Synthesis and structures of amido-functionalized N-heterocyclic nickel(II) carbene complexes

Article abstract of DOI:10.1016/j.jorganchem.2020.121543

A series of bis-bidentate nickel(II) complexes [Ni(R-bimy-CH₂CONH)₂] (bimyH = benzimidazole; R = Me (3), Et (4), Ph (5)) bearing amido-functionalized N-heterocyclic carbene ligands, and pincer-type nickel(II) complexes [Ni(Py-bimy-CH₂CONH)X] (X = Cl (6), Br (7)) bearing an amido- and pyridyl-functionalized N-heterocyclic carbene ligand were prepared. These complexes were characterized by NMR (1D and 2D) and single-crystal X-ray diffraction. Complexes 3-5 possess cis configuration, and the carbene ligands bound to the nickel atom through C2 carbon and NH nitrogen in a bis-bidentate coordination mode. In complexes 6 and 7, the pyridyl substituent was also N-bound to the nickel metal center resulting in a pincer-type coordination mode. As observed from the proton NMR spectra, the six-membered chelate rings in complexes 3-5 rendered the protons of the methylene moieties diastereotopic, and the cis configuration made the free rotation of the ethyl substituent in 4 and the phenyl substituent in 5 hampered by the adjacent substituent. The catalytic activity of these nickel complexes in Kumada cross-coupling of phenylmagnesium bromide with aryl chlorides was also investigated. The results indicated that pincer-type complexes 6 and 7 displayed excellent to moderate catalytic activity depending on the aryl chloride used.

Full text of DOI:10.1016/j.jorganchem.2020.121543

Journal of Organometallic Chemistry 927 (2020) 121543
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and structures of amido-functionalized N-heterocyclic
nickel(II) carbene complexes
Sheng-Zhi Lua, Hsueh-Hui Yang b, Wei-Ju Changa, Hsin-Hsueh Hsueha, Yong-Chieh Lina,
Fu-Chen Liua,∗, Ivan J.B. Lina, Gene-Hsiang Leec
a Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan, ROC
b Department of Medical Research, Hualien Tzu Chi Hospital, Buddhist Tzu Chi Foundation, Hualien, 970, Taiwan, ROC
c Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of bis-bidentate nickel(II) complexes [Ni(R-bimy-CH2CONH)2] (bimyH = benzimidazole; R = Me
(3), Et (4), Ph (5)) bearing amido-functionalized N-heterocyclic carbene ligands, and pincer-type nickel(II)
complexes [Ni(Py-bimy-CH2CONH)X] (X = Cl (6), Br (7)) bearing an amido- and pyridyl-functionalized N-
heterocyclic carbene ligand were prepared. These complexes were characterized by NMR (1D and 2D) and
single-crystal X-ray diffraction. Complexes 3-5 possess cis configuration, and the carbene ligands bound to
the nickel atom through C2 carbon and NH nitrogen in a bis-bidentate coordination mode. In complexes
6 and 7, the pyridyl substituent was also N-bound to the nickel metal center resulting in a pincer-type
coordination mode. As observed from the proton NMR spectra, the six-membered chelate rings in com-
plexes 3-5 rendered the protons of the methylene moieties diastereotopic, and the cis configuration made
the free rotation of the ethyl substituent in 4 and the phenyl substituent in 5 hampered by the adjacent
substituent. The catalytic activity of these nickel complexes in Kumada cross-coupling of phenylmagne-
sium bromide with aryl chlorides was also investigated. The results indicated that pincer-type complexes
6 and 7 displayed excellent to moderate catalytic activity depending on the aryl chloride used.
Received 14 July 2020
Revised 17 September 2020
Accepted 21 September 2020
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
Recently, N-heterocyclic carbene complexes containing donor
atoms (e.g. C, N, O, S, or P) have attracted considerable attention, as
Since the isolation of a stable N-heterocyclic carbene (NHC) by
Arduengo et al. in 1991 [1], the study of transition metal com-
plexes with NHC ligands has become one of the most exciting
research topics in organometallic chemistry. Numerous transition
metal-NHC complexes have been prepared and studied in the past
decades. The main reason for such a rich chemistry is the abil-
ity to tune the electronic and steric properties of the NHC lig-
ands by modifying substituents [2]. These properties make NHC
complexes catalytically active in various reactions [3]. For exam-
ple, palladium NHC complexes have been used as pre-catalysts
in numerous C-C cross-coupling reactions [4], which are of great
importance in organic chemistry. Although nickel NHC complexes
have not been studied as extensively as their palladium analogues,
a significant amount of work has been published recently [5] on
such diverse catalytic applications as Suzuki–Miyaura and Kumada
cross-coupling [6,7], olefin polymerization [8], C-H bond activation
[9], and reduction of carbon dioxide [10].
these complexes showed enhanced activity in many catalytic reac-
tions [11]. As continuation of our studies of transition metal com-
plexes with amido-functionalized NHC carbene ligands [12], here
we report the preparation and characterization of several amido-
functionalized NHC bis-bidentate and pincer-type nickel(II) carbene
complexes as well as the catalytic activity of these complexes in
Kumada cross-coupling of phenylmagnesium bromide with aryl
chlorides.
2. Results and discussion
2.1. Amido-functionalized NHC ligand precursors 1 and 2
The amido-functionalized NHC ligand precursors 1-acetamido-
3-phenylbenzimidazolium bromide (1), and 1-acetamido-3-
(2-pyridyl)benzimidazolium bromide (2) (Scheme 1), were
prepared from the reactions of 2-bromoacetamide with 1-
phenylbenzimidazole [13] and 1-(2-pyridyl)benzimidazole [14],
respectively, in refluxing acetonitrile. Both compounds are soluble
in water, MeOH, DMF, and DMSO; and slightly soluble in acetone,
∗
Corresponding author.
https://doi.org/10.1016/j.jorganchem.2020.121543
0022-328X/© 2020 Elsevier B.V. All rights reserved.

