Cyclopentadienyl nickel(ii) N, C-chelating benzothiazolyl NHC complexes: Synthesis, characterization and application in catalytic C-C bond formation reactions

Article abstract of DOI:10.1039/c6dt00252h

Cyclopentadienyl (Cp) Ni(ii) complexes [CpNiL][PF₆] containing hybrid N,C chelating benzothiazolyl NHC ligands (L₁ = 1-(2-benzothiazolyl)-3-methylimidazol-2-ylidene, 3a; L₂ = 1-(2-benzothiazolyl)-3-allylimidazol-2-ylidene, 3b; L₃ = 1-(2-benzothiazolyl)-3-benzylimidazol-2-ylidene, 3c) have been synthesized and fully characterized. The catalytic activity of 3a-3c in some C-C bond formation reactions has been examined. They are efficient catalysts for the homo-coupling of benzyl bromide in the presence of MeMgCl at r.t. with good functional group tolerance. Complex 3a is active in the catalytic oxidative homo-coupling of Grignard reagents with 1,2-dichloroethane as an oxidant at r.t.

Full text of DOI:10.1039/c6dt00252h

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Cyclopentadienyl Nickel (II) N,C-Chelating Benzoth
NHC Complexes: Synthesis, Characterization and
Application in Catalytic C-C Bond Formation Reactions
D
iaOI:
z
10
o
.10
l
39/C6DT00252H
y
l
a
b
a
*a,b,c
Jin Zhao*a,b
Wei Jie Teo, Zhe Wang, Fei Xue, T. S. Andy Hor,
a
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore
17543, Singapore.
1
b
Institute of Materials Research and Engineering, Agency for Science, Technology and
Research, 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore.
c
Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong SAR, China
Email addresses: zhaoj@imre.a-star.edu.sg; andyhor@hku.hk
Abstract
Cyclopentadienyl (Cp) Ni complexes [CpNiL][PF ] containing hybrid N,C chelating
6
benzothiazolyl NHC ligands (L
1
= 1-(2-benzothiazolyl)-3-methylimidazol-2-ylidene, 3a; L
2
=
1-(2-benzothiazolyl)-3-allylimidazol-2-ylidene, 3b;
L
3
=
1-(2-benzothiazolyl)-3-
benzylimidazol-2-ylidene, 3c) have been synthesized and fully characterized. The catalytic
activity of 3a-3c in some C-C bond formation reactions has been examined. They are
efficient catalysts for the homo-coupling of benzyl bromide in the presence of MeMgCl at r.t.
with good function group tolerance. Complex 3a is active in the catalytic oxidative homo-
coupling of Grignard reagent with 1,2-dichloroethane as oxidant at r.t.
1

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DOI: 10.1039/C6DT00252H
Introduction
Strong σ-electron donating property, high modularity and ease of handling render N-
heterocyclic carbene (NHC) a type of widely used ligands to support transition metal based
catalyst systems.[1] Nickel complexes containing NHC ligands have been proved to be
catalytically active for many organic transformations.[2] Due to their ease in synthesis and
high thermal stability, a variety of CpNi(II)-NHC complexes have been synthesized[3-5] and
recently significant interest has been drawn on their catalytic application in different organic
reactions.[2a-c,4,5] Among them, the use of CpNi(II) complexes containing mono-dentate NHC
ligands as pre-catalyst in C-C formation reactions,[4i,5] especially Suzuki-Miyaura coupling[5b-
g]
has been studied intensively. NHC ligands containing other donor functions can offer metal
complexes with enhanced stability and catalytic activity.[1g-h] Ni complexes with hybrid NHC
ligands (with C,N,C, N,C, or P,C coordination mode) are known to be generally good catalyst
for C-C bond formation reactions.[2a] However, the CpNi complexes with hybrid NHC ligand
are rare in the literature[3c,3l] and their catalytic efficiency is still unknown. Inspired by these
recent advances in the CpNi-NHC chemistry,[2-5] and as part of our continuous interest in the
[
6]
hybrid NHC ligand supported catalysis and the use of Ni in Grignard reagent involved C-C
[
7]
bond formation reactions, we herein report the synthesis and catalytic potential of a series
of Ni(II) complexes, [CpNiL][PF ] containing hybrid N,C chelating benzothiazolyl NHC
ligands L -L (1-(2-benzothiazolyl)-3-methylimidazol-2-ylidene, L ; 1-(2-benzothiazolyl)-3-
; 1-(2-benzothiazolyl)-3-benzylimidazol-2-ylidene, L
6
1
3
1
allylimidazol-2-ylidene, L
2
)
3 .
