Structure and redox tuning of gas adsorption properties in calixarene-supported Fe(ii)-based porous cages

Ag1+ incorporation via a Zr4+-anchored metalloligand: fine-tuning catalytic Ag sites in Zr/Ag bimetallic clusters for enhanced eCO2RR-to-CO activity

Attaining meticulous dominion over the binding milieu of catalytic metal sites remains an indispensable pursuit to tailor product selectivity and elevate catalytic activity. By harnessing the distinctive attributes of a Zr4+-anchored thiacalix[4]arene (TC4A) metalloligand, we have pioneered a methodology for incorporating catalytic Ag1+ sites, resulting in the first Zr-Ag bimetallic cluster, Zr2Ag7, which unveils a dualistic configuration embodying twin {ZrAg3(TC4A)2} substructures linked by an {AgSal} moiety. This cluster unveils a trinity of discrete Ag sites: a pair ensconced within {ZrAg3(TC4A)2} subunits and one located between two units. Expanding the purview, we have also crafted ZrAg3 and Zr2Ag2 clusters, meticulously mimicking the two Ag site environment inherent in the {ZrAg3(TC4A)2} monomer. The distinct structural profiles of Zr2Ag7, ZrAg3, and Zr2Ag provide an exquisite foundation for a precise comparative appraisal of catalytic prowess across three Ag sites intrinsic to Zr2Ag7. Remarkably, Zr2Ag7 eclipses its counterparts in the electroreduction of CO2, culminating in a CO faradaic efficiency (FECO) of 90.23% at -0.9 V. This achievement markedly surpasses the performance metrics of ZrAg3 (FECO: 55.45% at -1.0 V) and Zr2Ag2 (FECO: 13.09% at -1.0 V). Utilizing in situ ATR-FTIR, we can observe reaction intermediates on the Ag sites. To unveil underlying mechanisms, we employ density functional theory (DFT) calculations to determine changes in free energy accompanying each elementary step throughout the conversion of CO2 to CO. Our findings reveal the exceptional proficiency of the bridged-Ag site that interconnects paired {ZrAg3(TC4A)2} units, skillfully stabilizing *COOH intermediates, surpassing the stabilization efficacy of the other Ag sites located elsewhere. The invaluable insights gleaned from this pioneering endeavor lay a novel course for the design of exceptionally efficient catalysts tailored for CO2 reduction reactions, emphatically underscoring novel vistas this research unshrouds.

Metal-organic cages for gas adsorption and separation

The unique high surface area and tunable cavity size endow metal-organic cages (MOCs) with superior performance and broad application in gas adsorption and separation. Over the past three decades, for instance, numerous MOCs have been widely explored in adsorbing diverse types of gas including energy gases, greenhouse gases, toxic gases, noble gases, etc. To gain a better understanding of the structure-performance relationships, great endeavors have been devoted to ligand design, metal node regulation, active metal site construction, cavity size adjustment, and function-oriented ligand modification, thus opening up routes toward rationally designed MOCs with enhanced capabilities. Focusing on the unveiled structure-performance relationships of MOCs towards target gas molecules, this review consists of two parts, gas adsorption and gas separation, which are discussed separately. Each part discusses the cage assembly process, gas adsorption strategies, host-guest chemistry, and adsorption properties. Finally, we briefly overviewed the challenges and future directions in the rational development of MOC-based sorbents for application in challenging gas adsorption and separation, including the development of high adsorption capacity MOCs oriented by adsorbability and the development of highly selective adsorption MOCs oriented by separation performance.

Redox-Active Ruthenium-Organic Polyhedra with Tunable Surface Functionality and Porosities

Dinuclear ruthenium paddlewheel complexes exhibit high structural stability in redox reactions. The use of these chemical motifs for the construction of Ru-based metal-organic polyhedra (RuMOPs) provides a route for redox-active porous materials. However, there are few studies on the synthesis and characterization of RuMOPs due to the difficulty in controlling the assembly process via the ligand-exchange reaction of equatorial acetates of the diruthenium tetraacetate precursors with dicarboxylic acid ligands. In this study, we synthesized three novel cuboctahedral RuMOPs based on the Ru2(II/III)-paddlewheel units with different alkyl functionalizations on the benzene-1,3-dicarboxylate moieties. We evaluated the effect of external functionalization on the molecular packing and the porous and redox properties. The electrochemical measurements revealed the multielectron transferred redox process where the electron-donating/-withdrawing nature of the functional groups allows the control of the redox behavior.

