from mid-infrared to terahertz frequencies. The
combined advantage of strong polariton-field
confinement, anisotropic polariton propagation,
tunability by geometry and electric gating, as well
as the possibility of developing vd W heterostruc-tures (33), could open exciting new possibilities
for flatland infrared, thermal, and optoelectronic
applications, such as infrared chemical sensing,
planar hyperlensing (8, 18, 21), exotic optical coupling (10), and manipulation of near-field heat
transfer (2, 8). Real-space wavefront nanoimaging of in-plane hyperbolic polaritons, as demonstrated in our work, will be an indispensable
tool for verifying new design principles and for
REFERENCES AND NOTES
1. N. Yu et al., Science 334, 333–337 (2011).
2. A. V. Kildishev, A. Boltasseva, V. M. Shalaev, Science 339,
3. N. Yu, F. Capasso, Nat. Mater. 13, 139–150 (2014).
4. X. Ni, N. K. Emani, A. V. Kildishev, A. Boltasseva, V. M. Shalaev,
Science 335, 427 (2012).
5. D. Lin, P. Fan, E. Hasman, M. L. Brongersma, Science 345,
6. Y. Liu, X. Zhang, Appl. Phys. Lett. 103, 141101 (2013).
7. J. Lin et al., Science 340, 331–334 (2013).
8. J. S. Gomez-Diaz, A. Alù, ACS Photonics 3, 2211–2224
9. J. S. Gomez-Diaz, M. Tymchenko, A. Alù, Phys. Rev. Lett. 114,
10. C. L. Cortes, Z. Jacob, Nat. Commun. 8, 14144 (2017).
11. P. V. Kapitanova et al., Nat. Commun. 5, 3226 (2014).
12. Y. Yang et al., NPG Asia Mater. 9, e428 (2017).
13. A. A. High et al., Nature 522, 192–196 (2015).
14. D. N. Basov, M. M. Fogler, F. J. García de Abajo, Science 354,
15. T. Low et al., Nat. Mater. 16, 182–194 (2017).
16. A. Nemilentsau, T. Low, G. Hanson, Phys. Rev. Lett. 116,
17. S. Dai et al., Science 343, 1125–1129 (2014).
18. S. Dai et al., Nat. Commun. 6, 6963 (2015).
19. E. Yoxall et al., Nat. Photonics 9, 674–678 (2015).
20. J. D. Caldwell et al., Nat. Commun. 5, 5221 (2014).
21. P. Li et al., Nat. Commun. 6, 7507 (2015).
22. Z. Sun, Á. Gutiérrez-Rubio, D. N. Basov, M. M. Fogler,
Nano Lett. 15, 4455–4460 (2015).
23. A. J. Giles et al., Nano Lett. 16, 3858–3865 (2016).
24. See supplementary materials.
25. Z. Jacob, E. E. Narimanov, Appl. Phys. Lett. 93, 221109
26. O. Takayama, D. Artigas, L. Torner, Nat. Nanotechnol. 9,
27. S Law, V. Podolskiy, D. Wasserman, Nanophotonics 2, 103–130
28. A. Boltasseva, H. A. Atwater, Science 331, 290–291 (2011).
29. T. B. Hoffman, B. Clubine, Y. Zhang, K. Snow, J. H. Edgar,
J. Cryst. Growth 393, 114–118 (2014).
30. J. H. Edgar et al., J. Cryst. Growth 403, 110–113 (2014).
31. A. J. Giles et al., Nat. Mater. 17, 134–139 (2017).
32. P. Alonso-González et al., Science 344, 1369–1373 (2014).
33. S. Dai et al., Nat. Nanotechnol. 10, 682–686 (2015).
Funding: The authors acknowledge support from the European
Commission under the Graphene Flagship (GrapheneCore1, grant
no. 696656), the Marie Sklodowska-Curie individual fellowship
(SGPCM-705960), the Spanish Ministry of Economy, Industry, and
Competitiveness (national projects MAT2015-65525-R, MAT2015-
65159-R, FIS2014-60195-JIN, MAT2014-53432-C5-4-R, FIS2016-
80174-P, MAT2017-88358-C3-3-R and the project MDM-2016-0618
of the Maria de Maeztu Units of Excellence Programme),
the Basque government (PhD fellowship PRE-2016-1-0150), the
Regional Council of Gipuzkoa (project no. 100/16), and the
Department of Industry of the Basque Government (ELKARTEK
project MICRO4FA). Further, support from the Materials
Engineering and Processing program of the NSF, award number
CMMI 1538127, and the II−VI Foundation is greatly appreciated.
