Rehydrogenation of aminoboranes to amine-boranes using H2O: Reaction scope and mechanism

Article abstract of DOI:10.1002/ejic.201500117

Water has been successfully employed as a reagent with which to rehydrogenate aminoboranes (e.g., iPr₂N=BH₂, 2,2,6,6-Me₄C₅H₆N=BH₂, and also transient Me₂N=BH₂ derived from ¹/₂[Me₂N-BH₂]₂) to amine-boranes (e.g., iPr₂NH·BH₃, 2,2,6,6-Me₄C₅H₆NH·BH₃, Me₂NH·BH₃) in approximately 30 yield. The conversion to amine-boranes from the corresponding aminoboranes using this method represents an example of a metal-free, single-step route for the hydrogenation of the B=N bond. Deuterium labeling studies indicated that the protic hydrogen (N-H) on the rehydrogenated amine-borane was derived from H₂O, whereas the third hydridic hydrogen (B-H) on the amine-borane was generated from the formation of a postulated hydride-bridged intermediate H₂B(μ-H)(μ-NR₂)B(OH)H (R₂ = Me₂, iPr₂, 2,2,6,6-Me₄C₅H₆), which requires a second equivalent of the starting aminoborane, thus explaining the low yield. Formation of insoluble borates (B_xO_yH_z) provides a driving force for the reaction. Significantly, the yield can be increased by adding a sacrificial source of BH₃ (e.g., to ca. 53% for BH₃·THF) or by adding a separate source of H^- (e.g., to ca. 95% for LiBH₄) to complement the H⁺ (from H₂O) in a more atom-efficient reaction.

Full text of DOI:10.1002/ejic.201500117

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DOI:10.1002/ejic.201500117
Rehydrogenation of Aminoboranes to Amine–Boranes
Using H2O: Reaction Scope and Mechanism
Erin M. Leitao[a] and Ian Manners*[a]
Keywords: Water chemistry / Main group elements / Rehydrogenation / Amines / Boranes
Water has been successfully employed as a reagent with
which to rehydrogenate aminoboranes (e.g., iPr2N=BH2,
2,2,6,6-Me4C5H6N=BH2, and also transient Me2N=BH2 de-
rived from 1/2[Me2N-BH2]2) to amine–boranes (e.g.,
iPr2NH·BH3, 2,2,6,6-Me4C5H6NH·BH3, Me2NH·BH3) in ap-
proximately 30% yield. The conversion to amine–boranes
from the corresponding aminoboranes using this method rep-
resents an example of a metal-free, single-step route for the
hydrogenation of the B=N bond. Deuterium labeling studies
third hydridic hydrogen (B–H) on the amine–borane was
generated from the formation of a postulated hydride-
bridged intermediate H2B(μ-H)(μ-NR2)B(OH)H (R2 = Me2,
iPr2, 2,2,6,6-Me4C5H6), which requires a second equivalent
of the starting aminoborane, thus explaining the low yield.
Formation of insoluble borates (BxOyHz) provides a driving
force for the reaction. Significantly, the yield can be in-
creased by adding a sacrificial source of BH3 (e.g., to ca. 53%
for BH3·THF) or by adding a separate source of H– (e.g., to
indicated that the protic hydrogen (N–H) on the rehydroge- ca. 95% for LiBH4) to complement the H+ (from H2O) in a
nated amine–borane was derived from H2O, whereas the
more atom-efficient reaction.
