Article pubs.acs.org/JPCC
Fast Interchange of Coordinated and Guest Dimethylformamide Molecules in the Zinc(II) Lactate Terephthalate Metal−Organic Framework Marsel R. Gallyamov, Danil N. Dybtsev, Denis P. Pischur, Svetlana G. Kozlova, Nikolai K. Moroz,* and Vladimir P. Fedin Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation S Supporting Information *
ABSTRACT: Mobility of N,N-dimethylformamide (dmf) molecules in a homochiral metal−organic framework [Zn2(bdc)(S-lac)(dmf)]· dmf (bdc = 1,4-benzenedicarboxylate; S-lac = L-(−)-lactate) has been studied using 13C, 1H, and 2H solid-state NMR and DSC experiments. The compound exhibits a phase transition in the vicinity of 240 K, associated with disordering of the dmf molecules. In the high-temperature phase, the dmf molecules undergo intense diffusion accompanied by the exchange between the molecules coordinated with Zn and guest molecules in the framework pores. The activation energy of the molecular migration including exchange between coordinated and guest molecules was estimated to be 37 kJ/ mol.
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INTRODUCTION The postsynthetic modification approach is now widely recognized as a versatile tool to tune the composition and structure and to augment the functionalities of porous metal− organic coordination polymers.1,2 Metal−organic frameworks with nonbridging (terminal) thus labile ligands are valuable targets for such modification through the ligand exchange, especially when direct synthetic methods fail to achieve comparable results.3 A number of terminal ligand exchange reactions successfully demonstrated an introduction of reactive or catalytic centers, a modulation of sorption properties, proton conductivity, as well as a pore modification for metal nanoparticle stabilization.4−9 The exchange necessarily takes place between the coordination environment of the metal center (inner sphere ligands) and guest solvent molecules (outer sphere ligands). The systematic study of molecular substitution between the coordinated and guest solvent molecules is therefore fundamentally important in the development of the postsynthetic modification methodology. Also, a catalytic activation of various molecules by porous metal− organic coordination frameworks through the coordination to the metal cations encourages the investigation of ligand exchange mechanisms. In the present investigation we describe the solid-state NMR study of the ligand exchange between the coordinated and guest N,N-dimethylformamide (dmf) molecules in a homochiral [Zn2(bdc)(S-lac)(dmf)]·dmf porous framework. Dimethylformamide is one of the most popular solvents for the synthesis of porous metal−organic frameworks. The exchange © XXXX American Chemical Society
of inner- and outer-sphere dmf ligands in molecular complexes in solutions was extensively studied in the 1980s and 1990s.10−14 On the contrary, to the best of our knowledge, a similar process in crystalline metal−organic frameworks has not yet received any comparable attention, and our study hopefully provides an important insight on a way to a better understanding and control of the ligand substitution in solidstate materials in general.
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EXPERIMENTAL SECTION
Synthesis. The powdered sample of [Zn2(bdc)(S-lac)(dmf)]·dmf (1) was obtained and characterized according to procedures reported previously.15 The sample 1D with the deuterated dmf-d7 molecules was obtained by treating 1 in the dmf-d7 solution at 85 °C for 20 h. NMR Experiments. 13C MAS NMR spectra were obtained at room temperature and frequency of 125.76 MHz on a Bruker Avance III 500 MHz narrow-bore magnet spectrometer with a 4 mm HX probe at sample rotation frequency of 10 kHz and relaxation delay of 100 s. The 90° pulse with length of 4 μs was used. The SPINAL-64 (small phase incremental alternation with 64 steps) sequence at power of 84.3 W was used for Hdecoupling. The spectra were externally referenced to TMS. 2H NMR spectra dictated by the quadrupole interactions of deuterons in 1D were registered at a frequency of 76.77 MHz Received: March 24, 2015 Revised: October 15, 2015
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DOI: 10.1021/acs.jpcc.5b06349 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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temperature behavior of the dipole−dipole interactions of protons of dmf-h7 as well as quadrupole interactions of deuterons of dmf-d7, both of which are essentially responsive to molecular mobility, is required to understand the nature of this exchange. 1 H NMR. The room-temperature static 1H NMR spectra for 1 and 1D are plotted in Figure 2. The integral−intensity ratio
in the 200−300 K temperature range using a 5 mm solenoid probe and a solid-echo pulse sequence with the echo delay of 12 μs. The wide-line 1H NMR spectra resulting from the magnetic dipole−dipole proton interactions were recorded in the form of the first derivative of the absorption line in a low magnetic field (0.587 T) and 120−300 K temperature range by sweeping the frequency in the neighborhood of the Larmor frequency using a homemade NMR spectrometer with signal accumulation. DSC Measurements. DSC measurement were carried out with a differential scanning calorimeter NETZSCH DSC 204 F1 Phoenix by the heat flow method at a heating rate of 9 K/ min in 10 mL/min Ar flow. The powdered samples with mass of 15−25 mg, placed in opened aluminum crucibles, were used. The sensitivity and temperature scale were calibrated by melting of standard samples (C6H12, Hg, In). Netzsch Proteus Analysis software was applied to determine DSC peak areas and transition temperatures.
