Synthesis and catalytic activities of Ni complexes bearing a novel N–C–N pincer ligand containing NHC with a bicyclic motif

Article abstract of DOI:10.1016/j.jorganchem.2020.121200

A novel N–C–N pincer ligand bearing an NHC with a bicyclic framework, namely NHC(CH₂Py)₂, was developed and successfully applied to the syntheses of air- and moisture-stable silver and nickel complexes. The silver complex was identified as {[NHC(CH₂Py)₂]₂Ag}⁺(AgCl₂)^–, and the AgCl₂ anion was coordinated with one of four pyridines. Transmetallation from silver to nickel using NiCl₂(PPh₃)₂ in toluene and a subsequent anion exchange gave us {[NHC(CH₂Py)₂]NiCl}⁺(PF₆)^– and {[NHC(CH₂Py)₂]NiCl}⁺(BF₄)^– in good yields. These complexes were active and highly stable catalysts when used for Kumada-Tamao-Corriu coupling between 3-bromo-/3-chloropyridine and PhMgBr.

Full text of DOI:10.1016/j.jorganchem.2020.121200

Journal of Organometallic Chemistry 913 (2020) 121200
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and catalytic activities of Ni complexes bearing a novel
NeCeN pincer ligand containing NHC with a bicyclic motif
Shin Ando*, Nozomi Nakano, Hirofumi Matsunaga, Tadao Ishizuka**
Faculty of Life Sciences, Kumamoto University, 5-1 Oe-honmachi Chuo-ku, Kumamoto, 862-0973, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel NeCeN pincer ligand bearing an NHC with a bicyclic framework, namely NHC(CH2Py)2, was
developed and successfully applied to the syntheses of air- and moisture-stable silver and nickel com-
plexes. The silver complex was identified as {[NHC(CH2Py)2]2Ag}þ(AgCl2)e, and the AgCl2 anion was
coordinated with one of four pyridines. Transmetallation from silver to nickel using NiCl2(PPh3)2 in
toluene and a subsequent anion exchange gave us {[NHC(CH2Py)2]NiCl}þ(PF6)e and {[NHC(CH2Py)2]
NiCl}þ(BF4)e in good yields. These complexes were active and highly stable catalysts when used for
Kumada-Tamao-Corriu coupling between 3-bromo-/3-chloropyridine and PhMgBr.
Received 27 January 2020
Received in revised form
22 February 2020
Accepted 24 February 2020
Available online 28 February 2020
Keywords:
NHC
© 2020 Elsevier B.V. All rights reserved.
Pincer
Silver
Nickel
Coupling
1. Introduction
carbenic carbons that have a bicyclic architecture.
Our group developed our original NHCs, which we refer to as
Ligands equipped with multi-coordination sites are effective in
stabilizing metal complexes via chelating effects as well as in
controlling the electron density of the supporting metal ions [1].
Among them, N-heterocyclic carbenes (NHCs) possessing other
Lewis basic site(s) are being actively investigated because of their
high electron-donating ability [2]. NHCs are structurally diverse,
and numerous types of ligands containing one or more NHC motifs
have been developed. For example, CNC [3], PCP [4], CCC [5], and
NCN [6] pincer types of tridentate ligands containing at least one
NHC site have been effective in enhancing the activity of several
transitional metal catalysts. In these studies, the fine tuning of
additional Lewis basic sites and linker parts has often been
attempted, but the NHC core structure has typically been limited to
imidazolylidene, presumably because of the ease of synthesis
(Fig. 1). The development of pincer NHC ligands bearing varied core
NHC structures should expand the opportunity for the develop-
ment of NHC-metal complexes that show unique catalytic activity
[7]. Here, we report the development of a novel NCN type of pincer
ligand bearing our original NHC motif, which is equipped with non-
DHASI, and revealed that the aromatic ring on the bicyclic archi-
tecture has an effective shielding effect on bound metals [8]. Cu and
Ni complexes supported by original NHCs showed high catalytic
activity during the borylation of aryl bromide [8b], allylic arylation
with Grignard reagents [8c], Kumada-Tamao-Corriu (KTC) coupling
[8d], Suzuki-Miyaura coupling [8e], and Chan-Lam coupling [8f].
