Fluorinated butatrienes

Article abstract of DOI:10.1016/j.jfluchem.2010.06.017

Major improvements in the synthesis of 1,1,4,4-tetrafluorobutatriene (1) are presented. Despite many attempts to isolate new metal complexes of 1 only an iron complex containing a ligand which is composed of a partially hydrolyzed tetrafluorobutatriene-dimer and carbon monoxide could be isolated and characterized by X-ray crystallography. Certain metal centers and solvents accelerate the decomposition of 1. First attempts to synthesize 1,1-difluorobutatriene (2) are presented which underline the major challenges of a successful synthesis of 2.

Full text of DOI:10.1016/j.jfluchem.2010.06.017

Journal of Fluorine Chemistry 131 (2010) 1173–1181
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Fluorinated butatrienes
Christian Ehm, Floris A. Akkerman, Dieter Lentz *
Institut f u¨ r Chemie und Biochemie, Abt. Anorganische Chemie, Freie Universit a¨ t Berlin, Fabeckstraße 34-36, 14195 Berlin, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Major improvements in the synthesis of 1,1,4,4-tetrafluorobutatriene (1) are presented. Despite many
attempts to isolate new metal complexes of 1 only an iron complex containing a ligand which is
composed of a partially hydrolyzed tetrafluorobutatriene-dimer and carbon monoxide could be isolated
and characterized by X-ray crystallography. Certain metal centers and solvents accelerate the
decomposition of 1. First attempts to synthesize 1,1-difluorobutatriene (2) are presented which
underline the major challenges of a successful synthesis of 2.
Received 8 April 2010
Received in revised form 26 June 2010
Accepted 28 June 2010
Available online 6 July 2010
Keywords:
ß 2010 Elsevier B.V. All rights reserved.
Fluorinated cumulene
Metal complex
Iron
Triene
1
. Introduction
,1,4,4-Tetrafluorobutatriene 1 was first prepared by Martin
triene, its coordination chemistry and first attempts of the
synthesis of compound 2.
1
and Sharkey in 1959 [1]. Its chemistry remained almost unex-
plored due to the compound’s extreme instability, it polymerizes
even at ꢀ85 8C and is said to explode violently on warming to ꢀ5 8C
or contact with air. Even in inert solution it decomposes within a
few hours forming numerous unresolved oligomers. The known
chemistry of 1 is limited to a few derivatives also reported by
Martin and Sharkey, obtained by addition of bromine and chlorine,
and oxidation [1]. Later, Raman- and PE-spectroscopic data were
added [2,3] and its structure and experimental charge density were
elucidated by high-resolution X-ray diffraction [4]. Recently, the
2
. Results and discussion
2.1. Revised synthesis of tetrafluorobutatriene
An improved synthesis of 1 starting from commercially
available 1,1-difluoroethene (3) is depicted in Scheme 1.
first perfluorobutatriene complexes [IrCp*(PMe
3
)(C
4 4
F )] [5] and
2
The previously published syntheses of the precursor 2,2-
difluoroiodoethene (5) for the synthesis of 1 by Lacher and co-
workers [10,11] and later by Lentz and co-workers [12] could be
improved since both suffered from some disadvantages. The
elimination of hydrogen chloride from 1-chloro-2,2-difluoro-2-
iodoethylene (4) by potassium hydroxide in high boiling mineral
oil resulted in high conversion to the precursor 5 but due to
separation problems during the fractional condensation, yields
remained rather poor. The elimination of hydrogen chloride from 4
by potassium tert.-butoxide produces tert.-butanol and hence
involves the severe problem of separating the product from tert.-
butanol. Since tert.-butanol is a ball-shaped molecule, it passes
cooling traps even if they are cooled well beyond its boiling or
freezing point due to the high sublimation pressure of such
compounds [13,14]. Therefore purification has to be repeated
several times which consequently lowers the yield.
[
M-trans-(PPh ) (CO)(2,3-h -C F )Cl] (M = Rh, Ir) [6] were synthe-
3 2 4 4
sized by reduction of a sec.-perfluorobutyl ligand in the coordina-
tion sphere of iridium and from the free butatriene, respectively.
Additionally, 1 reacts with dienes forming the product of a Diels-
Alder reaction [7].
The only known partially fluorinated butatriene, 1,1-difluor-
obutatriene 2 was synthesized and spectroscopically characterized
by low temperature matrix-isolation experiments by Sander and
co-workers [8]. Until recently, none of the other partially
fluorinated butatrienes did appear in the literature. Recently, we
have studied all partially fluorinated butrienes as well as their
enyne isomers by computational chemistry methods [9]. Herein
we report on the optimization of the synthesis of tetrafluorobuta-
*
Corresponding author. Tel.: +49 30 83852695; fax: +49 30 83853310.
It turned out, that the elimination employing 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) in 2,6-lutidine results in a high
E-mail address: lentz@chemie.fu-berlin.de (D. Lentz).
0022-1139/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2010.06.017

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C. Ehm et al. / Journal of Fluorine Chemistry 131 (2010) 1173–1181
Scheme 1. Improved synthesis of tetrafluorofluorobutatriene (1).
conversion rate (Scheme 1, line 1), but large amounts of salt
precipitate during the reaction and incorporate substantial
amounts of the product, thereby lowering the yield.
the liquid nitrogen cooled trap, most likely carbon monoxide
resulting from further reactions of 1 with potassium hydroxide.
This can be caused by a too intense stream of starting material. The
use of technical grade potassium hydroxide proved to be useful,
presumably for two reasons: at first, the surface texture of the
technical potassium hydroxide is more rough compared to
chemical grade pallets. Hence the contact surface is increased.
Secondly, possible admixtures, e.g. potassium carbonate, could
play an important role. Consequential, the U-pipe has to have a
small diameter since larger ones result in larger amounts of used
potassium hydroxide and the contact surface is further increased.
However, an oversized contact surface results in a decrease of the
yield.
Summing up, there is a complex equilibrium between contact
time, contact surface, type of base and temperature which governs
the yield. The conditions mentioned above proved to give reliable
an overall yield of 42% of 1.
Martin and Sharkey stated that tetrafluorobutatriene explodes
when heated above its boiling point or in contact with air [1]. We
did not observe such an explosion when tetrafluorobutatriene (1 g)
was subjected to room temperature under its own vapor pressure.
