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Multiple Coordination Exchange for Room-Temperature Activation of Open-Metal Sites in Metal-Organic Frameworks Jinhee Bae, Jae Sun Choi, Sunhyun Hwang, Won Seok Yun, Dahae Song, Jae Dong Lee, and Nak Cheon Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07299 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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ACS Applied Materials & Interfaces

Multiple Coordination Exchange for Room-Temperature Activation of OpenMetal Sites in Metal-Organic Frameworks Jinhee Bae, Jae Sun Choi, Sunhyun Hwang, Won Seok Yun, Dahae Song, JaeDong Lee, and Nak Cheon Jeong* Department of Emerging Materials Science, DGIST, Daegu 42988, Korea

KEYWORDS: Metal-organic frameworks, open-metal sites, chemical activation, room-temperature activation, dichloromethane treatment, multiple coordination exchange, In-situ NMR, In-situ Raman

ABSTRACT: The activation of open coordination sites (OCSs) in metal-organic frameworks (MOFs), i.e., the removal of solvent molecules coordinated at the OCSs, is an essential step that is required prior to the use of MOFs in potential applications such as gas chemisorption, separation, and catalysis because OCSs often serve as key sites in these applications. Recently, we developed a “chemical activation” method involving dichloromethane (DCM) treatment at room temperature, which is considered to be a promising alternative to conventional thermal activation (TA), because it does not require the application of external thermal energy, thereby preserving the structural integrity of the MOFs. However, strongly coordinating solvents such as N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), and dimethyl sulfoxide (DMSO) are difficult to remove solely with the DCM treatment. In this report, we demonstrate a multiple coordination exchange (CE) process executed initially with acetonitrile (MeCN), methanol (MeOH), or ethanol (EtOH) and subsequently with DCM to achieve the complete activation of OCSs that possess strong extracoordination. Thus, this process can serve as an effective “chemical route” to activation at room temperature that does not require applying heat. To the best of our knowledge, no previous study has demonstrated the activation of OCSs using this multiple CE process, although MeOH and/or DCM has been popularly used in pretreatment steps prior to TA process. Using MOF-74(Ni), we demonstrate that this multiple CE process can safely activate a thermally unstable MOF without inflicting structural damage. Furthermore, on the basis of in situ 1H nuclear magnetic resonance (1H NMR) and Raman studies, we propose a plausible mechanism for the activation behavior of multiple CE.

INTRODUCTION Open coordination sites (OCSs), coordinatively unsaturated sites at an atom typically where Lewis base (LB) molecules can ligate with coordination bonding, are broadly observed in not only molecular substances1 but also non-molecular substances.2-6 The OCSs in these substances have been demonstrated to play substantial roles in both the structural stability and chemical functions of these compounds. Metal-organic frameworks (MOFs), an exciting class of crystalline non-molecular substances that are constructed of self-assembly by multiple links of coordination between inorganic nodes and rigid multitopic organic linkers, are also good examples that possess OCSs at their metal nodes. Interestingly, the “extracoordination” ability of OCSs in MOFs plays a substantial role in their potential applications, such as chemical separation,3,7-9 gas storage,2-3,10-19 heterogeneous catalysis,3,20-31 sensing,32-35 electronic conduction,36-40 and ion conduction,3,41-43 among others. To utilize the OCSs in MOFs for the aforementioned applications, an activation process to remove both precoordinated molecules (typically the solvents used during the MOF synthesis) at the OCSs and pore-filling guest molecules in the pores is ACS Paragon Plus Environment 1


