Fast H/D exchange of B, B′, B′-tribromoborazine in C 6D6 in the presence of aluminum tribromide: First evidence for an electrophilic substitution reaction of borazines in solution

Article abstract of DOI:10.1021/ic201240h

A solution of B,B′,B′-tribromoborazine (BrBNH)₃ in excess C₆D₆ in a sealed NMR tube shows no changes for over 14 months at room temperature but undergoes fast (within minutes) H/D exchange in the presence of AlBr₃ as a Lewis acid, as evidenced by ¹H, ²H, ¹¹B, and ²⁷Al NMR spectroscopy. The proposed electrophilic exchange mechanism is in agreement with the results of DFT computations. To our knowledge, this is the first example of the electrophilic substitution reaction of borazines in solution.

Full text of DOI:10.1021/ic201240h

ARTICLE
pubs.acs.org/IC
0
00
Fast H/D Exchange of B,B ,B -Tribromoborazine in C D in the
6
6
Presence of Aluminum Tribromide: First Evidence for an Electrophilic
Substitution Reaction of Borazines in Solution
,†
†
†
‡
‡
Alexey Y. Timoshkin,* Igor V. Kazakov, Anna S. Lisovenko, Michael Bodensteiner, and Manfred Scheer
†
Inorganic Chemistry Group, Department of Chemistry, Saint Petersburg State University, University Pr. 26, Old Peterhof,
1
98504, Russia
‡
Department of Inorganic Chemistry, University of Regensburg, 93040 Regensburg, Germany
S Supporting Information
b
0
00
ABSTRACT: A solution of B,B ,B -tribromoborazine (BrBNH) in excess
3
C D in a sealed NMR tube shows no changes for over 14 months at room
6
6
temperature but undergoes fast (within minutes) H/D exchange in
1
2
11
the presence of AlBr as a Lewis acid, as evidenced by H, H, B, and
3
2
7
Al NMR spectroscopy. The proposed electrophilic exchange mechanism is
in agreement with the results of DFT computations. To our knowledge, this is
the first example of the electrophilic substitution reaction of borazines in
solution.
’
INTRODUCTION
1
report on a fast H/D exchange between (BrBNH) and deutero-
3
benzene in solution, which is catalyzed by the Lewis acid AlBr3.
To our knowledge, this is the first example of the electrophilic
substitution reaction for tribromoborazine in solution.
Borazine is isoelectronic to benzene and has similar physical
properties but quite different chemical reactivity. While benzene
readily undergoes electrophilic substitution reactions, addition
reactions are more common for borazine. Dahl and Schaeffer
2
3
studied exchange reactions between borazine and a series of
deuterated compounds. The reaction of borazine with DCl
mostly leads to addition products, but the H/D exchange
reaction operates in parallel and leads to substitution of only
three (all imide) hydrogens to deuterium. In contrast to benzene,
no scrambling was observed due to 1,2-shifts, and so the final
’
EXPERIMENTAL DETAILS
7
BBr
3
was synthesized from elements by the standard procedure with
57.6% yield. Bromine impurities were removed by storing over liquid
mercury at room temperature for ca. 48 h. The obtained BBr was
3
purified by multiple (no less than two times) slow distillations in sealed
wholeglass systems in vacuum at room temperature (saturated vapor
pressure ca. 60 Torr).
Synthesis of (BrBNH) was carried out by the hot tube method
analogously to the synthesis of (ClBNH)
0
00
product of the substitution reaction is N,N ,N -trideuterobor-
azine. Treatment of borazine with ND Cl and DCN results in
4
3
fast NꢀH/NꢀD exchange, while treatment with D , B D , and
8
2
2 6
3
. All operations have been
NaBD results in BꢀH/BꢀD exchange. Such selectivity was
0
00
4
performed in a specially designed wholeglass apparatus. B,B ,B -Tribro-
moborazine was synthesized by passing gaseous BBr (7.60 g, 0.0303 mol)
over solid NH Br (4.47 g, 0.0456 mol) at 200ꢀ220 °C. Nitrogen was used
attributed to different exchange mechanisms. It was supposed
that NꢀH/NꢀD exchange proceeds via an electrophilic sub-
stitution reaction, but no evidence for this mechanism was
3
4
3
as a carrier gas. Colorless needle crystals of (BrBNH) were formed in the
3
4
provided. In previous work aluminum trihalides were used to
cold zone of the apparatus and purified by multiple (no less than 3 times)
sublimations in vacuum at 60ꢀ65 °C (saturated vapor pressure ca. 0.2
Torr). Yield: 0.229 g (7.2%).
