A Bis (BICAAC) Palladium(II) Complex: Synthesis and Implementation as Catalyst in Heck-Mizoroki and Suzuki-Miyaura Cross Coupling Reactions

Article abstract of DOI:10.1021/acs.inorgchem.0c03614

Carbenes are one of the most appealing, well-explored, and exciting ligands in modern chemistry due to their tunable stereoelectronic properties and a wide area of applications. A palladium complex (BICAAC)2PdCl2 with a recently discovered cyclic (alkyl)(amino)carbene having bicyclo[2.2.2] octane skeleton (BICAAC) was synthesized and characterized. The enhanced σ-donating and π-accepting ability of this carbene lend a hand to form a robust Pd-carbene bond, which allowed us to probe its reactivity as a precatalyst in Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions with low catalyst loading in open-air conditions. The diverse range of substrates was explored for both the cross-coupling reactions. To get a better understanding of the catalytic reactions, several analytical techniques such as field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and powder X-ray diffraction were employed in a conclusive manner.

Full text of DOI:10.1021/acs.inorgchem.0c03614

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Article
A Bis (BICAAC) Palladium(II) Complex: Synthesis and
Implementation as Catalyst in Heck-Mizoroki and Suzuki-Miyaura
Cross Coupling Reactions
Soumyadeep Chakrabortty, Mandeep Kaur, Manu Adhikari, Krishna K. Manar, and Sanjay Singh*
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ABSTRACT: Carbenes are one of the most appealing, well-
explored, and exciting ligands in modern chemistry due to their
tunable stereoelectronic properties and a wide area of applications.
A palladium complex (BICAAC)2PdCl2 with a recently discovered
cyclic (alkyl)(amino)carbene having bicyclo[2.2.2] octane skeleton
(BICAAC) was synthesized and characterized. The enhanced σ-
donating and π-accepting ability of this carbene lend a hand to
form a robust Pd-carbene bond, which allowed us to probe its
reactivity as a precatalyst in Heck-Mizoroki and Suzuki-Miyaura
cross-coupling reactions with low catalyst loading in open-air conditions. The diverse range of substrates was explored for both the
cross-coupling reactions. To get a better understanding of the catalytic reactions, several analytical techniques such as field-emission
scanning electron microscopy, high-resolution transmission electron microscopy, and powder X-ray diffraction were employed in a
conclusive manner.
cross-coupling reactions.6 In 2003, Gregory et al. first reported
INTRODUCTION
■
the Sonogashira reaction between alkyl electrophile and alkyl
Transition-metal-catalyzed cross-coupling reactions are one of
bromide under ambient conditions supported by the Pd-NHC
the most efficient and versatile methods for the formation of
complex;11b subsequently, Nolan and co-workers in 2004
C−C, C−O, C−N, C−S, and C−B bonds. Particularly, the
demonstrated the reactivity of an “unusual” N-heterocyclic
case of C−C bond-forming processes such as Heck−Mizoroki
carbene palladium complex toward the C−C coupling reaction
(HM), Negishi, Suzuki−Miyaura (SM), Kumada, and Stille
using a 2 mol % catalyst loading.12 An elegant report by Organ
reactions catalyzed by Pd complexes have gained prominen-
and co-workers in 2006 on the Pd-PEPPSI complex as an
ce.1a Palladium complexes by far dominate this research area,
excellent catalyst for different cross-coupling reactions clearly
followed by other precious transition metals (Rh, Ru, Ir, Cu,
authenticated the potential and suitability of carbenes as a
Ni, and Fe).1b,f These advancements of transition-metal
spectator ligand in such reactions.10
catalysis have largely been facilitated by the rational design
In 2005, Bertrand and co-workers reported on cyclic
of supporting ligands. Typically, sterically demanding and
(alkyl)(amino)carbenes (cAACs), which exhibit improved
electronically rich phosphines,1g,l various pincer ligands,1m and
nucleophilicity and electrophilicity as compared to typical
stable heterocyclic carbenes have been employed as the
NHCs.13 The better σ-donation and π-acceptor properties of
supportive ligands.2,3 In this regard, the application of N-
cAACs make them a superior ligand for the stabilization of
heterocyclic carbene-palladium (NHC-Pd) complexes has
unstable chemical species14 and elements in their unusual
made a paradigm change in the homogeneous catalysis for
oxidation states,15 although relatively few cAAC-metal
C−C bond-forming reactions.
