Bimetallic C-C Bond-Forming Reductive Elimination from Nickel

Article abstract of DOI:10.1021/jacs.6b00016

Ni-catalyzed cross-coupling reactions have found important applications in organic synthesis. The fundamental characterization of the key steps in cross-coupling reactions, including C-C bond-forming reductive elimination, represents a significant challenge. Bimolecular pathways were invoked in early proposals, but the experimental evidence was limited. We present the preparation of well-defined (pyridine-pyrrolyl)Ni monomethyl and monophenyl complexes that allow the direct observation of bimolecular reductive elimination to generate ethane and biphenyl, respectively. The sp³-sp³ and sp²-sp² couplings proceed via two distinct pathways. Oxidants promote the fast formation of Ni(III) from (pyridine-pyrrolyl)Ni-methyl, which dimerizes to afford a bimetallic Ni(III) intermediate. Our data are most consistent with the subsequent methyl coupling from the bimetallic Ni(III) to generate ethane as the rate-determining step. In contrast, the formation of biphenyl is facilitated by the coordination of a bidentate donor ligand.

Full text of DOI:10.1021/jacs.6b00016

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Article
Bimetallic C–C Bond Forming Reductive Elimination from Nickel
Hongwei Xu, Justin Blaise Diccianni, Joseph Katigbak, Chunhua Hu, Yingkai Zhang, and Tianning Diao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b00016 • Publication Date (Web): 22 Mar 2016
Downloaded from http://pubs.acs.org on March 23, 2016
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Bimetallic C–C Bond Forming Reductive
Elimination from Nickel
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Hongwei Xu, Justin B. Diccianni, Joseph Katigbak, Chunhua Hu, Yingkai Zhang, and Tianning Diao*
Department of Chemistry, New York University, 100 Washington Square East, New York, NY 10003
EꢀMail: diao@nyu.edu
ABSTRACT: Niꢀcatalyzed crossꢀcoupling reactions have found important applications in organic
synthesis. The fundamental characterization of the key steps in crossꢀcoupling reactions, including C–C
bond forming reductive elimination, represents a significant challenge. Bimolecular pathways were
invoked in early proposals, but the experimental evidence was quite limited. We present the preparation of
wellꢀdefined (pyridineꢀpyrrolyl)Ni monoꢀmethyl and phenyl complexes that allow the direct observation
of bimolecular reductive elimination to generate ethane and biphenyl, respectively. The sp3ꢀsp3 and sp2ꢀ
sp2 couplings proceed via two distinct pathways. Oxidants promote the fast formation of Ni(III) from
(pyridineꢀpyrrolyl)Niꢀmethyl, which dimerizes to afford a bimetallic Ni(III) intermediate. Our data is
most consistent with the subsequent methyl coupling from the bimetallic Ni(III) to generate ethane as the
rateꢀdetermining step. In contrast, the formation of biphenyl is facilitated by the coordination of a
bidentate donor ligand.
Introduction
Nickel complexes have emerged as appealing catalysts for crossꢀcoupling reactions (eq 1).1 While
contemporary Ni catalysts employing chelating ligands have started to solve important problems in
organic synthesis,2 catalyst design will benefit from advanced understanding of the reaction mechanism to
achieve more widespread utility.3 , 4 The diversity of pathways, depending on the ligand framework,
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highlights the challenges associated with characterizing the mechanism for Niꢀcatalyzed crossꢀcoupling
reactions. The two prevailing mechanisms include the Pdꢀlike “closedꢀshell” redox pathway via
Ni(0)/Ni(II) intermediates5 and the “openꢀshell” pathway featuring Ni(I)/Ni(III) intermediates.4,6
9
R3
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X
Ni catalyst
R: aryl, alkyl, etc.
M: B, Zn, Mg, Si, Zr, etc.
R3–M
+
(1)
R1 R2
R1 R2
In parallel with catalytic studies, historical and contemporary efforts have focused on stoichiometric
characterization of the fundamental steps of nickelꢀmediated C–C bond coupling. Several Ni(II) and
Ni(III) systems have been identified to undergo reductive elimination,6,7 whereas C–C bond formation
from isolated Ni(IV) has just been reported recently.8 Reductive elimination engaging two metal centers
has been determined to promote C–C coupling with Pd and Pt complexes,9 but such a bimetallic
mechanism with Ni has seldom been considered.10 Yamamoto observed transmetalation between two
Ni(II) intermediates prior to a “closedꢀshell” reductive elimination of biphenyl from Ni(II) to Ni(0)
(Scheme 1A).7aꢀd In contrast, early proposals by Kochi and Hegedus described a group transfer between
Ni(III) and Ni(II) as the key step, followed by reductive elimination from the Ni(III) intermediate
(Scheme 1B).6,11 This “openꢀshell” transmetalation was supported by minimal experimental evidence.12 In
light of the fundamental interest and potential applications of bimolecular mechanisms in Ni catalysis,
herein we present the direct observation of two distinct bimetallic pathways for Niꢀmediated sp3ꢀsp3 and
sp2ꢀsp2 C–C coupling. Spectroscopic and kinetic studies, in combination with DFT calculations, provide
evidence for highꢀvalent Ni intermediates. Our discovery of new bimetallic mechanisms for Niꢀmediated
C–C bond formation provides valuable insight for catalyst design in crossꢀcoupling reactions.
