Zhengtao Xu

Effective Mercury Sorption by Thiol-Laced Metal–Organic Frameworks: in Strong Acid and the Vapor Phase

Free-standing, accessible thiol (-SH) functions have been installed in robust, porous coordination networks to provide wide-ranging reactivities and properties in the solid state. The frameworks were assembled by reacting ZrCl4 or AlCl3 with 2,5-dimercapto-1,4-benzenedicarboxylic acid (H2DMBD), which features the hard carboxyl and soft thiol functions. The resultant Zr-DMBD and Al-DMBD frameworks exhibit the UiO-66 and CAU-1 topologies, respectively, with the carboxyl bonded to the hard Zr(IV) or Al(III) center and the thiol groups decorating the pores. The thiol-laced Zr-DMBD crystals lower the Hg(II) concentration in water below 0.01 ppm and effectively take up Hg from the vapor phase. The Zr-DMBD solid also features a nearly white photoluminescence that is distinctly quenched after Hg uptake. The carboxyl/thiol combination thus illustrates the wider applicability of the hard-and-soft strategy for functional frameworks.

White Light Emission and Second Harmonic Generation from Secondary Group Participation (SGP) in a Coordination Network

We describe a white emitting coordination network solid that can be conveniently applied as a thin film onto a commercial UV-LED lamp for practical white lighting applications. The solid state material was discovered in an exercise of exploring molecular building blocks equipped with secondary groups for fine-tuning the structures and properties of coordination nets. Specifically, CH(3)SCH(2)CH(2)S- and (S)-CH(3)(OH)CHCH(2)S- (2-hydroxylpropyl) were each attached as secondary groups to the 2,5- positions of 1,4-benzenedicarboxylic acid (bdc), and the resultant molecules (L1 and L2, respectively) were crystallized with Pb(II) into the topologically similar 3D nets of PbL1 and PbL2, both consisting of interlinked Pb-carboxyl chains. While the CH(3)S- groups in PbL1 are not bonded to the Pb(II) centers, the hydroxy groups in PbL2 participate in coordinating to Pb(II) and thus modify the bonding features around the Pb(II), but only to a slight and subtle degree (e.g., Pb-O distances 2.941-3.116 Å). Interestingly, the subtle change in structure significantly impacts the properties, i.e., while the photoluminescence of PbL1 is yellowish green, PbL2 features bright white emission. Also, the homochiral side group in PbL2 imparts significant second harmonic generation, in spite of its seemingly weak association with the main framework (the NLO-phore). In a broad perspective, this work showcases the idea of secondary group participation (SGP) in the construction of coordination networks, an idea that parallels that of hemilabile ligands in organometallics and points to an effective strategy in developing advanced functions in solid state framework materials.

Selective Ag(I) binding, H2S sensing, and white-light emission from an easy-to-make porous conjugated polymer.

Separating silver (Ag(+)) from lead (Pb(2+)) is one of the many merits of the porous polymer framework reported here. The selective metal binding stems from the well-defined chelating unit of N-heterocycles, which consists of a triazine (C3N3) ring bonded to three 3,5-dimethylpyrazole moieties. Such a rigid and open triad also serves as the distinct building unit in the fully conjugated 3D polymer scaffold. Because of its strong fluorescence and porosity (e.g., BET surface area: 355 m(2)/g), and because of the various types of metal species that can be readily taken up, this versatile framework is especially fit for functionalization. For example, with AgNO3 loaded, the framework solid exhibits a brown color in response to water solutions of H2S, even at the dilution of 5.0 μM (0.17 ppm); whereas cysteine and other biologically relevant thiols do not cause notable change in color. In another example, tunable white-light emission was produced when an Ir(III) complex was doped (e.g., about 0.02% of the polymer weight) onto the framework. Mechanistically, the bound Ir(III) centers become highly emissive in the orange-red region, complementing the broad, bluish emission from the polymer host to result in the overall white-light quality: the color attributes of the emission are therefore easily tunable by the Ir(III) dopant concentration. With this exemplary study, we intend to highlight metal uptake as an effective approach to modify and enrich the properties of porous polymer frameworks and to stimulate interest in further examining metal-polymer interactions in the context of sensing, separation, catalyzes, and other applications.

