Magnetic Bistability in Crystalline Organic Radicals: The Interplay of H

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Magnetic Bistability in Crystalline Organic Radicals: The Interplay of H-bonding, Pancake Bonding, and Electrostatics in HbimDTDA Michelle B. Mills, Tobie Wohlhauser, Benjamin Stein, Willem R. Verduyn, Ellen Song, Pierre Dechambenoit, Mathieu Rouzieres, Rodolphe Clérac, and Kathryn E. Preuss J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10370 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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Journal of the American Chemical Society

Magnetic Bistability in Crystalline Organic Radicals: The Interplay of H-bonding, Pancake Bonding, and Electrostatics in HbimDTDA. Michelle B. Mills,† Tobie Wohlhauser,†,‡ Benjamin Stein,† Willem R. Verduyn,† Ellen Song,† Pierre Dechambenoit,§,∆ Mathieu Rouzières,§,∆ Rodolphe Clérac,*,§,∆ and Kathryn E. Preuss*,† †Department

of Chemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada Institute of Chemical Technology, School of Engineering and Architecture of Fribourg, University of Applied Sciences and Arts Western Switzerland, CH-1705 Fribourg, Switzerland ‡

§CNRS,

CRPP, UMR 5031, F-33600 Pessac, France of Bordeaux, CRPP, UMR 5031, F-3360 Pessac, France

∆University

Supporting Information Placeholder ABSTRACT: The neutral radical 4-(2-benzimidazolyl)-

1,2,3,5-dithiadiazolyl (HbimDTDA) exhibits a first order phase transition around 270 K without symmetry breaking, preserving its orthorhombic Pbca space group between 340 and 100 K. Associated with this reversible single-crystal-to-single-crystal phase transition, thermal hysteresis of the magnetic susceptibility is observed. The low temperature (LT) phase is diamagnetic owing to pancake bonding between the -radicals. In the paramagnetic high temperature (HT) phase, the pancake bonds are broken and new electrostatic contacts are apparent. As a result of the dense 3D network of supramolecular contacts, which includes H-bonds, the HbimDTDA system provides the first example of magnetic bistability for a DTDA radical.

Systems that exhibit a hysteretic effect intrinsically possess memory. For example, in the case of thermal hysteresis, a bistable material adopts one of two different states, 0 and 1, at a given temperature, depending on its thermal history. It is then possible for such a system to retain information about its previous temperature – whether it was higher or lower prior to its present temperature. This memory effect is a technologically relevant property, manifesting in a wide range of system types, including mechanical, optical, ferroelectric, ferromagnetic, and natural systems.1

Understanding the connection between hysteresis (i.e., first-order phase transition) and structure is an important theme in modern materials design.

Figure 1. (Top) Line drawing and labelled ORTEP plot of HbimDTDA (100 K; thermal ellipsoids at 50%); (bottom) singly occupied molecular orbital (SOMO) and electrostatic potential (ESP) surface calculated at uB3LYP/6-31G(d,p)

Herein, we report a new organic radical, 4-(2benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA; Figure 1) exhibiting a memory effect near room temperature. The associated first-order phase transition is distinguished by reversible making/breaking of pancake bonds2 between pairs of -radicals, engendering reversible switching between diamagnetic and

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paramagnetic states. The system exhibits a thermal hysteresis centered at ca. 270 K. Characterization of the phase transition is reported using single crystal and powder X-ray diffraction (PXRD), magnetic measurements, and differential scanning calorimetry (DSC). For each experimental method, the phase transition is reversible and can be cycled with no evidence of degradation. HbimDTDA is a new member of a rare class of molecule-based material3 and is the first of its kind based on the DTDA heterocycle. HbimDTDA was prepared from 2-benzimidazolecarbonitrile using a modification of a typical synthetic protocol4 for the synthesis of DTDA radicals (see Supp. Info.). Single crystals of HbimDTDA were grown by sublimation under dynamic vacuum, affording dark purple blocks. All measurements were collected on crystalline material.

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Each HbimDTDA in the structure is part of a pancake bonded pair (mean inter-planar distance 3.12 Å). The trans-cofacial geometry of the pancake bonds, determined by overlap of the four lobes of each molecule’s singly occupied molecular orbital (SOMO; Figure 1), orients the molecules to create a 3D network of H-bonding, pancake bonding, and dispersion interactions (Figure S2). At 340 K, the H-bond contacts observed in the 100 K structure are present, albeit at an increased distance, consistent with typical H-bond thermal expansion5 (N…N distance 2.858(4) Å). However, the pancake bonds observed at 100 K are markedly absent at 340 K. The linear arrays defined by H-bond interactions have shifted with respect to one another along [010], resulting in breakage of the pancake bonds. At 100 K, the two DTDA ring centroids within a pancake bonded pair are superimposed when viewed perpendicular to the plane of the DTDA rings, whereas at 340 K, viewed from the same perspective, the DTDA ring centroids are shifted laterally by 2.56 Å (Figure S3). Not only have the pancake bonds been broken by a significant lateral shift, the mean inter-plane distance has increased to 3.68 Å (Figure S3). Thus, in breaking the pancake bonds, the DTDA rings have both shifted laterally and pulled apart.

