Synthesis, structure and conformational properties of fluoroformylchlorodifluoroacetyl disulfide, FC(O)SSC(O)CF2Cl: Conformational transferability in -C(O)SSC(O)- compounds

Article abstract of DOI:10.1002/ejic.200600604

Pure fluoroformylchlorodifluoroacetyl disulfide, FC(O)SSC(O)CF ₂Cl, has been prepared by the reaction of FC(O)SCl and CF ₂ClC(O)SH in quantitative yield. The conformational properties of the novel molecule have been studied by vibrational spectroscopy (IR - gas phase, Raman - liquid phase) and quantum chemical calculations (B3LYP and MP2 methods). The gaseous compound exhibits a conformational equilibrium at room temperature where the most stable form adopts a C₁ symmetry and a syn-periplanar (sp) orientation of both carbonyl groups with respect to the disulfide bond. A second form, observed in the IR spectrum of the vapor, corresponds to a conformer in which the carbonyl bond of the FC(O) moiety adopts an anti-periplanar (ap) position with respect to the S-S single bond, and gauche with respect to the ClC-C=O moiety in the chlorodifluoroacetyl group. The experimental free energy difference value ΔG⁰ = G ⁰_(ap-sp) - G⁰_(sp-sp) = 1.4(3) kcal/mol (IR) is reproduced well by the B3YLP/6-311+G(3df) (1.1 kcal/mol) and the MP2/6-31G* (1.8 kcal/mol) methods. In addition, the structure of a single-crystal, grown in situ, was determined by X-ray diffraction analysis at low temperature. The crystalline solid [monoclinic, P2₁/n, a = 5.579(3) A, b = 16.615(7), c = 8.455(4) A, β = 106.876(8)°] consists exclusively of molecules with the (sp-sp) conformation and the usual gauche orientation around the disulfide bond [φ(CS-SC) = 84.2°]. Conformational transferability is thus demonstrated once again for species that contain the -C(O)SSC(O)- group as part of a systematic program designed to analyze the behavior of this class of molecules. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600604

FULL PAPER
DOI: 10.1002/ejic.200600604
Synthesis, Structure and Conformational Properties of
Fluoroformylchlorodifluoroacetyl Disulfide, FC(O)SSC(O)CF2Cl:
Conformational Transferability in –C(O)SSC(O)– Compounds
Mauricio F. Erben,[a] Carlos O. Della Védova,*[a,b] Helge Willner,[c] and Roland Boese[d]
Keywords: Conformation analysis / Sulfur / Vibrational spectroscopy / X-ray diffraction
Pure fluoroformylchlorodifluoroacetyl disulfide, FC(O)SSC-
(O)CF2Cl, has been prepared by the reaction of FC(O)SCl
and CF2ClC(O)SH in quantitative yield. The conformational
properties of the novel molecule have been studied by vi-
brational spectroscopy (IR – gas phase, Raman – liquid
phase) and quantum chemical calculations (B3LYP and MP2
methods). The gaseous compound exhibits a conformational
equilibrium at room temperature where the most stable form
adopts a C1 symmetry and a syn-periplanar (sp) orientation
of both carbonyl groups with respect to the disulfide bond.
A second form, observed in the IR spectrum of the vapor,
corresponds to a conformer in which the carbonyl bond of
the FC(O) moiety adopts an anti-periplanar (ap) position with
respect to the S–S single bond, and gauche with respect to
the ClC–C=O moiety in the chlorodifluoroacetyl group. The
G0
= 1.4(3) kcal/mol (IR) is reproduced well by the
(sp–sp)
B3YLP/6-311+G(3df) (1.1 kcal/mol) and the MP2/6-31G*
(1.8 kcal/mol) methods. In addition, the structure of a single-
crystal, grown in situ, was determined by X-ray diffraction
analysis at low temperature. The crystalline solid [mono-
clinic, P21/n, a = 5.579(3) Å, b = 16.615(7), c = 8.455(4) Å, β =
106.876(8)°] consists exclusively of molecules with the (sp–
sp) conformation and the usual gauche orientation around
the disulfide bond [φ(CS–SC) = 84.2°]. Conformational trans-
ferability is thus demonstrated once again for species that
contain the –C(O)SSC(O)– group as part of a systematic pro-
gram designed to analyze the behavior of this class of mole-
cules.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
experimental free energy difference value ∆G0 = G0
–
(ap–sp)
relation to the study of protein folding.[1–5] The transferabil-
ity principle can be used to deduce the properties of large
molecules from smaller, readily accessible molecules whose
geometric parameters are experimentally known. Knowl-
edge of the structural properties and conformational prefer-
ence of simple molecules containing distinct functional
groups is therefore of prime interest.
