Half-sandwich NHC-nickel(ii) complexes as pre-catalysts for the fast Suzuki coupling of aryl halides: A comparative study

Article abstract of DOI:10.1039/c0dt00021c

Cationic half-sandwich nickel complexes of general formula [Ni(NHC)(NCMe)(η⁵-C₅R₅)](PF₆) [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) a, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) b; R = H, Me] were prepared from the reaction of their neutral homologues [Ni(NHC) Cl(η⁵-C₅R₅)] with 1 equiv. of KPF ₆ in acetonitrile at room temperature. The new cationic complexes [Ni(IPr)(NCMe)(η⁵-C₅Me₅)](PF₆) 3a, [Ni(IMes)(NCMe)(η⁵-C₅Me₅)](PF ₆) 3b and [Ni(IMes)(NCMe)(η⁵-C₅H ₅)](PF₆) 4b were obtained in high yield and were fully characterized by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, elemental analyses, and in the case of 3a by a single-crystal X-ray diffraction study. The neutral analogue of 3a, [Ni(IPr) Cl(η⁵-C₅Me₅)] 1a was also structurally characterized. Their geometries were compared and no significant structural differences were observed. Nevertheless solution NMR spectroscopy established that the acetonitrile ligand of the cationic species is labile in solution. This results in the absence of any rotational significant barrier about the nickel-carbene carbon bond at ambient temperature in solution in the sterically congested cationic complexes 3a and 3b, in contrast to their neutral analogues 1a and [Ni(IMes)Cl(η⁵-C₅Me₅)] 1b. The neutral and the cationic complexes catalyzed the cross-coupling of phenylboronic acid with aryl halides in the absence of co-catalysts or reductants. Surprisingly, the neutral or cationic nature of the complexes proved to have almost no influence on the reaction yields and rates. However, complexes bearing the bulky electron-rich pentamethylcyclopentadienyl ligand were much more active than those bearing the cyclopentadienyl ligand, and TOF of up to 190 h^-1, a high rate for nickel(ii) complexes under similar conditions, were observed with these species.

Full text of DOI:10.1039/c0dt00021c

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Half-sandwich NHC-nickel(II) complexes as pre-catalysts for the fast Suzuki
coupling of aryl halides: a comparative study† ‡
Vincent Ritleng,* Anna Magdalena Oertel and Michael J. Chetcuti*
Received 24th February 2010, Accepted 1st June 2010
First published as an Advance Article on the web 3rd August 2010
DOI: 10.1039/c0dt00021c
5
Cationic half-sandwich nickel complexes of general formula [Ni(NHC)(NCMe)(h -C5R5)](PF6)
[NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) a, 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene (IMes) b; R = H, Me] were prepared from the reaction of their neutral homologues
5
[Ni(NHC)Cl(h -C5R5)] with 1 equiv. of KPF6 in acetonitrile at room temperature. The new cationic
5
5
complexes [Ni(IPr)(NCMe)(h -C5Me5)](PF6) 3a, [Ni(IMes)(NCMe)(h -C5Me5)](PF6) 3b and
[Ni(IMes)(NCMe)(h -C5H5)](PF6) 4b were obtained in high yield and were fully characterized by 1H
5
and 13C NMR spectroscopy, IR spectroscopy, elemental analyses, and in the case of 3a by a
5
single-crystal X-ray diffraction study. The neutral analogue of 3a, [Ni(IPr)Cl(h -C5Me5)] 1a was also
structurally characterized. Their geometries were compared and no significant structural differences
were observed. Nevertheless solution NMR spectroscopy established that the acetonitrile ligand of the
cationic species is labile in solution. This results in the absence of any rotational significant barrier
about the nickel–carbene carbon bond at ambient temperature in solution in the sterically congested
5
cationic complexes 3a and 3b, in contrast to their neutral analogues 1a and [Ni(IMes)Cl(h -C5Me5)] 1b.
The neutral and the cationic complexes catalyzed the cross-coupling of phenylboronic acid with aryl
halides in the absence of co-catalysts or reductants. Surprisingly, the neutral or cationic nature of the
complexes proved to have almost no influence on the reaction yields and rates. However, complexes
bearing the bulky electron-rich pentamethylcyclopentadienyl ligand were much more active than those
bearing the cyclopentadienyl ligand, and TOF of up to 190 h-1, a high rate for nickel(II) complexes
under similar conditions, were observed with these species.
The chemistry of Ni-NHC complexes has been much less
Introduction
investigated than that of Pd-, Ru- or even Rh-NHC complexes.2
However, the last decade has seen the emergence of a number
of NHC-Ni(0) systems, generally generated in situ from a Ni(0)
compound such as Ni(COD)2 and a NHC ligand, or from a
Ni(II) compound, a NHC ligand and an excess of reductant.
