Crystal Engineering of a 4,6-c fsc Platform That Can Serve as a

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Crystal Engineering of a 4,6-c fsc Platform That Can Serve as a Carbon Dioxide Single-Molecule Trap Sameh K Elsaidi, Mona H Mohamed, Tony Pham, Lukasz Wojtas, Michael J. Zaworotko, and Brian Space Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01632 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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Crystal Growth & Design

Crystal Engineering of a 4,6-c fsc Platform That Can Serve as a Carbon Dioxide Single-Molecule Trap Sameh K. Elsaidi,†,‡,⊥ Mona H. Mohamed,†,‡,⊥ Tony Pham,† Lukasz Wojtas,† Michael J. Zaworotko§,* and Brian Space,†,* †

Department of Chemistry, CHE205, University of South Florida, 4202 E. Fowler Avenue, Tampa, FL, 33620, USA Chemistry Department, Faculty Of Science, Alexandria University, P.O.Box 426 Ibrahimia, Alexandria 21321, Egypt §Department of Chemical & Environmental Sciences, University of Limerick, Limerick, Republic of Ireland KEYWORDS: Crystal Engineering ·Metal-Organic Frameworks· Pore Size Control ·Design · Carbon Dioxide Trap ‡

ABSTRACT: 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, L1-L5, 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 fsc (four, six type c) nets. Pore size control in this family of fsc nets was exerted by varying the length of the ligand, whereas pore chemistry was defined via 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 fsc-1-NDS, fsc-2-NDS, fsc-3-NDS, fsc-4-NDS and fsc-5-NDS from L1-L5, respectively. Experimental CO2 sorption studies revealed higher isosteric heat of adsorption (Qst) for the smallest pore size material (fsc-1-NDS). Computational studies revealed that there is higher CO2 occupancy about the UMCs in fsc-1-NDS compared to other extended variants that were synthesized with NDS. SiF62- (SIFSIX) anions in fsc-2-SIFSIX 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 fsc nets reveal stronger affinity towards CO2 for fsc-2-SIFSIX vs. fsc-2-NDS along with higher Qst and CO2/N2 selectivity. The fsc platform reported herein offers a plethora of possible porous structures that are amenable to tuning of both pore size and pore chemistry.

INTRODUCTION Crystal engineering offers a bottom-up approach to materials design that focuses upon selecting molecules or ions in order to achieve a targeted outcome in terms of structure and properties.1-11 The success of such an approach is exemplified by two classes of multi-component materials: metal–organic materials (MOMs),6,10,12-15 and pharmaceutical co-crystals.16-18 In both instances, the inherent modularity of these multi-component materials creates a diversity of composition and a range of physicochemical properties that are not so readily achieved in existing classes of materials. Further, crystal engineering offers a degree of control over structure and facilitates systematic study of structure and function. With respect to MOMs, which are exemplified by metal–organic frameworks (MOFs),5 porous coordination polymers (PCPs)8 and porous coordination networks (PCNs),19 their ability to exhibit extra-large surface area has made them an attractive class of porous materials for gas purification and storage, catalysis, small molecule separations, and chemical sensing. In comparison to their purely inorganic analogues (e.g., zeolites), their amenability to fine-tuning of composition can result in an exceptional level of control over network topology, pore size, pore chemistry, and the resulting physicochemical properties. In such a context, the topology of a prototypal MOM network can serve as a blue-

print for families of networks or platforms based upon the same topology. In addition, the judicious selection of molecular building blocks (MBBs) allows for fine-tuning of both pore size and pore chemistry.20 Notable examples of MOM platforms include those based upon pcu,13 dia,21 tbo,22 mtn23 and rht10,24,25 topology. However, despite there being more than 20,000 MOMs already reported26 and a much greater range of permutations of MBBs yet to be investigated, most MOMs would not be considered to be members of a platform. Note, MBBs based on carboxylate moieties have tended to dominate the field to date. In this contribution, we introduce a versatile platform with 4,6-connected fsc (four, six type c) topology that is formed from linking two of the longest known and most widely studied MBBs: [M(pyridine)4L2] complexes, which can form square grid coordination polymers (CPs),27-29 and their pillared variants;29-34 and [Cu2(CO2R)4] square paddlewheel moieties, which were exploited in the prototypal MOFs, MOF-2,35 and HKUST-1, a MOF with tbo topology.22 These materials, which were first reported in the 1990s, remain attractive in terms of both design and properties. In particular, the simplicity of these prototypal MBBs and the availability of a large family of suitable ligands and pillars enable systematic control over both pore size and functionality in a manner that is not

