Ligand and Metal Effects on the Stability and Adsorption Properties of an Isoreticular Series of MOFs Based on T‐Shaped Ligands and Paddle‐Wheel Secondary Building Units

The synthesis of stable porous materials with appropriate pore size and shape for desired applications remains challenging. In this work a combined experimental/computational approach has been undertaken to tune the stability under various conditions and the adsorption behavior of a series of MOFs by subtle control of both the nature of the metal center (Co²⁺, Cu²⁺, and Zn²⁺) and the pore surface by the functionalization of the organic linkers with amido and N‐oxide groups. In this context, six isoreticular MOFs based on T‐shaped ligands and paddle‐wheel units with ScD_0.33 topology have been synthesized. Their stabilities have been systematically investigated along with their ability to adsorb a wide range of gases (N₂, CO₂, CH₄, CO, H_2, light hydrocarbons (C₁–C₄)) and vapors (alcohols and water). This study has revealed that the MOF frameworks based on Cu²⁺ are more stable than their Co²⁺ and Zn²⁺ analogues, and that the N‐oxide ligand endows the MOFs with a higher affinity for CO₂ leading to excellent selectivity for this gas over other species.     

New N,N-diamine ligands derived from (−)-menthol and their application in the asymmetric transfer hydrogenation

Natural (?)-menthol was applied to construction of mono-N-tosylated-1,2-diamine derivatives. The O-tosylation and elimination of the tosylate of the menthol intermediate led to trans-p-menth-2-ene. The unsaturated menth-2-ene was next transformed into a mixture of N-tosylaziridines, which upon reaction with sodium azide gave four isomeric azides. The reduction of the formed tosylazides on Pd/C gave new chiral mono-N-tosylated-1,2-diamines, which were used as ligands in the asymmetric transfer hydrogenation protocol on aromatic ketones and endocyclic imine.     

2‐Hydroxyazobenzenes to Tailor pH Pulses and Oscillations with Light

This paper evaluates the 2‐hydroxyazobenzene platform for tailoring proton concentration pulses and oscillations with monochromatic light. The easily prepared 2‐hydroxyazobenzenes exhibit large absorptions in the near‐UV range. Photoisomerization was investigated by UV/Vis absorption, ¹H NMR spectroscopy, and steady‐state fluorescence emission. In the whole investigated series, the trans stereoisomer of the 2‐hydroxyazobenzene motif provides the corresponding cis derivative with an action cross section in the 10³ M ^?1 cm^?1 range. At the same time, photoisomerization is accompanied by a significant pK drop of the phenol group. According to the phenyl‐substituent pattern, cis‐to‐trans thermal back‐isomerization can be tuned in the 10 ms–100 s range. Up to 2 units of reversible pH drops or pH oscillations on the 10 s timescale have been obtained by appropriately tailoring single‐wavelength illumination of 2‐hydroxyazobenzene solutions.     

Engineering micromechanics of soft porous crystals for negative gas adsorption

Framework materials at the molecular level, such as metal–organic frameworks (MOF), were recently found to exhibit exotic and counterintuitive micromechanical properties. Stimulated by host–guest interactions, these so-called soft porous crystals can display counterintuitive adsorption phenomena such as negative gas adsorption (NGA). NGA materials are bistable frameworks where the occurrence of a metastable overloaded state leads to pressure amplification upon a sudden framework contraction. How can we control activation barriers and energetics via functionalization of the molecular building blocks that dictate the frameworks'' mechanical response? In this work we tune the elastic and inelastic properties of building blocks at the molecular level and analyze the mechanical response of the resulting frameworks. From a set of 11 frameworks, we demonstrate that widening of the backbone increases stiffness, while elongation of the building blocks results in a decrease in critical yield stress of buckling. We further functionalize the backbone by incorporation of sp³ hybridized carbon atoms to soften the molecular building blocks, or stiffen them with sp² and sp carbons. Computational modeling shows how these modifications of the building blocks tune the activation barriers within the energy landscape of the guest-free bistable frameworks. Only frameworks with free energy barriers in the range of 800 to 1100 kJ mol⁻¹ per unit cell, and moderate yield stress of 0.6 to 1.2 nN for single ligand buckling, exhibit adsorption-induced contraction and negative gas adsorption. Advanced experimental in situ methodologies give detailed insights into the structural transitions and the adsorption behavior. The new framework DUT-160 shows the highest magnitude of NGA ever observed for nitrogen adsorption at 77 K. Our computational and experimental analysis of the energetics and mechanical response functions of porous frameworks is an important step towards tuning activation barriers in dynamic framework materials and provides critical design principles for molecular building blocks leading to pressure amplifying materials.

We characterise the elastic properties of molecular building blocks and how they impact the mechanical properties of soft porous crystals.     

Effet Mössbauer de 119Sn dans les systèmes SnSBaS et SnSTl2S

Tin-119 Mössbauer spectra have been recorded at liquid nitrogen and room temperatures for ternary sulfides isolated from the systems SnSBaS and SnSTl₂S. The chemical isomer shifts observed (2.8 – 3.4 mm/sec relative to BaSnO₃ at 293 K) are characteristic of divalent tin compounds. The data are consistent with the known structures of these compounds and are discussed with regard to the stereochemical activity of the tin(II) lone pair of electrons. For Tl₂Sn₂S₃ and Tl₄SnS₃ the Mössbauer parameters are interpreted in terms of direct population of conductance bands by nonbonding electron pairs.     

Cation mobility upon adsorption of methanol in NaY faujasite type zeolite: A molecular dynamics study compared to dielectric relaxation experiments

Molecular Dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY Faujasite system as a function of the loading. It has been shown that at low and intermediate loadings, SII cations can migrate toward the center of the supercage due strong interaction with the adsorbates, followed by hopping of SI' cations from the sodalite cage into the supercage to fill vacant SII sites. SI cations mainly remain trapped in their initial sites whatever the loading. At higher loading, only limited motion is observed for SII cations due to steric effects induced by the adsorbates within the supercage. These simulated results are in good agreement with those extracted by Complex Impedance Spectroscopy measurements, which provided the evolution of the number of extraframework cations in the different crystallographic sites as a function of the treatment temperature.     

Magnetic and Mössbauer investigation of the photomagnetic Prussian blue analogue Na(0.32)Co[Fe(CN)6](0.74).3.4H2O: cooperative relaxation of the thermally quenched state

Thanks to thermal quenching we investigated the relaxation of the metastable state of Na(0.32)Co[Fe(CN)6](0.74).3.4H2O at low temperature. A self-accelerated process has been observed in agreement with the cooperative character of the system, responsible for the large thermal hysteresis of the charge-transfer-induced spin transition. The mean-field analysis of the relaxation is discussed with respect to the equilibrium properties. A sizable deviation from mean-field behavior is observed at the beginning of the relaxation process, which might be attributed to a preliminary structural relaxation of the quenched state.