Metal-Cation-Independent Dynamics of Phenylene Ring in

Alessandro Marchetti , Juner Chen , Zhenfeng Pang , Shenhui Li , Daishun Ling , Feng Deng , Xueqian Kong. Advanced Materials 2017 29 (14), 1605895 ...
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Metal-Cation-Independent Dynamics of Phenylene Ring in Microporous MOFs: A 2H Solid-State NMR Study Alexander E. Khudozhitkov,†,‡ Daniil I. Kolokolov,*,†,‡ Alexander G. Stepanov,*,†,‡ Vsevolod A. Bolotov,§ and Danil N. Dybtsev‡,§ †

Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia ‡ Department of Physical Chemistry, Faculty of Natural Sciences,, Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russia § Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika Lavrentieva 3, Novosibirsk 630090, Russia S Supporting Information *

ABSTRACT: Mobility of the organic linkers in metal−organic frameworks (MOFs) is an established phenomenon. Knowledge of the details of linker motion in MOFs could provide a great deal of information about the linker structure and the way the guest molecules interact with the organic framework. However, the mobility of the organic linkers is poorly characterized. The extent of the influence of the metal cation or guest molecules on linker motion is still unknown for MOFs with identical topologies. In this work, we have analyzed the rotational dynamics of the phenylene ring fragments of terephthalate (1,4-benzenedicarboxylate, bdc) linkers in the series of MOFs [M2(bdc)2(dabco)]·G (M = Co2+, Ni2+, Cu2+, Zn2+; dabco =1,4-diazabicyclo[2.2.2]octane; G = none or dimethylformamide, DMF). We have established that the reorientational motion of the phenylene rings is performed by π-flipping of the plane of the ring about its C2 axis. The dynamics of the phenylene rings is insensitive to the variation of the metal cation, whereas the loading of the guest DMF molecules provides both a significant decrease of the rate of π-flips and an increase of the activation energy for the motion of the phenylene rings.

1. INTRODUCTION Porous metal−organic frameworks (MOFs) or coordination polymers constitute a unique class of materials built of metal cations or clusters and organic polytopic ligands in regular coordination frameworks. Many fascinating MOF properties, such as magnetism,1,2 luminescence,3−5 conductivity,6 electrical,7 and nonlinear optical8 properties, are directly associated with the framework composition and its crystal structure, as well as the local molecular motion of the building units. The influence of “static” features, namely, composition and structure, on the functionality of the material has been widely investigated, whereas the dynamics of the framework moieties and its effects on the properties have received only limited attention so far. A number of research groups have studied the dynamics of the organic linkers by solid-state NMR methods in an effort to clarify the effects of the crystal structure and/or guest molecules on the dynamics.9−14 Linker mobility has been shown to modulate framework luminescence intensity,15 mediate proton transfer, and represent the key factor in the negative coefficient of thermal expansion16−19 of various MOFs. The local motions of the linkers affect the flexibility of the whole © 2015 American Chemical Society

porous framework, which, in turn, plays a critical role in guest adsorption, the diffusion mechanism, and selective molecular detection.20−24 The detailed and systematic investigation of the dynamics of organic linkers and the limits under which it can be controlled is therefore an important direction for the multidisciplinary MOF research field, providing valuable insights into the nature of many important properties of such materials. 1,4-Diazabicyclo[2.2.2]octane- (dabco-) pillared metal terephthalates [M2(bdc)2(dabco)]·G (M = Co,25,26 Ni,27 Cu,28 Zn; bdc =1,4-benzenedicarboxylate, terephthalate; G = guest molecule) are microporous coordination polymers with some degree of framework flexibility.29 Such systems provide a convenient scaffolding for in-depth investigations of molecular motions because of their robust crystal structures and readily tunable chemical compositions. Earlier, the rotational and conformational dynamics of dabco in [Zn2(bdc)2(dabco)] were probed by solid-state NMR and calorimetric methods.30 Received: September 28, 2015 Revised: November 16, 2015 Published: November 17, 2015 28038

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Figure 1. 2H NMR experimental (left) and simulated (right) spectra of guest-free [M2(bdc)2(dabco)] samples, where M = (a) Zn, (b) Ni, (c) Cu, and (d) Co. The corresponding rate constants k for phenylene ring reorientation by two-site π-flips are shown near each simulated spectrum.

