Organometallic nickel(III) complexes relevant to cross-coupling and carbon-heteroatom bond formation reactions

Article abstract of DOI:10.1021/ja5024749

Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. In addition to Ni(0) and Ni(II) intermediates, several Ni-catalyzed reactions are proposed to also involve odd-electron Ni(I) and Ni(III) oxidation states. We report herein the isolation, structural and spectroscopic characterization, and organometallic reactivity of Ni(III) complexes containing aryl and alkyl ligands. These Ni(III) species undergo transmetalation and/or reductive elimination reactions to form new C-C or C-heteroatom bonds and are also competent catalysts for Kumada and Negishi cross-coupling reactions. Overall, these results provide strong evidence for the direct involvement of organometallic Ni(III) species in cross-coupling reactions and oxidatively induced C-heteroatom bond formation reactions.

Full text of DOI:10.1021/ja5024749

Article
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Organometallic Nickel(III) Complexes Relevant to Cross-Coupling and
Carbon−Heteroatom Bond Formation Reactions
Bo Zheng,† Fengzhi Tang,† Jia Luo,† Jason W. Schultz,† Nigam P. Rath,‡ and Liviu M. Mirica*,†
†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
S
* Supporting Information
ABSTRACT: Nickel complexes have been widely employed
as catalysts in C−C and C−heteroatom bond formation
reactions. In addition to Ni(0) and Ni(II) intermediates,
several Ni-catalyzed reactions are proposed to also involve
odd-electron Ni(I) and Ni(III) oxidation states. We report
herein the isolation, structural and spectroscopic character-
ization, and organometallic reactivity of Ni(III) complexes
containing aryl and alkyl ligands. These Ni(III) species
undergo transmetalation and/or reductive elimination reac-
tions to form new C−C or C−heteroatom bonds and are also competent catalysts for Kumada and Negishi cross-coupling
reactions. Overall, these results provide strong evidence for the direct involvement of organometallic Ni(III) species in cross-
coupling reactions and oxidatively induced C−heteroatom bond formation reactions.
uncommon mononuclear PdIII complexes.47−50 The described
INTRODUCTION
■
organometallic NiIII species exhibit structural and electronic
Nickel complexes have been employed in catalytic organo-
properties that suggest a metal-based radical description, in line
metallic transformations such as Negishi, Kumada, and Suzuki
with the presence of a NiIII center. These complexes are stable
cross-coupling reactions.1−6 By contrast to the thorough
at low temperature, yet they undergo rapid C−halide bond
mechanistic understanding of the widely used Pd-catalyzed
formation at room temperature, providing evidence that such
cross-coupling reactions, the mechanisms of Ni-catalyzed
NiIII species are the active intermediates in oxidatively induced
reactions are not fully understood, as Ni can undergo more
C−heteroatom bond formation reactions. The characterized
easily both one- and two-electron redox reactions, and the
NiIII(aryl)halide complexes undergo rapid transmetalation with
presence of paramagnetic species makes reactivity studies more
Grignard or organozinc reagents to yield detectable NiIII(aryl)-
difficult. While the involvement of NiIII (and NiI) oxidation
alkyl species followed by C−C bond formation, strongly
states in cross-coupling reactions7−31 and oxidatively induced
supporting their role as intermediates in Kumada and Negishi
C−heteroatom bond formation reactions32−36 is more
cross-coupling reactions. In addition, one-electron oxidation of
commonly accepted than for Pd, to the best of our knowledge
an isolated NiII(aryl)alkyl complex leads to rapid formation of
no organometallic NiIII species that can undergo C−C or C−
heteroatom bond formation reactions have been isolated or
characterized to date.
reductive elimination products, and both (tBuN4)NiII and
(
tBuN4)NiIII species are active catalysts for Kumada and Negishi
cross-coupling reactions. Overall, these studies provide for the
first time strong evidence for the direct involvement of
organometallic NiIII complexes in Ni-mediated cross-coupling
reactions and oxidatively induced C−heteroatom bond
formation reactions.
