Interconversion between Discrete and a Chain of Nanocages: Self-Assembly via a Solvent-Driven, Dimension-Augmentation Strategy

Using a ligand bearing a bulky hydrophobic group, a “shish kabob” of nanocages, has been assembled through either a one-fell-swoop or a step-by-step procedure by varying the dielectric constant of the assembly mixture. A hydrophobic solvent breaks down the chain to discrete nanocages, while a hydrophilic solvent reverses the procedure. Although the shish kabob of nanocages has exactly the same chemical composition and even the same Archimedean-solid structure as those of its discrete analogue, its gas-adsorption capacity is remarkably improved because assembly of a chain exposes the internal surface of an individual cage. This dimension-augmentation strategy may have general implications in the preparation of porous materials

Chromium(II) Metal–Organic Polyhedra as Highly Porous Materials

Herein we report for the first time the synthesis of Cr­(II)-based metal–organic polyhedra (MOPs) and the characterization of their porosities. Unlike the isostructural Cu­(II)- or Mo­(II)-based MOPs, Cr­(II)-based MOPs show unusually high gas uptakes and surface areas. The combination of comparatively robust dichromium paddlewheel units (Cr₂ units), cage symmetries, and packing motifs enable these materials to achieve Brunauer–Emmett–Teller surface areas of up to 1000 m²/g. Reducing the aggregation of the Cr­(II)-based MOPs upon activation makes their pores more accessible than their Cu­(II) or Mo­(II) counterparts. Further comparisons of surface areas on a molar (m²/mol cage) rather than gravimetric (m²/g) basis is proposed as a rational method of comparing members of a family of related molecular materials

Direct Measurement of Adsorbed Gas Redistribution in Metal–Organic Frameworks

Knowledge about the interactions between gas molecules and adsorption sites is essential to customize metal-organic frameworks (MOFs) as adsorbents. The dynamic interactions occurring during adsorption/desorption working cycles with several states are especially complicated. Even so, the gas dynamics based upon experimental observations and the distribution of guest molecules under various conditions in MOFs have not been extensively studied yet. In this work, a direct time-resolved diffraction structure envelope (TRDSE) method using sequential measurements by in situ synchrotron powder X-ray diffraction has been developed to monitor several gas dynamic processes taking place in MOFs: infusion, desorption, and gas redistribution upon temperature change. The electron density maps indicate that gas molecules prefer to redistribute over heterogeneous types of sites rather than to exclusively occupy the primary binding sites. We found that the gas molecules are entropically driven from open metal sites to larger neighboring spaces during the gas infusion period, matching the localized-to-mobile mechanism. In addition, the partitioning ratio of molecules adsorbed at each site varies with different temperatures, as opposed to an invariant distribution mode. Equally important, the gas adsorption in MOFs is intensely influenced by the gas–gas interactions, which might induce more molecules to be accommodated in an orderly compact arrangement. This sequential TRDSE method is generally applicable to most crystalline adsorbents, yielding information on distribution ratios of adsorbates at each type of site

Preparation of Core–Shell Coordination Molecular Assemblies via the Enrichment of Structure-Directing “Codes” of Bridging Ligands and Metathesis of Metal Units

A series of molybdenum- and copper-based MOPs were synthesized through coordination-driven process of a bridging ligand (3,3′-PDBAD, <b>L</b>^<b>1</b>) and dimetal paddlewheel clusters. Three conformers of the ligand exist with an ideal bridging angle between the two carboxylate groups of 0° (H₂α-<b>L</b>^<b>1</b>), 120° (H₂β-<b>L</b>^<b>1</b>), and of 90° (H₂γ-<b>L</b>^<b>1</b>), respectively. At ambient or lower temperature, H₂<b>L</b>^<b>1</b> and Mo₂(OAc)₄ or Cu₂(OAc)₄ were crystallized into a molecular square with γ-<b>L</b>^<b>1</b> and Mo₂/Cu₂ units. With proper temperature elevation, not only the molecular square with γ-<b>L</b>^<b>1</b> but also a lantern-shaped cage with α-<b>L</b>^<b>1</b> formed simultaneously. Similar to how Watson–Crick pairs stabilize the helical structure of duplex DNA, the core–shell molecular assembly possesses favorable H-bonding interaction sites. This is dictated by the ligand conformation in the shell, coding for the formation and providing stabilization of the central lantern shaped core, which was not observed without this complementary interaction. On the basis of the crystallographic implications, a heterobimetallic cage was obtained through a postsynthetic metal ion metathesis, showing different reactivity of coordination bonds in the core and shell. As an innovative synthetic strategy, the site-selective metathesis broadens the structural diversity and properties of coordination assemblies

