Frustrated Lewis pairs incorporating the bifunctional Lewis acid 1,1′-fc{B(C6F5)2}2: Reactivity towards small molecules

Article abstract of DOI:10.1039/c7dt04136e

Applications of the bifunctional ferrocenediyl Lewis acid 1,1′-fc{B(C₆F₅)₂}₂ in frustrated Lewis pair (FLP) chemistry are described. The coordination (or otherwise) of a range of sterically encumbered C-, N- and P-centred Lewis bases has been investigated, with lutidine, tetramethylpiperidine, PPh₃, P^tBu₃ and the expanded ring carbene 6Dipp being found to be sterically incapable of coordinate bond formation. The chemistry of a range of these FLPs in the presence of H₂O, NH₃, CO₂ and cyclohexylisocyanate (CyNCO) has been investigated, with the patterns of reactivity identified including simple coordination chemistry, E-H bond cleavage and C-B insertion.

Full text of DOI:10.1039/c7dt04136e

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Efficient extraction of sulfate from water using
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DOI: 10.1039/C7DT04136E
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ARTICLE
Frustrated Lewis pairs incorporating the bifunctional Lewis acid
1,1'-fc{B(C6F5)2}2: reactivity towards small molecules
Rémi Tirfoin,a Jessica Gilbert,a Michael J. Kellya and Simon Aldridgea*
Received 00th January 20xx,
Accepted 00th January 20xx
Applications of the bifunctional ferrocene-diyl Lewis acid 1,1'-fc{B(C6F5)2}2 in frustrated Lewis pair (FLP) chemistry are
described. The coordination (or otherwise) of a range of sterically encumbered C-, N- and P-centred Lewis bases has been
investigated, with lutidine, tetramethylpiperidine, PPh3, PtBu3 and the expanded ring carbene 6Dipp being found to be
sterically incapable of coordinate bond formation. The chemistry of a range of these FLPs in the presence of H2O, NH3, CO2
and cyclohexylisocyanate (CyNCO) has been investigated, with the patterns of reactivity identified including simple
coordination chemistry, E-H bond cleavage and C-B insertion.
DOI: 10.1039/x0xx00000x
www.rsc.org/
similar chemistry also occurs with PtBu3/B(C6F5)3, the
t
Introduction
formation of Bu3PŊN2OŊ1 is signalled by a maroon-to-amber
colour change and a shift of 300 mV in the FeII/III redox
The exploitation of frustrated Lewis pairs (FLPs) constitutes a
significant development in the fields of small molecule
activation and catalysis over the past decade.1-3 The
cooperative use of sterically unquenched Lewis acid and Lewis
basic sites facilitates transformations more typically associated
with transition metal complexes.3 Since the first example of
their use in the heterolytic cleavage of H2 was described in
2006,2 numerous applications have been reported in the
activation of other E-H bonds (e.g. E = B, C, N, O, Si), and in the
binding of small molecules, including atmospheric oxides (such
as CO, CO2, NO, N2O, and SO2).3 While the Lewis acid
component of a large number of these FLP systems is the
commercially available perfluoroaryl borane B(C6F5)3,
examples featuring other neutral boranes,4 heavier Group 13
analogues,5 borenium cations,6 and Lewis acids from the d-
block have also emerged.7
FLPs that contain a ferrocenyl reporter unit offer the
possibility for inducing a measurable electrochemical and/or
colorimetric response on small molecule capture,8,9 and
therefore have potential uses in detection. Two approaches in
this area are exemplified by the Fe(η5-C5Ph5)(η5-C5H4PtBu2)
/B(C6F5)3 system in which the Lewis basic phosphine unit
contains a ferrocenyl substituent,10 and by recent work from
our laboratory utilizing the ferrocenyl borane Fe(η5-C5H5){η5-
C5H4B(C6F5)2} [= FcB(C6F5)2, 1],11 a compound which was
previously reported by Piers and co-workers.12a In terms of
small molecule capture, the PtBu3/1 FLP binds N2O intact to
generate the ambiphilic adduct tBu3PŊN2OŊ1 (Scheme 1); while
potential.11,13
RB(C6F5)2
tBu
tBu
N
O
C6F5
C6F5
(R = C6F5, Fc)
P
N
B
N2O
PtBu3
tBu
R
Scheme 1 Binding of N2O by FLPs featuring PtBu3 and either B(C6F5)3 or 1.11,13
B(C6F5)2
B(C6F5)2
Fe
Fe
B(C6F5)2
1
2
Figure 1 Ferrocenyl and ferrocene-diyl based Lewis acids FcB(C6F5)2 (1) and
1,1'-fc{B(C6F5)2}2 (2) [Fc = Fe(η5-C5H5)(η5-C5H4); 1,1'-fc = Fe(η5-C5H4)2].
Although this N2O detection protocol illustrates the
potential of ferrocenyl boranes in FLP chemistry, we wanted to
probe the broader scope of this approach, by exploiting other
boranes incorporating metallocene scaffolds. In particular, we
hypothesized that 1,1'-fc{B(C6F5)2}2 (2) might be of interest,
due to (i) its potential to act as a bifunctional (potentially
chelating) Lewis acid to trap substrates bearing geometrically
divergent Lewis basic sites; and (ii) its stronger Lewis acidity, in
comparison to monofunctional 1 (Figure 1).12 This observation
reflects the π electron-withdrawing nature of the addition-
al -B(C6F5)2 group, and the fact that through-space donation
from the electron-rich iron centre to the individual pendant
borane function(s) is less pronounced when there are two such
groups.12
With this in mind, we set out to explore the potential for 2
to act as the Lewis acid component in FLPs featuring a range of
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C-, N- and P-donors, and subsequently to probe the use of ation with the much bulkier PtBu , for example, is known to
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3
such systems in the activation of small molecules such as H2O, constitute an FLP. With this in mind, the DinOteI:r1a0c.1t0io39n/Co7fD2T0w4i1t3h6Ea
NH3 and CO2. We report the results of these studies in this range of bulky C-, N- and P-donor Lewis bases was
manuscript.
investigated, with the aim of identifying FLP systems for
further reactivity studies.
(a) N-heterocyclic carbenes. 1 has previously been shown to
form classical Lewis acid-base adducts with the bulky N-aryl
substituted imidazolylidene donors IMes and IDipp, together
with the less strongly donating, backbone-chlorinated carbene
IMesCl2 (Figure 2).11a These NHCs were therefore thought to
Results and discussion
Alternative synthesis of 1,1'-fc{B(C6F5)2}2 (2)
HgCl
B(C6F5)2
B(C6F5)2
ClB(C6F5)2
Fe
Fe
provide a suitable comparative starting point for the
(2 equiv.)
HgCl
exploration of the coordination behaviour of 2. In addition, the
behaviour of 2 in the presence of the more sterically
encumbered (and more strongly σ-donating) expanded ring
systems 6Mes and 6Dipp was also to be probed.17
2
Scheme 2 Literature synthesis of 1,1'-fc{B(C6F5)2}2 (2).12b
Cl
Cl
Piers and co-workers have reported the synthesis of 2 from
1,1'-fc(HgCl)2 and ClB(C6F5)2 (Scheme 2).12b Generation of the
ClB(C6F5)2 precursor involves the synthesis of Me2Sn(C6F5)2
from LiC6F5, and its subsequent reaction with BCl3 at elevated
temperature/pressure.12c With this in mind, a more direct
synthesis from LiC6F5 was targeted, drawing on the route
which we have previously developed to prepare 1 from FcBBr2
(Scheme 3).14 The one-pot literature synthesis of bifunctional
1,1'-fc(BBr2)2 from ferrocene and BBr3,15 is followed by
reaction with slightly greater than four equivalents of LiC6F5,
N
N
N
N
N
N
Mes
Mes
Mes
Mes
Dipp
Dipp
IMesCl2
IMes
IDipp
N
N
N
N
Mes
Dipp
Mes
Dipp
6Mes
6Dipp
Figure 2 N-heterocyclic carbenes (NHCs) utilized in the current study.
n
generated in situ from BuLi and C6F5Br. Although LiC6F5 is
notoriously explosive above -40oC, the synthetic protocol that
we employ means that [as in the synthesis of Me2Sn(C6F5)2] it
never warms above -78oC, and so can be safely and reproduc-
bly used in the synthesis of 2. Following this procedure, 2 can
be isolated after extraction into hot hexane with a yield (ca.
