Experimental and theoretical studies of the potential interconversion of the amine-borane iPr2NH·BH(C6F5) 2 and the aminoborane iPr2N=B(C6F 5)2 involving hydrogen loss

Article abstract of DOI:10.1002/ejic.201100779

The amine-borane adduct iPr₂NH·BH(C₆F ₅)₂ (1) and the aminoborane iPr₂N=B(C ₆F₅)₂ (2) have been prepared and crystallographically characterised. Interconversion betwee

Full text of DOI:10.1002/ejic.201100779

FULL PAPER
DOI: 10.1002/ejic.201100779
Experimental and Theoretical Studies of the Potential Interconversion of the
Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2
Involving Hydrogen Loss and Uptake
Alasdair P. M. Robertson,[a] George R. Whittell,[a] Anne Staubitz,[a] Kajin Lee,[a]
Alan J. Lough,[b] and Ian Manners*[a]
Keywords: Main group elements / Amines / Boranes / Dehydrogenation / Hydrogenation
The amine–borane adduct iPr2NH·BH(C6F5)2 (1) and the ami-
noborane iPr2N=B(C6F5)2 (2) have been prepared and crys-
tallographically characterised. Interconversion between the
two compounds has been attempted using thermal and tran-
sition-metal-catalysed dehydrogenation and hydrogenation
protocols and the overall reaction thermodynamics were
probed by computational methods. Thermal dehydrogena-
tion of 1 was found to yield 2, together with uncharacterised
by-products. Treatment of 1 with the carbene 1,3-di-tert-but-
yl-4,5-dihydroimidazol-2-ylidene (3) under ambient condi-
tions did not lead to the elimination of hydrogen, but instead
to the loss of C6F5H to afford iPr2N=B(H)C6F5 (4). Attempts
to hydrogenate aminoborane 2 were unsuccessful with no re-
action observed either thermally or in the presence of transi-
tion-metal catalysts. A computational study of the intercon-
version between compounds 1 and 2 indicated a thermody-
namically unfavourable hydrogenation reaction, which was
inverse to that demonstrated for the analogous phosphane–
borane/phosphanylborane pair, iPr2PH·BH(C6F5)2 (5) and
iPr2P–B(C6F5)2 (6). The contrasting reactivity was attributed
to the different N–B and P–B π-bond strengths in 2 and 6,
respectively.
Introduction
Initial studies in the area of catalytic amine–borane dehy-
drogenation focussed primarily upon the use of rhodium
complexes as precatalysts, where the mechanism was sug-
gested to occur via reduction of the precatalyst to either
colloidal rhodium, Rh4–6 clusters or nanoparticles.[4–10]
More recently, the range of metal complexes reported to be
capable of catalytically dehydrogenating amine–boranes has
been significantly expanded. For example, homogeneous
systems based on Rh,[11] Ru,[12,13] Re,[14] Ni[15,16] and
Ir[17–19] are now known, along with those based on Group 2
and 4 metals,[20–25] and systems based on p-block metals.[26]
In addition, photoactivated catalysts of the first-row transi-
tion metals have also recently been reported.[27–29] Mechan-
istic insights into metal-catalysed dehydrocoupling pro-
cesses have also been provided through various studies dem-
onstrating the coordination of amine- and aminoboranes to
metal centres,[11,30–39] with the isolation of species impli-
cated in the catalytic cycle.[40]
Lewis base adducts of borane are well-studied com-
pounds, historically finding applications as reducing agents
in organic synthesis.[1–3] Of these species, amine–borane ad-
ducts, RxNH3–x·BH3, are an interesting subclass that can
exhibit unique reactivity due to the presence of both protic
and hydridic hydrogen atoms. The elimination of dihydro-
gen from molecules of this type affords new intra- or inter-
molecular boron–nitrogen bonds, and a wide range of cata-
lysts have now been reported which can promote such pro-
cesses under mild conditions (Scheme 1).[3]
Transition-metal-catalysed dehydrocoupling of amine–
borane adducts generally affords aminoboranes and boraz-
ines, with the products defined by both the choice of sub-
Scheme 1. Products of catalytic dehydrogenation of (a) secondary stituents on the adduct and the catalyst. These factors have
and (b) primary amine–borane adducts.
recently been exploited in the preparation of polyaminobor-
anes, a novel class of inorganic polymers, which can be re-
[a] School of Chemistry, University of Bristol,
garded as the boron–nitrogen analogues of polyolefins.[19,41]
Cantock’s Close, Bristol, BS8 1TS, United Kingdom
A significant driving force behind the development of
amine–borane dehydrocoupling processes is the potential
application of such adducts for hydrogen storage, with am-
monia–borane, NH3·BH3, attracting particular interest due
E-mail: Ian.Manners@Bristol.ac.uk
[b] Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, ON, Canada, M5S 3H6
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100779.
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I. Manners et al.
