Carbon-fluorine bond activation of tetrafluoroethylene on palladium(0) and nickel(0): Heat or lewis acidic additive promoted oxidative addition

Article abstract of DOI:10.1021/om400255t

The C-F bond cleavage reaction of tetrafluoroethylene (TFE; CF ₂=CF₂) with an M(0) complex (M = Pd, Ni) was investigated. The treatment of an M(0) precursor with TFE in the presence of the appropriate monodentate phosphine ligand led to a clean formation of the corresponding η²-TFE adduct (η²-TFE)M(PR₃) ₂. In the case of the Ni(0) species, in particular, the choice of phosphine ligands is crucial for the preparation of the desired η²-TFE complex: the use of either PCy₃ or P ⁱPr₃ resulted in the target adduct, while less sterically hindered phosphines such as PPh₃ and PⁿBu₃ gave the known octafluoronickelacyclopentane as a result of the oxidative cyclization of two TFE molecules. Thermolysis of both palladium and nickel η²-TFE adducts bearing PCy₃ as the ligand resulted in a C-F bond activation reaction and gave the corresponding (trifluorovinyl)metal fluorides, trans-(PCy₃)₂M(F)(CF=CF₂). The reaction of (η²-TFE)Pd(PPh₃)₂ with LiI as an additive allowed cleavage of the C-F bond in THF, even at room temperature, and gave trans-(PPh₃)₂Pd(I)(CF=CF₂) with a concomitant formation of lithium fluoride. Other metal halides, such as MgBr₂ and AlCl₃, also promoted the C-F bond cleavage of TFE. In addition, the use of either BF₃·Et₂O or B(C₆F₅)₃ exerted a similar accelerative effect on the C-F bond activation of TFE on either nickel or palladium. The molecular structures of a series of η²-TFE and trifluorovinyl complexes were unambiguously determined by means of X-ray crystallography. The resultant (trifluorovinyl)palladium or -nickel species have shown the potential to utilize a key intermediate in cross-coupling reactions with organometallic reagents to prepare a variety of trifluorovinyl compounds.

Full text of DOI:10.1021/om400255t

Article
pubs.acs.org/Organometallics
Carbon−Fluorine Bond Activation of Tetrafluoroethylene on
Palladium(0) and Nickel(0): Heat or Lewis Acidic Additive Promoted
Oxidative Addition
Masato Ohashi,*,† Mitsutoshi Shibata,† Hiroki Saijo,† Tadashi Kambara,† and Sensuke Ogoshi*,†,‡
†Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
‡JST, Advanced Catalytic Transformation program for Carbon utilization (ACT-C), Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: The C−F bond cleavage reaction of tetrafluoro-
ethylene (TFE; CF2CF2) with an M(0) complex (M = Pd,
Ni) was investigated. The treatment of an M(0) precursor with
TFE in the presence of the appropriate monodentate
phosphine ligand led to a clean formation of the corresponding
η2-TFE adduct (η2-TFE)M(PR3)2. In the case of the Ni(0)
species, in particular, the choice of phosphine ligands is crucial
for the preparation of the desired η2-TFE complex: the use of either PCy3 or PiPr3 resulted in the target adduct, while less
sterically hindered phosphines such as PPh3 and PnBu3 gave the known octafluoronickelacyclopentane as a result of the oxidative
cyclization of two TFE molecules. Thermolysis of both palladium and nickel η2-TFE adducts bearing PCy3 as the ligand resulted
in a C−F bond activation reaction and gave the corresponding (trifluorovinyl)metal fluorides, trans-(PCy3)2M(F)(CFCF2).
The reaction of (η2-TFE)Pd(PPh3)2 with LiI as an additive allowed cleavage of the C−F bond in THF, even at room
temperature, and gave trans-(PPh3)2Pd(I)(CFCF2) with a concomitant formation of lithium fluoride. Other metal halides,
such as MgBr2 and AlCl3, also promoted the C−F bond cleavage of TFE. In addition, the use of either BF3·Et2O or B(C6F5)3
exerted a similar accelerative effect on the C−F bond activation of TFE on either nickel or palladium. The molecular structures of
a series of η2-TFE and trifluorovinyl complexes were unambiguously determined by means of X-ray crystallography. The resultant
(trifluorovinyl)palladium or -nickel species have shown the potential to utilize a key intermediate in cross-coupling reactions with
organometallic reagents to prepare a variety of trifluorovinyl compounds.
