B-Methylated Amine-Boranes: Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions

Article abstract of DOI:10.1021/acs.inorgchem.5b01946

Although the dehydrogenation chemistry of amine-boranes substituted at nitrogen has attracted considerable attention, much less is known about the reactivity of their B-substituted analogues. When the B-methylated amine-borane adducts, RR'NH·BH₂Me (1a: R = R' = H; 1b: R = Me, R' = H; 1c: R = R' = Me; 1d: R = R' = iPr), were heated to 70 °C in solution (THF or toluene), redistribution reactions were observed involving the apparent scrambling of the methyl and hydrogen substituents on boron to afford a mixture of the species RR'NH·BH_3-xMe_x (x = 0-3). These reactions were postulated to arise via amine-borane dissociation followed by the reversible formation of diborane intermediates and adduct reformation. Dehydrocoupling of 1a-1d with Rh(I), Ir(III), and Ni(0) precatalysts in THF at 20 °C resulted in an array of products, including aminoborane RR'N=BHMe, cyclic diborazane [RR'N-BHMe]₂, and borazine [RN-BMe]₃ based on analysis by in situ ¹¹B NMR spectroscopy, with peak assignments further supported by density functional theory (DFT) calculations. Significantly, very rapid, metal-free hydrogen transfer between 1a and the monomeric aminoborane, iPr₂N=BH₂, to yield iPr₂NH·BH₃ (together with dehydrogenation products derived from 1a) was complete within only 10 min at 20 °C in THF, substantially faster than for the N-substituted analogue MeNH₂·BH₃. DFT calculations revealed that the hydrogen transfer proceeded via a concerted mechanism through a cyclic six-membered transition state analogous to that previously reported for the reaction of the N-dimethyl species Me₂NH·BH₃ and iPr₂N=BH₂. However, as a result of the presence of an electron donating methyl substituent on boron rather than on nitrogen, the process was more thermodynamically favorable and the activation energy barrier was reduced.

Full text of DOI:10.1021/acs.inorgchem.5b01946

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Article
pubs.acs.org/IC
B‑Methylated Amine-Boranes: Substituent Redistribution, Catalytic
Dehydrogenation, and Facile Metal-Free Hydrogen Transfer
Reactions
Naomi E. Stubbs, Andre Schafer, Alasdair P. M. Robertson, Erin M. Leitao, Titel Jurca, Hazel A. Sparkes,
́
̈
Christopher H. Woodall, Mairi F. Haddow, and Ian Manners*
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K.
S
* Supporting Information
ABSTRACT: Although the dehydrogenation chemistry of amine-boranes
substituted at nitrogen has attracted considerable attention, much less is
known about the reactivity of their B-substituted analogues. When the B-
methylated amine-borane adducts, RR′NH·BH2Me (1a: R = R′ = H; 1b: R
= Me, R′ = H; 1c: R = R′ = Me; 1d: R = R′ = iPr), were heated to 70 °C in
solution (THF or toluene), redistribution reactions were observed
involving the apparent scrambling of the methyl and hydrogen substituents
on boron to afford a mixture of the species RR′NH·BH3−xMex (x = 0−3).
These reactions were postulated to arise via amine-borane dissociation
followed by the reversible formation of diborane intermediates and adduct reformation. Dehydrocoupling of 1a−1d with Rh(I),
Ir(III), and Ni(0) precatalysts in THF at 20 °C resulted in an array of products, including aminoborane RR′NBHMe, cyclic
diborazane [RR′N−BHMe]2, and borazine [RN−BMe]3 based on analysis by in situ 11B NMR spectroscopy, with peak
assignments further supported by density functional theory (DFT) calculations. Significantly, very rapid, metal-free hydrogen
transfer between 1a and the monomeric aminoborane, iPr2NBH2, to yield iPr2NH·BH3 (together with dehydrogenation
products derived from 1a) was complete within only 10 min at 20 °C in THF, substantially faster than for the N-substituted
analogue MeNH2·BH3. DFT calculations revealed that the hydrogen transfer proceeded via a concerted mechanism through a
cyclic six-membered transition state analogous to that previously reported for the reaction of the N-dimethyl species Me2NH·
BH3 and iPr2NBH2. However, as a result of the presence of an electron donating methyl substituent on boron rather than on
nitrogen, the process was more thermodynamically favorable and the activation energy barrier was reduced.
polyaminoboranes, [RNH−BH2]n can be formed if the
precursor is ammonia-borane (R = H) or a sterically
unhindered primary amine-borane (R = Me, Et, nBu) and the
catalyst is selective for dehydropolymerization.2d,4e,5c,6a,14
Under most circumstances, elimination of two equivalents of
hydrogen occurs with ammonia-borane and primary amine-
boranes to yield borazines, [RN−BH]3 (Scheme 1).15
1. INTRODUCTION
Amine-boranes (RR′R″N·BH3, R, R′, R″ = H, alkyl or aryl) are
isoelectronic with alkanes and have been the recent focus of
intense attention as a result of their potential applications in
hydrogen storage and transfer, as well as precursors to new
inorganic materials.1 For example, polyaminoboranes, [RNH−
BH2]n, which are structurally analogous and isoelectronic to
polyolefins, represent an interesting class of polymers accessed
from amine-boranes that may possess useful piezoelectric or
preceramic properties.2 Dehydrocoupling of amine-boranes,
where the release of hydrogen is accompanied by the formation
of new B−N bonds, can be performed thermally,1c using
stoichiometric amounts of a hydrogen acceptor,1i−l,3 or much
more efficiently with a variety of Rh,4 Ir,2d,5 Ni,6 Ti,7 Fe,6c,8 Re,9
or Ru10 precatalysts as well as with other transition metal11 and
main-group species.12 Under these conditions, ammonia-
borane, NH3·BH3, as well as primary amine-boranes, RNH2·
BH3, and secondary amine-boranes, RR′NH·BH3 (R, R′ = alkyl,
aryl) readily eliminate one equivalent of hydrogen to yield
aminoboranes, RR′NBH2, which can either be stable as a
monomer or undergo oligomerization to yield linear RR′NH−
[BH2−RR′N]x−BH3 or cyclic [RR′N−BH2]x borazanes (x = 2
or 3) (Scheme 1).13 Alternatively, high molecular weight
In recent years amine-boranes have been shown to function
as hydrogen donors providing one or more equivalents of
hydrogen to acceptors such as imines,1k olefins,1l,2c,d amino-
boranes,1j a C4 cumulene,16 and azobenzene.17 We have
recently reported detailed kinetic and thermodynamic studies
of the hydrogen transfer reaction of Me2NH·BH3 and iPr2N
BH2 to yield Me2NBH2 and iPr2NH·BH3, where the former
aminoborane subsequently dimerizes to form [Me2N−BH2]2
(Scheme 2). Experimental data and density functional theory
(DFT) calculations determined that the hydrogen transfer step
was slightly endergonic (ΔG°exp = +10 7 kJ mol−1, ΔG°calc
=
+9.1 kJ mol−1) with the dimerization of Me2NBH2 providing
the driving force for the overall reaction (ΔG°exp = −28 14 kJ
Received: August 24, 2015
Published: November 4, 2015
© 2015 American Chemical Society
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Article
Scheme 1. Dehydrocoupling of Amine-Boranes
Scheme 2. Hydrogen Transfer between Me2NH·BH3 and
iPr2NBH2
2. RESULTS
2.1. Synthesis and Characterization of 1a−1d. The
synthesis of a series of B-methylated amine-boranes (1a−1d)
was carried out via salt metathesis with Li[BH3Me] and
[RR′NH2]Cl (R, R′ = H, Me, or iPr, Scheme 3).11d The former
species was prepared by the reaction of MeB(OH)2 with 1.5
equivalents of LiAlH4.24
Scheme 3. Synthesis of B-Methylated Amine-Boranes 1a−1d
(1a: R, R′ = H; 1b: R = H, R′ = Me; 1c: R, R′ = Me; 1d: R,
R′ = iPr)
mol−1, ΔG°calc = −20.0 kJ mol−1).1j The mechanistic pathway
for the hydrogen transfer was investigated and found to involve
a concerted process with a cyclic six-membered transition state
and an activation energy of +91
5 kJ mol−1 (ΔG°‡exp) or
+86.9 kJ mol−1 (ΔG°‡calc).