Scheme 1. Proposed Bimetallic Pathways for Ni-Mediated C–C Bond Formation
Results and Discussion
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Synthesis of mono-Methyl and mono-Phenyl Ni Complexes. LXꢀtype bidentate ligands are
beneficial to Niꢀcatalyzed crossꢀcoupling reactions,13 but fundamental understanding of the ligand effect
on C–C bond formation has been limited by the challenges associated with isolating Niꢀcarbyl
intermediates.14 We prepared (pyꢀMepyrr)Ni(acac), 1 (pyꢀMepyrr = 3,5ꢀdimethylꢀ2ꢀ(2ꢀpyridyl)pyrrole, acac
= acetylacetonate), which underwent methylation and phenylation with Grignard reagents to afford cis
and trans mixtures of (pyꢀMepyrr)Ni(CH3)(lut) 2 and (pyꢀMepyrr)Ni(Ph)(lut) 3 (lut = 2,4ꢀlutidine) (Scheme
2). The cis and transꢀisomers of 2 coꢀcrystallized into a highly disordered packing (Figure S1(B)).
Compound 3 did not form single crystals, so we prepared the alternative (pyꢀPhpyrr)Ni(acac) 7 and (pyꢀ
Phpyrr)Ni(Ph)(lut) 8. Single crystal Xꢀray diffraction established a square planar geometry for 2 and 8
(Figure S1 and S2). This geometry implies a lowꢀspin state for Ni and is consistent with the observed
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diamagnetic H NMR spectra. In solution, the assignments of cis- and transꢀisomers were confirmed by
NOESY and COSY experiments (Figure S4). The cis/trans ratio remained constant over time, indicating
that the preference for the transꢀgeometry was a thermodynamic effect.15 In the following studies, the
isomers were not separated, but used as a mixture.
Scheme 2. Syntheses of (py-Mepyrr)Ni Complexes
CH3MgBr
N
N
O
O
N
N
N
N
lut
CH3
lut
2,4-lutidine
40%
Ni
Ni
Ni
+
CH3
1:5
1
2-cis
2-trans
PhMgBr
2,4-lutidine (lut)
N
N
N
N
Ph
lut
3-trans
lut
Ni
Ni
+
40%
Ph
3-cis
1:5
Oxidant-Promoted Ethane Formation. The isolation of 2 has allowed us to carry out fundamental
studies on C–C bond formation from the wellꢀdefined monoꢀmethyl complex. We initiated our study by
evaluating 2 under Yamamoto conditions, where transmetalation followed by biphenyl coupling was
reported for (bpy)Ni(Ph)(Br) (bpy = bipyridine) in DMF.7c In contrast to this precedent, 2 remained stable
in DMFꢀd7 and C6D6 (Table 1, entries 1ꢀ2). We continued to explore the effect of external donor ligands
and oxidants. Immediate decomposition of 2 to CH4 was observed in the presence of five equivalents of
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bpy (entry 3).16 Addition of CH3I to 2 resulted in minor conversion upon heating (entry 4). Introducing O2
to 2 immediately furnished ethane in 19% yield (maximum theoretical yield = 50%) (entry 5).17 The yield
of ethane was increased to 43% with I2 as the oxidant, with concomitant formation of 10% CH3I (entry 6).
Addition of NBS (Nꢀbromosuccinimide) also led to high yield of ethane (entry 7). Notably, no methane
was observed under these oxidative conditions.
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Table 1. Formation of Ethane from 2
The selective ethane formation from 2 under oxidative conditions provided preliminary support for a
bimolecular reductive elimination process. We continued our investigation to obtain further evidence for a
bimolecular pathway. A crossover experiment was performed in which a mixture of 2 and 2-d3 in a 1:1
ratio reacted with I2 to generate a mixture of CH3CH3, CH3CD3, and CD3CD3 in a statistical distribution
(eq 2). The ratio of ethane to CH3I exhibited a linear dependence on [2] with a positive slope (Figure 1).
This influence of [2] on the product distribution reflects the requirement for two molecules of 2 to form
ethane, whereas the formation of CH3I is uniꢀmolecular in 2.
