Catalysis of Cross-Coupling and Homocoupling Reactions of Aryl Halides Utilizing Ni(0), Ni(I), and Ni(II) Precursors; Ni(0) Compounds as the Probable Catalytic Species but Ni(I) Compounds as Intermediates and Products

Article abstract of DOI:10.1021/acs.organomet.7b00446

Both Ni(0) and Ni(I) compounds are believed to exhibit cross-coupling catalytic properties under various conditions, and the compounds Ni(PPh₃)₄ and NiCl(PPh₃)₃ are compared as catalysts for representative Suzuki-Miyaura and Heck-Mizoroki cross-coupling reactions. The Ni(0) compound exhibits catalytic activities, for cross-coupling of chloro and bromoanisole with phenylboronic acid and of bromobenzene with styrene, yielding results which are comparable with those of many palladium-based catalysts, but our findings with NiCl(PPh₃)₃ are at this point unclear. It seems to convert to catalytically active Ni(0) species under Suzuki-Miyaura reaction conditions and is ineffective for Heck-Mizoroki cross-coupling. The paramagnetic Ni(I) compounds NiX(PPh₃)₃ (X = Cl, Br, I) are characterized for the first time by ¹H NMR spectroscopy and are found to exhibit broad meta and para resonances at δ 9-11 and 3-4, respectively, and very broad ortho resonances at δ 46; these resonances are very useful for detecting Ni(I) species in solution. The chemical shifts of NiCl(PPh₃)₃ vary with the concentration of free PPh₃, with which it exchanges, and are also temperature-dependent, consistent with Curie law behavior. The compound trans-NiPhCl(PPh₃)₂, the product of oxidative addition of chlorobenzene to Ni(PPh₃)₄ and a putative intermediate in cross-coupling reactions of chlorobenzene, is found during the course of this investigation to exhibit entirely unanticipated thermal lability in solution in the absence of free PPh₃. It readily decomposes to biphenyl and NiCl(PPh₃)₂ in a reaction relevant to the long-known but little-understood nickel-catalyzed conversion of aryl halides to biaryls. Ni(I) and biphenyl formation is initiated by PPh₃ dissociation from trans-NiPhCl(PPh₃)₂ and formation of a dinuclear intermediate, a process which is now better defined using DFT methodologies.

Full text of DOI:10.1021/acs.organomet.7b00446

Article
pubs.acs.org/Organometallics
Catalysis of Cross-Coupling and Homocoupling Reactions of Aryl
Halides Utilizing Ni(0), Ni(I), and Ni(II) Precursors; Ni(0) Compounds
as the Probable Catalytic Species but Ni(I) Compounds as
Intermediates and Products
Adeela Manzoor, Patrick Wienefeld, and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Peter H. M. Budzelaar*
Department of Chemical Sciences, Federico II University of Naples, Napoli 80126, Italy
S
* Supporting Information
ABSTRACT: Both Ni(0) and Ni(I) compounds are believed to exhibit cross-coupling catalytic properties under various
conditions, and the compounds Ni(PPh3)4 and NiCl(PPh3)3 are compared as catalysts for representative Suzuki−Miyaura and
Heck−Mizoroki cross-coupling reactions. The Ni(0) compound exhibits catalytic activities, for cross-coupling of chloro and
bromoanisole with phenylboronic acid and of bromobenzene with styrene, yielding results which are comparable with those of
many palladium-based catalysts, but our findings with NiCl(PPh3)3 are at this point unclear. It seems to convert to catalytically
active Ni(0) species under Suzuki−Miyaura reaction conditions and is ineffective for Heck−Mizoroki cross-coupling. The
paramagnetic Ni(I) compounds NiX(PPh3)3 (X = Cl, Br, I) are characterized for the first time by 1H NMR spectroscopy and are
found to exhibit broad meta and para resonances at δ 9−11 and 3−4, respectively, and very broad ortho resonances at δ 4−6;
these resonances are very useful for detecting Ni(I) species in solution. The chemical shifts of NiCl(PPh3)3 vary with the
concentration of free PPh3, with which it exchanges, and are also temperature-dependent, consistent with Curie law behavior.
The compound trans-NiPhCl(PPh3)2, the product of oxidative addition of chlorobenzene to Ni(PPh3)4 and a putative
intermediate in cross-coupling reactions of chlorobenzene, is found during the course of this investigation to exhibit entirely
unanticipated thermal lability in solution in the absence of free PPh3. It readily decomposes to biphenyl and NiCl(PPh3)2 in a
reaction relevant to the long-known but little-understood nickel-catalyzed conversion of aryl halides to biaryls. Ni(I) and
biphenyl formation is initiated by PPh3 dissociation from trans-NiPhCl(PPh3)2 and formation of a dinuclear intermediate, a
process which is now better defined using DFT methodologies.
While very successful for aryl and vinyl halides, this type of
cross-coupling turns out to be much less useful for alkyl halides
containing β-hydrogen atoms; although, there have been
reported a few examples of successful cross-coupling of primary
alkyl halides and tosylates with e.g. organoboron reagents.2
Although lagging considerably behind the development of
Pd-based catalysts, a number of Ni-based catalysts have also
been investigated and found to catalyze rapid cross-coupling of
ross-coupling reactions involving aromatic halides and
Cmain group metal-based nucleophiles are currently
utilized to form a wide variety of carbon−carbon and
carbon−heteroatom bond types.1 By and large, a significant
proportion of these reactions are catalyzed by Pd(0)
compounds of the type PdL2 (L = phosphine ligand) via
catalytic cycles which typically involve two-electron processes in
which the metal alternates between the 0 and the II oxidation
states as indicated in Scheme 1 (M = Pd) for a generic Suzuki−
Miyaura cross-coupling reaction.
Received: June 13, 2017
© XXXX American Chemical Society
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in the formation of Ni(I) compounds,7 which may or may not
be involved in a catalytic cycle.
