Metal-free dehydrogenation of amine-boranes by an N-heterocyclic carbene

Article abstract of DOI:10.1039/c3dt32988g

The dehydrogenation of primary and secondary amine-boranes (RNH ₂·BH₃ and R₂NH·BH₃; R = alkyl groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-ⁱPr₂C₆H₃) as a stoichiometric dehydrogenation agent. In the case of primary amine-boranes, carbene-bound adducts IPr·BH₂-NH(R)-BH₃ were obtained in place of the desired polymers [RNH-BH₂]_n. The secondary amine-borane ⁱPr₂NH·BH₃ participated in dehydrogenation chemistry with IPr to afford the aminoborane [ⁱPr₂NBH₂] and the dihydroaminal IPrH ₂ as products. Attempts to induce H₂ elimination from the arylamine-borane DippNH₂·BH₃ yielded a reaction mixture containing the known species IPr·BH₂NHDipp, IPr·BH₂NH(Dipp)-BH₃, free DippNH₂ and IPrH₂. The new hindered aryl-amine borane adduct Ar*NH ₂·BH₃ [Ar* = 2,6-(Ph₂CH) ₂-4-MeC₆H₂] underwent a reaction with IPr to give IPr·BH₃ and free Ar*NH₂, consistent with the presence of a weaker N-B dative bond in Ar*NH₂· BH₃ relative to its less hindered amine-borane analogues. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt32988g

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Metal-free dehydrogenation of amine–boranes by an
N-heterocyclic carbene†
Cite this: Dalton Trans., 2013, 42, 4625
Kyle J. Sabourin, Adam C. Malcolm, Robert McDonald, Michael J. Ferguson and
Eric Rivard*
The dehydrogenation of primary and secondary amine–boranes (RNH2·BH3 and R2NH·BH3; R = alkyl
groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-iPr2C6H3)
as a stoichiometric dehydrogenation agent. In the case of primary amine–boranes, carbene-bound
adducts IPr·BH2–NH(R)–BH3 were obtained in place of the desired polymers [RNH–BH2]n. The secondary
amine–borane iPr2NH·BH3 participated in dehydrogenation chemistry with IPr to afford the aminoborane
[iPr2NvBH2] and the dihydroaminal IPrH2 as products. Attempts to induce H2 elimination from the aryl-
amine–borane DippNH2·BH3 yielded a reaction mixture containing the known species IPr·BH2NHDipp,
IPr·BH2NH(Dipp)–BH3, free DippNH2 and IPrH2. The new hindered aryl-amine borane adduct Ar*NH2·BH3
[Ar* = 2,6-(Ph2CH)2-4-MeC6H2] underwent a reaction with IPr to give IPr·BH3 and free Ar*NH2, consistent
with the presence of a weaker N–B dative bond in Ar*NH2·BH3 relative to its less hindered amine–borane
analogues.
Received 12th December 2012,
Accepted 9th January 2013
DOI: 10.1039/c3dt32988g
www.rsc.org/dalton
Introduction
Amine–boranes (R3N·BH3; R = alkyl, aryl or H) represent a
widely explored chemical class1 with considerable recent atten-
tion devoted to the study of the parent system H3N·BH3 as a
chemical source of H2 for fuel cell technologies.2 In addition,
lithium amidoborate salts Li[NR2BH3] have been used in
organic chemistry as easy-to-handle and selective reducing
(H−) agents.3 Recently, the Manners group reported the syn-
thesis of novel linear B–N polymers [RNH–BH2]n via the con-
trolled dehydrogenative coupling of primary amine–boranes,
such as MeNH2·BH3, in the presence of Ir and Rh com-
plexes.4,5 This discovery not only represents an exciting
addition to the growing field of inorganic polymers,6 but
offers a new route for the high yield preparation of boron
nitride ceramics via the thermolysis of well-defined B–N
macromolecules.7
Scheme 1 Metal-catalyzed dehydrocoupling of amine–boranes ([M] = tran-
sition metal catalyst) and potential N-heterocyclic carbene-mediated dehydro-
genation chemistry.
nature of the B–N products formed was not investigated.8 If
related chemistry can transpire with primary amine–borane
adducts RNH2·BH3 then this would represent a potential
metal-free route to polyaminoboranes [RNH–BH2]n (Scheme 1).
In this paper we explore the use of N-heterocyclic carbenes
(NHCs) for the dehydrogenation of various amine–borane sub-
strates. Roesky and co-workers obtained evidence for the dehy-
drogenation of the parent adduct H3N·BH3 in the presence of
the carbene ItBu (ItBu = [(HCNtBu)2C:]), as the formation of
the hydrogenated aminal ItBuH2 was detected; however the
Results and discussion
Dehydrogenation of primary alkyl amine–borane adducts
Department of Chemistry, University of Alberta, 11227 Saskatchewan Dr., Edmonton,
AB, Canada, T6G 2G2. E-mail: erivard@ualberta.ca; Fax: +1 780 492 8231;
Tel: +1 780 492 4255
†CCDC 902241–902244. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt32988g
When the dehydrocoupling of amine–boranes is instigated by
transition metal catalysts, further loss of H2 from the B–N
polymers (e.g. [MeNH–BH2]n) or oligomers can occur with
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prolonged reaction times and/or elevated temperatures to give
the N-substituted borazines (e.g. [MeNBH]3); this process is
both thermodynamically and entropically favored.4,9 Given our
prior work with the N-heterocyclic carbene IPr (IPr
=
[(HCNDipp)2C:]; Dipp = 2,6-iPr2C6H3),10 we explored this
species as a dehydrogenation agent for the metal-free dehydro-
coupling of various amine–borane adducts (Scheme 1).11
We began our studies by combining methylamine borane
MeNH2·BH3 with one equivalent of IPr in toluene. Analysis of
1
the crude product mixture by H NMR spectroscopy (in C6D6)
revealed that the complete consumption of IPr had transpired
and that the expected dihydroaminal (HCNDipp)2CH2-
(IPrH2)12 had formed, consistent with the carbene-induced
dehydrogenation of MeNH2·BH3. However, in addition to
IPrH2, a second carbene-containing product was detected by
NMR spectroscopy suggesting that an alternative reaction
pathway to what is outlined in Scheme 1 had occurred. 11B
NMR data corroborated this finding as two new closely-spaced
broad resonances were detected at −14.0 and −14.7 ppm,
while the anticipated polyaminoborane [MeNH–BH2]n would
have yielded a broad 11B NMR resonance at −6.7 ppm.4a The
IPrH2 co-product was separated from the reaction mixture by
Fig. 1 Thermal ellipsoid plot (30% probability level) of IPr·BH2NH(Me)–BH3 (1).