respectively (Scheme 1b,c).3c,d In these reactions, either the
use of additives such as lithium iodide and silyl triflate or the
existence of a trimethylstannyl group bound to the rhodium
center could play an important role in the C−F bond activation
INTRODUCTION
■
The activation of C−F bonds by homogeneous transition-metal
complexes has been a fascinating subject in the field of
organometallic and organic chemistry, since it provides not only
of TFE; the formation of thermodynamically favored Li−F, Si−
new routes to metal−fluoride complexes under mild conditions
F, and Sn−F bonds might be a driving force behind these
but also novel potential synthetic routes to fluorinated organics
that are difficult to access by conventional reactions.1 Although
reactions. Inspired by such groundbreaking studies, we have
developed the first coupling reaction of TFE with aryl zinc
compounds to yield (α,β,β-trifluoro)styrene derivatives, in
striking developments have been made in recent years on the
intermolecular C−F bond activation of fluorinated olefins,2 the
alkenyl C(sp2)−F bond cleavage of perfluoroalkenes remains a
great challenge. In fact, among the perfluoroalkenes, only
hexafluoropropylene, CF3CFCF2, has been used as a
substrate in transition-metal-catalyzed C−F bond activation
reactions.2j,o−q In contrast, until 2010, no homogeneous
catalytic reactions that involve C−F bond cleavage of the
simplest perfluoroalkene, tetrafluoroethylene (1; TFE, CF2
CF2), had been reported, and C−F bond cleavage of TFE by a
transition metal had only been achieved in a few stoichiometric
reactions.3 In 1973, Kemmitt and co-workers reported that
treatment of an η2-TFE platinum complex with lithium iodide
at 95 °C gave a (trifluorovinyl)platinum complex in
quantitative yield (Scheme 1a).3b Similar C−F bond cleavage
reactions using either a dinuclear iridium or a mononuclear
rhodium complex were developed by Cowie and Booth,
which efficient C−F bond cleavage on palladium was achieved
by using lithium iodide as a coadditive.4 Moreover, we have
recently demonstrated the Pd0/PR3-catalyzed cross-coupling
reaction of fluoroalkenes with arylboronates, which required
neither an extraneous base to enhance the reactivity of
organoboronates nor a Lewis acid to promote the oxidative
addition of a C−F bond.5
Our next concern was to investigate how efficiently the
C(sp2)−F bond of TFE is cleaved under mild conditions. We,
therefore, started evaluating the effect of additives to the C−F
bond activation reaction. Among a variety of candidates with
Lewis acidity, boron Lewis acids in particular, such as BF3 and
Received: March 26, 2013
© XXXX American Chemical Society
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TFE (1)13 or a mixture of Pd2(dba)3 and PCy3 with 1 (Scheme
2a). Both reactions proceeded almost quantitatively, and
Scheme 1. Stoichiometric C−F Bond Cleavage on
Transition-Metal Complexes
Scheme 2. Formation of (η2-TFE)Pd(PR3)2 (2)
complex 2a was isolated in 92% yield. In contrast, the reaction
of Pd2(dba)3 with 1 in the presence of PPh3 (2 equiv, relative to
Pd atom) gave an equilibrium mixture of the corresponding η2-
TFE complexes supported by two triphenylphosphine ligands,
(η2-CF2CF2)Pd(PPh3)2 (2b) and (η2-dba)Pd(PPh3)2,14
which hampered the isolation of the desired product 2b.
Thus, the isolation of 2b was accomplished by the substitution
of a TFE molecule for the coordinated ethylene in (η2-CH2
CH2)Pd(PPh3)2 (Scheme 2a).15 On the other hand, treatment
B(C6F5)3, are well-known to show fluorophilicity, as evidenced
by the fact that in 1964 Olah et al. demonstrated the first
Friedel−Craft alkylation reaction of alkyl fluoride by employing
BF3.6 Furthermore, C−F bond activation promoted by the
addition of commercially available B(C6F5)3 has also been
developed in the past decade.7 It is assumed that the
fluorophilicity of the boron atom is the driving force behind
these reactions.
12c
of Pd(PtBu3)2 with 1 gave no products, probably due to the
greater bulkiness of PtBu3.