Results and Discussion
Synthesis and Characterization of the Benzothiazolyl imidazolium Salts and Ni Complexes
a-3c. The synthesis of the benzothiazolyl-imidazolium chloride salts 1a-1c was
3
6
c
accomplished by a previous described procedure. Through an anionic exchange reaction
2

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DOI: 10.1039/C6DT00252H
with NH
4
PF , 1a-1c were converted to the corresponding hexafluorophosphates 2a-2c
6
1
13
(
Scheme 1) in 70-84% yield. The H- and C-NMR spectra of 2a-2c in CD
3
CN show the
resonances of the NCHN protons at δ 9.32, 9.35, and 9.43 ppm and of the NCHN carbon at ca.
3
1
δ 150.60 ppm, respectively. Their P-NMR spectra give a septet signal centered at -143.8
ppm. Complexes 3a-3c with the general formula of [CpNiL][PF
6
] (L = L
1
, 3a; L= L
2
, 3b; L =
3
L , 3c) were prepared through the direct reaction between nickelocene and the corresponding
imidazolium salts 2a-2c (Scheme 1).
PF6
PF6
Ni
Cl
R
Ni
R
R
N
S
NH PF6
N
S
N
N
4
N
N
N
N
N
S
CH3OH
r.t., 8 h
CH3CN
o
8
0 C, 12 h
1
1
1
a. R=Methyl
b. R=Allyl
c. R=Benzyl
2a. R=Methyl
2b. R=Allyl
2c. R=Benzyl
3a. R=Methyl
3b. R=Allyl
3c. R=Benzyl
Scheme 1. Synthesis of 2a-2c and Ni complexes 3a-3c
Reactions of nickelocene with 1a-1c resulted in unstable green solid products which turned
yellow upon exposure to air and were not characterized in this work. In contrast, use of the
-
PF
6
salts resulted in air-stable green solids of complexes 3a-3c in good yield. Their
1
13
31
structures were characterized by H-, C- and P-NMR, ESI-MS and single-crystal X-ray
1
diffraction. Their purity was confirmed by elemental analysis. The disappearance of the H-
1
3
NMR signals of NCHC, and the downfield C-NMR signals of NCN carbene carbon at ca.
1
13
1
59 ppm in the H- and C-NMR spectra of 3a-3c are indicative of the Ni-Ccarbene bond
3
1
formation. The P-NMR spectra of 3a-3c give a septet peak with very similar chemical shift
as for 2a-2c, indicating the presence of non-coordinating [PF
]- ion in 3a-3c. The ESI-MS
6
+
spectra show the characteristic peaks of the [CpNiL] cation at m/z = 338.1 (for 3a), 364.1
(
for 3b) and 414.1 (for 3c) (+ve mode) and the [PF
6
]- anion at m/z 145 (-ve mode),
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supporting the formation of the ionic Ni(II)-NHC complexes. The carbene carbon signals of
DOI: 10.1039/C6DT00252H
3
a-3c are located in the range of those observed in the closely related ionic CpNi(II)-NHC
complexes where Ni center is bound to one mono-dentate NHC and one solvate,[3g-i] such as
] (IMes = 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene)[3g]
[
CpNi(IMes)(NCCH
3
)][PF
6
or two mono-dentate NHC ligands, [CpNi(IMes)(IMe)][PF ] (IMe = 1,3-dimethylimidazol-2-
6
ylidene),[5c] and the CpNi complexes containing NHC ligand with allyic N-substitution,
2
][3c]
[
CpNi(η -IAll)][BF
4
(IAll
=
1-methyl-3-allylimidazolin-2-ylidene
or
1,3-
diallylimidazolin-2-ylidene) in which one allylic double bond is nickel-coordinated. The
1
singlet H-NMR signals of the Cp ligand in 3a-3c at δ 5.97, 5.89 and 5.78 ppm, respectively,
locate at the lower field comparing with those observed in other CpNi(II)-NHC complexes[3-5]
.
2
Only [CpNi(η -IAll)][BF
4
] showed the comparable downfield signals of Cp ligand (δ 5.8-5.9
Cl
).[3c] The C-NMR signal of the Cp ligand of 3a-3c is in the range of those
13
ppm in CD
2
2
observed in other CpNi(II)-NHC complexes (δ 91-96 ppm).[3-5] Similar to those in the
CpNi(II) complexes with NHC bearing allylic N-substitution,[3c,3l,4i] the protons of the
terminal CH
2
groups of the allyl group in 3b give rise to two signals at δ 5.38 ppm (cis) and δ
5
.15 ppm (trans).
Molecular Structures of 3a-3c Determined by Single-Crystal X-Ray Diffraction. The green
single crystals of 3a-3c, suitable for X-ray diffraction, were obtained by cooling their
saturated CH
3
CN solution at -19 °C. The X-ray diffraction structural analysis confirms 3a-3c
+
are ionic complex. In the [CpNiL] cation, the benzothiazolyl imidazol-2-ylidene ligand
coordinates to the Ni(II) center in a bidentate N,C chelating mode, forming a five-member
ring as shown in Figures 1-3. The allyl donor in 3b is dangling. The key bond lengths and
angles of 3a-3c are given in Table 1.