All hydrogen atoms on the IPr unit and toluene solvate molecules have
been omitted for clarity. Selected bond lengths (Å) and angles (°): C(1)–B(1)
1.6128(18), B(1)–N(3) 1.5692(19), N(3)–B(2) 1.598(2), N(3)–C(4) 1.4825(18),
N(3)–H 0.949(19), B–H 1.105(17) to 1.18(2); C(1)–B(1)–N(3) 113.52(10), B(1)–
N(3)–C(4) 108.81(11), B(1)–N(3)–B(2) 116.07(12), B(2)–N(3)–C(4) 109.60(13);
C(1)–B(1)–N(3)–B(2) torsion angle = 62.49(16).
extraction with hexanes to leave a single carbene-containing B–N distance in 1 is identical within experimental error to the
product as a white solid. Recrystallization of this material from related internal B–N bond in MeNH–BH2–N(Me)H–BH3 [1.5730
(19) Å].13b Interestingly, compound 1 can be formally regarded
a toluene–hexanes solution (10 : 1) at −35 °C afforded colorless
crystals that were identified as the carbene-bound B–N–B as a donor–acceptor adduct of an aminoborane H2BNMeH
complex, IPr·BH2NH(Me)–BH3 (1) (Scheme 2; Fig. 1); relevant unit, that is itself derived from the dehydrogenation of
MeNH2·BH3.
X-ray crystallographic data is listed in Table 1.
As shown in Fig. 1, IPr·BH2NH(Me)–BH3 (1) adopts a gauche
In order to explore the generality of the carbene-induced
CIPr–B–N–B arrangement [torsion angle = 62.49(16)°] with a dehydrogenation chemistry described above, IPr was allowed
to react with the more hindered primary amine–borane
slightly elongated terminal B(2)–N(3) distance [1.598(2) Å] in
relation to the internal B(1)–N(3) linkage [1.5692(19) Å]. The iPrNH2·BH3. As with the methylated analogue MeNH2·BH3, for-
terminal B–N bond in 1 is similar in length to the dative B–N mation of the expected hydrogenated product IPrH2 was
interaction in MeNH2·BH3 [1.5936(13) Å],13a while the internal
observed and the NMR spectra of the isolated co-product were
consistent with the formation of the N-heterocyclic carbene
complex IPr·BH2N(iPr)H–BH3 (2). Compound 2 gave similar
spectral parameters as 1 and satisfactory combustion analyses
(C, H and N) were obtained. Despite repeated attempts, crystals
of 2 suitable for the production of high quality X-ray diffrac-
tion data could not be grown.
Scheme 2 outlines a potential reaction sequence for the for-
mation of 1 and 2 from the interaction of RNH2·BH3 with IPr
i
(R = Me and Pr). The overall reaction involves the generation
of equimolar quantities of IPrH2 and IPr·BH2NH(R)–BH3 with
the extrusion of free amine RNH2. The presence of a 1 : 1 ratio
between IPrH2 and the carbene–borane products 1 and 2 was
confirmed by 1H NMR spectroscopic analysis of the reaction
i
1
mixtures, while the formation of PrNH2 was noted in the H
and 13C{1H} NMR spectra when iPrNH2·BH3 and IPr were com-
bined in a J. Young NMR tube (in C6D6). Thus far, we have
been unable to detect or prepare the putative aminoborane
intermediates IPr·BH2NHR (R = Me and iPr) outlined in
Scheme 2.14
t
The tert-butylated adduct BuNH2·BH3 was then allowed to
react with IPr in toluene to verify the potential role of
increased steric bulk on the chemistry described above.
Scheme 2 Overall reaction for the synthesis of IPr·BH2NHR–BH3 (R = Me and
iPr; 1 and 2) with postulated reaction steps listed as eqn (1)–(3).
