Bis(triphenyl-λ5-phosphanylidene)ammonium fluoride: A reactive fluoride source to access the hypervalent silicates [Me nSiF5-n]- (n = 0-3)

Article abstract of DOI:10.1039/c3dt52617h

A new synthesis of bis(triphenyl-λ⁵-phosphanylidene) ammonium fluoride ((Ph₃PNPPh₃)F, abbreviated as (PNP)F), is described. The title compound has been fully characterized by multinuclear NMR spectroscopy, vibrational spectroscopy, elemental analysis and single crystal and powder X-ray diffraction for the first time. In the solid state (PNP)F exists as a covalent molecular compound, in which the fluoride ion is asymmetrically bonded to the two phosphorus atoms of the [PNP]⁺ cation. The phosphorus-fluorine bond with 181.98(13) pm is surprisingly long and the longest P-F bond in any phosphorane. (PNP)F can be assumed to be a very good source of reactive fluoride. To investigate the fluoride ion donating properties, (PNP)F was reacted with a range of different fluoromethylsilanes Me_nSiF_4-n (n = 0-4). Reactions of (PNP)F with the fluoromethylsilanes were performed in aceto- or propionitrile and in 1,2-dimethoxyethane under inert conditions. The resulting hypervalent fluoromethylsilicates [Me_nSiF_5-n]^- (n = 0-3) were fully characterized by multinuclear NMR and vibrational spectroscopy and single crystal X-ray diffraction. From the reaction of (PNP)F with Me ₄Si in acetonitrile, the starting materials were recovered unchanged. To aid the understanding of the experimental results the fluoride ion affinities (FIA) for these silanes have been calculated by DFT calculations on the PBE0/def2-TZVPP level of theory. The fluoride ion affinity in the series of Me_nSiF_4-n (n = 0-4) decreases with the number of methyl groups and is too low for Me₄Si to bind a fluoride ion under these reaction conditions.

Full text of DOI:10.1039/c3dt52617h

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Bis(triphenyl-λ5-phosphanylidene)ammonium
fluoride: a reactive fluoride source to access the
Cite this: Dalton Trans., 2014, 43,
4326
hypervalent silicates [MenSiF5−n]− (n = 0–3)†
Christoph Bolli,a Jorit Gellhaar,b Carsten Jenne,*a Mathias Keßler,a Harald Scherer,b
Helene Seegera and Rabiya Uzunb
A new synthesis of bis(triphenyl-λ5-phosphanylidene)ammonium fluoride ((Ph3PNPPh3)F, abbreviated as
(PNP)F), is described. The title compound has been fully characterized by multinuclear NMR spectroscopy,
vibrational spectroscopy, elemental analysis and single crystal and powder X-ray diffraction for the first
time. In the solid state (PNP)F exists as a covalent molecular compound, in which the fluoride ion is asym-
metrically bonded to the two phosphorus atoms of the [PNP]+ cation. The phosphorus–fluorine bond
with 181.98(13) pm is surprisingly long and the longest P–F bond in any phosphorane. (PNP)F can be
assumed to be a very good source of reactive fluoride. To investigate the fluoride ion donating pro-
perties, (PNP)F was reacted with a range of different fluoromethylsilanes MenSiF4−n (n = 0–4). Reactions
of (PNP)F with the fluoromethylsilanes were performed in aceto- or propionitrile and in 1,2-dimethoxy-
−
ethane under inert conditions. The resulting hypervalent fluoromethylsilicates [MenSiF5−n
]
(n = 0–3)
were fully characterized by multinuclear NMR and vibrational spectroscopy and single crystal X-ray dif-
fraction. From the reaction of (PNP)F with Me4Si in acetonitrile, the starting materials were recovered
unchanged. To aid the understanding of the experimental results the fluoride ion affinities (FIA) for these
silanes have been calculated by DFT calculations on the PBE0/def2-TZVPP level of theory. The
fluoride ion affinity in the series of MenSiF4−n (n = 0–4) decreases with the number of methyl groups and
is too low for Me4Si to bind a fluoride ion under these reaction conditions.
Received 22nd September 2013,
Accepted 8th October 2013
DOI: 10.1039/c3dt52617h
www.rsc.org/dalton
the solid state, many attempts have been made to approach
the properties of a “naked fluoride” ion in condensed phase
Introduction
The fluoride ion is an important reagent and intermediate in and thus its outstanding reactivity.
various reactions.1 For very reactive fluoride ion donors, the
Classical fluoride ion donors consist of a cation and a fluor-
term “naked fluoride” was introduced into the literature. The ide ion. In the solid state, the reactivity of a salt M+F− is
quest for a “naked fluoride” has driven several generations of enhanced by increasing the size of the cation M+ and thus
chemists to synthesize new compounds with increasing fluor- decreasing the lattice enthalpy.2 In solution the reactivity of a
ide ion donor properties and to understand the correlation fluoride ion depends on the degree of solvation, which is lower
between structure and reactivity. Although a “naked fluoride” in non-polar solvents (e.g. CH2Cl2, aromatics). While classical
ion does not exist in a chemical environment in solution or fluoride ion donors like CsF are insoluble in non-polar sol-
vents, fluoride ion donors containing large organic cations
possess improved solubility in organic solvents.3 Conse-
quently, a number of M+F− salts containing large cations have
been reported. The cation size can range from a small cesium
cation (cation volume 0.019 nm3) over medium size tetra-
methylammonium (0.121 nm3) and hexamethylguanidinium
aFachbereich C – Anorganische Chemie, Bergische Universität Wuppertal, Gaußstr.