We also successfully isolated some stable silver, copper, and nickel
complexes using DHASI ligands, and most of the structures were
confirmed by X-ray crystallography. It must be emphasized that
DHASIs have simple alkyl groups such as methyl, isopropyl, or
cyclohexyl on their nitrogen elements adjacent to the carbenic
carbon. In general, substituents on the nitrogen(s) of NHC ligands
must be bulky, particularly imidazolinylidene ligands, in order to
suppress the decomposition of metal complexes. On the other
hand, DHASIs have demonstrated a great capacity for the stabili-
zation of metal complexes even in the case of the silver complex in
N,N0-dimethyl-DHASI (DHASIMe). We envisioned a pincer ligand
with our motif showing good stability and activity during the
catalysis of transitional metal complexes, and decided to synthesize
N,N0-bis(pyridylmethyl)-DHASI, which is one of the most simple
examples of an NCN type of pincer ligand. The synthesized pincer
DHASI was applied to the syntheses of silver and nickel complexes,
and the nickel complex was found to be an active catalyst for KTC
coupling between 3-bromopyridine and PhMgBr.
* Corresponding author.
** Corresponding author.
E-mail addresses: ando@gpo.kumamoto-u.ac.jp (S. Ando), tstone@gpo.
kumamoto-u.ac.jp (T. Ishizuka).
https://doi.org/10.1016/j.jorganchem.2020.121200
0022-328X/© 2020 Elsevier B.V. All rights reserved.

2
S. Ando et al. / Journal of Organometallic Chemistry 913 (2020) 121200
2. Material and methods
diazadibenzo[h,k]tricyclo[5.2.2.2.2,6]undeca-8,10-diene
[8b]
(124 mg, 0.50 mmol) and a magnetic stir bar, and the resultant flask
was evacuated and backfilled with argon. 1,4-Dioxane (2.5 mL) was
added to the flask, and NaH (60% in oil, 100 mg, 2.50 mmol) was
added in small portions to the flask, followed by an addition of
freshly desalted 2-pyridylmethylchloride hydrochloride (246 mg,
1.50 mmol). The resultant flask was heated at reflux for 15 h under
argon, and then the reaction was quenched with sat. NH4Cl aq.
(20 mL) at 0 ꢁC. The mixture was extracted with EtOAc (15 mL ꢂ 3),
and the organic layers were washed with brine (30 mL), dried over
Na2SO4, filtered, and concentrated under reduced pressure. The
residue was purified by column chromatography (10e40% EtOAc/n-
hexane) on amino-silica gel (100 mL) to afford the title compound 2
(159 mg, 74%) as an off-white powder. mp: 176e178 ꢁC; 1H NMR
2.1. General procedure and chemicals
All reactions were carried out under an argon atmosphere with
freshly distilled solvents under anhydrous conditions, unless
otherwise noted. 1,4-Dioxane was distilled from sodium benzo-
phenone ketyl. Toluene was distilled from CaH2. Anhydrous THF
and acetonitrile were purchased and used without further purifi-
cation. PhMgBr was prepared from Mg and PhBr in THF, and the
resultant solution was titrated with 1,10-phenanthroline for the
determination of the concentration. Reagents were used without
further purification. Yields refer to chromatographically and spec-
troscopically (1H NMR) homogeneous materials, unless otherwise
stated.
All reactions were monitored by thin-layer chromatography
(TLC) carried out on 0.25-mm E. Merck silica gel plates (60Fꢀ254)
using UV-light (254 nm) for visualization or using phosphomo-
lybdic acid in EtOH and cerium sulfate in 15% sulfuric acid for
developing agents and heat for visualization. Fuji silysia silica gel
(PSQ60B) was used for flash chromatography.
Melting points were measured using a Yanako micro melting
point apparatus and are uncorrected. The NMR spectra were
recorded on Bruker Avance 600 (600 MHz) instruments and cali-
brated using solvent and TMS peaks as internal references. The
following abbreviations were used to indicate the multiplicities;
s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet, sep ¼ septet,
m ¼ multiplet, br ¼ broad, and app ¼ apparent. MS and HRMS (FAB)
were obtained with a JEOL JMS-700 mass spectrometer. High-
resolution mass spectra were obtained using EBE geometry.
