A hybrid organic/inorganic benzene

Article abstract of DOI:10.1002/anie.200805554

(Chemical Equation Presented) It isn't easy BN aromatic! 1,2-Dihydro-1,2-azaborine, a hybrid organic/inorganic benzene, is a stable aromatic molecule with features that are distinct from its isoelectronic organic (benzene) and inorganic (borazine) counterparts. Experimental structural, spectroscopic, and chemical data are fully supported by high-level calculations.

Full text of DOI:10.1002/anie.200805554

Angewandte
Chemie
DOI: 10.1002/anie.200805554
Boron Heterocycles
A Hybrid Organic/Inorganic Benzene**
Adam J. V. Marwitz, Myrna H. Matus, Lev N. Zakharov, David A. Dixon,* and Shih-Yuan Liu*
Dedicated to Professor Gregory C. Fu on the occasion of his 45th birthday
Benzene (c-C6H6) is arguably one of the most fundamentally
significant small molecules in chemistry. First discovered by
Faraday in 1825,[1] the study of benzene introduced the basic
concept of aromaticity and delocalization.[2] In addition to its
fundamental importance, benzene and its derivatives (arenes)
are ubiquitous in chemical research with numerous applica-
tions ranging from biomedical research to materials science.[3]
The inorganic isoelectronic relative of benzene, borazine (c-
B3N3H6),[4] has also played a pivotal role in fundamental as
well as applied chemistry. The isoelectronic and isostructural
À
=
relationship between the B N and C C bond and its
consequence on the aromaticity of borazine has been a
topic of discussion.[5] From a more applied perspective,
borazine serves as a precursor to BN-based ceramic materi-
als.[6] More recently, borazine has been implicated in chemical
hydrogen storage applications because it is formed as an
intermediate in the hydrogen release from ammonia–
borane.[7] Both benzene and borazine have been known for
more than 80 years, and consequently, their chemical and
physical properties have been thoroughly investigated. The
corresponding organic/inorganic (or organometalloidal)
hybrid structure containing carbon, boron, and nitrogen,
that is, 1,2-dihydro-1,2-azaborine 1, has thus far eluded
characterization.
The development of boron–nitrogen heterocycles such as
1,2-dihydro-1,2-azaborines (from hereon in, abbreviated as
1,2-azaborines) has been a relatively unexplored area of
research.[8] Dewar and White pioneered the chemistry of
monocyclic and ring-fused polycyclic 1,2-azaborine deriva-
tives in the 1960s.[9] Recently, contributions by the groups of
Ashe,[10] Piers,[11] and Paetzold,[12] as well as our group[13] have
further advanced the preparation of novel BN heterocycles
and sparked a renewed interest in the chemistry and proper-
ties of these compounds.[14] Despite the advances achieved to
date, and given the powerful tools made available by modern
chemical synthesis, it is surprising that a simple heterocycle
such as the parent 1,2-dihydro-1,2-azaborine 1 has remained
elusive. Dewar attempted its synthesis and isolation in 1967
but ultimately concluded that it “seems to be a very reactive
and chemically unstable system, prone to polymerization and
other reactions.”[15]
Herein, we describe the first isolation and characteriza-
tion of 1,2-dihydro-1,2-azaborine. Its successful preparation
allows a direct comparison of the physical and spectroscopic
properties of the series of an organic, inorganic, and now, an
organometalloidal benzene. The present study demonstrates
that 1,2-dihydro-1,2-azaborine 1 is not only isolable but it
actually exhibits remarkable stability, consistent with sub-
stantial aromatic character. Our experimentally determined
structural and spectroscopic properties are consistent with
values derived from high-level computations.
Scheme 1 illustrates our synthetic route to compound 1.
Coupling of the in situ-generated allylboron dichloride with
tert-butyldimethylsilyl allyl amine (TBS allyl amine) fur-
nished diene 2. Ring-closing metathesis of this intermediate
with the first-generation Grubbs catalyst produced an iso-
meric mixture of 3 and 3’ (60:40 ratio) in 82% yield.
Dehydrogenation of this mixture was carried out in the
presence of catalytic amounts of Pd/C to generate 4. Treat-
[*] M. H. Matus, Prof. Dr. D. A. Dixon
Department of Chemistry, University of Alabama
Tuscaloosa, AL 35487 (USA)
E-mail: dadixon@as.ua.edu
À
ment of heterocycle 4 with LiBHEt3 installed the B H
A. J. V. Marwitz, Dr. L. N. Zakharov, Prof. Dr. S.-Y. Liu
Department of Chemistry, University of Oregon
Eugene, OR 97403-1253 (USA)
functionality to give 5 in quantitative yield. Complexation of
1,2-azaborine 5 to {Cr(CO)3} produced the piano-stool adduct
6.[16] Subsequent removal of the N-protecting group gave 7 in
76% yield. Finally, decomplexation of 1 from {Cr(CO)3} was
accomplished using triphenylphosphine.