Scheme 1. Two-Electron Catalytic Cycle for Suzuki−
Miyaura Cross-Coupling Processes
This paper describes the early stages of an investigation into
the comparative use of Ni(0) and Ni(I) phosphine complexes
as catalysts for representative cross-coupling catalytic processes
of alkyl and aryl halides. In an attempt to address these issues,
we have begun to map out potentially relevant catalytic Ni(0)
8
and Ni(I) chemistry utilizing the Ni(0) compound Ni(PPh3)4
and the Ni(I) compound NiCl(PPh3)3.7a,9 While Suzuki−
Miyaura cross-coupling reactions of aryl electrophiles have been
catalyzed by a number of Ni-based catalyst systems,6b−q,x,y,7d−f
including some containing PPh3,6e,g,i,j,l−p preformed Ni(PPh3)4
has in fact rarely been utilized6n,o and thus the capabilities and
potential advantages of this readily available catalyst are largely
unknown. To our knowledge, very little is known also about the
catalytic chemistry of NiCl(PPh3)3, although Ni(I) complexes
of 1,1′-bis(diphenylphosphino)ferrocene (dppf) are receiving
considerable attention.7d−f
We have complemented our catalytic research by using NMR
spectroscopy to characterize the paramagnetic Ni(I) com-
pounds NiX(PPh3)3 (X = Cl, Br, I) and their very facile
exchange processes with free PPh3. To gain further mechanistic
information, we have also carried out NMR studies of the
oxidative-addition reactions of Ni(PPh3)4 with chloro and
bromobenzene, reactions which give the Ni(II) products trans-
NiPhX(PPh3)2 (X = Cl, Br).6k,n,p,s,10 Literature reports on the
thermal stabilities of these compounds differ substantial-
ly,6o,s,10b,e,h and we now show that trans-NiPhCl(PPh3)2 is
very stable in solution at room temperature in the presence of
free PPh3, but in the absence of added ligand, it decomposes
smoothly and rapidly to biphenyl and the Ni(I) compound
NiCl(PPh3)2. The latter finding has major mechanistic
implications for Ni-catalyzed conversion of aryl halides to
biaryls.8a,10d−f
alkyl halides.3 Interestingly, cross-coupling reactions of
secondary alkyl halides have also been accomplished,4 including
examples of stereoconvergent reactions in which both
enantiomers of a racemic reactant are converted to a single
resolved product.4j,l,m
In many earlier investigations of Ni-based cross-coupling
catalysts, it often seems assumed, implicitly or explicitly, that
two-electron catalytic cycles of the type shown in Scheme 1 (M
= Ni) are operational. However, stereoconvergent processes are
incompatible with such two-electron catalytic cycles, which
should result in the same configurational consequences for both
enantiomers, and mechanisms involving catalysis by Ni(I)
compounds and free radical intermediates have been proposed.
A candidate one-electron process is shown in Scheme 2.3o,p,u,v,4
Scheme 2. Possible Catalytic Cycle for Suzuki−Miyaura
Cross-Coupling Involving Ni(I)
RESULTS AND DISCUSSION
■
Syntheses and Properties of Ni(0), Ni(I), and Ni(II)
Coordination Compounds of PPh3. The ligand PPh3 was
chosen for reasons given above but also, in part, because PPh3
conveniently forms well-characterized nickel complexes in all
three oxidation states of interest, Ni(PPh3)4,8 NiX(PPh3)3 (X =
This catalytic cycle begins with a halonickel(I) compound
NiXLn and differs from the catalytic cycle shown in Scheme 1 in
that odd-electron intermediates5 are involved, and the metal
alternates between the I and III oxidation states. Unambiguous
mechanistic information has generally been difficult to come by,
however, as many Ni-based catalysts are as yet very poorly
defined. Indeed, very little is known about the possible roles of
Ni(I) species as catalysts or catalytic intermediates, and there is
a need for information on any kind concerning the chemistry of
Ni(I) compounds.
Although drawing considerably less attention than Pd-based
catalyst systems, a number of phosphine-stabilized, Ni-based
catalysts have been investigated for coupling of aromatic
electrophiles.6a−y While most of such studies utilized Ni(II)
precursors because of a dearth of useful Ni(0) candidates,3v it
seems generally to have been assumed that the actual catalytic
species were Ni(0) species formed in some way in situ. The
mechanism of Scheme 1 is commonly thought to apply,
although again there is currently available very little information
in most cases as to what the actual catalytic species are.
Interestingly, and possibly complicating the issue, phosphine
complexes of Ni(0) and Ni(II) can undergo comproportiona-
tion to Ni(I) species sufficiently readily that the presence of
both during an activation stage or in a catalytic cycle can result
Cl, Br, I),7a,9,10b,10 and NiX2(PPh3)2 (X = Cl, Br, I). While a
number of routes to Ni(0) and Ni(I) compounds have been
described, Ni(I) materials containing apparent impurities and
hence having significantly different colors have been reported,
and we therefore describe in some detail the procedures which
we have developed for synthesis and characterization.
11
j
The compounds NiX2(PPh3)2 (X = Cl, Br, I) are readily
prepared via reactions of the Ni(II) halides with PPh3; all
assume pseudotetrahedral structures and are paramagnetic (two
unpaired electrons) in solution and in the solid state.11
Interestingly, and in spite of their paramagnetism, all exhibit
useful 1H NMR spectra; broad but readily identified resonances
are observed at δ ∼19 (br), ∼1 (vbr), and ∼−5.5 (br) and have
been attributed on the basis of relative intensities, line widths,
and mechanism of spin delocalization to the meta, ortho, and
para hydrogens, respectively.12 Thus the meta resonance shifts
∼12 ppm downfield, and the ortho and para resonances shift
∼6 and ∼12.5 ppm upfield, respectively, from the correspond-
ing chemical shifts of diamagnetic PPh3 (δ ∼7), isotropic shifts
which have been interpreted in terms of unpaired spin on the
Ni(II) being delocalized into the ligand π system via a contact
interaction.12
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The Ni(0) compound Ni(PPh3)4 was readily prepared via
reduction of NiCl2(PPh3)2 by zinc powder in the presence of
excess PPh3 in DMF (eq 1).8a
compound NiCl(PPh3)2 (eq 3).7a,9a The degree of dissociation
does not seem to be known precisely although the bis-
phosphine compound has been isolated and characterized
crystallographically.9j
NiX2(PPh3)2 + Zn + 2PPh3 → Ni(PPh3)4 + ZnX2
(1)
NiX(PPh3)3 ⇌ NiX(PPh3)2 + PPh3
(3)
The orange-red, air-sensitive Ni(0) compound was charac-
terized by broad phenyl resonances at δ ∼7.0 and ∼7.8 in the
1H NMR spectrum and a very broad resonance at δ ∼24 in the
31P NMR spectrum (toluene-d8), in agreement with the
literature.8h The 31P resonance is broadened because Ni(PPh3)4
dissociates significantly to Ni(PPh3)3 plus free PPh3, and the
mixture engages in rapid exchange.8h
The Ni(I) compounds NiX(PPh3)3 (X = Cl, Br, I) are best
prepared via disproportionation reactions of Ni(PPh3)4 with
NiX2(PPh3)2 (eq 2).7a,9g,k,l
Since the resonances of free PPh3 are not observed in
solution spectra of NiCl(PPh3)3, it follows that exchange
between free and coordinated ligands must be rapid on the
NMR time scale, and it seemed plausible that the chemical
shifts might vary somewhat if the concentration of free PPh3
varied. We therefore carried out an NMR study of NiCl(PPh3)3
in which 1 equivalent of PPh3 had been added, finding that the
spectrum now exhibited resonances at δ 8.0, 6.4, and 5.7. Thus
all three averaged resonances shifted toward the chemical shifts
of diamagnetic PPh3 (δ 7.0−7.3), consistent with the
exchanged process hypothesized in eq 3, and it seems clear
that the broad resonances shown in Figure 3 are in fact
weighted averages of the resonances of comparable amounts of
free PPh3 and the bis-phosphine compound NiCl(PPh3)2.
