Cobalt pincer complex catalyzed Suzuki-Miyaura cross coupling – A green approach

Article abstract of DOI:10.1016/j.jorganchem.2016.11.005

A series of cobalt complexes with tridentate pincer ligands were synthesized to study their catalytic activity in Suzuki-Miyaura coupling reactions. Cobalt complexes, C-1, C-2, C-3 bearing asymmetrical PNCOP pincer ligand [C₆H₄-1-(NHPPh₂)-3-(OPPh₂)] (L-1) and symmetrical PNCNP, PNNNP pincer ligands [C₆H₄-2,6-(NHPPh₂)₂] (L-2) and [C₅H₃N-2,6-(NHPPh₂)₂] (L-3) were synthesized by the reaction of diphenylchlorophosphine with m-aminophenol, m-phenylenediamine and 2,6-diaminopyridine respectively in a 1:2 ratio in the presence of triethylamine as a base and tetrahydrofuran as solvent media. The synthesized complexes were examined for their C-C coupling efficiency in cross-coupling between phenyl boronic acid and para substituted bromobenzenes. Effect of variation of the ligand on the catalytic activity of cobalt pincer complex was explored based on the coupling yields. It is observed that as the number of ‘N’ atoms increases in the side arm of the ligand, the donating ability of the ligand increases which leads to the increased catalytic activity of the complex. The symmetrical PNNNP pincer complex (C-3) was found to be more effective as a catalyst among the complexes synthesized and reported in the present study.

Full text of DOI:10.1016/j.jorganchem.2016.11.005

Journal of Organometallic Chemistry 827 (2017) 41e48
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Cobalt pincer complex catalyzed Suzuki-Miyaura cross coupling e A
green approach
Lolakshi Mahesh Kumar, Badekai Ramachandra Bhat*
Catalysis and Materials Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Srinivasanagar, Surathkal, 575025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of cobalt complexes with tridentate pincer ligands were synthesized to study their catalytic
activity in Suzuki-Miyaura coupling reactions. Cobalt complexes, C-1, C-2, C-3 bearing asymmetrical
PNCOP pincer ligand [C6H4-1-(NHPPh2)-3-(OPPh2)] (L-1) and symmetrical PNCNP, PNNNP pincer ligands
[C6H4-2,6-(NHPPh2)2] (L-2) and [C5H3N-2,6-(NHPPh2)2] (L-3) were synthesized by the reaction of
diphenylchlorophosphine with m-aminophenol, m-phenylenediamine and 2,6-diaminopyridine
respectively in a 1:2 ratio in the presence of triethylamine as a base and tetrahydrofuran as solvent
media. The synthesized complexes were examined for their C-C coupling efficiency in cross-coupling
between phenyl boronic acid and para substituted bromobenzenes. Effect of variation of the ligand on
the catalytic activity of cobalt pincer complex was explored based on the coupling yields. It is observed
that as the number of ‘N’ atoms increases in the side arm of the ligand, the donating ability of the ligand
increases which leads to the increased catalytic activity of the complex. The symmetrical PNNNP pincer
complex (C-3) was found to be more effective as a catalyst among the complexes synthesized and re-
ported in the present study.