Figure 2. Excerpts from the crystal structure of HbimDTDA (top) at 100 K and (bottom) at 340 K; Hbonds are drawn in pink; pancake-bond contacts are drawn in green; select electrostatic contacts are drawn in red; pink ellipses highlight the presence and absence of a pancake bond; for alternate views, see Supp. Info. Figure S1.

Single crystal X-ray diffraction data were collected at 100 K (below the phase transition) and at 340 K (above the phase transition) using the same crystal. We examine these data in detail to compare structural differences between phases. At both temperatures, the space group is orthorhombic (Pbca), and molecular bond lengths and angles lie within a range that is typical of DTDA radicals, pancake-bonded or not. Subtle changes in molecular geometries (Table S1) and significant reorganizations in supramolecular contacts, consistent with a single-crystal-to-single-crystal phase transition, are apparent. At 100 K, H-bond contacts between the imidazole NH and the imidazole N atom of a neighboring molecule define linear arrays of HbimDTDA molecules along the [010] direction (N…N distance 2.831(4) Å; Figure 2).

Figure 3. Temperature dependence of the unit cell volume of HbimDTDA upon heating and cooling (lines connect data points to guide the eye); inset: temperature dependence of PXRD peak intensity upon heating and cooling (between 224 and 317 K in 3 K increments)

Short contacts between N- and S+ atoms contribute to a network of electrostatic interactions in both the 100 K and 340 K structures (Figure S2). However, a significant, new contact is observed in the 340 K structure, between both S atoms of one DTDA ring and an N atom of a neighboring DTDA (N-···S+ 3.381(5) and 3.428(5) Å; Figure 2). The geometry and distances of this contact are typical for dithiadiazolyls,6 and the


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stabilization imparted by this type of contact is frequently invoked when rationalizing the supramolecular structure of non-pancake bonded DTDA radicals.7 Although electrostatic contacts are often nondirectional, well-established exceptions are provided by -hole and -hole bonding,8 which exhibit geometric preferences that are readily understood from features of the molecular electrostatic surface potential (ESP). A rendering of the ESP for HbimDTDA (Figure 1) clearly shows a positive area in the center of the S-S bond, typical of DTDAs,7b explaining the geometry of the electrostatic contact and suggesting a -hole contribution (i.e., equal polarization of both S atoms due to N-S -bonds of the DTDA ring). Structural changes associated with the phase transition were tracked as a function of temperature using a single crystal and collecting diffraction data at 200, 230, 260, 270, 280, 290, and 320 K, followed by cooling and collecting at 290, 280, 270, 260, 230, and 200 K (see Supp. Info). A thermal hysteresis associated with a first order phase transition is clearly observed in the temperature dependence of the unit cell parameters (Figures 3 and S5; Table S1). Powder X-ray diffraction data collected at various intervals upon heating and cooling between 100 and 340 K confirm two unique phases, consistent with the above single-crystal diffraction analyses. A phase transition is clearly apparent between 260 and 280 K (Figure 3; inset) with a shift of the diffraction peaks, major changes of their intensities (Figure S6) and a thermal hysteresis (Figure S7).

Figure 4. Temperature dependence of the T product measured for HbimDTDA; Inset: a range of heating/cooling rates are plotted and the grey line indicates the anticipated T value for an ideal S =1/2, g =2 paramagnet.

The magnetic susceptibility of HbimDTDA was measured as a function of temperature between 1.8 and 400 K, at an applied field of 1 T. Below 220 K, the value

