Anna Gągor

Order–Disorder Transition and Weak Ferromagnetism in the Perovskite Metal Formate Frameworks of [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3] (M = Ni, Mn)

We report the synthesis, crystal structure, thermal, dielectric, Raman, infrared, and magnetic properties of hydrogen and deuterated divalent metal formates, [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3], where M = Ni, Mn. On the basis of Raman and IR data, assignment of the observed modes to respective vibrations of atoms is proposed. The thermal studies show that for the Ni compounds deuteration leads to a decrease of the phase transition temperature Tc by 5.6 K, whereas it has a negligible effect on Tc in the Mn analogues. This behavior excludes the possibility of proton (deuteron) movement along the N-H···O (N-D···O) bonds as the microscopic origin of the first-order phase transition observed in these crystals below 190 K. According to single-crystal X-ray diffraction, the dimethylammonium (DMA) cations are dynamically disordered at room temperature, because the hydrogen bonds between the NH2 (ND2) groups and the metal-formate framework are disordered. The highly dynamic nature of hydrogen bonds in the high-temperature phases manifests in the Raman and IR spectra through very large bandwidth of modes involving vibrations of the NH2 (ND2) groups. The abrupt decrease in the bandwidth and shifts of modes near Tc signifies the ordering of hydrogen bonds and DMA(+) cations as well as significant distortion of the metal-formate framework across the phase transition. However, some amount of motion is retained by the DMA(+) cation in the ferroelectric phase and a complete freezing-in of this motion occurs below 100 K. The dielectric studies reveal pronounced dielectric dispersion that can be attributed to slow dynamics of large DMA(+) cations. The low-temperature studies also show that magnetic properties of the studied compounds can be explained assuming that they are ordered ferrimagnetically with nearly compensated magnetic moments of Ni and Mn. IR data reveal weak anomalies below 40 K that arise due to spin-phonon coupling. Our results also show that due to structural phase transition more significant distortion of the metal-formate framework occurs for the deuterated samples.

Perovskite metal formate framework of [NH2-CH(+)-NH2]Mn(HCOO)3]: phase transition, magnetic, dielectric, and phonon properties.

We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH2-CH(+)-NH2][Mn(HCOO)3] (FMDMn). The anionic framework of [(Mn(HCOO)3(-)] is counterbalanced by formamidinium (FMD(+)) cations located in the cavities of the framework. These cations form extensive N-H···O hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R3̅c symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD(+) cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH2 groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD(+) cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD(+) cations is associated with significant distortion of the anionic framework. On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature Tc = 8.0 K.

Structural, magnetic and dielectric properties of two novel mixed-valence iron(II)–iron(III) metal formate frameworks

Two novel mixed-valence iron(II)–iron(III) formate frameworks templated by ethylammonium and diethylammonium cations have been prepared and characterized by DSC, X-ray diffraction and spectroscopic methods. We also report dielectric and magnetic properties of the obtained samples. Both MOFs crystallize in the P1c structure and exhibit magnetic order at 39 K. The analogue with diethylammonium cations undergoes a structural phase transition near 240 K into a triclinic phase. This transition has an order–disorder character and it is associated with pronounced dielectric anomaly. This compound is therefore the second discovered mixed-valence metal formate exhibiting multiferroic properties.

Temperature- and pressure-induced phase transitions in the niccolite-type formate framework of [H3N(CH3)4NH3][Mn2(HCOO)6]

We report the synthesis, crystal structure, thermal, pyroelectric, Raman, infrared and magnetic properties of [NH3(CH2)4NH3][Mn2(HCOO)6] niccolite. Our results show that this compound crystallizes in a trigonal structure (space group P1c) with dynamically disordered [NH3(CH2)4NH3]2+ cations. It undergoes a phase transition near Tc = 350 K. The low-temperature structure is polar (space group Cc) and pyroelectric measurements confirm that it exhibits ferroelectric properties. Detailed analysis of the structural changes shows that both the spatial arrangement of the [NH3(CH2)4NH3]2+ dipole moments and distortion of the manganese formate framework contribute to the spontaneous polarization within the (a, c) plane. Based on Raman and IR data, assignment of the observed modes to the respective vibrations of atoms is also proposed. Dynamic disorder of organic cations in the high-temperature phase manifests in the vibrational spectra through very large width of bands corresponding to vibrations of the NH3 groups. Ordering of these cations is clearly observed in the spectra through a pronounced decrease in their bandwidths below the phase transition temperatures. Low-temperature magnetic studies show that this compound is a weak ferromagnet below 9.0 K. We also report high-pressure Raman scattering studies of this compound, which reveal the presence of two pressure-induced phase transitions between 0.5 and 0.9 GPa and between 1.3 and 1.9 GPa.

