Rapid Suzuki-Miyaura cross-coupling reaction catalyzed by zirconium carboxyphosphonate supported mixed valent Pd(0)/Pd(II) catalyst

Article abstract of DOI:10.1002/aoc.5017

Mixed valent Pd(0)/Pd(II) nano-sized aggregates supported onto a chemically robust layered zirconium carboxyphosphonate framework is prepared and its catalytic activity in Suzuki-Miyaura cross coupling reaction is explored. The exceptionally high catalytic efficacy of the heterogeneous catalyst in Suzuki-Miyaura cross coupling reaction is signified by remarkably short reaction time 2?minutes and high turnover frequency of 1.3 x 10⁴?hr^?1. The catalyst can be recycled several times without significant loss of catalytic efficacy, while spectroscopic, structural and microscopic investigations suggest the integrity of the catalyst even after fifth catalytic cycle. The unique ability of the zirconium carboxyphosphonate framework to interact strongly with palladium in dual Pd(0)/Pd(II) oxidation states has been attributed to this remarkable augmentation of catalytic efficacy.

Full text of DOI:10.1002/aoc.5017

Received: 31 January 2019
DOI: 10.1002/aoc.5017
Revised: 22 April 2019
Accepted: 5 May 2019
F U L L P A P E R
Rapid Suzuki‐Miyaura cross‐coupling reaction catalyzed by
zirconium carboxyphosphonate supported mixed valent
Pd(0)/Pd(II) catalyst
Bagmita Bhattacharyya1 | Jyoti Prasad Biswas1 | Shashank Mishra2
| Nayanmoni Gogoi1
1 Department of Chemical Sciences,
Mixed valent Pd(0)/Pd(II) nano‐sized aggregates supported onto a chemically
robust layered zirconium carboxyphosphonate framework is prepared and its
catalytic activity in Suzuki‐Miyaura cross coupling reaction is explored. The
exceptionally high catalytic efficacy of the heterogeneous catalyst in Suzuki‐
Miyaura cross coupling reaction is signified by remarkably short reaction time
2 minutes and high turnover frequency of 1.3 x 104 hr−1. The catalyst can be
recycled several times without significant loss of catalytic efficacy, while spec-
troscopic, structural and microscopic investigations suggest the integrity of the
catalyst even after fifth catalytic cycle. The unique ability of the zirconium
carboxyphosphonate framework to interact strongly with palladium in dual
Pd(0)/Pd(II) oxidation states has been attributed to this remarkable augmenta-
tion of catalytic efficacy.
Tezpur University, Napaam
784028Sonitpur, Assam, India
2 Institut de recherches sur la catalyse et
l'environnement de Lyon, Université
Claude Bernerd Lyon 1, IRCELYON,
CNRS‐UMR 5256, 2 Avenue Albert
Einstein, 69626 Villeurbanne, France
Correspondence
Nayanmoni Gogoi, Department of
Chemical Sciences, Tezpur University,
Napaam, 784028, Sonitpur. Assam, India.
Email: ngogoi@tezu.ernet.in
Funding information
Science and Engineering Research Board,
Grant/Award Number: EMR/2016/002178
KEYWORDS
heterogeneous catalysis, layered zirconium carboxyphosphonate, Pd(0)/Pd(II) nanoaggregate
1 | INTRODUCTION
nanoparticles over a wide array of solid supports, e.g., car-
bon based materials,[9] silicious materials,[10] poly-
Enhancing the efficiency of heterogeneous catalysts to
match their homogeneous counterparts has emerged as
one of the most pressing challenge for furthering their
industrial applications.[1–3] Current outbursts of activities
to address this concern primarily relies on tuning the sur-
face area and electronic characteristics of active catalytic
species either by particle size miniaturization or by
manipulating interaction with support surface.[4–6] In this
regard, development of highly reactive heterogeneous
palladium‐based catalysts for cross coupling reactions
are of particular interest due to their relevance as indis-
pensable synthetic tools for chemist.[7,8] Concern over
limited catalyst reusability and contamination of residual
palladium in products led to the proliferation of heteroge-
neous catalysts for these cross coupling reactions. A fasci-
nating assortment of highly efficient heterogeneous
catalyst, prepared by immobilization of palladium
mers,[11] metal–organic frameworks[12,13] have already
been reported. Many of these heterogeneous catalysts
can be easily recycled and repeatedly used, afford prod-
ucts in high yields within a reasonably short reaction
time, require mild reaction conditions in green solvent
medium or even under solventless conditions. Nonethe-
less, in terms of catalyst reactivity, majority of reported
heterogeneous catalysts fare quite poorly as compared to
their homogeneous counterparts and this attribute
primarily manifests itself as typically longer reaction
completion time when heterogeneous catalyst are
employed.[14] For example, in case of Suzuki‐Miyaura
cross coupling reaction of aryl bromides with aryl boronic
acid, reaction completion time of as short as 1–2 mins
with concomitantly high turn‐over frequency have been
reported when adequately reactive homogeneous catalyst
are employed.[15–17] However, to the best of our
Appl Organometal Chem. 2019;e5017.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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BHATTACHARYYA ET AL.
knowledge, no heterogeneous catalyst which can effect
satisfactory conversion in less than 15 mins under con-
ventional heating has been reported so far.[18–21] Thus,
development of heterogeneous catalysts whose efficacies
in cross coupling reaction are comparable to that of
homogenous palladium‐based catalysts remains a highly
desirable objective.