Some bidentate phosphine ligands were found to allow the
formation of the corresponding η2-TFE complex. For example,
in the presence of either bis(dicyclohexylphosphino)ethane
(DCPE) or bis(dicyclohexylphosphino)butane (DCPB), CpPd-
(η3-allyl)16 reacted with 1 in toluene to give the desired product
(3a or 3b, respectively) in quantitative yield (Scheme 3a). In
In addition, zerovalent nickel complexes are assumed to be
promising candidates to accomplish the C−F bond cleavage of
TFE, since some of them are known to promote the oxidative
addition of the aromatic C(sp2)−F bond.8,9 However, the
traditional approach using Ni0L2 precursors had mostly been
unsuccessful because the expected η2-CF2CF2 intermediate
overreacted with further TFE molecules to give octafluoronick-
elacyclopentanes as a result of oxidative cyclization with
Ni(0).10 To overcome this circumstance, we started preparing
a novel η2-TFE nickel complex with bulkier supporting ligands,
since such a ligand is most certainly believed to circumvent the
overreaction. This concept was supported by the fact that η2-
CF2CF2 Ni(0) complexes coordinated by a tridentate ligand,
such as (tdt)Ni(η2-CF2CF2) (tdt = trans,trans,trans-cyclo-
dodeca-1,5,9-triene) and (triphos)Ni(η2-CF2CF2) (triphos =
CH3C(CH2PPh2)3), were unreactive toward the second TFE
molecule.10b,d,11 In addition, the use of ligands with strong σ-
donor abilities was anticipated to often allow the oxidative
addition of substrates that are otherwise unreactive. We thus
investigated the reaction of Ni(cod)2 with TFE in the presence
of PCy3 and IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene), with combined bulkiness and strong σ-donor abilities.
Herein, we give a detailed account of the C−F bond cleavage
reactions of TFE for both Pd(0) and Ni(0) species. These
reactions were found to be promoted by the addition of Lewis
acids to give a series of (trifluorovinyl)palladium or -nickel
complexes. In addition, isolation and molecular structures of
these trifluorovinyl complexes as well as η2-TFE palladium and
nickel adducts were also discussed.
Scheme 3. Formation of (η2-TFE)Pd(P−P) (3: P−P =
Bidentate Phosphine Ligand)
contrast, a higher reaction temperature was required to
generate the corresponding η2-TFE complex (η2-TFE)Pd(rac-
BINAP) (3c) when rac-BINAP was used as the bidentate ligand
(Scheme 3b).
Synthesis of η2-TFE Nickel Complexes Bearing
Phosphine Ligands. Exposing a toluene solution of a mixture
of Ni(cod)2 and 2 equiv of PCy3 to an atmosphere of TFE at
room temperature led to the clean formation of an η2-TFE
complex, (η2-CF2CF2)Ni(PCy3)2 (4a), and it was isolated in
64% yield (Scheme 4a). In contrast, as already reported by
Stone et al.,10b the use of PPh3 instead of PCy3 as a ligand
exclusively gave the octafluoronickelacyclopentane complex 5a
as a result of the oxidative cyclization of two molecules of TFE
with nickel (Scheme 4b). Systematic investigation into the
RESULTS AND DISCUSSION
■
Synthesis of η2-TFE Palladium Complexes Bearing
Phosphine Ligands. Formation of (η2-CF2CF2)Pd(PCy3)2
12
(2a) was accomplished by treating either Pd(PCy3)2 with
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product, (η2-TFE)Ni(IPr)2, was observed. Instead, IPr itself
was found to smoothly react with TFE to give the undesired
1,3-bis(2,6-diisopropylphenyl)-2-(perfluoroethylidene)-1H-imi-
dazole.5
Scheme 4. Formation of η2-TFE Nickel Complexes (4 and 6)
and an Octafluoronickelacyclopentane Complex (5a)
Molecular Structures of η2-TFE Palladium and Nickel
Complexes. The molecular structures of a series of η2-TFE
complexes, 2a,b, 3c, and 4a, were definitively determined by
means of X-ray crystallography (Figures 1 and 2). In these
complexes, the coordination geometry of the metal center is
regarded as a Y-shaped trigonal plane in which the midpoint of
the carbon−carbon double bond is assumed to be a
coordination center. The C1−C2 bond lengths of 1.419(4) Å
for 2a, 1.425(4) Å for 2b, 1.430(4) Å for 3c, and 1.4257(15) Å
for 4a fell within the range of previously reported CC bond
lengths of a coordinated TFE molecule (1.37(3)−1.45(2)
Å)11,18 but were significantly longer than that of a free TFE
molecule (1.311(3) Å).19 In addition, the normal angles
between the F1−C1−F2 and F3−C2−F4 planes, which is used
by Ibers to define how far the olefin substituents bend
backward,20 are 97.90(24)° for 2a, 103.35(21)° for 2b,
103.45(20)° for 3c, and 99.62(10)° for 4a. These values are
beyond the corresponding maximum that has been previously
observed in related η2-TFE complexes (70.4−84°),11,18c−f,k,l
indicating that the contribution of the back-donation from
palladium to the π* antibonding orbital of TFE must be very
large. Therefore, these complexes make some kind of
contribution to metallacyclopropane.