Ni(PPh3)4 + NiX2(PPh3)2 → 2NiX(PPh3)3
(2)
Although the overall process from Ni(II) to Ni(0) to Ni(I)
involves two steps, we find that it is not generally feasible to
prepare Ni(I) compounds directly from Ni(II) precursors
because mixtures of Ni(0) and Ni(I) are obtained.
1
The H chemical shifts of the Ni(I) compounds are also
temperature dependent, the meta resonance at δ ∼9 shifting
upfield, and the ortho and para resonances at δ ∼5 and ∼3
shifting downfield as the temperature increases. A plot of the
chemical shifts of NiCl(PPh3)3 vs T−1 are shown in Figure 2,
where the linearity expected of Curie law behavior13 is
observed.
The Ni(I) compounds NiX(PPh3)3 (X = Cl, Br, I) are also
tetrahedral in the solid state although they are believed to
dissociate in solution to the bis-phosphine species NiX-
(PPh3)2.7a,9a They are paramagnetic species with one unpaired
electron,7,9d a fact which has apparently discouraged others
from pursuing NMR studies of them. However, while their 31P
resonances are too broad to be observed, we find that all three
1
can readily be characterized by their H NMR spectra. As an
1
example, the H NMR spectrum of NiCl(PPh3)3 in toluene-d8
(Figure 1) exhibits resonances at δ ∼9.3 (br), ∼5.2 (vbr), and
Figure 2. Curie law plot for the phenyl resonances of NiCl(PPh3)3 in
⧫
■
▲
toluene-d8 ( = meta, = ortho, = para).
Figure 1. 1H NMR spectrum of NiCl(PPh3)3 in toluene-d8; * =
solvent resonances. The weak resonance at δ 7.70 has also been
observed in many spectra, including that of Ni(PPh3)4, and is
attributed to an unidentified diamagnetic PPh3 species. A COSY
experiment showed that its other phenyl resonances are obscured by
the solvent resonances; a sharp, weak resonance in 31P NMR spectra is
also attributed to this minor species.
The observed Curie law behavior suggests that dimerization
to spin-paired [NiCl(PPh3)2]2 is not significant over the
temperature range studied and that the extent of dissociation
remains essentially constant, and therefore presumably
complete. Thus the concentrations of free PPh3 and NiCl-
(PPh3)2 in solutions of NiCl(PPh3)3 are essentially identical,
and the intrinsic chemical shifts of the meta, ortho, and para
hydrogens of NiCl(PPh3)2 should be δ ∼11.3, ∼3.4, and ∼0.4,
respectively. We shall say more on this issue below.
The patterns of chemical shifts and line widths for the Ni(I)
compounds resemble those discussed above for the corre-
sponding tetrahedral Ni(II) compounds NiX2(PPh3)2, and it
seems very likely that the same mechanism for π electron
delocalization into the ligand π orbitals of the tetrahedral Ni(I)
species pertains, with meta, ortho, and para isotropic shifts being
∼3, ∼−2, and ∼−4 ppm, respectively. These shifts are
considerably less than the isotropic shifts observed for the
∼3.7 (br), attributed to the meta, ortho, and para hydrogens,
respectively, on the basis of relative intensities and line widths.
The corresponding meta, ortho, and para resonances of
NiBr(PPh3)3 and NiI(PPh3)3 are observed at δ 9−11, 4−5
and ∼3, respectively, and thus the spectra exhibit characteristic
resonances which are well outside the normal aromatic region
for diamagnetic compounds. The spectra differ also from those
of the Ni(II) species NiX2(PPh3)2 (X = Cl, Br, I) (see above)
and can be used to identify Ni(I) species in solution.7g,h
That said, we note that NiCl(PPh3)3 has been shown to
dissociate significantly in solution to the bis-phosphine Ni(I)
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Ni(II) compounds and are qualitatively consistent with the fact
that the Ni(II) compounds contain two unpaired electrons,
whereas the Ni(I) compounds contain only one and that there
is less unpaired-electron spin available for delocalization in the
latter.
An EPR spectrum of a solid sample was run at 77 K and
found to exhibit a resonance at g ≈ 2.22 (Figure 3), which is
consistent with the presence of Ni(I).7,9k,m
catalytic reactions which are still proceeding at the point of
termination and those whose rates have decreased to zero
before the point of termination. Indeed, the advantages of
reactions which are completed within a relatively short time
span, well before the point of termination, likewise remain
unappreciated. Thus use of isolated yields in comparisons of
various catalysts and ligand systems can be misleading.
Interestingly, catalysis of the Suzuki−Miyaura reaction by
Ni(PPh3)4 is approximately as effective as is catalysis by the
Pd(0) analogue, Pd(PPh3)4, investigated previously under
comparable conditions albeit with somewhat lower catalyst
loadings.14a Indeed, Ni(PPh3)4 is almost as effective for this
cross-coupling reaction as is the very good catalyst Pd-
[PBut3]2,14a suggesting that catalysis by nickel may be more
broadly useful than is commonly thought.
Shown in Figure 5 for purposes of comparison are analogous
reaction profiles for the same reaction utilizing NiCl(PPh3)3 as
catalyst. Experiments involving use of NiBr(PPh3)3 as catalyst
gave similar results, but use of NiCl2(PPh3)2 or a combination
of NiCl2(PPh3)2 and zinc powder produced no product after 24
h. In addition, no usefully successful cross-coupling reactions
involving alkyl halides were found using either Ni(PPh3)4 or
NiCl(PPh3)3. The apparent near equivalence of Ni(0) and
Ni(I) catalyst precursors surprised us, as we anticipated
Figure 3. EPR spectrum of NiCl(PPh3)3 at 77 K.
Catalytic Cross-Coupling Utilizing Ni(PPh3)4 and NiCl-
(PPh3)3. We have carried out an investigation of the catalytic
effectiveness of Ni(PPh3)4 for a representative Suzuki−Miyaura
cross-coupling reaction between phenylboronic acid and 4-
bromoanisole (eq 4); the latter was chosen to distinguish
between the desired product 4-methoxybiphenyl and biphenyl,
the sometimes observed homocoupling product of phenyl-
boronic acid.
1
significant differences in activities. We therefore utilized H
NMR spectroscopy in attempts to determine the nature of the
species present in live catalytic mixtures.7h Aliquots from cross-
coupling reactions involving Ni(0) and Ni(I) catalysts were
periodically removed, taken to dryness, and extracted with
toluene-d8. NMR spectra run immediately exhibited no
resonances attributable to Ni(I) species, and thus we assume
that in all cases catalysis involved Ni(0) as in Scheme 1. We
have been unable to determine just how the change in
oxidation state might have been effected, but we note that facile
interconversion between oxidation states has also been noted in
the analogous dppf catalyst system.7d−f
Figure 6 shows an analogous reaction profile for the Heck−
Mizoroki reaction of bromobenzene with styrene to give trans-
stilbene (eq 5). Reactions were again carried out in duplicate
and monitored by GC.
Duplicate or more runs were carried out in all cases, and
reaction profiles were generated by monitoring the loss of
starting compound and conversion to product by GC. Shown
in Figure 4 are representative reaction profiles for the formation
of 4-methoxybiphenyl from 4-chloro and 4-bromoanisole
utilizing Ni(PPh3)4 as catalyst.