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Figure 1. The dependence of [ethane]/[CH3I] ratio on [2]. Reaction conditions: [2] = 5–40 mM, [I2] = 20
mM, C6D6 = 0.6 mL, 22 ºC, 1 h. Internal standard = TMS2O. Estimated standard deviation for individual
points: ≤ 5%.
Experimental and Theoretical Characterization of Intermediates. (A) Spectroscopic Data. Our
subsequent spectroscopic studies on I2ꢀpromoted ethane formation from 2 provided structural information
on the Ni intermediates and shed some light on the reaction mechanism. The cyclic voltammetry of 2
exhibited two irreversible oxidation waves at 12 and 411 mV (vs. Fc/Fc+) (Figure S5). In a liquid N2 filled
cold well, 2 and I2 were mixed and the reaction mixture was immediately analyzed by EPR spectroscopy.
The EPR spectrum displayed a single species of an S = 1/2 state with a 1:1:1 superhyperfine splitting
(Figure 2, red).18 To determine the lifetime of this intermediate, we allowed the mixture to react at –20 ºC
for 10 minutes and examined it by EPR spectroscopy. The signal corresponding to the intermediate
decayed to zero. When NBS was used to replace I2 as the oxidant, an analogous intermediate was
observed (Figure 2, black). The features and splitting patterns for the two intermediates, generated from I2
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and NBS (red and black, respectively), are similar, but the g values are slightly different, reflecting two
distinct complexes with a similar structure.
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Figure 2. Xꢀband EPR spectra of the intermediate generated from the reaction of 2 with I2 (red) and the
reaction of 2 with NBS (black) recorded in toluene glass at 10 K. The blue spectrum (top) is the
simulation of the red spectrum. Spectroscopic parameters: gx = 2.20, gy = 2.14, gz = 2.03, Axx = 7.7 MHz,
Ayy = 63 MHz, Azz = 55 MHz. Microwave frequency = 9.380 GHz, power = 0.25 mW, modulation
amplitude = 1 mT/100 kHz.
The shortꢀlived Ni(III) intermediate detected by EPR spectroscopy prompted us to explore its
structure by DFT calculations. Geometry optimization of possible Ni(III) intermediates converged to (pyꢀ
Mepyrr)NiIII(CH3)(I)(lut), 4 (Figure 3). This structure features a square pyramidal geometry with pyridine
in the axial position and iodide trans to pyrrole. The spinꢀdensity plot reveals a Niꢀcentered radical
strongly coupled with the Nꢀatom of pyridine, whereas its couplings with pyrrole and lutidine are minor.
The EPR parameters of 4 were calculated using the ORCA package,19 and the computational g values and
Aiso reproduced the experimental spectra shown in Figure 2 (Table S1).
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Figure 3. Structures and Mulliken spinꢀdensity plots of 4 obtained from DFT calculations.
Variable temperature NMR experiments allowed us to identify the diamagnetic intermediates. In a
liquid nitrogenꢀfilled cold well, upon injection of I2, 2 rapidly evolved into a diamagnetic intermediate
with the concomitant formation of CH3I in 10% yield and minor paramagnetic species (Figure S7A).
While the complete conversion of this intermediate to ethane was fast at 22 ºC (Figure S7B), the
intermediate was stable on the NMR time scale at –20 ºC (Figure S8). Multiple singlet resonances in the
upfield region between 2.0 – 3.0 ppm are accompanied with several aromatic protons between 5.5 – 7.0
ppm. COSY experiments elucidate that there are a number of resonances overlap with the residual toluene
solvent (Figure S9). To determine the identity of this intermediate and assign the spectrum, we prepared
deuterium labeled (pyꢀMepyrr)Niꢀmethyl complexes, 2-d3, 2-d1, and 2-d7 (Figure 4). Complex 2-d7 is a
mixture of isotopologues and isotopomers. The overall deuterium incorporation in 2-d7 is 60% at each
position. When 2-d3 was treated with I2, four signals at 3.00, 2.94, 2.91, and 2.85 ppm disappeared
(Figure 5A). HSQC experiments revealed that the carbon chemical shifts corresponding to these proton
resonances were between 26.00 – 29.00 ppm (Figure S10). The concentration of these four proton
resonances, in combination with CH3I, accounted for 95% of the NiꢀCH3 from the starting 2. Moreover,
the overall integration of these four signals is half the overall integration of eight singlet peaks between
2.30 and 2.60 ppm (Figure S8). The reaction of 2-d1 led to the disappearance of two singlet 1H resonances
at 6.14 and 6.17 ppm, while all other resonances remained (Figure 5B). When 2-d7 was submitted to the
reaction conditions, all eight singlet resonances between 2.30 and 2.60 ppm diminished with small
residuals due to the incomplete deuterium incorporation to 2-d7 (Figure 5C).
