Fluorine-substituted arylphosphine for an NHC-Ni(I) system, air-stable in a solid state but catalytically active in solution

Article abstract of DOI:10.3390/molecules24183222

Monovalent NHC-nickel complexes bearing triarylphosphine, in which fluorine is incorporated onto the aryl groups, have been synthesized. Tris(3,5-di(trifluoromethyl)phenyl)phosphine efficiently gave a monovalent nickel bromide complex, whose structure was determined by X-ray diffraction analysis for the first time. In the solid state, the Ni(I) complex was less susceptible to oxidation in air than the triphenylphosphine complex, indicating greatly improved solid-state stability. In contrast, the Ni(I) complex in solution can easily liberate the phosphine, high catalytic activity toward the Kumada–Tamao–Corriu coupling of aryl bromides.

Full text of DOI:10.3390/molecules24183222

Communication
Fluorine-Substituted Arylphosphine for an
NHC-Ni(I) System, Air-Stable in a Solid State
but Catalytically Active in Solution
Kouki Matsubara *, Takahiro Fujii, Rion Hosokawa, Takahiro Inatomi, Yuji Yamada
and Yuji Koga
Department of Chemistry, Fukuoka University, 8-19-1 Nanakuma, Fukuoka 814-0180, Japan
* Correspondence: kmatsuba@fukuoka-u.ac.jp; Tel.: +81-92-871-6631
Academic Editor: Derek J. McPhee
Received: 22 August 2019; Accepted: 3 September 2019; Published: 4 September 2019
Abstract: Monovalent NHC-nickel complexes bearing triarylphosphine, in which fluorine is
incorporated onto the aryl groups, have been synthesized. Tris(3,5-di(trifluoromethyl)-
phenyl)phosphine efficiently gave a monovalent nickel bromide complex, whose structure was
determined by X-ray diffraction analysis for the first time. In the solid state, the Ni(I) complex was
less susceptible to oxidation in air than the triphenylphosphine complex, indicating greatly
improved solid-state stability. In contrast, the Ni(I) complex in solution can easily liberate the
phosphine, high catalytic activity toward the Kumada–Tamao–Corriu coupling of aryl bromides.
Keywords: monovalent nickel; fluorine-substituted phosphine; Kumada coupling; intermolecular
interaction; DFT calculations
1. Introduction
Recent developments in the field of nickel catalysis have opened up possibilities of new catalytic
processes directly involving monovalent nickel complexes in organometallic chemistry [1–6] along
with those involving conventional zerovalent nickel catalysts. Although limited to the structures of
specific ligands and complexes, monovalent nickel complexes are thermally stable and isolable [7].
Several studies have used well-defined nickel(I) complexes as catalyst precursors [8–14], and
reactions where nickel(I) complexes are the key intermediates have been proposed based on
theoretical calculations [15–20]. Recently, experimental results have also supported the catalyst
chemistry of nickel(I) [21–23]. One of the current bottlenecks in nickel(I) catalysis research is
developing catalytically active, air-stable nickel(I) catalyst precursors. It is safe to say that a
breakthrough here would go a long way in furthering our knowledge about nickel catalyst chemistry.
To date, phosphines with fluorine-substituted aryl groups have been frequently used due to
their attractive features in many catalytic reactions [24–31]. The induced electron-withdrawing
property of fluorine increases the π-acceptor nature of the phosphorus ligand, and it is expected that
the back-donation from the metal stabilizes the electron-rich, low-valent metal center. Cone-angles
relatively larger than those of phosphite ligands also provide kinetic stabilization. It may be noted
that tris(3,5-di(trifluoromethyl)phenyl)phosphine (P(Arm-CF3
metal complexes [24–28]. In this case, fluorine atoms can induce unique CF···π interactions between
CF groups and aromatic rings of ligand molecules to form solid ligand structures that stabilize
unsaturated metal complexes such as the “superstable” Pd(0) catalyst, [Pd(P(Arm-CF3 ] [29].
Recently we have synthesized three-coordinated mononuclear nickel(I) complexes
[Ni(IPr)Cl(L)] (IPr 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) by adding various
)3
) has additional unique properties in
3
)3)3
=
Molecules 2019, 24, 3222; doi:10.3390/molecules24183222
www.mdpi.com/journal/molecules

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monodentate or bidentate ligands L to the dinuclear nickel(I) complex, [Ni(IPr)Cl]2 [32–34]. The series
of complexes shown in Scheme 1 works efficiently under mild conditions in cross-coupling reactions,
such as Buchwald–Hartwig amination, Kumada–Tamao–Corriu coupling, and Suzuki–Miyaura
coupling [33,34]. These mononuclear complexes can regenerate the dinickel(I) halide complex in
solution, which can generate coordinatively unsaturated active intermediate, mononuclear Ni(I)
NHC complex, [Ni(NHC)X] [21]. When using weakly coordinating pyridine as the stabilizing ligand,
the stability of Ni(I) proved to be insufficient, whereas strongly withdrawing phosphite, P(OPh)3,
lowered its catalytic performance [34]. The complexes containing usual phosphines like PPh3 were
also smoothly oxidized even in the solid state. Therefore, to construct more stable and active nickel(I)
complexes, investigation of alternative ligands, such as the m-CF3-substituted arylphosphine (P(Arm-
CF3)3), is necessary. However, to the best of our knowledge, there has been no report on well-defined
nickel complexes bearing this phosphine ligand.