As can be seen, there was generally good mass balance
between reactant and product. Our reaction profiles also
provide information on the time scale over which the cross-
coupled products are formed. Most publications in the area of
catalytic cross-coupling reactions report isolated yields,
obtained on termination of reactions after arbitrary periods of
time.1−4 In such studies, little distinction can be made between
Again, excellent mass balance is observed, and again, catalysis
by Ni(PPh3)4 essentially matches that by Pd[PBut3]2,14b
although, perhaps surprisingly in view of effective catalysis of
Suzuki−Miyaura coupling by NiCl(PPh3)3, Heck−Mizoroki
Figure 4. Formation of 4-methoxybiphenyl and decay of 4-chloroanisole (a) and of 4-bromoanisole (b) using Ni(PPh3)4 as catalyst.
D
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Figure 5. Formation of 4-methoxybiphenyl and decay of 4-chloroanisole (a) and 4-bromoanisole (b) using NiCl(PPh3)3 as catalyst.
Figure 6. Formation of trans-stilbene and decay of bromobenzene
using Ni(PPh3)4 as catalyst.
coupling was not affected by NiCl(PPh3)3. We also assessed the
possibility of using NiCl(PPh3)3 and NiCl2(PPh3)2 in the
presence of zinc dust, which would reduce both to Ni(0)
species. Reaction profiles of these reactions were very similar to
that shown in Figure 6, indicating that catalysis of this Heck−
Mizoroki reaction does not require the use of preformed
Ni(PPh3)4. The reaction profiles are shown in the Supporting
Information (SI), Figures S1 and S2.
An NMR Study of the Oxidative-Addition Reactions of
Aryl Halides to Ni(PPh3)4 and of the Decomposition of
trans-NiPhCl(PPh3)2. As shown above, Ni(PPh3)4 is an
excellent catalyst for cross-coupling of aryl halides, presumably
via the type of catalytic cycle shown in Scheme 1 and involving
an oxidative step to yield an aryl-nickel intermediate. In
contrast, as mentioned above, Ni(PPh3)4 is found not to
catalyze cross-coupling of representative alkyl halides, and thus
this catalyst system differs fundamentally from nickel catalyst
systems reported elsewhere.3,4 The difference lies not in lack of
reactivity, however, as we have actually found that primary,
secondary, and even tertiary alkyl halides all react readily with
Ni(PPh3)4; they just do not give the anticipated alkyl-nickel
products. In an attempt to better understand these differences
in chemistry, we have carried out extensive NMR studies of
reactions of Ni(PPh3)4 with representative aryl and alkyl
halides. Our results with aryl halides are described below and
then related to catalytic biaryl formation. Reactions with alkyl
halides turned out to proceed in a very different fashion,
however, and will be described elsewhere.15
Figure 7. Stacked plot of the course of an oxidative-addition reaction
of a 4-fold excess of chlorobenzene to Ni(PPh3)4.
experiment involving reaction of Ni(PPh3)4 with a 4-fold
excess of chlorobenzene in toluene-d8 and monitored at 30 min
intervals for 8 h. The data presented in Figure 7 are for the
Ni(PPh3)4 starting material alone and for the reaction mixture
at 10, 30, 60, 180, and 360 min after initiation. Note that the
resonances of unreacted chlorobenzene are observed at δ 6.8−
6.85 (m) and 7.04−7.08 (m) and remain unchanged
throughout the reaction. Also shown is the CHD2 resonance
of the solvent at δ 2.09, useful as an internal standard for
chemical shift and relative intensity measurements.
As can be seen, the resonances of Ni(PPh3)4 at δ 6.93 and
7.35 disappear quickly as resonances of trans-NiPhCl(PPh3)2
appear at δ 6.23, 6.33, and 7.72. The other Ni-Ph and PPh3
resonances of trans-NiPhCl(PPh3)2 occur at δ 6.95 and 7.0−
The oxidative addition of chlorobenzene to Ni(PPh3)4 is
reported to proceed as in eq 6.10a,g
Ni(PPh3)4 + PhCl → trans‐NiPhCl(PPh3)2 + 2PPh3
(6)
Presented in Figure 7 is a stacked plot showing
representative 1H NMR spectra for an oxidative-addition
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7.1, respectively (COSY), and overlap significantly with the
chlorobenzene multiplet resonance at δ 7.04−7.08 and a broad
resonance of free PPh3 at δ ∼7.0. These result in the broad,
irregular feature observed in the region δ 6.9−7.1 in the earliest
spectra. The intensities of the three resonances of trans-
NiPhCl(PPh3)2 at δ 6.23, 6.33, and 7.72 cease growing within
the first hour, indicating that the oxidative-addition reaction
was completed as in eq 6. Furthermore, little change was noted
in the intensities or chemical shifts of the resonances of trans-
NiPhCl(PPh3)2 over several hours, and thus this compound is
reasonably stable under these conditions.
That said, the resonances of the free PPh3, 2 equivalents of
which are expected to be present on the basis of the
stoichiometry of eq 6, are identified as the broad resonances
at δ 7.05 and 7.29 in the spectrum obtained at 10 min, slightly
shifted from the positions of free PPh3 at δ 7.03 (br, o-H) and
7.32 (br, m-, p-H). This difference may seem only marginally
significant, but the trends apparently continue and, within 6 h
these resonances had shifted to δ 7.14 and 7.23 and were
sufficiently separated from other resonances that they could be
shown to have intensities comparable both to each other and,
as expected, to that of the resonance of trans-NiPhCl(PPh3)2
at δ 7.72 (all 12H). In addition, as time progressed, a triplet
with intensity about half those of the others and hence
attributable to the p-H emerged from around δ 7.0 and shifted
upfield to δ 6.90 after 6 h. Thus the free PPh3 in solution was
undergoing rapid exchange with a species, of very low initial but
growing concentration, and presumably of Ni(I), for which the
m-H lies downfield and the o- and p-H upfield of the positions
of the free ligand. A 31P NMR spectrum run after 6 h exhibited
only a single sharp resonance, at δ 21.6, attributable to trans-
NiPhCl(PPh3)2; there was no resonance visible for free PPh3.
A variable-temperature NMR experiment was therefore
carried out on a fresh solution of trans-NiPhCl(PPh3)2
containing 2 equivalents of free PPh3, formed like the sample
considered in Figure 7 by reaction of Ni(PPh3)4 with
Figure 8. 31P NMR spectra of a 1:2 mixture of NiPhCl(PPh3)2 and
PPh3 at 223, 233, and 243 K.
Figure 7, since each of the three resonances is shifting toward
the resonance positions of NiCl(PPh3)3 as time progresses.
A similar result has been reported for the 31P NMR spectrum
of trans-NiPhCl(PPh3)2 in the chlorobenzene in which it was
prepared from Ni(PPh3)4.6n A broadened resonance of the free
ligand was observed in this instance, and the broadening was
attributed to exchange with unreacted Ni(PPh3)4. This
rationale seems unlikely, however, given the presence of a
large excess of chlorobenzene and the rapidity with which the
oxidative-addition reaction occurs.