Geometric structures of symmetrically substituted non-
cyclic disulfides, XSSX, in the gas phase are characterized
by a gauche conformation around the S–S bond, with dihe-
dral angles φ(XS–SX) close to 90° (e.g. 90.76(6)° in
HSSH,[6] 87.7(4)° in FSSF,[7] 85.2(2) in ClSSCl,[8] 85.3(37)
in CH3SSCH3,[9] and 104.4(40)° in CF3SSCF3).[10] In this
conformation, the p-shaped lone pairs of the sulfur atoms
are perpendicular to each other and their mutual repulsion
is minimized. Furthermore, such a structure is favored by
Introduction
One of the most fundamental concepts in chemistry is
that of a functional group; the idea that a linked group
of atoms can exhibit a set of characteristic geometric and
chemical properties. This concept is especially useful to
understand the chemistry of biological macromolecules
which consist of polymeric combinations of a small number
of building blocks. From this empirical cornerstone of
chemistry – atoms and functional groups possess character-
istic and additive properties that in many cases exhibit a
remarkable transferability between different molecules – a
series of methodologies have been developed, especially in
[a] CEQUINOR (CONICET-UNLP), Departamento de Química,
Facultad de Ciencias Exactas, Universidad Nacional de La
Plata,
47 esq. 115 (B1900AJL), C. C. 962, La Plata, Buenos Aires, the anomeric effect by electron donation from the sulfur
República Argentina
lone pairs into the empty σ* orbitals of the opposing S–X
E-mail: carlosdv@quimica.unlp.edu.ar
bonds.[11,12] Disulfides with very bulky substituents, such as
[b] Laboratorio de Servicios a la Industria y al Sistema Científico
(UNLP-CIC-CONICET) Camino Centenario,
Gonnet, Buenos Aires, República Argentina
tBuSStBu, have dihedral angles which are considerably
larger than 90° [φ(CS–SC) = 128.2(27)].[13] On the other
hand, the dihedral angle for FC(O)SSC(O)F has been re-
ported to be 82.2(19).[14] Structural data for nonsymmetri-
cally substituted disulfides of the type XSSY are more
sparse in both experimental and theoretical terms. FC(O)-
[c] FB C, Anorganische Chemie, Bergische Universität Wuppertal,
Gaußstrasse 20, 47097 Wuppertal, Germany
[d] Institut für Anorganische Chemie, Universität Duisburg–Essen,
Universitätsstr. 5–7, 45117 Essen, Germany
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Fluoroformylchlorodifluoroacetyl Disulfide, FC(O)SSC(O)CF2Cl
FULL PAPER
SSCF3[15] and FC(O)SSCH3[16] have been studied in the gas tered at 1060 cm–1, is assigned to the F–Csp2 stretching
phase and display φ(CS–SC) dihedral angles of 95.0(27)° mode. In addition, the characteristic disulfide stretching
and 83.5(15)°, respectively.
mode is observed in the Raman spectrum of the liquid as a
Additionally, the presence of a carbonyl group attached signal of medium intensity at 535 cm–1.
to the S–S bond, as in FC(O)SSC(O)F,[17] FC(O)SSCH3,[15]
and FC(O)SSCF3,[16] may promote a conformational equi-
librium, but this depends on the relative orientation of the
C=O and S–S bonds. The syn-periplanar (sp) orientation,
with φ(SS–C=O) = 0°, is the prevailing form for these spe-
cies.
In order to gain additional experimental and theoretical
information about the structural and conformational be-
havior of acyl-substituted disulfides, we became interested
in molecules containing the –C(O)SSC(O)– fragment with
two carbonyl groups bonded to the disulfide bond. Thus,
fluoroformyltrifluoroacetyl disulfide, FC(O)SSC(O)CF3,
has been recently synthesized and its structural properties
studied in both the gas and condensed phases.[18] In the
present study, we have extended the analysis to the related
FC(O)SSC(O)CF2Cl molecule, a novel compound for
which the geometric structure and conformational proper-
ties have been determined by experimental and theoretical
methods.
Results
Synthesis, Characterization, and Physical Properties
Figure 1. Gas IR at 5.0 mbar (glass cell, 200 mm optical path
length, Si windows, 0.5 mm thick) and liquid Raman spectra for
FC(O)SSC(O)CF2Cl.
The synthesis of FC(O)SSC(O)CF2Cl was adapted from
that recently reported for FC(O)SSC(O)CF3,[18] by the reac-
tion of chlorodifluorothioacetic acid, CF2ClC(O)SH, with
fluorocarbonylsulfenyl chloride, FC(O)SCl, according to
Equation (1).
