EPR Insights into Switchable and Rigid Derivatives of the Metal

Article pubs.acs.org/JPCC

EPR Insights into Switchable and Rigid Derivatives of the Metal− Organic Framework DUT-8(Ni) by NO Adsorption Matthias Mendt,† Felix Gutt,† Negar Kavoosi,‡ Volodymyr Bon,‡ Irena Senkovska,‡ Stefan Kaskel,‡ and Andreas Pöppl*,† †

Institut für Experimentelle Physik II, Universität Leipzig, Linnéstrasse 5, 04103 Leipzig, Germany Department of Inorganic Chemistry, Technische Universität Dresden, Bergstrasse 66, 01062 Dresden, Germany

‡

S Supporting Information *

ABSTRACT: The metal−organic framework (MOF) DUT-8(Ni) (DUT = Dresden University of Technology) shows a structural transformation from a nonporous to a porous phase during the adsorption of gases. A rigid derivative of this material has recently been synthesized, where this “gate pressure like” flexibility is completely absent. This rigid derivative of DUT-8(Ni) always stays in the porous phase even in the absence of any adsorbate. This motivates the present investigation of the adsorption of nitric oxide (NO) on the flexible and rigid forms of DUT-8(Ni) by continuous wave electron paramagnetic resonance (EPR) spectroscopy at X-band frequency. The EPR signal of desorbed NO is measured at moderate temperatures and the decrease of its intensity indicates the adsorption of this gas within the porous phase of DUT-8(Ni) at low temperatures. An adsorption and desorption related hysteresis loop of the intensity of this signal is observed for the flexible but not for the rigid DUT-8(Ni). This difference might reflect the difference in the flexibility of both materials. Furthermore, EPR signals with electron spin S = 1/2 are measured, which can likely be attributed to Ni2+-NO adsorption complexes at defective paddle wheel units within the porous phase of DUT-8(Ni) with the unpaired electron sitting at the Ni2+ ion. The order of their g-tensor principle values allows a distinct characterization of the ligand environment of these ions. Defects for which the EPR signals indicate that at least one NDC (2,6-naphthalenedicarboxylate) ligand molecule does not coordinate to the paddle wheel are only observed for the rigid but not for the flexible DUT-8(Ni). In addition, the density of defective paddle wheel units with only one Ni2+ ion or a missing dabco (1,4-diazabicyclo[2.2.2]octane) ligand is indicated to be 1 order of magnitude larger in the rigid than in the flexible derivative of this MOF. The observed differences in the presence and amount of distinct defects might be related to the difference in the flexibility of both forms of the investigated material.

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responsible structural transitions like “breathing” MOFs2 or “gate pressure” MOFs.3,6 The latter type of switchable MOF materials is of special interest for applications like gas separation7 or sensors8 due to the distinct separation of “non-porous” or “close pore” and “porous” or “open pore”

INTRODUCTION Metal−organic frameworks are organic−inorganic hybrid compounds constructed of metal clusters which are connected by organic linker molecules forming 1D, 2D, or 3D crystalline porous networks.1 Reversible framework flexibility or switching as an answer to external stimuli like pressure, irradiation, or adsorption of guest molecules is a unique feature of the third generation of MOFs.2−5 In turn, these materials could be divided on several subclasses depending on the stimuli© 2016 American Chemical Society

Received: May 17, 2016 Revised: June 9, 2016 Published: June 9, 2016 14246

DOI: 10.1021/acs.jpcc.6b04984 J. Phys. Chem. C 2016, 120, 14246−14259

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The Journal of Physical Chemistry C states. The first representative of such type of materials, namely, ELM-11 (ELM − elastic layer-structured metal−organic frameworks), was reported by Kaneko and co-workers showing the “gate pressure” effect during the adsorption of various gases and alcohols.9−11 The similar effect, accompanied by an enormous unit cell volume change of nearly 250%, was recently discovered for DUT-8(Ni) (DUT-Dresden University of Technology).7,12,13 The crystal structure of DUT-8(Ni) consists of Ni2-paddle wheel units which are linked by 2,6NDC (2,6-naphthalenedicarboxylate) anions forming a 2D square-grid layer7 (Figure 1a,b). At the axial sites of the Ni2-

Hund’s coupling case (a).17 However, gaseous NO is still paramagnetic: All rotational levels of the first excited 2Π3/2 state have a magnetic moment since Σ and Λ are aligned parallel along the intramolecular axis. In a typical continuous wave cw EPR experiment at X-band frequency (f ≈ 9.5 GHz) and moderate gas pressures, the lowest rotational level of the 2Π3/2 state, which has a total angular momentum of J = 3/2, gives a typical nine line EPR pattern at a g-value of g = 0.777.18−20 Higher rotational levels with J > 3/2 cannot be observed at Xband frequency due to their small g-values.21 The high sensitivity of EPR for the lowest rotational level of the 2Π3/2 state of the free NO molecule allows the detection of even small amounts of desorbed NO gas in adsorption experiments.22−24 In recent work the detection of the EPR signal of desorbed NO gas was used to characterized the NO adsorption strength of the MOFs CPO-27(M) (M = Ni, Co), MIL100(M) (M = Fe, Al), and MIL-53(Al0.98Cr0.02),24 but NO can also be observed by EPR if it is adsorbed at some diamagnetic surface site. Then, the external electric field at that site might quench the orbital momentum.25 The electronic ground state of NO becomes paramagnetic leading to a characteristic EPR signal of an electron spin S = 1/2 at g-values near but smaller than the free electron one ge = 2.0023.23,25−27 In the case of the MOF DUT-8(Ni), another interesting aspect might be the possibility of NO molecules to form paramagnetic complexes with the Ni2+ ions.28−31 These might be defective species since the regular Ni2+ ions of the paddle wheel units in DUT-8 offer no open coordination sites to bind NO. One might speculate whether such defects are related to the difference in the flexibility of the normal and rigid derivatives of DUT-8(Ni), and the present contribution will give some insight into this question.

Figure 1. Crystal structure of the triclinic nonporous (a) and monoclinic porous (b) phase of DUT-8(Ni).

paddle wheels dabco (1,4-diazabicyclo[2.2.2]octane) ligands coordinate via their nitrogen atoms connecting the 2D layers into a 3D structure. At low gas pressure, the MOF stays in a triclinic nonporous phase (Figure 1a) but opens to a porous monoclinic phase (Figure 1b) above a certain threshold pressure to adsorb the corresponding gas. For DUT-8(Ni) this gate-pressure effect was observed during the adsorption of nitrogen (N2), xenon (Xe), and n-butane at temperatures T = 77 K, T = 165 K, and T = 272 K, as well as for carbon dioxide (CO 2 ) at room temperature and T = 195 K. 7,12−14 Interestingly, an intermediate porous structural phase of DUT-8(Ni) has been observed during the transformation from the nonporous to the porous phase triggered by the adsorption of ethane at T = 185 K and ethylene at T = 169 K.14 Simultaneously Li and co-workers synthesized the framework [Ni(2,6-NDC) (dabco)0.5], PXRD patterns of which completely match the DUT-8(Ni) “as made” phase, using a slightly modified synthetic procedure.15 In their report, a rigid derivative of DUT-8(Ni) was produced, which stays in the porous open form even after the removing of all guest molecules from the framework. Such a significant difference in the properties, resulting only from the minor changes in the synthesis conditions, prompted us to perform a deeper investigation of the flexibility origin of DUT-8(Ni) using a continuous wave (cw) electron paramagnetic resonance (EPR) technique. Due to its high sensitivity, EPR is well-suited to study paramagnetic defects in the crystal structure. Here the small radical nitric oxide (NO) is used as a magnetic probe to investigate the adsorption properties as well as the local adsorption sites of NO in the DUT-8(Ni) material with EPR. The NO molecule is accessible by EPR since it has one unpaired electron in an antibonding molecular 2Π state.16 In the absence of any external electric fields the 2Π1/2 ground state is diamagnetic since the components Σ = 1/2 and Λ = 1 of the electronic spin S and the orbital angular momentum L along the intramolecular axis are coupled antiparallel following