20, 42119 Wuppertal, Germany. E-mail: carsten.jenne@uni-wuppertal.de;
Tel: +49-202-439-2503
bInstitut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität
Freiburg, Albertstr. 21, 79104 Freiburg i. Br., Germany
†Electronic supplementary information (ESI) available: Synthesis and character- (0.202 nm3) cations to the large bis(tris(dimethylamino)-
λ5-phosphanylidene)ammonium, [(Me2N)3PNP(NMe2)3]+,
ization of [PNP][BF4], actual experimental spectra for (PNP)F, [PNP][Me3SiF2],
(0.476 nm3) cation.
[PNP][Me2SiF3], [PNP][MeSiF4], and [PNP][SiF5], details for the DFT calculations,
calculated fluoride ion affinities, thermodynamic estimations, and represen-
The synthesis and the crystal structure of the fluoride salts
tations and data for all crystal structures (pdf). CCDC 854540, 854541, 937494,
of the latter cation have been reported, but its reactivity has
not been broadly explored, presumably due to its expensive
937495, 937496, 937613, 954026, 955393, 955394 and 955395. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52617h
4326 | Dalton Trans., 2014, 43, 4326–4334
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synthesis. The comparable bis(triphenyl-λ5-phosphanylidene)-
ammonium cation ([Ph3PNPPh3]+, hereinafter abbreviated as
[PNP]+) is even larger (0.683 nm3) than all previously used
counter cations and thus is a very attractive cation for a reactive
fluoride source. The [PNP]+ cation has been widely used in in-
organic chemistry as a large weakly coordinating cation, there-
fore its synthesis is well established4 and [PNP]Cl is
commercially available. Although [PNP]Cl is a well-known
reagent and a large number of crystal structures containing
the [PNP]+ cation have been published,5 [PNP]+ salts contain-
ing small anions are rare, and the crystal structures of even the
simple halides [PNP]X (X = Cl,6 Br,7 I8) were reported only very
recently.
Several synthetic approaches for the synthesis of (PNP)F are
provided in the literature. However, all these reports lack a full
characterization of the claimed product.4b,9 Herein, the syn-
thesis of (PNP)F from [PNP][BF4] and KF and full spectroscopic
and structural characterization are reported. Exemplarily, the
reactivity of (PNP)F was tested by the reaction with a series of
H3CCN=H2O
½PNPꢀCl þ AgF ꢁꢁꢁꢁꢁꢁ! ðPNPÞF þ AgCl
ð3Þ
In the course of our work we tried to reproduce the syn-
thesis of (PNP)F according to all three published procedures.
Our attempts according to Berkessel and Brandenburg’s
method using an aqueous CH3CN solution resulted in the for-
mation of the [SiF6]2− anion and hydrogendifluoride [HF2]−,
which were both unambiguously identified by NMR.
Obviously, hydrogendifluoride is formed in aqueous solution,
which reacts with the glass surface of the reaction vessel to the
[SiF6]− anion. Furthermore, the obtained products in all reac-
tions starting from [PNP]Cl contained unreacted chloride as is
evident from several crystal structure determinations (see
section S.4.2 in the ESI†). Unfortunately, the simple test for
chloride by adding Ag[NO3] cannot be applied in this case,
because insoluble colorless [PNP][NO3] precipitated besides
AgCl. Thus reactions starting from [PNP]I with AgF in absolute
methanol were performed in a PFA vessel to exclude possible
side reactions with the glass surface. Nevertheless, in all reac-
tions the hydrogendifluoride anion, which contains a very
strong hydrogen bond,11 was obtained as the main product. It
should be noted that several commercial charges of AgF from
ChemPur and Apollo Scientific were used and all charges con-
tained at least traces of [HF2]− due to hydrolysis. We cannot
rule out that the synthesis in absolute methanol, in a comple-
tely water-free reaction, using very pure AgF, and starting from
the iodide [PNP]I may indeed give (PNP)F. However, we were
unable to prepare the fluoride (PNP)F according to the pub-
lished procedures and concluded that even traces of water led
to the formation of the hydrogendifluoride. From these
attempts three different crystal structures were obtained (see
section S.4.2 in the ESI†). All structures show the presence of
[HF2]− and the remaining chloride, which illustrates the syn-
thetic problems. Thus, it can be assumed that these previous
reports described the hydrogendifluoride [PNP][HF2]12 rather
than the fluoride (PNP)F. Grushin and Marshall generated
single crystals of (PNP)F from the reaction of [PNP]CN with
C6F6 and determined its room temperature crystal structure in
2008,9c when our work was already in progress. However, they
did not succeed in preparing pure bulk material and did not
report any further characterization.
silanes MenSiF4−n (n = 0–4). The resulting hypervalent fluoro-
−
methylsilicates [MenSiF5−n
]
themselves, in particular the
[Me3SiF2]− anion, have been proposed as alternative sources of
a “naked fluoride”.10 The fluoride ion affinity in the series of
MenSiF4−n (n = 0–4) decreases with the number of methyl
groups. The successful synthesis of the hypervalent fluoro-
−
methylsilicates [MenSiF5−n
]
(n = 0–3) will establish an experi-
mental threshold for the fluoride ion donor strength of
(PNP)F.
Results and discussion
Synthesis and characterization of the fluoride (PNP)F
Synthesis. The synthesis of the fluoride (PNP)F without
detailed characterization was claimed in the literature by
different authors. Martinsen and Songstad reacted [PNP]I with
AgF in absolute methanol and characterized the product by
C,H,N-elemental analysis and melting point determination
(eqn (1)).4b Douglas and Ruff performed a similar reaction but
started from [PNP]Cl and recrystallized the product from
CH2Cl2 (eqn (2)).9a According to elemental analysis the
product was claimed to be the dichloromethane adduct
(PNP)F·CH2Cl2. Berkessel and Brandenburg reported the syn-
thesis of (PNP)F in the CH3CN–H2O solvent system starting
from [PNP]Cl (eqn (3)).9b The yellowish product was character-
ized by NMR only. The reported 19F NMR chemical shift of
−50.3 ppm in CD3OD seems odd, since a 19F NMR chemical
shift of −148 ppm was obtained previously for [Me4N]F in
methanol.3a
We decided to prepare (PNP)F by a simple metathesis reac-
tion of [PNP][BF4] with KF in anhydrous methanol in analogy
to previously published procedures (eqn (4)).3b,k,13 Exclusion of
any traces of moisture is absolutely essential for its successful
preparation, otherwise the [HF2]− salt is formed.
