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 [MoO<sub>2</sub>(L)­(EtOH)] (<b>1α</b>, <b>1β</b>, and <b>1γ</b>) (L<sup>2–</sup> = 4-methoxy-2-oxybenzaldehyde isonicotinylhydrazonate) were synthesized by the reaction of H<sub>2</sub>L with the dioxobis­(acetylacetonato)­molybdenum­(VI) complex, [MoO<sub>2</sub>(acac)<sub>2</sub>], in ethanol. Removal of the coordinated ethanol molecule upon grinding or heating led to the solid-state transformation of the polymorphs <b>1α</b>, <b>1β</b>, or <b>1γ</b> into the coordination polymer [MoO<sub>2</sub>(L)]<sub><i>n</i></sub> (<b>2a</b>). The square inclusion complex [MoO<sub>2</sub>(L)]<sub>4</sub>⊃CH<sub>2</sub>Cl<sub>2</sub>­·4CH<sub>2</sub>Cl<sub>2</sub> (<b>2b⊃CH</b><sub><b>2</b></sub><b>Cl</b><sub><b>2</b></sub>­·<b>4CH</b><sub><b>2</b></sub><b>Cl</b><sub><b>2</b></sub>) was obtained by a self-assembly reaction in dichloromethane. Standard Gibbs energies of binding for molybdenum­(VI) compounds [MoO<sub>2</sub>(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 [MoO<sub>2</sub>(L)­(EtOH)] (<b>1α</b>, <b>1β</b>, and <b>1γ</b>), [MoO<sub>2</sub>(L)]<sub>4</sub>⊃CH<sub>2</sub>Cl<sub>2</sub>­·4CH<sub>2</sub>Cl<sub>2</sub> (<b>2b⊃CH</b><sub><b>2</b></sub><b>Cl</b><sub><b>2</b></sub>­·<b>4CH</b><sub><b>2</b></sub><b>Cl</b><sub><b>2</b></sub>), [MoO<sub>2</sub>(L)­(γ-pic)]­·γ-pic (<b>3</b>·<b>γ-pic</b>), [MoO<sub>2</sub>(L)­(py)] (<b>4</b>), and [MoO<sub>2</sub>(L)­(DMSO)] (<b>5</b>) 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.