N-methyl-anilinoborane: Monomer, dimers, molecular structure, and rearrangement into μ-(N-methyl-anilino)diborane(6). a convenient route to μ-(Amino)diboranes(6)

Article abstract of DOI:10.1002/zaac.201000420

N-Methyl-anilinoborane exists as a dimer in the solid state, shown by X-ray structural analysis, whereas in solution, a monomer-dimer equilibrium is present (¹H, ¹¹B and ¹³C NMR spectroscopy). The phenyl groups are in trans positions in the crystalline dimer. In solution, both possible isomers of the dimer exist in a trans/cis ratio of 3:2, in agreement with the energies for calculated optimized geometries [B3LYP/6-311+G(d,p) level of theory]. In solution, the monomer is rather reactive and disproportionates completely after several days to give a 1:1 mixture of bis(N-methyl-anilino)borane and μ-(N-methyl-anilino)diborane(6). This observation led to a convenient high yield synthesis of μ-(amino)diborane(6) derivatives, starting from bis(amino)boranes and borane/THF. Copyright

Full text of DOI:10.1002/zaac.201000420

ARTICLE
DOI: 10.1002/zaac.201000420
N-Methyl-anilinoborane: Monomer, Dimers, Molecular Structure, and
Rearrangement into μ-(N-Methyl-anilino)diborane(6). A Convenient Route
to μ-(Amino)diboranes(6)
Bernd Wrackmeyer,*[a] Elias Molla,[b] Peter Thoma,[a] Elena V. Klimkina,[a]
Oleg L. Tok,[a] Tobias Bauer,[a] and Rhett Kempe[a]
Keywords: Aminoboranes; μ-(Amino)diboranes(6); NMR spectroscopy; X-ray diffraction; Boron
Abstract. N-Methyl-anilinoborane exists as a dimer in the solid state, [B3LYP/6-311+G(d,p) level of theory]. In solution, the monomer is
shown by X-ray structural analysis, whereas in solution, a monomer– rather reactive and disproportionates completely after several days to
dimer equilibrium is present (1H, 11B and 13C NMR spectroscopy). The give a 1:1 mixture of bis(N-methyl-anilino)borane and μ-(N-methyl-
phenyl groups are in trans positions in the crystalline dimer. In solu- anilino)diborane(6). This observation led to a convenient high yield
tion, both possible isomers of the dimer exist in a trans/cis ratio of synthesis of μ-(amino)diborane(6) derivatives, starting from
3:2, in agreement with the energies for calculated optimized geometries bis(amino)boranes and borane/THF.
products R1R2N–BH2 (e.g. R1 or R2 = Ph), monomers or dimers
Introduction
(Scheme 1b), are difficult to obtain in pure state,[2] and there-
Aminoboranes of the general formula R1R2N–BH2 (mono-
fore, their properties in solution and in the solid state are not
mers or dimers) are well known and readily available by vari-
well studied.[2,3a] However, such aminoboranes deserve inter-
ous standard procedures.[1–3] It was shown that they can also
est, since dimers, if present, can readily dissociate because of
be obtained by transition metal catalyzed dehydrocoupling
the reduced basicity of the nitrogen atoms. The monomers thus
starting from the corresponding amine borane adducts
formed may be more reactive than dialkylaminoboranes owing
R1R2NH–BH3.[4,5] Potential applications of aminoboranes fo-
to reduced B–N(pp)π interactions, considering the aryl group
cus on the presence of B–H bonds, which invite to studies of
as a π acceptor competing successfully with the three-coordi-
aminoboranes as precursors of BN ceramic materials.[6] Re-
nate boron atom for the lone pair of electrons at the nitrogen
cently, some aspects of the synthetic potential of aminoboranes
atom. In this work, we report the molecular structure of N-
have been investigated,[7,8] of which the Pd-catalyzed C–B
methyl-anilinoborane dimer (12) and its behavior in solution.
