Coordination Covalent Frameworks: A New Route for Synthesis and Expansion of Functional Porous Materials

The synthetic approaches for fine-tuning the structural properties of coordination polymers or metal organic frameworks have exponentially grown during the past decade. This is due to the control over the properties of the resulting structures such as stability, pore size, pore chemistry and surface area for myriad possible applications. Herein, we present a new class of porous materials called Coordination Covalent Frameworks (CCFs) that were designed and effectively synthesized using a two-step reticular chemistry approach. During the first step, trigonal prismatic molecular building block was isolated using 4-aminobenazoic acid and Cr (III) salt, subsequently in the second step the polymerization of the isolated molecular building blocks (MBBs) takes place by the formation of strong covalent bonds where small organic molecules can connect the MBBs forming extended porous CCF materials. All the isolated CCFs were found to be permanently porous while the discrete MBB were nonporous. This approach would inevitably open a feasible path for the applications of reticular chemistry and the synthesis of novel porous materials with various topologies under ambient conditions using simple organic molecules and versatile MBBs with different functionalities that would not be possible using the traditional one-step approach

Coordination Covalent Frameworks: A New Route for Synthesis and Expansion of Functional Porous Materials

The synthetic approaches for fine-tuning the structural properties of coordination polymers or metal organic frameworks have exponentially grown during the past decade. This is due to the control over the properties of the resulting structures such as stability, pore size, pore chemistry and surface area for myriad possible applications. Herein, we present a new class of porous materials called Coordination Covalent Frameworks (CCFs) that were designed and effectively synthesized using a two-step reticular chemistry approach. During the first step, trigonal prismatic molecular building block was isolated using 4-aminobenazoic acid and Cr (III) salt, subsequently in the second step the polymerization of the isolated molecular building blocks (MBBs) takes place by the formation of strong covalent bonds where small organic molecules can connect the MBBs forming extended porous CCF materials. All the isolated CCFs were found to be permanently porous while the discrete MBB were nonporous. This approach would inevitably open a feasible path for the applications of reticular chemistry and the synthesis of novel porous materials with various topologies under ambient conditions using simple organic molecules and versatile MBBs with different functionalities that would not be possible using the traditional one-step approach

Crystal Engineering of a 4,6‑c fsc Platform That Can Serve as a Carbon Dioxide Single-Molecule Trap

We report herein a crystal engineering strategy that affords a new and versatile metal–organic material (MOM) platform that is tunable in terms of both pore size and functionality. This platform is comprised of two long-known molecular building blocks (MBBs) that alternate to form a cationic square grid lattice. The MBBs, [Cu­(AN)₄]²⁺ (AN = aromatic nitrogen donor), and [Cu₂(CO₂R)₄] square paddlewheel moieties are connected by five different fs, <b>L1</b>–<b>L5</b>, that contain both AN and carboxylate moieties. The resulting square grid nets formed from alternating [Cu­(AN)₄]²⁺ and [Cu₂(CO₂R)₄] moieties are pillared at the axial sites of the [Cu­(AN)₄]²⁺ MBBs with dianionic pillars to form neutral 3D 4,6-connected <b>fsc</b> (<b><u>f</u></b>our, <b><u>s</u></b>ix type <b><u>c</u></b>) nets. Pore size control in this family of <b>fsc</b> nets was exerted by varying the length of the ligand, whereas pore chemistry was defined by the presence of unsaturated metal centers (UMCs) and either inorganic or organic pillars. 1,5-Naphthalenedisulfonate (NDS) anions pillar in an angular fashion to afford <b>fsc-1-NDS</b>, <b>fsc-2-NDS</b>, <b>fsc-3-NDS</b>, <b>fsc-4-NDS</b>, and <b>fsc-5-NDS</b> from <b>L1-L5</b>, respectively. Experimental CO₂ sorption studies revealed higher isosteric heat of adsorption (<i>Q</i>_st) for the smallest pore size material (<b>fsc-1-NDS</b>). Computational studies revealed that there is higher CO₂ occupancy about the UMCs in <b>fsc-1-NDS</b> compared to other extended variants that were synthesized with NDS. SiF₆^2– (SIFSIX) anions in <b>fsc-2-SIFSIX</b> form linear pillars that result in eclipsed [Cu₂(CO₂R)₄] moieties at a distance of just 5.86 Å. The space between the [Cu₂(CO₂R)₄] moieties affords a strong CO₂ binding site that can be regarded as being an example of a single-molecule trap; this finding has been supported by modeling studies. Gas sorption studies on this new family of <b>fsc</b> nets reveal stronger affinity toward CO₂ for <b>fsc-2-SIFSIX</b> vs <b>fsc</b>-2<b>-NDS</b> along with higher <i>Q</i>_st and CO₂/N₂ selectivity. The <b>fsc</b> platform reported herein offers a plethora of possible porous structures that are amenable to tuning of both pore size and pore chemistry

