Donor-acceptor complexation and dehydrogenation chemistry of aminoboranes

Article abstract of DOI:10.1021/ic3018997

A series of formal donor-acceptor adducts of aminoborane (H ₂BNH₂) and its N-substituted analogues (H ₂BNRR′) were prepared: LB-H₂BNRR′₂- BH₃ (LB = DMAP, IPr, IPrCH₂ and PCy₃; R and R′ = H, Me or tBu; IPr = [(HCNDipp)₂C:] and Dipp = 2,6-iPr ₂C₆H₃). To potentially access complexes of molecular boron nitride, LB-BN-LA (LA = Lewis acid), preliminary dehydrogenation chemistry involving the parent aminoborane adducts LB-H₂BNH ₂-BH₃ was investigated using [Rh(COD)Cl]₂, CuBr, and NiBr₂ as dehydrogenation catalysts. In place of isolating the intended dehydrogenated BN donor-acceptor complexes, the formation of borazine was noted as a major product. Attempts to prepare the fluoroarylborane-capped aminoborane complexes, LB-H₂BNH ₂-B(C₆F₅)₃, are also described.

Full text of DOI:10.1021/ic3018997

Article
pubs.acs.org/IC
Donor−Acceptor Complexation and Dehydrogenation Chemistry of
Aminoboranes
Adam C. Malcolm, Kyle J. Sabourin, Robert McDonald, Michael J. Ferguson, and Eric Rivard*
Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, Canada T6G 2G2
S
* Supporting Information
ABSTRACT: A series of formal donor−acceptor adducts of
aminoborane (H2BNH2) and its N-substituted analogues
(H2BNRR′) were prepared: LB-H2BNRR′2-BH3 (LB =
DMAP, IPr, IPrCH2 and PCy3; R and R′ = H, Me or tBu;
IPr = [(HCNDipp)2C:] and Dipp = 2,6-iPr2C6H3). To
potentially access complexes of molecular boron nitride, LB-
BN-LA (LA = Lewis acid), preliminary dehydrogenation
chemistry involving the parent aminoborane adducts LB-
H2BNH2-BH3 was investigated using [Rh(COD)Cl]2, CuBr,
and NiBr2 as dehydrogenation catalysts. In place of isolating the intended dehydrogenated BN donor−acceptor complexes, the
formation of borazine was noted as a major product. Attempts to prepare the fluoroarylborane-capped aminoborane complexes,
LB-H2BNH2-B(C6F5)3, are also described.
Scheme 1. Donor−Acceptor Complexation of H2BNH2 and
INTRODUCTION
■
Proposed Dehydrogenation Chemistry
Ammonia borane (AB), H3N·BH3, has been extensively
explored as a chemical source of hydrogen1,2 and as a precursor
for the synthesis of boron nitride (BN) ceramics.3 H3N·BH3 is
also of fundamental interest because of its isoelectronic and
isolobal relationship with ethane, H3C−CH3. A key difference
between inorganic B−N analogues, such as AB, and their
hydrocarbon counterparts is the existence of polar B−N bonds
in the former species and the presence of protic (N−Hδ+) and
hydridic (B−Hδ−) residues which enable facile loss of H2 via
nonconventional Hδ−···δ+H interactions. Both the parent
species H2BNH2 and HBNH have been isolated under
cryogenic conditions;4,5 however unlike AB, they are elusive at
room temperature because of their propensity to spontaneously
polymerize.6
Boron nitride (BN) is a synthetic material that is isostructural
with diamond in its cubic form7 with comparable hardness and
thermal stability. These properties coupled with its electrically
insulating and thermal conducting behavior makes BN a
promising material for the microelectronics industry.8 To date,
most methods for preparing BN require high temperatures
(>1500 °C) and/or specialized equipment;9,10 however some
milder routes to BN are now emerging.11 Given our prior
experiences in stabilizing reactive molecular fragments (e.g.,
EH2 and H2EEH2; E = Si, Ge, and/or Sn) via a general donor−
acceptor stabilization protocol,12−14 we were interested in
accessing complexes of molecular BN which could later release
BN to form bulk boron nitride under mild conditions (Scheme
1). This paper reports a series of complexes of the general form
LB-H2BNH2-LA (LB = Lewis base; LA = Lewis acid),15 and
our attempts to generate metastable complexes of BN, LB-B
N-LA, via dehydrogenation chemistry.
RESULTS AND DISCUSSION
■
Synthesis of Donor−Acceptor Complexes of the
Parent Aminoborane (H2BNH2). Free H2BNH2 is a
challenging substrate to handle as it readily forms oligomeric
borazanes [H2BNH2]x (x = 2−5).16 Thus our goal was to
generate stable adducts of H2BNH2 from which productive
dehydrogenation chemistry could transpire (Scheme 1).
Starting from the known μ-aminodiborane, H2NB2H5 (1),15b
we were able to induce the nucleophilic scission of a bridging
B−H bond in 1 in the presence of the carbon-based donor,
IPrCH2 [IPrCH2 = (HCNDipp)2CCH2; Dipp = 2,6-
iPr2C6H3)]. IPrCH2 has recently been shown to be an effective
Lewis base in an analogous fashion as N-heterocyclic carbenes
(NHCs).17 Upon addition of IPrCH2 to a solution of 1, the
target complex, IPrCH2-H2BNH2-BH3 (2), precipitated from
solution and was subsequently isolated as a white solid in a 62%
yield (Scheme 2). The formation of the 2 was confirmed by
NMR (1H, 11B, 1H{11B}, GCOSY), and IR spectroscopy;
however because of extensive twinning within crystals of 2, only
Received: August 30, 2012
Published: November 15, 2012
© 2012 American Chemical Society
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It was found that the overall reaction pathway used to access
compound 2 could also be used to prepare H2BNH2 adducts
with alternate Lewis basic (LB) donors: LB-H2BNH2-BH3
(Scheme 2). When the strong σ-donor, tricyclohexylphosphine
(Cy3P), was added to 1 at room temperature, the known
phosphine-borane adduct, Cy3P·BH3,20 was formed as the
major phosphorus-containing product in a ca. 90% yield: 31P
NMR spectroscopy afforded a singlet at 22.9 ppm with
Scheme 2. Synthesis of the H2BNH2 Complexes (2−4)
Featuring Carbon-, Phosphorus-, and Nitrogen-Donors
1
resolvable coupling to boron (1:1:1:1 quartet, JP‑B = 64 Hz),
while a corresponding doublet resonance was detected by
11B{1H} NMR spectroscopy (−46.3 ppm, JB−P = 63 Hz).20
1
The minor soluble product formed in the reaction of Cy3P with
1 was present in about 10% yield and was tentatively assigned
as the previously unknown species Cy3P-H2BNH2-BH3 on the
basis of 31P and 11B NMR spectroscopy [11B NMR: δ = −21.0
(br) and −17.5 (br); 31P{1H} NMR: δ = 10.2 (br)]. This
species could be obtained in a much greater isolated yield if the
solution of 1 was first cooled to −35 °C prior to the addition of
solid PCy3. After workup of the reaction mixture, Cy3P-
H2BNH2-BH3 (3) was isolated as a colorless solid in a 61%
yield and identified by single-crystal X-ray crystallography
(Figure 1). The internal and terminal B−N bond lengths in 3
atom connectivity could be established by single-crystal X-ray
crystallography.
The presence of a coordinating IPrCH2 unit in 2 was verified
1
by H NMR spectroscopy, while the adjacent BH2−NH2-BH3
unit was located in the form of triplet (BH2) and quartet
resonances (BH3) in the proton-coupled 11B NMR spectrum
1
1
(triplet, δ = −14.7, JB−H = 95 Hz; quartet, δ = −21.9, JB−H
=
87 Hz). Linkage of the nucleophilic −CH2 group of IPrCH2 to
the -BH2- unit of the inorganic chain was substantiated by a
1H{11B} aoGCOSY study; the resonance corresponding to the
boron-bound hydrogen atoms of the -BH2- group at 1.40 ppm
showed off-diagonal coupling to a resonance at 1.88 ppm
belonging to the proximal terminal −CH2 residue of the
1
IPrCH2 donor. The H NMR resonance corresponding to the
central -NH2- unit in 2 could not be identified; however, an N−
H stretching band at 3340 cm−1 could be located by IR
spectroscopy.
The isolation of IPrCH2-H2BNH2-BH3 (2) in pure form was
thwarted by its decomposition in solvents in which it is soluble.
For example, compound 2 slowly decomposes in chlorinated
solvents (CH2Cl2, ClCH2CH2Cl, and CHCl3), while signifi-
cantly accelerated decomposition occurs when 2 is dissolved in
tetrahydrofuran (THF, decomposition half-life = ca. 24 h.). The
decomposition product in each case yields a clearly resolved
pentet at −40.6 ppm in the 11B NMR spectrum (1JB−H = 81
Hz), consistent with the presence of a BH4− anion. In addition,
Figure 1. Thermal ellipsoid plot (30% probability) of Cy3P-H2BNH2-
BH3 (3), with carbon-bound hydrogen atoms and dichloromethane
solvate omitted for clarity. Compound 3 cocrystallized with 4% Cy3P-
H2BNH2-BH2Cl.19 Selected bond lengths [Å] and angles [deg]: P−
B(1) 1.9730(17), B(1)−N 1.564(2), B(1)−H 1.155(18) and
1.086(18), N−H 0.847(19) and 0.93(2), N−B(2) 1.608(2), B(2)−
H 1.164(18), 1.157(19), and 1.12(2); P−B(1)−N 115.61(11), B(1)−
N−(B2) 114.84(13); P−B(1)−N−B(2) torsion angle = 174.29(12).