bond formation with aryl halides is particularly attractive.[7]
The latter part of this study prompted us to develop a new
The present knowledge on thermally induced reactions of
route to μ-(amino)diborane(6) derivatives.
secondary amines R1R2NH with Et3N–BH3 is summarized in
Scheme 1. In principle, this method appears to be the most
convenient route towards monomeric aminoboranes R1R2N–
BH2 in bulk quantities (Scheme 1a), if the groups R1 and R2
are secondary alkyl groups.[2] Sterically less demanding sub-
stituents lead to dimers (Scheme 1b). Problems are encoun-
tered if the rate of the reaction of R1R2N–BH2 with Et3N–BH3
is comparable to that of the starting amine R1R2NH with Et3N–
BH3, typical if one or both groups R1, R2 are aryl groups
(Scheme 1c).[2] Although appropriate changes in the experi-
Scheme 1. Thermally induced reaction of secondary amines with
mental procedure (e.g. a large excess of Et3N–BH3) help to
Et3N–BH3 to give aminoboranes, which may be monomers or dimers.
suppress the competitive formation of (R1R2N)2BH, the desired
In the case of R1 = Me, R2 = Ph, the reaction may proceed to give the
bis(N-methyl-anilino)borane.
* Prof. Dr. B. Wrackmeyer
Fax: +49-921-552157
E-Mail: b.wrack@uni-bayreuth.de
Results and Discussion
[a] Anorganische Chemie II
Universität Bayreuth
N-Methyl-anilinoborane, Structure and Properties
95440 Bayreuth, Germany
[b] Department of Chemistry
Jahangirnagar University
N-Methyl-anilinoborane (12) was obtained as described,[2]
Savar, Dhaka-1342, Bangladesh
and colorless crystals, grown from hexane, were studied by
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B. Wrackmeyer et al.
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X-ray diffraction. The molecular structure (Figure 1) shows a From the calculations of the optimized geometries, it follows
dimer, in which the phenyl groups are in trans positions. Inter- that trans-12 is slightly more stable than cis-1'2, and the energy
molecular interactions appear to be negligible. All bond of two molecules of 1 is slightly higher than that of 12. This
lengths and angles are in the expected ranges, similar as in reflects approximately the situation in solution, although the
other known molecular structures of aminoborane dimers.[3a,9] differences in energy are small (< 4 kJ·mol–1). After several
The four-membered ring is planar within the experimental er- hours, new signals can be detected, which arise from rear-
ror. Interestingly, the optimized calculated [B3LYP/6- rangement into bis(N-methyanilno)borane 2 and μ-(N-methyl-
1
311+G(d,p) level of theory][10] geometry of 12 reveals a non- anilino)borane(6) 3 (vide infra). The H{11B} NMR spectrum
planar ring (folding angle 15.1°), where all bond lengths and of 3 shows three 1H(BH) NMR signals in a ratio of 2:2:1, with
1
angles are in well agreement with the experimental data. Simi- the resonance for the unique bridging H nucleus at lowest
larly, the four-membered ring in the dimer of dimethylamino- frequency (see Exp. Sect.). This signal is split into a quintet
2
borane [Me2N–BH2]2 is planar in the solid state,[9] whereas the with J(1H(μ)B1H) = 8.5 Hz, in agreement with the calculated
1
calculated gas-phase geometry was found to be a minimum in values (–8.3 Hz and –8.7 Hz). Both H NMR signals for the
energy with a non-planar four-membered ring (folding angle terminal B–1H nuclei appear as doublets (2J(1H(μ)B1H) =
of 5°). In the parent aminoborane dimer [H2N–BH2]2, the cal- 8.5 Hz) without further fine structure. This is surprising at first
culated gas phase geometry reveals a planar ring. Although sight, since one would expect further splitting owing to two
packing forces in the crystal lattice may account for differences bond coupling between the different terminal B–1H nuclei.
between calculated and experimental structures, the driving However, according to calculations this coupling constant is
forces for non-planar four-membered rings are not readily ap- predicted to be small (–0.1 Hz), and therefore, the splitting
parent.
would not be resolved experimentally.
