Mobility and Reactivity of 4-Substituted TEMPO Derivatives in Metal

May 2, 2016 - In this work, we attempt to gain deeper understanding of guest–host interactions between adsorbed nitroxide radicals and MIL-53(Al), w...
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Mobility and Reactivity of 4‑Substituted TEMPO Derivatives in Metal−Organic Framework MIL-53(Al) A. S. Poryvaev,†,‡ A. M. Sheveleva,*,†,‡ D. I. Kolokolov,§,‡ A. G. Stepanov,§,‡ E. G. Bagryanskaya,‡,∥ and M. V. Fedin*,†,‡ †

International Tomography Center SB RAS, 630090 Novosibirsk, Russia Novosibirsk State University, 630090 Novosibirsk, Russia § Boreskov Institute of Catalysis SB RAS, Lavrentiev av. 5, 630090 Novosibirsk, Russia ∥ N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, 630090 Novosibirsk, Russia ‡

S Supporting Information *

ABSTRACT: Postsynthetic adsorption of stable nitroxide radical probes in diamagnetic nano/mesoporous metal−organic frameworks (MOFs) allows application of electron paramagnetic resonance (EPR) for studying structure, functions, and corresponding guest−host interactions in such MOFs. This approach was recently demonstrated using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) embedded in structurally flexible MIL-53(Al), with the mobility and reactivity of TEMPO reflecting structural dynamics of MOF. In the present work we embed three derivatives of TEMPO (4-oxo-TEMPO, 4-hydroxo-TEMPO, 4acetamido-TEMPO) in MIL-53(Al) and investigate structural “breathing” of this MOF with temperature to gain deeper understanding of underlying guest−host interactions. Different substituents in the piperidine ring lead to different mobility and reactivity of corresponding nitroxides. The obtained EPR data and molecular dynamics calculations show that the efficiency of nitroxide radical reaction with μ2-hydroxo group of MIL-53(Al) strongly correlates with the character of its molecular motion, and the reversibility of this reaction upon structural “breathing” is a general phenomenon. On the basis of these findings, practical suggestions on selecting spin probes for in-MOF EPR studies are formulated.

1. INTRODUCTION Metal−organic frameworks (MOFs) have drawn enormous attention during the past decade due to their fascinating properties and high potential for a wide variety of applications.1 The ability of MOFs for selective inclusion of molecules in their pores is one of the key properties for gas separation and storage, asymmetric catalysis, drug delivery, photogeneration of hydrogen, and so on.2−4 In addition, flexible MOFs might exhibit various structural transitions between different forms being induced by temperature, pressure, or sorption/desorption of guest molecules.5−9 Hydrogen bonding and guest−host interactions are generally crucial for such kind of phenomena and have therefore been intensively investigated.10,11 MIL-53(Al) is one of the brightest representatives of flexible MOFs with 3D structure undergoing temperature-induced structural transition with a significant hysteresis.12−14 The reversible changes (“breathing”) occur between large-pore (LP) and narrow-pore (NP) crystalline states with the cell volumes being 1427.5(3) and 863.9(2) Å3, respectively. Such transition is very sensitive to guest−host interactions, and sorption of small amounts of water or other molecules in the pores of MIL53(Al) suppresses the breathing behavior.15,16 Therefore, guest−host interactions in such type of systems were under special attention of many studies.17−19 © XXXX American Chemical Society

In particular, recently we have proposed postsynthetic adsorption of stable nitroxide radicals (nitroxides) and following electron paramagnetic resonance (EPR) detection as a perspective approach for studying breathing behavior itself as well as guest−host interactions in structurally flexible MOFs.14 The huge advantage of new approach is the ability to use the negligible amounts of guest molecules to probe the MOF’s interior, so that the structure is not perturbed by guests and breathing behavior is not suppressed. This opportunity owes to the high sensitivity of EPR, allowing the use of paramagnetic probes (nitroxide radicals) in concentrations of 1/1000 per pore (unit cell) of the MOF. It was initially anticipated that the mobility of nitroxide radical would change upon transition from LP to NP state, thus providing the means to monitor breathing of the MOF. Indeed, exactly this behavior was found for MIL-53(Al) and TEMPO radical ((2,2,6,6tetramethylpiperidin-1-yl)oxyl) used as the probe;14 however, in addition to changes in mobility upon LP ↔ NP conversion, unexpected changes in the magnetic susceptibility of the sample were observed. The effective amount of spins (second integral Received: March 22, 2016 Revised: April 29, 2016

