Reactions of amine-and phosphane-borane adducts with frustrated Lewis pair combinations of Group 14 triflates and sterically hindered nitrogen bases

Article abstract of DOI:10.1002/ejic.201000515

The ability of trialkyl Group 14 triflates in combination with amine and pyridine bases to dehydrogenate amine-and phosphane-borane adducts has been investigated. By using multinuclear NMR spectroscopy, it has been shown that Me₂NH ·BH₃ (11) is efficiently converted to [Me₂N-BH₂]2 (12) by the so-called "frustrated Lewis pair" (FLP) of nBu₃SnOTf (4, ^-OTf = ^-OSO₂CF₃) and 2,2,6,6-tetramethylpiperidine (6). Within the scope of the study, exchange of the Lewis acid effects the rate of dehydrogenation in the order: 4 gt; Me₃Si-OTf (2) gt; Et ₃SiOTf (3). Exchange of the Lewis base for 2,6-di-tert-butylpyridine (5) has also been shown to reduce the rate of reaction, whereas 1,3-di-tert-butylimidazol-2-ylidene (7) reacted directly with 2 to afford 1,3-bis-tert-butyl-4-(trimethylsilyl)imidazolium triflate (8[OTf]). For FLP combinations for which dehydrogenation reaction times are longer, detectable quantities of [H₂B(μ-H)(μ-NMe₂)BH₂] (14) are observed. Both the dehydrogenation reaction and competitive formation of this product are proposed to proceed by initial hydride abstraction by the Lewis acid, followed by deprotonation by the Lewis base, or combination with further dimethylamine-borane and elimination of [Me₂NH₂]OTf (18[OTf]), respectively. In contrast to 11, MeNH₂·BH ₃ (22) was not found to cleanly dehydrogenate to either [MeNH-BH ₂]₃ or [MeN-BH]₃ under the same conditions. An alternative reaction pathway was observed with either 2 or 4 and 6 with Ph ₂PH ·BH₃ (23), resulting in P-silylation or P-stannylation of the phosphane-borane, respectively.

Full text of DOI:10.1002/ejic.201000515

FULL PAPER
DOI: 10.1002/ejic.201000515
Reactions of Amine– and Phosphane–Borane Adducts with Frustrated Lewis
Pair Combinations of Group 14 Triflates and Sterically Hindered Nitrogen
Bases
George R. Whittell,[a] Edward I. Balmond,[a] Alasdair P. M. Robertson,[a] Sanjib K. Patra,[a]
Mairi F. Haddow,[a] and Ian Manners*[a]
Keywords: Amines / Boron / Dehydrogenation / Lewis acids / Lewis bases
The ability of trialkyl Group 14 triflates in combination with
amine and pyridine bases to dehydrogenate amine– and
phosphane–borane adducts has been investigated. By using
multinuclear NMR spectroscopy, it has been shown that
Me2NH·BH3 (11) is efficiently converted to [Me2N–BH2]2 (12)
by the so-called “frustrated Lewis pair” (FLP) of nBu3SnOTf
(4, –OTf = –OSO2CF3) and 2,2,6,6-tetramethylpiperidine (6).
Within the scope of the study, exchange of the Lewis acid
effects the rate of dehydrogenation in the order: 4 Ͼ Me3Si-
OTf (2) Ͼ Et3SiOTf (3). Exchange of the Lewis base for 2,6-
tions for which dehydrogenation reaction times are longer,
detectable quantities of [H2B(µ-H)(µ-NMe2)BH2] (14) are ob-
served. Both the dehydrogenation reaction and competitive
formation of this product are proposed to proceed by initial
hydride abstraction by the Lewis acid, followed by deproton-
ation by the Lewis base, or combination with further dimeth-
ylamine–borane and elimination of [Me2NH2]OTf (18[OTf]),
respectively. In contrast to 11, MeNH2·BH3 (22) was not
found to cleanly dehydrogenate to either [MeNH–BH2]3 or
[MeN–BH]3 under the same conditions. An alternative reac-
di-tert-butylpyridine (5) has also been shown to reduce the tion pathway was observed with either 2 or 4 and 6 with
rate of reaction, whereas 1,3-di-tert-butylimidazol-2-ylidene
(7) reacted directly with 2 to afford 1,3-bis-tert-butyl-4-(tri-
methylsilyl)imidazolium triflate (8[OTf]). For FLP combina-
Ph2PH·BH3 (23), resulting in P-silylation or P-stannylation of
the phosphane–borane, respectively.
By so preventing the adduct formation that is typical of
these compounds, heterolytic bond cleavage of an ad-
ditional substrate, such as hydrogen, becomes the dominant
reaction pathway. This chemistry is often described as that
of “frustrated Lewis pairs” (FLP) and has been shown to
facilitate further useful reactions. For instance, the phos-
phonium or ammonium hydridoborates formed from hy-
drogen cleavage are catalytically active in the hydrogenation
of imines, protected nitriles, aziridines,[40,41] enamines and
silylenol ethers.[42] The FLP approach has also proved suc-
cessful for the stoichiometric activation of many other small
molecules, including olefins[43] and CO2.[44,45] Based on
these reports we were intrigued as to whether the FLP ap-
proach would also permit the efficient dehydrogenation of
amine– and phosphane–borane adducts. In this respect we
were initially encouraged by reports that ammonia[46] and
catecholborane[47] can be successfully activated by this
method. During the course of our work, however, Miller
and Bercaw reported that the sterically bulky tertiary phos-
phane/borane Lewis pair of PtBu3 and B(C6F5)3 reacted
with amine–borane adducts to afford the targeted dehydro-
genated products and phosphonium hydridoborate salts.[48]
Furthermore, this topic has recently been the subject of a
theoretical study.[49]
1. Introduction
The dehydrogenation of amine–borane adducts is of in-
tense current interest for both the preparation of new polya-
minoborane materials[1,2] and for utilisation in hydrogen
storage[3] and hydrogen transfer applications.[4–32] Recent
work has shown that, in addition to the thermally induced
routes described in the 1950s that require temperatures
above 100 °C, efficient processes mainly involving transition
metal catalysis have been developed. The catalysts for these
reactions can be heterogeneous or homogeneous in nature
and have been shown to be successful for a variety of metal
centers from the d-block.[33,34] Other Group 13–15 Lewis
acid–Lewis base adducts such as phosphane–borane ad-
ducts have also been shown to undergo metal-catalyzed de-
hydrogenation to yield linear or cyclic oligomers or high-
molecular-weight polymers.[35–38]
Led by key discoveries by Stephan and co-workers, much
recent interest has focused on the metal-free activation of
small molecules by exploiting the reactivity of mixtures
containing sterically encumbered Lewis acids and bases.[39]
[a] School of Chemistry, University of Bristol, Cantock’s Close,
Bristol BS8 1TS, United Kingdom
With the exception of Bertrand’s (alkyl)(amino)-
carbenes[46] and some activity displayed by BPh3 systems
View this journal online at
E-mail: ian.manners@bristol.ac.uk
Fax: +44-117-929-0509
wileyonlinelibrary.com
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I. Manners et al.
