Carlotta Giacobbe

When long bis(pyrazolates) meet late transition metals: structure, stability and adsorption of metal–organic frameworks featuring large parallel channels

A family of bis(pyrazolato)-based metal-organic frameworks (MOFs) was isolated by reacting 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB) with a number of transition metal ions. Special attention was dedicated to their structural features, their thermal and chemical stability, as well as their spectroscopic and adsorption properties. The rod-like ligands, connecting Zn(II), Ni(II) and Fe(III) nodes, fabricate 3-D networks containing 1-D pervious channels. The combination of thermal analysis and variable-temperature XRPD demonstrated the remarkable thermal robustness of the three materials, which are stable in air up to at least 410 °C, and showed their structural response to increasing temperature. Specific experiments permitted us to test the chemical stability of the three species toward water as well as moderately acidic and basic solutions, the Ni(II) derivative being stable and hydrophobic in all the conditions assayed. The electronic transitions of both the ligand and the MOFs were investigated by solid-state UV-Vis absorption as well as by steady-state and time-resolved fluorescence analysis, which showed that the high fluorescence of the linker is perturbed in the three MOFs, suggesting high sensitivity to environmental changes. N2 adsorption measurements at 77 K allowed to estimate promising Langmuir specific surface areas, peaking at 2378 m2 g−1 in the case of the Ni(II) derivative. The best CO2 and CH4 uptake performances were achieved with the Fe(III)-based MOF. Indeed, adsorption experiments with CO2 revealed that a considerable amount, up to 40% wt, is adsorbed by the Fe(III) derivative under the mild conditions of 298 K and 10 bar.

Metal Azolate/Carboxylate Frameworks as Catalysts in Oxidative and C-C Coupling Reactions

The five metal azolate/carboxylate (MAC) compounds [Cd(dmpzc)(DMF)(H2O)] (Cd-dmpzc), [Pd(H2dmpzc)2Cl2] (Pd-dmpzc), [Cu(Hdmpzc)2] (Cu-dmpzc), [Zn4O(dmpzc)3]·Solv (Zn-dmpzc·S), and [Co4O(dmpzc)3]·Solv (Co-dmpzc·S) were isolated by coupling 3,5-dimethyl-1H-pyrazol-4-carboxylic acid (H2dmpzc) to cadmium(II), palladium(II), copper(II), zinc(II), and cobalt(II) salts. While Cd-dmpzc and Pd-dmpzc had never been prepared in the past, for Cu-dmpzc, Zn-dmpzc·S, and Co-dmpzc·S we optimized alternative synthetic paths that, in the case of the copper(II) and cobalt(II) derivatives, are faster and grant higher yields than the previously reported ones. The crystal structure details were determined ab initio (Cd-dmpzc and Pd-dmpzc) or refined (Cu-dmpzc, Zn-dmpzc·S, and Co-dmpzc·S) by means of powder X-ray diffraction (PXRD). While Cd-dmpzc is a nonporous 3D MAC framework, Pd-dmpzc shows a 3D hybrid coordination/hydrogen-bonded network, in which Pd(H2dmpzc)2Cl2 monomers are present. The thermal behavior of the five MAC compounds was investigated by coupling thermal analysis to variable-temperature PXRD. Their catalytic activity was assessed in oxidative and C-C coupling reactions, with the copper(II) and cadmium(II) derivatives being the first nonporous MAC frameworks to be tested as catalysts. Cu-dmpzc is the most active catalyst in the partial oxidation of cyclohexane by tert-butyl hydroperoxide in acetonitrile (yields up to 12% after 9 h) and is remarkably active in the solvent-free microwave-assisted oxidation of 1-phenylethanol to acetophenone (yields up to 99% at 120 °C in only 0.5 h). On the other hand, activated Zn-dmpzc·S (Zn-dmpzc) is the most active catalyst in the Henry C-C coupling reaction of aromatic aldehydes with nitroethane, showing appreciable diastereoselectivity toward the syn-nitroalkanol isomer (syn:anti selectivity up to 79:21).

Difluprednate: more than meets the eye.

Difluprednate, a FDA approved topical corticosteroid indicated for the treatment of inflammation and pain associated with ocular surgery, affords three polymorphic crystal forms (one hexagonal, sg. P65, and two distinct orthorhombic, both sg.s P212121, phases), whose preparation, thermal stability ranges, crystal structures and stereochemical preferences are here reported. Using DSC, single-crystal structural analysis and less conventional ab-initio X-ray powder diffraction methods, the rich structural and thermal behavior of three difluprednate polymorphs have been clarified, and the validity of previous complex and sometimes contradicting literature reports has been challenged. Complementary solution state NMR provided (1)H, (13)C and (19)F chemical shifts full assignment of the corresponding signals. These results allow us to precisely describe the selective isolation pathways toward three distinct crystal phases, and to define their structural and analytical data necessary for identification and easy and accurate quantification, by modern Rietveld analysis, of complex difluprednate polymorphic mixtures, often obtained as a result of poorly controlled (co)-precipitation methods.