Organometallic NiIII species have also been proposed as
catalytically active intermediates in metalloenzymes such as
methyl-coenzyme M reductase (MCR)37,38 and carbon
monoxide dehydrogenase/acetyl-CoA-synthetase (CODH/
ACS).39,40 For example, a NiIII-methyl species has been
characterized by EPR in MCR,41,42 and also structurally
characterized in two synthetic systems.43,44 Interestingly,
organometallic NiIII intermediates have also been recently
proposed to play a role in the C−H activation step during the
anaerobic oxidation of methane by methanotrophic arch-
aea.45,46
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [(tBuN4)NiIIIArX]+
Complexes. The NiII precursors (tBuN4)NiII(PhF)X (PhF =
p-fluorophenyl; X = Br: 1; X = Cl: 2) were prepared by the
oxidative addition of the corresponding aryl halides to
Ni(COD)2 in the presence of tBuN4 (Scheme 1).51 In these
Herein we report the isolation and characterization of a series
of NiIII(aryl)halide complexes stabilized by the tetradentate
ligand N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane
Received: March 11, 2014
Published: April 8, 2014
(
tBuN4), which was recently employed by us to stabilize
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Scheme 1. Synthesis of (tBuN4)NiII and (tBuN4)NiIII
complexes
The observed distorted octahedral geometries of the Ni(III)
centers are also in line with the Jahn−Teller-like distortions
expected for d7 ions. In both 1+ and 2+, the Ni−Npyridyl bond
that is trans to the aryl group is longer than the Ni−Npyridyl
bond trans to the halide, due to the stronger trans influence of
the C-donor ligand. The Ni−Caryl distance in 1+ (2.02 Å) is
slightly longer than those in the few other reported NiIII−aryl
complexes.55−58
Complexes [1+]PF6 and [2+]PF6 are paramagnetic and
exhibit effective magnetic moments μeff of 2.11−2.03 μb at −20
°C, corresponding to one unpaired electron.54 The EPR spectra
(77 K, 1:1 THF:PrCN glass) reveal rhombic signals and gave
=
complexes, tBuN4 acts as a tetradentate ligand, and the aryl and
halide group complete the distorted octahedral coordination
geometry of the NiII center, similar to other reported
2.160−2.162, along with superhyperfine coupling to the two
axial N donors (I = 1) observed in the gz direction (Figure 2A).
(
tBuN4)NiIIX2 complexes.52,53 The deviation from the octahe-
dral geometry likely results from the small ring size of the tBuN4
macrocyclic ligand and the bulky t-butyl N-substituents.47,48,50
This geometry is confirmed by single crystal X-ray diffraction
analysis of 1 that reveals an average axial Ni−Namine bond
length of 2.385 Å that is longer than the average equatorial Ni−
Npyridyl bond length of 2.036 Å (Figure 1A). Complexes 1 and 2
Figure 1. ORTEP representation of 1 (A) and the cation of [1+]PF6
(B) with 50% probability thermal ellipsoids. Selected bond lengths
(Å), 1: Ni1−N1 2.046; Ni1−N2 2.025; Ni1−N3 2.364; Ni1−N4
2.406; Ni1−C23 2.061; Ni1−Br1 2.643; 1+: Ni1−N1 1.965; Ni1−N2
1.928; Ni1−N3 2.323; Ni1−N4 2.305; Ni1−C23 2.018; Ni1−Br1
2.395.
are paramagnetic, suggesting an S = 1 ground state as expected
for octahedral high-spin NiII centers; this is confirmed by the
effective magnetic moment μeff of 2.89 μb for 1 determined by
the Evans method.54 Interestingly, cyclic voltammetry scans of
1 and 2 in MeCN reveal reversible oxidation waves at −450 and
−400 mV vs Fc+/Fc, respectively. These oxidative processes are
tentatively assigned to the NiII/NiIII couple and suggest that the
NiIII oxidation state should be accessible for this ligand system.