Nevertheless, pure tetrafluorobutatriene 1 polymerizes even at
ꢀ78 8C and rapidly at room temperature to give a slightly pink to
red polymer.
This problem can be solved in two different ways. For small
scale preparations one can remove solvent and volatiles and use
the content which do not pass a ꢀ40 8C cooling trap as solvent for
another elimination. While yields in the first run are approxi-
mately 75%, it increases in every further run to 85–90%. Regarding
the limited volatility of 2,6-lutidine this work-up is time
consuming. For larger scale preparations the salts are dissolved
in water to allow an easier separation of the volatile materials by
fractional condensation under vacuum. Finally the volatiles are
distilled yielding 85% of analytically pure 5. The reaction was
scaled up to a two-mole scale without any noticeable decrease in
yield.
Furthermore it turned out, that careful addition of bromine to 7
is important to obtain good yields of 8. Compound 8 is stable in air
and room temperature but within several weeks the color changes
19
to slightly pink. Nevertheless F NMR spectra showed no change
of the compound.
The reaction conditions for the syntheses of 1 were optimized as
follows: the elimination is performed over hot potassium
hydroxide in a U-shaped-glass-pipe under high vacuum. The U-
pipe is connected to two cooling traps, ꢀ78 8C to collect remaining
starting material and water and ꢀ196 8C to collect the product. It
turned out that:
Once prepared, it can only be transferred safely by
condensation at reduced pressure and must be stored at
ꢀ
196 8C. This method has so far prevented explosion and
polymerization.
1
st: decrease of the elimination temperature in the U-pipe to
8 8C,
nd: reduction of the stream of the starting material,
8
2
2.2. Attempted synthesis of 1,1-difluorobutatriene
3rd: use of technical potassium hydroxide flakes instead of
chemical grade pellets,
th: use of a thin U-pipe with an inner diameter of below 1 cm,
Several potential strategies for the synthesis of 1,1-difluor-
obutatriene are outlined below. All of them involve dehydroha-
logenation, dehalogenation or rearrangement steps.
4
results in reproducible yields of 80–100%.
The potential synthesis of 1,1-difluorobutatriene depicted in
Scheme 2 looks especially attractive as it starts from the
commercially available hydrofluorocarbon 1,1,1,3,3-pentafluoro-
Up to 2 g (ꢁ8 mmol) of the starting material can be converted
until the U-pipe is plugged up by the resulting molten potassium
hydroxide/water mixture.
1
butane (9) (Solkane 365 mfc).
It is very important that the pressure in the system must not
exceed 10 mbar otherwise volatiles are produced that even pass
Although the but-2-enes 10 and the alkyne 11 could be detected
in the reaction mixture on treatment of 9 with various bases like
ꢀ
2

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1175
Scheme 2. Attempted synthesis of 1,1-difluorobutatriene (2) by subsequent elimination of hydrogen fluoride from 1,1,1,3,3-pentafluorobutane (9).
[(Scheme_3)TD$FIG]
sulfate to the reaction mixture proved to be useful, under identical
conditions run without magnesium sulfate yielded only
a
41% [16].
An UV–vis spectrum of the enyne (13) showed bands at 214,
225 and 230 nm and the enyne was therefore subjected to gas
phase UV-irradiation but no rearrangement was observed after
2
4 h of irradiation (Scheme 4).
The experiences gained from the handling and synthesis of
Scheme 3. Palladium-catalyzed synthesis of 1,1-difluorobut-1-en-3-yne (13).
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tetrafluorobutatriene (1) advise a similar synthetic strategy for the
synthesis of 1,1-difluorobutatriene (2): gas phase hydrogen halide
elimination from a suitable precursor and immediate cooling to
ꢀ
196 8C.
A reasonable starting point was a synthetic route analogous to
the synthesis of
1 according to Scheme 5. However, the
dehydrobromination of 15 gives Z-1-bromo-4,4-difluorobutadiene
(16) but does not proceed further to yield 2.
Scheme 4. Attempted rearrangement of 1,1-difluorobut-1-en-3-yne (13).
It seemed necessary to use a brominated diene (19) which is
only capable of eliminating hydrogen bromide in the desired
modality (Scheme 6). Elimination of hydrogen bromide from 19
could only yield 2. After numerous unsuccessful attempts, it turned
out that only Negishi cross-coupling of 1,1-dibromo-2,2-difluor-
oethene (17) with vinylbromide affords isolable yields of the
desired diene 19 (Scheme 6).
potassium hydroxide, butyl lithium, LDA or potassium t-butoxide
19
by F NMR spectroscopy it turned out impossible to isolate 11 in
preparative amounts. In all cases a complex mixture of 9, 10 and 11
is obtained when stochiometric amounts of butyl lithium were
used. Using an excess of butyl lithium results in complete
decomposition without any hint for the formation of 2.
The synthesis of 18 as well as the synthesis of
a-halo-b,b-
Sander et al. reported the synthesis of 1,1-difluorobutatriene (2)
by laser induced (193 nm) rearrangement of 1,1-difluorobuta-1-
en-3-yne (13) within an argon matrix. The only other report on 13
is a patent by E.I. Du Pont de Nemours & Co. [15]. They claim its
synthesis by pyrolysis of 1-ethynyl-2,2,3,3-tetrafluorocyclobutane
and the use of its group I and II metal salts as an additive to fuels for
internal-combustion engines and as bactericides.
difluorovinylzinc compounds has previously been reported by
Burton [17,18]. It turned out that elimination of hydrogen bromide
from 19 over hot potassium hydroxide under high vacuum
conditions is not possible. The diene 19 passes the reaction zone
unchanged. Whether the failure is due to thermodynamic or
kinetic effects remains still unclear.
Since the elimination of hydrogen bromide over hot potassium
hydroxide proved unsuccessful, an alternative elimination from 19
in solution by a strong hindered base (DBU) was examined
A new synthesis of 13 was developed based on palladium-
catalyzed Stille cross-coupling of ethynyl-tri-n-butylstannane (12)
with 1,1-difluoro-2-iodo-ethene 5 (Scheme 3).The crude product
was purified by fractional condensation to give >95% pure 13
1
9
(Scheme 7). The reaction was monitored by F NMR spectroscopy.
