A Structurally Characterized Fluoroalkyne

Article abstract of DOI:10.1002/anie.201702401

The facile synthesis of a stable and isolable compound with a fluoroalkynyl group, M?C≡CF, is reported. Reaction of [Ru(C≡CH)(η⁵-C₅Me₅)(dppe)] with an electrophilic fluorinating agent (NFSI) results in the formation of the

Full text of DOI:10.1002/anie.201702401

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201702401
German Edition: DOI: 10.1002/ange.201702401
Fluoroalkynes
A Structurally Characterized Fluoroalkyne
Lewis M. Hall, David P. Tew, Natalie E. Pridmore, Adrian C. Whitwood, Jason M. Lynam,* and
John M. Slattery*
In memory of Roger Mawby 1939–2017
Abstract: The facile synthesis of a stable and isolable
compound with a fluoroalkynyl group, MꢀCꢁCF, is reported.
cally unstable. As fluorinated alkynes, RCꢁCF, are rare and
demonstrably unstable they have not been studied or utilized
extensively. Herein we report the synthesis of a stable,
isolable fluoroalkynyl complex that allows 1) a detailed
characterization of the RCꢁCF motif, where R in this case
5
Reaction of [Ru(CꢁCH)(h -C Me )(dppe)] with an electro-
5
5
philic fluorinating agent (NFSI) results in the formation of the
5
fluorovinylidene complex [Ru(=C=CHF)(h -C Me )(dppe)]-
5
5
[
N(SO Ph) ]. Subsequent deprotonation with LiN(SiMe )
is a metal-containing fragment, and 2) a preliminary study of
the reactivity of the fluoroalkynyl ligand.
2
2
3 2
5
affords the fluoroalkynyl complex [Ru(CꢁCF)(h -C Me )-
5
5
(
dppe)]. In marked contrast to the rare and highly reactive
We recently described how the electrophilic fluorination
[
6]
[7]
examples of fluoroalkynes that have been reported previously,
this compound can be readily isolated and structurally
characterized. This has allowed the structure and bonding in
the CCF motif to be explored. Further electrophilic fluorina-
of alkenyl-containing
and alkynyl-containing
ligands
[8]
within the coordination sphere of ruthenium may lead to
the rapid and selective formation of new fluorinated ligands,
including rare examples of fluorovinylidenes (Scheme 1a).
tion of this species yields the difluorovinylidene complex
5
[
Ru(C=CF )(h -C Me )(dppe)][N(SO Ph) ].
2
5
5
2
2
3
2
D
espite numerous examples of C(sp )ꢀF and C(sp )ꢀF
bonds in chemistry, which play a hugely important role in
materials, medicinal and agrochemical applications there
[1]
are few examples of compounds containing C(sp)ꢀF bonds.
Those that have been reported are often highly reactive and,
in marked contrast to fluoroalkanes, fluoroalkenes and
fluoroaromatic compounds, fluoroalkynes readily undergo
oligomerization reactions. For example, FCꢁCF is unstable at
[
2]
t
temperatures above ꢀ1968C and BuCꢁCF undergoes
[
3]
cyclotrimerization below 08C. Fluoroalkynes having per-
fluoroalkyl groups, however, appear to have greater stabil-
[
4]
ity. Recent calculations indicate that the source of this high
reactivity is a dimerization processes to (initially) give
Scheme 1. a) synthesis of fluorinated vinylidenes and b) proposed
synthetic strategy.
[
5]
a diradical. In the case of fluorinated alkynes, the transition
state for dimerization is at low energy compared to the
alkynes and the resulting diradical is thermodynamically
more stable than the starting compounds. In contrast, for the
non-fluorinated analogues, the barrier to dimerization is
much higher and the resulting diradicals are thermodynami-
This occurs via the novel outer-sphere electrophilic fluorina-
tion (OSEF) mechanism. Extension of this synthetic method-
ology promised to allow access to previously unexplored
fluorinated alkynyl complexes, Scheme 1b. The proposed
route involved electrophilic fluorination of a hydrogen-sub-
stituted alkynyl complex MCꢁCH to give the cationic
+
vinylidene complex [M=C=CHF] and subsequent deproto-
[*] L. M. Hall, Dr. N. E. Pridmore, Dr. A. C. Whitwood, Dr. J. M. Lynam,
nation to give the desired fluorinated alkynyl MCꢁCF. It was
Dr. J. M. Slattery
Department of Chemistry
The University of York
Heslington, York, YO10 5DD (UK)
E-mail: jason.lynam@york.ac.uk
john.slattery@york.ac.uk
anticipated that the metal center and ancillary ligands would
provide protection of the fluorinated alkyne functionality,
inhibiting the facile dimerization pathway in the organic
analogues. Herein we describe the successful implementation
of this strategy and characterization of a rare example of
a molecule with a stable CꢁCF group.
