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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Transition from a 1D Coordination Polymer to a Mixed-Linker Layered MOF Juan P. Vizuet, Thomas S. Howlett, Abigail L. Lewis, Zachary D. Chroust, Gregory T. McCandless, and Kenneth J. Balkus, Jr.*

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 03/29/19. For personal use only.

Department of Chemistry and Biochemistry, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080-3021, United States S Supporting Information *

ABSTRACT: A novel copper(II) metal−organic framework (MOF) has been synthesized by modifying the reaction conditions of a 1D coordination polymer. The 1D polymer is built by the coordination between copper and 2,2′-(1H-imidazole-4,5-diyl)di-1,4,5,6-tetrahydropyrimidine (H-L1). The geometry of H-L1 precludes its ability to form extended 3D framework structures. By adding 1,4-benzenedicarboxylic acid (H2BDC), a well-studied linker in MOF synthesis, we achieved the transition from a 1D polymer chain into porous 2D layered structures. Hydrogen bonding between L1 and BDC directs the parallel stacking of these layers, resulting in a 3D structure with one-dimensional channels accessible by two different pore windows. The preferred growth orientation of the crystal produces prolonged channels and a disparity in pore size distribution. This in turn results in slow diffusion processes in the material. Furthermore, an isoreticular MOF was prepared by substituting the BDC linker by 2,6-naphthalenedicarboxylic acid (H2NDC).

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INTRODUCTION Coordination polymers (CPs) were first defined by Bailar in 1964, when he compared a set of inorganic coordination complexes to organic polymers.1 In contrast to organic polymers, CPs are formed by the coordination bonds between organic linkers and metal ions. This leads to infinite networks growing along one, two, or three dimensions. 2,3 1D coordination polymers have been extensively studied and present potential applications in the areas of molecular electronics, nonlinear optics, and superconductivity.4,5 Particularly, two- and three-dimensional porous coordination polymers, also known as metal−organic frameworks (MOFs), have gained attention due to their inherent properties.6−8 MOFs have permanent porosity, high surface area, and tunable pore sizes.9−13 Due to these properties, MOFs have been applied in fields such as separation,14−22 storage,23−29 catalysis,30−36 energy storage,37−43 and sensing.44−50 While MOFs grow as infinite networks, nonextended porous structures are known as metal−organic polyhedra (MOPs), where the coordination bond angle hinders structure propagation.51−53 This angle can be controlled either by modifying the metal center54 or by choosing a linker with an appropriate geometry.55 Conversion of MOPs into MOF systems has been previously explored;56−59 in particular Li et al. reported the formation of a MOF system by treating a MOP with the 4,4′-bipyridine linker.60 This proves that is possible to create MOF structures derived from different types of coordination complexes by integration of another linker. © XXXX American Chemical Society

While most MOFs incorporate only one type of organic linker in the structure, mixed-linker MOFs incorporate two or more ligands in their structures and can be divided into two broad categories.61 The first category is comprised of MOFs that integrate isosteric ligands which play the same structural role but contain different functional groups: e.g., terephthalic acid and its derivatives.62−64 The second category includes MOFs where both ligands play different structural roles and the ratio between ligands is crucial to the assembly of the framework.65−67 The geometry of 2,2′-(1H-imidazole-4,5-diyl)di-1,4,5,6tetrahydropyrimidine (H-L1) (Figure 1a), although multidentate, does not lead to MOF structures when the species is coordinated to different metals. Metal−organic cubes (Co and In), metal−organic squares (Ni), and 1D coordination polymers (Cd) have been previously constructed using L1 as the linker.68 We discovered that the incorporation of 1,4benzenedicarboxylic acid (H2BDC) (Figure 1b), one of the most versatile linkers in the design and synthesis of MOFs,6 to the synthesis of an L1-based complex results in the formation of a new MOF system. Furthermore, we prove that an isoreticular MOF can be synthesized by substituting the BDC linker for 2,6-naphthalenedicarboxylic acid (H2NDC) (Figure 1c). Received: January 9, 2019

