On the feasibility of nickel-catalyzed trifluoromethylation of aryl halides

Article abstract of DOI:10.1021/om5008743

A computational screening of 42 bidentate phosphines (PP) has yielded promising candidates for Ph-CF₃ reductive elimination from Ni(II) complexes of the type [(PP)Ni(Ph)(CF₃)]. The computed barriers and synthetic accessibility consid

Full text of DOI:10.1021/om5008743

Article
pubs.acs.org/Organometallics
On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl
Halides
Jesus Jover,† Fedor M. Miloserdov,† Jordi Benet-Buchholz,† Vladimir V. Grushin,*,† and Feliu Maseras*,†,‡
́
†Institute of Chemical Research of Catalonia (ICIQ), Avgda. Països Catalans, 16, 43007 Tarragona, Catalonia, Spain
‡Departament de Química, Universitat Auton
̀
oma de Barcelona, 08197 Bellaterra, Catalonia, Spain
S
* Supporting Information
ABSTRACT: A computational screening of 42 bidentate phosphines (PP) has
yielded promising candidates for Ph−CF3 reductive elimination from Ni(II)
complexes of the type [(PP)Ni(Ph)(CF3)]. The computed barriers and synthetic
accessibility considerations have identified two PP ligands, dippf and dcypf (ΔG⧧ =
22.6 and 23.2 kcal/mol, respectively), for experimental studies with 1-Np (1-
naphthyl) in place of Ph. Ligand exchange of [(Ph3P)2Ni(1-Np)Cl] with dippf or
dcypf has cleanly produced [(dippf)Ni(1-Np)Cl] and [(dcypf)Ni(1-Np)Cl], the first
examples of trans square-planar 1,1′-ferrocenediyl backbone-based diphosphine metal
complexes devoid of M···Fe dative interactions. Treatment of these chlorides with CF3SiMe3/F−, AgCF3/MeCN or
[(Ph3P)3Cu(CF3)] does not furnish isolable or 19F NMR-detectable [(PP)Ni(1-Np)(CF3)] (PP = dippf, dcypf). Other
transformations have been observed instead, e.g., ligand exchange with the Ag and Cu complexes, the latter leading to
[(dcypf)Cu(CF3)], a rare example of well-defined CF3Cu(I) species. With CF3SiMe3/F−, indirect evidence has been obtained for
intermediacy of [(PP)Ni(1-Np)(CF3)] (PP = dippf, dcypf) and instantaneous decomposition via pathways other than C−CF3
reductive elimination. The first Ni(II) complexes with fluoride trans to a non-electron-deficient aryl, [(Cy3P)2Ni(1-Np)F] and
[(i-PrXantphos)Ni(1-Np)F], have been prepared and fully characterized. Surprisingly, [(Cy3P)2Ni(1-Np)F] can be produced
from [(Cy3P)2Ni(1-Np)Cl] and CsF rather than AgF that is conventionally used for the synthesis of late transition metal
fluorides via X/F exchange. While [(Cy3P)2Ni(1-Np)F] is unreactive toward CF3SiMe3, [(i-PrXantphos)Ni(1-Np)F] is readily
trifluoromethylated to produce robust [(i-PrXantphos)Ni(1-Np)(CF3)], a rare example of complexes of the type
[(PP)Ni(Ar)(CF3)] with PP other than dippe.
Scheme 1. Catalytic Cycle for Pd-Catalyzed
Trifluoromethylation of Aryl Halides
INTRODUCTION
■
Aromatic and heteroaromatic compounds bearing a trifluor-
omethyl (CF3) group on the ring are important building blocks
and intermediates for the synthesis of agrochemicals,
pharmaceuticals, and specialty materials.1−3 Metal-catalyzed/
mediated coupling of aryl halides with CF3 nucleophiles (eq 1)
is currently viewed3,4 as an alternative to the environmentally
unfriendly and limited in scope Swarts-reaction-based old
technology to produce trifluoromethylated aromatics. Since the
pioneering work of McLoughlin and Thrower5 in the 1960s,
considerable progress has been made in the area of Cu-
promoted/catalyzed trifluoromethylation of haloarenes.2,3,6−10
Apart from Cu, two more metals, Pd and Ni, are widely used in
a diversity of coupling reactions of aryl halides.11 A decade ago,
one of us12−14 explored the possibility of Pd-catalyzed
trifluoromethylation of haloarenes to show that Ar−CF3
reductive elimination from Pd(II) is the most problematic
elementary step of the proposed catalytic cycle (Scheme 1). For
example, [(PP)Pd(Ph)(CH3)] (PP = dppe, dppp) undergo
reductive elimination of toluene at 15−40 °C,16 whereas their
CF3 congeners produce PhCF3 in low to modest yield only
after many hours at 145 °C.13 This contrast is in full accord
with the previously recognized15 exceptional strength and
inertness of the late transition metal−perfluoroalkyl bonds. In
2006, however, it was reported14a that [(Xantphos)Pd(Ph)-
(CF3)] undergoes reductive elimination of PhCF3 in a highly
selective manner under mild conditions (50−80 °C). Since the
discovery of that first example of clean and facile Ar−CF3
reductive elimination from Pd, only three more ligands have
been found to promote this transformation, BrettPhos,17
Received: August 27, 2014
© XXXX American Chemical Society
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RuPhos,17,18 and (CF3)2PCH2CH2P(CF3)2.19 Of these four
ligands, only BrettPhos and RuPhos have been found17 suitable
for Pd-catalyzed trifluoromethylation of haloarenes (aryl
chlorides).
Even less progress has been made toward Ar−CF3 bond
formation at Ni. Perfluoroalkyl complexes of nickel are rare in
general.20,21 There has been only one type of CF3Ni aryls
reported in the literature. Vicic and co-workers21 have prepared
[(dippe)Ni(Ar)(CF3)] (dippe =1,2-bis(diisopropylphosphino)-
ethane; Ar = Ph, 3-Tol, 2-naphthyl, 4-anisyl) and demon-
strated21a that these complexes produce Ar2 and [(dippe)Ni-
(CF3)2] but not ArCF3 upon heating. The formation of the
corresponding trifluoromethylarenes in 11−22% yield was
observed only when the decomposition was performed in the
presence of PhZnBr, ZnBr2, or water.21a
in this database containing information for a total of 275
ligands. Analysis of the resulting LKB-PPscreen database with
principal component analysis (PCA) captures the effects of
changing backbones and substituents on ligand properties and
allows the grouping of ligands according to their chemical
similarity.
For the current study, we selected 35 of those ligands arising
from the combination of seven different backbones (Scheme 3)
Scheme 3. Backbones Selected for the Current Study
In the current work, we have attempted to identify ligands
that would promote Ar−CF3 reductive elimination from Ni(II).
Toward this goal, we have employed a rather nontraditional
methodology by first performing a broad screening of ligands
by computational means, followed by experimental work. This
combined study has produced a number of valuable insights
pertaining to organometallic fluorine chemistry in general and
Ni-catalyzed trifluoromethylation of aryl halides in particular.
COMPUTATIONAL STUDIES
■
Knowledge-Guided Computational Search of Efficient
Ligands. The experimental work by Vicic and co-workers21 has
proven that complexes with an aryl and CF3 on Ni(II) are
accessible, at least with dippe as the ancillary ligand. We have
probed the experimentally unsuccessful Ar−CF3 reductive
elimination from [(dippe)Ni(Ph)(CF3)]21a by DFT calcula-
tions (Scheme 2) to compute a prohibitively high free energy
barrier of 36.3 kcal/mol.
with five P-substituents: methyl (Me), tert-butyl (t-Bu), phenyl
(Ph), trifluoromethyl (CF3), and pentafluorophenyl (C6F5),
aiming to cover the maximum ligand space. Note that the
abbreviations presented in Scheme 3 refer to the backbones,
not particular ligands. Selected similar ligands containing i-Pr
on P were also included in the final set due to dippe being the
supporting ligand for the only synthesized and fully
characterized CF3Ni aryls.21 Although diphosphines bearing i-
Pr on the P atoms were not originally in LKB-PPscreen, they
could be easily imported from the data available in the LKB-PP
database. A complete description of the ligand selection
procedure and database expansion is detailed in the Supporting
Information.
Scheme 2. Computed Reductive Elimination Barriers for
(PP)Ni(Ph)CF3 Complexes
The selected diphosphines are highlighted on the LKB-
PPscreen map presented in Figure 1, showing the final set
spreading across most areas in the ligand space. The map was
generated by PCA24 and contains useful information regarding
ligand similarities (represented as proximity between the
ligands on the map) and trends observed when systematic
changes in the ligand structure are applied. The two first
principal components contain a combination of electronic (M−
L and σ-bonding descriptors) and steric effects (bite angles and
the He8wedge and nHe8 parameters), which place the ligands on
the southwest−northeast diagonal of the map. The He8wedge
and nHe8 descriptors are steric measures that capture the
interaction energy between a given ligand and a “wedge” of
eight helium atoms placed in approximate ligand positions of an
octahedral complex. The electron-rich ligands take up the top
right side of the map (t-Bu, filled squares), while the electron-
poor diphosphines are located in the southwest sector (CF3,
triangles). Likewise, the smaller ligands are in the southeast area
(dpm, blue markers), whereas the bulkier ones can be found in
the northwest zone (Xantphos and dpf, black and red markers).