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Figure 4. Deuterium labeled 2 for structure assignment of the diamagnetic intermediate.
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Figure 5. Comparison of the H NMR spectra of the intermediate generated from I2 reacting with proteo
and deuterio (pyꢀpyrr)Niꢀmethyl complexes 2-d3 (A), 2-d1 (B), and 2-d7 (C). Reaction conditions: [2] = 16
mM, [I2] = 16 mM, tolueneꢀd8 = 0.65 mL, –20 ºC. Internal standard = 1,3,5ꢀtrimethoxybenzene.
The 1H NMR resonances corresponding to the diamagnetic intermediate gradually decrease at –20 ºC,
while a singlet resonance at 0.820 ppm, corresponding to ethane, increases. The concentration of CH3I
remains constant. We were able to monitor the decrease of the intermediate and increase of ethane over 3–
4 hours. The concentration of the Ni–CH3 at 3.00 ppm exhibits an exponential decay (Figure 6A).20 The
concentration of this species, however, only accounts for a fraction of ethane. When all four NiꢀCH3
resonances identified in Figure 5A at 3.00, 2.94, 2.91, and 2.85 ppm are integrated, half of the total
concentration fit to a first order kinetic model (Figure 6B). The same model accommodates the
concentration of ethane.21 Other models, including second order kinetics, were fully considered but failed
to fit the experimental data. When the conversion of the intermediate to ethane was complete, the
resulting Ni species were NMR silent (Figure S7B). The EPR spectrum revealed the absence of any
radical species. Independently prepared (pyꢀpyrr)2NiII and the comparison of the spectra revealed that (pyꢀ
pyrr)2NiII is absent from the reaction (Scheme 3, cf. Figures S11ꢀS12). Alternatively, tetrahedral (pyꢀ
pyrr)Niꢀiodides are possible products (Scheme 3). Our attempts to prepare (pyꢀpyrr)Niꢀiodides via
multiple methods were unsuccessful, consistent with previous reports by Caulton.14b,22
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(B)
Figure 6. Reaction time courses for the consumption of the Ni–CH3 resonance at 3.00 ppm (2[5a]) (A)
and the conversion of 5 to ethane (B). The concentration of 5 ([5] = [5a] + [5b] + [5c]) was calculated by
dividing the total concentration of four NiꢀCH3 resonances at 3.0, 2.94, 2.91, and 2.85 ppm by 2 (see
discussions below). Reaction conditions: [2]0 = 17 mM, [I2]0 = 15 mM, tolueneꢀd8 = 0.65 mL, –20 ºC.
Internal standard = 1,3,5ꢀtrimethoxybenzene.
Scheme 3. Proposed Ni Products from the Reaction of 2 with I2
I
II
Ni
N
N
N
II
Ni
N
N
lut
or
I
N
II
II
Ni
N
N
N
N
Ni
not observed
I
Proposed Ni product
(B) Structural Assignment of Intermediate 4. The irreversible CV of 2 at 12 and 411 mV (vs. Fc/Fc+)
suggests that the highꢀvalent Ni species generated from electrochemical oxidation are unstable.
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Nevertheless, the data provides an estimation of the oxidation potentials for Ni(II)/Ni(III) and
Ni(III)/Ni(IV) transitions. Previous reports indicate that the oxidation potential of I2 is 140 mV (vs.
Fc/Fc+),23 implying that I2 is suitable for oxidizing 2 to Ni(III) instead of Ni(IV). This analysis is in
alignment with previous experimental and computational studies on the oxidation of Ni(II) to Ni(III) by
Br2.24 The EPR experiments provide further support for the oxidation of 2 to a Ni(III) intermediate upon
addition of I2. Replacing I2 with NBS leads to a minor shift of the g values while the overall shape of the
EPR signal remains. This observation suggests that the Ni(III) intermediate possesses a halide ligand.25
DFT calculations allow us to propose (pyꢀMepyrr)NiIII(I)(CH3)(lut) 4 as the possible structure (Figure 3).
The square pyramidal geometry is consistent with a Ni(III)ꢀhalide previously isolated by van Koten.25b
The calculated EPR parameters for 4 are in good agreement with the experimental values (Table S1). The
observed 1:1:1 superhyperfine splitting is due to the strong coupling of the unpaired electron on Ni with
the N of pyridine (I = 1) in the axial position. The lifetime of 4 at –20 ºC is short, as its EPR signal has
completely decayed after 10 minutes.