(600 MHz, CDCl3, 300 K)
d
¼ 8.53 (m, 2H), 7.65 (ddd, J ¼ 7.6, 7.6,
1.7 Hz, 2H), 7.34 (d, J ¼ 7.8 Hz, 2H), 7.20e7.12 (m, 8H), 7.04 (dd,
J ¼ 5.4, 3.2 Hz, 2H), 3.99 (d, J ¼ 14.2 Hz, 2H), 3.95 (s, 2H), 3.84 (d,
J ¼ 14.2 Hz, 2H), 3.53 (d, J ¼ 4.1 Hz, 1H), 3.35 (s, 2H), 3.15 (d,
J ¼ 4.1 Hz, 1H) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K)
d
¼ 159.5, 148.8, 142.0, 141.4, 136.5, 126.0, 125.7, 125.5, 124.5, 123.2,
122.1, 78.3, 69.0, 58.9, 48.6 ppm; HRMS (FABþ): m/z calcd. for
C
29H27N4: 431.2236 [MþH]þ; found 431.2239.
N,N0-Bis(2-pyridylmethyl)-imidazolinium chloride 3; meso-1,3-
Bis(2-pyridylmethyl)-4,5-dihydro- 4,5-(9,10-dihydroanthraceno)-
1H-imidazole-3-ium chloride. Compound 3: A flask was charged
with meso-3,5-bis(2-pyridylmethyl)-3,5-diazadibenzo[h,k]tricyclo
[5.2.2.2.2,6]undeca-8,10-diene (646 mg, 1.50 mmol) and a magnetic
stir bar. DME (37.5 mL) was added to the flask, followed by an
addition of NCS (401 mg, 3.0 mmol). The resultant flask was dark-
ened with aluminum foil and stirred for 16 h at ambient temper-
ature. The white precipitate was filtered, washed with Et2O
(5 mL ꢂ 3), and dried under reduced pressure to afford the title
compound as a white powder (656 mg, 94%). mp: 176e178 ꢁC; 1H
2.2. Syntheses of ligand and metal complexes
NMR (600 MHz, CDCl3, 300 K)
d
¼ 10.36 (s, 1H), 8.59 (ddd, J ¼ 4.9,
N,N0-Bis(2-pyridylmethyl)-imidazolidine 2; meso-3,5-Bis(2-
pyridylmethyl)-3,5-diazadibenzo[h,k]tricyclo[5.2.2.2.2,6]undeca-
8,10-diene. Compound 2: A flask was charged with meso-3,5-
1.1, 1.1 Hz, 2H), 7.80e7.75 (m, 8H), 7.36e7.30 (m, 4H), 7.22e7.15 (m,
6H), 5.17 (d, J ¼ 14.6 Hz, 2H), 4.98 (s, 2H), 4.65 (d, J ¼ 14.6 Hz, 2H),
4.54 (s, 2H) ppm; 13C{1H} NMR (151 MHz, CDCl3, 300 K)
d
¼ 160.4,
152.8,149.4, 138.6, 137.9, 137.1, 127.8, 127.6, 125.7, 125.3,124.7, 123.9,
65.4, 51.2, 44.5 ppm; HRMS (FABþ): m/z calcd. for C29H25N4:
429.2079 [MeCl]þ; found 429.2078.
[(Pincer NHC)2AgCl][AgCl2] 7; Complex 7: A flask was charged
with meso-1,3-Bis(2-pyridylmethyl)- 4,5-dihydro- 4,5-(9,10-
dihydroanthraceno)-1H-imidazole-3-ium Chloride (232 mg,
0.50 mmol), Ag2O (174 mg, 0.75 mmol), and a magnetic stir bar.
CH2Cl2 (5.0 mL) was added to the flask, which was then darkened
with aluminum foil and stirred for 16 h at ambient temperature.
The resultant mixture was filtered through a short pad of Celite,
washed with CH2Cl2, and the combined filtrate and washings were
concentrated under reduced pressure to ca. 3 mL. n-Hexane
(1.5 mL) was added to the flask to precipitate the title complex 7 as
a beige powder (280 mg, 98%). A single crystal for X-ray diffraction
analysis was grown by the slow diffusion of n-hexane into a CH2Cl2
solution. mp: 192e195 ꢁC (decomp.); 1H NMR (600 MHz, CDCl3,
300 K)
d
¼ 8.60 (dd, J ¼ 4.9, 0.6 Hz, 2H), 7.70 (ddd, J ¼ 7.6, 7.6, 1.7 Hz,
2H), 7.36 (d, J ¼ 7.8 Hz, 2H), 7.29e7.24 (m, 6H), 7.22e7.20 (m, 4H),
7.20e7.16 (m, 4H), 7.09 (dd, J ¼ 5.5, 3.2 Hz, 2H), 4.89 (d, J ¼ 15.3 Hz,
2H), 4.78 (s, 2H), 4.61 (d, J ¼ 15.3 Hz, 2H), 4.36 (s, 2H) ppm; 13C{1H}
NMR (151 MHz, CDCl3, 300 K)
d
¼ 155.0, 149.6, 139.3, 137.8, 137.2,
127.1, 127.0, 125.4, 124.9, 123.2, 122.7, 66.7, 54.5, 45.2 ppm (carbenic
carbon was not observed); HRMS (FABþ): m/z calcd. for
C
58H48N8Ag: 963.3053 [(NHC)2Ag]þ; found 963.3063.
