Syntheses and characterizations of linear triborazanes

Article abstract of DOI:10.1021/ic401844m

Reaction of the amine boranes NH₂(R)BH₃, where R = H, Me, and Bz, with ¹/₃ equiv of sodium hexamethyldisilazane produced the five-membered, linear aminoborane anions Na⁺[BH ₃N(R)HBH₂N(R)HBH₃^-], where R = H (1), Me (1Me), and benzyl (1Bz). Reactions of 1 and 1Me with ammonium chloride and methylammonium chloride, respectively, resulted in elimination of NaCl and H₂ to produce the linear triborazanes BH₃(RNHBH ₂)₂N(R)H₂, where R = H (2) and Me (2Me), with the structure of 2 crystallographically confirmed. The reactions of 1 and 1Me with pyridine-HCl produced the pyridine-capped aminoboranes H ₃B(RNHBH₂)₂(NC₅H₅), where R = H (3) and Me (3Me). 2 and 2Me proved to be stable up to 90 C but produced a mixture of products when heated above 90 C. 2 was selectively monochlorinated at the terminal boron when treated with 1 equiv of HCl and dichlorinated when reacted with a second 1 equiv of HCl.

Full text of DOI:10.1021/ic401844m

Article
pubs.acs.org/IC
Syntheses and Characterizations of Linear Triborazanes
William C. Ewing, Patrick J. Carroll, and Larry G. Sneddon*
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States
S
* Supporting Information
ABSTRACT: Reaction of the amine boranes NH2(R)BH3, where R = H, Me,
1
and Bz, with /3 equiv of sodium hexamethyldisilazane produced the five-
−
membered, linear aminoborane anions Na+[BH3N(R)HBH2N(R)HBH3 ],
where R = H (1), Me (1Me), and benzyl (1Bz). Reactions of 1 and 1Me
with ammonium chloride and methylammonium chloride, respectively, resulted
in elimination of NaCl and H2 to produce the linear triborazanes
BH3(RNHBH2)2N(R)H2, where R = H (2) and Me (2Me), with the structure
of 2 crystallographically confirmed. The reactions of 1 and 1Me with pyridine−
HCl produced the pyridine-capped aminoboranes H3B(RNHBH2)2(NC5H5),
where R = H (3) and Me (3Me). 2 and 2Me proved to be stable up to 90 °C
but produced a mixture of products when heated above 90 °C. 2 was selectively monochlorinated at the terminal boron when
treated with 1 equiv of HCl and dichlorinated when reacted with a second 1 equiv of HCl.
borane were prepared by amine displacement from BH3−THF with
their purities confirmed by 1H NMR. Pyridine−HCl and all
ammonium chloride salts (Sigma-Aldrich) were dried in vacuo with
heating prior to use. Fluorobenzene (Sigma-Aldrich) and acetonitrile
(Fisher) were dried over CaH2 and distilled prior to use. Glyme (1,2-
dimethoxyethane) and THF (Fisher) were dried over sodium/
benzophenone and distilled prior to use. Toluene and diethyl ether
(Fisher) were HPLC-grade and were used as received.
INTRODUCTION
■
The growth of aminoborane oligomers through the dehy-
drocoupling reactions of ammonia borane (AB, NH3BH3) has
been a topic of recent research owing to the interest in amine
boranes as potential hydrogen storage materials and/or
chemical precursors to boron nitride ceramics.1 While long-
chain oligomers and polymers have been shown to be products
of metal-catalyzed amine borane reactions,2 only a few small
linear oligomers have been characterized, including the four-
1
Physical Methods. The 11B NMR spectra at 128.4 MHz and H
NMR spectra at 400.1 MHz were obtained on a Bruker DMX-400
spectrometer equipped with appropriate decoupling accessories. All
11B chemical shifts are referenced to external BF3·O(C2H5)2 (0.0
ppm), with a negative sign indicating an upfield shift. All 1H chemical
shifts were measured relative to internal residual protons in the lock
solvents and are referenced to Me4Si (0.0 ppm). Elemental
microanalysis of many of the very hydroscopic products proved
difficult; however, satisfactory elemental analyses of 2Me and 1Bz
were obtained at the MicroAnalytical Facility at University of
California at Berkeley, Berkeley, CA. Thermal analysis was carried
out on a TA Instruments SDT model 2960 instrument capable of
simultaneous differential scanning calorimetry (DSC) and thermal
gravimetry (TGA) measurements.