The Ni−P bond distances (2.2453(3) and 2.2429(3) Å) in
4a are almost similar to the average of those observed in the
structurally characterized Ni−PCy3 complexes (2.211 Å).21 The
average Ni−C bond distance of 1.905(1) Å is slightly longer
than those found in (triphos)Ni(η2-CF2CF2) (1.86(2) Å)11
and (tmeda)Ni(η2-CF2CF2) (1.838(3) Å),18h which is
probably due to the steric repulsion between the TFE and
PCy3 molecules. In addition, unlike the case for the palladium
analogue 2a, the ligated CC bond in 4a is twisted by 23.5°
(cf. 1.4° in 2a) with respect to the P−Ni−P plane, which is
probably due to an avoidance of steric hindrance that is caused
by the smaller ionic radius of nickel.
reaction of Ni(cod)2 with TFE in the presence of a variety of
monodentate phosphines revealed that the reaction product
depends primarily on the bulkiness of phosphine ligands. That
is, a similar η2-TFE nickel complex (4b) was generated when
bulkier phosphines with a wider cone angle17 than ca. 160°,
such as isopropylphosphine, were employed as ligands (Scheme
4a). In contrast, reactions with the narrower phosphines such as
PPh3, PEt3, PBu3, and P(OMe)3 yielded the known
octafluoronickelacyclopentanes,10 regardless of the basicity of
the phosphine (Scheme 4b). Furthermore, neither tert-butyl
phosphine nor o-tolylphosphine gave any TFE-containing
products, probably due to their bulkiness.
The formation of an η2-TFE nickel complex could be
achieved by using bidentate phosphine ligands; for example,
treating Ni(cod)2 with an equimolar amount of DCPE under a
TFE gas atmosphere led to the formation of the η2-TFE
analogue (η2-TFE)Ni(dcpe) (6a) in quantitative yield (Scheme
4c). DCPB could also be applied to the same reaction
conditions to afford the corresponding complex (η2-TFE)Ni-
(dcpb) (6b). In contrast, reactions with both Ph2PCH2PPh2
and 2,2′-bipyridyl are known to give the corresponding
octafluoronickelacyclepentanes.10b
Dynamic Behavior in Solution of an η2-TFE Nickel
Complex. As mentioned above, an X-ray diffraction study
revealed that 4a has a pseudorotationally symmetrical structure
in the solid state and should be an AA′BB′XX spin system,
where A and B represent 19F nuclei and X represents 31P. The
19F NMR spectrum measured in toluene-d8 at −80 °C,
although it did not reach the slow-limit temperature, was
The attempt to employ IPr as a ligand in the preparation of a
η2-TFE nickel complex failed. When a mixture of Ni(cod)2 and
2 equiv of IPr was treated with TFE, no generation of a putative
Figure 1. ORTEP drawings of 2a with thermal ellipsoids at the 30% probability level. H atoms are omitted for clarity.
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Figure 2. ORTEP drawing of 4a with thermal ellipsoids at the 30% probability level. H atoms and solvent molecules (benzene) are omitted for
clarity.
consistent with the structure; two broadened signals with the
same intensity were observed at δ −112.8 and −140.0 (see also
the Supporting Information). These signals, however, broad-
ened and flattened with elevating temperature as they coalesced
into one signal at −20 °C, which implied that a fluxional
process involving either rotation or a twisting motion of the
coordinated TFE ligand had occurred (Figure 3). The 19F
gives an ORTEP drawing of the octafluoronickelacyclopentane
5a, showing a coordination geometry of the nickel center that is
slightly distorted in the square plane, as indicated by the sum of
the angles around nickel (369.1°) as well as by a twist between
the C1−Ni−C4 and P1−Ni−P2 planes (32.1°). The complex
5a essentially has the same stereochemical arrangement as the
PEt3 analogue (PEt3)2Ni(C4F8), which was previously fully
characterized by Burch et al.22 The 19F and 31P NMR spectra of
5a measured in C6D6 at room temperature displayed
resonances at δF −101 (t, JPF = 24.2 Hz) and δP 25.8 ppm
with the same coupling constant as the 19F nuclei (quin, JPF
=
24.2 Hz), which is attributable to α-CF2 and PPh3, respectively.
In addition, the β-CF2 signal was observed at −138 ppm as a
singlet signal. The observation of the apparent quintet of 31P
Figure 3. Fluxional behavior of 4a in solution.
nuclei as well as the apparently equivalent chemical shifts of 19
F
nuclei of both the α- and β-CF2 groups clearly indicated the
existence of rapid ring flipping of the nickelacyclopentane via a
transition state with a planar five-membered ring on the NMR
time scale.