These species were shown to be efficient catalysts in the amination
of aryl chlorides,9 C–S couplings,10 cross-coupling reactions of
fluorinated arenes,11 three-component couplings of unsaturated
hydrocarbons, aldehydes and silyl derivatives,12 and [2+2+2]
cycloadditions.13,14,15 The major disadvantages of these systems
are the air-sensitivity of the Ni(0) species and/or the necessity
of an excess of reductant,9b–d,12f ,13d which generates large amounts
of waste. To overcome this inconvenience, much recent effort has
been devoted to the development of well-defined air-stable Ni(II)
complexes bearing electron-rich NHC ligands3a,16 that are able
to catalyze organic reactions without external reductants. Such
species catalyze the amination of aryl halides,17 the hydrothio-
lation of alkynes,18 the base-free Michael addition19 and the a-
arylation of acyclic ketones17a (where no organometallic partner
is involved), as well as Suzuki,20 Kumada20c,21 and Negishi22 C–
C cross-couplings reactions, in which the organometallic reagent
(ArB(OH)2, ArMgX, ArZnX) is known to help reduce Ni(II) to
Ni(0).23,24
Since the first isolation of a stable imidazol-2-ylidene,1 N-
heterocyclic carbenes (NHCs) have become a very important
class of ligands in organometallic chemistry.2 NHCs behave like
typical strong s-donor3 ligands with non-negligible p-acceptor
abilities.4 These electronic characteristics are similar to those of
tertiary phosphines, and they show similar abilities to stabilize
the various oxidation states and coordinatively unsaturated in-
termediates that appear in catalytic reactions.2,5 However, NHCs
show superior performances in many aspects over traditional
trialkyl- and triarylphosphine ligands, including versatility, ready
preparation, thermal-, air- and moisture-stability, as well as non-
toxicity. In addition, NHCs exhibit superior qualities regarding
ligand dissociation6 and degradative cleavage,7 both of which are
less likely as compared to tertiary phosphines.8 Both properties
lead to a higher complex stability. Moreover, carbene complexes
have shown unprecedented catalytic activity under homogeneous
conditions in many important organic reactions.2
Laboratoire de Chimie Organome´tallique Applique´e, UMR CNRS 7509,
Ecole Europe´enne de Chimie, Polyme`res et Mate´riaux, Universite´ de
Strasbourg, 25 rue Becquerel, 67087 Strasbourg, France. E-mail: michael.
chetcuti@unistra.fr, vritleng@unistra.fr; Tel: (33) 3 6885 2631
† CCDC reference numbers 775553 (1a) and 767229 (3a). For crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
c0dt00021c
However, despite (i) the high importance of developing effi-
cient syntheses of biaryl compounds that are found in natural
products,25 drugs26 or materials,27 (ii) the much lower cost and
easier removal of nickel from the product28 (with respect to its
5
5
‡ Throughout this manuscript, Cp = h -C5H5, Cp* = h -C5Me5, IPr =
a = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IMes = b = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene.
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Table 1 1H NMR data of the cationic complexes 3a, 3b and 4b and of their neutral counterparts 1a, 1b and 2ba
Cp§
NCH CHN
Ar
Ref.
29
=
1ab
1.12 (Cp*)
6.64
7.39 (d, 2H, m-H, 3J = 7.6), 7.30 (t, 2H, p-H), 7.08 (d, 2H, m-H), 4.19 (m, 2H,
CHMe2), 2.29 (m, 2H, CHMe2), 1.61 (d, 6H, CHMe2, 3J = 6.8), 1.23 (d, 6H, CHMe2,
3J = 6.8), 1.06 (d, 6H, CHMe2, 3J = 6.8), 0.88 (d, 6H, CHMe2, 3J = 6.8)
7.62 (m, 2H, p-H), 7.50 (d, 4H, m-H, 3J = 7.5), 2.60 (br, 4H, CHMe2), 1.37 (d, 12H,
CHMe2, 3J = 6.9), 1.13 (d, 12H, CHMe2, 3J = 6.9)d
3ac
1be
0.88 (Cp*)
1.17 (Cp*)
7.51
6.22
This work
29
6.92 (br, 2H, m-H), 6.80 (br, 2H, m-H), 2.69 (br s, 6H, o-Me), 2.21 (s, 6H, p-Me), 1.81
(br s, 6H, o-Me)
3bc
2bf
4bf
0.96 (Cp*)
4.56 (Cp)
4.76 (Cp)
7.30
7.09
7.20
7.17 (s, 4H, m-H), 2.40 (s, 6H, p-Me), 2.12 (br s, 12H, o-Me)d
This work
30
This work
7.12 (s, 4H, m-H), 2.45 (s, 6H, p-Me), 2.18 (s, 12H, o-Me)
7.13 (s, 4H, m-H), 2.43 (s, 6H, p-Me), 2.12 (s, 3H, NCMe), 2.11 (s, 12H, o-Me)
a
d in ppm and J in Hz. b In toluene-d8 at 263 K. c In acetonitrile-d3. d Free CH3CN that results from exchange with CD3CN is seen as a singlet (at 1.96 ppm)
on the downfield side of the multiplet due to residual CHD2CN observed at 1.94 ppm. e In toluene-d8. f In chloroform-d1.
more widely used d10 counterpart, palladium), and (iii) the unique
properties of NHC ligands in comparison to phosphines,2 exam-
ples of NHC-Ni(II)-based catalysts for C–C coupling reactions
remain scarce.20,21,22
IPr, 3a; Cp§ = Cp*, NHC = IMes, 3b; Cp§ = Cp, NHC = IMes,
4b) were isolated as yellow to green air-stable solids in 71–84%
yield (eqn (1)). All complexes were fully characterized by 1H and
1
13C{ H} NMR spectroscopy (Tables 1 and 2), IR spectroscopy
Following our ongoing interest in NHC-Ni(II) compounds,6a we
have recently described the synthesis and dynamic behavior of air
and moisture stable neutral complexes [Ni(NHC)ClCp§] (Cp§ =
Cp, Cp*)‡ that bear aryl substituents on both NHC-nitrogen
atoms.29,30 Herein we report the high yield syntheses of some of
their cationic derivatives, [Ni(NHC)(NCMe)Cp§](PF6) by reaction
of the corresponding neutral species with KPF6 in acetonitrile.