1
the H NMR spectrum of the decomposition product contains
are 1.564(2) and 1.608(2) Å, respectively, and are consistent
with the presence of B−N single bonds. The Cy3P-H2BNH2-
BH3 array rests in a slightly canted anti conformation with a P−
B−N−B torsion angle of 174.29(12)°.
a downfield-positioned singlet at 8.18 ppm (in CDCl3), which
matches the backbone alkene C−H resonance in the
+
imidazolium cation [IPrCH3 ] (IPr = [(HCNDipp)2C:);
moreover diagnostic resonances due to the [IPrCH3]+ cation
can be located by 13C{1H} NMR spectroscopy.18 As a result,
compound 2 can only be obtained in about 90% purity with the
predominant contaminant being [IPrCH3]BH4 (see Supporting
Information, Figures S1 and S2).19 No further decomposition
occurs when 2 is stored in the solid state under N2, nor when 2
is mixed with arene solvents, such as toluene, in which 2 is only
sparingly soluble; attempts to purify 2 via recrystallization from
arene solvents failed because of the low solubility of this
compound in these solvent media. We are currently exploring
the fate of the extruded B−N product(s) formed during the
decomposition of 2 into [IPrCH3]BH4.
In contrast to IPrCH2-H2BNH2-BH3 2, the phosphine-
capped analogue 3 is indefinitely stable in both ethereal and
arene solvents, and can be crystallized from halogenated
hydrocarbon solvents without noticeable decomposition. When
a sample of 3 was heated to 100 °C in toluene, a reaction
mixture containing unreacted 3 (5%) and Cy3P·BH3 (85%)
(11B and 31P{1H} NMR) with traces of borazine ([HBNH]3,
6%) and cyclotriborazane ([H2BNH2]3, 4%) as soluble
products was obtained. A small amount of a white solid
remained that was not soluble in halogenated, arene or etheral
solvents, which possibly contained boron−nitrogen oligomers,
[H2BNH2]x and/or polymers.
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Dimethylaminopyridine (DMAP) is known to be an effective
Lewis base for the stabilization of electron-deficient main group
species.21 Thus we were interested in seeing if this donor could
be used to form complexes featuring the parent aminoborane
residue, H2BNH2. To this end, we treated the μ-aminodiborane
(1) with DMAP and gratifyingly observed the clean formation
of DMAP-H2BNH2-BH3 (4) as a white solid in a high isolated
yield of 75% (Scheme 2).
route to the target NHC-bound chain IPr-H2BNH2-BH3 (5)
was found (Scheme 3). Of the three reported complexes 2−4,
Scheme 3. Donor Substitution Chemistry Involving the
Aminoborane Complexes 2, 4, and 5
The solid state structure of 4 is presented in Figure 2, and
notably, the NDMAP-BH2-NH2-BH3 array in 4 adopts a gauche
the IPrCH2 derivative 2 has been shown to be the least
thermally stable (vide supra); thus we reasoned that the
IPrCH2-B bond in 2 would be more labile than the related
Cy3P-B and DMAP-B linkages in 3 and 4. Treatment of
IPrCH2-H2BNH2-BH3 (2) with 1 equiv of DMAP in benzene
yielded the DMAP adduct (4) and IPrCH2 as major products
by NMR spectroscopy. In an analogous fashion, ligand
exchange chemistry transpired between DMAP-H2BNH2-BH3
(4) and IPr to afford the new carbene-aminoborane adduct IPr-
H2BNH2-BH3 (5) in a 42% isolated yield; this species gave a
broad 11B{1H} NMR signal at −18.2 ppm due to coincident
−BH2- and −BH3 groups, with no resolvable coupling noted in
the proton-coupled 11B NMR spectrum (see Supporting
Information, Figure S6).19 Crystals of 5 suitable for X-ray
crystallography were grown from a saturated solution in
CH2Cl2/hexanes, and the refined structure is presented in
Figure 3.
Figure 2. Thermal ellipsoid plot (30% probability) of DMAP-
H2BNH2-BH3 (4), with carbon-bound hydrogen atoms and hexane
solvate molecules omitted for clarity. Selected bond lengths [Å] and
angles [deg]: N(2)−B(1) 1.5728(15), B(1)−N(1) 1.5720(17),
N(1)−B(2) 1.5939(16), B(1)−H 1.109(13), and 1.125(13), B(2)−
H 1.102(15), 1.145(14) and 1.147(17); N(2)−B(1)−N(1) 110.44(9),
B(1)−N(1)−B(2) 118.68(9); N(2)−B(1)−N(1)−B(2) torsion angle
= 65.71(13).
conformation about the B−N bond vector [torsion angle =
65.71(13)°]. The central H2B-NH2 bond distance in 4 is
1.5720(17) Å, which is the same within experimental error as
the B−N bond length in H3N·BH3 [1.58(2) Å].22 The B−N
bond length involving the terminally bound BH3 group in 4 is
1.5939(16) Å, and this value lies within the range of B−N
distances found in Shore’s inorganic butane analogue,
H3NBH2NH2BH3 [1.588−1.600 Å].15c,23 The 11B NMR
spectrum of 4 clearly shows the presence of -BH2- and -BH3
The reaction sequence outlined in Scheme 3 nicely illustrates
the relative donor strength of IPr > DMAP > IPrCH2 among
the adducts 2, 4, and 5. The successful syntheses of these
complexes and the phosphine-capped aminoborane Cy3P-
H2BNH2-BH3 (3) provide a suite of aminoborane donor−
acceptor adducts which could be potentially dehydrogenated to
afford trapped B−N species of higher bond orders (Scheme 1).
Synthesis of the N-Methylated Aminoborane Com-
plexes, LB-BH2NMe2-BH3. To more directly probe the
thermal lability of the terminal donor−acceptor interactions
in the aminoborane adducts LB-H2BNR2-BH3, a series of N-
methylated adducts were prepared to avoid competing
dehydrogenation chemistry that could transpire if N−H and
B−H residues were present within the same molecule. The
stable adduct DMAP-H2BNMe2-BH3 (7) was obtained through
a similar ring-opening pathway as used to prepare the H2BNH2
adducts (2−4) (Scheme 4). The required μ-dimethylaminodi-
borane (6)25a,b was synthesized by the reaction of the in situ
generated dimer [Me2NBH2]2 (11B NMR: triplet, δ = 4.75
1
groups in the form of triplet [δ = −3.7, JB−H = 100 Hz] and
1
quartet resonances [δ = −21.8 ppm, JB−H = 92 Hz],
respectively. Because of the low number of resonances in the
1H NMR spectrum of 4, the central -NH2- unit can be readily
located as a broad singlet at 2.19 ppm. Selectively decoupled
1H{11B} NMR experiments enabled the BH2 and BH3 groups
to be identified, as quadrupolar broadening of these resonances
by 11B nuclei (I = 3/2) was suppressed; the same technique was
also used to visualize the 3-bond coupling between the terminal
-BH3 unit and the adjacent -NH2- group in 4 (3JH−H = 4.4 Hz).
Our initial forays into donor−acceptor chemistry involved
the strong σ-donor IPr (IPr = [(HCNDipp)C:].12 In pursuit of
the carbene-capped complex IPr-H2B-NH2-BH3, IPr was
combined with the aminodiborane 1. Each time a new product
with a 11B NMR resonance at −18.2 ppm (broad) was detected
along with varying quantities (ca. 30−65%) of the known NHC
1
ppm, JB−H = 113 Hz) with 1 equiv of H3B·THF at 60 °C
(Scheme 4). To facilitate the formation of [Me2NBH2]2, the
dehydrocoupling of Me2NH·BH3, to generate [Me2NBH2]2,
was catalyzed by [Rh(COD)Cl]2.25c−g,26 Addition of 1 equiv of
DMAP to 6 caused ring-opening and precipitation of the
24
1
adduct IPr·BH3 (δ = −35 ppm, quartet, JB−H = 86 H, in
C6D6). As part of our efforts to obtain comparative donor
strengths within the LB-H2BNH2-BH3 series, an improved
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barrier about the central B−N bonds in these species. Notably,
Nutt and McKee reported that the gauche isomer of H3N-
H2BNH2-BH3 is more stable than the anti form by 11.2 kcal/
mol in the gas phase, while a rotational barrier of 13.1 kcal/mol
was estimated.28
DMAP-H2BNMe2-BH3 (7), IPrCH2-H2BNMe2-BH3 (8),
and IPr-H2BNMe2-BH3 (9) exhibit similar H2B-NMe2 bond
lengths of 1.5801(18), 1.585(8) avg., and 1.588(2) Å,
respectively. The terminal Me2N-BH3 bond lengths in these
chains are slightly elongated with respect to the internal B−N
bonds [e.g., B(2)−N(3) distance in 9 is 1.617(2) Å]. For
comparison, a related amine-borane chain, Me2NH-H2BNMe2-
BH was crystallographically characterized by Noth and co-
̈
3
workers, and a narrow B−N bond length range of 1.589(2) to
1.600(2) Å was noted for the internal and external B−N
linkages.29 Despite the similarity in the intrachain B−N bond
lengths between 7 and 8, it was experimentally confirmed that
DMAP is a stronger Lewis base than IPrCH2 in this system.
The reaction of 1 equiv of DMAP with IPrCH2-H2BNMe2-BH3
(8) quantitatively yielded DMAP-H2BNMe2-BH3 (7) along
with free IPrCH2 (eq 1), which parallels the ligand substitution
chemistry described already for the parent LB-H2BNH2-BH3
adducts (Scheme 2).
Figure 3. Thermal ellipsoid plot (30% probability) of IPr-H2BNH2-
BH3 (5), with carbon-bound hydrogen atoms and dichloromethane
solvate molecule omitted for clarity. B−H and N−H bond lengths
were constrained during refinement. Selected bond lengths [Å] and
angles [deg]: C(1)−B(1) 1.618(2), B(1)−N(3) 1.540(3), N(3)−B(2)
1.605(2), B(1)−H 1.13(2) and 1.15(2), B(2)−H 1.176(15),
1.192(15) and 1.179(15), N(3)−H 0.95(2), C(1)−B(1)−N(3)
116.93(15), B(1)−N(3)−B(2) 115.48(15), C(1)−B(1)−N(3)−B(2)
torsion angle = 179.90(19).