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DOI: 10.1021/acs.inorgchem.9b00077 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Schematic representation of the H-L1 (a), H2BDC (b), and H2NDC (c) linkers. Synthesis of {[Cu2(BDC)1.5(L1)]·xDMF·xH2O}n (2). A mixture containing Cu(NO3)2·3H2O (0.0720 g, 0.298 mmol), H-L1 (0.0340 g, 0.146 mmol), and 1,4-benzenedicarboxylic acid (H2BDC) (0.0380 g, 0.229 mmol) was dissolved in DMF (10 mL) and H2O (10 mL). The solution was placed in a capped 20 mL scintillation vial and heated at 80 °C for 12 h and then cooled to room temperature. Blue block crystals of 2 were collected by centrifugation and washed with DMF. The crystals were dried overnight at 80 °C. Yield: 0.0681 g, 70.8% based on H-L1. Anal. Calcd for Cu2(BDC)1.5(L1): C, 45.65; H, 3.47; N, 13.89. Found: C, 43.93; H, 3.34; N, 13.37. Synthesis of {[Cu2(NDC)1.5(L1)]·DMF·H2O}n (3). A mixture containing Cu(NO3)2·3H2O (0.0720 g, 0.298 mmol), H-L1 (0.0340 g, 0.146 mmol), and 2,6-naphthalenedicarboxylic acid (H2NDC) (0.0495 g, 0.229 mmol) was dissolved in DMF (10 mL) and H2O (10 mL). The solution was placed in a capped 20 mL scintillation vial and heated at 80 °C for 12 h and then cooled to room temperature. Blue block crystals of 3 were collected by centrifugation and washed with DMF. The crystals were dried overnight at 80 °C. Yield: 0.0411 g, 36.5% based on H-L1. Anal. Calcd for Cu2(NDC)1.5(L1): C, 51.20; H, 3.53; N, 12.36. Found: C, 48.38; H, 3.36; N, 11.82. X-ray Crystal Structure Analysis. In order to elucidate the crystal structure of the three compounds, single-crystal X-ray diffraction data sets (ω scans) were collected on a Bruker Kappa D8 Quest diffractometer, with Incoatec microfocus Mo Kα radiation source (λ = 0.71073 Å). For 1 and 3, green and blue crystalline fragments, respectively, were used to collect diffraction data, using a Photon II CPAD detector. In the case of 2 the data collection was performed using a blue crystalline fragment and a Photon 100 CMOS detector. All measurements were collected at a temperature of 100 K (temperature maintained with an Oxford Cryosystems cryostream apparatus). Following data collection, the data sets were integrated (Bruker SAINT), scaled (Bruker SADABS with multiscan absorption correction), and evaluated for space group determination (Bruker XPREP). SHELXT (intrinsic phasing method)71 was then used to create a preliminary structural model. This starting model was further developed through a series of refinement iterations using SHELXL2017.72 With the exception of the solvent molecules, all non-hydrogen atoms were refined anisotropically. In 1 only fully occupied H2O solvent molecules were located in the crystal structure. For 2, only the partially occupied DMF and H2O solvent molecules (not as “wellbehaved” in their position/orientation as the MOF in the average structure) were refined isotropically with restraints to keep the bond distances “chemically reasonable”. Hydrogen atoms bound to carbon atoms were refined in “riding” (AFIX) atomic sites, whereas hydrogen atoms bound to nitrogen atoms were only restrained by bond length (DFIX). For partially occupied H2O molecules, hydrogen atoms were restrained to orient toward “possible” neighboring H-bonding acceptors while an appropriate H−O−H bond angle was maintained. In the case of 3, DMF solvent molecules were modeled with two orientations with an occupancy ratio of ∼0.72:0.28 (positional disorder, anisotropic refinement). H2O molecules were modeled with two orientations with an occupancy ratio of ∼0.59:0.41 (positional disorder, isotropic refinement). Details of the crystal parameters, data collection, and refinement for 1 (CCDC 1874973), 2 (CCDC 1844994), and 3 (CCDC 1897921) are given in Table 1.

