1-Fluoro-2,4,6-trichloro-1,3,5-triazinium tetrafluoroborate: Synthesis, characterization, and ability to effect electrophilic aromatic substitution

Article abstract of DOI:10.1016/j.jfluchem.2003.08.010

The synthesis of 1-fluoro-2,4,6-trichloro-1,3,5-triazinium tetrafluoroborate, [(ClCN)₃F]⁺[BF₄] ^- (1), from (ClCN)₃, BF₃ and F₂ is reported. Compound 1 was shown to be a useful reagent for the quantitative fluorination of aromatic substrates such as benzene, chlorobenzene, nitrobenzene, and methoxybenzene.

Full text of DOI:10.1016/j.jfluchem.2003.08.010

Journal of Fluorine Chemistry 124 (2003) 229–232
Short communication
1-Fluoro-2,4,6-trichloro-1,3,5-triazinium tetrafluoroborate: synthesis,
characterization, and ability to effect electrophilic aromatic substitution
a,1
R. Eric Banks , Mohamed K. Besheesh , Wolfgang Fraenk , Thomas M. Klapotke
a
b
b,*
¨
aDepartment of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, UK
bDepartment of Chemistry, Ludwig-Maximilians University of Munich, Butenandtstrasse 5–13 (D), D-81377 Munich, Germany
Received 11 July 2003; received in revised form 27 August 2003; accepted 30 August 2003
Abstract
The synthesis of 1-fluoro-2,4,6-trichloro-1,3,5-triazinium tetrafluoroborate, [(ClCN)3F]þ[BF4]À (1), from (ClCN)3, BF3 and F2 is reported.
Compound 1 was shown to be a useful reagent for the quantitative fluorination of aromatic substrates such as benzene, chlorobenzene,
nitrobenzene, and methoxybenzene.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Electrophilic fluorination; N–F reagents; Fluorine; Fluoroaromatics; Trichlorotriazinium salts
1. Introduction
batch method employed originally [1,2] to convert cyanuric
chloride to the corresponding N-fluorotriazinium hexa-
fluoroarsenate, [(ClCN)3F]þ[AsF6]À (2), boron trifluoride
being used in place of arsenic pentafluoride to provide
the anionic moiety (Scheme 1). The tetrafluoroborate 1 is
a brilliant white, moisture-sensitive solid (hence best
manipulated in a dry-box under Ar or N2) which decom-
poses at 153–155 8C when heated progressively in a melt-
ing point apparatus. Like its hexafluoroarsenate analogue
[1], 1 is insoluble in CFCl3 but readily soluble in liquified
SO2; it is also quite soluble in acetonitrile and in nitro-
methane but reacts slowly with the former at room tem-
perature, hence our interest in the nitroalkane as a solvent
(see Section 2.1).
In view of the disorder problem encountered during an
X-ray diffraction study of the hexafluoroarsenate 2 [1,2],
no attempt has been made to undertake similar work with
[(ClCN)3F]þ[BF4]À (1) (complete reliable experimental
X-ray data are available for only one N-fluoro-sym-triazi-
nium salt, namely 1-fluoro-2,4,6-trimethoxy-1,3,5-triazi-
nium hexafluoroantimonate [7]). The identity of this new
salt rests, therefore, on its mode of synthesis, a good
elemental analysis (C, B, N), NMR data, and its ability to
act as a fluorine-plus (Fþ) source.
Although details of the synthesis and characterization of
the first N-fluoro-sym-triazinium salts were published in the
early 1990s [1–4] no information appears to have been
reported concerning their use as electrophilic fluorinating
agents [5].2 To provide a quantitative scale for the oxidizing
strength of these oxidative fluorinators we previously com-
puted the FPDE values (fluorine-plus detachment energy
values) quantum-chemically at the DFT B3LYP hybrid level
of theory at a 6-31G(d,p) basis [6]. Recently [6] we also
indicated the potential of 1-fluoro-2,4,6-trichloro-1,3,5-tria-
zinium tetrafluoroborate, [(ClCN)3F]þ[BF4]À (1), as a
reagent for effecting electrophilic ring-fluorination of car-
boaromatic compounds. The experimentation involved, now
detailed here, provides (to the best of our knowledge) the
first examples of C–F bond synthesis via the agency of an
N-fluorotriazinium salt.
2. Results and discussion
1-Fluoro-2,4,6-trichloro-1,3,5-triazinium tetrafluoroborate
(1)was preparedin excellentyieldbyadaptingthesmall-scale
The 19F NMR spectrum of 1, measured at 25 8C in SO2,
comprises an absorption at d(CFCl3) –141.7 (assignable to
BF4À) and one of one-quarter relative intensity shifted to
lower field at d(CFCl3) þ18.3, which is reasonable for the
cationic þNF moiety in 1. The corresponding d values for
a solution of 1 in CD3CN at À20 8C are À143.6 and
* Corresponding author. Tel.: þ49-89-2180-77491;
fax: þ49-89-2180-77492.
¨
E-mail address: tmk@cup.uni-muenchen.de (T.M. Klapotke).
1 Fax: þ44-161-285-1031.
2 For recent in-depth reviews of electrophilic fluorinating agents of the
N–F class, see [5].
0022-1139/$ – see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2003.08.010