During this study, a strategy based on both particle
size miniaturization and surface functionalization of the
catalytically active species by using a suitable solid
support has been adopted to enhance the efficacy of
heterogeneous catalyst in cross coupling reaction. The
solid support chosen for immobilization of palladium
2 | EXPERIMENTAL SECTION
2.1 | Materials and methods
Starting materials were purchased from commercial
sources and used without further purification. Solvents
were purified by conventional techniques and distilled
prior to use. Elemental analyses were performed on a
Perkin Elmer Model PR 2400 series II elemental analyzer.
Fourier transformed infrared spectra from 400 to 4000 cm
−1
were recorded on Perkin Elmer Frontier MIR‐FIR FT‐
IR spectrophotometer. ICP‐OES analysis was performed
using Perkin Elmer Optima 2100 DV Optical Emission
Spectroscopy analyzer. Thermogravimetric analysis was
performed with the help of Thermal analyzer (Model
TGA‐50 & DSC‐60, Shimadzu). NMR spectra were
recorded on a JEOL JNM‐ECS400 NMR spectrometer
operating at 400 MHz and samples were dissolved in deu-
terated solvents. Chemical shifts were reported in parts
per million downfield of Me4Si (TMS) as internal
standard. The scanning electron microscopy (SEM) anal-
yses were carried out using a JEOL JSM‐6390LV SEM,
equipped with an Energy‐Dispersive X‐ray analyzer.
The powder X‐ray diffraction patterns were recorded
on a Rigaku Multiflex instrument using a nickel‐filtered
CuKα (0.15418 nm) radiation source and scintillation
counter detector. The surface area and pore size distri-
bution of the compound were analyzed using BET
surface analyzer (QUANTACHROME NOVA 1000E).
The transmission electron microscopy analyses were
carried out using JEM‐2100, 200 kV, Jeol instrument.
X‐ray photoelectron spectroscopic (XPS) analyses were
performed in KRATOS Axis ultra DLD spectrometer
using Al Kα (1486.6 eV) radiation source. Magic angle
spinning NMR (MAS NMR) of 13C and 31P were
recorded with AVANCE III 500WB spectrometer using
adamantane (δ = 38 ppm) and H3PO4 (δ = 0 ppm) as
references, respectively.
nanoparticles is
a layered mesoporous zirconium
organophosphonate framework decorated with additional
L‐proline functionality. The L‐proline functionalized
phosphonate ligand, 1‐Phosphonomethyl‐pyrrolidine‐2‐
carboxylic acid can be easily synthesized by following
literature procedure and it provides an easy access to zir-
conium organophosphonate framework functionalized
with L‐proline (Scheme 1).[22] It is anticipated that large
surface area and intrinsic chemical robustness of the
zirconium organophosphonate framework will facilitate
palladium nanoparticle loading and improve reusability
of the catalyst, respectively.[23,24] Indeed, mesoporous
metal organophosphonates are widely recognized as a
versatile class of support materials for metal or metal
oxide nanoparticles.[25–28] Recent reports have established
the superior ability of amino acid‐functionalized zirco-
nium phosphonate frameworks in immobilization of
2–5 nm palladium nanoparticles resulting in high cata-
lytic activity.[29–31] Due to the layered structure of zirco-
nium organophosphonates, the amino acid groups lie
over the surface of the solid support in a well ordered
fashion which significantly accentuates their ability to
interact with substrates immobilized over the frame-
work.[32–34] Furthermore, several homogeneous as well
as heterogeneous palladium catalysts bearing L‐proline
display significantly enhanced reactivity in cross‐coupling
reactions.[35–39] Therefore, it is expected that immobilized
palladium nanoparticles will interact strongly with the
L‐proline functionality present over the surface of the
zirconium organophosphonate framework and thereby
augment the reactivity of the catalyst in cross coupling
reactions.
2.1.1 | Synthesis of
1‐Phosphonomethyl‐pyrrolidine‐2‐
carboxylic acid
1‐Phosphonomethyl‐pyrrolidine‐2‐carboxylic acid was
prepared by following
a
literature procedure
(Scheme 1).[22] To a mixture of L‐proline (1.1513 g,
10 mmol), phosphorus acid (0.90 g, 11 mmol) and
hydrochloric acid (30 ml), formaldehyde (1.47 ml,
40 mmol) was added dropwise under reflux condition.
The resultant mixture was further refluxed for 12 hrs.
Thereafter, the resulting solution was evaporated on a
water bath to obtain the product as a white solid. Yield:
SCHEME
carboxylic acid
1
Synthesis of 1‐Phosphonomethyl‐pyrrolidine‐2‐

BHATTACHARYYA ET AL.
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1.2 g (58%); FT‐IR (KBr, cm−1): υ = 3016 (b), 1887(w),
1705 (s), 1452 (vw), 1352 (s), 1250 (s), 1194 (s), 1151
(s), 1078 (s), 1005 (s), 925 (s), 919 (m), 721 (m), 640
(m), 459 (s).; 1H NMR (400 MHz, D2O, δ ppm):
1.87–2.39 (m, 4H), 3.22 (m, 2H), 3.39 (m, 1H), 3.77
(m, 1H), 4.25 (dd, 1H); 13C NMR (100 MHz, D2O, δ
ppm): 22.36, 27.78, 50.65, 56.68, 68.91, 171.27(‐COOH);
31P NMR (CDCl3, δ ppm): 7.74.