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
Scheme 1. Synthetic route to prepare the NHC salts 1-2 and the NHC nickel (II) complexes 3-7.
THF, and acetonitrile. In the proton NMR spectra of 1 and 2
(DMSO-d6), the diagnostic acidic benzimidazolium NCHN proton
appears at 10.39 and 10.60 ppm, respectively. These chemical
shifts are about 2.0 ppm downfield relative to the corresponding
of complex 5 occurring simultaneously in the reaction. Complex 5
could not be separated from the unidentified by-products. Eventu-
ally, complex 5 was prepared through an alternative method from
the reaction of Ni(OAc)2 with 1 in hot [Bu4N]Br (TBAB), which
is known as a basic metal precursor method [16]. Complexes
3-5 are air-stable solids and very soluble in DMSO, DMF, and
MeOH; but only slightly soluble in THF, acetonitrile, and acetone.
Attempts to prepare the corresponding mono-carbene complexes
failed. Only the bis-carbene complexes were observed and isolated
from the reactions when the ratio of the metal precursor to the
corresponding benzimidazolium salt was 1:1.
1
signals of the benzimidazoles. The JCH coupling constant of the
NCHN proton is 222.9 Hz for 1 and 198.2 Hz for 2. Azolium
1
salts JCH coupling constants have been used to estimate the
σ-donor ability of the NHC ligands [15]. The higher the value of
the coupling constant is, the poorer the σ-donating property of
a NHC ligand would be. This result suggested that the pyridyl
group would induce more σ-donating property than the phenyl
group. The methylene protons appear at 5.47 ppm in 1 and 5.43
ppm in 2. The amido protons of 1 (7.75 and 8.15 ppm) and 2 (7.76
and 8.07 ppm) are not equivalent due to a partial double bond
character of the C(O)-NH2 bond [12a], and these chemical shifts
are slightly downfield compared with those of 2-bromoacetamide
(7.25 and 7.62 ppm). The NCHN and carbonyl carbon signals
appear at 144.02 and 166.66 ppm in 1, and 147.56 and 166.52
ppm in 2, respectively, in the 13C NMR spectra.
The formation of the bis-bidentate nickel complexes is sug-
gested by the absence of the carbenic proton and one of the amido
proton resonances in their 1H NMR spectra. The remaining amido
proton appeared at 4.49 ppm in 3, 4.52 ppm in 4, and 4.62 ppm in
5. These chemical shifts are about 3.5 ppm upfield relative to the
signals of the NH2 protons of the parent benzimidazolium salts,
and consistent with the anionic nature of the amido ligand. Coor-
dination of the amido nitrogen and the carbene carbon atoms to
nickel produces a six-membered chelate ring, which renders the
protons of the methylene groups diastereotopic. Two well-resolved
doublets (4.75 and 5.19 ppm in 3, 4.78 and 5.09 ppm in 4, and
2.2. Nickel(II) bis-carbene complexes 3-5
4.42 and 4.50 ppm in 5) with a two-bond geminal coupling (2JHH
)
Treatment
of
NiCl2(CH3CN)2
ligand
and
K2CO3
with
the
of 16 Hz were observed in their proton NMR spectra. The signals
of these methylene protons were found to be fluxional. Variable-
temperature proton NMR spectra of complex 4 are shown in Fig. 1
to describe this behavior. At room temperature, two well-resolved
doublets at 4.78 and 5.09 ppm corresponding to the diastereotopic
protons of the two methylene groups were observed. As the tem-
perature increased, these two signals broadened and coalesced at
amido-functionalized
precursor
1-acetamido-3-
methylbenzimidazolium bromide [12a] or 1-acetamido-3-
ethylbenzimidazolium bromide [12b] in hot methanol yielded the
nickel(II) bis-carbene complex [Ni(R-bimy-CH2CONH)2] (R = Me
(3), Et (4)) shown in Scheme 1. When the same method was ap-
plied to prepare the nickel(II) bis-carbene complexes [Ni(Ph-bimy-
CH2CONH)2] (5), it failed due to the formation and decomposition
2

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
Fig. 1. Variable temperature 1H NMR spectra of 4 in DMSO–d6
about 100 C, and an average broad signal appeared. As the tem-
2.3. Pincer-type nickel(II) carbene complexes 6 and 7
º
perature increased further, this signal sharpened, and at 180 C it
º
appeared as a sharp singlet at 4.92 ppm. The fluxional behavior
of the methylene protons could be caused either by bond cleavage
(Ni-N or Ni-C) or fast vibration of the chelate rings at high tem-
perature. However, the cleavage of the Ni-N or Ni-C bond could be
energetically unfavorable, and our VT-NMR studies did not produce
any evidence to support it. As shown in Fig. 1, the NH proton reso-
nance shifted upfield gradually as the temperature was raised. This
result may suggest that the Ni-N bond weakens but the molecule
is still intact at the higher temperature. Thus, the fast vibration
of the chelate rings is more likely to account for the fluxional
behavior.
Similar to the syntheses of 3 and 4, we carried out the reaction
of NiCl2(CH3CN)2 with 1-acetamido-3-(2-pyridyl)benzimidazolium
bromide (2) and K2CO3 in refluxing methanol solution. During the
reaction, we observed the formation of pincer complex [Ni(Py-
bimy-CH2CONH)Cl] (6). However, it had decomposed before the
reaction was complete. After several attempts, we found that de-
composition was induced by K2CO3, a relative strong base. Thus,
we replaced potassium carbonate with sodium acetate, which is
a weaker base, and carried out the reaction in MeOH-DMF (1:3)
at 120 °C. Under these conditions complex 6 was isolated in 87%
yield. It is interesting to note that the strong base potassium car-
bonate is the only base that has been used to remove the amido
proton of the amido-functionalized carbene ligand precursors [17],
and the use of a weaker base such as sodium acetate has never
been reported before. To increase the solubility and compare the
catalytic activity, we also prepared the corresponding bromo ana-
log [Ni(Py-bimy-CH2CONH)Br] (7) from the reaction of Ni(OAc)2
with 2 in TBAB. Complexes 6 and 7 are air-stable and slightly
soluble in highly polar solvents such as DMSO, DMF, and H2O.
