Heterogeneous catalysts with programmable topologies generated by reticulation of organocatalysts into metal-organic frameworks: The case of squaramide

A well-established strategy to synthesize heterogeneous,metal-organic framework(MOF)catalysts that exhibit nanoconfinement effects,and specific pores with highly-localized catalytic sites,is to use organic linkers containing organocatalytic centers.Here,we report that by combining this linker approach with reticular chemistry,and exploiting three-dimensioanl(3D)MOF-structural data from the Cambridge Structural Database,we have designed four heterogeneous MOF-based catalysts for standard organic transformations.These programmable MOFs are isoreticular versions of pcu IRMOF-16,feu UiO-68 and pillared-pcu SNU-8X,the three most common topologies of MOFs built from the organic linker p.p'-terphenyldicarboxylic acid(tpdc).To synthesize the four squaramide-based MOFs,we designed and synthesized a linker,4,4’-((3,4-dioxocyclobut-1-ene-1,2-diyl)bis(azanedyil))dibenzoic acid(Sq_tpdc),which is identical in directionality and length to tpdc but which contains organocatalytic squaramide centers.Squaramides were chosen because their immobilization into a framework enhances its reactivity and stability while avoiding any self-quenching phenomena.Therefore,the four MOFs share the same organocatalytic squaramide moiety,but confine it within distinct pore environments.We then evaluated these MOFs as heterogeneous H-bonding catalysts in organic transformations:a Friedel-Crafts alkylation and an epoxide ring-opening.Some of them exhibited good performance in both reactions but all showed distinct catalytic profiles that reflect their structural differences.     

Template‐Free,Surfactant‐Mediated Orientation of Self‐Assembled Supercrystals of Metal–Organic Framework Particles

Mesoscale self‐assembly of particles into supercrystals is important for the design of functional materials such as photonic and plasmonic crystals. However, while much progress has been made in self‐assembling supercrystals adopting diverse lattices and using different types of particles, controlling their growth orientation on surfaces has received limited success. Most of the latter orientation control has been achieved via templating methods in which lithographic processes are used to form a patterned surface that acts as a template for particle assembly. Herein, a template‐free method to self‐assemble (111)‐, (100)‐, and (110)‐oriented face‐centered cubic supercrystals of the metal–organic framework ZIF‐8 particles by adjusting the amount of surfactant (cetyltrimethylammonium bromide) used is described. It is shown that these supercrystals behave as photonic crystals whose properties depend on their growth orientation. This control on the orientation of the supercrystals dictates the orientation of the composing porous particles that might ultimately facilitate pore orientation on surfaces for designing membranes and sensors.     

Lanthanide–Organic Framework Nanothermometers Prepared by Spray‐Drying

Accurate, noninvasive, and self‐referenced temperature measurements at the submicrometer scale are of great interest, prompted by the ever‐growing demands in the fields of nanotechnology and nanomedicine. The thermal dependence of the phosphor's luminescence provides high detection sensitivity and spatial resolution with short acquisition times in, e.g., biological fluids, strong electromagnetic fields, and fast‐moving objects. Here, it is shown that nanoparticles of [(Tb_0.914Eu_0.086)₂(PDA)₃(H₂O)]·2H₂O (PDA = 1,4‐phenylenediacetic acid), the first lanthanide–organic framework prepared by the spray‐drying method, are excellent nanothermometers operating in the solid state in the 10–325 K range (quantum yield of 0.25 at 370 nm, at room temperature). Intriguingly, this system is the most sensitive cryogenic nanothermometer reported so far, combining high sensitivity (up to 5.96 ± 0.04% K^?1 at 25 K), reproducibility (in excess of 99%), and low‐temperature uncertainty (0.02 K at 25 K).     

“Dual‐Template” Synthesis of One‐Dimensional Conductive Nanoparticle Superstructures from Coordination Metal–Peptide Polymer Crystals

Bottom‐up fabrication of self‐assembled structures made of nanoparticles may lead to new materials, arrays and devices with great promise for myriad applications. Here a new class of metal–peptide scaffolds is reported: coordination polymer Ag(I)‐DLL belt‐like crystals, which enable the dual‐template synthesis of more sophisticated nanoparticle superstructures. In these biorelated scaffolds, the self‐assembly and recognition capacities of peptides and the selective reduction of Ag(I) ions to Ag are simultaneously exploited to control the growth and assembly of inorganic nanoparticles: first on their surfaces, and then inside the structures themselves. The templated internal Ag nanoparticles are well confined and closely packed, conditions that favour electrical conductivity in the superstructures. It is anticipated that these Ag(I)‐DLL belts could be applied to create long (>100 μm) conductive Ag@Ag nanoparticle superstructures and polymetallic, multifunctional Fe₃O₄@Ag nanoparticle composites that marry the magnetic and conductive properties of the two nanoparticle types.     

Enhanced Spin Tunneling in a Molecular Magnet Mixed with a Superconductor

We report characterization and magnetic studies of mixtures of micrometer-size ribbons of Mn₁₂ acetate and micrometer-size particles of YBaCuO superconductor. Extremely narrow zero-field spin-tunneling resonance has been observed in the mixtures, pointing to the absence of the inhomogeneous dipolar broadening. It is attributed to the screening of the internal magnetic fields in the magnetic particles by Meissner currents flowing between superconducting grains surrounding the particles.     

Influence of deposition conditions on the protective behavior of tetraethyl orthosilicate sol–gel films on AA5754 aluminum alloy

The aim of this work is to compare the effect of two deposition methods, dip-coating and electrophoretic deposition, on the characteristics of tetraethyl orthosilicate (TEOS) sol–gel films on AA5754 aluminum alloy, especially in what concerns to their resistance against corrosion. The influence of pH bath on the hydrolysis of the system was analyzed. Moreover, the effect of some experimental parameters, such as deposition time and deposition voltage, was also evaluated. The results showed that pre-hydrolysis rate of the orthosilicate bath depends on the solution pH, and that at pH 2 complete hydrolysis of the solution was obtained after only 20 min. Moreover, it was observed that electrodeposited TEOS films provided better corrosion resistance than films obtained by dip-coating. The improved corrosion resistance was ascribed to a higher uniformity and density of the polysiloxane films and to an in situ modification of the aluminum–polysiloxane interface by the cathodic voltage.     

Thermal treatment effect on the mechanical,tribological and corrosion properties of Ni–W alloy obtained by direct and pulse plating electrodeposition

Ni–W electrodeposits have emerged as one of the most suitable alternatives to hard chromium mainly owing to their remarkable mechanical and tribological properties. Additionally, advanced technologies that require materials resistant to high temperatures could benefit from the use of Ni–W coatings. In this work, the effect of thermal treatment at different temperatures (300, 500, 700°C) on the characteristics of Ni–W coatings obtained by direct and pulse plating (PP) was studied. The morphology, composition, crystalline structure, hardness, wear rate, friction coefficient and corrosion resistance of the thermally treated coatings were analysed and compared with the performance of hard chromium coatings. The results indicate that the pulse-plated Ni–W coatings show better mechanical and tribological properties than the ones obtained by direct current. A significant improvement in hardness in Ni–W layers was achieved by thermal treatment, mainly in the films grown by PP, with minor changes in wear resistance and corrosion performance.