230
R.E. Banks et al. / Journal of Fluorine Chemistry 124 (2003) 229–232
Cl
Cl
N
N
N
N
CFCl
3
-
1
1 F
1 BF
3
BF
+
+
2
4
-196 to 20 ˚C
+
Cl
Cl
Cl
Cl
N
N
F
(1)
Scheme 1.
þ17.3 ppm (Table 1). The previously reported value of
À45.2 ppm for the hexafluoroarsenate [(ClCN)3F]þ[AsF6]À
(2) [1] is presumably incorrect due to decomposition of the
sample.
with 1, increased noticeably in overall intensity during the
next 2 h at the expense of the þNF absorption at d15.3, and
the initially colourless solution turned yellow.
Nitromethane (bp 101 8C), which has not to our know-
ledge featured previously in work with N–F reagents,
proved to be a satisfactory replacement for acetonitrile,
and reactions between 1 and our chosen aromatic sub-
strates were effected in this solvent, even at elevated
temperatures, without the complication experienced when
using CD3CN. Hence diversion of 1 down unwanted reac-
tion channels was eliminated (no attempt has yet been
made to determine the chemistry involved in attack of 1
on acetonitrile (possibly the formation of [CH3C ¼ NF]þ
occurs initially); the onset of reaction occurs at tem-
peratures above ca. 0 8C (determined using temperature-
programmed 19F NMR)).
2.1. Electrophilic fluorination of aromatics with
1-fluoro-2,4,6-trichloro-1,3,5-triazinium
tetrafluoroborate (1)
Small-scale individual (i.e. no competitive runs were
performed) reactions between 1 and benzene, methoxyben-
zene, chlorobenzene, and nitrobenzene were carried out, the
objective being simply to estimate the N–F reagent’s Fþ
transfer capability. Reaction mixtures were monitored using
19F NMR and by checking their abilities to liberate iodine
from aqueous potassium iodide.
Initially, NMR-scale experiments were conducted in acet-
onitrile-d3 with equimolar reactant ratios. Relative reaction
rates (C6H5OCH3 @ C6H6 @ C6H5NO2) and orientation of
attack (C6H5OCH3 ! 2:1 mixture of 4 and 2FC6H4OCH3;
C6H5NO2 ! 3FC6H4NO2) consistent with electrophilic
aromatic fluorination were established, but the 19F NMR
spectra of all the reaction mixtures contained a substantial
number of unassignable absorptions in addition to those
associated with ‘‘Fþ’’ attack on the aromatics under study.
This problem was resolved by dissolving the N-fluoro-
triazinium salt 1 in CD3CN at room temperature and mea-
suring the 19F NMR spectrum (probe temperature 27 8C) of
the solution immediately: the spectrum contained two
major peaks assignable to 1 [d(CFCl3) 15.3 ppm (þNF),
À146.7 ppm (BF4À)] and minor absorptions in the range
À1.0 to À110 ppm; the latter, which corresponded with
absorptions found in the spectra of reaction mixtures
involved in the fluorination of benzene and its derivatives
The results achieved using dilute solutions of 1 and an
aromatic subtrate (6.8 mmol lÀ1 of 1 and of C6H5X (X ¼ H,
OCH3, Cl, NO2)) in nitromethane reinforced our conclusion
based on the experiments conducted using acetonitrile as
solvent that it is a more powerful electrophilic fluoromating
agent than most of the ‘‘Fþ’’ equivalents of the N–F class
reported to date. In this respect, while 1 falls well short of the
À
inorganic N–F reagents NF4þXÀ (XÀ ¼ BF4À, AsF6
,
SbF6À) and FN2þAsF6À, which electrophilically fluorinate
nitrobenzene in anhydrous HF at À78 8C [8,9] and methane
in HF or pyridine-HF at room temperature [10], it appears to
be at least as reactive as N-fluorobis(trifluoromethylsulfo-
nyl)imide (3) and N-fluoropentachloropyridinium triflate
(4), hitherto judged to be the most powerful Fþ delivery
agents of the organic N–F class [11]. These two reagents
(3, 4) fluorinate benzene and its activated derivative
C6H5OCH3 under mild conditions, as does 1, but neither
has been demonstrated to attack deactivated benzenes under
non-forcing conditions (3 [12]: C6H5Cl/CDCl3, 22 8C, 24 h;
C6H5COCH3/CDCl3, 22 8C, 12 h; neat C6H5NO2, 22 8C,
12 h and 4 [13]: C6H5CO2CH3/CH2Cl2, 2 h, reflux tempera-
ture). By contrast, [(ClCN)3F]þ[BF4À] (1) attacks chloro-
benzene and (more slowly) nitrobenzene at ambient
temperature in nitromethane, consumption of the reagent
being complete (negative KI test) within 6 h at 70 8C in the
case of the chloro-aromatic (! a ca. 1:0.3:2 mixture of 2, 3,
and 4FC6H4Cl) and 90% (by 19F NMR) under the same
conditions with nitrobenzene (! a ca. 2:0.8 mixture of 3
Table 1
NMR data for [(ClCN)3F]þ[BF4]À (1)a
11B
13C
14N
19F
Reference
d (ppm)
d (ppm)
BF3ꢀOet2
þ1.0
TMS
MeNO2
À98b
CFCl3
þ173.4 (1)
À143.6 (4)
þ158.3 (2)
þ17.3 (1)
a CD3CN solution, À20 8C; relative intensities in parentheses.
b One resonance due to fast N–F exchange.