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required prior to using the MOFs. To date, several methods44 for the activation of MOFs have been developed: thermal activation (TA),45-46 solvent treatment,47-48 freeze-drying,49 supercritical carbon dioxide (CO2) exchange,48 and acid treatment.21,50-51 Among these methods, TA, which is typically performed by applying heat and vacuum, has been commonly used because supply of external thermal energy to exceed the bond dissociation and solvent vaporization energies is the most straightforward way for the removal of coordinating and pore-filling solvent molecules. However, often characterized by the negative influence of the TA process on the structural integrity of MOFs,48,52-54 TA method has evolved by combining it with the aforementioned solvent treatment method because replacement of pore-filling solvent with a lower boiling point solvent, e.g., dichloromethane (CH2Cl2, hereafter DCM), can aid in lowering the activation temperature in the TA process and thereby minimizing the potential structural damage.47 Thus, the idea for the use of DCM was confined to the removal of pore-filling solvents until a secondary function of DCM, i.e., scissoring the solvent coordination at the OCSs, was discovered.55 Recently, we communicated the discovery of the scissoring function of DCM. We demonstrated that coordinated methanol (MeOH), ethanol (EtOH), and acetonitrile (MeCN) molecules in MOFs could be removed solely by DCM treatment at room temperature without further TA process. In our previous study, the following two-step reaction mechanism was proposed: (i) coordination exchange (CE) of precoordinated solvent molecules with DCM, which can be bound by weak coordination of chlorine atoms through lone-pair electrons, and (ii) subsequent, spontaneous dissociation of the weak DCM coordination with low activation energy, which corresponds to the thermal energy at room temperature. (Hereafter, we refer to this process as “chemical activation” to describe a chemical method to activate OCSs.) Although those moderate-strength coordinations (i.e., with MeOH, EtOH, and MeCN) were readily removed by DCM treatment at room temperature, strong coordinations (i.e., with N,Ndimethylformamide (DMF), N,N-diethylformamide (DEF), and dimethyl sulfoxide (DMSO)) were difficult to dissociate solely with the DCM treatment. Here, we report “multiple coordination exchanges” that functions as an effective chemical activation of OCSs. The process was performed by the stepwise CE of strong-to-moderate coordinations followed by moderate-to-weak coordinations. As described below, soaking DMF-coordinated HKUST-1 (DMF-HK; HK = fully desolvated HKUST-1) in fresh MeCN for only 10 min at room temperature and subsequent repetition of the process several times leads to the complete exchange of DMF-coordination with MeCN-coordination. Subsequent soaking the MeCN-exchanged HK (hereafter MeCN-DMF-HK) in fresh DCM for 10 min at room temperature and repeating the process several times also leads to the complete removal of the coordinated MeCN at the OCSs. It is surprising that although MeOH and/or DCM have been popularly used in pretreatment steps prior to TA process to support the permanent porosity of MOFs by replacing pore-filling solvents with lower boiling point solvents,41,54,56-57 to the best of our knowledge, there has been no systematic study of ACS Paragon Plus Environment 2

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the function of stepwise “multiple CE”. Although several MOFs that possess OCSs should be suitable for this demonstration, we limited our studies to HKUST-1, MOF-74(Cu), and MOF-74(Ni). To our surprise, while the TA process at temperatures equal to or higher than 200 °C led to the complete collapse of MOF-74(Ni), the multiple CE allowed for complete activation of the MOF without structural damage. Furthermore, we prove the expandability of multiple CE. We demonstrate that MeOH and EtOH are also suitable reagents for the initial CE before moving forward to the next DCM treatment step. Additionally, we demonstrate that strongly coordinated DEF and DMSO molecules in HKUST-1 (hereafter DEF-HK and DMSO-HK) can be completely removed by multiple CE. In situ 1H nuclear magnetic resonance (1H NMR) spectroscopic analysis provides a direct observation of the CE of DMF with MeCN and the subsequent removal of coordinated MeCN by DCM. Finally, in situ Raman studies allowed us to suggest a plausible mechanism for multiple CE, providing compelling evidence for the coordination and dissociation of solvent molecules at the (CuII)2 center.

RESULTS AND DISCUSSION HKUST-1, a MOF that is constructed of three-dimensional links of paddle-wheel-like coordination between two Cu2+ ions and four 1,3,5-benzenetricarboxylate (BTC3-) ligands, is a good example of MOF that possesses a high concentration of OCSs. Our previous studies demonstrated the chemical activation function of DCM, using HKUST-1.55 However, strongly coordinated solvent molecules, such as DMF, were difficult to be completely removed by the activation with DCM (see Figure 1). DMF-HK was prepared by using a two-step process: the initial removal of precoordinated H2O and EtOH in the pristine HKUST-1 via TA at 150 °C under vacuum and the subsequent coordination of DMF via solvent treatment with pure DMF under moisture-free conditions (see Supporting Information, Section S1 for details). The direct DCM treatment of DMF-HK was examined by soaking the DMF-HK powder in pure DCM at room temperature for 10 min. Although this procedure was repeated 30 times using fresh DCM, the DMF-coordination bonds were not completely removed. The 1H NMR spectra, which were obtained after the samples were dissolved in deuterated sulfuric acid (D2SO4), indicate that DMF remained in the MOF even after 30 repetitions of the DCM treatment (see Section S2). With this observation, one might presume that a unique satisfactory method for the complete removal of strongly coordinated solvent molecules would be the TA. However, heating the MOF to exceed a critical temperature can be a highly risky way because structural damage or collapse of the MOF can occur. For example, we observed that the complete activation of DMF-HK required thermal energy corresponding to a temperature of 240 °C. However, when the sample was treated at a temperature slightly higher than 240 °C, the framework was seriously damaged, forming metallic copper (see Section S3). Thus, we conceived that stepwise multiple CE could be an effective and safe alternative method. We postulated that the replacement of strong ACS Paragon Plus Environment 3