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usually possible in existing classes of porous materials and is not easy to achieve in MOMs. We generate the fsc nets reported herein through the coordination of ligands L1-L5 (Figure 1) to Cu2+ cations in a 1-step process that facilitates the formation of [Cu(AN)4]2+ (AN = aromatic nitrogen donor) and [Cu2(CO2R)4] square paddlewheel moieties. The resulting square grid (sql) nets are comprised of alternating [Cu(AN)4]2+ and [Cu2(CO2R)4] moieties. Because the [Cu(AN)4]2+ MBBs are amenable to pillaring at their axial sites with dianionic pillars,29-34 this enables the formation of neutral 3D fsc nets in which pore size is controlled by the length of L1-L5. Further, the relative position of paddlewheel moieties is controlled by the nature of the anionic pillars. Six examples were prepared from HL1-HL5 and either 1,5-naphthalenedisulfonate (NDS) or SiF62- (SIFSIX) pillars. The resulting compounds, fsc-1NDS, fsc-2-NDS, fsc-3-NDS, fsc-4-NDS, fsc-5-NDS and fsc2-SIFSIX were characterized with respect to their single crystal X-ray structures and gas sorption performance. Notably, fsc-2-SIFSIX, an organic-inorganic hybrid material, was found to contain a binding site that represents a rare example of a single-molecule trap for CO2.36

Figure 1. The ligands (HL1-HL5) used herein for the isoreticular synthesis of fsc nets.

EXPERIMENTAL SECTION All reagents were used as purchased. Solvents were purified according to standard methods and stored in the presence of molecular sieves. Powder X-ray diffraction (PXRD) data were recorded at 298 K on a Bruker D8 Advance X-ray diffractometer at 20 kV, 5 mA for CuKα (λ = 1.5418 Å), with a scan speed of 0.5 s/step(6°/min) and a step size of 0.05° in 2θ. Calculated PXRD patterns were produced using Powder Cell for Windows Version 2.4 (programmed by W. Kraus and G. Nolze, BAM Berlin, 2000). Gas adsorption isotherms were collected on a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. A. Synthesis of Compounds. [Cu3(L1)4Cu(NDS)], fsc-1-NDS: Solvothermal reaction of 0.30 mmol (0.067 g) of Cu(NO3)2·2H2O, 0.40 mmol (0.049 g) of isonicotinic acid (HL1) and 0.10 mmol (0.036 g) of 1,5naphthalenedisulfonic acid (H2NDS) in 15 mL DMA/EtOH (2:1 ratio) at 85 °C for 3 days afforded blue cube-like crystals of fsc-1-NDS (53% yield based on the Cu(NO3)2·2H2O). [Cu3(L2)4(NDS)], fsc-2-NDS: Solvothermal reaction of 0.30 mmol (0.067 g) of Cu(NO3)2·2H2O, 0.40 mmol, (0.060 g) of 4-(pyridin-4-yl) acrylic acid (HL2) and 0.10 mmol (0.036 g) of 1,5-naphthalenedisulfonic acid (H2NDS) in 15 mL

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DMA/EtOH (2:1 ratio) at 85 °C for 3 days afforded blue cubelike crystals of fsc-2-NDS (60% yield based on Cu(NO3)2·2H2O). [Cu3(L3)4(NDS)], fsc-3-NDS: Solvothermal reaction of 0.30 mmol (0.067 g) of Cu(NO3)2·2H2O, 0.40 mmol, (0.075 g) of 4-(1H-imidazol-1-yl)benzoic acid (HL3) and 0.10 mmol (0.036 g) of 1,5-naphthalenedisulfonic acid (H2NDS) in 15 mL DMA/EtOH (2:1 ratio) at 85 °C for 3 days afforded blue cube-like crystals of fsc-3-NDS (67% yield based on Cu(NO3)2·2H2O). [Cu3(L4)4(NDS)], fsc-4-NDS: Solvothermal reaction of 0.30 mmol (0.067 g) of Cu(NO3)2·2H2O, 0.40 mmol, (0.080 g) of 4-(4-pyridinyl)benzoic acid (HL4)and 0.10 mmol (0.036 g) in 15 mL of 1,5-naphthalenedisulfonic acid (H2NDS) DMA/EtOH (2:1 ratio) at 85 °C for 3 days afforded blue cubelike crystals of fsc-4-NDS (62% yield based on Cu(NO3)2·2H2O). [Cu3(L5)4(NDS)], fsc-5-NDS: Solvothermal reaction of 0.30 mmol (0.067 g) of Cu(NO3)2·2H2O, 0.40 mmol (0.090 g)of 4-(2-(4-pyridyl)ethenyl) benzoic acid (HL5) and 0.10 mmol (0.036 g) of 1,5-naphthalenedisulfonic acid (H2NDS) in 15 mL DMA/EtOH (2:1 ratio) at 85 °C for 3 days afforded blue cube-like crystals of fsc-5-NDS (53% yield based on the Cu(NO3)2·2H2O). [Cu3(L2)4(SiF6)],fsc-2-SIFSIX: In a long thin test tube, copper hexafluorosilicate hexahydrate (CuSiF6·6H2O) (0.30 mmol, 0.094 g) in 3 mL of EtOH/DMA (2:1) was carefully layered over 4-(pyridin-4-yl) acrylic acid (HL2) (0.40 mmol, 0.060 g) in 3mL DMA. The tube was sealed and left undisturbed at room temperature. After two weeks blue cube-like crystals of fsc-2-SIFSIX were isolated (57% yield based on CuSiF6·6H2O). B. Structural Descriptions. TOPOS37 revealed that fsc-1-NDS, fsc-2-NDS, fsc-3-NDS, fsc-4-NDS, fsc-5-NDS, and fsc-2-SIFSIX exhibit 4,6connected fsc topology, a rare topology predicted by O’Keeffe and first reported in 2007.38 All six structures are comprised of two distinct MBBs that serve as alternating nodes in the bimodal fsc net. The copper paddlewheel moieties serve as 4connected square planar nodes that are arranged with alternating 6-connected [Cu(AN)4(X)] octahedral nodes (X is the bifunctional pillar) as presented in schemes 1 and 2. [Cu3(L1)4(NDS)], fsc-1-NDS, is sustained by isonicotinate linkers and NDS pillars and crystallizes in monoclinic space group C2/m (a = 17.6979(8) Å, b = 17.6261(8) Å, c = 18.8786(9) Å, β = 90.003(2), V = 5889.1(5) Å3) according to single crystal X-ray crystallography. The 6-connected Cu nodes exhibit distorted octahedral geometry (Cu-O = 2.340(5)–2.346(5) Å, Cu-N = 2.028(5)–2.038(6) Å with bond angles N-Cu-N = 89.7(2)–90.3(2)°, N-Cu-O =89.7(2)–90.1(2)° and O-Cu-O = 179.7(2)°). The Cu paddlewheel moieties serve as 4-connected nodes with Cu-O = 1.944(5)–1.980(6) Å and O-Cu-O = 89.0(2)–89.7(2)°. The distance apart and alignment of neighboring Cu centers of the square grid sheets are dependent on the nature of the pillar and the length of the linker. The NDS moieties in fsc-1-NDS link the Cu centers in an angular manner, resulting in the Cu centers being staggered with respect to each other. The shortest distance between the 6connected Cu centers is 11.106(3) Å while the Cu-Cu distance between adjacent paddlewheel moieties is 8.921(3) Å. The distance between the free oxygen atom of the NDS pillars and


