Stirring or milling? First synthesis of Rh(I)-(di-N-heterocyclic carbene) complexes both in solution and in a ball mill

Article abstract of DOI:10.1016/j.jorganchem.2020.121308

An environment-friendly, convenient, fast and solvent-free mechanochemical approach have been accomplished for the synthesis of several diimidazolium salts and the bridging dinuclear rhodium(I)–N-heterocyclic carbene complexes of the type [{RhCl(cod)}₂(μ-di-NHC)] derived from them. The compounds were synthesized also by the classical solvent method and the results of the two approaches were compared. A systematic study of both the mechanochemical and the solvent syntheses has also been carried out to determine the effects of various factors influencing the reactions. This is the first report on the mechanochemical synthesis of poly-NHC metal complexes as well as NHC–Rh complexes in ball mill. Comparative synthesis of several new, structurally well characterized alkylene-bridged diimidazolium salts and the respective dinuclear Rh(I)-(di-N-heterocyclic carbene) complexes, [{RhCl(cod)}₂(μ-di-NHC)], both by mechanochemical and traditional solvent methods, revealed advantages of the mechanochemical (ball-mill) synthesis, such as reduced solvent need, shorter reaction times and competitive or higher product yields.

Full text of DOI:10.1016/j.jorganchem.2020.121308

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Stirring or milling? First synthesis of Rh(I)-(di-N-heterocyclic carbene) complexes both
in solution and in a ball mill
Sourav De, Ferenc Joó, Henrietta Horváth, Antal Udvardy, Csilla Enikő Czégéni
PII:
S0022-328X(20)30210-2
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121308
JOM 121308
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 25 March 2020
Revised Date: 11 April 2020
Accepted Date: 16 April 2020
Please cite this article as: S. De, F. Joó, H. Horváth, A. Udvardy, Csilla.Enikő. Czégéni, Stirring or
milling? First synthesis of Rh(I)-(di-N-heterocyclic carbene) complexes both in solution and in a ball mill,
Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121308.
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1
Stirring or milling? First synthesis of Rh(I)-(di-N-heterocyclic
carbene) complexes both in solution and in a ball mill
Sourav Dea,b, Ferenc Joóa,c, Henrietta Horváthc, Antal Udvardya,*, Csilla Enikő Czégénic,*
aUniversity of Debrecen, Department of Physical Chemistry, P.O.Box 400, Debrecen, H-
4002 Hungary
bUniversity of Debrecen, Doctoral School of Chemistry
cMTA-DE Redox and Homogeneous Catalytic Reaction Mechanisms Research Group,
P.O.Box 400, Debrecen, H-4002 Hungary
*Corresponding authors
E-mail: nagy.csilla@science.unideb.hu; udvardya@unideb.hu
Dedicated to Professor László Kollár in recognition of his numerous outstanding
achievements in organometallic chemistry of metal carbonyl complexes and in catalytic
organic synthesis involving carbon monoxide.

2
Abstract
An environment-friendly, convenient, fast and solvent-free mechanochemical approach have
been accomplished for the synthesis of several diimidazolium salts and the bridging dinuclear
rhodium(I)-N-heterocyclic carbene complexes of the type [{RhCl(cod)}2(µ-di-NHC)] derived
from them. The compounds were synthesized also by the classical solvent method and the
results of the two approaches were compared. A systematic study of both the
mechanochemical and the solvent syntheses has also been carried out to determine the effects
of various factors influencing the reactions. This is the first report on the mechanochemical
synthesis of poly-NHC metal complexes as well as NHC-Rh complexes in ball mill.
Keywords
Ball mill, Mechanochemistry, di(N-heterocyclic carbene), Rhodium, Solvent-free

3
1.
Introduction
N-heterocyclic carbenes or NHCs have proven themselves as most powerful tools in the
domain of modern chemistry because of their outstanding potential and wide application in
the field of organometallic/coordination chemistry, catalysis, photophysics, medicine and
material science [1-5]. Hydroformylation and carbonylation were among the first processes
realized with rhodium-NHC complexes as the catalysts [6-9]. Poly-NHCs, of which di-NHCs
are the most common and abundant, have attained significant attractions in the last two
decades due to their allowance to form various organometallic compounds with diversity in
geometries. They are relatively easy to synthesize and their properties can be modified simply
by swapping the linker or changing the length of the linker, or placing various substituents
onto the linker [2, 10-13]. Transition metal complexes bearing NHCs as ligands have
obtained widespread application in several fields of chemistry. Rh-NHC complexes deserve
special mention due to their extensive usage in different domains of catalysis chemistry such
as hydrogenation, dehydrogenation, hydroamination, hydration, C-C cross-coupling etc. [1,
2-5, 10, 14-17]. Moreover, a recent study tells about the potential of Rh-NHC complexes as
anticancer drugs [18].
A number of excellent articles on the synthesis of NHC-metal complexes are already
available [3, 14-17, 19–26]. Still, the development of safer, more efficient and cleaner
synthetic methodology is on high demand. From the viewpoint of green chemistry, synthetic
methods under solvent-free or solvent-less conditions are of high desire as the energy-cost as
well as the waste-production are reduced. The traditional solvent synthesis has a number of
disadvantages such as long reaction time, high energy costs, furthermore it most often has a
negative impact on the environment due to the prevalent usage of volatile organic solvents.
Mechanochemistry has a long history in the everyday life and in laboratory-scale chemical
syntheses [27]. In the laboratory, mechanochemically assisted reactions are usually carried
out either in planetary or in vibrating ball mills, where the reagents are loaded in a milling jar
and movement of balls generates the mechanical force to the reagents [28, 29]. The
possibility to adjust several instrumental parameters makes the processes highly efficient and
reproducible [30-32]. Nevertheless, in some cases simple grinding together of the solid
reactants in a mortar with a pestle may also bring excellent results. Typically, such
procedures require solvents only for the workup and purification of the products. In the last

