Synthesis and catalytic activity in suzuki coupling of nickel complexes bearing n -butyl- and triethoxysilylpropyl-substituted NHC ligands: Toward the heterogenization of molecular catalysts

Article abstract of DOI:10.1021/om201101g

Cyclopentadienyl N-heterocyclic carbene (NHC) nickel complexes of general formula [Ni(R-NHC-nBu)XCp] [R-NHC-nBu = 1-butyl-3-methyl-, 1-isopropyl-3-butyl-, 1-phenyl-3-butyl-, 1-(2,4,6-trimethylphenyl)-3-butyl-, 1-(2,6- diisopropylphenyl)-3-butyl-imidazol-2-ylidene; X = Cl or I; Cp = η⁵-C₅H₅], which bear an n-butyl side-chain attached to one of the nitrogen atoms of the NHC ring, were synthesized as models for trialkoxysilylpropyl-substituted complexes. They were prepared by the direct reactions of nickelocene with the corresponding imidazolium salts (R-NHC-nBuHX). The new complexes [Ni(Me-NHC-nBu)ClCp] (1a), [Ni(iPr-NHC-nBu)ClCp] (1b), [Ni(Ph-NHC-nBu)ICp] (1c), [Ni(Mes-NHC-nBu)ICp] (1d), and [Ni(iPr₂Ph-NHC-nBu)ICp] (1e) were obtained in moderate to good yields and were fully characterized by standard spectroscopic techniques, and in the cases of 1a,b,d,e by single-crystal X-ray crystallography. The bulky electron-rich pentamethylcyclopentadienyl derivatives, [Ni(Mes-NHC-nBu) ICp*] (2d) and [Ni(iPr₂Ph-NHC-nBu)ICp*] (2e) (Cp* = η⁵-C₅Me₅), were prepared from reactions of in situ prepared [Ni(acac)Cp*] with the corresponding carbene precursors. Both Cp* complexes were also fully characterized spectroscopically, and their structures were established by single-crystal X-ray crystallography. All new complexes catalyzed the Suzuki-Miyaura cross-coupling of phenylboronic acid with aryl halides in the absence of cocatalysts or reductants. However, the small dialkyl-substituted species 1a and 1b proved to be the least efficient. In addition, in contrast to our previous results with the closely related diaryl-substituted species [Ni(Ar₂NHC)LCp^?] (L = Cl^-, NCMe (PF₆^-); Cp^? = Cp, Cp*), in which complexes that bear the electron-rich Cp* ligand were much more active than those bearing the Cp ligand, no substantial catalytic behavior differences were observed between the Cp complexes 1d,e and their Cp* counterparts 2d,e. A TOF of up to 352 h^-1, a so far unprecedented rate for nickel(II) complexes under similar conditions, was even observed with the Cp complex 1d. In view of these encouraging results, the triethoxysilylpropyl-substituted analogue of 1d, [Ni(Mes-NHC-TES)ClCp] (1d-TES) (Mes-NHC-TES = 1-(2,4,6-trimethylphenyl)-3-[3-(triethoxysilyl)propyl]imidazol-2- ylidene), was prepared, fully characterized, and tested catalytically. As it showed similar catalytic activity to 1d, it was heterogenized on alumina to give 1d-Al. The latter species, however, exhibited a greatly reduced catalytic activity compared to 1d and 1d-TES. Possible reasons for both the excellent activities of 1d and 1d-TES and the disappointing activity of 1d-Al are discussed.

Full text of DOI:10.1021/om201101g

Article
pubs.acs.org/Organometallics
Synthesis and Catalytic Activity in Suzuki Coupling of Nickel
Complexes Bearing n-Butyl- and Triethoxysilylpropyl-Substituted
NHC Ligands: Toward the Heterogenization of Molecular Catalysts
Anna Magdalena Oertel, Vincent Ritleng,* and Michael J. Chetcuti*
Laboratoire de Chimie Organomet
́
allique Appliquee
́ ́ ̀ ́
, UMR CNRS 7509, Ecole Europeenne de Chimie, Polymeres et Materiaux,
Universite
́
de Strasbourg, 25 Rue Becquerel, 67087 Strasbourg, France
S
* Supporting Information
ABSTRACT: Cyclopentadienyl N-heterocyclic carbene (NHC)
nickel complexes of general formula [Ni(R-NHC-nBu)XCp] [R-
NHC-nBu = 1-butyl-3-methyl-, 1-isopropyl-3-butyl-, 1-phenyl-3-
butyl-, 1-(2,4,6-trimethylphenyl)-3-butyl-, 1-(2,6-diisopropyl-
phenyl)-3-butyl-imidazol-2-ylidene; X = Cl or I; Cp = η5-C5H5],
which bear an n-butyl side-chain attached to one of the nitrogen
atoms of the NHC ring, were synthesized as models for trialkoxysilylpropyl-substituted complexes. They were prepared by the
direct reactions of nickelocene with the corresponding imidazolium salts (R-NHC-nBu·HX). The new complexes [Ni(Me-NHC-
nBu)ClCp] (1a), [Ni(iPr-NHC-nBu)ClCp] (1b), [Ni(Ph-NHC-nBu)ICp] (1c), [Ni(Mes-NHC-nBu)ICp] (1d), and
[Ni(iPr2Ph-NHC-nBu)ICp] (1e) were obtained in moderate to good yields and were fully characterized by standard
spectroscopic techniques, and in the cases of 1a,b,d,e by single-crystal X-ray crystallography. The bulky electron-rich
pentamethylcyclopentadienyl derivatives, [Ni(Mes-NHC-nBu)ICp*] (2d) and [Ni(iPr2Ph-NHC-nBu)ICp*] (2e) (Cp* = η5-
C5Me5), were prepared from reactions of in situ prepared [Ni(acac)Cp*] with the corresponding carbene precursors. Both Cp*
complexes were also fully characterized spectroscopically, and their structures were established by single-crystal X-ray
crystallography. All new complexes catalyzed the Suzuki−Miyaura cross-coupling of phenylboronic acid with aryl halides in the
absence of cocatalysts or reductants. However, the small dialkyl-substituted species 1a and 1b proved to be the least efficient. In
addition, in contrast to our previous results with the closely related diaryl-substituted species [Ni(Ar2NHC)LCp†] (L = Cl−,
NCMe (PF6 ); Cp† = Cp, Cp*), in which complexes that bear the electron-rich Cp* ligand were much more active than those
−
bearing the Cp ligand, no substantial catalytic behavior differences were observed between the Cp complexes 1d,e and their Cp*
counterparts 2d,e. A TOF of up to 352 h−1, a so far unprecedented rate for nickel(II) complexes under similar conditions, was
even observed with the Cp complex 1d. In view of these encouraging results, the triethoxysilylpropyl-substituted analogue of 1d,
[Ni(Mes-NHC-TES)ClCp] (1d-TES) (Mes-NHC-TES = 1-(2,4,6-trimethylphenyl)-3-[3-(triethoxysilyl)propyl]imidazol-2-
ylidene), was prepared, fully characterized, and tested catalytically. As it showed similar catalytic activity to 1d, it was
heterogenized on alumina to give 1d-Al. The latter species, however, exhibited a greatly reduced catalytic activity compared to 1d
and 1d-TES. Possible reasons for both the excellent activities of 1d and 1d-TES and the disappointing activity of 1d-Al are
discussed.
past decade has seen the emergence of a number of NHC-
nickel-based catalytic systems that have now found applications
in a vast number of organic transformations, including notably
in traditional C−C cross-coupling reactions,8−12 the amina-
tion13 and dehalogenation13b,14 of aryl halides, the oxidation of
secondary alcohols,15,16 C−S couplings,17 the activation of C−
F bonds,18,19 the hydrosilylation of internal alkynes,20 three-
component couplings of unsaturated hydrocarbons, aldehydes,
and silyl derivatives,21 and [2+2+2] cycloadditions.22−24 In this
context, we have recently reported that the neutral [Ni-
(Ar2NHC)ClCp†] and cationic [Ni(Ar2NHC)(NCCH3)Cp†]-
(PF6) [Ar2NHC = 1,3-diarylimidazol-2-ylidene, Cp† = Cp (η5-
INTRODUCTION
■
Since the first isolation of a stable imidazol-2-ylidene,1 N-
heterocyclic carbenes (NHCs) have become an important class
of ligands in organometallic chemistry.2 The easy preparation
and handling of their precursors, their high modularity, and
their strong σ-donor properties,3 which allow them to form
strong NHC−metal bonds that prevent ligand dissociation,4
have rendered them extremely popular as supporting ligands in
transition metal catalysis. NHC complexes have shown
unprecedented catalytic activity in many important organic
reactions.2,5
Although nickel offers significant potential advantages
compared to its more widely used d10 counterpart, palladium,
including a much lower cost6 and a reduced tendency to
aggregate and agglomerate into less active large metallic
nanoclusters,7 its chemistry with NHC ligands has been
much less studied than that of palladium.2b Nevertheless, the
Special Issue: F. Gordon A. Stone Commemorative Issue
Received: November 9, 2011
Published: February 2, 2012
© 2012 American Chemical Society
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C5H5), Cp* (η5-C5Me5)] half-sandwich complexes are efficient
precatalysts for the fast Suzuki coupling of aryl halides.25,26
These catalysts function without the addition of a reductant or
a cocatalyst, and TOFs of up to 190 h−1, a high rate for Ni(II)
complexes under similar conditions,9,27 have been observed.