Herein, we report the synthesis of a new 1D coordination polymer formed by the coordination of copper and H-L1. By incorporating BDC into the 1D polymer synthesis, a new twodimensional copper−MOF system was formed that integrated both linkers in the structure. Unique features in the MOF include large pores (∼8 Å), asymmetric five-coordinated metal centers, and uncoordinated oxygen atoms from the BDC linkers. Hydrogen bonding between the two linkers drives the 2D layers to stack, creating a 3D structure. The packing of the layers leads to the creation of internal channels and new secondary pores (∼5 Å). Furthermore, the incorporation of BDC implies that the properties of the MOF could be easily tuned via isoreticular chemistry69 or postsynthetic modification.70 To prove this concept, BDC was substituted with NDC in the reaction synthesis, leading to the formation of an isoreticular MOF. The crystal structures were solved by singlecrystal analysis and the main structural features are discussed below. Additionally, the results from FTIR, TGA, PXRD, and surface area analysis are presented and discussed.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All reagents and solvents were from different commercial sources and were used without further purification. The H-L1 linker was synthesized as reported elsewhere.68 Elemental analyses (C, H, N) were performed by Intertek USA, Inc. All samples were evacuated prior to the analysis. All infrared (IR) spectra were recorded using a Nicolet Avatar 360 FTIR spectrometer (Thermo Scientific) from pressed KBr pellets in the range of 400−4000 cm−1. Thermogravimetric analysis (TGA) was carried out using a Pyris 1 TGA instrument (PerkinElmer). The materials were heated to 100 °C, held for 1 h, and then heated to 600 °C at a rate of 5 °C/min and then to a 1000 °C at a rate of 10 °C/ min. The surface area and pore size distribution analysis were performed by Anton Paar QuantaTec Inc. in a Autosorb iQ instrument. The NLDFT (nonlinear density functional theory) adsorption model for zeolites with cylindrical pores was used for the pore analysis. The sample was evacuated at 180 °C for 6 h prior to the measurement. Argon at 87 K was used as the adsorbent, and the analysis took 147 h with the Dewar refilled during analysis. Powder Xray diffraction (PXRD) patterns were collected on a Ultima IV X-ray diffractometer (Rigaku) equipped with Cu Kα radiation, with a scan speed of 1°/min and a step size of 0.04° in 2θ. Synthesis of {[Cu(H-L1)Cl2]·H2O}n (1). A mixture containing Cu(NO3)2·3H2O (0.0240 g, 0.0993 mmol) and H-L1 (0.0340 g, 0.146 mmol) was dissolved in DMF (8.0 mL) and methanol (8.0 mL). Fifteen drops of concentrated HCl (0.75 mL, 24 mmol) were added to the mixture, and the solution was transferred to a 20 mL scintillation vial. The capped vial was heated at 80 °C for 12 h and then cooled to room temperature. Green bladelike crystals of 1 were collected by centrifugation and washed with DMF. The crystals were dried overnight at 80 °C. Yield: 0.0221 g, 57.8% based on Cu. Anal. Calcd for Cu(H-L1)Cl2: C, 35.99; H, 4.36; N, 22.90. Found: C, 35.05; H, 4.69; N, 21.84. B


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Inorganic Chemistry Table 1. Crystallographic Data for 1−3 chem formula formula wt T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (Mg m−3) cryst size (mm) F(000) μ(Mo Kα) (mm−1) θ range (deg) no. of rflns collected no. of indep rflns no. of params Rint Δρmax and Δρmin (e Å−3) goodness of fit (GOF) on F2 final R indices (I > 2σ(I)) final R indices (all data)

1

2

3

Cu(C11H16N6)Cl2·H2O 384.75 100(2) monoclinic P21/c 8.913(2) 14.075(2) 12.040(2) 98.934(5) 98.934(5) 98.934(5) 1492.1(5) 4 1.713 0.16 × 0.04 × 0.01 788 1.83 2.2−30.5 39901 4565 207 0.036 0.77 and −0.41 1.15 R = 0.031 Rw(F2) = 0.070 R = 0.034 Rw(F2) = 0.071