Investigating CO2 sorption in SIFSIX-3-M (M = Fe, Co, Ni, Cu, Zn) through computational studies

A combined Monte Carlo (MC) simulation and periodic density functional theory (DFT) study of CO2 sorption was performed in SIFSIX-3-M (M = Fe, Co, Ni, Cu, Zn), a family of hybrid ultramicroporous materials (HUMs) that consists of M2+ ions coordinated to pyrazine ligands and are pillared with SiF6 2 (\SIFSIX") anions. Grand canonical Monte Carlo (GCMC) simulations of CO2 sorption in all ve SIFSIX-3-M variants produced isotherms that are in good agreement with the corresponding experimental measurements. The theoretical isosteric heat of adsorption (Qst) for CO2 as obtained through canonical Monte Carlo (CMC) simulations are also in close agreement with the experimental values. Consistent with experiment, the simulations generated the following trend in the CO2 Qst: SIFSIX-3-Cu > SIFSIX-3-Ni > SIFSIX-3-Co > SIFSIX-Zn > SIFSIX-3-Fe. The magnitudes of the theoretical Qst and relative trend were further supported by periodic DFT calculations of the adsorption energy for CO2 within the respective HUMs. We attribute the observed Qst trend in SIFSIX-3-M to their di erences in pore size and lattice parameters. Speci cally, the sorption energetics decrease with increasing pore size and a/b lattice constant. Simulations of CO2 sorption in SIFSIX-3-Cu resulted in di erent pro les for the radial distribution function (g(r)) and dipole distribution than within the other analogues due to the smaller pore size and much shorter a/b unit cell lengths of the crystal structure; this is a direct consequence of the Jahn{Teller e ect. Although these HUMs are isostructural, notable di erences in the classical energy contributions for CO2 sorption were observed from the GCMC simulations. Overall, this study demonstrates that the CO2 Qst in SIFSIX-3-M can be controlled by the choice of the saturated metal, with values ranging from 42 to 54 kJ mol-1

Putting the Squeeze on CH₄ and CO₂ through Control over Interpenetration in Diamondoid Nets

