Different Breathing Mechanisms in Flexible ... - ACS Publications

Article Cite This: Chem. Mater. 2018, 30, 1667−1676

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Different Breathing Mechanisms in Flexible Pillared-Layered Metal− Organic Frameworks: Impact of the Metal Center Andreas Schneemann,†,‡,§,⊥ Pia Vervoorts,‡,§ Inke Hante,† Min Tu,†,|| Suttipong Wannapaiboon,†,‡,§ Christian Sternemann,# Michael Paulus,# D.C. Florian Wieland,#,○ Sebastian Henke,*,∇ and Roland A. Fischer*,‡,§ †

Lehrstuhl für Anorganische Chemie II, Organometallics and Materials Chemistry, Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany ‡ Department of Chemistry, Technische Universität München, Lichtenbergstrasse 4, 85748 Garching, Germany § Catalysis Research Centre, Technische Universität München, Ernst-Otto-Fischer Strasse 1, 85748 Garching, Germany || Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200f, 3001 Leuven, Belgium # Fakultät Physik/DELTA, Technische Universität Dortmund, 44221 Dortmund, Germany ○ Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Max-Planck-Strasse, 21502 Geesthacht, Germany ∇ Anorganische Chemie, Technische Universität Dortmund, Otto-Hahn Strasse 6, 44227 Dortmund, Germany S Supporting Information *

ABSTRACT: The pillared-layered metal−organic framework compounds M2(BME-bdc)2(dabco) (M2+ = Zn2+, Co2+, Ni2+, Cu2+; BME-bdc2− = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M2+. This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO2 pressure reveal different mechanisms for the np to lp phase transition during CO2 adsorption.

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Some MOFs exhibit marked structural flexibility, e.g. the socalled breathing, without breaking of chemical bonds and under retention of crystalline order.25−30 For example, these materials can undergo a phase transition from a less porous narrow pore (np) form to an expanded, more porous large pore (lp) form, or vice versa. The structural transition is triggered by an outer stimulus of various kinds, most prominently guest adsorption (liquid or gaseous),31−34 but also temperature change,35 mechanical pressure,36 or light.37 Among others, Susumu Kitagawa and co-workers pioneered the research on such kind of materials and suggested the term “soft porous crystals” (SPCs) for this highly responsive subclass of MOFs.26 One family of SPCs are the pillared-layered frameworks of the general composition M2L2P (whereas M2+ = divalent metal

INTRODUCTION Responsiveness is a feature not commonly associated with porous crystalline solid state materials, but rather with elastic organic and inorganic polymers.1,2 The established porous inorganic materials, e.g., zeolites, mesoporous silica, or activated charcoals, are inherently rigid structures and only few examples show some structural changes and adaptivity such as swelling/ compression upon outer stimuli (e.g., temperature or pressure).3,4 Two decades ago, a new class of crystalline porous materials called metal−organic frameworks (MOFs) emerged. On the basis of inorganic metal ions or clusters, which are interconnected by oligotopic organic linkers, MOFs feature an unsurpassed tailoring of internal surface areas,5−8 exceptional pore volumes,9−13 and ultra low densities.14 This set of unique properties together with a modular building block principle and the various strategies for chemical functionalization of MOF coordination space15−17 opens new horizons for material design.18−24 © 2018 American Chemical Society

Received: December 3, 2017 Revised: February 15, 2018 Published: February 16, 2018 1667

DOI: 10.1021/acs.chemmater.7b05052 Chem. Mater. 2018, 30, 1667−1676

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Chemistry of Materials cation; L2− = linear dicarboxylate linker; P = neutral nitrogen donor pillar).38 Dinuclear paddlewheel nodes (M2) are 4-fold coordinated by dicarboxylate linkers and are interconnected to yield two-dimensional M2L2 sheets of square grid (sql) topology, which are stacked by neutral nitrogen containing pillars. The prototypic compound Zn2(bdc)2(dabco) (bdc2− = 1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane), however, shows only a weak structural response upon adsorption of certain guest molecules (e.g., iso-propanol, benzene, or DMF).39,40 Interestingly, the guest responsiveness can be modulated and even tailored by chemical functionalization of the bdc2− linker.41,42 We showed in our previous works on frameworks of the type Zn2(fu-bdc)2(dabco), (fu-bdc2− = 2,5-functionalized-1,4-benzenedicarboxylate, Figure 1) that