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coordination with less strong or modest coordination would eventually result in the removal of the modest coordination by DCM treatment, as demonstrated in our previous study.55 Thus, if the initial exchange is feasible, multiple CE will be an efficient chemical method for the activation of OCSs in the absence of heat supply. A color change of DMF-HK was observed after sequential treatments of MeCN and DCM at room temperature, which provided initial evidence that the above postulate was successful because the color can be influenced by the coordination environment around the Cu2+ centers. The color of DMF-HK is slightly greenish blue. However, the color changed to sky blue after the MeCN treatment, and the sky blue color further turned to deep navy blue after DCM treatment (see optical microscopy images and ultraviolet-visible (UV-vis) spectra in Figure 2a). Notably, the color of DCM-MeCN-DMF-HK was the same as that of thermally activated pristine HKUST-1 (TA-HK), which certainly has no ligated solvent molecules. The UV-vis absorption of HKUST-1 is the results of ligand-to-metal charge transfer (LMCT) and dd transitions around the Cu2+ centers.58 The absorption band at energies greater than 2.5 eV (less than ca. 500 nm in wavelength) originate in the LMCT from the oxygen atoms in the carboxylate ligands to the Cu2+ ions, and the absorption band at energies less than 2.5 eV (greater than ca. 500 nm in wavelength) originate in d-d transition around the Cu2+ centers.58 The observed color changes are due to the shift of the d-d transition band. Figure 2a shows that the highest energy edge (inflection point) of the d-d transition band in DMF-HK is positioned ca. 565 nm. However, the edge was blueshifted to 547 nm after MeCN treatment, and the edge was further blueshifted to 455 nm after DCM treatment (see more information on the UV-vis absorption spectra of various solvent-coordinated HKUST-1 samples in Section S4). The band shift reflects the change in the chemical environment around the Cu2+ ions (ligand coordination or coordination-free state). More specifically, we speculate that the blueshift is a result of a partial decrease in the number of electrons and a loss of degeneracy in the d orbitals level rising from the changes in the solvent ligation and geometry around the Cu2+ centers during the CEs.58-59 More notably, the absorption spectrum of DCM20-DMF-HK (superscript numbers represent the repetitions of the treatment with a designated solvent), which was obtained after DMF-HK was directly treated with DCM 20 times, strikingly differs from that of TA-HK; however, the absorption spectrum of DCM-MeCN-DMF-HK is identical to that of TA-HK (see Figure 2). Based on this observation, we tentatively concluded that the sequential solvent treatments with MeCN and DCM ultimately led to the complete removal of solvent coordination, and that conceivable intermediate states with MeCN and DCM coordination are temporarily formed during the stepwise process (more information about these intermediate states is provided in the discussion of the 1H NMR and Raman studies presented below). To provide concrete evidence for the function of the multiple CE, we employed 1H NMR spectroscopic analysis. For a systematic study, we treated DMF-HK with MeCN, MeOH, and EtOH ACS Paragon Plus Environment 4

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(MeCN-DMF-HK, MeOH-DMF-HK, and EtOH-DMF-HK, respectively) then further treated these MeCN-DMF-HK, MeOH-DMF-HK, and EtOH-DMF-HK samples with DCM (DCM-MeCN-DMF-HK, DCM-MeOH-DMF-HK, and DCM-EtOH-DMF-HK, respectively). Each solvent treatment was performed by soaking each powder sample in pure solvent for 10 min at room temperature, and this process was repeated several times, monitoring the 1H NMR spectra at each step. Figure 3a shows the 1

H NMR spectra of DMF-HK, MeCN4-DMF-HK, and DCM5-MeCN4-DMF-HK, which were taken after

