Heterolytic activation of hydrogen using frustrated Lewis pairs containing tris(2,2′,2″-perfluorobiphenyl)borane

Article abstract of DOI:10.1039/c2dt30334e

The extremely sterically hindered borane tris(2,2′,2″- perfluorobiphenyl)borane (PBB) has been structurally characterised. In combination with bulky nitrogen bases, it forms the 'frustrated Lewis pairs' (FLPs) PBB/2,2,6,6-tetramethylpiperidine (TMP) (1),

Full text of DOI:10.1039/c2dt30334e

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Frustrated Lewis Pairs
Guest Editor: Douglas Stephan
Published in issue 30, 2012 of Dalton Transactions
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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
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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PAPER
Heterolytic activation of hydrogen using frustrated Lewis pairs containing
tris(2,2′,2′′-perfluorobiphenyl)borane†
Samantha C. Binding, Hasna Zaher, F. Mark Chadwick and Dermot O’Hare*
Received 14th February 2012, Accepted 28th March 2012
DOI: 10.1039/c2dt30334e
The extremely sterically hindered borane tris(2,2′,2′′-perfluorobiphenyl)borane (PBB) has been
structurally characterised. In combination with bulky nitrogen bases, it forms the ‘frustrated Lewis pairs’
(FLPs) PBB/2,2,6,6-tetramethylpiperidine (TMP) (1), PBB/1,4-diazobicyclo[2.2.2]-octane (DABCO) (2)
and PBB/2,6-lutidine (lut) (3). These novel, unquenched acid–base pairs have been shown to effect
facile room temperature heterolytic cleavage of dihydrogen to form the ammonium borate salts
[2,2,6,6-Me4C5H6NH2][HB(C12F9)3] (4) and [N(C2H4)3NH][HB(C12F9)3] (5), and lutidinium borate
[2,6-Me2C5H3NH][HB(C12F9)3] (6). Although these reactions are equilibria, the reverse reaction and
release of hydrogen gas was not apparent at temperatures up to 120 °C. The relative Lewis acidity of PBB
has been determined using the Gutmann–Beckett method.
adducts and their FLPs, or can be accessed through thermal dis-
Introduction
sociation of an adduct,14 both classical and frustrated reactivity
can be exploited.1,15,16,49
Although there has been extensive variation in the acidic and
In 2006, Welch and Stephan published the first example of a
metal-free system that reversibly and cleanly cleaves hydrogen.1
The zwitterionic phosphonium hydridoborate salt R2PH-(p-
C6F4)-BHR′2 [R = 2,4,6-C6Me3H2, R′ = C6F5] loses hydrogen
upon heating at 150 °C, and reacts with H2 gas at room tempera-
ture to regenerate the zwitterion. Prior to this exciting discovery,
there are few examples of non-metal mediated activation of
hydrogen,2–5 with the only non-metal p-block mediated heteroly-
tic splitting under ambient conditions involving metalloid germa-
nium compounds.6
basic partners, most investigations into the nature of the Lewis
acid involve changing C6F5 groups of tris(pentafluorophenyl)-
borane, B(C6F5)3, for substituents with different steric and
electronic requirements.38,45,49–52 Tris(2,2′,2′′-perfluorobiphenyl)-
borane (PBB) increases the steric bulk on the borane compared
to B(C6F5)3, while maintaining an electronegativity at the ortho-
position which is comparable to a fluorine substituent.54
PBB is a cocatalyst or activator in Ziegler Natta homogeneous
olefin polymerisation, exhibiting higher activities than its
B(C6F5)3 analogue and yielding more desirable polymers with
higher molecular weights and narrow polydispersities.51,52
Herein, we report the molecular structure of PBB and report its
use for the first time in forming frustrated Lewis pairs with a
series of nitrogen bases. We document the ability of these
systems to effect the heterolytic activation of dihydrogen and
their subsequent reactivity with carbon dioxide.