Cu2(C8H4O4)1.5(C11H15N6)·0.561C3H7NO)·0.75H2O 659.05 100(2) triclinic P1̅ 8.831(6) 9.932(7) 16.588(9) 89.91(2) 77.43(2) 78.109(8) 1388.1(15) 2 1.577 0.06 × 0.06 × 0.02 674 1.59 2.1−27.6 21789 6316 372 0.086 1.76 and −0.77 1.03 R = 0.059 Rw(F2) = 0.133 R = 0.106 Rw(F2) = 0.154

Cu2(C12H6O4)1.5(C11H15N6)·C3H7NO·H2O 770.73 100(2) triclinic P1̅ 8.738(2) 11.610(3) 16.083(5) 101.830(17) 91.448(16) 101.403(11) 1561.8(8) 2 1.639 0.22 × 0.14 × 0.01 792 1.43 2.4−30.5 49366 9552 476 0.047 1.18 and −0.66 1.03 R = 0.037 Rw(F2) = 0.133 R = 0.081 Rw(F2) = 0.091

Figure 2. Structure of 1. (a) Asymmetric unit of the coordination polymer (hydrogen atoms are omitted for clarity). (b) 1D polymer chain viewed along the b axis propagating along the c direction (solvent molecules are omitted for clarity).

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the geometric parameter τ = (β − α)/60, with τ = 0 being an ideal square pyramid and τ = 1 being an ideal trigonal bipyramid,73 the Cu metal center geometry in 1 is closer to an ideal square pyramid (τ = (161.77 − 158.46)/60 = 0.06). The Cu metal center puckers above a fitted CuN3Cl least-squares plane by ∼0.35 Å with the axial chlorine atom at a distance of ∼2.83 Å above this basal plane. As seen in Figure 2b, the bonding in 1 results in a one-dimensional coordination polymer propagating along the c direction. Additionally, hydrogen-bonding interactions are observed between the solvent H2O oxygen atom, O1W, and a neighboring linker

RESULTS AND DISCUSSION

{[Cu(H-L1)Cl2]·H2O}n (1). Compound 1 crystallizes in a monoclinic system, space group P21/c. The asymmetric unit (Figure 2a) contains a crystallographically unique copper metal center, coordinated to one H-L1 linker and two chlorine atoms. Water is also present in the structure. The coordination environment of copper can be described as a distorted fivecoordinate square pyramid, where the base of the pyramid is formed by three Cu−N bonds and one Cu−Cl bond, while the remaining Cu−Cl bond is located in the axial position. Using C


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Inorganic Chemistry

Figure 3. Structural fragments of 2. (a) Asymmetric unit of the BDC based 2D MOF (hydrogen atoms are omitted for clarity). (b) 1D polymeric chain viewed along the b axis propagating along the c direction (solvent molecules are omitted for clarity).

Figure 4. 3D structure of 2. (a) 2D layers viewed along the c axis propagating along the b axis. The red dashed circles highlight the interactions between linkers that drive the layer stacking (solvent molecules are omitted for clarity). Space-filling model seen along the (b) a axis and (c) b axis. The stacked 2D netlike layers result in channels propagating along the a direction that interconnect to smaller channels propagating along the b direction (solvent molecules (DMF and H2O) are omitted for clarity).

a distorted five-coordinate square-pyramidal environment where two Cu−N bonds and two Cu−O bonds form the base of the pyramid with a slightly longer Cu−O bond perpendicular to the base. Both bases are near planar (rootmean-square deviation of ∼0.1 Å on fitting to a CuN2O2 leastsquares plane) with the axial oxygen atom ∼2.45 Å above this basal plane. In comparison to 1, the copper centers in 2 are slightly more distorted, where Cu1 presents τ = (175.48 − 168.06)/60 = 0.12 while Cu2 presents τ = (174.86 − 165.97)/

H-L1 nitrogen atom, N4 (O1W···N4 interatomic distance of 2.739(2) Å) (Figure S1). {[Cu2(BDC)1.5(L1)]·xDMF·xH2O}n (2). Compound 2 crystallizes in a triclinic system, space group P1̅. The asymmetric unit, depicted in Figure 3a, contains two crystallographically unique copper centers (Cu1···Cu2 interatomic distance of 3.171(2) Å), each coordinated to one L1 linker and two different BDC linkers. Solvent molecules (DMF and water) are also present in the asymmetric unit. Both metal centers possess D


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Inorganic Chemistry

Figure 5. Structural fragments of 3. (a) Asymmetric unit of the NDC-based 2D MOF (hydrogen atoms are omitted for clarity). (b) 1D polymeric chain propagating along the [111] direction (solvent molecules are omitted for clarity).