4
two decades a huge increase in the applications of such methods in chemical research could
be seen, and the developments have been discussed in several recent reviews [28, 33-37].
Today, mechanochemistry ranks amongst the top ten world changing technologies according
to IUPAC [38]. In addition to introduction of grinding and milling into classical organic
synthesis to allow the design of solvent-less procedures [39], mechanochemical syntheses
have been more and more applied also in organometallic chemistry [34]. Procedures in
controlled atmospheres (H2, CO, etc.) have been developed [40] and various techniques have
been introduced to follow the reactions in situ for kinetic analysis of the syntheses [41]. With
regard to the subject of our present study, the mechanochemical synthesis of Pd(II)- and
Pt(II)-N-heterocyclic carbene complexes (with mortar and pestle, [42]) and the synthesis of
Ag(I)-, Au(I)-, Cu(II)- and Pd(II)-NHC complexes (in ball mills, [43]) deserve special
mention. In general, however, applications of mechanochemical methods for synthesis of
important organometallic catalysts are still rare.
In the present work, azolium salts serving as precursors to several new (-CH2-)n bridged (n=1
or 4) diimidazole-2-ylidene ligands and their bridging dinuclear Rh(I)-complexes have been
synthesized both via the classical solution synthesis and by solvent-free one-pot
mechanochemical method and the results are compared. To the best of our knowledge, this is
the first article to report the mechanochemical synthesis of di-NHC metal complexes as well
as the first Rh-NHC complexes synthesized in ball-mill.
2.
Results and discussion
2.1
General synthetic methods
To prepare the (-CH2-)n bridged (n=1,4) substituted diimidazolium salts 2a,b and 3a,b, the
corresponding unsubstituted diimidazoles 1a,b were first synthesized following the literature
[44,45]. Next, 1a,b were reacted with the corresponding benzylic chlorides in 1:2 ratio as
depicted in Scheme I. All the bridging dinuclear rhodium(I) complexes 4a,b and 5a,b were
prepared by reacting the diimidazolium chloride salts with [RhCl(cod)]2 and K2CO3 in 1:1:10
ratio (Scheme II).

5
Scheme I: Synthesis of the carbene ligand precursors 2a,b and 3a,b.
Scheme II: Synthesis of the metal complexes 4a,b, and 5a,b.
Some of the diimidazolium salts (with various halide counter-ions) have been already applied
for the preparation of gold (2a: [4]); palladium (2a: [46-48], 2b: [46]); ruthenium (2a: [49,
50], 2b: [49]); and iridium (2a: [51]) NHC-ligated complexes. 3b is also known from the
literature, however, it was synthesized only as part of a rotaxane and with iodide as the
counter ion [52]. All these previous syntheses were carried out in solution.
In our case, the classical solution synthesis of 2a,b and 3a,b as well as that of 4a,b and 5a,b
involved heating of solutions of the reaction partners (Schemes I and II) in the appropriate

6
solvent for a period of time during which the reactions completed. Conversely, the
mechanochemical synthesis of the same compounds consisted of milling together the reaction
partners in a planetary ball mill for the time sufficient to achieve high conversions; these
reactions were carried out in air. The successful formation of Rh-NHC complexes was
validated by the appearance of the characteristic Rh–Ccarbene doublet signal(s) at δ 180-185
13
ppm with J≃ 50 Hz in the C NMR spectra of the compounds. 4a,b and 5a,b are dinuclear
Rh(I) complexes in which two RhCl(cod) units are bridged by the appropriate carbene ligand
derived from 2a,b or 3a,b. The presence of two rhodium centers was confirmed by the two
doublet signals each in the region of δ 98-101 ppm and δ 68-71 ppm which are assigned to
Rh–Cvinyl carbons of coordinated cod. Interestingly, in the mechanochemical syntheses
described here, the dinuclear complexes 4a,b and 5a,b were exclusively obtained with no
sign of chelate [RhL] or macrocyclic [Rh2L2] species which are known from solution
syntheses [53, 54]. HR ESI-MS measurements also confirmed the composition of the
complexes. It should be added here, that none of the above ligands and complexes was
obtained previously by mechanochemical synthesis.
2.2
Structural characterization by single crystal X-ray diffraction
The solid state structures of the carbene ligand precursors 3a, and 3b, and metal complexes
4a, 4b and 5b have been determined by single crystal X-ray diffraction studies.
3a was dissolved in methanol in a small tube and saturated with KPF6. Then, in a
closed container, the tube was half immersed into diethyl ether and stored at –18°C. In four
weeks, colourless crystals, suitable for XRD measurements, appeared on the wall of the tube.
Crystals of 3b were obtained from a methanolic solution layered with diethyl ether.
Both 3a and 3b crystallized as triclinic (P1 space group). The asymmetric unit of 3a
contains a cationic NHC-precursor and two disordered PF-6 ions and a methanol (Figure 1)
while that of 3b contains half of the molecule and a chloride.
In the molecule of 3b, the imidazole centroids are long apart (7.070 Å, Figure S1);
capped sticks representation is shown on Figure 2. In the crystal, the molecules are packed in
a stair-like arrangement (Figure S2), in which the parallel imidazole ring planes are in a
distance of 3.877 Å while the distance of the arene planes is 3.649 Å. All of the bond lengths
and bond angles in the imidazole rings are as expected [55]. The supramolecular architecture
is further stabilized by π-π stacking interactions between the five- and six-membered rings.