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The Journal of Physical Chemistry C over EPR line of nitroxide) decreased by a factor of ∼3 upon transition from LP state to NP state, being assigned to the reaction of nitroxide with μ2-hydroxo group of MIL-53(Al) and formation of diamagnetic hydroxylamine (TEMPO-H). Remarkably, such reaction was found to be fully reversible, and upon transition back to LP state the number of spins in the sample was restored. The EPR data and molecular dynamics (MD) simulations have shown that in large pores (LP state) nitroxide undergoes molecular motion in a large solid angle, whereas in NP state it can be “trapped” in a set of orientations, some of which enable the weak hydrogen bonding between NO group of nitroxide and proton of μ2-hydroxo group of MIL53(Al) (OH···ON distance ∼1.1 Å). Upon reverse transition to LP state, the synergetic effect of increased temperature and volume available for tumbling leads to the restoration of TEMPO from hydroxylamine. In this work, we attempt to gain deeper understanding of guest−host interactions between adsorbed nitroxide radicals and MIL-53(Al), with a particular focus on the abovementioned reversible reaction of nitroxide with MOF. Previous study used the most known and widespread nitroxide radical TEMPO; however, on the basis of those results, one anticipates that the structure, size, and shape of nitroxide should strongly influence its mobility inside MOF and have a large impact on the efficiency of the interaction with μ2-hydroxo group. To verify this, in the present work, we study three derivatives of TEMPO (4-oxo-TEMPO or TEMPONE, 4-hydroxy-TEMPO or TEMPOL, 4-acetamido-TEMPO) embedded in MIL53(Al). Later we describe the developed experimental approaches, theoretical (MD) modeling, and experimental EPR data on these three systems and compare the results with those previously obtained for TEMPO. The observed correlation between structure, mobility, and reactivity of nitroxides in MIL-53(Al) supports the previously proposed mechanism and allows general suggestions on selection of nitroxides for in-MOF EPR studies.

Figure 1. Structures of studied nitroxide radicals.

10−4 M) and let to dry under ambient conditions. Afterward the sample was again thermally activated under vacuum (10−5 Torr, first 2 h at room temperature, next 12 h at 150 °C) and sealed in the quartz tube. The obtained concentrations of spin probes were close to 1 molecule per 1000 cells of MIL-53(Al), being controlled using continuous wave (CW) EPR at room temperature and the calibration sample (DPPH) with the known number of spins. 2.3. Molecular Dynamics Calculations. MD simulations were performed in the canonical (NVT) ensemble at 300 K using Gromacs-MD package.23 The equations of motion were integrated for 1 ns with a time step of 0.01 ps with accurate leapfrog stochastic dynamics integrator. The Ewald summation was used for calculating electrostatic interactions, whereas the short-range interactions were calculated with a cutoff distance of 11 Å. The local perturbation of the MIL-53(Al) structure by adsorbed nitroxide has been neglected for simplicity, and the electrostatic and van der Waals (vdW) terms for MIL-53(Al) were taken from the literature.24 An ab initio parametrization force field for radicals was taken from ref 25. 2.4. EPR Measurements. CW and pulse EPR spectra were measured at X-band (9 GHz) using the commercial spectrometer Bruker Elexsys E580 equipped with Oxford Instruments temperature control system (ER 4112HV with helium cryostat ER 4118CF-O). Modulation amplitude was 0.1 mT, mw frequency was ∼9.7 GHz, mw power of ∼0.2 to 0.002 mW was chosen to avoid saturation of spectral lines. For second integral measurements we used the signal of sapphire ring of standard X-band Bruker MD-5 resonator as a reference. Echo-detected (ED) spectra were obtained by recording the integrated spin−echo signal after two-pulse sequence (π/2 −τ−π−τ-echo). The integration window was 60 ns. The microwave pulse lengths were 24 and 48 ns, with the microwave power adjusted to provide π/2 and π pulses, respectively, except for 48/96 ns for TEMPONE and TEMPOL at 300 K. The value of time delay τ was varied from 150 to 500 ns. Theoretical modeling of EPR spectra was performed using EasySpin toolbox (version 5.0.2) for Matlab.26