Received 28 July 2016
Received in revised form
28 October 2016
Accepted 2 November 2016
Available online 5 November 2016
Keywords:
Suzuki-Miyaura cross-coupling
Pincer ligand
PNCOP
PNCNP
PNNNP
Biaryls
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
instances, with nickel [15e17], ruthenium [18,19] for Suzuki-
Miyaura cross-coupling reactions. Use of palladium metal in the
Pincer complexes have emerged over the last two decades as a
potential class of organometallic compounds capable of having
extended utility in the field of homogeneous catalysis [1,2]. Ever
since their introduction to the inorganic field in the late 1970s, the
pincer skeletons with their tridentate arrangement of donor sites
have been productively used in ligand-metal mediated catalysis,
inorganic chemistry and materials science [3]. Their exceptional
thermal stability often accompanied with the unique chemical
complexes, even though affords high yield due to its massive cat-
alytic activity, is an expensive affair due to the less abundance and
consequently heavy cost of the metal. Metals like Ni, Ru remain
close to Pd in its activity but are not appreciably environmentally
benign. Therefore, if quite an analogous outcome in terms of yield
and efficiency can be obtained using less expensive and substan-
tially benevolent metal complexes, the effort positively renders an
economical and a green approach towards the research study. In
their review article on non precious metal complexes with anionic
PCP pincer architecture, Murugesan and Kirchner [20] have pro-
vided an overview of the advancements in the pincer catalysis
employing cheap and abundant metals such as nickel, cobalt, and
iron with PCP pincer ligands which could result in the development
of novel, versatile, and efficient catalysts for atom-efficient catalytic
reactions. Their study also emphasized that the cobalt PCP pincer
complexes were not applied to any catalytic reactions. Cobalt
complex with NNN type pincer ligand has been reported to effi-
ciently catalyze polymerization of 1,3-butadiene [21]. The catalytic
involvement of cobalt pincer complexes in the cross coupling of aryl
halides and organoboron compounds is however unreported and
this fact made us opt for cobalt metal in the synthesis of pincer
stability of the
s-M-central atom and the rigid tridentate pincer
motif, avoids the dissociation of the metal from the ligand. This in
turn, makes the pincer metal interaction retain throughout the
catalytic reactions and can attribute to the enhanced catalytic ac-
tivity of the pincer complexes [4e7]. Suzuki-Miyaura cross-
coupling reaction shows functional group tolerance for a wide va-
riety of substrates which makes it an indispensible tool in organic
synthesis [8,9]. Research studies reported hitherto emphasize the
use of pincer complexes with mostly palladium [10e14] and some
* Corresponding author.
E-mail address: ram@nitk.edu.in (B.R. Bhat).
http://dx.doi.org/10.1016/j.jorganchem.2016.11.005
0022-328X/© 2016 Elsevier B.V. All rights reserved.

42
L.M. Kumar, B.R. Bhat / Journal of Organometallic Chemistry 827 (2017) 41e48
complexes for carrying out Suzuki coupling reactions. Pursuing our
research interest in the synthesis of pincer complexes and their
potential application on beneficial organic transformations, we
report in this paper, the synthesis of PNCOP pincer complex (C-1),
PNCNP pincer complex (C-2), PNNNP pincer complex (C-3) fol-
lowed by their novel application in cross coupling of aryl halides
and organoboron compounds with good yields of biaryls. Effect of
variation of the ligand on the catalytic activity of cobalt pincer
complex is being observed in this study report.
2.2.3. Synthesis of [C5H3N-2,6-(NHPPh2)2] (L-3)
To a suspension of 2,6-diaminopyridine (1 g, 9.2 mmol) in THF
(20 mL) was added triethylamine (1.85 g, 18.3 mmol). The mixture
was then cooled to 0 ꢀC and chlorodiphenylphosphine (4.04 g,
18.3 mmol) was added drop wise with stirring. The solution was
warmed to room temperature and set to reflux overnight. The so-
lution was then filtered, washed with anhydrous hexane
(2 ꢁ 10 mL) and the solvent was removed under vacuum to afford
ligand (L-3) as orange solid.
Yield: 87.2%.
1H NMR (400 MHz, CDCl3)
3H), 7.53e7.40 (m, 8H), 7.40e7.27 (m, 3H), 3.78e3.49 (br s, 2H, NH)
d 7.82e7.62 (m, 9H), 7.61e7.54 (m,
2. Experimental
(Supplementary Information Fig. S9).
2.1. Materials
31P{1H} NMR (161.8 MHz, DMSO):
(Supplementary Information Fig. S10).
d (ppm) 25.46 (s, PN)
Cobalt (II) acetate was purchased from Merck, India and used as
received. Other chemicals like m-aminophenol, m-phenylenedi-
amine, 2,6-diaminopyridine,chlorodiphenylphosphine, tetrahy-
drofuran (THF), triethylamine (Et3N), acetonitrile (ACN),
phenylboronic acid and aryl halides were purchased from Sigma-
Aldrich and used without further purification.