Phase transitions and chromium(III) luminescence in perovskite-type [C2H5NH3][Na0.5CrxAl0.5−x(HCOO)3] (x = 0, 0.025, 0.5), correlated with structural, dielectric and phonon properties

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C2H5NH3][Na0.5Cr0.5(HCOO)3] (EtANaCr), [C2H5NH3][Na0.5Al0.5(HCOO)3] (EtANaAl) and [C2H5NH3][Na0.5Al0.475Cr0.025(HCOO)3] (EtANaAlCr). These compounds crystallize in non-centrosymmetric monoclinic polar structures (space group Pn) and undergo order-disorder phase transitions upon heating to the monoclinic centrosymmetric structure (space group P21/n) at 369 (EtANaAl) and 373 K (EtANaCr). In principle, they are ferroelectric below these temperatures. In the high-temperature phase, ethylammonium (EtA+) cations are dynamically disordered over two symmetrically independent positions while upon cooling they begin to order. The ordering is accompanied by distortion of the metal formate framework. The hydrogen bonds (HBs) between the NH3+ group and NaO6 octahedral units are more robust than between the NH3+ group and CrO6 (AlO6) octahedral units and this feature explains a much stronger distortion of the former units and a weak effect of a trivalent cation type on the phase transition temperature. The dielectric studies have confirmed the occurrence of phase transitions of dipolar character and dipole relaxation processes. The optical studies show that EtANaCr and EtANaAlCr exhibit efficient Cr(iii)-based emission characteristics for intermediate-ligand field strength.

Improved properties of micronized genetically modified flax fibers.

The aim of this study was to investigate the effect of micronization on the compound content, crystalline structure and physicochemical properties of fiber from genetically modified (GM) flax. The GM flax was transformed with three bacterial (Ralstonia eutropha) genes coding for enzymes of polyhydroxybutyrate (PHB) synthesis and under the control of the vascular bundle promoter. The modification resulted in fibers containing the 3-hydroxybutyrate polymer bound to cellulose via hydrogen and ester bonds and antioxidant compounds (phenolic acids, vanillin, vitexin, etc.). The fibers appeared to have a significantly decreased particle size after 20h of ball-milling treatment. Micronized fibers showed reduced phenolic contents and antioxidant capacity compared to the results for untreated fibers. An increased level of PHB was also detected. Micronization introduces structural changes in fiber constituents (cellulose, hemicellulose, pectin, lignin, PHB) and micronized fibers exhibit more functional groups (hydroxyl, carboxyl) derived from those constituents. It is thus concluded that micronization treatments improve the functional properties of the fiber components.

Temperature-dependent IR and Raman studies of metal–organic frameworks [(CH3)2NH2][M(HCOO)3], M=Mg and Cd

Two metal-organic frameworks of [(CH3)2NH2][M(HCOO)3], where M=Mg and Cd, have been investigated by temperature-dependent IR and Raman methods in order to determine the nature of the phase transition. Our results indicate that phase transition in the Mg-compound is driven by ordering of the dimethylammonium cations. Additional X-ray diffraction and spectroscopic studies on Cd-compound as the function of temperature reveal that this compound does not undergo any structural phase transition. We attribute this behavior to the large size of the cavity occupied by the dimethylammonium cations and thus weak hydrogen bonding between these cations and formate ions.