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RESULTS AND DISCUSSION Coordinated and Guest dmf Molecules. According to the low-temperature (100 K) X-ray data15 of the metal−organic framework [Zn2(bdc)(S-lac)(dmf)], 5-coordinated Zn2+ ions, having a trigonal-bipyramidal coordination environment, are linked together by S-lac and bdc anions. The pore system is composed of intersected channels of ca. 5 Å diameter, filled with both coordinated and guest dmf molecules with interatomic Zn−O distances 2.102 and 5.288 Å, respectively, and the O−O distance between those dmf’s is 4.211 Å (see the Supporting Information, section SI 1, Figure S1). The dmf molecules seem to play some structure-supporting role since heating of 1 at ≈100 °C in vacuum results in a gradual solvent molecule release and partial deterioration of the crystallinity of the compound. 13 C NMR. The 13C MAS NMR spectra obtained at ambient temperatures (Figure 1) give no way to recognize two types of
Figure 2. Room-temperature wide-line 1H NMR spectra normalized on the same sample mass for 1 and 1D. The spectral component assigned to the dmf molecules is shown dashed.
for these spectra I1/I1D = 2.68(5) is in close proximity to the value of 2.75 expected for total dmf-d7 → dmf-h7 substitution when the number of protons per formula 8(bdc and S-lac) + 2 × 7(dmf) = 22 decreases to 8, while the substitution of only guest dmf molecules would give I1/I1D = 1.47. Then, the spectrum of 1D may be considered as the spectral component belonging to bdc and S-lac species. Ignoring, for a first approximation, a distinction in intermolecular dipolar coupling in both samples, the difference spectrum of 1 and 1D may be assigned to the dmf molecules in 1. The temperature evolution of the dmf 1H NMR spectra obtained with the above procedure is illustrated in Figure 3a. The temperature increase from 120 to 230 K results in a gradual narrowing of the spectra, apparently, due to molecular librations. A sharp spectrum narrowing in the interval of 240− 300 K is indicative of an effective suppression of the dipolar coupling as a result of development of the rotational or/and translational diffusion. The formation of a single narrow resonance line in this range suggests that both guest and coordinated dmf molecules are involved in the molecular motion. That is possible if a fast (in the NMR time scale) solvent exchange takes place. From the temperature dependence of the NMR line width (Figure 3b), the activation energy of the molecular motion, Ea, was estimated to be 37(3) kJ/mol (see the Supporting Information, section SI 4). This value must be considered as an upper limit of the ligand exchange activation energy since the molecular mobility includes migrations between different structural positions in the framework pores as well as the exchange between the coordinated and guest molecules. The characteristic frequencies of the ligand exchange exceed 104 s−1 above 250 K. Notice that the 1H NMR spectra of 1D, i.e., the spectra of the bdc and S-lac linkers, remain practically unchanged at all temperatures. DSC Measurements. The character of the above-described spectrum changes suggests a phase transition in the vicinity of 240 K. The phase transitions associated with an orientational disordering of polar molecules were found in a variety of porous MOFs.16−20 In order to determine the actual existence
Figure 1. Carbonyl and methyl fragments of the 1H-decoupled 13C MAS NMR spectrum (96 transients) for 1 at room temperature (for the complete spectrum see the Supporting Information, section SI 2).