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the Cu2+ ion of the paddlewheels is only 4.213(2) Å in fsc-1NDS and this allows for hydrogen bonding between NDS and aqua ligands with an O···O distance of 2.746(2) Å. There are no such interactions in the other variants, presumably because

of the longer lengths of L2-L5 (Figure 2). fsc-1-NDS exhibits square channels that are parallel to [0,0,1] with a pore limiting diameter (PLD) of 8.98 Å as defined by the longest diagonal of the channel after subtracting 3.5 Å for van der Waals radii.

Scheme 1. fsc nets are comprised from alternating [Cu(AN)4]2+ and [Cu2(CO2R)4] MBBs, the former of which can be pillared by linear SiF62- (SIFSIX) or angular 1,5-naphthalenedisulfonate (NDS) pillars.

Scheme 2. Self-assembled fsc net are pillared in linear and angular fashion pillaring using SIFSIX and NDS, respectively.

[Cu3(L2)4(NDS)], fsc-2-NDS, crystallizes in monoclinic space group C2/c (a = 22.1842(12) Å, b = 22.2031(11) Å, c= 20.5409(11) Å, β = 122.1590(15)°, V = 8565.3(8) Å3) and the angular linking mode of the NDS pillar means that the paddlewheel moieties are staggered in a manner similar to that of fsc-1-NDS. The Cu-Cu distance between adjacent paddlewheel moieties is 8.808(2) Å, whereas the distance between the 6-connected Cu centers is 10.960(2) Å. The 6-connected Cu nodes exhibit distorted octahedral geometry with bond distances of 2.160(7) Å and 1.930(9)–2.085(8)Å for Cu-O and Cu-N, respectively, while the N-Cu-N, N-Cu-O and O-Cu-O angles are 87.4(5)–92.7(4)°, 91.1(4)–96.4(3)° and 174.30(2)°,

respectively. The bond distances and angles of the 4connected Cu paddlewheel moieties are: Cu-O = 1.888(7)– 2.167(2) Å and O-Cu-O = 86.9(6)–90.4(6)°. The shortest distance between the free oxygen atom of NDS pillars and Cu2+ ion of the paddlewheels is 6.009 (2) Å. The square channels are parallel to [0,0,1] in fsc-2-NDS and they exhibit a PLD of 12.19 Å. [Cu3(L3)4(NDS)], fsc-3-NDS, crystallizes in orthorhombic space group Imma (a = 31.7864(12) Å, b = 33.3395(12) Å, c = 11.6773(4) Å, V = 12374.9(8) Å3). The distance between the 6-connected Cu centers is 11.677(25) Å whereas the Cu-Cu distance between paddlewheel moieties is 9.619(15) Å. The 6-