50%) broadly comparable to that reported by Piers (up to
40%).12b
The addition of IMes to 2 in a 2:1 molar ratio results in an
immediate colour change from violet to orange, and the
appearance of a 11B NMR resonance at δB = -14 ppm,
consistent with the formation of a tetra-coordinate borate
through coordination of the IMes donor.11a 1H NMR spectro-
scopy suggests that two molecules of NHC have been
assimilated and that the ‘normal’ C2-mode of carbene binding
pertains, with signals at δH = 7.46 and 7.59 ppm corresponding
to the backbone N(CH)2N protons (albeit with loss of the plane
of symmetry). The formation of a low symmetry adduct is also
BBr3
(1 equiv.),
BBr2
B(C6F5)2
LiC6F5
Fe
Fe
Fe
(2.0 equiv.)
toluene
45oC
consistent with the observation of ten distinct signals in the 19
F
NMR spectrum (indicating that the equivalence between the
ortho, meta and para fluorines of the two perfluorophenyl
rings had been lost); a similar pattern of 19F signals is observed
in the 19F spectrum of the corresponding monofunctional
adduct 1ŊIMes, and has been ascribed to restricted rotation
about the sterically congested B-C bonds on the timescale of
the NMR experiment.11a
1
BBr3
(2.7 equiv.),
hexanes (reflux)
BBr2
BBr2
B(C6F5)2
B(C6F5)2
LiC6F5
Fe
Fe
(4.1 equiv.)
In the case of 2, the molecular structure derived from X-ray
crystallography confirms the formulation of the adduct as 1,1'-
fc{B(C6F5)2ŊIMes}2 (2Ŋ(IMes)2, Figure 3). The solid-state
structure possesses approximate (non-crystallographic) C2
symmetry, and the fact that this adduct displays the same
number of 19F NMR resonances as 1ŊIMes presumably reflects
2
Scheme 3 (upper) Synthesis of 1 from FcBBr2, and (lower) modification for
the synthesis of 2 from 1,1'-fc(BBr2)2. Caution: the reaction temperature
must be kept below -40oC at all times when handling LiC6F5.
Interaction of Lewis Bases with 2
a
similar
symmetry
relationship
between
the
.
The sterically unencumbered tertiary phosphine Me3P is
known to form a 2:1 adduct with 2, through the formation of
two P→B coordinate bonds.12b The donor/acceptor chemistry
of 1 has been explored in greater depth,11a and the combin-
two -B(C6F5)2 IMes functions in solution. Bond metrics
associated with the borate functions (for example B-C
distances) are essentially identical to those measured for
1ŊIMes,11a reflecting the fact that there is relatively little
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(b) Nitrogen and phosphorus-centred donors. We have also
geometric perturbation around the boron centre induced by
the presence of the second borate function.
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probed the interactions of a range of teDrtOiaI:r1y0.p10h3o9s/pCh7DinTe0s41a3n6Ed
bulky N-donor Lewis bases with 2. Both lutidine (2,6-
dimethylpyridine) and tetramethylpiperidine give rise to FLPs
in benzene-d6 solution, as do PPh3 and PtBu3, the latter two
being characterized by unchanged 31P resonances at -5.4 and
61.9 ppm, respectively. By contrast, the combination of 2 and
PnBu3 can be shown to result in an immediate reaction, and
multinuclear NMR studies are consistent with classical adduct
formation. The single broad peak at δB = -11.0 ppm is
indicative of a four-coordinate borate centre, and the signal at
δP = -2.9 ppm is consistent with the 31P shift measured for the
.
Figure 3 Molecular structures of (left) 1,1'-fc{B(C6F5)2 IMes}2, 2Ŋ(IMes)2 and
known compound nBu3P B(C6F5)3.20 The chemical shift
Ŋ
.
difference between the meta and para 19F NMR signals, Δδm,p
is known to decrease on quaternization at boron, and has
therefore been widely used to probe the interactions of Lewis
bases with boranes containing the B(C6F5) moiety.21 In this
case, the decrease in the value of Δδm,p determined from 19F
NMR (from 10.7 ppm for 2 to 5.8 ppm) also supports the idea
that a classical phosphine-borane adduct has been formed.
(right) 1,1'-fc{B(C6F5)2 NH3}2, 2Ŋ(NH3)2, with thermal ellipsoids set at the 35%
probability level. Selected hydrogen atoms and solvate molecules omitted
and mesityl groups shown in wireframe format for clarity. Selected bond
lengths (Å) and angles (°): [for 2.(IMes)2] B(4)-C(60) 1.675(5), B(4)-C(3)
1.638(5), B(4)-C(14) 1.666(5), B(4)-C(47) 1.676(5), C(3)-B(4)-C(14) 113.9(3),
C(3)-B(4)-C(47) 106.0(3), C(14)-B(4)-C(47) 111.2(3); [for 2.(NH3)2] B(7)-N(8)
1.624(2), B(7)-C(5) 1.601(2), B(7)-C(9) 1.648(2), B(7)-C(20) 1.639(2), B(36)-
N(37) 1.627(2), B(36)-C(32) 1.600(2), B(36)-C(38) 1.649(2), B(36)-C(49)
1.638(2), C(5)-B(7)-C(9) 115.9(1), C(5)-B(7)-C(20) 113.3(1), C(9)-B(7)-C(20)
106.5(1), C(32)-B(36)-C(38) 114.4(1), C(32)-B(36)-C(49) 114.4(1), C(38)-B(36)-
C(49) 108.9(1).
The combination of PCy3 and 1 has previously been shown
by in situ NMR measurements to result in the formation of an
FLP in benzene-d6 solution.11a However, evidence can also be
obtained for the formation of a minor co-product over a
period of ca. 2 weeks at room temperature (Scheme 4a).
Although the overwhelmingly dominant species present in
solution at this point are still 1 and PCy3, the minor product
can be crystallized and shown by X-ray crystallography to be
the zwitterion Fc{B(F)(C6F5)(C6F4PCy3-4)} (3), derived from SNAr
attack by the phosphine at the para position of one of the
perfluorophenyl rings of the Lewis acid, with accompanying
migration of fluoride to the boron centre (Figure 4). Spectro-
scopically, this chemistry is characterised by diagnostic signals
in the 11B and 19F NMR spectra at δB = 2.0 ppm and δF = – 193.7
ppm associated with the boron-bound fluoride, and a shift in
the 31P NMR spectrum from the free phosphine at δP = 9.7
ppm to 39.9 ppm. Very similar NMR signals are observed for
the related system derived from the analogous activation of
B(C6F5)3, i.e. B(F)(C6F5)2(C6F4PCy3-4) (δB = -0.7 ppm, δP = 41.6
ppm).22
The reaction between IMes and 2 was also examined in a
1:1 molar ratio. However, monitoring by 19F NMR suggests that
this results in the formation of a mixture of the 1:1 adduct, the
2:1 adduct [i.e. 2Ŋ(IMes)2], and unreacted borane starting
material. Ten resonances due to 2Ŋ(IMes)2 are observed,
alongside signals corresponding to the free Lewis acid and
further signals, presumed to be derived from the 1:1 adduct.