A new signal at ꢀ99.5 ppm was detected close to that of
tetrafluorobutatriene 1. However, this resonance vanishes within
19
1
13
according to F, H and C NMR spectra. Addition of magnesium
[(Scheme_5)TD$FIG]
Scheme 5. Proposed synthesis of 1,1-difluorobutatriene by a bromination/double dehydrobromination sequence and observed reaction.

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C. Ehm et al. / Journal of Fluorine Chemistry 131 (2010) 1173–1181
Scheme 6. Palladium-catalyzed synthesis of 2-bromo-1,1-difluorobuta-1,3-diene (19) and attempted dehydrobromination.
[(Scheme_7)TD$FIG]
Scheme 7. Attempted synthesis of 1,1-difluorobutatriene by elimination of hydrogen bromide from 19 by treatment with DBU and unexpected formation of (21).
[(Scheme_8)TD$FIG]
Scheme 8. Proposed mechanism for the synthesis of 1,1-difluorobutatriene by bromination/dehydrobromination of 4-bromo-1,1,2-trifluorobut-1-ene and subsequent
elimination of zinc(II)halide.
5
an hour when the reaction mixture is subjected to room
temperature.
Neither reaction with tetracarbonyl(
h
-cyclopentadienyl)va-
5
nadium (26) [21], tricarbonyl(
h
-cyclopenta-dienyl)manganese
(27) [22], tricarbonyl(h -benzene)chromium (28) [23], tricarbo-
h -mesitylene)chromium (29) [24], dicarbonyl(h -cyclopen-
tadienyl)cobalt (30) [25] nor the chromo ethylene complex 31 [26]
gave successful reactions.
According to Rosenthal [27], the bis(trimethylsilyl)acetylene
ligand in the titanium complex (32) is easily removable for
example by substituted butatrienes. Similar reactions were
described by Suzuki using the ‘Negishi reagent’ dibutyldicyclo-
pentadienylzirconium [28]. A reaction with 1 was not observed.
6
Unfortunately, even at ꢀ50 8C the predominant signal of 3-
bromo-4,4,4-trifluorobut-1-ene 21 is rapidly formed. The elimina-
tion may take place and triene 2 is formed but further reaction with
DBU leads to defluorination. Since the diene 19 is the only fluorine
source in the reaction, it is most likely that DBU-hydrogen fluoride
is formed intermediately which then subsequently adds hydrogen
fluoride to the remaining diene 19. This finding is comparable to
6
5
nyl(
ꢂ
the hydrofluorination of alkenes by Olah’s reagent (x(HF)
pyridine) [19]. Butene 21 was isolated from the reaction mixture
19
1
13
by fractional condensation and identified by its F, H and
NMR spectra.
C
Reactions based on the reactivity of metal–metal bonds in
enneacarbonyldiiron (33), and bis[(dicarbonyl)(
5
h
-cyclopentadie-
Finally, 1,1,2-trifluoro-4-bromo-but-1-ene 22 was brominated
using bromine in dichloromethane. The reaction product 23 was
isolated by fractional condensation in excellent yield (95%) and
high purity (>99%) (Scheme 8).
The tribromo compound 23 was then subjected to high vacuum
elimination over hot potassium hydroxide. NMR spectroscopy of
the product revealed that only one elimination of hydrogen
bromide took place forming butene 25 and not the buta-1,2-diene
nyl)molybdenum] (34) successfully yield difluoroallene com-
plexes[29] but formation of tetrafluorobutatriene complexes
was not observed (Scheme 10).
Reactions with complexes containing labile ligands like Zeise’s
salt (35) [30], bis(triphenylphosphine)(cyclooctadiene)nickel (36)
[31] and tetraacetonitrilecopper(I)-tetrafluoroborate (37) [32] also
did not yield isolable tetrafluorobutatriene complexes, too. Instead
accelerated polymerization was observed for 35 and 36.
2
4 (Schemes 8 and 9).
[(cShem)_e9TD$FI]G
2.3. Attempts to synthesize metal complexes of
tetrafluorobutatriene
Photochemical displacement of a carbonyl ligand by THF and
reaction with the cumulene, a successful approach in the synthesis
of fluoroallene complexes [20], gave no evidence of the formation
of a tetrafluorobutatriene complex.
Scheme 9. Attempted synthesis of 1,1-difluorobutatriene by multiple
dehydrobromination of 23.

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C. Ehm et al. / Journal of Fluorine Chemistry 131 (2010) 1173–1181
1177
Scheme 10. Attempted synthesis of tetrafluorobutatriene metal complexes.
2
Finally, the copper(I) complexes (38)–(40) were examined
4 4 3 2
obtained directly from the triene are [Ir(h -C F )(CO)(PPh ) Cl]
(
Scheme 8) [32–34]. Tetrafluorobutatriene copper complex for-
and the analogous rhodium compound, as previously mentioned.
In one of the many attempts to react tetrafluorobutatriene with
enneacarbonyldiiron a few colorless crystals of an unexpected
product (41) were obtained (Scheme 11).
mation was not observed.
Several metals turned out to accelerate the polymerization. This
finding is significant for nickel, platinum and titanium. Further-
more, the stability of tetrafluorobuatriene 1 in different solvents
[(cSemh_11)T$DFIG]
increases in the row pentane < diethylether ꢃ THF < CH
CHCl
but have a decelerating effect on the reaction rate.
2
Cl
2
ꢃ
3
. High dilution conditions stabilize tetrafluorobutatriene 1
2.4. An unexpected iron complex of tetrafluorobutatriene
2
Unlike 1,1-difluoroallene and tetrafluoroallene which form
h
-
complexes by reactions with several transition metal complexes
35] similar reactions with tetrafluorobutatriene were unsuccess-
ful in most cases. The only tetrafluorobutatriene complexes
[
Scheme 11. Synthesis of iron complex 42.

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C. Ehm et al. / Journal of Fluorine Chemistry 131 (2010) 1173–1181
3. Conclusion
The instability of tetrafluorobutatriene as well as the accelera-
tion of its decomposition by certain metal centers makes its
chemistry challenging. Despite many attempts only an acciden-
tally synthesized iron complex could be isolated. Several possible
strategies for the successful synthesis of 1,1-difluorobutatrienes
were attempted experimentally, but none of them yielded isolable
amounts of this species. Further studies are ongoing in our
laboratory.
