Novel H2 activation by a tris[3,5-bis(trifluoromethyl)phenyl] borane frustrated Lewis pair

Article abstract of DOI:10.1039/c2dt30384a

Tris[3,5-bis(trifluoromethyl)phenyl]borane (1, BArF₁₈), has been synthesised on a practical scale for the first time. According to the Gutmann-Beckett method it is a more powerful Lewis acid than B(C ₆F₅)₃. It forms a 'frustrated Lewis pair' with 2,2,6,6-tetramethylpiperidine which cleaves H₂ to form a salt containing the novel anion [μ-H(BArF₁₈)₂]^-.

Full text of DOI:10.1039/c2dt30384a

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issue entitled:
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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COMMUNICATION
Novel H activation by a tris[3,5-bis(trifluoromethyl)phenyl]borane frustrated
2
Lewis pair†
Thomas J. Herrington, Alex J. W. Thom, Andrew J. P. White and Andrew E. Ashley*
Received 17th February 2012, Accepted 13th April 2012
DOI: 10.1039/c2dt30384a
Tris[3,5-bis(trifluoromethyl)phenyl]borane (1,
BArF ),
18
has been synthesised on a practical scale for the first
time. According to the Gutmann–Beckett method it is a
more powerful Lewis acid than B(C F ) . It forms a
6
5 3
‘
frustrated Lewis pair’ with 2,2,6,6-tetramethylpiperidine
which cleaves H to form a salt containing the novel anion
2
−
[
μ-H(BArF ) ] .
1
8 2
In recent years the concept and reactivity of “frustrated Lewis
pairs” (FLPs) continues to develop apace. Within these systems,
Fig. 1 Commonly used fluorinated aryl borates and their ‘parent’
Lewis acid boranes.
1
dative bond formation is restricted by steric encumbrance about
the donor and acceptor atoms which leads to ‘unquenched’ reac-
tivity. This enables the activation of small molecules such as
2
CO , and importantly the heterolytic cleavage of H , which has
2
2
led to application for the metal-free hydrogenation of polar
1
e
organic substrates (e.g. nitriles and imines), and even the weak
3
oxidant CO . Typically, FLPs consist of an electrophilic borane
2
(
most commonly B(C F ) or derivatives thereof), whose Lewis
6 5 3
Scheme 1 Synthesis of tris[3,5-bis(trifluoromethyl)phenyl]borane, (1).
acidity is promoted by electron-withdrawing substituents, in
combination with a hindered phosphine or amine e.g. Bu P or
t
3
2
,2,6,6-tetramethylpiperidine (TMP).
Tetraaryl borate anions [B(C F ) ] and tetrakis[3,5-bis-
was reported, and no subsequent reactivity studies have been
conducted.
−
6
5 4
−
(trifluoromethyl)phenyl]borate, [BArF ] , (Fig. 1) have widely
In continuation of our interest in FLP-H activation chemistry,
2
4
2
gained use as weakly-coordinating counterions for reactive cat-
we herein report a practical synthesis of BArF18 and communi-
cate preliminary findings of its Lewis acidic properties and dif-
fering reactivity with H in an FLP system, relative to B(C F ) .