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EXPERIMENTAL SECTION Synthesis and Characterization. DUT-8(Ni) Flexible. Ni(NO3)2·6H2O (0.407 g, 1.4 mmol) and 2,6-H2NDC (0.303 g, 1.4 mmol) were dissolved in 21 mL DMF. Dabco (0.1 g, 0.9 mmol) was dissolved in 9 mL methanol. The solutions were mixed in an ultrasonic bath for 10 min and the resulting solution was transferred into the 50 mL Teflon lined autoclave. Afterward the autoclave was heated for 48 h at 393 K. After cooling down, the resulting crystalline solid was washed with DMF and then the solvent was exchanged with dichloromethane within 3 days. The product was first activated at 393 K using dynamic vacuum for 4 h and then an evacuation was continued for 15 h. During the activation the color changed from green to yellow, which serves as a visual indicator of a structural transition in this particular case. Room temperature PXRD patterns of the “as made” and activated materials as well as nitrogen physisorption data at 77 K are given in Figures S1 and S2. DUT-8(Ni) Rigid. Ni(NO3)2·6H2O (0.870 g, 3.0 mmol), dabco (0.672 g, 6 mmol), and 2,6-H2NDC (0.588 g, 2.7 mmol) were dissolved in 60 mL DMF. The solutions were mixed in an ultrasonic bath for 10 min and the resulting suspension was transferred into a 50 mL Teflon lined autoclave. Afterward the autoclave was heated for 3 days at 408 K. After cooling down, the resulting crystalline solid was washed two times with DMF and then one time with ethanol. After that, the solid was filtered and the solvent was removed from the pores in dynamic vacuum at 443 K. After the activation, the color of the solid remained unchanged. Room temperature PXRD patterns measured from the “as made” and activated materials are 14247


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longer time (up to 15 min) showed no spectral changes indicating that thermal equilibrium was almost reached before each measurement. Simulations of the EPR signals were realized with the MATLAB toolbox EasySpin32 which employs the exact diagonalization of the spin Hamiltonian matrix. The signal of NO gas was simulated as described recently.20 The number of desorbed NO molecules at room temperature was estimated from the line width δBNOgas of the EPR signal of desorbed NO gas as determined by simulation, using a relation published earlier.20 For low temperatures this approach is not feasible since the temperature gradient in the sample tube prevents a direct determination of the amount of desorbed NO gas from its EPR line width applying kinetic gas theory and EPR line width theory.20 Therefore, several EPR samples with the same geometries but different room temperature mol densities nRT of pure NO gas were prepared. X-band cw EPR measurements of these samples were performed at the temperature T = 123 K. The homogeneous Lorentzian peakto-peak line widths δBNOgas of the corresponding NO gas signals were determined by simulation.20 The variation of nRT with these line widths follows in a good approximation a linear relation nRT (δBNOgas) (Figure S3). This relation was used to estimate from δBNOgas at T ≈ 125 K the mol numbers of desorbed NO gas in samples F62, F860, R60, and R800. The EPR intensities (INOgas of the signal of desorbed NO gas and IX of the signals of the EPR active species X = A, B, C, D, E) were determined by double integration of the simulations of the corresponding signals. If more than one species X contributed to the EPR spectrum, the relative intensities of the species X were determined by fitting the sum of the different simulated signals to the spectrum. The numbers NX (X = A, B, C, D, E) of spins contributing to the observed EPR signals of immobilized NO species were estimated by relating the corresponding signal intensities IX multiplied by T to a reference signal of an ultramarine sample with a known number of spins using a rectangular Bruker ER4105DR dual mode cavity in combination with an Oxford Instruments He ESR 900 cryostat. The stated multiplication by the temperature considers the temperature dependency of the magnetic susceptibility as expressed in Curie’s law. Almost all numbers of spins are given in units of moles per mass of the DUT-8(Ni) material in the respective samples. This mass differs from sample to sample. Therefore, the chosen unit ensures the comparability of the amounts of species adsorbed on the MOF material. However, the amount of desorbed NO might differ between the samples in this unit even if it is almost equal in units of moles (Table 1).

identical and a nitrogen physisorption experiment, performed at 77 K, results in a “type I” isotherm, typical for microporous materials (Figures S1 and S2). EPR Sample Preparation. In this work we consider three EPR samples of the flexible DUT-8(Ni) material called samples F0, F62, and F860 and three samples of its rigid derivative called samples R0, R60, and R800. Here the letters “F” and “R” are abbreviations for “flexible” and “rigid”, whereas the number afterward denotes the room temperature NO gas pressure in the corresponding EPR sample in units of mbar, assuming full desorption (Table 1). For each EPR sample a small amount Table 1. Mass m of the Activated Flexible DUT-8(Ni) or Its Activated Rigid Derivative in the Different EPR Samples, Moles of NNO of Loaded NO Molecules in the Sample Per Mass of the Activated DUT-8(Ni) Powder in That Sample, Room Temperature NO Gas Pressure pRT Assuming Full Desorption of NO Inside the Different EPR Sample Tubes, and Moles of Nads Per Mass DUT-8(Ni) of Adsorbed NO Molecules at Temperature T = 125 ± 2 K EPR sample F0 (flexible) F62 (flexible) F860 (flexible) R0 (rigid) R60 (rigid) R800 (rigid)

m (mg)

NNO (μmol/mg)

pRT (mbar)

Nads (μmol/mg)

5.6 10.4

0 0.28 ± 0.03a

0 62 ± 7

0.08/0.125b ± 0.03

5.7

7.1 ± 0.7

860 ± 90

4.4 5.5 4.0

0 0.51 ± 0.05 10.1 ± 1.0

0 60 ± 6 800 ± 80

5.4/6.6b ± 0.7 >0.46 10.0 ± 1.0

a

Note that for sample F62 the amount of MOF loaded is about twice that in the other samples, which has to be considered comparing NNO of sample F62 with R60 in the given units. bDuring adsorption/ desorption.

