Precursors: Synthesis, Reactivity, and Catalytic Application to the Suzuki-Miyaura Reaction

Article abstract of DOI:10.1021/acs.organomet.9b00834

The complexes (Cy2PC3H6PCy2)Ni(η2-arene) (arene = toluene (3), naphthalene (4)) have been synthesized and studied in detail. Displacement reactions of the toluene ligand afford the complexes (Cy2PC3H6PCy2)Ni(η2-styrene) (5), (Cy2PC3H6PCy2)Ni(PhCN) (6), (Cy2PC3H6PCy2)Ni(CO)2 (7), (Cy2PC3H6PCy2)Ni(PPh3) (8), (Cy2PC3H6PCy2)Ni(PCy3) (9), {(Cy2PC3H6PCy2)Ni}2(μ-H)2 (10), (Cy2PC3H6PCy2)Ni(η2-CO2) (11), and [(Cy2PC3H6PCy2)Ni]2(μ,η2-1,5-COD) (12). The relative rates of ArCl oxidative addition at Ni complexes 3, 4, 6, 8, and 12 have been evaluated experimentally, and the mechanism has been calculated by DFT. Complexes 3 and 4 are efficient catalysts for the Suzuki-Miyaura reaction between chloroarenes and Ar′B(OH)2.

Full text of DOI:10.1021/acs.organomet.9b00834

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Article
[(dcpp)Ni(η2‑Arene)] Precursors: Synthesis, Reactivity, and Catalytic
Application to the Suzuki−Miyaura Reaction
Florian D’Accriscio, Alexia Ohleier, Emmanuel Nicolas, Matthieu Demange, Olivier Thillaye Du Boullay,
Nathalie Saffon-Merceron, Marie Fustier-Boutignon, Elixabete Rezabal, Gilles Frison, Noel Nebra,
́
and Nicolas Mezailles*
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ABSTRACT: The complexes (Cy2PC3H6PCy2)Ni(η2-arene) (arene = toluene (3),
naphthalene (4)) have been synthesized and studied in detail. Displacement reactions
of the toluene ligand afford the complexes (Cy2PC3H6PCy2)Ni(η2-styrene) (5),
(Cy2PC3H6PCy2)Ni(PhCN) (6), (Cy2PC3H6PCy2)Ni(CO)2 (7), (Cy2PC3H6PCy2)Ni-
(PPh3) (8), (Cy2PC3H6PCy2)Ni(PCy3) (9), {(Cy2PC3H6PCy2)Ni}2(μ-H)2 (10),
(Cy2PC3H6PCy2)Ni(η2-CO2) (11), and [(Cy2PC3H6PCy2)Ni]2(μ,η2-1,5-COD) (12).
The relative rates of ArCl oxidative addition at Ni complexes 3, 4, 6, 8, and 12 have been
evaluated experimentally, and the mechanism has been calculated by DFT. Complexes 3
and 4 are efficient catalysts for the Suzuki−Miyaura reaction between chloroarenes and
Ar′B(OH)2.
(CH2)2PtBu2)Ni(η2-benzene)] complex, for which they
obtained structural information and showed that the arene
acts as a weak two-electron donor in an η2 coordination mode,
for either one or two Ni centers.10,11 Accordingly, Ni(I)
hydride species could be obtained upon treatment of the
benzene complex with H2, as documented by Jonas and
INTRODUCTION
■
The Suzuki−Miyaura reaction (SMR) represents an undeni-
able synthetic tool to build C−C bonds for both academic and
industrial purposes1 and typically involves Pd(0)/Pd(II) redox
scenarios. While this chemistry is largely developed for
palladium complexes, the use of nickel catalysts is simulta-
neously mechanistically intriguing2 and highly desirable due to
its higher abundance, economic issues, and essential proper-
ties.3 In recent years, important improvements have been
achieved in terms of substrate scope4 and catalyst loadings.5 In
turn, several reaction pathways are plausible in dealing with
nickel catalysis and precisely designed studies are essential to
discern the operative mechanism.2,3 In this sense, the
generation of electron-rich yet unsaturated metal fragments
seems to be crucial, facilitating reactivity at a metal center and
thus entering into the catalytic loop.6 A common strategy relies
on the stabilization of the desired fragment by a weakly
coordinating ligand that is readily displaced under mild
conditions. In 1974, Jonas reported the synthesis of [(Cy2P-
(CH2)nPCy2)Ni(η2-arene)] (n = 2, 3) complexes by the
stoichiometric reduction of the halide-bridged Ni(I)
dimers.7−9 In this seminal work, the formation of these
complexes were exclusively proved by elemental analyses,8 and
it was stated that arenes were readily displaced by several two-
electron donors such as alkenes and PPh3, without further
evidence.7
Wilke,7 with the Cy2P(CH2)nPCy2 (n = 2, 3) systems, and by
11
̈
Porschke with I.
