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Vapor and thermally induced solid-state structural transformations

Reactions or structural transformations in the solid-state are more difficult than in solution as they involve new bond formation or breakage with restricted molecular movements (Vittal, 2007).The external stimuli that usually promotes such structural transformations are heat, solvent vapor, mechanochemical force, light, etc. (Garay et al., 2007).Synthesis of the dioxomolybdenum(VI) complexes was carried out by using dichlorodioxomolybdenum(VI) ([MoO2Cl2]) and the corresponding nicotinoyl hydrazonato ligands derived from salicylaldehyde (L1), 3-methoxy-2- hydroxybenaldehyde (L2) or 4-methoxy- 2hydroxybenzaldehyde (L3). The mononuclear complexes[MoO2(HL1, 2)(MeOH)]Cl (1a, 2a), and [MoO2(HL2, 3)Cl] (2b and 3b), were synthesed and used as potential precursors for structural transformations in the solid state (Figure 1).Vapor induced transformation of [MoO2(HL2) (MeOH)]Cl (2a) afforded quantitatively and rapidly complex [MoO2(HL2)Cl] (2b). Upon vapor exposure the coordinated MeOH ligands were lost and the coordination sphere was simultaneously re-established by adjacent Cl– ligands. The structural transformations are primarily influenced by rearrangement of bonds and/or the removal of solvent only if the reacting groups of the molecules are closely and in correct orientation for the reaction to occur in the solid-state.Crystals of [MoO2(HL2)(MeOH)]Cl (2a) were further exposed to diffeerentsolvent vapors (water, acetonitrile, dichloromethane and i-propanol) in several independent sets. A sample of each set was analyzed ex-situ by ATR- IR spectroscopy andPXRD.The chemometric analysis using principal component analysis (PCA) was applied to analyze ATR-IR spectra obtained by reaction monitoring in order to provide an insight on the reaction profiles. The fastest transformation was in the case of water vapors (3 min) whereas the slowest one was in dichloromethane vapors (48 min).The complex [MoO2(HL2)Cl] (2b) was obtained as the final product no matter which solvent was used.This property can be useful for solvent detection in the atmosphere and similar compounds could have potential application as sensors for environmental monitoring. Thermally induced transformation resulted in the formation of the coordination polymer [MoO2(L2)]n.

molybdenum(VI) complexes ; aroylhydrazone ; vapour and thermally induced structural transformations

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