Oxidative Addition of Aryl Electrophiles to a Prototypical Nickel(0) Complex: Mechanism and Structure/Reactivity Relationships

Article abstract of DOI:10.1021/acs.organomet.7b00208

Detailed kinetic studies of the reaction of a model Ni⁰ complex with a range of aryl electrophiles have been conducted. The reactions proceed via a fast ligand exchange pre-equilibrium, followed by oxidative addition to produce either [Ni^IX(dppf)] (and biaryl) or [Ni^II(Ar)X(dppf)]; the ortho substituent of the aryl halide determines selectivity between these possibilities. A reactivity scale is presented in which a range of substrates is quantitatively ranked in order of the rate at which they undergo oxidative addition. The rate of oxidative addition is loosely correlated to conversion in prototypical cross-coupling reactions. Substrates that lead to Ni^I products in kinetic experiments produce more homocoupling products under catalytic conditions.

Full text of DOI:10.1021/acs.organomet.7b00208

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Article
pubs.acs.org/Organometallics
Oxidative Addition of Aryl Electrophiles to a Prototypical Nickel(0)
Complex: Mechanism and Structure/Reactivity Relationships
Sonia Bajo,†,§ Gillian Laidlaw,† Alan R. Kennedy,† Stephen Sproules,*,‡ and David J. Nelson*,†
†WestCHEM Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street,
Glasgow G1 1XL, U.K.
‡WestCHEM School of Chemistry, University of Glasgow, University Place, Glasgow G12 8QQ, U.K.
S
* Supporting Information
ABSTRACT: Detailed kinetic studies of the reaction of a model
Ni0 complex with a range of aryl electrophiles have been
conducted. The reactions proceed via a fast ligand exchange pre-
equilibrium, followed by oxidative addition to produce either
[NiIX(dppf)] (and biaryl) or [NiII(Ar)X(dppf)]; the ortho
substituent of the aryl halide determines selectivity between
these possibilities. A reactivity scale is presented in which a range
of substrates is quantitatively ranked in order of the rate at which
they undergo oxidative addition. The rate of oxidative addition is
loosely correlated to conversion in prototypical cross-coupling
reactions. Substrates that lead to NiI products in kinetic experiments produce more homocoupling products under catalytic
conditions.
Ni0 complexes are highly reactive toward oxidative
INTRODUCTION
■
addition,10 but there are selectivity issues if densely function-
Nickel catalysis is a rapidly growing field of synthetic chemistry;
it has the potential to allow different and exciting reactivity to
alized molecular scaffolds are used that contain several
potentially reactive sites.11 The number of accessible oxidation
be exploited and might replace palladium in some reactions.1
states complicates the mechanistic understanding of these
Nickel oxidatively adds to a wide range of electrophiles beyond
reactions: some studies have implicated NiI intermediates in
halides and triflates,2 and even ethers and fluorides can be
catalysis,12,13 while others have conducted experimental and/or
deployed in cross-coupling reactions.3−5 The ability of nickel to
computational studies to rule them out.14−16 Recently, Hazari
achieve oxidation states +1 and +3 has enabled elegant and
has detected NiI complexes in prototypical Suzuki−Miyaura
powerful tandem photocatalysis/cross-coupling methods;6,7
reactions, regardless of the Ni0, NiI, or NiII (pre)catalyst used.17
this reactivity manifold has the potential to be a rather general
A recent follow-up study showed that comproportionation
way to enable otherwise challenging C−O and C−N bond
competes with transmetalation in the cross-coupling of aryl
formations.8,9 However, our mechanistic understanding of
sulfamates and implicated NiI−sulfamyl and NiI−aryl com-
plexes as products of comproportionation.18 Schoenebeck has
nickel catalysis often lags far behind the development of useful
shown than the role and fate of NiI species are highly
synthetic protocols (Figure 1).
dependent on the choice of both ligand and electrophile, with
[NiX(bpy)] complexes being competent catalysts for some
reactions of aryl bromides and iodides.19 These studies shed
light on parts of a much larger puzzle, and much remains to be
done.
We have conducted a detailed study of the oxidative addition
of aryl halides to dppf−Ni0. This system was chosen for several
reasons. dppf is inexpensive and widely available in most
synthetic laboratories and so represents an accessible catalytic
system. Ni/dppf catalyst systems are competent for many
reactions, including Suzuki−Miyaura,17,18 Buchwald−Hart-
wig,20 and trifluoromethylthiolation.11,15,19 [Ni(COD)(dppf)]
(1) is a thermally stable Ni0 complex that can be prepared on a
Figure 1. Previous achievements in nickel catalysis and challenges
addressed here.
Received: March 17, 2017
© XXXX American Chemical Society
A
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sufficient scale in a straightforward manner.15 Finally, the
presence of 31P nuclides enables the use of NMR spectroscopy
to monitor reactions in the presence of excess aryl halide, which
swamps the 1H NMR spectra. A number of synthetic protocols
rely on the use of a ligand plus [Ni(COD)2],20 and therefore
[Ni(COD)(L)n] is formed in situ. More electron rich ligands
such as N-heterocyclic carbenes21,22 and trialkylphosphines are
often used in the reactions of more challenging substrates, but
complex 1 reacts with most electrophiles studied here (vide
infra).
