Nickel complexes of a pincer amidobis(amine) ligand: Synthesis, structure, and activity in stoichiometric and catalytic C-C bond-forming reactions of alkyl halides

Article abstract of DOI:10.1002/chem.200802059

The synthesis, properties, and reactivity of nickel(II) complexes of a newly developed pincer amidobis(amine) ligand (^McNN₂) are described. Neutral or cationic complexes [(^MeNN₂)NiX] (X = OTf (6), OC(O)CH₃ (7), CH₃CN (8), OMe (9)) were prepared by salt metathesis or chloride abstraction from the previously reported [( ^MeNN₂)NiCl] (1). The Lewis acidity of the {( ^McNN₂)Ni) fragment was measured by the 1H NMR chemical shift of the coordinated CH₃CN molecule in 8. Electrochemical measurements on 1 and 8 indicate that the electron-donating properties of NN₂ are similar to those of the analogous amidobis(phosphine) (pnp) ligands. The solid-state structures of 6-8 were determined and compared to those of 1 and [(^MeNN₂)NiEt] (3). In all complexes, the _MeNN₂ ligand coordinates to the Ni^II ion in a mer fashion, and the square-planar coordination sphere of the metal is completed by an additional donor. The coordination chemistry of ^MeNN ₂ thus resembles that of other three-dentate pincer ligands, for example, pnp and arylbis(amine) (ncn). Reactions of 2 with alkyl monohalides, dichlorides, and trichlorides were investigated. Selective C-C bond formation was observed in many cases. Based on these reactions, efficient Kumada-Corriu-Tamao coupling of unactivated alkyl halides and alkyl Grignard reagents with 1 as the precatalyst was developed. Good yields were obtained for the coupling of primary and secondary iodides and bromides. Double C-C coupling of CH₂Cl₂ with alkyl Grignard reagents by 1 was also realized. The scope and limitations of these transformations were studied. Evidence was found for a radical pathway in Ni-catalyzed C-C cross-coupling reactions, which involves Ni^Il alkyl intermediates.

Full text of DOI:10.1002/chem.200802059

FULL PAPER
DOI: 10.1002/chem.200802059
Nickel Complexes of a Pincer AmidobisACTHUNGTRENNNUG
ꢀ
and Activity in Stoichiometric and Catalytic C C Bond-Forming Reactions of
Alkyl Halides
[
a]
Oleg Vechorkin, Zsolt Csok, Rosario Scopelliti, and Xile Hu*
Abstract: The synthesis, properties, and
reactivity of nickel(II) complexes of a
gands. The solid-state structures of 6–8
were determined and compared to
were investigated. Selective CꢀC bond
formation was observed in many cases.
Based on these reactions, efficient
Kumada–Corriu–Tamao coupling of
unactivated alkyl halides and alkyl
Grignard reagents with 1 as the preca-
talyst was developed. Good yields were
obtained for the coupling of primary
and secondary iodides and bromides.
Double C–C coupling of CH Cl with
Me
newly developed pincer amidobis-
those of 1 and [( NN )NiEt] (3). In all
2
Me
Me
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(amine) ligand ( NN ) are described.
complexes, the
NN2 ligand coordi-
2
II
Neutral
or
cationic
complexes
nates to the Ni ion in a mer fashion,
and the square-planar coordination
sphere of the metal is completed by an
Me
[(
NN )NiX]
(X=OTf
(6),
2
OC(O)CH (7), CH CN (8), OMe (9))
3
3
were prepared by salt metathesis or
additional donor. The coordination
Me
chloride abstraction from the previous-
chemistry of NN thus resembles that
2
Me
ly reported [( NN )NiCl] (1). The
of other three-dentate pincer ligands,
2
2
2
Me
Lewis acidity of the {( NN )Ni} frag-
ment was measured by the H NMR
for example, pnp and arylbis
A
H
U
G
R
N
U
G
alkyl Grignard reagents by 1 was also
realized. The scope and limitations of
these transformations were studied.
Evidence was found for a radical path-
way in Ni-catalyzed C–C cross-coupling
2
1
(ncn). Reactions of 2 with alkyl mono-
halides, dichlorides, and trichlorides
chemical shift of the coordinated
CH CN molecule in 8. Electrochemical
3
measurements on 1 and 8 indicate that
the electron-donating properties of
Keywords: C–C coupling · coordi-
II
reactions, which involves Ni alkyl in-
nation modes
· cross-coupling ·
NN are similar to those of the analo-
termediates.
2
nickel · pincer ligands
gous amidobis(phosphine) (pnp) li-
Introduction
C species, and finally C–C reductive elimination to give the
3
2
desired products. Coupling of sp -, sp -, and sp-hybridized
carbon atoms is now readily achieved by using a wide range
of main-group organometallic nucleophiles and aryl, alkenyl,
and even certain activated alkyl (e.g., allylic, benzylic, and
Thanks to developments in Pd- and Ni-based catalysis, espe-
cially that of the Heck–Mizokori, Suzuki–Miyaura, Negishi,
Kosugi–Migita–Stille, Hiyama, and Kumada–Corriu–Tamao
types, metal-catalyzed C–C cross-coupling has become one
a-halocarbonyl) electrophiles (C-X, X=Cl, Br, I, OTf, OMs,
[1,2]
[1,2]
of the most powerful tools in organic synthesis.
A prevail-
etc.; OTf=triflate, OMs=mesylate).
Unactivated alkyl
0
0
ing mechanism for Pd - and Ni -catalyzed C–C coupling re-
actions involves first the oxidative addition of an organic
electrophile to a low-valent metal center, then transmetala-
tion of an organometallic nucleophile to the resulting X-M-
halides, especially those with b-hydrogen atoms, are, howev-
er, challenging electrophiles for this type of transformation
[3,4]
for the following reasons.
1) Oxidative addition of alkyl
halides is generally much slower than that of aryl and alken-
0
yl halides. A recent DFT study of model reactions on Pd
suggested that this difference in reactivity is due to the
[
a] O. Vechorkin, Dr. Z. Csok, Dr. R. Scopelliti, Prof. X. Hu
Laboratory of Inorganic Synthesis and Catalysis
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fꢁdꢁrale de Lausanne (EPFL)
ISIC-LSCI, BCH 3305, Lausanne 1015 (Switzerland)
Fax : (+41)21-693-9305
availability of the Ar-X p* orbitals for backbonding from
0
[5]
Pd . 2) If oxidative addition occurs, the resulting metal
alkyl species can undergo unproductive b-H elimination,
which is often both thermodynamically and kinetically fa-
[
6]
[7,8]
vorable. Recent work by Knochel et al., Kambe et al.,
E-mail: xile.hu@epfl.ch
[9–11]
[12,13]
[14]
Fu et al.,
Vicic et al.,
Cꢀrdenas et al., Van Koten
et al., Cahiez et al., Nakamura et al., and Martin and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802059.
[15]
[16]
[17]
Chem. Eur. J. 2009, 15, 3889 – 3899
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3889

[
18]
II
Fꢃrstner starts to address these challenges, and a number
of efficient catalytic systems are found for the coupling of
primary and secondary alkyl halides with alkyl nucleophiles.
Many of these systems employ Ni salts or complexes as cata-
ingly, reactions of Ni alkyl complexes with aryl halides, or
II
Ni aryl complexes with alkyl/aryl halides, did not give the
C–C coupled products. The metal-containing products of
these reactions were [( NN )Ni] halides, which could be
Me
2
[6–8,10,12–14]
0
II
Me
lyst precursors.
Although the classical Ni /Ni cata-
converted to a [( NN )Ni] alkyl compound by reaction with
2
[4]
[20]
lytic cycles were proposed in some cases, there are exam-
ples where two other mechanisms appear to be operational.
Kambe and co-workers postulated a nonradical and formal
an alkyl Grignard reagent [Eq. (2)].
components for a catalytic alkyl–alkyl coupling reaction be-
tween alkyl halides and Grignard nucleophiles are viable on
Thus, the principal
II
IV
Me
Me
Ni /Ni
mechanism for Ni-catalyzed Kumada–Corriu–
this [( NN )Ni] platform. We now show that [( NN )NiCl]
2 2
Tamao coupling in the presence of 1,3-butadiene addi-
is indeed a precatalyst for Kumada–Corriu–Tamao coupling
of unactivated alkyl halides, and present the scope and limi-
tations of this catalysis.
[7,8]
tives.
