- Email: [email protected]
Recent progress in perfluoroalkyl-phosphorus chemistry
Accepted Manuscript Title: Recent Progress in Perfluoroalkyl-Phosphorus Chemistry Author: N.V. Ignat’ev J. Bader K. Koppe B. Hoge H. Willner PII: DOI: Reference:
S0022-1139(14)00311-X http://dx.doi.org/doi:10.1016/j.jfluchem.2014.10.007 FLUOR 8455
To appear in:
FLUOR
Received date: Revised date: Accepted date:
1-8-2014 7-10-2014 10-10-2014
Please cite this article as: N.V. Ignat’ev, J. Bader, K. Koppe, B.H. , H. Willner, Recent Progress in Perfluoroalkyl-Phosphorus Chemistry, Journal of Fluorine Chemistry (2014), http://dx.doi.org/10.1016/j.jfluchem.2014.10.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Graphical Abstract (synopsis) The tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, provide convenient access to broad variety of perfluoroalkyl-phosphorus compounds that are of interest for various applications:
cr
ip t
conducting salts, ionic liquids, Brønsted and Lewis acid catalysts.
us
Graphical Abstract
(RF)2(O)PNP(O)(RF)2
M
H2 O
(RF)3PF2
(RF)P(O)(OH)2
Ac ce p
te
O Ca
d
H 2O
(RF)3P=O
(RF)2P(O)NR2
R 2NH
(RF)2 P(O)Cl (RF)2P(O)OH
an
NH 3
HF
H+(OH2)5[(RF)3PF3]-
MF Na
BH
M+ [(RF)3PF3]-
4
[(CH3 )3 Si] 2 NH
(RF )3P
(RF)3P=N Si(CH3)3
Recent Progress in Perfluoroalkyl-Phosphorus Chemistry *
N. V. Ignat’ev 1 , J. Bader 2, K. Koppe 3, B. Hoge 2, H. Willner 4
1
Merck KGaA, PM-ATI, Frankfurter Str. 250, D-64293, Darmstadt, Germany; E-mail: [email protected]
2
Universität Bielefeld, Anorganische Chemie, Universitätsstraße 25, 33615 Bielefeld, Germany
3
Heinrich-Heine-Universität Düsseldorf, Institut für Anorganische Chemie und Strukturchemie II, Universitätstrasse 1, 40225 Düsseldorf, Germany
4
Bergische Universität Wuppertal, Anorganische Chemie, Gauss Strasse 20, Page 1 of 22
2
D-42097 Wuppertal, Germany
1. Abstract Electrochemical fluorination (Simons process) provides a cheap industrial access to a series
ip t
of tris(perfluoroalkyl)difluorophosphoranes. These substances are a convenient starting material for the preparation of various perfluoroalkyl-phosphorus compounds. The
(FAP)
and
perfluoroalkyl-phosphinate
cr
preparation of a variety of new salts and ionic liquids with perfluoroalkyl-fluorophosphate anions
is
described.
The
us
tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, are strong Lewis acids which are of
an
interest for the application in catalysis. The syntheses of various derivatives of bis(perfluoroalkyl)phosphinic acids, (RF)2P(O)OH, are presented.
M
Keywords: Perfluoroalkyl phosphorus compounds; Electrochemical Fluorination; Conducting salts; Brønsted acids; Lewis Acids; Catalysis
d
2. Results and discussion
te
Phosphorus trifluoride, PF3, and phosphorus pentafluoride, PF5, were the first perfluorinated phosphorus compounds described in the literature [1,2]. Today PF5 is in
multi-ton
quantities
Ac ce p
produced
and
mostly
used
for the synthesis of lithium
hexafluorophosphate, LiPF6 – the most commonly used conducting salt in lithium-ion batteries [3]. A major drawback of LiPF6 based electrolytes is their poor stability at elevated temperatures due to the elimination of LiF and the generation of PF5: LiPF6
PF5 + LiF
[3]. LiPF6 is moisture sensitive (Scheme 1) [3]:
LiPF6 + H 2O
LiF + POF3 + 2 HF
Scheme 1 The limited thermal and hydrolytical stability of LiPF6 require a development of safer conducting salts for the application in Li-batteries. Perfluoroalkyl phosphorus compounds have been synthesized more than 60 years ago.
Page 2 of 22
3
R.N. Haszeldine et al. [4] have reported the preparation of tris(trifluoromethyl)phosphine, (CF3)3P, by the reaction of CF3I with white phosphorus in an autoclave at 200-220 °C. Surprisingly, the reaction of C2F5I and C3F7I with white phosphorus does not result in the formation of tri-substituted phosphines, (RF)3P [5,6]. Alternatively to the autoclave reaction,
(PhO)3P,
with
CF3Br/(Et2N)3P
(Ruppert
[(Et2N)3P-Br]+
reagent:
CF3Br, (Et 2N)3P
[CF3]-)
in
cr
hexamethylphosphoramide, HMPA (Scheme 2) [7].
(CF3)3P
us
(PhO)3P
ip t
tris(trifluoromethyl)phosphine, (CF3)3P, can be prepared by the reaction of triphenylphosphite,
HMPA
an
Scheme 2
The polar solvent hexamethylphosphoramide (HMPA) is required in this synthesis. The
M
reported yield of (CF3)3P is good (up to 85%), but the use of this toxic solvent and commercially not available trifluoromethyl bromide are serious drawbacks for this protocol.
d
Tris(trifluoromethyl)phosphine can be oxidized to (CF3)3PCl2 by Cl2 and converted to
te
(CF3)3PF2 by fluorination with ZnF2 [7].
Trifluoromethyl derivatives of phosphorus are not very stable compounds. Mahler has
Ac ce p
reported the gradual decomposition of (CF3)3PF2 to PF5 via elimination of difluorocarbene, :CF2 (Scheme 3) [8].
(CF3)3PF2
(CF3) 2PF3
(CF3)2PF3
CF3PF4
CF3PF4
PF5
+
+
+
CF2 CF2
CF2
Scheme 3 This process is slow at room temperature, but significantly accelerated at elevated temperatures. Difluorotris(trifluoromethyl)phosphorane, (CF3)3PF2 decomposes at 25 °C in the gas phase at a rate of 0.5% per month. At elevated temperatures (100 °C) the decomposition is much faster (40% within 17 hours) [8]. The handling of the reduction
Page 3 of 22
4
product P(CF3)3 requires special safety precautions because P(CF3)3 (b.p. 17 °C) reacts
(CF3)3P (b.p. 17 °C) Fig. 1
b) (C2F5)3P (b.p. 85-87 °C [11])
an
a)
us
cr
ip t
violently on contact with air (see Fig. 1, a) [9,10].
M
In contrast to (CF3)3P, the higher homologue (C2F5)3P is much more stable (Fig. 1, b). It can be handled by standard Schlenk techniques. Tris(pentafluoroethyl)phosphine can be readily
d
prepared by the reduction of tris(pentafluoroethyl)difluorophosphorane with NaBH4 or other
te
reagents (Scheme 4) [11].
NaBH4 solvent free
(C 2F5) 3P
Ac ce p
(C 2F5)3PF2
Scheme 4
a)
31
P NMR
Page 4 of 22
b)
us
cr
ip t
5
19
F NMR
an
Fig. 2: NMR spectra of tris(pentafluoroethyl)difluorophosphorane, (C2F5)3PF2 .
M
Tris(pentafluoroethyl)difluorophosphorane, (C2F5)3PF2, is nowadays produced in multi-ton quantities by the electrochemical fluorination (Simons process, ECF) of triethylphosphine
te
d
[12,13]. Tris(pentafluoroethyl)difluorophosphorane is a clear and colorless liquid. It can be distilled (b.p. 91-92 °C) at atmospheric pressure without decomposition. NMR spectroscopic
Ac ce p
data of (C2F5)3PF2 confirm a trigonal-bipyramidal structure with two equivalent fluorine atoms on the axial positions and the three C2F5 groups in the equatorial position (see Fig. 2). The first example of the electrochemical fluorination of an organophosphorus compound was reported by L.M. Yagupolskii et al. (Scheme 5) [14]
R3P=O
ECF HF
(RF)3P=O RF = C2-8 perfluoroalkyl
R = C2-8 alkyl Scheme 5 Tris(perfluoroalkyl)phosphine
oxides
were
identified
as
products
formed
in
the
electrochemical fluorination of trialkylphosphine oxides. Later it was found out that the main products
in
this
process
were
not
tris(perfluoroalkyl)phosphine
oxides
but
tris(perfluoroalkyl)difluorophosphoranes (Scheme 6) [15]. Page 5 of 22
6
ECF
R3P=O
(RF)3PF2
HF
R = C2-6 alkyl
Yield: 24-46%
+
F 2O
+
n H2
RF = C2-6 perfluoroalkyl
Scheme 6
ip t
The disadvantage of the electrochemical fluorination of trialkylphosphine oxides, however, is not only the low yield of the product, tris(perfluoroalkyl)difluorophosphorane, but especially
cr
the formation of an equimolar quantity of the toxic gas, F2O, that in combination with H2 (constantly produced during the ECF process) can cause a severe explosion. Using excludes
the
formation
of
F2O.
The
yield
us
trialkylphosphines
of
the
formed,
tris(perfluoroalkyl)difluorophosphoranes is much better in comparison to the ECF of
an
trialkylphosphine oxides (Table 1) [12,13]. This technology provides a convenient protocol for
M
the industrial production of (RF)3PF2. Tris(perfluoroalkyl)difluorophosphoranes are very reactive compounds. For example, (C2F5)3PF2 readily reacts with lithium fluoride forming the
d
corresponding lithium tris(perfluoroethyl)trifluorophosphate, Li[(C2F5)3PF3] (LiFAP) as an
+
LiF
Ac ce p
(C2F5)3PF2
te
analogue to lithium hexafluorophosphate, Li[PF6] (Scheme 7) [16,17]. Solvent
Li[(C 2F5) 3PF3] Solv LiFAP - proprietary material of BASF
Scheme 7
Table 1. Electrochemical Fluorination (ECF) of Trialkylphosphines
R3P
F
e-, Ni
RF
HF
P F
RF RF
R
RF
Yield, %
CH3
CF3
4
C2H5
C2F5
74
n-C3H7
n-C3F7
53
n-C4H9
n-C4F9
49 Page 6 of 22
7
i-C4H9
i-C4F9 , n-C4F9 s-C4F9 , t-C4F9
57
LiFAP can be prepared in situ in polar solvents such as dimethyl carbonate (DMC) or
ip t
ethylene carbonate (EC). These solvents are typical media for the preparation of electrolytes for Li-ion batteries. LiFAP is much more stable against hydrolysis in comparison to lithium
cr
hexafluorophosphate, Li[PF6]. After the addition of 500 ppm of water to a 1M solution of LiPF6 in EC:DMC (50:50 wt%) 250 ppm H2O were consumed within ca. 20 hours and a
us
corresponding amount (ca. 500 ppm) of HF was generated. In a similar experiment, after the addition of 1000 ppm of water to a 1M solution of Li[(C2F5)3PF3] (LiFAP) in an organic
an
carbonate mixture the system remained unchanged for three days; no water consumption and no HF formation were detected [17]. A solution of LiFAP in EC:DMC (50:50 wt%) is
M
slightly less conductive than a corresponding solution of Li[PF6], but the maximum conductivity (8.6 mS·cm-1) for the LiFAP electrolyte is reached with a 0.8 M solution. For
d
Li[PF6] in EC:DMC (50:50 wt%) the maximum conductivity (10.7 mS·cm-1) was reported using
te
a 1M solution [17]. Lithium bis(pentafluoroethyl)tetrafluorophosphate, Li[(C2F5)2PF4] has a
Ac ce p
symmetrical structure and exhibits a slightly better conductivity in organic solvents when compared to Li[(C2F5)3PF3] [16]. Due to the better thermal and hydrolytical stability, LiFAP electrolytes could provide an option to replace Li[PF6] electrolytes in Li-ion batteries. Tris(perfluoroalkyl)difluorophosphoranes react not only with alkali-metal fluorides, but
also with aqueous HF to the corresponding acid [H(OH2)n][(C2F5)3PF3] (HFAP). [18]. The reaction of (C2F5)3PF2 with 18.2 wt% aqueous HF proceeds within minutes and results quantitatively in the formation of [H(OH2)5][(C2F5)3PF3] (HFAP) as a pentahydrate (Scheme 8) [18]. HFAP is a convenient starting material for the preparation of a variety of metal-salts and ionic liquids with the FAP-anion [19].
