Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The Possibility of Immobilization on Magnetite for Ligand- and Base-Free Pd-Catalyzed Heck, Suzuki and Sonogashira Cross-Coupling Reactions in Water

Article abstract of DOI:10.1007/s10562-021-03552-5

A new versatile and recyclable NHC ligand precursor has been developed with ligand, base, and solvent functionalities for the efficient Pd-catalyzed Heck, Suzuki and Sonogashira cross-coupling reactions under mild conditions. Furthermore, NHC ligand precursor was immobilized on magnetite and its catalytic activity was also evaluated towards the coupling reactions as a heterogeneous catalyst. The NHC ligand precursor was prepared with imidazolium functionalization of TCT followed by a simple ion exchange by hydroxide ions. However, the results revealed an excellent catalytic activity for the both homogeneous and heterogeneous catalytic systems. 1.52?g.cm^?3 and 1194 cP was obtained for the density and viscosity of the NHC ligand precursor respectively. On the other hand, the heterogeneous type could be readily recovered from the reaction mixture and reused for several times while preserving its properties. Heterogeneous nature of the magnetic catalyst was studied by hot filtration, mercury poisoning, and three-phase tests. High to excellent yields were obtained for all entries for the both homogeneous and heterogeneous catalysts, which reflects the high consistency of the catalyst. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03552-5

Catalysis Letters
https://doi.org/10.1007/s10562-021-03552-5
Introduction of a Recyclable Basic Ionic Solvent with Bis‑(NHC)
Ligand Property and The Possibility of Immobilization on Magnetite
for Ligand‑ and Base‑Free Pd‑Catalyzed Heck, Suzuki and Sonogashira
Cross‑Coupling Reactions in Water
Qingwang Min1 · Penghua Miao1 · Deyu Chu1 · Jinghan Liu1 · Meijuan Qi1 · Milad Kazemnejadi2
Received: 7 December 2020 / Accepted: 23 January 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
A new versatile and recyclable NHC ligand precursor has been developed with ligand, base, and solvent functionalities for
the efcient Pd-catalyzed Heck, Suzuki and Sonogashira cross-coupling reactions under mild conditions. Furthermore, NHC
ligand precursor was immobilized on magnetite and its catalytic activity was also evaluated towards the coupling reactions
as a heterogeneous catalyst. The NHC ligand precursor was prepared with imidazolium functionalization of TCT followed
by a simple ion exchange by hydroxide ions. However, the results revealed an excellent catalytic activity for the both homo-
geneous and heterogeneous catalytic systems. 1.52 g.cm−3 and 1194 cP was obtained for the density and viscosity of the
NHC ligand precursor respectively. On the other hand, the heterogeneous type could be readily recovered from the reaction
mixture and reused for several times while preserving its properties. Heterogeneous nature of the magnetic catalyst was
studied by hot fltration, mercury poisoning, and three-phase tests. High to excellent yields were obtained for all entries for
the both homogeneous and heterogeneous catalysts, which refects the high consistency of the catalyst.
* Qingwang Min
minqw@dlpu.edu.cn
* Milad Kazemnejadi
miladkazemnejad@yahoo.com
1
School of Light Industry and Chemical Engineering, Dalian
Polytechnic University, Dalian 116034, PR China
2
Department of Chemistry, College of Science, Shiraz
University, 7194684795 Shiraz, Iran
Vol.:(0123456789)
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Q. Min et al.
Graphic Abstract
Keywords NHC ligand precursor · Homogeneous catalyst · Bis(nhc)pd complex · Ligand- · base · solvent-free · C–C cross-
coupling reactions
organic compounds, agrochemical, pharmaceutical, and fne
chemical industries [1].
1 Introduction
There are seven main Pd-catalyzed C–C cross-coupling
reactions including: Negishi, Suzuki–Miyaura, Stille,
Sonogashira, Heck-Mizoroki, Kumada, and Heck-Matsuda
reactions [2, 3].
Pd-catalyzed C–C cross-coupling reactions comprise one of
the most efcient and well-known methods for the of C–C
bond formation [1]. Powerful palladium-based catalysts
have enabled scientists to prepare various important and
vital compounds by C–C bond formation that were previ-
ously unavailable or require several tedious steps [2]. Thus,
cross-coupling reactions play a vital role in the synthesis of
The reaction between a halide or pseudo-halide and boronic
acids, boronate esters or orgaoboranes compound for the
assembly of biphenyl motifs catalyzed by the transition metal
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
is well known as Suzuki–Miyaura cross-coupling reaction [4].
Application in the synthesis of D159687 (an enzyme related
to infammatory and respiratory diseases) [5] and inhibitors of
p38 mitogen-activated protein (MAP) kinases [6], are some
examples for the application of Suzuki coupling reaction.
The Heck reaction is the Pd- catalyzed C–C cross-cou-
pling reaction between aryl, and alkenes, or vinyl halides (or
trifates) to give substituted alkenes [7]. It has widespread
application in pharmaceutical in both industrial and Lab-
scale syntheses [2, 8]. One example is its application in the
preparation of Idebenone, a compound similar to coenzyme
Q-10 used for Alzheimer’s disease, Huntington’s disease,
liver disease, and heart disease [9]. Ginkgolic acid [10], anti-
histaminic drug olopatadine [11], cafeine-styryl compounds
[12], and vascular endothelial growth factor (VEGF) inhibi-
tor axitinib [13] are some of the application of Heck reaction
in the preparation of biological molecules.
for Suzuki‐Miyaura and Heck coupling reactions [7], (3)
NiFe2O4@TABMA-Pd(0) (triazine bis[mercapto amine])
[21], (4) Pd@Hal-pDA-NPC (Hal=halloysite, pDA=poly-
dopamine, NPC=N-doped porous carbon monolayer) [22],
are some recent progresses for Pd-catalyzed C–C cross-cou-
pling reactions. Moreover, Zahoor et al., reviewed develop-
ment of green methodologies for Suzuki, Heck, Stille, and
Chan–Lam cross-coupling reactions [3].
Palladium complexes of bis-N-heterocyclic carbene
(NHC), have been successfully employed for a wide variety
of organic reactions, especially cross-coupling reactions due
to their strong σ-donor and weak π-acceptor properties [23].
Good catalytic activity, well stability, functionality toler-
ance, ease of preparation with cheap and accessible materi-
als, and ease of modifcation are some of the advantages of
N-heterocyclic carbene (NHC) ligands, which make them as
a promising ligand for Pd-based catalytic systems [24, 25].