Vicic, Phillips, Cꢀrdenas, and their co-workers re-
ported experimental and theoretical support for a radical-in-
I
III
itiated Ni /Ni catalytic cycle in Negishi alkyl–alkyl coupling
[12–14,19]
catalyzed by Ni terpyridyl complexes.
The reaction
pathways for Ni-catalyzed C–C coupling are therefore di-
verse. This can be advantageous as it may circumvent some
of the limitations imposed by conventional Pd catalysis.
We recently synthesized a new pincer-type amidobis-
Me
II
ACHTUNGTRENNUNG( amine) ligand ( NN ) and its Ni complexes including hy-
2
[20]
drocarbyl species (A). Herein, we describe the synthesis
Me
One unique feature of the [( NN )Ni] system is that the
2
II
Ni alkyl complexes can react with CH Cl and CHCl to
2
2
3
give doubly and triply coupled alkanes, respectively, in high
selectivity [Eqs. (3) and (4)]. This led to our earlier commu-
nication on the first Kumada–Corriu–Tamao coupling of
CH Cl with nBuMgCl, which forms nBuCH nBu as the
2
2
2
[20]
main product.
Herein, we report our expansion of this
study by employing other polychloroalkanes as electrophiles
and additional Grignard reagents as nucleophiles for these
intriguing transformations.
Results and Discussion
Me
Synthesis and characterization of complexes: [( NN )NiCl]
2
Me
Me
(
1), [( NN )NiMe] (2), [( NN )NiEt] (3), and
2 2
[( NN )NiPh] (4) were synthesized as reported earlier.
2
II
Me
[20]
and structure of analogous Ni acetate, triflate, acetonitrile,
and methoxide complexes. Our exploration of this ligand in
coordination chemistry and catalysis is inspired by previous
Reaction of 1 with CH MgCl often gave a small amount of
3
an unknown compound (<10%) besides 2. This compound
[21–23]
Me
work on pincer ligands.
We notice particularly the
is now identified as [( NN )MgCl ACHNUTRTGENNUNG( thf)] (5) by comparison
2
broad applications of pincer phosphine ligands (B and C) in
of its NMR spectrum with that of an independently pre-
late-transition-metal-mediated small-molecule activation
pared sample, which was synthesized by reaction of ligand
H NN with CH MgCl. Complex 5 is sparsely soluble in
2 3
[22–26]
Me
and homogeneous catalysis.
Pincer amine ligands are
less exploited for similar applications. One might expect a
mismatch between a soft late transition metal and the hard
amine donors, yet the work by Van Koten and others dem-
pentane, and by dissolution in pentane pure 2 with a negligi-
ble amount of 5 could be obtained.
We examined the thermal stability of nickel alkyl com-
plexes 2 and 3. Under an inert atmosphere and when dis-
solved in benzene, 2 does not undergo decomposition even
when heated at 1208C for days. Similarly, 3 is stable up to
808C; when heated at 1008C in benzene, it decomposed to
form insoluble solids. The stability of 3 against b-H elimina-
onstrated that stable pincer arylbis
D) of late transition metals could be made and showed
promising activity in C–H activation, transfer hydrogena-
ACHTUNGERTNNUNG( amine) (ncn) complexes
(
[
23,27,28]
tion, and CꢀC bond-forming reactions.
Our entry to
II
Ni-mediated C–C coupling was provided by Ni alkyl com-
plexes of the NN ligand. These compounds were thermal-
ly stable and the [( NN )NiEt] complex resisted b-H elimi-
nation. Preliminary study showed that the Ni methyl com-
Me
II
tion at 808C is noteworthy. Similar stability of Ni alkyl spe-
2
Me
cies was reported by Liang et al. on isoelectronic amido-
2
II
[25]
bis(phosphine) (pnp) complexes.
plex reacted with alkyl halides to give alkyl–alkyl coupled
products in high yields according to Equation (1). Interest-
Salt metathesis reactions of 1 produced the corresponding
Ni acetate (6) and triflate (7) complexes (Scheme 1). Ab-
II
3890
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3889 – 3899

Nickel Complexes of a Pincer Ligand
FULL PAPER
were observed for 1 in CH CN (Figure 1, top). The first re-
3
duction wave was irreversible (E =ꢀ1.5 V vs. ferrocene/
red
+
ferrocenium (Fc/Fc ), same below) and the second one
Me
2
Scheme 1. Synthesis of new nickel(II) complexes of the NN ligand.
straction of the chloride ligand in 1 by AgBF in CH CN
4
3
gave the cationic CH CN complex with BF as the counter
3
4
anion (8·BF ); alternatively, dissolution of 7 in CH CN gave
4
3
the same cation with triflate as the counter anion
Scheme 1). The latter reaction suggests that the triflate
(
ligand in 7 is labile. Reaction of 1 with NaOMe in THF led
II
to the Ni methoxide complex 9.
Me
II
All [( NN )Ni ] complexes have distinctive colors and
2
their electronic absorption spectra are shown in Figure S1 in
the Supporting Information. There is a significant difference
between the hydrocarbyl complexes (2–4) and the other de-
rivatives (1 and 6–8) in the visible spectra: the hydrocarbyl
complexes do not absorb between 600 and 800 nm, whereas
the spectra of the other compounds exhibit an absorption
maximum with a molar absorption coefficient of 100–
Figure 1. Cyclic voltammograms of complexes 1 (top; 2.1 mm) and 8
(bottom; 4 mm) recorded in CH
3
CN solutions at scan rates of 100 (1) and
ꢀ
1
4
00 mVs (8); the potential is referenced to the ferrocene/ferrocenium
couple.
ꢀ
1
ꢀ1
o’
200 Lcm mol .
(E =ꢀ1.94 V) was quasi-reversible. The oxidation wave oc-
The NMR spectra of 6–9 are consistent with the structural
curred at ꢀ0.12 V and its current height indicated that it
could be a two-electron oxidation (assuming the reduction is
one-electron). It is an irreversible oxidation even in the
presence of excess halide ions. When the chloride ion in 1
formation shown in Scheme 1. An effective C symmetry
2
v
Me
was observed for the protons of the NN ligand, which
2
suggests a fast rotation of the NiꢀO bond in 6, 7, and 9. The
Lewis acidity of a metal center can be measured by how
tightly it binds to a Lewis base, for example, crotonaldehyde
was substituted by a CH CN molecule, the cyclic voltammo-
3
gram changed dramatically. At scan rates higher than
[28,29]
ꢀ1
Me
or CH CN.
Previous studies employed CH CN to probe
400 mVs , complex [( NN )Ni ACHTUNGTNERNUG( CH CN)]BF (8·BF ) could
2 3 4 4
3
3
the Lewis acidity of group 10 metal pincer complexes. To
undergo one reversible reduction and one quasi-reversible
Me
II
o’
better compare the Lewis acidity of [( NN )Ni ] with
oxidation at E =ꢀ1.67 and ꢀ0.15 V, respectively (Figure 1,
2
them, we chose CH CN as well. The magnitude of the
bottom). At lower scan rates, the reduction remained rever-
3
downfield shift for the methyl protons of the coordinated
sible whereas the oxidation became irreversible. The elec-
Me
CH CN is proportional to the Lewis acidity of the complex.
tron-donating property of the NN ligand could be esti-
3
2
1
The H NMR signal for CH CN in 8 was observed at
mated by the oxidation potential of its metal complex, for
3
Me
2
.60 ppm in CDCl , 0.50 ppm higher than free CH CN. The
example, 1 and 8. Thus, the NN ligand is as good an elec-
3
3
2
Me
II
[
[
(
NN )Ni ] cation thus has a similar Lewis acidity to the
tron donor as the electron-rich amidobis(alkylphosphine)
2
II
[26]
o’
(pnp)Ni ] cation (d(H)=2.69 ppm for CH CN),
but is
ligand, which gave rise to a [(pnp)NiCl] complex with E =
3
II
[25]
more Lewis acidic than 2,6-bis(2-oxazolinyl)phenyl Ni and
ꢀ0.06 V.
The cyclic voltammogram of the methyl and
II
[(pocop)Ni ] complexes (pocop: diphosphinito; d(H)=2.4
phenyl Ni complexes showed an irreversible oxidation at
0.13 and 0.27 V in THF, respectively (Figure S2 in the Sup-
porting Information), which suggests an instability for a
[29,30]
and 2.2 ppm for CH CN, respectively).
3
The redox properties of 1, 2, 4, and 8 were probed by
cyclic voltammetry. Two reduction and one oxidation wave
III
Me
formal Ni alkyl or aryl species of the NN ligand.
