The perfluorinated alcohols c-C6F11OH, c-C6F10-1,1-(OH)2 and c-C6F10-1-(CF3)OH

Article abstract of DOI:10.1039/c8cc05148h

The thermally unstable α-fluoroalcohol undecafluorocyclohexanol (c-C₆F₁₁OH) was prepared by addition of hydrogen fluoride to the corresponding ketone. c-C₆F₁₀(CF₃)OH was obtained by protonation of its alkoxide [NMe₄]⁺[C₇F₁₃O]^-. Decafluorocyclohexane-1,1-diol (c-C₆F₁₀(OH)₂) was prepared by acidic workup of the corresponding alkoxide [NMe₄]⁺[C₆F₁₁O]^- with sulfuric acid, which yielded (c-C₆F₁₀(OH)₂) and fluorosulfonic acid. The structures of c-C₆F₁₀(CF₃)OH·2H₂O and of (c-C₆F₁₀(OH)₂) were elucidated by single-crystal X-ray and gas-phase electron-diffraction studies.

Full text of DOI:10.1039/c8cc05148h

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Schwaab, D. Kratzert, J. Schwabedissen, G. Stammler, N. W. Mitzel and I. Krossing, Chem. Commun.,
2018, DOI: 10.1039/C8CC05148H.
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DOI: 10.1039/C8CC05148H
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The perfluorinated alcohols c-C6F11OH, c-C6F10-1,1-(OH)2 and
c-C6F10-1-(CF3)OH
Jonas Schaab,a Miriam Schwab,a Daniel Kratzert,a Jan Schwabedissen,b Hans-Georg Stammler,b
Received 00th January 20xx,
Accepted 00th January 20xx
Norbert W. Mitzelb and Ingo Krossing*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: The thermally unstable α−fluoroalcohol undecafluoro- decafluoro-1-trifluoromethyl-cyclohexanol C6F10(CF3)COH (
cyclohexanol (c-C6F11OH) was prepared by addition of hydrogen On the way to we realized that undecafluorocyclohexanol
fluoride to the corresponding ketone. c-C6F10(CF3)OH was obtained C6F11OH ( ) might be accessible. Compound belongs to the
1).
1
2
2
by protonation of its alkoxide [NMe4]+[C7F13O]−. Decafluorocyclo- class of α-fluoroalcohols, known for their instability due to HF
hexane-1,1-diol (c-C6F10(OH)2) was prepared by acidic workup of the elimination.12–19 Very recently, the first crystal structure of an
corresponding alkoxide [NMe4]+[C6F11O]− with sulfuric acid, which α-fluoroalcohol was published.14 Since this reactive nature
yielded (c-C6F10(OH)2) and fluorosulfonic acid. The structures of could include interesting hydrogen bonding, we set out to
c-C6F10(CF3)OH ⋅ 2H2O and of (c-C6F10(OH)2) were elucidated by prepare and characterize this compound, preferably in different
single-crystal X-ray and gas-phase electron-diffraction studies.
phases.
Compound
fluorocyclohexanone (
thylammonium fluoride ([N(CH3)4]F) and (trifluoromethyl)tri-
methylsilane (F3C-Si(CH3)3) in an excess of 1,2-dimethoxyethane
(DME) for 15 h at ambient temperature (r.t., eq. 1). [N(CH3)4]+
1
was prepared in two steps by stirring deca-
) with equimolar amounts of tetrame-
Perfluorinated alcohols are known for their low nucleophili-
city,1 high H-bond donor strength2 and optical transparency.
This makes them interesting specialty solvents. They may also
be used to prepare a variety of weakly coordinating anions
(WCA) [M(ORF)n]– (ORF = fluorinated organic residue, M=B, Al,
n=4; M=Nb, Ta, n=6);3,4 such WCA are used to stabilize reactive
cations.5,6 The alcohol c-C6F10-1-(C6F5)OH served as precursor
for fluoroalcoholate groups in aluminate-type WCA to overco-
me some deficiencies noticed for [Al(ORF)4]– with RF = C(CF3)3
(disorder, twinning).7 However, the high steric demand allows
only three alkoxide groups to fit around the Al atom; the fourth
coordination site is occupied by a fluoride ion resulting in
[FAl{c-C6F10(C6F5)O}3]–.8,9 It is less prone to disorder in crystalline
compounds, but has due to the (Al−)F function a higher basicity
and coordinating ability. Due to the aromatic C6F5 group, this
WCA is not persistent towards moderate oxidants like NO+ or
3
[C7F13O]– (
removing DME in dynamic vacuum and recrystallization from
DCM (CH2Cl2). Acidic workup of with excess of conc. sulphuric
4) was obtained as colorless solid in 42% yield by
4
acid and purification by condensation afforded
liquid (60% yield; eq. 2 with R = CF3).