We now address the issue of the source of the Ni(I) species,
which was clearly present from the beginning and then slowly
increased in concentration. Conventional wisdom has long held
that oxidative-addition reactions of aryl halides to compounds
of Ni(0) and Pd(0) proceed via a concerted process similar to
that shown in Scheme 3.16
1
chlorobenzene. A room-temperature H NMR spectrum of
Scheme 3. Concerted Mechanism for Oxidative Addition of
Aryl Halides to M(0) Species
the sample was as in Figure 7, but a spectrum run at 223 K did
indeed exhibit weak, broad resonances at δ 10.20, 4.27, and
2.70. These chemical shifts compare well with the low-
temperature chemical shifts shown in Figure 2, and thus the
resonances are attributed to NiCl(PPh3)3. Consistent with the
exchange process discussed above, the three resonances were
much broader at 233 K and had shifted toward the
corresponding diamagnetic positions and were not observed
at higher temperatures.
This process would not, of course, result in the formation of
paramagnetic intermediates or products.
An alternative mechanism for oxidative addition of aryl
halides ArX (X = Cl, Br) to Ni(PEt3)4 that does involve
paramagnetic intermediates and products was proposed in 1979
by Tsou and Kochi (Scheme 4).17a In this, a single-electron
1
Complementing these H NMR data, a 31P NMR spectrum
run at room temperature revealed only the sharp singlet of
trans-NiPhCl(PPh3)2, with no resonance of PPh3 being
apparent. On cooling the sample to 243 K and then to 233
K, however, a broad resonance of free PPh3 did appear and then
narrow somewhat, and at 223 K, the spectrum exhibited sharp
resonances of comparable intensity for trans-NiPhCl(PPh3)2 at
δ 22.7 and for free PPh3 at δ −7.12 (Figure 8).
Scheme 4. Electron-Transfer/Radical-Pair Mechanism for
Oxidative Addition of Aryl Halides to M(0) Species
Thus the free PPh3 was undergoing ligand exchange with a
species which was evidently not trans-NiPhCl(PPh3)2 since its
1H and 31P resonances remained sharp and unshifted
throughout. The exchange partner was clearly of a nature
which could induce the pronounced line broadening observed
and is almost certainly the paramagnetic Ni(I) species
NiCl(PPh3)3 for which the 31P resonance is too broad to be
observed. Participation of the latter compound as the emerging
exchange partner would result in the assignments noted in
F
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transfer (SET) process yields a radical-ion pair which can
undergo combination within the solvent cage to give
NiArX(PEt3)2, which were observed and which are, of course,
identical to the products of oxidative addition via a concerted
process.
involved studies of trans-NiPhCl(PPh3)2 in the presence of 2
molar equivalents of PPh3. We therefore ran a room-
1
temperature H NMR spectrum of a sample of large, visually
pure orange crystals of trans-NiPhCl(PPh3)2 which had been
grown at −30 °C in a toluene/hexanes mixture containing free
PPh3. The NMR solution contained no added PPh3, however,
and to our surprise, a spectrum run immediately after mixing
(Figure 9) exhibited, in addition to the resonances of trans-
Alternatively the radical species can diffuse from the solvent
cage to give Ni(I) products NiX(PEt3)3, which were observed,
and aryl radicals which abstract a hydrogen atom from the
solvent SH to give arene products, which were also observed.
This paper has been much cited in the literature as evidence
for possible roles of Ni(I) species as intermediates for e.g. the
chemistry outlined in Scheme 2,3,4 and somewhat similar
observations have also been made for a dppf-Ni(0) system.7f
Although we had no a priori reason to suspect that our
observed oxidative addition of chlorobenzene to Ni(PPh3)4 to
give trans-NiPhCl(PPh3)2 did not proceed via the type of
concerted process shown in Scheme 3, the NMR studies
discussed above in connection with Figures 7 and 8 seemingly
required that a small amount of Ni(I) species was present from
the beginning. The Ni(I) could result from competitive
processes involving concerted and radical mechanisms, as in
Schemes 3 and 4, or it could result from a very slow secondary
reaction involving the Ni(II) product, trans-NiPhCl(PPh3)2.
Tsou and Kochi reported also that the Ni(I)/Ni(II) product
ratios decreased drastically in the order I > Br > Cl,17a and we
therefore examined reactions of Ni(PPh3)4 with bromo and
iodobenzene. Under reaction conditions analogous to those
used for chlorobenzene, we found that bromobenzene reacts
smoothly to give a good yield of trans-NiPhBr(PPh3)2,
Figure 9. 1H NMR spectrum of a solution containing trans-
NiPhCl(PPh3)2 (observable resonances denoted by arrows; others
obscured by solvent resonance at δ 6.99), biphenyl (#), and an
apparent Ni(I) species (see text); * = solvent resonances.
1
identified by H NMR resonances at δ 6.32 (1H, p-H of Ni-
NiPhCl(PPh3)2, relatively intense resonances of biphenyl at δ
7.40, 7.20, and ∼7.10 and broad new resonances at δ ∼10.6
(br), 4.1 (vbr and hence, barely perceptible above the baseline),
and 1.86 (br).
Ph), 6.22 (2H, m-H of Ni-Ph), and 7.74 (12 H, m, PPh), all
very similar to the corresponding resonances of trans-
NiPhCl(PPh3)2 and justifying the assignments. Interestingly,
the intensities of the resonances of trans-NiPhBr(PPh3)2
remained unchanged over several hours at 0 °C, although the
resonances of the free PPh3 were again clearly exchanging with
an unobserved species, presumably NiBr(PPh3)3. Thus the
chloro and bromo systems were very similar in their tendencies
to form diamagnetic species trans-NiPhX(PPh3)2 rather than
paramagnetic Ni(I) compounds NiX(PPh3)3.
In the case of the reaction of Ni(PPh3)4 with iodobenzene,
however, a resonance attributable to the NiPh group of
NiPhI(PPh3)2 was observed at δ 6.24 in the first spectrum run,
but it was too broad for J-coupling to be observed. All of the
other resonances in the spectrum were broadened as well, and
the resonances of the free PPh3 diverged from the diamagnetic
positions much more than was the case with the chloro and
bromobenzene systems. All of these observations, taken
together, indicate the presence of higher concentrations of
paramagnetic Ni(I) species and hence possibly a higher
propensity to the type of SET process shown in Scheme 4.
Thus both Ni(PEt3)4 and Ni(PPh3)4 undergo oxidative-
addition chemistry to yield both adducts of the type trans-
NiPhXL2 and Ni(I) halo compounds with proclivities to Ni(I)
increasing in the orders PhCl ≤ PhBr < PhI and Ni(PPh3)4 <
Ni(PEt3)4. However, formation of Ni(I) species occurs
concurrently with oxidative addition with Ni(PEt3)4 but not
necessarily with Ni(PPh3)4.