(Table 1) of the corresponding MOF was transferred into a conventional quartz glass X-band EPR tube. Then, the samples were activated under dynamic vacuum at pressures p < 10−4 mbar and temperature T = 393 K for about 16 h. After activation, the samples F0 and R0 were immediately sealed keeping the vacuum in the EPR tube. Then samples F62 and R60 were loaded with a small amount (Table 1) of nitric oxide using a special stainless steel vacuum line.24 The NO gas was condensed into the EPR tubes applying a cold trap where the sample volumes containing the MOF material were cooled at a temperature T = 77 K. From the NO gas pressure differences measured at the vacuum line, the moles of the loaded NO were calculated. The application of the cold trap during the sealing of the corresponding quartz glass tubes ensured that the entire amount of loaded NO was trapped within the EPR tubes. This was also verified by appropriate measurements of the NO gas pressure in the remaining lines. In the same way samples F860 and R800 were loaded with larger amounts of NO (Table 1). Spectroscopic Details. Cw EPR experiments were performed on a Bruker EMX micro (X-band, 9.4 GHz) spectrometer fitted with a Bruker ER 4119HS cylindrical cavity. Low temperature measurements were realized with an Oxford Instruments He cryostat ESR 900. In all experiments the modulation amplitude was set to 0.3 mT and the microwave power was adjusted such that no line shape distortion through saturation occurred. Before each measurement at a certain temperature at least 2 min passed to allow the system to reach thermal equilibrium. Test measurements performed after a

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RESULTS The room temperature spectrum of the sample F0 with the activated flexible DUT-8(Ni) and a low temperature spectrum measured for this sample at T = 8 K show no significant signals originating from the MOF (Figure S4a,b). Only at T = 8 K has a less intense broad signal been observed at a magnetic field position of about B = 100 mT. It might be related to single or clustered Ni2+ centers with total electron spin S ≥ 1 (Figure S4b). Due to the small signal intensity such centers might belong to defects or impurities and will not be discussed further in this paper. The room temperature X-band EPR spectrum of the sample R0 with the activated rigid DUT-8(Ni) (Figure S5a) shows a broad isotropic line at g = 2.49 ± 0.05 with a peak-to-peak line 14248


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The Journal of Physical Chemistry C width δBpp = 164 ± 40 mT. At temperature T = 9 K this line vanishes (Figure S5b). An analogous signal has been reported for calcined Ni/CeO2 samples.33 There this signal was attributed to small NiO domains showing the characteristic antiferromagnetism we have also observed in the measurements reported here. Therefore, we attribute this signal to a NiO impurity phase. It was also observed for samples R60 and R800 but will not be discussed further. In the case of an overlap of the signal of NiO with signals of interest, the former has been subtracted from the corresponding EPR spectra before further analysis. EPR spectra of the NO loaded samples F62, F860, R60, and R800 have been measured during complete temperature cycles including the cooling from room temperature to T = 7 K and the subsequent heating back to room temperature. In these measurements the EPR signal of the lowest rotational level of the 2Π3/2 state of desorbed NO gas18−20 has been observed for all four samples. Exemplary signals of NO gas in the samples F62 and F860 are shown in Figure 2a−d.

temperatures the signal intensity INOgas decreases drastically until it approaches zero at about Tdes = 93 ± 11 K. The heating (desorption) branch of INOgas has a maximum at Tmax = 149 ± 22 K. This value is larger than the value Tmax ≈ 132 K which has been approached during cooling. Between 104 K < T < 149 K, INOgas is clearly smaller in the desorption than in the adsorption branch. This hysteresis is also proven by the line widths δBNOgas of the EPR signals of desorbed NO, measured at T = 115 K during cooling and subsequent heating (Figure 2a,b). At this temperature, δBNOgas is clearly larger in the adsorption branch than in the desorption branch. For the latter case three lines can be observed, which correspond to the three ΔmJ = ± 1 transitions where ΔmJ is the change of the magnetic quantum number mJ of the total angular momentum J = 3/2 during the particular EPR transition.18 These three lines have been collapsed into one line in the adsorption branch at T = 115 K as a result of the higher NO gas pressure.20 This proves directly that more NO gas is desorbed at this temperature during the temperature driven adsorption than during the desorption. We have recently established a relation between the room temperature line width of the EPR signal of NO gas and its room temperature pressure.20 The homogeneous Lorentzian peak-to-peak line width of the room temperature EPR signal of NO gas in sample F62 has been determined by simulation to be δBNOgas = 11.6 ± 1.5 (Figure S6). This translates into a room temperature gas pressure of pRT = 51 ± 11 mbar20 meaning that NRT des = 0.23 ± 0.05 μmol/mg NO per DUT-8(Ni) stays in the gaseous phase outside the MOF (Table 1). This value indicates that nearly all NO is desorbed at room temperature (Table 1). About NRT ads = 0.05 ± 0.05 μmol/mg NO are adsorbed at T = 295 K. The amount of NO which is desorbed in sample F62 at T = 127 K can also be estimated from the line width of its EPR signal. This line width is δBNOgas = 13.5 ± 1.5 mT during cooling and δBNOgas = 8.7 ± 0.8 mT during heating as determined by simulation. From nRT (δBNOgas) (Figure S3) one can estimate for this temperature, that in sample F62 N127K ads = 0.08 ± 0.07 μmol/mg NO are adsorbed during cooling and N127K ads = 0.16 ± 0.06 μmol/mg NO are adsorbed during heating (Table 1). The first value is close to the amount NRT ads = 0.05 ± 0.05 μmol/mg of NO adsorbed at room temperature. This indicates that a significant adsorption of NO starts below T = 127 K. For the flexible DUT-8(Ni) sample F860 with much NO (Table 1) the EPR signal of desorbed NO gas has only been observed at temperatures 93 K < T ≤ 138 K. The absence of this signal at higher temperatures can be explained by its significant pressure broadening preventing its detection.20 However, δBNOgas becomes smaller during cooling and the EPR signal of desorbed NO gas becomes detectable at temperatures T ≤ 127 K. This indicates the adsorption of NO gas reducing the pressure broadening effect. Indeed, below T = 127 K, INOgas decreases with decreasing temperature until it approaches almost zero at Tdes = 104 ± 11 K (Figure 3b). For sample F62, a hysteresis occurs: The signal intensity INOgas is smaller in the desorption branch than in the adsorption branch at temperatures 104 K < T ≤ 127 K (Figure 3b). This difference is also reflected by the different line widths at T = 127 K which are δBNOgas = 59 ± 6 mT during cooling and δBNOgas = 31 ± 3 mT during heating (Figure 2c,d). According to the relation nRT (δBNOgas) (Figure S3) these values translate 127K into N127K ads = 5.4 ± 1.0 μmol/mg and Nads = 6.6 ± 0.8 μmol/

Figure 2. X-band EPR spectra of sample F62 measured at T = 115 K during cooling (a) and heating (b) and F860 measured at T = 127 K during cooling (c) and heating (d). The spectra show the EPR signal of desorbed NO gas.

For the flexible DUT-8(Ni) sample F62 with less NO (Table 1) the EPR signal of desorbed NO gas was observed at temperatures 82 K < T ≤ 295 K. The temperature dependence of its signal intensity INOgas is shown in Figure 3a. The cooling (adsorption) branch of INOgas has a maximum at Tmax = 132 ± 17 K. The decrease of INOgas at higher temperatures expresses the depopulation of the corresponding rotational ground state of the 2Π3/2 state in combination with Curie’s law.22 At lower 14249


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Figure 3. Intensities INOgas of the X-band EPR signal of NO gas in samples F62 (a), F860 (b), R60 (c), and R800 (d) measured at different temperatures during cooling (open squares, solid line) and heating (filled circles, dashed line).