ionic metal centres (e.g. in homogeneous olefin polymerisa-
4
tion). Their high stability in acidic and oxidative conditions is
2
6 5 3
attributed to the electron-withdrawing properties of their F-
substituents (which lower aromatic π-basicity and hence suscep-
tibility towards electrophilic B–C bond cleavage), and the
Na[BArF ] is synthesised via reaction of excess [3,5-bis-
24
(trifluoromethyl)phenyl]MgX (X = Cl, Br) with NaBF or
4
5
a,8
BF ·OEt ;
we reasoned that BArF18 should be an intermedi-
3
2
5
strength of their C–F bonds. Remarkably, in view of the rich
ate en route to the borate anion and decided to employ a rigid
6
chemistry developed for B(C F ) (which can be viewed as the
stoichiometry. Accordingly, the Grignard was generated via
6
5 3
−
i
Lewis acid ‘parent’ of [B(C F ) ] ) only one report exists for
metal-halogen exchange of PrMgCl and 1-bromo-3,5-bis-
6
5 4
the synthesis of the analogous tris[(3,5-trifluoromethyl)phenyl]
(trifluoromethyl)benzene in THF, which was subsequently
9
borane (BArF ), resulting from decomposition of the
reacted in situ with BF ·OEt2 (3 : 1) (Scheme 1). Facile
1
8
3
−
−6
[
BArF ] anion by the electrophilic platinum complex
trans-[(Ph P) Pt(Me)(OEt )] . Only X-ray crystallographic data
work-up followed by high vacuum sublimation (80 °C, 1 × 10
24
+
7
3
2
2
mbar)
afforded
tris[3,5-bis(trifluoromethyl)phenyl]borane
(
1, BArF ) in good yield (65–70%, 2–5 gram scale) as a free-
18
flowing white powder (Scheme 1).‡ The reaction solvent
appeared to be important; Grignard formation can also be con-
ducted in Et O, yet metathesis with BF ·OEt led to formation of
Department of Chemistry, Imperial College London, South Kensington,
London SW7 2AZ, UK. E-mail: a.ashley@imperial.ac.uk;
Tel: +44 (0)20 759 45810
2
3
2
−
11
[
BArF ] , as shown by B NMR spectroscopy. It is thought
†
Electronic supplementary information (ESI) available: Experimental
details, X-ray crystallographic details for 2·Et O, synthetic procedures
and NMR spectral data for 1 and 2, and free volume plots obtained from
X-ray crystal structures of C N·A (A = 1, B(C ). CCDC 868317.
24
2
the use of THF may retard the competitive addition of a fourth
Grignard equivalent by coordinating to 1 as it is formed in solu-
tion; indeed the sublimation step is required to remove THF
from the moderately labile adduct 1·THF, which is the actual
5
H
5
6 5 3
F )
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt30384a
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9019–9022 | 9019

product extracted immediately after the Grignard step, as evinced
1
19
11
by H, F and B NMR spectroscopy.
is practically insoluble in aliphatic hydrocarbons, moderately
1
so in aromatic solvents and displays optimum solubility in
CH Cl or CHCl ; this property contrasts with B(C F ) (soluble
2
2
3
6 5 3
in most common non-donor media). This behaviour may be
attributed to intermolecular H⋯F bond interactions between the
para proton and CF groups on neighbouring molecules in the
3
solid-state for 1; a distance of 2.63 Å is found in the reported
crystal structure [sum of vdW radii, r (F) + r (H) = 2.67 Å],
w
w
7
Scheme 2 Generation of 2 from heterolytic activation of H by 1 and
TMP.
2
,10
which would obviously be lacking for B(C F ) .
6
13
5 3
19
1
11
1
has been fully characterised by H, C, F and B NMR
spectroscopy; the latter shift (δ 68.1 ppm; CD Cl ) lends support
2
2
for a three-coordinate geometry in the solution-phase and is
noticeably deshielded in comparison with that found for
B(C F ) (δ 61.2 ppm; CD Cl ). Whilst B(C F ) has been
6
5 3
2
2
6 5 3
11
shown to be inert to pure oxygen at room temperature, admis-
sion of dry O to a CD Cl solution of 1 led to rapid decompo-
2
2
2
1
19
sition (numerous uncharacterisable resonances in the H, F and
B NMR spectra). Despite strongly electron-withdrawing CF3
groups in 1 (rationalised to contribute to the observed oxidative
11
−
stability of the [BArF ] anion), it is possible that the ortho-F
2
4
Fig. 2 (a) Diagram of 2·Et O. H atoms (except attached to B and N)
have been removed for clarity; thermal ellipsoids are shown at 50%
2
substituents in B(C F ) are more important in suppressing reac-
6
5 3
tion with O ; the absence of this structural feature in 1 might
probability. (b) View showing staggered geometry along the B2–H1–B1
2
then lead to the heightened reactivity observed for this trigonal
axis.
borane in this case. Interestingly, H O reversibly forms the
2
dative complex 1·OH ; the donor can be removed under vacuum
donor to B(C
bond, favoured by B(OC F ) .
6 5 3
Since Lewis acidity is a composite of both steric and elec-
tronic factors at the acceptor site, it would be useful to compare
6 5 3
F ) compared with the more ionic (harder) PvO
2
or through addition of 3 Å molecular sieves in CH Cl solution,
2
2
12
in contrast with the tightly bound analogue (C F ) B·OH .