complexes were investigated for their catalytic applications as
compared to a plethora of catalysts based on NHC-metal
In 1995, Hermann and co-workers reported the first Pd-
NHC complexes as highly active and robust catalysts for cross-
complexes. Nevertheless, the cAAC-metal complexes of Ru for
coupling reactions.4 Since then, Pd-NHC complexes came into
olefin metathesis16 and Au complexes for the coupling of
the limelight, and subsequently groups of Caddick,5 Gade,6
Nolan,7 Beller,8 Herrmann,9 and Organ10 have made a
Received: December 13, 2020
Published: April 12, 2021
significant breakthrough in the Pd-carbene catalyzed C−C
coupling reactions. In fact, various mono- and bis-NHC-Pd
complexes were synthesized and found to have a potent
activity in homogeneous catalysis (Figure 1).11 In 2002, Gade
and co-workers synthesized a palladium complex of oxazolinyl-
carbene and exploited its catalytic application for HM and SM
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Figure 1. A selection of carbene palladium chloride complexes used in C−C bond-formation reactions.
enamines and terminal alkyne and intramolecular hydro-
amination of alkyne are some prominent examples.17 Recently,
Bertrand and co-workers introduced bicyclic (alkyl)(amino)-
carbenes (BICAACs), as new members to the family of stable
carbenes, that have a bicyclo[2.2.2]octane framework.18a The
greater electrophilic and nucleophilic nature of BICAAC in
comparison to that of CAAC and NHC is experimentally
supported by ligand exchange reactions at the metal center and
the lower CO frequency of the metal carbonyl complex of
BICAAC.18a Further, these electronic factors are confirmed
from a computational study, and the calculated singlet−triplet
energy gap for BICAAC is smaller (45.7 kcal/mol) than that of
cAAC (49.2 kcal/mol) and NHC (68 kcal/mol)18b,c indicating
that BICAAC has superior ambiphilic characteristics than
NHC and is marginally better than cAACs. Because of these
features, BICAAC-metal complexes can be extrapolated as
potential catalysts to compare with the known metal-carbene
catalysts to pursue applications in homogeneous catalysis.
Herein, we report on the synthesis and characterization of a
(BICAAC)2PdCl2 complex and also evaluate the catalytic
activity of this complex in HM and SM cross-coupling
reactions under ambient conditions. The present complex,
(BICAAC)2PdCl2, showed good activity with catalyst loadings
lower than known biscarbene-PdCl2 complexes such as
“unusual”-NHC-PdCl2 and tetracyclic-NHC-PdCl2 com-
plexes.12,19 In addition, we also examined the fate of the
catalyst by recovering it after the completion of the reaction
using various analytical techniques to satisfy the curiosity to
check the robustness of the carbene-Pd bond and for any
further applications. This aspect was pursued keeping in view
that many cross-coupling reactions require the in situ
generation of catalytically active Pd (0) species of sufficient
stability and exclusion of air and moisture from the system to
prevent the irreversible formation of unreactive Pd-peroxo
species20 or precipitation of inactive Pd black.21
Scheme 1. Synthesis of the (BICAAC)2PdCl2 Complex
1
The product obtained was characterized by H and 13C{1H}
NMR spectroscopy. In the 1H NMR spectrum of
(BICAAC)2PdCl2 two broad septets corresponding to the
2,6-CH(CH3)2C6H3 group of carbene ligand appeared at 3.22
and 3.09 ppm. In the 13C{1H} NMR spectrum of Pd complex,
the signal for the carbene carbon nucleus was observed at
262.9 ppm, which is significantly upfield-shifted as compared
to BICAAC (334 ppm) and can be attributed to the strong
electrophilic nature of BICAAC in the Pd complex. The high-
resolution mass spectrometry (AP+) of the product showed a
signal at m/z = 763.3917 (calcd 763.3962 for C44H66N2PdCl),
which is attributed to [M-Cl]+.
