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Structural changes in copper based metal-organic framework catalyst induced by organic solvents
Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Structural changes in copper based metal-organic framework catalyst induced by organic solvents ⁎
Alena Kochubeia, Yunyao Zhanga, Zichun Wanga, William S. Priceb, Gang Zhengb, , Jun Huanga, a b
⁎
School of Chemical and Biomolecular Engineering, the University of Sydney, Darlington, NSW, 2008, Australia Nanoscale Organization and Dynamics Group, School of Science and Health, Western Sydney University, Penrith, NSW, 2751, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu-MOF NMR spectroscopy Structure change Organic solvents
Metal-organic frameworks (MOFs) are an emerging class of catalysts that posess large surface area due to intrinsic porosity, and high metal loading. However, they often lack catalytic properties due to the saturated coordination sphere of metal centres. Some MOFs have been found to exhibit good to excellent catalytic activity in a limited number of reactions, resulting from solvent induced changes of the coordination sphere. In this work the coordination sphere changes of Cu(bpy)(H2O)2(BF4)2(bpy) (bpy = 4,4-bipyridine) affected by common solvents with different dipole moments were studied by means of solution-state 1H and 19F NMR. It was found that organic molecules interact with the copper(II) centre and cause structural transformations of Cu-MOF in proportion to both the dipole moment and concentration of solvent. The sensitive NMR resonances reveal an exchange of the BF4¯ anions to DMSO (dimethylsulfoxide) molecules at axial positions of Cu(II) and even breakage of Cu-N and Cu-O bonds in equatorial positions with increasing DMSO concentration. Therefore, solvents with suitable dipole moments (1.7–3.9) can promote structural changes and, thus release highly active sites in the reaction, which can efficiently enhance the activity of Cu-MOF catalysts.
1. Introduction Metal-organic frameworks (MOFs) constructed by metal-containing nodes linked by organic polyatomic bridges [1,2] are a novel type of coordination networks [3] which were proposed and synthesised for the first time in the early 90 s [4]. Many MOFs possess highly ordered layered [5] or hierarchically organized [6] crystalline structures with intrinsic porosity and, therefore, large internal surface area. In addition, the pore size and functionalities can be purposely designed [7,8] and constructed via simple self-assembly processes [9,10] in a great chemical variety, since metal-containing and organic parts are interchangeable. Due to their unique properties, MOFs have already found applications in many fields, such as gas storage and separation [11], optics and ferroelectricity [12,13], energy conversion and storage [14], chemical and biosensing [15–17], drug delivery [18] and heterogeneous catalysis [19,20]. Initially, MOFs seemed to be superior to existing effective heterogeneous catalysts – zeolites [4]. However, the presence of organic linkers significantly lowers the thermal stability of catalytic materials [21,22], and some of the organic-inorganic lattices collapse with solvent removal [8,23]. Further, industrial-scale MOFs synthesis is currently prohibitively expensive [24,25]. Consequently, zeolites are still preferable for industrial gas-phase reactions catalysis
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[26,27], while MOFs are used only for niche areas such as fine chemicals [28] and individual enantiomers synthesis [29] which is usually conducted in condensed phase under mild conditions [19]. In spite of the potential for conducting heterogeneisation of transition metals homogeneous catalysts [30–32], to date only a small number of synthesised MOFs have been found to have catalytic properties [28]. In addition to pores and cavities size, often the activity is restricted by saturated metal centres coordination sphere [33]. Furthermore, it is noteworthy that the catalytic performance of a MOF depends not only on the shape and size of the reactants [34,35], but also on the properties of the solvent used. Hence, the catalytic reactivity of a specific MOF is frequently limited to only a small number of reactions. In case of truly heterogeneous catalysis when active sites do not leach in the bulk solution, such selectivity can be explained by a few reasons. (i) Solvent molecules occupy catalytic centres and, therefore, compete with reactants [36,37]. This is particularly the case for catalysis by MOF Lewis acid sites. For instance, Cu3(BTC)2 (BTC = benzene1,3,5-tricarboxylate) and Cu3(BDC)(bpy) (BDC = benzene-1,4-dicarboxylate) are good catalysts of aldehyde cyanosilylation and oxidative amidation of terminal alkynes, respectively, in non-polar hydrocarbon solvents but lack reactivity in polar solvents. (ii) Solvent molecules activate substrates for catalytic reactions [38]. In contrast to
Corresponding authors. E-mail addresses: [email protected] (G. Zheng), [email protected] (J. Huang).
