The Overlooked Potential of Sulfated Zirconia: Reexamining Solid Superacidity Toward the Controlled Depolymerization of Polyolefins.

Closed-loop recycling via an efficient chemical process can help alleviate the global plastic waste crisis. However, conventional depolymerization methods for polyolefins, which compose more than 50% of plastics, demand high temperatures and pressures, employ precious noble metals, and/or yield complex mixtures of products limited to single-use fuels or oils. Superacidic forms of sulfated zirconia (SZrO) with Hammet Acidity Functions (H0) ≤ - 12 (i.e., stronger than 100% H2SO4) are industrially deployed heterogeneous catalysts capable of activating hydrocarbons under mild conditions and are shown to decompose polyolefins at temperatures near 200 °C and ambient pressure. Additionally, confinement of active sites in porous supports is known to radically increase selectivity, coking and sintering resistance, and acid site activity, presenting a possible approach to low-energy polyolefin depolymerization. However, a critical examination of the literature on SZrO led us to a surprising conclusion: despite 40 years of catalytic study, engineering, and industrial use, the surface chemistry of SZrO is poorly understood. Ostensibly spurred by SZrO's impressive catalytic activity, the application-driven study of SZrO has resulted in deleterious ambiguity in requisite synthetic conditions for superacidity and insufficient characterization of acidity, porosity, and active site structure. This ambiguity has produced significant knowledge gaps surrounding the synthesis, structure, and mechanisms of hydrocarbon activation for optimized SZrO, stunting the potential of this catalyst in olefin cracking and other industrially relevant reactions, such as isomerization, esterification, and alkylation. Toward mitigating these long extant issues, we herein identify and highlight these current shortcomings and knowledge gaps, propose explicit guidelines for characterization of and reporting on characterization of solid acidity, and discuss the potential of pore-confined superacids in the efficient and selective depolymerization of polyolefins.

Multivariate Flexible Framework with High Usable Hydrogen Capacity in a Reduced Pressure Swing Process.

Step-shaped adsorption-desorption of gaseous payloads by flexible metal-organic frameworks can facilitate the delivery of large usable capacities with significantly reduced energetic penalties. This is desirable for the storage, transport, and delivery of H2, as prototypical adsorbents require large swings in pressure and temperature to achieve usable capacities approaching their total capacities. However, the weak physisorption of H2 typically necessitates undesirably high pressures to induce the framework phase change. As de novo design of flexible frameworks is exceedingly challenging, the ability to intuitively adapt known frameworks is required. We demonstrate that the multivariate linker approach is a powerful tool for tuning the phase change behavior of flexible frameworks. In this work, 2-methyl-5,6-difluorobenzimidazolate was solvothermally incorporated into the known framework CdIF-13 (sod-Cd(benzimidazolate)2), resulting in the multivariate framework sod-Cd(benzimidazolate)1.87(2-methyl-5,6-difluorobenzimidazolate)0.13 (ratio = 14:1), which exhibited a considerably reduced stepped adsorption threshold pressure while maintaining the desirable adsorption-desorption profile and capacity of CdIF-13. At 77 K, the multivariate framework exhibits stepped H2 adsorption with saturation below 50 bar and minimal desorption hysteresis at 5 bar. At 87 K, saturation of step-shaped adsorption occurs by 90 bar, with hysteresis closing at 30 bar. These adsorption-desorption profiles enable usable capacities in a mild pressure swing process above 1 mass %, representing 85-92% of the total capacities. This work demonstrates that the desirable performance of flexible frameworks can be readily adapted through the multivariate approach to enable efficient storage and delivery of weakly physisorbing species.

Adaptive Pore Opening to Form Tailored Adsorption Sites in a Cooperatively Flexible Framework Enables Record Inverse Propane/Propylene Separation.

