Organic radicals stabilization above 300 °C in Eu-based coordination polymers for solar steam generation.

Organic radicals feature unpaired electrons, and these compounds may have applications in biomedical technology and as materials for solar energy conversion. However, unpaired electrons tend to pair up (to form chemical bonds), making radicals unstable and hampering their applications. Here we report an organic radical system that is stable even at 350 °C, surpassing the upper temperature limit (200 °C) observed for other organic radicals. The system reported herein features a sulfur-rich organic linker that facilitates the formation of the radical centers; on the solid-state level, the molecules are crystallized with Eu(III) ions to form a 3D framework featuring stacks of linker molecules. The stacking is, however, somewhat loose and allows the molecules to wiggle and transform into sulfur-stabilized radicals at higher temperatures. In addition, the resulting solid framework remains crystalline, and it is stable to water and air. Moreover, it is black and features strong broad absorption in the visible and near IR region, thereby enhancing both photothermal conversion and solar-driven water evaporation.

A Bumper Crop of Boiling-Water-Stable Metal-Organic Frameworks from Controlled Linker Sulfuration.

The series of highly stable porous solids here feature systematic, regiospecific sulfur substitutions on the organic linkers for versatile functions. One major surprise lies in the controllable sequential reactions between sodium thiomethoxide (NaSMe) and octafluorobiphenyl-4,4'-dicarboxylic acid (H2bpdc-8F; this was readily made without precious metal catalysts). Namely, 3, 4, 6, and 8 methylthio-substitutions can be respectively achieved with regiospecificity (i.e., to produce the four molecules H2bpdc-3S5F, H2bpdc-4S4F, H2bpdc-6S2F, H2bpdc-8MS). A second surprise lies in their persistent formation of the UiO-67-type net with Zr(IV) ions, e.g., even in the case of the fully sulfurated H2bpdc-8MS. In addition to the remarkable breadth of functional control, all the Zr(IV)-based crystalline solids here are stable in boiling water (e.g., for 24 h) and in air as solventless, activated porous solids. Moreover, the thioether groups allow for convenient H2O2 oxidation to fine-tune the hydrophilicity and luminescence properties and improve proton conductivity.

Janus triple tripods build up a microporous manifold for HgCl2 and I2 uptake.

To boost the design of microporous solids, we integrated a two-faced shape (as in cucurbiturils and cyclodextrins) into the building blocks of framework materials. Reported herein is a planar tritopic carboxyl linker with secondary tripod donors sprouting off both sides at the core region. The two-faced, barrel-like core region imparts a rugged 3D character to the linker architecture, obviating close packing and creating complex-shaped cavities in an Eu(iii)-carboxylate network. The merits extend beyond the interesting shape of the multiple tripod: e.g., the two sets of sulfur tripods at the barrel region, together with the triazine center, offer a rich array of donors for adsorbing Hg(ii) ions. The microporous solid also removes iodine from vapor and water, and can be easily cycled in column chromatography.

A Thiol-Functionalized UiO-67-Type Porous Single Crystal: Filling in the Synthetic Gap.

Thiol groups (-SH) offer versatile reactivity for functionalizing metal-organic frameworks, and yet thiol-equipped MOF solids remain underexplored due to synthetic challenges. Building on the recent breakthrough using benzyl mercaptan as the sulfur source and AlCl3 for uncovering the thiol function, we report on the thiol-equipped linker 3,3'-dimercaptobiphenyl-4,4'-dicarboxylic acid and its reaction with Zr(IV) ions to form a UiO-67-type MOF solid with distinct functionalities. The thiol-equipped UiO-67 scaffold shows substantial stability toward oxidation, e.g., it can be treated with 30% H2O2 to afford oxidation of the thiol to the strongly acidic sulfonic function while maintaining the ordered porous MOF structure. The thiol groups also effectively take up palladium(II) ions from solutions to allow for comparative studies on catalytic activities and to help elucidate how the spatial configuration of the thiol groups can be engineered to impact the performance of heterogeneous catalysis in the solid state. Comparative studies on the stability in the solventless (activated) state also help to highlight the steric factor in stabilizing UiO-67-type frameworks.

Single-Crystalline UiO-67-Type Porous Network Stable to Boiling Water, Solvent Loss, and Oxidation.