H3COH
½PNPꢀ½BF4ꢀ þ KF ꢁꢁꢁ! ðPNPÞF þ K½BF4ꢀ
ð4Þ
The product was obtained in good yields as a white solid
and completely dried at 130 °C at 10−3 mbar. In the absence of
moisture (PNP)F can be stored without decomposition.14 At
low temperature (−40 °C) (PNP)F is soluble in absolute metha-
nol, acetonitrile, and propionitrile and slightly soluble in 1,2-
dimethoxyethane. (PNP)F is stable in methanol, but due to the
formation of very strong hydrogen bonds the reactivity is
H3COH
½PNPꢀI þ AgF ꢁꢁꢁ! ðPNPÞF þ AgI
ð1Þ
ð2Þ
1: H3COH
2: CH2Cl2
½PNPꢀCl þ AgF ꢁꢁꢁꢁꢁꢁ! ðPNPÞFꢂCH2Cl2 þ AgCl
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significantly reduced. Therefore solvents which are less prone
to the formation of hydrogen bonds are preferred. While
(PNP)F, which still contains one equivalent of methanol, is
stable in CH3CN at room temperature, solvent-free (PNP)F is
very reactive in solution. It reacts with the solvent CH3CN at
room temperature under the formation of the hydrogendifluor-
ide according to a previously described reaction (Fig. S3 and
S4†).15 When more resistant solvents like 1,2-difluorobenzene
or propionitrile are used, the fluoride starts to attack the
[PNP]+ cation at room temperature. N-Diphenylphosphin-
triphenylphosphazene and triphenylphosphinimine were identi-
fied as decomposition products (Fig. S10–S12†). A similar
decomposition has been observed for water-free [nBu4N]F.16
Spectroscopic characterization. The identity of (PNP)F was
unambiguously proven by elemental analysis, multinuclear
NMR spectroscopy, vibrational spectroscopy, powder diffrac-
tion and single crystal X-ray diffraction. All actual spectra are
included with the ESI.† The 1H and 31P NMR spectra of the
pure product at low temperature show the expected signals for
the [PNP]+ cation (Fig. S1 and S2†), the 19F NMR spectrum in
CD3CN at low temperature is in excellent agreement with a sol-
vated fluoride ion (Fig. S4†).3a,17 The Raman spectrum is indis-
tinguishable from that of other simple [PNP]+ salts in accord
with quantum-chemically calculated spectra (Fig. S5 and S8†).
In contrast the IR spectrum shows a significant shift of the IR-
active dominant PvNvP stretching vibration from 1260 cm−1
in [PNP][BF4] or other salts containing a free [PNP]+ cation to
1169 cm−1 in (PNP)F (Fig. 1 and S7†). Therefore a strong
cation–anion interaction and a significant structural change of
the PNP unit are expected.
Fig. 2 Molecular unit of the crystal structure of (PNP)F. Ellipsoids are
drawn at 50% probability and hydrogen atoms are drawn with arbitrary
radii. Bond lengths are given in pm.
(PNP)F is well reproduced by DFT calculations (see section
S.3.1 in the ESI†). The fluoride ion is located asymmetrically
between the two phosphorus atoms and bound to one phos-
phorus atom resulting in a trigonal bipyramidal environment
with the fluorine atom and one phenyl group in the axial posi-
tions around this phosphorus atom. The axial P–C bond
(188.7(2) pm) is significantly longer than the equatorial P–C
bonds (183.8(2) and 184.0(2) pm) in accord with a 3-center
4-electron F–P–C bond. The structure is reminiscent of the gas
phase structure of molecular Me4PF (P–Cax 188.4(8) pm, P–Ceq
182.6(4) pm).3i Interestingly, the P–F bond in (PNP)F
(181.98(13) pm) is even longer than the P–F bond in Me4PF
(175.3(6) pm)3i and therefore the longest known P–F bond in
any phosphorane (IUPAC: λ5-phosphane). The fluorine atom
shows another weak interaction with the second phosphorus
atom (264.59(14) pm), which is still much shorter than the
sum of the van der Waals radii (327 pm).18 The P1–N1 bond
(163.1(2) pm) is elongated by about 5 pm compared to a free
[PNP]+ cation and the P–N–P angle (127.0(1)°) is on the lower
end of the range of P–N–P angles5 due to the asymmetric inter-
action of the fluorine atom with both phosphorus atoms.
Crystal structure. The identity of (PNP)F was also confirmed
by a single crystal X-ray diffraction study at low temperature.
The obtained structure is very similar to the room temperature
crystal structure.9c In the solid state (PNP)F has a molecular
covalent structure (Fig. 2) in contrast to the ionic structures of
the other halides [PNP]X (X = Cl, Br, I)6–8 and the related com-
pound [(Me2N)3PNP(NMe2)3]F.3b,k The molecular structure of
Reactions of (PNP)F with MenSiF4−n (n = 0–4)
Syntheses and calculated FIAs. (PNP)F can be assumed to
be a very good source of reactive fluoride. To establish a
threshold for its fluoride donating properties, (PNP)F was
reacted with the series of different fluoromethylsilanes
MenSiF4−n (n = 0–4). The fluoride ion affinities (FIA) for these
silanes have been calculated (Table 1, also see section S.2 in
the ESI† for details). The FIA increases with an increasing
number of fluorine atoms. Thus, as expected methyl groups
lower the FIA while fluorine atoms increase the FIA of the
silanes. The FIA of Me4Si (120.6 kJ mol−1) was calculated to be
the lowest in the series and to be more than 60 kJ mol−1 lower
than that of Me3SiF. Reacting (PNP)F with these silanes would
allow the establishment of a threshold for the fluoride donor
Fig. 1 IR spectra (diamond ATR) of [PNP][BF4] (black, top) and (PNP)F (blue,
bottom). The bands due to the PvNvP stretching vibration are marked.
