Nickel(II) N-Heterocyclic Carbene Complexes: Versatile Catalysts for C–C, C–S and C–N Coupling Reactions

Article abstract of DOI:10.1002/ejic.201700057

A variety of Ni^II complexes with a wide range of electronic and steric properties, bearing picolylimidazolidene ligands (a–g) and Cp (Cp = η⁵-C₅H₅; 2a–f) or Cp* (Cp* = η⁵-C₅Me₅; 3a, c, g) groups, have been synthesised and characterised by using NMR spectroscopy and single-crystal X-ray crystallography. The complexes have been used as precatalysts for a wide range of catalytic transformations, which most likely involve a Ni⁰/Ni^II catalytic cycle. In particular, the new well-defined 2a, 2c, 3a and 3c complexes have demonstrated great efficiency and versatility towards Suzuki–Miyaura coupling reactions, hydroamination of activated olefins and C–S cross-coupling reactions of aryl halides and thiols under mild conditions.

Full text of DOI:10.1002/ejic.201700057

A Journal of
Accepted Article
Title: Nickel(II) N-Heterocyclic Carbene Complexes: Versatile Catalysts
for C-C, C-S and C-N Coupling Reactions
Authors: Pedro Sixto Valerga, Lourdes Benítez Junquera, Francys E.
Fernández, and M. Carmen Puerta
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
Nickel(II) N-Heterocyclic Carbene Complexes: Versatile Catalysts
for C-C, C-S and C-N Coupling Reactions
Lourdes BenítezJunquera, Francys E. Fernández, M. Carmen Puerta* and Pedro Valerga*[a]
Abstract:A variety of Ni(II) complexes with a wide range of
electronic and steric properties, bearing picolyl-imidazolidene ligands
(a-g) and Cp (2a-f) or Cp* (3a,c,g) groups, have been synthesised
and characterised using NMR spectroscopy and single crystal X-ray
crystallography. The complexes have been used as precatalysts for
a wide range of catalytic transformations most likely involving a
Ni0/NiII catalytic cycle. In particular, the new well-defined 2a, 2c, 3a
and 3c have demonstrated great efficiency and versatility towards
Suzuki-Miyaura coupling reactions, hydroaminations of activated
olefins and C-S cross-coupling reactions of aryl halides and thiols
under mild conditions.
advances in the use of Ni-NHC complexes in C-C coupling
reactions was published recently.[7] However, few examples of
Ni-NHC complexes bearing Cp and Cp* have been reported.
The first complex, CpNiCl(IMes), was synthesised by Abernethy
et al in 2000,[13]without testing its potential catalytic activity, and
since then several research groups have prepared similar
complexes bearing other monodentate NHC ligands.[4g,14,15] Ni
complexes with good donors triazolydene ligands have showed
appreciable catalytic activity in aryl-aryl cross-coupling reactions
but they undergo decomposition.[4g]Neutral [Ni(NHC)Cl(5-C5R5)]
and cationic [Ni(NHC)(NCMe)(5-C5R5)](PF6) [NHC
= 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene or 1,3-bis(2,4,6-
trimethylphenyl)-imidazol-2-ylidene; R = H, Me] were reported to
catalyze the cross-coupling of phenylboronic acid with aryl
halides in the absence of co-catalysts or reductants.[15]
Introduction
There is a continuous search for different Ni scaffolds that
will deliver versatile catalytic activity. The potential stabilisation
of Ni catalytic precursors provided by Cp and Cp* ligands, the
electronic and steric differences between these -ligands and
the previous knowledge from our research group
thatdemonstrated the use picolyl-imidazolydene ligands to tune
the steric and electronic nature of ruthenium complexes,[16] led
us towards the synthesis of [CpNi(NHC)]+ and [Cp*Ni(NHC)]+
bearing such ligands. Also, picolyl-imidazolydene ligands could
act as hemilabile groups able to provide the vacant site needed
in many catalytic reactions by decoordination of the pyridine ring.
Herein, we report the facile synthesis of [CpNi(NHC)]+ and
[Cp*Ni(NHC)]+, NHC = 1-(2-picolyl)-3-(R)-imidazol-2-ylidene (R =
Me (a), iPr (b), Ph (c), Mesityl (d)), 1-(2-picolyl)-3,4,5-(trimethyl)-
2-ylidene (e), 1-(2-picolyl)-4,5-(dichloro)-3-(methyl)-imidazol-2-
ylidene (f), 3-methyl-1-(2-picolyl)-benzoimidazol-2-ylidene (g).
The structures of three of these complexes were established by
single-crystal X-ray diffraction studies. The dynamic behaviour of
the metal complexes in solution and the activation free energy of
the process was determined by variable temperature (VT) 1H
NMR spectroscospy. More importantly, the preparation of stable
NiII complexes tuning the electronic and steric properties of the
metal centre proved to deliver complexes that showed excellent
activity and a wide substrate scope in Suzuki-Miyaura coupling
reactions, hydroamination of activated olefins and C-S cross
coupling of aryl halides. The reaction mechanism is likely to
involve a typical NiII to Ni0 reductive elimination pathway.