Increasing the stability of calixarene-capped porous cages through coordination sphere tuning

Chemically and thermally stable permanently porous coordination cages are appealing candidates for separations, catalysis, and as the porous component of new porous liquids. However, many of these applications have not turned to microporous cages as a result of their poor solubility and thermal or hydrolytic stability. Here we describe the design and modular synthesis of iron and cobalt cages where the carboxylate groups of the bridging ligands of well-known calixarene capped coordination cages have been replaced with more basic triazole units. The resultingly higher M-L bond strengths afford highly stable cages that are amenable to modular synthetic approaches and potential functionalization or modification. Owing to the robust nature of these cages, they are highly processable and are isolable in various physical states with tunable porosity depending on the solvation methods used. As the structural integrity of the cages is maintained upon high activation temperatures, apparent losses in porosity can be mediated by resolvation and crystallization or precipitation.

Stepwise assembly of thiacalix[4]arene-protected Ag/Ti bimetallic nanoclusters: accurate identification of catalytic Ag sites in CO2 electroreduction

The accurate identification of catalytic sites in heterogeneous catalysts poses a significant challenge due to the intricate nature of controlling interfacial chemistry at the molecular level. In this study, we introduce a novel strategy to address this issue by utilizing a thiacalix[4]arene (TC4A)-protected Ti-oxo core as a template for loading Ag1+ ions, leading to the successful synthesis of a unique Ag/Ti bimetallic nanocluster denoted as Ti8Ag8. This nanocluster exhibits multiple surface-exposed Ag sites and possesses a distinctive "core-shell" structure, consisting of a {Ti4@Ag8(TC4A)4} core housing a {Ti2O2@Ag4(TC4A)2} motif and two {Ti@Ag2(TC4A)} motifs. To enable a comprehensive analysis, we also prepared a Ti2Ag4 cluster with the same {Ti2O2@Ag4(TC4A)2} structure found within Ti8Ag8. The structural disparities between Ti8Ag8 and Ti2Ag4 provide an excellent platform for a comparison of catalytic activity at different Ag sites. Remarkably, Ti8Ag8 exhibits exceptional performance in the electroreduction of CO2 (eCO2RR), showcasing a CO faradaic efficiency (FECO) of 92.33% at -0.9 V vs. RHE, surpassing the FECO of Ti2Ag4 (69.87% at -0.9 V vs. RHE) by a significant margin. Through density functional theory (DFT) calculations, we unveil the catalytic mechanism and further discover that Ag active sites located at {Ti@Ag2(TC4A)} possess a higher εd value compared to those at {Ti2O2@Ag4(TC4A)2}, enhancing the stabilization of the *COOH intermediate during the eCO2RR. This study provides valuable insights into the accurate identification of catalytic sites in bimetallic nanoclusters and opens up promising avenues for efficient CO2 reduction catalyst design.

Porous Metal-Organic Cages Based on Rigid Bicyclo[2.2.2]oct-7-ene Type Ligands: Synthesis, Structure, and Gas Uptake Properties

Three new ligands containing a bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxydiimide unit have been used to assemble lantern-type metal-organic cages with the general formula [Cu4 L4 ]. Functionalisation of the backbone of the ligands leads to distinct crystal packing motifs between the three cages, as observed with single-crystal X-ray diffraction. The three cages vary in their gas sorption behaviour, and the capacity of the materials for CO2 is found to depend on the activation conditions: softer activation conditions lead to superior uptake, and one of the cages displays the highest BET surface area found for lantern-type cages so far.

Utilization of a Mixed-Ligand Strategy to Tune the Properties of Cuboctahedral Porous Coordination Cages

Ligand functionalization has been thoroughly leveraged to alter the properties of paddlewheel-based coordination cages where, in the case of ligand-terminated cages, functional groups are positioned on the periphery of synthesized cages. While these groups can be used to optimize solubility, porosity, crystal packing, thermal stability toward desolvation, reactivity, or optical activity, optimization of multiple properties can be challenging given their interconnected nature. For example, installation of functional groups to increase the solubility of porous cages typically has the effect of decreasing their porosity and stability toward thermal activation. Here we show that mixed-ligand cages can potentially address these issues as the benefits of various functional groups can be combined into one mixed-ligand cage. We further show that although ligand exchange reactions can be employed to obtain mixed ligand copper(II)-based cages, direct synthesis of mixed-ligand products is necessary for molybdenum(II) paddlewheel-based cages as these substitutionally inert clusters are resistant to ligand exchange. We ultimately show that highly soluble, highly porous, and thermally stable cuboctahedral cages are isolable by this strategy.

Porous nickel and cobalt hexanuclear ring-like clusters built from two different kind of calixarene ligands – new molecular traps for small volatile molecules

The formation and structural analysis of porous hexanuclear ring-like cluster complexes built from two different kind of calixarene ligands is presented, together with their stability and vapor solvent sorption properties.

