Three Polymorphic Forms of a Monomeric Mo(VI) Complex: Building Blocks for Two Metal–Organic Supramolecular Isomers. Intermolecular Interactions and Ligand Substituent Effects

Three polymorphic forms of the molybdenum­(VI) complex [MoO₂(L)­(EtOH)] (1α, 1β, and 1γ) (L^2– = 4-methoxy-2-oxybenzaldehyde isonicotinylhydrazonate) were synthesized by the reaction of H₂L with the dioxobis­(acetylacetonato)­molybdenum­(VI) complex, [MoO₂(acac)₂], in ethanol. Removal of the coordinated ethanol molecule upon grinding or heating led to the solid-state transformation of the polymorphs 1α, 1β, or 1γ into the coordination polymer [MoO₂(L)]_n (2a). The square inclusion complex [MoO₂(L)]₄⊃CH₂Cl₂­·4CH₂Cl₂ (2b⊃CH₂Cl₂­·4CH₂Cl₂) was obtained by a self-assembly reaction in dichloromethane. Standard Gibbs energies of binding for molybdenum­(VI) compounds [MoO₂(L)­(D)] with the sixth coordination site occupied by a nitrogen or an oxygen donor D were estimated using quantum chemical calculations. Crystal and molecular structures of the molybdenum­(VI) compounds [MoO₂(L)­(EtOH)] (1α, 1β, and 1γ), [MoO₂(L)]₄⊃CH₂Cl₂­·4CH₂Cl₂ (2b⊃CH₂Cl₂­·4CH₂Cl₂), [MoO₂(L)­(γ-pic)]­·γ-pic (3·γ-pic), [MoO₂(L)­(py)] (4), and [MoO₂(L)­(DMSO)] (5) were determined by the single crystal X-ray diffraction method. The compounds were further characterized by chemical analysis, thermogravimetric, and differential scanning calorimetry measurements, IR, UV–vis, one- and two-dimensional NMR spectroscopies, and the powder X-ray diffraction method.