The relative dearth of effective, nonpolluting, truly recyclable
and inexpensive catalysts for the syntheses of fine organic
chemicals on large scales28 generates an increasing demand for
supported versions of homogeneous catalysts.29 Among the
possible practical strategies for separating and recycling active
catalysts,30 the immobilization of homogeneous catalysts on
solid supports appears particularly attractive because it should
combine the advantages of heterogeneous catalysis (i.e., easy
product isolation and catalyst recycling) with the versatility of
homogeneous catalysis. Compared to expensive organic
polymers,31 inorganic metal oxide materials are common
supports for the heterogenization of molecular catalysts due
to their excellent thermal and chemical stability.32 In addition,
they possess high surface areas, and organic groups can be
robustly anchored on to the surface hydroxyl groups. As a N-
bonded n-butyl side arm could serve as a model for a three-
carbon linker between a carbene ring and a trialkoxysilane
functional group condensed on the surface of a metal oxide
(Scheme 1), we believed it was of interest (i) to replace one of
triethoxysilylpropyl-substituted derivative, the immobilization
of the latter onto alumina, and the catalytic activity of the
resulting heterogenized catalyst.
RESULTS AND DISCUSSION
■
a. Synthesis of [Ni(R-NHC-nBu)XCp] Complexes. The
Cp complexes [Ni(Me-NHC-nBu)ClCp] (1a), [Ni(iPr-NHC-
nBu)ClCp] (1b), [Ni(Ph-NHC-nBu)ICp] (1c), [Ni(Mes-
NHC-nBu)ICp] (1d), and [Ni(iPr2Ph-NHC-nBu)ICp] (1e)
all bear a dangling n-butyl side arm on their NHC ligands and
were obtained in respectable yields (49−68%) from the
reaction of nickelocene and the imidazolium salts a−e33,34 by
using the standard synthetic methods that were established for
other Ni(NHC)Cp complexes (Scheme 2).35−37 All complexes
were isolated as air-stable dark red to violet solids and were
characterized by 1H and 13C{1H} NMR spectroscopy,
elemental analysis, or electrospray mass spectroscopy, and in
the cases of 1a,b,d,e by X-ray crystallography.
The spectroscopic features of complexes 1a−e are typical of
neutral [Ni(NHC)XCp] species previously characterized by
us35 and others.36,37 The carbene carbon is observed in the
158−168 ppm range in the 13C NMR spectrum, while the Cp
carbon atoms appear at ca. 91−92 ppm, as it is the case for the
other neutral complexes of this type.35−37 The Cp protons
1
resonate as a singlet in the H NMR spectra of 1a and 1b, in
Scheme 1. n-Butyl Side Arm As a Model for a Three-Carbon
Linker between a NHC Ring and a Si(OR)3 Anchoring
Group
the same range as is observed for the Cp protons of the other
known alkyl-substituted complexes [Ni(Me2NHC)ICp]38
(Me2NHC = 1,3-dimethylimidazol-2-ylidene) and [Ni{Me-
NHC-(CH2)2CN}ICp] (Me-NHC-(CH2)2CN = 1-propylni-
trile-3-methylimidazol-2-ylidene),35b at 5.2−5.3 ppm. The Cp
protons of 1c−e appear between 4.8 and 4.9 ppm, as do those
of their mixed alkyl-aryl-substituted analogues,35b,37 and are
slightly downfield of the signals seen between 4.5 and 4.6 ppm
for [Ni(Ar2NHC)CpX] complexes that bear symmetrically
substituted diarylimidazol-2-ylidene ligands.35a,36 While the H
1
the NHC-aryl substituents of our complexes25 by an n-butyl
group and study its influence on the catalytic activity of the
resulting complexes and (ii) if the results were satisfying, to
prepare a trialkoxysilyl-functionalized analogue of the best n-
butyl-substituted catalyst precursor and then heterogenize the
latter.
Herein, we describe the synthesis of a series of cyclo-
pentadienyl [Ni(R-NHC-nBu)XCp] (R = Me, iPr, Ph, Mes,
iPr2Ph; X = Cl, I) and pentamethylcyclopentadienyl [Ni(Ar-
NHC-nBu)ICp*] (Ar = Mes, iPr2Ph) complexes that bear an n-
butyl side chain attached to one of the nitrogen atoms of the
NHC ring and discuss their structural and spectroscopic
features. Their catalytic activity in the Suzuki−Miyaura cross-
coupling of aryl halides with phenylboronic acid is then
presented and compared to the results we have previously
obtained under similar conditions with the related symmetri-
cally substituted [Ni(Ar2NHC)LCp†] species.25 The best TOF
reported for Ni(II) complexes in the absence of reductant and
cocatalyst9,25 was observed with one of the new complexes. We
also present the synthesis and catalytic activity of its
and 13C{1H} NMR spectra of 1a−c are straightforward, it is
noteworthy that 1b and 1c exhibit signals for diastereotopic
NCH2 and NCH2CH2 protons at ambient temperature. The
room-temperature 1H NMR spectra of the more sterically
congested compounds 1d and 1e are more complex and feature
a number of broad signals. For example, the two meta-hydrogen
atoms of the mesityl ring of 1d are isochronous but are
displayed as a broad singlet, while the two ortho-methyl groups
are not equivalent and appear as two broad singlets in a 3:3
relative integrated ratio, and the NCH2 protons are
diastereotopic and appear as two broad signals. At 263 K, all
signals are sharp: the meta-protons of the aromatic ring
resonate as two separate singlets in a 1:1 integrated ratio, and
the three methylene groups appear as three pairs of
nonequivalent multiplets. In the case of 1e, all signals are
sharp at 253 K. At this temperature, the methylene protons
resonate as three pairs of nonequivalent multiplets, the meta-
protons of the aromatic ring appear as two doublets in a 1:1
integrated ratio, and the two isopropyl groups are displayed as
four doublets in a 3:3:3:3 integrated ratio and two septets in a
Scheme 2. Preparation of the Cyclopentadienyl Nickel-NHC Complexes
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Scheme 3. Preparation of the Pentamethylcyclopentadienyl Nickel-NHC Complexes
1:1 relative integrated ratio. We can therefore conclude that
complexes 1d and 1e exist as single rotamers at low
temperature.
Both compounds exhibit diastereotopic NCH2 and
NCH2CH2 protons at ambient temperature, and all signals
are sharp. The two meta-hydrogen atoms and the two ortho-
methyl groups of the mesityl ring of 2d are not equivalent on
the NMR time scale and appear as four singlets in a 1:1 and 3:3
relative integrated ratio. Similarly, the 1H NMR spectrum of 2e
at room temperature displays four doublets and two
pseudoseptets in a 3:3:3:3 and 1:1 relative integrated ratio for
the ortho-isopropyl groups of the aromatic ring, as well as two
doublets in a 1:1 ratio for the meta-hydrogen atoms. Hence, in
contrast to their Cp analogues 1d and 1e, which exist as single
rotamers at low temperature, the sterically congested Cp*
complexes 2d and 2e already exist as single rotamers at ambient
temperature. This tends to confirm that the observed restricted
rotation about the Ni−carbene and N−C bonds in 1d,e and
2d,e are mainly due to steric factors. Nevertheless a minor
electronic component to this increased hindrance to free
rotation in 2d,e cannot be excluded since the downfield shift of
the 13C signal of the carbene carbons of 2d,e may indicate
increased π-back-donation from the more electron-rich nickel
atoms in these complexes.38,43
c. Structural Studies. The structures of [Ni(Me-NHC-
nBu)ClCp] (1a), [Ni(iPr-NHC-nBu)ClCp] (1b), [Ni(Mes-
NHC-nBu)ICp] (1d), [Ni(iPr2Ph-NHC-nBu)ICp] (1e), [Ni-
(Mes-NHC-nBu)ICp*] (2d), and [Ni(iPr2Ph-NHC-nBu)-
ICp*] (2e), each containing an asymmetric NHC ligand with
an n-butyl arm, were established by X-ray diffraction studies.
Crystals suitable for X-ray structure determination were grown
from toluene/pentane solutions at −32 °C for all complexes.