Modifications in bidentate phosphine ligands (PP) might
yield lower barriers for reductive elimination from their
complexes. It is widely known that both steric and electronic
parameters determine key ligand properties including their
ability to promote reductive elimination reactions.22 In general,
electron-deficient diphosphines with longer backbones and
bulkier substituents tend to lower reductive elimination
barriers. To identify bidentate phosphines that can facilitate
the desired Ar−CF3 bond formation at Ni(II), we employed
one of the University of Bristol Ligand Knowledge Bases23 that
provide a fast and rational way to select ligands, ensuring the
right modeling for a given process. In the current work, we have
used the LKB-PPscreen database,23d which has been constructed
to account for systematic variations on the backbone and P-
substituents of cis-chelating diphosphine ligands. Altogether, 25
different backbones and 11 substituents covering most of the
steric and electronic features of the ligands have been included
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Figure 1. Final ligand selection projected on the LKB-PPscreen map.
The dippe ligand used for the synthesis of [(dippe)Ni(Ph)-
(CF3)]21a lies on the middle right side of the map (orange
empty square), close to the small methyl-based electron-rich
ligands such as 1,3-bis(dimethylphosphino)propane (dmpp)
and 1,4-bis(dimethylphosphino)butane (dmpb). All of these
ligands are expected to produce high Ph−CF3 reductive
elimination barriers that should lower when moving to the
left side of the map.
Figure 2. Projected Ph−CF3 reductive elimination barriers (in kcal/
mol) on the LKB-PPscreen map with the lighter tones corresponding to
lower barriers.
and, more importantly, would render the Ni(0) center
incapable of activating the haloarene substrate via Ar−X
oxidative addition.25 Another option is electron-rich bulky
diphosphines that generally lead to intermediate barriers, e.g.,
21.7 and 24.1 kcal/mol computed for the i-Pr- and t-Bu-
substituted dpb, dpx, and dpf species. Some of such complexes
are expected to be reasonably accessible by synthesis. Although
Xantphos-based ligands could provide one of the best platforms
for Ar−CF3 reductive elimination from Pd(II),14 this is not the
case with nickel because the aryl and the CF3 in such complexes
are computed to be mutually trans, thereby impeding the
reductive elimination. For the complexes with the rest of the
ligands, both the cis and trans isomers were computed to
ensure the lowest energy conformer was used to compute the
reductive elimination. The cis complexes were found to be
more stable in all cases. Although the barrier for the t-
BuXantphos complex was also studied, we were unable to reach
convergence for the reductive elimination transition state.
On the basis of the computational studies, two non-
fluorinated ligands were identified as the most promising
candidates: 1,4-bis(di-tert-butylphosphino)butane (dtbpb, ΔG⧧
= 21.7 kcal/mol) and 1,1′-bis(diisopropylphosphino)ferrocene
(dippf, ΔG⧧ = 22.6 kcal/mol). These candidates were proposed
for the experimental work reported in the following section.
The synthetic studies, however, were not limited to these two
specific ligands. Other similar yet more synthetically accessible
diphosphines were also explored, even though they were
originally absent from the computational database used.
The computed ΔG⧧ values for the Ph−CF3 reductive
elimination from various complexes of the type [(PP)Ni(Ph)-
(CF3)] are presented in Table 1 and projected on the LKB-
Table 1. Reductive Elimination Barriers (in kcal/mol)
backbone
R
dpm
dpe
Dpp
dpb
dpx
dpf
Xantphos
Me
36.3
38.3
36.9
33.8
22.7
27.0
38.4
36.3
29.3
34.5
20.5
27.0
33.2
31.1
26.5
28.3
14.6
23.6
32.1
26.9
21.7
30.3
12.7
23.8
29.1
24.1
23.6
28.8
12.3
25.4
29.8
22.6
23.1
27.0
12.4
17.5
31.1
32.0
i-Pr
t-Bu
Ph
a
−
28.9
19.0
17.4
CF3
C6F5
a
The transition state for this species could not be correctly located.
PPscreen map in Figure 2. As can be seen from Figure 2, the
barrier changes in the range >35 to <15 kcal/mol when moving
through the ligand space, with the lighter colors on the map
corresponding to the lower free energy of activation. The
prohibitively high barrier of 36.3 kcal/mol computed for
[(dippe)Ni(Ph)(CF3)] (see above) is similar to those for the
dpe spacer and all of the Me−P derivatives. Moving to the
bottom left of the map, i.e., decreasing the electron-donating
ability of the ligand, lowers the barriers. Thus, for a given
backbone, the barriers computed for the CF3- and C6F5-
substituted phosphines are much lower than for their electron-
enriched analogues bearing Me, i-Pr, and t-Bu groups on the P
atoms. Therefore, the theory predicts that the most promising
ligands for the desired C−CF3 bond formation at Ni(II) should
be the CF3 derivatives of dpp, dpb, dpx, and dpf with the
barriers below 15 kcal/mol. The C6F5-substituted larger
backbone ligands (dpf and Xantphos) are predicted to be a
viable alternative with ΔG⧧ ≈ 18 kcal/mol. Such electron-poor
ligands, however, are not easily accessible by synthetic means
EXPERIMENTAL STUDIES
■
Of the computationally studied complexes of the type
[(PP)Ni(Ph)(CF3)], the most promising candidates were
considered for synthetic preparation in order to probe
experimentally the desired Ar−CF3 bond formation at the
Ni(II) center. The lowest energy transition state was computed
for the reductive elimination from [(dtbpb)Ni(Ph)(CF3)]
(21.7 kcal/mol; see above). Nonetheless, this complex was not
selected for the synthesis because of the low stability of seven-
C
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membered chelate rings and, as a consequence, likely formation
of polynuclear oligomeric species via coordination of the
bidentate dtbpb ligand to two Ni centers. Our synthetic effort
was therefore focused on [(dippf)Ni(Ph)(CF3)], the second
top candidate on the list. Given very close electronic and steric
parameters of the i-Pr and Cy (cyclohexyl) groups, we
considered including in the study 1, 1 ′-bis-
(dicyclohexylphosphino)ferrocene (dcypf), a commercially
available yet less costly close analogue of dippf. Therefore, we
computed the barrier to Ph−CF3 reductive elimination from
[(dcypf)Ni(Ph)(CF3)], even though dcypf was not in the initial
set of ligands for the DFT studies (Figures 1 and 2). The
calculated ΔG⧧ = 23.2 kcal/mol for the dcypf complex was very
close to that for the originally selected candidate, [(dippf)Ni-
(Ph)(CF3)] (22.6 kcal/mol).
in the NMR spectra of the product. As the paramagnetic
impurities could affect the reactivity of [(Ph3P)2Ni(1-Np)Cl] in
the next steps of the synthesis, we developed an alternative
isolation/purification procedure for this complex. After the
reaction, the solids were not washed with HCl but rather
extracted with hot 1,2-dichloroethane.29 Filtration of the extract
through Celite, followed by evaporation and addition of n-
hexane, afforded [(Ph3P)2Ni(1-Np)Cl] that was free from the
paramagnetism. A scaleup of this method furnished 6 g (76%
yield) of advantageously pure [(Ph3P)2Ni(1-Np)Cl] (Figure 3)
as orange crystals.
In order to prepare [(PP)Ni(Ph)(CF3)], where PP = dippf,
dcypf (as well as some other ligands; see below), we adopted
the synthetic strategy (Scheme 4) originally developed for
trifluoromethyl Pd(II) aryls12−14 and subsequently extended to
the preparation of [(dippe)Ni(Ar)(CF3)].21
Scheme 4. Synthetic Strategy for the Synthesis of Complexes
of the Type [(PP)Ni(Ar)(CF3)]
1
Figure 3. H NMR spectra of [(Ph3P)2Ni(1-Np)Cl] purified by the
literature8a (bottom) and our (top) procedures.