https://doi.org/10.1016/j.cattod.2019.04.031 Received 1 December 2018; Received in revised form 27 March 2019; Accepted 9 April 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Alena Kochubei, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.04.031
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aforementioned copper-based MOFs, Mn-MOF investigated by Horike et al., acts as Lewis acid in the catalytic Mukaiyama aldol reaction which is conducted in polar solvent DMF (dimethylformamide). This implies that DMF helps in activating the substrate, rather than obstruct catalytic sites [38]. (iii) The solvent interacts with the metal ion, altering the coordination sphere and, thus, affecting catalytic reactivity [39,40]. In case of [Cu3(BTC)2(H2O)3], referred to as HKUST-1, CH2Cl2 treatment leads to exchange water molecules in the apical position for dichloromethane molecules, a weakly bound and, therefore, highly mobile ligand, affording enhanced reactivity. As noted in our previous study [41], Cu(bpy)(H2O)2(BF4)2(bpy) (Cu-MOF) undergoes reversible structural changes during adsorption-desorption of organic molecules in the gas phase. It was found that the 11B chemical shift of BF4¯ shifts reversibly to high field (i.e., lower ppm) in proportion to the increasing dipole moment of the guest molecules what indicates the weakening of the Cu-F bond [41]. The above results lead to the questions: (i) how is the Cu(II) coordination sphere affected by solvents in the liquid phase, and (ii) will the copper ion environment be irreversibly changed by a guest compound with a sufficiently large dipole moment? In this study, four solvents with different dipole moments as tabulated in Table 1, including DMSO (dimethyl sulfoxide) which has one of the highest dipole moments among all organic solvents, were used to investigate the structural changes and changes in the Cu(II) coordination sphere in CuMOF in the liquid phase. A comparative study of the Cu-MOF - solvent mixtures was performed using solution state 1H and 19F NMR experiments. Additional experiments on solutions with different Cu-MOF concentrations and in a set of H2O-DMSO mixtures were performed to clarify the role of DMSO in coordination sphere changes.
Fig. 1. Coordination geometry of copper(II) centres in the Cu-MOF (dashed lines – bonds in the equatorial lattice of the layer, dashed arrows – hydrogen bonds between layers).
2. Experimental For the Cu-MOF synthesis hydrated copper(II) tetrafluoroborate and 4,4′-bipyridine were obtained from Sigma-Aldrich and ABCR Chemicals respectively. Deuterated solvents for NMR experiments: water-d2(99,9 atom % D), methanol-d4(99,96 atom % D), acetonitrile-d3(99,96 atom % D), dimethyl sulfoxide-d6(99.96 atom % D) and non-deuterated ethanol were purchased from Sigma-Aldrich. Crystalline Cu-MOF was synthesised following the procedure based on the literature [41,44]. Briefly, an aqueous solution of Cu(BF4)2•xH2O (0.309 g, 1.0 mmol) was added to a refluxing ethanol solution of 4,4′bipyridine (0.312 g, 2.0 mmol). After stirring for 4 h, a blue precipitate was filtered, washed with water and ethanol and dried at room temperature. Crystals of Cu-MOF were evacuated at 100 °C for 2 h and stored under argon, then, these two steps were repeated. Subsequently, the material was placed in a vacuum (< 10−2 mbar) at 100 °C for 12 h. Finally, it was sealed and kept in a glass tube for future use. For the NMR experiments in individual solvents, 14 mg of Cu-MOF was transferred into 5-mm NMR tubes, then about 0.6 mL of deuterated solvent
Fig. 2. 1H NMR spectra obtained on the Cu-MOF immersed in deuterated methanol, water, and acetonitrile, respectively, and on the Cu-MOF dissolved in deuterated DMSO (see Table 1).
