Ferrocenyl palladacycles derived from unsymmetrical pincer-type ligands: Evidence of Pd(0) nanoparticle generation during the Suzuki-Miyaura reaction and applications in the direct arylation of thiazoles and isoxazoles

Article abstract of DOI:10.1039/c9dt03465j

A new family of ferrocenyl-palladacycle complexes Pd(L¹)Cl (Pd1) and Pd(L²)Cl (Pd2) were synthesized and characterized by UV-visible, IR, ESI-MS, and NMR spectral studies. The molecular structures of Pd1 and Pd2 were determined by X-ray crystallographic studies. Palladacycle catalyzed Suzuki-Miyaura cross-coupling reactions were investigated utilizing the derivatives of phenylboronic acids and substituted chlorobenzenes. Mechanistic investigation authenticated the generation of Pd(0) nanoparticles during the catalytic cycle and the nanoparticles were characterized by XPS, SEM and TEM analysis. Direct C-H arylation of thiazole and isoxazole derivatives employing these ferrocenyl-palladacycle complexes was examined. The reaction model for the arylation reaction implicating the in situ generation of Pd(0) nanoparticles was proposed.

Full text of DOI:10.1039/c9dt03465j

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PAPER
Ferrocenyl palladacycles derived from
unsymmetrical pincer-type ligands: evidence
of Pd(0) nanoparticle generation during the
Suzuki–Miyaura reaction and applications in
the direct arylation of thiazoles and isoxazoles†
Cite this: Dalton Trans., 2019, 48,
17083
Ankur Maji,a Anshu Singh,a Aurobinda Mohanty,a Pradip K. Maji *b and
a
Kaushik Ghosh
*
A new family of ferrocenyl-palladacycle complexes Pd(L1)Cl (Pd1) and Pd(L2)Cl (Pd2) were synthesized
and characterized by UV-visible, IR, ESI-MS, and NMR spectral studies. The molecular structures of Pd1
and Pd2 were determined by X-ray crystallographic studies. Palladacycle catalyzed Suzuki–Miyaura cross-
coupling reactions were investigated utilizing the derivatives of phenylboronic acids and substituted
chlorobenzenes. Mechanistic investigation authenticated the generation of Pd(0) nanoparticles during the
catalytic cycle and the nanoparticles were characterized by XPS, SEM and TEM analysis. Direct C–H
arylation of thiazole and isoxazole derivatives employing these ferrocenyl-palladacycle complexes was
examined. The reaction model for the arylation reaction implicating the in situ generation of Pd(0) nano-
particles was proposed.
Received 26th August 2019,
Accepted 16th October 2019
DOI: 10.1039/c9dt03465j
rsc.li/dalton
site plays a significant role in thermal stability, potential
hemilability and catalytic activity.5 Along the same lines, the
Introduction
Over the past few decades, ferrocene has been the most use of the palladacycles of ligands having (N,N) donor groups
employed metallocene due to its unique electrochemical pro- present in an arm of ferrocene is limited and, to the best of
perties and its applications in medicinal chemistry.1 On the our knowledge, their catalytic applications have not been
other hand, ferrocene-based co-ordination complexes have reported in the literature.6
been utilized for molecular recognition and as cytotoxic
In fact, this class of ligands were utilized for the chelation
agents.2 Cyclometalated organometallic complexes based on assisted C–H bond functionalization of the C2 position of
6-phenyl-2,2′-bipyridine (NNC) have been widely utilized for ferrocene derivatives.7 Several transition metals like Ir, Rh, and
their catalytic and luminescence properties.3 Reports on ferro- Pd have been used for the activation of the C2–H of the ferro-
cenyl based NNC ligands and their metal complexes are scarce cene moiety and for the synthesis of ferrocene derivatives.8
in the literature; however, ferrocene based ligands having a
The Suzuki–Miyaura cross-coupling (SMC) reaction is the
combination of phosphorus, nitrogen and sulfur are known.4 traditional method for C–C bond formation. Palladium com-
The presence of a Pd–C σ-bond as the central or exterior donor plexes derived from phosphine-based ligands are the conven-
tional ligands for the SMC coupling reaction.9 At present
organometallic pincer type (NNC or NCN) palladium(II) com-
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667,
plexes with different types of donor atoms are widely employed
Uttarakhand, India. E-mail: ghoshfcy@iitr.ernet.in; Fax: +91-1332-273560;
as catalysts for coupling reactions.10 The use of ferrocene
Tel: +91-1332-275547
based organometallic complexes in such cross-coupling reac-
tions is less explored yet.11 Though the SMC reactions of aryl
bromides and iodides are available, reports on aryl chlorides
are not abundant due to the difficulty in the activation of C–Cl
bonds compared to C–Br or C–I bonds with low catalyst
loading (0.0001 mol%).12
bDepartment of Polymer and Process Engineering, Indian Institute of Technology
Roorkee, Saharanpur Campus, Saharanpur, UP 247001, India.
E-mail: pradip.fpt@iitr.ac.in
†Electronic supplementary information (ESI) available: Experimental section,
NMR, MS, and UV-visible spectral data and information on DFT calculations.
