Isolated Organometallic Nickel(III) and Nickel(IV) Complexes Relevant to Carbon-Carbon Bond Formation Reactions

Article abstract of DOI:10.1021/jacs.6b06862

Nickel-catalyzed cross-coupling reactions are experiencing a dramatic resurgence in recent years given their ability to employ a wider range of electrophiles as well as promote stereospecific or stereoselective transformations. In contrast to the extensively studied Pd catalysts that generally employ diamagnetic intermediates, Ni systems can more easily access various oxidation states including odd-electron configurations. For example, organometallic Ni^III intermediates with aryl and/or alkyl ligands are commonly proposed as the active intermediates in cross-coupling reactions. Herein, we report the first isolated Ni^III-dialkyl complex and show that this species is involved in stoichiometric and catalytic C-C bond formation reactions. Interestingly, the rate of C-C bond formation from a Ni^III center is enhanced in the presence of an oxidant, suggesting the involvement of transient Ni^IV species. Indeed, such a Ni^IV species was observed and characterized spectroscopically for a nickelacycle system. Overall, these studies suggest that both Ni^III and Ni^IV species could play an important role in a range of Ni-catalyzed cross-coupling reactions, especially those involving alkyl substrates.

Full text of DOI:10.1021/jacs.6b06862

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Article
Isolated Organometallic Nickel(III) and Nickel(IV) Complexes
Relevant to Carbon-Carbon Bond Formation Reactions
Jason W. Schultz, Kei Fuchigami, Bo Zheng, Nigam P. Rath, and Liviu M. Mirica
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06862 • Publication Date (Web): 06 Sep 2016
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Isolated Organometallic Nickel(III) and Nickel(IV) Complexes
Relevant to Carbon-Carbon Bond Formation Reactions
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Jason W. Schultz,† Kei Fuchigami,† Bo Zheng,† Nigam P. Rath,‡ and Liviu M. Mirica*,†
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† Department of Chemistry, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, United States
‡ Department of Chemistry and Biochemistry, University of Missouri-St. Louis, One University Boulevard, St. Louis, Missouri
63121-4400, United States
* e-mail: mirica@wustl.edu
ABSTRACT
Nickel-catalyzed cross-coupling reactions are experiencing a dramatic resurgence in
recent years given their ability to employ a wider range of electrophiles as well as promote
stereospecific or stereoselective transformations. In contrast to the extensively-studied Pd
catalysts that generally employ diamagnetic intermediates, Ni systems can more easily access
various oxidation states including odd-electron configurations. For example, organometallic NiIII
intermediates with aryl and/or alkyl ligands are commonly proposed as the active intermediates
in cross-coupling reactions. Herein, we report the first isolated NiIII-dialkyl complex and show
that this species is involved in stoichiometric and catalytic C-C bond formation reactions.
Interestingly, the rate of C-C bond formation from a NiIII center is enhanced in the presence of an
oxidant, suggesting the involvement of transient NiIV species. Indeed, such a NiIV species was
observed and characterized spectroscopically for a nickelacycle system. Overall, these studies
suggest that both NiIII and NiIV species could play an important role in a range of Ni-catalyzed
cross-coupling reactions, especially those involving alkyl substrates.
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INTRODUCTION
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Transition metal-catalyzed cross-coupling reactions have become indispensable in the
synthesis of a wide range of pharmaceuticals, materials, and fine chemicals.1-2 While Pd systems
are arguably the most commonly used catalysts in these transformations, Ni-based catalytic
systems have been developed recently for cross-coupling reactions given their broader substrate
scope and the ability to also employ alkyl electrophiles and nucleophiles.3-10 By comparison to
the Pd-mediated transformations that generally employ diamagnetic intermediates, the Ni
systems can more easily undergo one-electron redox reactions involving NiI and NiIII species.11-23
For example, a large fraction of Ni-catalyzed cross-coupling reactions are proposed to proceed
through organometallic NiIII di-alkyl/aryl intermediates that undergo rapid reductive elimination
to form a new C-C bond.20,24-45 However, to date no such NiIII di-alkyl/aryl complexes have been
isolated and characterized in detail.
