Fluorocarbanion chemistry. Tris(4-nitro-2,3,5,6-tetrafluorophenyl) methane and companions

Article abstract of DOI:10.1016/S0022-1139(99)00242-0

The titled compound (2) is prepared by oxidation of tris(4-amino-2,3,5,6-tetrafluorophenyl) methane with 98% H₂O₂ and trifluoroacetic anhydride. Compound 2, a strong carbon acid, forms long-persisting deep blue solutions of the anion

Full text of DOI:10.1016/S0022-1139(99)00242-0

Journal of Fluorine Chemistry 102 (2000) 185±188
Fluorocarbanion chemistry. Tris(4-nitro-2,3,5,6-tetra¯uorophenyl)
methane and companions
Robert Filler*, August E. Fiebig Jr., Braja K. Mandal
Department of Biological, Chemical, and Physical Sciences, Illinois Institute of Technology, Chicago, IL 60616-3793, USA
Received 28 June 1999; received in revised form 19 July 1999; accepted 16 August 1999
Dedicated to Professor Paul Tarrant on the occasion of his 85th birthday.
Abstract
The titled compound (2) is prepared by oxidation of tris(4-amino-2,3,5,6-tetra¯uorophenyl) methane with 98% H2O2 and tri¯uoroacetic
anhydride. Compound 2, a strong carbon acid, forms long-persisting deep blue solutions of the anion on reaction with alkalis, amines, and
alcohols. Oxidation of the anion of 2 with KMnO4 provides the corresponding triarylmethyl radical. Reaction of 2 with LiOCH3 in
methanol gives the stable blue lithium salt. The 4-cyano analog of 2 has also been prepared. # 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methane; Tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methide lithium salt; Tris(4-nitro-2,3,5,6-tetra¯uorophe-
nyl) methyl radical; Tris(4-cyano-2,3,5,6-tetra¯uorophenyl) methane
1. Introduction
acids. While tris(4-nitrophenyl) methane is prepared
directly, in low yield, from triphenylmethane and fuming
nitric acid, 2 is best synthesized from 1 in a three step
sequence (Scheme 1). Numerous attempts to prepare the
precursor tris(4-amino-2,3,5,6-tetra¯uorophenyl) methane
(4) by direct amination of 1 [7] gave inconsistent results,
with poor yields. We abandoned this approach and converted
1 to the hydrazino compound 3, which was reduced to 4 with
48% HI [8]. Oxidation of 4 with 98% H2O2 in tri¯uoroacetic
anhydride afforded 2 in 77% crude yield.
In an earlier paper [1] we reported the preparation and
ion-pair acidities of the carbon acid tris(penta¯uorophenyl)
methane (1) and its p-methyl and p-methoxy analogs. While
1 can be obtained by Friedel±Crafts alkylation of penta-
¯uorobenzene with CHCl3 [2], a more convenient route is
the reaction of tris(penta¯uorophenyl) methanol [3,4] with
PBr3 (Eq. (1)). Subsequent studies using dimsyl ion as base
[5,6] con®rmed the remarkable enhancement of equilibrium
acidities of triarylmethanes by poly¯uoroaryl substitution.
Generally, the replacement of each phenyl group by C6F5- or
XC6F4- increased the acidity by 5±6 pK units, resulting in
the of pKa values between 13.4 and 16.2.
HBr
ꢀC6F5†3COH ‡ PBr3 ! ‰ꢀC6F5†3Cd Brd‡Š ! ꢀC6F5†3CH
70%
1
(1)
2. Results and discussion
We now wish to report the synthesis and distinctive
chemistry of tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methane
(2), a much more acidic member of this series of carbon
* Corresponding author.
Scheme 1. Synthesis of tris(4-nitro-2,3,5,6-tetrafluorophenyl) methane.sc1
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 4 2 - 0

