Role of substituents present in bidentate ligand frame of Cu(I) catalysts on Sonogashira cross coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2020.121367

Cu(I) catalysts {[Cu(L^1?4)Cl(PPh₃)] where L^1?4 = condensed product of 2-(1-phenylhydrazinyl)-pyridine with different benzaldehydes} were synthesized and characterized by ¹H NMR, ³¹P NMR, UV–Vis and IR techniques. Complex 2 structure was authenticated by single crystal X-ray method. Different electron donating and withdrawing substituents are present in the ligand frame of Cu(I) catalysts and their role on Sonogashira reaction was investigated. The efficiency order of catalysts for the coupling reaction was found to be 2 > 1>3 > 4, clearly indicated the role of substituents present in the ligand frame was useful to effectively catalyze the Sonogashira reaction. The products were characterized using ¹H NMR and ¹³C NMR.

Full text of DOI:10.1016/j.jorganchem.2020.121367

Journal Pre-proof
Role of substituents present in bidentate ligand frame of Cu(I) catalysts on
Sonogashira cross coupling reactions
Sheela Kumari, Ashish Kumar Dhara, Anand Ratnam, Kiran Mawai, Virendra Kumar
Chaudhary, Aurobinda Mohanty, Kaushik Ghosh
PII:
S0022-328X(20)30269-2
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121367
JOM 121367
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 16 January 2020
Revised Date: 22 April 2020
Accepted Date: 2 June 2020
Please cite this article as: S. Kumari, A.K. Dhara, A. Ratnam, K. Mawai, V.K. Chaudhary, A. Mohanty,
K. Ghosh, Role of substituents present in bidentate ligand frame of Cu(I) catalysts on Sonogashira
cross coupling reactions, Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/
j.jorganchem.2020.121367.
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Role of substituents present in bidentate ligand frame of Cu(I) catalysts on
Sonogashira cross coupling reactions
Sheela Kumaria, Ashish Kumar Dharaa, Anand Ratnama, Kiran Mawaia, Virendra Kumar
Chaudharya, Aurobinda Mohantya and Kaushik Ghosha*
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee -247667,
INDIA
E-mail address: ghoshfcy@iitr.ac.in, ghoshfcy@gmail.com
Keywords Bidentate ligands; Cu(I) complexes; DFT studies; Sonogashira coupling; Single
X- ray crystal.
Abstract
Cu(I) catalysts {[Cu(L1-4)Cl(PPh3)] where L1-4 = condensed product of 2-(1-
phenylhydrazinyl)-pyridine with different benzaldehydes} were synthesized, characterized by
31
1H, P NMR, UV-Vis and IR techniques. Complex 2 structure was authenticated by single
crystal X-ray method. Different electron donating and withdrawing substituents are present in
the ligand frame of Cu(I) catalysts and their role on Sonogashira reaction was investigated.
The efficiency order of catalysts for the coupling reaction was found to be 2>1>3>4, clearly
indicated the role of substituents present in the ligand frame was useful to effectively catalyze
the Sonogashira reaction. The products were characterized using 1H and 13C NMR.
Introduction
Carbon-carbon bond formation using transition metal complexes received considerable
current attention and was developed as a resourceful device for the synthesis of important
molecules utilised in the pharmaceutical and agriculture industries. The reaction between sp2-
carbon of alkyl halides and sp- carbon of terminal alkynes was examined by Sonogashira,
Heck and Cassar back in the year 1975. [1-3] Heck and Cassar utilised palladium transition
metal to couple sp2-carbon of haloarenes and sp-carbon of terminal ethynes whereas
Sonogashira involved both palladium and copper metal based catalyst to couple haloarenes
and terminal ethynes. Sonogashira cross coupling reaction mainly requires base, Pd as