The solubility of complex 7 is only slightly better than that of
complex 6.
An additional interesting feature of the proton NMR spectrum
of complex 4 shown in Fig. 1 is that two methylene protons of the
ethyl groups are inequivalent at 3.59 and 4.31 ppm. As the single
crystal X-ray structure determination of complexes 3-5 (see below)
has confirmed the cis configuration, which results in steric hin-
drance and restricts the free rotation of the two ethyl substituents
in complex 4. The free rotation of these two ethyl groups can be
achieved at high temperature as shown in Fig. 1, where these two
signals first broaden and then coalesce at about 120 C. Finally, at
º
180 C an average broad signal appears at 4.00 ppm. The same
º
steric effect was also observed in the proton NMR spectrum of
complex 5, in which the phenyl group appeared as two broad sig-
nals at 7.88 and 7.36 ppm at room temperature, and re-appeared
In the proton NMR spectra of 6 and 7, the remaining amido
proton appeared at 3.73 and 4.15 ppm, respectively. These chem-
ical shifts are slightly upfield compared to those found in com-
plexes 3-5. Coordination of the amido nitrogen atom also pro-
duced a six-membered chelate ring in both cases; however, the
symmetric planar structure in solution made the two methylene
protons steric equivalent and only one proton signal (4.84 ppm
in 6 and 4.88 ppm in 7) was observed. Due to their low sol-
ubility, the 13C NMR spectra of these complexes have not been
acquired.
as a doublet at 7.86 ppm and a triplet at 7.37 ppm at 80 C (Fig.
º
S1 in the Supporting Information).
Upon the formation of the nickel carbene complexes, the C2
carbon signal shifted to the general carbonyl resonance region.
2D 13C-1H HMBC NMR technique was used to assign the car-
bonyl and C2 carbon signals unambiguously. For example, in the
2D NMR spectrum of 4 (Fig. S2 in the Supporting Information),
the carbon signal which had significant correlation with the amido
and methylene protons of the amido substituent was assigned to
the carbonyl group, while the carbon signal which has significant
correlation with the methylene protons of the ethyl substituent
was assigned to the C2 carbon. The C2 and carbonyl carbon
chemical shifts appeared at 178.55 and 170.83 ppm in 3, 178.43
and 170.90 ppm in 4, and 180.15 and 170.34 ppm in 5, respec-
tively. These C2 carbon chemical shifts are about 35 ppm down-
field relative to the corresponding signals of the benzimidazolium
salts.
2.4. Molecular structures of 3-7
Crystals of 3, 4, and 5 suitable for X-ray diffraction analyses
were obtained as described in the experimental section; all struc-
tures were found to contain solvent molecules in their unit cell.
Figs. 2-4 show the molecular structures of 3-5 with selected bond
distances and bond angles listed in the captions. Although the unit
cell of 4 contains two crystallographically different molecules, only
one molecule is displayed due to the difference is insignificant. In
3

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
Fig. 2. (a) Thermal ellipsoid plot of 3 at 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. (b) Extended structure of 3. Selected bond
˚
lengths (A) and angles ( ): Ni-C(11), 1.852(3); Ni-C(1), 1.854(3); Ni-N(1), 1.891(2); Ni-N(4), 1.897(2); N(1)-C(9), 1.307(4); N(4)-C(19), 1.316(4); C(11)-Ni-C(1), 92.83(12); C(11)-
º
Ni-N(1), 168.46(11); C(1)-Ni-N(1), 89.95(11); C(11)-Ni-N(4), 88.39(11); C(1)-Ni-N(4), 170.12(11); N(1)-Ni-N(4), 90.78(10); N(2)-C(1)-N(3), 105.7(2); N(5)-C(11)-N(6), 106.1(2).
˚
˚
each complex, the nickel atom is coordinated by two bidentate car-
bene ligands in cis configuration. The dihedral angles between the
two planes defined by the nickel atom and the coordinating atoms
in each bidentate ligand are 15.20° in 3, 12.69 and 15.65° in 4,
and 13.31° in 5. The trans-angles are 168.46(11) and 170.12(11)°
in 3, 169.89(8) and 171.46(8)° (one molecule), and 167.77(8) and
169.62(8)° (the other molecule) in 4, and 168.96(9) and 171.97(9)°
in 5, reflecting a slightly distorted square planar geometry for each
covalent radii of C (0.772 A) and N (0.70 A) [20], reflecting the
partial double bond character of the amido substituent. The six-
membered chelate rings adopt a boat conformation in all cases.
In addition, extended structures through hydrogen bonding in-
teractions have been found in all cases. In compound 3, two
molecules of 3 are connected to two water molecules through the
C=O groups to form a macrocycle. The O…O distances are 2.747
˚
and 2.846 A, respectively. In compound 4, a C=O group and an NH
˚
complex. The Ni-C bond distances (1.852(3)-1.8711(17) A) found in
group in one molecule are hydrogen-bonded to their neighboring
molecule to form a dimeric structure. The O…N distances are 3.044
complexes 3-5 are comparable to other nickel(II) cis bis(carbene)
˚
˚
complexes [17a-d,18]. The Ni-N bonds (1.8874(19)-1.9018(15) A) in
and 3.060 A, respectively. In complex 5, both solvent molecules
complexes 3-5 are longer than the sum of the covalent radii of
(CH3OH and water) are all involved in the hydrogen bonding in-
˚
˚
nickel and nitrogen atoms (Ni-N = 1.854 A) [19], but they are
teraction, and the O…O distances in the range of 2.703-2.843 A
shorter than those reported for the nickel(II) cis bis(carbene) com-
plexes with other amido-functionalized NHC ligands [17a-d] due
to the less stringent steric requirement imposed by the NH pro-
are observed. The intermolecular hydrogen bonding interaction be-
tween the C=O group of 5 and the water molecules produces a 1D
polymeric structure.
