Modulating the stacking modes of nanosized metal–organic frameworks by morphology engineering for isomer separation

Modulating different stacking modes of nanoscale metal–organic frameworks (MOFs) introduces different properties and functionalities but remains a great challenge. Here, we describe a morphology engineering method to modulate the stacking modes of nanoscale NU-901. The nanoscale NU-901 is stacked through solvent removal after one-pot solvothermal synthesis, in which different morphologies from nanosheets (NS) to interpenetrated nanosheets (I-NS) and nanoparticles (NP) were obtained successfully. The stacked NU-901-NS, NU-901-I-NS, and NU-901-NP exhibited relatively aligned stacking, random stacking, and close packing, respectively. The three stacked nanoscale NU-901 exhibited different separation abilities and all showed better performance than bulk phase NU-901. Our work provides a new morphology engineering route for the modulation of the stacking modes of nano-sized MOFs and improves the separation abilities of MOFs.

A morphology engineering method was utilized to modulate the stacking modes of three nano-NU-901 materials, leading to different separation abilities for isomers.     

Zinn

Ohne Zusammenfassung     

Thermal treatment of dense polyimide membranes

A study has been conducted to clarify the relationship between polymer structure, annealing temperature, and the extent of plasticization by high‐pressure CO₂ for two typical polyimide membranes; BTDA‐DAPI (poly(3,3′‐4,4′‐benzophenone tetracarboxylic–dianhydride diaminophenylindane) and 6FDA‐TMPDA (poly(2,2′‐bis(3,4′‐dicasrboxyphenyl) hexafluoropropane dianhydride–2,3,5,6‐tetramethyl‐1,4‐phenylenediamine). Both membrane materials are exposed to varying levels of thermal annealing at 200 and 250 °C. The effect of this heat treatment on free volume is examined using positron annihilation lifetime spectroscopy (PALS), whereas fluorescence spectroscopy is used to monitor changes in electronic structure. Results show that thermal annealing causes a reduction in both the size and number of free volume elements. A strong relationship is found between the fluorescence peak intensity for 6FDA‐TMPDA and both the membrane gas permeability and plasticization pressure. This correlation is most likely the result of the formation of charge transfer complexes, particularly at 250 °C. However, the formation of covalent crosslinks at these temperatures cannot be discounted. No fluorescence is observed for BTDI‐DAPI. Although thermal annealing has a significant effect on the extent of plasticization in both polymers, it is found that the rate of plasticization is unaffected by the annealing temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1879–1890, 2008     

Using linear programming to simulate service engineers

This paper describes the use of a network flow model to simulate a proposed spatial allocation of engineers to service requests. The model involves generating a pattern of demand and then determining an allocation of engineers to jobs which maximises the number of jobs immediately serviced. Within the maximum number of jobs the allocation pattern which minimises the amount of travelling is determined. The results obtained were compared with the allocation rules then in use.     

Exact and heuristic solution approaches for the mixed integer setup knapsack problem

We consider a class of knapsack problems that include setup costs for families of items. An individual item can be loaded into the knapsack only if a setup cost is incurred for the family to which it belongs. A mixed integer programming formulation for the problem is provided along with exact and heuristic solution methods. The exact algorithm uses cross decomposition. The proposed heuristic gives fast and tight bounds. In addition, a Benders decomposition algorithm is presented to solve the continuous relaxation of the problem. This method for solving the continuous relaxation can be used to improve the performance of a branch and bound algorithm for solving the integer problem. Computational performance of the algorithms are reported and compared to CPLEX.     

Controlling dark current in type-II superlattice photodiodes

Limiting the defect-mediated dark currents in type-II superlattice (T2SL) IR photodiodes remains the key challenge to focal plane arrays (FPAs) based on this material system. In spite of its larger effective mass to suppress tunneling and more than an order of magnitude longer Auger lifetime, the T2SL photodiode performance still lags behind that of the incumbent HgCdTe-based technology. The tunneling and generation–recombination currents can be strongly suppressed by employing a “W” T2SL structure and gradually increasing the energy gap in the depletion region. For maximum quantum efficiency, this graded-gap geometry is combined in a hybrid structure with two-constituent T2SL absorbers that exhibit roughly twice the diffusion length of the “W” structure. Finally, if the etch used to isolate neighboring pixels is stopped just beyond the junction in the graded-gap device, narrow-gap regions are not exposed and the total sidewall area is reduced by a factor of 20. We have combined all of these approaches to produce a 10.5 μm cutoff FPA with diffusion-limited performance and noise-equivalent differential temperature (NEDT) of 35 mK at 70 K.     