Subsequently, combinations of B(C6F5)3 with phosphines,7–11
bulky amines,12–14 pyridines,15–17 imines,12,18 carbonyls,19
carbenes,20–24 and phosphinoalkylboranes8 have all been used to
successfully cleave dihydrogen. This is attributed to their ability
to form ‘frustrated Lewis pairs’ (FLPs), where bulky substituents
on an electron pair acceptor and donor preclude the formation of
a dative bond by preventing close approach of their respective
acidic and basic centres.9 FLPs have been shown to activate an
important and significant range of small molecules, with
additions to carbonyls,25 CO2,26–32 N2O,33 and unsaturated
systems,10,25,30,34–39 the ring opening of heterocycles,16,24,40–43
and cleavage of disulfide bonds being reported.44 One of
their most attractive features is their ability to effect catalytic
hydrogenation of unsaturated substrates under mild
conditions.12,18,45–48 Where equilibria exist between Lewis
Results and discussion
PBB was synthesised from bromopentafluorobenzene according
to the procedure developed by Marks and co-workers.56 Colour-
less single crystals suitable for X-ray crystal structural analysis
were obtained by slow cooling of a concentrated hot hexane
solution to 20 °C. PBB crystallises in pairs of non-identical
molecules, and the crystal structure shows the propeller confor-
mation about a trigonal planar boron atom expected for ortho-
substituted triaryl boranes (Fig. 1).57 B–C bond lengths and
angles between the reference plane containing the BC3 fragments
and the mean plane of the attached C6F4 rings are expressed in
Inorganic Chemistry Laboratory, University of Oxford, South Parks
Road, Oxford, OX1 3QR. E-mail: dermot.ohare@chem.ox.ac.uk;
Tel: +01865 272686
†Electronic supplementary information (ESI) available: ORTEP drawing
and significant bond lengths for other PBB molecule in unit cell. CCDC
866505. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt30334e
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9061–9066 | 9061

Fig. 1 Molecular structure drawing of B(C12F9)3, PBB, showing 50%
thermal ellipsoids for one of the two crystallographically unique mol-
ecules. Fluorine atoms on two C12F9 groups have been omitted for
clarity.53 Selected bond lengths for displayed molecule: B(41)–C(58)
1.57(1); B(41)–C(100) 1.59(1); B(41)–C(105) 1.583(9) Å. Angle
between mean planes B(41)C3–C(58) 42.7(3)°; B(41)C3–C(100)
41.6(3)°; B(41)C3–C(105) 48.9(4)°.
Scheme 1 Reactions of tris(2,2′,2′′-perfluorobiphenyl)borane with
nitrogen bases and dihydrogen.
lutidinium borate [2,6-Me2C5H3NH][HB(C12F9)3] (6) respect-
ively. No coupling is observed between fluorine and boron
nuclei in the 1H NMR spectra, while fast exchange of H+
between nitrogen species in solution prevented NH signals being
observed in the salts (expected around 4.32,13 10.20,59 and
12.01 ppm16 for 4, 5 and 6 respectively).
Fig. 1. The B–C bond lengths ranging from 1.56(1) to 1.59(1) Å
are comparable to the B–C distances of 1.573(4) and 1.580(4) Å
seen in BMes3 (Mes = 2,4,6-C6H2Me3),57 and to the analogous
B–C bond length of 1.57(2) Å in bis(pentafluorophenyl)-
(2-perfluorobiphenyl)borane, BPB, (C6F5)2BC6F4(o-C6F5).58
The angles of rotation out of the reference plane containing the
BC3 fragment, ranging from 41.6(3)° to 48.9(4)°, are smaller
than the analogous 49.1° or 51° angles in BMes3, suggesting
more favourable pπ-donation from fluorine substituents into the
p-orbital of boron.57
To maximise the yields of 4 and 6 required heating of the
sealed reactions at 90 °C for 72 h. The maximum spectroscopic
yields of 4 and 6 were 28% and 29% respectively, determined
by 19F integration relative to residual starting material. Strong
BH doublets were observed by 11B NMR spectroscopy at
−18.41 ppm (4) and −18.42 ppm (6). These borohydride shifts
are more positive than the analogous shifts in [HB(C6F5)3]− at
−24.13 ppm and −24.7 ppm, and more similar to the −18.8 ppm
value observed for the borohydride [PhC2H4BH(C6F5)2]−.51 The
parent borane of this latter borohydride is found to be a stronger
Lewis acid than B(C6F5)3,51 suggesting PBB becomes more
Lewis acidic upon ortho-substitution of a fluorine for C6F5.
