Synthesis, reactivity, and catalytic application of a nickel pincer hydride complex

Article abstract of DOI:10.1021/om201279j

The nickel(II) hydride complex [(^MeN₂N)Ni-H] (2) was synthesized by the reaction of [(^MeN₂N)Ni-OMe] (6) with Ph₂SiH₂ and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. 2 was unstable in solution, and it decomposed via two reaction pathways. The first pathway was intramolecular N-H reductive elimination to give ^MeN₂NH and nickel particles. The second pathway was intermolecular, with H₂, nickel particles, and a five-coordinate Ni(II) complex [(^MeN₂N)₂Ni] (8) as the products. 2 reacted with acetone and ethylene, forming [( ^MeN₂N)Ni-OⁱPr] (9) and [(^MeN ₂N)Ni-Et] (10), respectively. 2 also reacted with alkyl halides, yielding nickel halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(^MeN₂N)Ni-Cl] (1) as catalyst, NaOⁱPr or NaOMe as base, and Ph₂SiH₂ or Me(EtO)₂SiH as the hydride source. The catalysis appears to operate via a radical mechanism.

Full text of DOI:10.1021/om201279j

Article
pubs.acs.org/Organometallics
Synthesis, Reactivity, and Catalytic Application of a Nickel Pincer
Hydride Complex
Jan Breitenfeld, Rosario Scopelliti, and Xile Hu*
́ ́
Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de
Lausanne (EPFL), SB-ISIC, BCH 3305, Lausanne, CH 1015, Switzerland
S
* Supporting Information
ABSTRACT: The nickel(II) hydride complex [(MeN2N)Ni-H] (2) was synthesized by
the reaction of [(MeN2N)Ni-OMe] (6) with Ph2SiH2 and was characterized by NMR and
IR spectroscopy as well as X-ray crystallography. 2 was unstable in solution, and it
decomposed via two reaction pathways. The first pathway was intramolecular N−H re-
ductive elimination to give MeN2NH and nickel particles. The second pathway was inter-
molecular, with H2, nickel particles, and a five-coordinate Ni(II) complex [(MeN2N)2Ni]
(8) as the products. 2 reacted with acetone and ethylene, forming [(MeN2N)Ni-OiPr] (9)
and [(MeN2N)Ni-Et] (10), respectively. 2 also reacted with alkyl halides, yielding nickel
halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(MeN2N)Ni-Cl] (1) as catalyst,
NaOiPr or NaOMe as base, and Ph2SiH2 or Me(EtO)2SiH as the hydride source. The catalysis appears to operate via a radical
mechanism.
and we are able to identify two interesting decomposition path-
ways.
INTRODUCTION
■
Transition-metal complexes of pincer ligands are heavily pur-
sued as catalysts or precatalysts, thanks to their enhanced stability
and their demonstrated ability to mediate unusual chemical
transformations.1−4 Within this context, many nickel pincer
complexes have been prepared,5−19 and some of them are shown
to be catalysts for C−C and C−S cross-coupling, double-bond
functionalization, and CO2 reduction reactions. Guan et al.
reported that a well-defined nickel PCP pincer hydride com-
plex was an efficient catalyst for hydrosilylation of aldehydes
and ketones as well as the reduction of CO2 with a borane.5,6,20
These reports provided some of the few examples in which
isolated nickel hydride species were catalytically compe-
tent.21−23 In general, nickel hydride species are highly reactive
and their roles in catalysis are largely hypothesized. The iso-
lation, reactivity, and catalytic application of nickel hydride
complexes are thus interesting.
Our group recently developed the pincer bis(amino)amide
ligand MeN2N.24,25 The nickel chloride complex [(MeN2N)Ni−
Cl] (1) is an excellent catalyst for cross-coupling of nonacti-
vated alkyl halides and direct C−H alkylation (Scheme 1).26−32
It was shown that β-H elimination was kinetically viable but
thermodynamically uphill for the corresponding nickel alkyl
species.33 This was thought to be one of the key factors for
efficient alkyl−alkyl coupling. The metal-containing product of
β-H elimination was the hydride complex [(MeN2N)Ni-H] (2),
which was also proposed as an intermediate for the isomer-
ization of [(MeN2N)Ni-iPr] (3) to [(MeN2N)Ni-nPr] (4).33
Preciously 2 was only observed in situ. Herein we report the
preparation and isolation of 2, its structure and reactivity, and
its involvement in the catalytic hydrodehalogenation of alkyl
halides. Like some other nickel hydride complexes, 2 is unstable
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [(MeN2N)Ni-H] (2)
and [(MeN2N)Ni-D] (2D). Two general methods are known for
the synthesis of Ni(II) hydride complexes: (1) N−H oxidative
addition of HNL2 ligands with Ni(0)8,11 and (2) reaction of
nickel halide complexes with a hydride donor such as LiAlH4 or
LiEt3BH.15 These two methods were attempted for the syn-
thesis of [(MeN2N)Ni-H] (2). No reaction occurred when
MeN2NH (5) was treated with Ni(COD)2; thus, the oxidative
addition route was unsuccessful. The hydride transfer reactions
were then examined. [(MeN2N)Ni-Cl] (1) did not react with
NaBH4. It reacted with LiAlH4, only leading to unidentifiable
products. As communicated earlier,33 reaction of 1 with LiEt3BH
produced [(MeN2N)Ni-H] (2), but only in solution. The
identity of 2 was confirmed by a diagnostic singlet at −22.81
ppm in the 1H NMR spectrum in C6D6. In solution, 2
decomposed within 1 h at room temperature to give a black
and metallic precipitate. The decomposition was faster if the
solvent was removed in vacuo. As a result, it was not possible to
isolate 2 in the solid form.
Silanes are sometimes used as hydride sources. No reaction
occurred between 1 and Ph2SiH2. However, when NaOMe was
added to a solution of 1 and Ph2SiH2 in C6D6, the solution
changed from brown to purple and then to red. A hydride
1
signal was observed in the H NMR spectrum of the red re-
action mixture. The purple color was indicative of [(MeN2N)Ni-
OMe] (6),25 suggesting that this species reacted with Ph2SiH2
Received: December 27, 2011
Published: February 22, 2012
© 2012 American Chemical Society
2128
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
Scheme 1. Cross-Coupling of Alkyl Halides and Direct Alkylation Reactions Catalyzed by Nickel Pincer Complex 1
to produce the hydride complex. Indeed, reaction of preisolated
6 with Ph2SiH2 proceeded smoothly to give 2. Due to the
instability of 2, isolation by solvent evaporation at room tem-
perature was unsuccessful, leading again to a black precipitate.