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During the past several years, we have employed tetradentate pyridinophane ligands to
stabilize uncommon organometallic PdIII and PdIV complexes capable of C-C and C-heteroatom
bond formations.46-50 In addition, we have recently used similar ligands to stabilize
organometallic NiIII species that can undergo C-heteroatom51-52 and C-C bond formation
reactions.51,53-55 Herein, we employ the ligand N,N′-di-methyl-2,11-diaza[3.3](2,6)pyridinophane
(MeN4) to stabilize the first isolated NiIII-dialkyl complex that allowed for its detailed
characterization and investigation of its C-C bond formation reactivity. Interestingly, while this
NiIII species undergoes C-C bond formation relatively slowly, it undergoes rapid and selective C-
C bond formation in the presence of one equivalent oxidant, suggesting the involvement of a
transient NiIV intermediate. Further support for such a species was provided by the isolation of a
NiIV metallacycle complex in which the rate of C-C bond formation is reduced. Overall, these
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studies report the isolation of NiIII–dialkyl and NiIII–aryl/alkyl complexes and provide strong
evidence for the potential involvement of organometallic NiIII and NiIV species in Ni-catalyzed
cross-coupling reactions, especially for transformations involving alkyl substrates.
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RESULTS AND DISCUSSION
Synthesis and Characterization of NiII/III Complexes. The red complex (MeN4)NiIIMe2, 1, was
prepared in 65% yield from the precursor (MeN4)NiIIBr2 via transmetalation with
methylmagnesium chloride (Scheme 1).56 The single crystal X-ray structure of 1 reveals a
square planar geometry for the Ni center that is bound to two pyridyl nitrogen atoms from the
MeN4 ligand and two methyl groups, with an average equatorial Ni-Npyridyl bond length of 1.973
Å and an average Ni-C bond length of 1.913 Å (Figure 1). As expected, 1 is diamagnetic, likely
due to the strong σ-donor methyl groups that favor the low-spin square planar geometry. The
cyclic voltammetry (CV) of 1 exhibits two oxidation processes at -1500 and -500 mV vs Fc+/Fc,
as well as a pseudo-reversible wave at ~0 V vs Fc+/Fc (Figure S1).56 The first two oxidation
events are tentatively assigned to NiII/NiIII couples that could correspond to complex 1 in which
the MeN4 ligand adopts different conformations,57 while the pseudo-reversible oxidation at ~0 V
is assigned to a NiIII/NiIV redox couple (vide infra).
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Scheme 1. Synthesis of (MeN4)NiMe2 Complexes
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Figure 1. ORTEP representation of 1 (top) and the cation of 2 (bottom), with 50% probability
thermal ellipsoids. Selected bond distances (Å), 1: Ni1-C1, 1.913; Ni1-C2, 1.913; Ni1-N1, 1.973;
Ni1-N2, 1.973; 2: Ni1-C1, 1.987; Ni1-C2, 1.978; Ni1-N1, 1.979; Ni1-N2, 1.996; Ni1-N3, 2.252;
Ni1-N4, 2.240.
Complex 1 can be easily oxidized with 1 equiv [Cp2Fe]PF6 (FcPF6) in THF at -50°C to
yield [(MeN4)NiIIIMe2]PF6, 2, as an orange-red solid.56 X-ray quality crystals of 2 were obtained
from THF/pentane, and the crystal structure shows the presence of a NiIII center that adopts a
distorted octahedral geometry, with an average axial Ni-Namine bond length of 2.246 Å and a
shorter average equatorial Ni-Npyridyl bond length of 1.988 Å (Figure 1). These values are
consistent with our previously reported organometallic (RN4)NiIII complexes (Ni-Namine bond
lengths of 2.212 - 2.431 Å, Ni-Npyridyl bond length of 1.903- 1.965 Å),51,53 as well as other
reported six-coordinate NiIII complexes.58-59 Similarly, an average Ni-CMe bond length of 1.983 Å
is consistent with the other structurally characterized NiIII-alkyl complexes reported (1.923,
1.994, 2.011 Å, 2.015 Å, and 2.039 Å).60-64 The CV of 2 in MeCN reveals a pseudo-reversible
oxidation wave at ~0 V vs. Fc+/Fc, followed by an irreversible oxidation at 0.36 V (Figure S2).