the samples were vacuum-treated at room temperature only to evacuate pore-filling solvent and subsequently dissolved in D2SO4.55 The peak for the three identical protons in BTC appears at 8.8 ppm, and the peaks for the two sets of the three identical CH3 protons and a formyl proton in DMF appear at ca. 2.8, 2.9, and 7.7 ppm, respectively. The peak for the three identical CH3 protons in MeCN appears at 2.1 ppm. As expected, while the intensity of the peak for DMF in the DMF-HK sample gradually decreased, the intensity of the peak for MeCN simultaneously increased during the MeCN treatment (see 1H NMR spectra presented in Section S5). Eventually, we observed the complete disappearance of the peaks for DMF after four repetitions of the MeCN treatment. The integral value of the MeCN peak reached the theoretical maximum (HBTC:HMeCN = 1:1.5). As also expected, the MeCN peak intensity gradually decreased during the DCM treatment and completely disappeared after five repetitions of the treatment. We confirmed this coordination exchange behavior using thermogravimetric analysis (see Section S6). We also observed that this behavior was generic for treatments with MeOH and EtOH instead of MeCN (see Figure 3b and 3c, and Section S5). The peak for the three identical CH3 protons in MeOH appears at 3.7 ppm, and the peaks for the three identical CH3 protons and the two identical CH2 protons in EtOH appear at 1.1 and 4.1 ppm, respectively. The 1H NMR spectra of the MeOH3-DMF-HK, EtOH9-DMF-HK, DCM9-MeOH3-DMF-HK, and DCM5-EtOH9-DMF-HK samples indicated that repetitive MeOH and EtOH treatments completely replaced the DMF coordination with MeOH and EtOH coordinations, respectively, and the following repetitive DCM treatments completely removed both MeOH and EtOH coordinations. We postulated that the removal rate of DMF from DMF-HK would correlate with the CE rate of MeCN, MeOH, and EtOH. With this postulation in mind, we plotted the amount of remaining DMF in DMF-HK at each step (see Section S5). The plot indicates that the CE rate decreased in the following order: MeOH > MeCN > EtOH. We also plotted the remaining amounts of MeCN, MeOH, and EtOH in the MeCN-HK, MeOH-HK, and EtOH-HK samples, respectively, during the DCM treatment to estimate the CE rates of DCM for those solvents. The results show that the rates are approximately inverse to the rates shown above: EtOH ≅ MeCN > MeOH. These observations allow us to estimate the “resistivity of CE (RCE)”, which is an expression of the CE rate, by combining (i) the coordination exchangeability influenced by the relative coordination strengths between the precoordinated and postcoordinated (guest) molecules, (ii) the number of guest molecules containable in the nanosized open cage, (iii) the ACS Paragon Plus Environment 5


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molecular motion and speed of the guest molecules, and (iv) the temperature used in the (guest) solvent treatments. Based on these parameters, we further speculate that the RCEs of the solvents decrease in the following order: DMF > MeOH > MeCN > EtOH > DCM. To provide more concrete evidence, we designed in situ 1H NMR experiments conducted in NMR tubes containing both the target HKUST-1 sample and deuterated solvent. More specifically, the DMFHK crystals were immersed in deuterated MeCN solvent (i.e., CD3CN). Then, the amount of DMF dissolved in the CD3CN solvent, which was dissociated from the DMF-HK crystals by CE with CD3CN, was monitored every 5 min (see Figure 4a). The quantity of the dissolved DMF was calibrated with an internal standard, CD2HCN, which was included in a commercial CD3CN solvent as an impurity. The peaks for DMF were initially absent in the CD3CN solvent but then increased as the exposure time increased (see Figure 4b). The integral value of the DMF peaks plateaued after 25 min, indicating the termination of the CE reaction. The termination of the CE reaction was also confirmed by analyzing the amount (ca. 1.8%) of DMF remaining in the crystals by taking an 1H NMR spectrum after the crystals were dissolved in D2SO4 (see the inset spectrum presented in Figure 4d). We also observed a similar pattern for the in situ CD2Cl2 treatment of the MeCN-HK crystals. Initially, the peak for MeCN was absent. However, the peak rapidly grew and eventually reached a plateau after 15 min (see Figure 4c and 4e). An NMR spectrum obtained from the reaction-terminated MeCN-HK crystals indicated that the amount of MeCN remaining in the crystal was only ca. 4.0% (see inset in Figure 4e). Similar patterns were also observed for the other in situ experiments which were performed with MeOD-d4 and EtOD-d6 (see Section S7). Thus, the in situ 1H NMR results provide compelling evidence for the superior performance of multiple CE in terms of the removal of strongly coordinating solvents at the OCSs. The phase purities of the DMF-HK, MeCN-DMF-HK, MeOH-DMF-HK, and EtOH-DMF-HK samples before and after the DCM treatments were determined via powder X-ray diffraction (PXRD) measurements (see Figure 3). The PXRD patterns indicated that the structural integrity of the MOF was well preserved even after strongly coordinated DMF was completely removed. To confirm this structural integrity, we also tested the N2 isotherms of DMF-HK, MeCN-DMF-HK, and DCM-MeCNDMF-HK. Whereas the DMF-HK and MeCN-DMF-HK samples were pretreated with TA at 150 °C prior to the isotherm measurements, the DCM-MeCN-DMF-HK sample was pretreated only by applying a vacuum at room temperature (see Sections S1 and S8 for details). The results show that, while the internal surface area of DMF-HK was only approximately 1406 m2·g−1, the internal surface area of MeCN-DMF-HK was 1863 m2·g−1, which indicates that the thermal energy at 150 °C is not sufficient to completely remove the pore-filling and/or coordinating DMF molecules. However, although DCM-MeCN-DMF-HK was only vacuum treated at room temperature, the internal surface