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(C) Structural Assignment of Intermediate 5. Lowꢀtemperature 1H NMR studies reveal a
diamagnetic intermediate 5 with a lifetime of 3–4 hours at –20 ºC. The assignment of the peaks is
facilitated by the deuterium labeling studies, 2D NMR experiments, and literature precedents. The
reaction of 2-d3 leads to the disappearance of four peaks at 3.00, 2.94, 2.91, and 2.85 ppm (Figure 5A),
suggesting four Ni–CH3 resonances exist for intermediate 5. Consistently, the total concentration of these
methyl groups, along with the small amount of CH3I byꢀproduct, accounts for 95% of the starting NiꢀCH3
from 2. The concentration of each individual Ni–CH3 resonance (3.00 ppm, for instance) undergoes first
order decay at –20 ºC (Figure 6A). Half of the overall concentration of the four Ni–CH3 resonances fit to
a first order kinetic model, which accounts for the formation of ethane (Figure 6B). These kinetic data are
consistent with two Ni–CH3 groups in each molecule of 5, which couple in an intramolecular fashion to
generate ethane.
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The 1H chemical shifts of Ni–CH3 and the 13C resonances of the corresponding Ni–CH3 are unusually
downfield for methyl resonances attached to metals (Figure 5A and Figure S10), implying an electronꢀ
deficient Ni center. Previous reports of highꢀvalent Ni complexes display an analogous trend (Scheme
4).8,26 The experiment with 2-d7 reveals that all eight methyl peaks between 2.30 and 2.60 ppm belong to
the pyꢀpyrr ligand (Figure 5C). The integrations suggest that the ratio of Ni–CH3/pyꢀpyrr is 1:1. These
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considerations, coupled with the kinetic data, lead us to propose the dimeric [(pyꢀpyrr)NiIII(CH3)(
µ-I)]2 as
the structure intermediate 5 (Scheme 5). The diamagnetic property of 5 arises from the antiferromagnetic
1
coupling of lowꢀspin Ni(III) (d7, S = 1/2) centers.27 The integrations of the H NMR resonances and 2D
NMR experiments suggest that three isomers exist. Duplicate experiments show different ratios of the
isomers, suggesting a distribution originated from a kinetic effect instead of thermodynamic equilibrium.
DFT calculations on possible stereoisomers converged to two symmetric structures that have the lowest
energy, 5a and 5b (Scheme 5). The Ni centers favor a square pyramidal geometry with pyridine
occupying the axial position. NMR data determined that the third isomer has two inequivalent Ni–CH3
and two sets of pyꢀpyrr ligands. DFT calculations converged to 5c, in which one Ni adapts a square
pyramidal geometry and the other is trigonal bipyramidal with an iodide and a methyl located at the axial
position (Scheme 5). The assignment of the 1H resonances for each isomer is presented in Scheme 5. The
assignments for 5c in brackets are arbitrary, since we do not have a way to distinguish between the two
inequivalent pyꢀpyrr ligands. The missing resonances are attributed to peaks underneath the residual
toluene solvent. The calculated chemical shifts of Ni–CH3 in each isomer are 3.75, 3.57, 2.87 and 2.77
ppm, respectively, in agreement with the experimental values.
Scheme 4. Reported 1H Chemical Shifts of High-Valent Ni Complexes
Scheme 5. Proposed Structures for Intermediate 5 and Assignments of 1H NMR Resonances
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Alternatively, Ni(IV)ꢀdimethyl complexes are considered as the structure of 5, and can be ruled out by
our data. (Pyꢀpyrr)NiIV(CH3)2(lut)(I) 5d is a possible intermediate (Scheme 6). The integration of Ni–
CH3/pyꢀpyrr for 5d is 2:1, inconsistent with the observed Ni–CH3/pyꢀpyrr ratio of 1:1 by NMR labeling
studies. (Pyꢀpyrr)2NiIV(CH3)2 5e would fit the kinetic and NMR data. The reductive elimination from 5e
to form ethane is expected to generate (pyꢀpyrr)2NiII as the resulting Ni species. The lack of (pyꢀpyrr)2NiII
after the reaction suggests that 5e is absent (Scheme 3, cf. Figures S11ꢀS12). In addition, the formation of
5e requires the transfer of the pyꢀpyrr ligand and methyl from one molecule of 4 to another. This process
is highly unlikely. The mixedꢀvalent (pyꢀpyrr)NiIV(CH3)2( -I)2NiII(pyꢀpyrr) 5f would fit the kinetic data.
µ
But the pyꢀpyrr ligands in any stereoisomers of 5f are inequivalent, inconsistent with the symmetry of the
1
three isomers determined by H NMR spectroscopy. Moreover, we performed DFT calculations on 5f,
which converged to an octahedral Ni(IV) bridged with a tetrahedral Ni(II). The tetrahedral Ni(II) is highꢀ
spin, contradictory to the observed diamagnetic NMR spectrum. The calculated Ni–CH3 1H NMR
chemical shifts are presented in Scheme 6, which are significantly more downfield compared to the
experimental values.