4626 | Dalton Trans., 2013, 42, 4625–4632
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Table 1 Selected crystallographic data for compounds 1, 3, 6 and 7
1
3·1.5THF
6
7·DCE
Empirical formula
Mr
C
35H53B2N3
C37H63B2N3O1.5
595.52
C12H22BN
191.12
C35H36BCl2N
552.36
537.42
T/K
λ/Å
173(1)
173(1)
173(1)
173(1)
1.54178 [Cu Kα]
Monoclinic
P21/n
1.54178 [Cu Kα]
Monoclinic
P21/n
0.71073 [Mo Kα]
1.54178 [Cu Kα]
Tetragonal
P42/mbc
Crystal system
Space group
Trigonal
ˉ
R3
a/Å
b/Å
c/Å
14.5000(2)
13.8866(2)
17.5606(2)
100.3528(6)
3478.36(8)
4
16.8935(5)
13.6955(4)
17.7459(5)
107.4379(18)
3917.1(2)
15.4951(12)
17.2578(2)
27.816(2)
20.4652(3)
β/°
V/Å3
5783.9(8)
18
0.988
0.055
6095.18(13)
8
1.204
Z
4
Dc/g cm−3
1.026
0.435
1.010
0.452
μ/mm−1
2.082
Crystal size/mm3
2θ Range for data collection/°
Index ranges, hkl
0.35 × 0.24 × 0.22
7.30–140.40
−17 to 17, −16 to 16,
−21 to 21
23 523
6657 (Rint = 0.0154)
0.9123–0.8623
6657/0/386
0.0488
0.55 × 0.32 × 0.10
8.30–137.80
−20 to 20, −16 to 16,
−21 to 21
0.49 × 0.32 × 0.13
5.26–51.64
−18 to 18, −18 to 18,
−33 to 33
0.27 × 0.19 × 0.14
7.24–140.10
−21 to 21, −21 to 21,
−20 to 22
Reflections collected
Independent reflections
Range of transmission factors
Data/restraints/parameters
R1 [I > 2σ(I)]a
26 141
13 920
39 954
7069 (Rint = 0.0358)
0.9554–0.7885
7069/0/349
0.0454
0.1344
0.205/−0.182
2343 (Rint = 0.0301)
0.9928–0.9735
2343/0/147
0.0406
0.1114
0.140/−0.172
2989 (Rint = 0.0180)
0.7536–0.6053
2898/0/206
0.0457
0.1351
0.593/−0.755
wR2 [all data]a
0.1515
0.368/−0.244
Δρ/e Å−3
a R1 = Σ||Fo| − |Fc||/Σ|Fo|; wR2 = [Σw(Fo2 − Fc2)2/Σw(Fo4)]1/2
.
Evidence for the dehydrogenation of tBuNH2·BH3 was noted by the increased steric bulk and electron donating nature of the
NMR spectroscopy as IPrH2 was identified12 as the major tBu group installed at nitrogen.
species after the volatiles were removed in vacuo; in addition,
ca. 10% of a new NHC-containing product was noted as a
minor product in the 1H NMR spectrum. The latter species
was identified as the expected B–N–B adduct IPr·BH2N(tBu)H–
BH3 (3) by comparison of the observed spectral parameters
with those obtained from an independently prepared sample
(vide infra). The formation of small quantities of 3 suggested
that the chemistry outlined in Scheme 2 was transpiring to a
t
significantly reduced degree for BuNH2·BH3 in comparison
with the less hindered analogues, MeNH2·BH3 and
ð4Þ
t
iPrNH2·BH3. When the reaction between BuNH2·BH3 and IPr
was monitored in situ by 11B NMR spectroscopy, a number of
In order to confirm the formation of IPr·BH2NH(tBu)–BH3
previously known products could be detected (eqn (4)), includ- (3) as a minor species during the abovementioned reaction
ing: the monomeric aminoborane [tBuNHvBH2] (35.2 ppm, t, between tBuNH2·BH3 and IPr, IPr·BH2NH(tBu)–BH3 (3) was
1
1JBH = 125 Hz, 7%),15a [tBuNBH]3 (25.9 ppm, d, JBH = 115 Hz, synthesized according to a related procedure used to generate
30%),15a,b IPr·BH2N(tBu)H–BH3 (−17.9, br, coincident BH2 and IPr·BH2NH2–BH3.16f Specifically, the tert-butylated complex 3
t
BH3 resonances, 12%, vide infra), BuNH2·BH3 (−21.9 ppm, q, was obtained as a moisture-sensitive solid in a 61% yield via
1JBH = 99 Hz, 35%), (tBuNH)B2H5 (−25.8 ppm, m, 8%),15c and the ring-opening of (tBuHN)B2H5 (4)16f with IPr (eqn (5)). Con-
two unknown species [−8.3 ppm (br s, 5%) and a pentet reson- clusive evidence for the formation of 3 was obtained by single-
ance at −36.4 ppm (1JBH = 82 Hz, 4%)15d consistent with a crystal X-ray diffraction (Fig. 2), which revealed the presence of
−
BH4 anion]. From this product distribution, and the for- a similar gauche disposed CIPr–B–N–B chain in 3 as in its
mation of IPrH2, it appears that the dehydrogenation of methylated congener 1. As with compounds 1 and 2, the proxi-
tBuNH2·BH3 occurred (eqn (1), Scheme 2). The presence of mal Dipp groups within the IPr ligand in 3 exhibit restricted
t
[tBuNBH]3 and BuNH2·BH3 is in line with a prior observation rotation on the NMR timescale, as evidenced by 1H and
t
that [tBuNHvBH2] converts into [tBuNBH]3 and BuNH2·BH3 13C{1H} NMR spectroscopy. In addition, the central BH2 hydro-
over time via transfer hydrogenation chemistry.15a,16e Thus it gen atoms in 3 exist in magnetically distinct environments
seems that further carbene coupling of IPr to transient (δ = 2.14 and 2.29 ppm; 1H NMR spectroscopy); a similar
[tBuNHvBH2] (eqn (2), Scheme 2) is largely suppressed due to spectral effect was noted in MeNH2·BH2NMe(H)–BH3.13b
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Dehydrogenation of the secondary amine–borane adduct
iPr2NH·BH3
It is evident from the preceding section that the steric bulk at
nitrogen plays an important role in dictating the outcome of
the carbene-induced dehydrogenation chemistry. Interestingly,
the reaction between the hindered secondary amine–borane
iPr2HN·BH3 and IPr yields IPrH2 and the known monomeric
aminoborane [iPr2NvBH2]18 after 24 h in toluene at room
temperature (eqn (6)). Due to the volatility of [iPr2NvBH2],
IPrH2 can be isolated from the reaction mixture in a 99% yield
by simply removing [iPr2NvBH2] in vacuo. This synthesis rep-
resents a significant improvement over our prior route to
IPrH2, via the reaction between [IPrH]Cl and Li[HBEt3], which
generates variable quantities of involatile IPr·BEt3 by-product
that is challenging to separate from IPrH2.12 The formation of
[iPr2NvBH2] as the sole B–N product implies that this species
is too weak of a Lewis acid to bind IPr. In support of this
notion, attempts to form IPr·BH2NiPr2 by the direct reaction of
excess IPr with in situ generated [iPr2NvBH2] (prepared via the
RhI catalyzed dehydrogenation of iPr2HN·BH3 in refluxing
toluene)18 failed to give any observable reaction. This result
suggests that if secondary amino–borane adducts IPr·BH2NR2
are formed in the abovementioned chemistry, they are likely
unstable relative to dissociation into IPr and free aminoborane
[H2BvNR2]. Notably, a formal donor–acceptor adduct of
H2BNMe2, IPr·BH2NMe2–BH3 (5), was recently synthesized,16f
and highlights the requirement of a Lewis acidic (BH3) group
coordinated to nitrogen in order to intercept a stable carbene
complex involving a secondary aminoborane H2BNR2 unit.