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energy difference between these two isomers has been calcu-
lated to be 29 kJ mol−1 (see section S.3.2 in the ESI†). Even at
−60 °C there was still chemical exchange between these two
isomers. The mechanism for this exchange at this temperature
is Berry-pseudo rotation. At higher temperatures also inter-
molecular exchange of fluorine atoms occurs, which empha-
sizes the fluorine donating capacity of these compounds. For
[MeSiF4]− and [SiF5]− only one 19F resonance was detected at
25 °C and even at temperatures below −60 °C. In these two
anions the chemical exchange between axial and equatorial
fluorine atoms is too fast. Thus, much lower temperatures
would be needed to distinguish between axial and equatorial
fluorine atoms by 19F-NMR spectroscopy.
Table 1 Calculated (PBE0/def2-TZVPP) fluoride ion affinities for the
silanes (FIA)a
Silane
FIA in kJ mol−1
SiF4
297.2
245.1
209.9
182.2
120.6
MeSiF3
Me2SiF2
Me3SiF
Me4Si
a See section S.2.3 in the ESI for details.
strength of (PNP)F in solution. Since salts containing the
difluorotrimethylsilicate anion [Me3SiF2]− found applications
as fluoride ion donors themselves,10 the successful synthesis
of this anion from (PNP)F and the silane Me3SiF would prove
that (PNP)F is a strong enough fluoride ion donor to generate
this common fluoride ion donor.
Therefore, (PNP)F was reacted with the fluoromethylsilanes
according to eqn (5) in aceto- or propionitrile and in 1,2-di-
methoxyethane (hereinafter abbreviated as DME). With the
exception of tetramethylsilane, the corresponding silicates
were formed and fully characterized by NMR and vibrational
spectroscopy and single-crystal X-ray diffraction. From the reac-
tion of (PNP)F with Me4Si in acetonitrile the starting materials
were recovered unchanged. Obviously, the FIA of Me4Si is too
low to bind a fluoride ion. The thermodynamics in the con-
densed phase were estimated (section S.3.4) and support the
experimental findings.
Crystal structures. The crystal structures of the ionic com-
pounds [PNP][MenSiF5−n] (n = 0–3) were determined by single-
crystal X-ray diffraction. Exemplarily, Fig. 3 shows part of the
crystal structures of [PNP][Me3SiF2] and Table 2 summarizes
the crystallographic parameters. This study presents the first
−
complete series of structurally determined [MenSiF5−n
]
(n =
0–3) anions with the same counter cation. While crystal struc-
tures containing the [Me3SiF2]−,3h,9,19 [Me2SiF3]−,20 and
[SiF5]− 21 anions with other counter cations have been pub-
lished before, this study reports the first structure of the
[MeSiF4]− anion. Single crystals suitable for X-ray diffraction
were obtained by layering an acetonitrile solution with diethyl
ðPNPÞF þ MenSiF4ꢁn ! ½PNPꢀ½MenSiF5ꢁnꢀ; n ¼ 0–4
ð5Þ
NMR spectroscopy. All fluoromethylsilicates proved to be
stable in propionitrile or DME solution for weeks even at room
temperature in sealed NMR tubes. For [MeSiF4]− slow fluorina-
tion of the cation was observed in DME–CD3CN solution. The
constitution of the fluoromethylsilicate anions in solution was
unambiguously established by multinuclear 1D and 2D NMR
experiments (see section S2 in the ESI†). The 29Si resonance
frequency of the fluoromethylsilicates decreases with an
increasing number of fluorine atoms in the anion: [Me3SiF2]−:
−65.3 ppm, [Me2SiF3]−: −75.3 ppm, [MeSiF4]−: −112.0 ppm
and [SiF5]−: −147.5 ppm. This trend is in analogy to the related
fluoromethylsilanes. NMR measurements were carried out also
at low temperatures, because of the expected dynamic behavior
of these anions in solution. For [Me3SiF2]− and [Me2SiF3]− no
19F-NMR signal at all could be detected at room temperature.
In contrast, at temperatures below −40 °C two different 19F
resonances were detected representing fluorine atoms in axial
and equatorial position respectively (Fig. S15†). For [Me2SiF3]−
this is expected for the energetically favored isomer, which
contains two fluorine atoms in the axial position and one flu-
orine atom in the equatorial position. In contrast, in the
lowest energy isomer of [Me3SiF2]− both fluorine atoms are in
the axial position. Obviously, in solution a second isomer,
which has one fluorine atom in the axial and one fluorine
Fig. 3 Part of the crystal structure of [PNP][Me3SiF2]. Ellipsoids are
drawn at 50% probability and hydrogen atoms are drawn with arbitrary
atom in the equatorial position, has to be present as well. The radii. Bond lengths are given in pm.
