Delineating the Role of Substituents on the Coordination Behavior of Aroylhydrazone Ligands in PdII Complexes and their Influence on Suzuki–Miyaura Coupling in Aqueous Media

Article abstract of DOI:10.1002/ejic.201900781

Reactions of piperonal aroylhydrazone ligands (HL1–HL4) with [PdCl₂(PPh₃)₂] in CH₃OH and CHCl₃ afforded four new Pd(II) complexes (1–4) featuring diverse coordination behaviors. These complexes 1–4 were fully characterized by elemental analyses, UV/Visible, FT-IR and ¹H/¹³C NMR spectra. The variant coordination modes of piperonal aroylhydrazone ligands with Pd(II) ion in these complexes were unambiguously confirmed by single-crystal XRD study. Though the ligands HL1–HL4 are of similar nature, one of them (HL4) possessing an electron withdrawing nitro group as substitution in the peripheral part of the hydrazone displayed an uncommon ligation towards the palladium ion. An experimentally observed molecular architects were in turn rationalized by density functional theory (DFT) calculations as well as natural bonding analysis (NBO). All these Pd(II) complexes were evaluated for their catalytic potential towards Suzuki–Miyaura cross-coupling reaction of sterically congested arylboronic acids with (hetero)arylhalides under an open-flask conditions in aqueous media with low catalyst loading. Advantageously, the selected catalyst was active up to five cycles without significant loss in activity.

Full text of DOI:10.1002/ejic.201900781

A Journal of
Accepted Article
Title: Delineating the role of substituents on the coordination behavior
of aroylhydrazone ligands in Pd(II) complexes and their influence
on the Suzuki-Miyaura coupling in aqueous media
Authors: Vignesh Arumugam, Chinnuswamy Shalini, Nallasamy
Dharmaraj, Werner Kaminsky, and Ramasamy Karvembu
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900781
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
FULL PAPER
Delineating the role of substituents on the coordination behavior
of aroylhydrazone ligands in Pd(II) complexes and their influence
on the Suzuki-Miyaura coupling in aqueous media
[b]
Arumugam Vignesh *[a, a’, b], Chinnuswamy Shalini , Nallasamy Dharmaraj[b], Werner Kaminsky[c], and
Ramasamy Karvembu* [a]
Abstract: Reactions of piperonal aroylhydrazone ligands (HL1–HL4)
with [PdCl2(PPh3)2] in CH3OH and CHCl3 afforded four new Pd(II)
complexes (1–4) featuring diverse coordination behaviors. These
complexes 1–4 were fully characterized by elemental analyses, UV-
visible, FT-IR and 1H/13C NMR spectra. The variant coordination
modes of piperonal aroylhydrazone ligands with Pd(II) ion in these
complexes were unambiguously confirmed by single-crystal XRD
study. Though the ligands HL1-HL4 are of similar nature, one of them
(HL4) possessing an electron withdrawing nitro group as substitution
in the peripheral part of the hydrazone displayed an uncommon
ligation towards the palladium ion. An experimentally observed
molecular architects were in turn rationalized by density functional
theory (DFT) calculations as well as natural bonding analysis (NBO).
All these Pd(II) complexes were evaluated for their catalytic potential
towards Suzuki-Miyaura cross-coupling reaction of sterically
congested arylboronic acids with (hetero)arylhalides under an open-
flask conditions in aqueous media with low catalyst loading.
Advantageously, the selected catalyst was active up to five cycles
without significant loss in activity.
Recently, Schiff base ligands like hydrazones,
diazabutadienes, semicarbazones, thiosemicarbazones, and
their derivatives are extensively used to design structurally novel
[6]
coordination compounds.
Among them, hydrazones are a
special class of Schiff base ligands due to their superior reactivity
to form stable complexes with most of the transition metals.[7] The
captivating properties of both the hydrazone ligands, as an
evident from their popularity and growing applications in various
fields are mainly dependent on their facile synthesis, stability, and
easy handling. Generally, hydrazones bind to metal ions as
bidentate NO-donor[8a] ligand to form five membered chelate ring.
If a third donor atom (D) is incorporated into such ligands, linked
to the carbonylic carbon via one or two intervening atoms, then
the DNO tri-coordination normally takes place. [8b] It is notable that
the nature of various donor atoms in Pd catalysts has a
substantial effect on their catalytic properties. Moreover, the
possibility of modulating structural, electronic and steric
properties of hydrazone complexes renders them as attractive
and diverse family of coordination compounds. These demanding
characteristics of hydrazone ligands and their Pd complexes
posed a challenge to chemists not only in terms of their synthesis
but also their structure and coordination model. Palladium
catalyzed Suzuki-Miyaura cross-coupling (SMC) reaction, [9] that
involves the construction of biaryl moiety through reaction of aryl
Introduction
Nature of the ligand and its coordinating atom(s) play vital
role on the stability and activity of the resulting metal complexes.[1]
Schiff base as a ligand[2] continues to be at the forefront of modern
coordination chemistry owing to its crucial importance in
numerous applications including catalysis,[3] pharmaceuticals[4]
and materials science.[5]
halides with arylboronic acids has evolved as one of the most
[9b]
prominent synthetic methodologies.
The significance of Pd
catalyzed cross-coupling reactions has been highlighted by the
award of 2010 Nobel Prize in chemistry to Professors Heck,
[10]
Negishi, and Suzuki.
Sterically hindered biaryls are pivotal
substructures found in numerous pharmaceutically active
compounds, [11] functional materials[12] and as ligands. [13]
A thorough scanning on the literature revealed that vast
majority of reports on Suzuki-Miyaura cross-coupling (SMC)
reaction utilized simple aryl halides and arylboronic acids as
coupling partners.[9,10b] On the other hand, only less attention was
paid to the SMC reaction of sterically congested arylboronic acids
with aryl halides using palladium catalysts (Figure 1).[14] The
development of SMC reaction of sterically demanding arylboronic
acids with aryl halides to construct highly substituted biaryls under
mild reaction conditions is an extremely arduous task. In addition,
the existing protocols for SMC reactions were hampered due to
serious limitations such as high catalyst loading to achieve good
yields and extended reaction time. Therefore, designing of new
palladium catalysts devoid of the aforementioned limitations is
highly desirable.
[a]
Dr. A. Vignesh, Prof. Dr. R. Karvembu
Department of Chemistry, National Institute of Technology,
Tiruchirapalli – 620015 E-mail : srivignesh3@gmail.com ; kar@nitt.edu
Website: https://www.rkarvembu.com
Present address: Key Laboratory of Engineering Plastics, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
Ms. C. Shalini, Prof. N. Dharmaraj
Inorganic & Nanomaterials Research Laboratory, Department of
Chemistry, Bharathiar University, Coimbatore – 641 046, India.
E-mail: dharmaraj@buc.edu.in
Website: http://ndharmaraj.wix.com/inrl
Dr. W. Kaminsky
Department of Chemistry, University of Washington, Seattle,
Washington 98195, USA. E-mail: wernerka@uw.edu
[a’]
[b]
[c]
Supporting information for this article is given via a link at the end of the
document.
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
FULL PAPER
group.[17] The palladium(II) hydrazone complexes 1–4 are an air-
stable and diamagnetic in nature.
Figure 1. Selected Pd based catalysts for SMC reaction of
sterically hindered coupling partners.
Scheme 1. Synthesis of the ligands (HL1–HL4) and their Pd(II)
complexes (1–4).
At present, our research group is focusing on new
hydrazone ligands and their transition metal complexes to explore
their biological [15] and catalytic applications. [16] In this connection,
we herein demonstrate how the change in the electronic property
of peripheral substitution in aroylhydrazone ligands could alter the
coordination chemistry of the resulting complexes with square-
planar d8 palladium(II) ions. Further, the catalytic potential of these
Pd(II) complexes was scrutinized towards the SMC reaction of
sterically hindered arylboronic acids with (hetero)arylhalides.