Figure 1. Molecular structure of 12 (ORTEP, 40 % probability, hydro-
gen atoms were omitted, except for the hydrogen atoms linked to bo-
ron). Selected bond lengths /Å and angles /°: N1–C1 1.4642(16), N1–
C7 1.4924(17), N1–B1 1.6155(18), C1–N1–C7 107.59(10), C1–N1–
B1 117.71(10), C7–N1–B1 113.30(11), C1–N1–B1 118.17(11), C7–
N1–B1 112.55(10), B1–N1–B1 86.56(10), N1–B1–N1 93.44(10).
When crystalline material of pure 12 was dissolved in
1
[D8]toluene, H-, 11B-, and 13C NMR spectra showed more
1
signals than expected. In the H NMR spectrum (Figure 2),
recorded immediately after dissolution, selective 11B decou-
pling at δ11B = 40 (for the monomer 1) led to sharpening of
1
two H(BH) NMR signals, as expected for the two different
1H(BH) nuclei. These signals are significantly broadened ow-
ing to slow rotation of the amino group about the B–N axis.
The energy of activation ΔG≠ for this process can be
low.[12] Thus, for the(3p1a0reKn) t aminoborane H2N–BH2 this rota-
Figure 2. 399.8 MHz 1H NMR spectra of freshly dissolved 12 (in
1
estimated[11] to ΔG≠
= 61 1 kJ·mol–1, which is rather
[D6]toluene, 23 °C), showing the region of H(BH) NMR signals.
tional energy barrier was calculated as ΔG≠
≈
117–
The 11B NMR spectra (Figure 3), measured within less than
143 kJ·mol–1, using different theoretical methods.[12a] The 10 min after dissolving crystals of 12, indicated the presence
1
analogous selective H{11B} experiment for δ11B = 3 (for the of the dimer 12 and the monomer 1, and there were already
dimer 12) caused sharpening of three 1H(BH) NMR signals, of fairly weak signals, assigned to bis(N-methyl-anilino)borane 2,
which two are in a 1:1 ratio [doublets with 2J(1HB1H) = 3.8 Hz and to μ-(N-methyl-anilino)diborane(6) 3. All NMR spectro-
(calcd. –3.9 Hz)] and belong to the isomer cis-1'2 as indicated. scopic assignments are in full agreement with literature
The dimers trans-12 and cis-1'2 are present in a ratio of ≈ 3:2. data,[13] and all chemical shifts agree well with those calculated
402
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A Convenient Route to μ-(Amino)diboranes(6)
on the basis of optimized geometries.[14] The liquid compounds μ-(Amino)diborane(6) Derivatives
2 and 3 were definitely not present in the solid 12, which had
Apparently, aminoboranes of type 1 and 2, provide an easy
a sharp melting point (65–66 °C, as reported[2]). The signals
for 2 and 3 were growing at the expense of those for dimers
and the monomer, and finally after several days, conversion of
1 (or its dimers) into a 1:1 mixture of 2 and 3 was almost
complete (> 95 %) (Scheme 2).
access to μ-(amino)diborane(6) derivatives, for which various
synthetic methods have been proposed, most of them tedious
and rather inefficient,[15] involving e.g. the handling of gaseous
diborane(6). There is a renewed interest in μ-(amino)dibo-
ranes(6) in the context of H3N–BH3 as a potential hydrogen
storage material[16] as well as μ-(amino)diborates(1–) as useful
ligands, an alternative to [BH4]–, in transition metal
complexes.[17] Thus, recently a straightforward synthesis of the
parent μ-(NH2)B2H5 from H3N–BH3 and BH3 in THF has been
reported.[18] Since bis(amino)boranes of type 2 are readily
accessible,[1–3] fairly unreactive, and stable for prolonged peri-
ods of time at ambient temperature, we performed preliminary
experiments in order to convert these compounds into the re-
spective μ-(amino)diboranes(6), as shown here for bis(diethyl-
amino)- (4a) and dipyrrolidinoborane (4b) (Scheme 3).
