Novel B(Ar′)2(Ar″) hetero-tri(aryl)boranes: A systematic study of Lewis acidity

Article abstract of DOI:10.1039/c5dt03854e

A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H₂. Under mild conditions eight of these will cleave H₂; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids.

Full text of DOI:10.1039/c5dt03854e

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: R. J. Blagg, T. R.
Simmons, G. R. Hatton, J. M. Courtney, E. L. Bennett, E. J. Lawrence and G. G. Wildgoose, Dalton Trans.,
2015, DOI: 10.1039/C5DT03854E.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/dalton

Page 1 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
Novel B(Ar')2(Ar'') hetero-tri(aryl)boranes: a systematic study of
Lewis acidity †
Robin J. Blagg,* Trevor R. Simmons, Georgina R. Hatton,‡ James M. Courtney, Elliot L.
Bennett, Elliot J. Lawrence and Gregory G. Wildgoose*
School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, U.K.
* g.wildgoose@uea.ac.uk ; r.blagg@uea.ac.uk
Abstract
A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-
bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been
studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under
mild conditions eight of these will cleave H2; the rate of cleavage depending on both the
electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow
comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different
series of boranes. Discrepancies in the correlation between these two types of measurements,
combined with structural characterisation of each borane, reveal that the twist of the aryl rings with
respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective,
and is an important consideration in the design of tri(aryl)boranes as Lewis acids.
Introduction
Since the initial report by Welch and Stephan 1 there has been rapid growth in studies of frustrated
Lewis pairs (FLPs).2-7 In the archetypal system the Lewis acid, B(C6F5)3, and Lewis base, P(tBu)3, are
combined and are restricted from forming a classical Lewis acid-base adduct due to their steric bulk.
Upon the addition of H2, however, this FLP heterolytic cleaves H2 to give protic and hydridic products.
FLPs in conjunction with H2 have found applications as mediators or catalysts for metal-free
hydrogenation of numerous functional groups including; aldehydes and ketones,8-10 N-heterocyclic
aromatics,11 imines and nitriles,12 and silyl enol ethers.13 FLPs have also been shown to react with
other small molecules such as oxides of carbon,14-16 nitrogen,17 and sulfur,18 along with alkenes and
alkynes.19, 20
Tris(pentafluorophenyl)borane, B(C6F5)3, is the most commonly used Lewis acidic component of
FLPs; although other electrophilic boranes have been used in FLPs (or suggested for use in FLPs),
such as other halogenated tri(aryl)boranes 9, 15, 21-23 including the stepwise-substitution series
B(C6Cl5)n(C6F5)3−n (n = 1-3),24, 25 and B{2,4,6-(CH3)3C6H2}n(C6F5)3−n (n = 1-3);26 and borenium cations
(commonly stabilised by N-heterocycle carbenes).27, 28 Not all Lewis acids used in FLPs have been
Page 1

Dalton Transactions
Page 2 of 23
View Article Online
DOI: 10.1039/C5DT03854E
boron based, with other examples including; Ingleson and co-workers’ carbon-based, water tolerant N-
methylacredinium salts,29 tri(aryl)aluminium analogues of classical boron based species,30 both
phosphorus(III) and phosphorus(V)-based species,31 and silicon based species.32, 33
We have previously introduced the concept of “combined electrochemical-frustrated Lewis pairs”,28, 34-
37 where the heterolytic cleavage of H2 by an FLP is coupled with in situ electrochemical oxidation of
the resulting Lewis acid-hydride, liberating two electrons, a proton, and regenerating the parent Lewis
acid. Hence, we have shown these systems to be electrocatalytic for the oxidation of H2 to yield two
protons and two-electrons overall – a key process for many hydrogen-based energy technologies.
To further develop our “combined electrochemical-frustrated Lewis pair” concept, we sought to
expand the range of tri(aryl)borane Lewis acids allowing us to probe the effects of further controlled
modification of the boranes. Our existing studies having focused on the archetypal B(C6F5)3,34, 35 its
perchlorinated analogue,36 three isomers of B{C6H3(CF3)2}3,37 along with the borenium cation
[(iPrN)2H2C3BC9H14]+.28 The stepwise substitution of the aryl rings would result in a range of 2:1
hetero-tri(aryl)boranes, in addition to the existing homo-tri(aryl)boranes. It was envisaged that by
altering the electronic and steric nature of successive aryl groups, that the Lewis acidic boranes would
exhibit varying properties (e.g. reactivates towards H2 activation, chemical tolerances, redox
properties). Such a systematic study could also allow for insights into the chemical origin of these
properties and provide further information toward a predictive model for reactivities of
tri(aryl)boranes.
Herein we report studies of four novel hetero-tri(aryl)boranes 2, 3, 5 and 6, incorporating
combinations of the pentafluorophenyl (C6F5, ArF5), 3,5-bis(trifluoromethyl)phenyl {3,5-(CF3)2C6H3,
ArF6}, and pentachlorophenyl (C6Cl5, ArCl5) halogenated-aryl rings; together with further studies of the
previously reported 24 tris(pentachlorophenyl)borane, 7, and the hetero-
(pentachlorophenyl)(pentafluorophenyl)boranes 8 and 9. Comparing these with the archetypal Lewis
acidic borane B(C6F5)3, 1, and B{3,5-(CF3)2C6H3}3, 4, allows for a comprehensive study of the effects
of stepwise substitution of the aryl rings (Figure 1). Both electrochemical and conventional NMR
methodologies are employed to quantify the Lewis acidity / electrophilicity of the boranes, and the
results of these methodologies compared. Finally, their potential to act as the Lewis acidic component
of an FLP {in combination with the Lewis base tri-tert-butylphosphine (tBu3P)} for the heterolytic
cleavage of H2 under mild conditions is studied.
Page 2

Page 3 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
Figure 1
Homo- and hetero-tri(aryl)boranes, B(Ar')2(Ar'') / BArHalx
Results&Discussion
The novel hetero-tri(aryl)boranes 3, 5 and 6, were all synthesised by broadly similar methods (Scheme
1), involving the reaction of a bis(aryl)haloborane with a suitable metal-aryl transfer agent. Reaction
of B{3,5-(CF3)2C6H3}2Br with Cu(C6F5) in CH2Cl2 at room temperature, or Zn(C6Cl5)2 in toluene at
+75 °C, led to synthesis of 3 and 5 respectively. In certain cases however, the impurity B{3,5-
(CF3)2C6H3}2(OH) was obtained (characterisation data in SI), due to reaction of the
bis(aryl)bromoborane precursor with trace water or hydroxide, fortunately 3 and 5 can be isolated by
sublimation or recrystallization, respectively. 6 is synthesised by the reaction of a toluene solution of
B(C6Cl5)2Cl with in situ generated Li{3,5-(CF3)2C6H3} in Et2O solution at −78 °C followed by slow
warming to room temperature.
Scheme 1
Synthetic routes for the synthesis of 3, 5 and 6
Applying this methodology for the synthesis of 2, by reaction of B(C6F5)2X (X = F, Cl) with M{3,5-
(CF3)2C6H3} (M = Li, Cu) under various conditions, we were unable to obtain 2 as anything other than
a minor component in a mixture of species. Therefore an alternative synthetic route was developed
(Scheme 2). Following on from the publication by Samigullin et al38 of a high yielding stepwise route
Page 3