Ammonia borane (AB; NH3·BH3) in particular has re-
ceived intense attention as a material for hydrogen storage
owing to its high hydrogen content (19.6 wt.-%), facile
hydrogen elimination under mild conditions in the presence
of a catalyst,[4] and high stability under ambient conditions
in the solid state. Recently, there has been extensive research
(over 100 reports since 2010) that involves studies of the
hydrolytic dehydrogenation of AB using catalysis,[5]
whereby H2O is used as a reagent to enhance the dehydro-
genation characteristics of AB along with transition-metal
nanoparticles,[6] stabilized by immobilization inside the
pores of a metal–organic framework,[7] by supporting on
various nanoshapes and powders,[8] by anions,[9] by graph-
ene or another carbon-based support,[10] or, alternatively, a
homogeneous catalyst.[11] The addition of two equivalents
of water relative to AB results in three equivalents of H2 as
illustrated in Equation (1).[12]
Introduction
Amine–boranes (RRЈHN·BH3; R, RЈ = alkyl, aryl, or H)
have been shown to be excellent reagents for the hydrogen-
ation and hydroboration of organic substrates[1] and are at-
tractive precursors to B–N materials and polymers
(Scheme 1).[2] More recently, success with catalytic dehydro-
genation protocols for amine–boranes has stimulated much
research into their potential use as hydrogen-storage and
transfer reagents.[3]
(1)
Although many methods for the release of hydrogen
from amine–boranes have been developed, efficient and
mild methods for regenerating the original adduct from the
spent fuel presents a challenge.[13] Ammonia borane can be
regenerated in a number of multistep procedures (digestion,
ammoniation, reduction), and there is a single example of
a one-pot reaction that uses hydrazine in liquid ammonia
as reported by Gordon and co-workers.[14] Baker and co-
workers have reported use of Lewis bases (pyridine, NH3)
to transform B–H bonds in polyborazylene and borazine to
BH3 with concomitant production of “B(NH2)3”.[15] There-
Scheme 1. Selected examples of amine–borane dehydrogenation at
ambient temperatures using different hydrogen acceptors or a cata-
lyst that yields polyaminoborane.
[a] School of Chemistry, University of Bristol,
Cantock’s Close, Bristol, BS8 1TS, United Kingdom
E-mail: Ian.Manners@bris.ac.uk
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500117.
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fore, other methods, particularly catalytic ones, are highly
Repeating the same reaction in dichloromethane yielded
desirable. Monomeric aminoboranes (e.g., iPr2N=BH2) similar results. However, use of THF, in which H2O is solu-
have been shown to accept H2 from amine–boranes as the ble, resulted in the formation of a second boron-containing
only report to date of hydrogenating unsaturated B–N species detected at δB = 19.1 ppm, which was suggested to
bonds in a single step (see Scheme 1 above).[16] We report be a borate complex (BxOyHz) that contained three-coordi-
here the formal addition of H2 to the B=N bond in amino- nate boron centers (see Figure S1 in the Supporting Infor-
boranes to yield amine–boranes in one step by using H2O. mation). Borate complexes are typically observed in the
These results are significant under the three regimes of (1) range of δB = 17–20 ppm by means of 11B NMR spec-
understanding the mechanism of the hydrolytic dehy- troscopy {e.g., B(OH)3 δB = 19.1 ppm; B(OMe)3 δB
=
drocoupling of amine–boranes using catalysts, (2) rehyd- 17.9 ppm; B(OEt)3 δB = 17.6 ppm; B(OiPr)3 δB = 17.1 ppm
rogenation of aminoboranes as models for spent fuel, and in THF, and the B(OH)3/[B(OH)4]– segment of the polybor-
(3) use of B–N compounds in metal-free catalysis.
ate, [H2en]2[B11O18(OH)]·7H2O (en = 1,2-diaminoethane):
δB = 18.3 ppm in D2O}.[19] The borate complex (BxOyHz)
obtained as a byproduct in the rehydrogenation reaction
was found to be insoluble in toluene and dichloromethane,
and was observed as a white precipitate when using those
solvents for the reaction. Concentrating the reaction mix-
ture from any of the three tried solvents and subsequent
Results and Discussion
1. Conversion of Aminoboranes to Amine–Boranes Using
H2O
1
investigation by H NMR spectroscopy in [D8]toluene re-
vealed signals characteristic of two different amine groups:
The addition of a stoichiometric or an excess amount (up
to 30 equiv.) of distilled H2O to a solution of R2N=BH2
(R2N=iPr2N or 2,2,6,6-tetramethylpiperidinyl, Me4C5H6N)
in toluene (ca. 0.3 m) with vigorous stirring at 20 °C[17] re-
sulted in the formal addition of H2 to the aminoborane,
which afforded the corresponding amine–borane
(R2NH·BH3, R2 = iPr2 or Me4C5H6) [Equation (2)].[18] This
result was indicated by 11B NMR spectroscopy, as the trip-
free R2NH and amine–borane adduct R2NH·BH3 (R2
=
iPr2 or Me4C5H6) (see Figure S2). The borate complex
(BxOyHz) was not soluble in [D8]toluene.
