Hydrogen activation by 2-boryl-N,N-dialkylanilines: A revision of Piers' ansa-aminoborane

Article abstract of DOI:10.1039/c2dt30926b

Two 2-[bis(pentafluorophenyl)boryl]-N,N-dialkylanilines reported here exemplify a new class of intramolecular frustrated B/N Lewis pairs. A structure closely related to this class structure was synthesized in 2003 by Piers et al. but was unable to activate H₂. The new aminoboranes can activate hydrogen at near ambient conditions; besides, one of them can hydrogenate imines and enamines in a catalytic fashion demonstrating the validity of the original Piers' approach to hydrogen activation with ansa-aminoboranes.

Full text of DOI:10.1039/c2dt30926b

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This article is published as part of the Dalton Transactions themed
issue entitled:
Frustrated Lewis Pairs
Guest Editor: Douglas Stephan
Published in issue 30, 2012 of Dalton Transactions
Articles in this issue include:
Communication
Hydrogen activation by 2-boryl-N,N-dialkylanilines: a revision of Piers’ ansa-aminoborane
Konstantin Chernichenko, Martin Nieger, Markku Leskelä and Timo Repo
Paper
Frustrated Lewis pair addition to conjugated diynes: Formation of zwitterionic 1,2,3-butatriene
derivatives
Philipp Feldhaus, Birgitta Schirmer, Birgit Wibbeling, Constantin G. Daniliuc, Roland Fröhlich,
Stefan Grimme, Gerald Kehr and Gerhard Erker
Paper
Fixation of carbon dioxide and related small molecules by a bifunctional frustrated pyrazolylborane
Lewis pair
Eileen Theuergarten, Janin Schlösser, Danny Schlüns, Matthias Freytag, Constantin G. Daniliuc,
Peter G. Jones and Matthias Tamm
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Volume 41 | Number 30 | 14 August 2012 | Pages 8999–9242
Featuring the themed issue: Frustrated Lewis Pairs
ISSN 1477-9226
COVER ARTICLE
Repo et al.
Hydrogen activation by 2-boryl-N,N-dialkylanilines: a revision of Piers’
ansa-aminoborane
1477-9226(2012)41:30;1-R

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COMMUNICATION
Hydrogen activation by 2-boryl-N,N-dialkylanilines: a revision of Piers’
ansa-aminoborane†
Konstantin Chernichenko, Martin Nieger, Markku Leskelä and Timo Repo*
Received 27th April 2012, Accepted 7th June 2012
DOI: 10.1039/c2dt30926b
Two
2-[bis(pentafluorophenyl)boryl]-N,N-dialkylanilines
reported here exemplify a new class of intramolecular frus-
trated B/N Lewis pairs. A structure closely related to this
class structure was synthesized in 2003 by Piers et al. but was
unable to activate H2. The new aminoboranes can activate
hydrogen at near ambient conditions; besides, one of them
can hydrogenate imines and enamines in a catalytic fashion
demonstrating the validity of the original Piers’ approach to
hydrogen activation with ansa-aminoboranes.
Frustrated Lewis pairs (FLPs) is a rapidly developing concept in
contemporary chemistry and catalysis.1 The idea of the substrate
activation by a mutual action of both a Lewis acid and base is
not new and has been successfully used in transition metal cata-
lysis.2 On the other hand, the frustrated Lewis pairs comprising
lightweight main-group elements and their reactivity have been
known for decades.3 Nevertheless, a real breakthrough in this
area was brought by Stephan et al. in 2006 in their pioneering
work on the activation of elemental hydrogen with phosphino-
borane (C6F5)2BC6F4PMes2.4 This initial finding boosted
research activity in this field leading to more than two hundred
papers on activation of hydrogen and other small molecules.5
Whereas a variety of small molecules have been activated by
FLPs, the most important achievement and practical application
is FLP-catalyzed hydrogenation of polar double bonds.6
In 2003, Piers et al. described an approach for hydrogen acti-
vation by a bridged frustrated aminoborane, emphasizing the
importance of the highly Lewis acidic bis(pentafluorophenyl)
boryl group. As an example, aminoborane 1 was synthesized
(Fig. 1).7 Attempts to activate hydrogen with 1 were unsuccess-
ful. This paper has presumably therefore been sparingly cited in
later publications.