After ca. 2 decades of latency, Johnson and Jones
independently expanded the range of arenes that are able to
stabilize low-valent P2Ni0 fragments.17−20 In an elegant
approach, Johnson proved the easy replacement of the
anthracene ligand from complexes of the types III17 and
IV18 to achieve the low-temperature NMR identification of
V,19 which displays η2 coordination of the fluorinated pyridines
prior to the C−X bond activation (X = H, F). In 2002, Jones
isolated complexes VI and VII stabilized by quinolines and
acridines, which are involved in the selective cleavage of Ar−
CN bonds.20 In addition, Love has recently demonstrated the
̈
ability of Porschke’s complex I to promote original C−
heteroatom bond activations of epoxides, oxaziridines, and
(thio)esters.21
Received: December 6, 2019
Two decades later, a family of related complexes has been
isolated with different electron-rich phosphines (platforms
I,10,11 II,12,13 and III14−16 in Chart 1).10−16 The Porschke
̈
group has particularly studied the chemistry of the [(tBu2P-
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Chart 1. Representative Examples of Stable Low-Valent [P2Ni0(η2-arene)] Fragments
Over the last five decades, this chemistry was almost
exclusively developed with monodentate phosphines and/or
chelating bis-dialkylphosphines (mostly focusing on the
“ethylene”-bridged species). In light of the effect of the bite
angle on the reactivity at the metal center,22 we decided to
focus on a bidentate ligand featuring a propyl bridge between
the two phosphorus moieties, Cy2P(CH2)3PCy2 (dcpp; 1), and
report here the synthesis and full characterization by
multinuclear NMR and X-ray analysis of [(Cy2P(CH2)3PCy2)-
Ni(η2-arene)] complexes 3 and 4 (arene = toluene and
naphthalene, respectively). In this contribution, we describe
several displacement reactions of the aromatic ligand to yield
the corresponding Ni(0) complexes 5−12. Among the results
presented, we show that complexes 3 and 4 are excellent
precursors for the synthesis of rare [(dcpp)Ni(L)] species,
where L = phosphine, 1,5-cyclooctadiene (COD), acetonitrile,
CO2. We provide an experimental comparative study of several
[(dcpp)Ni(L)] complexes for the oxidative addition of 4-
chlorobenzotrifluoride at the nickel center, associated with a
DFT study of the mechanism. Finally, the use of complex 4 as
Figure 1. Molecular views of 2Cl (a), 3 (b), and 4 (c). Thermal
ellipsoids are drawn at the 50% probability level, and hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and angles (deg):
2Cl, Ni1−P1 2.192(1), Ni1−P2 2.191(1), Ni1−Cl1 2.217(1), Ni1−Cl2
2.219(1), P1−Ni1−P2 97.50(3), Cl1−Ni1−Cl2 91.12(3), P1−Ni1−Cl1
176.44(3), P2−Ni1−Cl2 175.96(3); 3, Ni1−P1 2.147(1), Ni1−P2
2.156(1), Ni1−C1 2.001(4), Ni1−C2 1.993(4), C1−C2 1.424(6),
P1−Ni1−P2 104.09(4), C2−Ni1−C1 41.77(16); 4, Ni1−P1 2.151(1),
Ni1−P2 2.163(1), Ni1−C28 2.001(2), Ni1−C29 1.980(2), C28−C29
1.435(4), C33−C34 1.378(5), P1−Ni1−P2 103.31(3), C28−Ni1−P1
107.10(8), C28−Ni1−C29 42.27(10), C29−Ni1−P2 107.45(8).
an efficient catalyst for the SMR is evidenced, and a
mechanistic investigations via stoichiometric experiments
demonstrated the viability of a Ni(0)/Ni(II) catalytic cycle
for this transformation.
RESULTS AND DISCUSSION
■
Complexes 2X (X = Cl, Br) were readily synthesized by the
stoichiometric reaction of [NiX2·dme] with 1 in THF at room
temperature. The reactions of the nickel precursors with 1 did
not present new signals in 31P NMR, suggesting the formation
of complexes 2X with tetrahedral (Td) geometries in THF.
Complexes 2X were crystallized in excellent yields (from 93 to
97%). Note that when they are dissolved in CH2Cl2, crystals of
2x are diamagnetic, while they are still paramagnetic in THF,
pointing to a solvent-induced equilibrium in solution. The
structure of 2Cl is represented in Figure 1a (see data for 2Cl in
the Supporting Information). Complexes 2X both display a
square-planar (SP) geometry in the solid state, as shown by the
∑
angles value of 360.0° for 2Cl. This feature already points to an
increased flexibility of the bidentate ligand in comparison to
the related diphosphino-ethylene type ligands, for which only
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the SP complexes are observed in solution by NMR and in the
solid state by X-ray diffraction analysis.
toluene-d8 (down to −80 °C) but the fast equilibration
process could not be frozen or even slowed down.
Crystallization of 3 was successful upon cooling a concentrated
solution of the complex in toluene. The X-ray structure is
represented in Figure 1b. One of the main pieces of
information is the confirmation of the η2 coordination of the
toluene ligand. It can be noted that the coordinating bond of
toluene is only slightly elongated (1.424(6) Å, in comparison
to about 1.39 Å for free toluene), indicating a weak donation of
the nickel to the carbon−carbon bond, thus accounting for its
poor coordination.
The 2e− reduction of complexes 2X was subsequently
studied using monoelectronic reductants such as KC8 and Na/
naphthalenide at low temperatures (Scheme 1). Note that the
reduction of complex 2Cl by 2 equiv of Na sand in toluene was
studied by Nobile.23
Scheme 1. Synthesis of Complexes 2x, Further Reduction to
[(dcpp)Ni(η2-arene)] Species 3 and 4, and Styrene
a
Coordination
An alternative entry to the “(dcpp)Ni(arene)” platform was
achieved by reacting 2Br with 2 equiv of freshly prepared Na/
naphthalenide. After dropwise addition of the reducing agent
at −80 °C and subsequent stirring at room temperature
overnight, the reaction mixture turned from green to deep
yellow, accompanied by abundant precipitation of a yellow
solid containing a mixture of NaBr and “(dcpp)Ni0” species.
31P NMR of the reaction mixture evidenced the reduction of
2
Br and displayed two broad signals resonating at 15.3 and 21.0
ppm at room temperature. Satisfyingly, these signals resolved
into two doublets appearing at 15.2 and 20.9 ppm (2JP,P = 37.2
Hz) upon cooling at −60 °C, thus manifesting the presence of
a dynamic phenomenon in solution (see the Supporting
Information).13,15,16 1H NMR of the isolated 4 displayed the
presence of free naphthalene in solution despite abundant
washing of 4 with diethyl ether. Integration of the signals
attributed to the free naphthalene vs complexed naphthalene vs
the “(dcpp)Ni0” fragment gave approximately an 1:1:2 molar
ratio, thus suggesting an equilibrium between the expected
monometallic complex 4 and the bimetallic structure 4′’ in
solution (Scheme 2). The atom connectivity in 4 was
a
Reaction conditions: (i) 2 equiv of KC8 in toluene at −40 °C and
slow warming to rt overnight; (ii) 2 equiv of Na/naphthalenide in
THF at −80 °C, and slow warming to rt overnight; (iii) 1 equiv of
naphthalene in toluene at rt; (v) 1 equiv of styrene in toluene at rt.