2.1.4 | Typical procedure for
Suzuki‐Miyaura cross‐coupling reaction
0.24 mmol of the aryl boronic acid and 0.2 mmol of aryl
bromide were dissolved in 12 ml ethanol water (3:1) mix-
ture. To the above solution, 0.25 mmol of potassium car-
bonate and 0.5 mg (0.22 mol%) of the catalyst was added
and the reaction mixture was refluxed in a preheated
water bath for 2–10 minutes. The progress of the reaction
was monitored by using thin layer chromatography. After
completion of the reaction, the catalyst was removed by
filtration and ethanol was removed from the filtrate
under reduced pressure. The product was extracted from
the filtrate by using ethyl acetate and dried over Na2SO4.
Yields were determined from GC–MS by using toluene as
the internal standard. To investigate the role of the sol-
vents and the base used in the reaction mixture, tests
have been carried out using different solvents and bases.
It has been found that the use of 3:1 proportion of
C2H5OH:H2O system and K2CO3 as base under reflux
condition gives the maximum yield in minimum time.
2.1.2 | Synthesis of zirconium
carboxyphosphonate (ZrCP)
ZrOCl2.8H2O (322 mg, 1 mmol) was dissolved in water
to which CTAB (182 mg, 0.5 mmol) was added and
stirred to obtain a clear solution. 1‐Phosphonomethyl‐
pyrrolidine‐2‐carboxylic acid (104 mg, 1 mmol) was
dissolved in 5 mL distilled water and added drop
wise to the previous solution. The mixture was then
transferred to a Teflon lined container in an autoclave
and kept at 130 °C for 3 days. The precipitate obtained
was washed several times with distilled water and
then refluxed with acidified ethanol for 12 hrs. The
residue was then washed several times with ethanol to
remove the remaining acid and dried in vacuum
at 50 °C. Yield: 177 mg; Elemental analysis % found:
C, 11.59%; H, 4.31%; N, 2.04. FTIR (KBr, cm−1):
3408(br), 1734(w), 1621(m, s), 1425(w), 1116(m),
1024(m), 662(w), 475(br, w). TGA (10 °C/min under
air): Temperature range (% weight loss): 32–92 °C
(17%); 315–390 °C (88%).
2.1.5 | Catalyst recyclability test
p‐Bromo toluene (0.2 mmol) along with phenyl boronic
acid (0.24 mmol) under 3:1 C2H5OH: H2O solution at
80 °C in presence of 0.25 mmol of K2CO3 and
0.22 mol% of catalyst Pd@ZrCP was taken as a model
reaction for determination of the catalytic activity and
the recyclability of the catalyst. Reaction time was fixed
to 2 minutes for all the catalytic cycles. The reaction time
was monitored using a stopwatch and as soon as the time
completed the mixture was immediately filtered and the
filtered was further centrifuged to ensure no solid catalyst
remain in the solution. After that the catalyst was washed
with ethanol water solution for 4 times and then the
recovered catalyst was used directly for the reaction using
same quantity of starting reactants. The catalyst was
recycled upto 5 cycles. The reaction was monitored using
TLC and further verified by using GC–MS.
2.1.3 | Synthesis of Pd loaded zirconium
carboxyphosphonate (Pd@ZrCP)
250 mg of ZrCP was taken in a round bottom flask and
prepared a suspension of it in THF. A solution of Pd
(OAc)2 (50 mg, 2.2 mmol) in THF was taken and added
drop wise to the suspension. The mixture was stirred for
12 hrs. The resultant was washed several times with
THF and dried under vacuum. 250 mg of the dried prod-
uct was then dispersed in 10 ml water and ice‐cold water
solution of NaBH4 (100 mg, 2.6 mmol) was added drop
wise to it. The brown color of the compound changed to
black immediately. The suspension was further stirred
for 3 hrs after which it was washed several times with
water and dried under vacuum. FT‐IR (KBr, cm−1):
3383(br), 1625(m, s), 1408(w), 1069(m), 1012(m) 658(w),
464(s, w). Pd loading over the ZrCP is calculated to be
9.5% from ICP‐OES analysis.
2.1.6 | Palladium leaching test
To investigate the leaching of the catalyst in the reaction
medium, initially 4‐bromotoluene and phenyl boronic
acid was used as reactant in 3:1 C2H5OH:H2O solution
using 0.5 mg of the catalyst at 80 °C. As the reaction,
complete within 2 minutes, the reaction mixture was
immediately filtered through a Whatman filter paper
under hot condition which was followed by centrifuga-
tion to ensure the complete removal of any solid catalyst.
Then to the reaction mixture equivalent amount of

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BHATTACHARYYA ET AL.
4‐nitro bromo benzene, phenyl boronic acid and K2CO3
were added and reaction was monitored using TLC. No
new product has been observed after 2 h of reflux under
the same reaction conditions. It was further stirred for
another 4 hrs to ensure absence of any product formation.
The possibility of Pd leaching into the reaction mixture
was further analyzed with ICP‐OES analysis. During
standard heterogeneous Suzuki–Miyaura reactions of
4‐bromotoluene with phenyl boronic acid, 2 ml of mother
liquor was collected by using a syringe filter (Whatman
Puradisc 4, 4 mm diameter, 0.45 μm, PTFE) and the sol-
vent was evaporated under reduced pressure. The residue
was dissolved in HNO3 and analyzed by using ICP‐OES
which showed that concentration of Pd in reaction solu-
tion was less than the detection limit (i.e., 50 ppb). The
sample collected after the completion of the reaction also
shows concentration of Pd in reaction solution below the
detection limit.