Dalton Transactions
Page 4 of 23
View Article Online
DOI: 10.1039/C5DT03854E
for the synthesis of B{3,5-(CF3)2C6H3}2Br, we were able to successfully adapt their methodology for
the synthesis of B{3,5-(CF3)2C6H3}Br2. This required the generation of Li{3,5-(CF3)2C6H3} at −78 °C,
and it’s reaction in Et2O with BH3·SMe2 to give [Li(OEt2)n][H3B{3,5-(CF3)2C6H3}]. Hydride
abstraction using one equivalent Me3SiCl gave BH2{3,5-(CF3)2C6H3}, and reaction with excess
methanol converts the bis-hydride to the bis-methoxide B{3,5-(CF3)2C6H3}(OMe)2 (characterisation
data in SI). Conversion of the bis-methoxide to the bis-bromide was readily achieved by reaction with
excess BBr3, leading to the isolation of highly reactive B{3,5-(CF3)2C6H3}Br2 as a pale yellow oil.
Further reaction with two equivalents of Cu(C6F5) in CH2Cl2, results in the rapid generation of 2,
which may be purified via sublimation.
1. nBuLi, 3,5-(CF3)2C6H3Br
2. Me3SiCl
3. 3 MeOH
BH3(SMe2)
B{3,5-(CF3)2C6H3}(OMe)2
B{3,5-(CF3)2C6H3}Br2
B{3,5-(CF3)2C6H3}(OMe)2
B{3,5-(CF3)2C6H3}Br2
Et2O
2 BBr3
CxHy, 20°C, 1 h
2 Cu(C6F5)
CH2Cl2, 20°C, 1 h
B(C6F5)2{3,5-(CF3)2C6H3}
2
Scheme 2
Synthetic route for the synthesis of 2
The syntheses of 7-9 have previously been reported by Ashley et al,24 however we found a number of
minor changes to the published methodologies were necessary to reliably synthesise these compounds.
For the lithium-halogen exchange of C6Cl6 and nBuLi to generate LiC6Cl5, we found control of
temperature was critical with any temperature increase above −78 °C leading to an unacceptably high
generation of decomposition products. Additionally we found that due to the sparing solubility of
C6Cl6 in Et2O using as large a volume of solvent as practical was advantageous for initial lithiation. Of
equal importance was the addition of an anti-solvent (n-pentane) in approximately a 3:2 ratio to
precipitate out the aryl-lithium compound, so as to avoid the risk of undesirable Et2O cleavage
products, as previously reported by Ashley et al.24 These modifications allowed for the synthesis of 7
(by reaction of three equivalents of LiC6Cl5 with BCl3), and B(C6Cl5)2Cl (by reaction of two
equivalents of LiC6Cl5 with BCl3, precursor for 8), and B(C6Cl5)Br2 {by reaction of half an equivalent
of Zn(C6Cl5)2 with BBr3, precursor for 9), to be readily achievable. Compound 9 was obtained by
reaction of B(C6Cl5)Br2 with two equivalents of Cu(C6F5) and purification by sublimation as
previously reported. However, we were unable to reproduce the published synthesis of 8 by reaction of
B(C6Cl5)2Cl with Cu(C6F5) in toluene at +80 °C. We were however able to successfully synthesise 8
by reaction of B(C6Cl5)2Cl with freshly generated LiC6F5 in a toluene solution at −78 °C followed by
slow warming to room temperature, and extraction into n-hexane.
Page 4

Page 5 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
Structural Studies
X-ray crystal structures were obtained from single crystals of 5 (grown by slow diffusion of a saturated
CH2Cl2 solution into n-hexane at −25 °C), and 6 (grown from a saturated n-hexane solution at −25 °C)
(Figures 2a-b and S1a-b, Table 5).
Figure 2a
X-ray crystallographic structure of B{3,5-(CF3)2C6H3}2(C6Cl5) 5
Figure 2b
X-ray crystallographic structure of B(C6Cl5)2{3,5-(CF3)2C6H3} 6
(minor component of crystallographic disorder removed for clarity)
We have been unable to grow single crystals of 1-3, so instead, geometry optimised structures have
been calculated computationally {at the B3LYP/6-311+G(d,p) level of theory, see SI for further
details} (we previously reported the calculated structure of 134) (Figures S2a-c); calculated structures
of 4-8 do not show significant differences when compared to their X-ray crystal structures, thereby
validating this approach. {9 is an exception, due to the two ArF5 rings having significantly different
twist-angles (assumed to be due to crystal packing effects) in the crystal structure, a feature that is not
reproduced in the calculated structure.}
The crystallographic and calculated structures of 1-3, 5 and 6, along with the previously published
crystal structures of 4 39 and 7-9,24 all show similar features: a trigonal-planar boron centre and the
three aryl rings twisted with respect to the BC3 plane, leading to a propeller type conformation,
thereby minimising steric interactions between the aryl rings. The ortho-substituents would dominate
such steric interactions leading to the smallest (13-36°) twist occurring for ArF6 rings (ortho-H) the
largest (56-80°) for the ArCl5 rings (ortho-Cl) with the ArF5 rings (ortho-F) in the middle (22-52°), as
shown in Table 1.
Page 5

Dalton Transactions
Page 6 of 23
View Article Online
DOI: 10.1039/C5DT03854E
Table 1
Mean (standard deviation) twist-angles of aryl ring(s) from BC3 plane
ArF5
ArF6
ArCl5
B(C6F5)3 1 34
37.0° a
-
-
B(C6F5)2{3,5-(CF3)2C6H3} 2
B{3,5-(CF3)2C6H3}2(C6F5) 3
B{3,5-(CF3)2C6H3}3 4 39
B{3,5-(CF3)2C6H3}2(C6Cl5) 5
B(C6Cl5)2{3,5-(CF3)2C6H3} 6
B(C6Cl5)3 7 24
51.7° a
51.1° a
25.1° a
30.8° a
36(2)°
26(1)°
13(0)°
-
-
-
-
-
-
79.7° b
62(2)°
56(3)°
59(3)°
70(1)°
-
-
B(C6Cl5)2(C6F5) 8 24
B(C6F5)2(C6Cl5) 9 24
22(1)°
38(16)° c
-
-
a crystal structure not known, angles from optimised {B3LYP/6-311+G(d,p)} structure
b crystallographic symmetry leads to only one unique twist-angle
c each aryl ring has significantly different twist-angle: 52(1), 24(2)°
As the twist-angles of the aryl rings vary, changes in the electronic interactions of the ring and its
substituents with the boron centre can be implied. Firstly, large twist-angles orientate the rings such
that their ortho-substituents are above/below the boron centred trigonal plane, creating the potential
for through-space donation of any lone pair electron density on the ortho-substituents into the formally
vacant boron 2pz orbital, as we have shown to occur for the ortho-CF3 substituents of the 2,4- and 2,5-
isomers of 4.37 Secondly, as the twist-angle decreases, symmetry considerations imply an increase in
overlap between the boron 2pz orbital and the filled (2pz derived)
π orbitals of the aromatic ring (while
electronic effects via -bonding between the boron and aryl rings should be independent of the twist-
σ
angle of the aryl rings). In both these cases, donation of electron density into the boron 2pz orbital
would be expected to attenuate the borane’s Lewis acidity, influencing both its observed
electrochemical properties and reactivity.
Electrochemical Studies
Cyclic voltammograms were obtained at varying scan rates for 2, 3, 5-9 (Figure 3), at a glassy carbon
electrode, in the weakly-coordinating solvent CH2Cl2 using [nBu4N][B(C6F5)4] as the added electrolyte,
and compared with those previously reported by this group under the same conditions for 1 and 4
(reproduced in Figures S3a-b).37 Note that whilst electrochemical studies of 7-9 have been previously
reported,24 we have repeated the measurements herein to ensure they are comparable with our other
results. This is especially true considering our use of the weakly-coordinating anion [B(C6F5)4]− in the
electrolyte together with a glassy carbon macro-electrode, in contrast to the previously reported
voltammograms using an electrolyte containing the potentially non-innocent [BF4]− anion and a
platinum micro-electrode.
Page 6

Page 7 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
Figure 3
Experimental (line) and simulated (open circles) cyclic voltammograms for the reduction of: (a)
B(C6F5)2{3,5-(CF3)2C6H3} 2; (b) B{3,5-(CF3)2C6H3}2(C6F5) 3; (c) B{3,5-(CF3)2C6H3}2(C6Cl5) 5; (d)
B(C6Cl5)2{3,5-(CF3)2C6H3} 6; (e) B(C6Cl5)3 7; (f) B(C6Cl5)2(C6F5) 8; (g) B(C6F5)2(C6Cl5) 9. Shoulders (*) visible
at the higher scan rates due to trace impurity in the solvent/electrolyte
There are three general behaviours observed in the cyclic voltammograms recorded for boranes 1-9.
The first, for 2-4, showing completely irreversible reductions at all scan rates studied (50 mV.s−1 to 2.0
V.s−1), indicating vary rapid reaction/decomposition of the radical-anion intermediate generated upon
reduction of the parent borane.34 The second, for 1, 5 & 9, appear irreversible at slower scan rates yet
as the scan rates increase appear quasi-reversible as the kinetics of the homogeneous follow-on
Page 7