We found out that the reduction of 2X with KC8 leads to the
formation of two novel species characterized by two singlets
with a ca. 85/15 ratio, at 15.4 and 25.1 ppm in the 31P{1H}
NMR spectrum. Luckily, the minor species is much more
soluble than the major compound, which allowed their efficient
separation. The minor species is the dihydride dimer 11 (see
Scheme 3), as shown by the characteristic hydride shift at
Scheme 2. Equilibrium between (dcpp)Ni(naphthalene) 4
and the Bimetiallic Compound4′
1
−10.8 ppm in the H NMR spectrum. Complex 11 is likely
obtained because of the formation of small amounts of H2
when KC8 is suspended in toluene (although toluene was first
dried using a commercial purification system followed by
distillation over Na, with <10 ppm of H2O via a Karl Fisher
titration). Optimization of the yield of 3 (83% isolated yield)
can be achieved using a very concentrated medium which
favors its precipitation upon formation. After elimination of the
soluble dihydride complex 11, 3 can be extracted in toluene,
allowing elimination of both the KX salts and the graphite.
Complex 3 is reminiscent of the complex reported by Jonas,
[(dcpp)Ni(η2-benzene)], obtained in 37% yield from the
reduction of the corresponding Ni(I) precursor in benzene,
itself obtained by the comproportionation of a “(dcpp)NiII”
precursor and [(dcpp)Ni(η2-ethylene)].8 After isolation, 3 was
characterized with multinuclear NMR spectroscopy. However,
in contrast to the [{(tBu2P(CH2)2PtBu2)Ni}2(μ2,η2:η2-C6H6)]
definitively confirmed by X-ray diffraction of single crystals
obtained from a concentrated solution of 4 in toluene (see
Figure 1c). As expected, 4 crystallizes in a trigonal-planar
geometry around the Ni center (∑angles = 360.13°). The
structure shows the η2 coordination of the naphthalene ligand
through C28 and C29, displaying a significantly elongated C28−
C29 bond distance of 1.435(4) Å, and a P1−Ni1−P2 angle of
103.31(3)°, much larger than the angle observed in 2Cl
(97.50(3)°).
̈
system I studied by Porschke, for which a slow exchange of
benzene by C6D6 was observed at room temperature,10 the
To explore the relevance of complexes 3 and 4 as entries
into a SMR catalytic cycle, classical two-electron donors such
as alkenes (styrene and COD are presented here as
representative examples),24,25 CO,26−28 and phosphines were
reacted with complexes 3 and 4. Not surprisingly, and in
accord with statements by Jonas and Wilke, the 16-electron
Ni(0) complex 5 was obtained nearly quantitatively upon
addition of 1 equiv of styrene (92% yield; Scheme 1). The
complex presents the expected two doublets in the 31P{1H}
toluene ligand in 3 was rapidly exchanged by C6D6 even at 0
1
°C. Nevertheless, integration of the H signals of toluene vs
dcpp proved the coordination of one toluene per Ni fragment,
showing that the toluene does not act as a bridging ligand in
the present case. Moreover, the 31P NMR spectrum shows only
one signal at 15.4 ppm in toluene, for apparent equivalent
chemical environments of the phosphorus atoms, although the
coordination of the toluene should lead to a differentiation.
Low-temperature NMR experiments were carried out in
2
NMR spectrum (δ 28.2 and 19.1 ppm, JP,P = 25.4 Hz). The
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1
change in chemical shifts in the H NMR spectrum for the
alkene protons is significant (Δδ = −2.51 ppm for PhCHCH2,
Δδ = −3.12 and −2.79 ppm for PhCHCH2), revealing a strong
back-donation upon coordination.
Further displacement reactions were evaluated with complex
3 (Scheme 3). Mixing equimolar amounts of 3 and
Scheme 3. Reactivity of 3 with 2e− Donor Ligands
Figure 2. Molecular structures of 6 (a), 7 (b), 8 (c), and 12 (d).
Thermal ellipsoids are drawn at the 50% probability level, and
hydrogen atoms are omitted for clarity. Selected bond distances (Å)
and angles (deg): 6, Ni1−P1 2.147(1), Ni1−P2 2.169(1), Ni1−C1
1.866(2), Ni1−N1 1.919(2), N1−C1 1.223(3), P1−Ni1−P2 103.05(2),
C1−Ni1−N1 37.67(10), N1−C1−C2 137.8(2); 7, Ni1−P1 2.219(1),
Ni1−P2 2.220(1), Ni1−C1 1.768(4), Ni1−C2 1.768(4), C1−O1
1.151(5), C2O2 1.145(5), P1−Ni1−P2 101.82(4), C1−Ni1−C2
111.65(18); 8, Ni1−P1 2.145(1), Ni1−P2 2.152(1), Ni1−P3
2.113(1), P3−Ni1−P1 129.98(2), P3−Ni1−P2 123.52(2), P1−Ni1−P2
106.42(2); 12, Ni1−P1 2.143(1), Ni1−P2 2.142(1), Ni1−C1 1.990(4),
Ni1−C2 1.992(4), C1−C2 1.439(5), P2−Ni1−P1 103.79(4), C1−Ni1−
P2 107.83(11), C2−Ni1−P1 105.77(11), C1−Ni1−C2 42.36(14).
benzonitrile in toluene for 1 h at room temperature allowed
for the isolation of 6 as a pale yellow powder in 89% yield.