The formation of the (BICAAC)2PdCl2 complex was
confirmed by single-crystal X-ray diffraction studies. The
suitable crystals of the Pd complex were grown by layering its
dichloromethane solution with diethyl ether. The (BI-
CAAC)2PdCl2 complex crystallized in the monoclinic crystal
system with the P21/n space group (Table S1). The molecular
structure depicted in Figure 2 revealed the trans form of
(BICAAC)2PdCl2, and the Pd center adopted the square-
planar geometry. The metal center bound to two carbene units
through carbon C1 and C1′, and the remaining sites were
completed by two chloride groups. The Pd1−C1 (2.0297(3)
Å) and Pd1−Cl1(2.3247(8) Å) bond distances are almost
similar to the reported carbene-palladium-chloride complexes,
[(ethyl-cAAC-5)Pd(allyl)Cl]22 (Pd−C 2.030(2), Pd−Cl
2.374(1) Å); [(ethyl-cAAC-6)Pd(allyl)Cl]22 (Pd−C
2.058(2), Pd−Cl 2.397(1) Å); and [(NHC)2PdCl2] (Pd−C
2.019(13), Pd−Cl 2.289(4) Å).12 The Cl1−Pd1−C1′ and
Cl1−Pd1−Cl1′ bond angles in trans-(BICAAC)2PdCl2 are
90.90(4)° and 180.00(4)°, respectively, and stipulate the
square-planar geometry around the Pd(II) center in trans-
(BICAAC)2PdCl2 isomer.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of (BICAAC)2PdCl2.
The synthesis of the (BICAAC)2PdCl2 complex was performed
in a straightforward manner by the treatment of a BICAAC
solution in dry cold tetrahydrofuran (THF) with PdCl2 under
the exclusion of light at room temperature for 5−6 h (Scheme
1). The complex (BICAAC)2PdCl2 was obtained in 78% yield.
The UV−vis spectrum of (BICAAC)2PdCl2 in acetonitrile
showed two absorptions centered at 290 nm (4 × 102 L mol−1
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development of suitable catalysts that work under ambient
conditions has been a challenging task for the chemists in the
past decades.24 Along with that, the stability, sustainability, and
utilization of catalysts in low quantity have been key factors
that were taken into consideration.
To explore the properties of BICAAC, previously we
reported its reactivity with different hydroboranes.25 Very
recently, we reported the synthesis of two coordinated
complexes of Cu(I) and Au(I) with BICAAC ligand.26 As a
part of our continued interest in this direction, we evaluated
the Pd complex reported herein, (BICAAC)2PdCl2, as a
catalyst in Heck-Mizoroki (HM) and Suzuki-Miyaura cross-
coupling (SMCC) reactions under ambient conditions.
During optimizations for Heck-Mizoroki reaction, we used 4
mol % of catalyst and chose 4-bromo nitrobenzene and methyl
acrylate as model substrates in the presence of 1 mmol of
K2CO3 and 10 mol % of tetrabutylammonium bromide
(TBAB) at 110 °C in dimethylacetamide (DMA) as the
commonly used solvent for this rection (Table 1, entry 1). The
Figure 2. Single-crystal X-ray structure of trans-(BICAAC)2PdCl2.
Ellipsoids are shown at 50% probability levels. All hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and bond angles [deg]:
Pd1−C1 2.0297(3), Pd1−Cl1 2.3247(8), N1−C1 1.309(4); Cl1−
Pd1−C1 90.90(4), Cl1−Pd1−Cl1′ 180.00(4), Cl1−Pd1−C1′
89.10(5), C1′−Pd1−C1 180.0(4).
Table 1. Optimization Table for the Heck-Mizoroki
a
Reaction
cm−1) and 435 nm (1.9 × 102 L mol−1 cm−1), which
correspond to the intraligand charge transfer (ILCT) and
metal-to-ligand charge transfer (MLCT), respectively (see
Figure S4 in the Supporting Information). No d−d transition
was observed as expected. The field emission scanning electron
microscopic (FE-SEM) and energy-dispersive X-ray spectro-
scopic (EDX) measurements were also performed with the
(BICAAC)2PdCl2 complex to observe the surface morphology
and composition of the material. The crystallinity of the
material was also observed in the SEM images (see Figure S5
in the Supporting Information). The Pd/Cl ratio of the
complex was found to be 1:2 via an EDX analysis (Figure 3,
Table S2) consistent with the (BICAAC)2PdCl2 composition
of the complex.