Figure 6. 3D structure of 3. (a) 2D layers viewed along the origin propagating along the [011̅] direction. The red dashed circles highlight the interactions between linkers that drive the layer stacking (solvent molecules are omitted for clarity). Space-filling model seen along the (b) a axis and (c) b axis. The stacked 2D netlike layers result in channels propagating along the a direction that interconnect to smaller channels propagating along the b direction. Orange circles highlight the small channel accesses (solvent molecules (DMF and H2O) are omitted for clarity).

interatomic distance of 2.037 Å). These interactions, highlighted in Figure 4a by red dashed circles, drive the stacking of the 2D layers. Ultimately, the stacking of these 2D nets results in the formation of channels propagating along the a axis (Figure 4b). Additionally, the stacking of the layers produces smaller channels along the b direction that interconnect to the channels in the a direction (Figure 4c). Using Mercury software, the size aperture of both channels was calculated: along the a direction the calculated window size is 8.8 Å × 8.4 Å while along the b direction a window size of 5.0 Å × 6.2 Å is

60 = 0.15. As shown in Figure 3b, one BDC linker and two L1 linkers bond in an alternating sequence with the copper metal centers, forming a 1D polymeric chain that propagates along the c axis. A second BDC linker coordinates in a monodentate fashion to each copper center, connecting the polymeric chains and forming “netlike” 2D layers that extend along the b axis (Figure 4a). One of the uncoordinated oxygen atoms from this BDC presents hydrogen bonds with the amine hydrogens in L1 (H4N···O2 interatomic distance of 2.034 Å and H6N···O2 E


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Inorganic Chemistry

Figure 7. PXRD patterns of (a) 1, (b) 2, and (c) 3. The single-crystal pattern (black line), the as-synthesized pattern (red line) and the pattern after solvent removal (blue line) are shown.

Figure 8. TGA of (a) 1, (b) 2, and (c) 3 for both an as-synthesized sample (black line) and a solvent-free sample (red line).

and two different NDC linkers. Both DMF and water molecules are also present in this asymmetric unit. The coordination environment of the copper centers in 3 is almost identical with that in complex 2. The copper centers have a distorted five-coordinate square-pyramidal environment with two Cu−N bonds and two Cu−O bonds forming the base of the pyramid with a slightly longer Cu−O bond perpendicular to the base. Both bases are near planar (root-mean-square

observed. Solvent molecules (DMF and H2O) are located in the intersection of both channels (Figures S2 and S3). {[Cu2(NDC)1.5(L1)]·DMF·H2O}n (3). Compound 3 is analogous to compound 2, having the same topology and crystallizing in a triclinic system, space group P1̅. Figure 5a shows that the asymmetric unit also contains two crystallographically unique copper centers (Cu1···Cu2 interatomic distance of 3.1412(2) Å), each coordinated to one L1 linker F