31P{1H} NMR characterization of 6 shows two distinct signals
at 38.1 and 19.0 ppm resonating as doublets (2JP,P = 27.9 Hz),
thus indicating a desymmetrization of the “(dcpp)Ni0”
fragment by π coordination of the benzonitrile. Single crystals
of 6 were grown at −25 °C by slow diffusion of pentane into a
concentrated solution of 6 in THF. X-ray diffraction analysis
confirmed the trigonal-planar geometry around the Ni center
for 6 with a measured P1−Ni−P2 angle of 103.05(2)° and
shows the typical η2-coordination mode of the nitrile moiety
(elongated N1−C1 bond distance of 1.223(3) Å and N1−C1−
C2 angle of 137.8(2)°; Figure 2a).
particularly difficult. In fact, it was later shown that such a
reduction forms the [Ni(PPh3)4] complex, which dissociates
significantly to [Ni(PPh3)3] and free PPh3 in solution, engaged
in rapid exchange.32 Following a different synthetic approach,
i.e. the disproportionation of the bimetallic complex
[(Et2N)3Ti(μ-PCy2)Ni(PPh3)], Stephan and co-workers were
able to characterize this complex by X-ray analysis.33 A final
example was given in 2013 by Lu and colleagues, using the
tridentate ligand N(o-(NHCH2PiPr2)C6H4)3.34 To date, no X-
ray structures for other [Ni(PR3)3] complexes are known.
Moreover, Waterman and Hillhouse have shown that the
[Ni(tBu2P(CH2)2PtBu2)(PPh3)(N2)] complex of pseudo-Td
geometry was obtained from the {[(tBu2P(CH2)3PtBu2)-
Ni)2(μ,η2-C6H6)]} complex upon addition of PPh3 under N2.35
When 1 equiv of PPh3 was added to 3 under N2, the color
changed instantaneously from yellow to deep red. Two sets of
signals were observed in the 31P NMR spectrum, a triplet at
30.5 ppm (2JP,P= 85.1 Hz) and a doublet at 11.7 ppm,
corroborating the formation of a single new species, 8. The
complex, isolated in excellent 91% yield, was therefore
postulated to be a rare example of a NiL3 complex (L =
phosphine). Further addition of 1 equiv of PPh3 did not lead to
the formation of an 18-electron complex, as shown by the
presence of a sharp signal for free PPh3 in the 31P spectrum.
The ML3 trigonal-planar geometry was definitely proved by an
X-ray diffraction study on monocrystals obtained upon cooling
a concentrated solution of 8 (see structure in Figure 2c). The
sum of the angles of 359.91° was measured around the Ni
center. The geometry is slightly distorted, as the P3−Ni1−P1
and P3−Ni1−P2 angles are different (129.98(2) and
123.52(2)°, respectively). The increase in the bite angle
The 18-electron complex 7 was obtained after bubbling CO
into a solution of complex 3 at room temperature. This
complex was characterized by NMR spectroscopy showing one
triplet at 204.4 ppm for the CO moiety in the 13C NMR
2
spectrum, with JP,C = 2.3 Hz. The IR spectrum showed two
bands at 1918 and 1980 cm−1, as expected for the presence of
two CO groups in a Td environment. No loss of CO from this
complex could be observed even under prolonged vacuum.
Single crystals were grown from a well-concentrated THF
solution of 7 at −25 °C (see Figure 2b). X-ray diffraction
analysis proved the atom connectivity and the distorted Td
environment around the Ni center with slightly elongated C−
O bond distances of 1.151(5) and 1.145(5) Å in comparison
to free carbon monoxide (1.128 Å).29
The coordination of phosphines was more interesting, as
examples of ML3 complexes featuring three phosphines are
very rare and usually unstable.30 The complex [Ni(PPh3)3] has
been synthesized by Tolman and co-workers by the Zn
reduction of NiCl2 in the presence of PPh3.31 However, it was
described as being very unstable, and its characterization was
D
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provided by the propyl bridge is shown by the P1−Ni1−P2
angle of 106.42(2)°, which is much larger than that in the
related dtbpe complex reported by Waterman and Hillhouse
(93.62(2)°).35 This increase in the bite angle is therefore the
determining factor for the observation of the NiL3 geometry,
rather than the [NiL3(N2)] complex. A similar reaction
conducted with PCy3 also led to the formation of a single
complex 9 featuring the expected triplet and doublet patterns
recent years in the Ni-catalyzed transformations of CO2.44−48
However, only two examples of [(R3P)2Ni(η2-CO2)] com-
plexes have been structurally characterized to date, one with
PCy3 as ligand, known as Aresta’s complex,49 and the other
with P(iPr)3 as a ligand, reported by Johnson and co-
workers.50 Additionally, the X-ray structure of the [(tBu2P-
(CH2)2PtBu2)Ni(CO2)] complex was reported by Hillhouse
and co-workers.51 Here, CO2 was bubbled into a solution of 3,
resulting in a slight change in color from dark yellow to yellow.
Evaporation to dryness resulted in the isolation of 10 in an
excellent 93% yield, proving the strong coordination of the
CO2 moiety. The complex was characterized in 31P NMR by
two doublets at 43.5 and 7.6 ppm (2JP,P = 28.0 Hz), indicating
a significantly different chemical environment. The CO2
coordination was confirmed in the 13C NMR spectrum, in
which a signal was observed at 193.7 ppm. An IR spectrum was
performed on 10 in the solid state, giving a band at 1731 cm−1,
identical with the IR spectrum obtained by Nobile and co-
workers (who previously characterized this complex by EA and
IR).52 It is interesting to note that the reaction of
[(iPr2P(CH2)2PiPr2)Ni(μ-H)]2 or [(tBu2P(CH2)2PtBu2)Ni-
(μ-H)]2 with CO2 resulted in the reduction of CO2 into CO
with the concomitant oxidation of the phosphine moiety and
thus the formation of multiple Ni(0) complexes.48,53 It
therefore appears that the wider bite angle provided by the
propylene bridge between the phosphine moieties, in
comparison to the ethylene bridge, has a significant effect on
the stability of the CO2 complex.