Due to their low oxidation potentials, complexes 1 and 2 can
be readily oxidized with 1 equiv [Fc+]PF6 in THF at −50 °C to
yield the temperature-sensitive green products [(tBuN4)-
NiIII(PhF)Br]+, 1+, and [(tBuN4)NiIII(PhF)Cl]+, 2+, respec-
tively. Characterization by X-ray diffraction of crystals of
[1+]PF6 and [2+]PF6 (obtained from THF/pentane solutions)
confirms the identity of these species and shows the metal
centers adopt a distorted octahedral geometry (Figures 1B and
S46), with axial Ni−Namine bond lengths (2.305−2.323 Å) that
are greater than the axial Ni−Npyridyl bond lengths (1.903−
1.965 Å) and similar to the only two other six-coordinate
organometallic NiIII complexes reported by van Koten et al.55,56
Figure 2. (A) EPR spectra (red lines) of 1+ (top) and 2+ (bottom) in
1:1 THF:PrCN at 77 K, and the simulated EPR spectra (blue lines)
using the following parameters: 1+, gx = 2.272; gy = 2.180 (ABr = 16.0
G); gz = 2.026 (AN(2N) = 12.0 G, ABr = 11.7 G); 2+, gx = 2.282; gy =
2.175; gz = 2.028 (AN(2N) = 13.0 G). (B) DFT calculated Mulliken
spin density for 1+ (shown as a 0.005 isodensity contour plot), and the
relevant atomic and Ni orbital contributions to the spin density.
In addition, superhyperfine coupling to the Br atom (I = 3/2) is
observed for 1+ along the gy and gz directions. Taken together,
the observed structural and EPR parameters for [1+]PF6 and
[2+]PF6 strongly suggest the presence of a distorted octahedral
d7 NiIII center in a dz ground state.55−59 Density functional
2
theory (DFT) calculations support a metal-based radical
description for these complexes, and the calculated spin density
for 1+ shows the unpaired electron resides mostly (>98%) on
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the Ni center, with a major (74%) and minor (22%)
previously proposed by Hillhouse et al. for C−N bond
formation at a NiIII center.36 Overall, these experiments provide
strong evidence for the ability of NiIII(aryl)halide species to act
as the key intermediates in oxidatively induced C−heteroatom
bond formation from NiII precursors.32−36,60
2
2
2
contribution from the 3dz and 3dx −y Ni orbitals, respectively
2
2
(Figure 2B). The appreciable contribution from the 3dx −y
orbital also supports the observed superhyperfine coupling to
the equatorial Br ligand, while the calculated g values and
coupling constants for 1+ and 2+ are similar to the experimental
values (Table S16). In addition, the time-dependent DFT (TD-
DFT) calculated UV−vis spectra reproduce well the two
electronic absorptions observed for both 1+ and 2+ at ∼650 and
∼1000 nm (Figures S41 and S42), which are assigned to ligand-
to-metal charge transfer transitions (Tables S13 and S14).
C−Halide Bond Formation Reactivity of [(tBuN4)-
NiIIIArX]+ Complexes. The isolation of the organometallic
NiIII complexes [1+]PF6 and [2+]PF6 allowed the direct
investigation of their reactivity, as such NiIII species have
been proposed as intermediates in oxidatively induced C−
O,32,34,35 C−N,33,36 C−halide,60−62 and C−S63 bond formation
reactions from NiII precursors as well as H atom abstraction
reactions.59 Oxidation of 1 or 2 by [Fc+]PF6 in MeCN
generates the corresponding EPR-detectable NiIII species,
which then undergo C−halide bond formation to generate p-
fluorophenyl bromide (p-FPh-Br) or p-fluorophenyl chloride
(p-FPh-Cl) in up to 72% yield (Scheme 2). The reductive
C−C Bond Formation Reactivity of Organometallic
(
tBuN4)NiIII Complexes. In addition to C−heteroatom bond
formation reactions, organometallic NiIII species have been
proposed as catalytically active intermediates in Kumada and
Negishi cross-coupling reactions.7−31 In these transformations a
key NiIII-dihydrocarbyl species, formed either upon one-
electron oxidation of a NiII-dihydrocarbyl precursor or