This work presents a contribution in the line of the replacement
of catalytic systems based in Palladium by others based in less
expensive nickel and cobalt. Not less important is continue the
widespread trend to replace in the catalytic systems phosphine
ligands by others less aggressive for the environment.Transition
metal catalysed coupling reactions are among the most useful
tools for C-C, C-S and C-N bond formation.[1]Palladium-based
2
]
catalysts bearing phosphines[
and more recently, N-
heterocyclic carbene (NHC) ligands have been widely used in
these transformations because they display tolerance towards a
variety of functional groups and versatility in terms of reaction
conditions.[3]Numerous reports have been published regarding
the use of nickel catalysts in many catalytic processes, including
4
5
]
6 ]
cross-couplings,[ ]cycloisomerisations,[
cycloadditions,[
annulations, and others.[7]When using NHC ligands nickel-based
catalytic systems have recently shown excellent results in a wide
variety of transformations.[ 7 ] Reports of Ni-NHC complexes
acting as catalytic precursors in C-C coupling,[4,7] C-S cross
coupling,[ 8 ] C-F bond activation,[7] oxidation of secondary
9
10 ]
alcohols,[ ]hydrosilylation reactions,[
amination of aryl
halides[11] and cycloadditions[6] have been published.
5
5
The use of ƞ -C5H5 (Cp) or ƞ -C5Me5 (Cp*) ligands will lead
to more electron-rich catalytic precursors that may be more
stable due to the steric bulkiness of both ligands described for
many coupling reactions.[ 12 ] A review comprising the recent
[a]
L. Benítez Junquera, F. E. Fernández, M. C. Puerta and P. Valerga
Departamento de Ciencia de los Materiales, Ing. Metalúrgica y
Química Inorgánica
Results and Discussion
Universidad de Cádiz
Synthesis and characterization of the cationic complexes
Facultad de Ciencias, Campus Río San Pedro s/n, 11510, Puerto
Real, Cádiz, España
Fax: (+) 34956016288
[(η5-C5H5)Ni(picolyl-RI)]+
and
[(η5-C5Me5)Ni(picolyl-RI)]+:
Picolylimidazolium salts were obtained as previously described
in the literature[ 17 ] and used as picolyl-NHC precursors. The
initial approach for the synthesis of the cationic complexes [(η5-
C5H5)Ni(picolyl-RI)]+ was the direct addition of the
E-mail: carmen.puerta@uca.es, pedro.valerga@uca.es
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
picolylimidazolium salt to the nickel precursor. When
a
highly soluble in chlorinated solvents and CH3NO2, partially
soluble in acetone and THF, and insoluble in other solvents such
as hexane, diethyl ether and petroleum ether.
dimethoxyethane solution of the nickelocene and a was heated
under refluxed for 24h the corresponding bromide salt was
obtained as a green solid (Scheme 1). Further treatment of the
product with a methanolic solution of NaBPh4 led to complex 1a.
Scheme 1.Synthesis of complex 1a.
The cationic complex 1a was isolated as a green solid in
91% yield. However, the preparation of 1a analogues using
other picolylimidazolium salts was unsuccessful. Hence, the
transmetalation method was the route choice to afford the new
nickel(II) complexes 2a-f (Scheme 2). Silver carbene complexes
were prepared in situ by reaction of the appropriate
picolylimidazolium salt with a slight excess of silver oxide in 1,2-
Scheme 3.Synthesis of complexes 3a, c, g.
All complexes were fully characterised using H, 13C{1H}, 2D
1H-13C NMR spectroscopy and elemental analysis. The H NMR
spectra of the cationic complexes 2a-f and 3a,c,g lack the C2-H
imidazolium proton signal at 10-12 ppm, characteristic of the
free imidazolium salts indicating the effective coordination of the
1
1
dichloroethane
(DCE).
The
nickel
precursor
[(η5-
C5H5)Ni(COD)][BF4][18] was added to the corresponding silver
carbene solution, generating the new nickel picolyl-NHC
complexes 2a-f as green solids in good to high yields (60-93%).
The reaction was completed by heating overnight at 80ºC. All
the cyclopentadienyl nickel picolyl-NHC complexes are very
soluble in THF, acetone, and chlorinated solvents, but insoluble
in other solvents such as hexane, diethyl ether and petroleum
ether.
1
picolyl-NHC to the metal centre. It is important to note that H
NMR spectra of 2a-fshow the proton resonance of the
methylene bridge group as a singlet, in contrast to previously
reported picolyl-carbene complexes where the two diasterotopic
protons appeared as a doublet. This observation indicates that
the system is highly symmetric. A similar behaviour was
observed by Chetcuti and coworkers[12] while analysing the NMR
spectrum of [Ni(IMe)I(η5-C5H5)] (IMe = 1,3-dimethylimodazol-2-
ylidene). However, 3a,c,g, bearing the more sterically
encumbered ligand, (η5-C5Me5), showed the methylene bridge
group resonances as a pair of AB doublets at 5.6-5.7 ppm with
coupling constants of 14-15 Hz. In fact, analogous NMR features
for (η5-C5Me5)Ru chelating κ2C,N picolyl-NHC ligands[16] and
chelating κ2P,N-phosphinopicoline ligands[
20
]have been
observed. The (η5-C5H5) ligand in 1H NMR spectra of 2a-f
showed a proton resonance as a singlet at 5.11–5.70 ppm whilst
the analogous (η5-C5Me5) ligand in 3a,c,g appeared as a singlet
at 1.30-1.60 ppm.
Scheme 2.Synthesis of complexes 2a-f.