The purity of (BrBNH) was confirmed by EI mass spectrometry, H
and B NMR, X-ray crystallography, and vapor pressure measurements
(for more details on tensimetry studies, see ref 9). Single crystals of
ꢀ
abstract X from TiX in the presence of aromatic hydrocarbons,
4
6
+
resulting in arene complexes [TiCl -η Ar] (Ar = C Me H ,
x = 3, 4) undergoing fast H/D exchange with C D . However,
3
6
x
6ꢀx
1
6
6
3
1
1
toluene and C D do not undergo H/D exchange in the presence
6
6
4
of TiX and AlX .
4
3
Electrophilic substitution reactions for borazines were un-
known until 1999 when Chiavarino et al. reported gas-phase
alkylation of borazine. In the present communication we
Received: June 9, 2011
Published: August 05, 2011
5,6
r 2011 American Chemical Society
9039
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|Inorg. Chem. 2011, 50, 9039–9044

Inorganic Chemistry
ARTICLE
Table 1. Crystallographic Data
empirical formula
3
B Br
3 3 3
H N
M
r
317.18
cryst size [mm]
T [K]
0.40 ꢁ 0.28 ꢁ 0.18
123(1)
space group
cryst syst
a [Å]
Pnma
orthorhombic
14.1228(8)
13.8457(7)
4.0654(2)
794.95(7)
4
b [Å]
c [Å]
3
V [Å ]
Z
3
F
calcd [g/cm ]
2.650
Figure 1. Molecular structure of tribromoborazine in the crystal. Bond
lengths [Å] and angles [deg]: Br1ꢀB1 1.951(6), Br2ꢀB2 1.937(4),
ꢀ
1
μ [mm
]
15.136
B1ꢀN1 1.406(4), N1ꢀB2 1.419(5), N2ꢀB2 1.423(5), N1ꢀH1
Θ range [deg]
completeness to Θ
index range
3.24ꢀ30.51
0.979
0
0
1
.89(4), N2ꢀH2 0.86(6), B1ꢀN1ꢀB2 120.3(3), B2ꢀN2ꢀB2
0
20.6(4), Br1ꢀB1ꢀN1 119.6(2), N1ꢀB1ꢀN1 120.7(4), Br2ꢀB2ꢀ
ꢀ20 e h e 18
N2 119.1(3), N1ꢀB2ꢀN2 119.1(3), Br2ꢀB2ꢀN1 121.8(3).
ꢀ
14 e k e 19
5 e l e 5
ꢀ
3
(BrBNH) were grown over several days in a glass ampule in vacuum at
ca. 45 °C (with a temperature gradient of about 5 °C).
data/restrains/params
3825/0/53
Electron ionization mass spectra were measured using an MX-1321
apparatus with direct probe introduction. MS (EI, 30 eV, 120 °C):
unique reflns I > 2σ(I) (Rint
)
1233 (0.0367)
0.991
2
GOF in F
+
+
+
+
+
(
7
BrB
BrBNH)
3
100, Br
2
B
3
N
3
H
3
84, BrB
3
N
3
H
3
35, B
3
N
3
H
3
11. MS (EI,
R
1
1
/wR
2
(I > 2σ(I))
0.0323, 0.0655
0.0523, 0.0697
ꢀ1.060, 0.186
+
2+
0 eV, 120 °C): (BrBNH) 94; Br B N H 100; (BrBNH) 21;
3
2
3
3
3
3
R
/wR
2
(all data)
+
2+
2+
+
1
3
N
3
H
3
41; (Br
2
B
3
N
3
H
3
)
16; (BrB
3
N
3
H
3
)
14; B
3
N
3
H
3
22. H
ꢀ3
1
1
largest diff. [e Å ]
NMR (C
7.64 ppm.
Crystal structure analysis was performed on an Oxford Diffraction
6
D
6
): broad signal at 4.86 ppm. B NMR (C
6
D
6
) singlet at
2
1
2
trichloroborazine also feature planar structures as well as isoelec-
13
Gemini R Ultra CCD using molybdenum radiation (λ = 0.71073 Å). An
analytical absorption correction from crystal faces was applied. The
structure was solved by direct methods with the program SIR-97, and
tronic 1,3,5-tribromobenzene. BꢀN and BꢀBr bond distances
are in good agreement with previously reported values for sub-
14
stituted bromoborazines (BrBNMe) and (BrBNC F ) .