We report the synthesis, structure, and sorption properties of a family of eight diamondoid (<b>dia</b>) metal–organic materials (MOMs) that are sustained by Co­(II) or Zn­(II) cations linked by one of three rigid ligands: 4-(2-(4-pyridyl)­ethenyl)­benzoate (<b>1</b>), 4-(pyridin-4-yl)­benzoate (<b>2</b>), and 4-(pyridin-4-yl)­acrylate (<b>3</b>). Pore size control in this family of <b>dia</b> nets was exerted by two approaches: changing the length of the linker ligand from <b>1</b> to <b>3</b>, and using solvent as a template to control the level of interpenetration in nets based upon <b>1</b> and <b>3</b>. The resulting MOMs, dia-8i-<b>1</b>, dia-5i-<b>3</b>, dia-7i-<b>1</b>-Zn, dia-7i-<b>1</b>-Co, dia-4i-<b>3</b>-a, dia-4i-<b>3</b>-b, dia-4i-<b>2</b>, and dia-4i-<b>1</b>, exhibit 1D channels with pore limiting diameters (PLDs) of 1.64, 2.90, 5.06, 5.28, 8.57, 8.83, 11.86, and 18.25 Å, respectively. We selected <b>dia</b> nets for this study for the following reasons: their 1D channels facilitate study of the impact of pore size on gas sorption parameters in situations where pore chemistry is similar (pyridyl benzoate-type linkers) or identical (in the case of polymorphs), and their saturated metal centers eliminate open metal sites from dominating sorbent–solvate interactions and possibly masking the effect of pore size. Our data reveal that smaller pore sizes offer stronger interactions, as determined by the isosteric heat of adsorption (<i>Q</i>_st) and the steepness of the adsorption isotherm in the low-pressure region. The porous MOM with the smallest PLD suitable for physisorption, dia-7i-<b>1</b>-Co, was thereby found to exhibit the highest <i>Q</i>_st values for CO₂ and CH₄. Indeed, dia-7i-<b>1</b>-Co exhibits a <i>Q</i>_st for CH₄ of 26.7 kJ/mol, which was validated through grand canonical Monte Carlo simulation studies of CH₄ adsorption. This <i>Q</i>_st value is considerably higher than those found in covalent organic frameworks and other MOMs with unsaturated metal centers. These results therefore further validate the critical role that PLD plays in gas adsorption by porous MOMs

Highly Selective CO₂ Uptake in Uninodal 6‑Connected “mmo” Nets Based upon MO₄^2– (M = Cr, Mo) Pillars

A novel 4⁸.6⁷ topology metal–organic material (MOM) platform of formula [M­(bpe)₂(M′O₄)] (M = Co or Ni; bpe = 1,2-bis­(4-pyridyl)­ethene; M′ = Mo or Cr) has been synthesized and evaluated in the context of gas sorption. These MOMs have been assigned RCSR code <b>mmo</b> and are uninodal 6-connected nets. [Ni­(bpe)₂(MoO₄)], MOOFOUR-1-Ni, and its chromate analogue, CROFOUR-1-Ni, exhibit high CO₂ affinity and selectivity, especially at low loading. This behavior can be attributed to exceptionally high isosteric heats of adsorption (<i>Q</i>_st) of CO₂ in MOOFOUR-1-Ni and CROFOUR-1-Ni of ∼56 and ∼50 kJ/mol, respectively, at zero loading. These results were validated by molecular simulations which indicate that the electrostatics of these inorganic anions affords attractions toward CO₂ that are comparable to those of unsaturated metal centers

Highly Selective CO₂ Uptake in Uninodal 6‑Connected “mmo” Nets Based upon MO₄^2– (M = Cr, Mo) Pillars

A novel 4⁸.6⁷ topology metal–organic material (MOM) platform of formula [M­(bpe)₂(M′O₄)] (M = Co or Ni; bpe = 1,2-bis­(4-pyridyl)­ethene; M′ = Mo or Cr) has been synthesized and evaluated in the context of gas sorption. These MOMs have been assigned RCSR code <b>mmo</b> and are uninodal 6-connected nets. [Ni­(bpe)₂(MoO₄)], MOOFOUR-1-Ni, and its chromate analogue, CROFOUR-1-Ni, exhibit high CO₂ affinity and selectivity, especially at low loading. This behavior can be attributed to exceptionally high isosteric heats of adsorption (<i>Q</i>_st) of CO₂ in MOOFOUR-1-Ni and CROFOUR-1-Ni of ∼56 and ∼50 kJ/mol, respectively, at zero loading. These results were validated by molecular simulations which indicate that the electrostatics of these inorganic anions affords attractions toward CO₂ that are comparable to those of unsaturated metal centers

Highly Selective CO₂ Uptake in Uninodal 6‑Connected “mmo” Nets Based upon MO₄^2– (M = Cr, Mo) Pillars