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towards hydrogen,[50] FLPs almost entirely contain and elemental analysis, these data support the formation
B(C6F5)3 (1) or a derivative thereof as the Lewis acid. Al- of 1,3-bis-tert-butyl-4-(trimethylsilyl)imidazolium triflate
though the combination of large steric bulk and high Lewis (8[OTf]) as depicted in Scheme 1.
acidity no doubt explain this observation, the cost of this
compound compares to those of precious metal catalysts,
and herein lies a limitation. There is no a priori reason why
another Lewis acid may be ineffective as long as the afore-
mentioned steric and electronic requirements are met. We,
therefore, chose to initiate this study into the dehydrogena-
tion of amine– and phosphane–borane adducts by em-
–
–
ploying less expensive Me3SiOTf (2, OTf = OSO2CF3),
Et3SiOTf (3) and nBu3SnOTf (4) as the Lewis acids. 2,6-Di-
tert-butylpyridine (5), 2,2,6,6-tetramethylpiperidine (6) and
Scheme 1.
The molecular structure of 8[OTf] was confirmed by sin-
1,3-di-tert-butylimidazol-2-ylidene (7) were chosen as the gle-crystal X-ray diffraction, for which a thermal-ellipsoid
sterically encumbered Lewis bases. In addition to pre- plot is presented in Figure 1. The compound crystallised in
venting reaction with 2, 3 and 4, such steric bulk was de- the orthorhombic space group Pna21 with one well-sepa-
emed necessary to hinder competitive ligand-exchange rated ion pair per asymmetric unit. The N1–C3 and N2–
chemistry with the amine and phosphane–borane adducts C3 bond lengths are 1.336(3) and 1.333(3) Å, respectively,
of interest. With the exception of 5, these bases had pre- which are closer to those found in 1,3-bis-tert-butylimid-
viously been shown to be effective in the FLP activation azolium cation (9) [1.325(4) and 1.328(4) Å][53] than in the
of hydrogen.[51–54] During the course of our study, similar corresponding carbene (7) [1.3665(18) and 1.3660(18) Å].[58]
chemistry was reported for lutidine,[55] which also provided Furthermore, the N1–C3–N2 bond angle [109.9(2)°] is also
precedent for the use of a pyridine base.
more representative of 9 [107.4(3)°] than 7 [106.80(14)°].
These results clearly support the presence of a C3-bound
proton in 8[OTf]. The C1–C2 separation of 1.366(3) Å is
significantly longer than the corresponding distances in 9
and 7 [1.342(2) and 1.343(4) Å, respectively], but compar-
able to that in 1,3-diisopropyl-4-(trimethylsilyl)-2-[(trimeth-
ylsilyl)imino]imidazoline (10) [1.347(4) Å],[60] which also
contains a silylethylene backbone. It is presumably the ste-
ric pressure introduced by this substituent that is responsi-
2. Results and Discussion
2.1. Reactions of Trialkyl Group 14 Triflates with Lewis
Bases
The mixing of 2, 3 or 4 with a slight excess of either 5 or ble for the observed elongation of the C=C bond. The re-
6 in 1,2-dichlorobenzene at room temperature afforded no mainder of the metrical parameters are also consistent with
reaction as evidenced by 29Si and 119Sn NMR spec- the structure determined by spectroscopic means and are
troscopy.[56,57] These combinations of Lewis acids and bases included in the caption to Figure 1. It should be noted that
were therefore deemed appropriate for use in the FLP acti- this reaction is analogous to that observed between 7 and
vation of amine and phosphane–borane adducts. 1,3-Di- B(C6F5)3 when the reagents were allowed to stand at room
tert-butylimidazol-2-ylidene (7), however, afforded a pre- temperature.[53,54]
cipitate upon treatment with 2 in 1,2-dichlorobenzene.
In order to provide a point of reference for the current
Recrystallisation of the solid from CH2Cl2/hexane pro- study with respect to the literature, the abilities of the
duced crystals that were suitable for single-crystal X-ray dif- Group 14 triflates and N-donors to activate hydrogen were
fraction (vide infra), and these were also analysed by multi- investigated. The reaction of 2 and 6 with H2 (2 bar) in 1,2-
nuclear NMR spectroscopy. The 1H NMR spectrum exhib- dichlorobenzene was monitored by 29Si NMR spectroscopy.
ited a highly deshielded proton resonance at δ = 9.19 ppm, After 18 h at 50 °C, however, the only signal apparent was
which was taken as indicative of an imidazolium environ- that of the starting material 2 (δ = 43 ppm).[56] The same
ment. This signal integrated in a 1:1 ratio with respect to result was observed for the reaction employing 2 and 5; and
the olefin resonance at δ = 7.25 ppm, and 1:9 with regard when 4 and 6 were utilised, there was no conversion of the
to that assigned to the Me3Si group at δ = 0.49 ppm. These tin reagent (4) as evidenced by 119Sn NMR spectroscopy.[57]
data are consistent with protonation of the carbene centre At the time of this study there was no evidence to suggest
and silylation of the carbon backbone. Support for the for- that 5 was sufficiently nucleophilic to activate H2, so the
mer deduction was provided by the absence from the reaction was repeated with a Lewis acid known to facilitate
13C{1H} NMR spectrum of a signal that would be consid- this transformation with other Lewis bases. A solution of 1
ered characteristic of a two-coordinate carbon atom (δ = and 5 that was exposed to H2 (2 bar) only exhibited a reso-
213.2 ppm).[58] Further evidence for the latter can be taken nance corresponding to the borane (δ = 55 ppm)[61] after
from the resonance at δ = –6.2 ppm in the 29Si{1H} NMR ca. 1 h at room temperature. On heating at 50 °C for ca.