Page 7 of 22
8
RF (RF)3PF2
+ HF + 5 H2O
[H(OH2)5
]+
18.2% HF
F
RF
P F
F RF
Scheme 8
mixture of isomers with meridional and facial structures
ip t
R F = C 2F 5 , C 4F 9
HFAP is a strong Brønsted acid. The pentahydrate HFAP · 5 H2O corresponds to a 83%
cr
aqueous solution. This solution is stable at 0 °C for several months. The hydrolysis rate at
us
room temperature is about 1 mol% per month (RF = C2F5), while elevated temperatures accelerate the hydrolysis (Scheme 9) [20] resulting in the formation of corresponding
H+[(C2F5)3PF3]- x 5 H2O
135-140 °C, 20 h
an
bis(perfluoroalkyl)phosphinic acids.
(C2F5)2P(O)OH + C2F5H + 3 H2O + 3 HF
H+[(C4F9)3PF3]- x 5 H2O
M
Yield: 87%
135-140 °C, 20 h
(C4F9)2P(O)OH + C4F9H + 3 H2O + 3 HF Yield: 97%
d
Scheme 9
te
Bis(heptafluoropropyl)phosphinic acid (C3F7)2P(O)OH was prepared more than 50 years ago
Ac ce p
by a multistep procedure. The last step in this procedure was the hydrolysis of bis(heptafluoropropyl)trichlorophosphorane, (C3F7)2PCl3 [21]. The bis(pentafluoroethyl)phosphinic acid (C2F5)2P(O)OH was synthesized later by a similar protocol [22]. Bis(nonafluorobutyl)phosphinic acid, (C4F9)2P(O)OH, has been prepared by the hydrolysis of the corresponding phosphine oxide [23]. But, the protocol according to the Scheme 9 [20] is probably the most convenient one to prepare large quantities of bis(perfluoroalkyl)phosphinic acids. Today, (C2F5)2P(O)OH and (C4F9)2P(O)OH are produced industrially and are commercially available. In particular bis(nonafluorobutyl)phosphinic acid, (C4F9)2P(O)OH, is of interest for the application as a short chain fluorosurfactant to replace the persistent perfluorooctanoic
(PFOA)
and
perfluorooctanesulfonic
(PFOS)
acids.
Bis(nonafluorobutyl)phosphinic acid, (C4F9)2P(O)OH shows no signs of toxicity and is not bioaccumulative. Aqueous solutions of bis(nonafluorobutyl)phosphinic acid do not foam. In Page 8 of 22
9
mixtures with other strong foaming mist suppressants it is possible to reduce and adjust the foam layer. Bis(nonafluorobutyl)phosphinic acid is available in the European Union and registered by REACH under the trade name TividaTM FL 2100. Tris(perfluoroalkyl)difluorophosphoranes are strong Lewic acids. They are much
ip t
stronger than for example BF3 or PF5 and comparable to the acidity of AsF5 (see Table 2). [24]
cr
Table 2. Fluoride Ion Affinity (FIA) of various Lewis acids
F
F
+ F-
P
As
F
416.5
F
F F
F F
P F F P F
C2F5
389.3
F
F
357.1
F
te
F
+ F-
F F
d
F
F5C2
F5C 2
F
F
an
F5C2
F
M
F
+ F-
F As F F F P C2F5 C2F5 F
us
FIA [kJ/mol]
FIA = -ΔrG0 ; B3LYP/6-311G++(2d)
Ac ce p
Due to the strong Lewis acidity tris(perfluoroalkyl)difluorophosphoranes react with weak nucleophiles, such as chloride, resulting in the formation of chloro-fluoro phosphates [Kt][(RF)3PF2Cl] (Scheme 10) [25], or with acetate to form an acetoxy-phosphate [Kt][(RF)3PF2OC(O)CH3] (Kt = K+, [(Bu)4N]+) [26]. The direct synthesis of alkoxyphosphates [P(C2F5)3F2OR]- (R = alkyl) by the reaction of (C2F5)3PF2 with alcohols
or
alcoholates
is
not
very
selective
due
to
strong nucleophiles like the
high
reactivity
of
tris(perfluoroalkyl)difluorophosphoranes. The electrophilicity of the phosphorane (C2F5)3PF2 can be lowered by the formation of an adduct with organic bases, for instance with 4dimethylamino pyridine (Scheme 11) [27,28] This adduct is a handy starting material to prepare a variety of salts with functionalized perfluoroalkyl-phosphate anions of the structure [(C2F5)3PF2OR] (R = H, Et, CH2CF3, Ph, Ac, C2H4OH, etc.) (Scheme 11) [29].
[Kt] + Cl-
+
(RF)3PF2
R F = C2F5, C3F7
RT CH 3CN
[Kt][(R F) 3PF2Cl] [Kt] = organic cation Page 9 of 22
10
Scheme 10 F
F P F
C2F5 DMAP C2F5
F5C2
F
C2F5
P
F5C2
HX
N
F
F5C2 NMe2
DMAP: 4-Dimethylaminopyridine
[HDMAP]+
P F
C2F5 X
ip t
F5C2
F5C2
X = OH, OPh, OEt, OAc, OCH2CF3, OC2H4OH
cr
Scheme 11
The hydroxy and acetoxy phosphates are stable at room temperature but decompose at
us
elevated temperatures. The decomposition of these salts in an aqueous milieu results in the
an
formation of bis(perfluoroalkyl)phosphinic acid and gaseous products, thus forming an abundant foam [26].
M
Bis(perfluoroalkyl)phosphinic acids can be converted into derivatives which are of interest for various applications. For instance, bis(perfluoroalkyl)phosphinic acid chlorides
d
can be easily obtained by the reaction of bis(perfluoroalkyl)phosphinic acid with phosphorus
te
pentachloride or its derivatives. For the synthesis of (C2F5)2P(O)Cl the use of PhPCl4 as
[30].
Ac ce p
chlorinating reagent is more convenient in regard to isolation of the product (Scheme 12)
(C2F5)2P
O
OH
+ C6H5PCl 4
- HCl
(C2F5)2P
O Cl
+ C6H5P(O)Cl 2
Scheme 12
Bis(perfluoroalkyl)phosphinic acid chlorides are very reactive compounds. They can be easily converted to the corresponding bis(perfluoroalkyl)phosphinyl amides, (RF)2P(O)NRR’, which are little-known compounds. Only a few fluorinated phosphinic acid amides are described in the literature. Their syntheses are based on the reaction of phosphinic acid chlorides with amines. For example, bis(perfluoroalkyl)phosphinic acid amides (CnF2n+1)2P(O)NHR (R = H, CH3, C6H5, 3-FC6H4, 4-FC6H4) were prepared in moderate yields by the reaction of (CnF2n+1)2P(O)Cl (n = 1, 3, 4) with ammonia, methylamine and aromatic amines [31-33]. A Page 10 of 22
11
similar procedure was used to prepare secondary amides of bis(perfluoroalkyl)phosphinic acids (Scheme 13), which exhibit interesting surface properties [34].
O Cl
(RF) 2P
+ 2 RR'NH
RF = C2F5, C4F9
O NRR'
+ [RR'NH2]Cl
R = H, CH3, C4H9
ip t
(RF) 2P
R' = C4H9, CH2CH2OCH3, CH2CH=CH2, (CH2)4CH=CH2
cr
Scheme 13
Bis(pentafluoroethyl)phosphinic acid anhydride, (C2F5)2P(O)OP(O)(C2F5)2, reacts with
us
dibutylamine to give (C2F5)2P(O)N(C4H9)2. [35]. The disadvantage of this method is that one half of the anhydride is lost due to the formation of the dibutylammonium salt of
an
bis(pentafluoroethyl)phosphinic acid, [H2N(C4H9)2]+[(C2F5)2P(O)O]-. In contrast to that, the reaction of bis(pentafluoroethyl)phosphinic acid anhydride with hexamethyldisiloxane yields
M
the bis(pentafluoroethyl)phosphinic acid trimethylsilyl ester, (C2F5)2P(O)OSi(CH3)3, as the
d
only product (Scheme 14) [35].
te
[(C2F5)2P(O)]2O + (CH3)3SiOSi(CH3)3
2 (C2F5)2P(O)2OSi(CH3)3
Scheme 14
Ac ce p
Tris(perfluoroalkyl)phosphine oxides, (RF)3P=O, are also suitable starting materials for the preparation of bis(perfluoroalkyl)phosphinic acid amides. Burg and Sarkis reported a quantitative reaction of tris(trifluoromethyl)phosphine oxide with dimethyl amine [36]. Tris(nonafluorobutyl)phosphine oxide reacts with dimethyl amine resulting in the formation of a complex mixture of products [37]. This method is however limited to dimethyl amides. There is no protocol described to react phosphine oxides with other secondary amines to yield bis(perfluoroalkyl)phosphinic acid amides.The reaction of tris(perfluoroalkyl)phosphine oxides with primary amines leads to the selective formation of the corresponding amides in good to excellent yields (Scheme 15) [38].
(C2F3)3P=O
+
R NH2
- C2 F5H
(C2F3)2P(O)NHR
R = H, C 6H5, CH3C6 H 4, C6 H5CH 2, p-CH 3C6 H4 CH 2, p-CH 3 OC 6H 4CH2, C 6 H5CH 2CH 2, C4H9, C 8H 17
Page 11 of 22
12
Scheme 15 Tris(perfluoroalkyl)phosphine oxides and bis(perfluoroalkyl)phosphinyl chlorides, (RF)2P(O)Cl, readily react with hydrazines to form the corresponding hydrazides (Scheme 16 and 17) [38]. Within the reaction of (RF)2P(O)Cl with hydrazines, CaH2 is employed to trap the generated to
avoid
a
reaction
with
the
hydrazines
(Scheme
bis(perfluoroalkyl)phosphinic acids were so far unknown compounds.
+
R'R''N NH2
Hydrazides
of
(C2F3)2P(O)NH NR'R''
cr
(C2F3)3P=O
17).
ip t
HCl
- C 2F5 H
Scheme 16
+
R'HN NH2
CaH2
- HCl
M
R' = CH3, C6H5
(C2F3)2P(O)NH NHR'
an
(C2F3)2P(O)Cl
+
us
R' = CH3, C6H5, CH2C6H2-3,5-(t-C4H9)-4-OH ; R'' = H, CH3
2 HCl
CaCl2
+
2 H2
d
Scheme 17
te
The hydrolytic stability of bis(pentafluoroethyl)phosphinyl amides depends on the substituent R. Amides with a longer alkyl chain are more stable against hydrolysis in comparison to
Ac ce p
(C2F5)2P(O)NH2 which forms an ammonium salt in wet solvents. Therefore the reaction of bis(perfluoroalkyl)phosphinic chloride, (RF)2P(O)Cl, and bis(perfluoroalkyl)phosphinyl amide, (RF)2P(O)NH2,
yielding
[(RF)2P(O)]2NH,
should
the
be
corresponding
carried
out
in
bis(bis(perfluoroalkyl)phosphinyl) strictly
anhydrous
conditions
imide [39-41].