4

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Figure 1. ORTEP diagram of cation [CpNiL
1
]+ of 3a (50% probability ellipsoids). Hydrogen atoms
are omitted.
Figure 2. ORTEP diagram of cation [CpNiL
2
]+ of 3b (50% probability ellipsoids). Hydrogen atoms
are omitted.
Figure 3. ORTEP diagram of cation [CpNiL
3
]+ of 3c (50% probability ellipsoids). Hydrogen atoms
are omitted.
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Table 1. Selected bond length (Å) and angles (deg) of complexes 3a-3c
Bond length (Å)
3a
3b
3c
Ni-Cpcent
1.740
1.738
1.730
Ni-Ccarbene
1.868(2)
1.929(18)
138.04
83.83(9)
138.12
1.874(2)
1.9274(19)
139.89
83.82(9)
136.23
1.866(3)
1.925(2)
138.87
83.92(10)
137.16
Ni-Nbenzothiazole
Ccarbene-Ni-Cpcent
Ccarbene-Ni-Nbenzothiazole
Cpcent-Ni-Nbenzothiazole
The Ni center adopts a distorted pseudo-trigonal geometry. The sum of the angles with Ni as
vertex is 360° in all complexes with the Ccarbene-Ni-Nbenzothiazole angle of ca. 84° being
significantly smaller than the idealized angle. In the other neutral and ionic CpNi(II)-NHC
-
complexes, the Ccarbene-Ni-X or Ccarbene-Ni-L angles (X = halides or SR ; L= a neutral donor)
are in the range of 92°-98°.[3-5] The smaller bite angle observed in 3a-3c can be attributed to
the chelating effect. The Ni-Ccarbene bond lengths of 3a-3c (1.866-1.874 Å) are at the short
end of the range (1.85-1.92 Å) of the Ni-Ccarbene distance found in CpNi(II)-NHC
complexes[3-5] due to the chelating effect. Similar shortening of M-Ccarbene bond has been also
found in their CpMo analogues.[6c]
Catalytic Application of 3a-3c in C-C bond formation reactions
A preliminary survey of the catalytic activity of 3a in Kumada-Tamao-Corriu cross-
coupling reactions and the related reactions. Complex CpNi(IMes)Cl has been reported to
be an efficient pre-catalyst for the cross-coupling of aryl O-sulfamates with arylmagnesium
bromides under mild conditions.[5a] Due to the high activity and easy availability of Grignard
[
8]
reagent, and our recent success in the Ni catalysed cross-coupling of Grignard reagents with
organic halides[7a-b], we employed a similar condition to examine the catalytic activity of 3a
towards Kumada-Tamao-Corriu cross-coupling reactions (Scheme 2, Table S1 in ESI). After
1
h, 3a (1 mol%) gave only 22% and 40% yield of the cross-coupling product of 4-
bromoanisole with phenylmagnesium bromide or with p-tolylmagnesium bromide at r.t,
6

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respectively. However, a significant amount of homo-coupling product of Grignard reagent
DOI: 10.1039/C6DT00252H
has been observed.
a
Scheme 2. Kumada-Tamao-Corriu reactions catalysed by 3a ( Isolated yield).
In transition metal catalysed cross-coupling reactions, the homo-coupling products of the
organometallic reagents are unwanted by-product. However, selective oxidative homo-
coupling of aryl metal reagent has been received significant attention because this type of
reactions provides a straightforward protocol to symmetrical biaryls and polyaromatic
conjugated compounds.[9] The oxidative coupling reactions of aryl Grignard reagents have
been actively investigated using different transition metal based catalyst systems,[10] such as
Fe[10a-f], Cu[10g-h], Mn[10d,10i], Co[10f,10k-j] and Ru[10l]. The use of Ni catalysts is less common
comparing with their use in other types of C-C bond formation reactions.[10m] The results
shown in Scheme 2 prompted us to explore the activity of 3a in the oxidative homo-coupling
product of aryl Grignard reagent. A preliminary condition screening (Table S2 in ESI)
showed 3a (3 mol%) gave 81% yield of the homo-coupling product of p-tolylmagnesium
bromide at r.t after 1 h in the presence of 1,2-dichloroethane as oxidant (Scheme 3),
suggesting that CpNi(II)-NHC complexes can be used as catalyst for this type of reaction.
The results also indicate that the oxidative addition of organo halide to the reduced Ni center
could occur during the reaction.
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a
Scheme 3. Oxidative homo-coupling of aryl Grignard reagent catalysed by 3a ( Isolated yield).