H/D Exchange Studies. In the H NMR spectra of solution 1
3
6 5 3
2
full-matrix least-squares refinement on F in SHELXL-97 was carried
1
10
out. All hydrogen-atom parameters were refined with isotropic
displacements. CSD-423154 contains the supplementary crystallo-
graphic data for this paper. This data can be obtained from the Inorganic
Crystal Structure Database at Fachinformationszentrum Karlsruhe via
http://www.fiz-karlsruhe.de/icsd.html.
(
(BrBNH) concentration = 47 mmol/L) a broad signal, attrib-
3
11
uted to a NꢀH group, is observed at 4.86 ppm. The B NMR
spectrum shows a single broad peak at 27.7 ppm. No changes are
1
observed in the H NMR spectra after 14 months in a sealed glass
system stored in the dark at room temperature (Figure 2a). On
the contrary, solution 3, prepared beforehand and stored at the
same conditions, does not show any signal in the NꢀH region of
NMR Studies. NMR spectra were acquired on a Bruker Avance 400.
Sample preparation: C
6
D
6
was degassed by multiple freezingꢀcooling
cycles under vacuum, stored in a sealed glass ampule over molecular
sieves (4 Å), and distilled in vacuum before use. Solutions for the NMR
studies have been prepared by the following procedure. Solution 1:
1
the H NMR spectra (Figure 2b) but exhibits a corresponding
2
signal in H NMR spectra (Figure 2c), suggesting complete H/D
exchange between the NꢀH group of (BrBNH) and the CꢀD
3
(
2
BrBNH)
.65 mL, 0.0277 mol) in a sealed wholeglass system under vacuum,
which contained an NMR ampule, sealed under vacuum. Solution 2:
AlBr (0.1642 g, 0.6157 mmol) was dissolved in C (1.87 g, 2.13 mL,
.0223 mol) in a sealed wholeglass system under vacuum. The solution
3 6 6
(0.0402 g, 0.126 mmol) was dissolved in C D (2.33 g,
group of deuterobenzene. In order to study the rate of the
exchange, a freshly prepared solution of 35 mg of AlBr in 0.6 mL
3
of C D was added to an equal amount of solution 1 in a drybox.
6
D
6
6 6
3
1
After 5 min the signal at 4.86 ppm in the H spectrum has almost
completely disappeared and the corresponding signal in the H
NMR spectrum is clearly visible, indicating the exchange effec-
0
2
was divided into two parts and a NMR ampule. Solution 3: 0.64 mL of
solution 2 and 0.62 mL of solution 1 were mixed in a sealed wholeglass
system under vacuum, and part of it was sealed in a NMR ampule.
Further details on systems used for sample preparation can be found in
the Supporting Information (Figure 3S).
1
1
tively proceeds within minutes. No changes are observed in
B
27
11
and Al NMR; a single broad peak at 27.7 ppm in B NMR and
27
a single peak at 77.3 ppm in Al NMR are observed, which
indicates a similar chemical environment of boron and aluminum
atoms, which agrees with the presence of fully deuterated
tribromoborazine and aluminum tribromide.
’
RESULTS AND DISCUSSION
Structure of the Solid Compound. The crystal structure of
Formation of fully deuterated (BrBND) was confirmed by an
MS study of the solid residue after vaporization of the solvent
from solution 3 (see Figure 2S in the Supporting Information).
3
(
BrBNH) was established for the first time. Molecules (BrBNH)3
3
are essentially planar. The BꢀN distances inside the ring are slightly
unequal due to pronounced Br HꢀN hydrogen bonding in the
In addition, toluene, present in the source C D as an impurity
3
3 3
6 6
crystal. Major structural parameters are summarized in Figure 1;
(estimated toluene content is about 0.02%), also undergoes
complete H/D exchange of aromatic protons upon addition of
11
experimental details are given in Table 1. Both trifluoro- and
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Inorganic Chemistry
ARTICLE
1
1
2
Figure 2. NMR spectra in deuterobenzene: (a) H NMR of (BrBNH)
NMR of the solution of (BrBNH) after addition of AlBr
3
, (b) H NMR of the solution of (BrBNH)
3
after addition of AlBr
3
, and (c) H
3
3
.