The 13C{1H} NMR spectra of complexes 2a-f showed the
carbene carbon atom at 157.3-165.3 ppm (in CD3NO2). These
signals are upfield of the signals corresponding to 3a,c,g that
appeared at 171.2-172.3 ppm indicating increased –back-
donation from the more electron-rich nickel atom in the {(η5-
C5Me5)Ni} complexes in comparison to their {(η5-C5H5)Ni}
analogues.In addition to the spectroscopic data, X-ray structure
analyses of compounds 1a, 2c and 2f have been carried out.
Suitable crystals were grown from slow diffusion of pentane into
concentrated dichloromethane solutions of the complexes.
Figure 1-3 shows the ORTEP view of the cation in 1a, 2c and 2f,
respectively.
Attempts to prepare [(η5-C5Me5)Ni(picolyl-RI)]+ by direct
reaction
of
the
nickelocene
[(η5-C5Me5)2Ni]
and
picolylimidazolium salts were not successful. Thus, an
19
]
alternative nickel precursor, [(η5-C5Me5)Ni(acac)],[
was
employed, as described by Chetcuti et. al.,[12] in the synthesis of
complexes [Ni(NHC)X(η5-C5Me5)] (X = I, Cl). The starting
material [(η5-C5Me5)Ni(acac)] was prepared by reaction of
anhydrous [Ni(acac)2] with Li(η5-C5Me5) in THF according to the
procedure reported by Manriquez and coworkers.[19]The
appropriate picolylimidazoium salt was added to a suspension of
the in situ prepared nickel complex, and after stirring the
corresponding mixture for 16 h at 60 ºC, cationic complexes
3a,c,g were obtained (Scheme 3). The new NiIIpicolyl-NHC
complexes [(η5-C5Me5)Ni(picolyl-RI)]+3a,c,g were isolated as
In the case of [(η5-C5Me5)Ni(picolyl-RI)][Br] complexes 3a,c,g
all attempts of recrystallization were not successful. Thus, an
anion exchange process of bromide for tetraphenylborate was
completed. The compound 3c’ was collected as a green solid in
green
solids
in
high
yields
(70-80%).
All
the
pentamethylcyclopentadienyl nickel picolyl-NHC complexes are
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
almost quantitative yield after treatment of 3c in methanol with
an excess of NaBPh4. Recrystallization by slow diffusion of
pentane into a concentrated dichloromethane solution of 3c’
generated crystals suitable for X-ray structure determination.
Figure 4 shows the ORTEP view of the cation in 3c’.
Figure 4.ORTEP view of the cation in complex 3c’ with the thermal ellipsoids
at 50% probability.
Figure 1.ORTEP view of the cation in complex 1a with the thermal ellipsoids at
The molecular structures of these complexes are very
similar. A list of selected bond lengths and angles for all
complexes appear in Table 1. All complexes show a two-legged
piano stool geometry with the nickel atom bonded to a η5-C5H5
(1a, 2c, 2f) or η5-C5Me5 (3c’) group, and the corresponding
chelating κ2C,Npicolyl-NHC ligand. The nickel atom lies in the
centre of a distorted trigonal plane formed by the η5-C5R5 ring
centroid, the nitrogen N(3) of the pyridine moiety and the NHC
ligand; the sum of the angles is 360º in all structures. However,
the angles are far from their ideal 120º value for a trigonal
structure. Particularly, the bond angles formed by the
carbeniccarbon, the metal and the nitrogen atom N(3) of the
pyridine, C(1)-Ni-N(3), are 89.43(10), 91.72(14), 91.17(7) and
90.22(16) for 1a, 2c, 2f and 3c’, respectively. These values are
slightly shorter than those observed for closely related
[Ni(NHC)X(η5-C5R5)] complexes[14d,Fehler! Textmarke nicht
definiert.a,21],undoubtedly due to the chelating character of the
picolyl-NHC ligand.
50% probability.
Figure 2.ORTEP view of the cation in complex 2c with the thermal ellipsoids at
50% probability.
Table 1. Selected bond lengths (Å) and angles (deg) for complexes 1a, 2c, 2f,
and 3c’[a]
.
1a
2c
2f
3c’
Ni-Ccarbenic
Ni-N(3)
1.854(2)
1.919(2)
1.744(1)
89.48(10)
133.98(7)
135.53(6)
1.868(4)
1.911(3)
1.7459(6)
91.72(14)
133.1(1)
135.1(1)
1.868(18)
1.920(16)
1.7438(3)
91.17(7)
133.81(5)
134.69(6)
1.880(5)
1.922(4)
1.7483(6)
90.22(16)
132.56(11)
133.94(12)
[a]
Ni-Cp†
cent
C(1)-Ni-N(3)
N(3)-Ni-Cp†
[a
[a
cent
C(1)-Ni-Cp†
cent
Figure 3.ORTEP view of the cation in complex 2f with the thermal ellipsoids at
50% probability.
[a] Cp†cent= η5-C5H5 (1a, 2c, 2f) or η5-C5Me5 (3c’).
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
The nickel-carbene carbon bond lengths are not
significantly different from each other [1.854(2), 1.868(4),
1.868(18) and 1.880(5) Å for 1a, 2c, 2f and 3c’, respectively].
