New Thiadiazole Dioxide Bridging Ligand with a Stable Radical Form

Communication pubs.acs.org/crystal

New Thiadiazole Dioxide Bridging Ligand with a Stable Radical Form for the Construction of Magnetic Coordination Chains Dawid Pinkowicz,*,† Zhanyong Li,‡ Piotr Pietrzyk,† and Michał Rams§ †

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States § Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland ‡

S Supporting Information *

ABSTRACT: A new thiadiazole dioxide-type bridging ligand with the ability to form stable anionic radicals and the ability to chelate two metal centers in a symmetric fashion has been designed, synthesized, and fully characterized. Its paramagnetic potassium salt and an example of a CuII coordination chain were also obtained.

he first organic molecular magnet obtained by Miller et al.1 was based on TCNE•− radical anions. Since then several different families of organic spin-carriers were studied as potential candidates for the construction of new magnetic and/ or conductive materials.2 The most important ones are the aforementioned TCNE,3 TCNQ,4,5 α-nitronyl nitroxides,6−8 and verdazyl9,10 radicals. Studies of organic spin-carriers have significantly contributed to the field of molecule-based magnets by introducing new concepts and ideas, e.g., conductive molecular magnets11,12 or efficient “magnetic bridging” of lanthanide ions.13−15 The field of radical-based molecular magnets, however, is still underdeveloped compared to the lanthanide16 or transition metal systems17−20 mainly due to the sensitive nature of the open-shell organic molecules. Recently a few new stable (persistent) radicals and their complexes or molecules that form stable radicals were developed and reported.21,22 However, most of the radical ligands known so far are nonbridging and usually show blocking character. One that has drawn our particular attention due to the remarkable stability of the radical form and rigid structure was a chelating [1,2,5]thiadiazolo[3,4-f ][1,10]phenanthroline 1,1-dioxide (1,10-tdapO2) developed by Awaga et al.23,24 On the basis of this work, we have designed and developed a novel bischelating ligand specifically for bridging two metal centers that would undergo reduction to the persistent radical anion form. Such radical anion is expected to show strong magnetic coupling with the coordinated magnetic centers, while still allowing for the formation of extended coordination systems like coordination chains.25 The title bridging ligand is [1,2,5]thiadiazolo[3,4-f ][4,7]phenanthroline 1,1-dioxide (L) (Figure 1), and it belongs to the class of 3,4-substituted

T

© 2014 American Chemical Society

Figure 1. Structural formula of L.

1,2,5-thiadiazole 1,1-dioxide heterocycles.26,27 The main “improvement” in L compared to 1,10-tdapO2 in terms of its usability for the construction of extended coordination systems is the 4,7-phenanthroline-backbone that allows for bridging of two metal centers in a bis-chelating fashion with the use of both the thiadiazole and 4,7-phen nitrogen atoms while still maintaining all advantages of the aromatic thiadiazole dioxides.23 Because of the presence of a strongly electron withdrawing thiadiazole dioxide group, L can undergo two reversible reduction processes to a persistent27 radical anion L•− and a diamagnetic dianion L2−, respectively (Figure 2), under relatively mild conditions. L was obtained by a modification of the general procedure for 3,4-substituted 1,2,5-thiadiazole Received: June 20, 2014 Revised: August 7, 2014 Published: August 14, 2014 4878

dx.doi.org/10.1021/cg500906w | Cryst. Growth Des. 2014, 14, 4878−4881

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all atoms). Both LUMO and SOMO are identical and have a nodal plane at the thiadiazole N−C bonds (antibonding character) and show bonding character for the thiadiazole S−N and C−C bonds (C3−C4 and C9−C10). Electron paramagnetic resonance (EPR) spectrum of KL dissolved in degassed DMF is shown in Figure 3. It consists of

Figure 2. Cyclic voltammograms of 1 mM solutions of L (red line) and T (blue line) in 100 mM MeCN solution of n-Bu4NPF6 recorded at 100 mV/s.