With methylthio groups flanking the carboxyl groups, the 3,3',5,5'-tetrakis(methylthio)biphenyl dicarboxylate (TMBPD) linker forms a zirconium(IV) carboxylate porous framework featuring the topology of the UiO-67 prototype, i.e., with a face-centered-cubic array of the Zr6O4(OH)4 clusters. Thioether functionalization proves valuable because the ZrTMBPD crystal is found to be exceptionally stable not only upon long-term exposure to air but also in boiling water and a broad range of pH conditions. The hydrophobicity of the metal-organic framework can also be tuned by simple H2O2 oxidation, as illustrated in the water contact-angle measurement of the pristine and H2O2-treated ZrTMBPD solid.

Made in Water: A Stable Microporous Cu(I)-carboxylate Framework (CityU-7) for CO2, Water, and Iodine Uptake.

Using water as the sole solvent, the bifunctional molecule tetrakis(methylthio)-1,4-benzenedicarboxylic acid (TMBD) was reacted with Cu(CH3CN)4BF4 to form a robust microporous metal-organic framework (MOF, CityU-7) featuring Cu(I) ions being simultaneously bonded to the carboxyl and thioether donors. The MOF solid is stable in air and can be easily activated by heating, without the need for treatment with organic solvents. The subnanoscopic pores (ca. 0.6 nm) of the host net allow for uptake of CO2 and H2O but exhibit lesser sorption for N2 at 77 K. The microporous net can also be penetrated by I2 molecules.

Metalation Triggers Single Crystalline Order in a Porous Solid.

We report the dramatic triggering of structural order in a Zr(IV)-based metal-organic framework (MOF) through docking of HgCl2 guests. Although as-made crystals were unsuitable for single crystal X-ray diffraction (SCXRD), with diffraction limited to low angles well below atomic resolution due to intrinsic structural disorder, permeation of HgCl2 not only leaves the crystals intact but also resulted in fully resolved backbone as well as thioether side groups. The crystal structure revealed elaborate HgCl2-thioether aggregates nested within the host octahedra to form a hierarchical, multifunctional net. The chelating thioether groups also promote Hg(II) removal from water, while the trapped Hg(II) can be easily extricated by 2-mercaptoethanol to reactivate the MOF sorbent.

Bio-inspired stabilization of sulfenyl iodide RS-I in a Zr(IV)-based metal-organic framework.

A Zr(IV)-based metal-organic framework (MOF) appended with free-standing thiol (-SH) groups was found to react readily with I2 molecules to form sulfenyl iodide (S-I) units. In contrast to its solution chemistry of facile disproportionation into disulfide and I2, the sulfenyl iodide (SI) function, anchored onto the rigid MOF grid and thus prevented from approaching one another to undergo the dismutation reaction, exhibits distinct stability even at elevated temperatures (e.g., 90 °C). On a conceptual plane, this simple and effective solid host also captures the spatial confinement observed for the complex biomacromolecular scaffolds involved in iodine thyroid chemistry, wherein the spatial isolation and consequent stabilization of sulfenyl/selenenyl iodides are exerted by means of the protein scaffolds.

Room-temperature acetylene hydration by a Hg(II)-laced metal-organic framework.

Thiol (-SH) groups within a Zr(IV)-based metal-organic framework (MOF) anchor Hg(II) atoms; oxidation by H2O2 then leads to acidic sulfonate functions for catalyzing acetylene hydration at room temperature.

Tackling poison and leach: catalysis by dangling thiol-palladium functions within a porous metal-organic solid.

Self-standing thiol (-SH) groups within a Zr(IV)-based metal-organic framework (MOF) anchor Pd(II) atoms for catalytic applications: the spatial constraint prevents the thiol groups from sealing off/poisoning the Pd(II) center, while the strong Pd-S bond precludes Pd leaching, enabling multiple cycles of heterogeneous catalysis to be executed.

Pd uptake and H2S sensing by an amphoteric metal-organic framework with a soft core and rigid side arms.

Molecular components of opposite character are often incorporated within a single system, with a rigid core and flexible side arms being a common design choice. Herein, molecule L has been designed and prepared featuring the reverse design, with rigid side arms (arylalkynyl) serving to calibrate the mobility of the flexible polyether links in the core. Crystallization of this molecule with Pb(II)  ions led to a dynamic metal-organic framework (MOF) system that not only exhibits dramatic, reversible single-crystal-to-single-crystal transformations, but combines distinct donor and acceptor characteristics, allowing for substantial uptake of PdCl2 and colorimetric sensing of H2 S in water.