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connected Cu nodes display distorted octahedral geometry with bond distances of 2.408(2) Å and 1.998(2) Å for Cu-O and Cu-N, respectively. N-Cu-N, N-Cu-O and O-Cu-O angles are 89.1(2)–90.9(2)°, 89.11(7)–90.89(7)° and 180.0°, respectively. The bond distances within the 4-connected Cu paddlewheel are 1.947(2)–1.954(2) Å, whereas O-Cu-O angles range from 87.73(2)–90.4(2)°. The shortest distance between the free oxygen atom of the NDS pillars and the Cu2+ ion of the paddlewheels in fsc-3-NDS is 8.159(2) Å. The lower symmetry of L3 compared to the other ligands used herein results in two types of channels: one along [0,0,1] with a PLD 13.43Å and the other parallel to [1,0,0] with a PLD of 17.41Å. [Cu3(L4)4(NDS)], fsc-4-NDS, crystallizes in monoclinic space group C2/c (a= 26.189(3) Å, b= 26.223(3) Å, c = 21.117(3) Å, β = 127.232(5)°, V = 11547(3) Å3). The single crystal structure reveals that the paddlewheel moieties are once again in a staggered conformation and the Cu-Cu distance between adjacent paddlewheels is 9.055(2) Å. The distance between 6connected Cu centers is 10.885(2) Å. The 6-connected Cu nodes display octahedral geometry with bond distances of 2.357(5) and1.944(9)- 2.007(6) Å for Cu-O and Cu-N, respectively, and bond angles of 88.6(2)–91.4(2)°,89.3(2)–90.7(2)° and 178.8(2)° for N-Cu-N, N-Cu-O and O-Cu-O, respectively. Bond distances and angles within the 4-connected Cu paddlewheel moieties are 1.893(6)–1.979(4) Å and angles range from 88.9(2)–95.4(3)°. The NDS pillars and the Cu2+ ion of the paddlewheels are separated by a distance of 6.098(2) Å. fsc-4NDS exhibits square channels that are parallel to [0,0,1] with a PLD of 15.04 Å. [Cu3(L5)4(NDS)],fsc-5-NDS, crystallizes in monoclinic space group P2/c (a = 19.5016(16) Å, b = 21.8113(19) Å, c =

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19.7468(17) Å, β = 112.517(4,V = 7759.08 Å3). The paddlewheel moieties are staggered and the Cu-Cu distance between adjacent paddlewheels is 9.063(2) Å. The 6-connected Cu centers are separated by a distance of 10.912(2) Å and exhibit octahedral geometry with bond distances of 2.410(4) Å and 2.008(4)–2.012(3) Å for Cu-O and Cu-N, respectively. N-CuN, N-Cu-O and O-Cu-O bond angles are 89.8(2)°, 87.0(2)– 92.8(2)° and 175.45(2)°, respectively. The 4-connected Cu paddlewheel moieties display bond distances of 1.940(3)– 1.997(3) Å and O-Cu-O bond angles of 88.4(2)–90.0(2) °. The distance between the free oxygen atom of the NDS pillars and the Cu2+ ion of the paddlewheels is 8.083(2) Å. The square channels are parallel to [0,0,1] in fsc-5-NDS and they exhibit PLD of 16.26 Å. [Cu3(L2)4(SiF6)], fsc-2-SIFSIX, crystallizes in tetragonal space group P4/mmm (a = b = 15.650(9) Å, c = 8.348(7) Å, V = 2045(3) Å3). The anionic SIFSIX moieties pillar the cationic [Cu(AN)4]2+ centers in linear fashion, which has been observed in other SIFSIX pillared nets29 and necessitate eclipsed arrangement of the copper paddlewheel moieties with a Cu-Cu distance of only 5.86 Å (see Schemes 1 and 2). The distance between the 6-connected Cu centers is 8.348(4) Å. The 6connected Cu nodes exhibit octahedral geometry (Cu-F = 2.496(2) Å and Cu-N = 2.013(2)Å, ideal bond angles). The bond distances and angles of the 4-connected Cu paddlewheel moieties are Cu-O = 1.960(9)–1.977(9) Å and O-Cu-O = 86.9(2)–92.5(2)°, respectively. fsc-2-SIFSIX exhibits square channels parallel to [0,0,1] with a PLD of 12.15 Å.

Figure 2. Molecular views of the crystal structures of fsc-1-NDS through fsc-5-NDS showing the fine-tuning of the pore size of the fsc platform through the isoreticular synthesis approach.


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Figure 3. Illustration of the effect of the linker length on proximity of free oxygen atoms of the NDS pillar and the unsaturated Cu centers of the paddlewheel moieties.

RESULTS AND DISCUSSION The crystal engineering strategy detailed herein has taken advantage of two well-studied MBBs in order to combine them to develop a new porous MOM platform that is robust in terms of structure and is amenable to fine-tuning of pore size and pore chemistry. The MOM platform approach has proven to be particularly effective at addressing structure/function relationship in porous MOMs in order to optimize their performance. For example, pcu, dia and rht nets are MOM platforms that exhibit benchmark performance with respect to CO2 selectivity,10,24,25,30,31,33,39-41 methane binding energy40 and surface area,10 respectively. To put this in perspective, whereas there are now as many as 20,000 MOMs and counting,26 there are even now only a few readily accessible and fine-tunable platforms. Such platforms can facilitate exquisite control over structure and thereby enable systematic study of structure/function relationships in a manner that represents a paradigm shift from the screening approaches traditionally used in materials science. Unlike most common MOM platforms such as dia21 and pcu,13 which are mainly formed from a single node (metal ion/metal cluster) and one linker, the fsc nets described herein are comprised of two MBBs and two linkers and both can have an impact on pore size and/or pore chemistry.