FULL PAPER
to its high hydrogen content (19.6 wt.-%).[42,43] However, our current investigation. In this manuscript we report the
there are no reported examples of the resulting aminobo- synthesis and characterisation of the amine–borane adduct
ranes or borazines reacting directly with dihydrogen in the iPr2NH·BH(C6F5)2 (1) and the corresponding aminoborane
reverse reaction, although chemical regeneration may be iPr2N=B(C6F5)2 (2) and probe their dehydrogenation and
achieved by using other reagents.[44–48] The group of Rieger, hydrogenation chemistry, respectively. These species incor-
however, have reported a compound containing tethered porate electron-donating groups at nitrogen and electron-
amine and borane moieties that is capable of undergoing withdrawing groups at boron to help produce favourable
co-operative hydrogenation in
a
reversible fashion thermodynamics for reversible hydrogenation.[51] DFT
(Scheme 2).[49] The thermodynamics of hydrogen activation studies on the thermodynamics of dehydrogenation/hydro-
in this system have since been studied computationally by genation are also presented for the amine- and amino-
Pápai and co-workers, with ΔG for the hydrogenation in borane using identical methods to those applied by Stephan
toluene calculated at –12.5 kcalmol–1.[50] It should be and co-workers for phosphane–boranes/phosphanylbor-
noted, however, that in the context of aminoborane regen- anes,[52,53] allowing for a direct comparison between the re-
eration, hydrogen loss and uptake by this particular system lated systems.
does not involve making and breaking B–N bonds.
Results and Discussion
(a) Synthesis of iPr2NH·BH(C6F5)2 (1)
The synthesis of the amine–borane 1 was carried out by
slow addition of neat diisopropylamine to a toluene solu-
Scheme 2. Reversible H2 activation by an ansa-aminoborane.[49]
tion of HB(C6F5)2·SMe2 at –78 °C, in analogous fashion
to amine–borane synthesis from BH3·SMe2 (Scheme 4).[10]
Warming to 20 °C and removal of the solvent gave the
Due to the interest in aminoborane regeneration, we have
previously studied the thermodynamic parameters for hy-
product as a white solid in 92% yield and high purity, with
drogen loss/uptake from amine–boranes/aminoboranes
additional purification possible by recrystallisation from
computationally, primarily in the gas phase.[51] The free en-
toluene/hexanes.
ergy of dehydrogenation for simple adducts was shown to
be small and negative in character, with the entropic contri-
bution due to hydrogen release driving dehydrogenation.
The major factor in defining the reaction enthalpy of dehy-
Scheme 4. Synthesis of iPr2NH·BH(C6F5)2 (1).
drogenation/hydrogenation was also demonstrated to be the
strength of the B–N bond(s) in the respective reactants and
products, generally with aminoborane hydrogenation be-
coming increasingly favourable with electron-donating
groups at nitrogen and electron-withdrawing groups at
boron. However, solvent effects were also demonstrated to
have a significant influence on the thermodynamics of de-
hydrogenation. Calculations on this effect indicated varia-
tions in ΔG of up to 10 kcalmol–1 relative to the gas phase
values; a change which in some cases was of sufficient mag-
nitude to reverse the sign of ΔG.[51]
Analysis of the product by 11B NMR spectroscopy in
CDCl3 showed a single peak at δ = –14.8 ppm, which split
into a doublet on proton coupling (1JBH = 101 Hz). These
data are consistent with a tetra-coordinate boron centre
with a single hydrogen substituent.[55] 1H NMR spec-
troscopy similarly showed the expected peaks for two
closely related isopropyl groups [δ = 3.51 (2 H), 1.34 (6 H),
1.26 (6 H) ppm for 2ϫCH and 4ϫCH3 groups, which were
present as a multiplet and doublets, respectively] and single
N–H and B–H hydrogen atoms (δ = 4.60 and 3.64 ppm,
respectively). 13C NMR spectroscopy again showed two
peaks at δ = 51.5 and 19.9 ppm corresponding to reso-
nances of the isopropyl groups. However, no resonances
were apparent that corresponded to the carbon atoms of
the aromatic rings; a phenomenon which has been observed
before for the related Ph(Me)N=B(C6F5)2 by Knüppel and
co-workers.[56] 19F NMR spectroscopy, however, was consis-
In 2008, Stephan and co-workers reported the irrevers-
ible activation of dihydrogen by the phosphanylborane sys-
tems, tBu2P-B(C6F5)2 and Cy2P-B(C6F5)2 (Scheme 3).[52,53]
Scheme 3. Hydrogenation of R2P-B(C6F5)2.[52,53]
Aminoboranes and phosphanylboranes are formally iso- tent with the incorporation of pentafluorophenyl units, with
electronic, although the bonding within these systems is three resonances at δ = –133.1 (singlet), –157.7 (triplet, JFF
markedly different, with the 3p phosphorus centre leading = 19.7 Hz) and –162.8 (broad singlet) ppm. High-resolution
to poorer orbital overlap with the vacant 2p-orbital at electrospray ionisation mass spectrometry (ESI-MS) con-
boron, and more significant lone-pair character at phos- firmed the successful synthesis of the desired compound,
phorus.[54] The demonstration of hydrogen uptake by phos- [M – H]– = 446.1156 Da, with chemical impact ionisation
phanylboranes, coupled with our own computational stud- mass spectrometry indicating the presence of the expected
ies of aminoborane hydrogenation provided the impetus for fragments.
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Potential Amine–Borane/Aminoborane Interconversion
Crystals of 1 suitable for single-crystal X-ray diffraction (b) Synthesis of iPr2N=B(C6F5)2 (2)
were grown from toluene/hexanes at –40 °C (Figure 1). The
The preparation of the corresponding aminoborane 2
compound crystallised as colourless plates in the triclinic
was attempted by reaction of in situ prepared ClB(C6F5)2·
SMe2 with Li[NiPr2] in Et2O at –78 °C (Scheme 5). Upon
warming to 20 °C the solvent was removed under vacuum
and the product then extracted into hexanes and filtered.
The solvent was then removed under high vacuum, and the
resulting solids recrystallised from the minimum quantity
of hexanes to afford 2 as an off-white solid in 77% yield.
¯
space-group P1 with a single molecule per asymmetric unit.