The primary aim of this study was to construct the first
reactivity scale for oxidative addition to Ni0 that is based on a
series of kinetic experiments conducted under the same
conditions; to do this, it was important to study the oxidative
addition step, rather than consider prototypical catalytic
reactions in which the rate-determining step may not be
oxidative addition, may change with different substrates, or
where the (pseudo)halide will influence the rates of subsequent
steps in the catalytic cycle.23 Importantly, while it is possible to
appreciate which substrates undergo facile reaction and which
substrates may be challenging to deploy from a reading of the
synthetic chemistry literature, these typically consider proto-
typical reactions assessed by yield measurements;18,24 there is a
real need to also consider the rate at which key processes occur
because this provides valuable information about whether these
are feasible on a time scale relevant to synthetic reactions. These
quantitative data can then be used to select catalysts for
particular reactions by considering how quickly they will
activate the desired electrophile versus alternative sites in the
molecule. These data can also highlight when useful selectivity
cannot be achieved. To achieve the primary aim of this work, it
was important to first elucidate the mechanism of oxidative
addition to complex 1 and to elucidate the origin of NiI.
Table 1. Rate Constants for the Oxidative Addition of
Selected Aryl Halides to [Ni(COD)(dppf)] (1) (0.022 mol
L−1 in Benzene-d6)
entry
substrate
T (K)
kobs (10−4 s−1
)
[ArX] (mol L−1
)
1
2
o-MeC6H4I
303
323
323
323
293
293
323
323
323
323
5.04(7)
6.90(6)
1.9(2)
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.55
0.66
o-MeC6H4Br
o-MeC6H4Cl
p-MeC6H4Cl
p-F3CC6H4I
p-F3CC6H4Br
p-F3CC6H4Br
p-F3CC6H4Cl
p-F3CC6H4Cl
p-F3CC6H4Cl
3
4
2.26(3)
7.8(1)
5
6
0.238(4)
10.2(1)
2.39(3)
2.89(1)
3.42(6)
7
8
9
10
iodotoluene with 1 revealed both toluene and biaryl. Complex
1 does not undergo reaction with the internal standard used in
these reactions (OPCy3 or OPPh3) or the solvent, as confirmed
by control experiments in the absence of aryl halide. The
oxidative addition reaction is first order in aryl halide (Figure
2a).
Figure 2. Plots of kobs versus (a) concentration of p-F3CC6H4Cl (p-
F3C-Cl) in the oxidative addition of p-F3C-Cl to 1 and (b) the
reciprocal of the concentration of 1,5-cyclooctadiene (COD) in the
oxidative addition of p-F3CC6H4Br (p-F3C-Br) (0.44 mol L−1) to 1. All
reactions were carried out with 0.022 mol L−1 [Ni(COD)(dppf)] (1)
in benzene-d6 at 50 °C. Reactions were carried out in duplicate; each
replicate is plotted separately.
RESULTS AND DISCUSSION
■
Oxidative Addition of Aryl Halides. The oxidative
addition reactions of p-F3CC6H4X, o-MeC6H4X, and p-
MeC6H4Cl with [Ni(COD)(dppf)] (1) were studied initially
(Scheme 1 and Table 1; X = I, Br, Cl). Reactions with para-
Scheme 1. Oxidative Addition of Aryl Halides to
[Ni(COD)(dppf)] (1)
As expected, the oxidative addition rate decreases in the
order I > Br > Cl. Experiments at lower temperatures were
required to collect good-quality kinetic data for the reactions of
aryl iodides due to the rapidity of the reaction.28 The reaction
of o-chlorotoluene is ca. 10% slower than that of the para
isomer. The reaction of 1 with p-F3CC6H4Br was studied at 20
and 50 °C so that the rate constants could be used to quantify
the relative reactivity: this is 143:4:1 for I:Br:Cl (for the p-
trifluoromethyl substrates), which compares to 100:3:2 for the
reaction of halobenzenes with [Ni(PEt3)4] in THF29 and
400:2:1 with [Ni(PPh3)4] in benzene.30
Added COD inhibits the reaction (Figure 2b); therefore, it
must be displaced from the complex before oxidative addition
can occur. Trifluoromethylthiolation reactions catalyzed by 1
have been found to be faster than those catalyzed by a mixture
of [Ni(COD)2] and dppf, most likely due to the additional 1
equiv of COD inhibiting the latter reaction.15 For the oxidative
addition reactions considered here, a plot of ln[COD] versus
kobs has a slope of ca. −0.3 and Figure 2b shows a nonzero
intercept; thus, the kinetic behavior is not simply inverse first
order. Arenes are known to be competent ligands for Ni0
complexes,31 and so a plausible first step would be displacement
substituted aryl halides led to no diamagnetic complexes (as
determined by 31P NMR spectroscopy). Analysis of the isolated
products of these reactions by EPR spectroscopy revealed the
presence of [NiX(dppf)] (2-X) (vide infra).15,17,25 GC-FID and
GC-MS analysis of the reaction of p-F3CC6H4I with 1 showed
the presence of biaryl but no trifluorotoluene. The reactions of
o-halotoluenes with 1 led initially to [Ni(o-tol)X(dppf)] (3-
X);26,27 subsequent GC-MS analysis of the reaction of 2-
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of COD by the aryl halide. [1H,1H] EXSY experiments were
carried out to investigate the rate of exchange of free COD with
the COD bound to 1; no exchange was observed at 10−50 °C
in benzene-d6, ruling out a dissociative mechanism in which a
vacant site is generated.32 An associative mechanism is unlikely
due to the lack of space around the metal center;15,19 therefore,
an interchange mechanism in which incoming substrate
displaces COD is most likely.