Fig. 2 Thermal ellipsoid plot (30% probability level) of IPr·BH2NH(tBu)–BH3 (3).
All carbon-bound hydrogen atoms and THF solvate molecules have been
omitted for clarity. Selected bond lengths (Å) and angles (°): C(1)–B(1)
1.6088(16), B(1)–N(3) 1.5929(15), N(3)–B(2) 1.6162(17), N(3)–C(4) 1.5276(14),
B–H 1.110(16) to 1.125(13); C(1)–B(1)–N(3) 111.28(9), B(1)–N(3)–B(2) 110.21(9),
B(1)–N(3)–C(4) 113.25(9); C(1)–B(1)–N(3)–B(2) torsion angle = 69.27(12).
The 11B NMR spectrum of 3 contains a single broad peak at
−17.9 ppm due to coincident BH2 and BH3 resonances (identi-
fied with the aid of 1H{11B} NMR experiments).
ð5Þ
With IPr·BH2NH(tBu)–BH3 (3) in hand, we explored the
t
chemistry of this adduct with BuNH2 to verify if 3 can be con-
sumed in a reaction that is the reverse of eqn (3) in Scheme 2.
However when 3 was combined with BuNH2 in toluene, no
ð6Þ
t
reaction was observed at room temperature after three days;
heating the reaction mixture to 60 °C for a further three days
resulted in only a ca. 10% conversion of 3 into tert-butylbora-
zine [tBuNBH]3 with no evidence of other products resulting
Attempted dehydrocoupling of arylamine–boranes with an
N-heterocyclic carbene
t
from the dehydrogenation of BuNH2·BH3 (e.g. the products
listed in eqn (4)). Thus the low yield of 3 obtained in the reac- We also decided to investigate the potential dehydrogenation
tion of IPr with tBuNH2·BH3 does not appear to be due to com- of electronically and sterically distinct hindered arylamine–
peting reactivity between 3 and any tBuNH2 generated.
borane adducts. To begin, the known adduct DippNH2·BH3
19
The initial step of the reaction sequence in Scheme 2 (6, Fig. 3) was combined with one equivalent of IPr in toluene
involves the direct hydrogen transfer reaction between at room temperature. Analysis of the resulting reaction mixture
RNH2·BH3 and IPr to yield IPrH2 and transient aminoboranes by 1H, 11B and 13C{1H} NMR spectroscopy (C6D6) indicated the
H2BNRH as products (eqn (1)). In the presence of added IPr, it formation of the known species IPrH2 (31%),12 IPr·BH2NH-
is conceivable that the aminoborane adducts IPr·BH2NRH are Dipp (15%),14 IPr·BH2NH(Dipp)–BH3 (22%)14 and DippNH2
formed due to the electron-deficient nature of the boron (32%) as illustrated by eqn (7). Interestingly, this reaction pro-
center in H2BNRH (eqn (2)).14,17 For the cases where R = Me or duced identifiable products consistent with the reaction
iPr, once IPr·BH2NRH is formed, then the lone pair at nitrogen sequences (eqn (1)–(3)) outlined in Scheme 2. It is likely that a
could participate in borane abstraction chemistry with combination of the lower nucleophilicity of the nitrogen
unreacted RNH2·BH3 in solution to give the observed products center in IPr·BH2NHDipp reduces the degree of BH3 exchange
t
1 and 2 along with free amine RNH2 (eqn (3)). When R = Bu, between IPr·BH2NHDipp and DippNH2·BH3 (eqn (3)). In
this overall process appears to be significantly slower enabling addition, the BH2NHDipp array is concurrently Lewis acidic
additional oligomerization, disproportionation and dehydro- enough to enable the isolation of the stable carbene–amino-
genation chemistry to transpire with [tBuNHvBH2] at the borane adduct IPr·BH2NHDipp in the absence of exogenous
expense of forming IPr·BH2NH(tBu)–BH3 (3).
BH3 as a Lewis acid.