Dalton Transactions
Page 8 of 23
View Article Online
DOI: 10.1039/C5DT03854E
chemical decomposition step are outrun on the voltammetric timescale. Finally, 6-8, appear quasi-
reversible over the entire range of scan rates.
To quantify the observed redox behaviours we performed digital simulations of the experimental
voltammetric data, modelling the redox processes using an EC-mechanism 34 (i.e. a reversible,
heterogeneous electron transfer step followed by an irreversible, homogeneous chemical step
generating electro-inactive products – other postulated mechanisms produce a poor fit to the data).
These digital simulations allowed us to extract pertinent mechanistic parameters such as the formal
redox potentials and charge transfer coefficients (E° and
α respectively) and kinetic parameters for the
electron transfer (k0) and follow-on chemical step (kf) as shown in Table 2 (together with our
previously reported parameters for 1 and 4).
Table 2
Simulation parameters for the one-electron reductions of B(Ar')2(Ar")
BArHalx + e− == BArHalx•− BArHalx•− =>
D(BArHalx) =
D(BArHalx•−) /
cm2.s−1 a
decomposition
kf / s-1 b
9.2
E° / V vs [FeCp2]0/+
α
k0 / cm.s−1
0.379 1.45×10−2
0.300 8.45×10−3
0.387 4.85×10−3
0.419 4.56×10−3
0.512 2.69×10−1
0.415 1.14×10−2
0.425 1.20×10−2
0.416 1.13×10−2
0.445 5.59×10−2
0.85×10−5
1.41×10−5
1.18×10−5
3.76×10−5
2.54×10−5
1.44×10−5
1.22×10−5
1.26×10−5
1.68×10−5
B(C6F5)3 1 37
−1.52 0.01
−1.56 0.01
−1.57 0.01
−1.61 0.01
−1.70 0.01
−1.60 0.01
−1.54 0.01
−1.54 0.01
−1.58 0.01
B(C6F5)2{3,5-(CF3)2C6H3} 2
B{3,5-(CF3)2C6H3}2(C6F5) 3
B{3,5-(CF3)2C6H3}3 4 37
B{3,5-(CF3)2C6H3}2(C6Cl5) 5
B(C6Cl5)2{3,5-(CF3)2C6H3} 6
B(C6Cl5)3 7
≥ 30
≥ 25
≥ 25
0.36
0.056
≤ 10−5
≤ 10−5
0.88
B(C6Cl5)2(C6F5) 8
B(C6F5)2(C6Cl5) 9
a with exception of 7, all diffusion constants (D) obtained via 1H and/or 19F DOSY NMR spectroscopy
b kf values are modelled as a pseudo first-order process
These formal redox potentials are a measure of the electrophilicity of the LUMO (formally the vacant
boron 2pz orbital), and one might expect them to correlate to the Lewis acidity of the free borane; the
more negative the E° value, the less electrophilic the boron. When considering only the homo-
tri(aryl)boranes 1, 4 & 7, the observed redox potential imply that the net electron withdrawing effect
of the aryl rings increases (thereby making the boron more electrophilic) in the order ArF6 < ArCl5
ArF5. This is contrary to the description of ArCl5 as more electron withdrawing that ArF5 (due to back-
donation of the filled fluorine 2p orbitals into the aromatic * orbitals counteracting its high
electronegativity, this effect is much reduced for chlorine due to the poorer overlap of 3p orbitals with
the aromatic
orbitals; as shown by Hammett parameters: σparaCl = 0.227, σparaF = 0.062).24 We
≈
π
π
propose that this is due to through-space interaction between the ortho-Cl substituents with the boron
centre in 7 (B···Cl ca. 3.1 Å), quenching the electrophilicity by donation of electron density from the
chlorine lone pairs into the formally empty boron 2pz orbital. The orientation {large twist-angle,
56(3)°, orientating ortho-Cl above/below the BC3 trigonal-plane} and size of the chlorine 3p orbitals,
Page 8

Page 9 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
make such interaction much more favourable than for 1; an effect which might also be expected to
occur to a varying degree for all the other boranes incorporating ArCl5 substituents, 4, 5, 8 & 9.
Further discussion of the trends in E° is given below, but a linear trend can clearly be observed for the
series 1-4: B(ArF5)x(ArF6)y (the stepwise substitution of ArF5 with ArF6). Similar experiments
performed for the series B{2,4,6-(CH3)3C6H2}n(C6F5)3−n (n = 1-3)26 at a Pt disc in
thf/[nBu4N][B(C6F5)4], displayed a pronounced linear change in E varying by ca. 500 mV as each
½
mesityl ring was substituted with a C6F5 ring.
Additionally, since the reversibility of the voltammograms is dependent on the stability of the radical-
anion intermediate generated upon reduction of the parent borane, and as we have previously
reported,34, 37 it is assumed that the decomposition pathway proceeds via interaction of the radical-
anion with the solvent, it may be implied that the slower the rate of decomposition (quantified by the
rate constant kf), the more steric shielding is present around the boron centre to stabilise the radical
anion. Hence, the presence of ArCl5 ring(s) reduces kf values by up to five orders of magnitude due to
its high steric bulk (due to the ortho-Cl substituents and the associated high twist-angles), compared to
the ArF5 and ArF6 rings alone.
MeasurementsofLewisacidity
To-date common methods for the quantification of Lewis acidity have been based on spectroscopic
techniques. One such technique, the “Gutmann-Beckett method”,40, 41 involves adduct formation
between the Lewis acid of interest and the Lewis base triethylphosphine-oxide (Et3PO); and
measurement of its 31P chemical shift (commonly reported as the “acceptor number” – a normalised
proxy for the observed chemical shift relative to that of free Et3PO in hexane). With the rational that
increased Lewis acidity, results in de-shielding (and thereby an increase in chemical shift) of the
bound phosphorus atom. A comprehensive study of Lewis acidic boron compounds was recently
published by Sivaev and Bregadze,42 identifying a number of electronic effects which may influence
the measurement of Lewis acidity by this method. As a measure of Lewis acid-base adduct formation
the “Gutmann-Beckett method” is also somewhat limited by steric effects, and is blind to any
associated electronic influences present in the trigonal planar parent borane that are no longer present
in the tetrahedral adduct (such as through-space donation from ortho-substituents into the boron 2pz
orbital, as discussed above for the ArCl5 substituents). Considering that all our Lewis acids are boron
based, we will also consider both the 11B chemical shifts of the free boranes and their Et3PO adducts as
tools to measure Lewis acidity.
Spectral data for Et3PO adducts of 1-6, 8, 9, are detailed in Tables 3 and S1. (As previously reported,24
due to its steric bulk 7 will not form an adduct with Et3PO, clearly identifying a limit on such
methodology.)
Page 9

Dalton Transactions
Page 10 of 23
View Article Online
DOI: 10.1039/C5DT03854E
Table 3
Measurements of Lewis acidity: 31P and 11B NMR chemical shifts (in CD2Cl2) for the
Et3POB(Ar')2(Ar") adducts
δP(adduct) / ppm δB(adduct) / ppm
B(C6F5)3 1 a
+77.15
+78.01
−2.46
+0.27
+2.95
+4.27
+4.37
+6.06
B(C6F5)2{3,5-(CF3)2C6H3} 2
B{3,5-(CF3)2C6H3}2(C6F5) 3
B{3,5-(CF3)2C6H3}3 4
B{3,5-(CF3)2C6H3}2(C6Cl5) 5
B(C6Cl5)2{3,5-(CF3)2C6H3} 6
B(C6Cl5)3 7
+77.68
+78.80
+76.7 (br)
+76.9 (v.br)
no adduct formation
B(C6Cl5)2(C6F5) 8 b
B(C6F5)2(C6Cl5) 9 c
+74.70
+75.35
+2.27
−0.31
all values herein have been re-measured by the authors, internally referenced to δP (Et3PO) = +50.70 ppm
a previously reported in reference 42 over the range δP +77.0 to +78.1
b previously reported in reference 24 as δP +74.5 and δB +0.3
c previously reported in reference 24 as δP +75.8 and δB −1.1
Thereby we have three measures to quantify the Lewis acidity of different boranes: the 11B chemical
shift of the free borane, and the 31P and 11B chemical shifts of the Lewis acid-base adduct
Et3POB(Ar')2(Ar"), together with the standard reduction potential (E°) of the free borane which
measures the electrophilicity of the formally vacant 2pz orbital of the boron (Table 2). Comparing the
variation in these measures across the stepwise substitution of aryl rings in the boranes 1-9 (Figure 4),
together with considering any correlation between all four measures (Figure S5), allows for general
trends (and outliers) to be identified.
Page 10