7
Figure 1. Capped sticks representation of the molecular structure of 3a in the solid state
(disordered PF-6 counter ions and a methanol molecule are omitted for clarity).
Selected bond lengths and angles: N1–C1:1.328(3) Å; N2–C1:1.378(3) Å; N3–C15:1.328(3)
Å; N4–C15:1.326(3) Å; C2=C3:1.336(3) Å; C16=C17:1.341(4) Å; N1–C14:1.355(3) Å; N3–
C14:1.356(3) Å; N1–C1–N2:108.6(2)°; N1–C1–N2:108.1(2)°; N1–C14–N3:109.85(19)°;
N2–C4–C5:113.6(2)°; N4–C18–C19:111.2(2)°.
Figure 2. Capped sticks representation of the molecular structure of 3b in the solid state
(counter ions are omitted for clarity). Selected bond lengths and angles: N1–C1:1.333(2) Å;
N2–C1:1.327(2) Å; C2=C3:1.341(3) Å; N1–C1–N2:108.38(15)°; N2–C4–C5:112.10(14)°;
N1–C14–C15:111.94(15)°.
Compound 2b (as a dinitrate salt) in its co-crystal with [Cu(NO3)2(H2O)2] has been
previously studied by X-ray diffraction by Doimeadios [56]. Although the solid state
structures of 3a, and 3b, discussed above, are very similar to that of 2b published in ref. [56],
however, the bond lengths and angles cannot be quantitatively compared due to the large
error of the structure of 2b (R1 = 12.89%).

8
Crystallization of 5b was attempted by several methods, unfortunately the best
crystals (obtained by slow evaporation of its solution in CH2Cl2) were still of rather bad
quality. 5b crystallized in monoclinic P21/n space group, the unit cell contains half of the
molecule. After refinement of the best dataset, the error still remained large (R1 = 17.51%,
wR2=41.25%) so while the molecular model (Figure 3) proved to be suitable, the bond
distances and bond angles cannot be evaluated.
Figure 3. Capped sticks representation of the molecular structure of 5b in the solid state.
The Rh(I)-complexes 4a and 4b were crystallized from benzene solutions layered with
diethyl ether and were isolated as yellow crystals not sensitive to air and moisture. Both
complexes crystallize in the monoclinic crystal system, however the space groups are
different (C2/c for 4a, and P21/c for 4b). Capped sticks representations of the molecular
structures are shown on Figure 4, while the selected bond distances and angles can be found
in Table 1.
Figure 4. Capped sticks representation of the molecular structure of the Rh(I)-complexes 4a
(left) and 4b (right) in the solid state.

9
Table 1. Selected bond lengths and bond angles for the solid state structures of the Rh(I)-
complexes 4a and 4b.
4a
4b
Rh1–C1
Rh2–C21
Rh1–Cl1
Rh2–Cl21
Rh1–Rh2
C2–C3
2.0125(18)
2.0261(16)
2.3775(4)
2.3828(15)
6.938
2.025(3)
2.030(3)
2.3808(8)
2.3669(18)
8.277
1.336(2)
1.339(3)
1.336(4)
1.332(4)
C22–C23
N2–C11–N3
N1–C1–N2
N3–C21–N4
N1–C4–C5
N4–C24–C25
C1–Rh1–Cl1
C21–Rh2–Cl21
110.60(3)
103.91(13)
103.53(14)
112.08(14)
114.18(15)
88.79(4)
–
104.4(2)
104.1(2)
112.2(2)
113.2(2)
91.28(8)
89.36(8)
89.76(5)
There are only a few publications in the literature on the molecular structures of
[{RhCl(cod)}2(µ-di-NHC)] dinuclear Rh(I)-complexes [57-60]. Similar to the published
structures, both 4a and 4b consist of two RhCl(cod) fragments connected by the methylene-,
or butylene-bridged di-NHC ligands (Figure 4). In 4a, the Rh–Ccarbene bond distances are
2.0125(18) Å and 2.0261(16) Å, while in 4b the same bonds are 2.030(3) Å and 2.025(3) Å
long. These bond lengths are consistent with the typical Rh–Ccarbene distances in Rh(I)-NHC
complexes, and refer to σ bonds with very little back-donation [55]. In 4a, the two imidazole
rings are at a dihedral angle of 87.47°, in contrast, in 4b, these planes are almost parallel with
a dihedral angle of 7.84°.
In the crystal of 4a, there are no interactions between the phenyl groups of the
neighboring molecules, and the centroids of the imidazole rings are 3.719 Å and 5.021 Å
apart (Figure S3). In the case of 4b, the flexibility of the butylene bridge allows π-π stacking
interactions between the imidazole rings (their distance is 3.654 Å) and also between the
phenyl rings with a distance of 4.534 Å (Figures S4 and S5).
On the basis of the Rh1–Rh2 distances (6.938 Å in 4a and 8.277 Å in 4b) no Rh–Rh
bonding interactions can be assumed in these complexes.
It has previously been observed, that the Rh(I)-complexes of di-NHC ligands with a
methylene bridge and aliphatic N-substituents preferred chelate coordination [53, 54]. In
contrast, 4a contains the N-benzyl-substituted di-NHC ligand (2a) in bridging position

10
between the two RhCl(cod) moieties. Only two examples of similar structure have been
determined so far [57, 58].
2.3
Mechanochemical synthesis of diimidazolium salts 2a,b and 3a,b and the respective
[{RhCl(cod)}2(µ-di-NHC)] complexes 4a,b and 5a,b. Comparison to solution synthesis.
The main aim of the present work was the exploration of the usefulness of mechanochemical
synthesis in the field of Rh(I)-NHC complexes and its comparison to the solution methods of
synthesis. For this purpose, the syntheses were also carried out by using a planetary ball mill.
Initially, the influence of ball milling conditions like milling time, frequency of milling and
size of bearing balls were investigated (Tables 2 and 3). To do so, a milling cycle of 4
minutes (2 minutes milling, followed by another 2 minutes pause) to avoid the overheating,
was used. Ligand 2b (Table 2) was taken as reference, and 10 pieces of bearing balls with 5
mm diameter (in the following: 10 Ø 5 mm) were employed to check the effects of frequency
and milling time. The yield was found to be increased with the rise of frequency (Table 2,
entries 2, 5) and after an initial increase, it levelled off with longer milling time (Table 2,
entries 1-3). In contrast, the amount and the size of balls influenced significantly the yield
(Table 2, entries 4-8). It was finally found that a mixture of 10 Ø 5 mm and 10 Ø 8 mm balls
yielded the best result (71 % isolated yield) after 90 cycles at 550 rpm frequency (Table 2,
entry 8).
Table 2. Optimization of milling conditions for the synthesis of 2ba from 1b and benzyl
chloride.
Entry
Diameter of
Number of
balls
Frequency
(rpm)
Cyclesb
Yield (%)c
balls (Ø mm)
1
2
3
4
5
6
7
8
5
10
10
350
350
350
550
550
550
550
550
45
90
135
90
90
90
90
90
42
58
59
65
68
67
70
71
5
5
10
5
5
10
20
8
10
8
20
5 + 8
10 + 10