Quantum Chemical Calculations
In a first step, the potential function for the internal rota-
CF2ClC(O)SH + FC(O)SCl Ǟ FC(O)SSC(O)CF2Cl + HCl
(1)
tion around the S–S bond was derived by structure optimi-
The new compound is a colorless liquid with the charac- zations at fixed φ(CS–SC) dihedral angles, whereas near-sp
teristic overpowering sulfenylcarbonyl odor. In the liquid mutual orientations were supposed for both C=O double
or gaseous state, the compound is stable for days at room bonds and the S–S bond. The potential function obtained
temperature. The vapor pressure, measured in a small sec- with the B3LYP/6-31G* method is shown in Figure 2. Min-
tion of the vacuum line (total volume ca. 15 mL) with a ima occur at dihedral angles around 75°, whereas a rather
capacitance manometer over the temperature range 247– flat maximum in the region of a trans C–S–S–C skeleton is
291 K, follows the equation logp = 5.857 – 2383/T (p/bar, observed, with imaginary frequencies, at φ(CS–SC) = 180°.
T/K), and gives an extrapolated normal boiling point of
407 K (134 °C).
Besides the two enantiomeric forms that are related by
rotation around the disulfide bond, several conformations
In the 19F NMR spectrum of the liquid, two singlets with are feasible, in principle, for FC(O)SSC(O)CF2Cl, but they
an intensity ratio of 1:2 were observed. The more intense depend on the orientation of the C=O bonds of the FC(O)
signal is located at δ = –62.6 ppm, whereas the second sig- and the CF2ClC(O) groups. Each of them can be syn-peri-
nal appears at δ = 42.3 ppm. The related trifluoromethyl planar (sp) or anti-periplanar (ap) relative to the S–S bond.
species shows corresponding signals at δ = –74.9 ppm This leads to four possible conformers: (sp–sp), (ap–sp),
[CF3C(O)– group] and 41.4 ppm [FC(O)– group].[18]
(sp–ap), and (ap–ap) [the first orientation refers to the
Additional evidence for the identity of FC(O)SSC(O)- FC(O) group and the second to the CF2ClC(O) group, see
CF2Cl comes from the IR spectrum of the vapor and the Scheme 1].
Raman spectrum of the liquid (Figure 1 and Table SI1 in
The potential functions for the internal rotation around
the Supporting Information). Two intense bands in the car- both S–C bonds were calculated (B3LYP/6-31G*) by full
bonyl stretching region at 1847 and 1771 cm–1 are charac- geometry optimization at fixed φ(SS–C=O) torsional angles
teristic of the FC=O and CF2ClC=O groups, respectively. (Figure 3). As expected, both curves possess minima for sp
The strongest band in the IR spectrum of the vapor cen- [φ(SS–C=O) = 0°] and ap [φ(SS–C=O) = 180°] orientations.
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M. F. Erben, C. O. Della Védova, H. Willner, R. Boese
FULL PAPER
ers correspond to stable structures and agree with respect
to the conformational preference. In the lowest-energy con-
former, both carbonyl bonds adopt an sp orientation with
respect to the S–S bond, whereas the second most stable
conformer possesses an ap orientation of the FC(O) group
with respect to the S–S single bond. Moreover, structures
with ap orientation of the CF2ClC(O) group with respect
to the S–S bond are considerably higher in energy (∆G0Ϸ
4.0 kcal/mol or even more) and are not expected to be de-
tectable in our experiments.
Table 1. Calculated relative energies (corrected by zero-point en-
ergy) and vibrational frequencies of the C=O stretching modes
[cm–1] with IR intensities [km/mol] in parentheses for FC(O)-
SSC(O)CF2Cl.
Conformer[a] Method of calculation
∆E0
ν(FC=O) ν(O=CCF2Cl)
(sp–sp)
(ap–sp)
(sp–ap)
(ap–ap)[e]
B3LYP/6-31G*
B3LYP/6-311+G(3df)
MP2/6-31G*
0.00[b] 1922 (231)
0.00[c] 1892 (249)
0.00[d] 1894 (177)
1851 (192)
1827 (214)
1784 (127)
Figure 2. Calculated potential function (B3LYP/6-31G*) for the in-
ternal rotation around the S–S bond in FC(O)SSC(O)CF2Cl.