This inference is supported by the 11B NMR spectrum, which
features a sharp signal at -13.7 ppm, consistent with the
borate functions in the 2:1 adduct, alongside a broad signal
consistent with uncomplexed three-coordinate borane
centres. The observation of a mixture of products suggests
that IMes binding at the two borane functions is essentially
independent, consistent with the conformationally flexible
nature of the ferrocene-diyl core.
Given the diagnostic nature of the 11B and 19F signals
associated with adduct formation, these measurements were
then used to determine the course of the reactions of 2 with
other NHCs. Thus, signals in the range δB = -10 to -13 ppm
were observed on addition of two equivalents of IMesCl2,
IDipp or 6Mes (with the solution in each case turning an
orange colour), with the respective 19F NMR spectra displaying
a similar ten-resonance pattern to that measured for 2.(IMes)2.
By contrast, on addition of 6Dipp to 2, the deep maroon colour
The combination of PCy3 with 2 was therefore investigated
in a 2:1 molar ratio. While there was no immediate colour
change on mixing, 1H, 11B, 19F and 31P NMR spectra measured
(a)
(b)
PCy3
PCy3
F
F
F
F
1
of the solution is retained, and H, 11B and 19F NMR spectra
F
F
F
F
C6F5
could be used to establish that no substantive reaction has
taken place. In particular, the 11B and 19F signals are unshifted
from those of the starting material 2, and the 1H NMR
spectrum consists of the superposition of the signals due to
6Dipp and 2. In this case alone it appears that the NHC and 2
constitute an unquenched FLP, an observation consistent with
the larger steric demands of the 6Dipp ligand (%Vbur = 50.8 vs.
36.5, 44.5 and 42.2 for IMes, IDipp and 6Mes).18,19
B(C6F5)2
C6F5
B
B
PCy3
slow
Fe
F
F
Fe
Fe
B
C6F5
C6F5
1
3
Scheme 4 (a) Minor reaction pathway of PCy3 with 1, proceeding via
nucleophilic attack at the para C-F bond of a C6F5 group to give 3. (b)
Proposed structure of the product of the reaction of 2 with PCy3.
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is therefore potentially suggestive of a weak secondary
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23
donor/acceptor interaction (Scheme 4b).DOI: 10.1039/C7DT04136E
Interestingly, and in contrast to PCy3/1, NMR data for
PCy3/2 show no evidence of unreacted 2 (i.e. FLP formation
does not occur) - an observation consistent with the enhanced
electrophilicity of 2 compared to 1. It is also noteworthy that
(despite the 2:1 molar ratio) PCy3 does not appear to react at
the second B(C6F5)2 unit in 2. Potentially, this can be attributed
to the reduced electrophilicity of the remaining -B(C6F5)2 unit
on conversion of the first to an electron-donating fluoroborate
function.9h
Figure 4 Molecular structure of Fc{B(F)(C6F5)(C6F4PCy3-4)}, 3, with thermal
ellipsoids set at the 35% probability level; hydrogen atoms for clarity.
Selected bond lengths (Å) and angles (°): B(7)-F(8) 1.426(2), B(7)-C(3)
1.613(3), B(7)-C(9) 1.660(3), B(7)-C(20) 1.682(3), C(3)-B(7)-C(9) 110.1(2),
C(3)-B(7)-C(20) 114.0(2), C(9)-B(9)-C(20) 108.2(2).
Reactions of small molecules with FLPs derived from 2
With the potential for 2 to function as an FLP in conjunction
with sterically encumbered bases established, we focussed on
exploring the chemistry of such systems in small molecule
activation. In common with 1, we find that combinations of 2
with any of PPh3, PtBu3, or 6Dipp (or for that matter the N-
donors lutidine or tetramethylpiperidine) do not activate H2
either under ambient conditions or on heating to 350 K. Such
observations are consistent with the electron-donating nature
of the ferrocene-diyl substituent and the consequently
reduced Lewis acidity at boron compared to B(C6F5)3.17,24 As
such, we hypothesized that the bifunctional nature of Lewis
acid 2 might be better suited to the cleavage (or capture) of
small molecules which yield Lewis bases that can be bound in a
chelating fashion (Figure 5). We therefore examined the
reactivity of 2 towards H2O, NH3 and CO2 (plus CO2 surrogates
of the type RNCO).
after 1 h show that a reaction has taken place. A broad upfield-
shifted resonance is observed in the 11B NMR spectrum at δB =
2.6 ppm, and a signal at δP = 40.1 ppm in the 31P NMR
spectrum; both are suggestive of an SNAr type reaction similar
to that observed for PCy3/1. The 19F NMR spectrum is more
complex. In the case of Fc{B(F)(C6F5)(C6F4PCy3-4)} (3), six
resonances are observed, corresponding to the boron-bound
fluoride, the ortho, meta and para fluorines of the
unreacted -C6F5 moiety, and two signals for the para -C6F4PCy3
moiety. If analogous reactivity were to take place at both
borane functions in 2, then either six or twelve signals might
be expected, depending on whether the two borate units are
symmetry-related on the NMR timescale. On the other hand, if
reaction were to take place at only one of the two borane
units, then (superficially) nine signals would be expected, with
the additional three arising from the perfluoroaryl rings
associated with the unreacted borane centre. In this case
C6F5
C6F5
C6F5
C6F5
B
B
O
1
twelve 19F signals are observed, but paradoxically, the H and
L
X
Fe
Fe
31P NMR data are consistent with the take-up of only one
phosphine unit. Half of the added PCy3 remains unreacted.
Of the twelve 19F resonances observed, six can readily be
assigned to fluorine environments analogous to those in 3, on
the basis of their very similar pattern of chemical shifts
(including a resonance at -194.9 ppm, indicative of a boron-
bound fluoride). The additional six signals, come in three pairs,
suggesting that they arise from the ortho, meta and para
fluorines of an unreacted -B(C6F5)2 moiety, in which the
equivalence of the two perfluoroaryl rings has been lost. Such
inequivalence is presumably caused by restricted rotation
about the B-CCp bond, which might arise either as a result of
the increased steric bulk of the peripheral borate centre, or
due to an interaction between the borane and the boron-
bound fluoride (Scheme 4b). Some evidence for the latter
comes from analysis of the magnitude of the Δδm,p splittings
derived from the 19F NMR data (using in the case of
the -B(C6F5)2 unit, the mean chemical shift difference between
meta and para resonances). Δδm,p has a value of 10.7 ppm for
starting material 2, this being reduced to 4.4 ppm for the C6F5
group of the borate function (cf. 3.6 ppm for the analogous
C6F5 unit in 3). That determined for the –B(C6F5)2 group is 7.2
ppm (i.e. intermediate between borate and ‘free’ borane), and
O
B
B
C6F5
C6F5
C6F5
C6F5
Figure 5 Postulated binding motifs in small molecule capture/activation
using FLPs containing 2.