IR (KBr);
n 3441, 3076, 1589, 1483, 1180, 1069, 959, 754, 727, 694,
552 cmꢂ1
.
MS-ESI: (m/z): 478.4 (Supplementary Information Fig. S11).
Elemental analysis calculated for C29H25N3P2 (Mr ¼ 477.1): C,
72.95; H, 5.28; N, 8.80. Found: C, 72.15; H, 5.03; N, 8.56%.
2.3. Synthesis of complexes
2.2. Synthesis of ligands (L-1, L-2 and L-3)
2.3.1. Synthesis of [Co(COOCH3){C6H4-1-(NHPPh2)-3-(OPPh2)}](C-
1)
2.2.1. Synthesis of [C6H4-1-(NHPPh2)-3-(OPPh2)](L-1)
In a round bottomed flask, m-aminophenol (1 g, 9.2 mmol,) was
stirred in THF (20 mL). Triethylamine (1.85 g, 18.3 mmol) was added
to the RB flask and stirred well. The mixture was then cooled to 0 ꢀC
and chlorodiphenylphosphine (4.04 g, 18.3 mmol) was added drop
wise with stirring. The solution was warmed to room temperature
and set to reflux overnight. The solution was then filtered, washed
with anhydrous hexane (2 ꢁ 10 mL) and the solvent was removed
under vacuum to afford ligand (L-1) as yellow solid.
Cobalt acetate (0.13 g, 0.5 mmol) in THF (3 mL) was refluxed for
4 h with L-1 (0.25 g, 0.5 mmol) in THF (5 mL). Complex obtained
was filtered and washed with ether. Yield: 79.6%.
IR (KBr);
n .
3052, 1435, 1132, 1052, 997, 754, 727, 692 cmꢂ1
MS-ESI: (m/z): 595.4 [M]þ (Supplementary Information Fig. S4),
Elemental analysis calculated for C32H27CoNO3P2(Mr ¼ 594.1): C,
64.66; H, 4.58; N, 2.36 Found: C, 63.95; H, 4.41; N, 2.18%.
Yield: 83.0%.
2.3.2. Synthesis of [Co(COOCH3){C6H5-1,3-(NHPPh2)2}](C-2)
Cobalt acetate (0.13 g, 0.5 mmol) in THF (3 mL) was refluxed for
4 h with L-2 (0.25 g, 0.5 mmol) in THF (5 mL). Complex obtained
was filtered and washed with ether. Yield: 77.4%.
1H NMR (400 MHz, CDCl3)
d
7.89e7.75 (m, 9H), 7.65e7.43 (m,
15H), 4.02 (br s, 1H, NH). (Supplementary Information Fig. S1).
31P{1H} NMR (161.8 MHz, DMSO):
(ppm) 25.5 (s, PN), 31.3 (s,
PO) (Supplementary Information Fig. S2).
IR (KBr); 3350, 3177, 3055,1607,1489, 1437, 1177,1126,982, 752,
725, 694, 557, 527 cmꢂ1
d
IR (KBr);
n 3055, 2361, 1589, 1435, 1142, 1057, 1026, 993, 756,
694, 563 cmꢂ1
.
n
MS-ESI: (m/z): 594.2 [M]þ (Supplementary Information Fig. S8),
Elemental analysis calculated for C32H28CoN2O2P2 (Mr ¼ 593.1): C,
64.76; H, 4.76; N, 4.72. Found: C, 63.89; H, 4.52; N, 4.43%.
.
MS-ESI: (m/z): 477.0 (Supplementary Information Fig. S3).
Elemental analysis calculated for C30H25NOP2 (Mr ¼ 477.1): C,
75.46; H, 5.28; N, 2.93. Found: C, 74.90; H, 5.30; N, 2.59%.
2.3.3. Synthesis of [Co(COOCH3)2{C5H3N-2,6-(NHPPh2)2}](C-3)
Cobalt acetate (0.13 g, 0.5 mmol) in THF (3 mL) was refluxed for
4 h with L-3 (0.25 g, 0.5 mmol) in THF (5 mL). Complex obtained
was filtered and washed with ether. Yield: 82.3%.