R.E. Banks et al. / Journal of Fluorine Chemistry 124 (2003) 229–232
231
F
+
. . .
Cl
Cl
Cl
F
NO2
NO2
(1)
(0.3)
(2)
C6H6
(2)
C6H5NO2
F
F
C6H5Cl
[(ClCN)3F]+[BF4]–
(1)
F
(0.8)
C6H5
O
CH3
CH3
O
CH3
F
O
F
(2)
(1)
F
Scheme 2. Electrophilic fluorination reactions of [(ClCN)3F]þ[BF4]À (1) in dry nitromethane. After ca. 6 h the consumption of reagent 1 was complete,
indicating a quantitative reaction. The relative molar ratios of the products formed are given in parentheses.
and 2FC6H4NO2) Since a swift exothermic reaction occurs
between methoxybenzene and 1 at room temperature, solu-
tions containing these reactants in CD3CN or CH3NO2 were
prepared at À20 8C (this precaution does not appear to have
been necessary in the cases of fluorination with 3 [12] or
4 [13]). When allowed to warm up to room temperature,
the colourless solutions turned yellow at about 0 8C then
became brownish-violet during the next 2 h, by which time 1
had been completely consumed (negative KI test) and
4 and 2FC6H4OCH3 were present in the ratio of 2:1
(Scheme 2).
3.2. Caution
The safe handling of elemental fluorine requires a high
level of expertise and special apparatus [14]. Only experi-
mentalists with appropriate know-how and equipment, plus
facilities for dealing with F2 and HF burns [14], should
attempt to prepare 1.
Care must be taken when using nitromethane to avoid
accidents arising from inhalation of its vapour, fire and
explosion [15].
3.3. Preparation of 1-fluoro-2,4,6-trichloro-1,3,5-
triazinium tetrafluoroborate (1)
3. Experimental
Several reactions were carried out on a 4–5 mmol scale. In
a typical experiment, boron trifluoride (0.271 g, 4.0 mmol)
and difluorine (0.151 g, 4.0 mmol) were condensed sepa-
rately, in vacuo, onto a frozen solution of cyanuric chloride
(0.738 g, 4.0 mmol) in trichlorofluoromethane (20 cm3)
contained in a pre-passivated (with F2) T316 stainless steel
autoclave (120 cm3) cooled to À196 8C (liq. N2). The
reaction vessel was sealed, warmed to room temperature
during 3 h, then stored at that point for 5 days (an excessive
time). Volatile material was removed from the reaction vessel
under vacuum, leaving a white residual solid that was shown
to be virtually pure 1-fluoro-2,4,6-trichloro-1,3,5-triazinium
3.1. General experimental procedures
Raman and NMR data for [(s-ClCN)3F]þ[BF4] (1) were
obtained using a Perkin-Elmer 2000 NIR FT instrument and
a Jeol Eclipse 400 spectrometer, respectively; 19NMR ana-
lysis of reaction mixtures produced via the interaction of 1
with benzene and its derivatives was performed with a
Bruker AC-200 machine (188.8 MHz). Elemental analyses
were performed with a Vario EL analyser (for C and N) and
by ICP (for B). The mp of 1 was determined using a Bu¨chi
B-540 apparatus.

232
R.E. Banks et al. / Journal of Fluorine Chemistry 124 (2003) 229–232
tetrafluoroborate (nc) (1; 1.10 g, 3.8 mmol, 95% yield), mp
(decomp.) 153–155 8C. Anal. Calcd. for C3BCl3F5N: C,
12.42; B, 3.72, N, 14.47: Found. C, 12.40; B, 3.58; N,
14.61%. 19F NMR (376.0 MHz, CD3CN at À20 8C; CFCl3
ref.): d 17.3 (s, 1F, NF), À143.6 (s, 4F, BF4). 13C NMR
(67.9 MHz, CD3CN at À20 8C; TMS): d 173.4 (s, C-1),
158.3 (s, C-3.5).
Acknowledgements
Financial support by the University of Munich (LMU),
UMIST and the Fonds der Chemischen Industrie are grate-
fully acknowledged. We thank Ms. Carmen Nowak for
preparing the diagrams and figures.
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1-fluoro-2,4,6-trichloro-1,3,5-triazinium
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