2
Chem. Eur. J. 2009, 15, 3889 – 3899
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3891

X. Hu et al.
Me
II
Structure of complexes: The solid-state structures of com-
plexes 6–8 were determined by X-ray crystallography
The overall structure of these [( NN )Ni ] complexes is
2
similar to that of their counterparts supported by anionic
ncn and pnp ligands. For the Ni Cl complexes, the Ni–Cl
II
(
Figure 2). The key structural parameters for these com-
distances are comparable in 1, [{N(SiMe CH PPh ) }NiCl]
2
2
2 2
[21]
(
2
10,
.1636(11) ꢄ),
nyl}NiCl] (12, 2.2388(5) ꢄ). The Ni–N
and 12 are also close (1.9909(13) ꢄ in the latter); however,
the Ni–N(amide) bond in 1 is slightly shorter than in 10 and
1 (1.924(2) and 1.895(3) ꢄ, respectively). For the Ni alkyl
2.1703(6) ꢄ),
[{N(o-C H PPh ) }NiCl]
(11,
6
4
2 2
[25]
and [{2,6-bis({dimethylamino}methyl)phe-
[33]
AHCTUNGTRENNUNG( amine) distances in
1
AHCTUNGTRENNUNG
II
1
complexes, the Ni–C distance in 3 is nearly identical to that
in [{N(o-C H PPh ) }NiMe] (13, 1.967(11) ꢄ) and [{N(o-
Figure 2. Crystal structures of complexes 6 (left), 7 (middle), and 8
right). The thermal ellipsoids are displayed at 50% probability.
6
4
2 2
(
C H PPh ) }NiACTHUNGTRENNUNG
6
4
2 2
AHCTUNGTRENNUNG
[25]
plexes, together with those of 1 and 3, are summarized in
(1.967(8) and 1.966(2) ꢄ, respectively).
II
Table 1 and Table 2. The coordination geometry of Ni is
approximately square-planar in all five complexes. The
Me
II
Reactions of [( NN )Ni Me] with alkyl halides: We report-
2
Me
pincer NN ligand binds to Ni in the expected mer fashion,
ed previously that [( NN )NiMe] (2) reacted with alkyl hal-
2
2
and the fourth coordination site is occupied by an additional
donor. The two phenyl rings deviate slightly from coplanar,
ides according to Equation (1). The rate of the reaction fol-
lowed the orders: RI>RBr>RCl and benzyl>octyl>cyclo-
hexyl. There was evidence for a radical process in these re-
with an angle of 22.22 (1), 21.66 (3), 25.49 (6), 7.51 (7), and
2
[20]
1
6.958 (8), respectively. The amide nitrogen atom is sp hy-
actions. We propose that 2 first reacts with an alkyl halide
bridized, as the sums of the three bonds around it are all
close to 3608. The Ni–N(amide) distance is about 1.83 ꢄ in 1
and 6–8, but increases to 1.89 ꢄ in 3. This is consistent with
to undergo a one-electron oxidation, forming a transient
alkyl radical, which quickly recombines with the metal to
AHCTUNGTRENNUNG
form a Ni bis ACHNTUGRNENUG( alkyl) complex. Reductive C–C elimination
[31]
II
a high trans influence of alkyl ligands. The Ni–N
bonds are always about 0.1 ꢄ shorter than the correspond-
ing Ni–N(amine) bonds, which suggests some degree of
A
H
U
G
R
N
U
G
then gives a Ni halide complex and the C–C coupled prod-
uct. Consistent with this radical-rebound mechanism, 2 does
not react with trimethylsilyl triflate, an electrophile prone to
SN2 oxidative addition. Curiously, we find a large steric in-
fluence in the reaction rate even among primary alkyl hal-
ides. For instance, reaction with C H Cl takes place at room
ACHTUNGTRENNUNG
p donation from the amide lone pair.
Replacement of an anionic fourth ligand by a neutral
donor (e.g., from 7 to 8) causes no appreciable structural
2
5
Me
II
change in the {( NN )Ni } fragment. The acetate ligand in
temperature, but reactions with nC H Cl and nC H Cl re-
3 7 8 17
2
1
6
is coordinated in an h fashion, a common coordination
mode for mononuclear Ni acetate complexes. The Ni–O
quire a temperature of 60 and 1008C, respectively. The
origin of this difference is currently under investigation by
DFT methods. It is noted that 2 reacts with nC H Cl at a
II
[32]
distance is slightly shorter in 6 (1.890(3) ꢄ) than in 7
2
5
(
1.946(2) ꢄ).
rate that is comparable to that with CH Cl , thus indicating
2 2
that nC H Cl is not an inter-
mediate in the latter reaction,
2
5
II
[a]
Table 1. Selected bond lengths for [(MeNN
Complex
2
)Ni ] complexes.
[
b]
Ni–N(amide)
A
H
U
G
R
N
U
G
Ni–N
A
H
U
G
R
N
U
G
Ni–X
as no nC H Cl was detected
2
5
Me
[c]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
2
2
2
2
2
)NiCl] (1)
1.835(2)
1.8907(18)
1.831(3)
1.830(2)
1.825(5)
1.956(2)
1.976(2)
1.957(3)
1.9357(17)
1.944(5)
2.2029(7)
1.959(2)
1.890(3)
1.946(2)
1.875(6)
during the reaction.
Me
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
)NiEt] (3)
)Ni(OC(O)CH )] (6)
3
The reaction between 2 and
aryl halides was much slower
and did not afford the C–C
coupled organic compounds.
Me
[c]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
Me
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
)Ni
)Ni
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OTf)] (7)
Me
[c]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(NCCH )]BF (8·BF )
3
4 4
[
a] Bond lengths are in ꢄ; data for 1 and 3 are from ref. [20]. [b] X is the fourth ligand on Ni besides the tri-
dentate NN chelate. [c] Average of two independent molecules in the asymmetric unit.
The
phenyl
complex
2
Me
[
( NN )NiPh] (4) did not
2
react with either alkyl or aryl
halides. It appears that only
alkyl–alkyl coupling could be
Me
II
[a]
Table 2. Selected bond angles for [( NN
2
)Ni ] complexes.
(amine)-Ni-N-
(amide)av.
Complex
N
A
H
U
G
R
N
U
G
N
X
A
H
U
G
R
N
U
G
C-N
C
ACTHUNGTRENUNNG( amide)- Ni-N ACHTUNGTRNENUNG( amide)-
[
b]
A
T
N
R
N
N
av.
C
av.
Me
II
mediated by the [( NN )Ni ]
2
Me
[c]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
2
2
2
2
2
)NiCl] (1)
86.26(8)
84.47(8)
85.63(13)
87.33(5)
86.8(2)
94.04(6)
95.51(10)
93.27(18)
92.65(5)
93.3(2)
127.2(2)
127.93(19)
127.6(3)
128.8(3)
128.8(5)
116.12(16)
113.93(17)
116.1(2)
115.22(13)
115.5(4)
system, in contrast to conven-
tional Pd and Ni catalysis, in
which the selectivity is higher
Me
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
)NiEt] (3)
)Ni(OC(O)CH )] (6)
3
Me
[c]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
Me
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
)Ni
)Ni
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OTf)] (7)
[
1]
Me
for aryl–aryl coupling.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(NCCH )]BF
3
4
[
c]
Me
(
8·BF
a] Bond angles are in degrees; data for 1 and 3 are from ref. [20]. [b] X is the fourth ligand on Ni besides the
tridentate NN chelate. [c] Average of two independent molecules in the asymmetric unit.
4
)
Because [( NN )MgCl
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(thf)]
(
5) is a byproduct in the syn-
[
2
thesis of 2, it is hard to com-
3892
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Chem. Eur. J. 2009, 15, 3889 – 3899

Nickel Complexes of a Pincer Ligand
FULL PAPER
pletely prevent the contamination of a bulk sample of 2 by
conduct the catalysis at the same temperature. Octyl iodide
was chosen as the test substrate because it could react with
2 at room temperature and it contains b-hydrogen atoms.
