Crystalline organization and bonding of N-perfluoroacylpyridiniumaminides

Article abstract of DOI:10.1016/S0022-1139(99)00047-0

N-Perfluoroacylpyridiniumaminides are stable crystalline solids characterized by a potentially useful set of properties. The molecular geometries resulting from the determination of the crystal structures of three members of the series suggest that, due to the presence of the perfluoroalky chains, the electron density on the formally negatively charged nitrogen of the ylide system is reduced. The N-N and the OC-N bonds in the systems are shortened and the whole molecules are stabilized as compared to non-fluorinated N-alkanoylpyridiniumaminides. The packing in the crystals can be qualitatively understood as a compromise between two distinct effects: the tendency to segregate, of the fluorocarbon groups on one hand and the bipyridinium blocks on the other, as in the case of 4b or c, the formation of weak interactions involving the pyridinium hydrogens and the most effective hydrogen bond accepting atoms (i.e. O and N).The latter effect leads to disorder of the perfluoroalkyl chains in 4a. In 4b and c, even at room temperature, a surprisingly ordered arrangement of the segregated perfluoropropyl chains is observed, showing a clear tendency to helicoidal conformation related to fluorine-fluorine repulsion.

Full text of DOI:10.1016/S0022-1139(99)00047-0

Journal of Fluorine Chemistry 97 (1999) 183±190
Crystalline organization and bonding of
N-per¯uoroacylpyridiniumaminides
Tullio Caronna, Eleonora Corradi, Stefano Valdo Meille1,
Barbara Novo, Giuseppe Resnati*, Giancarlo Sidoti
Dipartimento di Chimica, Politecnico, 7, via Mancinelli, I-20131 Milano, Italy
Received 7 December 1998; received in revised form 8 February 1999; accepted 8 February 1999
Dedicated to Prof. Yoshiro Kobayashi on the occasion of his 75th birthday
Abstract
N-Per¯uoroacylpyridiniumaminides are stable crystalline solids characterized by a potentially useful set of properties. The molecular
geometries resulting from the determination of the crystal structures of three members of the series suggest that, due to the presence of the
per¯uoroalky chains, the electron density on the formally negatively charged nitrogen of the ylide system is reduced. The N±N and the OC±
N bonds in the systems are shortened and the whole molecules are stabilized as compared to non-¯uorinated N-alkanoylpyridinium-
aminides. The packing in the crystals can be qualitatively understood as a compromise between two distinct effects:
1. the tendency to segregate, of the ¯uorocarbon groups on one hand and the bipyridinium blocks on the other, as in the case of 4b or c,
2. the formation of weak interactions involving the pyridinium hydrogens and the most effective hydrogen bond accepting atoms (i.e. O and
N).
The latter effect leads to disorder of the perfluoroalkyl chains in 4a. In 4b and c, even at room temperature, a surprisingly ordered arrangement
of the segregated perfluoropropyl chains is observed, showing a clear tendency to helicoidal conformation related to fluorine±fluorine
repulsion. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Pyridine; Fluorine; Crystal structure; Conformation; Bonding
1. Introduction
In this way, 4-cyano-N-(per¯uorobutanoyl)pyridinium-1-
aminide (4a) was isolated (50% yield) starting from 4-
cyanopyridine (1a) and similarly N,N0-bis(per¯uorobuta-
noyl)-4,40-bipyridine-1,10-diium-1,10diaminide (4b) and
N, N0- bis(per¯uorobutanoyl)-(E)-1, 2-(dipyridinium-1, 10-
diaminide-4,40-diyl)ethene (4c) were formed (Scheme 2)
starting from 4,40-bipyridine (1b) and (E)-1,2-(4,40-dipyr-
idyl)ethene (1c). For the latter compounds containing the
bipyridinium motif applications can be envisaged as elec-
trochromic displays [10], as agrochemicals [11], as electron
acceptors [12] or as electron transfer catalysts [13].
In general, per¯uoroalkyl chains are known to impart
unique physical and chemical properties to molecules.