The former reversible redox events suggests that the NiIV oxidation state may be accessible, yet
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the one-electron oxidized species derived from 2 likely undergoes a rapid chemical reaction such
as reductive elimination (vide infra), and the resulting Ni product could give rise to the higher
potential irreversible oxidation (i.e., an ECE electrochemical mechanism). The EPR spectrum of
complex 2 exhibits a pseudo-axial signal with a gx, gy, and gz of 2.228, 2.210, and 2.014,
respectively, and superhyperfine coupling was observed in the gz direction (A2N = 14.3 G) due to
the two axial N donors (I =1) coupling to the NiIII center (Figure 2), consistent with the presence
of a NiIII metal center in a dz2 ground state.22,58-59,65-66 Furthermore, density functional theory
(DFT) calculations and the calculated spin density support a metal-based radical description for 2
in which the unpaired electron resides mostly on the Ni center (Figure 2). Importantly, 2
represents to the best of our knowledge the first isolated NiIII–dialkyl complex. With this
compound in hand, we can now probe directly its reactivity, especially since such organometallic
NiIII species have been proposed for the past few decades as key intermediates in cross-coupling
and other C-C bond formation reactions.24-45
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Exp
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Figure 2. (Top) EPR spectrum (black line) of 2 in PrCN at 77K, and the simulated EPR
spectrum (red line) using the following parameters: gx= 2.228, gy= 2.210, gz= 2.014 (A2N = 14.3
G). (Bottom) DFT calculated Mulliken spin density for 2 (shown as a 0.05 isodensity contour
plot), and the relevant atomic and Ni orbital contributions to the spin density.
C-C Bond Formation Reactivity of NiII/III Complexes. The organometallic reactivity of 1 and
2 was then investigated under different reaction conditions. First, a solution of 1 in MeCN slowly
generates ethane in 11% yield in 8 hours at room temperature under N2, along with a detectable
amount of methane (Scheme 2a). The reductive elimination of ethane from 1 most likely
proceeds through a classical NiII/Ni0 mechanism, as suggested by the formation of an observable
amount of Ni black. While performing the reductive elimination at elevated temperatures does
lead to formation of ethane in larger yields, appreciable amounts of methane were also observed;
thus, all reactivity studies were performed at RT to limit the side reactions. By comparison, the
addition of 1 equiv of a mild oxidant such as ferrocenium hexafluorophosphate (FcPF6) or
acetylferrocenium hexafluorophosphate (AcFcPF6) to a solution of 1 under N2 increases the
ethane yield to 61 % in 8 hours, yet methane is also produced in 24 % yield. Interestingly,
addition of 2 equiv AcFcPF6 to a solution of 1 in MeCN resulted in clean formation of ethane in
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88% yield in 30 min at RT under N2 (Scheme 2a). Since the oxidation potential of AcFcPF6
(~0.25 V vs. Fc+/Fc)67 is more positive than the potential corresponding to the reversible
oxidation observed for 1 and tentatively assigned to the NiIII/NiIV redox couple (vide supra), the
rapid formation of ethane suggests the involvement of a NiIV intermediate. The C-C bond
formation reactivity of the isolated NiIII complex 2 was also probed, to reveal that 2 in MeCN
slowly generates 54% ethane after 24 hours, along with an appreciable amount of methane
(Scheme 2b). By comparison, addition of 1 equiv AcFcPF6 to a solution of 2 in MeCN afforded
ethane cleanly in 84 % yield after 24 hours, further supporting the potential involvement of a
NiIV species in the ethane elimination step (Scheme 2b). The slower reactivity observed for 2 in
presence of 1 equiv AcFcPF6 vs. that of 1 in presence of 2 equiv AcFcPF6 is proposed to be due to
the 6-coordinate geometry of the NiIII center in 2 – a geometry expected to be maintained in a