the dmf molecules: any splitting of the resonance lines, associated with chemical inequality of the coordinated and guest dmf molecules, is not observed in any of the carbonyl (163.3 ppm) and methyl (30.8; 36.7 ppm) resonances. The half-widths of these lines are about 100 Hz. That means that the expected splitting is less than 0.5 ppm which seems rather surprising, particularly for carbonyl groups, half of which have short Zn−O contacts. On the other hand, this fact may be associated with the existence of the exchange between the coordinated and guest molecules, averaging chemical shifts over all dmf molecules. Therefore, the investigation of the B
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spectra are presented in the Supporting Information, section SI 5). With the exception of very low temperatures, the spectrum of the stationary dmf-d7 molecule with rapidly reorienting CD3 groups is expected as a superposition of two doublet lines with the integral intensity ratio of 6:1 and differed widely from quadrupole splitting, corresponding to the movable methyl deuterons and fixed formyl deuterons, respectively.21 The spectra of this sort are observed in the low-temperature phase of 1D up to 230 K (Figure 5a). From spectrum simulations, the
Figure 3. (a) Wide-line 1H NMR absorption spectra of the dmf molecules in 1 at various temperatures (see also the Supporting Information, section SI 3, Figure S3). (b) Temperature dependence of the spectrum rms width (○) and fit to experimental data, calculated for Ea = 37 kJ/mol and preexponential factor of 8 × 1011 s−1 (solid line). A related dependence for bdc and S-lac species (sample 1D) is added for comparison (▲).
of a phase transition, thermodynamic properties of the title compound were investigated by DSC in the temperature range from 213 to 285 K (Figure 4). The thermal anomaly with the
Figure 5. Experimental (○) and simulated (solid lines) 2H NMR spectra for 1D at 220 (a) and 300 (b) K. The dashed lines are the weighted contributions of different deuteron groups. Inset: Temperature dependence of the spectrum rms width in the high-temperature phase; the relative dependence calculated for the activation parameters determined from the 1H NMR measurements is shown solid. Figure 4. Temperature dependence of the DSC signal for [Zn2(bdc)(S-lac)(dmf)]·dmf.
quadrupole coupling constants at 220 K were estimated to be 46 and 138 kHz for the methyl and formyl deuterons, respectively, which are in reasonable agreement with the earlier dmf-d7 NMR study (a decrease close to 10% of both constants in our case may be assigned to the above-mentioned molecular librations).21 The development of molecular motion in the high-temperature phase changes the spectrum shape remarkably, and above 280 K, the spectrum takes the form indicative of substantial averaging of the quadrupolar couplings (Figure 5b). The spectrum transformation, as a function of the dmfjump frequency with interaction parameters estimated for the low-temperature phase, may in principle be described basing on known stochastic approaches. 22 However, the lack of information about dmf orientations in the disordered hightemperature phase presents obstacles to such a description, and we interpreted the spectrum fine structure observed above 280 K in terms of quadrupolar couplings averaged by the molecular motion. Under diffusion in the low-symmetry crystal, the effective, time-averaged quadrupole coupling tensors of the two methyl groups of the planar (or dynamically planar) dmf-d7 molecule may become nonequivalent if the set of the
maximum position at 253.0 K and Tonset = 248.6 K was observed. The thermal effect was estimated to be 5.3(5) J/g corresponding to an entropy increase ΔS = 9.5−11.5 J K−1 mol−1. This entropy growth could be assigned to the increase of the number of the guest dmf positions by exp(ΔS/R) = 3−4 times. The observed anomaly is offset to high temperatures by several degrees as compared with that in the NMR measurements. This shift may be explained by the differences in the operating conditions of the static NMR and dynamic DSC measurements rather than by the inaccuracy in the NMR temperature scale, not exceeding by our estimates 0.1 K in the considered temperature region. It could be assumed that the intense diffusive mobility of the guest dmf molecules, including their exchange with the dmf ligands, is an inherent characteristic of the disordered high-temperature phase only. 2 H NMR. Some of the supporting evidence for fast exchange between coordinated and guest molecules comes from analysis of the static 2H NMR spectra for 1D (the 2H MAS NMR C
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both these extreme mechanisms and proceeds through simultaneous shortening of Zndmf (guest) and elongation of Zndmf (ligand) distances resulting in an intermediate 4 + 2 coordination of Zn2+ cation. Noteworthy, the abovementioned exchange is registered for the disordered hightemperature phase. Whether this disorder state is a prerequisite for such facile solvent exchange or not remains an open question.