(M = Co2+, Ni2+, Cu2+, Zn2+; G = none or dimethylformamide) by a solid-state 2H NMR method. The goal of the study was to determine the effects of different metal cations and the loading

This article reports a thorough study of the rotational dynamics of the phenylene rings of the terephthalate (bdc) linkers in a series of MOFs of the form [M2(bdc)2(dabco)]·G 28039

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Figure 3. Top: Main axes of the dipolar interaction between a 2H nucleus (purple) and the metal centers (green) (black arrows). Bottom: Influence of the electron−nuclear dipolar interaction on the line shape of the 2H NMR spectrum.

were synthesized by a slight modification of a procedure published earlier for [Zn2(bdc)2(dabco)]·4DMF.31 Deuterated terephthalic acid D4-H2bdc (143 mg, 0.84 mmol), dabco (47 mg, 0.42 mmol), and 0.84 mmol of the desired metal(II) nitrate [Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, Ni(NO3)2·6H2O, or Co(NO3)2·6H2O] in DMF (10 mL) were placed into a screw-cap glass vial and sonicated for 10 min. The reaction vessel was then heated in an oven for 40 h at 100 °C for M = Zn, Ni, and Co or at 120 °C for M = Cu. The resulting material was isolated as a colorless (M = Zn), blue (M = Cu), green (M = Ni), or blue-green (M = Co) crystalline powder in moderate to good yields. The phase purity of the obtained compounds was confirmed by powder X-ray diffraction. The preactivation of the solvated compounds was performed by moderate heating (ca. 60 °C) in a dynamic vacuum (2 mbar) for several hours. 2.2. Sample Preparation. The crystalline powder of the deuterated MOF (∼0.06 g) was placed into a special glass cell of 5-mm diameter and 3-cm length. Then, the cell was connected to the vacuum line and activated at 383 K for 6 h under a vacuum. For guest-free materials, the cell was then sealed off with a flame torch, while the sample itself was maintained in liquid nitrogen to prevent possible damage to the material by the hot flame. In the case of DMF-filled samples, an additional saturation of the materials was performed prior to the sealing: The guest-free material was impregnated with liquid DMF for 1 day and then dried on a filter. According to thermogravimetric analysis, the amount of adsorbed guest molecules for each sample was found to be approximately four molecules per MOF formula unit. 2.3. NMR Measurements. 2H NMR experiments were performed at 61.4 MHz on a Bruker Avance-400 spectrometer using a high-power probe with a 5-mm horizontal solenoid coil. Spectra were obtained by Fourier transformation of the quadrature-detected phase-cycled quadrupole echo arising from two different echo sequences.

Figure 2. (a−c) Possible motions of the phenylene ring and corresponding simulated spectra: (a) free rotation about the C2 axis, (b) librations, and (c) π-flips of the phenylene ring. (d) Experimental spectrum of [Zn2(bdc)2(dabco)] at 392 K.

of guest molecules such as dimethylformamide (DMF) on the motion of the terephthalate phenylene group. We have established that the loading of DMF molecules in the MOF channels decreases the rate and increases the activation barriers of the motion, whereas the variation of the metal cations has a much weaker effect on the activation energy of the motion. The weak influence of the nature of the metal cation on the dynamics of the phenylene rings in porous networks of [M2(bdc)2(dabco)] potentially allows one to finely adjust the composition and physical properties of the MOFs while maintaining the fundamental dynamics of the linker fragments and associated functionalities on the same level.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Materials. The title metal terephthalate coordination polymers [M2(bdc)2(dabco)] (M = Zn, Ni, Co, Cu) 28040

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Figure 4. 2H NMR experimental (left) and simulated (right) spectra of [M2(bdc)2(dabco)]·4DMF samples, where M = (a) Zn, (b) Ni, (c) Cu, and (d) Co. The rate constants k for phenylene ring reorientation by two-site π-flips are shown near each simulated spectrum.

on the standard formalism.35−40 Details are given in the Supporting Information (SI).

Because the spectra of zinc samples are formed by pure quadrupole interactions, a standard quadrupole echo sequence32,33 was used (90x−τ1−90y−τ2−acquisition−t) where τ1 = 20 μs, τ2 = 17.6 μs, and the repetition delay is t = 0.4−1 s. Because the other samples contained paramagnetic centers, the “EXORCYCLE” echo sequence was used (90x−τ1−90φ−τ2−acquisition−t).34 The duration of the π/2 pulse was equal to 1.6 μs. The temperature of the samples was controlled with a variabletemperature BVT-3000 unit with a precision of 1 K. Before the measurements, the samples were allowed to come to thermal equilibrium at the given temperature for at least 15 min. 2.4. Modeling of Spectra. Modeling of spectra was performed with a homemade FORTRAN program based

3. RESULTS Figure 1 shows 2H NMR spectra at 253−393 K of deuterated phenylene fragments of terephthalate linkers for [M2(bdc)2(dabco)]·G (M = Zn2+, Co2+, Ni2+, Cu2+) (see also Figure S2, SI). For Zn2(bdc)2(dabco)] below 250 K, 2H NMR spectra with a Pake powder pattern (Q0 ≈ 176 kHz, η0 ≈ 0), characteristic of the static (on the 2H NMR time scale, τC = [(3/4)2πQ0]−1 ≫ 10−6 s)41 phenylenes, could be observed. Above 250 K, the line shape continuously evolved with temperature, displaying the 28041