6
5 3
2
In order to better understand the reactivity of 1, Lewis acidity
assessments were performed by employing the Gutmann–
the steric profile of 1 with B(C
state structure of the latter has been reported. Fortunately the pyr-
idine adducts, C N·A (A = 1, B(C ), have been crystallo-
F ) ; however, to date no solid-
6 5 3
3
1
Beckett (Et PO probe; P NMR) and Childs (trans-crotonalde-
3
1
hyde; H NMR of H resonance) methods; both rely on respect-
5
H
5
F )
6 5 3
3
ive chemical shift differences (Δδ) upon complexation of the
graphically characterised for both boranes in which both have
probe to the Lewis acid, which is proportional to the Lewis acid
virtually identical B–N bond lengths (1.63 Å, within e.s.u.), per-
mitting valid comparison. Excision of the pyridine ligand
1
3
16
strength of the acceptor site. The results, compared with data
acquired for B(C F ) , are tabulated in Table 1. It can be seen
enabled a comparison of the relative free volume from the
B centre, at a given radius, for the remaining pyramidalised
borane fragments.† The results show that 1 is less hindered in
the 2–4 Å region (i.e. that occupied by the pyridine molecule),
as anticipated from the smaller size of the ortho-H in 1 relative
6
5 3
that 1 displays a Lewis acidity ca. 6% greater than that for
B(C F ) using the former method, which contrasts markedly
6
5 3
with a ca. 38% reduction observed employing the Childs. A
14
linear correlation is usually documented between methods,
although an increasing number of boron systems oppose this
1
1
to the ortho-F in B(C
spectroscopic data (an electronic probe at the B atom)
supports the Gutmann–Beckett assignment that 1 is more Lewis
acidic than B(C
Addition of 1 to 2,2,6,6-tetramethylpiperidine (TMP) in
CD Cl (1 : 1) demonstrated the formation of an FLP, as evi-
F ) ; in conjunction with the B NMR
6 5 3
15
15a
observation. Notably, Britovsek et al. reported a non-linear
trend for the series B(C F ) (OC F ) (x = 1–3), where prefer-
this
6
5 3−x
6 5 x
ence for Et PO binding over crotonaldehyde is observed as x
F ) .
6 5 3
3
5b
1
increases.
This was rationalised using Pearson’s HSAB prin-
ciple where the covalent (softer) CvO bond is a preferable
2
2
1
19
11
denced by unchanged resonances in the H, F and B NMR
spectra relative to the species in isolation. Subsequent admission
3
1
1
Table 1
P and H NMR spectral data derived for Lewis acidity
measurements of 1 and B(C
6 5 3
F )
of H (1 atm) led to the rapid precipitation of a white solid, and
2
1
H NMR spectroscopy revealed exactly half of the initial TMP
Et
3
PO
trans-Crotonaldeyde
11
remained in solution, whereas B NMR showed complete con-
sumption of 1, indicating complete sequestration of the borane.
Elemental analysis of the solid was consistent with the molecular
3
1
a
b
1
a
c
Lewis acid
None
P NMR/ppm
Δδ/ppm
H NMR/ppm
Δδ/ppm
50.7
77.3
78.9
—
26.6
28.2
6.85
7.93
7.52
—
1.08
0.67
formula unit (1) (TMP)(H ) (2, Scheme 2).‡ Remarkably,
B(C
6
F
5
)
3
2
2
1
H activation occurs even in Et O, and led to the generation of
2 2
large single crystals suitable for study by X-Ray diffraction,
a
b
CD
2
2
Cl
Cl
2
at room temperature. Δδ = [Et
3
PO(coordinated) − Et
Cl )].
3
PO-
c
which solved as the novel [TMPH][μ-H(BArF ) ]·Et O
18 2
2
(CD
2
)]. Δδ = [H
3
(coordinated) − H
3
(CD
2
2
(
2·Et O; Fig. 2). The anion geometry approximates to D3
2
9020 | Dalton Trans., 2012, 41, 9019–9022
This journal is © The Royal Society of Chemistry 2012

symmetry, and the bridging borohydride unit is virtually linear
BĤB = 176.3°). The B–H bond lengths (1.40 and 1.42 Å) are
(
17
similar to those for seen in Li[μ-H(BEt ) ] (1.376(6) Å) yet
distinct from [TMPH][H–B(C F ) ] (1.18(2) Å); the longer
3
2
1
8
6
5 3
bonds reflect the electron-deficient B–H–B interactions relative
to terminal B–H. The aryl rings adopt an almost staggered con-
formation (torsion angles 58.7–61.5°). The [TMPH] cation
shows H-bonding to an Et O molecule with N⋯O and H⋯O
2
separations of 2.869(4) and 1.97 Å respectively, the N–H⋯O
angle being ca. 178°. Evidently the ammonium ion binds the
neutral O atom in preference to the charged borohydride anion.