Performing the rehydrogenation reaction of iPr2N=BH2
on a larger scale (15 mmol aminoborane) in toluene allowed
for the isolation of each product. In concentrated solutions
(Ͼ0.3 m) vigorous evolution of gas (H2) was observed as
let signal characteristic of the aminoborane at δB
=
well as immediate precipitation of white borate solids at
34.8 ppm (R2 = iPr2) or δB = 36.3 ppm (R2 = Me4C5H6)
was absent after 10 min., and the appearance of a quartet
at δB = –21.3 ppm (R2 = iPr2) or δB = –21.8 ppm (R2 =
Me4C5H6) assigned to the corresponding amine–borane
was observed as the only soluble species in toluene (Fig-
ure 1).
[18a]
20 °C. The amine–borane iPr2NH·BH3
was separated
from the insoluble borate solids by means of filtration, and
the residual solution of amine–borane in toluene was iso-
lated by concentrating under vacuum to give a conversion
of approximately 30% relative to iPr2N=BH2. In a dilute
THF solution with an internal standard [B(OiPr)3], the con-
version of iPr2N=BH2 to iPr2NH·BH3 by using 1 equiv. of
H2O at 20 °C was monitored by means of 11B NMR spec-
troscopy. After 10 min, all of the aminoborane was con-
sumed, and 20% iPr2NH·BH3 (δB = –20.9 ppm) along with
80% BxOyHz (δB = 19.1 ppm) were observed in the 11B
NMR spectrum. Surprisingly, repeating the same reaction
under a closed system (with buildup of H2 gas) yielded 16%
iPr2NH·BH3, 58% BxOyHz, and 26% of a boron-containing
species (δB = 0.3 ppm) postulated to be iPr2NH·B(OH)3 as
determined by the relative integration of the NMR spectro-
scopic signals.[20]
(2)
To facilitate the determination of the identity of the
BxOyHz byproduct(s) formed, the solid obtained from the
reaction mixture was dissolved in DMSO, the only readily
available solvent (other than H2O) that would solubilize the
borates. By means of 11B NMR spectroscopy (in [D6]-
DMSO), two signals were now observed in the 11B NMR
spectrum, a sharp peak at δ = 0.3 ppm and a broad peak
at δ = 19.1 ppm, similar to those previously observed in the
11B NMR spectrum from the reaction monitored in situ of
iPr2N=BH2 with H2O at 20 °C in THF (Figure 2, a, bot-
tom). The assignment at δ = 0.3 ppm, suggested to be
iPr2NH·BxOyHz, is supported by the 11B NMR spectrum
([D6]DMSO) of a separate synthesis of the complex through
Figure 1. 11B NMR spectra of (a) 0.30 m solution iPr2N=BH2 in
toluene and (b) 30 min. after addition of 1 equiv. H2O at 20 °C.
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the direct reaction of iPr2NH and BxOyHz [sourced from Hydrogen Tracking Using Deuterium Labeling
either commercial B(OH)3 or the resulting solids obtained
To investigate the source of the protic hydrogen (N–H)
from mixing equimolar amounts of B(OH)3 with
and hydridic hydrogen atoms (B–H) during the hydrogen-
BH3·THF] in THF at 20 °C (Figure 2, a, top).
ation of aminoboranes to amine–boranes, isotopic labeling
studies were performed. Addition of D2O to
Me4C5H6N=BH2 or iPr2N=BH2 in [D8]toluene afforded
the respective products Me4C5H6ND·BH3 or iPr2ND·BH3
on the basis of (1) the absence of the “NH” signal in the
1H NMR spectrum (Figure 3, a, i and ii) and (2) the pres-
ence of a quartet in the 11B NMR spectra that corre-
sponded to a “BH3” group (Figure 3, b, i and ii).
Figure 2. 11B NMR spectra in [D6]DMSO of (a) the reaction of
iPr2NH with BxOyHz [from B(OH)3 or solids obtained from
BH3·THF with B(OH)3] in THF (top, blue), borates isolated from
the reaction of iPr2N=BH2 with H2O in toluene (bottom, red), at
20 °C; and (b) B(OH)3 purchased from Alfa Aesar (top, blue),
Figure 3. (a) 1H{11B} NMR spectra (* [D8]toluene) and (b) 11B
NMR spectra after 1 h of (i) Me4C5H6N=BH2 with H2O yielding
Me4C5H6NH·BH3; (ii) Me4C5H6N=BH2 with D2O yielding
Me4C5H6ND·BH3; and (iii) iPr2N=BD2 with H2O yielding
iPr2NH·BD3.