X-ray crystal structure data have been deposited in the Cambridge
Crystallographic Data Centre and the deposition number for complex Pd1 is
CCDC 1939017 and for complex Pd2 is CCDC 1939018. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/C9DT03465J
Compared to well-recognized traditional cross-coupling
reactions, the palladium-catalyzed direct arylation via C–H
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bond activation is observed to be more reliable and environ- Synthesis of palladacycles
mentally friendly.13 Direct arylation of isoxazoles and thiazoles
Palladium complexes Pd(L1)Cl (Pd1) and Pd(L2)Cl (Pd2) were
synthesized using the Na2[PdCl4] metal salt and neutral
ligands L1H and L2H in an equimolar ratio in methanol
(shown in Scheme 1). Coordination of ligands with the metal
was analyzed primarily based on UV-visible, IR, and NMR spec-
tral studies. The absorption spectra of all the complexes were
recorded in dichloromethane at room temperature (Table S1
and Fig. S1†). The absorption bands of L1H near 332, 290 and
251 nm were assigned to n → π* and π → π* transitions of the
ligand. The absorption band of L1H near 484 nm was assigned
to ligand-to-metal (iron) charge transfer (LMCT) transitions.20
Complex Pd1 showed a broad band near 532 nm due to ligand
to metal (palladium) charge transfer (LMCT) transitions
(shown in Fig. S1†). In the case of L2H, the peaks near 255 and
325 nm were assigned to intra-ligand n → π* and π → π* tran-
sitions and that at 435 nm was assigned to ligand to metal
(iron) charge transfer transitions. Complex Pd2 showed a
broad band near 535 nm due to the ligand to metal (palla-
dium) charge transfer (LMCT) transition (Fig. S1†).20 During
IR spectral studies coordination of nitrogen to the metal
center resulted in a shift in ν(–HCvN). For complex Pd1 the
stretching frequency of the ν(–HCvN) shift was around 35 cm−1
and for complex Pd2 a shift of 10 cm−1 was observed (shown
in Fig. S2–S5†).20 The complexes are diamagnetic and dis-
played well-resolved 1H-NMR and 13C-NMR spectra in d6-
DMSO solution (shown in ESI Fig. S8–S12†). The 1H-NMR
signal of the imine of Pd1 was shifted from ∼7.07 ppm to
∼7.10 ppm and for Pd2 the doublet proton of pyridine hydro-
gen shifted from ∼8.68 to 9.06 ppm due to complexation.19
provides important molecules having potential use in medic-
inal chemistry.14 The function of supporting ligands in such
arylation reactions has been proved to play a significant role
during the catalytic process. Among the ligands reviewed, it is
important to note that phosphine-free ligands, particularly
nitrogen-based ligands, have generated considerable interest
due to their air stability and high catalytic performance. In the
literature 1,10-phenanthroline (phen)/Pd(OAc)2 could efficien-
tly catalyze the direct C–H arylation reaction of imidazoles
with aryl halides to give regioselective products.15 Palladium
complexes derived from nitrogen-based ligands facilitate
reductive elimination steps in direct arylation reactions.14
Moreover, organometallic palladium complexes could not only
facilitate reductive elimination steps but also generate Pd(0)
nanoparticles during the catalytic reactions.16 Very few reports
have described direct arylation via sp2 (C–H) bond activation
catalyzed by in situ generated Pd(0) nanoparticles.17
In the present study, we have designed and synthesized
ferrocene-based organometallic palladium complexes. We
would like to mention here that the iron atom in the metalloli-
gands containing ferrocene arms decreases their donor and/or
increases their acceptor properties, which in turn causes a
change in the catalytic activity of the C-coordinated Pd
complexes.11,16b These complexes were utilized for SMC cross-
coupling reactions. During our investigation of the mechanism
of the catalytic process, the generation of Pd(0) nanoparticles
was indicated. In situ generated Pd(0) nanoparticles were ana-
lyzed by field-emission scanning electron microscopy
(FE-SEM), transmission electron microscopy (TEM) and X-ray
photoelectron spectroscopy (XPS). We have investigated direct
arylation via sp2 (C–H) bond activation and performed aryla-
tion reactions of isoxazoles and thiazoles with different aryl
halides.
1
The H-NMR signal of the –CH2– singlet proton shifted from
Results and discussion
Synthesis of ligands
Ligands (L1H and L2H) were synthesized by the condensation
of ferrocenecarboxaldehyde, 2-(1-phenylhydrazinyl)pyridine18
and 2-((1-phenylhydrazinyl)methyl)pyridine19 in an equimolar
ratio respectively. After 3 hours deep red colored precipitates
of ligands L1H and L2H were obtained and characterized by
1
UV-visible, IR and NMR spectroscopy. The H-NMR spectra of
L1H and L2H showed one doublet at around ∼8.0 ppm due to
the adjacent proton of pyridine18,19 and for L2H one singlet
was observed at ∼4.0 ppm due to the presence of the –CH2–
proton.16 The 1H-NMR peaks confirmed the formation of
Schiff bases L1H and L2H, respectively. The ESI-mass spectral
study was performed for both the ligands in the acetonitrile
solution. Ligands L1H and L2H showed peaks at m/z =
382.0305 (for the [L1H]+ ion) and m/z = 395.1137 (for the
[L2H]+ ion) respectively which authenticate the formation of
Schiff bases (spectra are shown in ESI Fig. S15 and S16†).
Scheme 1 Schematic representation of synthesized complexes Pd1 (a)
and Pd2 (b).
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5.22 ppm to 5.75 ppm due to the metal–ligand coordination of
Pd2. In the NMR spectral splitting of ferrocene, protons were
observed due to carbon–metal bond formation.17 ESI-MS ana-
lysis of Pd1 and Pd2 was performed in the acetonitrile solution
and m/z peaks at 527.0384 [Pd1 + CH3CN–Cl]+ for Pd1 and at
534.9662 [Pd2 + H−]+ for Pd2 were recorded. Spectra are shown
in ESI Fig. S14–S16.†
Crystal structures of Pd1 and Pd2
The single crystals of Pd1 and Pd2 suitable for X-ray diffraction
were grown by slow evaporation in a dichloromethane and
methanol (1 : 3) mixture. The crystal structure data and refine-
ment parameters for complexes Pd1 and Pd2 are given in ESI
Table S2.† Selected bond distances and bond angles are shown
in ESI Tables S3 and S4.† The ORTEP diagrams (30% prob-
ability of ellipsoids) of complexes Pd1 and Pd2 are shown
in Fig. 1 and 2. The Pd(1)–N(1) bond lengths of Pd1 and Pd2
are 2.125(5) and 2.155(4) Å respectively which are higher
than those reported in our previous report,19c however, con-
sistent with those reported by Sokolov and co-workers.21 The
Fig. 3 Ball-and-stick representation of intermolecular halogen-
bonding and C–H⋯π interaction in infinite chains. Hydrogen atoms are
omitted for clarity. Color code: carbon, black; nitrogen, blue; chlorine,
light green, Pd, white; and XBs: dotted greenish-blue.