A novel 4⁸.6⁷ topology metal–organic material (MOM) platform of formula [M­(bpe)₂(M′O₄)] (M = Co or Ni; bpe = 1,2-bis­(4-pyridyl)­ethene; M′ = Mo or Cr) has been synthesized and evaluated in the context of gas sorption. These MOMs have been assigned RCSR code <b>mmo</b> and are uninodal 6-connected nets. [Ni­(bpe)₂(MoO₄)], MOOFOUR-1-Ni, and its chromate analogue, CROFOUR-1-Ni, exhibit high CO₂ affinity and selectivity, especially at low loading. This behavior can be attributed to exceptionally high isosteric heats of adsorption (<i>Q</i>_st) of CO₂ in MOOFOUR-1-Ni and CROFOUR-1-Ni of ∼56 and ∼50 kJ/mol, respectively, at zero loading. These results were validated by molecular simulations which indicate that the electrostatics of these inorganic anions affords attractions toward CO₂ that are comparable to those of unsaturated metal centers

Highly Selective CO₂ Uptake in Uninodal 6‑Connected “mmo” Nets Based upon MO₄^2– (M = Cr, Mo) Pillars

A novel 4⁸.6⁷ topology metal–organic material (MOM) platform of formula [M­(bpe)₂(M′O₄)] (M = Co or Ni; bpe = 1,2-bis­(4-pyridyl)­ethene; M′ = Mo or Cr) has been synthesized and evaluated in the context of gas sorption. These MOMs have been assigned RCSR code <b>mmo</b> and are uninodal 6-connected nets. [Ni­(bpe)₂(MoO₄)], MOOFOUR-1-Ni, and its chromate analogue, CROFOUR-1-Ni, exhibit high CO₂ affinity and selectivity, especially at low loading. This behavior can be attributed to exceptionally high isosteric heats of adsorption (<i>Q</i>_st) of CO₂ in MOOFOUR-1-Ni and CROFOUR-1-Ni of ∼56 and ∼50 kJ/mol, respectively, at zero loading. These results were validated by molecular simulations which indicate that the electrostatics of these inorganic anions affords attractions toward CO₂ that are comparable to those of unsaturated metal centers

Theoretical Investigations of CO₂ and CH₄ Sorption in an Interpenetrated Diamondoid Metal–Organic Material

Grand canonical Monte Carlo (GCMC) simulations of CO₂ and CH₄ sorption and separation were performed in dia-7i-1-Co, a metal–organic material (MOM) consisting of a 7-fold interpenetrated net of Co²⁺ ions coordinated to 4-(2-(4-pyridyl)­ethenyl)­benzoate linkers. This MOM shows high affinity toward CH₄ at low loading due to the presence of narrow, close fitting, one-dimensional hydrophobic channelsthis makes the MOM relevant for applications in low-pressure methane storage. The calculated CO₂ and CH₄ sorption isotherms and isosteric heat of adsorption, <i>Q</i>_st, values in dia-7i-1-Co are in good agreement with the corresponding experimental results for all state points considered. The experimental initial <i>Q</i>_st value for CH₄ in dia-7i-1-Co is currently the highest of reported MOM materials, and this was further validated by the simulations performed herein. The simulations predict relatively constant <i>Q</i>_st values for CO₂ and CH₄ sorption across all loadings in dia-7i-1-Co, consistent with the one type of binding site identified for the respective sorbate molecules in this MOM. Examination of the three-dimensional histogram showing the sites of CO₂ and CH₄ sorption in dia-7i-1-Co confirmed this finding. Inspection of the modeled structure revealed that the sorbate molecules form a strong interaction with the organic linkers within the constricted hydrophobic channels. Ideal adsorbed solution theory (IAST) calculations and GCMC binary mixture simulations predict that the selectivity of CO₂ over CH₄ in dia-7i-1-Co is quite low, which is a direct consequence of the MOM’s high affinity toward both CO₂ and CH₄ as well as the nonspecific mechanism shown here. This study provides theoretical insights into the effects of pore size on CO₂ and CH₄ sorption in porous MOMs and its effect upon selectivity, including postulating design strategies to distinguish between sorbates of similar size and hydrophobicity