Electronic and steric parameters of functionalities are deeply
in¯uenced by nearby per¯uoroalkyl chains resulting in
signi®cant changes of the properties of the individual mole-
cules (electron distribution, conformation) and of the bulk
material (thermal and chemical stability, crystallinity, sur-
face tension, water and oil resistance) [14]. For these
reasons, we thought that the presence of per¯uoroalkyl
As a part of our studies on the reactivity of per¯uoro-cis-
2,3-dialkyloxaziridines [1]2[2±9], we observed that when a
wide variety of pyridine derivatives 1 are reacted with
per¯uoro-cis-2-n-butyl-3-n-propyloxaziridine (2), a mixture
of corresponding N-oxides 3 and N-(per¯uorobutanoyl)pyr-
idinium-1-aminides 4 is produced (Scheme 1) [4,5]. The
formation of these products can be rationalized by consider-
ing that pyridine derivatives 1 behave as nucleophiles with
the oxaziridine 2 and attack either the oxygen or the nitrogen
atom of the three-membered ring. Thus the oxaziridine 2
works as either oxygenating or aminating agent to give the
N-oxides 3 and the N-aminides 4, respectively.
*Corresponding author. Tel.: +39-02-239-93032; fax: +39-02-239-
93080; e-mail: fluorine@dept.chem.polimi.it
1Co-corresponding author
2Perfluoro cis-2,3-dialkyloxaziridine are powerful yet selective oxidiz-
ing agents and can also work as aminating agents: see [1] and successive
papers.
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 0 4 7 - 0

184
T. Caronna et al. / Journal of Fluorine Chemistry 97 (1999) 183±190
Scheme 1.
chains in the N-aminides 4 could induce speci®c and useful
features and we undertook a systematic study of the proper-
ties of these molecules. The mass spectra fragmentations
under EI and FAB conditions [15] and the photophysical
behavior (solvatochromic effects and ¯uorescence) [16] of
some N-aminides 4 have been studied. In this paper we
present the characterization in the solid state of the N-
aminide 4a and of N,N0-diaminides 4b and c.
2. Results and discussion
Fig. 1. Molecular structures of compounds 4a (1a), 4b (1b) and 4c (1c). In
the case of 4a only one of the three different structures is shown. The
disorder on the ethylenic bridge in 4c is modeled by splitting carbon atoms
over two positions.
N-(Per¯uorobutanoyl)pyridinium-1-aminides 4a±c are
colorless crystalline compounds, inde®nitely stable in air
and in light. They were routinely puri®ed through chroma-
tography on silica gel.
The re®ned crystal structures of 4a±c (see Fig. 1(a)±(c))
share many common features. Molecular dimensions
(Table 1) suggest that the bonding in these systems is best
described considering that the negative charge formally on
the aminide nitrogen N2 (Scheme 2 and Figs. 1, 2 and 3) is
partly distributed to the carbonyl oxygen by mesomeric
effect (as also shown by mass spectra fragmentations [15])
and to the per¯uoroalkyl chain by inductive effect. On the
other hand, the positive charge of the ylide is stabilized by
the aromatic system [28,29]. Bonds around N2 appear
somewhat shorter, and therefore bond orders should be
larger, than those in similar aminide systems without the
strong electron-withdrawing per¯uoroalkyl chains. The
weakest bond in N-aminides 4, namely the zwitterionic
Non-¯uorinated N-acylpyridiniumaminides rapidly
decompose at room temperature while non-¯uorinated N-
benzoyl analogues are completely stable [17±27]. This is
consistent with the fact that a benzoyl residue can delocalize
the negative charge of the aminide nitrogen more effectively
than an acyl residue. The inde®nite stability of N-(per¯uor-
obutanoyl)pyridinium-1-aminides 4 proves that the per-
¯uoroalkyl residue brings about a high stabilization of
the molecule, probably via its strong electron withdrawing
ability.
Ê
Ê
N1±N2 bond, measures 1.404(8) A in 4b, 1.399(4) A in
Ê
4c, while the average value is 1.407(7) A in 4a. These values
are hardly affected by the C5±N1±N2±C6 torsion angle and
are slightly but consistently shorter than those reported for
related molecules where the per¯uoropropyl group is
Ê
replaced by other groups (1.416(5) A [30] and 1.412(6) A
Ê
[31]). The aminide C6±N2 bond in 4a±c are 1.298(8)
Ê
(average value over 4a-A±a-C), 1.298(9) and 1.307(4) A,
respectively. These values are also marginally shorter than
Ê
the 1.317(5) A value found in a compound where the
per¯uoropropyl group is replaced by an electron-poor phe-
Ê
nyl ring [30] and clearly smaller than the 1.340(8) A value
found in a system with a scarcely electron-withdrawing
Scheme 2.