2.2.2. Synthesis of [C6H4-1,3-(NHPPh2)2] (L-2)
m-phenylenediamine (1 g, 9.3 mmol), THF (20 mL) was taken in
a RB flask, to which triethylamine (1.87 g, 18.5 mmol) was added
and stirred well. The mixture was then cooled to 0 ꢀC and chlor-
odiphenylphosphine (4.08 g, 18.5 mmol) was added drop wise with
stirring. The solution was warmed to room temperature and set to
reflux overnight. The solution was then filtered through a short
plug of celite, washed with anhydrous hexane (2 ꢁ 10 mL) and the
solvent was removed under vacuum to afford ligand (L-2) as
brownish black solid.
IR (KBr);
567 cmꢂ1
MS-ESI: (m/z): 655.1 [M]þ (Supplementary Information
Fig. S12), Elemental analysis calculated for 33H31CoN3O4P2
n 3446, 3051, 1562, 1435, 1138, 1057, 995, 756, 729, 694,
.
C
(Mr ¼ 654.1): C, 60.56; H, 4.77; N, 6.42. Found: C, 59.85; H, 4.43; N,
6.28%.
Yield: 78.9%.
2.4. General procedure for the Suzuki reaction
1H NMR (400 MHz, CDCl3)
d 7.82e7.62 (m, 8H), 7.61e7.54 (m,
4H), 7.53e7.40 (m, 12H), 4.02 (br s, 2H, NH) (Supplementary
Aryl halide (1.0 mmol) was added to a mixture of phenylboronic
acid (1.3 mmol), cobalt pincer complex (0.005 mmol) and base
(2.0 mmol) in 5 mL of solvent and heated to 80 ꢀC. The mixture was
then cooled to room temperature and the organic phase analyzed
by gas chromatography.
Information Fig. S5).
31P{1H} NMR (161.8 MHz, DMSO):
(Supplementary Information Fig. S6).
d (ppm) 25.47 (s, PN).
IR (KBr);
n 3360, 3055, 2922, 2365, 1607, 1437, 1184, 1121, 1022,
995, 750, 725, 694, 559, 529 cmꢂ1
.
MS-ESI: (m/z): 477.1 (Supplementary Information Fig. S7).
Elemental analysis calculated for C30H26N2P2 (Mr ¼ 476.1): C,
75.62; H, 5.50; N, 5.88. Found: C, 75.01; H, 5.41; N, 5.69%.
2.5. Characterization methods
The C, H and N contents of the compounds were determined by

L.M. Kumar, B.R. Bhat / Journal of Organometallic Chemistry 827 (2017) 41e48
43
Thermoflash EA1112 series elemental analyzer. Magnetic suscepti-
bility measurement was recorded on a Sherwood Scientific mag-
netic susceptibility balance (UK). Thermal analysis was carried out
(EXSTAR-6000) from room temperature to 700 ꢀC at a heating rate
of 10 ꢀC/min. The electronic spectrum of the complex was recorded
in analytikjena SPECORD S600. FT-IR spectra were recorded on a
Bruker-Alpha ECO-ATR FTIR spectrometer. 1H NMR (400 MHz)
spectrum was recorded in Bruker AV 400 instrument using TMS as
internal standard. 31P{1H} NMR (161.8 MHz) spectrum was recor-
ded in Varian instrument using H3PO4 as internal standard. Mo-
lecular mass of the compounds was determined using a Waters Q-
Tof micro mass spectrometer with an ESI source. The coupling re-
action product analysis was carried out using Gas Chromatography
(GC) (Shimadzu 2014, Japan), siloxane Restek capillary column
(30 m length and 0.25 mm diameter) and Flame Ionization De-
tector. The column temperature was increased at the rate of 10 ꢀC/
min. Nitrogen gas was used as the carrier gas.
corresponding to the presence of one unpaired electron. The li-
gands which give low-spin complexes usually have donor atoms of
low electronegativity such as P, As or C capable of forming
p bonds
[23]. Complexes, C-1 and C-2 are low spin four coordinate com-
plexes whose magnetic moments very well matches to the calcu-
lated range of 2.1-2-9 B.M for square planar Co(II) species and
magnetic moment of C-3 very much lies in the specified range for
five co-ordinate cobalt(II) complexes [24,25].