Also, the values are comparable with similar Ni-NHC
complexes.[14d,Fehler! Textmarke nicht definiert.a,21]
1
VT NMR Studies: Taking as a representative example the H
NMR spectra of 2c and 3c it was evident that at room
temperature the methylene bridge protons of the 1-(2-picolyl)-3-
(phenyl)-imidazol-2-ylidene ligand appeared as a broad singlet
for 2c and as a pair of doublets for 3c. The doublets are part of
an AB spin system that are magnetically non-equivalent at room
temperature for 3c which is bearing the more sterically hindered
Cp* ligand.
Variable-temperature data from 298 to 193 K was collected
on CD2Cl2 solution of 2c. At low temperatures the methylene
bridge protons appeared as a doublet that coalesce into a
singlet above 264 K (Figure 5), most likely due to an averaging
of the anisotropy of the methylene group environment through
small angle oscillations about the N-Ni and N-Ccarbene bonds
coupled with oscillations of the phenyl group on the N-Car bond.
The rates (k) (s-1) of the process at different temperatures were
simulated (See supporting information for details). Finally, using
an Eyring plot, a free energy of activation (ΔGǂ)of 35 ± 1 kJ mol-1
for the dynamic process in solution was determined.
Variable-temperature 1H NMR data from 268 to 353 K on
CD3NO2 solution of 3cwere obtained. A similar dynamic process
was observed in comparison to 2c. However, in this case at
room temperature the methylene protons are non-equivalent, but
above 345 K the doublets coalesce into a broad singlet (Figure
S1, Supporting info). This behaviour indicates that the more
sterically constrained complex, 3c, has more restricted angles of
oscillations and C-Nar bond rotations. Previous studies on the
rotational barriers on metal-NHC complexes have shown that
steric effects have a predominant role in such dynamic process
in line with our observations.[14d]
Figure 5. Stacked 1H NMR spectra ilustrating the evolution of the CH2-bridge
signal of the picoly-imidazolydene ligand in 2c.
Catalytic activity in Suzuki-Miyaura coupling reactions:
Suzuki-Miyaura cross coupling reaction is one of the most useful
methods for the formation of C-C bonds.[1,4,6]Traditionally, this
reaction has been carried out using aryl halides as electrophiles,
aryl boronic acids as nucleophiles and Pd based catalysts.[22]
Some nickel complexes have shown activity towards this
transformation, including recently described nickel complexes
bearing monodentate NHC ligands.[7]Aiming for the generation
of new and versatile catalysts, complexes 2a-f and 3a,c,g were
synthesised and tested on Suzuki-Miyaura reactions. Initial
studies focused on the optimization of reaction conditions,
catalysts loading and solvent. Thus, we have studied the
reaction of 4-bromoacetophenone with phenyl boronic acid in
the presence of 2a as a model to optimise the reaction
conditions (see Supporting Information). The reaction optimised
conditions were used to complete a catalyst screening (Table 2).
The results indicate that complexes 2a-d and 3a,c led to the
desired cross-coupling product (A) in good to high yields (Table
2), but complex 2f and 3g yielded the homocoupling product
(biphenyl) (B). It is possible to obtain products of aldol
condensation or alpha-arylation of ketones reactions. However,
these products were not observed. Furthermore, considering
the results it is clear that cyclopentadienyl nickel complexes
(entries 1-6) showed better catalytic activity than its
Furthermore, it is possible that the averaging process of the
methylene protons might be due to
a
conformational
isomerisation of the ‘chair-boat’ type, similar to that occurring in
cyclohexene which has a typical energy of activation of ca. 23
KJ mol-1 (Scheme 4). The six-membered metallacycle of
compounds 2c and 3c are stereochemically more crowed, which
explains the higher activation energy obtained for 2a.
Scheme 4. ‘chair-boat’ metallacycle isomerisation
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
pentamethylcyclopendienyl analogues (entries 7-9) in terms of
yields and selectivity towards the cross-coupling product.
Besides, cyclopentadienyl nickel complexes bearing Me, iPr and
Ph groups (2a, 2b and 2c, respectively) as wingtip substituents
in the NHC moiety (entries 1-3) showed higher conversion
towards the desired product (A) in comparison with their mesityl
(2d), 4,5-dimethyl (2e) and 4,5-dichloro (2f) substituted
analogues. This indicates that electronic effects in the NHC
ligands ruled by the 4,5-position substituents play an important
role in the catalytic activity, although considering that the mesityl
substituted NHC led to lower yields we cannot discard that steric
properties also have an effect in the catalytic performance of the
new nickel complexes. Entry 3 clearly indicates that 2c is the
most active complex showing high levels of selectivity.
Entry
1
R
Yield [%][b]
91
COMe
2
3
4
5
6
COH
CF3
CN
90
85
83
85
73
Me
OMe
Table 2.Suzuki-Miyaura catalyst screening[a]
.
[a] Reaction conditions: substrate (1 mmol), phenylboronic acid (1.3 mmol),
K3PO4 (2.6 mmol), 2c (5 mol%), toluene (3 mL), 1h, 110ºC. [b] Product yield
and selectivity determined by GC-MS and 1H NMR using 1,3,5-
trimethoxybenzene as an internal standard.
cat (5 mol%)
O
Br
A
B
To understand if any steric effect in the substrate will affect
the performance of 2c in the coupling reactions, a comparison
between 2-, 3- and 4-bromoacetophenone was completed
O
B(OH)2
K3PO4 tolueno
110ºC
(Table
substitutedbromoacetophenone derivatives led to lower
reactions yields (entries and 2) in comparison to p-
4).
