Syntheses, characterisation and solid-state study of alkali and ammonium BArF salts

Article abstract of DOI:10.1039/c7ra05928k

A new synthetic protocol for synthesising a number of BArF derivatives has been developed. Single crystal X-ray analysis of an array of alkali metal and ammonium salts has allowed the determination of the coordination sphere and/or the map of short contacts of the positively charged atoms. The increasing number of coordination bonds and/or short contacts between the alkali metal cation and the surrounding atoms has been rationalised in terms of the size of the alkali metal centre. It has also been demonstrated that an increase in the number of coordination bonds and/or short contacts translates into longer M-F distances. In the case of the ammonium BArF salts, the N-B distances are shorter than the M-B distances in the alkali metal BArF salts, indicating stronger interactions between the cationic nitrogen and the anionic boron than those between the boron and the alkali metal centres. Finally, a study of the structures of alkali metal hydrated and THF-solvated BArF salts showed that the interactions between the metal centre and the surrounding atoms depend not only on the size of the alkali metal centre but also on the occupancy of the first coordination sphere.

Full text of DOI:10.1039/c7ra05928k

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PAPER
Syntheses, characterisation and solid-state study of
alkali and ammonium BArF salts†
Cite this: RSC Adv., 2017, 7, 32833
a
a
Lucas Carreras,
Jordi Benet-Buchholza and Anton Vidal-Ferran
Laura Rovira,a Monica Vaquero,
Ignasi Mon,a Eddy Martin,a
´
ab
*
A new synthetic protocol for synthesising a number of BArF derivatives has been developed. Single crystal X-
ray analysis of an array of alkali metal and ammonium salts has allowed the determination of the
coordination sphere and/or the map of short contacts of the positively charged atoms. The increasing
number of coordination bonds and/or short contacts between the alkali metal cation and the
surrounding atoms has been rationalised in terms of the size of the alkali metal centre. It has also been
demonstrated that an increase in the number of coordination bonds and/or short contacts translates
into longer M–F distances. In the case of the ammonium BArF salts, the N–B distances are shorter than
the M–B distances in the alkali metal BArF salts, indicating stronger interactions between the cationic
nitrogen and the anionic boron than those between the boron and the alkali metal centres. Finally,
a study of the structures of alkali metal hydrated and THF-solvated BArF salts showed that the
interactions between the metal centre and the surrounding atoms depend not only on the size of the
alkali metal centre but also on the occupancy of the first coordination sphere.
Received 26th May 2017
Accepted 21st June 2017
DOI: 10.1039/c7ra05928k
rsc.li/rsc-advances
phenyl]borate anion ([B(3,5-(CF3)2C6H3)4]ꢀ, hereaer BArF)3e
has gained signicance within this group of uorine-containing
Introduction
anions because of its stability and weak interaction with
cations.
The term weakly coordinating anion (WCA) appeared in the
literature in the 1990s and was used to describe anions that
interacted weakly with cations.1 This qualifying term was used
in the past for counterions such as [ClO4]ꢀ, [PF6]ꢀ, [SbF6]ꢀ,
New and important applications for the BArF anion have
been reported in recent years and have led to classical anions
being replaced.3b–d,5–13 The BArF anion has been used in organic
transformations as phase transfer catalyst,3b–d Lewis acid cata-
lyst,5,6 in electrochemistry,7 in lithium batteries,8 as a compo-
nent of ionic liquids9 and as a remover of lanthanide ions.10
Regarding applications in enantioselective catalysis, the BArF
anion has been widely used as a counterion in Ir-mediated
hydrogenations,11 and as regulation agent or additive for
supramolecular catalysts.12,13
The synthesis of NaBArF was rst reported by the Kobayashi
group, who carried out the addition of arylmagnesium deriva-
tives to boron reagents.3b–e Bergman and co-workers14 subse-
quently described more efficient synthetic protocols that
avoided the use of magnesium metal as reagent. Analogous
synthetic protocols have been developed for LiBArF3h,13c and
KBArF.3h,15 It has also been demonstrated that RbBArF12d or
CsBArF12d can be efficiently prepared by displacing the sodium
cation in NaBArF with rubidium or cesium salts.16 The fact that
RbBArF and CsBArF have lower solubility in water than NaBArF
is the driving force behind the exchange of the cations bonded
with the BArF moiety. Our group has pioneered the syntheses of
a number of enantiopure ammonium BArF salts by displace-
ment of the chloride with BArF anions in the corresponding
amine hydrochlorides.12b,d
[AsF6]ꢀ, [BF4]ꢀ, [CF3SO3]ꢀ, carborane anions (i.e., [HCB11
-
H11ꢀnXn]ꢀ with X ¼ halogen) and [AlCl4]ꢀ, since these anions
proved to have a weakly coordinating character in aqueous
solutions due to the delocalisation of their negative charge over
all their groups or atoms.