2
(at 38.3 and at 6.0 ppm respectively, with JP,P = 85.1 Hz),
pointing to a similar trigonal-planar geometry. This complex,
featuring only alkylphosphine ligands, proved much more
soluble than complex 8 and was reluctant to crystallization.
Competitive coordination experiments between PPh3 and
PCy3 were conducted. Mixing 3 with 1 equiv of each
phosphine resulted in the appearance of the signal of complex
8 in the 31P{1H} NMR spectrum together with free PCy3.
Moreover, PCy3 from complex 9 was quantitatively displaced
by PPh3 to form 8 in toluene.
From another standpoint, a reactivity similar to that of
“(diphosphine)Ni” 14e− fragments may in some instances be
observed from the bimetallic Ni(I) bridging dihydride
complexes. These complexes were shown to eliminate H2
upon addition of a better two-electron donor.36−41 In turn,
these complexes have been obtained by three different routes
starting typically from the readily available Ni(II) precursors
(routes b and c in Scheme 4) and in one instance by the
reaction of H2 with the Ni(0) benzene complex (Scheme 4,
route a).11
The arene ligand in Ni(0) complexes 3 and 4 is obviously
readily displaced under mild conditions. The easy access to
several Ni(0) complexes allowed us to evaluate their efficiency
in the oxidative addition elementary step. We selected p-CF3-
PhCl as substrate due to its electron-poor character, along with
the very convenient presence of a CF3 group allowing for easy
19F NMR reaction monitoring. Its reaction with complex 3
resulted in the fast formation (15 min at room temperature) of
a single new species, the complex [(dcpp)NiCl(p-CF3-C6H4)]
(13) isolated as a yellow powder in ca. 90% yield. It is
characterized by two doublets at 19.3 and 5.9 ppm (2JP,P = 53.0
Hz) in 31P NMR and by a singlet appearing at −62.4 ppm in
19F NMR. In addition to this very diagnostic NMR pattern, the
structure of 13 was definitively confirmed by X-ray diffraction
of single crystals that were obtained by cooling at −30 °C a
concentrated solution of 13 in a THF/Et2O mixture of
solvents. Complex 13 crystallizes in a square-planar geometry
around the Ni center (see Figure 3). As may be expected, the
Ni1−P1 bond distance of 2.255(1) Å in 13 (trans to the
aromatic ring) is much longer than the Ni1−P2 bond distance
(2.152(1) Å; trans to chloride). A moderate P1−Ni−P2 bite
angle of 97.81(2)° (vs 104.09(4), 103.31(3), and 106.42(2)°
in 3, 4, and 8, respectively) is measured that once again
illustrates the flexibility of the dcpp ligand.
With the product complex 13 fully characterized, oxidative
addition was performed with other Ni(0) complexes: 4, 6, 8,
and 12. The relative rates of the oxidation reaction were
compared at 60 °C in THF (Table 1). The η2-arene Ni(0)
complexes 3 and 4 produced the Ni(II) species 13
quantitatively within 5 min (time to record the first NMR
spectrum). Complex 6 bearing the benzonitrile ligand required
a longer time in comparison to complex 8, bearing PPh3, to
reach full conversion into 13 (entry 3 vs entry 4). Finally,
oxidative addition from the isolated complex 12 was very slow
(9 days), most likely because of the extremely poor solubility
Scheme 4. Synthesis of Bridging Dihydride Complexes
Complex 11 was previously synthesized by the reduction
route c (Scheme 4) by Jonas and Wilke7 and mentioned to be
synthesized by route a by Jonas starting from [(dcpp)Ni(η2-
benzene)].8 Accordingly, we obtained it quantitatively upon
bubbling H2 into a solution of complex 3 in toluene for a few
minutes at room temperature. A visible color change from
yellow to red was indicative of the fast reaction, which was
followed by 31P NMR, showing only the appearance of the
signal at 25.1 ppm for complex 11.23 Proof on the lability of H2
in complex 11 was provided by the clean formation of 12 upon
addition of 1,5-cyclooctadiene (COD) under mild conditions.
Alternatively, complex 12 can be prepared in excellent yield
from 1 and [Ni(COD)2] in THF (see the Supporting
Information) or from 3 by adding 0.5 equiv of COD (Scheme
3). Suitable crystals for X-ray diffraction analysis were obtained
by slow diffusion of diethyl ether into a concentrated solution
of 12 in THF at −25 °C and proved the dimeric structure
through double η2 coordination of the olefins to the
“(dcpp)Ni0” fragment (Figure 2d).
Finally, the coordination of CO2 was studied. In fact, CO2
has been shown not only to coordinate some Ni(0) fragments
but also to be coupled with alkynes in the coordination sphere
of the metal.42,43 Significant results have been obtained in
E
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oxidative addition and in turn will influence the overall
catalytic cycle.
In line with our interest in Ni-mediated cross-coupling
events, along with their associated mechanisms,54 we
wondered whether these kinetics only depend on the strength
of the Ni−L bond. In order to answer this question, the
mechanism of the oxidative addition was evaluated by DFT
calculations. Two mechanisms are to be envisaged for the
oxidative addition of ArCl starting from a 16-electron complex
[(dcpp)Ni(L)], as it implies the displacement of ligand L:
dissociative vs associative (Table 2). If the dissociative
mechanism occurs, the same 14-electron intermediate
[(dcpp)Ni] will be formed from any complex, and the kinetics
only depends on the strength of the Ni−L bond. On the other
hand, in the associative mechanism, the kinetics depends on
the energy of the 18-electron transition state [(dcpp)Ni(L)(p-
CF3-PhCl], in which ArCl substitutes the L ligand. We thus
computed the two processes starting from complexes 3, 4, 6,
and 8 and [(dcpp)Ni(C2H4)] as a model for complex 12
(Table 2).