˚
tons. The N-C(O) bond distances (1.304(3)-1.322(3) A) in these
Crystals of 6 and 7 suitable for single crystal X-ray diffraction
analyses were obtained from a DMSO/ether/EA mixed solvent sys-
complexes are shorter than the sum of the individual single bond
4

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
Fig. 3. (a) Thermal ellipsoid plot of 4 at 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. (b) Extended structure of 4. Selected bond
˚
lengths (A) and angles ( ): Ni(1)-C(12), 1.8613(18); Ni(1)-C(1), 1.8677(18); Ni(1)-N(3), 1.8904(17); Ni(1)-N(6), 1.8907(15); C(34)-Ni(2), 1.8711(17); C(23)-Ni(2), 1.8640(18); N(9)-
º
Ni(2), 1.8934(15); Ni(2)-N(12), 1.9018(15); N(3)-C(11), 1.322(3); N(6)-C(22), 1.310(2); N(9)-C(33), 1.311(2); N(12)-C(44), 1.313(2); C(12)-Ni(1)-C(1), 95.02(8); C(12)-Ni(1)-N(3),
171.46(8); C(1)-Ni(1)-N(3), 88.42(8); C(12)-Ni(1)-N(6), 87.92(7); C(1)-Ni(1)-N(6), 169.89(8); N(3)-Ni(1)-N(6), 90.03(7); N(1)-C(1)-N(2), 105.97(16); N(5)-C(12)-N(4), 106.01(15);
N(11)-C(34)-N(10), 106.02(15); N(8)-C(23)-N(7), 106.00(15); C(23)-Ni(2)-C(34), 95.71(7); C(23)-Ni(2)-N(9), 88.41(7); C(34)-Ni(2)-N(9), 169.62(8); C(23)-Ni(2)-N(12), 167.77(8);
C(34)-Ni(2)-N(12), 88.45(7); N(9)-Ni(2)-N(12), 89.47(7).
tem. Complexes 6 and 7 are isostructural, and except for the Ni-
X (X = Cl, Br) bond distance, both complexes have similar bond
distances and angles. Thus, only the molecular structure of 6 is
shown in Fig. 5, and the selected bond distances and bond angles
are listed in the captions. The molecular structure of 7 is shown
in Fig. S3 of the supporting information. In both compounds, the
nickel atom is surrounded by a carbene ligand and a halide ligand.
The carbene ligand coordinates to a nickel atom through a carbene
carbon and two nitrogen atoms, one from the pyridyl substituent
and the other from the amido group, to form a tridentate pincer
complex, in which the carbene carbon is trans to a halide ligand.
The nickel atom is almost coplanar with the plane defined by the
four coordinated atoms, and the distance above the defined plane
pincer-type nickel complexes [21]. The Ni-Namido bonds (1.840(3)
˚
˚
A in 6 and 1.838(7) A in 7) in both complexes are shorter than
those found in complexes 3-5, due to a weak trans influence of
the pyridyl group in both complexes. The N-C(O) bond distance is
˚
˚
1.325(4) A in 6 and 1.328(12) A in 7, which is slightly longer than
those found in complexes 3-5. The Ni-Npy bond distance is 1.932(3)
˚
˚
A in 6 and 1.933(8) A in 7, which is within the range observed
in other nickel complexes containing Ni-Npy bond [21]. The Ni-Cl
˚
bond distance is 2.2117(10) A in 6, and the Ni-Br bond distance
˚
is 2.3520(15) A in 7, these Ni-X (X = Br, Cl) bond distances are
comparable to other nickel complexes containing the Ni-X (X = Cl,
Br) bonds [21,22]. No extended hydrogen bonding interaction was
found in either structure.
˚
˚
is 0.039 A in 6 and 0.041 A in 7. The C-Ni-X (X = Cl or Br) bond
angle is linear in the structures of both complexes; it is 177.73(11)°
in 6 and 177.4(3)° in 7. The N-Ni-N bond angle is 170.72(12)° in 6
2.5. Catalytic studies
˚
and 170.1(3)° in 7. The Ni-C bond distance is 1.790(3) A in 6 and
The nickel carbene complexes are well-known catalysts for Ku-
mada cross-coupling reaction of organic halides with Grignard
˚
1.799(9) A in 7; both are slightly shorter than those found in other
5

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
Fig. 4. (a) Thermal ellipsoid plot of
5
at 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. (b) Extended structure of 5. Selected
˚
bond lengths (A) and angles ( ): Ni-C(16), 1.859(2); Ni-C(1), 1.859(2); Ni-N(1), 1.8874(19); Ni-N(4), 1.8926(19); N(1)-C(15), 1.310(3); N(4)-C(30), 1.304(3); C(16)-Ni-C(1),
º
94.31(9); C(16)-Ni-N(1), 171.97(9); C(1)-Ni-N(1), 89.23(9); C(16)-Ni-N(4), 88.57(9); C(1)-Ni-N(4), 168.96(9); N(1)-Ni-N(4), 89.31(8); N(2)-C(1)-N(3), 105.63(18); N(5)-C(16)-
N(6), 105.93(18).
