Polymorph-Dependent Molecular Valence Tautomerism Synchronized

Apr 10, 2009 - A newly designed valence tautomeric (VT) complex exhibits a VT interconversion synchronized with polymorphic crystal-melt phase transit...
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1980

Chem. Mater. 2009, 21, 1980–1988

Polymorph-Dependent Molecular Valence Tautomerism Synchronized with Crystal-Melt Phase Transitions Daisuke Kiriya,† Ho-Chol Chang,*,† Kohei Nakamura,† Daisuke Tanaka,† Ko Yoneda,† and Susumu Kitagawa*,†,‡ Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, and Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto UniVersity, 69 Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ReceiVed February 24, 2009

This paper describes a molecular valence tautomeric (VT) bistability synchronized with polymorphic crystal-melt phase transitions of a newly designed VT complex, [Co(C9Espy)2(3,6-DTBQ)2] (CoC9Espy, C9Espy ) dinonyl-pyridine-3,5-dicarboxylate and 3,6-DTBQ ) 3,6-di-tert-butyl-semiquinonate (3,6DTBSQ) or catecholate (3,6-DTBCat)). Two polymorphic crystals which are the thermodynamically stable phase (K1) and the kinetically stable phase (K2), commonly containing a [low spin-CoIII(C9Espy)2(3,6DTBSQ)(3,6-DTBCat)] (ls-[CoIII]) tautomer with a low-spin CoIII atom and mixed-valence ligands. These polymorphic crystals show a molecular VT interconversion that is synchronized with crystal-to-melt phase transitions at different temperatures, 368.2 and 362.6 K for the ls-[CoIII]⊂K1 and the ls-[CoIII]⊂K2 phases, respectively. Interestingly, the ls-[CoIII]⊂K2 obtained from a melt of a [high spin-CoII(C9Espy)2(3,6DTBSQ)2] (hs-[CoII]) tautomer with a high-spin CoII atom with two 3,6-DTBSQ exhibits a doublemelting phenomenon that includes, in part, the thermodynamically unfavorable hs-[CoII]-to-ls-[CoIII] VT interconversion. Eventually, three types of VT interconversions synchronized with macroscopic crystalmelt phase transitions appear either in equilibrium or nonequilibrium conditions: (1) the thermodynamically stable ls-[CoIII]⊂K1 to the hs-[CoII]⊂melt, (2) the metastable ls-[CoIII]⊂K2 to the hs-[CoII]⊂melt, and (3) the relaxing process of the metastable hs-[CoII]⊂melt to the thermodynamically stable ls-[CoIII]⊂K1. A new strategy for simultaneous control of molecular states and macroscopic phases using thermodynamically and kinetically formed polymorphic crystalline phases is presented.

1. Introduction Extensive studies have been performed on bistable molecules, which can transform their physical properties with the aid of an external stimulus, for developing moleculebased devices such as those used in information storage and sensors.1-12 More recently, experimental efforts have concentrated on the construction of supramolecular assemblies * To whom correspondence should be addressed. Phone: 81 75 383 2733 (S.K.), 81 11 706 3479 (H.-C.C.). Fax: 81 75 383 2732 (S.K.), 81 11 706 3447 (H.-C.C.). E-mail: [email protected] (S.K.), [email protected] (H.-C.C.). † Department of Synthetic Chemistry and Biological Chemistry. ‡ Institute for Integrated Cell-Material Sciences.

(1) Feringa, B. L. Molecular Switches; WILEY-VCH: Weinheim, 2001. (2) (a) Kahn, O.; Launary, J. P. Chemtronics 1988, 3, 140–151. (b) Gu¨tlich, P.; Goodwin, H. A. Top. Curr. Chem. 2004, 233–235. (c) Real, J. A.; Gasper, A. B.; Niel, V.; Mun˜oz, M. C. Coord. Chem. ReV. 2003, 236, 121–141. (3) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348–3391. (4) Irie, M. Chem. ReV. 2000, 100, 1685–1716. (5) Beauvais, L. G.; Shores, M. P.; Long, J. R. J. Am. Chem. Soc. 2000, 122, 2763–2772. (6) Halder, G. J.; Kepert, C. J.; Mouvaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762–1765. (7) Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Domingo, N.; Cavallini, M.; Biscarini, F.; Tejada, J.; Rovira, C.; Veciana, J. Nat. Mater. 2003, 2, 190–195. (8) Ohkoshi, S.; Arai, K.; Sato, Y.; Hashimoto, K. Nat. Mater. 2004, 3, 857–861. (9) Kahn, O.; Martinez, C. J. Science 1998, 279, 44–48. (10) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277–283. (11) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature (London) 2000, 408, 541–548.

of bistable molecules, which exhibit a synchronic transformation of molecular states and macroscopic assembled phases accompanied by macroscopically detectable events. For example, bistable photochromic molecules (biphenanthrylidene,1,13,14 fulgide,15 diarylethene,4,16 azobenzene,17 etc.) have been examined for their ability to induce macroscopic mechanical effects accompanied with the photochromic bistability. Alternatively, synchronicity has been examined on solid-solid,18-20 solid-liquid crystal,21,22,22 and liquid crystal-melt23 phase transitions of some spin crossover iron complexes. The concept of synchronic bistability has received considerable attention not only from purely scientific interests but also from commercial interests,24 while an important key step in the related field is to rationally control a state which is defined as a combination of bistable molecular modules and their assembled states toward versatile physicochemical functionality. (12) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172–1175. (13) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M. Chem. ReV. 2000, 100, 1789–1816. (14) van Delden, R. A.; Koumura, N.; Harada, N.; Feringa, B. L. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4945–4949. (15) Yokoyama, Y. Chem. ReV. 2000, 100, 1717–1739. (16) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Nature (London) 2007, 446, 778–781. (17) Ikeda, T. J. Mater. Chem. 2003, 13, 2037–2057.