were added and resulting mixture was analysed. For the NMR experiments on dissolved Cu-MOF solutions in DMSO, a Cu-MOF/DMSO stock solution was prepared by dissolving ca. 23 mg in 1 mL deuterated DMSO and then 0.6 mL of the stock solution was transferred into a 5 mm Wilmad NMR tube. 1 mL 1/10 diluted solution was made from the stock solution and then 1 mL 1/100 diluted solution was made from the 1/10 diluted solution. 0.6 mL of each diluted solution was transferred into a 5 mm Wilmad NMR tube. For the NMR experiments on CuMOF in DMSO:H2O mixtures, first, the solutions of DMSO:H2O were prepared by mixing the DMSO-d6 with water in proportions 1:2, 1:4, 1:6, 1:8, 1:12. Then the solutions were dispensed into 5 mm Wilmad NMR tubes, Cu-MOF was added into tubes and the resulting mixtures were analysed. It worth to note that the mixtures were not filtered so suspended solid particles could be present. All the 1H and 19F NMR spectra were recorded on a Bruker Avance 400 spectrometer at 298 K at 400 and 376 MHz, respectively. The 1H signals were referenced against trimethylsilylpropanoic acid (TSP). For 1D 1H and 19F NMR experiments, acquisition times were 0.3 s (1H) and 1.0 s (19F), recovery delays were 1.0 s (1H) and 0.02 s (19F), number-of-scans was 32 (1H) and 256
Table 1 Dipole moments of the solvents [42]. Solvent1
Dipole moment (D)2
H2 O CH3OH CH3CN (CH3)2SO
1.85 1.70 3.92 3.96
In all NMR experiments, except 19 F NMR spectra in mixtures DMSO:H2O and spin-lattice relaxation time measurements, deuterated solvents were used. 2 The listed dipole moments are for non-deuterated solvents. According to previous research [43], the deuteration does not affect significantly polarity of the bonds and molecules dipole moment. 2
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bipyridine and BF4¯ moieties. Taking into consideration 1H NMR spectra of binuclear copper(II) complexes [50,51] in solutions and the fact that Fermi contact coupling and dipolar coupling caused by paramagnetic nuclei affect both chemical shift and line broadening in 1H NMR spectra in general [52], very broad signal at about 48 ppm in methanol and acetonitrile and significantly narrower signal at 18.8 ppm in DMSO-d6 were assigned to 4,4′-bipyridine protons. The 1H NMR data indicate that the DMSO molecules interact with metal sites of Cu-MOF so that the Cu-N and CuO distances become longer and, therefore, deshielding effect of copper (II) centre weakens. Considering the chemical shifts of 4,4′-bipyridine itself, some changes in the structure of Cu-MOF happen when the dipole moment of solvent molecule is higher than 3.92 D. 19 F NMR measurements were conducted to investigate the interactions between solvent molecules and the axially positioned ligands. As shown in Fig. 3, broad 19F peaks of BF4¯ were observed for the Cu-MOF immersed in D2O, CD3OD and CD3CN in the range -100 – -250 ppm; a sharp 19F peak of BF4¯ was observed for the Cu-MOF fully dissolved in DMSO at -146.8 ppm. The significant line broadening due to enhanced spin-spin relaxation signifies the paramagnetic centre proximity to BF4¯ [53]. This means that the Cu-F bonds and, therefore, ‘solid’ Cu-MOF molecules, were observed in all solvents except deuterated DMSO. The bonds between copper(II) centres and BF4− ions were broken, the BF4− ions were replaced by DMSO molecules in deuterated DMSO, and thus free dissolved BF4− ions, with longer spin-spin relaxation times, were probed. As per the discussion above, we conclude that the breakage of the Cu-F bonds in Cu-MOF can be caused by their interactions with the solvent with dipole moment higher than 3.92 D. In order to elucidate whether DMSO molecules are able to interact with the copper ion equatorially, exchanging H2O and 4,4′-bipyridine molecules, further 1H NMR measurements were performed on the 23, 2.3, and 0.23 mg/mL Cu-MOF/DMSO solutions (Fig. 4a–c). A broad bipyridine peak was observed at ca. 19 ppm for the 23 mg/mL sample; the bipyridine peak shifted toward high field and become narrower with concentration decreasing. This peak reached 8.5 ppm for the 0.23 mg/mL solution, which can be attributed to non-bound (i.e. dissolved) 4,4′-bipyridine molecules in DMSO [54]. At the same time, the signals of water and DMSO moved from 4.3 to 3.3 ppm and from 2.9 to 2.6 ppm, respectively, when the Cu-MOF/DMSO solution was diluted from 23 mg/mL to 0.23 mg/mL (Fig. 4a–c). For comparison, pure dimethyl sulfoxide-d6 (Fig. 4d) gave the peaks of dissolved water and residual non-deuterated DMSO at around 3.33 ppm and 2.50 ppm, respectively [48], while there is no other peaks in a range 6–25 ppm. The data presented in Fig. 4a-d allow one to conclude that in very diluted
Fig. 3. 19F NMR spectra of Cu-MOF immersed in D2O (a), CD3OD (b), and CD3CN (c) and Cu-MOF fully dissolved in deuterated DMSO (d).
