The perfluorinated alcohols (F5C6)(F 3C)2COH and (F5C6)(F 10C5)COH: Synthesis, theoretical and acidity studies, spectroscopy and structures in the solid state and the

Article abstract of DOI:10.1039/c0cp02596h

The syntheses of the perfluorinated alcohols (F₅C ₆)(F₃C)₂COH (1) and (F₅C ₆)(C₅F₁₀)COH (2) are described. Both compounds were prepared in reasonable yields (1: 65%, 2

Full text of DOI:10.1039/c0cp02596h

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PAPER
The perfluorinated alcohols (F5C6)(F3C)2COH and (F5C6)(F10C5)COH:
synthesis, theoretical and acidity studies, spectroscopy and structures in
the solid state and the gas phasew
a
a
Nils Trapp,z Harald Scherer,z Stuart A. Hayes,yb Raphael J. F. Berger,yb
Agnes Ku
¨
tt,zc Norbert W. Mitzel,yb Jaan Saamezc and Ingo Krossingz*a
Received 19th November 2010, Accepted 21st January 2011
DOI: 10.1039/c0cp02596h
The syntheses of the perfluorinated alcohols (F5C6)(F3C)2COH (1) and (F5C6)(C5F10)COH (2) are
described. Both compounds were prepared in reasonable yields (1: 65%, 2: 85%) by reacting the
corresponding ketone with C6F5MgBr, followed by acidic work-up. The alcohols were
characterized by NMR, vibrational spectroscopy, single-crystal X-ray diffraction, acidity
measurements and gas-phase electron diffraction. A combination of appropriate 2D NMR
experiments allowed the unambiguous assignment of all signals in the 19F spin systems, of which
that of 2 was especially complex. High acidity of the alcohols is indicated by acidity
measurements as well as the calculated gas phase acidities. It is also supported by the crystal
structure of 2, which exhibits only a single weak intermolecular hydrogen bridge with an Oꢀ ꢀ ꢀO
distance of 301 pm. This shows the low donor strength of the oxygen atom in the compound,
which is partly compensated through formation of two intramolecular CFꢀ ꢀ ꢀH contacts of 220
and 232 pm length to the proton not involved in the hydrogen bridge. The pKa values in
acetonitrile are 22.2 for 1 and 22.0 for 2; their calculated gas phase acidities are 1367 and
1343 kJ molꢁ1 (MP2/TZVPP level).
class is small. Thus straightforward syntheses of alcohols from
Introduction
common fluorinated precursors, which can be carried out in
standard glass vessels, are desirable as most laboratories are
not equipped for direct fluorination techniques. Perfluorinated
tertiary alcohols can be prepared by reacting perfluorinated
ketones with perfluorinated organometallic reagents5,6 and
SbF5-catalyzed rearrangement of fluorooxiranes.7 The com-
mercially available nonafluoro-tert-butanol can be obtained
by CsF-catalyzed disproportionation of fluoroacetone,8 by
halogen exchange from (F3C)2(Cl3C)COH,9 or even from the
extremely toxic perfluoroisobutene.10–12 More recently, fluoro-
alkylsilanes such as F3CSi(CH3)3 were successfully used to
transfer fluoroalkyl groups to carbonyl compounds,13,14
anhydrides or activated esters of carboxylic acids,15 usually
in the presence of fluoride as a catalyst.
Partly fluorinated and perfluorinated alcohols are used as
specialty solvents, owing to their low nucleophilicity,1 high
H-bond donor strength2 and optical transparency. It was
recently shown, for example, that the oxidation of olefins by
hydrogen peroxide is accelerated in 1,1,1,3,3,3-hexafluoro-2-
propanol as a solvent by a factor of 105, if compared to
1,4-dioxane.3 Perfluorinated tertiary alcohols, such as nona-
fluoro-tert-butanol, share these qualities and additionally exhibit
exceptional chemical robustness, which can be exploited e.g. in
the synthesis of large and stable weakly coordinating anions.4
However, the number of easily available compounds in this
a Albert-Ludwigs-Universitat Freiburg, Institut fur Anorganische und
¨
¨
Analytische Chemie and Freiburger Materialforschungszentrum
FMF, Albertstr. 21, D-79104 Freiburg, Germany.