Crystallographic data and data collection parameters are listed
in Table S1 for complexes 1a,b and in Table S2 for complexes
1d,e and 2d,e (see the Supporting Information). Key bond
lengths and bond angles are collected in Table 1 for complexes
1
Similar dynamic behavior and temperature-dependent H
NMR spectra have been previously observed for the very
closely related [Ni(Mes-NHC-nBu)BrCp]37 and [Ni{Mes-
NHC-(CH2)4CN}ICp]35b (Mes-NHC-(CH2)4CN = 1-(2,4,6-
trimethylphenyl)-3-(butylnitrile)imidazol-2-ylidene) com-
plexes, as well as for the symmetrically substituted and sterically
congested Cp* complex [Ni(Mes2NHC)ClCp*] (Mes2NHC =
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene),38 and have
been attributed to restricted rotation about the Ni−carbene and
N−C bonds. The origin of these restricted rotations is not
obvious, and hydrogen bonding between the halide and one of
the NCH2 protons has been invoked as a possible explanation
for [Ni(Mes-NHC-nBu)BrCp].37 However, such restricted
rotation is not observed in the closely related analogue
[Ni{Mes-NHC-(CH2)3CN}ClCp] (Mes-NHC-(CH2)3CN =
1-(2,4,6-trimethylphenyl)-3-(propylnitrile)imidazol-2-yli-
dene),39 which bears a small but more electronegative chloride
instead of a more bulky bromide or iodide, and with
[Ni(Mes2NHC)ClCp],36,40 which bear the less bulky Cp
ligand instead of the large Cp*. Furthermore, there are no
especially short H···X interactions in any of the solid-state
structures reported here (vide infra). Thus, in the cases of 1d
and 1e, it is probably the presence of the voluminous iodide
and its steric interactions with the Ar-NHC-nBu and Cp groups
that hamper the free rotation about the Ni−carbene and N−C
bonds. As regards the Ni−carbene bond in particular, a minor
electronic component cannot be excluded since a number of
studies show that π-bonding with NHC ligands may be far from
negligible.41
b. Synthesis of [Ni(Ar-NHC-nBu)ICp*] Complexes. The
Cp* complexes [Ni(Mes-NHC-nBu)ICp*] (2d) and [Ni-
(iPr2Ph-NHC-nBu)ICp*] (2e) were obtained from the
reaction of in situ prepared [Ni(acac)Cp*]42 with the
imidazolium iodides d and e by using the procedure recently
developed in our laboratory (Scheme 3).38 Both complexes
were isolated as dark red crystals in moderate yields (30−34%).
Good spectroscopic data were obtained, and both molecular
structures were established by X-ray diffraction studies.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 1a and 1b with Esd’s in Parentheses
1a
1b (molecule A)
1b (molecule B)
Ni−C(1)
Ni−Cl
Ni−Cpcent
1.878(2)
2.1947(6)
1.768
1.887(5)
2.1926(15)
1.769
1.863(5)
2.1859(18)
a
b
1.759
c
b
Ni−CCp av
2.126
2.133
2.137
The spectroscopic features of complexes 2d and 2e are
similar to those of the neutral [Ni(NHC)XCp*] species
previously characterized by us.38 Thus, in their 13C NMR
spectra, the signals of the carbene carbons are downfield of the
signals seen for their corresponding Cp derivatives 1d,e and are
observed in the 175−178 ppm range. The Cp* carbon atoms
appear at ca. 101−102 (C5Me5) and 10−11 (C5Me5) ppm, as is
C(1)−Ni−Cl
C(1)−Ni−Cpcent
Cl−Ni−Cpcent
95.26(7)
132.8
93.66(16)
133.8
93.86(17)
134.2
131.9
132.4
b
131.9
a
Cpcent = centroid of the Cp group. The Cp ring in molecule B is
rotationally disordered over two equally populated sites, and the
positional esd's are relatively large. The centroid and the average Ni−
c
Ccp distance is based on all 10 Cp carbons. Average Ni−C distance to
the case for the other complexes of this type.38 In their H
1
the Cp ring.
NMR spectra, the Cp* protons resonate as a singlet at ca. 1.3−
1.5 ppm and are located in between the signals seen at ca. 1.1−
1.2 ppm for [Ni(Ar2NHC)ClCp*] complexes and at 1.60 ppm
for [Ni(Me2NHC)ICp*].38
1a,b and in Table 2 for complexes 1d,e and 2d,e. Complex 1b
crystallizes in the P21/c space group with two independent
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 1d, 1e, 2d, and 2e with Esd's in Parentheses
1d
1e
2d
2e
Ni−C(1)
Ni−I
1.886(4)
2.5222(7)
1.761
1.884(3)
2.5099(5)
1.759
1.891(3)
2.5187(5)
1.761
1.888(6)
2.5374(9)
1.765
a
Ni−Cp†
cent
b
Ni−CCp† av
C(1)−Ni−I
C(1)−Ni−Cpcent
I−Ni−Cpcent
2.124
2.131
2.139
2.138
97.28(13)
134.8
95.53(10)
134.7
96.43(11)
135.7
95.18(19)
137.0
127.9
129.7
127.5
127.1
a
b
Cp†cent = centroid of the Cp or Cp* group. Average Ni−C distance
to the Cp or Cp* ring.
molecules (A and B) in the unit cell; one of these (molecule A)
exhibits some disorder of the n-butyl arm and the other
(molecule B) some rotational disorder of the Cp* ligand, but
the general geometric features of both molecules are similar.
Figure 1 shows molecule A of 1b with only one position of the
Figure 2. Molecular structures of [Ni(Mes-NHC-nBu)ICp] (1d) and
of [Ni(iPr2Ph-NHC-nBu)ICp] (1e) showing all non-H atoms.
Ellipsoids are shown at the 50% probability level, and key atoms are
labeled.
Figure 1. Molecular structures of [Ni(Me-NHC-nBu)ClCp] (1a) and
of [Ni(iPr-NHC-nBu)ClCp] (1b) showing all non-H atoms. Ellipsoids
are shown at the 50% probability level, and key atoms are labeled.
Only molecule A of 1b, with one position of the n-butyl arm, is shown.
n-butyl arm. The structure of 1a is also depicted in Figure 1.
The molecular structures of 1d and 1e are shown in Figure 2,
and those of complexes 2d and 2e in Figure 3. Note that
complex 2e exhibits some disorder of the n-butyl arm: only one
position is depicted in Figure 3.
The molecular structures of these complexes are closely
related to each other and to those established for similar
[Ni(NHC)XCp†] complexes,35b,36−39 in particular for the
asymmetrically substituted [Ni{Me-NHC-(CH2)2CN}ICp]35b
and [Ni{Mes-NHC-(CH2)3CN}ClCp].35b,39 The nickel atom
is bonded to a η5-Cp (1a,b,d,e) or Cp* (2b,d) group, a NHC
Figure 3. Molecular structures of [Ni(Mes-NHC-nBu)ICp*] (2d) and
of [Ni(iPr2Ph-NHC-nBu)ICp*] (2e) showing all non-H atoms.
Ellipsoids are shown at the 50% probability level, and key atoms are
labeled. Only one position of the n-butyl arm of 2e is shown.
moiety, and a halide ligand in all molecules; if one consider the
Cp or Cp* group as a single ligand, the metal lies at the center
of a pseudotrigonal plane. The sum of the angles subtended at
the nickel atom by the Cp or Cp* ligand centroid, the halogen
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Table 3. Suzuki−Miyaura Cross-Coupling of 4′-Bromoacetophenone with Phenylboronic Acid Catalyzed by Complexes 1a−2e,
a
1d-TES, and 1d-Al
b
b
entry
cat. (mol %)
t (min)
conversion (%)
A/B ratio
c
1
1a (1)
60
60
60
60
60
60
15
60
60
60
60
60
81
80/20
80/20
c
2
1b (1)
87
de
,
f
3
1d (3)
78
86
91
99
88
53
88
88
97
43
B n.d.
d
d
c
f
4
1d (3)
B n.d.
f
5
1d (1)
B n.d.
f
6
1d (1)
B n.d.
c
f
7
1d (1)
B n.d.
c
f
8
1d (0.5)
1e (3)
B n.d.
d
d
c
f
9
B n.d.
f
10
11
12
2e (1)
B n.d.
f
1d-TES (1)
1d-Al (3)
B n.d.
c
f
B n.d.
a
Reaction conditions: 4′-bromoacetophenone (1.0 mmol), phenylboronic acid (1.3 mmol), K3PO4 (2.6 mmol), catalyst (1−3 mol %), toluene (3.0
b
c
d
mL), 110 °C. NMR yield, average value of three runs. Schlenk tube loaded in a glovebox with H2O < 0.5 ppm, O2 < 0.5 ppm. Reaction run under
Schlenk tube techniques. Run at 90 °C. n.d. = not detected.
e
f
atom, and the carbene carbon atom of the NHC ligand is
indeed almost exactly 360°. There are, however, significant
departures from the idealized 120° angles of a trigonal-planar
structure for all six molecules (Tables 1 and 2). The carbenoid
carbon C(1) and the halide Cl (1a,b) or I (1d,e and 2d,e)
subtend an angle of 95.5 2° at the nickel atom. These values
are in the range of those observed for the closely related
[Ni{Me-NHC-(CH2)2CN}ICp]35b and [Ni{Mes-NHC-
(CH2)3CN}ClCp],35b,39 for which values of 93.4(2)°/92.9(2)
° and 98.41(9)° have been determined, respectively.
