Frustrated Lewis pairs derived from N-heterocyclic carbenes and Lewis acids

Article abstract of DOI:10.1039/b908737k

The chemistry of frustrated Lewis pairs derived from N-heterocyclic carbenes and a number of Lewis acids has been probed. The combination of 1,3-bis[2,6-(di-iso-propyl)phenyl]-1,3-imidazol-2-ylidene (IDipp) (1) with B(C₆F₅)₃

Full text of DOI:10.1039/b908737k

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www.rsc.org/dalton | Dalton Transactions
Frustrated Lewis pairs derived from N-heterocyclic carbenes and Lewis acids†
Preston A. Chase,a Austin L. Gille,b Thomas M. Gilbertb and Douglas W. Stephan*a
Received 5th May 2009, Accepted 19th June 2009
First published as an Advance Article on the web 23rd July 2009
DOI: 10.1039/b908737k
The chemistry of frustrated Lewis pairs derived from N-heterocyclic carbenes and a number of Lewis
acids has been probed. The combination of 1,3-bis[2,6-(di-iso-propyl)phenyl]-1,3-imidazol-2-ylidene
(IDipp) (1) with B(C6F5)3 was shown to give the classical Lewis acid–base adduct (IDipp)B(C6F5)3 (2)
which was unreactive. In contrast, the combination 1,3-di-tert-butyl-1,3-imidazol-2-ylidene (3) with
B(C6F5)3 proved to form a frustrated Lewis pair, and reacts with H2 to give the salt [ItBuH][HB(C6F5)3]
(4) in high yield. In a similar fashion, addition of (3) to a series of amine-borane adducts including
H3NB(C6F5)3 (5), PhH2NB(C6F5)3 (6) and PhH2NB(C6F5)3 (7) led to deprotonation and formation of
imidazolium amido-borate salts [ItBuH][H2NB(C6F5)3] (8), [ItBuH][PhHNB(C6F5)3] (9) and
[ItBuH][Ph2NB(C6F5)3] (10). Similar reactions of the amine-borane adducts EtNH2B(C6F5)3 (11)
tBuNH2B(C6F5)3 (12) and Et2NHB(C6F5)3 (13) gave the amidoboranes EtHNB(C6F5)2 (14)
tBuHNB(C6F5)2 (15) and Et2NB(C6F5)2 (16) respectively, with liberation of C6F5H and an equivalent of
unreacted carbene. Mechanistically these reactions are thought to proceed through transient
imidazolium salts similar to (8)–(10). In addition, the combination of carbene (3) and the cation
[CPh3]+ in frustrated Lewis pair chemistry has been probed. Reaction of this combination at room
temperature results in the immediate formation of [C3H2N2tBu2(C6H5)CPh2][B(C6F5)4] (17) stemming
from carbene attack at a para-carbon of one of the phenyl rings of trityl. Addition of carbene to the
benzyl amine adduct of [CPh3][B(C6F5)4] results in the formation of [ItBuH][B(C6F5)4] (18) and the
secondary amine Ph3CNHCH2Ph (19), thus providing the first example of an all carbon-based
frustrated Lewis pair. Crystallographic data for (2), (4), (9) and (13) are reported.
Introduction
In exploring the reactivity of Lewis acids and bases, we have
recently unveiled the concept of “frustrated Lewis pairs”.1,2 The
particular systems we have focused on are contrived such that
steric encumbrance “frustrates” classical Lewis acid–base adduct
formation. These combinations deliver unique reactivity as a
result of the unquenched Lewis acidity and basicity. In our
initial discovery of such systems, we showed that the mixture
of bulky phosphine and borane centers in the complex (2,4,6-
Me3C6H2)2P(C6F4)B(C6F5)2 is capable of reversibly activating
dihydrogen (Scheme 1).3 Subsequently we showed that heterolytic
Scheme 1 Activation of H2 by frustrated Lewis pairs.
activation of H2 can be accomplished with simple mixtures of
bulky phosphines and boranes as in tBu3P/B(C6F5)3.4 Since that
1 atm of H2. However, in contrast to [tBu3PH][HB(C6F5)3], this
time, we and others have exploited this activation of hydrogen for
simple modification allows for the facile liberation of H2. On
the catalytic hydrogenation of imines, nitriles, aziridines, enamines
the other hand, altering the Lewis base has also been probed.
Most recently, we have shown that lutidine/B(C6F5)3 exists in an
equilibrium with free base and acid, thus exhibiting classical and
and silylenolethers.5–12
In exploring the range of Lewis acids and bases that exhibit
such reactivity, we have recently prepared the Lewis acid B(p-
C6F4H)3.13 This Lewis acid in combination with P(C6H4Me)3
rapidly forms [(C6H4Me)3PH][HB(p-C6F4H)3] at 25 ◦C under
frustrated Lewis pair reactivity.14 This and the activation of small
molecules by frustrated Lewis pairs has been recently reviewed.15
In this paper, we further probe the range of Lewis bases and acids
that afford such unique frustrated Lewis pair reactivity. To this
end, it is noted carbenes ligands are strong sigma donors and have
become increasingly ubiquitous in catalysis.16–20 In addition, we
note the recent work of Bertrand and co-workers who showed that
amino-alkyl carbenes react directly with H2, NH321 and P422,23 as a
result of the donor and acceptor properties of carbene-carbon. We
began our efforts with an examination of N-heterocyclic carbenes
(NHCs). Herein, we show that bulky NHCs in combination with
aDepartment of Chemistry, University of Toronto, 80 St. George St., Toronto,
Ontario, Canada M5S3H6. E-mail: dstephan@chem.utoronto.ca
bDepartment of Chemistry and Biochemistry, Northern Illinois University,
DeKalb, Illinois, 60115, USA
† Electronic supplementary information (ESI) available: Optimized Carte-
sian coordinates and energies. CCDC reference numbers 689780–689783.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b908737k
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7179–7188 | 7179
©

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3
the Lewis acid B(C6F5)3 exhibits the ability to activate H-H and
N-H bonds. Replacing the B(C6F5)3 with [CPh3]+ also shows N-H
bond splitting but other reactions are hampered by preferential
nucleophilic attack of the NHC on one of the phenyl rings.
The reactivity is also considered in the light of DFT calculation.
Preliminary communications by ourselves8 and the research group
of Tamm24 have described a portion of this chemistry.
2 ¥ m-ArCF), -166.3 (t, 1F, JFF = 21 Hz, m-ArCF). 13C NMR
not obtained due to decomposition. Anal Calc’d: C 60.02, H 4.03,
N 2.95. Found: C 59.51, H 4.14, N 2.95.
Synthesis of [ItBuH][HB(C6F5)3] (4)
A solution of B(C6F5)3 (0.282 g, 0.55 mmol) in toluene (4 mL)
was placed in a 25 mL flask equipped with a sealable Teflon tap.