An initial screen showed that coupling of octyl iodide with
5
. However, no reaction was found between 5 and alkyl hal-
ides, CH Cl , and CHCl , thus ruling out the involvement of
2
2
3
5
in Equations (1), (3), and (4). The reactions of 2 with
CH Cl and CHCl [Eqs. (3) and (4)] are particularly inter-
CH MgCl and nBuMgCl with 9 mol% 1 in THF gave the
2
2
3
3
esting because they involve the cleavage of two and three
desired product in 69 and 30% yield, respectively. Changing
the solvent to dimethylacetamide (DMA) improved the
yields, more for the b-H-containing nBuMgCl (65% yield,
CꢀCl bonds to form new CꢀC bonds at the same carbon
center. We examined the reactions of 2 with other alkyl di-
chlorides and polychlorides and the results are summarized
entry 1, Table 4) than for CH MgCl (74% yield, entry 2,
Table 4). Therefore, DMA was chosen as the solvent for the
3
Me
in Table 3. Although [( NN )NiCl] (1) was formed as the
2
Me
[a]
Table 3. Organic products identified in reactions of [( NN
with di- and polychloroalkanes.
2
)NiMe] (2)
Table 4. Catalytic coupling of octyl iodide with alkyl Grignard reagents.
[a]
cat: 1
octyl-I+alkyl-MgClƒƒ!octyl–alkyl
[
b]
Entry
1
Substrate
Conditions
RT, 5 min
Product
Yield [%]
Entry
RMgCl
Yield [%]
1
2
65
74
55
CH MgCl
3
3
77
4
54
2
RT, 24 h
30
4
5
6
0
0
3
4
RT, 24 h
1008C, 24 h
CH
CH
3
CH
2
CH
2
Cl
5
6
[
a] 0.6 mmol (1.2 equiv) of RMgCl in THF (1–3m) was added dropwise
2
=CH
2
to a DMA (0.75 mL) solution of 1 (9 mol%) and octyl iodide (0.5 mmol)
at ꢀ208C. The reaction time was 30 min. [b] GC yields relative to the or-
ganic halides.
5
6
608C, 24 h
–
–
–
–
CH
3
CHCl
2
RT, 24 h
[
a] See Experimental Section for conditions.
rest of the catalysis runs. Bulkier primary Grignard reagent
iBuMgCl could be coupled in good yield (entry 3, Table 4),
whereas benzyl Grignard reagent gave a lower yield (54%,
entry 4, Table 4). Coupling was ineffective for secondary
and tertiary Grignard reagents (entries 5 and 6, Table 4).
The same protocol was then used for the coupling of
other substrates. Primary and secondary alkyl iodides could
be coupled to primary alkyl Grignard reagents in 70–80%
yields (entries 1–3, 5, and 6, Table 5). An exception is the
only metal-containing product, no doubly or triply alkylated
organic compounds were produced in these reactions. In-
stead, small amounts of radically coupled products can be
identified. For instance, 2 reacted with benzotrichloride to
give
entry 1, Table 3), derived presumably from reductive deha-
logenation of the dimerization product, 1,1,2,2-tetrachloro-
a 55% yield of 1,2-dichloro-1,2-diphenylethylene
(
1
,2-diphenylethylene. A similar reaction was reported earli-
[34]
er for sodium formate in the presence of Pd/C.
Similar
coupling of cyclohexyl iodide with CH MgCl, which gave
3
radical dimerization and/or dehalogenation reactions are re-
sponsible for the production of other olefinic compounds
only 39% yield (entry 7, Table 5). Benzyl Grignard reagent
again gave lower coupling yields (entries 4 and 8, Table 5).
Tertiary alkyl iodide could not be coupled (entry 9, Table 5).
Primary alkyl bromide could also be used to give compa-
rable yields (entries 10–16, Table 5). Coupling of benzyl bro-
mide was sluggish due to the formation of the PhCH CH Ph
(
entries 2–4, Table 3). We were not able to identify any or-
ganic products for the reactions of 2 with other unactivated
alkyl dichlorides (entries 5 and 6, Table 3). Thus, multiple
C–C coupling is only efficient in the reactions of 2 with
CH Cl and CHCl .
2
2
dimer (entry 17, Table 5). The yield also dropped when sec-
ondary alkyl bromide was used (entry 18, Table 5). Coupling
of alkyl chlorides was ineffective (entries 19–21, Table 5).
Lowering the catalyst loading only slightly decreased the
yields. For instance, with 3 mol% loading of 1, coupling of
PhCH CH Br with CH MgCl gave PhCH CH CH in 65%
2
2
3
Catalytic C–C coupling of alkyl monohalides: Based on
Equations (1) and (2), a catalytic C–C coupling of alkyl hal-
ides and alkyl Grignard reagents with 1 as the precatalyst
seems plausible. We examined whether 1 would be active
for this Kumada–Corriu–Tamao type of coupling. Because
2
2
3
2
2
3
yield, and coupling of octyl bromide with nBuMgCl gave do-
decane in 70% yield (entries 10 and 15, Table 5, see foot-
notes).
the transmetalation of 1 with CH MgCl to give 2 has a
3
higher yield when carried out at low temperature and be-
cause previous experiments showed that coupling of CH Cl
As alkyl–aryl or aryl–aryl coupling was not observed be-
2
2
II
with nBuMgCl had the best yield at ꢀ208C, we decided to
tween Ni hydrocarbyl compounds and organic halides, no
Chem. Eur. J. 2009, 15, 3889 – 3899
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www.chemeurj.org
3893

X. Hu et al.
Table 5. Catalytic coupling of alkyl halides with alkyl Grignard re-
agents.
hexyl iodide was coupled with nBuMgCl, the yields for
[
a]
C H and C H C H were 58 and 25%, respectively
1
2
26
6
11
4
9
1
2
cat: 1
1
2
alkyl -X+alkyl -MgClƒƒ!alkyl –alkyl
[
(Eq. (6)]. Thus, alkyl iodides react faster than alkyl bro-
1
2
[b]
Entry
1
R -X
R MgCl
Yield [%]
70
mides, and primary halides react faster than secondary hal-
ides during the catalysis. This would be consistent with the
turnover-determining step of the catalysis being the C–X ac-
tivation of alkyl halides by the nickel alkyl intermediates.
2
3
CH
3
MgCl
80
75
When
bromomethylcyclopropane
was
coupled
to
nC H MgCl, 1-nonene was formed as the major product,
5
11
thus giving further evidence for the intermediacy of an alkyl
radical in the C–X activation step [(Eq. (7)]. Coupling of
certain alkene-containing substrates gave mostly noncyclized
products [(Eq. (8)]. This was probably due to a slower rate
of cyclization for alkyl radicals derivatized from these sub-
4
5
6
7
8
9
47
76
78
39
10
0
5
ꢀ1
strates (kꢁ10 s ) than for the ring opening of the cyclo-
propanyl methyl radical (kꢁ10 s ), which competed with
the cross-coupling reaction. Similar observations were re-
CH
CH
3
MgCl
7
ꢀ1
[14]
[14]
ported earlier by Cꢀrdenas et al.
[c]
10
1
75
80
1
3
MgCl
1
2
3
58
23
1
14
15
16
17
18
19
88
87
78
32
45
[
[
d]
e]
3
CH MgCl
[f]
CH
3
MgCl
4/25
2
0
1
0
0
2
[
a] 0.6 mmol (1.2 equiv) of RMgCl in THF (1–3m) was added dropwise
to a DMA (0.75 mL) solution of 1 (9 mol%) and organic halides
0.5 mmol) at ꢀ208C. The reaction time was 30 min. [b] GC yields rela-
tive to the organic halides. [c] 70% yield when 3 mol% of 1 was used.
d] 65% yield when 3 mol% of 1 was used. [e] PhCH CH Ph was formed
(
[
2
2
in 62% yield. [f] 4% yield at ꢀ208C, and 25% yield at 608C.
attempts were made to test the activity of 1 in catalytic
alkyl–aryl and aryl–aryl coupling.