186
R. Filler et al. / Journal of Fluorine Chemistry 102 (2000) 185±188
While the large ®eld/inductive effect of C6F5- and XC6F4-
on a Perkin-Elmer 257 grating spectrometer or a Perkin-
Elmer 2000 FTIR spectrometer. Ultraviolet and visible
spectra were measured on a Perkin-Elmer Lambda-19 spec-
trometer. Mass spectra were recorded on a Varian CH7 mass
spectrometer. Electron spin resonance spectra were obtained
with a standard Varian X-band esr spectrometer with
100 KHz ®eld modulation.
groups causes the pronounced exaltation in acidities [5], the
added resonance effect of the nitro groups in the methide ion
of 2 contributes a major enhancement in acidity, though not
as great as might be anticipated. Previous studies [9]
demonstrated that the ®rst p-NO2 group introduced into
triphenylmethane sharply increases the acidity, while suc-
cessive substitutions of the second and third p-nitro groups
result in a marked attenuation, due to steric inhibition of
resonance and saturation effects. Thus, 2 reacts with NaOH
and amines to form long-persisting intense cobalt blue
solutions of the anion (Eq. (2)). Moreover, 2 also readily
protonates alcohols, including tri¯uoroethanol (pKa 11.3),
but not penta¯uorophenol (pKa 5.5). By comparison, tris(4-
nitrophenyl) methane (pKa 12.7), [9] reacts with NaOH to
form a blue solution of anion which fades in a short time. On
the basis of these observations and the previous studies, the
pKa of 2 is estimated to be in the range of 6±7. The
remarkable stability of the anion suggested that 2 might
form a stable free radical. Earlier, we developed a simple
method for generating stable triarylmethyl radicals directly
from triarylmethanes by alkaline permanganate oxidation of
the corresponding carbanion (Eq. (2)) [10].
3.1. Tris(pentafluorophenyl) methane (1)
Tris(penta¯uorophenyl) methanol (5.3 g, 0.0l mol) was
placed in a 100 ml three-necked ¯ask ®tted with a re¯ux
condenser and magnetic stirrer. Phosphorus tribromide
(2.7 g, 0.0l mol) was added slowly over a period of
15 min. The mixture was heated at 1308C and stirred
constantly for 3 h. After cooling, carbon tetrachloride and
water were added to the oily reaction mixture. The mixture
was stirred and the carbon tetrachloride layer was separated.
After drying over magnesium sulfate, the carbon tetrachlor-
ide layer was evaporated to dryness in vacuo. A small
amount of methanol was added to the dried mass to cause
a white solid to separate immediately. The solid was recrys-
tallized from methanol to give 3.6 g (70%) of pure white
product, m.p. l58±l59.58C. Anal. Calcd. for C19HF15: C,
44.38; H, 0.19; Found: C, 44.19; H, 0.44; IR 3000 cm 1; 1H
NMR d 6.20 (m).
3.2. Tris(4-hydrazino-2,3,5,6-tetrafluorophenyl) methane
(3) (nc)
(2)
The radical is extracted with oxygen-free toluene to form
a yellowish-brown solution. In this way, 2 is easily con-
verted to the radical 5, whose ESR spectrum is shown in
Fig. 1. Hyper®ne coupling constants for 5 and its non-
¯uorinated analog are listed in Table 1. The reaction of 2
with lithium methoxide in methanol gives the stable deep
blue lithium salt, lmax 608 nm (propylene carbonate).
Further studies on this salt will be published separately.
Finally, another interesting member of this series, tris(4-
cyano-2,3,5,6-tetra¯uorophenyl) methane (7) was prepared
in low yield by reaction of the 4-bromo analog (6) with
cuprous cyanide (Eq. (3)). Compound 7 behaves much like 2
in forming colored solutions (orange-brown) when treated
with weak bases.
A mixture of tris(penta¯uorophenyl) methane (8.04 g,
15.6 mmol), 7.14 g (15.9 mmol) of 99±100% hydrazine
hydrate and 40 ml of p-dioxane was heated under re¯ux
for 21.5 h and then poured into 100 ml of water. The
resulting mixture was extracted several times with ether
and the combined ethereal layers thoroughly washed with
water. The ethereal layer was then extracted with 3 N
hydrochloric acid and the combined acid layers were
washed with ether to remove any neutral material. The acid
layer was carefully made basic with sodium hydroxide
solution and then extracted with ether. The resulting ethereal
layer was washed with water, dried over anhydrous mag-
nesium sulfate, and evaporated in vacuo to give a dark oil
which solidi®ed upon treatment with benzene. The crude
solid was crystallized twice from benzene using Darco G-60
charcoal for decolorization to give large golden needles.
After drying in a vacuum desiccator, the material weighed
7.53 g (87.5%). This compound does not have a character-
istic melting point since temperatures of melting or decom-
position ranging from 728C to 1498C were obtained,
depending on the rate of heating; IR (nujol): 3346 and
3262 (w, N±H), 1658 (m, aromatic C=C), 1497 (s, aromatic
ring), 1467 (vs, aromatic ring), 1003 and 982 cm 1 (s, C±F);
1H NMR (CH3CN): d 3.88 [broad (17.5 Hz wide at half
height) s, 6, ±NH2), d 5.84 [broad (9 Hz wide at half height)
s, 3, NH), and d 6.11 (s, 1, CH); Accurate mass (by high
(3)
3. Experimental
General: All starting materials and solvents were of the
highest quality available. NMR spectra were measured on a
Varian HA-60 spectrometer. Infrared spectra were recorded