1
Scheme 6. Ni(IV) Complexes Ruled Out as the Structure of 5 and the Corresponding Ni–CH3 H
NMR Chemical Shifts Predicted by DFT Calculations
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Mechanism of Oxidant-Promoted Ethane Formation. In the studies presented above, we explored
and characterized the C–C bond formation process from (pyꢀpyrr)Ni monoꢀmethyl complex 2. The lack of
reactivity of 2 in DMF contradicts Yamamoto’s “closedꢀshell” pathway.7 The absence of ethane in the
presence of CH3I reveals a pathway distinct from simple nucleophilic substitution28 or oxidation of Ni(II)
by CH3I.2a Instead, 2 undergoes reductive elimination to generate ethane in the presence of oxidants. This
reaction is highly selective, since methane is absent from the reaction. The I2ꢀpromoted ethane formation
is bimolecular, evident in the crossover experiment and the increased [ethane]/[CH3I] ratio as a function
of [2].
On the basis of the structural assignment of the intermediates above, we propose the mechanism for
I2ꢀpromoted ethane formation (Scheme 7). The process is initiated by the oxidation of 2 to afford a Ni(III)
intermediate, assigned as (pyꢀMepyrr)NiIII(I)(lut)(CH3) 4 (step i) (Scheme 7). Intermediate 4 has a short
lifetime and rapidly undergoes further conversion. Four possible pathways are considered for the
subsequent ethane formation. In the free radical pathway (Scheme 7, pathway A), 4 undergoes a Ni–
carbon bond homolysis to eject a methyl radical, which dimerizes to generate ethane. This pathway is
inconsistent with the absence of methane after the reaction, since methane is an expected product from the
ejection of a methyl radical.29 Moreover, monitoring the reaction by 1H NMR spectroscopy clearly reveals
the conversion of organometallic Ni–CH3 intermediates to ethane. The radical mechanism is inconsistent
with the clean first order kinetics. In comparison, the reaction of 2 with bpy is speculated to form a
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methyl radical, which results in methane as the major product (Table 1, entry 3) (see below for further
discussion about ligandꢀpromoted radical ejection).
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Scheme 7. Proposed Mechanism for I2-Promoted Reductive Elimination of Ethane
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An alternative mechanism involves a methyl transfer from 4 to 2 to afford a Ni(III)ꢀdimethyl
intermediate (pathway B). This pathway is a reminiscent of Kochi and Hegedus’ proposal.6 The rapid
decay of paramagnetic intermediate 4 into EPR silent species is contradictory to a Ni(III)ꢀdimethyl
intermediate. Instead, a diamagnetic intermediate 5 converts to ethane over 3–4 hours at –20 ºC. Pd(III)
has been reported to undergo disproportionation to afford Pd(IV), followed by reductive elimination to
form ethane.9a We evaluated the analogous pathway (pathway C). The possible Ni(IV) complexes,
generated from disproportionation, can be ruled out as the structure of intermediate 5 based on direct and
circumstantial evidences presented above (Scheme 6).
The NMR and kinetic data fit well to [(pyꢀpyrr)NiIII(CH3)(
µ-I)]2 5, which is generated from the
dissociation of 2,4ꢀlutidine from 4 (step ii, pathway D). The reductive elimination from 5 to furnish
ethane is first order, and is the rateꢀdetermining step (step iii). The precise transition state for the C–C
bond formation is unknown. We speculate that the dissociation of the µ-iodide occurs prior to methyl
coupling. The lack of S = 1/2 species after the reaction is evident from the EPR studies, which implies a
Ni(II) as the resulting Ni product. The broad NMR signals suggest that the resulting Ni(II) is highꢀspin.
The competing reductive elimination of CH3I from 4 (step vi) takes place in parallel with dimerization
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(step ii). Once 5 is formed, the bridging
µ-iodide inhibits CH3I reductive elimination, consistent with the
5
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constant [CH3I] after the initial period of the reaction. The favorable formation of ethane relative to CH3I
is a result of the fast dimerization (step ii).
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Donor Ligand-Promoted Bimolecular Biphenyl Formation. (PyꢀMepyrr)Ni(Ph)(lut), 3, underwent
C–C bond formation via a different pathway, relative to 2. Complex 3 remained intact in DMFꢀd7 and
C6D6 (Table 2, entries 1ꢀ2). Addition of bpy afforded biphenyl in 49% yield upon heating (maximum
theoretical yield = 50%) (entry 3). Neither addition of CH3I (entry 4) nor O2 (entry 5) to 3 afford
biphenyl or toluene. Repeating the experiment in tolueneꢀd8 resulted in the absence of benzene and
phenol. Addition of I2 resulted in PhI in high yield (entry 6).
