On the reaction of perfluoro-aza-propene with oxygen difluoride and fluorination of bis(trifluoromethyl)-hydroxylamine

Article abstract of DOI:10.1016/S0022-1139(99)00135-9

The reactions of perfluoro-aza-propene CF₃NCF₂ with an excess of oxygen difluoride OF₂ and the fluorination of bis(trifluoromethyl)-hydroxylamine (CF₃)₂NOH are discussed. Additionally, we present density functional calculations (B3LYP/6-31G*) on bis(trifluoromethyl)-hypofluorite (CF₃)₂NOF, which clearly show that this molecule is a model substance for the discussion of negative hyperconjugation.

Full text of DOI:10.1016/S0022-1139(99)00135-9

Journal of Fluorine Chemistry 99 (1999) 145±149
On the reaction of per¯uoro-aza-propene with oxygen di¯uoride
and ¯uorination of bis(tri¯uoromethyl)-hydroxylamine
Rolf Minkwitz*, Stefan Reinemann, Ralf Ludwig1
È
Dortmund, Fachbereich Chemie der Universitat, Anorganische Chemie, Otto-Hahn Str. 6, 44227 Dortmund, Germany
Received 17 November 1998; accepted 14 June 1999
Abstract
The reactions of per¯uoro-aza-propene CF3NCF2 with an excess of oxygen di¯uoride OF2 and the ¯uorination of bis(tri¯uoromethyl)-
hydroxylamine (CF3)2NOH are discussed. Additionally, we present density functional calculations (B3LYP/6-31G*) on bis(tri¯uor-
omethyl)-hypo¯uorite (CF3)2NOF, which clearly show that this molecule is a model substance for the discussion of negative
hyperconjugation. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Bis(tri¯uormethyl)-amino-hypo¯uorite; Fluorine; Fluorination of per¯uoro (2,4-dimethyl-3-oxa-2,4-diazapentane); Density functional calcula-
tions; NBO analysis
1. Introduction
ꢀCF3†2NOF ‡ CF3NCF2!ꢀCF3†2NONꢀCF3†
(6)
2
Only in few cases are addition reactions of OF2 described
in the literature [1±6]. Recently, we have studied the reaction
of OF2 with per¯uoroacetone (CF3)2CO, which resulted in
the formation of per¯uoro(isopropyl)-tri¯uoroacetyl-perox-
ide (CF3)2C(F)OOC(O)CF3 (1) [7].
‡
The preparation of Cs NꢀCF3† from CF3NCF2 and CsF
2
in (3) is already described in the literature [11]. It makes no
difference whether monomeric CF3NCF2 or the dimeric
form (CF3)2NC(F)==NCF3 is used as starting reagent.
In contradiction to CF3OOF, which is preparable by
reacting an excess of OF2 with carbonyl ¯uoride COF2
and ¯uorination of CF3OOH [12,13], the isolation of
(CF3)2NOF has not yet been reported.
78ꢁC=‡25ꢁC
CsF
2ꢀCF3†CO ‡ OF2
!
ꢀCF3†2CꢀF†OOCꢀO†CF3 ‡CF4
(1)
Inthefollowingreport,wediscussthereactionsofCF3NCF2
with an excess of OF2 and the ¯uorination of (CF3)2NOH. In
orderto®ndahighquality explanationfortheformationofthe
observed reaction products we started intensive calculations,
ab initio (HF/6-31G*)-calculations as well as density func-
tional calculations on the B3LYP/6±31G*-level.
We have also reported a new synthesis of perfluoro(2,4-
dimethyl-3-oxa-2,4-diazapentane) (CF3)2NON(CF3)2 and
its characterisation by electron distribution, photo electron
spectra and density functional calculations (2) [8±10]
78ꢁC 25ꢁC
2CF3NCF2 ‡ OF2
!
ꢀCF3†2NONꢀCF3†
(2)
2
CsF
The reaction is catalysed by small amounts of CsF and can
be explained by an ionic mechanism with the intermediate
formation of (CF3)2NOF (3±6).
2. Experimental
(CF3)2NOH is prepared according to the literature method
from NH3 and CF3NO [14]. OF2 was provided by Prof.
È
Sartori, Universitat Duisburg.
‡
CF3NCF2 ‡ CsF!Cs NꢀCF3†
(3)
(4)
2
‡
OF2 ‡ Cs NꢀCF3† !ꢀCF3†2NOF ‡ CsF
ꢀCF3†2NOF ‡ Cs NꢀCF3† !ꢀCF3†2NONꢀCF3† ‡ CsF
2
2.1. Reaction of CF3NCF2 with OF2
‡
2
2
(5)
0.1 mmol CsF were placed in a 30 ml stainless steel
reactor and treated with elemental ¯uorine (250 mbar). After
removing the ¯uorine 1 mmol CF3NCF2 and 5 mmol OF2
*Corresponding author. Fax: +49-231-7553810
1Physikalische Chemie
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 3 5 - 9