A wide variety of WCAs are known2 and of these, borate-
based anions (i.e. [BPh4]ꢀ and its derivatives)3 deserve special
mention. The [BPh4]ꢀ anion was used as counterion in Fe-based
olen polymerisation catalysts.4 The BPh4 group is prone to
hydrolysis and sensitive to decomposition.2 All these drawbacks
were solved by attaching uorine-containing substituents to the
phenyl groups.
Several uorine-containing derivatives of [BPh4]ꢀ have been
developed (i.e., [B(C6F5)4]ꢀ (ref. 3a), [B(3,5-(CF3)2C6H3)4]ꢀ (ref.
3e) and [B(CF3)4]ꢀ (ref. 3j)). The tetrakis[3,5-bis(triuoromethyl)
aInstitute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science
¨
and Technology, Av. Paısos Catalans 16, 43007 Tarragona, Spain. E-mail: avidal@
iciq.cat
b
´
ICREA, Pg. Lluıs Companys 23, 08010 Barcelona, Spain
† Electronic supplementary information (ESI) available: Experimental details,
spectroscopic and crystallographic data. CCDC 1546291–1546302. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra05928k
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1
and H NMR data were in agreement with those previously re-
ported for LiBArF$4H2O.3h
Potassium tetrakis[3,5-bis(triuoromethyl)phenyl]borate,
(KBArF)
Potassium bis(trimethylsilyl)amide (0.18 g, 0.85 mmol) was
dissolved in 50 mL of diethyl ether. This solution was added
dropwise over a solution of HBArꢁF$(EtO2)2 (0.86 g, 0.85 mmol)
in 30 mL of diethyl ether at ꢀ20 C. Aer stirring overnight at
ꢀ20 ꢁC, the solvent was concentrated under reduced pressure to
dryness. The solid was dissolved in 6 mL of diethyl ether and the
solution was cooled at ꢀ78 ꢁC. Hexane (20 mL) was slowly added
over this solution to precipitate the desired product. The solvent
was ltered-off via cannula and dried under reduced pressure to
afford the product KBArF as a white solid. Isolated 0.61 g, 80%
Fig. 1 Ammonium BArF derivatives.
Though alkali metal and some ammonium-based BArF salts
were known in the literature, their structures in the solid-state
had not been systematically determined and studied.17 More-
over, neither had systematic characterisation data of these
compounds in solution ever been reported.18 Thus, we report
herein our efforts to develop efficient methods for the prepa-
ration of a number of BArF salts and to characterise in the solid-
state and in solution an array of alkali [i.e., LiBArF (1), NaBArF
(2), KBArF (3), RbBArF (4) and CsBArF (5)] and ammonium BArF
derivatives (structures 6–8, see Fig. 1). The present article
provides valuable information on the spatial atomic arrange-
ment, distances and geometries together with the coordination
bonds and/or short contacts of the positively charged atoms in
the solid-state.
1
yield. H NMR (C4D8O, 400 MHz): d 7.79 (br s, 8H), 7.58 (br s,
4H) ppm. 13C{1H} NMR (C4D8O, 126 MHz): d 162.8 (q, JC–B ¼ 50
Hz), 135.6, 130.0 (qm, JC–F ¼ 32 Hz), 125.5 (q, JC–F ¼ 272 Hz),
118.2 (m) ppm. 19F{1H} NMR (C4D8O, 376 MHz): d ꢀ63.2
(24F) ppm. 11B{1H} NMR (C4D8O, 128 MHz): d ꢀ8.6 ppm. IR
(neat, cmꢀ1): 1612, 1357, 1281, 1187, 1124. Mp $ 350 ꢁC.20
Content in K by ICP-OES: calcd 4.33%, found 4.40%. IR and 1H
NMR data were in agreement with those previously reported.15
(R)-(1-Phenylethyl)ammonium tetrakis[3,5-
bis(triuoromethyl)-phenyl]borate, (7)
NaBArF (0.28 g, 0.32 mmol) was mixed with (R)-(1-phenylethyl)
ammonium hydrochloride21 (0.05 g, 0.32 mmol) in 5 mL of Et2O
and allowed to react overnight. Filtration and evaporation of the
solvent yielded the target compound as a white solid (0.30 g,
95% yield). 1H NMR (CD2Cl2, 400 MHz): d 7.73 (br s, 8H), 7.58–
7.52 (m, 7H), 7.44–7.39 (m, 2H), 5.91 (br s, 3H), 4.90 (q, J ¼
6.9 Hz, 1H), 1.91 (d, J ¼ 6.9 Hz, 3H) ppm. 13C{1H} NMR (CD2Cl2,
100 MHz): d 162.2 (q, JC–B ¼ 49 Hz), 135.2, 133.7, 132.1, 130.9,
129.3 (qm, JC–F ¼ 31 Hz), 126.7, 125.1 (q, JC–F ¼ 272 Hz), 117.9
(m), 56.9, 20.1 ppm. 19F{1H} NMR (CD2Cl2, 376 MHz): d ꢀ63.3
(24F) ppm. 11B{1H} NMR (CD2Cl2, 128 MHz): d ꢀ8.6 ppm. IR
(neat, cmꢀ1): 3683, 3606, 3323, 3281, 2857, 1612, 1492, 1353,
1275, 1098. Mp ¼ 149–154 C. [a]D +1.0 (c 1.03, CH3OH). MS
(ESI): m/z calcd for C8H12N: 122.1; found: 122.1 [M ꢀ BArF]+.