4. Experimental
4.1. General techniques
All manipulations of air and moisture sensitive compounds
were performed on a standard vacuum line in flame dried flasks
under an atmosphere of argon. The argon, purification grade 4.8,
was provided by LINDE. Solvents were distilled under argon from
sodium (toluene, pentane, hexane, and benzene), sodium/benzo-
phenone (THF, DME, diethyl ether, and tert.-butylmethyl ether) or
phosphorous pentoxide (DMF, dichloromethane, chloroform,
tetrachloromethane, and acetonitrile). Solvents were stored over
Fig. 1. Molecular structure (ORTEP [36]) of compound 41.
˚
sodium–potassium alloy (ethers and hydrocarbons) or 3 A
The crystal and molecular structure of 41 was elucidated by X-
molecular sieve (DMF, acetonitrile and chlorohydrocarbons) in
flasks with Normag or Young plug valve and transferred to the
reaction vessels via standard vacuum techniques (condensation) or
Schlenk technique. Air-sensitive compounds were stored and
weighted in a glovebox (MBraun Labmaster sp). Unless otherwise
noted all manipulations were carried out using standard Schlenk
techniques in an argon counterflow to exclude moisture and air.
Cooling of reaction mixtures up to ꢀ125 8C was performed by
ethanol/liquid nitrogen mixtures. Lower temperatures up to
ꢀ160 8C were obtained by iso-pentane/liquid nitrogen mixtures.
Unless otherwise noted all reaction temperatures are internal
temperatures, especially involving exothermic reactions at low
temperatures.
ray crystallography (Fig. 1). The iron atom is almost octahedrally
coordinated by two carbonyl ligands in the trans positions and two
other carbonyl ligands in cis position to the two carbon atoms of
the ferracyclohexadiene-ring. The bond angle C10–Fe1–C11 of
1
63.97(12)8 of the carbonyl ligands trans to each other deviates
significantly from linearity. All carbon atoms within the ring are
2
sp -hybridized (angles range from 116.65(12)8 to 130.4(2)8) and
the ring itself is almost planar. The C2–Fe–C8 bond angle of
5.50(11)8 is the smallest in the ferracyclohexadiene-ring. The iron
9
˚
˚
carbon bonds Fe–C8 (2.029(3) A) and Fe–C2 (2.054(3) A) are
slightly different and longer than the distances to the carbonyl
ligands.
As expected for solid carbon acids two molecules are
interconnected by hydrogen bridging bonds.
Commercially available chemicals were used without further
purification. NMR measurements were carried out on a JEOL
Lambda 400 spectrometer at 20 8C (if not indicated differently).
Temperatures below room temperature were reached by
evaporation of liquid nitrogen. Temperature sensitive com-
pounds were transferred into the precooled spectrometer
without interruption of the cold chain. Air and moisture sensitive
compounds were measured in sealed NMR tubes (Young) or
1
9
The F NMR and IR-spectra of 41 are in accordance with the
molecular structure obtained by X-ray diffraction. Despite many
further attempts the synthesis of 41 remained irreproducible.
One can only speculate about the mechanism of the formation
of 41. There exist at least two principally different pathways. First,
dimerization of tetrafluorobutatriene, coordination of the dimer,
CO insertion and partial hydrolysis. Second, iron promoted
dimerization forming a metallacycle, formation of the four
membered ring, CO insertion and partial hydrolysis. As already
found by Wilkinson, tetrafluoroethene reacts with iron carbonyls
forming a ferracyclopentane by oxidative addition [37]. Similar
metallacycles have been obtained for phosphine, bipyridyl and
cyclooctadiene complexes of nickel [38]. To our knowledge, a CO
insertion into the five-membered fluorinated metallacycle has not
been observed up to now. In contrast, CO insertion into
hydrocarbons plays an important role in industrial processes
flame sealed 4-mm-Duran-glass-tubes. Chemical shifts
d are
given by definition as dimensionless number. The absolute
values of the coupling constants are given in Hertz (Hz),
regardless of their signs. Multiplicities are abbreviated as singlet
(s), doublet (d), triplet (t), quartet (q), and multiplet (m). Spectra
1
13
were referenced with internal standards: for H and C NMR
1
9
119
(solvent signal) and external standards for F and
(CFCl and tetramethyltin). Measurement frequencies are
399.65 MHz ( H), 100.40 MHz C), 376.00 MHz ( F) and
Sn NMR
3
1
13
19
(
1
19
148.95 MHz ( Sn).
Crystal structure data were collected on a Bruker SMART-CCD-
1000-TM diffractometer with Mo K
-radiation at ꢀ100 8C. A
(Monsato process, hydroformylation) and occurs even in non-
fluorinated butatriene complexes [39]. Fe(CO) (CO–CF –CF –CF
4
2
2
2
–
a
CO) has been synthesized from perfluoroglutaryl chloride and
structurally characterized by X-ray diffraction [40]. The subse-
quent hydrolysis of one of the metal-bound fluorocarbyl groups is
well known going back to work by Kemmitt [41] in the 1960s and
has been recently reviewed by Hughes [42]. Recently, a partially
hydrolyzed trimer of tetrafluoroallene as well as a manganese
complex of the tetrafluoroallene dimer were structurally charac-
terized [43].
suitable crystal was selected on an installation described in the
literature using a microscope, mounted onto a glass fiber using
silicon grease and transferred into the cold gas stream of the
diffractometer [44]. Empirical absorption correction was done
with SADABS [45]. Structure solution and refinement was done
using the least square refinement method implemented in the
SHELX program suite [46]. Pictures were created using ORTEP
[36].

C. Ehm et al. / Journal of Fluorine Chemistry 131 (2010) 1173–1181
1179
4
4
.2. General working procedures
yield of up to 100% was determined by weighting in a Young-valve
tube.
1
9
.2.1. 1,1-Difluoro-2-iodoethylene (5)
F NMR (CD
Ref. [11].
2 2
Cl , 193 K): d = 96.1 (s). Full spectroscopic data see
1
00 g (0.6 mol) iodinemonochloride (ICl) in a flame dried 2-L
flask with a Normag type teflon plug valve and a magnetic stir bar
were cooled to ꢀ196 8C and evacuated (3 times). The reaction
vessel and the difluoroethylene gas flask were connected to the
vacuum line. At 0 8C (ice bath) 40 g (0.62 mol) difluoroethylene
was added in portions over the vacuum line, monitored by a
vacuum gauge. Then 300 mL of 2,6-lutidine were added at 0 8C
followed by the fast addition of 110 g (0.72 mol) 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU). The valve at the reaction was closed
and the mixture slowly warmed to room temperature and stirred
overnight. All volatiles were condensed into a ꢀ196 8C cooling trap.