A proposed low-energy alternative to the separation of alkanes from alkenes by energy-intensive cryogenic distillation is separation by porous adsorbents. Unfortunately, most adsorbents preferentially take up the desired, high-value major component alkene, requiring frequent regeneration. Adsorbents with inverse selectivity for the minor component alkane would enable the direct production of purified, reagent-grade alkene, greatly reducing global energy consumption. However, such materials are exceedingly rare, especially for propane/propylene separation. Here, we report that through adaptive and spontaneous pore size and shape adaptation to optimize an ensemble of weak noncovalent interactions, the structurally responsive metal-organic framework CdIF-13 (sod-Cd(benzimidazolate)2) exhibits inverse selectivity for propane over propylene with record-setting separation performance under industrially relevant temperature, pressure, and mixture conditions. Powder synchrotron X-ray diffraction measurements combined with first-principles calculations yield atomic-scale insight and reveal the induced fit mechanism of adsorbate-specific pore adaptation and ensemble interactions between ligands and adsorbates. Dynamic column breakthrough measurements confirm that CdIF-13 displays selectivity under mixed-component conditions of varying ratios, with a record measured selectivity factor of α ≈ 3 at 95:5 propylene:propane at 298 K and 1 bar. When sequenced with a low-cost rigid adsorbent, we demonstrated the direct purification of propylene under ambient conditions. This combined atomic-level structural characterization and performance testing firmly establishes how cooperatively flexible materials can be capable of unprecedented separation factors.

Best Practices in the Characterization of MOF@MSN Composites.

Research on permanently porous nanomaterials has gripped the attention of materials chemists for decades. Mesoporous silica nanoparticles (MSNs) and metal-organic frameworks (MOFs) are two of the most studied classes of materials in this field. Recently, explorations into embedding MOFs within the mesopores of MSNs have aimed to create composites that are greater than the sum of their parts. While initial progress has been promising, it has become clear that the characterization of these MOF@MSN composite materials represents a significant challenge that is often overlooked, leading to an unfortunate ambiguity in the field. The greatest difficulty lies in determining whether the product of a synthesis is simply a physical mixture of the two materials or truly the targeted composite, with MOF exclusively crystallized in the pores or on the surfaces of the MSN. This challenge is aggravated by the dramatically different porosity and composition of the components, often resulting in ambiguous information from common characterization techniques. This Viewpoint will address this challenge by calling attention to the mentioned issues and proposing a standardized approach to characterizing these materials. In particular, the use of powder X-ray diffraction, gas physisorption, and electron microscopy with systematic control experiments and data analysis is outlined. This approach can provide the information needed to clearly validate the architecture of an apparent MOF@MSN composite.

A Porous Chalcogen-Bonded Organic Framework.

The manner of bonding between constituent atoms or molecules invariably influences the properties of materials. Perhaps no material family is more emblematic of this than porous frameworks, wherein the namesake modes of connectivity give rise to discrete subclasses with unique collections of properties. However, established framework classes often display offsetting advantages and disadvantages for a given application. Thus, there exists no universally applicable material, and the discovery of alternative modes of framework connectivity is highly desirable. Here we show that chalcogen bonding, a subclass of σ-hole bonding, is a viable mode of connectivity in low-density porous frameworks. Crystallization studies with the triptycene tris(1,2,5-selenadiazole) molecular tecton reveal how chalcogen bonding can template high-energy lattice structures and how solvent conditions can be rationalized to obtain molecularly programmed porous chalcogen-bonded organic frameworks (ChOFs). These results provide the first evidence that σ-hole bonding can be used to advance the diversity of porous framework materials.

Cooperative Carbon Dioxide Adsorption in Alcoholamine- and Alkoxyalkylamine-Functionalized Metal-Organic Frameworks.

A series of structurally diverse alcoholamine- and alkoxyalkylamine-functionalized variants of the metal-organic framework Mg2 (dobpdc) are shown to adsorb CO2 selectively via cooperative chain-forming mechanisms. Solid-state NMR spectra and optimized structures obtained from van der Waals-corrected density functional theory calculations indicate that the adsorption profiles can be attributed to the formation of carbamic acid or ammonium carbamate chains that are stabilized by hydrogen bonding interactions within the framework pores. These findings significantly expand the scope of chemical functionalities that can be utilized to design cooperative CO2 adsorbents, providing further means of optimizing these powerful materials for energy-efficient CO2 separations.

Influence of Metal Substitution on the Pressure-Induced Phase Change in Flexible Zeolitic Imidazolate Frameworks.