Scheme 4. Synthesis of μ-Dimethylaminodiborane (6) and
the Corresponding H2BNMe2 Adducts 7−9
Theoretical studies (B3LYP/cc-pVDZ)19 were performed on
DMAP-H2BNH2-BH3 (4), DMAP-H2BNMe2-BH3 (7), and the
structurally truncated model complexes, ImMe2CH2-H2BNH2-
BH3 (2′) and ImMe2CH2-H2BNMe2-BH3 (8′) [ImMe2CH2 =
(HCNMe)2CCH2]. The density functional theory (DFT)
calculations indicate the presence of polar and dative internal
and terminal B−N bonds (Bδ+-Nδ−), while the accompanying
LB-BH2 interactions also show dative character; furthermore,
there is reasonable agreement between the X-ray structural data
and the calculated bond lengths and angles.19 The Wiberg bond
indices for the central B−N bonds in 2′, 4, 7, and 8′ are nearly
identical to each other (0.60 to 0.69) and are only slightly
higher in value than the indices found for the terminal −NR2-
BH3 bonds in the same compounds; these results mirror the
structural data obtained by X-ray crystallography. Although
some variations are present, the B−N bonds in each of the
studied chains are derived from orbital hybrids of sp3character,
consistent with the tetrahedral geometries present at B and N.
In addition, significant intramolecular B−Hδ−···δ+H−N hydro-
gen bonding interactions within the −BH2NH2BH3 arrays in 2′
and 4 are present, as illustrated by the delocalized molecular
orbital shown in Figure 5.
Dehydrocoupling Chemistry of the Parent Amino-
borane Adducts. As indicated in Scheme 1, we were
interested in accessing complexes bearing the unsaturated B−
N entities, HBNH and BN as encapsulated units for the
future development of low temperature routes to bulk boron
nitride. Given the successful synthesis of a family of donor−
acceptor complexes LB-H2BNH2-BH3 (compounds 2−5), we
then expanded our investigations to include their dehydrogen-
ation chemistry. Because of the thermal instability of the
IPrCH2 analogue, 2, we focused our studies on the more stable
desired linear adduct, DMAP-H2BNMe2-BH3 (7), from a
hexanes/THF mixture as a pale yellow solid in a 63% yield.
Using the same methodology, stable adducts involving IPrCH2
and IPr donors (8 and 9) were also be prepared (Scheme 4).
All of the N-methylated analogues 7−9 show indefinite stability
in the solid state, as well as in refluxing toluene and THF,
respectively.
The crystallographically determined structures of adducts 7−
9 are shown collectively in Figure 4. The central H2BNMe2
units in 7 and 8 (L = DMAP and IPrCH2) bear structurally
similar gauche LB−B−N−B conformations as the parent
H2BNH2 adduct 4; however, an anti conformation was found
within the IPr adduct, IPr-H2BNMe2-BH3 (9) [CIPr−B−N−
Bterminal torsion angle = 178.38(12)°]. This difference may in
part be due to the increased steric bulk of IPr, relative to the
IPrCH2 and DMAP donors, which cause the H2BNMe2BH3
chain to take on an alternate low energy conformation. Shore
and co-workers have recently determined that for the
compound H3N-H2BNH2-BH3 both anti and gauche con-
formers could be selectively crystallized depending on the
absence or presence of 18-crown-6 in the crystalline lattice.27
Shore’s observations, in conjunction with the presence of both
anti and gauche conformations in our adducts LB-H2BNR2-BH3
(R = H and Me) imply that there is a low rotational energy
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Figure 4. Thermal ellipsoid plots (30% probability) of DMAP-H2BNMe2-BH3 (7), IPrCH2-H2BNMe2-BH3 (8), and IPr-H2BNMe2-BH3 (9) with
carbon-bound hydrogen atoms and solvate molecules omitted for clarity; compound 8 contains a disordered NMe2BH3 unit. Selected bond lengths
[Å] and angles [deg]: Compound 7: N(2)−B(1) 1.5722(19), B(1)−N(1) 1.5801(18), N(1)−C(1) 1.4835(16), N(1)−C(2) 1.4800(17), N(1)−
B(2) 1.6020(19), B(1)−H 1.115(14) and 1.104(15), B(2)−H 1.130(15), 1.130(16), and 1.126(18); N(2)−B(1)−N(1) 111.32(11), B(1)−N(1)−
B(2) 114.53(11); N(2)−B(1)−N(1)−B(2) torsion angle = 60.99(15). Compound 8; values involving a disordered NMe2BH3 unit in square
brackets: C(2)−B(1) 1.659(3), B(1)−N(3A) 1.589(6) [1.581(6)], N(3A)−C(5A) 1.507(13) [1.470(14)], N(3A)−C(6A) 1.496(12) [1.441(13)],
N(3A)−B(2A) 1.596(14) [1.585(17)], C(2)−B(1)−N(3A) 106.2(4) [120.4(4)], B(1)−N(3A)−B(2A) 120.5(8) [106.9(9)]; C(2)−B(1)−N(3A)−
B(2A) torsion angle = 52.9(7) [59.3(8)]. Compound 9: C(1)−B(1) 1.639(2), B(1)−N(3) 1.588(2), N(3)−C(4) 1.477(2), N(3)−C(5) 1.475(2),
N(3)−B(2) 1.617(2), B(1)−H 1.146(18) and 1.130(18), B(2)−H 1.12(2), 1.16(2), and 1.16(2); C(1)−B(1)−N(3) 114.39(12), B(1)−N(3)−B(2)
110.63(13); C(1)−B(1)−N(3)−B(2) torsion angle = 178.38(12).
[HBNH]3 (11B NMR: d, δ = 30.6 ppm, JB−H = 141 Hz),30
1
implying that thermal dehydrogenation of 4 had transpired.
Interestingly the observed 3:1 ratio of the 11B NMR signals for
DMAP·BH3 and borazine is lower than the 1:1 ratio expected if
decomposition according to eq 2 was the sole process. The
nature of this dehydrogenation reaction will be discussed in
detail later on in this paper.
Figure 5. HOMO-18 for ImMe2CH2-H2BNH2-BH3 (2′) as derived
from a DFT study showing orbital overlap between B−H and N−H
bonding orbitals.
Recently Liu and co-workers reported that transition metal
DMAP adduct, 4. It has been well established in the literature
that Rh(I) complexes are very effective at inducing the rapid
catalytic dehydrocoupling of amine-borane adducts
(R2NH·BH3; R = alkyl, aryl or H).25,26 To our surprise,
when DMAP-H2BNH2-BH3 (4) was combined with sub-
stoichiometric quantities of [Rh(COD)Cl]2 in THF, no
reaction was evident at both room temperature or in refluxing
THF by in situ 11B NMR spectroscopy. A small quantity of a
black powder was generated, presumably precipitated Rh metal
or clusters; however in each case unreacted DMAP-H2BNH2-
BH3 was recovered and identified by NMR spectroscopy.
The DMAP adduct (4) is completely stable at room
temperature in THF and in chlorinated solvents, which is in
contrast to IPrCH2-BH2NH2-BH3 (2) which decomposes in
these solvents over time. However, heating of a solution of 4 in
refluxing benzene affords the known adduct, DMAP·BH3 (11B
halides (MX and MX2, M = Fe, Ni, Co, Cu; X = F, Cl, Br, and
I) are effective precatalysts for the dehydrogenation of
substituted amine-boranes, RH2B·NH2R (R = alkyl).31 To
induce H2 elimination from DMAP-H2BNH2-BH3 (4) we
conducted a series of additional dehydrogenation trials using
the protocol developed by the Liu group.31 When 4 was reacted
with substoichiometric quantities of DME·NiBr2 (16 mol %) at
room temperature in THF for two days, 11B NMR spectros-
copy of the reaction mixture showed about a 50% conversion of
4 into DMAP·BH3 and borazine. When either CuBr or
Me2S·CuBr were investigated as catalysts, compound 4 was
consumed entirely after two days in both cases, yielding
DMAP·BH3 and borazine as major soluble products (eq 2)
along with the formation of an unidentified broad singlet at 5.5
ppm (ca. 5% by integration). This result, when taken with the
thermolysis of 4, imply that the DMAP adduct (4) can undergo
one formal dehydrogenation event, which results in the formal
1
NMR: quartet, δ = −13.9 ppm, JB−H = 93 Hz) and borazine
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extrusion of HBNH in the form of its trimer, borazine
[HBNH]3.
Scheme 6. Synthesis and Catalytic Dehydrogenation of
DMAP-H2BNH(tBu)-BH3 (11)
The generation of borazine in the dehydrogenation of 4
represents a potentially useful synthetic route to this hetero-
cycle. Borazine has been utilized as a precursor for the low-
pressure synthesis of boron nitride;32 however, most routes to
this B−N species yield only moderate amounts of this volatile
compound and require extensive purification steps.33 It should
be mentioned that replacement of THF in the above
dehydrogenation chemistry with a less volatile solvent (such
as triglyme) will be needed to effectively isolate borazine in
pure form because of its similar boiling point as THF; such
experiments are ongoing in our laboratories.
At this time, we do not know the mechanism by which H2
elimination from DMAP-H2BNH2-BH3 occurs; however, there
exists three possible pathways (Scheme 5): (a) direct H2 loss
combination of NMR spectroscopy, elemental analysis, and
single-crystal X-ray crystallography (Figure 6). Comparable
Scheme 5. Possible Dehydrogenation Reaction Pathways for
H2BNH2 Adducts
Figure 6. Thermal ellipsoid plot (30% probability) of DMAP-
H2BNH(tBu)-BH3 (11), with carbon-bound hydrogen atoms omitted
for clarity. Selected bond lengths [Å] and angles [deg]: N(2)−B(1)
1.583(2), B(1)−N(1) 1.593(2), N(1)−B(2) 1.623(2), N(1)−C(1)
1.519(2), B(1)−H 1.11(2) and 1.11(2), B(2)−H 1.14(2), 1.17(2) and
1.16(2); N(2)−B(1)−N(2) 108.95(13), B(1)−N(1)−C(1)
112.87(12), B(1)−N(1)−B(2) 110.57(14); N(2)−B(1)−N(1)−
B(2) torsion angle = 76.66(18).
