Hydrogel supported vinylimidazole based PEPPSI-Pd-NHC catalysts: The catalytic activities in Heck and Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1016/j.molstruc.2020.127948

Increasing environmental awareness and limited raw material resources have increased the importance of catalysts by leading to green technology. For this purpose, there is a need for efficient, new, selective, reusable, environmentally friendly catalysts

Full text of DOI:10.1016/j.molstruc.2020.127948

Journal Pre-proof
Hydrogel supported vinylimidazole based PEPPSI-Pd-NHC catalysts: The catalytic
activities in Heck and Suzuki-Miyaura coupling reactions
Cihangir Boztepe, Asım Künkül, Nevin Gürbüz
PII:
S0022-2860(20)30273-8
DOI:
https://doi.org/10.1016/j.molstruc.2020.127948
MOLSTR 127948
Reference:
To appear in: Journal of Molecular Structure
Received Date: 9 November 2019
Revised Date: 29 January 2020
Accepted Date: 20 February 2020
Please cite this article as: C. Boztepe, Ası. Künkül, N. Gürbüz, Hydrogel supported vinylimidazole based
PEPPSI-Pd-NHC catalysts: The catalytic activities in Heck and Suzuki-Miyaura coupling reactions,
Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.127948.
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Credit authorship contribution statement
Cihangir Boztepe : Conceptualization, Methodology, Investigation,
Writing - original draft, Writing-review & editing, Funding acquisition.
Asım Künkül: Conceptualization, Investigation, Writing - original draft,
Supervision, Writing - review & editing.
Nevin Gürbüz: Conceptualization, Methodology, Investigation,Resources,
Writing - original draft

Hydrogel supported vinylimidazole based PEPPSI-Pd-NHC catalysts: The
catalytic activities in Heck and Suzuki-Miyaura coupling reactions
Cihangir Boztepea, Asım Künkülb, Nevin Gürbüzc
aDepartment of Biomedical Engineering, Faculty of Engineering, İnönü University, Malatya,
44280, Turkey
(e-mail: cihangir.boztepe@inonu.edu.tr)
bDepartment of Chemical Engineering, Faculty of Engineering, İnönü University, Malatya,
44280, Turkey
(email: asim.kunkul@inonu.edu.tr)
cDepartment of Chemistry, Catalysis Research and Application Center, İnönü University,
Malatya, 44280, Turkey
(email: nevin.gurbuz@inonu.edu.tr)
Correspondence to: Cihangir Boztepe (E-mail: cihangir.boztepe@inonu.edu.tr , Tel: +90 422
377 37 24)
ABSTARCT
Increasing environmental awareness and limited raw material resources have increased the
importance of catalysts by leading to green technology. For this purpose, there is a need for
efficient, new, selective, reusable, environmentally friendly catalysts and catalytic systems. In
this study, PEPPSI (pyridine enhanced precatalyst preparation, stabilization, and initiation)
type and vinylimidazole (VI) based palladium complexes of N-heterocyclic carbenes
(PEPPSI-Pd-NHCs) which have catalytic activity in homogeneous Suzuki and Heck reactions
were prepared. To obtain environmentally safe polymeric heterogeneous catalysts, this novel
PEPPSI-Pd-NHC complexes were polymerized with Acrylamide (AAm) and 2-acrylamido-2-
methylpropane sulfonic acid (AMPS) by free radical polymerization. Characterization of
PEPPSI-Pd-NHC complexes and polymer supported catalysts were made by using various
elemental and structural analyses techniques. The synthesized polymeric supported

heterogeneous catalysts have high activity, stability and reusability for Suzuki and Heck
reaction of aryl bromides in aqueous media.
Keywords: Heterogeneous catalysts, Suzuki-Miyaura and Heck reactions, Polymer-based
catalysts, PEPPSI-Pd-NHC.
1. Introduction
Carbon-carbon (C-C) bond formation reactions are important reactions in the industry. Many
chemicals used in industries such as pharmaceuticals, agriculture, food, paint, and plastics
have C-C bonds. For decades, N-heterocyclic carbenes (NHCs) and their transition metal
complexes have been widely used as catalysts in organometallic chemistry and particularly in
C-C bond formation reactions. NHCs are key components in catalysis which have become
common ligands in organometallic and inorganic chemistry due to their tunable steric and
electronic properties. [1,2]
PEPPSI type (PEPPSI= Pyridine-Enhanced Precatalyst
Preparation, Stabilization, and Initiation) Pd-NHC complexes are easily handled, air and
moisture stable complexes which have attracted much attention as efficient catalysts in
catalytic formation of C-C and carbon-heteroatom bonds, due to their strong σ- donation and
weak π-accepting abilities [3,4]. Generally, they have been used widely as homogeneous
catalysts in many C-C bon forming reactions such as Suzuki–Miyaura (often shortened to the
Suzuki reaction), Heck-Mizoraki, Sonogashira, Stille, Kumada and Negishi reactions [5,6].
Suzuki reaction is the most commonly used C-C bond-forming method which occurs between
boronic acid derivatives and organic electrophiles to produces the valuable biaryl products
such as pharmaceutics, polymers, organic materials, semiconductors and agrochemicals [7,8].
Suzuki reaction has many advantages such as the ability to tolerate a wide range of organic
functional groups, mild reaction conditions, stability, ease of preparation and low toxicity of
the organoborane compounds. Due to its superior properties, the Suzuki reaction is widely
used in C-C bond forming reactions [9, 10]. The palladium-catalyzed Heck reaction, one of
the most utilized cross-coupling reactions, is carbon-carbon bond-forming processes between
the aryl/vinyl halides and olefins under basic conditions in the synthetic organic chemistry
[11,12]. Heck reaction has been widely used as a tool for organic synthesis due to various
aryl sources have been utilized and many types of ligands have been explored for the
palladium catalysts. The high thermal stability of NHCs makes these complexes particularly