Figure 3. EPR spectrum of the potassium salt KL dissolved in anhydrous and oxygen-free DMF (black line) and its simulation (red line). For details see text.

1,1-dioxide heterocycles,28,29 namely, a condensation reaction between the suitable α-diketone (4,7-phenanthroline-5,6dione30,31) and sulfamide in dry ethanol under inert gas atmosphere (for details see Supporting Information (SI)). During an unsuccessful attempt to purify crude L by vacuum sublimation (10 mbar, 285 °C, 15 h) instead of an orange product, yellow needle crystals formed which proved to be [1,2,5]thiadiazolo[3,4-f ][1,10]phenanthroline (T), a thiadiazole congener of L (monoclinic P21/c by sc-XRD). However, L was successfully purified by recrystallization from hot MeCN and isolated as orange needle crystals (monoclinic P21/c by scXRD). Selected crystallographic parameters for both compounds are summarized in Table S1, asymmetric units are shown in Figure S5, and selected bond lengths are presented in Table S3 in SI. The cyclic voltammogram of L is similar to those of other 3,4-substituted [1,2,5]thiadiazole 1,1-dioxides23,26 with two reversible reduction peaks at −499 and −1229 mV vs ferrocene (red line in Figure 2). The thiadiazole congenere T exhibits only one reversible redox event in the MeCN electrochemical window at −1957 mV vs ferrocene (blue line in Figure 2). The low value of the first reduction potential for L suggests that it could be reduced chemically by potassium iodide. Indeed, the reaction of L with a large excess of KI in MeCN led to a dark purple crystalline potassium salt KL that is sparingly soluble in MeCN (for details see SI). The salt crystallizes in monoclinic space group P21/c (for details see Table S1 and for asymmetric unit see Figure S5 in SI) with potassium ions coordinated to the nitrogen (2.819−2.903 Å) and oxygen atoms (2.684−2.720 Å) of the L•− radical anion. The anions are organized in chainlike stacks along the a crystallographic axis by significant π−π interactions with two different distances between the centroids of the adjacent 4,7-phen backbones: 3.828 and 3.476 Å (Figure S6 in SI) and the shortest C−C distance between two adjacent L•− anions of 3.209 Å along the stack. Analysis of the respective bond lengths of the neutral L and the radical L•− (Table S3 in SI) leads to similar conclusions as previously drawn for other congeners23,24,32 that upon reduction the thiadiazole ring is the most affected part of the molecule. The thiadiazole N−C bonds are elongated by ca. 0.036 Å, whereas the thiadiazole S−N and C−C bonds are significantly shortened by ca. 0.055 and 0.082 Å, respectively. These observations are in agreement with the shapes of the LUMO of the neutral ligand and the SOMO of the reduced species (Figure S7 in SI; B3LYP correlation and exchange functionals33,34 with 6-311++G(2d,2p) basis set for

67 lines in contradiction to the previously reported [1,2,5]thiadiazolo[3,4-f ][1,10]phenanthroline 1,1-dioxide analogue.23,24 The complex hyperfine feature indicates that the unpaired electron is delocalized over the whole molecule. Fitting of the spectrum resulted in the following spinHamiltonian parameters: giso = 2.0029 ± 0.0002, two N with aN = 0.332 mT, two N with aN = 0.101 mT, two H with aH = 0.163 mT, and two sets of nearly equivalent H atoms with aH = 0.033(9) and 0.033(8) mT. This hydrogen coupling scheme remains in agreement with the NMR nuclear shielding values for the neutral L (for details see SI). The Mulliken population analysis shows that the spin density redistribution is strongly uneven. The majority of the spin density is localized around the two nitrogen atoms of the thiadiazole ring (0.25 at each). The remaining two nitrogen atoms of the 4,7-phen backbone bear 0.038 of the spin density each (Figure S4 and Table S2 in SI). Magnetic properties of KL were investigated in the 400−2 K temperature range. The χT vs T and χ vs T dependencies are presented in Figure 4. The χT(T) product decreases with lowering of the temperature from 0.198 cm3 K/mol, which is much below the expected spin-only value of 0.375 and reaches 0.008 cm3 K/mol below 120 K. This level corresponds to 2.1% of noninteracting S = 1/2 spins, most probably related to defects in the crystal structure of KL.23 Such behavior suggests very strong antiferromagnetic interactions between the adjacent