Further, the level of diversity is particularly high because the MBBs and the linkers both belong to different categories: [Cu(AN)4X]2+ is based upon a saturated metal center (SMC), whereas the [Cu2(CO2R)4] paddlewheel is an unsaturated metal center (UMC); SIFSIX is an inorganic pillar, whereas NDS and L1-L5 are all organic linkers. We herein address the structural outcomes of linker substitution on pore size and of pillar substitution on pore geometry. The effect of pore geometry, size, and chemistry on gas adsorption performance is also detailed. A. Effect of Linker Substitution. The pore size control through varying the ligand length and its impact on the gas sorption properties are studied herein. Similar to other reported isoreticular porous MOFs,42-44 systematic control over the pore size has been performed by changing the linker length from L1 to L5. The exploitation of NDS for angular pillaring of [Cu(AN)4]2+ moieties connected by the organic ligands, L1-L5, has afforded five isostructural MOMs: fsc-1-NDS, fsc-2-NDS, fsc-3-NDS, fsc-4-NDS,and fsc-5-NDS with PLDs of 8.98, 12.19, 13.43, 15.04, and 16.26 Å, respectively (Figure 2). Permanent porosity for fsc-1-NDS, fsc-2NDS, fsc-4-NDS and fsc-5-NDS was confirmed via CO2 adsorption measurements at 195 K which revealed Langmuir surface areas of 680, 757, 713, and 1350 m2 g-1, respectively. fsc-3-NDS was found to exhibit no permanent porosity, presumably because of framework collapse upon activation.



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Figure 4: Single-component CO2 and N2 adsorption isotherms for (a) fsc-1-NDS, (b) fsc-2-NDS, (c) fsc-2-SIFSIX and (d) fsc-5-NDS.

The variable PLDs of the fsc nets reported herein facilitates the systematic evaluation of the effect of the pore size on gas sorption properties. The CO2 uptakes of fsc-1-NDS, fsc-2NDS, fsc-4-NDS and fsc-5-NDS at 298 K and 1.0 atm were observed to be 88 cm3 g-1 (99 cm3 cm-3 ), 55 cm3 g-1 (47 cm3 cm3 ), 62 cm3 g-1 (46 cm3 cm-3) and 73 cm3 g-1 (61 cm3 cm-3), respectively (Figure 4 for fsc-1-NDS, fsc-2-NDS, and fsc-5NDS; Figure S18 for fsc-4-NDS). The isosteric heats of adsorption (Qst) of CO2 were calculated using adsorption data collected at 273 and 298 K according to the virial equation (see SI, Figures S24-S28). Figure 5 shows that the CO2 Qst for fsc-1-NDS is nearly steady across all loadings and is greater than that of fsc-2-NDS and fsc-5-NDS even though all three MOMs exhibit the same functionality. We attribute the higher affinity of fsc-1-NDS towards CO2 to the synergy between the primary and secondary binding sites which is available only for fsc-1-NDS as a result of the shorter distance between the oxygen atom of the sulfonate group and the copper paddlewheels (Figure 3). Similar observations for the impact of pore size on CO2 adsorption has been reported by us 33,40,45 and others6,46, in particular Qst and CO2/N2 selectivity (see SI, Figure S43). Note, the experimental CO2 Qst values reported herein for fsc-1-NDS, fsc-2-NDS, and fsc-5-NDS are in good agreement to those produced from molecular simulation in the

respective variants (see SI, Figures S39, S41-S42). Because of the small pore size for fsc-1-NDS, this MOM was also found to exhibit a sharp increase at low loading for H2 at 77 K (see SI, Figure S20).


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Figure 5. CO2 isosteric heats of adsorption (Qst) of fsc-1-NDS, fsc-2-NDS and fsc-5-NDS.

Computational studies were performed on fsc-1-NDS, fsc-2-NDS, and fsc-5-NDS to model the effects that pore chemistry and pore size have on the interaction between the sorbed CO2 molecules and the framework, especially the copper paddlewheels in the respective MOMs. We believe that performing modeling studies on these NDS variants were sufficient to reach one of the conclusions of the manuscript. The simulations revealed that the open-metal Cu2+ ion sites are the first to be occupied in these MOMs. Figure 6 shows the radial distribution function, g(r), of CO2 carbon atoms about the Cu2+ ions of the copper paddlewheels in fsc-1-NDS, fsc-2-NDS, and fsc-5-NDS at 273 K and 0.05 atm. For these three compounds, a nearest-neighbor peak can be observed at distance characteristic of such interactions. This peak corresponds to the sorption of CO2 molecules onto the Cu2+ ions with a Cu2+– C distance of about 3.3 Å. Note, this distance is comparable to what was observed for CO2 sorption onto the copper paddlewheels in MOFs with rht topology through previous molecular simulation studies.47,48 Further, this distance is also comparable to what was discerned through neutron powder diffraction (NPD) studies investigating CO2 sorption in HKUST-1, a prototypal MOF with copper paddlewheels, where Cu2+–C distances in the range of 3.0 to 3.2 Å were observed.49 A very large peak can be seen at that distance for fsc-1-NDS, while much smaller peaks were observed for fsc-2-NDS and fsc-5NDS. This signifies that there is a much greater affinity for CO2 molecules sorbing onto the Cu2+ ions of the copper paddlewheels in fsc-1-NDS compared to fsc-2-NDS and fsc-5NDS. The high CO2 occupancy about the copper paddlewheels in fsc-1-NDS relative to fsc-2-NDS and fsc-5-NDS can be explained by the following. In fsc-1-NDS, as the CO2 molecules bind onto the Cu2+ ions of the Cu paddlewheels, they can also interact with the RSO3– oxygen atoms of the NDS group that is very nearby. Indeed, a favorable, synergistic interaction can be seen between the CO2 oxygen atoms and the Cu2+ ions and between the CO2 carbon atoms and the decorating oxygen atoms of the NDS pillar in fsc-1-NDS (Figure 7).