The nickel−carbene bond lengths lie in the narrow ranges of
1.863(5)−1.886(6) Å for the complexes with R = alkyl and X =
Cl (1a,b, Table 1) and of 1.884(3)−1.891(4) Å for the
complexes with R = aryl and X = I (1d,e and 2d,e, Table 2),
and are therefore not significantly different from each other.
These values are comparable to those reported for [Ni{Me-
NHC-(CH2)2CN}ICp] (1.884(6)/1.866(7) Å)35b and [Ni-
{Mes-NHC-(CH2)3CN}ClCp] (1.882(3) Å),35b as well as for
[Ni(NHC)XCp*] complexes (1.880(4)−1.906(3) Å).38 The
Ni−Cl distances of 2.1947(6) Å in 1a and of 2.1926(6)/
2.1859(18) Å in 1b are close to the Ni−Cl distances observed
in related [Ni(NHC)ClCp] complexes.35b,36a−c,37 The Ni−I
distances in 1d and 1e (2.5222(7) and 2.5099(5) Å) are
comparable to those observed in the more sterically congested
Cp* complexes 2d and 2e (2.5187(5) and 2.5374(9) Å) and to
those reported for [Ni(Me2NHC)ICp],38 [Ni{Me-NHC-
(CH2)2CN}ICp],35b and [Ni(Me2NHC)ICp*],38 where values
of 2.5006(6), 2.5083(10)/2.4979(10), and 2.5212(5) Å were
observed, respectively.
of these complexes and thus no evidence of any intramolecular
hydrogen bonding. In addition there is no evidence of any
intermolecular hydrogen bonding in any of the unit cells. The
crystal structure of 1e exhibits unusually large channels that
nevertheless appear to be void of potential solvent molecules.
d. Suzuki−Miyaura Cross-Coupling of Aryl Halides.
The catalytic activities of the new half-sandwich nickel(II)
complexes 1a−2e, which bear an n-butyl arm on their NHC
ligand, were examined for the coupling of 4′-bromoacetophe-
none with phenylboronic acid under the standard conditions
established with their symmetrically substituted analogues
[Ni(Ar2NHC)LCp†],25 i.e., with 1.3 equiv of PhB(OH)2 and
2.6 equiv of K3PO4 in toluene at 90−110 °C (Table 3). No
additive such as PPh3 was necessary, as is often the case in other
systems.9c−f,27a,c All complexes are catalytically active and give
the desired product 4-acetylbiphenyl in good to excellent yield
in 15 to 60 min, which are fast reaction times for nickel-
catalyzed Suzuki couplings. Typical reaction times, indeed,
usually range under similar conditions from 1−2 h at best to
12−24 h at worst.9,27
Relatively good conversion to the coupling product was
observed with 3 mol % of 1d at 90 °C (Table 3, entry 3).
Increasing the temperature to 110 °C slightly improved the
yield (entry 4), but more interestingly, decreasing the catalyst
loading 1 mol % did not affect the reaction and 91% conversion
could be observed after 60 min with 1d (entry 5). This is in
sharp contrast to what had been observed with the symmetri-
cally substituted analogues [Ni(Ar2NHC)LCp†], which bear a
small chloride or acetonitrile ligand instead of a voluminous
iodide, and for which decreasing the catalyst loading led to
decreased conversions, thereof indicating fast catalyst deactiva-
tion.25 Nevertheless, it is worth mentioning that significant
yield variations were observed between each of the three
catalytic runs that were conducted for these table entries
(entries 3−5). Since these reactions had been conducted by
using standard Schlenk techniques, we presumed that these
variations were due to small amounts of O2, which was entering
the catalytic reactor when the solvent was added (see
Experimental Section) and which was progressively deactivating
the highly air-sensitive active species. To prevent such O2
In all six structures, the η5-C5R5 rings exhibit structural
distortions with significant variations in both the CCp†−CCp†
and Ni−CCp† distances. Such variations have been occasionally
observed in other Ni(η5-C5R5) systems and arise from “allyl-
ene”, “diene”, or intermediate distortions (between “ally-ene”
and “diene”) of the η5-C5R5 ring.44,45 Thus, “diene” distortions
are observed in complexes 1a, 1b, 1e, 2d, and 2e, and
intermediate distortions are seen in complex 1d (see Scheme
S1 in the Supporting Information).
In contrast to what has been observed in some related
complexes,37 there are no short intramolecular contacts in any
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contamination, we then decided to load the catalytic reactors in
a glovebox with H2O and O2 levels below 0.5 ppm (see
Experimental Section). Under these conditions, no irregular
yield variations were observed. Indeed, we even observed
significant improvements. Thus, with a catalyst loading of 1 mol
% of 1d, the coupling reaction was almost quantitative after 60
min (entry 6), and a conversion of 88% was observed after only
15 min of reaction (entry 7), leading to a turnover frequency
(TOF) of 352 h−1. This is even better than the TOF of 190 h−1
we recently reported with [Ni{(iPr2Ph)2NHC}(NCMe)Cp*]-
(PF6) [(iPr2Ph)2NHC = 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene]25 and is, to our knowledge, the best TOF
observed so far in Suzuki−Miyaura cross-coupling catalyzed by
a Ni(II) complex in the absence of reductant and
cocatalyst.9,25,27,46 Nevertheless, we note that further decreasing
the catalyst loading to 0.5 mol % led to a conversion of only
53% after 60 min (entry 8), which indicates that the limits of
our system in terms of turnovers lie close to the 1 mol % range.
In contrast to [Ni{(iPr2Ph)2NHC}ClCp], which bears bulky
iPr2Ph groups and gave better results than [Ni(Mes2NHC)-
ClCp],25 no improvement was observed with 1e compared to
1d (entry 9 vs entry 4). In addition, in contrast to the bulky,
electron-rich [Ni(Ar2NHC)LCp*] complexes, which gave
much better results than their Cp analogues,25 no enhanced
catalysis was registered with the Cp* complex 2e (entry 10 vs
entry 5). Finally, a greatly decreased selectivity was noted with
the small species, 1a and 1b, that bear an alkyl instead of an aryl
substituent on the NHC ring, and a chloride instead of an
iodide on the nickel atom. Significant amounts of the
homocoupling product, 4,4′-diacetylbiphenyl, were indeed
formed with these complexes (entries 1 and 2).
Table 4. Suzuki−Miyaura Cross-Coupling of Aryl Chlorides
and Bromides with Phenylboronic Acid Catalyzed by 1d and
a
1e
a
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.3
A few comments concerning the reaction mechanism can be
made. We have suggested25 that the active catalytic species is a
Ni(0) complex that is generated by initial reduction of the
Ni(II) precursor by homocoupling of phenylboronic acid.27,47
Such initial reduction to a highly air-sensitive Ni(0) species is
further corroborated by the observed catalyst deactivation when
small amounts of O2 accidentally entered the Schlenk tubes.
Also, in our previous report, the absence of a significant
reactivity difference between the neutral [Ni(Ar2NHC)ClCp†]
complexes and the cationic [Ni(Ar2NHC)(NCMe)Cp†](PF6)
species, which bear labile acetonitrile ligands, suggested that the
necessary generation of a vacant site might arise through Cp or
Cp* ring slippage rather than halide or acetonitrile dissocia-
tion.25,48 The significant stabilization of the active species with
the bulky iodide-containing catalyst precursors 1d−2e tends to
confirm this hypothesis. The n-butyl arm indeed seems too
small and too far from the metal center to exert such an
influence. Moreover, the absence of improvement with 1e and
2e (with respect to 1d) shows that neither a possible faster
reduction from the starting Ni(II) complex to the active Ni(0)
species (as may be expected with the more electron-rich Cp*
species) nor an even better steric protection of the active center
(as expected with a iPr2Ph substituent instead of a Mes
substituent and/or a Cp* ligand instead of a Cp ligand) seems
to play a significant role anymore in the presence of the highly
protecting iodide ligand.
mmol), K3PO4 (2.6 mmol), 1d or 1e (1−3 mol %), toluene (3 mL),
b
c
d
e
110 °C, 1 h. NMR yield. Isolated yield. 22% of biphenyl. 12% of
f
g
dehalogenation product. GC-MS yield. 2 h of reaction.
chloroacetophenone with both catalysts (entries 1 and 2).
The electron-donating substrates 4-bromotoluene and 4-
chlorotoluene were however converted to the desired coupling
products in lower yields (entries 3 and 4), and even lower
yields were observed with the sterically hindered 2-bromo- and
2-chlorotoluene (entries 5 and 6), which is rather disappointing
since other Ni(II)-based catalysts convert these substrates very
efficiently.9,27 Nevertheless, good conversion was obtained for
the cross-coupling of bromonaphthalene with phenylboronic
acid (entry 9). In the case of 4′-bromoaniline (entry 7), side
reactions occurred in addition to the desired coupling, as large
amounts of biphenyl (resulting from the homocoupling of
phenylboronic acid) and of aniline (resulting from the
dehalogenation of 4′-bromoaniline) were detected. Finally, no
reaction occurred with the electronically deactivated 1-bromo-
4-nitrobenzene (entry 8).