Adjacent molecules within the unit cell were in close prox-
imity to one another, as exemplified by the intermolecular
C(10)···F(2) separation of 3.231 Å, and the atom connectiv-
ity within each molecule confirmed the formulation based
on spectroscopic methods. The angles around B(1) and
N(1) summed to 438 and 448°, respectively, which were
consistent with the expected tetrahedral geometries at
each centre. The B(1)–N(1) bond length of 1.628(3) Å
falls within the range observed for both tertiary amine–
secondary borane adducts [1.578(8)–1.676(3) Å][57–60] and
secondary amine–tris(pentafluorophenyl)borane adducts
[1.613(3)–1.653(4) Å],[61–64] and is therefore unremarkable.
The C(1)–N(1)–B(1)–C(7) dihedral angle of 179.32° in-
ferred a staggered conformation upon the molecule, as ex-
pected on the basis of steric arguments.
Scheme 5. Synthesis of iPr2N=B(C6F5)2 (2).
The 11B NMR spectrum of the product showed a single
signal at δ = 33.2 ppm, which remained as a singlet on pro-
ton coupling. The shift and multiplicity of this signal are
consistent with the postulated trigonal planar boron envi-
ronment with no hydrogen substituents.[55] 1H NMR spec-
troscopy of the product showed two peaks at δ = 3.32 and
0.90 ppm, which appeared as a septet and a doublet, respec-
tively, and were consistent with the presence of two chemi-
cally equivalent isopropyl groups. The 13C NMR spectrum
was also consistent with this assignment with peaks appar-
ent at δ = 51.2 and 23.3 ppm. The equivalence of the two
isopropyl groups is also in line with the expected planar
molecular structure. As for amine–borane 1, no resonances
were observed for the carbon atoms of the aromatic rings,
but 19F NMR spectroscopy confirmed the inclusion of the
pentafluorophenyl moieties within the product structure.
Recrystallisation of 2 from hexanes at –40 °C afforded
crystals suitable for single-crystal X-ray diffraction, which
confirmed the formulation based on spectroscopic data
Figure 1. Molecular structure of 1 with thermal ellipsoids at the
30% probability level. Selected bond lengths [Å] and angles [°]:
N(1)–B(1) 1.628(3), N(1)–C(1) 1.537(3), N(1)–C(4) 1.536(3), C(7)–
B(1) 1.640(4), C(13)–B(1) 1.632(4), H(1N)–N(1)–C(4) 103.2(14),
H(1N)–N(1)–C(1) 103.9(14), C(4)–N(1)–C(1) 111.26(18), H(1N)–
N(1)–B(1) 107.7(13), C(4)–N(1)–B(1) 117.91(18), C(1)–N(1)–B(1)
111.46(18), H(1B)–B(1)–N(1) 107.1(10), H(1B)–B(1)–C(13)
109.9(11), N(1)–B(1)–C(13) 111.7(2), H(1B)–B(1)–C(7) 108.6(11),
N(1)–B(1)–C(7) 111.82(19), C(13)–B(1)–C(7) 107.74(19).
This geometry has obvious implications with regard to
the intramolecular elimination of hydrogen from this mole-
cule. Should the solid-state structure persist in solution, di-
rect combination of the proton and hydride would be im-
possible without rotation around the B–N bond into an en-
ergetically disfavoured conformation. It should be noted,
however, that thermal elimination of hydrogen from some
amine–borane adducts is believed to occur via an intermo-
lecular route followed by rapid dissociation of the dimeric
product.[10,65,66]
Figure 2. Molecular structure of 2 with thermal ellipsoids at the
30% probability level. Selected bond lengths [Å] and angles [°]:
N(1)–B(1) 1.376(4), N(1)–C(1) 1.493(4), N(1)–C(4) 1.498(4), C(7)–
B(1) 1.603(5), C(13)–B(1) 1.600(5), B(1)–N(1)–C(1) 125.3(3), B(1)–
N(1)–C(4) 121.2(3), C(1)–N(1)–C(4) 113.5(2), N(1)–B(1)–C(13)
124.7(3), N(1)–B(1)–C(7) 121.6(3), C(13)–B(1)–C(7) 113.6(3).
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(Figure 2). The compound crystallised as colourless needles centres, particularly those with their own inherent steric de-
in the monoclinic space group P21/c with one molecule per mands, i.e. IrH2POCOP. Secondly, the presence of highly
asymmetric unit.
electron-withdrawing pentafluorophenyl groups at boron is
As observed in the crystals of 1, adjacent molecules were likely to significantly reduce the hydridic character of the
packed in close proximity to one another, leading to a short hydrogen atoms at boron, certainly making hydrogen loss
intermolecular separation of 2.961 Å between F(5) and less favourable, but also possibly making B–H activation at
F(8). The angles around B(1) and N(1) both summed to a metal centre less likely. Both factors are likely to result in
360° and the C(1)–N(1)–B(1)–C(7) dihedral angle was a high kinetic barrier to dehydrogenation, leading to negli-
179.94°, all of which were consistent with the planar gible conversion under ambient conditions, despite the
C2BNC2 core expected for monomeric aminoboranes. favourable thermodynamics of the desired transformation
Furthermore, the planes defined by the two C6F5 groups (vide infra).