Bis(bis(perfluoroalkyl)phosphinyl)imide salts can be prepared in better yield in a one-pot synthesis starting from bis(perfluoroalkyl)phosphinic acid chloride, (RF)2P(O)Cl and ammonia (Scheme 18) [41]. (C2F5)2P (C2F5)2P
O Cl
+ 6 NH3
- 70 °C Et2O
O
O O
NH2 O
(C2F5)2P N P(C 2F5)2 NH4
(C 2F5 )2P(O)Cl + 2 N(Et)3 Et2 O
(C2F5)2P
O N P(C 2F5)2
Et3NH
Yield: 79%
Page 12 of 22
13
Scheme 18 Treating the triethylammonium salt with concentrated sulphuric acid results in the liberation of the free acid, bis(bis(perfluoroalkyl)phosphinyl)imide [(RF)2P(O)]2NH (H-FPI) (Scheme 19) [41]. O
N P(C2F5)2
Et3NH
H2SO4
(C2F5)2P
- [Et 3NH] [HSO4 ]
O
N P(C2F5)2 H Yield: 97%
cr
(C2F5)2P
O
ip t
O
Scheme 19
us
Bis(bis(perfluoroalkyl)phosphinyl)imide [(RF)2P(O)]2NH (H-FPI) represents the phosphorus
an
analogue of the well-known bis(trifluoromethylsulfonyl)imide, (CF3SO2)2NH (H-TFSI; DesMarteau imide) (42]. It is a colorless, hygroscopic solid which melts at 47 °C and
M
decomposes at 196°C. It is hydrolytically stable in an aqueous solution for several weeks [41]. [(C2F5)2P(O)]2NH is a strong Brønsted acid with a pKa value of 1.9. This value is well
d
comparable to the pKa value of 1.7 for (CF3SO2)2NH [42]. The acidic strength based on the
te
measurements of the N-H stretching frequencies of tri-n-octylammonium salts in CCl4 solution also show quite similar acidic properties of neat [(C2F5)2P(O)]2NH and (CF3SO2)2NH
Ac ce p
[41]. Due to the strong acidic properties of [(C2F5)2P(O)]2NH (H-FPI) it is of interest as a Brønsted acid catalyst [43]. Rare-earth metal salts of FPI can be used as Lewis acid catalysts. Many organic salts of the FPI-anion are ionic liquids; some of them are liquid at room temperature [44]. (C2F5)3PF2
reacts
with
(CH3)3N
under
hydride
abstraction,
forming
the
hydridophosphate anion [(C2F5)3PF2H]-. The intermediately formed imminium ion is stabilized by
a
second
molecule
(CH3)3N
to
give
the
isolable
product
[(CH3)2NCH2N(CH3)3]+[(C2F5)3PF2H]- (A) (Scheme 20) [45]
Page 13 of 22
14
F F5C2
P F
F
Me
C2F5 C2F5
+ NMe3
N
F5C 2
CH2
F5C2
Me
C2F5
P
H
F
NMe3
F NMe3
F5C2
A
C2F5 H
F
cr
Scheme 20
P
ip t
Me2N
F5C 2
us
Salt (A) serves as reagent to transfer the (CH3)2NCH2-moiety to any stronger base than (CH3)3N, for instance (CH3)3P (Scheme 21): F P
C2F5
C2F5
P(CH 3)3
P
C2F5
+ N(CH3)3 H F [(CH3)2NCH2P(CH 3)3]+ C2F5
M
+ H F [(CH3)2NCH 2N(CH 3)3]+ C2F5
F
an
C2F5
A
Scheme 21
d
Salts of the tris(perfluoroethyl)hydridodifluorophosphate anion, [(C2F5)3PF2H]-, can be
te
prepared directly in a one-pot reaction of the tris(perfluoroethyl)difluorophosphorane with
Ac ce p
LiAlH4 (Scheme 22) [45] and the desired chloride salt:
(C2F5)3PF2
+ LiAlH4
THF
- "AlH3"
Li+
F F5C2 F5C2
F
P F
C2F5
[Kat]Cl
H
- LiCl
[Kat]+
F5C2 F5C2
P F
C2F5 H
[Kat]+ = tetraphenylphosphonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-butyl-2,3-dimethylimidazolium, N-hexylpyridinium. Scheme 22
Surprisingly, the reaction of tris(perfluoroalkyl)difluorophosphoranes with NaBH4 proceeds completely
different
and
results
in
the
formation
of
corresponding
tris(perfluoroalkyl)phosphines, (RF)3P (Scheme 4) [11]. Tris(perfluoroalkyl)phosphines, (RF)3P, are much weaker Lewis acids in comparison to tris(perfluoroalkyl)difluorophosphoranes. However, (RF)3P reacts with strong bases such as BuLi, t-BuOK or CH3MgCl Page 14 of 22
15
via the formal formation of corresponding phosphoranides (B) (Scheme 23) [11, 46]. These phosphoranides (B) represent thermally labile compounds which can be used for the nucleophilic transfer of C2F5 groups (Scheme 23) [11]. In the presence of B(OCH3)3 this reaction leads to the formation of pentafluoroethyl borate that can be converted to
cr
Et2O, -60° C
(C2F5)2P C4H 9 + LiF + C2F4 C4H 9 C 2F 5 RT P Li+ C 2F5 B(OCH3)3 C 2F 5 HF/KHF2 Li [C2F5B(OCH 3)3] K C 2F5BF 3 B
Yield: 58% on (C2F5)3P
an
Scheme 23
us
(C 2F 5) 3P + BuLi
ip t
pentafluoroethyltrifluoroborate [11, 47].
The reaction of tris(pentafluoroethyl)phosphine, (C2F5)3P with benzophenone in the presence
M
of t-BuOK proceeds similarly [11] and results in the formation of the corresponding pentafluoroethylated alcoholate (C) that can be converted into pentafluoroethylated alcohol
te
d
with HCl (Scheme 24).
Ac ce p
(C 2F5)3P + C 6H5-C(O)-C 6H 5 + t-BuOK
OK 20° C THF
C 6H5-C-C 6H5
OH HCl
C2F5
C 6H5-C-C 6H5 C2F5
C
Scheme 24
Anhydrous CsF is a strong base. It reacts with tris(pentafluoroethyl)phosphine, (C2F5)3P, to form the corresponding phosphoranide (Scheme 25) [48].
F5C2
F5C2
P C2F5
CsF, -30°C CH3CN
δ (31P) 15.6 ppm
Cs+
F5C2 F5C2 P F5C2 F
δ (31P) -33.9 ppm Scheme 25
Page 15 of 22
16
Cs+[(C2F5)3PF]- is stable in acetonitrile at -30° C and was characterised by NMRspectroscopy [48]. In contrast to trimethylamine (see Scheme 20), (CH3)2NCH2F is a fluoride-ion donor. The reaction of fluoromethyldimethylamine with tris(pentafluoroethyl)phosphine results in the
the
phosphorane
(E).
This
phosphorane
reacts
with
C2F5
P C2F5 +
Me Me
N
C2F5
F5C2
CH2F
H2C
P
F5C2
F
Me
F5C2
Me
F5C2
us
C2F5
second
molecule
cr
fluoromethyldimethylamine to give salt (G) (Scheme 26). [48]
a
ip t
formation of the corresponding phosphoranide salt (D) which undergoes rapid conversion to
N
C2F5 P
F5C2
E
an NMe2
F
M
C2F5 F Me2 N P
F5C2
F5C2
te
d
G
+
C2F5 F NMe2 H2C P F
NMe2
F
D
F5C2
of
N
Me Me
N CH2F
Me Me
F
Scheme 26
Ac ce p
In solution, the zwitter-ion (G) undergoes a 1,3-methyl shift to the thermodynamically more stable zwitter-ionic salt (H) (Scheme 27). The subsequent reaction with the strong base P(CH3)3 gives rise to the formation of the zwitterionic phosphonium phosphate (Scheme 27) [48]. F5C2 F5C2
F
P
F Me2 N
C2F5
G
NMe2
RT
CH2 Cl2
F5C2 F5C2
F P
F Me N
C2F5
H
NMe3
PMe 3
F5C2
- NMe3
F5C2
F P
F Me N
PMe3
C2F5
Scheme 27 The advantage of (RF)3PF2 compared to PF5 is not only its high Lewis acidity (compare Table 2). In contrast to PF5, (C2F5)3PF2 represents a liquid (b.p. 91-92 °C) which can be easily handled using a syringe. [49]. These properties make tris(pentafluoroethyl)difluorophosphorane attractive for the application as a Lewis acid catalyst. (C2F5)3PF2 was tested as Page 16 of 22
17
a catalyst in various reactions: cyano-silylation of aldehydes and ketones (Scheme 28), Strecker reaction (Scheme 29) and Diels-Alder reaction (Scheme 30) [50].
(C 2 F5) 3PF2
C6H5C(O)CH3 + (CH3)3SiCN
CN C6H5COSi(CH3)3
(cat.)
R.T., 5 min
C6H5C(O)H + (CH3)3SiCN
(C 2F5 )3 PF2 (cat.) R.T., 10 min
Yield: 91%
ip t
CH3
C6H5CH(CN)OSi(CH 3)3
(C 2 F5) 3PF2 (cat.) R.T., 10 min
p CH3OC6H5CH(CN)OSi(CH 3)3 Yield: 94%
us
p CH3OC6H5C(O)H + (CH3)3SiCN
cr
Yield: nearly quantitative
Catalyst loading below 1 mol%
R1
C
+
R
3
4
NH R + (CH3)3SiCN
R2
M
O
an
Scheme 28: Cyano-silylation of aldehydes and ketones
(C 2F5 )3 PF2 (cat.) R.T., 5 min
R1 R
CN
2
N
R3 R4
R 2 = H, CH 3 ; R3 = H, CH 3
Catalyst loading: 0.1 - 1 mol%
te
R 4 = C4H9, C6H5, p-ClC6H4
d
R 1 = C4H9, C6H5, p-CH3OC6H4, p-NO2C6H4, C3H3O
Ac ce p
Scheme 29: Strecker reaction
+
(C 2F5)3PF2 (cat)
+
CH 2 Cl2, 0°C, 15 min
O
Catalyst loading: 1 mol%
O
O
4-acetyl-2-methyl4-acetyl-1-methylcyclohex-1-en cyclohex-1-en Yield: 75% O
+
+
(C2F5)3PF2 (cat) Solvent free, -7°C, 2h
O
endoCatalyst loading: 0.14 mol%
exoO
2-acetyl-bicyclo[2.2.1]hept-5-ene Yield: 67%
Scheme 30: Diels-Alder reaction Page 17 of 22
18
The examples above demonstrate the activity of (C2F5)3PF2 as a Lewis acid catalyst. Even a low catalyst loading of 0.1 to 1 mol% of tris(pentafluoroethyl)difluorophosphorane yields in a quantitative conversion at room temperature or below [50]. Tris(pentafluoroethyl)difluoro-
ip t
phosphorane, (C2F5)3PF2, can be immobilized on activated carbon or graphite in order to generate a solid Lewis acid catalyst (see Fig. 3) which can be regenerated after use. The
cr
example below demonstrates the use of this catalyst in a Strecker synthesis (Scheme 31)
M
an
us
[50].