Elemental analysis calcd (%) for C41H24BF24N: C 48.75, H 2.45,
N 1.42; found: C 48.12, H 2.49, N 1.49.
Details on the determination of single crystal X-ray diffrac-
tion structures and general structural comments were included
in the ESI† of this article.
Experimental
For the general information on the preparation methods for the
BArF salts, see the ESI.† The spectroscopic data of the BArF salts
prepared using known methods can also be found in the ESI.†
Lithium tetrakis[3,5-bis(triuoromethyl)phenyl]borate
etherate, (LiBArF$(EtO2)2)
Lithium bis(trimethylsilyl)amide (0.212 g, 1.23 mmol) was dis-
solved in 30 mL of diethyl ether. This solution was added
dropwise over a solution of HBArF$(EtO2)2 (ref. 19) (1.24 g, 1.23
25
ꢁ
ꢁ
mmol) in 40 mL of diethyl ether at ꢀ20 C. Aer stirring over-
ꢁ
night at ꢀ10 C, the solvent was concentrated under reduced
pressure to dryness. The solid was dissolved in 6 mL of diethyl
ꢁ
ether and the solution was cooled at ꢀ78 C. Hexane (20 mL)
was slowly added over this solution to precipitate the desired
product. The solvent was ltered-off via cannula and dried
under reduced pressure to afford the product LiBArF$(Et2O)2 as
1
a white solid. Isolated 1.11 g, 89% yield. H NMR (CD2Cl2, 400
Results and discussion
Syntheses of alkali metal and ammonium BArF derivatives
MHz): d 7.72 (br s, 8H), 7.57 (br s, 4H), 3.69 (q, J ¼ 7.1 Hz, 8H),
1.25 (t, J ¼ 7.1 Hz, 12H) ppm. 13C{1H} NMR (CD2Cl2, 126 MHz):
d 162.2 (q, JC–B ¼ 50 Hz), 135.2, 129.3 (qm, JC–F ¼ 32 Hz), 125.0 Though the preparation of LiBArF and KBArF was known in the
(q, JC–F ¼ 272 Hz), 117.9 (m), 67.0, 14.7 ppm. 19F{1H} NMR literature,3h,13c,15 these syntheses require the use of organo-
(C4D8O, 376 MHz): d ꢀ63.2 (24F) ppm. 11B{1H} NMR (C4D8O, magnesium intermediates and aqueous work-ups. Thus, we
128 MHz): d ꢀ8.6 ppm. 7Li NMR (D2O, 194 MHz): d ꢀ0.4 ppm. IR envisaged that these compounds could alternatively be
(neat, cmꢀ1): 1624, 1611, 1354, 1274, 1111, 1101. Mp ¼ 145– prepared via an acid–base reaction (Scheme 1) between the
148 ꢁC. Content in Li by ICP-AES: calcd 0.68%, found 0.67%. IR corresponding acid of BArF (9) and the corresponding alkali
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only a singlet for the signals of the BArF anion with very small
differences in the chemical shis (see Table 1). 1H was in
agreement with the proposed structures and 13C{1H} NMR
showed the expected set of signals for the four equivalent 3,5-
bis(triuoromethyl)phenyl groups and the corresponding
organic moieties in the case of the ammonium BArF salts. IR
spectroscopy also conrmed the structure of the BArF salts with
intense bands in the C]C stretching (ca. 1600–1650 cmꢀ1) and
C–F stretching (ca. 1100–1350 cmꢀ1) vibration regions. Intense
bands in the N+–H stretching region (ca. 2700–3000 cmꢀ1
,
broad) were also observed for the ammonium BArF salts.
Scheme 1 Syntheses of (a) LiBArF$(Et2O)2 and (b) KBArF.