The efcient application of Pd-NHC complexes has been well
known in organometallic chemistry and catalysts, especially in
the last decade. Various modifcations have been reported for
Pd-NHC complexes that could be point to some recently: (1)
N,Nʹ-bridged binuclear NHC palladium complex for Suzuki
reaction [23], (2) 2-hydroxyethyl substituted N-coordinate-
Pd(II)(NHC) for direct arylation reaction [26], (3) [(PDCR)
Pd(MeCN)](PF6)2] for electrochemical activation of CO2 [27],
(4) SPIONs-bis(NHC)-palladium(II) for C–C cross-coupling
reactions [24], and (5) bi-nuclear NHC–palladium(II) com-
plexes for Suzuki–Miyaura cross-coupling reactions [25].
In this work, a NHC ligand precursor with high basicity
has been developed with bis-NHC pincer ligand property. In
this way, a homogeneous water soluble NHC ligand precursor
consisting of a counter anion of hydroxyl anions and imidazole
cations on a triazine framework was prepared and character-
ized. The basic NHC ligand precursor could play the role of
a ligand through a bis-(NHC) moiety, efective solvent and
base. Pd ions could be coordinated via a bis-NHC moiety of
the NHC ligand precursor and used as an efcient catalyst for
the C–C cross-coupling reactions. The prominent advantage
of the NHC ligand precursor was its recyclability from the
aqueous phase and reused for several runs with preservation
of its properties. Moreover, the NHC ligand precursor could be
immobilized on magnetite and used as a heterogeneous mag-
netically recyclable nanocatalyst for the C–C cross-coupling
reactions. The heterogeneous catalyst could be recycled for
several consecutive runs with an insignifcant reactivity loss.
Another important type of C–C cross-coupling reaction is
coupling of Csp-Csp2 between terminal acetylenic substrates
with a vinyl, aryl halide or trifates catalyzed by Pd along with
Cu as a co-catalyst for mainly construction of aryl acetylenes
is known as Sonogashira reaction [14]. Sonogashira reaction
has also a vital application in synthetic organic chemistry,
in the formation of pharmaceutical and complex biological
compounds from simple precursors [14, 15], and in the con-
struction of olopatadine hydrochloride [11] and GRN-529 (a
highly selective mGluR5 negative allosteric modulator) mol-
ecules [16]. Traditionally, it was catalyzed by a Pd catalyst
along with a Cu as a co-catalyst. However, the presence of
Cu persuades the by-product of Glaser-type homo-coupling
between two terminal acetylenes [14]. Thus, the eforts con-
ducted toward the development of copper-free methodologies.
In addition, coupling reactions have a special place in
the synthesis of organic compounds. For example, direct
sp3 arylation of 7-chloro-5-methyl-[1,2,4]triazolo[1,5-a]
pyrimidine [17], enantioselective dicarbofunctionalization
of unactivated alkenes [18], desulfonative cross-coupling
of benzylic sulfone derivatives with 1,3-oxazoles [19], and
cross-coupling reactions promoted by biaryl phosphorinane
ligands [20] are some of the recent application of Pd-cata-
lyzed cross-coupling reactions in organic synthesis.
Traditionally, these reactions catalyzed by a Pd salt in the
presence of toxic and expensive phosphine ligand and Cu
co-catalyst (in the case of Sonogashira coupling), that lack of
catalyst recovery, unwilling by-products, toxic solvents disad-
vantages, limited its widespread application [1]. In this way,
and note to their vital application in industry and medicine,
numerous methodologies have been developed and reported
for construction of various types of Pd-catalyzed C–C cross-
coupling reactions, that can be point to: (1) Pd/GO/Fe3O4/
PAMPS (GO=graphene oxide, PAMPS=poly 2-acrylamido-
2-methyl-1-propansulfonic acid) for Suzuki–Miyaura cou-
pling reaction [4], (2) [Pd(1‐tritylimidazole)2Cl2] complex
2 Experimental
2.1 Materials and Instruments
All the chemicals were obtained from Sigma Aldrich
or Merck companies and used as received without
1 3

Q. Min et al.
further purifcation. Wang resin, polymer-bound 4-iodo-
benzene, was purchased from NovaBiochem containing
0.64–1.1 mmol g−1 loading, 100–300 mesh, and cross-linked
with 1% divinylbenzene used for three-phase test. All sol-
vents were distilled (under Ar atmosphere) before use. All
other reagents were of analytical grade. Progress of the
coupling reactions were monitored by thin layer chroma-
tography (TLC) on silica gel or gas chromatography (GC)
on a Shimadzu-14B gas chromatograph with N2 fow as a
carrier gas and equipped with an HP-1 capillary column.
Anisole was used as an internal standard for quantitative
analyses. The viscosity of TAIm[OH] and TAIm[I] was
obtained on a SMART L+PPR Fungilab instrument. FTIR
analyses were recorded on a JASCO FT/IR 4600 spectropho-
tometer using KBr disk. The NMR spectra, 1H (250 MHz)
and 13C (62.9 MHz), were obtained using a Bruker Avance
DPX-250 spectrometer in deuterated solvents of DMSO-d6
or CDCl3, and TMS as an internal standard. XPS analyses
were conducted on a XR3E2 (VG Microtech) with anode
X-ray source using Al-Kα=1486.6 eV. Energy-dispersive
X-ray spectroscopy (EDX) analysis was conducted on
a feld emission scanning electron microscope, FESEM
JOET 7600F, equipped with a spectrometer for energy dis-
persion of X-rays from Oxford instruments. The X-ray dif-
fraction (XRD) studies of the nanoparticles were recorded
on a Bruker AXS D8-advance X-ray difractometer using
Cu-Kα radiation. Elemental analysis (C,H,N,S) was per-
formed using a PerkinElmer-2004 apparatus. Transmission
electron microscopy (TEM) images were taken on a Philips
EM208 microscope operating at 100 kV. The size distribu-
tion of the nanoparticles was measured using the dynamic
light scattering (DLS) method using a HORIBA-LB550
instrument. Field emission scanning electron microscopy
(FE-SEM) image was taken on a MIRA3 TESCAN instru-
ment. Thermogravimetric analysis (TGA) of the samples
was performed using a NETZSCH STA 409 PC/PG under
inert atmosphere (N2) with a heating rate of 10 °C min−1 in
the temperature range of 25–850 °C. The magnetic behavior
of the NPs was conducted on a Lake Shore vibrating sample
magnetometer (VSM) at room temperature. ICP analyses
were conducted on a VARIAN VISTA-PRO CCD simultane-
ous ICP-OES instrument.
the solution was gradually turns into yellow. Then, 1-meth-
ylimidazole (15.0 mmol) was added to the mixture, then
refuxed for 24 h. [TAIm]OH (2) was extracted to n-BuOH
(3×10 mL). The resulting organic phases were placed in a
vacuum oven (65 °C) for 24 h.