Extensive studies on hydrogen bonding in fluorinated alcohols
were published.16–19 In summary, the presence of electron-
withdrawing groups adjacent to the hydroxyl function makes
the oxygen a weaker Lewis donor (weaker hydrogen bond
acceptor), while the proton is a stronger Lewis acceptor
(stronger hydrogen bond donor). However, this higher Lewis
acidity does not compensate for the reduced basicity of the
oxygen atom. As a result, there is weaker intermolecular
hydrogen bonding between the fluorinated alcohols themselves.
Often steric hindrance additionally reduces the extent of
E-mail: krossing@uni-freiburg.de; Fax: +49 76 1203 6001
b Universitat Bielefeld, Lehrstuhl fur Anorganische Chemie und
Strukturchemie, Universitatsstr. 25, D-33615 Bielefeld, Germany.
¨
¨
¨
c Institute of Chemistry, University of Tartu, 14a Ravila Street,
50411 Tartu, Estonia.
w Electronic supplementary information (ESI) available. CCDC
reference number 800116. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c0cp02596h
z Preparation, spectroscopy, X-ray diffraction, calculations.
y Gas-phase electron diffraction.
z Acidity measurements.
c
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intermolecular hydrogen bonding. At the same time strong
interactions are observed between the fluorinated alcohol and
a donor molecule (such as ethers). For example, hexafluoro-
isopropanol forms a discrete complex with tetrahydrofuran
and other basic solvents.2
Spectroscopic analyses
Infrared and Raman spectra of neat liquid 1 and solid 2 are in
good agreement with calculated bands (Fig. 2 and Tables
S1–S3 in ESIw) and show the expected signals in the range
of C–F and C–C bands. The main O–H stretching modes were
recorded at 3610 cm–1 for 1 and 3623 cm–1 for 2. Both signals
exhibit shoulders, indicating the existence of associated structures
in their standard state. In agreement with this observation,
dimers were found in the crystal structure of 2 (see below).
Results and discussion
(F5C6)(F3C)2COH (1) was prepared by a Grignard-type reaction
originally described elsewhere,6 avoiding the erratically explosive
C6F5Li,20 with slight optimizations to increase the total yield.
C6F5MgBr was reacted with excess gaseous hexafluoroacetone
in diethyl ether and then hydrolyzed with aqueous HCl. The neat
product can only be obtained by distillation of the resulting ether
adduct 1ꢀOEt2 from concentrated H2SO4 in 65% overall yield.
(F5C6)(C5F10)COH (2) was formed in a similar reaction (100 g
scale) from the Grignard reagent and perfluorocyclohexanone,
i.e. with no volatile reagents (Fig. 1). The product is a colorless
powder, which was purified by sublimation (85% overall yield).
Both alcohols are soluble in ethers, toluene, chloroform,
dichloromethane, acetonitrile, fluorobenzene and 1,2-difluoro-
benzene. 2 is soluble in hexane. Other solvents were not tested.
Experimental procedures are described in detail in the ESI.w
NMR spectroscopic characterization
The 19F-NMR spectrum of 1, recorded at room temperature in
CDCl3, shows the typical chemical shifts to be expected for
pentafluorophenyl groups at ꢁ160.2 ppm for the signal of the
meta-fluorine atoms and at ꢁ148.9 ppm for the fluorine atom
in the para-position. The ortho-fluorine atoms are not chemi-
cally equivalent and display two broad resonances (FWHM E
300 Hz) at ꢁ138.4 ppm and ꢁ132.8 ppm. This indicates
a strongly hindered rotation of the pentafluorophenyl ring
also responsible for the broadening of the signal of the
meta-fluorine atoms. The resonance of the two CF3 groups
is observed at ꢁ76.2 ppm. All CF3 fluorine atoms are chemically
Fig. 1 Addition of the C6F5MgBr Grignard reagent to the fluorinated ketones, leading to compounds 1 and 2 after acidic work-up.