FT-IR spectra of the palladium complexes (1–4) were
compared with that of free hydrazone ligands (HL1–HL4). The
spectra of free ligands showed the N–H stretching as a medium
intensity band in the region 3208–3234 cm-1and the amide C=O
as a strong band in the region 1650–1660 cm-1.[18a] The C=N
stretching of ligands HL1–HL4 was observed as a strong band in
the 1567–1597 cm-1range.[18b] The spectra of palladium
complexes 1–3 did not display any band assignable to neither N–
H nor C=O group of the amide functionality and thus revealed the
fact that the ligands underwent enolisation followed by
deprotonation prior to coordination with palladium ion in
complexes 1–3.[18c] A strong band observed in the spectra of
complexes 1–3 in the range of 1478–1543 cm-1 was assigned to
the coordinated azomethine group (HC=N). Appearance of a new
band in the spectra of complexes 1–3 in the region of 1171–1172
cm-1 indicated the presence of an imidolate functionality (N=C–O)
in them. In sharp contrary to the above, the spectrum of complex
4 exhibited any absorption due to C=O stretching. However, the
spectrum exhibited two strong bands at 1590 and 1529 cm-1
corresponding to the coordination of deprotonated amide nitrogen
and the free azomethine groups, respectively.[17] Hence, we
inferred that the nature of coordination of ligand HL4 in complex
4 is quite different from the rest of the complexes containing the
ligands HL1 – HL3.
The 1H NMR spectra of the free hydrazone ligands HL1-HL4
showed the resonance due to amide N-H proton as a sharp singlet
at 9.61, 9.36, 9.60, and 9.46 ppm, respectively. However, the
spectra of complexes 1–4 did not register any such signal and
indicated that the ligands underwent either deprotonation of
enolized imidol –OH in complexes 1–3 or deprotonation of amide
N–H in complex 4 upon coordination with Pd(II) ion.[19a] A singlet
observed at 8.76, 8.33, 8.86, and 8.82 ppm was attributed to the
azomethine proton of the free ligands HL1–HL4. Likewise,
complexes 1–4 registered their signal for the azomethine proton
in the region 9.42–8.11 ppm.[19a] The resonance due to methyl
group (CH3) present in the coordinated ligand in complex 2 and
Results and Discussion
Synthetic routes utilized for the novel hydrazone ligands
(HL1–HL4) and their Pd(II) complexes (1–4) are depicted in
Scheme 1. Condensation of piperonal and substituted aroyl
hydrazides
afforded
the
corresponding
piperonal
aroylhydrazones (HL1–HL4) as shown in Scheme 1. The
palladium complex 1 was obtained by treating an equimolar
quantity of piperonal benzhydrazone (HL1) with [PdCl2(PPh3)2] in
presence of Et3N as base. By following the same procedure,
complex 2 was prepared by using the hydrazone HL2 bearing an
electron donating –CH3 group as peripheral substituent. As such,
the reaction of HL3 (having –Cl as peripheral substituent) with
[PdCl2(PPh3)2] afforded complex 3. Surprisingly, the reaction of
HL4 with palladium precursor afforded complex 4 with an
unexpected monoanionic monodentate coordination of hydrazone
ligand through the deprotonated amide nitrogen (N) atom. The
remarkable difference in the ligation mode of hydrazone HL4
towards the Pd(II) ion in complex 4 could be accounted as follows.
The presence of a strong electron withdrawing –NO2 group at
para position with respect to amide functionality in HL4 could have
enhanced the acidity of the amide proton compared to the same
present in ligands HL1–HL3 and thus facilitated its removal in the
former ligand prior to coordination with palladium. [17] Generally,
the N-centre is softer than the O-centre in deprotonated amide
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
FULL PAPER
that of free ligand HL2 registered a singlet at 2.53 and 2.34 ppm,
respectively.[17] Further, the signals corresponding to the aromatic
protons of the free ligand and their corresponding complexes 1–4
are unexceptional. Also, the 13C NMR spectra of HL1–HL4 and
their Pd(II) complexes 1–4 registered the expected signals in the
appropriate regions. The signals for amide carbons in HL1–HL3
and complexes 1–3, were seen in the region of 163.4–159.6 and
165.8–159.1 ppm, respectively.[19b] Interestingly, the spectrum of
complex 4 registered the amide carbonyl at 197.0 ppm which is a
clear indication for the non-involvement of this group in
coordination to the metal in this case. Further, the appearance of
a signal at 157.3 ppm in the spectrum of complex 4 reflected that
the azomethine is also not involved in coordination. Appearance
of a signal at 21.1 and 21.3 ppm in the case of HL2 and its
complex 2 was assigned to the methyl group.[17] The resonances
corresponding to all the aromatic carbons present in HL1–HL4
and their complexes 1–4 were noticed in the region of 100.8–
146.6 ppm.[19b]
(HL4). The unique coordination behavior of ligand HL4 compared
to ligands HL1–HL3 towards the palladium ion in the same
precursor is quite understandable from the electronic nature of –
NO2 group present in the architect of the former ligand. The
strongly electron attracting –NO2 group in ligand HL4 could have
prevented the normal coordination behavior of this particular
ligand by preventing the enolisation of amide functionality. On the
other hand, the –NO2 group in HL4 caused the proton attached
with the amide nitrogen to be more acidic and thus facilitated its
deprotonation to inculcate an exclusive monobasic and
monodentate type of coordination by HL4 towards Pd(II) ion in
complex 4.
The molecular structures of the ligands HL1–HL3 and
complexes 1–4 were unambiguously determined by single-crystal
X-ray diffraction study (Figure 2 and 3). Suitable single-crystals
of HL1–HL3 and complexes 1–4 were obtained by the slow-
evaporation method. Selected bond lengths and angles of
complexes 1–4 are provided in Table 1. Crystal data and structure
refinement parameters for the ligands HL1–HL3 and Pd(II)
complexes 1–4 are summarized in Table 12 and 13, respectively.
The ligand HL1 was crystallized in orthorhombic system with
P2(1)2(1)2(1) space group whereas HL2 and HL3 were crystallized
in monoclinic system with P2(1)/n and P2(1) space group. However,
complexes 1and 4 were crystallized in monoclinic system with P
21/c space group whereas complex2 was in P21/n space group.
But, complex 3 was crystallized in triclinic system with P-1 space
group. The molecular structure of complexes 1 and 2 consists of
discrete units which correspond to [Pd(L1)2] or [Pd(L2)2].
HL1
In complexes 1 and 2, palladium ion bound to the oxygen
and azomethine nitrogen atoms of the two mutually trans
deprotonated hydrazone ligands HL1 or HL2 in monoanionic
bidentate form and hence formed two compartments of five
membered chelate rings. Similarly, the ligand HL3 was attached
with the Pd(II) centre in complex 3 through azomethine nitrogen
atom and deprotonated imidol oxygen atom to form a five
membered chelate ring, satisfying only two of the four
coordination sites around palladium with 1:1 stoichiometry.
Further, a chloride ion and phosphorous atom of coordinated
triphenylphosphine occupied the third and fourth coordination
sites around the palladium(II) ion and completes NOClP square-
planar environment. Fascinatingly, the molecular structure of
complex 4 established a different coordination behavior of ligand
HL4 compared to that of the other similar ligands HL1–HL3 in
complexes 1–3. Herein, deprotonated amide nitrogen
coordination with the devoid of enolisation with carbonyl group, a
set of PPh3 occupying trans positions and a chloride ion formed
NClP2 square-planar environment around Pd(II) ion. This type of
coordination exhibited by the aroylhydrazone ligand (HL4) is not
reported in the literature more than once.[17] A significant
lengthening of Pd–N bond (2.023(2)Å) in complex 4 is attributed
to the steric constraints imposed by the two triphenylphosphine
ligands and the monodentate nature of the hydrazone ligand
HL2
HL3
Figure 2. ORTEP diagram of aroylhydrazone ligands HL1–HL3
with thermal ellipsoids at the 50% probability level.
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
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Based on the single-crystal XRD data, the geometry around the
Pd(II) ion was identified to be square-planar in complexes 1 and
The observed bond lengths and angles in the complexes were
compared with previously reported similar palladium complexes
and found consistent.[16, 17, 19c,d.]
2
and distorted square-planar in complexes
3 and 4.