Crystal structure description and analysis
One of the key structural features of the BArF anion is that its
negative charge is distributed over its skeleton, in which the
electronegative uorine atoms play a major role in accommo-
dating the single negative charge of the boron.
metal bis(trimethylsilyl)amide (i.e., LiN(SiMe3)2 or KN(SiMe3)2,
respectively) in a low boiling point organic solvent.
The low boiling point of the liquid components of the
mixture would facilitate the isolation of the desired compound
by evaporation in vacuo. This synthetic procedure was followed
and LiBArF$(Et2O)2 and KBArF were isolated in high yield. It
should be noted that this preparation method does not require
separating the target compound from other salts.
The preparation of RbBArF, CsBArF and the ammonium
BArF derivatives is well documented and relies on a counter-
anion exchange reaction around the alkali metal or ammonium
groups.12d The difference in the solubility of the target BArF
derivative and the by-product of the reaction (i.e. the salt arising
from the combination of the cation initially bound to the BArF-
containing reagent and the anion initially bound to the alkali
metal or ammonium groups) constitutes the driving force of the
reaction and shis the equilibrium. The preparation methods
for the required starting materials and BArF salts have been
detailed in the ESI.†
The distribution of this single negative charge over a multi-
tude of atoms renders the BArF anion both weakly nucleophilic
and basic, which poses questions about the coordination
sphere (CS) of the cationic species and about the number of
weak interactions (or short contacts) required for the single
positive charge of the alkali metal or ammonium cation to be
compensated. Regarding the alkali metal BArF salts, we
reasoned that the different charge densities in the alkali metal
cationic series (i.e., Li+, Na+, K+, Rb+ and Cs+) arising from the
differences in the cationic radii would translate into a different
number of interactions between the uorine atoms or solvate
molecules and the cation in each alkali metal BArF derivative. In
terms of the ammonium BArF derivatives, we reasoned that the
size and topology of the backbones of the substituents at
nitrogen would affect the number of interactions between the
electronegative atoms present in the BArF anion or solvate
molecules and the nitrogen cation. We considered that deter-
mining the coordination spheres, the map of short contacts and
the space-charge compensation map for the positively charged
atom in a number of alkali metal BArF and ammonium BArF
derivatives would be highly useful for understanding the role of
these interesting derivatives in catalytic processes,3b–d,5,6,11–13
electrochemistry7,8 and other applications.9,10 Thus, we turned
our attention to obtaining single crystals suitable for X-ray
analysis for the whole series of BArF derivatives and to per-
forming the corresponding structural studies by X-ray analysis.
Single crystals suitable for X-ray analysis were obtained for the
whole series of alkali metal BArF salts and the ammonium
derivatives studied. Molecules of solvate were incorporated into
the crystal structure in some structures (i.e., LiBArF$4THF,
NaBArF$6THF, CsBArF$7THF and CsBArF$DCM). Moreover,
residual water molecules from the solvent were incorporated
into the crystal structure in some cases (i.e., LiBArF, NaBArF,
KBArF, RbBArF and 7).
Characterisation in solution of alkali metal and ammonium
BArF derivatives
The whole array of BArF salts was characterised using standard
spectroscopic techniques (IR and 1H, 7Li, 11B, 13C, 19F and 133Cs
NMR spectroscopy). 19F{1H} and 11B{1H} NMR spectra showed
Table 1 11B{1H} and 19F{1H} NMR chemical shifts (ppm) for the alkali
metal (in C4D8O) and ammonium (in CD2Cl2) BArF salts
Entry
Compound
11B{1H}a
19F{1H}b
1
2
3
4
5
6
7
8
LiBArF$(Et2O)2
NaBArF
KBArF
RbBArF
ꢀ8.6
ꢀ8.6
ꢀ8.6
ꢀ8.5
ꢀ8.6
ꢀ8.5
ꢀ8.5
ꢀ8.6
ꢀ63.2
ꢀ63.1
ꢀ63.2
ꢀ63.3
ꢀ63.1
ꢀ63.0
ꢀ63.3
ꢀ63.3
CsBArF
6
7
8
Alkali metal BArF salts. The detailed single crystal X-ray
diffraction study not only conrmed the structure of the
compounds but, as intended, also provided important infor-
mation on how the BArF moiety and solvent molecules interact
with the alkali metal (selected distance values in the solid-state
a 11B{1H} chemical shis are quoted in ppm relative to trimethyl borate
(TMB) in C4D8O. b 19F{1H} chemical shis are quoted in ppm relative to
triuoroacetic acid (TFA) in C4D8O.