BxOyHz synthesized from BH3·THF and H2O or BH3·THF and
commercial B(OH)3 (bottom, red), at 20 °C.
Similarly, the 11B NMR spectroscopic resonance of
BxOyHz, (Figure 2, b, bottom; δB = 19.1 ppm, [D6]DMSO),
produced by the controlled addition of either H2O to
BH3·THF (1 m solution) or B(OH)3 to BH3·THF at 20 °C,
correlates well with the 11B NMR spectroscopic signal ob-
tained from measuring the spectrum of a commercial sam-
These observations indicated that the protic hydrogen
added to nitrogen in the amine–borane product originated
from the added H2O, and that the hydridic hydrogen intro-
duced at boron was derived from another source.[24] As the
addition of H2 occurred irrespective of the type of solvent
(THF, toluene, CH2Cl2), including deuterium-labeled sol-
vents (e.g., [D8]toluene, [D8]THF), we suspected that the
hydridic hydrogen was derived from the aminoborane sub-
strate. To confirm our postulation, we treated the labeled
substrate iPr2N=BD2 with H2O, and this yielded the antici-
pated product iPr2NH·BD3 (Figure 3, a and b, iii).
ple (Alfa Aesar) of B(OH)3 (Figure 2, b, top; δB
=
19.1 ppm, [D6]DMSO).[21] The evidence provided by these
controlled reactions therefore suggests that the borate pre-
cipitate formed during the reaction contains three coordi-
nate boron centers (δB = 19.1 ppm), which, in the presence
of an excess amount of amine, results in adduct formation
(δB = 0.3 ppm).
The substituted cyclodiborazane [Me2N–BH2]2 can also
Other Protic Sources
[22]
be rehydrogenated to yield Me2NH·BH3
by using H2O
under elevated temperature (100 °C, 1 h) in toluene, which
To test the generality of this reaction and to gain further
is necessary to promote the monomerization that precedes insight into the mechanism, we treated other protic solvents
the H2 addition (see Figure S3 in the Supporting Infor- with the iPr2N=BH2 substrate. To this end, various alcohols
mation).[16,23] However, both the linear diborazane (MeOH, EtOH, iPrOH) and acids (HCl, TfOH) were em-
Me2NH–BH2–NMe2–BH3, and cyclotriborazane [MeNH– ployed. For the reaction with MeOH, other than the unre-
BH2]3 remained unchanged even upon heating (100 °C, acted aminoborane, the major boron-containing product
24 h) in the presence of an excess amount of H2O in tolu- was B(OMe)3 as determined by means of 11B NMR spec-
ene. Polymeric aminoborane [MeNH-BH2]n and further cy- troscopy (δB = 17.5 ppm). Minor amounts of iPr2NH·BH3
clic species such as borazine (NH–BH)3 produced B(OH)3 (1%) and (iPr2N)2BH (3%) were also observed. The pro-
(or a derivative such as BxOyHz, δB = 19.1 ppm) upon stir- portion of diaminoborane [(iPr2N)2BH] produced increased
ring at 20 °C for 16 h in THF, as determined by means of with an increase in steric bulk on the added alcohol (32%
11B NMR spectroscopy (see Figure S4).
with EtOH and 58% with iPrOH) relative to the unreacted
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aminoborane. Furthermore, a negligible amount of amine– Me4C5H6NH·BH3) relative to the same reaction in the ab-
borane was detected as the steric bulk on the alcohol in- sence of LiBH4 (44%). The addition of LiBH4 as a sacrifi-
creased, as determined by 11B NMR spectroscopy after cial source of H– to a reaction of polyaminoborane
30 min at 20 °C. The product ratios observed suggest that [MeNH–BH2]n with H2O resulted in minor production of
dissociation of the adduct is more prevalent with alcohols, MeNH2·BH3 (10%) after 4 h at 20 °C in THF upon com-
and that formation of the iPr2NH·B(OR)3 becomes much plete consumption of the LiBH4, thereby leaving 89% poly-
less favorable when the steric bulk of the R substituent is mer and 1% (MeNH2)2BH, as shown by in situ 11B NMR
increased. This was supported by the observation that the spectroscopy (see Figure S6). Repeating the same reaction
attempted synthesis of iPr2NH·B(OiPr)3 was unsuccessful with the unsubstituted polymer [NH2–BH2]n resulted in a
on the basis of 11B NMR spectroscopic analysis of a reac- minor amount of NH3·BH3 (relative to unreacted LiBH4);
tion mixture that contained equimolar amounts of iPr2NH however, this reaction was significantly hindered by the re-
and B(OiPr)3 in THF at 20 °C. As anticipated, the addition duced solubility of the polymer in THF.