B3LYP/6-31G*
B3LYP/6-311+G(3df)
MP2/6-31G*
0.89 1903 (379)
1.14 1866 (453)
1.76 1881 (301)
1847 (190)
1826 (204)
1780 (127)
B3LYP/6-31G*
B3LYP/6-311+G(3df)
MP2/6-31G*
4.48 1923 (245)
3.74 1893 (267)
6.05 1897 (183)
1827 (250)
1804 (288)
1759 (163)
Scheme 1. Representation of the conformers of FC(O)SSC(O)-
CF2Cl.
B3LYP/6-31G*
MP2/6-31G*
5.29 1901 (327)
7.93 1872 (250)
1826 (295)
1759 (195)
The calculated maxima for near-perpendicular orientations
[φ(SS–C=O) Ϸ 90°] have similar energy values in both cases,
which are characterized as torsional transition states (TS,
Nimag = 1). The geometries of the four minima were fully
optimized, including frequency calculations, with the
B3LYP method and with the use of the 6-31G* and 6-
311+G(3df) basis sets and the MP2/6-31G* approximation.
Calculated relative energies (corrected for zero-point en-
ergy) and vibrational frequencies of the C=O stretches with
their IR intensities are collected in Table 1. All three com-
putational methods predict that the four different conform-
[a] First orientation (sp or ap) refers to FC(O) group, second orien-
tation (sp or ap) to CF2ClC(O) group. [b] E0 = –1820.807875
hartree. [c] E0 = –1817.714699 hartree. [d] E0 = –1821.140101
hartree. [e] The (ap–ap) was not calculated at the B3LYP/6-
311+G(3df) approximation.
In regard to the chlorodifluoromethyl group, the pre-
dominant conformer possesses a gauche structure with
ClC–C=O values close to 80°; the C–F bond deviates from
the eclipsed orientation [φ(FC–C=O) = 42°]. From the po-
tential energy curve obtained by the rotation of the CF2Cl
group around the C–C single bond (Figure 4) both cis
Figure 3. Potential energy curves for FC(O)SSC(O)CF2Cl as a
function of the φ(SS–C=O) dihedral angles calculated with the
B3LYP/6-31G* approximation. [᭺: φ(SS–C1=O1), ᭹: φ(SS–
C2=O2), for atom numbering see Figure 5].
Figure 4. Potential energy curves for FC(O)SSC(O)CF2Cl as a
function of the φ(ClC–C=O) dihedral angle calculated with the
B3LYP/6-31G* approximation.
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Fluoroformylchlorodifluoroacetyl Disulfide, FC(O)SSC(O)CF2Cl
FULL PAPER
[φ(ClC–C=O) = 0°] and trans [φ(ClC–C=O) = 180°] forms
correspond to torsional transition states (Nimag = 1). The
calculated barrier heights (corrected for zero-point energies)
are 1.1 and 2.5 kcal/mol, respectively.
Vibrational Spectra
The IR spectrum of the vapor and the Raman spectrum
of liquid FC(O)SSC(O)CF2Cl are shown in Figure 1. Both
spectra are similar to those of FC(O)SSC(O)CF3.[18] A ten-
tative assignment of the observed bands was performed by
comparison with the calculated spectrum and the approxi-
mate description of modes is based on the calculated dis-
placement vectors for the fundamental modes of vibration,
as well as on comparisons with the spectra of related mole-
cules, especially FC(O)SSC(O)F[17] and CF2ClC(O)Cl.[19]
Experimental and calculated [B3LYP/6-311+G(3df)] fre-
quencies for the (sp–sp) and (ap–sp) conformers, together
with the tentative assignments, are given in the Supporting
Information (Table SI1).
The ab initio calculations indicate that the (sp–sp) con-
former is more stable than the (ap–sp) conformer. The vi-
brational spectra are consistent with this prediction, but the
presence of a second conformer in the vapor phase becomes
apparent. It is known that the ν(C=O) normal mode of car-
bonyl compounds is very sensitive to conformational prop-
erties.[20,21] As observed in Figure 5, two intense bands oc-
cur in the IR spectrum of FC(O)SSC(O)CF2Cl vapor at
1847 and 1771 cm–1, whereas a third band of low intensity
appears at 1827 cm–1. The first two bands are assigned to
the C=O stretching modes of the FC=O and CF2ClC=O
Figure 5. C=O vibrational stretching region in the IR spectrum of
gaseous FC(O)SSC(O)CF2Cl at 5.0 mbar (glass cell, 200 mm op-
tical path length, Si windows, 0.5 mm thick).
located at 1060 cm–1, which can be assigned to this mode
in the main (sp–sp) conformer; the second band, at
1089 cm–1, can be assigned to the same mode of the less
stable (ap–sp) form. Quantum chemical calculations cor-
rectly reproduce this frequency shift of 19 cm–1. According
to the calculations, the other fundamental modes of the (sp–
sp) and (ap–sp) conformers either differ by less than 2 cm–1
or have too low an intensity to be observed in our experi-
ments.
