Département de chimie moléculaire

The asym. CrIII polyoxometalate complex Cs10[($\gamma$-SiW10O36)2(Cr(OH)(H2O))3]·17H2O (1) was synthesized in H2O under atm. pressure from the trinuclear precursor [Cr3(CH3COO)7(OH)2] and the divacant ligand [$\gamma$-SiW10O36]8-. Complex 1 is built up of two [$\gamma$-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 1st excited state located at 11 cm-1. These results were 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 electrochem. studies showed that 1 is stable in aq. media in a 1-7 pH range. This stability is chem. confirmed by the study of the reactivity of 1 with LaIII cations, which gave the Cs4[($\gamma$-SiW10O36)2(Cr(OH)(H2O))3(La(H2O)7)2]·20H2O compd. (2). Indeed, during the synthetic process of this 3d-4f system, the integrity of the [($\gamma$-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 revealed that in the solid state the rare earth cations connect these subunits, affording a 3d-4f double-chain monodimensional system. [on SciFinder(R)]