Characterization data for 2: Yellow oil, density: 2.22 g.
cm−3, 1H-NMR (250 MHz, D2O) δ(ppm): 4.50 (s, 9H, CH3),
7.32 (d, 3H, J = 6.25 Hz, Im-H), 1.96 (d, 3H, J = 6.25 Hz,
Im-H), 8.98 (s, 3H, Im-H); 13C-NMR (62.9 MHz, D2O)
δ(ppm): 38.0, 117.7, 126.2, 144.6; EDX (%wt, average of
fve points)=C 26.55, N 18.88, I 44.57.
2.2.2 Preparation of 1,1′,1′’‑(1,3,5‑Triazine‑2,4,6‑Triyl)
Tris(3‑Methyl‑1H‑Imidazol‑3‑ium) Hydroxide
(TAIm[OH]) (3)
Exchange of iodide counter ions with hydroxide was pre-
formed via a simple ion exchange using Ag2O (as a part
of Hofmann elimination). Ag2O was also prepared in this
study according to a simple procedure described elsewhere
[28]. [TAIm]OH (2, 1.0 mL) was added to 20 mL of distilled
water, then Ag2O (8.0 mmol, 1.8 g) was added to the mix-
ture in one step. The reaction was performed under refux
conditions for 8 h. The sediment (AgI) was fltered and the
product was extracted to n-BuOH (3×10 mL). The result-
ing organic phases were dried in a vacuum oven (65 °C).
Scheme 1 shows a schematic view for the preparation of
[TAIm]OH.
1
Characterization data for 3: Yellow oil, H-NMR
(250 MHz, D2O) δ(ppm): 4.14 (s, 9H, CH3), 7.86 (d, 3H,
J=7.00 Hz, Im-H), 8.10 (d, 3H, J=7.00 Hz, Im-H), 8.98 (s,
3H, Im-H); 13C-NMR (62.9 MHz, D2O) δ(ppm): 38.7, 118.8,
126.6, 145.6; EDX (%wt)=C 49.33, N 35.76, O 14.91.
2.2.3 Determination of Hydroxyl Group in TAIm[OH]
The hydroxyl content in TAIm[OH] was measured by an
acid titration assay. In this test, 1.0 mL of the NHC ligand
precursor3 was added to 50 mL of distilled water and stirred
vigorously for 10 min (pH=13.88). The resulting solutions
was titrated with acetic acid 1.0 M in the presence of phe-
nolphthalein indicator at room temperature under air condi-
tions. About 11.96 mL acetic acid was consumed at the end
point (pH=7.0). At the same time, a blank was also titrated
and the total volume of acetic acid was recorded. Based on
the experiment, 1.0 mL of NHC ligand precursor contain
12.0 mmol hydroxyl group. So, with assumption of three
equivalent of −OH ions in a mmol of TAIm[OH], there are
4 mmol TAIm[OH] in a 1.0 mL of NHC ligand precursor.
Based on the Mw of the NHC ligand precursor, it can be
concluded that the density is equal to 1.55 g.cm−3.
2.2 Methods
2.2.1 Preparation of 1,1′,1′’‑(1,3,5‑Triazine‑2,4,6‑Triyl)
Tris(3‑Methyl‑1H‑Imidazol‑3‑ium) Iodide (TAIm[I]) (2)
At frst, for the preparation of cyanuric iodide (1), 2, 4,
6-trichloro-1,3, 5-triazine (TCT, 0.37 g, 2.0 mmol) was
added to 20 mL of dry acetone at room temperature. Then,
NaI (1.5 g, 10 mmol) was added to the mixture. The fask
was sealed and the mixture was stirred for 12 h. The color of
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
Scheme 1 The preparation of TAIm[OH] (3) and TAIm[OH]-Pd (4)
2.2.4 Preparation of Fe3O4@SiO2‑TAIm[OH]‑Pd
deionized water and EtOH (respectively), and dried into
oven (60 °C) overnight (Scheme 2).
Fe3O4@SiO2Cl was prepared through three steps according
to a procedure described elsewhere [8, 29]. Next, complex
4 was prepared with dissolution of Pd(OAc)2 (1.0 mmol) to
5.0 mL EtOH, then, TAIm[OH] (3.0 mmol, 0.73 mL) was
added to the mixture and stirred for 4 h at room temperature.
The solid was fltered, washed with warm EtOH and deion-
ized water, dried into oven and stored as a gray powder.
TAIm[OH]-Pd was immobilized on Fe3O4@SiO2 in one step
as follows: Fe3O4@SiO2Cl (100 mg) was sonicated in 20 mL
toluene at room temperature for 20 min. Then, TAIm[OH]-
Pd (10.0 mg) was added to the mixture and refuxed for
24 h under N2 atmosphere. Then, the mixture was allowed
to cooled to room temperature, and the product was sepa-
rated by applying an external magnetic feld, washed with
Characterization data for 4: 1H-NMR (400 MHz, CDCl3)
δ(ppm): 3.04 (s, 6H, CH3), 4.00 (s, 3H, CH3), 6.67–6.69 (m,
2H), 6.93–6.95 (m, 2H), 7.46 (s, 2H), 7.67–7.68 (m, 2H),
8.63 (s, 1H); 13C-NMR (100 MHz, CDCl3) δ(ppm): 24.8,
36.4, 41.7, 112.8, 114.9, 121.4, 137.6, 162.3, 166.2, 170.2,
173.2; EDX (%wt, average of fve points)=C 40.12, N 25.06,
O 13.22, Pd 21.57.
2.2.5 General Procedure for Heck Cross‑Coupling Reactions
Catalyzed by Fe3O4@SiO2‑TAIm[OH]‑Pd and Pd(OAc)2/
TAIm[OH]
2.2.5.1 Method 1 (Heterogeneous Catalyst) In a sealed tube
equipped with a magnetic stirrer, aryl halide (1.0 mmol),
alkene (1.3 mmol), and catalyst 9 (0.01 g, 0.2 mol% Pd)
Scheme 2 Preparation of
Fe3O4@SiO2-TAIm[OH]-Pd
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Q. Min et al.
were mixed/dissolved in H2O (2.0 mL). The mixture was
refuxed for appropriate time which monitored by TLC
(eluent: n-hexane/ ethyl acetate, 4:1). Upon reaction com-
pletion, the mixture allowed to cooled to room tempera-
ture, then, DCM (15.0 mL) was added to the mixture. The
catalyst was separated magnetically, and the organic phase
was separated, washed with deionized water (3×10 mL),
and dried over Na2SO4. Finally, the solvent was evaporated
under reduced pressure, and the product was purifed and
isolated by fash chromatography or a short column of silica
gel to give the corresponding pure coupling product.
three Cl sites in TCT were functionalized with iodide in 2,
and subsequently with methyl imidazole in 3. In addition,
the complete elimination of iodide in the EDX spectrum
of 3, confrmed the successfully ion exchange reaction.