The decomposition was avoided by conducting the reac-
tion and isolation at −70 °C (Scheme 2). Higher yields were
Scheme 2. Synthesis of the Hydride Complex 2
Figure 1. X-ray structure of complex 2. There are two molecules in the
asymmetric unit; only one is shown. The thermal ellipsoids are
displayed at 50% probability; two toluene solvent molecules and all
hydrogen atoms except the hydride ligand are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ni1−N1, 1.936(2); Ni1−
N2, 1.881(2); Ni1−N3, 1.935(2); Ni1−H1A, 1.50(2); N1−Ni1−N3,
174.53(8); N2−Ni1−H1A, 177.8(10).
obtained when ambient light was excluded. In a typical pro-
cedure, Ph2SiH2 was added to a concentrated solution of 6 in
toluene to give 2 in solution. Addition of precooled pentane to
this solution produced 2 as an orange precipitate, which could
be isolated in a yield of 67%. Solid samples of 2 can be stored at
−28 °C in the dark. When it was exposed to light or warmed to
room temperature, 2 gradually turned black.
environment. The average Ni−H distance is 1.505(20) Å,
comparable to that of [(PCP)Ni-H] (Ni−H = 1.42(3) Å).15
The [(MeN2N)Ni-D] complex (2D) was synthesized
similarly, starting from 6 and Ph2SiD2. A deuteride signal was
The characteristic hydride signal was found at −22.8 ppm in
1
the H NMR spectrum of 2 in C6D6. This signal is shifted
upfield compared to those for the analogous [(POCOP)Ni-H]
(−8 ppm),5 [(PCP)Ni-H] (−10 ppm),15 and [(PNP)Ni-H]
(−18 ppm)11 complexes, probably indicating a more electron
rich nickel center. The IR spectrum of 2 in C6H6 shows a Ni−H
vibrational band at 1768 cm−1. The solid-state molecular struc-
ture of 2 was established by a single-crystal X-ray diffraction
study (Figure 1). The nickel center is in a square-planar ligand
2
observed at −22.8 ppm in the H NMR spectrum of 2D.
Decomposition of [(MeN2N)Ni-H] (2) and [(MeN2N)Ni-D]
(2D). When it was freshly prepared in solution at room tem-
perature, 2 decomposed readily to give the protonated ligand
MeN2NH (5) and a black metallic precipitate (Scheme 3). We
suspected that the decomposition was due to N−H reductive
2129
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
Scheme 3. N−H Elimination from Complex 2
elimination. The isolated and purified sample of 2 decomposes
in the same way when dissolved in solution at room tem-
perature, but the decomposition rate is slower. To ascertain if
the N−H hydrogen in MeN2NH originated from the hydride
ligand of 2, we examined the decomposition of the deuteride
complex 2D.
An independent sample of MeN2ND (5D) was prepared by
reaction of the Li complex [MeN2NLi]2 (7)24 with D2O. 5D
shows a 2H NMR signal at δ 7.41 ppm for the anilino hydrogen.
Figure 2. X-ray structure of complex 8. The thermal ellipsoids are
displayed at 50% probability. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ni1−N1, 2.150(6); Ni1−
N2, 1.991(5); Ni1−N3, 2.204(5); Ni1−N5, 2.023(6); Ni1−N6,
2.109(5); N1−Ni1−N3, 142.0(2); N3−Ni1−N6, 111.6(2); N1−
Ni1−N6, 101.6(2); N2−Ni1−N5, 177.8(2).
2
The decomposition of 2D was then followed by H NMR.
A signal at δ 7.4 ppm became visible within 24 h, indicating the
formation of 5D. This result confirms that 2 and 2D de-
compose via N−H/D reductive elimination. It is interesting to
note that analogous [(PNP)Ni-H] complexes are stable against
N−H elimination.11
The formation of 8 was not observed. This result suggests that
the formation of 8 proceeds through an intermolecular process,
which is much slower than N−H elimination in dilute solu-
tions. The latter also suggests that N−H elimination is intra-
molecular. On the basis of the aforementioned observations
and reasoning, we propose that 8 was produced by a bimolec-
ular combination of 2 to form H2 and Ni(I) species (Scheme 5).
The latter was unstable and underwent a disproportionation
reaction to give 8 and nickel particles.
Examination of the NMR spectrum of the product mixture
from the decomposition of 2 revealed that a paramagnetic
species (8) and H2 were also formed. A second decomposition
pathway appears to exist. We initially hypothesized that the
paramagnetic species was a Ni(I) species, produced by a
combination reaction of two molecules of 2 to give H2. We
then attempted to prepare the Ni(I) species independently. Re-
action of 1 with a chemical reductant such as Na/naphthalene
yielded a paramagnetic nickel species whose NMR spectrum is
identical with that of 8 (Scheme 4). The molecular structure
This hypothesis is further supported by inhibition and iso-
topic exchange tests. When the decomposition of 2 was exa-
mined under 7 bar of H2, the formation of 8 was no longer
observed, even in concentrated solutions. Only N−H elimination
was found. When the decomposition of 2 was followed under
D2, [(MeN2N)Ni-D] (2D) and MeN2ND (5D) were observed
Scheme 4. Formation of Complex 8 from Reduction of
Complex 1
2
by H NMR. These results suggest that the bimolecular com-
bination of 2 is reversible, and the equilibrium can be shifted by
hydrogen pressure (Scheme 5).
Despite many attempts, our efforts to trap the Ni(I) species
using additional donor ligands such as CO and phosphine were
fruitless.
Reactivity of [(MeN2N)Ni-H] (2). Nickel hydride complexes
are known to insert into double bonds (CC and CO).
These reactions are key steps in Ni-catalyzed hydrosilylation
and hydrogenation reactions.34 The reactions of 2 with acetone
and ethylene were studied in order to examine its possible
utility in reduction catalysis (Scheme 6).