Pd(1)–N(3) bond distances of Pd1 and Pd2 are 1.986(5)(Å) and
2.015(6) (Å), respectively, which are higher than the literature
values.16b,19c The Pd(1)–C(11) bond lengths of Pd1 and Pd2 are
1.968(6) and 1.972(4) Å respectively which are consistent with
those reported by Singh and co-workers and Sokolov and co-
workers.16b,21 The N3–Pd1–Cl1 and C11–Pd1–N1 bond angles
of complex Pd1 are 176.48(15)° and 160.27(22)° respectively
which show distorted square planar geometry and are in good
agreement with theoretical values. C–H⋯Cl secondary inter-
actions result in the formation of chains as shown in Fig. 3.
Fig. 1 The ORTEP diagram (30% probability level) of complex Pd1.
Hydrogen atoms are omitted for clarity.
Suzuki–Miyaura reaction
The catalytic activity of complexes Pd1 and Pd2 was examined
towards the Suzuki–Miyaura reactions of aryl chloride deriva-
tives with phenylboronic acids. Reports to date on pallada-
cycles as powerful catalysts for Suzuki–Miyaura reactions have
mainly focused on phosphorus, sulfur, nitrogen and selenium-
containing structures, including N-heterocyclic carbene (NHC)
compounds.16b,21–24 With Pd1 as the precatalyst, the coupling
reaction between 4-chlorobenzaldehyde (1a) and phenyl-
boronic acid (2a) chosen as the model reaction is shown in
Scheme 2. After careful optimization of the reaction conditions
[1,1′-biphenyl]-4-carbaldehyde (3) was isolated and its yield
was 95% in DMF : H2O (2 : 1) at 80 °C when K2CO3 was utilized
as the base as shown in Scheme 2 and Table 1.
Initially, the catalytic reactions studied with various bases
such as KOH, NaOH, K2CO3, Na2CO3, CH3COONa, KtOBu and
Fig. 2 The ORTEP diagram (30% probability level) of complex Pd2.
Hydrogen atoms are omitted for clarity of the structure.
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Pd2. Both electron-withdrawing phenylboronic acids (4-Cl and
4-F) react with 4-COCH3 and 4-CN chlorobenzenes to give pro-
ducts in very good to excellent yields (Table 2, 3ba, 3bb, 3ca
and 3cb). The reactions of heteroarenes 3-chloropyridine and
2-chloroquinoline with (4-Cl and 4-F) phenylboronic acids also
provided cross-coupled products in very good to excellent
yields (Table 2, 3bc, 3bd, 3cc and 3cd). Interestingly, 3-NO2
chlorobenzene smoothly coupled with (4-Cl and 4-F) phenyl-
boronic acids to give products in excellent yields (Table 2, 3be
and 3ce). Both electron-withdrawing phenylboronic acids (4-Cl
and 4-F) react with 2-NO2 chlorobenzene to give products in
moderate yields (Table 2, 3bf and 3cf).
According to the literature, the best results for coupling of
aryl chlorides were achieved with palladium complexes derived
from phosphorus-containing pincer complexes or with palla-
dacycles modified with carbenes.11,21–23 Our results are com-
parable with the data reported by Richards and coworkers.11a
Nevertheless our catalysts are more active than other catalysts
reported in the literature.21,23,24
Scheme 2 Reaction optimization for the Pd-catalyzed Suzuki–Miyaura
reaction.
Table 1 Optimization of solvents and bases in the presence of Pd1
Entry
Base
Solvent
%Yield
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
NaOH
Na2CO3
NaOAc
KOAc
K3PO4
NaHCO3
KtOBu
Et3N
K2CO3
K2CO3
Toluene : water (2 : 1)
Ethanol : water (2 : 1)
Water
DMF : water (2 : 1)
DMSO : water(2 : 1)
Benzene : water (2 : 1)
NMP : water
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
DMF : water (2 : 1)
81
78
25
95
91
82
89
61
65
95
74
75
65
55
59
74
NRa
NRb
9
10
11
12
13
14
15
16
17
18
Probable reaction pathway
During SMC reactions catalyzed by Pd1 and Pd2, the solution
turned black immediately after mixing the reactants.16 We
accomplished several experiments to recognize the catalytically
active species in our reaction. We speculate that the active cata-
lytic species was not the complex, but rather some in situ gen-
erated species in the reaction mixture. We performed X-ray
photoelectron spectroscopic (XPS) analysis for investigating
the surface compositions of the palladium species (shown in
Fig. 4). The 3d core-level lines of Pd1 fitted with two main
peaks, where Pd 3d5/2 and 3d3/2 peaks at 342.7 eV and 337.4 eV
Reaction conditions: Pd1 (0.0001 mol%), 2a (1.0 mmol), 3a
(1.2 mmol), base (2 mmol), solvent (4.5 mL), 80 °C, 4 h. a Only the
ligand (0.01 mol%). b Only in the presence of (0.01 mol%) Na2[PdCl4].