(19F), and short excitation pulse durations of 2 μs for 1H and 5 μs for 19F were used to achieve the homogenous exciatation bandwidths of ca. 0.44 MHz (1H) and ca. 0.18 MHz (19F).
3. Results and discussion As noted previously [45], the grid sheets of Cu-MOF consist of JahnTeller octahedrons where every copper ion is “surrounded” by two bare and two H-bonded 4,4′-bipyridine molecules in equatorial positions, and bound with two BF4¯ in axial positions (Fig. 1). The layers are linked via hydrogen bonds between coordinated water molecules and the fluorine atoms on the BF4¯. This structure provides the opportunity for flexible structural changes during adsorption-desorption of compounds [46,47]. The 1D 1H and 19F NMR spectra were obtained on the Cu-MOF immersed in deuteraeted water, methanol and acetonitrile and on the Cu-MOF dissolved in deuteratd DMSO. The dipole moments of the four solvents are listed in the Table 1. As shown in Fig. 2, a coordinated water peak can be observed at around 5 ppm in each of the four spectra. The signals are broad and shift down field by about 2.0 ppm compared to the signal of dissolved water [48,49] due to the deshielding effect of the paramagnetic Cu(II) atom and the hydrogen bonds with the 4,4′-
Fig. 4. 1H NMR spectra of 23 mg/mL (a), 2.3 mg/mL (b), and 0.23 mg/mL (c) Cu-MOF/DMSO-d6 solutions and pure DMSO-d6 (d). 3
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Fig. 5. 19F NMR spectra of Cu-MOF dissolved in solvent mixtures with different DMSO:H2O molar ratios (a–e) and in pure H2O (f).
solution (0.23 mg/ml; Fig. 4c) the Cu-N and Cu-O bonds in equatorial position of copper(II) are broken and the H2O and 4,4′-bipyridine molecules contained were released into the bulk solution. Therefore, DMSO molecules may have the capability to interact with the Cu(II) equatorially; when the DMSO:Cu(II) ratio is high enough the coordinated water molecules and 4,4′-bipyridine ligands can be replaced by DMSO molecules, resulting in the breakdown of Cu-MOF. To clarify whether DMSO molecules are favourable for chemisorption and can cause irreversible breakage of Cu-F bonds even in presence of competing solvent molecules, 19F NMR measurements were performed on Cu-MOF dissolved/suspended in a series of DMSO:H2O mixtures with different molar ratios. As shown in Fig. 5, a sharp peak from dissolved BF4¯ with splitting (caused by coupling with the boron isotopes) was observed for all DMSO:H2O mixtures except for the partial disappearance of splitting observed for the 1:12 mixture. It can be inferred that the DMSO molecules, even at relatively low concentration, can interact with Cu-MOF, thus causing irreversible breakage of the CuF bonds and exchanging the BF4¯ ions in the Cu(II) coordination sphere.
4. Conclusions In summary, this study reveals that polar organic molecules interact with copper (II) active sites in Cu-MOF resulting in lengthening of Cu-F, Cu-O and Cu-N bonds. Furthermore, when solvent with dipole moment higher than 3.92 D, e.g., DMSO, was used, the exchange of apical BF4¯ ligands with solvent molecules in the Cu-MOF took place, while other bonds still existed unless the Cu-MOF concentration was very low. Experiments in DMSO:H2O mixtures confirmed that even very low DMSO concentrations (less than 10 mol%) is effective for apical position activation. The results of this investigation may be helpful for design of catalytic experiments in the future.
Acknowledgements This work was supported by the Australian Research Council (DP180104010) and the University of Sydney SOAR fellow.