If the overall reaction is considered as a reversible ligand
exchange (with rate constants k1 and k−1) followed by
irreversible oxidative addition (with rate constant k2), the
steady-state approximation can be used to derive eq 1. At low
concentrations of COD, k2 ≫ k−1[COD] and the expression
can be simplified to eq 2. At high [COD], k−1[COD] ≫ k2 and
eq 3 can be derived. The alternative, an equilibrium
approximation in which the ligand exchange rate does not
influence the rate of reaction, is not consistent with our results
because this should lead to good inverse first-order behavior in
[COD] across all COD concentrations (i.e., eq 3), not just at
higher [COD]. Our results are consistent with an increasing
contribution from the k−1[COD] term as [COD] increases and,
therefore, with eq 1.
aryl halides to a Pd0 complex via a Meisenheimer intermediate
had ρ = 5.2.36
Our results are consistent with a change in rate-determining
step or with the contributions of different processes to
determining the overall rate. A plausible explanation is that
the electron-poor aryl halides which are better η2 ligands for
electron-rich metal centers displace the COD more readily or
that the equilibrium for ligand exchange lies further toward a
[Ni(η2-ArX)(dppf)] complex. However, the oxidative addition
step from this intermediate may be faster for electron-rich aryl
halides. These two competing effects were revealed in a DFT
study of the oxidative addition of aryl halides to [Pd(PtBu3)2]
via [Pd(η2-arene)(PtBu3)] complexes.37 To further probe this
possibility, the reactions of p-MeOC6H4Br with 1 in the
presence of different concentrations of COD were monitored
by 31P NMR experiments. By krel for each experiment being
defined as kobs divided by kobs in the absence of added COD,
the plot in Figure 4 was constructed. This shows that the more
−d[1]/dt = (k1k2[1][ArX])/(k−1[COD] + k−2)
(1)
−d[1]/dt = k1[1][ArX]kobs = k1[ArX]
(2)
−d[1]/dt = (K1k2[1][ArX])/[COD]kobs
= (K1k2[ArX])/[COD]
(3)
Figure 3. Hammett treatment for the oxidative addition of p-YC6H4Cl
(0.44 mol L−1) to [Ni(COD)(dppf)] (1) (0.022 mol L−1) in benzene-
d6 at 50 °C. Each experiment was performed in duplicate, and each
replicate is plotted separately.
COD displacement by alkene ligands is faster than oxidative
addition. 1 reacts with 1 equiv of styrene at room temperature
within a few minutes to form an equilibrium mixture that
consists of a 1:20 mixture of 1 and [Ni(η2-styrene)(dppf)].
Kinetic studies with 20 equiv of p-chlorostyrene at 50 °C
yielded an analogous species immediately after substrate
addition; this species then decayed via a pseudo-first-order
process with a rate similar to that for p-chlorotoluene
([2.63(1)] × 10−4 and [2.39(3)] × 10−4 s−1, respectively).
This is consistent with a pre-equilibrium followed by rate-
determining oxidative addition. The oxidative addition of
alkene-containing aryl halides to [Ni(PEt3)4] is occurs via
[Ni(η2-alkene)(PEt3)2] and a “ring-walking” process.33 As the
rate of ligand exchange is involved in the rate expression (eq 1),
it might be expected that substrates bearing good ligands (such
as 4-chlorostyrene) might react much more quickly than
simpler aryl halides. However, the nickel center must still leave
the alkene to “ring-walk” to the C−X bond prior to oxidative
addition, which presents an energetic barrier.
Figure 4. Plot of krel (kobs divided by kobs in the absence of COD)
versus [COD] for the reactions of (i) p-F3CC6H4Br (filled red circles)
and (ii) p-MeOC6H4Br (open black circles) with 1 in benzene-d6
(0.022 mol L−1) at 50 °C. The concentration of aryl halide was 0.44
mol L−1 in each case. All reactions were performed in duplicate, and all
replicates are plotted.
A Hammett analysis34 was carried out using the following
para-substituted substrates: chloroanisole, chlorotoluene, chlor-
obenzene, methyl chlorobenzoate, and chloro(trifluoromethyl)-
benzene.35 The resulting plot shows that for electron-rich
substrates ρ = 1.2 and that there appears to be an opposite
trend for electron-poor substrates (Figure 3).
This outcome differs significantly from a number of relevant
examples in the literature. The oxidative addition of aryl
chlorides to [Ni(PEt3)4] via rate-determining electron transfer
had ρ = 5.4.29 For oxidative addition to [Ni(PPh3)4], substrates
with electron-donating substituents had similar reaction rates
suggestive of a common rate-determining step such as ligand
dissociation from nickelwhile a set of electron-withdrawing
substrates had ρ = 8.8.30 The SNAr-like oxidative addition of
electron rich substrate, which is a poorer ligand for the
electron-rich d10 Ni0 complex, is more sensitive to the addition
of COD; 2 equiv of COD (relative to 1) decreases the rate of
oxidative addition of p-F3CC6H4Br by 58% but decreases the
rate of oxidative addition of p-MeOC6H4Br by 71%. These
results are consistent with our proposal for the switch in rate-
determining step.