NiIV species, which should undergo reductive elimination slower than a 4- or 5-coordinate Ni
center generated upon direct two-electron oxidation of the square planar NiII complex 1.68-69 For
all these reactivity studies proposed to involve high-valent Ni species, the final Ni product was
determined to be the paramagnetic species [(MeN4)NiII(MeCN)2]2+, which could be crystallized
out of the reaction mixture.56
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Scheme 2. C-C Bond Formation Reactivity of the (MeN4)NiMe2 Complexes
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Additional mechanistic studies were employed to probe the observed C-C bond formation
reactivity. Crossover experiments employing a 1:1 mixture of 1 and (MeN4)NiII(CD3)2, 1-d6, in
MeCN revealed that upon addition of 1 equiv AcFcPF6 a 1:1 mixture of CH3CH3 and CH3CD3
was generated in ~20% yield for each product after 8 hours (Scheme 3). Given the typical yield
of ~60% ethane upon one-electron oxidation of 1 under the same conditions, the result suggests a
CH3CH3:CH3CD3:CD3CD3 ratio of 1 : 1 : 1. By comparison, the addition of 2 equiv AcFcPF6 to a
1:1 mixture of 1 and (MeN4)NiII(CD3)2, 1-d6, in MeCN rapidly generates in 30 min ~40%
CH3CH3 along with a very small amount of CH3CD3 (Scheme 3). Given the typical yield of 88%
ethane upon two-electron oxidation of 1, this result suggests that almost no crossover occurs
under these conditions, to give a CH3CH3:CH3CD3:CD3CD3 ratio of 1 : ~0 : 1, while the small
amount (<5%) of CH3CD3 detected most likely arises from transient formation of 2 through
partial one-electron oxidation of 1. Importantly, the yields of ethane under all of the investigated
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conditions are not affected significantly in the presence of the radical trap 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) and no TEMPO-Me was detected,56 suggesting that no
free methyl radicals are formed in the reaction mixture.
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Scheme 3. Crossover Reactivity Studies of (MeN4)NiIIMe2
Based on the reactivity studies mentioned above, two different mechanisms are proposed
for the observed C-C bond formation reactions. The slower ethane formation reactivity observed
upon one-electron oxidation of the NiII complex 1, or even slower when starting from the
isolated NiIII complex 2, is proposed to proceed through a methyl group transfer step that would
require the dissociation of one of the amine N donors, thus limiting the reaction rate (Scheme
4A). Such a slow methyl group transfer between two Ni centers is supported by the formation of
the ethane products CH3-CH3 : CH3-CD3 : CD3-CD3 in a 1:1:1 ratio, and is similar to our
analogous high-valent Pd systems that have been investigated in detail.46-48 Formation of a
transient [(MeN4)NiIVMe3]+ intermediate 3 is then expected to undergo reductive elimination of
ethane (Scheme 4A). By comparison, the two-electron oxidation of the NiII complex 1 or one-
electron oxidation of the isolated NiIII complex 2 is proposed to directly generate a reactive
dicationic [(MeN4)NiIVMe2]2+ species 4 that should rapidly generate ethane upon reductive
elimination (Scheme 4B). In this case no appreciable crossover ethane products should be
generated, unless a small fraction of the NiII precursor is only partially oxidized to the NiIII
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complex 2. The rate of ethane formation should be particularly increased when the square planar
NiII complex 1 is rapidly oxidized by two electrons, as that could generate a transient and more
reactive 4- or 5-coordinate NiIV species formed before the conformation change of the MeN4
ligand to become a tetradentate ligand.48 This is also supported by the CV of 1 (Figure S1) that
reveals two oxidation events assigned to the NiII/NiIII couple and likely corresponding to two
conformations of MeN4 ligand binding in a tridentate and bidentate fashion to the Ni center,