H +
NH2 + (CH3)3SiCN
te
O
d
Fig. 3
(C2 F5 )3 PF2 on C (cat.) CH2 Cl2, R.T., 5 min
Ac ce p
Catalyst (18 wt% of (C2F5)3PF2 on coal) loading: 0.1 to 1 mol% calculation on (C2 F5 )3 PF2
N
H N H Yield: 97%
Scheme 31
The catalyst (18 wt% of (C2F5)3PF2 on activated coal; Scheme 31) can be regenerated by filtration and used again with only a minimal loss of activity [50]. The acidity of bis(trifluoromethyl)phosphinic acid, (CF3)2P(O)OH has been studied by
H.J. Emeleus et al. [51]. They have found that (CF3)2P(O)OH is a stronger Brønsted acid than HBr and sulfuric acid. The high acidity of bis(perfluoroalkyl)phosphinic acids, (RF)2P(O)OH, could be proven by studying their catalytic activity in the acylation of β–naphtol with acetic acid anhydride as a model reaction. (Scheme 32, Table 3) [52]
Page 18 of 22
19
OH + (CH3CO)2O
OC(O)CH3
(RF )2P(O)OH (Cat)
+ CH3C(O)OH
Scheme 32
Entry a)
Catalyst (mol %)
Time (min)
Solvent
1
(C2F5)2P(O)OH (1 %)
30
CH2Cl2
96
2
(C4F9)2P(O)OH (1 %)
30
CH2Cl2
97
3
CF3SO3H (1 %)
4
without catalyst
cr
us
an
M
30
30
Yield (%)
CH2Cl2
95
CH2Cl2
< 10%
The substrate (β-naphthol) was treated with Ac2O (1.2 equiv.) in the presence of the catalyst at room temperature.
te
d
a)
ip t
Table 3. Acylation of β–naphtol with (Ac)2O in the presence of Brønsted acid catalyst [52]
Ac ce p
These results demonstrate that bis(perfluoroalkyl)phosphinic acids, (RF)2P(O)OH, are valuable Brønsted acid catalysts with a catalytic activity comparable to that of trifluoromethane sulfonic acid, CF3SO2OH.
Conclusion
The tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, provide convenient access to broad variety of perfluoroalkyl-phosphorus compounds that are of interest for various applications: conducting salts, ionic liquids, Brønsted and Lewis acid catalysts.
Acknowledgement Authors thank Dr. P. Barthen (Heinrich-Heine University of Düsseldorf) for the preparation and testing of some metal-FAP salts. Page 19 of 22
20
References. [1] E. Tassel, J. Chem. Soc., Abstr. 58 (1890) 1052. [2] H. Moissan: Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, 138 (1904) 789.
ip t
[3] T. Nakajima and H. Groult (Eds.), Fluorinated Materials for Energy Conversion, ELSEVIER, 2005, p. 140. [5] H.J. Emeleus, J. D. Smit, J. Chem. Soc. (1959) 375-381.
cr
[4] F.W. Bennet, H.J. Emeleus, R.N. Haszeldine, J. Chem. Soc. (1953) 1565-1571.
us
[6] A.H. Cowley,T. A. Furtsch, D.S. Dierdorf, J. Chem. Soc. D, Chem. Comm. (1970) 523524. [8] W. Mahler, Inorg. Chem. 2 (1963) 230. [9] W. Mahler, Inorg. Chem. 18 (1979) 352.
an
[7] M. Görg, G.V. Röschenthaler, A.A. Kolomeitsev, J. Fluorine Chem. 79 (1996), 103-104.
M
[10] S. A. Hayes, R. J. F. Berger, B. Neumann, N. W. Mitzel, J. Bader, B. Hoge, Dalton Trans. 39 (2010) 5630–5636.
[11] U. Welz-Biermann, N. Ignatyev, M. Weiden, M. Schmidt, U. Heider, A. Miller, H. Willner,
d
P. Sartori, WO 2003/087113, Merck Patent GmbH, Darmstadt, Germany.
te
[12] N. Ignat’ev and P. Sartori, J. Fluorine Chem. 103 (2000) 57-61. [13] U. Heider, V. Hilarius, P. Sartori, N. Ignatiev, EP 1037896 B1; US 6,264,818;
Ac ce p
JP 3387059; WO 2000/21969, Merck Patent GmbH, Darmstadt, Germany. [14] L.M. Yagupolskii, V.N. Zavatskii, V.Y. Semenii, K.N. Bildinov, P.V. Serebrov, A.A. Goncharenko, A.A. Kirsanov, M.I. Lyapunov, N.G. Feshchenko, Chemical Abstracts, 89 (1978) 43766j, 666; Chemical Abstracts, 90 (1979) 87670y, 645. [15] V.Y. Semenii, V.A. Stepanov, N.V. Ignat'ev, G.G. Furin, L.M. Yagupolskii, Zh. Obschei. Khim. (Russ.), 55 (1985) 2716-2720. [16] N.V. Ignat’ev, H. Willner, P. Sartori, J. Fluorine Chem. 130 (2009) 1183-1191. [17] M. Schmidt, U. Heider, A. Kuehner, R. Oesten, M. Jungnitz, N. Ignat´ev, P. Sartori, J. Power Sources, 97-98 (2001) 557-560. [18] N. Ignatyev, M. Schmidt, A. Kühner, V. Hilarius, U. Heider, A. Kucheryna, P. Sartori, H. Willner, EP 1399453 B; WO 2003/002579; US 7,094,328, Merck Patent GmbH, Darmstadt, Germany. [19] N.V. Ignat’ev, U. Welz-Biermann, A. Kucheryna, G. Bissky, H. Willner, J. Fluorine. Chem. 126 (2005) 1150-1159. [20] U. Welz-Biermann, N. Ignatyev, M. Weiden, U. Heider, A. Kucheryna, H. Willner, Page 20 of 22
21
WO 2003/087110; US 7,202,379, Merck Patent GmbH, Darmstadt, Germany. [21] H. J. Emeleus, J. D. Smit, J. Chem Soc. (1959) 375-381. [22] T. Mahmood, J. M. Shreeve, Inorg. Chem, 25 (1986) 3128-3131. [23] T. Mahmood, J.-M. Bao, R.L. Kirchmeier, J. M. Shreeve, Inorg. Chem, 27 (1988) 29132916.
ip t
[24] S. Steinhauer, J. Bader, H.-G. Stammler, N. Ignat‘ev, B. Hoge, Angew. Chem. Int. Ed. 53 (2014) 5206 – 5209.
[25] N. Ignatiev, G. Bissky, H. Willner, WO 2008/092489; US 8,211,277; EP 2114965 B1,
cr
Merck Patent GmbH, Darmstadt, Germany.
[26] N. Ignatyev, W. Hierse, W. Wiebe, H. Willner, DE 10 2011 111 490 A1, Merck Patent
us
GmbH, Darmstadt, Germany.
[27] B. Hoge, J. Bader, N. Ignatyev, M. Schulte, WO 2012/041431, Merck Patent
an
GmbH, Darmstadt, Germany.
[28] J. Bader, N. Ignat'ev, B. Hoge, Inorg. Chem. (2014) DOI: 10.1021/ic500857b. [29] B. Hoge, J. Bader, N. Ignatyev, M. Schulte, W. Hierse, W. Wiebe, H. Willner,
M
WO 2012/048772, Merck Patent GmbH, Darmstadt, Germany. [30] N. Ignatyev, E. Aust, D. Bejan, H. Willner, WO 2010/009791 A1, EP 2303898 B1, Merck
d
Patent GmbH, Darmstadt, Germany.
[31] L. M. Yagupol'skii, N. V. Pavlenko, N. V. Ignat'ev, G. I. Matyushecheva, V. Ya. Semenii,
te
Zh. Obsch. Khim. 54 (1984) 334-339.
[32] N. V. Pavlenko, G. I. Matyushecheva, V. Y Semenii., L. M. Yagupol'skii, Zh. Obsch.
Ac ce p
Khim. 55 (1985) 1586-1590.
[33] R. G. Cavell, T. L. Charlton, W. Sim, J. Am. Chem. Soc. 93 (1971) 1130-1137. [34] N. Ignatyev, R. Friedrich, S. Schellenberg, K. Koppe, W. Frank, DE 10 2012 022441 A1, Merck Patent GmbH, Darmstadt, Germany. [35] N. Ignatyev, W. Wiebe, H. Willner, WO 2013/013766 A1, Merck Patent GmbH, Darmstadt, Germany.
[36] A. B. Burg, A. J. Sarkis, J. Am. Chem. Soc. 87 (1965) 238-241. [37] T. Mahmood, J. M. Bao, R. L. Kirchmeier, J. M. Shreeve, Inorg. Chem. 27 (1988) 29132916. [38] D. Bejan, V. Dinou, N. Ignat’ev, E. Bernhardt, H. Willner, Phosphorus, Sulfur and Silicon and Related Elements (2014), accepted. [39] N. V. Pavlenko, G. I. Matyushecheva, V. Y. Semenii, L. M. Yagupol’skii, J. General Chem. USSR, 55 (1985) 1586-1590. [40] N. Ignatiev, U. Weltz-Biermann, M. Heckmeier, G. Bissky, H. Willner, WO 2006/128563; US 8,106,217; EP 1888605 B1, Merck Patent GmbH, Darmstadt, Germany. Page 21 of 22
22
[41] D. Bejan, H. Willner, N. Ignatiev, C.W. Lehmann, Inorg. Chem. 47 (2008) 9085-9089. [42] J.F. Foropoulos, D.D. DesMarteau, Inorg. Chem. 23 (1984) 3720-3723. [43] D. Bejan, Dissertation, University of Wuppertal (2010), Germany. [44] D. Bejan, N.V. Ignat´ev, H. Willner, J. Fluorine Chem. 131 (2010) 325-332. [45] N. Ignatyev, M. Schulte, J. Bader, B. Hoge, WO 2011/072810 A1, Merck Patent GmbH, [46] O. Shyshkov, Dissertation, Bremen University (2006), Germany.
ip t
Darmstadt, Germany.
Fluorine Chemistry, Paris, 2013.
cr
[47] N.V. Ignat’ev, A. Miller, H. Willner, Oral Presentation, 17th European Symposium on [48] N. Allefeld, B. Neumann, H.-G. Stammler, G.-V. Röschenthaler, N. Ignat’ev, B. Hoge,
us
Chemistry - A European Journal, 20 (2014) 7736-7745.
[49] N. Ignatyev, D. Dejan, H. Willner, WO 2010/012359 A1, Merck Patent GmbH,
an
Darmstadt, Germany.
[50] N. Ignat’ev, V. Pichugina-Bernhardt, B. Bittner, F. Schlüter, E. Bernhardt, H. Willner, Oral presentation, 21th Winter Fluorine Conference, Florida, 2013.
M
[51] R.H. Haszeldine, R.C. Paul, J. Chem. Soc. (1955) 563-574. [52] N.V. Ignat'ev, D. Bejan, H. Willner, Chimica Oggi/Chemistry Today. 29 (2011) N 5, 32-
d
34.
Ac ce p
te
Highlights
1. The phosphoranes, (RF)3PF2, are produced by means of Electrochemical Fluorination. 2. Li[(C2F5)3PF3] possesses an advantageous properties. 3. Bis(perfluoroalkyl)phosphinic acids are efficient Brønsted acid catalysts. 4. The tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, are useful Lewis acid catalysts. 5. The phosphoranes, (RF)3PF2, are useful material to produce variety of chemicals.