11
b
aSynthesis was performed with 0.526 mmol of 1b and 1.05 mmol of benzyl chloride in air. 1 cycle= 2 min
milling + 2 min pause. cIsolated yield.
In the case of the metal complexes, 4b was taken as a reference (Table 3) with 10 Ø 5 mm
and 10 Ø 8 mm balls and the best yield (74 %) was observed after 45 cycles at 550 rpm
frequency (Table 3, entry 4). Under these optimized conditions, during the synthesis of 4b,
the outside surface temperature of the milling jar was regularly checked with the use of a
remote infrared thermometer. It was found that by the end of the 45th cycle (i.e. in 3 h
reaction time), the surface temperature of the jar increased to 34.1 °C from 23.6 °C measured
at the start of milling.
Table 3: Optimization of milling conditions for the synthesis of metal complex 4ba.
Entry
Diameter of
Number of
balls
Frequency
(rpm)
Cyclesb
Yield (%)c
balls (Ø mm)
1
2
3
4
5 + 8
5 + 8
5 + 8
5 + 8
10 + 10
10 + 10
10 + 10
10 + 10
350
350
350
550
45
90
69
71
72
74
135
45
aSynthesis was performed with 0.338 mmol of 2b, 0.338 mmol of [RhCl(cod)]2 and 3.38 mmol of K2CO3 in air.
b1 cycle= 2 min milling + 2 min pause. cIsolated yield.
As the volume of the milling jar was fixed (12.5 mL), the influence of the amount of starting
material for the preparation of diimidazolium salt 2b from 1b, and metal complex 4b from 2b
was examined (Tables 4 and 5). It was observed that 100 mg of 1b (Table 4, entry 2) and 150
mg of ligand precursor 2b (Table 5, entry 3) as reactants resulted in the highest yields.
Table 4: Effect of the reactant amount for the formation of ligand 2ba in its mechanochemical
synthesis from 1b and benzyl chloride.
Entry
Amount of 1b (mg)
Yield (%)b
1
2
3
4
50
58
71
68
62
100
150
200

12
aReactions were performed with 1b and benzyl chloride (2 mol equivalent), ball mill, mixed Ø 5 and Ø 8 mm
b
balls, 550 rpm, 90 cycles. Isolated yield.
Table 5: Effect of the reactant amount for the formation of metal complex 4ba in its
mechanochemical synthesis from 2b and [RhCl(cod)]2.
Entry
Amount of 2b (mg)
Yield (%)b
1
2
3
4
50
61
67
74
70
100
150
200
aReactions were performed with 2b, [RhCl(cod)]2 (1 mol equivalent) and K2CO3 (10 mol equivalent), ball mill,
mixed Ø 5 and Ø 8 mm balls, 550 rpm 90 cycles. bIsolated yield.
In addition, the influence of inert milling aids like quartz, silica, alumina and Celite Hyflo
Supercel were also investigated. However, no positive influence was observed.
In the case of solution syntheses, the impact of reaction time and temperature were initially
checked for the preparation of carbene ligand precursor 2b and metal complex 4b taken as
references (Tables 6 and 7). According to the results, overnight reactions at high temperature
(80 °C to reflux temperature; Table 6, entry 4; Table 7, entry 4) provided the best yields.
Table 6. Effect of the reaction temperature and time for the formation of 2b in its solution
synthesis from 1b and benzyl chloride.a
Entry
Temperature (°C)
Time (h)
Yield (%)b
1
2
3
4
room temperature
50
6
0
6
6
6
reflux temperature
reflux temperature
49
76
overnight
aReaction was performed in acetonitrile (5 mL) with 0.526 mmol of 1b and 1.05 mmol of benzyl chloride.
bIsolated yield.

13
Table 7. Effect of the reaction temperature and time for the formation of metal complex 4b in
its solution synthesis from 2b and [RhCl(cod)]2.a
Entry
Temperature (°C)
Time (h)
Yield (%)b
1
2
3
4
room temperature
3
0
0
50
80
80
3
3
18
77
overnight
aReaction was performed in toluene (10 mL) with 0.338 mmol of 2b, 0.338 mmol of [RhCl(cod)]2 and 3.38
mmol of K2CO3. bIsolated yield.
To compare the mechanochemical method with the classical solvent approach, the synthesis
of 2b and 4b were carried out maintaining the same reaction time, i.e. 6 h for the preparation
of diimidazolium salt 2b (Table 2, entry 8 vs Table 6, entry 3) and 3 h for the synthesis of the
Rh(I)-complex (Table 2, entry 4 vs Table 6, entry 3). The comparison clearly shows that the
ball mill synthesis results in higher yields than the solution method (71 % vs 49 % for the
carbene precursor 2b; and 74 % vs 18 % for complex 4b). In addition, since the
mechanochemical synthesis requires solvents only for extraction and purification, the overall
solvent need is significantly reduced – up to 50 % in the small scale syntheses of ligands and
complexes described in this study (see Experimental part).
3.
Conclusion
Convenient and efficient novel mechanochemical methods of synthesis for two (-CH2-)n-
bridged diimidazolium salts (precursors to di-NHC ligands) and four bridging dinuclear
rhodium(I)-N-heterocyclic carbene complexes of the type [{RhCl(cod)}2(µ-di-NHC)] were
developed which are characterized with shorter reaction times and substantially reduced need
of organic solvents compared to the classical solution syntheses of the same compounds.
Together with the simplicity of the procedure, zero-solvent condition, and possible
multipurpose applications, directed us to the conclusion that the mechanochemical (ball mill)
synthesis of metal complexes in several cases may prove superior in comparison to the
classical solution synthesis approach. In addition, the solid state structures of two new
diimidazolium salts (3a and 3b) and three of the new [{RhCl(cod)}2(µ-di-NHC)] complexes,
4a, 4b, 5b) were determined by single crystal X-ray diffraction.