Metal-organic frameworks that display step-shaped adsorption profiles arising from discrete pressure-induced phase changes are promising materials for applications in both high-capacity gas storage and energy-efficient gas separations. The thorough investigation of such materials through chemical diversification, gas adsorption measurements, and in situ structural characterization is therefore crucial for broadening their utility. We examine a series of isoreticular, flexible zeolitic imidazolate frameworks (ZIFs) of the type M(bim)2 (SOD; M = Zn (ZIF-7), Co (ZIF-9), Cd (CdIF-13); bim- = benzimidazolate), and elucidate the effects of metal substitution on the pressure-responsive phase changes and the resulting CO2 and CH4 step positions, pre-step uptakes, and step capacities. Using ZIF-7 as a benchmark, we reexamine the poorly understood structural transition responsible for its adsorption steps and, through high-pressure adsorption measurements, verify that it displays a step in its CH4 adsorption isotherms. The ZIF-9 material is shown to undergo an analogous phase change, yielding adsorption steps for CO2 and CH4 with similar profiles and capacities to ZIF-7, but with shifted threshold pressures. Further, the Cd2+ analogue CdIF-13 is reported here for the first time, and shown to display adsorption behavior distinct from both ZIF-7 and ZIF-9, with negligible pre-step adsorption, a ∼50% increase in CO2 and CH4 capacity, and dramatically higher threshold adsorption pressures. Remarkably, a single-crystal-to-single-crystal phase change to a pore-gated phase is also achieved with CdIF-13, providing insight into the phase change that yields step-shaped adsorption in these flexible ZIFs. Finally, we show that the endothermic phase change of these frameworks provides intrinsic heat management during gas adsorption.

An Allosterically Regulated, Four-State Macrocycle.

Macrocycles capable of host-guest chemistry are an important class of structures that have attracted considerable attention because of their utility in chemical separations, analyte sensing, signal amplification, and drug delivery. The deliberate design and synthesis of such structures are rate-limiting steps in utilizing them for such applications, and coordination-driven supramolecular chemistry has emerged as a promising tool for rapidly making large classes of such systems with attractive molecular recognition capabilities and, in certain cases, catalytic properties. A particularly promising subset of such systems are stimuli-responsive constructs made from hemilabile ligands via the weak-link approach (WLA) to supramolecular coordination chemistry. Such structures can be reversibly toggled between different shapes, sizes, and charges based upon small-molecule and elemental-anion chemical effectors. In doing so, one can deliberately change their recognition properties and both stoichiometric and catalytic chemistries, thereby providing mimics of allosteric enzymes. The vast majority of structures made to date involve two-state systems, with a select few being able to access three different states. Herein, we describe the synthesis of a new allosterically regulated four-state macrocycle assembled via the WLA. The target structure was made via the stepwise assembly of ditopic bidentate hemilabile N-heterocyclic carbene thioether (NHC,S) and phosphino thioether (P,S) ligands at PtII metal nodes. The relatively simple macrocycle displays complex dynamic behavior when addressed with small-molecule effectors, and structural switching can be achieved with several distinct molecular cues. Importantly, each state was fully characterized by multinuclear NMR spectroscopy and, in some cases, single-crystal X-ray diffraction studies and density functional theory computational models. This new structure opens the door to complex multicue switching reminiscent of multistate chemoswitches that could be important in controlling stoichiometric and catalytic transformations as well as generating molecular logic systems.

General and Direct Method for Preparing Oligonucleotide-Functionalized Metal-Organic Framework Nanoparticles.

Metal-organic frameworks (MOFs) are a class of modular, crystalline, and porous materials that hold promise for storage and transport of chemical cargoes. Though MOFs have been studied in bulk forms, ways of deliberately manipulating the external surface functionality of MOF nanoparticles are less developed. A generalizable approach to modify their surfaces would allow one to impart chemical functionality onto the particle surface that is independent of the bulk MOF structure. Moreover, the use of a chemically programmable ligand, such as DNA, would allow for the manipulation of interparticle interactions. Herein, we report a coordination chemistry-based strategy for the surface functionalization of the external metal nodes of MOF nanoparticles with terminal phosphate-modified oligonucleotides. The external surfaces of nine distinct archetypical MOF particles containing four different metal species (Zr, Cr, Fe, and Al) were successfully functionalized with oligonucleotides, illustrating the generality of this strategy. By taking advantage of the programmable and specific interactions of DNA, 11 distinct MOF particle-inorganic particle core-satellite clusters were synthesized. In these hybrid nanoclusters, the relative stoichiometry, size, shape, and composition of the building blocks can all be independently controlled. This work provides access to a new set of nucleic acid-nanoparticle conjugates, which may be useful as programmable material building blocks and as probes for measuring and manipulating intracellular processes.