1 as a colorless
+ 7
NO2 . Yet, it has proven to stabilize unusual cations (e.g.
[Ag2Se12]2+ 10 or Ag/P clusters11). A perfluorinated alcohol for
WCA-synthesis of lower steric demand than that of c-C6F10-1-
(C6F5)OH and reduced disposition for disorder could be
a.Albert-Ludwigs-Universität Freiburg, Institut für Anorganische und Analytische
Chemie and Freiburger Materialforschungszentrum FMF, Albertstr. 21, D-79104
Freiburg, Germany.
E-Mail: krossing@uni-freiburg.de; Fax: +49 76 1203 6001
b.Universität Bielefeld, Lehrstuhl für Anorganische und Strukturchemie,
Centrum für Molekulare Materialien, CM2
Universitätsstr. 25, D-33615 Bielefeld, Germany.
E-Mail: mitzel@uni-bielefeld.de
Electronic Supplementary Information (ESI) available. CCDC reference number
1839041, 1839048, 1839049 and 1848204. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/x0xx00000x
The synthesis of
stoichiometric addition of [N(CH3)4]+F– to
solid alkoxide [N(CH3)4]+[C6F11O]– (
) as an intermediate (eq. 1),
2
was attempted in analogy to that of
1 by
3
. This yielded the
5
but acidic workup did not proceed analogously and afforded
20
decafluorocyclohexane-1,1-diol (
6
)
in 63% yield (eq. 3). In
addition, fluorosulfate ([FSO3]–) was formed in a – to our
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J. Name., 2013, 00, 1-3 | 1
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knowledge – unprecedented way. Rather than only protonating high-field shifted by −9.9 ppm, relative to the alkoxide, whereas
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DOI: 10.1039/C8CC05148H
the alkoxide to
generated a diol (!) and the anion [FSO3]–.
2
, strongly dehydrating concentrated H2SO4 all other 13C signals are only slightly high-field shifted.
At r.t., the 19F NMR spectrum of
6 in CD2Cl2 shows a single
A hypothetic reaction mechanism is the following: proto- broad coalescence signal (−132 ppm, width 1600 Hz, see ESI)
nation of [N(CH3)4]+[C6F11O]– gives reacts due fast exchanging axial and equatorial fluorine atoms. The 13
as intermediate.
next in a concerted manner with H2SO4 to
and HSO3F (calcd:§ resonances are not affected by this dynamics and the α-carbon
rG° = –12 kJ mol–1). Alternatively, eliminates HF atom resonates at 90.8 ppm. Expectedly, the remaining 13C
rG° = +32 kJ mol–1), which fluorinates chemical shifts are between 110.2 and 107.8 ppm (see ESI). The
and HSO3F. Compound
then dehydrates 1H resonance of the OH groups is found at 3.94 ppm.
. NMR studies of in H2SO4 independently
proved the latter, a quite unexpected reaction (cf. ESI).
Surprisingly, the reaction did not result in formation of . To
compare the stability of to those of other recently prepared
α-fluoroalcohols,12–19 it was prepared in this way12–15 by
addition of HF to (eq. 4).
2
2
C
6

rH° = –9;

2
(calcd:§

rH° = +78; 
H2SO4, resulting in
H2SO4 leading to
3
3
6
3
The 19F NMR spectrum of
2 in CFCl3 at ambient temperature
also contains only two very broad and overlapping coalescence
signals. In contrast, at 223 K the different 19F signals are well
separated, but there are twice as many than expected. 2D 19F
2
2
1
COSY and J-19F,13C-HSQC correlation spectra (cf. ESI) proved
3
the presence of two isomers of the α-fluoroalcohol in solution:
one with axial and one with equatorial OH group. The molar
ratio of these isomers at 223 K in CFCl3 is 2:1 (axial:equatorial).