Figure 4. Magnetic susceptibility for KL measured at HDC = 10 kOe, shown as χT product. The blue dashed line shows the level expected for isolated spins S = 1/2 with g = 2.00. 4879



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L•− radical anions. We used a number of simple models that may be correlated with the crystal structure of KL (see Figure S8 in the SI). The Bleaney−Bowers model for two interacting 1/2 spins (H = −2JS1S2, dimer model) does not follow the experimental points if g = 2 is fixed, or leads to a nonphysical value of g larger than 2.6. The experimental data could not be fitted using the Bonner−Fisher model (S = 1/2 chain) as well. Introduction of an interchain interaction in the mean field approximation leads to zJ′ constant of the order of J, which is also not acceptable and does not agree with the crystal structure. The model of a chain with two alternating exchange constants J1 and J2, which we approximated by numerical calculations for a closed ring of 14 S = 1/2 spins was also tested. Such model corresponds well with the crystal structure, but in order to fit the experimental data it is necessary to introduce a very strong ferromagnetic interchain interaction. Detailed description of these attempts can be found in the SI. The magnetic behavior of the KL salt might arise from the dimerization of the adjacent radical anions.35 This will be investigated in the future using a single crystal sample and reported separately (it is important to note that the observed behavior is not a purity issue−the purity has been confirmed by two complementary techniques: elemental analysis and powder X-ray diffraction). The ligand L was tested as a potential bis-chelating bridging building block for the construction of paramagnetic dinuclear, multinuclear, or extended coordination assemblies. Combination of L with CuIICl2·2H2O in a 1:1 ratio in reagent grade MeCN leads to small greenish-brown block crystals of CuIICl2(L)·MeCN CuL (triclinic P1̅ by sc-XRD; for details see Table S1, for asymmetric unit see Figure S5 and for bond list see Table S3 in the Supporting Information). CuL forms straight, slightly waved 1-D coordination chains with L bridging two CuII ions in a bis-chelating fashion (Figure 5). Each CuII

nitrogen atoms (there are no entries in the Cambridge Structural Database reporting similar coordination mode). Magnetic properties of CuL were investigated in the 300−2 K temperature range at 1 kOe external magnetic field and in the 50 to 0 kOe magnetic field range at 1.8 K. The χT vs T dependence is presented in Figure 6. The χT(T) curve remains

Figure 6. Magnetic properties of CuL (main: at 1 kOe - black cricles; inset: at 1.8 K - black points) per one CuII center. Green line: the best fit to the Bonner−Fisher model see text).

constant at 0.374 cm−3·K·mol−1 (slightly lower than the expected spin-only value for one isolated S = 1/2 CuII center with g = 2.1) down to 50 K and then shows a rapid decrease probably due to weak intrachain antiferromagnetic interactions between the neighboring Cu II centers (the interchain interactions can be neglected because of the significant interchain Cu···Cu distances larger than 7.966 Å). The data were fitted using the Bonner−Fisher chain model (green line in Figure 6; H = −2JΣSiSi+136,37) assuming g = 2.1 (fixed). The best fit gave the exchange constant JCuCu = −1.8(1) cm−1, suggesting the antiferromagnetic nature of the magnetic interactions transmitted through the diamagnetic L bridge along the chain. The magnetization curve at 1.8 K does not show any signs of spontaneous magnetization, and its almost linear behavior confirms the antiferromagnetic ground state (Figure 6 inset). In conclusion, a new bis-chelating bridging ligand with the ability to form very stable radical anions has been designed, synthesized, and characterized together with its potassium salt KL and the first example of its coordination chain CuL. The 4,7-phenanthroline backbone provides very efficient pathways for very strong magnetic exchange through the π−π stacking interactions in KL. The position of the donor atoms allows for bis-chelating coordination and for the formation of extended coordination compounds. In such systems, very strong antiferromagnetic interactions between the radical and the coordinated metal centers are expected due to the large spin density carried by the donor atoms and the potential direct overlap of the appropriate magnetic orbitals of the metal and SOMO of the radical. In particular, the use of the radical form in combination with selected metal centers (S ≠ 1/2) may allow for the construction of high-temperature single chain magnets with a low lying magnetic ground state. Further work on the design and synthesis of the coordination chains based on the radical anion L•− is currently in progress.