Catalytic application of 3a-3c in the homo-coupling of benzylic halides. Bibenzyl
compounds and their derivatives can be found in a wide array of naturally occurring products
and pharmaceuticals.[11] It is, thus, important to have a facile entry to symmetrical bibenzyl
derivatives through the catalytic homo-coupling of benzylic halides.[12] Some Ni-based
catalyst systems supported by different ligand are known to be active for this type of
reactions. [12d-h] For example, POCOP-nickel (II) pincer complexes could catalyse the homo-
coupling reactions of benzyl halides in the presence of Zn, giving the coupled products in
high yields at 115 °C after 15 h.[12g] NiCl
(PPh
) catalysed the reaction with moderate to
2
3
2
good activity in the presence of Mg, Mn or Zn at r. t. for 18 h.[12h] As shown in Table 2, under
the conditions similar to the Kumada-Tamao-Corriu cross-coupling reactions shown in
Scheme 2, 3a-3c (1 mol%) could catalyse the reaction between benzylic bromide and
MeMgCl at r.t. affording bibenzyl in 70-80% yield after 1 h with 100% conversion of
benzylic bromide (entries 1-3). The reduction product and cross-coupling product were
observed in GC-MS as side product. No reaction occurred in the absence of catalyst (entry 4).
Stoichiometric amount of Grignard reagent was required for the reaction. No bibenzyl was
detected when only 0.2 equivalent of MeMgCl was used (entry 5). When complex
CpNi(IMes)Cl was used as the catalyst (entry 6), the yield of bibenzyl was significantly
lower and the reaction was incomplete.
a
Table 2. Homo-coupling of benzyl bromide catalysed by 3a-3c
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Yield of
Recovered benzyl
Entry
Catalyst
b
b
bibenzyl (%)
bromide (%)
1
2
3
4
3a
3b
3c
-
71 (71)
81 (78)
76 (74)
n.d.
0
0
0
100
86
37
d
3
b
n.d.
5
6
CpNi(IMes)Cl
51
a
Reaction conditions: Benzyl bromide (0.5 mmol), MeMgCl (0.6 mmol, 1.0 M in
b
THF), catalyst (1 mol%) in THF (2 mL), r.t. 1 h. Isolate yields and NMR yields. The
c
d
isolated yields are given in parentheses. n.d. = not detected. 0.1 mmol of MeMgCl
used.
The scope of the benzylic halide substrates was studied using 3b as catalyst and the results
are summarized in Table 3. The electron-donating groups (entries 1,2) and electron
withdrawing groups (entries 3-5) on the para-position did not influence the activity of the
catalyst 3b gave similar high yields of bibenzyl products. It is peculiar that the reaction was
tolerant to the nitrile and ester functional groups (entries 4,5). The homo-coupling reaction
3
2
also proceeded selectively on the sp halide instead of the sp halide (entries 6,7). The homo-
coupling of (1-bromoethyl)benzene led to a parallel amount of diastereomeric products (entry
8
) with 77% yield. Reaction of a sterically hindered substrate (entry 9) required an additional
hour of reaction to reach full conversion and gave a yield comparable to the less sterically
hindered substrates. When the less reactive chlorinated substrates were used (entries 10-13),
the reactions required 24 h to achieve full conversion and the yield of bibenzyl products was
generally low as expected.
a
Table 3. Homo-coupling of benzyl halides catalysed by 3b
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DOI: 10.1039/C6DT00252H
1
2
b
Entry
X
Time (h)
Yield (%)
R
R
i
1
2
3
4
5
6
7
8
4- Pr
H
H
H
H
H
H
H
Me
H
H
H
H
H
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
1
1
83
88
t
4- Bu
4-CF
4-CN
4-COOMe
4-Br
3
1
77
1
85c
70c
81
1
1
2-Br
1
83
77 [1:1]d
c
H
1
e
9
2-Me
H
2
8
7
f
24
24
24
24
44
65
24
16
1
1
1
1
0
1
2
3
f
f
f
2
4
-Me
-Me
4
-CF3
a
Reaction conditions: Benzylic halide (0.5 mmol), MeMgCl (0.6 mmol, 1.0 M in THF), 3b
b
(
1 mol%), THF (2 mL) r.t. 1 h. Isolated yields; Complete conversion for all reactions.
c
e
f
d
NMR yield. Diastereomeric ratio [meso:dl] based on NMR. 87% conversion after 1 h. 24
h. 3 equiv. MeMgCl was used.