Thermally Promoted C−F Bond Activation of TFE on
Palladium(0) or Nickel(0). Before the effect of additives on
the C−F bond cleavage of TFE was evaluated, control
experiments were carried out in the absence of any additive.23
Although complex 2a remained intact in a THF solution at
room temperature, heating the THF solution at 100 °C under a
N2 atmosphere initiated a C−F bond activation of TFE to yield
the expected (trifluorovinyl)palladium(II) fluoride 7 in 45%
yield (Scheme 5). NMR observation revealed the concomitant
NMR spectrum of 4a at room temperature exhibited a single
but broadened resonance at δ −131.7 assignable to the
coordinated TFE. At that temperature, the 31P signal of 4a
appeared at δ 32.6 with a complicated coupling pattern, which
indicated that the 19F atoms A and B become chemically
equivalent but are still magnetically inequivalent, resulting in an
AAA′A′XX′ spin system.
Molecular Structure of Octafluoronickelacyclopen-
tane 5a and Its Fluxional Behavior in Solution. Figure 4
Scheme 5. Generation of Trifluorovinylpalladium(II) and
Trifluorovinylnickel(II) Fluorides via Direct Oxidative
Addition of TFE
Figure 4. ORTEP drawing of 5a with thermal ellipsoids at the 30%
probability level. H atoms are omitted for clarity.
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formation of a palladium 2-perfluorobutenyl species (8).24 The
recovery of Pd(PCy3)2 (26%) indicated the existence of a
coordination−dissociation equilibrium of TFE to palladium
under the reaction conditions. Thus, this reaction was
conducted under a TFE atmosphere (1 atm), resulting in an
improvement in the yield of 7. In contrast, thermolysis of the
palladium triphenylphosphine complex 2b under the same
conditions resulted in its rapid decomposition to give palladium
black and a free TFE molecule; the C−F bond activation did
not proceed at all.4
higher temperature (100 °C) did not give the expected
oxidative addition products (Scheme 7).
Scheme 7. Reaction of (η2-TFE)M(dcpe) with LiI
In addition, the nickel analogue 4a was found to cleave the
C−F bond of TFE at 100 °C under a nitrogen atmosphere,
giving the corresponding (trifluorovinyl)nickel fluoride 9 in
70% yield (Scheme 5). However, conducting this reaction
under a TFE atmosphere (1.0 atm) resulted in the formation of
the difluorophosphorane F2PCy3 (δF −64.2 (d), δP −23.4 (t),
JPF = 650 Hz); complex 9 was not obtained at all. The
characteristic upfield-shifted resonances attributable to a
fluorine adjacent to a group 10 metal appeared at δ −317.9
(br s) and −385.7 (t, JPF = 41.4 Hz) in the 19F NMR spectra of
7 and 9, respectively. In the 19F NMR spectrum of 9, three
resonances assignable to the trifluorovinyl moiety were
detected at δ −92.4, −133.3, and −159.1. Attempts to carry
out the corresponding C−F bond activation on nickel or
palladium supported by a dcpe ligand failed; neither 3a nor 6a
gave any trifluorovinyl complex even at 100 °C.
Metal Halide Promoted C−F Bond Activation of TFE
on M(0) (M = Pd, Ni). We next investigated which additives
can effectively cleave the C−F bond of TFE on η2-TFE
complexes. The C−F bond activation of TFE can be achieved
through treatment with other metal halides. The reaction of 2a
with a half equimolar amount of magnesium bromide afforded
the corresponding trifluorovinyl complex trans-(PCy3)2Pd(Br)-
(CFCF2) (12) in quantitative yield (Scheme 8). In contrast,
Scheme 8. Metal Halide Promoted C−F Bond Activation of
TFE
LiI-Promoted C−F Bond Activation of TFE on
Palladium(0) or Nickel(0). As reported in a previous
communication,4 the addition of lithium iodide to a THF
solution of 2b at room temperature resulted in the very smooth
oxidative addition of a C−F bond, within a few minutes, to
afford trans-(PPh3)2Pd(I)(CFCF2) (10b) with the concom-
itant formation of lithium fluoride (Scheme 6). The rate of the
the reaction with aluminum chloride required a prolonged
reaction time, and after 120 h, the starting material 2a was
quantitatively converted into trans-(PCy3)2Pd(Cl)(CFCF2)
(13). On the other hand, the nickel complex 4a indeed reacted
with MgBr2 or AlCl3 more smoothly than the palladium
analogue 2a, to give the corresponding (trifluorovinyl)nickel
halides trans-(PCy3)2Ni(X)(CFCF2) (14, X = Br; 15, X =
Cl) in excellent yield. Thus, the nature of group 10 metals
apparently influenced the reactivity in cleaving the C−F bond
of TFE. In addition, the factor determining the reactivity, of
course, depended on not only the nature of the group 10 metals
but also the metal halides employed. For example, in the
transformation of 4a into 15, the reaction with magnesium
chloride was very slow (room temperature, 24 h), and the use
of lithium chloride gave a complicated mixture, including the
desired complex 15. A series of trifluorovinyl complexes were
identified on the basis of the characteristic 19F resonances of the
trifluorovinyl group. It should be mentioned that a related
(trifluorovinyl)palladium chloride complex, trans-(PPh3)2Pd-
(Cl)(CFCF2), has recently been isolated and structurally
defined by Lu, Shen, and co-workers.25
Molecular Structures of Trifluorovinyl Metal Halides.