respectively, and similarly to what was observed previously for analogous Pd complexes.57 In
addition, we cannot exclude the possibility that methyl group exchange could occur at the NiII
center in 1. However, if such methyl exchange is rapid, the formation of a statistical mixture of
(MeN4)NiII(CH3)2 : (MeN4)NiII(CH3)(CD3) : (MeN4)NiII(CD3)2 in a 1:2:1 ration is expected, which
upon reductive elimination would generate a 1:2:1 mixture of CH3-CH3 : CH3-CD3 : CD3-CD3,
which was not observed experimentally. Finally, particularly surprising is the limited reactivity
of the isolated NiIII complex 2, although several previous studies of Ni-catalyzed cross-coupling
reactions have proposed NiIII di-hydrocarbyl species as the key intermediates undergoing rapid
C-C bond formation.24-45 While the limited reactivity of 2 could be due to the use of a
tetradentate ligand that generates a coordinatively-saturated Ni center, it may be important to
consider that in some Ni systems an increased C-C bond formation reactivity could be due to the
formation of reactive transient NiIV species. This may be especially relevant to alkyl-alkyl cross-
coupling transformations, as the presence of two strong -donor alkyl groups on the Ni center
should facilitate the formation of a NiIV species under catalytic conditions.8,70-71
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Scheme 4. Proposed Mechanisms for One-electron (A) and Two-electron (B) Oxidation of 1
and Subsequent C-C Bond Formation (the bold methyl group in 3 represents an isotopically
labeled group that could lead to crossover products)
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Synthesis and Characterization of a NiIV Complex. In order to provide further evidence for
the proposed NiIV intermediate 4 mentioned above, we set out to synthesize a more stable NiIV
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species supported by the MeN4 ligand. Previously, use of the cycloneophyl group (-CH2CMe2-o-
C6H4-) has been shown to limit the extent of C-C reductive elimination from NiIV and PdIV
intermediates.15,17,72-76 As such, the NiII red complex (MeN4)NiII(cycloneophyl), 5, was prepared
in 63% yield from the (PMe3)2NiII(cycloneophyl) precursor77 via ligand exchange with MeN4
(Scheme5),56 and its single crystal X-ray structure reveals a square planar geometry for the NiII
center with an average equatorial Ni-Npyridyl bond length of 1.967 Å and an average Ni-C bond
length of 1.917 Å (Figure 3), similar to the bond lengths observed for 1. In addition, the CV of 5
shows well-defined oxidation waves at -1.17 and 0.25 V vs. Fc+/Fc, which are assigned to the
NiII/NiIII and NiIII/NiIV redox couples, respectively. Indeed, oxidation of 5 with 1 equiv FcPF6 in
THF at -50°C produces [(MeN4)NiIII(cycloneophyl)]PF6, 6, as an orange solid in 79% yield
(Scheme 5). The NiIII center in 6 adopts a distorted octahedral geometry with an average axial
Ni-Namine bond length of 2.254 Å and a shorter average equatorial Ni-Npyridyl bond length of
1.999 Å. As for the Ni-C bond lengths, the Ni-Csp3 bond is slightly longer then the Ni-Csp2 bond
(1.973 Å vs 1.920 Å), as observed previously for other high-valent Ni and Pd(cycloneophyl)
complexes.72,74-76 Similarly to 2, the EPR spectrum of 6 exhibits a pseudo-axial signal with
observable superhyperfine coupling in the gz direction (AN = 14.5 G).56
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Scheme 5. Synthesis of (MeN4)Ni(cycloneophyl) Complexes 6 and 7
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Figure 3. ORTEP representation of 5 (top) and the cation of 6 (bottom) with 50% probability
thermal ellipsoids. Selected bond distances (Å), 5: Ni1-C1, 1.879; Ni1-C8, 1.954; Ni1-N1, 1.936;
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Ni1-N2, 1.997; 6: Ni1-C17, 1.973; Ni1-C18, 1.920; Ni1-N1, 2.001; Ni1-N2, 1.997; Ni1-N3,
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2.241; Ni1-N4, 2.266.