C2H5OH:H2O (3:1) in presence of catalyst Pd@ZrCP
(0.5 mg, 0.22 mol%) and BrPhCONH@SiO2 (200 mg)
were refluxed at 80 °C for 2 mins. After this the suspen-
sion was analyzed by TLC and the solid was separated
under vacuum in hot condition, washed with EtOH
and finally extracted with CH2Cl2. Another reaction
carried out under the same reaction condition but with-
out the addition of 4‐bromotoluene to the reaction
medium. In this case also the solid was recovered by
hot filtration followed by washing with C2H5OH, and
finally extracted with CH2Cl2. The recovered solids from
both the reaction mixtures were hydrolyzed with 2 M
KOH/H2O (1.7 g in 10 ml EtOH, 5 ml H2O) at 90 °C for
3 days. The resulting solution was neutralized with 10%
HCl v/v (9.1 ml), extracted with CH2Cl2 followed by ethyl
acetate, concentrated, and the resulting mixture was ana-
1
lyzed by H NMR.
2.1.8 | Mercury poisoning test
2.1.7 | Three phase test
A solution of 4‐bromoacetopheneone (0.2 mmol, 34 mg)
phenyl boronic acid (0.24 mmol, 29 mg), K2CO3
(0.25 mmol, 34 mg) and Pd@ZrCP (0.5 mg, 0.22 mol%)
was heated at 80 °C for 3 minutes. The progress of the
reaction as this point was monitored by TLC & GC–MS.
After that metallic mercury (300 equiv) was added to
the reaction mixture. The reaction was continued for
another 10 minutes under the same reaction condition
and the mother liquor was recovered by using a syringe
filter (Whatman Puradisc 4, 4 mm diameter, 0.45 μm,
PTFE). The recovered solution was concentrated under
reduced pressure to remove ethanol, extracted with ethyl
acetate and dried over Na2SO4 and finally analyzed with
GC–MS.[41]
Initially, aminopropyl‐modified Silica (SiO2‐C3H6NH2)
was prepared by slight modification of a reported
procedure.[40] To a suspension of SiO2 (100–200 mesh size,
3 g) in dry toluene a solution of 3‐Aminopropyl-
trimethoxysilane (APTMS) (4.8 ml, 27 mmol) and pyridine
(3 ml, 36.9 mmol) were added dropwise through a
dropping funnel under N2 atmosphere. The resulting mix-
ture was refluxed for 24 hrs. After that, the suspension was
filtered and Soxhlet extracted with CH2Cl2 for another
24 hrs. The resulting solid was dried under vacuum at
room temperature to obtain the product as white powder.
Yield: 3.25 g; FT‐IR: 3448 (b), 2927 (w, b), 1638 (w, b), 1088
(s), 791 (s), 574 (w), 466(s).
For carrying out the three phase test, a procedure
developed by Corma et al. was adopted.[41] For this pur-
pose a solid supported aryl bromide was initially prepared
A solution of 4‐Bromobenzoylchloride (0.800 g, 3.6 mmol)
was dissolved in dry THF(10 ml) in a round bottom flask
with aminopropyl trimethoxy silane modified silica (SiO2‐
C3H6NH2) (1 g) and pyridine (404 μL, 5 mmol) under N2
atmosphere. The mixture was stirred for 12 hrs at 40 °C
after which the resulting suspension was filtered and
washed 3 times by 20 ml 5% (v/v) HCl in water, followed
by 20 ml of 0.2 M aqueous K2CO3, 2 washes each with
distilled water and with 20 ml of ethanol. The solid was
finally washed with large excess of CH2Cl2 and finally
dried in air. 1.1 g solid is recovered and assigned as
BrPhCONHC3H6@SiO2. FT‐IR: 3447 (b), 2926 (w, b),
1639 (w, b), 1068 (s), 791 (s), 463 (s).
3 | RESULTS AND DISCUSSION
On account of their versatile characteristics, layered
mesoporous zirconium carboxy aminophosphonates have
found several applications as catalyst support. Herein, we
have employed a surfactant assisted sol–gel technique to
prepare a mesoporous ZrIV organocarboxyphosphonate,
which was subsequently used as a support matrix for
palladium mixed valent Pd(0)/Pd(II) nano‐sized
aggregates. Hydrothermal reaction of ZrOCl2.8H2O with
1‐Phosphonomethyl‐pyrrolidine‐2‐carboxylic
acid
in
presence of cetyltrimethylammonium bromide (CTAB) at
130 °C for 3 days followed by treatment with ethanolic HCl
to remove CTAB, yielded ZrIV organocarboxyphosphonate,
ZrCP as an off white solid (Scheme 2). Thereafter, the
surfactant free ZrIV organocarboxyphosphonate, ZrCP
was impregnated with Pd(OAc)2 for a definite period.
In the next step, 4‐Bromo toluene (0.2 mmol), phenyl
boronic acid (0.24 mmol), K2CO3 (0.25 mmol) in

BHATTACHARYYA ET AL.
5 of 13
SCHEME 2 Synthesis of ZrCP and Pd@ZrCP
Reduction of absorbed Pd(II) by NaBH4 eventually resulted
in palladium incorporated ZrIV carboxyphosphonate
composite, Pd@ZrCP where Pd(0)/Pd(II) nano‐sized
aggregates are homogenously dispersed over the ZrIV
carboxyphosphonate framework. Both the pristine ZrIV
organocarboxyphosphonate, ZrCP and the palladium
incorporated composite, Pd@ZrCP were characterized by
various analytical, spectroscopic, microscopic and textural
studies such as elemental analysis, FT‐IR, MAS NMR,
powder X‐ray diffraction, SEM, TEM, ICP‐OES, XPS, TGA
and BET surface area analysis.