of acid HX (X = Cl, OSO2CF3) resulted in the formation
of the addition product iPr2NH·BH2X.[25] On the basis of
these results, we postulate that the first step in the mecha-
nism of the rehydrogenation of aminoboranes is the ad-
dition of H–OH across the N=B bond (Scheme 2).
Figure 4. 11B NMR spectrum of isolated (a) Me4C5H6NH·BH3 and
(b) BxOyHz species from the reaction of Me4C5H6N=BH2 with
1 equiv. BH3·THF and 1 equiv. H2O in toluene at 20 °C.
Mechanistic Considerations
To further explore the redistribution of H– and HO– be-
tween boron centers, additional experiments were per-
formed in the absence of amine.
The reaction of BH3·THF with B(OH)3 in THF resulted
in vigorous bubbling (presumably loss of H2 gas), and a
white precipitate, assigned as BxOyHz, was formed. Isola-
Scheme 2. Postulated mechanism of H2 addition to R2N=BH2
tion of the latter followed by dissolution in [D6]DMSO gave
(R = Me2, iPr2, Me4C5H6) using H2O.
a broad singlet at δ = 19.1 ppm by means of 11B NMR
spectroscopy (see Figure S7 in the Supporting Infor-
mation). The 1H NMR spectrum showed two broad signals
at δ = 6.5 and 3.4 ppm in [D6]DMSO, which integrated in
Addition of an Alternative Hydride Source
a 2:3 ratio, thus indicating the presence of two “B(OH)”
To remove the necessity of hydride loss from the amino- environments.[26] Scrambling of the substituents on boron
borane substrate, we performed analogous reactions in the was also detected during reactions of iPr2N=BH2 with
presence of a sacrificial hydride source (e.g., BH3·THF, B(OH)3.[27] A substantial amount of iPr2NH·BH3 (40%)
LiBH4). The reaction of equimolar amounts of BH3·THF, formed after 48 h at 20 °C together with a minor amount
H2O, and Me4C5H6N=BH2 in toluene at 20 °C gave rise to of (iPr2N)2BH as the soluble products. A significant
the amine–borane Me4C5H6NH·BH3 on the basis of in situ amount of white precipitate assigned as iPr2N·BxOyHz (δB
11B NMR spectroscopic analysis (Figure 4, a). Isolation by = 0.3 ppm, [D6]DMSO) was also formed, and approxi-
filtration and subsequent 11B NMR spectroscopic analysis mately 30% of iPr2N=BH2 was left unreacted.
of the precipitate formed suggest, once again, the formation
These results with B(OH)3 in place of water in the reac-
of borate species, BxOyHz (Figure 4, b, see also Figure 3, a). tion were not surprising as this species also functions as a
The isolated yield of the amine–borane product was signifi- source of protons. As the identity of the BxOyHz species is
cantly increased relative to the reaction in the absence of difficult to assign definitively owing to its low solubility and
the BH3·THF at 53%. Similarly, in reactions of R2N=BH2 crystallinity, we suggest a potential route to formation of
(R2 = iPr2 in THF, see Figure S5 in the Supporting Infor- the amine–borane and B(OH)3 products to be through the
mation; R2 = Me4C5H6 in toluene) with H2O in the pres- formation of intermediate H2B(μ-H)(μ-NR2)B(OH)H (R2 =
ence of 1 equiv. of LiBH4 at 20 °C the final isolated yield Me2, iPr2, Me4C5H6; Scheme 2). The first step in the rehyd-
of the amine–borane was increased (95% for rogenation is proposed to be the addition of H–OH across
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2,2,6,6-Me4C5H6N=BH2[32] were synthesized by means of literature
methods and purified by sublimation or distillation prior to use.