In CpNi(II)-NHC complexes catalysed Buchwald-Hartwig arylaminations and Suzuki-
Miyaura coupling reactions, low valent Ni(0) and Ni(I) species stabilized by NHC ligand
have been proposed to be active species or intermediate.[4a-b,5c] Although neither Ni(0)-NHC
nor Ni(I)-NHC species have been isolated by using CpNi(II)-NHC complexes as precursor,
they could been obtained through other synthetic routes and their application in C-C bond
formation reactions has been studied.[13] It has been reported recently that CpNi(I)-NHC
complexes, CpNi(IPr) and CpNi(SIPr) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene; SIPr =1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene), isolated from the
reaction between NaCp and [(µ-Cl)Ni(IPr)]
2
or [(µ-Cl)Ni(SIPr)] , could be used as pre-
2
1
0

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catalyst for Suzuki-Miyaura cross-coupling reactions.[13h] T-shape three-coordinate nickel(I)
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DOI: 10.1039/C6DT00252H
halide bearing two bulky NHC ligands, Ni(IPr)
2
Cl, has been proven to be a reaction
intermediate in Ni(IPr)
2
Cl
2
catalysed Kumada-Tamao-Corriu cross-coupling reactions.[13i]
The analogous complex Ni(IMes)
2
Br has been found to be active for both Kumada-Tamao-
Corriu and Suzuki-Miyaura cross-coupling reactions.[13j] For both reactions, Ni(I)(IMes)
Ar
n
formed through transmetalation was proposed to be the active species capable of reacting
with aryl halide to form a Ni(III) diaryl halide.[13j] Reaction of [(µ-Cl)Ni(IPr)]
2
with
{
CH(SiMe
3
)
2
}MgCl could afford a two coordinate nickel (I) alkyl complex
}.[13k] The use of sterically encumbered Grignard reagent prevents from
(
IPr)Ni{CH(SiMe
)
3 2
the formation of Ni(0) dimer. (IPr)Ni{CH(SiMe
3
2
) } could react with benzyl bromide to give
Ni(II) alkyl halide, bibenzyl and 1,1-bis(trimethylsilyl)-2-phenylethane through a radical
oxidative addition mechanism and a Ni(III) dialkyl bromide complex has been proposed to be
an intermediate.[13k] Although the catalytic reaction mechanism has not been investigated in
this work, on the basis of the mechanistic studies for Ni-catalysed homo- and cross-coupling
reactions of organic halides involving Ni(I) and Ni(III) species[14] and the results mentioned
above[13], a tentative reaction mechanism for the homo-coupling of benzylic halides catalysed
by 3a-3c has been proposed (Scheme 4). Before the beginning of catalytic cycle, Ni(I) methyl
species A supported by the bidentate hybrid NHC ligand could be generated from the
reactions between 3a-3c with MeMgCl. Oxidative addition of benzyl halide onto A gives
Ni(III) species B. Subsequently reacting with MeMgCl, B is converted to another Ni(III)
species C. The elimination of ethane from C gives a Ni(I)-benzyl intermediate D. Through
another oxidation addition process of the benzyl halide, D is converted to a Ni(III) bisbenzyl
species E. E undergoes reductive elimination to give bibenzyl product and a Ni(I) halide F. F
reacts further with MeMgCl to regenerate A. The higher efficiency of 3a-3c compared with
CpNi(IMes)Cl indicates the beneficial effect of chelating ligand on catalyst performance
1
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which has been observed in many Ni-NHC catalysed C-C formation reactions.[2a] In this
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DOI: 10.1039/C6DT00252H
system, the bidentate hybrid NHC ligand could stabilize the highly reactive Ni(I) and Ni(III)
species more efficiently. Furthermore, the presence of N donor in the hybrid NHC ligand
could result in a more facile oxidation addition, while the steric pressure caused by the
bidentate ligand could favour the reduction elimination process. When PhMgBr was used to
replace MeMgCl, a mixture containing bibenzyl, the cross-coupling product, and the homo-
coupling product of PhMgBr could be observed in GC-MS after 1 h using 3a as catalyst. The
proposed catalytic cycle is also consistent with this observation.
3
3
Scheme 4. Tentative reaction mechanism of 3a-3c catalysed sp -sp homo-coupling of
benzylic halides
Conclusions
A series of air-stable CpNi(II) complexes 3a-3c with bidentate hybrid NHC ligands have
been synthesized and fully characterized. A preliminary survey of their application in C-C
1
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bond formation reactions shows 3a is active for the catalytic oxidative homo-coupling of
DOI: 10.1039/C6DT00252H
Grignard reagent. In the presence of a Grignard reagent, complexes 3a-3c are efficient
catalysts for the homo-coupling of benzyl bromides under facile conditions (r.t., 1 h and 1
mol% of catalyst loading) and with good functional groups tolerance. Further exploration of
the catalytic applications of CpNi-NHC complexes towards C-C bond formation reactions is
currently underway.
Experimental
Materials and physical measurements. All commercial chemicals were used as purchased.
All preparations and manipulations were performed using standard Schlenk techniques under
an Argon atmosphere. Solvents were dried by standard procedures. Nickelocene,[3d]
CpNiCl(IMes)Cl[3d] and benzothiazolyl-imidazolium chloride salts 1a-1c[6c] were prepared
according to the literature methods. Elemental analyses for C, H, and N were performed on a
1
Perkin-Elmer PE 2400 CHNS elemental analyzer and Elementar vario MICRO Cube. H and
13C NMR were measured at r.t. with Bruker ACF300 300 MHz and AMX500 500 MHz FT
NMR spectrometers. Electrospray ionisation mass spectrometric (ESI-MS) analysis was
performed on a Finnigan LCQ quadrapole ion trap mass spectrometer and Brucker amaZon X
ion trap mass spectrometer.