10.1021/cm900543r CCC: $40.75  2009 American Chemical Society Published on Web 04/10/2009

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Scheme 1. Synchronic Bistability of CoCnOpy and VT Bistability of CoC9Espy with Nonyl Ester Groups

Regarding macroscopic phases in the synchronic systems reported so far, the synchronic bistabilities have been realized between two thermodynamically stable macroscopic phases of two different molecular states at given temperatures. On the other hand, it is well-known that two or more polymorphic25-27 crystalline phases can appear in a given compound, resulting from thermodynamic and kinetic influences that lead to two or more different arrangements of the molecules of that compound in the solid state. Polymorphic crystals have different structures and, hence, in effect, could show different thermodynamic stabilities and physicochemical properties.26,28-32 In fact, some polymorphic spin cross(18) Hayami, S.; Shigeyoshi, Y.; Akita, M.; Inoue, K.; Kato, K.; Osaka, K.; Takata, M.; Kawajiri, R.; Mitani, T.; Maeda, Y. Angew. Chem., Int. Ed. 2005, 44, 2–6. (19) Tokoro, H.; Matsuda, T.; Miyashita, S.; Hashimoto, K.; Ohkoshi, S. J. Phys. Soc. Jpn. 2006, 75, 08004. (20) To¨rnroos, K. W.; Hostettler, M.; Chernyshov, D.; Vangdal, B.; Bu¨rgi, H.-B. Chem.sEur. J. 2006, 12, 6207–6215. (21) Hayami, S.; Moriyama, R.; Shuto, A.; Maeda, Y.; Ohta, K.; Inoue, K. Inorg. Chem. 2007, 46, 7692–7694. (22) Seredyuk, M.; Gaspar, A. B.; Ksenofontov, V.; Galyametdinov, Y.; Kusz, J.; Gu¨tlich, P. AdV. Funct. Mater. 2008, 18, 2089–2101. (23) Fujigaya, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 2003, 125, 14690–14691. (24) (a) Halford, B. Chem. Eng. News 2007, 85, 34–35. (b) Bhatt, J. C.; Busch, B. D.; Bybell, D. P.; Cottrell, R. F.; Deyoung, A.; Liu, C.; Telfer, S. J.; Thornton, J. E.; Vetterling, W. T. U.S. Patent 7166558, January 23, 2007. (25) Dunitz, J. D.; Bernstein, J. Acc. Chem. Res. 1995, 28, 193–200. (26) Bernstein, J.; Davey, R. J.; Henck, J.-O. Angew. Chem., Int. Ed. 1999, 38, 3440–3461. (27) Braga, D.; Grepioni, F. Chem. Soc. ReV. 2000, 29, 229–238. (28) Basavoju, S.; Desiraju, G. R. Chem. Commun. 2005, 2439–2441. (29) Bernstein, J. Nat. Mater. 2005, 4, 427–428. (30) Mutai, T.; Satou, H.; Araki, K. Nat. Mater. 2005, 4, 685–687. (31) Awaga, K.; Tanaka, T.; Shirai, T.; Fujimori, M.; Suzuki, Y.; Yoshikawa, H.; Fujita, W. Bull. Chem. Soc. Jpn. 2006, 79, 25–34. (32) Itkis, M.,E.; Chi, X.; Cordes, A. W.; Haddon, R. C. Science 2002, 296, 1443–1445.

over and photochromic crystals show different types of bistable profiles.33-39 However, each polymorphic crystal has generally been obtained independently from solution, and the polymorphic structures are retained during the molecular interconversion,33-39 preventing the appearance of polymorphic transition coupled with molecular bistability. Thus, this background motivates us to experimentally achieve a new bistable molecular assembly within interconvertible polymorphic phases under controllable conditions. Our most recent investigations demonstrated that a series of cobalt complexes containing alkyl chains exhibit valence tautomeric (VT) interconversion synchronized with macroscopic crystal-crystal40 and crystal-melt41 phase transitions. These studies demonstrated that the introduction of alkyl chains leads to not only flexible assembled structures but also the synchronic bistability of molecular VT interconversion and macroscopic phase transformation. For example, alkoxy-functionalized VT complex, [CoIII(CnOpy)2(3,6DTBQ)2] in Scheme 1 (CoCnOpy, CnOpy ) 3,5-dialkoxypyridine, and 3,6-DTBQ ) 3,6-di-tert-butyl-semiquinonate (33) Matouzenko, G. S.; Bousseksou, A.; Lecocq, S.; Koningsbruggen, P. J.; Perrin, M.; Kahn, O.; Collet, A. Inorg. Chem. 1997, 36, 5869–5879. (34) Machivie, M.; Guionneau, P.; Le`tard, J.-F.; Chasseau, D. Acta Crystallogr. 2003, B59, 479–486. (35) Galet, A.; Gaspar, A. B.; Mun˜oz, M. C.; Levchenko, G.; Real, J. A. Inorg. Chem. 2006, 45, 9670–9679. (36) Ren, X. M.; Nishihara, S.; Akutagawa, T.; Noro, S.; Nakamura, T. Inorg. Chem. 2006, 45, 2229–2234. (37) Morimoto, M.; Kobatake, S.; Irie, M. Chem.sEur. J. 2003, 9, 621– 627. (38) Kobatake, S.; Kuma, S.; Irie, M. J. Phys. Org. Chem. 2007, 20, 960– 967. (39) Naumov, P.; Ohnishi, Y. Acta Crystallogr. 2004, B60, 343–349. (40) Kiriya, D.; Chang, H.-C.; Kitagawa, S. Dalton Trans. 2006, 1377– 1382. (41) Kiriya, D.; Chang, H.-C.; Kitagawa, S. J. Am. Chem. Soc. 2008, 130, 5515–5522.