All their chemical shifts could be assigned (see ESI). Compounds
2
and 6
can best be distinguished by the 13C chemical shifts of
their α-carbon atoms (
2
: 100.9 ppm;
6
: 90.8 ppm). Additionally,
1
the α-carbon 13C of
2
shows a large doublet splitting of JF-C
=
At r.t.,
3 was stirred in an excess of anhydrous hydrogen
253 Hz.
fluoride (aHF) for 15 min. Later, excess aHF was slowly removed
at −78 °C in vacuum. The resulting colorless powder is stable
under inert conditions below −30 °C and under its own vapor
pressure even at 20°C. Solutions in HF or trichlorofluorome-
thane (CFCl3) are also stable. Otherwise
verse of eq. 4. No evidence for the presence of
in the gaseous phase (cf. ESI). Only two other isolated α-fluoro-
alcohols are currently known, trifluoromethanol (CF3OH)17,18
and heptafluorocyclobutan-1-ol (C4F7OH).13,14
Figure 1a shows the molecular structure of [N(CH3)4]+
[C7F13O]−
) in the crystalline state. The two anions in the
(4
asymmetric unit are approximately perpendicular to each
other. The [NMe4]+ cations form hydrogen bonds to the oxygen
and fluorine atoms. The closest contacts exist between the
oxygen atom and three of the methyl-hydrogen atoms from one
cation and to one hydrogen atom from the other cation (Figure
1a). The second formula unit coordinates analogously (see ESI).
The shortest OH contacts between cations and anions at
2.37(3) Å and 2.51(3) Å are long and represent only minor inter-
actions. Both anions in the asymmetric unit adopt chair con-
formations with the CF3-group in equatorial position (O axial).
According to DFT-calculations§ the CF3 group prefers, due to its
steric demand, this position by 20 kJ mol−1 in energy relative to
an axial position.
2
decomposes in a re-
was detected
2
19F NMR spectra of
4 in CD2Cl2 show the signal of the CF3
group in the typical range21 at −73.1 ppm. 19F NMR signals of the
equatorial fluorine atoms of the CF2 groups are at −135.1,
−138.8 and −142.2 ppm, those of the axial fluorine atoms at
−116.5 (two overlapping signals) and −122.7 ppm. All signals are
multiplets. The higher-order spin system (AA’BB’CC’DD’EFG3)
prevents a direct measurement of coupling constants. The
conspicuously large splitting of the CF2 resonances with ≥270 Hz
is the sum of more than a single coupling constant, but
2
dominated by the respective JF-F-coupling. Assignments were
derived from 2D 19F COSY and J-19F,13C-HSQC spectra (cf. ESI).
1
The CF3 group gives a13C resonance at 124.6, the α-carbon atom
at 84.3 ppm. Other 13C chemical shifts are between 108.5 and
114.1 ppm (cf. ESI).
1H NMR spectra of
4.00 ppm. The spin system is similar to that of
1
in CD2Cl2 show the OH resonance at
, but with an
4
additional proton’s spin. In the 19F NMR spectrum the CF3 group
resonates at −70.8 ppm. The signal of the axial fluorine atoms
are fully separated at −119.1, −120.7 and −124.0 ppm, the
Figure 1a (left): Section of the crystal structure of 4. Selected bond lengths (Å) and angles
(°): O1–C1 1.328(3), O2–C8 1.328(3), C1–C7 1.557(3), C8–C14 1.549(3), C7–F11 1.338(3),
signals of the equatorial fluorine atoms at −132.2, −139.5 and C7–F12 1.343(3), C7–F13 1.345(3), O1···H19A_$1 2.37(3), O1···H19B_$2 2.51(3),
F2···H20B_$1 2.28(3); O1-C1-C7 110.5(2), O1-C1-C6 110.2(2), C7-C1-C2 108.7(2) ), C7-C1-
C6 109.5(2), F1-C2-F2 106.1(2), O2-C8-C14 110.6(2), O2-C8-C13 110.5(2), O2-C8-C9
110.8(2), C14-C8-C9 109.1(2). Symmetry codes: $1 = x, ½−y, ½−z, $2 = ½−X, −½+Y, ½−Z
Figure 2b (right): Section of the crystal structure of 5. Selected bond lengths (Å) and
−142.1 ppm. The shift differences to corresponding signals of
the alkoxide are more pronounced for fluorine atoms closer to
the α-carbon atom. The α-carbon 13C resonance (74.4 ppm) is
angles (°): O1–C1 1.257(1), C1–F11 1.489(1), C1–C2 1.563(2), F9···H12A_$1 2.47(3),
2 | J. Name., 2012, 00, 1-3
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O1···H9C_$1 2.48(3), O1···H11B_$1 2.44(3), O1···H9A_$2 2.41(3), O1···H11A_$2 2.48(3),
O1-C1-C2 115.2(1), O1-C1-F11 114.0(9), C2-C1-F11 100.2(8) C2-C1-C6 109.9(9), C3-C4-C5
113.0(9), O1-C1-C2 115.2(1), F1-C2-F2 106.9(9), F3-C3-F4 107.2(9). Symmetry codes: $1
= 1−x, 1−y, 1−z, $2 = x, 1+y, z. Displacement ellipsoids are drawn at 50% probability level.