Catalytic Applications of the (BICAAC)2PdCl2 Com-
plex in C−C Coupling Reactions. The palladium-catalyzed
C−C cross-coupling reactions and C−H bond activation are
the most coherent and effective methods, which led to the vast
improvement to the methodologies applied in synthetic
organic chemistry.23 In this aspect, aryl-substituted α,β-
unsaturated esters, olefins, and biaryl systems having ortho,
meta, or para functionalities are supposed to be the key
intermediates in the synthesis of natural products, pharma-
ceuticals, and precursors for material chemistry.23c Hence, the
b
entry catalyst (mol %)
solvent
DMA
DMA
DMA
DMA
DMA
DMF
base
time
yield (%)
1
4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NEt3
6 h
82
78
78
45
52
75
70
42
22
2
2
10 h
10 h
10 h
24 h
10 h
10 h
10 h
10 h
10 h
10 h
10 h
10 h
10 h
10 h
10 h
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.5
0.1
0.1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
DCE
toluene
1,4-dioxane
H2O
DMA
DMA
DMA
DMA
DMA
DMA
c
NR
50
61
30
48
42
35
NaH
tBuOK
KOH
NaOAc
Na2CO3
a
Typical reaction conditions: catalyst (BICAAC)2PdCl2, 1.0 mmol 4-
bromonitrobenezene, 1.1 mmol methyl acrylate, 1.0 mmol base, 10
b
c
mol % TBAB. Isolated yield. NR = No reaction.
Figure 3. EDX spectrum of (BICAAC)2PdCl2.
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Table 2. Heck-Mizoroki Reaction (Substrate Scope and Yields)
a
Aryl bromide (1.0 mmol), substituted olefin (1.1 mmol), catalyst 0.5 mol %; K2CO3 (1.0 mmol), TBAB (10 mol %), dimethylacetamide (4 mL),
10−12 h.
progress of the reactions was monitored by thin-layer
chromatography (TLC), and the product was characterized
DMA is the most suitable solvent (Table 1, entries 6−10).
Next, optimizations were performed by changing bases (such
1
t
by H and 13C{1H} NMR spectra. The expected product 1a
as NEt3, NaH, BuOK, KOH, NaOAc, and Na2CO3) with
was obtained with 82% isolated yield after 6 h. Decreasing the
catalyst loading to 2 mol % resulted in the formation of 1a in
78% yield after 10 h (Table 1, entry 2). Rewardingly, lowering
the catalyst loading to 0.5 mol % did not affect the yield of 1a
(Table 1, entry 3) and resulted in turnover number (TON)
value of 156. However, the use of 0.1 mol % catalyst gave 1a in
45% yield after 10 h, and with extended time of 24 h the yield
reached up to 52% (Table 1, entries 4 and 5). The obtained
turnover number (TON) value with our catalyst is slightly
better and comparable to the previously reported catalyst-
s.12,19,24a,27a,b
Taken together, we chose the catalyst loading of 0.5 mol %
and the reaction time of 10 h for further optimization.
Screening of the model reaction in different solvent mediums
(dimethylformamide (DMF), 1,2-dichloroethane (DCE),
toluene, 1,4-dioxane, and H2O) led to the observation that
DMA as the solvent under the above reaction conditions that
revealed K2CO3 as the appropriate base (Table 1, entries 11−
16). Overall, we found the best-optimized reaction protocol
with a combination of 0.5 mol % (BICAAC)2PdCl2, 1 mmol
K2CO3, and 10 mol % TBAB as an additive in DMA for 1
mmol of 4-bromonitrobenzene. Not surprisingly, when the
reaction was performed with lower than 10 mol % TBAB (or in
its absence) only traces of 1a formed along with deposition of
Pd black. In the literature, it has been well-documented that
the presence of TBAB suppresses the catalyst degradation and
inhibits the formation of Pd black.28a,d Noticeably, the report
by Ananikov and co-workers employed higher amounts of
TBAB as an additive (1 equiv with respect to the substrate) for
a Pd-NHC complex for the HM reaction.29 To extend the
substrate scope for the catalyst, reactions were performed by
varying the olefinic as well as the aryl bromide part having
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a
Table 3. Optimization Table for the Suzuki-Miyaura Cross-Coupling Reaction
b
entry
catalyst (mol %)
solvent
DMA
DMA
base
yield (%)
1
2
3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
K2CO3
NEt3
tBuOK
85
10
67
51
50
90
60
73
56
97
DMA
4
5
6
7
8
9
10
DMA
DMA
DMF
toluene
DME
KOH
NaOAc
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
1,4-dioxane
H2O
a
Typical reaction conditions: catalyst (BICAAC)2PdCl2 0.5 mol %, 1.0 mmol 4-bromonitrobenezene, 1.1 mmol phenylboronic acid, 1.0 mmol
b
base. Isolated yield.