All characteristic peaks observed in the FT‐IR spec-
trum of the pristine ZrIV carboxyphosphonate framework,
ZrCP are also observed in FT‐IR spectrum of Pd@ZrCP
without any significant deviation and this clearly indi-
cates the structural integrity of the host matrix in the
composite. IR spectra of both the materials feature an
intense and sharp peak at 1024 cm−1 which can be attrib-
uted to Zr‐O‐P stretching vibration. Similarly, the peak
observed at 1116 cm−1 is characteristic of P=O stretching
vibration. The intense band observed at ~1625 cm‐1 can
be assigned to C=O stretching vibration associated with
the ‐COOH functionality present within the ligand.[42]
Absence of any band near 2260‐2340 cm‐1 region indi-
cates absence of free P‐OH groups in the pristine frame-
work as well as the composite. Furthermore, complete
removal of CTAB, used for the synthesis of the mesopo-
rous compound is validated by the absence of any peak
in the region 2850‐2930 cm‐1 (Figure S1, S2).
Broad peaks observed in the powder X‐ray diffraction
pattern of both ZrCP and Pd@ZrCP indicate
disordered and amorphous nature of the materials
(Figure 1).[43] Furthermore, all peaks observed in the
powder X‐ray diffraction pattern of ZrCP are present
in the diffraction pattern of Pd@ZrCP and it establishes
the integrity of the support within the composite mate-
rial. Apart from the peaks corresponding to the pristine
framework, characteristic peak for (100) plane of metal-
lic palladium is observed at 2θ = 39.4° (JCPDS‐87‐0641)
in the powder X‐ray diffraction pattern of Pd@ZrCP.
FIGURE 1 Powder X‐ray diffraction pattern of ZrCP and
Pd@ZrCP
Thermogravimetric analysis of both the pristine frame-
work, ZrCP and the composite Pd@ZrCP showed rapid
weight loss when heated up to 95 °C and this can be
attributed to the desorption of water molecules occluded
within the host. On heating beyond 100 °C, a gradual
weight loss is observed upto 450 °C for both ZrCP
and Pd@ZrCP and this can be attributed to the loss
of coordinated water molecules as well as the decompo-
sition of the organic substituents (Figure S3).
Solid state magic angle spinning (MAS) 13C and 31P
NMR spectra of both the pristine framework, ZrCP and
its composite Pd@ZrCP were recorded to probe the
interaction between immobilized palladium nanoparti-
cles with the solid support. 13C MAS NMR spectra of
the ZrCP show characteristic peaks for all the carbon
atoms of the amino carboxyphosphonate ligand. Peaks
observed at 24.1 and 29.4 ppm can be, respectively,
assigned to γ‐ and β‐carbon atoms of the pyrrolidine ring.
The α‐ and δ‐carbon atoms of pyrrolidine ring resonate at
70.9 and 61.3 ppm, respectively. Signals for the methylene
carbon atom attached to phosphonate group and that of
the carboxyl carbon atom appear at 52.9 and 173.1 ppm,

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BHATTACHARYYA ET AL.
respectively (Figure 2). 13C MAS NMR spectrum of
Pd@ZrCP showed significant variation in the spectral
region corresponding to the carboxyl carbon atom of the
phosphonate ligand. Unlike to pristine framework, the
carboxyl carbon atom in Pd@ZrCP resonates as an
intense signal centered at 180.0 ppm with two weak
shoulders at 172.9 and 188.0 ppm. While the peak at
172.9 can be associated to carboxyl carbon peak observed
in the pristine framework, peaks at 180.0 and 188.0 ppm
can be attributed to carboxyl groups having moderate
to strong interaction with immobilized palladium nano-
particles (Figure 3). The higher intensity of the peak
at 180.0 ppm suggests that majority of the carboxyl
group in Pd@ZrCP lies at relatively close proximity to
the palladium nanoparticles while a small fraction of the
carboxyl group remain free from any interaction with
embedded nanoparticles. 31P MAS NMR spectra of ZrCP
shows one broad peak centered at 4.0 ppm and this
can be easily assigned to the phosphorous atom of
1‐phosphonomethylproline (Figure S11). Loading Pd
nanoparticles onto ZrCP does not trigger any significant
deviation in the chemical shift of phosphorous as the cor-
responding peak is observed at 4.2 ppm (Figure S12).[33]
The surface morphology of both the pristine frame-
work and its palladium nanocomposite was studied using
scanning electron microscopy (SEM). SEM images show
presence of micrometer size disordered amorphous parti-
cles in both ZrCP and Pd@ZrCP and surface morphol-
ogy of the pristine framework does not change after
loading palladium nanoparticles (Figure 4). Elemental
mapping of Pd@ZrCP shows the presence of palladium
on the surface of the framework (Figure S4). EDX spec-
trum shows the presence of Pd in the framework with a
percentage loading of 8.5% (Figure S5). EDS mapping
show the uniform distribution of palladium over the sur-
face in Pd@ZrCP (Figure S4(a) & (b)). From the ICP‐
OES, it was found that the amount of loading of Pd is
about 9.5% (0.89 mmol Pd/g) which is also in agreement
with the value found in the surface EDS analysis. The
high loading of palladium in ZrCP as compared to those
reported for other metal organophosphonate support can
be attributed to the presence of dangling free carboxylate
groups of L‐proline groups on the surface of the ZrCP,
which strongly interact with the embedded metal
nanoparticles.[27,28]
TEM images (Figure 5) of Pd@ZrCP show palladium
nanoparticles of average size 5 nm are homogeneously dis-
tributed over the zirconium carboxyphosphonate frame-
work (Figure S13). The selected area electron diffraction
(SAED) pattern shows the (100), (200), (222), and (311)
planes which corresponds to the fcc structure of the metal-
lic palladium (Figure S6). The N2 absorption/desorption
isotherm of ZrCP show hysteresis loop and a sudden jump
at P/P0 = 0.90 and this can be mainly attributed to the lay-
ered structure. Moreover, the BJH pore size analysis of the
pristine framework revealed that the BJH pore volume is
0.893 cc/g and pore radius is 11.0 nm and this establish
the mesoporous character of the solid support. BET sur-
face area analysis reveals that of ZrCP and Pd@ZrCP
revealed considerable reduction in surface area of the pris-
tine framework upon loading palladium nanoparticles
from 211 m2/g to 149 m2/g (Figure S10 & S12).