In the UVeVisible spectra (Fig. 1), there is a shift in the ab-
sorption band from the ligand to the complex perceptibly indi-
cating the complex formation. The absorption bands observed in
the spectrum of the free ligand have shifted to lower energy region
in the spectra of complexes due to the coordination of the ligand
with metal ion. All the transitions are majorly due to intra ligand
charge transfer transitions (ILCT) and ligand to metal charge
transfer transitions (LMCT).
The thermal analysis (TGA) result of the complexes is repre-
sented in the Fig. 2. The TG curves obtained are typical curves
representing single step dissociation of the complexes. The disso-
ciation of the ligands is observed well after the temperature of
500 ꢀC indicating the exceptional stability of the pincer complex
formed. The TGA curves also support the proposed structures of the
complexes with respect to the weight loss of the ligands and re-
sidual mass as cobalt oxide.
3. Results and discussion
3.1. Characterization studies
The procedures for the synthesis of the ligands and the com-
plexes are shown in Schemes 1 and 2 respectively. The ligands and
the complexes were characterized by FTIR, mass spectra and
elemental analysis. All the results were in full agreement with the
proposed structure. 1H NMR spectral analysis of the ligands also
substantiates the structures proposed for the ligands.
FTIR analysis of the ligands L-1, L-2 and L-3 showed medium
intensity N-H stretching indicating the bond formation between N
and P. Formation of bond between O and P in L-1 is indicated by the
absence of broad peak for O-H stretching. A sharp peak at 694 cmꢂ1
is assigned to the P-N stretching in both the ligands and the com-
plexes [22]. Formation of the complex was further confirmed
through magnetic susceptibility study, UVeVisible spectroscopy
and Thermal analysis.
3.2. Catalytic activity studies
Pincer complexes have been considerably employed to catalyze
cross coupling of aryl halides with organoborons since their advent
to the field of catalysis. Researchers have explored different ways to
enhance the catalytic activity for C-C cross coupling using pincer
complexes, however with palladium as the chief catalyzing metal.
An optional and alternative method to complement the existing
approach of catalysis is emphatically the use of 3d transition metal
complexes. The synthesized complexes using cobalt as the cata-
lyzing metal explicitly corroborates the green approach and pro-
vides an alternative inexpensive catalyst for Suzuki cross coupling.
The reaction between 4-bromobenzonitrile and phenyl boronic
acid was chosen as a model reaction to evaluate the catalytic
The magnetic susceptibility measurements in the solid state
show that the complexes are paramagnetic at room temperature
with magnetic moment values 2.34, 2.41 and 1.96 BM respectively
Scheme 1. Synthesis of ligands: a) PNCOP pincer ligand (L-1); b) PNCNP pincer ligand (L-2); c) PNNNP pincer ligand (L-3).

44
L.M. Kumar, B.R. Bhat / Journal of Organometallic Chemistry 827 (2017) 41e48
Scheme 2. Synthesis of complexes C-1, C-2 and C-3.
Fig. 1. UVeVis spectrum of: a) Ligand L-1 and complex C-1 (b) Ligand L-2 and complex C-2 (c) Ligand L-3 and complex C-3.
activity of the Co-pincer catalyst (C-3) in the Suzuki coupling of aryl
halides with phenyl boronic acid. Various factors including solvent,
catalyst loading, base, temperature and time were screened to
optimize the reaction conditions. The results are summarized in
Tables 1e3. Amongst the solvents tested (Fig. 3), highest catalytic
activity was observed with dioxane as the solvent media (Table 1,

L.M. Kumar, B.R. Bhat / Journal of Organometallic Chemistry 827 (2017) 41e48
45
acetonitrile as a solvent may be attributed to the property of
acetonitrile to act in itself as a ligand with its exceptional coordi-
nating property. The other parameter considered for optimization
is the reaction temperature. As illustrated in Table 1 (entries 6 & 7),
80 ꢀC is the best temperature for the chosen reaction in acetonitrile
media.