Crystal Structure
Because simple covalent disulfides are liquids or gases at
groups in the most abundant (sp–sp) conformer. Compari- ambient temperatures and because they are frequently labile
son with the calculated frequencies then allows the assign- species, very little is known about their structures in the
ment of the third band to the FC=O group in the (ap–sp) solid state. Only with the development of special crystalli-
form. Thus, the calculated wavenumber difference [B3LYP/ zation techniques has it become possible to extend detailed
6-311+G(3df)] for the C=O stretching mode of the FC(O) structural studies to the crystalline state. By using the in
group [ν(FC=O), Table 1] between the (sp–sp) and the (ap– situ crystallization technique developed at Essen,[22] an ap-
sp) forms is +26 cm–1, a value that is in good agreement propriate single-crystal of FC(O)SSC(O)CF2Cl was grown
with the experimentally observed value of +20 cm–1. No at 193 K. The compound crystallizes in the monoclinic sys-
significant difference is expected in the wavenumbers of the tem (P21/n spatial group) with the following unit cell dimen-
CF2ClC=O stretching mode between the (sp–sp) and (ap– sions: a = 5.579(3) Å, b = 16.615(7) Å, c = 8.455(4) Å, and
sp) forms; the calculated difference is about 2 cm–1. Accord- α = γ = 90° and β = 106.876(8)°, Z = 4 (for the full crystal-
ingly, the stretching mode of the (ap–sp) conformer is as- lographic data and treatment information, see Table SI2 in
signed to the band at 1771 cm–1, which is masked by the the Supporting Information). Only the (sp–sp) conformer is
band of the corresponding stretching mode of the (sp–sp) observed in the crystal, with a gauche orientation around
form. The conformational composition was derived from the S–S bond. The structure of the molecule is shown in
the integrated intensities of the C=O vibrations for the (sp– Figure 6, and Table 2 includes the main geometric param-
sp) and (ap–sp) forms, with the intensities calculated by the eters derived from the structure refinement, as well as those
B3LYP/6-311+G(3df) method taken into account (Table 1). obtained from quantum chemical calculations. The overall
This analysis leads to a composition of 92(5)% of the more crystal packing, as viewed along the ab plane, is shown in
stable (sp–sp) form at ambient temperature (the estimated Figure 7.
error limit includes uncertainties in the measured areas and
in the calculated intensities).
Intermolecular interactions that are dominated by F···F
contacts are common for fluorinated molecules where there
A conformational equilibrium was also evident in the F– is no other choice for the stabilization of the packing. Ac-
C(O) stretching region [ν(Csp2–F), Table SI1 given in the cording to quantum chemical calculations, F···F contacts in
Supporting Information] through the presence of two bands aromatic systems can contribute up to 14 kcal/mol of local
in the IR spectrum of the vapor. The more intense band is stabilization energy.[23] In the present case, the chlorodifluo-
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M. F. Erben, C. O. Della Védova, H. Willner, R. Boese
FULL PAPER
Figure 6. Molecular model with atom numbering scheme for the
single-crystal structure of FC(O)SSC(O)CF2Cl.
Table 2. Experimental and calculated geometric parameters for the
gauche (sp–sp) conformer of FC(O)SSC(O)CF2Cl.[a]
Parameter
X-ray[b]
B3LYP
6-31G* 6-311+G(3df)
MP2/6-31G*
S1–S2
S1–C1
S2–C3
C1–C2
C1=O1
C3=O2
(C2–F)mean
C3–F4
C–Cl
S2–S1–C1
S1–S2–C3
S1–C1=O1
S2–C3=O2
S1–C1–C2
F–C2–F
S2–C3–F4
(F–C–C)mean 109.5 (3)
Cl–C–C 110.7 (2)
(Cl–C–F)mean 110.0 (3)
2.029 (1)
1.759 (4)
1.796 (3)
1.556 (4)
1.179 (4)
1.165 (4)
1.335 (4)
1.342 (4)
1.739 (3)
99.7 (1)
2.074
1.790
1.824
1.553
1.196
1.182
1.340
1.342
1.785
99.8
101.4
125.7
129.7
112.1
108.5
106.6
109.6
110.0
109.6
2.9
2.050
1.785
1.813
1.557
1.187
1.176
1.339
1.347
1.773
101.4
100.4
126.2
130.5
112.2
108.0
106.3
110.0
109.6
109.6
0.9
2.049
1.774
1.794
1.539
1.211
1.194
1.354
1.358
1.753
98.2
Figure 7. Stereoscopic illustration of the crystal packing of FC-
(O)SSC(O)CF2Cl at 193 K.