146
R. Minkwitz et al. / Journal of Fluorine Chemistry 99 (1999) 145±149
were condensed. During 48 h the reaction mixture is
warmed from 788C to 258C in 108C steps. At 1608C
OF2 and CF4 were condensed into a cooled glass ampoule
( 1968C). The remaining reaction products are trapped by
fractional condensation. They were characterised by 19F-
NMR, 13C-NMR and vibrational spectra.
3. Results and Discussion
The reaction of CF3NCF2 with an excess of OF2 in the
temperature range of 78 to +258C results in the formation of
(CF3)2NON(CF3)2, CF3NO and CF4 (7). CF3NO and CF4 can
be regarded as direct decomposition products of the inter-
mediate (CF3)2NOF. CF3NO and CF4 cannot be detected if
CF3NCF2 and OF2 are used in a ratio of 2 : 1 [8,9].
Yields (wt.%): (CF3)2NON(CF3)2: 77; CF3NO: 12; CF4:
11.
2.2. Reaction of (CF3)2NOH with F2
The ¯uorination of (CF3)2NOH and F2 in a ratio of 1 : 1
occurs above 258C. Hydrogen ¯uoride is detected by IR
spectra in gaseous phase. However no hints for isolated
(CF3)2NOF are obtained. (CF3)2NF, CF3NF2, F2CO,
CF3OF, CF3NO and CF4 are identi®ed as reaction products
by 19F-NMR, 13C-NMR and vibrational spectra.
At 788C a ®vefold excess of ¯uorine is needed to
abstract HF. The products are identical with those for the
1 : 1 reaction.
The ¯uorination experiments lead to the same basic
interpretation as the reaction of CF3NCF2 with OF2.
(CF3)2NOF is formed intermediately, but its stability
does not allow isolation and it decomposes into CF3NO
and CF4.
2.2.1. (CF3)2NOH: F2 1:1
0.1 mmol CsF were placed in a 30 ml stainless steel
reactor and treated with elemental ¯uorine (250 mbar).
After removing the ¯uorine 1 mmol (CF3)2NOH, 2 ml
CCl3F and 1 mmol F2 were condensed. During 24 h the
reaction mixture is warmed slowly from 788C to 258C.
After separating the reaction products they were charac-
terised as described above.
2.2.2. (CF3)2NOH: F2 1:5
0.1 mmol CsF were placed in a 30 ml stainless steel
reactor and treated with elemental ¯uorine (250 mbar).
After removing the ¯uorine 1 mmol (CF3)2NOH, 2 ml
CCl3F and 5 mmol F2 were condensed. After 24 h at
788C the excess of ¯uorine was removed by pumping
at 1968C. After separating the reaction products they were
characterised as described above.
Raman Spectra (Jobin-Yvon T 64000, Ar+-Laser from
Spectra Physics, ꢀ = 514.5 nm).
IR-spectra (Bruker IFS 113v).
3.1. Calculations
In order to prove this thesis we started intensive calcula-
tions on (CF3)2NOF. In Table 1 the geometry parameters
of (CF3)2NOF (HF/6-311G* und B3LYP/6-311G*) are
presented. The two methods lead to extremely different
values, especially the oxygen±¯uorine bond length deviates
by nearly 20 pm. (rOF HF 6-311G* 137.9 pm, rOF B3LYP 6-311G*
NMR-spectra (Bruker DPX 300) at 608C in CFCl3
with CFCl3 as external standard.
157.7 pm). The density functional method leads to an
O±F-distance (157.7 pm), which is not explainable by
a classic O±F single bond. In dioxygen di¯uoride FOOF,
a model molecule for negative hyperconjugation, a similar
2.2.3. Ab initio calculations
The structure of (CF3)2NOF was calculated ab initio with
the Hartree±Fock-Method (HF) [15] and density functional
with the Becke3LYP-Method [16,17] by using the GAUS-
SIAN-94-Package [18]. All calculations were run with 6±
31G* basis sets. B3LYP presents a Density Functional
Theory (DFT) method using hybrid functionals. For
B3LYP the Becke-3-parameter gradient corrected exchange
functional is combined with the gradient-corrected correla-
tion LYP functional by Lee, Yang and Parr. The DFT
method is used because it achieves signi®cantly greater
accuracy than Hartree±Fock theory by including some of
the effects of electron correlation.
Table 1
Calculated bond lengths (pm) and angles (8) for (CF3)2NOF
HF 6-311G*
B3LYP 6-311G*
rOF
137.9
157.7
rNO
rNC
rCF
132.4
128.3
143.6/144.2
129.6±130.8
108.8
147.8/147.0
131.7±133.4
108.6
FON
ONC
CNC
114.2
117.1
120.7
120.2

