Encapsulating Inorganic Acetylene, HBNH, Using Flanking Coordinative Interactions

Article abstract of DOI:10.1002/anie.201504867

A stable donor-acceptor coordination complex of the elusive parent inorganic iminoborane HBNH (a structural analogue of acetylene) is reported. This species was generated via thermally induced N₂ elimination/1,2-H migration from a hydrido(azido)borane adduct NHC·BH₂N₃ (NHC=N-heterocyclic carbene) in the presence of a fluorinated triarylborane. The mechanism of this process was also investigated by computational and isotopic labeling studies. This transformation represents a new and potentially modular route to unsaturated inorganic building blocks for advanced material synthesis.

Full text of DOI:10.1002/anie.201504867

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201504867
German Edition: DOI: 10.1002/ange.201504867
Iminoborane
Hot Paper
Encapsulating Inorganic Acetylene, HBNH, Using Flanking
Coordinative Interactions
Anindya K. Swarnakar, Christian Hering-Junghans, Koichi Nagata, Michael J. Ferguson,
Robert McDonald, Norihiro Tokitoh, and Eric Rivard*
Abstract: A stable donor–acceptor coordination complex of
the elusive parent inorganic iminoborane HBNH (a structural
analogue of acetylene) is reported. This species was generated
via thermally induced N2 elimination/1,2-H migration from
a hydrido(azido)borane adduct NHC·BH2N3 (NHC = N-het-
erocyclic carbene) in the presence of a fluorinated triarylbor-
ane. The mechanism of this process was also investigated by
computational and isotopic labeling studies. This transforma-
tion represents a new and potentially modular route to
unsaturated inorganic building blocks for advanced material
synthesis.
a formal frustrated Lewis pair (FLP)[11] interacting with
HBNH. By judicious modification of the capping LB and LA
groups it is expected that a solution-phase route to boron
nitride would be possible via mild dehydrogenation of
[12]
=
LB·HB NH·LA,
followed by BN extrusion (Scheme 1);
Scheme 1. Potential route to bulk boron nitride via an HBNH adduct;
LA=Lewis acid, LB=Lewis base.
ꢀ
I
minoboranes (RB NR) are isoelectronic to alkynes; how-
ever, they display greatly enhanced reactivity due to the
typically BN is prepared at temperatures exceeding 9008C.[8]
Herein we also introduce a general method to form B–N (and
possibly other element–nitrogen) multiple bonds by the
energetically favored loss of N2 from inorganic hydridoazide
[1,2]
À
higher polarity of B N bonds,
and thus self-oligomerize
when smaller R groups are present.[3] This is typified by the
ꢀ
parent iminoborane HB NH which only exists in the
condensed phase as its iconic trimer, borazine [HBNH]3
(also termed “inorganic benzene”).[4] Despite these chal-
lenges, monomeric HBNH is of considerable fundamental
interest and has only been isolated via matrix isolation below
40 K.[5,6] Furthermore HBNH is likely a key intermediate in
the laser-induced preparation of nanodimensional boron
nitride (BN) from H3N·BH3 dehydrogenation;[7] boron nitride
is of great value to the materials community due to its
insulating properties and ability to withstand harsh external
conditions.[8] Recent breakthroughs by the laboratories of
Bertrand, Braunschweig, and Stephan have demonstrated
that both N-heterocyclic carbene (NHC) and cyclic-
(alkyl)amino carbene (CAAC) donors can bind functional-
precursors.[13] Key intermediates en route to our LB·HB
=
NH·LA complex were isolated and structurally characterized,
and the mechanism of iminoborane adduct formation was
probed by both computational and isotope-labeling studies.
The N2 elimination process central to the current study
was discovered in an attempt to form the electrophilic
azidoborane adduct IPr·B(OTf)2N3 from the addition of two
equivalents of MeOTf[14] to the known azidoborane[15]
IPr·BH2N3 (1) (IPr= [(HCNDipp)2CD], Dipp = 2,6-iPr2C6H3;
OTfÀ = OSO2CF3). It was hoped that reducing IPr·B(OTf)2N3
would afford an unstable boron(I) species that would yield
oligomeric adducts of [IPr·(BN)]x after loss of dinitrogen.[16]
However, when IPr·BH2N3 (1) was reacted with MeOTf, gas
evolution was noted and a new product was formed that
contained a nitrogen-bound methyl group. This product
proved to be thermally unstable in solution at room temper-
ature, yet at À358C colorless crystals suitable for X-ray
crystallography were obtained, revealing the generation of
ized iminoboranes such as ClBNSiMe3 and tBuBNtBu as 1:1
[9]
=
adducts LB·RB NR’ (LB = Lewis base).