Fig. 2 Experimental (solid lines) and calculated (DFT-BP86/SV(P), dotted lines) vibrational spectra of (a) compound 1 and (b) 2.
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equivalent, but there are two slightly different 5J(19F,19F)
coupling constants to the ortho-fluorine atoms. In the
1H-NMR spectrum one broad signal without observable
splitting is found at 4.13 ppm. Its linewidth (FWHM = 8 Hz)
rules out 19F,1H-couplings bigger than 4 Hz. The carbon
chemical shifts could be easily assigned by 13C{1H}- and
2D-13C,19F-correlation spectra.
groups of the cyclohexyl ring are not chemically equivalent.
The spin systems in the aromatic and the aliphatic part of the
molecule are of higher order, resulting in complex splitting
patterns. For the resonances of the CF2 groups these patterns
are dominated by the large 2J(19F,19F) couplings with absolute
values larger than 270 Hz, making them easy to identify. The
13C,19F-correlation spectrum (Fig. 3A) allows us to determine
which fluorine resonances belong to the same CF2 group. The
19F signals at ꢁ113.6 ppm and ꢁ131.2 ppm correlate to the
same 13C resonance at 112.0 ppm (2), the 19F resonances at
NMR spectra of 2 were recorded in toluene-d8 solutions at
room temperature. The 19F NMR spectrum displays nine
signals due to the fact that the fluorine atoms of the CF2
Fig. 3 2D NMR correlation spectra of 2 in toluene-d8 at RT (Avance II+ 400 MHz). A: 13C,19F HSQC (1/(2J) = 1.25 ms); B: 19F NOESY
(mixing time: 0.8 s, negative intensities in solid black); C: 19F,1H COSY; lower left: Labelling of the atoms in the cyclohexyl ring of 2 as used in the
NMR discussion (R = C6F5).
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ꢁ117.7 ppm and ꢁ135.8 ppm to the 13C signal at 109.0 ppm
(3) and the 19F signals at ꢁ121.3 ppm and ꢁ140.7 ppm to the
13C resonance at 108.1 ppm (4), respectively. Because of their
intensities, which are half of the integrals of the other aliphatic
fluorine resonances, the signals at ꢁ121.3 ppm and ꢁ140.7 ppm
are assigned to the CF2 group (4) of the cyclohexyl ring.
Typical values of the 13C and 19F chemical shifts are
detected for the pentafluorophenyl group. The resonance of
the meta-fluorine atoms is found at ꢁ159.0 ppm, of the
para-fluorine atom at ꢁ146.7 ppm and of the ortho-fluorine
atoms at ꢁ132.9 ppm. The signal of the ortho-fluorine atoms
shows a significant line broadening, indicating a hindered
rotation of the pentafluorophenyl ring. The linewidth of this
signal precludes resolving of the splitting pattern, except for
a triplet splitting of 53 Hz that cannot be explained solely by
the spin system of the pentafluorophenyl group and must thus
be attributed to an additional coupling to two fluorine atoms
of the cyclohexyl ring. The absolute magnitude of this formal
5J(19F,19F) coupling strongly indicates a major contribution
of through-space interactions of the coupling fluorine
atoms. Given the structure of the molecule, the likeliest
coupling partners responsible for this triplet splitting of the
ortho-fluorine resonances are the axial fluorine atoms of the
CF2 group 2.
In a 19F,19F COSY spectrum (see ESIw) the signal of
the ortho-fluorine atoms correlates to the resonances at
ꢁ113.6 ppm and ꢁ131.2 ppm, which must hence be assigned
to the CF2 group 2, leaving the fluorine signals at ꢁ117.7 ppm
and ꢁ135.8 ppm for the CF2 group 3 of the cyclohexyl ring.