transmetalation of a NiIII-monohydrocarbyl species, is assumed
to undergo reductive elimination to generate the cross-coupled
product. However, while the formation of a transient
bis(trifluoromethyl)NiIII species has been reported recently by
Vicic et al.,64 no NiIII-dihydrocarbyl species has been isolated to
date and observed to undergo C−C bond formation. In order
to probe the reactivity of such organometallic Ni species, the
complex (tBuN4)NiII(PhF)Me, 4, was synthesized by trans-
metalation of 1 with 1 equiv MeMgCl in THF (Scheme 3). The
Scheme 3. C−C Bond Formation Reactivity of
Organometallic (tBuN4)NiII and (tBuN4)NiIII Complexes
Scheme 2. C−halide Bond Formation Reactivity of
Organometallic (tBuN4)NiII and (tBuN4)NiIII Complexes
elimination from the 1+ or 2+ species is proposed to yield an
unstable NiI species that undergoes rapid disproportionation to
Ni0 and a [(tBuN4)NiII(halide)(MeCN)]+ species, as observed
experimentally.54 Complex 2+ is less stable than 1+ in solution
as it likely undergoes ligand exchange and reduction more
easily; thus, most reactivity experiments were performed using
isolated or in situ generated 1+. When an MeCN solution of
[1+]PF6 is warmed up to RT, p-FPh-Br is formed in 54% yield
(Scheme 2). In order to probe the mechanisn of reductive
elimination, a 1:1 mixture of 1 and (tBuN4)NiII(PhMe)Cl, 3, in
MeCN was treated with [Fc+]PF6 (1 equiv per Ni), and analysis
of the reaction mixture reveals the formation of p-FPh-Br and
p-MePh-Cl as well as p-FPh-Cl and p-MePh-Br.54 Moreover,
similar yields of the C−halide bond formation products were
observed when a 1:1 mixture in THF of [1+]PF6 and
[(tBuN4)NiIII(PhMe)Cl]PF6, [3+]PF6, was warmed up to RT,
while no ligand exchange was observed at the NiIII center when
the mixture was kept at −50 °C for 8 h. The formation of the
crossover products suggests an initial halide dissociation step
followed by C−halide bond formation, similar to what was
single crystal X-ray analysis of 4 reveals a square planar
geometry for the NiII center with the two pyridyl N atoms and
the two organic ligands bonded to Ni (Figure 3), in contrast to
the distorted octahedral geometry of complexes 1 and 2. The
lack of axial interactions in 4 is likely due to the presence of two
strong σ-donor organic ligands. In addition, 4 is diamagnetic, as
expected for a square planar low-spin NiII d8 center and in
contrast to the high-spin paramagnetic complexes 1 and 2.
Gratifyingly, when 4 is oxidized by [Fc+]PF6 in MeCN, the
C−C bond formation product, p-FPh-Me, is generated in 61%
yield (Scheme 3). A small amount of fluorobenzene (PhF) is
also produced during the oxidatively induced C−C bond
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Figure 3. ORTEP representation of 4 with 50% probability thermal
ellipsoids. Selected bond lengths (Å): Ni1−N1 1.987; Ni1−N2 1.975;
Ni1−C23 1.921; Ni1−C24 1.902.
formation reactivity of 1, and PhF most likely forms by the
reaction of 3+ with the trace amount of water present in the
solvent.54 ESI-MS analysis of the oxidation of 1 performed at
−50 °C reveals an m/z peak at 520.2524 that was assigned to
[(tBuN4)NiIII(PhF)Me]+, 4+ (calculated m/z: 520.2507, Figure
S11), while oxidation of (tBuN4)NiII(PhF)(13CH3), 13C-4, with
1 equiv [Fc+]PF6 generates a species with an m/z peak at
521.2544 corresponding to [(tBuN4)NiIII(PhF)(13CH3)]+ (cal-
culated m/z: 521.2545, Figure S11). A similar yield of p-FPh-
Me (63%) is obtained from the in situ transmetalation of 1 with
1 equiv MeMgCl followed by oxidation with 1 equiv [Fc+]PF6
(Scheme 3). The intermediacy of the NiIII(aryl)alkyl species 4+
during C−C bond formation is further supported by the direct
transmetalation of the 1+ with 1 equiv MeMgCl at −50 °C to
generate the orange species 4+, which upon warming to RT
generates p-FPh-Me in 48% yield (Scheme 3).