The occurrence of the C−F bond cleavage of TFE was
unambiguously confirmed by X-ray crystallography of 7 and 9,
the results of which were previously reported in our recent
communication (Figure 5).5 Each group 10 metal center in 7
and 9 possessed an approximate square-planar geometry with a
trans alignment of the PCy3 ligands in the solid state. The
fluoride and trifluorovinyl ligands were also situated in mutually
trans positions, and the trifluorovinyl and either palladium or
nickel coordination planes are almost orthogonal (dihedral
Scheme 6. LiI-Promoted C−F Bond Activation of TFE
C−F bond activation on palladium in the presence of lithium
iodide was found to depend on the ligand that coordinates to
the palladium. When the reaction of 2a with lithium iodide was
conducted, it took more than 4 days to consume 2a while a
similar trifluorovinyl complex, trans-(PCy3)2Pd(I)(CFCF2)
(10a), was also generated. Furthermore, oxidative addition to
nickel was also accelerated by the addition of an equimolar
amount of LiI to afford trans-(PCy3)2Ni(I)(CFCF2) (11a).
In these reactions, lithium iodide would act as a Lewis acid to
enhance the elimination ability of fluorine. This is supported by
the fact that both MgBr2 and AlCl3 can accelerate the C−F
bond activation of TFE (vide infra), although another
possibility of basic iodide attacking the M−F center (M =
Pd, Ni) cannot be completely ruled out. The high lattice
enthalpy of LiF(s) might also be important for the occurrence
of the oxidative addition at room temperature.
As anticipated by the fact that a thermally promoted C−F
bond activation of TFE did not occur at all, treating the η2-TFE
dcpe complexes 3a and 6a with lithium iodide in THF even at a
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C6D6 solution of 2a allowed the oxidative addition of the C−F
bond to the palladium to give trans-(PCy3)2Pd(BF4)(CF
CF2) (16) in quantitative yield (Scheme 9). The (trifluoro-
vinyl)palladium complex 16 was isolated in 58% yield, and its
molecular structure was determined by X-ray crystallography.
Scheme 9. BF3·Et2O-Promoted C−F Bond Activation of TFE
One of the most significant structural features of 16 was that
one of the fluorine atoms in the tetrafluoroborate bridges
between the palladium and boron atoms (Figure 7). Thus,
Figure 5. ORTEP drawing of 9 with thermal ellipsoids at the 30%
probability level. H atoms are omitted for clarity.
angle: 87.73(13)° for 7; 88.93(21)° for 9). The Pd−F bond
length of 2.139(5) Å in 7 and the Ni−F bond length of
1.833(4) Å in 9 lie within the ranges of the corresponding
metal−fluoride bonds (1.947(4)−2.1024(17) Å for palladium
and 1.836(3)−1.916(3) Å for nickel, respectively).2,26,27
The ORTEP drawing of 10b shows that palladium adopts a
square-planar coordination geometry and is coordinated with
two PPh3 ligands in a trans manner (Figure 6). Similar to the
Figure 7. ORTEP drawing of 16 with thermal ellipsoids at the 30%
probability level. H atoms are omitted for clarity.