spectrum. The 19F NMR spectrum indicated the inclusion 18 h, however, two new signals were observed in the 11B
of the triflate ion in the product by the presence of a singlet NMR spectrum, replacing that corresponding to the start-
at δ = 78.2 ppm.[59] In combination with mass spectrometry ing borane. The predominant signal (δ = –26 ppm; JBH
=
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Reactions of Amine- and Phosphane-Borane Adducts
Figure 1. Molecular structure of 8[OTf] with thermal ellipsoids at the 50% probability level. Hydrogen atoms have been omitted for
clarity. Selected bond lengths [Å] and angles [°]: C1–C2 1.366(3), C1–Si1 1.895(2), C1–N1 1.410(3), C2–N2 1.372(3), C3–N1, 1.336(3),
C3–N2 1.333(3); N1–C1–C2 104.7(2), C1–C2–N1 109.62(2), N1–C3–N2 109.9(2), C1–N1–C3 108.40(19), C2–N2–C3 107.38(19).
87 Hz), representing 80% of the soluble boron present, was 5. Repeating the reaction with 6 but with 3 as the Lewis
consistent with the formation of [HB(C6F5)3]– and thus H2
activation.[50] It seems logical to conclude, therefore, that
it is the lower electrophilicity of the Group 14 species that
precludes the heterolysis of H2 in this study. Compounds
such as amine–borane adducts, however, may still be ame-
nable to dehydrogenation by these combinations of reagents
as the bonds to be cleaved are already polarised to facilitate
the reaction.
acid resulted in a modest lengthening of t1/2 to 36.2 min
under the same conditions. Finally, when 4 was used with
the same amine, the reaction was ca. 86% complete by the
time that the first NMR spectrum could be acquired (ca.
7 min). In all the reactions, when the formation of appropri-
ate triflate salt and silane or stannane was taken into ac-
count, the product distribution was qualitatively the same
as outlined above. It is noteworthy that the amount of the
side product (14) formed relative to the product of dehydro-
genation (12) became larger as t1/2 increased, and this ob-
servation will be discussed in due course. In summary, the
rate of the dehydrogenation increases as both the expected
electro- and nucleophilicity of Lewis acid and base, respec-
tively, increase.
2.2. Reactions of Dimethylamine– and Methylamine–Borane
Adducts with Mixtures of Trialkyl Group 14 Triflates and
N-Donor Bases
In previous investigations on amine–borane dehydroge-
nation we and others have found Me2NH·BH3 (11) a conve-
nient substrate to study, as it readily converts to [Me2N–
BH2]2 (12) in the presence of a broad range of cata-
lysts.[34,62] A solution of 11 with a slight excess of 2 and 6
was prepared in 1,2-dichlorobenzene and the evolution of
the mixture monitored by 11B NMR spectroscopy. During
the course of the reaction, the spectra obtained were typical
of Me2NH·BH3 dehydrogenation, displaying resonances as-
signable to the starting material [δ = –14 ppm (q, JBH
96 Hz)], 12 [δ = 4 ppm (t, JBH = 111 Hz)] and the intermedi-
ate aminoborane, Me2N=BH2 {13 [δ = 35 ppm (t, JBH
127 Hz)]}.[63] A side product assigned as [H2B(µ-H)(µ-
=
=
NMe2)BH2] {14 [δ = –19 ppm (td, JBH = 129 Hz, JBH
=
Scheme 2.
31 Hz)]}[64] was also detected, albeit in small quantity. Re-
action progress was accompanied by the precipitation of a
When proposing a mechanism for the above transforma-
colourless solid that was characterised as 2,2,6,6-tetrameth- tion, three possible alternatives need to be considered.
ylpiperidinium triflate ([6·H][OTf]) by 1H and 19F NMR Firstly, the Lewis acid and base may interact with the
spectroscopy.[65] The final reaction mixture was further ana- amine–borane in a concerted manner, similar to what has
lysed by 29Si NMR spectroscopy, and this confirmed the been proposed for the activation of H2 by phosphanes and
expected conversion of 2 to Me3SiH [15 (δ = –15 ppm)] boranes.[67] Secondly, the Lewis base may deprotonate at
(Scheme 2).[66] The time taken for conversion of half the the nitrogen atom with the Lewis acid abstracting a hydride
starting material (t1/2) was 10.3 min, and this was length- from the resulting anion in a subsequent step. Alternatively,
ened considerably (3.66 h) on exchanging the Lewis base for the converse is possible with hydride abstraction preceding
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I. Manners et al.
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deprotonation. Mixtures of either 5 or 6 with 11 in 1,2- tion effectively consumes ammonia–borane, but without
liberation of hydrogen, and is analogous to what we observe
with 4 and 11 (Scheme 3). In our dehydrogenation reac-
tions, however, the presence of a strong base provides a low-
activation-energy route to the formal dehydrogenation
product. In the absence of this reagent or when deproton-
ation is slow, the boronium cation can interact with another
molecule of 11 to afford the observed side reaction.
