Engineering functional group decorated ZIFs to high-performance Pd@ZIF-92 nanocatalysts for C(sp2)-C(sp2) couplings in aqueous medium

Article abstract of DOI:10.1016/j.jcat.2020.09.024

Herein the first ever Pd²⁺-decorated ZIF-92 nanostructure is readily accessed by post-synthesis modification (PSM) of the zeolitic imidazolate framework ZIF-90. Thus, the imidazolate linker of the ZIF-90 is functionalized with the hydroxyethylimino group, followed by anchoring the palladium catalytically active sites through a tight coordination of Pd²⁺ ions to both the N and O atoms of ZIF-92. The acquired data (by PXRD, EDS, SEM, XPS, ICP-AES) concerning the crystalline structure, elemental mapping and network configuration of the Pd@ZIF-92 catalyst confirmed its structure, before and after the cross-coupling reactions and following recycling. The Pd@ZIF-92 showed excellent catalytic activity (>99%) and chemoselectivity in Suzuki-Miyaura coupling proceeding in neat water, at very low Pd loading (0.009 mol%). Comparison of its performance with that of many reported Pd nanocatalysts operating in aqueous/protic media testifies to its superiority. The appealing gateway to this innovative, stable, robust, easily recoverable and reusable Pd@ZIF-92 nanostructure recommends its beneficial utilization in C(sp²)-C(sp²) cross-couplings and other sustainable, environmentally-friendly processes.

Full text of DOI:10.1016/j.jcat.2020.09.024

Journal Pre-proofs
Engineering functional group decorated ZIFs to high-performance
Pd@ZIF-92 nanocatalysts for C(sp2)-C(sp2) couplings in aqueous medium
Li-Xin You, Bai-Bei Zhao, Shan-Xin Yao, Gang Xiong, Ileana Dragutan,
Valerian Dragutan, Fu Ding, Ya-Guang Sun
PII:
S0021-9517(20)30392-4
https://doi.org/10.1016/j.jcat.2020.09.024
YJCAT 13912
DOI:
Reference:
To appear in:
Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
10 August 2020
21 September 2020
23 September 2020
Please cite this article as: L-X. You, B-B. Zhao, S-X. Yao, G. Xiong, I. Dragutan, V. Dragutan, F. Ding, Y-G.
Sun, Engineering functional group decorated ZIFs to high-performance Pd@ZIF-92 nanocatalysts for C(sp2)-
C(sp2) couplings in aqueous medium, Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.2020.09.024
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Engineering functional group decorated ZIFs to high-performance
Pd@ZIF-92 nanocatalysts for C(sp2)-C(sp2) couplings in aqueous
medium
Li-Xin Youa, Bai-Bei Zhaoa, Shan-Xin Yaoa, Gang Xionga, Ileana Dragutanb, Valerian
Dragutanb,*, Fu Dinga, Ya-Guang Suna,c,*
a Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, Shenyang
University of Chemical Technology, Shenyang, China
b Institute of Organic Chemistry, Romanian Academy, Bucharest, Romania
c Key Laboratory of Resource Chemical Technology and Materials, Ministry of Education,
Shenyang University of Chemical Technology, Shenyang, China
ABSTRACT
Herein the first ever Pd2+-decorated ZIF-92 nanostructure is readily accessed by post-synthesis
modification (PSM) of the zeolitic imidazolate framework ZIF-90. Thus, the imidazolate linker
of the ZIF-90 is functionalized with the hydroxyethylimino group, followed by anchoring the
palladium catalytically active sites through a tight coordination of Pd2+ ions to both the N and
O atoms of ZIF-92. The acquired data (by PXRD, EDS, SEM, XPS, ICP-AES) concerning
the crystalline structure, elemental mapping and network configuration of the Pd@ZIF-92
catalyst confirmed its structure, before and after the cross-coupling reactions and following
recycling. The Pd@ZIF-92 showed excellent catalytic activity (>99%) and chemoselectivity in
Suzuki-Miyaura coupling proceeding in neat water, at very low Pd loading (0.009 mol%).
Comparison of its performance with that of many reported Pd nanocatalysts operating in
aqueous/protic media testifies to its superiority. The appealing gateway to this innovative, stable,
robust, easily recoverable and reusable Pd@ZIF-92 nanostructure recommends its beneficial
utilization in C(sp2)-C(sp2) cross-couplings and other sustainable, environmentally-friendly
processes.
Keywords:
1-st Pd-ZIF-92 heterogeneous catalyst; MOFs, PSM; C-C cross-couplings; low palladium
loading; benign water medium.