entropically favored process, which is driven by the higher vibrational entropy of the lp phase compared to the np phase. Only few studies exist on the influence of the metal center on the framework flexibility of a SPC material,49 with single and mixed metal MIL-53 materials as the prominent example (MIL = Matériaux de l′Institut Lavoisier, M(OH)bdc; M3+ = Al3+, Fe3+, Cr3+, ...).50−53 Klein et al. studied the metal ion substitution on the guest induced flexibility of M2(ndc)2(dabco) SPCs (M2+ = Co2+, Cu2+, Ni2+, Zn2+; ndc2− = 2,6-naphthalenedicarboxylate; DUT-8).54 All four congeners DUT-8(M) show distinct variations of gas adsorption properties and different degrees of flexibility, presumably caused by the specific electronic structures of the different metal ions. Herein we present a related study on the M2(BMEbdc)2(dabco) series abbreviated as 1(M) (M2+ = Co2+, Ni2+, Cu2+, Zn2+). The synthesis and detailed characterization of the new homologues 1(Co), 1(Ni), and 1(Cu) is documented (note 1(Cu) has been implemented in a CO2 selective membrane before,55 but the structural chemistry has not been investigated yet). The structural flexibility and gas sorption properties, as well as the thermoresponsive breathing behavior of this series of functionalized SPCs are studied and significant differences with respect to the reference compound 1(Zn) are apparent. In particular, we observed different breathing mechanisms of these compounds during CO2 adsorption.

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EXPERIMENTAL SECTION

General Remarks. All chemicals were purchased from commercial suppliers and were used without further purification (e.g., SigmaAldrich, Fluka, Alfa Aesar, ABCR and others). Linker Synthesis. The linker was prepared via Williamson ether synthesis of dimethyl-2,5-dihydroxy-1,4-benzenedicarboxylate with 1bromo-2-methoxyethane. Detailed synthesis procedures have been published elsewhere.41 MOF Synthesis and Composition. Material 1(Zn) was prepared according to previously published protocols.41 For all other 1(M), the synthesis were adapted from literature-known procedures for the nonfunctionalized M2(bdc)2(dabco) frameworks.56−58 M(NO3)2·xH2O (Co(NO3)2·6H2O, Ni(NO3)2·6H2O, or Cu(NO3)2·3H2O, respectively), H2BME-bdc and dabco were combined with DMF in a beaker (for detailed amounts, see Table S1). The mixture was sonicated until everything was dissolved. The solution was left to settle and after 20 min a precipitate formed, which was removed by filtration. The filtrate was collected in a 25 mL screw jar, which was sealed and subsequently heated for 48 h at elevated temperature (for T, see Table S1). A colored precipitate was formed: dark violet 1(Co), dark green 1(Ni), or green 1(Cu). The samples were activated via solvent exchange and drying. First, the mother liquor was replaced by fresh DMF, and the mixture was vigorously stirred for 30 min and afterward left to settle for 24 h. Subsequently, the DMF was replaced by CHCl3 and the suspension was again vigorously stirred for 30 min and left to settle for 24 h. This last step was repeated one more time. The solid was then filtered off with a frit and washed three times with chloroform and afterward dried at 130 °C in vacuo. Microcrystalline powders were obtained, which were dark violet, light green or turquoise for 1(Co), 1(Ni), and 1(Cu), respectively. Elemental analysis for 1(Co). Found: C 47.09%, H 4.49%, N 3.57%. Theoretical: C 47.78%, H 5.19%, N 3.28%. For 1(Ni). Found: C 43.46%, H 4.65%, N 3.16%. Theoretical: C 47.81%, H 5.19%, N 3.28%. For 1(Cu). Found: C 45.07%, H: 4.88%, N 3.05%. Theoretical: C 47.27%, H 5.13%, N 3.24%. Note: The difference between theoretical and measured values arises from small contaminations, i.e., the formation of small amounts of metal oxides during solvothermal synthesis. However, the effects of these minor impurities on the materials properties (framework flexibility, gas sorption, thermal behavior, etc.) are neglectable.

Figure 1. (a) Depiction of the building blocks used for the synthesis of M2(fu-bdc)2(dabco) materials. (b) Side view in a single cavity of the extended 3D network structure. (c) Schematic illustration of the lp → np phase transition of the M2(fu-bdc)2(dabco) materials.

attachment of pendant alkoxy chains of varying functionality to the bdc2− linker has a drastic influence on the gas sorption properties43,44 and structural flexibility of these materials.41,45 Depending on the type of functionality attached to the linker, a guest-induced, fully reversible phase transition from an expanded lp to contracted np phase occurs upon adsorption of CO2 from the gas phase. By the choice of the pendent side chains this pore contraction/expansion can be adjusted over a considerable range of CO2 pressure. Going further, it is even possible to fine-tune the flexible behavior in this class of SPCs by the preparation of mixed-linker materials.43,46 The most outstanding example among this series of functionalized ZnSPCs is Zn2(BME-bdc)2(dabco), 1(Zn), (BME-bdc2− = 2,5bis(2-methoxyethoxy)-1,4-benzenedicarboxylate), which shows solvent and guest-induced flexibility47 and a massive, reversible expansion from np to lp upon thermal treatment.48 The driving force of the lp → np pore contraction are attractive dipolar and dispersion interactions of the flexible side chains with neighboring side chains and with the backbone of the framework. Hence, the framework contraction after solvent removal is favored enthalpically. Upon readsorption of guest molecules (solvent or gas) the material returns to the expanded lp phase to provide more pore space for guest adsorption and to maximize the guest−framework interactions. Contrary, the heat induced np → lp transition of guest free 1(Zn) is an 1668