8(Ni) sample F860 with a similar amount of loaded NO (Table 1). The temperature dependence of the intensity INOgas for sample R800 is shown in Figure 3d. No adsorption/desorption related hysteresis loop has been resolved. Below Tdes = 121 ± 5 K almost all NO has been adsorbed. This value is larger than Tdes ≈ 104 K as determined for the equivalent flexible DUT8(Ni) sample F860. At T = 126 K the line width of the EPR signal of desorbed NO in sample R800 is δBNOgas = 2.8 ± 0.3 mT. One can estimate from this value that the amount of adsorbed NO at T = 126 K is N126K ads = 10.0 ± 1.0 μmol/mg (Figure S3). This is clearly more than N127K ads as determined for the equivalent flexible DUT-8(Ni) sample F860 (Table 1). All EPR signals which we will attribute to immobilized NO species have been measured for the samples F62, F860, R60, and R800 within a full temperature cycle starting with cooling from room temperature down to T ≈ 10 K and followed by the subsequent heating up to room temperature. In all cases two EPR signals of immobilized NO species measured in the same sample at the same temperature but during cooling and heating were identical. Therefore, we only report and discuss signals which have been measured during cooling (Figures S7, S8, S9, S10). Spectral simulations of the EPR spectra of samples F62, F860, R60, and R800 in the field range 260 mT < B < 320 mT revealed up to five paramagnetic species with electron spin S = 1/2 for the NO adsorbed DUT-8(Ni) samples. We will call these species, as well as their corresponding signals, A, B, C, D, and E. The simulation of each signal is defined by seven values: its relative intensity; the three principle values gxx, gyy, and gzz of its g-tensor; and an anisotropic broadening model described by the values δgxx, δgyy, and δgzz which are the full widths at half height of three independent Gaussian distributions assumed for the three g-tensor principle values. Using a least-squares procedure, each measured spectrum has been fitted by a sum

mg adsorbed NO, respectively (Table 1). This proves directly that at T = 127 K less NO was adsorbed during cooling than heating. The EPR spectra of the rigid DUT-8(Ni) sample R60 with less NO show the signal of desorbed NO gas at temperatures 121 K ≤ T ≤ 295 K. The corresponding intensity INOgas reveals no hysteresis comparing the adsorption and desorption branch (Figure 3c). Its maximum occurs at Tmax = 185 ± 25 K. Below this temperature INOgas distinctly decreases until the EPR signal of NO gas has vanished at Tdes = 124 ± 9 K. Both temperatures Tmax and Tdes are clearly larger than for sample F62. This indicates that for the rigid DUT-8(Ni) sample R60 the adsorption of NO happens at higher temperatures than for the flexible DUT-8(Ni) sample F62, although both samples have been loaded with an equal amount of NO in units of moles (Table 1). The room temperature EPR signal of desorbed NO in sample R60 has a line width δBNOgas = 11.2 ± 1.3 mT h which translates into a room temperature NO gas pressure pRT = 49.2 ± 10 mbar.20 Therefore, NRT ads = 0.11 ± 0.11 μmol/mg NO is adsorbed for sample R60 at T ≈ 295 K (Table 1). The latter value is larger than NRT ads ≈ 0.05 μmol/mg as determined for the equivalent flexible DUT-8(Ni) sample F62 (Table 1). At T = 124 K the line width of the EPR signal of desorbed NO in sample R60 is δBNOgas = 0.7 ± 0.3 mT. Again, one can derive from the relation nRT (δBNOgas) (Figure S3) that almost all NO, 124K > 0.46 μmol/mg, are adsorbed at this namely, Nads temperature. This is significantly more than we have determined at T = 127 K for the equivalent flexible DUT8(Ni) sample F62 (Table 1). For the rigid DUT-8(Ni) sample R800 with a large amount of loaded NO (Table 1), the EPR signal of desorbed NO gas has been detected at temperatures 100 ≤ T ≤ 136 K. This range is comparable to the one observed for the flexible DUT14250


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Figure 4. EPR spectra (lowest) and simulations (above) of samples F62 measured at T = 82 K (a), F860 measured at T = 62 K (b), R60 measured at T = 79 K (c), and R800 measured at T = 82 K (d).

Table 2. Samples Where the Corresponding Species Are Present and g-Tensor Principle Values of the Different EPR Active Species Which Have Been Attributed to Ni2+-NO Adsorption Complexes species

gxx

samples

A B C D

F62, F860, R60, R800 F860, R60, R800c R60, R800 R60, R800

E NO@Ni(65%)-Y Zeolitea Ni2+-NO@SiO2b

R60d -

2.169 2.145 2.137 2.336 2.354 2.325 2.171 2.156

± ± ± ± ± ± ±

gyy 0.009 0.012 0.018 0.005e 0.004f 0.003 0.002

2.169 2.199 2.137 2.336 2.320 2.325 2.171 2.184

± ± ± ± ± ± ±

gzz 0.009 0.038 0.018 0.005e 0.004f 0.003 0.002

2.355 ± 0.014 2.332 ± 0.038 2.294 ± 0.043 gzz (Table 2). The corresponding gzz singularities are not resolved at any temperatures due to their large line widths or the superposition with other signals. One can still estimate by spectral simulations for both species an upper bound gzz < 2.23. The observed relation gxx,yy > gzz is indicative for a 3d9 Ni+ or 3d7 Ni3+ ion with the unpaired electron in the dz2 orbital. Similar signals have been observed for Ni3+ in an S = 1/2 low spin state with a (dz2)1 ground state configuration.40−42 Equivalent signals of Ni-doped LiF and NaF crystals have also been attributed to Ni+ ions in a tetragonal compressed octahedral ligand field with the unpaired electron in the dz2 orbital.39 For this case one can derive the second-order relations37−39

(1)

8λ Δ2

should hold approximately for this system. Here, λ is the spin− orbit coupling constant, Δ1 is the energy difference between the (dx2−y2)1 ground state configuration and the (dyz)1 and (dxz)1 configurations of the corresponding excited states, whereas Δ2 is the energy difference between the ground state configuration and the (dxy)1 excited state configuration. It follows that gxx,yy < gzz as observed for species A. Therefore, we attribute species A to a Ni2+-NO adsorption complex where the unpaired electron is in the dx2−y2 orbital at the Ni2+ ion. The EPR spectra of the flexible DUT-8(Ni) sample F860 with much NO, which have been measured at different temperatures between T = 7 K and T = 294 K, are given in Figure S8. The spectrum which has been measured at T = 62 K and its simulation are shown in Figure 4b. Again, the spectra show the signal of species A at temperatures T ≥ 137 K. For temperatures T ≤ 127 K this signal changes: A new bump occurs at a field position g = 2.25 ± 0.02 (Figures 4b, S8). It was not possible to explain the overall signal by a single S = 1/2 species even with an orthorhombic g-tensor. In addition, leastsquares fits of this signal by the sum of two contributing paramagnetic species with electron spin S = 1/2, having both axial symmetric g-tensors, do not lead to satisfying results. The assumption that the observed signal has contributions from two paramagnetic S = 1/2 species, one with an axial symmetric gtensor, and the second with an orthorhombic g-tensor, leads to a fit of high accuracy (Figure 4b). The former can be assigned to species A whereas the latter is a new species B. Its g-tensor principle values are given in Table 2. Since it has an orthorhombic g-tensor it might be attributed to a Ni2+-NO adsorption complex where the ligand environment of the Ni2+ ion has lower than axial symmetry. Temperature dependent EPR spectra of the rigid DUT8(Ni) sample R60 with less NO are summarized in Figure S9. A representative spectrum measured at T = 79 K and its simulation are shown in Figure 4c. Assuming that solely species with electron spin S = 1/2 contribute to the observed signals, the latter can only be explained by at least five species. The high field edge singularity of the measured spectrum at T = 79 K has a shoulder at B = 310 mT and is slightly split into two minima at B = 314 mT (Figure 4c). A sum of at least three different S = 1/2 signals is necessary to explain this part of the spectrum. One of the minima matches the field position of the gxx singularity of species B and might therefore be attributed to B. If one assumes that species A also contributes to the observed signal, the additional presence of a new S = 1/2