Fig. 1 Ansa-aminoboranes prepared by Piers (1), by our group (4, 5)
and reported herein (2, 3).
demonstrating that Piers et al. were very close to successful
hydrogen activation using a solely main-group compound
(Fig. 1).
Recently, we have reported the ansa-aminoboranes 4 and 5,
which efficiently transfer molecular hydrogen to imines and
other nitrogen-containing compounds in a catalytic fashion.8,9
Asymmetric hydrogenations have been attempted by introducing
chiral amines into frameworks of the catalysts (e.g. (S)-5) but
only fair enantioselective inductions (up to 38% ee) were
achieved. We attributed this to excessive conformational freedom
of the catalyst molecules (Fig. 1). In this regard removal of the
methylene junction between the phenylene ring and the amino
groups may lead to more rigid molecules with superior asym-
metric effectiveness. To the best of our knowledge, 1 is the only
2-[bis(pentafluorophenyl)boryl]-aniline that has been aimed for
H2 activation, although some other structures with nitrogen-con-
taining groups in the ortho-position to the bis(pentafluorophe-
nyl)boryl moiety are known.10 Hence, the achiral ansa-
aminoboranes 2 and 3 were prepared to test their reactivity
towards hydrogen and evaluate potential catalytic activity in
hydrogenations.
Herein we report two new compounds closely related to the
original ansa-aminoborane 1. In these molecules the diphenyl-
amino moiety was substituted with more basic 2,2,6,6-tetra-
methylpiperidino-1 (TMP) (2) or dimethylamino (3) groups.
Both 2 and 3 readily activate H2 at ambient conditions,
Results
The ansa-aminoboranes 2 and 3 were prepared in high yield by
the standard approach8,9 from bis(pentafluorophenyl)boron
chloride 1111 and o-N,N-dialkylaminophenyllithiums 9 and 10
(Scheme 1). A one-step synthesis starting from iodobenzene and
LiTMP was previously reported to give 1-(2-iodophenyl)-
2,2,6,6-tetramethylpiperidine 7 in 81% yield.12 Since no exact
Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55,
FIN-00014, University of Helsinki, Finland. E-mail: timo.repo@helsinki.fi;
Fax: +358 (9)19150198; Tel: +358 (9)19150194
†Electronic supplementary information (ESI) available: Experimental
procedures and crystallographic data in CIF. CCDC 875800 (2), 875801
(2H2) and 875802 (3). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt30926b
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9029–9032 | 9029

Scheme 1 Synthesis of aminoboranes 2 and 3.
Scheme 2 Hydrogen activation by the aminoboranes 2 and 3: for-
mation of ammonium borohydrides 2H2 and 3H2. ORTEP drawing (dis-
placement parameters drawn at 50% probability level) of the molecule
of 2H2 (hydrogen atoms except for NH and BH are omitted for clarity).†
procedure was reported for 7, we were able to synthesize it in ca.
30% reproducible yield only. The respective lithium compounds
9 and 10 were isolated from the reaction of 7 or N,N-dimethyl-2-
bromoaniline 8 with butyllithium in hexane at 0 °C. It is worth
noting that isolation of pure 9 and 10 is beneficial as the sub-
sequent reaction with (C6F5)2BCl 11 gives 3 in high yield, and 2
almost quantitatively.
toluene and isolated as white crystals, from which the crystal
structure was determined (Scheme 2). Attempts to dehydrogenate
2H2 back into 2 were unsuccessful; heating it for 4 days at
110 °C in toluene-d8 led to formation of a minute (2.6 mol% by
19F NMR) amount of 2. The irreversible activation of hydrogen
by 2 is in sharp contrast with the previously reported 4, whose
corresponding hydrogen adduct 4H2 starts to release hydrogen
within minutes at 110 °C and is fully converted to 4 upon
heating for 24 h in an open system.8 The structure of 2H2 in a
solid state represents the zwitterionic structure typical for those
ansa-ammonium borohydrides reported previously by us9 and
Erker et al.14 The counterparts of the intramolecular ion pair are
oriented towards each other forming an almost plain 6-mem-
bered pseudo-ring with the 1.65(3) Å H–H distance (Scheme 2,
Table 2).
