Nickel(II) complexes with N,O-donor thiopseudourea ligands: Syntheses, structures, and catalytic applications in Kumada–Corriu cross-coupling reactions

Article abstract of DOI:10.1002/aoc.6261

Synthesis, characterization, and physical properties of benzyl-N′-(4-R-benzoyl)-N-(2,6-diisopropylphenyl)carbamimidothioates, HL¹ (R = H) and HL² (R = Cl), and their nickel(II) complexes having the general molecular formula [Ni(L^1/2)₂] (1 and 2) have been reported. Elemental analysis, magnetic susceptibility, solution electrical conductivity, and various spectroscopic (IR, UV–Vis, and ¹H NMR) measurements were used to characterize HL¹, HL², and the two complexes (1 and 2). The molecular structures of all four compounds were determined by single-crystal X-ray crystallographic studies. The structures of HL¹ and HL² showed the imino-ketone form of both compounds. In each of 1 and 2, the six-membered chelate ring forming iminolate-O and azomethine-N donor two (L^1/2)^? ligands form a square-planar trans-N₂O₂ coordination environment around the metal center. The spectroscopic characteristics of HL¹, HL², 1, and 2 are consistent with their molecular structures. Both complexes were successfully employed as efficient catalysts in Kumada–Corriu C-C cross-coupling reactions of aryl bromides with phenylmagnesium bromide. The reactions provided biaryl products in good to excellent yields with a good substrate scope.

Full text of DOI:10.1002/aoc.6261

Received: 22 December 2020
DOI: 10.1002/aoc.6261
Revised: 5 March 2021
Accepted: 26 March 2021
F U L L P A P E R
Nickel(II) complexes with N,O-donor thiopseudourea
ligands: Syntheses, structures, and catalytic applications in
Kumada–Corriu cross-coupling reactions
Kinche Shakunthala
Srinivas Keesara
| Ankit Kumar Srivastava
| Samudranil Pal
| G. Narendra Babu
|
School of Chemistry, University of
Synthesis, characterization, and physical properties of benzyl-N0-(4-R-ben-
zoyl)-N-(2,6-diisopropylphenyl)carbamimidothioates, HL1 (R = H) and HL2
(R = Cl), and their nickel(II) complexes having the general molecular formula
[Ni(L1/2)2] (1 and 2) have been reported. Elemental analysis, magnetic suscep-
tibility, solution electrical conductivity, and various spectroscopic (IR, UV–Vis,
and 1H NMR) measurements were used to characterize HL1, HL2, and the two
complexes (1 and 2). The molecular structures of all four compounds were
determined by single-crystal X-ray crystallographic studies. The structures of
HL1 and HL2 showed the imino-ketone form of both compounds. In each of
Hyderabad, Hyderabad, 500046, India
Correspondence
Samudranil Pal, School of Chemistry,
University of Hyderabad, Hyderabad
500046, India.
Email: spal@uohyd.ac.in
Funding information
University Grants Commission, Grant/
Award Numbers: 187/(CSIR-UGC NET
June 2017), F.540/2/CAS-111/2016(SAP-I),
F.4-2/2006(BSR)/13-1125/2013(BSR),
F.25-1/2014-15(BSR)/5-129/2007(BSR);
Department of Science and Technology
(DST), New Delhi, Grant/Award Number:
SR/FST/CST-030/2014(C); Ministry of
Education, Government of India, Grant/
Award Number: F11/9/2019-U3(A);
Council of Scientific and Industrial
Research, India, Grant/Award Number:
09/414(1025)/2012-EMR-I
1
and 2, the six-membered chelate ring forming iminolate-O and
azomethine-N donor two (L1/2)− ligands form a square-planar trans-N2O2 coor-
dination environment around the metal center. The spectroscopic characteris-
tics of HL1, HL2, 1, and 2 are consistent with their molecular structures. Both
complexes were successfully employed as efficient catalysts in Kumada–Corriu
C C cross-coupling reactions of aryl bromides with phenylmagnesium bro-
mide. The reactions provided biaryl products in good to excellent yields with a
good substrate scope.
K E Y W O R D S
crystal structure, Kumada–Corriu reaction, nickel(II), NO-donor, thiopseudourea
1 | INTRODUCTION
particular attention.[2] Generally, complexes of palla-
dium, the 4d-block transition metal in Group 10, are
Transition metal catalyzed cross-coupling reaction lead-
ing to the formation of C C bond is a powerful and
widely used tool for the synthesis of natural products and
a variety of complex and structurally intricate organic
compounds with potential for use as agrochemicals,
pharmaceuticals, and functional materials.[1,2] The biaryl
frameworks are widespread and play important roles in
deciding the properties and activities of these types of
compounds. Hence, in recent years, the syntheses of
biaryls via C C cross-coupling reactions have attracted
found to be the most efficient catalysts for the various
cross-coupling reactions. However, due to the high cost
of palladium, there is a continuous quest for less expen-
sive and environment friendly alternative to these palla-
dium catalysts. As a result, complexes of nickel, the
lightest and inexpensive member of Group 10, have
attracted immense attention as catalysts for cross-
coupling reaction.[3–6] The first nickel catalyzed C C
cross-coupling reaction using alkyl or aryl halides, and
Grignard reagents were reported independently by
Appl Organomet Chem. 2021;e6261.
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https://doi.org/10.1002/aoc.6261

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SHAKUNTHALA ET AL.