R. Filler et al. / Journal of Fluorine Chemistry 102 (2000) 185±188
187
Fig. 1. Electron spin resonance spectrum of the tris(4-nitro-2,3,5,6-tetrafluorophenyl) methyl radical.
resolution mass spectrometry): 550.076546. Calcd.:
550.07747.
3.4. Tris(4-nitro-2,3,5,6-tetrafluorophenyl) methane (2) (nc)
To a solution of 12 ml of tri¯uoroacetic anhydride in
40 ml of dichloromethane, was added dropwise, 5 ml of
98% hydrogen peroxide. (Caution! This powerful oxidant is
hazardous and should be handled with great care.) After the
exothermic reaction subsided, the mixture was re¯uxed for
40 min. A solution of 6.00 g (11.9 mmol) of tris(4-amino-
2,3,5,6-tetra¯uorophenyl) methane in 160 ml of dichloro-
methane was added to the re¯uxing oxidizing mixture over a
period of 2.7 h. The reaction mixture turned lime-green after
the start of the addition. 10 min after completion of the
addition, 4 ml of tri¯uoroacetic anhydride was added. After
an additional 2.6 h of re¯ux, 3 ml of 98% hydrogen per-
oxide, followed by 8 ml of tri¯uoroacetic anhydride was
added slowly to the re¯uxing mixture. The green reaction
mixture turned to a golden yellow. After an additional 3 h of
re¯ux, 3.5 ml of 98% hydrogen peroxide, followed by 10 ml
of tri¯uoroacetic anhydride was added dropwise to the
mixture and the solution was allowed to re¯ux under
nitrogen for 14.8 h.
3.3. Tris(4-amino-2,3,5,6-tetrafluorophenyl) methane (4)
from tris(4-hydrazino-2,3,5,6-tetrafluorophenyl) methane
The hydrazine reduction method of Birchall and
Haszeldine [8] was applied to this synthesis. A solution
of20.0 g(36.3 mmol)oftris(4-hydrazino-2,3,5,6-tetra¯uoro-
phenyl) methane in 136 ml of 48% hydroiodic acid was
re¯uxed with stirring for 44 h. The resulting solution was
poured into 300 ml of water to yield an oil which solidi®ed.
The solid was ®ltered off, washed with water, dried, and
crystallized from methylcyclohexane to yield 10.7 g
(58.2%) of product as light yellow leaves, m.p. 170±
1738C (lit. [7] m.p. 170±1728C); IR (CHCl3): 3484 (w,
N±H), 3392 (m, N±H), 1668 (s, N±H), 1606 (w, aromatic
C=C), 1517 (s, aromatic ring), 1503 (vs, aromatic ring),
1013 and 976 cm 1 (vw, C±F); 1H NMR (CDCl3): d 6.10 (s,
1, CH) and 4.00 (broad s, 6, NH2).
The mixture was then poured into 300 ml of water, the
layers were separated and the aqueous phase extracted three
times with dichloromethane. The combined organic phases
were washed three times with water, dried over anhydrous
magnesium sulfate, and evaporated in vacuo to a crude oil.
Trituration of the oil with hexane and hexane±dichloro-
methane mixtures in the cold gave 5.48 g (77.5%) of crude
yellow powder.
Table 1
Hyperfine coupling constants for the tris(4-nitrotetrafluorophenyl)- and
tris(4-nitrophenyl) methyl radicals
Position (atom)
(4±O2NC6F4)3C.
a (G)
(4±O2NC6H4)3C.
a (G)
para (N)
ortho (F or H)
meta (F or H)
0.55
1.85
1.35
0.69
2.52
1.14
A 1.00 g sample of this material was chromatographed on
a 18.5 Â 2 cm2 column (ca. 13 g) of Baker Analyzed silica