Table 4. Suzuki-Miyaura Cross-Coupling Reaction (Substrate Scope and Yields)
a
b
Aryl bromide (1.0 mmol), boronic acid (1.1 mmol), catalyst 0.5 mol %; K2CO3 (1.0 mmol), solvent (4 mL), 10−12 h. DMA as reaction
c
medium. Water as the reaction medium.
different positional substituents (ortho, meta, and para). The
substrates with electron-withdrawing groups such as NO2, CN,
and CHO on aryl bromide led to the desired products with a
high yield (71−83%) (Table 2, 1a−1f). Gratifyingly, tert-butyl
acrylate and styrene as the coupling partners were also
investigated, which led to the formation of the respective
products (2a−2d and 3a−3f) (Table 2). In all the cases of
1a−1f and 2a−2d the (E)-methyl cinnamates and for 3a−3f
(E)-stilbene derivatives were the only products, confirmed by
the trans coupling constant (3JH−H = 16 Hz) of the olefinic
hydrogens that appeared at ∼6.0−7.5 ppm in the H NMR
spectra. Interestingly, no significant difference in the catalytic
1
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performance was observed among the ortho-, meta-, and para-
substituted aryl bromide substrates under the same reaction
conditions, and good to moderate product yields were
obtained.
Encouraged with the results of the HM reaction, we further
evaluated the catalytic activity of (BICAAC)2PdCl2 for the
SMCC reaction keeping in view that the SM reaction has
previously been studied well with NHC-Pd catalysts.7e,12,19
The reaction between phenylboronic acid and 4-nitro-
bromobenzene in the presence of K2CO3 as the base with
0.5 mol % of the catalyst gave the desired product 4a in 85%
yield (Table 3, entry 1) with a TON of 194. The use of other
bases such as Cs2CO3, CsF, or K3PO4 failed to activate the
catalyst, as no product formed in these reactions.
the BICAAC ligand was still associated with the Pd centers in
the recovered material, but due to the poor solubility of the
material, we used the techniques such as field emission
scanning electron microscopy (FE-SEM), EDX, and HR-TEM
to characterize this material.
The SEM images of the recovered material showed a block-
shaped morphology (see Figure S7 in the Supporting
Information), which is not akin to the SEM images of
(BICAAC)2PdCl2 (see Figure S5 and Table S2). In addition,
the EDX analysis in conjunction with SEM confirmed the
presence of palladium in the gray material (see Figure S8 in the
Supporting Information). The HR-TEM images of the material
clearly depicted the formation of fine, uniform, and small-sized
palladium nanoparticles of 5−8 nm average diameter (Figure
4), which was further confirmed by the powder X-ray
Further, bases such as NEt3, tBuOK, KOH, and NaOAc were
found to be less efficient than K2CO3 (Table 3, entries 2−5).
To identify the suitable reaction medium, we examined the
reaction in different solvents like DMA, DMF, toluene, DME,
and 1,4-dioxane and found DMA as the appropriate reaction
medium (Table 3, entries 6−9). Interestingly, during the
revision of the manuscript, we found water as the best solvent
for this reaction (Table 3, entry 10).