Although a wide range of N-substituted amine-boranes are
known and have been investigated extensively,4a to date, far
fewer examples of B-substituted amine-boranes have been
studied. The geometries and dissociation energies of NH3·
BHxMe3−x and MexNH3−x·BH3 (x = 0−3) were theoretically
predicted by Boutalib and co-workers, who suggested that the
stability of the amine-borane increased upon inclusion of a
methyl group at nitrogen with a corresponding decrease when
boron possessed methyl substituents.18 Dixon and co-workers
investigated the dehydrogenation energies of the same set of
amine-boranes via DFT calculations and reported that having a
methyl substituent at boron led to an exothermic dehydrogen-
ation reaction, whereas at nitrogen the process was close to
thermoneutral.19 In addition, our group has previously reported
that the B-pentafluorophenyl substituted amine-borane,
iPr2NH·BH(C6F5)2, undergoes thermal dehydrogenation at
100 °C to yield the aminoborane iPr2NB(C6F5)2.20 We have
also synthesized a series of B-thioaryl substituted amine-
boranes, including iPr2NH·BH2SR (R = Ph, C6F5), which
underwent both thermal and catalytic dehydrogenation to yield
B-thioaryl substituted aminoborane, iPr2NBHSR.21 Synthesis
of two B-methylated amine-boranes, Me3N·BH2Me and
Me2NH·BH2Me, has been previously reported by Paul and
Roberts,22 and Beachley and Washburn,23 although no further
studies of their reactivity were described.
The B-methylated amine-boranes were isolated as either
colorless solids (1a, 1b, 1d) or as a liquid (1c), and selected
characterization data is given in Table 1.25 As expected, one
signal was observed in the 11B NMR spectra in the range −8.5
to −15.1 ppm as a triplet (1JBH = 94−96 Hz), for 1a−1d in
CDCl3 (Figures S1, S3, S6, and S9). The observed chemical
shift and coupling pattern was indicative of a four-coordinate
boron center with two hydrogen substituents. Comparison of
the 11B NMR chemical shifts of 1a−1d to the analogous amine-
boranesammonia-borane, NH3·BH3; N-methyl amine-bor-
ane, MeNH2·BH3; N-dimethyl amine-borane, Me2NH·BH3;
and N-diisopropyl amine-borane, iPr2NH·BH3indicated that
a downfield shift was observed upon the replacement of a
hydrogen for a methyl group at boron (Table 1).4a The
observed 1H (Figures S2, S4, S7, and S10) and 13C (Figures S5,
S8, and S11) NMR spectra were unremarkable, but consistent
with the assigned structures.
Recrystallization of 1a from a solution of Et2O/hexanes at
−40 °C yielded small, colorless crystals suitable for study by
single crystal X-ray diffraction, which corroborated the
connectivity suggested by the other spectroscopic data (Figure
1a).25 In an analogous fashion, crystals of 1b and 1d were
grown that were also suitable for X-ray diffraction (Figures 1b
and 2).25
Herein, we report the synthesis of a series of B-methylated
amine-boranes (1a, NH3·BH2Me; 1b, MeNH2·BH2Me; 1c,
Me2NH·BH2Me;22,23 1d, iPr2NH·BH2Me) and describe studies
of their reactivity at elevated temperatures as well as toward a
range of well-established dehydrocoupling catalysts and the
hydrogen acceptor, iPr2NBH2. While our work was in
progress, Liu and co-workers independently reported the
synthesis of 1a and 1b, and determined that the catalytic
dehydrocoupling of these species in the presence of CoCl2 at
80 °C yielded borazine, [RN−BMe]3 (R = H or Me) as the sole
product after 36 h.11d
The B−N bond length for 1a was determined to be 1.614(3)
Å, which was similar to that of 1b (1.605(2) Å) within
experimental error (Table 2). On the other hand, the inclusion
of two sterically bulky isopropyl groups at nitrogen (in 1d)
significantly lengthened the B−N bond to 1.6333(6) Å.
Although the increased electron donating ability of the
isopropyl groups would be expected to strengthen the dative
bond, the steric effect of the bulky alkyl groups dominates,
thereby weakening the B−N bond. The B−N bond length for
NH3·BH3 was determined to be 1.58(2)27 by neutron
diffraction, which was considered similar to that observed for
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Table 1. Yields and 11B and H NMR Spectroscopic Data of Selected Amine-Boranes in CDCl3
Article
4a,26
1
amine-borane
yield/%
δB
1JBH
δH(B−CH3)
δH(N−CH)
ref
NH3·BH3
1a
86
41
89
46
71
18
90
31
−21.6
−15.1
−18.8
−12.3
−15.1
−8.5
95
95
94
93
96
94
97
96
−
−0.11
−
−0.18
−
−0.19
−
−
−
4a
this work
4a
MeNH2·BH3
1b
2.54
2.52
2.59
2.55
3.23
3.34
this work
26
Me2NH·BH3
1c
this work
4a
iPr2NH·BH3
1d
−21.1
−14.6
−0.18
this work
Longer-distance, noncovalent intermolecular interactions
were observed for 1a between adjacent amine-boranes, with
lengths in the range 2.08−2.34 Å between the hydridic and
protic hydrogen atoms on boron and nitrogen, respectively
(Figure 3 and Table 3). These distances were shorter than
Figure 1. Molecular structure of compounds (a) 1a and (b) 1b with all
non-H atom thermal ellipsoids drawn at the 50% probability level.
Figure 3. Association of the molecules of 1a via dihydrogen
intermolecular interactions in the solid state with thermal ellipsoids
at the 50% probability level. All hydrogen atoms bonded to carbon are
omitted for clarity.
Figure 2. Molecular structure of compound 1d with all non-H atom
thermal ellipsoids drawn at the 50% probability level. All hydrogen
atoms bonded to carbon were omitted for clarity.
Table 2. B−N Bond Lengths of Selected Amine-Boranes by
X-ray Diffraction
Table 3. H−H Bond Distances (D(HN−HB)) and Angles
(∠HHN and ∠HHB) between Protic and Hydridic
Hydrogens on B-Methylated Amine-Boranes 1a, 1b, and 1d
amine-borane
B−N bond length/Å
ref
a
NH3·BH3
1.58(2)
27
amine-borane
D(HN−HB)/Å
∠HHN/deg
∠HHB/deg
1a
1.614(3)
this work
28
1a
2.076
2.133
2.160
2.342
2.253
2.130
2.119
2.163
161.9
140.5
143.6
167.2
137.5
166.0
151.9
174.2
147.4
123.4
102.4
92.6
MeNH2·BH3
1b
1.5936(13)
1.605(2)
this work
28
Me2NH·BH3
1d
1.5965(13)
1.6333(6)
this work
1b
1d
94.9
a
Determined by neutron diffraction.
99.3
131.0
144.1
the amine-boranes 1a, 1b, and 1d. However, a significantly
longer B−N bond length was found compared to the analogous
amine-boranes without a methyl group at boron (1.5936(13)
and 1.5965(13) Å for MeNH2·BH3 and Me2NH·BH3,
respectively),28 as determined by X-ray diffraction. This was
likely a consequence of an increase in electron density at boron
induced by the electron donating methyl group, which reduced
the strength of the B−N dative bond. Both the boron and
nitrogen atoms of 1b were found to exhibit distorted
tetrahedral geometries, consistent with approximate sp3 hybrid-
ization, and the C1−N1−B1−C2 chain was found to adopt a
gauche conformation with a dihedral angle of 178.27°.
twice the van der Waals radius for two hydrogen atoms (2.4 Å).
Dihydrogen intermolecular interactions were also detected for
amine-boranes 1b and 1d (Figures S61 and S62 and Table 3).