The PPh3 ligands in [(Ph3P)2Ni(1-Np)Cl] were easily
displaced with more electron-donating dippf and dcypf (eq
2) as well as with i-PrXantphos and PCy3.27 Otherwise poorly
THF-soluble [(Ph3P)2Ni(1-Np)Cl] easily dissolved upon
stirring in THF containing dcypf (1.2 equiv) for 30 min. The
31P NMR spectrum of the resultant deep red solution indicated
full conversion of the starting complex (δ = 21.3 ppm) and the
formation of a new species resonating at −5.4 ppm. Singlet
resonances from the displaced PPh3 (−5.5 ppm) and unreacted
dcypf (−9.3 ppm) were also observed. From this solution,
[(dcypf)Ni(1-Np)Cl] was isolated in 69% yield and fully
characterized, including by single-crystal X-ray diffraction. To
our surprise, [(dcypf)Ni(1-Np)Cl] was found to be exclusively
trans in both solution and the solid state (Figure 4). This
complex is the first trans square-planar group 10 metal chelate
derived from the 1,1′-ferrocenediyl backbone.30,31 The singlet
resonance observed in the 31P NMR spectrum of isolated
[(dcypf)Ni(1-Np)Cl] in the temperature range of +25 to −90
°C indicated that the complex retains the trans geometry in
solution.32,33 The dippf derivative, [(dippf)Ni(1-Np)Cl], was
prepared similarly (91% yield) and also found to be exclusively
trans in solution (31P NMR: s, 1.1 ppm) and in the crystal
(Figure 4). Alternatively, [(dcypf)Ni(1-Np)Cl] could be
Synthesis and Characterization of [(dcypf)Ni(1-Np)Cl]
and [(dippf)Ni(1-Np)Cl] (1-Np = 1-naphthyl). The key
chloro Ni(II) aryl intermediates could be prepared via ligand
exchange reactions of [(Ph3P)2Ni(Ar)Cl] or oxidative addition
of ArCl to [Ni(COD)2] in the presence of the desired
phosphine ligand. The σ-aryl ligand of choice was 1-naphthyl
(1-Np) because its Ni(II) derivatives are more robust and
convenient to work with than the phenyl analogues.26 Cassar’s
catalyst, [(Ph3P)2Ni(1-Np)Cl],27 is a particularly easily
accessible and versatile, air-stable complex.26−28 The replace-
ment of the Ph ligand in [(PP)Ni(Ph)(CF3)] with 1-Np was
justified by additional calculations showing that the barriers to
the Ar−CF3 reductive elimination for Ar = 1-Np and Ph are
similar, 23.2 vs 22.6 and 27.7 vs 23.2 kcal/mol for PP = dippf
and dcypf, respectively. The slightly higher barriers computed
for the 1-Np complexes are apparently due to the extra energy
cost of the bulkier naphthyl group adopting the proper
conformation in the transition state. Originally, [(Ph3P)2Ni(1-
Np)Cl] was prepared by Cassar27 from [(Ph3P)2NiCl2] and 1-
chloronaphthalene in the presence of a manganese−iron alloy
as a reducing agent. The product was then purified by
recrystallization from hot benzene. Later Brandsma et al.28a
modified this procedure by replacing the Mn alloy with Zn dust
in excess and using NiCl2·6H2O and PPh3 in place of premade
[(Ph3P)2NiCl2]. The excess of Zn dust was removed by
washing with hydrochloric acid and the resulting complex was
used without further purification. We noticed, however, that
this procedure that has been widely used by others28b−d
furnished [(Ph3P)2Ni(1-Np)Cl] contaminated with paramag-
netic species, likely Ni(I), as was manifested by broadened lines
synthesized via oxidative addition of 1-chloronaphthalene to
Ni(0) in the presence of dcypf. Adding [Ni(COD)2] to a
solution of dcypf in toluene-d8 resulted in almost complete
instantaneous disappearance of the 31P NMR signal from the
D
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We then attempted the synthesis of [(dcypf)Ni(1-Np)F] for
use in the preparation of the targeted complex [(dcypf)Ni(1-
Np)(CF3)] upon treatment with CF3SiMe3. No reaction
occurred between [(dcypf)Ni(1-Np)Cl] and AgF in benzene
under sonication12−14,35 or CsF in THF. With anhydrous
[Me4N]+F− in THF, however, [(dcypf)Ni(1-Np)Cl] did react
to give [(dcypf)Ni(1-Np)F], which was identified by its 19F and
31P NMR parameters (see above). After 10 min at room
temperature, only ca. 10% of [(dcypf)Ni(1-Np)F] was
produced at 20% conversion of the starting chloride. After 22
h, the desired product had decomposed completely at full
conversion of [(dcypf)Ni(1-Np)Cl], and a complex reaction
mixture was produced that contained large quantities of
naphthalene and binaphthyl but no fluoronaphthalene. In a
similar experiment but in the presence of Ph3PO as an internal
standard, the yield of [(dcypf)Ni(1-Np)F] was ca. 5% at 75%
conversion after 18.5 h. The treatment of the reaction mixture
with CF3SiMe3 resulted in instantaneous disappearance of
[(dcypf)Ni(1-Np)F], but the 19F and 31P NMR spectra did not
display any signals that could be assigned to [(dcypf)Ni(1-
Np)(CF3)]. Again, GC-MS analysis of the reaction solution
indicated the formation of binaphthyl and naphthalene and no
fluoronaphthalene, trifluoromethylnaphthalene, and pentafluor-
oethylnaphthalene. A similar behavior was observed for
[(dippf)Ni(1-Np)Cl] that underwent ca. 80% conversion
after 2.5 h of reaction with [Me4N]+F− in THF to give
[(dippf)Ni(1-Np)F] in ca. 20% yield. On addition of CF3SiMe3
to the resultant solution, this fluoride complex reacted at 70%
and full conversion after 1 and 14 h, respectively. 19F NMR
analysis of the reaction mixture indicated the presence of
unreacted CF3SiMe3, CHF3, FSiMe3, and a small quantity of a
Ni−CF3 species that we carefully assign to [(dippf)Ni(CF3)2]
(−20.0 ppm, t, JF−P = 29 Hz) by analogy with the data
reported21a for [(dippe)Ni(CF3)2] (−20.0 ppm, dd, JF−P = 23.8
and 30.8 Hz). The low yield and concentration of the species
that we tentatively formulate as [(dippf)Ni(CF3)2] prevented
observation of its 31P NMR signal. GC-MS analysis of the
reaction mixture showed the presence of naphthalene and
binaphthyl but not trifluoromethylnaphthalene.
Figure 4. ORTEP drawings of trans-[(dcypf)Ni(1-Np)Cl]·2THF
(left) and trans-[(dippf)Ni(1-Np)Cl]·C6H6 (right) with the THF and
C6H6 molecules and all H atoms omitted for clarity and thermal
ellipsoids drawn at the 50% probability level.
free ligand at −9.3 ppm and appearance of a new resonance at
23.1 ppm, which was assigned to [(dcypf)Ni(COD)].34 The
treatment of the latter (generated in situ) with 1-chloronaph-
thalene at 50 °C for 15 h produced [(dcypf)Ni(1-Np)Cl] in
98% yield (31P NMR).
Reactions of [(dcypf)Ni(1-Np)Cl] and [(dippf)Ni(1-
Np)Cl] with CF3 Sources. Having prepared and characterized
[(PP)Ni(1-Np)Cl] (PP = dcypf, dippf), we attempted their
transformation to the desired trifluoromethyl complexes
[(dippf)Ni(1-Np)(CF3)] and [(dcypf)Ni(1-Np)(CF3)]. The
treatment of the chloro complexes with CF3SiMe3 in the
presence of CsF in THF12−14 led to a complex reaction mixture
not containing the targeted product in 19F NMR-detectable
quantities. After 21 h of reaction of [(dcypf)Ni(1-Np)Cl] with
CF3SiMe3 (3 equiv) and CsF (3 equiv) in THF at room
temperature, ca. 70% conversion of the nickel complex was
observed (31P NMR). This reaction gave rise to a species (ca.
25% yield) displaying a doublet (JP−F = 39 Hz) at −1.1 ppm
and a broadened singlet at −365.5 ppm in the 31P and 19F
NMR spectra, respectively. The 19F NMR singlet, however,
resolved into a triplet with the same coupling constant of 39 Hz
after stirring the mixture with freshly added CsF and CF3SiMe3
for 20 h at room temperature. We therefore assigned those
signals to [(dcypf)Ni(1-Np)F]. In the 31P NMR spectrum, an
unassigned broadened resonance at ca. 23.8 ppm (ca. 20%) and
a singlet at −9.3 ppm (free dcypf; ca. 20%) were also observed.
In addition to the aforementioned triplet at −365.5 ppm, the
19F NMR spectrum displayed resonances at −416.4 and −419.8
ppm (apparently from other Ni−F species), −157.6 ppm
(FSiMe3), −78.8 ppm (CHF3), and numerous unidentified
low-intensity peaks in the −49 to −84 ppm region.
Importantly, small quantities (ca. 2%) of C2F5H (−139.4
ppm, d, JF−H = 52 Hz, 2F; −86.3 ppm, s, 3F) were also
detected. The reaction of [(dippf)Ni(1-Np)Cl] with CF3SiMe3
in the presence of CsF in THF occurred similarly. After 17 h,
the formation of ca. 10−15% of [(dippf)Ni(1-Np)F] (31P
NMR δ: 5.7 ppm, d; 19F NMR δ: −362.0 ppm, t, JP−F = 39 Hz)
was observed at 75% conversion. The formation of Ni−F
complexes and C2F5H suggested that NiCF3 species might have
been produced as unstable intermediates that underwent α-F-
elimination to give difluorocarbene species. GC-MS analysis of
the reaction mixture revealed the formation of binaphthyl and
naphthalene. Fluoronaphthalene, trifluoromethylnaphthalene,
and pentafluoroethylnaphthalene were not detected.