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(3,6-DTBSQ) or 3,6-di-tert-butyl-catecholate (3,6-DTBCat)) exhibits the VT interconversion between [low spin-CoIII(CnOpy)2(3,6-DTBSQ)(3,6-DTBCat)] (ls-[CoIII]) and [high spin-CoII(CnOpy)2(3,6-DTBSQ)2] (hs-[CoII]) synchronized with the phase transition between the thermodynamically stable crystalline phase and melt. To obtain polymorphic crystals of a VT complex, we designed the pyridine ancillary ligand with nonyl ester moieties (Scheme 1). The ligand possesses hydrogen-bonding (HB) sites with long alkyl chains,therebyconstructingHB-basedpolymorphiccrystals.42-45 According to these strategies, we aimed to synthesize a new VT complex [CoIII(C9Espy)2(3,6-DTBSQ)(3,6-DTBCat)] (CoC9Espy, Scheme 1; C9Espy ) dinonyl-pyridine-3,5dicarboxylate) and reveal the thermodynamic basis for the VT interconversion synchronized with phase transition in between thermodynamically or kinetically favored polymorphic crystals and isotropic melts. 2. Experimental Section 2.1. Materials. 3,5-Pyridinedicarboxylic acid (Tokyo Chemical Industry Co.), thionyl chloride (Wako Pure Chemical Industries Ltd.), and 1-nonanol (Aldrich) were used without further purification. Co2(CO)8 was purchased from Lancaster Synthesis Ltd. and 3,6-di-tert-butyl-benzoquinone was prepared by the published procedure.46 All synthetic operations and measurements were performed under N2 atmosphere, using Schlenk line techniques. 2.2. Synthesis of Dinonyl-pyridine-3,5-dicarboxylate (C9Espy). 47 3,5-Pyridinedicarboxylic acid (1.0 g, 6.0 mmol) was suspended in SOCl2 (4.4 mL, 60.0 mmol), and this mixture was heated to reflux for 23 h. Subsequently, excess SOCl2 was evaporated under vacuum, and the remaining white solid was dissolved in 20 mL of CH2Cl2. 1-Nonanol (2.65 g, 18.4 mmol) was added to this solution in an ice bath, and then the solution was allowed to warm to room temperature and was subsequently heated to reflux for 3 h. The reaction was quenched by adding 50 mL of aqueous NaOH (1 M), and the organic material was extracted with CH2Cl2. The combined organic layers were washed with water and then dried with Na2SO4 and MgSO4. After filtration and evaporation of the solvent and residual 1-nonanol in vacuo, the crude product was obtained, resulting in a brown viscous oil. Purification by column chromatography (silica gel, hexane/ethyl acetate ) 28/1, Rf ) ca. 0.5) yielded the product as a light yellow oil. Yield 1.75 g (4.17 mmol, 70%). 1H NMR (500 MHz, CDCl3) δ 0.84-0.87 (t, 6H), 1.25-1.34 (m, 20H), 1.39-1.45 (m, 4H), 1.74-1.80 (m, 4H), 4.35-4.37 (t, 4H), 8.83 (t, 1H), 9.34 (d, 2H). 2.3. Synthesis of [Co(C9Espy)2(3,6-DTBQ)2] (CoC9Espy, K1 and K2 Phases). The complex CoC9Espy was prepared by similar procedures to those in our previous report;41 Co2(CO)8 was dissolved in 22 mL of toluene, and 3,6-di-tert-butyl-benzoquinone and C9Espy were added at room temperature under N2 atmosphere. The mixture was stirred for 3 h at 353 K and dried under reduced (42) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem. ReV. 1998, 98, 1375– 1405. (43) Braga, D.; Giaffreda, S. L.; Grepioni, F.; Maini, L.; Polito, M. Coord. Chem. ReV. 2006, 250, 1267–1285. (44) Moulton, B.; Zaworotko, M. J. Chem. ReV. 2001, 101, 1629–1658. (45) Desiraju, G. R. Angew. Chem., Int. Ed. 2007, 46, 8342–8356. (46) (a) Belostotskaya, I. S.; Komissarova, N. L.; Dzhuaryan, E´. V.; Ershov, V. V. IzV. Akad. Nauk. SSSR 1972, 7, 1594–1596. (b) Karpyuk, A. D.; Starosel’skaya, L. F.; Petrov, E´. S.; Beletskaya, I. P. IzV. Akad. Nauk. SSSR 1985, 1, 218–220. (47) Dijkstra, H. P.; Chuchuryukin, A.; Suijkerbuijk, B. M. J. M.; Klink, G. P. M.; Mills, A. M.; Spek, A. L.; Kotena, G. AdV. Synth. Catal. 2002, 344, 771–780.