of the T product is ca. 0.03 cm3 K mol-1 and changes little with temperature, consistent with diamagnetic, pancake-bonded [HbimDTDA]2 dimers (Figure 4). Above 220 K, the T product increases dramatically with temperature, to 0.28 cm3 K mol-1 at 290 K, consistent with a phase transition to a paramagnetic (non-pancake-bonded) species. Above 290 K, the T product increases with temperature at a significantly lower rate, reaching 0.31 cm3 K mol-1 at 400 K. Although the T product continues to increase, by 400 K it has not reached the expected value for one unpaired electron per molecule in an ideal Curie-type system (C = 0.375 cm3 K mol-1 for S = 1/2 and giso = 2), implying that antiferromagnetic coupling dominates the intramolecular interactions, even in the high temperature phase. Between 220 and 290 K, the value of the T product is different upon heating than upon cooling, resulting in a thermal hysteresis loop (T = 9 K, centered at 270 K). Cycling the temperature using a range of heating/cooling rates, from a rapid temperature change (15 K min-1) to a very slow temperature change (0.14 K min-1), the hysteresis loop appears almost unaltered. DSC data were collected on HbimDTDA, cycling the temperature between 223 and 353 K, using a range of heating/cooling rates (0.5 to 20 K/min; Figure S8). A first order phase transition is observed. Integration of the endo- and exotherm peaks associated with the phase transition reveals an average change in enthalpy of 0.9 kJ mol-1 upon heating and 0.6 kJ mol-1 upon cooling, consistent with similar phase transitions in which pancake bonding is disrupted between organic radicals.9 As already seen by X-ray diffraction and magnetic measurements, a thermal hysteresis is observed at the transition by DSC (Figure S9), with no sign of degradation after extensive cycling. Molecule-based crystals with “dynamic” properties,1a i.e., exhibiting physical properties that can be switched by external stimuli, are drawing attention as potential molecular devices and functional materials, particularly if they exhibit hysteresis and therefore memory. Wellknown examples include Fe(II) spin-crossover (SCO) complexes,10 in which a low-spin/high-spin switching can be effected by a change in temperature and, with sufficient cooperativity (i.e., elastic interactions between complexes), can exhibit magnetic bistability.11 “Dynamic” behavior is uncommon in crystals of molecular organic radicals,12 and the observation of magnetic bistability in organic radicals has been reported for only a few species.13 One report is based on a nitroxide3a and one on a phenalenyl (PLY),3b and the remaining few are derivatives of 1,3,2-dithiazole (DTA).3c,3d,3h,14 In all examples, magnetic bistability manifests as a crystallographic phase transition between a diamagnetic and paramagnetic phase. In the low



temperature (LT), diamagnetic form, pancake bonding between -radicals dominates the structure. In the high temperature (HT) structure, the -radicals are not pancake bonded, however there must be a set of competing intermolecular contacts that stabilizes the crystal lattice. In some cases, electrostatic forces have been identified as a primary factor.15 The key role played by supramolecular interactions to generate the cooperativity necessary for hysteresis cannot be overstated.16 In HbimDTDA, a dense network of short contacts between radical molecules induces enough cooperativity to the system to allow a first order phase transition between a diamagnetic LT phase dominated by pancake bonds and a paramagnetic HT phase which is stabilized by supramolecular interactions. The competition between these two phases leads to the first example of bistability in a DTDA radical system, and although the width of the hysteresis loop is not large, the phase transitions occur remarkably close to room temperature. Future endeavors include the exploration of lightinduced switching below the phase transition temperature and the synthesis of the selenium-containing analogue, with the goal of increasing the pancake bond enthalpy (to shift the phase transition to higher temperature) and increasing polarizability and thus the -hole contribution to the electrostatic contacts (to widen the hysteresis loop). ASSOCIATED CONTENT

Supporting Information. CIFs for the structure of HbimDTDA at 100, 320, and 340 K, and upon heating and cooling at 200, 230, 260, 270, 280, and 290 K; DSC data; experimental details; Table S1; Figures S1-S9. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

[email protected]; [email protected] Funding Sources

The authors declare no competing financial interests. K.E. P. received operational funding from the Natural Science and Engineering Research Council (NSERC) of Canada Discovery Grant 2014-05655 and the Canada Research Chairs program 950-230174, and equipment funding from the Canada Foundation for Innovation (CFI) John R. Evans Leaders Fund (JELF) 32866 and the Ontario Research Fund (ORF) for Small Infrastructure 460548. M.B.M. received an Ontario Graduate Scholarship (OGS) J5223. E.S. received an NSERC Undergraduate Student Research Award (USRA) 337944. R.C., P.D. and M.R. received funding from the University of Bordeaux, the Région Nouvelle Aquitaine, the CNRS, the MOLSPIN COST action CA15128 and the GdR MCM-2: Magnétisme et Commutation Moléculaires.