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boranes have weaker B–N bonds when compared with their
alkyl amine–boranes.21 This effect in conjunction with the sig-
nificant steric bulk about nitrogen within the Ar*NH2 unit,
likely facilitates the direct borane exchange reaction between
Ar*NH2·BH3 and IPr in place of productive dehydrogenation
chemistry.23 We are currently exploring dehydrogenation
chemistry as a route towards low coordinate B–N systems.16f,24
ð8Þ
Conclusions
Fig. 3 Thermal ellipsoid plots (30% probability level) for DippNH2·BH3 (6) and
Ar*NH2·BH3 (7) with all carbon-bound hydrogen atoms and solvate molecules
omitted for clarity. Selected bond lengths (Å) and angles (°): Compound 6:
C(1)–N 1.4633(14), B–N 1.6197(17), N–H 0.911(15) and 0.936(15), B–H 1.085
(15) to 1.134(14); C(1)–N–B 116.31(10). Compound 7: C(1)–N 1.465(2),
B–N 1.641(3), N–H 0.929(19), B–H 1.13(2) and 1.20(3); C(1)–N–B 116.50(15);
a crystallographic mirror plane bisects 7.
The N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp =
2,6-iPr2C6H3) is an effective dehydrogenation agent for N-alkyl-
amine–boranes. Specifically, the primary amine–borane
adducts RNH2·BH3 (R = Me and iPr) participate in clean chem-
istry with IPr to yield the hydrogenated aminal IPrH2 and the
carbene-capped B–N–B chains, IPr·BH2NH(R)–BH3 as pro-
t
ducts; in the case of the more hindered adduct BuNH2·BH3 a
dramatic reduction in the amount of IPr·BH2NH(R)–BH3
formed was noted. The secondary amine–borane adduct
iPr2NH·BH3 interacts with IPr to give the dehydrogenated amino-
borane [iPrNvBH2] and IPrH2. Due to a likely reduction in
B–N bond strength, the arylamine–borane Ar*NH2·BH3 partici-
pates in a BH3 exchange reaction with IPr, with no sign of pro-
ductive dehydrogenation chemistry occurring. This study
indicates that N-heterocyclic carbenes are effective stoichio-
metric dehydrogenation agents for amine–boranes, thus
opening up new vistas of reactivity in terms of non-metal
mediated dehydrogenation chemistry.8,25
ð7Þ
To explore if a sterically encumbered arylamine–borane
adduct could participate in BH3 exchange chemistry with IPr,
we targeted the preparation of Ar*NH2·BH3 (Ar*
= 2,6-
(Ph2CH)2-4-MeC6H2). The synthesis of Ar*NH2·BH3 (7) was
accomplished in quantitative yield from the treatment of
20
Ar*NH2 with a THF solution of H3B·THF. Compound 7 was
Experimental section
General
obtained as an air-stable colorless solid that decomposes
under N2 with gas evolution at ca. 165 °C.21 Crystals of 7 of
suitable quality for single-crystal X-ray crystallography were All reactions were performed using standard Schlenk tech-
obtained from a ClCH2CH2Cl–hexanes solution at −35 °C, and niques under an atmosphere of nitrogen or in a nitrogen-filled
the refined structure is presented in Fig. 3 along with that of glovebox (Innovative Technology, Inc.). Solvents were dried
the less hindered Dipp analogue DippNH2·BH3 (6).
using a Grubbs-type solvent purification system26 manufac-
The B–N bond length in Ar*NH2·BH3 (7) was found to be tured by Innovative Technology, Inc., degassed (freeze–pump–
1.641(3) Å, which is highly elongated when compared to the thaw method), and stored under an atmosphere of nitrogen
i
t
t
dative B–N interaction in Me2NH·BH3 [1.5936(13) Å],13a and is prior to use. PrNH2, BuNH2, BuNH2·BH3 and H3B·THF were
longer than the B–N bond length in DippNH2·BH3 [1.6197(17) purchased from Sigma Aldrich; iPrNH2 and tBuNH2 were
Å]. Based on these metrical parameters it was anticipated that distilled prior to use, and all other chemicals were used
the B–N bond in 7 might be considerably weaker in relation to as received. IPr,27 Ar*NH2,20 iPr2NH·BH3,18 iPrNH2·BH3,28
the B–N linkages found within alkylamine–borane adducts R′ (tBuHN)B2H5,16f and DippNH2·BH3 were prepared according
19
NH2·BH3 (R′ = alkyl group).
to literature procedures. 1H, 11B{1H}, and 13C{1H} NMR spectra
In accordance with the presence of a weaker B–N inter- were recorded with either Varian iNova 400 or VNMRS-500
action in Ar*NH2·BH3 (7), this species interacted with one spectrometers and referenced externally to SiMe4 (1H and
equivalent of IPr to yield IPr·BH3 and the hindered aryl- 13C{1H}) and Et2O·BF3 (11B). Elemental analyses were per-
22
1
amine Ar*NH2 (eqn (8)), as determined by H, 11B and 13C{1H} formed by the Analytical and Instrumentation Laboratory at
NMR spectroscopy. It has been observed that arylamine– the University of Alberta. Infrared spectra were recorded using
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Dalton Transactions
Nicolet IR100 FTIR as Nujol mulls between NaCl plates. (ArC), 124.7 (ArC), 124.2 (ArC), 122.9 (–N–CH–), 43.3 (N(CH3)2),
Melting points were obtained in sealed glass capillaries under 28.9 (iPr), 28.8 (iPr), 26.0 (iPr), 25.9 (iPr), 23.0 (iPr), 22.9 (iPr).
nitrogen using a MelTemp melting point apparatus and are Anal. Calcd for C28H45N3B2: C, 75.52; H, 10.19; N, 9.44. Found:
uncorrected.
C, 75.50; H, 9.97; N, 9.27. Mp (°C): 181–182.
