Competitive Intramolecular Hydrogen Bonding: Offering Molecules a Choice.

The conformational preferences of N-((6-methylpyridin-2-yl)carbamothioyl)benzamide were studied in solution, the gas phase and the solid state via a combination of NMR, density functional theory (DFT) and single crystal X-ray techniques. This acyl thiourea derivative can adopt two classes of low energy conformation, each stabilized by a different 6-membered intramolecular hydrogen bond (IHB) pseudoring. Analysis in different solvents revealed that the conformational preference of this molecule is polarity dependent, with increasingly polar environments yielding a higher proportion of the minor conformer containing an NH…N IHB. The calculated barrier to interconversion is consistent with dynamic behaviour at room temperature, despite the propensity of 6-membered IHB pseudorings to be static. This work demonstrates that introducing competitive IHB pathways can render static IHBs more dynamic and that such systems could have potential as chameleons in drug design.

Tectonically-driven oxidant production in the hot biosphere.

Genomic reconstructions of the common ancestor to all life have identified genes involved in H2O2 and O2 cycling. Commonly dismissed as an artefact of lateral gene transfer after oxygenic photosynthesis evolved, an alternative is a geological source of H2O2 and O2 on the early Earth. Here, we show that under oxygen-free conditions high concentrations of H2O2 can be released from defects on crushed silicate rocks when water is added and heated to temperatures close to boiling point, but little is released at temperatures <80 °C. This temperature window overlaps the growth ranges of evolutionary ancient heat-loving and oxygen-respiring Bacteria and Archaea near the root of the Universal Tree of Life. We propose that the thermal activation of mineral surface defects during geological fault movements and associated stresses in the Earth's crust was a source of oxidants that helped drive the (bio)geochemistry of hot fractures where life first evolved.

Hybridising inorganic materials with fluorescent BOPHY dyes: A structural and optical comparative study.

This study presents the design and characterization of new monochromatic light-harvesting systems based on inorganic porous materials hybridized with organic dye molecules within their structure. A new fluorescent BOPHY dye was prepared, characterized optically and used as both reference and synthetic precursor for two alkoxysilane derivatives that were incorporated separately within a silica structure. The dyes, one bearing one alkoxysilane group and the other one two, were co-condensed with tetraethyl orthosilicate to form a hybrid organo-silica framework, where they are found at specific locations. The structure of the new materials was analysed by powder XRD and TEM, which confirmed the presence of the hexagonal pore arrangement typical of mesoporous MCM-41 silica particles. The steady-state and time-resolved analysis showed that the particles where the dyes are most dispersed within the framework retain the highest fluorescence quantum yield, up to 0.63, in the green-yellow region of the visible spectrum. On the other hand, increasing the content of BOPHY units in the solid matrix seem to favour non-radiative deactivation pathways and aggregation phenomena, which lower the efficiency of light emission. The materials also exhibit interesting properties, such as a dual excited-state decay and fluorescence anisotropy. The short fluorescence lifetime, about 2 ns, matches the typical singlet lifetime of BOPHY dyes, whereas the long component, up to 20 ns, is attributed to delayed fluorescence, which could take place via charge recombination. Optical anisotropy experiments revealed that all materials show polarised light emission to a significant extent and, for most samples, it was also possible to determine a polarisation transfer decay trace, from 400 to 800 ps This is ascribed to the occurrence of energy migration between neighbouring dye units within the silica structure.

Selective Gas Uptake and Rotational Dynamics in a (3,24)-Connected Metal-Organic Framework Material.

The desolvated (3,24)-connected metal-organic framework (MOF) material, MFM-160a, [Cu3(L)(H2O)3] [H6L = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid)], exhibits excellent high-pressure uptake of CO2 (110 wt% at 20 bar, 298 K) and highly selective separation of C2 hydrocarbons from CH4 at 1 bar pressure. Henry's law selectivities of 79:1 for C2H2:CH4 and 70:1 for C2H4:CH4 at 298 K are observed, consistent with ideal adsorption solution theory (IAST) predictions. Significantly, MFM-160a shows a selectivity of 16:1 for C2H2:CO2. Solid-state 2H NMR spectroscopic studies on partially deuterated MFM-160-d12 confirm an ultra-low barrier (∼2 kJ mol-1) to rotation of the phenyl group in the activated MOF and a rotation rate 5 orders of magnitude slower than usually observed for solid-state materials (1.4 × 106 Hz cf. 1011-1013 Hz). Upon introduction of CO2 or C2H2 into desolvated MFM-160a, this rate of rotation was found to increase with increasing gas pressure, a phenomenon attributed to the weakening of an intramolecular hydrogen bond in the triazine-containing linker upon gas binding. DFT calculations of binding energies and interactions of CO2 and C2H2 around the triazine core are entirely consistent with the 2H NMR spectroscopic observations.

Gas adsorption and structural diversity in a family of Cu(II) pyridyl-isophthalate metal-organic framework materials.

A family of Cu(II)-based metal-organic frameworks (MOFs) has been synthesized using three pyridyl-isophthalate ligands, H2L1 (4'-(pyridin-4-yl)biphenyl-3,5-dicarboxylic acid), H2L2 (4''-(pyridin-4-yl)-1,1':4',1''-terphenyl-3,5-dicarboxylic acid) and H2L3 (5-[4-(pyridin-4-yl)naphthalen-1-yl]benzene-1,3-dicarboxylic acid). Although in each case the pyridyl-isophthalate ligands adopt the same pseudo-octahedral [Cu2(O2CR)4N2] paddlewheel coordination modes, the resulting frameworks are structurally diverse, particularly in the case of the complex of Cu(II) with H2L3, which leads to three distinct supramolecular isomers, each derived from Kagomé and square nets. In contrast to [Cu(L2)] and the isomers of [Cu(L3)], [Cu(L1)] exhibits permanent porosity. Thus, the gas adsorption properties of [Cu(L1)] were investigated with N2, CO2 and H2, and the material exhibits an isosteric heat of adsorption competitive with leading MOF sorbents for CO2 [Cu(L1)] displays high H2 adsorption, with the density in the pores approaching that of liquid H2This article is part of the themed issue 'Coordination polymers and metal-organic frameworks: materials by design'.

Tuning the coordination chemistry of a Cu(II) complex at high-pressure.

Application of pressure on the Cu-complex CuAsp causes Jahn-Teller Cu-O bonds to be compressed, increasing the coordination environment from [4 + 1] to [4 + 2], highlighted by a discontinuity on compression of these bonding interactions.

A metal-organic framework with a covalently prefabricated porous organic linker.

We report here the synthesis of a metal-organic framework comprising an organic cage linker with covalently prefabricated, intrinsic porosity. The network can be compared to a porous rock salt structure where the pores are partially filled by charge-balancing cations.

A homochiral three-dimensional zinc aspartate framework that displays multiple coordination modes and geometries.

A homochiral three-dimensional zinc aspartate has been synthesised via reflux and solvothermal methods, which due to the coordinative flexibility of zinc displays three distinct coordination geometries and three coordination modes of aspartic acid to zinc.

Nitric oxide chemisorption in a postsynthetically modified metal-organic framework.

Postsynthetic metal-organic framework (MOF) derivatization introduces accessible secondary amine functionalities that react with nitric oxide (NO) to form N-diazenium diolates. This is in contrast to the parent MOF that binds NO essentially irreversibly at open metal coordination sites.