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Inorganic Chemistry

in an alternating sequence with the copper metal centers, forming a 1D polymeric chain that propagates along the [111] crystallographic direction. As in the case of 2, complex 3 has a second NDC linker that coordinates in a monodentate fashion to each copper center, connecting the polymeric chains and forming “netlike” 2D layers that extend along the [011̅] direction (Figure 6a). The uncoordinated oxygen atoms from this NDC form hydrogen bonds with the amine hydrogens in L1 (H4N···O4 interatomic distance of 2.021 Å and H6N···O4 interatomic distance of 2.066 Å). These interactions, highlighted in Figure 6a by red dashed circles, drive the stacking of the 2D layers. Ultimately, the stacking of these 2D nets results in the formation of channels propagating along the a axis (Figure 6b). Using Mercury software, the size aperture of these channels was calculated to be 8.9 Å × 5.2 Å. The stacking of the layers in this MOF also produces smaller channels along the b direction that interconnect to the channels in the a direction (Figure 6c). Unlike the case for 2, the entrances to these channels are very narrow with a calculated window size of 5.9 Å × 3.2 Å. Solvent molecules (DMF and H2O) are located in the intersection of both channels (Figures S4 and S5). PXRD and Thermal Analysis. Powder X-ray diffraction was performed to confirm the purity of the three complexes. The good fit between the simulated and experimental patterns indicates phase purity in the bulk samples of 1 (Figure 7a), 2 (Figure 7b), and 3 (Figure 7c). In the case of 2, the peak at 9° has an intensity higher than expected, corresponding to the 010 plane. A similar effect is observed for 3, where the peak at 8.6° has a higher intensity, corresponding to the 011̅ plane. The diffraction pattern after solvent removal for all structures is also shown. For 1, evacuation at 140 °C for 12 h induced a change in the crystal structure to an unknown phase, while 2 and 3 remain largely unchanged after evacuation at 180 °C for 12 h. Solvent removal in 2 and 3 was accompanied by a slight change in color from blue to dark blue. Additionally, thermogravimetric analysis (TGA) was performed to examine the thermal stability of all the compounds.

Figure 9. FTIR spectra of 2 (a) as synthesized and (b) evacuated.

Figure 10. Argon adsorption (black squares)/desorption (red circles) isotherm of 2. The inset shows the pore size distribution of 2.

deviation of ∼0.1 Å on fitting to a CuN2O2 least-squares plane) with the axial oxygen atom ∼2.45 Å above this basal plane. The copper centers in 3 have a similar distortion, with Cu1 having τ = (174.49 − 166.95)/60 = 0.13 while Cu2 presents τ = (175.37 − 169.20)/60 = 0.10. Bond distances around the copper centers of 1−3 are included in Table S1. Figure 5b shows one NDC linker and two L1 linkers bonding

Figure 11. Space-filling model of 2 viewed along the (a) a axis and (b) b axis. Preferred growth along the a axis results in large channels without creating new access through larger pores (solvent molecules (DMF and H2O) are omitted for clarity). G


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Inorganic Chemistry

Figure 12. (a) CO2 adsorption (black circles)/desorption (white circles) at 35 °C. (b) CO2 adsorption at 35 °C (black circles), 100 °C (red squares), and 200 °C (blue diamonds).

Surface Area Analysis and Pore Size Distribution. To elucidate the surface area of 2, the sample was sent to Quantachrome Instruments for 87 K argon adsorption− desorption analysis. The isotherm, as seen in Figure 10, presents low-pressure hysteresis even after several days of analysis with long (>10 min) equilibrium times. This indicates slow diffusion inside the material, which is unexpected considering the 3.4 Å kinetic diameter of argon, its low reactivity, and the expected presence of large pores in the structure. The calculated BET (Brunauer−Emmett−Teller) surface area of 2 was 334.24 m2/g. To elucidate the cause of slow diffusion, the information provided from the pore size distribution was used. The DFT (density functional theory) pore size distribution shows a predominance of smaller pores (5.5 Å) over the larger pores (8.3 Å) (inset in Figure 10). The disparity in the pore distribution is an indication that the material has a favored growth orientation: in particular, layer stacking (propagation along the a axis) has preference over layer growth (propagation along the b axis). Furthermore, any growth along the b axis results in the appearance of both types of pores while propagation along the a axis only increases the number of smaller pores present in the structure. The preferred growth orientation of the crystal was confirmed by Xray diffraction. Single crystals of 2, representative of the bulk from two different samples, were oriented along the different crystallographic axes. The analysis of the “tablet” shaped crystals showed that the length of the crystal (its longest dimension) is growth along the a axis while the thickness of the crystal (its smallest dimension) is growth along the b axis. As a consequence of this morphology, diffusion is slowed by two different causes: prolonged internal pore channels in the crystal and few larger pores to access (Figure 11). Another factor that slows diffusion rates is the analysis temperature. To prove that the low-pressure hysteresis seen in the argon isotherm was due to the problems described above, carbon dioxide (CO2) adsorption−desorption analysis at 35 °C was performed. The results are shown in Figure 12a, where no hysteresis is observed. CO2 isotherms at different temperatures (35, 100, and 200 °C) are presented in Figure 12b.