were tilted with respect to this core (78.39 and 81.25°),
Following only moderate success in thermal and metal-
which would preclude any substantial orbital overlap be- catalysed dehydrogenation of amine–borane 1, alternative
tween the electron-deficient π-system and the B=N bond. approaches were also investigated. Stephan and co-workers
This orientation, however, is what would be expected to previously demonstrated the elimination of pentafluoroben-
minimise steric interactions between the two rings. The zene from secondary amine adducts of tris(pentafluoro-
presence of bulky substituents on both B and N was also phenyl)borane, i.e. R2NH·B(C6F5)3 (R = Et, tBu), using a
manifested in a narrowing of the C–B–C and C–N–C angles carbene as a catalytic Lewis base.[63] We therefore investi-
(113.60 and 113.48°, respectively) away from the ideal value gated the employment of the same system with 1, in which
(120°). The B=N bond length of 1.376(4) Å is within the elimination of H2 might be expected to be feasible and
range formed by unconstrained aminoboranes containing possibly entropically more favourable than elimination of
organic substituents at both
B
and
N [1.365(5)– pentafluorobenzene. Using a 10-mol-% loading of 1,3-di-
1.441(2) Å].[67–73] In addition, this represents a significant tert-butyl-4,5-dihydroimidizol-2-ylidene (3), we observed
length contraction of 15% from 1, which is consistent with complete consumption of the adduct by 11B NMR spec-
an increase in bond order and change in hybridisation state. troscopy over 24 h. This was accompanied by the growth of
a new peak at δ = 33.2 ppm, which appeared as a doublet
in the proton-coupled spectrum (1JBH = 129 Hz). The prod-
(c) Studies of the Interconversion between 1 and 2
uct of this reaction was identified as iPr2N=BH(C6F5) (4),
produced via elimination of pentafluorobenzene, which was
also detected in the H NMR spectrum as a complex mul-
(i) Dehydrogenation of 1: Generally dehydrogenations of
amine–borane adducts can be carried out under either ther-
mal or catalytic conditions with the relative kinetics and
thermodynamics of the process being influenced by the sub-
stitution pattern at nitrogen and boron. Initially, dehydroge-
nation of 1 was attempted in toluene solution at 50 °C, with
no conversion of 1 apparent over 24 h. Heating an identical
solution to 100 °C for 70 h was then attempted. Under
these conditions the adduct was shown by 11B and 19F
NMR analysis to be unstable, partially dehydrogenating to
produce the aminoborane 2, in low yield (23%), along with
other unidentified boron-containing products which proved
to be inseparable from the desired product (Scheme 6).
1
tiplet centred at 5.7 ppm when the reaction was repeated in
1
C6D6 (Scheme 7). H, 13C and 19F NMR spectra of the re-
action product, a colourless oil, were also consistent with
the assigned structure.
Scheme 7. Reaction of amine–borane adduct 1 with carbene 3.
Previous work has also demonstrated the use of frus-
trated Lewis pairs (FLPs)[76] in stoichiometric[77,78] and
catalytic[25] quantities in the dehydrogenation of various
amine–borane adducts. We therefore attempted the dehy-
drogenation of amine–borane 1 using the combination of
nBu3SnOTf/TMP (TMP = 2,2,6,6-tetramethylpiperidine) at
Scheme 6. Thermal dehydrogenation of 1.
Attempts to prepare the aminoborane under milder con- 20 °C. Over 65 h at this temperature however, no reaction
ditions via the use of transition metal catalysts were unsuc- occurred, with only unreacted 1 being observed in the 11B
cessful, with no dehydrogenative reaction prevailing at NMR spectrum. Again, in this case it is likely that the lack
20 °C with [Rh(μ-Cl)(cod)]2 (cod = 1,5-cyclooctadiene), col- of reactivity can be attributed to the reduced hydridic char-
loidal Rh,[10,74] “Cp2Ti”,[23] IrH2POCOP (POCOP = κ3-1,3- acter of the hydrogen at boron relative to those found in
(PtBu2)2C6H3)[19,75] or the Fagnou catalyst system (KOtBu adducts of BH3, which would hinder hydride abstraction by
and RuCl2L2 [L = iPr2PCH2CH2NH2]).[12] We postulate the Lewis acid.
that this is due to two main factors which prevent the catal-
(ii) Hydrogenation of 2: Of perhaps greater interest than
ysis. Firstly, the adduct is highly sterically encumbered, with the dehydrogenation of the amine–borane, 1, was the poten-
large groups at both boron and nitrogen. These are likely tial hydrogenation of aminoborane 2. Treatment of a tolu-
to impair coordination to coordinatively unsaturated metal ene solution of 2 with 4 bar of H2 at 20 °C, conditions
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Potential Amine–Borane/Aminoborane Interconversion
closely related to those employed in the hydrogenation of metries, as expressed by the calculated H–N–B–H and C–
tBu2P-B(C6F5)2 and Cy2P-B(C6F5)2,[79] resulted in no reac- B–N–C dihedral angles. The former agree to within 3σ of
tion after 24 h. Despite attempted hydrogenations at 20 °C those determined experimentally, and the latter (175.3 and
and 100 bar H2 over 24 h and later 430 h, and at 100 °C 179.9° for 1 and 2, respectively) are also in good agreement
over 24 h at the same pressure, there was no evidence for the (Ͻ4% deviation).
hydrogenation of 2 by 11B NMR spectroscopy (Scheme 8).
Calculations on the dehydrogenation thermodynamics of
1 to yield 2 indicate that this process is favoured under stan-
dard conditions with ΔG = –5.6 kcalmol–1 (Table 1).
Furthermore, this value would be expected to become more
negative with increasing temperature, due to the positive ΔS
term that results from the liberation of H2. In this sense, the
calculations are broadly in line with the observed results,
although at elevated temperatures alternative reaction path-
ways clearly compete with dehydrogenation of 1.[85]
Scheme 8. Attempted hydrogenation of aminoborane 2.