Nevertheless, there are cross-peaks of the ortho-fluorine reso-
nances to both the fluorine signals of the CF2 group 2. An
analysis of the splitting patterns of the signals in the 1D 19F-
NMR spectrum shows clearly that the 53 Hz coupling is found
in the resonance at ꢁ113.6 ppm, which is therefore assigned to
the fluorine atoms in the axial position of these CF2 groups.
An independent confirmation of this assignment together
with additional information about the orientation of the
aliphatic fluorine atoms can be obtained by homonuclear
19F NOE measurements, which are not yet commonly used
in 19F-NMR.21 In fact, to our knowledge it is only the fifth
example in the literature where homonuclear 19F NOEs are
used as an analytical tool.22–25 In the 19F,19F NOESY
(Fig. 3B) only one cross-peak is found between the resonance
at ꢁ113.6 ppm (2a) and signals of the neighbouring CF2 group
3, whereas for the associated signal at ꢁ131.2 ppm (2e) there
are cross-peaks to both resonances of this CF2 group. This
unambiguously proves that the signal at ꢁ113.6 ppm must be
assigned to the fluorine atom in the axial position of the CF2
group 2 and the signal at ꢁ131.2 ppm to the corresponding
fluorine atom in the equatorial position. The one CF2 fluorine
signal of CF2 group 3 that shows a cross-peak to the signal at
ꢁ113.6 ppm, and consequently, belongs to the fluorine atom in
the equatorial position of this CF2 group, is observed at
ꢁ135.8 ppm (3e). The resonance at ꢁ117.7 ppm (3a) must
hence be the signal of the fluorine atom in the axial position of
this CF2 group. Finally there is a NOE cross-peak between the
signal at ꢁ113.6 ppm (2a) and the resonance at ꢁ121.3 ppm
(4a) of CF2 group 4, which is assigned to the fluorine atom in
the axial position, leaving the signal at ꢁ140.7 ppm (4e) for the
corresponding equatorial fluorine atom. All other NOE cross-
peaks are in accordance to this assignment. By this means, all
aliphatic fluorine resonances and the positions of the fluorine
atoms in the cyclohexyl ring are clearly assignable. It should be
noted that as a result, the signals of the fluorine atoms in axial
and equatorial positions can be distinguished by their chemical
shift, which is about 18 ppm to higher field for the resonances
of the fluorine atoms in the equatorial position. Unfortunately
the cross-peaks of the resonance at ꢁ113.6 ppm (2a) to the
ortho-fluorine signal are dispersive, which indicates magneti-
zation transfer via spin–spin coupling for these cross-peaks,
superimposing potential NOE.
1
The H NMR spectrum shows the OH proton signal as a
triplet at 3.14 ppm. In the 19F,1H COSY-NMR spectrum
(Fig. 3C) cross-peaks of this proton signal to the ortho-fluorine
resonances and to the fluorine signals of 2a, 2e and 3a are
detected. The cross-peak to the ortho-fluorine signal is the
most intense, and unlike the others it shows no triplet splitting
in the proton dimension but a doublet with the middle signal
of the triplet missing. Taking the mechanism of magnetization
transfer for COSY-type experiments into account, this proves
that the 19F,1H-coupling constant of 5.3 Hz is the active
coupling for this cross-peak and hence is the coupling between
the proton and the ortho-fluorine atoms. For formal
5J(19F,1H) coupling constants of this order of magnitude in
such a structure, a decisive contribution of through-space
coupling is most likely.26
Thus a conformer with the proton pointing in the direction
of the perfluorophenyl group should be favored in solution as
it is also found for the calculated molecular structures of 2
discussed below. A 19F,1H HOESY (see ESIw) contributes
additional confirmation for this interpretation. However, this
spectrum also demonstrates that conformers with the proton
directed to the cyclohexyl ring exist as well.