Dr. D. P. Tew
School of Chemistry
University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
5
Reaction of [Ru(CꢁCH)(h -C Me )(dppe)] (1), with NFSI
5
5
in toluene solution at ꢀ788C followed by warming to room
temperature resulted in selective fluorination at the coordi-
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201702401.
5
nated alkynyl ligand and formation of [Ru(h -C Me )(=C=
5
5
CHF)(dppe)][N(SO Ph) ], 2[N(SO Ph) ], Scheme 2. The for-
2
2
2
2
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+
Scheme 2. i) +NFSI, toluene, ꢀ788C ! RT; ii)+ LiN(SiMe ) , ꢀHN(SiMe ) , ꢀLi[N(SO Ph) ], THF, ꢀ788C ! RT. Structures of the 2 cation and
3
2
3
2
2
2
+
3
are shown with thermal ellipsoids at 50% probability and hydrogen atoms [except H(4A) and H(4B)] are omitted for clarity. In the 2 cation the
hydrogen and fluorine atoms of the vinylidene are disordered over two positions.
mation of the vinylidene ligand was confirmed by the
spectra of a C D solution of 3 were essentially unchanged
6 6
3
observation of resonances at d = 362.8 (d,
and 173.9 (d, JCF = 236.2 Hz) ppm in the C{ H} NMR
J
= 43.5 Hz)
over a period of four months at room temperature and,
although some new resonances of low intensity appeared in
CF
13
1
1
1
19
spectrum and doublets at d = 7.32 and ꢀ235.8 ppm in the H
the F NMR spectrum over this time, the long-term stability
1
9
2
and F NMR spectra, respectively ( J = 80.2 Hz). Salt
of a fluoroalkyne in a condensed phase is without prece-
FH
[
10]
metathesis with NaPF resulted in the formation of 2[PF ]
dent. The spectroscopic parameters for 3 are more closely
related to fluorinated alkynes such as FCꢁCF than to the
6
6
which was structurally characterized by single crystal X-ray
[9]
5
[11]
diffraction, confirming the assignments of the NMR spectra.
fluorocarbyne complexes [Mo(h -C Me )(CO) (ꢁCF)] and
5
5
2
[12]
Conversion of 2[N(SO Ph) ] into the desired alkynyl
[(PNP)RhꢁCF][CHB Cl ]
which are to our knowledge
the only other structurally characterized examples of a C(sp)ꢀ
2
2
11
11
5
complex [Ru(CꢁCF)(h -C Me )(dppe)] (3), was achieved by
5
5
i
ꢀ
deprotonation with LiN(SiMe ) in THF solution at ꢀ788C
F bond (PNP = {2- Pr P-4-Me-C H } N ). For example, 3
2 6 4 2
3
2
1
9
followed by warming to room temperature. The presence of
shows a resonance in the F NMR spectrum at d = ꢀ189.4
F
1
the alkynyl group in 3 was confirmed by a band in the IR
( J = 332.2 Hz) ppm and the corresponding resonance in
CF
ꢀ
1
1
5
spectrum at 2148 cm , resonances at d = 185.6 and 111.4 (d,
FCꢁCF is at d = ꢀ261.3 ( J = 287.3 Hz) ppm, but [Mo(h -
F
CF
1
13
1
1
JCF = 332.2 Hz) ppm in the C{ H} NMR spectrum, and
C Me )(CO) (ꢁCF)] (d = 78.15 ( J = 556 Hz) ppm) and
5
5
2
F
CF
4
1
a triplet resonance at d = ꢀ189.4 (t, J = 4.8 Hz) ppm in
[(PNP)RhꢁCF][CHB Cl ] (d = 66.2 ( J = 470 Hz) ppm)
FP
11
11
F
CF
1
9
the F NMR spectrum. The structure of 3 was confirmed by
single-crystal X-ray diffraction, which unambiguously showed
the presence of the fluorinated alkynyl ligand.