2. EXPERIMENTAL SECTION 2.1. Materials. The TEMPO derivatives used were 4-oxoTEMPO (TEMPONE), 4-hydroxy-TEMPO (TEMPOL), and 4-acetamido-TEMPO, all obtained from Aldrich and used as received. Their chemical structures are illustrated in Figure 1. The hydrothermal synthesis of MIL-53(Al) was performed according to procedure previously described and analyzed using TGA and PXRD methods.15,20,21 2.2. Sample Preparation. In this study we used two different approaches for embedding nitroxides into the pores of MOF: (i) sorption of radicals from the gas phase and (ii) the impregnation method. Gas-Phase Sorption. The weighted quantity of MOF (20 mg, polycrystalline powder) was placed in a tube equipped with vacuum valve. Prior to adsorption of radical, the MOF was evacuated at 10−5 Torr for 2 h at room temperature; then, it was maintained for 12 h at 150 °C (activation procedure). The adsorption of nitroxide into MOF was performed at room temperature from the gas phase (equilibrium vapor pressure at room temperature is known to be 0.8 Torr for TEMPO and is noticeably smaller for TEMPO derivatives (0.0117 Torr for TEMPONE));22 then, the sample was sealed in the quartz tube. Impregnation Method. First, MOF was activated under vacuum at 150 °C (see above). Then, the polycrystalline powder was saturated with an excess of dilute solution of nitroxide dissolved in a volatile diethyl ether (concentration 2 × B

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3. RESULTS AND DISCUSSION 3.1. Molecular Dynamics Calculations. Previous study on MIL-53(Al) with TEMPO embedded (TEMPO@MIL53(Al)) has shown that MD simulations of radical tumbling inside the pores of MOF are in good qualitative agreement with anisotropic motion deduced by simulation of multifrequency CW EPR spectra.14 In particular, the ranges of enforced/ suppressed orientations in LP state are well reproduced as well as the immobilization of radical upon transition of MOF to NP state. Therefore, we started our investigation of TEMPO derivatives in MIL-53(Al) from theoretical MD computation of their mobility depending on structure. We have selected three derivatives of TEMPO; two of them (TEMPONE and TEMPOL) have very small deviations from the structure of TEMPO (see Figure 1), whereas the third one (4-acetamidoTEMPO) has a bulky substituent in the fourth position of piperidine ring. First, the mobility of three nitroxides was studied in the LP state of MIL-53(Al). Figure 2 shows the structure of MIL-

Figure 3. MD calculations of θ angle (between NO group and Z axis) for TEMPONE (a), TEMPOL (b) and 4-acetamido-TEMPO (c). Left column shows temporal dependence θ(t), whereas right column shows angular distribution averaged over time.

∼0.1 ns. TEMPOL does not exhibit similar jumps between two orientations but has one extra accessible orientation with θ ≈ 90°, which is, however, less probable than θ ≈ 20 and 160°. 4-acetamido-TEMPO is trapped with θ ≈ 50 and 130° and undergoes only librational motion. This is not surprising because 4-acetamido-TEMPO turns out to be too big due to the bulky substituent to undergo considerable reorientations in the cavity. Thus, the mobility of three studied radicals in MIL53(Al) is noticeably different. With respect to the amplitude of librations, it decreases in the series TEMPONE > TEMPOL > 4-acetamido-TEMPO, as one can see from θ(t) dependences and the widths of corresponding angular distributions in Figure 3. In the case of NP state of MIL-53(Al), the size of the pore becomes comparable to the size of each of three nitroxides; therefore, local perturbation of MOF structure introduced by nitroxide is quite strong and precise calculations are difficult; however, neglecting changes in local structure of MOF and treating this as approximation, we still can conclude that TEMPONE and TEMPOL are trapped in orientations with piperidine ring of radical being closely parallel to the long diagonal of the lozenge (horizontal direction in Figure 4). Within the plane containing long diagonal of the lozenge, NO group of the trapped radical can have different orientations. As two representative cases, we consider the orientations across and along the nanochannel of MIL-53(Al). When NO group is directed across the channel (Figure 4a), it is remote from the μ2-hydroxo group of the MOF; at the same time, when NO group is directed along the nanochannel (Figure 4b), the