Author contributions: P.L. and R.H. conceived the study. Sample
fabrication was performed by I.D. and S.V., coordinated by S.V.,
and supervised by F.C. and L.E.H. P.L. performed the experiments
and simulations. F.J.A.-M. and A. Y.N. contributed to the
simulations. S.L. and J.H.E. grew the isotopically enriched boron
nitride. R.H. coordinated and supervised the work. P.L. and
R.H. wrote the manuscript with the input of all other co-authors.
Competing interests: R.H. is cofounder of and on the scientific
advisory board of Neaspec GmbH, a company producing
scattering-type near-field scanning optical microscope
systems, such as the one used in this study. The remaining
authors declare no competing financial interests. Data and
materials availability: All data are available in the manuscript or
the supplementary materials.
Materials and Methods
Figs. S1 to S14
11 October 2017; accepted 10 January 2018
Nitrogen fixation and
reduction at boron
Marc-André Légaré,1,2 Guillaume Bélanger-Chabot,1,2 Rian D. Dewhurst,1,2
Eileen Welz,3 Ivo Krummenacher,1,2 Bernd Engels,3 Holger Braunschweig1,2*
Currently, the only compounds known to support fixation and functionalization of dinitrogen
(N2) under nonmatrix conditions are based on metals. Here we present the observation
of N2 binding and reduction by a nonmetal, specifically a dicoordinate borylene. Depending
on the reaction conditions under which potassium graphite is introduced as a reductant,
N2 binding to two borylene units results in either neutral (B2N2) or dianionic ([B2N2]2–)
products that can be interconverted by respective exposure to further reductant or to air.
The 15N isotopologues of the neutral and dianionic molecules were prepared with 15N-labeled
dinitrogen, allowing observation of the nitrogen nuclei by 15N nuclear magnetic resonance
spectroscopy. Protonation of the dianionic compound with distilled water furnishes a
diradical product with a central hydrazido B2N2H2 unit. All three products were characterized
spectroscopically and crystallographically.
The element nitrogen is essential for life on Earth and makes up over three-quarters of the atmosphere by volume, yet its common elemental form (dinitrogen, N2) is extreme- ly stable and thus difficult to utilize. Nature
overcomes this through the nitrogen-binding en-
zymes called nitrogenases (1, 2), whereas industry
relies on the energy-intensive, transition-metal–
catalyzed Haber-Bosch process (3, 4) to convert
dinitrogen to ammonia for the production of fer-
tilizer. In the century since the discovery of the
Haber-Bosch process, a number of transition-metal
(TM) species have been found to bind (and even
functionalize) N2 at low temperatures (5–14). The
rare ability of certain transition-metal complexes
to bind N2 is attributed to their advantageous
combination of unoccupied and occupied d or-
bitals, which are of appropriate energy and sym-
metry to synergically accept electron density
from and backdonate to N2 (Fig. 1A). The latter
process, termed p backdonation, weakens the
N-N bond while simultaneously strengthening
the metal-nitrogen bond and is thus crucial in
effective N2 binding and activation. In contrast to
transition metals, main-group compounds gen-
erally lack the combination of empty and filled
orbitals required to form bonds of s and p sym-
metry, respectively, and thus very few are able to
provide p backbonding. Accordingly, N2 binding
by p-block compounds is currently restricted to
a handful of highly reactive, most often strongly
Lewis acidic, species generated in the gas phase
or under matrix isolation conditions (15–18). Of
the main-group elements, only the strongly re-
ductive element lithium reacts with N2 at room
temperature (albeit slowly) to give an isolable
product, the binary nitride Li3N (19).
Over the past decade, advances in the chemistry of low-valent, low-coordinate main-group
elements have indicated that these compounds,
often bearing reactive lone pairs of electrons as
well as vacant orbitals at the same atom, effectively mimic transition metals in many reactions
(20). Undoubtedly the most well-developed examples of these are the singlet carbenes (:CR2),
particularly the s-donating and p-accepting carbenes developed by Bertrand and co-workers
(21). The combination of filled and empty orbitals proximal in space and energy in these compounds has facilitated binding and activation of
a number of challenging small molecules such
as H2, NH3, and P4 under mild conditions. Base-stabilized borylenes (22, 23), compounds containing a monovalent boron atom and one or two
neutral donor groups (:BRLn; n = 1, 2; R = anionic
substituent; L = neutral donor; Fig. 1B), are a
1Institute for Inorganic Chemistry, Julius-Maximilians-
Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
2Institute for Sustainable Chemistry & Catalysis with Boron,
Julius-Maximilians-Universität Würzburg, Am Hubland,
97074 Würzburg, Germany. 3Institute for Physical and
Theoretical Chemistry, Julius-Maximilians-Universität Würzburg,
Emil-Fischer-Strasse 42, 97074 Würzburg, Germany.
*Corresponding author. Email: email@example.com