Convenient Detection of Pd(II) by a Metal–Organic Framework with Sulfur and Olefin Functions

A highly specific, distinct color change in the crystals of a metal-organic framework with pendant allyl thioether units in response to Pd species was discovered. The color change (from light yellow to orange/brick red) can be triggered by Pd species at concentrations of a few parts per million and points to the potential use of these crystals in colorimetric detection and quantification of Pd(II) ions. The swift color change is likely due to the combined effects of the multiple functions built into the porous framework: the carboxyl groups for bonding with Zn(II) ions to assemble the host network and the thioether and alkene functions for effective uptake of the Pd(II) analytes (e.g., via the alkene-Pd interaction). The resultant loading of Pd (and other noble metal) species into the porous solid also offers rich potential for catalysis applications, and the alkene side chains are amenable to wide-ranging chemical transformations (e.g., bromination and polymerization), enabling further functionalization of the porous networks.

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.

Mixed-Valence CuIICuI15I17 Cluster Builds up a 3D Metal−Organic Framework with Paramagnetic and Thermochromic Characteristics

Four cubane-like Cu4I4 units are assembled around an iodine atom to form the giant, mixed-valent Cu(II)Cu(I)15I17 cluster. The Cu(II)Cu(I)15I17 cluster and a bipyrazole linker form a 3D open framework with paramagnetic and thermochromic properties. This paper also touches on the resemblance of this cluster to the self-similar object of a Sierpinski tetrahedron.

Conjugated porous polymers for photocatalytic applications

Conjugated porous polymers (CPPs) are a class of fully crosslinked polymers defined by high surface area and porosity in the nanometer range, having been traditionally developed for applications such as gas storage, sensing and (photo)catalysis. As these materials are comprised of extended π-conjugation, their ability to act as light harvesters, and in turn photocatalysts, has come to prominence. The insoluble nature of CPPs allows them to be employed as photocatalysts under heterogeneous conditions, replacing traditional homogeneous systems. This Perspective highlights the current state-of-the-art CPPs along with a view to their applications as heterogeneous photocatalysts for a wide range of chemical transformations including hydrogen production, organic synthesis and photopolymerization, just to name but a few.

Extraction of palladium from nuclear waste-like acidic solutions by a metal–organic framework with sulfur and alkene functions

We report a robust metal–organic framework (MOF) for convenient recovery of Pd(II) from acidic nitric solutions which emulate high-level liquid wastes (HLLW) generated from the reprocessing of spent nuclear fuel. The framework solid (ASUiO-66) was constructed from Zr(IV) ions and the multifunctional linker 2,6-bis(allylsulfanyl)terephthalic acid (H2L), and features the well-known UiO-66 topology. Herein the robust Zr(IV)-carboxylate bonds impart structural strength to the host net, while the alkene and thioether units provide efficient and selective binding of the Pd(II) ions. For example, over 95% of the Pd(II) ions can be adsorbed from a simulated HLLW (1.0 M HNO3, containing about 20 different types of metal elements), with Ag(I) being the only other metal ion taken up significantly by the ASUiO-66 sorbent. Moreover, the adsorbed Pd(II) species can be effectively stripped by a dilute solution of thiourea (0.01 M); and the regenerated framework solid can be used for additional cycles of Pd extraction, with the sorption capacity for Pd(II) being little changed (38–41 mg g−1). The isotherm adsorption data fit well with the Langmuir model with a saturation capacity of 45.4 mg g−1, being equivalent to each octahedral cage in the UiO-66 net containing roughly one Pd(II) ion. In a broader perspective, the alkene and thioether combination could be anchored onto other sorbent systems (e.g., porous polymers and resins) to impart versatile adsorption properties for the retrieval of noble metal ions.

Coordination networks from a bifunctional molecule containing carboxyl and thioether groups.

The bifunctional molecule tetrakis(methylthio)-1,4-benzenedicarboxylic acid (TMBD) interacts with the increasingly harder metal ions of Cu (I), Cd (II), and Zn (II) to form the coordination networks of Cu 2TMBD, CdTMBD, and Zn 4O(H 2O) 3(TMBD) 3, where the carboxyl group consistently bonds to metal ions, while the softer methylthio group binds with preference to the softer metal ions (i.e., chelation to Cu (+), single-fold coordination to Cd (2+), and nonbonding to Zn (2+)). Diffuse-reflectance spectra show that the metal-thioether interaction is associated with smaller electronic band gaps of the solid-state networks.

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.