FIGURE 2 13C MAS NMR spectrum of ZrCP

BHATTACHARYYA ET AL.
7 of 13
FIGURE 3 13C MAS NMR spectrum of Pd@ZrCP
FIGURE 4 SEM images of ZrCP (left) & Pd@ZrCP (right)
FIGURE 5 TEM images of Pd@ZrCP
The influence of loading palladium nanoparticles on
the electronic environment of the constituent atoms of
the ZrCP was further probed by using X‐ray photo elec-
tron spectroscopy (XPS). The overall XPS spectrum of
the pristine framework, ZrCP shows characteristic peaks
of all the constituent elements present in the framework
(Figure S14 and S18). Peaks at 132.8 eV and 133.6 eV cor-
responds the 2p3/2 and 2p1/2 electrons of pentavalent
phosphorus (P5+) atom present in ZrCP (Figure S16).
Upon the incorporation of palladium nanoparticle to the

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BHATTACHARYYA ET AL.
ZrCP framework, no reasonable shift in the binding ener-
gies of 2p3/2 and 2p1/2 electrons of P5+ are observed as the
corresponding peaks appear at 132.5 eV and 133.3 eV
(Figure S20), respectively. Furthermore, for the pristine
framework, peaks corresponding to 3d5/2 and 3d3/2 elec-
trons of tetravalent Zr appear at 182.4 eV and 184.8 eV
(Figure S15). Whereas upon loading of palladium to the
framework, peaks corresponding to the 3d5/2 and 3d3/2
electrons of Zr4+ are shifted to a lower binding energies
of 181.1 eV and 184.1 eV, respectively (Figure S19). Two
peaks observed in the C 1s region of XPS spectrum of
ZrCP at 284.8 eV and 287.2 eV are characteristic of car-
bon atoms involved in C‐C and C=O bonds, respectively
(Figure S17). In case of the palladium loaded framework,
peaks corresponding to 1s electron of carbon atoms
involved in C‐C and C=O bonding are observed at
284.1 eV and 287.1 eV, respectively (Figure S21).
nanoparticles can be primarily attributed to strong inter-
action with L‐proline. It is pertinent to note here that
presence of PdO within the composite has been ruled
out as characteristic peak for PdO at 2θ = 34° is not
observed in the powder X‐ray diffraction pattern of
Pd@ZrCP.[36] Therefore, presence of Pd2+ within the
composite has been attributed to the coordination of car-
boxylate groups of the zirconium carboxyphosphonate
framework to the palladium atoms on the surface of the
nanoparticles. This hypothesis is also supported by the
observation of a strong signal in the 13C MAS NMR of
Pd@ZrCP at 180.0 ppm which has been attributed to
carboxylate groups coordinated to palladium.
4 | SUZUKI‐MIYAURA CROSS
COUPLING REACTION
XPS of the Pd@ZrCP was investigated further in the
Pd(3d) region to explore presence of palladium as well
as its electronic state over the framework (Figure 6). Inci-
dentally, binding energy of palladium 3d and binding
energy of Zirconium 2p electrons coincide and, therefore,
three distinct peaks were observed at 333.2, 340.2 and
346.6 eV. After deconvolution of the parent peaks, peaks
at 332.8 eV and 346.6 eV can be, respectively, assigned
to 2p3/2 and 2p1/2 electrons of tetravalent zirconium. Sim-
ilarly, peaks at 334.7 eV and 339.9 eV can be assigned to
3d5/2 and 3d3/2 electrons of metallic palladium, respec-
tively. However, apart from these peaks an additional
set of peaks appeared at binding energies 336.2 eV and
341.5 eV and these indicate the presence of palladium
in Pd(II) state. Thus, the XPS spectrum of palladium in
Pd@ZrCP established the presence of palladium in both
Pd(0) and Pd (II) oxidation state while the % proportion
of Pd(II) based on peak area is 28%. Presence of
unreduced Pd(II) on the surface of metallic palladium
Palladium has been used extensively as a catalyst in C‐C
cross coupling reactions. Due to the presence of mixed
valent Pd(0)/Pd(II) nanoparticles of average size 5 nm,
Pd@ZrCP was anticipated to act as a highly active cata-
lyst in C‐C coupling reactions. Therefore, the catalytic
activity of Pd@ZrCP was tested in Suzuki‐Miyaura cross
coupling reaction of aryl bromides with aryl boronic
acids. Initial catalytic investigations carried out by using
4‐bromotoluene and phenyl boronic acids as substrates
showed satisfactory conversion within
a span of
2 minutes. Thereafter, the above reaction was carried
out in different conditions to optimize the base, solvent
and temperature and the results are summarized in
Table 1. By using 0.22 mol% of the catalyst and K2CO3
as a base, excellent yields (above 95%) were obtained
within 2 mins at 80 °C in aqueous ethanolic medium.