reagents [23]. In this study, the catalytic activities of complexes
3-7 were initially tested in the coupling reactions of phenylmag-
nesium bromide with chlorobenzene. A typical experiment is de-
scribed in the experimental section. As shown in Table 1, the
alkyl-substituted complexes 3 and 4 were the least active catalysts,
achieving only 37% yield after 6 h (entries 1 and 2). The phenyl-
substituted complex 5 had a better catalytic activity resulting in
49% yield (entry 3). In comparison, both pincer-type complexes
6 and 7 displayed high catalytic activity as the coupling product
was obtained in 94 and 96% yields (entries 4 and 5). The different
halide substituent in these two complexes did not have a signifi-
cant effect on their catalytic activity.
bromide and chlorobenzene, the mixture in THF changed color
to orange brown immediately, and a homogeneous solution was
formed. The color change indicated reduction of the nickel(II) com-
plex into a more active nickel(0) species [24]. It has been reported
that the pincer-type complexes possess excellent catalytic perfor-
mance in many coupling reactions [25]. During the reaction, the
hemilabile pyridyl substituent of these two pincer-type complexes
could stabilize the coordinatively unsaturated intermediate, which
improves the catalytic properties through the reversible coordina-
tion of the hemilabile substituent to the metal center resulting in
higher catalytic activity.
As complexes 6 and 7 proved to be effective catalysts for
the cross-coupling reaction of phenylmagnesium bromide with
chlorobenzene, they were further tested on substituted aryl chlo-
It is interesting to note that complexes 6 and 7 alone are
not soluble in THF; however, upon addition of phenylmagnesium
6

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
Table 1
Kumada cross-coupling reactions of aryl chloride with PhMgBr.
Entrya
Catalyst
R
Yield (%)b
1
3
4
5
6
7
6
7
6
7
6
7
H
37
36
49
94
96
5
2
H
3
H
4
H
5
H
6
CN
CN
OMe
OMe
Me
Me
7
5
8
55c
55c
51d
52d
9
10
11
a
Reaction conditions: 1.0 mmol aryl chloride, 2.0 mmol phenylmagnesium bromide, 2 mmol% catalyst, 10 mL THF, room
temperature.
b
GC yield, an average yield of two runs.
32% of biphenyl was also obtained.
26% of biphenyl was also obtained.
c
d
3. Conclusions
In summary, we have prepared several amido-functionalized
NHC nickel carbene complexes, which include cis bis-bidentate
complexes 3-5 and pincer-type complexes 6 and 7. The prepa-
ration of complex 6 was unique, as acetate anion, a weak base,
could deprotonate the C2-H and one of the amido protons. The
chelating rings rendered the protons of the methylene moieties di-
astereotopic in complexes 3-5, and the cis configurations resulted
in steric hindrance, which prevents the free rotation of the ethyl
and phenyl substituents in 4 and 5, respectively, in solution. The
catalytic activity of these nickel complexes was examined in Ku-
mada cross-coupling reaction of phenylmagnesium bromide with
aryl chlorides, in which complexes 6 and 7 displayed similar cat-
alytic activities resulting in excellent to moderate yields.
4. Experimental
4.1. General procedures
THF was dried and freshly distilled prior to use. Other sol-
vents and chemicals were of analytic reagent grade and used
as received. Compounds NiCl2(CH3CN)2 [26], 1-acetamido-
3-methylbenzimidazolium
bromide
[12a],
1-acetamido-3-
Fig. 5. Thermal ellipsoid plot of 6 at 50% probability level. Hydrogen atoms and sol-
ethylbenzimidazolium bromide [12b], 1-phenylbenzimidazole [13],
and 1-(2-pyridyl)benzimidazole [14] were synthesized according
to the literature methods. Elemental analyses were carried out on
a Hitachi 270-30 spectrometer. 1H NMR spectra (δ (TMS) = 0.00
ppm) were recorded on either a Bruker Avance DPX300 or a
Bruker Avance II 400 spectrometer operating at 300.13 and 400.13
MHz, respectively. 13C NMR spectra were recorded on the same
spectrometers operating at 75.46 and 100.61 MHz, respectively.
Infrared spectra were recorded on a Jasco FT/IR-460 Plus spec-
trometer with 4 cm−1 resolution. GC spectra were obtained with
GC 9800 gas chromatograph equipped with a FID detector.
˚
vent molecules are omitted for clarity. Selected bond lengths (A) and angles ( ): Ni-
º
C(1), 1.790(3); Ni-N(4), 1.840(3); Ni-N(3), 1.932(3); Ni-Cl(1), 2.2117(10); N(4)-C(14),
1.325(4); C(1)-Ni-N(4), 89.27(14); C(1)-Ni-N(3), 81.81(13); N(4)-Ni-N(3), 170.72(12);
C(1)-Ni-Cl(1), 177.73(11); N(4)-Ni-Cl(1), 91.93(10); N(3)-Ni-Cl(1), 96.91(8); N(2)-
C(1)-N(1), 107.4(3).
rides (R-C6H4Cl, R = CN, Me, OMe) as substrates. Both com-
plexes displayed very low activity in the reaction with 4-
chlorobenzonitrile and only 5% yield was observed (entries 11
and 12). The low yield is caused by the cyano substituent of 4-
chlorobenzonitrile, which induces a side reaction, therefore, reduc-
ing the formation of the desired coupling product. The yields were
better with methoxy- and methyl-substituted chlorobenzene. Thus,
55 % of 4-methoxybiphenyl and 32% of biphenyl (entries 13 and
14), and 51% of 4-methylbiphenyl and 26% of biphenyl (entries 15
and 16), respectively, were obtained.