Figure 6: Radial distribution function, g(r), of CO2 carbon atoms about the Cu2+ ions of the copper paddlewheels in fsc-1-NDS, fsc2-NDS, and fsc-5-NDS and fsc-2-SIFSIX at 273 K and 0.05 atm. Note, the g(r) are normalized to a total magnitude of unity over the distance examined.

Figure 7: A molecular illustration of the CO2 molecule interaction between Cu paddlewheels and the decorating oxygen atoms of NDS moieties in fsc-1-NDS. Atom colors: C = cyan, H = white, N = blue, O = red, S = yellow, Cu = tan.

The CO2 molecules are held onto very tightly at this site, which explains the high CO2 occupancy in this MOM. This also explains the high CO2 adsorption enthalpy in this MOM at low loading compared to other members that were synthesized with the NDS pillar. The use of isonicotinic acid (HL1) as the linker causes the NDS group to become closer to the Cu paddlewheel, thus allowing for this strong interaction to be possible. In contrast, using HL2 and HL5 as the linkers cause the NDS group to become farther away from the paddlewheel (Figure 3). As a result, the CO2 molecules cannot interact with the RSO3– ions at the same time that they sorb onto the copper paddlewheels in fsc-2-NDS and fsc-5-NDS. Hence, the Cu2+ ions and the RSO3– ions behave as two independent sites in these two MOMs. Sorption onto a single Cu2+ ion of a copper paddlewheel as observed in fsc-2-NDS and fsc-5-NDS results in a weaker interaction compared to the cooperative interaction between the copper paddlewheels and the NDS groups as observed in fsc-1-NDS. B. Effect of Pillar Substitution. The orientation of the unsaturated metal centers of the copper paddlewheels can be modulated by the pillar,33 e.g. if the NDS angular pillar in fsc-2-NDS is substituted by an SiF62- pillar as in fsc-2-SIFSIX (Figure 8) then the [Cu2(CO2R)4] moieties from adjacent 2D square grid sheets are aligned to afford Cu···Cu distances of only 5.86 Å. The short distance and the alignment of the UMCs of the paddlewheel moieties in fsc-2SIFSIX afford the possibility of synergetic interactions with the adjacent [Cu2(CO2R)4] moieties, as supported by computational studies. Indeed, the space between the two [Cu2(CO2R)4] moieties represents a strong CO2 binding site that can be regarded as an example of a single-molecule trap. A recently reported example by Zhou’s group36 of a singlemolecule trap for CO2 exhibits a different fashion of interaction where each oxygen atom of the adsorbed CO2 molecule can coordinate to one Cu2+ ion of adjacent copper paddlewheels. The reported structure relies on a certain type of organic carboxylate linker which places a limit on the versatility of this material. In addition, the reported structure contains a twist between two copper paddlewheel units that are facing each other due to the twist of the phenyl groups of the organic linkers. On the other hand, the single-molecule trap for CO2 in fsc-2-SIFSIX is dependent on the dianionic pillar, which can easily be modulated. Indeed, the facile synthesis of fsc-2SIFSIX and the simplicity of modifying the pillar offers a wide range of possible fsc structures.


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Permanent porosity was confirmed via CO2 adsorption measurements at 195 K, which reveal Langmuir surface areas of 833 and 757 m2 g-1 for fsc-2-SIFSIX and fsc-2-NDS, respectively (see SI, Figures S14-S15). CO2, CH4 and N2 adsorption isotherms were collected at 298 K and it was discovered that fsc-2-SIFSIX displayed high affinity towards CO2, as exemplified by the higher CO2 uptakes at the low-pressure regions compared to CH4 and N2 (see Figure 4c). The CO2 uptakes in fsc-2-SIFSIX and fsc-2-NDS at 298 K and 1.0 atm were measured to be 69 cm3 g-1 (54 cm3 cm-3) and 55 cm3 g-1 (47cm3 cm-3), respectively, whereas the CH4 and N2 uptakes are 15.5 and 16.3 cm3 g-1, and 4.8 and 6.0 cm3 g-1, respectively. fsc-2SIFSIX therefore outperforms fsc-2-NDS with respect CO2 sorption. In order to rationalize these observations, the isosteric heats of adsorption (Qst) of CO2 for the two MOMs were calculated using adsorption data collected at 273 and 298 K according to the virial equation (see SI). Figure 9 reveals that the CO2 Qst for fsc-2-SIFSIX was found to be at least 4 kJ mol-1 greater than that of fsc-2-NDS across all loadings. The CO2 Qst values presented in Figure 9 for fsc-2-SIFSIX were found to be in good agreement to those determined from computer simulation. As mentioned earlier, the Qst at the low loading region is attributed to the UMCs of the [Cu2(CO2R)4] as the first binding site. The use of SiF62- as a pillar for designing the fsc-2SIFSIX structure creates a dual site trap between the unsaturated metal centers in which a single CO2 molecule might interact simultaneously with two Cu2+ cations from the adjacent [Cu2(CO2R)4] moieties. The ideal adsorbed solution theory (IAST) selectivities for both structures were calculated for 50:50 CO2/CH4 and 10:90 CO2/N2 mixtures based on the experimental gas adsorption isotherms. The CO2/CH4 selectivities for fsc-2-SIFSIX and fsc-2-NDS at 298 K and 1.0 atm were found to be 35 and 7, respectively, while those for CO2/N2 were calculated to be 201 and 24, respectively (Figures S44-S45). These data further validate the importance of tuning the UMCs and the judicious choice of the pillar to create a material with high affinity towards CO2 as exemplified by the high Qst and selectivity of CO2.

