7. M. A. Tanatar, J. Paglione, C. Petrovic, L. Taillefer, Science 316,
8. H. Pfau et al., Nature 484, 493–497 (2012).
9. Y. Machida et al., Phys. Rev. Lett. 110, 236402 (2013).
10. J. K. Dong, Y. Tokiwa, S. L. Bud’ko, P. C. Canfield, P. Gegenwart,
Phys. Rev. Lett. 110, 176402 (2013).
11. R. W. Hill, C. Proust, L. Taillefer, P. Fournier, R. L. Greene,
Nature 414, 711–715 (2001).
12. J. Crossno et al., Science 351, 1058–1061 (2016).
13. N. Wakeham et al., Nat. Commun. 2, 396 (2011).
14. S. A. Hartnoll, Nat. Phys. 11, 54–61 (2015).
15. J. A. N. Bruin, H. Sakai, R. S. Perry, A. P. Mackenzie, Science
339, 804–807 (2013).
16. V. Eyert, Ann. Phys. (Berlin) 11, 650–704 (2002).
17. C. N. Berglund, H. J. Guggenheim, Phys. Rev. 185, 1022–1033
18. V. N. Andreev, A. V. Chudnovskii, A. V. Petrov, E. I. Terukov,
Phys. Status Solidi, A Appl. Res. 48, K153–K156 (1978).
19. D. W. Oh, C. Ko, S. Ramanathan, D. G. Cahill, Appl. Phys. Lett.
96, 151906 (2010).
20. B. S. Guiton, Q. Gu, A. L. Prieto, M. S. Gudiksen, H. Park, J. Am.
Chem. Soc. 127, 498–499 (2005).
21. Q. Gu, A. Falk, J. Wu, L. Ouyang, H. Park, Nano Lett. 7,
22. S. Lee et al., J. Am. Chem. Soc. 135, 4850–4855 (2013).
23. P. Kim, L. Shi, A. Majumdar, P. L. McEuen, Phys. Rev. Lett. 87,
24. S. Lee et al., Nat. Commun. 6, 8573 (2015).
25. J. D. Budai et al., Nature 515, 535–539 (2014).
26. A. K. McCurdy, H. J. Maris, C. Elbaum, Phys. Rev. B 2,
27. X. Tan et al., Sci. Rep. 2, 466 (2012).
28. J. Zaanen, Nature 430, 512–513 (2004).
29. M. M. Qazilbash et al., Science 318, 1750–1753 (2007).
30. M. M. Qazilbash et al., Phys. Rev. B 74, 205118 (2006).
31. V. J. Emery, S. A. Kivelson, Phys. Rev. Lett. 74, 3253–3256
32. J. Merino, R. H. McKenzie, Phys. Rev. B 61, 7996–8008 (2000).
33. G. Pálsson, G. Kotliar, Phys. Rev. Lett. 80, 4775–4778 (1998).
This work was supported by the U.S. Department of Energy (DOE)
Early Career Award DE-FG02-11ER46796. Parts of this work
were performed at the Molecular Foundry, a Lawrence Berkeley
National Laboratory user facility supported by the Office of
Science, Basic Energy Sciences, U.S. DOE, under contract
DE-AC02-05CH11231, and used facilities of the Electronic Materials
Program at LBNL supported by the Office of Science, Basic Energy
Sciences, U.S. DOE, under contract DE-AC02-05CH11231. O.D.
acknowledges funding from the U.S. DOE, Office of Science, Basic
Energy Sciences, Materials Sciences and Engineering Division.
C.K. was partially supported by the Tsinghua-Berkeley Shenzhen
Institute. K.H. and X.Z. were supported by U.S. DOE, Basic Energy
Sciences Energy Frontier Research Center (DoE-LMI-EFRC) under
award DOE DE-AC02-05CH11231. K.H. also acknowledges public
sector funding from A*STAR of Singapore (M4070232.120) and
Pharos Funding from the Science and Engineering Research Council
(grant 152 72 00018). J.H. acknowledges support from the National
Science Foundation of China (grant 11572040) and the Thousand
Young Talents Program of China. Simulation work by J.H. at Oak
Ridge National Laboratory was supported by DOE Basic Energy
Sciences award DE-SC0016166. Theoretical calculations were
performed using resources of the National Supercomputer Center in
Guangzhou and the Oak Ridge Leadership Computing Facility. We
thank R. Chen, D. F. Ogletree, E. Wong, J. Budai, and A. Said for
technical assistance and helpful discussions. J. W. conceived the
project; S.L. and J.S. synthesized the materials; S.L., K.H., K.L., and
K. W. fabricated the devices; S.L. and K.H. performed the thermal and
electrical measurements; C.K. performed Auger electron
spectroscopy; F. Y., S.A.H., K.H., C.D., J.J.U., and X.Z. helped with data
analysis and theoretical understanding; J.H. and O.D. performed the
modeling of thermal conductivity from first-principles phonon
dispersions; and all authors contributed to writing the manuscript.