The reactions of p-F3CC6H4Br with 1 at temperatures from
20 to 50 °C (inclusive) were used to obtain activation
parameters: ΔH⧧ = 24(1) kcal mol−1 and ΔS⧧ = 0(3) cal K−1
mol−1. The displacement of COD is involved in the overall
reaction; therefore, ΔS⧧ is the sum of terms for COD exchange
C
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comparisons in the literature are only qualitative, consider
only halides,29,30 or rely on the use of prototypical reactions
which have complex catalytic cycles and which might be
influenced by various other factors.24,53−55 Liu has reported a
computational study of the activation of various Ph−X bonds
by [Ni(PMe3)], showing that the barrier decreases in the order
X = OMe > F > OAc > OMs > NHAc > OTs > SAc > Cl >
OTf > Br.56 Schoenebeck has used DFT calculations to
compare the reactivity of different electrophiles toward dppf−
Ni0, finding that the barrier decreases in the order OMe > OPiv
> OTs > OMs > OTf.11 Most of these studies consider
reactions via mono- rather than bisphosphine Ni0 complexes;
thus, they cannot necessarily be extrapolated to dppf−Ni0 or
other such species. Here, this gap in the literature is addressed
by quantitatively ranking these substrates in the order of the
rate at which they undergo oxidative addition to [Ni(COD)-
(dppf)] (1).
Scheme 2. Mechanisms for the Oxidative Addition of Aryl
Halides to [Ni(COD)(dppf)] (1)
The following p-F3CC6H4X substrates were prepared or
purchased: X = OTf, OTs, OSO2NMe2, OPiv, OCO2Et,
OCONEt2, OMe, F. Many of these substrates underwent
reaction only very slowly at 50 °C and so were studied at 70 °C
in toluene-d8 instead (Table 2).57 Rate constants for the
Table 2. Rate Constants for the Oxidative Addition of
Selected Aryl (Pseudo)halides (0.44 mol L−1) to
[Ni(COD)(dppf)] (1) (0.022 mol L−1 in Benzene-d6 at 293
or 323 K or in Toluene-d8 at 343 K)
and oxidative addition. The small ΔS⧧ rules out bimetallic
oxidative addition. Three common mechanisms for the
oxidative addition of aryl halides to zerovalent d10 transition
metals are illustrated in Scheme 2.38 Oxidative addition to
anionic Pd0 is known;39 however, anionic Ni0 intermediates are
not considered here because no ions are added to these
reactions, the medium is of low polarity, and it is known that
the addition of anions to the oxidative addition reactions of
dppf−Pd0 complexes does not lead to a rate enhancement.40,41
On the basis of literature reports, the shallow Hammett plot is
not consistent with electron transfer mechanisms or an SNAr-
type mechanism (ρ > ca. 4). A radical chain mechanism42 can
be excluded because the oxidative addition reaction proceeds in
the presence of 1 equiv of TEMPO. A concerted three-center
transition state is consistent with our results and with the
relatively low ΔS⧧ (cf. 2(1) cal K−1 mol−1 for the oxidative
addition of aryl iodides to [Pd(PPh3)4]);43 such transition
entry
X in substrate (p-F3CC6H4X)
T (K)
kobs (10−4 s−1
)
a
1
I
293
293
323
323
323
343
343
343
343
343
343
343
343
7.8(1)
0.238(4)
10.2(1)
2.39(3)
0.82(1)
10.6(4)
4.9(2)
a
2
Br
a
3
Br
a
4
Cl
5
OTs
6
OTs
7
OCO2Et
OTf
8
4.62(2)
2.79(7)
1.9(1)
9
OCONEt2
XOSO2NMe2
OPiv
10
11
12
13
1.0(1)
b
OMe
<0.01
b
F
<0.01
a
b
From Table 1. Estimated from ca. 25% conversion after 48 h.
states have been characterized using DFT techniques.11,15
A
related concerted five-center transition state for the oxidative
addition of aryl sulfamates to dppf−Ni0 has also been
reported.18
fluoride and the methyl ether are estimated (ca. 25%
conversion after 48 h) because the reaction is too slow to
monitor in situ.58 The tosylate and bromide were each studied
at two temperatures, so that kobs could be used to construct a
reactivity scale (Figure 5).
This has allowed a range of electrophiles to be ranked in the
order of their rate of oxidative addition to Ni0 for the first time.
It shows that most electrophiles will undergo reaction with 1 in
the presence of aryl ethers and aryl fluorides; it also allows the
identification of situations where synthetically useful selectivity
can be achieved between electrophiles (i.e., >10-fold rate
difference). Figure 5 also allows for comparisons with palladium
catalysis; the rate of oxidative addition to Pd0 typically
decreases in the order I > Br ≈ OTf > OTs > Cl.41,59−61 In
reactions with 1, the rate of oxidative addition decreases in the
order Br > Cl > OTs > OTf.
Oxidative Addition of Other Aryl Electrophiles. A great
strength of nickel catalysis is the ability to deploy a range of
substrates beyond the halides and triflates typically used in
palladium catalysis.1−3,44 Nickel has been used to achieve the
cross-coupling of aryl sulfonates and sulfamates at room
temperature,45,46 enable the deployment of carbamates
which are useful directing groups for ortho-lithiation
reactions)in cross-coupling catalysis,47 and even to allow
aryl ethers48,49 and aryl fluorides4,50 to be used in cross-
coupling. Esters can be used in cross-coupling51 and
decarboxylative cross-coupling52 reactions, depending on the
regioisomer.
There has not yet been a quantitative experimental
comparison of the reactivity of this varied palette of substrates
in oxidative addition to Ni0; this limits our understanding of the
reactivity of Ni0. The best examples of experimental
Aryl sulfamates undergo oxidative addition to 1 more slowly
than aryl carbamates, and tosylates undergo oxidative addition
more slowly than chlorides. These results are in contrast to
D
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Figure 5. Reactivity scale for the oxidative addition of aryl electrophiles to [Ni(COD)(dppf)] (1).
reactions conducted by Houk, Garg, and Snieckus55 and by
Monteiro,53 respectively. However, the literature studies
considered the outcomes of cross-coupling reactionsnot
just the reaction of the substrate with Ni0and used
[NiCl2(PCy3)2] as the catalyst, which may exhibit different
selectivity. The stability of complexes of the form [Ni(Ar)X-
(L)n] under the reaction conditions, and the rate at which they
undergo transmetalation, may be a function of ligand X.