General procedure for the synthesis of Benzothiazolyl-Imidazolium Hexafluorophosphate
(
2a-2c). To a solution of benzothiazolyl-imidazolium chloride salts 1a-1c (1.0 mmol) in
methanol (40 mL) was added NH PF (173 mg, 1.1 mmol). The solution was stirred at r.t. for
h. The precipitate was collected and washed with methanol (2 x 20 mL). The
characterization data for 2a-2c are as follows.
4
6
8
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3

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DOI: 10.1039/C6DT00252H
Ha
Hd
Hb
N
S
Hc
Proton labelling of the benzothiazolyl moiety for NMR signal assignments
For 1-(2-benzothiazolyl)-3-methylimidazolium hexafluorophosphate (2a). White solid (Yield:
1
2
53 mg, 70%). H NMR(500 MHz, CD
3
CN): δ (ppm) = 9.32 (s, 1H, NCHN), 8.12 (d, J = 8.0
Hz, 1H, H
a
), 8.08-8.06 (m, 2H, =CHN(CH
3
) and H
d
), 7.67 (dt, J = 8.0 Hz, 1.0 Hz, 1H, H
c
),
1
3
7
.62-7.59 (m, 2H, =CHN(benzothiazole) and H
b
), 3.99 (s, 3H, N(CH
3
)). C NMR(125.77
MHz, CD CN): δ (ppm) = 154.91 (NCS), 150.60 (NCN), 136.90, 134.58, 128.97, 128.23,
3
31
1
26.31, 124.51, 123.77, 121.72, 37.82 (N-CH
3
). P NMR(202.44 MHz, CD CN): δ (ppm) =
3
+
]- = 145.1. Anal. Calcd for
-
143.78 (sep, PF ). ESI-MS (in CH
6
3
CN: m/z): [L
1
H] = 216.2, [PF
6
C
11
H
10
F
6
3
N PS: C, 36.57; H 2.79; N, 11.63; S, 8.88. Found: C, 36.16; H, 3.03; N, 11.58; S,
9
.10.
For 1-(2-benzothiazolyl)-3-allylimidazolium hexafluorophosphate (2b). Off-white solid
1
(
Yield: 325 mg, 84%). H NMR(500 MHz, CD
3
CN): δ (ppm) = 9.35 (s, 1H, NCHN), 8.13-
), 8.07 (d, J = 8.5 Hz, 1H, H ), 7.69-7.64 (m, 2H,
), 7.61 (dt, J = 8.0 Hz, 1.0 Hz, 1H, H ), 6.10 (ddt, J = 7.0 Hz,
CH=CH ), 4.92 (d, J = 6.5 Hz,
CN): δ (ppm) = 154.91 (NCS), 150.59
8
.12 (m, 2H, =CHN(Allyl) and H
a
d
=
CHN(benzothiazole) and H
c
b
6
.5 Hz, 6.0 Hz, 1H, -CH
2
CH=CH
2
), 5.54-5.51 (m, 2H, -CH
2
2
2
H, -CH
2
CH=CH
2
). 13C NMR(125.77 MHz, CD
3
(NCN), 136.39, 134.65, 130.72, 128.99, 128.28, 124.98, 124.54, 123.79, 123.17, 122.14,
3
1
5
3.55. P NMR(202.44 MHz, CD
3
CN): δ (ppm) = -143.77 (sep, PF
6
). ESI-MS (in CH
3
CN:
+
]- = 145.1. Anal. Calcd for C13
m/z): [L
2
H] = 242.2, [PF
6
H
12
F
6
3
N PS: C, 40.31; H,3.12; N,
1
0.85; S, 8.28. Found: C, 40.06; H,3.35; N, 11.02; S, 8.52.
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For 1-(2-benzothiazolyl)-3-benzylimidazolium hexafluorophosphate (2c). White solid (Yield:
DOI: 10.1039/C6DT00252H
1
3
54 mg, 81%). H NMR(500 MHz, CD
H, =CHN(Bn) and H ), 8.06 (d, J = 8.0 Hz, 1H, H
), 7.52-7.45 (m, 5H, Ph), 5.48 (s, 2H, -CH
CN): δ (ppm) = 154.91 (NCS), 150.56 (NCN), 136.39, 134.64,
3
CN): δ (ppm) = 9.43 (s, 1H, NCHN), 8.13-8.10 (m,
2
a
d
), 7.67 (t, J = 8.5 Hz, 1H, H ), 7.63-7.59
c
1
3
(
m, 2H, =CHN(benzothiazole) and H
NMR(125.77 MHz, CD
33.74, 130.61, 130.37, 130.02, 129.00, 128.29, 124.88, 124.53, 123.78, 122.34, 54.87. P
b
2
Ph). C
3
3
1
1
NMR(202.44 MHz, CD
3
CN): δ (ppm) = -143.78 (sep, PF
6
). ESI-MS (in CH CN: m/z):
3
+
]- = 145.1. Anal. Calcd for C17
[
L
3
H] = 292.2, [PF
6
H
14
F
6
N
3
PS: C, 46.69; H,3.23; N, 9.61; S,
7
.33. Found: C, 46.74; H,3.40; N, 9.60; S, 7.74.