The structure of one of these cationic complex was established by
a single-crystal X-ray diffraction study and was compared to that
of its neutral counterpart. No remarkable structural differences
were observed in the solid state but solution NMR spectroscopy
did reveal that the acetonitrile ligand in the cations is labile; this
might lead to differences in the catalytic behaviour. Having all
these complexes in hand, we thus decided to check the respective
influence of (i) their cationic or neutral nature and (ii) the presence
of the bulky electron-rich pentamethylcyclopentadienyl ligand or
of the less bulky and electron poorer cyclopentadienyl on their
activity in Suzuki–Miyaura couplings.
All the complexes catalyzed the cross-coupling of aryl halides
with phenyl boronic acid in the absence of co-catalysts or
reductants. Surprisingly, the neutral or cationic nature of the
complexes proved to have almost no influence on the reaction
yields and rates. However, complexes bearing the bulky electron-
rich pentamethylcyclopentadienyl ligand proved to be much more
active than those bearing the cyclopentadienyl ligand, and among
the best TOFs reported for Ni(II)-catalyzed Suzuki cross-coupling
reactions under similar conditions were observed with these
complexes.20
and elemental analyses.
(1)
1
1
The H and 13C{ H} NMR spectra of the cationic complexes
3a, 3b and 4b at ambient temperature are straightforward: all show
the presence of one h -Cp* or -Cp group and of the IPr or IMes
5
ligand. As for the neutral complexes 1a,29 1b29 and 2b,16e,30 the
spectra reveal that an effective plane of symmetry that bisects the
molecule exists in solution on the NMR time scale. This effective
mirror plane contains the acetonitrile ligand, the nickel and the
NHC carbene carbon atom, as well as the Cp* or Cp ring centroid.
Nevertheless, in contrast to the sterically congested neutral
Cp* species 1a and 1b, which possess a rotation barrier of
65–67 kJ mol-1 about the nickel–carbene carbon bond,29 the
corresponding cationic complexes 3a and 3b show a greatly
reduced rotation barrier at room temperature. Thus, the 1H NMR
spectrum of 1b at ambient temperature displays two singlets in
a 1 : 1 integrated ratio for the four meta-hydrogen atoms of the
two mesityl groups, as well as three singlets, in a 3 : 3 : 3 relative
integrated ratio, for the four ortho- and the two para-methyl
groups.29 In contrast, its cationic analogue 3b displays only one
singlet for all the meta-hydrogen atoms and two singlets in a 3 : 6
relative integrated ratio for the ortho- and para-methyl groups (the
bigger signal being slightly broadened, Table 1). Similarly, the
1H NMR spectrum of 3a displays only one doublet for the meta
aromatic ring protons, one broad signal for the CH protons of
the isopropyl groups and two doublets for the isopropyl methyls
(the central carbon of each isopropyl group is diastereotopic). All
these signals are doubled in the 1H NMR spectrum of 1a (Table 1).
This doubling of the aryl substituent signals of 1a and 1b is also
Results and discussion
Synthesis and characterization of the cationic complexes
[Ni(NHC)(NCMe)Cp§]+PF6
-
When acetonitrile solutions of the neutral complexes,
[Ni(NHC)ClCp§] (Cp§ = Cp*, NHC = IPr, 1a; Cp§ = Cp*, NHC =
IMes, 1b; Cp§ = Cp, NHC = IMes, 2b)‡ were treated with 1
equivalent of KPF6, the chloride was abstracted and the cationic
1
observed in their 13C NMR spectra (Table 2). The H and 13C
-
complexes, [Ni(NHC)(NCCH3)Cp§]+PF6 (Cp§ = Cp*, NHC =
NMR spectra of the less bulky Cp species 2b and 4b are similar.
8154 | Dalton Trans., 2010, 39, 8153–8160
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1
Table 2 13C{ H} NMR data of the cationic complexes 3a, 3b and 4b and of their neutral counterparts 1a, 1b and 2ba
Cp§
NCN NCH CHN Ar
Ref.
=
1ab 102.1 (C5), 10.1 (Me5) 180.0 123.1
3ac 105.6 (C5), 9.8 (Me5) 172.3 128.6
1bb 102.2 (C5), 9.9 (Me5) 177.2 124.1
3bc 105.6 (C5), 9.5 (Me5) 170.2 126.8
149.6 and 145.8 (o-CAr), 138.0 (ipso-CAr), 130.1 and 125.9 (m-CAr), 125.7 (p-CAr), 28.7 29
(CHMe2), 27.2 and 26.9 (CHMe2), 23.9 and 22.7 (CHMe2)
147.6 (o-CAr), 137.0 (ipso-CAr), 131.6 (p-CAr), 125.4 (m-CAr), 29.5 (CHMe2), 27.1 and This work
22.9 (CHMe2)d
138.6 and 134.8 (o-CAr), 138.4 (p-CAr), 137.8 (ipso-CAr), 130.5 and 129.3 (m-CAr), 21.4 29
(o-Me), 20.6 (p-Me), 18.6 (o-Me)
140.4 (ipso-CAr or p-CAr), 137.0 (p-CAr or ipso-CAr), 136.4 (o-CAr), 130.3 (m-CAr), 21.1 This work
(p-Me), 18.9 (o-Me)d
2be 92.3 (C5H5)
4bc 94.3 (C5H5)
167.2 124.6
160.2 127.1
139.3 (p-CAr or ipso-CAr), 136.8 (ipso-CAr or p-CAr), 136.1 (o-CAr), 129.4 (m-CAr), 21.4 This work
(p-Me), 18.6 (o-Me)
140.9 (p-CAr or ipso-CAr), 136.9 (ipso-CAr or p-CAr), 136.4 (o-CAr), 130.3 (m-CAr), 21.3 This work
(p-Me), 18.4 (o-Me)d
a
d in ppm. b In benzene-d6. c In acetonitrile-d3. d Free CH3CN that results from exchange with CD3CN is seen as two singlets (at 1.77 and 118.3 ppm)
overlapping with the multiplet due to residual CHD2CN at 1.32 ppm and with the singlet due to CD3CN at 118.3 ppm. e In chloroform-d1.