Herein we use a donor–acceptor approach[10] to isolate the
first stable molecular adduct containing HBNH. Specifically
this parent iminoborane is sandwiched between Lewis basic
(LB) and Lewis acidic (LA) units to yield a stable complex of
=
the formal boraiminium adduct [IPr·HB NH(Me)]OTf (2)
=
the form LB·HB NH·LA, which could also be viewed as
(Scheme 2; Figure 1).
Compound 2 adopts a trans HBNH configuration in the
solid state with a B–N distance of 1.361(5) Š, a value that is
=
[*] A. K. Swarnakar, Dr. C. Hering-Junghans, Dr. M. J. Ferguson,
Dr. R. McDonald, Prof. Dr. E. Rivard
slightly elongated with respect to the B N bond lengths found
Department of Chemistry, University of Alberta
11227 Saskatchewan Drive, Edmonton, Alberta, T6G 2G2 (Canada)
E-mail: erivard@ualberta.ca
Homepage: http://www.chem.ualberta.ca/~erivard/
K. Nagata, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Uji, Kyoto, 611-0011 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504867.
Scheme 2. Synthesis of 2 starting from 1 and MeOTf; adding extra
equivalents of MeOTf to 1 led to the same products.
10666
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 10666 –10669

Angewandte
Chemie
Figure 2. Molecular structure of IPr·BH2N3·BArF (3) with thermal
ellipsoids presented at a 30% probability level; all carbon-bound
3
=
Figure 1. Molecular structure of [IPr·BH NH(Me)]OTf (2) with thermal
ellipsoids presented at a 30% probability level. All carbon-bound
hydrogen atoms and OTfÀ counterion have been omitted for clarity.
Selected bond lengths [Š] and angles [8] with parameters associated
with a second molecule in the asymmetric unit listed in square
brackets: C(1B)–B(1B) 1.574(5) [1.568(5)], B(1B)–N(3B) 1.366(5)
[1.361(5)], N(3B)–C(4B) 1.478(5) [1.460(5)]; C(1B)-B(1B)-N(3B)
121.3(4) [122.1(4)].
hydrogen atoms have been omitted for clarity. Selected bond lengths
[Š] and angles [8]: C(1)–B(1) 1.610(2), B(1)–N(3) 1.599(2), N(3)–N(4)
1.253(2), N(4)–N(5) 1.134(2), N(3)–B(2) 1.656(2); N(5)-N(4)-N(3)
177.68(18).
bond [1.134(2) Š] in relation to the internal N(3)–N(4)
linkage [1.253(2) Š], consistent with the accumulation of
triple bond character. Interestingly, the bond lengths and the
overall geometry of the H2B–N3 unit in 3 remain unperturbed
in relation to those found in the precursor IPr·BH2N3 (1).[15] In
addition to the B–H stretching mode at 2467 cmÀ1, a diagnos-
tic azide IR n(N3) band at 2134 cmÀ1 was noted for 3, which
matches well the related stretches at 2189 and 2175 cmÀ1
found in the bis-silylated azide [(Me3Si)2NNN]B(C6F5)4.[19]
IPr·BH2N3·BArF3 (3) slowly decomposes at room temper-
ature, both in solution (within 24 h) and in the solid state
(4 days). As a result we decided to explore the thermolysis of
3 in more detail.
in the abovementioned CAAC and NHC iminoborane
adducts LB·RBNR’ [1.300(3) to 1.340(5) Š] (LB = Lewis
base).[9] The CIPr–B interaction in 2 [1.571(7) Š avg.] is nearly
the same value within experimental error (3s) as the related
coordinative bond in CAAC·BrB NSiMe3 [1.606(4) Š],[9b]
=
À
indicating that a similar C B bonding environment is likely
present in both species.
Compound 2 can be viewed as an NHC adduct of
methylated HBNH, and accordingly we thought that the N2
elimination/1,2-H migration protocol in Scheme 1 could be
extended to include other Lewis acidic entities.[17] Given our
The clean conversion of IPr·BH2N3·BArF (3) to a new
3
prior successes using W(CO)5 as
a
Lewis acid,[10c,d,h]
carbene-containing product was accomplished by heating
a solution of 3 in toluene to 808C for 12 h.[20] The resulting
highly lipophilic colorless solid afforded an IR spectrum
devoid of an azide band, suggesting that N2 loss occurred.