4.2. X-ray structure determination
Crystallographic data collections were carried out on a Nonius
KappaCCD diffractometer with a graphite-monochromatored Mo
˚
Kα radiation (λ = 0.71073 A) at 150(2) or 296(2) K. Cell parameters
were retrieved and refined using the DENZO–SMN [27] software on
7

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
all reflections. Data reduction was performed with the DENZO–SMN
[27] software. Empirical absorption correction was based on the
symmetry-equivalent reflections and was applied to the data us-
ing the SORTAV [28] program. Structural analysis was made using
the SHELXTL program on a personal computer. The structure was
solved using the SHELXS-97 [29] program and refined using the
SHELXL-97 [30] program by a full–matrix least–squares method on
F2 values. All non-hydrogen atoms were refined anisotropically. Ex-
cept for the amido hydrogens in 5, which were found and refined
isotropically, all the other hydrogen atoms were fixed at calculated
positions and refined using a riding mode. Crystal data and struc-
ture refinement for the different compounds are given in support-
ing information. Crystal data, details of data collection and struc-
ture refinement for compounds 4-7 (CCDC 2014566-2014570) can
be obtained from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
NiCl2(CH3CN)2 (105.8 mg, 0.5 mmol), K2CO3 (276.0 mg, 2.0
mmol), and MeOH (20 mL). The solution was refluxed for 1.5
h. The green solution changed color to yellow with formation
of a yellow precipitate. After cooling, the solvent was removed,
and the solid was washed with 100 mL of deionized water, then
re-dissolved in methanol and filtered through a pad of Celite. The
solvent was removed resulting in 180.0 mg of light yellow solid
(0.41 mmol, 83% yield). Slow diffusion of ether into a solution
of compound in methanol - chloroform afforded orange crystals
suitable for X-ray diffraction analysis. Mp: 324-329°C. 1H NMR
(DMSO–d6): δ 7.80 (d, J = 7.7 Hz, 1H, C6H4), 7.55 (d, J = 7.7 Hz,
1H, C6H4), 7.35 (m, 2H, C6H4), 5.19 (d, J = 16.3 Hz, 1H, CH2), 4.75
(d, J = 16.3 Hz, 1H, CH2), 4.49 (s, 1H, NH), 3.31 (s, 3H, CH3). 1H
NMR (DMSO–d6, 120 C): δ 7.70 (d, J = 6.7 Hz, 1H, C H ), 7.52
º
6
4
(d, J = 7.5 Hz, 1H, C6H4), 7.36 (m, 2H, C6H4), 4.94 (br s, 2H, CH2),
3.89 (br s, 1H, NH), 3.38 (s, 3H, CH3). 13C NMR (DMSO–d6): δ
178.55 (NCN), 170.83 (C=O), 134.57, 133.85 (C=C), 123.94, 123.56,
111.37, 111.01 (C6H4), 50.64 (CH2), 33.89 (CH3). IR (KBr): 3330(vw),
3203(vw), 3059(vw), 3035(vw), 2941(vw), 2925(vw), 2865(vw),
1624(s), 1586(vs), 1460(w), 1433(vw), 1385(s), 1344(vw), 1312(w),
1257(vw), 1207(vw), 1174(vw), 1150(vw), 1130(vw), 1092(vw),
1026(vw), 1012(vw), 982(vw), 927(vw), 848(vw), 796(vw), 757(w),
733(vw), 689(vw), 584(vw), 571(vw), 558(vw) cm−1. Anal. Calc.
For C20H22N6O3Ni: C, 53.01; H, 4.89; N, 18.55. Found: C, 52.93; H,
4.96; N, 18.44%.
4.3. Preparations of complexes
4.3.1. Preparation of 1-acetamido-3-phenylbenzimidazolium bromide
(1)
1-Phenylbenzimidazole
(2.042
g,
10.5
mmol),
2-
bromoacetamide (1.518 g, 11.0 mmol) and acetonitrile (25 mL)
were placed into a 50 mL flask. The mixture was refluxed for
2
days followed by the solvent removal. The remaining solid
was washed thrice with 20 mL portions of dichloromethane. The
washed solid was dried resulting in 3.089 g of white powder (9.3
mmol, 89% yield). 1H NMR (DMSO–d6, 400 MHz,): δ 10.39 (s,
1H, NCHN), 8.15 (br s, 1H, NH2), 8.09 (d, J = 8.0 Hz, 1H, C6H4),
7.86 (m, 3H, Ph, C6H4), 7.75 (m, 6H, Ph, C6H4, NH2), 5.47 (s, 2H,
CH2). 13C NMR (DMSO–d6, 100 MHz): δ 166.66 (C=O), 144.02
(NCN), 133.46, 132.25 (C=C), 131.15, 131.08, 130.99 (Ph), 127.94,
127.65 (C6H4), 125.56 (Ph), 114.66, 113.97 (C6H4), 49.29 (CH2). IR
(KBr): 3313(vw), 3294(vw), 3141(w), 2945(m), 2802(vw), 2742(vw),
2506(vw), 1838(vw), 1704(vs), 1693(s), 1566(w), 1561(w), 1406(w),
1385(s), 1306(vw), 1257(w), 1207(vw), 1081(vw), 1029(vw),
1020(vw), 862(vw), 781(w), 761(m), 694(vw), 669(vw), 662(vw),
and 585(vw) cm−1. Anal. Calc. For C15H14 BrN3O: C, 54.23; H, 4.25;
N, 12.65. Found: C, 54.30; H, 4.01; N, 12.64%.