Kiriya et al. pressure to give a purple precipitate. This product was dissolved in toluene/acetonitrile (1:10, v/v) and then the solution was subjected to slow evaporation under a steady flow of N2 gas at 298 K. Dark violet crystals (K1 phase) were obtained after filtration. On the other hand, recrystallization from toluene/acetonitrile at 253 K gave polymorphic crystals (K2 phase) that were suitable for crystallographic analysis. [Co(C9Espy)2(3,6-DTBQ)2] (CoC9Espy, K1 phase): 37% yield. Anal. Calcd for C78H122CoN2O12: C, 69.98; H, 9.19; N, 2.09. Found: C, 69.78; H, 9.00; N, 1.94. 2.4. Physical Measurements. Elemental analysis was performed on a Flash EA 1112 series (Thermo Finnigan instrument). Microscopic analysis was carried out on the samples between two glass slides using a BX51 microscope (Olympus) with an LK-600 hot stage (Linkam) under N2 atmosphere. Different scanning calorimetric measurements were measured on a DSC 822e (Mettler) under N2 atmosphere, where the sample was placed between an aluminum DSC pan and lid. Variable-temperature X-ray diffraction measurements were carried out with Cu KR radiation equipped with a RINT2000 diffractometer (Rigaku). Magnetic susceptibilities were recorded over the temperature range 5-400 K at 1 T with a superconducting quantum interference device (SQUID, Quantum Design). All values were corrected for diamagnetism using Pascal’s constants.48 1H NMR spectroscopy was performed in CDCl3 using an A-500 spectrometer (JEOL), where chemical shifts were determined with respect to CHCl3 (δ ) 7.24) as an internal standard. 2.5. X-ray Crystallographic Data. Crystallographic measurements for the K1 and K2 phases of CoC9Espy were performed on a Rigaku mercury diffractometer with a CCD two-dimensional detector using Mo KR radiation equipped with a graphite monochromator. Data were collected at 293 K (K1 phase) and 213 K (K2 phase). The sizes of the unit cells were estimated from the reflections collected on the setting angles of 18 frames by changing ω by 0.3° (K1 phase) and 0.5° (K2 phase) for each frame. Two or three different χ settings were used, and ω was changed by 0.3° (K1 phase) and 0.5° (K2 phase), per frame. Intensity data were collected in 2400 (K1 phase) and 1440 (K2 phase) frames (exposure time ) 60 (K1 phase) and 50 (K2 phase) s/image). An empirical absorption correction using the program REQABA49 was performed. The structures were solved by direct method (SIR 200250 (K1 phase) and SIR 9751 (K2 phase)). Restraints of DFIX and DANG in SHELXL-9752 were applied to atoms belonging to the disordered alkyl carbons (DFIX; C38-C43, C45-C54, C72-C79, C103-C108, C140-C147, C204-C210, C230-C237, DANG; ∠C75-C77, ∠C76-C78, ∠C77-C79, ∠C204-C206) to achieve convergence during least-squares refinement in the K1 phase. The K2 phase was refined on F2 in SHELXL-97.52 The crystallographic data are summarized in Table 1. CCDC-713278 (K1 phase) and CCDC713277 (K2 phase) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge in the Supporting Information and from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (48) Kahn, O. Molecular Magnetism; WILEY-VCH: Weinheim, 1993. (49) Jacobson, R. A. REQABA Empirical Absorption Correction, Version 1.1-0310 1998; Molecular Structure Corp.: The Woodlands, TX, 1996-1998. (50) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2003, 36, 1103. (51) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (52) Sheldrick, G. M. SHELXL97; University of Go¨ttingen: Germany, 1997.

Polymorph-Dependent Molecular Valence Tautomerism Table 1. Crystallographic Data and Structure Refinement Parameters for the K1 and K2 Phases of CoC9Espy

formula formula weight crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) density (g/cm3) Z T (K) µ (mm-1) R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b GOF a

K1 phase

K2 phase

C78H122CoN2O12 1338.74 triclinic P1j 14.635(5) 28.619(10) 30.882(10) 103.760(4) 100.123(5) 95.685(4) 12238(7) 1.089 6 293(1) 0.265 0.1032 0.3349 1.035

C78H122CoN2O12 1338.74 triclinic P1j 11.516(3) 13.794(3) 13.869(4) 72.746(13) 76.944(15) 69.631(13) 1953.9(8) 1.138 1 213(2) 0.277 0.0669 0.1874 1.055

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.