A few further observations are noteworthy. During the
catalysis, the color of the reaction mixture resembles that of
Concerning the mechanism of the catalysis, stoichiometric
reactions of 1 and 2 suggest that the first step is the transme-
II
2
and 3. The electronic absorption spectrum of the solution
talation of 1 by a Grignard reagent to form a Ni alkyl inter-
mixture for the catalytic coupling of octyl bromide and
mediate, which then reacts with an alkyl halide to form the
C–C coupled product and 1 again, thereby reentering the
catalyst cycle (Figure 3). Further support for this proposal
comes from the fact that when 9 mol% of 2 was used as the
CH MgCl under 9 mol% of 1 was similar to those of the
3
Me
Me
[
( NN )Ni] hydrocarbyls, but not the other [( NN )Ni] de-
2
2
rivatives (Figure S3 in the Supporting Information). This
suggests that the resting state of the catalyst is the nickel
alkyl species. When an equal mixture of octyl iodide and
C H CH CH Br was coupled with nBuMgCl, the yields for
precatalyst for the coupling of octyl iodide with CH MgCl,
3
nonane was formed in 79% yield, comparable to the result
obtained with 1. Although Ni has been widely used for
Kumada–Corriu–Tamao coupling, this is the first time an
6
5
2
2
C H and C H C H were 66 and 44%, respectively
12
26
6
5
6
13
II
[(Eq. (5)]. When an equal mixture of octyl iodide and cyclo-
isolable Ni alkyl intermediate has been shown to be a com-
3894
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Chem. Eur. J. 2009, 15, 3889 – 3899

Nickel Complexes of a Pincer Ligand
FULL PAPER
reaction scheme operates for other Ni-catalyzed Kumada–
Corriu–Tamao coupling reactions remains to be seen. There
is, however, one major difference between the two systems:
the activation of alkyl halide involves radical intermediates
[7]
here but not in the Kambe system.
Catalytic C–C coupling of alkyl polyhalides: It was shown
earlier that 1 was a precatalyst for the double C–C coupling
[20]
of CH Cl with nBuMgCl at ꢀ208C (entry 1, Table 6).
A
2
2
Figure 3. Proposed mechanism for alkyl–alkyl coupling catalyzed by 1.
[
a]
Table 6. Double C–C coupling of CH
2 2
Cl with Grignard reagents.
cat: 1
2
RMgCl+CH Cl
2
2
ƒƒƒ!RCH
solvent
2
R
petent intermediate in the catalytic cycle. This is in contrast
to the Ni terpyridyl system, in which a Ni alkyl compound
[b]
Entry
RMgCl
Mol%
of catalyst
t [h]
Yield [%]
II
was catalytically incompetent for the Negishi-type alkyl–
alkyl coupling, but its one-electron reduced species was ef-
1
2
3
4
5
6
6
3
6
3
3
3
0.1
0.3
0.3
0.3
0.3
0.3
59
68
81
51
23
69
[12]
fective. It is different from the systems developed by Fu
and co-workers, for which a radical pathway was exclud-
[c]
[
11]
[
d]
ed. The mechanism is also different from the classical C–
0
C coupling mechanism by Pd , in which the activation of or-
ganic halides proceeds prior to transmetalation, rather than
the reverse in the current system.
7
8
4.5
12
0.3
0.3
30
49
The scope and limitations of the catalysis are in line with
this proposed mechanism. A low temperature (e.g., ꢀ208C)
assures the efficiency of transmetalation and the stability of
Ni alkyl intermediates, yet under such conditions only
weaker alkylꢀBr and alkylꢀI bonds, but not alkylꢀCl bonds,
9
1
1
3
9
3
3
0.3
0.5
0.3
0
6
0
0
1
can be activated. Therefore, no satisfactory cross-coupling of
alkyl chlorides could be obtained. The tridentate ligand
12
0.3
0.5
0
4
Me
NN remains on the metal center throughout the catalytic
[e]
2
1
3
0
II
cycle, including at the Ni alkyl stage. Such a four-coordi-
nate square-planar Ni species is coordinatively rather satu-
[
a] 0.8 mmol of RMgCl (1–2m solution in THF) as the limiting reagent;
RMgCl was added dropwise to a CH Cl (1.5 mL, 25 equiv) solution of 1.
b] GC yields relative to the Grignard reagents. [c] C 11MgBr in THF
was used. [d] C 11MgBr in ether was used. [e] 3 mol% [(dme)NiCl
was used as the precatalyst.
II
2
2
rated, and its subsequent reaction with alkyl halides would
be sensitive to steric demands. This may lead to the ineffi-
ciency for the cross-coupling of bulky Grignard reagents
and/or alkyl halides. The activation of organic halides goes
through a single-electron transfer step, which can explain
why aryl halides were not active. According to electrochem-
istry studies, Ni–Me is easier to oxidize than Ni–Ph (see
above). This may explain why aryl Grignard reagents were
not effective.
[
5
H
5
H
2
]
wide range of alkyl Grignard reagents were then examined
for this reaction. Gratifyingly, other primary alkyl Grignard
reagents worked. For example, coupling of CH Cl2 with
2
nC H MgCl by using 3 mol% catalyst loading gave 68% of
5
11
The mechanism in Figure 3 is quite similar to that pro-
posed for the Ni-catalyzed cross-coupling of unactivated
alkyl halides with alkyl Grignard reagents in the presence of
nC H -CH -nC H (entry 2, Table 6). The yield could be in-
5 11 2 5 11
creased to 81% if 6 mol% of catalyst was used (entry 3,
Table 6). nC H MgCl in THF gave the best results; chang-
5
11
1
,3-butadiene. Kambe et al. suggested that in the latter cat-
ing to nC H MgBr in THF/ether lowered the yields (en-
5 11
tries 4 and 5, Table 6). Coupling of CH Cl with nC H MgCl
2 2 8 17
0
alysis, the NiCl precatalyst was first reduced to Ni by a
2
Grignard reagent, then reacted with 2 equivalents of 1,3-bu-
tadiene to give a bis-p-allyl Ni complex. The allyl species
gave 69% of nC H -CH -nC H (entry 6, Table 6). Bulkier
8 17 2 8 17
alkyl Grignard reagents could be used but gave lower yields.
II
then reacted again with the Grignard reagent to form a nu-
For instance, iBuMgCl coupled with CH Cl to give iBu-
2
2
II
cleophilic Ni alkyl anion, which activated alkyl halides by
CH -iBu in 30% yield with 4.5 mol% precatalyst (entry 7,
2
IV
oxidative addition to form a Ni bis
A
H
U
G
E
N
N
(alkyl) intermediate.
Table 6). Increasing the precatalyst loading to 12 mol% im-
proved the yield to 49% (entry 8, Table 6). BenzylMgCl did
not work, as it gave mainly the homocoupling product
C H CH CH C H (60% yield, entry 9, Table 6). Coupling
Subsequent reductive elimination afforded the coupling
[7]
product and completed the catalytic cycle. Both systems
involve first transmetalation by Grignard reagents to give a
6
5
2
2
6
5
II
nucleophilic Ni alkyl intermediate, then activation of alkyl
was not efficient with a secondary or tertiary Grignard re-
IV
II
halides to give a formal Ni bis
A
H
U
G
E
N
N
(alkyl) species, and finally
agent (entries 10–12, Table 6). When another soluble Ni
productive C–C reductive elimination. Whether this general
complex, for example, [(dme)NiCl2] (dme=dimethoxy-
Chem. Eur. J. 2009, 15, 3889 – 3899
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www.chemeurj.org
3895

X. Hu et al.
ethane), was used as the precatalyst, only a trace amount of
the doubly coupled product was formed (entry 13, Table 6),
thus confirming the unique activity of 1 for this catalysis.
The overall results suggest that the double C–C coupling be-
tween CH Cl and a primary alkyl Grignard reagent cata-
Table 7). A radical dimerization process is likely to be op-
erational in these reactions.
Conclusion
2
2
lyzed by 1 is general and rapid. The yields are remarkably
high considering the fact that these alkyl Grignard reagents
We have described the synthesis, characterization, structure,
Me
II
contain b-hydrogen atoms. The ability of [( NN )Ni] alkyl
and reactivity of a series of Ni compounds of the newly de-
2
Me
species to resist b-hydrogen elimination is likely a key factor
for this high efficiency. The catalysis does not tolerate well
the steric bulkiness of the Grignard reagents, as bulkier pri-
mary Grignard reagents give lower yields, and secondary
and tertiary Grignard reagents cannot be coupled.
veloped pincer amidobis ACHTUNGETRNNUNG( amine) ligand NN . The coordi-
2
Me
II
nation chemistry of NN on Ni is comparable to that of
2
the analogous pincer amidobis(phosphine) (pnp) and
arylbis ACHUTNGRENNUG( amine) (ncn) ligands, whereas its electron-donating
ability is similar to that of alkyl pnp. The Lewis acidity of
Me
Although its detailed mechanism warrants further studies,
based on stoichiometric reactions, the double coupling catal-
ysis is proposed to proceed through similar sequences as in
the Kumada–Corriu–Tamao coupling: transmetalation of 1
the [( NN )Ni] cation is comparable to that of its isoelec-
2
tronic [(pnp)Ni] complex. A significant difference in the
electronic properties of the two classes of pincer ligands
(NN2 versus pnp) is, however, expected. Replacing phos-
phine with amine donors makes the ligand “harder” and
more suitable for stabilizing higher-oxidation-state metal
II
by a Grignard reagent gives a Ni alkyl species, which reacts
with CH Cl to give the coupling product and regenerate 1.