To explore why triflates react more slowly than tosylates,
activation parameters were determined for these two oxidative
addition reactions. ΔH⧧ (kcal mol−1) increases in the order Br
24(1) < OTs 28(2) < OTf 31(2). ΔS⧧ is almost 0 for bromide
and tosylate (0(3) and 8(5) cal K−1 mol−1, respectively) yet
larger for triflate (16(7) cal K−1 mol−1), indicative of a change
in the nature of the transition state. Complex 1 reacts with 1-
naphthyl triflate to form a species consistent with [Ni(1-
naphthyl)(OTf)(dppf)], but the 31P{1H} signals are very broad,
suggesting a fluxional species; reaction in the presence of tetra-
n-octylammonium bromide yields the same signals initially, but
a pair of sharp doublets appears later. The latter are consistent
with the formation of [NiBr(1-naphthyl)(dppf)]. Aryl triflates
form cationic complexes upon oxidative addition to [Pd-
(PPh3)4] or [Pd(dppf)(η2-methyl methacrylate)], and the
results here are consistent with a similar scenario with complex
1. Alternatively, there may be different contributions from
three- and five-centered transition states for OTs and OTf.
Either way, the larger value of ΔS⧧ results from the fact that the
triflate moiety is more loosely bound or is outer sphere, and so
the oxidative addition TS is likely to be quite different.
Products of Oxidative Addition. Complex 1 reacts with
para-substituted aryl (pseudo)halides to form NiI products plus
biaryl; the latter products were detected by GC-FID and/or
GC-MS analysis. Archetypal [NiCl(dppf)] (2-Cl), which has
been isolated and spectroscopically characterized by Schoene-
beck,15 gives a spectroscopic benchmark from which to
compare the other NiI species in the series.62 1H NMR analysis
of samples from kinetic experiments with p-trifluoromethyl-
phenyl chloride and p-chlorotoluene revealed characteristic
signals corresponding to this species.17 Complex 2-Cl was
isolated from oxidative addition reactions repeated on a slightly
larger scale. It gave an EPR signal characteristic of a d9 metal
with giso = 2.172, significantly larger than that of the free
electron (ge = 2.0023).63−67 Hyperfine coupling from two
equivalent 31P nuclei (I = 1/2, 100% abundance) of the
chelating dppf ligand gives a triplet splitting pattern (Figure 6).
[NiBr(dppf)] (2-Br) and [NiI(dppf)] (2-I) were also detected
in the 1H NMR spectra of the corresponding kinetic
experiments and were characterized by methods including X-
ray crystallography and EPR spectroscopy. The solid-state
structures of 2-Br and 2-I obtained under our crystallization
Figure 6. X-band EPR spectra for [NiX(dppf)] complexes (2-X)
recorded at ambient temperature. Experimental data are depicted by
the black traces; simulations are depicted by the red lines. The
simulation parameters can be found in Table S5 in the Supporting
Information.
conditions were found to be centrosymmetric halide-bridged
dimers (see Figure 7 for (2-Br)2 and the Supporting
Information for (2-I)2); the X-ray crystal structures of the
monomer of 2-Br and the dimer (2-I)2 have recently been
reported.18,19 The preference for the larger halides to dimerize
is reflected in their EPR spectra, where comparatively weaker
signals were obtained for 2-Br and 2-I dissolved in THF. The
use of a coordinating solvent was necessary to generate a
measurable amount of monomeric species from the diamag-
netic dimeric form. Noticeably, there is a significant g shift
commensurate with the larger spin−orbit coupling constants of
E
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Figure 7. Molecular structure of [Ni(μ-Br)(dppf)]2 (2-Br)2
determined by X-ray crystallography, with thermal ellipsoids drawn
at 50% probability and hydrogen atoms excluded.
Figure 8. Partial 31P NMR spectra from the reaction of [Ni(COD)-
(dppf)] (1) (0.022 mol L−1) with o-MeC6H4I (0.44 mol L−1) in
benzene-d6 at 50 °C, before substrate addition (top), after 1 h
(middle), and after 2.5 h (bottom). Ph3PO is an internal standard.
these heavier halides and a concomitant decrease in the 31P
hyperfine coupling constant in comparison to 2-Cl, given the
more covalent bonding with Br and I (see Table S5 in the
Supporting Information).
Several new NiI complexes, generated in situ in toluene, were
similarly characterized by EPR at ambient temperature (Figure
6). The sulfamate complex 2-OSO2NMe2 has been observed
spectroscopically in a recently published study.18 As all
substrates other than the halides provide an oxygen donor
ligand, g and A remain invariant across the series, nearly
identical with those of the archetypical 2-Cl (Table S5 in the
Supporting Information). Unlike the halides, the availability of
adjacent donor atoms presents the possibility of a bidentate
coordination mode, changing the symmetry from trigonal
planar to pseudotetrahedral. Only the EPR spectra of 2-OPiv
displayed overlapping signals; the simulation was achieved by
including 16% of a second species with smaller g = 2.144 but
the same A value. This signal could arise from tetrahedral NiI,68
as pivalate is more likely than the other substrates to chelate the
metal ion. It is possible that the shift in g for the other ligands is
not large enough to distinguish the tetrahedral complex from
the dominant trigonal complex. A more detailed spectroscopic
study is currently underway to elucidate the molecular and
electronic structures of these new NiI complexes. Nevertheless,
these data are sufficient at this point to show that the products
of oxidative addition of p-F3CC6H4X compounds to 1 are NiI
complexes 2-X.