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Importantly, the reversible redox event present in the CV of 6 at E1/2 = 0.21 V vs. Fc+/Fc
suggests that the corresponding NiIV complex should be accessible, and oxidation of 6 with 1
equiv NOPF6 in MeCN at -40°C yielded the desired orange-red complex
[(MeN4)NiIV(cycloneophyl)](PF6)2, 7 (Scheme 5). While several attempts to structurally
characterize 7 were unsuccessful, the complex was fully characterized by 1D and 2D 1H/13C-
NMR (Figures S12-S17), which reveal significantly downfield shifted resonances for 7
compared to 5 and suggest the presence of a more oxidized metal center.56 The downfield shift is
most notable for the methylene protons of the cycloneophyl group that move from 1.24 ppm in 5
to 5.33 ppm in 7. In addition, the presence of a NiIV center was confirmed by X-ray
photoelectron spectroscopy (XPS), which shows an increase of the Ni 2p3/2 and 2p1/2 binding
energies of ~0.6 eV between 6 and 7 and is consistent with the presence of a more oxidized Ni
center in 7 vs 6 (Figure 4).78 The use of the cycloneophyl bidentate ligand does indeed limit the
rate of reductive elimination from both 6 and 7, as the 1,1-dimethylbenzocyclobutane product
was obtained in only 10 % and 38 % yield, respectively, after 48 hours at RT.56 Overall, these
results confirm the formation of a NiIV complex supported by the MeN4 ligand and provide strong
support for the potential involvement of NiIV species in the oxidatively-induced C-C bond
formation reactions described above. Finally, it is important to note that the MeN4 ligand system
supports the formation of Ni complexes in three different oxidation states (II, III, and IV) and
thus could be employed in comparative reactivity studies aimed at deciphering the role of each
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oxidation state in various organometallic reactions, especially those relevant to cross-coupling
reactions.
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3000
6 (NiIII)
7 (NiIV)
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Bonding Energy (eV)
Figure 4. X-ray photoelectron spectra of the Ni binding energy region for complexes 6 (black
line) and 7 (red line). Selected bonding energies (eV), 6: 2ꢀ , 854.43; 2ꢀ , 871.79. 7: 2ꢀ ,
ꢁ/ꢂ
ꢁ/ꢂ
ꢃ/ꢂ
854.97; 2ꢀꢃ/ꢂ, 872.46.
Catalytic Reactivity of Ni Complexes. In addition to stoichiometric C-C bond formation
studies, we have also investigated the ability of the (MeN4)Ni complexes to catalyze cross-
coupling reactions. Gratifyingly, 1 was shown to be an active catalyst for the Kumada coupling
of aryl iodides with aryl or alkyl Grignard reagents. For example, the reaction of iodotoluene
with phenylmagnesium bromide or 1-hexylmagnesium bromide affords the corresponding
coupled products in 96% and 60% unoptimized yields, respectively, in 1 hour at RT (Scheme 6).