catalyst and CuI as co-catalyst. [4-8] 1-10 mol% of palladium catalyst generally consumed
for Sonogashira reaction. As Pd metal is very costly so its usage at higher gage in industries
is circumscribed.[9] However, among all transition metals copper was found to be best choice
for such reactions as Cu metal is cheap, easily available and biologically relevant. [14]
In 1964, Weingarten and other researchers, published that if copper salts have good solubility
at moderate temperature, output of the reaction will be high. [10-15] Several additives were
utilized in the coupling reactions so as to enrich the output of the reaction. [16-22] Varous
reports in the literature described Sonogashira reaction catalysed by copper based catalyst
supported by different bidentate ligands depicted in Scheme 1.
Scheme 1. Ligands used in the literature.
The C-C coupling reactions are performed using different catalytic systems. In 2001,
Venkataraman and coworkers, reported coupling of haloarenes and terminal alkynes using 1,
10 phenanthroline ligand. The reaction was catalyzed by [Cu(phen)(PPh3)Br] catalyst in
presence of K2CO3 and toluene solvent. [23] Guo et al. utilized ethylene diamine as a ligand
and CuI for the Sonogashira reactions. [24] Fuo and his coworkers developed global friendly
method for Sonogashira reactions using CuBr, 1,10-phenanthroline as a ligand, TBAB as
phase transfer catalyst. [25] Li’s group used DABCO (1,4-Diazabicyclo[2.2.2]octane) as a
ligand in combination with CuI, Cs2CO3 to perform coupling reaction between terminal
phenylethynes and haloarenes.[26] Coupling reaction between terminal ethynes and alkyl

halides was also achieved by Sekar and his group employing N,N’-dibenzyl BINAM and CuI
as a catalyst. [27] Bolm’s group performed Sonogashira coupling reaction in presence of
dioxane solvent and Cs2CO3 base using [Cu(DMEDA)2]Cl2.H2O as a catalyst. [28] Various
researchers have utilised different ligands such as triphenylphosphine [23, 29-30], 1,3-
diphenylpropane-1,3-dione [31], rac-BINOL [32], N, N-dimethylglycine [33] with Cu(I)
metal salts to perform coupling reactions.
To our best knowledge, a single report by Venkataraman and co-workers depicted the
formation of Cu(I) complex which was derived from 1,10 phenanthroline ligand and utilised
10 mol% of catalyst to perform Sonogashira reaction. [23] Bolm and coworkers clearly
mentioned that electron donating group containing complex are more stable compared to
electron withdrawing group. The electron donating group containing complex increase the
reactivity of coupling reactions by stabilizing intermediate species.[34] Inspired from
literature, we synthesized Cu(I) complexes{[Cu(L1-4)PPh3Cl], where L are bidentate ligands}
from the bidentate Schiff bases which contains electron donating and withdrawing groups at
benzaldehyde part shown in Scheme 2. All mentioned complexes were well characterized by
different spectral techniques and utilised for Sonogashira coupling reactions. Most of the
research groups performed the reaction with 10 mol% of catalyst whereas 8 mol% of catalyst
is utilised in the present work. Electron donating and withdrawing substituents present in
ligand frame of Cu(I) complexes effect the rate of reaction. The order of efficiency of
catalysts for the coupling reaction was found to be 2>1>3>4. This is due to stabilisation of
intermediate species more by electron donating substituents present in ligand frame of Cu(I)
catalysts compared to withdrawing groups. This order clearly indicated the role of
substituents present in the ligand frame was useful to effectively catalyze the Sonogashira
1
coupling reaction. The formation of isolated products were characterized using H and 13C
NMR spectral techniques.

N
N
N
N
N
N
CuCl, PPh3
Acetonitrile
H
H
Cl
Cu
R
H
NMe2
Cl
Metal complex
1
PPh3
2
3
4
NO2
R
R
Scheme 2: Cu(I) complexes used in the present work.
Results and Discussion
Syntheses
The bidentate ligands L1, L2, L3, L4 were formed by treating 2-(1-phenylhydrazinyl)-pyridine
[35] with benzaldehyde, 2,2-dimethylaminobenzaldehyde, 4-chloro benzaldehyde, 4-nitro
benzaldehyde. The obtained Schiff bases were treated with CuCl in one step and in another
step equimolar solution of triphenylphosphine was added and it resulted in formation of
yellow green compound. These compounds were recrystallized using dichloromethane.
The structure of complex 2 is displayed in Figure 1. The bond angles and lengths are shown
in Table 1. The –NN donor atoms present in bidentate ligand binds to CuCl and on other side
phosphorous atom of triphenylphosphine get attached to metal centre offered tetrahedral
geometry to complex.
Figure 1. ORTEP diagram of complex 2. Hydrogen atoms are omitted for lucidity.