Scheme 3 shows a mechanistic view for this transformation,
where iodide counter ion attack to Ag2O to give TAIm[OAg]
intermediate. Then, in the presence of a water molecule,
AgOH is formed and provides the desire product. Another
prove for this process, was the formation of AgI sediments,
which characterized by FTIR analysis. It worth noted that
the reaction was failed with TCT and didn’t found any
product for 4 (Scheme 1). This could be directly related to
the diference of atomic radius of Cl (175 pm) and iodide
(198 pm), wherein iodine binds more weakly to the imida-
zolium cation than chlorine. However, with cyanuric iodide,
the reaction processed efciently.
2.2.5.2 Method 2 (Homogeneous Catalyst) In a sealed tube
equipped with a magnetic stirrer, aryl halide (1.0 mmol),
alkene (1.3 mmol), and Pd(OAc)2 (1.0 mol%) were mixed
and dissolved in TAIm[OH] (2.0 mL). The mixture was
refuxed for the appropriate time which monitored by TLC
(eluent: n-hexane/ ethyl acetate, 4:1). After the comple-
tion of the reaction, 10 mL of water and 10 mL of CH2Cl2
was added to the mixture. The aqueous phase was further
extracted with another 10 mL CH2Cl2. The organic phase
was dried over MgSO4, and the resulting coupling product
was obtained after removal of the solvent under reduced
pressure followed by fash chromatography.
X-ray difraction patterns of Fe3O4, Fe3O4@SiO2, and
Fe3O4@SiO2-TAIm[OH]-Pd is shown in Fig. 1. Surface
functionalization of magnetite with silica shell don’t change
the crystal structure of Fe3O4 NPs, however, the peaks inten-
sity and sharpness decreased slightly and an amorphous
peak at 2θ=13.1° indicate the presence of amorphous silica
shell coated on magnetite in agreement with the literature
(Fig. 1a,b) [2, 8]. Further functionalization of Fe3O4@SiO2
with TAIm[OH]-Pd causes to more reduction in peaks inten-
sity slightly (Fig. 1c). However, no change in the pattern
and the peaks position, confrmed that the crystal structure
of Fe3O4 remain intact during functionalization (Fig. 1c). A
newly formed peak at 2θ=9.1° indicating a synergetic efect
between amorphous silica shell and TAIm[OH]-Pd, which
shifted the amorphous peak to the lower degrees, consist-
ence with the literature [30]. This shift is a prove for the
further successful functionalization of Fe3O4@SiO2 with
TAIm[OH]-Pd complex.
The Suzuki and Sonogashira cross-coupling reactions
were accomplished with a same procedure described for
Heck reaction. Phenylboronic acid (1.0 mmol) and phenyl
acetylene (1.0 mmol) was used for Suzuki and Sonogashira
cross-coupling reactions respectively. TAIm[OH] could be
extracted by two efective approaches: (i) removal of water
under reduced pressure from the residue of aqueous phase;
and (ii) extraction by pure n-BuOH (3 × 6 mL) and then,
removal of n-BuOH under reduced pressure. However, in the
present study, the frst approach was served for the recycling
of TAIm[OH]. The properties of TAIm[OH] was studied in
each cycle by FTIR, elemental EDX analyses and viscose
meter before use.
From the TEM image of Fe3O4@SiO2-TAIm[OH]-Pd
nanoparticles, the particles are nearly spherical in shape with
a homogeneous morphology and an average size of 30 nm
(Fig. 2A). FE-SEM image of Fe3O4@SiO2-TAIm[OH]-
Pd showed a homogeneous morphology for the NPs in
agreement with the TEM image (Fig. 2B). Also, the size
distribution of the NPs was studied by DLS analysis. The
results showed that the main of NPs have a size of between
25–27 nm, in agreement with the TEM image (Fig. 2C).
Magnetic behavior of the samples was studied by VSM
analysis. The results represent a superparamagnetic behavior
3 Results and Discussion
3.1 Catalyst Characterization
According to the acid titration assay, there is 12.0 mmol
hydroxyl group in 1.0 mml of the imidazolium ionic com-
pound. This gives a density equal to 1.55 g.cm−3 for the
NHC ligand precursor 3, which is completely in agreement
with the measured density by a standard instrument (1.52 g.
cm−3). Also, the viscosity of TAIm[OH] was found to be
1194 cP. Table 1 shows the general physical properties of
TAIm[OH]. FTIR, 1H-NMR, 13C-NMR and EDX analyses
were completely confrmed the chemical structures of 2–4
compounds (ESI, Figs. S1-S4, and Scheme 1), which all
Table 1 Physical properties of TAIm[OH] NHC ligand precursor
Physical
Mw
Color/
Density
Viscosity
(cP)
properties
(g.mol−1
)
appearance (g.cm−3
)
Value
375.4
Yellow oil
1.52a
1194
aBased on apparatus. 1.55 g.cm−3 from the acid titration assay
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
Scheme 3 A mechanistic view for the ion exchange reaction (as a part of Hofmann elimination method) during the preparation of TAIm[OH]
NHC ligand precursor
for Fe3O4, Fe3O4@SiO2 and catalyst 9, with a zero coerciv-
ity for all of them (Fig. 2D). The saturation magnetization
of 69, 40, and 20 emu g−1 was found for Fe3O4, Fe3O4@
SiO2, and catalyst 9 respectively. This magnetization reduc-
tion in each step is directly related to the immobilization
of a diamagnetic shell in each step, that dilute the applied
magnetic feld [8]. Although, a large decrease in magneti-
zation was observed, however it is a strong confrmation
for successful immobilization of magnetite with organic
compounds (Fig. 2D-c). Despite this reduction, the cata-
lyst could be collected from the reaction mixture less than
1.0 min completely.
To manifest and confrm the elements in the catalyst and
oxidation state of Pd, an overall survey and high resolution
XPS analysis was performed on the catalyst 9. As shown in
Fig. 3a, all expected elements were detected in their cor-
responding binding energies. An important point was the
detection of small amount of Cl in the XPS spectrum of 9,
which was in agreement with the corresponding EDX analy-
sis and the proposed structure sketched for 9 in Scheme 2
(Fig. 3a). The high resolution XPS 3d-Pd analysis confrmed
the presence of both Pd(0) and Pd(+2) simultaneously in the
catalyst. As shown in Fig. 3b, the peaks at 337.25 eV and
342.55 eV were assigned to the binding energies of 3d5/2
and 3d3/2, correspond to+2 oxidation state of Pd [31]. Also,
two peaks with smaller intensity at 335.1 eV and 340.3 eV
demonstrated that a small amount of Pd are in zero oxidation
state [31, 32]. This was a positive point in a cross-coupling
reaction with a possible oxidative-addition and reductive-
elimination stages, that was seen for Pd catalyzed C–C
cross-coupling reactions.