This is the first example of H cleavage by an FLP to produce a
2
bridging borohydride salt.
The only other example of arylborane-mediated H activation
2
in ethereal solvent is by the FLP (Fmes) BH/DABCO [Fmes =
2
2
,4,6-tris(trifluoromethyl)phenyl, DABCO = diazobicyclo[2.2.2]-
19
octane]; here the Lewis acidity remains unquenched because
the steric bulk around the B centre in (Fmes) BH prevents Et O
2
2
binding. In contrast, both 1 and B(C F ) coordinate Et O;
6
5 3
2
11
1
·OEt is observable in Et O solution as shown in the B NMR
2 2
1
5
Fig. 3 H NMR spectrum of 2 (pyridine-d ). Solvent peaks denoted
spectrum (δ 19.0 ppm), a shift indicative of four-coordinate
boron. However, whilst B(C F ) forms a strong adduct that
quenches FLP activity, 1·OEt must dissociate sufficiently at
−
5
by *, ▲ and ■ denote 1·pyridine-d and [1-H] respectively.
6
5 3
2
0
2
ambient temperature to allow participation of 1 in H heterolysis.
This is analogous to the behaviour exhibited by the 2,6-
2
oxygen donors H O and Et O more reversibly under comparable
2 2
conditions. BArF18 forms an FLP in the presence of TMP which
−
dimethylpyridine/B(C F ) FLP, which cleaves H only upon
6
5 3
2
1
reacts rapidly with H to form the anion [μ-H(BArF ) ] and
2
18 2
2
dissociation of the weakly-bound classical adduct.
has been crystallographically characterised. This is the first
example of a bridging borohydride resultant from FLP-mediated
H heterolysis. Interestingly this reaction also proceeds in Et O,
An unambiguous structural assignment of 2 by NMR spec-
troscopy was hampered by the insolubility of the compound in
most non-donor media; only using 1,2-difluorobenzene at 80 °C
2
2
which is believed to be an example of a system operating on the
classical/frustrated Lewis pair borderline. Current research is
focusing on the use of different Lewis base partners whilst
exploring catalytic hydrogenation chemistry of these FLP/H2
protocols.
(
a solvent with a high dielectric constant reported to dissolve
2
2
1
poorly soluble ionic salts) was a H NMR spectrum obtained
that correctly reproduced the 1 : TMP ratio in 2. Despite this, we
1
11
were unable to observe any resonances in either the H or
B
NMR spectra which could be assigned to the B(μ-H)B unit in 2.
This property is reminiscent of the related [(C F ) B(μ-H)B-
6
5 3
1
19
(
C F ) ] anion; here low temperature H and F NMR (<183 K)
provided the only means of identification, with B NMR unable
to distinguish a B(μ-H)B environment. IR spectroscopy of 2
Acknowledgements
6
5 3
11
2
3
We thank Imperial College (Junior Research Fellowships; AEA
and AJWT), the EPSRC (EP/P505550/1), and the Royal Society
and its deutero analogue, obtained from D and TMP : 1 (1 : 2),
2
(Research Grant RG100606) for funding this research. AEA also
also failed to unambiguously reveal a B–H(D) stretch.
thanks Dr George Britovsek for sponsorship in the Imperial
College Junior Research Fellowship scheme, and the Royal
Commission for the Exhibition of 1851 for a Research
Fellowship.