14
4.
Experimental
4.1.
General information
The Rh-metal precursor [RhCl(cod)]2 was prepared as described in [61]. Synthesis of 1,1ʹ-
di(imidazole-1-yl)methane, 1a and 1,4-di(imidazole-1-yl)butane, 1b were done according to
the literature [44, 45]. 1,1ʹ-methylene-bis(3-benzyl-imidazolium)dichloride, 2a [62] and 1,1ʹ-
(butane-1,4-diyl)bis(3-benzyl-imidazolium)dichloride, 2b [46] are known compounds,
however, in addition to the traditional solvent synthesis, we obtained them also in reactions
in a ball mill. The identity and purity of these four compounds were checked by correlating
their respective 1H, 13C and ESI-MS spectra to those available in the literature.
All other chemicals and solvents were purchased from Sigma-Aldrich, Alpha Aesar,
Merck, Molar Chemicals Kft. and VWR International and employed as received without
further purification. Analytical thin-layer chromatography (TLC) was carried out on
Kieselgel 60 F254 plates from Merck and the plates were visualised under UV fluorescence
light at 254 nm. The column chromatography was executed on silica gel from Sigma-Aldrich
(70-230 mesh, 63-200 µm).
Reactions in ball mill were carried out with the use of a planetary milling instrument
model ‘RETSCH PM 100’ with a stainless steel jar (12.5 mL) and G100 ball bearings (Ø 5
mm and Ø 8 mm) operated at room temperature. In the generally used protocol, 1 cycle
consisted of 2 min milling followed by 2 min cooling at ambient temperature; after an initial
warming period the stabilized temperature of the milling jar in a 45 cycles procedure was
around 34 °C, estimated by the use of a remote infrared thermometer.
13
1H and C{1H} NMR spectra were recorded at room temperature on a Bruker DRX
13
360 instrument. MeOD (δ= 49.15 ppm) and CD2Cl2 (δ= 54.00 ppm) were used as C NMR
1
internal standards for the ligands and for the metal complexes, respectively, while the H
NMR spectra were referenced to TMS and residual non-deuterated solvent peaks. High-
resolution electrospray ionization mass spectra (HR ESI-MS) were recorded on a Bruker
maXis II MicroTOF-Q type Qq-TOF-MS instrument and controlled by Compass Data
Analysis 4.4 software from Bruker. CHN elemental analysis was done using an Elementar
Vario Micro microanalyzer.
Single crystals were examined on a Bruker D8 Venture diffractometer (SC-XRD) and
data processing was managed by Olex2 software [63] including SHELX programs [64]. The

15
molecular images were prepared by the Mercury CSD-4.3.0 software [65]. The
crystallographic data (excluding the structure factors) for 3a, 3b, 4a, 4b, 5b were deposited at
Cambridge Crystallographic Data Centre, as CCDC-1990551 and 1981017-1981020.
4.2.
Synthesis of 1,1ʹ-methylene-bis(3-benzyl-imidazolium)dichloride (2a) and 1,1ʹ-
(butane-1,4-diyl)bis(3-benzyl-imidazolium)dichloride (2b)
2a and 2b were synthesized in a slightly modified, more convenient way than described
earlier [62, 46]. (For characterization see Supplementary Material).
Solution synthesis:
A mixture of 1a (100 mg, 0.675 mmol) or 1b (100 mg, 0.526 mmol) and benzyl chloride (for
1a: 155 µL, 1.35 mmol; for 1b: 121 µL, 1.052 mmol) was dissolved in CH3CN (5 mL) in a
Schlenk tube and stirred overnight under reflux temperature until the appearance of a white
precipitate. The resulting hot solution was filtered and the precipitate was washed with 2×5
mL cold CH3CN, and once with 5 mL acetone, respectively, and then dried under vacuum.
White powder. Yield: 78% for 2a (211 mg); 76% for 2b (177 mg).
Mechanochemical synthesis:
A 12.5 mL ball milling jar was charged with 10 Ø 5 mm and 10 Ø 8 mm stainless steel balls,
1a (100 mg, 0.675 mmol) or 1b (100 mg, 0.525 mmol) and benzyl chloride (for 1a: 155 µL,
1.35 mmol; for 1b: 121 µL, 1.052 mmol). The mixtures were milled over a period of 90
cycles (1 cycle= 2 min milling + 2 min pause) at 550 rpm. Afterwards, the jar and the bearing
balls were washed twice with 5 mL methanol and the resulting solution was filtered through
Celite Hyflo Supercel. Evaporation of methanol yielded 2a (74%, 199 mg) or 2b (71%, 165
mg) as a white powder.
4.3.
Synthesis of 1,1ʹ-methylene-bis(3-(2,4,6-trimethylbenzyl)imidazolium)dichloride (3a)
and 1,1ʹ-(butane-1,4-diyl)bis(3-(2,4,6-trimethylbenzyl)imidazolium)dichloride (3b)
Solution synthesis:
A mixture of 1a (100 mg, 0.675 mmol) or 1b (100 mg, 0.526 mmol) and 2-(chloromethyl)-
1,3,5-trimethylbenzene (for 1a: 228 mg, 1.35 mmol; for 1b: 177 mg, 1.052 mmol) was
dissolved in CH3CN (5 mL) in a Schlenk tube and stirred overnight under reflux temperature
until the appearance of a white precipitate. The resulting hot solution was filtered and the
precipitate was washed with 2×5 mL cold CH3CN, and once with 5 mL acetone, respectively,
and dried under vacuum. White powder. Yield: 70 % for 3a (228 mg) and 86% for 3b (238
mg).