4.2.5. Attempted synthesis of 1,1-difluorobutatriene (2) by multiple
hydrogen fluoride elimination from 1,1,1,3,3-pentafluorobutane (25)
10 mmol (1.5 g) 1,1,1,3,3-pentafluorobutane was diluted with
the appropriate solvent (see Table 2.4.2) in a flame dried Young-
valve 50-mL-flask. With the exception of butyllithium and LDA
(addition temperature, reaction time see Table) all bases were
added at room temperature. The reaction was monitored by NMR
spectroscopy (see Table for results).
Physical data of observed intermediates:
3
portions of 50 mL of water were added to the reaction mixture
1
9
and the mixture was stirred for 5 min. Again all volatiles were
condensed into the trap. 1,1-difluoro-2-iodoethylene was distilled
off (b.p. 35 8C) to yield 97 g (85%) of the pure product
1,1,1,3,3-Pentafluorobutane (9):
= ꢀ64.15 (3F, m), ꢀ88.95 (2F, m).
F
NMR (CDCl
3
,
20 8C):
d
1
9
1,1,1,3-Tetrafluorobut-2-ene (10):
isomer 1:
= ꢀ59.16 (3F, m), ꢀ83.2 (1F, m). Isomer 2:
= ꢀ57.84 (3F, m), ꢀ83.3 (1F, m).
F NMR (CDCl , 20 8C):
3
(purity > 99%). A scale up of the reaction to 2 mol iodine
d
monochloride gave an equal yield.
d
1
19
19
H NMR (CDCl
3
, 20 8C):
d
= 4.82 (dd). F NMR (CDCL
3
, 20 8C):
1,1,1-Trifluorobut-2-yne (11):
F
NMR (CDCl
3
,
20 8C):
5
1
d
= ꢀ71.36 (dd); ꢀ75.79 (dd). Full spectroscopic data see Ref. [11].
d
d
= ꢀ50.41 (3F, q, J(F–H) = 4 Hz). H NMR (CDCl
3
, 20 8C):
5
= 1.95 (3H, q, J(H–F) = 4 Hz).
4.2.2. 1,1,4,4-Tetrafluorobuta-1,3-diene (7)
The previously published method was modified as follows. Zinc
4.2.6. Synthesis of 1,1-difluorobuta-1-en-3-yne (13)
was carefully activated by treatment with dilute hydrogen chloride
and subsequently washed with water, ethanol and acetone and
finally dried in high vacuum. Only the finest powder was employed
in the following synthesis. 1,1-Difluoro-2-iodoethylene (13 g,
In a flame dried Young-valve 50-mL-flask 7.56 g n-butyl-
stannylacetylene (24 mmol, 1.04 eq.) and 4.37 g 1,1-difluoro-2-
iodoethylene (23 mmol) were added to 250 mg palladium(II)a-
cetate (5 mol%), 750 mg triphenylphosphine (12.5 mol%) and 1 g
magnesium sulfate in 20 mL DMF. The reaction vessel was
heated to 80 8C (bath-temperature) for 45 min. The color of the
reaction mixture changes to black during heating. The reaction
mixture was subjected to fractional condensation in high
vacuum (10 mbar) over a ꢀ78 8C cooling trap. The product,
1.4 g (66% yield) of a colorless gas, was collected in a ꢀ196 8C
cooling trap and stored in a Young-valve flask in the fridge at
4 8C.
6
8.45 mmol) was added dropwise to activated zinc powder (8 g)
in DMF (50 mL) at room temperature. An exothermic reaction
19
occurs. F NMR measurement revealed complete conversion to
the zinc compound [47]. The zinc reagent was transferred to a
ꢀ3
second flask containing [Pd(PPh
3
)
4
] (2 g, 1.7 mmol, 2.5 mol%) and
1
,1-difluoro-2-iodo ethylene (13 g, 68.45 mmol) via a small
diameter teflon tube. The reaction temperature was maintained
at 75 8C for 4 h. Meanwhile the product was collected as a colorless
liquid in a trap kept at ꢀ78 8C which was connected to the reflux
1
9
3
F NMR (CDCl , 20 8C):
3
d
= ꢀ76.53 (1F, d, J(F–H) = 22.8 Hz),
ꢀ3
1
condenser. Fractional condensation under vacuum (10 mbar)
ꢀ81.70 (1F, s) no geminal F–F coupling observable. H NMR (CDCl ,
3
3
yielded 5.2 g (61%) in the trap kept at ꢀ120 8C.
20 8C):
d = 5.22 (1H, dm, J(H–F) = 22.8 Hz), all other coupling
1
19
13
1
3 3
H NMR (CDCl , 20 8C): d = 4.52 (m, 2H). F NMR (CDCL ,
constants could not be resolved, 3.58 (1H, m). C{ H} NMR (CDCl ,
20 8C): d = 163.9 (1C, dd, CF2, J(C–F) = 293 Hz, 300 Hz), 81.2 (1C,
3
2
2
0 8C):
d
= ꢀ86.77 (m); ꢀ88.09 (m). Full spectroscopic data see Ref.
[
4].
dd, J(C–F) = 4 Hz, 9 Hz), 71.9 (1C, dd, J(C–F) = 4 Hz, 12 Hz), 64.4 (1C,
J(C–F) = 19.5 Hz, 43 Hz).
4.2.3. 1,4-Dibromo-1,1,4,4-tetrafluorobuta-1,3-diene (8)
The previously published method was modified as follows.
4.2.7. Attempted rearrangement of 1,1-difluorobuta-1-en-3-yne (13)
1 mmol (ca. 100 mg) 13 was condensed into a Normag-valve 2-
L-flask and irradiated at room temperature for 4 h. All volatiles
were condensed onto CDCl3 in a 4-mm-glass-tube. F NMR
revealed only starting material.