Furthermore weak n(N–H) and n(B–H) vibrations emerged
at 3370 and 2511 cmÀ1, respectively, while the corresponding
N–H and B–H resonances were located at 5.44 and 4.00 ppm
in the 1H{11B} NMR spectrum of the product with an
integration ratio of 1:1. The N–H group yields a doublet
resonance due to 3JHH coupling with an adjacent B–H group,
supporting the formation of an iminoborane HBNH array. X-
IPr·BH2N3 (1) was combined with THF·W(CO)5; however,
no reaction occurred. Fortunately the sterically demanding
fluoroarylborane BArF (ArF = 3,5-C6H3(CF3)2)[18] binds to
3
the azide moiety in 1 to yield IPr·BH2N3·BArF (3) as
3
a colorless solid (Scheme 3, Figure 2).
The most notable structural feature of IPr·BH2N3·BArF
3
(3) is the substantial shortening of the terminal N(4)–N(5)
ray crystallography conclusively identified the product as the
F
=
donor–acceptor iminoborane complex IPr·HB NH·BAr 3 (4)
(Scheme 3, Figure 3). The central iminoborane B–N distance
in 4 is 1.364(2) Š and is in line with the presence of a double
bond (Ærcov(B = N) = 1.38 Š).[21] The capping IPr and BArF
3
=
units in 4 form a steric sheath about the central HB NH unit
and help enforce a trans core geometry [C(1)-B(1)-N(3)-B(2)
torsion angle = 179.26(12)8] which minimizes the IPr···BArF
3
intramolecular repulsions. The CIPr–B distance in 4 is
1.596(2) Š and is shorter than the dative C–B interaction in
ImiPr2·tBuB NtBu [1.648(2) Š; ImiPr2 = (HCNiPr)2CD],[9c]
=
illustrating the lower steric demand of the HBNH unit in 4.
Scheme 3. Preparation of the azidoborane 3 and its conversion into
the iminoborane complex 4; for related 15N and 2H labeling studies,
see the Supporting Information.
The terminal N–BArF bond in 4 is 1.5708(18) Š and lies in
3
the range of B–N single bonds noted in IPr·BH2NH2BH3
Angew. Chem. Int. Ed. 2015, 54, 10666 –10669
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10667

.
Angewandte
Communications
F
3
=
Figure 3. Molecular structure of IPr·HB NH·BAr (4) with thermal
Figure 4. 1H{11B} NMR N–H resonances from a 1:1 mixture of
ellipsoids presented at a 30% probability level. All carbon-bound
hydrogen atoms have been omitted for clarity. Selected bond lengths
[Š] and angles [8]: C(1)–B(1) 1.596(2), B(1)–N(3) 1.364(2), N(3)–B(2)
1.5708(18); C(1)-B(1)-N(3) 123.45(13), B(1)-N(3)-B(2) 130.72(12),
N(3)-B(1)-H(1B) 122.3(10), B(1)-N(3)-H(3N) 117.3(12).
IPr·HB 15NH·BArF3 and 4.
=
(1) shows a high negative partial charge of À0.54 on the
internal Nazide atom, thus explaining the electrophilic attack at
this site by MeOTf and BArF . Within IPr·BH2N3·BArF (3)
3
3
[1.540(3)–1.605(2) Š];[22] of note, the capping N–BArF inter-
the borane-bound nitrogen [N(3) in Figure 2] has significant
lone pair character, in line with the mesoionic form drawn in
3
action in IPr·BH2N3·BArF3 (3) is 1.599(2) Š.[23]
As outlined in Scheme 3, the formation of 4 is postulated
to occur via N2 loss from 3 followed by a 1,2-hydride shift
from boron to nitrogen (vide infra). A related process has
been observed by Paetzold,[1a] who prepared iminoboranes
Scheme 3. The computed energies for the conversion of 3 into
F
IPr·HB NH·BAr (4) are À64.5 kcalmolÀ1 (DrH8(298 K))
=
3
and À75.6 kcalmolÀ1 (DrG8(298 K)), while the estimated
activation barrier for N2 loss from 3 is 31.3 kcalmolÀ1.[26,29]
À
ꢀ À
=
via alkyl-group migration (R2N B(R’)-N!R2N-B N R’)
involving a transient boranitrene.[24] Cummins and Fox also
observed nitrogen extrusion from the azidoborate salt nBu4N-
[(N3)B(C6F5)3] in the presence of (THF)U[N(tBu)Ar]3 (Ar=
NBO analysis gives rise to a charge of the central HB NH
fragment in 4 of À0.13. The B–N linkage in 4 can be
formulated as a double bond, with significant polarization of
the s and p components towards N (78% for each). More-
over, the Wiberg bond index (WBI) for this linkage (1.32)
supports the presence of multiple-bond character; accord-
ingly, the Kohn–Sham orbitals reveal a LUMO having B–N
p* character, while contributions to the BN double bond
appear in the HOMOÀ7 (Figure S6).[26]
3,5-Me2C6H3) to yield the uranium(V) nitride nBu4N-
[25]
=
[(F5C6)3B·N U{N(tBu)Ar}]3.
