Abstract: 1. Introduction Metal-organic frameworks (MOFs) have drawn enormous attention during last decade.[2] Particular interest are flexible MOFs which might exhibit various structural transitions between different forms being induced by temperature or sorption/desorption of guest molecules.[3] MIL-53(Al) is one of the brightest representatives of flexible MOFs with 3D structure undergoing temperature-induced structural transition with a significant hysteresis.[3,4] Recently we have proposed post-synthetic adsorption of stable nitroxide radicals and following electron paramagnetic resonance (EPR) detection as a perspective approach for studying breathing behavior and guest-host interactions in structurally-flexible MOFs.[1] In this work we attempt to gain deeper understanding of guest-host interactions between adsorbed nitroxides and MIL-53(Al). In the present work we study three derivatives of TEMPO (4-oxo-TEMPO, 4-hydroxy-TEMPO, 4-acetamido-TEMPO) embedded in MIL-53(Al). 2. Experimental Part CW 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 ~9.7 GHz, mw power ~0.2 - 0.002 mW was chosen to avoid saturation of spectral lines. For second integral measurements we used X-band Bruker MD-5 resonator with sapphire ring inside as a reference. Theoretical modeling of EPR spectra was performed using EasySpin toolbox (Version 5.0.2) for Matlab.26 3. Results and discussion We begin our investigation of TEMPONE, TEMPOL and 4-acetamido-TEMPO in MIL-53(Al) from theoretical MD computation of their mobility depending on structure. First, the mobility of three nitroxides was studied in the LP state of MIL-53(Al). Fig.1 clearly demonstrates that the mobility of three studied radicals in MIL-53(Al) is principally different. TEMPONE tends to conform to the orientations having NO bond parallel to the Z axis of MIL-53(Al), whereas TEMPOL tends to occupy orientations perpendicular. Molecular motion 4-acetamido-TEMPO can be characterized by small-angle librations. Figure. 1. (a) Structure of MIL-53(Al). Illustration of radical’s motion in LP(b) and NP(b΄) of MIL-53(Al) according to MD simulations: (b, b΄) TEMPONE; (c, c΄) TEMPOL; (d, d΄) 4-acetamido-TEMPO. In case of NP state of MIL-53(Al) we 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 (see Fig.1). NO group of the trapped radical can have different orientations. That is orientations across and along the nanochannel of MIL-53(Al). When NO group is directed across the channel, it is remote from the μ2-hydroxo group of the MOF; at the same time, when NO group is directed along the nanochannel, the complex formation between radical and MIL-53(Al) via μ2-hydroxo group is quite possible (distance NO…OH is ~ 2 Å for TEMPONE and TEMPOL). 4-acetamido-TEMPO remains immobile in NP state, the only possible orientation refers to NO group oriented across the nanochannel. Thus, MD calculations predict that TEMPONE and TEMPOL undergo fast restricted reorientational motion in LP state and become immobilized in NP state, possibly forming complexes with μ2-hydroxo group of the MOF. Instead, 4-acetamido-TEMPO is essentially immobile in both states, and the formation of complexes with MOF is hindered. Fig.2 shows the CW EPR spectra of three 4-substituted-TEMPO@MIL-53(Al) samples and corresponding simulations. Figure. 2. X-band EPR (mw9.7 GHz) spectra of TEMPO derivatives in MIL-53(Al) prepared with impregnation method (a) TEMPONE (b) TEMPOL (c) 4-acetamido-TEMPO. All spectra are normalized. Simulations are shown in red. EPR spectra of 4-acetamido-TEMPO@MIL-53(Al) show that this nitroxide remains essentially immobile even at room temperature. Room-temperature spectra of TEMPONE and TEMPOL in MIL-53(Al) strongly differ from those at 80 K. Reasonable agreement between simulated and experimental spectra of in LP state was achieved assuming restricted molecular motion of radical (MOMD model). The best agreement for TEMPONE at LP state was obtained using g = [2.009, 2.0067, 2.002] and A = [0.65, 0.53, 3.44] mT, λ =1.0, τc=2.7 ns. In case of TEMPOL simulations yielded g = [2.01, 2.0074, 2.0028] and A = [0.68, 0.59, 3.55] mT, λ=-0.3, τc= 7 ns. Shorter value of τc found for TEMPONE shows that it is more mobile inside cavity compared to TEMPOL, in good agreement with MD results. 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 CW EPR spectra of radicals were measured relative to signal of sapphire ring. We conclude that under transition ~93% TEMPONE radicals and ~35% of TEMPOL radicals convert to EPR-silent form. In previous work we supposed that changes in magnetic susceptibility upon LPNP conversion refer to formation of hydroxylamine in narrow pores of MIL-53(Al). As was shown by EPR and MD above, TEMPONE shows the most appropriate orientation in LP state, and thus during structural transition it is more likely to be trapped with orientation favoring the interaction of NO group with μ2-hydroxo group of MIL-53(Al). The preferred orientation for TEMPOL less promotes interaction of NO group with μ2-hydroxo group, resulting in the smaller fraction of EPR-silent hydroxylamine formed in NP state. 4. Conclusions In this work we have studied the guest-host interactions between “breathing” MOF MIL-53(Al) and series of nitroxides 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 TEMPONE, TEMPOL and 4-acetamido-TEMPO. We have found a clear correlation between the angular distribution 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.[1] Acknowledgments This work was supported by the Russian Foundation for Basic Research (No. 14-03-00224) and the RF President’s Grant (MD-276.2014.3). References 1) A. M. Sheveleva, D. I Kolokolov, et al., J. Phys. Chem. Lett. 2014, 5, 20−24 2) H. C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112, 673-674. 3) Y. F. Yue, et al. J. Phys. Chem. C 2015, 119, 9442-9449. 4) Y. Liu, J. H. Her, A. Dailly, et al., J. Am. Chem. Soc. 2008, 130, 11813-11818 5) T. Loiseau, C. Serre, C. Huguenard, et al., Chem. Eur. J. 2004, 10, 1373-1382. 6) S. Stoll, A. Schweiger, Epr. J. Magn. Reson. 2006, 178, 42-55

Cite: Sheveleva A. , Poryvaev A.S. , Kolokolov D.I. , Stepanov A.G. , Bagryanskaya E.G. , Fedin M.V.
Mobility and Reactivity of 4-substituted TEMPO Derivatives in Metal-Organic Framework MIL-53(Al)
18th International Zeolite Conference 19-24 Jun 2016