Cooperative Electronic and Structural Regulation in a Bioinspired Allosteric Photoredox Catalyst.

Herein, we report the first allosteric photoredox catalyst regulated via constructively coupled structural and electronic control. While often synergistically exploited in nature, these two types of control mechanisms have only been applied independently in the vast majority of allosteric enzyme mimics and receptors in the literature. By embedding a model of photosystem II in a supramolecular coordination complex that responds to chloride as an allosteric effector, we show that distance and electronic control of light harvesting can be married to maximize allosteric regulation of catalytic activity. This biomimetic system is composed of a Bodipy photoantenna, which is capable of transferring excited-state energy to a photoredox pair, wherein the excitation energy is used to generate a catalytically active charge-separated state. The structural aspect of allosteric regulation is achieved by toggling the coordination chemistry of an antenna-functionalized hemilabile ligand via partial displacement from a Rh(I) structual node using chloride. In doing so, the distance between the antenna and the central photoredox catalyst is increased, lowering the inherent efficiency of through-space energy transfer. At the same time, coordination of chloride lowers both the charge of the Rh(I) node and the reduction potential of the Rh(II/I) couple, to the extent that electronic quenching of the antenna excited state is possible via photoinduced electron transfer from the metal center. Compared to a previously developed system that operates solely via electronic regulation, the present system demonstrates that coupling electronic and structural approaches to allosteric regulation gives rise to improved switching ratios between catalytically active and inactive states. Contributions from both structural and electronic control mechanisms are probed via nuclear magnetic resonance, X-ray diffraction, electrochemical, spectroelectrochemical, and transient absorption studies. Overall, this work establishes that intertwined electronic and structural regulatory mechanisms can be borrowed from nature to build stimuli-responsive inorganic materials with potential applications in sensing, catalysis, and photonic devices.

Allosteric supramolecular coordination constructs.

Coordination chemistry is regularly used to generate supramolecular constructs with unique environments around embedded components to affect their intrinsic properties. In certain cases, it can also be used to effect changes in supramolecular structure reminiscent of those that occur within stimuli-responsive biological structures, such as allosteric enzymes. Indeed, among a handful of general strategies for synthesizing such supramolecular systems, the weak-link approach (WLA) uniquely allows one to toggle the frameworks' structural state post-assembly via simple reactions involving hemilabile ligands and transition metal centers. This synthetic strategy, when combined with dynamic ligand sorting processes, represents one of the few sets of general reactions in inorganic chemistry that allow one to synthesize spatially defined, stimuli-responsive, and multi-component frameworks in high to quantitative yields and with remarkable functional group tolerance. The WLA has thus yielded a variety of functional systems that operate similarly to allosteric enzymes, toggling activity via changes in the frameworks' steric confinement or electronic state upon the recognition of small molecule inputs. In this Perspective we present the first full description of the fundamental inorganic reactions that provide the foundation for synthesizing WLA complexes. In addition, we discuss the application of regulatory strategies in biology to the design of allosteric supramolecular constructs for the regulation of various catalytic properties, electron-transfer processes, and molecular receptors, as well as for the development of sensing and signal amplification systems.

Turning on catalysis: incorporation of a hydrogen-bond-donating squaramide moiety into a Zr metal-organic framework.

Herein, we demonstrate that the incorporation of an acidic hydrogen-bond-donating squaramide moiety into a porous UiO-67 metal-organic framework (MOF) derivative leads to dramatic acceleration of the biorelevant Friedel-Crafts reaction between indole and ß-nitrostyrene. In comparison, it is shown that free squaramide derivatives, not incorporated into MOF architectures, have no catalytic activity. Additionally, using the UiO-67 template, we were able to perform a direct comparison of catalytic activity with that of the less acidic urea-based analogue. This is the first demonstration of the functionalization of a heterogeneous framework with an acidic squaramide derivative.