We attempted to extend this methodology to the de-
hydrogenation of MeNH2·BH3 (22) with the aim of achiev-
ing selective removal of either 1 or 2 equiv. of H2. The reac-
tions of 22 with a mixture of 2 and 6, by using both 1 and
2 mol-equiv. of the latter, however, afforded only a complex
mixture of products as evidenced by 11B NMR spec-
troscopy. Nonetheless, it is noteworthy that 2 equiv. of the
FLP were required to consume all of 22 and that the dehy-
drogenation products [MeNH–BH2]3 and [NMe–BH]3 were
not significant components of the reaction mixture.[63]
dichlorobenzene showed no signs of reaction under condi-
tions comparable to those of dehydrogenation, as evidenced
by 11B NMR spectroscopy. When, for example, stannyl tri-
flate 4 was treated with 11, however, conversion to 14 was
observed in the absence of any dehydrogenation by 11B
NMR spectroscopy. Only a broad resonance at δ ≈ 0 ppm
and accounting for trace amounts of the soluble boron
present was also observed in the 11B NMR spectrum. We
tentatively assign this signal to the boronium cation,
[H2B(NHMe2)(OTf)] (16), on account of the similarity in
chemical shift with that of [H2B(NH3)(OEt2)]+ (δ =
0.21 ppm).[8] Attempts to isolate a model for 16 by hydride
abstraction from Me3N·BH3 (17), however, failed. During
the course of the reaction of 4 with 11, colourless crystals
were observed to precipitate from the solution, and these
were later characterised as [Me2NH2]OTf (18[OTf]) by sin-
1
gle-crystal X-ray diffraction[68] and H and 19F NMR spec-
troscopy. When these results are taken in conjunction with
the formation of nBu3SnH (19), from the initial hydride ab-
straction and as evidenced by a signal at δ = –89 ppm (JSnH
= 1526 Hz) in the 119Sn NMR spectrum,[69] the full reaction
stoichiometry can be described (Scheme 3).
2.3. Reactions of Diphenylphosphane–Borane Adduct with
Mixtures of nBu3SnOTf and Me3SiOTf with 2,2,6,6-
Tetramethylpiperidine
In light of our results regarding the dehydrogenation of
secondary amine–borane adducts, we ventured to investi-
gate the analogous reaction of Ph2PH·BH3 (23). This reac-
tion was of particular interest as phosphane–borane ad-
ducts are only sluggishly dehydrogenated by transition
metal catalysts, thus necessitating high temperatures and
Scheme 3.
Baker, Dixon and co-workers have previously investi- long reaction times. As the mixture of 4 and 6 had been
gated the reaction of H3N·BH3 (20) with the Lewis acid 1.[8] shown to dehydrogenate 11 in the shortest period of time,
They found that initial hydride abstraction was followed by compound 23 was treated with this FLP reagent in slight
interaction of the resulting borenium cation with another excess. The white precipitate that so formed was collected
1
molecule of 20. Subsequent loss of H2 then afforded the and demonstrated to be [6·H][OTf] on the basis of H and
coupled species [H2B(µ-H)(µ-NH2)BH2NH3]+ (21) as the 19F NMR spectroscopy. Removal of all volatiles from the
[HB(C6F5)3]– salt, which could catenate further (Scheme 4). filtrate afforded a colourless oil, which was also analysed
At Lewis acid additions of Ͼ 10 mol-%, however, the con- by multinuclear NMR spectroscopy. The absence of reso-
centration of the cyclic cation 21 appeared high enough for nances at δ = –40.0, 1.7 and 172 ppm in the 11B, 31P and
the rate of an alternative reaction pathway to become sig- 119Sn NMR spectra, respectively, indicated complete con-
nificant (Scheme 5). It should be noted that this side-reac- sumption of both 4[70] and 23.[71] However, there were no
Scheme 4.
Scheme 5.
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Reactions of Amine- and Phosphane-Borane Adducts
signals in the former two spectra that could be assigned to
any of the expected dehydrogenation products, Ph2PH–
BH2–PPh2–BH3, [Ph2P–BH2]3 and [Ph2P–BH2]4.[36] Further
evidence for the occurrence of a reaction other than dehy-
drogenation was apparent from the observation of phos-
phorus-tin coupling (JP-Sn = 196 Hz) in both the 31P and
119Sn NMR spectra. Unfortunately, the oily nature of the
product prevented further purification of the material and
thus hampered elucidation of the structure. In order to ob-
tain a crystalline product, we repeated the experiment using
2 as the Lewis acid. On employing the same procedure as
utilised with stannyl triflate 4, we also obtained an oil, but
which crystallised on cooling and could be sublimed. The
crystals obtained from the latter procedure were of a quality
suitable for single-crystal X-ray diffraction, and this enabled
the unambiguous determination of the molecular structure
as Ph2P(SiMe3)·BH3 (24). This formulation is also consis-
tent with the data from multinuclear NMR spectroscopy
and mass spectrometry. Compound 24 crystallised in the
monoclinic space group P21/n with one molecule per asym-
metric unit. There were no short intermolecular contacts,
and a thermal ellipsoid plot along with selected metrical
parameters is included in Figure 2. All bond lengths and
angles are as expected, and the structural solution is in-
cluded solely for the purpose of supporting product identifi-
cation. The similarities in the 11B and 31P NMR spectra for
24 (δB = –39 ppm, δP = –23 ppm) and the product obtained
with 4 (δB = –36 ppm, δP = –20 ppm) suggest that an
analogous product was obtained when the latter was used
as the Lewis acid. The reaction chemistry is depicted in
Scheme 6.
Scheme 6.
In contrast to the reaction with dimethylamine–borane
(11), compound 23 neither reacts with Lewis acid 2 nor is
deprotonated by Lewis base 6 in isolation. These results are
consistent with functionalisation of the phosphane occur-
ring by a concerted mechanism.
3. Conclusions
The FLP formed by nBu3SnOTf (4) with 2,2,6,6-tetra-
methylpiperidine (6) was found to dehydrogenate Me2NH·
BH3 (11) both cleanly and rapidly to afford [Me2N–BH2]2
(12), in addition to nBu3SnH (19) and 2,2,6,6-tetrameth-
ylpiperidinium triflate ([6·H][OTf]). Formally exchanging
the Lewis acid for either Me3SiOTf (2) or Et3SiOTf (3) still
resulted predominantly in dehydrogenation, but ac-
companied by increasing relative quantities of [H2B(µ-H)(µ-
NMe2)BH2] (14). A similar trend was also observed on ex-
changing the base for the less nucleophilic 2,6-di-tert-butyl-
pyridine (5). The formation of both products is proposed
to proceed by hydride abstraction from the amine–borane
adduct by the Lewis acid. Selectivity is then determined by
the relative activation energies for deprotonation of the re-
sulting boronium cation vs. reaction with further 11. In
contrast to the reactivity with 11 and in the presence of 6,
compounds 2 and 4 both functionalised Ph2PH·BH3 (23)
by a formal deprotonation/substitution reaction. Studies
suggest, however, that the E–O (E = Si or Sn) and P–H
bonds are cleaved in a concerted process.