Figure 2. (a) Structure of MIL-53(Al) in LP state. (b) Orientation of nitroxide in the local frame (θ is the angle between NO group and Z axis) and sketches of preferred orientations in LP state for all three TEMPO derivatives (according to MD simulations). Colored ellipsoids highlight the regions of librational motion (vide infra).

53(Al), defines relevant coordinate system, and sketches preferred orientations of three studied nitroxides according to MD data. Figure 3 details this information by showing corresponding temporal dependences of calculated θ angles (between NO group of radical and Z axis). One observes that for each nitroxide there are two preferred orientations, where radical resides exhibiting intensive librations on the picosecond scale (appearing as “noise”). Depending on initial conditions, the radical occupies one of these two preferred orientations, as shown by black and green traces (Figure 3). TEMPONE can jump between these two orientations (θ ≈ 20 and 160°) with characteristic time of C

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3.2. Incorporation of TEMPO Derivatives in MIL53(Al). Our previous study has demonstrated that TEMPO radicals can be adsorbed into the pores of MIL-53(Al) from the gas phase; following this approach we obtained samples of TEMPO@MIL-53(Al) that exhibited nearly 100% LP↔NP switching of the framework with temperature and successfully investigated them using EPR;14 however, in the case of 4substituted TEMPO derivatives, preparation of samples with radicals embedded in MOF was not as straightforward. Compared with TEMPO, the gas-phase sorption of its 4substituted derivatives in MIL-53(Al) resulted in observation of strong unwanted EPR signals with g ≈ 2.0026, which were superimposed with the expected signals of nitroxide@MOF (see the Supporting Information for details). Moreover, thermal LP↔NP breathing was strongly suppressed. Both observations can be rationalized assuming that TEMPOL, TEMPONE, and 4-acetamido-TEMPO form clusters during gas-phase sorption in MOF, so that some radicals are homogeneously distributed inside the framework, whereas some others reside in exchange-coupled aggregates. The latter leads simultaneously to the appearance of strong singlet in EPR spectrum and suppression of breathing behavior of the MOF. The above explanation is very reasonable: first, it is known that adsorption of guest molecules in rather small concentrations can suppress breathing of MIL-53(Al).15 Second, compared with volatile TEMPO, its 4-substituted derivatives have stronger intermolecular interactions in the solid phase and lower vapor pressure, possibly leading to the lower diffusivity and higher tendency for clusterization in MOF.22,27 Therefore, we implemented another approach, namely, the impregnation method using volatile diethyl ether as a solvent (which was later removed from MOF prior to experiments; see the Experimental Section). This approach led to much better results for 4-substituted TEMPO derivatives and also worked perfectly for unsubstituted TEMPO. The strong unwanted

Figure 4. Two representative locations of TEMPO derivatives in narrow pores of MIL-53(Al) with long (on the left, (a)) and short (on the right, (b)) distances from NO-group to μ2-hydroxo group of MIL53(Al).

complex formation between radical and MIL-53(Al) via μ2hydroxo group is quite possible (distance NO···OH is ∼2 Å for TEMPONE and TEMPOL). These results are very similar to the previous findings for unsubstituted TEMPO.14 With regard to 4-acetamido-TEMPO that was immobile even in LP state, the situation does not principally change upon transition to NP state. The only possible orientation refers to NO group oriented across the nanochannel, similar to the LP state. Thus, MD calculations predict that TEMPONE and TEMPOL have two predominant orientations in LP state and undergo fast librational motion, whereas in the NP state they become immobilized, possibly forming complexes with μ2hydroxo group of the MOF. Instead, 4-acetamido-TEMPO is essentially immobile in both states, and the formation of complexes with MOF is hindered.