3
membered parent diborazane NH3BH2NH2BH3 and its N-
methyl-, N,N-dimethyl-, and N,N-(1,4-C4H8)diborazane deriv-
atives.4 We also recently reported5 the syntheses of the five-
−
membered, linear anionic chain [BH3NH2BH2NH2BH3 ] (1)
and both linear [BH3NH2BH2NH2BH2NH2BH3−] and
−
branched [HB(NH2BH3)3 ] seven-membered chains via
anionic AB chain growth initiated by deprotonation with
Verkade’s base (VB).6 Herein, we report a greatly improved
route to the 1 anion, as well as to its N-methylated and N-
benzylated derivatives Na+[BH3N(R)HBH2N(R)HBH3 ] (R =
−
CH3, 1Me; R = CH2C6H5, 1Bz). We further demonstrate that
1 and 1Me can be readily converted to the first examples of
linear triborazanes BH3(RNHBH2)2N(R)H2, where R = H (2)
and Me (2Me), by their reactions with ammonium and methyl
ammonium chloride, respectively.
Na+[BH3NH2BH2NH2BH3−] (1). NaHMDS (11.0 mmol) was
added to a suspension of AB (1.02 g, 33.0 mmol) in fluorobenzene
(30 mL) and the mixture stirred at 50 °C for 24 h. The solution was
next diluted with 30 mL of toluene and frit-filtered. The retained solid
was washed with toluene (2 × ∼20 mL) and diethyl ether (3 × ∼20
1
mL) and then dried in vacuo. The H and 11B NMR spectra of the
EXPERIMENTAL SECTION
■
resultant white solid (1.03 g, 10.8 mmol, 96%) were consistent with
5
−
Unless otherwise noted, all reactions and manipulations were
performed in dry glassware under a nitrogen or argon atmosphere
using the high-vacuum or inert-atmosphere techniques described by
Shriver and Drezdzon.7
Materials. Ammonia borane (AB; Aviabor, 97% minimum purity)
was ground into a free-flowing powder using a commercial coffee
grinder. Sodium hexamethyldisilazane (NaHMDS; 0.6 M in toluene),
BH3−tetrahydrofuran (THF) (1 M in THF), methylamine (2 M in
THF), HCl (1 M in diethyl ether), and benzylamine were used as
received (Sigma-Aldrich). Methylamine borane and benzylamine
the literature values for [BH3NH2BH2NH2BH3 ].
Na+[H3BN(CH3)HBH2N(CH3)HBH3−] (1Me). NaHMDS (11.1
mmol) was added to a suspension of methylamine borane (1.50 g,
33.4 mmol) in fluorobenzene (30 mL) and the mixture stirred at 60
°C for 24 h. In a nitrogen-filled glovebag, the solution was diluted with
30 mL of toluene and frit-filtered. The retained solid was washed with
Received: July 17, 2013
Published: August 26, 2013
© 2013 American Chemical Society
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Article
toluene (2 × ∼20 mL) and diethyl ether (3 × ∼20 mL) and then dried
in vacuo. Yield: 1.03 g (10.8 mmol, 96%). 11B NMR (THF-d8, ppm, J
in Hz): −3.3 (t, J = 98, BH2), −18.2 (q, J = 87, 2BH3). Density
functional theory (DFT)/gage invariant atomic orbital (GIAO)-
calculated [B3LYP/6-311G(d)] 11B NMR shifts: S,R isomer, −5.6,
HCCH2CH2N−), 3.15 (br s, NH2), 2.87 (s, BH2), 2.09 (s, NH2), 1.79
(quin, J = 4, BH2), 1.12 (t, J = 5, BH3).
H3B[(CH3)NHBH2]2(NC5H5) (3Me). A solution of 1Me (0.10 g,
0.78 mmol) in acetonitrile (15 mL) was cooled at −20 °C and then
treated with pyridine−HCl (0.92 g, 0.78 mmol). The solution
immediately became cloudy. After stirring for 10 min, acetonitrile
was vacuum-evaporated and fluorobenzene (∼10 mL) was then
vacuum-transferred to the flask. The mixture was warmed to room
temperature and quickly filtered and the retained solid washed with
cold fluorobenzene. The solvent was vacuum-evaporated from the
filtrate to leave the product as a white solid. Yield: 0.13 g (0.68 mmol,
87%). 11B NMR (CD3CN, ppm, J in Hz): −0.4 (t, J = 109, BH2), −4.0
(t, J = 105, BH2), −18.2 (q, J = 90, BH3). DFT/GIAO-calculated
[B3LYP/6-311G(d)] 11B NMR shifts: S,R isomer, −1.3, −6.9, −17.7
1
−20.0, −23.4 ppm; R,R isomer, −5.7, −19.1, −19.1 ppm. H{11B}
NMR (THF-d8, ppm, J in Hz): 2.60 (s, 2NH2), 2.11 (d, J = 6, 2CH3),
1.62/1.35 (q, J = 4, BH2, diastereomers), 1.22 (d, J = 3, BH3).