Table 1. Bond lengths (Å) and angles (º) of complex 2.
Bond distance (Å)
Cu(1)-N(1)
Cu(1)-N(3)
Cu(1)-Cl(1)
N(2)-N(3)
N(3)-C(4)
N(4)-C(6)
2.057(3)
2.199(3)
2.2905(11)
1.406(4)
1.290(5)
1.437(6)
Cu(1)-P(1)
P(1)-C(1)
P(1)-C(2)
P(1)-C(3)
C(4)-C(5)
N(4)-C(7)
2.2026(10)
1.823(4)
1.829(4)
1.830(4)
1.437(5)
1.450(6)
Bond angle (⁰)
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-Cl(1)
Cl(1)-Cu(1)-P(1)
76.59(12)
100.45(9)
116.30(4)
N(3)-Cu(1)-P(1)
N(1)-Cu(1)-P(1)
C(1)-P(1)-Cu(1)
117.14(8)
125.88(9)
115.35(13)
The crystal evidences and structural factors related to complex 2 is prescribed in Table S3.
The crystal structure of complex 2 comprised of one copper metal centre attached to one
pyridine nitrogen (N1), one imine nitrogen (N3), one phosphorus atom P(1) of
triphenylphosphine and one chlorine (Cl1) atom. All donor atoms attached to central metal
offered tetrahedral geometry to the complex.
Electronic properties
1
13
The H and C NMR of ligands (L1-L4) were recorded in CDCl3 solvent. Cu(I) complexes
1
are diamagnetic as these contains d10 configuration. The H NMR spectra of all four
complexes (1-4) were taken in deuterated DMSO solvent and their related NMR plots are
placed in supporting file. Crystal structure of complex 2 clearly showed that the complexes
consist of one phosphine group which can give 31P NMR. So, 31P NMR plot for PPh3
molecule attached to metal centre is located in supporting file. The IR spectra was collected
between 400-4000 cm-1 range for all four ligands and their complexes placed in supporting
information. The UV-Visible spectra of all mentioned ligands and their complexes were done

in acetonitrile solvent. The UV-Visible plots and the molar absorption coefficient values of
ligands as well as complexes is given in Figures S17-S18 and Table S2 of supporting file.
The ESI-MS spectra of all complexes were taken in acetonitrile solvent and the relevant plots
of complexes 1-4 are given in supplementary file (Figure S72-S75).
Theoretical calculations
All the theoretical data correlated to complexes (1-4) are depicted in the Figure S71 and
Table S4 of supplementary file.
Catalytic studies
Sonogashira coupling was done using Cu(I) catalyst. To carry out the coupling reaction,
Scheme 3 was followed. Catalyst and K2CO3 were dissolved in toluene solvent. Substituted
phenylacetylenes and the haloarenes was poured to above solution. Now, the solution was
heated under nitrogen environment for 20 h at 110°C temperature
Scheme 3. Schematic drawing of Sonogashira reaction using catalyst 1-4.
Sonogashira coupling reaction was carried out using CuCl metal salt. When catalyst was
replaced by CuCl metal salt for coupling of iodobenzene and phenylacetylene in presence of
K2CO3 and toluene at 110°C, no reaction was observed.
Catalyst effect was studied for Sonogashira reaction. Optimisation was done with all the
complexes and it was observed that 2 complex is more effective compared to other
complexes and the isolated yields obtained for all four complexes is depicted in Figure 2.