The combination of 2 and PtBu3 reacts very readily with
water via O-H bond cleavage. The dark maroon colour of 2 in
benzene-d6 solution turns to a yellow-orange, and over 24
hours the formation of a yellow crystalline product is
1
observed. In situ H, 11B, 19F and 31P NMR studies give an
indication as to the nature of the reaction taking place. Thus,
the 31P spectrum shows a single peak at δP = 58.3 ppm
corresponding to the phosphonium cation, [HPtBu3]+, and the
19F NMR spectrum shows three resonances, shifted from those
in the starting material, and with a significant decrease in Δδm,p
compared to 2 (4.4 cf. 10.7 ppm). A large upfield shift in the
11B NMR signal is also observed (to δB = 2.0 ppm),
characteristic of the transformation of a three-coordinate
borane to a four-coordinate borate. Moreover, single crystals
suitable for X-ray crystallography could be obtained by
recrystallization from chloroform. The solid-state structure so
obtained confirms the nature of the product as [HPtBu3][1,1'-
fc{B(C6F5)2}2(µ-OH)] (i.e. [HPtBu3][2Ŋ(µ-OH)]), featuring
a
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hydroxide ion bridging symmetrically between the two boron 1,1'-fc{B(C6F5)2ŊNH3}2 [i.e. 2Ŋ(NH3)2]. In order toVicewonArftiicrlme Ontlhinee
DOI: 10.1039/C7DT04136E
centres in the anionic component. (Scheme 5 and Figure formation of this adduct structurally, a higher yielding
6).25,26 From a geometric perspective, the hydroxide binding synthesis was developed via condensation of liquid ammonia
motif is similar to that observed in the zwitterionic onto solid 2. An immediate colour change was observed from
cobaltocenium system Co(C5H4BF2)2(µ-OH) reported by deep purple to pale yellow, and (after venting the excess
Herberich and co-workers, with the B-O bond distances being ammonia) crystals suitable for X-ray crystallography could be
comparable for the two adducts [1.569(2)/1.563(2) vs. grown from diethyl ether solution. The structure so obtained
1.521(3)/1.556(3) Å, respectively].27
(Figure 3) confirms the formation of 2Ŋ(NH3)2. Geometrically
the structure has much in common with that of 2Ŋ(IMes)2 (also
Figure 3), with the slightly less pyramidalized nature of the BC3
fragments [Σ(∠C-B-C) = 336.7o (mean) cf. 330.0o (mean) for
2.(IMes)2] presumably reflecting the differing donor
capabilities of ammonia and IMes.
C6F5
C6F5
B
Stoich.
H2O
B(C6F5)2
B(C6F5)2
H
P
OH
Fe
Fe
tBu
tBu
tBu
PtBu3
B
C6F5
C6F5
2
Clearly this phosphine-based FLP system does not have
sufficient unquenched reactivity to deprotonate ammonia. On
the other hand, NHCs have been shown to be basic enough to
deprotonate the adduct H3NŊB(C6F5)3.30 Imidazolylidene donors
form (unreactive) classical donor/acceptor adducts with 2, but
the combination with the bulkier expanded ring NHC 6Dipp
constitutes an FLP. Attempts were therefore made to activate
ammonia using the 6Dipp/2 system: however, these also
resulted in the isolation of 2Ŋ(NH3)2. Presumably the 2:1 adduct
is formed very rapidly, and essentially irreversibly, on exposure
of the FLP to ammonia; in the absence of the possibility for
chelation (due to ‘blocking’ of the second Lewis acid function
by NH3), 6Dipp is not sufficiently basic to deprotonate the
bound NH3. Given that ItBu will deprotonate H3NŊB(C6F5)3,30
and that expanded ring NHCs are typically more basic than
their imidazolylidene counterparts,31 this observation again
reflects the weaker intrinsic Lewis acidity of the individual
boron centres in 2 compared to B(C6F5)3.12a
Scheme 5 Reaction of PtBu3/2 with water.24
Figure 6 Molecular structure of [HPtBu3][1,1'-fc{B(C6F5)2}2(µ-OH)],
[HPtBu3][2.(µ-OH)], with thermal ellipsoids set at the 35% probability level;
hydrogen atoms (except those originating from H2O) and solvate molecules
omitted for clarity. Selected bond lengths (Å) and angles (°): B(7)-O(8)
1.569(2), B(9)-O(8) 1.563(2), B(7)-C(3) 1.596(2), B(7)-C(37/48)
1.660(2)/1.664(2), B(9)-C(10)1.593(2), B(9)-C(15/26) 1.652(2)/1.662(2), B(7)-
O(8)-B(9) 139.9(1), C(3)-B(7)-C(37) 109.1(1), C(3)-B(7)-C(48) 116.2(1), C(37)-
B(7)-C(48) 105.7(1), C(10)-B(9)-C(15) 109.4(1), C(10)-B(9)-C(26) 114.7(1),
C(15)-B(9)-C(26) 107.4(1).
FLP systems featuring 1 as the Lewis acid component are
not able to bind/activate CO2 (in contrast to PtBu3/B(C6F5)3, for
example).31 With this in mind, analogous FLPs containing 2
were targeted, it being not only a stronger Lewis acid, but also
bifunctional, and hence potentially complementary to the
Given the vast disparity in the pKas of H2O and [HPtBu3]+
(ca. 31.2 and 11.4, respectively in DMSO)28,29 the cleavage of
water by PtBu3/2, presumably relies very heavily from a
thermodynamic viewpoint on the role of 2 as a chelating Lewis
acid to sequester OH-. With this in mind, we were also
interested in examining the behaviour of this FLP in the
presence of ammonia (pKa ca. 41 in DMSO),28 hypothesizing
Lewis basic component formed in the reaction of
a
phosphine/NHC with CO2 (Figure 5). However, benzene
solutions containing either the PtBu3/2 or tetramethylpiperi-
dine/2 FLP, show no propensity to bind CO2, either at room
temperature or 350 K. Both tertiary phosphines and secondary
amines have previously been shown to be capable of binding
CO2 in conjunction with B(C6F5)3, and the failure of systems
containing 2 to act in a similar manner presumably also
reflects its weaker Lewis acidity.32
The 6Dipp/2 FLP features the strongest Lewis base under
investigation, and it was postulated that this system might
therefore be capable of binding CO2. In our hands, however,
we find that this reaction does not proceed cleanly, and when
using benzene as the solvent, significant precipitate is formed.
In previous examples of CO2 capture by NHC-containing FLPs,
an initial activation step is observed involving nucleophilic
attack by the carbene at the carbon atom of CO2, before
trapping of the resulting betaine derivative by the Lewis acid
component.32c The reaction of 6Dipp with CO2 was therefore
investigated in the absence of 2 in an attempt to shed light on
the course of the reaction. Exposure of a benzene solution of
-
that a similar mode of binding might be possible with NH2 .
Thus, a ‘free-pump-thaw’ degassed solution of PtBu3/2 was
exposed to NH3 (at 1 atm. pressure) and stirred for 4 hours.
The resulting yellow crystalline product was crystallized from a
concentrated diethyl ether solution. However, the isolated
material does not give rise to any 31P NMR signals, and in situ
measurements of the reaction mixture show a single peak at
δP = 61.9 corresponding to free PtBu3. The 11B NMR spectrum
of the isolated product shows a single sharp resonance at δB =
-8.7 ppm, and the 19F NMR spectrum shows three signals at δF
=-133.3, -156.2 and -162.7 ppm (Δδm,p = 6.5 ppm), diagnostic
1
of a binding event occurring at both borane centres. The H
NMR signal corresponding to the NH unit integrates for six
protons, suggesting formation of the bis-ammonia adduct,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Journal Name
CyNCO
(5 equiv.)
room temp.
C6F5
6Dipp to CO2 (at ca. 1 atm. pressure), results in immediate
formation of an off-white precipitate. Moreover, investigation
of this material by 1H NMR (in bromobenzene-d5), reveals
signals characteristic of the formamidinium cation [(6Dipp)H]+,
together with three multiplet resonances at 6.15, 5.14 and
5.10 ppm (integrating to 1H each), corresponding to the
alkenic protons of an RCH=CH2 unit. These observations are
consistent with ring opening of the 6Dipp heterocycle via
deprotonation at the C5 position (Scheme 6), in a manner
analogous to that observed previously for 6Dipp in the
presence of other electrophiles (e.g. H+, [LAu]+).33 Although the
product in this case could not be crystallographically character-
ized, peak envelopes at m/z = 405 and 447 in the positive and
negative ESI-MS spectra, respectively, are consistent with the
formation of the proposed cationic and anionic components.