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area of the sample was 1840 m2·g−1. Thus, these PXRD and isotherm results provide indisputable evidence for the safety of multiple CE strategy in terms of structural integrity. In our previous report, we demonstrated that the presence or absence of coordinated solvent was reflected in the vibrational mode of Cu–Cu bonding.55 Therefore, we hypothesized that the type of coordinated solvent would also be reflected in this vibration mode. To test this hypothesis, we monitored the Raman spectra of several HKUST-1 samples in which the Cu2+ centers were coordinated with DMF, MeCN, MeOH, and EtOH. A wetted HKUST-1 sample in DCM, a thermally activated HKUST-1 sample, and a DCM-treated MeCN-DMF-HK sample were also tested for comparison. To avoid exposure to a moist atmosphere, we prepared the Raman samples by sealing them in disc-shaped quartz containers under the moisture-free conditions of an Argon-charged glovebox. As hypothesized, the spectra exhibited different Raman shifts in the range of 171–228 cm-1 depending on the type of coordinated solvent (see Section S9). On the basis of these observations, we further hypothesized that the successive substitution of solvent coordination would also be reflected in the vibration mode. To this end, we designed an in situ Raman experiment to monitor continuous changes in the Raman shift of the Cu–Cu vibration during the sequential treatment of the DMF-HK crystals with EtOH and DCM (see Figure 5). While the stretching vibration of DMF-HK appeared at the shift of approximately 175 cm−1, the vibration was shifted to approximately 185 cm-1 after the sample was exposed to EtOH. The vibration was further shifted to approximately 213 cm-1 when EtOH was displaced by DCM. However, when the sample was exposed to ambient air, the vibration at 213 cm-1 further shifted to approximately 228 cm-1, indicating the formation of open-state (CuII)2 centers. This observation implies that the spontaneous dissociation of coordinated DCM arises only with the thermal energy at room temperature, which is consistent with the trend observed in the above NMR results. More interestingly, upon further exposure of the sample to ambient air, the vibration mode of the open-state (CuII)2 (228 cm-1) redshifted to 168 cm-1, indicating the coordination of H2O molecules in the ambient atmosphere. Therefore, the presence of the Raman shift at 213 cm-1 and the transitions of the Raman shift provide compelling evidence that strongly supports the CE behavior of the OCSs. Meanwhile, using Badger’s rule,60 we estimate that the Cu−Cu bond strength increases in the following order: DMF-HK < EtOH-HK < DCMHK < open-state-HK. Given that bond strength is, in general, inversely proportional to the bond length, the aforementioned estimation agrees well with the theoretically calculated Cu–Cu bond lengths. The calculated Cu–Cu bond length increases in the following order: open-state-HK < DCM-HK < EtOH-HK < DMF-HK (see Section S10). We further simulated the bond dissociation energies (BDEs) of the Cu– solvent bonds to understand the relationship between the Cu−Cu bond strength and the BDEs of the Cu– solvent bonds. The simulation exhibits an increase of the BDE in the order of open-state-HK < DCMHK < EtOH-HK < DMF-HK. Thus, we speculate that the stretching vibration frequency and Cu–Cu bond strength are inversely proportional to the BDE of the Cu–solvent bond. ACS Paragon Plus Environment 7