1. Introduction
With ever increasing demands for protection of the environment particular attention
has been paid to more sustainable, innovative processes which, by using benign
materials, less energy input and minimizing the newly-generated chemical waste, fulfil
many key criteria of green chemistry. Progress in this respect also regards the efficiency,
accessibility and recyclability of the employed catalysts and milder reaction conditions
(especially use of less toxic solvents) to diminish the final environmental impact. Along
these lines, immobilizing the catalyst on solid supports has since long been adopted as
it complies with some of the above requests while ensuring a lower cost of the operation
by catalyst reuse and reduction of harmful residues through prevention of metal leaching.
Catalyst supports have in turn progressed to more advanced, better suited variants. A
prominent place among the newest generations of such versatile materials is currently
taken by metal-organic frameworks (MOFs), constructed from metal-ions or clusters,
coordinated by multidentate organic linkers [1-10]. A special class of MOFs emerging
as newest generation is represented by the zeolitic imidazolate frameworks (ZIFs) [11].
Endowed with high surface area, enhanced thermal and chemical stability, unusual
robustness, adjustable nanoporosity and excellent tunability conferred by the organic
functional groups covalently attached to the imidazolate linker, ZIFs have rapidly gained
a plethora of fruitful applications in many fields, such as gas separation, drug delivery,
sensing devices and notably in heterogeneous catalysis [12,13].Most studied and applied
in metal-based catalysis are ZIF-8 and ZIF-67 while ZIF-92 is only rarely encountered.
As a further asset, post-synthesis modification (PSM) [14,15] of ZIFs and MOFs is
instrumental in expanding their chemical diversity and potential applications.
Advantages of the PSM approach consist in: (a) high-throughput optimal
functionalization, independent of the framework synthesis conditions; (b) initial
structure can be modified to produce further arrangements with the same topology yet
different properties; (c) diverse substitution patterns allow a plurality of the functional
units introduced into a single framework thus enabling an efficient tuning of its
characteristics; (d) purification and isolation of the modified products are facile because
the chemical derivatization is performed directly on crystalline solids. In catalysis, active
centers are connected with backbones of the ZIFs and MOFs by covalent, coordinate
covalent, and combination thereof.
In view of the pivotal role of Pd in a variety of metal-catalyzed organic reactions [16-

21], engineering palladium catalysts has been arduously pursued, particularly in C(sp2)-
C(sp2) cross-coupling reactions [22-24]. The well-established Suzuki-Miyaura reaction
between aryl halides and arylboronic acids [23] represents a preferred methodology for
synthesis of not only conventional biaryl molecules [24a,b] or intricate cyclic and
heterocyclic frameworks [24c,d] but quite recently for advanced applications such as
aggregation-induced emission luminogens (AIEgens) [24e], multifunctional organic electronics
based on semiconducting 2D coordination polymers accessed by an interfacial modified Suzuki
protocol [24f] However, the standard procedures using homogeneous catalysts suffer
from waste of noble metal which, too difficult to be recovered, also give rise to highly
toxic residues in the products and generate substantial palladium leaching in the
environment. Progress towards heterogeneous catalysts made possible recovery and
recycling while maintaining a high catalytic activity. Also utilization of water as a
suitable reaction medium has received increasing attention over the last few decades
[25]. In comparison with organic solvents, commonly used in the palladium-catalyzed
Suzuki-Miyaura cross-couplings, water has proved quite advantageous since it is not
toxic or flammable, highly abundant and, most importantly, environment-friendly.
In our previous work [26], taking advantage by the PSM protocol, the Fe3O4@La-
MOF-Schiff base-Pd nanocatalyst has been prepared from Fe3O4@La-MOF through
condensation with pyridine-2-carboxaldehyde to the imine derivative of Fe3O4@La-
MOF-Schiff base that was subsequently functionalized by coordinating PdCl2 to the
available nitrogen atoms. This Fe3O4@La-MOF-Schiff base-Pd configuration proved to
be very active, quite stable, magnetically recoverable and reusable in C(sp2)-C(sp2)
cross-coupling reactions. Quantitative yields of biaryls (>99%) have been attained at low
Pd loadings. Synergistic cooperativity between the oxophilic La nodes and the organic
linker in the La-MOF has been also demonstrated. In another investigation [27] we
designed a well-defined Pd@Eu-MOF nanocatalyst which displayed a high activity and
stability in Suzuki-Miyaura reactions. TEM and XPS measurements on the as-prepared
and recycled Pd@Eu-MOF confirmed that it has maintained its original structure and
morphology while no Pd NPs agglomeration was observed.
The present research extends the PSM strategy for constructing a new Pd-ZIF
ensemble taking into account that Pd on ZIFs is largely underexplored in C(sp2)-C(sp2)
cross-couplings [28,29]. Here unveiled are the preparation, characterization, and
catalytic properties of a novel, innocuous heterogeneous nanocatalyst, Pd@ZIF-92. This
is, to the best of our knowledge, the first report on the use of ZIF-92 as support in Pd