BF3·Et2O served as a Lewis acid to promote the C−F bond
cleavage, and in addition, the resultant tetrafluoroborate served
as a Lewis base in order to stabilize the square-planar geometry
of the palladium(II) center in 16 by coordination of the
fluorine atom to the palladium. The same coordination mode of
the tetrafluoroborate anion in the Pd(II) complex has been
reported in three different citations in the literature.28 The Pd−
F4 bond distance of 2.161(3) Å in 16 is quite a bit shorter than
those observed in the other reported PdII−F−BF3 complexes
(2.241(2)−2.355(5) Å), while the Pd−F4−B bond angle of
141.9(5)° was almost equal to the upper range of the observed
values (123.5(7)−141.9(4)°).28 The B−F4 bond distance of
1.446(7) Å in 16 was found to be slightly longer than the
average of the other three nonbridged B−F bonds (1.377(7)
Å). We found the CC bond distance of the coordinated
trifluorovinyl group (1.297(8) Å for 16) to be equal within the
margin of error to those observed in Hg(CFCF2)2, (η5-
C5Me4Et)2Ti(OH)(CFCF2), (PPh3)Au(CFCF2), [Ir2(η1-
CFCF2)(CH3)(CO)2(μ-Cl)(dppm)2](CF3SO3), and Ph3Ge-
(CFCF2) (1.312(6), 1.331(4), 1.297(14), 1.298(9), and
1.230(8) Å, respectively)29 as well as that in trans-(PPh3)2Pd-
(Cl)(CFCF2) (1.270(4) Å).25
Figure 6. ORTEP drawing of 10b with thermal ellipsoids at the 30%
probability level. H atoms are omitted for clarity.
case for 7, the trifluorovinyl group was almost orthogonal to the
coordination plane (dihedral angle: 87.12(19)°). We also
carried out an X-ray diffraction study of the (trifluorovinyl)
nickel iodide 11a. Whereas the structure refinement was not
sufficient due to a deterioration in the quality of the crystal
analyzed, two PCy3 ligands in 11a coordinated to the nickel
center in a trans manner, and the trifluorovinyl and iodide
ligands were bound to the nickel center to form a square-planar
geometry.
Boron Derivative Promoted C−F Bond Activation of
TFE. We found that the C−F bond cleavage of TFE was also
promoted by the addition of boron derivatives with Lewis
acidity. The addition of an equimolar amount of BF3·Et2O to a
The NMR spectra of 16 revealed that its molecular structure
in solution is consistent with that observed in the solid state, in
F
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Article
which the tetrafluoroborate anion interacted with the palladium
center. The variable-temperature 19F NMR spectra of 16
demonstrated the fluxionality of the tetrafluoroborate. The 19F
signal assignable to the tetrafluoroborate anion, measured in
toluene-d8 at room temperature, was observed as a broad signal
at δ −172, and there was an obvious chemical shift to upper
magnetic field from that of the free tetrafluoroborate anion
(NaBF4; ca. δ −150 to −153).30 With a decrease in the
temperature, the peak broadened and decoalesced at −40 °C,
and then it split into two signals with an intensity ratio of 3:1 at
−90 °C, while the spectrum measured at that temperature did
not reach the slow-limit spectrum. This temperature depend-
ence of the spectra strongly indicates the site-exchange process
of the fluorine atoms on the tetrafluoroborate between the
bridging Pd−F−B site and the terminal B−F sites.
DABCO and pyridine is probably caused by the difference in
the pKa values of their conjugate acids ([DABCO-H]+, 8.82;
[pyridine-H]+, 5.17);31,32 the stronger Lewis base DABCO is
more amenable to complexation with BF3. The treatment of
18-BF4 with DABCO, however, did not give a transformation
to 7.
As for the reaction with BF3·Et2O, the addition of an
equimolar amount of B(C6F5)3 to a C6D6 solution of 2a
promoted a C−F bond cleavage, and this reaction proceeded
within a few minutes. Monitoring of the initial stage of the
reaction by means of 19F NMR spectroscopy indicated the
generation of [FB(C6F5)3]− anion as a result of the abstraction
of a fluorine atom, suggesting the formation of a cationic
trifluorovinylpalladium complex (19; Scheme 11). The
A similar procedure could be applied to the nickel analogue
4a, giving trans-(PCy3)2Ni(BF4)(CFCF2) (17), which was
characterized on the basis of the observation of a similar
fluxional behavior of the tetrafluoroborate anion in variable-
temperature 19F NMR spectra (Scheme 9).
Scheme 11. B(C6F5)3-Promoted C−F Bond Activation of
TFE
The (trifluorovinyl)palladium tetrafluoroborate complex 16
was found to be stable in either toluene or benzene solution
even when the temperature was elevated to 100 °C. However, it
showed reactivity toward a Lewis base that contains a nitrogen
atom, and the nature of the Lewis base was apparently
influenced by the choice of the two different types of reactions.