orientation quadrupole coupling tensors in all sites visited during the motion for one group is distinguished from that for the other group. If this is the case, the expected spectrum will consist of three doublet lines with the intensity ratio 3:3:1. In the case of the exchange between the guest and coordinated molecules, these lines must be common for both types of molecules. With this in mind, we attempted to describe the high-temperature spectra in the framework of the fast-motion approach as a superposition of three components corresponding to the different motion-averaged quadrupole tensors. This procedure implies the consideration of three independent parameters for each spectral component: quadrupole coupling constant - q; asymmetry parameter - η; a Lorentzian broadening of spectral lines, associated with fluctuations of the quadrupole coupling in the course of the molecular jumps, - β. The number of fitting parameters may be reduced to 5 assuming that (i) the β values are the same for all resonance lines and that (ii) the motion-induced transformation of the quadrupole tensor of the formyl deuteron is similar to that for the methyl deuterons trans to the formyl deuteron since the directions of the main axes of both axial tensors in the starting (motionless) dmf positions are close to parallel. In other words, we supposed that, for these tensors, the η values are the same and the ratio of the quadrupole coupling constants is identical to that in the low-temperature phase. As could be seen from Figure 5b, the solvent exchange model with the above simplifications allowed us to obtain a satisfactory fit to the experimental data. From the spectra simulation, the following parameters of the effective interactions were obtained: q = 35.8 kHz, η = 0.79 for the formyl deuteron; q = 11.9 kHz, η = 0.79 and q = 6.4 kHz, η = 0.65 for the methyl deuterons in trans- and cis-position to the formyl deuteron, respectively. In the high-temperature phase, the temperature behavior of the widths of 2H spectra considered as a whole is similar to that observed for the 1H NMR spectra and may be assigned to the same activation energy of the molecular motion (inset in Figure 5). The mobility of outer-sphere molecules and the metal coordination sphere susceptibility toward the ligand exchange in the solid state are generally considered to be lower than those in solution. On the other hand, the metal−organic framework structure may stabilize unusual coordination numbers and polyhedrons, such as the trigonal bipyramidal environment of the Zn(II) cation in our case, supposedly susceptible to a local ligand rearrangements. Also, the guestaccessible space in the channels of [Zn2(bdc)(S-lac)(dmf)] is more capacious than the average volume of the dmf molecule,23 which may suggest an ability of guest molecules to displace from their equilibrium positions and enter into the ligand substitution reaction. As a result, the estimated activation energy for the solvent dmf exchange in 1 (Ea < 37 kJ/mol) is even lower than the typical value for the dmf ligand substitution in solutions, such as Ea = 54.3 kJ/mol for [Zn(Me6tren)dmf]2+ (Me6tren = 2,2′,2″-tris(N,N-dimethylamino)triethylamine) featuring the same 5-coordinated Zn(II) cation.10 The mechanism of the dmf exchange in 1 remains uncertain and may involve the temporary association of a guest dmf molecule to the Zn2+ coordination sphere (the associative mechanism) as well as temporary detachment of the dmf ligand into the framework pore (the dissociative mechanism). These mechanisms imply the changes of coordination number of metal cations from 5 to 6 or 4 in the transition states, respectively, which are both very common for Zn2+ coordination chemistry. It is quite likely, however, that the actual solvent exchange process combines
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CONCLUSION The mobility of solvent N,N-dimethylformamide molecules in the metal−organic framework [Zn2(bdc)(S-lac)(dmf)]·dmf (bdc = 1,4-benzenedicarboxylate; S-lac = S-lactate) has been studied with 13C, 1H, and 2H NMR as well as DSC methods. The compound demonstrates a phase transition around T = 240 K, associated with the order−disorder state of the dmf molecules. Above this temperature the solvent molecules exhibit intensive diffusion movement, accompanied by the fast exchange between the coordinated and guest dmf molecules in the framework pores. By our estimates, the characteristic frequencies of the solvent interchange exceed 104 s−1 above 250 K. The activation energy of the molecular migration including the diffusion and interchange processes was estimated to be 37 kJ/mol.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06349. Coordinated and guest dmf molecules in [Zn2(bdc)(Slac)(dmf)]·dmf (SI 1); 13C MAS NMR for [Zn2(bdc)(Slac)(dmf)]·dmf; wide-line 1H NMR spectra for the samples 1 and 1D; activation parameters for the dmf migration; 2H MAS NMR spectra of the dmf-d7 molecules in the sample 1D (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS DND and VPF acknowledge the grant of the Government of the Russian Federation (PN 14.Z50.31.0006, leading scientist M. Schröder).
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