This short series of electronically activated and deactivated
aryl halides shows that the reaction scope of the [Ni(Ar-NHC-
n-Bu)ICp] catalyst precursors 1d and 1e is of the same order as
that of the related symmetric complexes [Ni(Ar2NHC)LCp*]
25
−
(Ar = Mes, L = Cl; Ar = iPr2Ph, L = NCMe, PF6 ) species.
Thus, a decreased activity was noticed for both types of
complexes with the electron-donating substrates. Nevertheless,
compared to the active species generated by the symmetric
catalysts [Ni(Ar2NHC)LCp*] bearing either a chloride or an
acetonitrile ligand, those formed by the n-butyl compounds
1d,e, bearing a voluminous iodide ligand, are much more stable
In view of these unexpected but interesting results, some
aspects of the catalytic potential of the new [Ni(Ar-NHC-
nBu)ICp] catalyst precursors 1d,e were examined with a short
series of aryl bromides and chlorides bearing electron-
withdrawing and electron-donating substituents (Table 4).
Good yields of 4-acetylbiphenyl were obtained from 4′-
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and allow the reduction of the catalyst loading, which is crucial
in the perspective of a heterogenized version. Moreover, these
species are able to catalyze Suzuki−Miyaura cross-coupling in
the absence of additives, which is also a key point for a potential
heterogenized version of this catalyst.
broad singlet, and the NCH2 protons as one broad signal. Thus,
in comparison to 1d, which bears a bulky iodide ligand, the
triethoxysilyl-functionalized complex 1d-TES, which carries a
smaller chloride ligand, shows greatly reduced rotational
barriers at room temperature. This result confirms the prevalent
role played by the voluminous iodide ligand in complexes 1d,e
in hampering free rotation about the Ni−carbene and N−C
bonds. However, since the very closely related complex
[Ni{Mes-NHC-(CH2)3CN}ClCp],35b,39 which bears an alkyl-
nitrile side arm, and the symmetric compound [Ni-
(Mes2NHC)ClCp]36 do not show any rotational barrier at all
on the NMR time scale at room temperature, this also
demonstrates that despite its far-off position to the nickel atom,
the bulky dangling triethoxysilylpropyl arm is noninnocent with
respect to the dynamic processes in 1d-TES and may to some
extent play a protecting role. Thus, although 1d-TES does not
carry a protecting iodide ligand, we still decided to investigate
its catalytic activity. Satisfyingly, 1d-TES also shows good
activity for the coupling of phenylboronic acid with 4′-
bromoacetophenone; 97% conversion to 4-acetylbiphenyl was
observed after 60 min of reaction at 110 °C with a catalyst
loading of 1 mol % (Table 3, entry 11). In light of these results,
we decided to heterogenize it and study the catalytic activity of
the resulting supported complex.
f. Immobilization of [Ni(Mes-NHC-TES)ClCp] (1d-TES)
onto Alumina and Catalytic Activity of the Hetero-
genized Complex. The complex [Ni(Mes-NHC-TES)ClCp]
(1d-TES) was covalently grafted onto alumina by condensation
of the triethoxysilyl groups with the surface hydroxyls by
refluxing 1d-TES in the presence of alumina in toluene for 4 h.
The alumina-tethered complex 1d-Al was obtained as a pink
solid, which was characterized by standard solid-state spectros-
copies (see the Experimental Section). The data suggested that
complex 1d-TES was intact and had been successfully
immobilized onto the alumina surface.
e. Synthesis and Catalytic Activity of Trialkoxysilyl-
propyl-Substituted Complexes. Since the presence of an
iodide ligand bound to the nickel atom seemed crucial for a
stable catalytically active species, initial studies focused on the
syntheses of the iodo complexes [Ni(Ph-NHC-TMS)ICp] (1c-
TMS), [Ni(Mes-NHC-TMS)ICp] (1d-TMS), and [Ni(iPr2Ph-
NHC-TMS)ClCp] (1e-TMS). These were obtained by direct
reaction, in refluxing DME, of nickelocene with 1-phenyl-3-[3-
(trimethoxysilyl)propyl]imidazolium iodide,33 1-(2,4,6-trime-
thylphenyl)-3-[3-(trimethoxysilyl)propyl]imidazolium iodide,33
or 1-(2,6-diisopropylphenyl)-3-[3-(trimethoxysilyl)propyl]-
imidazolium iodide, respectively.33 Unfortunately, purification
of the trimethoxysilyl-functionalized 1c-, 1d-, and 1e-TMS
complexes proved to be very difficult, as neither crystallization
nor column chromatography could be employed for these
species. The complexes remained immobilized onto the silica or
alumina by condensation of the trimethoxysilyl groups with the
1
surface hydroxyl groups. Thus, their H NMR spectra always
revealed important traces of the ligand precursor and of several
hydrolysis products.
Consequently, subsequent studies focused on the synthesis
of triethoxysilyl-functionalized complexes, which proved to be
more robust toward hydrolysis than trimethoxysilyl-substituted
compounds. Thus, reaction of nickelocene with 1-(2,4,6-
trimethylphenyl)-3-[3-(triethoxysilyl)propyl]imidazolium
chloride (d-TES)49 in DME at reflux afforded [Ni(Mes-NHC-
TES)ClCp] (1d-TES), which was isolated as a violet oil of
satisfying purity in 71% yield after column chromatography
1
(Scheme 4). The new complex was characterized by H and
13C{1H} NMR spectroscopy and electrospray mass spectros-
copy.
The catalytic activity of the immobilized complex 1d-Al was
next evaluated for the coupling of 4′-bromoacetophenone with
phenylboronic acid under the standard conditions established
for its nongrafted analogues. The desired product, 4-
acetylbiphenyl, was obtained in only 43% yield after 1 h of
reaction with 3 mol % of 1d-Al (Table 3, entry 12). This
conversion is markedly lower than those observed with 1−3
mol % of the nongrafted analogues 1d−2e, which bear an
iodide ligand (Table 3, entries 3−10). It is also much lower
than that observed with 1d-TES, which does not bear an iodide
ligand but bears the very bulky dangling triethoxysilylpropyl
arm (Table 3, entry 11). The disappointing activity observed
with 1d-Al may thus be attributed to the instability of the active
species generated by 1d-Al in the absence of any protecting
group; 1d-Al indeed does not bear an iodide ligand, and the
bulky dangling anchoring arm of 1d-TES may have lost its
(supposed) protecting properties in 1d-Al, as it is now
condensed to the alumina surface and is therefore less mobile.
However, the activity of 1d-Al is also lower than that observed
for [Ni(Mes2NHC)ClCp], for which 68% conversion were
observed after 30 min of reaction with a catalyst loading of 3
mol %.25 Moreover, despite a better selectivity, it is also lower
than what is seen for the alkyl-substituted and chloride-
containing species 1a,b (Table 3, entries 1 and 2). This
disappointing activity may thus also be attributed (at least
partially) to the heterogenization itself, which may render the
active sites more difficult to reach by the substrates.
Scheme 4. Preparation of the Triethoxysilyl-Functionalized
Nickel-NHC Complex 1d-TES
1
The H and 13C{1H} NMR spectra of complex 1d-TES at
ambient temperature present no anomalies and show the
presence of one η5-Cp group and of the functionalized NHC
ligand. The carbene carbon is observed at 162.9 ppm in the 13
C
NMR spectrum, while the Cp carbon atoms appear at 91.8
1
ppm. The Cp protons resonate at 4.72 ppm in the H NMR
spectrum. These values are close to those observed for its n-
butyl-substituted analogue 1d (vide supra). Nevertheless, in
1
contrast to the room-temperature H NMR spectrum of 1d, in
which the two meta-hydrogen atoms of the mesityl ring are
equivalent on the NMR time scale and are displayed as one
broad singlet, the two ortho-methyl groups appear as two broad
singlets in a 3:3 relative integrated ratio, and the NCH2 protons
are diastereotopic, the room-temperature 1H NMR spectrum of
1d-TES displays the two meta-hydrogen atoms of the mesityl
ring as a sharp singlet, the two ortho-methyl groups as one
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made with the aid of DEPT 13C spectra for all complexes and 1H/13C
HSQC correlation for complex 2e. The chemical shifts are referenced
to residual deuterated solvent peaks. Chemical shifts (δ) and coupling
constants (J) are expressed in ppm and Hz, respectively. The 1H NMR
variable-temperature experiments with complexes 1d and 1e were
recorded at 400 MHz in CDCl3 from 298 to 253 K. Electrospray mass
spectra were recorded on a MicrOTOF 66 mass spectrometer.
Elemental analyses were performed by the Service d’Analyses, de
CONCLUSION
■
Direct reaction of imidazolium salts bearing a N-bound n-butyl
or triethoxysilylpropyl arm with nickelocene or in situ prepared
[Ni(acac)Cp*] allowed the preparation of the corresponding
[Ni(R-NHC-nBu)XCp] (1a−e), [Ni(Ar-NHC-nBu)ICp*]
(2d,e), and [Ni(Mes-NHC-TES)ClCp] (1d-TES) complexes
in moderate to good yields. The latter was grafted onto
alumina.