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Table 2 Crystallographic parameters for (PNP)F, [PNP][Me3SiF2], [PNP][Me2SiF3], [PNP][MeSiF4], and [PNP][SiF5]a
(PNP)Fb
[PNP][Me3SiF2]
[PNP][Me2SiF3]
[PNP][MeSiF4]
[PNP][SiF5]
CCDC number
Formula
M
854540
854541
C39H39F2NP2Si
649.74
937495
C38H36F3NP2Si
653.71
937613
C37H33F4NP2Si
657.67
937494
C36H30F5NP2Si
661.64
C36H30FNP2
557.55
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P2/c
a/pm
b/pm
c/pm
α/°
918.35(10)
2071.5(2)
1725.64(18)
90
1141.01(4)
1956.15(7)
1502.66(6)
90
1161.03(4)
1949.23(6)
1455.43(4)
90
1060.2(2)
4030.2(8)
1612.9(3)
90
1579.07(3)
1384.68(3)
1738.11(3)
90
β/°
119.671(6)
90
2.8524(5)
100(2)
91.026(2)
90
3.3534(2)
100(2)
92.148(1)
90
3.29149(18)
103(2)
108.20(3)
90
6.55(1)
123.105(1)°
90
3.18347(11)
100(2)
γ/°
U/nm3
T/K
150(2)
Z
4
4
4
8
4
μ(Mo-Kα)/mm−1
No. of data collected
No. of unique data
Rint
0.186
19 863
4333
0.0582
0.0374, 0.0800
0.0581, 0.0889
0.206
89 985
14 011
0.0217
0.0348, 0.0946
0.0432, 0.1001
0.214
47 618
11 592
0.0330
0.0398, 0.0942
0.0595, 0.1039
0.220
31 975
15 083
0.0309
0.0530, 0.1144
0.0754, 0.1259
0.231
50 173
6130
0.0603
0.0448, 0.0923
0.0640, 0.0994
R1, wR2 (I > 2σ(I))c
R1, wR2 (all data)
a The crystallographic parameters for the other crystal structures are given in the ESI. b For structural data at 293 K see ref. 9c. c R1 = ∑||Fo| −
|Fc||/∑|Fo|, wR2 = (∑[w(Fo2 − Fc2)2]/∑[wFo4])1/2
.
ether. In the case of [PNP][SiF5] a half diethyl ether molecule
crystallized as a solvate molecule and caused a disorder of the
[SiF5]− anion (see section S.4.2 in the ESI†). Crystal structures
of the pentafluorosilicate anion [SiF5]− often suffer from dis-
order and only a limited number of well determined [SiF5]−
structures is known.21 However, replacement of the diethyl
ether by t-butyl methyl ether resulted in the crystallization of
solvate-free and ordered [PNP][SiF5].
All structures contain separated [PNP]+ cations and
−
[MenSiF5−n
]
(n = 0–3) anions. The shortest cation–anion con-
tacts are weak Si–F⋯H–C interactions. Thus it can be assumed
that the [PNP]+ cation has only a marginal influence on the
structure of the anions and thus only coordinates very weakly.
Fig. 4 shows the structures of all anions as determined by
X-ray diffraction together with experimentally and theoretically
obtained averaged bond lengths and angles. In all anions the
more electronegative fluorine atoms occupy axial positions in
accord with VSEPR rules22 and quantum-chemical calculations
(Fig. 4 and section S.3.2 in the ESI†). However, with the excep-
tion of [SiF5]−, different stereo isomers may exist, where the
methyl groups are in axial or equatorial position, respectively.
According to DFT calculations the experimentally determined
structures with the methyl groups in equatorial positions are
calculated to be 20–30 kJ mol−1 more stable than the stereo
isomers with the methyl groups in axial positions (see section
S.3.2 in the ESI†).
Fig. 4 Structures of the anions [MenSiF5−n] (n = 0–3) as determined by
X-ray diffraction. Ellipsoids are drawn at 50% probability and hydrogen
atoms are drawn with arbitrary radii. Averaged experimental and calcu-
lated (PBE0/def2-TZVPP in italics) bond lengths (in pm) and angles (°)
are given.
The trends of the experimental averaged bond lengths are
visualized in Fig. 5 together with quantum-chemically calcu- weakens and elongates all bonds in these anions. Therefore
lated values. The calculations reproduce the experimental the fluoride ion donor properties increase (or the FIA
values well, only the Si–Meeq bond lengths are overestimated. decreases) with the number of methyl groups. The axial Si–F
In general axial bonds are 5 to 10 pm longer than the respect- bond length in [Me3SiF2]− is about 13 pm longer than in
ive bonds to the same ligand in the equatorial position. Repla- [SiF5]−. Therefore the good fluoride ion donor properties of
cement of fluorine atoms by electron donating methyl groups [Me3SiF2]− can be explained by the relatively weak Si–F
4330 | Dalton Trans., 2014, 43, 4326–4334
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ESI†).4b SiF4 and Me4Si were commercially available. IR spectra
were recorded on a Nicolet Magna-IR 760 equipped with a
Diamond ATR attachment. FT-Raman spectra were recorded in
flame-sealed capillaries on a Bruker Vertex 70 IR spectrometer
equipped with a Bruker RAM II Raman module using a highly
1
sensitive Ge detector and a Nd:YAG-Laser (1064 nm). H, 13C,
19F, 29Si, and 31P NMR spectra were measured on a Bruker
Avance II 400 WB spectrometer in 5 mm NMR tubes. Chemical
shifts are given with respect to Me4Si (1H, 13C, 29Si), CFCl3
(19F), and H3PO4 (85%) (31P).
Synthesis of (PNP)F. Potassium fluoride was dried under
vacuum at 250 °C for 8 h, ground in the glove box, and again
dried under vacuum at 250 °C for another 4 h. Anhydrous
methanol (3 ml) was added to a mixture of [PNP][BF4] (2.10 g,
3.36 mmol) and freshly dried KF (0.25 g, 4.30 mmol). The reac-
tion mixture was stirred for 1 h at room temperature and fil-
trated. The solid residue was washed with 1 ml of methanol
and again filtrated. The volume of the combined filtrates was
reduced by about 1/3 under vacuum and anhydrous diethyl
ether (2 ml) was added. The precipitate was filtered off and
dried under vacuum for 15 h at 80 °C. The residue was finely
ground and dried for another 6 h at 130 °C. The product
remained as a colorless solid (1.52 g, 2.73 mmol, 81%,
mp. 170 °C (dec.)). IR (diamond, cm−1): ˜ν = 403 (vw), 450 (m),
474 (w), 496 (s), 531 (s, sh), 547 (w), 576 (m), 618 (vw), 689 (vs,
sh, br), 718 (vs), 747 (m), 801 (vw), 856 (vw), 929 (vw), 997 (w),
1027 (w), 1067 (vw), 1086 (vw), 1107 (s, br), 1170 (vs, sh, br),
1264 (s, br, sh), 1434 (s), 1481 (m), 1588 (vw, sh), 3061 (m, br).