Synthesis of IPr·BH2NH(iPr)–BH3 (2). To a solution of IPr
(0.065 g, 0.17 mmol) in toluene was added iPrNH2·BH3
X-ray crystallography
Crystals of appropriate quality for X-ray diffraction studies (0.012 g, 0.17 mmol) and the reaction mixture was stirred for
were removed from a vial in a glovebox and immediately 8 h. The volatiles were then removed in vacuo to yield a pale
covered with a thin layer of hydrocarbon oil (Paratone-N). yellow powder, and the crude product was washed with 3 ×
A suitable crystal was then selected, attached to a glass fiber, 3 mL of hexanes to yield 2 as a colorless solid that was recrys-
and quickly placed in a low-temperature stream of nitrogen.29 tallized from THF at −35 °C (0.064 g, 80%). 1H NMR
All data were collected using a Bruker APEX II CCD detector/ (400 MHz, C6D6): δ = 7.17–7.05 (m, 6H, ArH), 6.45 (s, 2H,
3
D8 diffractometer using Cu or Mo Kα radiation, with the N–CH–), 3.05 (septet, JHH = 6.9 Hz, 1H, CH(CH3)2), 2.98
crystal cooled to −100 °C. The data were corrected for absorp- (m, 4H, CH(CH3)2), 2.20 (s, 2H, –BH2–, assignment made by
tion through Gaussian integration from indexing of the crystal selective 1H{11B} decoupling), 2.04 (s, 3H, –BH3, assignment
1
3
faces. Structures were solved using the direct methods30 made by selective H{11B} decoupling), 1.48 (d, JHH = 6.5 Hz,
3
(SHELXD: 1, 3, 6 and 7). Refinements were completed using 6H, CH(CH3)2), 1.47 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.08
the program SHELXL-97.31 Where possible, boron and nitro- (d, JHH = 6.5 Hz, 3H, –NCH(CH3)2), 0.99 (d, JHH = 6.5 Hz, 6H,
3
3
3
gen bound hydrogen atoms were isotropically refined, other- CH(CH3)2), 0.98 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 0.79 (d,
wise were assigned positions based on the sp2 or sp3 3JHH = 6.5, 3H, –NCH(CH3)2); the NH group was not located.
hybridization geometries of their attached carbon, nitrogen or 11B NMR (128 MHz, C6D6): δ = −16.5 (br, –BH2–), −17.9 (br,
boron atoms and were given thermal parameters 20% greater –BH3). 13C{1H} NMR (100 MHz, C6D6): δ = 146.5 (ArC), 146.3
than those of their parent atoms.
(ArC), 133.9 (ArC), 130.8 (ArC), 124.8 (ArC), 124.1 (ArC), 122.6
(N–CH–), 53.5 (–NCH(CH3)2), 28.9 (iPr), 28.8 (iPr), 26.0 (iPr),
23.0 (iPr), 22.8 (iPr), 21.1 (iPr), 19.2 (iPr). Anal. Calcd for
Special refinement conditions
Compound 3: Attempts to refine peaks of residual electron C30H49N3B2: C, 76.12; H, 10.43; N, 8.88. Found: C, 75.54;
density as disordered or partial-occupancy solvent tetrahydro- H, 10.52; N, 8.43. Mp (°C): 181–182.
furan oxygen or carbon atoms were unsuccessful. The data
Reaction of tBuNH2·BH3 with IPr. To a solution of IPr
were corrected for disordered electron density through use of (0.196 g, 0.505 mmol) in toluene was added tBuNH2·BH3
the SQUEEZE procedure,32 as implemented in PLATON.33 (0.044 g, 0.51 mmol) and the resulting clear yellow solution
A total solvent-accessible void volume of 1174.0 Å3 with a total was stirred for 8 h. Analysis of the reaction mixture by
electron count of 244 (consistent with 6 molecules of solvent 11B NMR spectroscopy indicated the presence of the follow-
tetrahydrofuran, or 1.5 molecules per formula unit of the ing species (with relative 11B integration values listed):
1
molecule of interest) was found in the unit cell.
11B (128 MHz, toluene): δ = 35.2 ([tBuNHvBH2], t, JBH
=
125 Hz, 7%), 25.9 ([tBuNBH]3, d, JBH = 115 Hz, 30%), −8.3
1
Synthetic procedures
(unknown species, br s, 5%), −17.9 (IPr·BH2NH(tBu)–BH3 (3),
Synthesis of IPr·BH2NH(CH3)–BH3 (1). To a solution of IPr br s, coincident BH2 and BH3 resonances, 12%), −21.9
1
(0.242 g, 0.624 mmol) in toluene was added MeNH2·BH3 (tBuNH2·BH3, quartet, JBH = 99 Hz, 35%), −25.8 ([(tBuNH)-
1
(0.028 g, 0.62 mmol) and the reaction mixture was stirred for B2H5], m, 8%), −36.4 (BH4−, pentet, JBH = 82 Hz, 4%). The
8 h. The volatiles were then removed in vacuo to yield a pale volatiles were then removed from the reaction mixture to yield
yellow powder, and the crude product was washed with 3 × a pale yellow powder (0.169 g) that contained a mixture of
3 mL of hexanes to yield 1 as a colorless solid (0.113 g, 82%). IPrH2 and 3 (90 : 10 ratio) by 1H NMR spectroscopy.