suitable for Heck coupling reactions due to Heck coupling reactions often require high
temperatures [13-15].
NHC catalysts are homogeneous catalysts and their major disadvantage is that they are not
easily separated from the reaction medium. Increased environmental awareness and limited
raw material resources have led to the shift towards green technology, which has increased the
importance of heterogeneous catalysts [16,17]. So, there is a need for effective, selective,
reusable, environmentally friendly heterogeneous catalysts and catalytic systems [18,19]. The
best way to combine the advantages of homogeneous and heterogeneous catalysts is to bind
soluble and catalytic active metal complexes onto suitable materials. The polymer-supported
catalysts obtained by placing on a matrix can combine the advantages of homogeneous
catalyst (such as chemical activity and selectivity) and heterogeneous catalyst (such as
stability, ease of separation and reusability) [20, 21]. Hydrogels have been great interest in
many fields such as physics, chemistry, biotechnology and nanotechnology due to having a
larger surface area compared to the bulk states of metals. Hydrogels are three-dimensional
cross-linked hydrophilic polymers which can hold large amounts of water in aqueous media
[22, 23]. Unlike conventional carrier materials, hydrogels provide a very large surface area to
metals thanks to their three-dimensional network structures. Their metal composites are used
as catalysts, sensors, antimicrobial materials, magnetic drug delivery materials, and electronic
materials [24, 25]. Hydrogel supported catalysts provide a cheaper, environmentally friendly
reaction medium to the reactants in water without the need for organic and toxic solvents and
they can be reused easily by regenerating [26, 27].
In the literature, many types of hydrogel-palladium composites were prepared and their
catalytic activity in organic transformation reactions was investigated. Generally,
immobilization and stabilization of palladium nanoparticles in support materials is required
for high catalytic activity. Their agglomeration in the support material and leaching into the
medium reduces catalytic activity. [28-30]. Copolymerization of metal containing monomers
is an alternative method to synthesis of highly stable polymer supported metal nanoparticles.
In principle, some metal-containing monomers which have vinyl groups can be polymerized
with co-monomers. [31-33]. In the literature, there is only one study on the polymerization of
PEPPSI-Pd-NHC complexes to obtain the hydrogel-palladium catalyst. Boztepe et al. have
been synthesized dimethyl benzimidazole and vinylimidazole based PEPPSI-Pd-NHC
complexes which have catalytic activity in homogeneous Suzuki reactions. And then, they
have been prepared hydrogel supported PEPPSI-Pd-NHC catalysts from them by

polymerization method as a new technique. The catalytic activities of these polymeric
catalysts were used in Suzuki reactions and it has been explored that the vinylimidazole based
polymeric catalysts showed better catalytic activity and stability in aqueous media [34].
In this study, catalytically active, vinylimidazole based and novel PEPPSI-Pd-NHC
complexes 2a-f were prepared by using Vinylimidazole-based salts 1a-f. Then, these
homogenous catalysts were converted to polymer-supported heterogeneous catalysts by
polymerization of them with acrylamide (AAm) and 2-acrylamido-2-methyl-1-
propanesulfonic acid (AMPS) monomers. The chemical composition analysis of PEPPSI-Pd-
NHC complexes was made by using NMR, FT-IR, and elemental analyses while the chemical
and morphological analysis of hydrogel supported catalysts were made by using FT-IR, SEM,
SEM-EDX, TEM, TGA analysis techniques. To investigation the catalytic activities of all
catalyst systems, they were used in Suzuki and Heck coupling reactions of aryl bromides.
2. Experimental Section
All chemicals were commercial products. All reactions for the preparation of
Vinylimidazolium salts (1a- 1f) and palladium-(NHC)-PEPPSI complexes (PEPPSI-Pd-NHC
complexes, 2a-2f) were carried out under air. PdCl2, pyridine and solvents were purchased
from Sigma-Aldrich and used as obtained. Acrylamide (AAm) and 2-acrylamido-2-
methylpropane sulfonic acid (AMPS) were purchased from Merck and they were crystallized
from an ethanol solution.
2.1. Synthesis of azolium salts, NHC-palladium-pyridine complexes and hydrogel-based
heterogeneous PEPPSI-Pd-NHC catalysts
Vinylimidazole-based salts (1a- 1f) were prepared according to known methods (Scheme 1)
[35-37]. The palladium-NHC-pyridine (PEPPSI-Pd-NHC) complexes (2a-2f) were
synthesized with the method reported by Organ (Scheme 2) [38]. Molecular structures of
them were determined by NMR and FT-IR analyses. The poly(AAm-co-AMPS) supported
PEPPSI-Pd-NHC heterogeneous catalysts were prepared according to published procedure
(Scheme 3) [34].
Homogeneous and heterogeneous Suzuki and Heck coupling reactions were performed
according to published method [34].
2.2. NMR Spectroscopy