On the other hand, dissolution occurred readily in pyridine-d
5
1
19
11
to give readily assignable H (Fig. 3), F and B NMR spectra,
+
which corresponded to the species [1·pyridine-d ], [TMPH] ,
5
−
and the borohydride anion [1-H] (1 : 1 : 1 ratio). The latter was
cleanly resolved by a diagnostic terminal BH (1 : 1 : 1 : 1 quartet)
1
1
in the H NMR (δ = 4.53 ppm, J
= 84 Hz), accompanied
1-BH
1
Notes and references
11
by an intense doublet in the B NMR spectrum (δ = −7.2 ppm,
J11-BH = 84 Hz). This confirms the composition of 2, and
1
1
‡
Data for compound 1: H NMR (CD
2
Cl
2
, 400 MHz): δ 8.24 (s, 3H,
Cl , 101 MHz):
1
3
1
para-H), δ 8.02 (s, 6H, ortho-H). C{ H} NMR (CD
δ 142.8 (s, br, ipso-C), δ 138.3 (s, ortho-C), δ 132.2 (q, JCF = 33.5 Hz,
meta-C), δ 127.0 (s, para-C), δ 123.7 (q,
2
2
reveals the behaviour of the anion in donor media as both a
2
−
1
source of terminal borohydride [1-H] , and a labile equivalent of
J
CF = 272.9 Hz, CF ).
3
1
9
11
the Lewis acid 1.
F NMR (CD
2
Cl
2
, 376 MHz): δ −63.4 (s, CF
3
). B NMR (CD
18 9
28 MHz): δ 68.1 (s, br). HRMS (EI, m/z): for BC24F H Calcd:
1
2 2
Cl ,
1
6
50.0510. Found: 650.0510. IR (KBr, cm− ): 1615 (m), 1607 (m), 1385
Conclusions
(m), 1283 (s), 1227 (m), 1169 (s), 1127 (s), 909 (m), 844 (w), 720 (m),
08 (w), 683 (m), 657 (m). Anal. Calcd for C24 BF18: C 44.34;
H 1.40; N 0.00. Found: C 44.22; H 1.29; N 0.00.
7
H
9
The facile synthesis of tris[3,5-bis(trifluoromethyl)phenyl]
borane (1) has been achieved on a multi-gram scale. Gutmann–
Beckett measurements indicate BArF18 to be a stronger Lewis
acid than the ubiquitous B(C F ) , yet it appears to bind the
1
Data for 2: H NMR (C
H), δ 7.73 (s, 12H, ortho-H), δ 4.00 (br, 2H, NH2), 1.57 (m, 2H, CH
6
H
4
F
2
, 400 MHz, 353 K): δ 7.84 (s, 6H, para-
),
): 3274 (m), 3234
2
−1
1.46 (m, 4H, CH ), 1.23 (s, 12H, CH ). IR (KBr, cm
2
3
(m), 3095 (m), 3034 (m), 2983 (m), 1616 (m), 1577 (w), 1459 (w),
6
5 3
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9019–9022 | 9021

1
6
365 (s), 1279 (s), 1165 (s), 1126 (s), 900 (s), 841 (m), 710 (s), 682 (s),
49 (s). Anal. Calcd for C57 36N: C 47.43; H 2.72; N 0.97.
8 (a) K. Fujiki, M. Kashiwagi, H. Miyamoto, A. Sonoda, J. Ichikawa,
H. Kobayashi and T. Sonoda, J. Fluorine Chem., 1992, 57, 307–321;
(b) M. Brookhart, B. Grant and A. F. Volpe, Organometallics, 1992, 11,
3920–3922.
9 It has been advised that metal-halogen exchange should be used for
Grignard formation from halogeno(trifluoromethyl)benzenes, instead of
direct synthesis using Mg, since the latter protocol has led to explosions
(presumably due to Mg insertion into C–F bonds), especially in large-
scale syntheses. See: N. A. Yakelis and R. G. Bergman, Organometallics,
2005, 24, 3579–3581 and references contained therein.
39 2
H B F
Found: C 47.34; H 2.63; N 1.02.
Crystal data for 2·Et
M = 1517.63, triclinic, P1ˉ (no. 2), a = 12.0325(5), b = 15.7928(8), c =
2
19 2 9 4
O: (C48H B F36)(C H20N)·C H10O,
1
3
1
1
7.3620(9) Å, α = 90.233(4), β = 92.367(4), γ = 100.933(4)°, V =
236.4(3) Å , Z = 2, D = 1.557 g cm , μ(Mo-Kα) = 0.164 mm , T =
c
73 K, colourless tablets, Oxford Diffraction Xcalibur 3 diffractometer;
3 209 independent measured reflections (Rint = 0.0305), F refinement,
3
−3
−1
2
1 2
R (obs) = 0.0822, wR (all) = 0.2509, 7684 independent observed
absorption-corrected reflections [|F
parameters.
o o
| > 4σ(|F |), 2θmax = 57°], 1009
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