[(Pincer NHC)NiCl][PF6] 8; Complex 8. A flask was charged with
[(pincer NHC)2AgCl][AgCl2] 7 (120 mg, 0.21 mmol), NiCl2(PPh3)2
(275 mg, 0.42 mmol), and a magnetic stir bar. The resultant flask
was evacuated and backfilled with argon (repeated three times),
Fig. 1. Design of the NCN pincer ligand with a DHASI core structure.

S. Ando et al. / Journal of Organometallic Chemistry 913 (2020) 121200
3
and toluene (20 mL) was added to the flask. The reaction mixture
was then heated at reflux for 15 h under argon, and the supernatant
was removed by decantation. Toluene was added to the flask, and
the flask was sonicated for 10 min, followed by decantation of the
supernatant. This washing process was repeated twice. KPF6
(39 mg, 0.21 mmol) and CH3CN (21 mL) were added to the flask,
which contained a residual green nickel complex, and the mixture
was stirred for 24 h at ambient temperature. The reaction mixture
was then filtered through a short pad of Celite, washed with CH3CN
(2 mL ꢂ 3), and the combined filtrate and washings were concen-
trated under reduced pressure. The residual yellow powder was
reprecipitated from Et2O/CH3CN. The obtained powder was washed
with H2O, filtered, and dried under reduced pressure to afford the
title complex 8 (102 mg, 73%) as a pale yellow powder. A single
crystal for X-ray diffraction analysis was grown by the slow diffu-
sion of Et2O into a CH3CN solution. mp: 239e242 ꢁC (decomp.); 1H
chloromethylpyridine hydrochloride in the presence of NaH as a
base, and a subsequent oxidation of imidazolidine 2 to imidazoli-
nium chloride 3 was accomplished via NCS in a 94% yield. The
synthesized ligand precursor was briefly evaluated using KTC
coupling between 3-halogenopyridines and PhMgBr (Table 1). A
comparison with DHASIBnꢃHCl 6 [8a], which has a similar steric
environment, was performed to examine the effects of additional
coordination. Both NHC precursors were treated with n-BuLi in the
presence of Ni(acac)2 to form NHCeNi complexes in situ, and were
directly used as catalysts. Although the reaction between 3-
bromopyridine and PhMgBr can be catalyzed using both nickel
complexes, the coupling using DHASIBn required a much longer
reaction time than the reaction with the developed pincer ligand.
The coupling using 3-chloropyridine 4b would not proceed using a
DHASIBn-Ni catalyst, while the novel pincer NHCeNi complex was
capable of the reaction. These observations indicate that as a ligand
for NHCeNi catalysis, the DHASI with additional coordination sites
is superior to the monodentate DHASI.
Encouraged by the above promising results, we next attempted
the preparation of an air-stable NHCeNi precatalyst for easy
handling (Scheme 2). The imidazolinium chloride 3 was treated
with Ag2O in CH2Cl2 at 25 ꢁC to afford a silver complex 7 in a 98%
yield (see the following section for structural confirmation in the
solid state). With the NCN pincer NHC-silver complex 7 in hand, we
attempted an NHC transmetalation from silver to nickel. Although
the treatment of the silver complex 7 with various nickel salts in
CH2Cl2 at room temperature was fruitless, refluxing in toluene with
NiCl2(PPh3)2 gave a poorly soluble green complex [9]. After the
removal of triphenylphosphine by extraction with toluene, the
resultant complex was treated either with NaBF4 or KPF6 in
acetonitrile to exchange a counter anion to tetrafluoroborate or
hexafluorophosphate (74% for complex 8 and 73% for complex 9
respectively).