Figure 5. X-band (νmw ≈ 9.7 GHz) CW and ED EPR spectra of TEMPO derivatives in MIL-53(Al) prepared with impregnation method (a) TEMPONE, (b) TEMPOL, and (c) 4-acetamido-TEMPO. All spectra are normalized. Simulations are shown in red, and temperature and obtained ξ values are indicated on the plot. The values of tensor components used: TEMPONE − g = [2.009, 2.0067, 2.002], A = [0.65, 0.53, 3.44] mT; TEMPOL − g = [2.01, 2.0074, 2.0028], A = [0.68, 0.59, 3.55] mT; 4-acetamido-TEMPO − g = [2.095, 2.0068, 2.0026], A = [0.57, 0.46, 3.40] mT. D

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state; therefore, the contribution of the former is diminished at τ = 500 ns. Thus, the validity of subtraction procedure for elucidation of pure EPR spectrum in NP state is cross-checked by ED EPR at long τ delays. Reasonable agreement between simulated and experimental EPR spectra of all three radicals in MOF was obtained in a model of quasi-librational radical motions accompanied by slow (or even absent) diffusive rotation, having a minor influence on EPR spectra. Librations lead to the partial averaging of the gand A-tensor components, and account of this partial averaging provides good agreement with experimental EPR data.29,30 Quasi-librations have been taken into account in a simplest model requiring only one additional simulation parameter ξ.31 In the NP state we considered A- and g-tensors to obey equation T′x,y,z(T) = ξ·Tx,y,z + (1 − ξ)·Tiso, where T = g- or Atensors, Tx,y,z are true principle values of the tensor, and Tiso = (Tx + Ty + Tz)/3 is the isotropic value. This model phenomenologically describes isotropic librations without any preferred axis; however, in the LP state the account of anisotropy of librations for TEMPONE and TEMPOL allowed better agreement with experiment than in a model of isotropic librations. The best fits were obtained assuming that x components of tensors were not averaged, whereas y,z components obeyed equations T′y,z(T) = ξ·Ty,z + (1 − ξ)·Tyz with Tyz = (Ty + Tz)/2. The rotational correlation time (τc ≈ 80−200 ns) had a small impact on the line width, and in general spectra could also be simulated in complete absence of rotational motion (full data sets are given in the Supporting Information). Note that simulations in large pores required larger amplitude of librations (smaller ξ values) for TEMPONE compared with TEMPOL (ξ = 0.4 vs 0.6), in perfect agreement with the results of MD calculations. Simulations in the narrow pores were accomplished using ξ = 0.9 for all three nitroxides, implying that the radicals are almost completely immobilized and partial averaging of tensors due to librations is small. In addition to mobility changes induced by transition from LP to NP state, we examined the changes in magnetic susceptibility of each sample. Second integrals over the whole CW EPR spectra of radicals in LP and mixed LP/NP states were measured relative to the reference signal of sapphire ring (Figure S5). TEMPONE demonstrates the largest change of magnetic susceptibility upon LP↔LP/NP conversion reaching ∼41%. The same value for TEMPOL is only ∼14%, and no change of magnetic susceptibility is found for 4-acetamidoTEMPO. Taking into account that the conversion from LP to NP state for TEMPONE and TEMPOL was found above to be 44 and 40%, respectively, we arrive at the values of δ ≈ 93% (TEMPONE) and δ ≈ 35% (TEMPOL) for the conversion to EPR silent form in NP state (obviously, no such conversion occurs for 4-acetamido-TEMPO, i.e., δ = 0). Thus, this conversion depth parameter δ indicates the true fraction (percentage) of radicals converted to diamagnetic form upon LP → NP switching of their host pores. In previous work we supposed that quantitative changes in magnetic susceptibility upon LP↔NP conversion refer to the formation of hydroxylamine in narrow pores of MIL-53(Al). In this case we anticipate strong correlation between the mobility/ localization of radical in LP state and the changes of magnetic susceptibility upon transition to NP state. As was previously revealed by EPR and MD, both TEMPONE and TEMPOL in LP state exhibit librational motion around preferred orientations with NO group close to μ2-hydroxo group of MIL-53(Al), whereas 4-acetamido-TEMPO is trapped in orientation with