from a central -H2BNH2- unit to give an encapsulated HB
NH moiety; (b) loss of H2 from a terminal −NH2−BH3 unit
followed by a hydride shift to give a similar LB-HBNH-BH3
adduct, which later decomposes to give borazine and LB·BH3
(LB = Lewis base); (c) redistribution of DMAP-H2BNH2-BH3
to give DMAP·BH3 and transient H2BNH2 that either
undergoes self-oligomerization (to give insoluble [H2BNH2]x)
or further dehydrogenation to yield borazine. Recent theoretical
studies have indicated a possible preference for pathway b over
pathway a given a calculated lower activation energy for H2 loss
via a terminal BH3 group.34 Support for pathway c is found in
recent studies from the Manners group wherein both thermal
and IrI catalyzed redistribution of Me3N-BH2NMe2BH3 to give
Me3N-BH3 and [Me2NBH2]2 (presumably via transient
Me2NBH2) was reported.15e We are currently exploring
the synthesis of selectively deuterated analogues of 4 to further
probe which dehydrogenation pathway is operating.
spectral features were observed in 11 as found in the related
DMAP adducts 4 and 7; however, the existence of magnetically
inequivalent hydrides within the internal BH2 group in 11 was
1
noted in the H NMR spectrum; the assignment of these
distinct B−H hydrides (at 2.64 and 2.95 ppm) was made using
1
selective H{11B} decoupling experiments at the resonant 11B
frequency for the BH2 group. Weller and co-workers also noted
similar inequivalence of the internal B−H groups in MeNH2-
BH2-NMe(H)-BH3.36
Synthesis of DMAP-H2BN(tBu)-BH3 and Dehydrogen-
ation Chemistry. We then decided to add steric bulk to the
H2BNR2 unit via the synthesis of the tert-butylated adduct,
DMAP-H2BNH(tBu)-BH3 (11). It was hypothesized that the
tBu group would also increase the electron density on nitrogen
via inductive effects resulting in a stronger bonding interaction
with the terminal BH3 unit, and in turn, provide access to the
novel the dehydrogenated adduct DMAP-HBN(tBu)-BH3.
DMAP-H2BNH(tBu)-BH3 (11) was synthesized by ring-
opening of the μ-aminodiborane (tBu)NHB2H5 (10)35a with
DMAP (Scheme 6). The tert-butylated aminoborane complex
11 precipitated as a pure white solid (73% yield) from the
hexanes/THF reaction mixture and was characterized by a
As shown in Figure 6, compound 11 crystallizes in a gauche
conformation [NDMAP−B−N−Bterminal torsion angle =
76.66(18)°], and has a coordinative NDMAP-BH2 bond length
[1.583(2) Å] that is similar in value, within experimental error
(3σ), as the respective NDMAP-B bonds present in the parent
adduct DMAP-H2BNH2-BH3 (4) [1.5728(15) Å], and the
methylated analogue DMAP-H2BNMe2-BH3 (7) [1.5722(19)
Å].
With the tert-butyl substituted adduct 11 in hand, we decided
to explore dehydrogenative chemistry to access the iminobor-
ane adduct DMAP-HBN(tBu)-BH3. Combining 11 with a
catalytic amount of Me2S·CuBr in benzene (for 12 h at room
temperature) resulted in the formation of DMAP·BH3 and the
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μ-aminodiborane (tBu)NHB2H5 (10) in 15 and 21% yields,
respectively, with 64% unreacted 11, as determined by 11B
NMR spectroscopic analysis of the reaction mixture. We then
reacted 11 with a catalytic amount of [Rh(COD)Cl]2 in
benzene at room temperature, and noted the formation of a
series of products by in situ 11B NMR spectroscopy (Scheme
6): specifically, the presence of [tBuNBH]3 (12, 5%, d, −25.2
attempted to independently synthesize the adducts Cy3P-
H2BNH2-B(C6F5)3 and IPr-H2BNH2-B(C6F5)3 via a different
synthetic route. Scheer and co-workers reported the dehydro-
genative coupling of the phosphine complex, H3P·W(CO)5
with trimethylamine-alane or -gallane adducts [Me3N·EH3; E =
Al and Ga] to give the novel donor−acceptor complexes,
Me3N-H2E-PH2-W(CO)5.14a Therefore we explored analogous
chemistry involving the known borane adducts IPr·BH3 and
Cy3P·BH3, and the ammonia adduct H3N·B(C6F5)3 to
potentially afford the aminoborane complexes, LB-H2BNH2-
B(C6F5)3 (eq 3). However when these reactions were
1
ppm, JB−H = 143 Hz), [tBuNH-BH2]3 (13, 31%, t, −4.7 ppm,
1JB−H = 95 Hz), poly-tert-butylborazylene (14, 11%, br s,
−30.1),35 DMAP·BH3 (31%), and 10 (21%) was observed. The
formation of large quantities of the known cycloborazane
trimer, [tBuNH-BH2]3 is interesting as this product is not
generated when 11 is kept in benzene in the absence of Rh
catalyst. One possible pathway to this species could involve
transfer hydrogenation chemistry between N-tert-butyl borazine
[tBuNBH]3 and 11 to yield the hydrogenated trimer [tBuNH-
BH2]3 and products stemming from the dehydrogenation of
11.37 Interestingly, our findings also corroborate studies
conducted by Wright and co-workers, who showed that the
dehydrocoupling of tBuH2N·BH3 with Al(NMe2)3 as a catalyst
resulted in mixtures of the trimers 12 and 13, and related B−N
oligomers (such as 14).35b
conducted at room temperature or in refluxing solvent (toluene
or THF), no reaction occurred. Furthermore, attempts to
induce dehydrogenation chemistry in the presence of the
dehydrogenation catalysts [Rh(COD)Cl]2 and CuBr failed to
yield any observable chemistry. At the moment we are
exploring salt metathesis routes to the desired arylfluorobor-
ane-terminated adducts.
Attempted Syntheses of B(C6F5)3-Terminated Amino-
borane Adducts, LB-H2BNH2-B(C6F5)3. Given the lack of
success in isolating dehydrogenated adducts of the general form
LB-H2BNH2-BH3, we decided to target complexes in which the
terminal BH3 group was replaced by the arylfluoroborane Lewis
acid, B(C6F5)3, LB-H2BNH2-B(C6F5)3. Not only would this
confine any dehydrogenation chemistry to the central H2BNH2
CONCLUSION
■
In summary we have prepared formal donor−acceptor adducts
of the parent aminoborane H2BNH2, LB-H2BNH2-BH3 (LB =
IPrCH2, Cy3P, DMAP and IPr). Initial studies have
demonstrated that the DMAP adduct DMAP-H2BNH2-BH3
can undergo a single dehydrogenation event upon heating, or in
the presence of metal catalysts, to yield borazine and the
involatile adduct DMAP·BH3. In addition, the preparation of
N-substituted adducts LB-H2BNR2-BH3 was reported along
with comparative thermolysis chemistry and theoretical
bonding analyses. Attempts to replace terminal BH3 Lewis
acid groups with the stronger Lewis acid B(C6F5)3 to yield
complexes of the general form LB-H2BNH2-B(C6F5)3 were
unsuccessful. Future work will involve exploring alternate
pathways toward intercepting molecular BN as metastable
adducts, LB-BN-LA, and the use of these species as
molecular sources of bulk boron nitride.
38
unit but also the high Lewis acidity of B(C6F5)3 might
encourage the eventual formation of a metastable complex of
boron nitride LB-BN-B(C6F5)3 following the dehydrogen-
ation chemistry outlined in Scheme 1.
To investigate if the terminal BH3 unit in the aminoborane
adducts LB-H2BNH2-BH3 could be displaced by B(C6F5)3,
Cy3P-H2BNH2-BH3 (3) was combined with 1 equiv of
B(C6F5)3 in a 10:1 toluene/THF solvent mixture. The reaction
was monitored in situ by NMR, and after 3 h, numerous
products were observed by 31P and 11B NMR spectroscopy,
with no sign of the potential products [Cy3P(C6F4)BF(C6F5)2],
the [HB(C6F5)3]− anion, [HBNH]3, [H2BNH2]3 or Cy3P·BH3
noted;20,38c commensurate with the above data, the resulting
19F{1H} NMR spectrum contained many resonances from
−130 to −170 ppm (>30 signals). Unfortunately, equally
complicated spectra were obtained when the related LB-
H2BNH2-BH3 adducts were reacted with B(C6F5)3, thus clean
BH3/B(C6F5)3 exchange involving these species does not
appear to be feasible.
Positing that BH3/B(C6F5)3 Lewis acid exchange might
transpire in a system where competing dehydrogenation
chemistry cannot occur, we reacted the methylated amino-
borane adduct DMAP-H2BNMe2-BH3 (7) with B(C6F5)3.
Reaction of 7 with B(C6F5)3 in toluene yielded a mixture of
products by 11B NMR. The mixture contained unreacted 7 (ca.
53%), small quantities of [Me2N-BH2]2 (8%), two minor
unknown products [20 ppm (br s, 7%) and 11.5 ppm (d, 1JB−H
= 101 Hz, 7%)], and a doublet at −25 ppm (1JB−H = 95 Hz, ca.
25%) corresponding to the [HB(C6F5)3]− anion.39 It is evident
that hydride transfer from a BH2 or BH3 group in 7 to B(C6F5)3
occurred; however, the nature of the corresponding cation
(partnered with [HB(C6F5)3]−) is unknown at this time.
Because of the lack of clean BH3/B(C6F5)3 exchange
chemistry involving the adducts 3 and 4 and B(C6F5)3 we
EXPERIMENTAL SECTION
■
General Procedures. All reactions were performed using standard
Schlenk line techniques under an atmosphere of nitrogen or in an inert
atmosphere glovebox (Innovative Technology, Inc.). Solvents were
dried using Grubbs-type solvent purification system manufactured by
Innovative Technology, Inc.,40 degassed (freeze−pump−thaw meth-
od) and stored under an atmosphere of nitrogen prior to use.