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Table 2. Formation of Biphenyl from 3
The high yield of biphenyl in the presence of bpy prompted us to undertake a further characterization
of the reaction pathway and the intermediate. The crossover experiment between 3 and (pyꢀMepyrr)Ni(pꢀ
Tol)(lut) afforded a mixture of biphenyl, 4ꢀphenyltoluene, and 4,4'ꢀdimethylbiphenyl in a ratio
comparable to the statistical distribution (eq 3). While one equivalent of phenanthroline was capable of
fully displacing lutidine from 3 to afford the pentaꢀcoordinated (pyꢀMepyrr)Ni(Ph)(phen) 10 (Figure
S3(B)), ligand substitution between 3 and bpy favored dissociation (eq 4). The equilibrium constant was
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determined to be 0.019 ± 0.002 at 22 ºC, by integrating the H resonances of 3 and 6 in the presence of
different concentrations of 2,4ꢀlutidine. The full conversion of 3 to 6 was achieved by removing 2,4ꢀ
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lutidine under vacuum followed by reꢀdissolving in C6D6 five times. At 22 ºC, 6 evolved biphenyl in full
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conversion after 2 h.
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(3)
(4)
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EPR analyses provided additional insight to the mechanism of this transformation. The reaction of
(pyꢀMepyrr)NiII(Me)(lut), 2, with 1 equiv. of (bpy)Ni0(cod) (eq 5) exhibited a rhombic signal
corresponding to an S = 1/2 Ni species at 50 K in toluene glass (Figure 7, red spectrum). An organic
radical is present, which could result from the decomposition of Ni(I). This paramagnetic species is
assigned to (pyꢀMepyrr)NiI(bpy), generated inꢀsitu by the comproportionation of Ni(II) with Ni(0). The
EPR spectrum of the reaction mixture of 3 with bpy displayed the same species as (pyꢀMepyrr)NiI(bpy)
(Figure 7, blue spectrum).
I
N
N
N
N
2 + (bpy)Ni0(cod)
Ni
(5)
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Figure 7. Xꢀband EPR spectra recorded in toluene glass at 50 K for the reaction mixture of 3 with bpy
(blue) and the comproportionation reaction (eq 5) (red). The black spectrum is the simulation of the red
spectrum: gx = 2.27, gy = 2.11, gz = 2.04, Axx = 3.7 MHz, Ayy = 26 MHz, Azz = 8.5 MHz.
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The statistical distribution of products from the crossover experiment (eq 3) provides crucial evidence
for a bimolecular pathway for bpyꢀpromoted biphenyl formation. Increasing the coordination number of a
transition metal complex has been reported to trigger the ejection of alkyl/aryl radical via steric
interactions.30 Free phenyl radicals are not involved here, since benzene is not an observed product.
Instead, a phenyl group transfer from 6 to 3 can be proposed to generate a Ni(III)ꢀdiphenyl intermediate,
which undergoes reductive elimination. This hypothesis is consistent with (pyꢀMepyrr)NiI(bpy) as the
resulting Ni species; however, other pathways cannot be ruled out. For example, bpy may bridge between
two molecules of 3 to facilitate the bimolecular reductive elimination. Our ongoing efforts focus on
elucidation of the precise role of bpy in this reaction by engaging in DFT calculations.
Conclusion
Collectively, we have established two bimolecular pathways for Niꢀmediated C–C bond formation.
The sp3ꢀsp3 coupling of Ni–CH3 to form ethane is initiated by oxidation to generate Ni(III), which
dimerizes to afford the diamagnetic, bimetallic Ni(III)–CH3 intermediate. The resulting Ni(III)–CH3
dimer undergoes rateꢀdetermining reductive elimination to afford ethane (Scheme 7). The bimolecular
pathway is verified by crossover experiments and the increased [ethane]/[CH3I] ratio as increasing [Ni–
CH3]. The initial Ni(III) intermediate is characterized by EPR spectroscopy, and evidence for the
1
bimetallic [Ni(III)–CH3]2 include H NMR experiments and kinetic studies. The experimental data are in
good agreement with DFT calculations. Biphenyl formation, in contrast, is triggered by the coordination
of a bidentate donor ligand (eq 4). While EPR characterization of the resulting Ni(I) species supports a
phenyl transfer between two Ni(II) intermediates to generate a Ni(III)ꢀdiphenyl intermediate, the precise
mechanism requires further investigation. These two bimetallic reductive elimination pathways presented
here highlight the complexity of Niꢀmediated C–C bond formation reactions. The elucidation of the
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bimolecular mechanism provides important insight for future design of active Ni catalysts for crossꢀ
coupling and homoꢀcoupling reactions.
