Hysteretic Spin Crossover between a Bisdithiazolyl Radical and Its

Published on Web 10/21/2010

Hysteretic Spin Crossover between a Bisdithiazolyl Radical and Its Hypervalent σ-Dimer Kristina Lekin,† Stephen M. Winter,† Laura E. Downie,‡ Xuezhao Bao,§ John S. Tse,| Serge Desgreniers,‡ Richard A. Secco,§ Paul A. Dube,⊥ and Richard T. Oakley*,† Department of Chemistry, UniVersity of Waterloo, Waterloo, Ontario N2L 3G1, Canada, Department of Physics, UniVersity of Ottawa, Ottawa, Ontario K1N 6N5, Canada, Department of Earth Sciences, UniVersity of Western Ontario, London, Ontario N6A 5B7, Canada, Department of Physics and Engineering Physics, UniVersity of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada, and Brockhouse Institute for Materials Research, McMaster UniVersity, Hamilton, Ontario L8S 4M1, Canada Received August 11, 2010; E-mail: [email protected]

Abstract: The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal j 21m and consists of π-stacked modifications. The R-phase crystallizes in the tetragonal space group P4 radicals, tightly clustered about 4j points and running parallel to c. The β-phase belongs to the monoclinic space group P21/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S · · · S-S · · · S σ-bonds. Variabletemperature magnetic susceptibility χ measurements confirm that R-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S ) 1/2 centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (Tv) 380 K, TV ) 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimerto-radical conversion in which the hypervalent S · · · S-S · · · S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that R-1a behaves as a Mott insulator, but the ambienttemperature conductivity σRT increases from near 10-7 S cm-1 at 0.5 GPa to near 10-4 S cm-1 at 5 GPa. The value of σRT for β-1a (near 10-4 S cm-1 at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σRT increases in a manner which parallels the behavior of R-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S · · · S-S · · · S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S · · · Se-Se · · · S σ-bonds.

Introduction

Neutral radicals are open-shell molecules, and the unpaired electrons found in them are capable, in principle, of acting as carriers of charge1 and/or sources of magnetic coupling.2 Interest †

University of Waterloo. University of Ottawa. § University of Western Ontario. | University of Saskatchewan. ⊥ McMaster University. (1) (a) Haddon, R. C. Nature 1975, 256, 394. (b) Haddon, R. C. Aust. J. Chem. 1975, 28, 2333. (c) Haddon, R. C. Aust. J. Chem. 1975, 28, 2343. (2) (a) Kinoshita, M.; Turek, P.; Tamura, M.; Nozawa, K.; Shiomi, D.; Nakazawa, Y.; Ishikawa, M.; Takahashi, M.; Awaga, K.; Inabe, T.; Maruyama, Y. Chem. Lett. 1991, 1225. (b) Tamura, M.; Nakazawa, Y.; Shiomi, D.; Nozawa, K.; Hosokoshi, Y.; Ishikawa, M.; Takahashi, M.; Kinoshita, M. Chem. Phys. Lett. 1991, 186, 401. (c) Chiarelli, R.; Novak, M. N.; Rassat, A.; Tholence, J. L. Nature 1993, 363, 147. (d) Alberola, A.; Less, R. J.; Pask, C. M.; Rawson, J. M.; Palacio, F.; Oliete, P.; Paulsen, C.; Yamaguchi, A.; Farley, R. D.; Murphy, D. M. Angew. Chem., Int. Ed. 2003, 42, 4782. ‡