1H NMR and 13C NMR spectra were recorded with a Bruker Avance III 400 MHz NMR
spectrometer with sample solutions prepared in DMSO. The chemical shifts were reported in
d units downfield from the internal reference (Me4Si).
2.2.1.1-Vinyl-3-(2-methylbenzyl)imidazolium chloride, 1a
o
1
Yield: 84% (2.29 g). m.p.: 189-190 C; v(CN) = 1634 cm-1. H NMR (δ, DMSO): 2.32 (s, 3H,
NCH2C6H4-CH3); 5.49 (s, 2H, NCH2C6H4-CH3); 5.43 and 6.01 (dd, 2H, J = 8 Hz,
NCH=CH2); 7.26-7.37 (m, 4H, NCH2C6H4-CH3); 7.27 and 7.52 (dd, 1H, J = 8 Hz,
13
NCH=CH2); 7.89 ve 8.29 (s, 2H, NCH=CHN); 9.54 (s, 1H, NCHN). C NMR (δ, DMSO):
18.6 (NCH2C6H4-CH3); 50.5 (NCH2C6H4-CH3); 108.7 (NCH=CH2); 119.3 and 135.7
(NCH=CHN); 129.0 (NCH=CH2); 123.5, 126.5, 126.9, 128.9 and 132.2 (NCH2C6H4-CH3);
136.6 (NCHN). Anal. calcd for C13H15N2Cl: C, 66.52; H, 6.44; N, 11.93. Found: C, 66.54;
H, 6.49; N, 11.89.
2.2.2.1-Vinyl-3-(3,5-dimethylbenzyl)imidazolium chloride, 1b
Yield: 84% (2.29 g). m.p.: 189-190 oC; v(CN) = 1634 cm-1. 1H NMR (δ, DMSO): 2.27 (s, 6H,
NCH2C6H3(CH3)2-3,5); 5.37 (s, 2H, NCH2C6H3(CH3)2-3,5); 5.43 and 5.97 (dd, 2H, J = 12 ve
8 Hz, NCH=CH2); 7.04 and 7.07 (s, 2H, NCH2C6H3(CH3)2-3,5); 7.29 and 7.34 (dd, 1H, J = 8
Hz, NCH=CH2); 7.93 and 8.23 (s, 2H, NCH=CHN); 9.58 (s, 1H, NCHN). 13C NMR (δ,
DMSO): 20.6 (NCH2C6H3(CH3)2-3,5); 52.0 (NCH2C6H3(CH3)2-3,5); 108.8 (NCH=CH2);
119.4 and 135.5 (NCH=CHN); 128.9 (NCH=CH2); 123.3, 126.1, 130.1 and 134.2
(NCH2C6H2(CH3)2-3,5); 138.2 (NCHN). Anal. calcd for C14H17N2Br: C, 57.35; H, 5.84; N,
9.55. Found: C, 57.31; H, 5.92; N, 9.59.
2.2.3.1-Vinyl-3-(2,3,5,6-tetramethylbenzyl)imidazolium chloride, 1c
o
1
Yield: 84% (2.29 g). m.p.: 189-190 C; v(CN) = 1634 cm-1. H NMR (δ, CDCl3): 2.20 (s, 6H,
NCH2C6H(CH3)4-2,6); 2.24 (s, 6H, NCH2C6H(CH3)4-3,5); 5.69 (s, 2H, NCH2C6H(CH3)4-
2,3,5,6); 5.37 and 6.07 (dd, 2H, J = 12 ve 8 Hz, NCH=CH2); 6.92 (s, 1H, NCH2C6H(CH3)4-
2,3,5,6); 7.06 and 8.18 (s, 2H, NCH=CHN); 7.52 and 7.56 (dd, 1H, J = 8 Hz, NCH=CH2);
11.17 (s, 1H, NCHN). 13C NMR (δ,CDCl3): 15.6 (NCH2C6H(CH3)4-2,6); 20.4
(NCH2C6H(CH3)4-3,5); 48.9 (NCH2C6H(CH3)4-2,3,5,6); 109.7 (NCH=CH2); 119.5 and 135.1
(NCH=CHN); 128.9 (NCH=CH2); 121.1, 127.8, 133.6 and 134.0 (NCH2C6H(CH3)4-2,3,5,6);

136.1 (NCHN). Anal. calcd for C16H21N2Cl: C, 69.42; H, 7.65; N, 10.12. Found: C, 69.51; H,
7.68; N, 10.09.
2.2.4.1-Vinyl-3-(4-t-butylbenzyl)imidazolium bromide, 1d
o
1
Yield: 84% (2.29 g). m.p.: 189-190 C; v(CN) = 1634 cm-1. H NMR (δ,CDCl3): 1.21 (s, 9H,
NCH2C6H4-(C(CH3)3); 5.51 (s, 2H, NCH2C6H4--(C(CH3)3); 5.25 and 5.94 (dd, 2H, J = 8 Hz,
NCH=CH2); 7.32 and 7.38 (d, 4H, J = 8 Hz, NCH2C6H4--(C(CH3)3); 7.35 and 7.43 (dd, 1H, J
13
= 8 Hz, NCH=CH2); 7.40 and 7.86 (s, 2H, NCH=CHN); 11.24 (s, 1H, NCHN). C NMR
(δ,CDCl3): 31.2 (NCH2C6H4-(C(CH3)3); 34.5 (NCH2C6H4-(C(CH3)3); 53.3 (NCH2C6H4-
(C(CH3)3); 109.6 (NCH=CH2); 119.4 and 136.3 (NCH=CHN); 128.8 (NCH=CH2); 122.3,
126.4, 128.4 and 129.8 (NCH2C6H4-CH3); 136.6 (NCHN). Anal. calcd for C16H21N2Br: C,
59.82; H, 6.59; N, 8.72. Found: C, 59.84; H, 7.02; N, 8.68.
2.2.5.1-Vinyl-3-(2,3,4,5,6-pentamethylbenzyl)imidazolium chloride, 1e
o
1
Yield: 84% (2.29 g). m.p.: 189-190 C; v(CN) = 1634 cm-1. H NMR (δ,DMSO): 2.20 (s, 6H,
NCH2C6(CH3)5-2,6); 2.22 (s, 6H, NCH2C6(CH3)5-3,5); 2.23 (s, 3H, NCH2C6(CH3)5-4); 5.52
(s, 2H, NCH2C6(CH3)5-2,3,4,5,6); 5.38 and 6.02 (dd, 2H, J = 8 ve 6 Hz, NCH=CH2); 7.78
and 8.33 (s, 2H, NCH=CHN); 7.30 and 7.34 (dd, 1H, J = 8 Hz, NCH=CH2); 9.25 (s, 1H,
13
NCHN). C NMR (δ,DMSO): 16.8 (NCH2C6(CH3)5-2,6); 17.10 (NCH2C6(CH3)5-3,5); 17.40
(NCH2C6(CH3)5-4); 49.0 (NCH2C6(CH3)5-2,3,4,5,6); 108.8 (NCH=CH2); 119.3 ve 135.4
(NCH=CHN); 129.3 (NCH=CH2); 123.4, 126.7, 133.4 and 134.1 (NCH2C6H2(CH3)3-2,4,6);
136.6 (NCHN). Anal. calcd for C17H23N2Cl: C, 70.21; H, 7.97; N, 9.63. Found: C, 70.23; H,
7.92; N, 9.59.
2.2.6.1-Vinylbenzyl-3-N-(2-methyl)phthalimideimidazolium chloride, 1f
Yield: 84% (2.29 g). m.p.: 189-190 oC; v(CN) = 1634 cm-1. 1H NMR (δ,DMSO): 5.44 and 5.46
(dd, 2H, J = 8 and 12 Hz, NCH=CH2); 6.05 (s, 2H, NCH2N(CO)2C6H4); 7.34-7.39 (m, 4H,
NCH2N(CO)2C6H4); 7.35 and 7.39 (dd, 1H, J = 8 Hz, NCH=CH2); 7.40 and 8.26 (s, 2H,
NCH=CHN); 9.56 (s, 1H, NCHN). 13C NMR (δ,DMSO): 50.6 (NCH2N(CO)2C6H4); 109.8
(NCH=CH2); 119.4 ve 135.3 (NCH=CHN); 129.0 (NCH=CH2); 124.0, 124.2 ve 131.8
(NCH2N(CO)2C6H4); 167.1 (NCH2N(CO)2C6H4); 136.6 (NCHN). Anal. calcd for
C14H13N3O2Cl: C, 58.04; H, 4.17; N, 14. 50. Found: C, 58.01; H, 4.22; N, 14.48.