catalysis. The choice of ZIF-92 as support for Pd catalysts has multiple reasons. We
expect that ZIF-92, bearing an ethanolimine function outside the framework matrix, will
provide a stable bidentate coordination mode of the palladium active sites resulting from
both oxophilic and nitrophilic abilities of palladium to bind to oxygen and nitrogen of
the ethanolimine motif. Furthermore, this type of functionalization will confer a
favourable exposure of the palladium active sites to the approaching reagents. The
functionality arranged evenly on the outer shell of the framework may reasonably
control the distribution of palladium on the surface of the catalyst and prevent metal
aggregation during the catalytic process. Such distinct features of the Pd active sites
enable our Pd@ZIF-92 to attain a high catalytic activity at very low catalyst loading.
Synthesis of Pd@ZIF-92 involves three reaction steps, i. e. (i) obtaining the zeolitic
imidazolate framework ZIF-90 [30,31], (ii) post-modification of ZIF-90 to ZIF-92 [32],
followed by (iii) anchoring of palladium into ZIF-92 through complex formation
(Scheme 1). The rationale of our concept is to devise an active, robust and easy
accessible Pd catalyst, supported on the hydrophilic ZIF-92. We prove that this new
nanostructure, with uniform size and unique morphology, exhibits an excellent activity,
chemoselectivity and recyclability in C(sp2)-C(sp2) cross-couplings in pure water as an
environment-friendly solvent.
Scheme 1 Synthesis route of Pd@ZIF-92.
2. Experimental
2.1. Synthesis of the catalyst (Scheme 1)
ZIF-90 [29-31] and ZIF 92 [31] were prepared according to literature: ZIF-90 by reaction
of Zn(NO3)2⸳6H2O with imidazole-2-carboxaldehyde and ZIF 92 by condensation of the
aldehyde group in ZIF-90 with 2-hydroxyethylamine. Synthesis of Pd@ZIF-92 was carried out
in methanol using ZIF-92 and PdCl2 (see Support Info). As only Pd@ZIF-8 has been