T. Caronna et al. / Journal of Fluorine Chemistry 97 (1999) 183±190
185
Table 1
Bond distances, bond angles and torsion angles in 4a (the three independent molecules are labelled as A±C, respectively), 4b and 4c
Ê
Bond distance (A)
4a A
4a B
4a C
4b
4c
N1±N2
N2±C6
1.406(7)
1.409(7)
1.407(7)
1.404(8)
1.298(9)
1.219(9)
1.533(11)
1.515(14)
1.517(17)
1.399(4)
1.307(4)
1.233(4)
1.540(5)
1.516(5)
1.526(5)
1.327(8)
1.224(7)
1.537(9)
1.551(12)
1.506(10)
1.278(9)
1.187(10)
1.554(11)
1.483(11)
1.529(8)
1.288(8)
1.229(8)
1.524(10)
1.524(11)
1.529(9)
C6±O
C6±C7
C7±C8
C8±C9
Bond angle (8)
C6±N2±N1
O± C6±N2
O± C6±C7
N2± C6±C7
C6±C7±C8
C7±C8±C9
Torsion angle (8)
C5±N1±N2±C6
C1±N1±N2±C6
N1±N2±C6±C7
N2±C6±C7±C8
C6±C7±C8±C9
C6±C7±C8±F7
111.9(5)
110.9(6)
112.0(5)
115.1(6)
117.0(3)
132.9(6)
117.7(6)
109.4(6)
110.5(7)
120.4(10)
130.9(8)
117.1(8)
111.9(7)
112.7(7)
117.5(8)
130.7(7)
116.1(6)
113.1(6)
112.6(7)
113.4(9)
132.3(7)
116.9(7)
110.7(7)
114.5(8)
114.7(11)
133.2(3)
116.5(3)
110.2(3)
113.3(3)
117.7(3)
138.2(6)
47.9(8)
79.8(8)
103.9(8)
177.3(6)
85.5(9)
98.7(7)
84.4(8)
34.1(1)
149.8(7)
177.5(6)
90.6(9)
19.6(5)
162.1(3)
173.8(3)
91.0(4)
174.9(5)
81.9(7)
174.5(6)
85.3(8)
155.3(11)
179.4(9)
170.6(9)
177.6(9)
173.0(9)
176.9(9)
170.1(11)
174.1(12)
172.4(3)
172.0(3)
group [31]. Consistent with the above discussion, the ami-
nide system (C7±OC6±N2±N1) has an almost planar struc-
ture in all three compounds implying a bond order of C6±N2
signi®cantly greater than unity.
In the closely related per¯uoropropylglutaramide [32],
the angles around the amide carbon, namely O=C±N, N±C±
C and C±C=O, measure 125.9(1), 115.9(1) and 118.2(1),
respectively. In non-¯uorinated aliphatic amides these
values are typically 121.6, 115.7 and 122.5, while the
average values of the three angles in systems 4a±c are
132.0, 111.0 and 116.9. This implies that in our systems
the electrostatic repulsion between negatively charged O
and N atoms substantially widens the O=C±N angle as
compared to per¯uoropropylglutaramide [32] where it is
already four degrees wider than in non-¯uorinated aliphatic
amides. Moreover, the widening of the O=C±N angle in both
per¯uoroglutaramide and in 4a±c is probably related to the
arrangement of the per¯uoropropyl chain which is nearly
Fig. 2. Crystal packing of compound 4a viewed as a projection along the a-axis.

186
T. Caronna et al. / Journal of Fluorine Chemistry 97 (1999) 183±190
Fig. 3. Crystal packing of compound 4b viewed as a projection along the b-axis.
Ê
(1.514 A [33] and 1.549(1) A [32], respectively). Our ami-
nides 4 show a bond length close to that of per¯uoropro-
Ê
pylglutaramide (1.540(5) and 1.538 A considering 4c
and the average value over 4a and b, respectively). The
lengthening observed in aminides 4 with respect to
the average amide value is surprisingly small if the
negative charge present on the aminide moiety of 4 is
considered.