Na+[H3BN(CH2C6H5)HBH2N(CH2C6H5)HBH3−] (1Bz). NaHMDS
(0.9 mmol) was added to a suspension of benzylamine borane (0.32 g,
2.6 mmol) in fluorobenzene (20 mL) and the mixture stirred at 60 °C
for 24 h. In a nitrogen-filled glovebag, the solution was diluted with
toluene (∼20 mL) and frit-filtered. The retained solid was washed with
toluene (2 × 15 mL) and cold diethyl ether (3 × ∼15 mL) and then
dried in vacuo for 12 h. Yield: 0.12 g (0.4 mmol, 49%). Anal. Calcd: C,
60.97; H, 8.77; N, 10.16. Found: C, 59.62; H, 8.89; N, 9.94. 11B NMR
(CD3CN, ppm, J in Hz): −2.6 (t, J = 89, BH2), −17.2 (q, J = 92,
2BH3). DFT/GIAO-calculated [B3LYP/6-311G(d)] 11B NMR shifts:
S,R isomer, −10.0, −17.4, −24.4 ppm; R,R isomer, −13.3, −20.0,
−20.0 ppm. 1H{11B} NMR (CD3CN, ppm, J in Hz): 7.28 (m,
2CH2C6H5), 3.49/3.40 (m, 2CH2C6H5, diastereomers), 2.52 (s,
2NH), 1.85/1.77 (s, BH2, diastereomers), 1.24 (d, J = 3, 2BH3).
Attempted Synthesis of Na+[H3BN(CH3)2BH2N(CH3)2BH3−].
NaHMDS (1.4 mmol) was added to a suspension of dimethylamine
borane (0.25 g, 4.2 mmol) in fluorobenzene (20 mL) and the mixture
stirred at 60 °C for 12 h. Analysis by 11B NMR indicated no formation
of oligomeric products but instead showed a large peak near 28 ppm.
BH3(NH2BH2)2NH3 (2). NH4Cl (0.78 g, 14.6 mmol) was added to a
solution of 1 (1.36 g, 14.3 mmol) in glyme (30 mL) and the mixture
stirred at room temperature for 24 h. The solution was then filtered
and the retained solid washed with glyme (4 × ∼20 mL). The filtrate
was vacuum-evaporated until ∼5 mL of glyme remained. The product
was precipitated from the concentrated glyme solution by the addition
of diethyl ether (∼25 mL), then filtered, and washed with diethyl ether
(3 × ∼20 mL). The remaining white solid was dried in vacuo for 18 h
to remove all traces of glyme. Yield: 1.06 g (11.9 mmol, 84%), 11B
NMR (THF-d8, ppm, J in Hz): −10.5 (t, J = 95, BH2), −12.4 (t, J =
104, BH2), −22.3 (q, J = 95, BH3). DFT/GIAO-calculated [B3LYP/6-
1
ppm; S,S isomer, −0.6, −7.5, −23.2 ppm. H{11B} NMR (CD3CN,
ppm, J in Hz): 8.64 (d, J = 5, HCCH2CH2N−), 8.24 (m,
HCCH2CH2N−), 7.77 (m, HCCH2CH2N−), 3.45 (s, NH), 2.87/
2.75 (s, BH2, diastereomers), 2.34 (m, NH), 2.20 (d, J 6, CH3), 2.01
(d, J = 6, CH3), 1.57/1.24 (s, BH2, diastereomers), 1.12 (s, BH3).
Attempted Thermal Cyclization of 2 and 2Me. A suspension of
2 (0.04 g, 0.4 mmol) in fluorobenzene (∼10 mL) was heated at 100
°C for 12 h while being monitored by 11B NMR. The solvent was
vacuum-evaporated and the residue taken up in ether (∼10 mL) and
filtered. The ether was vacuum-evaporated, leaving 12 mg of a white
solid, which 11B and 1H NMR analyses identified as a ∼1:1 mixture of
AB and cyclotriborazane (CTB).8
A solution of 2Me (0.02 g, 0.2 mmol) in fluorobenzene (∼10 mL)
was heated at 100 °C for 12 h while being monitored by 11B NMR.