Catalyst loading for the reaction was also considered for opti-
mization. 0.5 mol% of the catalyst proved to be well efficient in
catalyzing the reaction in acetonitrile media (Table 1, entry 7). In-
crease in catalyst loading to 0.75 mol% and1.0 mol% was found not
to increase the yield appreciably (Table 1, entries 9 & 10); the fact
which substantiates that the product yield is not very sensitive to
catalyst loading at higher concentration. Since no remarkable dif-
ference in the yield was observed when reactions were performed
under argon or open air (Table 1, Entries 7 & 11), it was decided to
carry out all further reactions under air.
The effect of bases on the product yield was examined for the
Fig. 2. TGA curves of the complexes C-1, C-2 and C-3.
Table 1
Solvent and temperature study for the Suzuki coupling of 4-bromobenzonitrile and phenyl boronic acid using catalyst C-3.
Entry
Solvent
Base
Temperature (ꢀC)
Yield (%)a
1
2
3
4
5
6
7
MeOH
EtOH
EtOH:H2O (1:1)
1,4-dioxane
Toluene
ACN
ACN
ACN
ACN
ACN
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
50
60
90
100
100
70
80
80
80
80
80
54
72
49
84
75
73
80
71
82
83
78
8b
9c
10d
11e
ACN
Reaction conditions: 4-bromobenzonitrile (1.0 mmol), Phenylboronic acid (1.3 mmol), K2CO3 (2.0 mmol), catalyst (0.5 mol%), solvent (5.0 mL), 16 h under inert atmosphere.
a
GC yields.
Catalyst (0.25 mol%).
Catalyst (0.75 mol%).
Catalyst (1.0mol%).
b
c
d
e
Atmospheric condition.
entry 4). Acetonitrile was the second most productive solvent in
terms of conversion (Table 1, entry 7). Moderate catalyst activities
were found in other solvents such as methanol, ethanol, ethanol/
aqueous mixture and toluene (Table 1, entries 1e3 and 5). Aceto-
nitrile was chosen as the optimum solvent for carrying out the
reactions considering the lower reaction temperature of acetoni-
trile (at 80 ꢀC) and since the difference in yield between dioxane
and acetonitrile is quite marginal. The enhanced activity of
coupling reaction. Cs2CO3 was found to be the most effective base
(Table 2, entry 3) followed by the organic base Et3N (Table 2, entry
5). Slightly lower yields were obtained when K2CO3, Na2CO3, KOtBu
were used as base (Table 2, entries 1, 2 and 4).
The proceedings of the reactions were traced by GC. Most of the
4-bromobenzonitrile was converted by the completion of 14 h.
Nevertheless, dependence of product yield on the reaction time
was scrutinized for a period of 20 h (Fig. 4). Yield variance is
apparent to a notable extent up to 16 h of the reaction time, further
advance in reaction time showed no much effect on the percentage
conversion of the product, owing to which the reaction time for the
catalytic conversion was optimized for 16 h.
Further, the cross coupling reaction was extended to the
coupling between phenyl boronic acid and different aryl halides
using the optimized reaction conditions. The results of these re-
actions are summarized in Table 3.
As can be seen from the results, various substituted aromatic
halides underwent Suzuki coupling effectively. Electron with-
drawing groups on the aryl halides accelerated the process of
conversion to biaryls whereas presence of electron donating groups
brought about lesser conversion. Lowest conversion was observed
Table 2
Study of effect of base on Suzuki coupling of 4-bromobenzonitrile and phenyl
boronic acid using catalyst C-3.
Entry
Solvent
Base
Yield (%)a
1
2
3
4
5
ACN
ACN
ACN
ACN
ACN
K2CO3
Na2CO3
Cs2CO3
KOtBu
Et3N
80
76
87
65
83
Reaction conditions: 4-bromobenzonitrile (1.0 mmol), Phenylboronic acid
(1.3 mmol), base (2.0 mmol), catalyst (0.5 mol%), solvent (5.0 mL), 16 h.
a
GC yields.