3. Results and Discussion A view of the CoC9Espy molecules in the K1 phase is shown in Figure 1a. There are six molecules in the unit cell in which four molecules labeled as I to IV in Figure 1a are crystallographically independent. All of four Co atoms adopt a distorted octahedral symmetry and are surrounded by two nitrogen donors of the C9Espy ancillary ligand in the axial direction and four oxygen donors of the two chelating 3,6DTBQ on the same plane, where Co2 and Co3 are on the inversion center of its molecules. It was found that the alkyl chains highly disorder so as to fill the void spaces, making positioning difficult without using the restrains (see the Experimental Section). Bond lengths to the Co center are often indicative of the oxidation state of the Co atom and redox-active dioxolene ligands.53-55 All four independent molecules in the K1 phase have a trans structure with Co-N bond lengths in the region of 1.929(2)-1.941(2) Å and Co-O bond lengths in the region of 1.843(2)-1.896(2) Å as listed in Table 2 (see also the Supporting Information). These structural parameters around the Co atoms in the four independent molecules are very similar to each other and characteristic of a [low spin-CoIII(C9Espy)2(3,6-DTBSQ)(3,6DTBCat)] (ls-[CoIII]) tautomer.41,53-55 For the molecules I and IV, the ligands with shorter and longer C-O bond lengths can be crystallographically assigned as 3,6-DTBSQ and 3,6-DTBCat form,53,55 respectively, while for the molecules II and III, the crystallographic distinction between 3,6-DTBSQ and 3,6-DTBCat was impossible due to the presence of crystallographic inversion center on each Co atom (see the Supporting Information). The crystals of the K1 phase show temperature-independent χMT values characteristic of the S ) 1/2 spin state up to 355 K (Figure 2a), which provides a strong indication of the ls-[CoIII] tautomer in the crystalline K1 phase (ls[CoIII]⊂K1). Upon further heating the ls-[CoIII]⊂K1, the (53) Buchanan, R. M.; Pierpont, C. G. J. Am. Chem. Soc. 1980, 102, 4951– 4957. (54) Pierpont, C. G. Coord. Chem. ReV. 2001, 216-217, 99–125. (55) Adams, D. M.; Dei, A.; Rheingold, A. L.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 8221–8229.

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magnetic susceptibility abruptly increased to 2.92 cm-1 K mol-1 at 370 K, indicating a drastic change in the molecular states. It is well-known that differences in the ligand field nature of octahedral Co(III) and Co(II) result in the accompanying change in magnetism as they shift from the ls[CoIII] tautomer to the hs-[CoII] tautomer with an additional contribution to the change in magnetism added by the formation of a second radical 3,6-DTBSQ ligand (Scheme 1).53-64 Therefore, the observed magnetic data clearly show a ls-[CoIII]-to-hs-[CoII] VT interconversion upon heating. In the course of subsequent cooling, the χMT value first gradually and then abruptly returns to the value characteristic of the ls-[CoIII] tautomer at 343 K, with an attendant hysteresis of approximately 25 K. Clear coincidence was observed between the thermal hysteresis loop in the magnetic measurement and the endo- (Tc1 ) 368.2 K, ∆H ) 84.3 kJ/ mol, ∆S ) 229 J/mol K) and exothermic peaks (348 K, ∆H ) 77.7 kJ/mol) that appeared in differential scanning calorimetry (DSC) curves in Figure 2b. Variable-temperature X-ray diffraction (XRD) experiments and optical microscopic studies were carried out to shed light on the macroscopic events around these temperatures. When the purple crystal of the ls-[CoIII]⊂K1 between the two glass slides in Figure 3a was heated, the crystal-melt phase transition was observed at Tc1 and the whole region was covered by a green melt as shown in Figure 3a-c. Upon cooling the melt at 1 K min-1, part of the green melt started to crystallize and eventually the whole region was covered by the violet crystalline phase shown in Figure 3d-f. XRD data in Figure 3g supports the melting and crystallization processes during the first heating and cooling processes. Here we noted that a recrystallized crystalline phase (K2 phase) obtained from the melt has the XRD pattern dissimilar to that of the ls-[CoIII]⊂K1 phase. The above results clearly show that CoC9Espy demonstrates the VT interconversion synchronized with the ls-[CoIII]⊂K1to-hs-[CoII]⊂melt phase transition and afforded the polymorphic crystalline phase, K2, after successive cooling. A single crystal of the ls-[CoIII]⊂K1 was obtained at 298 K, while recrystallization from toluene/acetonitrile solution at 253 K afforded a single crystal of the ls-[CoIII]⊂K2, which gives a simulated XRD pattern identical with that in Figure 3g (top, see the Supporting Information). The structural characterization of the ls-[CoIII]⊂K2 at 213 K shows that the Co atom has a coordination environment similar to that (56) Gu¨tlich, P.; Dei, A. Angew. Chem., Int. Ed. 1997, 36, 2734–2736. (57) Pierpont C. G.; Kitagawa, S. Inorganic Chromotropism; Kodansha/ Springer: Tokyo, 2007; pp 116-142. (58) Pierpont, C. G.; Buchanan, R. M. Coord. Chem. ReV. 1981, 38, 45– 87. (59) Hendrickson, D. N.; Pierpont, C. G. Top. Curr. Chem. 2004, 234, 63–95. (60) (a) Abakumov, G. A.; Cherkasov, V. K.; Bubnov, M. P.; E´llert, O. G.; Dobrokhotova, Zh. B.; Zakharov, L. N.; Struchkov, Yu. T. Dokl. Akad. Nauk 1993, 328, 332–335. (61) Shultz, D. A. Magnetism: Molecules to Materials II; Wiley-VCH: Weinheim, 2001; 281-306. (62) Dei, A.; Gatteschi, D.; Sangregorio, C.; Sorace, L. Acc. Chem. Res. 2004, 37, 827. (63) Evangelio, E.; Ruiz-Molina, D. Eur. J. Inorg. Chem. 2005, 295, 7– 2971. (64) Sato, O.; Cui, J. A.; Matsuda, R.; Tao, J.; Hayami, S. Acc. Chem. Res. 2007, 40, 361–369.