Page 11 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
Figure 4
B(ArF6)y(ArCl5)z 4-7, and (c) B(ArF5)x(ArCl5)z 7-9 & 1.
Variation of E°(borane), δB(borane), δP(adduct) and δB(adduct); for (a) B(ArF5)x(ArF6)y 1-4, (b)
Page 11

Dalton Transactions
Page 12 of 23
View Article Online
DOI: 10.1039/C5DT03854E
The series B(ArF5)x(ArF6)y, 1-4, shows linear variations in all four measures across the series, with
δB(borane), δP(adduct) and δB(adduct) all increasing as ArF5 rings are substituted with ArF6 rings.
−E°,
The series B(ArF6)y(ArCl5)z, 4-7, shows clear trends only for
ArF6 rings are substituted with ArCl5 rings (with exception of 5, which appears higher than the trend
−E° showing a general decrease as the
otherwise implies). For δB(borane), δP(adduct) and δB(adduct), any trends are relatively poor (further
emphasised by the fact that an adduct of 7 cannot be formed).
Finally, the series B(ArF5)x(ArCl5)z, 7-9, 1, shows generally linear variations with δP(adduct) increasing,
while −
E°, δB(borane) and δB(adduct) all decrease as ArCl5 rings are substituted with ArF5 rings.
However, compound 9, while consistent with the general trends, is clearly an outlier for −E° and
δB(borane) in which cases its higher and lower (respectively) than the trends would otherwise imply.
The two most prominent outliers, in both cases where −E° is higher than the trends otherwise imply, 5
& 9 (circled in Figure 4), incorporate a single ArCl5 ring with a large (>70°) twist-angle, leading to the
ortho-Cl substituents being located above/below the boron centre. As previously suggested (vide-
supra) this orientation allows for significant through-space donation of electron density into the
formally vacant boron 2pz orbital,37 thereby reducing the electrophilicity (increasing −E°) of the boron
from that predicted otherwise by the trend {the observation of 9 as an outlier in the general trend of
δB(borane), could also be rationalised similarly}. It should be noted that 9 does not appear as an outlier
in plots of δP(adduct) and δB(adduct) (considering the poor trends of the 4-7 series, 5 cannot be
identified as an outlier or not), since the geometry changes at the boron centre and the explicit filling
of the boron 2pz orbital, eliminates the possibility for electronic effects due to ortho-substituents.
Similar interactions, leading to the “Gutmann-Beckett method” failing to give adequate measures of
Lewis acidity, have been previously noted for ferrocenyl substituted silicon cations; where the cationic
silicon interacts with the ferrocenyl backbone, an interaction that is quenched upon adduct formation.43
Considering the correlations between all six combinations of the four measurements (see Figures S5
for correlation plots), for the entire data set (1-9), there is little evidence of correlation between any of
the potential measures of Lewis acidity with themselves nor with the measure of electrophilicity, –E°.
However, there are generally linear correlations between all six combinations of measures for the
B(ArF5)x(ArF6)y, 1-4, series (which it should be noted, are the only compounds which do not
incorporate ArCl5 substituents).
This lack of clear correlation between any pair of measures of Lewis acidity, particularly that of E° (i.e.
electrophilicity of the boron centre) and δP(adduct) (i.e. the “Gutmann-Beckett” acceptor number)
(Figure S5b),is not surprising given that the “Gutmann-Beckett method” is highly dependent on steric
constraints and adduct formation that will not occur for the most sterically bulky Lewis acids, such as
7, and the 2,4- and 2,5- isomers of 4;37 whereas, electron transfer is not influenced to any appreciable
extent by the sterics surrounding the boron centre in these tri(aryl)boranes, where the pendant
conjugated aryl systems ensure electron transfer to even the most crowded boron centres.37 What these
correlations do reveal is that the electrochemical measurements clearly and easily identify outliers for
Page 12

Page 13 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
the trends, and that where these occur there is an obvious, but often unconsidered electronic effect due
to the twist-angle of the aryl rings affecting either the donation of electron density into the vacant 2pz
orbital on boron from ortho-substituents on the rings, or π-electron density from the aryl rings or both.
H cleavagebyFLPs
2
The ability of these boranes to act as the Lewis acidic component of an FLP for the cleavage of H2 was
screened, in combination with the Lewis base P(tBu)3 in dichloromethane at room temperature, with
the reactions monitored by 1H and 11B NMR spectroscopy (see Figures S6-S14).
In addition to the now well studied 1, under these mild conditions, 2, 3, 5, 6, 8 & 9, irreversibly
heterolytically cleave H2 to generate terminal-tri(aryl)borohydrides; as shown by the observation of a
1:1:1:1 quartet in the 1H NMR spectra and a doublet in the 11B NMR spectra with 1JHB coupling of 85-
95 Hz (Table 4). (H2 cleavage has also been previously reported for the Lewis acids 8 & 9, with the
Lewis bases tmp & lutidine at elevated temperatures,25, 44 and as an equilibrium process with thf as the
Lewis base.23) As previously reported,21, 37 while 4 rapidly cleaves H2 it does not lead to generation of
a terminal-hydride, but instead to the bridging-hydride species [
associated with [
-H(4)2]− could not be distinguished in either the 1H or 11B NMR spectra (despite
[(tBu)3PH][ -H(4)2] remaining soluble in CD2Cl2). Finally, while 7/P(tBu)3 and 7/thf FLPs have
µ
-H(4)2]−. Resonances unequivocally
µ
µ
previously been shown to cleave H2, albeit at elevated temperatures,23, 45 under these mild conditions
there was no evidence for reaction of H2 with the FLP 7/P(tBu)3 over a period of five days.
Table 4
1H and 11B NMR spectral data for the terminal hydride products from the FLP cleavage of H2
δH(−HBArHalx) / ppm δB(−HBArHalx) / ppm
1JHB / Hz
93.9
B(C6F5)3 1
+3.61
+3.69
+3.71
−25.3
B(C6F5)2{3,5-(CF3)2C6H3} 2
B{3,5-(CF3)2C6H3}2(C6F5) 3
B{3,5-(CF3)2C6H3}3 4
B{3,5-(CF3)2C6H3}2(C6Cl5) 5
B(C6Cl5)2{3,5-(CF3)2C6H3} 6
B(C6Cl5)3 7
−19.9
86.1
−14.7
86.1
no terminal-hydride formation
+4.22
+4.24
−10.0
−8.6
86.0
88.0
no reaction
−14.3
B(C6Cl5)2(C6F5) 8
+4.11
+3.94
86.1
90.0
B(C6F5)2(C6Cl5) 9
−19.6
all values herein have been re-measured by the authors
Since all our reactions were performed under the same conditions and monitored throughout, it is
possible to make qualitative descriptions as to the kinetics and mechanism(s) of H2 cleavage by these
boranes. We observed the reaction with 1 as the Lewis acid reaching completion fastest (ca. 5 hours
following H2 addition), with the reaction time increasing in the order, 1 < 2
≈ 9 < 3 < 8 < 5 ≈ 6 (ca. 23%
conversion, after ca. 96 hrs). Cleavage with 1-3 was near quantitative, while the Lewis acids 5, 6, 8 &
9 led to a mixture of products by 11B NMR spectroscopy (the by-products giving signals in the range
δB +6 to −3 ppm, corresponding to four-coordinate boron centres – we speculate that some of these
Page 13