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ACKNOWLEDGMENT

The contributions of Ms. Melissa G. Ignacio, Prof. Dmitriy V. Soldatov, and Dr. Gzregorz Szymanski to the collection of powder X-ray diffraction data are gratefully acknowledged. T. W. thanks Prof. Olimpia Mamula-Steiner for supervision in Fribourg. REFERENCES (1) (a) Sato, O. Dynamic Molecular Crystals with Switchable Pphysical Properties. Nat. Chem. 2016, 8, 644-656. (b) Silverberg, J. L.; Na, J.-H.; Evans, A. A.; Liu, B.; Hull, T. C.; Santangelo, C. D.; Lang, R. J.; Hayward, R. C.; Cohen, I. Origami Structures with a Critical Transition to Bistability Arising from Hidden Degrees of Freedom. Nat. Mater. 2015, 14, 389-393. (c) Gibbs, H. M.; McCall, S. L.; Venkatesan, T. N. C. Differential Gain and Bistability Using a Sodium-Filled Fabry-Perot Interferometer. Phys. Rev. Lett. 1976, 36, 1135-1138. (d) Horiuchi, S.; Kagawa, F.; Hatahara, K.; Kobayashi, K.; Kumai, R.; Murakami, Y.; Tokura, Y. Above-Room-Temperature Ferroelectricity and Antiferroelectricity in Benzimidazoles. Nat. Commun. 2012, 3, 1308. (e) Jiles, D. C.; Atherton, D. L. Theory of Ferromagnetic Hysteresis. J. Magn. Magn. Mater 1986, 61, 48-60. (f) Lebar, T.; Bezeljak, U.; Golob, A.; Jerala, M.; Kadunc, L.; Pirš, B.; Stražar, M.; Vučko, D.; Zupančič, U.; Benčina, M.; Forstnerič, V.; Gaber, R.; Lonzarić, J.; Majerle, A.; Oblak, A.; Smole, A.; Jerala, R. A Bistable Genetic Switch Based on Designable DNA-Binding Domains. Nat. Commun. 2014, 5, 5007. (2) (a) Preuss, K. E. Pancake bonds: -Stacked Dimers of Organic and Light-Atom Radicals. Polyhedron 2014, 79, 1-15. (b) Mulliken, R. S.; Person, W. B. Molecular Complexes: A Lecture and Reprint Volume; John Wiley & Sons, Inc.: New York, 1969. (3) (a) Shultz, D. A.; Fico Jr., R. M.; Boyle, P. D.; Kampf, J. W. Observation of a Hysteretic Phase Transition in a Crystalline Dinitroxide Biradical That Leads to Magnetic Bistability. J. Am. Chem. Soc. 2001, 123, 10403-10404. (b) Itkis, M. E.; Chi, X.; Cordes, A. W.; Haddon, R. C. Magneto-Opto-Electronic Bistability in a Phenalenyl-Based Neutral Radical. Science 2002, 296, 1443-1445. (c) Brusso, J. L.; Clements, O. P.; Haddon, R. C.; Itkis, M. E.; Leitch, A. A.; Oakley, R. T.; Reed, R. W.; Richardson, J. F. Bistabilities in 1,3,2-Dithiazolyl Radicals. J. Am. Chem. Soc. 2004, 126, 8256-8265. (d) Brusso, J. L.; Clements, O. P.; Haddon, R. C.; Itkis, M. E.; Leitch, A. A.; Oakley, R. T.; Reed, R. W.; Richardson, J. F. Bistability and the Phase Transition in 1,3,2-Dithiazolo[4,5-b]pyrazin-2-yl. J. Am. Chem. Soc. 2004, 126, 14692-14693. (e) Clarke, C. S.; Jornet, J.; Deumal, M.; Novoa, J. J. The Origin of the Bistability in the Thiazyl Radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA): A First Principles Bottom-Up Investigation of the Magnetic Properties of Its High Temperature Polymorph. Polyhedron 2009, 28, 1614-1619. (f) Clarke, C. S.; Jornet-Somoza, J.; Mota, F.; Novoa, J. J.; Deumal, M. Origin of the Magnetic Bistability in Molecule-based Magnets: A First-Principles Bottom-Up Study of the TTTA Crystal. J. Am. Chem. Soc. 2010, 132, 17817-17830. (g) Vela, S.; Mota, F.; Deumal, M.; Suizu, R.; Shuku, Y.; Mizuno, A.; Awaga, K.; Shiga, M.; Novoa, J. J.; Ribas-Arino, J. The Key Role of Vibrational Entropy in the Phase Transitions of Dithiazolyl-Based Bistable Magnetic Materials. Nat. Commun. 2014, 5, 4411. (h) Bates, D.; Robertson, C. M.; Leitch, A. A.; Dube, P. A.; Oakley, R. T. Magnetic Bistability in Naphtho-1,3,2Dithiazolyl: Solid State Interconversions of a Thiazyl -Radical and Its N-N -Bonded Dimer. J. Am. Chem. Soc. 2018, 140, 3846-3849. (4) Del Bel Belluz, P.; Cordes, A. W.; Kristof, E. M.; Kristof, P. V.; Liblong, S. W.; Oakley, R. T. 1,2,3,5-Diselenadiazolyls as Building Blocks for Molecular Metals. Preparation and Structures of [PhCN2Se2]+PF6- and [PhCN2Se2]2. J. Am. Chem. Soc. 1989, 111, 9276-9278. (5) Dougherty, R. C. Temperature and Pressure Dependence of Hydrogen Bond Strengths: A Perturbation Molecular Orbital Approach. J. Chem. Phys. 1998, 109, 7372-7378.


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Magnetic Bistability in Crystalline Organic Radicals: The Interplay of H

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