Stabilizing Porosity in Organic Cages through Coordination Chemistry

The number of studies concerning the permanent porosity of molecular materials, especially porous organic cages (POCs) and porous coordination cages (PCCs), have increased substantially over the past decade. The work presented here outlines novel approaches to the preparation of porous molecular structures upon metalation of nonporous, amine-based organic cages. Reduction of the well-known CC3 and CC1 imine-based POCs affords nonporous, highly flexible amine cages. These materials can be endowed with significant levels of structural rigidity via post-synthetic metalation of their ethylenediamine-type binding pockets. The hybrid metal-organic cages accessed through this approach combine aspects of POC and PCC chemistry, with structures of this type providing a potentially promising new direction for the design and development of porous molecular materials with tunability in overall charge, metal cation, porosity, and solubility.

Synthesis and Characterization of an Isoreticular Family of Calixarene-Capped Porous Coordination Cages

Functionalization of permanently porous coordination cages has been used to tune phase, surface area, stability, and solubility in this promising class of adsorbents. For many cages, however, these properties are intricately tied together, and installation of functional groups, for example, to increase solubility often leads to a decrease in surface area. Calixarene-capped cages offer the advantage in that they are cluster-terminated cages whose solid-state packing, and thus surface area, is typically governed by the nature of the capping ligand rather than the bridging ligand. In this work we investigate the influence of ligand functionalization on two series of isoreticular Ni(II)- and Co(II)-based calixarene-capped cages. The two types of materials described are represented as octahedral and rectangular prismatic coordination cages and can be synthesized in a modular manner, allowing for the substitution of dicarboxylate bridging ligands and the introduction of functional groups in specific locations on the cage. We ultimately show that highly soluble cages can be obtained while still having access to high surface areas for many of the isolated phases.

Synthesis, characterization, and polymerization of capped paddlewheel porous cages

Although paddlewheel-based structures are common among permanently porous metal-organic materials, suitable strategies for the isolation of metal node-terminated, capped paddlewheel-based cage structures remain limited. We explored the use of chelating dicarboxylate ligand derivates (esp) for the isolation of trimesate-linked cages, Mo12(btc)4(esp)6, that are structural analogs of the small octahedral pore of HKUST-1. The porosity of these novel cages is appreciably higher than that of previously reported structures of this type. We also demonstrate that pillaring the isolated cage with DABCO generated an amorphous polymer that featured exceptional thermal stability and enhanced porosity.

Gas Storage in Porous Molecular Materials

Molecules with permanent porosity in the solid state have been studied for decades. Porosity in these systems is governed by intrinsic pore space, as in cages or macrocycles, and extrinsic void space, created through loose, intermolecular solid-state packing. The development of permanently porous molecular materials, especially cages with organic or metal-organic composition, has seen increased interest over the past decade, and as such, incredibly high surface areas have been reported for these solids. Despite this, examples of these materials being explored for gas storage applications are relatively limited. This minireview outlines existing molecular systems that have been investigated for gas storage and highlights strategies that have been used to understand adsorption mechanisms in porous molecular materials.

Soluble imidazolium-functionalized coordination cages for efficient homogeneous catalysis of CO2 cycloaddition reactions

The soluble metal–organic cage ImBDC-Co can be employed as a homogeneous catalyst for the CO₂ cycloaddition reaction, and shows higher catalytic activity than its heterogeneous counterparts. Moreover, the soluble cage can realize recovery and reuse without activity loss.

Using Low-Pressure Methane Adsorption Isotherms for Higher-Throughput Screening of Methane Storage Materials

A useful correlation between the low-pressure (up to 1.2 bar), low-temperature (195 K) and high-pressure (up to 65 bar), room temperature (298 K) methane storage properties of a range of porous materials is reported. Methane isotherms under these two sets of conditions show a remarkable agreement in both equilibrium adsorption and deliverable capacities for materials with pore volumes that are less than approximately 0.80 cm³/g. This trend holds well for the suite of metal-organic frameworks and porous coordination cages we studied, in addition to a zeolite and porous organic cage. Although it is well known that gravimetric gas storage capacity trends with gravimetric surface area, the 1.2 bar, 195 K excess adsorption capacity of a given framework is a better indicator of its room temperature, 65 bar capacity. Given the significantly smaller sample quantities needed for low-pressure measurements, greater accessibility to researchers around the world, accuracy of the measurement, and higher throughput, we envision this method as a rapid screening tool for the identification of methane storage materials. As excess/total adsorption and gravimetric/volumetric adsorption can be interconverted by simple utilization of the scalar quantities of pore volume or density, respectively, this method can be easily adapted to obtain both gravimetric and volumetric total adsorption capacities for a given adsorbent. In terms of volumetric methane adsorption, we further investigate the relationship between crystallographic and bulk density for the adsorbents studied here. With this analysis, it becomes apparent that in the absence of novel synthetic approaches, reported volumetric storage capacities should be viewed as an optimistic upper limit for a given material and not necessarily a true reflection of its actual adsorption properties as most MOFs have bulk densities that are less than half of their crystallographic values.