=
To gain insight into the mechanism by which IPr·HB
NH·BArF (4) is formed, the deuterium-labeled analogue
3
IPr·BD2N3·BArF (3-d) was synthesized (Scheme 3). Subse-
3
quent thermolysis of 3-d at 808C yielded the isotopomer
IPr·DB ND·BAr (4-d) as confirmed by NMR and IR
Given the presence of hydridic (B–H) and acidic (N–H)
F
3
=
=
residues within the parent iminoborane adduct IPr·HB
spectroscopy, supporting a 1,2-H migration process. The rate
of conversion of 3 to 4 was monitored by NMR spectroscopy
(758C, C6D6) and displayed a first-order dependence on the
concentration of IPr·BH2N3·BArF3 (3). Notably thermolysis of
the deutero analogue 3d did not yield any discernable H/D
kinetic isotope effect (Figure S4),[26] suggesting that N2 loss
and formation of a transient nitrene is the rate-determining
step.
NH·BArF (4), we attempted the dehydrogenation[12] of this
3
species to yield the first molecular adduct of boron nitride
IPr·B N·BAr . However, when 4 was treated with 2 mol%
F
3
ꢀ
of the active aminoborane dehydrogenation catalyst
[(cod)RhCl]2 (cod = 1,5-cyclooctadiene) at room temperature
and later at 908C, only the starting material could be
recovered. Increasing the catalyst loading to 20 mol%, and
prolonged heating to 1408C (for 96 h) led to decomposition of
4 into an unidentifiable mixture of products. The lower
reactivity of 4 in relation to other unsaturated B–N systems
In order to facilitate the recording of an 15N{1H} NMR
spectrum, the N-labeled adduct IPr·HB 15NH·BArF was
15
=
3
prepared as a 1:1 mixture with unlabeled 4 (Scheme S2).[26]
can be traced to the high degree of steric protection about the
Interestingly the N–H group in IPr·HB 15NH·BArF gave
HB NH unit (Figure S10) due to the bulky flanking IPr
15
[26]
=
=
3
rise to a doublet of doublet resonance in its 1H NMR
and BArF groups; in fact 4 can be handled in air (but
3
1
spectrum (Figure 4) with a JHÀ
value of 69.6 Hz; for
decomposes in water) and remains unchanged in the presence
of nBuLi, K[N(SiMe3)2], Ph3C[B(C6F5)4], MeOTf, and even
elemental I2.
In conclusion, we have discovered a novel Lewis acid
induced N2 elimination/hydride-shift process to yield the first
stable adduct of the parent iminoborane HBNH. By lowering
15N
1
comparison, the -NH2 group in 4-nitroaniline yields a JHÀ
15N
of 86.3 Hz.[27] An 15N{1H} NMR resonance for 4-N15 was
located at 155.4 ppm, and is downfield in relation to the 15N
NMR resonance in borazine [HBNH]3 (À278 ppm).[28]
To better understand the bonding and reactivity trends
observed, computations were carried out (pbe0/cc-pVDZ
level).[26] Natural bond orbital analysis (NBO) of IPr·BH2N3
=
the steric bulk of the capping groups within LB·HB NH·LA
complexes we hope to encourage H2 loss and the subsequent
10668 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 10666 –10669

Angewandte
Chemie
133, 777; d) S. M. I. Al-Rafia, A. C. Malcolm, R. McDonald,
formation of bulk boron nitride in a solution-based approach.
The generality of this method will also be explored across the
main group to obtain new unsaturated species of potential use
in controlled, low-temperature, advanced material synthe-
ses.[30]
M. J. Ferguson, E. Rivard, Angew. Chem. Int. Ed. 2011, 50, 8354;
Angew. Chem. 2011, 123, 8504; e) T. Yamaguchi, A. Sekiguchi,
M. Driess, J. Am. Chem. Soc. 2010, 132, 14061; f) R. S. Ghadwal,
H. W. Roesky, Acc. Chem. Res. 2013, 46, 444; g) A. C. Filippou,
B. Baars, O. Chernov, Y. N. Lebedev, G. Schnakenburg, Angew.
Chem. Int. Ed. 2014, 53, 565; Angew. Chem. 2014, 126, 576; h) E.