Surface-Specific Functionalization of Nanoscale Metal-Organic Frameworks.

A method for modifying the external surfaces of a series of nanoscale metal-organic frameworks (MOFs) with 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA) is presented. A series of zirconium-based nanoMOFs of the same topology (UiO-66, UiO-67, and BUT-30) were synthesized, isolated as aggregates, and then conjugated with DOPA to create stably dispersed colloids. BET surface area analysis revealed that these structures maintain their porosity after surface functionalization, providing evidence that DOPA functionalization only occurs on the external surface. Additionally, dye-labeled ligand loading studies revealed that the density of DOPA on the surface of the nanoscale MOF correlates to the density of metal nodes on the surface of each MOF. Importantly, the surface modification strategy described will allow for the general and divergent synthesis and study of a wide variety of nanoscale MOFs as stable colloidal materials.

Small molecule regulation of self-association and catalytic activity in a supramolecular coordination complex.

Herein, we report the synthesis and characterization of the first weak-link approach (WLA) supramolecular construct that employs the small molecule regulation of intermolecular hydrogen bonding interactions for the in situ control of catalytic activity. A biaryl urea group, prone to self-aggregation, was functionalized with a phosphinoalkyl thioether (P,S) hemilabile moiety and incorporated into a homoligated Pt(II) tweezer WLA complex. This urea-containing construct, which has been characterized by a single crystal X-ray diffraction study, can be switched in situ from a rigid fully closed state to a flexible semiopen state via Cl(-) induced changes in the coordination mode at the Pt(II) structural node. FT-IR and (1)H NMR spectroscopy studies were used to demonstrate that while extensive urea self-association persists in the flexible semiopen complex, these interactions are deterred in the rigid, fully closed complex because of geometric and steric restraints. Consequently, the urea moieties in the fully closed complex are able to catalyze a Diels-Alder reaction between cyclopentadiene and methyl vinyl ketone to generate 2-acetyl-5-norbornene. The free urea ligand and the semiopen complex show no such activity. The successful incorporation and regulation of a hydrogen bond donating catalyst in a WLA construct open the doors to a vast and rapidly growing catalogue of allosteric catalysts for applications in the detection and amplification of organic analytes.

Allosteric regulation of supramolecular oligomerization and catalytic activity via coordination-based control of competitive hydrogen-bonding events.

Herein, we demonstrate that the activity of a hydrogen-bond-donating (HBD) catalyst embedded within a coordination framework can be allosterically regulated in situ by controlling oligomerization via simple changes in coordination chemistry at distal Pt(II) nodes. Using the halide-induced ligand rearrangement reaction (HILR), a heteroligated Pt(II) triple-decker complex, which contains a catalytically active diphenylene squaramide moiety and two hydrogen-bond-accepting (HBA) ester moieties, was synthesized. The HBD and HBA moieties were functionalized with hemilabile ligands of differing chelating strengths, allowing one to assemble them around Pt(II) nodes in a heteroligated fashion. Due to the hemilabile nature of the ligands, the resulting complex can be interconverted between a flexible, semiopen state and a rigid, fully closed state in situ and reversibly. FT-IR spectroscopy, (1)H DOSY, and (1)H NMR spectroscopy titration studies were used to demonstrate that, in the semiopen state, intermolecular hydrogen-bonding between the HBD and HBA moieties drives oligomerization of the complex and prevents substrate recognition by the catalyst. In the rigid, fully closed state, these interactions are prevented by steric and geometric constraints. Thus, the diphenylene squaramide moiety is able to catalyze a Friedel-Crafts reaction in the fully closed state, while the semiopen state shows no reactivity. This work demonstrates that controlling catalytic activity by regulating aggregation through supramolecular conformational changes, a common approach in Nature, can be applied to man-made catalytic frameworks that are relevant to materials synthesis, as well as the detection and amplification of small molecules.

Influence of donor point modifications on the assembly of chalcogen-bonded organic frameworks.

Incremental, single-atom substitutions of Se-based chalcogen bond (Ch-bond) donors with stronger donating Te centers were implemented in two new triptycene tris(1,2,5-chalcogenadiazole) tectons. The appreciably more favorable Ch-bonding ability of the Te-based donors promotes assembly of low-density networks and more stable Ch-bonded organic frameworks (ChOFs).