Ê
perpendicular to the O=C±N group so that short non-bond-
ing interatomic distances can occur between the amide
nitrogen and oxygen on one side and the ¯uorine atoms
on the a-carbon on the other side. In the case of 4c the
F1. . .O and the F2. . .N distances measure 2.634(4) and
Ê
2.595(3) A, respectively. It is noteworthy that these values
are shorter than the shortest inter¯uorine distances in the
per¯uoroalkyl chain (F2. . .F4: 2.679(3) and F2. . .F6
Ê
2.696(3) A) all of which appear to involve the F2 atom
The disorder of the per¯uoropropyl chain found in 4a is
somewhat reduced in 4b while it is virtually absent in 4c. In
4b and c the per¯uoroalkyl chain deviates from the exact
trans planar arrangement showing a clear tendency to
spiralization. The torsion angles on the CF2±CF2 bond
are 170.1(11)8 in 4b and 172.4(3)8 in 4c, while those on
the CF2±CF3 bond are 174.1(12)8 and 172.0(3)8, respec-
tively. These values can be compared with the dihedrals of
helical poly(tetra¯uoroethylene) [34,35] and of per¯uoro-
butane (central bond), which are twisted by about 158 from
the trans conformation according to experimental studies
and recent accurate molecular orbital calculations [36]. No
tendency to helicity was found in per¯uoropropylglutar-
amide [32] which has a strictly planar CF2±CF2 chain,
probably because of the requirements of the intermolecular
hydrogen bonding characterizing that system. We must also
which is the atom closest to the negatively charged N2. The
F1. . .O and the F2. . .N distances may result from steric
effects but possibly also from electronic, inductive and
hyperconjugative effects. They may be related to the elec-
tron-withdrawing action of the per¯uoroalkyl chains and
they probably play a key role in determining the unexpected
stability of N-acylpyridiniumaminides 4. While in non-
¯uorinated pyridiniumaminides stability is usually a con-
sequence of an effective delocalization of the negative
charge formally located on N2 [17±27], in N-per¯uoro-
acylpyridiniumaminides 4 stability is related to a set of short
interatomic distances involving ¯uorine atoms.
Moreover, the average length reported for the bond
between the acyl group and the a-carbon is shorter
in hydrocarbon amides than in per¯uoropropylglutaramide

T. Caronna et al. / Journal of Fluorine Chemistry 97 (1999) 183±190
187
note that the CF2±CF2 bond distances for the planar com-
Ê
pound (1.552(2) and 1.549(1) A, [32]) are longer than those
Ê
found in 4b and c (1.515(14) and 1.516(5) A for CF2±CF2;
of disorder which has been recently found in related systems
[37,38]. In our case it was modeled, somewhat roughly but
successfully, by splitting the benzyl carbon atoms over two
positions (C3A and C3B) with site occupancy factor of 30%
and 70%, respectively.
As shown in Figs. 3 and 4, alternating layers of segre-
gated per¯uoroalkyl chains and extended, quasi-planar
bipyridinium fragments are present in the packings of 4b
and c. In order to achieve optimum packing of both the
aromatic component and the per¯uoroalkyl chains, mole-
cules are tilted with respect to the a±c crystallographic
plane. From the data obtained at room temperature, the
per¯uoropropyl chains are unusually ordered in 4c with
Ê
1.517(17) and 1.526(5) A for CF2±CF3, respectively) and
also in 4a. The large estimated errors in 4a and b limit the
signi®cance of this comparison and a particular caution has
to be used before conclusions are made if we remember that
bond length corrections for torsional disorder will increase
the bond length values.
The orientation of the aminide plane in 1c with respect to
the pyridinium system is likely to be determined by packing
effects, since widely different tilts were also found in related
compounds [1b, c]. The N1±N2±C6 angle correlates qua-
litatively with the values of the C5±N1±N2±C6 torsion
angle which are 34.1(1)8 and 19.6(5)8 in 4b and in c,
respectively, and 138.2(6), 79.8(8) and 98.7(7) in the three
units of 4a. Values of this torsion angle close to 08 allow the
formation of intramolecular interaction (see Fig. 1(c))
involving the hydrogen on C5 and the acyl oxygen
Ê
intermolecular distances as short as 2.95 A. These observa-
tions are consistent with other recent reports [32] showing
that highly ordered arrangement of per¯uoroalkyl chains in
crystal lattice can be achieved when these chains are con-
nected to sterically appropriate units resulting in relatively
high cohesive energy. Crystallization of aliphatic ¯uorocar-
bons should be favored over hydrocarbons by higher dis-
persive interactions while the dif®culty in attaining order in
the case of long ¯uoroalkyl chains may well be due to
Ê
(C5. . .O 2.736(4) A in 4c).