Neither of them shows an observable rotational barrier at ambient
temperature.
These results suggest that substitution of a bulky chloride by a
smaller acetonitrile molecule would reduce the steric congestion
in the cationic complexes bearing the bulky Cp* and allow
free rotation about the nickel–carbene carbon bond at ambient
temperature. However a comparison of the molecular structures
of 1a and 3a shows that they are very similar (vide infra). We thus
tend to believe that this observed free rotation at room temperature
in the cationic species is the direct result of ligand exchange and a
dynamic process rather than of a purely dynamic process. Indeed,
free CH3CN that results from exchange with CD3CN is seen in the
1
1H and 13C { H} NMR spectra of 3a, 3b, and 4b, indicating that
the acetonitrile ligand of the cationic species is labile in solution.
A dynamic rotational process would be much more facile for
the two coordinate intermediate presumably present during the
acetonitrile ligand exchange.
Fig. 1 Molecular structure of 1a showing all non-H atoms. Ellipsoids are
shown at the 50% probability level and key atoms are labelled.
It is noteworthy that the carbene carbon atoms in complexes
3a, 3b and 4b appear at 172.3, 170.2 and 160.2 ppm (in CD3CN),
respectively, in the 13C NMR spectrum of these complexes. These
signals are slightly upfield of the signals seen at 180.0, 177.2
(both in C6D6) and 167.2 ppm (in CDCl3), respectively, for their
neutral derivatives (Table 2). The apparent absence of a significant
rotational barrier about the nickel–carbene carbon bond in the
cationic Cp* species 3a and 3b at ambient temperature is thus
probably mainly due to the lability of acetonitrile in solution, but
there may also be a minor electronic component.29
The IR spectra of the cationic complexes are rather surprising
as weak n(CN) stretches are observed for solid samples of all
three complexes at 2281 (3a), 2294 (3b) and 2299 cm-1 (4b). These
values are consistent with those reported for other Ni(II)–N-bound
acetonitrile complexes.31
Fig. 2 Molecular structure of the cationic part of 3a showing all non-H
atoms. Ellipsoids are shown at the 50% probability level and key atoms are
labelled.
Structural studies of complexes 1a and 3a
Crystals of the cationic complex 3a and of its neutral precursor 1a
suitable for X-ray structure determination were grown from cold
acetonitrile–toluene (3a) and toluene (1a) solutions. The molecular
structures of 1a and of the cationic part of 3a are shown in similar
orientations in Fig. 1 and 2, respectively. Crystallographic data
and data collection parameters are listed in Table 3, and a list of
selected bond lengths and angles for both complexes appear in
Table 4.
The molecular structure of these complexes are strikingly
5
similar. Both feature a nickel atom bonded to a h -Cp* group,
a IPr moiety, and a chloride (1a) or an acetonitrile (3a) ligand
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Table 3 X-Ray crystallographic data and data collection parameters for
complexes 1a and 3a
(L = Cl) and in 3a [L = N(3)] does not appear to be correlated to
the rotation barrier variation.
The C(1)–Ni–Cp§cent angle spans the range of 142 0.5◦ for
the Cp* species 1a and 1b,29 which show a rotation barrier about
the nickel–carbene carbon bond. In contrast, the Cp species 2a,17b
2b16e and [Ni(1,3-dimethylimidazol-2-ylidene)ICp],29 as well as the
Cp* complex, [Ni(1,3-dimethylimidazol-2-ylidene)ICp*], exhibit
a smaller angle of 134.2 1.8◦ and do not show an observable
rotation barrier.29 This angle seems thus to be correlated to
the presence or absence of a rotation barrier about the nickel–
carbene carbon bond. However, despite the C(1)–Ni–Cp*cent angle
of 140.0◦ (Table 4) measured in the cationic derivative 3a, a greatly
reduced rotational barrier is observed in this complex, but as
discussed earlier, this behaviour is probably due to acetonitrile
ligand dissociation.
Compound
1a
3a
Empirical formula
Mr
C37H51ClN2Ni
617.96
Monoclinic
Cc
11.9474(8)
31.263(3)
10.6055(5)
93.510(4)
3953.8(5)
4
1.038
0.581
173(2)
Prism, red
0.30 ¥ 0.10 ¥ 0. 05
11, 37, 13
—
C39H54N3Ni·F6P
768.53
Orthorhombic
Pnma
13.7019(5)
18.0171(7)
16.5430(7)
90
4083.9(3)
4
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Dc/Mg m-3
m/mm-1
1.250
0.571
173(2)
T/K
Crystal form, colour
Crystal size/mm
h, k, lmax
Block, dark yellow
0.20 ¥ 0.18 ¥ 0.15
17, 23, 21
0.568, 0.784
26294
0.0698 (3025)
0.1316 (4829)
1.090
The nickel–carbene carbon bond lengths are not significantly
˚
different from each other [Ni–C(1) = 1.900(4) A (1a); 1.911(4)
T
min, Tmax
(3a)]. These value are comparable to those reported for the closely
Reflns collected
R (reflections)
wR2 (reflections)
GOF on F2
14280
29
˚
related neutral mesityl analogue 1b [1.906(3) A] and for its
0.0597 (5068)
0.1499 (7141)
0.965
16e
˚
Cp derivative 2b [1.917(9) A], but are somewhat longer than
17b
˚
that reported for 2a [1.8748(11) A]. The Ni–Cl distance of
˚
2.2094(13) A in 1a is close to the Ni–Cl distance observed in 1b,
where a value of 2.1962(9) has been registered,29 but is slightly
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) for 1a and 3a
longer than those reported for the neutral Cp derivatives 2a
16e
and 2b, where values of 2.1876(3)17b and 2.185(2) A have been
˚
1a
3a
observed, respectively. The acetonitrile ligand of 3a is linear [N(3)–
C(2)–C(3) = 179.4(5)◦] but the Ni–N(3) bond is nevertheless not
perfectly co-linear with this axis [Ni–N(3)–C(2) = 170.1(4)◦].