The solution was freeze–pump–thaw cycled (¥3), placed under
an atmosphere of N2 and frozen in liquid N2. Under a strong
flow of N2, the Teflon tap was quickly replaced with a septa
and a solution of ItBu (0.100 g, 0.55 mmol) in toluene (4 mL)
was added quickly via syringe. The Teflon tap was replaced, the
flask evacuated and placed under an atmosphere of H2. The
flask was sealed, the reaction stirred at -78 ◦C for 2 hours and
then slowly warmed overnight. The white, turbid reaction mixture
was pumped dry to give a white powder. X-Ray quality crystals
were grown from layering pentane onto a CH2Cl2 solution of the
Experimental
General considerations
All manipulations were performed on a double manifold N2
(H2)/vacuum line with Schlenk-type glassware or in an N2-filled
inert atmospheres glove box. The N2 and H2 gases were dried
by passage through a Dririte column. Solvents (Aldrich) were
dried using an Innovative Technologies solvent system (toluene,
hexanes, pentane, CH2Cl2). NMR spectra were obtained on a
Bruker Avance 300 MHz spectrometer and spectra were referenced
to residual solvent (1H, 13C) or externally (11B; BF3OEt2; 19F;
CFCl3; 31P; 85% H3PO4). NMR solvents were purchased from
Cambridge Isotopes, dried over CaH2 (CD2Cl2 and C6D5Br)
or Na/benzophenone (toluene-d8), vacuum distilled prior to
◦
1
product and cooling to -35 C. Yield: 0.372 g (97%). H NMR
(CD2Cl2): d 8.20 (t, 1H, 4JHH = 1.8 Hz, N2CH), 7.50 (d, 2H, 4JHH
=
1
1.8 Hz, =CH), 3.63 (1:1:1:1 q, 1H, JHB = 89 Hz, BH), 1.70 (s,
18H, tBu). 13C{ H} NMR (CD2Cl2): d 148.7 (d, 1JCF = 244 Hz, o-
1
ArCF), 138.5 (d, 1JCF = 244 Hz, p-ArCF), 137.2 (d, 1JCF = 245 Hz,
m-ArCF), 129.8 (s, N2CH), 126.2 (b, BC), 121.3 (s, NCH), 61.8
(s, C(CH3)3), 30.2 (s, C(CH3)3). 11B NMR (CD2Cl2): d -25.3 (d,
˚
use and stored over 4 A molecular sieves in the glovebox. All
amines were purchase from Aldrich; liquid amines were dried
1JBH = 89 Hz, BH). 19F NMR (CD2Cl2): d -133.9 (d, 6F, JFF
=
˚
over 4 A molecular sieves and solid reagents were used as
21 Hz, o-ArCF), -164.9 (t, 3F, JFF = 21 Hz, p-ArCF), -167.5 (m,
6F, m-ArCF). Anal. Calc’d: C 50.17, H 3.19, N 4.03. Found: C
50.39, H 3.03, N 4.10.
received. 1,3-bis[2,6-(di-iso-propyl)phenyl]-1,3-imidazol-2-ylidene
(IDipp) (1) and 1,3-di-tert-butyl-1,3-imidazol-2-ylidene (ItBu) (3)
and were purchased from Strem Chemicals and used as received.
B(C6F5)3 and [Ph3C][B(C6F5)4] were generously provided by Nova
Chemicals. The adducts H3NB(C6F5)325 (5), EtNH2B(C6F5)37 (11)
and tBuNH2B(C6F5)3 (12), were prepared as previously described.
Synthesis of PhNH2B(C6F5)3 (6)
To a solution of B(C6F5)3 (0.313 g, 0.61 mmol) in dry CH2Cl2
(2 mL) was added a solution of PhNH2 (0.0.57 g, 0.61 mmol)
in dry CH2Cl2 (2 mL). The vial was rinsed with CH2Cl2 (2 ¥
2 mL) and the reaction was stirred for 2 h. Solvent was removed
in vacuo to obtain a white powder. Yield: 0.358 g, 97%. 1H NMR
(C6D5Br): d 6.92 (m, 3H, overlapping m- and p-ArCH), 6.82 (m,
Synthesis of (IDipp)B(C6F5)3 (2)
A solution of B(C6F5)3 (0.100 g, 0.20 mmol) in toluene (5 mL) was
cooled to -78 ◦C. To the cooled solution was added (1) (0.076 g,
0.20 mmol) in toluene (9 mL) via syringe. The reaction was stirred
for 5 min. then allowed to warm to room temperature over 2 h.
The solvent was removed in vacuo to give the product as a light
brown solid. Note: prolonged stirring led to formation of multiple
other products. Yield 0.161 g (91%). X-Ray quality crysta◦ls were
2H, o-ArCH), 6.69 (br s, 2H, NH2). 13C{ H} NMR (C6D5Br):
1
1
1
d 147.6 (d, JCF = 235 Hz, o-ArCF), 140.2 (d, JCF = 226 Hz,
1
p-ArCF), 136.6 (d, JCF = 239 Hz, m-ArCF), 133.8 (s, ArCH),
129.6 (s, ArCH), 128.6 (s, ArCH), 121.9 (s, ArCH) 115.6 (br s,
BArC). 11B NMR (C6D5Br): d -5.3 (br s). 19F NMR (C6D5Br): d
-132.8 (d, 6F, 3JFF = 21 Hz, o-ArCF), -154.6 (t, 3F, 3JFF = 21 Hz,
p-ArCF), -161.8 (m, 6F, m-ArCF). Anal. Calc’d: C 47.64 H 1.17
N 2.31. Found: C 47.28 H 1.17 N 2.50.