46
L.M. Kumar, B.R. Bhat / Journal of Organometallic Chemistry 827 (2017) 41e48
Table 3
Catalytic activity study for complexes.
Entry
X
R
Yield (%)a
C-1
C-2
C-3
1
2
3
4
5
6
7
8
9
10
Br
H
CN
50
81
68
77
75
36
53
69
87
64
54
83
70
80
81
40
54
71
90
68
56 (51.6)
89 (86.4)
76 (72.6)
84 (81.3)
87 (85.2)
41 (36.0)
59 (54.7)
76 (73.0)
92 (90.2)
71 (66.8)
OCH3
COCH3
NHCOCH3
F
CH3
OH
I
CN
CHO
Cl
Reaction conditions: Aryl halide (1.0 mmol), Phenylboronic acid (1.3 mmol), Cs2CO3 (2.0 mmol), catalyst (0.5 mol%), solvent (5.0 mL), 16 h.
a
GC yields, average of 3 trials (Isolated yield).
when aryl halide with fluorine atom in the para position was being
employed which perhaps occurs due to the destabilization of the
aromatic ring due to the competing inductive and resonance effects
generally shown by halides attached directly on the aromatic ring.
Presence of iodide is preferred as a better leaving group among the
halides which is reflected in the results. (Entry 2 and 9, Table 3).
Among the three catalysts reported in this paper, highest cata-
lytic activity is shown by complex C-3 which possesses tridentate
PNNNP pincer ligand. When one of the side arm substituent is
oxygen, it results in PNCOP pincer complex C-1 which shows
comparatively lesser activity. The results of this study imply the
effect of the donating capability of the atoms linked to the main
donor atom ‘P’. As the number of ‘N’ atoms increases in the side arm
of the ligand, the donating ability of the ligand increases which
consequently results in the increased catalytic activity of the
complex (Fig. 5).
3.3. Mechanism consideration
Mechanism of the cobalt(II) catalyzed CeC coupling reaction is
quite a complex supposition and requires comprehensive study in
understanding the process. The catalytic metal component Co(II)
getting reduced to either Co(0) or Co(I) is both a possibility, and
speculating on the existent active species formed in the catalytic
process is quite difficult. We tentatively assume that the catalyst
Fig. 3. Effect of solvent on C-3.
Fig. 4. Effect of reaction time on the product yield.
Fig. 5. Graphical representation of % GC conversion of various substituted aryl halides.

L.M. Kumar, B.R. Bhat / Journal of Organometallic Chemistry 827 (2017) 41e48
47
Fig. 6. Possible mechanism for C-C coupling by C-3.
precursor (a) gets reduced to active Co(0) species (b) in the pres-
Acknowledgements
ence of phenylboronic acid and base (Fig. 6). The coupling process
may then start with the oxidative addition of aryl halide to the
Co(0) center to form an R2-Co(II) intermediate (c). Subsequent
nucleophilic substitution of the halide group and transmetallation
between phenylboronic acid and R2-Co(L)-CO3 (d) would result in
biaryl R1-Co(L)-R2 species (e). In the last step, reductive elimination
supposedly takes place yielding the target biaryl (f) and also
regenerating Co(0) species [26].
The author, Mrs. Lolakshi Mahesh Kumar thanks National
Institute of Technology Karnataka, Surathkal for research fellow-
ship. We also thank Dr. Reddy's Institute of Life Sciences, Hyderabad
for NMR and Mass analysis.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2016.11.005.
4. Conclusions
References
In conclusion, the method proposes the use of low cost, abun-
dant and environmentally benign cobalt metal in the synthesis of
complex which can be efficiently used as a catalyst in cross
coupling reaction. Synthesis of the complexes is simple and can be
achieved in a facile manner from the commercially available
starting materials. The present study has led to the consideration of
the role of ligands and their electron donating ability in enhancing
the catalytic activity of a complex. It is observed that as the number
of ‘N’ atoms increases in the side arm of the ligand, the donating
ability of the ligand increases which consequently results in the
increased catalytic activity of the complex.
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