Preparation of Core–Shell Coordination Molecular Assemblies via the Enrichment of Structure-Directing “Codes” of Bridging Ligands and Metathesis of Metal Units

A series of molybdenum- and copper-based MOPs were synthesized through coordination-driven process of a bridging ligand (3,3′-PDBAD, <b>L</b>^<b>1</b>) and dimetal paddlewheel clusters. Three conformers of the ligand exist with an ideal bridging angle between the two carboxylate groups of 0° (H₂α-<b>L</b>^<b>1</b>), 120° (H₂β-<b>L</b>^<b>1</b>), and of 90° (H₂γ-<b>L</b>^<b>1</b>), respectively. At ambient or lower temperature, H₂<b>L</b>^<b>1</b> and Mo₂(OAc)₄ or Cu₂(OAc)₄ were crystallized into a molecular square with γ-<b>L</b>^<b>1</b> and Mo₂/Cu₂ units. With proper temperature elevation, not only the molecular square with γ-<b>L</b>^<b>1</b> but also a lantern-shaped cage with α-<b>L</b>^<b>1</b> formed simultaneously. Similar to how Watson–Crick pairs stabilize the helical structure of duplex DNA, the core–shell molecular assembly possesses favorable H-bonding interaction sites. This is dictated by the ligand conformation in the shell, coding for the formation and providing stabilization of the central lantern shaped core, which was not observed without this complementary interaction. On the basis of the crystallographic implications, a heterobimetallic cage was obtained through a postsynthetic metal ion metathesis, showing different reactivity of coordination bonds in the core and shell. As an innovative synthetic strategy, the site-selective metathesis broadens the structural diversity and properties of coordination assemblies

Preparation of Core–Shell Coordination Molecular Assemblies via the Enrichment of Structure-Directing “Codes” of Bridging Ligands and Metathesis of Metal Units

A series of molybdenum- and copper-based MOPs were synthesized through coordination-driven process of a bridging ligand (3,3′-PDBAD, <b>L</b>^<b>1</b>) and dimetal paddlewheel clusters. Three conformers of the ligand exist with an ideal bridging angle between the two carboxylate groups of 0° (H₂α-<b>L</b>^<b>1</b>), 120° (H₂β-<b>L</b>^<b>1</b>), and of 90° (H₂γ-<b>L</b>^<b>1</b>), respectively. At ambient or lower temperature, H₂<b>L</b>^<b>1</b> and Mo₂(OAc)₄ or Cu₂(OAc)₄ were crystallized into a molecular square with γ-<b>L</b>^<b>1</b> and Mo₂/Cu₂ units. With proper temperature elevation, not only the molecular square with γ-<b>L</b>^<b>1</b> but also a lantern-shaped cage with α-<b>L</b>^<b>1</b> formed simultaneously. Similar to how Watson–Crick pairs stabilize the helical structure of duplex DNA, the core–shell molecular assembly possesses favorable H-bonding interaction sites. This is dictated by the ligand conformation in the shell, coding for the formation and providing stabilization of the central lantern shaped core, which was not observed without this complementary interaction. On the basis of the crystallographic implications, a heterobimetallic cage was obtained through a postsynthetic metal ion metathesis, showing different reactivity of coordination bonds in the core and shell. As an innovative synthetic strategy, the site-selective metathesis broadens the structural diversity and properties of coordination assemblies