Complex 1 reacts with the carbonate (p-F3CC6H4OCO2Et)
at 70 °C to form a transient (unidentified) diamagnetic
complex (δP 22.4 ppm) and signals consistent with NiI (δH 12.5
ppm, ω1/2 = ca. 200 Hz). The formation of a paramagnetic
species was evidenced by a weak signal in the EPR spectrum of
this reaction mixture in toluene (Figure 6). The g and A values
matched the other spectra in this study, consistent with an O-
donor carbonate ligand completing the NiI coordination sphere
(Table S5 in the Supporting Information).
Reactions with ortho-substituted aryl halides lead to species
consistent with [Ni(Ar)X(dppf)] (e.g., Figure 8; OPPh3 is an
internal standard),26,27 confirmed by comparison with an
authentic sample of [NiCl(o-tol)(dppf)] (3-Cl). However,
these complexes do not persist under the reaction conditions,
and the concentration present at the end of the reaction does
not account for all of the 1 that was present at the start. GC-MS
analysis of reaction mixtures reveals the presence of both
toluene and a species consistent with 2,2′-dimethylbiphenyl.
Experiments were carried out to identify the factors that
influence the stability of [Ni(Ar)X(dppf)]; 1- and 2-
bromonaphthalenes have similar electronic properties but
different steric environments around the C−Br bond. The
oxidative addition reactions of 1- and 2-bromonaphthalene are
(respectively) 4 and 6 times faster than that of p-
(trifluoromethyl)bromobenzene in benzene-d6 at 20 °C (kobs
= [0.97(5)] × 10−4, [1.59(4)] × 10−4, and [0.238(4)] × 10−4
s−1, respectively), consistent with previously observed reactivity
differences.18,49 The 1-isomer reacts with 1 to form a NiII
complex; GC analysis showed naphthalenepotentially from
decomposition of this complex in the GC inletand traces of
1,1′-binaphthyl. The 2-isomer reacts to produce no diamagnetic
phosphine-containing species, while GC analysis revealed no
naphthalene but quantities of 2,2′-binaphthyl. Only para-
magnetic products were obtained when complex 1 was exposed
to o-bromofluorobenzene or p-bromofluorobenzene; thus, an
ortho substituent larger than fluorine is necessary to stabilize
NiII. There has been discussion of this “ortho effect” in the
literature, with the most likely explanation being steric
protection of the nickel center.69,70
Origin of NiI Products. [Ni(Ar)X(L)n] complexes are
more stable when Ar is ortho-substituted,71−73 although ligand
choice is crucial.74 A thorough understanding of the effect of
structure on the reactivity of these types of complex is
important because these serve as intermediates, not only in
nickel-catalyzed cross-coupling but also in the reductive cross-
coupling of aryl and alkyl electrophiles,75 and as potential
intermediates in tandem nickel/photoredox-mediated reac-
tions6,7,76 or in new reactions currently under development.77
The oxidative addition of chlorobenzene or 2-chloronaph-
thalene to [Ni(COD)(dppf)] (1) leads to [NiCl(dppf)] (2-
Cl),15,17 while o-chlorotoluene produces [NiCl(o-tol)(dppf)]
(3-Cl).26 On the basis of experiments described here, it is
proposed that [NiCl(Ar)(dppf)] complexes always form
initially and that NiI arises from a series of comproportionation
and disproportionation events. This is consistent with a recent
computational study of this process.18
A solution of 1 was exposed to p-F3CC6H4I at 10 °C and
warmed to 27 °C in the magnet of the NMR spectrometer, at
which point two new species were identified by 31P{1H} NMR
spectroscopy (Figure 9). One of these is consistent with
[NiI(p-F3CC6H4)(dppf)], while the other is as yet unidentified
(δP ∼31 ppm); both are consumed as the reaction progresses,
to yield no diamagnetic phosphorus-containing species. The
expected NiII product of oxidative addition is obtained initially
but reacts rapidly in a subsequent process. The unidentified
signal is not [Ni(dppf)2] (δP 15 ppm)78 or [NiI2(dppf)]
(paramagnetic). Attempts to synthesize a model [Ni-
F
DOI: 10.1021/acs.organomet.7b00208
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Comproportionation leads to [NiX(dppf)] (2-X) and a NiI−
aryl species (Scheme 3a).18 Notably, isolable [Ni(Ar)(L)n]
Scheme 3. Proposed Mechanism for the Formation of
[NiX(dppf)] (2-X) from the Oxidative Addition of Aryl
(Pseudo)halides to [Ni(COD)(dppf)] (1)
Figure 9. Partial 31P{1H} NMR spectrum from exposing [Ni(COD)-
(dppf)] (1) (0.022 mol L−1) in benzene-d6 to p-F3CC6H4I (0.44 mol
L−1) at 283 K and warming to 300 K.
Figure 10. Kinetic profile for the reaction of 1 equiv of [Ni(COD)-
(dppf)] (1) (black) and 1 equiv of [NiCl(o-MeC6H4)(dppf)] (3-Cl)
(red) to form [NiCl(dppf)] (2-Cl) in 1/1 THF/benzene-d6 at 50 °C.