The substrate scope for SMCC was elaborated with aryl
bromide bearing different electron-withdrawing and electron-
donating substituents (Table 4). All the aryl bromides
containing electron-withdrawing substituents (NO2, CN, F,
CHO) at ortho, meta, or para positions afforded the respective
products in good yields (72−85%) (Table 4, 4a−4g). Of note,
no significant dissimilarity in the catalytic activity was noticed
with respect to the positional substitution. In addition, 2-
bromonaphthalene also afforded C−C coupled product 4h
with 72% yield. The p-OCH3- and p-CH3-substituted aryl
bromides also furnished the desired products 4i and 4j in high
yields of 85% and 80%, respectively. Also, the coupling of p-
OCH3-substituted phenyl boronic acid with 4-nitrobromoben-
zene gave 4k in 91% yield. p-OH and 1,4-dibromo-substituted
aryl bromides gave the corresponding products 4l and 4m in
good yields. The heterocyclic substrate 5-bromo-2-furaldehyde
also smoothly afforded 4n in 84% yield. We compared the
catalytic activity of the present complex, (BICAAC)2PdCl2,
with some of the notable examples of NHC-PdCl2 complexes
known to catalyze Heck-Mizoroki and Suzuki-Miyaura cross-
coupling reactions (see Figure S6 in the Supporting
Information).
Figure 4. HR-TEM images of the material recovered after the
completion of catalysis.
diffraction pattern of the gray material where Pd(111) and
Pd(200) planes of the material were found (see Figure S9 in
the Supporting Information).31 On the basis of these results,
we infer that, once (BICAAC)2PdCl2 catalyzed the coupling
reactions, it does not stay as the BICAAC-coordinated
palladium complex; rather, it slowly formed fine catalytically
inactive palladium nanoparticles. These observations indirectly
support the homogeneous reaction, where the catalyst lived
sufficiently long to complete the reaction, and the degenerated
catalyst at the completion of the reaction did not show any
reactivity.
Notably, when some of these catalytic reactions were
performed for durations longer than specified in Tables 2
and 4, the formation of trace quantities of gray precipitate was
observed for Heck-Mizoroki and Suzuki-Miyaura reactions.
Such observations motivated us to investigate if the catalyst
operated in a heterogeneous manner. For this purpose, a
mercury drop test was performed for the reaction between p-
nitrobromobenzene and methyl acrylate in a model reaction
(see Figure S10 in the Supporting Information). The absence
of a significant effect of mercury on the metal-catalyzed
CONCLUSION
■
In summary, we have demonstrated the synthesis and
characterization of the first BICAAC-palladium complex,
(BICAAC)2PdCl2. This complex is stable under ambient
conditions without the exclusion of moisture or oxygen.
Moreover, this complex acts as a potential catalyst in cross-
coupling reactions (Suzuki-Miyaura and Heck-Mizoroki). All
the catalytic reactions were performed under open-air
conditions, and broad substrates scope (irrespective of the
ortho, meta, and para substitution pattern) of aryl bromides
were demonstrated with good to moderate yields of the
reaction indicated the homogeneous nature of the reaction.30
A
similar observation was found in the case of Suzuki-Miyaura
coupling between p-nitrobromobenzene and phenylboronic
acid. We recovered the gray material from a scaled-up reaction
employed to prepare 1a and used it as a catalyst for the fresh
HM and SMCC reactions to check the activity of this material
for the scope of catalyst recyclability. Unfortunately, no
catalytic activity was detected under the optimized reaction
condition. In view of this, we were prompted to check whether
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products. This complex is found to have better catalytic activity
with a low catalyst loading as compared to a few previously
reported carbene-PdCl2 complexes.12,19 The homogeneous
process for the catalytic reaction was supported by the Hg-
drop test. Other evidence from FE-SEM, HR-TEM, and PXRD
indirectly support the homogeneous reaction pathway for these
reactions. At present, our attention is focused on the synthesis
of new metal complexes of BICAAC ligand to explore their
reactivity studies and catalytic applications in organic trans-
formations.