Introduction of Functionalized Mesopores to Metal–Organic Frameworks via Metal–Ligand–Fragment Coassembly

Introduction of functionalized mesopores into microporous metal–organic frameworks (MOFs) can endow them with suitable properties for applications in gas storage, separation, catalysis, and drug delivery. However, common methods for functionalization (including pre- and post-synthetic modifications) of the internal surface of a MOF reduce the pore size of the MOF because the additional functional groups fill up the pores. We present a metal–ligand–fragment coassembly strategy for the introduction of (meso)­pores functionalized with various substituent groups on the ligand fragments. Astonishingly, this new functionalization strategy <i>increases</i> the pore volume of a MOF instead of reducing it. Since the ligand fragments are often readily available or easily prepared, the new procedure for synthesis of the modified MOFs becomes much easier and more applicable than existing approaches. Remarkably, mesopores can be generated conveniently and controllably by the coassembly of a ligand and its fragment containing the desired functional groups. The fragment/ligand ratio has been optimized to preserve the parent structure and to promote maximum mesopore introduction, which has led to a systematic evaluation of the effectiveness of a series of functional groups for the adsorption of guest molecules

Metal–Organic Frameworks Based on Previously Unknown Zr₈/Hf₈ Cubic Clusters

The ongoing study of zirconium– and hafnium–porphyrinic metal–organic frameworks (MOFs) led to the discovery of isostructural MOFs based on Zr₈ and Hf₈ clusters, which are unknown in both cluster and MOF chemistry. The Zr₈O₆ cluster features an idealized Zr₈ cube, in which each Zr atom resides on one vertex and each face of the cube is capped by one μ₄-oxygen atom. On each edge of the cube, a carboxylate from a porphyrinic ligand bridges two Zr atoms to afford a 3D MOF with a very rare (4,12)-connected <b>ftw</b> topology, in which two types of polyhedral cages with diameters of ∼1.1 and ∼2.0 nm and a cage opening of ∼0.8 nm are found. The isostructural Zr– and Hf–MOFs exhibit high surface areas, gas uptakes, and catalytic selectivity for cyclohexane oxidation

Metal–Organic Frameworks Based on Previously Unknown Zr₈/Hf₈ Cubic Clusters

The ongoing study of zirconium– and hafnium–porphyrinic metal–organic frameworks (MOFs) led to the discovery of isostructural MOFs based on Zr₈ and Hf₈ clusters, which are unknown in both cluster and MOF chemistry. The Zr₈O₆ cluster features an idealized Zr₈ cube, in which each Zr atom resides on one vertex and each face of the cube is capped by one μ₄-oxygen atom. On each edge of the cube, a carboxylate from a porphyrinic ligand bridges two Zr atoms to afford a 3D MOF with a very rare (4,12)-connected <b>ftw</b> topology, in which two types of polyhedral cages with diameters of ∼1.1 and ∼2.0 nm and a cage opening of ∼0.8 nm are found. The isostructural Zr– and Hf–MOFs exhibit high surface areas, gas uptakes, and catalytic selectivity for cyclohexane oxidation

Metal–Organic Frameworks Based on Previously Unknown Zr₈/Hf₈ Cubic Clusters

The ongoing study of zirconium– and hafnium–porphyrinic metal–organic frameworks (MOFs) led to the discovery of isostructural MOFs based on Zr₈ and Hf₈ clusters, which are unknown in both cluster and MOF chemistry. The Zr₈O₆ cluster features an idealized Zr₈ cube, in which each Zr atom resides on one vertex and each face of the cube is capped by one μ₄-oxygen atom. On each edge of the cube, a carboxylate from a porphyrinic ligand bridges two Zr atoms to afford a 3D MOF with a very rare (4,12)-connected <b>ftw</b> topology, in which two types of polyhedral cages with diameters of ∼1.1 and ∼2.0 nm and a cage opening of ∼0.8 nm are found. The isostructural Zr– and Hf–MOFs exhibit high surface areas, gas uptakes, and catalytic selectivity for cyclohexane oxidation