communicated previously for C(sp2)-C(sp2) couplings [25,28], this is the first synthesis of
Pd@ZIF-92 and first application of ZIF-92 as catalyst support in such processes. Additionally,
upgrading hydrophilicity from ZIF-90 to ZIF-92, by linker functionalization, has opened ways
to a greener platform for eco-friendly, heterogeneous Pd catalysis for cross-coupling reactions
in water.
2.2. Characterization of supports and the catalyst
Powder X-ray diffraction (PXRD) patterns of the samples were collected on an X-
ray diffractometer (BRUKER D8 ADVANCE) with Cu-Kalpha radiation. N2 gas
sorption experiments were carried out on a V-Sorb 2800 TP volumetric gas sorption
instrument. Gas chromatograph (GC) analyses were performed on an Agilent
Technologies 7890A gas chromatograph. X-ray photoelectron spectroscopy (XPS) was
run on Thermo VG ESCALAB250. Inductive coupled plasma atomic emission spectroscopy
(ICP-AES) was determined on Thermo IRIS Advantage. Scanning electron microscopy
(SEM) images were taken with a German ZEISS SUPRA 55 high resolution field
emission scanning electron microscope. Energy dispersive X-ray spectroscopy (EDS) was
taken on a German ZEISS SUPRA-55 high resolution field emission scanning electron
1
microscope with an Oxford-AztecX-Max80 X-ray energy spectrometer. H NMR
spectra were recorded on a Bruker BioSpin GmbH AVANCE III 500 MHz spectrometer
operating at 500 MHz.
2.3. General reaction conditions for the Suzuki-Miyaura cross-coupling reaction
The typical experimental procedure for the Suzuki-Miyaura coupling reactions using the
Pd@ZIF-92 as catalyst is as follows. A mixture of aryl halide (1.0 mmol), boronic acid (1.2
mmol), base (2.0 mmol, the indicated species), solvent (6.0 mL, as indicated) and catalyst (8
mg) were placed in a Schlenk tube and stirred in air, at the appropriate temperature for the
certain time (as indicated in Tables 1). After completion of the reaction, the mixture was
extracted three times with ethyl acetate. The product yield was determined by GC analysis using
hexadecane as the internal standard.
3. Results and discution
3.1. Catalyst characterization
This is the first synthesis of Pd@ZIF-92 and first application of ZIF-92 as catalyst
support in such processes. Also, upgrading hydrophilicity from ZIF-90 to ZIF-92, by
linker functionalization, has opened ways to a greener platform for eco-friendly,
heterogeneous Pd catalyst. The prepared Pd@ZIF-92 was characterized by means of

various techniques. The powder X-ray diffraction pattern (PXRD) (Fig. 1) revealed that
the uniform crystalline state of the ZIF-90 backbones was kept during the post-
modification experiments and during the catalytic cycles run with Pd@ZIF-92 (fresh
and after recycling). From the N2 adsorption-desorption isotherms (Fig. S1) a slight
decrease of the Brunauer-Emmett-Teller (BET) surface areas (from 1573.8 of ZIF-90 to
1152.7 m2g-1 of Pd@ZIF-92) (Langmuir Surface Area) was observed after PSM,
correlated with the presence of the imino functional groups.
Fig. 1. PXRD of different samples.
Fig. 2. EDS spectrum of Pd@ZIF-92.
To map the distribution of the different constituent atoms (N, Cl, Zn, and Pd) and get
more direct evidence on the catalyst structure, we have applied EDS (Fig. 2). EDS
mappings confirmed that these elements are evenly distributed within the frame, as
evidenced also by SEM images (Fig. 3). XPS spectra of Pd@ZIF-92 (Fig. 4), run before

and after reaction, further revealed that all Pd species within Pd@ZIF-92 are in the +2
oxidation state, corresponding to the binding energies of 337.3 and 342.5 eV of Pd3d5/2
and Pd3d3/2 (before the reaction) and to 337.2 and 342.6 eV of the Pd3d5/2 and Pd3d3/2
levels (after reaction). The ICP-AES measurement of Pd@ZIF-92 indicated that the
palladium content is 0.12 wt%. Results of the above determinations confirm a successful
synthesis of the catalyst and that the catalyst structure is essentially preserved during the
reaction.
Fig. 3. SEM image (a) and elemental mappings of Pd@ZIF-92 show the presence of O,
N, Zn, Cl and Pd atoms (b-f).
Fig. 4. XPS spectra of Pd@ZIF-92: before the reaction (left) and after the reaction (right).
3.2. Catalytic activity
The catalytic profile of the Pd@ZIF-92 was investigated in the Suzuki-Miyaura
cross-coupling reaction of aryl halides with arylboronic acid (Table 1). In this model
reaction and under Pd@ZIF-92 catalysis, the optimal experimental conditions have been
explored using different solvents and bases. Best results were obtained in pure water as
the solvent, with K2CO3 as the base (yield >99%) (Table 1, entry 1,2). When instead of