Next we attempted the synthesis of [(dcypf)Ni(1-Np)(CF3)]
36
from [(dcypf)Ni(1-Np)Cl] and AgCF3 or [(Ph3P)3Cu-
(CF3)].7d The reaction between [(dcypf)Ni(1-Np)Cl] and
[(Ph3P)3Cu(CF3)] in benzene reached >95% conversion after
1 h to give [(dcypf)Cu(CF3)] (ca. 50% yield) and a nickel
fluoride (ca. 30% yield), which we formulate as [(Ph3P)2Ni(1-
Np)F] on the basis of the NMR data (19F: −336.9 ppm, t, JP−F
= 45 Hz; 31P: 14.7 ppm, d, JP−F = 45 Hz). This reaction,
however, did not involve CF3 transfer from Cu to Ni but rather
resulted in phosphine ligand exchange leading to [(dcypf)Cu-
(CF3)]. The latter was also prepared independently from
[(Ph3P)3Cu(CF3)] and dcypf (eq 3), isolated, and fully
characterized. The 19F and 31P NMR spectra of [(dcypf)Cu-
(CF3)] displayed a triplet (−25.7 ppm, JP−F = 17 Hz) and a
quartet (−7.7 ppm, JP−F = 17 Hz), respectively, in full accord
with the solid-state structure obtained by single-crystal X-ray
diffraction (Figure 5). No reaction occurred between [(dcypf)-
Ni(1-Np)Cl] and [(dcypf)Cu(CF3)] in C6D6 at room
temperature. At 50 °C, only a slow reaction was observed,
giving rise to CHF3, [(dcypf)Ni(1-Np)F] (see above), and
minute quantities of unidentified species, likely perfluoroalkyl
Ni and Cu derivatives (19F NMR). Trifluoromethylnaphthalene
was not produced.
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or even with CsF (50 °C) in THF (eq 4). This fluoride
complex is exclusively trans in solution (31P NMR δ: 17.6, d,
J
P−F = 45 Hz; 19F NMR δ: −368.7, t, JP−F = 45 Hz) and in the
solid state (Figure 6). As expected,12 a number of Ni−F···H−C
Figure 5. ORTEP drawing of [(dcypf)Cu(CF3)] with all H atoms
omitted and thermal ellipsoids drawn at the 50% probability level.
Stirring [(dcypf)Ni(1-Np)Cl] with AgCF3 in MeCN for 10
min produced a yellow precipitate that was poorly soluble in
CH2Cl2, MeCN, benzene, and DMF. The 31P NMR spectrum
of the supernatant displayed a number of weak broad signals.
Two resonances at −23.4 and −26.6 ppm that could be
assigned to Ni−CF3 species were observed in the 19F NMR
spectrum. As the peak at −23.4 ppm disappeared, the one at
−26.6 ppm grew in intensity. Importantly, both Ni−CF3
resonances were singlets, suggesting that the dcypf ligand and
Cl originally on Ni had been transferred to Ag to give
[(dcypf)AgCl] (which precipitated out). The liquid phase
would then contain CF3 nickel species that we tentatively
formulate as [(MeCN)2Ni(1-Np)(CF3)] and [(MeCN)2Ni-
(CF3)2(1-Np)]−. Heating an aliquot of this solution under
argon at 60 °C for 24 h did not produce 1-NpCF3.
Unexpectedly, however, exposure of another aliquot of this
solution to air produced 1-NpCOF (ca. 4%) and 1-NpCF3 (ca.
2%) after 3 h. In order to find out if the formation of these
products was caused by oxygen or moisture, another portion of
the solution was treated with deaerated water and another one
with dry air. While neither 1-NpCF3 nor 1-NpCOF was
produced upon addition of O2-free water,37 dry air prompted
the formation of both. Apparently, oxidatively induced
reductive elimination from Ni38 led to the formation of 1-
NpCF3. The other product, 1-NpCOF, was likely formed via α-
F-elimination that competed with the C−CF3 reductive
elimination from phosphine-free Ni(III). After the facile
hydrolysis of the resultant difluorocarbene on Ni to CO with
adventitious water and subsequent migratory insertion, 1-
NpCO-F reductive elimination would produce the acid fluoride
final product. This reaction pathway to ArCOF has been
established for phosphine-stabilized CF3Pd aryls.12,35a,c
Figure 6. ORTEP drawing of trans-[(Cy3P)2Ni(1-Np)F] showing
intermolecular F···H−C contacts, with thermal ellipsoids drawn at the
50% probability level.
contacts shorter than the sum of the van der Waals radii of H
and F (2.67 Å) were found in the crystal structure of
[(Cy3P)2Ni(1-Np)F]. These contacts include two intermolec-
ular interactions of the F ligand with the C−H bonds of the σ-
naphthyl in the 4 (2.38 Å) and 5 (2.43 Å) positions and four
F···H−C contacts involving the cyclohexyl rings on the P
centers (2.56, 2.59, 2.61, and 2.63 Å).
Unfortunately, yet unsurprisingly,12 the thermal decom-
position of [(Cy3P)2Ni(1-Np)F] did not result in C−F bond
formation. The lack of reactivity of this Ni(II) fluoride toward
CF3SiMe3, however, was totally unexpected, considering our
experience in the successful preparation of Pd−CF3,12−14 Cu−
39
CF3,7d and Rh−CF3 complexes from the corresponding
fluorides. No reaction was observed between [(Cy3P)2Ni(1-
Np)F] and CF3SiMe3 (5 equiv) in THF at room temperature.
At 60 °C, CHF3 and FSiMe3 were produced in a sluggish
reaction as the only 19F NMR-observable species. Attempts to
enhance the reactivity of the Ni−F bond toward CF3SiMe3 with
such additives as KHF2 (30 mol %), [Me4N]+F− (30 mol %), or
t-BuOH (2 equiv) were unsuccessful.
In contrast, [(i-PrXantphos)Ni(1-Np)F] smoothly reacted
with CF3SiMe3 to give [(i-PrXantphos)Ni(1-Np)(CF3)]
quantitatively. First, the fluoride was prepared from the
reaction of [(Ph3P)2Ni(1-Np)Cl] with i-PrXantphos and
[Me4N]+F− in THF. This fluoro complex was isolated in 66%
yield and found to be trans in the solid state and in solution
(31P NMR δ: 6.7 ppm, d, JP−F = 41 Hz; 19F NMR δ: −372.5
ppm, br s). The two structurally characterized solvates of [(i-
PrXantphos)Ni(1-Np)(F)], one with a water molecule and one
with a molecule of CH2Cl2 (Figure 7), display, in addition to
conventionally observed12,40 F···HC contacts, stronger H-
bonding interactions with the molecules of CH2Cl2 and H2O.
Being a much stronger hydrogen bond donor, water forms a
considerably shorter contact to the fluorine (F···HOH = 1.74
Å) than dichloromethane (F···HCHCl2 = 2.05 Å). As a result,
the Ni−F bond in the H2O adduct (1.923(4) Å) is longer than
in the dichloromethane solvate (1.868(2) Å), which naturally
CF3 and F Ni Complexes with Cy3P and i-PrXantphos
Ancillary Ligands. As mentioned above, little is known about
tertiary phosphine-stabilized nickel aryls bearing a CF3 ligand,21
and only one such complex, [(dippe)Ni(2-Np)(CF3)], has
been structurally characterized.21a The poor stability and
apparent propensity of [(dcypf)Ni(1-Np)(CF3)] and [(dippf)-
Ni(1-Np)(CF3)] to undergo α-F-elimination (see above)
prompted us to attempt the synthesis of CF3Ni aryls with
other phosphine ligands. First, we found that [(Cy3P)2Ni(1-
Np)Cl], preisolated or generated in situ from [(Ph3P)2Ni(1-
Np)Cl] and Cy3P,27 could be converted to [(Cy3P)2Ni(1-
Np)F] upon treatment with [Me4N]+F− (room temperature)
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Figure 7. ORTEP drawing of trans-[(i-PrXantphos)Ni(1-Np)F]·H2O
and trans-[(i-PrXantphos)Ni(1-Np)F]·CH2Cl2 with thermal ellipsoids
drawn at the 50% probability level.
Figure 8. ORTEP drawing of trans-[(i-PrXantphos)Ni(1-Np)(CF3)]
with the solvent molecules and all H atoms omitted and thermal
ellipsoids drawn at the 50% probability level.
barriers of 47.7 and 32.0 kcal/mol to Ar−CF3 reductive
elimination from the Ni center of [(i-PrXantphos)Ni(Ar)-
(CF3)] for Ar = 1-Np and Ph, respectively. It is noteworthy that
a closely related palladium complex, [(i-PrXantphos)Pd(Ph)-
(CF3)], does produce PhCF3 on heating at 100 °C, albeit
slowly and with modest selectivity.14b
translates into the shorter Ni−C bond distance trans to F···
HOH (1.848(6) Å) than to F···HCHCl2 (1.896(3) Å).
The fluoride [(i-PrXantphos)Ni(1-Np)F] appeared to be
remarkably thermally stable, displaying no signs of decom-
position at 110 °C in xylenes. Only at 140 °C did sluggish
decomposition begin to reach full conversion after 3 days. No
C−F bond formation took place, and the only signal observed
in the 19F NMR spectrum of the resultant solution was a
doublet at −94.8 ppm with a large coupling constant of 1036
Hz, pointing to the characteristic12 P−F bond formation. Other
products of the thermal decomposition of [(i-PrXantphos)Ni-
(1-Np)F] included naphthalene, binaphthyl, and 9,9-dimethyl-
xanthene (GC-MS).