Tamao et al.[3] and Corriu and Masse[4] in 1972. Since
then, a large number of nickel complexes with a variety
of ligands such as Schiff bases, monophosphines/
diphosphines/triphosphines, diamines, β-diketonates, N-
heterocyclic carbenes, and various pincer ligands have
been employed for this cross-coupling reaction.[5,6]
Thiourea, isothiourea, and thiopseudourea derivatives
are of considerable interest for their complexation
behaviors and applications as catalysts as well as for their
biological and biochemical properties leading to various
therapeutic and diagnostic applications.[7–9] In this work,
we have examined the coordination behavior of two
mode. Interestingly, the present thiopseudourea ligand
system (Ln)− behaves in the opposite way and acts as
O,N-donor in the two complexes having the general for-
mula [Ni(Ln)2] (1 and 2 where n = 1 and 2, respectively)
(Scheme 1). Herein, we report the syntheses, characteri-
zation, and X-ray structures of HL1/2 and their complexes
[Ni(L1/2)2] (1 and 2). Application of both complexes as
effective catalysts in Kumada–Corriu C C cross-coupling
reactions of aryl bromides with phenylmagnesium bro-
mide has been demonstrated.
thiopseudoureas
benzyl-N0-(4-R-benzoyl)-N-(2,-
2 | EXPERIMENTAL
2.1 | Materials
6-diisopropylphenyl)carbamimidothioates (HLn, where
n = 1 and 2 for R = H and Cl, respectively) toward
nickel(II) (Scheme 1) and the catalytic activities of the
resulting complexes in Kumada–Corriu cross-coupling
reaction. The deprotonated thiopseudourea system (Ln)−
can act as a bidentate six-membered chelate ring forming
ligand in two ways: via iminolate-O and thioether-S coor-
dination or via iminolate-O and azomethine-N coordina-
tion. Considering the moderately soft character of
nickel(II) and the sterically crowded environment of the
azomethine-N, it was anticipated that (Ln)− may prefer
the O,S-coordination mode rather than the O,N-
coordination mode. In a somewhat similar situation, it
has been found that N,N-dialkyl/diaryl-N0-(aroyl)
thioureates (L)− provide square-planar nickel(II) com-
plexes [NiL2], where (L)− coordinates the metal center
through the O-atom and the less crowded S-atom instead
of the N-atom containing two alkyl groups.[7] It may be
noted that like the present ligand system, each of these
thioureates (L)− also forms six-membered chelate ring at
the metal center in either O,S- or O,N-coordination
Aryl halides and phenylmagnesium bromide for the
cross-coupling reactions were procured from Sigma-
Aldrich and were used as received. The N-(4-R-benzoyl)-
N0-(2,6-diisopropylphenyl)thioureas (R = H and Cl) were
prepared in about 90% yields from 4-R-benzoyl chlorides
(R = H and Cl), NH4SCN, and 2,6-diisopropylaniline by
following a reported procedure.[9] All other chemicals
used in this work were of reagent grade available com-
mercially and were used without further purification. All
solvents used were purified by standard methods.[10]
2.2 | Physical measurements
Elemental (CHN) analyses were performed on a Thermo
Finnigan Flash EA1112 series elemental analyzer. A
Sherwood scientific balance was used for the magnetic
susceptibility measurements. A Digisun DI-909 conduc-
tivity meter was used to measure the electrical conductiv-
ities in solution. The infrared spectra were recorded with
a Bruker TENSOR II FT-IR spectrophotometer. A
Shimadzu UV-3600 UV–Vis–NIR spectrophotometer was
1
used to collect the electronic spectra. The H NMR spec-
tra were recorded using a Bruker NMR spectrometer.
2.3 | Synthetic procedures
2.3.1 | Benzyl-N0-(4-R-benzoyl)-N-
(2,6-diisopropylphenyl)
carbamimidothioates (HLn)
Benzyl bromide (0.36 ml, 518 mg, 3 mmol) was added
dropwise to
a
solution of N-(4-R-benzoyl)-N0-(2,-
SCHEME 1 Syntheses of HL1/2 and [Ni(L1/2)2] (1 and 2):
(i) PhCH2Br and NaH in tetrahydrofuran at 0ꢀC for 3 h.
(ii) Ni(OAc)2ꢁ4H2O in MeOH–CHCl3 (1:1) at 50ꢀC for 24 h
6-diisopropylphenyl)thiourea (R = H or Cl) (3 mmol)
and NaH (60% in mineral oil, 200 mg, 5 mmol) in tetra-
hydrofuran (50 ml) and stirred at 0ꢀC for 3 h. The

SHAKUNTHALA ET AL.
3 of 11
reaction mixture was then neutralized with aqueous
NH4Cl, and the organic layer was extracted with ethyl
acetate (2 × 50 ml), dried over MgSO4, filtered, and
finally, the filtrate was evaporated to dryness in vacuum
to obtain the crude product. It was then purified by col-
umn chromatography on silica gel using ethyl acetate–n-
hexane mixture (1:9) as the eluent. The solution obtained
was evaporated to dryness and the product was isolated
as white solid.
H22,25), 1.42 (9) (d, 6H, Me-protons of dipp), 1.33 (9) (d,
6H, Me-protons of dipp).
[Ni(L2)2] (2): Yield 690 mg (70%). Anal. Calcd for
C54H56N4O2Cl2S2Ni (986.76) (%): C, 65.73. H, 5.72. N,
5.68. Found (%): C, 65.46. H, 5.85. N, 5.58. Selected IR
band (cm−1): 1579 (C N). UV–Vis (CHCl3; λmax (nm) (ε
[M−1 cm−1])): 600sh (56), 502 (152), 400sh (2.3 × 103),
357sh (1.2 × 104), 330sh (1.8 × 104), 304 (2.3 × 104),
1
252 (1.7 × 104). H NMR (CDCl3; 500 MHz, δ [J], ppm
HL1: Yield 1.1 g (85%). Anal. Calcd for C27H30N2OS
(430.59) (%): C, 75.31. H, 7.02. N, 6.51. Found (%): C,
75.26. H, 7.12. N, 6.63. Selected IR bands (cm−1): 3165
(NH), 1653 (C O), 1598 (C N). 1H NMR (CDCl3.
500 MHz, δ [J], ppm [Hz]): 12.45 (s, 1H, NH), 8.38 (8) (d,
2H, H1,5), 7.55 (8) (t, 1H, H3), 7.48 (8) (dd ! t, 2H, H2,4),
7.39–7.35 and 7.29–7.23 (both m, 3H, 3H, H11–15,19), 7.21
(8) (d, 2H, H18,20), 4.55 (s, 2H, H9a,b), 3.14 (7) (sep, 2H,
[Hz]): 7.42 (8) (t, 1H, H19), 7.24–7.16 (m, 7H, H1,5,11–15),
6.99 (9) (d, 2H, H2,4), 6.61 (8) (d, 2H, H18,20), 4.30 (s, 2H,
H
9a,b), 4.11 (7) (sep, 2H, H22,25), 1.40 (7) (d, 6H, Me-
protons of dipp), 1.33 (7) (d, 6H, Me-protons of dipp).