Gas-phase electron diffraction investigation of 1
GED (gas-phase electron diffraction) data were recorded for 1
and its molecular structure was refined making use of the
SARACEN method27 of structural refinement. An important
structural feature of this compound is the possibility of
intra-molecular hydrogen bonding, however, the positions of
hydrogen atoms are usually not well-determined by GED due
to the relatively low electron-scattering cross-section of the
hydrogen atom. On the other hand, as there are no elements
heavier than fluorine present in 1, it was possible that the
diffraction from hydrogen might not be completely lost, thus,
models with and without the hydrogen atom were tested.
Despite the lack of any heavy atoms that would have domi-
nated the electron scattering, the two models gave an equally
good fit of the theoretical and experimental intensities, with
almost identical R-factors and refined heavy-atom parameters.
In the model containing the hydrogen atom, the uncertainties
on the parameters describing the hydrogen atom positions
were equal to the applied restraints, reinforcing the conclusion
that there is no information in the experimental data in this
regard.
The refined geometric molecular parameters, for which
structural information could be obtained, and the refined
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Table 1 Ten selected (of 34) independent parameters (p) for which information was obtained in the GED study and dependent parameters (d)
that were restrained during the GED refinement, as well as restraints and their uncertainties. The atom numbering is shown in Fig. 5
Independent parameters
GED (rg)
GED (rh1
)
Restraint
MP2/TZVPPa (re)
p1
p3
p6
p8
p21
p23
p25
p29
p32
p33
r C–C average (aromatic)
1.404(2)
1.556(5)
1.409(5)
1.339(1)
113.2(4)
31.0(15)
110.6(4)
107.5(2)
86.2(9)
1.400(2)
1.549(5)
1.409(5)
1.340(1)
113.4(4)
29.8(19)
110.2(4)
107.2(2)
85.8(8)
1.391
1.538
1.398
1.332
112.8
31.5
111.0
108.0
84.6
r C–C average (aliphatic)
r C–O
r C–F average
+ C(1)–C(12)–O
f C(4)–C–C–O
+ C(1)–C(12)–C average
+ F–C–F
f C(1)–C–C–F(16)
112.8(5)
31.5(100)
84.6(50)
67.3(50)
f C(1)–C–C–F(20)
65.4(23)
66.7(22)
67.3
Dependent parameters
d1
d2
r C–O minus C–C aromatic (p6 minus p1)
r C–C aromatic minus C–F (p1 minus p8)
R-Factor
0.006(5)
0.065(3)
6.52%
0.009(5)
0.060(4)
6.60%
0.007(5)
0.059(5)
0.007
0.059
—
a
Optimized in C1 symmetry.
values are displayed in Table 1, whilst the molecular scattering
intensity for 1 is shown in Fig. S5 of the ESI.w The radial-
distribution curve (RDC, Fig. 4) exhibits five distinct peaks
(the first of which comprises all of the bonded distances), two
shoulders and an elongated tail. Each of these features can be
interpreted as having the potential to provide some structural
information, whilst the remaining features, such as the peak
shapes and relative heights, may contain further information
that will generally be less certain. In any case, the molecule is
too large for a full structural refinement based on the GED
data alone, thus a combination of constraints and restraints
was applied, along the lines of the SARACEN method27,28 of
structural refinement. The constraints used were based on the
results of ab initio calculations and are in line with the local
symmetry suggested by solution NMR spectroscopy, namely
pseudo-C2v symmetry for the perfluorophenyl group (the
deviation from this symmetry is due to independent placing
of the ortho-fluorine atoms), and assumption of a single
internal F–C–F angle for the two CF3 groups. The molecular
structure (Fig. 5) was thus defined in terms of 34 independent
parameters (a full description of the model and the indepen-
dent parameters used is provided in ESIw). Of these, only six
parameters were refined without being directly restrained and
a further three parameters refined with only loose restraints,
yielding uncertainties significantly smaller than the corres-
ponding restraint uncertainty, therefore indicating the influence
of the experimental data in the refined structure. However, due
to the similarity of the bonded distances, as evident from the
radial-distribution curve, the aromatic C–C, the C–O and
the C–F distances are correlated, and the differences between
the aromatic C–C distance, and the C–O and C–F distances
were restrained to calculated values, as summarized in Table 1.