Dalton Transactions
Page 14 of 23
View Article Online
DOI: 10.1039/C5DT03854E
could be due to trace moisture and the products of its reaction with the FLP). One feature of note in
the 1H NMR spectra recorded for 1-3 is initially the borohydride resonances are observed as sharp
signals before broadening (for 1 and 2, without changing δH; for 3, initially observed at ca. +3.3 ppm
and gradually shifting to ca. +3.7 ppm before the signal broadens) to eventually resolve into the
characteristic broad 1:1:1:1 quartet. Further, the dissolved H2 is not observed until the signal has
stopped shifting (while it is clearly observed as the reaction progresses for 5, 6, 8 & 9). These
observations may provide further clues as to the mechanism of FLP H2 cleavage, and we intend to
investigate them further in later work.
The reactivity of these boranes towards heterolytic H2 cleavage unsurprisingly depends on both the
Lewis acidity and electrophilicity of the boron centre and the steric bulk of the substituents; hence the
lack of reactivity under these conditions of the bulky 7 despite its apparently favourable
electrophilicity. However, as we have discovered herein, the correlation between the various aryl
substituents and the Lewis acidity – and even the steric buttressing around the boron centre – is by no
means simple. There are subtle steric and electronic influences at work (e.g. through space interactions
of ortho-substituents with the vacant 2pz orbital affecting both steric orientation of these rings and the
electronics of the borane) that this report has highlighted, that require careful consideration when
designing potential new Lewis acidic boranes for FLP reactions.
Conclusions
We have synthesised four new hetero-tri(aryl)boranes, 2, 3, 5 & 6. Which, together with three known
homo-tri(aryl)boranes, 1, 4 & 7 and two known hetero-tri(aryl)boranes, 8 & 9, give a series of nine
compounds linked by stepwise substitution of their aryl rings. Their redox chemistry has been
investigated electrochemically, along with their suitability as Lewis acid components of FLP systems
for the heterolytic cleavage of H2 under mild conditions.
Rapid electrochemical techniques and analysis of the position and shape of the resulting
voltammograms allow for the electrophilicity to be quantified along with a qualitative description of
the steric shielding around the boron centre. While the correlations both between different
spectroscopic measures of Lewis acidity, and between measures of Lewis acidity and electrochemical
measures of electrophilicity are generally poor, the electrochemical measurements easily allow us to
identify outliers from trends within specific series of substituted boranes. These arise due to electronic
effects (such as, interactions of ortho-substituents and aryl π orbitals, with the formally vacant boron
2pz orbital) are not always obvious from spectroscopic measurements of Lewis acidity. Where these
electronic interactions are identified, we have found them to correspond to a higher than average twist-
angle between the boron trigonal-plane. Whilst steric buttressing and inductive/mesomeric through-
bond effects are often considered in the design of new boranes for FLP studies, the degree of twist
angle of each aryl ring and the resulting potential for through-space electronic effects is shown to be
Page 14

Page 15 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
an important but often unconsidered factor in determining the Lewis acidity and reactivity of these
compounds.
While all but one of these compounds cleaves H2 under mild conditions as part of an FLP, the rate of
cleavage depends on both the Lewis acidity/electrophilicity of the boron centre and its surrounding
steric bulk. Establishing that while increasing the formal Lewis acidity should increases the rate of
reaction, it must be balanced by preventing too much steric bulk around the active site that might
inhibit the rate of reaction. However, while these new boranes do not appear to cleave H2 as rapidly as
the archetype 1, these slower reactions afford the opportunity to observe signals arising from potential
intermediates formed during H2 cleavage by NMR spectroscopy. A comprehensive kinetic
investigation of these reactions as part of our future work could possibly lead to a greater
understanding of the mechanism of H2 cleavage by FLPs.
Experimental
All reactions and manipulations were performed under an atmosphere of dry oxygen-free N2, using
either standard Schlenk techniques or in an MBraun UNIlab glovebox. All solvents were dried prior to
use by refluxing over an appropriate drying agent {Na/benzophenone for n-pentane, n-hexane,
petroleum ether (b.p 40-60 °C) and diethyl ether; Na for toluene; CaH2 for dichloromethane},
collected by distillation under a dry oxygen-free N2 atmosphere and stored over 4 Å molecular sieves
prior to use.
NMR Spectra were obtained on a Bruker Avance DPX-500 spectrometer; for 1H spectra residual
protio-solvent was used as an internal standard; for 13C the solvent resonance(s) were used as an
internal standard;46 for 19F spectra CFCl3 was used as an external standard; for 11B spectra BF3·Et2O
was used as an external standard; for 31P spectra 85% H3PO4 was used as an external standard.
High resolution mass spectrometry was performed by the EPSRC Mass Spectrometry Service at the
University of Swansea. Elemental analyses were performed by the Elemental Analysis Service at
London Metropolitan University.
Single crystals of 5 were grown by slow diffusion of a saturated CH2Cl2 solution into n-hexane at
−
25 °C, single crystals of 6 were grown from a saturated n-hexane solution at
suitable crystals were selected, encapsulated in a viscous perfluoropolyether and mounted on an
Agilent Technologies Xcaliber-3 single crystal X-ray diffractometer using Mo K radiation where the
−25 °C. For 5 and 6,
α
crystals were cooled to 140 K during data collection and a full sphere of data collected. The data was
reduced and an absorption correction performed using Agilent Technologies CrysAlisPro version
171.37.35.47 Using Olex2,48 the structures were solved and space group assigned with
SuperFlip/EDMA using charge flipping,49 and then refined with the ShelXL version 2014/7 refinement
program using least squares minimisation.50
Page 15

Dalton Transactions
Page 16 of 23
View Article Online
DOI: 10.1039/C5DT03854E
Table 5
Crystallographic Data for 5 and 6.
B{3,5-(CF3)2C6H3}2(C6Cl5) 5
C22 H6 B Cl5 F12
686.33
B(C6Cl5)2{3,5-(CF3)2C6H3} 6
empirical formula
formula weight
temperature / K
crystal system
space group
a / Å
C20 H3 B Cl10 F6
722.53
140(1)
140(1)
Monoclinic
C2/c
Monoclinic
P21/n
11.5422(8)
13.1500(8)
16.6630(10)
90.0
8.5532(17)
9.742(2)
b / Å
c / Å
30.812(8)
90.0
α / °
β / °
103.799(7)
90.0
90.871(18)
90.0
γ / °
volume / Å3
2456.1(3)
2567.1(10)
4
Z
4
ρcalc / mg.mm-3
ꢀ / mm-1
1.856
1.870
0.696
1.142
F(000)
1344.0
1408.0
crystal size / mm3
radiation
0.1 × 0.05 × 0.05
Mo Kα (λ = 0.71073 Å)
6.688 to 52.724°
0.1 × 0.05 × 0.05
Mo Kα (λ = 0.71073 Å)
5.764 to 52.0°
2Θ range for data
collection
index ranges
−11 ≤ h ≤ 14, −16 ≤ k ≤ 16, −20 −10 ≤ h ≤ 9, −12 ≤ k ≤ 12, −38 ≤
≤ l ≤ 20
l ≤ 36
reflections collected
9849
20074
independent reflections
2506 [R int = 0.0534, R sigma
0.0572]
=
5027 [R int = 0.2721, R sigma
0.3383]
=
data / restraints /
parameters
goodness-of-fit on F2
2506 / 0 / 183
5027 / 75 / 357
1.009
1.013
final R indexes [I≥2σ(I)] R1 = 0.0417, wR2 = 0.0808
R1 = 0.0891, wR2 = 0.1457
R1 = 0.2819, wR2 = 0.2313
0.54 / −0.69
final R indexes [all data] R1 = 0.0755, wR2 = 0.0925
largest diff. peak / hole /
e.A-3
0.38 / −0.27
CCDC 1418145 (5), and 1418144 (6) contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Electrochemical studies were carried out using a Metrohm Autolab ꢀ-II, PGSTAT30 or PGSTAT302N
potentiostat linked to a computer running Metrohm Autolab NOVA version 1.11 software, in
conjunction with a three electrode cell comprising: a glassy carbon disc working electrode
(Bioanalytical Systems, Inc., ca. 7.0 mm2 area calibrated using the [FeCp2]0/+ redox couple), a
platinum wire (99.99% purity) counter electrode, and a silver wire (99.99% purity) pseudo-reference
electrode. All working electrodes were polished with 0.3 ꢀm α-alumina and dried prior to use. All
electrochemical measurements were performed at ambient temperature under a dry N2 atmosphere
Page 16