H3N·BH3, tBuNH2·BH3, B(C6F5)3, and Cy3P were purchased from
Aldrich and used as received. Me2NH·BH3, H3B·THF, and
dimethylaminopyridine (DMAP) were purchased from Alfa Aesar
and used as received. 1,3-Bis-(2,6-diisopropylphenyl)-imidazol-2-
ylidene (IPr),41 1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-methyli-
dene (IPrCH2),17b H3N·B(C6F5)3,38c IPr·BH3,24 and Cy3P·BH3
42
were prepared following literature procedures. 11B and 19F{1H} NMR
1
spectra were collected on a Varian iNova-400 spectrometer, while H
and 13C{1H} NMR spectra were either collected on Varian iNova 400
or Varian VNMRS-500 spectrometers. Samples were referenced
1
externally to SiMe4 (1H, H{11B}, 13C{1H}), F3B·OEt2 (11B, 11B{1H})
and CFCl3 (19F{1H}). Elemental analyses were performed by the
Analytical and Instrumentation Laboratory at the University of
Alberta. Infrared spectra were recorded on a Nicolet IR100 FTIR
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Table 1. Crystallographic Data for Compounds 3−5, 7−9, and 11
b
3
4
5
7
8
9
11
formula
C
19.50H43B2Cl3NP
C7H17B2N3
164.86
C28H45B2Cl2N3
516.19
C9H21B2N3
192.91
C31H51B2ClN3
522.82
C32H54B2N3
502.40
C11H25B2N3
220.96
fw
450.49
cryst. dimens. (mm)
0.34 × 0.29 × 0.26 0.23 × 0.13 ×
0.36 × 0.33 ×
0.19
0.38 × 0.36 ×
0.31
0.61 × 0.36 ×
0.27
0.23 × 0.23 ×
0.15
0.33 × 0.15 ×
0.04
0.06
cryst. syst.
space group
unit cell
a (Å)
monoclinic
monoclinic
triclinic
orthorhombic
monoclinic
monoclinic
monoclinic
P21/c
P21/n
P1
P212121
P21/n
P21/n
P21/c
̅
10.9130 (6)
18.3557 (10)
13.6997 (8)
11.2392 (2)
7.4511 (1)
12.1976 (2)
9.3058 (3)
9.4670 (3)
20.3326 (7)
77.8380 (13)
85.7537 (13)
61.4232 (15)
1536.99 (9)
2
8.4065 (6)
10.2412 (5)
12.9371 (7)
24.8848 (13)
12.7575 (2)
19.0656 (3)
13.1625 (2)
8.0183 (2)
18.0715 (4)
9.9072 (2)
b (Å)
10.0395 (8)
14.3963 (11)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
110.9230 (10)
101.0881 (9)
99.8480 (10)
93.3871 (9)
91.7241 (13)
2563.3 (2)
4
1002.41 (3)
4
1215.01 (16)
4
3248.4 (3)
4
3195.91 (9)
4
1434.93 (6)
4
ρcalcd (g cm−3
)
1.171
1.092
1.115
1.055
1.069
1.044
1.023
μ (mm−1
)
0.430
0.496
2.034
0.062
0.140
0.440
0.448
T (K)
173(1)
55.32
173(1)
140.02
1822
173(1)
173(1)
55.04
173(1)
50.50
173(1)
141.44
20925
173(1)
141.86
9552
2θmax (deg)
137.20
total data
22370
10182
10623
22840
unique data (Rint)
5933 (0.0424)
4489
1822 (0.0000)
1637
5335 (0.0117)
4978
2782 (0.0354)
2418
5886 (0.0286)
4738
5834 (0.0184)
5320
2706 (0.0393)
2149
observed data
[I > 2σ(I)]
params.
232
140
393
150
376
346
171
a
R1 [I > 2σ(I)]
0.0422
0.0336
0.0621
0.0359
0.0570
0.0555
0.0533
a
wR2 [all data]
0.1252
0.0956
0.1675
0.0886
0.1725
0.1623
0.1564
difference map Δρ
0.300/−0.204
0.156/−0.146
0.313/−0.319
0.165/−0.120
0.386/−0.627
0.424/−0.436
0.397/−0.216
(e Å−3
)
a
b
2
4
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo − Fc2)2/∑w(Fo )]1/2. Flack parameter = −1.6(19).
spectrometer as Nujol mulls between NaCl plates. Melting points were
measured in sealed glass capillaries under nitrogen using a MelTemp
melting point apparatus and are uncorrected.
with the data integration program SAINT (version 7.68A),50 using all
reflection data (exactly overlapped, partially overlapped, and non-
overlapped). The refined value of the twin fraction (SHELXL-97
BASF parameter) was 0.292(2).
X-ray Crystallography. Crystals suitable for X-ray diffraction
studies were removed from a vial (in a glovebox) and immediately
coated with a thin layer of hydrocarbon oil (Paratone-N). A suitable
crystal was then mounted on a glass fiber, and quickly placed in a low
temperature stream of nitrogen on the X-ray diffractometer.43 All data
were collected using a Bruker APEX II CCD detector/D8
diffractometer using Mo Kα or Cu Kα radiation, with the crystals
cooled to −100 °C. The data were corrected for absorption through
Gaussian integration from the indexing of the crystal faces.44 Crystal
structures were solved using direct methods (3, 4, 5, 7, 8, 9, and 11:
SHELXD45,46), and refined using SHELXS-97 (Table 1). The
assignment of hydrogen atoms positions were based on the sp2 or
sp3 hybridization geometries of their attached carbon atoms, and were
given thermal parameters 20% greater than those of their parent
atoms.
Special Refinement Conditions. Compound 3. Attempts to
refine peaks of residual electron density as disordered or partial-
occupancy solvent dichloromethane chlorine or carbon atoms were
unsuccessful. The data were corrected for disordered electron density
through use of the SQUEEZE procedure47 as implemented in
PLATON.48 A total solvent-accessible void volume of 734 Å3 with a
total electron count of 248 (consistent with 6 molecules of solvent
dichloromethane, or 1.5 molecules per formula unit of the Cy3P-
BH2NH2-BH3 molecules) was found in the cell.
Compound 5. Distance restraints (by use of the SHELXL SADI
instruction) were applied to the disordered isopropyl group for the
following bonds: C32−C37A and C32−C37B; C37A−C38A, C37A−
C39A, C37B−C38B, and C37B−C39B. Distance restrains (SADI)
were also applied to the N−H and B−H bonds: B1−H1B1 and B1−
H1BB; B2−H2BA, B2−H2BB, B2−H2BC; N3−H3NA, N3−H3NB.
The disordered solvent dichloromethane molecule was subjected to
both C−Cl distance restraints (SADI) and “rigid bond” restraints by
use of the SHELXL DELU instruction. Finally, an “anti-bumping”
restraint was applied to the H1BB···H2SB distance between one of the
hydrogen atoms of the minor orientation of the disordered solvent
dichloromethane and one of the hydrogen atoms attached to B1.
Compound 8. The B1−N3A and B1−N3B distances were
restrained to be approximately equal by use of SHELXL SADI
instruction during refinement. The C1S−C1S′ distance was restrained
to be a target value of 1.50(2) Å during refinement.
Compound 9: Distances within the disordered solvent n-hexane
molecule were restrained during refinement: d(C1SA-C1SA′) =
d(C1SA-C2SA) = d(C2SA-C3SA) = d(C1SB-C1SB′) = d(C1SB-
C2SB) = d(C2SB-C3SB) = 1.52(2) Å; d(C1SA···C2SA′) =
d(C1SA···C2SA′) = d(C1SA···C3SA) = 2.46(2) Å (primed atoms
are related to unprimed ones via the crystallographic inversion center
(0, 0, 1/2)).
Compound 4. The crystal used for data collection was found to
display non-merohedral twinning. Both components of the twin were
indexed with the program CELL_NOW.49 The second twin
component can be related to the first component by 180° rotation
about the [1 0 1] axis in real space and about the [5/6 0 1] axis in
reciprocal space. Integrated intensities for the reflections from the two
components were written into a SHELX-97 HKLF 5 reflection file
Synthetic Procedures. Synthesis of Aminodiborane,
NH2B2H5 (1). Ammonia-borane (61.7 mg, 2.00 mmol) was taken up
as a slurry in 15 mL of hexanes, and H3B·THF (2.00 mL, 1.0 M
solution in THF, 2.00 mmol) was added. The resulting cloudy solution
was stirred for 3 days at room temperature to yield a colorless solution.
The solution volume was concentrated by half under vacuum and
filtered. The presence of 1 can be determined by diluting a small
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aliquot of the reaction mixture with C6D6 for 11B NMR analysis. 11B
NMR (128 MHz, C6D6): δ = −26.6 ppm [d of t, 1JBH = 27 Hz (B−H
bridging) and 138 Hz (B−H terminal)]. In all subsequent reactions,
the formation of 1 was assumed to be quantitative for the purpose of
calculating reactant quantities and product yields.
10 Hz, PCy3), 26.2 (PCy3). 11B (128 MHz, C6D6): δ = −17.5 (br,
-BH2NH2BH3), −21.0 (br, -BH2NH2BH3). 31P{1H} (161 MHz,
C6D6): δ = 10.2 (br). IR (Nujol/cm−1): 3233 (w, νNH), 2299 (w,
νBH), 2204 (w, νBH), 2190 (w, νBH). Anal. Calcd. for C18H40B2NP:
C, 66.91; H, 12.48; N, 4.33. Found: C, 66.93; H, 12.40; N, 4.14. Mp
(°C) 117−119.
Synthesis of IPrCH2-H2BNH2-BH3 (2). A solution of IPrCH2
(0.859 g, 2.13 mmol) in 3 mL of hexanes was added to a purified
solution of NH2B2H5 (1) (made from 66.0 mg of H3N·BH3, 2.14
mmol) in 5:1 hexanes/THF to give a white slurry which was stirred at
room temperature for 2 h. The mother liquor was decanted, and the
white precipitate was washed with 2 × 6 mL portions of hexanes and
toluene. The product was dried under vacuum, to give 2 as a white
powder (0.587 g, 62%).
Synthesis of DMAP-H2BNH2-BH3 (4). A solution of p-
dimethylaminopyridine (0.0918 g, 0.75 mmol) in 5 mL of 5:1
hexanes/THF was added to a solution of NH2B2H5 (made from 23.2
mg of H3N·BH3, 0.75 mmol) in 10 mL of 5:1 hexanes/THF solvent
mixture. The resulting mixture clouded to give a white slurry after 8 h.
The mother liquor was then decanted, and the white solid was washed
with 6 mL portions of hexanes, diethyl ether and toluene. The product
was dried under vacuum, giving 4 as a white powder (0.0920 g, 74%).
Crystals of 4 suitable for X-ray crystallography (colorless needles)
were obtained by cooling a saturated dichloroethane/Et2O solution to
−35 °C.