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Inorganic Chemistry
ARTICLE
Scheme 1. Proposed H/D Exchange via Electrophilic Substitution Reactions
1
9
AlBr . Signals at 7.02ꢀ7.13 ppm corresponding to protons of the
standard Gaussian 03 program package on the High-Performance
Computing cluster of St. Petersburg State University. Density
functional theory in the framework of the hybrid three-parameter
3
aromatic ring completely disappear after AlBr addition, while
3
the singlet at 2.10 ppm corresponding to protons of the aliphatic
20
CH group of toluene is observed after 14 months after addition
exchange functional of Becke with the gradient-corrected correla-
tion functional of Lee, Yang, and Parr (B3LYP) with an all-
3
2
21
of AlBr . According to H NMR spectra, the presence of small
3
22
amounts of CH D groups is detected both in the source solvent
electron TZVP basic set was used throughout. The basis sets are
(311/1) for H, (62111/411/1) for B and N, (73211/6111/1) for
Al, and (842111/63111/511) for Br. All structures have been fully
optimized and verified to be minima or transition states (TS) by
subsequent vibrational analysis. Intrinsic reaction coordinate (IRC)
scans confirmed that obtained TS are connecting reactant and
products. Such level of theory has been previously used by us to
2
and in the sample after addition of AlBr . Although we cannot
3
completely rule out H/D exchange in CH groups of toluene, its
3
rate is much slower compared to the fast exchange of aromatic
protons. In our opinion, this indicates a fast electrophilic
substitution of the aromatic protons between C D and toluene.
6
6
+
The proposed mechanism (Scheme 1) includes [Br B N H D]
ions which are analogous to Wheland intermediates. Such
arenium” ions as C H7 were experimentally isolated with bulky
counterions and structurally characterized by Reed et al.
3
3 3 3
1
5
23
study borazine complexes.
+
+
“
The relative energies of the (B N H ) ions suggest that the
6
3 3 7
16
ꢀ1
nitrogen-protonated isomer is by 133 kJ mol more stable than
the boron-protonated one. Thus, in acidic media, hydrogen
exchange is expected to occur only at the nitrogen center.
The optimized structures of the compounds involved in
hydrogen exchange and the reaction profile for tribromoborazine
and borazine are given in Figure 3. Predicted structural parameters
+
Recently, borazinium ions [R B N H ] were obtained via reac-
3
3 3 4
tion of donorꢀacceptor complexes R B N H AlBr with HBr
3
3
3
3
3
3
and structurally characterized by the N €o th group as tetrabromo-
1
7
aluminate salts.
In order to start a series of electrophilic substitution reactions,
generation of active HBr or DBr species is needed and may be
achieved in two ways.
+
ꢀ
4
for gaseous borazinium tetrabromoaluminates [Br B N H ] [AlBr ]
and [B N H ] [AlBr ] agree well with those experimentally found in
3
3
3
4
+
ꢀ
4
3
3
7
+
ꢀ
+
(
1) Reaction of AlBr with traces of water (for example, from
solid borazinium salts [Et B N H ] [AlBr ] and [(t-Bu) B N H ]
3
3 3 3 4 4 3 3 3 4
ꢀ17
the used solvent C D ), which is unlikely due to careful
6
6
[AlBr4] (see Table 2S, Supporting Information, for details).
We optimized TS connecting HAlBr Br B N H and
[Br B N H ] [AlBr ] (by analogy with benzene, called π
and σ complexes, respectively). Two conformers (which are
different in the location of the AlBr moiety around the hetero-
cycle) have been considered for both σ and π complexes
(Figure 3a). IRC scan confirmed that the obtained TS is
experimental procedures.
2) HBr abstraction from reaction of (BrBNH) or C D with
4
3 3 3
3 3 3 3
+
ꢀ
(
3
6 6
3
3
3
4
4
AlBr due to either intra- or intermolecular HBr/DBr
3
elimination. In the latter case formation of AlBr C D
2
6
5
3
intermediate could not be ruled out, analogously to
possible formation of AlCl C H discussed by Olah
2
6 5
1
8
et al.
connecting isomer 1 of π-HAlBr
Br B N H and isomer
3 3 3 3
4
3 3 3
+
ꢀ
1 of σ-[Br B N H ] [AlBr ] .