Hydrogen transfer to carbonyls and imines from a hydroxycyclopentadienyl ruthenium hydride: evidence for concerted hydride and proton transfer.

Reaction of ([2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)CO)](2)H)Ru(2)(CO)(4)(mu-H) (6) with H(2) formed [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COH)Ru(CO)(2)H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at -10 degrees C showed second-order kinetics with Delta H(double dagger) = 12.0 kcal mol(-1) and Delta S(double dagger) = -28 eu. The rate of reduction was not accelerated by CF(3)CO(2)H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 gave individual kinetic isotope effects k(RuH)/k(RuD) = 1.5 +/- 0.2 and k(OH)/k(OD) = 2.2 +/- 0.1 for PhCHO reduction at 0 degrees C. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of k(OHRuH)/k(ODRuD) = 3.6 +/- 0.3. [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COSiEt(3))Ru(CO)(2)H] (12) and NEt(4)(+)[2,5-Ph(2)-3,4-Tol(2)(eta(4)-C(4)CO)Ru(CO)(2)H](-) (13) were unreactive toward PhCHO under conditions where facile PhCHO reduction by 8 occurred. PhCOMe was reduced by 8 30 times slower than PhCHO; MeN=CHPh was reduced by 8 26 times faster than PhCHO. Cyclohexene was reduced to cyclohexane by 8 at 80 degrees C only in the presence of H(2.) Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed.     

Asymmetric diosmium sawhorse complexes

Three asymmetric diosmium(I) carbonyl sawhorse complexes have been prepared by microwave heating. One of these complexes is of the type Os₂(μ‐O₂CR)(μ‐O₂CR′)(CO)₄L₂, with two different bridging carboxylate ligands, while the other two complexes are of the type Os₂(μ‐O₂CR)₂(CO)₅L, with one axial CO ligand and one axial phosphane ligand. The mixed carboxylate complex Os₂(μ‐acetate)(μ‐propionate)(CO)₄[P(p‐tolyl)₃]₂, ( 1 ), was prepared by heating Os₃(CO)₁₂ with a mixture of acetic and propionic acids, isolating Os₂(μ‐acetate)(μ‐propionate)(CO)₆, and then replacing two CO ligands with two phosphane ligands. This is the first example of an Os₂ sawhorse complex with two different carboxylate bridges. The syntheses of Os₂(μ‐acetate)₂(CO)₅[P(p‐tolyl)₃], ( 3 ), and Os₂(μ‐propionate)₂(CO)₅[P(p‐tolyl)₃], ( 6 ), involved the reaction of Os₃(CO)₁₂ with the appropriate carboxylic acid to initially produce Os₂(μ‐carboxylate)₂(CO)₆, followed by treatment with refluxing tetrahydrofuran (THF) to form Os₂(μ‐carboxylate)₂(CO)₅(THF), and finally addition of tri‐p‐tolylphosphane to replace the THF ligand with the P(p‐tolyl)₃ ligand. Neutral complexes of the type Os₂(μ‐O₂CR)₂(CO)₅L had not previously been subjected to X‐ray crystallographic analysis. The more symmetrical disubstituted complexes, i.e. Os₂(μ‐formate)₂(CO)₄[P(p‐tolyl)₃]₂, ( 8 ), Os₂(μ‐acetate)₂(CO)₄[P(p‐tolyl)₃]₂, ( 4 ), and Os₂(μ‐propionate)₂(CO)₄[P(p‐tolyl)₃]₂, ( 7 ), as well as the previously reported symmetrical unsubstituted complexes Os₂(μ‐acetate)₂(CO)₆, ( 2 ), and Os₂(μ‐propionate)₂(CO)₆, ( 5 ), were also prepared in order to examine the influence of axial ligand substitution on the Os—Os bond distance in these sawhorse molecules. Eight crystal structures have been determined and studied, namely μ‐acetato‐1κO:2κO′‐μ‐propanoato‐1κO:2κO′‐bis[tris(4‐methylphenyl)phosphane]‐1κP,2κP′‐bis(dicarbonylosmium)(Os—Os) dichloromethane monosolvate, [Os₂(C₂H₃O₂)(C₃H₅O₂)(C₂₁H₂₁P)₂(CO)₄]·CH₂Cl₂, ( 1 ), bis(μ‐acetato‐1κO:2κO′)bis(tricarbonylosmium)(Os—Os), [Os₂(C₂H₃O₂)₂(CO)₆], ( 2 ) (redetermined structure), bis(μ‐acetato‐1κO:2κO′)pentacarbonyl‐1κ²C,2κ³C‐[tris(4‐methylphenyl)phosphane‐1κP]diosmium(Os—Os), [Os₂(C₂H₃O₂)₂(C₂₁H₂₁P)(CO)₅], ( 3 ), bis(μ‐acetato‐1κO:2κO′)bis[tris(4‐methylphenyl)phosphane]‐1κP,2κP‐bis(dicarbonylosmium)(Os—Os) p‐xylene sesquisolvate, [Os₂(C₂H₃O₂)₂(C₂₁H₂₁P)₂(CO)₄]·1.5C₈H₁₀, ( 4 ), bis(μ‐propanoato‐1κO:2κO′)bis(tricarbonylosmium)(Os—Os), [Os₂(C₃H₅O₂)₂(CO)₆], ( 5 ), pentacarbonyl‐1κ²C,2κ³C‐bis(μ‐propanoato‐1κO:2κO′)[tris(4‐methylphenyl)phosphane‐1κP]diosmium(Os—Os), [Os₂(C₃H₅O₂)₂(C₂₁H₂₁P)(CO)₅], ( 6 ), bis(μ‐propanoato‐1κO:2κO′)bis[tris(4‐methylphenyl)phosphane]‐1κP,2κP‐bis(dicarbonylosmium)(Os—Os) dichloromethane monosolvate, [Os₂(C₃H₅O₂)₂(C₂₁H₂₁P)₂(CO)₄]·CH₂Cl₂, ( 7 ), and bis(μ‐formato‐1κO:2κO′)bis[tris(4‐methylphenyl)phosphane]‐1κP,2κP‐bis(dicarbonylosmium)(Os—Os), [Os₂(CHO₂)₂(C₂₁H₂₁P)₂(CO)₄], ( 8 ).     

