Predesign and Systematic Synthesis of 11 Highly Porous Coordination Polymers with Unprecedented Topology

We propose and validate a simple strategy of vertex connection that can be used for framework design and pore size/type modulation to prepare a mother structure and another 10 highly porous isoreticular frameworks with unprecedented topology. Importantly, the potential accessible pore volumes (57–71%), pore sizes (6.8–11. 2 Å; 17.0–29.0 Å; 12.5–22.8 Å; 11.9–24.5 Å), and the pore shapes of this series of highly porous frameworks were simultaneously and systematically tuned. Interestingly, the pore size of IIa [Zn₄O­(L²)₂(BDC)_0.5]­{(CH₃)₂NH₂} decreased a little less than that of IIc [Zn₄O­(L²)₂(2,6-NDC)_0.5]­{(CH₃)₂NH₂}; however, its selectivity of CO₂ toward CH₄ increased by almost two times

Behavior of Binary Guests in a Porous Coordination Polymer

Controlling the condensed state of multiple guests in nanoporous media is critical to many applications, but an understanding of this phenomenon in pores smaller than a few nanometers is still unavailable. In this work, we investigate the aggregation state of binary guests, poly­(ethylene glycol) (PEG), and long-chain normal alkanes, in subnanometer channels of a porous coordination polymer (PCP) by monitoring their thermal transition behaviors. PEG and alkanes are immiscible in the bulk and their melting transitions are not affected by each other. Meanwhile, in the PCP nanochannels, the transition temperature and the heat of the binary–guest system were significantly different from when PEG or an alkane was individually included. This suggests the formation of microscopically segregated domain structures of PEG and alkane in the host crystal. The transition behaviors gradually varied by changing the introduction ratio between PEG and alkane, and thus the aggregation states of the two guests were successfully controlled by the simple variation of relative amounts. This methodology offers a promising route to control spatial configurations of multiple guest molecules in nanoporous matrices for advanced applications

Controlled Synthesis of Anisotropic Polymer Particles Templated by Porous Coordination Polymers

Nonspherical polymer particles have been efficiently prepared in different morphological crystals of porous coordination polymers (PCPs) by in situ radical polymerization of styrene and methylmethacrylate, followed by removal of the host PCP frameworks in aqueous tetrasodium ethylenediaminetetraacetate (Na-EDTA) solution. In this replication process, the isolated vinyl polymer particles retained the size and morphologies of the original PCP particles, although the polymer chains were not stabilized by cross-linking. This morphological retention of vinyl polymers after the isolation from the PCP matrixes was ensured by the rigidity and porosity of the polymers, which was confirmed by DSC and adsorption measurements. The unconventional assembly of polymer chains in the particles is of interest from the viewpoints of functional properties of the polymer particles

Radical Copolymerization Mediated by Unsaturated Metal Sites in Coordination Nanochannels

Radical copolymerization of methyl methacrylate (MMA) and styrene was performed in [Tb­(1,3,5-benzenetrisbenzoate)]_<i>n</i> with coordinatively unsaturated metal sites (UMS) immobilized along the one-dimensional nanochannels. A drastic increase in the proportion of MMA units in the resulting copolymers was obtained compared with that obtained from the corresponding solution polymerization systems. Simultaneous coordination of MMA to the UMS is the key to increasing the MMA proportion during the copolymerization in the nanochannels, which was demonstrated by variable temperature IR measurements and several controlled experiments

Opening of an Accessible Microporosity in an Otherwise Nonporous Metal–Organic Framework by Polymeric Guests

The development of highly porous metal–organic frameworks (MOFs) is greatly sought after, due to their wide range of applications. As an alternative to the development of new structures, we propose to obtain new stable configurations for flexible MOFs by insertion of polymeric guests. The guests prevent the otherwise spontaneous closing of the host frameworks and result in stable opened forms. Introduced at a fraction of the maximal capacity, polymer chains cause an opening of the occupied nanochannels, and because of the MOF reticular stiffness, this opening is propagated to the neighboring nanochannels that become accessible for adsorption. Composites were obtained by in situ polymerization of vinyl monomers in the nanochannels of an otherwise nonporous MOF, resulting in homogeneously loaded materials with a significant increase of porosity (<i>S</i>_BET = 920 m²/g). In addition, by limiting the accessible configurations for the framework and forbidding the formation of a reactive intermediate, the polymeric guest prevented the thermal degradation of the host MOF even at very low loading (as low as 3 wt %) and increased its stability domain by more than 200 °C

Integration of Intrinsic Proton Conduction and Guest-Accessible Nanospace into a Coordination Polymer

We report the synthesis and characterization of a coordination polymer that exhibits both intrinsic proton conductivity and gas adsorption. The coordination polymer, consisting of zinc ions, benzimidazole, and orthophosphate, exhibits a degree of flexibility in that it adopts different structures before and after dehydration. The dehydrated form shows higher intrinsic proton conductivity than the original form, reaching as high as 1.3 × 10^–3 S cm^–1 at 120 °C. We found that the rearranged conduction path and liquid-like behavior of benzimidazole molecules in the channel of the framework afforded the high proton conductivity. Of the two forms of the framework, only the dehydrated form is porous to methanol and demonstrates guest-accessible space in the structure. The proton conductivity of the dehydrated form increases by 24 times as a result of the in situ adsorption of methanol molecules, demonstrating the dual functionality of the framework. NMR studies revealed a hydrogen-bond interaction between the framework and methanol, which enables the modulation of proton conductivity within the framework