In addition to these parameters, the C–C–O angle could also
be refined without a restraint, converging to 115.8(8)1 or
114.2(8)1 depending on whether the respective model for
vibrational motion adopted was rg or rh1. Only the first of
these values is significantly larger than the calculated value of
112.81, however, the disagreement between the two structure
types gives a more faithful estimate of the parameter uncertainty
than just the standard deviation obtained from the least-squares
refinement (which assumes a particular model). In the end, this
parameter was restrained to the calculated value, as shown in
Table 1, without any detrimental effect on the quality of the
refinement.
Theoretical calculations suggest two possible conformers, as
evident by local minima on the potential energy surface, which
are shown in Fig. 5: one with the hydrogen atom in the cis
conformation with respect to the perfluorinated phenyl moiety
(Fig. 5a) and another in the trans position (Fig. 5b). Calculations
at both the MP2 and B3LYP levels of theory predict the cis
Fig. 4 Experimental and difference (experimental minus theoretical)
radial-distribution curves. Molecular-scattering intensities were multiplied
by sꢀexp[(2 ꢂ 10–6 s2)/(ZF – fF)(ZO – fO)] prior to Fourier transformation.
Fig. 5 Molecular geometries of 1, calculated to be minima on the
potential energy surface, and their relative electronic energies (MP2/
TZVPP). Atom numbering is as used in the GED refinement.
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conformer to have the lowest energy, but the calculated energy
difference is smaller than can be accurately calculated and, as
mentioned above, the two conformers could not be distin-
guished by GED. However, as can be seen from Table 1, the
calculated results are in good agreement with those from GED,
thus it seems that the MP2 method performs well for this type of
chemical system. Full details of the structural model, refined
parameter values (rh1), molecular geometries for both the rh1
and rg structure types, data analysis parameters, correlation
matrix and up-hill curves are provided as ESI.w
formation of only a single weak hydrogen bridge. Correspond-
ingly, in the molecule which acts as hydrogen donor, the C6F5
group is rotated by 81.71 with respect to the C6F10 group,
while in the other groups both are orthogonal with an angle of
89.71.
Expectedly, in the MP2/TZVPP calculation of the monomer
without any hydrogen bonding, the angle lies between both
values with 85.01. The calculated contact distances between the
fluorine atoms in the ortho position of the C6F5 ring and the
2-position of the C6F10 ring are 243 and 255 pm, which is in
good accordance with the crystal structure (243 to 263 pm).
The proton not involved in the intermolecular hydrogen bond
additionally exhibits two rather short Hꢀ ꢀ ꢀF contacts at 220
pm (with the fluorine atom in the ortho position of C6F5,
Oꢀ ꢀ ꢀF: 250 pm) and 232 pm (with a fluorine atom in C6F10,
Oꢀ ꢀ ꢀF: 273 pm) in the crystal structure. These interactions are
comparable to the Hꢀ ꢀ ꢀF interactions in 1 (Hꢀ ꢀ ꢀF (calc., MP2/
TZVPP): 246 and 184 pm, Oꢀ ꢀ ꢀF (calc.) 271 and 260 pm) and
in good agreement with the MP2/TZVPP minimum geometry
(which features Oꢀꢀ ꢀF distances of 250 and 269 pm, respectively).
Apart from the global minimum, a Cs symmetric local minimum
only 13 kJ mol–1 higher in energy was found (Fig. 7).
Crystal structure analysis of 2 and results from computations
Compound 2 crystallized at room temperature from a hexane
solution in the form of colorless needles. The space group was
determined as monoclinic P21/c with a = 12.05 A, b = 15.16 A,
c = 15.99 A, b = 113.91 and Z = 8.