The aminoborane 3, containing the four-membered B–N ring,
slowly produces the respective intramolecular ammonium boro-
hydride 3H2 in C6D6 at the constant rate of about 8 mol% h−1
upon exposure to 2 bar of hydrogen. Thus 3 was totally con-
verted into 3H2 within 12 h. When hydrogen is vented off and
the solution of 3H2 is left standing under Ar in an open system,
it slowly dehydrogenates back to 3 with the rate of about 10%
per day. Thus 3 exhibits reversibility of hydrogen uptake at room
temperature without any need of thermal promotion for dehydro-
genation of 3H2. The thermal behaviour of the borohydride 3H2
is more complicated and is a subject of a separate study which
will be published elsewhere.
The aminoborane 2 was isolated as deeply coloured yellow
crystals. NMR data, especially the 19F-NMR spectrum, which
contains resonances at −127.2, −149.3 and −162.1 ppm, are
consistent with three-coordinated boron species and reveal the
“frustration” between the B and N centres. Colours from yellow
to deep bloody red are typical for bridged phosphino-13 and ami-
noboranes,8,9 and the aminoborane 2 is not an exception in this
respect. In contrast, the N,N-dimethylaminoborane 3 was isolated
as white crystals. In addition, NMR measurements (in ppm 13C:
δ = 47.9 (N(CH3)2); 10B: δ = 9.0; 19F: δ = −156.6 (p-F), −163.4
(m-F)) evidence a B–N bonding. It was not clear from the NMR
data whether the B–N bonding is intra- or intermolecular, taking
into account that the dimethylamino group is sterically benign.
Both structures, 2 and 3, were determined using the X-ray dif-
fraction method (Fig. 2). A four-membered ring with the B–N
bond lengths of 1.771(3) and 1.741(3) Å for two different mol-
ecules of 3 in a unit cell was found. The formation of four-mem-
bered B–N rings was recently reported for the products of
hydroboration of enamines with Piers’ borane HB(C6F5)2.14 In
contrast to 3, the X-ray diffraction method showed no evidence
of the B–N bonding in 2. Evidently, extreme steric hindrance of
the 2,2,6,6-tetramethylpiperidinyl moiety is responsible for this
effect.
Though both 2 and 3 activate molecular hydrogen under 2 bar
pressure and at room temperature, they do this in a different
fashion. The “truly frustrated” 2 absorbs H2 instantly upon
exposure. The resultant borohydride 2H2 was produced in
Catalytic activity of the prepared aminoboranes/ammonium
borohydrides in the hydrogenation of imines and enamines was
studied. No reaction occurred in an attempted hydrogenation of
Fig. 2 ORTEP drawings (displacement parameters drawn at 50% probability level) of the aminoboranes 1 reported by Piers et al.,7 and 2–3 (hydro-
gen atoms are omitted for clarity, one of two crystallographically independent molecules of 3 is shown).†
9030 | Dalton Trans., 2012, 41, 9029–9032
This journal is © The Royal Society of Chemistry 2012

Table 1 Hydrogenation of 16 and 17 catalyzed by the aminoborane 3a
Loading of 3,
Table 2 Comparison of geometries of the B–CvC–N frames of 2,
2H2 and 3, based on X-ray diffraction data
Substrate %mol
Temperature, °C Time, h Conversion, %
2
2H2
3c
16
16
16
16
17
17
4
110
80
100
80
25
80
36
18
18
22
18
1
30
70
81
100
15
d(B–N), Å
3.031(4)
126.6(2)
119.1(2)
76.1(2)
2.941(2)
123.1(1)
116.9(1)
86.0(1)
74.3(1)
−0.05(2)
−0.33(2)
1.771(3) [1.741(3)]
93.0(2) [92.2(2)]
101.7(2) [101.7(2)]
89.7(1) [88.7(1)]
85.6(1) [87.6(1)]
10
10
15
5
∠(CvCB), °
∠(CvCN), °
N-dihedral ∠, °a
B-dihedral ∠, °a
d(N+H), Åb
49.4(2)
5
100
d(B−H), Åb
a 0.25 mmol of the substrate and a catalytic amount of 3 were placed in a
Schlenk tube and stirred under 2 bar H2 and respective conditions.