Raman (40 mW, 1000 scans, cm−1): ˜ν = 238 (vw), 423 (vw), 618
(w), 665 (w), 1001 (vs), 1029 (m), 1110 (w), 1184 (vs), 1589 (s),
3058 (s). 1H NMR (400.17 MHz, CD3CN, 298 K): δ = 7.49 (m,
m-H, [PNP]+), 7.59 (m, o-H, [PNP]+), 7.68 (m, p-H, [PNP]+); 19F
NMR (376.53 MHz, CD3CN, 298 K) δ = −71.4; 31P{1H} NMR
(161.99 MHz, CD3CN, 298 K) δ = 20.8 (s). Found: C, 77.1; H,
5.5; N 2.4 Calc. for C36H30FNP2: C, 77.5; H, 5.4; N, 2.5%.
Synthesis of [PNP][Me3SiF2] in propionitrile. Me3SiF
(334 mg, 3.62 mmol) was condensed onto a frozen mixture of
(PNP)F (337 mg, 0.60 mmol) and propionitrile (4.5 ml) in an
H-tube-glass-vessel equipped with J. Young Teflon-in-glass
valves. The reaction mixture was warmed up and stirred at
−40 °C for 1 h. Subsequently, the solvent and all volatiles were
removed under vacuum at room temperature. The remaining
colorless solid was ground and dried under vacuum at room
temperature (331 mg, 0.51 mmol, 84%). IR (diamond, cm−1):
˜ν = 441 (m), 495 (vs, sh), 520 (vs), 546 (m), 616 (vw), 689 (vs,
sh), 720 (vs), 744 (vw), 763 (m), 814 (s, br), 859 (w), 996 (m, sh),
1028 (w), 1113 (vs, sh), 1179 (w), 1267 (vs, br, sh), 1288 (vw),
Fig. 5 Experimental and calculated (PBE0/def2-TZVPP) averaged bond
lengths for the anions [MenSiF5−n] (n = 0–4).
bond.3h,9 In contrast to the Si–Fax bond, the influence of the
number of fluorine atoms on the Si–Feq and Si–Meeq bond
lengths is less pronounced. In the hypothetical [Me4SiF]−
anion an extremely long Si–Meax bond of 201.8 pm is calcu-
lated (cf. the Si–Me bond length in Me4Si is 185 pm).23
Conclusion
(PNP)F has been prepared as a bulk material for the first time
from [PNP][BF4] and KF. It has been unambiguously character-
ized by multinuclear NMR and vibrational spectroscopy,
elemental analysis, and X-ray diffraction. In the solid state
(PNP)F has a covalent structure according to its crystal struc-
ture and in agreement with the spectroscopic findings and
quantum-chemical calculations. The fluoride donor properties
of (PNP)F were investigated by the reactions with the series of
fluoromethylsilanes MenSiF4−n (n = 0–4). With the exception of
Me4Si, the silane with the lowest FIA, the fluoromethylsilicates
[MenSiF5−n] (n = 0–3) were formed. These were fully character-
ized and their crystal structure determinations yield the first
consecutive series of structurally characterized fluoromethyl-
silicates [MenSiF5−n] (n = 0–3) with the same counter cation.
Experimental
General remarks
Air and moisture sensitive solid reagents were manipulated 1435 (s, sh), 1481 (w), 1588 (w, sh), 2974 (vw, br), 3055 (w, sh).
using standard vacuum and Schlenk techniques or in a glove Raman (50 mW, 1000 scans, cm−1): ˜ν = 235 (w), 278 (w),
box with an atmosphere of dry argon (H2O < 1 ppm, O2
<
586 (vw), 617 (w), 663 (m), 1001 (vs), 1029 (m), 1111 (m), 1180
1 ppm). The solvents 1,2-dimethoxyethane, methanol, propio- (w, br), 1587 (s, sh), 2896 (w), 3063 (s, br). 1H NMR
3
nitrile, acetonitrile-d3, diethyl ether, t-butyl methyl ether and (400.17 MHz, CH3CH2CN, 193 K): δ = −0.17 (t, JH–F = 8 Hz,
Me4Si were dried over CaH2 and distilled prior to use. [Me3SiF2]−), 7.49 (m, m-H, [PNP]+), 7.58 (m, o-H, [PNP]+), 7.67
MeSiF3,24 Me2SiF2 24 and Me3SiF24 were prepared by published (m, p-H, [PNP]+); 19F NMR (376.53 MHz, CH3CH2CN, 193 K) δ =
procedures. [PNP][BF4] was synthesized from [PNP]Cl and −60.5 (s, Si–F(ax),
1JF–Si = 200 Hz, [Me3SiF2]−), −158.7 (s, Si–
Na[BF4] following a published procedure (section S1 in the
F
(eq), [Me3SiF2]−). 29Si NMR (79.50 MHz, CD3CN, 193 K)
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δ
=
−65.3 (1JSi–F
=
200 Hz, [Me3SiF2]−); 31P{1H}-NMR dried under vacuum at room temperature (348 mg, 0.53 mmol,
(161.99 MHz, CH3CH2CN, 298 K) δ = 20.8 (s, [PNP]+).