2.4 | X-ray crystallography
H
22,25), 1.25 (7) (d, 6H, Me-protons of dipp), 1.18 (7) (d,
X-ray quality single crystals of HL1 and HL2 were
obtained by slow evaporation of their solutions in ethyl
acetate–n-hexane (1:10) mixture, whereas those for [Ni
(L1)2] (1) and [Ni(L2)2] (2) were grown by slow evapora-
tion of their solutions in chloroform–methanol (1:1) mix-
ture. Unit cell determination and intensity data collection
for each crystal were carried out at room temperature
(298 K) on a Bruker D8 Quest diffractometer fitted with a
Photon 100 CMOS area detector and an Incoatec Micro-
focus Source for graphite monochromated Mo Kα radia-
tion (λ = 0.71073 Å). Data acquisition, integration, and
reduction were performed using the APEX3 software
package.[11] The SADABS program[12] was employed for
an empirical absorption correction. In the case of HL1,
some residual absorption effect was treated with an addi-
tional correction using the XABS2 program.[13] All four
structures were solved by direct method and refined on
F2 using full-matrix least-squares procedures. The non-
hydrogen atoms were refined anisotropically. The hydro-
gen atoms were included at idealized positions and
refined as riding atoms with relative isotropic thermal
parameters of their parent atoms. The SHELX-97 pro-
grams[14] available in the WinGX software suite[15] were
used for structure solution and refinement. Mercury
package[16] was used to prepare the structural illustra-
tions. Selected crystallographic data for all four structures
are listed in Table 1.
6H, Me-protons of dipp).
HL2: Yield 1.2
g
(86%). Anal. Calcd for
C27H29N2ClOS (465.03) (%): C, 69.73. H, 6.29. N, 6.02.
Found (%): C, 69.85. H, 6.35. N, 6.12. Selected IR bands
(cm−1): 3199 (NH), 1686 (C O), 1598 (C N). H NMR
1
(CDCl3. 500 MHz, δ [J], ppm [Hz]): 12.40 (s, 1H, NH),
8.29 (9) (d, 2H, H1,5), 7.44 (9) (d, 2H, H2,4), 7.40–7.33
and 7.30–7.24 (both m, 3H, 3H, H11–15,19), 7.22 (8) (d,
2H, H18,20), 4.52 (s, 2H, H9a,b), 3.13 (7) (sep, 2H, H22,25),
1.26 (7) (d, 6H, Me-protons of dipp), 1.18 (7) (d, 6H,
Me-protons of dipp).
2.3.2 | [Ni(L1/L2)2] (1 and 2)
A methanol solution (30 ml) of Ni(OAc)2ꢁ4H2O (249 mg,
1 mmol) was added to a chloroform solution (30 ml) of
HLn (2 mmol), and the mixture was stirred at 50ꢀC for
24 h. The reaction mixture was then evaporated under
reduced pressure, and the solid obtained was washed
with water and then dried in a vacuum desiccator. The
dry solid was dissolved in chloroform (15 ml). To this
chloroform solution methanol (15 ml) was added with
stirring. The light brown solid separated was collected by
filtration and dried in air.
[Ni(L1)2] (1): Yield 630 mg (69%). Anal. Calcd for
C54H58N4O2S2Ni (917.87) (%): C, 70.66. H, 6.37. N, 6.10.
Found (%): C, 70.65. H, 6.52. N, 6.15. Selected IR band
(cm−1): 1589 (C N). UV–Vis (CHCl3; λmax (nm) (ε
[M−1 cm−1])): 600sh (47), 502 (131), 395sh (2.7 × 103),
355sh (1.3 × 104), 330sh (1.9 × 104), 303 (2.8 × 104),
2.5 | General procedure for the
Kumada–Corriu cross-coupling reaction
1
253 (2.1 × 104). H NMR (CDCl3; 500 MHz, δ [J], ppm
A mixture of aryl bromide (1.0 mmol), phenylmagnesium
bromide (1.2 mmol), and the nickel complex catalyst
(1 or 2) (0.05 or 0.1 mol%) in tetrahydrofuran (2 ml) was
stirred at room temperature (25ꢀC) under nitrogen
[Hz]): 7.42 (9) (t, 1H, H19), 7.28 (8) (t, 1H, H3), 7.23–7.16
(m, 7H, H1,2,4,5,11,13,15), 7.04 (9) (dd ! t, 2H, H12,14), 6.74
(9) (d, 2H, H18,20), 4.35 (s, 2H, H9a,b), 4.15 (9) (sep, 2H,

4 of 11
SHAKUNTHALA ET AL.