Et3N, where K2CO3 and Na2CO3 provided a coupled product in respectively were assignable to the presence of the Pd(II)
a yield of 95% (isolated yield), are summarized in Table 1. The species.11b,23,24 After treatment with bases and substrates in
SMC reaction was very limited in neat water due to the insolu- the presence of the Pd1 catalyst for 4 h, the peaks shifted from
bility of the catalyst (Table 2, entry 3). This problem was over- 342.7 to 343.0 eV (for Pd 3d5/2) and from 337.4 eV to 337.8 eV
come by adding organic solvents to dissolve the catalyst in the (for Pd 3d3/2), clearly indicating the presence of Pd(II) species
reaction medium. Control experiments revealed that the SMC in the reaction mixture.24 However, the two new peaks at 342.5
reaction does not proceed in the presence of only ligand (L1H) eV (for Pd 3d5/2) and 336.4 eV (for Pd 3d3/2) indicated the pres-
(Table 1, entry 17). Commonly available Na2[PdCl4] was also ence of Pd(0) species.11b,23,24 Hence, we proposed that the Pd(II)
found to be ineffective (Table 1, entry 18).
complex was converted to some new species containing Pd(0),
We further scrutinized the substrate scope of the reactions which was important for catalytic activity. To recognize the
of various haloarenes with organoboronic acids in the pres- species containing Pd(0) we performed FE-SEM and TEM ana-
ence of complexes Pd1 and Pd2. The reactions gave good lysis. Electron microscopic data clearly showed the formation
results for activated aryl chlorides (Table 2), and the TON was of palladium nanoparticles in the solution mixture. From the
up to 95 × 104 (Table 2, 3aa–3ae and 3al–3ap). Unactivated size distribution curve, the average size of these nanoparticles
hetero-chloroarenes also worked using complexes Pd1 and Pd2 was estimated to be 3–4 nm (shown in Fig. 5a and b). The
but with moderate yields of up to 65% to 78% (Table 2, 3af– SAED pattern revealed the polycrystalline nature of palladium
3ah, 3aq–3as, 3au and 3av). These catalysts were also utilized nanoparticles (shown in Fig. 5d). Due to the generation of
for the sequential cross-coupling reaction of 2,6-dichloropyri- high surface area Pd(0) exhibited excellent catalytic activity.
dine which afforded 2,6-diphenylpyridine and 2,6-bis(4-ethyl- Hence, this experiment showed that Pd1 is the pre-catalyst of
phenyl)pyridine in moderate yields of 65%–71% (Table 2, 3ah this catalytic cycle. For obtaining a better insight into the cata-
and 3as). Sterically hindered 1-naphthyl chloride resulted in lytic cycle, we performed the Hg poisoning test. Under opti-
yields of 85% to 91% (Table 2, 3ak and 3at).
mized reaction conditions, we obtained [1,1′-biphenyl]-4-car-
We next explored the substrate scope of the reactions of the baldehyde as a coupled product in 20% yield after 4 h in the
electron-withdrawing phenylboronic acids (4-Cl and 4-F) with presence of 1 drop of Hg. This reaction confirms that palla-
different haloarenes in the presence of complexes Pd1 and dium complex Pd1 is reduced under thermal conditions to
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Table 2 Substrate scope for the Pd-catalyzed Suzuki–Miyaura coupling of chlorobenzene derivatives and aryl boronic acids
Reaction conditions: 1 (0.0001 mol%), 2a (1.0 mmol), 3a (1.2 mmol), base (2 mmol), solvent (4.5 mL), 80 °C, 4–8 h. a PhB(OH)2 2.4 mmol, catalyst
loading (0.0002 mol%) and K2CO3 4 mmol.
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Fig. 4 The XPS spectra of the 3d level of palladium and deconvolution peaks of the Pd species: deconvoluted (red), Pd(II) (magenta and dark
yellow), Pd(0) (blue), (a) precatalyst Pd1 and (b) precatalyst Pd1 after treatment with K2CO3, [1,1’-biphenyl]-4-carbaldehyde and bromobenzene in
DMF:H2O at 80 °C after 4 h.
Fig. 5 (a) The SEM image of NPs obtained from catalyst Pd1. (b) The TEM image of NPs obtained respectively during SMC (scale bar 50 and 20 nm).
(c) EDX analysis of NPs obtained during SMC. (d) The SAED pattern of NPs obtained during SMC. (e) The particle size distribution of spherical Pd
nanoparticles obtained during the SMC reaction.
form the palladium(0) species or cluster, followed by oxidative intermediate Pd(II). Then, reductive elimination gives rise to
addition of haloarenes to the palladium(0) species.10e Then, the coupled product and regenerates the active species
transmetalation occurs with phenylboronic acid to give rise to (Scheme 3).9,20
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Scheme 3 The proposed catalytic cycle.
Palladium-catalyzed direct C–H arylation reaction under
aerobic conditions
zoles or isoxazoles with aryl halides. Thus, the reaction of thia-
zole and bromobenzene was studied and the reaction con-
ditions such as catalyst loading, temperature, reaction time,
and base were optimized (Scheme 4 and Fig. 6). The reaction
was performed in the presence of various concentrations of
palladium catalyst Pd1, and it was found that the 0.1 mol%
catalyst was efficient to complete the transformation of
4-methylthiazole. Lowering the catalyst loading to 0.02% and
0.05% led to a decrease in the conversion of 4-methylthiazole.
This reaction was also screened at various temperatures, reveal-
ing that the performance of the catalyst was best in the temp-
erature range of 140–160 °C (shown in Fig. 6).
Arylated thiazoles and isoxazoles are known to exhibit a variety
of interesting biological properties25 and they can be syn-
thesized via the transition metal-catalyzed arylation of thia-
After the optimization of catalyst loading and temperature,
Scheme 4 Screening of reaction conditions for the direct arylation the catalytic reaction was monitored at different time intervals
reaction of 4-methylthiazole with bromobenzene.
in the presence of bases. From these studies, screening the
Fig. 6 The reaction profile of palladium-catalyzed (Pd1) direct arylation of 4-methylthiazole with aryl bromides. (a) The effect of catalyst loading.
(b) The effect of temperature. (c) The effect of time. (d) The effect of the amount of pivalic acid. All the optimization reactions are done three times.