Compound (7) (5.2 g, 40 mmol) was condensed onto bromine
6.4 g, 42 mmol) in CH Cl (10 mL) in a 100 mL glass flask. The
(
2
2
19
reaction mixture was rapidly warmed to room temperature and
stirred for 5 h at ambient temperature. 5 mL of sodium
thiosulfate saturated water was added and the mixtures were
stirred for 5 min. The layers were separated, the water phase
extracted 3 times with 2 mL of dichloromethane. Fractional
condensation of the combined organic phases under vacuum
yielded (8) (9.87 g, 82%) as a colorless liquid in the trap kept at
4.2.8. Synthesis of E-1,4-dibromo-1,1-difluoro-2-butene (15)
A flame dried Young-valve 25-mL-flask, equipped with a
magnetic stir bar, was charged with 2.63 g (29 mmol) 1,1-
difluorobutadiene. Bromine (31 mmol, 4.96 g) was added in small
portions by condensation. Distillation via a spinning band column
at reduced pressure of 12 mbar yielded 4.08 g (56%) 15 in 97%
purity.
ꢀ
60 8C.
H NMR (CDCL
1
19
3 3
, 20 8C): d = 6.33 (m). F NMR (CDCL , 20 8C):
d
= ꢀ50.30 (m). Full spectroscopic data see Ref. [11].
1
9
3
F NMR (CDCl
3
, 20 8C):
d
= ꢀ46.83 (2F, CF
2
Br–, dd, J(F–
= 6.33 (1H,
4
1
4
.2.4. 1,1,4,4-Tetrafluorobuta-1,2,3-triene (1)—general procedure for
elimination in the gas phase over hot potassium hydroxide
F) = 9.7 Hz, J(F–H) = 1.9 Hz). H NMR (CDCl , 20 8C): d
BrC–CH– dtt, J(H–trans-H) = 15.2 Hz, J(H–CH
J(H–F) = 1.9 Hz), 6.10 ( H,
H) = 15.2 Hz, J(H–F) = 9.7 Hz, J(H–CH
dd, J(H–H) = 7.2 Hz, J(H–H) = 1.3 Hz).
20 8C):
F) = 24.4 Hz), 115.7 (1C, t, J(C–F) = 301 Hz), 28.0 (1C, s).
3
3
3
H
2
2
Br) = 7.2 Hz,
J(H-trans-
Br) = 1.3 Hz), 3.94 (2H,
4
1
3
Compound (8) (1 g, 3.5 mmol) was passed over technical grade
KOH which was filled into a U-shaped tube with a diameter of no
more than 1 cm and heated to 88 8C by evaporation under vacuum
F
2
BrC–CH 55 , dtt,
3
4
2
3
4
13
1
C{ H} NMR (CDCl
3
,
ꢀ3
1
2
(
ꢀ
10 mbar). The volatile materials were collected in traps kept at
d
= 131.0 (1C, t, CF
2
1
,
J(C–F) = 7.5 Hz), 129.7 (1C, t, J(C–
78 8C (water) and ꢀ196 8C (3). No repetition was necessary. A

1
180
C. Ehm et al. / Journal of Fluorine Chemistry 131 (2010) 1173–1181
3
4
.2.9. Attempted elimination of hydrogen bromide from
129.5 (1C, q, CF
3
, J(C–F) = 2 Hz), 125.0 (1C, s), 122.6 (1C, s) 46.50
2
E-1,4-dibromo-1,1-difluoro-2-butene (15)
(1C, q, J(C–F) = 34 Hz).
The attempted elimination was performed according to the
general procedure described in Section 4.2.4. A temperature
of 90 8C was employed. The product was identified by NMR data as
4.2.12. Synthesis of 1,2,4-tribromo-1,1,2-trifluorobutane (23)
In a flame dried 25-mL-Schlenk flask bromine (1.6 g, 10 mmol)
was added dropwise at room temperature to a stirred solution of
1.89 g (10 mmol) 4-bromo-1,1,2-trifluorobut-1-ene in 10 mL
dichloromethane. The solution was stirred for 1 h. Fractional
1
-bromo-4,4-difluorobutadiene (16).
19
3
F NMR (CDCl
3
, 20 8C):
d
= ꢀ81.82 (1F, dddd, J(F-trans-
2
4
5
H) = 24.3 Hz, J(F–F) = 16.7 Hz, J(F–H) = 1.7 Hz, J(F–F) = 1.1Hz),
2
3
4
ꢀ3
ꢀ
83.67 (1F, dddd, J(F–F) = 16.7 Hz, J(F-cis-H) = 1.4 Hz, J(F–
condensation in high vacuum (10 mbar) yielded 3.3 g (95%) (23)
5
1
H) = 1.1 Hz, J(F–F) = 1.4Hz). H NMR (CDCl
3
, 20 8C):
d
= 6.59 (1H,
in >99% purity in the cooling trap kept at ꢀ30 8C.
3
3
4
19
–
HC 55 CHBr, dddd, J(H–H) = 10.8 Hz, J(H-cis-H) = 7.4 Hz, J(H–
F NMR (CDCl
3
, 17 8C):
d
= ꢀ59.00 (1F, m, CF
2
Br), ꢀ117.54 (1F,
4
3
1
F) = 1.4 Hz, J(H–F) = 1.1 Hz), 6.17 (1H, 55 CHBr, dddd, J(H-cis-
H) = 7.4 Hz, J(H–H) = 1.2 Hz, J(H–F) = 1.4 Hz, J(H–F) = 1.1 Hz),
m, CFBr). H NMR (CDCl
m, CH Br), 2.93 (1H, m, CH
(CDCl , 20 8C): = 119.05 (1C, ddd, CF
J(C–F) = 33 Hz), 104.90 (1C, ddd, CFBr, J(C–F) = 266 Hz, J(C–
F) = 31 Hz, 28 Hz), 41.70 (1C, d, CH , J(C–F) = 20 Hz), 23.90 (1C, m,
CH Br).
3
, 20 8C):
d = 3.65 (1H, m, CH Br), 3.55 (1H,
2
4
5
5
13
1
2
2
), 2.80 (1H, m, CH
2
). C{ H} NMR
2
Br, J(C–F) = 310 Hz, 312 Hz,
3
3
1
5
.38 (1H,
F
3
2
C 55 CH, dddd,
J(H-trans-F) = 24.3 Hz,
J(H-cis-
3
d
4
13
1
2
1
2
F) = 1.4 Hz, J(H–H) = 10.8 Hz, J(H–H) = 1.2 Hz).