16212

9

J. AM. CHEM. SOC. 2010, 132, 16212–16224

is therefore growing in the development of radicals as building blocks for single-component electronic, magnetic, and even magnetoelectronic materials.3 Critical to the pursuit of these ventures, however, is the need to understand the complex relationships between solid-state structure and magnetic and electronic function. For example, the strong charge correlation associated with the half-filled (f ) 1/2) band structure of an array of weakly interacting radicals gives rise to a large on-site Coulomb potential U,4 as a result of which simple (tight-binding) band theory fails, and such materials fall into Mott insulating (3) (a) Rawson, J. M.; Alberola, A.; Whalley, A. J. Mater. Chem. 2006, 16, 2560. (b) Hicks, R. G. Org. Biomol. Chem. 2007, 5, 1321. (c) Awaga, K.; Tanaka, T.; Shirai, T.; Umezono, Y.; Fujita, W. C. R. Chimie 2007, 10, 52. (d) Awaga, K.; Tanaka, T.; Shirai, T.; Fujimori, M.; Suzuki, Y.; Yoshikawa, H.; Fujita, W. Bull. Chem. Soc. Jpn. 2006, 79, 25. (e) Hicks, R. G. Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds; John Wiley & Sons: New York, 2010. (4) Hubbard, J. Proc. R. Soc. (London) 1963, A276, 238. 10.1021/ja106768z  2010 American Chemical Society

Hysteretic Spin Crossover in Bisdithiazolyl Radical Chart 1

states.5 Relatively high, albeit not metallic, conductivity can nonetheless be achieved in these strongly correlated systems. Haddon has reported a series of spirophenalenyl radicals,6 for which f ) 1/4, and variable-temperature structural and transport property studies7 on these materials have provided insight into their electronic and magnetic behavior. Activated conductivity has also been observed for resonance-stabilized bisdithiazolyl radicals 1 (Chart 1),8 which are true f ) 1/2 systems, and improved conductivity,9 as well as magnetic ordering, both antiferromagnetic (AFM)10 and ferromagnetic (FM),11 has been achieved by the replacement of sulfur by selenium, as in 2, 3, and 4. For the latter systems, the effect on charge and spin transport of the structural changes induced by chemical pressure (molecular modification)12 and physical pressure13 has been explored. (5) (a) Mott, N. F. Proc. Phys. Soc. A 1949, 62, 416. (b) Mott, N. F. Metal-Insulator Transitions; Taylor and Francis: London, 1990. (6) (a) Chi, X.; Itkis, M. E.; Patrick, B. O.; Barclay, T. M.; Reed, R. W.; Oakley, R. T.; Cordes, A. W.; Haddon, R. C. J. Am. Chem. Soc. 1999, 121, 10395. (b) Pal, S. K.; Itkis, M. E.; Tham, F. S.; Reed, R. W.; Oakley, R. T.; Haddon, R. C. Science 2005, 309, 281. (c) Mandal, S. K.; Samanta, S.; Itkis, M. E.; Jensen, D. W.; Reed, R. W.; Oakley, R. W.; Tham, F. S.; Donnadieu, B.