[MeNH–BH2]n (Mn = 180000, PDI = 1.2) and [NH2–BH2]n were
synthesized by literature methods.[33] Distilled H2O was obtained
from a Millipore-Q distillation apparatus in house.
the N=B bond. Subsequent formation of H2B(μ-H)(μ-
NR2)B(OH)H from the addition of a second equivalent of
aminoborane would allow for the facile redistribution ob-
served at boron, as upon the dissociation of H2B(μ-H)(μ-
NR2)B(OH)H with the addition of H2O amine–borane and
BH(OH)2 are released. The presence of free amine in solu-
tion would allow for facile formation of an adduct with
BH(OH)2 [e.g., R2NH·BH(OH)2], and repeating the same
process for another equivalent of R2N=BH2 and H2O
would yield a second equivalent of R2NH·BH3 as well as
B(OH)3. The three-coordinate borates (BxHyOz) are then
presumably formed by the loss of H2 in reactions of
B(OH)3, B(OH)2H, B(OH)H2, and free BH3 in solution.
NMR spectra were recorded with JEOL JNM-ECP300 or JNM-
LA300 spectrometers. Chemical shifts were reported relative to re-
sidual solvent peaks (1H and 13C) or to external standards:
BF3·OEt2 (11B). Integration of 11B NMR spectra was performed
using MestraNova version 9.0.1.
General Procedure for H2O Addition Reactions: At least 1 equiv. of
H2O (120 μL, 7 mmol; 279 μL, 15 mmol; 16 μL, 0.88 mmol) was
added to a solution of aminoborane Me4C5H6N=BH2 (20 mL,
0.33 m in toluene) or iPr2N=BH2 (40 mL, 0.38 m in toluene) or
[Me2N-BH2]2 (50 mg, 0.44 mmol in toluene) with an automated pi-
pette and the mixture was stirred for 60 min at 20 °C {or 100 °C
in the case of ([Me2N-BH2]2)}. Vigorous bubbling was observed
immediately and precipitate formed (also observed in CH2Cl2, and
some cloudiness was apparent in THF). An aliquot was removed
for NMR spectroscopy: 11B NMR (96 MHz, ppm): δB = –21.8 (q,
JHB = 96 Hz, Me4C5H6NH·BH3) or δB = –21.3 (q, JHB = 97 Hz,
iPr2NH·BH3) or δB = –14.3 (q, JHB = 96 Hz, Me2NH·BH3). The
insoluble products were isolated by filtration (0.312 g from
Me4C5H6N=BH2 reaction; 0.660 g from iPr2N=BH2 reaction), and
Conclusion
We have successfully demonstrated the conversion of
aminoboranes, R2N=BH2 (R2 = Me2, iPr2, Me4C5H6), to
amine–boranes, R2NH·BH3, by using H2O as a reagent. In-
vestigations of the mechanism using deuterium labeling
studies and model reactions suggest that the protic
hydrogen (N–H) in the amine–borane product is derived
from the H2O, whereas the hydridic hydrogen (B–H) is de- the residual mixture was concentrated to dryness under vacuum to
isolate the amine–borane (0.453 g, 44% Me4C5H6NH·BH3;
0.524 g, 30% iPr2NH·BH3). 11B NMR of BxHyOz (96 MHz, [D6]-
DMSO, ppm): δB = 0.5 (br., s) and 19.0 (s) (from Me4C5H6N=BH2
reaction) or δB = 0.4 (br. s) and 18.9 (s) (from iPr2N=BH2 reaction)
ppm. Further characterization of BxHyOz was not possible owing
to its insolubility in solvents other than DMSO.
Variation of the Number of Equivalents of H2O: Adding a sub-stoi-
chiometric amount of H2O (0.5 equiv.) to iPr2N=BH2 in dilute tol-
uene solution at 20 °C resulted in 8% conversion after 1 h, 24%
after 24 h, and 34% after 72 h. By comparison, the addition of
rived from the aminoborane substrate and the amine–bor-
ane is formed through a hydride-bridged intermediate
[H2B(μ-H)(μ-NR2)B(OH)H]. The addition of a sacrificial
source of hydride (e.g., BH3·THF or LiBH4) resulted in a
substantial increase in yield and hence, coupled with the
addition of H2O, provides a potential avenue to explore the
rehydrogenation of other unsaturated B=N compounds.