C{ H} NMR
1
(CDCl
3
, 20 8C):
d
= 158.0 (1C, dd, CF
4
2
55 , J(C–F) = 300 Hz, 293 Hz),
2
3
1
22.6 (1C, dd, J(C–F) = 5.4 Hz, 2.1 Hz), 108.3 (1C, dd, J(C–
2
2
F) = 12.0 Hz, 3.7 Hz), 80.0 (1C, dd, J(H–H) = 30.6 Hz, 15.3 Hz).
4
.2.13. Attempted synthesis of 4-bromo-3,4,4-trifluorobuta-1,2-diene
(24)
Attempt 1: The attempted elimination was performed according
to the general procedure described in Section 4.2.4. A temperature
4.2.10. 2-Bromo-1,1-difluorobuta-1,3-diene (19)
In a flame dried 50-mL-Schlenk flask, equipped with a magnetic
stir bar, were added 2.2 g (10 mmol) 1,1-dibromo-2,2-difluor-
oethylene, 654 mg activated zinc dust (finest powder) and 30 mL
DMF. A mild exothermic reaction occurred. The solution was
stirred overnight. The solution was then transferred via a teflon
pipe to another 50-mL-Schlenk flask, equipped with a magnetic stir
bar and 225 mg palladium(II)acetate and 655 mg triphenylpho-
sphine (25 mol%).The reaction mixture was frozen in liquid
nitrogen and degassed. 1.5 g vinylbromide which was synthesized
according to literature methods were condensed onto the reaction
mixture. The reaction mixture was heated to 60 8C (bath-
temperature) for 2.5 h. Afterwards the mixture was subjected to
fractional condensation over cooling traps (ꢀ40 8C, ꢀ100 8C and
1
9
1
13
of 90 8C was employed. According to F, H and C NMR data 3,4-
dibromo-3,4,4-trifluorobut-1-ene (25) is formed by a single
hydrogen bromide elimination.
Physical data for 3,4-dibromo-3,4,4-trifluorobut-1-ene (25):
1
9
F NMR (CDCl
3
, 17 8C):
m, CFBr). H NMR (CDCl , 20 8C):
m, CH Br), 2.93 (1H, m, CH ), 2.80 (1H, m, CH
(CDCl , 20 8C): = 131.45 (1C, d, CH 55 , J(C–F) = 18 Hz), 121.00 (d,
d
= ꢀ59.2 (1F, m, CF
2
Br), ꢀ125.35 (1F,
1
3
d
= 3.65 (1H, m, CH
2
Br), 3.55 (1H,
1
3
1
2
2
2
). C{ H} NMR
2
3
d
3
1
55 CH
2 2
, J(C–F) = 10 Hz), 118.50 (1C, ddd, CF Br, J(C–F) = 314 Hz,
2
1
310 Hz, J(C–F) = 35 Hz), 104.90 (1C, ddd, CFBr, J(C–F) = 266 Hz,
2
2
J(C–F) = 32 Hz, 29 Hz), 41.70 (1C, d, CH , J(C–F) = 20 Hz), 23.90
ꢀ
196 8C). The product was collected in the ꢀ100 8C trap (1.52 g,
(1C, m, CH Br).
2
8
0% purity). The calculated yield was 72%.
19
3
F NMR (CDCl
3
5
, 20 8C):
d
= ꢀ81.03 (1F, dddd, J(F–F) = 23.9 Hz,
4.2.14. General procedure for the generation of metal–THF complexes
of 26–30 and metal–ethene complex 31 and attempted synthesis of
tetrafluorobutatriene complexes thereof
4
5
J(F–H) = 1.6 Hz, J(F–H) = 2.1 Hz, J(F–H) = unresolved), ꢀ84.23
3
4
5
5
(1F, dddd, J(F–F) = 23.9 Hz, J(F–H) = 2.4 Hz, J(F–H) = 1.8 Hz, J(F–
1
3
H) = 0.8 Hz). H NMR (CDCl
H) = 16.3 Hz, J(H–H) = 10.3 Hz, J(H–F) = 2.4 Hz, J(H–F) = 1.6 Hz,
CH 55 ), 5.51 (1H, ddd, J(H–H) = 16.3 Hz, J(H–F) = 0.8 Hz, J(H–
H) = 0.9 Hz, 55 CH
3
, 20 8C):
d
= 6.44 (1H, dddd, J(H–
A photoreactor with water-cooled Pyrex-light, circulation pipe
and magnetic stir bar was charged with 0.25 mmol (1 eq.) of the
metal complex. The reactor was evacuated for several minutes and
afterwards frozen in liquid nitrogen. 50–100 mL pentane and 5–
10 mL THF were condensed into the reactor (depending on the size
of the reactor). The solvent was degassed again by melting/
freezing. The solution was irradiated at ꢀ78 8C with a mercury-
high-pressure light (Philips HPK-125) for 1 h.
3
4
4
3
5
2
3
2
5
, trans-H), 5.34 (1H, dddd, J(H–H) = 10.3 Hz,
5
2
2
J(H–F) = 2.1 Hz, J(H–F) = 1.8 Hz, J(H–H) = 0.9 Hz, 55 CH , cis H).
13
1
1
C{ H} NMR (CDCl
3
,
20 8C):
d
= 154.8 (1C, dd, CF2, J(C–
C 55 ), 119.4 (1C, dd, J(C–
F) = 11.6 Hz, 3.5 Hz), 83.0 (1C, J(C–F) = 32 Hz, 24 Hz, CBr).
F) = 292 Hz, 288 Hz), 126.1 (1C, s, H
2
The reaction mixture was frozen in liquid nitrogen and
0.5 mmol tetrafluorobutatriene (A) or 1,1-difluorobut-1-en-3-
yne (B) or ethylene (C) were condensed into the reactor. The
solution was warmed to room temperature overnight. The solution
was filtered under inert condition over a G4-frit and the solvent
was removed in high vacuum (for A and B). NMR spectra of the
residue revealed no fluorine containing complex.
4
2
.2.11. Attempted elimination of hydrogen bromide from
-bromo-1,1-difluorobuta-1,3-diene (19)
Attempt 1: The attempted elimination was performed according
to the general procedure described in Section 4.2.4. A temperature
of 90 8C was employed. According to F NMR data the starting
compound 19 was recovered unchanged.
19
Attempt 2: A flame dried 4-mm-Duran-glass-tube was charged
under argon with 0.05 mL (0.3 mmol) DBU and 0.25 mL toluene-d .
The solution of C was frozen in liquid nitrogen again and
0.5 mmol tetrafluorobutatriene were condensed into the reactor.