However, the reaction did not proceed when only water
was used as solvent or in presence of organic base,
triethylamine. Nevertheless, under the optimized reaction
conditions, the turnover frequency (TOF) of the catalyst,
Pd@ZrCP in Suzuki‐Miyaura cross coupling reaction of
4‐bromotoluene and phenyl boronic acids is found to be
1.3 x 104 hr−1, considering the total amount of palladium
present. The catalytic efficacy of Pd@ZrCP under con-
ventional heating condition is compared with several
recently reported heterogeneous catalysts for Suzuki‐
Miyaura cross coupling reaction (Table 2). Pd@ZrCP
outperforms all reported state‐of‐the‐art heterogeneous
catalyst in terms of reaction completion time as well as
TOF as considerably good yields is obtained within a span
of two minutes even when 0.22 mol% catalyst is used.
Thereafter, cross coupling reaction between several other
substituted bromoaryls and p‐methoxyphenyl boronic
acid were investigated by using Pd@ZrCP as catalyst
under the optimized reaction conditions. The catalyst is
found to give satisfactory yields within 2 mins in case of
FIGURE 6 XPS spectrum of Pd(3d) region Pd@ZrCP

BHATTACHARYYA ET AL.
9 of 13
TABLE 1 Optimization of reaction condition for Pd@ZrCP catalyzed Suzuki‐Miyaura reaction
Entry
Solvent
Base
Catalyst
Time (min)
Temp (° C)
Yield (%)
1
C2H5OH:H2O (3:1)
C2H5OH:H2O (3:1)
C2H5OH:H2O (3:1)
C2H5OH:H2O (3:1)
C2H5OH:H2O (3:1)
C2H5OH:H2O (2:1)
C2H5OH:H2O (1:1)
Toluene
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na3PO4
NaOH
Na2CO3
Cs2CO3
NEt3
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
ZrCP
180
180
20
2
RT
40
60
80
80
80
80
80
80
80
80
80
80
80
80
80
0
0
2
3
23
93
0
4
5
90
2
6
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
Pd@ZrCP
83
78
trace
78
0
7
2
8
2
9
CH3OH
2
10
11
12
13
14
15
16
H2O
2
C2H5OH:H2O (3:1)
C2H5OH:H2O (3:1)
C2H5OH: H2O (3:1)
C2H5OH: H2O (3:1)
C2H5OH: H2O (3:1)
C2H5OH:H2O (3:1)
2
78
81
75
45
0
2
2
2
2
KOH
2
80
Reaction condition: 0.24 mmol phenyl boronic acid; 0.2 mmol 4‐bromo toluene; 0.25 mmol base; catalyst 0.5 mg (0.22 mol%)
TABLE 2 Comparison of the present catalyst with some reported catalysts for the Suzuki‐Miyaura cross coupling reaction
Catalyst
Solvent
Temperature(°C)
Time (Minute)
Yield(%)
Reference
Pd@ZrCP
EtOH/H2O
DMF
80
80
2–10
59–98
81–100
54–99
92–100
43–99
This work
Fe3Pd2(OH)2[PA]8(H2O)4
Pd NPs at NHC at ZIF‐8
Pd‐Fe BN nanosheet
Pd‐amino functionalized SiO2
20–60
18
19
20
21
EtOH/H2O
EtOH/H2O
H2O
90–120
80
15–1440
30–240
30–1440
80
almost all the substrates tested during this investigation.
All the para and meta substituted aryl bromides with
phenyl boronic acid as coupling partner gave good
yields up to 98%. However, p–NO2, p‐COCH3, m‐COCH3
substituted aryl bromides as well as bulky aryl bromides
require slightly longer time of 8–10 mins for completion
of the reaction. Furthermore, relatively poor yields
were observed in case of all reactions involving p–OCH3
substituted boronic acid (Table 3).
by using p‐bromo toluene and phenyl boronic acid as
substrates. For this purpose, the heterogeneous catalyst,
Pd@ZrCP was isolated after each catalytic run and washed
with acetone/ethanol and used directly for the next
catalytic run. It is observed that the catalyst has excellent
recyclability in Suzuki‐Miyaura cross coupling reaction as
no significant loss of conversion efficiency was observed
up to fifth cycle. The recovered catalyst did not show any
sign of deactivation as the TOF during fifth catalytic cycle
was found to be 1.3 x 104 hour−1. The IR spectrum of the
catalyst isolated after second and fifth catalytic cycles are
The recyclability of the catalyst, Pd@ZrCP in
Suzuki‐Miyaura cross coupling reaction was also evaluated

10 of 13
BHATTACHARYYA ET AL.
TABLE 3 Suzuki‐Miyaura cross coupling reaction with different substrates under optimization condition
Sl no.