In the reaction between 2 and H2, the borohydride salt formed
after 48 h stirring at 20 °C has the same spectroscopic features as
products of the reactions of 1 and 3 with hydrogen. Upon
cooling to −20 °C, 5 precipitated as a white solid, an analytically
pure sample can be isolated by decanting the supernatant, and
washing with pentane. The five 19F NMR resonances from the
C6F5 rings each divide in a 1 : 1 ratio, indicating restricted C–C
rotation between perfluorinated rings and resulting in inequiva-
lent C12F9 environments. Following isolation and characteris-
ation, 5 was redissolved in toluene and again placed under a
hydrogen atmosphere and heated to 90 °C. The 19F NMR
spectra after 24 hours indicated complete loss of the initial salt
The reactions of PBB with an equimolar amount of each
of the nitrogen bases 2,2,6,6-tetramethylpiperidine (TMP), 1,4-
diazobicyclo[2.2.2]octane (DABCO) and 2,6-lutidine (lut) in
1
toluene were monitored by H, 19F and 11B NMR spectroscopy.
No adduct formation was apparent in all three cases, with reson-
ances characteristic of the separate starting materials consistent
with formation of the new, coloured ‘frustrated Lewis pairs’
PBB/TMP (1), PBB/DABCO (2) and PBB/lut (3) (Scheme 1).
Introduction of hydrogen (1 atm) to the NMR samples of each of
these systems resulted in immediate reaction (as observed by
19F NMR) at room temperature. The 19F NMR spectra of the
anions suggest immediate formation of a tetrahedral borohydride
anion, [HPBB]−, with nine new 19F resonances accompanying
the dominant seven of unreacted PBB. The spectroscopic sig-
natures of the borohydride species exhibit similar features to
those seen for [MePBB]−,55,56 and from this we can infer for-
mation of the ammonium borate salts [2,2,6,6-Me4C5H6NH2]
[HB(C12F9)3], (4), and [N(C2H4)3NH][HB(C12F9)3] (5), and
9062 | Dalton Trans., 2012, 41, 9061–9066
This journal is © The Royal Society of Chemistry 2012

CO2 to the FLP, nor insertion into the B–H bond being clearly
apparent. These results contrast to the B(C6F5)3 analogues of
4 and 6 which successfully reduce carbon dioxide via insertion
into the B–H bond,56,57 and to the unquenched acid–base
systems B(C6F5)3/PtBu3 and (C6H2Me3)2P(C6F4)B(C6F5)2 which
can activate CO2 directly, forming new B–O and P–C bonds.26,28
The formates 7 and 8 were heated under an atmosphere of
CO2, and the reappearance of the borohydride doublet was
observed via 11B NMR spectroscopy at 145 °C, indicating rela-
tively facile decarboxylation, and supporting the failure of the
reverse insertion reaction. Dehydrogenation to the FLP was not
observed however.
As the PBB ortho-C6F5 groups render the boron centre less
sterically accessible than in B(C6F5)3, we therefore carried out
NMR studies to determine how this affects the Lewis acidity of
the borane. The Lewis acidity of boranes is commonly deter-
mined via two NMR methods. Using the Gutmann–Beckett
method, which has been adapted to solutions of boranes by Brit-
ovsek et al.,62–66 the difference in 31P{1H} chemical shift
between uncoordinated Et3PO (50.41 ppm) and the Et3PO →
Lewis acid adduct (80.68 ppm) is measured, giving a Δδ =
30.27 ppm and an Acceptor Number (AN) for PBB of 87.90.‡65
This indicates PBB is a stronger Lewis acid than B(C6F5)3
(AN 79.8).61,62 The same trend was observed by Marks et al.
using Child’s method, where deshielding of proton H3 in trans-
crotonaldehyde occurs upon coordination to a Lewis acid.67–69
Hence it is postulated that the o-C6F5 substituent has a greater
inductively withdrawing effect through the sigma framework
than an o-F, causing boron to be more deshielded in PBB anions
compared to B(C6F5)3.