Page 22 of 22
S0022-1139(14)00311-X http://dx.doi.org/doi:10.1016/j.jfluchem.2014.10.007 FLUOR 8455
To appear in:
FLUOR
Received date: Revised date: Accepted date:
1-8-2014 7-10-2014 10-10-2014
Please cite this article as: N.V. Ignat’ev, J. Bader, K. Koppe, B.H. , H. Willner, Recent Progress in Perfluoroalkyl-Phosphorus Chemistry, Journal of Fluorine Chemistry (2014), http://dx.doi.org/10.1016/j.jfluchem.2014.10.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Graphical Abstract (synopsis) The tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, provide convenient access to broad variety of perfluoroalkyl-phosphorus compounds that are of interest for various applications:
cr
ip t
conducting salts, ionic liquids, Brønsted and Lewis acid catalysts.
us
Graphical Abstract
(RF)2(O)PNP(O)(RF)2
M
H2 O
(RF)3PF2
(RF)P(O)(OH)2
Ac ce p
te
O Ca
d
H 2O
(RF)3P=O
(RF)2P(O)NR2
R 2NH
(RF)2 P(O)Cl (RF)2P(O)OH
an
NH 3
HF
H+(OH2)5[(RF)3PF3]-
MF Na
BH
M+ [(RF)3PF3]-
4
[(CH3 )3 Si] 2 NH
(RF )3P
(RF)3P=N Si(CH3)3
Recent Progress in Perfluoroalkyl-Phosphorus Chemistry *
N. V. Ignat’ev 1 , J. Bader 2, K. Koppe 3, B. Hoge 2, H. Willner 4
1
Merck KGaA, PM-ATI, Frankfurter Str. 250, D-64293, Darmstadt, Germany; E-mail: [email protected]
2
Universität Bielefeld, Anorganische Chemie, Universitätsstraße 25, 33615 Bielefeld, Germany
3
Heinrich-Heine-Universität Düsseldorf, Institut für Anorganische Chemie und Strukturchemie II, Universitätstrasse 1, 40225 Düsseldorf, Germany
4
Bergische Universität Wuppertal, Anorganische Chemie, Gauss Strasse 20, Page 1 of 22
2
D-42097 Wuppertal, Germany
1. Abstract Electrochemical fluorination (Simons process) provides a cheap industrial access to a series
ip t
of tris(perfluoroalkyl)difluorophosphoranes. These substances are a convenient starting material for the preparation of various perfluoroalkyl-phosphorus compounds. The
(FAP)
and
perfluoroalkyl-phosphinate
cr
preparation of a variety of new salts and ionic liquids with perfluoroalkyl-fluorophosphate anions
is
described.
The
us
tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, are strong Lewis acids which are of
an
interest for the application in catalysis. The syntheses of various derivatives of bis(perfluoroalkyl)phosphinic acids, (RF)2P(O)OH, are presented.
M
Keywords: Perfluoroalkyl phosphorus compounds; Electrochemical Fluorination; Conducting salts; Brønsted acids; Lewis Acids; Catalysis
d
2. Results and discussion
te
Phosphorus trifluoride, PF3, and phosphorus pentafluoride, PF5, were the first perfluorinated phosphorus compounds described in the literature [1,2]. Today PF5 is in
multi-ton
quantities
Ac ce p
produced
and
mostly
used
for the synthesis of lithium
hexafluorophosphate, LiPF6 – the most commonly used conducting salt in lithium-ion batteries [3]. A major drawback of LiPF6 based electrolytes is their poor stability at elevated temperatures due to the elimination of LiF and the generation of PF5: LiPF6
PF5 + LiF
[3]. LiPF6 is moisture sensitive (Scheme 1) [3]:
LiPF6 + H 2O
LiF + POF3 + 2 HF
Scheme 1 The limited thermal and hydrolytical stability of LiPF6 require a development of safer conducting salts for the application in Li-batteries. Perfluoroalkyl phosphorus compounds have been synthesized more than 60 years ago.
Page 2 of 22
3
R.N. Haszeldine et al. [4] have reported the preparation of tris(trifluoromethyl)phosphine, (CF3)3P, by the reaction of CF3I with white phosphorus in an autoclave at 200-220 °C. Surprisingly, the reaction of C2F5I and C3F7I with white phosphorus does not result in the formation of tri-substituted phosphines, (RF)3P [5,6]. Alternatively to the autoclave reaction,
(PhO)3P,
with
CF3Br/(Et2N)3P
(Ruppert
[(Et2N)3P-Br]+
reagent:
CF3Br, (Et 2N)3P
[CF3]-)
in
cr
hexamethylphosphoramide, HMPA (Scheme 2) [7].
(CF3)3P
us
(PhO)3P
ip t
tris(trifluoromethyl)phosphine, (CF3)3P, can be prepared by the reaction of triphenylphosphite,
HMPA
an
Scheme 2
The polar solvent hexamethylphosphoramide (HMPA) is required in this synthesis. The
M
reported yield of (CF3)3P is good (up to 85%), but the use of this toxic solvent and commercially not available trifluoromethyl bromide are serious drawbacks for this protocol.
d
Tris(trifluoromethyl)phosphine can be oxidized to (CF3)3PCl2 by Cl2 and converted to
te
(CF3)3PF2 by fluorination with ZnF2 [7].
Trifluoromethyl derivatives of phosphorus are not very stable compounds. Mahler has
Ac ce p
reported the gradual decomposition of (CF3)3PF2 to PF5 via elimination of difluorocarbene, :CF2 (Scheme 3) [8].
(CF3)3PF2
(CF3) 2PF3
(CF3)2PF3
CF3PF4
CF3PF4
PF5
+
+
+
CF2 CF2
CF2
Scheme 3 This process is slow at room temperature, but significantly accelerated at elevated temperatures. Difluorotris(trifluoromethyl)phosphorane, (CF3)3PF2 decomposes at 25 °C in the gas phase at a rate of 0.5% per month. At elevated temperatures (100 °C) the decomposition is much faster (40% within 17 hours) [8]. The handling of the reduction
Page 3 of 22
4
product P(CF3)3 requires special safety precautions because P(CF3)3 (b.p. 17 °C) reacts
(CF3)3P (b.p. 17 °C) Fig. 1
b) (C2F5)3P (b.p. 85-87 °C [11])
an
a)
us
cr
ip t
violently on contact with air (see Fig. 1, a) [9,10].
M
In contrast to (CF3)3P, the higher homologue (C2F5)3P is much more stable (Fig. 1, b). It can be handled by standard Schlenk techniques. Tris(pentafluoroethyl)phosphine can be readily
d
prepared by the reduction of tris(pentafluoroethyl)difluorophosphorane with NaBH4 or other
te
reagents (Scheme 4) [11].
NaBH4 solvent free
(C 2F5) 3P
Ac ce p
(C 2F5)3PF2
Scheme 4
a)
31
P NMR
Page 4 of 22
b)
us
cr
ip t
5
19
F NMR
an
Fig. 2: NMR spectra of tris(pentafluoroethyl)difluorophosphorane, (C2F5)3PF2 .
M
Tris(pentafluoroethyl)difluorophosphorane, (C2F5)3PF2, is nowadays produced in multi-ton quantities by the electrochemical fluorination (Simons process, ECF) of triethylphosphine
te
d
[12,13]. Tris(pentafluoroethyl)difluorophosphorane is a clear and colorless liquid. It can be distilled (b.p. 91-92 °C) at atmospheric pressure without decomposition. NMR spectroscopic
Ac ce p
data of (C2F5)3PF2 confirm a trigonal-bipyramidal structure with two equivalent fluorine atoms on the axial positions and the three C2F5 groups in the equatorial position (see Fig. 2). The first example of the electrochemical fluorination of an organophosphorus compound was reported by L.M. Yagupolskii et al. (Scheme 5) [14]
R3P=O
ECF HF
(RF)3P=O RF = C2-8 perfluoroalkyl
R = C2-8 alkyl Scheme 5 Tris(perfluoroalkyl)phosphine
oxides
were
identified
as
products
formed
in
the
electrochemical fluorination of trialkylphosphine oxides. Later it was found out that the main products
in
this
process
were
not
tris(perfluoroalkyl)phosphine
oxides
but
tris(perfluoroalkyl)difluorophosphoranes (Scheme 6) [15]. Page 5 of 22
6
ECF
R3P=O
(RF)3PF2
HF
R = C2-6 alkyl
Yield: 24-46%
+
F 2O
+
n H2
RF = C2-6 perfluoroalkyl
Scheme 6
ip t
The disadvantage of the electrochemical fluorination of trialkylphosphine oxides, however, is not only the low yield of the product, tris(perfluoroalkyl)difluorophosphorane, but especially
cr
the formation of an equimolar quantity of the toxic gas, F2O, that in combination with H2 (constantly produced during the ECF process) can cause a severe explosion. Using excludes
the
formation
of
F2O.
The
yield
us
trialkylphosphines
of
the
formed,
tris(perfluoroalkyl)difluorophosphoranes is much better in comparison to the ECF of
an
trialkylphosphine oxides (Table 1) [12,13]. This technology provides a convenient protocol for
M
the industrial production of (RF)3PF2. Tris(perfluoroalkyl)difluorophosphoranes are very reactive compounds. For example, (C2F5)3PF2 readily reacts with lithium fluoride forming the
d
corresponding lithium tris(perfluoroethyl)trifluorophosphate, Li[(C2F5)3PF3] (LiFAP) as an
+
LiF
Ac ce p
(C2F5)3PF2
te
analogue to lithium hexafluorophosphate, Li[PF6] (Scheme 7) [16,17]. Solvent
Li[(C 2F5) 3PF3] Solv LiFAP - proprietary material of BASF
Scheme 7
Table 1. Electrochemical Fluorination (ECF) of Trialkylphosphines
R3P
F
e-, Ni
RF
HF
P F
RF RF
R
RF
Yield, %
CH3
CF3
4
C2H5
C2F5
74
n-C3H7
n-C3F7
53
n-C4H9
n-C4F9
49 Page 6 of 22
7
i-C4H9
i-C4F9 , n-C4F9 s-C4F9 , t-C4F9
57
LiFAP can be prepared in situ in polar solvents such as dimethyl carbonate (DMC) or
ip t
ethylene carbonate (EC). These solvents are typical media for the preparation of electrolytes for Li-ion batteries. LiFAP is much more stable against hydrolysis in comparison to lithium
cr
hexafluorophosphate, Li[PF6]. After the addition of 500 ppm of water to a 1M solution of LiPF6 in EC:DMC (50:50 wt%) 250 ppm H2O were consumed within ca. 20 hours and a
us
corresponding amount (ca. 500 ppm) of HF was generated. In a similar experiment, after the addition of 1000 ppm of water to a 1M solution of Li[(C2F5)3PF3] (LiFAP) in an organic
an
carbonate mixture the system remained unchanged for three days; no water consumption and no HF formation were detected [17]. A solution of LiFAP in EC:DMC (50:50 wt%) is
M
slightly less conductive than a corresponding solution of Li[PF6], but the maximum conductivity (8.6 mS·cm-1) for the LiFAP electrolyte is reached with a 0.8 M solution. For
d
Li[PF6] in EC:DMC (50:50 wt%) the maximum conductivity (10.7 mS·cm-1) was reported using
te
a 1M solution [17]. Lithium bis(pentafluoroethyl)tetrafluorophosphate, Li[(C2F5)2PF4] has a
Ac ce p
symmetrical structure and exhibits a slightly better conductivity in organic solvents when compared to Li[(C2F5)3PF3] [16]. Due to the better thermal and hydrolytical stability, LiFAP electrolytes could provide an option to replace Li[PF6] electrolytes in Li-ion batteries. Tris(perfluoroalkyl)difluorophosphoranes react not only with alkali-metal fluorides, but
also with aqueous HF to the corresponding acid [H(OH2)n][(C2F5)3PF3] (HFAP). [18]. The reaction of (C2F5)3PF2 with 18.2 wt% aqueous HF proceeds within minutes and results quantitatively in the formation of [H(OH2)5][(C2F5)3PF3] (HFAP) as a pentahydrate (Scheme 8) [18]. HFAP is a convenient starting material for the preparation of a variety of metal-salts and ionic liquids with the FAP-anion [19].