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Chemistry of Materials

Figure 2. Powder X-ray diffraction patterns of 1(Co) (purple), 1(Ni) (green), 1(Cu) (blue), and 1(Zn) (gray) of the as-synthesized (left) and activated, dried (right) samples. The abbreviations as and dry denote the as-synthesized and dried states of the material, respectively. The simulated powder pattern of 1(Zn)as is shown as well. The patterns have been normalized and vertically offset for the sake of clarity. Analytical Methods. Elemental analyses were measured on a vario EL instrument provided by Elementar Hanau in the Microanalytical Laboratory of the Department of Analytical Chemistry at the RuhrUniversität Bochum. Liquid-phase NMR (nuclear magnetic resonance) spectra were measured on a Bruker Avance DPX 200 spectrometer (1H, 200 MHz) at 293 K. 1H NMR spectra of digested MOFs were recorded in 0.5 mL DMSO-d6 and 0.05 mL of DCl/D2O (20%). Chemical shifts are given relative to TMS (tetramethylsilane) and are referenced to the solvent signals as internal standards. Infrared spectra were recorded on a Bruker Alpha-P FT-IR situated in a glovebox. For all measurements, the ATR mode (sample stage with a diamond crystal) of the spectrometer was used and measurements with 48 scans were performed. Thermogravimetric analyses (TGAs) were recorded on a Netzsch STA 409 PC TG-DSC apparatus with a heating rate of 5 K/min and the samples were placed in preweighted, clean Al2O3 crucibles. All measurements were performed in a stream of N2 gas with constant flow rates of 20 mL/min. Differential scanning calorimetry (DSC) curves were recorded on the same instrument. Special heating/cooling programs were applied, the samples were heated as close to the decomposition temperature as possible (the decomposition temperature has been determined by TGA) and cooled down to 50 °C again. This cycle was repeated two more times. The DSC curves were analyzed using Proteus Analysis software package. SEM Micrographs of the guest-free MOF powders were recorded using a JEOL JSM 7500F Field Emission Electron Microscope, using an acceleration voltage of 1 kV. The samples were placed on a carbon tape attached to the sample holder. Standard Powder X-ray diffraction (PXRD) measurements were performed on a PanAlytical X’Pert Pro with Cu Kα radiation in Bragg−Brentano geometry with an automatic divergence slit and a position sensitive detector using a continuous scan mode in the range of 2θ = 5−50°. The samples were measured on zero-background silica substrates cut along the (510) plane. For measurements of the as-synthesized samples 1(M)as, the microcrystalline powders were taken straight from the synthesis solution and measured while still slightly wet from the solvent. For measurements of the dried samples 1(M)dry, the substrate was covered with a thin film of grease and the dried samples were distributed on the grease. Additional PXRD data were recorded at beamline I11 of Diamond Light Source (Didcot, UK) with a monochromatic X-ray beam (λ = 0.826952 Å) using the high resolution multianalyzing crystals (MAC) detector. Samples were sealed in glass capillaries before the measurements. In situ PXRD data were measured at beamline BL09 at the DELTA synchrotron facilities in Dortmund with a monochromatic X-ray beam (λ = 0.4596 Å) using a MAR345 image plate detector.59 The in situ PXRD patterns recorded during CO2

adsorption/desorption were measured at 195 K using an in situ cell equipped with a closed cycle helium cryostat.47 The measurement cell was connected to a CO2 gas container and a vacuum pump to adjust the pressure and a manometer was used to measure the set pressure. The variable-temperature PXRD (VT-PXRD) patterns were recorded using an Anton Parr heating stage also at beamline BL09 at DELTA. The stage was sealed with a graphite dome and dynamic vacuum was applied during the measurements. The obtained data were integrated using the program package Fit2D.60 PXRD patterns were fit with the Pawley method61 using the TOPAS Academic software package (version 5). Space groups and starting values for the unit cell parameter refinement were taken from the published crystallographic data of compound 1(Zn).48 Profile shapes were fitted using a Thompson-Cox-Hastings profile.62 N2 and CO2 sorption experiments were performed on 50−60 mg of outgassed samples (130 °C for minimum 3 h in vacuo) using a Belsorb-Max from Microtrac-Bel with optimized protocols and gases of 99.9995% purity in the Laboratory of Industrial Chemistry at the Ruhr-University Bochum.