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the 2H NMR line shape increases with increasing magnitude of the electron spin.42 To model the spectrum line shape influenced by the paramagnetic center, we used a general approach reported earlier13,42 (see also the Supporting Information for technical details). This approach requires information on the mutual arrangement of paramagnetic centers and deuterium sites that can be retrieved from crystallographic XRD data (see Figure 3). By taking into account the dipolar interactions of deuterium with eight neighboring paramagnetic sites, this approach allowed us to perfectly fit the experimental line shapes observed for the cobalt-, nickel-, and copper-based materials (see Figure 1b−d). The fitting results clearly show that the dynamics of the phenylene ring of the terephthalic fragments is fully described by the π-flip model for all studied materials. Figure 4 shows 2H NMR spectra for the materials loaded with DMF guest molecules (see also Figure S3, SI). The spectral line shapes evolve with temperature in a manner similar to that for the materials without guest molecules. This means that the reorientational mechanism of phenylene ring motion is not influenced by the presence of the guest molecules in the pores of the studied MOFs. For guest-loaded materials, the 2H NMR line shape of the phenylene ring displays involvement in reorientational motion only above 343 K, which indicates that the mobility of the phenylene ring is considerably hindered compared to that in the guest-free materials. In the presence of DMF, the temperature at which the phenylene mobility becomes detectable by 2H NMR line shape analysis increases by 90−120 K (see Figure 5).

4. DISCUSSION The simulation of 2H NMR line shape for the phenylene rings within a broad temperature range allowed for the analysis of the kinetic parameters of the identified motion. Figure 6 shows Arrhenius plots for the rate constants of the phenylene π-flipping for all MOFs studied. It can be seen that there is no strong difference between the flipping rates of the phenylene rings in the studied MOFs with different metal cations in both guest-free and DMF-loaded materials. The observed rates obey the Arrhenius law and can thus be characterized by an activation energy and a pre-exponential factor of the identified motion (see Figure 6 and Figure S4, SI). Comparison of the rates for different MOFs (see Table 1) clearly shows that the nature of the metal cation influences the mobility of the phenylene ring in the terephthalate linkers to a lesser extent than the presence of guest molecules. In fact, for the guest-free materials, the variation of the activation barrier lies within 4 kJ·mol−1, which is close to the experimental error of 2 kJ·mol−1 for the determination of activation energies. The pre-exponential factors show a stronger variation: k0 for the Co-based material is 20−30 times lower than the values for the Ni- and Zn-based materials and ∼3 times lower than the value for the Cu-based material. However, these differences in k0 do not affect the rotational rates as strongly as the guest loading does (see Figure S4, SI). The weak dependence of the motion parameters on the type of metal can be explained by the fact that the phenylene ring is not directly connected to the metal cation but, rather, is connected through the oxygen atom; that is, the possible impact of the particular cation is reduced with the distance. Such a conclusion can likely be extended to other MOF materials based on metal oxides as inorganic nodes, including the most popular MOF families based on metal carboxylates. For

Figure 5. 2H NMR spectra of [M2(bdc)2(dabco)] and [M2(bdc)2(dabco)]·4DMF (M= Zn2+, Co2+, Ni2+, Cu2+) at the temperature threshold of phenylene ring involvement in reorientational motion by π-flipping.

molecular mobility of the deuterated fragments of the MOF. Being fixed in the para position, these terephthalate phenylenes can only rotate about their 2-fold C2 axes. Overall, three different models of the motion can be proposed: a free rotation about the C2 axis (Figure 2a), a reorientation of the phenylene ring by π-flipping (Figure 2c), and librations of the phenylene ring in a limited sector (Figure 2b). Spectra that are expected for these motions are notably different. Therefore, the comparison of the experimental spectrum with those expected for certain molecular motions could provide information about the real reorientational mechanism of the phenylene fragment. In the present case, the observed line shapes agree with the two-site π-flip model. The rates of phenylene ring flips can be assessed on the basis of the best-fit parameters at each temperature. The 2H NMR line shapes of the [Co2(bdc)2(dabco)] sample are considerably different from those of the zinc material because of the paramagnetic interaction. The Co2+, Ni2+, and Cu2+ cations have nonzero electron spins, and the dipolar interaction between these spins and the nuclear spin of deuterium distort the spectral line shapes, making them asymmetric. The influence of the paramagnetic interaction on 28042

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Figure 6. Arrhenius plots of the rate constants of π-flipping motion for [M2(bdc)2(dabco)]·G MOFs, where M = (a) Zn, (b) Ni, (c) Cu, and (d) Co. Experimental data are given by symbols: colored, guest-free; solid black, G = DMF. Arrhenius fits are indicated by dashed lines.