Figure 4. EPR spectra (red lines) of 4+ (top) and 13C-4+ (bottom) in
3:1:5 THF:Et2O:PrCN at 77 K, and the simulated EPR spectra (blue
lines) using the following parameters: 4+, gx = 2.317; gy = 2.265; gz =
13
2.009 (AN(2N) = 11.2 G); C-4+, gx = 2.318 (A = 17.0 G); gy =
13C
13C
2.265; gz = 2.009 (AN(2N) = 11.2 G, A = 5.2 G), including
superhyperfine coupling due to the 13C−methyl group bonded to the
NiIII center.
the formation of such a NiIII species.54 A similar NiIII(aryl)alkyl
species is generated upon the reaction of 1+ with octylzinc
bromide, as observed by EPR (Figure S28). Overall, these
studies suggest the formation of a detectable NiIII(aryl)alkyl
intermediate that undergoes C−C reductive elimination, and
therefore it is expected to be catalytically active in cross-
coupling reactions (see below).
The yield of p-FPh-Me during these oxidatively induced
reductive elimination reactions (57
4%) shows little
dependence on the initial concentration of 1 (1−200 mM).
Moreover, the oxidation of 4 with [Fc+]PF6 in the presence of
the radical trap TEMPO does not change the yield of p-FPh-
Me (61 1% yield) and <2% of the Me-TEMPO adduct is
observed by GC-MS.54 The EPR spectra of 4 in the presence of
the spin trap DMPO, either in the absence or presence of
FcPF6, reveal no radical adduct detectable by EPR, except for
the radical impurity present in the DMPO (Figure S17). In
addition, the reaction of [1+]PF6 with MeMgCl in the presence
of DMPO also does not generate any DMPO radical adduct.54
Overall, these results suggest that the reactions occurring
during the C−C reductive elimination of p-FPh-Me through
the [(tBuN4)NiIII(PhF)Me]+ intermediate most likely do not
follow a radical mechanism.
Cross-Coupling Catalytic Reactivity of (tBuN4)Ni Com-
plexes. The organometallic NiII and NiIII complexes described
herein are active catalysts for Kumada and Negishi cross-
coupling reactions (Scheme 4). For example, the coupling of
iodotoluene with PhMgBr is accomplished in up to 72%
unoptimized yield using either 1 or 1+ as the catalyst, while the
Scheme 4. Catalytic Activity of (tBuN4)NiII and (tBuN4)NiIII
Complexes in Kumada and Negishi Cross-Coupling
Reactions
Interestingly, 4+ exhibits a rhombic EPR spectrum with
superhyperfine coupling to the two axial N atoms in the z
direction, while the EPR spectrum of [(tBuN4)NiIII(PhF)-
(13CH3)]+ (13C-4+), formed upon the treatment of 1+ with
13CH3MgI, reveals broader signals in both the x and z
directions due to additional superhyperfine coupling to the 13
C
nuclei (I = 1/2) of the methyl group (Figure 4 and Table S7).
The observed superhyperfine coupling to the 13C nuclei of the
equatorial Me ligand is likely due to the appreciable
2
2
contribution of the Ni 3dx −y orbital (22%) to the singly
occupied molecular orbital (Figure 2B) and thus strongly
suggests the presence of a Ni−Me bond. In addition, the DFT
calculated EPR parameters and UV−vis spectrum of 4+
reproduce well the experimental results and further support
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Kumada coupling of iodoheptane with an alkyl Grignard
generates the product in 27% unoptimized yield using 1+ as the
catalyst.54 In addition, both 1 and 1+ can catalyze the Negishi
coupling of 1-octylzinc bromide with iodoethylbenzene in 23%
and 20% unoptimized yields, respectively (Scheme 4), and a
similar yield was obtained when iodopropylbenzene was used
instead.54 Interestingly, when 1 is treated with 1 equiv
iodoethylbenzene, the EPR spectrum of the resulting solution
reveals the presence of two NiIII signals that are tentatively
assigned to a NiIII(aryl)halide and a NiIII(aryl)alkyl species, by
comparison to the EPR spectra of analogous NiIII complexes
described above (Scheme 5 and Figure S29). This observation
for such reactions nor does it imply how such NiIII can be
generated under catalytic conditions (i.e., whether these species
are formed by radical or nonradical mechanisms). However, we
consider these studies open up the possibility of detailed
mechanistic studies of various cross-coupling reactions, which
are the focus of our current research efforts.