That is, the formation of the Lewis acid−base adduct took place
by the addition of a half equimolar amount of 1,4-
diazabicyclo[2.2.2]octane (DABCO) to a benzene solution of
16, giving 7 in quantitative yield with a concomitant generation
of (DABCO)·(BF3)2 (Scheme 10). The use of NEt3, instead of
observation of a set of 19F resonances with characteristic JFF
coupling constants attributable to a trifluorovinyl group also
supported the formation of 19. Complex 19 was unstable in
solution and decomposed gradually at room temperature. This
instability may stem from either a vacancy in the coordination
site of the square-planar geometry on palladium as a result of
the formation of the [FB(C6F5)3]− anion33 with a much weaker
coordinating ability, or it may be the result of the insufficient
occupancy of the resultant vacant site in 19 by a ligand, such as
benzene, used as a solvent, or the result of a perfluorophenyl
ring in the counteranion that could not sufficiently stabilize the
metal center. Thus, we conducted successive treatments of 2a
with B(C6F5)3 followed by pyridine, giving the stable cationic
complex 18-FB(C6F5)3. Unlike 19, 18-FB(C6F5)3 was stable
enough to be isolated, and indeed, a benzene or toluene
solution of 18-FB(C6F5)3 remained intact at room temperature
for 1 day.
Scheme 10. Reactivity of 16 toward DABCO and Pyridine
DABCO, led to a decrease in the yield of 7 to 30% even after a
prolonged reaction time (23 h). As already mentioned above,
(trifluorovinyl)palladium fluoride 7 could be obtained by the
thermally promoted oxidative addition of a C−F bond of TFE
on palladium (Scheme 5), whereas an undesired side reaction
inevitably took place to give 8 and hamper the isolation of 7. In
contrast, this reaction enabled the selective formation of 7, as
depicted in Scheme 10.
On the other hand, treatment of 16 with an equimolar
amount of pyridine in benzene resulted in the rapid substitution
of pyridine for tetrafluoroborate on palladium to give a cationic
trifluorovinyl complex, [trans-(PCy3)2Pd(py)(CFCF2)]-
[BF4] (18-BF4) (Scheme 10). In the 19F NMR spectrum of
18-BF4, a resonance assignable to the tetrafluoroborate anion at
δ −154.2 (s) appeared at almost the same region as the signal
for NaBF4, clearly indicating that it lies out of the coordination
sphere of palladium. The difference in the product between
The molecular structure of 18-FB(C6F5)3 was unambigu-
ously determined by means of X-ray crystallography, and an
ORTEP drawing of the cationic part of 18-FB(C6F5)3 is shown
in Figure 8. The drawing clearly demonstrates that the
palladium center possesses a square-planar geometry with a
trans alignment of the trifluorovinyl and pyridine ligands, and
the Pd−N bond distance was 2.103(2) Å. In the crystal lattice,
the [FB(C6F5)3]− anion lies out of the coordination sphere of
1
the palladium. In the H NMR spectrum of 18-FB(C6F5)3
measured in C6D6, the five signals assignable to the
coordinating pyridine molecule were observed to be
inequivalent, indicating that neither rapid coordination−
dissociation equilibrium nor rotation of the coordinating
pyridine about the Pd−N bond took place in solution.
G
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Article
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grant-in-Aid for Scientific
Research (A) (No. 21245028), Grant-in-Aid for Young
Scientists (A) (No. 25708018), and Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” (No. 23105546) from
MEXT. M.O. also acknowledges The Noguchi Institute.
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CONCLUSIONS
■
In conclusion, we demonstrated the preparation of a series of
η2-TFE palladium and nickel complexes, (η2-TFE)M(PR3)2 (M
= Ni, Pd). Heating both palladium and nickel η2-TFE
complexes supported by PCy3 ligands allowed C−F bond
activation to take place in order to generate the corresponding
(trifluorovinyl)metal fluorides trans-(PCy3)2M(F)(CFCF2).
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metal halides such as LiI, MgBr2, and AlCl3, giving the
corresponding (trifluorovinyl)metal halides. Furthermore, the
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intermediates in the cross-coupling reaction with organo-
metallic reagents to prepare a variety of trifluorovinyl
compounds. Further theoretical studies on the reaction
mechanism are ongoing in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving detailed experimental
procedures, analytical and spectral data for all new compounds,
and crystallographic data for 2a,b, 3c, 4a, 5a, 9, 10b, 16, and
18-FB(C6F5)3. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
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Ed. 2010, 49, 2933−2936. (w) Kuehnel, M. F.; Schloeder, T.; Riedel,
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Lentz, D. Angew. Chem., Int. Ed. 2012, 51, 2218−2220. (x) Kuehnel,
M. F.; Holstein, P.; Kliche, M.; Krueger, J.; Matthies, S.; Nitsch, D.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ohashi@chem.eng.osaka-u.ac.jp (M.O.); ogoshi@
chem.eng.osaka-u.ac.jp (S.O.).
■
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dx.doi.org/10.1021/om400255t | Organometallics XXXX, XXX, XXX−XXX