2
2
Catalytic coupling experiments were also carried out on
other dichloroalkanes with 3 mol% of 1 as the precatalyst.
No doubly C–C coupled products were formed, even with
CH CHCl . This is consistent with the results from stoichio-
metric reactions of 2 and dichloroalkanes (Table 3). In some
cases, the organic products could be identified. For instance,
ions. This might be the origin for the enhanced activity of
Me
[( NN )Ni] alkyl complexes towards alkyl halides relative
2
[25]
to the [(pnp)Ni] alkyl complexes. A Ni bis ACHUTNGRNEGUN( alkyl) species
with a formal oxidation state of +III or +IV is the pre-
sumed intermediate in such a reaction. Furthermore, com-
pared to pnp complexes, intramolecular metal-to-ligand
3
2
coupling of benzotrichloride with CH MgCl gave 1,2-di-
p backbonding is diminished in NN complexes. Therefore,
3
2
chloro-1,2-diphenylethylene in 25% yield (entry 1, Table 7),
NN complexes might be more active in p-bond activation
2
and coupling of benzodichloride with CH MgCl gave stil-
chemistry, which we plan to explore in the near future.
3
Me
bene, styrene, and [Ph
A
H
U
G
R
N
U
G
The [( NN )NiMe] (2) complex reacted cleanly with
3
2
2
respectively (entry 2, Table 7). Coupling of 1,1-dichloro-3,3-
alkyl halides, CH Cl , and CHCl to give C–C coupled prod-
ucts in high yields. The reactions of 2 with other di- and poly-
chloroalkanes were also probed but a different reactivity
2
2
3
1
1
1
dimethylbutane with nBuMgCl (R L ) gave BuR R Bu in
2
[20]
42% yield (entry 3, Table 7),
and coupling of Ph CCl
2
2
with MeMgCl gave Ph C=CPh in 70% yield (entry 4,
pattern was observed. Based on stoichiometric reactions of
2
2
Me
2
and [( NN )NiCl] (1), catalytic Kumada–Corriu–Tamao
2
coupling of unactivated alkyl halides and alkyl Grignard re-
agents was achieved by using as 1 the precatalyst. Good
yields were obtained for this challenging type of cross-cou-
pling that suffered from the inertness of alkyl halides and
the decomposition of catalytic intermediates via facile b-H
elimination. The catalysis was rapid and primary and secon-
dary iodides and bromides were tolerated. Although Ni-cat-
alyzed alkyl–alkyl coupling has been reported by a number
of groups, complex 1 is one of the very few isolated and
well-characterized (pre)catalysts known. Complex 2 is the
[
a]
Table 7. Catalytic coupling of alkyl polyhalides.
[
b]
Entry
RCl
x
R’MgCl
Product
Yield [%]
25
1
CH
3
MgCl
8
II
2
3
CH
CH
3
MgCl
16
first isolable Ni alkyl species that has been shown to be cat-
alytically competitive. The resting state of the catalysis is the
Ni alkyl species. Transmetalation occurred before the activa-
tion of organic halides in the catalytic cycle. The mechanism
of this catalysis is therefore unique. This underscores the vi-
ability of several distinctive reaction pathways for Ni-cata-
lyzed C–C coupling reactions, which can be beneficial in
terms of substrate scope and selectivity.
22
3
MgCl
70
40
[
c]
4
The double C–C coupling between CH Cl and Grignard
2
2
reagents catalyzed by 1 has been further investigated. Long-
chain primary alkyl Grignard reagents could be successfully
coupled in 60–80% yields; bulkier primary alkyl Grignard
reagents could also be used but with lower yields. Extension
[
a] 0.4 mmol of RMgCl (1–2m solution in THF) was added dropwise to a
THF (1.5 mL) solution of polyhalides (1 equiv with respect to the
number of CꢀCl bonds) and 1 (3 mol%). [b] GC and/or NMR yields rel-
ative to the organic polyhalides. [c] Reported in ref. [20].
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Chem. Eur. J. 2009, 15, 3889 – 3899

Nickel Complexes of a Pincer Ligand
FULL PAPER
Me
Me
to secondary and tertiary alkyl Grignard reagents was not
successful, probably for steric reasons. Coupling of other di-
and polychloroalkanes with Grignard reagents was attempt-
ed, and in several cases radical dimerization products were
produced.
A
H
U
G
R
N
U
G
2
)Ni
A
H
U
G
R
N
U
G
2
)NiCl] (1; 50 mg, 0.14 mmol) was dis-
solved in THF (5 mL) and 10 mol excess of NaOAc (118 mg, 1.40 mmol)
was added. This mixture was vigorously stirred for 6 h and the resulting
precipitate was isolated by filtration. A 10 mol excess of NaOAc (118 mg,
1
.40 mmol) was added again and the mixture was stirred for another 6 h.
The precipitate was isolated by filtration and the filtrate was evaporated.
The green solid residue was washed with a mixture of pentane and ben-
zene (5:1, 3ꢅ2 mL), and dried under vacuum (42 mg, 79%). Single crys-
tals suitable for X-ray crystallography were obtained by cooling a tolu-
ene/pentane solution (1:4) of 6 at ꢀ358C.
6 6
H NMR (400.13 MHz, C D ): d=7.30 (d, J=8.4 Hz, 2H), 6.95 (t, J=
Future work in this area includes kinetic and computa-
tional studies on the mechanism of the reactions between 2
and alkyl monohalides, CH Cl , and CHCl , and the applica-
2
2
3
Me
tion of ligand NN in other metal-catalyzed bond-forming
1
2
reactions.
8
(
C
1
.1 Hz, 2H), 6.48 (dd, J=7.7, 1.0 Hz, 2H), 6.32 (t, J=7.7 Hz, 2H), 2.64
s, 12H; N
1
3
3 2 3
) ), 2.04 ppm (s, 3H; COCH ); C NMR (100.62 MHz,
arom
arom
arom
6
D
3
), 146.6 (C ), 145.8 (C ), 127.7 (C H),
arom
arom
arom
19.6 (C H), 116.0 (C H), 115.3 (C H), 49.3 (N ), 24.6 ppm
A
H
U
G
R
N
N
(CH
3
)
2
(
COCH ); elemental analysis calcd (%) for C18 NiO : C 58.10, H
3
H
23
N
3
2
Experimental Section
6.23, N 11.29; found: C 57.44, H 6.22, N 11.37.
Me
2
ACTHNGUTRENNU[G ( NN )Ni ACHTUNGRETNNNU(G OTf)] (7): A solution of AgOTf (73 mg, 0.39 mmol) in THF
Chemicals and reagents: All manipulations were carried out under an
(2 mL) was added to a solution of 1 (100 mg, 0.39 mmol) in THF (3 mL).
The resulting solution was stirred for 30 min and then the white precipi-
tate of AgCl was isolated by filtration. The filtrate was evaporated and
the brown solid residue was washed with pentane (3 mL) and dried
under vacuum (99 mg, 75%). Single crystals suitable for X-ray crystallog-
inert N
Solvents were purified with a two-column solid-state purification system
Innovative Technology, NJ, USA) and transferred to the glove box with-
2
(g) atmosphere with standard Schlenk or glove box techniques.
(
out exposure to air. Deuterated solvents were purchased from Cambridge
Isotope Laboratories, Inc., and were degassed and stored over activated
raphy were obtained by diffusion of pentane into a benzene solution of 7.
3
ꢄ molecular sieves. All other reagents were purchased from commer-
1
H NMR (400.13 MHz, [D
6
]DMSO): d=7.27 (dd, J=8.2, 1.4 Hz, 2H),
cial sources and were degassed by standard freeze–pump–thaw proce-
dures prior to use. Ligand H NN
7
2
1
4
.13 (dd, J=8.2, 1.7 Hz, 2H), 6.98 (td, J=7.5, 1.4 Hz, 2H), 6.84 (m, 2H),
Me
Me
2
and complexes [( NN
2
)NiCl] (1),
13
6
.60 ppm (s, 12H); C NMR (100.62 MHz, [D ]DMSO): d=143.4, 137.3,
Me
Me
Me
[
(
NN
2
)NiMe] (2), [( NN
2
)NiEt] (3), and [( NN
2
)NiPh] (4) were pre-
arom
arom
arom
arom
28.8, 124.1 (C H), 120.5 (C H), 120.0 (C H), 115.4 (C H),
3 3
4.1 ppm (N(CH ); elemental analysis calcd (%) for C17 NiSO F :
[
20, 35]
13
pared as described previously.