The
use
of
ortho
and
meta
Entry
Catalyst
2a
Time [min]
Yield [%][b]
Selectivity A/B[b]
100/0
1
bromoacetophenone. The results indicate that the catalytic unit
may have some steric bulk probably due to the presence of the
phenyl group in the picolyl-imidazolydene ligand.
1
2
3
4
5
6
7
8
9
90
60
60
90
90
90
90
90
90
88
80
91
56
63
2
2b
100/0
2c
100/0
Table 4.Suzuki-Miyaura coupling of bromoacetophenones.catalysed by 2c.
2d
100/0
2e
100/0
2f
0/100
3a
66
58
1
90/10
3c
95/5
Entry
1
Substrate
Product
Yield [%][b]
3g
0/100
[a] Reaction conditions: 4-bromoacetophenone (1 mmol), phenylboronic acid
(1.3 mmol), K3PO4 (2.6 mmol), catalyst loading (5 mol%), toluene (3 mL),
110ºC. [b] Product yield and selectivity determined by GC-MS and 1H NMR
using 1,3,5-trimethoxybenzene as an internal standard.
3’-bromoacetophenone
74
The tolerance to different functional groups in the substrate
sheds light onto the versatility of any catalyst. In this case,
several aryl bromides substituted with electron-withdrawing and
electron-donating groups were coupled with phenyl boronic acid
using 2c (Table 3).The presence of electron-withdrawing
fragments, from moderate (-CHO, -COMe) to strong (-CF3, -CN)
yielded the desired products in good to excellent yields (entries
1-4). Besides, the presence of electron-donating groups did not
affect detrimentally the activity of 2c and good yields were
obtained of the desired coupling products with methyl and
methoxy groups as substituents (entries 5-6).
2
2’-bromoacetophenone
76
[a] Reaction conditions: substrate (1 mmol), phenylboronic acid (1.3 mmol),
K3PO4 (2.6 mmol), 2c (5 mol%), toluene (3 mL), 110ºC. [b] Product yield and
selectivity determined by GC-MS and 1H NMR using 1,3,5-trimethoxybenzene
as an internal standard.
The use of non-aromatic bromides as substrates has proved
to be more challenging in terms of leading to the desired Suzuki-
Miyaura coupling products. Thus, we have assessed the scope
of 2c using allyl bromide and cinnamyl acetate as substrates
(Table 5). The results indicate that 2c is capable of giving the
Table 3.Suzuki-Miyaura coupling with aryl bromides by 2c[a]
.
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
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Scheme 5.Catalytic hydroamination of activated olefins.
corresponding allyl benzene in good yields (entry 1). Also, when
a nonaromatic acetate instead of the typically used halides was
used as substrate very good yields of the desired coupling
product were generated (entry 2).
The versatility of the new Ni-NHC complexes was further
assessed for the cross-coupling of amines and activated olefins.
An initial screening of the activity of the Ni-NHC complexes was
completed using morpholine and ethyl acrylate as substrates
under typical hydroamination conditions (Table 6). The results
indicate that catalyst 2a,c and 3a,c lead to the desired coupling
products in good yields (entries 1,3,7 and 8). The presence of
bulkier substituents on the imidazolidene ligand decreased the
activity of the Ni-NHC complexes as was previously observed in
the Suzuki-Miyaura reactions.
Table 5.Suzuki-Miyaura coupling with allyl bromides and esters by 2c.
Entry
1
R
H
R1
H
X
Yield [%][b]
65
Table 6.Catalyst screening for the hydroamination of morpholine with ethyl
Br
acrylate [a]
.
2
Ph
H
OOCMe
82
H
[a] Reaction conditions: substrate (1 mmol), phenylboronic acid (1.3 mmol),
K3PO4 (2.6 mmol), 2c (5 mol%), toluene (3 mL), 110ºC. [b] Product yield and
selectivity determined by GC-MS and 1H NMR using 1,3,5-trimethoxybenzene
as an internal standard.
Entry
Catalyst
2a
Yield [%][b]
The reaction mechanism has not been elucidated. However,
some comments can be made taking as reference the
mechanistic aspects already reported for related compounds.[7a]
The significant reactivity difference, in terms of reaction yields
and selectivity, between the Cp and Cp* nickel complexes
suggest that the necessary vacant site might arise through the
dissociation of the pyridine ring rather than Cp or Cp* ring
slippage. As the Cp species 2a,c showed the best results on
yields and selectivity, steric effects may influence the reaction
mechanism. This hypothesis supports the observation that the
presence of the more sterically hindered Cp* ring led to lower
yields and selectivity. The electronic stabilisation expected from
the more electron rich Cp* ligand does not generate better
catalytic activity likely because its stronger donor properties
strengthen the Ni-N bond. Our results differ from other CpNi-
NHC and Cp*Ni-NHC complexes where slippage of the Cp* and
Cp ring has been observed.[23]
1
2
3
4
5
6
7
8
9
66
-
2b
2c
49
-
2d
2e
6
2f
-
3a
66
54
43
3c
3g
[a] Reaction conditions: secondary amine (0.5 mmol), activated olefin (1.0
mmol), AgOTf (10 mol%), cat (5 mol%), acetonitrile (3 mL), room temperature,
24 h. [b] Product yield and selectivity determined by GC-MS and 1H NMR
using 1,3,5-trimethoxybenzene as an internal standard.
