The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL¹) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL²) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe^II(HL)₂](ClO₄)₂, [Fe^IIL₂] and [Fe^IIIL₂]ClO₄ (HL = HL¹ or HL²) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe^III(L¹)₂]ClO₄ is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe^III(L¹)₂]ClO₄ and [Fe^III(L²)₂]ClO₄ are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the ⁶A₁ ↔ ²T₂ transition curve is sigmoidal with T_½ ∼280 K and the SCO is virtually complete. As regards [Fe^III(L¹)₂]ClO₄, Mössbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.

Flexible metal–organic frameworks (MOFs) are promising materials in gas-related technologies. Adjusting the material to processes requires understanding of the flexibility mechanism and its influence on the adsorption properties. Herein, we present the mechanistic understanding of CO₂-induced pore-opening transitions of the water-stable MOF JUK-8 ([Zn(oba)(pip)]_n, oba^2– = 4,4′-oxybis(benzenedicarboxylate), pip = 4-pyridyl-functionalized benzene-1,3-dicarbohydrazide) as well as its potential applicability in gas purification. Detailed insights into the global structural transformation and subtle local MOF–adsorbate interactions are obtained by three in situ techniques (XRD, IR, and ¹³CO₂-NMR). These results are further supported by single-crystal X-ray diffraction (SC-XRD) analysis of the solvated and guest-free phases. High selectivity toward carbon dioxide derived from the single-gas adsorption experiments of CO₂ (195 and 298 K), Ar (84 K), O₂ (90 K)_, N₂ (77 K), and CH₄ (298 K) is confirmed by high-pressure coadsorption experiments of the CO₂/CH₄ (75:25 v/v) mixture at different temperatures (288, 293, and 298 K) and in situ NMR studies of the coadsorption of ¹³CO₂/¹³CH₄ (50:50 v/v; 195 K).

We report a high proton-conducting material prepared for the first time by economical and environmentally-friendly mechanochemistry. Structural elucidation of the material from powder X-ray diffraction data reveals the details of the solid-state reaction. The reaction represents a new synthetic strategy towards materials related to fuel cell technology.