Figure 2. Effect of catalyst (1-4) on coupling of iodobenzene and phenylacetylene.
Solvents and bases effect were studied using complex 2 shown in Figures 3 and 4. Reactions
were carried out in different solvents and bases. Using K2CO3, toluene as a base and solvent,
isolated yields for desired product is high compared to other solvents and bases.
Figure 3. Effect of solvents during coupling of iodobenzene and phenylacetylene using
catalyst 2.
Figure 4. Effect of bases during coupling of iodobenzene and phenylacetylene using catalyst
2.
90
80
70
60
50
40
30
20
10
2
3
4
5
6
7
8
Catalyst 2 loading (mol%)

Figure 5 Effect of catalyst 2 loading on Sonogashira coupling reaction.
Different mol% of catalyst 2 such as 2, 4, 6 and 8 mol% were employed to study the
reactivity of Sonogashira coupling reaction. The obtained % yield with each mol% is shown
in Figure 5. The yield increases with increase in mol% of catalyst 2. With 2, 4, 6 and 8 mol%
of catalyst 2, % yield are 19%, 43%, 59% and 85%. Maximum yield was obtained when 8
mol% of catalyst was used. We also examined the catalytic activity of other copper based
catalyst in this reaction. When Cu(I) complex having 1,10 phenanthroline ligand i,e.
[Cu(phen)(PPh3)Cl] [36] was employed for the coupling of iodobenzene and phenylacetylene
under optimized reaction conditions, 58% yield was obtained. Maximum amount of coupled
product was gained in 20h, when 8 mol% of catalyst was employed during the reaction.
Sonogashira reaction was studied with different substituents of phenylacetylene and
1
13
haloarenes. The attained product was characterized using H and C NMR spectral studies.
The NMR plots and the spectral data of the desired products are deposited in the supporting
file. The isolated yields obtained with different substituents of phenylacetylene and aryl
halides consuming catalyst 2 are shown in Table 2.
During the reaction electron donating groups present in the metal complex will help in
stabilization of Cu(III) intermediate shown in the mechanism. The lone pair of -NMe2 group
can interacts with p orbitals of benzene ring that further donates its electron to p orbital of -
imine group due to which electron donating tendency of ligand increases and form stable
Cu(III) intermediate during progress of the reaction, but such type of interactions are not
possible in case of electron withdrawing groups. That’s why isolated yields obtained for all
the complexes follows the order 2>1>3>4. In complex 2, the electron donating power of –
NMe2 is more than –H, Cl, NO2. That’s why ended up with better isolated yields. The reason
behind it is stabilization of Cu(III) intermediate during the mechanism.

Table 2: Reactions of phenylacetylenes with haloarenes using catalyst 2.
%
S.No Phenylacetylenes
1.
Haloarenes
Products
Yielda
(P1)
(P2)
85
92
2.
3.
(P3)
83
87
4.
5.
(P4)
(P5)
79
86
6.
(P6)
7.
8.
(P7)
(P8)
(P9)
(P10)
83
81
73
86
69
82
70
79
9.
10.
11.
12.
13.
14.
(P11)
(P12)
(P13)
(P14)

(P15)
(P16)
88
83
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
(
P17)
80
75
73
69
78
63
72
60
(P18)
(P19)
(P20)
(P21)
(P22)
(P23)
(P24)
Reaction conditions: Phenylacetylenes (2.5 mmol), Haloarenes (2.0 mmol), catalyst (8
mol%), base (2.0 mmol), Toulene. Temperature (110°C) under inert atmosphere for 20h. (a)
represents the isolated yields.
2.5 Mechanistic Pathway