Complex 1
Complex 2
Complex 3
Complex 4
Figure 3. ORTEP diagram of Pd(II) complexes 1–4 with thermal ellipsoids at the 50% probability level. For clarity, hydrogen atoms
were omitted in complex 4.
the LUMO is mainly located on the metal ions. But, for complexes
DFT calculations were performed to better understand the
coordination mode of hydrazone ligands in Pd(II) complexes 1–4
at the B3LYP/6-31G* (for non-metals)/SDD (for palladium (II) with
effective core potential). Optimized geometries for experimentally
isolated (1–4) and hypothetically proposed (3’ and 4’) Pd(II)
complexes are shown in Figure 4 and 5.The frontier molecular
orbital diagrams of complexes 1–4 along with their HOMO and
LUMO energies, and energy gaps are shown in Figure 6 and
Table 2. The HOMO-LUMO energy differences for the palladium
complexes are in the range of 3.00–3.45 eV. Mulliken population
analysis is used to calculate the percentage contributions of
various groups to each molecular orbital in complexes 1–4. For
complexes 1 and 2, the HOMO is mainly located on the ligand and
3 and 4, the HOMO and LUMO are located on both metal ion and
ligand. In addition, we have compared the energies of
experimentally isolated (3 and 4) and hypothetical complexes (3’
and 4’). The total energies (Hartree) of complexes 3 and 3’ were
-2996.948297 and -2873.415051, respectively. Similarly,
-
3777.855840 and -2363.199506 were the energies of complexes
4 and 4’, respectively. The observed E (kcal/mol) between the
pair of complexes 3, 3’ and 4, 4’ were 77520 and 887701 kcal/mol,
respectively and thus revealed that the hypothetically proposed
complexes 3’ and 4’ were destabilized compared with
experimentally isolated complexes 3 and 4. Hence, the observed
coordination behavior of the ligands HL1–HL4 in the palladium
complexes 1–4 were attested by DFT studies.
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
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Table 1. Selected bond lengths (Å) and angles (°) for complexes 1–4.
1
2
3
4
Pd(1)–N(2)
Pd(1)–N(1)
Pd(1)–O(1)
Pd(1)–N(2)#
Pd(1)–O(1)#
Pd(1)–P(1)
Pd(1)–P(2)
Pd(1)–Cl(1)
O(1)–Pd(1)–N(2)
O(1)#1-Pd(1)-N(2)#1
O(1)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
P(1) –Pd(1)–Cl(1)
N(2)–Pd(1)– N(2)#
O(1)–Pd(1)– O(1)#
N(2)–Pd(1)–P(1)
O(1)–Pd(1)–Cl(1)
N(1)– Pd(1)–Cl(1)
P(1)–Pd(1)–P(2)
1.993(2)
----
1.9905(16)
1.993(2)
1.9905(16)
----
----
----
79.89(7)
79.89(7)
----
----
----
180.0
180.00(7)
----
----
----
----
1.988(3)
----
1.995(2)
1.988(3)
1.995(2)
----
----
----
80.05(9)
80.05(9)
----
----
----
180.00(12)
180.00(5)
----
----
----
----
2.0646(15)
----
1.9974(13)
----
----
2.2747(9)
----
2.2781(8)
78.46(6)
----
96.32(4)
----
----
----
----
174.63(4)
172.88(4)
----
----
2.023(2)
----
----
----
2.3120(7)
2.3298(7)
2.3243(7)
----
----
----
90.85(7)
88.04(3)
----
----
----
----
178.55(8)
170.54(3)
----
Complex 1
Complex 2
Complex 3
Figure 4. Optimized geometries for an experimentally isolated Pd(II) complexes 1–4
Complex 4
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
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3’
4’
Figure 5. Optimized geometries for hypothetical Pd(II) complexes 3’ and 4’.
Figure 6. HOMO-LUMO energy profile diagrams of the Pd(II) complexes (1–4).
To gain more insight into the unusual bonding observed in
complex 4, natural bonding analysis (NBO) was performed. The
NBO analysis also exposed the existence of strong bonding
interactions with Wiberg bond indices (WBI) of 0.467 and 0.473
for Pd-N and Pd-O bonds respectively, are in comparable with
complexes 1 and 2. The Pd-Cl and Pd-P bonds have stronger
interaction than Pd-N bonds for complexes 3 and 4 (Figures 7a-
d; Table 3). We have carried out topological analysis of
complexes 1-4, the Laplacian plot of electron density shows the
bond critical points (bcps) between the Pd-N, Pd-O, Pd-P and Pd-
Cl bonds (Figure 7e). Variety of topological parameters at bond
critical points (bcps) for complexes 1-4 are listed in Table 4. The
natural population analysis (NPA) of complex 4 shows that the
charge on metal center is negative (qPd = -0.032), whereas other
complexes (1-3) bear positive charge. This indicates that the
metal center in complex 4 acts as an acceptor by flow of electron
density from phosphine units. Further, the less positive natural
charges of phosphorous atoms observed in complexes 3 and 4
highlighted the presence of more electron density around them
(Table 5). Electrostatic potential surfaces (ESP) of the ligands
after the removal of protons were computed to visualize the
electron density on the coordinating atoms of hydrazone moiety
(Figure 8). ESP surfaces of L1 and L2 have almost same electron
density over the hydrazone moiety. When a moderately electron
withdrawing chlorine atom is attached in L3, the electron density
was observed to be little reduced. In similar manner, strong
electron withdrawing nitro group in L4 strongly suppressed the
electron density over the hydrazone moiety that thus led to a weak
coordination of ligand to palladium. Coordination of the hydrazone
ligand to palladium via a single atom also eased the steric loading
around the metal ion and thus paved way for the two phosphine
ligands to occupy their positions around the palladium ion in
complex 4 with only one coordinated hydrazone ligand.
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
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Figure 7. Selected NLMO of complex 4 as obtained from NBO analysis [(a) Pd-N bond, (b) Pd-Cl bond (c,d) Pd-P bonds]; (e) Laplacian
plot of the electron density, ∇2ρ(r) with negative contours shown as red lines and positive contours shown as black lines for 4 in Pd-P-
N plane (blue dots indicates bcps).
to less reports available on sterically hindered coupling partner.
Table 2. HOMO-LUMO energies calculated for complexes 1–4.
We began with the identification of most suitable reaction
Complex
HOMO (eV)
LUMO (eV)
E (eV)
parameters for the SMC reaction of sterically hindered arylboronic
acids with aryl halides (Table 6). Primarily, p-chlorobenzonitrile
and 2,6-dimethylphenylboronic acid were chosen as model
substrates and complex 3 was used as test catalyst in ethanol
medium in presence of a base. As expected, the catalytic reaction
was completed smoothly to yield the expected product. Our
attempted SMC reaction by excluding either the palladium
catalyst or a base did not progress any way. Among the various
bases tested, only NaOH did facilitate the higher quantity of the
coupled product. Our next objective in this study was to find the
best solvent that could effectively mediate the coupling reaction.
To realize this, a series of polar and non-polar solvents were
tested and arrived at a conclusion that water was the best one for
this catalytic reaction. In general, temperature of catalytic
reactions does have a promising effect on the reaction outcome
and therefore, the experiment was performed over a range of
temperatures including room temperature. As no coupled product
was formed at room-temperature, it was decided to increase the
temperature. During the scanning over a wide range of
temperatures in the range of 50 to 100 °C (Table 7), prominent
result of the product yield was observed only at 80 °C. Next, we
turned our attention to screen the catalytic potential of four
different palladium complexes synthesized in this work and
compared with that of commercially available palladium salts such
as PdCl2(PPh3)2, and PdCl2 as catalysts (Table 6, entries 22-24).
1
2
3
4
-5.33
-1.89
3.44
-5.27
-1.83
3.44
-5.41
-1.96
3.45
-5.16
-2.16
3.00
Table 3. Wiberg bond indices (WBI) of complexes 1-4 obtained
from NBO analysis.
Complex
Pd-N
1
2
3
4
0.467
0.467
0.415
0.453
0.638
0.671
0.432
-
Pd-O
0.473
0.474
Pd-P
-
-
-
-
0.557
0.618
Pd-Cl
Catalytic activity
With our understanding on the influence of peripheral
substitution of hydrazone ligands on their coordination ability
towards palladium ion in complexes 1–4, we moved ahead to find
out their trend in catalytic performance in SMC reaction of
sterically congested arylboronic acids with aryl halides. The
reason for the choice of the arylboronic acids herein is mainly due
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
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Figure 8. ESP surfaces of ligands after removal of protons (a) L1, (b) L2, (c) L3 and (d) L4.