Crystals suitable for X-ray crystallography were obtained by
Synthesis of IPr·BH2NH(tBu)–BH3 (3). To a solution of
cooling a saturated solution of 1 in a 10 : 1 toluene–hexanes (tBuHN)B2H5 (0.071 mmol in 5 mL of a 5 : 1 hexanes–THF
mixture to −35 °C. 1H NMR (400 MHz, C6D6): δ = 7.19–7.06 mixture) was added IPr (0.277 g, 0.0698 mmol) and the
3
(m, 6H, ArH), 6.46 (s, 2H, N–CH–), 3.05 (septet, JHH = 7.2 Hz, mixture was heated at 60 °C for two days to yield a yellow solu-
2H, CH(CH3)2), 2.93 (septet, 3JHH = 6.8 Hz, 2H, CH(CH3)2), 2.18 tion. The solution was cooled to −35 °C for 24 h to yield 3 as
3
(d, JHH = 6.0 Hz, 3H, –NH(CH3)), 1.96 (s, 2H, –BH2–, assign- colorless prisms (0.209 g, 61%). Crystals obtained from this
ment made by selective 1H{11B} decoupling), 1.94 (s, 3H, –BH3, method were suitable for X-ray crystallography. 1H NMR
assignment made by selective 1H{11B} decoupling), 1.45 (500 MHz, C6D6): δ = 7.18 (d, JHH = 8.0 Hz, 4H, ArH), 7.08
3
3
3
3
(d, JHH = 6.8 Hz, 6H, CH(CH3)2), 1.43 (d, JHH = 6.8 Hz, 6H, (t, JHH = 8.0 Hz, 2H, ArH), 6.48 (s, 2H, N–CH–), 3.01 (m, 4H,
CH(CH3)2), 0.97 (d, JHH = 6.8 Hz, 6H, CH(CH3)2), 0.96 (d, JHH CH(CH3)2), 2.29 (br s, 1H, –BH2NH(tBu)BH3, assignment made
3
3
= 6.8 Hz, 6H, CH(CH3)2); the NH group was not located. by selective H{11B} decoupling), 2.14 (br s, 1H, –BH2NH(tBu)-
1
11B NMR (128 MHz, C6D6): δ = −14.0 (br, –BH2–), −14.7 (br, BH3, assignment made by selective 1H{11B} decoupling), 1.91
–BH3). 13C{1H) NMR (100 MHz, C6D6): δ = 146.30 (ArC), 146.28 (br s, 3H, –BH2NH(tBu)BH3, assignment made by selective
ArC), 133.8 (ArC), 130.8 (ArC), 129.3 (ArC), 128.5 (ArC), 125.6 1H{11B} decoupling), 1.46 (overlapping doublets, 3JHH = 6.5 Hz,
4630 | Dalton Trans., 2013, 42, 4625–4632
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12H, –CH(CH3)2), 1.34 (br s, 1H, –NH(tBu)), 1.08 (s, 9H,
–C(CH3)3), 0.99 (d, JHH = 7.5 Hz, 6H, CH(CH3)2), 0.98 (d,
2 (a) G. Alcaraz and S. Sabo-Etienne, Angew. Chem., Int. Ed.,
2010, 49, 7170; (b) C. W. Hamilton, R. T. Baker, A. Staubitz
and I. Manners, Chem. Soc. Rev., 2009, 38, 279;
(c) C. A. Jaska, K. Temple, A. J. Lough and I. Manners,
Chem. Commun., 2001, 962; (d) N. Blaquiere, S. Diallo-
Garcia, S. I. Gorelsky, D. A. Black and K. Fagnou, J. Am.
Chem. Soc., 2008, 130, 14034; (e) M. E. Bluhm,
M. G. Bradley, R. Butterick, III, U. Kusari and L. G. Sneddon,
J. Am. Chem. Soc., 2006, 128, 7748; (f) T. J. Clark,
C. A. Russell and I. Manners, J. Am. Chem. Soc., 2006, 128,
9582; (g) R. T. Baker, J. C. Gordon, C. W. Hamilton,
N. J. Henson, P.-H. Lin, S. Maguire, M. Murugesu,
B. L. Scott and N. C. Smythe, J. Am. Chem. Soc., 2012, 134,
5598; (h) D. Pun, E. Lobkovsky and P. J. Chirik, Chem.
Commun., 2007, 3297; (i) F. H. Stephens, V. Pons and
R. T. Baker, Dalton Trans., 2007, 2613; ( j) R. Komm,
R. A. Geanangel and R. Liepins, Inorg. Chem., 1983, 22,
1684; (k) W. Luo, P. G. Campbell, L. N. Zakharov and
S.-Y. Liu, J. Am. Chem. Soc., 2011, 133, 19326.
3
3JHH = 7.5 Hz, 6H, CH(CH3)2). 11B NMR (160 MHz, C6D6): δ =
−17.9 (br, Δν1/2 = 168 Hz, –BH2– and –BH3 overlapping).
13C{1H} NMR (125 MHz, C6D6): δ = 146.6 (ArC), 146.4 (ArC),
134.0 (ArC), 130.7 (ArC), 128.3 (ArC), 124.9 (ArC), 124.0 (ArC),
122.5 (N–CH), 53.7 (C(CH3)3), 29.0 (CH(CH3)2), 28.7
(CH(CH3)2), 28.3 (CH(CH3)2), 26.0 (C(CH3)3), 22.9 (CH(CH3)2),
22.8 (CH(CH3)2). Anal. Calcd for C31H51N3B2: C, 76.39;
H, 10.55; N, 8.62. Found: C, 75.25; H, 10.62; N, 8.50. Mp (°C):
110–111.
i
Preparation of IPrH2 from the reaction of Pr2NH·BH3 with
IPr. To a solution of IPr (0.143 g, 0.369 mmol) in toluene was
added iPr2NH·BH3 (0.043 g, 0.37 mmol) and the resulting clear
yellow solution was stirred for 8 h. The volatiles were removed
in vacuo to yield a pale yellow powder that was redissolved in
5 mL of hexanes, and the solution was filtered. Removal of the
solvent from the filtrate afforded spectroscopically pure IPrH2
(pale yellow solid; 0.142 g, 99%) as determined by 1H and
13C{1H} NMR spectroscopy.12
3 L. Pasumansky, C. T. Goralski and B. Singaram, Org.
Process Res. Dev., 2006, 10, 959.