2.2.7.Dichloro[1-(vinyl)-3-(2-methylbenzyl)-imidazol-2-ylidene](pyridine)palladium (II), 2a
o
1
Yield: 74% (0.42 g). m.p.: 241-242 C; v(CN) = 1436 cm-1. H NMR (δ, DMSO): 2.42 (s, 3H,
NCH2C6H4-CH3); 5.76 (s, 2H, NCH2C6H4-CH3); 5.22 and 5.79 (dd, 2H, J = 8 Hz,
NCH=CH2); 7.17-7.32 (m, 4H, NCH2C6H4-CH3); 7.36 and 7.99 (s, 2H, NCH=CHN); 8.11 and
8.09 (dd, 1H, J = 8 Hz, NCH=CH2); 7.55 (t, 2H, J = 8 Hz, NC5H5-3,5); 7.99 (t, 1H, J = 8 Hz,
NC5H5-4); 8.78 (t, 2H, J = 8 Hz, NC5H5-2,6). 13C NMR (δ, DMSO): 19.7 (NCH2C6H4-CH3);
52.2 (NCH2C6H4-CH3); 104.2 (NCH=CH2); 119.3 ve 136.8 (NCH=CHN); 129.5 (NCH=CH2);
125.4, 126.5, 126.5, 128.6, 130.8, 133.5, 134.4, 139.4 ve 151.4 (NCH2C6H4-CH3 ve NC5H5);
152.5 (Pd-Ccarbene). Anal. calcd for C18H20N3Cl2Pd: C, 47.44; H, 4.42; N, 9.22. Found: C,
47.51; H, 4.50; N, 9.19.
2.2.8.Dichloro[1-(vinyl)-3-(3,5-dimethylbenzyl)-imidazol-2-ylidene](pyridine)palladium (II),
2b
Yield: 74% (0.42 g). m.p.: 241-242 oC; v(CN) = 1436 cm-1. 1H NMR (δ, DMSO): 2.24 (s, 6H,
NCH2C6H3(CH3)2-3,5); 5.65 (s, 2H, NCH2C6H3(CH3)2-3,5); 5.09 and 5.32 (dd, 2H, J = 8 ve 4
Hz, NCH=CH2); 6.69 ve 7.16 (s, 2H, NCH=CHN); 6.92 and 7.07 (s, 2H, NCH2C6H3(CH3)2-
3,5); 8.06 and 8.10 (dd, 1H, J = 8 Hz, NCH=CH2); 7.25 (m, 2H, NC5H5-3,5); 7.67 (m, 1H,
NC5H5-4); 8.96 (t, 2H, J = 8 Hz, NC5H5-2,6). 13C NMR (δ, DMSO): 21.3 (NCH2C6H3(CH3)2-
3,5); 55.1 (NCH2C6H3(CH3)2-3,5); 103.4 (NCH=CH2); 118.2 and 134.5 (NCH=CHN); 126.9
(NCH=CH2); 122.1, 124.6, 127.1, 130.2, 133,4, 138.0, 138.6 and 152.1 (NCH2C6H2(CH3)2-
3,5 ve NC5H5); 152.7 (Pd-Ccarbene). Anal. calcd for C19H22N3Cl2Pd: C, 48.58; H, 4.72; N, 8.95.
Found: C, 48.51; H, 4.82; N, 8.90.
2.2.9.Dichloro[1-(vinyl)-3-(2,3,5,6-tetramethylbenzyl)-imidazol-2-ylidene]
(pyridine)palladium (II), 2c
o
1
Yield: 74% (0.42 g). m.p.: 241-242 C; v(CN) = 1436 cm-1. H NMR (δ,DMSO): 1.28 (s, 9H,
NCH2C6H4-(C(CH3)3); 5.77 (s, 2H, NCH2C6H4--(C(CH3)3); 5.20 and 5.74 (dd, 2H, J = 8 Hz,
NCH=CH2); 7.37-7.44 (m, 4H, NCH2C6H4--(C(CH3)3); 7.40 and 7.96 (s, 2H, NCH=CHN);
8.10 and 8.14 (dd, 1H, J = 8 Hz, NCH=CH2); 7.55 (t, 2H, J = 8 Hz, NC5H5-3,5); 8.01 (t, 1H,
J = 8 Hz, NC5H5-4); 8.84 (t, 2H, J = 8 Hz, NC5H5-2,6). 13C NMR (δ, DMSO): 31.2