Scheme 1. Previously reported mononuclear Ni(I)-NHC halide complexes [8].
In the course of our research, we have found a desirable ligand (fluorine-substituted
monodentate phosphine) to stabilize three-coordinated nickel(I) complex. Intriguingly, when using
P(Arm-CF3)3, the obtained monovalent nickel complex in the crystalline state did not suffer air oxidation
for some minutes, while simultaneously maintaining the catalytic activity in the Kumada–Tamao–
Corriu cross-coupling reaction in solution under an inert-gas atmosphere. Here, we have reported
the synthetic procedure, structures, and properties of the nickel(I) complexes and results in a
Kumada–Tamao–Corriu cross-coupling reaction. This is the first example of a well-defined nickel
complex bearing the m-CF3-substituted triarylphosphine.
2. Results
2.1. Preparation and Characterization of Ni(I) Complexes
The bromide analogue of [Ni(IPr)Cl]2 reported in the literature [35] is expected to be more stable
than its chloride analogue and can be transformed into mononuclear nickel(I)-IPr complex in
conjunction with the phosphorus ligand. The reactions of [Ni(IPr)Br]2 (1) with triarylphosphines,
triphenylphosphine (PPh3), tris(4-(trifluoromethyl)phenyl)phosphine
(P(Arp-CF3)3),
tris(3,5-
bis(trifluoromethyl)phenyl)phosphine (P(Arm-CF3)3), and tris(pentafluorophenyl)phosphine (P(C6F5)3)
were conducted in a tetrahydrofuran (THF) solution at room temperature. As expected, the
corresponding mononuclear nickel(I) complexes, [NiBr(IPr)(PAr3)] (PAr3 = PPh3 (2a), P(Arp-CF3)3 (2b),
and P(Arm-CF3)3 (2c)), were successfully obtained in 42, 46, and 57% yields (Scheme 2), respectively,
after recrystallization. However, P(C6F5)3 did not coordinate to nickel, resulting in the complete
recovery of the starting materials under the reaction conditions. This may be attributed to steric
repulsion between a pair of diisopropylphenyl groups of the IPr ligand and pentafluorophenyl
groups of P(C6F5)3, where the cone angle (184°) is greater than that of P(Arm-CF3)3 (160°).
Scheme 2. Reaction of dinickel(I) complex 1 with phosphines.

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The obtained Ni(I) complexes were paramagnetic and the EPR resonances were observed even
at room temperature (see Supporting Information), similar to the previous chloride analogue,
[NiCl(IPr)(PPh
measurement of 2c showed a favorable value of χ
The value did not change over the temperature range until 300 K, according to Curie′s rule. The
3
)] [33]. Moreover, a SQUID (superconducting quantum interference device)
3
−1
M
T = 0.47 cm ·K·mol at 50 K, assigned as S = 1/2.
3
−1
previous study also showed similar values (χ
M
T = 0.35–0.52 cm ·K·mol ), which are somewhat higher
3
−1
than the theoretical value of 0.375 cm ·K·mol when S = 1/2, attributed to spin-orbital angular
momentum.
1
The H-NMR spectral measurement for the crystals of 2a–2c dissolved in C
6
D6 revealed a partial
regeneration of the starting complex 1 accompanied by free triarylphosphine from these products. A
similar phenomenon is observed in a solution of the previous Ni(I) chloride analogues, suggesting
the existence of coordination-elimination equilibrium of weakly coordinating triarylphosphine in
solution [34]. It is notable that the ratio of the regenerated complex 1 from the mononuclear complex
2c was only slightly smaller than the ratio from 2a.
2.2. X-Ray Crystallography and Theoretical Studies
The structures of complexes 2a, 2b, and 2c were confirmed by X-ray crystallography by using
single crystals after recrystallization. Figure 1 shows representative examples of the structures of 2a
and 2c. The result from the crystal of 2b was insufficient to discuss the detailed structure,
unfortunately. There were no significant differences between the 2a and 2c structures, as far as the
bond lengths and angles of nickel–bromine bonds are concerned. (Table 1). On the other hand, some
differences were observed in the bond lengths between the nickel–phosphorus and the nickel–
carbene carbon atoms: Ni(1)–P(1), 2.201(1) (2a) and 2.182(1) (2c); Ni(1)–C(1), 1.918(5) (2a), and 1.936(3)
Å (2c). Similar differences between the metal–phosphorus atom in platinum(II) complexes bearing
PPh
3
and P(Arm-CF3
) were reported as ca. 0.02 Å [36]. The extension of the nickel–carbon bond in 2c
3
appears to be in balance with the strength of the back-donation to the nickel–phosphorus bond.
A space-filling model of 2c was depicted in Figure 1c,d to show the coordination sphere of nickel.
The substituents of P(Arm-CF3
around nickel. In particular, one CF
)
3
largely occupied the remaining spaces other than IPr and bromine
on the aryl group is sandwiched between the two isopropyl
3
groups contained in the two wingtip groups of IPr, which hampers the free rotation of the Ni–P axis.