The new, broad resonances resemble those of NiCl(PPh3)3
(Figure 1) in that an apparent meta resonance (δ ∼10.6) is
broad and shifted to low field, an apparent para resonance (δ
∼1.86) is comparably broad and shifted to high field, and an
apparent ortho resonance (δ ∼4.1) is much broader and is
shifted upfield from the diamagnetic positions. These chemical
shifts differ significantly from the averaged chemical shifts of
NiCl(PPh3)3, i.e. of a 1:1 mixture of NiCl(PPh3)2 and PPh3,
shown in Figure 1 (δ ∼9.3, ∼5.2, and ∼3.7), and we believe
that the new Ni(I) species appearing in Figure 9 is actually
NiCl(PPh3)2. The chemical shifts agree reasonably well with
those calculated above on the assumption of averaged chemical
shifts for NiCl(PPh3)3.
Of significance to discussions of catalytic biaryl formatio-
n,8a,10d−f the resonances of trans-NiPhCl(PPh3)2 shown in
Figure 9 disappeared from the spectrum within 1 h as the
resonances of biphenyl intensified. Thus while trans-NiPhCl-
(PPh3)2 decomposes to biphenyl and Ni(I) extremely slowly at
298 K in toluene solution in the presence of PPh3, conversion is
complete within minutes in its absence. Although the broad,
1
sometimes overlapping resonances in H NMR spectra render
accurate integrations difficult, the changes seem consistent with
the stoichiometry of the decomposition reaction being as in eq
7.
2trans‐NiPhCl(PPh3)2 → 2NiCl(PPh3)2 + Ph2
(7)
Intrinsic Instability of trans-NiPhCl(PPh3)2 and the
Relevance Thereof to Catalytic Biaryl Formation. Para-
magnetic Ni(I) species may also arise as products of a
secondary reaction following the oxidative-addition step, and
we note that the experiments described in Figures 7 and 8
These observations differ radically from those reported for
the very similar compounds trans-NiPhX(PEt3)2 (X = Cl, Br),
which seemingly are indefinitely stable in solution at room
temperature,17 but which, unlike trans-NiPhCl(PPh3)2, react
G
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rapidly with aryl halides via a radical-chain process to give
biaryls and NiX(PEt3)2.17a Our observations do, however,
seemingly rationalize several apparently incongruent observa-
tions in the literature concerning trans-NiPhCl(PPh3)2 and
confirm others. For instance, Hidai et al.10a report the original
syntheses of trans-NiPhCl(PPh3)2 and trans-NiPhBr(PPh3)2 via
oxidative additions of chloro and bromobenzene to Ni(PPh3)4
but make no mention of latent instability of the products at
room temperature in solution. This paper is frequently cited as
the preferred route to the compounds trans-NiPhX(PPh3)2 (X
= Cl, Br), but apparently on the basis of this report, it seems
frequently assumed that the compounds are thermally stable in
solution.
Scheme 5. Dinuclear Route to Biaryls via Nickel Catalysis (L
= PPh3)
Confusing the issue, while several authors prepare trans-
NiPhX(PPh3)2 by allowing oxidative-addition reaction mixtures
to stir for many hours, it is also been reported that the isolated
products inexplicably undergo decomposition to biphenyl
within minutes.6o,s,10b,e,h These apparent incongruities have
received little attention but are now readily rationalized.
Products maintained in reaction mixtures can be stable for
many hours because of the free PPh3, which remains in
solution, and isolated products contaminated with free PPh3
will likewise seem stable if the PPh3 contaminant is not
recognized as being present. Alternatively, as our Figure 9
shows, pure trans-NiPhCl(PPh3)2 is very unstable in solution at
room temperature. Consistent with our findings, Ni(I) species
are also cited as products of decomposition of NiPhX-
(PPh3)2,6s,10h although just how the Ni(I) species were
identified is not always reported.
Related to the chemistry of eqs 6 and 7, reactions of aryl
halides with Ni(0) compounds such as Ni(PPh3)4 have often
been cited as good catalytic routes to biaryls at 50−80
°C.8a,10d−f The reactions are generally believed to involve
conversion of a Ni(0) catalyst to an aryl-nickel(II) intermediate
followed somehow by reductive elimination of biaryl to
regenerate the Ni(0) catalyst. In some cases, the active Ni(0)
species is generated from Ni(II) precursors either electro-
chemically or via reduction by zinc metal.
Several mechanisms have been considered for biaryl
formation, including radical-chain processes, reductive elimi-
nation from diaryl Ni(II) and diaryl Ni(IV) compounds, and
homolysis to aryl radicals which couple.10e,f,18a,b The last
mentioned seems very unlikely in the present case because the
aryl radicals would certainly abstract hydrogen from the
deuterated solvent toluene to yield benzene rather than
biphenyl. To our knowledge, the inhibitory effect of added
PPh3 on the clean conversion of trans-NiPhCl(PPh3)2 to Ni(I)
and biphenyl has not been noted and related to mechanistic
considerations, and it seems reasonable to interpret our
observations in terms of a mechanism in which PPh3
dissociation to give NiPhCl(PPh3) is followed by reaction of
the latter with a molecule of trans-NiPhCl(PPh3)2 to give
chloride-bridged dinuclear species (Scheme 5).
The dinuclear species could in principle eliminate biphenyl
directly and produce Ni(I), or it could undergo metathesis to
give NiPh2(PPh3)2 and NiCl2(PPh3)2, followed by the reductive
elimination and comproportionation steps shown. The system
could be made catalytic in nickel if carried out in the presence
of zinc dust, which would convert the Ni(I) product to Ni(0)
catalyst. We have carried out DFT calculations to assess the
feasibility of these processes.
NiCl(PPh3)2 and Biphenyl. Potentially relevant paths leading
to biphenyl formation, summarized in Schemes 6 and 7, were
a
Scheme 6. Oxidative Addition of PhCl to NiL4
a
Calculated relative free energies per Ni atom (298 K) in kcal/mol.
Thick blue arrows indicate oxidative-addition transition states. In
either case, the final product is trans-NiPhClL2.
explored using closed-shell restricted density-functional meth-
ods: optimization with Turbomole,19 b-p20,21/SV(P),22 single-
point energies at Gaussian 0923 M0624/cc-pVTZ,25−27 and
PCM(THF). For Ni(II) species, triplet states were considered
as well (energies included in the SI) but were in most cases
found to be higher in energy. No broken-symmetry solutions
were found for dimeric Ni(II) compounds. We found that some
form of dispersion correction is essential and that different
choices (DFT-D3,28 M06, M06L24) produce rather different
results. Here we report data based on M06 results; a motivation
is provided in the SI. Due to this method sensitivity, it seems
likely that the error margins in the results are larger than usual
for DFT studies. Numbers mentioned below and in the figures
are free energies.
In view of the SET mechanism proposed by Kochi,17a we
also briefly explored potential SET intermediates using triplet
and open-shell singlet electronic structures, but no plausible
intermediates could be located. This does not rule out SET as a
reaction mechanism, but in our opinion it suggests that
involvement of long-lived SET intermediates is not very likely.