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Figure 1. (a) Four independent molecules I-IV in the K1 phase of CoC9Espy at 293 K and (b) that in the K2 phase at 213 K. Hydrogen atoms are omitted for clarity. Color code: green (Co), red (O), blue (N), and gray (C). Table 2. Selected Bond Lengths (Å) for the K1 and K2 Phases of CoC9Espy K1 phase

Co4-O25 Co4-O27 Co4-N5 C158-O25 C172-O27

molecule I 1.895(2) Co1-O2 1.845(2) Co1-O4 1.939(2) Co1-N2 1.309(5) C(2)-O2 1.354(5) C16-O4 molecule II 1.860(2) Co2-O14 1.938(3) 1.322(3) C81-O14 molecule III 1.867(2) Co3-O20 1.936(2) 1.325(5) C120-O20 molecule IV 1.896(2) Co4-O26 1.846(2) Co4-O28 1.929(3) Co4-N6 1.319(3) C159-O26 1.349(4) C173-O28

Co1-O1 Co1-N1 C1-O1

1.868(2) 1.944(2) 1.324(3)

Co1-O1 Co1-O3 Co1-N1 C1-O1 C15-O3 Co2-O13 Co2-N3 C80-O13 Co3-O19 Co3-N4 C119-O19

1.883(2) 1.843(2) 1.941(2) 1.315(5) 1.363(5) 1.868(2) 1.324(3) 1.858(2) 1.325(5) 1.884(2) 1.852(2) 1.938(3) 1.310(3) 1.349(4)

K2 phase Co1-O2

1.860(2)

C2-O2

1.326(3)

as in the ls-[CoIII]⊂K1 (Figure 1b). The axial Co-N length is 1.944(2) Å and equatorial Co-O lengths are 1.860(2) and 1.868(2) Å, which are similar to those for the ls-[CoIII]⊂K1 (Table 2). The structural distinction between 3,6-DTBSQ and 3,6-DTBCat is lost with the imposed inversion symmetry on the Co atom.

Figure 2. (a) Temperature-dependent magnetic susceptibility and (b) DSC curves observed for the first heating process (red line) on the ls-[CoIII]⊂K1 and the first cooling (red line) and the second heating processes on the ls-[CoIII]⊂K2 (green line) at a scan rate of 1 K min-1.

In contrast to the similar intramolecular coordination environments in the two polymorphic phases, remarkable differences are observed in their crystal packing structures. The ls-[CoIII]⊂K1 forms a two-dimensional layered structure, which is formed by HB interactions between Cs H3,6-DTBQ(II) · · · OdCC9Espy(I), CsHC9Espy(II) · · · OdCC9Espy(IV), CsHC9Espy(IV) · · · OdCC9Espy(II), and CsH3,6-DTBQ(IV) · · · Od CC9Espy(III) as illustrated in Figure 4a-c. All of the observed

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Figure 3. Snapshots (a-c) of the melting process around 367 K and (d-f) the crystallization process around 347 K for CoC9Espy between two glass slides at a scan rate of 1 K min-1. The green region is the hs-[CoII] tautomer in the melt. (g) Temperature-dependent XRD patterns for CoC9Espy: the original crystals in the K1 phase (bottom, red line, 296 K) and the melt (middle, blue line, 373 K) and the crystals in the K2 phase obtained by recrystallization at a scan rate of 1 K min-1 (top, green line, 293 K).

Figure 4. (a) Schematic illustration of HBs, where dioxolene moieties are described as nonspecific valence states, (b) the HB unit (orange line), and (c) the projection of the layered structure of the ls-[CoIII]⊂K1 along the a-axis. Alkyl chains are omitted for clarity. (d) Schematic illustration of the HBs, (e) the HB (orange line) with paired alkyl chains, and (f) the projection of whole assembled structure of the ls-[CoIII]⊂K2 along the a-axis. The tert-butyl moieties are omitted for clarity.

HBs listed in Table 3 have the structural parameters in the range of typical HBs as reported previously.65,66 On the other hand, the assembled structure of the ls-[CoIII]⊂K2 depicted in Figure 4d-f demonstrates a one-dimensional HB network based on C-HC9Espy · · · OdCC9Espy along the c-axis (Table 3).65,66 The one-dimensional HB network are further organized with the aid of paired alkyl chains of the neighboring

molecules (Figure 4e,f). XRD studies showed that the crystalline phase obtained from the melt at a cooling rate of 1 K min-1 afforded an XRD pattern corresponding to the ls-[CoIII]⊂K2 phase (see the Supporting Information). These results demonstrated that significant differences between the ls-[CoIII]⊂K1 and the ls-[CoIII]⊂K2 are found in their assembled structures including the HB networks, which is

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Table 3. Geometric Data Concerning the HBs Found in the ls-[CoIII]⊂K1 and the ls-[CoIII]⊂K2 C· · ·O (Å) ls-[CoIII]⊂K1 C-H3,6-DTBQ(II) · · · OdCEspy(I) 3.575 C-HC9Espy(II) · · · OdCC9Espy(IV) 3.472 C-HC9Espy(IV) · · · OdCC9Espy(IV) 3.435 3.603 C-H3,6-DTBQ(IV) · · · OdCC9Espy(III) ls-[CoIII]⊂K2 C-HC9Espy · · · OdCC9Espy 3.385

∠C-H · · · O (deg)

∠H · · · OdC (deg)