19
If so, the F signals from the CF
3
groups could not be equivalently observed. The averaged equivalent
19
F signal of 2c may be provided by the fast equilibrium of elimination and the coordination of
phosphine. Because this complex has only one phosphine ligand, intramolecular CF···π interaction
between phosphine ligands did not exist in the crystal structure.
N(1)
N(1)
C(1)
C(1)
Ni(1)
Ni(1)
Br(1)
P(1)
Br(1)
P(1)
2a
2c
(a)
(b)

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IPr
Ni
F
Br
P
Br
P
F
(c)
(d)
Figure 1. ORTEP drawing of (a) 2a and (b) 2c (50% probability of thermal ellipsoids. Hydrogen atoms
and solvent molecule (THF in the crystal of 2c) are omitted for clarity). Views of space-filling model
of 2c from (c) the side and (d) the bottom are illustrated (pink: Br; gray: C; light green: F; orange: P;
light yellow: Ni).
Table 1. Representative bond lengths and angles of 2a and 2c.
2a
2c
Ni(1)–Br(1)
Ni(1)–P(1)
Ni(1)–C(1)
2.301(1)
2.201(1)
1.918(5)
138.4(1)
114.9(1)
106.44(4)
2.3016(8)
2.182(1)
1.936(3)
137.1(1)
113.5(1)
109.29(4)
Lengths (Å)
Angles (°)
C(1)–Ni(1)–Br(1)
C(1)–Ni(1)–P(1)
Br(1)–Ni(1)–P(1)
The distribution of the single electron-occupied molecular orbital (SOMO) of 2a and 2c was
investigated by performing single-point density functional theory (DFT) calculations (B3LYP/6-
31G(d,p) level) implemented in the Gaussian 16 program package [37] at the fixed geometries given
by the crystallographic coordinates. Similar electron distributions were given in both complexes as
shown in Figure 2a,b. The unpaired electron in SOMO is distributed mainly in one d orbital of nickel,
resulting in the formation of two σ*(d-n
phosphorus atom and the NHC unit, and the π*(d-p) orbital between nickel and bromine. Because of
the electron-withdrawing property of the CF groups, the acceptor ability of the arylphosphine may
σ
) orbitals with σ-type non-bonding orbitals (n ) of the
σ
3
be enhanced in 2c. The ratio of the unpaired electron in the nickel d-orbital was 34.9% in 2c, lower
than that of 2a (41.4%), whereas that in the phosphorus atom was 12.4% and 10.6% in 2c and 2a,
respectively. Interestingly, in the case of bromine, the figure was much larger in 2c (28.3%) than that
in 2a (21.7%). The reason for this may be the fact that the energy level of NHC–Ni–PPh
providing the main component of SOMO is lower-shifted upon the substitution of the CF
3
unit in 2c
3
group into
arylphosphine to become closer to the 4p orbital of bromine. This can lead to a greater occupation of
the 4p orbital in the Ni–Br π*-orbital component of SOMO as compared to that of 2a.
2c
2a
(a)
(b)

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Figure 2. Kohn–Sham orbitals of SOMO corresponding to a density isosurface value of 0.02 au in (a)
2a and (b) 2c obtained from single-point DFT calculation with B3LYP/6-31G(d,p) level using the
crystallographic coordinates.
2.3. Oxidation Potentials
It was found that the stability of 2c in air was improved more than that of 2a. In fact, in the solid
state, it had not been degraded by oxidation for a period of time: the surface crystal color of 2c began
to change gradually after several minutes upon oxidation in air, whereas 2a was oxidized as soon as
it was exposed to air. On the other hand, the resistance to oxidation could be evaluated by cyclic
voltammogram (CV), which was measured with THF solutions of 2a or 2c, an excess amount of free
phosphine (2 equiv), and Bu
4
NPF
6
. The free phosphine (PPh
3
for 2a and P(Arm-CF3) for 2c) was added
3
in order to suppress the generation of the Ni(I) dimer 1 with the liberation of phosphine in
equilibrium. As shown in Figure 3, in the CV measured using Ag/AgCl as a reference electrode, the
one-electron oxidation potential of 2a was 1.0 V, and that of 2c was higher, 1.3 V. These were not
oxidation potentials of the free phosphines. Therefore, it is believed that the oxidation resistance of
the nickel center is shown to be improved by the fluorine-substituted phosphine, which may also be
reflected in the stability in the solid state, along with the fluorine-derived inter- or intramolecular
interactions.
Figure 3. Redox behavior of NiBr(IPr)(PPh
3
) (2a) (blue line) and 2c (red line) in THF with Bu
4
NPF
6
(working electrode: Pt; counter electrode: Pt; scan rate: 100 mV·s−1).
2.4. Catalytic Performance for Cross-Coupling Reaction
The Kumada–Tamao–Corriu coupling of aryl bromides using complexes 2a, 2b, and 2c were
representatively conducted under the same reaction conditions. Because the equilibrium ratio of the
1
regenerated dimer 1 observed in the H-NMR spectrum was only slightly different between 2a and
2c, it is expected that the catalytic activity of these complexes will not change. As shown in Scheme
3, 0.5 mol% of 2a–2c were added to the reaction media in the presence of 5 equiv of the corresponding
triarylphosphine, and the mixture was stirred under an inert gas atmosphere at room temperature
for 18 h. The addition of the excess amount of triarylphosphine can shift the ligand-elimination
equilibrium from the dimer 1 toward the mononuclear complexes 2. In the reaction of 4-
bromotoluene with phenylmagnesium bromide, the product 1-methyl-4-phenylbenzene was
successfully obtained in excellent yields (86, 94, and 89% using 2a, 2b, and 2c, respectively). More
inactive substrate (4-bromoanisole) also produced 1-methoxy-4-phenylbenzene in high yields (76, 79,
and 91% with 2a, 2b, and 2c, respectively). These results indicated that the reactions proceeded
efficiently under these conditions, regardless of the nature of triarylphosphines, as expected.