In addition to the initial hydrogenation experiments, it
was also of interest to investigate the potential catalytic hy-
drogenation of aminoborane 2. Although no literature pre-
cedent for such hydrogenations exists, examples are of
course abundant for alkenes, the formally isoelectronic
C=C analogues. Transition-metal-catalysed olefin hydro-
genation is an exceptionally well-developed field within
which various metals have now been shown to be effec-
tive,[80] with the recent focus mainly on enantioselective hy-
drogenations.[81,82] With a view to probing the catalytic hy-
drogenation of 2 we selected representative examples of
Table 1. Thermodynamics of hydrogen release from 1 and 5.[a]
1 Ǟ 2 + H2
5 Ǟ 6 + H2
ΔEreaction
ΔHreaction
ΔGreaction
2.7
4.9
–5.6
31.0
33.1
22.7
ΔH formation π-bond
–29.7
–16.6
[a] All energies are in kcalmol–1, gas phase at 1 bar. ΔG was calcu-
lated at 298.15 K. ΔE is the zero-point-corrected electronic energy,
homo- and heterogeneous rhodium catalysts, Rh(PPh3)3Cl and was calculated at 0 K.
(Wilkinson’s catalyst),[83] and Rh/Al2O3, for initial investi-
The data for 1 should be contrasted with that calculated
gation. Catalytic alkene hydrogenation can often be
achieved at room temperature under 1 bar or less of
H2,[83,84] but in our own system, no reaction prevailed using
3 mol-% of either catalyst at 20 °C over 24 h under 1 or
100 bar of H2 according to analysis by 11B NMR spec-
troscopy.
for the dehydrogenation of 5 to afford 6, which is unfavour-
able with ΔG = 22.7 kcalmol–1. Consistent with this result
is the reported reverse reaction for tBu2P-B(C6F5)2, to yield
tBu2PH·BH(C6F5)2, which occurs over 2 d at 60 °C.[52]
Although no free energy value was reported for
this process, the enthalpy change was calculated (ΔH =
–41.4 kcalmol–1)[53] and found to be broadly in agreement
with that for the iPr analogue (ΔH = –33.1 kcalmol–1).
The key to understanding the contrasting thermodynam-
ics of these amine and phosphane–borane dehydrogenation
reactions lies in the relative strengths of the σ-bonds in the
starting materials and the π-bonds of the dehydrogenated
products. While the σ-bonds are of similar strength for both
adducts (ΔHσ = –29.0 kcalmol–1 for 1, and –32.0 kcalmol–1
for 5), (Table 2), the π-bond[86] of the phosphanylborane 6
is much weaker (ΔHπ = –16.6 kcalmol–1),[87] leading to an
overall relative stabilisation of the phosphane–borane 5.
This is not the case for the aminoborane, where the strength
of the π-bond (29.7 kcalmol–1) is almost the same as that
of the σ-bond in the amine–borane. The reduced bond en-
ergy for the P–B π-bond, when compared to that for N–B,
may be explained by a combination of poorer overlap of
the 3p-orbital on phosphorus, relative to the 2p-orbital on
nitrogen, with the 2p-orbital on boron [see Supporting In-
formation, Figure SI1; for comparison of molecular orbitals
(MOs) comprising the π-bonds in 2 and 6], and the large
(d) DFT Calculations
With a view to understanding the experimentally ob-
served behaviour of 1 and 2 towards dehydrogenation and
hydrogenation, respectively, we performed DFT calcula-
tions on this system. We have previously reported related
data for these compounds as part of a larger study aimed
at correlating the substituents at boron and nitrogen with
the thermodynamic reversibility of amine–borane dehydro-
genation.[51] In order to permit a direct comparison with
the phosphane–borane systems described by Stephan and
co-workers (Scheme 3) and thereby to promote understand-
ing of the markedly different thermodynamic behaviour of
the two systems, we performed the DFT calculations re-
ported here using the same method, and at the same level
of theory.[52,53] In instances where particular data has not
been reported for the phosphorus-containing compounds,
it has been calculated for the direct analogues of 1 and 2,
namely iPr2PH·BH(C6F5)2 (5) and iPr2P-B(C6F5)2 (6).
The structures of both 1 and 2 were optimised at the
MPW1K level of theory and all pertinent bond lengths and
angles were in good agreement with those obtained by X-
ray crystallographic methods (see Supporting Information,
section 6). This was exemplified by the B–N bond lengths
Table 2. Thermodynamics of σ-bond formation in 1 and 5.[a]
1
5
ΔH formation σ-bond
–29.0
–32.0
(1.626 and 1.384 Å for 1 and 2, respectively) and gross geo- [a] All energies in kcalmol–1, gas phase.
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I. Manners et al.