1H NMR (400 MHz, CDCl3): δ = 7.52 (t, 4H, 3JHH = 7.5 Hz, ArH),
7.34 (d, 2H, 3JHH = 7.5 Hz, ArH), 7.00 (s, 2H, −N-CH), 2.56 (septet,
3
3
4H, JHH = 6.5 Hz, −CH(CH3)2), 1.88 (br t, 2H, JHH = 4.0 Hz,
IPrCH2), 1.41 (s, 2H, -BH2NH2BH3, assignment made by selective
1H{11B} decoupling), 1.35 (d, 12H 3JHH = 6.5 Hz, −CH(CH3)2), 1.16
3
1H NMR (400 MHz, CDCl3): δ = 8.08 (d, 2H, JHH = 7.3 Hz,
3
ArH), 6.57 (d, 2H, 3JHH = 7.3 Hz, ArH), 3.13 (s, 6H, -N(CH3)2), 2.79
(d, 12H, JHH = 6.5 Hz, −CH(CH3)2), 1.06 (s, 3H, -BH2NH2BH3,
assignment made by selective 1H{11B} decoupling); the −NH2- group
was not located. 13C{1H} (125 MHz, CDCl3): δ = 162.3 (ArC), 145.7
(ArC), 131.4 (ArC), 130.8 (ArC), 124.7 (-N-CH), 121.3 (-N-C(CH2)-
N-), 28.9 (−CH(CH3)2), 25.7 (−CH(CH3)2), 22.6 (IPrCH2). 11B
(s, 2H, -BH2NH2BH3, assignment made by selective 1H{11B}
3
decoupling), 2.19 (br s, 2H, -BH2NH2BH3), 1.26 (t, 3H, JHH = 4.4
Hz, -BH2NH2BH3, assignment made by selective 1H{11B} decoupling).
13C{1H} (125 MHz, CDCl3): δ = 155.8 (ArC), 147.4 (ArC), 106.2
(ArC), 39.6 (Ar−N(CH3)2). 11B (128 MHz, CDCl3): δ = −3.7 (t, 1JBH
= 100 Hz, -BH2NH2BH3), −21.8 (q, 1JBH = 92 Hz, -BH2NH2BH3). IR
(Nujol/cm−1): 3301 (w, νNH), 2364 (w, νBH), 2297 (w, νBH), 2243
(w, νBH). Mp (°C) 138−139. Despite repeated attempts, combustion
analyses gave consistently low values for nitrogen content (lower by
ca. 2%). See the Supporting Information, Figures S3 and S4 for copies
of the NMR spectra of 4.19
1
(128 MHz, CDCl3): δ = −14.7 (br t, JBH = 95 Hz, -BH2NH2BH3),
−21.9 (br q, 1JBH = 87 Hz, -BH2NH2BH3). IR (Nujol/cm−1): 3340 (s,
νNH), 2351 (s, νBH), 2304 (s, νBH), 2219 (s, νBH). Anal. Calcd. for
C28H45B2N3: C, 75.52; H, 10.19; N, 9.44. Found: C, 74.02; H, 11.24;
N, 8.49. Mp (°C) 148−149. Compound 2 was always obtained with
about 10% of [IPrCH3]BH4 as a contaminant (vide infra); thus
satisfactory elemental analyses could not be obtained (see the
Supporting Information, Figures S1 and S2).19
Thermolysis of DMAP-H2BNH2-BH3 (4). Ten milligrams of 4 was
dissolved in 0.5 mL of benzene and sealed in a J. Young NMR tube
under an atmosphere of nitrogen. The sample was heated at 100 °C
for 24 h. The in situ 11B NMR analysis showed the presence of
Decomposition of IPrCH2-H2BNH2-BH3 (2) in Solution. To
investigate its decomposition, compound 2 (75.3 mg, 0.168 mmol)
was dissolved in 8 mL of THF and stirred at room temperature. An
aliquot of solution was removed and analyzed by 11B NMR
spectroscopy every hour. Initial measurements (<15 min.) showed
no signs of decomposition. After 24 h about 50% had decomposed
into [IPrCH3][BH4] by integration of the 11B NMR spectrum. After
48 h about 90% had decomposed into the same product. Volatiles
1
DMAP·BH3 and borazine (doublet at 30.6 ppm, JBH = 141 Hz) in a
3:1 ratio by integration of peaks, with trace (<5%) starting material.
The volatiles were removed in vacuo and the white residue (ca. 5 mg)
was identified as DMAP·BH3 by NMR.
NMR Data for DMAP·BH3:51 1H NMR (400 MHz, CDCl3): δ =
were removed in vacuo, and white residue was analyzed by 1H and 11
B
3
3
8.05 (d, 2H, JHH = 7.6 Hz, ArH), 6.49 (d, 2H, JHH = 7.6 Hz, ArH),
NMR to yield a crude sample of [IPrCH3][BH4], containing about
3.08 (s, 6H, -N(CH3)2). 11B (128 MHz, CDCl3): δ = −13.9 (q, 1JBH
93 Hz, -BH3).
=
10% of 2.
1
NMR Data for [IPrCH3][BH4]. H NMR (400 MHz, CDCl3): δ =
3
Dehydrogenation of DMAP-H2BNH2-BH3 (4). (i) To a solution
of 4 (34.2 mg, 0.207 mmol) in 5 mL of THF was added about 1.0 mg
of [Rh(COD)Cl]2 (1 mol %). The solution was initially clear yellow,
and turned black-green after 2 h. An aliquot of the solution was then
analyzed by 11B NMR at the initiation of reaction and after 60 h,
showing only the presence of 4 and no other soluble products. The
mother liquor was decanted and dried in vacuo to recover 71% of
starting material.
(ii) 20.7 mg (0.126 mmol) of 4 and 6.2 mg of NiBr2·DME (0.02
mmol, 16 mol % cat., DME = dimethoxyethane) were dissolved in 6
mL of THF to yield a golden brown solution that quickly turned black.
The mixture was stirred for 48 h to yield a clear colorless solution with
black precipitate. 11B NMR analysis of this clear solution showed that
50% of the starting material remained, with the new products
DMAP·BH3 (40%) and borazine (10%) formed.
(iii) 19.8 mg (0.120 mmol) of 4 and 8.1 mg of CuBr·SMe2 (32 mol
%) were dissolved in 6 mL of THF to yield a light yellow solution that
quickly turned black after 10 min. The mixture was stirred for 48 h to
give a clear colorless solution with a black metallic precipitate. 11B
NMR analysis of this clear solution showed no signs of starting
material, with the presence of DMAP·BH3 (65%) and borazine (30%),
and one unidentified product (5%, br s, 5.5 ppm) detected.
(iv) 21.5 mg (0.130 mmol) of 4 and 6.0 mg of CuBr (32 mol %)
were dissolved in 6 mL of THF to yield a pale yellow solution that
turned black after 2 h. The mixture was stirred for 48 h to give a clear
colorless solution with a black metallic precipitate. 11B NMR of this
clear solution showed no signs of starting material, with the presence
8.18 (s, 2H, −N-CH), 7.65 (t, 4H, JHH = 8.0 Hz, ArH), 7.44 (d, 2H,
3JHH = 8.0 Hz, ArH), 2.34 (septet, 4H, JHH = 6.8 Hz, −CH(CH3)2),
3
2.14 (s, 3H, IPrCH3), 1.32 (d, 12H, 3JHH = 6.8 Hz, −CH(CH3)2), 1.23
3
1
(d, 12H, JHH = 6.8 Hz, −CH(CH3)2), 0.33 (q, 4H, JBH = 81.5 Hz,
−
1
BH4−). 11B (128 MHz, CDCl3): δ = −40.6 (pentet, JBH = 81 Hz,
BH4 ). 13C{1H} (125 MHz, CDCl3): δ = 145.7 (−N−C-N), 144.9
(ArC), 132.6 (ArC), 129.0 (ArC), 126.2 (ArC), 125.4 (-N-CH), 29.2
(−CH(CH3)2), 24.6 (−CH(CH3)2), 23.4 (−CH(CH3)2), 10.9
(IPrCH3).
Synthesis of Cy3P-H2BNH2-BH3 (3). To a cold (−35 °C) solution
of 1 (made from 0.0696 g of H3N·BH3, 2.25 mmol) in 15 mL of 5:1
hexanes/THF was added PCy3 (630 mg, 2.25 mmol). The resulting
mixture was stirred at reduced temperature for 1 h, which clouded to
give a white slurry after 16 h at room temperature. The mother liquor
was then decanted, and the white solid was washed with two 6 mL
portions of 5:1 hexanes/THF. The product was dried under vacuum,
giving 3 as a white powder (0.440 g, 61%). Crystals of 3 suitable for X-
ray crystallography (colorless prisms) were obtained by cooling a
saturated dichloromethane/hexanes solution to −35 °C.
1H NMR (400 MHz, C6D6): δ = 2.51 (br s, 3H, -BH2NH2BH3,
1
assignment made by selective H{11B} decoupling), 2.42 (br s, 2H,
-BH2NH2BH3, assignment made by selective 1H{11B} decoupling),
3
2.20 (br s, 2H, -NH2), 1.96 (q, 3H, JHH = 12.0 Hz, P(C6H11)3), 1.84
3
3
(d, 6H, JHH = 13.0 Hz, P(C6H11)3), 1.59 (d, 6H, JHH = 9.2 Hz,
P(C6H11)3), 1.51 (d, 3H, 3JHH = 9.6 Hz, P(C6H11)3), 1.25 (q, 6H, 3JHH
= 12.0 Hz, P(C6H11)3), 1.02 (m, 9H, P(C6H11)3). 13C{1H} (125 MHz,
1
2
C6D6): δ = 31.2 (d, JPC = 29 Hz, PCy3), 28.5 (PCy3), 27.4 (d, JPC
=
12913
dx.doi.org/10.1021/ic3018997 | Inorg. Chem. 2012, 51, 12905−12916

Inorganic Chemistry
Article
of DMAP·BH3 (65%) and borazine (30%), and one unidentified
product (5%, br s, 5.5 ppm) detected.