Fluorobenzene was vacuum-evaporated and the remaining white solid
1
redissolved in THF-d8. Analysis by 11B and H NMR identified this
solid as a ∼3:1 mixture of methylamine borane and tri-N-
methylcyclotriborazane.4b,c
Chlorination of 2. A solution of 2 (0.05 g, 0.6 mmol) in glyme
(∼10 mL) was cooled at −30 °C, while a solution of HCl−etherate
(0.6 mmol) was added via syringe. After the solution was stirred for 15
min at −30 °C, 11B NMR analysis indicated formation of the
monochlorinated product 2Cl. Attempts to isolate the product by
solvent vacuum evaporation resulted in product decomposition. 11B
NMR (glyme, ppm, J in Hz): −6.6 (d, J = 98, BH2), −10.9 (t, J = 101,
BH2), −12.1 (t, J = 123, BH2). DFT/GIAO-calculated [B3LYP/6-
311G(d)] 11B NMR shifts for 1-ClB3N3H11: −6.1, −10.6, −11.4 ppm.
In a separate reaction, after 1 equiv of HCl−etherate (0.6 mmol)
had been added to a sample of 2 (0.05 g, 0.6 mmol) in glyme at −30
°C, a second 1 equiv of HCl−etherate (0.6 mmol) was added and the
mixture stirred for an additional 15 min at −30 °C. 11B NMR analysis
then indicated formation of a dichlorinated product, but attempts to
isolate the product by solvent vacuum evaporation resulted in product
decomposition. 11B NMR (CD3CN, ppm, J in Hz): −1.6 (d, J = 129,
BH), −7.5 (t, J = 113, BH2), −12.2 (t, J = 96, BH2). The DFT/GIAO-
calculated 11B NMR shifts of the three possible isomers of Cl2B3N3H12
are given in Figure S5 in the Supporting Information (SI).
Computational Studies. DFT/GIAO/NMR calculations were
performed using the Gaussian 09 program.9 Geometry optimizations
were carried out at the B3LYP/6-311G(d) level. All 11B NMR
chemical shifts were calculated at the B3LYP/6-311G(d) level using
the GIAO option in Gaussian 09 and are referenced to BF3−OEt2
using an absolute shielding constant of 102.2 ppm. Frequency analyses
were carried out to ensure that each minimized structure was a true
minimum with no imaginary frequencies.
1
311G(d)] 11B NMR shifts: −12.8, −13.5, −21.0 ppm. H{11B} NMR
(CD3CN, ppm, J in Hz): 4.69 (br s, NH3), 2.97 (br s, NH2), 2.15 (br s,
NH2), 2.08 (m, BH2), 1.97 (quin, J = 4, BH2), 1.31 (t, J = 4, BH3).
H3B[(CH3)NHBH2]2N(CH3)H2 (2Me). CH3NH3Cl (0.26 g, 3.9
mmol) was added to a solution of 1Me (0.5 g, 3.9 mmol) in
acetonitrile (∼20 mL) and the mixture stirred at 60 °C with careful
monitoring by 11B NMR. After ∼6 h, 1Me had been consumed, with
the 11B NMR spectrum showing only 2Me resonances. In a nitrogen-
filled glovebag, the solution was frit-filtered and the retained solid
washed with acetonitrile (2 × ∼20 mL). The solvent from the
combined filtrates was evaporated on a high-vacuum line until
crystallization was observed (∼24 h). The crystals were washed two
times with cold ether and then dried in vacuo. Yield: 0.5 g (3.4 mmol,
88%), Anal. Calcd: C, 27.58; H, 15.43; N, 32.16. Found: C, 27.90; H,
15.71; N, 31.93%. 11B NMR (THF-d8, ppm, J in Hz): −5.1 (t, J = 105,
BH2), −7.3 (t, J = 108, BH2), −18.9 (q, J = 91, BH3). DFT/GIAO-
calculated [B3LYP/6-311G(d)] 11B NMR shifts: S,S isomer, −8.2,
−9.2, −17.5 ppm; R,S isomer, −6.8, −8.6, −21.4 ppm. 1H{11B} NMR
(THF-d8, ppm, J in Hz): 5.17 (br s, NH2), 3.35 (br s, NH), 2.67 (br s,
NH), 2.40 (t, J = 6, CH3), 2.18 (d, J = 6, CH3), 2.11 (d, J = 6, CH3),
1.91 (m, BH2), 1.80 (m, BH2), 1.29 (d, J = 4, BH3).