3 3 3 4 4
In both cases formation of HBr/DBr in the presence of excess
AlBr starts the classical catalytic cycle generating arenium ions
Analogous computations have been made for the borazine
3
+
C D H . Subsequent H/D scrambling leads to DBr, which in
system (Figure 3b). In this case upon optimization isomer 2 of π-
6
6
+
ꢀ
turn will react with (BrBNH) to form borazinium ion (BrB-
HAlBr
4
B
3
N
3
H
6
converged to σ-[B
3
N
3
H
7
] [AlBr
4
] (isomer 2).
and
3
3 3 3
+
NH) D and eventually to the fully deuterated (BrBND) , since
We also obtained the TS connecting π-HAlBr
4
B N H
3 3 6
3
3
3 3 3
ꢀ
] , as confirmed by IRC scan.
4
+
the tribromoborazine to deuterobenzene ratio is 1:446.
isomer 1 of σ-[B
N
3
3
H
7
] [AlBr
Computational Studies. To shed more light into the me-
chanism of H/D exchange, quantum chemical computations
were performed for gaseous AlBr ꢀHBrꢀ(BrBNH) and
In both systems, σ complexes are more stable than π com-
ꢀ
1
plexes (by 11 and 41 kJ mol for tribromoborazine and borazine,
ꢀ
1
respectively). Note that for (BrBNH) the TS lies only 25 kJ mol
3
3
3
ꢀ
1
AlBr ꢀHBrꢀ(HBNH) systems without taking into account
higher than the most stable σ complex and 14 kJ mol higher than
the most stable π complex. The small activation energy indicates
3
3
solvation effects. All computations were carried out using the
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Inorganic Chemistry
ARTICLE
ꢀ
1
3
Figure 3. Energetic profiles for the AlBr -catalyzed hydrogen exchange for (a) tribromoborazine and (b) borazine. Relative energies are in kJ mol .
B3LYP/TZVP level of theory.
ꢀ
1 24,25
that the hydrogen/deuterium exchange should not be kinetically
hindered even at room temperature. These theoretical predictions
are in good agreement with the observed experimental data on H/D
exchange.
electrophilic exchange by Volkov et al. (45ꢀ55 kJ mol ).
Their quantum chemical computations indicate that the dimeric
form of the aluminum trihalide is more likely to serve as a
catalytic species for the hydrogen exchange between HCl
and C H . The activation energies for HCl/C H dimer-
catalyzed exchange predicted at the MP2/LANL2DZ(d)+ level
of theory are only 20 and 29 kJ mol (for Al Cl and Ga Cl ,
Barriers for the monomer-catalyzed exchange
(by AlCl and GaCl ) are by about 25 kJ mol higher.
According to a recent theoretical study, even higher oligomeric
ions M3X10 and M4X13 are expected to be formed in excess of
In the case of (HBNH) the barrier from the σ complex is much
3
6
6
6 6
ꢀ
1
ꢀ1
higher (42 kJ mol ) than that from the π complex (1 kJ mol ),
ꢀ
1
suggesting a π-type transition state. However, the value of
2
6
2
6
ꢀ
1
24,25
4
2 kJ mol is also not prohibitingly high to disallow H/D exchange
respectively).
ꢀ
1
in the case of borazine, suggesting that such electrophilic substitu-
tion reactions in solution are characteristic for borazine chemistry in
general. These barriers are quite small, so the exchange reaction is
expected to be operational even at room temperature.
3
3
ꢀ
ꢀ
26
group 13 metal trihalides in solution. It is expected that in
borazine systems barriers would also be smaller if higher oligo-
Theoretical activation energies obtained in the present work
are comparable with those predicted for the C H ꢀHClꢀAlCl
meric forms of AlBr were involved.
6
6
3
3
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Inorganic Chemistry
ARTICLE
As pointed out by one of the reviewers “ref 3 may be
considered to indicate electrophilic substitution upon reaction
with DCl as it demonstrates that (a) the product B N H 3DCl
does not decompose to borazine and (b) once formed it does not
exchange with borazine. This leaves little room for other inter-
pretations than an electrophilic substitution by DCl”. Indeed,
observations of Dahl and Schaeffer allow one to exclude an
additionꢀelimination mechanism for the exchange but do not
provide a direct hint to the electrophilic substitution pathway. In
our experiment, however, fast H/D exchange with deuteroben-
(5) Chiavarino, B.; Crestoni, M. E.; Fornarini, S. J. Am. Chem. Soc.
1999, 121, 2619.
(
6) Chiavarino, B.; Crestoni, M. E.; Marzio, A. D.; Fornarini, S.; Rosi, M.