Mesures Physiques et de Spectroscopie Optique, UMR CNRS 7177,
1
Institut de Chimie, Universite
NMR were performed by the Unite
Solide, UMR CNRS 8181, Universite
́
de Strasbourg. DRIFT and H MAS
Single-crystal X-ray diffraction studies of complexes 1a,b,d,e
and 2d,e revealed that these compounds have geometric
features common to all [Ni(R2NHC)XCp†] compounds.
Nevertheless, 1H NMR data of the iodide-containing complexes
[Ni(Ar-NHC-nBu)ICp†] 1d,e and 2d,e revealed restricted
rotation about the Ni−carbene and N−C bonds at room
temperature. The origin of these hampered rotations is believed
to be mainly steric in nature and is attributed to the presence of
the voluminous iodide ligand. Accordingly, in contrast to 1d, its
triethoxsilylpropyl analogue, [Ni(Mes-NHC-TES)ClCp] (1d-
TES), which carries a smaller chloride ligand, shows greatly
reduced (but not equal to zero) rotation barriers at room
temperature.
́
de Catalyse et de Chimie du
de Lille I, France. Vibrational
́
frequencies are expressed in cm−1. ICP-AES was performed by the
Laboratoire de Chimie Analytique et Miner
Institut Pluridisciplinaire Hubert Curien, Universite
́
ale, UMR CNRS 7178,
de Strasbourg.
́
Commercial compounds were used as received. The imidazolium salts
a,33 b,34 c−e,33 and c, d, e-TMS,33 nickelocene,35a and [Ni(acac)-
Cp*]38 were prepared according to published procedures. The
imidazolium salt d-TES was prepared according to a slightly modified
version of microwave-assisted procedures that have been reported for
the synthesis of other silane-substituted imidazolium halides.49
Synthesis of [Ni(Me-NHC-nBu)ClCp] (1a). Nickelocene (553
mg, 2.93 mmol) and 1-butyl-3-methylimidazolium chloride a (339 mg,
1.94 mmol) were refluxed for 4 h in thf (20 mL). The solvent was then
removed in vacuo, the residue was extracted with hot toluene (20 mL),
and the extract was filtered through Celite, which was subsequently
rinsed with toluene (3 × 5 mL) until the washings were colorless. The
resulting solution was then concentrated under vacuum and passed
though a neutral silica chromatography column and eluted with ethyl
acetate/pentane (8.5:1.5) to give a red solution. After solvent removal,
crystallization from toluene/pentane (1:2) at −32 °C gave 1a (282
mg, 0.948 mmol, 49%) as dark red needles. ES-MS: m/z [M]+ calcd
The new complexes [Ni(Ar-NHC-nBu)ICp†] 1d,e and 2e
efficiently catalyze the cross-coupling of phenylboronic acid
with aryl halides in the absence of cocatalysts or reductants.25
Moreover, in contrast to the results observed with their
symmetric analogues [Ni(Ar2NHC)LCp†] (L = Cl−; NCMe,
25
−
PF6 ), no substantial activity difference was observed
between the Cp complexes 1d,e and their Cp* counterpart
2e, and a TOF of up to 352 h−1 was even observed with the Cp
complex 1d. The significant stabilization of the active species,
which allowed this very high TOF for a nickel(II) complex
under similar conditions, is attributed to the presence of the
voluminous iodide ligand, which would play a highly protecting
role.
1
for C13H19N2Ni 261.090, found 261.089. H NMR (CDCl3, 300.13
MHz): δ 6.91 (s, 2H, NCH), 5.22 (s, 5H, C5H5), 4.75 (br, 2H,
NCH2), 4.33 (s, 3H, NCH3), 1.94 (br, 2H, NCH2CH2), 1.51 (m, 2H,
3
CH2CH3), 1.05 (t, 3H, CH3, J = 7.4). 13C{1H} NMR (CDCl3, 75.47
MHz): δ 159.8 (NCN), 123.5 (NCH), 121.8 (NCH), 91.7 (C5H5),
51.7 (NCH2), 38.9 (NCH3), 33.2 (NCH2CH2), 20.2 (CH2CH3), 14.0
(CH3).
Despite the absence of a protecting iodide ligand, the
triethoxsilylpropyl-substituted complex 1d-TES is as efficient as
1d. This surprisingly high activity with respect to what is
observed with other chloride-containing complexes25 may be
attributed to the presence of the bulky dangling triethoxsilyl-
propyl arm. This group may indeed play a protecting role, as
suggested by its noninnocent behavior with respect to the
dynamic processes in 1d-TES. Accordingly, the activity drop
observed with the alumina-tethered compound 1d-Al may be
attributed, at least partially, to the absence of a protecting
group.
Thus, our quest to heterogenize half-sandwich NHC-
nickel(II) complexes [Ni(NHC)LCp†] onto a metal oxide
and to evaluate the effect of the substitution of one aryl
substituent on the NHC ring by an n-butyl arm led us to the
serendipidous discovery of the important role that may be
played by the “spectator” ligand L or a bulky dangling NHC
side arm in protecting the active species.
Synthesis of [Ni(iPr-NHC-nBu)ClCp] (1b). Nickelocene (176 mg,
0.932 mmol) and 1-isopropyl-3-butylimidazolium chloride b (189 mg,
0.932 mmol) were refluxed in DME (9 mL) for 2 h. The resulting dark
red solution was cooled to room temperature, and the solvent
removed in vacuo. Column chromatography over neutral silica with
diethyl ether as eluent afforded 1b (175 mg, 0.538 mmol, 58%) as dark
red crystals after solvent evaporation. ES-MS: m/z [M]+ calcd for
C15H23N2Ni 289.121, found 289.122. 1H NMR (CDCl3, 300.13
3
3
MHz): δ 6.96 (d, 1H, NCH, J = 2.1), 6.94 (d, 1H, NCH, J = 2.1),
3
6.47 (septet, 1H, NCHMe2, J = 6.8), 5.22 (s, 5H, C5H5), 4.90 and
4.70 (2 m, 2 × 1H, NCH2), 2.01 and 1.88 (2 m, 2 × 1H, NCH2CH2),
1.52 (d + m, 6 H+ 2H, NCHMe2 and CH2CH3), 1.06 (t, 3J = 7.5, 3H,
CH3). 13C{1H} NMR (CDCl3, 75.47 MHz): δ 158.2 (NCN), 122.2
(NCH), 118.1 (NCH), 91.8 (C5H5), 53.8 (NCHMe2), 51.7 (NCH2),
33.3 (NCH2CH2), 23.8 (NCHMe2), 20.3 (CH2CH3), 14.0 (CH3).
Synthesis of [Ni(Ph-NHC-nBu)ICp] (1c). Nickelocene (123 mg,
0.651 mmol) and 1-phenyl-3-butylimidazolium iodide c (213 mg,
0.649 mmol) were refluxed for 16 h in DME (12 mL). The solution
slowly turned from dark green to dark red. The solvent was then
removed in vacuo, the residue was extracted with toluene (10 mL), and
the extract was filtered through Celite, which was rinsed with toluene
until the washings were colorless. The resulting solution was
concentrated to dryness and recrystallized from toluene/pentane
(1:4) to afford 1c (194 mg, 0.430 mmol, 66%) as red crystals, which
were washed with pentane (3 × 3 mL) and dried under vacuum. Anal.
Calcd for C18H21IN2Ni: C, 47.94; H, 4.69; N, 6.21. Found: C, 48.00;
H, 4.82; N, 6.06. 1H NMR (CDCl3, 300.13 MHz): δ 8.07 (d, 2H, Ph),
7.57 (m, 3H, Ph), 7.15 (d, 1H, NCH, 3J = 1.9), 7.11 (d, 1H, NCH, 3J =
1.9), 5.17 (m, 1H, NCH2), 4.90 (s, 5H, C5H5), 4.58 (m, 1H, NCH2),
2.01 and 1.91 (2 m, 2 × 1H, NCH2CH2), 1.55 (m, 2H, CH2CH3), 1.07
EXPERIMENTAL SECTION
■
General Comments. All reactions were carried out using standard
Schlenk or glovebox techniques under an atmosphere of dry argon.
Solvents were distilled from appropriate drying agents under argon
prior to use. Unless otherwise specified, solution NMR spectra were
recorded at 298 K on FT-Bruker Ultra Shield 300 and FT-Bruker
Spectrospin 400 spectrometers operating at 300.13 or 400.14 MHz for
1H and at 75.47 or 100.61 MHz for 13C{1H}. A 1H 2D COSY
spectrum was obtained for complex 2e to help in the 1H signal
assignments. The assignments of the 13C{1H} NMR spectra were
3
(t, 3H, CH3, J = 7.3). 13C{1H} NMR (CDCl3, 75.47 MHz): δ 165.9
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3
(NCN), 140.9 (ipso-CAr), 129.1 and 126.5 (o-/m-CAr), 128.6 (p-CAr),
123.7 and 122.6 (NCH), 92.0 (C5H5), 53.2 (NCH2), 32.9
(NCH2CH2), 20.3 (CH2CH3), 14.1 (CH3).