0.79(3), C1–O1 1.380(3), C1–O2 1.406(3), C7–O3 1.401(3), C7–O4 1.387(3), H1···O3
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2.05(2), O4–H4 0.78(2), H4···O2_$1 2.01(3), O2–H2 0.79(2), H2_$1···O1 2.15(3); O1-C1-
DOI: 10.1039/C8CC05148H
O2 108.8(2), O3-C7-O4 113.3(2), O1–H1···HO3 154.2(2), O4–H4···O2_$1 169.3(2),
O2_$1–H2···O1 142.1(2). Symmetry codes: $1 = ½−x, +y, −½+z.
C–C and C–F bonds are of typical lengths,7,14 the C–O bond at
1.328(3) Å is relatively short (see later).
Figure 1b displays the molecular structure of [N(CH3)4]+
[C6F11O]– (
5). Like in 4, Weak hydrogen bonds between O···H
und F···H (2.44(3)–2.48(3) Å) connect anions and cations. The
oxygen atom in equatorial position is favored over the axial by
4 kJ mol−1 in energy (DFT).§ C–C and C–F bond lengths are similar
to those of
4 A
and as expected for perfluoro-alkylalcohols.7,14
notable exception is the long C–F bond (1.489(1) Å) at the α-
carbon. In turn, the geminal C–O bond is very short (1.257(1) Å).
Assignments of O and F positions were proven after structure
refinement by reverse assignments and
analyzing the residual electron density
(see S-Fig. 18, ESI). They also agree with
DFT results. The combination of short
geminal C–O and long C–F bonds is
Figure 4: Asymmetric unit of the structure of co-crystals of 1 and H2O. Displacement
ellipsoids are drawn at 50% probability level. Selected structural parameters (Å/°) of
hydrogen bridges: O2−H 0.79(3), H···O5 1.92(3), O2···O5 2.643(3). O2−H···O5 153(3),
O5−H 0.75(4), H···O3 2.28(4), O5···O3 2.938(3), O5−H···O3 116(3), O5−H 0.82(3), H···O1
2.22(3), O5···O1 2.938(3), O5−H···O1 150(3), O3−H 0.76(3), H···O4 1.98(3), O3···O4
2.705(3), O3−H···O4 160(3), O1−H 0.73(4), H···O4 1.99(4), O1···O4 2.711(3), O1−H···O4
168(4)°, O4−H 0.82(4), H···O2´ 2.18(4), O2´···O4 2.843(3), O4−H···O2´ 137(3)°.
probably induced by conjugation of the oxygen lone pair with
the antibonding *-orbital of the C–F bond (see left inset). By
contrast, the α-C–F bond of heptafluorocyclobutanol at 1.352 Å
is rather normal.14
The three of the four hydrogen bonds donated by the water
molecules are thus considerably longer [HH2O···O(1 : 2.22(3),
2.18(4), 2.28(4) Å, O···O: 2.938(3), 2.843(3), 2.938(3) Å] than the
)
Hydrogen bonds from 2.01(3) Å (H4···O2) up to 2.15(3) Å
(H3···O4), shorter than those in hexafluoro-cyclobutan-1,1-
diol,14 connect the molecules of
6. They form zigzag-chains in
hydrogen bonds of protons donated by
1 (H(1 ···OH2O: 1.92(3),
)
1.99(4), 1.98(3), O···O: 2.643(3), 2.711(3), 2.705(3) Å), the
fourth one bonds to a water molecule (HH2O···OH2O: 2.28(3),
O···O: 2.920(3) Å)
four pillar-stacks; their backbones aggregate in fluorinated do-
mains (ESI). The C–O bond lengths within the diol range from
1.380(3) to 1.406(3) Å and are ~0.15 Å longer than in
5.