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niques, or under an atmosphere of argon within an MBraun
glovebox. All solvents were dried via a Grubbs-type solvent purifi-
cation system. Deuterated solvents were purchased from Sigma Al-
drich and distilled from potassium (C6D6) or CaH2 (CDCl3). Diiso-
propylamine was purchased from Sigma Aldrich and distilled from
CaH2 prior to use. [Rh(μ-Cl)(cod)]2, Cp2TiCl2 and dichlorobis[2-
(diisopropylphosphanyl)ethylamine]ruthenium were purchased
from Sigma Aldrich and used without further purification. Dihy-
drogen was purchased from BOC in 99.995% purity and used as
received. IrH2POCOP[75] and HB(C6F5)2·SMe2[89] were synthesised
barrier to inversion at phosphorus. Both factors result in
an increased lone pair character at phosphorus, and also
therefore, reduced P–B π-bonding.[54]
Stephan and co-workers note that, based on their own
calculations, the π-bond is very much polarised towards the
phosphorus atom in tBu2P–B(C6F5)2 and a similar finding
has previously been reported for tBu2P–B(CF3)2.[52,88] The
limited participation of the boron 2p-orbital in this MO
suggests that it would dominate the corresponding anti-
bonding orbital, enabling coordination of H2 to boron. In- via literature methods. B(C6F5)3 was synthesised via the method of
Lancaster and co-workers and sublimed at 100 °C at 2ϫ10–2 mbar
deed, the authors calculated the activation energy for a hy-
drogenation that proceeded in this manner and stated that
prior to use.[90] 1,3-Di-tert-butyl-4,5-dihydroimidazol-2-ylidene (3)
it was in reasonable agreement with the observed rate.[52]
was synthesised via deprotonation of the corresponding imid-
azolium BF4– salt with a stoichiometric quantity of KOtBu in THF,
It is noteworthy that such an interaction is less likely in
aminoboranes, as the greater participation of the boron 2p-
orbital in B=N π-bonding would destabilise H2 binding in
before extracting into toluene and removing the solvent.
NMR spectra were recorded with JEOL JNM-ECP300/400 or
JNM-LA300 spectrometers. Chemical shifts were reported relative
the transition state.
to residual solvent peaks (1H and 13C) or to external standards:
Given the favourable thermodynamics for the dehydro-
CFCl3 (19F); BF3·Et2O (11B). Elemental analysis was performed
genation of 1 to give 2, unfavourable kinetics clearly play a
significant role in the experimentally observed chemistry.
It would be expected that either inter- or intramolecular
with a Eurovector EA 3000 Elemental Analyser by Desmond Davis
of the University of Bristol Microanalysis Laboratory. ESI mass
spectra were recorded with a Bruker Daltonics Apex IV Fourier-
pathways would be hindered by the bulky substituents at N
and B, which prevent close approach between molecules, or
rotation about the B–N bond so as to bring both hydrogen
atoms into close proximity.
transform Ion Cyclotron resonance mass spectrometer and EI and
CI mass spectrometry were carried out with a VG Analytical Auto-
Spec mass spectrometer, all at the University of Bristol.
X-ray Structural Characterisation: Diffraction data were collected
with a Bruker-Nonius KappaCCD using graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). The data were integrated and
scaled using the Denzo-SMN package.[91] The structures were
solved and refined with the SHELXTL-PC V6.1 software pack-
age.[92] Refinement was by full-matrix least-squares on Fo2 using all
data (negative intensities included). Molecular structures are pre-
sented with thermal ellipsoids at a 30% probability level and all
hydrogen atoms attached to carbon are omitted for clarity. In all
structures, hydrogen atoms bonded to carbon were included in cal-
culated positions and treated as riding atoms, whereas those at-
tached to boron or nitrogen were located and refined with isotropic
thermal parameters.
Conclusions
The amine–borane adduct, iPr2NH·BH(C6F5)2 (1) and
the aminoborane iPr2N=B(C6F5)2 (2) were prepared, with
both species isolated in high yields. Dehydrogenation of the
amine–borane to the corresponding aminoborane was
found to be possible at 100 °C, although this also resulted
in the formation of other unidentified products. Despite
screening common dehydrocoupling catalysts, this transfor-
mation could not be achieved in a catalytic manner. We
attribute the sluggish reactivity to a combination of the iPr2NH·BH(C6F5)2 (1): To a solution of HB(C6F5)2·SMe2 (1.00 g,
2.45 mmol) in toluene (30 mL) at –78 °C was added dropwise neat
iPr2NH (0.25 g, 2.45 mmol). The mixture was stirred at –78 °C for
30 min before warming to 20 °C and stirring for a further 1 h. The
solvent was then removed under high vacuum to yield the product
as an off-white solid. X-ray quality crystals of 1 were attained by
recrystallisation from toluene/hexane at –40 °C. Yield: 1.00 g, 92%.
high steric bulk of the adduct itself, which may impair coor-
dination to catalytic centres, and also to deactivation of the
hydrides at boron due to the electron-withdrawing effect of
the pentafluorophenyl groups. Hydrogenation of the ami-
noborane 2 was also attempted without success under a
variety of thermal and catalytic conditions.
A theoretical study of the amine–borane/aminoborane
system revealed a thermodynamically unfavourable hydro-
genation reaction, the opposite to the situation found for
1
11B NMR (96 MHz, CDCl3): δ = –14.8 [d, JBH = 101 Hz,
1
BH(C6F5)2] ppm. H NMR (300 MHz, CDCl3): δ = 4.60 (br. s, 1
H, N-H), 3.51 (multiplet, 2 H, C-H), 1.34 (d, 3JHH = 6.61 Hz, 6 H,
3
CH3), 1.26 (d, JHH = 6.97 Hz, 6 H, CH3) ppm. 1H {11B} NMR
the analogous phosphane–borane/phosphanylborane sys- (300 MHz, CDCl3): δ = 4.60 (br. s, 1 H, N-H), 3.64 (s, 1 H, BH),
3
3
3.51 (sept, JHH = 6.79 Hz, 2 H, C-H), 1.34 (d, JHH = 6.61 Hz, 6
tem. The calculations indicated that the lack of reactivity
of the aminoborane under conditions where analogous
phosphanylboranes are hydrogenated can be attributed pri-
marily to the different relative strengths of the N–B and P–
B π-bonds in aminoborane 2 and phosphanylborane 6.