gxx , yy ≈ ge +

6λ Δ3

(2)

gzz ≈ ge

where Δ3 is the energy difference between the (dz2)1 ground state configuration and the (dyz)1 and (dxz)1 excited state configurations, but the signals D and E have not been observed in any EPR spectra of the activated DUT-8(Ni) samples F0 and R0. Therefore, they have to be again related to the presence of NO. We suppose that signals D and E are related to Ni2+-NO complexes with the unpaired electron in the dz2 orbital at the Ni2+ ion. Interestingly, the signal of species D becomes slightly orthorhombic at low temperatures T < 32 ± 11 K as can be seen in Figure 5a. The corresponding g-tensor principle values are given in Table 2. This indicates that the ground state of the Ni2+ ion containing the unpaired electron in the dz2 orbital adopts lower than axial symmetry at low temperatures. Temperature dependent EPR spectra of the rigid DUT8(Ni) sample R800 with much NO are shown in Figure S10. A representative spectrum at T = 82 K and its simulation are shown in Figure 4d. The experimental spectrum seems to be a sum of two signals of S = 1/2 species with gxx,yy > gzz and gxx,yy < gzz. The first one has the same gxx,yy values as species D but a larger line width (Table 2, Figure S11) and we attribute it to species D. As for sample R60, it becomes orthorhombic at temperatures T < 32 ± 11 K adopting the same g-tensor 14252


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EPR signals of species B and C for samples R60 and R800 could not be uniquely determined from the simulations owing to the poor resolution of each single signal B and C in the measured spectra: Within the error intervals of the g-tensor principle values of species B and C given in Table 2, leastsquares fits of the measured EPR spectra lead to several different solutions which differ substantially in the relative fractions of signals B and C. However, it turns out that the sum NB + NC could be determined with high confidence. Spectra of all samples show at temperatures T ≤ 62 K a signal which is typical for a paramagnetic species with electron spin S = 1/2 and a nearly axially symmetric g-tensor with principle values gxx,yy ≈ 1.95 ± 0.4 > gzz (Figures S7, S8). We attribute it to NO molecules adsorbed at diamagnetic sites.23−27 Unfortunately the gzz powder edge singularity, which is most sensitive to the electric field at the adsorption site, is not resolved. However, already the small values for gxx and gyy indicate relatively small electric field gradients25 at the adsorption site of these NO species compared to typical values for metal ion NO complexes (Mn+-NO, M = Na,26 Al,23,24 Mg,43 Zn44). Therefore, we attribute this signal to NO molecules weakly physisorbed at some nonmetallic sites of DUT-8(Ni). We call it signal NOads. Its broad line width might reflect a large inhomogeneous distribution of different adsorption sites, the presence of which has recently been indicated for different MOFs by infrared (IR) spectroscopy.24 This broadening forbids an estimation of the gzz value as well as the largest principle value of the 14N hyperfine interaction which typically occurs in a direction parallel to the molecular π*y orbital where the unpaired electron resides.

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DISCUSSION Ni2+ has a 3d8 electron configuration and can therefore occur in a high spin triplet or a low spin singlet ground state. It follows that the total electronic spin of the two magnetically coupled Ni2+ ions of the DUT-8(Ni) paddle wheel units can attain values S = 0, 1, or 2. All EPR spectra of the activated samples F0 and R0 show no signals which might be attributed to corresponding S = 1 or S = 2 states of the framework Ni2+ dimers. Indeed it has recently been indicated by first principle calculations that for both porous as well as nonporous phases of DUT-8(Ni) a low spin S = 0 ground state of these Ni2+ pairs is favored,45 but one cannot conclude that the EPR data have verified this theoretical result: Electron spins with integer values might still show no EPR signal at X-band frequencies if the zero field splitting of this spin system is larger than 80 GHz as we have confirmed by spectral simulations. For the flexible DUT-8(Ni) samples, F62 and F860 hysteresis loops of the amount of adsorbed NO have been observed by EPR of desorbed NO gas. During cooling less NO has been adsorbed than during the subsequent heating. This indicates that the structural transformation from the nonporous to the porous phase has actually occurred during NO adsorption as has also been indicated by a partial color change from yellow to green for sample F860 during a slow cooling procedure (Figure S12). In particular, the adsorption of some NO within the porous phase of the flexible DUT-8(Ni) has been proven this way. The absence of such hysteresis for samples R60 and R800 (Figure 3c,d) indicates that the rigid DUT-8(Ni) shows no such framework flexibility. For the flexible DUT-8(Ni) sample F62 with less NO the temperature dependence of INOgas (Figure 3a) indicates a significant adsorption of NO at temperatures T < 127 K

Figure 5. EPR spectra (lower) and simulations (upper) of samples R60 measured at T = 21 K (a) and R800 measured at T = 21 K (b).

principle values (Figure 5b, Table 2). The latter signal with gxx,yy < gzz could not be simulated by a single S = 1/2 species, but an unraveling determination of all contributing species from the experimental spectrum is not possible without any constraints. We suppose that the species observed for sample R60 are also present in sample R800. Therefore, the experimental spectra of sample R800 have been fitted by a sum of the signals of species A, B, C, and D keeping their g-tensor principle values in the error intervals determined for the former three samples (Figure 4d, Figure 5b). The line widths and relative intensities have not been constrained to any interval. The signal of species E does not verifiably contribute to the experimental spectra and has been omitted in the fits. In that way all spectra of sample R800 could be fitted with high accuracy. For all four samples F62, F860, R60, and R800 the EPR spectra which have been attributed to Ni2+-NO adsorption complexes have been fitted for all temperatures by the sum of up to five species A to E. According to eq 2, gzz = 2.0 has been assumed for the simulations of signals D and E. The corresponding line width parameter has been arbitrarily chosen to be δgzz = 0.1 mT. In that way the numbers NA, NB, NB + NC, ND, and NE of spins contributing to the different species have been estimated from the relative intensities as determined from the fits. Their temperature dependencies are shown in Figure 6a−d. The numbers NB and NC of spins contributing to the 14253


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Figure 6. Number of spins contributing to the signals of species A, B, C, and D in samples F62 (a), F860 (b), R60 (c), and R800 (d). (a) NA (squares), (b) NA (squares), NB (triangles), (c and d) NA (squares), NB+NC (triangles), ND (circles). Due to the bad resolution of signals B and C in samples R60 and R800, only the sum NB+NC could be determined with confidence.