Experimental gas-phase structures of free molecules of
1
The extreme affinity of to water became obvious in a first
1
and
6
were determined by means of gas electron diffraction26
attempt to determine its crystal structure by crystallizing the
liquid material transferred in vacuum into a sealed capillary di-
rectly on an X-ray diffractometer. This sample turned out to be
augmented by quantum-chemical calculations (PBE0/cc-pVT,
details see ESI). Figure 5 shows the radial distribution curves,
Table 1 lists selected structure parameters. The C−O and C−F
distances were refined27 in a group for both molecules with the
differences fixed at the quantum-chemically determined values
(see ESI for details). Parameters regarding hydrogen atoms
positions were regularized by quantum-chemical values in a
way described earlier.28
a water adduct of the stoichiometry (1)3(H2O)2 (Figure 4). The
structural parameters of all three crystallographically indepen-
dent molecules are very similar and agree well with parameters
of the gas-phase structure of the isolated molecule (vide infra).
In the molecular assembly 1 has the role of a weak acceptor but
reasonably good donator for hydrogen bonds.25
Table 1: Selected structural parameter of the gas-phase structures of 1 and 6. Distances
rg are given in Å and angles and dihedrals in degree. Errors are given as 3.
Parameter
1
6
OH axial
1.368(1)
OH equatorial
rg(C–O)
rg(O–H)
1.421(1)
1.386(1)
0.986(13)
109.3(19)
0.994(13) 0.986(13)
111.9(11) 108.0(19)
(C–O–H)
(O–C–(O/C) 110.0(10)
(HOC(O/C) 62.1(11)
111.2(10)
178.0(30) 65.2(30)
Figure 3: Section of the crystal structure of 6. Displacement ellipsoids are drawn at 50%
probability level. Selected structural parameters (Å/°) of hydrogen bridges: O1–H1
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b) U. P. Preiss, G. Steinfeld, H. Scherer, A. M. T. ErVleie,wBA.rtBicelenOknmlinile,
The fluorinated scaffolds of
1 and 6 are similar to those in gas-
A. Kraft and I. Krossing, Z. Anorg. Allg. Chem., 2013, 639, 714.
DOI: 10.1039/C8CC05148H
phase structures of previously investigated perfluorocyclo-
hexane29 and perfluoromethylcyclohexane.30 Both structures
exhibit no molecular symmetry. Strikingly different are their
C−O bonds: they are shorter in
steric demand for the CF3 vs. OH group.
5. a) I. Krossing and I. Raabe, Angew. Chem., Int. Ed. Engl., 2004, 43,
2066–2090; b) T. A. Engesser, M. R. Lichtenthaler, M. Schleep
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8. N. Trapp, Dissertation, Universität Freiburg, 2008.
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Ed., 2011, 50, 11253–11256.
6 than in 1 indicating a higher
The C−O distance in c-C6F10-1-(C6F5)OH at rg = 1.409(5) Å
indicates the steric demand of a pentafluorophenyl to be higher
than that of an OH group but less than that of a CF3 group.
Interestingly, the C−O distances in the diol are clearly different.
The axial C−O bond is about 0.02 Å longer than that the equa-
torial. This finding and the fact that the dihedral angle HOCO/C
of the equatorial OH group is the same within the error limits as
the corresponding parameter of the CF3-substituted alcohol,
emphasizes the conjugational effect mentioned above.
11. M. Elsayed Moussa, M. Fleischmann, E. V. Peresypkina, L. Dütsch,
M. Seidl, G. Balázs and M. Scheer, Eur. J. Inorg. Chem., 2017,
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Figure 5: Experimental (o) and model (—) radial distribution curves of 1 (left) and 6 (right)
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Acknowledgement
We thank the Magres-Verbund of the University Freiburg for
the NMR measurements. This work was financially supported by
Deutsche Forschungsgemeinschaft, joint grant KR2046/29-1
and MI477/33-1 and core-facility GED@BI grant MI477/21-2.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
§
Quantum chemical calculation were done at the (RI-)BP86/
def2-TZVPP22-level with the program Turbomole.23,24
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