Scheme 4. Prototypical Reactions To Assess the Effect of
Electrophile Identity on Reactivity in Catalysis Mediated by
[Ni(COD)(dppf)] (1)
(Ar)2(dppf)] complex via the addition of o-MeC6H4MgCl to
[NiCl(o-MeC6H4)(dppf)] at −50 °C in the NMR spectrometer
led only to decomposition, while repeating the experiment
(with o-MeC6H4MgCl or MesMgBr) in the presence of COD
led only to the smooth formation of complex 1 via reductive
elimination. The reaction of 1 with biphenylene79 led to the
formation of a new species that was not consistent with the
unknown species in Figure 9 (δP ∼13 ppm). Notably, very few
[Ni(Ar)2(L)] complexes have been reported with bidentate
ligands (such as [Ni(Mes)2(bpy)]);80,81 however, some trans-
[Ni(Ar)2(L)2] species69,82−84 of often limited stability85 are
known.
[NiCl(o-MeC6H4)(dppf)] (3-Cl) is relatively stable in
solution at room temperature and at slightly elevated
temperatures26 but decomposes when it is heated in the
presence of 1.86 The production of biaryl is therefore
predominantly via a comproportionation step to form [NiCl-
(dppf)] (2-Cl). An alternative mechanism could be envisaged,
in which ligand scrambling87 between two molecules of 3-Cl
leads to [Ni(o-MeC6H4)2(dppf)] and [NiCl2(dppf)], which is
then followed by reductive elimination to form biaryl and
comproportionation to form two molecules of [NiCl(dppf)];88
this process is known to be energetically favorable overall, on
the basis of calculated values of ΔGrxn.19 However, kinetic
experiments show that such a process is much less competent
than the reaction between 3-Cl and 1: the comproportionation
reaction between 1 and 3-Cl was profiled (Figure 10); 25% of
complex 1 is consumed but all of 3-Cl reacts. When the
reaction is repeated in the presence of added COD, no
comproportionation occurs. Other studies have reported the
comproportionation of [Ni(dppf)(L)n] complexes with [Ni-
(Ar)X(dppf)] but have not reported kinetic data.17,18 Our data
show that the comproportionation is mediated by Ni0 but does
not consume most of the Ni0.
complexes are rather rare.89 We propose that disproportiona-
tion of the latter leads to a Ni0 complex that is recaptured by
COD and a bi(aryl) complex that undergoes reductive
elimination (Scheme 3b). Alternatively, hydrogen atom
abstraction from the solvent might lead to [Ni0(dppf)] and
arene (Scheme 3b)); arene and biaryl byproducts have been
observed in various oxidative addition reaction mixtures during
this work and in catalytic reactions in the literature.18 The
following evidence is consistent with this proposal: (i) the
presence of 1 significantly increases the rate at which 3-Cl is
converted to 2-Cl, via comproportionation, (ii) the addition of
o-MeC6H4MgCl to [NiCl(dppf)] in the presence of COD leads
to 1 without detectable intermediates, (iii) attempts to form
[Ni(Ar)2(dppf)] in an analogous manner, using o-
MeC6H4MgCl or MesMgBr in the presence of COD, lead to
1 without detectable intermediates, and (iv) the comproportio-
nation reaction is not stoichiometric in complex 1.90 Both the
disproportionation and hydrogen atom abstraction mechanisms
appear to be operative, although the former is favored in
reactions of aryl halides without ortho substitutents (vide
supra).
The fact that the major pathway relies on 1 requires careful
consideration of the kinetics of the oxidative addition reaction.
Complex 1 is involved in a pseudo-first-order oxidative addition
step in which it is consumed (with observed rate constant
A proposal for the mechanism of the formation of 2-X from
oxidative addition reactions is presented in Scheme 3.
G
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Organometallics
Article
the lack of a buildup of the NiII complex. This explains the good
fit of the experimental data for [1] versus time to a pseudo-first-
order process. The observed rate constants obtained are
sufficient to quantify and compare the rates at which different
aryl (pseudo)halides undergo reaction with 1.91
Table 3. Results from Suzuki−Miyaura Reactions with 1-
And 2-Naphthyl (Pseudo)halides Catalyzed by
a
[Ni(COD)(dppf)] (1)
substrate
1-Br
[1] (mol %)
NapPh
NapNap
NapH
10
10
10
10
10
10
10
97
96
95
64
19
24
14
<1
<1
<1
3
3
3
2
5
9
3
8
d[1]/dt = −kobsOA[1] − kobsCom[1][Ni(Ar)X(dppf)]
1-OTs
1-OTf
(4)
1-OCO2Et
1-OCONEt2
1-OSO2NMe2
1-OPiv
1
Relationship between Oxidative Addition and Catal-
ysis. The rate and selectivity of oxidative addition will have
implications for catalysis. Previous studies of [NiCl(dppf)]
suggest that it is an inferior catalyst at best17 and completely
inactive at worst.15 The reactions of 1- and 2-naphthyl
substrates were considered to allow the effects of both
electrophile identity and ortho-substitution pattern to be
considered.92 The model reactions in Scheme 4 were
considered; the outcomes were quantified by GC-FID analysis
calibrated for each analyte (Table 3).
