Counter Anion Effects on the Formation and Structural Transformations of Mo(vi)-Hydrazone Coordination Assemblies: Salts, Solvates, Co-Crystals, and Neutral Complexes

Complex salts [1H]X and [1H](XA)(0.5)center dot 2MeOH, and co-crystals [1H]X center dot 0.5VA (X = chloride or bromide, XA = chloranilate or bromanilate, VA = o-vanillin azine), comprising [MoO2(HL)(MeOH)](+) ([1H](+)) cation (H2L = 3-methoxysalicylaldehyde isonicotinoyl hydrazone), were prepared either by solution-based synthesis or by mechanochemical synthesis. Whereas [1H]X salts were extremely sensitive to humidity, their stability could be reinforced by the azine incorporation into the complex network. Solvent-mediated transformations of [1H]X led to methanol co-ligand replacement and afforded complexes [MoO2(HL)X] (2Cl center dot MeOH, 2Cl, and 2Br center dot 0.5MeCN). However, solvates [1H](XA)(0.5)center dot 2MeOH, under the same conditions, gave stable complexes [1H](XA)(0.5) in which methanol remained coordinated. The differences in the assembly's behavior were attributed to the packing arrangements, the relative orientation of cations and anions, and interactions between them. Polymorph [MoO2(L)(MeOH)] (1), not attainable by other routes, was the only product when compounds [MoO2(HL)X] were treated with a weak base at low temperatures. Tetranuclear [MoO2(L)](4) and polynuclear [MoO2(L)](n) (2) supramolecular isomers, concomitantly crystallized when the reaction was conducted solvothermally. All of the complexes were characterized using X-ray diffraction methods (SCXRD and PXRD), spectroscopic methods (ATR-IR and solution-state and solid-state MAS NMR), and elemental and thermal analyses. The cytotoxicity of the different types of compounds against THP-1 and HepG2 cells was also evaluated.