The two pyridinium rings are coplanar both in 4b and c. In
the case of 4c, the connecting ethylenic group shows a kind
Fig. 4. Crystal packing of compound 4c viewed as a projection along the b-axis.

188
T. Caronna et al. / Journal of Fluorine Chemistry 97 (1999) 183±190
Fig. 5. X-ray powder diffraction patterns of diaminide 4b recorded at 258C (5a) and at 808C (5b).
entropic effects related to the deviations from the trans
planar arrangement of per¯uoroalkyl chains.
trary, no signi®cant interactions between the two aromatic
rings were found (distances between the rings are around
Ê
Indeed, at higher temperatures 4c shows a high entropy
phase. This is apparent from differential scanning calori-
metry investigations which, in addition to the melting
transition at 1928C (ÁHˆ7.8 kJ/mol), reveal another
endotherm at 688C (ÁHˆ5.4 kJ/mol). X-ray powder spectra
recorded at 258C and 808C between the two ®rst-order
transitions (Fig. 5) do not suggest a mesophase but rather
a second crystalline phase. We can speculate that the
hydrocarbon layers remain ordered, while the per¯uoroalkyl
chains are conformationally disordered.
Contrary to the cases of 4b and c, the crystal packing of
4a clearly shows no segregation between per¯uoroalkyl
chains and the pyridinium fragments (see Fig. 2). This fact
is probably related to the different substitution pattern and to
the smaller size of the pyridinium as compared to the
bipyridinium system. Analysis of intermolecular distances
suggests that weak intermolecular hydrogen bonds work as
a driving force for the crystal packing. Such interactions
involve mainly the nitrile nitrogen but also the negatively
charged nitrogen and oxygen atoms of the aminide moiety
as acceptors of the hydrogen atoms bound to pyridinium
4.5 A and they are tilted by 258 or 758 depending on the
pair).
3. Conclusions
The presence of the per¯uoroalkyl chains in N-acylpyr-
idiniumaminides 4 has pronounced effects on the structure
of the molecules resulting in their unexpected stability.
These effects can be ultimately traced back to the well-
known ``group electronegativity'' [40] of per¯uoroalkyl
chains and are also mediated by a network of short inter-
molecular distances. Finally, highly ordered arrangement of
per¯uoroalkyl chains in the crystal lattice of 4b and c should
be noted and related to the similar examples found in mixed
per¯uorocarbon±hydrocarbon systems [41].
4. Experimental
4.1. Preparation of compounds 4a±c
Ê
carbons (e.g. OB. . .H5C 2.41 A and OB. . .H5C-C5C: 164.78;
Ê
N3A. . .H2B 2.47 A; N3A. . .H2B-C2B: 137.748, where A, B,
Aminides 4a±c were prepared as described in [4,5]. On
reaction with the oxaziridine 2, 4-cyanopyridine (1a)
afforded 4a (50%)) and 4-cyanopyridine N-oxide (3a)
(42% yield). In a similar way, the reaction of 4,40-bipyridine
(1b) with one equivalent of the oxaziridine 2 afforded a
mixture of the corresponding mono-N-aminide (55% yield)
and mono-N-oxide (35% yield). Further reaction of the
and C subscripts identify atoms of the three independent
molecules) [39]. The unexpected occurrence of three dif-
ferent molecular structures in the crystals of 4a must relate
to such speci®c interactions. It is also noteworthy that the
terminal CF3 groups are located quite close to the CN group
Ê
of adjacent molecules (N3. . .F6: 3.27(3) A). On the con-

T. Caronna et al. / Journal of Fluorine Chemistry 97 (1999) 183±190
189
Table 2
Crystal data collection and refinement parameters for 4a±4c
4a
4b
4c
Crystal data
Formula
C10H4F7N3O
C18H8F14N4O2
578.28
C20H10F14N4O2
604.32
1
M (g mol
Ê
a (A)
)
315.16
8.3956(5)
19.2256(6)
22.9847(6)
90.0
6.0907(6)
7.9307(9)
12.6481(12)
73.260(7)
82.145(7)
71.486(8)
554.09(0.11)
0.4Â0.2Â0.8
1.732
5.2799(5)
8.1806(5)
13.0764(8)
86.439(4)
89.355(6)
84.184(5)
560.81(0.07)
0.8Â0.2Â0.6
1.789
Ê
b (A)
Ê
c (A)
ꢀ (deg.)
