By Daniël L. J. Broere1 and
Patrick L. Holland2
The high stability of elemental ni- trogen (N2) is reflected in the name that Lavoisier coined for the element hat makes up the seemingly inert component of the atmosphere, azote, meaning “no life.” Ironically, chemical processes that convert this robust molecule into ammonia (NH3) are essential to
keep the billions of people on our planet
alive. Both the industrial Haber-Bosch
process and natural nitrogenase enzymes
use iron (Fe) to catalyze this challenging
chemical transformation. Transition metals
(M) weaken or break the strong triple bond
(N;N) by donating electrons from their
atomic d orbitals into the antibonding p
orbitals of N2. Main-group elements such
as boron (B) lack accessible d orbitals, so
their ability to weaken N2 would seem to be
very limited. However, on page 896 of this
issue, Légaré et al. (1) show that modification of the electronic environment of the B
atom can enable N2 binding and reduction
at a B center.
The efforts of synthetic chemists over
many decades have shown that complex
ions of many metals can interact with N2.
More specifically, the right combination of
empty and filled d orbitals on the metal
can accept electrons from N2, and simultaneously weaken the N;N bond through
p-backdonation, which moves electron density from the metal into antibonding orbitals of N2 (see the left panel of the figure).
Légaré et al. used a borylene compound,
which is a monovalent B atom with a lone
pair of electrons, to bind N2 (see the middle
panel of the figure). Although most boron
compounds have B in the +3 oxidation
state, borylenes formally have the +1 oxidation state, making them very electron-rich.
In 2011, Bertrand and co-workers (2)
demonstrated that these intrinsically reactive boron(I) species can be isolated
by binding cyclic (alkyl)(amino)carbenes
(CAACs), which stabilize the extra electrons
on B. These borylenes mimic transition
metals by binding carbon monoxide (CO)
(3) and releasing it upon irradiation with
light (4, 5). Because CO binding typically
depends on p-backdonation, this finding
suggested that borylenes might also bind
N2, albeit by using different orbitals than
transition metals do. Recent work demonstrated binding of N2 to an unstabilized
borylene under matrix conditions at 10 K
(6). However, even at these extremely low
temperatures, which are used to observe
the most reactive and unstable molecules,
N2 binding was very weak.
Légaré et al. show how to make a reactive
borylene that can bind N2 to form an adduct
that is stable enough to isolate (see the right
panel of the figure). The resulting product
consists of an N2 fragment bound by two bor-
ylenes. In contrast to the linear M–N–N–M
units typically found in transition-metal
complexes, the B–N–N–B unit has a zigzag
structure, which makes the product some-
what resemble an organic azo compound
(C–N=N–C). However, quantum-mechanical
calculations imply an electronic structure in
which the borylene accepts electron density
from N2 with simultaneous p-backdonation
from its filled p orbital. As a result, the N;N
bond is reduced to a bond order of ~1.5 in the
borylene adduct. Although there is not yet ev-
idence for the reversibility of N2 binding, the
bound N2 molecule can be further reduced
with potassium and protonated to generate
a diborahydrazine, which has a N–N single
bond and new N–H bonds.
The work by Légaré et al. presents a remarkable example of an isolated compound
in which a nonmetal can bind and reduce
N2, and underlines the potential of main-group elements for small-molecule activation. It also raises the question: What is next
for borylenes? Classical steps in transition-metal catalysis involve two-electron redox
changes on the metal center. However, the
borylenes described by Légaré et al.
readily undergo one-electron redox changes.
Binding of cooperative (7) or redox-active
(8) ligands to a borylene could assist in facilitating multielectron transformations, a
strategy that has met with recent success
in complexes of first-row transition metals
that perform one-electron redox chemistry
(9). The work by Légaré et al. also could
lead to pathways to use borylenes for the
conversion of N2 into amines and other N-containing organic compounds with Earth-abundant boron catalysts. j
1. M.-A. Légaré et al ., Science 359, 896 (2018).
2. R. Kinjo, B. Donnadieu, M. A. Celik, G. Frenking, G. Bertrand,
Science 333, 610 (2011).
3. F. Dahcheh, D. Martin, D. W. Stephan, G. Bertrand, Ange w.
Chem. Int. Ed. 53, 13159 (2014).
4. H. Braunsch weig et al ., Nature522, 327 (2015).
5. H. Braunsch weig etal., J.Am.Chem.Soc.139, 1802 (2017).
6. K. Edel, M. Krieg, D. Grote, H. F. Bettinger, J. Am. Chem. Soc.
139, 15151 (2017).
7. J.R.Khusnutdinova, D.Milstein, Ange w. Chem. Int. Ed.54,
8. P. J. Chirik, K. Wieghardt,Science 327, 794 (2010).
9. J. I. van der Vlugt, Eur.J.Inorg.Chem.3, 363 (2012).
Boron compounds tackle dinitrogen
A borylene compound can match transition metals by activating the strong N2 bond
1Debye Institute for Nanomaterials Science, Utrecht University,
Universiteitsweg 99, 3584 CG Utrecht, Netherlands.
2Department of Chemistry, Yale University, New Haven, CT
06511, USA. Email: firstname.lastname@example.org; email@example.com
Metal N2 complex
Empty d orbital accepts electron
density from N2 and flled d orbital
donates electron density into N2
Borylene N2 complex
Empty sp2 orbital accepts electron
density from N2 and flled p orbital
donates electron density into N2
Reduction of bound N2
After reduction and protonation,
N2 forms diborahydrazine
(R represents an aryl group).
d orbital R
23 FEBRUARY 2018 • VOL 359 ISSUE 6378 871
Binding N2 at boron
Transition metals (M) bind N2 by accepting and donating
electron density using d orbitals. Légaré et al. now show that
borylenes (B) can bind N2 using p orbitals.