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To address the wide applicability of multiple CE, we also examined DEF- and DMSO-coordinated HKUST-1 (hereafter DEF-HK and DMSO-HK) samples. DEF-HK and DMSO-HK were prepared via the TA of pristine HKUST-1 at 150 °C and the subsequent coordination of pure DEF and DMSO, respectively (see Section S1). Before testing the multiple CE, we first tested the direct DCM treatment to determine if the coordinated DEF and DMSO could be removed only by DCM treatment. However, the resulting 1H NMR spectra showed that 4% and 46% of DEF and DMSO, respectively, remained in the samples even after 30 repetitions of DCM treatment (see Section S11), indicating that the RCEs of DEF and DMSO are similar to or stronger than the RCE of DMF. However, repetitive MeCN treatments of DEF-HK and DMSO-HK completely replaced the DEF and DMSO coordinations with MeCN coordination (see Figure 6 and Section S11). The subsequent DCM treatments also completely removed the MeCN coordination from the HKUST-1 samples. The phase purities of the DEF-HK and DMSOHK samples before and after the solvent treatments were tested with PXRD measurements. The PXRD patterns indicated that the structural integrities of the MOFs were well preserved, and showed patterns that were similar to those observed in DMF-HK (see Figure 6). To determine if this multiple CE could be further expanded to other MOFs, we tested DMFcoordinated pristine MOF-74(Cu) and MOF-74(Ni). Crystalline MOF-74(Cu) and MOF-74(Ni) powders were synthesized from 2,5-dihydroxyterephthalic acid (DOBDC) and Cu(NO3)2⋅3H2O and Ni(NO3)2⋅6H2O, respectively, in DMF (the experimental details are presented in Section S1). Prior to testing multiple CE with these MOFs, we examined their TA behaviors to obtain the minimum temperatures required for complete removal of coordinated DMF and to test their structural integrities. The 1H NMR results show that MOF-74(Cu) required a temperature of approximately 200 °C for complete removal of coordinated DMF and that MOF-74(Ni) required a temperature higher than 240 °C (see Section S12). After TA at 200 °C, the framework of MOF-74(Cu) remained intact (see Section S12). However, the framework of MOF-74(Ni) collapsed even though the coordinated DMF molecules were not fully dissociated at this temperature (see Section S12). Meanwhile, the coordinated DMF decomposed during the TA process, forming dimethylamine (DMA).61 The formed DMA molecules strongly coordinated to the OCSs, interfering with the completion of the activation (see Section S13). Next, we tested the multiple CE process expecting that multiple CE would be a safe alternative for activation. The coordinated DMF molecules in both MOF-74(Cu) and MOF-74(Ni) were completely removed after sequential CEs with MeCN and DCM. In terms of structural integrity, the multiple CE exhibited behavior that differs from that of the TA. The framework of thermally unstable MOF-74(Ni) remained intact after multiple CE processes (see Figure 7). We also observed a similar pattern in MOF74(Mg) (see Section S14). Therefore, we tentatively conclude that this multiple CE is a superior strategy for the safe activation of thermally unstable MOFs. We also observed that MeOH-exchanged MOFACS Paragon Plus Environment 8

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74(Ni) was comparably stable at elevated temperatures (see Section S15).62-63 Although more comprehensive studies are required to fully understand the role of the CE, extracoordination must be a substantial factor that exerts an influence on the structural integrity of the MOFs.

CONCLUSIONS In summary, we designed a multiple CE process for the mild activation of OCSs occupied by strong coordination bonds. For the example examined, the scissoring of strong DMF-coordination, which requires high thermal energy for TA, could be safely performed by two-step CE with MeCN, MeOH, or EtOH initially and DCM subsequently. The multiple CE process was clearly demonstrated not only by monitoring the dissolution and evolution of 1H NMR peaks for the precoordinating and postcoordinating solvent molecules at the OCSs but also by observing successive changes in the Raman shift of the Cu– Cu vibration. The in situ Raman studies enabled us to suggest the presence of DCM coordination at the OCSs as well as transitional coordination changes. The in situ NMR and Raman analytical techniques— which we employed for the first time in MOF studies—could be realized due to the permanent porosity of MOFs that allows the free transport of guest solvent molecules. We also demonstrated that the multiple CE was effective in the removal of strongly coordinating DEF and DMSO. In our previous report, we suggested that direct DCM treatment could be applied to the activation of MOF-74. However, the direct activation method solely with DCM treatment has not been succeeded because the DMF molecules strongly coordinate to the metal ion centers. Instead, we found that the multiple CE was a powerful method for the safe activation of thermally unstable MOFs, such as MOF-74(Ni). We anticipate that this multiple CE will be useful for the MOF industry, and the resulting guidelines (including in situ NMR and Raman techniques) will prove transferrable or adoptable to other studies of MOFs containing ligand-accessible metal ions.

ASSOCIATED CONTENT Supporting Information Experimental details, theoretical studies, UV-vis absorption data, PXRD data, 1H-NMR data, BrunauerEmmett-Teller (BET) data, and Raman data. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Ministry of Science, ICT, and Future Planning (MSIP) of Korea under the auspices of the Basic Science Research Program sponsored by the National Research Foundation (NRF) (Grant No. NRF-2016R1A2B2014918) and by the DGIST R&D Program (Grant No. 16-01ACS Paragon Plus Environment 9


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Table of Contents (TOC) Artwork

An efficient room-temperature activation of open-metal sites achieved by multiple coordination exchange is a safe method to retain the crystalline structure of metal-organic framework materials.



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FIGURES

Direct CE Pathway (less effective) Coordination Exchange with CH2Cl2 H O C NMe2

Spontaneous Dissociation ClCH Cl 2

Multiple CE Pathway (more effective) 1st Coordination Exchange with CH3CN

2nd Coordination Exchange with CH2Cl2 NCCH 3

Figure 1. Schematic illustration of direct and multiple CEs for II the chemical activation of the paddle-wheel-like (Cu )2 node within HKUST-1. Hydrogen atoms bound to the carbon atoms in the benzene moieties are omitted for the sake of clarity.