Conversions were determined by 1H-NMR of crude reaction mixtures.
a Angle between the least squares planes of the phenylene bridge and the
TMP (NCC) or B(C6F5)2 (BCC) groups. b A declination of hydrogen
atoms from the B–CvC–N plane. c Values for the second
crystallographically independent molecule are in brackets.
by Piers and lately by Pápai to a low basicity of the diphenyl-
amino group. The power of a frustrated Lewis pair stems from
the basicity of the basic and the acidity of the acidic counter-
parts, of which both are crucial for activation of H2. This has
been studied theoretically.15 Besides, in our recent paper we
have systematically studied a series of structurally homological
intramolecular frustrated Lewis pairs. It was shown that gradual
decreasing of the basicity of the amino group (from 4 to 5)
makes the respective ammonium borohydride less stable and
hydrogen liberation more facile. This translates into a dramatic
rise of the catalytic activity of the respective aminoboranes, for
example 5. Eventually, we have reached the point on the basicity
scale of the amine part where the respective ammonium boro-
hydride, the product of H2 activation, was no longer stable at
ambient conditions.9 A relation between the acidity of boranes
and reversibility of hydrogen uptake was a matter of an exper-
imental study as well.16 On the other hand, the mutual geometry
of the acid and base counterparts can substantially improve not
only kinetic parameters of hydrogen splitting,13 but also thermo-
dynamic stability of hydrogen adducts. Thus enhanced thermo-
dynamic stability of ortho-anilinium borohydrides was
emphasized by Pápai et al.; the calculated ΔG value for hydro-
genation of 1 is +7.1 kcal mol−1. This value is much lower than
expected based on evaluation of the Lewis acid and base
strengths apart from their intramolecular nature and the mutual
geometry. Similarly, surprisingly high thermodynamic stability
was calculated for the ansa-ammonium borohydride 4H2.15
In addition, other frustrated ansa-B/N systems containing an
ethylene or ethynylene bridge and constrained geometry were
theoretically designed and claimed to be promising candidates
for activation of small molecules, particularly methane.17
Fig. 3 Substrates used in the attempted catalytic hydrogenation with
the aminoboranes 2, 3 or ammonium borohydrides 2H2, 3H2.
imines 12–16 (Fig. 3) using 2 or 2H2 as the catalyst (5 mol%,
toluene, 110 °C, 2 bar H2, 16 h). Moreover, no reactivity was
observed when equimolar amounts of the borohydride 2H2 and
benzaldehyde or benzonitrile were heated for 24 h at 110 °C
in C6D6.
The aminoborane 3 was tried as a catalyst for the hydrogen-
ation of imines 12–16 and an enamine 17 (4 mol%, toluene,
110 °C, 2 bar H2, 36 h). As a result, only 16 and 17 were cataly-
tically hydrogenated (Table 1, Fig. 3). To figure out the reason
for such a strong difference in reactivity, 12–17 were mixed with
equimolar amounts of the ammonium borohydride 3H2 in C6D6.