E-mail: lsy@uoregon.edu
[**] Correspondence concerning X-ray crystallography should be
directed to Lev Zakharov (lev@uoregon.edu). Correspondence
concerning computational calculations should be directed to David
Dixon. Funding was provided by the U.S. Department of Energy
(S.Y.L., DE-FG36-08GO18143; D.A.D., DE-FC36-05GO15059), and
the National Science Foundation (A.J.V.M., DGE-0549503). We
thank Ms. Raluca Craciun for preparing the ESP maps.
The use of {Cr(CO)3} as a temporary “protecting group”
was necessary because efforts toward cleaving the N TBS
bond directly from 5 were unsuccessful. Compound 1 proved
to be difficult to isolate, owing to its high volatility. However,
we ultimately accomplished its isolation (10% yield) by
fractional vacuum transfer in the presence of a low-boiling
À
Supporting Information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805554.
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Figure 2 shows a direct comparison of the
UV/Vis absorption spectra of benzene, bor-
azine, and 1.[18] The spectrum for compound 1
displays lmax at 269 nm (e = 15632mÀ1 cmÀ1),
with two smaller transitions at 219 nm (e =
8495mÀ1 cmÀ1)
and
at
205 nm
(e =
7459mÀ1 cmÀ1). The absorbance at 269 nm is
only slightly red-shifted relative to the a band
of benzene (255 nm, e = 977mÀ1 cmÀ1), but
absorbs much more strongly than benzene.
Benzeneꢀs strongest absorption band is
located at 208 nm (e = 12380mÀ1 cmÀ1). In
contrast to benzene and 1,2-azaborine 1, the
absorption spectrum of borazine shows only a
very weak band at 203 nm (e = 1299mÀ1 cmÀ1)
and negligible absorbance at higher wave-
lengths. The electronic spectrum of 1 is
Scheme 1. Synthesis of 1,2-dihydro-1,2-azaborine 1.
consistent with
a
delocalized aromatic
reaction solvent, isopentane. The inherent efficiency of the
decomplexation reaction (84% yield) was measured by
1H NMR spectroscopy against an internal standard. The
melting point of 1 is À458C, which is slightly higher than that
of borazine (À588C) but considerably lower than that of
benzene (58C). We found 1,2-dihydro-1,2-azaborine 1 to be a
relatively stable heterocycle. 1H NMR spectroscopy of a 0.7m
solution of 1 in CD2Cl2 showed no appreciable degradation
when the solution was heated to 608C for five days.
Furthermore, 1 is stable to chromatography on silica gel and
is relatively nonpolar (Rf = 0.4 with pentane as eluent).
We characterized compound 1 by using NMR, UV/Vis,
and IR spectroscopy, and high-resolution mass spectrometry.
The data are consistent with the proposed structure of 1.
Figure 1 depicts the 1H NMR spectrum of heterocycle 1. The
Figure 2. Absorption spectra of 1, benzene, and borazine. All sub-
strates are 10À4 m in pentane.
system and differs substantially from the behavior exhibited
by borazine.
Our experimentally determined spectroscopic data are
supported by electronic structure calculations.[19] The chem-
ical shifts in 1H, 13C, 11B, and 14N NMR spectroscopy of
benzene, compound 1, and borazine were calculated at the
density functional theory (DFT) B3LYP/Alhrichs-vtzp level
(see Table S1 in the Supporting Information). The excitation
energies and oscillator strengths for the prediction of the UV/
Vis spectra of benzene, 1,2-azaborine 1, and borazine were
calculated with time dependent DFT (TD-DFT) at the
B3LYP/aug-cc-pVDZ//MP2/cc-pVTZ and B3LYP/aug-cc-
pVTZ//MP2/cc-pVTZ levels, and with equations of motion
CCSD (EOM-CCSD/aug-cc-pVDZ//MP2/cc-pVTZ) level
(See Table S2 in the Supporting Information). The IR
stretching frequencies of the benzene triad were calculated
at the MP2/cc-pVTZ level (see Table S3 in the Supporting
Information). The calculated spectroscopic signatures agree
well with our experimental results, thus further supporting
our structural assignment of compound 1.
Figure 1. 1H NMR spectrum of 1 in CD2Cl2.
proton signals were unambiguously assigned through 2D
NMR spectroscopy experiments.[17] The C H resonances all
À
lie in the aromatic region and display the characteristic
À
pattern of a 1,2-azaborine. The B H resonance appears
upfield of the other resonances and is split into a broad
quartet (1JBH = 130 Hz) by the 11B atom (S = 3/2). The
coupling of the N H proton with the 14N nucleus (S = 1)
À
results in a triplet (1JNH = 57 Hz) that is observed for solutions
of 1 in CD3CN, C6D6, CD3OD, and CD2Cl2.