CH2CH3), 0.94 (t, 3H, CH3, J = 7.2). 13C{1H} NMR (C6D6, 75.47
MHz): δ 175.4 (NCN), 138.1 and 138.0 (o-CAr), 136.6 and 134.6
(ipso-/p-CAr), 130.1 and 128.3 (m-CAr), 124.6 and 119.9 (NCH),
101.3 (C5Me5), 53.7 (NCH2), 32.3 (NCH2CH2), 23.7 (o-Me), 20.6 (p-
Me), 20.3 (CH2CH3), 18.4 (o-Me), 13.8 (CH3), 10.7 (C5Me5).
Synthesis of [Ni(iPr2Ph-NHC-nBu)ICp*] (2e). To a freshly
prepared suspension of [Ni(acac)Cp*]38 (1.00 mmol) in thf (10
mL) was added 1-(2,6-diisopropylphenyl)-3-butylimidazolium iodide e
(323 mg, 0.783 mmol) in a single portion, and the reaction mixture
was stirred at reflux for 3 h. The resulting red-violet suspension was
cooled to room temperature, and the solvent removed in vacuo. The
residue was extracted with toluene (10 mL) and filtered through silica,
which was rinsed with toluene (3 × 10 mL) until the washings were
colorless. Concentration to ca. 1 mL followed by addition of pentane
(10 mL) and crystallization at −32 °C afforded 2e (160 mg, 0.264
mmol, 34%) as a dark red solid, which was washed with pentane (3 ×
3 mL) and dried under high vacuum. Anal. Calcd for C29H43IN2Ni: C,
Synthesis of [Ni(Mes-NHC-nBu)ICp] (1d). Nickelocene (380 mg,
2.01 mmol) and 1-(2,4,6-trimethylphenyl)-3-butylimidazolium iodide
d (753 mg, 2.03 mmol) were refluxed for 20 h in DME (30 mL). The
solution slowly turned from dark green to dark red. The solvent was
then removed in vacuo, and unreacted nickelocene was extracted with
pentane (3 × 5 mL). Column chromatography over neutral silica with
diethyl ether/pentane (7:3) and then toluene as eluents afforded 1d
(612 mg, 1.24 mmol, 62%) as a deep red-violet solid after solvent
evaporation. ES-MS: m/z [M]+ calcd for C21H27N2Ni 365.1522, found
365.1503. 1H NMR (CDCl3, 263 K, 400.14 MHz): δ 7.16 (s, 2H, m-H
and NCH), 6.99 (s, 1H, m-H), 6.87 (s, 1H, NCH), 5.47 (m, 1H,
NCH2), 4.88 (s, 5H, C5H5), 4.36 (m, 1H, NCH2), 2.47 (s, 3H, o-Me),
2.42 (s, 3H, p-Me), 2.01 and 1.90 (2 m, 2 × 1H, NCH2CH2), 1.74 (s,
3H, o-Me), 1.48 and 1.44 (2 m, 2 × 1H, CH2CH3), 1.05 (t, 3H, CH3,
1
3J = 7.3). H NMR (CDCl3, 298 K, 400.14 MHz): δ 7.15 (s, 1H,
1
57.55; H, 7.16; N, 4.63. Found: C, 57.62; H, 7.06; N, 4.28. H NMR
(CDCl3, 300.13 MHz): δ 7.44 (m, 2H, m-H), 7,21 (dd, 1H, p-H, 3J =
NCH), 7.03 (br, 2H, m-H), 6.86 (s, 1H, NCH), 5.44 (br, 1H, NCH2),
4.89 (s, 5H, C5H5), 4.42 (br, 1H, NCH2), 2.49 (br, 3H, o-Me), 2.42 (s,
3H, p-Me), 2.00 (br, 2H, NCH2CH2), 1.74 (br, 3H, o-Me), 1.51 (br,
3
3
6.4), 7.13 (d, 1H, NCH, J = 1.8), 6.87 (d, 1H, NCH, J = 1.8), 5.38
and 4.78 (2 m, 2 × 1H, NCH2), 4.28 (septet, 1H, CHMe2-A, 3J = 6.8),
2.05 and 1.83 (2 m, 2 × 1H, NCH2CH2), 1.98 (septet, 1H, CHMe2-B,
3
2H, CH2CH3), 1.07 (t, 3H, CH3, J = 7.3). 13C{1H} NMR (CDCl3,
3
75.47 MHz): δ 166.4 (NCN), 139.1 and 136.9 (ipso-/p-CAr), 137.9
and 134.4 (o-CAr), 130.1 and 128.6 (br, m-CAr), 124.4 and 122.5
(NCH), 91.8 (C5H5), 53.6 (NCH2), 33.1 (NCH2CH2), 21.3 (p-Me),
20.1 (CH2CH3), 18.0 (br, o-Me), 14.1 (CH3).
3J = 6.8), 1.59 (m, 2H, CH2CH3), 1.42 (d, 3H, CHMe2-A, J = 6.8),
3
1.30 (s, 15H, C5Me5), 1.24 (d, 3H, CHMe2-B, J = 6.8), 1.12 (d, 3H,
3
3
CHMe2-B, J = 6.8), 1.08 (t, 3H, CH2CH3, J = 7.4), 0.87 (d, 3H,
CHMe2-A, J = 6.8). 13C{1H} NMR (CDCl3, 75.47 MHz): δ 178.3
3
Synthesis of [Ni(iPr2Ph-NHC-nBu)ICp] (1e). Nickelocene (566
mg, 3.00 mmol) and 1-(2,6-diisopropylphenyl)-3-butylimidazolium
iodide e (1.25 g, 3.03 mmol) were refluxed for 39 h in DME (40 mL).
The solution slowly turned from dark green to dark red. The solvent
was then removed in vacuo, and unreacted nickelocene was extracted
with pentane (4 × 5 mL). Column chromatography over neutral silica
with diethyl ether/pentane (7:3) as eluent afforded 1e (1.060 g, 1.98
mmol, 66%) as a violet solid after solvent evaporation. ES-MS: m/z
[M]+ calcd for C24H33N2Ni 407.1992, found 407.1946. 1H NMR
(NCN), 148.4 and 145.8 (o-CAr), 136.4 (ipso-CAr), 129.9 (m-CAr),
126.8 (NCH), 125.2 (m-CAr), 123.7 (p-CAr), 119.9 (NCH), 101.8
(C5Me5), 54.7 (NCH2), 33.2 (NCH2CH2), 29.3 (CHMe2-A), 28.2
(CHMe2-B), 28.0 (CHMe2-B), 25.9 (CHMe2-A), 23.2 (CHMe2-A),
23.0 (CHMe2-B), 20.6 (CH2CH3), 14.3 (CH3), 10.8 (C5Me5).
Synthesis of [Ni(Mes-NHC-TES)ClCp] (1d-TES). Nickelocene
(126 mg, 0.667 mmol) and 1-(2,4,6-trimethylphenyl)-3-[3-
(triethoxysilyl)propyl]imidazolium chloride (d-TES) (285 mg, 0.667
mmol) were refluxed for 30 min in DME (10 mL). The resulting dark
red suspension was cooled to room temperature, and the solvent
removed in vacuo. Column chromatography over neutral silica with
diethyl ether/pentane (7:3) as eluent afforded 1d-TES (261 mg, 0.474
mmol, 71%) as a violet oil after solvent evaporation. ES-MS: m/z [M]+
calcd for C26H39N2NiO3Si 513.2078, found 513.2059. 1H NMR
(CDCl3, 300.13 MHz): δ 7.17 (s, 1H, NCH), 7.08 (s, 2H, m-H), 6.82
(s, 1H, NCH), 5.00 (br, 2H, NCH2), 4.72 (s, 5H, C5H5), 3.87 (q, 6H,
3
(CDCl3, 253 K, 400.14 MHz): δ 7.54 (t, 1H, p-H, J = 6.8), 7.46 (d,
1H, m-H, 3J = 6.8), 7.26 (d, 1H, m-H, 3J n.r.), 7.16 (d, 1H, NCH, 3J =
3
1.7), 6.93 (d, 1H, NCH, J = 1.7), 5.48 (m, 1H, NCH2), 4.80 (s, 5H,
3
C5H5), 4.37 (m, 1H, NCH2), 3.41 (sept, 1H, CHMe2, J = 6.8), 2.01
and 1.91 (2 m, 2 × 1H, NCH2CH2), 1.72 (sept, 1H, CHMe2, 3J = 6.8),
3
1.46 (d, 3H, CHMe2, J = 6.8 and m, 2H, CH2CH3), 1.20 (d, 3H,
3
3
3
CHMe2, J = 6.8), 1.05 (d, 3H, CHMe2, J = 6.8 and t, 3H, CH3, J =
7.2), 0.97 (d, 3H, CHMe2, 3J = 6.8). 1H NMR (CDCl3, 298 K, 300.13
MHz): δ 7.54 (t, 1H, p-H, 3J = 6.8), 7.47 (br, 1H, m-H), 7.26 (br, 1H,
m-H), 7.15 (d, 1H, NCH, 3J = 1.7), 6.92 (d, 1H, NCH, 3J = 1.7), 5.49
(br, 1H, NCH2), 4.82 (s, 5H, C5H5), 4.44 (br, 1H, NCH2), 3.52 (br,
1H, CHMe2), 2.01 (br, 2H, NCH2CH2), 1.78 (br, 1H, CHMe2), 1.49
(br, 5H, CHMe2 and CH2CH3), 1.22 (br, 3H, CHMe2), 1.08 (t, 3H,
3
Si(OCH2CH3)3, J = 6.9), 2.43 (s, 3H, p-Me), 2.25 (m, 2H, CH2),
3
2.11 (br, 6H, o-Me), 1.26 (t, 9H, Si(OCH2CH3)3, J = 6.9), 0.79 (m,
2H, CH2Si). 13C{1H} NMR (CDCl3, 75.47 MHz): δ 162.9 (NCN),
139.2 and 137.0 (ipso-/p-CAr), 136.2 (br, o-CAr), 129.2 (m-CAr), 123.4
and 123.1 (NCH), 91.8 (C5H5), 58.7 (Si(OCH2CH3)3), 54.3 (NCH2),
25.2 (CH2), 21.4 (p-Me), 18.5 (Si(OCH2CH3)3), 18.4 (o-Me), 7.9
(CH2Si).