Page 7 of 22
8
RF (RF)3PF2
+ HF + 5 H2O
[H(OH2)5
]+
18.2% HF
F
RF
P F
F RF
Scheme 8
mixture of isomers with meridional and facial structures
ip t
R F = C 2F 5 , C 4F 9
HFAP is a strong Brønsted acid. The pentahydrate HFAP · 5 H2O corresponds to a 83%
cr
aqueous solution. This solution is stable at 0 °C for several months. The hydrolysis rate at
us
room temperature is about 1 mol% per month (RF = C2F5), while elevated temperatures accelerate the hydrolysis (Scheme 9) [20] resulting in the formation of corresponding
H+[(C2F5)3PF3]- x 5 H2O
135-140 °C, 20 h
an
bis(perfluoroalkyl)phosphinic acids.
(C2F5)2P(O)OH + C2F5H + 3 H2O + 3 HF
H+[(C4F9)3PF3]- x 5 H2O
M
Yield: 87%
135-140 °C, 20 h
(C4F9)2P(O)OH + C4F9H + 3 H2O + 3 HF Yield: 97%
d
Scheme 9
te
Bis(heptafluoropropyl)phosphinic acid (C3F7)2P(O)OH was prepared more than 50 years ago
Ac ce p
by a multistep procedure. The last step in this procedure was the hydrolysis of bis(heptafluoropropyl)trichlorophosphorane, (C3F7)2PCl3 [21]. The bis(pentafluoroethyl)phosphinic acid (C2F5)2P(O)OH was synthesized later by a similar protocol [22]. Bis(nonafluorobutyl)phosphinic acid, (C4F9)2P(O)OH, has been prepared by the hydrolysis of the corresponding phosphine oxide [23]. But, the protocol according to the Scheme 9 [20] is probably the most convenient one to prepare large quantities of bis(perfluoroalkyl)phosphinic acids. Today, (C2F5)2P(O)OH and (C4F9)2P(O)OH are produced industrially and are commercially available. In particular bis(nonafluorobutyl)phosphinic acid, (C4F9)2P(O)OH, is of interest for the application as a short chain fluorosurfactant to replace the persistent perfluorooctanoic
(PFOA)
and
perfluorooctanesulfonic
(PFOS)
acids.
Bis(nonafluorobutyl)phosphinic acid, (C4F9)2P(O)OH shows no signs of toxicity and is not bioaccumulative. Aqueous solutions of bis(nonafluorobutyl)phosphinic acid do not foam. In Page 8 of 22
9
mixtures with other strong foaming mist suppressants it is possible to reduce and adjust the foam layer. Bis(nonafluorobutyl)phosphinic acid is available in the European Union and registered by REACH under the trade name TividaTM FL 2100. Tris(perfluoroalkyl)difluorophosphoranes are strong Lewic acids. They are much
ip t
stronger than for example BF3 or PF5 and comparable to the acidity of AsF5 (see Table 2). [24]
cr
Table 2. Fluoride Ion Affinity (FIA) of various Lewis acids
F
F
+ F-
P
As
F
416.5
F
F F
F F
P F F P F
C2F5
389.3
F
F
357.1
F
te
F
+ F-
F F
d
F
F5C2
F5C 2
F
F
an
F5C2
F
M
F
+ F-
F As F F F P C2F5 C2F5 F
us
FIA [kJ/mol]
FIA = -ΔrG0 ; B3LYP/6-311G++(2d)
Ac ce p
Due to the strong Lewis acidity tris(perfluoroalkyl)difluorophosphoranes react with weak nucleophiles, such as chloride, resulting in the formation of chloro-fluoro phosphates [Kt][(RF)3PF2Cl] (Scheme 10) [25], or with acetate to form an acetoxy-phosphate [Kt][(RF)3PF2OC(O)CH3] (Kt = K+, [(Bu)4N]+) [26]. The direct synthesis of alkoxyphosphates [P(C2F5)3F2OR]- (R = alkyl) by the reaction of (C2F5)3PF2 with alcohols
or
alcoholates
is
not
very
selective
due
to
strong nucleophiles like the
high
reactivity
of
tris(perfluoroalkyl)difluorophosphoranes. The electrophilicity of the phosphorane (C2F5)3PF2 can be lowered by the formation of an adduct with organic bases, for instance with 4dimethylamino pyridine (Scheme 11) [27,28] This adduct is a handy starting material to prepare a variety of salts with functionalized perfluoroalkyl-phosphate anions of the structure [(C2F5)3PF2OR] (R = H, Et, CH2CF3, Ph, Ac, C2H4OH, etc.) (Scheme 11) [29].
[Kt] + Cl-
+
(RF)3PF2
R F = C2F5, C3F7
RT CH 3CN
[Kt][(R F) 3PF2Cl] [Kt] = organic cation Page 9 of 22
10
Scheme 10 F
F P F
C2F5 DMAP C2F5
F5C2
F
C2F5
P
F5C2
HX
N
F
F5C2 NMe2
DMAP: 4-Dimethylaminopyridine
[HDMAP]+
P F
C2F5 X
ip t
F5C2
F5C2
X = OH, OPh, OEt, OAc, OCH2CF3, OC2H4OH
cr
Scheme 11
The hydroxy and acetoxy phosphates are stable at room temperature but decompose at
us
elevated temperatures. The decomposition of these salts in an aqueous milieu results in the
an
formation of bis(perfluoroalkyl)phosphinic acid and gaseous products, thus forming an abundant foam [26].
M
Bis(perfluoroalkyl)phosphinic acids can be converted into derivatives which are of interest for various applications. For instance, bis(perfluoroalkyl)phosphinic acid chlorides
d
can be easily obtained by the reaction of bis(perfluoroalkyl)phosphinic acid with phosphorus
te
pentachloride or its derivatives. For the synthesis of (C2F5)2P(O)Cl the use of PhPCl4 as
[30].
Ac ce p
chlorinating reagent is more convenient in regard to isolation of the product (Scheme 12)
(C2F5)2P
O
OH
+ C6H5PCl 4
- HCl
(C2F5)2P
O Cl
+ C6H5P(O)Cl 2
Scheme 12
Bis(perfluoroalkyl)phosphinic acid chlorides are very reactive compounds. They can be easily converted to the corresponding bis(perfluoroalkyl)phosphinyl amides, (RF)2P(O)NRR’, which are little-known compounds. Only a few fluorinated phosphinic acid amides are described in the literature. Their syntheses are based on the reaction of phosphinic acid chlorides with amines. For example, bis(perfluoroalkyl)phosphinic acid amides (CnF2n+1)2P(O)NHR (R = H, CH3, C6H5, 3-FC6H4, 4-FC6H4) were prepared in moderate yields by the reaction of (CnF2n+1)2P(O)Cl (n = 1, 3, 4) with ammonia, methylamine and aromatic amines [31-33]. A Page 10 of 22
11
similar procedure was used to prepare secondary amides of bis(perfluoroalkyl)phosphinic acids (Scheme 13), which exhibit interesting surface properties [34].
O Cl
(RF) 2P
+ 2 RR'NH
RF = C2F5, C4F9
O NRR'
+ [RR'NH2]Cl
R = H, CH3, C4H9
ip t
(RF) 2P
R' = C4H9, CH2CH2OCH3, CH2CH=CH2, (CH2)4CH=CH2
cr
Scheme 13
Bis(pentafluoroethyl)phosphinic acid anhydride, (C2F5)2P(O)OP(O)(C2F5)2, reacts with
us
dibutylamine to give (C2F5)2P(O)N(C4H9)2. [35]. The disadvantage of this method is that one half of the anhydride is lost due to the formation of the dibutylammonium salt of
an
bis(pentafluoroethyl)phosphinic acid, [H2N(C4H9)2]+[(C2F5)2P(O)O]-. In contrast to that, the reaction of bis(pentafluoroethyl)phosphinic acid anhydride with hexamethyldisiloxane yields
M
the bis(pentafluoroethyl)phosphinic acid trimethylsilyl ester, (C2F5)2P(O)OSi(CH3)3, as the
d
only product (Scheme 14) [35].
te
[(C2F5)2P(O)]2O + (CH3)3SiOSi(CH3)3
2 (C2F5)2P(O)2OSi(CH3)3
Scheme 14
Ac ce p
Tris(perfluoroalkyl)phosphine oxides, (RF)3P=O, are also suitable starting materials for the preparation of bis(perfluoroalkyl)phosphinic acid amides. Burg and Sarkis reported a quantitative reaction of tris(trifluoromethyl)phosphine oxide with dimethyl amine [36]. Tris(nonafluorobutyl)phosphine oxide reacts with dimethyl amine resulting in the formation of a complex mixture of products [37]. This method is however limited to dimethyl amides. There is no protocol described to react phosphine oxides with other secondary amines to yield bis(perfluoroalkyl)phosphinic acid amides.The reaction of tris(perfluoroalkyl)phosphine oxides with primary amines leads to the selective formation of the corresponding amides in good to excellent yields (Scheme 15) [38].
(C2F3)3P=O
+
R NH2
- C2 F5H
(C2F3)2P(O)NHR
R = H, C 6H5, CH3C6 H 4, C6 H5CH 2, p-CH 3C6 H4 CH 2, p-CH 3 OC 6H 4CH2, C 6 H5CH 2CH 2, C4H9, C 8H 17
Page 11 of 22
12
Scheme 15 Tris(perfluoroalkyl)phosphine oxides and bis(perfluoroalkyl)phosphinyl chlorides, (RF)2P(O)Cl, readily react with hydrazines to form the corresponding hydrazides (Scheme 16 and 17) [38]. Within the reaction of (RF)2P(O)Cl with hydrazines, CaH2 is employed to trap the generated to
avoid
a
reaction
with
the
hydrazines
(Scheme
bis(perfluoroalkyl)phosphinic acids were so far unknown compounds.
+
R'R''N NH2
Hydrazides
of
(C2F3)2P(O)NH NR'R''
cr
(C2F3)3P=O
17).
ip t
HCl
- C 2F5 H
Scheme 16
+
R'HN NH2
CaH2
- HCl
M
R' = CH3, C6H5
(C2F3)2P(O)NH NHR'
an
(C2F3)2P(O)Cl
+
us
R' = CH3, C6H5, CH2C6H2-3,5-(t-C4H9)-4-OH ; R'' = H, CH3
2 HCl
CaCl2
+
2 H2
d
Scheme 17
te
The hydrolytic stability of bis(pentafluoroethyl)phosphinyl amides depends on the substituent R. Amides with a longer alkyl chain are more stable against hydrolysis in comparison to
Ac ce p
(C2F5)2P(O)NH2 which forms an ammonium salt in wet solvents. Therefore the reaction of bis(perfluoroalkyl)phosphinic chloride, (RF)2P(O)Cl, and bis(perfluoroalkyl)phosphinyl amide, (RF)2P(O)NH2,
yielding
[(RF)2P(O)]2NH,
should
the
be
corresponding
carried
out
in
bis(bis(perfluoroalkyl)phosphinyl) strictly
anhydrous
conditions
imide [39-41].