EDX analysis of the NPs confirmed the presence of
all expected elements including C, N, O, Si, Fe, Pd, and
Cl in agreement with the suggested structure for Fe3O4@
SiO2-TAIm[OH]-Pd (Scheme 2, and Fig. 3c). More impor-
tantly, there is 2.08%wt Pd in the catalyst, that is completely
Fig. 1 XRD patterns of a Fe3O4, b Fe3O4@SiO2, and c Fe3O4@SiO2-
TAIm[OH]-Pd
1 3

Q. Min et al.
Fig. 2 a TEM image, b FE-
SEM image, and c DLS of
catalyst 9. d VSM curves of a
Fe3O4, b Fe3O4@SiO2, and c
catalyst 9
Fig. 3 a An overall survey and b high resolution (normalized, energy-corrected) XPS analysis, c EDX, and d TGA analyses of catalyst 9
consistence with ICP analysis (2.12%wt). The percentage
of all other elements has been shown in the inset table in
Fig. 3c.
weight loss in the temperature span of 300–750 °C (starts
at 300 °C with a gentle slope) with 34% weight loss. This
weight loss could be directly related to the organic moieties
(containing Pd complex) immobilized on magnetite, com-
pletely in agreement with the EDX analysis. Another small
Thermal behavior of 9 was shown in Fig. 3d. The NPs
demonstrated good thermal stability and there is a main
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
weight loss occurs below 300 °C, that could be attributed to
the adsorbed water molecule on the surface or trapped in the
crystal structure of the NPs (Fig. 3d). Totally, 37% weight
loss was observed until 850 °C, which suggest a good ther-
mal stability for the NPs.
parameters for the both heterogeneous (Scheme 4, method 1)
and homogeneous (Scheme 4, method 2) catalytic systems.
It should be noted that the reactions were performed in the
absence of any basic reagent.
Figure 4a shows the efect of solvent on method 2. The
high efciency of 86–88% was obtained for water, EtOH,
MeOH, DMF, and DMSO solvents for 85 min. The low
efciency of 55% and 75% was obtained for CH3CN and
toluene respectively (Fig. 4a). It seems that the properties
such as dielectric constant, polarity, and being protic for the
solvents, provide better conditions for the coupling reaction.
The reaction under solvent-free conditions didn’t show any
satisfactory results (30% for 85 min, 30% for 120 min, at
120 °C). Efect of temperature was evaluated in the next step
for the both methods (Fig. 4b). Method 1 (using Fe3O4@
SiO2-TAIm[OH]-Pd) gave the high possible efciency at
refux conditions. Decrease of temperature, decreases the
efciency linearly (Fig. 4b). On the other hand, for method
2 using TAIm[OH], there is not any product at tempera-
tures below 40 °C; and 120 °C was the premium tempera-
ture with 90% isolated yield (Fig. 4b). Raising temperature
didn’t efect on the reaction efciency. Finally, the catalyst
amount of 9 and Pd(OAc)2 was studied for methods 1 and
2 (Fig. 4c,d). According to ICP analysis, there is 2.12%wt
Pd in the catalyst. So, based on amount of starting material
for cross-coupling reactions, it could be concluded that the
3.2 Optimization of Reaction Parameters
To fnd optimum conditions for the C–C cross-coupling
reactions catalyzed by Fe3O4@SiO2-TAIm[OH]-Pd and
TAIm[OH], the reaction parameters such as solvent, tem-
perature, and catalyst amount were studied. For this goal,
the Heck coupling reaction of iodobenzene with styrene
was chosen as a model reaction to study of the reaction
Scheme 4 The model cross-coupling reaction of iodobenzene with
styrene for optimization of reaction parameters via method 1 and
method 2 catalyzed by Fe3O4@SiO2-TAIm[OH]-Pd and Pd(OAc)2/
TAIm[OH] respectively
Fig. 4 a Solvent efect over the Heck cross-coupling reaction of
iodobenzene with styrene via method 1. conditions: Solvent (2.0 mL,
except the case of “free”), Fe3O4@SiO2-TAIm[OH]-Pd (as a cata-
lyst), Refux (except the case of “free”), 85 min. b Efect of tempera-
ture over the Heck cross-coupling reaction of iodobenzene with sty-
rene via method 1 and method 2 (85 min for method 1, and 60 min
for method 2). Efect of c Fe3O4@SiO2-TAIm[OH]-Pd amount and d
Pd(OAc)2 over the Heck cross-coupling reaction of iodobenzene with
styrene via the method 1 and method 2, respectively (85 min, H2O,
Ref. for method 1, and 60 min, 120 °C for method 2)
1 3

Q. Min et al.
reactions were catalyzed in the presence of 0.2 mol% Pd
of Fe3O4@SiO2-TAIm[OH]-Pd. The results demonstrated
that 0.2 mol% and 1.0 mol% were the premium amounts of
catalyst for method 1 (catalyst 9) and 2 (Pd(OAc)2)
Table 2 Heck cross-coupling reaction of aryl halides with alkenes catalyzed by Fe3O4@SiO2-TAIm[OH]-Pd and Pd(OAc)2/ TAIm[OH]
Entry
X
R1
R2
Product
Time (min)
Method 1
Yield (%)
Method 1
Method 2
Method 2
1
2
3
4
5
6
7
8
I
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2 Bu
12a
85
60
80
220
100
110
280
40
55
175
45
60
180
50
86
82
70
82
74
60
92
84
80
90
85
78
85
80
70
90
80
85
80
70
85
90
88
80
90
90
86
75
95
90
75
96
90
84
98
95
89
90
90
82
95
84
94
85
70
90
95
95
85
90
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
I
Br
Cl
I
100
250
120
135
300
60
77
200
60
70
200
65
70
220
75
90
95
120
260
135
66
60
70
80
4-MeO
4-MeO
4-MeO
4-NO2
4-NO2
4-NO2
4-CN
4-CN
4-CN
4-Me
4-Me
4-Me
4-Br
4-Cl
H
H
H
4-MeO
4-NO2
4-CN
4-OH
4-Br
12b
12c
12d
12e
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
60
200
55
75
65
12f
12 g
12 h
n
80
240
110
45
55
60
12i
12j
12 k
12 l
12 m
I
I
I
I
60
Bold numbers represent the number of compounds (or products)
aReaction conditions: Aryl halide (1.0 mmol), alkene (1.3 mmol), sealed tube
bFor method 1: Fe3O4@SiO2-TAIm[OH]-Pd (0.01 g, 0.2 mol% Pd), H2O (2.0 mL), Ref
cFor method 2: Pd(OAc)2 (1.0 mol%), TAIm[OH] (2.0 mL), 120 °C
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
respectively. The relationship between catalyst amount mol%
and reaction isolated yield was shown in Fig. 4c,d.