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Figure 8: (a) and (b) shows the pore decoration of fsc-2-SIFSIX and fsc-2-NDS, respectively, along [0,0,1]. (c) Topological representation of fsc topology nets reported herein. The red balls represent the 6-connected nodes while the green balls represent the 4connected copper paddlewheels.

Figure 9: CO2 isosteric heats of adsorption (Qst) of fsc-2SIFSIX and fsc-2-NDS. The computational study performed herein revealed that the substitution of NDS with SiF62- has affected the CO2 molecule interaction about the Cu paddlewheels when comparing


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the sorption properties between fsc-2-SIFSIX and fsc-2-NDS. The simulations revealed that there are more CO2 molecules sorbed onto the copper paddlewheels for fsc-2-SIFSIX than fsc-2-NDS. This can be observed in the g(r) plot as shown in Figure 6, as a larger nearest-neighbor peak can be seen for fsc2-SIFSIX compared to fsc-2-NDS. Examination of the crystal structure of fsc-2-SIFSIX revealed that using SiF62- as the pillar causes adjacent paddlewheels to be arranged close to each other, with a Cu2+–Cu2+ distance of approximately 5.86 Å between the neighboring paddlewheels. This close distance between nearby paddlewheels allows the CO2 molecules to interact with the Cu2+ ions of adjacent paddlewheels in a favorable fashion. Indeed, in fsc-2-SIFSIX, the CO2 molecules can coordinate to two Cu2+ ions of adjacent paddlewheels simultaneously through its oxygen atom to yield a very favorable interaction (Figure 10). Note, the nearest-neighbor peak for fsc-2-SIFSIX was shifted to about 3.5 Å. This is because the proximity of the nearby Cu2+ ion acts as a counterforce, pulling the CO2 molecule in the other direction, and thus leading to a larger CO2-Cu2+ interaction distances.

dipole magnitude region (see SI, Figure S33). This signifies that there are more CO2 molecules sorbing to the SiF62– ions in comparison to the RSO3– ions of the NDS group. A closer view of the modeled structure in fsc-2-SIFSIX revealed that the CO2 molecules can coordinate to two equatorial fluorine atoms of the SiF62– group simultaneously, which results in a strong interaction between the CO2 molecule and the pillar (see SI, Figure S36). A similar interaction was observed about the SiF62- ions in [Cu(bpy)2SiF6]n (SIFSIX-1-Cu) through modeling studies.50 In contrast, a single CO2 molecule cannot coordinate to the two terminal oxygen atoms of the RSO3– ions coincidentally in fsc-2-NDS due to steric constraints provided by the linkers in this MOM. As a result, only one CO2 molecule can coordinate to one RSO3– oxygen atom in fsc-2-NDS, which is a weaker interaction compared to coordination to two equatorial fluorine atoms simultaneously as observed in fsc-2-SIFSIX (see SI, Figure S37). Note, the difference in the CO2 molecule interaction between the two pillars are also reflected in the calculated selectivities at high loadings, as the CO2/N2 and CO2/CH4selectivities are much higher for fsc-2-SIFSIX relative to fsc-2-NDS at 298 K and 1.0 atm (see SI, Figures S44-S45).

CONCLUSION

Figure 10: A molecular illustration of the CO2 molecule interaction between adjacent paddlewheels in fsc-2-SIFSIX. Atom colors: C = cyan, H = white, N = blue, O = red, Cu = tan.

The simulation studies revealed that this interaction corresponds to a binding energy of ca. 35 kJ mol-1, which is in good agreement to the experimental initial Qst value that was calculated for fsc-2-SIFSIX (see SI, Figure S40). The CO2 molecules are held onto more tightly by adjacent Cu2+ ions in fsc-2SIFSIX, which explains why there is a much greater occupancy of CO2 molecules about the copper paddlewheels in this MOM relative to the fsc-2-NDS. In fsc-2-NDS, the NDS pillar causes the copper paddlewheels to be much farther apart from each other. As a result, a single CO2 molecule can only coordinate to one Cu2+ ion of a copper paddlewheel in fsc-2-NDS, which is a weaker Cu2+–CO2 interaction compared to that in fsc-2-SIFSIX. Modeling studies have also added insights into the interaction between the CO2 molecules and the pillars between fsc-2SIFSIX and fsc-2-NDS. These interactions are observed in the simulations at higher loadings after the open-metal sites are filled. Calculation of the normalized dipole distribution for CO2 molecules sorbed about the anionic pillars in fsc-2SIFSIX and fsc-2-NDS at 273 K and 1.0 atm revealed that there is a higher population of CO2 molecules for fsc-2SIFSIX compared to fsc-2-NDS across the indicated induced