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Page 15 of 20

(a) Stepwise Multiple CE TA-HK

K-M (Normalized)

1.0

DCM-MeCN-DMF-HK

0.5

MeCN-DMF-HK DMF-HK

0.0 400

600

800

1000

1200

1400

(b) Direct CE TA-HK

1.0

K-M (Normalized)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DCM20-DMF-HK 2

DCM -DMF-HK

0.5

DMF-HK

0.0 400

600

800

1000

1200

1400

Wavelength (nm)

Figure 2. Diffuse-reflectance UV-vis absorption spectra of (a) DMF-HK (black curve), MeCN-DMF-HK (sky blue curve), and DCM-MeCN-DMF-HK (blue curve) powders and (b) DMF-HK 2 20 (black curve), DCM -DMF-HK (green curve), and DCM DMF-HK (orange curve) powders. Superscript numbers in the labels indicate the number of repetitions of the DCM treatment. The absorption spectrum of the TA-HK sample (red dashed curve) is also displayed for comparison.



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(a) Multiple CE with intermediate MeCN H

Page 16 of 20

(d) Multiple CE with intermediate MeCN

CO2H H

HO2C

CO2H

HCON(CH3)2 (DMF)

H 5

CH3CN

4

5

4

DCM -MeCN -DMF-HK

DCM -MeCN -DMF-HK

MeCN4-DMF-HK

MeCN4-DMF-HK

DMF-HK

DMF-HK

8

6

4

10

2

(b) Multiple CE with intermediate MeOH

20

30

40

50

60

(e) Multiple CE with intermediate MeOH

CH3OH DCM9-MeOH3-DMF-HK

DCM9-MeOH3-DMF-HK

3 MeOH -DMF-HK

3 MeOH -DMF-HK

DMF-HK

DMF-HK

8

6

4

10

2

(c) Multiple CE with intermediate EtOH CH3CH2OH

20

30

40

50

(f) Multiple CE with intermediate EtOH CH3CH2OH

DCM5-EtOH9-DMF-HK

DCM5-EtOH9-DMF-HK

EtOH9-DMF-HK

EtOH9-DMF-HK

DMF-HK

DMF-HK

8

6

4

60

10

2

20

δ (ppm)

30

40

50

60

2θ (º)

1

Figure 3. (a-c) H NMR spectra and (d-f) PXRD patterns of DMF-coordinated HKUST-1 before and after initial solvent treatment with (a, d) MeCN, (b, e) MeOH, and (c, f) EtOH and subsequent solvent treatment with DCM. Superscript numbers in the labels indicate the number of repetitions of the corresponding solvent treatment. The NMR spectra were taken after the powder samples were completely dissolved in D2SO4.


Page 17 of 20

1

(a) Liquid-phase in situ H NMR NMR Tube

CD3CN

DMF

H O C NMe2

NCCD

3

DMF-HK

(b) Exchange of DMF with MeCN-d3 HCON(CH3)2 (DMF)

(c) Exchange of MeCN with CD2Cl2

CD2HCN impurity (internal standard)

CDHCl2 impurity (internal standard)

(v)

(v)

(iv)

(iv)

(iii)

(iii)

(ii)

(ii)

(i)

(i)

3.2

3.0

2.8

2.6

2.0 1.9

5.4

2.2

(v)

Normalized amount of MeCN

(iii)

Remaining amount of DMF (1.8%) (ii)

0

5

10

15

20

6

25

1.8

(e) Exchange of MeCN with CD2Cl2

(iv)

8

2.0

δ (ppm)

(d) Exchange of DMF with MeCN-d3

(i)

CH3CN

5.2

δ (ppm)

Normalized amount of DMF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 δ (ppm)

30

35

2

40

45

50

(ii)

(iv)

(v)

Remaining amount of MeCN (4.0%)

(i)

8

6

4

2

δ (ppm)

0

Exposure time (min)

(iii)

5

10

15

20

25

30

35

40

45

50

Exposure time (min) 1

Figure 4. (a) Schematic illustration of the in situ H NMR experiments designed for directly monitoring the CEs. The scheme displays the CE of precoordinated DMF with postcoordinating deuterated acetonitrile, MeCN-d3 (CD3CN). The II in situ NMR setup is for monitoring the amount of DMF molecules that dissociated from the (Cu )2 node and thereby dissolved in MeCN-d3. Hydrogen atoms bound to the carbon atoms in the benzene moieties are omitted for the sake of 1 clarity. (b, c) In situ time-course H NMR spectra of (b) the MeCN-d3 (CD3CN) solvent containing DMF-HK crystals and (c) the DCM-d2 (CD2Cl2) solvent containing MeCN-DMF-HK crystals. The CD2HCN and CDHCl2 impurities included in the corresponding MeCN-d3 and DCM-d2 solvents were used as internal standards for the quantitative analysis of DMF and MeCN molecules, respectively. (d, e) Plots of the increase in the amount of dissolved (d) DMF and (e) MeCN against the exposure time of the DMF-HK and MeCN-DMF-HK crystals to MeCN-d3 and DCM-d2 solvents, respectively. The insets 1 display the ex situ H NMR spectra of the reaction-terminated (d) DMF-HK and (e) MeCN-DMF-HK crystals, which were taken after the crystals were completely dissolved in D2SO4.