Table 4. Electron density, ρ(r), Lagrangian kinetic energy, G(r), Potential energy density, V(r), Total energy density, H(r), Laplacian of
the electron density, ∇2ρ(r), Electron localization function, ELF and Ellipticity, ɛ in a.u of selected bond critical points (bcps) in 1-4.
BCP
ρ(r)
G(r)
V(r)
H(r)
∇2ρ(r)
ELF
ɛ
1
2
3
4
Pd-N
Pd-O
Pd-N
Pd-O
Pd-N
Pd-O
Pd-N
Pd-O
Pd-N
Pd-O
Pd-P
P1-Cl
Pd-N
Pd-P
Pd-Cl
Pd-P
0.096
0.087
0.022
0.084
0.303
0.090
0.011
0.011
0.064
0.060
0.091
0.071
0.076
0.067
0.054
0.067
0.162
0.151
0.037
0.153
0.055
0.155
0.047
0.076
0.144
0.154
0.069
0.076
0.141
0.080
0.076
0.078
-0.186
-0.162
-0.43
-0.023
-0.010
-0.006
-0.009
-0.297
-0.014
-0.007
-0.005
-0.002
0.004
0.559
0.562
0.124
0.577
-0.966
0.560
0.159
0.281
0.567
0.638
0.153
0.253
0.561
0.270
0.284
0.264
0.114
0.098
0.017
0.084
0.980
0.101
0.001
0.001
0.041
0.028
0.376
0.178
0.070
0.139
0.078
0.140
6.610
6.492
7.552
6.378
0.063
6.457
2.065
0.832
6.942
8.641
2.902
5.903
11.921
7.171
10.542
7.163
-0.162
-0.353
-0.170
-0.055
-0.081
-0.146
-0.150
-0.099
-0.089
-0.143
-0.093
-0.081
-0.091
-0.030
-0.013
-0.001
-0.093
-0.005
-0.012
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10.1002/ejic.201900781
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Among these catalysts, an unusually formed complex 4 exhibited
higher catalytic activity than others. Catalyst loading tests
performed using 0.1 mol% of palladium complex 4 yielded higher
quantities (96%) of the coupled product (Table 8). But such a high
catalyst content involving expensive metal in organic reactions
may lead to palladium inactive black formation and hinder the
catalysis.[20] To gain the advantage of less expense as well as free
from block formation, the reaction was conducted with an
extremely low catalyst loading (0.01 mol%) ended up with
successful result.
Table 5. Calculated natural charges (qPd, qN, qO, qP and qCl) of 1-
4.
Complex
qPd
qN
qO
qP
qCl
1
2
3
4
0.590
0.589
0.232
-0.032
-0.259
-0.258
-0.269
-0.375
-0.643
-0.644
-0.634
-
-
-
-
-
1.205
1.170
-0.437
-0.439
Table 6. Screening studies for the SMC reaction.a
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10.1002/ejic.201900781
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Table 7. Effect of temperature on SMC reaction.
and furnished exclusively the expected di-arylated heterocycles
without any trace of mono-arylated products.
To bring out the merit of the developed protocol for industrial
application, we executed a gram-scale synthesis using 10 mmol
of p-chlorobenzonitrile (1a) and 13 mmol of (2,6-
dimethoxyphenyl)boronic acid (2c), in which case 78 % of 3c
(Scheme 2) was realized and thus fulfilled our objective.
Entry
Temperature (°C)
Isolated yield (%)
1
2
3
4
5
6
50
60
70
80
90
65
71
79
88
83
81
100
Scheme 2. Gram-scale synthesis of 3c.
Table 8. Effect of catalyst loading.
During the entirety of these catalytic reactions, on no
occasion we were left with a homo-coupled product. In order to
identity the present catalytic reaction is homogeneous or
heterogenous catalysis, we carried out mercury poisoning test[9e]
under the optimized conditions by reacting 1a or 1c with 2a in
presence of catalyst 4 and Hg(0). The yield of the coupled product
3a or 3i did not affect by the addition of an excess of Hg(0) and
Entry
Catalyst loading
(mol %)
0.1
Isolated Yield
TON
(%)
96
90
82
1
2
3
960
9000
82000
0.01
0.001
The optimized conditions reported herein are also suitable for
electron-rich aryl halides. Encouraged by the preliminary results
discussed above, we further investigated the substrate scope of
arylboronic acids as well as aryl halides for the selected SMC
reaction. Notably, all the aryl halides and arylboronic acids utilized
in this study were challenged and sterically hindered substrates.
At the outset, the reaction of p-chlorobenzonitrile (1a) with a
series of sterically hindered arylboronic acids such as (2,6-
dimethylphenyl)boronic acid (2a), (2,3-dimethylphenyl)boronic
acid (2b), (2,6-dimethoxyphenyl)boronic acid (2c) and
naphthalen-2-ylboronic acid (2d) in presence of complex 4 as
catalyst provided an expected coupled products 3a–3d in good
yields (Table 9).Significantly, the reaction of 1-chloro-4-
nitrobenzene (1b) with arylboronic acids (2a–2d) underwent
successfully and yielded the coupled products in 87–95 % (Table
9, entries 3e–3h). Next, we turned our attention to utilize
heteroaryl halides as one of the coupling partners in SMC reaction
under the same optimized conditions. It is noteworthy to mention
here that the nitrogen heterocyclic compounds are considered as
problematic substrates for SMC reaction owing to its tendency to
bind with the metal ion and thus deactivating the catalyst.[16b] In
this connection, 2-chloropyridine and 2-chloroquinoline (1c and
1d) were coupled with arylboronic acids (2a–2d), wherein the
respective 2-arylated heterocycles were obtained in good to
excellent yields (75–89%) (Table 10, entries 3i–3p). In contrast to
the previous observations that this nitrogen heterocyclic
compound tends to bind with the catalyst and thus deactivating it,
but in our case, they underwent the coupling smoothly to give the
respective products.
thus proved the active palladium catalyst should be
a
homogeneous species and not palladium nano-particles.
Furthermore, the recyclability of catalyst 4 was examined by the
reaction
of
4-chlorobenzontrile
(1a)
with
(2,6-
dimethylphenyl)boronic acid (2a) under the optimized conditions.
The first cycle provided 87% of 3a followed by 82% in the next
run. Third and fourth cycles gave 74% and 59% of the same
heterocyclic product. The formation coupled product 3a was
drastically reduced after the fourth catalytic cycle (Figure 9).
Given the unique tendency of the new catalytic system, we
were curious about arriving at the symmetrical bis SMC reaction
of 2,5-dichloropyridine (1e) with arylboronic acids (Table 11,
entries 3q–3t) in presence of catalyst 4 under the optimized
reaction conditions. To our great surprise, the symmetrical bis
SMC reaction too was effected successfully and the
corresponding 2,6-diarylated pyridines were obtained in 79–84%
yields. It is significant to state here that this protocol is selective
Figure. 9 Re-usability of catalyst 4.
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10.1002/ejic.201900781
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Table 9. SMC reaction of aryl halides with arylboronic acids.[a,b]
Table 11. Bis SMC reaction of 2, 6-dichloropyridine with
arylboronic acids.[a,b]
Conclusions
Conclusively, high-yield synthetic route to aroylhydrazone
ligands HL1–HL4 and their respective palladium(II) complexes 1–
4 featuring different coordination behavior is described. All the
complexes have been fully characterized through elemental
analyses, UV-visible, FT-IR and 1H/13C NMR spectra. Final proof
for the molecular structure of ligands HL1–HL3 and complexes
1–4 were established by single-crystal XRD analyses revealed
that an unusual coordination mode exhibited by one out of the four
different piperonal hydrazone ligands (HL4) possessing strong
electron withdrawing –NO2 group, an uncommon observation in
the coordination chemistry of hydrazones in literature. The effect
of electronic properties of peripheral substituent on the aroyl ring
of the piperonal hydrazones on their coordination towards
palladium ion in these complexes was discussed. A theoretical
study presented in this work showed good agreements between
the calculated and experimental data. In addition, these palladium
complexes showed remarkable activity as catalysts in SMC of
sterically hindered substrates in aqueous media without any
externally added oxidant, additive or phase transfer reagents.