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are summarised in the discussion that follows and in Table 2). P4/n but is not isostructural to the structure of lithium and
The asymmetric unit in the structure of LiBArF contains sodium BArF salts. In the asymmetric unit we identied
a quarter of one BArF molecule (corresponding to S4 symmetry), a quarter of one BArF molecule (corresponding to S4 symmetry),
a quarter of one lithium atom and 0.68 highly disordered a quarter of a potassium atom and 1/8 of a water molecule. The
molecules of water. The water molecules crystallised in two BArF anion displayed as expected a slightly distorted tetrahedral
disordered ways forming two types of structures. In Fig. 2A, an geometry. The potassium atom is surrounded by eight uorine
octahedral geometry can be observed which is formed by two atoms (F3, F4 and their symmetry equivalents) and one oxygen
water oxygen atoms (O1 and O20), and four uorine atoms from atom (O1W) from a water molecule (see Fig. 2E). The shortest
0
0
˚
˚
neighbouring CF3 groups (F4, F4A, F4B and F4C). In Fig. 2B, K–F distances in KBArF are 2.61 A (F3 ) and 2.73 A (F4 ) and the
˚
a trigonal bipyramidal geometry can be observed. In this case K–O distance is 2.81 A (O1W, see Table 2). RbBArF crystals were
three water oxygen atoms (O1, O2 and O2A) are coordinated in isostructural to those obtained for KBArF (compare Fig. 2E with
a central plane to the lithium atom and two uorine atoms (F4 F). The asymmetric unit contains a quarter of one BArF mole-
and F4A) are placed in the axial positions of the trigonal cule, a quarter of one rubidium atom and a quarter of one water
bipyramid. The octahedral geometry is minor (28%), while the molecule. The BArF anion is located on a rotation–reection
trigonal bipyramidal geometry is major (72%). The BArF cation axis, presents S4 symmetry and displays a slightly distorted
˚
is disordered in two orientations and shows a slightly distorted tetrahedral geometry. The shortest Rb–F distances are 2.99 A
˚
˚
tetrahedral geometry. Interestingly, a higher solvation around (F3) and 2.93 A (F4), and the Rb–O distance is 2.94 A (O1W, see
the Li cation translates into a longer Li–F distance (i.e., 2.287(3) Table 2). Finally, the crystal structure of CsBArF (see Fig. 2G)
˚
A in structures LiBArF-A or LiBArF-B with two or three solvation was studied. The asymmetric unit of this salt contains a quarter
˚
molecules of water, respectively, and 3.34 A in structure of a BArF molecule showing a S4 symmetry, a quarter of
LiBArF$4H2O).22 This observation indicates that an increased a cesium atom located on a four-fold rotation axis and a quarter
water solvation leads to swarming of the ion coordination of a CH2Cl2 molecule. The BArF cation is disordered in two
sphere and translates into a separation of the anion and the orientations and presents a slightly distorted tetrahedral
cation. The single crystal structure obtained for NaBArF is iso- geometry. In this case, the crystal symmetry changes from the
structural to that obtained for LiBArF. An analysis of the P4/n space group observed in the other BArF salts to the P4/ncc
structure collected at room temperature has already been re- space group. Since the cesium atom is larger than potassium
ported.23 However, our discussion uses the data collected by us and rubidium, a higher number of interactions with the
at 100 K to aid comparison. The asymmetric unit of NaBArF surrounding atoms were observed. Twelve interactions with the
contains a quarter of one BArF molecule (corresponding to S4 uorine atoms F3, F4 and F5 (and their symmetry equivalents)
symmetry), a quarter of one sodium atom and 0.59 highly and two with the chlorine atoms from the crystallisation solvent
˚
disordered molecules of water. As in the case of LiBArF, NaBArF with distances ranging from 3.04 to 3.81 A (see Fig. 2G and
adopts an octahedral geometry (Fig. 2C), where the two oxygen Table 2) were observed. It is interesting to note the high number
atoms from the water molecules (O1 and O20) and the four of interactions observed for the cesium derivative (fourteen),
uorine atoms (F4–F4C) interact with the sodium atom, and which is one of the highest reported in the literature for
a trigonal bipyramidal geometry (Fig. 2D), where the sodium a cesium centre.24
atom interacts with two uorine atoms (F4 and F4A) and with
Ammonium BArF salts. Single crystals suitable for X-ray
the three oxygen atoms from the water molecules (O1, O2 and analysis were obtained for compounds 6–8 and crystallo-
O2A). In contrast to LiBArF, the octahedral geometry is major graphic studies conrmed the structures of the compounds (see
(62%), while the trigonal bipyramidal geometry is minor (38%). Fig. 3). Moreover, interesting information was obtained
The BArF cation is disordered in two orientations and shows regarding the short contact interactions between some atoms in
a slightly distorted tetrahedral geometry. The crystal structure the BArF moieties and the cationic nitrogen atoms. Relevant
obtained for KBArF (see Fig. 2E) corresponds to the space group information is listed in the discussion that follows and in
Table 2 Selected distances (A˚) for alkali metal BArF saltsa
Entry
Distance
LiBArF
NaBArF
KBArF
RbBArF
CsBArF
1
2
3
4
5
6
7
8
9
M–B
M–X
7.509(4)
7.546(1)
7.891(4)
O1 – 2.814(9)
7.994(9)
O1 – 2.9392(8)
8.025(3)
Cl1S – 3.807(3)
O1 – 1.874(9)
O2 – 1.930(8)
O20 – 1.856(15)
F40 – 2.287(3)
F4 – 3.490(4)
F400 – 2.520(2)
O1 – 2.193(5)
O2 – 2.274(17)
O20 – 2.258(7)
F4 – 2.395(2)
F40 – 2.995(11)
M–F
F3 – 2.873(18)
F30 – 2.615(4)
F4 – 2.792(2)
F40 – 2.728(6)
F10 – 3.160(3)
F3 – 2.997(2)
F30 – 3.040(2)
F4 – 2.925(18)
F20 – 3.063(19)
F3 – 3.070(11)
F4 – 3.671(6)
F5 – 3.282(8)
F60 – 3.042(10)
a
(0) the prime code corresponds to disordered atoms.