The plane that contains the imidazol-2-ylidene ring is almost
orthogonal to the Cl–Ni–Cp*cent plane and makes an angle of 86.3◦
with it in the neutral complex 1a. The two halves of the molecule
are thus not related by a mirror plane, bur are not too far off from
mirror symmetry, and the two aryl groups approximately eclipse
each other. In contrast a crystallographically imposed mirror plane
bisects the cation of 3a and the PF6- anion, so this angle is precisely
90◦ in 3a.
Ni–C(1)
1.900(4)
2.2094(13)
2.155
2.061(5)
2.202(5)
1.430
1.911(4)
1.881(4)
2.143
2.063(5)
2.166(3)
1.429
Ni–La
Ni–C(Cp*) (av)
Ni–C(Cp*) (min)
Ni–C(Cp*) (max)
C(Cp*)–C(Cp*) (av)
C(Cp*)–C(Cp*) (min)
C(Cp*)–C(Cp*) (max)
1.399(7)
1.471(8)
1.392(5)
1.466(7)
Ni–N(3)–C(2)
N(3)–C(2)–C(3)
C(1)–Ni–L
C(1)–Ni–Cp*cent
L–Ni–Cp*cent
—
—
170.1(4)
179.4(5)
97.54(17)
140.0
93.85(13)
143.0
123.1
Both aryl rings in 1a are close to perpendicular to the
imidazolylidene ring (the plane of all aromatic carbon atoms in
each aryl ring makes angles of 83.1 and 86.3◦ with the carbene
plane). The corresponding angle in 3a is 85.7◦.
122.5
a L = Cl for 1a and N(3) for 3a.
in a two-legged piano stool geometry. If one considers the Cp*
group as a single ligand, the nickel atom lies at the centre of a
trigonal plane formed by the ring centroid, the carbenoid carbon
atom C(1) of the NHC ligand, and the chloride Cl or the nitrogen
N(3) of the acetonitrile ligand; the sum of all angles subtended by
these atoms is equal to 360◦ in both structures. However there
are significant departures from the idealized 120◦ angles of a
trigonal structure. The carbenoid carbon C(1) and the chloride
atom Cl of 1a subtend an angle of 93.8(1)◦ at the nickel atom, and
similarly the carbenoid carbon C(1) and the nitrogen atom N(3) of
3a subtend a slightly larger angle of 97.5(2)◦. These angles are in
the range of those observed for the closely related neutral mesityl
analogue 1b,29 for its Cp derivative 2b16e and for the Cp derivative
of 1a, [Ni(IPr)ClCp] 2a,17b for which values of 95.3(1), 98.4(2) and
93.86(3)◦ have been registered, respectively. The Cp complexes
2a and 2b, which show the biggest difference for this angle
(4.5◦), both do not show an observable rotation barrier about the
nickel–carbene carbon bond in solution at ambient temperature.
Hence, the difference of 3.6◦ between the C(1)–Ni–L angle in 1a
In both complexes, the Cp* ring exhibits structural distortions,
as there are significant variations in the Ni–Cp* carbon distances,
˚
which range from 2.061(5) to 2.202(5) A in 1a and from 2.063(5)
˚
to 2.166(3) A in 3a, as well as fluctuations in the aromatic C–C
˚
distances, which extend from 1.3991(7) to 1.471(8) A in 1a and
˚
from 1.392(5) to 1.466(7) A in 3a. Such variations arise from
“allyl–ene” distortions in the Cp* ligand and have been previously
observed in other Cp*Ni systems.32
There are no abnormally short non-bonded contacts or unusual
packing features in molecule 1a. However, all the acetonitrile
-
methyl protons in 3a interact with fluorine atoms of the PF6
anion: the hydrogen atom on the mirror plane undergoes two C–
˚
H ◊ ◊ ◊ F interactions of 2.441 A, while the two other protons each
-
interact with one fluorine atom of another PF6 group (each C–
˚
H ◊ ◊ ◊ F = 2.571 A).
In conclusion, there appear to be no significant structural
differences between 1a and 3a that might explain the low rotation
barrier in 3a. This behaviour is thus more likely due to the labile
nature of the acetonitrile ligand in solution (vide supra).
8156 | Dalton Trans., 2010, 39, 8153–8160
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Table 5 Optimization of reaction conditions for the Suzuki–Miyaura
cross-coupling of 4¢-bromoacetophenone with phenylboronic acid cat-
alyzed by 3b a
Table 6 Suzuki–Miyaura cross-coupling of 4¢-bromoacetophenone with
phenylboronic acid catalyzed by 1a–4b a
Entry
Cat. (mol%)
t/min
Conv.b (%)
Select.b (5/6)
PhB(OH)2 K3PO4
Entry Solvent (equiv.)