(NCH2C6H4-(C(CH3)3); 34.7 (NCH2C6H4-(C(CH3)3); 53.8 (NCH2C6H4-(C(CH3)3); 104.0
(NCH=CH2); 119.8 and 133.5 (NCH=CHN); 128.8 (NCH=CH2); 123.7, 125.4, 125.8, 128.8,
133.3, 139.5 and 151.3 (NCH2C6H4-CH3 and NC5H5); 152.0 (Pd-Ccarbene). Anal. calcd for
C21H26N3Cl2Pd: C, 50.67; H, 5.26; N, 8.44. Found: C, 50.70; H, 5.36; N, 8.49.
2.2.10. Dichloro[1-(vinyl)-3-(-4-t-buthylthylbenzyl)-imidazol-2-ylidene](pyridine)palladium
(II), 2d
o
1
Yield: 74% (0.42 g). m.p.: 241-242 C; v(CN) = 1436 cm-1. H NMR (δ, CDCl3): 2.18 (s, 6H,
NCH2C6H(CH3)4-2,6); 2.20 (s, 6H, NCH2C6H(CH3)4-3,5); 5.84 (s, 2H, NCH2C6H(CH3)4-
2,3,5,6); 5.08 and 5.30 (dd, 2H, J = 12 Hz, NCH=CH2); 6.98 (s, 1H, NCH2C6H(CH3)4-
2,3,5,6); 6.31 and 7.06 (s, 2H, NCH=CHN); 8.12 and 8.16 (dd, 1H, J = 8 Hz, NCH=CH2);
7.31 (t, 2H, J = 8 Hz, NC5H5-3,5); 7.72 (t, 1H, J = 8 Hz, NC5H5-4); 8.98 (t, 2H, J = 4 Hz,
NC5H5-2,6).13C NMR (δ, CDCl3): 15.9 (NCH2C6H(CH3)4-2,6); 20.5 (NCH2C6H(CH3)4-3,5);
50.2 (NCH2C6H(CH3)4-2,3,5,6); 103.7 (NCH=CH2); 117.4 and 134.5 (NCH=CHN); 129.0
(NCH=CH2); 121.0, 124.5, 132.8, 133.4, 134.5, 138.2 and 151.3 (NCH2C6H(CH3)4-2,3,5,6
and NC5H5); 151,5 (Pd-Ccarbene). Anal. calcd for C21H26N3Cl2Pd: C, 50.67; H, 5.26; N, 8.44.
Found: C, 50.70; H, 5.36; N, 8.49.
2.2.11. Dichloro[1-(vinyl)-3-(2,3,4,5,6-pentamethylbenzyl)-imidazol-2-
ylidene](pyridine)palladium (II), 2e
Yield: 74% (0.42 g). m.p.: 241-242 oC; v(CN) = 1436 cm-1. 1H NMR (δ,DMSO): 2.20 (s, 12H,
NCH2C6(CH3)5-2,3,5,6); 2.24 (s, 3H, NCH2C6(CH3)5-4); 5.73 (s, 2H, NCH2C6(CH3)5-
2,3,4,5,6); 5.22 and 5.79 (dd, 2H, J = 8 ve 16 Hz, NCH=CH2); 6.59 and 7.84 (s, 2H,
NCH=CHN); 8.07 and 8.11 (dd, 1H, J = 8 Hz, NCH=CH2); 7.58 (t, 2H, J = 8 Hz, NC5H5-
3,5); 8.02 (t, 1H, J = 8 Hz, NC5H5-4); 8.87 (t, 2H, J = 4 Hz, NC5H5-2,6). 13C NMR
(δ,DMSO): 17.0 (NCH2C6(CH3)5-2,6); 17.1 (NCH2C6(CH3)5-3,5); 17.4 (NCH2C6(CH3)5-4);
50.5 (NCH2C6(CH3)5-2,3,4,5,6); 104.4 (NCH=CH2); 118.8 and 136.0 (NCH=CHN); 127.5
(NCH=CH2); 121.8, 125.4, 133.2, 133.4, 134.1, 139.5 and 151.3 (NCH2C6H2(CH3)3-2,4,6
and NC5H5); 151,7 (Pd-Ccarbene). Anal. calcd for C22H28N3Cl2Pd: C, 51.63; H, 5.51; N, 8.21.
Found: C, 51.61; H, 5.58; N, 8.18.

2.2.12. Dichloro[1-(vinyl)-3-N-(2-methylphtalimide)-imidazol-2-ylidene](pyridine)palladium
(II), 2f
Yield: 74% (0.42 g). m.p.: 241-242 oC; v(CN) = 1436 cm-1. 1H NMR (δ,DMSO): 5.22 and 5.24
(dd, 2H, J = 8 Hz, NCH=CH2); 6.25 (s, 2H, NCH2N(CO)2C6H4); 7.84 and 7.95 (s, 2H,
NCH=CHN); 7.85-7.97 (m, 4H, NCH2N(CO)2C6H4); 8.00 and 8.04 (dd, 1H, J = 8 Hz,
NCH=CH2); 7.59 (t, 2H, J = 8 Hz, NC5H5-3,5); 7.99 (t, 1H, J = 8 Hz, NC5H5-4); 8.98 (t, 2H, J
= 8 Hz, NC5H5-2,6). 13C NMR (δ,DMSO): 52.2 (NCH2N(CO)2C6H4); 104.7 (NCH=CH2);
118.4 and 135.3 (NCH=CHN); 126.9 (NCH=CH2); 124.0, 125.4, 132.1, 133.7, 139.1 and
152.4 (NCH2N(CO)2C6H4 and NC5H5); 167.1 (NCH2N(CO)2C6H4); 167.7 (NCHN); 154,1
(Pd-Ccarbene). Anal. calcd for C19H18N4Cl2O2Pd: C, 44.60; H, 3.55; N, 10.95. Found: C, 44.51;
H, 3.60; N, 10.87.
2.3. Instrumentation
IR spectra were recorded with a PerkinElmer Spectrum as KBr in the range of 4000 – 450
cm−1 on a Fourier transform spectrometer. Scanning electron microscopy (SEM) images were
recorded using a Bruker model VEGA3 FESEM. Chemical composition analysis was
performed with an EDAX analyzer coupled with a scanning electron microscope (SEM,
Hitachi Ltd., S-4700). Transmittance electron microscopy (TEM) imaging was performed
using a Jeol JEM 1400 at 100 kV. Thermogravimetric analysis (TGA) was carried out under
air up to 800 °C at 10 °C/min using a STA504.
2.4. General procedure for cross coupling reactions
For the homogeneous Suzuki coupling reactions; 1.00 mmol aryl bromide, 1.50 mmol
phenylboronic acid, 2.50 mmol K2CO3, 1 mmol % Pd-NHC (2a-2f ) catalysts, 2.5 mL DMF,
2.5 mL H2O were added to a small round bottom flask. The mixture was vigorously stirred at
22 oC for 3 hours under oxygen atmosphere.
For the homogeneous Heck coupling reactions; 1.00 mmol aryl bromide, 1.50 mmol styrene,
2.50 mmol K2CO3, 1 mmol% Pd-NHC (2a-2f ) catalysts, 2.5 mL DMF, 2.5 mL H2O were
added to a small round bottom flask. The mixture was vigorously stirred at 80 oC for 3 hours
under oxygen atmosphere.
For the heterogeneous Suzuki coupling reactions; 1.00 mmol aryl bromide, 1.50 mmol
phenylboronic acid, 2.50 mmol K2CO3, 1 mmol% Pd-NHC (3a-3f) catalysts, 5 mL H2O were