80%). IR (diamond, cm−1): ˜ν = 434 (w), 448 (vw), 496 (vs, br),
Synthesis of [PNP][Me3SiF2] in 1,2-dimethoxyethane. Me3SiF 531 (vs), 545 (vs), 617 (w), 689 (vs), 721 (vs), 747 (w), 761 (m),
(308 mg, 3.56 mmol) and 1,2-dimethoxyethane (monoglyme) 804 (w), 853 (m, br), 997 (m), 1027 (w), 1112 (vs), 1182 (w),
(5 ml) were condensed onto a frozen mixture of (PNP)F 1266 (s, br), 1287 (m), 1437 (m), 1482 (w), 1588 (w), 3059 (w).
(311 mg, 0.56 mmol) in an H-tube-glass-vessel equipped with Raman (50 mW, 1000 scans, cm−1): ˜ν = 616 (w), 663 (m),
J. Young Teflon-in-glass valves. The reaction mixture was 724 (vw), 1001 (vs), 1029 (m), 1111 (m), 1163 (w), 1183 (w),
warmed up and stirred at −40 °C for 1 h. Subsequently, the 1439 (vw), 1576 (m), 1589 (s), 2903 (vw, CH3), 3062 (s). 1H NMR
solvent and all volatiles were removed under vacuum at room (400.17 MHz, CD3CN, 298 K): δ = −0.08 (s, 3H, [MeSiF4]−), δ =
temperature. The remaining colorless solid was ground and 3.30 (s, 9H, H3CO(CH2)2OCH3), 3.47 (s, 6H, H3CO-
dried under vacuum at room temperature (339 mg, 0.52 mmol, (CH2)2OCH3),7.50 (m, m-H, [PNP]+), 7.60 (m, o-H, [PNP]+), 7.69
93%).
(m, p-H, [PNP]+); 13C NMR (100.62 MHz, CD3CN, 298 K): δ =
Synthesis of [PNP][Me2SiF3] in acetonitrile. Me2SiF2 3.5 ([Me2SiF3]−), 127.5 (ipso-C, [PNP]+), 130.1 (m-C, [PNP]+),
(102 mg, 1.08 mmol) and acetonitrile (2 ml) were condensed 132.1 (o-C, [PNP]+), 134.4 (p-C, [PNP]+); 19F NMR (376.53 MHz,
onto a frozen mixture of (PNP)F (100 mg, 0.18 mmol) in an CD3CN, 193 K) δ = −111.6 (s, [MeSiF4]−); 29Si NMR (79.50 MHz,
H-tube-glass-vessel equipped with J. Young teflon-in-glass CD3CN, 298 K)
valves. The reaction mixture was warmed up and stirred at (161.99 MHz, CD3CN, 298 K) δ = 21.8 (s, [PNP]+).
−40 °C for 30 min. Subsequently, the solvent and all volatiles Synthesis of [PNP][SiF5]·1/2DME in 1,2-dimethoxyethane.
were removed under vacuum at −40 °C. The remaining color- SiF4 (56 mg, 0.54 mmol) and 1,2-dimethoxyethane (2 ml) were
less solid was ground and dried under vacuum at room temp- condensed onto frozen mixture of (PNP)F (100 mg,
δ =
−68.2 ([MeSiF4]−); 31P{1H} NMR
a
erature (105 mg, 0.16 mmol, 89%). IR (diamond, cm−1): ˜ν = 0.18 mmol) in an H-tube-glass-vessel equipped with J. Young
421 (w), 443 (w), 458 (vw), 501 (vs), 532 (vs), 546 (vs), 600 (s), teflon-in-glass valves. The reaction mixture was warmed up
618 (w), 663 (w), 691 (vs), 721 (vs), 748 (w), 766 (m), 779 (m), and stirred at −40 °C for 50 min. Subsequently, the solvent
842 (m, br), 931 (vw, br), 997 (m), 1027 (w), 1074 (vw), and all volatiles were removed under vacuum at room tempera-
1114 (vs), 1161 (vw), 1184 (w), 1267 (s, br), 1287 (m), 1438 (m), ture. The remaining colorless solid was ground and dried
1482 (w), 1588 (w, br), 1660 (vw, br), 2943 (vw, br, CH3), under vacuum at room temperature (105 mg, 0.11 mmol,
3063 (w). Raman (50 mW, 1000 scans, cm−1): ˜ν = 129 (vw), 176 64%). IR (diamond, cm−1): ˜ν = 446 (m, sh), 478 (w), 495 (w),
(vw), 234 (w), 356 (vw), 492 (vw), 617 (w), 663 (m), 725 (vw), 530 (s), 553 (w), 689 (vs, br), 719 (s), 744 (vw), 759 (vw), 778 (w),
1001 (vs), 1029 (m), 1110 (m), 1181 (w), 1576 (m), 1588 (s), 793 (vw), 879 (vs, br, sh), 997 (w), 1028 (vw), 1113 (vs, br),
2896 (vw, CH3), 3063 (s). 1H NMR (400.17 MHz, CD3CN, 1164 (vw), 1188 (w), 1243 (vs, br, sh), 1280 (vw), 1438 (vs), 1483
298 K): δ = −0.15 (s, [Me2SiF3]−), 7.50 (m, m-H, [PNP]+), 7.59 (m), 1588 (w, sh), 2809 (vw), 2883 (w), 2928 (m). Raman
(m, o-H, [PNP]+), 7.68 (m, p-H, [PNP]+); 13C NMR (100.62 MHz, (50 mW, 1000 scans, cm−1): ˜ν = 242 (w), 617 (vw), 666 (w), 1001
CH3CH2CN, 193 K): δ = 3.5 ([Me2SiF3]−), 127.5 (ipso-C, [PNP]+, (vs), 1029 (m), 1110 (m), 1184 (vw), 1587 (s, sh), 3060 (s).