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, 1982. (d) Pal, S. K.; Itkis, M. E.; Tham, F. S.; Reed, R. W.; Oakley, R. T.; Haddon, R. C. J. Am. Chem. Soc. 2008, 130, 3942. (7) (a) Haddon, R. C.; Sarkar, A.; Pal, S. K.; Chi, X.; Itkis, M. E.; Tham, F. S. J. Am. Chem. Soc. 2008, 130, 13683. (b) Bag, P.; Itkis, M. E.; Pal, S. K.; Donnadieu, B.; Tham, F. S.; Park, H.; Schlueter, J. A.; Siegrist, T.; Haddon, R. C. J. Am. Chem. Soc. 2010, 132, 2684. (c) Huang, J.; Kertesz, M. J. Am. Chem. Soc. 2003, 125, 13334. (d) Huang, J.; Kertesz, M. J. Am. Chem. Soc. 2007, 129, 1634. (e) Bohlin, J.; Hansson, A.; Stafstrom, S. Phys. ReV. B 2006, 74, 155111. (8) (a) Beer, L.; Brusso, J. L.; Cordes, A. W.; Haddon, R. C.; Itkis, M. E.; Kirschbaum, K.; MacGregor, D. S.; Oakley, R. T.; Pinkerton, A. A.; Reed, R. W. J. Am. Chem. Soc. 2002, 124, 9498. (b) Beer, L.; Britten, J. F.; Brusso, J. L.; Cordes, A. W.; Haddon, R. C.; Itkis, M. E.; MacGregor, D. S.; Oakley, R. T.; Reed, R. W.; Robertson, C. M. J. Am. Chem. Soc. 2003, 125, 14394. (c) Leitch, A. A.; Reed, R. W.; Robertson, C. M.; Britten, J. F.; Yu, X.; Secco, R. A.; Oakley, R. T. J. Am. Chem. Soc. 2007, 129, 7903. (9) (a) Brusso, J. L.; Derakhshan, S.; Itkis, M. E.; Kleinke, H.; Haddon, R. C.; Oakley, R. T.; Reed, R. W.; Richardson, J. F.; Robertson, C. M.; Thompson, L. K. Inorg. Chem. 2006, 45, 10958. (b) Brusso, J. L.; Cvrkalj, K.; Leitch, A. A.; Oakley, R. T.; Reed, R. W.; Robertson, C. M. J. Am. Chem. Soc. 2006, 128, 15080. (10) Leitch, A. A.; Brusso, J. L.; Cvrkalj, K.; Reed, R. W.; Robertson, C. M.; Dube, P. A.; Oakley, R. T. Chem. Commun. 2007, 3368. (11) Robertson, C. M.; Leitch, A. A.; Cvrkalj, K.; Reed, R. W.; Myles, D. J. T.; Dube, P. A.; Oakley, R. T. J. Am. Chem. Soc. 2008, 130, 8414. (12) (a) Leitch, A. A.; Yu, X.; Winter, S. M.; Secco, R. A.; Dube, P. A.; Oakley, R. T. J. Am. Chem. Soc. 2009, 131, 7112. (b) Robertson, C. M.; Leitch, A. A.; Cvrkalj, K.; Myles, D. J. T.; Reed, R. W.; Dube, P. A.; Oakley, R. T. J. Am. Chem. Soc. 2008, 130, 14791.