Fig. 2 19F NMR spectra showing formation of [HPBB]− resonances
(★) of (a) 4 after heating at 90 °C for 24 h, (b) 6 after heating at 90 °C
for 24 h, (c) 5 after 24 h at 20 °C, and (d) 5 after heating at 90 °C for 3
days. Unlabelled resonances correspond to unreacted PBB.
Scheme 2 Reaction scheme showing the formation of the boroformate
salts [TMPH][HCO2PBB], (7) and [lutH][HCO2PBB], (8).
Experimental
[DABCOH][HPBB] (5), and the growth of nine new resonances
(Fig. 2d). A shift of the borohydride doublet from −18.58 ppm
to −22.92 ppm (1JBH = 94.3 Hz) suggests the boron nucleus in
the new species is less strongly deshielded and further identifi-
cation and characterisation of this product is underway.
Unlike mixtures of B(C6F5)3 and TMP, which cleave H2 quan-
titatively at 20 °C,13 FLP 1 requires heating and does not go
to completion. This corroborates work by Soós et al. where
it was found TMP in combination with the modified borane
(C6F5)2BMes yields only one sixth of the hydrogen-cleaved
product compared to its DABCO analogue.59
The adduct of lut–B(C6F5)3 exists in equilibrium with the dis-
sociated acid–base pair at room temperature,16 yet despite this,
still undergoes reaction with H2 at room temperature in 87%
yield. The increased bulkiness of 3 on the other hand causes it to
exist purely in the unquenched state, and again requires heating
to effect a reaction with dihydrogen.
The CO2 insertion reactivity of the salts 4 and 6 was sub-
sequently investigated. As a comparison, the anticipated product
formates [TMPH][HCO2PBB], (7), and [lutH][HCO2PBB], (8),
were independently synthesised by adding 1 : 1 solutions
of TMP–formic acid or lut–formic acid in toluene to PBB
(Scheme 2) and were characterised by NMR spectroscopy.
However, preliminary investigations into the reactivity of 1, 3, 4
and 6 with carbon dioxide shows only degradation of the boro-
hydride signals in the 19F NMR spectrum, with no binding of
General data
Air and moisture sensitive reactions were performed on a dual-
manifold vacuum/N2 line using standard Schlenk techniques, or
in a N2 filled MBraun Unilab glovebox. Hexane, pentane and
toluene were dried using a Braun SPS-800 solvent purification
system. Et2O was dried at reflux over Na/benzophenone and dis-
tilled under N2. Dry solvents were stored under N2 over K
mirrors in oven dried ampoules with a Rotaflo cap. H2 gas
(>99.95% dry) from Sigma Aldrich and CO2 gas (99.99%) from
ARGO International Ltd were passed directly into a dual mani-
fold Schlenk line. Deuterated NMR solvents were purchased
from Goss Scientific, dried and freeze–pump–thaw degassed
(×3) over the appropriate drying agent: C7D8 (99.6% D)(K),
CD2Cl2 (99.8% D)(activated 3 Å molecular sieves). All other
organic reagents were purified by conventional methods unless
otherwise stated. BCl3 (1.0 M in hexanes), nBuLi (1.6 M in
hexanes), 2,2,6,6-tetramethylpiperidine (>99%), and 1,4-diazabi-
cyclo[2.2.2]octane (DABCO) were purchased from Sigma
Aldrich, and 2,6-lutidine (>98%) from Alfa Aesar. Bases were
dried as follows: TMP and lutidine were freeze–pump–thaw
degassed (×3) and stored over 3 Å molecular sieves in the glove
‡AN = [Δδ]/[δ(Et3PO → SbCl5) − δ(Et3PO in hexane)] × 100; Δδ =
δ(Et3PO → LA in CD2Cl2) − δ(Et3PO uncoordinated in hexane); 3 : 1
excess of LA to Et3PO.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9061–9066 | 9063

1
3
box. DABCO was sublimed under vacuum. H, 19F, 31P and 11B
6.56 (2 H, d, JHH = 7.7, CH), 2.39 (6 H, s, CH3); 19F NMR
NMR measurements were recorded on 300 MHz Varian
(282 MHz, C7D8) δ ppm −119.84 (3F, s, br), −132.05 (3 F, s,
1
3
4
VX-Works spectrometers. H shifts are referenced internally to
br), −137.57 (6 F, s, br), −143.73 (3 F, td, JFF = 20.7, JFF =
residual proteo-solvent, relative to TMS (δ = 0); 19F, 31P and 11
B
9.5), −149.25 (3 F, t, 3JFF = 21.3), −150.66 (3 F, td, 3JFF = 21.3,
4JFF = 6.5), −160.82 (6 F, t, 3JFF = 17.2).