(equiv.) T/◦C t/min (%)
Conv.b Select.b
(5/6)
1
2
3
4
5
6
7
8
1a (3)
3a (3)
3a (3)
1b (3)
3b (3)
2a (3)
2a (3)
2b (3)
2b (3)
4b (3)
4b (3)
1b (1)
3a (1)
15
10
15
15
15
15
30
15
30
15
30
60c
60c
92
95
98
92
89
79
87
54
68
55
68
63
61
93/7
94/6
92/8
95/5
94/6
100/0
100/0
100/0
100/0
100/0
100/0
99/1
1
2
3
4
5
6
DME
DME
DME
CH3CN 1.3
Toluene 1.3
Toluene 1.3
1.1
1.1
1.3
2.2
2.2
2.6
2.6
2.6
2.6
25
82
82
85
90
90
60
120
120
120
120
40
22
82
90
32
100
100
100/0
100/0
100/0
100/0
96/4
96/4
9
a Reaction conditions: 4¢-bromoacetophenone (1 mmol), phenylboronic
acid (1.1–1.3 mmol), K3PO4 (2.2–2.6 mmol) as base, 3b (3 mol%), solvent
(3 mL). b According to NMR.
10
11
12
13
98/2
Catalytic Suzuki–Miyaura coupling reactions
a Reaction conditions: 4¢-bromoacetophenone (1 mmol), phenylboronic
acid (1.3 mmol), K3PO4 (2.6 mmol) as base, 1a–4b (1–3 mol%), toluene
(3 mL), 90 ◦C. b According to NMR. c Run at 110 ◦C.
Initial studies focussed on the reaction of 4¢-bromoacetophenone
with various amounts of phenylboronic acid and K3PO4 in the
presence of the cationic Cp* complex 3b (3 mol%) under various
solvent and temperature conditions to optimize the reaction
conditions (Table 5). All reactions were run without any additive
such as PPh3 as is often the case.20c–f ,i,23a,c
electron-rich Cp* complexes 1 and 3. These latter species gave
much better results overall in terms of rates and conversions,
but with a slightly decreased selectivity, as the Cp species did
not produce the homocoupling product 6 (entries 1–5 vs. 6–11).
Conversions up to 95% were obtained in the case of 2a after only
10 min, and 98% after 15 min, leading to turnover frequencies
(TOF) of 190 and 131 h-1 (entries 2 and 3). These are among
the best TOFs observed so far in Suzuki–Miyaura cross-coupling
catalyzed by a Ni(II) complex in the absence of reductant and co-
catalyst.20,23 We are indeed aware of only one example where a
higher TOF of 261 h-1 has been observed (with a pincer-type bis-
NHC-nickel(II) catalyst), for the coupling of 4-bromobenzonitrile
with phenylboronic acid.20a,33
The first runs were conducted in DME. Low conversion to the
coupling product 5 was observed with 1.1 equiv. of PhB(OH)2
and 2.2 equiv. of K3PO4 at room temperature (Table 5, entry 1).
Increasing the temperature to 82 ◦C greatly improved the yield
(entry 2), and increasing the PhB(OH)2 and K3PO4 loadings to
1.3 and 2.6 equiv., respectively, led to an even better conversion
of 90% after 2 h (entry 3). Under these conditions, acetonitrile
was not effective (entry 4), whereas toluene proved to be the
best solvent. Full conversion was indeed observed in this solvent,
though with a slightly decreased selectivity, as small amounts of the
homocoupling product 4,4¢-acetylbiphenyl 6 were detected (entry
5). In the latter solvent, it even proved to be possible to decrease
the reaction time to only 40 min to reach full conversion (entry 6).
The catalytic activities of the various half-sandwich NHC-
nickel(II) complexes 1–4 were next examined under the standard
conditions established with complex 3b, i.e.: with 1.3 equiv. of
PhB(OH)2 and 2.6 equiv. of K3PO4 in toluene at 90 ◦C (Table 6).
All complexes are catalytically active and give the desired product
5 in moderate to excellent yield in 15 min, which is a very
fast reaction time for nickel-catalyzed Suzuki couplings. Typical
reactions times, indeed, usually range under similar conditions
from 1–2 h at best to 12–24 h at worst.20,23
Longer reaction times with the Cp species 2 and 4 afforded only
slightly improved conversions (entries 7, 9 and 11). Attempts to
decrease the catalyst loadings of the Cp* species 1b and 3a to
1 mol% led to only 63 and 61% conversion, respectively, after 1 h
◦
at 110 C (entries 12 and 13). These results indicate fast catalyst
deactivation.
The precise reaction mechanism remains to be elucidated.
Nevertheless, a few comments can be made. The absence of
significant reactivity difference between the cationic and the
neutral complexes suggest that the necessary creation of a vacant
site might arise through Cp or Cp* ring slippage rather than
acetonitrile or chloride dissociation. We have indeed observed
many cases of Cp* ring slippage in NiCp* complexes,34 and our
recent work with CpNi–NHC complexes has also shown that Cp
ligands are much more labile than expected in such systems.6a In
addition, it is noteworthy that traces of biphenyl were observed in
all cases. This suggests initial reduction of the Ni(II) precursor to a
Ni(0) active species by the homocoupling of phenylboronic acid via
a mechanism previously postulated for related Pd(II) species24 and
Surprisingly, the neutral or cationic nature of the complexes
had almost no influence on the reaction yields and rates (Table 6,
entries 1, 4, 8 and 9 vs. 3, 5, 10 and 11, respectively). A marked
difference is observed between the Cp-complexes 2a and 2b (entries
6–9), which may be explained by the better steric protection
of the active site by the more bulky IPr ligand. Although still
present, this influence is less prominent with the more bulky,
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for other nickel based systems.23 Initial reduction to an unstable
Ni(0) species is further corroborated by the very high air sensitivity
of the active species, and the fast colour change of the reaction
medium (after only 1–2 min at 90 ◦C).
