Semiconductive and magnetic one-dimensional coordination polymers of Cu(II) with modified nucleobases

Four new copper(II) coordination complexes, obtained by reaction of CuX2 (X = acetate or chloride) with thymine-1-acetic acid and uracil-1-propionic acid as ligands, of formulas [Cu(TAcO)2(H2O)4]·4H2O (1), [Cu(TAcO)2(H2O)2]n (2), [Cu3(TAcO)4(H2O)2(OH)2]n·4H2O (3), and [Cu3(UPrO)2Cl2(OH)2(H2O)2]n (4) (TAcOH = thymine-1-acetic acid, UPrOH = uracil-1-propionic acid) are described. While 1 is a discrete complex, 2-4 are one-dimensional coordination polymers. Complexes 2-4 present dc conductivity values between 10(-6) and 10(-9) S/cm(-1). The magnetic behavior of complex 2 is typical for almost isolated Cu(II) metal centers. Moderate-weak antiferromagnetic interactions have been found in complex 3, whereas a combination of strong and weak antiferromagnetic interactions have been found in complex 4. Quantum computational calculations have been done to estimate the individual "J" magnetic coupling constant for each superexchange pathway in complexes 3 and 4. Compounds 2-4 are the first known examples of semiconductor and magnetic coordination polymers containing nucleobases.

Carbonaceous electrode materials for supercapacitors

Supercapacitors have been widely studied around the world in recent years, due to their excellent power density and long cycle life. As the most frequently used electrode materials for supercapacitors, carbonaceous materials attract more and more attention. However, their relatively low energy density still holds back the widespread application. Up to now, various strategies have been developed to figure out this problem. This research news summarizes the recent advances in improving the supercapacitor performance of carbonaceous materials, including the incorporation of heteroatoms and the pore size effect (subnanopores' contribution). In addition, a new class of carbonaceous materials, porous organic networks (PONs) has been managed into the supercapacitor field, which promises great potential in not only improving the supercapacitor performances, but also unraveling the related mechanisms.

Post‐synthetic Modification of MOFs

Post‐synthetic modification is increasingly recognised as an important and versatile tool in the preparation of functionalised metal organic frameworks (MOFs). The process involves one or more reactions on a pre‐formed MOF, and it can be used to prepare MOFs that are not accessible by direct combination of metal and linker. This review explores the methods and strategies that have been developed for post‐synthetically modifying MOFs, concentrating on four classes of reaction: covalent transformations of the linker, coordination of a metal centre to a linker, modification of the inorganic part of the MOF and exchange of counter‐ions. Examples of the use of the modified MOFs are given, with a focus on their utility in catalysis.

Microporous organic polymers with ketal linkages: synthesis, characterization, and gas sorption properties

A series of microporous organic polymers with ketal linkages were synthesized based on the condensation of aromatic acetyl monomers with pentaerythritol. Fourier transform infrared and solid-state cross-polarization/magic-angle-spinning (13)C NMR spectroscopy were utilized to confirm the ketal linkages of the resulting polymers. The morphology can be observed from scanning electron microscopy and transmission electron microscopy images. The materials possess Brunauer-Emmet-Teller specific surface area values ranging from 520 to 950 m(2) g(-1), and the highest hydrogen sorption capacity is up to 1.96 wt % (77 K and 1.0 bar), which is superior to that of most of microporous organic polymers. The facile and cost-effective preparation process and excellent gas sorption properties make these kinds of materials promising candidates for practical applications.

Absorption of CO2 and CS2 into the Hofmann-type porous coordination polymer: electrostatic versus dispersion interactions

Absorption of CO2 and CS2 molecules into the Hofmann-type three-dimensional porous coordination polymer (PCP) {Fe(Pz)[Pt(CN)4]}n (Pz = pyrazine) was theoretically explored with the ONIOM(MP2.5 or SCS-MP2:DFT) method, where the M06-2X functional was employed in the DFT calculations. The binding energies of CS2 and CO2 were evaluated to be -17.3 and -5.2 kcal mol(-1), respectively, at the ONIOM(MP2.5:M06-2X) level and -16.9 and -4.4 kcal mol(-1) at the ONIOM(SCS-MP2:M06-2X) level. It is concluded that CS2 is strongly absorbed in this PCP but CO2 is only weakly absorbed. The absorption positions of these two molecules are completely different: CO2 is located between two Pt atoms, whereas one S atom of CS2 is located between two Pz ligands and the other S atom is between two Pt atoms. The optimized position of CS2 agrees with the experimentally reported X-ray structure. To elucidate the reasons for these differences, we performed an energy decomposition analysis and found that (i) both the large binding energy and the absorption position of CS2 arise from a large dispersion interaction between CS2 and the PCP, (ii) the absorption position of CO2 is mainly determined by the electrostatic interaction between CO2 and the Pt moiety, and (iii) the small binding energy of CO2 comes from the weak dispersion interaction between CO2 and the PCP. Important molecular properties relating to the dispersion and electrostatic interactions, which are useful for understanding and predicting gas absorption into PCPs, are discussed in detail.