The correlation between oxidative addition rate (see Figure
5) and reaction outcome is neither linear nor perfect, but it
shows that any substrate which undergoes oxidative addition
more slowly than the aryl carbonate is unlikely to yield
favorable results in catalysis under these conditions. In contrast
to the measured rates of oxidative addition, the triflate and
tosylate substrates achieve very similar results in catalysis;
similarly, the carbonate performs much more poorly than the
triflate, despite the oxidative addition step occurring at a similar
rate. The identity of the electrophile has an effect on catalysis at
stages other than oxidative addition. It will affect subsequent
steps in the catalytic cycle such as transmetalation: a different
[Ni(Ar)X(dppf)] complex is formed which will undergo
transmetalation (or be converted to the hydroxide complex)93
at a different rate.23 The oxidative addition rate is measured
under carefully controlled conditions where relatively few
factors can be changed; in contrast, the optimization of catalytic
reactions requires careful study of the effects of concentration,
solvent, catalyst loading, base identity and quantity, coupling
partner identity and quantity, and so forth. The poor
performance of the more reluctant electrophiles in cross-
coupling reactions shows that the oxidative addition rate plays a
significant role in determining the feasibility of catalytic
reactions. In addition, substrates that lead to NiI products
more readily (i.e., 2-naphthyl substrates) show an increased
2
1
1-Br
1
1
1
1
1
24
32
34
11
2
<1
<1
<1
<1
2
1-OTs
1-OTf
1
1-OCO2Et
1-OCONEt2
1-Br
10
10
10
10
10
10
10
1
93
94
96
40
25
17
18
29
30
33
20
6
6
5
2
3
1
1
2
1
1
3
4
1-OTs
1-OTf
1-OCO2Et
1-OCONEt2
1-OSO2NMe2
1-OPiv
5
11
4
1-Br
1-OTs
1
2
<1
<1
1-OTf
1
2
1-OCO2Et
1-OCONEt2
1
1
a
The balance of the material is unreacted naphthyl (pseudo)halide.
Reaction conditions: 10 mol % of [Ni(COD)(dppf)] (1), 2 equiv of
PhB(OH)2, 3 equiv of K3PO4, toluene, 90 °C, 4 h. All outcomes were
quantified using calibrated GC-FID analysis.
kobsOA) and a second-order process in which it mediates
comproportionation and the formation of biaryl (with observed
rate constant kobsCom). Therefore, the consumption of 1 can be
approximated to eq 4. However (i) the latter process does not
consume 1, (ii) [NiII(Ar)X(dppf)] is always very low for para-
substituted aryl halides because it is not observed by 31P NMR
spectroscopy under the reaction conditions, and (iii) this
second process is much faster than oxidative addition due to
Scheme 5. Summary of the Outcomes of This Study
H
DOI: 10.1021/acs.organomet.7b00208
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
propensity to form deleterious homocoupling side products.
The mechanism outlined above may well be the means by
which NiI forms in a multitude of other reactions.
ACKNOWLEDGMENTS
■
This research was generously funded by the Engineering and
Physical Sciences Research Council (EP/M027678/1). D.J.N.
thanks the University of Strathclyde for a Chancellor’s
Fellowship. G.L. thanks the EPSRC and the University of
Strathclyde for a Vacation Bursary. We thank Dr. John
Parkinson for running [1H,1H] EXSY experiments, Professor
Jonathan Percy for access to laboratory equipment and
chemicals, and Professor Robert Mulvey and Dr. Allan Watson
for helpful discussions. We are grateful to Craig Irving, Patricia
Keating, Alexander Clunie, and Gavin Bain for their assistance
with technical and analytical facilities.
CONCLUSIONS
■
The outcomes of this study are summarized below and in
Scheme 5. These advance our understanding of mechanism and
structure/activity relationships in nickel catalysis. Most
importantly, we present the most detailed reactivity scale to
date for the oxidative addition of aryl electrophiles to a model
nickel catalyst. Further studies are underway to map different
ligand systems that are relevant to nickel catalysis onto this
scale. We have investigated the mechanism of the oxidative
addition to [Ni(COD)(dppf)] (1); COD displacement is
necessary to allow the reaction to occur but is not rate-
determining. This is important because many synthetic
protocols use catalysts generated in situ from ligands and
[Ni(COD)2]. [NiX(dppf)] arises primarily from compropor-
tionation of [Ni(Ar)X(dppf)] with [Ni(COD)(dppf)], leading
to the formation of biaryl. The propensity of substrates to form
[NiX(dppf)] is a factor of the steric effect of the ortho
substituent, and in catalytic reactions, substrates that lead to NiI
produce more unwanted homocoupling side products. Further
studies are currently underway in our laboratory with the aim of
extending and applying this reactivity scale. The raw data
underpinning this study (NMR spectroscopy data for character-
isation and kinetic studies, EPR spectroscopy data for
paramagnetic complexes, GC-FID data from catalysis) is
available from http://dx.doi.org/10.15129/85c810eb-cdb3-
420c-8957-d392ad76ce11. Crystallographic data for (2-Br)2
and (2-I)2 can be retrieved from the CCDC (1478811 and
1478812) via www.ccdc.cam.ac.uk/structures/.
ABBREVIATIONS
■
dppf, 1,1′-bis(diphenylphosphino)ferrocene; bpy, 2,2′-bipyri-
dine; COD, 1,5-cyclooctadiene
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
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met.7b00208.
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*E-mail for S.S.: Stephen.Sproules@glasgow.ac.uk.
*E-mail for D.J.N.: david.nelson@strath.ac.uk.
ORCID
Alan R. Kennedy: 0000-0003-3652-6015
Stephen Sproules: 0000-0003-3587-0375
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The authors declare no competing financial interest.
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I
DOI: 10.1021/acs.organomet.7b00208
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Organometallics
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J
DOI: 10.1021/acs.organomet.7b00208
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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In contrast, the higher molecular weights of the naphthalene
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straightforward, and both isomers of naphthol are readily available.
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K
DOI: 10.1021/acs.organomet.7b00208
Organometallics XXXX, XXX, XXX−XXX