NMR (600 MHz, acetone-d6, 300 K)
d
¼ 8.93 (d, J ¼ 5.6 Hz, 2H), 8.10
(dd, J ¼ 7.6, 7.6 Hz, 2H), 7.82 (d, J ¼ 7.6 Hz, 2H), 7.46e7.38 (m, 4H),
7.21 (dd, J ¼ 5.4, 3.1 Hz, 2H), 7.08 (br s, 2H), 6.84e6.77 (m, 2H), 5.41
(d, J ¼ 16.0 Hz, 2H), 5.26 (d, J ¼ 16.0 Hz, 2H), 5.10 (s, 2H), 4.89 (s, 2H)
ppm; 13C{1H} NMR (151 MHz, acetone-d6, 300 K)
d
¼ 157.0, 155.8,
141.1, 140.0, 138.8, 128.2, 127.3, 126.4, 126.1, 125.8, 124.5, 69.4, 50.7,
46.5 ppm (carbenic carbon was not observed); HRMS (FABþ): m/z
calcd. for C29H24N4NiCl: 521.1043 [MePF6]þ; found 521.1072.
[(Pincer NHC)NiCl][BF4] 9; Complex 9. Complex 9 was synthe-
sized using the same procedure detailed in the synthesis of com-
plex 8 except for the usage of NaBF4 instead of KPF6. The title
complex 8 was obtained as a greenish yellow powder (95 mg, 74%).
A single crystal for X-ray diffraction analysis was grown by the slow
diffusion of Et2O into a CH3CN solution. mp: 237e239 ꢁC (decomp.);
1H NMR (600 MHz, acetone-d6, 300 K)
d
¼ 8.93 (d, J ¼ 5.6 Hz, 2H),
8.10 (ddd, J ¼ 7.6, 7.6, 1.1 Hz, 2H), 7.83 (d, J ¼ 7.6 Hz, 2H), 7.45e7.39
(m, 4H), 7.21 (dd, J ¼ 5.4, 3.1 Hz, 2H), 7.08 (br s, 2H), 6.84e6.77 (m,
2H), 5.41 (d, J ¼ 16.0 Hz, 2H), 5.26 (d, J ¼ 16.0 Hz, 2H), 5.11 (s, 2H),
4.89 (s, 2H) ppm; 13C{1H} NMR (151 MHz, acetone-d6, 299 K)
3.2. Structures of silver and nickel complexes in a solid state
d
¼ 157.0, 155.9, 141.1, 140.0, 138.9, 128.2, 127.3, 126.4, 126.2, 125.9,
124.5, 69.4, 50.7, 46.5 ppm (carbenic carbon was not observed);
HRMS (FABþ): m/z calcd. for C29H25N4NiCl: 521.1043 [MeBF4]þ;
found 521.1049.
Silver complexes of NHCs that have halide anions are aptly
known to be very complex bonding motifs in the solid state; [10]
therefore, we confirmed the structure by X-ray diffraction. The
structure of complex 7 (CCDC# 1974276) in the solid state con-
tained an [(NHC)eAge(NHC)]þ motif and a AgCl2 anion bound to a
nitrogen of the pyridine ring, and the absence of an AgeAg inter-
action was the most characteristic point of this complex (Fig. 2).
This rare binding system had been previously reported only by
Danopoulos and co-workers using an NHC equipped with a 2-
pyridylmethyl group on the nitrogen of an imidazolylidene ligand
(Fig. 3a and Table 2) [11]. Lee and coworkers reported a related
silver complex with an NHC that possessed a pyrazole instead of a
pyridine (Fig. 3b) [12]. The bent structure of (AgX2)e in this binding
motif deviates from the standard structure where the (AgX2)e is
approximately linear in most [(NHC)2Ag]þ(AgX2)e complexes with
an AgeAg interaction. The XeAgeX angles of the two precedents
are 154.5ꢁ (complex 10 reported by Danopoulos) and 139.8ꢁ
(complex 11 reported by Lee), respectively, and the Cl1eAg2eCl2 in
complex 4 is 123.4ꢁ. The distances between the silver and the ni-
trogen were 2.47 Å in complex 10, 2.35 Å in complex 11, and 2.35 Å
in complex 7 (Ag2eN3). The twist angles of the two NHC planes
were 33.5ꢁ in complex 10 and 24.2ꢁ in complex 7, while complex 11
was almost planer around the NHCeAgeNHC moiety (2.0ꢁ). In
contrast to these differences, the distances between carbenic car-
bon and silver were similar (2.08e2.11 Å).