View Article Online
Fc
B(C6F5)2
O
DOI: 10.1039/C7DT04136E
B
C6F5
Fe
N
N
(with or
without PtBu3)
Cy
Cy
O
O
O
1
4
C6F5
C6F5
O
O
Fc
Fc
B
N
B
N
C6F5
Cy
C6F5
Cy
N
N
Cy
Cy
Scheme 7 Reaction of CyNCO with the PtBu3/1 FLP (or with 1 alone), leading
to the formation of 4.
6Dipp
H
6DippH
CO2
N
O
N
O
N
O
N
O
Dipp
Dipp
Dipp
Dipp
N
N
Dipp
Dipp
Figure 7 Molecular structure of 4, with thermal ellipsoids set at the 35%
probability level; hydrogen atoms and solvate molecules omitted, and
cyclohexyl groups shown in wireframe format for clarity. Selected bond
lengths (Å): C(9)-C(12) 1.452(2), C(12)-O(13) 1.288(2), O(13)-B(14) 1.534(2),
B(14)-N(15) 1.533(3), N(15)-C(16) 1.332(3), C(16)-N(17) 1.475(2), C(12)-
N(17) 1.340(3), C(16)-O(24) 1.215(3).
Scheme 6 Ring opening of 6Dipp in the presence of CO2.
In the absence of clean reactivity leading to the trapping of
carbon dioxide by FLPs containing 2, attention was turned to
more reactive CO2 surrogates, such as isocyanates. The
reaction of PtBu3/2 with CyNCO gives a complex mixture of
products, and so to simplify matters, the analogous chemistry
employing monofunctional 1 was probed. However, as in the
case of the PtBu3/2 system, the 31P NMR spectrum measured in
situ implies no net involvement of the phosphine, with an
unshifted resonance being observed at δP = 61.9 ppm. A
control reaction (without PtBu3) generates the same product
(albeit more slowly), which gives rise to signals in the 11B and
19F NMR spectra consistent with the formation of a four-
coordinate boron centre. In addition, a peak envelope centred
at m/z = 780 in the ESI-MS spectrum is consistent with the
assimilation of two molecules of CyNCO (Scheme 7). Crystals of
the product, 4, were obtained by slow cooling of a solution in
benzene-d6, and X-ray crystallography was used to unambig-
uously determine the connectivity associated with insertion of
two molecules of CyNCO into the CCp-B bond of 1 (Figure 7).
The molecular structure is based around a non-planar six-
membered CNCNBO heterocycle featuring a pendant carbonyl
function. Analysis of the bond lengths within the heterocycle
suggest that there is some degree of delocalized π character
within the O(13)-C(12)-N(17) unit [d{C(12)-O(13)} = 1.288(2) Å]
and also within the amido function defined by N(15)-C(16)-
O(24) [d{N(15)-C(16)} = 1.332(3) Å]. The relatively long C(16)-
N(17) distance [1.475(2) Å], on the other hand, suggests less
significant delocalization between these two fragments.
The insertion of heterocumulenes of this sort into reactive
B-C bonds is by no means unprecedented. Cowley and co-
workers have reported the insertion of dicyclohexyl
carbodiimide into the B-C bond of PhBCl2 to give the
benzamidinate system {PhC(CyN)2}BCl2 (Scheme 8a),34,35 and a
similar initial step in the reaction of 1 with CyNCO would
generate a closely-related BNCO four-membered ring via
migration of the more nucleophilic ferrocenyl fragment (cf.
C6F5; Scheme 8b). The more electrophilic nature of the boron
centre in this intermediate then presumably facilitates
nucleophilic attack by a second equivalent of the isocyanate,
with subsequent ring-closure generating the observed 6-
membered heterocyclic product.
Ph
Ph
(a)
Cl
B
Cy
Cy
Cy
1,3 Ph shift
CyNCNCy
N
N
Cl
PhBCl2
N
C
N
B
Cy
Fc
Cl
Cl
Fc
Fc
O
(b)
Cy
N
Cy
Cy
CyNCO
CyNCO
O
N
O
N
O
1
4
C
B
B
B
C6F5
N
C6F5
C6F5
C6F5
CyN
C6F5
C
C6F5
Cy
O
Scheme 8 (a) Mechanism for the insertion of a carbodiimide into the B-C
bond of PhBCl2 as reported by Cowley and co-workers;34 (b) potential
mechanism for the double insertion of cyclohexyl isocyanate into the B-CCp
bond of 1 to give 4.
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
0.058 mmol) were dissolved in the minimum benzene required
to dissolve all solid material (ca. 5 mL), and the reaction
View Article Online
DOI: 10.1039/C7DT04136E
Experimental
CAUTION: SOLUTIONS CONTAINING LiC6F5 ARE KNOWN TO
EXPLODE AT TEMPERATURES IN EXCESS OF -40oC.
mixture stirred for 24 h at room temperature. Volatiles were
then removed in vacuo, and the product recrystallized from
THF/hexane as single crystals suitable for X-ray crystal-
ography. Yield: 0.056 g, 65%.
General considerations
All manipulations were carried out using standard Schlenk line
or dry-box techniques under an atmosphere of argon or
dinitrogen, respectively. Solvents were degassed by sparging
with argon and dried by passing through a column of the
appropriate drying agent using a commercially available Braun
SPS. NMR spectra were recorded in chloroform-d, benzene-d6,
bromobenzene-d5 or THF-d8, which were dried over molecular
sieves, potassium, calcium hydride or calcium hydride
respectively, and stored under argon in Teflon valve ampoules.
NMR samples were prepared under argon in 5 mm Wilmad
1
Characterizing data for 2Ŋ(IMes)2: H NMR (500 MHz, THF-d8,
20oC) : δ 7.59, 7.46 (d, 2JH-H = 2.1 Hz, each 2H, backbone CH of
IMes), 6.99, 6.90, 6.70, 6.45 (s, each 2H, IMes aromatic CH),
3.80, 3.55, 3.22, 2.77 (br s, each 2H, CH of Cp), 2.40, 2.24, 2.20,
2.18, 1.98, 1.81 (s, each 6H, Me of IMes). 11B NMR (128 MHz,
THF-d8, 20oC ): δ -14 (sharp). 19F NMR (377 MHz, THF-d8, 20oC):
3
3
δ -117.8 (d, JFF = 24.5 Hz, ortho-CF of C6F5), -119.0 (d, JFF
=
24.5 Hz, ortho-CF of C6F5), -123.9 (d, 3JFF = 24.5 Hz, ortho-CF of
C6F5), -128.1 (d, 3JFF = 24.5 Hz, ortho-CF of C6F5), -163.3 (t, 3JFF
=
3
507-PP tubes fitted with J. Young Teflon valves. H and 13C
1
19.5 Hz, para-CF of C6F5), -164.1 (t , JFF = 20.0 Hz, para-CF of
C6F5), -167.1 (t, 3JFF = 22.1 Hz, meta-CF of C6F5), -167.4 (t, 3JFF
=
NMR spectra were recorded on Varian Mercury-VX-300 or
Bruker AVII-500 spectrometers and referenced internally to
residual protio-solvent (1H) or solvent (13C) resonances and are
reported relative to tetramethylsilane (δ = 0 ppm). 11B, 19F and
31P NMR spectra were referenced with respect to Et2O.BF3,
CFCl3 and 85% aqueous H3PO4, respectively. Chemical shifts
are quoted in δ (ppm) and coupling constants in Hz. UV-vis
spectra were collected on a Scintio UV S-2100 UV/Vis
spectrometer. Elemental analyses were carried out at London
3
22.1 Hz, meta-CF of C6F5), -167.9 (t, JFF = 22.1 Hz, meta-CF of
C6F5), -168.6 (t, JFF = 22.1 Hz, meta-CF of C6F5). Elemental
3
microanalysis: calc. (for C76H56B2F20FeN4) C 61.53, H 3.81, N
3.78; meas. C 61.88, H 4.01, N 3.66. Crystallographic data:
2Ŋ(IMes)2Ŋ3/2(THF)Ŋ3/2(pentane), C88H83B2F20FeN4O1.5
,
Mr
=
1678.05, orthorhombic, P 2ac 2ab, a = 11.4405(1), b =
22.3759(2), c = 32.0362(2) Å, V = 8201.0(1) Å3, Z = 4, ρc = 1.359
Mg m-3, T = 150 K, λ = 1.54180 Å. 17080 reflections collected,
16778 independent [R(int) = 0.029] used in all calculations. R1
= 0.0421, wR2 = 0.1252 for observed unique reflections
[I>2σ(I)] and R1 = 0.0428, wR2 = 0.1263 for all unique
reflections. Max. and min. residual electron densities 0.84 and
-0.49 e Å3. CCDC reference: 1582718.