Figure 3. View of complex 13. Thermal ellipsoids are drawn at the
50% probability level. For clarity reasons, the hydrogen and
disordered atoms have not been displayed. Selected bond distances
(Å) and angles (deg): Ni1−P1 2.255(1), Ni1−P2 2.152(1), Ni1−C28
1.922(2), Ni1−Cl1 2.219(1), P1−Ni1−P2 97.81(2), C28−Ni1−P2
89.19(5), C28−Ni1−Cl1 86.43(5), Cl1−Ni1−P1 87.52(2).
Table 1. Comparative Study of the Oxidative Addition of 4-
a
Chlorobenzotrifluoride to Complexes 3, 4, 6, 8, and 12
It is interesting to note that the bond strength between Ni
and toluene is by far the weakest (ΔG = 16.2 kcal/mol vs
27.8−36.2 kcal/mol for the other ligands). In particular, it is
striking that naphthalene is ca. 11 kcal/mol more strongly
bound than toluene and is closer to the binding of ethylene
(27.8 vs 32.9). Importantly, the relative reactivities of the PPh3
complex 8 and PhCN complex 6 cannot be rationalized by
only considering the Ni−L bond strength. Indeed, the Ni−
PPh3 strength is higher than that of Ni−PhCN (36.2 vs 29.5
kcal/mol), despite leading to a faster reaction. On the other
hand, it is consistent with the energy barrier to reach the 18-
electron complexes, which is 9 kcal/mol lower for [(dcpp)-
Ni(PPh3)(p-CF3-PhCl)] (TS-X) in comparison to [(dcpp)-
Ni(PhCN)(p-CF3-PhCl)]. The highest energy barrier is found
with the best acceptor of the series C2H4 (G = 32.9 kcal/mol).
Overall, the calculations nicely reproduce the trends in the
kinetics and relative rates observed for the oxidative addition.
Moreover, they provide additional insight, as an associative
mechanism for the oxidative addition with complexes 3, 4, and
entry
complex
L ligand
toluene
naphthalene
PhCN
PPh3
COD
t required to reach completion
1
2
3
4
5
3
4
6
8
<5 min
<5 min
60 min
15 min
9 days
12
a
All reaction mixtures were heated at 60 °C in a J. Young NMR tube
under a nitrogen atmosphere starting from 19.2 μmol of the
“(dcpp)Ni0” complex, 4-chlorobenzotrifluoride (1 equiv; 19.2
μmol), and 0.6 mL of THF. Yields were determined by 19F or 31P
NMR.
of the complex, even at 60 °C. It is therefore obvious that the
nature of the ligand stabilizing the 14-electron fragment
[(dcpp)Ni] has a very significant effect on the kinetics of the
Table 2. DFT Evaluation of Ni−L Bond Strength and Energies of Oxidative Addition (kcal/mol) in 4-Chlorobenzotrifluoride
a
complex
L
Ni−L bond strength G (dissociative)
energy barrier G (associative)
3
4
6
8
toluene
naphthalene
PhCN
PPh3
C2H4
16.2
27.8
29.5
36.2
32.9
12.6
19.3
32.6
23.1
34.0
b
model of 12
a
b
Calculations at the ωB97X-D/def2-TZVPP//ωB97X-D/6-31G(d)-def2-TZVP level (see the Supporting Information for details). Based on a
potential energy surface scan of the reaction. No TS could be located at the ωB97X-D/6-31G(d)-def2-TZVP level.
F
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Table 3. Optimization of the Reaction Conditions for the “(dcpp)Ni0”-Catalyzed Suzuki−Miyaura Reaction
a f
,
entry
R1
R2
cat.
solvent
THF
THF
THF
THF
base (equiv)
additive (equiv)
T (°C)
t (h)
yield (%)
b
1
2
3
4
5
6
7
8
Me (14a)
Me (14a)
Me (14a)
Me (14a)
Me (14a)
Me (14a)
CF3 (14e)
CF3 (14e)
Me (14a)
CF3 (14e)
CF3 (14e)
CF3 (14e)
CF3 (14e)
H (15a)
H (15a)
H (15a)
H (15a)
H (15a)
H (15a)
F (15h)
F (15h)
H (15a)
F (15h)
F (15h)
F (15h)
F (15h)
3
3
3
3
3
3
4
4
4
K2CO3 (3.0)
K2CO3 (3.0)
Cs2CO3 (3.0)
tBuOK (3.0)
K3PO4 (3.0)
K3PO4 (1.2)
K3PO4 (3.0)
K3PO4 (3.0)
K3PO4 (3.0)
K3PO4 (3.0)
K3PO4 (3.0)
K3PO4 (3.0)
KOH (1.5)
none
none
none
none
none
none
none
H2O (3.0)
H2O (3.0)
H2O (3.0)
H2O (3.0)
H2O (18.0)
H2O (1.5)
25
60
60
60
60
60
80
80
80
80
60
60
80
39.0
87.0
24.0
24.0
24.0
24.0
4.5
4.5
16.0
4.5
22
b
73
b
5
b
11
b
THF
THF
>99
b
41
c
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
82
c
>99
c
9
>99
d
c
10
11
12
13
4
>99
d
c
4
e
2.0
2.0
4.5
80
c
4
60
c
4
>99
a
b
Reactions performed in Schlenk flasks under a nitrogen atmosphere using 3 mol % of catalyst loading if not otherwise specified. Yields
c
determined by GC-MS analysis using n-decane as an internal standard. Yields determined by 19F NMR using 1-fluoroheptane as an internal
standard. Catalyst loading: 1 mol %. Catalyst loading: 2 mol %. 1.2 equiv of the boronic acid was used in THF (entries 1−6) vs 2.0 equiv in 1,4-
dioxane (entries 7−13).
d
e
f
8 (13−23 kcal/mol) is preferred by the difference with a more
energetically demanding dissociative mechanism with com-
plexes 6 and [(dcpp)Ni(C2H4)] (>29 kcal/mol).
be achieved as efficiently using 1.5 equiv of KOH and 1.5 equiv
of distilled H2O (entry 13 vs entry 8).