Open-atmospheric conditions, low catalyst loading, water as
solvent, scalability and re-usability are the key advantages of this
new catalytic protocol.
Table 10. SMC reaction of heteroaryl halides with arylboronic
acids.[a,b]
Experimental Section
General Information. All the reactions and manipulations for the
synthesis of ligands and their palladium complexes were carried
out under an open atmosphere. Elemental analyses (C, H, and N)
were performed on Elemental analyzer instrument. IR spectra
(4000–400 cm−1) of the compounds were recorded on FT-IR
spectrophotometer. Melting points were determined by melting
1
point apparatus and are uncorrected. H and 13C NMR spectra
were recorded in CDCl3 as solvent on 400 and 100 MHZ
instruments, respectively. The following abbreviations were used
for multiplicity: s = singlet, d = doublet, t = triplet, m = multiplet,
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and dd = doublet of doublets. Mass spectra were recorded in
mass spectrometer. [PdCl2(PPh3)2] (97%), PdCl2, and arylboronic
acids (96-98%) were purchased from commercial suppliers and
used as received. All reagent grade chemicals were used without
further purification unless otherwise specifically mentioned.
Solvents were purified and dried according to standard
procedures.[21]
Elemental analysis (%) calculated for C15H11N3O5; C, 57.51; H,
3.54; N, 13.41. Found (%) C, 57.47; H, 3.58; N, 13.44. Selected
IR bands (KBr, ν in cm−1): 3234 (N–H), 1660 (C=O), 1597 (C=N),
1504 (NO2). 1H NMR (CDCl3, δ ppm): 9.46 (s, 1H, N–H), 8.82 (s,
1H, HC=N), 7.53 (dd, J = 4.0, 2.0 Hz, 2H, ArH), 7.38 – 7.36 (m,
4H, ArH), 7.05(s, 1H, ArH), 6.04(s, 2H, Piperonyl CH2). 13C NMR
(CDCl3, δ ppm): 158.3, 152.7, 142.2, 134.4, 132.6, 130.7, 128.1,
127.9, 125.9, 125.7, 122.2, 107.7, 101.8.
General procedure for the synthesis of ligands HL1– HL4:
General procedure for the synthesis of palladium complexes
1- 4:
The aroylhydrazone ligands HL1–HL4 were synthesized
from equimolar quantity of piperonal with appropriate
aroylhydrazides such as benzhydrazide (HL1), p-toluichydrazide
(HL2), p-chlorobenzhydrazide (HL3) and p-nitrobenzhydrazide
(HL4) in ethanol. The reaction mixture was then refluxed on a
water-bath for 6 h and poured into crushed ice. The corresponding
hydrazones formed as pale-yellow color solid were filtered,
washed repeatedly with distilled water and re-crystallized from
methanol with 72–86 % yield. The identity of the ligands was
checked by various analytical techniques.
To a methanolic solution (20–30 mL) of appropriate ligands
(HL1–HL4) (1 equiv.) was added a chloroform solution of
[PdCl2(PPh3)2] (1 equiv.) followed by four drops of triethylamine.
Then the reaction mixture was refluxed for 8–10 h and kept at
room-temperature for crystallization. Needle like reddish brown
crystals suitable for X-ray studies were obtained on slow
evaporation.
[Pd(L1)2] (complex 1). Yield: 88%. M.p. 216–219 °C. Elemental
analysis (%) calculated for C30H22N4O6Pd; C, 56.22; H, 3.46; N,
8.74. Found (%) C, 56.15; H, 3.49; N, 8.69. UV-visible (solvent:
DMSO, nm): 269, 307, 324, 351, 370, 371, 378, 390. Selected IR
bands (KBr, ν in cm−1): 1682 (C=N−N=C), 1505 (C=N), 1172
(N=C–O). 1H NMR (CDCl3, δ ppm): 8.11 (s, 1H, HC=N), 7.90 (d,
J = 3.6 Hz, 2H, ArH), 7.68 (d, J = 8.8 Hz, 1H, ArH), 7.45 (t, J = 7.4
Hz, 2H, ArH), 7.35 (t, J = 7.2 Hz, 1H, ArH), 7.03 (d, J = 8.8 Hz,
(E)-N'-(benzo[d][1,3]dioxol-5-ylmethylene)benzohydrazide (HL1).
Yield 85% M.p.195–197 °C. Elemental analysis (%) calculated for
C15H12N2O3; C, 67.16; H, 4.51; N, 10.44. Found (%); C, 67.19; H,
4.47; N, 10.43. Selected IR bands (KBr, ν in cm−1): 3226 (N–H),
1660 (C=O), 1574 (C=N). H NMR (CDCl3, δ ppm): 9.61 (s, 1H,
N–H), 8.76 (s, 1H, HC=N), 7.88 (d, J = 7.6 Hz, 1H, ArH), 7.62 (s,
2H, ArH), 7.44 (d, J = 8.4 Hz, 3H, ArH), 6.88 (d, J = 8.4 Hz, 2H,
1
ArH), 5.96 (s, 2H, Piperonyl CH2). 13C NMR (CDCl3, δ ppm): 163.4, 2H, ArH), 5.91 (s, 2H, Piperonyl CH2).13C NMR (CDCl3, δ ppm):
148.6, 146.6, 145.7, 139.7, 138.6, 135.1, 128.8, 127.7, 127.1,
126.5, 106.9, 101.7, 100.8.
163.1, 160.7, 136.5, 129.9, 128.7, 127.4, 127.3, 126.8, 126.7,
119.9, 119.5, 116.0, 115.8.
(E)-N'-(benzo[d][1,3]dioxol-5-ylmethylene)-4-
[Pd(L2)2] (complex 2). Yield: 81%. M.p. 237–239 °C. Elemental
analysis (%) calculated for C32H26N4O6Pd; C, 57.45; H, 3.92; N,
8.37. Found (%) C, 57.40; H, 3.95; N, 8.41. UV-visible (solvent:
DMSO, nm): 266, 280, 293, 325, 358, 369, 370, 378. Selected IR
bands (KBr, ν in cm−1): 1650 (C=N−N=C), 1543 (C=N), 1171
(N=C–O). 1H NMR (CDCl3, δ ppm): 9.32 (s, 1H, HC=N), 8.00 (d,
J = 7.2 Hz, 2H, ArH), 7.95 (d, J = 6.0 Hz, 2H, ArH), 7.72 (d, J =
7.2 Hz, 1H, ArH), 7.50 (t, J = 6.2 Hz, 1H, ArH), 7.39 (dd, J = 5.6,
methylbenzohydrazide (HL2). Yield 86%. M.p.206–208°C.
Elemental analysis (%) calculated for C16H14N2O3; C, 68.08; H,
5.00; N, 9.92. Found (%) C, 68.10; H, 5.04; N, 9.94. Selected IR
bands (KBr, ν in cm−1): 3229 (N–H), 1659(C=O), 1591 (C=N). 1H
NMR (CDCl3, δ ppm): 9.36 (s, 1H,N–H), 8.33 (s, 1H, HC=N), 7.57
(t, J = 6.6 Hz, 1H, ArH), 7.51 (d, J = 8.0 Hz, 2H, ArH), 7.44 (s,1H,
ArH), 7.20 (s,1H, ArH), 6.77 (d, J = 8.0 Hz, 2H, ArH), 6.05 (s, 2H,
Piperonyl CH2), 2.34 (s, 3H, CH3). 13C NMR (CDCl3, δ ppm): 161.2, 6.0 Hz, 1H, ArH), 5.95 (s, 2H, Piperonyl CH2), 2.53 (s, 3H,
158.6, 132.8, 132.7, 132.5, 132.3, 129.4, 128.1, 125.8, 125.7,
122.1, 116.6, 110.2, 21.1.
CH3).13C NMR (CDCl3, δ ppm): 163.2, 160.1, 137.0, 135.6, 130.9,
129.7, 129.5, 128.9, 128.6, 128.3, 126.1, 120.4, 119.6, 21.3.