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Fig. 2 Crystal structure of the alkali metal BArF derivatives. Some phenyl rings from the BArF unit have been omitted for the sake of clarity. Colour
scheme: C: black, H: blue, B: orange, F: green, Li: violet, O: red; Na/K/Rb/Cs: purple. Atomic displacement ellipsoids are drawn at a 50%
probability level. For LiBArF: octahedral (A) and trigonal bipyramidal (B) geometries, in which fluorine atoms F4A, F4B and F4C are symmetry
equivalent to F4. For NaBArF: octahedral (C) and trigonal bipyramidal (D) geometries, in which fluorine atoms F4, F4A, F4B and F4C are symmetry
equivalent. For KBArF: see structure (E) for the geometry, in which fluorine atoms F3D, F3E, F3F and F3G are symmetry equivalent to F3 and F4A,
F4B and F4C to F4. For RbBArF: see structure (F) for the geometry, in which fluorine atoms F3D, F3E, F3F and F3G are symmetry equivalent to F3
and F4A, F4B and F4C to F4. For CsBArF: see structure (G) for the geometry, in which fluorine atoms F3D, F3E, F3F, F3G, F4A, F4B, F4C, F5A, F5B
and F5C are symmetry equivalent to F3, F4 and F5, respectively.
Fig. 3 Crystal structure of the ammonium BArF derivatives 6–8 ((A) structure of 6; (B) structure of 7; (C) structure of 8). Colour scheme: C: black,
H: blue, B: orange, F: green, N: purple, O: red. Atomic displacement ellipsoids are drawn at a 50% probability level.
Table 3. The asymmetric unit in the structure of 6 contains two hydrogen atoms from the nitrogen and the uorine atoms.
halves of the BArF anion and the dibenzylammonium coun- Additional short contacts were observed for the hydrogen atoms
terion, with a slightly distorted Td symmetry (see Fig. 3A). The from the ammonium benzyl aromatic rings and uorine atoms.
two independent BArF cations have a slightly distorted tetra- (R)-(1-Phenylethyl)ammonium BArF (7) crystallised in the space
hedral geometry and show only C2 symmetry because the two group C2 with one ammonium cation, two half BArF anions and
aromatic rings present in the asymmetric unit display different one water molecule in the asymmetric unit. The two indepen-
orientations. Short contacts were observed between the dent BArF structures have a slightly distorted tetrahedral
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Table 3 Selected shortest distances (A˚) for ammonium BArF salts 6–8
relevant features of the crystal structures studied are described
in the discussion that follows. THF-solvated LiBArF crystallised
with four molecules of solvent around the Li atom in a tetrahe-
dral manner (see Fig. 4A).26 THF-solvates of NaBArF and
CsBArF27 could also be successfully crystallised. In the case of
the NaBArF THF-solvate, six molecules of THF in a slightly
distorted octahedral geometry around the Na were observed,
whereas for CsBArF, seven molecules of THF were displayed
around the Cs centre in a monocapped trigonal prismatic
geometry. As expected, the coordination number of the metal
centres increased as did the size of the metal. Not surprisingly,
the M–B distances for the THF-solvates became longer as the
Entry
Distance
6
7
8
1
2
3
N–B
N–F
N–O
6.724(2)
3.058(2)
—
7.477(3)
2.959(3)
2.705(4)
6.778(6)
2.995(5)
—
geometry and show only C2 symmetry because the two aromatic
rings in the asymmetric unit present different orientations.