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RESULTS AND DISCUSSION Synthesis and Solvent-Dependent Phase Behavior. The series of functionalized pillared-layered MOFs 1(Co), 1(Ni), 1(Cu), and 1(Zn) of the general formula M2(BMEbdc)2(dabco), were synthesized according to protocols of the known parent compounds.57,58,63 The obtained microcrystalline powders were dried by solvent exchange (details in the Experimental Section) and their structural integrity and complete activation was proven by PXRD (Figure 2), NMR (Figures S1−S4), IR (Figures S5 and S6) and TGA studies (Figure S7). The crystallites are in a size range that exclude nano- and mesoscopic size effects on the materials properties (see Figure S18).64,65 Except 1(Zn), all as-synthesized (as) materials are intensely colored, predominantly because of d-d transitions or charge transfer bands. Some color change during activation was observed, which we ascribe to solvation effects and to the structural changes upon solvent removal (see below). The PXRD patterns of the different 1(M)as phases suggest that all materials are phase pure and isoreticular (Figure 2). Analyses of the diffraction patterns with the Pawley method61 confirm that 1Co(as, 1(Ni)as and 1(Cu)as crystallize in the tetragonal space group P4/mmm and feature the characteristic 1669


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Chemistry of Materials Table 1. Crystallographic Data of the As-Synthesized lp and Dried np Phases of 1(Zn), 1(Co), 1(Ni), and 1(Cu)a space group a (Å) b (Å) c (Å0 α (deg) β (deg) γ (deg) V (Å3) Z d/D

1(Co)as

1(Co)dry

1(Ni)as

1(Ni)dry

1(Cu)as

1(Cu)dry

1(Zn)as

1(Zn)dry

P4/mmm 10.9757(5) 10.9757(5) 9.5477(14) 90 90 90 1150.2(2) 1 1

C2/m 18.610(6) 10.899(3) 9.508(3) 90 91.83(17) 90 1927.6(11) 2 0.57

P4/mmm 10.92568(17) 10.92568(17) 9.34461(17) 90 90 90 1115.47(4) 1 1

C2/m 18.221(5) 11.483(8) 9.288(5) 90 94.19(4) 90 1938.6(19) 2 0.63

P4/mmm 10.83917(8) 10.83917(8) 9.63535(8) 90 90 90 1132.04(2) 1 1

P21/m 18.681(5) 11.398(4) 9.512(3) 90 92.265(15) 90 2023.7(13) 2 0.61

C2/m 16.72(2) 14.11(3) 9.69(1) 90 91.78(1) 90 2284(6) 2 0.84

P21/m 18.616(2) 10.750(1) 9.643(1) 90 91.156(5) 90 1929(4) 2 0.58

a The data of 1(Zn) have been taken from ref 41 and 48. d/D represents the ratio of the short M2−M2 (d) and the long M2−M2 (D) distance across the pore. (See Figure 1c). The relationship between d and D and the unit cell parameters are illustrated in the Figure S19.

lp structure with a maximized unit cell volume (d/D = 1, Table 1; d = short diagonal across the pore; D = long diagonal across the pore, see Figure 1). 1(Zn)as already features a slightly contracted monoclinic structure with a d/D ratio of only 0.84. The M2-dabco-M2 distance (equal to unit cell parameter c) varies across the series, which originates from slightly different metal ion radii, different M-dabco bond lengths and different M-M distances in the paddlewheel units. The values observed for the 1(M)as materials are in agreement with the c-axis length observed for the related M 2 (bdc) 2 (dabco), M 2 (1,4ndc)2(dabco), and M2(2,6-ndc)2(dabco) parent compounds (see Table S5). Differences in the unit cell volumes of 1(Co)as, 1(Ni)as, and 1(Cu)as can be ascribed to the varying M2-dabcoM2 distances as well as varying M2-carboxylate bond distances. After removal of the DMF guest molecules followed by activation in vacuo at elevated temperature, the PXRD patterns of all four materials 1(M)dry change drastically (Figure 2). As already observed for 1(Zn), shifts of the prominent lowest angle reflection to higher angles suggest a significant contraction of these frameworks upon drying. Interestingly, the patterns of 1(M)dry feature large variations with the utilized metal ion, suggesting different np structures for each material. Pawley refinement of the unit cell parameters against the experimental diffraction patterns clearly manifests the structural differences of the individual 1(M)dry materials (Table 1). The d/D ratios are the smallest for 1(Co)dry (0.57) followed by 1(Zn)dry (0.58). For 1(Cu)dry a ratio of 0.61 is found, and the highest ratio was found for 1(Ni)dry, amounting to 0.63. The volume change per M2(BMEbdc)2(dabco) unit is depicted in Figure 3. The strongest change was found for 1(Co) with −16.2%. 1(Zn) showed a cell volume change of −15.6% upon drying and 1(Ni) of −13.1%. Interestingly, 1(Cu) displays the lowest volume reduction of only −10.6%, even though the d/D ratios suggest that 1(Ni) should exhibit the least pronounced volume contraction. This is attributed to the significant increase of β from 90° to 94.19° when going from 1(Ni)as to 1(Ni)dry, which also contributes to the volume decrease of 1(Ni). In order to prove the reversibility of the solvent induced phase transition, PXRD patterns of the 1(M)dry species were recorded after immersing them in DMF. The patterns are shown in the Figure S13 and confirm full reversibility of the phase transitions. We suppose that orbital directing effects on the basis of the different electron configurations of the metal ions (3d7 for Co2+, 3d8 for Ni2+, 3d9 for Cu2+, and 3d10 for Zn2+) are the key factor to understand the distinct phase behaviors of the four isoreticular compounds studied here. Naturally, the transition