shifts were referenced externally to CFCl3, 85% H3PO4 (δ = 0)
and BF3·OEt2 respectively. J values are given in Hz.
NMR scale synthesis of [2,2,6,6-Me4C5H6NH2][HB(C12F9)3]
4, [N(C2H4)3NH][HB(C12F9)3] 5, and [2,6-Me2C5H3NH][HB-
(C12F9)3] 6. These compounds were synthesised in a similar
manner and thus only one preparation is described. A Young’s
tap NMR tube containing the FLP 1 (0.040 mmol) in 0.7 ml
d8-toluene was freeze–thaw–degassed (×3) and backfilled with
H2 (1 atm). The sealed reaction was heated to 90 °C for 3 days.
4. 11B NMR (96 MHz, C7D8) δ ppm −18.41 (d, 1JBH = 82.2);
1H NMR (300 MHz, C7D8) δ ppm 3.31 (1 H, s, br, BH), 1.39 (2
X-ray data collection, reduction, solution and refinement
A typical crystal was mounted on MiTeGen MicroMounts using
perfluoropolyether oil and cooled rapidly to 150 K in a stream of
N2 using an Oxford Cryosystems CryoStream unit.70 Data were
collected with an Enraf-Nonius KappaCCD diffractometer, using
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Raw
frame data were reduced using the DENZO-SMN package.71
Intensity data were corrected using multi-scan method with
SCALEPACK (within DENZO-SMN). The structure was solved
using direct methods with SIR9272 and refined using full-matrix
least squares refinement on all F2 data using the CRYSTALS
program suite.73,74 The structure was found to be a non-merho-
dal twin due to a 180 degree rotation about the a-axis.75 In
general distances and angles were calculated using the full var-
iance–covariance matrix; dihedral angles were calculated using
PLATON.76
3
3
H, quin, JHH = 5.4, CH2), 1.12 (4 H, t, JHH = 5.4, CH2), 0.89
(12 H, s, CH3); 19F NMR (282 MHz, C7D8) δ ppm −130.12
(3 F, s, br), −139.38 (3 F, dd, 3JFF = 20.7, 4JFF = 13.0), −141.98
3
3
(3 F, d, JFF = 21.6), −142.22 (3 F, d, JFF = 22.4), −155.51
(3 F, t, 3JFF = 21.1), −159.01 (3 F, t, 3JFF = 22.4), −162.72 (3 F,
3
3
t, JFF = 21.1), −163.91 (3 F, t, JFF = 21.1), −165.03 (3 F, td,
3JFF = 21.0, 4JFF = 6.9).
5. 11B NMR (96 MHz, C7D8) δ ppm −18.58 (d, 1JBH = 86.3);
1H NMR (300 MHz, C7D8) δ ppm 13.07 (1 H, s, br, NH), 2.27
(12H, s, br, CH2); 19F NMR (282 MHz, C7D8) δ ppm −138.43
(3 F, dd, 3JFF = 21.6, 4JFF = 12.5), −142.44 (3 F, d, 3JFF = 22.0),
Synthesis of tris(2,2′,2′′-perfluorobiphenyl)borane (PBB).55
B(C12F9)3 PBB was synthesised according to a literature pro-
cedure and single crystals were grown by slow cooling of a hot
hexane solution to 20 °C.