Figure 6. Proposed mechanism for Sonogashira coupling. [34]
Mechanistic pathway for Sonogashira coupling reaction is given in Figure 6. In the first step
of mechanism, base deprotonated the terminal alkyne. The Cu(I) metal complex bound to
deprotonated phenylacetylene and gave rise to (b) step. In the next step, alkyl halide attached
to (b) intermediate and gave rise to (c) step. The oxidation state of Cu(I) altered to Cu(III). At
last, reductive elimination occurred and formed desired products.
Conclusions
To study a new methodology for Sonogashira coupling reaction, four new mononuclear
copper complexes were synthesized and characterized. These complexes have different
substituents on the ligand frame. Molecular structure of representative complex 2 was
determined by single crystal X-ray diffraction. These complexes were utilised as catalysts for
Sonogashira coupling reactions. The order of efficiency of catalysts for the coupling reaction
was found to be 2>1>3>4. This order clearly indicated -NMe2 group present in the ligand
frame was useful to effectively catalyze the Sonogashira coupling reaction.
Experimental Section
Materials and Measurements
The quality of all chemicals were of high standard. The solvents utilized during reactions
were purified by distillation. Reagents such as substituted benzaldehydes, CuCl,
triphenylphosphine were consumed as obtained.
Methods and instrumentation
IR spectra were analysed using KBr pellets with Thermo Nikolet Nexus FT-IR spectrometer.
The UV-Vis spectra were obtained from Thermo Scientific UV-Visible spectrophotometer.
1H, 13C and 31P NMR spectral data were collected by Jeol, 400 MHz spectrometer using
deuterated solvents. All synthesis was done at room temperature and the catalytic reactions

were performed under inert atmosphere at 110°C. ESI-MS spectra of all 1-4 compounds were
taken using Brüker microTOF-Q II mass spectrometer.
Syntheses of ligands
The bidentate ligands L1, L2, L3, L4 were produced by the reaction of 2-(1-
phenylhydrazinyl)-pyridine [35, 37] with benzaldehyde, 2, 2- dimethylaminobenzaldehyde,
4-chloro benzaldehyde, 4-nitro benzaldehyde. The synthesized ligands were given in the
1
13
scheme 2. The H, C NMR, IR, UV-Visible plots and their relevant data are placed in the
supporting file for all ligands (L1-L4).
Syntheses of metal complexes: [Cu(L1-4)Cl(PPh3)] complexes were prepared using the
resulting method given below.
[Cu(L1)(PPh3)Cl] (1): L1 and CuCl was dissolved in acetonitrile and ligand L1 was poured to
metal solution under inert environment. After half an hour of stirring, same equivalent of
triphenylphosphine was added to it. The solution was stirred for one day. Yellow colored
compound was precipitated out. The compound was recrystallized using dichloromethane.
Yield (76%). Theoretical. calcd. for C36H30ClCuN3P (634.61): C, 68.13; H, 4.76; Cl, 5.59;
Cu, 10.01; N, 6.62; P, 4.48. IR (KBr pellet, ʋmax ꢀ cm-1): 1590 (ʋC=N), 507, 694, 755(ʋPPh3).
UV-Vis (Acetonitrile; λmax /nm (ε, M-1cm-1)): 228(65000), 277(25500), 327(45000). 31P
NMR (400 MHz, (CD3)2SO): δ 30.17 ppm.1H NMR (400 MHz, (CD3)2SO): δ 7.92 (bs, 3H),
7.83-7.80(m, 6H), 7.59-7.39 (m, 21H) ppm. {HRMS: [1-Cl-] (C36H30CuN3P m/z =598.1223)}
[Cu(L2)(PPh3)Cl] (2): Same procedure was followed as given for complex 1 but L2 was used
instead of L1. Yield (74%). Theoretical. calcd. for C38H35ClCuN4P (677.68): C, 67.35; H,
5.21; Cl, 5.23; Cu, 9.38; N, 8.27; P, 4.57. IR (KBr pellet, ʋmax ꢀcm-1): 1587(ʋC=N), 510, 696,
758(ʋPPh3).UV-Vis (Acetonitrile; λmax/nm (ε, M-1cm-1)): 254(29330), 323(32830),
31
354(48350). P NMR (400 MHz, (CD3)2SO): δ 30.82 ppm.1H NMR (400 MHz, (CD3)2SO):