1
obtained by cooling a toluene/hexanes solution to -35 C. H
NMR (toluene-d8, -60 ◦C): d 6.97 (d, 1H, 3JHH = 7.6 Hz, ArH),
3
3
6.92 (t, 1H, JHH = 7.0 Hz, ArH), 6.82 (t, 1H, JHH = 7.6 Hz,
ArH), 6.58 (d, 2H, 3JHH = 7.6 Hz, 2 ¥ ArH), 6.37 (d, 1H, 3JHH
=
7.7 Hz, ArH), 6.32 (s, 1H, =CH), 6.20 (s, 1H, =CH), 3.26–3.02
(overlapping m, 3H, 3 ¥ CH(CH3)2), 1.72 (br m, 1H CH(CH3)2),
1.37 (d, 3H, 3JHH = 6.6 Hz, CH(CH3)2), 1.31 (d, 3H, 3JHH = 6.4 Hz,
CH(CH3)2), 0.95 (d, 6H, 3JHH = 6.7 Hz, 2 ¥ CH(CH3)2), 0.90 (br,
Synthesis of Ph2NHB(C6F5)3 (7)
To a solution of B(C6F5)3 (0.316 g, 0.62 mmol) in dry CH2Cl2
(2 mL) was added solid Ph2NH (0.104 g, 0.62 mmol). On addition,
the solution turned light orange. The vial was rinsed with CH2Cl2
(2 ¥ 2 mL) and the reaction was stirred for 2 h. Solvent was
removed in vacuo to obtain a light yellow solid. Yield: 0.416 g,
99%. 1H NMR (C6D5Br): d 7.15 (t, 4H, 3JHH = 8 Hz, m-ArCH),
3
3
2H, JHH = 6.6 Hz, 2 ¥ CH(CH3)2), 0.83 (d, 3H, JHH = 6.3 Hz,
CH(CH3)2), 0.62 (d, 3H, JHH = 6.6 Hz, CH(CH3)2). 11B{ H}
3
1
NMR (toluene-d8, -60 ◦C): d -15.6 (s). 19F NMR (toluene-d8, -60
3
C): d -110.6 (d, 1F, JFF = 24 Hz, o-ArCF), -119.4 (dofd, 1F,
3JFF = 52, 29 Hz, o-ArCF), -126.0 (d, 1F, 3JFF = 24 Hz, o-ArCF),
-127.8 (d, 1F, 3JFF = 25 Hz, o-ArCF), -132.8 (dofd, 1F, 3JFF = 51,
6.98 (d, 4H, JHH = 8 Hz, o-ArCH), 6.89 (t, 2H, JHH = 8 Hz,
3
3
p-ArCH), 6.11 (br s, 1H, NH). 13C{ H} NMR (C6D5Br): d 148.0
1
3
1
1
25 Hz, o-ArCF), -133.5 (d, 1F, JFF = 24 Hz, o-ArCF), -157.1
(d, JCF = 250 Hz, o-ArCF), 144.1 (d, JCF = 261 Hz, p-ArCF),
142.8 (s, ArCN), 137.2 (d, 1JCF = 261 Hz, m-ArCF), 129.2 (s, m-
ArCH), 121.9 (s, p-ArCH), 118.4 (s, o-ArCH), 113.3 (br s, ArCB).
(m, 2F, 2 ¥ p-ArCF), -158.2 (t, 1F, 3JFF = 21 Hz, p-ArCF), -162.5
(m, 1F, m-ArCF), -163.4 (m, 2F, 2 ¥ m-ArCF), -164.3 (m, 2F,
7180 | Dalton Trans., 2009, 7179–7188
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©

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11B NMR (C6D5Br): not observed. 19F NMR (C6D5Br): d -127.6
(d, 6F, JFF = 22 Hz, o-ArCF), -143.8 (t, 3F, JFF = 21 Hz,
p-ArCF), -159.9 (m, 6F, m-ArCF). Anal. Calc’d: C 52.89, H 1.63,
N 2.06. Found: C 52.41, H 1.77, N 2.09.
129.4, 128.8, 127.4, 125.6, 122.7, 122.1, 121.0, 120.0, 119,0, 117.8,
60.4 (s, C(CH3)3), 29.0 (s, C(CH3)3). 11B NMR (C6D5Br): -7.6 (s).
3
3
3
19F NMR (C6D5Br): d -131.4 (d, 2F, JFF = 23 Hz, m-ArCF),
-131.8 (d, 2F, 3JFF = 25 Hz, o-ArCF), -132.1 (d, 2F, 3JFF = 26 Hz,
3
o-ArCF), -162.0 (t, 2F, JFF = 21 Hz, p-ArCF), -162.3 (t, 1F,
3JFF = 21 Hz, p-ArCF), -166.4 (m, 2F, m-ArCF), -166.6 (m, 2F,
m-ArCF), -167.8 (m, 2F, m-ArCF). Anal. Calc’d: C 57.16, H 3.63,
N 4.88. Found: 57.03, H 3.50, N 4.85.
Synthesis of [ItBuH][H2NB(C6F5)3] (8)
To a solution of H3NB(C6F5)3 (0.050 g, 0.09 mmol) in toluene
(3 mL) was added ItBu (0.017 g, 0.09 mmol) in hexanes (2 mL).
After addition, a light yellow solid precipitated from solution. The
vial was rinsed with hexanes (3 ¥ 2 mL) and the reaction was stirred
for 5 minutes. The stirring was stopped and the solid was allowed
to settle for 30 minutes after which the solvent was decanted. The
remaining solid was dried to give the product as pale yellow solid.
Yield 0.064 g, 95%. 1H NMR (CD2Cl2): d 9.53 (s, 1H, N2CH), 7.42
Synthesis of Et2NHB(C6F5)3 (13)
To a solution of B(C6F5)3 (0.300 g, 0.59 mmol) in dry CH2Cl2
(3 mL) was added neat Et2NH (0.123 g, 1.68 mmol). The reaction
was stirred for 2 h. Solvent was removed in vacuo to obtain a white
powder. Yield: 0.340 g, 99%. X-Ray quality crystals were obtained
by layering pentane onto a toluene solution of the product and
cooling to -35◦C. 1H NMR (C6D5Br): d 5.20 (br, 1 H, NH), 3.18
(sextet, 1H, 3JHH = 7.0 Hz, NCHH), (septet, 1H, 3JHH = 7.0 Hz,
1
(s, 2H, =CH), 4.00 (br s, 2H, NH2), 1.72 (s, 18H, tBu). 13C{ H}
1
NMR (CD2Cl2): d 148.6 (d, JCF = 244 Hz, o-ArCF), 140.1 (d,
1
1JCF = 244 Hz, p-ArCF), 137.5 (d, JCF = 245 Hz, m-ArCF),
134.4 (s, N2CH), 128.7 (b, BC), 119.9 (s, NCH), 61.4 (s, C(CH3)3),
30.4 (s, C(CH3)3). 11B NMR (CD2Cl2): d -10.0 (br s, BNH2). 19F
NMR (CD2Cl2): d -135.5 (br, 6F, o-C6F5), -161.2 (br, 3F, p-C6F5),
-166.0 (br, 6F, m-C6F5). Anal. Calc’d: C 49.04, H 3.41, N 5.92.
Found: C 49.33, H 3.50, N 6.12.