Other nickel complexes have been used as catalyst for the
Suzuki-Miyaura couplings given the need to find cheaper
alternatives to the widely used Pd catalysts. Most of the reported
[24]
The substrate scope of the catalytic system was assessed
testing different activated olefins and secondary amines. The
products showed anti-Markovnikov regiochemistry across all the
activated olefinic substrates. Moderate to good yields were in all
cases obtained (Table 7). Particularly, the most activated olefin,
acrylonitrile, led to higher yields disregarding the differences on
the secondary amines (entries 1,4 and 7). Besides, when
morpholine was used as substrate higher reaction yields were
observed in comparison to pyperidine and pyrrolidine.
examples are derivatives of NiCl2(PCy3)2 and more recently a
few Ni-NHC complexes have been described.[7] However, 2c
has shown a wider substrate tolerance and does not require the
addition of reducing agents[25] to facilitate the required reduction
of NiII to Ni0. Only a few reports in literature describe Ni-NHCs
catalytic systems for Suzuki-Miyaura couplings that do not
require the use of reducing agents.[26]
Catalytic activity in C-N cross coupling reactions: Transition
metal catalysed hydroamination of olefins has been widely
explored using palladium catalysts (Scheme 5).[2a,b]However,
few examples have been reported of nickel catalysed
hydroamination of olefins probably due the difficult stabilisation
of the highly reactive Ni species.[11e]
Table 7. Catalytic hydroamination of activated olefins with complexes 2a,c and
3a,c.
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
Yield [%][b]
Entry
Catalyst
2a
Yield [%][b]
Entry
X
n
R
Product
2a
75
2c
3a
77
3c
1
2
3
4
5
6
99
13
92
35
11
0
2b
1
2
3
4
5
6
7
8
9
O
2
2
2
2
2
2
1
1
1
CN
89
80
2c
O
COOMe
COOEt
CN
63
66
72
50
51
65
50
62
40
49
77
42
45
60
41
37
70
66
71
65
68
73
52
37
69
54
71
62
54
78
59
27
2d
O
2e
2f
CH2
CH2
CH2
CH2
CH2
CH2
[a] Reaction conditions: thiophenol (1.1 mmol), 4-iodotoluene (1.0 mmol),
NaOtBu (1.2 mmol), cat (5 mol%), DMF (3 mL), 100ºC, 24 h. [b] Product yield
and selectivity determined by GC-MS and 1H NMR using 1,3,5-
trimethoxybenzene as an internal standard.
COOMe
COOEt
CN
To assess the substrate scope of the C-S cross coupling
reaction catalysed with 2a,c several electron-rich aryl halides
and thiolphenols were evaluated (Table 9). Electron-poor aryl
halides were not used because they may undergo the
nucleophilic substitution reaction instead of the metal-catalysed
reductive elimination pathway. The results indicate that altering
the electronic and steric nature of the aryl iodides did not have a
significant effect in the reaction yields (entries 2-6). All products
were generated in good to quantitative yields. Selectivity
towards the iodo derivative was observed on the coupling of 1-
chloro-4-iodobenzene with thiophenol and 4-chlorothiophenol
affording the corresponding 4-chlorophenyl thioether as the only
product (entries 6 and 20). As expected, aryl bromides and
chlorides generally led to lower yields of the corresponding
thioether in comparison to the equivalent aryl iodide (entries 9-
13). Besides, the presence of electron-donor or electron-
withdrawing groups in the para position of the thiophenol did not
affect significantly the yields of C-S coupling product (entries 14-
23). Furthermore, a heteroaromatic aryl bromide generated the
desired coupling product in moderate to quantitative yields
depending on the thiophenol electronic nature (entries 10, 19
and 23). Finally, in the absence of the nickel catalyst no coupling
products and no significant differences were observed in the
performance of 2a in comparison to the more sterically hindered
2c.
COOMe
COOEt
[a] Reaction conditions: secondary amine (0.5 mmol), activated olefin (1.0
mmol), AgOTf (10 mol%), cat (5 mol%), acetonitrile (3 mL), room temperature,
24 h. [b] Product yield and selectivity determined by GC-MS and 1H NMR
using 1,3,5-trimethoxybenzene as an internal standard.
Regarding the reaction mechanism,it is important to note
that no significant differences were observed between the CpNi-
NHC (2a,c) and Cp*Ni-NHC complexes (3a,c) which suggest Cp
or Cp* ring slippage to generate the required coordination
vacancy.[27]
Catalytic activity in C-S cross coupling reactions: The
synthesis of compounds containing C(aryl)-S bonds has proven
to be fundamental in biological, pharmaceutical and material
science applications.[28] Palladium, cooper, nickel, cobalt and
iron-based catalytic systems have been successful in C-S cross
coupling reactions. In the particular case of nickel, high catalyst
loadings are often required[29] with the exception of the use of
well-defined Ni-NHC complex reported by Nicasio and
coworkers.[8b]Recently, Zhang et. al.[8a]described the used ofNi-
NHC catalyst for C-S couplings. The presence of a bidentate
ligand should prevent the formation of anionic or bridging
thiolate complexes. Taking into consideration the good results
obtained in the C-C and C-N coupling reactions with the new Ni-
NHC catalytic system,thenaturalconsequence was to check their
performance in the catalytic thiolation of aryl halides with thiols.