4. Experimental
4.1. General Considerations
All manipulations of air- and moisture-sensitive materials were car-
ried out either under inert gas (nitrogen or argon) or in vacuo by
using standard Schlenk-line and glove-box techniques. All reaction
solvents were dried and degassed by means of a Grubbs-type sol-
vent purification system immediately prior to use.[72] Deuterated
solvents for NMR spectroscopy were purchased from the Cam-
bridge Isotope Laboratories, dried and degassed by inert-gas distil-
lation from appropriate drying agents (CaH2 for CDCl3 and NaK
for C6D6), or used as received (D3COD). Tris(pentafluorophenyl)-
Figure 2. Molecular structure of 24 with thermal ellipsoids at the
50% probability level. Hydrogen atoms attached to carbon atoms
have been omitted for clarity. Selected bond lengths [Å] and angles
[°]: P1–B1 1.9389(17), P1–C1 1.8170(15), P1–C7 1.8172(15), P1–Si1
2.2831(6); C1–P1–C7 105.35(7), C1–P1–B1 112.83(7), C7–P1–B1
112.65(7), C1–P1–Si1 103.95(5), C7–P1–Si1 110.01(5), B1–P1–Si1
111.55(6).
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I. Manners et al.
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borane was purchased from Boulder Scientific and purified in ac-
cordance with the literature.[73] Methylamine–borane,[63] diphenyl-
phosphane–borane[71] and 1,3-bis-tert-butylimidazol-2-ylidene[58]
were prepared according to literature methods. The remainder of
the chemicals used within this research were purchased from Sigma
Aldrich Ltd. NMR spectra were recorded at ambient temperature
with either a Jeol JNM-ECP300/400 or JNM-LA300 spectrometer.
Chemical shifts are reported relative to residual protio solvent (1H),
the deuterio solvent itself (13C) or external BF3·Et2O (11B), 85%
H3PO4 (31P), CFCl3 (19F), Me4Si (29Si) and Me4Sn (119Sn) stan-
dards. Spectra recorded in 1,2-dichlorobenzene and CH2Cl2 were
acquired unlocked. Elemental analysis was performed with an
Eurovector EA 3000 Elemental Analyser by Des Davis of the Uni-
versity of Bristol Microanalysis Laboratory. Mass spectrometry
employing either electron ionisation (EI) or chemical
ionisation (CI), were carried out with a VG Analytical AutoSpec
mass spectrometer at the University of Bristol.
nBu3SnOTf (4) and 6 with H2: No reaction was apparent by 119Sn
NMR spectroscopy at ambient temperature after 2 h or at 50 °C
after a further 18 h. Only unreacted 4 was observed in solution
(
119Sn NMR: δ = 151 ppm).[70]
B(C6F5)3 (1) and 5 with H2: No reaction was apparent by 11B NMR
spectroscopy at ambient temperature after 2 h with only 1 (11B
NMR: δ = 58.6 ppm) being observed in solution.[61] The mixture
was then heated to 50 °C for a further 18 h, after which analysis
by 11B NMR spectroscopy exhibited two signals: a minor peak cor-
responding to an unknown product (δ = 3.5 ppm) and a major
peak arising from [HB(C6F5)3]– (δ = –26 ppm; JBH = 87 Hz).[50]
4.4.1. Dehydrogenation of Me2NH·BH3 with Mixtures of Trialkyl
Group 14 Triflates and N-Donor Bases
All dehydrogenation reactions were carried out as described in de-
tail below for the reaction of Me2NH·BH3 (11) with Me3SiOTf (2)
and 2,2,6,6-tetramethylpiperidine (6). The percentage composition
values were calculated from integration of the 11B NMR spectra.
To a solution of 11 (0.025 g, 0.42 mmol) in 1,2-Cl2C6H4 (0.35 mL)
was added a solution of 2 (0.086 g, 0.39 mmol) and 6 (0.057 g,
0.40 mmol) in further 1,2-dichlorobenzene (0.35 mL). The reaction
mixture was then stirred at 20 °C for 2 h, before the components
of the mixture were identified by 11B NMR spectroscopy. 11B NMR
(9.25 MHz, 1,2-Cl2C6H4): δ = –22.2 (trace), –18.6 {td, JBH = 129,
4.2. Reactions of Trialkyl Group 14 Triflates with Lewis Bases
In 1,2-dichlorobenzene (0.7 mL), 0.4 mmol of the respective Lewis
acid and Lewis base were mixed. The solutions were then stirred
at 20 °C for 2 h before identification of the components by 29Si or
119Sn NMR spectroscopy. Each of the Lewis acids, Me3SiOTf (2)
[66]