added to a small round bottom flask. The mixture was vigorously stirred at 80 oC for 2 hours
under oxygen atmosphere.
For the heterogeneous Heck coupling reactions; 1.00 mmol aryl bromide, 1.50 mmol styrene,
2.50 mmol K2CO3, 1 mmol% Pd-NHC (3a-3f) catalysts, 5 mL H2O were added to a small
o
round bottom flask. The mixture was vigorously stirred at 90 C for 4 hours under oxygen
atmosphere.
All reactions were monitored on an Schimadzu 2010 Plus GC-MS system by GC-FID with a
HP-5 column of 30 m length, 0.32 mm diameter and 0.25 ꢀm film thickness.The conversion
of substrates to products was estimated by the area of GC chromatogram peaks. The yields are
based on aryl halide.
3. Results and Discussion
3.1. Synthesis of vinylimidazole based salts (1a-f)
NHC ligand precursors (1a-f) was synthesized according to published procedure [39]. The
1H- NMR shifts of 1a, 1b, 1c, 1d, 1e, and 1f were similar to other characterized imidazolium
1
salts [40-42]. In the H NMR spectra of 1a, 1b, 1c, 1d, 1e, and 1f, NCHN protons were
observed as sharp singlets at 9.54, 9.58, 11.17, 11.24, 9.25 and 9.56 ppm respectively. The
iminocarbones (NCHN) were detected as typical singlets in the 1H decoupled mode at 138.6,
138,2, 136.1, 136.6, 136.6 and 136.6 ppm respectively.
Scheme 1. Synthesis of vinylimidazole based NHC ligand precursors (1a-f)
3.2. Synthesis of vinylimidazole based PEPPSI-Pd-NHC complexes (2a-f)
1
For the H NMR spectra of the PEPPSI-Pd-NHC complexes (2a-f), sharp peaks were not
observed in the lower field region belonging to the acidic iminoprotons of imidazolium salts

(NCHN). Similarly, in the 13C NMR spectra, iminocarbons of imidazolium salts (NCHN)
were not observed in the range of δ = 136-139 ppm. This observation was confirmed by a
previous study [43]. In the 1H NMR and 13C NMR spectra of the complexes 2a-f, loss of the
characteristic peak of the acidic iminoprotons (NCHN) and iminocarbons (NCHN) signal
suggests the formation of palladium-NHC-PEPPSI complexes. Additionally, characteristic
13
carbenic carbons in compounds 2a-f appeared in the C NMR spectra as deshielded singlets
at 152.5, 152.7, 152.0, 151.5, 151.7 and 154.1 ppm respectively. In order to demonstrate the
utility of these PEPPSI-Pd-NHC complexes, we used them as co-catalysts in Suzuki and Heck
coupling reactions, which are common industry-applicable processes. The IR data also clearly
indicated the presence of (CN) vibration at 1429, 1427, 1444, 1446, 1430 and 1446 cm-1 for
2a-f.
Scheme 2. Synthesis of PEPPSI-Pd-NHC complexes (2a-f)
3.3. Synthesis and characterization of hydrogel-based heterogeneous PEPPSI-Pd-NHC
catalysts
The poly(AAm-co-AMPS) supported catalysts (3a, 3b, 3c, 3d, 3e and 3f) containing
vinylimidazole based PEPPSI-Pd-NHC complexes were synthesized as shown in Scheme 3.

Scheme 3. Synthesis of poly(AAm-co-AMPS) supported PEPPSI-Pd-NHC catalyst.
To characterize the chemical structures of PEPPSI-Pd-NHC complexes and polymeric
samples infrared spectra were recorded. FT-IR spectrums of PEPPSI-Pd-NHC complexes,
poly(AAm-co-AMPS) hydrogel and poly(AAm-co-AMPS) supported PEPPSI-Pd-NHC
catalysts are shown in Fig. 1. PEPPSI-Pd-NHC complexes (curves 2a, 2b, 2c, 2d, 2e and 2f)
include similar absorption peaks. In the FT-IR spectra of PEPPSI-Pd-NHC complexes,
imidazole ring peaks appeared at around 1429 cm-1, at around 1214 cm-1 and around 966 cm-1
due to C-N, =C-N, and ring stretching vibrations respectively. The characteristic peaks of the
vinyl group appeared at around 3096 cm-1, 1647 cm-1 and 1396 cm-1 due to C-H stretching,
terminal C=C stretching and benzene ring C=C-C stretching respectively. The characteristic
peaks of methyl groups at various positions on the benzene ring appeared in the range of 903
cm-1 and 847 cm-1.
In the FT-IR spectra of poly(AAm-co-AMPS), the wide bands were observed in the range of
3000-3600 cm-1 due to -NH stretching of acrylamide and -OH stretching vibrations of the
sulfonic acid. The broad peak at 1654 cm-1 shows the characteristic carbonyl bond (–C = O)
vibration of amide I and amide II groups. The C-H stretching appeared at around 1444 cm-1.
The aliphatic C-N stretching appeared at around 1177 cm-1 and 1153 cm-1. The peak observed
at 1033 cm-1 corresponds to the stress vibration of S = O of the AMPS units.
At the FT-IR curves of poly(AAm-co-AMPS) based PEPPSI-Pd-NHCs (3a, 3b, 3c, 3d, 3e,
and 3f), due to the addition of C = C-H groups of PEPPSI-Pd-NHC complex to the structure,
a new peak is observed at around 3190 cm.-1 In addition, the wide peak at 1654 cm-1 of amide
I and amide II turns into a sharp peak at 1652 cm.-1 This is due to the bond formed between
the oxygen atom in carbonyl and Pd (II). At 1408 cm,-1 the new peak observed as the C-N
vibration of pyridine. The C-N peaks of the aromatic imidazole group at around 1429 cm-1
shifts to 1452 cm-1 due to the interaction of palladium with carbonyl groups in the polymer
chain.

Fig. 1. FT-IR spectrums of PEPPSI-Pd-NHC complexes, Poly(AAm-co-AMPS) (Crude) and
polymer-supported PEPPSI-Pd-NHC catalysts
To obtain information about the porosity and identify the surface morphology of the
synthesized PEPPSI-Pd-NHC-containing hydrogels, SEM images of the freeze-dried samples
are depicted in Fig. 2. In Fig. 2, it is seen that the catalysts, which are swollen in water, have
micro-pores connected to each other from the inside and can provide a very large surface area
to the reactants thanks to this three-dimensional network structure. In addition, hydrogels
having this surface morphology increase the permeability of the aqueous solution. This
facilitates the adsorbed interaction with the adsorbent.

Fig. 2. SEM images of freeze-dried polymer supported PEPPSI-Pd-NHC catalysts
Fig. 3 shows the SEM-EDX spectra of poly(AAm-co-AMPS) supported PEPPSI-Pd-NHC
catalysts. It was determined from the EDX spectra of the catalysts that the palladium content
by mass ranged from 5.14 to 6.04%. This is proof that the polymerization of PEPPSI-Pd-NHC
complexes with monomers is performed in high yield.