1
NCHH), 0.99 (t, 6H, 3JHH = 7.0 Hz, NCH2CH3). 13C{ H} NMR
1
1
(C6D5Br): d 148.8 (d, JCF 237 Hz, o-ArCF), 147.7 (d, JCF
=
1
257 Hz, o-ArCF), 146.7 (d, JCF = 236 Hz, o-ArCF), 140.4 (d,
1JCF = 251 Hz, p-ArCF), 139.6 (d, 1JCF = 253 Hz, p-ArCF), 137.6
(d, 1JCF = 254 Hz, m-ArCF), 136.8 (d, 1JCF = 251 Hz, 2 overlapping
m-ArCF), 117.3 (br s, ArCB), 113.5 (br s, ArCB), 47.9 (s, NCH2),
14.3 (s, NCH2CH3). 11B NMR (C6D5Br): d -4.2 (br s). 19F NMR
(C6D5Br): d -126.9 (d, 2F, 3JFF = 20 Hz, o-ArCF), -128.2 (br s, 2F,
o-ArCF), -139.3 (br s, 2F, o-ArCF), -154.1 (t, 1F, 3JFF = 21 Hz,
Synthesis of [ItBuH][PhN(H)B(C6F5)3] (9)
To a solution of PhNH2B(C6F5)3 (0.052 g, 0.086 mmol) in C6D5Br
(0.5 mL) was added ItBu (0.015 g, 0.086 mmol) in an NMR tube.
The reaction was mixed. After 16 hours, the solvent was removed
in vacuo, triturated with pentane and the solvent was removed in
vacuo. The product was obtained as a white solid. Yield 0.063 g
3
p-ArCF), -156.0 (t, 2F, JFF = 21 Hz, p-ArCF), -160.8 (br m,
2F, m-ArCF), -162.0 (m, 2F, m-ArCF), -162.2 (m, 2F, m-ArCF).
Anal. Calc’d: C 45.16, H 1.89, N 2.39; Found: C 44.68, H 2.37, N
2.25.
1
(94%). H NMR (C6D5Br): d 7.75 (s, 1H, N2CH), 6.96 (t, 2H,
3JHH = 8 Hz, m-ArCH), 6.75 (s, 2H, =CHN), 6.60, (d, 2H, 3JHH
=
8 Hz, o-ArCH), 6.35 (t, 1H, 3JHH = 8 Hz, p-ArCH), 4.26 (s, 1H,
NMR scale preparation of EtN(H)B(C6F5)2 (14)
1
NH), 1.18 (s, 18H, C(CH3)3). 13C{ H} NMR (C6D5Br): d 152.4
A solution of EtNH2B(C6F5)3 (0.012 g, 22 mmol) in C6D5Br
(0.3 mL) was added to a solution of ItBu (0.004 g, 22 mmol)
in C6D5Br (0.3 mL). The reaction solution was placed in an NMR
tube and NMR spectra were obtained within 5 minutes of mixing.
1
1
(s, N2CH), 148.2 (d, JCF = 235 Hz, o-ArCF), 138.2 (d, JCF
=
1
248 Hz, p-ArCF), 136.3 (d, JCF = 239 Hz, m-ArCF) 128.4 (s,
m-ArCH), 128.2 (s, =CH), 119.9 (s, =CHN), 114.4 (s, o-ArCH),
114.1 (br s, BArC), 112.6 (s, p-ArCH), 60.1 (s, C(CH3)3), 28.7 (s,
C(CH3)3). 11B NMR (C6D5Br): d -9.7 (br s). 19F NMR (C6D5Br): d
-133.2 (d, 6F, 3JFF = 22 Hz, o-ArCF), -161.9 (t, 3F, 3JFF = 21 Hz,
p-ArCF), -165.9 (m, 6F, m-ArCF). Anal. Calc’d: C 53.52, H 3.47,
N 5.35. Found: C 53.25, H 3.42, N 5.23.
1H NMR (C6D5Br): d 5.41 (br s, 1H, NH), 2.92 (quin, 2H, 3JHH
=
3
7.0 Hz, NCH2), 1.00 (t, 3H, JHH = 7.0 Hz, NCH2CH3). d 11B
NMR (C6D5Br): d 33.3 (br s). 19F NMR (C6D5Br): d -132.0 (m,
2F, o-ArCF), -132.6 (m, 2F, o-ArCF), -150.8 (t, 1F, 3JFF = 21 Hz,
3
p-ArF), -152.2 (t, 1F, JFF = 21 Hz, p-ArF), -160.9 (m, 2F, m-
ArCF), -161.2 (m, 2F, m-ArCF).
Synthesis of [ItBuH][Ph2NB(C6F5)3] (10)
To a solution of Ph2NHB(C6F5)3 (0.137 g, 0.20 mmol) in hexane
(4 mL) and benzene (2 mL) was added a solution of ItBu (0.036 g,
0.20 mmol) in hexanes (2 mL). A white solid precipitated from
solution immediately on addition. The ItBu vial was rinsed with
hexanes (2 mL) then benzene (2 mL). The reaction was stirred for
16 hours after which all solids were removed in vacuo. Hexanes
(4 mL) was added to the solid and the solvent was removed in
vacuo to obtain the product as a white powder. Yield 0.170 g,
Synthesis of tBuN(H)B(C6F5)2 (15)
To a slurry of tBuNH2B(C6F5)3 (0.160 g, 0.27 mmol) in dry hexanes
(4 mL) was added a solution of ItBu (1 mg, 6 mmol, 2 mol%) in
dry hexanes (2 mL). The reaction was stirred at room temperature
for 2 h. Solvent was removed in vacuo to obtain a white powder.
1
Yield 0.111 g, 98%. H NMR (C6D5Br): d 5.63 (br s, 1 H, NH),
1
1
1.10 (s, 9H, tBuH). 13C{ H} NMR (C6D5Br): d 147.3 (d, JCF
=
1
1
98%. H NMR (C6D5Br): d 7.62 (br s, 1H, N2CH), 7.21 (d, 4H,
244 Hz, o-ArCF), 145.3 (d, JCF = 242 Hz, o-ArCF), 141.5 (d,
1JCF = 256 Hz, p-ArCF), 141.1 (d, 1JCF = 253 Hz, p-ArCF), 137.1
(d, 1JCF = 251 Hz, 2 ¥ overlapping m-ArCF), 111.2 (br s, ArCB),
52.8 (s, C(CH3)3), 31.3 (s, C(CH3)3). 11B NMR (C6D5Br): d 32.7
(br s). 19F NMR (C6D5Br): d -130.8 (m, 2F, o-ArCF), -133.2
3JHH = 8 Hz, o-ArCH), 6.98 (t, 4H, 3JHH = 8 Hz, m-ArCH), 6.77
(s, 2H, =CHN), 6.68 (t, 2H, 3JHH = 8 Hz, p-ArCH), 1.17 (s, 18H,
1
C(CH3)3). 13C{ H} NMR (C6D5Br): d 154.4 (s, N2CH), 149.9 (d,
1JCF = 240 Hz, o-ArCF), 148.3 (d, 1JCF = 240 Hz, o-ArCF), 138.7
(d, 1JCF = 230 Hz, p-ArCF), 136.7 (d, 1JCF = 245 Hz, m-ArCF),
3
(m, 2F, o-ArCF), -151.6 (t, 1F, JFF = 21 Hz, p-ArF), -153.0
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(t, 1F, 3JFF = 21 Hz, p-ArF), -161.3 (overlapping m, 4F, m-ArCF).