temperatures than the boiling point (Figure 3c), almost the entire NO can adsorb within the porous phase. For samples F860 and R800 with much NO the maximum of INOgas(T) cannot be observed, since the pressure broadening of the EPR signal of desorbed NO prevents its detection at high temperatures. However, one can estimate from the EPR line width of desorbed NO that at T ≈ 125 K about f cooling F860 = 76 ± 15% of all NO in the flexible DUT-8(Ni) sample F860 is = 90 ± 10% is adsorbed during cooling, whereas f heating F860 adsorbed during the subsequent heating (Table 1). This difference shows that adsorption of NO within the pores has heating actually occurred. The values f cooling F860 and f F860 are significantly cooling heating larger than the values f F62 and f F62 obtained at lower NO loading. This indicates that for higher NO pressures a larger fraction of the MOF material is transformed into the porous phase at T ≈ 125 K and consequently the amount of NO adsorbed in the pores is higher. This also indicates that for the sample F860 most NO is adsorbed within the pores and not on the outer surface of the MOF. Otherwise one would expect ≥ f cooling f cooling F62 F860 since adsorption sites on the outer surface of the MOF should be equally accessible for both samples F62 and F860. For the rigid DUT-8(Ni) sample R800 about f R800 = 90 ± 10% of all NO is adsorbed at T ≈ 125 K (Table 1). The relation f R60 ≈ f R800 holds. So we do not observe for the rigid DUT-8(Ni) samples a correlation between the total amount of loaded NO and the fraction of NO which has been adsorbed at T ≈ 125 K. This result is in line with the proposed absence of flexibility for the rigid DUT-8(Ni) material and further indicates that even for sample R800 the largest possible amount of NO which can be adsorbed has not been reached. Comparing samples F860 and R800 with much NO one can recognize that the fraction of adsorbed NO at T ≈ 125 K in the desorption branch is almost equal for both samples, since f heating F860 ≈ f R800. This is an additional indication that for sample F860

whereas for the equivalent rigid DUT-8(Ni) sample R60 the adsorption of NO starts at higher temperatures 160 K < T < 180 K (Figure 3c). According to the EPR line width of = 30 ± 15% of all NO in sample desorbed NO, less than f cooling F62 F62 is adsorbed during cooling at T ≈ 125 K, whereas more than f R60 = 90% of all NO in sample R60 is adsorbed at this temperature (Table 1). This indicates additionally that for sample F62 the nonporous phase is predominantly present at high temperatures preventing the adsorption of NO, whereas for sample R60 the porous phase is the most abundant phase at = 45 ± all temperatures. During the subsequent heating, f heating F62 11% of all NO in sample F62 is adsorbed at T ≈ 125 K (Table indicating again the presence 1). This value is larger than f cooling F62 of the adsorption triggered structural transformation between is still smaller the nonporous and porous phases, but f heating F62 than f R60. This observation indicates that for the flexible DUT8(Ni) sample F62 a fraction of NO has not been adsorbed within the porous phase during cooling. Otherwise, one would ≈ f R60. One explanation might be that not expect that f heating F62 enough pores of the flexible DUT-8(Ni) have opened for sample F62 to allow the adsorption of as much NO as adsorbed on the rigid DUT-8(Ni) material in sample R60. However, we have to say that a part of NO is likely to freeze out for sample F62 in the temperature range 82 K < T < 115 K. We have observed by EPR for a sample of pure NO gas with a room temperature gas pressure pNOgas = 47 mbar that INOgas decreases to zero at such temperatures (Figure S13). This observation is in line with the known boiling point of NO which adopts T = 122 K at pNOgas = 800 mbar, T = 100 K at pNOgas = 50 mbar, and T = 90 K at zero gas pressure.46 Thus, for sample F62 the adsorption of NO occurs in a temperature range where the condensation of this gas also takes place (Figure 3a). Therefore, a portion of NO was presumably not adsorbed at low temperatures but has frozen out. Since for the rigid DUT8(Ni) sample R60 the adsorption of NO occurs at higher 14254


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The Journal of Physical Chemistry C the NO has opened a sufficient pore volume in the flexible DUT-8(Ni) to adsorb as much NO as for sample R800. The presented results confirm the PXRD data (Figure S1) which show that the flexible activated DUT-8(Ni) stays at room temperature in the nonporous phase whereas the rigid activated DUT-8(Ni) stays in the porous phase. In addition, the nitrogen adsorption experiments at T = 77 K (Figure S2) are consistent with the EPR data of desorbed NO. They show for the flexible DUT-8(Ni) a significant hysteresis loop in the adsorption and desorption isotherms indicating an adsorption induced structural transformation from the nonporous to the porous phase at some threshold pressure. For the rigid DUT8(Ni) the nitrogen adsorption isotherms are of type I and show no such hysteresis, indicating constant porosity of the material at all pressures. In addition to the EPR signal of desorbed NO gas, signals have been observed which we have attributed to the sum of up to five species A to E of Ni2+-NO complexes, but there are no coordinatively unsaturated sites at the regular Ni2+ ions assuming a defect-free, ordered DUT-8(Ni) structure. Therefore, species A to E are most likely NO moieties adsorbed at defective sites of the regular Ni2+-paddle wheels or Ni2+-NO species of some impurity phase. This is also supported by the number of spins contributing to the signals of the different species. Based on the sum formula the number density of regular Ni2+ ions in DUT-8(Ni) should be NNi = 3.04 μmol/ mg. The number of loaded NO molecules in sample R800 is NNO ≈ 10 μmol/mg (Table 1), but the total number of spins contributing to signals A to E is less than NNi‑NO ≈ 10 nmol/mg as observed for sample R800 at T = 21 K (Figure 6d). This number is more than 2 orders of magnitude smaller than NNi and NNO for the sample R800, which indicates the defective or impurity related nature of species A to E. Their fraction of the total amount of Ni2+ ions in the MOF frameworks is approximately 0.5% or below. Therefore, these species cannot be attributed to regular framework ion sites. For all samples the small fraction of NO contributing to species A to E and the small fraction of NO contributing to desorbed NO at temperatures T < 125 K (Table 1, Figure 3a− d) indicate that most NO in the samples becomes nondetectable for EPR at low temperatures. We have proven experimentally that desorbed NO freezes out at temperatures T < 115 (Figure S13). According to the temperature dependencies of INOgas (Figure 3a−d) a significant amount of NO might freeze out only for sample F62. So, there must be other possibilities which explain the EPR silence of most of the NO at low temperatures. NO might form diamagnetic N 2 O 2 adsorption complexes at the outer surface or within the pores of the DUT-8(Ni). It might also be possible that NO is still mobile inside the pores of the porous phase but the time between two collisions of an NO molecule with another or with the surface of the host material might be so short that its EPR signal becomes too broad to be detectable anymore. Indeed, the presence of a diamagnetic phase of adsorbed NO was also observed for NO adsorbed in ZSM-5 and Na-A zeolites.23 Interestingly, no lifetime broadening has been observed for all signals A to E at all temperatures up to room temperature. This means that the lifetime of these species should be longer than nanoseconds even at room temperature. Indeed, the proposed changes of the electronic configuration of the Ni2+ ions within the Ni2+-NO adsorption complexes are more characteristic for a chemisorption than physisorption of the NO at the Ni2+, which might explain the long lifetime of these

species at ambient temperatures. Nevertheless, the backward and forward reactions Ni 2 + + NO ↔ Ni 2 +−NO