show a marked difference. Furthermore, a comparison
between the spectroscopic and structural parameters for
fluorinated and non-fluorinated alkynes (Table 1) shows that
the presence of fluorine substituents shortens the CꢁC bond
and there is a commensurate blue shift in the CꢁC stretch in
Complex 3 proved to be stable for extended periods in
1
31
1
both solution and the solid state. The H and P{ H} NMR
[
a]
Table 1: Structural comparisons between fluorinated and non-fluorinated alkyne/ alkynyl compounds.
n CꢀH or CꢀF [cmꢀ
1
Compound
CꢁC [ꢀ]
CꢀF [ꢀ]
CꢀH [ꢀ]
RuꢀC
ꢀ]
n CꢁC
]
ꢀ
1
[
[cm ]
[
14]
[14]
[15]
[15]
HCꢁCH
FCꢁCH
1.2084
.208
1.198(3)
.204
1.197(3)
.195
1.202(3)
.230
1.187(4)
.224
1.0570
1.076
–
–
–
1974
2085
2240
2357
2437
2591
1925
2056
2148
2307
3374 and 3287
3525 and 3421
1
[
16]
[16]
[16]
[17]
[17]
1.279(5)
1.271
1.053(5)
1.074
3358 1057, 586 and 583
3450, 1119 and 651
1349 and 794
1419 and 820
3269
1
[
18]
FCꢁCF
1.276(2)
1.278
1
[
19]
[
Ru]ꢀCꢁCH
Ru]ꢀCꢁCF 3
1
–
2.015(2)
2.000
2.036(3)
2.017
1
1.075
3489
1068
1115
[
1.324(4)
1
1.295
[a] Unscaled, calculated values at the (RI)-pbe0/def-SV(P) level in italics.
2
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the vibrational spectra. Although the crystallographically
determined CꢁC bond lengths in the alkynyl groups of 1 and 3
are statistically indistinguishable, the CꢁC stretch of the
alkynyl ligand exhibits the same blue shift on fluorination
observed in the alkyne analogues. These data support the
suggestion that the compound is best viewed as containing
a fluorinated alkynyl ligand and that the CꢀF bond in 3 is
closely related to the fluoroalkynes.
To assess whether the stability of 3 arises from a thermo-
dynamic stabilization of the CꢀF bond, the bond dissociation
energies (BDEs) for a range of CꢀH and CꢀF bonds with sp,
2
3
sp , and sp hybridization have been calculated (see Support-
[
13]
ing Information for methodology and calibration).
The
resulting data are shown graphically in Figure 1, in which the
Scheme 3. a) DFT-calculated ((RI-)PBE0-D3/def2-TZVPP//(RI-)BP86/
SV(P) level in CH Cl (e=8.93) using the COSMO model) energies for
2
2
alkynes and their corresponding diradical dimerization products. The
data for R =R =H and R =F and R =H show good agreement with
1
2
1
2
[
5]
those previously reported. b) Calculated structure of the diradical
arising from the dimerization of 3.
Figure 1. Plot of calculated CꢀF bond dissociation energy versus
calculated CꢀH bond dissociation energy. The dotted line represents
a 1:1 relationship between the parameters. Insert shows expansion
tions observed in the metal-free analogues. This is supported
by preliminary mechanistic investigations, which showed that
ꢀ1
between 510–560 kJmol
.
°
DG298 for dimerization of the sterically less encumbered
5
model complex [Ru(CꢁCF)(h -C Me )(PH C H PH )] is
5
5
2
2
4
2
ꢀ1
dotted line represents a 1:1 relationship between CꢀH and Cꢀ + 80 kJmol , similar to that for the dimerization of FCꢁCF
2
3
ꢀ1
F BDEs. Systems with sp - and sp -hybridized bonds appear
above the line, indicating that, as expected, the CꢀF bonds in
these systems are stronger than the corresponding CꢀH
(+ 68 kJmol ). However, potential energy surface scans (at
the PBE0/def2-TZVPP//BP86/SV(P) level) suggest that when
the full dppe ligands are included, this creates a sterically
more crowded transition state for dimerization of 3, with an
bonds. However, the same trend in relative BDEs is not
predicted for compounds containing CꢁCH and CꢁCF
groups: in these cases, the calculated BDE for the CꢀH and
CꢀF bonds are essentially identical. This is also true for
ꢀ1
electronic energy barrier around 50 kJmol higher in energy
than the model system.
