electrochemical Properties of Molecular Icosanuclear and Bidimensional Hexanuclear Cu(II) Azido Polyoxometalates

Two new Cu(II) azido polyoxometalates compounds have been synthesized, and their structures were determined
by X-ray crystallography. The compound Na14[SiW9O34Cu3(N3)2(OH)(H2O)]2·24H2O (1) is built from two [SiW9O34Cu3(μ1,1,3-N3)2(μ-OH)(H2O)]7- subunits where the copper centers, connected by two azido ligands and one hydroxo
group, form a nearly equilateral triangle. The two subunits are related by an inversion center and connected via the
two μ1,1,3-N3 ligands in an end-to-end fashion, affording a hexanuclear Cu(II) cluster. Linkage of these fragments
via CuOW bonds leads to a bidimensional arrangement of the polyoxometalate units. The complex LiK14Na9[P8W48O184Cu20(N3)6(OH)18]·60H2O (2) consists of two {Cu5(OH)4}6+ and two {Cu5(OH)2(μ1,1,3,3-N3)}7+ subunits
connected via four μ-OH and four μ1,1-N3 additional ligands, the 20 copper centers being encapsulated in the
[P8W48O184]40- crown polyoxotungstate ligand. 1 represents the first multidimensional compound based on azido
polyoxometalate (POM) units, and 2 represents by far the largest azido POM complex isolated to date. Magnetic
measurements revealed an overall antiferromagnetic behavior for both compounds. Nevertheless, the study of the
variation of the magnetization with the applied field indicates that 1 possesses a triplet ground state, which can be
attributed to weak ferromagnetic interaction between the S = 1/2 triangular subunits. The stability of 1 and 2
evidenced by UV−vis spectroscopy and gel filtration chromatography, in particular at pH 5, has allowed a detailed
study of their redox and electrocatalytic properties. For both compounds, the stability of the Cu(II)/Cu(I) couple is
remarkable compared with the observations made in other Cu(II)-substituted POMs. Electrochemical quartz crystal
microbalance measurements clearly demonstrate that the formation of the Cu(I) species occurs neatly without the
formation of Cu(0). The accumulation of such Cu(II) centers within the complexes is a favorable condition to envision
applications involving several electrons. The electrocatalytic reduction of dioxygen and hydrogen peroxide was
achieved efficiently and has shown that the reactivity increases with the nuclearity and/or the Cu/W ratio of the
POM complex. The dioxygen reduction is an overall four-electron process with water as the final product. Finally,
the reduction of the W centers triggers a strong electrocatalysis of solvent reduction.