homogeneous systems, respectively, for C–C cross-coupling
reactions. It should be noted that for both systems, no base
was used due to the basic intrinsic nature of the TAIm[OH]
in the both catalytic systems. Table 2 shows the results of
this study for the Heck reaction. As shown in the table, a
wide range of substrates bearing electron donor and electron
withdrawing substituent can be coupled with styrene and
3.3 Catalytic Activity
The catalytic activity of Fe3O4@SiO2-TAIm[OH]-Pd and
Pd(OAc)2/ TAIm[OH] was studied as two heterogeneous and
Table 3 Suzuki cross-coupling reaction of aryl halides with phenylboronic acid catalyzed by Fe3O4@SiO2-TAIm[OH]-Pd and Pd(OAc)2/
TAIm[OH]
Entry
X
R
Product
Time (min)
Method 1
Yield (%)
Method 1
Method 2
Method 2
1
2
3
4
5
6
7
8
I
H
H
H
4-MeO
4-MeO
4-MeO
4-NO2
4-NO2
4-NO2
4-CN
4-CN
4-CN
4-Me
4-Me
4-Me
4-Br
4-Cl
14a
35
60
175
60
65
190
33
45
190
65
80
225
80
110
240
85
120
85
100
245
95
30
40
160
50
50
170
25
35
160
50
66
185
70
95
220
75
100
80
95
220
90
110
240
96
90
90
84
80
70
95
92
85
94
78
68
85
75
65
85
70
90
80
70
95
84
60
96
96
90
90
88
82
98
95
90
94
90
82
88
90
80
85
80
90
90
78
95
90
80
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
I
Br
Cl
I
Br
Cl
14b
14c
14d
14e
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
14f
14 g
14 h
1-Naphthyl
1-Naphthyl
1-Naphthyl
2-Pyrimidyl
2-Pyrimidyl
2-Pyrimidyl
14i
115
260
Bold numbers represent the number of compounds (or products)
aReaction conditions: Aryl halide (1.0 mmol), phenylboronic acid (1.3 mmol), sealed tube
bFor method 1: Fe3O4@SiO2-TAIm[OH]-Pd (0.01 g, 0.2 mol% Pd), H2O (2.0 mL), Ref
cFor method 2: Pd(OAc) (1.0 mol%), TAIm[OH] (2.0 mL), 120 °C
1 3

Q. Min et al.
Table 4 Sonogashira cross-coupling reaction of aryl halides with phenyl acetylene catalyzed by Fe3O4@SiO2-TAIm[OH]-Pd and Pd(OAc)2/
TAIm[OH]
Entry
X
R
Product
Time (min)
Method 1
Yield (%)
Method 1
Method 2
Method 2
1
2
3
4
5
6
7
8
I
H
H
H
4-MeO
4-MeO
4-MeO
4-NO2
4-NO2
4-NO2
4-CN
4-CN
4-CN
4-Me
4-Me
4-Me
4-Br
4-Cl
16a
120
180
200
150
210
230
100
165
190
120
190
220
155
200
240
150
250
190
135
150
95
100
145
160
130
170
200
100
150
175
110
175
190
140
180
210
140
200
180
120
135
90
95
90
84
95
88
80
92
88
80
96
90
82
90
88
82
92
85
90
95
92
96
80
80
97
94
90
95
92
84
98
94
90
96
95
90
94
90
85
96
92
92
95
95
98
84
80
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
Cl
I
Br
I
Br
Cl
I
Br
Cl
16b
16c
16d
16e
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
16f
16 g
16 h
1-Naphthyl
1-Naphthyl
1-Naphthyl
3-Pyrimidyl
3-Pyrimidyl
3-Pyrimidyl
16i
220
180
200
180
Bold numbers represent the number of compounds (or products)
aReaction conditions: Aryl halide (1.0 mmol), phenyl acetylene (1.3 mmol), sealed tube
bFor method 1: Fe3O4@SiO2-TAIm[OH]-Pd (0.01 g, 0.2 mol% Pd), H2O (2.0 mL), Ref
cFor method 2: Pd(OAc)2 (1.0 mol%), TAIm[OH] (2.0 mL), 120 °C
n-butyl acrylate alkenes. In general, the homogeneous sys-
tem has better efciency in terms of product yield (11–14%
better efciency) and time (20–25 min less time than the
heterogeneous system) than the heterogeneous system using
Fe3O4@SiO2-TAIm[OH]-Pd.
Three other notable points from Table 2 are: (1) Aryl hal-
ides containing electron withdrawing groups such as 4-NO2
and 4-CN provide better efciency than aryl halides con-
taining electron donor substituent such as 4-MeO and 4-Me
(for example compare 12b with 12c, and 12i with 12j) com-
pletely in agreement with the reported Pd-catalyzed C–C
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
Table 5 Designed control
Entry
Catalyst
Base
–
Solvent
Time (min)
Yield (%)
experiments for cross-coupling
reaction of iodobenzene with
styrene through method 2
1
2
3
4
TAIm[I] (2)/Pd(OAc)2
TAIm[I] (2)/Pd(OAc)2
TAIm[I] (2)/Pd(OAc)2
TAIm[OH] (3)/Pd(OAc)2
TAIm[OH] (3)/Pd(OAc)2
Pd(OAc)2
–
–
–
–
60
60
60
60
60
60
60
12
85
75
90
90
–
b
K3PO4b
K2CO3
–
5
K3PO4
–
–
–
6c
7d,e
DMF (or H2O)
–
TAIm[OH]-Pd (4)
85
aReaction conditions: Iodobenzene (1.0 mmol), styrene (1.3 mmol), Pd(OAc)2 (1.0 mol%), TAIm[OH]
(2.0 mL), Ref., sealed tube
b2.0 mmol
cPd(OAc)2: 1.0 mol%, DMF/H2O (2.0 mL)
d0.01 mmol (5.2 mg)
eThe efciency reached to 90 for 65 min
cross-coupling reactions in literature [2, 7], (2) the reactivity
of aryl halides as a leaving group was as order of: I>Br>Cl
(Table 2) and (3) Styrene showed a higher activity for the
coupling reaction than n-butyl acrylate in a completely same
reaction conditions (for example, compare 12a and 12 h). As
will be shown in following, these results could be extended
to Suzuki and Sonogashira suitably. Moreover, the results
suggest a mechanism including oxidative-addition and
reductive elimination for both catalytic systems.
and unlikely (according to the proposed mechanism). The
catalytic activity of the TAIm[OH] (3)/Pd(OAc)2 mixture
was also studied in the presence and in the absence of K3PO4
as a base. As shown in Table 5, entries 4,5, the results in the
presence and absence of K3PO4 did not difer and for both
reactions 90% efciency was obtained for 1 h. The results
confrm well that there is no need for any external base in
the coupling reactions.