Fig. 3. EDX spectra of freeze-dried polymer supported PEPPSI-Pd-NHC catalysts
TEM images are given in Fig. 4 to obtain information about the size and distribution of
palladium nanoparticles in the structure of poly (AAm-AMPS) support. TEM images showed
that the palladium were well-dispersed through the support surface and their average size
distributions are approximately 10 nm.

Fig. 4. TEM images of polymer supported PEPPSI-Pd-NHC catalysts
To thermogravimetric analysis (TGA) of Poly(AAm-co-AMPS) (crude) and Poly(AAm-co-
AMPS)-based PEPPSI-Pd-NHC complexes, the change in the mass of the sample was
measured versus temperature increase. As a result of the analysis of each sample, the
temperature-% mass curve is obtained and the thermograms obtained are given in Fig. 5. In
Fig. 5, the Poly (AAm-co-AMPS) polymer showed a very different thermogram curve
compared to catalysts. Due to the absence of an inorganic structure in the structure of poly
(AAm-AMPS) polymer, the mass loss in the thermogram curve was sharp and clear. This
o
mass loss occurs between about 200 and 230 C and 85% of the structure is seen to undergo
thermal degradation. The simple mass loss between 20-100 oC corresponds to about 10% and
this mass loss is due to moisture. When the thermogram curves of the catalysts are examined,
it is seen that the thermal stability increases with the introduction of PEPPSI-Pd-NHC
complex. The reason for this change is that it turns from an organic structure to an

organometallic structure. The mass loss of the catalysts at a temperature between about 20 and
120 oC is caused by moisture. The resistance to thermal degradation of them lasts up to about
190 oC. After this temperature, mass loss occurs in two stages. The decomposition occurring
in the temperature range 230-320oC, where the first mass loss occurs, corresponds to the
degradation of cross-linked aliphatic chains. At the temperature between 330 and 480oC, the
second mass loss occurs and the aromatic groups are decomposed. Thermal decomposition of
aromatic benzene structures and imidazole and pyridine groups of the hetero-aromatic
structure occurs within this temperature range. It can be seen that the residual amount of
catalysts appears to be about 6%. This shows the palladium oxide remaining in the structure.
Fig. 5. TGA thermograms of Poly(AAm-co-AMPS) polymer (Crude) and polymer supported
PEPPSI-Pd-NHC catalysts
From FT-IR, SEM, EDX, TEM and TGA analyzes, it can be seen that PEPPSI-Pd-NHC
complexes are successfully polymerized.
3.4. Coupling Reactions
Both homogeneous and heterogeneous Suzuki and Heck reactions carried out the conversion
to the desired product. At the end of the reactions, low amount biaryl coupling product was
found as a by-product.

3.4.1.Homogeneous Suzuki-Miyaura and Heck coupling reactions of aryl Bromides
The homogeneous reaction conditions optimized considering to published method [43, 44]. It
has been examined the whether the electronic properties of the reactant had a significant
effect upon the homogeneously catalyzed coupling reactions. A number of aryl bromides were
also successfully coupled to phenylboronic acid and styrene, respectively. The yields of
homogeneous catalyzed Suzuki and Heck coupling reactions were shown in Table 1 and
Table 2, respectively. When Table 1 is examined, it can be seen that both the electron-rich
(bromoacetophenone and bromobenzaldehyde) and, especially, the electron-poor non-
activated aryl bromides (bromobenzene, bromotoluene, and bromoxylene) were coupled
successfully with phenylboronic acid in good yields under mild reaction conditions. Table 2
showed that the, both the electron-rich and electron-poor non-activated aryl bromides were
coupled successfully with styrene in good yields under 80 oC . When the activity of PEPPSI-
Pd-NHC complexes in the homogeneous Suzuki and Heck reaction is compared, the catalysts
can achieve high yields at room temperature in the homogeneous Suzuki reaction, while at
least 80°C is required to achieve high yields in the homogeneous Heck reaction. This can be
due to the fact that the electronic properties of the acidic phenylboronic acid are different
from styrene.
Table 1. The homogeneous Suzuki coupling reaction of aryl bromides with phenylboronic
acid catalyzed by PEPPSI-Pd-NHC complexes
Yield (%)
Catalyst
100
99
99
100
99
84
78
85
86
88
90
71
69
73
88
83
89
79
83
85
89
84
91
2a
2b
2c
2d
2e
2f
100
100
100
100
100
99
100
Reaction conditions:1.00 mmol aryl bromide, 1.50 mmol phenylboronic acid, 2.50 mmol K2CO3, 1 mmol%
Pd-NHC, 2.5 mL DMF, 2.5 mL H2O, 22oC, 3 h. The purity of compounds is checked by GC, yields are based
on aryl bromide derivatives.

Table 2. The homogeneous Heck coupling reaction of aryl bromides with styrene catalyzed
by PEPPSI-Pd-NHC complexes
Yield (%)
Catalyst
87
88
93
90
84
96
85
84
90
92
87
94
78
80
88
84
85
90
71
74
77
80
75
87
65
71
74
69
70
77
2a
2b
2c
2d
2e
2f
Reaction conditions:1.00 mmol aryl bromide, 1.50 mmol styrene, 2.50 mmol K2CO3, 1 mmol% Pd-NHC, 2.5
mL DMF, 2.5 mL H2O, 80oC, 3 h. The purity of compounds is checked by GC, yields are based on aryl
bromide derivatives.
3.4.2.Heterogeneous Suzuki-Miyaura and Heck coupling reactions of aryl bromides in
aqueous media
Palladium-catalyzed coupling reactions such as Suzuki and Heck reactions are generally
carried out in an inert atmosphere in organic solvents. poly(AAm-AMPS) supported
catalysts which included various types of PEPPSI-Pd-NHC complexes has been used for
heterogeneous Suzuki and Heck coupling reactions in water. Firstly, the heterogeneous
reaction conditions optimized considering the procedure of our previous work [34]. For this
purpose, catalyst 3a was used in the optimization of heterogeneous catalytic reaction
conditions. Optimized reaction conditions of Suzuki and Heck coupling were shown under
Table 3 and Table 4, respectively.
In Table 4, the catalytic activity of the poly(AAm-co-AMPS)-PEPPSI-Pd-NHC catalysts
used in the heterogeneous catalytic Suzuki reactions in the electron-withdrawing substrates
(conversion 87-100%) is seen to be higher than the catalytic activity of the electron-donating
substrates (conversion 81-94%). In addition, all catalysts have high catalytic activity on each
substrate. When the catalytic activity of the catalysts is compared, it can be seen that the