Rivard, Dalton Trans. 2014, 43, 8577.
[11] D. W. Stephan, Acc. Chem. Res. 2015, 48, 306.
[12] a) C. W. Hamilton, R. T. Baker, A. Staubitz, I. Manners, Chem.
Soc. Rev. 2009, 38, 279; b) E. M. Leitao, J. Titel, I. Manners, Nat.
Chem. 2013, 5, 817; c) M. Krieg, F. Reicherter, P. Haiss, M.
Strçbele, K. Eichele, M. – J. Treanor, R. Schaub, H. F. Bettinger,
Angew. Chem. Int. Ed. 2015, 54, 8284; Angew. Chem. 2015, 127,
8402.
[13] For related rearrangements involving organic azides, see:
a) B. V. Rokade, K. R. Prabhu, J. Org. Chem. 2012, 77, 5364;
b) G. Albertin, S. Antoniutti, D. Baldan, J. Castro, S. García-
Fontµn, Inorg. Chem. 2008, 47, 742.
[14] A related H/OTf exchange process is known within zinc(II)
hydride chemistry: P. A. Lummis, M. R. Momeni, M. W. Lui, R.
McDonald, M. J. Ferguson, M. Miskolzie, A. Brown, E. Rivard,
Angew. Chem. Int. Ed. 2014, 53, 9347; Angew. Chem. 2014, 126,
9501.
Acknowledgements
This work was supported by the NSERC of Canada (Discov-
ery Grant for E.R) and the Canada Foundation for Innova-
tion (CFI). C.H.-J. acknowledges the Alexander von Hum-
boldt Foundation for a Feodor Lynen Postdoctoral Fellow-
ship, while K.N. thanks the Japan Society for the Promotion of
Science (JSPS) for a PhD fellowship. We also thank Dr. Tapas
Purkait, Olena Shynkaruk, and William Torres Delgado for
experimental assistance, Compute Canada for providing
computational resources, and Prof. Alex Brown and Dr.
Urmi Bhusan Bhakta for helpful discussions.
Keywords: boron · carbenes · donor–acceptor systems ·
[15] A. Solovyev, Q. Chu, S. J. Geib, L. Fensterbank, M. Malacria, E.
Lacôte, D. P. Curran, J. Am. Chem. Soc. 2010, 132, 15072.
[16] For a recent computational study, see: M. R. Momeni, L.
Shulman, E. Rivard, A. Brown, Phys. Chem. Chem. Phys.
2015, 17, 16525.
multiple bonds · nitrogen
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10666–10669
Angew. Chem. 2015, 127, 10812–10816
[17] a) S. M. I. Al-Rafia, R. McDonald, M. J. Ferguson, E. Rivard,
Chem. Eur. J. 2012, 18, 13810; b) N. E. Stubbs, T. Jurca, E. M.
Leitao, C. H. Woodall, I. Manners, Chem. Commun. 2013, 49,
9098.
[18] E. L. Kolychev, T. Bannenberg, M. Freytag, C. G. Daniliuc, P. G.
Jones, M. Tamm, Chem. Eur. J. 2012, 18, 16938.
[19] A. Schulz, A. Villinger, Chem. Eur. J. 2010, 16, 7276.
[20] Photolysis of 3 (Hg lamp, 200 W) for 3 hrs. in toluene or CH2Cl2
gave the new product, 4, along with unknown by-products.
[21] P. Pyykkç, M. Atsumi, Chem. Eur. J. 2009, 15, 12770.
[22] A. C. Malcolm, K. J. Sabourin, R. McDonald, M. J. Ferguson, E.
Rivard, Inorg. Chem. 2012, 51, 12905.
[1] a) P. Paetzold, Adv. Inorg. Chem. 1987, 31, 123; b) H. Nçth,
Angew. Chem. Int. Ed. Engl. 1988, 27, 1603; Angew. Chem. 1988,
100, 1664; c) P. Paetzold, Pure Appl. Chem. 1991, 63, 345;
d) R. C. Fischer, P. P. Power, Chem. Rev. 2010, 110, 3877; For
related M-B ꢀ NR chemistry, see: e) H. Braunschweig, R. D.