Not Limited to Iron: A Cobalt Heme–NO Model Facilitates N–N Coupling with External NO in the Presence of a Lewis Acid to Generate N2O

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N₂O. We previously reported that a heme Fe–NO model engages in this N?N bond‐forming reaction with NO. We now demonstrate that (OEP)Co^II(NO) similarly reacts with 1 equiv of NO in the presence of the Lewis acids BX₃ (X=F, C₆F₅) to generate N₂O. DFT calculations support retention of the Co^II oxidation state for the experimentally observed adduct (OEP)Co^II(NO?BF₃), the presumed hyponitrite intermediate (P^.+)Co^II(ONNO?BF₃), and the porphyrin π‐radical cation by‐product of this reaction, and that the π‐radical cation formation likely occurs at the hyponitrite stage. In contrast, the Fe analogue undergoes a ferrous‐to‐ferric oxidation state conversion during this reaction. Our work shows that cobalt hemes are chemically competent to engage in the NO‐to‐N₂O conversion reaction.     

Molecular Pivot‐Hinge Installation to Evolve Topology in Rare‐Earth Metal–Organic Frameworks

Linker desymmetrization has been witnessed as a powerful design strategy for the discovery of highly connected metal–organic frameworks (MOFs) with unprecedented topologies. Herein, we introduce molecular pivot‐hinge installation as a linker desymmetrization strategy to evolve the topology of highly connected rare‐earth (RE) MOFs, where a pivot group is placed in the center of a linker similar to a hinge. By tuning the composition of pivot groups and steric hindrances of the substituents on various linker rotamers, MOFs with various topologies can be obtained. The combination of L‐SO₂ with C_2v symmetry and 12‐connected RE₉ clusters leads to the formation of a fascinating (4,12)‐c dfs new topology. Interestingly, when replacing L‐SO₂ with a tetrahedra linker L‐O, the stacking behaviors of RE‐organic layers switch from an eclipsed mode to a staggered stacking mode, leading to the discovery of an intriguing hjz topology. Additionally, the combination of the RE cluster and a linker [(L‐(CH₃)₆)] with more bulky groups gives rise to a flu topology with a new 8‐c inorganic cluster. The diversity of these RE‐MOFs was further enhanced through post‐synthetic installation of linkers with various functional groups. Functionalization of each linker with acidic and basic units in the mesoporous RE‐based PCN‐905‐SO₂ allows for efficient cascade catalytic transformation within the functionalized channels.