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) (L2– = 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.