Figure 5. Fragment of the crystal structure of CuL showing the coordination chain -CuII-L-CuII-L- (Cu - orange, S - yellow, Cl - green, C - gray, N - blue, O - red, H - white).

center (occupying an inversion center) is in a strongly distorted octahedral environment with two Cl− anions coordinated in the apical positions (av. Cu−Cl distance of 2.26 Å) and the equatorial plane occupied by N atoms of L. Both CuII centers show strong Jahn−Teller distortion along the N3−Cu−N4 bonds with an average Cu−N distance of 2.52 Å. The crystal structure is stabilized by quite weak and infrequent π−π interactions between the adjacent 4,7-phen backbones with the shortest distance of 3.757 Å and other weak van der Waals contacts involving interstitial MeCN molecules (see Figure S9 in SI for crystal packing). It is important to note that CuL represents the first example of a thiadiazole dioxide group coordinated directly to a metal center with the use of its 4880



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(11) Wang, W.-Z.; Wu, Y.; Ismayilov, R. H.; Kuo, J.-H.; Yeh, C.-Y.; Lee, H.-W.; Fu, M.-D.; Chen, C.-h.; Lee, G.-H.; Peng, S.-M. Dalton Trans. 2014, 43, 6229−6235. (12) Gu, H.; Zhang, X.; Wei, H.; Huang, Y.; Wei, S.; Guo, Z. Chem. Soc. Rev. 2013, 42, 5907−5943. (13) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. Nat. Chem. 2011, 3, 538−542. (14) Demir, S.; Zadrozny, J. M.; Nippe, M.; Long, J. R. J. Am. Chem. Soc. 2012, 134, 18546−18549. (15) Meihaus, K. R.; Corbey, J. F.; Fang, M.; Ziller, J. W.; Long, J. R.; Evans, W. J. Inorg. Chem. 2014, 53, 3099−3107. (16) (a) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Chem. Rev. 2013, 113, 5110−5148. (b) Ren, M.; Pinkowicz, D.; Yoon, M.; Kim, K.; Zheng, L.-M.; Breedlove, B. K.; Yamashita, M. Inorg. Chem. 2013, 52, 8342−8348. (17) Nowicka, B.; Korzeniak, T.; Stefańczyk, O.; Pinkowicz, D.; Chorąży, S.; Podgajny, R.; Sieklucka, B. Coord. Chem. Rev. 2012, 256, 1946−1971. (18) Waldmann, O. Coord. Chem. Rev. 2005, 249, 2550−2566. (19) (a) Sieklucka, B.; Podgajny, R.; Korzeniak, T.; Nowicka, B.; Pinkowicz, D.; Kozieł, M. Eur. J. Inorg. Chem. 2011, 305−326. (b) Pinkowicz, D.; Rams, M.; Nitek, W.; Czarnecki, B.; Sieklucka, B. Chem. Commun. 2012, 48, 8323−8325. (20) Sieklucka, B.; Podgajny, R.; Pinkowicz, D.; Nowicka, B.; Korzeniak, T.; Balanda, M.; Wasiutynski, T.; Pelka, R.; Makarewicz, M.; Czapla, M.; Rams, M.; Gawel, B.; Lasocha, W. CrystEngComm 2009, 11, 2032−2039. (21) Fatila, E. M.; Clérac, R.; Rouzières, M.; Soldatov, D. V.; Jennings, M.; Preuss, K. E. J. Am. Chem. Soc. 2013, 135, 13298−13301. (22) Morgan, I. S.; Peuronen, A.; Hänninen, M. M.; Reed, R. W.; Clérac, R.; Tuononen, H. M. Inorg. Chem. 2013, 53, 33−35. (23) Shuku, Y.; Suizu, R.; Awaga, K. Inorg. Chem. 2011, 50, 11859− 11861. (24) Shuku, Y.; Suizu, R.; Domingo, A.; Calzado, C. J.; Robert, V.; Awaga, K. Inorg. Chem. 2013, 52, 9921−9930. (25) Zhang, W.