TABLE 1 Selected crystallographic data for HL1, HL2, [Ni(L1)2] (1), and [Ni(L2)2] (2)
Compound
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
HL1
HL2
1
2
C27H30N2OS
430.59
C27H29N2OClS
465.03
C54H58N4O2S2Ni
917.87
C54H56N4O2Cl2S2Ni
986.76
Triclinic
Triclinic
Triclinic
Triclinic
ꢀ
P1
ꢀ
P1
ꢀ
ꢀ
P1
P1
9.591(2)
10.479(2)
13.109(3)
72.223(7)
86.174(9)
85.211(7)
1249.1(5)
2
10.8061(4)
11.2035(5)
12.1252(5)
73.437(2)
71.818(1)
71.212(2)
1292.25(9)
2
9.2963(10)
10.5788(11)
13.5217(14)
68.971(3)
80.553(4)
80.439(4)
1216.0(2)
1
8.4743(6)
12.5662(9)
12.8658(9)
97.861(2)
106.032(2)
97.242(2)
1284.96(16)
1
b (Å)
c (Å)
α (ꢀ)
β (ꢀ)
γ (ꢀ)
V (Å3)
Z
ρ (g cm−3
μ (mm−1
)
1.145
1.195
1.253
1.275
)
0.149
0.249
0.529
0.606
Reflections collected
Reflections unique
Reflections I ≥ 2σ(I)
Parameters
17,449
31,846
34,745
45,530
4228
4526
4259
4463
2289
3848
3451
4215
280
293
290
299
R1, wR2 (I ≥ 2σ(I))
R1, wR2 (all data)
GOF on F2
0.0909, 0.1645
0.1962, 0.2038
1.048
0.0470, 0.1182
0.0557, 0.1243
1.052
0.0549, 0.1465
0.0702, 0.1599
1.078
0.0342, 0.0911
0.0364, 0.0929
1.049
Δρmax, Δρmin (e Å−3
)
0.403, −0.308
0.432, −0.440
0.915, −0.860
0.401, −0.397
atmosphere for the required time. After that, the solvent
was removed under reduced pressure, and the residue
was purified by column chromatography on silica gel
using n-hexane as the eluent to isolate the cross-coupled
product.
diamagnetic character. The complexes are insoluble in
most of the common organic solvents such as methanol,
ethanol, acetonitrile, dimethylsulfoxide, and N,N-
dimethylformamide. However, they readily dissolve in
dichloromethane and chloroform and provide brown
solutions. In solution, they are electrically non-
conducting. The use of bivalent nickel starting material
for synthesis and the diamagnetic and nonelectrolytic
character indicate that both 1 and 2 are square-planar
nickel(II) complexes.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and some properties
Reactions of equimolar amounts of benzyl bromide and
N-(4-R-benzoyl)-N0-(2,6-diisopropylphenyl)thiourea
3.2 | X-ray structures
(R = H and Cl) in presence of excess (1.7 times) NaH in
tetrahydrofuran provided HL1 and HL2 in high yields
(ꢂ85%) (Scheme 1). The complexes 1 and 2 were synthe-
sized in good yields (ꢂ70%) by reacting one mole equiva-
lent of Ni(OAc)2ꢁ4H2O with two mole equivalents of HL1
and HL2, respectively, in MeOH–CHCl3 (1:1) mixture.
The elemental analysis data of all four compounds are in
good agreement with the corresponding molecular for-
mulas. Magnetic susceptibility measurements with pow-
dered samples of both complexes revealed their
1
2
ꢀ
All of HL , HL , 1, and 2 crystallize in the triclinic P1
space group. The asymmetric units of HL1 and HL2 con-
tain one complete molecule of the corresponding com-
pounds. On the other hand, each of the two complex
molecules is centrosymmetric, and the metal center
resides at the inversion center. Thus, the asymmetric
units of both 1 and 2 have half of the corresponding com-
plex molecules. The molecular structures of HL1 and HL2
are illustrated in Figure 1, whereas those of 1 and 2 are

SHAKUNTHALA ET AL.
5 of 11
FIGURE 2 Perspective views of [Ni(L1)2] (1) (top) and [Ni
(L2)2] (2) (bottom). The thermal displacement ellipsoids of the
nonhydrogen atoms are drawn at the 40% probability level. Only
selected atoms are labeled for clarity
FIGURE 1 Molecular structures of HL1 (top) and HL2
(bottom). All nonhydrogen atoms are represented by their thermal
ellipsoids at the 40% probability level. Only selected atoms are
labeled for clarity
interaction. The remaining intramolecular bond lengths
and angles are unexceptional in both structures.
The structures of 1 and 2 confirm the deprotonation
of the migratory proton and the monoanionic state of the
ligands (L1)− and (L2)− (Figure 2). In the complexes, the
C(7) O(1) bond length is ꢂ0.04 Å longer and C(7) N
(1) bond length is ꢂ0.06 Å shorter compared with the
corresponding bond lengths in HL1 and HL2 (Table 2).
Hence, the imino-ketone fragment of the ligand is in the
iminolate state in both 1 and 2.[18,19] In contrast to HL1
and HL2, the C(8) N(1) is longer and C(8) N(2) is
shorter in 1 and 2 (Table 2) indicating the single and dou-
ble bond characters, respectively. In each centrosymmet-
ric complex, the ligand coordinates the metal center
through the iminolate-O and the azomethine-N atoms
and forms a six-membered chelate ring with bite angle
close to 90ꢀ (Table 3). Because of the inversion center at
the coordinated metal atom, two ligands form a near per-
fect square-planar trans-NiN2O2 coordination geometry
(Figure 2). The Ni N and Ni O bonds lengths are very
similar in 1 and 2, and they are within the ranges
shown in Figure 2. Selected bond parameters are listed in
Tables 2 and 3.
Both HL1 and HL2 can have two forms amide and
imino-ketone (Scheme 1) due to amine-imine tautomer-
ism.[17] The X-ray structures revealed that both molecules
exist in the imino-ketone form (Figure 1). The shorter C
(8) N(1) bond length compared with the C(8) N(2) bond
length indicates the double bond character of the former
(Table 2). The C(7) O(1) and C(7) N(1) bond lengths
are consistent with double and single bond characters,
respectively. There is an intramolecular N HꢁꢁꢁO hydro-
gen bond involving the amine and the keto-O leading to
the formation of a planar six-membered ring (rms devia-
tions: 0.02 and 0.08 Å for HL1 and HL2, respectively) in
each structure. The NꢁꢁꢁO distance and the N HꢁꢁꢁO
angle are 2.638(3) Å and 128ꢀ for HL1 and 2.677(2) Å and
126ꢀ for HL2. It appears that the imino-ketone form is
favored over the amide form due to this N HꢁꢁꢁO

6 of 11
SHAKUNTHALA ET AL.