3
4
3
−142.65 (3 F, dd, JFF = 23.0, JFF = 7.0), −155.05 (3 F, t, JFF
3
3
= 21.0), −159.71 (3 F, t, JFF = 22.6), −162.15 (3 F, t, JFF
=
3
4
21.0), −163.68 (3 F, tt, JFF = 21.7, JFF = 6.5), −164.80 (3 F,
td, 3JFF = 21.6, 4JFF = 7.8).
Crystal structure determination of PBB
6. 11B NMR (96 MHz, C7D8) δ ppm −18.42 (d, 1JBH = 84.3);
ˉ
Crystal data. C36F27B, triclinic (P1), a = 10.4698(3), b =
1H NMR (300 MHz, C7D8) δ ppm 7.05 (1 H, t, JHH = 7.6,
3
16.4472(5), c = 20.0203(6) Å, α = 77.4208(15), β = 90.0050
(15), γ = 80.2753(16)°, V = 3313.93(17) Å3, Z = 4, λ =
0.71073 Å, T = 150(2) K, μ = 0.219 mm−1, Dcalc = 1.916 Mg
3
CH), 6.53 (2 H, d, JHH = 7.6, CH), 3.53 (1 H, s, br, BH),
2.16 (6 H, s, CH3); 19F NMR (282 MHz, C7D8) δ ppm −129.62
(3 F, s, br), −140.55 (3 F, dd, 3JFF = 20.7, 4JFF = 12.1), −141.75
m−3, 14 572 independent reflections [R(int) = 0.049]; R1
0.0868, wR2 = 0.2264 [I > 2σ(I)]. CCDC 866505.
=
3
3
(3 F, d, JFF = 23.3), −142.27 (3 F, d, JFF = 22.4), −156.07
3
3
(3 F, t, JFF = 21.1), −159.40 (3 F, t, JFF = 22.0), −163.48
3
3
Synthesis of (2,2,6,6-Me4C5H6N)B(C12F9)3, 1, [N(C2H4)3N]-
B(C12F9)3, 2, and (2,6-Me2C5H3NH)B(C12F9)3, 3. These com-
pounds were synthesised in a similar manner and thus only one
preparation is described. B(C12F9)3 (37.8 mg, 0.040 mmol) and
TMP (5.6 mg, 0.040 mmol) were dissolved in 0.7 ml d8-toluene
in a vial to give a yellow (1 and 3) or peach-orange (2) solution
which was transferred to a Young’s tap NMR tube.
(3 F, t, JFF = 21.1), −164.23 (3 F, t, JFF = 21.1), −165.39
(3 F, t, 3JFF = 22.0).
Scaled-up attempted synthesis of 4 and 6, and bulk synthesis
of 5. Solutions of 1, 2 and 3 (0.314 mmol) in 3 ml toluene were
each freeze–thaw–degassed three times in a Rotaflo ampoule,
and backfilled with 1 atm hydrogen. The sealed reactions were
stirred at room temperature for 15 hours yielding pink (4), amber
(5) and pale yellow (6) solutions respectively. 5 was isolated
as a white powder by decanting the toluene and washing
with −20 °C pentane. Yield 62 mg, 18%. Anal. Calcd for
C42H14BF27N: C, 47.1; H, 1.3; N, 2.6. Found: C, 47.0; H, 1.2;
N, 2.6.
1. 11B{1H} NMR (96 MHz, C7D8) δ ppm 70.0 (s, br); H
1
NMR (300 MHz, C7D8) δ ppm 1.54 (2 H, m, CH2), 1.23 (4 H, t,
3JHH = 6.0, CH2), 1.03 (12 H, s, CH3); 19F NMR (282 MHz,
C7D8) δ ppm −119.59 (3 F, s, br), −131.82 (3 F, s, br), −137.32
3
4
(6 F, s, br), −143.49 (3 F, td, JFF = 20.9, JFF = 9.3), −149.02
3
3
4
(3 F, t, JFF = 21.3), −150.42 (3 F, td, JFF = 21.2, JFF = 6.5),
−160.57 (6 F, t, 3JFF = 17.1).