The scope of the SMR was then studied under the optimized
conditions: i.e., 3 mol % of catalyst 4, 1:2:3 ratio of
ArCl:Ar′B(OH)2:K3PO4 at 80 °C overnight in the presence
of 3 equiv of water (Table 4). First, 4-tolyl chloride (14a) was
used as a model substrate and catalyst 4 proved efficient for
both electron-rich (entries 1−5) and electron-poor (entries 6−
8) arylboronic acids 15a−h to produce the corresponding
biaryls 16aa−ah in excellent yields (>90%). More precisely,
while electron-rich arylboronic acids 15a−e led to full
conversion into biaryls 16aa−ae, the introduction of
electron-withdrawing substituents in the aromatic ring resulted
in slightly lower yields of biaryls 16af−ah. This phenomenon
may be due to the lower nucleophilic character of fluorinated
arylboronic acids and their higher propensity to undergo
protodeborylation reactions.56,57
Motivated by the recent interest in organofluorine
compounds in industry,58 along with the important challenge
of developing the SMR involving highly fluorinated arylboronic
acids,59 our efforts were focused toward the coupling of a
family of chloroarenes 14a−e and arylboronic acids 15f,g
bearing F or CF3 groups in the arene (see Table 4, entries 6−
16). Satisfyingly, 4 catalyzed the synthesis of biaryls 16af−ah,
16bg−eg and 16bh−eh in nearly quantitative yields
(according to 19F NMR analysis). The compounds were
isolated in moderate to excellent yields (26−96%) after
purification by flash chromatography. Finally, the scope of the
transformation is not influenced by the electronic character of
the substituents of the chloroaromatic and covers several
functional groups such as ethers (entries 9 and 13), ketones
(entries 10 and 14), and esters (entries 11 and 15).
These calculations clearly point out that both complexes 3
and 4 should be the most efficient precatalysts of the series for
coupling processes for which oxidative additions are difficult.
Accordingly, we have recently reported on the excellent
catalytic activity of complex [(dcpp)Ni(η2-toluene)] (3) in the
Negishi type cross-coupling of aryl chlorides and ArZnCl
salts.54b Stoichiometric reactions and DFT calculations allowed
for elucidation of the mechanism that involves a Ni(0)/Ni(II)
redox scenario. We logically wondered whether this mecha-
nism is still operative when different transmetalating agents are
used such as arylboronic acids, involved in the Suzuki−
Miyaura reaction (SMR).
First of all, the reaction conditions were optimized (see
Table 3). With 3 mol % of the precatalyst, under nearly
stoichiometric conditions (1:1.2 ArCl:Ar′B(OH)2), the
reaction requires mild heating to achieve decent yield in a
rather long time (entry 2) using K2CO3 (3 equiv) as the base.
K3PO4 emerged as a much more efficient base (entry 5 vs
entries 2−4). Most remarkably, during this optimization
process the critical role of the residual amounts of water
contained in the base was evidenced. In fact, the use of
extensively dried K3PO4 in 1,4-dioxane only produced 82% of
coupled product 16eh (entry 7), whereas a quantitative yield
of 16eh was reached upon addition of 3 equiv of distilled water
(K3PO4:H2O ratio equal to 1:1, entry 8). It even allowed
reducing the catalyst charge to 1 mol % in the favorable
coupling of 14e and 15h (entry 10 vs entry 8). However, an
increase in the amount of added water had a negative effect on
the reaction (compare entries 11 and 12). These observations
are consistent with a recent report by Grimaud and co-workers,
who showed the need for residual H2O and a base in order to
gradually generate OH− anions and the transmetalating [Ar-
B(OH)3]− species.55 Finally, the coupling of 14e and 15h can
In order to verify the plausibility of a Ni(0)/Ni(II) redox
scenario during the 4-catalyzed SMR, stoichiometric experi-
ments were performed. As shown above, complex 4 cleanly
reacts with 1 equiv of 4-chlorobenzotrifluoride (14e) in THF
G
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Table 4. Scope of the Catalytic SMR
CONCLUSION
■
In conclusion, we have presented here an improved synthesis
of complex 3 and the novel complex 4, [(Cy2P(CH2)3PCy2)-
Ni(η2-arene)] (with arene = toluene and naphthalene,
respectively). The arene ligands in these complexes are readily
displaced by stronger two-electron donors. Displacement
reactions with styrene, PhCN, CO, H2, and COD (complexes
5, 6, 7, 11, and 12) were conducted with complex 3. Moreover,
rare examples of NiL3 of the form [(diphosphine)Ni(PR3)]
were also isolated by this method (complexes 8 and 9). In
addition, a very rare example of a [(diphosphine)Ni(CO2)]
complex featuring the weak ligand CO2 has also been obtained
efficiently. The capability of complexes 3 and 4 to generate
under very mild conditions 1 equiv of the electron-rich, 14-
electron “(dcpp)Ni” fragment has been shown experimentally,
by the oxidative addition of ArCl to Ni. Complexes 3 and 4
were therefore tested as catalysts in the coupling of
chloroarenes with arylboronic acids (Suzuki−Miyaura reac-
tion). We evidenced the critical effect of controlled amounts of
water in combination with the base in order to facilitate the
transmetalation step. The optimization of the reaction
conditions allowed for the preparation of a broad family of
biaryls in excellent yields, including challenging biaryls bearing
perfluorinated groups (F and CF3). Preliminary stoichiometric
reactions suggest the involvement of a Ni(0)/Ni(II) pathway
for the SMR. Studies of the catalytic applications of complexes
3 and 4 in other cross-coupling processes are currently
undergoing in our laboratories and will be reported in due
course.