4
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Structural changes in copper based metal-organic framework catalyst induced by organic solvents ⁎
Alena Kochubeia, Yunyao Zhanga, Zichun Wanga, William S. Priceb, Gang Zhengb, , Jun Huanga, a b
⁎
School of Chemical and Biomolecular Engineering, the University of Sydney, Darlington, NSW, 2008, Australia Nanoscale Organization and Dynamics Group, School of Science and Health, Western Sydney University, Penrith, NSW, 2751, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu-MOF NMR spectroscopy Structure change Organic solvents
Metal-organic frameworks (MOFs) are an emerging class of catalysts that posess large surface area due to intrinsic porosity, and high metal loading. However, they often lack catalytic properties due to the saturated coordination sphere of metal centres. Some MOFs have been found to exhibit good to excellent catalytic activity in a limited number of reactions, resulting from solvent induced changes of the coordination sphere. In this work the coordination sphere changes of Cu(bpy)(H2O)2(BF4)2(bpy) (bpy = 4,4-bipyridine) affected by common solvents with different dipole moments were studied by means of solution-state 1H and 19F NMR. It was found that organic molecules interact with the copper(II) centre and cause structural transformations of Cu-MOF in proportion to both the dipole moment and concentration of solvent. The sensitive NMR resonances reveal an exchange of the BF4¯ anions to DMSO (dimethylsulfoxide) molecules at axial positions of Cu(II) and even breakage of Cu-N and Cu-O bonds in equatorial positions with increasing DMSO concentration. Therefore, solvents with suitable dipole moments (1.7–3.9) can promote structural changes and, thus release highly active sites in the reaction, which can efficiently enhance the activity of Cu-MOF catalysts.
1. Introduction Metal-organic frameworks (MOFs) constructed by metal-containing nodes linked by organic polyatomic bridges [1,2] are a novel type of coordination networks [3] which were proposed and synthesised for the first time in the early 90 s [4]. Many MOFs possess highly ordered layered [5] or hierarchically organized [6] crystalline structures with intrinsic porosity and, therefore, large internal surface area. In addition, the pore size and functionalities can be purposely designed [7,8] and constructed via simple self-assembly processes [9,10] in a great chemical variety, since metal-containing and organic parts are interchangeable. Due to their unique properties, MOFs have already found applications in many fields, such as gas storage and separation [11], optics and ferroelectricity [12,13], energy conversion and storage [14], chemical and biosensing [15–17], drug delivery [18] and heterogeneous catalysis [19,20]. Initially, MOFs seemed to be superior to existing effective heterogeneous catalysts – zeolites [4]. However, the presence of organic linkers significantly lowers the thermal stability of catalytic materials [21,22], and some of the organic-inorganic lattices collapse with solvent removal [8,23]. Further, industrial-scale MOFs synthesis is currently prohibitively expensive [24,25]. Consequently, zeolites are still preferable for industrial gas-phase reactions catalysis
⁎
[26,27], while MOFs are used only for niche areas such as fine chemicals [28] and individual enantiomers synthesis [29] which is usually conducted in condensed phase under mild conditions [19]. In spite of the potential for conducting heterogeneisation of transition metals homogeneous catalysts [30–32], to date only a small number of synthesised MOFs have been found to have catalytic properties [28]. In addition to pores and cavities size, often the activity is restricted by saturated metal centres coordination sphere [33]. Furthermore, it is noteworthy that the catalytic performance of a MOF depends not only on the shape and size of the reactants [34,35], but also on the properties of the solvent used. Hence, the catalytic reactivity of a specific MOF is frequently limited to only a small number of reactions. In case of truly heterogeneous catalysis when active sites do not leach in the bulk solution, such selectivity can be explained by a few reasons. (i) Solvent molecules occupy catalytic centres and, therefore, compete with reactants [36,37]. This is particularly the case for catalysis by MOF Lewis acid sites. For instance, Cu3(BTC)2 (BTC = benzene1,3,5-tricarboxylate) and Cu3(BDC)(bpy) (BDC = benzene-1,4-dicarboxylate) are good catalysts of aldehyde cyanosilylation and oxidative amidation of terminal alkynes, respectively, in non-polar hydrocarbon solvents but lack reactivity in polar solvents. (ii) Solvent molecules activate substrates for catalytic reactions [38]. In contrast to
Corresponding authors. E-mail addresses: [email protected] (G. Zheng), [email protected] (J. Huang).