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Scheme 3. Kumada–Tamao–Corriu coupling of aryl bromides using 2a–2c as catalysts.
3. Discussion
As noted above, apparently 2c is more stable in air than 2a in the crystalline state. It is believed
that the stability in the crystalline state is derived from several factors. One of them is a steric effect.
X-ray crystallography demonstrated that the bulky meta-CF3 substituted phosphine buried the
coordination sphere around the nickel center other than those of the NHC and bromine, kinetically
blocking the facile access of small molecules such as dioxygen. As is supportde by the discussion
above, more hindered, ortho-fluorine substituted P(C6F5)3 was difficult to coordinate to nickel.
Additionally, the increase in the oxidation potential from Ni(I) to Ni(II) for 2c should lead to the
resistance of the nickel center to oxidation in air, compared with 2a. However, in solution, the stability
of the complex derived from the phosphine should be greatly reduced by its facile elimination to
form the Ni(I) dimer 1, enabling a similar catalyst performance of 2c to that of 2a in Kumada–Tamao–
Corriu coupling of aryl bromides via the dinickel reaction pathway [9].
As indicated in the literature, the presence of intermolecular interactions in the crystals formed
with fluorine atoms may not be negligible [29]. This hypothesis for the specific intermolecular
interactions was strongly supported by the results of X-ray crystallography. It should be noted that
there are characteristic, multiple short contacts involving fluorine atoms between 2c molecules, such
as the F···H interaction observed with hydrogens of aryl para-C–H in phosphine and isopropyl CH3
groups in IPr, as well as many F···F interactions (Figure 4b and ESI). Such F···F interactions have been
visualized as a strong dispersed interaction (i.e. van der Waals interaction) in perfluoropolymers [38]
and perfluoroalkanes [39]. Interestingly, such short contacts were mostly distributed in layers,
suggesting that these interactions could be a driving force to induce and subsequently stabilize the
crystal packing. In this packing structure, the CF···π interaction was not observed, in contrast to the
palladium chemistry [29]. On the other hand, in a crystal of 2a, the fewer intermolecular short contacts
were observed only between C–H groups (Figure 4a and ESI). It was difficult to evaluate the strength
of the intermolecular interactions experimentally, because thermal measurements such as differential
scanning calorimetry (DSC) or thermo gravimeter (TG) may break the metal–phosphorus and metal–
carbon bonds more easily. However, it is believed that the higher oxidation potential and these steric
and electronic factors specifically derived from meta-CF3 substituted phosphine contribute to the air-
stable nature of 2c in the solid state compared with that of 2a, at present.

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(a)
(b)
Figure 4. Crystal packing views from the (010) planes depicted using crystallographic data of (a) 2a
and (b) 2c (pink: Br; gray: C; light green: F; red: O; orange: P; light yellow: Ni). Short contacts that are
shorter than van der Waals radii are addressed as red lines. A blue dashed line in (b) highlighted a
layer where many contacts containing F atoms (light green) are gathered.
4. Materials and Methods
4.1. Materials
Super-dehydrated grade THF, toluene, and hexane were used as solvents as purchased from
WAKO Pure Chemical Industries, Ltd., Tokyo, Japan. Benzene-d6 was distilled from sodium
benzophenone ketyl and stored under a nitrogen atmosphere. Organic reagents used for coupling
reactions were distilled just before use or used as purchased. N-Heterocyclic carbene (IPr) [40] and
nickel dimer, [Ni(IPr)(μ-Br)]2 (1) [35], were prepared according to the literature methods.
4.1.1. Ni(IPr)(PPh3)Br (2a)
Complex 1a (66.5 mg, 0.0630 mmol) and PPh3 (33.0 mg, 0.126 mmol) were dissolved in THF (1
mL) at room temperature, and the mixture was stirred for 5 min to give a light-yellow solution.
Hexane (4 mL) was then added to the obtained solution, and the solution was stored at −30 °C. Yellow
1
crystals were obtained in a 42% yield (42.0 mg) after filtration. H-NMR (C6D6): δ 11.0 (brs), 8.5 (brs),
4.5 (brs), 1.7 (brs). Anal. calcd. for C45H51N2PNiBr: C 68.46%, H 6.51%, N 3.55%; Found: C 67.78%, H
6.47%, N 3.51%.
4.1.2. Ni(IPr)(PAr(p-CF3)3)Br (2b)
Complex 1a (52.5 mg, 0.0500 mmol) and PAr(p-CF3)3 (46.9 mg, 0.100 mmol) were dissolved in THF
(1 mL) at room temperature, and the mixture was stirred for 5 min to give an orange-red solution.