16
Mechanochemical synthesis:
A 12.5 mL ball milling jar was charged with 10 Ø 5 mm and 10 Ø 8 mm stainless steel balls,
1a (100 mg, 0.675 mmol) or 1b (100 mg, 0.526 mmol) and 2-(chloromethyl)-1,3,5-
trimethylbenzene (for 1a: 228 mg, 1.35 mmol; for 1b: 177 mg, 1.052 mmol). The mixtures
were milled over a period of 90 cycles (1 cycle= 2 min milling + 2 min pause) at 550 rpm.
Afterwards, the jar and the bearing balls were washed with 2×5 mL methanol and the
resulting solution was filtered through Celite Hyflo Supercel. Evaporation of methanol
yielded 3a (68%, 222 mg) or 3b (78%, 216 mg) as a white powder.
3a:
1H NMR (360 MHz, MeOD), δ/ppm: 9.45 (s, 2H, NCHN), 8.21 (s, 2H, CHimid) 7.79 (s, 2H,
CHimid), 7.23 (s, 4H, CHAr), 6.89 (s, 2H, NCH2N), 5.74 (s, 4H, ArCH2N), 2.53 (18H, CH3);
13C{1H} NMR (90 MHz, MeOD), δ/ppm: 140.17 (NCNimid), 138.52 (CAr), 129.76 (CHAr),
125.46 (CAr), 123.32 (CHimid), 122.58 (CHimid), 58.88 (NCH2N), 48.18 (ArCH2N), 19.97
(CH3), 18.56 (CH3);
Elemental analysis: Calc. for: C27H34N4Cl2 × 4.5 H2O: C, 57.24; H, 7.65; N, 9.89. Found: C,
57.38; H, 6.89; N, 11.56;
HR ESI-MS: m/z for [M-2Cl,H]: Calculated: 413.2700, Found: 413.2700.
3b:
1H NMR (360 MHz, MeOD), δ/ppm: 9.09 (s, 2H, NCHN), 7.91 (s, 2H, CHimid), 7.63 (s, 2H,
CHimid) 7.21 (s, 4H, CHAr), 5.68 (s, 4H, ArCH2N), 4.48 (s, 4H, N-CH2-CH2-CH2-CH2-N),
2.51 (18H, CH3), 2.12 (s, 4H, N-CH2-CH2-CH2-CH2-N);
13C{1H} NMR (90 MHz, MeOD), δ/ppm: 139.88 (NCNimid), 138.36 (CAr), 135.74 (CAr),
129.72 (CHAr), 126.07 (CAr), 122.82 (CHimid), 122.33 (CHimid), 48.93 (-CH2-CH2-CH2-CH2-),
47.63 (ArCH2N), 26.82 (-CH2-CH2-CH2-CH2-), 20.02 (CH3), 18.61 (CH3);
Elemental analysis. Calc. for C30H40N4Cl2 × 4.5H2O: C, 59.20; H, 8.11; N, 9.21. Found: C,
59.27; H, 8.05; N, 9.41;
HR ESI-MS: m/z for [M-Cl]: Calculated: 491.2936, Found: 491.2935.
4.4.
Synthesis of bridging di-nuclear Rh(I)-metal complexes 4a ,4b, 5a, 5b
Solution synthesis:
150 mg of 2a (0.374 mmol) or 2b (0.338 mmol) or 3a (0.309 mmol) or 3b (0.284 mmol) was
dissolved in 10 mL of toluene in a Schlenk tube under inert atmosphere followed by the

17
addition of [RhCl(cod)]2 (for 2a: 184 mg, 0.374 mmol; for 2b: 167 mg, 0.338 mmol; for 3a:
152 mg, 0.309 mmol; for 3b: 140 mg, 0.284 mmol) and K2CO3 (for 2a: 517 mg, 3.74 mmol;
for 2b: 467 mg, 3.38 mmol; for 3a: 427 mg, 3.09 mmol; for 3b: 392 mg, 2.84 mmol) in one
portion. The solutions were stirred overnight at 80 °C; the final conversion was checked with
TLC. The resulting solutions were then filtered, the residues were washed with 2×5 mL
toluene and the filtrates were collected. The combined filtrates were evaporated to dryness
and the residues were purified using silica gel column chromatography with a CH2Cl2 and
EtOAc mixture (1:1) as eluent and dried under vacuum. Yellow powders. Yield 80% for 4a
(245 mg), 77% for 4b (224 mg), 51% for 5a (143 mg) and 65% for 5b (174 mg).
Mechanochemical synthesis:
A 12.5 mL ball milling jar was charged with 10 Ø 5 mm and 10 Ø 8 mm stainless steel balls,
150 mg of 2a (0.374 mmol) or 2b (0.338 mmol) or 3a (0.309 mmol) or 3b (0.284 mmol),
[RhCl(cod)]2 (for 2a: 184 mg, 0.374 mmol; for 2b: 167 mg, 0.338 mmol; for 3a: 152 mg,
0.309 mmol; for 3b: 140 mg, 0.284 mmol) and K2CO3 (for 2a: 517 mg, 3.74 mmol; for 2b:
467 mg, 3.38 mmol; for 3a: 427 mg, 3.09 mmol; for 3b: 392 mg, 2.84 mmol) and the
mixtures were milled over a period of 45 cycles (1 cycle= 2 min milling + 2 min pause) at
550 rpm. Afterwards, the jar and the bearing balls were washed with 2×5 mL
dichloromethane and the resulting solutions were filtered. The filtrates were evaporated to
dryness, and the residues were purified using silica gel column chromatography with a
CH2Cl2 and EtOAc mixture (1:1) as eluent and dried under vacuum. Yellow powders. Yield
71%, for 4a (218 mg), 74% for 4b (215 mg), 58% for 5a (162 mg) and 69% for 5b (185 mg).
4a:
1H NMR (360 MHz, CD2Cl2), δ/ppm: 7.98 (s, 2H, CHimid) 7.66 (s, 2H, CHimid) 7.43 (s, 10H,
CHAr), 6.75 (s, 2H, NCH2N), 5.87 (s, 4H, ArCH2N), 5.14-5.13 (m, 4H, cod-CHvinyl), 3.50 (s,
2H, cod-CHvinyl), 3.34 (s, 2H, cod-CHvinyl), 2.67-1.82 (m, 16H, cod-CHallyl);
1
13C{1H} NMR (90 MHz, CD2Cl2), δ/ppm: 184.23 (d, JRh–C = 51.9 Hz , NCNimid), 136.39
(CAr), 128.79 (CHAr), 128.03 (CHAr), 122.05 (CHimid), 121.50 (CHimid), 99.84 (d, J = 5.9 Hz,
1
cod-CHvinyl), 99.20 (d, J = 6.6 Hz, cod-CHvinyl), 69.73 (d, JRh–C = 13.8 Hz, cod-CHvinyl),
1
68.64 (d, JRh–C = 13.8 Hz, cod-CHvinyl), 63.31 (NCH2N), 54.58 (ArCH2N), 33.62 (cod-
CHallyl), 32.17 (cod-CHallyl), 29.20 (cod-CHallyl), 28.40 (cod-CHallyl);
Elemental analysis. Calc. for C37H44N4Cl2Rh2: C, 54.10; H, 5.40; N, 6.82. Found: C, 54.22;
H, 5.55; N, 6.76;
HR ESI-MS: m/z for [M-Cl]: Calculated: 785.1359, Found: 785.1357.