Bis(bis(perfluoroalkyl)phosphinyl)imide salts can be prepared in better yield in a one-pot synthesis starting from bis(perfluoroalkyl)phosphinic acid chloride, (RF)2P(O)Cl and ammonia (Scheme 18) [41]. (C2F5)2P (C2F5)2P
O Cl
+ 6 NH3
- 70 °C Et2O
O
O O
NH2 O
(C2F5)2P N P(C 2F5)2 NH4
(C 2F5 )2P(O)Cl + 2 N(Et)3 Et2 O
(C2F5)2P
O N P(C 2F5)2
Et3NH
Yield: 79%
Page 12 of 22
13
Scheme 18 Treating the triethylammonium salt with concentrated sulphuric acid results in the liberation of the free acid, bis(bis(perfluoroalkyl)phosphinyl)imide [(RF)2P(O)]2NH (H-FPI) (Scheme 19) [41]. O
N P(C2F5)2
Et3NH
H2SO4
(C2F5)2P
- [Et 3NH] [HSO4 ]
O
N P(C2F5)2 H Yield: 97%
cr
(C2F5)2P
O
ip t
O
Scheme 19
us
Bis(bis(perfluoroalkyl)phosphinyl)imide [(RF)2P(O)]2NH (H-FPI) represents the phosphorus
an
analogue of the well-known bis(trifluoromethylsulfonyl)imide, (CF3SO2)2NH (H-TFSI; DesMarteau imide) (42]. It is a colorless, hygroscopic solid which melts at 47 °C and
M
decomposes at 196°C. It is hydrolytically stable in an aqueous solution for several weeks [41]. [(C2F5)2P(O)]2NH is a strong Brønsted acid with a pKa value of 1.9. This value is well
d
comparable to the pKa value of 1.7 for (CF3SO2)2NH [42]. The acidic strength based on the
te
measurements of the N-H stretching frequencies of tri-n-octylammonium salts in CCl4 solution also show quite similar acidic properties of neat [(C2F5)2P(O)]2NH and (CF3SO2)2NH
Ac ce p
[41]. Due to the strong acidic properties of [(C2F5)2P(O)]2NH (H-FPI) it is of interest as a Brønsted acid catalyst [43]. Rare-earth metal salts of FPI can be used as Lewis acid catalysts. Many organic salts of the FPI-anion are ionic liquids; some of them are liquid at room temperature [44]. (C2F5)3PF2
reacts
with
(CH3)3N
under
hydride
abstraction,
forming
the
hydridophosphate anion [(C2F5)3PF2H]-. The intermediately formed imminium ion is stabilized by
a
second
molecule
(CH3)3N
to
give
the
isolable
product
[(CH3)2NCH2N(CH3)3]+[(C2F5)3PF2H]- (A) (Scheme 20) [45]
Page 13 of 22
14
F F5C2
P F
F
Me
C2F5 C2F5
+ NMe3
N
F5C 2
CH2
F5C2
Me
C2F5
P
H
F
NMe3
F NMe3
F5C2
A
C2F5 H
F
cr
Scheme 20
P
ip t
Me2N
F5C 2
us
Salt (A) serves as reagent to transfer the (CH3)2NCH2-moiety to any stronger base than (CH3)3N, for instance (CH3)3P (Scheme 21): F P
C2F5
C2F5
P(CH 3)3
P
C2F5
+ N(CH3)3 H F [(CH3)2NCH2P(CH 3)3]+ C2F5
M
+ H F [(CH3)2NCH 2N(CH 3)3]+ C2F5
F
an
C2F5
A
Scheme 21
d
Salts of the tris(perfluoroethyl)hydridodifluorophosphate anion, [(C2F5)3PF2H]-, can be
te
prepared directly in a one-pot reaction of the tris(perfluoroethyl)difluorophosphorane with
Ac ce p
LiAlH4 (Scheme 22) [45] and the desired chloride salt:
(C2F5)3PF2
+ LiAlH4
THF
- "AlH3"
Li+
F F5C2 F5C2
F
P F
C2F5
[Kat]Cl
H
- LiCl
[Kat]+
F5C2 F5C2
P F
C2F5 H
[Kat]+ = tetraphenylphosphonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-butyl-2,3-dimethylimidazolium, N-hexylpyridinium. Scheme 22
Surprisingly, the reaction of tris(perfluoroalkyl)difluorophosphoranes with NaBH4 proceeds completely
different
and
results
in
the
formation
of
corresponding
tris(perfluoroalkyl)phosphines, (RF)3P (Scheme 4) [11]. Tris(perfluoroalkyl)phosphines, (RF)3P, are much weaker Lewis acids in comparison to tris(perfluoroalkyl)difluorophosphoranes. However, (RF)3P reacts with strong bases such as BuLi, t-BuOK or CH3MgCl Page 14 of 22
15
via the formal formation of corresponding phosphoranides (B) (Scheme 23) [11, 46]. These phosphoranides (B) represent thermally labile compounds which can be used for the nucleophilic transfer of C2F5 groups (Scheme 23) [11]. In the presence of B(OCH3)3 this reaction leads to the formation of pentafluoroethyl borate that can be converted to
cr
Et2O, -60° C
(C2F5)2P C4H 9 + LiF + C2F4 C4H 9 C 2F 5 RT P Li+ C 2F5 B(OCH3)3 C 2F 5 HF/KHF2 Li [C2F5B(OCH 3)3] K C 2F5BF 3 B
Yield: 58% on (C2F5)3P
an
Scheme 23
us
(C 2F 5) 3P + BuLi
ip t
pentafluoroethyltrifluoroborate [11, 47].
The reaction of tris(pentafluoroethyl)phosphine, (C2F5)3P with benzophenone in the presence
M
of t-BuOK proceeds similarly [11] and results in the formation of the corresponding pentafluoroethylated alcoholate (C) that can be converted into pentafluoroethylated alcohol
te
d
with HCl (Scheme 24).
Ac ce p
(C 2F5)3P + C 6H5-C(O)-C 6H 5 + t-BuOK
OK 20° C THF
C 6H5-C-C 6H5
OH HCl
C2F5
C 6H5-C-C 6H5 C2F5
C
Scheme 24
Anhydrous CsF is a strong base. It reacts with tris(pentafluoroethyl)phosphine, (C2F5)3P, to form the corresponding phosphoranide (Scheme 25) [48].
F5C2
F5C2
P C2F5
CsF, -30°C CH3CN
δ (31P) 15.6 ppm
Cs+
F5C2 F5C2 P F5C2 F
δ (31P) -33.9 ppm Scheme 25
Page 15 of 22
16
Cs+[(C2F5)3PF]- is stable in acetonitrile at -30° C and was characterised by NMRspectroscopy [48]. In contrast to trimethylamine (see Scheme 20), (CH3)2NCH2F is a fluoride-ion donor. The reaction of fluoromethyldimethylamine with tris(pentafluoroethyl)phosphine results in the
the
phosphorane
(E).
This
phosphorane
reacts
with
C2F5
P C2F5 +
Me Me
N
C2F5
F5C2
CH2F
H2C
P
F5C2
F
Me
F5C2
Me
F5C2
us
C2F5
second
molecule
cr
fluoromethyldimethylamine to give salt (G) (Scheme 26). [48]
a
ip t
formation of the corresponding phosphoranide salt (D) which undergoes rapid conversion to
N
C2F5 P
F5C2
E
an NMe2
F
M
C2F5 F Me2 N P
F5C2
F5C2
te
d
G
+
C2F5 F NMe2 H2C P F
NMe2
F
D
F5C2
of
N
Me Me
N CH2F
Me Me
F
Scheme 26
Ac ce p
In solution, the zwitter-ion (G) undergoes a 1,3-methyl shift to the thermodynamically more stable zwitter-ionic salt (H) (Scheme 27). The subsequent reaction with the strong base P(CH3)3 gives rise to the formation of the zwitterionic phosphonium phosphate (Scheme 27) [48]. F5C2 F5C2
F
P
F Me2 N
C2F5
G
NMe2
RT
CH2 Cl2
F5C2 F5C2
F P
F Me N
C2F5
H
NMe3
PMe 3
F5C2
- NMe3
F5C2
F P
F Me N
PMe3
C2F5
Scheme 27 The advantage of (RF)3PF2 compared to PF5 is not only its high Lewis acidity (compare Table 2). In contrast to PF5, (C2F5)3PF2 represents a liquid (b.p. 91-92 °C) which can be easily handled using a syringe. [49]. These properties make tris(pentafluoroethyl)difluorophosphorane attractive for the application as a Lewis acid catalyst. (C2F5)3PF2 was tested as Page 16 of 22
17
a catalyst in various reactions: cyano-silylation of aldehydes and ketones (Scheme 28), Strecker reaction (Scheme 29) and Diels-Alder reaction (Scheme 30) [50].
(C 2 F5) 3PF2
C6H5C(O)CH3 + (CH3)3SiCN
CN C6H5COSi(CH3)3
(cat.)
R.T., 5 min
C6H5C(O)H + (CH3)3SiCN
(C 2F5 )3 PF2 (cat.) R.T., 10 min
Yield: 91%
ip t
CH3
C6H5CH(CN)OSi(CH 3)3
(C 2 F5) 3PF2 (cat.) R.T., 10 min
p CH3OC6H5CH(CN)OSi(CH 3)3 Yield: 94%
us
p CH3OC6H5C(O)H + (CH3)3SiCN
cr
Yield: nearly quantitative
Catalyst loading below 1 mol%
R1
C
+
R
3
4
NH R + (CH3)3SiCN
R2
M
O
an
Scheme 28: Cyano-silylation of aldehydes and ketones
(C 2F5 )3 PF2 (cat.) R.T., 5 min
R1 R
CN
2
N
R3 R4
R 2 = H, CH 3 ; R3 = H, CH 3
Catalyst loading: 0.1 - 1 mol%
te
R 4 = C4H9, C6H5, p-ClC6H4
d
R 1 = C4H9, C6H5, p-CH3OC6H4, p-NO2C6H4, C3H3O
Ac ce p
Scheme 29: Strecker reaction
+
(C 2F5)3PF2 (cat)
+
CH 2 Cl2, 0°C, 15 min
O
Catalyst loading: 1 mol%
O
O
4-acetyl-2-methyl4-acetyl-1-methylcyclohex-1-en cyclohex-1-en Yield: 75% O
+
+
(C2F5)3PF2 (cat) Solvent free, -7°C, 2h
O
endoCatalyst loading: 0.14 mol%
exoO
2-acetyl-bicyclo[2.2.1]hept-5-ene Yield: 67%
Scheme 30: Diels-Alder reaction Page 17 of 22
18
The examples above demonstrate the activity of (C2F5)3PF2 as a Lewis acid catalyst. Even a low catalyst loading of 0.1 to 1 mol% of tris(pentafluoroethyl)difluorophosphorane yields in a quantitative conversion at room temperature or below [50]. Tris(pentafluoroethyl)difluoro-
ip t
phosphorane, (C2F5)3PF2, can be immobilized on activated carbon or graphite in order to generate a solid Lewis acid catalyst (see Fig. 3) which can be regenerated after use. The
cr
example below demonstrates the use of this catalyst in a Strecker synthesis (Scheme 31)
M
an
us
[50].