The H−H−N angles were determined to be relatively linear
(137.6−174.2°), in contrast to the corresponding H−H−B
angles indicating a bent geometry, with values in the range
92.7−147.4°. Together with the dihydrogen distances, these
types of linear H−H−N bond and bent H−H−B bond
geometry were similar to those observed for dihydrogen
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intermolecular interactions in other crystallographically charac-
terized amine-boranes.27−29
Scheme 5. Thermolysis of 1a in Toluene at 70 °C
2.2. Thermally Induced Redistribution and Dehydro-
genation Reactions of 1a−1d. To determine the thermal
stability of the B-methylated amine-boranes, a solution of 1a−
1d in either a coordinating (tetrahydrofuran, THF) or a weakly
coordinating (toluene) solvent was monitored by 11B NMR
spectroscopy at ambient (20 °C) and elevated (70 °C)
temperatures over various periods of time. The assignment of
the product signals detected by 11B NMR spectroscopy was
further supported by literature chemical shift data (δB,lit), where
available, as well as DFT calculations of 11B NMR chemical
elevated temperatures was then investigated; a toluene solution
of 1b−1d was heated to 70 °C until quantitative consumption
of the amine-borane was detected. As the steric bulk of the alkyl
groups increased at nitrogen, the reaction time for complete
consumption of the B-methylated amine-borane increased from
48 h (1b) to 170 h (1c) to 500 h (21 days) (1d), as monitored
by 11B NMR spectroscopy.
shifts (δB,calc) and JBH coupling constants (Table S4).25
1
2.2.1. Thermally Induced Redistribution and Dehydrogen-
ation Reactions of 1a. The thermal stability of 1a at ambient
temperature was investigated by 11B NMR spectroscopy. No
reaction was observed for 1a in a THF solution after 170 h at
20 °C (Figure S16). However, under analogous conditions in a
toluene solution, significant amounts of products derived from
the redistribution of the methyl and hydrogen substituents at
boron were observed; in addition to unreacted 1a (ca. 40%),
the appearance of singlet, doublet, and quartet peaks at −6.1,
−9.5, and −22.3 ppm, respectively, were detected by 11B NMR
spectroscopy at 20 °C after 170 h. The product peaks were
postulated to correspond to B-trimethyl amine-borane, NH3·
BMe3 [δB,exp −6.1 (s)] [δB,calc −7.0] (ca. 10%), B-dimethyl
Similar to the case of 1a, thermolysis of 1b in toluene at 70
°C led to the formation of a complex mixture of redistribution
and dehydrogenation products by 11B NMR spectroscopy,
although the process was slower (48 h). These products
consisted of MeNHBMe2 [δB,exp 45.0 (s)] [δB,calc 47.4] [δB,lit
45.7]30 (ca. 30%), tentatively assigned cyclic oligo(B-methyl
aminoborane) [MeNH−BHMe]x [δB,exp −3.2 (d, br)] (ca.
20%), N-trimethyl triborazane [MeNH−BH2]3 [δB,exp −7.0
(m)] [δB,lit −5.4 (t, 1JBH1 = 107 Hz)]32 (ca. 20%), and MeNH2·
BH3 [δB,exp −18.4 (q, JBH = 96 Hz] [δB,lit −18.8 (1JBH = 94
Hz)]4a (ca. 30%) (Scheme 6, Figure S26). Analogous
redistribution and dehydrogenation products were detected
for the thermolysis reaction of 1c (Figure S27), and for 1d, only
dehydrogenated derivatives iPr2NBMe2 [δB,exp 44.0 (s)]
1
amine-borane, NH3·BHMe2 [δB,exp −9.5 (d, JBH = 107 Hz)]
[δB,calc −9.8 (1JBH = 97 Hz)] (ca. 30%), and NH3·BH3 [δB,exp
−22.3 (q, 1JBH = 91 Hz)] [δB,lit −21.6 (q, 1JBH = 95 Hz)]4a (ca.
10%) (Scheme 4, Figure S17).
1
[δB,calc 47.2] (ca. 30%) and iPr2NBHMe [δB,exp 39.2 (d, JBH
Scheme 4. Redistribution of Methyl and Hydrogen
Substituents at Boron for 1a in Toluene at 20 °C
= 119 Hz)] [δB,calc 41.4 (1JBH = 122 Hz)] (ca. 40%) were
1
formed together with iPr2NH·BH3 [δB,exp −21.4 (q, JBH = 99
Hz)] [δB,calc −21.1 (1JBH = 97 Hz)]4a in toluene at 70 °C
(Figure S28). Similar reactivity was also observed for the
thermolysis of 1b−1d in the more coordinating solvent, THF,
at 70 °C over a period of 170−340 h (Figures S29−S31).
2.3. Catalytic Dehydrocoupling Reactions of B-
Methylated Amine-Boranes 1a−1d. In an attempt to
favor dehydrocoupling over redistribution reactions, the use
of transition metal catalysts was explored. Specifically, the
reactivity of the B-methylated amine-boranes 1a−1d toward
previously established dehydrocoupling catalysts [Rh(COD)(μ-
Cl)]2 (COD = 1,5-cyclooctadiene),4a IrH2(POCOP) (POCOP
= κ3-1,3-(OPtBu2)2C6H3),2d,5a and skeletal nickel6a was
investigated.
2.3.1. Catalytic Dehydrocoupling of 1a−1d with [Rh-
(COD)(μ-Cl)]2. A THF solution of 1a was treated with 2.5 mol
% [Rh(COD)(μ-Cl)]2 (5 mol % Rh) at 20 °C, and the reaction
course was monitored by 11B NMR spectroscopy. After 1 h,
complete consumption of the starting material was observed
with the quantitative formation of the dehydrocoupled product,
B-trimethyl borazine [NH−BMe]3 (Scheme 7a, Figure S32).
The quantitative conversion was similarly reported by Liu and
co-workers,11d whereby 1a in diglyme with 5 mol % CoCl2
yielded borazine [NH−BMe]3 with the exception that their
reaction was heated at 80 °C and required 36 h to reach
completion. Upon addition of [Rh(COD)(μ-Cl)]2 to the THF
solution of 1a−1d, a change from a transparent yellow solution
to a black suspension was observed. This was consistent with
reduction of the Rh(I) precatalyst to rhodium colloids, the
likely active catalyst based on previous studies on amine-borane
dehydrocoupling with [Rh(COD)(μ-Cl)]2.4a,33
As well as the products arising from methyl and hydrogen
redistribution at boron for 1a, a broad 11B NMR peak with no
detectable proton coupling was observed at δB,exp 46.3 (s). This
was assigned to B-dimethyl aminoborane, NH2BMe2 [δB,calc
48.8] [δB,lit 47.1]30 (ca. 10%), which was postulated to arise as a
result of the instability of NH3·BHMe2 toward hydrogen loss, as
predicted by DFT calculations.19
The redistribution reaction of 1a in toluene was also studied
at elevated temperature (70 °C). After 24 h, 11B NMR
spectroscopy showed that 1a had been completely consumed
with the formation of the dehydrocoupled product B-trimethyl
borazine [NH−BMe]3 [δB,exp 34.3 (s)] [δB,calc 34.5] [δB,lit
36.0]31 (ca. 10%). Other products observed were NH2
BMe2 (ca. 40%) and the redistribution product NH3·BHMe2
(ca. 30%). Minor amounts of NH3·BMe3 (ca. 10%), and
[NH2−BH2]x [δB,exp −13.9 (m)] [δB,lit −10.7 (in solid state)]5c
(ca. 10%) were also detected (Scheme 5, Figure S18). Similar
redistribution products were observed when heating 1a to 70
°C in THF after 24 h (Figure S19).