An initial screening of the Ni-NHC catalysts (2a-f) was
carried out with thiophenol and 4-iodotoluene as representative
substrates (Table 8). In order to minimise the formation of
undesired phenyl disulfide the experiments were conducted with
a slight excess of the aryl iodide. The results indicate that 2a
and 2c are again the most active catalysts (entries 1 and 3)
leading to almost quantitative yields.
Table 9. Catalysed C-S cross coupling of thiophenol and its derivatives with
aryl and alkyl halides using 2a,c[a]
.
Yield [%][b]
2a
Table 8. Catalysed C-S cross coupling of thiophenol and 4-iodotoluene with
Entry
R1
Halide
aryl and alkyl halides using 2a,c[a]
.
2c
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
a Ni(0) complex as part of a reductive elimination pathway
although other options can not be totally discarded. Further
studies would be required in this regard. Finally, given the
comparable performance of 2a and 2c it is likely that the needed
vacant site for the catalytic reaction is generated via Cp ring
slippage.
1
H
H
85
99
99
80
67
95
99
45
90
80
<5
27
99
99
98
99
81
99
99
90
89
25
89
92
96
75
59
98
99
32
81
71
35
15
99
99
96
99
80
99
80
89
89
21
2
3
H
4
H
5
H
Conclusions
6
H
We have prepared and characterised new examples of NiII
complexes bearing picolyl-NHC supported by hydrocarbylCp
and Cp* ligands. The change of the cyclopentadienyl (Cp) group
for the more electron-rich pentamethylcyclopentadienyl ligand
allowed the generation of complexes with a wide range of
electronic and steric properties. Complexes 1a, 2c, 2f and 3c’
were unequivocally characterised using single crystal X-ray
crystallography.
1H NMR spectra revealed an interesting dynamic behaviour
of the studied compounds in solution that can be attributed to
restricted oscillations and rotations of the Ni-Ccarbene bond and/ or
the existence of a ‘chair-boat’ conformational isomerisation in
the metalacycles.
The new nickel complexes showed activity in several
catalytic transformations. Particularly, the well-defined
[CpNi(picolyl-NHC]BF4 complex (2c) exhibited activity and wide
tolerance to substrates in Suzuki-Miyaura reactions,
hydroamination of activated olefins and C-S cross coupling
reactions of thiols with aryl halides. The reaction conditions
described were mild, the yields were good to excellent and the
catalyst loadings of 5 mol% showed that the new NiIIcomplex is
a versatile catalytic precursor for many applications. The
reaction mechanism probably involves Ni0/NiII catalytic cycle
where the appropriate coordination vacancy is generated either
due to the hemilability of the picolyl-NHC ligand or ring slippage
of the cyclopentadienyl ligand.
8
H
9
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Me
Me
Me
Experimental Section
Preparation of complex 1a.Nickelocene[31](0.56 g, 3.0 mmol) and a
(0.80 g, 3.1 mmol) were refluxed in dimethoxyethane for 24 h. The
mixture was filtered; the resulting solid washed with petroleum ether and
dried under vacuum. The solid was added to a NaBPh4 (1.50 g, 4.4
mmol) solution in methanol and was stirred at room temperature for 1 h.
The precipitated was filtered, washed with ethanol and petroleum ether,
and dried under vacuum. Complex 1a (1.16 g, 91% yield) was obtained
as a green microcrystalline solid.1H NMR (400 MHz, [D6]acetone, 298 K,
TMS): δ = 8.60 (d, 3J(H,H) = 4.98 Hz, 1H, CHpyridine), 7.85 (t, 3J(H,H) =
7.63 Hz, 1H, CHpyridine), 7.60 (d, 3J(H,H) = 7.63 Hz, 1H, CHpyridine), 7.49 (d,
[a] Reaction conditions: secondary amine (0.5 mmol), activated olefin (1.0
mmol), AgOTf (10 mol%), cat (5 mol%), acetonitrile (3 mL), room temperature,
24 h. [b] Product yield and selectivity determined by GC-MS and 1H NMR
using 1,3,5-trimethoxybenzene as an internal standard.
In terms of the reaction mechanism no further experiments
were conducted. However, a recent review collect different
mechanistic possibilities in cross-coupling reactions catalyzed by
similar systems[30] and we can hypothesise that the CpNi-NHC
complexes 2a,c act as catalyst precursors like other Ni(II)
complexes previously reported. The active species most likely is
3
3J(H,H) = 1.76 Hz, 1H, CHimidazole backbone), 7.16 (t, J(H,H) = 5.57 Hz, 1H,
CHpyridine), 7.11 (d, 3J(H,H) = 2.05 Hz, 1H, CHimidazole backbone), 5.66 (s, 5H,
C5H5), 5.47 (s, 2H, CH2), 3.70 ppm (s, 3H, CH3); 13C{1H} NMR (400 MHz,
[D6]acetone, 298 K, TMS): δ = 164.86 (q, 1J(B,C) = 49.37 Hz, BPh4),
160.85 (Ni-C), 160.56 (Cpyridine), 156.77 (Cpyridine), 140.19 (Cpyridine), 136.97
(d, 2J(B,C) = 10.85 Hz, BPh4), 126.50 (Cpyridine), 125.99 (BPh4), 125.21
(Cpyridine), 124.81 (Cimidazol), 124.12 (Cimidazol), 122.24 (BPh4), 93.40 (C5H5),
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
FULL PAPER
54.72 (CH2), 38.29 ppm (CH3); elemental analysis calc. (%) for
C39H36BN3Ni, C 76.01, H 5.89, N 6.82; found: C 76.12, H 5.91, N 6.79.
