Framework Fluxionality of Organometallic Oxides : Synthesis, Crystal Structure, EXAFS, and DFT Studies on [{Ru(η⁶-arene)}₄Mo₄O₁₆] Complexes

Reactions of the molybdates Na₂MoO₄•2 H₂O and (<i>n</i>Bu₄N)₂[Mo₂O₇] with [{Ru(arene)Cl₂}₂] (arene=C₆H₅CH₃, 1,3,5-C₆H₃(CH₃)₃, 1,2,4,5-C₆H₂(CH₃)₄) in water or organic solvents led to formation of the triple-cubane organometallic oxides [{Ru(η⁶-arene)}₄Mo₄O₁₆], whose crystal and molecular structures were determined. Refluxing triple cubane [{Ru(η⁶-C₆H₅CH₃)}₄Mo₄O₁₆] in methanol caused partial isomerization to the windmill form. The two isomers of [{Ru(η⁶-C₆H₅CH₃)}₄Mo₄O₁₆] were characterized by Raman and Mo K-edge X-ray absorption spectroscopy (XAS), both in the solid-state and in solution. This triple-cubane isomer was also used as a spectroscopic model to account for isomerization of the p-cymene windmill [{Ru(η⁶-1,4-CH₃C₆H₄CH(CH₃)₂)}₄Mo₄O₁₆] in solution. Using both Raman and XAS techniques, we were then able to determine the ratio between the windmill and triple-cubane isomers in dichloromethane and in chloroform. Density functional calculations on [{Ru(η⁶-arene)}₄Mo₄O₁₆] (arene=C₆H₆, C₆H₅CH₃, 1,3,5-C₆H₃(CH₃)₃, 1,4-CH₃C₆H₄CH(CH₃)₂, C6(CH₃)₆) suggest that the windmill form is intrinsically more stable, provided the complexes are assumed to be isolated. Intramolecular electrostatic interactions and steric bulk induced by substituted arenes were found to modulate but not to reverse the energy difference between the isomers. The stability of the triple-cubane isomers should therefore be accounted for by effects of the surroundings that induce a shift in the energy balance between both forms.