Résumé
Organometallic oxides of general formula [Mo4O16{Ru(arene)}4] display two isomeric forms, referred to as the "triple cubane" and "windmill" structures. In a previous study (Laurencin et al. Chem. Eur. J. 2004, 10, 208), we had shown that the arene ligand plays an important role on the relative stability of the two isomers. In this work, new experiments and DFT calculations have been carried out in order to try to rationalize and further control the isomerization process in solution. The synthesis and spectroscopic study of [Mo4O16{Os(p-cymene)}4] shows that when ruthenium is replaced by osmium, the equilibria in CHCl3 and CH2Cl2 can be altered. In the case of [Mo4O16{Ru(p-cymene)}4], 1H NMR experiments carried out on CD2Cl2/CD3OD solutions reveal that the solvent, through its polarity and molecular form, has a strong impact on the isomerization. DFT calculations were performed on [Mo4O16{M(arene)}4] (M = Ru, Os ; arene = benzene, toluene, mesitylene, p-cymene, hexamethylbenzene). The change in relative energy of the computed structures by switching from ruthenium to osmium is found to be in line with the experimental observations. Calculations also show that the dielectric properties of the solvent, as well as the facility with which it can access the oxometallic core, directly influence the relative stability of both isomers. This work should thus open the way to further studies of organometallic oxides, by allowing their rational preparation and a better understanding of their properties, thanks to a precise analysis of their interaction with the solvation environment.