ARTICLES

In addition to the charge correlation problem, all radical-based materials suffer from a tendency of the radicals to dimerize, either by the formation of localized σ-bonds or through multicenter π-π bonds.14 In the case of one-dimensional π-stacked structures,15 this process can be described in terms of a charge-density-wave (CDW) or Peierls distortion.16 The consequent pairing of spins, which is facilitated at low temperatures, quenches magnetic interactions and, to the extent that an electronic band model applies, gives rise to the opening of a band gap at the Fermi level. The resulting diamagnetic materials may have a lower conductivity than the parent radical, but there are cases where association actually improves conductivity.17 There are also a few reports of magnetically bistable radicals, that is, systems which display temperature regimes within which the dimer (S ) 0) and radical (S ) 1/2) states can coexist.17-19 The magnetic hysteresis which is observed with these spin-crossover phase transitions has led to interest in the potential use of bistable radicals in information storage devices.20 In some cases, hysteresis is apparent in several channels (magnetic, electronic and optical), and these discoveries have fueled the pursuit of multifunctional materials.21 In addition to the phase changes associated with dimer-to-radical interconversions, there are numerous examples of radicals and radical dimers that crystallize as non-interconvertable polymorphs.22 Herein we report the preparation and structural characterization of the bisdithiazolyl radical 1 (R1 ) Et, R2 ) F), hereafter (13) Mito, M.; Komorida, Y.; Tsuruda, H.; Tse, J. S.; Desgreniers, S.; Ohishi, Y.; Leitch, A. A.; Cvrkalj, K.; Robertson, C. M.; Oakley, R. T. J. Am. Chem. Soc. 2009, 131, 16012. (14) Tian, Y.-H.; Huang, J.; Kertesz, M. Phys. Chem. Chem. Phys. 2010, 12, 5084. (15) (a) Andrews, M. P.; et al. J. Am. Chem. Soc. 1991, 113, 3559. (b) Bryan, C. D.; Cordes, A. W.; Goddard, J. D.; Haddon, R. C.; Hicks, R. G.; MacKinnon, C. D.; Mawhinney, R. C.; Oakley, R. T.; Palstra, T. T. M.; Perel, A. S. J. Am. Chem. Soc. 1996, 118, 330. (16) Peierls, R. C. Quantum Theory of Solids; Oxford University Press: London, 1955; p 108. (17) Chi, X.; Itkis, M. E.; Kirschbaum, K.; Pinkerton, A. A.; Oakley, R. T.; Cordes, A. W.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 4041. (18) (a) Fujita, W.; Awaga, K. Science 1999, 286, 261. (b) Fujita, W.; Awaga, K.; Matsuzaki, H.; Okamoto, H. Phys. ReV. B 2002, 65, 064434. (c) McManus, G. D.; Rawson, J. M.; Feeder, N.; van Duijn, J.; McInnes, E. J. L.; Novoa, J. J.; Burriel, R.; Palacio, F.; Oliete, P. J. Mater. Chem. 2001, 11, 1992. (d) Fujita, W.; Awaga, K.; Nakazawa, Y.; Saito, K.; Sorai, M. Chem. Phys. Lett. 2002, 352, 348. (e) Shultz, D. A.; Fico, R.; Boyle., P. D.; Kampf, J. W. J. Am. Chem. Soc. 2001, 123, 10403. (19) (a) Barclay, T. M.; Cordes, A. W.; George, N. A.; Haddon, R. C.; Itkis, M. E.; Mashuta, M. S.; Oakley, R. T.; Patenaude, G. W.; Reed, R. W.; Richardson, J. F.; Zhang, H. J. Am. Chem. Soc. 1998, 120, 352. (b) Brusso, J. L.; Clements, O. P.; Haddon, R. C.; Itkis, M. E.; Leitch, A. A.; Oakley, R. T.; Reed, R. W.; Richardson, J. F. J. Am. Chem. Soc. 2004, 126, 8256. (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. J. Am. Chem. Soc. 2004, 126, 14692. (20) Rota, J.-B.; Le Guennic, B.; Robert, V. Inorg. Chem. 2010, 49, 1230. (21) Itkis, M. E.; Chi, X.; Cordes, A. W.; Haddon, R. C. Science 2002, 296, 1443. (22) (a) Alberola, A.; Clarke, C. S.; Haynes, D. A.; Pascu, S. I.; Rawson, J. M. Chem. Commun. 2005, 4726. (b) Bond, A. D.; Haynes, D. A.; Pask, C. M.; Rawson, J. M. J. Chem. Soc., Dalton Trans. 2002, 25. (c) Banister, A. J.; Bricklebank, N.; Clegg, W.; Elsegood, M. R. J.; Gregory, C. I.; Lavender, I.; Rawson, J. M.; Tanner, B. K. J. Chem. Soc., Chem. Commun. 1995, 679. (d) Banister, A. J.; Bricklebank, N.; Lavender, I.; Rawson, J. M.; Gregory, C. I.; Tanner, B. K.; Clegg, W.; Elsegood, M. R. J.; Palacio, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 2533. (e) Leitch, A. A.; McKenzie, C. E.; Oakley, R. T.; Reed, R. W.; Richardson, J. F.; Sawyer, L. D. Chem. Commun. 2006, 1088. (f) Kinoshita, M. Jpn. J. Appl. Phys. 1994, 33, 5718. (g) Tamura, M.; Hosokoshi, Y.; Shiomi, D.; Kinoshita, M.; Nakazawa, Y.; Ishikawa, M.; Sawa, H.; Kitazawa, T.; Eguchi, A.; Nishio, Y.; Kajita, K. J. Phys. Soc. Jpn. 2003, 72, 1735. (h) Fatila, E. M.; Jennings, M. C.; Goodreid, J.; Preuss, K. E. Acta Crystallogr. 2010, C66, o264. J. AM. CHEM. SOC.

9

VOL. 132, NO. 45, 2010 16213

ARTICLES

Lekin et al.