BH3(NH2BH2)2(NC5H5) (3). A solution of 1 (0.10 g, 1.1 mmol) in
acetonitrile (15 mL) was cooled at −20 °C and then reacted with
pyridine−HCl (0.12 g, 1.1 mmol). The solution immediately became
cloudy. After stirring for 10 min, the solution was quickly filtered and
the retained solid washed with cold acetonitrile. The solvent was
vacuum-evaporated from the filtrate to leave the product as a white
solid. Yield: 0.11 g (0.74 mmol, 68%). 11B NMR (CD3CN, ppm, J in
Hz): −3.3 (t, J = 107, BH2), −9.6 (t, J = 102, BH2), −21.7 (q, J = 90,
BH3). DFT/GIAO-calculated [B3LYP/6-311G(d)] 11B NMR shifts:
−5.4, −11.0, −21.5 ppm. 1H{11B} NMR (CD3CN, ppm, J in Hz): 8.62
(d, J = 1.2, HCCH2CH2N−), 8.25 (m, HCCH2CH2N−), 7.77 (t, J = 7,
Collection and Reduction of Crystallographic Data. X-ray-
quality crystals of 2Me formed as the material was under high vacuum.
Crystallographic data and structure refinement information are
summarized in Table 1. The X-ray intensity data were collected on
a Bruker APEXII CCD area detector employing graphite-mono-
chromatized Mo Kα radiation. Rotation frames were integrated using
SAINT,10 producing a list of unaveraged F and σ(F2) values that were
then passed to the SHELXTL11 program package for further
processing and structure solution on a Dell Pentium 4 computer.
The intensity data were corrected for Lorentz and polarization effects
and for absorption using SADABS.12
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Table 1. Crystallographic Data
Scheme 1. Syntheses of 1, 1Me, and 1Bz
2Me
empirical formula
C3B3H20N3
fw
130.65
cryst class
space group
Z
monoclinic
P21/c (No. 14)
4
a, Å
5.7671(4)
16.7609(12)
10.4260(8)
103.302(4)
980.76(12)
0.885
b, Å
c, Å
β, deg
V, Å3
D
calc, Mg/m3
μ, mm−1
0.051
λ(Mo Kα), Å
cryst size, mm
F(000)
0.71073
0.26 × 0.14 × 0.04
296
2θ angle, deg
temperature, K
hkl collected
4.70−50.74
143(1)
−6 ≤ h ≤ 6, −19 ≤ k ≤ 20, −12 ≤ l ≤ 12
26815
precipitated from the reaction solution by the addition of
toluene. 1 was isolated as an air-stable solid that was very
soluble in glyme, N,N-dimethylformamide, and water and
no. of measd reflns
no. of unique reflns
no. of param
1796 (Rint = 0.0207)
163
1
slightly soluble in acetonitrile and THF. Both the 11B and H
R indices (F > 2σ)
R indices (all data)
R1 = 0.0340, wR2 = 0.0911
R1 = 0.0375, wR2 = 0.0945
1.056
NMR spectra of 1 were consistent with the previously reported
values.5
b
GOF
VB proved to be inactive for inducing N-alkylated amine
borane oligomerizations, but the reactions of methylamine
final difference peaks, e/Å3
+0.185, −0.104
a
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑w(Fo − Fc2)2/∑w(Fo )2}1/2
.
2
2
1
borane and benzylamine borane with /3 equiv of NaHMDS
b
GOF = {∑w(Fo − Fc2)2/(n − p)}1/2, where n = number of
2
readily produced the 1Me and 1Bz (Scheme 1; R = CH3, Bz)
five-membered chains. 1Me was obtained in an excellent 96%
isolated yield, but 1Bz was obtained in a much lower 49% yield
owing to the formation of a side product, exhibiting a downfield
(∼28 ppm) doublet resonance in its 11B NMR spectrum
characteristic of (R2N)2BH-type compounds.15,16 Reaction of
(CH3)2NHBH3 with NaHMDS produced no oligomers, but
instead gave a single product, again exhibiting a doublet 11B
NMR resonance near 28 ppm consistent with (Me2N)2BH
formation.15,16
reflections and p = number of parameters refined.
Solution and Refinement of the Structures. The structures
were solved by direct methods (SHELXS-9713), and refinement was by
full-matrix least squares based on F2 also using SHELXL-97. All
reflections were used during the refinement (values of F2 that were
experimentally negative were replaced with F2 = 0). All non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were refined
isotropically.
RESULTS AND DISCUSSION
1Me and 1Bz are chiral compounds and are synthesized as
mixtures of diastereomers. The DFT-optimized structures of 1
and the two possible diastereomers of 1Me are shown in Figure
1, and their 11B and 1H NMR spectra are compared in Figures 2
and 3.
