Magnetism, Electron Paramagnetic Resonance, Electrochemistry, and Mass Spectrometry of the Pentacopper(II)-Substituted Tungstosilicate [Cu₅(OH)₄(H₂O)₂(A<i>-</i>α-SiW₉O₃₃)₂]^10-, A Model Five-Spin Frustrated Cluster

Abstract

The dimeric, pentacopper(II)-substituted tungstosilicate [Cu5(OH)4(H2O)2(A-α-SiW9O33)2]10- (1) has been characterized by single-crystal X-ray diffraction, elemental analysis, IR, electrochemistry, magnetic measurements, electron paramagnetic resonance (EPR), and mass spectrometry (MS). Magnetization and high-field EPR measurements reveal that the pentameric copper core {Cu5(OH)4(H2O)2}6+ of 1 exhibits strong antiferromagnetic interactions (Ja = −51 ± 6 cm-1, Jb = −104 ± 1 cm-1, and Jc = −55 ± 3 cm-1) resulting in a spin ST = 1/2 ground state. EPR data show that the unpaired electron spin density is localized on the spin-frustrated apical Cu2+ ion with gzz = 2.4073 ± 0.0005, gyy = 2.0672 ± 0.0005, gxx = 2.0240 ± 0.0005, and Azz = −340 ± 20 MHz (−0.0113 cm-1). 1 can therefore be considered as a model system for a five-spin, electronically coupled, spin-frustrated system. Polyanion 1, which is stable over a wide pH domain (pH 1−7), was characterized by cyclic voltammetry (CV) in a pH 5 medium. Its CV was constituted by an initial two-step reduction of the Cu2+ centers to Cu0 through Cu+, followed at more negative potential by the redox processes of the W centers. Controlled potential coulometry of 1 allows for the reduction of the five Cu2+ centers, as seen by consumption of 10.05 ± 0.05 electrons per molecule. Polyanion 1 triggers efficiently the electrocatalytic reduction of nitrate and nitrite, and it also catalyzes the reduction of N2O. To our knowledge, this is the first example of N2O catalytic reduction by a polyoxoanion. Fourier transform ion cyclotron resonance MS was used to unambiguously assign the molecular weight of the solution-phase species 1 and the oxidation states of the Cu atoms in the central {Cu5(OH)4(H2O)2}6+ core. Infrared (IR) multiphoton dissociation MS/MS of 1 showed evidence of a condensation process similar to bronze formation at low irradiation intensity. Higher IR intensity resulted in the formation of stable fragments consistent with those previously observed in the solution chemistry of polyoxoanions