General procedure for the synthesis of nickel complexes (3a-3c). A solution of 2a, 2b or 2c
(0.1 mmol) in acetonitrile (5 mL) was added to nickelocene (23 mg, 0.12 mmol). The solution
o
was stirred at 80 C for 12 h. The solution slowly turned from dark green to brownish-green.
The solvent was then removed under vacuum. The residue was washed with ethanol (3 x 5
mL) and then redissolved in acetonitrile (10 mL) and filtered. The solvent of the filtrate was
evaporated to yield a green solid.
1
For 3a. Bright green solid (Yield: 34 mg, 70%). H NMR(500 MHz, CD
3
CN): δ (ppm) = 8.07
)), 7.66 (t, J = 7.5 Hz, 1H, H ),
), 7.25 (d, J = 2.0 Hz, 1H, =CHN(benzothiazole)), 5.97 (s, 5H,
(
d, J = 8.0 Hz, 1H, H
.58-7.55 (m, 2H, H
Cp), 3.77 (s, 3H, N(CH
a
), 7.97 (d, J = 2.0 Hz, 1H, =CHN(CH
3
c
7
b
and H
d
)). 13C NMR(125.77 MHz, CD
CN): δ (ppm) = 175.71 (NCS),
3
3
1
3
58.61 (NCN), 149.67, 131.19, 129.44, 128.45, 127.66, 125.14, 121.34, 118.97, 93.44 (Cp),
31
9.04 (N-CH
3
). P NMR(202.44 MHz, CD
3
CN): δ (ppm) = -143.76 (sep, PF
6
). ESI-MS (in
CH
3
CN: m/z): [CpNiL
1
]+ = 338.1, [PF ]- = 145.1. Anal. Calcd for C16
6
H
14
F N
6 3
PSNi: C, 39.70;
H,2.92; N, 8.68; S, 6.62. Found: C, 39.50; H, 3.33; N, 8.66; S, 6.50.
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DOI: 10.1039/C6DT00252H
For 3b. Bright green solid (Yield: 33 mg, 65%). H NMR(500 MHz, CD CN): δ (ppm) =
3
8
.08 (d, J = 8.0 Hz, 1H, H
.55 (m, 2H, H and H
), 5.89 (s, 5H, Cp), 5.38 (d, J = 10.5 Hz, 1H, -CH
a
), 8.02 (s, 1H, =CHN(Allyl)), 7.66 (t, J = 7.5 Hz, 1H, H
), 7.27 (s, 1H, =CHN(benzothiazole)), 6.11-6.02 (m, 1H, -
CH=CH ), 5.15 (d, J = 17 Hz,
). C NMR(125.77 MHz, CD CN): δ (ppm) =
c
), 7.58-
7
b
d
CH
2
CH=CH
2
2
2
13
1
1
1
H, -CH
2
CH=CH
2
), 4.66 (s, 2H, -CH
2
CH=CH
2
3
75.86 (NCS), 158.71 (NCN), 149.68, 132.89, 131.19, 129.46, 127.69, 127.27, 125.17,
31
21.32, 119.54, 118.88, 93.76 (Cp), 53.56 (N-C-CH=CH
2
). P NMR(202.44 MHz, CD
3
CN):
δ (ppm) = -143.77 (sep, PF
6
). ESI-MS (in CH
3
CN: m/z): [CpNiL
2
]+ = 364.1, [PF ]- = 145.1.
6
Anal. Calcd for C18
N, 8.11; S, 6.12.
H
16
F N
6 3
PSNi: C, 42.39; H,3.16; N, 8.24; S, 6.29. Found: C, 42.50; H,3.28;
1
For 3c. Dark green solid (Yield: 40 mg, 71%). H NMR(500 MHz, CD
3
CN): δ (ppm) = 8.09
), 7.60-7.51 (m,
), 7.46-7.26 (m, 6H, =CHN(benzothiazole) and Ph), 5.78 (s, 5H, Cp), 5.29 (s,
(
d, J = 8.0 Hz, 1H, H
H, H and H
H, -CH
49.71, 136.15, 131.21, 130.15, 129.47, 127.82, 127.72, 127.54, 125.19, 121.33 119.80,
a
), 8.05 (s, 1H, =CHN(Bn)), 7.66 (t, J = 7.5 Hz, 1H, H
c
2
2
1
9
b
d
1
3
2
Ph). C NMR(125.77 MHz, CD CN): δ (ppm) =176.34 (NCS), 158.77 (NCN),
3
3
1
3.79 (Cp), 54.87 (N-C-Ph). P NMR(202.44 MHz, CD
3
CN): δ (ppm) = -143.77 (sep, PF
6
).