No reaction occurred between 3H2 and imines 12–15 at room
temperature or upon heating for 3 h at 50 °C. In contrast, 16 and
17 react instantly at room temperature, though with different
outcome. The enamine 17 produces smoothly N-cyclohexyl-
piperidine (17H2) and the aminoborane 3. No interactions were
found between 3 and the starting enamine 17 or the product
amine 17H2 by NMR. On the other hand, as complete consump-
tion of the borohydride 3H2 occurred upon mixing with imine
16 (1 : 1), the complex mixture comprising the reduced imine
16H2 coordinated to aminoborane 3 was produced. The coordi-
nation between 3 and the amine 16H2 or the imine 16 explains
the longer reaction times (compared to 17) and the requirement
of thermal promotion to break the Lewis acid–base adduct
(Table 1).
The ansa-aminoborane 2 exemplifies the exceptionally reac-
tive frustrated Lewis pair. Possessing the extremely hindered
TMP-moiety, the aminoborane 2 does not form the B–N bonded
ring, revealing FLP reactivity in full strength. Other structurally
close compounds have either partially quenched reactivity due to
the B–N bond formation caused by less hindered amino substitu-
ents (3 or reported by Erker et al.)14 or diminished reactivity due
to a lower basicity of the diphenylamino group in 1. Therefore,
aminoborane 2 activates H2 producing the extremely stable
adduct 2H2. Comparison of the structures of 2 and 2H2 in a
solid state revealed that activation of H2 causes a minor change
in geometry of the B–CvC–N frame (Table 2). Thus the rigid
Discussion
Experimental results reported herein and published by Piers
et al. represent three different examples of 2-[bis(perfluorophenyl)-
boryl]-anilines. The aminoborane 1 does not produce an intra-
molecular B–N bonding and does not activate hydrogen. While
frustration of the borane and amine parts is caused, apparently,
by steric reasons, the inability to split hydrogen was attributed
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9029–9032 | 9031

+
geometry of 2 is optimal for H2 activation, making 2H2 particu-
larly stable due to a large contribution of the entropic factor to
the free energy. The formation of the 6-membered pseudo-ring
of 2H2 is thermodynamically more favourable than the respect-
ive 7-membered ring as evident by different dehydrogenation
behaviour of 2H2 and 4H2, mentioned previously. This suggestion
requires further experimental verification by direct calorimetric
studies which are in progress. Additional stability of the resultant
ammonium borohydride 2H2 is supported by its inability to
hydrogenate imines or even benzaldehyde. Apparently, an impor-
tant step in the hydrogenation with ansa-ammonium borohydrides
is the breakage of the “dihydrogen” bond and the resultant ring-
opening of the six-membered pseudo-ring. In the case of 2H2 this
step is complicated by rigidity of the B–CvC–N frame and
strong electrostatic attraction between counterparts of the intra-
molecular ion pair. Nevertheless, there are some differences
between 2 and 2H2 in orientation of the amine and boryl parts.
Due to steric repulsion, the TMP- and (C6F5)2B-groups have a
sliding orientation in 2 and are turned by 76° and 49° respectively
relative to the plane of the bridging C6H4 ring (Table 2). In the
borohydride 2H2 these angles are 86° and 74° respectively,
demonstrating geometry close to Cs-symmetric.
NHMe2 group to the substrate’s –NvC– bond is required, in
the case of enamines the catalyst protonates the β-carbon, which
is much more accessible. This consideration requires further
experimental and theoretical support.
In conclusion, two new ansa-aminoboranes bridged with a
phenylene ring were prepared. The aminoborane 2 containing a
highly basic and sterically hindered 2,2,6,6-tetramethylpiperidine
moiety instantly activates H2 at ambient conditions producing
the extremely stable and unreactive ammonium borohydride
2H2. The ansa-aminoborane 3 containing a smaller N,N-
dimethylamino group produces an intramolecular Lewis adduct
comprising a four-membered B–N ring. It features reversible H2
activation at room temperature, which is remarkable. 3 or the
respective ammonium borohydride 3H2 efficiently catalyzes
hydrogenation of some imines and enamines, demonstrating
selectivity to non-hindered substrates. The aminoboranes 2 and 3
exemplify the validity of the approach to hydrogen activation by
the first intramolecular B/N frustrated pair proposed by Piers
et al. back in 2003.
