The focus of this work was on the synthesis, characterization and catalytic activity of Mo(VI) complexes with hemilabile polydentate ligands that show a high tendency to form discrete small-sized self-assembled structures. A series of mononuclear, [MoO₂(L^1 or 3)(MeOH)] (1a and 3a) and [MoO₂(L¹)(EtOH)] (1b), and dinuclear, [MoO₂(L^1–3)]₂ (1–3), complexes was synthesised by using 2-aminobenzoylhydrazone ligands derived from salicylaldehyde (H₂L¹), 3-methoxy-2-hydroxybenzaldehyde (H₂L²) and 4-methoxy-2-hydroxybenzaldehyde (H₂L³). Different reaction conditions led to the formation of several crystalline forms of the dinuclear cyclic species [MoO₂(L^1 or 3)]₂ (1-α, 1-β, 1-γ, 1·CH₃CN, 3-α, 3-β and 3·2CH₃CN), a feature which is observed for the first time in the case of the metallosupramolecular compounds incorporating cis-MoO₂ ²⁺ subunits. These complexes were tested as catalysts for cyclooctene epoxidation under eco-friendly reaction conditions by using aqueous tert-butyl hydroperoxide as an oxidant without the addition of an organic solvent. The use of the hemilabile ligands provided the opportunity to employ relatively stable coordinatively saturated dinuclear Mo(VI) complexes, which in turn offered access to highly reactive pentacoordinated species in the presence of olefins. Lastly, the hemilabile ligand induced catalytic enhancement was addressed by using density functional theory calculations.

Synthesis of hydrazones was performed via both conventional and solvent-free routes using the corresponding hydrazide (isonicotinic hydrazide, nicotinic hydrazide, 2-aminobenzhydrazide or 4-aminobenzhydrazide) and appropriate aldehyde (salicylaldehyde, 3-methoxysalicylaldehyde or 4-methoxysalicylaldehyde). A systematic study dedicated to solvatomorphism or polymorphism screening resulted in the formation of twelve novel crystalline forms, and eight of these were characterized via the single crystal X-ray diffraction method. In all studied structures, the molecules were assembled into endless supramolecular chains, discrete rings, chains of rings or nets. The mechanochemical synthesis employing liquid-assisted grinding was also applied and the nicotinic- and isonicotinic-based hydrazones were found to form readily from their corresponding precursors. The chemometric study using principal component analysis for mechanochemical synthesis monitoring was implemented for the first time to provide an insight into the reaction profiles. A thoughtful combination of ex situ powder X-ray diffraction and chemometric analysis was essential to identify a stepwise mechanism for the hydrazone formation via an intermediate phase. In five investigated reactions the first principal component accounted for at least 75% of the total variance, whereas in the case of two reactions this component accounted for 69.72 and 46.23% of the total variance. The hydrazones were also evaluated for cytotoxic activity in vitro. All compounds exhibited weak to moderate cytotoxicity against THP-1 and no cytotoxicity against HepG2 cells. Substantial antibacterial activity was obtained against Moraxella catarrhalis while no growth inhibition of Staphylococcus aureus, Enterococcus faecalis and Escherichia coli was observed.

This article describes the selective cleavage of coordination bonds by mechanochemical methods and the further application of the thus obtained precursors for a facile preparation of new coordination compounds. In the class of dioxomolybdenum(VI) coordination compounds, [MoO₂L(ROH)], where L stands for a tridentate dianionic ONO ligand and ROH represents different alcohol molecules, mechanical treatment induces an exclusive cleavage of the molybdenum–alcohol bond, which can thus be considered as a mechanosensitive bond. Alcohol removal can also be accomplished by heating. Both grinding and heating resulted in highly reactive, coordinatively unsaturated compounds, an orange amorphous pentacoordinated [MoO₂L] (I) and the brown polymeric [MoO₂L]_n ((I)_nnn), respectively. Even though both I and (I)_nn are stable at room temperature, they can be interconverted using only solvent-free techniques, a conversion followed by a colour change of the sample. The tendency of such unsaturated complexes to complete their coordination spheres was exploited for the efficient solution and solvent-free syntheses of octahedral molybdenum complexes with selected O- and N-donating ligands. Both approaches herein used, solution and solvent-free, have proved to be superior under specific conditions and their respective advantages and weaknesses are discussed.

We present a solvent-free thermo-mechanochemical approach for the direct coupling of carboxylic acids and amines, which avoids activators and additives. Detailed analysis of the reactions by ex situ and in situ monitoring methods led to the observation, isolation, and characterisation of multicomponent crystalline intermediates that precede the formation of amides. We applied our methodology for the quantitative synthesis of the active pharmaceutical ingredient moclobemide.

The cocrystal of 4-oxopimelic acid and 4,4′-bipyridine was isolated in two polymorphic forms depending upon the solvent of crystallisation. Polymorphs exhibit hydrogen-bonded chains formed between the carboxyl and pyridine moieties while different packing arrangements of chains result from C–H⋯O interactions. Solid-state synthesis and interconversion conditions were investigated by grinding and thermal methods.