Dewhurst, A. Schneider, Chem. Rev. 2010, 110, 3924.
[2] P. Paetzold, C. V. Plotho, Chem. Ber. 1982, 115, 2819.
[3] P. Paetzold, A. Richter, T. Thijssen, S. Würtenberg, Chem. Ber.
1979, 112, 3811.
[4] A. Stock, E. Pohland, Ber. Dtsch. Chem. Ges. 1926, 59, 2210.
[5] a) R. Sundaram, S. Scheiner, A. K. Roy, T. Kar, J. Phys. Chem. C
2015, 119, 3253; b) M. H. Matus, D. J. Grant, M. T. Nguyen,
D. A. Dixon, J. Phys. Chem. C 2009, 113, 16553.
[6] a) E. R. Lory, R. F. Porter, J. Am. Chem. Soc. 1973, 95, 1766;
b) C. A. Thompson, L. Andrews, J. Am. Chem. Soc. 1995, 117,
10125; c) F. Zhang, P. Maksyutenko, R. I. Kaiser, A. M. Mebel,
A. Gregusova, S. A. Perera, R. J. Bartlett, J. Phys. Chem. A 2010,
114, 12148.
[7] H. Liu, P. Jin, Y.-M. Xue, C. Dong, X. Li, C.-C. Tang, X.-W. Du,
Angew. Chem. Int. Ed. 2015, 54, 7051; Angew. Chem. 2015, 127,
7157.
[8] a) R. T. Paine, C. K. Narula, Chem. Rev. 1990, 90, 73; b) A. K.
Geim, I. V. Grigorieva, Nature 2013, 499, 419; c) L. Liu, J. Park,
D. A. Siegel, K. F. McCarty, K. W. Clark, W. Deng, L. Basile,
J. C. Idrobo, A. – P. Li, G. Gu, Science 2014, 343, 163.
[9] a) F. Dahcheh, D. Martin, D. W. Stephan, G. Bertrand, Angew.
Chem. Int. Ed. 2014, 53, 13159; Angew. Chem. 2014, 126, 13375;
b) F. Dahcheh, D. W. Stephan, G. Bertrand, Chem. Eur. J. 2015,
21, 199; c) H. Braunschweig, W. C. Ewing, K. Geetharani, M.
Schäfer, Angew. Chem. Int. Ed. 2015, 54, 1662; Angew. Chem.
2015, 127, 1682.
[10] a) U. Vogel, A. Y. Timonshkin, M. Scheer, Angew. Chem. Int.
Ed. 2001, 40, 4409; Angew. Chem. 2001, 113, 4541; b) P. A.
Rupar, M. C. Jennings, P. J. Ragogna, K. M. Baines, Organo-
metallics 2007, 26, 4109; c) S. M. I. Al-Rafia, A. C. Malcolm,
S. K. Liew, M. J. Ferguson, E. Rivard, J. Am. Chem. Soc. 2011,
[23] One could also depict the bonding in 4 as the zwitterion
F
=
NHC(+)-HB NH-B(À)Ar 3; in this case polar two center, two
electron CNHC-B and N-BArF bonds are still present.
3
[24] For a related study on boranitrenes, see: M. Filthaus, L.
Schwertmann, P. Neuhaus, R. W. Seidel, I. M. Oppel, H. F.
Bettinger, Organometallics 2012, 31, 3894.
[25] A. R. Fox, C. C. Cummins, J. Am. Chem. Soc. 2009, 131, 5716.
[26] For full details, see the Supporting Information. CCDC 1403519
(2), 1403520 (3) and 1403521 (4) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre.
[27] T. Axenrod, P. S. Pregosin, M. J. Wieder, E. D. Becker, R. B.
Bradley, G. W. A. Milne, J. Am. Chem. Soc. 1971, 93, 6536.
[28] B. Wrackmeyer, O. L. Tok, Z. Naturforsch. B 2007, 62, 220.
[29] To estimate the activation barrier, the N3–N4 bond length in 3
was scanned. The extrusion of N2 is accompanied by a 1,2-H-shift
and formation of 4, see the Supporting Information (Figure S9)
for further information.
[30] T. K. Purkait, A. K. Swarnakar, G. B. De Los Reyes, F. A.
Hegmann, E. Rivard, J. G. C. Veinot, Nanoscale 2015, 7, 2241.
Received: May 28, 2015
Published online: July 17, 2015
Angew. Chem. Int. Ed. 2015, 54, 10666 –10669
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10669