(3)

have some dynamic equilibrium which depends on the temperature T and the concentration cNO of nonbonded NO species which have steric access to the free Ni2+ sites. Lower temperatures and higher concentrations cNO should shift this equilibrium to higher numbers of the NO adsorption complexes. In a next step we want to investigate the nature of the species A to E based on the temperature dependencies of their number of spins (Figure 6a−d). Each of these species might belong to defect sites on the outer surface of DUT-8(Ni) or an impurity phase or to defect sites within the porous phase of the rigid DUT-8(Ni), flexible DUT-8(Ni), or a porous impurity phase. Combinations of these possibilities might also hold for a single species defined by its EPR signal. Since we do not see any EPR signal of desorbed NO for sample R60 at temperatures T < 120 K, the change of the total number of desorbed NO should be smaller than the detection limit NLimit = 0.5 nmol/mg (Figure S14). However, the sum of the number of spins contributing to signals B and C for sample R60 increases approximately Δ(NB + NC) = 1.4 nmol/mg during the cooling from T = 116 K to T = 12 K (Figure 6c) which is distinctly larger than NLimit. Therefore, if species B or C should be defects on the outer surface of the MOF material or of some impurities, the observed increase of NB + NC might be explained by a rearrangement of diamagnetic N2O2 adsorbed at that surface, but not by an adsorption from the gaseous phase of NO. However, in that case one might expect that significant amounts of species B and C have already formed at higher temperatures, where more molecules of desorbed NO have steric access to these surface sites. However, for samples R60 and R800 NB + NC starts to increase significantly during cooling in the temperature range T < 130 K (Figure 6c,d), where the adsorption of most of the NO has finished (Figure 3c,d). Therefore, we attribute both species to Ni2+-NO defect sites in the pores of the rigid DUT-8(Ni), although we cannot fully rule out that they might be formed within a porous impurity phase. Since we have not observed species C for samples F62 and F860 at any temperature, it seems to occur only in the rigid but not the flexible DUT-8(Ni). For the same reason we can exclude the presence of significant amounts of the rigid DUT8(Ni) in samples F62 and F860. Species B has also been detected for the flexible DUT-8(Ni) sample F860 (Figure 4b), so it seems to occur in both variations of this MOF. For sample F860 the temperature dependence NB (Figure 6b) shows a similar correlation with the temperature range where NO becomes adsorbed (Figure 3b) as observed for NB + NC in the case of samples R60 and R800. Therefore, we attribute species B additionally to defect centers within the porous phase of the flexible DUT-8(Ni). We have not observed species D and E for the NO loaded flexible DUT-8(Ni) samples F62 and F860. Therefore, we likewise attribute both species to Ni2+-NO defect sites which occur only in the NO loaded rigid DUT-8(Ni) material. Again, the absence of species D and E in the samples F62 and F860 shows that there are not significant amounts of rigid DUT8(Ni) in these samples. For the rigid DUT-8(Ni) sample R800 with much NO the number of spins ND contributing to species D has the same temperature dependence as NB + NC (Figure 6d). For the rigid DUT-8(Ni) sample R60 with less NO, ND 14255


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Figure 7. Schematic drawings of the possible defect models, which might explain the EPR spectra: (a) NO-ax-1Nimissing Ni2+ in the paddlewheel and coordination of the NO along the axial directionthis model might explain species A, B, and C; (b) NO-ax-2Nimissing of the dabco molecule and coordination of the NO in the axial directionthis model might explain species A, B, and C; (c) NO-eq-1Nimissing of NDC2− and Ni2+ and coordination of the NO in the equatorial planethis model might explain species C and D; (d) NO-eq-2Nibroken bond between the Ni2+ and the NDC ligand and coordination of the NO in the equatorial planethis model might explain species C and D. It was not possible to deduce from the EPR data whether the bonds to the dabco and NDC ligands are only broken as in the drawings (a) an (d) or these ligands are completely missing as in the drawings (b) and (c). The defect models in the present sketches have been assumed just for illustration.

and NE also increase in the same temperature range as NB + NC but approach distinctly smaller values at low temperatures (Figure 6c). Therefore, we likewise attribute both species D and E to defect sites in the pores of the rigid DUT-8(Ni). Comparing NB + NC and ND for the rigid DUT-8(Ni) at different NO loading indicates that the formation of species C or B is favored with respect to species D (Figure 6c,d). Species A has been observed for all four samples where it is the most abundant species of immobilized NO at all temperatures. Its number of spins NA shows for samples F860 and R800 a stepwise increase in a temperature range (Figure 6b,d), where the adsorption of NO occurs (Figure 3b,d) and where the numbers of spins contributing to species B, C, and D start to increase (Figure 6b,d). Therefore, we likewise attribute species A to Ni2+-NO defect sites within the porous phase of the flexible and rigid DUT-8(Ni). For samples F62 and R60 with less NO, such a step in NA(T) is absent and NA increases continuously with decreasing temperature. Due to the small total amount of NO in these samples, the change in cNO after the adsorption of NO within the pores might not be large enough to shift reaction 3 toward the products to such an extent that NA(T) shows a stepwise increase in that temperature range. For the rigid DUT-8(Ni) sample R800 with much NO, NA increases by an amount ΔNA ≈ 1 nmol/mg while cooling from T = 100 K to T = 12 K. This also supports the proposed assignment of species A to Ni2+ related defect sites within the pores. If species A was solely related to sites at the outer surface of DUT-8(Ni), the amount of desorbed NO, which is smaller than NLimit = 0.5 nmol/mg in that temperature range (Figure S14), would presumably be an upper limit for ΔNA. We have also observed a considerable amount of species A and of species C at temperatures where a significant adsorption of NO has not started yet (Figure 3a−d, Figure 6a−d). For

example, the rigid DUT-8(Ni) sample R60 with less NO shows up to NA ≈ 1 nmol/mg spins contributing to species A at T = 181 K. Indeed, we have observed the signal of species A for all NO loaded samples at room temperature. At T = 295 K the amount of species A is equal for both flexible DUT-8(Ni) samples F62 and F860 (NA ≈ 0.07 nmol/mg). This indicates that all corresponding adsorption sites, which are accessible for NO at room temperature, have already been saturated for sample F62. The same might hold for species C, since both rigid DUT-8(Ni) samples R60 and R800 show at T = 295 K NC = 0.16 ± 0.07 nmol/mg spins contributing to species C. Therefore, a distinct number of defects, which are related to species A and C, might be localized at the outer surface of the DUT-8(Ni). However, for the flexible DUT-8(Ni) samples some fractions of a porous phase, which are present at high temperatures, might also explain the occurrence of species A at T = 295 K. Therefore, one can still not conclude unambiguously from the EPR data whether species A and C are related solely to defects within the porous phase or additionally to defects at the outer surface of the MOF material. The observed amount of species A is approximately 1 order of magnitude larger for the rigid DUT-8(Ni) samples R60 and R800 than for the flexible DUT-8(Ni) samples F62 and F860, respectively (Figure 6a−d). We have already gotten evidence that for the flexible DUT-8(Ni) sample F860 as much NO as for the rigid DUT-8(Ni) sample R800 has become adsorbed within the porous phase, whereas the presence of significant traces of the rigid DUT-8(Ni) in samples F62 and F860 can be excluded. Therefore, the results indicate that the density of defective species A is 1 order of magnitude larger in the rigid compared to the flexible DUT-8(Ni). Up to now, we have attributed all observed species A, B, C, D, and E to Ni2+-NO complexes which are formed at defective Ni2+ sites within the pores of the DUT-8(Ni) material. The 14256