[29Si NMR (79.50 MHz, 1,2-Cl2C6H4): δ = 43 ppm], Et3SiOTf
(3)[56] [29Si NMR (79.50 MHz, 1,2-Cl2C6H4): δ = 44 ppm] and
JBH = 31 Hz, [H2B(µ-H)(µ-NMe2)BH2] (14), 3%}, –14.3 (q, JBH
=
nBu3SnOTf (4)[70] 119Sn NMR (98.16 MHz, 1,2-Cl2C6H4): δ =
[
96 Hz, 11, 8%), 4.1 [t, JBH = 111 Hz, (Me2NBH2)2 (12), 89%] ppm.
151 ppm] showed no reactivity with either 2,6-di-tert-butylpyridine
(5) or 2,2,6,6-tetramethylpiperidine (6). Reaction of 2 (0.057 g,
0.26 mmol) with 1,3-di-tert-butylimidazol-2-ylidene (7, 0.043 g,
0.24 mmol) in 1,2-Cl2C6H4 resulted in the rapid formation of a
colourless precipitate. The solid was collected by filtration and
recrystallised from CH2Cl2/hexane to afford pure 1,3-di-tert-butyl-
4-(trimethylsilyl)imidazolium triflate (8[OTf], 0.075 g, 78%) as
Reaction of 11 with 2 and 5: 11B NMR (9.25 MHz, 1,2-Cl2C6H4):
δ = –19.5 (td, JBH = 129, JBH = 31 Hz, 14, 18%), –15.3 (q, JBH
=
96 Hz, 11, 27%), –1.5 (br. s, 24%), 3.3 (t, JBH = 111 Hz, 12, 31%)
ppm. Reaction of 11 with 3 and 6: 11B NMR (9.25 MHz, 1,2-
Cl2C6H4): δ = –22.1 (br. s, 4%), –18.6 (td, JBH = 129, JBH = 31 Hz,
14, trace), –13.9 (q, JBH = 96 Hz, 11, 5%), 4.2 (t, JBH = 111 Hz,
12, 87%) ppm. Reaction of 11 with 4 and 6: 11B{1H} NMR
(9.25 MHz, 1,2-Cl2C6H4): δ = –18.7 (td, JBH = 129, JBH = 31 Hz,
1
colourless crystals. H NMR (400 MHz, CDCl3): δ = 9.19 [d, JHH
= 4 Hz, 1 H, N(CH)N], 7.24 [d, JHH = 4 Hz, 1 H, C(CH)N], 1.78
(s, 9 H, CH3), 1.74 (s, 9 H, CH3), 0.49 (s, 9 H, SiCH3) ppm.
13C{1H} NMR (100.53 MHz, CDCl3): δ = 136.7 [s, N(CH)N],
134.2 [s, C(CH)N], 128.7 [s, CC(N)Si], 120.8 (q, JCF = 321 Hz,
OTf), 61.8 [s, NC(CH3)3], 60.5 [s, NC(CH3)3], 30.7 [s, C(CH3)3],
29.8 [s, C(CH3)3], 1.3 [s, Si(CH3)3] ppm. 19F NMR (376.12 MHz,
CDCl3): δ = –78.2 (s, OTf) ppm. 29Si{1H} NMR (79.50 MHz,
CDCl3): δ = –6.2 [s, Si(CH3)3] ppm. C15H29F3N2O3SSi (401.55): C
44.76, H 7.26, N 6.96; found C 44.49, H 7.08, N 7.22. MS (EI):
m/z (%) = 253.2 (100) [M – OTf]+, 197.1 (15) [M – OTf – C4H8]+,
14, 2%), 4.1 (t, JBH = 111 Hz, 12, 91%), 27.2 ppm [d, JBH
=
148 Hz, BH(NMe2)2, 7%].[74]
4.4.2. Dehydrogenation of Me2NH·BH3 with Me3SiOTf and 2,2,6,6-
Tetramethylpiperidine for the Isolation of 2,2,6,6-Tetramethylpiper-
idine Triflate: To a solution of 11 (0.025 g, 0.42 mmol) in 1,2-
Cl2C6H4 (0.35 mL) was added a solution of 2 (0.086 g, 0.39 mmol)
and 6 (0.057 g, 0.40 mmol) in further 1,2-Cl2C6H4 (0.35 mL) at
20 °C. The mixture was stirred for 2 h before being filtered through
a glass-fibre pad. The white solid so collected was then dried in
vacuo. 1H NMR (399.7 MHz, D3COD): δ = 1.83–1.76 (m, 2 H,
CH2); 1.67–1.63 (m, 4 H, CH2); 1.42 (s, 12 H, CH3) ppm. 19F NMR
(376.12 MHz, D3COD): δ = –80.1 (s, OTf) ppm. A spectroscopi-
cally identical product was attained through the stoichiometric re-
action of trifluoromethanesulfonic acid and 2,2,6,6-tetramethyl-
piperidine in 1,2-dichlorobenzene.
181.1 (64) [M – OTf – SiC3H8]+, 141.1 (35) [M – OTf – C4H8
–
C4H8]+, 125.1 (55) [M – OTf – SiC3H8 – C4H8]+, 69.0 (44) [M –
OTf – SiC3H8 – C4H8 – C4H8]+.
4.3. Attempted H2 Heterolysis with Mixtures of Trialkyl Group 14
Triflates and N-Donor Bases
All reactions with dihydrogen were attempted in a manner analo-
gous to that described with Me3SiOTf (2) and 2,2,6,6-piperidine
(6): A solution of 2 (0.081 mg, 0.36 mmol) and 6 (0.056 g,
0.39 mmol) was prepared in 1,2-Cl2C6H4 (0.7 mL) in an NMR tube
fitted with a greaseless stopcock (J. Young). The solution was then
frozen at –196 °C, the tube evacuated and warmed to ambient tem-
perature before hydrogen was admitted (2 bar). After 2 h at ambi-
ent temperature, the 29Si NMR spectrum only indicated the present
of 2 (δ =43 ppm).[66] The solution was then heated at 50 °C for 18 h
before the analysis was repeated.
4.4.3. Determination of Reaction Rates for the Dehydrogenation of
Me2NH·BH3 with Mixtures of Lewis Acids and Lewis Bases: The
relative rates of dehydrogenation of 11 by using various mixtures
of Lewis acids and Lewis bases were determined by recording 11B
NMR spectra as a function of time. The relative concentrations of
reagent and product were subsequently determined by integration
of the corresponding signals. The reactions were performed in
NMR tubes fitted with greaseless stopcocks (J. Young) on a
0.4 mmol scale in 1,2-Cl2C6H4 (0.7 mL).
2 and 2,6-Di-tert-butylpyridine (5) with H2: No reaction was appar-
ent by 29Si NMR spectroscopy at ambient temperature after 2 h or
at 50 °C for a further 18 h. Only unreacted 2 was observed in solu-
tion (29Si NMR: δ = 43 ppm).[66]
4.4.4. Reactions of Me2NH·BH3 with N-Donor Lewis Bases: Com-
pound 11 (0.023 g, 0.4 mmol) was combined with either 5 or 6
(0.4 mmol) in 1,2-Cl2C6H4 (0.7 mL) and the mixture stirred at
20 °C for 2 h. In both cases no reactions were observed by 11B
3972
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 3967–3975

Reactions of Amine- and Phosphane-Borane Adducts
NMR spectroscopy, with only 11 being present in solution
(–14 ppm).[63]
4.6.2. Reaction of Ph2PH·BH3 with Me3SiOTf: To a solution of 23
(0.075 g, 0.62 mmol) in CH2Cl2 (1 mL) was added a solution of 2
(0.16 g, 0.68 mmol) in further CH2Cl2 (1 mL) and the mixture
stirred at 20 °C for 2 h. 31P and 11B NMR spectroscopy indicated
that no reaction had occurred, with 23 being the only apparent
species in solution. 11B{1H} NMR (96.25 MHz, CH2Cl2): δ = –41.1
(d, JBP = 45 Hz, BH3) ppm. 31P{1H} NMR (121.44 MHz, CH2Cl2):
δ = 1.5 (br. m, 23) ppm.