Synthesis of IPrCH2-H2BN(CH3)2-BH3 (8). A solution of IPrCH2
(0.988 g, 2.45 mmol) in 3 mL of hexanes was added to a solution of
N(CH3)2B2H5 (prepared from 0.145 g of Me2NH·BH3 as described
previously, 2.47 mmol). The initially clear and colorless solution
turned to a pale yellow slurry, and the reaction mixture was stirred at
room temperature for 4 h. The mother liquor was decanted, and the
precipitate was washed with hexanes (2 × 4 mL). The product was
dried under vacuum to give 8 as a pale yellow powder (0.988 g, 85%).
Crystals of 8 suitable for X-ray crystallography (yellow prisms) were
grown by cooling a saturated hexanes/dichloroethane solution to −35
°C.
Synthesis of IPr-H2BNH2-BH3 (5) from the Reaction of 4 and
IPr. IPr (110.0 mg, 0.284 mmol) and 4 (18.9 mg, 0.115 mmol) were
taken up in 5 mL of benzene, and stirred at room temperature to yield
a clear golden solution after 8 h. The volatiles were removed in vacuo
to yield a pale yellow solid, and the solid was washed with 10 mL of
hexanes, and then 5 mL of a 1:1 hexanes/benzene solvent mixture.
The remaining solid was dried under vacuum to yield 5 as a white solid
(20.9 mg, 42%). X-ray quality crystals were obtained by cooling a
hexanes/dichloromethane solution of 5 to −35 °C.
1H NMR (500 MHz, CDCl3): δ = 7.17 (t, 4H, 3JH−H = 7 Hz, ArH),
7.08 (d, 2H, 3JHH = 7 Hz, ArH), 6.42 (s, 2H, -N-CH), 2.90 (septet, 4H,
3JHH = 6.8 Hz, −CH(CH3)2), 2.25 (s, 2H, -BH2NH2BH3, assignment
1H NMR (500 MHz, CDCl3): δ = 7.52 (t, 4H, 3JHH = 7.1 Hz, ArH),
7.33 (d, 2H, 3JHH = 7.1 Hz, ArH), 6.98 (s, 2H, −N-CH), 2.58 (septet,
3
3
4H, JHH = 7.0 Hz, −CH(CH3)2), 1.96 (br t, 2H, JHH = 6.0 Hz,
IPrCH2), 1.97 (s, 2H, -BH2N(CH3)2BH3, assignment made by
1
made by selective H{11B} decoupling), 2.11 (s, 3H, -BH2NH2BH3,
assignment made by selective 1H{11B} decoupling), 1.85 (s, 2H,
selective H{11B} decoupling), 1.82 (s, 6H, -BH2N(CH3)2BH3), 1.39
1
3
3
3
-BH2NH2BH3), 1.43 (d, 12H, JHH = 6.8 Hz, −CH(CH3)2), 0.98 (d,
(d, 12H, JHH = 7.0 Hz, −CH(CH3)2), 1.16 (d, 12H, JHH = 7.0 Hz,
−CH(CH3)2), 1.05 (s, 3H, -BH2N(CH3)2BH3, assignment made by
selective 1H{11B} decoupling). 13C{1H} (125 MHz, CDCl3): δ = 162.3
(ArC), 145.7 (ArC), 131.2 (ArC), 131.0 (ArC), 124.6 (-N-CH), 121.2
(-N-CCH2), 51.6 (-N(CH3)2), 29.0 (−CH(CH3)2), 25.8 (−CH-
(CH3)2), 22.6 (IPrCH2). 11B (128 MHz, CDCl3): −7.7 (br t, 1JBH = 90
3
12H, JHH = 6.7 Hz, −CH(CH3)2). 13C{1H} (125 MHz, CDCl3): δ =
146.1 (ArC), 133.7 (ArC), 130.8 (ArC), 128.3 (ArC), 124.5 (-N-CH),
122.7 (ArC), 28.8 (−CH(CH3)2), 25.8 (−CH(CH3)2), 22.9 (−CH-
(CH3)2). 11B{1H} (128 MHz, CDCl3): δ = −18.2 (br). IR (Nujol/
cm−1): 3211 (w, νNH), 2444, 2170, 2201, 2197 (w, νBH). Mp (°C):
144−145. Despite repeated attempts, combustion analyses gave
consistently low values for nitrogen content (lower by ca. 2%). See
the Supporting Information, Figures S5 and S6 for copies of the NMR
spectra of 5.19
1
Hz, -BH2N(CH3)2BH3), −13.1 (br q, JBH = 80 Hz, -BH2N-
(CH3)2BH3). IR (Nujol/cm−1): 2335 (w, νBH), 2295 (w, νBH).
Anal. Calcd. for C30H49B2N3·(0.5 ClCH2CH2Cl): C, 71.21; H, 9.83; N,
8.04. Found: C, 72.27; H, 10.17; N, 7.90. Mp (°C) 158−159.
Thermolysis of IPrCH2-H2BN(CH3)2-BH3 (8). A solution of 8 (ca.
10 mg) in 0.5 mL of C6D6 was sealed in a J. Young NMR tube under
an atmosphere of nitrogen, and heated to 65 °C for 24 h. 11B and 1H
NMR spectroscopy showed no signs of decomposition. The sample
was then heated at 100 °C for 24 h. The reaction mixture was analyzed
by 11B and 1H NMR spectroscopy. The only identifiable soluble
product by in situ 11B NMR spectroscopy was the dimer [Me2NBH2]2
Synthesis of Dimethylaminodiborane, (CH3)2NB2H5 (6).
Me2NH·BH3 (0.162 g, 2.75 mmol) was dissolved in 6 mL of 5:1
hexanes/THF and combined with 1.0 mg of [Rh(COD)Cl]2.
Immediately the solution turned clear yellow, and within 2 h the
solution was dark black-green. The mixture was stirred for 16 h at
room temperature and filtered through a plug of silica; the presence of
[Me2NBH2]2 was confirmed by 11B NMR spectroscopy which showed
a triplet at 4.75 ppm (1JBH = 113 Hz).52 One equivalent of H3B·THF
(2.7 mL, 1.0 M solution in THF, 2.7 mmol) was added, and the clear
colorless mixture was heated at 60 °C for 8 h to yield 6 by 11B NMR
1
(triplet at 4.75, JBH = 113 Hz), with trace starting material (9% by
integration).
Synthesis of IPr-H2BN(CH3)2-BH3 (9). A solution of N-
(CH3)2B2H5 was prepared as described above (from 0.82 mmol of
Me2NH·BH3 and 0.81 mmol of H3B·THF). To this clear and colorless
solution (approximately 10 mL), was added 6 mL of a hexanes
solution of IPr (308 mg, 0.79 mmol). The golden brown mixture was
initially opaque, but became clear after 12 h. The volatiles were
removed in vacuo, yielding crude 9 as a light brown powder (321 mg,
88%). The product was further purified by recrystallization from a
saturated solution of 1:1 THF/hexanes at −35 °C (96.0 mg, 26%).
Crystals obtained by this method (light yellow prisms) were suitable
for X-ray crystallography.
(128 MHz, THF): δ = −17.5 (br t, 1JBH = 147 Hz). The observed 11
B
NMR resonance for 6 was in agreement with data reported previously
for this compound.25d,g In all subsequent reactions, the formation of 6
was assumed to be quantitative for the purpose of calculating reactant
quantities and product yields.
Synthesis of DMAP-H2BN(CH3)2-BH3 (7). A solution of p-
dimethylaminopyridine (0.336 g, 2.75 mmol) in 5 mL of 5:1
hexanes:THF was combined with a 15 mL solution of N(CH3)2B2H5
prepared in situ (from 2.74 mmol of Me2NH·BH3). The initially clear
colorless solution turned to a pale yellow slurry, and the reaction
mixture was stirred for 8 h at room temperature. The volatiles were
removed under vacuum, and the solid material was washed with 4 mL
portions of hexanes, diethyl ether, and benzene. The volatiles were
then removed under vacuum to give 7 as a pale yellow solid (0.332 g,
63%). Crystals of 7 suitable for X-ray crystallography (colorless
prisms) were grown by cooling a concentrated hexanes/THF solution
to −35 °C.
1H NMR (400 MHz, C6D6): δ = 7.19 (t, 4H, 3JHH = 8.0 Hz, ArH),
7.10 (d, 2H, 3JHH = 7.9 Hz, ArH), 6.54 (s, 2H, −N-CH), 2.98 (septet,
3
4H, JHH = 6.8 Hz, −CH(CH3)2), 2.20 (s, 2H, -BH2N(CH3)2BH3,
assignment made by selective 1H{11B} decoupling), 2.09 (s, 6H,
-BH2N(CH3)2BH3), 2.00 (s, 3H, -BH2N(CH3)2BH3, assignment made
3
by selective 1H{11B} decoupling), 1.44 (d, 12H, JHH = 6.7 Hz,
3
−CH(CH3)2), 0.99 (d, 12H, JHH = 6.7 Hz, −CH(CH3)2). 13C{1H}
3
1H NMR (500 MHz, CDCl3): δ = 8.13 (d, 2H, JHH = 6.0 Hz,
(125 MHz, C6D6): δ = 161.9 (N-C-N), 146.2 (ArC), 135.4 (ArC),
130.6 (ArC), 124.3 (-N-CH), 123.2 (ArC), 54.6 (-BH2N(CH3)2BH3),
29.1 (−CH(CH3)2), 26.2 (−CH(CH3)2), 22.4 (−CH(CH3)2). 11B
(128 MHz, C6D6): δ = −10.0 (br s, -BH2N(CH3)2BH3), −11.3 (br s,
-BH2N(CH3)2BH3). IR (Nujol/cm−1): 2438 (w, νBH), 2334 (w,
νBH), 2269 (w, νBH), 2237 (w, νBH). Anal. Calcd. for C29H47B2N3:
C, 75.83; H, 10.31; N, 9.15. Found: C, 75.97; H, 10.77; N, 8.28. Mp
(°C): 179−180.