■
We recently demonstrated14 that strong bases, such as
bis(dimethylamino)naphthalene (Proton Sponge), can pro-
mote ammonia borane hydrogen release through anionic
dehydropolymerization reactions. We proposed5,14 that these
As reported,5 the 11B NMR spectrum (Figure 2a) of 1
consists of a triplet at −8.8 ppm and a quartet at −22.3 ppm in
a 1:2 ratio. The spectra of 1Me (Figure 2b) and 1Bz (Figure S1
in the SI) are similar, but, as predicted by the DFT/GIAO
chemical shift calculations, with peaks shifted slightly downfield
relative to 1 as a result of the N-alkylation. The internal BH2
−
reactions involve initial deprotonation to form BH3NH2 ,
followed by reaction of this anion with another AB to form the
−
BH3NH2BH3 anion. This new anion can then react with
successive equivalents of AB with H2 elimination to produce
H3BNH2BH2(NH2BH3)x chains. Reaction of only 3 equiv of
−
AB with VB (3 days at room temperature) allowed isolation
(74% yield) and structural characterization of the [VBH+] salt
protons of 1 appear as a single pentet at 1.78 ppm in its H
1
−
of the [BH3NH2BH2NH2BH3 ] anion (1). It was further
NMR spectrum, but because of 1Me and 1Bz both being
diastereotopic mixtures, their internal BH2 protons give rise to
two peaks, at 1.62 and 1.35 ppm for 1Me and at 1.85 and 1.77
ppm for 1Bz (Figures 3 and S1 in the SI). Unlike the BH2
protons, the CH3 protons of 1Me do not show diastereomeric
differences but appear as a single doublet (split by the adjacent
N−H) at 2.11 ppm.
While the cyclic dehydrotrimer of AB (CTB) has been
known for decades,17 no linear triborazanes have heretofore
been reported. Nevertheless, we found that the parent
triborazane 2 and its methyl derivative 2Me were readily
produced (Scheme 2) in 84 and 88% isolated yields by the
shown that 1 could be converted to both seven-membered
linear and chain-branched anionic oligomers by reaction with
an additional AB equivalent, thus providing strong evidence for
the proposed anionic chain growth mechanism.5
We now report that 1 can be produced in both higher yields
and larger scales with faster reaction times when NaHMDS is
used as the base (Scheme 1). In a typical preparation, the
reaction of 3 equiv of AB (33.0 mmol) with 1 equiv of
NaHMDS (11.0 mol) in a fluorobenzene solution for 24 h at
50 °C gave a 96% isolated yield (10.8 mol) of salt 1. The salt
was largely insoluble in fluorobenzene and could be completely
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1
Figure 3. Comparison of the H{11B} NMR spectra of 1 (top) and
1Me (bottom). y = residual protons in THF-d8.
Scheme 2. Syntheses of 2 and 2Me
Figure 1. DFT-optimized [B3LYP/6-311G(d)] geometries of 1 and
the R,R/S,S and S,R isomers of 1Me..
which the starting material was consumed (∼5−6 h) so that
workup could be immediately undertaken. When 2Me was
allowed to remain in solution for longer periods, decomposition
resulted to form a mixture of products that included
methyldiborazane (BH3NH(CH3)BH2N(CH3)H2)4 and meth-
ylamine borane.
Similar reactions of the sodium borazane salts with other
Lewis base hydrochlorides afforded base-capped borazane
chains. For example, when pyridine−HCl was reacted with 1
and 1Me in cold acetonitrile, H2 and NaCl were quickly
eliminated to form the pyridine-substituted derivatives 3 and
3Me (Scheme 3). Similar capped aminoborane oligomers (L−
H2BNH2BH3) have been recently reported as the product of
the reactions of Lewis bases with μ-aminodiborane.18
Figure 2. Comparisons of the 11B NMR spectra of (a) 1, (b) 1Me, (c)
2, (d) 2Me, (e) 3, and (f) 3Me.
glyme solution reactions of 1 and 1Me with NH4Cl and
CH3NH3Cl, respectively.
2 was soluble and stable in glyme and water at room
temperature and slightly soluble in THF and acetonitrile.