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The Journal of Physical Chemistry C order of the g-tensor principle values gxx,yy < gzz of species A and C (Table 2) indicates that these moieties are related to Ni2+NO complexes where the remaining unpaired electron is in the dx2 − y2 orbital of the Ni2+ ion (eq 1). The EPR data are consistent in the following two defect models for species A or C. We cannot deduce from the data which model applies for which species: NO-ax-1Ni. NO might bind along the fourfold axial symmetry axis to a defective paddle wheel unit with only one Ni2+ ion (Figure 7a). A similar defect has recently been identified in a MOF material with Cu2+ paddle wheels.47 The dz2 orbital of the Ni2+ and the πy* MO of the NO molecule will then form a bonding dz2 + πy* MO leading to an effective (dz2 + π*y )2(dx2−y2)1(dz2 − π*y )0 electronic ground state configuration.30 NO-ax-2Ni. The paddle wheel unit contains the whole Ni2+ dimer but lacks one dabco ligand at whose coordination site NO binds to the Ni2+ (Figure 7b). As for the case of NO-ax1Ni, this Ni2+-NO adsorption complex may have an effective (dz2 + π*y )2(dx2−y2)1(dz2 − π*y )0 ground state configuration. In this defect model the second Ni2+ ion of the paddle wheel unit must be in the low spin S = 0 state. Otherwise the dipole− dipole interaction between the S = 1 state of the second Ni2+ and the S = 1/2 at the Ni2+-NO complex would be larger than 3 GHz, as one can estimate from the distance r ≈ 2.6 Å between the two Ni2+ ions and the corresponding formula.48 This would change the spectra of the Ni2+-NO complexes significantly and prevent the observation of the well-defined powder pattern of the S = 1/2 species as we do here. Species B features an orthorhombic g-tensor with gxx < gyy < gzz (Table 2). Since gyy is closer to gxx than gzz, we assume that this moiety is likewise formed at Ni2+ sites of defective paddle wheels where either the dabco (NO-ax-2Ni) or the second Ni2+ ion (NO-ax-1Ni) is missing, but where an additional distortion at the four equatorial oxygen binding sites is present that alters the local C4 symmetry of the metal unit. We denote this species NO-or. The order of the g-tensor principle values gzz < gxx,yy of species D and E (Table 2) is reversed compared to species A, B, and C. This indicates that for species D and E the remaining unpaired electron resides in the dz2 orbital of the Ni2+ ion.37−39 Consequently, both species D and E might be explained by one of the following two defect models, but we cannot deduce from the EPR data which model applies for which species: NO-eq-1Ni. There might be defective paddle wheel units which contain only one Ni2+ ion and do not coordinate to all four NDC ligands. Instead, NO can bind in the pseudo equatorial plane to the Ni2+ ion (Figure 7c). Now the bonding MO might be effectively a superposition of the π*y MO of the NO molecule and the dx2−y2 orbital of the Ni2+ ion. The unpaired electron might remain in the dz2 orbital and the effective electronic ground state configuration of the Ni2+-NO complex might be (dx2−y2 + π*y )2(dz2)1(dx2−y2 − π*y )0. NO-eq-2Ni. There might be paddle wheel units with two Ni2+ ions where again at least one NDC ligand is not coordinating to the metal unit. In its place NO coordinates in the pseudo equatorial plane to one of the Ni2+ ions (Figure 7d). As explained for the defect model NO-eq-1Ni, the effective electronic ground state configuration of the Ni2+-NO complex might be (dx2−y2 + πy*)2(dz2)1(dx2−y2 − πy*)0. As in case NO-ax2Ni, the second Ni2+ must be in its low spin S = 0 state. The EPR signal of species D becomes slightly orthorhombic at temperatures T < 21 K (Figure 5a,b). This indicates that the ground state of the Ni2+ ion having the unpaired electron

adopts less than axial symmetry at low temperatures. Indeed, the coordination of an NO molecule in the equatorial plane of a defective paddle wheel unit to a Ni2+ ion, as in defect models NO-eq-1Ni and NO-eq-2Ni, should distort the C4 symmetry of the metal unit. In total, the EPR results point out two factors which might be related to the difference in the framework flexibility of the two considered derivatives of DUT-8(Ni). One might speculate that the nonporous phase is stabilized by π−π stacking between NDC ligands. EPR has indicated that only in the rigid but not in the flexible DUT-8(Ni) defective paddle wheel units with missing NDC ligands are present. The partial absence of such ligands in the rigid DUT-8(Ni) might weaken the total attractive force between the NDC ligands originating from π−π stacking. This might prevent the stabilization of the nonporous phase, and the rigid DUT-8(Ni) stays porous even in the absence of any adsorbates. EPR has also indicated that the density of defective paddle wheel units with only one Ni2+ ion or with a missing dabco ligand is 1 order of magnitude larger in the rigid than in the flexible DUT-8(Ni). Such defects might stabilize the open porous structure preventing the transformation into the contracted, nonporous structure in the absence of any adsorbates.

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CONCLUSION Cw EPR investigation of NO adsorption is an ideal tool to detect tiny but important structural differences in switchable MOFs such as DUT-8(Ni). These factors are decisive in governing switchable vs nonswitchable (rigid) behavior. The EPR signal of desorbed NO and its temperature dependence indicated the adsorption of NO within the pores of the MOF materials at low temperatures, showing a hysteresis loop for the flexible DUT-8(Ni) indicating a structural transformation from the nonporous to the porous phase triggered by the presence of NO. The absence of such hysteresis for the rigid DUT-8(Ni) is in line with the proposed rigidity of this material observed by nitrogen adsorption, staying porous at all temperatures. Up to five distinct paramagnetic Ni2+-NO adsorption complexes with electron spin S = 1/2 were observed studying the porous phases of the flexible and rigid DUT-8(Ni) materials. Their number densities are less than 0.5% of the number density of regular Ni2+ ions in the DUT-8 structure indicating their defective nature. The most abundant species occurs in both materials. Its unpaired electron resides in a dx2−y2 orbital at the Ni2+ ion which might indicate that NO bonds along the axial symmetry axis to the Ni2+ ion of a defective paddle wheel, where either a dabco ligand or the second metal ion is missing. The number density of this species is 1 order of magnitude larger in the rigid than in the flexible DUT-8(Ni). This finding might be related to the difference in the flexibility of these materials. Interestingly, not for the flexible but only for the rigid DUT8(Ni) material two Ni2+-NO adsorption species were observed, where the unpaired electron resides in a dz2 orbital at the Ni2+ ion. A defect model where an NO molecule bonds in the equatorial plane of a defective paddle wheel unit to the Ni2+ ion can explain both species. This indicates that at least one NDC linker does not coordinate to these units. The absence of some NDC ligands in the rigid DUT-8(Ni) might weaken the total attractive force between the NDC ligands originating from π−π stacking. This might also explain why rigid DUT-8(Ni) stays porous even in the absence of any adsorbates. 14257


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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.jpcc.6b04984. Room temperature PXRD data, nitrogen sorption isotherms at 77 K, additional EPR related figures, line width parameters of the g-tensor principle values, photos of the NO loaded flexible DUT-8(Ni) at room temperature and 77 K, and estimation of the detection limit of desorbed NO by cw EPR (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 341 97-32608. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) in the frame of its priority program SPP 1362.

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ABBREVIATIONS EPR, electron paramagnetic resonance; DUT, Dresden University of Technology; NO, nitric oxide; NDC, 2,6naphthalenedicarboxylate; dabco, 1,4-diazabicyclo[2.2.2]octane; ELM, elastic layer-structured metal−organic framework; PXRD, powder X-ray diffraction; cw, continuous wave

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REFERENCES

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