3
CH3, J = 7.2), 1.03 (br, 6H, CHMe2). 13C{1H} NMR (CDCl3, 75.47
MHz): δ 167.9 (NCN), 148.4 and 145.5 (br, o-CAr), 136.7 (ipso-CAr),
130.2 (p-CAr), 126.1 and 122.0 (NCH), 124.9 and 123.4 (br, m-CAr),
91.6 (C5H5), 54.0 (NCH2), 33.1 (NCH2CH2), 28.7 and 28.2 (b,
CHMe2), 26.9 and 26.1 (br, CHMe2), 23.7 and 22.6 (br, CHMe2), 20.1
(CH2CH3), 14.1 (CH3).
Immobilization of [Ni(Mes-NHC-TES)ClCp] (1d-TES) onto
Alumina. Alumina (3.70 g, 36.3 mmol) was introduced into a
Schlenk tube and dried under vacuum overnight at 110 °C. A solution
of 1d-TES (200 mg, 0.364 mmol) in toluene (15 mL) was added, and
the resulting suspension refluxed for 4 h with vigorous stirring. After
cooling the suspension to room temperature, toluene was removed by
filtration and the solid washed with hot CH2Cl2 (6 × 10 mL). The
resulting pink solid was then dried overnight under high vacuum to
give the supported complex 1d-Al. ICP-AES: Ni, 2.84 mg·g−1 (0.0484
Synthesis of [Ni(Mes-NHC-nBu)ICp*] (2d). To a freshly
prepared suspension of [Ni(acac)Cp*]38 (0.851 mmol) in thf (5
mL) was added 1-(2,4,6-trimethylphenyl)-3-butylimidazolium iodide d
(315 mg, 0.851 mmol) suspended in thf (5 mL), and the reaction
mixture was stirred at reflux for 3 h. The resulting deep violet
suspension was cooled to room temperature, and the solvent removed
in vacuo. The residue was extracted with toluene (5 mL) and filtered
through silica, which was rinsed with toluene (4 × 5 mL) until the
washings were colorless. Concentration to ca. 1 mL followed by
addition of pentane (2 mL) and crystallization at −32 °C afforded 2d
(145 mg, 0.257 mmol, 30%) as a dark red solid, which was washed
1
mmol·g−1). H MAS NMR (400.14 MHz, 14 kHz, 128 transients): δ
6.9 (NCH and m-H), 4.6 (Cp and NCH2), 3.4 (p-Me and
Si(OCH2CH3)3−n), 2.1 (o-Me), 1.3 (Si(OCH2CH3)3−n). DRIFT (64
scans): ν(O−H); 3509 (s); ν(Csp2−H); 3131 (m), 3087 (m);
ν(Csp3−H) 2979 (m), 2930 (s), 2890 (s), 2831 (m); ν(CC) 1610
(w), 1565 (w), 1488 (m), 1456 (m).
General Procedure for Suzuki−Miyaura Cross-Coupling
Reactions Using Standard Schlenk Techniques. A Schlenk tube
equipped with a septum was charged with aryl halide (1.0 mmol),
phenylboronic acid (1.3 mmol), K3PO4 (2.6 mmol), and catalyst (1.0−
3.0 mol %) before being put under an atmosphere of argon. Toluene
(3 mL) was injected, and the mixture immediately heated with
1
with pentane (3 × 3 mL) and dried under high vacuum. H NMR
(C6D6, 300.13 MHz): δ 6.87 (s, 1H, m-H), 6.66 (s, 1H, m-H), 6.38 (s,
1H, NCH), 6.00 (d, 1H, NCH), 4.93 (m, 2H, NCH2), 2.95 (s, 3H, o-
Me), 2.14 (s, 3H, p-Me), 1.89 (m, 1H, NCH2CH2), 1.57 (s, 3H, o-
Me), 1.46 (s, 15H, C5Me5), 1.39 (m, 1H, NCH2CH2 and m, 2H,
2837
dx.doi.org/10.1021/om201101g | Organometallics 2012, 31, 2829−2840

Organometallics
Article
vigorous stirring by putting the Schlenk into an oil bath at 90 or 110
°C. After 15−120 min, the reaction was stopped by cooling the
reaction to room temperature and allowing air to enter in the Schlenk
tube. GC yields were calculated by using tetradecane as an internal
standard. NMR yields were determined by removing a sample with a
syringe, drying it under vacuum, extracting the residue with CDCl3,
and filtering the solution into the NMR tube. In a standard workup,
the solvent was completely removed under vacuum. The residue was
extracted with a 1:1 mixture of diethyl ether/water (20 mL). The
organic layer was separated, and the aqueous layer extracted with
another 10 mL portion of diethyl ether. The combined extracts were
washed with water (2 × 10 mL) and dried over Na2SO4. The solvent
was removed under reduced pressure, and the crude material was
purified by column chromatography over SiO2 with pentane/ethyl
acetate as eluent to give the desired product. All yields are the average
values of three runs.
General Procedure for Suzuki−Miyaura Cross-Coupling
Reactions Using Standard Glovebox Techniques. In a glovebox
with levels of H2O and O2 < 0.5 ppm, a Schlenk tube equipped with a
glass stopper was charged with aryl halide (1.0 mmol), phenylboronic
acid (1.3 mmol), K3PO4 (2.6 mmol), and a toluene solution (3 mL)
containing 1.0−3.0 mol % of catalyst. The Schlenk tube was then taken
outside the glovebox, and the mixture immediately heated with
vigorous stirring by putting the Schlenk tube into an oil bath at 110
°C. After 15−120 min, the reaction was stopped by cooling the
reaction to room temperature and allowing air to enter in the Schlenk
tube. GC and NMR yields were determined as for the reactions carried
out under standard Schlenk techniques. Standard workups were similar
to those of the reactions carried out under standard Schlenk
techniques.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vritleng@unistra.fr, michael.chetcuti@unistra.fr. Tel: +
33 3 68 85 26 31.
■
ACKNOWLEDGMENTS
■
A.M.O. thanks the Minister
̀
e de l’Enseignement Super
́
ieur et de
la Recherche for a doctoral fellowship. We are grateful to the
CNRS and the University of Strasbourg for their financial help.
We appreciate the assistance of Mr. Michel Schmitt in
obtaining the VT NMR data for complexes 1d,e and the 2D
1H COSY and 1H/13C HSQC correlations for complex 2e, and
of Dr. Lydia Brelot for all the structural determinations listed
here. We also acknowledge Dr. Reg
́
is M. Gauvin for allowing
the DRIFT and H MAS NMR measurements of 1d-Al at the
Universite de Lille I.
1
́
DEDICATION
■
M.J.C. wishes to acknowledge the immense contributions of
Professor F. Gordon A. Stone to organometallic chemistry as an
inspirational mentor and researcher, especially during its golden
age in the 1970s and early 1980s. This article is dedicated to his
memory.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Scheme S1 illustrating the distortions in the η5-C5R5 ligands of
complexes 1a−2e, Tables S1 and S2, giving X-ray crystallo-
graphic data and data collection parameters for complexes
1a,b,d,e and 2d,e, and CIF files giving X-ray structural data,
including data collection parameters, positional and thermal
parameters, and bond distances and angles for complexes
1a,b,d,e and 2d,e. This material is available free of charge via
the Internet at http://pubs.acs.org. Crystallographic data
(excluding structure factors) have also been deposited in the
Cambridge Crystallographic Data Centre as Supplementary
publications nos. CCDC 846657 (1a), 846658 (1b), 846659
(1d), 846660 (1e), 846661 (2d), and 846662 (2e). Copies of
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