δ 8.42 (s, 1H), 8.08-7.83(m, 13H), 7.63-7.56 (m, 9H), 7.33 (s, 3H), 7.16 (s, 1H), 6.61(s, 2H),
3.06(s, 6H) ppm. {HRMS: [2-Cl-] (C38H35CuN4P m/z =641.1836)}
[Cu(L3)(PPh3)Cl] (3): Above procedure was followed but L3 was used instead of L1. Yield
(71%). Theoretical. calcd. for C36H29Cl2CuN3P (669.061): C, 66.86; H, 4.85; Cl, 5.33; Cu,
9.56; N, 6.32; O, 2.41; P, 4.67. IR (KBr pellet, ʋmax ꢀ cm-1): 1588 (ʋC=N), 508, 694,
752(ʋPPh3). UV-Vis (Acetonitrile; λmax/nm (ε, M-1cm-1)): 231(43000), 274(17500),
31
333(37600). P NMR (400 MHz, (CD3)2SO): δ 30.97 ppm.1H NMR (400 MHz, (CD3)2SO):
δ 7.99 (s, 2H), 7.60-7.41(m, 8H), 7.28-7.19 (m, 16H), 7.05 (d, 2H), 6.25 (bs, 1H) ppm.
{HRMS: [3-Cl-] (C36H29ClCuN3P m/z =632.1044)}
[Cu(L4)(PPh3)Cl] (4): Same procedure was followed as given for complex 1 but L4 was used
instead of L1. Yield (72%). Theoretical. calcd. for C37H32ClCuN4O2P (679.61): C, 64.63; H,
4.37; Cl, 10.60; Cu, 9.50; N, 6.28; P, 4.63. IR (KBr pellet, ʋmax ꢀ cm-1): 1587 (ʋC=N), 505,
31
694, 742 (ʋPPh3). UV-Vis (Acetonitrile; λmax/nm (ε, M-1cm-1)): 262(32830), 389(36500). P
NMR (400 MHz, (CD3)2SO): δ 30.35 ppm.1H NMR (400 MHz, (CD3)2SO): δ 8.08 (s, 5H),
7.79(s, 2H), 7.67-7.53 (m, 9H), 7.38-7.25 (m, 12H), 7.15-7.11 (m, 1H) ppm. {HRMS: [4-Cl-]
(C37H32ClCuN4O2P m/z =643.1164)}
Single X-ray Crystallography
Crystal of complex 2 was attained by gentle vaporization of dichloromethane solvent. The
data was collected on a Bruker Kappa Apex-II CCD diffractometer by using graphite
monochromated Mo-Kα radiation (λ= 0.71073 Å) at 293 K. The structure was solved using
WinGX software. ORTEP structure was attained using MERCURY software.
Catalytic studies

In a round bottom flask, 8% mole percent of catalyst, K2CO3 (2.0 mmol), phenylacetylene
(2.5 mmol), aryl halide (2.0 mmol), and 20ml toluene was taken. The whole solution was
heated at 110-120°C for 20 h under N2 atmosphere. The solution was cooled to room
temperature and filtered to remove insoluble impurities. Filterate was evaporated and the
1
solid was then passed through column. The desired compound was characterized by H and
13C NMR.
DFT studies
The DFT studies were performed using Gaussian 03 software. HOMO and LUMO plots were
drawn using Chemcraft software. The time dependent DFT studies were performed to analyse
the electronic transitions.
Acknowledgements
KG is thankful to CSIR, New Delhi, India for financial assistance No. 01(2942)/18/EMR-II
dated 01-05-2018). SK and VKC are thankful to CSIR, AKD and KM are thankful to UGC,
AR and AM are thankful to MHRD, India for financial assistance.
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Highlights:
•
•
Sonogashira coupling reactions were performed using Cu(I) complexes.
All four Cu(I) complexes were supported by bidentate ligands. The ligands contain
electron donating and withdrawing group at the benzaldehyde part.
•
•
The structure of complex 2 was determined by single crystal X-ray technique.
The electron donating group in the complex helps in stabilization of Cu(III)
intermediate during coupling reaction. The Cu(I) complex having electron donating
groups will provide better efficiency of the reaction compared to electron
withdrawing groups.
•
Catalytic efficiency of complex 2 was found to be the best among the complexes.