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À
À
The aromaticity of benzene and borazine has been a topic
of intense discussion, and borazine is generally considered
substantially less aromatic than benzene.[5] What is the
aromatic character of the organic/inorganic hybrid 1? Ashe
and co-workers showed that 1,2-azaborines readily undergo
electrophilic aromatic substitution reactions.[20] We recently
provided structural data highlighting that 1,2-azaborines have
delocalized structures consistent with aromaticity.[13b] To
complete the analysis, Table 1 contains the magnetic and
whether the N H proton is protic and the B H group is
À
hydridic. The N H functionality in 1 undergoes a deuterium
À
exchange in CD3OD [Eq. (3)]. The disappearance of the N H
resonance in the 1H NMR spectrum of 1 was monitored over
the course of approximately 24 h. The rate constant for
exchange in CD3OD was determined to be kHD = 7(Æ2) ꢁ
10À7 mÀ1 s . The B H group is typically hydridic in character.
À1
À
For instance, Fu and co-workers demonstrated that the
negatively charged 1H-boratabenzene reduces n-dodecanal
to dodecanol.[24] However, no reaction between 1 and
benzaldehyde occurred over the course of several days at
608C [Eq. (4)]. In contrast, complete consumption of benzal-
dehyde with borazine was observed within 24 h, furnishing
reduced benzaldehyde derivatives. The unreactive nature of 1
relative to borazine indicates that the chemical reactivity of 1
more closely resembles that of benzene than borazine.
Table 1: Chemical shift and stabilization energy data for benzene, 1,2-
azaborine 1, and borazine.[a]
Entry
1
À
1
2
3
4
5
6
d(N H, H NMR)
8.44,[b] [7.8]
4.9,[b] [5.4]
[À5.62]
5.63,[b] [5.3]
4.4,[b] [5.0]
[À2.02]
1
À
d(B H, H NMR)
NICS (0)
NICS (1)
[À8.76]
[À10.39]
[34.1]
[À7.27]
[À3.01]
RSE [kcalmolÀ1
]
[21 (Æ2)]
[3.0]
[c]
DHf(298) [kcalmolÀ1
]
[20.5]
[À119.0]
[a] Numbers in brackets are calculated values. [b] Experimental chemical
shift [ppm] in CD2Cl2. [c] Value of 10.0 kcalmolÀ1 taken from referen-
ce [5a] on the basis of a different reaction scheme.
Although we were unable to determine the crystal
structure of 1, we were able to obtain the structure of its
chromium(0) tricarbonyl adduct 7, and thus assess the
coordinating ability of 1. Single crystals of 7 suitable for X-
ray diffraction were grown from an ether solution of 7 by slow
evaporation.[25] Figure 3 lists selected structural parameters of
energetic data of 1,2-azaborine 1 together with those data for
À
benzene and borazine for direct comparison. The N H and
À
B H chemical shifts of 1,2-azaborine 1 are significantly
downfield shifted compared to the corresponding signals for
borazine (Table 1, entries 1,2). The nucleus-independent
chemical shift (NICS)[21] values of benzene, 1,2-azaborine 1,
and borazine indicate a trend of decreasing aromaticity going
from benzene to borazine. These chemical shift patterns are
consistent with 1,2-azaborine possessing substantial aromatic
character. Finally, we have predicted the resonance stabiliza-
tion energy (RSE)[22] of 1,2-azaborine 1 to be 21 kcalmolÀ1
(Table 1, entry 5). This value was derived computationally
(see the Supporting Information for details) through Equa-
tions (1) and (2), which reveal that the RSE of 1,2-azaborine 1
is approximately 13 kcalmolÀ1 less than that of benzene
(RSE = 34.1 kcalmolÀ1). The heat of formation of 1 (Table 1,
entry 6) was calculated using the same accurate approach
used for predicting the heats of formation of benzene and
borazine.[23] Thus, by all accounts (reactivity, structure,
chemical shift, and stabilization energy), 1,2-azaborines can
be considered significantly aromatic.
Figure 3. Molecular structure of 7, in direct comparison with the
known benzene chromium(0) tricarbonyl complex 8. Numbers in
brackets are computed values optimized at the B3LYP/DGDZVP2 level.