Hexane (4 mL) was then added to the obtained solution, and the solution was stored at −30 °C. Orange
1
crystals were obtained in a 57% yield (63.7 mg) after filtration. H-NMR (C6D6): δ 11.0 (brs), 8.4 (brs),
19
4.5 (brs), 1.8 (brs). F-NMR (C6D6): δ −53.6. Anal. calcd. for C48H48N2F9PNiBr: C 58.03%, H 4.87%, N
2.82%; Found: C 58.48%, H 4.48%, N 2.72%.
4.1.3. Ni(IPr)(PAr(m-CF3)3)Br (2c)
Complex 1a (44.0 mg, 0.418 mmol) and PAr(m-CF3)3 (62.0 mg, 0.835 mmol) were dissolved in THF
(1 mL) at room temperature, and the mixture was stirred for 5 min to give an orange-red solution.
Hexane (4 mL) was then added to the obtained solution, and the solution was stored at −30 °C. Orange
1
crystals were obtained in a 57% yield (63.7 mg) after filtration. H-NMR (C6D6): δ 8.3 (brs), 8.0 (brs),
19
31
7.5 (brs), 4.5 (brs), 2.0 (brs). F-NMR (C6D6): δ −63.1. P-NMR (C6D6): No signal. Anal. calcd. for
C51H45N2F18PNiBr: C 51.15%, H 3.79%, N 2.34%; Found: C 51.61%, H 4.02%, N 2.29%.

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4.2. Methods
All experiments were carried out under an inert gas atmosphere using standard Schlenk
techniques and a glove box (MBraun UniLab, München, Germany) unless otherwise noted. Column
chromatography of organic products was carried out using silica gel (Kanto Kagaku, silica gel 60 N
1
(spherical, neutral), Tokyo, Japan). The H-NMR spectra were taken with a Bruker Avance-III400 Y
plus 400 MHz spectrometer (Bruker BioSpin, Billerica, MA, USA) at room temperature. Chemical
shifts (δ) were recorded in ppm from the solvent signal. The magnetic properties of the materials
were investigated using a Quantum Design MPMS-5S superconducting quantum interference device
(SQUID) magnetometer (Quantum Design Inc., San Diego, CA, USA). The elemental analysis was
carried out with J-Science CHN Corder JM-11 (J-Science Lab Co. Ltd., Kyoto, Japan), equipped with
AUTO-SAMPLER, using tin foil, where the samples were held in a glove box. The X-band EPR
measurements were collected with a Bruker EMX Plus spectrometer (Bruker BioSpin, Billerica, MA,
USA) equipped with a continuous flow N2 cryostat.
4.2.1. Kumada–Tamao–Corriu Coupling of Aryl Bromides
In a typical example, 4-bromotoluene (136.8 μL, 0.80 mmol), triphenylphosphine (5.3 mg, 0.020
mmol), and 2a (4.1 mg, 2.0 μmol) were dissolved in THF (1 mL). After stirring for 5 min, phenyl
magnesium chloride THF solution (0.60 mL, 1.2 mmol) was added to the solution. After 18 h, water
(20 mL) was added. The organic layer was extracted with dichloromethane (20 mL × 4). The residual
product was purified with silica gel column chromatography eluting with hexane to give 4-
methoxybiphenyl (white solid; 115.6 mg, yield 86%).
4.2.2. X-Ray Crystallography
Single crystals of 2a, 2b, and 2c for X-ray diffraction were grown at −30 °C from toluene/hexane
(2a) and THF/hexane (2b and 2c) solutions. All the data were obtained at 125 (2a), 151 (2b), and 180
K (2c) using a Rigaku Saturn CCD diffractometer with a confocal mirror and graphite-
monochromated Mo Kα radiation (λ = 0.71070 Å). Data reduction of the measured reflections was
performed using the software package CrystalStructure [41]. The structures were solved by direct
2
methods (SHELXT-2014) [42] and refined by full-matrix least-squares fitting based on F , using the
program SHELXL-2014 [43]. All non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were located at ideal positions and included in the refinement but
were restricted to riding on the atom to which they were bonded. Unfortunately, the refinement for
the crystal 2b cannot be completed, and only the preliminary structure was shown in the supporting
information. CCDC 1947417–1947418 contains the supplementary crystallographic data of 2a and 2c
for this paper. A copy of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/cgi-
bin/catreq.cgi.
4.2.3. Theoretical Details
All the DFT calculations were performed utilizing the GAUSSIAN 16 Rev. A.03 program
package (Gaussian Inc., Wallingford, CT, USA) [37]. The B3LYP functional was employed with a
standard split valence-type basis set, 6-31G(d,p). The single-point calculations to obtain SOMOs were
carried out using crystallographic coordinates without geometry optimization, and done with a tight
self-consistent field (SCF) convergence criterion. All the computation was carried out using the
computer facilities at Research Institute for Information Technology, Kyushu University, Fukuoka,
Japan.
4.2.4. Electrochemistry
The cyclic voltammogram of 2a and 2c were recorded on an ALS/chi electrochemical analyzer
Model/610A with a platinum working electrode, a silver wire reference electrode, and a platinum
−1
wire counter electrode, with a scan rate of 50 mV·s . The analyte solutions of these complexes were
prepared with a 0.1 M solution of tetra-n-butyl ammonium perchlorate in acetonitrile. Ferrocene was

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used as an internal standard and the potential reported here is referenced to the ferrocene/ferricinium
couple.