Figure 3. Depiction of the reduced cell volume changes after the materials transitioned from the as-synthesized (lp) phase to the evacuated (np) phase. The reduced cell volumes correspond to the volume per M2(BME-bdc)2(dabco) formula unit.

from the lp to the np phase requires a distortion of the coordination sphere from the ideal square pyramidal geometry. The energy required for this distortion is balanced by the more attractive dispersion and dipolar interactions between the organic building units in the np phase. However, a distortion of the coordination environment of the metal ion is more favorable for the closed shell Zn2+ system (3d10) compared to the open shell Cu2+ system (3d9).66 Consequently, the np phase of 1(Cu) is less contracted than the np phase of 1(Zn), because the dipolar and dispersion interactions only allow for a minor distortion of the stiffer Cu2+ coordination sphere. Our data further suggest that 1(Co) is as tolerant to distortions as 1(Zn), while 1(Ni) lies in between 1(Zn)/1(Co) and 1(Cu). A comprehensive understanding of the influence of the electronic structure of the metal center on the framework flexibility of the M2(BME-bdc)2(dabco) materials, however, can only be obtained by sophisticated computational modeling of the materials and their responsive behavior. Such an endeavor is beyond the scope of this work and will be addressed in future studies. Sorption Properties. The sorption properties of the activated materials 1(M)dry were characterized using N2 and CO2 physisorption at 77 and 195 K, respectively. In previous studies on related functionalized pillared-layered frameworks41 it was observed that the implementation of side chains at the fubdc2− linker induced a high selectivity toward CO2 adsorption 1670


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Figure 4. CO2 physisorption isotherms measured at 195 K on materials 1(Co) (purple squares), 1(Ni) (green circles), 1(Cu) (blue triangles), and 1(Zn) (gray diamonds). (a) Linear pressure axis, (b) logarithmic pressure axis to highlight the low-pressure behavior. Filled symbols represent adsorption, empty symbols desorption. Lines are a guide to the eye.

Figure 5. Progression of the reduced cell volume and the d/D ratio calculated from in situ PXRD data. The data were recorded during adsorption at 195 K as a function of CO2 pressure for materials 1(Co) (purple), 1(Ni) (green), 1(Cu) (blue), and 1(Zn) (gray). Large, swelling, intermediate and narrow pores are abbreviated by lp, sp, ip, and np, respectively. The lines only represent a guide to the eye. The data of 1(Zn) have been taken from ref 47.

over N2. From the N2 physisorption isotherms at 77 K (Figure S14) we see that all four derivatives barely adsorb any N2 and are essentially nonporous toward N2. This is different for CO2 at 195 K (Figure 4). All materials show significant CO2 uptake, reaching saturation values at p = 1000 mbar of 80.2, 79.4, 90.2, and 89.3 cm3/g for 1(Zn), 1(Co), 1(Ni), and 1(Cu), respectively. Importantly, a stepped isotherm is observed for CO2 sorption in 1(Zn) and 1(Cu). The steps at 40 mbar (for 1(Cu)) and 250 mbar (for 1(Zn)) are attributed to the np → lp phase transition during CO2 uptake.47 The isotherms of 1(Co) and 1(Ni), however, feature a substantially different shape and do not show distinct steps. Instead, 1(Ni) and 1(Co) display a fairly smooth increase in uptake over a very wide pressure range and the slope of the isotherm is much less steep. Nevertheless, the uptake at saturation (p(CO2) ∼ 1000 mbar) is similar for all four compounds, suggesting that 1(Co) and 1(Ni) must also be present in the lp phase at saturation, to

provide the larger pore space needed to host the CO2 guest molecules. The np → lp phase transition, however, must follow a different mechanism for 1(Co) and 1(Ni) than for 1(Cu) and 1(Zn). Upon CO2 desorption, all four materials show a hysteretic behavior, which again points toward a flexible adsorbent. 1(Zn) shows a step in the desorption isotherm, ascribed to inverse lp → np transition starting at 0.24 bar, which finishes at 0.11 bar. On the contrary, the lp → np transition cannot be detected for 1(Cu) because CO2 remains in the lp phase down to the minimum pressure of the desorption experiment (∼0.02 bar); however, the shape of the desorption isotherm gives rise to the assumption, that the transition to the np phase will occur close to this pressure. For 1(Co) and 1(Ni) again no sharp steps are apparent during desorption, rather a fairly gradual decrease of the adsorbed CO2 is visible, but again with a significant hysteresis compared to the adsorption branch of the isotherms. 1671