water, N,N-dimethylformamide (DMF), toluene or EtOH were used as solvents (Table
1, entry 9-11) and Cs2CO3, NaOH, t-C4H9ONa or Et3N as alternative bases (Table 1,
entry 12-15), the yields decreased significantly.
Table 1 Catalytic activity of Pd@ZIF-92 nanocatalyst in Suzuki-Miyaura cross-
coupling of aryl halides with arylboronic acids.a
R1
R2
X
Pd@ZIF-92
+
R1
R2
Base, Solvent
B(OH)2
Entry
Halide
(R1/X)
H/Br
H/I
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
H/Br
Arylboronic
acid (R2)
Temperature
Yield
(%)
>99b
>99
95
88
76
70
50
27
86
22
<5
78
80
81
83
Solvent
Base
Time (h)
(oC)
80
80
80
80
80
70
60
50
80
80
80
80
80
80
80
80
80
80
80
80
90
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
DMF
Toluene
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
NaOH
Et3N
NaOC4H9-t
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
6
6
5
4
2
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
12
CH3/Br
COCH3/Br
H/Br
H/Br
COCH3/Cl
COCH3/Cl
92c
>99d
71
>99
19
48
COCH3
CH3
H
H
a
Reaction conditions: aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), base (2.0 mmol),
solvent (6.0 mL), Pd@ZIF-92 (8.0 mg, 0.009 mol%); b See Fig. S2; c See Fig. S3; d See Fig. S4.
The choice of pure water as the reaction medium was deliberate in consideration not only
of the hot ongoing research for benign catalysis in aqueous systems [33] and the impressive,
recent efforts devoted to Suzuki-Miyaura C-C coupling proceeding in cleaner media [34,35a].
Water being fundamentally distinct from organic solvents implies different reactivities and

selectivities of reactions run in this medium. Also importantly, the structure assumed by each
ZIF depends primarily on the solvent used [35b]. Or, ZIFs have recognized water absorption
capacities [35c] and this absorbed water facilitates the Pd catalytic cycle (Scheme 2). As
expected, water led, to our satisfaction, to the best performance. Mixed water-solvent systems
(water-DMSO, water-DMF, water-alcohols, etc.) [36,37] or mere organic solvents (MeOH,
toluene, etc), detrimental to the environment, can occasionally give poorer results [38ab].
On investigating the influence of reaction temperature and time, lower yields have
been recorded when decreasing either of these reaction parameters (Table 1, entry 3-8).
The highest yield, more than 99% (Table 1, entry 1,2) was attained in reactions
o
proceeding in water, at 80 C, for 6 h, with 8 mg catalyst loading, that is a lower Pd
content than for most Suzuki-Miyaura reactions so far reported demonstrating the
excellent catalytic activity of our Pd@ZIF-92.
The effect of substituents on the reaction has also been considered. Results show that
the bromo and iodo derivatives led to excellent product yields (Table 1, entry 1,2,17）
while chloride substituents had a rather negative impact (Table 1, entry 20); however,
when the reaction temperature was increased by 10 oC, and time from 6 to 12 h, even in
this case a better yield could be attained (Table 1, entry 21).
Our study was next extended to substituted arylboronic acids with electron-donating
(4-Me; Table 1, entry 19) and electron-withdrawing substituents (4-acetyl; Table 1, entry
18), Results confirmed a higher yield for the former (>99% for 4-Me) vs. the latter (4-
COMe, 71%) indicating, as foreseen, that our Pd@ZIF-92 nanocatalyst performs better
in the case of arylboronic acids with electron-donating substituents.
A comparison of the catalytic activity of Pd@ZIF-92 with that of diverse other Pd
nanocatalysts supported on MOFs or organic polymers, in the cross-coupling reaction of
bromobenzene with phenylboronic acid, is relevant for the performance of our optimized
catalytic system in pure water and low palladium loading (Table 2). Noteworthy,
Pd@ZIF-92 nanocatalyst provided more than 99% yield of biaryls when operating in pure
water, under moderate reaction conditions, at very low palladium loading, as compared to other
catalysts. It can be observed that the nature of the catalysts and solvents are of paramount
importance for the product yield and necessary reaction conditions (temperature and
time) (Table 2). As seen from Table 2, our data are comparable with the excellent results
reported for Pd-dendritic catalysts (Entry 14). We assume that the rationally designed
configuration ensuing PSM has allowed an even distribution of the Pd catalytic sites on
the surface of the support, hence a diminished aggregation of Pd during the catalytic