ArH), 7.14 (t, 3H, 3JHH = 8 Hz, ArH), 3.35 (d, 2H, 3JHH = 8 Hz,
CH2), 1.82 (t, 1H, 3JHH = 8 Hz, NH).
Satisfactory analytical data could not be obtained.
Synthesis of Et2NB(C6F5)2 (16)
X-Ray data collection and reduction
To a solution of Et2NHB(C6F5)3 (0.096 g, 0.19 mmol) in hexanes
(4 mL) and benzene (2 mL) was added ItBu (1 mg, 0.006 mmol,
3 mol%). The reaction was stirred for 12 hours after which the
solvent was removed in vacuo. The light oil was dissolved in
hexanes and solvent removed in vacuo to give the product as a light
orange oil. Yield 0.075 g, 96%. 1H NMR (C6D5Br): d 3.05 (q, 4H,
Crystals were manipulated and mounted in capillaries in a glove-
box, thus maintaining a dry, O2-free environment for each crystal.
Diffraction experiments were performed on a Siemens SMART
System CCD diffractometer. The data (4.5◦ < 2q < 45–50.0◦) were
collected in a hemisphere of data in 1329 frames with 10 second
exposure times. The observed extinctions were consistent with the
space groups in each case. A measure of decay was obtained by
re-collecting the first 50 frames of each data set. The intensities of
reflections within these frames showed no statistically significant
change over the duration of the data collections. The data were
processed using the SAINT and SHELXTL processing packages.
An empirical absorption correction based on redundant data was
applied to each data set. Subsequent solution and refinement was
performed using the SHELXTL solution package. The crystal
data are summarised in Table 1.
3
3JHH = 7.0 Hz, NCH2), 0.98 (t, 6H, JHH = 7.0 Hz, NCH2CH3).
1
1
13C{ H} NMR (C6D5Br): d 145.0 (d, JCF = 246 Hz, o-ArCF),
1
1
140.5 (d, JCF = 257 Hz, p-ArCF), 136.4 (d, JCF = 257 Hz,
m-ArCF), 111.0 (br s, BArC), 43.4 (s, NCH2), 14.1 (s, NCH2CH3).
11B NMR (C6D5Br): d 33.5 (br s). 19F NMR (C6D5Br): d -132.1
3
(m, 4F, o-ArCF), -152.9 (t, 2F, JFF = 21 Hz, p-ArCF), -161.1
(m, 4F, m-ArCF). Anal. Calc’d: C 46.08, H 2.42, N 3.36. Found:
C 46.23, H 2.39 N 3.24.
Synthesis of [ItBu(C6H5)CPh2][B(C6F5)4] (17)
Structure solution and refinement
To a stirred solution of [CPh3][B(C6F5)4] (0.153 g, 0.17 mmol) in
toluene (4 mL) was added ItBu (0.030 g, 0.17 mmol) in toluene
(2 mL). The mixture was stirred for 2 hours then all volatiles were
removed. The resulting light yellow solid was dissolved in CH2Cl2
(2 mL) then hexanes (10 mL) was added dropwise over 2 minutes.
A white solid precipitated and the mixture was stirred overnight.
The solution was decanted, and the solid dried in vacuo. Yield:
Non-hydrogen atomic scattering factors were taken from the
literature tabulations.27 The heavy atom positions were determined
using direct methods employing the SHELXTL direct methods
routine. The remaining non-hydrogen atoms were located from
successive difference Fourier map calculations. The refinements
were carried out by using full-matrix least squares techniques on F,
minimizing the function w (Fo - Fc)2 where the weight w is defined
as 4Fo2/2s (Fo2) and Fo and Fc are the observed and calculated
structure factor amplitudes, respectively. In the final cycles of each
refinement, all non-hydrogen atoms were assigned anisotropic
temperature factors in the absence of disorder or insufficient
data. In the latter cases atoms were treated isotropically. C-H
atom positions were calculated and allowed to ride on the carbon
to which they are bonded assuming a C-H bond length of
1
3
0.147 g, 80%. H NMR (CD2Cl2): d 7.50 (d, 1H, JHH = 4 Hz,
3
NCH), 7.44 (d, 1H, JHH = 4 Hz, NCH), 7.39 (m, 6H, m-ArH,
p-ArH), 7.22 (m, 4H, o-ArH), 6.80 (dofd, 2H, JHH = 10 Hz, 3 Hz,
=CH), 5.90 (t of t, 1H, JHH = 3 Hz, ItBuCH), 5.78 (dofd, 2H,
JHH = 10 Hz, 3 Hz, =CH), 1.81 (s, 9H, tBuH), 1.77 (s, 9H, tBuH).
1
13C{ H} NMR (CD2Cl2): d 148.2 (d, JCF = 283 Hz, o-CF), 146.3
(s, =CPh2), 144.5 (s, =C(CH)2), 140.6 (s, ipso-PhC), 138.2 (d,
JCF = 250 Hz, p-CF), 136.3 2 (d, JCF = 249 Hz, m-CF), 130.4 (s,
PhCH), 130.0 (s, =CH), 128.2 (s, PhCH), 128.0 (s, PhCH), 125.4
(s, N2C), 123.3 (s, =CH), 120.8 (s, NCH), 118.9 (s, NCH), 65.7
(s, tBuC), 63.4 (s, tBuC), 38.1 (s, ItBu-CH), 31.1 (s, tBuCH), 30.6
(s, tBuCH). 11B NMR (CD2Cl2): d -16.1. 19F NMR (CD2Cl2): d
-131.7 (br s, 8F, o-CF), -161.9 (t, 4F, 3JFF = 20 Hz, p-CF), -165.8
(m, 8F, m-CF). Anal. Calc’d for C54H36BF20N2: C 58.77, H 3.29,
N 2.54. Found: C 59.02, H 3.12, N 2.44.
˚
0.95 A. H-atom temperature factors were fixed at 1.10 times
the isotropic temperature factor of the C-atom to which they
are bonded. The H-atom contributions were calculated, but not
refined. The locations of the largest peaks in the final difference
Fourier map calculation as well as the magnitude of the residual
electron densities in each case were of no chemical significance. In
the case of (9), a twofold disorder of the cation was modelled
employing constrained C-N and C-C distances of 1.44 and
˚
1.54 A, respectively. In this portion of the model hydrogen atom
Formation of [ItBuH][B(C6F5)4] (18) and Ph3CNHCH2Ph (19)
contributions were not included. Additional details are provided
in the electronic supplementary information. ESI.†
In the glove box, a solution of NH2CH2Ph (3 mg, 27 mmol) in
C6D5Br (0.2 mL) was added to a solution of [CPh3][B(C6F5)4]
(25 mg, 27 mmol) in C6D5Br (0.3 mL). The reactants were mixed
for 20 minutes after which the solution was added to solid ItBu
(5 mg, 27 mmol). NMR spectra were obtained within 5 minutes of
mixing. Signals for both [ItBuH][B(C6F5)4] and Ph3CNHCH2Ph26
were found and confirmed by comparison to literature values.