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Pawley analysis clearly shows that the lattice parameters of the np phase continuously change with increasing CO2 pressure until a certain threshold pressure (800 mbar for 1(Co)dry and 400 mbar for 1(Ni)dry) at which new peaks belonging to the lp phase appear. This swelling behavior is also clearly visible in the progression of the d/D ratio with increasing CO2 pressure (see Figure 5). To emphasize the swelling behavior of the np phase of these two materials, we denoted this continuously expanding phase swelling pore (sp) phase hereafter. Noteworthy, no ip phase is identified in these materials. In the case of 1(Co)dry, the sp phase expands gradually by 8.05% in the range from 0 to 800 mbar CO2 pressure (on average 1% expansion per 100 mbar CO2 pressure). For 1(Ni)dry a similar gradual swelling of 4.4% is observed in the range from 0 to 400 mbar CO2 pressure (on average 1.1% expansion per 100 mbar CO2). After reaching the threshold pressure (400 mbar for 1(Ni) and 800 mbar for 1(Co)) the lp phase appears in the diffraction patterns. Interestingly, from this point on neither the cell parameters for the sp nor for the lp phase change significantly, just the Bragg intensities of the lp phase increase, while the intensities of the expanded sp phase decrease. This demonstrates that the expanded sp phase directly transforms to the lp phase without forming a transient ip phase. After reaching 1000 mbar in both cases, the lp phase is present as a single phase and the phase transition is complete. The in situ X-ray diffraction experiments show a similar trend during the desorption process. In the case of 1(Co)dry and 1(Ni)dry the lp phase is the dominant phase until a pressure of 100 mbar and 200 mbar is reached, respectively. For material 1(Cu)dry, it is necessary to reach 0 mbar CO2 pressure to return to the np phase. Figure 6 shows a schematic depiction of the two distinct processes observed for the CO2 adsorption induced phase

Based on these data we propose different transition mechanisms between the np and lp phases, depending on nature the metal ion of the material. Compounds 1(Zn) and 1(Cu) feature a sharp, switching-like transition from one distinct phase to the other, whereas compounds 1(Ni) and 1(Co) likely show a continuous swelling process. To prove our assumptions, PXRD data were collected during CO2 adsorption at beamline BL9 of the synchrotron facility DELTA (Dortmund, Germany). The activated samples were placed on a sample holder equipped with a cryostat and diffraction patterns were recorded at 195 K, whereas the samples were exposed to a CO2 atmosphere of controllable pressure from p = 0 mbar to p = 1000 mbar. The in situ PXRD patterns for materials 1(Co)dry, 1(Ni)dry, and 1(Cu)dry can be found in the Figure S11. Measurements on 1(Zn)dry have been reported elsewhere.47 Note that the phase transition pressures determined by the volumetric gas adsorption studies (see above) are not directly comparable to the transition pressures seen in the in situ PXRD measurements, because a dynamic setup using a CO2 gas flow of controlled pressure and very short equilibration times (few minutes) were used for the in situ PXRD measurements. On the other hand, a static setup with much longer equilibration times (minutes to hours) was used for the volumetric gas physisorption measurements. The in situ PXRD data support our previous assumptions drawn from the CO2 physisorption studies. 1(Zn)dry and 1(Cu)dry show three distinct phases during CO2 adsorption: The already discussed np and lp phases and an additional intermediate pore (ip) phase. At the beginning of the CO2 sorption experiment, both materials are present in their np phases. Upon reaching a certain threshold CO2 pressure (100 mbar for 1(Cu)dry and 400 mbar for 1(Zn)dry), two new sets of reflections appear, which belong to the lp phase and a distinct ip phase. The ip phases feature unit cell volumes and d/D ratios, which lie between the values of the respective np and lp phases of these materials (Figure 5). In the case of 1(Cu)dry, the np phase disappears at a CO2 pressure of 200 mbar. From 200 to 400 mbar the ip and lp phases are present in parallel and at 500 mbar the transition process is completed and only the lp phase remains until saturation at 1000 mbar. For 1(Zn)dry the np, ip, and lp phase are present simultaneously in the diffraction patterns from 400 to 900 mbar. After reaching 1000 mbar, the transition of the materials is complete and solely the lp phase is present. Noticeably, the ip phase of 1(Cu)dry and 1(Zn)dry is never present as a single phase (neither during adsorption nor during desorption) and the reflections of this phase are of much lower intensity than the reflections of the dominant np or lp phases. In a previous study on 1(Zn)dry we could prove that the ip phase is only a transient intermediate, which occurs during the transition from the np to the lp phase.47 An additional step for a hypothetical filling of the ip phase is not observed in the isothermal gas adsorption measurements. We note that the unit cell parameters and d/D ratios are almost constant for the individual np, ip, and lp phases of 1(Cu)dry and 1(Zn)dry, proving that the np → lp phase transition proceeds via a distinct switching from np to ip to lp. A very different phase behavior is observed for 1(Co)dry and 1(Ni)dry. For these two materials the Bragg peaks corresponding to the np phase shift continuously with increasing CO2 pressure, conforming a continuous expansion (i.e., swelling) of the np phase, rather than a discrete switching from np to ip to lp.