The TGA curves for 1 (Figure 8a), 2 (Figure 8b), and 3 (Figure 8c) reveal that the solvent molecules can be removed when the complexes are heated. In 1, 3 wt % loss from H2O occurred between 120 and 140 °C, corresponding to the calculated weight in the single-crystal analysis. Decomposition of the complex started at 170 °C and finished at 600 °C. When the sample was evacuated before the analysis, the unknown phase appeared to be slightly more stable, with decomposition starting at 250 °C (red line in Figure 8a). In the case of 2, removal of both DMF and H2O occurred in the range of 150− 240 °C. The weight loss is in accordance with the data obtained from single-crystal analysis (5 wt %). The framework started decomposing at 300 °C, presenting two different rates of decomposition before and after 320 °C. The different decomposition rates are derived from the binding of two linkers in the framework (Cu−O and Cu−N).61 Removing the solvent before TGA analysis (red line in Figure 8b) shows a faster decomposition rate, although two rates are still observable. In the isoreticular complex 3, a decomposition temperature of 280 °C was observed. Furthermore, two different decomposition rates before and after 310 °C were observed as well, even in the case of an evacuated sample (red line in Figure 8c). The remaining product after the decomposition of 1 and 2 was confirmed to be copper(II) oxide by PXRD (Figures S6 and S7). In the case of 3, a mixture of copper(I) and copper(II) oxide was observed (Figure S8). FTIR Spectra. An FTIR measurement was performed for 2 to confirm the hydrogen bonding between linkers, the driving force behind the stacking of the layers. Ideally, the L1 linker should present a single signal in the 3300 cm−1 region, corresponding to the secondary amine groups. Nevertheless, the spectra of an as-synthesized sample of 2 (Figure 9a) presents two distinct signals in that region. This can be accounted for by the hydrogen bonding between one of the secondary amines in L1 and the free C−O bond from BDC. Further evidence of hydrogen bonding is provided once the sample is evacuated (Figure 9b). Due to the loss of water, which also presents hydrogen bonding with the C−O bond, the interaction between linkers increases, as evidenced by the intensified amine signals. Additionally, solvent removal can be confirmed by FTIR. The signal at 1673 cm−1 of the as-synthesized sample can be attributed to the carbonyl stretch of DMF. The lack of this signal in the evacuated sample shows that the solvent was removed without an overall change to the structure, as other peaks remained intact. FTIR of as-synthesized and evacuated 1 and 3 can be found in Figures S9 and S10 in the Supporting Information.

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CONCLUSION Three new structures, a 1D coordination polymer and two isoreticular 2D mixed-linker MOFs, were successfully synthesized and characterized. By incorporating a second linker in the synthesis of the 1D polymer, we were able to form porous twodimensional layers. In addition, hydrogen bonding between the two different linkers promotes layer stacking, resulting in a three-dimensional structure with internal pore channels. These H


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Inorganic Chemistry

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types of MOFs are very unusual and present appealing possibilities in regard to their design and applications. While the current synthesis method leads to preferential growth along one direction, we are studying methods to separate the layers and to promote growth into other directions. We are currently exploring the synthesis of other isoreticular structures with different BDC and L1 derivatives, along with different metal salts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00077. Additional figures as described in the text, FTIR spectra of 1 and 3, and selected bond distances for 1−3 (PDF) Accession Codes

CCDC 1844994 and 1874973 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kenneth J. Balkus, Jr.: 0000-0003-1142-3837 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Robert A. Welch foundation (Grant No. AT-1153) and CONACYT. We also thank Riaz Ahmad from Anton Paar QuantaTec Inc. for the surface area measurement.

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REFERENCES

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