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effect of bases on this reaction revealed that KF, NatOBu, KtOBu 6b–6d, 6g, 6k, and 6l). Substrates with electron-donating
and KOAc were inefficient and led to lower conversion of groups formed the desired coupled products in moderate
4-methylthiazole. The use of bases such as NaHCO3 and K3PO4 yields of up to 75–81% (shown in Table 3, 6b–6d, 6g, 6k, and
resulted in better conversion of 4-methylthiazole and higher 6l). Heteroaryl bromide 3-bromopyridine also resulted in
conversion was observed in the case of K2CO3 and Na2CO3 88–92% yield (Table 3, 6h) when Pd1 and Pd2 were used as cat-
(Table S7†). This reaction was also investigated with various sol- alysts. Sterically hindered 1-naphthyl bromide resulted in a
vents in the presence of catalyst Pd1 (Table S7†), which revealed yield of 86% to 92% (Table 3, 6m). In the case of meta-elec-
that the use of dimethylacetamide (DMA) as a solvent led to the tron-donating haloarene the yield was 70% to 75% (Table 3,
formation of the product in 94% yield. Control experiments 6i); however, in the case of meta-nitro bromobenzene a better
showed that no product was obtained in the absence of catalyst yield of 75% to 81% was achieved (Table 3, 6j). This is due to
Pd1 or in the presence of only a ligand and a base (Table S7,† the steric influence and electronic effects. The catalytic per-
entry 16). Other palladium sources like Na2PdCl4 also failed to formances of Pd1 and Pd2 are comparable, but Pd2 gives rise
produce alkylated products under the optimized conditions to better yields due to the flexibility of the methylene (–CH2–)
(Table S7,† entry 17). However, low conversion occurs in the group. The catalytic efficiencies of Pd1 and Pd2 were compared
absence of PivOH acid (Table S7,† entry 18).
with palladium(II) complexes described previously. Liu and co-
Several other halides were reacted under these optimized workers reported camphyl-based α-diimine palladium com-
conditions. Substituted bromobenzene reacted smoothly plexes for the arylation of thiazoles at 2 mol% loading, which
resulting in the arylation of 4-methylthiazole in very good is greater than those of Pd1 and Pd2.26 Recently the same
yields. It was observed that aryl halides with different func- group has synthesized bulky bis(imino)acenaphthene (BIAN)-
tional groups such as chloro, fluoro, acetyl, cyano, aldehyde supported Pd PEPPSI complexes for the direct arylation of
and nitro gave excellent yields of 88–95% (shown in Table 3, azoles, which are comparable with Pd1 and Pd2.27
Table 3 The palladium-catalyzed direct arylation of 4-methylthiazole with aryl bromides
Reaction conditions: Arene (1.2 mmol), aryl bromide (1 mmol), palladium complexes (0.1 mol%), PivOH (0.3 mmol), K2CO3 (2 mmol) and DMA
(2 mL), 140 °C for 10 h in an aerobic environment. a Instead of 4-bromobenzaldehyde, 4-bromo acetophenone and 4-bromobenzonitrile, reaction
was done with 4-chlorobenzaldehyde, 4-chloroacetophenone and 4-chlorobenzonitrile respectively, catalyst loading 0.2 mol%.
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Other heterocycles like isoxazole derivatives also reacted
efficiently with electron-donating and electron-deficient bro-
mides to give the corresponding coupling products. Sterically
hindered 3,5-dimethylisoxazole could be coupled efficiently
with chloro, aldehyde, fluoro, acetyl, cyano, and nitro groups
which gave excellent yields of 78% to 91%. Sterically hindered
1-naphthyl bromide resulted in yields of 79% to 85% (Table 4).
Doucet and co-workers established the ligand-free arylation
of 3,5-dimethylisoxazole with a catalyst loading of 0.1 mol%
to 0.001 mol% which is comparable to Pd1 and Pd2.28,29 Liu
and co-workers reported palladium complexes as catalysts
for arylation of sterically hindered 3,5-dimethylisoxazole at
0.1 mol% loading, which is comparable with Pd1 and Pd2.27,30
Probable mechanism
The reaction pathway was monitored by X-ray photoelectron
spectroscopic (XPS) analysis. After treatment with the base and
the substrate in the presence of Pd1, for 8 h, we performed XPS
analysis which showed that the major Pd 3d5/2 peak shifted to
a binding energy of 342.8 eV and the 3d3/2 peak shifted to
337.7 eV which were assigned to other Pd(II) species. In
addition, two new peaks appeared at 341.8 eV and 336.5 eV
which were assigned to Pd 3d5/2 and Pd 3d3/2 respectively of the
Pd(0)11b,23,24 species (shown in Fig. 7). These peaks confirm
that both Pd(II) and Pd(0) species are present during the cata-
lytic cycle similar to our results obtained during the SMC reac-
Fig. 7 The XPS spectra of the Pd 3d level and deconvolution peaks of
the Pd species: experimental deconvoluted (red), Pd(II) (magenta and
dark yellow), and Pd(0) (blue) precatalyst Pd1 after treatment with
K2CO3, 4-methylthiazole and bromobenzene in DMA at 140 °C for 8 h.
tion described previously. We next performed the TEM analysis
of the reaction mixture of arylation of 4-methylthiazole. The
generation of palladium nanoparticles was observed in the
reaction (Fig. 8). The average size of these nanoparticles was
estimated to be 10–12 nm. The SAED pattern of the reaction
Table 4 The palladium-catalyzed direct arylation of 3,5-dimethylisoxazole with aryl bromides
Reaction conditions: Arene (1.2 mmol), aryl bromide (1 mmol), palladium complex (0.1 mol%), PivOH (0.3 mmol), K2CO3 (2 mmol) and DMA
(2 mL), 140 °C for 10 h in an aerobic environment.
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Fig. 8 (a) The TEM image of NPs obtained during arylation of 4-methylthiazole (scale bar 100 nm). (b) The SAED pattern of NPs obtained during
arylation of 4-methylthiazole. (c) The particle size distribution of spherical Pd nanoparticles obtained during arylation of 4-methylthiazole.
mixture of arylation of 4-methylthiazole shows the polycrystal-
line nature of palladium nanoparticles. Based on the data
obtained from XPS and TEM analysis, we speculate on the
probable reaction pathway for the direct arylation of thiazoles
and oxazoles. These experimental results showed that complex
Pd1 is not the actual catalytically active species in this reaction.