In addition, the reaction of chlorotoluene with phenylmagnesium bromide yields the coupled
product in 62% yield, although longer reaction time were needed for full conversion. By
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contrast, the coupling of 1-iodooctane with 1-hexylmagnesium bromide yielded only 20% of the
coupled product (Scheme 6).56
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Scheme 6. Kumada Cross-coupling Reactions Catalyzed by 1
R-X
R'MgX
PhMgBr
Time, h Yield a
p-tolyl-I
p-tolyl-Cl
p-tolyl-I
octyl-I
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1
96 %
62 %
60 %
20 %
PhMgBr
hexylMgBr
hexylMgBr
1
a Yields were determined using GC-FID with decane as the internal standard; no coupled products were
observed in these reactions in the absence of 1.56
The reduced catalytic reactivity of 1 observed in the presence of an alkyl halide was
further probed in a stoichiometric reaction. Addition of CD3I to 1 in MeCN led to rapid
formation of the NiIII species 2, as observed by EPR (Scheme S22), followed by its slow decay
and formation of ethane in 67% yield after 24 hours, along with 20% CD3CH3 (Scheme 7). When
the reaction of 1 with CD3I was performed in presence of the radical trap TEMPO, the TEMPO-
CD3 adduct was detected by GC in 85% yield and thus confirming the formation of CD3 radicals
in the reaction mixture (Scheme 7).56 This result provides strong support for the ability of alkyl
halides to undergo single-electron transfer (SET) from the NiII complexes and generate high-
valent Ni intermediates, as proposed previously in several mechanistic studies of Ni-catalyzed
cross-coupling reactions.34,51,79 In the absence of the radical trap, it is expected that the generated
alkyl radical could abstract an H atom from the solvent, or it could react with the formed NiIII
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species 2 and yield side products such as CD3CH3 (Scheme 8). Such radical side reactions could
lead to the limited catalytic reactivity observed for 1 in alkyl-alkyl Kumada coupling, in which
the side products octane and 2-hexyl-THF were observed at the end of the reaction.56 Overall,
these reactivity studies show that the investigated (MeN4)Ni complexes are catalytically
competent in cross-coupling reactions, and strong evidence is also provided for the involvement
of high-valent organometallic Ni species, both NiIII and NiIV, in cross-coupling reactions,
especially those involving alkyl electrophiles.
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Scheme 7. C-C Bond Formation Reactivity from the oxidation of (MeN4)NiIIMe2 with CD3I.
Scheme 8. Proposed Mechanism for the Oxidation of 1 by CD3I and Subsequent C-C Fond
Formation.
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CONCLUSION
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8
In conclusion, this report describes the use of the ligand N,N′-di-methyl-2,11-
diaza[3.3](2,6)pyridinophane (MeN4) to stabilize the first isolated NiIII-dialkyl complex, along
with its detailed characterization and C-C bond formation reactivity. The [(MeN4)NiIIIMe2]+
species was shown to undergo C-C bond formation relatively slowly, yet clean formation of the
C-C coupled product ethane was observed in the presence of an one-electron oxidant, suggesting
the involvement of a transient NiIV intermediate. Further support for such an intermediate was
provided by the isolation of a NiIV metallacycle complex in which the rate of C-C bond
formation is reduced. In addition, the (MeN4)NiIIMe2 complex was also shown to be a competent
catalyst in Kumada coupling reactions. Overall, the unprecedented isolation of a NiIII–dialkyl
complex provided an opportunity to directly probe the involvement of organometallic NiIII and
NiIV species in C-C bond formation reactions, and Ni-catalyzed cross-coupling reactions in
particular.
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Supporting Information
Synthetic details, spectroscopic characterization, stoichiometric and catalytic reactivity studies,
computational details, and crystallographic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
mirica@wustl.edu
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGEMENTS
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We thank the National Science Foundation (CHE-1255424) for support. The purchase of the
Bruker EMX-PLUS EPR spectrometer was supported by the National Science Foundation (MRI,
CHE-1429711).
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TOC Graphic
5
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Me
N
Me
N
7
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N
N
Me
Me
Me
Me
[Fc+
]
NiII
NiIII
N
N
N
THF, -50°C
N
Me
[(MeN4)NiIIIMe2]+
X-ray, EPR, ESI-MS
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Me
(
MeN4)NiIIMe2
MeCN
RT, 24 h
2 eq [AcFc+
MeCN, RT, 30 min
]
1 eq [AcFc+
MeCN, RT, 24 h
]
CH3-CH3
CH3-CH3
+
CH4
84-88 %
54 %
30 %
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