Metropolitan University. BrC6F5, BuLi, PtBu3, PPh3, lutidine,
n
tetramethylpiperidine, ammonia and carbon dioxide were
sourced commercially. 1,1'-fc(BBr2)2, FcB(C6F5)2 (1), and NHCs
(IMes, IDipp, IMesCl2, 6Mes and 6Dipp) were prepared by
literature routes.14,15,36
In situ NMR data for 2Ŋ(IMesCl2)2: 11B NMR (128 MHz,
benzene, 20oC): δ -13 (sharp). 19F NMR (377 MHz, benzene,
Syntheses of novel compounds
3
20oC): δ -117.0 (d, JFF = 16.0 Hz, ortho-CF of C6F5), -118.9 (d,
1,1'-fc{B(C6F5)2}2 (2): To a solution of bromopentafluoro-
benzene (5.89 mL, 47.5 mmol) in hexane (30 mL) at -78˚C, was
added dropwise nBuLi (29.7 mL of a 1.6 M solution, 47.5 mmol)
and the mixture stirred at -78˚C for 45 min. CAUTION: LiC6F5 IS
KNOWN TO EXPLODE AT TEMPERATURES ABOVE -40oC. A
solution of 1,1'-fc(BBr2)2 (6.10 g, 11.6 mmol) also in hexane
(150 mL), was then added dropwise at -78oC, and the resulting
mixture allowed to warm slowly to room temperature over a
period of 12 h. After removal of volatiles in vacuo the product
was extracted into hot hexane. Further removal of the volatiles
in vacuo, and washing of the resulting solid with pentane,
yielded a dark purple solid which was dried thoroughly, and
found to be of sufficient purity for subsequent chemistry
without further recrystallization. Yield: 5.45 g, 54%. 1H, 11B and
19F NMR data agree with literature values.12b
General procedure for investigating the formation of Lewis
base adducts of 2: The procedure is exemplified for the case of
IMes in a 2:1 molar ratio. This and other Lewis bases were
probed in both 2:1 and 1:1 molar ratios. In the case of 1,1'-
fc{B(C6F5)2ŊIMes}2, 2Ŋ(IMes)2, the product was isolated and
characterized by standard spectroscopic, analytical and
crystallographic methods. For other Lewis bases, in situ 11B and
19F NMR data (based on those measured for 2Ŋ(IMes)2) were
used to determine whether or not a classical Lewis acid/base
adduct was formed. IMes (0.035 g, 0.116 mmol) and 2 (0.100 g,
3JFF = 16.0 Hz, ortho-CF of C6F5), -122.3 (d, 3JFF = 16.0 Hz, ortho-
CF of C6F5), -128.5 (d, 3JFF = 16.0 Hz, ortho-CF of C6F5), -159.3 (d,
3JFF = 22.5 Hz, para-CF of C6F5), -162.0 (d, 3JFF = 22.5 Hz, para-CF
3
of C6F5), -164.3 (d, JFF = 20.9 Hz, meta-CF of C6F5), -165.4 (d,
3JFF = 20.9 Hz, meta-CF of C6F5), -166.3 (d, 3JFF = 20.9 Hz, meta-
CF of C6F5), -166.5 (d, 3JFF = 20.9 Hz, meta-CF of C6F5).
In situ NMR data for 2Ŋ(6Mes)2: 11B NMR (128 MHz, benzene,
20oC): δ -11 (sharp). 19F NMR (377 MHz, benzene, 20oC): δ -
3
3
112.6 (d, JFF = 29.7 Hz, ortho-CF of C6F5), -118.8 (d, JFF = 27.4
Hz, ortho-CF of C6F5), -119.9 (d, 3JFF = 24.5 Hz, ortho-CF of C6F5),
-123.0 (d, 3JFF = 25.4 Hz, ortho-CF of C6F5), -153.3 (t, 3JFF = 19.4
Hz, para-CF of C6F5), -158.3 (t , 3JFF = 19.4 Hz, para-CF of C6F5), -
3
3
161.2 (t, JFF = 21.3 Hz, meta-CF of C6F5), -163.6 (t, JFF = 23.6
Hz, meta-CF of C6F5), -164.2 (t, 3JFF = 23.9 Hz, meta-CF of C6F5),
-165.6 (t, 3JFF = 21.4 Hz, meta-CF of C6F5).
In situ NMR data for 2Ŋ(PnBu3)2: 11B NMR (128 MHz, benzene,
20oC): δ -11 (broad). 19F NMR (377 MHz, benzene, 20oC): δ -
126.2 (broad s, ortho-CF of C6F5), -157.9 (t, 3JFF = 20.3 Hz, para-
CF of C6F5), -163.7 (t, 3JFF = 20.3 Hz, meta-CF of C6F5). 31P NMR
(162 MHz, benzene, 20oC): δ -2.9.
In situ NMR data from the reaction of 2 with PCy3: (500 MHz,
chloroform-d, 20oC): δ 4.42, 4.38, 4.22, 4.09, 3.65, 3.29, 3.11,
3.08 (broad s, each 1H, C5H4B), 2.65 (m, 3H, Cy), 2.10-1.22
(overlapping m, 30 H, Cy). 11B NMR (128 MHz, benzene-d6,
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
20oC): δ 32 (very broad), 3 (broad). 19F NMR (377 MHz, independent [R(int) = 0.026] used in all calcuVlaietwioAnrtisc.le ORnline
=
1
benzene-d6, 20oC): δ -123.4 (s, 2F, C6F4), -129.3 (s, orthoCF of 0.0326, wR2 = 0.804 for observed unique reflections [I>2σ(I)]
DOI: 10.1039/C7DT04136E
B(C6F5)2), -123.0 (s, orthoCF of B(C6F5)2), -131.4 (s, 2F, C6F4), - and R1 = 0.0351, wR2 = 0.0822 for all unique reflections. Max.
131.9 (s, orthoCF of BF(C6F4PCy3)(C6F5)), -156.3 (broad s, para- and min. residual electron densities 0.31 and -0.39 e Å3. CCDC
CF of B(C6F5)2), -156.7 (broad s, paraCF of B(C6F5)2), -160.1 (s, reference: 1576950.
paraCF of BF(C6F4PCy3)(C6F5)), -163.5 (s, metaCF of B(C6F5)2), - 2Ŋ(NH3)2: A Schlenk flask containing 2 (0.100 g, 0.114 mmol)
163.9 (s, metaCF of B(C6F5)2), -164.5 (s, metaCF of was cooled to -60 °C and sufficient NH3 condensed to allow all
BF(C6F4PCy3)(C6F5)), -194.9 (s, BF). 31P NMR (162 MHz, of the solid material to dissolve. The solution was stirred, and
benzene-d6, 20oC): δ 40.1.