1JCP = 109 Hz), 129.9 (m-C, [PNP]+), 132.5 (o-C, [PNP]+), 134.1 1H NMR (400.17 MHz, CD3CN, 298 K): δ = 3.30 (s, 3H, H3CO-
(p-C, [PNP]+); 19F NMR (376.53 MHz, CH3CH2CN, 193 K) δ = (CH2)2OCH3), 3.47 (s, 2H, H3CO(CH2)2OCH3), 7.49 (m, 12H,
3
−66.3 (sep, JF–H = 8 Hz, Si–F(ax), [Me2SiF3]−), −132.9 (s, Si– m-H, [PNP]+), 7.59 (m, 12H, o-H, [PNP]+), 7.68 (m, 6H, p-H,
F
(eq), [Me2SiF3]−); 29Si NMR (79.50 MHz, CD3CN, 298 K) δ = [PNP]+); 19F NMR (376.53 MHz, CD3CN, 298 K) δ = −138.4 (s,
1
−75.3 ([Me2SiF3]−); 31P{1H} NMR (161.99 MHz, CD3CN, 298 K) [SiF5]−, JF–Si = 145 Hz); 29Si (79.50 MHz, CD3CN, 298 K) δ =
δ = 21.5 (s, [PNP]+).
−147.5 ([SiF5]−, 1JSi–F = 145 Hz).
Synthesis of [PNP][Me2SiF3] in 1,2-dimethoxyethane.
Me2SiF2 (102 mg, 1.08 mmol) and 1,2-dimethoxyethane (2 ml)
were condensed onto a frozen mixture of (PNP)F (100 mg,
X-ray crystallography
General remarks. The single-crystal X-ray structure determi-
0.18 mmol) in an H-tube-glass-vessel equipped with J. Young nations were carried out on a Bruker APEX-II diffractometer
Teflon-in-glass valves. The reaction mixture was warmed up equipped with a CCD area detector or an Oxford Diffraction
and stirred at −40 °C for 30 min. Subsequently, the solvent Xcalibur Eos Gemini ultra diffractometer using Mo-Kα
and all volatiles were removed under vacuum at −40 °C. The (0.71073 Å) radiation. The crystals were mounted onto a cryo
remaining colorless solid was ground and dried under vacuum loop using fluorinated oil and frozen in the cold nitrogen
at room temperature (105 mg, 0.16 mmol, 89%).
stream of the goniometer. The structures were solved by direct
Synthesis of [PNP][MeSiF4]·3/2DME in 1,2-dimethoxy- methods (SHELXS).25 Subsequent least-squares refinement on
ethane. 1,2-Dimethoxyethane (5 ml) and MeSiF3 (231 mg, F2 (SHELXL 97-2) located the positions of the remaining atoms
2.31 mmol) were condensed onto a frozen mixture of (PNP)F in the electron density maps.25 All non-hydrogen atoms were
(370 mg, 0.66 mmol) in an H-tube-glass-vessel equipped with refined anisotropically. The hydrogen atoms were placed in
J. Young Teflon-in-glass valves. The reaction mixture was calculated positions using a riding model and were refined iso-
warmed up and stirred at −50 °C for 1 h. Subsequently, the tropically in blocks. The data were corrected for absorption
solvent and all volatiles were removed under vacuum at room (semi-empirical from equivalents). Graphical representations
temperature. The remaining colorless solid was ground and of the structures were prepared with the program DIAMOND.26
4332 | Dalton Trans., 2014, 43, 4326–4334
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DFT calculations
6 (a) C. Knapp and R. Uzun, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2010, 66, o3185; (b) B. A. Correia Bicho,
C. Bolli, C. Jenne and H. Seeger, Acta Crystallogr., Sect. E:
Struct. Rep. Online, 2013, 69, o1435–o1436.
7 C. Knapp and R. Uzun, Acta Crystallogr., Sect. E: Struct. Rep.
Online, 2010, 66, o3186.
8 M. A. Beckett, P. N. Horton, M. B. Hursthouse and
J. L. Timmis, Acta Crystallogr., Sect. E: Struct. Rep. Online,
2010, 66, o319.
DFT (PBE0 functional) calculations were performed with the
def2-TZVPP basis sets as implemented in the program TURBO-
MOLE.27 Frequency calculations were performed analytically
with the AOFORCE module at the same level.28 The calculated
structures are true minima on the energy hyper surface as
shown by the absence of imaginary frequencies.
9 (a) W. Douglas and J. K. Ruff, J. Organomet. Chem., 1974,
65, 65–69; (b) A. Berkessel and M. Brandenburg, Org. Lett.,
2006, 8, 4401–4404; (c) V. V. Grushin and W. J. Marshall,
Organometallics, 2008, 27, 4825–4828.
10 (a) D. A. Dixon, W. B. Farnham, W. Heilemann, R. Mews
and M. Noltemeyer, Heteroat. Chem., 1993, 4, 287–295;
(b) G.-V. Röschenthaler, E. Lork, G. Bissky and
A. Kolomeitsev, Z. Kristallogr. - New Cryst. Struct., 2002, 217,
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W.-D. Stohrer and R. Mews, Inorg. Chem., 2002, 41, 4715–
4721; (d) E. Lork, D. Viets, M. Müller and R. Mews,
Z. Anorg. Allg. Chem., 2004, 630, 2692–2696.
Acknowledgements
This work was supported by the Deutsche Forschungs-
gemeinschaft (DFG) and the Fonds der Chemischen Industrie
(FCI). We are grateful to Prof. Dr Maik Finze and Dr Jan
Sprenger, Universität Würzburg, for performing elemental ana-
lyses, Uwe Pelz and Prof. Dr Harald Hillebrecht, Universität
Freiburg, for obtaining powder diffraction data, and Prof. Dr
Ingo Krossing, Universität Freiburg, for providing compu-
tational resources.
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12 D. T. Martin and S. Bywater, Makromol. Chem., 1992, 193,
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