H, CH3), 1.26 (d, JHH = 6.97 Hz, 6 H, CH3) ppm. 13C NMR
3
(75 MHz, CDCl3): δ = 51.5 (s, CH), 19.9 (s, CH3) ppm. 19F NMR
(283 MHz, CDCl3): δ = –133.1 (s, C6F5), –157.7 (t, JFF = 19.7 Hz,
C6F5), –162.8 (br. s, C6F5) ppm. MS (ESI–): m/z (%) = 446.1156
(5) [M – H]–. MS (CI+): m/z (%) = 345.0 (10) [M – iPr2NH – H]+,
169.0 (47) [H2C6F5]+, 102.1 (97) [iPr2NH2]+, 86.1 (100) [iPr2NH –
CH3]+. C18H16BF10N (447.12): calcd. C 48.35, H 3.61, N 3.13;
found C 49.31, H 3.52, N 3.07.
Experimental Section
General: All manipulations were carried out under an atmosphere
of nitrogen gas using standard vacuum line and Schlenk tech-
iPr2N=B(C6F5)2 (2): To a solution of HB(C6F5)2·SMe2 (1.00 g,
2.45 mmol) in Et2O (5 mL) was added dropwise a 2 m solution of
5284
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 5279–5287

Potential Amine–Borane/Aminoborane Interconversion
HCl in Et2O (1.22 mL, 2.45 mmol) at 0 °C. The mixture was then
stirred for 1 h before warming to 20 °C and stirring for 16 h. All
volatiles were then removed under vacuum, with a view to remov-
ing any residual HCl, yielding a colourless oil which was immedi-
ately redissolved in Et2O (5 mL). A solution of lithium diisopro-
pylamide (2.44 mmol) in Et2O (5 mL) was prepared and added
dropwise to the mixture at –78 °C. The reaction mixture was stirred
for 30 min before warming to 20 °C and stirring for a further 2 h.
The reaction was then monitored by 11B NMR spectroscopy, indi-
cating complete consumption of the starting material, and 84%
conversion to iPr2N=B(C6F5)2. The solvent was removed under
high vacuum to yield an off-white solid, which was extracted with
hexanes (50 mL), and filtered through glass-fibre filter paper, be-
fore the solvent was once again removed yielding the desired prod-
uct as a white solid. The product was recrystallised from the mini-
mum volume of hexanes at –40 °C, which produced crystals suit-
able for X-ray diffraction. Yield: 0.84 g, 77%. 11B NMR (96 MHz,
at 5.7 ppm, a complex multiplet, was apparent and assigned as
pentafluorobenzene [authentic sample purchased from Sigma Ald-
rich, 1H NMR shift (C6D6): 5.85 ppm]. The 19F NMR spectrum
was also consistent with the stoichiometric formation of pentafluo-
robenzene.
Attempted Catalytic Dehydrogenation of iPr2NH·BH(C6F5)2 (1):
Dehydrogenation experiments were carried out according to pre-
viously published literature which was appropriate for the catalyst
in use. The reactions are summarised below (Table 3).
Table 3. Experimental conditions used in the attempted catalytic
dehydrogenation of 1.
Catalyst
Loading
[mol-%]
Solvent
Time
[h]
Temp.
[°C]
IrH2POCOP
Rh colloids
[Rh(μ-Cl)(cod)]2
Fagnou’s catalyst[a]
[Cp2Ti]
1
5
5
10
100
THF
18
18
70
18
70
25
45
25
25
25
toluene
toluene
Et2O
1
C6D6): δ = 33.2 [s, B(C6F5)2] ppm. H NMR (300 MHz, C6D6): δ
3
3
= 3.32 (sept, JHH = 6.79 Hz, 2 H, C-H), 0.90 (d, JHH = 6.79 Hz,
toluene
12 H, CH3) ppm. 1H {11B} NMR (300 MHz, C6D6): δ = 3.32 (sept,
3JHH = 6.79 Hz, 2 H, C-H), 0.90 (d, JHH = 6.79 Hz, 12 H,
3
[a] RuCl2L2 [L = iPr2PCH2CH2NH2] and 30 equiv. KOtBu.
CH3) ppm. 13C NMR (75 MHz, C6D6): δ = 51.2 (s, CH), 23.3 (s,
CH3) ppm. 19F NMR (283 MHz, C6D6): δ = –130.3 (s, C6F5),
–153.0 (t, JFF = 20.56 Hz, C6F5), –161.1 (br. s, C6F5) ppm. MS
(CI+):m/z(%)=446.1140(4)[M+H]+,430.2(43)[M–Me]+,278.2(32)
[M – C6F5]+, 93.1 (100) [Toluene (Solvent)]. C18H14BF10N (445.11):
calcd. C 48.57, H 3.17, N 3.15; found C 49.04, H 3.63, N 3.59.
In all cases, analysis of the crude product mixtures by 11B/19F
NMR spectroscopy indicated no formation of aminoborane 2 over
the time scales indicated.
Reaction of iPr2NH·BH(C6F5)2 (1) with nBu3SnOTf/2,2,6,6-Tet-
ramethylpiperidine: To a solution of iPr2NH·BH(C6F5)2 (100 mg,
0.22 mmol) in toluene (1.25 mL) was added a mixture of nBu3-
SnOTf (97 mg, 0.22 mmol) and 2,2,6,6-tetramethylpiperidine
(31.6 mg 0.22 mmol) at 20 °C, and the mixture was then stirred for
70 h in a glovebox. The crude mixture was then analysed by 11B
NMR spectroscopy, indicating the presence of only unreacted start-
ing material in solution.