allowed to warm to 20oC, while ammonia gas was vented to
3: A mixture of PCy3 (0.053 g, 0.19 mmol) and 1 (0.100 g, 0.19 waste. The resulting yellow solid was isolated, and single
mmol) were dissolved in benzene-d6 (2 mL) at which point crystals suitable for X-ray crystallography were obtained from
analysis by 1H, 11B and 31P NMR spectroscopy revealed an a concentrated ether solution stored at -30oC. Yield: 0.089 g,
1
unquenched mixture of Lewis acid and Lewis base 86%. H NMR (500 MHz, benzene-d6, 20oC): δ 3.89 (s, 4H,
components. After stirring for 10 d, a brown oil formed at the C5H4), 3.55 (s, 4H, C5H4) 3.41 (b s, 6H, NH). 19F NMR (377 MHz,
bottom of the reaction vessel; volatiles were removed in benzene-d6, 20oC): δ -133.3 (d, 3JFF = 9.2 Hz, ortho-CF of C6F5), -
vacuo, and the resulting residue dissolved in chloroform-d (5 156.2 (t, 3JFF = 20.4 Hz, para-CF of C6F5), -162.7 (t, 3JFF = 22.4 Hz,
mL) and layered with hexane. A very small quantity of orange meta-CF of C6F5). 11B NMR (128 MHz, benzene-d6, 20oC): -9
crystals was formed, which was isolated by filtration and dried (sharp). Elemental microanalysis: calc. (for C34H14B2F20FeN2) C
in vacuo. Yield: <5%. 1H NMR (500 MHz, chloroform-d, 20oC): δ 44.94, H 1.55, N 3.08%; meas. C 44.89, H 1.56, N 3.28%. MS
4.42 (broad s, 2H, C5H4B), 4.18 (broad s, 5H, Cp), 3.35 (broad s, (ESI): 908.1. Crystallographic data: 2Ŋ(NH3)2Ŋ3/2(C6H6),
2H, C5H4B), 2.90 (m, 3H, PCH of Cy), 2.13-1.27 (overlapping m, C43H23B2F20FeN2, Mr = 1025.10, triclinic, P-1, a = 10.4515(3), b =
30 H, CH2 of Cy). 11B NMR (96 MHz, chloroform-d, 20oC): δ 2 13.5721(3), c = 14.2616(4) Å, α = 94.087(2), β = 93.027(3), γ =
(broad). 19F NMR (282 MHz, chloroform-d, 20oC): δ -124.7 (m, 100.948(2)o, V = 1976.6(1) Å3, Z = 2, ρc = 1.722 Mg m-3, T = 150
2F, C6F4), -132.0 (m, 2F, C6F4), -133.3 (d, 3JFF = 20.9 Hz, ortho-CF K, λ = 1.54180 Å. 41166 reflections collected, 8188 independ-
of C6F5), -162.2 (t, 3JFF = 20.9 Hz, para-CF of, C6F5), -165.8 (m, ent [R(int) = 0.021] used in all calculations. R1 = 0.0259, wR2 =
meta-CF of C6F5), -193.7 (broad s, BF). 31P NMR (121 MHz, 0.0656 for observed unique reflections [I>2σ(I)] and R1
=
chloroform-d, 20oC): δ 39.9. Crystallographic data: 3ŊCHCl3, 0.0270, wR2 = 0.0664 for all unique reflections. Max. and min.
C41H43BCl3F10FeP, Mr = 929.76, triclinic, P-1, a = 11.7155(3), b = residual electron densities 0.32 and -0.34 e Å3. CCDC
12.6410(4), c = 15.6649(3) Å, α = 83.830(2), β = 79.824(2), γ = reference: 1576951.
66.833(3)º, V = 2097.4(1) Å3, Z = 2, ρc = 1.472 Mg m-3, T = 150 Reaction of 6Dipp with CO2: 6-Dipp (0.100 g, 0.247 mmol) was
K, λ = 1.54180 Å. 8635 reflections collected, 8635 independent dissolved in minimal benzene, the solution freeze-pump-thaw
[R(int) = 0.019] used in all calculations. R1 = 0.0357, wR2 = degassed, and the evacuated headspace backfilled with CO2 to
0.0824 for observed unique reflections [I>2σ(I)] and R1
=
1 bar pressure. The reaction mixture was stirred for 4 h, and
0.0374, wR2 = 0.0838 for all unique reflections. Max. and min. the resulting precipitate then isolated by filtration, washed
residual electron densities 0.54 and -0.51 e Å3. CCDC with pentane and dried in vacuo. The product was dissolved in
reference: 954137.
0.6 mL of bromobenzene-d5 and transferred to a J. Young’s
1
[HPtBu3][2Ŋ(µ-OH)]: 2 (0.100 g, 0.114 mmol) was dissolved in tube for analysis by multinuclear NMR. H NMR (500 MHz,
minimal benzene and combined with PtBu3 (0.023 g, 0.114 bromobenzene-d5, 20oC): (signals due to cation) δ 7.19 (m, 6H,
mmol). One drop of deionized water was added and the aromatic CH of Dipp), 3.61 (m, 4H, NCH2), 3.10 (sept, 3JHH = 7.1
mixture stirred for 1 h. Volatiles were removed in vacuo and Hz, 4H, CH of iPr), 2.39 (m, 2H, CH2), 1.38 (d, 3JHH = 7.1 Hz, 24H,
single crystals of the product obtained from a concentrated Me of iPr); (signals due to anion) 7.19 (m, 6H, Ar), 6.15 (m, 1H,
1
chloroform solution stored at -30 °C. Yield: 0.097 g, 78%. H alkene CH), 5.14 (m, 1H, alkene CH2), 5.10 (broad s, 1H, alkene
3
i
NMR (500 MHz, chloroform-d, 20 ˚C): δ 5.46 (broad s, 1H, OH), CH2), 4.35 (d, JHH = 6.5 Hz, 2H, CH2), 3.10 (m, 4H, CH of Pr),
4.06 (d, 1JPH = 461 Hz, 1H, PH), 3.91 (s, 4H, Cp), 3.68 (s, 4H, Cp), 1.24 (d, 24H, Me of Pr). ESI-MS (m/z): (pos) 405 [6-DippH]+;
i
3
1.55 (d, JPH = 15.2 Hz, 27H, tBu). 11B NMR (128 MHz, (neg) 447 [C29H39N2O2]-.
chloroform-d, 20oC): δ 1 (broad). 19F NMR (377 MHz, 4: 1 (0.100 g, 0.189 mmol) and excess (>5 equiv.) of CyNCO
chloroform-d, 20oC): δ -132.4 (d, JFF = 10.7 Hz, ortho-CF of were dissolved in benzene-d6 and the resulting solution
3
C6F5), -160.4 (t, 3JFF = 20.4 Hz, para-CF of C6F5), -164.8 (t, 3JFF
=
transferred to a Young’s NMR tube, for spectroscopic analysis
19.5 Hz, meta-CF of C6F5). 31P NMR (121 MHz, chloroform-d, over a period of one week. Storage at 6oC resulted in the
20˚C): δ 58.3. ESI-MS (m/z): (pos) 203.2 [HPtBu3]+; (neg) 921.0 formation of orange-red crystals. 1H NMR (500 MHz,
[2.(µ-OH)]-.
Elemental
microanalysis:
calc.