A
H
U
G
R
N
N
3
)
2
H
20
N
3
1
Physical methods: The H and C NMR spectra were recorded at 293 K
on a Bruker Avance 400 spectrometer. H NMR chemical shifts were ref-
C 44.18, H 4.36, N, 9.09; found: C 44.76, H 4.44, N 9.14.
1
Me
2 3 4 4
AHCTUNGTRENNUG[ ( NN )Ni ACHTUNGTERNNUNG( CH CN)]BF (8): A solution of AgBF (56 mg, 0.39 mmol) in
CH
CH
then the white precipitate of AgCl was isolated by filtration. The filtrate
was evaporated and the green solid was washed with benzene (2 mL) and
pentane (3 mL), and dried under vacuum (119 mg, 94%). Single crystals
erenced to residual solvent as determined relative to Me
4
Si (d=0 ppm).
The C{ H} chemical shifts are reported in ppm relative to the carbon
resonance of [D ]DMSO (d=39.52 ppm), CD CN (d=118.26 ppm), or
(d=128.02 ppm). GC–MS measurements were conducted on a
3
CN (2 mL) was added to a solution of 1 (100 mg, 0.39 mmol) in
1
3
1
3
CN (3 mL). The resulting green solution was stirred for 30 min and
6
3
6 6
C D
Perkin–Elmer Clarus 600 chromatograph equipped with a Clarus 600T
mass spectrometer. GC measurements were conducted on a Perkin–
Elmer Clarus 400 chromatograph with a flame ionization detector. UV/
Vis measurements were carried out with a Varian Cary 50 Bio spectro-
photometer controlled by Cary WinUV software. Elemental analyses
were performed on a Carlo Erba EA 1110 CHN instrument at EPFL. X-
ray diffraction studies were carried out in the EPFL Crystallographic Fa-
cility. Data collections were performed at 140(2) K on a four-circle kappa
goniometer equipped with an Oxford Diffraction KM4 Sapphire charge-
coupled device. Data were reduced by CrysAlis PRO. The absorption
correction was applied by using
SHELXTL program was used for structure solution, refinement, and geo-
metrical calculation. Cyclic voltammetric measurements were recorded
in a glove box by an IviumStat electrochemical analyzer that was con-
3
suitable for X-ray crystallography were obtained by cooling a CH CN/
ether (1:4) solution of 8 at ꢀ358C.
1
3
H NMR (400.13 MHz, CD CN): d=7.32 (dd, J=7.8, 1.0 Hz, 2H), 7.24
1
(
m, 4H), 6.49 (m, 2H), 3.54 ppm (s, 12H); H NMR (400.13 MHz,
CDCl ): d=7.40 (dd, J=8.2, 1.0 Hz, 2H), 7.05 (m, 4H), 6.59 (td, J=8.2,
.0 Hz, 2H), 2.98 (s, 12H; (CH ), 2.60 ppm (br, 3H; CH CN);
CN): d=145.7 (C ), 142.6 (C ), 129.5
3
1
N
A
H
U
G
R
N
U
G
3
)
2
3
1
3
arom
arom
C NMR (100.62 MHz, CD
3
arom
arom
arom
arom
[
36]
(C H), 121.4 (C H), 119.3 (C H), 118.8 (C H), 50.4 ppm (N-
ACHTUNGTREN(NUGN CH ) ); elemental analysis calcd (%) for C H N NiBF : C 49.03, H
5
[
37]
a
semiempirical method.
The
3
2
18 23
4
4
.26, N 12.71; found: C 48.97, H 5.34, N 12.26.
Me
[
38]
2
ACTHUNGTNERNUN[G ( NN )Ni ACHTUNGRETNNNU(G OMe)] (9): Compound 1 (150 mg, 0.43 mmol) was dissolved
in THF (2 mL) and a solution of NaOMe (23 mg, 0.43 mmol) in THF
(1 mL) was added. The mixture was stirred for 1 h and the solvent was
evaporated. The solid residue was dissolved in benzene and the resulting
solution was filtered to remove NaCl. The filtrate was evaporated to give
9 as the crude product. It was recrystallized from toluene/pentane (3:1,
2 mL) at ꢀ358C (105 mg, 71%).
2
nected to a glassy carbon working electrode (surface area=0.07 cm ), a
platinum wire auxiliary electrode, and an Ag/AgNO
electrode filled with acetonitrile and [nBu [PF ] (0.1m). All potentials
were referenced to Fc/Fc as internal standard. The temperature of reac-
tions below room temperature was regulated by a Julabo FT-902 chiller.
3
(0.01m) reference
4
]
A
H
U
G
E
N
N
6
+
1
Synthetic methods
H NMR (400.13 MHz, C D ): d=7.39 (dd, J=8.5, 1.4 Hz, 2H), 6.95 (td,
6
6
Me
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[( NN
2
)MgCl
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(thf)] (5): A 3.0m solution of CH
3
MgCl in THF (0.26 mL,
(200 mg, 0.78 mmol) in
J=7.2, 1.4 Hz, 2H), 6.59 (dd, J=7.9, 1.4 Hz, 2H), 6.40 (td, J=7.2,
1.4 Hz, 2H), 3.35 (s, 3H; OCH
(100.62 MHz, C ): d=147.9 (C ), 147.5 (C ), 128.0 (C H), 119.8
(C H), 114.9 (C H), 114.1 (C H), 53.8 (OCH
Me
13
0
.78 mmol) was added to a solution of H NN
2
3 3 2
), 2.53 ppm (s, 12H; N ACHUTNGERTNNUGN( CH ) ); C NMR
arom
arom
arom
THF (5 mL) under stirring. After 1 h, the solvent was evaporated and the
solid residue was dissolved in a minimum quantity of benzene and fil-
tered. Pentane was added to the filtrate and a precipitate was formed.
The precipitate was collected, washed with pentane, and dried under
6 6
D
arom
arom
arom
3
), 49.0 ppm (N-
A
T
N
R
N
U
3
)
2
23 3
H N
N 12.21; found: C 59.26, H 6.62, N 11.82.
Me
vacuum (204 mg, 83%).
2
Reaction of [( NN )NiMe] (2) with alkyl monohalides: In a typical ex-
periment, 2 (0.015 mmol) was loaded into an NMR tube along with C D
6 6
1
H NMR (400.13 MHz, C
6
D
6
): d=7.84 (dd, J=8.2, 1.0 Hz, 2H), 7.12 (td,
J=7.2, 1.7 Hz, 2H), 7.00 (dd, J=7.9, 1.4 Hz, 2H), 6.64 (td, J=7.9,
(0.6 mL). Alkyl halide (0.15 mmol, 10 equiv or 0.015 mmol, 1 equiv) was
added to this solution and the reaction was periodically monitored by
H NMR spectroscopy. If the reaction did not occur at room temperature,
1
.4 Hz, 2H), 3.54 (b, 4H; THF), 2.55 (s, 12H), 1.31 ppm (b, 4H; THF);
1
3
arom
arom
1
C NMR (100.62 MHz,
C
6
D
6
): d=147.1 (C ), 142.2 (C ), 127.1
arom
arom
arom
arom
(
(
C
H), 119.9 (C H), 114.9 (C H), 113.9 (C H), 68.3
the solution was heated at 60 or 1008C. The identification and quantifica-
THF
THF
1
CH
2
CH
2
O
), 45.8 (N
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(CH
3
)
2
), 25.2 ppm (CH
2
CH
2
O
).
tion of products were achieved by H NMR spectroscopy. The results are
Chem. Eur. J. 2009, 15, 3889 – 3899
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3897

X. Hu et al.
Me
summarized in Table S1 in the Supporting Information. Yields are re-
ferred to the formation of Ni halides. For example, reaction of 2 with
2 3 4 4
Crystallographic details for [( NN )Ni ACHTUNGTERNUN(GN NCCH )]BF ACHTUNGTNNEGRU( 8·BF ): A total of
II
31050 reflections (ꢀ22ꢂhꢂ21, ꢀ7ꢂkꢂ7, ꢀ47ꢂlꢂ47) were collected at
T=140(2) K in the range of 2.80 to 26.378 of which 7783 were unique
(Rint =0.0467); MoKa radiation (l=0.71073 ꢄ). The structure was solved
by the Direct method. All non-hydrogen atoms were refined anisotropi-
cally and hydrogen atoms were placed in calculated idealized positions.