The studies that used separate sources of H– and H+ also
indicated that heterolytic cleavage of H2 is desirable for the
addition to unsaturated B=N compounds. Interestingly, the 1 equiv. of H2O under identical conditions yielded 27% after 1 h,
80% after 24 h and was completely converted to iPr2NH·BH3 by
48 h. Conversely, addition of 3 equiv. of H2O under identical condi-
tions resulted in complete conversion to iPr2NH·BH3 in less than
1 h.
model results suggested that during catalytic dehydrocou-
pling reactions of amine–boranes in aqueous media, a por-
tion of the evolved H2 probably arises from the uncatalyzed
reaction of the aminoborane intermediate with H2O to re-
generate the substrate, which, in turn, can further eliminate
H2 catalytically. Unfortunately, in the presence of H2O
alone, concomitant formation of borates occurs. The forma-
tion of this undesirable, thermodynamically stable byprod-
uct would need to be avoided for this reaction to be very
useful for amine–borane spent-fuel regeneration.
Preparation of iPr2NH·BxOyHz: iPr2NH (2 mL, 14 mmol) was
added dropwise to a solution of B(OH)3 in THF (912 mg, 14 mmol,
20 mL of THF) with a syringe. The resulting suspension slowly
became more opaque with a white precipitate forming over time.
The mixture was stirred for 24 h at 20 °C before concentrating to
dryness to isolate an insoluble white solid [1.953 g, 84% based on
iPr2NH·B(OH)3]. 11B NMR (96 MHz, [D6]DMSO): δB = 0.4 (br.
1
s) and 19.1 (s) ppm. H NMR (400 MHz, [D6]DMSO): δH = 6.41
[br. s, 3 H, B(OH)], 3.30 [sept, JHH = 6 Hz, 2 H, CH(CH3)2], 2.04
(ca. 4 H, s, unknown), 1.15 [d, JHH = 6 Hz, 7 H, CH(CH3)2] ppm.
13C NMR (76 MHz, [D6]DMSO): δC = 207.0 (unknown), 46.8
[CH(CH3)2], 31.2, 19.5 [CH(CH3)2] ppm. Further characterization
(X-ray, MS) of iPr2NH·BxOyHz was not possible owing to its insol-
ubility in solvents other than DMSO.
Experimental Section
General: All manipulations were carried out under an atmosphere
of nitrogen gas using standard vacuum line and Schlenk tech-
niques, or under an atmosphere of argon within an MBraun
glovebox. All solvents were dried by means of a Grubbs-type sol-
vent purification system and/or over molecular sieves (3 Å). Deu-
terated solvents were purchased from Sigma Aldrich Ltd and dis-
tilled from CaH2 ([D8]THF, [D8]toluene, [D6]DMSO) or used as is
(D2O). Me2NH·BH3 was purchased from Sigma Aldrich Ltd and
sublimed before use. B(OH)3 (99%) was purchased from Alfa Aesar
and used without further purification. iPr2NH·BH3,[18a] 2,2,6,6-
Me4C5H6NH·BH3,[18b,28] Me2NH-BH2-NMe2–BH3,[29] [MeNH-
Addition of ROH to iPr2N=BH2 (R = Me, Et, iPr): An equivalent
of anhydrous ROH (0.077–0.14 mL, 2 m) was added to a solution
of aminoborane iPr2N=BH2 (5 mL, 0.38 m in toluene). The reac-
tion was stirred for 30 min. before an aliquot was removed for
analysis by 11B NMR spectroscopy. 11B NMR (96 MHz): δ = R
= Me: 66% iPr2N=BH2 (δB = 34.9 ppm), 3% (iPr2N)2BH (δB
27.2 ppm), 30% B(OMe)3 (δB = 17.9 ppm), 1% iPr2NH·BH3 (δB
=
=
BH2]3,[30] [Me2N-BH2]2,[18a] [NH-BH]3,[18a] iPr2N=BH2,[31] and –20.9 ppm); R = Et: 51% iPr2N=BH2 (δB = 34.9 ppm), 32%
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
132, 13332–13345; c) H. C. Johnson, T. N. Hooper, A. S. Wel-
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(iPr2N)2BH (δB = 26.9 ppm), 17% B(OEt)3 (δB = 17.6 ppm); R =
iPr: 41% iPr2N=BH2 (δB = 34.9 ppm), 58% (iPr2N)2BH (δB
27.0 ppm), 1% B(OiPr)3 (δB = 17.1 ppm).