Paths for PhCl oxidative addition are shown in Scheme 6.
The first dissociation step from NiL4 (L = PPh3) is facile; in
fact, with a calculated dissociation energy of only ∼2 kcal/mol,
solutions of NiL4 should already contain a significant amount of
NiL3. After this dissociation, a transition state exists for direct
C−Cl oxidative addition without going through an intervening
arene complex; although, the barrier calculated for this is rather
high (38 kcal/mol). A lower-energy path goes through NiL2
and its arene complex, with an effective barrier of 27 kcal/mol.
DFT Calculations on the Oxidative Addition of PhCl to
Ni(PPh3)4 and the Conversion of trans-NiPhCl(PPh3)2 to
H
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Article
a
open up (G), trap a phosphine ligand (H), and break apart into
L2NiCl2 and LNiPh2 (I). Biphenyl formation, indicated by bold
red arrows in Scheme 7, can take place at this stage or after first
binding a second phosphine molecule (J). Alternatively,
reductive elimination could proceed directly from asymmetric
dimer F. Our results suggest that all three variations are
possible, although the path via J is likely preferred.
Scheme 7. Reactions Studied
Geometries of the three relevant transition states are shown
in Figure 10. Mononuclear elimination from 14-e− intermediate
I proceeds with a barrier of only 5 kcal/mol, while elimination
from 16-e− J has a somewhat higher barrier of 6 kcal/mol,
which is still a very fast reaction at room temperature. We
conclude that any LnNiPh2 species formed from trans-NiPhClL2
will not survive for long. Even elimination from asymmetric
dimer F has a barrier of only 11 kcal/mol. Thus under
conditions where trans-NiPhClL2 undergoes intermolecular Cl/
Ph exchange, fast formation of biphenyl would be expected.
This explains the role of free PPh3 in stabilizing solutions of
trans-NiPhClL2; it opposes dissociation of L, which is the first
step toward Cl/Ph exchange. In particular, the main effect is
not conversion of LNiPh2 to potentially more stable L2NiPh2:
our results show that even L2NiPh2 would eliminate rapidly
once formed.
The last statement, concerning the very low thermal stability
of NiPh2(PPh3)2, gives pause for thought. We have noted above
the very significant differences in chemistry between the Ni-
PEt3 and Ni-PPh3 systems, and note also that Ni(II)
compounds of the type NiAr2(2,2-bipyridine) were shown
some years ago to be too thermally stable to participate in
known cross- and homocoupling catalytic cycles involving bipy-
type ligands.3c This latter finding was shortly thereafter
conflated to infer that “reductive elimination from diorgano−
Ni complexes is a facile process when the formal oxidation state
of the metal is...not two”,3e a conclusion which is clearly
incorrect. Nickel chemistry can readily involve formal oxidation
states 0−IV and which couple(s) is(are) involved in any
particular catalytic process may depend to a very great extent
on the ligand system(s) used.
a
Calculated relative free energies (298 K) in kcal/mol. From species C
on, energies are per pair of Ni atoms. Thick red arrows indicate
reductive-elimination transition states. No calculations were attempted
for species D and G.
Scheme 7 summarizes plausible reaction paths leading from
the initial oxidative-addition product trans-NiPhClL2 (A) to
biphenyl and Ni(I) species. Since this scheme involves
binuclear species, starting from C, energies are given per two
Ni atoms, relative to A.
For biphenyl formation to occur, some form of Ph/Cl
exchange is needed. Complex A can dissociate one phosphine
ligand (B), bind to a second molecule of A giving singly
bridged dimer C, and lose one more phosphine to form a
doubly bridged dimer in one of two ways. The preferred dimer
(E) has two chloride bridges, but forming this is nonproductive.
The alternative is a Cl/Ph bridged dimer (F), which then can
SUMMARY
■
The Ni(0) compound Ni(PPh3)4 is found to be a surprisingly
interesting catalyst for representative Suzuki−Miyaura and
Heck−Mizoroki cross-coupling reactions such as the cross-
coupling of chloro and bromoanisole with phenylboronic acid
to give 4-methoxybiphenyl and of bromobenzene with styrene
Figure 10. Geometries of transition states for C−C coupling from NiPh2L, NiPh2L2, and [NiPhClL]2; bond lengths in Å.
I
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to be at δ 6.95. A prominent 12H resonance at δ 7.72 (br) is assigned
to the coordinated PPh3 of trans-NiPhCl(PPh3)2, with the other PPh3
resonances overlapping at δ 7.0−7.1. Similar experiments were carried
out at lower temperatures and, at room temperature, with
bromobenzene.
General Experimental Methodologies for Suzuki−Miyaura
Reactions. 4-Bromoanisole (0.187g, 1.0 mmol) in 3 mL of dioxane
was added under argon to a mixture of 0.107 g of Ni(PPh3)4 (0.1
mmol), 0.18 g of phenylboronic acid (1.5 mmol), and 0.625 g of
Cs2CO3 (2 mmol). The mixture was stirred at 60 °C for 4 h. Stirring
was stopped periodically to allow the undissolved base to settle, and
aliquots of 0.1 mL were taken, diluted by 10 mL of solvent and
analyzed by GC. Hexadecane was used as an internal standard. A
similar procedure was used for studies of trans-NiPhCl(PPh3)2,
NiCl(PPh3)3, and NiBr(PPh3)3.
to give trans-stilbene. This catalytic system can also be
generated by treating the air-stable NiCl2(PPh3)2 with zinc
dust and works well for Heck−Mizoroki but not Suzuki−
Miyaura cross-coupling reactions utilizing phenylboronic acid;
the latter may react preferentially with the zinc. A mechanistic
NMR study of the oxidative-addition reaction of chlorobenzene
to Ni(PPh3)4 shows that the diamagnetic, putative cross-
coupling intermediate trans-NiPhCl(PPh3)2 forms quickly and
virtually quantitatively, but a small amount of paramagnetic
species also forms and slowly increases in concentration. The
latter turns out to be the paramagnetic Ni(I) compound
NiCl(PPh3)3, which is prepared separately and characterized for
1
the first time by NMR spectroscopy. Its H NMR spectrum
exhibits broad meta and para resonances at δ ∼9.3 and ∼3.7,
respectively, and a very broad meta resonance at δ ∼5.2,
resonances which are well-separated from normal chemical
shifts of diamagnetic complexes of PPh3 and hence are very
useful for detecting this and similar Ni(I) species in solution.
The compound trans-NiPhCl(PPh3)2 is found to exhibit
entirely unanticipated thermal lability in solution in the absence
of free PPh3. It readily decomposes to biphenyl and
NiCl(PPh3)3 in a reaction relevant to the long-known but
little understood nickel-catalyzed conversion of aryl halides to
biaryls. Biphenyl formation is initiated by PPh3 dissociation
from Ni(PPh3)4 and formation of a dinuclear intermediate, a
process which is now better defined using DFT methodologies.
The fact that the analogous compound trans-NiPhCl(PEt3)2 is
stable17 is probably to be related to the fact that the smaller,
better donor ligand PEt3 does not dissociate significantly under
the same conditions.