https://doi.org/10.1016/j.cattod.2019.04.031 Received 1 December 2018; Received in revised form 27 March 2019; Accepted 9 April 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Alena Kochubei, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.04.031
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aforementioned copper-based MOFs, Mn-MOF investigated by Horike et al., acts as Lewis acid in the catalytic Mukaiyama aldol reaction which is conducted in polar solvent DMF (dimethylformamide). This implies that DMF helps in activating the substrate, rather than obstruct catalytic sites [38]. (iii) The solvent interacts with the metal ion, altering the coordination sphere and, thus, affecting catalytic reactivity [39,40]. In case of [Cu3(BTC)2(H2O)3], referred to as HKUST-1, CH2Cl2 treatment leads to exchange water molecules in the apical position for dichloromethane molecules, a weakly bound and, therefore, highly mobile ligand, affording enhanced reactivity. As noted in our previous study [41], Cu(bpy)(H2O)2(BF4)2(bpy) (Cu-MOF) undergoes reversible structural changes during adsorption-desorption of organic molecules in the gas phase. It was found that the 11B chemical shift of BF4¯ shifts reversibly to high field (i.e., lower ppm) in proportion to the increasing dipole moment of the guest molecules what indicates the weakening of the Cu-F bond [41]. The above results lead to the questions: (i) how is the Cu(II) coordination sphere affected by solvents in the liquid phase, and (ii) will the copper ion environment be irreversibly changed by a guest compound with a sufficiently large dipole moment? In this study, four solvents with different dipole moments as tabulated in Table 1, including DMSO (dimethyl sulfoxide) which has one of the highest dipole moments among all organic solvents, were used to investigate the structural changes and changes in the Cu(II) coordination sphere in CuMOF in the liquid phase. A comparative study of the Cu-MOF - solvent mixtures was performed using solution state 1H and 19F NMR experiments. Additional experiments on solutions with different Cu-MOF concentrations and in a set of H2O-DMSO mixtures were performed to clarify the role of DMSO in coordination sphere changes.
Fig. 1. Coordination geometry of copper(II) centres in the Cu-MOF (dashed lines – bonds in the equatorial lattice of the layer, dashed arrows – hydrogen bonds between layers).
2. Experimental For the Cu-MOF synthesis hydrated copper(II) tetrafluoroborate and 4,4′-bipyridine were obtained from Sigma-Aldrich and ABCR Chemicals respectively. Deuterated solvents for NMR experiments: water-d2(99,9 atom % D), methanol-d4(99,96 atom % D), acetonitrile-d3(99,96 atom % D), dimethyl sulfoxide-d6(99.96 atom % D) and non-deuterated ethanol were purchased from Sigma-Aldrich. Crystalline Cu-MOF was synthesised following the procedure based on the literature [41,44]. Briefly, an aqueous solution of Cu(BF4)2•xH2O (0.309 g, 1.0 mmol) was added to a refluxing ethanol solution of 4,4′bipyridine (0.312 g, 2.0 mmol). After stirring for 4 h, a blue precipitate was filtered, washed with water and ethanol and dried at room temperature. Crystals of Cu-MOF were evacuated at 100 °C for 2 h and stored under argon, then, these two steps were repeated. Subsequently, the material was placed in a vacuum (< 10−2 mbar) at 100 °C for 12 h. Finally, it was sealed and kept in a glass tube for future use. For the NMR experiments in individual solvents, 14 mg of Cu-MOF was transferred into 5-mm NMR tubes, then about 0.6 mL of deuterated solvent
Fig. 2. 1H NMR spectra obtained on the Cu-MOF immersed in deuterated methanol, water, and acetonitrile, respectively, and on the Cu-MOF dissolved in deuterated DMSO (see Table 1).
