Experimental and Computational Study of the Framework Fluxionality of Organometallic Derivatives of Polyoxometalates: Analysis of the Effect of the Metal and of the Solvent

Organometallic oxides of the general formula [Mo 4 O 16 {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 showed that the arene ligand plays an important role in the relative stability of the two isomers. In this work, new experiments and DFT calculations have been carried out to try to rationalize and further control the isomerization process in solution. The synthesis and spectroscopic study of [Mo 4 O 16 {Os( p -cymene)} 4 ] showed that, when ruthenium is replaced by osmium, the equilibria in CHCl 3 and CH 2 Cl 2 can be altered. In the case of [Mo 4 O 16 {Ru( p -cymene)} 4 ], 1 H NMR experiments carried out in CD 2 Cl 2 /CD 3 OD solutions revealed that the solvent, through its polarity and molecular form, has a strong impact on the isomerization. DFT calculations were performed on [Mo 4 O 16 {M(arene)} 4 ] (M = Ru, Os; arene = benzene, toluene, mesitylene, p -cymene, hexamethylbenzene). The change in relative energy of the computed structures upon switching from ruthenium to osmium was found to be in line with the experimental observations. Calculations also showed that the dielectric properties of the solvent, as well as the facility with which it can access the oxometallic core, directly influence the relative stabilities of the two 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, through a precise analysis of their interaction with the solvation environment.