2.3. Typical procedure for a KTC coupling reaction
PhMgBr was freshly prepared from PhBr and Mg turnings in THF
and titrated using 1,10-phenanthroline for determination of the
concentration. Ni complex and a magnetic stir bar were added to a
flask. An arylhalide in THF was added to the flask under an Ar at-
mosphere. The resultant flask was cooled in an ice bath, and
PhMgBr in THF (2.5 eq.) was added dropwise at the same temper-
ature. The reaction solution was stirred at ambient temperature
until TLC monitoring indicated the complete consumption of the
substrate, and the reaction was then quenched with sat. NH4Cl aq.
at 0 ꢁC. The resultant mixture was extracted with EtOAc (three
times), and the combined organic layers were washed with brine,
dried over NaSO4, filtered, and concentrated under reduced pres-
sure. The crude residue was purified by a column chromatography
on silica gel to afford the cross-coupled product.
3. Results and discussion
3.1. Syntheses of a novel NCN pincer-type NHC ligand, a silver
complex, and nickel complexes
The obtained NCN pincer NHCeNi complexes were subse-
quently analyzed by X-ray diffraction (CCDC numbers are 1974274
for complex 8 and 1974275 for complex 9). Complexes 8 and 9 were
recrystallized from n-hexane/acetonitrile into yellow plates
We started the synthesis of a new pincer ligand from imidazo-
lidine 1 (Scheme 1), which we had previously reported [8b]. Two
pyridylmethyl groups were introduced using freshly desalted 2-

4
S. Ando et al. / Journal of Organometallic Chemistry 913 (2020) 121200
Scheme 1. Synthesis of an NCN pincer ligand with a DHASI coordination site.
Table 1
Effects of additional coordination sites.
Fig. 2. ORTEP diagram of complex 7. Thermal ellipsoids are shown at the 50% proba-
bility level (CCDC 1974276). H atoms are omitted for clarity.
envelope conformation with a 5.8ꢁ bend toward the aromatic ring
on the bicyclic architecture. The steric maps and %Vbur of these
complexes were also calculated using the SambVca program
developed by Nolan and Caballo (Fig. 5) [13], which indicated that
the bicyclic architecture resulted in a slight increase in the steric
pressure (%Vbur: 61.4 on complex 8; 58.1 on complex 12). This steric
pressure from the bicyclic architecture might have caused the
higher catalytic activity and stability of the nickel complex during
the KTC coupling shown in Table 4 (vide infra).
Entry
Substrate
Ligand
Y (eq.)
Time (h)
Yield (%)
1
2
3
4
4a
4a
4b
4b
3
6
3
6
2.5
2.5
3.0
3.0
2.5
20.5
6.0
75
64
67
21.0
NRa
aNo reaction.
3.3. Structures of nickel complexes in a solution state
(complex 8, Fig. 4) or blocks (complex 9), respectively. Both struc-
tures are basically identical in the solid state except for the counter
anions. We compared the structure of complex 8 with a similar
nickel complex 12 that was reported by Lee and co-workers
(Table 3) [12]. These complexes bind to Ni with a carbene and
two nitrogens of pyridine rings, and the coordination geometry of
nickel is square planar in both structures. Bond lengths around the
bound nickel as well as the twist angles between the N1eCeN2
planer and a planer around the bound nickel are located in close
proximity. One of the most characteristic differences was observed
In the X-ray diffraction studies of Ni complexes 8 and 9, pairs of
asymmetric conformers were observed in a unit cell. In theory, the
two pyridylmethyl groups are unequivalent in a single conformer in
a solid state. In a solution state, however, the NMR spectra of these
complexes showed symmetric features (see the supplementary
material), which suggested the presence of a rapid conformational
exchange between the enantiomeric conformers in an NMR time
scale at 300 K. The experimental observations from silver complex
7 showed the same situation, but we could not predict the structure
in a solution state at this point, because the NHCeAg complexes
with a halogen counter anion have a substantial amount of possible
equilibria [10].
around
a heterocycle containing a carbenic carbon, which
amounted to the imidazolylidene ring on complex 12 was planer,
while the imidazolinylidene of DHASI appeared to have an
Scheme 2. Syntheses of NCN pincer NHCeNi complexes.

S. Ando et al. / Journal of Organometallic Chemistry 913 (2020) 121200
5
Fig. 3. Structure of NHC-AgX complexes related to complex 7.