In another experiment, two solvents, DMF and H2O,
were used instead of TAIm[OH], and no detectable efcien-
cies were observed for either (Table 5, entry 6). The results
show that TAIm[OH], in addition to its high solubility, plays
the role of ligand and base well. In another control test, the
catalytic activity of 4 was evaluated towards Heck model
reaction. The reaction aforded 85% of 12a for 1 h; but the
efciency reached to 90% for 65 min (Table 5, entry 7).
This slight diference may be directly related to the time
required for the formation of the Pd complex in the mixture.
In addition, the reaction can take place after the formation
of the frst species of palladium complex, which could be
another reason for the slight diference in the catalytic activ-
ity of compound 4 with a mixture of 3/ Pd(OAc)2. Also, this
test clearly confrmed the base functionality for TAIm[OH]
moiety.
Signifcant results for Heck reaction also led to the evalu-
ation of catalyst capability for Suzuki and Sonogashira cou-
pling reactions. The results for these reactions were also
signifcant for both systems, and good to excellent efcien-
cies were achieved for all substrates. Tables 3 and 4 show the
results of this study. Similar to the Heck reaction, aryl hal-
ides containing electron withdrawing groups provided better
efciency than the electron donor type for both Sonogashira
and Suzuki reactions. The results of the homogeneous sys-
tem showed that TAIm[OH] has multiple functions as a sol-
vent, base and ligand and catalyzes the coupling reactions
well, only in the presence of Pd (OAc)2.
3.4 Control Experiments
In order to clarify the ligand and base role of TAIm[OH],
several control reactions were performed over the Heck
model reaction. Initially, compound 2 (2.0 mL) was used
instead of 3 to the Heck model coupling reaction. Efciency
dropped to 12% for an hour (Table 5, entry 1).
3.5 Reusability
One of the main factor for a heterogeneous catalyst, which
distinguishes it from the homogeneous type is its recycla-
bility with the preservation of catalytic activity [33]. For a
heterogeneous catalyst with magnetite solid support bearing
immobilized metal complex, this factor is directly related to
the metal leaching amount, stability during reaction medium,
and deactivation. In this way, the heterogeneous nature of
Fe3O4@SiO2-TAIm[OH]-Pd was evaluated by three experi-
ments of (i) hot fltration, (ii) mercury poisoning, and (iii)
Another interesting point is that the efciency in the pres-
ence of K3PO4 and K2CO3 bases created 85% and 75% ef-
ciency for one hour at 120 °C, respectively (Table 5, entries
2,3). The results clearly show the base role and NHC-ligand
carbene property for TAIm[OH] and also show that TAIm[I]
still has some basicity (refer to 12% yield in the absence of
base, entry 1), but the formation of HI seems very difcult
1 3

Q. Min et al.
Scheme 5 The three-phase test of 4-iodobenzene bounded Wang resin, n-butyl acrylate, and iodobenzene (in test 2) in the presence of Fe3O4@
SiO2-TAIm[OH]-Pd
three-phase tests. All experiments were done on the Heck
model reaction (Scheme 4) under optimized reaction condi-
tions. For hot fltration test, the magnetic catalyst was fltered
magnetically after 40 min of the reaction time with 38% con-
version (based on GC analysis). The reaction was allowed
to processed for further 60 min. The coupling conversion
reached to 40% after that time, which strongly confrmed
the absence of any metal leaching to the mixture and het-
erogeneous performance of the catalyst. Mercury poisoning
test was carried out by addition of 320 molar equivalents
of Hg(0) vs. Pd content after 40 min of the reaction time.
The reaction mixture was stirred for another 60 min. The
reaction conversion was determined by GC analysis and
found to be 38%. Hg(0) was adsorbed by metal sites on the
catalyst surface (or by amalgamation) and deactivates the
active sites and suppressed the catalytic activity immedi-
ately [29]. No progress in the reaction immediately after
addition of Hg(0) confrm this phenomena as well as the
heterogeneous nature of the catalyst in agreement with the
hot fltration test. Finally, the three-phase test, as a promis-
ing test for evaluation of a heterogeneous performance of a
catalyst was applied over the catalyst (9) [29]. As shown in
Scheme 5, a Wang resin-bound aryl iodide was reacted with
n-butyl acrylate in water under refux conditions in the pres-
ence of Fe3O4@SiO2-TAIm[OH]-Pd (0.01 g, 0.2 mol% Pd).
Didn’t found any coupling product under these conditions,
and at the end of the reaction, the catalyst was recovered
magnetically. Moreover, the hydrolysis of the Wang resin by
trifuoroacetic acid (TFAA) led to the formation of 4-iodo
benzoic acid (Scheme 5, test 1). In another experiment, iodo-
benzene was also added to the mixture (Scheme 5, test 2).
The experiment gives 87% conversion for 12 h for 95 min
and no coupling product was detected after acid hydroly-
sis of the Wang resin. The results from the three-phase test
clearly confrmed the heterogeneous nature and performance
of the catalyst for the C–C coupling reactions, completely
in agreement with the hot fltration and Hg(0) poisoning
that suggest no considerable metal leaching occurs in the
medium.
Finally, the recyclability of the heterogeneous catalyst
(9) was evaluated over the Heck, Suzuki and Sonogashira
cross-coupling reactions for the preparation of 12a, 14a, and
16a coupling products, respectively. All experiments were
accomplished under optimized conditions (Tables 2–4). Fig-
ure 5a,d show the results of recycling and Pd metal leach-
ing for the three couplig reactions. As evident in the fgure,
insignifcat activity loss was observed for all the reactions,
as 6%, 4%, and 5% loss in product isolated yield for Heck,
Suzuki, and Sonogashira cross-coupling reactions respect-
vely. Furthermore, didn’t detect any Pd metal leaching by
ICP analysis until cycle 4 for all coupling reactions in the
residue (Fig. 5b). The leaching amount was slighlty incresed
and reached to 2.5%, 1.8%, and 2.6% for Heck, Suzuki, and
Sonogashira cross-coupling reactions respectively for 8th
cycle. The recovered catalyst after 8th cycle was charater-
ized by TEM analysis. As shown in Fig. 5c, a same mor-
phology and particle size distributon was observed same as
the corresonding freshly prepared catalyst. In addition, the
oxidation state of Pd sites in the recovered catalyst was stud-
ied by high resolution XPS analysis. The results confrmed
that the oxidation state of Pd ative sites remained intact and
consist of a mixture of 0 and+2 oxidation states (Fig. 5d).
The results clearly confrmed the stability and high catalytic
activity of the catalyst during the coupling reactions.