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ASSOCIATED CONTENT
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Supporting Information. All experimental procedures, additional discussions and figures, details of DFT
calculations, NMR spectra, and crystallographic data (PDF, CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
diao@nyu.edu
Funding Sources
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank Brian Schaefer, Valerie Schmidt, and Nadia Leonard (Chirik lab, Princeton University) for
assistance in recording EPR spectra and Michelle Li for preparing the pyꢀPhpyrr ligand. This work was
supported by the NYUꢀMRSEC Program funded by National Science Foundation under award number
DMRꢀ1420073 and New York University.
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+
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16. In this rigorously dry system, the source of the protons is unclear. Control experiments reveal that 2
reacts rapidly with trace amount of water to form methane. We were able to characterize one of the
resulting Ni species to be (pyꢀMepyrr)2Ni(bpy) 9 (Figure S3(A)).
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18. The elevated baseline is an artifact from close g values for the rhombic spectrum. This effect is not
pronounced in THF (Figure S6). For an example of baseline lifting due to close g values, see:
Hearshen, D. O.; Hagen, W. R.; Sands, R. H.; Grande, H. J.; Crespi, H. L.; Gunsalus, I. C.; Dunham,
W. R. J. Magn. Reson. 1986, 69, 440.
19. Neese, F. Wiley Interdisciplinary Reviews: Computational Molecular Science 2012, 2, 73.
20. Kinetic fitting was carried with COPASI software: Hoops, S.; Sahle, S.; Gauges, R.; Lee, C.; Pahle,
J.; Simus, N.; Singhal, M.; Xu, L.; Mendes, P.; Kummer, U. Bioinformatics 2006, 22, 3067.
21. The early points of ethane slightly deviate from the fitting. We attribute this error to imperfect
shimming at the beginning of the experiments.
22. See Supporting Information for attempted experiments to prepare (pyꢀpyrr)Ni(halide).
23. Nelson, I. V.; Iwamoto, R. T. J. Electroanal. Chem. 1964, 7, 218.
24. (a) Higgs, A. T.; Zinn, P. J.; Simmons, S. J.; Sanford, M. S. Organometallics 2009, 28, 6142. (b) Renz,
A. L.; Pérez, L. M.; Hall, M. B. Organometallics 2011, 30, 6365.
25. For stable Ni(III) halide complexes, see: (a) Oguro, K.; Wada, M.; Sonoda, N. J. Organomet. Chem.
1979, 165, C10. (b) Grove, D. M.; Van Koten, G.; Zoet, R.; Murrall, N. W.; Welch, A. J. J. Am.
Chem. Soc. 1983, 105, 1379. (c) Gray, L. R.; Higgins, S. J.; Levason, W.; Webster, M. J. Chem. Soc.,
Dalton Trans. 1984, 459. (d) Grove, D. M.; Van Koten, G.; Mul, P.; Zoet, R.; Van der Linden, J. G.
M.; Legters, J.; Schmitz, J. E. J.; Murrall, N. W.; Welch, A. J. Inorg. Chem. 1988, 27, 2466.
26. (a) Klein, H.ꢀF.; Bickelhaupt, A.; Jung, T.; Cordier, G. Organometallics 1994, 13, 2557. (b) Klein, H.ꢀ
F.; Bickelhaupt, A.; Lemke, M.; Sun, H.; Brand, A.; Jung, T.; Röhr, C.; Flörke, U.; Haupt, H.ꢀJ.
Organometallics 1997, 16, 668.
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27. The same effect is present in previous dimeric Ni(III) complexes: (a) Hikichi, S.; Yoshizawa, M.;
Sasakura, Y.; Komatsuzaki, H.; Moroꢀoka, Y.; Akita, M. Chem. Eur. J. 2001, 7, 5011. (b) Lee, C.ꢀM.;
Chiou, T.ꢀW.; Chen, H.ꢀH.; Chiang, C.ꢀY.; Kuo, T.ꢀS.; Liaw, W.ꢀF. Inorg. Chem. 2007, 46, 8913. (c)
Kuwamura, N.; Kitano, K.; Hirotsu, M.; Nishioka, T.; Teki, Y.; Santo, R.; Ichimura, A.; Hashimoto,
H.; Wright, L. J.; Kinoshita, I. Chem. Eur. J. 2011, 17, 10708.
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28. For recent examples, see: (a) Csok, Z.; Vechorkin, O.; Harkins, S. B.; Scopelliti, R.; Hu, X. J. Am.
Chem. Soc. 2008, 130, 8156. (b) Wotal, A. C.; Ribson, R. D.; Weix, D. J. Organometallics 2014, 33,
5874.
29. Pieck, R.; Steacie, E. W. R. Can. J. Chem. 1955, 33, 1304.
30. Fernández, I.; Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Organometallics 2007, 27, 109.
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