The solution was stirred overnight. After a work-up similar to A
and B no evidence for the formation of a fluorine containing species
8
The tube was frozen in liquid nitrogen and degassed. Approxi-
mately 50 mg of diene 19 were condensed into the tube. The tube
was flame sealed and unfreezed at ꢀ80 8C. A variable temperature
NMR study was performed (ꢀ80 8C up to +10 8C). Predominantly
1
9
was found by F NMR, too.
1
9
compound 21 is formed, a small signal in the low temperature
F
4.2.15. Attempted synthesis of a tetrafluorobutatriene complex of 32
In a flame dried 10-mL-Normag-valve flask, equipped with a
magnetic stir bar, were added 0.1 mmol of titanium complex 32
and 5 mL of solvent (either toluene, THF, diethylether, tert.-
butylmethylether, 1,2-dimethoxyethane, acetonitrile, dichloro-
methane, chloroform or pentane). The solution was frozen in
liquid nitrogen and degassed. 0.25 mmol tetrafluorobutatriene
NMR spectra is observable at ꢀ99.5 ppm which might result from
the formation of 2.
Physical data for 3-bromo-4,4,4-trifluorobut-1-ene 21:
19
3
1
F NMR (CDCl
NMR (CDCl , 20 8C):
3
C{ H} NMR (CDCl , 20 8C): d = 129.5 (1C, q, CF , J(C–F) = 284 Hz);
3
, 20 8C):
d
= ꢀ72.75 (3F, d, J(F–H) = 7.1 Hz). H
3
d
= 5.53 (1H, m) 4.80 (2H, m), 3.84 (1H, m).
13
1
1
3

C. Ehm et al. / Journal of Fluorine Chemistry 131 (2010) 1173–1181
1181
2
were condensed into the reaction vessel. The solution was warmed
R
1
values were 0.0457 (I > 2
s
(I)). The final wRðF Þ values were
very slowly to room temperature. Already below ꢀ80 8C the
0.1031 (I > 2 (I)). The final R
s
1
values were 0.0833 (all data). The
19
2
2
solution turned black. F NMR spectra showed no evidence for
the formation of a fluorine containing titanium complex. A run
employing toluene in a Normag-NMR-tube was monitored starting
from ꢀ100 8C. Nevertheless no evidence for a fluorine containing
titanium species was found either.
final wRðF Þ values were 0.1200 (all data). The goodness of fit on F
was 1.020. Empirical absorption correction (SADABS), least squares
refinement (SHELXL-97).
Acknowledgments
4
.2.16. General procedure for attempted synthesis of a
We appreciate the support from the Deutsche Forschungsge-
meinschaft (DFG) and the Graduate School (GK1582) ‘‘Fluorine as a
Key Element’’. Dicyclopentadienenyltitanium bis(trimethylsilyl)a-
cetylene complex was a gift from the group of Prof. U. Rosenthal
(Catalysis, Leibnitz Institut f u¨ r Katalyse e.V.). 1,1,1,3,3-Pentafluor-
obutane was a gift of Honeywell.
tetrafluorobutatriene complexes of 33–35, 37
In a flame dried 25-mL-Schlenk flask, equipped with a magnetic
stir bar, was added 0.1 mmol of the metal complex and 10 mL of
solvent (please see Scheme 2.6.4 and 2.6.5 for details). The reaction
mixture was frozen in liquid nitrogen and degassed. 0.25 mmol of
tetrafluorobutatriene were condensed into the reaction vessel. The
solution was warmed to ꢀ30 8C and argon was added to the vessel
for pressure compensation. The reaction mixture was stirred
overnight at room temperature. After removal of all volatiles in
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8
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9
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0
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[
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39) 0.1 mmol (dimethylsulfide)copper(I) bromide and
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(
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0
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8
8
19
d -toluene or d -THF. F NMR spectra showed no evidence for the
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4.2.18. Preparation of 41
In an 8-mm-Duran-glass-tube tetrafluorobutatriene (5 mmol)
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[
[
[
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was condensed onto liquid nitrogen cooled enneacarbonyl diiron
24 mg, 0.065 mmol) in 1 mL of wet dichloromethane. The tube
(
was flame sealed and slowly warmed to room temperature. The
enneacarbonyl diiron slurry turned into a clear solution within 1 h.
After 3 days 2 colorless air-sensitive crystals were isolated from a
grey solid. One crystal was used for X-ray crystallography and the
second one for NMR- and IR-spectroscopy.
[
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[
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4
19
F NMR (d8-THF, CFCl
1F, –CF
–), ꢀ110 (1F, –CF
090 (s), 2072 (s) cm
Crystal structure analysis: A suitable crystal was selected using a
3
): ꢀ52 (1F, 55 CF
2
), ꢀ74 (1F, 55 CF ), ꢀ74
2
[
(
2
2
2
–) ppm; IR (diethylether): 2139 (s),
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[
[
[
ꢀ1
.
39] P. Mathur, V.D. Avasare, S.M. Mobin, J. Clust. Sci. 20 (2009) 399–415.
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microscope, mounted onto glass fiber using silicon grease, and
transferred into the cold gas stream of a diffractometer. BRUKER-
˚
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[
[
[
43] D. Lentz, M.Z. Paschke, Anorg. Allg. Chem. 630 (2004) 973–976.
44] M. Veith, H. Baernighausen, Acta Cryst. B 30 (1974) 1806–1813.
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Inc., Madison, WI, 1996, R.H. Blessing, Acta Crystallogr., Sect. A 51 (1995) 33.
AXS, SMART-CCD, Mo Ka, l = 0.71073 A, T = 133 K.
Crystal data for 41: C13
H
2
F
6
FeO
7
, M = 440.00, monoclinic,
˚
˚
˚
a = 6.8946(19) A, b = 13.457(4) A, c = 15.899(4) A,
a = 90.008,
3
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Institut f u¨ r Anorganische Chemie der Universit a¨ t, Tammanstrasse 4, D-3400
G o¨ ttingen, Germany, 1998.
˚
b
= 90.926(7)8,
g
= 90.008, V = 1475.0(7) A , T = 173(2) K, space
ꢀ1
group P2
1
/n, Z = 4,
m
(Mo K
a
) = 1.133 mm , 11736 reflections
measured, 4220 independent reflections (Rint = 0.0447). The final
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