1
Figure 3. 128.4 MHz H coupled 11B NMR spectrum of the dimer 12
(in [D8]toluene, 23 °C), measured less than 10 min after dissolution,
showing a triplet signal for 1, a doublet signal for 2, a poorly resolved
triplet for 12, and the typical doublet signals of a triplet for 3. A quartet
signal (unlabeled) may arise from an amine borane adduct
(Me(Ph)NH–BH3) because of traces of hydrolysis.
Scheme 3. μ-(Diethylamino)- and μ-(pyrrolidino)diborane(6) from the
reactions of bis(amino)boranes or aminoborane dimers with BH3/THF.
Scheme 2. Rearrangement of N-methyl-anilinoborane 1 into μ-N-
methyl-anilino-diborane.
The fast reaction of 4a,b with BH3/THF under mild condi-
The inspection of 13C NMR spectra (Figure 4) gave analo- tions suggests that the first step involves exchange to give
1
gous information, which confirms the evidence from H NMR R1 N–BH2 as a short lived monomer, which is trapped immedi-
2
spectra (Figure 2) that 12 in solution is accompanied by the ately by BH3 to give μ-(amino)diboranes(6) 6a,b. Even with-
isomer 1'2, in which the phenyl groups occupy cis positions.
out optimizing the reaction conditions, only minor quantities
Figure 4. 100.4 MHz 13C{1H} NMR spectrum of a solution of 12 in [D8]toluene, 23 °C), showing the regions of the 13C(Ph-ipso) and the
13C(NMe) resonances, 10 min (bottom traces) and 12 h after dissolution (top traces). Spurious signals (unlabeled) appear after several hours.
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B. Wrackmeyer et al.
ARTICLE
borane(6) (6b) were prepared by adding three equivalents of BH3/THF
to the respective bis(amino)borane at –78 °C, and warming the mixture
to room temp. The reactions were essentially complete after 1 h at
room temp. NMR spectra (1H, 11B, and 13C NMR) were measured
from solutions in [D8]toluene with samples in 5 mm tubes at 23
1 °C, using Bruker ARX-250 and AMX-500, and Varian Inova 400
spectrometers. Chemical shifts (ppm) are given with respect to Me4Si
[δ1H (C6D5CD2 H) = 2.08; δ13C (C6D5CD3) = 20.4], external BF3–
OEt2 [δ11B = 0 for Ξ(11B) = 32.083971 MHz]; chemical shifts δ1H are
given to 0.03, δ13C to 0.1; and δ11B to 0.3. The melting points
(< 5 %) of the aminoborane dimers 5a,b were formed, accord-
ing to the 11B NMR spectra. In contrast, starting from the dim-
ers 5a,b, also readily available,[1–3] the analogous conversion
into 6a,b proceeded slowly (up to 24 h), even upon heating.
Under these conditions, the reactions were accompanied by
formation of (BuO)2BH and B(OBu)3 due to THF-cleavage.
The proposed intermediacy of R1 N–BH2 as a monomer is fur-
2
ther supported by the observation that sBu2N–BH2 7, which
has no tendency for dimerization,[2] reacts smoothly at room
temperature, in spite of the bulky sBu groups, with BH3/THF (uncorrected) were determined using a Büchi 510 melting point appa-
ratus. Calculations were carried out using the Gaussian 03 (Revi-
sion.B02) program package[10] by optimizing geometries at the
B3LYP/6-311+G(d,p) level of theory. The structures correspond to
minima in energy as indicated by the absence of imaginary frequencies
by computing force constants and vibrational frequencies. Chemical
shifts (GIAO[19]) and coupling constants[20] were calculated at the same
level of theory.
to afford μ-(sBu2N)B2H5 8 in essentially quantitative yield
after several hours (Scheme 4). It should be noted that 7 sur-
vives in a toluene solution heating at 110 °C for several days,
without any trace of disproportionation. The same is true for
the dimers 5a,b. Therefore, it can be safely assumed that the
N-methyl-anilino group in 1 favors disproportionation into 2
and 3. In previous work[2] (see also Scheme 1), formation of
μ-(N-methylanilino)diborane(6) 3 was not observed owing to
the presence of Et3N which, in the absence of a stronger base
like R1R2NH, traps all “free” BH3. In comparison with Et3N,
THF is a weaker base towards BH3 and releases BH3 for reac-
tions with amino- and bis(amino)boranes.