18
4b:
1H NMR (360 MHz, CD2Cl2), δ/ppm: 7.45-7.33 (m, 4H, CHimid + 10H, CHAr), 7.13 (d, 1JH-H
=
1.80 Hz, 1H, N-CH2-CH2-CH2-CH2-N), 7.04 (d, 1JH-H = 1.80 Hz, 1H, N-CH2-CH2-CH2-CH2-
1
N), 6.77 (d, JH-H = 1.80 Hz, 1H, N-CH2-CH2-CH2-CH2-N), 6.54 (d, 1JH-H = 1.80 Hz, 1H, N-
1
CH2-CH2-CH2-CH2-N), 5.99 (d, JH-H = 14.76 Hz, 2H, ArCH2N), 5.69-5.57 (m, 2H,
ArCH2N), 4.98-4.88 (m, 4H, cod-CHvinyl), 4.86-4.82 (m, 2H, cod-CHvinyl), 4.45-4.32 (s, 2H,
cod-CHvinyl), 2.54-2.35 (m, 8H, cod-CHallyl), 2.23 (s, 4H, N-CH2-CH2-CH2-CH2-N), 2.01-1.94
(m, 8H, cod-CHallyl);
13C{1H} NMR (90 MHz, CD2Cl2), δ/ppm: 182.67 (d, 1JRh–C = 51.2 Hz , NCNimid), 182.21 (d,
1JRh–C = 50.6 Hz , NCNimid), 136.91 (CAr), 136.89 (CAr), 128.71 (CHAr), 128.68 (CHAr),
128.36 (CHAr), 128.22 (CHAr), 127.95 (CHAr), 121.30 (CHimid), 121.28 (CHimid), 120.45
1
1
(CHimid), 120.35 (CHimid), 98.21 (d, JRh–C = 6.61, cod-CHvinyl), 98.18 (d, JRh–C = 6.61, cod-
CHvinyl), 98.04-97.88 (m, cod-CHvinyl), 68.64 (d, 1JRh–C = 13.77, cod-CHvinyl), 68.60 (d, 1JRh–C
= 13.78, cod-CHvinyl), 67.96 (d, 1JRh–C = 13.79, cod-CHvinyl), 67.92 (d, 1JRh–C = 13.79 Hz, cod-
CHvinyl), 54.55 (ArCH2N), 54.48 (ArCH2N), 50.34 (-CH2-CH2-CH2-CH2-), 50.25 (-CH2-CH2-
CH2-CH2-), 32.95-32.86 (m, cod-CHallyl) 29.03 (-CH2-CH2-CH2-CH2-), 28.95 (cod-CHallyl),
28.68 (cod-CHallyl), 27.95 (cod-CHallyl), 27.52 (cod-CHallyl);
Elemental analysis. Calc. for C40H50N4Cl2Rh2 × 0.5H2O: C, 55.06; H, 5.89; N, 6.42. Found:
C, 52.26; H, 5.81; N, 6.30;
HR ESI-MS: m/z for [M-Cl]: Calculated: 827.1829, Found: 827.1828.
5a:
1
1H NMR (360 MHz, CD2Cl2), δ/ppm: 7.76 (d, JH-H = 1.80 Hz, 2H, CHimid), 7.52 (s, 2H,
CHimid), 6.93 (s, 4H, CHAr), 6.23 (d, 1JH-H = 2.05 Hz, 2H, NCH2N), 5.85 (d, 1JH-H = 14.08 Hz,
2H, ArCH2N) 5.46 (d, 1JH-H = 14.08 Hz, 2H, ArCH2N), 5.09-5.00 (m, 2H, cod-CHvinyl), 4.98-
4.96 (m, 2H, cod-CHvinyl), 3.51-3.49 (m, 4H, cod-CHvinyl), 2.59-2.36 (m, 6H, cod-CHallyl),
2.29 (s, 6H, CH3), 2.27 (s, 12H, CH3), 2.16-1.96 (m, 10H, cod-CHallyl);
1
13C{1H} NMR (90 MHz, CD2Cl2), δ/ppm: 183.44 (d, JRh–C = 50.55 Hz , NCNimid), 138.62
(CAr), 138.38 (CAr), 129.34 (CHAr), 127.98 (CAr), 120.58 (CHimid), 120.16 (CHimid), 99.52 (d,
1JRh–C = 6.61 Hz, cod-CHvinyl), 99.02 (d, 1JRh–C = 6.61 Hz, cod-CHvinyl), 69.7 (d, 1JRh–C =14.44
1
Hz , cod-CHvinyl), 67.82 (d, JRh–C = 14.44 Hz, cod-CHvinyl), 63.92 (NCH2N), 48.76
(ArCH2N), 33.59 (cod-CHallyl), 32.44 (cod-CHallyl), 29.18 cod-CHallyl), 28.50 (cod-CHallyl),
20.74 (CH3), 19.81 (CH3);