were added and resulting mixture was analysed. For the NMR experiments on dissolved Cu-MOF solutions in DMSO, a Cu-MOF/DMSO stock solution was prepared by dissolving ca. 23 mg in 1 mL deuterated DMSO and then 0.6 mL of the stock solution was transferred into a 5 mm Wilmad NMR tube. 1 mL 1/10 diluted solution was made from the stock solution and then 1 mL 1/100 diluted solution was made from the 1/10 diluted solution. 0.6 mL of each diluted solution was transferred into a 5 mm Wilmad NMR tube. For the NMR experiments on CuMOF in DMSO:H2O mixtures, first, the solutions of DMSO:H2O were prepared by mixing the DMSO-d6 with water in proportions 1:2, 1:4, 1:6, 1:8, 1:12. Then the solutions were dispensed into 5 mm Wilmad NMR tubes, Cu-MOF was added into tubes and the resulting mixtures were analysed. It worth to note that the mixtures were not filtered so suspended solid particles could be present. All the 1H and 19F NMR spectra were recorded on a Bruker Avance 400 spectrometer at 298 K at 400 and 376 MHz, respectively. The 1H signals were referenced against trimethylsilylpropanoic acid (TSP). For 1D 1H and 19F NMR experiments, acquisition times were 0.3 s (1H) and 1.0 s (19F), recovery delays were 1.0 s (1H) and 0.02 s (19F), number-of-scans was 32 (1H) and 256
Table 1 Dipole moments of the solvents [42]. Solvent1
Dipole moment (D)2
H2 O CH3OH CH3CN (CH3)2SO
1.85 1.70 3.92 3.96
In all NMR experiments, except 19 F NMR spectra in mixtures DMSO:H2O and spin-lattice relaxation time measurements, deuterated solvents were used. 2 The listed dipole moments are for non-deuterated solvents. According to previous research [43], the deuteration does not affect significantly polarity of the bonds and molecules dipole moment. 2
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bipyridine and BF4¯ moieties. Taking into consideration 1H NMR spectra of binuclear copper(II) complexes [50,51] in solutions and the fact that Fermi contact coupling and dipolar coupling caused by paramagnetic nuclei affect both chemical shift and line broadening in 1H NMR spectra in general [52], very broad signal at about 48 ppm in methanol and acetonitrile and significantly narrower signal at 18.8 ppm in DMSO-d6 were assigned to 4,4′-bipyridine protons. The 1H NMR data indicate that the DMSO molecules interact with metal sites of Cu-MOF so that the Cu-N and CuO distances become longer and, therefore, deshielding effect of copper (II) centre weakens. Considering the chemical shifts of 4,4′-bipyridine itself, some changes in the structure of Cu-MOF happen when the dipole moment of solvent molecule is higher than 3.92 D. 19 F NMR measurements were conducted to investigate the interactions between solvent molecules and the axially positioned ligands. As shown in Fig. 3, broad 19F peaks of BF4¯ were observed for the Cu-MOF immersed in D2O, CD3OD and CD3CN in the range -100 – -250 ppm; a sharp 19F peak of BF4¯ was observed for the Cu-MOF fully dissolved in DMSO at -146.8 ppm. The significant line broadening due to enhanced spin-spin relaxation signifies the paramagnetic centre proximity to BF4¯ [53]. This means that the Cu-F bonds and, therefore, ‘solid’ Cu-MOF molecules, were observed in all solvents except deuterated DMSO. The bonds between copper(II) centres and BF4− ions were broken, the BF4− ions were replaced by DMSO molecules in deuterated DMSO, and thus free dissolved BF4− ions, with longer spin-spin relaxation times, were probed. As per the discussion above, we conclude that the breakage of the Cu-F bonds in Cu-MOF can be caused by their interactions with the solvent with dipole moment higher than 3.92 D. In order to elucidate whether DMSO molecules are able to interact with the copper ion equatorially, exchanging H2O and 4,4′-bipyridine molecules, further 1H NMR measurements were performed on the 23, 2.3, and 0.23 mg/mL Cu-MOF/DMSO solutions (Fig. 4a–c). A broad bipyridine peak was observed at ca. 19 ppm for the 23 mg/mL sample; the bipyridine peak shifted toward high field and become narrower with concentration decreasing. This peak reached 8.5 ppm for the 0.23 mg/mL solution, which can be attributed to non-bound (i.e. dissolved) 4,4′-bipyridine molecules in DMSO [54]. At the same time, the signals of water and DMSO moved from 4.3 to 3.3 ppm and from 2.9 to 2.6 ppm, respectively, when the Cu-MOF/DMSO solution was diluted from 23 mg/mL to 0.23 mg/mL (Fig. 4a–c). For comparison, pure dimethyl sulfoxide-d6 (Fig. 4d) gave the peaks of dissolved water and residual non-deuterated DMSO at around 3.33 ppm and 2.50 ppm, respectively [48], while there is no other peaks in a range 6–25 ppm. The data presented in Fig. 4a-d allow one to conclude that in very diluted
Fig. 3. 19F NMR spectra of Cu-MOF immersed in D2O (a), CD3OD (b), and CD3CN (c) and Cu-MOF fully dissolved in deuterated DMSO (d).