(E)-N'-(benzo[d][1,3]dioxol-5-ylmethylene)-4-
[Pd(L3)(Cl)(PPh3)] (complex 3). Yield: 90% . M.p. 219–222 °C.
Elemental analysis (%) calculated for C33H25Cl2N2O3PPd; C,
56.15; H, 3.57; N, 3.97. Found (%) C, 56.09; H, 3.49; N, 3.90. UV-
visible (solvent: DMSO, nm): 268, 309, 324, 342, 362, 369,371,
377. Selected IR bands (KBr, ν in cm−1): 1681 (C=N−N=C), 1587
(C=N), 1437 (PPh3), 1172 (N=C–O). H NMR (CDCl3, δ ppm):
9.42 (s, 1H, HC=N), 8.51 (s,1H, ArH), 8.10 (d, J = 8.0 Hz, 2H,
ArH), 8.08 (d, J = 8.8 Hz, 2H, ArH), 7.99 (d, J = 7.2 Hz, 6H, ArH),
chlorobenzohydrazide (HL3). Yield 79%. M.p.217–219 °C.
Elemental analysis (%) calculated for C15H11ClN2O3; C, 59.52; H,
3.66; N, 9.25. Found (%) C, 59.55; H, 3.64; N, 9.23. Selected IR
bands (KBr, ν in cm−1): 3208 (N–H), 1650 (C=O), 1567 (C=N). 1H
NMR (CDCl3, δ ppm): 9.60 (s, 1H, N–H), 8.86 (s, 1H, HC=N), 7.50
(dd, J = 4.0, 2.0 Hz, 2H, ArH), 7.36 – 7.34 (m, 3H, ArH), 7.51 (d,
J = 8.0 Hz, 2H, ArH), 7.21 (s, 1H, ArH), 7.00 (s, 1H, ArH), 6.03 (s,
1
2H, Piperonyl CH2). 13C NMR (CDCl3, δ ppm): 159.6, 158.2, 140.7, 7.95 (t, J = 7.2 Hz, 2H, ArH), 7.64 (t, J = 7.6 Hz, 6H, ArH), 7.57-
132.5, 131.4, 129.1, 128.0, 125.8, 125.6, 123.2, 122.2, 119.09,
101.9.
7.50 (m, 3H, ArH) 5.85 (s, 2H, Piperonyl CH2). 13C NMR (CDCl3,
δ ppm): 165.8, 163.8, 139.3, 129.8, 129.0, 128.9, 128.7, 128.5,
128.3, 127.2, 127.2, 127.1, 127.1, 125.3, 124.8, 122.6, 120.3,
119.7.
(E)-N'-(benzo[d][1,3]dioxol-5-ylmethylene)-4-
nitrobenzohydrazide (HL4). Yield 72%. M.p.207–209 °C.
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FULL PAPER
[Pd(L4)(Cl)(PPh3)2] (complex 4). Yield: 85% . M.p. 209–211 °C.
Elemental analysis (%) calculated for C51H40ClN3O5P2Pd; C,
62.59; H, 4.12; N, 4.29. Found (%) C, 62.51; H, 4.16; N, 4.25. UV-
visible (solvent: DMSO, nm): 265, 281, 309, 331, 369, 371.
9.34 (s, 1H, HC=N), 8.06 (s, 1H, ArH), 7.93 (t, J = 10.2 Hz, 9H,
ArH), 7.63 (t, J = 8.0 Hz, 9H, ArH), 7.56 (d, J = 8.0 Hz, 4H, ArH),
7.48 (d, J = 7.2 Hz, 12H, ArH), 7.39 (d, J = 7.2 Hz, 2H, ArH), 5.91
(s, 2H, Piperonyl CH2).13C NMR (CDCl3, δ ppm): 197.0, 159.1,
Selected IR bands (KBr, ν in cm−1): 1730 (C=O) 1674 (C=N−N=C), 157.3, 134.3, 132.5, 131.6, 130.9, 129.9, 128.5, 127.9, 126.1,
1590 (C=N), 1435 (PPh3), 1485 (NO2). 1H NMR (CDCl3, δ ppm):
125.7, 125.6, 122.0, 120.3, 111.6, 102.0.
Table 12. Crystal data and structure refinement for ligands HL1–HL3.
HL1
HL2
HL3
CCDC Number
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
1874119
C16 H18 N2 O5
318.32
110(2)
1.54178
1874118
C17 H18 N2 O4
314.33
1874117
C16 H15 Cl N2 O4
334.75
150(2)
150(2)
0.71073
Monoclinic
P2(1)/n
0.71073
Monoclinic
P2(1)
Orthorhombic
P2(1)2(1)2(1)
6.2978(2)
13.3981(5)
18.0205(7)
90
10.922(2)
6.5745(12)
21.897(4)
90
10.849(3)
6.4524(18)
10.951(3)
90
b (Å)
c (Å)
α (°)
β (°)
γ (°)
90
90
98.035(2)
90
1557.0(5)
4
91.309(3)
90
766.5(4)
2
Volume (Å3)
Z
1520.54(10)
4
1.39
Density (cal.) (Mg/m−3)
1.341
1.450
Absorption
(mm−1)
coefficient 0.872
0.097
0.272
F(000)
672
664
348
Crystal size (mm3)
Reflections collected
0.18 x 0.05 x 0.04
15878
0.48 x 0.28 x 0.19
15982
0.52 x 0.24 x 0.05
8933
Independent reflections
2213[R(int) = 0.0502]
3572 [R(int) = 0.0277]
0.9819 and 0.9551
3464 [R(int) = 0.0258]
0.9866 and 0.8717
Max.
and
min. 0.9659 and 0.8588
transmission
Refinement method
Full-matrix least-squares on Full-matrix least-squares on Full-matrix least-squares on F2
F2
F2
Data
/
restraints
/
2213 / 0 / 211
3572 / 0 / 211
3464 / 1 / 210
parameters
Goodness-of-fit on F2
Final R indices
[I > 2σ(I)]
0.921
1.036
1.031
R1 = 0.0263, wR2 = 0.0634
R1 = 0.0398, wR2 = 0.1032
R1 = 0.0282, wR2 = 0.0706
R indices
(all data)
Largest diff. peak
and hole (e.Å-3)
R1 = 0.0287, wR2 = 0.0653
0.157 and -0.226
R1 = 0.0455, wR2 = 0.1083
0.296 and -0.271
R1 = 0.0301, wR2 = 0.0721
0.157 and -0.204
Single-crystal X-ray diffraction studies:
proposed structure. The structure was completed by difference
Fourier synthesis with SHELXL97.[24] Scattering factors are from
Waasmair and Kirfel.[25] Hydrogen atoms were placed in
geometrically idealized positions and constrained to ride on their
parent atoms with C---H distances in the range 0.95-1.00
Angstrom. Isotropic thermal parameters Ueq were fixed such that
they were 1.2Ueq of their parent atom Ueq for CH's and 1.5Ueq of
their parent atom Ueq in case of methyl groups. All non-hydrogen
atoms were refined anisotropically by full-matrix least-squares.
Data collection: Single-crystal X-ray diffraction data of
complexes 1–4 were collected at 19oC on single crystal X-ray
diffractometer using Mo-radiation. Crystal-to-detector distance
was 40 mm and exposure time was 3 seconds per frame for all
sets. The data was integrated and scaled using SAINT / SADABS
from within the APEX2 software package by Bruker (Version 2.1-
4).[22] Solution by direct methods (SHELXS, SIR97[23]) produced a
complete heavy atom phasing model consistent with the
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10.1002/ejic.201900781
European Journal of Inorganic Chemistry
Table 13 Crystal data and structure refinement for complexes 1–4.