4.4.5. Reactions of Me2NH·BH3 with Trialkyl Group 14 Triflates:
Compound 11 (0.023 g, 0.4 mmol) was combined with either 2, 3
or 4 (0.4 mmol) in 1,2-Cl2C6H4 (0.7 mL) and the mixture stirred at
20 °C for 2 h. In each case, 29Si or 119Sn NMR spectroscopy indi-
cated a mixture of unreacted Lewis acid along with the correspond-
ing silane or stannane produced by hydride abstraction at the boron
atom. For 2: 29Si NMR (79.5 MHz, 1,2-Cl2C6H4): δ = –16
4.6.3. Reaction of Ph2PH·BH3 with 2,2,6,6-Tetramethylpiperidine:
(Me3SiH) ppm.[66] For 3: 29Si NMR (79.5 MHz, 1,2-Cl2C6H4): δ = To a solution of 23 (0.075 g, 0.62 mmol) in CH2Cl2 (1 mL) was
–2 (Et3SiH) ppm.[56] For 4: 119Sn NMR (98.16 MHz, 1,2-Cl2C6H4):
δ = –89 (d, JSnH = 1526 Hz, nBuSnH) ppm.[69] 11B NMR spec-
troscopy indicated that the sole boron-containing product pro-
added a solution of 6 (0.1 g, 0.68 mmol) in further CH2Cl2 (1 mL)
and the mixture stirred at 20 °C for 2 h. 31P NMR and 11B NMR
spectroscopy indicated that ca. 5% of 23 had undergone an phos-
duced was [H2B(µ-H)(µ-NMe2)BH2] (14). 11B NMR (96.25 MHz, phane–amine exchange to afford 6·BH3,[75] with the concomitant
1,2-Cl2C6H4): δ = –19 (td, JBH = 129, JBH = 31 Hz) ppm.[64] When
the reaction mixture was allowed to stand for ca. 1 d, colourless
crystals formed that were suitable for single-crystal X-ray analysis.
Comparison of the unit-cell parameters with the literature[68] dem-
onstrated these to consist of [Me2NH2]OTf, and this formulation
liberation of diphenylphosphane. 11B{1H} NMR (96.25 MHz,
CH2Cl2): δ = 41.1 (d, JPB = 45 Hz, Ph2PH·BH3), –22.8 (s, 6·BH3)
ppm. 31P{1H} NMR (121.44 MHz, CH2Cl2): δ = 1.5 (br. m,
Ph2PH·BH3), –39.7 (s, Ph2PH) ppm.[76]
4.6.4. Reaction of Ph2PH·BH3 with nBu3SnOTf and 2,2,6,6-Tetra-
methylpiperidine: To a solution of 23 (0.1 g, 0.5 mmol) in CH2Cl2
(1 mL) was added a solution of nBu3SnOTf (4, 0.242 g, 0.55 mmol)
and 6 (0.077 g, 0.55 mmol) in further CH2Cl2 (1 mL). The mixture
was stirred at 20 °C for 2 h, resulting in the rapid formation of a
colourless precipitate. After a further 2 h of stirring, the solid was
collected by filtration and the solvent removed in vacuo to afford
nBu3SnPPh2·BH3 as a colourless oil (25, 0.22 g, 92%). 11B{1H}
1
is consistent with the NMR spectra of the bulk sample. H NMR
(300 MHz, D3COD): δ = 2.67 (s, 6 H, Me) ppm. 19F NMR
(282.09 Hz, D3COD): δ = –80.0 (s, OTf) ppm.
4.5.1. Reaction of MeNH2·BH3 with 1 equiv. of Me3SiOTf and
2,2,6,6-Tetramethylpiperidine: To a solution of MeNH2·BH3 (22,
0.023 g, 0.5 mmol) in CH2Cl2 (0.75 mL) was added a solution of
Me3SiOTf (2, 0.121 g, 0.55 mmol) and 2,2,6,6-tetramethylpiper-
idine (6, 0.076 g, 0.55 mmol) in further CH2Cl2 (0.75 mL). The NMR (96.25 MHz, C6D6): δ = –36.4 (br. s, BH3) ppm. 31P{1H}
mixture was then stirred at 20 °C for 2 h before study by 11B NMR
spectroscopy. 11B{1H} NMR (96.25 MHz, CH2Cl2): δ = –23.2
(4%), –22.2 (17%), –19.9 (3%), –19.0 (22, 6%),[63] –18.3 (4%), –8.3
to –4.7 (overlapping singlets, 41%), –1.8 (13%), 28.8 (2%), 32.4
([MeN–BH]3, 10%),[63] 41.2 (trace) ppm.
NMR (121.44 MHz, C6D6): δ = –20.1 (s, JSnP = 196 Hz,
1
nBu3SnPPh2) ppm. H NMR (300 MHz, C6D6): δ = 7.83–7.70 (m,
4 H, Ph), 7.12–6.97 (m, 6 H, Ph); 1.52–1.39 (m, 2 H, nBu), 1.27–
1.13 (m, 4 H, nBu), 0.80 (t, JHH = 7.16 Hz, 3 H, nBu) ppm.
13C{1H} NMR (75.43 MHz, C6D6): δ = 134.0 (d, JCP = 8.65 Hz,
Ph), 130.4 (s, Ph), 129.3 (d, JCP = 9.23 Hz, Ph), 29.6 (s, nBu), 27.8
(s, nBu), 14.1 (s, nBu), 12.3 (s, nBu) ppm. 119Sn{1H} NMR
(98.16 MHz, C6D6): δ = 17.1 (d, JSnP = 196 Hz, nBu3SnPPh2) ppm.