138.20 135.59 134.71 143.28

134.50 133.61 133.36 146.74

138.67

133.77

the main reason for the observed polymorphism of CoC9Espy. The temperature-dependent magnetic susceptibility and DSC analysis was carried out for the obtained ls-[CoIII]⊂K2 phase. Upon the heating at 1 K min-1 an abrupt variation of χMT was observed (Figure 2a), and at the same time an endothermic peak was observed at Tc2 ) 362.6 K (Figure 2b). These results clearly support that the ls-[CoIII]⊂K2 exhibits ls-[CoIII]-to-hs-[CoII] VT interconversion on a heating process. It should be noted that the observed endothermic peak, Tc2 ) 362.6 K, for the ls-[CoIII]⊂K2 is clearly lower than the value Tc1 ) 368.2 K, for the ls-[CoIII]⊂K1 phase. Further microphotographic studies depicted in Figure 5a-e visually show the unprecedented sequence of state changes of the ls-[CoIII]⊂K2 during the heating process. The ls[CoIII]⊂K2 in Figure 5a, which is obtained by recrystallization from the melt at a cooling rate of 1 K min-1, melts at Tc2 to give a green isotropic melt shown in Figure 5b. Interestingly, the crystals gradually crystallize from the melt upon holding the temperature at 363.2 K for 60 min (Figure 5c). The crystallization process proceeded further by holding the temperature at 363.2 K for 600 min (Figure 5d). This crystal was shown to be the recrystallized ls-[CoIII]⊂K1 by the XRD pattern (see the Supporting Information) and melts at 367 K when the sample in Figure 5d was reheated from 363.2 K (Figure 5e). This behavior can be understood as a double-melting phenomenon,67 while the unusual color changes coupled with the double-melting process are noteworthy. The DSC curve in Figure 5f was obtained using the same temperature controlling profile with that for the

Figure 6. (a) Temperature- and time-dependent magnetic susceptibility obtained by heating the ls-[CoIII]⊂K2 from 200 K. (b) Temperature profile for the time-dependent magnetic susceptibility measurements. The heating rate was at 1 K min-1.

microphotographic study and demonstrates that two endothermic peaks appear at 362.7 and 367.4 K. The former peak corresponds to the ls-[CoIII]⊂K2-to-melt phase transition while the latter is assigned to the recrystallized ls-[CoIII]⊂K1to-melt phase transition. It would be of interest to monitor the molecular states accompanied with the double-melting process. Figure 6a shows time-dependent magnetic susceptibility for the ls[CoIII]⊂K2 on the heating process, where the temperature was changed as in the case of the microscope observation and DSC analysis (Figure 6b). As shown in Figure 6a, the χMT value first abruptly increases to 2.50 cm3 K mol-1 at around Tc2 (at 145 min), which corresponds to 81%68 VT interconversion from the ls-[CoIII] tautomer to the hs-[CoII] tautomer synchronized with the ls-[CoIII]⊂K2-to-melt phase transition. Surprisingly, keeping the temperature at 363.2 K results in a gradual decrease in the χMT value as a function of time, and it eventually reaches a value of 1.09 cm3 K mol-1 (77% ls-[CoIII])68 after 640 min (780 min as whole in Figure 6a). The above results clearly demonstrate the occurrence of reverse VT interconversion, where thermo-

Figure 5. Snapshots of the double-melting behavior of CoC9Espy on the second heating process: microphotograph taken at (a) 303 K, (b) 363 K, (c) 363.2 K after 60 min, (d) 363.2 K after 600 min, and (e) 373 K. (f) DSC thermogram of CoC9Espy during the second heating process, including the temperature holding at 363.2 K for 600 min. All heating rates are at 1 K min-1.

Polymorph-Dependent Molecular Valence Tautomerism

Figure 7. Schematic Gibbs free energy versus temperature diagram for CoC9Espy. TVT(melt) is the VT equilibrium temperature in the melt, and Tc1 and Tc2 are the temperature of the synchronic bistability of K1 and K2 phases, respectively. TVT(K1) and TVT(K2) are the supposed VT equilibrium temperature in the K1 and K2 crystalline phases, respectively. TVT(K1) and TVT(K2) could not be observed because the melting (Tc1 and Tc2) occurs at lower temperature than TVT(K1) and TVT(K2).

dynamically unfavorable hs-[CoII]-to-ls-[CoIII] interconversion occurs at 363.2 K. Upon further heating at 1 K min-1, an abrupt VT interconversion from the ls-[CoIII] tautomer to the hs-[CoII] tautomer was observed at around Tc1 (at 145 min). All of these results show first melting (the ls[CoIII]⊂K2 f the melt), recrystallization (the melt f the ls-[CoIII]⊂K1), and second melting (the ls-[CoIII]⊂K1 f the melt) synchronized with the ls-[CoIII]-to-hs-[CoII], the hs-[CoII]-to-ls-[CoIII], and the ls-[CoIII]-to-hs-[CoII] VT interconversions, respectively. The observed molecular VT bistability synchronized with the double-melting behaviors can be rationally explained using the G-T diagram shown in Figure 7. In the diagram, TVT(melt) refers to a supposed VT equilibrium temperature in