TABLE 2 Selected bond lengths (Å) and angles (ꢀ) for HL1, HL2, 1, and 2
Compound
C(7) O(1)
C(7) N(1)
C(8) N(1)
C(8) N(2)
C(8) S(1)
HL1
HL2
[Ni(L1)2] (1)
1.278(3)
[Ni(L2)2] (2)
1.272(2)
1.239(4)
1.373(4)
1.314(4)
1.326(4)
1.766(3)
1.229(2)
1.364(2)
1.312(2)
1.327(2)
1.768(2)
1.308(3)
1.309(2)
1.341(3)
1.343(2)
1.320(4)
1.314(2)
1.774(3)
1.771(2)
thioureates,[7] which are somewhat analogous to the pre-
sent ligands, the opposite O,S-coordination was preferred
over the O,N-coordination. In these complexes, reso-
nance delocalization of the negative charge of the ligand
is possible only in the O,S-coordination mode but not in
the O,N-coordination mode. Thus, it appears that the
coordination mode of the reported thioureates as well as
the present deprotonated thiopseudoureas is dictated by
the delocalization of the ligand negative charge and not
by the steric crowding around the coordinating atom.
TABLE 3 Metal centered bond lengths (Å) and angles (ꢀ) for 1
and 2
Complex
1
2
Ni(1) O(1)
1.8215(19)
1.911(2)
1.8215(19)
1.911(2)
91.58(9)
180.000(1)
88.42(9)
88.42(9)
180.0
1.8342(12)
1.9000(13)
1.8342(12)
1.9000(13)
91.64(5)
180.00(7)
88.36(5)
88.36(5)
180.00(9)
91.64(5)
Ni(1) N(2)
Ni(1) O(10)a
Ni(1) N(20)a
O(1) Ni(1) N(2)
O(1) Ni(1) O(10)a
O(1) Ni(1) N(20)a
N(2) Ni(1) O(10)a
N(2) Ni(1) N(20)a
O(10)a Ni(1) N(20)a
3.3 | Spectroscopic properties
Infrared spectra of HL1, HL2, 1, and 2 were recorded in
the range 4000–500 cm−1 using the ATR mode
(Figures S1 and S2). The spectra display several bands of
various intensities. No attempt was made to assign all the
bands except for the following few. Both HL1 and HL2
show a somewhat broad band at ꢂ3180 cm−1 due to the
N H stretching. Two medium intensity bands at ꢂ1670
and 1598 cm−1 are assigned to the C O and C N
stretches, respectively.[18,19] In contrast, neither of 1 and
2 displays the N H and C O stretching bands. Absence
of these bands indicates the deprotonation of HL1 and
HL2 and formation of the iminolate states (L1)− and (L2)−
in the complexes. A medium intensity band observed at
ꢂ1584 cm−1 for both complexes is attributed to the metal
coordinated C N stretching.[18,19]
The electronic absorption spectra of 1 and 2 were
recorded using their chloroform solutions. The spectral
profiles of the two complexes are very similar (Figure 3).
In each spectrum, two low intensity absorptions, a shoul-
der at 600 nm and a peak at 502 nm, are followed by
three strong shoulders at ꢂ400, ꢂ355, and 330 nm and
two very strong peaks at 303 and ꢂ250 nm. Square-
planar nickel(II) complexes are reported to exhibit weak
spin-allowed ligand field transitions in the range 645–
460 nm.[19a,b,20] Thus, the first two weak absorptions are
assigned to ligand field transitions. Considering that each
of HL1 and HL2 in chloroform displays an absorption
91.58(9)
aSymmetry transformations used to generate equivalent atoms: −x, −y,
−z + 2 for 1 and −x, −y, −z for 2.
observed for nickel(II) square-planar complexes with
ligands having comparable coordinating atoms.[19,20]
A
possible explanation for the observed O,N-
coordination mode instead of the O,S-coordination mode
of (L1/2 −
in 1 and 2 despite relatively more steric
)
crowding around N-atom compared with that around S-
atom is as follows. The delocalization of the negative
charge by resonance is possible only in the O,N-
coordination mode, but not in the O,S-coordination mode
(Scheme 1). Further the C O and C N bond lengths in
the six-membered chelate rings of the two complexes are
in the range 1.272(2)–1.343(2) Å. However, the average C
(8) S(1) bond is ꢂ0.46 Å longer than these bonds in
HL1/2 as well as in the corresponding complexes 1 and
2 (Table 2). Hence, it is very likely that the effective delo-
calization of the negative charge of the ligand with the
formation of a relatively more uniform six-membered
chelate ring leads to the O,N-coordination mode over the
O,S-coordination mode of (L1/2 −
in the present com-
)
plexes. In the previously reported square-planar
nickel(II) complexes with N,N-dialkyl/diaryl-N0-(aroyl)

SHAKUNTHALA ET AL.
7 of 11
FIGURE 3 Electronic spectra of [Ni
(L1)2] (1) (—) and [Ni(L2)2] (2) (------) in
chloroform
FIGURE 4 1H NMR spectra of HL1
(top) and [Ni(L1)2] (1) (bottom) in CDCl3
maximum at ꢂ295 nm followed by a shoulder at
ꢂ260 nm both being very strong, three strong shoulders
observed for the complexes are attributed to ligand-to-
metal charge transfer transitions[19,20] and the last two
very strong highest energy peaks are assigned to intra-
ligand transitions.
The proton NMR spectra of HL1, HL2, 1, and 2 were
recorded in CDCl3. The chemical shift data (numbering

8 of 11
SHAKUNTHALA ET AL.