process. This resulted in a reduced noble metal leaching and a favourable utilization of
the Pd@ZIF-92 nanocatalyst in cross-coupling reaction, thus allowing a low catalyst
loading. Of a particular relevance are the fast-growing advancements on Suzuki cross-
coupling in aqueous media offering superb examples of highly efficient palladium
catalysts, even at the ppm or sub-ppm Pd loadings [51].
Table 2 Comparison of the catalytic behaviour of Pd@ZIF-92 with related Pd
nanostructures in the reaction of bromobenzene with phenylboronic acid.
Entry
Catalyst
Reaction
Conditions
80oC, 6h
80oC, 6h
90oC, 2h
rt, 8h
Solvent
Yield (%)
Ref.
1
2
3
4
5
Pd@ZIF-92a
[SmPd2[L]2Cl4]n[H3O]n
PdNPs/PzCb
H2O
H2O
H2O
H2O
H2O
>99
99
94
89
59
This work
[5h]
[39]
[40]
[41]
PS-Pd-salenc
Pd/MOP-bipy
70oC, ˗
50oC, 1h
70oC, 1h
50oC, 4h
75oC, 1h
6
7
8
9
CL-salen-Pd(II)d
Cell-Sb-Pd(II) catalyst
[EuPd2[L]2Cl4]n[H3O]n
Fe3O4@SiO2-Dendrimer-Pd
Fe3O4@-SiO2/isoniazide/Pde
EtOH-H2O
EtOH-H2O
EtOH-H2O
EtOH-H2O
99
99
99
97
96
96
94
90.9
100
86
73
99
99
99
95
81
94
92
[42]
[43]
[5h]
[44]
[45]
[46]
[47]
[48]
[49]
[25e]
[5i]
[27]
[26]
[5h]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25oC, 1.5h EtOH-H2O
f
Pd(0/II)/CS-bigua@Fe3O4
25oC, 2h
80oC, 1h
EtOH-H2O
EtOH-H2O
Pd-Fe3O4/rGOg
Pd/g-C3N4
25oC, 0.5h EtOH-H2O
FcDPdNPsh
RT, 1h
70oC, 3h
80oC, 9h
80oC, 6h
80oC, 0.5h
50oC, 4h
80oC, 1h
80oC, 6h
95oC, 4h
95oC, 4h
95oC, 4h
THF-H2O
DMF-H2O
DMF-H2O
EtOH
EtOH
EtOH
EtOH
EtOH
C6H5CH3
C6H5CH3
C6H5CH3
Pd/Pr-MOF
Pd/Sm-MOF,
PdNPs/Eu-MOF
Fe3O4@La-MOF-Pd
[TbPd2[L]2Cl4]n[H3O]n
HCPs-Bipy-Pdi
Pd/Eu-MOF
[50]
[27]
[38b]
[38b]
[38b]
Polymer
Pd/Eu-MOF
Pd/Tb-MOF
Pd/Gd-MOF
91
a
b
c
0.009 mol% Pd.
Catalyst is bispyrazole-based cryptand 3 mol %; TBAB.
e
supported Pd-salen. d CL = cellulose; 0.5 mol% Pd. 0.2 mol% Pd. f 0.2 mol% Pd. g 2 wt% Pd.
h Pd : Dendron = 5 : 1. i Hypercrosslinked polymers.
The role of water as the solvent for the exceptional outcome of the cross-coupling
reaction with our catalyst is striking. In the case of the easier ionizable bromo- and iodo-
aryl derivatives we suppose that water, abundantly adsorbed on the surface of ZIF-92,