Palladium complex Pd1 under thermal conditions form the pal-
ladium(0) species or palladium(0) clusters in equilibrium.15a
Due to the in situ formation of the Pd(0) species or the Pd(0)
cluster and the high surface area, Pd1 shows excellent catalytic
activity. The oxidative addition of haloarenes gives rise to inter-
mediate II. Then ligand exchange occurs with potassium salts
of pivalic acid to give intermediate III. The C–H bond cleavage
step of 4-methylthiazole using pivalate generates intermediate
IV via the CMD transition state,16a,30,31 which undergoes reduc-
tive elimination furnishing the arylated product and regenerat-
ing the Pd(0) intermediate (I) (Scheme 5).
Homogeneous and heterogeneous tests
The SMC reaction of 4-chlorobenzaldehyde with phenylboronic
acid using catalyst Pd1 in the presence of mercury and PPh3
under optimum conditions gives rise to 20% and 37% con-
verted products.
The direct C–H arylation reaction of 4-methylthiazole with
bromobenzene catalyzed by Pd1 in the presence of mercury Scheme 5 The proposed catalytic cycle.
17092 | Dalton Trans., 2019, 48, 17083–17096
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and PPh3 under optimum conditions produces cross-coupled India. Infrared spectra were recorded as KBr pellets on a
products in 17% and 31% yields. These results show the possi- Nicolet NEXUS Agilent 1100 FT-IR spectrometer, using 50
bility of the existence of homogeneous Pd(0) species.22 scans, and the frequencies were reported in cm−1. Electronic
Furthermore, to establish the nature of the catalysts, hot fil- spectra were recorded in CH2Cl2 and CH3OH with an
tration experiments were performed. In the case of the SMC Evolution 600, Thermo Scientific UV-visible spectrophotometer
reaction under standard conditions in the presence of Pd1 using cuvettes of 1 cm path length. A field emission scanning
after 1 h, the reaction mixture was filtered. Then the filtrate electron microscope (MIRA3 TESCAN, USA) at an accelerating
(reaction mixture) was further continuously stirred under the voltage of 3–10 kV was used for the morphological characteriz-
same reaction conditions. After 4 h the maximum conversion ation of the catalytically active species. Transmission electron
dropped from 94% (4 h) (standard conditions) to 63% (4 h). microscopy (TEM, Tecnai G2 20SeTWIN, FEI Netherlands) was
This experiment showed that some of the palladium was used for the morphological characterization of the catalytically
leached from in situ generated NPs. These results suggest that active species. An X-ray photoelectron spectrophotometer (XPS;
the catalytic process is essentially homogeneous.10d,11b The ULVAC – PHI, INC, Japan) was used for oxidation state calcu-
direct arylation of 4-methylthiazole with bromobenzene in the lations and surface morphology determination. 1H-NMR and
presence of Pd1 under standard conditions resulted in a 13C-NMR spectra were recorded with
a Bruker AVANCE
decrease in conversion from 80% to 59% (standard con- 500.13 MHz spectrometer and a JEOL 400 MHz spectrometer,
ditions). These results suggest that the catalytic process is and chemical shifts were reported in δ (ppm) units relative to
essentially homogeneous.
tetramethylsilane (TMS) as the internal standard. Proton coup-
ling patterns are described as singlet (s), doublet (d), triplet
(t), quartet (q), multiplet (m), and broad (br). ESI-MS experi-
ments were performed on a Brüker micrOTOFTM-Q-II mass
spectrometer.
Conclusions
In summary, we have successfully synthesized and character-
ized ferrocenyl Schiff bases and organometallic palladium com-
X-ray crystallography
plexes Pd1 and Pd2. The molecular structures of Pd1 and Pd2 Yellow crystals of Pd1 and Pd2 were obtained by slow evapor-
were authenticated by single crystal X-ray diffraction. Air and ation of solutions of the complexes in acetonitrile. X-ray data
moisture-stable complexes Pd1 and Pd2 effectively catalyzed collection and processing for complexes were performed on a
the Suzuki–Miyaura cross-coupling reaction with a TON of up Bruker Kappa Apex-II CCD diffractometer by using graphite
to 95 × 104. Investigation of the mechanism indicated that the monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 296 K for
palladium complexes acted as pre-catalysts and generation of Pd1 and at 293 K for Pd2. Crystal structures were solved by
Pd(0) nanoparticles was proposed; Pd(0) nanoparticles were direct methods. Structure solutions, refinement and data
characterized by XPS, SEM, and TEM analysis. Complexes Pd1 output were carried out with the SHELXTL program.32–34 All
and Pd2 were found to be efficient catalysts for direct regio- non-hydrogen atoms were refined anisotropically. Hydrogen
selective Csp2–H functionalization reactions between different atoms were placed in geometrically calculated positions and
heteroarenes (4-methylthiazole and 3,5-dimethylisoxazole) and refined using a riding model. Images were created with the
aryl halides under aerobic conditions with a TON of up to 9.6 × DIAMOND program.35
102. These reactions showed excellent functional group toler-
ance and a wide range of aryl bromides and various azoles were
Synthetic procedures
Synthesis of L1H. A methanolic (10 mL) solution of 2-pyridyl
utilized. The generated Pd(0) nanoparticles were also found to
be the active catalyst in these reactions; however, their size and phenylhydrazine (0.370 g, 2.0 mmol) was placed in a 50 mL
shape were different from our previous results obtained for round bottom flask and stirred at room temperature for 5 min.