H +
NH2 + (CH3)3SiCN
te
O
d
Fig. 3
(C2 F5 )3 PF2 on C (cat.) CH2 Cl2, R.T., 5 min
Ac ce p
Catalyst (18 wt% of (C2F5)3PF2 on coal) loading: 0.1 to 1 mol% calculation on (C2 F5 )3 PF2
N
H N H Yield: 97%
Scheme 31
The catalyst (18 wt% of (C2F5)3PF2 on activated coal; Scheme 31) can be regenerated by filtration and used again with only a minimal loss of activity [50]. The acidity of bis(trifluoromethyl)phosphinic acid, (CF3)2P(O)OH has been studied by
H.J. Emeleus et al. [51]. They have found that (CF3)2P(O)OH is a stronger Brønsted acid than HBr and sulfuric acid. The high acidity of bis(perfluoroalkyl)phosphinic acids, (RF)2P(O)OH, could be proven by studying their catalytic activity in the acylation of β–naphtol with acetic acid anhydride as a model reaction. (Scheme 32, Table 3) [52]
Page 18 of 22
19
OH + (CH3CO)2O
OC(O)CH3
(RF )2P(O)OH (Cat)
+ CH3C(O)OH
Scheme 32
Entry a)
Catalyst (mol %)
Time (min)
Solvent
1
(C2F5)2P(O)OH (1 %)
30
CH2Cl2
96
2
(C4F9)2P(O)OH (1 %)
30
CH2Cl2
97
3
CF3SO3H (1 %)
4
without catalyst
cr
us
an
M
30
30
Yield (%)
CH2Cl2
95
CH2Cl2
< 10%
The substrate (β-naphthol) was treated with Ac2O (1.2 equiv.) in the presence of the catalyst at room temperature.
te
d
a)
ip t
Table 3. Acylation of β–naphtol with (Ac)2O in the presence of Brønsted acid catalyst [52]
Ac ce p
These results demonstrate that bis(perfluoroalkyl)phosphinic acids, (RF)2P(O)OH, are valuable Brønsted acid catalysts with a catalytic activity comparable to that of trifluoromethane sulfonic acid, CF3SO2OH.
Conclusion
The tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, provide convenient access to broad variety of perfluoroalkyl-phosphorus compounds that are of interest for various applications: conducting salts, ionic liquids, Brønsted and Lewis acid catalysts.
Acknowledgement Authors thank Dr. P. Barthen (Heinrich-Heine University of Düsseldorf) for the preparation and testing of some metal-FAP salts. Page 19 of 22
20
References. [1] E. Tassel, J. Chem. Soc., Abstr. 58 (1890) 1052. [2] H. Moissan: Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, 138 (1904) 789.
ip t
[3] T. Nakajima and H. Groult (Eds.), Fluorinated Materials for Energy Conversion, ELSEVIER, 2005, p. 140. [5] H.J. Emeleus, J. D. Smit, J. Chem. Soc. (1959) 375-381.
cr
[4] F.W. Bennet, H.J. Emeleus, R.N. Haszeldine, J. Chem. Soc. (1953) 1565-1571.
us
[6] A.H. Cowley,T. A. Furtsch, D.S. Dierdorf, J. Chem. Soc. D, Chem. Comm. (1970) 523524. [8] W. Mahler, Inorg. Chem. 2 (1963) 230. [9] W. Mahler, Inorg. Chem. 18 (1979) 352.
an
[7] M. Görg, G.V. Röschenthaler, A.A. Kolomeitsev, J. Fluorine Chem. 79 (1996), 103-104.
M
[10] S. A. Hayes, R. J. F. Berger, B. Neumann, N. W. Mitzel, J. Bader, B. Hoge, Dalton Trans. 39 (2010) 5630–5636.
[11] U. Welz-Biermann, N. Ignatyev, M. Weiden, M. Schmidt, U. Heider, A. Miller, H. Willner,
d
P. Sartori, WO 2003/087113, Merck Patent GmbH, Darmstadt, Germany.
te
[12] N. Ignat’ev and P. Sartori, J. Fluorine Chem. 103 (2000) 57-61. [13] U. Heider, V. Hilarius, P. Sartori, N. Ignatiev, EP 1037896 B1; US 6,264,818;
Ac ce p
JP 3387059; WO 2000/21969, Merck Patent GmbH, Darmstadt, Germany. [14] L.M. Yagupolskii, V.N. Zavatskii, V.Y. Semenii, K.N. Bildinov, P.V. Serebrov, A.A. Goncharenko, A.A. Kirsanov, M.I. Lyapunov, N.G. Feshchenko, Chemical Abstracts, 89 (1978) 43766j, 666; Chemical Abstracts, 90 (1979) 87670y, 645. [15] V.Y. Semenii, V.A. Stepanov, N.V. Ignat'ev, G.G. Furin, L.M. Yagupolskii, Zh. Obschei. Khim. (Russ.), 55 (1985) 2716-2720. [16] N.V. Ignat’ev, H. Willner, P. Sartori, J. Fluorine Chem. 130 (2009) 1183-1191. [17] M. Schmidt, U. Heider, A. Kuehner, R. Oesten, M. Jungnitz, N. Ignat´ev, P. Sartori, J. Power Sources, 97-98 (2001) 557-560. [18] N. Ignatyev, M. Schmidt, A. Kühner, V. Hilarius, U. Heider, A. Kucheryna, P. Sartori, H. Willner, EP 1399453 B; WO 2003/002579; US 7,094,328, Merck Patent GmbH, Darmstadt, Germany. [19] N.V. Ignat’ev, U. Welz-Biermann, A. Kucheryna, G. Bissky, H. Willner, J. Fluorine. Chem. 126 (2005) 1150-1159. [20] U. Welz-Biermann, N. Ignatyev, M. Weiden, U. Heider, A. Kucheryna, H. Willner, Page 20 of 22
21
WO 2003/087110; US 7,202,379, Merck Patent GmbH, Darmstadt, Germany. [21] H. J. Emeleus, J. D. Smit, J. Chem Soc. (1959) 375-381. [22] T. Mahmood, J. M. Shreeve, Inorg. Chem, 25 (1986) 3128-3131. [23] T. Mahmood, J.-M. Bao, R.L. Kirchmeier, J. M. Shreeve, Inorg. Chem, 27 (1988) 29132916.
ip t
[24] S. Steinhauer, J. Bader, H.-G. Stammler, N. Ignat‘ev, B. Hoge, Angew. Chem. Int. Ed. 53 (2014) 5206 – 5209.
[25] N. Ignatiev, G. Bissky, H. Willner, WO 2008/092489; US 8,211,277; EP 2114965 B1,
cr
Merck Patent GmbH, Darmstadt, Germany.
[26] N. Ignatyev, W. Hierse, W. Wiebe, H. Willner, DE 10 2011 111 490 A1, Merck Patent
us
GmbH, Darmstadt, Germany.
[27] B. Hoge, J. Bader, N. Ignatyev, M. Schulte, WO 2012/041431, Merck Patent
an
GmbH, Darmstadt, Germany.
[28] J. Bader, N. Ignat'ev, B. Hoge, Inorg. Chem. (2014) DOI: 10.1021/ic500857b. [29] B. Hoge, J. Bader, N. Ignatyev, M. Schulte, W. Hierse, W. Wiebe, H. Willner,
M
WO 2012/048772, Merck Patent GmbH, Darmstadt, Germany. [30] N. Ignatyev, E. Aust, D. Bejan, H. Willner, WO 2010/009791 A1, EP 2303898 B1, Merck
d
Patent GmbH, Darmstadt, Germany.
[31] L. M. Yagupol'skii, N. V. Pavlenko, N. V. Ignat'ev, G. I. Matyushecheva, V. Ya. Semenii,
te
Zh. Obsch. Khim. 54 (1984) 334-339.
[32] N. V. Pavlenko, G. I. Matyushecheva, V. Y Semenii., L. M. Yagupol'skii, Zh. Obsch.
Ac ce p
Khim. 55 (1985) 1586-1590.
[33] R. G. Cavell, T. L. Charlton, W. Sim, J. Am. Chem. Soc. 93 (1971) 1130-1137. [34] N. Ignatyev, R. Friedrich, S. Schellenberg, K. Koppe, W. Frank, DE 10 2012 022441 A1, Merck Patent GmbH, Darmstadt, Germany. [35] N. Ignatyev, W. Wiebe, H. Willner, WO 2013/013766 A1, Merck Patent GmbH, Darmstadt, Germany.
[36] A. B. Burg, A. J. Sarkis, J. Am. Chem. Soc. 87 (1965) 238-241. [37] T. Mahmood, J. M. Bao, R. L. Kirchmeier, J. M. Shreeve, Inorg. Chem. 27 (1988) 29132916. [38] D. Bejan, V. Dinou, N. Ignat’ev, E. Bernhardt, H. Willner, Phosphorus, Sulfur and Silicon and Related Elements (2014), accepted. [39] N. V. Pavlenko, G. I. Matyushecheva, V. Y. Semenii, L. M. Yagupol’skii, J. General Chem. USSR, 55 (1985) 1586-1590. [40] N. Ignatiev, U. Weltz-Biermann, M. Heckmeier, G. Bissky, H. Willner, WO 2006/128563; US 8,106,217; EP 1888605 B1, Merck Patent GmbH, Darmstadt, Germany. Page 21 of 22
22
[41] D. Bejan, H. Willner, N. Ignatiev, C.W. Lehmann, Inorg. Chem. 47 (2008) 9085-9089. [42] J.F. Foropoulos, D.D. DesMarteau, Inorg. Chem. 23 (1984) 3720-3723. [43] D. Bejan, Dissertation, University of Wuppertal (2010), Germany. [44] D. Bejan, N.V. Ignat´ev, H. Willner, J. Fluorine Chem. 131 (2010) 325-332. [45] N. Ignatyev, M. Schulte, J. Bader, B. Hoge, WO 2011/072810 A1, Merck Patent GmbH, [46] O. Shyshkov, Dissertation, Bremen University (2006), Germany.
ip t
Darmstadt, Germany.
Fluorine Chemistry, Paris, 2013.
cr
[47] N.V. Ignat’ev, A. Miller, H. Willner, Oral Presentation, 17th European Symposium on [48] N. Allefeld, B. Neumann, H.-G. Stammler, G.-V. Röschenthaler, N. Ignat’ev, B. Hoge,
us
Chemistry - A European Journal, 20 (2014) 7736-7745.
[49] N. Ignatyev, D. Dejan, H. Willner, WO 2010/012359 A1, Merck Patent GmbH,
an
Darmstadt, Germany.
[50] N. Ignat’ev, V. Pichugina-Bernhardt, B. Bittner, F. Schlüter, E. Bernhardt, H. Willner, Oral presentation, 21th Winter Fluorine Conference, Florida, 2013.
M
[51] R.H. Haszeldine, R.C. Paul, J. Chem. Soc. (1955) 563-574. [52] N.V. Ignat'ev, D. Bejan, H. Willner, Chimica Oggi/Chemistry Today. 29 (2011) N 5, 32-
d
34.
Ac ce p
te
Highlights
1. The phosphoranes, (RF)3PF2, are produced by means of Electrochemical Fluorination. 2. Li[(C2F5)3PF3] possesses an advantageous properties. 3. Bis(perfluoroalkyl)phosphinic acids are efficient Brønsted acid catalysts. 4. The tris(perfluoroalkyl)difluorophosphoranes, (RF)3PF2, are useful Lewis acid catalysts. 5. The phosphoranes, (RF)3PF2, are useful material to produce variety of chemicals.
Page 22 of 22