singlet was much weaker (Figure S1), implying that the contribution from clusterized radicals was small but still the conversion from LP to NP state with temperature was not complete for each of three TEMPO derivatives, implying the presence of regions with high local concentrations of nitroxide. As a result, the spectra at low temperatures were contributed by radicals in both LP and NP states. Thus, contrary to TEMPO, it is challenging to prepare MIL53(Al) with 4-substituted TEMPO derivatives homogeneously distributed inside the framework. The presence of inhomogeneities, that is, high local concentrations of radical, leads to the partial suppression of thermal switching from LP to NP state. Nevertheless, as will be shown later, it is possible to subtract undesired contributions of nonbreathing fraction in EPR data and analyze the mobility and reactivity of adsorbed nitroxides in the breathing mode. 3.3. EPR Study. As was previously mentioned, the framework MIL-53(Al) undergoes temperature-induced transitions between LP and NP states with large hysteresis around room temperature, so that at room temperature LP state can be created by heating MOF up to 400 K and subsequent cooling back to 300 K, whereas NP state can be reached by cooling MOF to 80 K and subsequently heating back to 300 K.28 In all of our experiments we followed this strategy to obtain LP and NP states of MIL-53(Al) embedded with nitroxides and investigate them using EPR at room temperature. The problem of partial LP → NP switching discussed in the previous section can be remedied because the EPR spectra in NP state drastically differ from those in LP state, as was previously demonstrated for TEMPO.14 The spectra in NP state are typical of immobilized nitroxides and are generally much broader than those in LP state. Because the spectrum in pure LP state is easily measured at high temperature in each case, it can be subtracted with variable weight from the corresponding spectrum in mixed LP/NP state, providing that sharp features characteristic of LP state completely disappear, thus finally resulting in the pure EPR spectrum of radical in NP state. Figure 5 shows room-temperature CW EPR spectra of three 4-substituted-TEMPO@MIL-53(Al) samples in LP and NP states obtained in this way. Upon subtraction of nonbreathing contribution, we have found that thermal LP → NP switching efficiency was ∼44% for TEMPONE@MIL-53(Al) and ∼40% for TEMPOL@MIL-53(Al) in the prepared samples. Note that the previously described procedure of disentangling the contributions of breathing and nonbreathing fractions can be fruitfully verified by application of pulse echo-detected (ED) EPR (sequence π/2 − τ − π − τ − echo). This method is almost free from contributions of highly concentrated and clusterized radicals because they relax within a dead time of pulse experiment (this is clear from comparison of ED and CW EPR spectra at 80 K, Figure 5); however, at room temperature ED EPR lines are broadened compared with CW EPR (by ∼0.1 mT) due to pulse detection and relatively fast transverse relaxation. Noticeably, the obtained spectra of nitroxides in LP and NP state of MOF are very close for both detection schemes. Moreover, room-temperature ED EPR spectra of mixed LP/NP state are different at τ = 150 and 500 ns, and the spectrum at τ = 500 ns closely corresponds to the spectrum in pure NP state obtained by subtraction procedure (Supporting Information). This occurs because highly concentrated radicals located in unswitched large pores relax faster than isolated radicals located in switched narrow pores of mixed NP/LP E

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The Journal of Physical Chemistry C NO group remote from μ2-hydroxo group of MOF. This explains well why the conversion depth δ is high for TEMPONE and TEMPOL and zero for 4-acetamidoTEMPO. At the same time, δ is much higher for TEMPONE compared with TEMPOL (δ ≈ 93 vs 35%) despite the fact that their mobility in large pores seems to be very similar (Figure 3). Closer consideration of MD data shows that the distance λ between oxygen of NO group of radical and hydrogen of μ2hydroxo group of MIL-53(Al) is strongly different for TEMPONE and TEMPOL (Figure 6). Note that each λ

distances are short enough, the conversion to diamagnetic hydroxylamine upon LP → NP transition is high and vice versa.