Complexes
CCDC Number
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
1
2
3
4
1019330
C30H22N4O6Pd
640.92
100(2) K
0.71073 Å
Monoclinic
P 21/c
1019332
C32H26N4O6Pd
668.97
100(2) K
0.71073 Å
Monoclinic
P 21/n
989143
C33H25Cl2N2O3PPd
705.82
100(2) K
0.71073 Å
Triclinic
P-1
1019331
C51H40ClN3O5P2Pd
978.65
100(2) K
0.71073 Å
Monoclinic
P 21/c
17.1505(15)
6.3425(6)
24.257(2)
90
101.749(4)
90
2583.3(4)
4
1.648
11.1487(4)
5.7641(2)
21.9213(7)
90
94.755(2)
90
1403.86(8)
2
1.583
10.239(3)
12.038(5)
12.385(5)
78.405(17)
83.188(15)
88.370(15)
1484.8(9)
2
14.2043(8)
27.2807(14)
11.5011(6)
90
96.396(3)
90
4429.0(4)
4
1.468
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Density (cal.) (Mg/m−3)
1.579
Absorption
coefficient 0.773
0.715
0.897
0.604
(mm−1)
F(000)
1296
0.13x 0.10 x 0.06
53763
680
712
2000
0.15 x 0.09 x 0.07
80882
Crystal size (mm3)
Reflections collected
Independent reflections
0.08x 0.07 x 0.03
26256
3456
0.50 x 0.25 x 0.20
49728
7464 [R(int) = 0.0235]
6395
[R(int)
=
[R(int)
=
11068 [R(int) = 0.0609]
0.0455]
0.0550]
Max.
and
min. 0.9551 and 0.9062
0.9789 and 0.9450
0.8410 and 0.6627
0.9589 and 0.9148
transmission
Refinement method
Full-matrix
least- Full-matrix
squares on F2
least- Full-matrix
squares on F2
least- Full-matrix
least-
squares on F2
6395 / 0 / 373
squares on F2
Data
/
restraints
/
3456 / 0 / 197
7464 / 0 / 379
11068 / 0 / 568
parameters
Goodness-of-fit on F2
Final R indices
[I > 2σ(I)]
1.002
1.018
1.080
1.033
R1 = 0.0309, wR2 = R1 = 0.0448, wR2 = R1 = 0.0228, wR2 = R1 = 0.0424, wR2 =
0.0673 0.0929 0.0564 0.0887
R1 = 0.0697, wR2 = R1 = 0.0796, wR2 = R1 = 0.0272, wR2 = R1 = 0.0717, wR2 =
R indices
(all data)
0.0813
0.1078
0.0611
0.1008
Largest diff. peak
0.544 and -0.718
1.009 and -0.321
0.375 and -0.412
1.285 and -0.803
and hole (e.Å-3)
Computational details:
conditions.The identity of the recovered catalyst was checked with
DFT calculations were carried out with the program package Thin layer chromatography.The fresh catalyst was used as
Gaussian 09 Revision C.01.[26] Ground state geometries were reference and found that the Rf values are same.
optimized without symmetry constraints using the hybrid Becke-
Lee-Yang-Parr (B3LYP)[27] functional in combination with 6-
31G(d)[28] basis set for non-metals and SDD[29] basis set with
effective core potential for metal ions have been used for all the
calculations. The initial geometry coordinates have been extracted
from the crystal structure of the molecule.
Spectral data of products listed in table 9 - 11:
Entry 3a: 2',6'-dimethyl-[1,1'-biphenyl]-4-carbonitrile.[30a] 1H NMR:
8.25 (d, J = 6.0 Hz, 2H), 7.95 (d, J = 6.0 Hz, 2H), 7.50 (t, J = 5.8
Hz, 1H), 7.41 (d, J = 6.0 Hz, 2H), 2.49 (s, 6H). 13C NMR: 148.3,
137.0, 130.2, 129.7, 128.9, 128.7, 128.4, 125.8, 120.5, 20.5.
Entry 3b: 2',3'-dimethyl-[1,1'-biphenyl]-4-carbonitrile.[16a] 1H NMR:
8.22 (d, J = 6.0 Hz, 1H), 8.03 (d, J = 6.4 Hz, 2H), 7.97 (d, J = 5.6
Hz, 2H), 7.51 (t, J = 5.4 Hz, 1H), 7.41 (d, J = 5.6 Hz, 1H ) 2.50 (s,
3H), 2.44 (s, 3H). 13C NMR: 137.4, 137.1, 130.8, 130.0, 129.5,
128.9, 128.3, 125.3, 125.2, 119.8, 119.6, 23.5, 20.4.
Experimental procedure for the cross-coupling reactions:
Entry 3c: 2',6'-dimethoxy-[1,1'-biphenyl]-4-carbonitrile.[30b] 1H NMR:
7.67 (d, J = 8.8 Hz, 2H), 7.45 (t, J = 7.4 Hz, 1H), 7.27 (d, J = 7.2
Hz, 2H), 7.04 (d, J = 8.8 Hz, 2H), 3.86 (s, 6H). 13C NMR: 159.4,
130.4, 130.3, 128.8, 128.2, 125.7, 122.1, 117.8, 114.4, 55.5.
Entry 3d: 4-(naphthalen-2-yl)benzonitrile.[30c] 1H NMR: 8.16 (s, 1H),
7.92 (d, J = 7.2 Hz, 2H), 7.67 (d, J = 8.0 Hz, 2H), 7.46 (t, J = 7.4
Hz, 2H), 7.38 (d, J = 7.2 Hz, 2H), 7.34 (d, J = 8.0 Hz, 2H). 13C NMR:
146.4, 136.5, 129.9, 128.7, 127.4, 127.3, 126.8, 126.7, 119.9,
119.5, 116.0, 115.8.
A mixture of (hetero)aryl halides (2 mmol), arylboronic acids (2.6
mmol), NaOH (2.0 mmol), water (20 mL) and complex 4 (0.01
mol%) was stirred at 80 °C under aerobic conditions. After the
completion, the crude reaction mixture was workup with
dichloromethane and the organic layer was collected and dried over
anhydrous sodium sulfate. The resulting mixture was purified by
alumina column chromatography using n-hexane and ethyl acetate
(50:1) as eluents. The identity of the obtained products was
1
Entry 3e: 2,6-dimethyl-4'-nitro-1,1'-biphenyl. H NMR: 8.27 (d, J =
1
confirmed by H and 13C NMR and the data were compared with
6.0 Hz, 2H), 7.94 (d, J = 8.0 Hz, 2H), 7.50 (t, J = 5.9 Hz, 1H), 7.40
literature reports.[30] The recovered catalyst (complex 4) was dried
and used in successive cycles under the identical reaction
(d, J = 6.0 Hz, 2H), 2.48 (s, 6H). 13C NMR: 140.8, 129.7, 129.0,
This article is protected by copyright. All rights reserved.

10.1002/ejic.201900781
European Journal of Inorganic Chemistry
FULL PAPER
128.5, 125.6, 125.4, 120.4, 119.9, 21.0. HRMS (ESI): calculated for 7.35 (t, J = 7.4 Hz, 1H), 2.50 (s, 12H). 13C NMR: 158.1, 134.2, 132.5,
C14H13NO2 [M+H]+: 228.26; found: 227.16.
131.0, 128.4, 128.0, 127.9, 21.1.
Entry 3f: 2,3-dimethyl-4'-nitro-1,1'-biphenyl.[16a] 1H NMR: 8.00 (d, J Entry 3r: 2,6-bis(2,3-dimethylphenyl)pyridine. 1H NMR: 8.46 (d, J =
= 7.2 Hz, 2H), 7.94 (d, J = 6.0 Hz, 2H), 7.74 (d, J = 7.2 Hz, 2H), 8.8 Hz, 2H), 7.80 (d, J = 9.2 Hz, 2H), 7.76 (d, J = 7.6 Hz, 2H), 7.50
7.50 (t, J = 6.2 Hz, 1H), 7.40 (dd J = 5.6, 6.0 Hz, 1H), 2.50 (s, 3H). (t, J = 7.6 Hz, 2H), 7.41 (t, J = 7.4 Hz, 1H), 2.57 (s, 6H), 2.10 (s,
13C NMR: 147.4, 135.4, 132.9, 130.0, 129.8, 128.9, 128.3, 125.3, 6H). 13C NMR: 158.3, 132.5, 132.5, 130.3, 128.2, 128.0, 127.9,
121.6, 119.6, 23.3, 21.1.
125.8, 125.7, 23.8, 20.5. HRMS (ESI): calculated for C21H21N
1
Entry 3g: 2,6-dimethoxy-4'-nitro-1,1'-biphenyl. H NMR: 7.95 (d, J [M+H]+: 288.41; found: 288.45.