Table 1. Activation Energies (Ea) and Pre-exponential Factors (k0) for the Reorientational Motion of the Phenylene Rings by Two-Site π-Flips in [M2(bdc)2(dabco)]·G Materials without DMF

a

−1

compound

Ea (kJ·mol )

[Zn2(bdc)2(dabco)] [Ni2(bdc)2(dabco)] [Cu2(bdc)2(dabco)] [Co2(bdc)2(dabco)]

± ± ± ±

36 36 34 32

2 2 2 2

with DMF k0a

−1

(s )

7 × 10 10 × 1011 1 × 1011 0.3 × 1011 11

−1

Ea (kJ·mol )

k0a (s−1)

± ± ± ±

0.9 × 1011 3 × 1011 8 × 1011 3 × 1011

47 53 53 55

2 4 7 4

Standard error for pre-exponential factors is 50%.

[M2(bdc)2(dabco)] with the available data for other materials with similar types of structures, such as MOF-5. The barrier for phenylene ring flips for guest-free MOF-5 was found to be 47 ± 8 kJ·mol−1,10,46 which is significantly higher than the barriers obtained in the present work for [M2(bdc)2(dabco)]. This effect seems to be defined by the electronic factor. Indeed, the steric factor is assumed to decrease with an increase in the distance between terephthalic fragments. Alternatively, the relatively high Ea value reported for MOF-5 could be due to the presence of a doubly interpenetrated MOF-5 phase, which is very difficult to track down and identify by conventional analytical methods.47,48 In fact, it was recently noted14 that the organization of the MOF structure strongly impacts both the activation barrier and the pre-exponential factor of the linker motion, but the number of MOF structures studied so far is not sufficient to provide a systematic view.

example, a recent study of the dynamics of the phenylene fragments in MIL-53 based on chromium and aluminum hydroxide chain units14 also revealed that the activation energies of the rotational motion were very similar: Ea(Cr) = 41 ± 1 kJ·mol−1 and Ea(Al) = 37 ± 1 kJ·mol−1. The minor discrepancy in the activation energies could be due to the slight differences in the crystal structures of these MIL-53 frameworks. The observed variations of the motional parameters for the [M2(bdc)2(dabco)] materials discussed herein can be explained similarly. The adsorption of guest molecules into [M2(bdc)2(dabco)] results in a decrease of the rate of the π-flips by 2 orders of magnitude and a notable increase in the activation energies of phenylene ring motion. Indeed, as can be seen in Table 1, the average activation energy for the guest-free molecules is equal to 34 kJ mol−1 compared to 52 kJ·mol−1 for the samples saturated with DMF. This result is rather expectable because it has been shown that an increase in the packing density of guest molecules reduces the molecular motion of the linkers and leads to an increase in the activation energy.11,14 Alternatively, the increase in the activation barrier might be provided by the direct interaction of the guest molecules with the phenylene rings. Indeed, it has been shown that the phenylene rings could be the adsorption site for CO2.43−45 It is interesting to compare the activation parameters of the phenylene motion observed for the title frameworks

5. CONCLUSIONS A study of the rotational dynamics of the phenylene rings of the terephthalate linkers in the series of MOFs [M2(bdc)2(dabco)]· G (M = Co2+, Ni2+, Cu2+, Zn2+; G = none or dimethylformamide, DMF) was performed. An investigation of the effects of various factors on the mobility of the phenylene rings reveals the following: The electronic structure of the metal cation has a relatively weak effect on the parameters of the motion: 28043

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The rates and activation energies of the motion are similar for materials with different metal cation centers. The main parameters of motion are controlled mostly by the geometry of the framework and the presence of guest molecules. The loading of the DMF molecules into the MOF channels results in a dramatic decrease of the rate by at least 2 orders of magnitude and an increase of the activation energy of the motion of the phenylene ring of the terephthalate linker from 32−36 to 47−55 kJ·mol−1. A weak dependence of the dynamics of the phenylene rings on the metal cation in porous networks implies that the composition and physical properties of metal−organic frameworks can be tuned while the fundamental linker dynamics and associated functionalities are maintained almost on the same level. At the same time, the strong dependence of the rate of rotational motion of the linkers on the guest loading implies that the rate of the motion could be controlled by the loading of guests.

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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.5b09435. Details of 2H NMR spectra simulation and additional experimental 2H NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.I.K.). *E-mail: [email protected] (A.G.S.). Fax: +7 383 330 8056. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Russian Foundation for Basic Research (Grant 14-03-91333). V.A.B. and D.N.D. acknowledge a grant from the Government of the Russian Federation (PN 14.Z50.31.0006).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b09435 J. Phys. Chem. C 2015, 119, 28038−28045

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DOI: 10.1021/acs.jpcc.5b09435 J. Phys. Chem. C 2015, 119, 28038−28045