19
Elemental analysis. Calc. for C43H56N4Cl2Rh2 × 4.5H2O: C, 52.34.03; H, 6.64; N, 5.68.
Found: C, 52.10; H, 5.64; N, 5.74;
HR ESI-MS: m/z for [M-Cl]: Calculated: 869.2298, Found: 869.2303.
5b:
1H NMR (360 MHz, MeOD), δ/ppm: 6.99-6.91 (m, 2H, CHimid + 4H, CHAr) ), 6.27 (d, 1JH-H
=
1
1.84 Hz, 1H, CHimid), 6.18 (, JH-H = 1.84 Hz, 1H, CHimid), 5.90-5.83 (m, 2H, N-CH2-CH2-
CH2-CH2-N), 5.49-5.45 (m, 2H, N-CH2-CH2-CH2-CH2-N), 4.99-4.97 (m, 4H, CHAr), 4.87-
4.77 (m, 2H, cod-CHvinyl), 4.42-4.32 (m, 2H, cod-CHvinyl), 3.56-3.52 (m, 2H, cod-CHvinyl),
3.42-3.37 (m, 2H, cod-CHvinyl), 2.55-2.39 (m, 8H, cod-CHallyl), 2.33-2.28 (m, 18H, CH3),
2.19-1.99 (m, 4H, N-CH2-CH2-CH2-CH2-N), 2.04-1.97 (m, 8H, cod-CHallyl);
13C{1H} NMR (90 MHz, CD2Cl2), δ/ppm: 181.86 (d, 1JRh–C = 50.55 Hz , NCNimid), 181.63 (d,
1JRh–C = 50.54 Hz , NCNimid), 138.46 (CAr), 129.22 (CHAr), 128.38 (CAr), 119.97 (CHimid),
1
118.97 (CHimid), 118.87 (CHimid), 97.92 (d, JRh–C =6.57 , cod-CHvinyl), 97.88-97.75 (m, cod-
1
1
CHvinyl), 67.87 (d, JRh–C = 14.45 Hz, cod-CHvinyl), 67.84 (d, JRh–C =13.79 Hz , cod-CHvinyl),
1
67.81 (d, JRh–C =14.45 Hz , cod-CHvinyl), 50.56 (-CH2-CH2-CH2-CH2-), 50.44 (-CH2-CH2-
CH2-CH2-), 48.71 (ArCH2N), 48.67 (ArCH2N), 33.27 (cod-CHallyl), 33.21 (cod-CHallyl), 29.19
(cod-CHallyl), 29.07 (cod-CHallyl), 28.63 (CH2-CH2-CH2-CH2-), 27.98 (cod-CHallyl), 27.77
(cod-CHallyl), 20.76 (CH3), 19.76 (CH3), 19.73 (CH3);
Elemental analysis. Calc. for C46H62N4Cl2Rh2 × 0.5H2O: C, 57.75; H, 6.64; N, 5.86. Found:
C, 57.61; H, 6.54; N, 5.68;
HR ESI-MS: m/z for [M-Cl]: Calculated: 911.2768, Found: 911.2751
Appendix – Supplementary Material
Characterization data for 2a and 2b; Experimental details for SC-XRD structure
determinations of 3a, 3b, 4a, 4b, 5b; Crystallographic data of compounds 3a, 3b, 4a, 4b, 5b
(Table S1); Packing diagrams and π-π stackings in crystals of 3a, 3b, 4a, 4b (FiguresS1-S7);
1H and 13C{1H} NMR spectra of 2a,b, 3a,b, 4a,b, and 5a,b (Figures S8-S15).
Acknowledgement
The research was supported by the EU and co-financed by the European Regional
Development Fund (under the projects GINOP-2.3.2-15-2016-00008 and GINOP-2.3.3-15-
2016-00004), and by the Thematic Excellence Programme of the Ministry for Innovation and

20
Technology of Hungary (ED-18-1-2019-0028), within the framework of the Vehicle Industry
thematic programme of the University of Debrecen. The financial support of the Hungarian
National Research, Development and Innovation Office (FK-128333) is greatly
acknowledged. SD is thankful to the Stipendium Hungaricum scholarship programme and the
Government of India for supporting his PhD study.
Conflicts of Interest: The authors declare no conflicts of interest.

21
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Graphical Abstract
Synopsis
Comparative synthesis of several new, structurally well characterized alkylene-bridged
diimidazolium salts and the respective dinuclear Rh(I)-(di-N-heterocyclic carbene)
complexes, [{RhCl(cod)}2(µ-di-NHC)], both by mechanochemical and traditional solvent
methods, revealed advantages of the mechanochemical (ball-mill) synthesis, such as reduced
solvent need, shorter reaction times and competitive or higher product yields.

29
Highlights
•
•
•
•
•
First mechanochemical synthesis of Rh(I)-NHC complexes
Synthesis of new alkylene bridged diimidazolium salts as NHC ligand precursors
Synthesis of four new [{RhCl(cod)}2(µ-di-NHC)] complexes
Structural characterization by spectroscopic methods and SC-XRD (5 new structures)
Comparison of the ball-mill and the traditional solution syntheses