Nanostructured Drugs Embedded into a Polymeric Matrix: Vinpocetine/PVP Hybrids Investigated by Debye Function Analysis

Microcrystalline vinpocetine, coground with cross-linked polyvinylpyrrolidone, affords hybrids containing nanosized drug nanocrystals, the size and size distributions of which depend on milling times and drug-to-polymer weight ratios. Using an innovative approach to microstructural characterization, we analyzed wide-angle X-ray total scattering data by the Debye function analysis and demonstrated the possibility to characterize pharmaceutical solid dispersions obtaining a reliable quantitative view of the physicochemical status of the drug dispersed in an amorphous carrier. The microstructural properties derived therefrom have been successfully employed in reconciling the enigmatic difference in behavior between in vitro and in vivo solubility tests performed on nanosized vinpocetine embedded in a polymeric matrix.

Elucidating the CO2 adsorption mechanisms in the triangular channels of the bis(pyrazolate) MOF Fe2(BPEB)3 by in situ synchrotron X-ray diffraction and molecular dynamics simulations

The structural origin of the remarkable performance of the metal–organic framework Fe2(BPEB)3 as a CO2 adsorbent (40.5% of the host weight at 298 K and 10 bar) was investigated by combining advanced experimental and computational tools. In situ and operando high-resolution and high-energy powder X-ray diffraction experiments were carried out at 273 and 298 K dosing CO2 in the pressure range of 0–9 bar and 0–21 bar, respectively. Juxtaposing a conventional approach for structural determination and refinement with total scattering analysis enabled us to unveil the adsorption sites occupied by the gaseous probe and the host–guest interactions at 298 K and all the assayed pressures. These studies were complemented by molecular dynamics simulations to support the location of the guest molecules inside the host channels and characterize the energetics of the host–guest and guest–guest interactions. Our molecular-level insight into the nature of the host–guest interactions in Fe2(BPEB)3, a rigid host without exposed metal sites or functional groups with particular affinity to CO2 on the ligand, provides key information on a debated topic, namely the chemical and structural properties a host should possess for efficient CO2 adsorption under rather mild conditions.

Impact of Structural Polymorphism on Ionic Conductivity in Lithium Copper Pyroborate Li6CuB4O10

The search for high Li-ion conducting ceramics has regained tremendous interest triggered by the renaissance of the all-solid-state battery. Within this context we herein reveal the impact of structural polymorphism of lithium copper pyroborate Li6CuB4O10 on its ionic conductivity. Using combined in situ synchrotron X-ray and neutron powder diffraction, a structural and synthetic relationship between α- and β-Li6CuB4O10 could be established and its impact on ionic conductivity evolution was followed using electrochemical impedance spectroscopy. We show that the high temperature form of Li6CuB4O10 exhibits a high Li-ion conductivity (2.7 mS cm-1 at 350 °C) and solve its crystal structure for the first time. Our results emphasize the significant impact of structural phase transitions on ionic conductivity and show possible high Li-ion mobility within borate based compounds.

Structure and properties of robust bipyrazolato-based coordination polymers

Porous coordination polymers (PCPs) represent a consolidated research line for numerous disciplines, from chemistry to physics, materials science, biology and even pharmacology. As a matter of fact, the clever combination of tailored structural features and key functional properties renders PCPs potentially suitable for technologically advanced applications. As shown also by our work [1-4], polyazolato-based PCPs are particularly promising in this respect, because they typically juxtapose, to interesting structures and functions, high thermal and chemical stability. Along this research line, the present contribution focuses upon the family of bipyrazolato-based PCPs isolated by reacting the ditopic ligand 1,4-bis(4-acetylydopyrazolyl) benzene (H2L) with a number of transition metal ions. Special concern is dedicated to structural aspects, retrieved by state-of-the-art XRPD methods applied to laboratory data, thermal and chemical robustness, adsorption properties. All the materials possess 3-D networks: while NiL and Fe2L3 feature non interpenetrated architectures, interpenetration is found in ZnL. Despite this, all show 1-D pervious channels, the shape of which depends on the stereochemistry of the metal ions: square, rhombic, or triangular channels are observed for ZnL, NiL, or Fe2L3, accounting for 49, 75 or 64% of the total volume. Coupling DSC to in situ variable temperature XRPD allowed to demonstrate i) the remarkable thermal robustness of the materials, stable in air up to 310 °C; ii) their stability along consecutive cycles of thermal activation; iii) the existence of permanent porosity; iv) the response of the unit cell parameters to temperature increase. Solutionphase tests allowed to assess the chemical stability toward a number of common solvents at room temperature for a few days. Finally, N2 adsoption measurements at 77 K allowed to estimate promising BET specific surface areas above 1000 m2/g.