termed 1a. This material is dimorphic, crystallizing in one case (R-1a) as π-stacked radicals and in the other (β-1a) as π-stacked dimers; in the latter structure the dimers consist of pairs of radicals laced together by hypervalent four-center six-electron (4c-6e) S · · · S-S · · · S σ-bonds, as shown in Chart 1.23 Variabletemperature magnetic and conductivity measurements confirm that R-1a behaves as a Mott insulator, as do other radicals of this type,8 with a room-temperature conductivity σRT near 10-6 S cm-1. The lateral mode of dimerization found for β-1a is rare for a sulfur-based bisdithiazolyl radical22e but has been observed for a number of selenium-containing variants 2,24 all of which show a remarkable increase in conductivity with applied pressure. In the case of 2 (R1 ) Me, R2 ) H), the structural and concomitant electronic changes have recently been shown to involve a buckling of the dimeric unit, which causes a sudden closure of the HOMO-LUMO gap, ultimately resulting in metallization of the otherwise semiconducting material.25 In order to compare the structural and transport properties of hypervalent σ-dimers based on 1 (sulfur) and 2 (selenium), we have performed a detailed analysis of the structure and conductivity of β-1a as a function of pressure and temperature. The results of this work have revealed a very different response to elevated pressure and temperature for radicals of type 1 relative to those of type 2. In contrast to the latter, in which the intermolecular S · · · Se-Se · · · S bond remains intact with both increased pressure and temperature, the S · · · S-S · · · S bond of β-1a opens very readily. At ambient pressure, moreover, the dimer-to-radical interconversion for β-1a displays magnetic hysteresis with a narrow window (5 K) of bistability. The solidstate structural and electronic factors underlying the differing behavior of the S · · · E-E · · · S σ-dimers (E ) S, Se) are discussed in the light of DFT calculations on model systems. Results Synthesis. The procedure (Scheme 1) for the preparation of 1a is based on methods developed for related radicals.8 Alkylation of 2,4,6-trifluoropyridine (5) with ethyl triflate produces N-ethyl-2,4,6-trifluoropyridinium triflate (6) which, upon treatment with gaseous ammonia, affords the corresponding 2,6-diaminopyridinium triflate (7). Double Herz cyclization of the latter with sulfur monochloride in refluxing acetonitrile (MeCN) then furnishes the desired bisdithiazolylium framework in the form of the triflate salt [1a][OTf]. Cyclic voltammetry on solutions of this salt in MeCN, with 0.1 M n-Bu4NPF6 as supporting electrolyte and Pt wire electrodes, confirms the expected series (-1/0, 0/+1, +1/+2) of redox processes, and the half-wave potentials of these couples, which are listed in Table 1, are consistent with those seen for related radicals.8 Based on these potentials, chemical reduction of [1a][OTf] was (23) Hypervalent 4c-6e bonds involving combinations of S, Se, and/or Te are well known for systems in which the terminal chalcogens are saturated. See, for example: (a) Nakanishi, W.; Hayashi, S.; Toyota, S. Chem. Commun. 1996, 371. (b) Nakanishi, W.; Hayashi, S.; Arai, T. Chem. Commun. 2002, 2416. (c) Nakanishi, W.; Hayashi, S.; Morinaka, S.; Sasamori, T.; Tokitoh, N. New J. Chem. 2008, 32, 1881. (d) Sharma, S.; Selvakumar, K.; Singh, V. P.; Zade, S. S.; Singh, H. B. Phosphorus, Sulfur Silicon Relat. Elem. 2008, 183, 827. (24) (a) Beer, L.; Brusso, J. L.; Haddon, R. C.; Itkis, M. E.; Kleinke, H.; Leitch, A. A.; Oakley, R. T.; Reed, R. W.; Richardson, J. F.; Secco, R. A.; Yu, X. J. Am. Chem. Soc. 2005, 127, 1815. (b) Leitch, A. A.; Yu, X.; Robertson, C. M.; Secco, R. A.; Tse, J. S.; Oakley, R. T. Inorg. Chem. 2009, 48, 9874. (25) Tse, J. S.; Leitch, A. A.; Yu, X.; Bao, X.; Zhang, S.; Liu, Q.; Jin, C.; Secco, R. A.; Desgreniers, S.; Ohishi, Y.; Oakley., R. T. J. Am. Chem. Soc. 2010, 132, 4876. 16214

J. AM. CHEM. SOC.

9

VOL. 132, NO. 45, 2010

Scheme 1

Table 1. Electrochemical Half-Wave Potentialsa and EPR

Parameters for Derivatives of 1 R1, R2 Et, Hb

E1/2(-1/0), V E1/2(0/+1), V E1/2(+1/+2), V aN (S), mT aN (R1), mT aH (CH2), mT aR (R2), mT g-value d

Et, Clc

-0.95 -0.146 1.292 0.318 0.061