catalytic activity of catalyst 3f is highest on all substrates. This can be said to be caused by
the -CHO and -NH functional groups in the phthalimide compound present in the 3f catalyst.
Table 3. The heterogeneous Suzuki coupling reaction of aryl bromides with phenylboronic acid
catalyzed by poly(AAm-co-AMPS) supported PEPPSI-Pd-NHC catalysts
Yield (%)
Catalyst
92
91
88
91
93
95
99
96
88
91
84
85
81
93
87
89
92
85
89
94
90
91
89
87
88
93
3a
3b
3c
3d
3e
3f
87
89
94
100
Reaction conditions:1.00 mmol aryl bromide, 1.50 mmol phenylboronic acid, 2.50 mmol K2CO3, 1 mmol%
Pd-NHC, 5 mL H2O, 80oC, 2h. The purity of compounds is checked by GC, yields are based on aryl bromide
derivatives.
When Table 4 is examined, it is seen that the catalysts used in heterogeneous catalytic Heck
reactions have yields between 64-89% in electron-withdrawing substrates and yields
between 48-74% in electron-providing substrates. It can be said that the activities of the
catalysts are not as high as the catalytic activities in the Suzuki reaction. This is because the
interaction of anionic poly (AAm-co-AMPS) support with styrene (such as electronics,
diffusion, and adsorption) is less than that of phenylboronic acid. Catalyst 3f also has the
highest catalytic activity in the Heck reaction.

Table 4. The heterogeneous Heck coupling reaction of aryl bromides with styrene catalyzed by
poly(AAm-co-AMPS)-PEPPSI-Pd-NHC catalysts
Yield (%)
Catalyst
64
71
69
74
73
87
69
82
83
75
71
89
57
69
61
59
63
73
54
48
61
63
69
74
49
55
60
64
57
69
3a
3b
3c
3d
3e
3f
Reaction conditions:1.00 mmol aryl bromide, 1.50 mmol styrene, 2.50 mmol K2CO3, 1 mmol% Pd-NHC, 5
mL H2O, 90oC, 4 h. The purity of compounds is checked by GC, yields are based on aryl bromide
derivatives.
Catalysts increase the reaction rate by changing the reaction mechanism. In the
homogeneous catalytic reaction, it is known that the catalytic activity and selectivity of the
catalyst which is in the same phase as the substrates can interact directly with the reactive
molecules without any limiting effect. In heterogeneous catalytic reactions, because the
catalyst is present in different phases with the reagents, it leads to decreases in catalytic
activity due to limiting effects such as adsorption, heat transfer rate, diffusion, surface
tension, film formation on the catalyst surface. Homogeneous catalytic Suzuki reactions
have high yields at room temperature in about 3 hours, while heterogeneous catalytic
reactions have high yield at a temperature of 80°C for at least 2 hours. This is caused by the
use of pure water as a solvent in heterogeneous catalytic reactions. The fact that the catalyst
is in a different phase with the reagents causes limiting effects such as reduced interaction
with the reagents, increased heat transfer resistance to the catalyst, molecular diffusion
resistance, more complex reaction cycle (adsorption-surface reaction-desorption). Therefore,
in order to achieve high yields in the heterogeneous reaction, the reaction temperature must
be increased. The same applies to the Heck reaction.

In heterogeneous catalytic reactions, the reusability of the catalysts without loss of catalytic
activity is very important in economic and ecological terms. Catalysts are easily separated
from the reaction medium by simple filtration and can be reused. Thus, they reduce reaction
costs and reduce heavy metal waste. To test this, the reusability of catalyst 3f in the
heterogeneous Suzuki and Heck reaction of bromobenzaldehyde was investigated.
After the coupling reaction was complete, the 3f catalyst was filitrated, washed with ethanol
and reused for the next run of the coupling reaction. The reactions were carried out five
times with the same catalyst and the percent yield graphs versus the number of uses are
shown in Fig. 6. From Fig. 6, it can be said that the catalyst retains its catalytic activity after
re-use in both coupling reactions. In all reusability test reactions, palladium black species
were not detected in the reaction medium. We can say that palladium does not leach from
the polymeric support into the reaction medium.
Fig. 6. A recyclability test for the 3f catalyst in coupling reactions of bromobenzaldehyde.
A: Suzuki reactions, B: Heck reactions. (The reaction conditions are the same as the
optimized coupling reactions).
4. Conclusion
Vinylimidazole based PEPPSI-Pd-NHC complexes were successfully synthesized and they
showed good catalytic performance in homogeneous Suzuki and Heck coupling reactions of
various aryl bromides. These catalytically active complexes were successfully polymerized
with AAm, AMPS and MBAAm monomers to obtain hydrogel supported PEPPSI-Pd-NHC
catalysts. The microporous-structured poly(AAm-AMPS) support material provided the high
dispersion of palladium active sites. Hydrogel-based PEPPSI-Pd-NHC catalysts showed high
catalytic activity in heterogeneous Suzuki and Heck coupling reactions of various aryl

bromides in aqueous media. These catalysts had higher catalytic activity in Suzuki reactions
than Heck reactions. Catalyst 3f had the highest catalytic activity in heterogeneous Suzuki and
Heck coupling reactions. This can be said to be caused by the presence of the anionic –CHO
and –NH functional groups contained in the 2f complex. In Suzuki and Heck coupling
reactions, the 3f catalyst was easily recovered and reused five times without significant loss of
catalytic activity. It can be suggested that poly(AAm-AMPS) supported PEPPSI-Pd-NHC
catalysts based on vinylimidazole can be applied to various palladium-catalyzed reactions in
the context of green chemistry.
Acknowledgments
This study was supported by the Technological and Scientific Research Council of Turkey
TÜBİTAK (Project No: 116Z042).
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Highlights
• Vinylimidazole-based PEPPSI-type palladium-complexes were synthesized.
• This novel PEPPSI-Pd-NHC complexes were polymerized by free radical polymerization.
• The complexes were tested as catalysts in Suzuki and Heck reaction in aqueous media.