Suzuki–Miyaura cross-coupling reactions. Studies on other cata- Ferrocene carboxaldehyde (0.428 g, 2.0 mmol), also dissolved
lytic activities of these interesting organometallic palladium in methanol (10 mL), was added dropwise with stirring. Then
complexes are under progress.
the reaction mixture was stirred for another 3 h. A red color
solid was filtered off and washed with cold methanol. Yield:
585 mg 77% (L1H).
Selected IR data: (KBr, ν/cm−1): 1585 (νCvN) UV-visible
[CH2Cl2, λmax/nm (ε/M−1 cm−1)]: 484(3950), 335(22 850), 290
Experimental
The reported methods were used for the synthesis of 2-pyridyl (20 850), 255(23 950). Anal. calcd for C22H19FeN3: C, 69.31; H,
phenylhydrazine18 and 2-picolylphenylhydrazine.19 Aryl bro- 5.02; N, 11.02; found: C, 69.13; H, 5.12; N, 11.15. 1H-NMR
mides and chlorides, phenylboronic acids, and K2CO3 were (400 MHz, δ/ppm, CDCl3): 8.10 (d, J = 4.7, 1H); 7.61 (t, J = 3.9,
procured from Avra, TCI Chemicals (India) Pvt. Ltd. 8.0 Hz, 4H); 7.47 (t, J = 7.4 Hz, 1H); 7.24 (s, 2H); 7.07 (s, 1H);
Ferrocenecarboxaldehyde was purchased from Sigma-Aldrich 6.75–6.72 (m, 1H), 4.55 (s, 2H), 4.30 (s, 2H), 4.11 (s, 5H).
(USA). Solvents used for spectroscopic studies were of HPLC 13C-NMR (400 MHz, δ/ppm, CDCl3): 158.5; 147.6; 139.4; 138.5;
grade and purified by standard procedures before use. 137.5; 130.5; 129.9; 128.4; 115.3; 109.5; 81.1; 69.6; 69.1; 67.1.
Palladium chloride was obtained from Arora Matthey, Kolkata, ESI-MS (CH3CN): m/z = 382.0305 for the [L1H]+ ion.
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Synthesis of L2H. A methanolic (10 mL) solution of 2-picolyl- acetate (2 × 10 mL). The combined extract was washed with
phenylhydrazine (0.370 g, 2 mmol) was placed in a 50 mL water and dried over anhydrous Na2SO4. The solvent of the
round bottom flask and stirred at room temperature for 5 min. extract was removed under reduced pressure with a rotary evap-
Ferrocene carboxaldehyde (0.428 g, 2.0 mmol), also dissolved orator to obtain the product. The crude products were purified
in methanol (10 mL), was added dropwise with stirring. Then by column chromatography on silica gel using hexane/ethyl
the reaction mixture was stirred for another 3 h. A red color acetate as an eluent. The isolated cross-coupled products were
solid was filtered off and washed with cold methanol. Yield: authenticated by 1H- and 13C-NMR spectra displayed in
596 mg 75% (L2H). Selected IR data: (KBr, ν/cm−1): 1594 Fig. S22–S85 in the ESI.†
(νCvN) UV-visible [CH2Cl2, λmax/nm (ε/M−1 cm−1)]: 439(1588),
General procedure for the arylation reaction
324(29 530), 252(17 540). Anal. calcd for C23H21FeN3: C, 69.89;
1
H, 5.35; N, 10.63; found: C, 69.73; H, 5.47; N, 10.57. H-NMR An oven-dried flask (10 mL) was charged with 4-methylthiazole
(400 MHz, δ/ppm, CDCl3): 8.67 (d, J = 4.5, 1H); 7.63 (t, J = 7.6, (1.2 mmol) or 3,5-dimethylisoxazole (1.2 mmol), an aryl halide
9.2 Hz, 1H); 7.36–7.29 (m, 4H); 7.23 (s, 2H); 7.15 (d, J = 7.8 Hz, (1.0 mmol), a base (2.0 mmol), an acid additive (0.30 mmol), a
1H); 6.93 (t, J = 8.0, 6.6, 1H), 5.22 (s, 2H), 4.55 (s, 2H), 4.25 (s, catalyst (0.1 mol%) and 2 mL of solvent. The flask was placed
2H), 3.99 (s, 5H). 13C-NMR (400 MHz, δ/ppm, CDCl3): 156.6; in a pre-heated oil bath at 140 °C under aerobic conditions,
149.8; 147.6; 137.0; 132.9; 129.1; 122.4; 120.7; 120.2; 114.2; and the reaction mixture was stirred. After completion of the
82.0; 69.1; 68.9; 67.7; 51.9(–CH2–). ESI-MS (CH3CN): m/z = reaction, the mixture was cooled to room temperature and
395.1137 for the [L2H]+ ion.
quenched by adding 10 mL of water. This mixture was
extracted with dichloromethane (3 × 15 mL). The combined
organic layer was washed with water and dried over anhydrous
Syntheses of complexes Pd1 and Pd2
0.10 mmol ligand (L1H: 0.038 g and L2H: 0.039 g) was stirred Na2SO4. The solvent of the extract was removed under reduced
in 40 mL of methanol for 15 min. Na2[PdCl4] (0.029 g, pressure with a rotary evaporator to obtain the product. The
0.10 mmol) was added to it. The mixture was stirred further crude products were purified by column chromatography on
for 6 h at 60 °C. The red solid formed was collected by fil- silica gel using hexane/ethyl acetate as an eluent. The isolated
tration and air-dried overnight to obtain complexes Pd1 and cross-coupled products were authenticated by 1H and 13C NMR
Pd2.
spectra displayed in Fig. S86–S137 in the ESI.† GC-MS mass
The single crystals of each of the two complexes were spectra of some respective direct arylation products are shown
obtained from a mixture of dichloromethane : methanol (1 : 3). in Fig. S138–S141.†
Complex Pd1. Yield: 35 mg 67%, Selected IR data: (KBr, ν/
XPS analysis. The sample for XPS analysis was prepared as
cm−1): 1620 (νCvN) UV-visible [CH2Cl2, λmax/nm (ε/M−1 cm−1)]: follows: after the Suzuki–Miyaura reaction of 4-chlorobenzalde-
529(2250), 371(5530), 285(14 420), 239(17 710). Anal. calcd for hye with phenylboronic acid in the presence of Pd1 under opti-
C22H18ClFeN3Pd: C, 50.61; H, 3.47; N, 8.05; found: C, 50.67; H, mized conditions, the reaction mixture was dropped onto a
1
3.35; N, 8.23. H-NMR (400 MHz, δ/ppm, DMSO): 7.18 (s, 1H); glass slide. Then the sample was dried under air. The resulting
6.79–6.72 (m, 4H); 6.66 (d, J = 5.8, 2H); 6.10 (s, 1H); 6.05 (t, J = sample was then used for XPS analysis.