Table 2
Selected bond length and angles of complex 7, complex 10 [11], and
complex 11 [12].
Distance (Å)/angle
(deg)
Complex 8
Complex 10
[11]
Complex 11
[12]
Fig. 4. ORTEP diagram of complex 8. Thermal ellipsoids are shown at the 50% prob-
ability level (CCDC 1974274). H atoms and counter anion are omitted for clarity.
C
carbeneeAg1
2.11 (1), 2.08 2.07 (4), 2.07 2.09 (9), 2.09
(1)
(6)
(8)
NeAg2
2.35 (6)
2.47 (4)
175.9 (2)
154.5 (3)
2.35 (1)
179.5 (5)
139.8 (2)
C
carbeneeAg1eCcarbene 173.3 (4)
Cl1eAg2eCl1 123.4 (1)
coupling (entry 4). Even with this lowered catalyst loading system,
complex 9 showed better activity than complex 12 (entry 5), and the
bicyclic motif effectively stabilized the catalytically active nickel
species. To demonstrate this stabilizing effect, we attempted the
experiment shown in Fig. 6. In this experiment, we performed the
same coupling using a 0.5 mol% catalyst for 24 h, and then an
additional 3-bromopyridine and PhMgBr solution was added to the
reaction flask and stirred at 25 ꢁC for a further 42 h (Fig. 6a). After the
isolation procedure, we obtained the product in a 69% yield, sug-
gesting that the active species maintained its level of activity in the
reaction vessel after 24 h. Further investigation using a second
addition (first addition was after 24 h and second was addition after
66 h, Fig. 6b) obviously indicated that the catalyst remained active
even after 66 h, although the activity seemed to decrease somewhat.
These data verified that our NCN pincer NHC preserved the activity
of the bound nickel under the reaction conditions for as long as
several days.
3.4. Application of the nickel complexes as a catalyst for Kumada-
Tamao-Corriu coupling
With the well-defined novel NHCeNi complexes in hand, we
investigated their catalytic activity during KTC coupling between 3-
bromopyridine 4a and PhMgBr. Both complexes 8 and 9 were acti-
vated in situ presumably by a Grignard reagent, and the resultant
complex catalyzed the KTC coupling shown in Table 4 with a 0.5 mol
% catalyst loading, which resulted in 52 and 64% yields, respectively
(entries 1 and 2). Complex 12 was also a catalytically active complex,
but its activity was somewhat lower than complexes possessing the
DHASI core structure (entry 3). Albeit with the requirement of longer
reaction time, 0.1 mol% of complex 9 was capable of catalyzing this
Table 3
Comparison of an NCN pincer with imidazolylidene-Ni complex 12.
Distance (Å)/angle (deg)
Complex 8
1.84 (3)
1.93 (2), 1.93 (2)
2.23 (7)
110.4 (2)
87.4 (9), 87.4 (8)
179.2 (7)
Complex 12
CeNi
1.84 (3)
1.94 (4), 1.94 (3)
2.24 (1)
106.9 (3)
86.7 (1), 87.0 (2)
179.0 (1)
N3eNi, N4eNi
NieCl
N1eCeN2
CeNieN3, CeNieN3
CeNieCl

6
S. Ando et al. / Journal of Organometallic Chemistry 913 (2020) 121200
Fig. 5. The steric maps of a) complex 8 and b) complex 12. Calculation parameters: Bondi radii scaled by 1.17; 3.5 Å for the sphere radius; 0.0 Å for the distance from the center of the
sphere; 0.10 Å for the mesh spacing.
Table 4
4. Conclusions
KTC coupling using an NCN pincer NHCeNi precatalyst.
We developed a novel NCN pincer NHC ligand that bears a
bicyclic architecture on its non-carbenic carbons. The synthe-
sized pincer ligand was successfully applied to the syntheses of
silver and nickel complexes, and we unambiguously confirmed
the structures of complexes 7, 8, and 9 via X-ray crystallography.