1
12: m.p. 65–66 °C, H NMR (399.8 MHz): δ = 2.51 (s, 6 H, NMe),
3.69 (s, 4 H, BH2) 7.5–6.80 (m, NPh, overlap with signals from mono-
mer and other dimer). 13C NMR (125.8 MHz): δ = 56.2 (NMe), 154.5
(NPh-i), 124.7 (NPh-o), 129.0 (NPh-m), 124.3 (NPh-p). 11B NMR
1
(128.4 MHz): δ = 4.3 (t, J(11B,1H) = 117 Hz).
1
1'2: H NMR (399.8 MHz): δ = 2.78 (s, 6 H, NMe), 3.81, 3.48 [d, d,
2
4 H, J(1HB1H) = 3.8 Hz, BH2], 7.5–6.80 (m, NPh, overlap with sig-
nals from monomer and other dimer). 13C NMR (125.8 MHz): δ =
56.3 (NMe), 155.9 (NPh-i), 125.8 (NPh-o), 128.9 (NPh-m), 124.6
(NPh-p). 11B NMR (128.4 MHz): δ = 4.3.
1
1: H NMR (399.8 MHz): δ = 2.50 (s, 3 H, NMe), 4.68, 4.84 (s, s,
br, 2 H, BH2), 7.5–6.80 (m, NPh, overlap with signals from dimers).
13C NMR (125.8 MHz): δ = 43.4 (NMe), 149.5 (NPh-i), 121.5 (NPh-
o), 128.8 (NPh-m), 125.5 (NPh-p). 11B NMR (128.4 MHz): δ = 40.0
Scheme 4. Quantitative conversion of the strictly monomeric aminobo-
rane 7 into μ-(amino)diborane(6) (8).
1
(t, J(11B,1H) = 134 Hz).
2: 1H NMR (399.8 MHz): δ = 2.70 (s, 6 H, NMe), 3.71 (s, 1 H, BH),
6.73–7.10 (m, 10 H, NPh). 13C NMR (125.8 MHz): δ = 39.0 (NMe),
149.9 (NPh-i), 122.7 (NPh-o), 128.8 (NPh-m), 125.8 (NPh-p). 11B
NMR (d 128.4 MHz): δ = 29.6 (1J(11B,1H) = 122 Hz).
Conclusions
The title compound is a dimer in the solid state, whereas
in solution a monomer–dimer equilibrium is present. Reduced
BN(pp)π bonding in the N-methyl-anilinoborane monomer ap-
parently gives rise to disproportionation. In the absence of an-
other base, borane BH3 is trapped by bis(N-methyl-anilino)bo-
rane and N-methyl-anilinoborane to give finally μ-(N-methyl-
anilino)diborane(6). Using BH3/THF as the source for borane,
it proved possible to convert readily available, otherwise fairly
unreactive and stable bis(amino)boranes into the respective re-
active μ-(amino)diborane(6) derivatives.
3: 1H NMR (399.8 MHz): δ = 0.80 (quintet, 1 H, 2J(1HB1H) = 8.5 Hz
2
μ-HB2), 1.92, 2.05 (d, d, 4 H, J(1HB1H) = 8.5 Hz BH2), 2.55 (s, 3 H,
NMe), 6.70–7.00 (m, 5 H, NPh). 13C NMR (125.8 MHz): δ = 56.4
(NMe), 152.5 (NPh-i), 122.7 (NPh-o), 128.8 (NPh-m), 125.8 (NPh-p).