In summary, we report a novel, versatile MOM platform with tunable pore size and chemistry for selective CO2 capture. Our materials are sustained by pillaring a fine-tunable cationic paddlewheel square grid lattice via divalent anionic pillars, such as NDS and SiF62-, to afford a new family of 3D 4,6-c fsc nets. We were able to fine-tune the synergy between the primary and secondary binding sites as well as adjusting pore size by using linkers with different lengths to optimize the distance between the UMCs and the decorating oxygen atoms of the NDS pillars. Gas sorption data in conjunction with computational studies validate a strong affinity toward CO2 across all loadings in case of fsc-1-NDS as expressed by a nearly steady Qst value. The use of longer linkers obliterates that synergy, and as a result, both binding sites behave independently, thus causing a significant drop in the CO2 Qst at higher loading due to the weaker interactions. We have also successfully demonstrated the effect of pillar geometry on tuning the UMCs through the substitution of angular NDS pillar with linear SiF62- pillar that tunes the alignment of the unsaturated copper paddlewheel moieties with a short Cu-Cu distance of 5.86 Å. This space represents a single-molecule trap, which closely fits one CO2 molecule. Our gas sorption data reveals the strong affinity of fsc-2-SIFSIX toward CO2 as exemplified by a high Qst value at low loading and a high CO2/N2 and CO2/CH4 selectivity at 298 K and 1.0 atm. Computational studies attribute this behavior to the dual interaction of CO2 with the unsaturated Cu2+ centers from the adjacent [Cu2(CO2R)4] moieties. We therefore anticipate that other fsc nets containing SiF62- and other inorganic pillars will further improve Qst and selectivity and studies to address this matter are currently underway in our laboratory. This study therefore offers a prototypal model that demonstrates the crucial role of tuning the UMCs, the pore chemistry, and pore size for highly selective gas adsorption in porous materials.

ACKNOWLEDGMENTS M.H.M. acknowledges support from the Schlumberger Foundation and its Faculty for the Future Fellowship program. B.S. acknowledges the National Science Foundation (Award No. CHE-1152362), the computational resources that were made


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available by a XSEDE Grant (No. TG-DMR090028), and the use of the services provided by Research Computing at the University of South Florida.

ASSOCIATED CONTENT Supporting Information Additional gas adsorption isotherms, crystallographic tables, and modeling details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Michael J. Zaworotko: [email protected] *Brian Space: [email protected]

Author Contributions ⊥These

authors equally contributed.

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(38) Bi, M.; Li, G.; Hua, J.; Liu, Y.; Liu, X.; Hu, Y.; Shi, Z.; Feng, S. Crystal Growth & Design 2007, 7, 2066. (39) Elsaidi, S. K.; Mohamed, M. H.; Schaef, H. T.; Kumar, A.; Lusi, M.; Pham, T.; Forrest, K. A.; Space, B.; Xu, W.; Halder, G. J.; Liu, J.; Zaworotko, M. J.; Thallapally, P. K. Chemical Communications 2015. (40) Elsaidi, S. K.; Mohamed, M. H.; Wojtas, L.; Chanthapally, A.; Pham, T.; Space, B.; Vittal, J. J.; Zaworotko, M. J. Journal of the American Chemical Society 2014, 136, 5072. (41) Schoedel, A.; Cairns, A. J.; Belmabkhout, Y.; Wojtas, L.; Mohamed, M.; Zhang, Z.; Proserpio, D. M.; Eddaoudi, M.; Zaworotko, M. J. Angewandte Chemie International Edition 2013, 52, 2902. (42) Zhang, Y.-B.; Zhou, H.-L.; Lin, R.-B.; Zhang, C.; Lin, J.-B.; Zhang, J.-P.; Chen, X.-M. Nature Communications 2012, 3, 642. (43) Guillerm, V.; Ragon, F.; Dan-Hardi, M.; Devic, T.; Vishnuvarthan, M.; Campo, B.; Vimont, A.; Clet, G.; Yang, Q.; Maurin, G.; Férey, G.; Vittadini, A.; Gross, S.; Serre, C. Angewandte Chemie International Edition 2012, 51, 9267.

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For Table of Contents Use Only


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Crystal Engineering of a 4,6-c fsc Platform That Can Serve as a Carbon Dioxide Single-Molecule Trap Sameh K. Elsaidi, Mona H. Mohamed, Tony Pham, Lukasz Wojtas, Michael J. Zaworotko, and Brian Space

We report the crystal engineering of a highly versatile 4,6-c fsc platform that is tunable in terms of both pore size and functionality. The pore size control was exerted by varying the length of the ligand, whereas pore chemistry was implemented via unsaturated metal centers (UMCs) and the use of either inorganic or organic pillars.


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Crystal Engineering of a 4,6-c fsc Platform That Can Serve as a

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