The reaction of 2 with acetone was sluggish. The reaction took
12 h to complete; the expected insertion product, [(MeN2N)Ni−
OiPr] (9), was formed in ca. 50% yield. 9 was independently
prepared by reaction of [(MeNN2)Ni-Cl] (1) with NaOiPr in
76% yield.
of 8 was determined by a single-crystal X-ray diffraction study
(Figure 2). Surprisingly, it is not a Ni(I) species, but a Ni(II)
complex coordinated to two MeN2N ligands ([(MeNN2)2Ni]).
The nickel ion is in a five-coordinate, distorted-trigonal-bipyramidal
coordination environment. One of the pincer ligands is
bidentate, with a noncoordinating NMe2 donor (Ni−N >
3.7 Å). The formation of 8 from 1 likely occurred via a Ni(I)
intermediate that was produced by reduction of 1. The Ni(I)
species was not stable, and it underwent a disproportionation
reaction to form 8 and nickel particles.
A few experiments were carried out to probe how 8 may be
formed from 2. First, reaction of 5 with 2 did not lead to 8 and
H2. Second, 2 did not react with a base such as NEt3 to give H2,
which suggested that deprotonation of 2 with the anilide moiety of
a second molecule of 2 was not a likely reaction pathway.
Third, we found that the decomposition of 2 was significantly
slower in dilute solutions. In addition, in dilute solutions, de-
composition occurred only via N−H reductive elimination.
The reaction of 2 with ethylene was faster. When a solution
of 2 in C6D6 was treated with ethylene (6 bar), [(MeN2N)-
Ni-Et] (10) was formed in minutes. 10 was prepared earlier by
reaction of 1 with EtMgCl.24 After the reaction, 2 was no longer
observed, consistent with the earlier conclusion that β-H elim-
ination is thermodynamically uphill and the equilibrium lies
largely on the side of nickel alkyl complexes. The rapid reaction
of 2 with an olefin to form a nickel alkyl complex also provides
2130
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
Scheme 5. Second Decomposition Pathway of Complex 2 That Led to H2, Nickel Particles, and Complex 8
Scheme 6. Reactivity of Complex 2 with Acetone, Ethylene,
and Alkyl Halides
The catalytic reduction of tetradecyl chloride was used as the
test reaction. Table 1 summarizes the influence of various
reaction parameters. With toluene as solvent, NaOiPr as base,
and Ph2SiH2 as hydride source, the reduction gave a quanti-
tative yield of tetradecane (entry 1, Table 1). Two equivalents
of silane and base were required for full conversion. Reduction
in other solvents such as THP (tetrahydropyran), THF, DMA,
or dioxane gave lower yields (entries 2−5, Table 1). No
reduction was observed using Et3SiH (entries 6−8, Table 1).
Reduction using PHMS or PhSiH3 as the hydride source was
less efficient (entries 9−12, Table 1). Me(EtO)2SiH could be a
good hydride source, but only in conjunction with NaOMe as
base and THP as solvent (entries 13 and 14, Table 1). Under
these conditions, tetradecane was also formed quantitatively
(entry 14, Table 1). Even with the best systems, the loading of
catalyst is in the range of 8−10% (entries 1 and 14, Table 1).
A reaction time of 5−6 h was sufficient.
The scope of the hydrodehalogenation was explored using
the Ph2SiH2/NaOiPr/toluene protocol (Table 2). Not only
primary alkyl halides but also secondary and tertiary alkyl
halides could be reduced (entries 1−7, Table 2). The minimum
catalyst loading for a high yield decreased in the order Cl >
Br > I (e.g. compare entries 1−3, Table 2). The order correlates
with the bond strength. The yields were between 73% and 99%.
Aryl halides could also be reduced, but only with 39−57%
yields (entries 8−10, Table 2). For the reduction of aryl halides,
THP was used as the solvent and Me(EtO)2SiH was used as
the hydride source. These changes were made to ensure that
benzene did not originate from Ph2SiH2 or toluene.
evidence that 2 is an intermediate in the isomerization of
[(MeN2N)Ni-iPr] (3) to [(MeN2N)Ni-nPr].
The reactions of 2 with alkyl halides were instantaneous. In
these reactions, alkyl halides were reduced to the corresponding
alkanes, and nickel halide complexes were formed as the metal-
containing species (Scheme 6). Alkyl iodides, bromides, and
chlorides, but not fluorides, were reduced. The reaction rates fol-
lowed the order alkyl iodides > bromides > chlorides.
Catalytic Hydrodehalogenation of Alkyl Halides.
Hydrodehalogenation is an important reaction in organic syn-
thesis.35 Tin-based reduction methods are selective and effi-
cient. However, the toxicity associated with tin reagents has
prompted the developments of alternative methods, such as
transition-metal-catalyzed reduction using organic silanes.35,36
The formation and reactivity of 2 constitute a stoichiometric
reaction cycle for the reduction of alkyl halides by silane
(Figure 3). We reasoned that the Ni−Cl complex 1 could be
As reduction of alkyl halides by 2 appears to be faster than
reactions of 2 with alkene or ketone, we thought this reduction
method might tolerate unsaturated functional groups. Indeed, a
C−C double bond was tolerated, with a 64% yield in reduction
(entry 11, Table 2). The reaction was accompanied by a small
degree of isomerization (∼5% trans to cis). Alkyl bromides with
ether and amide groups were selectively reduced in modest
yields (entries 12 and 13, Table 2). Unfortunately, ketone and
ester groups were not tolerated (entries 14 and 15, Table 2).
Overall the group tolerance is modest, probably because the re-
actions were run under basic conditions.
Mechanism of Catalytic Hydrodehalogenation. The
overall catalytic cycle should be similar to that shown in Figure 3.
All the nickel-containing species, i.e., the nickel chloride, hydride,
and alkoxide complexes, have been isolated and characterized.
Their reactivity is consistent with the proposed catalytic cycle.
The byproduct of hydride formation, Ph2SiHOiPr, was
identified by GC-MS. The catalytic reduction of tertiary alkyl
halides such as adamantyl chloride and bromide suggests that
the reduction does not occur via a SN2 type mechanism. The
reaction is more likely to go through radical or carbenium
intermediates. To differentiate these two possibilities, reduction
with radical- and carbenium-probe substrates was examined
(Scheme 7).
Figure 3. Proposed catalytic cycle for hydrodehalogenation reactions
of organic halides. X = halide.
used as a catalyst. 1 could react with an alkoxide to from a nickel
alkoxide complex (e.g., 9), which could then react with a silane to
form the Ni−H complex 2. The latter could react with an alkyl
halide to give the reduction product and regenerate the catalyst.