Fig. 3B shows the structure obtained using only one half of the
expanded BArF anions. A short contact was observed between
the asymmetric hydrogen atom from the counterion and
a uorine atom from the BArF group. Additional short contacts
were observed between the water molecule and a uorine atom,
and between the water molecule and the phenyl moiety from the
cation. Remarkably, the water molecule forms a hydrogen bond
˚
˚
size of the cation increased (7.94 A for LiBArF$4THF, 8.74 A for
˚
NaBArF$6THF and 9.09 A for CsBArF$7THF). As a direct effect of
the longer M–B distances for the THF-solvates compared with
the alkali metal derivatives, no remarkable short contacts were
observed between the uorine atoms and the corresponding
alkali metal cation in any of the three structures studied.
Discussion on solid-state structures of BArF salts. The
detailed single crystal X-ray diffraction study that was per-
formed provided important information about their structure. A
few trends were observed regarding the distances between the
boron atom of the BArF motif and the cationic alkali metal or
ammonium centres. For the series of alkali metal BArF salts
(Table 2), the M–B distance became longer as the ionic radius of
the cation increased. In terms of the ammonium BArF salts
(compounds 6–8), it is interesting to point out that the
ammonium-based structures had an N–B distance ranging from
˚
with the cationic nitrogen atom (N1–O1W ¼ 2.71 A). (S,S)-Bis-(1-
phenylethyl)ammonium BArF (8) crystallised in the triclinic
space group P1. The asymmetric unit in the structure contains,
in this case, a whole BArF molecule which has C1 symmetry with
a slightly distorted tetrahedral geometry for the carbon atom
and the secondary ammonium counterion which has a slightly
distorted molecular Td symmetry. The solid-state structure of
the salt is displayed in Fig. 3C. In this case, no remarkable short
contacts were observed. The steric hindrance of the methyl
substituents in the ammonium could account for the lack of
interactions with the BArF moiety.
˚
6.72 to 7.48 A (see Table 3), which is shorter than the B–Li
Tetrahydrofuran-solvated alkali metal BArF salts. Having
determined the distances and geometries within the crystal
structures of alkali metal BArF salts, we turned our attention to
performing a similar study on a set of solvates from the same
derivatives. We anticipated that the geometries and map of
short contacts between the BArF anion and the solvated alkali
metal cations would be different from those obtained for the
alkali metal BArF structures. The oxophilicity of alkali metal
cations25 led us to reason that crystallisation of the BArF salts in
an oxygen-containing solvent such as tetrahydrofuran (THF)
would lead to the corresponding solvates. Crystals suitable for
X-ray analysis were obtained following this strategy. The most
˚
distance (7.51 A; see Table 2), despite the fact that the lithium
had a smaller cationic radius than that of NH4+ (radii for Li+ and
+
28
˚
˚
NH4 are described as 0.79 A and 2.11 A, respectively). More-
over, a trend between the steric bulkiness around the nitrogen
atom and the N–B distances could not be established: the N–B
distance of the primary ammonium BArF derivative 7 was
higher than those of the secondary ammonium BArF derivatives
6 and 8 (compare distances in entry 1 of Table 3). Other relevant
structural parameters in the alkali metal BArF salts are the
number of coordination bonds and/or short contacts identied
in the crystal structure. A representation of the number of these
two types of interaction compared with the ionic radius of the
Fig. 4 Crystal structure of alkali metal BArF solvates ((A) structure of LiBArF$4THF; (B) structure of NaBArF$6THF; (C) structure of CsBArF$7THF).
Colour scheme: C: black, H: blue, B: orange, F: green, Li: violet, Na: purple, Cs: light green, O: red. Atomic displacement ellipsoids are drawn at
a 50% probability level.
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Fig. 5 (Left) The number coordination bonds or/and short contacts around the cationic metal alkali atom compared with the ionic radius of the
metal, (right) the average distance between the fluorine atoms and the cationic metal alkali atom compared with the ionic radius of the metal.
metal28 is shown on Fig. 5 (le) (this number has been calcu- yielding unreported structural details of the structures of
lated for LiBArF and NaBArF on the basis of the ratio of the KBArF, RbBArF, CsBArF, 6, 7 and 8. The increasing number of
octahedral and trigonal bipyramidal structures indicated in the coordination bonds and/or short contacts between the alkali
discussion and in the ESI†). As expected, the trend observed is metal cation and the surrounding atoms has been rationalised
that the number of coordination bonds and short contacts in terms of the size of the alkali metal centre. A study of the
increases as the ionic radius increases (from ca. ve in LiBArF to structures of alkali metal hydrated and THF-solvates (i.e.,
fourteen in CsBArF). Interestingly, the increased number of LiBArF$4H2O, LiBArF$4THF, NaBArF$6THF and CsBArF$7THF)
interactions between the uorine and the alkali metal cation showed that the interactions between the metal centre and the
translates into longer M–F distances for cesium, which is the surrounding atoms depend not only on the size of the alkali
largest cation in the series (Fig. 5 (right)). Interestingly, two metal centre but also on its solvation.