To rationalize the structural and spectroscopic changes
observed upon the introduction of fluorine into alkynyl
systems, the electronic structures of FCꢁCF, HCꢁCF, HCꢁ
compounds 1 and 3.
The similarity between the spectroscopic parameters and
CꢀF BDE of 3 and compounds RCꢁCF presumably indicates
5
5
CH, [Ru(h -C H )(PH ) (CꢁCF)], and [Ru(h -C H )(PH ) -
5
5
3
2
5
5
3 2
that the enhanced stability of the ruthenium-substituted
compound is not thermodynamic in nature. This is supported
by additional calculations which demonstrate that the puta-
tive diradical arising from the dimerization of 3 (Scheme 3) is
(CꢁCH)] were optimized using explicitly correlated Brueck-
[20]
ner coupled cluster theory (BCCD(T)-F12).
Brueckner
orbitals (BOs) derived from these calculations were qualita-
tively similar to the Kohn–Sham orbitals obtained from
ꢀ
1
5
thermodynamically more stable (DG298 = ꢀ18 kJmol ) than
two molecules of 3. The corresponding dimerization of 1 is
a treatment of FCꢁCF, HCꢁCF, HCꢁCH, [Ru(h -C H )-
5
5
5
(PPh ) (CꢁCF)], and [Ru(h -C H )(PPh ) (CꢁCH)] at the
3
2
5
5
3 2
ꢀ
1
thermodynamically disfavored (DG298 =+ 131 kJmol ), mir-
(RI)-PBE0/def2-TZVPP level, although the energies
obtained from the BOs were closer to those expected on
the basis of photoelectron spectroscopy giving confidence
that the resulting wavefunctions were effectively modeling
the electronic structure of the molecules (Figure 2).
[
5]
roring the behavior of alkynes RCꢁCF and RCꢁCH. We
therefore suggest that the presence of the bulky dppe and Cp*
ligands kinetically stabilize the CꢁCF linkage by inhibiting
the intermolecular dimerization and oligomerization reac-
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5
Figure 2. Brueckner orbitals for FCꢁCF, HCꢁCF, HCꢁCH, [Ru(h -C H )(PH ) (CꢁCH)], and [Ru(h -C H )(PH ) (CꢁCF)]. Energies are in electron-
5
5
3
2
5
5
3 2
volts.
An examination of the frontier orbitals provides an
explanation for some of the observed changes on incorpo-
ration of fluorine into sp-hybridized systems. For example, the
highest occupied orbital in all instances has CꢀC p-bonding
character, but is CꢀF p-antibonding. This interaction is
therefore expected to weaken CꢀF bonding, in a similar
p antibonding. A net CꢀC p-bond order of two might be
predicted on this basis; however, as the orbitals 12 and 13 are
largely based on fluorine, their effect on decreasing the CꢀC
bond order is expected to be small, which may explain the
shortening of the CꢁC bond on fluorine incorporation.
5
Inspection of the orbitals for [Ru(h -C H )(PH ) (CꢁCF)]
5
5
3 2
5
way that the occupancy of the doubly degenerate p* orbitals
and [Ru(h -C H )(PH ) (CꢁCH)] revealed that the CꢀF
5
5
3 2
in F weakens the FꢀF bond. This provides a ready explan-
interactions within the alkynyl moiety showed broadly the
same features (notably orbitals 52–54, which correspond to
a set of three 4d orbitals with antibonding interactions with
the CCF p system and display the same antibonding CꢀF
2
ation for the weaker-than-expected CꢀF bond in sp-hybrid-
ized systems. In FCꢁCF there are a total of 12 electrons
involved in p bonding (orbitals 10–15). Of these, eight are
involved in CꢀC p-bonding interactions (orbitals 10, 11, 14,
and 15) whereas those in orbitals 12 and 13 are CꢀC
interactions as the FCCF frontier orbitals), although the
decreased symmetry results in a breaking of the degeneracy of
4
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the p system. This, coupled with the mirroring of the
In conclusion, it has been demonstrated that the versatile
analogous spectroscopic parameters with FCꢁCF reinforces
nature of OSEF may be used to prepare a novel fluorine-
substituted alkynyl ligand which acts as model for more
reactive non-metal-substituted compounds. Detailed insight
into the structure of fluorinated alkynes has been obtained
and the ready preparation of this compound has demon-
strated that it may be used for further organometallic
chemistry.
the notion that 3 is best viewed as a ruthenium-substituted
fluorinated alkyne.