2. 11B{1H} NMR (96 MHz, C7D8) δ ppm 70.0 (s, br); H
Synthesis of [2,2,6,6-Me4C5H6NH2][HCO2B(C12F9)3] 7, and
[2,6-Me2C5H3NH][HCO2B(C12F9)3] 8. Formic acid (0.2 ml,
5.19 mmol) was added dropwise to a stirring solution of 2,6-
lutidine (0.6 ml, 5.19 mmol) in Et2O (20 ml). The solvent was
removed in vacuo to leave a white powder ([TMPH][O2CH]) or
colourless oil ([lutH][O2CH]). This formate salt (0.0314 mmol)
was dissolved in d8-toluene (0.7 ml) and added to PBB (30 mg,
0.0314 mmol).
1
NMR (300 MHz, C7D8) δ ppm 2.39 (12 H, s); 19F NMR
(282 MHz, C7D8) δ ppm −119.63 (3 F, s, br), −131.82 (3 F, s,
3
4
br), −137.37 (6 F, s, br), −143.44 (3 F, td, JFF = 20.7, JFF
=
=
3
3
9.5), −149.00 (3 F, t, JFF = 21.6), −150.41 (3 F, td, JFF
21.55, 4JFF = 6.0), −160.57 (6 F, t, 3JFF = 17.2).
3. 11B{1H} NMR (96 MHz, C7D8) δ ppm 67.0 (br, s); 1H
3
NMR (300 MHz, C7D8) δ ppm 7.01 (1 H, t, JHH = 7.7, CH),
9064 | Dalton Trans., 2012, 41, 9061–9066
This journal is © The Royal Society of Chemistry 2012

1
7. 11B NMR (96 MHz, C7D8) δ ppm −0.8 (s, br); H NMR
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8 P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme and
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9 G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129,
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10 J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem., Int.
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11 D. P. Huber, G. Kehr, K. Bergander, R. Fröhlich, G. Erker, S. Tanino,
Y. Ohki and K. Tatsumi, Organometallics, 2008, 27, 5279.
12 P. A. Chase, T. Jurca and D. W. Stephan, Chem. Commun., 2008,
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13 V. Sumerin, F. Schulz, M. Nieger, M. Leskelä, T. Repo and B. Rieger,
Angew. Chem., Int. Ed., 2008, 47, 6001.
14 C. Jiang, O. Blacque, T. Fox and H. Berke, Organometallics, 2011, 30,
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15 S. J. Geier, A. L. Gille, T. M. Gilbert and D. W. Stephan, Inorg. Chem.,
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16 S. J. Geier and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 3476.
17 C. B. Caputo, S. J. Geier, D. Winkelhaus, N. W. Mitzel, V. N. Vukotic,
S. J. Loeb and D. W. Stephan, Dalton Trans., 2012, 41, 2131.
18 P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew. Chem., Int.
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19 M. Lindqvist, N. Sarnela, V. Sumerin, K. Chernichenko, M. Leskelä and
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20 G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller and G. Bertrand,
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7433.
22 D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones and
M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428–7432.
23 D. Holschumacher, C. Taouss, T. Bannenberg, C. G. Hrib,
C. G. Daniliuc, P. G. Jones and M. Tamm, Dalton Trans., 2009, 6927.
24 S. Kronig, E. Theuergarten, D. Holschumacher, T. Bannenberg,
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26 C. M. Mömming, E. Otten, G. Kehr, R. Fröhlich, S. Grimme,
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2009, 48, 6643.
27 G. Ménard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 1796.
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29 S. Chakraborty, J. Zhang, J. A. Krause and H. Guan, J. Am. Chem. Soc.,
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30 C. M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich and G. Erker,
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32 J. Boudreau, M.-A. Courtemanche and F.-G. Fontaine, Chem. Commun.,
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33 E. Otten, R. C. Neu and D. W. Stephan, J. Am. Chem. Soc., 2009, 131,
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34 C. M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich, B. Schirmer,
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35 M. A. Dureen, C. C. Brown and D. W. Stephan, Organometallics, 2010,
29, 6422.