4.3.4. Preparation of [Ni(Et-bimy-CH2CONH)2] (4)
Complex 4 was prepared analogously to complex 3 from 1-
acetamido-3-ethylbenzimidazolium bromide (284.2 mg, 1.0 mmol),
NiCl2(CH3CN)2 (106.0 mg, 0.5 mmol), K2CO3 (276.0 mg, 2.0 mmol),
and MeOH (20 mL) as a light yellow solid (190.0 mg, 0.41 mmol,
82% yield). Slow diffusion of ether into a methanol solution of the
compound afforded orange crystals suitable for X-ray diffraction
analysis. Mp: 292-296°C. 1H NMR (DMSO–d6): δ 7.83 (d, J = 7.9
Hz, 1H, C6H4), 7.63 (d, J = 7.9 Hz, 1H, C6H4), 7.34 (m, 2H, C6H4),
5.09 (d, J = 16.4 Hz, 1H, CH2(amide)), 4.78 (d, J = 16.4 Hz, 1H,
CH2(amide)), 4.52 (br s, 1H, NH), 4.31 (m, 1H, CH2(ethyl)), 3.59 (m,
1H, CH2(ethyl)), 1.19 (t, J = 7.1 Hz, 3H, CH3). 1H NMR (DMSO–d6,
180 C): δ 7.68 (d, J = 7.9 Hz, 1H, C H ), 7.54 (d, J = 7.9 Hz,
º
6
4
1H, C6H4), 7.36 (m, 2H, C6H4), 4.92 (s, 2H, CH2(amide)), 4.00 (q,
J = 5.6 Hz, 2H, CH2(ethyl)), 3.72 (br s, 1H, NH), 1.36 (t, J = 7.1
Hz, 3H, CH3). 13C NMR (DMSO–d6): δ 178.43 (NCN), 170.90 (C=O),
134.28, 133.57 (C=C), 123.99, 123.66, 111.31, 111.25 (C6H4), 50.79
(CH2(amide)), 42.79 (CH2(ethyl)), 15.35 (CH3). IR (KBr): 3340(vw),
3246(vw), 3065(vw), 3034(vw), 2988(vw), 2965(vw), 2931(vw),
2867(vw), 1618(s), 1589(vs), 1476(vw), 1449(w), 1396(m), 1385(m),
1336(vw), 1305(w), 1270(vw), 1237(vw), 1207(vw), 1191(vw),
1169(vw), 1111(vw), 1081(vw), 1034(vw), 1012(vw), 979(vw),
930(vw), 881(vw), 845(vw), 818(vw), 776(vw), 758(w), 745(w),
719(vw), 612(vw) cm−1. Anal. Calc. for C22H25N6O2.5Ni: C, 55.96;
H, 5.34; N, 17.80. Found: C, 55.90; H, 5.18; N, 17.85%.
4.3.2. Preparation of 1-acetamido-3-(2-pyridyl)benzimidazolium
bromide (2)
A mixture of 1-(2-pyridyl)benzimidazole (3.123 g, 16.0 mmol)
and 2-bromoacetamide (2.208 g, 16.0 mmol) in THF (25 mL) was
refluxed for 2 days to form a white precipitate. The precipitate
was isolated by filtration and washed with THF repeatedly. The
resulting white solid was dried to result in 3.902 g of white
powder (11.7 mmol, 73% yield). 1H NMR (DMSO–d6, 400 MHz): δ
10.60 (s, 1H, NCHN) 8.78 (d, J = 4.8 Hz, 1H, py), 8.45 (m, 1H, py),
8.29 (td, J = 7.8 Hz, J=1.8 Hz, 1H, py), 8.07 (m, 3H, py+NH2), 7.76
(m, 4H, C6H4+NH2), 5.43 (s, 2H, CH2). 13C NMR (DMSO–d6, 100
MHz): δ 166.52 (C=O), 150.07 (py), 147.56 (NCN), 143.81, 141.20
(py), 132.63 (C=C), 129.64 (C=C), 128.18, 127.81 (C6H4), 125.79
(py), 117.62 (C6H4), 116.22 (py), 114.65 (C6H4), 49.42 (CH2). IR
(KBr): 3786(w), 3691(vw), 3349(s), 3172(vw), 2954(m), 2808(vw),
2687(vw), 2604(vw), 2382(vw), 2299(vw), 1684(vs), 1559(w),
1484(vw), 1463(vw), 1441(vw), 1399(w), 1355(vs), 1342(vw),
1334(vw), 1308(w), 1286(vw), 1261(m), 1218(w), 1180(vw),
1157(vw), 1132(vw), 1099(vw), 1051(w), 890(vw), 854(w), 827(vw),
786(m), 757(vw), 743(vw), 721(vw), 672(w), 573(vw), and 512(vw)
cm−1. Anal. Calc. For C14 H13BrN4O: C, 50.47; H, 3.93; N, 16.82.
Found: C, 50.43; H, 4.00; N, 16.79%.
4.3.5. Preparation of [Ni(Ph-bimy-CH2CONH)2] (5)
A
50 mL flask was charged with 1-acetamido-3-
phenylbenzimidazolium bromide (332.2 mg, 1.0 mmol), Ni(OAc)2
(88.2 mg, 0.5 mmol), and TBAB (1.0 g). The solution was heated
to 130 C for 3 h. The solid dissolved yielding a green solution.
º
The solution was cooled to room temperature and an orange
precipitate was formed. The resulting solid was washed with
100 mL of deionized water, and then dissolved in methanol and
filtered through a pad of Celite. After the solvent was removed,
90.2 mg of light yellow solid (0.16 mmol, 32% yield) was obtained.
Crystals suitable for X-ray diffraction analysis were isolated from
methanol solution. Mp: 303-306°C. 1H NMR (DMSO–d6): δ 7.88
(br s, 2H, Ph), 7.50 (d, J = 8.0 Hz, 1H, C6H4), 7.36 (br s, 2H, Ph),
7.26 (t, J = 7.1 Hz, 1H, C6H4), 7.19 (m, 3H, Ph+C6H4), 4.62 (br
4.3.3. Preparation of [Ni(Me-bimy-CH2CONH)2] (3)
A
50 mL flask was charged with 1-acetamido-3-
methylbenzimidazolium bromide (270.8 mg, 1.0 mmol),
8

S.-Z. Lu, H.-H. Yang, W.-J. Chang et al.