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: + 44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Sanjay Singh − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali, Mohali
140306, Punjab, India; orcid.org/0000-0003-0806-
3105; Email: sanjaysingh@iisermohali.ac.in
Authors
EXPERIMENTAL SECTION
Soumyadeep Chakrabortty − Department of Chemical
Sciences, Indian Institute of Science Education and Research
Mohali, Mohali 140306, Punjab, India; orcid.org/0000-
0001-6043-4722
Mandeep Kaur − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali, Mohali
140306, Punjab, India
Manu Adhikari − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali, Mohali
140306, Punjab, India
Krishna K. Manar − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali, Mohali
140306, Punjab, India
■
Materials and Methods. The synthesis of (BICAAC)2PdCl2 was
performed under an inert atmosphere of dry nitrogen using standard
Schlenk techniques or a glovebox, where O2 and H2O levels were
maintained below 1 ppm. All other reactions were performed under
ambient atmospheric conditions in glassware dried at 150 °C in a hot
air oven. Solvents were purified by an MBRAUN solvent purification
system MB SPS-800 and were used directly from the SPS system. All
chemicals were purchased from Merck and used without further
purification. The bicyclic (alkyl)(amino) carbene (BICAAC) was
prepared following the literature procedure reported by Bertrand and
co-workers.18a and was characterized using H and 13C{1H} NMR
1
methods. High-resolution mass spectrometry (HRMS) for (BI-
CAAC)2PdCl2 was performed with a Waters SYNAPT G2-S
instrument. The H and 13C{1H} NMR spectra were recorded with
1
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03614
Bruker 400 MHz spectrometer with tetramethylsilane (TMS) as an
external reference; chemical shift values are reported in parts per
million. For absorption spectroscopy measurements, an LABINDIA
UV−vis spectrophotometer 3000+ was used. FE-SEM and EDX
analyses were performed with a JEOL JSM 7600F field emission
electron microscope. The HR-TEM measurement was done with a
JEOL F200 with an electron acceleration energy of 200 kV.
Synthesis of (BICAAC)2PdCl2. BICAAC (0.31 g, 1.00 mmol)
was dissolved in cold THF (10 mL), and subsequently solid palladium
chloride (0.09 g, 0.50 mmol) was added to this solution. The
resulting mixture was stirred for ∼5−6 h in the exclusion of light.
This was followed by the removal of all volatiles under vacuum.
Further, the brown powder residues were dissolved in 1 mL of
dichloromethane, and then 10 mL of diethyl ether was added to
precipitate the impurities. The colored solution was filtered and
evaporated, which afforded the [(BICAAC)2PdCl2] complex in 78%
yield (0.3 g). The crystals were obtained after [(BICAAC)2PdCl2]
was dissolved in the dichloromethane and diethyl ether layered onto
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Science and Engineer-
ing Research Board (SERB) New Delhi, India (Grant No.
CRG/2020/004563). A postdoctoral fellowship to M.K. from
IISER Mohali is gratefully acknowledged. The National-
Postdoctoral Fellowship from DST-SERB, India (PDF/2017/
001078), to K.K.M. is gratefully acknowledged. Authors thank
IISER Mohali for NMR, HRMS, single-crystal, and powder X-
ray facility and for infrastructural support. We thank Dr. A. Roy
Choudhury and Mr. M. Joshi for PXRD measurements, Dr. A.
Venkatesan for providing the FE-SEM and EDX facilities, and
Dr. U. K. Gautam for the HR-TEM measurements.
1
this solution. mp 240 °C. H NMR (400 MHz, CDCl3) δ = 7.40−
7.31 (m, 3H, Ar−H), 3.22 (sept, 1H, CH(CH3)2, 3JH−H = 8 Hz), 3.09
3
(sept, 1H, CH(CH3)2, JH−H = 8 Hz), 1.82−1.69 (m, 3H), 1.61 (s,
1H), 1.46−1.42 (m, 10H), 1.33−1.30 (m, 1H), 1.22−1.18 (m, 7H),
1.00−0.98 (m, 3H), 0.93−0.92 (m, 3H). 13C{1H} NMR (100 MHz,
CDCl3) δ = 262.9, 146.4, 146.3, 140.6, 128.4, 125.1, 125.0, 65.5, 65.3,
52.8, 52.7, 43.7, 40.8, 37.5, 36.9, 33.5, 32.9, 32.8, 32.4, 28.7, 28.6,
28.4, 27.6, 27.5, 26.6, 26.5, 23.8, 23.5, 20.7, 20.4. HRMS (AP+) m/z
calculated for C44H66N2PdCl: 763.3962 [M-Cl]+; found: 763.3917.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03614.
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Information on catalytic reactions and characterization
(NMR, UV−vis, FE-SEM, EDX, HR-TEM graphics)
(PDF)
Accession Codes
CCDC 2003312 contain the supplementary crystallographic
data for this paper. This data can be obtained free of charge via
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