DISCUSSION
■
Using the LKB-PPscreen database,23d we have studied by
computational means 42 bidentate phosphines as ancillary
ligands (PP) for Ph−CF3 reductive elimination from complexes
of the type [(PP)Ni(Ph)(CF3)]. Remarkably low barriers of ca.
15−18 kcal/mol have been computed for this process for
certain diphosphines PP bearing CF3 or C6F5 groups on the
phosphorus atoms. Experimental work toward the synthesis of
such electron-deficient phosphines and the corresponding
nickel complexes was not pursued, however, because Ni(0)
bearing such ligands is unlikely to undergo Ar−X oxidative
addition, the first key step in Ni-catalyzed coupling reactions of
aryl halides.25 Apart from the fluorinated phosphines, the most
promising ligand, dtbpb (21.7 kcal/mol), was disregarded due
to the propensity of long-backbone diphosphines to form
oligomeric metal complexes. We finally settled on the next two
candidates, [(dippf)Ni(Ph)(CF3)] and [(dcypf)Ni(Ph)(CF3)],
with the computationally predicted Ph−CF3 reductive elimi-
nation barriers of 22.6 and 23.2 kcal/mol, respectively. The
experimental work was done, however, with σ-1-naphthyl rather
than σ-phenyl complexes because of the easy accessibility and
enhanced stability of the starting material, Cassar’s catalyst
[(Ph3P)2Ni(1-Np)Cl]. The use of the naphthyl derivatives was
justified by the similar ΔG⧧ values computed for Ar = 1-Np and
Ph, 23.2 vs 22.6 and 27.7 vs 23.2 kcal/mol for dippf and dcypf,
respectively.
Unlike [(Cy3P)2Ni(1-Np)F], which appeared unreactive
toward Ruppert’s reagent (see above), [(i-PrXantphos)Ni(1-
Np)F] readily reacted with CF3SiMe3 to give [(i-PrXantphos)-
Ni(1-Np)(CF3)]. The latter was also more conveniently
prepared in 53% isolated yield via a one-pot method from
[(Ph3P)2Ni(1-Np)Cl], i-PrXantphos, CsF, and CF3SiMe3 in
THF (Scheme 5). Like its Pd analogue,14b [(i-PrXantphos)Ni-
(1-Np)(CF3)] was found to be trans in the solid state (Figure
8) and in solution (31P NMR δ: 18.3 ppm, q, JP−F = 14 Hz; 19
NMR δ: −14.0 ppm, t, JP−F = 14 Hz).
F
The decomposition of [(i-PrXantphos)Ni(1-Np)(CF3)] in
xylenes at 120 °C was slow (16 h) and poorly selective, not
giving rise to 1-NpCF3. All signals in the 19F NMR spectrum of
the reaction mixture were broadened, probably because of the
formation of paramagnetic Ni species. The presence of the
previously observed doublet at −95 ppm with JP−F = ca. 1050
Hz (see above) and a Ni−F resonance at −390 ppm pointed to
α-F-elimination and subsequent P−F bond formation. Addi-
tional support for this reaction pathway was provided by the
detection (19F NMR and GC-MS) of small quantities (ca. 5%)
of difluoromethylnaphthalene that must have been produced
from a difluorocarbene intermediate. The lack of C−CF3 bond
formation in this reaction is consistent with the high computed
The synthesis of [(PP)Ni(1-Np)(CF3)] (PP = dcypf, dippf),
however, appeared to be a much greater challenge than
originally expected. Although both [(dcypf)Ni(1-Np)Cl] and
Scheme 5. Synthesis of [(i-PrXantphos)Ni(1-Np)(CF3)]
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[(dippf)Ni(1-Np)Cl] were easily prepared from [(Ph3P)2Ni(1-
Np)Cl] and the corresponding diphosphine, numerous
attempts to trifluoromethylate the Ni−Cl bond with
CF3SiMe3/CsF, AgCF3/MeCN, or [(Ph3P)3Cu(CF3)] were
unsuccessful. With the Ag and Cu reagents, facile phosphine
ligand transfer to the coinage metal occurred. The resultant
copper complex, [(dcypf)Cu(CF3)] (Figure 5), a rare example
of adequately characterized well-defined CF3Cu(I) spe-
cies,7c−e,41 was isolated but failed to trifluoromethylate
[(dcypf)Ni(1-Np)Cl].
Nonselective reactions occurred upon treatment of [(dcypf)-
Ni(1-Np)Cl] or [(dippf)Ni(1-Np)Cl] with CF3SiMe3/CsF to
give naphthalene, binaphthyl, and Ni−F complexes that likely
emerged via α-F-elimination from unstable NiCF3 intermedi-
ates. The poorly stable fluorides [(PP)Ni(1-Np)F] (PP = dippf,
dcypf) generated in situ from [Me4N]+F− and the correspond-
ing Ni chloro complex readily reacted with CF3SiMe3.
However, no formation of the desired complexes [(PP)Ni(1-
Np)(CF3)] was observed (19F NMR); naphthalene and
binaphthyl but not trifluoromethylnaphthalene were again
detected in the reaction solutions. This outcome parallels the
recently reported20l formation of biphenyl but not benzotri-
fluoride in the reactions of [(Ph3P)2Ni(O2CCF3)(CF3)] with
PhMgBr, PhLi, or Ph2Zn in an attempt to prepare [(Ph3P)2Ni-
(Ph)(CF3)]. The lack of C−CF3 formation from [(PP)Ni(1-
Np)(CF3)] (PP = dippe, dcype) is likely due to the existence of
less energetically demanding pathways. In addition to the
aforementioned α-F-elimination, these processes likely involve
transmetalation leading to [(PP)Ni(CF3)2] (detected in some
cases) and [(PP)Ni(1-Np)2] that reductively eliminates
binaphthyl. Single electron transfer prompting radical processes
to give naphthalene are also conceivable. A detailed study of
such side transformations, however, was beyond the scope of
the current project.
To gain deeper insight into trifluoromethyl Ni(II) aryls, we
have explored the possibility of preparing [(PP)Ni(1-Np)-
(CF3)], where PP = (Cy3P)2 or i-PrXantphos. Although these
studies have not yielded a CF3−Ar bond forming system,
interesting results have been obtained that certainly merit
discussion. Both 1-naphthyl Ni(II) fluorides prepared and
isolated in the current work, trans-[(Cy3P)2Ni(1-Np)F] and
trans-[(i-PrXantphos)Ni(1-Np)F], represent the first examples
of fully characterized fluoro nickel aryls lacking electron-
withdrawing substituents on the aromatic ring. Numerous
complexes of the type trans-[(R3P)2Ni(Ar)F] reported in the
literature have been synthesized via Ar−F oxidative addition of
polyfluorinated (hetero)arenes to R3P-stabilized Ni(0),42
including for R = Cy.43 The enhanced electron-deficiency/π-
acidity of the σ-(hetero)aryl trans to the fluoro ligand in the
resultant d8 square-planar Ni(II) complex is expected12,35c,44 to
contribute considerably to the stabilization of the Ni−F bond.
Since this stabilizing factor is strongly diminished for Ni(II)
complexes with F trans to a conventional σ-aryl such as Ph or
Np, it was not clear at all if trans-[(Cy3P)2Ni(1-Np)F] and
trans-[(i-PrXantphos)Ni(1-Np)F] could exist. As we have
found in the current work, not only can they be generated
and detected, but are also isolable in good yield for full
characterization. Another remarkable finding is the formation of
[(Cy3P)2Ni(1-Np)F] from the corresponding chloride and CsF
in THF, as in the vast majority of cases AgF must be used in
order to exchange a heavier halide on a late transition metal
center for fluoride. Particularly striking is the difference in the
reactivity of trans-[(Cy3P)2Ni(1-Np)F] and trans-[(i-
PrXantphos)Ni(1-Np)F] toward CF3SiMe3. Numerous repor-
ted7d,12,14,39,45 examples of efficient trifluoromethylation of a
transition metal−fluorine bond with CF3SiMe3 suggested that
both Ni fluorides should undergo this transformation as easily.
Surprisingly, however, trans-[(Cy3P)2Ni(1-Np)F] appeared to
be unreactive toward CF3SiMe3 even at elevated temperatures
and even in the presence of such promoters as KHF2,
[Me4N]+F−, and t-BuOH. While the mechanism of F/CF3
exchange between transition metal fluorides and CF3SiMe3
remains unknown, coordination of the fluoride ligand on the
metal to the Si atom is likely the initiation step of the CF3
transfer. We believe that the reaction of CF3SiMe3 with trans-
[(Cy3P)2Ni(1-Np)F] bearing two bulky phosphines might be
precluded by steric constraints. In contrast, less crowded trans-
[(i-PrXantphos)Ni(1-Np)F] readily reacted with CF3SiMe3 to
give trans-[(i-PrXantphos)Ni(1-Np)(CF3)], which, in full
accord with the theoretical predictions, is reluctant to undergo
C−CF3 reductive elimination.