5. Conclusions
In summary, the Ni(I)-IPr complex bearing meta-CF3 substituted triarylphosphine ligand was
successfully isolated. Although the “superstable” Pd(0) catalyst has been derived using palladium
and phosphine, such a triarylphosphine complex using nickel has been determined here for the first
time. In the solid state, steric bulk, appropriate electron-withdrawing properties, and the presence of
intermolecular interactions make nickel less susceptible to oxidation in air where the
triarylphosphine is coordinated. Particularly, intermolecular multiple interactions including those
between F and H, and F and F were found to be gathered in layers in the crystalline state. On the
other hand, it is possible to maintain the same catalytic activity as the analogous PPh3 complex, since
the phosphine can be easily eliminated in solution, resulting in the generation of active species. In the
future, we plan to work on the development of other useful catalytic reactions and detailed
mechanistic research using this Ni(I) complex.
Supplementary Materials: The supplementary materials are available online. Figure S1–S8: NMR spectra for the
complexes, Figure S9: χmolT vs T plot, Figure S10: ESR spectrum, Figure S11–S12: Packing views of the complexes
(expanded views), details of the Kumada coupling reactions including NMR spectra of products.
Acknowledgments: This work was financially supported by JSPS KAKENHI Grant No. 19K05489. The authors
specially thank Dr. Shinji Kanegawa, Kyushu University, for conducting SQUID measurement of the nickel(I)
complexes.
Author Contributions: T.F. and T.I. conceived and designed the experiments; R.H. performed the experiments;
Y.K. analyzed the data; Y.Y. contributed theoretical calculations; K.M. managed this research and wrote the
paper.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Tasker, S.Z.; Stanley E.A.; Jamison, T.F. Recent advances in homogeneous nickel catalysis. Nature 2014, 509,
299–309. doi:10.1038/nature13274
2.
Gu, J.; Wang, X.; Xue, W.; Gong, H. Nickel-catalyzed reductive coupling of alkyl halides with other
electrophiles: Concept and mechanistic considerations. Org. Chem. Front. 2015, 2, 1411–1421.
doi:10.1039/c5qo00224a
3.
4.
Tobisu, M.; Chatani, N. Cross-Couplings Using Aryl Ethers via C–O Bond Activation Enabled by Nickel
Catalysts. Acc. Chem. Res. 2015, 48, 1717–1726. doi:10.1021/acs.accounts.5b00051
Ritleng, V.; Henrion, M.; Chetcuti, M.J. Nickel N-Heterocyclic Carbene-Catalyzed C–Heteroatom Bond
Formation, Reduction, and Oxidation: Reactions and Mechanistic Aspects. ACS Catal. 2016, 6, 890−906.
doi:10.1021/acscatal.5b02021
5.
6.
7.
8.
9.
Zimmermann, P.; Limberg, C. Activation of Small Molecules at Nickel(I) Moieties. J. Am. Chem. Soc. 2017,
139, 4233−4242. doi:10.1021/jacs.6b12434
Inatomi, T.; Koga, Y.; Matsubara, K. Dinuclear Nickel(I) and Palladium(I) Complexes for Highly Active
Transformations of Organic Compounds. Molecules 2018, 23, 140–160. doi:10.3390/molecules23010140
Lin, C.-Y.; Power, P.P. Complexes of Ni(I): A “rare” oxidation state of growing importance. Chem. Soc. Rev.
2017, 46, 5347–5399. doi:10.1039/C7CS00216E
Schley, N.D.; Fu, G.C. Nickel-Catalyzed Negishi Arylations of Propargylic Bromides: A Mechanistic
Investigation. J. Am. Chem. Soc. 2014, 136, 16588−16593. doi:10.1021/ja508718m
Miyazaki, S.; Koga, Y.; Matsumoto, T.; Matsubara, K. A new aspect of nickel-catalyzed Grignard cross-
coupling reactions: Selective synthesis, structure, and catalytic behavior of a T-shape three-coordinate
nickel(I) chloride bearing a bulky NHC ligand. Chem. Commun. 2010, 46, 1932–1934. doi:10.1039/B924716E
10. Davies, C.J.E.; Page, M.J.C.; Ellul, E.; Mahon, M.F.; Whittlesey, M.K. Ni(I) and Ni(II) ring-expanded N-
heterocyclic carbene complexes: C–H activation, indole elimination and catalytic hydrodehalogenation.
Chem. Commun. 2010, 46, 5151–5153. doi:10.1039/C0CC01335H

Molecules 2019, 24, 3222
10 of 11
11. Zhang, K.; Conda-Sheridan, M.; Cooke, S.R.; Louie, J. N-Heterocyclic Carbene Bound Nickel(I) Complexes
and Their Roles in Catalysis. Organometallics 2011, 30, 2546–2552. doi:10.1021/om200090d
12. Klein, A.; Kaiser, A.; Sarkar, B.; Wanner, M.; Fiedler, J. The Electrochemical Behaviour of Organonickel
Complexes: Mono-, Di- and Trivalent Nickel. Eur. J. Inorg. Chem. 2007, 965–976. doi:10.1002/ejic.200600865
13. Zhang, X.; Xie, X.; Liu, Y. Nickel-catalyzed cyclization of alkyne-nitriles with organoboronic acids
involving anti-carbometalation of alkynes. Chem. Sci. 2016, 7, 5815–5820. doi:10.1039/C6SC01191H
14. Cao, Z.-C.; Xie, S.-J.; Fang, H.; Shi, Z.-J. Ni-Catalyzed Cross-Coupling of Dimethyl Aryl Amines with
Arylboronic Esters under Reductive Conditions. J. Am. Chem. Soc. 2018, 140, 13575–13579.