Journal of Organometallic Chemistry 927 (2020) 121543
s, 1H, NH), 4.50 (d, J = 16.2 Hz, 1H, CH2), 4.42 (d, J = 16.2 Hz,
added to the reaction mixture, followed by extraction with ether
(3×20 mL). The organic fractions were combined and dried with
anhydrous magnesium sulfate. After the volatiles were removed in
vacuo, a light brown solid was isolated. The product yield was cal-
culated by integration the area of peaks in a gas chromatogram.
1H, CH2). 1H NMR (DMSO–d6, 100 C): δ 7.86 (d, J = 7.6 Hz, 2H,
º
Ph), 7.44 (d, J = 8.0 Hz, 1H, C6H4), 7.37 (t, J = 7.6 Hz, 2H, Ph),
7.28 (t, J = 7.5 Hz, 1H, C6H4), 7.22 (t, J = 7.2 Hz, 2H, Ph+C6H4),
7.17 (d, J = 7.9 Hz, 1H, C6H4), 4.46 (s, 2H, CH2), 4.16 (br s, 1H,
NH). 13C NMR(DMSO–d6): δ 180.15 (NCN), 170.34 (C=O), 137.26
(Ph), 133.50, 133.27 (C=C), 129.41, 128.70, 126.23 (Ph), 124.24,
123.96, 110.78, 110.61 (C6H4), 50.23 (CH2). IR (KBr): 3332(vw),
3200(vw), 3095(vw), 3067(vw), 3036(vw), 2971(vw), 2925(vw),
2888(vw), 1621(vs), 1591(vs), 1501(vw), 1473(w), 1457(vw),
1416(w), 1385(s), 1303(m), 1284(w), 1243(vw), 1215(w), 1191(vw),
1158(vw), 1092(w), 1070(w), 1048(w), 1018(w), 911(vw), 889(vw),
842(vw), 812(vw), 785(vw), 755(m), 748(m), 697(m), 626(vw),
606(vw), 497(vw) cm−1. Anal. Calc. for C30H28N6O4Ni: C, 60.53; H,
4.74; N, 14.12. Found: C, 60.44; H, 4.80; N, 14.05%.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgments
This work was supported by the Ministry of Science and Tech-
nology of the ROC through Grant MOST 108- 2113-M-259 -003.
4.3.6. Preparation of [Ni(Py-bimy-CH2CONH)Cl] (6)
1-Acetamido-3-(2-pyridyl)benzimidazolium bromide (333.2 mg,
1.0 mmol), NiCl2(CH3CN)2 (211.4 mg, 1.0 mmol), NaOAc (164.4 mg,
2.0 mmol), and 20 mL of MeOH - DMF (1:3, v/v) were placed
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2020.
121543.
into a 50 mL flask. The mixture was refluxed at 120 C for
º
24 h yielding a yellow-brown precipitate. The solvents were re-
moved, and the resulting solid was washed with methanol sev-
eral times. After methanol was removed, 300.0 mg of yellow
solid (0.87 mmol, 87% yield) was isolated. Crystals suitable for
X-ray diffraction analysis were obtained from DMSO - ether -
ethyl acetate. Mp: 309-311°C. 1H NMR (DMSO–d6): δ 8.84 (br s,
1H, py), 8.36 (d, J = 7.7 Hz, 1H, C6H4), 8.29 (m, 2H, py), 7.83
(d, J = 7.4 Hz, 1H, C6H4), 7.58 (m, 2H, C6H4), 7.52 (t, J = 6.4
Hz, 1H, py), 4.84 (s, 2H, CH2), 3.73 (br s, 1H, NH). IR (KBr):
3333(vw), 3121(vw), 3101(vw), 3068(vw), 2965(vw), 2930(vw),
1596(vs), 1533(vw), 1482(s), 1477(s), 1427(vw), 1385(m), 1366(w),
1269(vw), 1259(vw), 1241(vw), 1203(vw), 1095(vw), 1023(vw),
1012(vw), 908(vw), 856(vw), 766(w), 740(w), 732(vw), 617(vw)
cm−1. Anal. Calc. for C14 H11 ClN4ONi: C, 48.68; H, 3.21; N, 16.22.
Found: C, 48.60; H, 3.03; N, 15.90%.
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1.0 mmol), Ni(OAc)2 (176.1 mg, 1.0 mmol), and TBAB (1.0 g) were
placed into a 50 mL flask. The solution was heated to 130 C for 3
º
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dried, resulting in 257.2 mg orange solid (0.66 mmol, 66% yield).
Crystals suitable for X-ray diffraction analysis were obtained from
DMSO - ether - acetone. Mp: 342-344°C. 1H NMR (DMSO–d6): δ
8.61 (d, J = 5.7 Hz, 1H, py), 8.34 (m, 3H, py+C6H4), 7.84 (d, J = 7.1
Hz, 1H, C6H4), 7.56 (m, 2H, C6H4), 7.52 (t, J = 6.3 Hz, 1H, py), 4.88
(s, 2H, CH2), 4.15 (br s, 1H, NH). IR (KBr): 3323(vw), 3117(vw),
3100(vw), 3082(vw), 3070(vw), 3031(vw), 2962(vw), 2927(vw),
2851(vw), 1598(vs), 1536(vw), 1508(vw), 1482(m), 1424(vw),
1383(vw), 1366(w), 1340(vw), 1303(vw), 1259(w), 1240(vw),
1201(vw), 1143(vw), 1098(w), 1055(w), 1021(w), 914(vw), 862(vw),
803(w), 763(w), 745(w), 711(vw), 620(vw), 587(vw) cm−1. Anal.
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43.19; H, 2.90; N, 14.35%.
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