188
R. Filler et al. / Journal of Fluorine Chemistry 102 (2000) 185±188
gel powder No. 3405. The column was eluted with a 50:50
hexane±dichloromethane mixture, and the ®rst 300 ml of
eluted solvent were evaporated in vacuo to give 0.136 g
(13.6%) of material as a white powder, m.p. ca. 1758C. The
next 475 ml of eluted solvent yielded 0.321 g (32.1%) of
product as a slightly yellow solid, m.p. 186±1878C. Further
elution with 2 l of the mixed solvent yielded only traces of
dark sticky solids; FTIR(KBr): 2924 (C±H), l627 (m, aro-
matic C=C), 1556 (vs, aromatic ring or ±NO2),1500 (s,
aromatic ring or NO2), l358 (s, ±NO2), and 1030 (s) and
m.p. 136±1398C (lit. [11] m.p. 139±139.58C); IR (CHCl3):
1637 (w, aromatic C=C), 1488 (vs, aromatic ring), 1464 (m,
1
aromatic ring) 1013 and 962 cm (w, C±F); 1H NMR
(CDCl3): d 6.31 (s, C±H).
3.7. Tris(4-cyano-2,3,5,6-tetrafluorophenyl) methane (7)
(nc)
A mixture of 2.57 g (3.69 mmol) of tris(4-bromo-2,3,5,6-
tetra¯uorophenyl) methane, 1.32 g (14.8 mmol) of cuprous
cyanide, and 100 ml of puri®ed N,N-dimethylformamide
was re¯uxed for 10 h. A mixture of 6.5 g ferric chloride and
26 ml of 1 N hydrochloric acid was added to the blood red
reaction mixture, which was heated under re¯ux for an
additional hour. The reaction mixture was poured onto ca.
300 ml of crushed ice and the resulting solid was collected
by ®ltration. The crude solid was dissolved in benzene, the
solution was ®ltered and mixed with hexane to yield an
orange solid. Three crystallizations from benzene±hexane
(once using Darco G-60 charcoal) gave 0.209 g (10.6%) of
product, m.p. 197±2028C (dec.); IR (CHCl3): 2250 (w,
C>N), 1651 (w, aromatic C=C), 1496 (vs, aromatic ring)
1
1
1005 cm (s, C±F); H NMR (CDCl3): d 6.39 (s, C±H);
Anal. Calcd. for C19HF12N3O6: C, 38.34; H, 0.17; N, 7.06.
Found: C, 37.93; H, 0.41; N, 6.57.
3.5. Tris(4-nitrophenyl) methane
Triphenylmethane (60.0 g, 0.246 mol) was added in
small portions over 45 min to 600 ml of well-stirred 90%
fuming nitric acid maintained at ca. 408C. The yellow
reaction mixture was then stirred at 388C to 468C for
35 min and poured onto 2 l of crushed ice. The crude solid
was ®ltered off, washed with water, with saturated aqueous
sodium bicarbonate solution until the ®ltrate was basic, and
then with water until the ®ltrate was neutral. The dry solid
was crystallized four times from toluene (500±600 ml each
time) to give 21.5 g (23.1%) of product as yellow leaves,
m.p. 214±2168C (lit. m.p. 212±2138C [12]; IR (nujol): 3100
and 3067 (w, aromatic C±H), 1606 and 1596 (s, aromatic
1
1
and 1014 cm (w, C±F); H NMR (CDCl3): d 6.41 (s, C±
H); Anal. Calcd for C22HF12N3: C, 49.37; H, 0.19. Found:
C, 49.11; H, 0.44.
Acknowledgements
1
C=C), 1517 and 1348 (vs, NO2), and 842 and 747 cm
(s, NO2).
We thank Dr. Shrikant V. Kulkarni for conducting the
ESR studies on tris(4-nitro-2,3,5,6-tetra¯uorophenyl)
methyl radical.
3.6. Tris(4-bromo-2,3,5,6-tetrafluorophenyl) methane (6)
The Friedel-Crafts reaction developed by Beckert and
Lowe [2] was applied, with modi®cation, to this synthesis. A
150-ml Hoke stainless steel gas-sampling cylinder equipped
with a Hoke No. 343 valve was used as the reaction vessel.
The bomb was dried overnight in a small oven at ca. 908C
and cooled under a nitrogen stream. When cool, the bomb
was charged with 17.5 g (0.131 mol) of anhydrous alumi-
num chloride powder, 10.02 g (43.74 mmol) of 2,3,5,6-
tetra¯uorobromobenzene and 1.74 g (14.6 mmol) of chloro-
form. The valve threads were wrapped with Te¯on pipe joint
tape and the valve assembly was screwed down tight on the
bomb. The bomb was inverted several times to mix the
contents and then heated in an upright electric furnace at 84±
l358C for 7.3 h. After the reactor cooled, the hydrogen
chloride gas was bled off and the violet-colored solids were
broken up and extracted with approximately 1 l of carbon
tetrachloride. The carbon tetrachloride was evaporated in
vacuo to leave a crude yellow solid which was crystallized
twice (once using Darco G-60 charcoal) from hexane to give
2.27 g (22.4%) of product as a white microcrystalline solid,
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