oxygen atom in the carbonyl group bonded to the other
sulfur atom. Following Figure 6, these 1,4 S···O distances
for FC(O)SSC(O)CF2Cl are labeled as S1···O2 and S2···O1,
with values of 3.016 Å and 3.078 Å, respectively. The φ(SS–
C=O) dihedral angles around the corresponding C–S bonds
are 4.8 and 0.8°, respectively. Following Burling et al.,[28]
these nearly planar conformations favor electronic delocal-
ization of the nonbonded π electrons of the sulfur atom,
which results in an increased attractive interaction between
the sulfur and oxygen atoms.
99.1 (1)
99.4
126.7 (2)
131.1 (3)
111.8 (2)
107.1 (2)
106.7 (2)
126.0
129.7
112.9
108.5
106.6
108.7
110.3
109.9
2.0
φ(SS–C=O1)
φ(SS–C=O2)
φ(CS–SC)
0.8 (4)
4.8 (3)
84.2 (2)
2.0
77.4
68.8
2.7
83.5
78.9
2.9
71.1
77.2
φ(ClC–C=O) 102.3 (4)
Discussion
[a] See Figure 6 for atom numbering. [b] Uncertainties are σ values.
According to the vibrational spectra, the title molecule
romethyl groups interact through C–F···F–C contact which exists in the gas phase as a mixture of two conformers that
measure 2.817 Å. This value is within the range found in differ in the relative orientation of the FC(O) group and the
comparable structural studies, in which C–F···F–C contacts S–S single bond, with the sp conformer prevailing over the
of 2.777 and 2.868 Å are identified as packing motifs.[24,25] ap form. At room temperature, the more stable conformer
The sum of the van der Waals radii would suggest 2.7 Å as accounts for 92(5)% of the vapor species. The standard
a contact distance, but this does not obviously hold for car- Gibbs free-energy difference [∆G0 = G0
– G0(sp–sp)] de-
(ap–sp)
bon-bonded fluorine.
rived from the IR spectrum of the vapor (1.4 kcal/mol) is
The investigation of short nonbonded intramolecular 1,4 reproduced satisfactorily at the B3LYP/6-311+G(3df),
S···O contacts have attracted much attention because S···O B3LYP/6-31G*, and MP2/6-31G* levels of calculation. On
distances in the range 2.77–3.16 Å have been observed in the other hand, only the sp conformation is observed for
crystals.[26] These values are much shorter than the sum of the mutual orientation of the CF2ClC=O double bond and
the sulfur and oxygen van der Waals radii (3.3 Å).[27] The the S–S single bond, with a calculated mean value of ∆G0
presence of such short S···O contacts in these compounds, = G0(sp–ap) – G0(sp–sp) of 4.7 kcal/mol. This behavior parallels
each of which crystallizes in a different packing environ- that recently reported for FC(O)SSC(O)CF3, with ∆G0 =
ment, indicates that the conformational feature results from G0
– G0
= 1.14(15) kcal/mol from the IR spec-
(ap–sp)
(sp–sp)
a nonbonded intramolecular interaction. Computational trum of the matrix isolated vapor and sp orientation of the
results suggest that electronic conjugation gives rise to the S–S single bond with respect to the C=O double bond in
observed S···O close contact.[28] In molecules which contain the –C(O)CF3 moiety.[18]
the –C(O)SSC(O)– moiety, there are two 1,4 S···O distances,
The chlorodifluoroacetyl group adopts a gauche orienta-
defined by one sulfur atom of the disulfide bond and the tion in the crystal, with a corresponding dihedral angle
4422
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 4418–4425

Fluoroformylchlorodifluoroacetyl Disulfide, FC(O)SSC(O)CF2Cl
FULL PAPER
φ(ClC–C=O) of 102.3(4)° [φ(F3C3–C=O) = 136.4(3)°]. As rily reproduce the molecular dimensions of the crystalline
already commented in this paper, quantum chemical calcu- solid.
lations predict a different behavior for a molecule with the
(sp–sp) conformation, with φ(ClC–C=O) and φ(F3C3–
C=O) dihedral angles of 78.9 and 160.7°, respectively. The Experimental Section and Instrumentation
predominant conformer found experimentally for chlorodi-
fluoroacetyl chloride [CF2ClC(O)Cl] in the gas phase pos-
General Procedure: Volatile materials were manipulated in a glass
vacuum line equipped with two capacitance pressure gauges (221
AHS-1000 and 221 AHS-10, MKS Baratron, Burlington, MA),
three U-traps and valves with PTFE stems (Young, London, UK).