substantially accelerate the transmetalation step of the palladium catalytic cycle, in a
concerted manner with the base (Scheme 2) [52].
Pd@ZIF-92
ZIF-92
PdCl2
This work
R1-X
R1-R2
Oxidative Addition
Reductive Elimination
Linker, Base
O
Cl
Cl
Pd(II)
N
R1
Cl
R1
O
N
O
N
Cl
Cl
Pd(IV)
Pd(IV)
R2
Pd@ZIF-92
Cl
OH
Y
B
Y
X
OH
OH
R1
O
N
Cl
Cl
Transmetalation
Pd(IV)
OH
Water, Base
Water, Base
X
Y
B
Y
R2
OH
R2
OH
B
Y
Y
Scheme 2 Effect of water and the base on the Pd catalytic cycle.
Fig. 5. Recycling of Pd@ZIF-92 in cross-coupling of bromobenzene (1.0 mmol) with
phenylboronic acid (1.2 mmol) in water (6.0 mL), K2CO3 (2.0 mmol), 80 oC, 6 h.
Moreover, also in the reductive elimination step water may enhance the effect of the
base, in corroboration with the increased hydrophilicity of ZIF-92 [53]. By contrast, the
low yield achieved with chloro derivatives can be accounted for by admitting the well-

known greater stability of C-Cl bonds, as against C-Br and C-I bonds, in the oxidative
addition step [54].
To prove that Pd@ZIF-92 behaves as a heterogeneous system, a heterogeneity test
was performed. First, the reaction between bromobenzene and phenylboronic acid was
carried out under our standard conditions and, as anticipated, resulted in a high yield (>
99%). Then, the supernatant was added to a mixture of p-bromotoluene and
phenylboronic acid, without adding the catalyst. As no product was detected, and
moreover no leakage of Pd ions could be identified by ICP-AES in the clear water
filtrate, the important conclusion from this test is that catalysis by Pd@ZIF-92 occurs
heterogeneously [52f,55]. By recycling experiments, it was found that Pd@ZIF-92 could
be reused in five consecutive runs without a significant decrease of the catalytic activity
(Fig. 5).
4. Conclusions
This contribution enables a straightforward access to the first example of Pd@ZIF-92, a
highly efficient, heterogeneous nanocatalyst and its valorization in green Suzuki-Miyaura
cross-couplings. Assets of the new hydrophilic Pd@ZIF-92 nanostructure are its availability by
a facile and inexpensive synthetic procedure, its robustness, recoverability and reusability. The
catalyst showed an excellent activity in eco-friendly cross-couplings in neat water, at quite low
palladium loading, and under mild reaction conditions. It can be easily separated and reused
over more than five reaction cycles. Quite importantly, the preferred solvent employed in the
catalytic reaction is water as sole component and not the usual solvent systems that create
problems in recycling. Cooperative effects of the palladium active sites, ZIF support, ligand
and water ensure a synergistic control of the catalyst activity and reaction chemoselectivity.
This environmentally benign and economical option is in line with modern concepts of green
chemistry and emerging strategies for sustainable technologies. The hydrophilic propensity of
ZIF-92, resulting from functionalization of the ZIF-90 linker, provides a greener opportunity
for heterogeneous catalysis in water. Our concise and efficient procedure may open alternative
ways for advances in Pd catalysis in C-C cross-couplings and related organic transformations.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgement

This work was supported by the National Natural Science Foundation of China (21671139),
the Distinguished Professor Project of Liaoning province (2013204), Scientific Research
Project of Education Department of Liaoning Province (LDF2017001), Program for Liaoning
Innovative Research Team in University (LT2018012) and the Science and Technology
Innovation Program for Middle-aged and Youth Talents of Shenyang (RC180086). ID and VD
acknowledge support from the Institute of Organic Chemistry of the Romanian Academy.
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Graphical Abstract
HO
PdCl2
NH2
MeOH
MeOH, 60 ℃
ZIF-92
Pd@ZIF-92
ZIF-90

Highlights
 First Pd@ZIF-92 nanocatalyst for efficient C(sp2)-C(sp2) cross-
coupling reactions.
 Post-synthetic modification of ZIF-90 to ZIF-92 and Pd@ZIF-92.
 High catalytic activity and recyclability for reactions in pure water.
 Environment-friendly, stable and robust nanocatalyst for cross-
couplings.