The asymmetric unit contains a dimer of 2 (Fig. 6) con-
nected by an asymmetric hydrogen bridge. All contacts fall
into the characteristic range for weak O–Hꢀ ꢀ ꢀO bridges29,30
with H2ꢀ ꢀ ꢀO1: 223.1(1) pm, H2–O2: 88(3) pm (corrected for
libration), O1ꢀ ꢀ ꢀO2: 301.2(1) pm and an O–H–O angle of
166(3)1. The H atom not involved in the H bonding points
away from the hydrogen bridge with a H–O–H angle of
119.1(1)1 (both hydrogen atoms were found on the Fourier
map and refined without positional constraints). This bonding
scenario is in agreement with the considerations from earlier
studies, already described above. Perfluorinated alcohols tend
to be poor donors for intermolecular H bonding due to the
presence of electron-withdrawing groups near the oxygen
atom. In the absence of a donor and in combination with
the high steric demand of the C12F15 moiety, this leads to the
This is in good agreement with the NMR spectroscopic
results. As discussed above, these indicate a dynamic system in
solution, in which the global minimum with the H atom
pointing in the direction of the pentafluorophenyl group is
slightly favoured.
Another crystal structure (2a) could be recorded, in which 2
co-crystallized with n-butylammonium bromide and one water
molecule. The crystal quality was very poor, hence the structure
is regarded as preliminary and shall not be discussed in much
detail. However, it exhibits a very short hydrogen bridge
between the hydroxyl group of 2 and the water molecule
(Oꢀ ꢀ ꢀO distance: 254 pm, cf. 250–260 pm in H9O4+). This is
another clear indication of the low basicity of the alcohol and
underlines its potential in further syntheses. More details of
this structure are deposited with the ESI.w
Acidities of the perfluorinated alcohols
Table 2 shows calculated and experimental acidity values of
compounds 1 and 2 in comparison with those of the acidic
perfluorinated alcohols nonafluoro-tert-butanol
3
and
perfluoro-1-adamantanol C10F15OH 4. For experimental pKa
values in acetonitrile of a wider range of acids, including non-
fluorinated species, see ref. 31 and 32.
The acidities of 1–4 are governed on one hand by the
intrinsic properties of the compounds and on the other hand
by solvent effects. The gas-phase acidities are influenced by the
intrinsic properties only. The solvent acidities (pKa values) are
influenced both by intrinsic properties and solvent effects. The
following intrinsic properties are important:
(1) the inductive effect of the b-fluorine atoms; (2) release of
steric strain via shortening of the Ca–O bond upon deprotonation
and simultaneous elongation of the Ca–Cb bond (see ref. 35 for
examples of the high importance of steric strain release on
acidity of strained polyfluorinated alcohols); (3) possible weak
intramolecular hydrogen bonding in the neutral molecules
1 and 2; (4) steric shielding of the anionic centre in the anions
Fig. 6 Asymmetric unit of the crystal structure of 2. Thermal
ellipsoids drawn at 50% probability level. Distances are given in pm,
libration-corrected distances are indicated by an asterisk: O1–O2
301.2(3) pm, O1–H1 93(4)*, O2–H2 88(3)*, O1–C 141.7(1), O2–C
140.0(1), O1–H2–O2 166.3(2)1.
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Fig. 7 H–F contacts and F–F contacts in (a) the C1 global minimum geometry of 2; (b) another local minimum structure with Cs symmetry.
Geometries were calculated at the MP2/TZVPP level (distances given in pm).
weakest. All of measured compounds have high homo-
conjugation constants in acetonitrile. This is an indication of
unfavorable solvation of the perfluoroalcoholate anions in
acetonitrile, so that the anions are stabilized via hydrogen
bonding with the corresponding neutral compounds (which
are also very strong hydrogen bond donors) present in the
solution.