R. Minkwitz et al. / Journal of Fluorine Chemistry 99 (1999) 145±149
147
Table 2
Comparison of experimental [19] and calculated bond lengths (pm) and angles (8) for O2F2
Experimental
HF/6-31G*
B3LYP/6-31G*
MP2/6-31G*
MP4SDQ/6-31G*
rOF
rOO
157.5
121.7
109.5
87.5
136.7
131.0
105.8
84.1
149.6
126.5
108.3
86.7
149.3
129.2
106.9
85.7
147.0
132.0
106.2
85.7
FOO
FOOF
Table 3
Calculated bond lengths (pm) and angles (8) for (CF3)2NOF (B3LYP/6-311G*) and comparative compounds
(CF3)2NOF
CF3OF [24]
142.1
(CF3)2NONO [22]
N2O5 [23]
(CF3)2NOCH3 [21]
rOF
157.6
128.3
rNO
157.2
141.0
115.6
142.6
107.6
112.1
121.5
149.2
118.3
142.4
rNC
147.3
108.6
117.1
120.2
142.9
109.4
108.1
118.0
NOF
ONC
CNC
O±F-distance of 158.2 pm was detected by electron
distribution [19].
the results of the Hartree±Fock-calculations do not corre-
spond with the experimentally determined bond lengths of a
hyperconjugated system. The results of the B3LYP-calcula-
tions are comparable with the MP2-or MP4SDQ-results,
which harmonise with the experimental values [20].
Obviously correlation effects have to be considered in
calculations on a hyperconjugated molecule.
In Table 2 the structural parameters for O2F2, obtained
from the B3LYP 6-31G*-calculations, are compared with
those previously determined by Oberhammer and Mack
from HF/6-31G*, MP2/6-31G* and MP4SDQ/6-31G* cal-
culations [20]. The simple example of O2F2 makes clear that
Fig. 1. Calculated structure of (CF3)2NOF (B3LYP 6-311G*). (a) View on the C±N±C-plane; (b) view along the C±N-bond.

148
R. Minkwitz et al. / Journal of Fluorine Chemistry 99 (1999) 145±149
TheB3LYP-calculationsalso®ttheexperimentalvaluesfor
which leads to an O±F-bond weakening and a very long
O±F-distance of 157.7 pm. The corresponding NBOs are
shown in Fig. 2. A transmittance of electron density from a
bonding orbital or a lone pair into a non-bonding orbital is
called `negative hyperconjugation'.
known (CF3)2NOX-compounds e.g. (CF3)2NON(CF3)2
[8,10]. It has to be concluded that this method leads to
authentic bond lengths and angles for the target molecule
(CF3)2NOF.
In Table 3 the B3LYP-structural parameters for (CF3)2-
NOF are compared with those of CF3OF, (CF3)2NONO,
N2O5 and (CF3)2NOCH3. The calculated structure of
(CF3)2NOF is presented in Fig. 1.
To summarise it has to be pointed out that from the
density functional calculations we obtained a good explana-
tion for experimental results. It was shown that (CF3)2NOF
is model substance for negative hyperconjugation. A pos-
sible method for the isolation of this molecule can be seen in
the low temperature radical combination of (CF3)2NOꢂ
and Fꢂ radicals. O2F2 is synthesised on a similar way
[30±32].
The C±N±O-and C±N±C-angles of 117.1 and 120.78
indicate a nearly planar arrangement in (CF3)2NOF. For
(CF3)2NOCH3 a C±N±O-angle of 108.18 was determined by
electron distribution [21]. The comparison with
(CF3)2NONO [22] and N2O5 [23] shows, that in (CF3)2NOF
a short N±O-bond of 128.3 pm is present. These facts
suggest signi®cant contribution of a `double bond no bond'
component shown by the mesomeric forms in (8) and give
an explanation for the observed formation of CF3N==O and
CF4 in the discussed experiments.
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