Attempted Thermal Dehydrogenation of iPr2NH·BH(C6F5)2 (1): A
solution of iPr2NH·BH(C6F5)2 (0.05 g, 0.11 mmol) in toluene
(3 mL) was heated to 50 °C for 24 h before analysis by 11B NMR
spectroscopy, which indicated that no reaction had ocurred. The
solution was then heated to 100 °C for 70 h, before further analysis
by 11B NMR spectroscopy. 11B NMR (96 MHz, Toluene): δ = 32.5
[br. s, iPr2N=B(C6F5)2, 23%], 19.3 (br. s, 52%), –0.57 (br. s, 24%)
and –25.6 (s, 1%) ppm.
Attempted Hydrogenation of iPr2N=B(C6F5)2 (2): All attempted hy-
drogenations were performed according to the following general
procedure. A solution of iPr2N=B(C6F5)2 (50 mg, 0.11 mmol) in
toluene (3 mL) was prepared and placed into a stainless steel auto-
clave (200 mL). The system was then pressurised with 100 bar of
hydrogen, and stirred at 20 °C for 24 h. The pressure was then
vented before transfer of the autoclave to the glovebox prior to
sample analysis by 11B NMR spectroscopy.
19F NMR spectroscopy also confirmed the presence of the desired
aminoborane 2 as a component of the reaction mixture: 19F NMR
(283 MHz, Toluene): δ = –130.3 (s, C6F5), –153.0 (t, JFF
=
20.56 Hz, C6F5), –161.1 (br. s, C6F5) ppm.
Reaction of iPr2NH·BH(C6F5)2 (1) with 1,3-Di-tert-butyl-4,5-dihy-
droimidazol-2-ylidene (3): To a solution of iPr2NH·BH(C6F5)2
(0.5 g, 1.12 mmol) in toluene at 20 °C was added 10 mol-% 1,3-di-
tert-butyl-4,5-dihydroimidizol-2-ylidene (0.02 g, 0.11 mmol). The
mixture was then stirred at 20 °C for 24 h, leading to complete
consumption of the starting material, and the appearance of a new
peak at δ = 33.2 ppm in the 11B NMR spectrum. The solvent was
then removed under high vacuum to yield the product,
iPr2N=BH(C6F5), 4, as a clear oily liquid. Yield: 0.20 g, 65%. 11B
In the case of the catalytic experiments, the catalysts were added
in the solid phase to the toluene solutions within the glovebox im-
mediately prior to sealing the autoclave.
For the reactions performed above ambient temperature, the vessel
was pressurised at 20 °C to a value that would ultimately afford the
desired pressure at the final temperature.
1
NMR (96 MHz, C6D6): δ = 33.2 [d JBH = 129 Hz, B(C6F5)H]. 1H
3
(300 MHz, C6D6): δ = 5.11 (br. q, 1 H, BH), 3.52 (sept, JHH
=
3
Table 4. Experimental conditions used in the attempted hydrogena-
tion of 2.
6.79 Hz, 1 H, CH), 2.97 (sept, JHH = 6.79 Hz, 1 H, CH), 1.13 (d,
3JHH = 6.79 Hz, 6 H, CH3), 0.86 (d, JHH = 6.79 Hz, 6 H, CH3)
3
ppm. 1H {11B} (300 MHz, C6D6): δ = 5.11 (s, 1 H, BH), 3.52 (sept,
Temp.
[°C]
Catalyst
Loading
[mol-%]
Time
[h]
H2, pressure
[bar]
3JHH = 6.79 Hz, 1 H, CH), 2.97 (sept, JHH = 6.79 Hz, 1 H, CH),
3
3
3
1.13 (d, JHH = 6.79 Hz, 6 H, CH3), 0.86 (d, JHH = 6.79 Hz, 6 H,
CH3). 19F NMR (283 MHz, C6D6): δ –132.9 (m, C6F5), –155.2 (t,
JFF = 19.48, C6F5), –162.4 (td, JFF = 22.61, JFF = 10.02 Hz,
C6F5) ppm. 13C NMR (75 MHz, C6D6): δ = 52.8 (s, CH), 46.0 (s,
CH), 27.4 (s, CH3), 22.0 (s, CH3) ppm. MS (CI+): m/z (%) = 278.1
(6) [M – H]+, 169.0 (60) [C6F5H2]+, 102.1 (73) [iPr2NH2]+, 86.1
(100) [iPr2NH – CH3]+.
25
25
25
100
25
25
25
25
45
none
none
none
none
Rh(PPh3)3Cl
Rh/Al2O3
Rh(PPh3)3Cl
Rh/Al2O3
Rh colloids
N/A
N/A
N/A
N/A
3
3
3
3
5
24
24
430
24
24
24
24
24
24
1
100
100
100
1
1
100
100
100
Conducting the identical reaction in C6D6 allowed us to obtain the
1H NMR spectrum of the crude mixture, wherein after 24 h a peak
Eur. J. Inorg. Chem. 2011, 5279–5287
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5285

I. Manners et al.
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Acknowledgments
A. P. M. R. and A. S. acknowledge the Engineering and Physical
Sciences Research Council (EPSRC) for funding. I. M. thanks the
EPSRC for financial support, the European Union for a Marie
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Potential Amine–Borane/Aminoborane Interconversion
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Published Online: October 27, 2011
Eur. J. Inorg. Chem. 2011, 5279–5287
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