We are grateful for the financial support from the Academy of
Finland (139550). We thank M. Lindqvist and Dr V. Sumerin for
corrections and remarks during preparation of the manuscript.
The N,N-dimethylaminoborane 3 is an example of a sterically
benign ortho-borylaniline. The absence of steric repulsion facili-
tates formation of a four-membered B–N ring. Though the basi-
city of the dimethylamino group is substantially lower than that
of the TMP, and some reactivity is quenched by the intramolecu-
lar Lewis B–N adduct, 3 is still able to activate H2 at near
ambient conditions. The progress of hydrogen uptake was found
to be linear with time under constant pressure, perhaps due to
the rate-limiting character of the B–N ring dissociation. The for-
mation of four-membered (C6F5)2B–N adducts was reported pre-
viously by Erker et al. and the B–N ring dissociation energy was
Notes and references
1 (a) D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535;
(b) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46.
2 For example, alkyne activation by a nucleophile/gold cation (trans-
addition): (a) A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180;
(b) A. Corma, A. Leyva-Pérez and M. J. Sabater, Chem. Rev., 2011, 111,
1657.
3 (a) H. C. Brown, H. I. Schlesinger and S. Z. Cardon, J. Am. Chem. Soc.,
1942, 64, 325; (b) G. Wittig and A. Rückert, Liebigs Ann. Chem., 1950,
566, 101; (c) G. Wittig and O. Bub, Liebigs Ann. Chem., 1950, 566, 113;
(d) G. Wittig and H. Schloeder, Liebigs Ann. Chem., 1955, 592, 38.
4 G. C. Welch, R. R. San Juan, J. D. Masuda and D. W. Stephan, Science,
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5 On the moment of writing, 237 publications containing the concept
“frustrated Lewis pair” can be found through SciFinder.
6 D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie,
S. J. Geier, J. M. Farrell, C. C. Brown, Z. M. Heiden and G. C. Welch,
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measured to be 13–14 kcal mol−1 14
The ease of hydrogen
.
release from the ammonium borohydride 3H2 is remarkable. Evi-
dently, the formation of the B–N bond facilitates the shift of the
equilibrium to 3.
Reversibility of hydrogen uptake by 3 at room temperature
resembles those of 5, which was found to be the most active
FLP catalyst for the hydrogenation of various imines (TOF ca.
100 h−1). While imines 12–15 were smoothly hydrogenated with
the ansa-aminoboranes 4 and 5 as catalysts, the imine 16 was
not hydrogenated catalytically due to coordination of the sub-
strate to the catalyst.8,9 In contrast, the ansa-aminoborane 3 was
able to hydrogenate 16 but not 12–15, demonstrating the oppo-
site selectivity to non-hindered imines. Even though the
dimethylamino group is one of the smallest possible secondary
amino-groups, 3H2 is much more hindered than 4H2 containing
the TMPCH2 group; the diminished accessibility of the
ammonium borohydride catalyst 3H2 for a substrate is caused by
the tighter ortho-connection of the amine and boryl parts and by
constrained geometry of the 6-membered pseudo-ring in com-
parison to the 7-membered one in 4H2. In this respect the
smooth hydrogenation of the quite hindered enamine 17 is
remarkable. A possible explanation is the influence of steric
factors during proton transfer from the catalyst (3H2) to a sub-
strate, which is considered to be the first step in the catalytic
cycle. Indeed, while in the case of imines the protonation occurs
at the nitrogen atom and hence the proximity of the catalyst’s
7 R. Roesler, W. E. Piers and M. J. Parvez, J. Organomet. Chem., 2003,
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9032 | Dalton Trans., 2012, 41, 9029–9032
This journal is © The Royal Society of Chemistry 2012