4. CONCLUSIONS In this work we have studied the guest−host interactions between “breathing” MOF MIL-53(Al) and series of nitroxide radicals adsorbed into its pores. We implemented MD simulations and experimental/theoretical X-band EPR studies to address mobility and reactivity of three derivatives of TEMPO, namely, 4-oxo-TEMPO (TEMPONE), 4-hydroxoTEMPO (TEMPOL), and 4-acetamido-TEMPO. We have found a clear correlation between the mobility and preferred locations of this series of nitroxides and their ability to form complexes with μ2-hydroxo groups of MIL-53(Al), and previous study on unsubstituted TEMPO perfectly agrees with this correlation.14 The in-depth understanding of this reversible reaction of formation/decomposition of corresponding hydroxylamines is still to be reached; however, the presently reported results confirm the general character of such phenomenon (now observed for TEMPO, TEMPONE, and TEMPOL in MIL-53(Al)), including its full reversibility. Such reversible H-bond formation is quite unusual, and the present study clearly shows that the observed phenomena depend on the structure of nitroxide, which in turn influences both its mobility and sterical accessibility to μ2-hydroxo groups of MIL53(Al). Apart from gaining insights into the mechanism of radical interaction with MOF, the present study of series of nitroxides revealed several issues that have to be considered for future inMOF EPR studies using nitroxide probes. In particular, volatility of the radical is a key issue because the best samples were obtained using volatile TEMPO, whereas all three of its 4substituted derivatives did not form homogeneous distribution in the void of the MOF, leading to the suppression of the breathing behavior in the fractions overconcentrated with radical and complicating interpretation of EPR data. At the same time, the easiest approach for experimental monitoring of structural dynamics of MOF is provided by second integral over the EPR spectrum, and the amplitude of its change is higher for TEMPONE than for TEMPO. Thus, both issues have to be carefully considered and suitable trade-off has to be found.

Figure 6. Conversion depth δ to diamagnetic form upon transition to NP state versus λ, where λ is the distance between oxygen of NO group of nitroxide and hydrogen of μ2-hydroxo group of MIL-53(Al). Linear correlation for three studied TEMPO derivatives and TEMPO itself is shown. Principal difference in location of TEMPONE and TEMPOL with respect to μ2-hydroxo group is illustrated on top (perpendicular cross-section of MOF compared with Figure 2a is shown). The interaction of NO group with the closest OH-group of MIL-53(Al) is highlighted.

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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.6b02966. Complementary EPR data and description (Figures S1− S5 and S7 and Table S1). Complementary MD data (Figure S6). (PDF)

value shown in Figure 6 refers to the maximum of the corresponding distance distribution (see Figure S6). In this orientation of TEMPOL its OH group interacts with another μ2-hydroxo group, leading to a translational displacement of TEMPOL compared with TEMPONE along the nanochannel. As a result, λ ≈ 1.7 Å for TEMPONE and λ ≈ 3.0 Å for TEMPOL in corresponding preferred orientations. If we calculate the same value for unsubstituted TEMPO based on our previous MD data,14 we obtain λ ≈ 2.3 Å. Completing the data set by λ ≈ 3.4 Å for 4-acetamido-TEMPO and by previously measured value δ ≈ 66% for unsubstituted TEMPO, we can plot the correlation between δ and λ for four nitroxides (Figure 6). Obviously, we observe closely linear dependence δ(λ). Whether such linearity is local or not, the observed general trend perfectly supports the above conclusions on correlation between radical localization/mobility in LP state of MIL-53(Al) and its reactivity upon transition to NP state. If the mobility of radical favors orientations with NO group directed toward the μ2-hydroxo group and if the resulting NO···HO

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AUTHOR INFORMATION

Corresponding Authors

*M.V.F.: E-mail: [email protected]. Tel: +7 383 3301276. *A.M.S.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation (No. 14-13-00826). Theoretical MD study was supported by the Russian Foundation for Basic Research (No. 14-03-00224) and by FASO Russia (project 0333-2014-0001). F

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DOI: 10.1021/acs.jpcc.6b02966 J. Phys. Chem. C XXXX, XXX, XXX−XXX