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the melt, which was calculated to be 326 K from the temperature-dependent χMT of the supercooled melt during the cooling process (see the Supporting Information).41 According to the experimentally observed sequence, Tc2 < Tc1, it is revealed that the K1 phase is the thermodynamically stable form and the K2 phase obtained after cooling from the melt is the thermodynamically metastable form as shown in the diagram.26 This relationship can rationally explain the experimental facts as follows: the thermodynamically stable ls-[CoIII]⊂K1 transforms to the hs-[CoII]⊂melt at Tc1. Upon cooling, the supercooled hs-[CoII]⊂melt is transformed into the metastable ls-[CoIII]⊂K2, which melts into the hs[CoII]⊂melt at Tc2 accompanied by synchronic VT interconversion. Because the hs-[CoII]⊂melt obtained at Tc2 is in the metastable state, the thermodynamically stable ls-[CoIII]⊂K1 begins to grow. This is because free energies decrease in the order ls-[CoIII]⊂K2 > hs-[CoII]⊂melt > ls-[CoIII]⊂K1 in the Tc2-Tc1 range. As a whole, CoC9Espy exhibits the three types of synchronic bistabilities attributed to the macroscopic double-melting behavior (Scheme 2): (1) VT interconversion synchronized with thermodynamically stable ls-[CoIII]⊂K1 to the thermodynamically stable hs[CoII]⊂melt at Tc1 in equilibrium condition (Process 1), (2) VT interconversion synchronized with the metastable ls[CoIII]⊂K2-to-the metastable hs-[CoII]⊂melt at Tc2 in nonequilibrium condition (Process 2), and (3) reverse VT interconversion synchronized with the relaxation69,70 from the metastable hs-[CoII]⊂melt-to-the thermodynamically stable ls-[CoIII]⊂K1 in the Tc1-Tc2 range (Process 3). In addition to the above three types of synchronic bistabilities during the double-melting process, the ls-[CoIII]⊂K2 phase was obtained from the stable hs-[CoII]⊂melt as shown in Scheme 2 as Process 4. According to the G-T diagram, important features for the synchronic bistabilities in both equilibrium and nonequilib-

Scheme 2. Processes Involving VT Synchronized with Polymorphic Crystal-Melt Phase Transitions

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rium processes can be obtained. First the relation, TVT(melt) < Tc1 and Tc2, implies significant enthalpic stabilization of the K1 and K2 crystalline phases constructed from the HB and weakly packed alkyl moieties. This enthalpic effect leads to a significant decrease of the Gibbs energy curve for the ls[CoIII]⊂K1 and the ls-[CoIII]⊂K2 below that for the ls[CoIII]⊂melt. Second, the introduction of the alkoxy chains leads to an increase in the Gibbs energy slope (dG/dT ) -S) of the melts as a positive entropic effect,41,71 leading to the appearance of Tc1 and Tc2 at temperatures lower than TVT(K1) and TVT(K2). Third, the metastable ls-[CoIII]⊂K2 is thermally stable enough; otherwise, the ls-[CoIII]⊂K2 would relax into the ls-[CoIII]⊂K1 at a lower temperature than the melting point of the ls-[CoIII]⊂K2. In the present case, the sufficient HB and alkoxy chain networks would be the main role for the thermal stability of the ls-[CoIII]⊂K2. These three features account for the unprecedented synchronicity between the molecular VT and macroscopic, polymorphic crystalmelt phase transformations. 4. Conclusion In conclusion, a new VT complex, CoC9Espy, shows thermodynamically stable and metastable polymorphic crystals, the main differences between them being found (65) Desiraju, G. R. Acc. Chem. Res. 1991, 24, 290–296. (66) Desiraju, G. R. Chem. Commun. 2005, 2995–3001. (67) Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1980; Vol. 3. (68) The χMT values for the ls-[CoIII] and the hs-[CoII] tautomer were fixed to 0.54 and 2.96 cm3 K mol-1, respectively, which were experimentally obtained at 200 and 400 K. (69) Ito, S.; Inabe, H.; Morita, N.; Ohta, K.; Kitamura, T.; Imafuku, K. J. Am. Chem. Soc. 2003, 125, 1669–1680. (70) Handa, Y. P.; Roovers, J.; Wang, F. Macromolecules 1994, 27, 5511– 5516.

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in their assembled structures with HBs. Both of the polymorphic crystals transform into the hs-[CoII]⊂melt at different temperatures; therefore, CoC9Espy exhibits molecular VT interconversions synchronized with crystalmelt phase transitions in equilibrium and nonequilibrium conditions. Interestingly, CoC9Espy in the metastable polymorphic crystalline phase shows the double-melting phenomenon, which leads to an unusual synchronic VT interconversion, ls-[CoIII] f hs-[CoII] f ls-[CoIII] f hs[CoII]. The present studies have also shown that the doping of both alkoxy chains and ester moieties are the key factors for exhibiting unusual double-melting phenomenon synchronized with VT. This direct experimental evidence and the strategy for linking molecular bistability and polytypic crystalline phases will open a new perspective on a wide range of bistable materials. Acknowledgment. The authors thank M. Yamasaki (Rigaku) for helping with the structural analysis. This work was supported by a Grant-In-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. D.K. is grateful to JSPS Research Fellowships for Young Scientists. Supporting Information Available: Bond lengths and angles of the ls-[CoIII]⊂K1 and the ls-[CoIII]⊂K2. The simulated and observed XRD patterns for the ls-[CoIII]⊂K2 phase obtained from the melt, XRD pattern for the ls-[CoIII]⊂K1 phase obtained from the melt during the double-melting process, and estimation of TVT(melt) (PDF). Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. CM900543R (71) Sorai, M.; Saito, K. Chem. Rec. 2003, 3, 29–39.