-X.; Ishikawa, R.; Breedlove, B.; Yamashita, M. RSC Adv. 2013, 3, 3772−3798. (26) Linder, T.; Badiola, E.; Baumgartner, T.; Sutherland, T. C. Org. Lett. 2010, 12, 4520−4523. (27) Mirífico, M. V.; Caram, J. A.; Gennaro, A. M.; Cobos, C. J.; Vasini, E. J. J. Phys. Org. Chem. 2011, 24, 1039−1044. (28) Wright, J. B. J. Org. Chem. 1964, 29, 1905−1909. (29) Vorreither, H. K.; Ziegler, E. Monatsh. Chem. 1965, 96, 216− 219. (30) Bonhôte, P.; Wrighton, M. S. Synlett. 1997, 897−898. (31) D’Alessandro, D. M.; Keene, F. R. Dalton Trans. 2006, 1060− 1072. (32) Xie, Y.; Shuku, Y.; Matsushita, M. M.; Awaga, K. Chem. Commun. 2014, 50, 4178−4180. (33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (34) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789. (35) (a) Decken, A.; Mailman, A.; Passmore, J.; Rautiainen, J. M.; Sherer, W.; Scheidt, E.-W. Dalton. Trans. 2011, 40, 868−879. (b) Shuvaev, K. V.; Decken, A.; Grein, F.; Abedin, T. S. M.; Thompson, L. K.; Passmore, J. Dalton. Trans. 2008, 4029−4037. (36) Kahn, O. Molecular Magnetism; Wiley: New York, 1993; p 380. (37) Estes, W. E.; Gavel, D. P.; Hatfield, W. E.; Hodgson, D. J. Inorg. Chem. 1978, 17, 1415−1421.

ASSOCIATED CONTENT

S Supporting Information *

Experimental details; synthetic procedures with additional spectral and structural characterization for L, T, KL, and CuL; PXRD patterns and thermogravimetric analyses for L, KL, and CuL (Figures S1−S3); additional details on the EPR, magnetic characterization, and DFT calculations for KL and CuL; asymmetric units for L, T, KL, and CuL; additional structural diagrams and crystallographic information files. This material is available free of charge via the Internet at http:// pubs.acs.org. CCDC 997099−997102 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. pl. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.P. gratefully acknowledges the financial support of the European Commission within the Marie Curie International Outgoing Fellowship, project MultiCyChem (Grant Agreement No. PIOF-GA-2011-298569) and the START fellowship of the Foundation for Polish Science (2013 edition). Part of the research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Project ATOMIN, Contract No. POIG.02.01.00-12-023/08). Magnetic measurements were partially performed using the equipment purchased from the “Large Research Infrastructure Fund” of the Polish Ministry of Science and Higher Education (Decision No. 6350/IA/158/ 2013.1; Contract No. CRZP/UJ/254/2013). D.P. gratefully acknowledges Prof. Barbara Sieklucka (Jagiellonian University, Kraków, Poland) and Prof. Kim R. Dunbar (Texas A&M University, College Station, USA) for the support of this work.

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New Thiadiazole Dioxide Bridging Ligand with a Stable Radical Form

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