2.2.2. Thermally Induced Redistribution and Dehydrogen-
ation Reactions of 1b−1d. In contrast to the redistribution of
the methyl and hydrogen substituents at boron observed for 1a
in toluene at 20 °C, no corresponding reaction was detected for
solutions of 1b−1d in either coordinating (THF) or weakly
coordinating (toluene) solvents after 170 h by 11B NMR
spectroscopy (Figures S20−S25). The stability of 1b−1d at
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Scheme 6. Thermolysis of 1b in Toluene at 70 °C
Scheme 7. Catalytic Dehydrocoupling of (a) 1a and (b) 1b in
Scheme 8. Catalytic Dehydrocoupling of (a) 1c and (b) 1d in
THF at 20 °C ([M] = 2.5 mol % [Rh(COD)(μ-Cl)]2, 5 mol
THF at 20 °C ([M] = 2.5 mol % [Rh(COD)(μ-Cl)]2, 5 mol
a
a
% IrH2(POCOP), 10 mol % Skeletal Nickel)33
% IrH2(POCOP), 10 mol % Skeletal Nickel)
a
Species that appear with only specific substrates and precatalysts are
labeled as follows: (red) with 1c and [Rh(COD)(μ-Cl)]2 or skeletal
nickel, (blue) with 1d and [Rh(COD)(μ-Cl)]2 or skeletal nickel,
(purple) with 1d and [Rh(COD)(μ-Cl)]2.
a
Species that appear with only specific substrates and precatalysts are
labeled as follows: (red) with 1a and IrH2(POCOP) or skeletal nickel,
(blue) with 1b and IrH2(POCOP) or skeletal nickel, (purple) with 1b
and skeletal nickel, and (green) with 1b and [Rh(COD)(μ-Cl)]2 or
skeletal nickel.
redistribution product iPr2NBMe2 (ca. 20%) (Scheme 8b,
Figure S35).
2.3.2. Catalytic Dehydrocoupling of 1a−1d with
IrH2(POCOP) and Skeletal Nickel. Similar to the dehydrocou-
pling of 1a with [Rh(COD)(μ-Cl)]2, the reaction of 1a with 5
mol % IrH2(POCOP) in THF at 20 °C required 1 h for
complete consumption of amine-borane and yielded [NH−
BMe]3 (ca. 90%). However, B-methylbis(amino)borane
(NH2)2BMe [δB,exp 32.1 (s)] [δB,calc 31.0] (ca. 10%) was also
detected as a second product by 11B NMR spectroscopy
(Scheme 7a, Figure S36). In contrast, the reaction of 1a with 10
mol % skeletal nickel in THF at 20 °C was significantly slower
than for both [Rh(COD)(μ-Cl)]2 and IrH2(POCOP) as
precatalysts, with complete consumption of 1a detected after
24 h together with the formation of the same two products as
for IrH2(POCOP), namely borazine [NH−BMe]3 (ca. 80%)
and (NH2)2BMe (ca. 20%) (Scheme 7a, Figure S37).
In contrast to the rapid and quantitative dehydrocoupling
reactivity observed for 1a, the reaction of 1b in THF at 20 °C
with 2.5 mol % [Rh(COD)(μ-Cl)]2 (5 mol % Rh) resulted in a
slower conversion of the amine-borane (24 h for complete
consumption). In this case, two products were formed: B-
trimethyl-N-trimethyl borazine [MeN−BMe]3 [δB,exp 35.7
(s)]34 [δB,calc 36.4] (ca. 80%) and B-methyl-N-methylbis-
(amino)borane (MeNH)2BMe [δB,exp 31.0 (s)] [δB,calc 30.2]
[δB,lit 31.7]30 (ca. 20%), as observed by 11B NMR spectroscopy
(Scheme 7b, Figure S33). In this case, the formation of the two
products differs from the results described by Liu and co-
workers for the Co-catalyzed dehydrogenation of 1b, as under
more forcing conditions (5 mol % CoCl2, 80 °C, 36 h),
borazine was observed as the sole product.11d
In contrast to the results for 1a, a significantly faster reaction
was observed for 1b with 5 mol % IrH2(POCOP) than for
[Rh(COD)(μ-Cl)]2 in THF at 20 °C, with complete
consumption of the amine-borane observed after 1 h rather
than 24 h. However, a more complex array of products was
formed, including B-methyl-N-methyl aminoborane, MeNH
BHMe [δB,exp 41.9 (d, 1JBH = 116 Hz)] [δB,calc 44.0 (1JBH = 122
Hz)] (ca. 20%), cyclic B-methyl-N-methyl diborazane
The catalytic dehydrogenation of 1c with 2.5 mol %
[Rh(COD)(μ-Cl)]2 (5 mol % Rh) in THF at 20 °C proceeded
at a similar rate to that of 1b, although the product mixture
showed a further increase in complexity. Complete con-
sumption of amine-borane was observed after 24 h with the
formation of Me2NBHMe (ca. 30%), the corresponding
dimer, cyclic B-dimethyl-N-tetramethyl diborazane, [Me2N−
1
[MeNH−BHMe]2 [δB,exp 1.4 (d, JBH = 114 Hz)] [δB,calc −0.8
(1JBH = 111 Hz)] (ca. 20%), and [MeN−BMe]3 (ca. 60%)
(Scheme 7b, Figure S38). Not only was the reaction of 1b with
10 mol % skeletal nickel in THF at 20 °C slower than that for
IrH2(POCOP) (24 h), a wider range of products was formed,
including MeNHBHMe (ca. 10%), MeNHBMe2 (ca.
10%), [MeNH−BHMe]2 (ca. 30%), [MeN−BMe]3 (ca. 20%),
and (MeNH)2BMe (ca. 30%) (Scheme 7b, Figure S39).
As with the case of [Rh(COD)(μ-Cl]2 as a precatalyst, the
reaction of 1c with 5 mol % IrH2(POCOP) in THF at 20 °C
was complete after 24 h. Moreover, in contrast to the formation
of an array of products as in the former case, only two products
were present by 11B NMR spectroscopy: Me2NBHMe (ca.
60%) and [Me2N−BHMe]2 (ca. 40%) (Scheme 8a, Figure
S40). Similarly, complete consumption of 1c was observed by
11B NMR spectroscopy after 24 h with 10 mol % skeletal nickel
in THF at 20 °C, with the formation of Me2NBHMe (ca.
1
1
BHMe]2 [δB,exp 4.9 (d, JBH = 110 Hz) and 4.4 (d, JBH = 109
Hz)] [δB,calc 2.4 (1JBH = 110 Hz)] (ca. 40%), and B-methyl-N-
dimethylbis(amino)borane, (Me2N)2BMe [δB,exp 30.3 (s)]
[δB,calc 33.5] (ca. 20%) together with an unidentified product
[δB,exp 3.8 (m)] (ca. 10%) being observed by 11B NMR
spectroscopy (Scheme 8a, Figure S34). The cyclic diborazane,
[Me2N−BHMe]2, was observed by 11B NMR spectroscopy as
two doublet peaks, consistent with the presence of cis and trans
isomers.
The reaction of the sterically encumbered B-methylated
amine-borane 1d with 2.5 mol % [Rh(COD)(μ-Cl)]2 (5 mol %
Rh) in THF at 20 °C was observed to be substantially slower
than for 1a−1c, with complete consumption of 1d requiring
120 h. The products detected by 11B NMR spectroscopy were
iPr2NBHMe (ca. 80%), and the presumed, dehydrogenated
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40%), [Me2N−BHMe]2 (ca. 30%), (Me2N)2BMe (ca. 10%),
and an unassigned product [δB,exp 0.9 (m)] (ca. 20%) (Scheme
8a, Figure S41).
tentatively assigned oligomeric/polymeric species had been
consumed. The only products detected were [MeN−BMe]3
(ca. 90%), MeNHBMe2 (ca. 5%), and (MeNH)2BMe (ca.
5%) (Figure S47). Again, it appears that any oligo/poly(B-
methyl-N-methyl aminoborane) formed readily undergoes
further dehydrogenation and depolymerization under the
reaction conditions.
No reaction was observed following the attempted
dehydrogenation of 1d with 5 mol % IrH2(POCOP) in THF
at 20 °C after 120 h by 11B NMR spectroscopy (Figure S42). In
contrast, 1d underwent dehydrogenation to yield the amino-
borane, iPr2NBHMe, as the sole product, using 10 mol %
skeletal nickel in THF at 20 °C, as observed by 11B NMR
spectroscopy. Nevertheless, the reaction proceeded significantly
slower compared to the reaction of 1d with [Rh(COD)(μ-
Cl)]2 as a precatalyst, with 70% conversion being observed after
210 h (Scheme 8b, Figure S43).