The residual peak and hole electron densities were 1.353 and
1
3 2
0 equivalents of CH CH Cl in benzene took place at room temperature
and finished within 18 h to give 1 (near quantitative) and propane (35%
remained in solution). Heating a benzene solution of 2 in the presence of
1
0 equivalents of propyl chloride at 608C for 18 h gave 33% conversion;
at 1008C the reaction finished in 4 h and produced 1 (near quantitative)
and n-butane (30% remained in solution). The yields of the propane and
ꢀ
3
ꢀ1
ꢀ1.632 eA , respectively. The absorption coefficient was 1.041 mm
.
1
butane were determined by NMR integration references to 1. H NMR
The least-squares refinement converged normally with residuals of
2
(
in C
6
D
6
): propane (0.82, 1.25 ppm), butane (0.85, 1.25 ppm).
R(F)=0.0613,
wR(F )=0.1108,
and
GOF=1.268
(I>2s(I)).
Me
C H BF N Ni; M =440.92; space group Pca21; orthorhombic; a=
Reaction of [( NN
2
)NiMe] (2) with alkyl polyhalides: Same as the reac-
18 23
4
4
w
3
1
1
7.8380(7), b=5.7806(2), c=37.8747(12) ꢄ; V=3905.4(2) ꢄ ; Z=8;
calcd =1.500 Mgm .
tion with alkyl monohalides. The identification of organic products was
made by GC–MS or NMR spectroscopy, and quantification was made by
GC, with decane or dodecane as an internal standard. See the Supporting
Information for GC–MS, NMR, and GC data.
ꢀ
3
4
CCDC 704456 (7), 704457 (6), and 704458 (8·BF ) contain the supple-
mentary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Typical procedure for the synthesis of R-R’ from R’-X and RMgCl: A vial
was charged with a certain amount of 1, R’-X, and DMA (0.75 mL).
After the solution reached the desired temperature, a solution of RMgCl
in THF was added dropwise under vigorous stirring. The reaction was al-
lowed to proceed for a certain period of time, and then the solution was
warmed or cooled to room temperature. A mixture of distilled water
(
15 mL), hydrochloric acid (25%, 1 mL), and dodecane (internal stan-
dard, 60 mL, 0.265 mmol) or decane (internal standard, 60 mL,
.422 mmol) was added to the reaction mixture. The resulting solution
was extracted with diethyl ether (3ꢅ10 mL) and the organic phase was
separated, dried over MgSO , and filtered. The loading of the catalyst
Acknowledgements
0
We thank Julie Risse and Aswin Yellepeddi for experimental assistance.
The EPFL-ISIC mass spectrometry facility provided assistance in mass
spectrometric measurements. We thank Profs. Jꢁrꢆme Waser and Karl
Gademann for insightful discussions, and the Waser group for use of the
GC–MS instrument. This work is supported by the EPFL.
4
and the reaction time are shown in Table 4 and Table 5. The organic
products were identified by GC–MS, and the yields were determined by
GC with decane or dodecane as the internal standard. See the Supporting
Information for GC–MS, NMR, and GC data.
Typical procedure for the synthesis of RCH
A vial was charged with a certain amount of 1 and CH
2
R from CH
2
Cl
2
and RMgCl:
Cl (1.5 mL).
[
1] Metal-Catalyzed Cross-Coupling Reactions, 2nd ed. (Eds.: A. de
Meijere, F. Diederich), Wiley-VCH, Weinheim, 2004.
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2
2
After the solution reached the desired temperature, a solution of RMgCl
was added dropwise under vigorous stirring. The reaction was allowed to
proceed for a certain period of time, and then the solution was warmed
to room temperature. A mixture of distilled water (15 mL), hydrochloric
acid (25%, 1 mL), and dodecane (internal standard, 60 mL, 0.265 mmol)
was added to the reaction mixture. The resulting solution was extracted
with diethyl ether (3ꢅ10 mL) and the organic phase was separated, dried
[
[
2
005, 117, 680–695; Angew. Chem. Int. Ed. 2005, 44, 674–688.
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time are shown in Table 6. The organic products were identified by GC–
MS. The yield of RCH R was determined by GC with decane or dodec-
4
, and filtered. The loading of the catalyst and the reaction
1
998, 110, 2512–2515; Angew. Chem. Int. Ed. 1998, 37, 2387–2390.
2
[
7] J. Terao, H. Watanabe, A. Ikumi, H. Kuniyasu, N. Kambe, J. Am.
Chem. Soc. 2002, 124, 4222–4223.
ane as the internal standard. See the Supporting Information for GC–
MS, NMR, and GC data.
[
[
8] J. Terao, N. Kambe, Bull. Chem. Soc. Jpn. 2006, 79, 663–672.
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Ed. 2002, 41, 1945–1947; c) J. R. Zhou, G. C. Fu, J. Am. Chem. Soc.
Me
2
Crystallographic details for [( NN )Ni ACHUTTNGENRNUG( OAc)] (6): A total of 30213 re-
flections (ꢀ10ꢂhꢂ10, ꢀ15ꢂkꢂ15, ꢀ39ꢂlꢂ39) were collected at T=
1
40(2) K in the range of 2.76 to 26.378 of which 6862 were unique (Rint
=
0
.0349); MoKa radiation (l=0.71073 ꢄ). The structure was solved by the
Direct method. All non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were placed in calculated idealized positions. The residu-
2
2
003, 125, 12527–12530; d) B. Saito, G. C. Fu, J. Am. Chem. Soc.
007, 129, 9602–9603.
ꢀ
3
al peak and hole electron densities were 1.615 and ꢀ0.487 eA , respec-
[
10] a) J. R. Zhou, G. C. Fu, J. Am. Chem. Soc. 2003, 125, 14726–14727;
b) N. A. Strotman, S. Sommer, G. C. Fu, Angew. Chem. 2007, 119,
ꢀ
1
tively. The absorption coefficient was 1.166 mm . The least-squares re-
2
finement converged normally with residuals of R(F)=0.0441, wR(F )=
3
626–3628; Angew. Chem. Int. Ed. 2007, 46, 3556–3558.
0
.1046, and GOF=1.049 (I>2s(I)). C18
group P2 orthorhombic; a=8.3469(2), b=12.7857(4), c=
1.5964(9) ꢄ; V=3372.00(16) ꢄ ; Z=8; 1calcd =1.466 Mgm
23 3 2 w
H N NiO ; M =522.58; space
[
[
11] C. Fischer, G. C. Fu, J. Am. Chem. Soc. 2005, 127, 4594–4595.
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1 1 1
2 2 ;
3
ꢀ3
3
.
Me
2
Crystallographic details for [( NN )NiOTf] (7): A total of 16903 reflec-
tions (ꢀ8ꢂhꢂ8, ꢀ17ꢂkꢂ17, ꢀ24ꢂlꢂ23) were collected at T=
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1
40(2) K in the range of 2.80 to 26.378 of which 2107 were unique (Rint
=
0
.0529); MoKa radiation (l=0.71073 ꢄ). The structure was solved by the
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2737.
Direct method. All non-hydrogen atoms were refined anisotropically and
hydrogen atoms were placed in calculated idealized positions. The residu-
ꢀ
3
al peak and hole electron densities were 0.340 and ꢀ0.388 eA , respec-
ꢀ
1
tively. The absorption coefficient was 1.129 mm . The least-squares re-
2
finement converged normally with residuals of R(F)=0.0344, wR(F )=
0
.0597, and GOF=1.076 (I>2s(I)).
17 20 3 3 3 w
C H F N O SNi; M =462.13;
space group Pnma; orthorhombic; a=7.0081(3), b=14.5619(6), c=
[17] M. Nakamura, K. Matsuo, S. Ito, B. Nakamura, J. Am. Chem. Soc.
2004, 126, 3686–3687.
3
ꢀ3
1
9.5131(9) ꢄ; V=1991.34(15) ꢄ ; Z=4; 1calcd =1.541 Mgm .
3898
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3889 – 3899

Nickel Complexes of a Pincer Ligand
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Received: October 6, 2008
Published online: February 19, 2009
Chem. Eur. J. 2009, 15, 3889 – 3899
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3899