(19F), and short excitation pulse durations of 2 μs for 1H and 5 μs for 19F were used to achieve the homogenous exciatation bandwidths of ca. 0.44 MHz (1H) and ca. 0.18 MHz (19F).
3. Results and discussion As noted previously [45], the grid sheets of Cu-MOF consist of JahnTeller octahedrons where every copper ion is “surrounded” by two bare and two H-bonded 4,4′-bipyridine molecules in equatorial positions, and bound with two BF4¯ in axial positions (Fig. 1). The layers are linked via hydrogen bonds between coordinated water molecules and the fluorine atoms on the BF4¯. This structure provides the opportunity for flexible structural changes during adsorption-desorption of compounds [46,47]. The 1D 1H and 19F NMR spectra were obtained on the Cu-MOF immersed in deuteraeted water, methanol and acetonitrile and on the Cu-MOF dissolved in deuteratd DMSO. The dipole moments of the four solvents are listed in the Table 1. As shown in Fig. 2, a coordinated water peak can be observed at around 5 ppm in each of the four spectra. The signals are broad and shift down field by about 2.0 ppm compared to the signal of dissolved water [48,49] due to the deshielding effect of the paramagnetic Cu(II) atom and the hydrogen bonds with the 4,4′-
Fig. 4. 1H NMR spectra of 23 mg/mL (a), 2.3 mg/mL (b), and 0.23 mg/mL (c) Cu-MOF/DMSO-d6 solutions and pure DMSO-d6 (d). 3
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Fig. 5. 19F NMR spectra of Cu-MOF dissolved in solvent mixtures with different DMSO:H2O molar ratios (a–e) and in pure H2O (f).
solution (0.23 mg/ml; Fig. 4c) the Cu-N and Cu-O bonds in equatorial position of copper(II) are broken and the H2O and 4,4′-bipyridine molecules contained were released into the bulk solution. Therefore, DMSO molecules may have the capability to interact with the Cu(II) equatorially; when the DMSO:Cu(II) ratio is high enough the coordinated water molecules and 4,4′-bipyridine ligands can be replaced by DMSO molecules, resulting in the breakdown of Cu-MOF. To clarify whether DMSO molecules are favourable for chemisorption and can cause irreversible breakage of Cu-F bonds even in presence of competing solvent molecules, 19F NMR measurements were performed on Cu-MOF dissolved/suspended in a series of DMSO:H2O mixtures with different molar ratios. As shown in Fig. 5, a sharp peak from dissolved BF4¯ with splitting (caused by coupling with the boron isotopes) was observed for all DMSO:H2O mixtures except for the partial disappearance of splitting observed for the 1:12 mixture. It can be inferred that the DMSO molecules, even at relatively low concentration, can interact with Cu-MOF, thus causing irreversible breakage of the CuF bonds and exchanging the BF4¯ ions in the Cu(II) coordination sphere.
4. Conclusions In summary, this study reveals that polar organic molecules interact with copper (II) active sites in Cu-MOF resulting in lengthening of Cu-F, Cu-O and Cu-N bonds. Furthermore, when solvent with dipole moment higher than 3.92 D, e.g., DMSO, was used, the exchange of apical BF4¯ ligands with solvent molecules in the Cu-MOF took place, while other bonds still existed unless the Cu-MOF concentration was very low. Experiments in DMSO:H2O mixtures confirmed that even very low DMSO concentrations (less than 10 mol%) is effective for apical position activation. The results of this investigation may be helpful for design of catalytic experiments in the future.
Acknowledgements This work was supported by the Australian Research Council (DP180104010) and the University of Sydney SOAR fellow.
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