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(a)

H O C NMe2

ν = 175 cm

-

OHC H 2 5

185 cm

-1

H2 O

ClCH Cl 2

213cm

-1

228cm

-1

168cm

-1

1

(b)

Cu-Cu coordinated with H2O DMF EtOH DCM OS 168 175 185 213 228

Expose to ambient air (coordination of moist H2O Expose to ambient air (dissociation of DCM

C-H3

C-Cl

C-H (Ar) C-N-C C-C

(DMF) 663

(DCM) 703

(BTC) 742 826

(DMF) 864

(EtOH) -1 880 cm

(v) H2O-coord.

(iv) Open-state

Expose to DCM (iii)

(iii) DCM-coord.

Expose to EtOH

(ii) EtOH-coord. (ii) (i) DMF-coord.

DMF-HK

(i)

(i)

180 210 240

650

700

750

800

850

Raman Shift (cm-1)

Figure 5. (a) Illustration of the DMF-coordinated, EtOHcoordinated, DCM-coordinated, extracoordination-free II (open-state), and H2O-coordinated (Cu )2 centers in HKUST1. (b) Successive changes of the in situ Raman spectra of DMF-coordinated HKUST-1 during the sequential exposure to EtOH, DCM, and ambient air.


Page 19 of 20

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(a) Multiple CE of DEF-HK H

CO2H H

H2 C

O N

H HO2C

CO2H

(b) Multiple CE of DMSO-HK H HO2C

5

5

10

DCM -MeCN -DMSO-HK

5

10

MeCN -DEF-HK

MeCN -DMSO-HK

DEF-HK

DMSO-HK

4

2

8

6

(c) Multiple CE of DEF-HK

2

(d) Multiple CE of DMSO-HK

5

5

DCM -MeCN -DMSO-HK

MeCN -DEF-HK

5

MeCN -DMSO-HK

DEF-HK

DMSO-HK

5

DCM -MeCN -DEF-HK

20

4

δ (ppm)

δ (ppm)

10

CH3CN

H

5

6

CH3

(DMSO)

CO2H

CH3CN

DCM -MeCN -DEF-HK

8

O S

H3C

CH3

CH2 H3C (DEF)

H

CO2H H

30

40

50

10

10

60

10

20

2θ (º)

30

40

50

60

2θ (º)

1

Figure 6. (a, b) H NMR spectra and (c, d) PXRD patterns of (a, c) DEF- and (b, d) DMSO-coordinated HKUST-1 before and after initial MeCN treatment and subsequent DCM treatment. Superscript numbers in the labels indicate the number of repetitions of the corresponding solvent treatment. The NMR spectra were taken after the powder samples were completely dissolved in D2SO4.



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(a) Multiple CE of MOF-74(Cu)

(b) Multiple CE of MOF-74(Ni)

CO2H OH

H HO

H

H CO2H

HO HCON(CH3)2 (DMF) 20

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CH3CN

CO2H OH H CO2H

20

HCON(CH3)2 (DMF) 20

DCM -MeCN -MOF-74(Cu)

CH3CN

20

DCM -MeCN -MOF-74(Ni)

20

20

MeCN -MOF-74(Cu)

MeCN -MOF-74(Ni)

Pristine-MOF-74(Cu)

Pristine-MOF-74(Ni)

8

6

4

2

8

6

4

δ (ppm)

δ (ppm)

(c) Multiple CE of MOF-74(Cu)

(d) Multiple CE of MOF-74(Ni)

20

20

DCM -MeCN -MOF-74(Ni)

20

MeCN -MOF-74(Cu)

20

MeCN -MOF-74(Ni)

Pristine-MOF-74(Cu)

Pristine-MOF-74(Ni)

DCM -MeCN -MOF-74(Cu)

10

20

30

40

2

20

20

50

60

10

20

2θ (º)

30

40

50

60

2θ (º)

1

Figure 7. (a, b) H NMR spectra and (c, d) PXRD patterns of DMF-coordinated (a, c) MOF-74(Cu) and (b, d) MOF-74(Ni) before and after initial MeCN treatment and subsequent DCM treatment. Superscript numbers in the labels indicate the number of repetitions of the corresponding solvent treatment. The NMR spectra were taken after the powder samples were completely dissolved in D2SO4.


Metal Sites in Metal-Organic Frameworks - ACS Publications

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