3
ArH), 6.52 (d, 2H, JHH = 6.0 Hz, ArH), 3.10 (s, 6H, -BH2N-
(CH3)2BH3), 2.55 (s, 2H, -BH2N(CH3)2BH3, assignment made by
1
selective H{11B} decoupling), 2.23 (s, 6H, -N(CH3)2), 1.32 (t, 3H,
3JHH = 4.4 Hz, -BH2N(CH3)2BH3, assignment made by selective
1H{11B} decoupling). 13C{1H} (125 MHz, CDCl3): δ = 155.8 (ArC),
148.6 (ArC), 105.5 (ArC), 50.0 (-BH2N(CH3)2BH3), 39.6 (-N-
(CH3)2). 11B (128 MHz, CDCl3): δ = 1.8 (br t, -BH2N(CH3)2BH3),
1
−12.5 (q, JBH = 91 Hz, -BH2N(CH3)2BH3). IR (Nujol/cm−1): 2394
Synthesis of tert-Butylaminodiborane, (tBu)HNB2H5 (10).
tBuNH2·BH3 (174 mg, 2.00 mmol) was taken up as a slurry in 10
mL of hexanes, and H3B·THF (2.00 mL, 1.0 M solution in THF, 2.00
mmol) was added to the stirring mixture. The resulting cloudy
solution was stirred for 3 days at room temperature to yield a colorless
solution. The solution volume was concentrated by half under vacuum
and filtered. The presence of (tBu)NHB2H5 can be determined by
diluting a small aliquot with C6D6 for 11B NMR analysis. (11B NMR
(w, νBH), 2354 (w, νBH), 2330 (w, νBH), 2281 (w, νBH). Anal.
Calcd. for C9H21B2N3: C, 56.04; H, 10.97; N, 21.78. Found: C, 55.98;
H, 10.94; N, 21.43. Mp (°C): 131−131.5.
Attempted Thermolysis of DMAP-H2BN(CH3)2-BH3 (7). Ten
milligrams of 6 dissolved in 1 mL of THF was sealed in a J. Young
NMR tube under an atmosphere of nitrogen and heated to 100 °C for
24 h. 11B NMR analysis of the reaction mixture showed only the
presence of starting material with no decomposition.
1
128 MHz, C6D6): δ = −26.4 ppm [t of d, JBH = 30 Hz (B−H
12914
dx.doi.org/10.1021/ic3018997 | Inorg. Chem. 2012, 51, 12905−12916

Inorganic Chemistry
Article
bridging) and 127 Hz (B−H terminal)]. This 11B resonance matched
the value found in the literature,35a in which 10 was synthesized in a
different manner as reported here. In all subsequent reactions, the
formation of 10 was assumed to be quantitative for the purpose of
calculating reactant quantities and product yields.
(2) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010,
110, 4079.
(3) (a) Hu, M. G.; Geanangel, R. A.; Wendlandt, W. W. Thermochim.
Acta 1978, 23, 249. (b) Eddy, C. R.; Sartwell, B. D. J. Vac. Sci. Technol.
A. 1995, 13, 2018 , and references therein. (c) Frueh, S.; Kellett, R.;
Mallery, C.; Molter, T.; Willis, W. S.; King’ondu, C.; Suib, S. L. Inorg.
Chem. 2011, 50, 783.
(4) Kwon, C. T.; McGee, H. A., Jr. Inorg. Chem. 1970, 9, 2458.
(5) (a) Lory, E. R.; Porter, R. F. J. Am. Chem. Soc. 1973, 95, 1766.
(b) Thompson, C. A.; Andrews, L. J. Am. Chem. Soc. 1995, 117, 10125.
(c) Thompson, C. A.; Andrews, L.; Martin, J. M. L.; El-Yazal, J. J. Phys.
Chem. 1995, 99, 13839.
(6) For theoretical studies, see: Himmel, H.-G. Eur. J. Inorg. Chem.
2003, 2153.
(7) (a) Wentorf, R. H., Jr. J. Chem. Phys. 1957, 26, 956. (b) Vel, L.;
Demazeau, G.; Etourneau, J. Mater. Sci. Eng., B. 1991, 10, 149 , and
references therein.
Synthesis of DMAP-H2BN(tBu)H-BH3 (11). A solution of p-
dimethylaminopyridine (0.329 g, 2.70 mmol) was added to a solution
of 10 (made from 2.00 mmol tBuH2N·BH3) in 10 mL of 5:1 hexanes/
THF. The resulting mixture clouded to give a white slurry after 8 h.
The mother liquor was then decanted, and the white solid was washed
with 4 mL portions of 5:1 hexanes/THF. The product was dried under
vacuum, giving 11 as a white powder (0.321 g, 73%). Crystals of 11
suitable for X-ray crystallography (colorless prisms) were obtained by
cooling a saturated THF/hexanes solution to −35 °C.
3
1H NMR (400 MHz, CDCl3): δ = 8.07 (d, 2H, JHH = 7.6 Hz,
ArH), 6.53 (d, 2H, 3JHH = 7.6 Hz, ArH), 3.10 (s, 6H, -N(CH3)2), 2.95
1
(br s, 1H, -BH2NtBuHBH3, assignment made by selective H{11B}
decoupling), 2.64 (br s, 1H, -BH2N(tBu)HBH3, assignment made by
(8) Solozhenko, V. L.; Leonidov, V. Y. Russ. J. Phys. Chem. 1988, 62,
1646.
(9) Rudolpho, S. Am. Ceram. Soc. Bull. 2000, 79, 50.
selective 1H{11B} decoupling), 1.80 (br s, 1H, -NH(tBu)), 1.31 (s, 9H,
3
-NC(CH3)3), 1.08 (d, 3H, JHH = 3.6 Hz, -BH2N(tBu)HBH3,
assignment made by selective 1H{11B} decoupling). 13C{1H} (125
MHz, CDCl3): δ = 155.6 (ArC), 147.8 (ArC), 106.3 (ArC), 54.6
(-NC(CH3)3), 39.4 (-N(CH3)2), 28.3 (-NC(CH3)3). 11B (128 MHz,
(10) (a) Zedlitz, R.; Heintze, M.; Schubert, M. B. J. Non-Cryst. Solids
1996, 198−200, 403. (b) Pan, Z.; Sun, H.; Zhang, Y.; Chen, C. Phys.
Rev. Lett. 2009, 102, 055503. (c) Watanabe, K.; Taniguchi, T.; Kanda,
H. Nat. Mater. 2004, 3, 404. (d) Shapoval, S. Y.; Petrashov, V. T.;
Popov, O. A.; Westner, O. A.; Yoder, M. D., Jr.; Lok, C. K. C. Appl.
Phys. Lett. 1990, 57, 1885. (e) Weber, A.; Bringmann, U.; Nikulski, R.;
Klages, C.-P. Diamond Relat. Mater. 1993, 2, 201. (f) Ichiki, T.;
Yoshida, T. Appl. Phys. Lett. 1994, 64, 851. (g) Ichiki, T.; Momose, T.;
Yoshida, T. Jpn. J. Appl. Phys. 1994, 75, 1330.
1
CDCl3): δ = −3.4 (br t, -BH2NHtBuBH3), −21.7 (q, JBH = 91 Hz,
-BH2NHtBuBH3). IR (Nujol/cm−1): 3211 (w, νNH), 2388 (w, νBH),
2357 (w, νBH), 2281 (w, νBH), 2255 (w, νBH). Anal. Calcd. for
C11H25B2N3: C, 59.79; H, 11.40; N, 19.02. Found: C, 59.41; H, 11.09;
N, 18.63. Mp (°C): 132−133.
Dehydrogenation of 11 with [Rh(COD)Cl]2. To a solution of 11
(56.9 mg, 0.26 mmol) in 5 mL of toluene was added about 1.0 mg of
[Rh(COD)Cl]2 (1 mol %). The solution was initially clear yellow, and
turned black after 3 h. After 4 h the reaction was analyzed by in situ
11B NMR spectroscopy; the observed products were [HBNtBu]3 (12,
5%, d, −25.2 ppm, 1JB−H = 143 Hz), [H2BNHtBu]3 (13, 31%, t, −4.7
(11) (a) Kubota, Y.; Watanabe, K.; Tsuda, O.; Tanighuchi, T. Science
2007, 317, 932. (b) Xiong, Z.; Wu, G.; Chua, Y. S.; Hu, J.; He, T.; Xu,
W.; Chen, P. Energy Environ. Sci. 2008, 1, 360.
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1
ppm, JB−H = 95 Hz), poly-tert-butylborazylene (14, 11%, br s,
−30.1),35 DMAP·BH3 (31%), and 10 (21%).
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF format. Further details are given in
Figures S1−S10 and Tables S1−S5. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: erivard@ualberta.ca.
Notes
The authors declare no competing financial interest.
Pawlis, S.; Schurmann, M.; Jurkschat, K. Eur. J. Inorg. Chem. 2011, 344.
̈
(i) Abraham, M. Y.; Wang, Y.; Xie, Y.; Wie, P.; Schaefer, H. F., III;
ACKNOWLEDGMENTS
■
Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2011, 133, 8874.
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada (Discovery
Grant for E.R.; CGS-M for A.C.M.), the Canada Foundation
for Innovation (CFI), Alberta Innovates-Technologies 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 Canada Summer Jobs (CJS) and the Rupperts-
land Institute. We would like to thank Ms. Nupur Dabral, Dr.
Tiffany MacDougall, Dr. Mickey Richards, and Mr. Mark
(j) Ghadwal, R. S.; Azhakar, R.; Roesky, H. W.; Propper, K.; Dittrich,
̈
B.; Goedecke, C.; Frenking, G. Chem. Commun. 2012, 48, 8186.
(k) Wang, Y.; Robinson, G. H. Inorg. Chem. 2011, 50, 12326.
(15) For pioneering studies in this area, see: (a) Schlesinger, H. I.;
Ritter, D. M.; Burg, A. B. J. Am. Chem. Soc. 1938, 60, 2297. (b) Burg,
A. B.; Randolph, C. L., Jr. J. Am. Chem. Soc. 1949, 71, 3451. For more
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Am. Chem. Soc. 2011, 133, 19322.
Miskolzie for their assistance with obtaining H{11B} NMR
1
(16) Bodekker, K. W.; Shore, S. G.; Bunting, R. K. J. Am. Chem. Soc.
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spectra. We would also like to thank the reviewers for their
detailed and insightful comments regarding our manuscript.
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