CONCLUSIONS
■
All data accrued in the current study prompt us to look at the
development of Ni-catalyzed nucleophilic trifluoromethylation
of aryl halides as a big challenge. The DFT computational
screening of a broad variety of bidentate phosphines as auxiliary
ligands for Ar−CF3 reductive elimination from Ni(II) has
identified a number of viable candidates. Two of them, dippf
and dcypf, were selected for experimental work on the basis of
the computed barriers and synthetic accessibility consider-
ations. However, the seemingly straightforward synthesis of
[(PP)Ni(Ar)(CF3)] (PP = dippf, dcypf; Ar = 1-Np) has failed
after numerous attempts using three different nucleophilic CF3-
transferring reagents under a variety of conditions. In a way, the
outcome of our studies echoes the famous Jagger−Richards line
“You can’t always get what you want, but if you try sometime
you just might find you get what you need”.46 We note that this
is frequently the case with innovative science explorations in
general and organometallic fluorine research in particular.12
Although Ar−CF3 reductive elimination from Ni(II) still
remains unachieved, our work on the current project has
yielded the following key learnings:
1. While in theory some CF3Ni(II) aryls might have low
barriers to the desired Ar−CF3 reductive elimination
(e.g., [(dippf)Ni(Ph)(CF3)] and [(dcypf)Ni(Ph)-
(CF3)]), other lower activation energy pathways exist
for the same systems, as evidenced by experimental facts.
2. Nucleophilic trifluoromethylation of the Ni−X bond, the
second elementary step in the proposed catalytic loop
(shown for Pd in Scheme 1), may represent a much
bigger problem than originally thought.
3. At least in some cases, Ni(II) fluoro complexes can be
formed from the corresponding heavier halides upon
treatment with CsF rather than with AgF, which is
conventionally used for the synthesis of late transition
metal fluorides via X/F exchange.
4. Ni(II) complexes with fluoride trans to a non-electron-
deficient aryl can exist and be stable.
5. 1,1′-Ferrocenediyl backbone-based diphosphine ligands
can coordinate to Ni(II) in a trans manner without Ni···
Fe dative interactions.
6. Complexes of the type [(PP)Ni(Ar)(CF3)] can be
prepared for diphosphines PP other than dippe.
H
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Cy2P), 0.31 (m, 2H, Cy2P), 0.60 (m, 2H, Cy2P), 0.76 (m, 2H, Cy2P),
0.84−1.93 (m, 32H, Cy2P), 3.01 (br m, 2H, CH in Cy2P), 3.20 (br m,
2H, CH in Cy2P), 4.18 (m, 2H, Cp), 4.23 (m, 2H, Cp), 4.34 (m, 2H,
Cp), 4.63 (m, 2H, Cp), 7.20 (m, 1H, C10H7Ni), 7.30 (m, 1H,
C10H7Ni), 7.41 (d, J = 8.0, Hz 2H, C10H7Ni), 7.57 (m, 2H, C10H7Ni),
7.64 (d, J = 8.1 Hz, 2H, C10H7Ni), 11.10 (d, J = 8.1 Hz, 1H, C10H7Ni).
31P{1H} NMR (C6D6, 162 MHz), δ: −5.5 (s, major isomer, 96%),
−9.2 (s, minor isomer, ca. 4%).33 X-ray quality single crystals of trans-
[(dcypf)Ni(1-Np)Cl]·2THF were obtained by slow evaporation of its
solution in THF (Figure 4).
EXPERIMENTAL SECTION
■
Anhydrous NiCl2 (98% purity) was purchased from Aldrich. 1-
Chloronaphthalene (Aldrich, technical grade) was found by H NMR
1
to contain 3% of 2-chloronaphthalene. Zn dust (≥98% purity; <10
μm) was purchased from Aldrich and used without additional
activation. Deuterated solvents were purchased from Deutero.
Anhydrous oxygen-free benzene, benzene-d6, Et2O, xylenes, THF,
THF-d8, and n-hexane were distilled or vacuum-transferred from Na/
OCPh2. MeCN was distilled from P2O5 under argon. CD2Cl2 was
vacuum-transferred from CaH2. All purified deoxygenated dry solvents
were stored in a glovebox over freshly activated 4 Å molecular sieves.
Celite AW Standart Super-Cel NF (Aldrich) for filtration in a glovebox
was dried at 150 °C under vacuum for 8 h and stored in a glovebox.
Cesium fluoride was dried at 100 °C under vacuum for 5 h and stored
in a glovebox. All other chemicals and solvents were purchased from
Aldrich, Alfa Aesar, Apollo Scientific, and VWR and used as received,
including ethanol (96% HPLC grade), 1,2-dichloroethane (DCE;
≥99.0% purity), and n-hexane. NMe4F,47 i-PrXantphos,48 [(Cy3P)2Ni-
(1-Np)Cl],27 and [(PPh3)3Cu(CF3)]7d were synthesized following the
literature procedures. NMR spectra were recorded on Bruker Avance
400 Ultrashield and Bruker Avance 500 Ultrashield spectrometers. For
NMR experiments performed in nondeuterated solvents, 31P and 19F
chemical shifts were referenced to the chemical shifts of free PPh3
(−5.5 ppm), CHF3 (−78.8 ppm), and 4,4′-difluorobiphenyl (−116.4
ppm). Single-crystal X-ray diffraction studies were carried out with a
Bruker Apex DUO diffractometer equipped with an IμS microfocus
source. An Agilent Technologies 7890A chromatograph equipped with
a 5975C MSD unit was used for GC-MS analysis. Elemental analyses
were performed by the Microanalysis Center at the Complutense
University of Madrid.
[(Ph3P)2Ni(1-Np)Cl]. A 100 mL two-neck flask equipped with a
reflux condenser, gas inlet, and a Teflon-coated magnetic stir-bar was
charged with NiCl2 (1.30g, 10 mmol), PPh3 (5.77g, 22 mmol), and
ethanol (60 mL). The mixture was stirred under reflux in an argon
atmosphere for 1 h to produce a dark green suspension of
[(Ph3P)2NiCl2]. 1-Chloronaphthalene (3.25g, 2.7 mL, 20 mmol)
followed by Zn dust (1.31g, 20 mmol) were added to the agitated
reaction mixture. A yellow precipitate was immediately produced. The
reaction mixture was stirred under reflux for an additional 3 h and
allowed to cool to room temperature. The product was isolated in air.
The yellow solid was separated, washed with ethanol (100 mL),
dissolved in hot DCE (50 mL), and filtered through a pad of Celite.
The Celite was then washed with extra DCE (100 mL). Evaporation of
the combined DCE filtrate and washings produced an oily orange
solid, which was treated with n-hexane (300 mL) and kept at −32 °C
for 15 h. After warming to room temperature, the orange product was
separated by filtration, washed with n-hexane (2 × 100 mL), and dried
under vacuum. The yield of [(Ph3P)2Ni(1-Np)Cl] was 5.67 g (76%).
This complex is air-stable in the solid state but slowly decomposes in
CH2Cl2 solutions in air. NMR samples were prepared in anhydrous,
O2-free solvents inside a glovebox. Anal. Calcd for C46H37ClNiP2 (%):
C, 74.1; H, 5.0. Found: C, 73.5; H, 5.1. 1H NMR (CD2Cl2, 500 MHz),
δ: 6.45 (t, J = 7.5 Hz, 1H, C10H7Ni), 6.73 (d, J = 8.1 Hz, 1H,
C10H7Ni), 6.95 (m, 1H, C10H7Ni), 7.02−7.08 (m, 2H, C10H7Ni),
7.11−7.17 (m, 13H, 12H − PPh3, 1H − C10H7Ni), 7.25 (t, J = 7.4 Hz,
6H, PPh3), 7.47 (m, 12H, PPh3), 9.15 (d, J = 8.0 Hz, 1H, C10H7Ni).
31P{1H} NMR (C6D6, 203 MHz), δ: 21.7 (s).
[(dippf)Ni(1-Np)Cl]. In a glovebox, a mixture of [(Ph3P)2Ni(1-
Np)Cl] (224 mg, 0.30 mmol), dippf (151 mg, 0.36 mmol), and
benzene (6 mL) was agitated with a Teflon-coated magnetic stir-bar
for 0.5 h to produce small needle-shaped crystals. Extra benzene (50
mL) was added to dissolve the crystals, and the reaction mixture was
filtered through a pad of Celite. The flask was rinsed with benzene (3
× 10 mL), and the washings were filtered through the same pad. After
evaporation of the combined filtrate and the washings to ca. 3 mL with
a flow of argon and addition of n-hexane (5 mL), the precipitated
crystals were separated, washed with n-hexane (3 × 3 mL), and dried
under vacuum. The yield of [(dippf)Ni(1-Np)Cl] was 175 mg (91%).
