Michalina Anioła

Giant Negative Area Compressibility Tunable in a Soft Porous Framework Material

A soft porous material [Zn(L)2(OH)2]n·Guest (where L is 4-(1H-naphtho[2,3-d]imidazol-1-yl)benzoate, and Guest is water or methanol) exhibits the strongest ever observed negative area compressibility (NAC), an extremely rare property, as at hydrostatic pressure most materials shrink in all directions and few expand in one direction. This is the first NAC reported in metal-organic frameworks (MOFs), and its magnitude, clearly visible and by far the highest of all known materials, can be reversibly tuned by exchanging guests adsorbed from hydrostatic fluids. This counterintuitive strong NAC of [Zn(L)2(OH)2]n·Guest arises from the interplay of flexible [-Zn-O(H)-]n helices with layers of [-Zn-L-]4 quadrangular puckered rings comprising large channel voids. The compression of helices and flattening of puckered rings combine to give a giant piezo-mechanical response, applicable in ultrasensitive sensors and actuators. The extrinsic NAC response to different hydrostatic fluids is due to varied host-guest interactions affecting the mechanical strain within the range permitted by exceptionally high flexibility of the framework.

NH⋯O and OH⋯O interactions of glycine derivatives with squaric acid

Four new hydrogen-bonded complexes of a simple amino acid glycine (GLY) and its methyl derivatives – sarcosine (N-methylglycine, SAR), dimethylglycine (DMG) and betaine (N,N,N-trimethylglycinium, BET) with squaric acid (3,4-dihydroxy-3-cyclobuten-1,2-dione, H2SQ) are synthesized and characterized by X-ray diffraction, FTIR and NMR spectroscopy. The complexes differ in stoichiometry, interaction with H2SQ and the hydrogen-bonding system. Methylation of glycine gradually reverses the dissociation of squaric acid in co-crystals. This process is correlated with the number of N–H⋯O bonds to the squaric acid oxygens and the N–H donor capability of glycine derivatives. The effect of the presence of methyl groups on the proton and carbon-13 chemical shifts is studied. The DFT calculations are performed for one unit of complexes and stable zwitterionic structures of SAR, DMG and BET in complex with H2SQ, except the GLY unit, are suggested, which appears as an uncharged (neutral) form. The experimental and computed vibrational spectra of complexes studied are reported.

Negative linear compression and expanding NH⋯N bonds in an imidazoline compound

The 3-dimensional network of NH⋯N hydrogen bonds and Cl⋯Cl hydrogen contacts in the crystal structure of 2-(3′-chlorophenyl)imidazoline exhibits an anomalous hydrostatic compression. The lengthening of hydrogen bonds NH⋯N and some CH⋯N contacts as well as their supramolecular architecture lead to anomalous expansion of the crystal along [x] and [y] on increasing pressure to 0.1 GPa. The mechanism of this phenomenon is due to the ‘stiffness’ of the NH⋯N and Cl⋯Cl interactions and ‘softness’ of other van der Waals contacts. Above 0.1 GPa all crystal directions become compressed. However, up to 1.20 GPa, the crystal remains in the same orthorhombic phase of polar space group Fdd2.

Pressure-preferred symmetric reactions of 4,4′-bipyridine hydrobromide

High pressure and temperature trigger symmetric chemical reactions of 4,4′-bipyridine hydrobromide monohydrate (44′biPyHBr·H2O) in methanol solution. Above 0.1 GPa and 423 K, the 4,4′-bipyridinium dibromide salt (44′biPy2HBr) precipitates, while the 4,4′-bipyridine free base remains dissolved in the methanol–water mixture. At 0.35 GPa and 473 K, both pyridine moieties are N-methylated and N,N-dimethylbipyridinium dibromide (44′biPy2CH3Br) is formed. None of the high-pressure compounds of 44′biPy2HBr and 44′bPy2CH3Br are solvated, which contrasts with the strong preference of analogous 1,4-diazabicyclo[2.2.2]octane (dabco) monosalts and disalts to form solvates at high-pressure. The high-pressure reactivity of 44′biPyHBr is analogous to that of 1,4-diazabicyclo[2.2.2]octane hydrobromide (dabcoHBr); however, dabcoHBr is asymmetrically N-methylated at one amine site only. This asymmetric mono N-methylation of dabcoHBr has been associated with strong electrostatic interactions between the transannular N-atoms.