2131
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
a
Table 1. Catalytic Reduction of Tetradecyl Chloride Using 1 as Catalyst
b
entry
silane
Ph2SiH2
base
solvent
cat. loading (mol %)
time (h)
amt of silane (equiv)
conversion/yield (%)
1
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOMe
NaOiPr
NaOiPr
NaOiPr
NaOMe
NaOiPr
NaOiPr
NaOMe
toluene
THP
8
10
9
6
5
6
6
6
6
5
5
5
6
5
5
6
5
2
2
2
2
2
1
2
2
2
1
2
2
3
2
100/99
77/18
100/23
100/53
51/12
0/0
2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Et3SiH
3
THF
4
DMA
dioxane
toluene
THP
10
8
5
6
6
7
Et3SiH
10
10
10
6
1/1
8
Et3SiH
THP
1/1
9
PMHS
THP
33/32
35/20
29/27
82/25
30/30
100/100
10
11
12
13
14
PhSiH3
toluene
THP
PhSiH3
10
10
10
10
PhSiH3
THP
Me(EtO)2SiH
Me(EtO)2SiH
toluene
THP
a
b
See the Experimental Section for conditions. GC yields.
First, we examined the catalytic reduction of cycloheptyl
reactions might be developed. One possibility is to produce a
nickel hydride complex from the nickel halide complex through
a hydride transfer reaction, and the hydride complex may react
with an olefin to form a nickel alkyl complex. The latter could
react with an alkyl halide to complete a catalytic cycle for hy-
droalkylation of olefin (Figure 4 (right)).
bromide (Scheme 7). If a carbenium intermediate is involved,
skeletal rearrangement should take place, leading to methyl-
cyclohexane. This was observed for hydrodehalogenation using
AlCl3/Et3SiH.37 In the current case, only cycloheptane was pro-
duced, ruling out the involvement of a carbenium intermediate.
Next, we examined the reduction of 6-bromohex-1-ene and
2-iodohept-1-ene (Scheme 7). Only methyl- and 1,2-di-
methylcyclopentane were formed. In principle, two reaction
pathways can lead to the formation of these compounds. The
first involves the insertion of the Ni−H bond into the terminal
olefin, followed by an intramolecular substitution of the halide.
However, insertion of 2 into an olefin is slow compared to
reduction. This reaction pathway can thus be excluded. The
second reaction pathway involves a single-electron transfer
(SET) from 2 to the organic halide to generate an alkyl radical,
which undergoes a fast ring-closing addition to the double
bond, before receiving a hydrogen radical.38 The SET likely oc-
curs through a Cl-bridged, inner-sphere process, as the reduc-
tion potentials of alkyl halides are in the range of −1 V vs
SCE39 and the oxidation potential of (MeN2N)Ni complexes are
in the range of 0.3 V vs SCE.25 We currently favor this reaction
pathway. Alternatively, the reactions might proceed through a
σ-bond metathesis type transition state involving Ni−H and
R−X. It would be interesting to probe this possibility by
computational methods.
In the current study, we demonstrated the production of the
nickel hydride complex 2 from the nickel chloride complex 1
and the reaction of ethylene with 2 to form the corresponding
nickel ethyl complex. The stoichiometric reaction cycle for the
hydroalkylation of olefin was thus in place. We then tried to
turn the reaction catalytic, using 10 mol % 1 as catalyst,
NaOMe as base, Me(EtO)2SiH as hydride source, decene as
olefin, and dodecyl iodide as electrophile. To suppress the com-
peting reduction of alkyl halide, the silane was added slowly as
the limiting reagent. The catalysis was not efficient, with a yield
of about 10% for hydroalkylation. The reduction product, dode-
cane, was produced in a yield of about 5%. The majority of the
starting materials, i.e., decene and dodecyl iodide, remained
unreacted. The inefficiency of the catalysis might originate from
the instability of the nickel hydride complex 2, as described
above.
CONCLUSION
■
In summary, we developed a clean synthetic route to the nickel
hydride complex [(MeN2N)Ni-H] (2) and determined its crys-
tal structure. 2 decomposed in solution via two pathways,
including intramolecular N−H reductive elimination and a bi-
molecular combination reaction. The former reaction led to the
protonated ligand and nickel particles, while the latter reaction pro-
duced the five-coordinate Ni(II) species [(MeN2N)2Ni] (8), H2,
and nickel particles. The bimolecular combination reaction was
suppressed in the presence of H2. Complex 8 was indepen-
dently synthesized by reduction of [(MeN2N)Ni-Cl] (1) and
was characterized by X-ray crystallography. 2 inserted into
ethylene and acetone to form the corresponding nickel alkyl
and alkoxy complexes. 2 rapidly reduced alkyl halides to form
alkanes. On the basis of the reactivity of 2, a catalytic hydro-
dehalogenation method was developed. A large number of
organic halides could be reduced by silane using 1 as the catalyst.
The reduction of alkyl halide was found to proceed via a radical
Stoichiometric reactions of 2 with cycloheptyl bromide and
6-bromohex-1-ene were also examined, and the results were
similar to those for the catalytic reductions.
Hydroalkylation of Olefin. Pincer complex 1 was shown
earlier to be a catalyst for cross-coupling of alkyl halides with
alkyl Grignard reagents.26 This type of cross-coupling is chal-
lenging because metal alkyl intermediates tend to undergo
unproductive β-H elimination.30,40−42 The proposed reaction
sequence for the alkyl−alkyl coupling is shown in Figure 4 (left).
A nickel halide complex first reacts with a Grignard reagent to
form a nickel alkyl complex, which reacts with an alkyl halide to
give the coupling product and regenerate the nickel halide com-
plex.30 We thought that if a nickel alkyl complex could be
generated from a nickel halide complex using a reagent other
than an alkyl Grignard reagent, alternative alkyl−alkyl coupling
2132
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
Table 2. Scope of Catalytic Hydrodehalogenation of Organic
Halides
EXPERIMENTAL SECTION
■
a
Chemicals and Reagents. All manipulations were carried out
under an inert N2(g) atmosphere using standard Schlenk or glovebox
techniques. Solvents were purified using a two-column solid-state
purification system (Innovative Technology, Amesbury, MA, USA)
and transferred to the glovebox without exposure to air. Deuterated
solvents were purchased from Cambridge Isotope Laboratories, Inc.,
and were degassed and stored over activated 3 Å molecular sieves.