CONCLUSION
■
The studies reported herein provide for the first time strong
evidence that organometallic NiIII species are catalytically
relevant in Ni-catalyzed cross-coupling reactions, which have
been proposed to involve such NiIII intermediates for more
than three decades. The use of the tetradentate tBuN4 ligand
allowed the isolation and characterization of several NiIII(aryl)
and NiIII(aryl)alkyl complexes and thus provided a unique
opportunity to investigate their reactivity in oxidation, trans-
metalation, and reductive elimination steps that are essential for
the catalytic cross-coupling reactions. A detailed understanding
of the reactivity of organometallic odd-electron Ni complexes
requires a range of spectroscopic and mechanistic studies that
are essential for the development of more efficient and selective
catalysts for various C−C and C−heteroatom organic trans-
formations.
Scheme 5. Proposed Formation of Two NiIII Species upon
the Reaction of 1 with Phenethyl Iodide
is in line with the well-established radical scavenging ability of
NiII complexes to generate NiIII species in Kharasch radical
additions, as reported by van Koten et al.65,66 In addition, for
both Kumada and Negishi cross-coupling reactions, formation
of adducts between the organometallic reagent and the solvent
(e.g., THF) was observed,54 supporting radical mechanisms for
both cross-coupling reactions, and also as observed recently by
Fu et al.15
ASSOCIATED CONTENT
■
S
* 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.
Since the seminal work of Kochi et al. more than three
decades ago,8 many experimental7−9,14−21,24,25,27,30,67 and
computational reports68−70 have proposed the involvement of
organometallic NiIII intermediates in cross-coupling reactions.
For example, Fu et al. proposed a radical mechanism for Ni-
catalyzed cross-coupling reactions that involves NiI and NiIII
intermediates,15 while Cardenas et al. have proposed the
involvement of NiIII(Ar)X intermediates in transmetalation
reactions of alkyl zinc reagents with aryl halides in Negishi
cross-coupling.19,20 Moreover, Vicic et al. have proposed a NiIII-
dialkyl species as an intermediate in alkyl−alkyl Negishi
coupling,16,17 while Hu et al. have recently proposed a radical
bimetallic oxidation mechanism for alkyl−alkyl Kumada
coupling that involves two different NiIII intermediates.67 The
involvement of alkyl radical species that generate a NiIII(aryl)-
alkyl species has also been proposed in the cross-electrophile
coupling of aryl halides with alkyl halides by Weix et al.27 The
reactivity studies of the organometallic (tBuN4)NiIII complexes
reported herein provide strong evidence for these proposed
reactions at NiIII centers, such as the ability of an alkyl halide to
oxidize a NiII-aryl species, most likely through a radical
mechanism, as well as for the involvement of more than one
NiIII intermediate in these cross-coupling catalytic reactions.67
It is important to note that a large number of mechanisms
have been proposed for the various Ni-catalyzed cross-coupling
reactions,1−6 and several of these mechanisms have suggested
the involvement of NiIII intermediates.7−31 We have employed
herein the described (tBuN4)NiIII complexes in stoichiometric
transmetalation and reductive elimination reactions as well as
Kumada and Negishi cross-coupling reactions that strongly
supports their involvement in such catalytic transformations.
This study does not attempt to provide an overall mechanism
AUTHOR INFORMATION
■
Corresponding Author
mirica@wustl.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the American Chemical Society Petroleum Research
Fund (52988-ND3) and the National Science Foundation
(CHE-1255424) for support. L.M.M. is also supported by a
Sloan Research Fellowship.
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