Methanolysis of MoCl5 in the presence of different alkaline agents; molecular structures of the polynuclear molybdenum(V) methoxides and electron charge density distribution from X-ray diffraction study of the new K–Mo cluster

The influence of the solution total alkalinity and the base cation and anion types on the structure and properties of molybdenum oxo-methoxides formed in reaction of MoCl5 in methanol as following: MoCl5 + CH3OH + M+A−, where M+ = K+ or Na+, and A− = OCH3− or OH− was studied. The investigation has shown that hetero-bimetallic oxo-methoxides containing Na or K ions – [NaMo2O2(OCH3)7(CH3OH)] (1), [KMo2O2(OCH3)7(CH3OH)2] (2) – are formed in alkaline media with sodium or potassium methoxides taken as a base. Single crystal X-ray structure determination (XRD) of the complexes 1 and 2 revealed that their crystal structures can be considered as consisting of anionic [Mo2O2(OCH3)7]− and cationic [Na(CH3OH)]+ or [K(CH3OH)2]+ fragments assembled into different supramolecular aggregates. When Na, K or Mg methoxides are used as a base, while the medium remains to be acidic, regardless the cation type the same compounds are formed, namely, [Mo2Cl2(OCH3)8] (3) or [Mo2Cl4(OCH3)6] (4) (in the absence of alkali). When KOH or NaOH are used instead of respective methoxides it has been shown that at weakly acidic (close to neutral) media the octanuclear molybdenum cluster [Mo8O8(μ3-O)4(μ2-O)4(μ3-OCH3)2(μ2-OCH3)4Cl2(CH3OH)4]·6CH3OH (5·6CH3OH) is formed as a main product. The DFT calculations of the mechanism of complex 5 formation were carried out. The process of octanuclear Mo(V) cluster formation from initially formed binuclear ones was shown to be a thermodynamically favorable. The detailed analysis within QTAIM theory of an electron density distribution function ρ(r) in the crystal of 2 obtained directly from its X-ray diffraction data has been performed. This analysis has provided the direct and convincing experimental evidence of the Mo–Mo bond presence in a dimolybdenum clusters and, in addition, allowed to estimate the energy of some important interatomic interactions in 2 and related clusters.

For the reaction of MoCl5 methanolysis in the presence of base of major importance are the resultant acidity and the type of alkaline agent used. Molecular structures of a row of Mo(V) oxoalkoxides, mechanisms of their formation and the direct experimental evidence (based on charge density analysis) of Mo–Mo bonding in polynuclear molybdenum oxoalkoxides are presented.Figure optionsDownload as PowerPoint slide