Synthesis, structure and optical properties of two novel luminescent polar dysprosium metal–organic frameworks: [(CH3)2NH2][Dy(HCOO)4] and [N2H5][Dy(HCOO)4]

We report the synthesis, crystal structures, Raman, infrared, electron absorption and emission spectra of two novel luminescent MOFs, [(CH3)2NH2][Dy(HCOO)4] and [N2H5][Dy(HCOO)4]. These data show that [(CH3)2NH2][Dy(HCOO)4] and [N2H5][Dy(HCOO)4] crystallize in polar space groups of Pna21 and Pca21 symmetry, respectively. The polar properties of [N2H5][Dy(HCOO)4] come from the arrangement of template cations that possess an internal dipole moment as well as from the symmetry of the framework. In [(CH3)2NH2][Dy(HCOO)4], the Dy(III)–formate framework is centrosymmetric and the polar properties of this compound arise from the arrangement of the DMA+ cations. Vibrational studies reveal that the position of the ν(NN) band, which is very sensitive to coordination changes, is characteristic for hydrazine molecules forming onium ions. Optical studies show that both compounds exhibit emission of Dy(III) ions, in spite of full concentration of dysprosium ions in the studied compounds. Our data also show that radiative decay times, 57 μs for [(CH3)2NH2][Dy(HCOO)4] and 96 μs for [N2H5][Dy(HCOO)4], are very long compared to lifetimes observed in inorganic compounds. We have also preformed Judd–Ofelt analysis of the optical absorption spectrum of the hydrazinium analogue that provides information on various optical parameters such as electric-dipole oscillator strengths, Judd–Ofelt parameters Ωλ, branching ratios and spontaneous emission probabilities.

Metal–organic framework in an L-arginine copper(II) ion polymer: structure, properties, theoretical studies and microbiological activity

A novel 1D polymeric copper(II) complex with L-arginine and a linear bridged 4,4′-bipyridine with a formula of {[Cu(L-Arg)2(μ-4,4′-bpy)]Cl2·3H2O}∞ (1) (where L-Arg = L-arginine, 4,4′-bpy = 4,4′-bipyridine) was synthesized. The crystal structure and properties of the product were characterized using X-ray diffraction, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), spectroscopic techniques (FT-IR, Raman, NIR-vis-UV electronic and EPR), magnetic methods, and microbiological examinations. The crystals of 1 crystallized in a trigonal system and a space group of P3221 was characterized with a = b = 12.31 A, c = 18.45 A, V = 2420 A3, Z = 3, α = β = 90° and γ = 120°. The N and O donor atoms of trans-chelated L-Arg zwitterions and two N atoms of the 4,4′-bpy molecule form a tetragonal distorted octahedral geometry around the copper(II) ions with static character (T = 0.748). The diffuse-reflectance electronic spectrum of 1 is characteristic of the [CuN2N′2O2] chromophore. The EPR spectrum of frozen 1 (at 77 K) dissolved in water is related to the N2O2 set (g⊥ = 2.057, g‖ = 2.258 and A‖ = 169 G). The structure of the [Cu(L-Arg)2(μ-4,4′-bpy)]2+ model complex was optimized at the B3LYP and B3LYP-D3 levels. The calculations of the atomic spin densities on the atoms in the doublet state of the model complex revealed that, with regard to the ligands, the spin population is distributed mainly over the oxygen and nitrogen atoms of L-arginine. The antimicrobial activities were examined against the Gram-positive and Gram-negative bacteria strains: Streptococcus mutans, Enterococcus hirae, Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Salmonella enterica, Shigella flexneri; and fungi: Saccharomyces cerevisiae, Candida albicans. Complex 1 exhibited strong antimicrobial activity against bacteria and fungi, both in their growth inhibition as well as in microbial killing.

Structural and vibrational properties of imidazo[4,5-c]pyridine, a structural unit in natural products.

The molecular structures and vibrational properties of 1H-imidazo[4,5-c]pyridine in its monomeric and dimeric forms are analyzed and related to the experimental results derived from the XRD, IR, and Raman studies. The theoretical data are discussed on the basis of DFT quantum chemical calculations using the B3LYP correlation functional and 6-311G(2d,2p) basis set. This compound crystallizes in the non-centrosymmetric orthorhombic space group Fdd2. The asymmetric unit contains one molecule of 1H-imidazo[4,5-c]pyridine and disordered molecules of solvents. The molecules are organized in hydrogen-bonded chains propagating along the [1 0 -3] direction. The stability of the dimeric form arising from charge delocalization and the existence of an N-H···N intermolecular hydrogen bond has been analyzed using the natural bond orbital approach. The normal modes, which are unique for the imidazopyridine skeleton, have been identified. The spectra of other compounds containing the imidazopyridine unit have been analyzed.