Materials and Methods
Figs. S1 to S11
Tables S1 to S3
3 May 2016; accepted 22 December 2016
Synthesis and characterization of
the pentazolate anion cyclo-N5ˉ
Chong Zhang,1 Chengguo Sun,2 Bingcheng Hu,1† Chuanming Yu,1 Ming Lu1†
Pentazole (HN5), an unstable molecular ring comprising five nitrogen atoms, has been of
great interest to researchers for the better part of a century. We report the synthesis
and characterization of the pentazolate anion stabilized in a (N5)6(H3O)3(NH4)4Cl salt.
The anion was generated by direct cleavage of the C–N bond in a multisubstituted
arylpentazole using m-chloroperbenzoic acid and ferrous bisglycinate. The structure was
confirmed by single-crystal x-ray diffraction analysis, which highlighted stabilization of
the cyclo-N5ˉ ring by chloride, ammonium, and hydronium. Thermal analysis indicated the
stability of the salt below 117°C on the basis of thermogravimetry-measured onset
Pentazole (HN5) and its anion (cyclo-N5ˉ) have been identified as potential constit- uents of materials with high energy den- sity, and accordingly they are candidates for possible applications in both military
and civilian contexts (1–3). Generally, cyclo-N5ˉ
has been stabilized only at low temperature,
through conjugation with an aromatic ring bearing a strong electron-donating group (4–7). In
this conjugated structure, the C–N bond is much
stronger than either the N–N single bond or
N=N double bond (8). The selective cleavage of
the C–N bond in arylpentazoles while keeping
cyclo-N5ˉ intact still presents a great challenge.
Several elegant methodologies have been applied
to this problem, including the use of electrospray
negative-ion mass spectrometry for selective C–N
bond cleavage or, more recently, radical anion to
activate the C–N bond (9–11). However, to date,
all attempts to prepare the solid form of cyclo-N5ˉ via the cleavage of this C–N bond have proven unsuccessful (12–16).
In our previous studies, we found that the formation of cyclo-N5ˉ from arylpentazoles proceeded
more efficiently upon increasing the number of
electron-donating groups at the meta/para-position
of the aryl groups (17). We then considered adding
a reagent to stabilize the cyclo-N5ˉ immediately
after cleavage of the aryl-pentazole bond. After
hundreds of experiments targeting efficient C–N
bond cleavage, we succeeded in isolating a stable
salt, (N5)6(H3O)3(NH4)4Cl (fig. S1), prepared by
the rupture of the C–N bond in 3,5-dimethyl-4-
hydroxyphenylpentazole (HPP) through treatment
with m-chloroperbenzoic acid (m-CPBA) and ferrous bisglycinate [Fe(Gly)2].
In our synthesis planning, Fe(Gly)2 played a
dual role as both a cyclo-N5ˉ stabilizer and a
m-CPBA mediator. When an aqueous solution
of Fe(Gly)2 (2.5 equivalents) was added to a
stirred solution of HPP (1 equivalent) in acetonitrile and methanol (v/v, 1/1) at −45°C, no chemical reaction occurred, which indicated that the
ferrous complex was insensitive to HPP and
unlikely to destroy the five-membered nitrogen
ring in the HPP molecule. After adding m-CPBA
(4 equivalents) in cold methanol, cyclo-N5ˉ was
readily detected in the solution by electrospray
ionization (ESI) mass spectrometry: The intense
negative ion peak could be observed at a mass/
charge ratio m/z of 70.09 (figs. S2 to S7). Upon
completion of the reaction, the insoluble materials were eliminated by filtration. The collected
filtrate was evaporated under vacuum to furnish
a dark-brown solid. The pure product could be
isolated through silica gel column chromatography with an acceptable yield (19%) to give
(N5)6(H3O)3(NH4)4Cl as an air-stable white solid.
Primary structural confirmation came from
single-crystal x-ray diffraction analysis. The pentazolate salt crystallized in the cubic space group
Fd-3m with a cell volume of 5801.0 ± 0.5 Å3 (18).
As seen in the ellipsoid plot of the pentazolate salt
in Fig. 1A, the pentagonal N5ˉ ring comprises five
nitrogen atoms in a perfectly planar arrangement,
as evident from the torsion angles (N1′-N1-N2-N3,
0°; N1-N2-N3-N2′, 0°). Each N atom offers a
p-orbital electron to form a conjugated p56 bond
together with another single electron, which in
principle fulfills the geometric criterion of aromaticity. Relevant bond distances and angles
are shown in tables S2 and S3. The N–N bond
lengths in cyclo-N5ˉ are 1.309 Å, 1.310 Å, 1.310 Å,
1.324 Å, and 1.324 Å; the average N–N bond distance (1.315 Å)—intermediate between N–N
single bond lengths (hydrazine, 1.452 Å) (19)
and N=N double bond lengths (trans-diamine,
1.252 Å) (20)—is slightly shorter than both the
experimental N–N bond distance for 4-dimethyl-
aminophenylpentazole (average 1.323 Å) (21, 22)
1School of Chemical Engineering, Nanjing University of Science
and Technology, Nanjing, Jiangsu 210094, China. 2School of
Chemical Engineering, University of Science and Technology
Liaoning, Anshan, Liaoning 114051, China.
*These authors contributed equally to this work. †Corresponding
author. Email: firstname.lastname@example.org (B.H.); email@example.com.