2.4. Attempted Dehydropolymerization of 1a and 1b
with Skeletal Nickel. We have previously prepared high
molecular weight poly(N-methyl aminoborane), [MeNH−
BH2]n, using stoichiometric amounts of skeletal nickel via the
dehydropolymerization of MeNH2·BH3.6a We explored analo-
gous reactions of 1a and 1b in an attempt to prepare the
currently unknown B-methylated polyaminoborane, [NHR−
BHMe]n (R = H or Me).
2.5. Hydrogen Transfer Reactions of 1a−1d with
iPr2NBH2. Following previous work from our group on
hydrogen transfer from amine-boranes to N-diisopropyl
aminoborane, iPr2NBH2,1i,j the ability for B-methylated
amine-boranes 1a−1d to act as hydrogen donors was
investigated. Thus, a stoichiometric amount of iPr2NBH2
(in THF) was added to solid 1a at 20 °C and monitored by 11
B
NMR spectroscopy. After 10 min, complete hydrogenation of
iPr2NBH2 to iPr2NH·BH3 was observed, along with the
formation of borazine [NH−BMe]3 and oligo/polyaminobor-
35
ane [NH2−BHMe]2,3,or x
,
products expected from dehydro-
genation/oligomerization of NH2BHMe (Scheme 10a,
Scheme 10. Hydrogen Transfer of (a) 1a with iPr2NBH2
in THF at 20 °C and (b) 1a with iPr2NBH2 in THF at 20
°C in the Presence of Two Equivalents of Cyclohexene (Cy
= C6H11)
First, 1a was treated with 100 mol % skeletal nickel in THF
at 20 °C and the reaction was monitored by 11B NMR
spectroscopy. After 1 h, ca. 40% conversion of 1a to borazine
[NH−BMe]3 (ca. 20%) and (NH2)2BMe (ca. 10%) was
observed and, in addition, a broad peak that was tentatively
assigned to be oligo/poly(B-methyl aminoborane) [NH2−
BHMe]x [δB,exp −7.8 to −9.1 (br)]35 (ca. 10%) (Scheme 9a,
Scheme 9. Attempted Dehydropolymerization of (a) 1a and
(b) 1b with Stoichiometric Amounts of Skeletal Nickel in
THF at 20 °C
Figure S48). No substantial change in the reaction mixture
was detected by 11B NMR spectroscopy after 1 h (Figure S49).
However, each set of 11B and 11B{1H} NMR spectra also
showed the presence of several other peaks, of which some
could not be successfully assigned. Nonetheless, one very minor
product, generally detected at a level corresponding to ca. 1%,
1
was identified as iPr2NBHMe [δB,exp 39.6 (d, JBH = 122
Figure S44). After allowing the reaction to continue for 24 h,
both 1a and the tentatively assigned [NH2−BHMe]x were no
longer detected, with [NH−BMe]3 (ca. 90%) and (NH2)2BMe
(ca. 10%) being the only products observed (Figure S45).
These results suggest that, if oligo/poly(B-methyl amino-
borane) is indeed formed, this material undergoes dehydrogen-
ation and depolymerization much more readily than the N-
methyl analogue, [MeNH−BH2]n.36
Hz)], the major dehydrogenation product formed in the
catalytic dehydrogenation of 1d with Rh and Ni precatalysts
(see section 2.3). The formation of this species was surprising
as it represents a cross-product where the methyl substituent on
boron presumably arises from the amine-borane substrate, 1a.
Another unexpected product was identified as borazine, [NH−
BH]3 [δB,exp = 29.4 (d, 1JBH = 141 Hz)] [δB,lit = 30.2 (d, 1JBH
=
141 Hz)].4a
Next, we explored the analogous reaction of 1b with 100 mol
% skeletal nickel in THF at 20 °C. After 1 h, 80% conversion
was observed by 11B NMR spectroscopy with the formation of
MeNHBHMe (ca. 10%) and [MeNH−BHMe]2 (ca. 20%)
along with unidentified products [δB,exp 40.8 (s)] (ca. 10%),
[δB,exp 2.2 (s)] (ca. 10%), [δB,exp −0.2 (d)] (ca. 20%), and [δB,exp
−2.6 (br)] (ca. 10%) (Scheme 9b, Figure S46). The peaks
detected between δB 2.2 and −2.6 ppm were tentatively
assigned to oligo/poly(B-methyl-N-methyl aminoborane),
[MeNH−BHMe]x.35 After 24 h, the 11B NMR spectrum
indicated that 1b, MeNHBHMe, [MeNH−BHMe]2, and the
Transient aminoboranes that exist sufficiently long in
solution can be trapped through hydroboration with cyclo-
hexene to yield, for the case of NH2BH2, B-dicyclohexyl
aminoborane, NH2BCy2 (Cy = C6H11).1i,37 To investigate
whether hydrogen transfer between 1a and iPr2NBH2 occurs
via formation of a transient aminoborane, NH2BHMe,
amine-borane 1a was added to one equivalent of iPr2NBH2
and two equivalents of cyclohexene in THF at 20 °C, and the
subsequent reaction was monitored by 11B NMR spectroscopy.
After 1 h, hydrogenation of iPr2NBH2 to iPr2NH·BH3 was
observed, together with the formation of the hydroborated
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aminoborane species, B-methylcyclohexyl aminoborane,
NH2BMeCy [δB,exp 46.7 (s)] [δB,calc 49.2] (Scheme 10b,
Figure S50). Consistent with these results, the interception of
NH2BHMe by cyclohexene almost completely prevents the
formation of the oligomer, [NH2−BHMe]x.
As with the case of 1a as a hydrogen donor, a stoichiometric
amount of iPr2NBH2 (in THF) was added to solid 1b at 20
°C. The reaction was monitored by 11B NMR spectroscopy
with 90% hydrogenation of iPr2NBH2 to iPr2NH·BH3
detected after 10 min and subsequent formation of MeNH
BHMe, [MeNH−BHMe]2, and (MeNH)2BMe (Scheme 11a,
Figure S51). No further reaction was detected by 11B NMR
spectroscopy after 1 h (Figure S52).
equivalent of iPr2NBH2 and two equivalents of cyclohexene
in THF at 20 °C after 1 h by 11B NMR spectroscopy (Figure
S56).
In contrast to the very rapid (1a and 1b) and slower (1c)
hydrogen transfer between 1a−1c and iPr2NBH2, no
reaction was observed between a stoichiometric amount of
iPr2NBH2 and the sterically encumbered B-methylated
amine-borane, 1d, in THF at 20 °C after 1 h (Figure S57).
However, after 24 h, a small amount of hydrogen transfer was
detected, as shown by 11B NMR spectroscopy with hydro-
genation of iPr2NBH2 to iPr2NH·BH3 and concomitant
dehydrogenation of 1d to iPr2NBHMe (Scheme 11c, Figure
S58). The reaction progressed until an apparent equilibrium
was established1j after 56 days with 95% hydrogenation of
iPr2NBH2 (Figure S59).
Scheme 11. Hydrogen Transfer of (a) 1b, (b) 1c, and (c) 1d
with iPr2NBH2 in THF at 20 °C
3. DISCUSSION
3.1. Thermally Induced Redistribution and Dehydro-
genation Reactions of 1a−1d. The B-methylated amine-
borane, 1a, was observed to undergo methyl and hydrogen
redistribution at boron at ambient (20 °C, in toluene) and
elevated (70 °C, in toluene or THF) temperatures that resulted
in the formation of NH3·BMe3, NH3·BHMe2, and NH3·BH3
(see section 2.2, Schemes 4 and 5).
The redistribution of substituents at boron in three-
coordinate boranes is well-known, and the mechanism has
been proposed to occur via a diborane intermediate.38
However, limited research has involved studies of analogous
four-coordinate boron species,39 with no reports of redis-
tribution of alkyl substituents at boron as part of an amine-
borane.40 Benton and Miller investigated the reaction between
two amine-boranes with different halogen substituents at boron
(selected from Me3N·BX3 (X = F, Cl, Br, I)) and reported that
when the solution was heated at 50 °C (CH2Cl2), 70 °C
(CHCl3), and 100 °C (C6H5Cl) for 6 h, no redistribution was
observed.41 However, when the reaction was heated to 160 °C
in the gas phase, redistribution of the halogen substituents was
noted. In addition, labeling studies where one of the amine-
boranes was 10B enriched (and the other consisted of natural
abundance boron, ca. 80% 11B, ca. 20% 10B) showed evidence
for redistribution of 10B between the amine-borane substrates,
suggesting redistribution occurred via B−N bond cleavage.