doi:10.1021/jacs.8b08779
15. Szatkowski, L.; Hall, M.B. Dehalogenation of chloroalkanes by nickel(I) porphyrin derivatives, a
computational study. Dalton Trans. 2016, 45, 16869–16877. doi:10.1039/C6DT02632J
16. Lim, C.-H.; Kudisch, M.; Liu, B.; Miyake, G.M. C–N Cross-Coupling via Photoexcitation of Nickel–Amine
Complexes. J. Am. Chem. Soc. 2018, 140, 7667−767. doi:10.1021/jacs.8b03744
17. Lin, X.; Phillips, D.L. Density Functional Theory Studies of Negishi Alkyl–Alkyl Cross-Coupling Reactions
Catalyzed by a Methylterpyridyl-Ni(I) Complex. J. Org. Chem. 2008, 73, 3680–3688. doi:10.1021/jo702497p
18. Phapale, V.B.; Guisán-Ceinos, M.; Buñuel, E.; Cárdenas, D.J. Ni-Catalyzed Cascade Cyclization–Kumada
Alkyl–Alkyl Cross-Coupling. Chem. Eur. J. 2009, 15, 12681–12688. doi:10.1002/chem.201300882
19. Jones, G.D.; Martin, J.L.; McFarland, C.; Allen, O.R.; Hall, R.E.; Haley, A.D.; Brandon, R.J.; Konovalova, T.;
Desrochers, P.J.; Pulay, P.; et al. Ligand redox effects in the synthesis, electronic structure, and reactivity of
an alkyl-alkyl cross-coupling catalyst. J. Am. Chem. Soc. 2006, 128, 13175–13183. doi:10.1021/ja063334i
20. Ren, H.; Li, G.-F.; Zhu, B.; Lv, X.-D.; Yao, L.-S.; Wang, X.-L.; Su, Z.-M.; Guan, W. How Does Iridium(III)
Photocatalyst Regulate Nickel(II) Catalyst in Metallaphotoredox-Catalyzed C–S Cross-Coupling?
Theoretical and Experimental Insights. ACS Catal. 2019, 9, 3858–3865. doi:10.1021/acscatal.9b00375
21. Inatomi, T.; Fukahori, Y.; Yamada, Y.; Ishikawa, R.; Kanegawa, S.; Koga, Y.; Matsubara, K. Ni(I)–Ni(III)
cycle in Buchwald–Hartwig amination of aryl bromide mediated by NHC-ligated Ni(I) complexes. Cat. Sci.
Technol. 2019, 9, 1784–1793. doi:10.1039/C8CY02427H
22. Zarate, C.; Yang, H.; Bezdek, M.J.; Hesk, D.; Chirik, P.J. Ni(I)-X Complexes Bearing a Bulky α-Diimine
Ligand: Synthesis, Structure, and Superior Catalytic Performance in the Hydrogen Isotope Exchange in
Pharmaceuticals. J. Am. Chem. Soc. 2019, 141, 5034–5044. doi:10.1021/jacs.9b00939
23. Charboneau, D.J.; Brudwig, G.W.; Hazari, N.; Lant, H.M.C.; Saydjari, A.K. Development of an Improved
System for the Carboxylation of Aryl Halides through Mechanistic Studies. ACS Catal. 2019, 9, 3228–3241.
doi:10.1021/acscatal.9b00566
24. Howell, J.A.S.; Lovatt, J.D.; McArdle, P.; Cunningham, D.; Mainome, E.; Gottlieb, H.E.; Goldschmidt, Z.
The effect of fluorine, trifluoromethyl and related substitution on the donor properties of triarylphosphines
towards [Fe(CO)4]. Inorg. Chem. Commun. 1998, 1, 118–120. doi:10.1016/S1387-7003(98)00021-5
25. Howell, J.A.S.; Fey, N.; Lovatt, J.D.; Yates, P.C.; McArdle, P.; Hursthouse, M.B.; Light, M.E. Effect of fluorine
and trifluoromethyl substitution on the donor properties and stereodynamical behaviour of
triarylphosphines. J. Chem. Soc., Dalton Trans. 1999, 17, 3015–3028. doi:10.1039/A903211H
26. Zuccaccia, D.; Belpassi, L.; Rocchigiani, L.; Tarantelli, F.; Macchioni, A. A Phosphine Gold(I) π-Alkyne
Complex: Tuning the Metal−Alkyne Bond Character and Counterion Position by the Choice of the
Ancillary Ligand. Inorg. Chem. 2010, 49, 3080–3082. doi:10.1021/ic100093n
27. Kolter, M.; Böck, K.; Karaghiosoff, K.; Koszinowski, K. Anionic Palladium(0) and Palladium(II) Ate
Complexes. Angew. Chem. Int. Ed. 2017, 56, 13244–13248. doi:10.1002/anie.201707362
28. Cutillas, N.; Martínez, A.; Yellol, G.S.; Rodríguez, V.; Zamora, A.; Pedreño, M.; Donaire, A.; Janiak, C.; Ruiz,
J. Anticancer C,N-Cycloplatinated(II) Complexes Containing Fluorinated Phosphine Ligands: Synthesis,
Structural Characterization, and Biological Activity. Inorg. Chem. 2013, 52, 13529–13535.
doi:10.1021/ic401973k
29. Jakab, A.; Dalicsek, Z.; Holczbauer, T.; Hamza, A.; Pápai, I.; Finta, Z.; Timári, G.; Soós, T. Superstable
Palladium(0) Complex as an Air- and Thermostable Catalyst for Suzuki Coupling Reactions. Eur. J. Org.