Anal. Calcd for C32H43ClFeNiP2: C, 60.1; H, 6.8. Found: C, 59.7; H,
1
6.6. H NMR (C6D6, 400 MHz), δ: 0.00 (q, J = 7.9 Hz, 6H, CH3 in
iPr2P), 0.75 (q, J = 5.9 Hz, 6H, CH3 in i-Pr2P), 1.01 (q, J = 6.5 Hz, 6H,
CH3 in i-Pr2P), 1.59 (q, J = 8.3 Hz, 6H, CH3 in i-Pr2P), 1.82 (m, 2H,
CH in i-Pr2P), 3.43 (m, 2H, CH in i-Pr2P), 4.17 (m, 2H, Cp), 4.20 (m,
2H, Cp), 4.31 (m, 2H, Cp), 4.57 (m, 2H, Cp), 7.22 (t, J = 7.4 Hz, 1H,
C10H7Ni), 7.33 (t, J = 7.4 Hz, 1H, C10H7Ni), 7.47 (m, 2H, C10H7Ni),
7.66 (m, 2H, C10H7Ni), 11.26 (d, J = 8.3 Hz, 1H, C10H7Ni). 31P{1H}
NMR (C6D6, 162 MHz), δ: 1.3 (s). X-ray quality single crystals of
trans-[(dippf)Ni(1-Np)Cl]·C6D6 precipitated out of the NMR sample
solution in the tube after 3 days at room temperature (Figure 4).
[(Cy3P)2Ni(1-Np)F]. (A) In a glovebox, a 50 mL screw-cap FEP
tube was charged with a Teflon-coated magnetic stir-bar, [(PPh3)2Ni-
(1-Np)Cl] (373 mg, 0.50 mmol), PCy3 (280 mg, 1.00 mmol), and
THF (20 mL). After stirring for 5 min, the starting complex had
completely dissolved due to the formation of [(Cy3P)2Ni(1-Np)Cl].
+
After the addition of NMe4 F− (65 mg, 0.7 mmol) and agitation for 20
h complete conversion of [(Cy3P)2Ni(1-Np)Cl] (s, 14.4 ppm) to
[(Cy3P)2Ni(1-Np)F] (d, 17.6 ppm, J = 45 Hz) was observed by 31P
NMR. The fluoro complex was isolated in the glovebox. The reaction
mixture was diluted with THF (35 mL) and filtered through a pad of
Celite. The reaction tube was rinsed with THF (2 × 10 mL), and the
washings were filtered through the same Celite pad. After evaporation
of the combined filtrate and the washings with a flow of argon to ca. 5
mL, the mixture was kept at −32 °C for 1 day to complete the
precipitation. The yellow solid was separated, washed with Et2O (3 ×
5 mL), and dried under vacuum to give 298 mg of crude
[(Cy3P)2Ni(1-Np)F]. For further purification, the crude product was
dissolved in benzene (15 mL) at ca. 70 °C and filtered hot through a
pad of Celite, which was then washed with hot benzene (2 × 3 mL).
The combined filtrate and the washings were allowed to cool to room
temperature and treated with ether (15 mL). The yellow micro-
crystalline solid was separated, washed with ether (2 × 5 mL), and
dried under vacuum. The yield of the thus isolated [(Cy3P)2Ni(1-
Np)F] containing ca. 0.4 molecule of C6H6 per Ni (1H NMR) was 230
mg (58%). Anal. Calcd for C46H73FNiP2·0.4C6H6: C, 72.9; H, 9.5.
1
Found: C, 72.8; H, 9.2. H NMR (THF-d8, 500 MHz), δ: 0.75−2.19
[(dcypf)Ni(1-Np)Cl]. In a glovebox, a mixture of [(Ph3P)2Ni(1-
Np)Cl] (224 mg, 0.30 mmol), dcypf (208 mg, 0.36 mmol), and THF
(6 mL) in a 20 mL vial was agitated with a Teflon-coated magnetic
stir-bar for 0.5 h and then filtered through a short Celite plug. The vial
was rinsed with THF (2 × 2 mL), and the washings were filtered
through the same plug. After evaporation of the combined filtrate and
the washings with a flow of argon, the oily residue was treated with
THF (2 mL) and n-hexane (10 mL) and kept at −22 °C for 1 day.
The precipitated red crystals were separated, washed with n-hexane (2
× 3 mL), and dried under vacuum. The yield of [(dcypf)Ni(1-Np)Cl]
was 166 mg (69%). Anal. Calcd for C44H59ClFeNiP2: C, 66.1; H, 7.4.
(m, 6PCy3, 66H), 6.86 (t, J = 7.5 Hz, 1H, C10H7Ni), 7.08 (d, J = 7.9
Hz, 1H, C10H7Ni), 7.23 (t, J = 7.4 Hz, 1H, C10H7Ni), 7.30 (s, C6H6),
7.37 (t, J = 7.5 Hz, 1H, C10H7Ni), 7.44 (d, J = 8.1 Hz, 1H, C10H7Ni),
7.68 (d, J = 6.8 Hz, 1H, C10H7Ni), 9.87 (d, J = 8.3 Hz, 1H, C10H7Ni).
31P{1H} NMR (THF-d8, 203 MHz), δ: 17.6 (d, JP−F = 45 Hz). 19F
NMR (THF-d8, 377 MHz), δ: −368.7 (t, JP−F = 45 Hz). X-ray quality
single crystals of [(Cy3P)2Ni(1-Np)F] were grown by slow
evaporation of its solution in THF inside the glovebox (Figure 6).
(B) In a glovebox, a 20 mL vial was charged with a Teflon-coated
magnetic stir-bar, [(Cy3P)2Ni(1-Np)Cl] (50 mg, 0.06 mmol), PCy3
(18 mg, 0.06 mmol), CsF (58 mg, 0.38 mmol), and THF (4 mL). The
vial was sealed and brought out. After agitation for 24 h at 60 °C (oil
1
Found: C, 66.6; H, 7.3. H NMR (C6D6, 400 MHz), δ: 0.17 (m, 2H,
I
dx.doi.org/10.1021/om5008743 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
bath) ca. 85% conversion of [(Cy3P)2Ni(1-Np)Cl] (s, 14.4 ppm) to
[(Cy3P)2Ni(1-Np)F] (d, 17.6 ppm, J = 45 Hz) was detected (31P
NMR).
2.93 (sep, J = 6.8 Hz, 2H, CH in i-iPr2P), 5.27 (d, J = 7.0 Hz, 1H,
C10H7Ni), 6.16 (t, J = 7.5 Hz, 1H, C10H7Ni), 6.69 (d, J = 7.7 Hz, 2H,
Xantphos), 6.93 (t, J = 7.6 Hz, 2H, Xantphos), 6.97 (d, J = 8.1 Hz, 1H,
C10H7Ni), 7.27 (d, J = 7.6 Hz, 2H, Xantphos), 7.33 (t, J = 7.6 Hz, 1H,
C10H7Ni), 7.48 (d, J = 7.9 Hz, 1H, C10H7Ni), 7.81 (t, J = 7.6 Hz, 1H,
C10H7Ni), 11.77 (d, J = 8.3 Hz, 1H, C10H7Ni). 31P{1H} NMR (C6D6,
162 MHz), δ: 18.5 (q, JP−F = 14 Hz). 19F NMR (C6D6, 377 MHz), δ:
−13.3 (t, JP−F = 14 Hz).
[(i-PrXantphos)Ni(1-Np)F]. In a glovebox, a 50 mL screw-cap FEP
tube was charged with a Teflon-coated magnetic stir-bar, [(PPh3)2Ni-
(1-Np)Cl] (373 mg, 0.50 mmol), i-PrXantphos (266 mg, 0.60 mmol),
and THF (20 mL). After stirring for 15 min, the starting complex had
dissolved due to the formation of dark red [(i-PrXantphos)Ni(1-
+
Np)Cl]. After the addition of NMe4 F− (1.4 equiv, 65 mg, 0.70 mmol)
[(dcypf)Cu(CF3)]. In a glovebox, a mixture of [(PPh3)3Cu(CF3)]
(138 mg, 0.15 mmol), dcypf (87 mg, 0.15 mmol), and THF (4 mL)
was agitated with a Teflon-coated magnetic stir-bar in a 20 mL vial for
1 h. The yellow reaction mixture was filtered through a pad of Celite.
The vial was rinsed with THF (2 × 2 mL), and the washings were
filtered through the same pad. After evaporation of the combined
filtrate and the washings with a flow of argon, the oily residue was
treated with n-hexane (5 mL) and the mixture was kept at −32 °C for
1 day to complete the precipitation. The yellow solid was separated,
washed with n-hexane (2 × 3 mL) and ether (2 × 4 mL), and dried
under vacuum to give 47 mg of [(dcypf)Cu(CF3)] (44% yield). Anal.
Calcd for C35H52CuF3FeP2: C, 59.1; H, 7.4. Found: C, 58.8; H, 7.2. 1H
NMR (C6D6, 400 MHz), δ: 1.07−1.85 (m, 32H, Cy2P), 2.03 (m, 12H,
Cy2P), 4.05 (m, 4H, Cp), 4.17 (brs, 4H, Cp). 31P{1H} NMR (C6D6,
162 MHz), δ: −7.7 (q, JP−F = 17 Hz). 19F NMR (C6D6, 377 MHz), δ:
−25.7 (t, JP−F = 17 Hz). X-ray quality single crystals of [(dcypf)Cu-
(CF3)] were obtained by layering its THF solution (8 mg in 0.2 mL)
with n-hexane in a 5 mm NMR tube (Figure 5).
