Résumé
Recently synthesized copper octacyanomolybdate molecules present interesting photomagnetic properties. Before irradiation, the molecules behave as noncoupled CuII paramagnetic ions (with central diamagnetic MoIV ion), whereas after irradiation (406 nm) they behave as superparamagnetic molecules with CuII ions coupled to the molybdenum ion. The proposed mechanism to explain these photomagnetic properties is based on the photoinduced charge transfer from MoIV (S = 0) to CuII (S = 1/2) leading to the formation of MoV (S = 1/2) and CuI (S = 0) with strong ferromagnetic coupling between MoV and the other CuII spin carriers. This paper presents X-ray magnetic circular dichroism (XMCD) measurements at the molybdenum L2,3 edges. Two bimetallic molecules have been investigated: [Mo(CN)2(CN-CuL)6]8+, L being tris(2-amino)ethylamine and [Mo(CN)6(CN-CuL′2)2], with L′ being N,N′,dimethyl ethylene diamine (labeled MoCu2-Meen). In both cases, before irradiation the XMCD signal is null as expected for diamagnetic MoIV (S = 0). After irradiation, an XMCD signal appears, which directly demonstrates the formation of spin density on the Mo ion. After reaching room temperature, the photoinduced spectral features disappear, indicating the reversibility of the photoinduced process. In the case of MoCu2-Meen, the XMCD experiments allow the observation of an X-ray-induced metastable state made of high-spin MoIV ion (S = 1). The application of the sum rule for isotropic spectra shows that there is no variation of the molybdenum oxidation state during the X-ray-induced magnetic process. From the Mo L2,3 XMCD signal of the X-ray-photoexcited MoCu2-Meen, we obtain an orbital magnetic moment equal to 0.13 μB and a spin magnetic moment equal to 1.22 μB at T = 10 K and H = 6 T. These results demonstrate that the Mo ion in the X-ray-photoinduced excited state of the MoCu2-Meen complex is high-spin MoIV (S = 1).