Page 17 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
(using either a custom-built electrochemical cell or within a glovebox), in CH2Cl2 containing 50 mM
[nBu4N][B(C6F5)4] as the supporting electrolyte and between 1.5 and 2.5 mM of the analyte species of
interest. Cyclic voltammetric measurements were, where possible, iR-compensated using positive-
feedback to within 85 5% of the uncompensated solution resistance (ca. 666 Ω
). [nBu4N][B(C6F5)4]
was synthesised according to published methods.51 All potentials were referenced to the [FeCp2]0/+
redox couple, which was added as an internal standard. Simulations of electrochemical processes were
performed using ElchSoft DigiElch version 7.096 software.52
B{3,5-(CF3)2C6H3}2Br,38 Cu(C6F5),53 Zn(C6Cl5)2, B(C6Cl5)3 7, and B(C6F5)2(C6Cl5) 9,24 were
synthesised as previously reported. Characterisation data for the intermediate B{3,5-
(CF3)2C6H3}(OMe)2, and the impurity B{3,5-(CF3)2C6H3}2(OH) is detailed in the supplementary
information. All other reagents were obtained from commercial suppliers and used as supplied.
B{3,5-(CF3)2C6H3}Br2 nBuLi (6.25 cm3, 10 mmol, 1.6 M in hexanes) was slowly added to a cooled
(−78 °C) solution of 3,5-bis(trifluoromethyl)bromobenzene (1.72 cm3, 10 mmol) in Et2O (20 cm3) and
stirred for 10 min. BH3·SMe2 (0.95 cm3, 10 mmol) is added, and after 10 min the reaction mixture
warmed to room temperature and stirred for a further 30 min. Me3SiCl (1.27 cm3, 10 mmol) is added
to the clear orange solution, resulting in the rapid formation of a white precipitate, the mixture is
stirred for 30 min. Methanol (1.2 cm3, 30 mmol) is slowly added, resulting in rapid evolution of H2,
and the mixture stirred for an hour. Volatiles are removed in vacuo to give a cloudy white oil; the
intermediate B{3,5-(CF3)2C6H3}(OMe)2 is extracted into 25 cm3 petroleum ether and isolated as a
clear pale yellow solution by filtration (via cannula). BBr3 (2.0 cm3, 20.8 mmol) is added and the
mixture stirred for 30 min, the mixture is once again filtered (via cannula) isolating a clear pale yellow
solution, all volatiles are removed in vacuo to give the product as a highly reactive yellow oil. Yield
1.63 g (4.2 mmol, 42%)
1H NMR (500.21 MHz, CD2Cl2, 25 ºC,
δ
): +8.66 (s, 2H, ArF6 2,6-H), +8.20 (s, 1H, ArF6 4-H); 11B
NMR (160.49 MHz, CD2Cl2, 25 ºC,
(s, 6F, ArF6 3,5-CF3).
δ
): +57.3 (br.s); 19F NMR (470.67 MHz, CD2Cl2, 25 ºC,
δ): −63.3
B(C6F5)2{3,5-(CF3)2C6H3} 2 Solutions of freshly prepared B{3,5-(CF3)2C6H3}Br2 (1.62 g, 4.2
mmol) in 10 cm3 CH2Cl2 and Cu(C6F5) (2.30 g, 10 mmol) in 40 cm3 CH2Cl2 are combined, and stirred
for ca. 1 hour. The reaction mixture is filtered (via cannula) to remove the blue-grey precipitate,
isolating the clear pale yellow solution, volatiles are removed in vacuo to give the off-white solid
product. Yield: 1.78 g (3.2 mmol, 77%). The product can be further purified by sublimation at 10−1
mbar / 90 °C.
1H NMR (500.21 MHz, CD2Cl2, 25 ºC,
δ
): +8.22 (s, 1H, ArF6 4-H), +8.15 (s, 2H, ArF6 2,6-H); 11B
NMR (160.49 MHz, CD2Cl2, 25 ºC,
δ
): +63.5 (br.s); 13C{1H} NMR (125.78 MHz, CD2Cl2, 25 ºC,
δ):
Page 17

Dalton Transactions
Page 18 of 23
View Article Online
DOI: 10.1039/C5DT03854E
+148.3 (br.d, 1JCF = 248 Hz, ArF5 2,6-C), +145.2 (br.d, 1JCF = 260 Hz, ArF5 4-C), +141.3 (br.s, ArF6 1-
C), +138.4 (br.d, 1JCF = 255 Hz, ArF5 3,5-C), +137.6 (br.q, 3JCF = 1.8 Hz, ArF6 2,6-C), +132.2 (q, 2JCF
33.5 Hz, ArF6 3,5-C), +128.9 (sept., 3JCF = 3.7 Hz, ArF6 4-C), +123.7 (q, 1JCF = 273 Hz, ArF6 3,5-CF3);
=
19F NMR (470.67 MHz, CD2Cl2, 25 ºC, ): −63.3 (s, 6F, ArF6 3,5-CF3), −127.3 (m, 4F, ArF5 2,6-F),
δ
−145.2 (t, 3JFF = 18.7 Hz, 2F, ArF5 4-F), −160.4 (m, 4F, ArF5 3,5-F). HRMS-APCI (m/z): [M]+ calc. for
C20H3BF16, 558.0070; found, 558.0063. Elemental analysis (calc. for C20H3BF16): C 43.17 (43.03), H
0.57 (0.54).
B{3,5-(CF3)2C6H3}2(C6F5) 3 Solutions of B{3,5-(CF3)2C6H3}2Br (0.72 g, 1.0 mmol) in 10 cm3
CH2Cl2 and Cu(C6F5) (0.23 g, 1.0 mmol) in 1.0 cm3 CH2Cl2 are combined, and stirred for ca. 1 hour.
The reaction mixture is filtered (via cannula) to remove the precipitate, isolating the clear pale yellow
solution, volatiles are removed in vacuo to give the off-white solid product. Yield: 0.46 g (0.75 mmol,
75%). The product can be further purified by sublimation at 10−1 mbar / 110 °C.
1H NMR (500.21 MHz, CD2Cl2, 25 ºC,
NMR (160.49 MHz, CD2Cl2, 25 ºC,
): +66.0 (br.s); 13C{1H} NMR (125.78 MHz, CD2Cl2, 25 ºC,
+148.3 (br.d, 1JCF = 250 Hz, ArF5 2,6-C), +144.9 (br.d, 1JCF = 260 Hz, ArF5 4-C), +142.3 (br.s, ArF6 1-
δ
): +8.22 (s, 2H, ArF6 4-H), +8.07 (s, 4H, ArF6 2,6-H); 11B
δ
δ):
C), +138.5 (br.d, 1JCF = 255 Hz, ArF5 3,5-C), +137.7 (br.q, 3JCF = 2.3 Hz, ArF6 2,6-C), +132.3 (q, 2JCF
33.0 Hz, ArF6 3,5-C), +127.9 (sept., 3JCF = 3.7 Hz, ArF6 4-C), +123.8 (q, 1JCF = 273 Hz, ArF6 3,5-CF3),
=
+113.2 (br.s, ArF5 1-C); 19F NMR (470.67 MHz, CD2Cl2, 25 ºC, ): −63.4 (s, 12F, ArF6 3,5-CF3),
δ
−126.0 (m, 2F, ArF5 2,6-F), −146.0 (tt, 3JFF = 19.9 Hz, 4JFF = 4.2 Hz, 1F, ArF5 4-F), −160.1 (m, 2F,
ArF5 3,5-F). HRMS-APCI (m/z): [M − H]+ calc. for C22H5BF17, 603.0211; found, 603.0201. Elemental
analysis (calc. for C22H6BF17): C 43.86 (43.73), H 1.08 (1.00).
B(C6Cl5)2Cl
to −78 °C. nBuLi (35.5 cm3, 50 mmol, 1.41 M in hexanes) was added, and the cooled reaction mixture
stirred for 4 hours to give a clear golden solution. Cooled (−78 °C) n-pentane (
300 cm3) was added to
C6Cl6 (14.24 g, 50 mmol) was suspended in Et2O (≥
300 cm3) and the slurry cooled
≥
give a fine white precipitate of LiC6Cl5; after 30 min, BCl3 (25 cm3, 25 mmol, 1.0 M in heptane) was
slowly added, and the reaction mixture slowly warmed to room temperature and stirred for 12 hours.
Removal of volatiles in vacuo, gave a pale orange solid; the product was extracted with toluene (3×
100 cm3) and isolated by filtration to give an amber solution. Removal of volatiles in vacuo, washing
with n-hexane, and drying in vacuo, gave B(C6Cl5)2Cl as a pale yellow solid. Yield: 8.80 g (16.1 mmol,
64.4%).
Characterisation data as previously reported in reference 24.
B{3,5-(CF3)2C6H3}2(C6Cl5) 5 B{3,5-(CF3)2C6H3}2Br (1.00 g, 1.93 mmol) and Zn(C6Cl5)2 (0.55 g,
0.97 mmol) are combined and suspended in 10 cm3 toluene. The reaction vessel is sealed and heated at
ca. 75 °C for ca. 72 hours. Once cooled volatiles are removed in vacuo, and the product extracted into
Page 18