General procedure for the catalytic arylation of thiols with aryl
halides: Thiol (1.1 mmol), aryl halide (1.0 mmol), catalyst 5 mol% (0.05
mmol), sodium tert-butoxide (1.2 mmol), 1,3,5-trimethoxybenzene (0.25
mmol) and dimethylformamide (3 mL) were placed on a 10 mL vial and
stirred at 100ºC for 24 h. Aliquots (0.2 mL) were taken at fixed times,
quenched in diethyl ether (3 mL), and filtered through a short pad of SiO2.
The filtrate was subject to GC-MS and 1H NMR analysis. All data
reported is an average of at least two runs.
Preparation of complex 2b.
A dichloroethane suspension of 3-
isopropyl-1-(2-picolyl)imidazolium bromide b (0.28 g, 1.0 mmol) and
Ag2O (0.12 g, 0.5 mmol) was stirred in the dark at room temperature for
3h. The pale yellow suspension was filtered through a pad of Celite into
(η5-C5H5)Ni(COD)][BF4] (0.22 g, 0.7 mmol) and stirred at 80ºC for 16 h.
The reaction mixture was filtered through a pad of Celite to remove the
silver salt. The solvent was removed under reduced pressure. After
washing the resulting solid with petroleum ether, complex 2b (0.17 g,
60% yield) was obtained as a green microcrystalline solid.1H NMR (400
MHz, [D3]nitromethane, 298 K, TMS): δ = 8.59 (d, 3J(H,H) = 5.58 Hz, 1H,
CHpyridine), 7.91 (t, 3J(H,H) = 7.91 Hz, 1H, CHpyridine), 7.68 (d, 3J(H,H) =
7.62 Hz, 1H, CHpyridine), 7.54 (d, 3J(H,H) = 2.05 Hz, 1H, CHimidazole backbone),
7.21 (t, 3J(H,H) = 5.57 Hz, 1H, CHpyridine), 7.16 (d, 3J(H,H) = 2.06 Hz, 1H,
CHimidazole backbone), 5.65 (s, 5H, C5H5), 5.54 (s, 2H, CH2), 4.42 (m, 1H,
CH(CH3)2), 1.48 ppm (d, 3J(H,H) = 6.74 Hz, 6H, CH3); 13C{1H} NMR (400
MHz, [D3]nitromethane, 298 K, TMS): δ = 160.51 (Ni-C), 159.83 (Cpyridine),
157.32 (Cpyridine), 140.32 (Cpyridine), 126.37 (Cpyridine), 125.20 (Cpyridine),
124.86 (Cimidazol), 119.09 (Cimidazol), 93.69 (C5H5), 55.13 (CH2), 53.83
(CH(CH3)2), 28.67 ppm (CH(CH3)2); elemental analysis calc. (%) for
C17H20BF4N3Ni, C 49.58, H 4.89, N 10.20; found: C 49.71, H 4.88, N
10.23.
X-ray crystallography: CCDC 1424252-1424256 contain the
crystallographic supplementary data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cifExperimental
Details.supplementary data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cifExperimental Details.
Acknowledgements
We thank the Spanish MICINN (Projects CTQ2007-60137/ BQU
and CTQ2010-15390) and “Junta de Andalucía“ (PAIDI-FQM188
and Project of Excellence P08-FQM-03538) for financial support.
L.B.J. acknowledges fellowship support from “Junta de
Andalucía“ (ref. 2009-126).
Preparation of complex 3c.A suspension of [(η5-C5Me5)Ni(acac)] (0.38
g, 1 mmol) and 3-phenyl-1-(2-picolyl)imidazolium bromide c (0.32 g, 1.0
mmol) in THF was stirred at 60 ºC for 16 h. The resulting crude solution
was filtered through a pad of Celite and concentrated up to 3-5 mL. After
precipitation with petroleum ether (10 mL) complex 3c (0.46 g, 92% yield)
was obtained as a green microcrystalline solid. 1H NMR (400 MHz,
[D3]nitromethane, 298 K, TMS): δ = 8.47 (d, 3J(H,H) = 5.65 Hz, 1H,
CHpyridine), 7.97 (d, 3J(H,H) = 7.71 Hz, 2H, CHphenyl), 7.95 (t, 3J(H,H) =
7.89 Hz, 1H, CHpyridine), 7.78 (d, 3J(H,H) = 7.71 Hz, 1H, CHpyridine), 7.70 (d,
Keywords:nickel complexes • hydroamination • C-C bond
formation • C-S coupling• N-Heterocyclic carbenes
3
3J(H,H) = 1.61 Hz, 1H, CHimidazole backbone), 7.65 (t, J(H,H) = 7.68 Hz, 2H,
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
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10.1002/ejic.201700057
European Journal of Inorganic Chemistry
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Efficient and versatile Ni(II)-NHC
complexes for Suzuki Miyaura
coupling reactions, hydroaminations of
activated olefins and C-S cross
coupling of aryl halides and thiols.
Lourdes BenítezJunquera, Francys E.
Fernández, M. Carmen Puerta* and
Pedro Valerga*
Page No. – Page No.
Nickel(II) N-Heterocyclic Carbene
Complexes: Versatile Catalysts for C-
C, C-S and C-N Coupling Reactions
This article is protected by copyright. All rights reserved.