Reactant 1
Reactant 2
Product
Time (min)
Yield
2
98%
(2a)
(2b)
2
98%
10
8
95%
98%
(2c)
(2d)
8
2
2
96%
93%
95%
(2e)
(2f)
(2 g)
8
4
80%
89%
(2 h)
(2i)
2
2
87%
65%
(2j)
(2 k)
2
2
2
59%
89%
88%
(2 l)
(2 m)
(2n)
2
79%
(2o)
Reaction condition: 0.24 mmol Aryl boronic acid; 0.2 mmol Aryl halide; 0.25 mmol K2CO3; catalyst Pd@ZrCP 0.5 mg (0.22 mol%); solvent: EtOH:H2O (3:1).
in good agreement with the IR spectrum of the fresh
catalyst and all characteristic peaks are observed at nearly
identical wave numbers (Figure 7). Furthermore, the
powder X‐ray diffraction pattern of the catalyst isolated
after fifth catalytic run shows a small peak at 2θ = 39.4°,
characteristic of metallic palladium present in the catalyst
and thereby establish the remarkable stability as well as
reusability of the present catalyst (Figure S22).
of Suzuki‐Miyaura cross coupling reaction, FE‐TEM
images of the catalyst isolated after fifth catalytic
run were recorded. FE‐TEM images clearly reveal that
palladium nanoparticles of average size 5 nm are present
in the recovered catalyst and the morphology of the
particles do not change significantly after 5th catalytic
cycle (Figure 8).
Furthermore, leaching test carried out to investigate
the possible leaching of palladium from the composite
to the reaction solution clearly established that negligible
In order to investigate the change in size and
morphology of the palladium particles during the course

BHATTACHARYYA ET AL.
11 of 13
keeping the soluble reactant Ph(B(OH)2), insoluble
aryl bromide BrPhCONHC3H6@SiO2 and the catalyst
Pd@ZrCP in three different phases. If the catalytic system
remains heterogeneous and no leaching of palladium
take place during the reaction, the supported aryl
bromide will not participate in cross coupling reaction.
A soluble aryl bromide, p‐bromotoluene was also added
to the reaction mixture to ensure presence of an active
species and thus mimic the exact reaction condition.
During the three phase test, the 4‐bromotoluene reacts
with phenyl boronic acid to yield 90% cross coupling
product within 2 mins. However, the immobilized aryl
bromide did not undergo cross coupling reaction which
1
is established by H NMR analysis (Figure S30 & S31) of
the immobilized substrate recovered from the three phase
test mixture.
Mercury poisoning test was also carried out to
investigate the active species present in the catalytic
cycle. As reported the catalytic system consist of both
Pd(0) and Pd(II) species, the active species that take part
in the reaction cycle can be investigated by the mercury
poisoning test. The reaction of 4‐bromoacetophenone
with phenylboronic acid was taken as the model reaction
for this test as this substrate takes comparatively longer
reaction time than other substrates. The progress of the
reaction was monitored by GC–MS, revealed that after
FIGURE 7 FT‐IR spectra of catalyst Pd@ZrCP as prepared and
after 5th
amount of palladium leached out of the composite,
Pd@ZrCP even after five consecutive catalytic run.
ICP‐OES analysis of the organic product isolated after
fifth catalytic run did not show any detectable amount
of palladium. Furthermore, ICP‐OES measurements of
the reaction solution showed that the concentration for
palladium in reaction solution, both during as well as
after completion of the reaction is below the detection
limit (50 ppb) of the measurement.
In order to gain insight into the mechanism as well as
further establish the heterogeneity of the catalytic
species, three phase test has been carried out by using a
previously described procedure.[40] For this purpose,
4‐bromobenzoylchloride was covalently immobilized on
modified silica to prepared a solid supported aryl bromide
substrate, BrPhCONHC3H6@SiO2. Thereafter, Suzuki‐
Miyaura cross coupling reaction was carried out by
3
minutes 45% conversion occur. Elemental mercury
was then added to the reaction mixture and the reaction
was further allowed to proceed for another 10 mins.
GC–MS analysis of the reaction mixture recovered at this
point revealed only 50% conversion and this shows that
the active species that present in the catalytic cycle is
Pd(0) not Pd(II). However, the small difference in
conversion before and after addition of elemental
mercury may be attributed to the presence of Pd(II) in
the catalytic species as evidenced by XPS analysis
discussed above.
FIGURE 8 TEM images of Pd@ZrCP after 5th catalytic cycle

12 of 13
BHATTACHARYYA ET AL.
SCHEME 3 Mechanism of Suzuki‐
Miyaura cross coupling reaction
It is well established that the Suzuki‐Miyaura cross
mixed valent Pd(0)/Pd(II) catalyst in cross‐coupling
coupling reaction passes through oxidative addition,
transmetallation and reductive elimination steps during
its catalytic cycle. During the catalytic cycle palladium
switches oxidation state from Pd(0) to Pd(II). Palladium
leaching test, three phase test and mercury poisoning test
suggest heterogenous nature of the present catalyst in
Suzuki‐Miyaura cross coupling reaction. Based on
results, we suggest a plausible mechanism for Suzuki‐
Miyaura cross coupling reaction catalyzed by Pd@ZrCP
(Scheme 3). The remarkably short reaction completion
time, recyclability without any loss of catalytic efficacy
and resistance to metal leaching of the present catalyst
can be attributed to the interaction of surface L‐proline
groups with the embedded palladium nanoparticles.
reactions.
ACKNOWLEDGMENTS
Generous financial support from the Science and Engi-
neering Research Board (Grant No. EMR/2016/002178)
is gratefully acknowledged. Authors are thankful to C.
Lorentz and C. Chabanier at IRCELYON for solid‐state
NMR and XPS studies, respectively.
ORCID
Shashank Mishra https://orcid.org/0000-0003-2846-4221
Nayanmoni Gogoi https://orcid.org/0000-0001-7398-6200
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