General Experimental Methodologies for Heck−Mizoroki
Reactions. Pyridine and acetonitrile were dried overnight over
molecular sieves. Ni(PPh3)4 (0.11g, 0.1 mmol) was added under argon
to a mixture of 0.16 g of bromobenzene (1 mmol), 0.42 g of styrene (4
mmol), and 0.32 g of pyridine (4 mmol) in 3 mL of acetonitrile; the
mixture was stirred at 65 °C for 4 h. Aliquots of 0.1 mL were taken at
specified intervals, diluted by 10 mL of acetonitrile and analyzed by
GC. Naphthalene was used as an internal standard.
NMR Studies of Suzuki−Miyaura Reactions. A mixture of 0.18
g of phenylboronic acid (1.5 mmol), 0.088 g of NiCl(PPh3)3 or 0.107
g of Ni(PPh3)4 (both 0.1 mmol), and 0.625 g Cs2CO3 (2 mmol) in a
test tube sealed with a septum was purged with argon. A solution of
0.187 g of 4-bromoanisole (1 mmol) in 3 mL of dioxane was added,
and the mixture was stirred at 60 °C. Stirring was stopped periodically
to allow the undissolved base to settle, and aliquots of 0.5 mL were
taken, pumped down to dryness, dissolved in deuterated toluene, and
1
analyzed by H NMR spectroscopy. A similar experiment was carried
out using 0.69 g of trans-NiPhCl(PPh3)2 (0.1 mmol) in the presence
of 0.026 g of PPh3 (0.1 mmol).
MethodsComputational. Geometries were optimized using
Turbomole19 coupled to the external Baker optimizer29,30 via the
BOpt package;31 at this stage we used the Turbomole b-p
functional20,21 in combination with the def-SV(P) basis22 and the RI
approximation.32 The nature of all stationary points was checked using
a vibrational analysis (no imaginary frequencies for minima, exactly
one for transition states). Improved single-point energies were then
calculated using Gaussian 09,23 the M06 functional,24 and the cc-pVTZ
basis set25−27 obtained from the EMSL library.33,34 The PCM
method35−38 was used for modeling the solvent (THF). To obtain
final free energies, thermal corrections from the b-p/SV(P) vibrational
analysis (enthalpy and entropy, 298 K, 1 bar) were added to the above-
mentioned final electronic energies; the entropy contribution was
scaled by 0.67 to account for reduced freedom in solution.39,40
EXPERIMENTAL SECTION
■
General Procedures. All syntheses were carried out under a dry,
deoxygenated argon or nitrogen atmosphere with standard Schlenk
line techniques. Argon was deoxygenated by passage through a heated
column of a BASF copper catalyst and then dried by passing through a
column of 4A molecular sieves. Handling and storage of air-sensitive
compounds were carried out in an LC Technology LCBT-1 benchtop
purged glovebox. NMR spectra were recorded on Bruker AV500 or
1
AV600 MHz spectrometers with H NMR data being referenced to
TMS via the residual proton signals of the deuterated solvent, and EPR
spectra were run on powder samples at 77 K on a Bruker EMX
spectrometer equipped with a Bruker-8/2.7 electromagnet and a
Bruker ER4119HS resonant cavity. GC experiments were carried out
using a Varian 3900 GC equipped with a CP 8400 autosampler, 0.32
mm Varian fused silica column, and an FID; the injector temperature
was set at 250 degrees, the initial column temperature was at 140
degrees, and the detector temperature was at 250 degrees. Hexadecane
was added as an internal standard, and GraphPad Software Prism
Version 3.02 was used for curve fitting. The compounds Ni(PPh3)4,8
NiX(PPh3)3 (X = Cl, Br, I),7a,9,10b NiX2(PPh3)2 (X = Cl, Br, I),11 and
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00446.
10g
trans-NiPhCl(PPh3)2 were prepared as in the literature.
Figures S1 and S2; a brief discussion of the computa-
tional methodologies tested and utilized; tables of the
free energies of various relevant species calculated at
different levels (Table S1); tables of the contributions to
final free energies (Table S2) (PDF)
Cartesian coordinates for all of the calculated structures
(XYZ)
NMR Study of Oxidative Addition of Chlorobenzene to
Ni(PPh3)4. In a typical procedure, a solution of 48 mg of Ni(PPh3)4
(43 μmol) in 0.7 mL of toluene-d8 was transferred into an NMR tube;
the tube was sealed by a septum and parafilm, and 25 μL of
chlorobenzene (0.16 mmol, 4.0 equiv) was added. The sample was
shaken briefly, and NMR spectra were run at various temperatures
using as internal standard the toluene-d8 signal at δ 2.09. Although 1H
and 31P data have been reported for diamagnetic trans-NiPhCl(PPh3)2
in CD2Cl2,10g we have carried out all of our reactions in toluene-d8, in
which the chemical shifts are different and thus needed to be
determined. The resonances of the p- and m-H of the Ni-Ph group are
observed at δ 6.33 (t, J = 7.5 Hz) and 6.23 (t, J = 7.5 Hz), respectively,
while the resonance of the corresponding o-H was obscured by
stronger PPh3 resonances but was determined via a COSY experiment
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for M.C.B.: bairdmc@chem.queensu.ca.
*E-mail for P.H.M.B.: petrushenricusmaria.budzelaar@unina.it.
J
DOI: 10.1021/acs.organomet.7b00446
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Organometallics
Article
(m) Lu, Z.; Fu, G. C. Angew. Chem., Int. Ed. 2010, 49, 6676.
(n) Wilsily, A.; Tramutola, F.; Owston, N. A.; Fu, G. C. J. Am. Chem.
Soc. 2012, 134, 5794.
(5) (a) Baird, M. C. Chem. Rev. 1988, 88, 1217. (b) Brown, T. L. In
Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: New
York, 1990; p 67.
ORCID
Michael C. Baird: 0000-0002-1497-3511
Notes
The authors declare no competing financial interest.
(6) For examples, see (a) Tamao, K.; Sumitani, K.; Kumada, M. J.
Am. Chem. Soc. 1972, 94, 4374. (b) Percec, V.; Bae, J.-Y.; Hill, D. H. J.
Org. Chem. 1995, 60, 1060. (c) Saito, S.; Sakai, M.; Miyaura, N.
Tetrahedron Lett. 1996, 37, 2993. (d) Indolese, F. Tetrahedron Lett.
1997, 38, 3513. (e) Saito, S.; Oh-tani, S.; Miyaura, N. J. Org. Chem.
ACKNOWLEDGMENTS
■
M.C.B. and P.H.M.B. acknowledge financial support from
NSERC, and Professors Bruce Hill and Michael Mombour-
quette are thanked for running and interpreting the EPR
spectrum of NiCl(PPh3)3.
̂
1997, 62, 8024. (f) Galland, J.-C.; Savignac, M.; Genet, J.-P.
Tetrahedron Lett. 1999, 40, 2323. (g) Inada, K.; Miyaura, N.
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