11B NMR (128.4 MHz): δ = –17.0 (t, d, 1J(11B,1H) = 131 Hz, 29 Hz).
1
6a: H NMR (399.8 MHz): δ = 0.07 (m, 1 H, μ-HB2), 1.30 (m, 4 H,
BH2), 2.56, 0.89 (q, t, 4 H, 6 H, NEt2). 13C NMR (125.8 MHz): δ =
1
51.6, 10.5 (NEt2). 11B NMR (89.3 MHz): δ = –18.7 (t, d, J(11B,1H) =
128.6 Hz, 31.3 Hz).
6b: 13C NMR (125.8 MHz): δ = 61.3, 24.7 (pyrrolidine). 11B NMR
1
(89.3 MHz): δ = –18.3 (t, d, J(11B,1H) = 131 Hz, 29 Hz).
Experimental Section
Starting Materials, Measurements, and Calculations
1
8: H NMR (399.8 MHz): δ = 0.28 (m, 1 H, μ-HB2), 1.49 (m, 4 H,
H2B; overlap with signals from sec-butyl groups). 13C NMR
(125.8 MHz): δ = 65.7 29.6, 18,9, 12.2, 12.1 (N-sBu; some signals are
broadened, preventing the resolution of all signals for the two diastere-
N-methyl-anilinoborane (1), bis(N-methyl-anilino)borane (2), bis(di-
ethylamino)borane (4a), dipyrrolidinoborane (4b), diethylaminoborane
dimer (5a), pyrrolidinoborane dimer (5b), and bis(sec-butyl)aminobo-
rane (7) were prepared as described,[2] and a THF solution of BH3/
THF (1 m) (Aldrich) was used as received. The 1:1 mixture containing
2 and μ-(N-methyl-anilino)diborane(6) (3) was obtained as a colorless
oil by dissolving crystals of 12 in toluene, keeping the solution at r. t.
1
omers). 11B NMR (89.3 MHz): δ = –20.1 (t, d, J(11B,1H) = 128 Hz,
33 Hz).
X-ray Structural Analysis of 12
[21]
for 8 d, and removing all readily volatile materials in vacuo The X-ray crystal structural analysis of 12
was carried out for a
(10–1 Torr). Samples of μ-(diethylamino)- (6a) and μ-(pyrrolidino)di-
single crystal (selected in perfluorinated oil[22] at room temperature) at
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A Convenient Route to μ-(Amino)diboranes(6)
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada,
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133(2) K using a STOE IPDS II system equipped with an Oxford
Cryostream low-temperature unit; wavelength: λ = 0.71069 Å. For-
mula weight: 360.15. Crystal system: monoclinic; space group: P21/
n. Unit cell dimensions: a = 5.8260(8) Å, b = 15.8580(16) Å, c =
7.80.10(8) Å, α = 90°, β = 110.161°(8), γ = 90°. Volume V =
676.56(13) Å3. Z = 2. Absorption coefficient μ = 0.067 mm–1. F(000):
256. Crystal size: 0.42 × 0.39 × 0.34 mm. Theta range for data collec-
tion: 1.28 to 26.145°. Index ranges: –7 ≤ h ≤ 7, –19 ≤ k ≤ 19, –9 ≤ l
≤ 9; reflections collected: 8869, independent reflections: 989 [R(int) =
0.0544]. Completeness to θ = 25.65°: 99.4 %. Data/restraints/parame-
ters: 1271/0/96; goodness-of-fit on F2: 1.028. Final R indices [I>2σ
(I)] R1 = 0.0415, wR2 = 0.1118; R indices (all data): R1 = 0.0544,
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Acknowledgement
This work was supported by the Deutsche Forschungsgemeinschaft. E.
M. thanks the Alexander-von-Humboldt-Stiftung for a scholarship. We
gratefully acknowledge the gift of some samples of aminoboranes from
the late Professor R. Köster.
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Received: November 24, 2010
Published Online: January 12, 2011
Z. Anorg. Allg. Chem. 2011, 401–405
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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