More importantly, the recyclability of TAIm[OH] was
studied in each recycling. In this way, the model reaction
of iodobenzene with styrene was chosen and in each cycle,
EDX, density, and viscosity values were determined. The
results were tabulated in Table 6. As shown in Table 6,
the NHC ligand precursor retained its properties well in
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
Fig. 5 a Recyclability and b leaching studies of catalyst 9 over the
Heck (reaction of iodobenzene with styrene, preparation of 12a),
Suzuki (reaction of iodobenzene with phenylboronic acid, preparation
of 14a), and Sonogashira (reaction of iodobenzene with phenylacety-
lene, preparation of 16a) coupling reactions under optimized condi-
tions. c TEM image and d high resolution XPS analysis (energy cor-
rected) of the recovered catalyst 9 after 8th reaction cycle of Heck
cross-coupling reaction of iodobenzene with styrene under optimized
conditions
Table 6 Recyclability studies of
Physical properties
EDX elemental analysis (%wt)
Density
Viscosity (cP)
Yield (%)
TAIm[OH] over the reaction of
iodobenzene with styrene
(g.cm−3 c
)
C
O
N
Ib
Fresh
49.33
49.47
49.62
49.76
49.90
50.05
50.22
50.39
50.52
35.76
35.52
35.25
34.96
34.80
34.55
34.20
33.94
33.68
14.91
14.95
15.05
15.18
15.16
15.18
15.28
15.33
15.36
0.00
0.06
0.08
0.10
0.14
0.22
0.30
0.34
0.44
1.52
1.52
1.52
1.54
1.53
1.57
1.57
1.57
1.58
1194
1194
1194
1194
1197
1198
1198
1200
1208
90
89
89
88
88
89
87
87
88
1th cycle
2th cycle
3th cycle
4th cycle
5th cycle
6th cycle
7th cycle
8th cycle
aReaction conditions: Iodobenzene (1.0 mmol), styrene (1.3 mmol), H2O (2.0 mL), Pd(OAc)2 (1.0 mol%),
recovered (except fresh) TAIm[OH] (2.0 mL), Ref., sealed tube, 60 min
bIn this study, iodide was detected after 1th run due to the use of iodobenzene substrate
cBased on apparatus. 1.55 g.cm−3 from the acid titration assay
successive cycles, which a very little efciency loss per
cycle (only 2% efciency loss after the eighth cycle) was
observed for the reaction. A very important result was the
presence of iodide ions from the frst cycle in the EDX
analysis of the recovered TAIm[OH], which indicates that
the structure of the catalyst is directly involved in the reac-
tion. The presence of iodide increased slowly and increased
linearly from the ffth cycle onwards. As will be shown in
the next section (Mechanistic Studies), the halide ion (from
the aryl halide substrate) is replaced by the used HO− ions
in the reductive-elimination step. The presence of iodide
is also responsible for the increase in density and viscos-
ity, especially in the ffth and later cycles. However, under
homogeneous and heterogeneous conditions, catalysts can
1 3

Q. Min et al.
subsequently the stability of the NHC ligand precursor dur-
ing consecutive recycles. As shown in Fig. 6, an almost iden-
tical FTIR spectrum than the freshly prepared was obtained
for the recovered TAIm[OH].
3.6 Mechanism Studies
According to the obtained evidences and the mechanisms
reported for Heck and Sonogashira reactions [34, 35],
three mechanisms was proposed for the Heck, Sonoga-
shira, and Suzuki coupling reactions catalyzed by Fe3O4@
SiO2-TAIm[OH]-Pd or TAIm[OH]-Pd. The results of XPS
analysis showed that the coordinated Pd ions in TAIm[OH]-
Pd have a mixture of 0 and+2 oxidation states. Therefore,
according to the expectations and mechanisms reported
for catalytic coupling reactions with Pd-based systems, the
coupling reaction includes oxidative-addition and reductive-
elimination steps. According to the proposed mechanism
Fig. 6 FTIR spectrum of the recovered TAIm[OH] over the reaction
of iodobenzene with styrene after 8th run
be used as long as there are hydroxide groups in the catalyst,
or in other words, the catalyst is active.
In addition, FTIR spectrum of the recovered TAIm[OH]
after 8th run confrmed preservation of the structure and
Scheme 6 The proposed reaction mechanisms for Fe3O4@SiO2-TAIm[OH]-Pd or TAIm[OH]-Pd catalyzed a Sonogashira, b Heck, and c Suzuki
cross-coupling reactions
1 3

Introduction of a Recyclable Basic Ionic Solvent with Bis-(NHC) Ligand Property and The…
for Heck coupling reaction (Scheme 6a), the reaction ini-
tially begins with an oxidative-addition in the presence of
aryl halide and the formation of the intermediate I. In the
presence of an alkene, a π-complex II is formed and then
an insertion is performed (Intermediate III). With a β-H
elimination, a double bond is formed, followed by forma-
tion of a π-complex IV. The role of hydroxyl groups in the
NHC ligand precursor as a base at this stage was proved by
the tasks of: (i) reductive elimination, (ii) formation of the
Heck product, and (iii) returns the catalyst to the cycle. The
Sonogashira reaction is also performed according to the tra-
ditional mechanisms reported for the Pd-catalyzed Sonoga-
shira reaction including oxidative-addition and reductive-
elimination steps (Scheme 6b) [34–37].
on magnetite with the preservation of its catalytic activity.
The heterogeneous catalyst was also characterized by TGA,
VSM, FE-SEM, TEM, XPS, EDX, and DLS analyses. How-
ever, the results revealed excellent catalytic activity for the
both homogeneous and heterogeneous catalytic systems.
The Pd sites in two catalytic systems were in a mixture of
0 and+2 oxidation states, which eliminates the need to any
pre-activation or pre-reduction of Pd sites for an oxidative-
addition and reductive elimination steps. The heterogene-
ous nature of the heterogeneous catalyst was confrmed by
hot fltration, mercury (0) poisoning, and three-phase tests.
Finally, the heterogeneous catalyst could be recycled for at
least 8 consecutive runs for all three C–C coupling reactions
with insignifcant metal leaching and catalytic activity loss.
As shown by the results obtained from the recovery of
TAIm[OH] (Table 5, the presence and increase of iodine
in successive cycles), the NHC ligand moiety promotes the
reductive-elimination steps by absorbing the halide through
its triazine framework. This result is also refected in the
mechanisms, and as shown in the schemes, the halide ion
replaced by the used HO− groups in the TAIm[OH] structure
in the reductive-elimination step.
Supplementary Information The online version contains supplemen-
tary material available at https://doi.org/10.1007/s10562-021-03552-5.
Acknowledgement This work was sponsored in part by Natural Sci-
ence Foundation of Liaoning Province (No. 20170520296).
Compliance with Ethical Standards
Subsequently, a similar mechanism was proposed for
Suzuki cross-coupling according to the literature [34, 36].
The oxidation-addition reaction leads to the preparation of
intermediate I (Scheme 6c). The basicity nature of the cata-
Conflict of interest There are no conficts to declare.
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