J. Am. Chem. Soc. 1999, 121, 11204.
7) Handbuch der Praeparativen Anorganischen Chemie, Bd. 3; Brauer, G.,
Ed.; Enke Verlag: Stuttgart, 1981; p 706.
8) Brown, C. A.; Laubengayer, A. W. J. Am. Chem. Soc. 1955,
7, 3699.
9) Kazakov, I. V.; Timoshkin, A. Y. Russ. J. Inorg. Chem., submitted
3
3
6
3
(
(
7
(
for publication.
(10) (a) Clark, R. C.; Reid, J. S. Acta Crystallogr. 1995, A51,
887–897. (b) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano,
G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (c) Sheldrick, G. M.
Acta Crystallogr. 2008, A64, 112–122.
zene was simultaneously observed for both (BrBNH) and
3
aromatic protons of toluene. Taking into account that electro-
philic substitution pathway is well established for aromatic
hydrocarbons, our observations provide a direct hint to the
operation of the electrophilic substitution pathway, which is
supported by computational studies.
(11) Anand, B.; N €o th, H.; Schwenk-Kircher, H.; Troll, A. Eur.
J. Inorg. Chem. 2008, 3186.
(12) Coursen, D. L.; Hoar, J. L. J. Am. Chem. Soc. 1952, 74, 1742.
(13) Milledge, H. J.; Pant, L. M. Acta Crystallogr. 1960, 13, 285.
(14) Anand, B.; N €o th, H.; Schwenk-Kircher, H.; Troll, A. Eur.
’
CONCLUSIONS
J. Inorg. Chem. 2008, 3186.
In this report we demonstrated a fast H/D exchange between
(15) Wheland, G. W. J. Am. Chem. Soc. 1942, 64, 900.
0
00
B,B ,B -tribromoborazine and deuterobenzene catalyzed by the
(16) Reed, C. A.; Kim, K.-C.; Stoyanov, E. S.; Stasko, D.; Tham, F. S.;
Lewis acid AlBr . Theoretical studies support the hypothesis that
Mueller, L. J.; Boyd, P. D. W. J. Am. Chem. Soc. 2003, 125, 1796.
(17) Cem €u nd, B.; G €u nther, B.; N €o th, H. ARKIVOC 2008, 5, 136.
(18) Olah, G. A.; T €o r €o k, B.; Joscher, J. P.; Bucsi, P.; Esteves, P. M.;
Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 2002, 124, 11379.
3
the exchange proceeds via an electrophilic substitution mechan-
ism, which should be operational for the parent borazine as well.
Our observations on the catalytic role of AlBr open wide
3
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
perspectives for studying electrophilic exchange reactions of
borazines in solution.
Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian03, revision B.05; Gaussian, Inc.: Wallingford, CT,
’
ASSOCIATED CONTENT
S
Supporting Information. Optimized structures, xyz co-
b
ordinates, total energies, standard enthalpies and standard
entropies for all considered compounds, description of the glass
system used for sample preparation for NMR studies, mass
spectra for the product obtained after solvent evaporation,
comparison of the theoretically predicted and experimental bond
distances in borazinium salts (10 pages). This material is available
free of charge via the Internet at http://pubs.acs.org.
’
AUTHOR INFORMATION
2
004.
Corresponding Author
(
(
(
20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
21) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B. 1988, 37, 785.
22) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005,
*E-mail: alextim@AT11692.spb.edu.
7
, 3297.
’
ACKNOWLEDGMENT
(23) Lisovenko, A. S.; Timoshkin, A. Y. Inorg. Chem. 2010, 49,
0357.
1
The authors are grateful to Christine Thoms for help with
(
24) Volkov, A. N.; Timoshkin, A. Y.; Suvorov, A. V. Int. J. Quantum
Chem. 2004, 100, 412.
25) Volkov, A. N.; Timoshkin, A. Y.; Suvorov, A. V. Int. J. Quantum
mass-spectral studies and technical assistant Georgine St €u hler
and Dr. G ꢀa bor Bal ꢀa zs for help with NMR studies. The authors
acknowledge Saint Petersburg State University for research grant
(
Chem. 2005, 104, 256.
1
2.37.139.2011 and the Alexander von Humboldt Foundation
(26) Timoshkin, A. Y. Russ. J. Inorg. Chem. 2009, 54, 87.
for general support. We thank reviewers for helpful comments.
’
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dx.doi.org/10.1021/ic201240h |Inorg. Chem. 2011, 50, 9039–9044