Synthesis, structures, and theoretical investigation of three polyoxomolybdate-based compounds: self-assembly, fragment analysis, orbital interaction, and formation mechanism

Abstract

Three Mo₈- and Mo₁₂-based polyoxoanion compounds (1–3), containing [Mo₈O₂₆]⁴⁻ and [(SiO₄)(Mo₁₂O₃₆)]⁴⁻ anions, were successfully synthesized by a one-pot hydrothermal method, and structurally characterized by an X-ray crystallographic method. 1, [Cu(L′)₄]₂[Mo₈O₂₆] (L′ = 1-methyl-imidazole), contained rarely observed Mo₈-based oblate shell [Mo₈O₂₆]⁴⁻ polyoxoanion, 2, [L′′H]₄[SiMo₁₂O₄₀]·5[L′′]·5H₂O (L′′ = benzotriazole), and 3, [L′′H]₄[SiMo₁₂O₄₀]·2H₂O, contained Mo₁₂-based spherical shell α-Keggin [(SiO₄)(Mo₁₂O₃₆)]⁴⁻ polyoxoanions. Interestingly, SiO₄⁴⁻ fragment is disordered in 2 but is ordered in 3. The observation that 1–3 have different polyoxoanion structures but crystallize in order in the same solution system is rare for POM synthetic chemistry, and is of interest for understanding the formation mechanism and self-assembly process of Mo-based polyoxoanions. Fragment analysis based on the clathrate structural model, i.e. [Mo₈O₂₆]⁴⁻ anion modeled as [MoO₄]₂⁴⁻@Mo₆O₁₈, and [(SiO₄)(Mo₁₂O₃₆)]⁴⁻ anion modeled as [SiO₄]⁴⁻@[Mo₁₂O₃₆] and/or [SiO₄]⁴⁻@[Mo₃O₉]₂@[Mo₆O₁₈], and orbital interaction analysis based on extended Hückel theory calculation were employed successfully to account for the formation mechanism and self-assembly process of fragments and two Mo-based polyoxoanions in 1–3. The calculated large charge transfer and/or high electronic polarization between fragments provide electronic structural information on structural stability and help to interpret the fragment–fragment self-assembly process.