Computational methods
Optimizations were performed with the GAUSSIAN (G98) suite.28
All molecules examined were fully optimized without constraints
at the HF level using either the 3-21G basis set or a modified
version of the 6-31+G(d) basis set where peripheral H atoms
were treated using the 3-21G basis set. Examination of the
optimized structures by analytical frequency analysis at this level
demonstrated them to be minima (no imaginary frequencies).
The structures were reoptimized using a two-layer ONIOM
approach,29–33 using the MPW1K DFT model34–36 and various
1
(18): H NMR (C6D5Br): d 7.75 (br s, 1H, N2CH), 6.80 (s, 2H,
=CH), 1.19 (s, 18H, tBuH) 11B NMR (C6D5Br): d -16.4. 19F NMR
(C6D5Br): d -130.9 (br s, 8F, o-CF), -161.4 (t, 4F, 3JFF = 21 Hz,
p-CF), -164.3 (m, 8F, m-CF).
1
3
(19): H NMR (C6D5Br): d 7.56 (d, 6H, JHH = 8 Hz, ArH),
3
7.34 (d, 2H, JHH = 8 Hz, ArH), 7.27–7.16 (m, 8H, overlapping
7182 | Dalton Trans., 2009, 7179–7188
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Table 1 Crystallographic dataa
(2)
(4)
(9)
(13)·0.25C6H14
Formula
C45H36BF15N2
900.57
Triclinic
P-1
9.1610(11)
11.9932(14)
19.725(2)
78.645(2)
85.995(2)
75.689(2)
2058.3(4)
2
25
1.453
0.0616
0.132
19967
C29H22BF15N2
694.30
Monoclinic
P21/c
15.4897(14)
9.7960(9)
20.1789(19)
C35H27BF15N3
785.41
Monoclinic
C2/c
18.552(2)
18.677(2)
22.029(2)
C23.5H13.5BF15N
605.66
Triclinic
P-1
10.7771(12)
12.3780(14)
19.156(2)
72.8740(10)
74.3570(10)
85.354(2)
2351.7(5)
4
-100
1.711
0.0513
0.183
22668
8258
730
0.0533
Formula weight
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
107.0520(10)
109.055(2)
3
˚
V (A )
2927.3(5)
4
25
1.575
0.0662
0.159
27461
5167
7215.1(13)
8
25
1.446
0.1676
0.139
34157
6356
Z
T (◦C)
d(calc) (g cm-3)
R(int)
Abs coeff, m (cm-1)
Data collected
2
2
Data Fo > 3s(Fo
)
7218
568
0.0533
0.1160
0.890
Variables
R (>3s)b
Rw (>3s)c
GOF
428
497
0.0444
0.1249
0.928
0.0769
0.1999
0.928
0.1231
0.820
ꢀ
ꢀ
ꢀ
ꢀ
1
2
a
2
2
2
Data collected with Mo Ka radiation (l = 0.71069 A). b R = (Fo - Fc)/ Fo. c Rw={ [w(Fo - Fc 2]/ [w(Fo) ]} .
˚
)
41
basis sets ranging from 6-31+G(d) to 6-311+G(d,p) for the high
levels, and the 3-21G basis set for the low levels.37 Single point
energies using these structures and the M0638 model (Table 2) were
determined using the GAMESS program.39,40 Relative energies
were corrected using unscaled zero point energies (ZPEs) from
the frequency analysis. Optimized Cartesian coordinates for
compounds investigated are available as ESI.†
˚
˚
1.663(5) A. This compares with the B-C distance of 1.6407(18) A,
found in the B(C6F5)3 adduct of 1,3,4,5-tetramethyl-1,3-imidazol-
˚
2-ylidene described by Power et al. and the distance of 1.649(3) A
observed by Tamm and co-workers,24 for the abnormal NHC
isomer containing a imidazolium-4-yl carbene-B(C6F5)3 adduct.
The longer B-C bond in (2) is consistent with the increased steric
demand of the isopropyl-substituted arene substituents on the N
of the carbene. This adduct (2) is robust exhibiting no reaction with
H2 or olefin. Nonetheless, it is not stable to for extended periods
in solution, decomposing to a complex mixture of unidentified
products.
Results and discussion
Frustrated Lewis pair: NHC’s and B(C6F5)3
Screening reactions of N-heterocyclic carbenes with Lewis acids
for frustrated Lewis pair reactivity began with the reaction of
1,3-bis[2,6-(di-iso-propyl)phenyl]-1,3-imidazol-2-ylidene (IDipp)
(1) with B(C6F5)3. The reaction was monitored by 11B NMR
spectroscopy revealing the formation of a new species (2) that
exhibited a resonance at -15.6 ppm. The 19F NMR spectrum of (2)
gave rise to 15 separate resonances inferring considerable molec-
ular dissymmetry, which was thought to result from restricted
1
rotation of the C6F5 rings. Similarly the H NMR spectrum of
(2) is also quite complex. These data suggest the formation of a
simple Lewis adduct formulated as (IDipp)B(C6F5)3 (Scheme 2).
An X-ray crystallographic study of (2) (Fig. 1) confirms this
formulation. The B-C dative bond in (2) was found to be
Scheme 2 Formation of the carbene-borane adduct (2).
Fig. 1 ORTEP drawing of (2).
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The analogous reaction of 1,3-di-tert-butyl-1,3-imidazol-2-
ylidene (3) with B(C6F5)3 was examined. At room temperature
a complex mixture of products was observed, one of which was
subsequently identified as a product of carbene rearrangement.24
However, these bypro◦ducts were not formed when the reaction
was performed at -60 C in toluene. Indeed 1H, 19F and 11B NMR
spectra revealed that no reaction at all was observed consistent
with the generation of a frustrated Lewis pair. It is clear that
the steric demands of the t-butyl substituents on N preclude
coordination of C to B.