Table 7 Suzuki–Miyaura cross-coupling of aryl bromides and chlorides
with phenylboronic acid catalyzed by 1b and 3a a
To get some further insight into the roles played by the
different reactive species present in the medium and in particular
by the phenylboronic acid, we conducted a series of control
experiments. The first one was carried out in the absence of
the phenylboronic acid but in otherwise unchanged conditions.
No reaction was observed, nor was there any colour change
of the reaction medium. The 4¢-bromoacetophenone and the
catalyst precursor were found to be unchanged by 1H NMR
analysis of the crude mixture. A second control experiment was
run in the absence of 4¢-bromoacetophenone. In this case, the
colour of the reaction medium changed in a couple of minutes
Entry
Cat.
R
X
Conv.b (%)
Select.b (7/8)
1
2
3
4
5
6
7
8
3a
1b
3a
1b
3a
1b
3a
1b
COMe
COMe
OMe
OMe
OMe
OMe
Me
Cl
Cl
Br
Br
Cl
Cl
Br
Br
92
91
70
21
93/7
100/0
90/10
88/12
—
92/8
100/0
100/0
1
0
as observed during the typical catalytic runs. Unfortunately H
12c
31d,e
49d,e
NMR analysis of the medium gave a complicated spectrum
that could not be interpreted. Finally a last control experiment
was run in the absence of base. A rapid colour change was
observed, and signals of the catalyst precursor were not observable
Me
a Reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.3 mmol),
K3PO4 (2.6 mmol) as base, 1b or 3a (3 mol%), toluene (3 mL), 90 ◦C, 1 h.
b According to NMR. c Isolated yield. d Run at 110 ◦C. e GC yield.
1
any more by H NMR; nevertheless no coupling had occurred.
These control experiments demonstrate the crucial role played by
phenylboronic acid in the generation of the catalytically active
species (presumably a Ni(0) species as discussed above), as well as
the necessary presence of K3PO4 for this nickel complex to achieve
the cross-coupling reaction.
The better results observed with complexes 1 and 3 bearing
the bulky Cp* ligand, as compared to the Cp species 2 and 4,
may be explained by a better stabilization of the coordinatively
unsaturated sites through steric protection and electronic donation
and/or by a faster or more efficient reduction of the initial Ni(II)
species by the electron-rich ligand.
As the cationic Cp* species 3a presents the best results in
terms of rate (Table 6, entries 2 and 3) and the neutral Cp*
species 1b, the best results in terms of rate vs. selectivity (entry 4),
the reaction scopes of these two complexes have been examined
with a short series of aryl bromides and chlorides bearing
electron-withdrawing and electron-donating substituents. Results
are presented in Table 7.
acetonitrile. Single-crystal X-ray diffraction studies established
the molecular geometries of the neutral and cationic analogues
1a and 3a. No remarkable structural differences were observed.
Nevertheless, solution NMR spectroscopy established that in
contrast to the sterically congested neutral Cp* species 1a and 1b,
the corresponding cationic complexes 3a and 3b show a greatly
reduced barrier to the nickel–carbene bond rotation at ambient
temperature. In addition, NMR spectroscopy also established that
the acetonitrile ligand of the cationic species is labile in solution.
Hence, the absence of a rotation barrier in 3a and 3b is most
probably due to the labile nature of the acetonitrile ligand in
solution rather than to the smaller size of acetonitrile as compared
to the chloride.
The neutral and the cationic complexes [NiII(NHC)LCp§] (L =
Cl- or NCMe, PF6 ) 1–4 catalyse the Suzuki–Miyaura cross-
-
Excellent yields of 4-acetylbiphenyl were obtained from 4¢-
chloroacetophenone with both catalysts (Table 7, entries 1 and 2).
In addition, a remarkable increase of selectivity was observed with
1b compared to the result obtained with 4¢-bromoacetophenone
(Table 6, entry 4 vs. Table 7, entry 2). It is noteworthy that
excellent selectivity was also observed with 4-bromotoluene
(entries 7 and 8). Electron-donating substrates were however
converted to the desired coupling products in lower yields (entries
3–8) and, excepting 4-bromoanisole, which was converted to
4-methoxybiphenyl in 70% by 3a and only 21% by 1b, the
neutral catalyst precursor was slightly more active with these
substrates.
couplings of aryl halides and phenylboronic acid in the presence of
K3PO4 as the sole additive. The bulky electron-rich Cp* species are
much more efficient than the Cp complexes, and one of the highest
rates for nickel(II)-based catalysts in the absence of a co-catalyst
or reductant was observed for the coupling of phenylboronic acid
and 4¢-bromoacetophenone with 3a. Although the novel cationic
species possess a labile acetonitrile ligand, no substantial benefit
was observed during the catalytic reactions. The necessary creation
of a vacant site might arise through Cp or Cp* ring slippage
rather than acetonitrile or chloride dissociation. The high rates
observed with the Cp* complexes is believed to be derived at
least partially from the presence of the bulky electron-rich Cp*
ligand, which may both facilitate the probable initial reduction to
Ni(0) by phenylboronic acid and stabilize the coordinatively un-
saturated sites through steric protection and electronic donation.
Mechanistic studies are currently under way to try to confirm
these hypotheses. Catalytic studies with other half-sandwich Ni-
NHC complexes that bear a weakly coordinating dangling arm
that may stabilize the coordinatively unsaturated active site (and
hence allow for a better catalyst stability) are also underway.