Table 2 Experimental pKa and logKAHA values in acetonitrile;
experimental and calculated (MP2/TZVPP) gas phase acidities (GA,
kJ molꢁ1) of fluorinated alcohols including compounds 1 and 2
Compound
pKa (MeCN) logKAHA GA (exp.) GA (calc.)
(F3C)2(F5C6)COH 1 22.15
4.4
—
—
1367
1343
1366
1328
C12F15OH 2
(F3C)3COH 3
C10F15OH 4
22.00
20.5531
18.25
4.7
4.831
4.8
135533
132134
Conclusions
and the –OH group in the neutrals; (5) polarizability of the
fluorocarbon moiety of the alcohol. Possible negative hyper-
conjugation should also be mentioned (see ref. 33), although
its effect is small.
The perfluorinated alcohols (F5C6)(F3C)2COH (1) and
(F5C6)(C5F10)COH (2) were synthesized and fully characterized;
compound 2 is available in a convenient 100 g scale reaction
and easily purified by sublimation. 2 exhibits a weak inter-
molecular OH bridge and two short intramolecular CFꢀ ꢀ ꢀH
contacts, indicative of the low donor strength and thus high
acidity of the compound. These interactions complicated the
already non-trivial NMR studies even further; however, the
spectra could still be fully assigned and interpreted using a
combination of (2D-)NMR experiments, crystal structure data
and computations. Both compounds, as well as their metal
alkoxides (which will be discussed in detail in an upcoming
publication), are promising precursors for further chemistry
with perfluorinated ligands. There are especially interesting
prospects for crystallography, as the number of rather rigid,
stable perfluorinated ligands such as in 2 is still limited. While
not quite as acidic as e.g. (F3C)3COH, the examined alcohols
are still very suitable as ligands in stable and weakly coordi-
nating anions exhibiting no crystallographic disorder, as will
be shown in upcoming publications.
The first two properties are the most important both in the
gas phase and in solution. The number of b-fluorine atoms is 6
in 1, 4 in 2, 9 in (F3C)3COH 3 and 6 in perfluoroadamantanol
4. Release of steric strain is an acidifying factor in perfluoro-
adamantanol, but probably not in the other three alcohols (see
ref. 34). In the gas phase, where polarizability and size are very
important for higher acidity, 2 is a markedly stronger acid
than 1 (by 24 kJ molꢁ1) and the rather bulky perfluoroada-
mantanol 4 is the strongest (calc. GA 15 kJ molꢁ1 higher than
that of 2) of all measured alcohols. Even though 3 has the
largest number of b-fluorine atoms (and is clearly a stronger
acid than 1 and 2 in acetonitrile), in the gas phase it is
significantly weaker than 2 and comparable to 1. There is also
the possibility of weak intramolecular hydrogen bond formation
in compounds 1 and 2 (Fig. 6).
In acetonitrile solution, in addition to the intrinsic effects,
solvation of the anions becomes important. The anions of all
four alcohols have localized charges and even with a poor
H-bonding donor such as MeCN, there is some solvent effect.
Also, steric shielding of the anionic centre comes into play.
Looking at the geometries of the anions, the –Oꢁ centre seems
to be the most shielded in 2 and the least shielded in perfluoro-
adamantanol 4 (Fig. 7 in ESIw). Taking into account these and
the above considerations, it appears logical that in MeCN
perfluoroadamantanol 4 is the most acidic of the three com-
pounds, followed by (F3C)3COH 3, and 1 and 2 are the
Acknowledgements
The authors gratefully acknowledge financial support from the
DFG, FCI and the University of Freiburg as well as from the
Estonian Science Foundation (grants No 7374 and 8162). We
also thank Prof. Ivo Leito for fruitful discussions and correc-
tions and Prof. Gerhard Hagele for helpful advice concerning
¨
the NMR studies.
c
6190 Phys. Chem. Chem. Phys., 2011, 13, 6184–6191
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