reviews by Plenio comprehensively discussed the coordination
The detailed structural information in the solid-state
chemistry of the CF-units to alkali metal centres on the basis of provided herein regarding an array of alkali metal and ammo-
empirical and computational data in the solid-state and in nium BArF derivatives may well allow a better understanding of
solution. This structural information provided useful data to the role of BArF derivatives in catalysis, electrochemistry and
validate our results.29 When considering the shortest M–F supramolecular processes and broaden their applicability in
distances in the alkali metal BArF salts (see Table 2, entries 5–9), future processes of interest.
it is observed that all distance values are below the upper
threshold for CF–M bond lengths reported by Plenio.29a On the
other hand, the average M–F distances (Fig. 5 (right)) for the Na,
Acknowledgements
K and Cs derivatives are in agreement with the CF–M distance
distribution statistics reported in the above mentioned reviews.
For the THF-solvates LiBArF$4THF, NaBArF$6THF and
CsBArF$7THF, the number of solvate molecules around the
metal centres increased as the ionic radius of the metal became
larger.
The authors would like to thank MINECO (CTQ2014-60256-P
and Severo Ochoa Excellence Accreditation 2014–2018 SEV-
2013-0319) and the ICIQ Foundation for nancial support. L.
C. thanks MINECO for a FPI-SO pre-doctoral fellowship. L. R.
and I. M. thank ICIQ for pre-doctoral fellowships. Dr M. V. is
grateful for the nancial support received from the CELLEX
Foundation.
Conclusions
Notes and references
A new synthetic protocol for the efficient preparation of lithium
and potassium BArF has been developed by reaction between
HBArF$(Et2O)2 and the corresponding lithium or potassium
bis(trimethylsilyl)amide. This new synthesis constitutes an
unreported, short, efficient and practical means of synthesising
LiBArF and KBArF. The method paves the way for the prepara-
tion of new metal BArF derivatives. Moreover, a number of alkali
metal and ammonium BArF derivatives have been fully char-
acterised both in solution and in the solid-state by using stan-
dard spectroscopic techniques.
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an aza-macrocycle respectively. For KBArF, six crystal
structures were found (CCDC ref. codes: FUPBEZ and
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(coordinated to a selenoether macrocyle), OJAMET and
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OJAMUJ and OJEBAI (coordinated to an aza-macrocycle),
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found, but only one relevant structure was found
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`
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22 Interestingly LiBArF crystallised also as a tetrahydrate
(LiBArF$4H2O). This specic structure has been reported
previously (see ref. 3h) but the crystallographic study was
performed at 173 K. The crystallographic data obtained at
100 K in the present work for LiBArF$4H2O were used for
comparison with the LiBArF structures measured under
identical conditions. In the case of the LiBArF$4H2O, the
lithium atom is coordinated to four H2O molecules in
a tetrahedral fashion, and Li–F and B–Li distances are
longer in the tetrahydrate LiBArF$4H2O than in LiBArF-A
or LiBArF-B (see ESI†).
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16 Though seminal pieces of work on alkali metal BArF salts
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¨
indicate that these compounds can be prepared by cation 24 Porschke et al. have reported a sixteen-coordinated cesium
exchange (see ref. 3e, 3h and 5), detailed preparation
centre. For example, see: D. Pollak, R. Goddard and
¨
K.-R. Porschke, J. Am. Chem. Soc., 2016, 138, 9444–9451.
methods and isolated yields are not provided.
17 This statement is made on the basis of the structures 25 A. E. H. Wheatley, Chem. Soc. Rev., 2001, 30, 265–273.
checked in the CCDC† database. A query was made in the 26 The measured LiBArF$4THF sample was a multicomponent
CCDC database. The reponse to this query provided all the
crystal structures including the [BArF]ꢀ sub-structure (2060
results) submitted to the CCDC until February 2017 {CSD
crystal formed by a minimum of two crystals (ratio 51 : 49).
Additionally only a limited number of reections could be
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collected. Due to these difficulties, the data obtained is less 28 P. F. Lang and B. C. Smith, Dalton Trans., 2010, 39, 7786–
precise. 7791.
27 The CsBArF$7THF sample was an inversion twin and 29 (a) H. Plenio, Chem. Rev., 1997, 97, 3363–3384; (b) H. Plenio,
ꢀ
presented high disorder in the solvent molecules and CF3
units. As a result, the bond lengths for a reduced number
of C–C bonds are less precise.
ChemBioChem, 2004, 5, 650–655.
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