scheme in Scheme 1) are given in Section 2 under the
respective compounds, and the spectra of HL1 and 1 are
depicted in Figure 4, whereas the spectra of HL2 and
2 are shown in Figure S3. The spectrum of each of the
two complexes 1 and 2 clearly indicates that in solution
also the two ligands are equivalent as observed in the
corresponding X-ray structure. Both thiopseudoureas
HL1 and HL2 display a singlet at ꢂ12.43 ppm due to the
NH group. As expected, neither of 1 and 2 shows this sig-
nal due to the deprotonation of the NH group and the
monoanionic state of the ligand. The tertiary H-atoms of
the two isopropyl groups of HL1 and HL2 appear as a
two-proton septate at ꢂ3.14 ppm, whereas the two-
proton septate for the same H-atoms is shifted downfield
by ꢂ1 ppm in the cases of 1 and 2. Unlike the single sig-
nal for the tertiary H-atoms, the methyl protons of the
two isopropyl groups resonate as two closely spaced six-
proton doublets for all four compounds. These two
doublets are observed at ꢂ1.25 and 1.18 ppm for the
thiopseudoureas (HL1 and HL2), while they move slightly
downfield to ꢂ1.41 and 1.33 ppm for the complexes
(1 and 2). The methylene protons of the benzyl group
appear as a two-proton singlet at ꢂ4.54 ppm for HL1 and
HL2. However, in contrast to the protons of the isopropyl
groups, the two-proton singlet due to the benzylic methy-
lene protons in the complexes (1 and 2) appears slightly
upfield at ꢂ4.33 ppm. The aromatic protons of HL1 and
HL2 resonate in the range 7.21–8.38 ppm whereas those
for 1 and 2 are observed in relatively upfield and some-
what narrow range of 6.61–7.42 ppm with the expected
splitting patterns.
3.4 | Catalytic studies
Optimization of the reaction conditions for C C cross-
coupling was performed using 1-bromo-
4-methoxybenzene (1.0 mmol) and phenylmagnesium
bromide (1.2 mmol) as the model substrates and the com-
plexes 1 and 2 as catalysts in 2 ml of solvent (toluene and
or tetrahydrofuran) (Table 4). All the reactions were car-
ried out at room temperature (25ꢀC). Using toluene as
solvent and 0.05 mol% of 1 under nitrogen atmosphere,
80% yield was obtained in 24 h (Entry 1). In the mixed
solvent toluene–tetrahydrofuran (1:1), the yield decreased
considerably (Entry 2). In contrast, better yield (88%) was
obtained in much less time in pure tetrahydrofuran
(Entry 3). However, if the reaction was conducted in air
keeping all other conditions the same as in Entry 3, the
yield decreased drastically (Entry 4). Doubling of the cat-
alyst loading increased the yield but by a very small
amount (4%). Both 1 and 2 showed good and comparable
catalytic activities and provided the coupling product in
essentially the same yield (88 and 90%) under the same
conditions (Entries 3 and 6). The homo-coupling product
biphenyl was also produced but in very little amount
(ꢂ4%). Therefore, the conditions used for Entry 3 were
found to be optimal for reactions of phenylmagnesium
bromide with aryl bromides using 1 or 2 as catalyst.
The substrate scope of the above coupling reaction
was probed using 1 as the catalyst. The results are sum-
marized in Table 5. Reactions of phenylmagnesium bro-
mide with para-substituted bromobenzenes (4-F, 4-Cl,
4-Me, and 4-OMe) and 2-bromonaphthalene produced
TABLE 4 Optimization of reaction conditions
Entry
Catalyst
Catalyst loading (mol%)
Solvent
Toluene
Toluene–THF (1:1)
THF
Time (h)
Yielda (%)
1
2
3
4
5
6
[Ni(L1)2] (1)
[Ni(L1)2] (1)
[Ni(L1)2] (1)
[Ni(L1)2] (1)
[Ni(L1)2] (1)
[Ni(L2)2] (2)
0.05
0.05
0.05
0.05
0.10
0.05
24b
24b
6b
12c
6b
80
68
88
40
92
90
THF
THF
THF
6b
Note: BrC6H4OMe: 1.0 mmol; phenylmagnesium bromide: 1.2 mmol; solvent: 2 ml.
aIsolated yield.
bIn nitrogen.
cIn air.

SHAKUNTHALA ET AL.
9 of 11
TABLE 5 Coupling reactions of
phenylmagnesium bromide with
various aryl bromides
Entry
Aryl bromide
Product
Time (h)
Yielda (%)
1
2
3
4
5
6
7
8
9
1-Bromo-4-fluorobenzene
1-Bromo-4-chlorobenzene
1-Bromo-4-methylbenzene
1-Bromo-4-methoxybenzene
2-Bromonaphthalene
4-Fluorobiphenyl
4-Chlorobiphenyl
4-Methylbiphenyl
4-Methoxybiphenyl
2-Phenylnaphthalene
3-Methylbiphenyl
3-Methoxybiphenyl
2-Methylbiphenyl
2-Methoxybiphenyl
6
96b
94b
93b
88b
86b
92b
88b
86c
83c
6
6
6
6
1-Bromo-3-methylbenzene
1-Bromo-3-methoxybenzene
1-Bromo-2-methylbenzene
1-Bromo-2-methoxybenzene
12
12
24
24
Note: Aryl bromide: 1.0 mmol; phenylmagnesium bromide: 1.2 mmol; THF: 2 ml.
aIsolated yield.
b[Ni(L1)2]: 0.05 mol%.
c[Ni(L1)2]: 0.1 mol%.
the desired products in 86–96% yields in 6 h (Entries 1–
5). The reactions of meta-substituted bromobenzenes
(3-Me and 3-OMe) with phenylmagnesium bromide also
provided high yields (92% and 88%) of the
corresponding products (Entries 6 and 7) but in double
reaction time (12 h) compared with that (6 h) required
for the reactions of the para-substituted bromobenzenes
with the same Grignard reagent. In contrast, the cross-
coupling reactions of ortho-substituted bromobenzenes
(2-Me and 2-OMe) with phenylmagnesium bromide
require much longer reaction time (24 h) and double
catalyst loading (0.1 mol%) compared with those needed
for the coupling of para- and meta-substituted
bromobenzenes with phenylmagnesium bromide to
4 | CONCLUSIONS
In summary, two thiopseudoureas HL1 and HL2 and their
corresponding nickel(II) complexes [Ni(L1)2] and [Ni
(L2)2] have been successfully synthesized and character-
ized. The microanalytical data and diamagnetic and non-
electrolytic characteristics of the two complexes indicate
+2 oxidation state and square-planar coordination envi-
ronment of the metal center in each complex. The X-ray
structures of HL1/2 showed the imino-ketone tautomeric
form of both thiopseudoureas in the solid state, whereas
the X-ray structures of [Ni(L1/2)2] confirmed the
iminolate-O and azomethine-N donor coordination mode
of the ligands (L1/2 −
and a square-planar trans-N2O2
)
afford
the
products
2-methylbiphenyl
and
coordination geometry for both complexes. The spectro-
scopic features of all four compounds corroborate the
corresponding molecular structures. Both complexes
exhibit efficient catalytic activity toward Kumada–Corriu
cross-coupling reactions of phenylmagnesium bromide
with a variety of aryl bromides at room temperature. The
reactions provided good to excellent yields of the biaryls.