In a similar manner to that suggested by Benton and Miller
for B-halogenated amine-boranes,41 the redistribution of the
methyl and hydrogen substituents at boron for B-methylated
amine-boranes was postulated to arise through dissociation of
the B−N bond to yield free amine and borane (Scheme 12, step
A). Subsequent dimerization of two electron deficient borane
molecules would lead to a diborane intermediate (Scheme 12,
step B). Redistribution of methyl and hydrogen at boron could
be rationalized by cleavage and formation of a bond between
boron and the bridging moiety (Scheme 12, step C).
Retrodimerization of a hydrogen−methyl-bridged diborane
would yield two boranes (Scheme 12, step D) that reassociate
with free amine to reform two different amine-boranes
(Scheme 12, step E). In principle, this process could be
Similar to the case of the hydrogen transfer reaction between
1a and iPr2NBH2, trapping of the likely transient monomeric
aminoborane intermediate, in this instance, MeNHBHMe,
was attempted with cyclohexene. Under analogous conditions,
1b was added to one equivalent of iPr2NBH2 and two
equivalents of cyclohexene in THF at 20 °C. After 1 h,
hydrogenation of iPr2NBH2 to iPr2NH·BH3 was detected,
along with the formation of [MeNH−BHMe]2 and a peak at
δB,exp 45.4 (s), assigned to MeNHBMeCy [δB,calc 47.3], as
well as minor amounts of MeNHBHMe by 11B NMR
spectroscopy (Figure S53).
Slower hydrogen transfer was detected between 1c and
iPr2NBH2, with 45% hydrogenation after 10 min in THF at
20 °C by 11B NMR spectroscopy. As well as the formation of
iPr2NH·BH3, the dehydrogenated product Me2NBHMe was
also detected (Scheme 11b, Figure S54). After 1 h, further
hydrogenation (72%) was observed as well as the appearance of
the cyclic diborazane, [Me2N−BHMe]2 (Figure S55). In the
case of 1c, the presence of cyclohexene resulted in no new
products corresponding to the trapped aminoborane, Me2N
BMeCy, being observed for the reaction of 1c with one
Scheme 12. Postulated Mechanism of the Methyl−Hydrogen Exchange Reaction of 1a
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Figure 4. Calculated relative Gibbs free energies G298 (in kJ mol−1) for the methyl−hydrogen exchange reaction of 1a. Calculations were performed
at the M06-2X/cc-pVTZ level of theory; solvent corrections for THF were applied.25
repeated to yield the full range of possible amine-boranes with
different combinations of methyl and hydrogen substituents at
boron.
and toluene (δB,exp = −15.7) suggests that such an effect, if it
exists, has only a small effect on the B−N bond strength. The
B-methylated amine-boranes 1a−1d were also calculated to be
slightly more stable as a solution in THF than in toluene, by
8.0−13.8 kJ mol−1 (Table S5).25 As a more polar solvent, THF
probably provides a solution environment in which the amine-
borane with polar N−H and B−H bonds is more stable,
thereby providing a possible explanation why redistribution was
not observed for 1a in THF at 20 °C.
Redistribution of methyl and hydrogen substituents in the B-
methylated amine-boranes 1b−1d was not detected in toluene
at 20 °C after 170 h, but scrambling was observed at 70 °C,
with a rate for complete consumption of the amine-borane in
the order 1b > 1c > 1d (see section 2.2.2). The electron
donating alkyl groups at nitrogen would be expected to
strengthen the dative bond between the nitrogen and boron,
resulting in an increased resistance to dissociation. This was
supported by DFT calculations for 1b and 1c, where the bond
dissociation energies in toluene increased from +124.9 kJ mol−1
for 1a to +142.9 and +150.5 kJ mol−1 for 1b and 1c,
respectively (Table 4). However, the bond dissociation energy
This proposed mechanism was probed by DFT calculations
as a THF solution with the dissociation energy of the B−N
bond of 1a calculated to be +70.7 kJ mol−1 (Figure 4, step A).
The diborane, formed from two BH2Me moieties (Figure 4,
step B), can be bridged by either two hydrogen atoms or a
hydrogen atom and a methyl group (+44.0 kJ mol−1 relative to
the dihydrogen-bridged diborane). A dimethyl-bridged dibor-
ane was also considered to be a possible intermediate, but as
expected, the corresponding calculated energy was considerably
higher (+114.9 kJ mol−1), implying that the species is unlikely
to be formed during the redistribution reaction. The activation
energy associated with the interconversion of the dihydrogen-
bridged and hydrogen−methyl-bridged diborane intermediates,
where cleavage of a B−H bridge bond and rotation of one
BH2Me moiety around the remaining B−H bond occurs to lead
to the formation a new B−C bridging bond, was calculated to
be +112.9 kJ mol−1 (Figure 4, step C). The energy for the
transition state for the retrodimerization of the diborane to two
boranes was determined to be +66.9 kJ mol−1 (Figure 4, step
D), with an exergonic reassociation of ammonia with the free
boranes to form amine-boranes, NH3·BH3 (−85.9 kJ mol−1)
and NH3·BHMe2 (−74.4 kJ mol−1) (Figure 4, step E).
Redistribution of methyl and hydrogen substituents at boron
was observed for 1a in toluene but not in THF at 20 °C. Based
on previous observations of oxygen-containing species
coordinating to the protic hydrogen on nitrogen in amine-
boranes and their dehydrocoupling products,29a,43 a possible
explanation involves stabilization of the amine-borane via
hydrogen bonding to the donating THF solvent (Scheme 13).
However, DFT calculations determined that 1a·THF was
higher in energy than the corresponding dihydrogen-bridged
diborane (formed in step B in Figure 4) by +19.5 kJ mol−1 and
similar 11B NMR chemical shifts for 1a in THF (δB,exp = −16.3)
Table 4. Amount of Time Required for Complete
Consumption of B-Methylated Amine-Boranes 1a−1d in
Toluene at 70 °C and Calculated Bond Dissociation
a
Energies (D0) of 1a−1d
time required for complete
amine-
borane
consumption of 1a−1d in
b
toluene at 70 °C/h
calcd D0 of B−N bond/kJ mol−1
1a
1b
1c
1d
24
48
+124.9
+142.9
+150.5
+128.7
170
500
a
Calculations were performed at the M06-2X/cc-pVTZ level of
theory; solvent corrections for toluene were applied.25 Based on 11B
b
NMR spectroscopy.
for 1d (+128.7 kJ mol−1) was calculated to be close to that for
1a. This was not unexpected as the B−N bond length for 1d
determined by X-ray diffraction was elongated compared to 1a
and 1b (Table 2), where the steric effect of the isopropyl
groups on nitrogen appeared to have a greater influence on the
bond distance than their electron donating characteristic. The
much lower reactivity of 1d toward redistribution despite the
Scheme 13. Structure of Postulated THF Stabilized B-Methyl
Amine-Borane (1a·THF)
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Table 5. Comparison of Percentage Hydrogenation of iPr2NBH2 to iPr2NH·BH3 Using Hydrogen Donating Amine-Boranes
a
1a−1c and RR′NH·BH3 (R, R′ = H, Me) in THF at 20 °C
hydrogenation of iPr2NBH2/%45
b
c
amine-borane
after 10 min
after 1 h
ΔG‡ /kJ mol−1
ΔG298 /kJ mol−1
ref
NH3·BH3
1a
89
100
56
90
4
98
100
91
−
+64.7
−
−23.6
−
−20.0
+9.1
45
this work
45
MeNH2·BH3
1b
−
92
+74.1
+86.9
+76.0
this work
1j
Me2NH·BH3
1c
16
45
72
−14.7
this work
Calculations performed at the M06-2X/cc-pVTZ level of theory; solvent corrections for THF were applied.25 Activation energy of hydrogen
a
b
c
transfer. Gibbs free energy of hydrogen transfer.
apparently weaker B−N bond was likely a consequence of the
unfavorable formation of the redistribution products, iPr2NH·
BHMe2 and iPr2NH·BMe3, on steric grounds. Indeed, neither
species was detected on thermolysis at 70 °C after 500 h.
