Structural, Magnetic, EPR, and Electrochemical Characterizations of a Spin-Frustrated Trinuclear Cr^III Polyoxometalate and Study of Its Reactivity with Lanthanum Cations

The asymmetric CrIII polyoxometalate complex Cs10[(γ-SiW10O36)2(Cr(OH)(H2O))3]·17H2O (1) has been synthesized in water under atmospheric pressure from the trinuclear precursor [Cr3(CH3COO)7(OH)2] and the divacant ligand [γ-SiW10O36]8−. Complex 1 is built up of two [γ-SiW10O36]8− Keggin units sandwiching a trinuclear {(CrIII(OH)(H2O))3} fragment where the paramagnetic centers are bridged by three μ-OH ligands forming a nearly isosceles triangle. The magnetic properties of this spin-frustrated system have thus been interpreted considering a 2-J Hamiltonian showing that the CrIII ions are antiferromagnetically coupled and that 1 possesses an S = 3/2 ground state with an S = 1/2 first excited state located at 11 cm−1. These results have been confirmed by EPR spectroscopy measurements (Q-band), which have also enabled the quantification of the electronic parameters characterizing the quadruplet spin ground state. The magnitude of the magnetic exchange interactions and the nature of the ground state are discussed in light of previously reported isosceles triangular S = 3/2 clusters. UV−visible and electrochemical studies have shown that 1 is stable in aqueous media in a 1−7 pH range. This stability is chemically confirmed by the study of the reactivity of 1 with LaIII cations, which has allowed the isolation of the Cs4[(γ-SiW10O36)2(Cr(OH)(H2O))3(La(H2O)7)2]·20H2O compound (2). Indeed, during the synthetic process of this 3d−4f system, the integrity of the [(γ-SiW10O36)2(Cr(OH)(H2O))3]10− building unit constituting 1 is maintained despite the high oxophilic character of the LaIII ions. The single crystal X-ray diffraction study of 2 has revealed that in the solid state the rare earth cations connect these subunits, affording a 3d−4f double-chain monodimensional system.