However, at even mildly elevated temperatures, 2 decomposed
in ethereal solvents to give 11B NMR spectra characteristic of
trigonal B−O compounds (singlets, ∼18−20 ppm).15 For the
reaction of 1Me with CH3NH3Cl, it was necessary to carefully
monitor the reaction by 11B NMR to determine the point at
The DFT-optimized structures of 2 and 3 and the two
possible diastereomers of both 2Me and 3Me are shown in
Figures 4 and S2 in the SI. In agreement with their proposed
linear structures, the 11B NMR spectra (Figure 2c−f) of all of
these compounds displayed two low-field triplets and a high-
field quartet in 1:1:1 ratios with the resonances of 2Me and
3Me again shifted downfield, as predicted by the DFT/GIAO
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Scheme 3. Syntheses of 3 and 3Me
1
Figure 5. Comparison of the H{11B} NMR spectra of 3 (top) and
3Me (bottom). * = residual H2O; x = residual protons in CD3CN.
Figure 6. Crystallographically determined structure of 2Me (R,S).
Selected distances (Å) and angles (deg): B1−N2, 1.5838(13); C7−
N2, 1.4820(13); N2−B3, 1.5819(13); B3−N4, 1.5766(13); C8−N4,
1.4845(12); N4−B5, 1.5749(13); B5−N6, 1.5898(14); N6−C9,
1.4820(13); B1−N2−C7, 111.58(8); B1−N2−B3, 115.65(7); C7−
N2−B3, 108.45(8); N2−B3−N4, 111.06(7); B3−N4−C8, 112.24(8);
C8−N4−B5, 108.90(7); B3−N4−B5, 112.68(7); N4−B5−N6,
108.41(7); B5−N6−C9, 113.43(8).
Figure 4. The N−B−N, B−N−B, and C−N−B angles (108−
115°) are all consistent with sp3 hybridization. In contrast to
the structures of 15 and H3BNH2BH2NH3,3 where the B−N
distances were greater at the terminal boron atoms (∼1.59 Å)
than at the internal boron atoms (∼1.57 Å), the B−N lengths
in 2Me are reasonably homogeneous, averaging 1.581 Å. This
value is similar to the B−N length in methylamine borane
[1.587(3) Å]19 but longer than the 1.549 Å average B−N bond
length reported for the methyldiborazane H3BN(CH3)HBH2N-
(CH3)H2.4
Intermolecular dihydrogen bonding between hydridic B−H
(δ−) and protonic N−H (δ+) hydrogen atoms has been shown
to be important in many boron−nitrogen−hydrogen com-
pounds, as characterized by B−H---H−N dihydrogen-bonding
distances of ∼1.7−2.2 Å and with H---H−N angles of 117−
171°, usually larger than the B−H---H angles, 95−120°.4,5,20
The ORTEP plot presented in Figure 7 shows the head-to-tail
orientation of two chains that is adopted by 2Me (R,S) in the
solid state, with B1−H hydrogen atoms on each chain directed
toward the N6−H or N4−H hydrogen atoms on the adjacent
chain. The most significant interactions are between the N6−
H6b and both B1−H1a and B1−H1c hydrogen atoms, with the
H6b---H1a and H6b---H1c distances [2.06(2) and 2.20(2) Å]
and the N6−H6b---H1a [151.7(11)°] and N6−H6b---H1c
[150.5(11)°] angles, as well as the H6b---H1a−B1 [97.7(7)°]
and H6b---H1c−Ba [90.9(8)°] angles, falling in the ranges
indicated above for B−H---H−N bonding.20
Figure 4. DFT-optimized [B3LYP/6-311G(d)] geometries of 2 and
the S,S/R,R and S,R isomers of 2Me.
chemical shift calculations, compared to those of 2 and 3. The
1
comparison of the H NMR spectra of 3 and 3Me in Figure 5
shows that the two sets of internal BH2 resonances observed in
3 (at 2.87 and 1.79 ppm) became four resonances in the
spectrum of 3Me, owing to its synthesis as a diastereomeric
mixture. However, despite the fact that 2Me should also be a
diastereomeric mixture, the expected two sets of BH2
resonances were not resolved and only three CH3 resonances
1
were evident in its H NMR spectrum (Figure S2 in the SI).