a
b−d
entry
R1
R2
yield (%)
b
1
2
3
4
5
6
7
8
Me (14a)
Me (14a)
Me (14a)
Me (14a)
Me (14a)
Me (14a)
Me (14a)
Me (14a)
OMe (14b)
Ac (14c)
CO2Me (14d)
CF3 (14e)
OMe (14b)
Ac (14c)
CO2Me (14d)
CF3 (14e)
H (15a)
>99 (90)
>99 (86)
>99 (81)
>99 (78)
b
1-Np (15b)
2-Np (15c)
4-Me (15d)
4-OMe (15e)
4-CF3 (15f)
3,5-CF3 (15g)
4-F (15h)
3,5-CF3 (15g)
3,5-CF3 (15g)
3,5-CF3 (15g)
3,5-CF3 (15g)
4-F (15h)
b
b
b
>99 (90)
>99 (95)
b
e
e
90 (68)
95 (69)
95 (26)
90 (73)
95 (66)
95 (41)
90 (72)
95 (45)
e
e
9
e
10
11
12
13
14
15
16
e
e
e
e
4-F (15h)
4-F (15h)
4-F (15h)
e
95 (96)
95 (78)
e
a
Reactions were performed in Schlenk flasks under a nitrogen
atmosphere starting from the appropriate aryl chloride (14a−e; 0.5
mmol), the corresponding arylboronic acid (15a−h; 1 mmol), dried
K3PO4 (1.5 mmol), distilled water (1.5 mmol), catalyst 4 (3 mol %),
b
and 1 mL of dry 1,4-dioxane. Yields determined by GC-MS analysis
c
using n-decane as an internal standard. Yields determined by 19F
d
NMR using 1-fluoroheptane as an internal standard. Isolated yields
are provided in parentheses (average of two independent runs). 16af
ASSOCIATED CONTENT
* Supporting Information
e
■
sı
was partially contaminated with the 4,4′-bis(trifluoromethyl)biphenyl
homocoupling product.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00834.
at room temperature within 2 h to form complex 13 (Scheme
5). In agreement with Grimaud’s findings,55 complex 13
General procedures, materials and methods, synthesis
and characterization of complexes, Suzuki−Miyaura
cross-coupling reactions, comparative study for the
oxidative addition at Ni(0) complexes, mechanistic
study of stoichiometric reactions, NMR spectral data,
X-ray structural determinations, and DFT calculations
(PDF)
Scheme 5. Stoichiometric Mechanistic Investigations:
Evidence for a Ni(0)/Ni(II) Pathway
a
Cartesian coordinates of the calculated structures (XYZ)
Accession Codes
CCDC 1960209−1960216 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
a
Reaction conditions: (i) 1 equiv of 4-chlorobenzotrifluoride (14e)
and stirring 2 h in THF at rt; (ii) 1 equiv of 4-fluorophenylboronic
acid (15h), 1.5 equiv of K3PO4 + 1.5 equiv of H2O, and stirring 2 h in
THF at rt.
AUTHOR INFORMATION
Corresponding Author
remained unreacted in the presence of K3PO4 (1.5 equiv) +
H2O (1.5 equiv), thus ruling out the plausible formation of a
coupling-competent ArNiIIOH key intermediate. The coupling
reaction thus seems to involve the formation of the
corresponding borate [Ar-B(OH)3]−. Accordingly, instanta-
neous creation of coupled product 16eh was achieved upon
addition of 4-fluorophenylboronic acid (15h, 1 equiv) to the
reaction mixture via fast transmetalation and subsequent
reductive elimination. These observations provide some
support for an operative catalytic loop involving Ni(0) and
Ni(II) intermediates.
■
́
́
́
Nicolas Mezailles − Laboratoire Heterochimie Fondamentale et
́
́
Appliquee, Universite Paul Sabatier, CNRS, 31062 Toulouse,
France; orcid.org/0000-0001-8735-6285;
Email: mezailles@chimie.ups-tlse.fr
Authors
́
́
Florian D’Accriscio − Laboratoire Heterochimie Fondamentale
́
́
et Appliquee, Universite Paul Sabatier, CNRS, 31062 Toulouse,
France
H
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Alexia Ohleier − Laboratoire Heterochimie Fondamentale et
́
́
Appliquee, Universite Paul Sabatier, CNRS, 31062 Toulouse,
France
́
Emmanuel Nicolas − Laboratoire Chimie Moleculaire, Ecole
Polytechnique, CNRS, 91128 Palaiseau Cedex, France;
orcid.org/0000-0002-0017-5500
Matthieu Demange − Laboratoire Chimie Moleculaire, Ecole
Polytechnique, CNRS, 91128 Palaiseau Cedex, France
Olivier Thillaye Du Boullay − Laboratoire Heterochimie
Fondamentale et Appliquee, Universite Paul Sabatier, CNRS,
31062 Toulouse, France
Nathalie Saffon-Merceron − Institut de Chimie de Toulouse
ICT-FR2599, Universite Paul Sabatier, CNRS, 31062
́
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́
́
́
́
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Toulouse Cedex, France
́
́
Marie Fustier-Boutignon − Laboratoire Heterochimie
Fondamentale et Appliquee, Universite Paul Sabatier, CNRS,
31062 Toulouse, France
Elixabete Rezabal − Laboratoire Chimie Moleculaire, Ecole
Polytechnique, CNRS, 91128 Palaiseau Cedex, France;
orcid.org/0000-0003-0397-6140
Gilles Frison − Laboratoire Chimie Moleculaire, Ecole
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́
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́
́
Noel Nebra − Laboratoire Heterochimie Fondamentale et
́
́
Appliquee, Universite Paul Sabatier, CNRS, 31062 Toulouse,
France
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.9b00834
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
The authors are grateful to the CNRS and Universite de
Toulouse for general financial support, the ANR program
(ANR-12-BS07-0016-01; fellowship to F.D.), and the Region
Midi-Pyrenees (fellowship to A.O.). E.N. and M.D. acknowl-
edge the Ecole Polytechnique and CNANO Ile de France for
their respective Ph.D. grants. We also thank CalMip (CNRS,
Toulouse, France) for access to calculation facilities. This
article is dedicated to the memory of Prof. P. Le Floch.
́
́
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https://dx.doi.org/10.1021/acs.organomet.9b00834
Organometallics XXXX, XXX, XXX−XXX