Unless otherwise noted, all other reagents and starting materials were
purchased from commercial sources and used without further
purification. Liquid compounds were degassed by standard freeze−
pump−thaw procedures prior to use. The complexes [(MeN2N)-
Ni−Cl] (1), [(MeN2N)Ni-OMe] (6), and [MeN2NLi]2 (7) were
prepared as previously reported.24,25
Physical Methods. The 1H and 13C{1H} NMR spectra were
recorded at 293 K on a Bruker Avance 400 spectrometer. 1H, 2H, and
13C{1H} NMR chemical shifts were referenced to residual solvents as
determined relative to Me4Si (δ 0 ppm). GC-MS measurements were
conducted on a Perkin-Elmer Clarus 600 GC equipped with a Clarus
600T MS. GC measurement was conducted on a Perkin-Elmer Clarus
400 GC with an FID detector. Elemental analyses were performed on
a Carlo Erba EA 1110 CHN instrument at EPFL. The temperature of
reactions below room temperature was regulated by a Julabo FT-902
chiller. X-ray diffraction studies were carried out at the EPFL Cry-
stallography Facility. The data collections for both crystal structures
were performed at low temperature (100(2) K) using Mo Kα radiation
on a Bruker APEX II CCD diffractometer equipped with a kappa
geometry goniometer. The data were reduced by EvalCCD43 and then
corrected for absorption.44 The solution and refinement was
performed by SHELX.45 The structure was refined using full-matrix
least squares based on F2 with all non-hydrogen atoms anisotropically
defined. Hydrogen atoms were placed in calculated positions by means
of the “riding” model.
Synthesis of [(MeN2N)Ni-H] (2). In the dark, to a purple solution
of 6 (138 mg, 0.401 mmol) in 2 mL of toluene was added at −70 °C a
solution of Ph2SiH2 (74 mg, 0.40 mmol) in 0.5 mL of toluene. The
solution turned dark red and was stirred for 30 min before a precooled
solution of pentane (−70 °C, ∼5 mL) was added. A fine orange
precipitate formed. The solution was removed by syringe, and the
precipitate was washed with 4 × 5 mL of precooled pentane (−70 °C).
After drying in vacuo at −30 °C for 12 h, 2 was obtained as a solid
(84 mg, 0.27 mmol, 67% yield). Single crystals suitable for X-ray
analysis were obtained 1 day after layering a toluene solution of 2 with
pentane at −70 °C.
1H NMR (400 MHz, C6D6): δ 7.53 (d, J = 8.2 Hz, 2H,CHaryl), 7.08
(t, J = 7.6 Hz, 2H, CHaryl), 6.48 (dt, J = 14.8, 7.5 Hz, 4H, CHaryl), 2.71
(s, 12H, N(CH3)2), −22.81 ppm (s, 1H, Ni−H). 13C{1H} NMR (101
MHz, C6D6): δ 148.24 (Caryl), 147.69 (Caryl), 128.49 (Caryl), 119.84
(Caryl), 114.36 (Caryl), 114.23 (Caryl), 52.88 ppm (CH3). Anal. Calcd
for 2 and 0.075 toluene (C16H20N3Ni): C, 61.39; H, 6.44; N, 13.42.
Found: C, 61.44; H, 6.44; N, 12.35. The discrepancy in the nitrogen
content is probably due to the decomposition of 2. IR (in C6D6):
1768 cm−1 for Ni−H.
a
For alkyl halides, the Ph2SiH2/NaOiPr/toluene combination was
applied; for aryl halides, the Me(EtO)2SiH/NaOMe/THP combina-
tion was applied. See the Experimental Section for details. GC yields.
b
Scheme 7. Reduction of Radical- and Carbenium-Probe
Substrates
Synthesis of [(MeN2N)Ni-D] (2D). The synthesis was the same as
for 2, except that Ph2SiD2 was used as the silane. Yield: 60%.
1H NMR (400 MHz, C6D6): δ 7.53 (d, J = 8.2 Hz, 2H, CHaryl), 7.08
(t, J = 7.6 Hz, 2H, CHaryl), 6.48 (dt, J = 14.8, 7.5 Hz, 4H, CHaryl), 2.71
ppm (s, 12H, N(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 148.24
(Caryl), 147.69 (Caryl), 128.49 (Caryl), 119.84 (Caryl), 114.36 (Caryl),
114.23 (Caryl), 52.88 ppm (CH3). 2H NMR (61 MHz, C6H6):
δ −22.82 ppm (s, 1H, Ni-D).
Synthesis of [(MeN2N)D] (5D). To a solution of 7 (10 mg,0.019
mmol) in 1.0 mL of C6H6 or C6D6 was added 0.2 mL of D2O. The
organic layers were separated and dried over MgSO4.
mechanism. The nickel hydride complex 2 belongs to a small
group of isolated and defined nickel hydride species that are
catalytically active. Its decomposition pathways and reactivity
could serve as useful references when considering the roles of
nickel hydrides in catalytic transformations.
1H NMR (400 MHz, C6D6): δ 7.54 (d, J = 7.7 Hz, 2H, CHaryl), 6.98
(dd, J = 14.3, 7.3 Hz, 4H, CHaryl), 6.87 (t, J = 7.4 Hz, 2H, CHaryl),
2
2.43 ppm (s, 12H, N(CH3)2). H NMR (61 MHz, C6H6): δ 7.41
ppm (N-D).
2133
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
Figure 4. (left) Proposed reaction sequence for alkyl−alkyl Kumada coupling catalyzed by 1. (right) Hypothetical catalytic cycle for hydroalkylation
of olefins. X = halide.
1
Synthesis of [(MeN2N)2Ni] (8). To a solution of naphthalene
(37 mg, 290 mmol) in 5 mL of THF was added Na (7 mg, 304 mmol)
at room temperature. The solution turned green and was stirred over-
night at room temperature. After filtration, the solution was added to a
solution of 1 (100 mg, 287 mmol) in 10 mL of THF. The solution
turned dark and was stirred for 2 h. The solvent was removed by
vacuum. The solid residue was extracted with 3 × 10 mL of toluene
and filtered through Celite. The solvent was reduced to about 0.5 mL,
and the red solution was stored at −28 °C overnight. Complex 8 was
then obtained as a solid (70 mg, 0.123 mmol, 43% yield). Single
crystals suitable for X-ray analysis were obtained from a pentane solu-
tion of 8 at −28 °C.