Page 19 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
CH2Cl2 giving a pale green solution. The solid is precipitated by, addition of n-hexane, concentration
in vacuo, and cooling at −25 °C. The micro-crystalline pale green solid was isolated and dried in
vacuo. Yield: 0.46 g (0.64 mmol, 33%).
1H NMR (500.21 MHz, CD2Cl2, 25 ºC,
δ
): +8.20 (s, 2H, ArF6 4-H), +8.13 (s, 4H, ArF6 2,6-H); 11B
NMR (160.49 MHz, CD2Cl2, 25 ºC, δ):
δ
): +65.7 (br.s); 13C{1H} NMR (125.78 MHz, CD2Cl2, 25 ºC,
+137.4 (br.q, 3JCF = 3.7 Hz, ArF6 2,6-C) +135.4 (s, ArCl5 4-C), +132.9 (s, ArCl5 2,6/3,5-C), +132.3 (q,
2JCF = 33.0 Hz, ArF6 3,5-C), +131.4 (s, ArCl5 2,6/3,5-C), +127.7 (sept., 3JCF = 3.7 Hz, ArF6 4-C), +123.6
δ
(q, 1JCF = 273 Hz, ArF6 3,5-CF3); 19F NMR (470.67 MHz, CD2Cl2, 25 ºC, ): −63.2 (s, 12F, ArF6 3,5-
CF3). HRMS-APCI (m/z): [M]+ calc. for C22H6BCl5F12, 685.8784; found, 685.8773. Elemental analysis
(calc. for C22H6BCl5F12): C 38.62 (38.49), H 0.81 (0.88).
B(C6Cl5)2{3,5-(CF3)2C6H3} 6 nBuLi (1.56 cm3, 2.2 mmol, 1.41 M in hexanes) was slowly added to a
cooled (−78 °C) solution of 3,5-bis(trifluoromethyl)bromobenzene (0.38 cm3, 2.2 mmol) in Et2O (50
cm3) and left to stir for 1 hour. B(C6Cl5)2Cl (1.23 g, 2.3 mmol) was dissolved in toluene (20 cm3),
cooled (−78 °C), and slowly added to the reaction mixture, which was left to stir at −78 °C for 3 hours
and then slowly warmed to room temperature over 18 hours. Removal of volatiles in vacuo, gave a
pale yellow solid. This was extracted with n-hexane (2× 50 cm3), which after removal of volatiles in
vacuo, gave a light yellow powder. Yield 0.78 g (1.1 mmol, 50%).
1H NMR (500.21 MHz, CD2Cl2, 25 ºC,
δ
): +8.15 (s, 1H, ArF6 4-H), +8.03 (s, 2H, ArF6 2,6-H); 11B
NMR (160.49 MHz, CD2Cl2, 25 ºC, δ):
δ
): +66.0 (br.s); 13C{1H} NMR (125.78 MHz, CD2Cl2, 25 ºC,
+137.3 (br.q, 3JCF = 2.3 Hz, ArF6 2,6-C) +137.0 (s, ArCl5 4-C), +133.5 (s, ArCl5 2,3,5,6-C), +132.5 (q,
2JCF = 33.4 Hz, ArF6 3,5-C), +128.4 (sept., 3JCF = 3.8 Hz, ArF6 4-C), +123.7 (q, 1JCF = 273 Hz, ArF6 3,5-
δ
CF3); 19F NMR (470.67 MHz, CD2Cl2, 25 ºC, ): −63.3 (s, 6F, ArF6 3,5-CF3). HRMS-APCI (m/z):
[M]+ calc. for C20H3BCl10F6, 721.7058; found, 721.7054. Elemental analysis (calc. for C20H3BCl10F6):
C 33.36 (33.24), H 0.50 (0.42).
B(C6Cl5)2(C6F5) 8
nBuLi (1.40 cm3, 1.97 mmol, 1.41 M in hexanes) was slowly added to a
cooled (−78 °C) solution of bromopentafluorobenzene (0.25 cm3, 2.0 mmol) in toluene (50 cm3) and
stirred for 15 minutes. A solution of B(C6Cl5)2Cl (1.09 g, 2.0 mmol) in toluene (20 cm3) was then
slowly added to the reaction mixture, which was subsequently left to warm to room temperature over
18 hours. The solution was filtered through celite (via cannula), and the volatiles removed in vacuo to
give a sticky amber solid. This was then extracted with n-pentane (5 cm3), which when cooled (−78 °C)
precipitated the product as a pale yellow powder. Yield: 0.28 g (0.41 mmol, 21%).
Characterisation data as previously reported in reference 24.
Page 19

Dalton Transactions
Page 20 of 23
View Article Online
DOI: 10.1039/C5DT03854E
Acknowledgements
G.G.W. thanks the Royal Society for financial support by a University Research Fellowship. The
research leading to these results has received funding from the European Research Council under the
ERC Grant Agreements no. 307061 (PiHOMER) and 640988 (FLPower).
We acknowledge the use of the EPSRC funded National Chemical Database Service hosted by the
Royal Society of Chemistry, and the EPSRC UK National Mass Spectrometry Facility (NMSF) at the
University of Swansea. We thank the Research Computing Service at the University of East Anglia for
access to the high performance computing cluster.
ReferencesandFootnotes
†
Electronic supplementary information (ESI) available: characterisation data for the impurity
B{3,5-(CF3)2C6H3}2(OH), characterisation data for the intermediate B{3,5-(CF3)2C6H3}(OMe)2;
further details on the X-ray crystallographic studies of 5, 6; further details on the DFT
calculations; reproductions of the previously published cyclic voltammograms of 1 and 4, cyclic
voltammograms of B{3,5-(CF3)2C6H3}2(OH); further details for the NMR characterisation of
the Et3PO adducts of 1-9, and correlation plots between the four different potential measures of
Lewis acidity; further details for the H2 cleavage by 1-9/P(tBu)3, including the time resolved 1H
and 11B NMR spectra monitoring the reactions. CCDC 1418145 (5), 1418144 (6). For ESI and
crystallographic data in CIF or other electronic format see DOI:…
‡
Current address: School of Chemistry, The University of Manchester, Oxford Road, Manchester,
M13 9PL, U.K.
1
2
3
4
5
6
7
8
9
G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129, 1880-1881.
D. W. Stephan, Dalton Trans., 2009, 3129-3136.
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46-76.
D. W. Stephan, Comp. Inorg. Chem. II, 2013, 1, 1069-1103.
D. W. Stephan and G. Erker, Chem. Sci., 2014, 5, 2625-2641.
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2015, 54, 6400-6441.
D. W. Stephan, J. Am. Chem. Soc., 2015, 137, 10018-10032.
T. Mahdi and D. W. Stephan, J. Am. Chem. Soc., 2012, 136, 15809-15812.
Á. Gyömöre, M. Bakos, T. Földes, I. Pápai, A. Domján & T. Soós, ACS Catalysis, 2015, 5,
5366-5372.
10
11
D. J. Scott, T. R. Simmons, E. J. Lawrence, G. G. Wildgoose, M. J. Fuchter and A. E. Ashley,
ACS Catalysis, 2015, 5, 5540-5544.
T. Mahdi, J. N. del Castillo and D. W. Stephan, Organometallics, 2013, 32, 1971-1978.
Page 20