2-methoxybiphenyl in slightly lower yields of 86% and
83%, respectively (Entries 8 and 9). The observed
increase in the reaction time and the catalyst loading
with the change of the substituent position on the
bromobenzene from para to meta to ortho is in all like-
lihood to be due to the gradual increase of the steric
hindrance. It may be noted that room temperature
cross-coupling reactions for the synthesis of biaryls
using nickel complexes as catalysts are not very com-
mon.[5,6] The present catalyst system not only works at
room temperature but also provides as good as or
improved yields when compared with the yields
reported so far for nickel complex catalyzed reactions.
ACKNOWLEDGMENTS
K.S. and A.K.S. thank the University Grants Commission
(UGC), New Delhi, for research fellowships (Nos.
187/(CSIR-UGC NET June 2017) and F.25-1/2014-15
(BSR)/5-129/2007(BSR), respectively). G.N.B. thanks the
Council of Scientific and Industrial Research (CSIR),

10 of 11
SHAKUNTHALA ET AL.
10, 443. h) U. S. Kanchana, E. J. Diana, T. V. Mathew, G.
Anilkumar, Appl. Organomet. Chem. 2020, 34, e5983.
[2] a) S. P. Stanforth, Tetrahedron 1998, 54, 263. b) J. Hassan, M.
Sévignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 2002,
102, 1359. c) G. P. McGlacken, L. M. Bateman, Chem. Soc. Rev.
2009, 38, 2447. d) H. Aldemir, R. Richarz, T. A. M. Gulder,
Angew. Chem., Int. Ed. 2014, 53, 8286. e) J. Wencel-Delord, A.
Panossian, F. R. Leroux, F. Colobert, Chem. Soc. Rev. 2015, 44,
3418. f) S. Yuan, J. Chang, B. Yu, Top. Curr. Chem. 2020,
378, 23.
New Delhi, for a research fellowship (No. 09/414(1025)/
2012-EMR-I). S.K. thanks the UGC, New Delhi, for the
Dr. D. S. Kothari postdoctoral fellowship (No. F.4-2/2006
(BSR)/13-1125/2013(BSR)). We thank the Ministry of
Education, Government of India for financial assistance
(Grant No. F11/9/2019-U3(A)) to the University of
Hyderabad–Institute of Excellence. Financial and infra-
structural supports from the UGC (through the CAS pro-
gram, Grant No. F.540/2/CAS-111/2016(SAP-I)) and the
Department of Science and Technology (DST), New
Delhi (through the FIST program, Grant No. SR/FST/
CST-030/2014(C)), are gratefully acknowledged.
[3] K. Tamao, K. Sumitani, M. Kumada, J. Am. Chem. Soc. 1972,
94, 4374.
[4] R. J. P. Corriu, J. P. Masse, J. Chem. Soc., Chem. Commun.
1972, 144.
[5] a) B. M. Rosen, K. W. Quasdorf, D. A. Wilson, N. Zhang, A.-
M. Resmerita, N. K. Garg, V. Percec, Chem. Rev. 2011, 111,
1346. b) X. Hu, Chem. Sci. 2011, 2, 1867. c) M. M. Heravi, P.
Hajiabbasi, Monatsh. Chem. 2012, 143, 1575. d) S. Z. Tasker,
E. A. Standley, T. F. Jamison, Nature 2014, 509, 299. e) Z. Li,
L. Liu, Chin. J. Catal. 2015, 36, 3. f) M. M. Heravi, V.
Zadsirjan, P. Hajiabbasi, H. Hamidi, Monatsh. Chem. 2019,
150, 535. g) K. E. Poremba, S. E. Dibrell, S. E. Reisman, ACS
Catal. 2020, 10, 8237.
[6] a) V. P. W. Böhm, T. Weskamp, C. W. K. Gstöttmayr, W. A.
Herrmann, Angew. Chem., Int. Ed. 2000, 39, 1602. b) F.-L.
Qing, R. Wang, B. Li, X. Zheng, W.-D. Meng, J. Fluorine Chem.
2003, 120, 21. c) Z.-X. Wang, L. Wang, Chem. Commun. 2007,
2423. d) J. Berding, T. F. van Dijkman, M. Lutz, A. L. Spek, E.
Bouwman, Dalton Trans. 2009, 6948. e) N. S¸ahin, H. E. Moll,
AUTHOR CONTRIBUTIONS
Kinche Shakunthala: Investigation. Ankit Kumar
Srivastava: Investigation. G. Narendra Babu: Investiga-
tion. Srinivas Keesara: Investigation. Samudranil Pal:
Conceptualization; supervision.
DATA AVAILABILITY STATEMENT
CCDC 2049212–2049215 contain the supplementary crys-
tallographic data for HL1, HL2, [Ni(L1)2] (1), and [Ni
(L2)2] (2). These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html
or
from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Addi-
tional data (Figures S1–S8) that support the findings of
this study are available in the supplementary material of
this article.
_
D. Sémeril, D. Matt, I. Özdemir, C. Kaya, L. Toupet, Polyhe-
dron 2011, 30, 2051. f) W. Dai, J. Xiao, G. Jin, J. Wu, S. Cao,
ꢁ
J. Org. Chem. 2014, 79, 10537. g) V. Kuchtanin, L. Klešcˇíkova,
ꢁ
M. Šoral, R. Fischer, Z. Ru˚ žicˇkova, E. Rakovský, J. Moncoľ, P.
Segľa, Polyhedron 2016, 117, 90. h) L. Iffland, A. Petuker, M.
van Gastel, U.-P. Apfel, Inorganics 2017, 5, 78. i) M. Mastalir,
K. Kirchner, Monatsh. Chem. 2017, 148, 105. j) K. Matsubara,
H. Yamamoto, S. Miyazaki, T. Inatomi, K. Nonaka, Y. Koga,
Y. Yamada, L. F. Veiros, K. Kirchner, Organometallics 2017,
36, 255.