Figure 6. Schematic depiction of the different phase transition processes upon CO2 adsorption in materials 1(Cu)dry and 1(Zn)dry in contrast to materials 1(Co)dry and 1(Ni)dry. Large, swelling, intermediate, and narrow pores are abbreviated by lp, sp, ip, and np, respectively.

transitions in these compounds. For 1(Ni)dry and 1(Co)dry a gradual swelling of the pore (sp) is observed followed by a transformation to the lp phase at a certain threshold pressure. On the contrary, 1(Cu)dry and 1(Zn)dry show a distinct switching between three different states: np, ip, and lp. Interestingly, for these compounds the ip phase is never present by itself but only in parallel with at least one of the two other phases. This fact strongly suggests that the ip phase is 1672


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Chemistry of Materials only a kinetic intermediate and not a thermodynamically stable phase. Similar to the variations in their solvent-dependent phase behavior, the striking differences in the CO2 adsorption behavior are consequences of the electron configuration of the applied metal ion and the electronic structure of the corresponding flexible MOFs. Thermoresponsivity. As previously reported, 1(Zn)dry as well as some mixed linker derivatives undergo the np → lp phase transition also by heating in the absence of guest adsorption/desorption.46,48 To analyze if the other 1(M)dry frameworks, containing different metal centers, also possess this rare property,35,67 we conducted TG-DSC experiments and variable-temperature (VT) PXRD measurements. In Figure 7

Table 2. Summary of the Thermodynamic Data for Materials 1(Co)dry, 1(Ni)dry, 1(Cu)dry, and 1(Zn)dry Obtained from DSC (Cycle 2)a Tnp→lp (°C) ΔHnp→lp (kJ mol−1) ΔSnp→lp (J K−1 mol−1 Tlp→np (°C) ΔHlp→np (kJ mol−1) ΔSlp→np (J K−1 mol−1)

1(Co)dry

1(Ni)dry

1(Cu)dry

1(Zn)dry

219 16.4 33.3 213 −15.4 −31.7

203 10.3 21.6 190 −11.4 −24.6

151 7.2 17.0 126 −8.3 −20.8

216 14.2 29.0 204 −15.3 −32.1

T, ΔH, and ΔS are the phase transition temperature, enthalpy, and entropy, respectively, for the pore opening (np → lp) and contraction (lp → np). a

temperature of 203 °C was found. Finally, with 151 °C the temperature for the np → lp phase transition of 1(Cu)dry is significantly lower than for all other materials under study here. Accordingly, the phase transition enthalpies (determined via peak integration) vary depending on the chosen metal center, with the same trend as observed for the temperatures. A transition enthalpy of ΔHnp→lp = 14.2 kJ mol−1 was determined for 1(Zn)dry, similar to 1(Co)dry with ΔHnp→lp = 16.4 kJ mol−1. ΔHnp→lp is lower for 1(Ni)dry and amounts to 10.3 kJ mol−1. Compound 1(Cu)dry differs significantly from the other materials, with a much lower ΔHnp→lp of 7.2 kJ mol−1. We ascribe these differences in phase transition temperatures and their corresponding enthalpies to the different magnitudes of contraction of the individual 1(M)dry compounds. Further, the stronger contraction of 1(Co)dry and 1(Zn)dry implies the presence of stronger dipolar and dispersion interactions in the np state. In turn, this gives rise to a higher energy difference between the np and the lp phase. The energy difference is directly visible in higher transition temperatures and larger transition enthalpies for the more contracted materials. From the phase transition enthalpies and the phase transition temperatures, the corresponding phase transition entropies were calculated (see Table 2). Naturally, the np → lp phase transition is associated with a gain in entropy primarily due to an enhanced movement of the side chains in the more spacious lp phase. Consequently, entropy is the driving force for the temperature-induced np → lp phase transitions in these 1(M)dry materials, whereas the adsorption enthalpy of CO2 is the driving force for the related phase transitions during gas adsorption. A hysteretic effect for the reverse lp → np transition is evident from the DSC curves. The transition temperatures Tlp → np are substantially lower than Tnp → lp (see Table 2). The enthalpies ΔHlp → np, however, match very well to ΔHnp → lp, confirming a largely reversible process. Importantly, supplementary VT-PXRD measurements for compounds 1(Co)dry, 1(Ni)dry and 1(Cu)dry (see Figure S12) confirm that the materials undergo fully reversible np → lp → np phase transitions with a hysteresis during the heating−cooling cycle. A kinetic intermediate phase (i.e., an ip phase as observed during CO2 adsorption in 1(Cu)dry and 1(Zn)dry) is not observed in the VT-PXRD experiment, which is in accordance with faster transition kinetics at elevated temperature.