Structuralization of Ca²⁺-Based Metal–Organic Frameworks Prepared via Coordination Replication of Calcium Carbonate

The emergence of metal–organic frameworks (MOFs) as potential candidates to supplant existing adsorbent types in real-world applications has led to an explosive growth in the number of compounds available to researchers, as well as in the diversity of the metal salts and organic linkers from which they are derived. In this context, the use of carbonate-based precursors as metal sources is of interest due to their abundance in mineral deposits and their reaction chemistry with acids, resulting in just water and carbon dioxide as side products. Here, we have explored the use of calcium carbonate as a metal source and demonstrate its versatility as a precursor to several known frameworks, as well as a new flexible compound based on the 2,5-dihydroxybenzoquinone (H₂dhbq) linker, Ca­(dhbq)­(H₂O)₂. Furthermore, inspired by the ubiquity and unique structures of biomineralized forms of calcium carbonate, we also present examples of the preparation of superstructures of Ca-based MOFs via the coordination replication technique. In all, the results confirm the suitability of carbonate-based metal sources for the preparation of MOFs and further expand upon the growing scope of coordination replication as a convenient strategy for the preparation of structuralized materials

pH-Dependent Interpenetrated, Polymorphic, Cd²⁺- and BTB-based Porous Coordination Polymers with Open Metal Sites

Two polymorphic porous coordination polymers constructed from Cd²⁺ and benzene-1,3,5-tribenzoate and having interpenetrated two-dimensional (2D) (6,3) net topology and three-dimensional (3D) (10,3)-b net topology structures were synthesized. The number of single honeycomb-type layers interpenetrated dictates the dimensionality of the crystal structures of these two phases. The 2D structure has two interpenetrated layers, whereas the 3D structure has four interpenetrated layers. Interestingly, these two polymorphic forms selectively adsorb CO₂ over N₂, C₂H₄, and C₂H₆. Moreover, diffuse reflectance Fourier-transform infrared spectroscopy of CO₂ adsorbed on these two polymorphic phases indicates strong interaction between CO₂ and the open metal sites present on Cd²⁺ ions

Preparation of Porous Polysaccharides Templated by Coordination Polymer with Three-Dimensional Nanochannels

Polymerization of monosaccharide monomers usually suffers from the production of polysaccharides with ill-defined structures because of the uncontrolled random reactions among many reactive hydroxyl groups on saccharide monomers. In particular, rational synthesis of polysaccharides with porosity approximating molecular dimensions is still in its infancy, despite their usefulness as drug carriers. Here, we disclose an efficient synthetic methodology for the preparation of polysaccharides with controllable mesoporosity in the structure, utilizing [Cu₃(benzene-1,3,5-tricarboxylate)]_<i>n</i> (HKUST-1; <b>1</b>) as templates. Cationic ring-opening polymerization of 1,6-anhydro glucose was performed in nanochannels of <b>1</b>, followed by removal of the host frameworks, giving polysaccharide particles as replicas of the original molds. Nitrogen adsorption measurement revealed that the obtained polysaccharide particles contained high mesoporosity in the structure, which could be controlled systematically depending on the polymerization conditions. Because of the large specific surface area, tunable porosity and particle size, we could also demonstrate the capabilities of our polysaccharides for loading and releasing of a drug molecule and protein

Absorption of CO₂ and CS₂ into the Hofmann-Type Porous Coordination Polymer: Electrostatic versus Dispersion Interactions

Absorption of CO₂ and CS₂ molecules into the Hofmann-type three-dimensional porous coordination polymer (PCP) {Fe­(Pz)­[Pt­(CN)₄]}_<i>n</i> (Pz = pyrazine) was theoretically explored with the ONIOM­(MP2.5 or SCS-MP2:DFT) method, where the M06-2X functional was employed in the DFT calculations. The binding energies of CS₂ and CO₂ were evaluated to be −17.3 and −5.2 kcal mol^–1, respectively, at the ONIOM­(MP2.5:M06-2X) level and −16.9 and −4.4 kcal mol^–1 at the ONIOM­(SCS-MP2:M06-2X) level. It is concluded that CS₂ is strongly absorbed in this PCP but CO₂ is only weakly absorbed. The absorption positions of these two molecules are completely different: CO₂ is located between two Pt atoms, whereas one S atom of CS₂ is located between two Pz ligands and the other S atom is between two Pt atoms. The optimized position of CS₂ agrees with the experimentally reported X-ray structure. To elucidate the reasons for these differences, we performed an energy decomposition analysis and found that (i) both the large binding energy and the absorption position of CS₂ arise from a large dispersion interaction between CS₂ and the PCP, (ii) the absorption position of CO₂ is mainly determined by the electrostatic interaction between CO₂ and the Pt moiety, and (iii) the small binding energy of CO₂ comes from the weak dispersion interaction between CO₂ and the PCP. Important molecular properties relating to the dispersion and electrostatic interactions, which are useful for understanding and predicting gas absorption into PCPs, are discussed in detail