MS-CI+ (70 eV): m/z (%) = 490.2 (10) [M+], 477.1 (38) [MH+–
BH3], 291.1 (86) [nBu3Sn], 187.1 (100) [H2PPh2+]; satisfactory ele-
mental analysis could not be obtained due to the oily nature of the
product.
4.5.2. Reaction of MeNH2·BH3 with 2 equiv. of Me3SiOTf and
2,2,6,6-Tetramethylpiperidine: To
a solution of 22 (0.023 g,
0.5 mmol) in dichloromethane CH2Cl2 (0.75 mL) was added a solu-
tion of 2 (0.242 g, 1.1 mmol) and 6 (0.152 g, 1.1 mmol) in CH2Cl2
(0.75 mL). The mixture was then stirred at 20 °C for 2 h before
analysis by 11B NMR spectroscopy. 11B{1H} NMR (96.25 MHz,
CH2Cl2): δ = –22.8 (2%), –22.0 (2%), –18.2 (trace), –8.9 (49%),
–5.5 (4%), –1.7 (13%), –1.0 (22%), 29.5 (trace), 32.4 ([MeN–BH]3,
1%),[63] 41.3 (6%).
4.7. X-ray Structural Characterization
Single-crystal X-ray structural study was performed with a CCD
4.6.1. Reaction of Ph2PH·BH3 with Me3SiOTf and 2,2,6,6-Tetra- Bruker SMART APEX diffractometer equipped with an Oxford
methylpiperidine: To
a
solution of Ph2PH·BH3 (23, 0.25 g,
Instruments low-temperature attachment. Data were collected at
1.23 mmol) in CH2Cl2 (1 mL) was added a solution of Me3SiOTf
100(2) K by using graphite-monochromated Mo-Kα radiation (λα =
(2, 0.31 g, 1.36 mmol) and 2,2,6,6-tetramethylpiperidine (6, 0.19 g, 0.71073 Å). The frames were indexed, integrated, and scaled by
1.36 mmol) in further CH2Cl2 (1 mL). The mixture was stirred at using the SMART and SAINT software package,[77] and the data
20 °C for 2 h, upon which a colourless solid was seen to precipitate.
After a further 2 h, the solid was collected by filtration and the
solvent removed in vacuo to yield a colourless oil that solidified on
cooling to –18 °C. The solid was sublimed at 45 °C (0.005 Torr) to
afford crystals of (Me3Si)PPh2·BH3 (24, 0.28 g, 82%). 11B NMR
(96.25 MHz, CDCl3): δ = –39.4 (dq, JBP = 35, JBH = 97 Hz) ppm.
31P{1H} NMR (121.44 MHz, CDCl3): δ = –23.4 (br. m, Me3-
were corrected for absorption by using the SADABS program.[78]
Pertinent crystallographic data for compounds 8[OTf] and 24 are
summarized in Table 1. The structure was solved and refined by
using the SHELX suite of programs.[79] All molecular structures
were generated by using ORTEP-3 for Windows Version 2.02.[80]
The hydrogen atoms bonded to carbon atoms were included in geo-
metrically calculated positions in the final stages of the refinement
SiPPh2) ppm. 1H NMR (300 MHz, CDCl3): δ = 7.68–7.60 (m, 4 and were refined according to the typical riding model, whereas
H, Ph), 7.47–7.40 (m, 6 H, Ph), 0.38 (d, JHP = 6 Hz, 9 H, Me3Si) the hydrogen atoms attached to boron atoms were located and re-
ppm. 13C{1H} NMR (75.43 MHz, CDCl3): δ = 133.2 (d, JCP
8 Hz, Ph), 130.3 (s, Ph), 128.8 (d, JCP = 9 Hz, Ph), –2.4 (d, JCP
=
=
fined with isotropic thermal parameters. All non-hydrogen atoms
were refined with anisotropic thermal parameters. CCDC-760847
(8[OTf]) and -761915 (24) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
9 Hz, Me3Si) ppm. 29Si{1H} NMR (59.60 MHz, CDCl3): δ = 5.2
(d, JSiP = 44 Hz, Me3SiPPh2) ppm. MS-CI+ (70 eV): m/z (%) =
271.1 (75) [M+ – H], 258.1 (100), [M+ – BH3], 187.1 (44) [H2PPh2+]; charge from The Cambridge Crystallographic Data Centre via
satisfactory elemental analysis could not be obtained.
www.ccdc.cam.ac.uk/data_request/cif.
Eur. J. Inorg. Chem. 2010, 3967–3975
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3973

I. Manners et al.
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8[OTf] and 24.
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8[OTf]
24
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
[C14H29N2Si][CF3O3S] C15H22BPSi
402.55
orthorhombic
Pna21
17.5409(9)
9.6250(5)
12.1972(6)
90
272.20
monoclinic
P21/n
8.9019(4)
15.6900(8)
11.7475(6)
90
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[21]
[22]
[23]
[24]
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α [°]
β [°]
90
90
95.635(3)
90
1632.85(14)
4
1.107
2.24
γ [°]
V [Å3]
2059.27(18)
4
1.298
Z
ρcalcd. [gcm–3]
µ [mm–1]
2.57
F(000)
856
584
Reflections
collected
18162
4225
4016
236
14637
3765
3026
178
independent
observed [IϾ2σ(I)]
No. of variables
Goodness-of-fit
Final R indices
[IϾ2σ(I)][a]
R indices (all data)[a]
1.052
1.017
[26]
R1 = 0.0416
wR2 = 0.1105
R1 = 0.0473
wR2 = 0.1123
R1 = 0.0358
wR2 = 0.0783
R1 = 0.0503
wR2 = 0.0848
[27]
[28]
[29]
[a] R1 = Σ ||Fo| – |Fc||/Σ|Fo| with Fo2Ͼ2σ(Fo2); wR2 = [Σw(|Fo2| –
2
|Fc |)2/Σ|Fo2|2].
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Acknowledgments
I. M. thanks the Engineering and Physical Sciences Research
Council (EPSRC) for financial support, the European Union for a
Marie Curie Chair and the Royal Society for a Wolfson Research
Merit Award. A. P. M. R. would like to acknowledge the EPSRC
for a doctoral training award. S. K. P. thanks the European Union
for a Marie Curie Postdoctoral Fellowship.
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