a close approach of the Lewis acid and base is precluded by steric
demands. However, attractive interactions result in an “encounter
complex” which reacts subsequently with H2. A similar situation
was also computed for the case ItBu and B(C6F5)3.24 It is also
interesting to note that this product (4), fails to transfer H2 to
=
tBuN CPhH in contrast to [R3PH][HB(C6F5)3] where catalytic
reduction of imine was reported.7,8
Activation of N-H bonds
In an effort to further probe the activation of bonds by car-
bene/borane combinations, a series of amine adducts including
Activation of H2
25
H3NB(C6F5)3 (5), PhH2NB(C6F5)3 (6) and PhH2NB(C6F5)3 (7)
were prepared. Reactions of these adducts with the carbene
(3) were studied. In each case, these reactions resulted in the
deprotonation of the Lewis acid bound amine, giving rise to the
new species (8)–(10), respectively (Scheme 4). These imidazolium
salts exhibit imidazolium protons at 9.53, 7.75 and 7.62 ppm,
Given the previously established activation of H2 by phosphine-
borane frustrated Lewis pairs, H2 was added to the present
mixture of carbene and borane.24 At -78 ◦C this addition results
in the clean formation of the salt [ItBuH][HB(C6F5)3] (4) in
97% isolated yield (Scheme 3) as proved by the observation of
1H NMR resonances at 8.20 and 3.63 ppm attributable to the
imidazolium and hydridoborate hydrogen atoms, respectively. In
addition the 19F and 11B NMR spectra were consistent with the
presence of the borate anion [HB(C6F5)3]-. X-Ray crystallography
also confirmed the formulation of (4) (Fig. 2). The structural
data for (4) is as expected8,42 with the closest ion-ion contact
being between the BH hydride and a proton of the imidazolium
1
respectively, while the 11B{ H} NMR resonances were observed
at -10.0, -9.7 and -7.6 ppm, respectively. These latter shifts are
in accord with the formation of the amido-borate anions. In the
case of [ItBuH][H2NB(C6F5)3] (8) and [ItBuH][PhHNB(C6F5)3]
(9) simple 19F NMR spectra are observed consisting with the
expected ortho, meta and para fluorines of the anion. In con-
trast, for (10) [ItBuH][Ph2NB(C6F5)3], a much more complicated
spectrum was observed at room temperature consistent with
the proposition of restricted rotation of the diphenylamido
group. The nature of these salts was supported by an X-ray
crystallographic study of (9) (Fig. 3) despite a twofold disorder
˚
tBu (2.592 A). The species (4) is thermally robust showing no
evidence of decomposition upon heating to 150 ◦C for 4 days.
The formation of (4) is interesting in that neither N-heterocyclic
carbenes nor B(C6F5)3 alone react with H2.4,6,21 Computational
studies of the reactions of phosphine and borane6,43 suggest that
Scheme 3 Activation of H2 by borane and carbene.
Scheme 4 Activation of NH bonds by borane and carbene.
Fig. 2 ORTEP drawing of (4).
Fig. 3 ORTEP drawing of (9).
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of the [ItBu]+ cation. The anion was fully ordered and revealed
a B-N distance is 1.532(8) A, significantly shorter than those
centered around 33 ppm consistent with the three-coordinate
monomeric amidoboranes. The steric congestion in these products
as well as the dissymmetry of the primary amido-derivatives results
in restricted rotation about the B-N bond as is shown by the
19F NMR spectra. Restricted rotation is attributed in part to
the significant p-character of the B-N interaction resulting from
donation of the N lone pair to the empty B pz-orbital.47,48
The apparent regeneration of the carbene in the formation
of (14)–(16) suggests that its role is catalytic in these reactions.
Indeed this was confirmed as these products could be formed
efficiently in the presence of 2–3 mol% of (3) in 2 hours at 25 ◦C.
Given the formation of (7)–(9), it is reasonable to suggest that
the role of the carbene is to first deprotonate the coordinated
amine, generating a transient imidazolium amido-borate salt. The
resulting electron rich and sterically crowded amido-borate is then
apparently susceptible to reaction with the imidazolium resulting
in the elimination of C6F5H affording the products (14)–(16) and
regenerating the carbene. This result stands in contrast to the work
of Bertrand et al. who found NHC carbenes alone unreactive
towards NH bonds.21 It is noteworthy however, that Clyburne and
co-workers did report carbene C–HN interactions between IMes
and HNPh2.49
˚
seen in the amido borates [Na(OEt2)4][H2N{B(C6F5)3}2] (1.628(3)
44
45
˚
˚
and 1.636(3) A), [LiOEt2][C4H4NB(C6F5)3] (1.576(4) A) and
46
˚
[HNEt3][indolateB(C6F5)3] (1.565(4) A). The formation of these
salts results from the basicity of the carbene and its inability to
displace amine from the coordination sphere of B(C6F5)3.
In the same vein the amine-borane adducts EtNH2B(C6F5)3 (11)
tBuNH2B(C6F5)3 (12) and Et2NHB(C6F5)3 (13) were prepared.
The spectra data for the adducts (11) and (12) were in accord with
those previously published. In the latter case the 19F NMR data
were consistent with 2:1 inequivalence of the C6F5 rings, suggesting
NH ◊ ◊ ◊ F interactions similar to those previously described by
Lancaster and co-workers.25 Indeed this notion was subsequently
confirmed by X-ray crystallography of (13) (Fig. 4) in which the
NH moiety is positioned between two of the arene rings giving
rise to interactions with the two of the ortho-fluorines at distances
˚
˚
of 2.108 A and 2.107 A. The B-N bond distance is typical at
˚
1.650(5) A.
Frustrated Lewis pair: NHC’s and [CPh3]+
We have also explored the utility of the cation [CPh3]+ in frustrated
Lewis pair chemistry as it is isolobal with Lewis acidic boranes.
Reaction of [CPh3][B(C6F5)4] with the bulky NHC (3) in toluene at
room temperature results in an immediate reaction affording a new
ionic product (17). New signals in the 1H NMR spectrum at d 6.80
and 5.78 are resolved into doublets of doublets, each integrating
for 2 protons. Additionally, COSY analysis reveals that a triplet
of triplets at 5.90 ppm is coupled to both of the aforementioned
signals. Two singlets at 1.81 and 1.77 ppm, are attributable to in-
1
equivalent t-butyl groups of (3). The 13C{ H} NMR spectrum also
displays separate resonances for the tert-butyl groups at 65.7 and
63.4 and 31.1 and 30.6 ppm for the quaternary and terminal methyl
groups, respectively. The NHC backbone carbons are observed at
120.8 and 118.9 ppm. Collectively these data result in the formu-
lation of (17) as [C3H2N2tBu2(C6H5)CPh2][B(C6F5)4] (Scheme 6).
The steric bulk of the NHC results in restricted rotation about
the link to the cyclohexadiene fragment resulting in the molecular
dissymmetry. Such nucleophilic substitutions at the para-position
of a phenyl ring of the trityl cation have been described before. For
example bulky phosphines have been shown to result in analogous
chemistry.50 While in some cases, migration of the para-hydrogen
Fig. 4 ORTEP drawing of (13).