5.0, 4.8, 1H); 3.48 (s, 1H); 3.36 (d, J = 8.5, 2H); 3.30 (s, 5H).
FE-SEM and TEM analyses. The sample for TEM analysis
13C-NMR (400 MHz, δ/ppm, DMSO): 157.7; 140.9; 135.2; 132.1; was prepared as follows: after the Suzuki–Miyaura reaction of
131.7; 130.2; 117.2; 115.0; 109.8; 90.73; 74.9; 70.76; 68.14; 67.8. 4-chlorobenzaldehye with phenylboronic acid in the presence
ESI-MS (CH3CN): 527.0384 for the [Pd1 + CH3CN–Cl]+ ion.
of Pd1, the reaction mixture was diluted with DMF and water.
Complex Pd2. Yield: 41 mg 76%, Selected IR data: (KBr, ν/ Then one drop was dropped onto a copper grid. Then the
cm−1): 1603 (νCvN) UV-visible [CH2Cl2, λmax/nm (ε/M−1 cm−1)]: sample was dried under air. The resulting sample was then
532(1460), 359(2620), 272(16 060), 244(21 800). Anal. calcd for used for TEM analysis. For FE-SEM the same technique was
C23H20ClFeN3Pd: C, 51.52; H, 3.76; N, 7.84; found: C, 51.41; H, followed except that instead of the copper grid, the sample was
1
3.69; N, 7.97. H-NMR (400 MHz, δ/ppm, DMSO): δ 9.05 (d, J = prepared on a glass slide. The same process was also followed
4.9 Hz, 1H), 8.72 (s, 1H), 7.91 (t, J = 8.4 Hz, 1H), 7.66 (d, J = 7.4 for the sample preparation from direct C–H arylation reaction
Hz, 1H), 7.43 (d, J = 7.0 Hz, 1H), 7.18 (t, J = 7.5 Hz, 2H), 7.11 (s, catalyzed by Pd1 for TEM analysis.
2H), 6.84 (t, J = 7.0 Hz, 1H), 5.75 (s, 2H), 4.86 (s, 1H), 4.60 (s,
Hg poisoning test. To carry out the Hg poisoning test, cata-
1H), 4.56 (s, 1H), 4.38 (s, 5H). ESI-MS (CH3CN): 534.9662 for lyst Pd1 (0.0001 mol%) was stirred with 1 drop of Hg in an
the [Pd2 + H−]+ ion.
oven-dried RB flask. After that, 4-chlorobenzaldehyde
(1.0 mmol), phenylboronic acid (1.2 mmol) and K2CO3
(2 mmol) were added to the flask and the reaction was carried
General procedure for the SMC reaction
The Pd(II) complex (0.0001 mol%) in DMF (3 mL) was added to out under optimum conditions. Reaction progress was moni-
a mixture of chloro substituted benzene (1.0 mmol), aryl tored by TLC. The reaction was further continued for 4 hours
boronic acid (1.2 mmol), and K2CO3 (2 mmol) in H2O (1.5 mL) at 80 °C. The same procedure was followed for direct C–H ary-
taken in a 20 mL round bottom flask. After stirring for 4–8 h at lation reaction catalyzed by Pd1, with 4-methylthiazole
80 °C, the reaction mixture was cooled and quenched by (1.2 mmol), an aryl halide (1.0 mmol), K2CO3 (2.0 mmol), an
adding 20 mL of water. The mixture was extracted with ethyl acid additive (0.30 mmol), catalyst Pd1 (0.1 mol%) and 3 mL of
17094 | Dalton Trans., 2019, 48, 17083–17096
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solvent at 140 °C. After standard workup of the reaction
mixture, % yield was determined using 1H-NMR.
5 A. Togni, C. Breutel, A. Schnyder, F. Spindler, H. Landert
and A. Tijani, J. Am. Chem. Soc., 1994, 116, 4062.
PPh3 poisoning test. In an oven-dried RB flask, catalyst Pd1
(0.0001 mol%) was stirred with PPh3 (5 mol%), 4-chlorobenzal-
dehyde (1.0 mmol), phenylboronic acid (1.2 mmol) and K2CO3
(2 mmol) and the reaction was carried out under optimum
conditions. Reaction progress was monitored by TLC. The reac-
tion was further continued for 4 hours at 80 °C. The same pro-
cedure was followed for direct C–H arylation reaction catalyzed
by Pd1, with 4-methylthiazole (1.2 mmol), an aryl halide
(1.0 mmol), K2CO3 (2.0 mmol), an acid additive (0.30 mmol),
catalyst Pd1 (0.1 mol%) and 3 mL of solvent at 140 °C. Then
5 mol% PPh3 was added and the reaction was carried out for
10 h under optimum conditions. After standard workup of the
reaction mixture, % yield was determined using 1H-NMR.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
KG thanks the of Scientific and Industrial Research, India,
New Delhi (01(2942)/18/EMR-II dated 01-05-2018) for financial
assistance. Ankur Maji, Anshu Singh, and Aurobinda Mohanty
also thank the MHRD for their fellowships. We thank Prof.
Marilyn Olmstead for her help in solving crystal structure.
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