Instead, the exclusive formation of their dehydrogenated
derivatives iPr2NBMe2 and iPr2NBHMe was evidenced,
together with iPr2NH·BH3 (Figure S28). This suggests that, in
the case of 1d, the reaction was driven by the subsequent
dehydrogenation of the initially formed, sterically disfavored
redistribution products and the very slow amine-borane
consumption rate was a likely consequence.
3.2. Catalytic Dehydrocoupling Reactions of 1a−1d.
The reactivity of the B-methylated amine-boranes 1a−1d with
respect to catalytic dehydrocoupling/dehydrogenation varied
significantly (see section 2.3). As previously noted, DFT
calculations highlight that the inclusion of a methyl group at
boron energetically favors dehydrogenation.19 Favorable
kinetics were also apparent, with shorter reaction times being
observed for B-methylated amine-borane dehydrocoupling/
dehydrogenation, compared to analogous amine-boranes with-
out a methyl group at boron. For example, treatment of 1a with
2.5 mol % [Rh(COD)(μ-Cl)]2 precatalyst for 1 h in THF led
to the release of two equivalents of hydrogen to form the
borazine [NH−BMe]3 quantitatively at ambient temperature
by 11B NMR spectroscopy. In comparison, the catalytic
dehydrocoupling of MeNH2·BH3 with the same precatalyst
yielded N-trimethyl borazine, [MeN−BH]3, but elevated
temperatures (45 °C) and an extended reaction time (ca.
48−84 h) were required.4a
We have previously reported that IrH2(POCOP) functions
as an efficient dehydropolymerization catalyst for MeNH2·BH3
to form high molecular weight poly(N-methyl aminoborane),
[MeNH−BH2]n, in THF at 20 °C within 20 min.5c However,
treatment of 1a and 1b with 5 mol % IrH2(POCOP) resulted in
two equivalents of hydrogen being released to yield borazine
products, [RN−BMe]3 (R = H, Me). No reaction was observed
for 1d with a catalytic amount of the iridium complex, which is
presumably the result of both the precatalyst and substrate
being too sterically encumbered for a catalytic reaction to take
place. Successful dehydrocoupling was observed for 1c in THF
at 20 °C within 24 h, a surprising result in comparison to the
analogous amine-borane without a methyl substituent at boron,
Me2NH·BH3, which was reported to undergo very slow
catalytic dehydrocoupling, with less than one equivalent of
hydrogen being released after 48 h in THF at 25 °C.5b Similar
to the case of [Rh(COD)(μ-Cl)]2 as a precatalyst, the increased
reactivity of IrH2(POCOP) toward B-methylated amine-
boranes 1a−1c compared to the N-methyl analogues was
postulated to be due to the favorable dehydrocoupling/
dehydrogenation kinetics and thermodynamics arising from
the presence of the electron donating methyl group at boron.
The use of catalytic amounts of skeletal nickel resulted in
successful dehydrocoupling of all the B-methylated amine-
boranes 1a−1d but with reduced selectivity. Since 1d only
reached ca. 70% conversion after 120 h, poisoning of the nickel
surface was postulated to occur before complete conversion of
1d to iPr2NBHMe was achieved. The poisoning of
heterogeneous catalysts by species with B−H bonds has been
demonstrated by our group in the case of Rh colloids.44 Based
on the previous results, boranes arising from dissociation of the
B-methylated amine-borane adducts likely lead to hydrogen gas
release and concomitantly form a boron-containing layer on the
nickel surface, rendering the catalyst inactive.
3.3. Attempted Dehydropolymerization of 1a and 1b
with Skeletal Nickel. Our group has previously reported the
reaction of MeNH2·BH3 with a stoichiometric amount of
skeletal nickel, whereby after 2 h in THF at 20 °C high
molecular weight poly(N-methyl aminoborane) was isolated
with 60% yield.6a With the aim of isolating the first high
molecular weight polyaminoborane with non-hydrogen sub-
stituents at boron, the amine-boranes 1a and 1b were therefore
also treated with a stoichiometric amount of skeletal nickel in
THF at 20 °C. In the case of 1a, partial consumption of the
amine-borane after 1 h was detected, together with the
presence of the tentatively assigned oligomer/polymer,
[NH2−BHMe]x, observed as a broad peak between −7.8 and
−9.1 ppm by 11B NMR spectroscopy. However, after 24 h, both
the tentatively assigned oligomer/polymer and the B-
methylated amine-borane were no longer detectable and the
final products were borazine, [NH−BMe]3, and bis(amino)-
borane (NH2)2BMe. A similar observation was noted for the
reaction of 1b with 100 mol % skeletal nickel in THF at 20 °C.
The methyl group at boron presumably increases the hydridic
nature of the hydrogen cosubstituents at boron, favoring further
dehydrogenation of oligo/poly(B-methyl aminoborane) to
borazine. Thus, the oligomer/polymer was only observed as
an intermediate before undergoing further dehydrocoupling to
the more thermodynamically favorable borazine.
3.4. Hydrogen Transfer Reactions of 1a−1d with
iPr2NBH2. Hydrogen transfer was detected between B-
methylated amine-boranes 1a−1d and iPr2NBH2 in THF at
20 °C, although the reaction progressed dramatically slower in
the case of 1d. To compare the hydrogen donating abilities of
1a−1c to amine-boranes without a methyl group at boron, the
percentage hydrogenation of iPr2NBH2 to iPr2NH·BH3 was
determined after 10 min and 1 h by relative integration in the
11B NMR spectra (Table 5).1j,45 After 10 min, at least 90%
hydrogenation of iPr2NBH2 to iPr2NH·BH3 was detected
10886
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Inorganic Chemistry
Article
using 1a and 1b as hydrogen donors. For amine-boranes
without a methyl group at boron, the percentage hydrogenation
decreased significantly from 89% (for NH3·BH3) to 4% (for
Me2NH·BH3) after 10 min. In comparison, for analogous
amine-boranes with a methyl group at boron, larger values and
a less substantial decrease were observed from 100% (for 1a) to
45% (for 1c).
Although 1d undergoes hydrogen transfer with iPr2NBH2
at a markedly slower rate than for 1a−1c, subsequent reactivity
of the resulting aminoborane iPr2NBHMe was not observed,
with only this species and iPr2NH·BH3 being detected as
products by 11B NMR spectroscopy (Scheme 11c). This system
reached an apparent equilibrium after 56 days with the
percentage hydrogenation of iPr2NBH2 determined to be
ca. 95%. Interestingly, although this equilibrium required a
significantly longer time to establish, the percentage hydro-
genation was substantially greater when compared to that for
the reaction of Me2NH·BH3 with iPr2NBH2 (54%), which
required 18 h in THF at 20 °C.1i,j This suggests that, although
the sterically encumbered nature of 1d results in a slow
reaction, the favorable thermodynamics enables the reaction to
reach near completion.
The increased hydrogen donating ability observed for 1a−1d
toward iPr2NBH2 is presumably a consequence of the
presence of the electron donating methyl group at boron. The
enhanced hydridic character of the hydrogen substituents at
boron would be expected to lead to faster dehydrogenation
reactions. The hydrogen transfer from 1a−1d also appears to
be thermodynamically more favorable, which is in agreement
with the previous work by Dixon and co-workers where
dehydrogenation was calculated to be more exergonic as the
number of methyl substituents at boron increased.19
calculated for 1a−1c (+64.7 to +76.0 kJ mol−1) (Table 5).
Hydrogen transfer from Me2NH·BH3 to iPr2NBH2 was
reported to be slightly endergonic (+9.1 kJ mol−1), in contrast
to the Gibbs free energies determined for 1a−1c, which were
exergonic (−14.7 to −23.6 kJ mol−1). Prior and subsequent to
the formation of the cyclic transition state, encounter
complexes were located, which were attributed to the presence
of the cyclic hydrogen bonding. Although these complexes were
determined to be slightly endergonic compared to initial and
final products, the activation energy was effectively reduced,
further favoring the hydrogen transfer reaction. This shows
that, both kinetically and thermodynamically, B-methylated
amine-boranes 1a−1c have an improved ability to function as
hydrogen donors toward the aminoborane iPr2NBH2 than
Me2NH·BH3.