Synthesis of Ar*NH2·BH3 (Ar* = 2,6-(Ph2CH)2-4-MeC6H2)
(7). To
a
cold (−35 °C) solution of Ar*NH2 (0.416 g,
4 (a) A. Staubitz, A. Presa Soto and I. Manners, Angew. Chem.,
Int. Ed., 2008, 47, 6212; (b) A. Staubitz, M. E. Sloan,
A. P. M. Robertson, A. Friedrich, S. Schneider, P. J. Gates,
J. Schmedt auf der Günne and I. Manners, J. Am. Chem.
0.946 mmol) in THF was added H3B·THF (0.50 mL,
0.95 mmol, 1.0 M in THF), and the mixture was stirred for 4 h.
The volatiles were removed in vacuo to yield 7 as a white solid
(0.424 g, 98%). Crystals suitable for X-ray crystallography were
Soc.,
2010,
132,
13332;
(c)
R.
Dallanegra,
obtained by cooling
a
saturated dichloroethane–hexanes
A. P. M. Robertson, A. B. Chaplin, I. Manners and
A. S. Weller, Chem. Commun., 2011, 47, 3763.
5 (a) M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey
and K. I. Goldberg, J. Am. Chem. Soc., 2006, 128, 12048;
(b) J. Choi, A. H. R. MacArthur, M. Brookhart and
A. S. Goldman, Chem. Rev., 2011, 111, 1761.
6 I. Manners, Angew. Chem., Int. Ed. Engl., 1996, 35, 1602.
7 (a) R. T. Paine and C. K. Narula, Chem. Rev., 1990, 90, 73;
(b) T. Wideman, E. E. Remsen, E. Cortez, V. L. Chlanada
and L. G. Sneddon, Chem. Mater., 1998, 10, 412.
8 A. Jana, C. Schulzke and H. W. Roesky, J. Am. Chem. Soc.,
2009, 131, 4600.
1
(10 : 1) solution of 7 to −35 °C. H NMR (400 MHz, C6D6): δ =
7.08–6.94 (m, 20H, Ar–PhH), 6.70 (s, 2H, Ar–CH(Ph)2), 6.34
(s, 2H, ArH), 4.66 (br s, 2H, Ar*NH2), 2.88 (br s, 3H,
Ar*NH2BH3, assignment made by broadband 1H{11B} decou-
pling), 1.69 (s, 3H, –Ar*CH3). 11B NMR (128 MHz, C6D6): δ =
−16.1 (br, –BH3). 13C{1H} NMR (100 MHz, C6D6): δ = 143.5
(ArC), 142.1 (ArC), 136.1 (ArC), 136.0 (ArC), 135.8 (ArC), 130.4
(ArC), 130.0 (ArC), 129.1 (ArC), 128.8 (ArC), 127.5 (ArC), 50.8
(CH(Ph)2), 43.5 (CH(Ph)2), 21.0 (Ar*CH3). Anal. Calcd for
C33H32NB: C, 87.41; H, 7.11; N, 3.09. Found: C, 87.07; H, 7.10;
N, 3.18. Mp (°C): 165 (decomp.).
9 (a) A. P. M. Robertson, R. Suter, L. Chabanne,
G. R. Whittell and I. Manners, Inorg. Chem., 2011, 50,
12680; (b) J. R. Vance, A. P. M. Robertson, K. Lee and
I. Manners, Chem.–Eur. J., 2011, 17, 4099.
Acknowledgements
This work was supported by the Natural Sciences and Engin- 10 For selected examples, see: (a) K. C. Thimer,
eering Research Council (NSERC) of Canada, the Canada
Foundation for Innovation (CFI), Alberta Innovates-Technology
Futures (New Faculty Award to E. R.), and Suncor Energy
Inc. (Petro-Canada Young Innovator Award to E. R.). K. J. S.
received support from the Ruppertsland Institute and the
Canada Summer Jobs (CSJ) program.
S. M. I. Al-Rafia, M. J. Ferguson, R. McDonald and
E. Rivard, Chem. Commun., 2009, 7119; (b) S. M. I. Al-Rafia,
A. C. Malcolm, R. McDonald, M. J. Ferguson and E. Rivard,
Angew. Chem., Int. Ed., 2011, 50, 8354; (c) S. M. I. Al-Rafia,
M. J. Ferguson and E. Rivard, Inorg. Chem., 2011, 50,
10543; For the use of NHCs as ligands in the main group,
see: (d) N. Kuhn and A. Al-Sheikh, Coord. Chem. Rev., 2005,
249, 829; (e) R. Wolf and W. Uhl, Angew. Chem., Int. Ed.,
2009, 48, 6774; (f) Y. Wang and G. H. Robinson, Inorg.
Chem., 2011, 50, 12326.
Notes and references
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L. Monnier, Tetrahedron, 1999, 55, 1197.
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This journal is © The Royal Society of Chemistry 2013
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14 Our attempts to directly prepare the intermediates 23 For the preparation of carbene–borane adducts via
i
t
IPr·BH2NHR (R = Me, Pr, Bu and Dipp) via the reaction of
IPr·BH2I with Li[NHR] have been unsuccessful. The Dipp-
analogue IPr·BH2–NHDipp was recently synthesized via an
alternate route: S. M. I. Al-Rafia, R. McDonald,
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11B NMR shift of −36.4 ppm (pentet, JBH = 81 Hz) was
−
noted for the BH4 ions in Sr(BH4)2·2THF (in d8-THF): 27 L. Jafarpour, E. D. Stevens and S. P. Nolan, J. Organomet.
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4632 | Dalton Trans., 2013, 42, 4625–4632
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