(3). Isolated [(MeN2N)Ni-D] (2D) under H2 Pressure by H NMR. A
sapphire NMR tube was charged with 2 (7 mg, 0.02 mmol) in 0.4 mL
of C6D6 with a sealed capillary containing CDCl3/TMS as external
1
standard. The NMR tube was pressurized with 6 bar of H2. H NMR
spectra were recorded. After 12 h, 2 completely decomposed. At no
time was the characteristic signal of 8 at 82.41 ppm observed.
Reaction of 2 with Ethylene. A sapphire NMR tube was charged
with 2 (7 mg, 0.02 mmol) in 0.4 mL of C6D6 with 0.5 μL of toluene as
the internal standard. The red solution was then pressurized with 6 bar
of ethylene pressure and shaken for 5 min to dispense ethylene in
1
solution. H NMR spectra were recorded. After about 30 min, 2 was
completely consumed. The product 10 was identified by the following
NMR signals.
Anal. Calcd for 8 (C32H40N6Ni): C, 67.74; H, 7.11; N, 14.81.
Found: C, 68.02; H, 7.27; N, 14.73.
1H NMR (400 MHz, C6D6): δ 7.61 (d, J = 8.1 Hz, 2H, CHaryl), 7.06
(t, J = 7.4 Hz, 2H, CHaryl), 6.63 (d, J = 7.7 Hz, 2H, CHaryl), 6.45 (t, J =
7.4 Hz, 2H, CHaryl), 2.38 (s, 12H, N(CH3)2), 0.91 (t, J = 7.7 Hz, 3H,
CH2CH3), −0.46 ppm (q, J = 7.6 Hz, 2H, CH2CH3). The yield was
calculated using the signal of toluene as standard (60%). No signals
from 2 remained.
Synthesis of [(MeN2N)Ni-OiPr] (9). To a suspension of NaOiPr
(59 mg, 0.71 mmol) in 2 mL of THF was added a brown solution of 1
(214 mg, 0.614 mmol) in 2 mL of THF. The solution was stirred for
1 h and turned dark blue. The solvent was removed by vacuum. The
solid residue was extracted with 3 × 20 mL of pentane and filtered
through Celite. The solvent was reduced to about 0.5 mL, and the
solution was stored at −35 °C overnight to yield a blue crystalline
solid. After filtration and washing with pentane (3 × 3 mL), 9 was
obtained as a blue solid. Yield: 76% (163 mg, 0.438 mmol).
1H NMR (400 MHz, C6D6): δ 7.41 (d, J = 8.1 Hz, 2H, CHaryl), 6.94
(t, J = 7.2 Hz, 2H, CHaryl), 6.59 (d, J = 7.6 Hz, 2H, CHaryl), 6.39 (t, J =
7.3 Hz, 2H, CHaryl), 3.40 (br s, 1H, CH(CH3)2), 2.51 (br s, 12H,
Reaction of 2 with Acetone. A J. Young NMR tube was charged
with 2 (11 mg, 0.046 mmol) in 0.6 mL of C6H6 with a sealed capillary
(TMS in CDCl3) as the external standard. Then acetone (3 μL, 0.05
1
mmol) was added. H NMR spectra were recorded. After 12 h, 2 was
completely consumed. All volatiles were removed by vacuum, and the
residue was dissolved in C6D6. The product 9 was identified by com-
paring its NMR spectrum with an independently synthesized sample of 9.
The yield was calculated by integration of the N(CH3)2 signal of 2
(2.71 ppm) before acetone addition and the N(CH3)2 signal of 9
(2.51 ppm), using the signal of TMS as the standard (58%).
Reaction of Cyclohexyl Chloride with 2. A J. Young NMR tube
was charged with a solution of 2 (10 mg, 0.032 mmol, 1 equiv.) in
0.3 mL of C6D6 and a sealed capillary containing H2O/DMSO-d6
as internal standard (2.84 ppm). A solution of cyclohexyl chloride
(10 mg, 0.085 mmol, 2 equiv) in 0.3 mL of C6D6 was added. Then 1H
NMR spectra were recorded periodically every 4 min over a period of
1 h. The disappearance of 2 and C6H11Cl was observed as the char-
acteristic signals at 2.71, −22.81, and 3.65 ppm diminished. The
formation of the products (1 and cyclohexane) was observed as the
characteristic signals at 2.57 and 1.40 ppm appeared.
Optimization of Catalytic Reduction of Alkyl Halides (Table 1).
In a flask charged with 2 equiv of base (NaOiPr or NaOMe) a solution
of 8−10 mol % of 1 in 0.5 mL of solvent, 1 equiv of C14H29Cl, and
1 equiv of C12H26 (internal standard) were added in sequence.
A solution of 1−3 equiv of silane in 0.5 mL of solvent was added by
syringe pump over 30 min. After 6 h, the reaction was quenched by
addition of 1 mL of water, followed by addition of 3 mL of Et2O. The
organic layer containing the products was separated, and the aqueous
layer was further extracted with Et2O (3 × 3 mL). Conversion was
1
N(CH3)2), 1.44 ppm (br s, 6H, CH(CH3)2). H NMR (400 MHz,
Tol-d8, −40 °C): δ 7.33 (d, J = 8.3 Hz, 2H, CHaryl), 6.91 (m, 2H,
CHaryl), 6.45 (d, J = 7.9 Hz, 2H, CHaryl), 6.36 (t, J = 7.5 Hz, 2H,
CHaryl), 3.41−3.27 (m, 1H, CH(CH3)2), 2.41 (s, 12H, N(CH3)2),
1.46 ppm (d, J = 5.4 Hz, 6H, CH(CH3)2). 13C{1H} NMR (101 MHz,
C6D6): δ 148.26 (Caryl), 147.95 (Caryl), 128.19 (Caryl), 120.13 (Caryl),
115.25 (Caryl), 114.34 (Caryl), 63.14 (OC(CH3)2), 49.60 (CH3), 29.44
ppm (OC(CH3)2). Anal. Calcd for 7 (C19H27N3NiO): C, 61.32; H,
7.31; N, 11.29. Found: C, 61.45; H, 7.77; N, 11.97. The nitrogen
content is higher than the theoretical value, indicating the presence of
a small amount of impurity which is not detectable by NMR.