The ready availability of 3 offered the opportunity to
explore the reactivity of the MCꢁCF group and, specifically,
to incorporate a second fluorine atom into the complex and
form a difluorinated vinylidene ligand. Free vinylidenes are at
a much higher energy than the corresponding alkynes and it is
only in the coordination sphere of some metals where this is
[
21]
reversed.
The lowest difference in energy between the Acknowledgements
alkyne/vinylidene pair is for the difluorovinylidene and
difluoroacetylene pair (ca. 130 kJmol ).
ꢀ1
[21,22]
Furthermore
We are grateful to the University of York (PhD studentships
to LMH and NEP) and EPSRC (grants EP/H011455 and EP/
K031589/1) for funding. DPT thanks the Royal Society for
a University Research Fellowship. Fruitful discussions with
Professors Robin Perutz, Youichi Ishii and Odile Eisenstein
are also gratefully acknowledged.
the isomerization of difluorovinylidene to difluoroacetylene
ꢀ
1
has a much more substantial barrier (ca. 136 kJmol ) than
the hydrogen-containing analogues, which is believed to be
due to the antiaromatic transition state for fluorine migration
[
22c]
and the commensurate requirement to cleave a CꢀF bond.
As a consequence of this high barrier to isomerization to its
parent alkyne, difluorovinylidene can be observed in low
Curiously, this stability has not Conflict of interest
resulted in an extensive chemistry of difluorovinylidene as
a ligand, indeed the only examples are in dimeric complexes
[
23]
temperature matrices.
The authors declare no conflict of interest.
[
24]
prepared through defluorination reactions.
[
6,7]
Using OSEF methodology,
the fluoroalkynyl ligand in
Keywords: alkynes · alkynyls · electrophilic fluorination ·
3
may be readily converted into a difluorovinylidene. Reac-
fluorine · vinylidenes
tion of 3 with NFSI in THF at ꢀ788C resulted in formation of
5
[
Ru(h -C Me )(=C=CF )(dppe)][N(SO Ph) ],
(4[N-
5
5
2
2
2
(
SO Ph) ]), Scheme 4, which was characterized in the same
2 2
manner as 2[N(SO Ph) ] and 3, Scheme 2. Resonances at
2
2
1
9
dꢀ134.0 (s) ppm in the F NMR spectrum and at d 366.7 (t,
3
1
1
J
= 22.5 Hz) and d 231.3 (t, J = 277.6 Hz) ppm in the
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CF
CF
3
1
C{ H} NMR spectrum of 4[N(SO Ph) ] are again consistent
2
2
with the presence of the difluorinated vinylidene ligand. The
structure of 4[PF ] was also confirmed by single-crystal X-ray
6
diffraction (Scheme 4).
2
432 – 2506.
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4
[
[
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[
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9] Complex 2·PF co-crystallized as two forms, one green with no
6
solvent of crystallization and one orange as a CH Cl solvate.
2
2
Only the bond metrics for the green form are discussed in the
manuscript. Crystal data and CIFs for all compounds (including
both solvates) are provided in the Supporting Information.
CCDC 1535738 – 1535741, contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
The Cambridge Crystallographic Data Centre.
+
Scheme 4. i) + NFSI, THF, ꢀ788C ! RT. Structure of the 4 cation
shown with thermal ellipsoids at 50% probability and hydrogen atoms
omitted for clarity.
[10] Perfluoromethylacetylene has been reported to undergo no
change in the gas phase at 258C at 10 cmHg over one month or at
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2
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[
Manuscript received: March 6, 2017
Final Article published: && &&, &&&&
7
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Fluoroalkynes
Fancy a fluorine? In contrast to other
fluorinated alkynes, structurally charac-
L. M. Hall, D. P. Tew, N. E. Pridmore,
A. C. Whitwood, J. M. Lynam,*
terized {MꢀCꢁCF} exhibits considerable
long-term stability. Structural, spectro-
scopic and computational analyses reveal
that this longevity is underpinned by
kinetic stabilization by a half-sandwich
ruthenium substituent. Subsequent fluo-
rination provides facile access to a rare
example of a difluorovinylidene complex.
J. M. Slattery*
&&&& — &&&&
A Structurally Characterized Fluoroalkyne
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!