General strategy for the synthesis of rigid weak-link approach platinum(II) complexes: tweezers, triple-layer complexes, and macrocycles.

Air-stable, heteroligated platinum(II) weak-link approach (WLA) tweezer and triple-layer complexes that possess P,X-Aryl hemilabile ligands (P^ = Ph2PCH2CH2-, X = chalcoethers or amines) have been synthesized via the halide-induced ligand rearrangement (HILR) reaction, using a one-pot, partial chloride-abstraction method. The approach is general and works with a variety of phosphine-based hemilabile ligands; when a P,S-Ph ligand is used as the relatively strongly chelating ligand, heteroligated complexes are formed cleanly when an ether- (P,O-Ph), amine- (P,N-Ph2), or fluorinated thioether-based (P,S-C6F4H) hemilabile ligand is used as the weakly chelating counterpart. The HILR reaction has also been used to synthesize bisplatinum(II) macrocycles free of oligomeric material without having to resort to the high-dilution conditions typical for macrocycle synthesis. This approach is complementary to the traditional WLA to the synthesis of macrocyclic complexes which typically proceeds via fully closed, chloride-free intermediates. The structures of the complexes may be toggled between semiopen (with only one chelating ligand) and fully closed (with both ligands chelating) via the abstraction and addition of chloride.

Multivariate zeolitic imidazolate frameworks with an inverting trend in flexibility.

Through systematic linker substitution in a flexible zeolitic imidazolate framework (ZIF) with step-shaped adsorption-desorption, structural intermediates between the known open and closed phases were isolated. Reflecting this, modulative sorption behaviour with an inverting adsorption pressure trend-in which the step pressure decreases and then increases again with increasing mixed linker concentration-is observed, highlighting how linker substitution modifies the energetic landscape of framework flexibility.

Structural resolution and mechanistic insight into hydrogen adsorption in flexible ZIF-7.

Flexible metal-organic frameworks offer a route towards high useable hydrogen storage capacities with minimal swings in pressure and temperature via step-shaped adsorption and desorption profiles. Yet, the understanding of hydrogen-induced flexibility in candidate storage materials remains incomplete. Here, we investigate the hydrogen storage properties of a quintessential flexible metal-organic framework, ZIF-7. We use high-pressure isothermal hydrogen adsorption measurements to identify the pressure-temperature conditions of the hydrogen-induced structural transition in ZIF-7. The material displays narrow hysteresis and has a shallow adsorption slope between 100 K and 125 K. To gain mechanistic insight into the cause of the phase transition correlating with stepped adsorption and desorption, we conduct powder neutron diffraction measurements of the D2 gas-dosed structures at conditions across the phase change. Rietveld refinements of the powder neutron diffraction patterns yield the structures of activated ZIF-7 and of the gas-dosed material in the dense and open phases. The structure of the activated phase of ZIF-7 is corroborated by the structure of the activated phase of the Cd congener, CdIF-13, which we report here for the first time based on single crystal X-ray diffraction measurements. Subsequent Rietveld refinements of the powder patterns for the gas-dosed structure reveal that the primary D2 adsorption sites in the dense phase form D2-arene interactions between adjacent ligands in a sandwich-like adsorption motif. These sites are prevalent in both the dense and the open structure for ZIF-7, and we hypothesize that they play an important role in templating the structure of the open phase. We discuss the implications of our findings for future approaches to rationally tune step-shaped adsorption in ZIF-7, its congeners, and flexible porous adsorbents in general. Lastly, important to the application of flexible frameworks, we show that pelletization of ZIF-7 produces minimal variation in performance.

Metal-Organic Framework Nanoparticles.

Due to their well-defined 3D architectures, permanent porosity, and diverse chemical functionalities, metal-organic framework nanoparticles (MOF NPs) are an emerging class of modular nanomaterials. Herein, recent developments in the synthesis and postsynthetic surface functionalization of MOF NPs that strengthen the fundamental understanding of how such structures form and grow are highlighted; the internal structure and external surface properties of these novel nanomaterials are highlighted as well. These fundamental advances have resulted in MOF NPs being used as components in chemical sensors, biological probes, and membrane separation materials, as well as building blocks for colloidal crystal engineering.