Figure 7. Differential scanning calorimetry (DSC) traces of materials 1(Co)dry (purple), 1(Ni)dry (green), 1(Cu)dry (blue), and 1(Zn)dry (gray). The top branch represents the heating and the bottom branch the cooling curve. Three consecutive heating/cooling cycles were performed. Only the second cycle is shown here. The curves have been background-corrected for clarity.

the DSC traces of all four materials are displayed. Three heating−cooling cycles were measured to investigate the reversibility of the temperature induced phase transitions. The first and the second cycles slightly differed (most likely due to partial adsorption of ambient water from the air during sample preparation, which is released during the first heating cycle), however, the second and third cycles revealed almost identical results for 1(Co)dry and 1(Ni)dry (see Figures S15 and S16). In the case of 1(Cu)dry, a systematic decrease in the peak intensity and the peak onset temperature is apparent in the second and third cycles. TGA data reveal significantly lower decomposition temperature for 1(Cu)dry (∼270 °C) than for the other 1(M)dry materials (decomposition between 320 and 350 °C; see Figure S7). Hence, we attribute the loss in peak area over the course of three heating/cooling cycles to a partial thermal decomposition of 1(Cu)dry. Similar to 1(Zn)dry, the other 1(M)dry materials display an endothermic peak for the np → lp phase transition on the heating branch and an exothermic peak for the corresponding lp → np transition on the cooling branch of the DSC trace. The transition temperature was identified by the onset of the DSC signal (see Table 2). For 1(Co)dry and 1(Zn)dry similar transition temperatures Tnp→lp were found, amounting to 219 and 216 °C, respectively. For 1(Ni)dry, a slightly lower

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CONCLUSION In this study, we investigated transitions between narrow and large pore phases in a series of pillared-layered M2(BMEbdc)2(dabco) MOFs as a function of different stimuli, namely 1673


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Wharmby and Dr. Sneha R. Bajpe for their help with the synchrotron PXRD experiments at DLS. The authors acknowledge Mrs. Susanne Buse for measuring gas adsorption isotherms.

solvent and gas sorption as well as temperature. Strikingly, we found fundamentally different phase behaviors depending on the incorporated metal center. The magnitude of pore contraction upon desolvation depends on the metal center and decreases in the sequence 1(Co) ≈ 1(Zn) > 1(Ni) > 1(Cu). Interestingly, a similar trend is observed for the thermally induced lp → np transition of the guest-free samples. The phase transition enthalpy, as well as the transition temperature decrease in the same sequence 1(Co) ≈ 1(Zn) > 1(Ni) > 1(Cu). Furthermore, isothermal gas sorption and in situ X-ray diffraction under variable CO2 pressure reveal different phase transition mechanisms as a function of the metal center. 1(Cu) and 1(Zn) switch between distinct narrow, intermediate, and large pore phases, whereas 1(Co) and 1(Ni) first show a gradual swelling of the narrow pore phase, followed by a transition to the large pore phase. Future studies will focus on the preparation of mixed metal MOFs which might allow for precise tuning of the transition temperatures, pressures and energies, which previously proved useful for other MOF systems.51,52 Additionally, we aim to prepare thin film devices of these flexible MOFs, which could be useful for sensing applications,68 where the signal transduction cascade is initiated by the guest-/temperature-induced volume change or pore transformation.69

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05052. 1 H NMR spectra of digested MOFs, IR spectra, TGA curves, Pawley refinements of PXRD patterns, in situ PXRD patterns, VT-PXRD patterns, SEM, and further information (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Suttipong Wannapaiboon: 0000-0002-6765-9809 Roland A. Fischer: 0000-0002-7532-5286 Present Address ⊥

A.S. is currently at Sandia National Laboratories, Livermore, CA 94551-0696, United States Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by DFG research unit FOR 2433 (MOF Switches). A.S. gratefully acknowledges the research cluster SusChemSys for a doctoral fellowship. The project “Sustainable Chemical Synthesis (SusChemSys)” is cofinanced by the European Regional Development Fund (ERDF) and the state of North Rhine-Westphalia, Germany, under the Operational Programme “Regional Competitiveness and Employment” 2007−2013. S.H. acknowledges support by the Alexander von Humboldt Foundation. The authors thank DELTA Dortmund for allocation of beamtime at beamline BL9 as well as the Diamond Light Source (visit EE9225) for powder X-ray diffraction experiments. Additional thanks go to Dr. Michael T. 1674


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