The stoichiometric reactions of these latter adducts with
the carbene (3) results in the formation of the amidoborane
EtHNB(C6F5)2 (14) tBuHNB(C6F5)2 (15) and Et2NB(C6F5)2 (16)
respectively. The 19F and 1H NMR spectra of the resulting
reaction mixtures reveal the presence of these products together
with an equivalent of C6F5H and an equivalent of unreacted (3)
(Scheme 5). The 11B NMR spectra of (14)–(16) show broad signals
Scheme 5 Catalytic activation of NH bonds by borane and carbene.
Scheme 6 Reaction of trityl-cation with (3).
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Table 2 Computational data: energetics [M06/6-311++G(d,p), cor-
to the formally cationic carbon follows affording a phosphonium
cation, such migration does not occur for the NHC derivative.
Attempts to exploit other known frustrated Lewis pair reactions
using the trityl/carbene pair were complicated by the rapidity of
this para-substitution. For example, neither alkene activation with
1-hexene51 nor ring opening of THF4,24,52 were observed between
-95 ◦C and room temperature. Instead only the formation of
(17) was observed. Similarly attempts to activate H2 with the
[CPh3][B(C6F5)4]/(3) were also circumvented by the preferred
para attack reactivity. However, evidence for FLP-like reactivity
was obtained in the reaction of an amine adduct of the trityl
cation with the bulky NHC (Scheme 7). Similar to reactions
with the B(C6F5)3/(3) mixture, addition of benzyl amine to
[CPh3][B(C6F5)4], results in the formation of the benzylamine
adduct of the trityl cation. Addition of a stoichiometric amount
of (3) affords products derived from deprotonation of the NH
group, giving the known compounds [ItBuH][B(C6F5)4] (18) and
the secondary amine Ph3CNHCH2Ph (19) as shown by 1H NMR
data. This is, to our knowledge, the first example of an all carbon-
based frustrated Lewis pair.
rected for zero point energy, kcal mol-1]
Energy/kcal
Reactant
Products
mol-1
(6) + (3)
(11)+ (3)
[ItBuH][EtHNB(C6F5)3]
[ItBuH][PhHNB(C6F5)3]
-10.5
-22.0
11.5
[ItBuH][EtHNB(C6F5)3] (14) + C6F5H + ItBu
[ItBuH][PhHNB(C6F5)3] (C6F5)2BNHPh + C6F5H + ItBu 20.8
is expected to be more pronounced for the more-donating
alkylamine systems as compared to arylamine derivatives, thus
accounting for the differing reactivity.
Computation data (Table 2) were obtained by considering
reactions of (6) and (11) with (3). Formation of the ion pairs
is exothermic for both amine-boranes, with the arylamidoborate
anion being more exothermic, presumably a result of the electron
withdrawing nature of the phenyl ring, stabilizing the anion.
Remarkably the model indicates that the dearylation step is almost
exactly as endothermic as ion formation is exothermic, making
the overall formation of C6F5H and ethylamidoborane slightly
endothermic and the formation of C6F5H and phenylaminoborane
slightly exothermic. As this is precisely counter to experimental ob-
servation, this aspect continues to be the subject of computational
study.
The structures of the ion pairs were also probed. The imida-
zolium cation was placed on the face of the pseudo-tetrahedron
around boron formed by three C6F5 substituents to consider the
delivery of a proton to a B-C6F5 bond. In this geometry, there was
no computational evidence for protonation suggesting “backside”
attack of the B-C is not an operative mechanism. Instead, it seems
that the carbene may mediate the protonation of the B-C bond
in the transient amine-borane encouraging liberation of C6F5H
(Scheme 9).
Scheme 7 Reaction of trityl-amine adduct with (3).
Computational and mechanistic considerations
In an effort to gain some insight into the N-H activation
reactions, the reactions of amine-borane adducts with the carbene
(3), these systems were probed computationally. The proposed
mechanism outlined in Scheme 8 involves deprotonation of the
amine moiety to form the ion pairs, followed by (for alkylamine-
boranes) transfer of the proton from carbene cation to the
B-C6F5 environment, leading to loss of C6F5H and generation
of the amido-borane. This suggests that localization of negative
charge on the amidoborate anion at boron as a result of the
electron-withdrawing substituents makes the boron/B-C bond
susceptible to protonation. Such a distribution of electron density
Scheme 9 Proposed ion-pairing geometry prompting protonation of a
C6F5 substituent.
This view is consistent with the notion that the strength of
the B-C6F5 bond in the amido-borate anion should decrease with
increasing donor ability of the amine, while the strength of the
=
resulting B N p bond in the product should increase. The relative
donor strengths of ethyl- and phenylamine to the B center was
examined by calculating the energies of the dative bond-breaking
reactions:
(C6F5)3B - NH2R → (C6F5)3B + NH2R (R = Et, Ph)
The values were 29.7 and 22.0 kcal mol-1 for R = Et and
Ph, respectively, consistent with expectations. The strength of
the p bond in the two aminoboranes was also calculated in
the usual way,53–56 by comparing the energies of conformations
of amido-boranes in which the plane of the NHR moiety was
Scheme 8 Mechanism considered computationally.
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fixed perpendicular to that of the BC2 moiety.57 The values were
calculated to be 31.3 and 22.9 kcal mol-1 for R = Et and Ph,
respectively, consistent with a stronger p bond for the more
donating amine substituent. Overall, these factors dictate that the
liberation of C6F5H from the ethylamine-borane reaction should
be exothermic by ca. 1.6 kcal mol-1, while the phenylamine-borane
reaction should be exothermic by ca. 0.9 kcal mol-1. While the
trend in these observations is similar to experimental observation,
clearly the models do not account for subtle positional effects
between cations and anions in the ion pairs which are manifested
in the kinetics associated with the mechanism.
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Conclusions
In conclusion, the combinations of the N-heterocyclic carbenes
and a Lewis acid, either B(C6F5)3 or trityl cation have been
shown to react as frustrated Lewis pairs. H-H bond cleavage as
well as N-H bond cleavage has been achieved by these systems.
The N-H bond activation is shown to depend on the amine
substituents and in the case of electron donating amines, loss
of C6F5H to give amino-boranes is observed. Mechanistically,
carbene mediates this latter reaction via a transient imidazolium
amidoborate species. Nonetheless, computational studies infer
the presence of additional subtleties controlling this elimination
process. We are continuing to study these and related frustrated
Lewis pairs. In particular we are interested in the activation of a
variety of small molecules and the application of these systems in
catalytic processes as well as mechanistic details.
Financial support from NSERC of Canada is gratefully ac-
knowledged. DWS is grateful for the award of a Canada Research
Chair.
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57 Optimization led to structures containing pyramidal geometries around
N, so the comparison involves two issues: the energy associated with
loss of the p bond and that associated with pyramidalization of the
nitrogen. One generally assumes that the latter is identical between
molecules in such a comparison. One could fix the B-NHR moiety to
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7188 | Dalton Trans., 2009, 7179–7188
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The Royal Society of Chemistry 2009
©