= 6.4 Hz, 2H), 7.86 (d, J = 6.8 Hz, 2H), 7.51 (t, J = 6.0 Hz, 1H), 7.41 Entry 3s: 2,6-bis(2,6-dimethoxyphenyl)pyridine. 1H NMR: 7.90 (d, J
(d, J = 5.6 Hz, 1H), 3.86 (s, 6H). 13C NMR: 147.5, 132.8, 130.1, = 8.4 Hz, 2H), 7.76 (d, J = 7.6 Hz, 2H), 7.73 (d, J = 8.4 Hz, 2H),
129.0, 128.3, 125.3, 121.9, 121.3, 55.5. HRMS (ESI): calculated for 7.75 (t, J = 7.4, Hz, 2H), 7.41 (t, J = 7.4 Hz, 1H), 3.85 (s, 12H). 13
C
C14H13NO4 [M+H]+: 260.26; found: 260.19.
NMR: 158.2, 152.8, 130.7, 128.1, 127.9, 125.8, 125.7, 56.2.HRMS
Entry 3h: 2-(4-nitrophenyl)naphthalene. 1H NMR: 8.24 (s, 1H), 7.98 (ESI): calculated for C21H21NO4 [M+H]+: 352.40; found: 352.43.
(d, J = 7.6 Hz, 2H), 7.78 (d, J = 8.0 Hz, 2H), 7.72 (d, J = 8.0 Hz, Entry 3t: 2,6-di(naphthalen-2-yl)pyridine.[14g] 1H NMR: 8.23(s, 2H),
2H), 7.60 (t, J = 7.4 Hz, 1H), 7.46 (t, J = 7.2 Hz, 1H), 7.40 (d, J = 8.02 (dd, J = 8.4, 1.6 Hz, 1H), 7.99 (d, J = 8.4 Hz, 1H), 7.94-7.88
7.2 Hz, 2H). 13C NMR: 148.0, 142.6, 136.3, 132.3, 130.8, 129.5, (m, 4H), 7.55-7.50 (m, 4H), 7.45-7.29 (m, 4H). 13C NMR: 158.1,
129.0, 128.7, 124.0, 123.9, 120.7, 120.2. HRMS (ESI): calculated 145.3, 134.8, 134.2, 132.0, 129.8, 128.2, 127.9, 127.8, 125.8,
for C14H13NO4 [M+H]+: 250.28; found: 250.25.
125.7, 125.4, 122.1, 101.8.
Entry 3i: 2-(2,6-dimethylphenyl)pyridine.[30d] 1H NMR: 7.94 (d, J =
7.6 Hz, 2H), 7.90 (d, J = 8.0 Hz, 1H), 7.70 (d, J = 8.0 Hz, 1H), 7.62
(t, J = 7.6 Hz, 2H), 7.51 (t, J = 7.0 Hz, 1H), 2.49 (s, 6H). 13C NMR:
148.3, 138.2, 137.0, 129.6, 129.5, 128.5, 127.3, 125.6, 120.3, 21.2.
1
Acknowledgements
Entry 3j: 2-(2,3-dimethylphenyl)pyridine. H NMR: 8.33 (d, J = 8.0
Hz, 1H), 7.82 (d, J = 8.0 Hz, 2H), 7.54 (t, J = 7.6 Hz, 1H), 7.45 (t, J
= 6.6 Hz, 1H), 7.40 (dd, J = 7.2, 8.0 Hz, 1H), 7.28 (t, J = 7.2 Hz,
1H), 2.47 (s, 3H), 2.15 (s, 3H).13C NMR: 144.5, 131.2, 130.2, 129.8,
129.7, 129.1, 128.8, 128.7, 127.1, 121.1, 120.5, 24.8, 21.3. HRMS
(ESI): calculated for C13H13N[M+H]+: 184.25; found: 184.10.
Dr. A. Vignesh thanks the Science & Engineering Research Board
(SERB), New Delhi, India, for the award of National Post-Doctoral
Fellowship (File Number: PDF/2017/000231, from September 2017
to February 2018) and Chinese Academy of Sciences President’s
International Fellowship Initiative for the award of post-doctoral
fellowship (Grant No. 2018PM0012 from April 2018 to till date). Ms.
C. Shalini thanks the Department of Science and Technology
(DST), New Delhi, India, for the award of Promotion of University
Research and Scientific Excellence (PURSE) – Phase-II fellowship
1
Entry 3k: 2-(2,6-dimethoxyphenyl)pyridine. H NMR: 7.85 (d, J =
8.4 Hz, 2H), 7.79 (d, J = 7.6 Hz, 1H), 7.54 (t, J = 7.6 Hz, 2H), 7.44
(t, J = 7.2, Hz, 1H), 7.00 (d, J = 8.4 Hz, 1H), 3.85 (s, 6H).13C NMR:
159.7, 148.2, 129.7, 128.6, 127.1, 122.9, 120.4, 116.7, 114.3, 55.3.
HRMS (ESI): calculated for C13H13NO2 [M+H]+: 216.25; found:
216.31.
(Official
Memorandum
Number:
BU/DST
PURSE
(II)
Entry 3l: 2-(naphthalen-2-yl)pyridine. 1H NMR: 8.20 (s, 1H), 8.01 (t,
J = 4.2 Hz, 1H), 7.84 (d, J = 7.6 Hz, 2H), 7.77 (d, J = 2.4 Hz, 2H),
/APPOINTMENT/13).
7.55 (t, J = 7.8 Hz, 2H), 7.46-7.43(m, 2H), 7.29-7.23 (m, 1H). 13
C
NMR: 143.7, 137.1, 136.4, 129.7, 128.5, 125.0, 122.9, 122.5, 120.5,
120.4, 119.6, 116.5, 111.7. HRMS (ESI): calculated for
C15H11N[M+H]+: 206.26; found: 206.14.
Conflicts of interest
Entry 3m: 2-(2,6-dimethylphenyl)quinoline.[30e] 1H NMR: 8.18 (d, J =
7.6 Hz, 2H), 7.81 (d, J = 7.6 Hz, 1H), 7.68 (d, J = 8.0 Hz, 1H), 7.53
(t, J = 7.8 Hz, 2H), 7.44 (dd, J = 7.2, 6.4 Hz, 2H), 7.21 (t, J = 7.0
Hz, 1H), 2.40 (s, 6H). 13C NMR: 145.9, 136.8, 133.6, 130.9, 130.6,
129.7, 129.4, 128.7, 127.6, 121.1, 120.9, 120.5, 19.7.
There are no conflicts to declare
Keywords: Pd(II) complexes. Coordination mode. XRD-
Studies. DFT. Suzuki coupling.
Entry 3n: 2-(2,3-dimethylphenyl)quinoline.[16b] 1H NMR: 8.58 (d, J =
8.4 Hz, 1H), 8.34 (d, J = 8.0 Hz, 1H), 7.61 (dd, J = 1.6, 5.2 Hz, 1H),
7.58-7.51 (m, 2H), 7.35 (d, J = 7.6 Hz, 1H), 6.82 (d, J = 2.4 Hz,
1H), 6.72 (t, J = 8.8 Hz, 1H), 6.68 (t, J = 4.2 Hz, 1H), 2.47 (s, 3H),
2.04 (s, 3H). 13C NMR: 141.0, 132.9, 132.7, 132.5, 132.3, 129.4,
128.1, 125.8, 125.7, 122.1, 116.7, 110.2, 102.0, 21.1, 20.7.
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A. Vignesh*, C. Shalini, N.
Dharmaraj, W.Kaminsky, and R.
Karvembu*
Page No. – Page No.
Title Delineating the role of substituents
on the coordination behavior of
aroylhydrazone ligands in Pd(II)
complexes and their influence on the
Suzuki-Miyaura coupling in aqueous
media
Text for Table of Content
Key Topic: Palladium(II) hydrazone complexes
Reaction of aroylhydrazone ligands with [PdCl2(PPh3)2] afforded four new complexes
of stoichiometric variation. Influence of peripheral substituents of aroylhydrazone on
the coordination diversity were investigated. Catalytic potential of these complexes
were tested towards the Suzuki coupling of sterically challenged arylboronic acids
with (hetero)aryl halides under an open-flask conditions and low catalyst loading.
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