ORCID
Kinche Shakunthala https://orcid.org/0000-0002-1473-
4940
Ankit Kumar Srivastava https://orcid.org/0000-0002-
3195-5112
G. Narendra Babu https://orcid.org/0000-0001-6370-
1371
Srinivas Keesara https://orcid.org/0000-0003-1991-2790
Samudranil Pal https://orcid.org/0000-0003-0217-8841
[7] a) R. del Campo, J. J. Criado, E. Garcia, M. R. Hermosa, A.
ꢁ
Jiménez-Sanchez, J. L. Manzano, E. Monte, E. Rodríguez-
ꢁ
Fernandez, F. Sanz, J. Inorg. Biochem. 2002, 89, 74. b) G.
Binzet, G. Kavak, N. Külcü, S. Özbey, U. Flörke, H. Arslan,
J. Chem. 2013, 536562. and references therein
[8] a) T. Iwamoto, T. Watano, M. Shigekawa, J. Biol. Chem. 1996,
271, 22391. b) T. P. Prakash, A. Püschl, M. Manoharan,
Nucleos. Nucleot. Nucl. Acids 2007, 26, 149. c) M. Ferreira, L. S.
REFERENCES
~
Assunçao, A. H. Silva, F. B. Filippin-Monteiro, T. B.
ꢁ
Creczynski-Pasa, M. M. Sa, Eur. J. Med. Chem. 2017, 129, 151.
[1] a) X. Chen, K. M. Engle, D.-H. Wang, J.-Q. Yu, Angew. Chem.,
Int. Ed. 2009, 48, 5094. b) R. Jana, T. P. Pathak, M. S. Sigman,
Chem. Rev. 2011, 111, 1417. c) C. C. C. J. Seechurn, M. O.
Kitching, T. J. Colacot, V. Snieckus, Angew. Chem., Int. Ed.
2012, 51, 5062. d) S. Xu, E. H. Kim, A. Wei, E. Negishi, Sci.
Technol. Adv. Mater. 2014, 15, 044201. e) A. de Meijere, S.
Bräse, M. Oestreich (Eds), Metal Catalyzed Cross-Coupling
Reactions and More, Wiley-VCH 2014. f) M. Busch, M. D.
Wodrich, C. Corminboeuf, ACS Catal. 2017, 7, 5643. g) I.
Kanwal, A. Mujahid, N. Rasool, K. Rizwan, A. Malik, G.
Ahmad, S. A. A. Shah, U. Rashid, N. M. Nasir, Catalysts 2020,
d) J. Merad, J.-M. Pons, O. Chuzel, C. Bressy, Eur. J. Org.
Chem. 2016, 5589.
[9] a) K. Srinivas, P. Srinivas, P. S. Prathima, K. Balaswamy, B.
Sridhar, M. M. Rao, Catal. Sci. Technol. 2012, 2, 1180. b) S.
Thogiti, S. P. Parvathaneni, S. Keesara, J. Organomet. Chem.
2016, 822, 165.
[10] D. D. Perrin, W. L. F. Armarego, D. P. Perrin, Purification of
Laboratory Chemicals, 2nd ed., Pergamon 1983.
[11] APEX3, Program for Data Collection on Area Detectors,
Bruker AXS Inc., Madison, Wisconsin, USA, 2016.

SHAKUNTHALA ET AL.
11 of 11
[12] G. M. Sheldrick, SADABS, program for area detector absorp-
tion correction, University of Göttingen, Göttingen, Germany,
1997.
[13] S. Parkin, B. Moezzi, H. Hope, J. Appl. Crystallogr. 1995,
28, 53.
[20] a) A. la Cour, M. Findeisen, R. Hazell, L. Hennig, C. E. Olsen,
O. Simonsen, J. Chem. Soc., Dalton Trans. 1996, 3437. b) C.
Lochenie, S. Schlamp, A. P. Railliet, K. Robeyns, B. Weber, Y.
Garcia, Cryst. Eng. Comm. 2014, 16, 6213.
[14] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112.
[15] L. J. Farrugia, J. Appl. Crystallogr. 2012, 45, 849.
[16] F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P. M.
Cabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. van de
Streek, P. A. Wood, J. Appl. Crystallogr. 2008, 41, 466.
[17] N. Phukan, J. B. Baruah, Cryst. Eng. Comm. 2016, 18, 3877.
[18] a) S. Das, S. Pal, J. Organomet. Chem. 2004, 689, 352. b) S. Das,
S. Pal, J. Organomet. Chem. 2006, 691, 2575. c) A. R. B. Rao, S.
Pal, J. Organomet. Chem. 2013, 731, 67.
[19] a) A. Mukhopadhyay, G. Padmaja, S. N. Pal, S. Pal, Inorg.
Chem. Commun. 2003, 6, 381. b) A. Mukhopadhyay, S. Pal,
Polyhedron 2004, 23, 1997. c) A. Mukhopadhyay, S. Pal, Eur.
J. Inorg. Chem. 2006, 4879. d) A. Mukhopadhyay, S. Pal, Eur.
J. Inorg. Chem. 2009, 4141. e) T. Ghosh, A. Mukhopadhyay,
K. S. C. Dargaiah, S. Pal, Struct. Chem. 2010, 21, 147. f) G. N.
Babu, S. Pal, J. Chem. Sci. 2017, 129, 1105.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Shakunthala K,
Srivastava AK, Babu GN, Keesara S, Pal S. Nickel
(II) complexes with N,O-donor thiopseudourea
ligands: Syntheses, structures, and catalytic
applications in Kumada–Corriu cross-coupling
reactions. Appl Organomet Chem. 2021;e6261.
https://doi.org/10.1002/aoc.6261