ꢃ (deg.)
ꢄ (deg.)
95.089(5)
90.0
Ê 3
V (A )
3695.34(0.21)
0.3Â0.4Â0.5
1.699
Size (mm)
1
ꢅcalcd. (g cm
Z
)
12
1
1
Crystal system
Space group
F(0 0 0)
Monoclinic
P21/n
Triclinic
P-1
Triclinic
P-1
1872
1.692
286
1.786
300
1.797
1
)
ꢆ (Cu Kꢀ) (mm
Data collection
Scan range (ꢂ) (8)
Measured reflections
3.00/57.44
6696
3.65/56.73
1956
3.39/57.14
3036
Unique reflections
5046
1464
1519
Observed reflections with Iꢀ2ꢇ(I)
Absorption correction
3086
None
947
Crystal face indexing
1260
Crystal face indexing
Structure solution and refinement
Refined parameters
R1
569
174
193
0.0935
0.2528
1.069
0.1073
0.3107
1.050
0.0488
0.1270
1.060
wR2
Goodness-of-fit
Ê
Áꢅ (max/min) (e A
3
)
0.583/ 0.428
0.592/ 0.385
0.204/ 0.222
[3] A. Arnone, D.D. DesMarteau, B. Novo, V.A. Petrov, M. Pregnolato,
G. Resnati, J. Org. Chem. 61 (1996) 8805.
mono-N-aminide with one equivalent of oxaziridine 2
afforded a mixture of the di-N-aminide 4b (55% yield)
and the N-aminide-N0-oxide (35% yield). The di-N-aminide
4b was alternatively obtained in one step by treating the
4,40-bipyridine (1b) with two equivalents of the oxaziridine
2. Similar results were found in the reaction of (E)-1,2-di(4-
pyridyl)ethene (1c) with the oxaziridine 2.
[4] C. Balsarini, B. Novo, G. Resnati, J. Fluorine Chem. 80 (1996) 31.
[5] R. Bernardi, B. Novo, G. Resnati, J. Chem. Soc., Perkin Trans. 1
(1996) 2517.
 Â
[6] J.-P. Begue, A. M'Bida, D. Bonnet-Delpon, B. Novo, G. Resnati,
Synthesis (1996) 399.
[7] A. Arnone, B. Novo, M. Pregnolato, G. Resnati, M. Terreni, J. Org.
Chem. 62 (1997) 6401.
[8] A. Arnone, P. Metrangolo, B. Novo, G. Resnati, Tetrahedron 54
(1998) 7831.
4.2. X-ray crystal structure determination
[9] M. Tingoli, A. Temperini, L. Testaferri, M. Tiecco, G. Resnati,
Carbohydrate Lett. 3 (1998) 39.
Crystal structure analysis of 4a±c was performed at
ambient conditions on a Siemens P4 diffractometer, with
[10] C.L. Bird, A.T. Kuhn, Chem. Soc. Rev. 70 (1981) 49.
[11] L.A. Summers, The Bipyridinium Herbicides, Academic Press,
London, 1980.
Ê
graphite monochromated Cu Kꢀ radiation (ꢁˆ1.54178 A)
[12] L.A. Kelly, M.A.J. Rodgers, J. Phys. Chem. 98 (1994) 6386.
[13] V.G. Koshechko, L.A. Kiprianova, L.I. Fileleeva, Tetrahedron Lett.
33 (1993) 6677.
using a ꢂ±2ꢂ scan technique. The structures were solved by
direct methods using SIR92 [42] and re®ned by full-matrix
least squares on F2 using SHELXL93 [43]. Crystal data
collection and re®nement parameters are reported in
Table 2.
[14] R.E. Banks, B.E. Smart, J.C. Tatlow (Eds.), Organofluorine
Chemistry, Plenum Press, New York, 1994.
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