Alkali-metal azides interacting with metal-organic frameworks

Interactions between alkali-metal azides and metal-organic framework (MOF) derivatives, namely, the first and third members of the isoreticular MOF (IRMOF) family, IRMOF-1 and IRMOF-3, are studied within the density functional theory (DFT) paradigm. The investigations take into account different models of the selected IRMOFs. The mutual influence between the alkali-metal azides and the π rings or Zn centers of the involved MOF derivatives are studied by considering the interactions both of the alkali-metal cations with model aromatic centers and of the alkali-metal azides with distinct sites of differently sized models of IRMOF-1 and IRMOF-3. Several exchange and correlation functionals are employed to calculate the corresponding interaction energies. Remarkably, it is found that, with increasing alkali-metal atom size, the latter decrease for cations interacting with the π-ring systems and increase for the azides interacting with the MOF fragments. The opposite behavior is explained by stabilization effects on the azide moieties and determined by the Zn atoms, which constitute the inorganic vertices of the IRMOF species. Larger cations can, in fact, coordinate more efficiently to both the aromatic center and the azide anion, and thus stabilizing bridging arrangements of the azide between one alkali-metal and two Zn atoms in an η(2) coordination mode are more favored.

Progress on first-principles-based materials design for hydrogen storage

This article briefly summarizes the research activities in the field of hydrogen storage in sorbent materials and reports our recent works and future directions for the design of such materials. Distinct features of sorption-based hydrogen storage methods are described compared with metal hydrides and complex chemical hydrides. We classify the studies of hydrogen sorbent materials in terms of two key technical issues: (i) constructing stable framework structures with high porosity, and (ii) increasing the binding affinity of hydrogen molecules to surfaces beyond the usual van der Waals interaction. The recent development of reticular chemistry is summarized as a means for addressing the first issue. Theoretical studies focus mainly on the second issue and can be grouped into three classes according to the underlying interaction mechanism: electrostatic interactions based on alkaline cations, Kubas interactions with open transition metals, and orbital interactions involving Ca and other nontransitional metals. Hierarchical computational methods to enable the theoretical predictions are explained, from ab initio studies to molecular dynamics simulations using force field parameters. We also discuss the actual delivery amount of stored hydrogen, which depends on the charging and discharging conditions. The usefulness and practical significance of the hydrogen spillover mechanism in increasing the storage capacity are presented as well.

Attaching molecular hydrogen to metal cations: perspectives from gas-phase infrared spectroscopy

In this perspective article we describe recent infrared spectroscopic investigations of mass-selected M(+)-H(2) and M(+)-D(2) complexes in the gas-phase, with targets that include Li(+)-H(2), B(+)-H(2), Na(+)-H(2), Mg(+)-H(2), Al(+)-H(2), Cr(+)-D(2), Mn(+)-H(2), Zn(+)-D(2) and Ag(+)-H(2). Interactions between molecular hydrogen and metal cations play a key role in several contexts, including in the storage of molecular hydrogen in zeolites, metal-organic frameworks, and doped carbon nanostructures. Arguably, the clearest view of the interaction between dihydrogen and a metal cation can be obtained by probing M(+)-H(2) complexes in the gas phase, free from the complicating influences of solvents or substrates. Infrared spectra of the complexes in the H-H and D-D stretch regions are obtained by monitoring M(+) photofragments as the excitation wavelength is scanned. The spectra, which feature full rotational resolution, confirm that the M(+)-H(2) complexes share a common T-shaped equilibrium structure, consisting essentially of a perturbed H(2) molecule attached to the metal cation, but that the structural and vibrational parameters vary over a considerable range, depending on the size and electronic structure of the metal cation. Correlations are established between intermolecular bond lengths, dissociation energies, and frequency shifts of the H-H stretch vibrational mode. Ultimately, the M(+)-H(2) and M(+)-D(2) infrared spectra provide a comprehensive set of benchmarks for modelling and understanding the M(+)···H(2) interaction.

Incorporating magnesium and calcium cations in porous organic frameworks for high-capacity hydrogen storage

We propose incorporating a bi-functional group consisting of magnesium or calcium cations and a 1,2,4,5-benzenetetroxide anion (C6H2O4(4-)) in porous materials to enhance the hydrogen storage capacity. The C6H2O4M2 bifunctional group is highly stable and polarized, and each group provides 18 (M = Mg) or 22 (M = Ca) binding sites for hydrogen molecules with an average binding energy of ca. 10 kJ mol(-1) per hydrogen molecule based on RIMP2/ TZVPP calculations. Two porous materials (PAF-Mg or PAF-Ca) constructed with the bi-functional groups show remarkable improvement in hydrogen uptakes at normal ambient conditions. At 233 K and 10 MPa, the predicted gravimetric uptakes are 6.8 and 6.4 wt% for PAF-Mg and PAF-Ca respectively. This work reveals that fabricating materials with large numbers of binding sites and relatively low binding energies is a promising approach to achieve high capacity for on-board storage of hydrogen.

Finding MOFs for highly selective CO2/N2 adsorption using materials screening based on efficient assignment of atomic point charges

Electrostatic interactions are a critical factor in the adsorption of quadrupolar species such as CO(2) and N(2) in metal-organic frameworks (MOFs) and other nanoporous materials. We show how a version of the semiempirical charge equilibration method suitable for periodic materials can be used to efficiently assign charges and allow molecular simulations for a large number of MOFs. This approach is illustrated by simulating CO(2) and N(2) adsorption in ~500 MOFs; this is the largest set of structures for which this information has been reported to date. For materials predicted by our calculations to have promising adsorption selectivities, we performed more detailed calculations in which accurate quantum chemistry methods were used to assign atomic point charges, and molecular simulations were used to assess molecular diffusivities and binary adsorption isotherms. Our results identify two MOFs, experimentally known to be stable upon solvent removal, that are predicted to show no diffusion limitations for adsorbed molecules and extremely high CO(2)/N(2) adsorption selectivities for CO(2) adsorption from dry air and from gas mixtures typical of dry flue gas.