Computational Details. All the structures have been fully
optimized in tetrahydrofuran using the Gaussian09 package49 with
the B97D density functional.50 The standard 6-31G+(d)51 basis set
was used for all H, C, O, and F atoms; the Stuttgart triple-ζ basis set
(SDD),52 along with the associated electron core potentials, was
employed for Ni and Fe. Free energies in solvent are computed with
the (IEF-PCM) continuum dielectric solvation model53 with the radii
and nonelectrostatic SMD terms developed by Truhlar and co-
workers.54 In all cases, frequency calculations were carried out to
ensure the nature of stationary points and transition states and to allow
the calculation of the free energy for all the species. For these
frequency calculations, the pressure was always 1 atm and the
temperature 25 °C unless otherwise noted.
and agitation for 13.5 h, complete conversion of [(i-PrXantphos)Ni(1-
Np)Cl] (s, 6.2 ppm) to [(i-PrXantphos)Ni(1-Np)F] (d, 6.7 ppm, J =
41 Hz) was observed by 31P NMR. The product was isolated in the
glovebox. The reaction mixture was filtered through a pad of Celite.
The reaction tube was rinsed with THF (3 × 5 mL), and the washings
were filtered through the same Celite pad. After evaporation of the
combined filtrate and the washings with a flow of argon to ca. 2 mL, n-
hexane (25 mL) was added, and the mixture was kept at −18 °C for 1
day to complete the precipitation. The orange solid was separated,
washed with Et2O (5 mL) and n-hexane (2 × 5 mL), and dried under
vacuum to give 215 mg of crude [(i-PrXantphos)Ni(1-Np)F] (66%).
For additional purification, a solution of the complex (28 mg) in
benzene (3 mL) was filtered through a pad of Celite and evaporated
with a flow of argon, the oily residue was treated with n-hexane (1
mL), and the orange solid was separated and dried under vacuum. The
yield of the extra purified [(i-PrXantphos)Ni(1-Np)F] was 25 mg
(59%). Anal. Calcd for C37H47FNiOP2: C, 68.6; H, 7.3. Found: C,
68.9; H, 7.2. 1H NMR (THF-d8, 500 MHz), δ: 1H {31P} NMR (C6D6,
400 MHz), δ: −0.34 (d, J = 7.1 Hz, 6H, CH3 in i-Pr2P), 0.99 (d, J = 7.0
Hz, 6H, CH3 in i-Pr2P), 1.13 (d, J = 6.8 Hz, 6H, CH3 in i-Pr2P), 1.35
(d, J = 6.8 Hz, 6H, CH3 in i-Pr2P), 1.40 (s, 3H, CH3 in Xantphos),
1.62 (s, 3H, CH3 in Xantphos), 1.85 (sep, J = 6.8 Hz, 2H, CH in i-
Pr2P), 2.65 (sep, J = 6.9 Hz, 2H, CH in i-Pr2P), 5.98 (d, J = 7.0 Hz,
1H, C10H7Ni), 6.24 (t, J = 7.5 Hz, 1H, C10H7Ni), 6.87 (d, J = 7.5 Hz,
2H, Xantphos), 6.94−7.02 (m, 3H, Xantphos and C10H7Ni), 7.32 (d, J
= 7.7 Hz, 2H, Xantphos), 7.37 (m, 1H, C10H7Ni), 7.50 (d, J = 8.1 Hz,
2H, C10H7Ni), 7.75 (t, J = 7.5 Hz, 1H, C10H7Ni), 12.41 (d, J = 8.2 Hz,
1H, C10H7Ni). 31P{1H} NMR (C6D6, 162 MHz), δ: 6.8 (d, JP−F = 41
Hz). 19F NMR (C6D6, 377 MHz), δ: −372.5 (brs). X-ray quality single
crystals of [(i-PrXantphos)Ni(1-Np)F]·CH2Cl2 were grown by slow
evaporation of its solution in dichloromethane. X-ray quality single
crystals of [(i-PrXantphos)Ni(1-Np)F]·H2O were grown by crystal-
lization of the crude product from n-hexane in a system containing
adventitious water (Figure 7).
ASSOCIATED CONTENT
■
S
[(i-PrXantphos)Ni(1-Np)(CF3)]. In a glovebox, a mixture of
[(PPh3)2Ni(1-Np)Cl] (149 mg, 0.20 mmol), i-PrXantphos (89 mg,
0.20 mmol), CsF (61 mg, 0.40 mmol), THF (5 mL), and CF3SiMe3
(59 μL, 0.40 mmol) in a 20 mL vial was agitated with a Teflon-coated
magnetic stir-bar. After 22 h, full conversion of CF3SiMe3 was
observed (19F NMR). Additional amounts of CsF (61 mg, 0.40 mmol)
and CF3SiMe3 (149 μL, 1.00 mmol) were added, and the reaction
mixture was agitated for 3.5 h to reach full conversion of [(i-
PrXantphos)Ni(1-Np)Cl] (31P NMR). The product was isolated in
the glovebox. The reaction mixture was filtered through a pad of
Celite. The vial was rinsed with THF (2 × 2 mL), and the washings
were filtered through the same pad. After evaporation of the combined
filtrate and the washings with a flow of argon to ca. 1 mL and addition
of ether (3 mL) to the oily residue, red-orange crystals formed. The
solid was separated, washed with ether (2 × 3 mL), and dried under
vacuum to give 73 mg of crude [(i-PrXantphos)Ni(1-Np)(CF3)]
(53%) in the form of X-ray quality crystals (Figure 8). For additional
purification, a solution of the crude product (28 mg) in benzene (2.5
mL) was filtered through a pad of Celite that was then washed with
benzene (2 × 1 mL). Evaporation of the combined filtrate and the
washings, followed by vacuum drying, gave 26 mg of extra purified [(i-
PrXantphos)Ni(1-Np)(CF3)] (49%). Anal. Calcd for C38H47F3NiOP2:
* Supporting Information
Full details of crystallographic and computational studies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: vgrushin@iciq.es.
*E-mail: fmaseras@iciq.es.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Bianca K. Munoz and Maxim A. Novikov for
̃
preliminary studies toward the synthesis of [(Cy3P)2Ni(1-Np)
F]. This work was supported by the ICIQ Foundation and the
Spanish Government (Grants CTQ2011-25418 and CTQ2011-
27033 and Severo Ochoa Excellence Accreditation 2014-2018
SEV-2013-0319). F.M. thanks the Government of Spain
(MINECO) for the FPI Ph.D. Scholarship (BES-2012-
054922). J.J. thanks the Government of Spain (MINECO)
for a Juan de la Cierva Fellowship (JCI-2011-10134).
1
C, 65.4; H, 6.8. Found: C, 65.4; H, 6.7. H{31P} NMR (C6D6, 400
MHz), δ: −0.47 (d, J = 7.1 Hz, 6H, CH3 in i-Pr2P), 0.70 (d, J = 6.8 Hz,
6H, CH3 in i-Pr2P), 1.27 (d, J = 6.9 Hz, 6H, CH3 in i-Pr2P), 1.30 (s,
3H, CH3 in Xantphos), 1.38 (d, J = 6.8 Hz, 6H, CH3 in i-Pr2P), 1.60
(s, 3H, CH3 in Xantphos), 1.99 (sep, J = 7.0 Hz, 2H, CH in i-Pr2P),
J
dx.doi.org/10.1021/om5008743 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Am. Chem. Soc. 2012, 134, 6548. (l) Ye, Y.; Sanford, M. S. J. Am. Chem.
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the dcypf complex. However, the purified [(Ph3P)2Ni(1-Np)Cl] used
in the synthesis of [(dcypf)Ni(1-Np)Cl] displayed only one singlet in
the 31P NMR spectrum, suggesting the absence of [(Ph3P)2Ni(2-Np)
Cl]. It is therefore more plausible that the extra 31P NMR signal at
−9.3 ppm observed for the isolated [(dcypf)Ni(1-Np)Cl] is from the
nucleus of the uncoordinated P atoms of [(dcypf-κ1P)2Ni(1-Np)Cl]
and/or oligomeric [(μ-dcypf)Ni(1-Np)Cl]n, whose other signals
overlap with the intense singlet from [(dcypf)Ni(1-Np)Cl]. Indeed,
on cooling the sample of [(dcypf)Ni(1-Np)Cl] to −90 °C (THF-d8),
the minor 31P NMR peak at −9.3 ppm was replaced with five weak
singlet resonances at 55.9, −5.5, −5.8, −8.5, and −9.0 ppm. A set of
two doublets that would be expected from cis-[(dcypf)Ni(1-Np)Cl]
was not observed within the temperature range used (25−−90 °C).
The formation of [(PP-κ1P)2Ni(1-Np)Cl] and/or oligomeric [(μ-PP)
Ni(1-Np)Cl]n is believed to be less likely for PP = dippf than for
bulkier PP = dcypf.
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