(1). Monitoring the Decomposition. In situ Generated
[(MeN2N)Ni-D] (2D) by 2H NMR. A J. Young NMR tube was
charged with a solution of 6 (25 mg, 0.073 mmol) in 1 mL of
C6H6. Ph2SiD2 (14 mg, 0.073 mmol) was added, and the solu-
2
tion immediately turned dark red. H NMR spectra were re-
corded to follow the disappearance of the [Ni]D signal (−22.79
ppm) and the appearance of the ND signal (7.42 ppm).
(2). Isolated [(MeN2N)Ni-D] (2D) by 2H NMR. A J. Young NMR tube
was charged with a red solution of 2D (10 mg, 0.032 mmol) in 0.6 mL
2
of C6H6. H NMR spectra were recorded to follow the disappearance
of the [Ni]D signal (−22.79 ppm) and the appearance of the ND
signal (7.42 ppm).
2134
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
determined by GC. The products and yields were determined by
GC-MS.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xile.hu@epfl.ch.
Typical Procedure for Catalytic Reduction of Alkyl Halides
(Table 2). In a flask charged with NaOiPr (25 mg, 0.31 mmol, 2
equiv), a solution of 1 (3.7 mg, 0.011 mmol) in 0.5 mL of toluene,
substrate (0.15 mmol, 1 equiv), and internal standard (0.15 mmol,
1 equiv) were added in sequence. A solution of Ph2SiH2 in 0.5 mL of
toluene was added by syringe pump over 30 min. After 6 h, the re-
action was quenched by addition of 1 mL of water, followed by ad-
dition of 3 mL of Et2O. The organic layer containing the products
was separated, and the aqueous layer was further extracted with Et2O
(3 × 3 mL). Conversion was determined by GC. The products and
yields were determined by GC-MS.
Notes
The authors declare no competing financial interest.
Biography
Typical Procedure for Catalytic Reduction of Aryl Halides
(Table 2). In a flask charged with NaOMe (30 mg, 0.56 mmol,
3 equiv), a solution of 1 (5.7 mg, 0.016 mmol) in 0.5 mL of THP,
substrate (0.19 mmol, 1 equiv), and internal standard (0.13 mmol,
0.7 equiv) were added in sequence. A solution of Me(EtO)2SiH (52 mg,
0.39 mmol, 2 equiv) in 0.5 mL of THP was added by syringe pump
over 30 min. After 6 h, the reaction was quenched by addition of 1 mL
of water, followed by addition of 3 mL of Et2O. The organic layer
containing the products was separated, and the aqueous layer was
further extracted with Et2O (3 × 3 mL). Conversion was determined
by GC. The products and yields were determined by GC-MS.
Procedure for Hydroalkylation of Decene. In a flask charged
with NaOMe (30 mg, 0.56 mmol, 2 equiv), a solution of 1 (10 mg,
0.029 mmol), mesitylene (35 mg, 0.29 mmol, internal standard),
dodecyl iodide (85 mg, 0.29 mmol, 1 equiv), and decene (400 mg,
3.81 mmol, 10 equiv) in 0.5 mL of toluene were added. A solution of
Me(EtO)2SiH (80 mg, 0.58 mmol, 2 equiv) in 0.5 mL of toluene was
added by syringe pump to the above solution over 30 min. After 4 h,
the reaction was quenched by addition of 3 mL of water, followed by
addition of 10 mL of Et2O. The organic layer containing the products
was separated, and the aqueous layer was further extracted with Et2O
(2 × 5 mL). Conversion was determined by GC. The products and
yields were determined by GC-MS.
Crystallographic Details for 2. A total of 28 696 reflections
(−12 < h < 12, −14 < k < 12, −22 < l < 25) were collected at T = 100(2)
K in the range 3.06−27.50°, 9325 of which were unique (Rint = 0.0604),
with Mo Kα radiation (λ = 0.710 73 Å). The structure was solved by
direct methods. All non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were placed in calculated idealized positions. The
residual peak and hole electron densities were 0.600 and −0.535 e Å−3,
respectively. The absorption coefficient was 0.948 mm−1. The least-
squares refinement converged normally with residuals of R(F) = 0.0433
and Rw(F2) = 0.0996 and GOF = 1.061 (I > 2σ(I)). Crystal data for
Xile Hu was born in 1978 in Putian, China. He received a B.S. degree
from Peking University (2000) and a Ph.D. degree from the University
of California, San Diego (2004; advisor Prof. Karsten Meyer). He then
carried out a postdoctoral study at the California Institute of
Technology (advisor Prof. Jonas Peters) before joining the faculty of
́
́ ́
the Ecole Polytechnique Federale de Lausanne (EPFL) as a tenure-
track assistant professor in 2007. His research interests span from
organometallic chemistry, synthetic methodology, and asymmetric
catalysis to biomimetic and biospeculated coordination chemistry to
electrocatalysis and artificial photosynthesis. Thanks to his inter-
national experience, he speaks three languagesMandarin, English,
and Frenchall with confusing accents.
ACKNOWLEDGMENTS
■
This work was supported by the EPFL and the Swiss National
Science Foundation (Project No. 200021_126498).
REFERENCES
C H N Ni: M = 406.20, space group P1, triclinic, a = 9.9932(12) Å,
̅
23 29
3
w
■
b = 11.0590(14) Å, c = 19.449(3) Å, α = 76.138(13)°, β = 83.944(10)°,
γ = 85.539(12)°, V = 2072.1(5) Å3, Z = 4, ρcalcd = 1.302 Mg/m3.
Crystallographic Details for 8. A total of 6570 reflections (−15 <
h < 15, −16 < k < 16, −24 < l < 24) were collected at T = 100(2) K in
the range 3.12−27.56°, 6570 of which were unique (Rint = 0.0000), with
Mo Kα radiation (λ = 0.710 73 Å). The structure was solved by direct
methods. All non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were placed in calculated idealized positions. The residual
peak and hole electron densities were 1.651 and −1.771 e Å−3, respec-
tively. The absorption coefficient was 0.709 mm−1. The least-squares
refinement converged normally with residuals of R(F) = 0.0855 and
Rw(F2) = 0.2316 and GOF = 1.194 (I > 2σ(I)). Crystal data for
C32H40N6Ni: Mw = 567.41, space group P21/n, monoclinic,
a = 12.066(3) Å, b = 12.7657(18) Å, c = 18.651(3) Å, α = 90°, β =
93.517(18)°, γ = 90°, V = 2867.4(9) Å3, Z = 4, ρcalcd = 1.314 Mg/m3.