The hydrogen transfer reaction of 1a with iPr2NBH2
resulted in the detection of very small quantities of the
unexpected product, iPr2NBHMe, with substituents appa-
rently derived from each of the reactants. In addition, the
borazine [NH−BH]3, with no methyl groups at boron, was also
formed as another surprising product. The mechanism of
formation for these species is unclear,47 and is the subject of
ongoing studies.
4. SUMMARY
Detailed studies of the reactivity of a series of B-methylated
amine-boranes with different substituents (H, Me, iPr) at
nitrogen have revealed a range of interesting, unexpected,
complex, and potentially useful chemistry. First, redistribution
of methyl and hydrogen substituents at boron was observed in
solution at ambient and elevated temperatures. This was
proposed to arise from dissociation of the amine-borane adduct
with subsequent redistribution of the substituents in the borane
via formation of a diborane intermediate. To our knowledge,
this represents the first demonstration of redistribution
reactions at a four-coordinate boron center under mild
conditions.
To further probe the mechanism of hydrogen transfer, DFT
studies were conducted for 1a−1c with iPr2NBH2 (Figure 5,
Rapid dehydrocoupling of B-methylated amine-boranes with
Rh(I), Ir(III), and Ni(0) precatalysts was detected to yield a
variety of products including aminoboranes, cyclic diborazanes,
and borazines, based on in situ 11B NMR spectroscopy with
peak assignments supported by DFT calculations. In
comparison to the well-studied catalytic dehydrocoupling of
N-methylated amine-boranes, faster dehydrocoupling/dehydro-
genation reactions were observed for the B-methylated
analogues. This is suggested to be a consequence of the
increased hydridic nature of the hydrogen atoms at boron
induced by the electron donating methyl group.
Attempts to synthesize high molecular weight polyaminobor-
anes with a methyl substituent at boron were made via catalytic
dehydropolymerization of 1a and 1b. However, any oligomeric
or polymeric B-methylated species that formed under the
reaction conditions appeared to readily undergo further
dehydrogenation to yield mainly the B-methylated borazine,
[RN−BMe]3 (R = H, Me). We are continuing our efforts in
this area with the aim to increase the yield of poly(B-methyl
aminoborane) and other analogous materials under conditions
where further dehydrocoupling is absent.
Figure 5. Calculated relative Gibbs free energies G298 (in kJ mol−1) for
the hydrogen transfer reaction of 1a with iPr2NBH2. Calculations
performed at the M06-2X/cc-pVTZ level of theory; solvent
corrections for THF were applied.25 All hydrogen atoms bonded to
carbon were omitted for clarity.
Figures S65 and S66).25 In each case, a concerted mechanism
with a transition state of a six-membered ring was determined,
indicating that both hydrogen atoms were transferred in the
same step. An analogous transition state was identified for the
hydrogen transfer reaction of Me2NH·BH3 with iPr2NBH2.1j
In our previously investigated reaction of Me2NH·BH3 with
iPr2NBH2, the activation energy associated with the
formation of the six-membered cyclic transition state was
found to be +86.9 kJ mol−1,46 which was higher than that
Very rapid hydrogenation of iPr2NBH2 to iPr2NH·BH3
was observed for the B-methylated amine-borane, 1a, with
complete hydrogen transfer observed after 10 min in THF at 20
°C, with very good hydrogenation rates also observed for 1b
and 1c. The hydrogen donating ability of B-methylated amine-
10887
DOI: 10.1021/acs.inorgchem.5b01946
Inorg. Chem. 2015, 54, 10878−10889

Inorganic Chemistry
Article
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membered transition state. In addition, the reaction was
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borane iPr2NBH2 than the previously investigated amine-
borane, Me2NH·BH3. Our ongoing studies focus on transfer
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unsaturated organic substrates.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01946.
̈
Am. Chem. Soc. 2012, 134, 3598−3610. (h) Zahmakiran, M.; Ozkar, S.
Inorg. Chem. 2009, 48, 8955−8964. (i) Chen, Y.; Fulton, J. L.;
Linehan, J. C.; Autrey, T. J. Am. Chem. Soc. 2005, 127, 3254−3255.
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(b) Dietrich, B. L.; Goldberg, K. I.; Heinekey, D. M.; Autrey, T.;
Linehan, J. C. Inorg. Chem. 2008, 47, 8583−8585. (c) Staubitz, A.;
Sloan, M. E.; Robertson, A. P. M.; Friedrich, A.; Schneider, S.; Gates,
Details of all experiments along with NMR spectra, mass
spectra, elemental analysis, crystallographic information
and DFT studies (PDF)
Crystallographic information for 1a, CH8BN (CIF)
Crystallographic information for 1b, C2H10BN (CIF)
Crystallographic information for 1d, C7H20BN (CIF)
P. J.; Schmedt auf der Gunne, J. S.; Manners, I. J. Am. Chem. Soc. 2010,
̈
132, 13332−13345. (d) Stevens, C. J.; Dallanegra, R.; Chaplin, A. B.;
Weller, A. S.; Macgregor, S. A.; Ward, B.; McKay, D.; Alcaraz, G.;
Sabo-Etienne, S. Chem. - Eur. J. 2011, 17, 3011−3020.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Ian.Manners@bristol.ac.uk.
■
(6) (a) Robertson, A. P. M.; Suter, R.; Chabanne, L.; Whittell, G. R.;
Manners, I. Inorg. Chem. 2011, 50, 12680−12691. (b) Vogt, M.; de
Notes
Bruin, B.; Berke, H.; Trincado, M.; Grutzmacher, H. Chem. Sci. 2011,
̈
The authors declare no competing financial interest.
2, 723−727. (c) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am.
Chem. Soc. 2007, 129, 1844−1845.
(7) (a) Sloan, M. E.; Staubitz, A.; Clark, T. J.; Russell, C. A.; Lloyd-
Jones, G. C.; Manners, I. J. Am. Chem. Soc. 2010, 132, 3831−3841.
(b) Helten, H.; Dutta, B.; Vance, J. R.; Sloan, M. E.; Haddow, M. F.;
Sproules, S.; Collison, D.; Whittell, G. R.; Lloyd-Jones, G. C.;
Manners, I. Angew. Chem., Int. Ed. 2013, 52, 437−440.
ACKNOWLEDGMENTS
■
N.E.S., A.S., A.P.M.R., and I.M. acknowledge EPSRC for
funding. A.S. is grateful to the Deutsche Forschungsgemein-
schaft (DFG) for a postdoctoral research fellowship. E.M.L. and
T.J. acknowledge the EU for Marie Curie Postdoctoral
Fellowships.
(8) (a) Vance, J. R.; Robertson, A. P. M.; Lee, K.; Manners, I. Chem. -
Eur. J. 2011, 17, 4099−4103. (b) Vance, J. R.; Schafer, A.; Robertson,
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A. P. M.; Lee, K.; Turner, J.; Whittell, G. R.; Manners, I. J. Am. Chem.
Soc. 2014, 136, 3048−3064. (c) Baker, R. T.; Gordon, J. C.; Hamilton,
C. W.; Henson, N. J.; Lin, P.-H.; Maguire, S.; Murugesu, M.; Scott, B.
L.; Smythe, N. C. J. Am. Chem. Soc. 2012, 134, 5598−5609.
(d) Bhattacharya, P.; Krause, J. P.; Guan, H. J. Am. Chem. Soc. 2014,
136, 11153−11161. (e) Luo, W.; Campbell, P. G.; Zakharov, L. N.;
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(34) (a) Borazine [MeN−BMe]3 was independently synthesized
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1
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(35) Assignment of [RNH−BHMe]x (R = H, Me) was made by
comparing the difference in 11B chemical shifts of NH3·BH2Me [δB,exp
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DOI: 10.1021/acs.inorgchem.5b01946
Inorg. Chem. 2015, 54, 10878−10889