The proposed linear triborazane structure for 2Me was
confirmed by the crystallographic determination depicted in the
ORTEP diagram in Figure 6. As discussed earlier, 2Me is
formed as a mixture of diastereomers with the structure in
Figure 6 corresponding to the calculated R,S isomer shown in
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130 °C, and a final char yield of ∼45%.22 The thermal behavior
of 2 is notably different from that observed for CTB, which
decomposes via three exothermic steps between 150 and 220
°C.23 This difference suggests that 2 does not undergo
cyclization to CTB during its solid-state pyrolysis, which is in
agreement with recent computational work that predicted a
high activation energy of cyclization (49.4 kcal/mol).24
In noncoordinating solvents, solutions of 2 heated to 80 °C
also showed no trace of cyclization. After the solutions were
held at 100 °C for 12 h in fluorobenzene, 11B NMR analysis
showed the formation of equivalent small amounts of CTB and
AB but a larger amount of a white solid that, when dissolved in
DMSO, was shown by 11B NMR to contain 2, oligomerized AB,
and borohydride anion. Heating 2Me at 100 °C in
fluorobenzene similarly resulted in the formation of a ∼3:1
mixture of methylamine borane and N-methylcyclotriborazane
(Scheme 4).4b,c
Figure 7. ORTEP drawing showing interchain dihydrogen interactions
in 2Me (R,S). Selected distances (Å) and angles (deg): H6b---H1a,
2.06(2); H6b---H1c, 2.20(2); H4---H1a, 2.49(2); H4---H1b, 2.41(2);
N6−H6b---H1a, 151.7(11); H6b---H1a−B1, 97.7(7); N6−H6b---H1c,
150.5(11); H6b---H1c−B1, 90.8(8); N4−H4---H1a, 142.3(9); H4---
H1a−B1, 98.0(7); N4−H4---H1b; 144.4(9); H4---H1b−B1, 102.7(7).
Scheme 4. Chlorination Reactions of 2
Despite the fact that the enthalpy of dehydrogenative
cyclization of 2 to CTB was calculated to be −12.2 kcal/
mol,21 2 is surprisingly thermally stable. The TGA measure-
ments shown in Figure 8 showed that in the solid state 2
Recently, Shore et al. reported the synthesis of NH3BH2Cl
via the treatment of AB with anhydrous HCl at −40 °C. He
then showed that the reaction of NH3BH2Cl with sodium
amide produced CTB.25
Figure 8. TGA (left axis, blue ) and DSC (right axis, red ---)
analyses of 2. Measurements were carried out on 2.32 mg of 2 under
nitrogen with a ramp rate of 10 °C/min.
The reaction of 2 with 1 equiv of HCl in glyme at −30 °C
yielded a product (2Cl) that exhibited three triplets in its 11B
NMR spectrum (Figure 9b), with the disappearance of the
quartet in the spectrum of 2 and the emergence of a new triplet
at −6.6 ppm consistent with terminal-boron chlorination. The
DFT/GIAO-calculated shifts also strongly support the
formation of a 1-ClBH2(NH2BH2)2NH3 product. When a
second 1 equiv of HCl was added to a glyme solution
containing 2Cl at −30 °C, 11B NMR showed the disappearance
(Figure 9c) of the higher field triplet resonance and the
emergence of a new doublet at −1.6 ppm with the new
doublet−triplet−triplet pattern (Figure S4b in the SI) possible
for any of the 1,1-, 1,3-, and 1,5-Cl2B3N3H12 dichlorinated
products. A comparison of the experimentally observed shifts
for these resonances with the DFT/GIAO-calculated 11B NMR
chemical shifts for each isomer (Figure S5 in the SI) ruled out
formation of the 1,1-dichlorinated product, but the predicted
shifts for the 1,3 and 1,5 derivatives are nearly coincident and
thus either product is possible. Reactions with additional
undergoes a two-step decomposition with an initial ∼30%
weight loss beginning at ∼110 °C, followed by a second ∼30%
loss beginning at ∼142 °C. Decomposition is complete by 200
°C, with an overall char yield of 39%. A complete conversion to
boron nitride would correspond to an 84.2% char; thus,
decomposition must involve the loss of molecular fragments
larger than H2. The apparent endothermic peak in the DSC at
135 °C and the overlapping exothermic process peaking at
∼160 °C most probably arise from a combination of 2
decomposition and decomposition-product evaporation/sub-
limation that is accompanying the initial weight loss. The
second overlapping endothermic process near 170 °C could
then result from further product evaporation/sublimation
associated with the second weight loss event. The TGA of 2
is similar to that of AB, which showed an initial weight loss
beginning at 110 °C, followed by a second loss with onset at
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the U.S. Department of Energy and
the National Science Foundation for their support of this
project. We also thank the National Science Foundation for an
instrumentation grant (Grant CHE-0840438) used for the
purchase of the X-ray diffractometer employed in these studies
and Dr. Andrew McGhie for his assistance with thermal
analyses.
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, figures of DFT-
■
S
optimized geometries, tables of the coordinates and energies of
1
all DFT-optimized structures, the H NMR spectrum of 2Me,
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
1
the H and 11B NMR spectra of 1Bz, and the proton-coupled
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revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: lsneddon@sas.upenn.edu.
■
Notes
The authors declare no competing financial interest.
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