Conclusions
In summary, cationic Ni(II)-NHC complexes of formula
[Ni(NHC)(NCCH3)Cp§](PF6) (Cp§ = Cp*, NHC = IPr 3a;
Cp§ = Cp*, NHC = IMes 3b; Cp§ = Cp, NHC = IMes 4b)
were isolated in high yields from the reaction of their neutral
counterparts [Ni(NHC)ClCp§] 1a, 1b and 2b with KPF6 in
8158 | Dalton Trans., 2010, 39, 8153–8160
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General procedure for the Suzuki coupling reactions
Experimental
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 vigorous stirring by putting the Schlenk
tube in an oil-bath at 90 ◦C. After 10–60 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 internal standard. NMR yields were determined by
removing a sample with a syringe, drying it under high vacuum,
extracting the residue with CDCl3 and filtering the solution in
the NMR tube. In a standard work up, the solvent was removed
completely 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 value
of at least two runs.
General
All reactions were carried out using standard Schlenk techniques
under an atmosphere of dry argon. Solvents were distilled from
appropriate drying agents under argon prior to use. Solution
NMR spectra were recorded at 298 K on a FT-Bruker Ultra
Shield 300 spectrometer operating at 300.13 MHz for 1H, and at
75.47 MHz for 13C { H}. DEPT 13C spectra were recorded for all
1
complexes to helpinthe 13C signal assignments. The chemical shifts
are referenced to the residual deuterated solvent peaks. Chemical
shifts (d) and coupling constants (J) are expressed in ppm and
Hz respectively. IR spectra of complexes 3a, 3b and 4b were
recorded on a FT-IR Nicolet 380 spectrometer with KBr pellets.
Vibrational frequencies are expressed in cm-1. Elemental analyses
were performed by the Service d’Analyses, de Mesures Physiques et
de Spectroscopie Optique, UMR CNRS 7177, Institut de Chimie,
Universite´ de Strasbourg. Commercial compounds were used as
received. Complexes [Ni(IPr)ClCp*] 1a,29 [Ni(IMes)ClCp*] 1b,29
[Ni(IPr)ClCp] 2a17b and [Ni(IMes)ClCp] 2b16e,30 were prepared
according to published methods.
Synthesis of [Ni(IPr)(NCMe)Cp*](PF6) (3a)
X-Ray diffraction studies. Structure determination and refinement
KPF6 (94 mg, 0.511 mmol) was added to a suspension of 1a
(316 mg, 0.511 mmol) in acetonitrile (5 mL). The mixture was
stirred for 1 h at room temperature. During the first 15 min, the
colour changed from violet to dark yellow. The reaction medium
was filtered through Celite, concentrated to ca. 1 mL, and treated
with diethyl ether (3 mL) to yield an orange–brown solid after
standing at -28 ◦C for 1 h. The mother-liquor was removed by
syringe, and the solid washed with diethyl ether (3 ¥ 1 mL), and
dried under vacuum to give 3a (280 mg, 0.364 mmol, 71%). Anal.
Calc. for C39H54F6N3NiP: C, 60.95; H, 7.08; N, 5.47. Found: C,
Single crystals of 1a and 3a suitable for X-ray diffractions studies
were selected from batches of crystals obtained at –28 ◦C from
toluene (1a) and acetonitrile–toluene (3a) solutions. Diffraction
data for all crystals were collected at 173(2) K on a Nonius Kappa-
CCD area detector diffractometer using graphite-monochromated
˚
Mo-Ka radiation (l = 0.71073 A). A summary of crystal data,
data collection parameters, and structure refinements is given in
Table 1. Cell parameters were determined from reflections taken
from one set of ten frames (1.0◦ steps in phi angle), each at
20 s exposure. All structures were solved using direct methods
with SHELXS-97 and refined against F2 for all reflections using
the SHELXL-97 software.35 Multiscan absorption corrections
(MULscanABS in PLATON) were applied.36 All non-hydrogen
atoms were refined with anisotropic displacement parameters.
Hydrogen atoms were generated according to stereochemistry and
refined as fixed contributors using a riding model in SHELXL-97.
1
61.37; H, 7.22; N, 5.41%. H NMR (CD3CN, 300.13 MHz): see
1
Table 1. 13C{ H} NMR (CD3CN, 75.47 MHz): see Table 2. FT-IR:
n(CN) 2281 (w), n(P–F) 838 (s).
Synthesis of [Ni(IMes)(NCMe)Cp*](PF6) (3b)
Complex 3b was prepared from [Ni(IMes)ClCp*] 1b by a pro-
cedure similar to that used for 3a, and was isolated as an
analytically pure yellow–brown solid in 79% yield. Anal. Calc.
for C33H42F6N3NiP: C, 57.92; H, 6.19; N, 6.14. Found: C, 57.80;
H, 6.19; N, 6.02%. 1H NMR (CD3CN, 300.13 MHz): see Table 1.
Acknowledgements
We thank the Ministe`re de l’Enseignement Supe´rieur et de la
Recherche (doctoral fellowship to A. M. O.), the CNRS and the
Universite´ de Strasbourg for financial support of this work. Dr
Lydia Brelot is gratefully acknowledged for her work in the X-ray
structure resolutions.
1
13C{ H} NMR (CD3CN, 75.47 MHz): see Table 2. FT-IR: n(CN)
2294 (w); n(P–F) 839 (s).
Notes and references
Synthesis of [Ni(IMes)(NCMe)Cp](PF6) (4b)
1 A. J. III Arduengo, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
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Complex 4b was prepared from [Ni(IMes)ClCp] 2b by a procedure
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pure green solid in 84% yield. Anal. Calc. for C28H32F6N3NiP: C,
2 (a) Recent reviews of NHC ligands and their applications in catalysis
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75.47 MHz): see Table 2. FT-IR: n(CN) 2299 (w); n(P–F) 842 (s).
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