Page 21 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
12
P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew. Chem., Int. Ed., 2007, 46, 8050-
8053.
13
14
H. Wang, R. Fröhlich, G. Kehra and G. Erker, Chem. Commun., 2008, 5966-5968.
G. Ménard, T. M. Gilbert, J. A. Hatnean, A. Kraft, I. Krossing and D. W. Stephan,
Organometallics, 2013, 32, 4416-4422.
15
Z. Lu, Y. Wang, J. Liu, Y. jian Lin, Z. H. Li and H. Wang, Organometallics, 2013, 32, 6753-
6758.
16
17
18
A. E. Ashley, A. L. Thompson and D. O'Hare, Angew. Chem., Int. Ed., 2009, 48, 9839-9843.
E. Otten, R. C. Neu and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 9918-9919.
M. Sajid, A. Klose, B. Birkmann, L. Liang, B. Schirmer, T. Wiegand, H. Eckert, A. J. Lough, R.
Fröhlich, C. G. Daniliuc, S. Grimme, D. W. Stephan, G. Kehr and G. Erker, Chem. Sci., 2013, 4,
213-219.
19
20
21
J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem., Int. Ed., 2007, 46, 4968-4971.
M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396-8397.
T. J. Herrington, A. J. W. Thom, A. J. P. White and A. E. Ashley, Dalton Trans., 2012, 41,
9019-9022.
22
23
24
S. C. Binding, H. Zaher, F. M. Chadwick and D. O'Hare, Dalton Trans., 2012, 41, 9061-9066.
D. J. Scott, M. J. Fuchter and A. E. Ashley, Angew. Chem., Int. Ed., 2014, 53, 10218-10222.
A. E. Ashley, T. J. Herrington, G. G. Wildgoose, H. Zaher, A. L. Thompson, N. H. Rees, T.
Krämer and D. O’Hare, J. Am. Chem. Soc., 2011, 133, 14727-14740.
D. O’Hare and A. Ashley, US Pat. 2012/0283340 A1, 2012.
25
26
S. A. Cummings, M. Iimura, C. J. Harlan, R. J. Kwaan, I. V. Trieu, J. R. Norton, B. M.
Bridgewater, F. Jäkle, A. Sundararaman and M. Tilset, Organometallics, 2006, 25, 1565-1568.
J. M. Farrell, J. A. Hatnean and D. W. Stephan, J. Am. Chem. Soc., 2012, 134, 15728-15731.
E. J. Lawrence, T. J. Herrington, A. E. Ashley and G. G. Wildgoose, Angew. Chem., Int. Ed.,
2014, 53, 9922-9925.
27
28
29
30
31
32
E. R. Clark and M. J. Ingleson, Angew. Chem., Int. Ed., 2014, 53, 11306-11309.
G. Ménard and D. W. Stephan, Angew. Chem., Int. Ed., 2012, 53, 8272-8275
J. M. Bayne and D. W. Stephan, Chem. Soc. Rev., 2015, DOI: 10.1039/c5cs00516g.
A. Schäfer, M. Reißmann, A. Schäfer, W. Saak, D. Haase and T. Müller, Angew. Chem., Int.
Ed., 2011, 50, 12636-12638.
33
34
35
B. Waerder, M. Pieper, L. A. Körte, T. A. Kinder, A. Mix, B. Neumann, H.-G. Stammler and N.
W. Mitzel, Angew. Chem., Int. Ed., 2015, DOI: 10.1002/anie.201504171.
E. J. Lawrence, V. S. Oganesyan, G. G. Wildgoose and A. E. Ashley, Dalton Trans., 2013, 42,
782-789.
E. J. Lawrence, V. S. Oganesyan, D. L. Hughes, A. E. Ashley and G. G. Wildgoose, J. Am.
Chem. Soc., 2014, 136, 6031-6036.
Page 21

Dalton Transactions
Page 22 of 23
View Article Online
DOI: 10.1039/C5DT03854E
36
37
E. J. Lawrence, R. J. Blagg, D. L. Hughes, A. E. Ashley and G. G. Wildgoose, Chem. Eur. J.,
2015, 21, 900-906.
R. J. Blagg, E. J. Lawrence, K. Resner, V. S. Oganesyan, T. J. Herrington, A. E. Ashley and G.
G. Wildgoose, Dalton Trans., 2015, DOI: 10.1039/c5dt01918d.
38
39
40
41
42
43
K. Samigullin, M. Bolte, H.-W. Lerner and M. Wagner, Organometallics, 2014, 33, 3564-3569.
W. V. Konze, B. L. Scott and G. J. Kubas, Chem. Commun., 1999, 1807-1808.
U. Mayer, V. Gutmann and W. Gerger, Monatshefte für Chemie, 1975, 106, 1235-1257.
M. A. Beckett, G. C. Strickland, J. R. Holland & K. S. Varma, Polymer, 1996, 37, 4629-4631.
I. B. Sivaev and V. I. Bregadze, Coord. Chem. Rev., 2014, 270-271, 75-88.
A. R. Nödling, K. Müther, V. H. G. Rohde, G. Hilt and M. Oestreich, Organometallics, 2014,
33, 302-308.
44
45
46
H. Zaher, A. E. Ashley, M. Irwin, A. L. Thompson, M. J. Gutmann, T. Krämera and D. O'Hare,
Chem. Commun., 2013, 49, 9755-9757.
A. L. Travis, S. C. Binding, H. Zaher, T. A. Q. Arnold, J.-C. Buffet and D. O’Hare, Dalton
Trans., 2013, 42, 2431-2437.
G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A. Nudelman, B. M. Stoltz, J. E.
Bercaw, and K. I. Goldberg, Organometallics, 2010, 29, 2176-2179.
CrysAlisPro, Agilent Technologies, Yarnton, U.K.
47
48
L. J. Bourhis, O. V. Dolomanov, R. J. Gildea, J. A. K. Howard and H. Puschmann, Acta
Crystallogr., Sect A: Fundam. Crystallogr., 2015, 71, 59-75; O. V. Dolomanov, L. J. Bourhis, R.
J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339-341.
L. Palatinus and G. Chapuis, J. Appl. Crystallogr., 2007, 40, 786-790; L. Palatinus. and A. van
der Lee, J. Crystallogr. Cryst., 2008, 41, 975-984; L. Palatinus, S. J. Prathapab and S. van
Smaalen, J. Appl. Crystallogr., 2012, 45, 575-580.
49
50
51
G. M. Sheldrick, Acta Crystallogr., Sect C: Cryst. Struct. Commun., 2015, 71, 3-8; G. M.
Sheldrick, Acta Crystallogr., Sect A: Fundam. Crystallogr., 2008, 64, 112-122.
S. Lancaster, ChemSpider Synthetic Pages, 2003, DOI:10.1039/SP215,
http://cssp.chemspider.com/215; T. E. Krafft, Boulder Scientific Company, US Pat. 5,679,289,
1997, Eur. Pat. 1,186,608, 2002; R. J. LeSuer, C. Buttolph and W. E. Geiger, Anal. Chem.,
2004, 76, 6395-6401.
52
53
DigiElch-Professional, ElechSoft, Kleinromstedt, Germany.
A. Cairncross, W. A. Sheppard and E. Wonchoba, Org. Synth., 1979, 59, 122.
Page 22

Page 23 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT03854E
Novel B(Ar')2(Ar'') hetero-tri(aryl)boranes: a systematic study of
Lewis acidity
Robin J. Blagg,* Trevor R. Simmons, Georgina R. Hatton, James M. Courtney, Elliot L.
Bennett, Elliot J. Lawrence and Gregory G. Wildgoose*
School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, U.K.
* g.wildgoose@uea.ac.uk ; r.blagg@uea.ac.uk
Tableof ContentsEntry
H2
P(tBu)3
[(tBu)3PH][HB(Ar')2(Ar'')]
[B(Ar')2(Ar'')]
solvent
B(Ar')2(Ar'')
+ e
F
decomposition
Cl
CF3
F
F
Cl
Ar =
F
Cl
Cl
CF3
F
Cl
Nine homo- and hetero-tri(aryl)boranes related by stepwise aryl substitutions, are studied for H2
cleavage as part of an FLP, and comparative Lewis acidity/electrochemical measurements discussed.