=
[3]
Addition of HCl (in Et2O) to iPr2N=BH2: An equivalent of HCl
in Et2O (0.96 mL, 2 m) was added to a solution of aminoborane
iPr2N=BH2 (5 mL, 0.38 m in toluene). The reaction was stirred for
1 h before an aliquot was removed for analysis by means of 11B
NMR spectroscopy. 11B NMR (96 MHz, ppm): δB = –7.5 (t, JHB
= 121 Hz).[25] The only product observed was the addition product
across the N=B bond, namely, iPr2NH·BH2Cl (identical characteri-
zation to the literature[25]), hence there was no apparent Cl/H
scrambling on the boron.
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Addition of HOTf (in Et2O) to iPr2N=BH2: An equivalent of HOTf
in Et2O (1.9 mL, 1 m) was added to a solution of aminoborane
iPr2N=BH2 (5 mL, 0.38 m in toluene). The reaction was stirred for
1 h before an aliquot was removed for analysis by 11B NMR spec-
troscopy. 11B NMR (96 MHz, ppm): δB = –2.8 (br. t, JHB
=
131 Hz).[25] The only product observed was the addition product
across the N=B bond, namely, iPr2NH·BH2OTf (identical charac-
terization to the literature[25]), hence there was no OTf/H scram-
bling on the boron.
Addition of BH3·THF and H2O to Me4C5H6N=BH2: BH3·THF
(1 m) followed by H2O (36 μL) under nitrogen was added to a solu-
tion of the aminoborane (5 mL, 0.33 m in toluene). Vigorous bub-
bling was observed immediately, and a precipitate formed. An ali-
quot was removed for NMR spectroscopy: 11B NMR of
Me4C5H6NH·BH3 (96 MHz, [D8]toluene): δB = –22.6 ppm (q, JHB
= 97 Hz).[18b] Isolated yield of Me4C5H6NH·BH3 = 53%. 11B NMR
of BxHyOz (96 MHz, [D6]DMSO): δB = 0.5 (br., s) and 19.0 (s)
ppm. Further characterization of BxHyOz was not possible owing
to its insolubility in solvents other than DMSO.
[7]
[8]
Addition of LiBH4 and H2O to Me4C5H6N=BH2: LiBH4 (44 mg)
followed by H2O (36 μL) under nitrogen was added to a solution
of the aminoborane (5 mL, 0.33 m in toluene). Vigorous bubbling
was observed immediately and a precipitate formed. The solid pre-
cipitate and dissolved Me4C5H6NH·BH3 were separated by cannula
filtration and both dried further under vacuum. 11B NMR of
Me4C5H6NH·BH3 (96 MHz, [D8]toluene): δB = –22.6 ppm (q, JHB
= 97 Hz).[18b] Note: 11B NMR of Li[Me4C5H6NBH3] (96 MHz): δB
= –25 ppm (q, JHB = 86 Hz). Isolated yield of Me4C5H6NH·BH3
= 95%. 11B NMR of BxHyOz (96 MHz, [D6]DMSO): δB = –36.6
ppm (p, JHB = 81 Hz; LiBH4).[34] Further characterization of
BxHyOz was not possible owing to its insolubility in solvents other
than DMSO.
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Acknowledgments
E. M. L. is grateful to the European Union (EU) for a Marie Curie
postdoctoral fellowship and I. M. acknowledges the Engineering
and Physical Sciences Research Council (EPSRC) for funding. The
authors also thank Naomi Stubbs for help preparing the revised
manuscript.
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FULL PAPER
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(D2O).
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1
[26]
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The H NMR spectrum in [D6]DMSO of commercial B(OH)3
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to BxOyHz (major) and amine–borate (minor).
Received: February 3, 2015
[21] The 11B NMR spectrum of B(OH)3 in D2O shows a signal at
δB = 18.6 ppm. The 1H NMR spectrum of B(OH)3 shows a
Published Online: ᭿
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Date: 10-04-15 13:22:00
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FULL PAPER
Amine–Boranes
E. M. Leitao, I. Manners* ................ 1–8
The conversion of aminoboranes to amine–
boranes using water has been demonstrated
and the mechanism investigated using
deuterium labeling studies and model reac-
tions.
Rehydrogenation of Aminoboranes to
Amine–Boranes Using H2O: Reaction
Scope and Mechanism
Keywords: Water chemistry / Main group
elements / Rehydrogenation / Amines /
Boranes
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