(1) The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen,
C. M., Eds.; Elsevier: Amsterdam, 2007.
(2) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750−3781.
(3) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759−
1792.
(4) van der Vlugt, J. I.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48,
8832−8846.
(5) Chakraborty, S.; Krause, J. A.; Guan, H. Organometallics 2009, 28,
582−586.
(6) Chakraborty, S.; Zhang, J.; Krause, J. A.; Guan, H. R. J. Am. Chem.
Soc. 2010, 132, 8872−8873.
(7) Zhang, J.; Medley, C. M.; Krause, J. A.; Guan, H. R.
Organometallics 2010, 29, 6393−6401.
(8) Ozerov, O. V.; Guo, C. Y.; Fan, L.; Foxman, B. M.
Organometallics 2004, 23, 5573−5580.
ASSOCIATED CONTENT
(9) Fan, L.; Ozerov, O. V. Chem. Commun. 2005, 4450−4452.
(10) Liang, L. C.; Chien, P. S.; Lin, J. M.; Huang, M. H.; Huang, Y. L.;
Liao, J. H. Organometallics 2006, 25, 1399−1411.
(11) Liang, L. C.; Chien, P. S.; Lee, P. Y. Organometallics 2008, 27,
3082−3093.
■
S
* Supporting Information
Figures giving NMR spectra of new complexes and reaction
mixtures and an IR spectrum of 2 and CIF files giving
crystallographic data for 2 and 8. This material is available free
of charge via the Internet at http://pubs.acs.org.
(12) Liang, L. C.; Chien, P. S.; Huang, Y. L. J. Am. Chem. Soc. 2006,
128, 15562−15563.
2135
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136

Organometallics
Article
(13) Adhikari, D.; Mossin, S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.;
Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676−3682.
(14) Adhikari, D.; Pink, M.; Mindiola, D. J. Organometallics 2009, 28,
2072−2077.
(15) Boro, B. J.; Duesler, E. N.; Goldberg, K. I.; Kemp, R. A. Inorg.
Chem. 2009, 48, 5081−5087.
(16) Pandarus, V.; Zargarian, D. Organometallics 2007, 26, 4321−
4334.
(17) Levina, V. A.; Rossin, A.; Belkova, N. V.; Chierotti, M. R.;
Epstein, L. M.; Filippov, O. A.; Gobetto, R.; Gonsalvi, L.; Lledos, A.;
Shubina, E. S.; Zanobini, F.; Peruzzini, M. Angew. Chem., Int. Ed. 2011,
50, 1367−1370.
(18) Wang, Z. X.; Wang, L. Chem. Commun. 2007, 2423−2425.
(19) Stol, M.; Snelders, D. J. M.; Godbole, M. D.; Havenith, R. W. A.;
Haddleton, D.; Clarkson, G.; Lutz, M.; Spek, A. L.; van Klink, G. P. M;
van Koten, G. Organometallics 2007, 26, 3985−3994.
(20) Huang, F.; Zhang, C. G.; Jiang, J. L.; Wang, Z. X.; Guan, H. R.
Inorg. Chem. 2011, 50, 3816−3825.
(21) Rigo, P.; Bressan, M.; Basato, M. Inorg. Chem. 1979, 18,
860−863.
(22) Green, M. L. H.; Munakata, H. J. Chem. Soc., Dalton Trans.
1974, 269−272.
(23) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 6777−&.
(24) Csok, Z.; Vechorkin, O.; Harkins, S. B.; Scopelliti, R.; Hu, X. L.
J. Am. Chem. Soc. 2008, 130, 8156−8157.
(25) Vechorkin, O.; Csok, Z.; Scopelliti, R.; Hu, X. L. Chem. Eur.
J. 2009, 15, 3889−3899.
(26) Vechorkin, O.; Hu, X. L. Angew. Chem., Int. Ed. 2009, 48, 2937−
2940.
(27) Vechorkin, O.; Proust, V.; Hu, X. L. J. Am. Chem. Soc. 2009, 131,
9756−9766.
(28) Vechorkin, O.; Barmaz, D.; Proust, V.; Hu, X. L. J. Am. Chem.
Soc. 2009, 131, 12078−12079.
(29) Vechorkin, O.; Proust, V.; Hu, X. L. Angew. Chem., Int. Ed. 2010,
49, 3061−3064.
(30) Hu, X. L. Chem. Sci. 2011, 2, 1867−1886.
(31) Vechorkin, O.; Scopelliti, R.; Hu, X. L. Angew. Chem., Int. Ed.
2011, 50, 11777−11781.
(32) Complex 1 is now commercially available from Sigma-Aldrich
bearing the synonym “Nickamine”. The name reflects the combination
of nickel and amine ligands.
(33) Breitenfeld, J.; Vechorkin, O.; Corminboeuf, C.; Scopelliti, R.;
Hu, X. L. Organometallics 2010, 29, 3686−3689.
(34) Chakraborty, S.; Guan, H. R. Dalton Trans. 2010, 39, 7427−
7436.
(35) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2002, 102,
4009−4091.
(36) Yang, J.; Brookhart, M. J. Am. Chem. Soc. 2007, 129, 12656−
12657.
(37) Doyle, M. P.; McOsker, C. C.; West, C. T. J. Org. Chem. 1976,
41, 1393−1396.
(38) Fossey, J. S.; Lefort, D.; Sorba, J. Free Radicals in Organic
Chemistry; Wiley: New York, 1995.
(39) Andrieux, C. P.; Gallardo, I.; Saveant, J. M.; Su, K. B. J. Am.
Chem. Soc. 1986, 108, 638−647.
(40) Netherton, M. R.; Fu, G. C. Adv. Synth. Catal. 2004, 346, 1525−
1532.
(41) Frisch, A. C.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 674−
688.
(42) Rudolph, A.; Lautens, M. Angew. Chem., Int. Ed. 2009, 48,
2656−2670.
(43) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M.
J. Appl. Crystallogr. 2003, 36, 220−229.
(44) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33−38.
(45) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(46) Lim, B. S.; Fomitchev, D. V.; Holm, R. H. Inorg. Chem. 2001, 40,
4257−4262.
2136
dx.doi.org/10.1021/om201279j | Organometallics 2012, 31, 2128−2136