sesses a gauche orientation of the chlorine atoms [φ(ClC–
C=O) = 104.6(10)°],[29] whereas a second stable conformer
where the C–Cl bond eclipses the C=O bond is higher in
The vacuum line was connected to an IR cell (optical path length
energy by ∆G0 = 1.1(3) kcal/mol.[29] From a study of the 200 mm, Si windows 0.5 mm thick) contained in the sample com-
partment of an FTIR instrument (Impact 400D, Nicolet, Madison,
WI). This allowed us to monitor the purification processes and to
follow the course of the reactions. The pure compound was stored
in flame-sealed glass ampules under liquid nitrogen in a long-term
Dewar vessel. The ampules were opened with an ampule key on
the vacuum line, an appropriate amount of the compound was
taken out for the experiments, and then the ampules were flame-
sealed again.[40]
Raman spectrum of the liquid at different temperatures, a
∆H0 value of 1.03(11) kcal/mol was determined, whereas
for the sample dissolved in liquid xenon, this value de-
creases to 0.71(17) kcal/mol.[19] For the free molecule of the
title compound, this last conformation corresponds to a ro-
tational transition state; the calculated barrier is only
1.1 kcal/mol. Thus, the barrier to rotation appears to be
quite low for the chlorodifluoroacetyl moiety, and the dis-
crepancy between the X-ray structure and the calculated
geometrical properties probably reflects the distortions in-
duced by the crystal packing.
Transferability is then evident in the conformational
properties of FC(O)SSC(O)CF2Cl. Extensive studies of car-
bonylsulfenyl compounds with the general formula XC(O)-
SY have established the preference for a syn-periplanar con-
formation around the C–S bond.[20,30–33] In the case of di-
sulfides (so that Y corresponds to an –SR group), the pre-
ferred mutual orientation of the C=O and S–S bonds is also
sp.[12,14–16] It has also been established experimentally that
the ap conformation appears as a second stable form that
makes appreciable contributions to the vapor at room tem-
perature when X is a fluorine atom,[14,17,34,35] whereas only
the sp conformation is adopted by species containing the
CF3C(O)S moiety.[36–38]
CF2ClC(O)SH was synthesized by the reaction of either
CF2ClC(O)Cl or [CF2ClC(O)]2O (98% Aldrich) with H2S (98%
Linde, Germany) in a metal reactor.[41] This method was adapted
from the literature procedure for the synthesis of CF3C(O)SH.[42]
CF2ClC(O)Cl was synthesized by the reaction of chlorodifluoro-
acetic acid (98% Merck) with PCl5 according to the usual method.
FC(O)SCl was obtained by the reaction of commercial ClC(O)SCl
(Aldrich 95%) with SbF5 following the reported method.[43,44]
Conventional vacuum techniques were used to condense equimolar
quantities (typically 2.5 mmol) of CF2ClC(O)SH and FC(O)SCl
into a 6 mm o.d. glass ampule. The vessel was flame-sealed and
placed in a –40 °C ethanol bath. At this temperature, the reaction
proceeded rapidly, as judged by the disappearance of the yellow
color [due to FC(O)SCl] of the reaction mixture. The mixture was
then warmed to –10 °C and preserved at that temperature for about
1 h. Subsequently the products were separated by “trap-to-trap”
condensation through traps held at –30 °C, –60 °C and –196 °C.
Pure FC(O)SSC(O)CF2Cl was retained as a colorless liquid in the
–60 °C trap. The yield was nearly quantitative and, apart from the
HCl generated in the reaction, only minor quantities of OCS and
SiF4 were observed as byproducts in the U-trap at –196 °C.
Breitzer et al.[39] have reported the analysis of the crystal
structures of several disulfides. A plot of the S–S bond
length versus the disulfide torsion angles reveals the greatest
density of data points in the bond length region 2.00–
2.06 Å and torsion angles between 75 and 90°. A similar
pattern has also been observed for the structures of gaseous
disulfides. In accordance with this trend, the disulfide bond
length and torsion angle in crystalline FC(O)SSC(O)CF2Cl
are 2.029(1) Å and 84.2(2)°, respectively.