(for chloroform-d, 20oC): δ 4.89 (t, JHH = 2.1 Hz, 2H, Cp), 4.80 (t,
3
C55H46B2F20FeOP) C 54.49, H 3.83%; meas. C 54.88, H 4.01%. 3JHH = 2.1 Hz, 2H, Cp), 4.18 (s, 5H, Cp), 4.02 (tt, JHH = 12.0 Hz,
3
3
4
Crystallographic
data:
[HPtBu3][2Ŋ(µ-OH)]Ŋ3/2(benzene), 4JHH = 3.4 Hz, 1H, CH of Cy), 3.04 (tt, JHH = 12.0 Hz, JHH = 3.4
C55H46B2F20FeOP, Mr = 1211.37, triclinic, P-1, a = 11.8245(4), b Hz, 1H, CH of Cy), 2.70–1.05 (20H, CH2 of Cy). 19F NMR (282
= 13.6477(4), c = 15.8558(4) Å, α = 84.214(2), β = 88.541(2), γ = MHz, chloroform-d, 20oC): δ -133.2 (d, 3JFF = 25.2 Hz, ortho-CF
87.111(3)º, V = 2542.0(1) Å3, Z = 2, ρc = 1.583 Mg m-3, T = 150 of C6F5), -156.0 (t, JFF = 20.9 Hz, para-CF of C6F5), -163.1 (td,
3
K, λ = 1.54180 Å. 27110 reflections collected, 10487 3JFF = 21.9 Hz, 4JFF = 9.6 Hz, meta-CF of C6F5). 11B NMR (96 MHz,
8 | J. Name., 2012, 00, 1-3
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chloroform-d, 20oC): δ 1. 13C NMR (126 MHz, chloroform-d,
View Article Online
DOI: 10.1039/C7DT04136E
Conflicts of interest
There are no conflicts to declare.
1
20oC): 178.1 (CO), 149.4 (Fc quaternary C), 148.1 (d, JCF
=
=
243.5 Hz, C6F5), 140.3 (d, 1JCF = 254.2 Hz, C6F5), 137.0 (d, 1JCF
255.6 Hz, C6F5), 75.0 (C5H4), 72.8 (C5H4), 70.7 (Cp), 61.8 (CH of
Cy), 50.0 (CH of Cy), 31.2, 30.1, 28.9, 27.9, 26.5, 24.9 (CH2 of
Cy). ESI-MS (m/z): (pos) 780 [M]+; accurate mass: calc. (for
C36H31BF10FeN2O2, [M]+) 781.1747; meas. 781.1748.
Crystallographic data: 4ŊC6H6, C42H37BF10FeN2O2, Mr = 858.41,
triclinic, P-1, a = 10.9221(1), b = 11.1191(1), c = 17.0196(2) Å, α
= 94.976(1), β = 95.697(1), γ = 112.976(1)o, V = 1875.7(1) Å3, Z
= 2, ρc = 1.520 Mg m-3, T = 150 K, λ = 1.54180 Å. 55367
reflections collected, 8562 independent [R(int) = 0.030] used in
all calculations. R1 = 0.0389, wR2 = 0.0842 for observed unique
reflections [I>2σ(I)] and R1 = 0.0648, wR2 = 0.0926 for all
unique reflections. Max. and min. residual electron densities
0.67 and -0.57 e Å3. CCDC reference: 1576592.
Acknowledgements
We acknowledge funding from the EPSRC (studentship to MJK:
EP/P505216/1), and Dr Jamie Hicks (Oxford) for help with
crystallography.
Notes and references
1
Initial identification of steric frustration in Lewis acid/base
complex formation: H.C. Brown, H.I. Schlesinger and S.Z.
Cardon, J. Am. Chem. Soc., 1942, 64, 325.
Initial demonstration of the use of FLPs in small molecule
activation: G.C. Welch, R.R.S. Juan, J.D. Masuda and D.W.
Stephan, Science, 2006, 314, 1124.
2
Crystallography
3
4
5
6
7
8
9
For a recent review, see: D.W. Stephan and G. Erker, Angew.
Chem., Int. Ed., 2015, 54, 6400.
See, for example: T.J. Herrington, A.J.W. Thom, A.J.P. White
and A.E. Ashley, Dalton Trans., 2012, 41, 9019.
See, for example: C. Appelt, J.C. Slootweg, K. Lammertsma
and W. Uhl, Angew. Chem., Int. Ed., 2013, 52, 4256.
See, for example: J.M. Farrell, J.A. Hatnean and D.W.
Stephan, J. Am. Chem. Soc., 2012, 134, 15728.
See, for example: A.M. Chapman, M.F. Haddow and D.F.
Wass, J. Am. Chem. Soc., 2011, 133, 18463.
C.R. Wade, A.E.J. Broomsgrove, S. Aldridge and F.P. Gabbaï,
Chem. Rev., 2010, 110, 3958.
Data for 2Ŋ(IMes)2, [HPtBu3][2Ŋ(µ-OH)], 2Ŋ(NH3)2, 3 and 4 were
collected on a Oxford Diffraction Supernova diffractometer at
150 K. Data collection and reduction were carried out using
CrysAlisPro or Denzo/Scalepack, structure solution using
Superflip, and refinement using CRYSTALS or SHELXT-2014/7.37
Complete details of all structures are contained within the
respective CIFs which have been deposited with the CCDC
(reference numbers: 954137, 1576950-1576952 and 1582718).
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Conclusions
The bifunctional Lewis acid 1,1'-fc{B(C6F5)2}2 (2) is shown to
form frustrated Lewis pairs with the sterically encumbered N-
donor bases lutidine and tetramethylpiperidine, together with
PPh3, PtBu3 and the expanded ring carbene 6Dipp. In the case
of PCy3, attack at the para-CF bond of one of the C6F5 groups
leads to nucleophilic aromatic substitution with accompanying
migration of fluoride to boron. The more facile nature of this
chemistry in comparison to that observed for the mono-
functional analogue FcB(C6F5)2 (1) can be attributed to the
more electron deficient nature of the -B(C6F5)2 functions in 2.
The PtBu3/2 FLP can be shown to cleave one of the O-H
bonds in water to give a tertiary phosphonium salt in which
the anion features a symmetrically bound ‘inverse-chelated’ B-
O(H)-B unit. Attempts to bring about analogous N-H bond
activation with ammonia are frustrated by the weaker
Brønsted acidity of the N-H bond and/or the inability to
generate a similar B-N(H)2-B motif due to the (less reversible)
formation of strong H3N→B donor/acceptor bonds. Neither
PtBu3/1 or PtBu3/2 shows any propensity to activate carbon
dioxide, in contrast to the combination of PtBu3 and the
stronger Lewis acid B(C6F5)3. Attempts to use the stronger
Lewis base 6Dipp lead to ring opening of the carbene brought
about by coordination to the electrophilic C-atom of CO2, while
the use of the more reactive heterocumulene CyNCO can be
shown to lead to insertion into the C-B bond linking the borane
function to the ferrocene backbone.
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ARTICLE
Journal Name
Aldridge, Dalton Trans., 2013, 42, 12836; (t) P. Thilagar, D. 30 P.A. Chase and D.W. Stephan, Angew. Chem., Int. Ed., 2008,
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10 A. Ramos, A. J. Lough and D. W. Stephan, Chem. Commun.,
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11 (a) M.J. Kelly, J. Gilbert, R. Tirfoin and S. Aldridge, Angew.
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35 See also: (a) G.A. Pierce, D. Vidovic, D.L. Kays, N.D. Coombs,
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25 2 captures/cleaves H2O over a relatively short timeframe (ca.
1 h). Prolonged exposure to excess water, however, leads to
the formation of boric acid, pentafluorobenzene and
ferrocene, each of which could be characterized by
multinuclear NMR. Such chemistry mirrors the behaviour 1
towards water, which also proceeds via B-C bond
cleavage.12a
26 For an earlier example of O-H bond cleavage in water by an
FLP, see: R. Roesler, W.E. Piers and M. Parvez, J. Organomet.
Chem., 2003, 680, 218.
27 G.E. Herberich, U. Englert, A. Fischer and D. Wiebelhaus,
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28 http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf
Retrieved 24 September 2017.
29 M.R. Netherton and G.C. Fu, Org. Lett., 2001, 3, 4295.
10 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7DT04136E
For Table of Contents
Applications of the bifunctional Lewis acid 1,1'-fc{B(C6F5)2}2 in FLP chemistry are described,
including reactions towards H2O, NH3, CO2 and cyclohexylisocyanate.