Chem. 2015, 1, 60–66. doi:10.1002/ejoc.201403214
30. Korenaga, T.; Ko, A.; Uotani, K.; Tanaka, Y.; Sakai, T. Synthesis and Application of 2,6-Bis(trifluoromethyl)-
4-pyridyl Phosphanes: The Most Electron-Poor Aryl Phosphanes with Moderate Bulkiness. Angew. Chem.
Int. Ed. 2011, 50, 10703–10707. doi:10.1002/anie.201104588

Molecules 2019, 24, 3222
11 of 11
31. Stark, M.J.; Shaw, M.J.; Fadamin, A.; Rath, N.P. Synthesis, structural characterization and catalytic activity
of indenyl complexes of ruthenium bearing fluorinated phosphine ligands. J. Organomet. Chem. 2017, 847,
41–53. doi:10.1016/j.jorganchem.2017.03.043
32. Dible, B.R.; Sigman, M.S.; Arif, A.M. Oxygen-Induced Ligand Dehydrogenation of a Planar Bis-μ-
Chloronickel(I) Dimer Featuring an NHC Ligand. Inorg. Chem. 2005, 44, 3774–3776. doi:10.1021/ic050445u
33. Nagao, S.; Matsumoto, T.; Koga, Y.; Matsubara, K. Monovalent Nickel Complex Bearing a Bulky N-
Heterocyclic Carbene Catalyzes Buchwald-Hartwig Amination of Aryl Halides under Mild Conditions.
Chem. Lett. 2011, 40, 1036–1038. doi:10.1246/cl.2011.1036
34. Matsubara, K.; Fukahori, Y.; Inatomi, T.; Tazaki, S.; Yamada, Y.; Koga, Y.; Kanegawa, S.; Nakamura, T.
Monomeric Three-Coordinate N-Heterocyclic Carbene Nickel(I) Complexes: Synthesis, Structures, and
Catalytic Applications in Cross-Coupling Reactions. Organometallics 2016, 35, 3281–3287.
doi:10.1021/acs.organomet.6b00419
35. Laskowski, C.A.; Bungum, D.J.; Baldwin, S.M.; Del Ciello, S.A.; Iluc, V.M.; Hillhouse, G.L. Synthesis and
Reactivity of Two-Coordinate Ni(I) Alkyl and Aryl Complexes. J. Am. Chem. Soc. 2013, 135, 18272–18275.
doi:10.1021/ja4095236
36. Clarke, M.L.; Ellis, D.; Mason, K.L.; Orpen, A.G.; Pringle, P.G.; Wingad, R.L.; Zaher, D.A.; Baker, R.T. The
electron-poor phosphines P{C
hydroformylation. A comparison of the coordination chemistry of P{C
6
H
3
(CF
3
)
2
-3,5}
3
and P(C
6
F
5
)
3
do not mimic phosphites as ligands for
(CF -3,5} and P(C and the
6
H3
3
)2
3
6
F5)3
unexpectedly low hydroformylation activity of their rhodium complexes. Dalton Trans. 2005, 7, 1294–1300.
doi:10.1039/B418193J
37. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone,
V.; Petersson, G.A.; Nakatsuji, H. et al., Gaussian 16 (Revision A.03). Gaussian, Inc.: Wallingford, CT, USA,
2016.
38. Hasegawa, T. Understanding of the intrinsic difference between normal- and perfluoro-alkyl compounds
toward total understanding of material properties. Chem. Phys. Lett. 2015, 627, 64–66.
doi:10.1016/j.cplett.2015.03.046
39. Pollice, R.; Chen, P. Origin of the immiscibility of alkanes and perfluoroalkanes. J. Am. Chem. Soc. 2019, 141,
3489–350. doi:10.1021/jacs.8b10745
40. Jafarpour, L.; Stevens, E.D.; Nolan, S.P. A sterically demanding nucleophilic carbene: 1,3-Bis(2,6-
diisopropylphenyl)imidazol-2-ylidene). Thermochemistry and catalytic application in olefin metathesis. J.
Organomet. Chem. 2000, 606, 49–54. doi:10.1016/S0022-328X(00)00260-6
41. Crystal Structure. Version. 4.2: Crystal Structure Analysis Package. Rigaku Co.: Tokyo, Japan, 2015; pp. 196–
8666.
42. Sheldrick, G.M. SHELXT—Integrated space-group and crystal-structure determination. Acta Cryst. 2015,
A71, 3–8. doi:10.1107/S2053273314026370
43. Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Cryst. 2015, C71, 3–8.
doi:10.1107/S2053229614024218
Sample Availability: Not available.
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