The two computational methods we have used predict
dihedral angles for the molecular skeleton with values sim-
ilar to those obtained from the X-ray analysis. The methods
are less successful in the reproduction of the lengths of
some of the bonds around sulfur. Thus, even with large ba-
sis sets [6-311+G(3df)], the B3LYP method predicts the S–
X bonds to be too long (by up to 0.026 Å for the S1–C1
bond). While these bonds are better described by the MP2
method with the modest 6-31G* basis set, this method fails
to reproduce the C=O double bond lengths well, which are
also made out to be too long (0.032 Å for the C1 = O1
bond). In practice, however, the contraction of polar bonds
is to be expected upon crystallization and compensation for
X-ray Diffraction at Low Temperature: An appropriate crystal of
FC(O)SSC(O)CF2Cl ca. 0.3 mm in diameter was obtained on the
diffractometer at a temperature of 193(2) K with a miniature zone
melting procedure with the use of focused infrared laser radia-
tion.[22] The diffraction intensities were measured at low tempera-
tures with a Nicolet R3m/V four-circle diffractometer. Intensities
were collected with graphite–monochromatized Mo-Kα radiation
with the ω-scan technique. The crystallographic data, conditions
and some features of the structure are listed in Table SI2 (Support-
ing Information). The structure was solved by Patterson syntheses
and refined by full-matrix least-squares methods on F with the
SHELXTL-Plus program.[45] Absorption correction details are
given in Table SI2 (Supporting Information). All atoms were as-
signed anisotropic thermal parameters. Further details of the crys-
tal structure investigation may be obtained from the Fachinforma-
tionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Ger-
many (Fax: +49-7247-808-666, E-mail: crysdata@fiz-karlsruhe.de)
on quoting the depository number CSD-416694.
Vibrational Spectroscopy: Gas-phase infrared spectra were recorded
with a resolution of 1 cm–1 in the range 4000–400 cm–1 with a
this phenomenon allows both methodologies to satisfacto- Bruker IFS 66v FTIR instrument and Raman spectra of liquid
Eur. J. Inorg. Chem. 2006, 4418–4425
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4423

M. F. Erben, C. O. Della Védova, H. Willner, R. Boese
FULL PAPER
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trometer. The liquid sample was contained in a 4 mm glass capil-
lary and the spectrum excited with 500 mW of 1064 nm radiation
from a Nd:YAG laser (ADLAS, DPY 301, Lübeck, Germany).
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were held in flame-sealed, thin-walled 3 mm o.d. tubes placed in-
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Avance DRX-300 spectrometer operating at 282.41 MHz. The
spectrum was measured with the sample at room temperature with
a mixture of CD3CN and CFCl3 as an external lock and reference,
respectively.
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performed with the GAUSSIAN03 program package[46] under the
Linda parallel execution environment with two coupled PC’s. MP2
and B3LYP methods with standard basis sets up to 6-311+G(3df)
and gradient techniques were used for the geometry optimizations
and calculation of the vibrational properties. Transition states were
optimized by the Synchronous Transit-guided Quasi-Newton
(STQN) method, and torsional barrier heights were calculated
from the relative energies of the TS and the stable structure with
the zero-point energies of the species taken into account. All the
computed TS structures show only one imaginary frequency, which
corresponds to the torsion involved in the conformational transi-
tion.
[22] R. Boese, M. Nussbaumer, “In Situ Crystallisation Tech-
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cle): Observed and calculated vibrational data for (sp–sp) and (ap–
sp) conformers; X-ray crystallographic data and structural refine-
ment details.
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Forschungsgemeinschaft is gratefully acknowledged. The Argentin-
ean authors thank the ANPCYT-DAAD for the German-Argen-
tinean cooperation Awards (PROALAR) and the DAAD Regional
Program of Chemistry for Argentina. They also thank the Consejo
Nacional de Investigaciones Científicas y Técnicas (CONICET),
the Comisión de Investigaciones Científicas de la Provincia de Bue-
nos Aires (CIC), República Argentina. They are indebted to the
Facultad de Ciencias Exactas, Universidad Nacional de La Plata,
República Argentina for financial support. M.F.E. and C.O.D.V.
would like to express their gratitude to Holger Pernice, Plácido
García, Mike Finze, Stefan Balters, and Stefan von Ahsen for
friendship and valuable help in the laboratory work during their
stay in Duisburg. C.O.D.V. especially acknowledges the DAAD,
which generously sponsors the DAAD Regional Program of Chem-
istry for the Republica Argentina to support Latin-American stud-
ents to study for their PhD in La Plata.
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Eur. J. Inorg. Chem. 2006, 4418–4425

Fluoroformylchlorodifluoroacetyl Disulfide, FC(O)SSC(O)CF2Cl
FULL PAPER
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Received: June 1, 2006
Published Online: September 7, 2006
Eur. J. Inorg. Chem. 2006, 4418–4425
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4425