]+ = 414.1, [L
H] = 292.1, [PF
+
]- = 145.1. Anal. Calcd for
ESI-MS (in CH
3
CN: m/z): [CpNiL
3
3
6
C
22
H
18
F
6
3
N PSNi: C, 47.17; H,3.24; N, 7.50; S, 5.72. Found: C, 47.26; H,3.48; N, 7.19; S,
5
.45.
General procedure for the catalytic cross-coupling of 4-bromoanisole with Aryl Grignard
reagents. A Schlenk tube, placed under an argon atmosphere, was charged with 4-
bromoanisole (0.5 mmol), 3a (0.005 mmol, 1 mol %), toluene (1 mL) at r.t. under stirring.
Aryl Grignard reagent (0.6 mmol, 1.0 M in THF) was added dropwise within 30 min and the
resulted reaction mixture was stirred at r.t. for another 30 min. The reaction was quenched
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with a drop of HCl (aq., 2 M) and solvent removed under reduced pressure. The product was
DOI: 10.1039/C6DT00252H
isolated by flash chromatography on silica gel with hexane as the eluent.
General procedure for catalytic homo-coupling of Aryl Grignard reagents. A Schlenk tube,
placed under an argon atmosphere, was charged with 3a (0.005 mmol, 1 mol %), 1,2-
dichloroethane (0.62 mmol) and THF (2 mL) at r.t. under stirring. Aryl Grignard reagent (0.5
mmol, 1.0 M in THF) was added and the resulted reaction mixture was stirred at r.t. for 1 h.
The reaction was quenched with a drop of HCl (aq., 2 M) and solvent removed under reduced
pressure. The product was isolated by flash chromatography on silica gel with hexane as the
eluent.
General procedure for catalytic homo-coupling of benzyl halides. In a typical example, after
standard cycles of evacuation and back-fill with pure Argon, complex 3b (2.5 mg, 0.005
mmol) was introduced into a 25 mL-Schlenk tube equipped with a magnetic stir bar. To the
catalyst were added THF (2 mL), internal standard 1,3,5-trimethoxybenzene (42 mg, 0.25
mmol) and benzyl bromide (59.5 µL, 0.5 mmol) at r.t. under stirring. Methylmagnesium
chloride solution (3.0 M in THF, 0.2 mL, 0.6 mmol) was added and the resulted reaction
mixture was stirred at r.t. for 1 h. The reaction was quenched with a drop of HCl (aq., 2 M)
and solvent removed under reduced pressure. The product was isolated by flash
chromatography on silica gel with hexane as the eluent. NMR yields were calculated using
1
,3,5-trimethoxybenzene as the internal standard.
X-ray Crystallography. Diffraction measurements were conducted at 100 K on a Bruker D8
Venture X-Ray diffractometer by using Mo Kα radiation (λ = 0.71073 Å) and a Photon 100
detector. The data were corrected for Lorentz and polarization effects with the Apex 2 suite
of programs and for absorption effects with SADABS.[15] Structure solutions and refinements
were performed by using the programs Bruker Apex 2. The structures were solved by direct
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methods to locate the heavy atoms, followed by difference maps for the light non-hydrogen
DOI: 10.1039/C6DT00252H
atoms. Anisotropic thermal parameters were refined for the rest of the non-hydrogen atoms.
Hydrogen atoms were placed geometrically and refined isotropically. Crystal data and
structure refinement for complexes 3a-3c are given in Table S3 in the ESI.
Acknowledgements
We acknowledge the Agency for Science, Technology and Research (A*Star) of Singapore
for financial support (WBS No. R-143-000-566-305). We thank Ms. G. K. Tan for X-ray
diffractometry assistance.
Notes and references
Electronic Supplementary Information (ESI) available: For the catalytic reaction results, the
characterization data for the bibenzyl products, the crystallographic data for complexes 3a-3c
and the crystal data in CIF and CCDC reference numbers 1441071 (3b), 1441072 (3c) and
441073 (3a).
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DOI: 10.1039/C6DT00252H
Cyclopentadienyl Nickel (II) N,C-Chelating Benzothiazolyl
NHC Complexes: Synthesis, Characterization and Application
in Catalytic C-C Bond Formation Reactions
a
b
a
*a,b,c
*a,b
Wei Jie Teo, Zhe Wang, Fei Xue, T. S. Andy Hor,
Jin Zhao
a
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore
17543, Singapore.
1
b
Institute of Materials Research and Engineering, Agency for Science, Technology and
Research, 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore.
c
Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong SAR, China
Email addresses: zhaoj@imre.a-star.edu.sg; andyhor@hku.hk
Graphical Abstract