We carried out preliminary reactivity studies of hetero-
cycle 1. In particular, we were interested in determining
complex 7 together with the known bond lengths for the
benzene chromium(0) tricarbonyl complex 8[26] for direct
À
comparison. The geometries in 7, including the Cr CO and
À
C O distances, are strikingly similar to that for the benzene
analogue 8, indicating that the coordination behavior of 1,2-
azaborine 1 is similar to that of benzene. This is further
supported by the nearly identical carbonyl stretching fre-
quencies between the two complexes measured by FT-IR
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vacuum to provide 1 as a clear and colorless liquid (0.006 g, 10%,
yield determined by H NMR spectroscopy in CD3OD with hexam-
spectroscopy. The binding energy of the 1,2-azaborine ring to
1
{Cr(CO)3} in 7 is calculated at the DFT B3LYP/DZVPZ level
to be À54.4 kcalmolÀ1, which is essentially the same as the
value for the benzene derivative (À54.9 kcalmolÀ1). On the
other hand, the binding energy of the corresponding borazine
complex is only À42.7 kcalmolÀ1.
ethylbenzene as an internal standard). m.p. À46–À458C; 1H NMR
(600 MHz, CD2Cl2): d = 8.44 (t, 1JNH = 57 Hz, 1H), 7.70 (br app. t,
1H), 7.40 (app. t, 3JHH = 6.5 Hz, 1H), 6.92 (d, 3JHH = 10.7 Hz, 1H),
6.43 (t, 3JHH = 6.3 Hz, 1H), 4.9 ppm (br q, 1JBH = 128 Hz, 1H);
13C NMR (75 MHz, CD2Cl2): d = 144.5, 134.7, 131.6, 112.1 ppm;
11B NMR (192.5 MHz, CD2Cl2): d = 31.0 ppm (d, 1JBH = 131 Hz);
FT-IR (thin film): n˜ = 3398, 3027, 3008, 2525, 1613, 1533, 1453, 1427,
1350, 1216, 1162, 1109, 894, 820, 715, 579 cmÀ1; UV/Vis (penta-
ne): lmax = 269 nm. HRMS (EI) calcd for C4H6BN (M+): 79.059330;
found: 79.059269.
Electronic structure calculations of compound 1 at the
B3LYP/DZVP2 level show that, in contrast to benzene and
borazine, the HOMO of 1,2-azaborine is not degenerate. The
HOMO–LUMO gap in 1 (5.32 eV) is smaller than those for
benzene (6.55 eV) and borazine (7.91 eV). This trend is
consistent with the electronic spectra (Figure 2). The orbital
diagram of the HOMO of 1 is shown in Figure 4A. We also
Received: November 13, 2008
Published online: December 22, 2008
Keywords: aromaticity · benzene · boron ·
.
density functional calculations · heterocycles
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at the 3 and 5 positions of the 1,2-azaborine ring, which is
consistent with the experimental results, in which electro-
philic aromatic substitutions occur exclusively at these
positions of the heterocycle. The ESP diagram shows a
À
positive electrostatic potential (blue color) at the N H group,
À
which is consistent with the observed protic nature of the N
H proton.
In summary, we prepared 1,2-dihydro-1,2-azaborine 1, a
hybrid organic/inorganic benzene that had thus far eluded
characterization. The structural, spectroscopic, and chemical
data presented in this work were fully supported by high-level
calculations, and indicated that 1,2-dihydro-1,2-azaborine is a
stable aromatic molecule with features that are distinct from
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Experimental Section
1: In a glove box (under a nitrogen atmosphere), complex 7 (0.150 g,
0.698 mmol), triphenylphosphine (0.915 g, 3.49 mmol), and isopen-
tane (3.0 mL) were combined in a Schlenk tube and sonicated at room
temperature for 3 h. Isopentane and 1 were transferred under vacuum
to a cold trap at À788C. Residual isopentane was removed under
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[17] See the Supporting Information for the COSY and HETCOR
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[25] Crystallographic data for 7: C7H6BCrNO3; Mr = 291.12; crystal
size 0.20 ꢁ 0.14 ꢁ 0.09 mm; monoclinic; space group P21/c; a =
11.5311(8), b = 6.9443(5), c = 12.2196(8) ꢂ; b = 115.946(1)8; V=
[18] The electronic spectra of benzene, compound 1, and borazine
were measured under identical conditions ([substrate] = 10À4
m
879.86(11) ꢂ3;
Z = 4;
F(000) = 432;
1calc = 1.623 gcmÀ3
l = 0.71073 ꢂ;
;
m(MoKa) =
T= 173(2) K;
in pentane) to ensure a direct comparison.
1.267 mmÀ1
;
[19] For computational details of this work, see the Supporting
Information. For previously reported calculations on compound
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Schleyer, Chem. Rev. 2005, 105, 3842 – 3888.
2Vmax = 54.008; 9413 reflections measured [Rint = 0.0262];, 1915
reflections observed; 142 refined parameters; R1 = 0.0293,
wR2 = 0.0728 for reflections with I > 2s(I); R1 = 0.0345,
wR2 = 0.0765, and GOF = 1.027 for all data; max/min residual
electron density + 0.439/À0.201 eꢂÀ3. CCDC 709282 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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