The silver complex 7 in the solid state is a rare [(NHC)2Ag][AgCl2]
type of binding system that is without AgeAg interaction. The
NHCeNi complexes 8 and 9 with PF6 and BF4 counter anions
were also analyzed, and the results indicated that the binding
systems were almost identical to the reported NCN
imidazolylidene-Ni complex 12 with the exception of differences
between imidazolinylidene and imidazolylidene. Calculation us-
ing the SambVca program suggested that our DHASI-based pincer
ligand has a somewhat higher %Vbur than complex 12. The well-
defined nickel complexes were used as precatalysts for KTC
coupling between 3-bromo-/3-chloropyridine and PhMgBr,
Entry
Precatalyst
X (mol %)
Y (M)
Time (h)
Yield (%)
where the complex 9 was found to be the best catalyst among the
three complexes we tested. The robustness of an in situ-formed
catalyst was also investigated, and the performance of complex 9
showed it to be a long-lived active species. Further investigations
to find a unique reaction for the synthesized novel NCN pincer
NHCeNi complex are being conducted in our laboratory.
1
2
3
4
5
8
9
12
9
12
0.5
0.5
0.5
0.1
0.1
0.4
0.4
0.4
0.2
0.2
2.5
2.5
2.5
48
52
64
29
65
41
48

S. Ando et al. / Journal of Organometallic Chemistry 913 (2020) 121200
7
Fig. 6. Investigation of the robustness of the in situ-formed catalyst.
39e51;
Declaration of competing interest
(k) C. Gunanathan, D. Milstein, Chem. Rev. 114 (2014) 12024e12087;
(l) M. Albrecht, G. van Koten, Angew. Chem. Int. Ed. 40 (2001) 3750e3781;
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
(m) M.E. van der Boom, D. Milstein, Chem. Rev. 103 (2003) 1759e1792.
ꢀ
[2] V. Charra, P. de Fremont, P. Braunstein, Coord. Chem. Rev. 341 (2017) 53e176.
[3] K. Inamoto, J. Kuroda, E. Kwon, K. Hiroya, T. Doi, J. Organomet. Chem. 694
(2009) 389e396.
[4] A. Eizawa, K. Arashiba, H. Tanaka, S. Kuriyama, Y. Matsuo, K. Nakajima,
K. Yoshizawa, Y. Nishibayashi, Nat. Commun. 8 (2017).
[5] (a) A.R. Naziruddin, Z.J. Huang, W.C. Lai, W.J. Lin, W.S. Hwang, Dalton Trans. 42
(2013) 13161e13171;
Acknowledgements
This research was financially supported in part by JSPS KAKENHI
Grant Numbers 18K06583 and 19K06998. We thank Mr. Tatsuya
Uehashi for the initial studies on the synthesis of the pincer NHC
ligand precursor and the silver complex. We are grateful to Dr.
Hiroyasu Sato at the Rigaku Corporation for assistance with the
crystallographic analysis.
(b) A.R. Chianese, A. Mo, N.L. Lampland, R.L. Swartz, P.T. Bremer, Organome-
tallics 29 (2010) 3019e3026;
(c) T.R. Helgert, T.K. Hollis, E.J. Valente, Organometallics 31 (2012) 3002e3009.
[6] (a) A.G. Nair, R.T. McBurney, M.R.D. Gatus, D.B. Walker, M. Bhadbhade,
B.A. Messerle, J. Organomet. Chem. 845 (2017) 63e70;
(b) C. Chen, H. Qiu, W. Chen, J. Organomet. Chem. 696 (2012) 4166e4172;
(c) A.M. Magill, D.S. McGuinness, K.J. Cavell, G.J. Britovsek, V.C. Gibson,
A.J. White, D.J. Williams, A.H. White, B.W. Skelton, J. Organomet. Chem.
617e618 (2001) 546e560.
Appendix A. Supplementary data
[7] Y. Jiang, C. Gendy, R. Roesler, Organometallics 37 (2018) 1123e1132.
[8] (a) S. Ando, H. Matsunaga, T. Ishizuka, Tetrahedron 69 (2013) 1687e1693;
(b) S. Ando, H. Matsunaga, T. Ishizuka, J. Org. Chem. 80 (2015) 9671e9681;
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2020.121200.
(c) S. Ando, H. Matsunaga, T. Ishizuka, Asian J. Org. Chem.
3 (2014)
1058e1061;
(d) S. Ando, M. Mawatari, H. Matsunaga, T. Ishizuka, Tetrahedron Lett. 57
(2016) 3287e3290;
(e) S. Ando, H. Matsunaga, T. Ishizuka, J. Org. Chem. 82 (2017) 1266e1272;
(f) S. Ando, Y. Hirota, H. Matsunaga, T. Ishizuka, Tetrahedron Lett. 60 (2019)
1277e1280.
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