36 J.-B. Sortais, T. Voss, G. Kehr, R. Fröhlich and G. Erker, Chem.
Commun., 2009, 7417.
37 A. Stirling, A. Hamza, T. A. Rokob and I. Papai, Chem. Commun., 2008,
3148.
38 A. Stute, G. Kehr, R. Fröhlich and G. Erker, Chem. Commun., 2011, 47,
4288.
(300 MHz, C7D8) δ ppm 8.05 (1 H, s, br, O2CH), 5.92 (2 H, br,
NH), 0.97 (2 H, s, CH2), 0.88 (4 H, t, br, CH2), 0.68 (12 H, br,
CH3); 19F NMR (282 MHz, C7D8) δ ppm −124.13 (3 F, s, br),
−136.05 (3 F, dd, 3JFF = 23.2, 5JFF = 10.2), −136.11 (3 F, s, br),
−137.06 (3 F, s, br), −155.01 (3 F, s, br), −156.12 (3 F, t, 3JFF
=
3
20.3), −158.27 (3 F, t, JFF = 19.5), −164.38 (3 F, s), −164.60
(3 F, s).
1
8. 11B NMR (96 MHz, C7D8) δ ppm 0.18 (br, s); H NMR
(300 MHz, C7D8) δ ppm 12.53 (1 H, br, NH), 8.08 (1 H, s,
O2CH), 6.82 (1 H, t, 3JHH = 7.9), 6.17 (2 H, d, 3JHH = 7.9), 2.03
(12 H, s, CH3); 19F NMR (282 MHz, C7D8) δ ppm −123.79
(3 F, s, br), −136.32 (3 F, dd, 3JFF = 23.6, 5JFF = 10.4), −136.44
(3 F, s, br), −136.95 (3 F, s, br), −155.48 (3 F, t, JFF = 21.0),
156.04 (3 F, t, JFF = 20.9), −158.42 (3 F, t, JFF = 21.3),
3
3
3
−164.51 (3 F, br), −164.74 (3 F, t, br, 3JFF = 20.0).
Lewis acidity determination. Gutmann–Beckett method:60,61
An NMR tube is charged with PBB and Et3PO in a 3 : 1 molar
ratio in dry CD2Cl2 with a sealed capillary insert of uncoordi-
nated Et3PO in CD2Cl2 and the 31P NMR spectrum was recorded
at 20 °C. 31P{1H} NMR Et3PO reference δ = 50.4 ppm; (Et3PO)
B(C6F5)3 reference adduct δ = 77.0 ppm; reference shift Δδ =
26.6 ppm. (Et3PO)B(C12F9)3 adduct δ = 80.7 ppm, shift Δδ =
30.27.‡
Childs method:67 To an NMR tube charged with PBB in
0.6 ml dry CD2Cl2, trans-crotonaldehyde (5 mg, 0.0059 ml,
0.07 mmol) in 0.2 ml CD2Cl2 was added via vacuum transfer to
the frozen sample, and the temperature of the reaction main-
1
tained below −20 °C. The H NMR spectrum was recorded at
−20 °C and the difference in shift of the vinylic proton H3
measured: δ(H3 uncoordinated) 6.88 ppm; δ(H3 coordinated)
7.86 ppm; Δδ = 0.98 ppm.
Conclusion
Three novel FLP systems capable of affecting metal-free hydro-
gen-splitting have been prepared using the bulky borane PBB in
combination with the nitrogen bases TMP, 2,6-lutidine and
DABCO. The vastly increased bulk of PBB compared to
B(C6F5)3 reduces the reactivity of its analogous frustrated Lewis
pairs significantly. We continue to investigate activation of small
molecules by these unquenched donor–acceptor adducts and
intend to report matters in due course.
Acknowledgements
Assistance with crystallographic refinement was provided by
Dr Amber L. Thompson, Chemical Crystallography, Oxford.
Assistance with the NMR experiments was provided by Dr Nick
H. Rees.
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43 G. C. Welch, R. Prieto, M. A. Dureen, A. J. Lough, O. A. Labeodan,
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