Expedient synthesis of N[sbnd]Oxy-Heterocyclic Carbenes (NOHC) ligands and metal complexes using mechanochemistry

Article abstract of DOI:10.1016/j.jorganchem.2021.121914

1,3-Di(benzyloxy)imidazol-2-ylidenes (or NOHCs) possess structures related to N-heterocyclic carbenes (NHCs), with potentially useful applications including the stabilization of metals. In order to prepare NOHC metal complexes, a multi-step synthesis invo

Full text of DOI:10.1016/j.jorganchem.2021.121914

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Expedient synthesis of N-Oxy-Heterocyclic Carbenes (NOHC)
ligands and metal complexes using mechanochemistry
´
¨
´
Aneta Wroblewska , Gaetan Lauriol , Grzegorz Mloston ,
´ ´
Xavier Bantreil , Frederic Lamaty
PII:
S0022-328X(21)00235-7
DOI:
Reference:
https://doi.org/10.1016/j.jorganchem.2021.121914
JOM 121914
To appear in:
Journal of Organometallic Chemistry
Received date:
Revised date:
Accepted date:
27 March 2021
25 May 2021
31 May 2021
´
¨
´
Please cite this article as: Aneta Wroblewska , Gaetan Lauriol , Grzegorz Mloston , Xavier Bantreil ,
´ ´
Frederic Lamaty , Expedient synthesis of N-Oxy-Heterocyclic Carbenes (NOHC) ligands and
metal complexes using mechanochemistry, Journal of Organometallic Chemistry (2021), doi:
https://doi.org/10.1016/j.jorganchem.2021.121914
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Highlights

Mechanochemistry was used to prepare 1-hydroxyimidazole 3-oxides and O-alkylated
derivatives

Novel NOHC silver complexes were mechanosynthesized in a fast and efficient multi-step
process


Gold and copper complexes were prepared by mechanotransmetalation
Ag complex 5 displays cytotoxic activity against HCT116 cell line.

Expedient synthesis of N-Oxy-Heterocyclic Carbenes (NOHC) ligands and
metal complexes using mechanochemistry
Aneta Wróblewska,*1 Gaëtan Lauriol,3 2 Xavier Bantreil,3 Frédéric
Lamaty*3
1Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza
112, 90-363 Lodz, Poland
2Department of Organic and Applied Chemistry, University of Lodz, Tamka 12, 91-403 Lodz,
Poland
3IBMM, Univ Montpellier, CNRS, ENSCM, Montpellier, France
* awroblew@cbmm.lodz.pl, frederic.lamaty@umontpellier.fr
Dedicated to Prof. Elena Shubina on the occasion of her 70th birthday
Abstract
1,3-Di(benzyloxy)imidazol-2-ylidenes (or NOHCs) possess structures related to N-heterocyclic
carbenes (NHCs), with potentially useful applications including the stabilization of metals. In
order to prepare NOHC metal complexes,
a
multi-step synthesis involving
mechanochemistry, and making use of ball-milling, was designed. A library of unprecedented
NHOC metal complexes were tested for their cytotoxic activities against HCT116 colorectal
cancer cell line. The tests demonstrated that the studied complexes displayed a similar
activity, comparable with that one exhibited by the ligand. However, a remarkable activity
was found in the case of silver complex 5 with an IC50 value of 0.75 μM, comparable to that
of doxorubicin.
Graphical abstract

Mechanochemistry was employed as an efficient technique for the preparation of NOHCs
and their metallic complexes.
Keywords
Mechanochemistry; N-oxy-heterocyclic carbenes (NOHC); Silver complexes; Cytotoxicity;
HCT116
1. Introduction
Mechanochemistry, defined as the use of mechanical forces to synthesize or degrade
molecules, is currently recognized as an innovative technology that can facilitate the
practical and sustainable preparation of both organic and inorganic compounds.1 This
approach, possibly involving mechanical milling, with the use of ball-mills, enables the
development of solvent-free syntheses in a very efficient way.2,3 Along with organic
molecules, synthesized mechanochemically, ligands and discrete metal complexes have a
place of choice since many advantages of this solid-state synthesis have found a direct
illustration in this area.4-7 It is worth citing (i) shorter reaction time (ii) elimination or
significant decrease of the amount of reaction solvent (iii) facilitated purification resulting
from more easily attained reaction completion. In most cases, metallic complexes are
isolated by crystallization, and the combination of mechanosynthesis with this recovery
technique makes it a very powerful methodology, useful in organic and organometallic
chemistry as well.
N-heterocyclic carbenes (NHC) and their corresponding complexes have found an essential
place in the area of coordination chemistry. Since the discovery of 1,2-bis(adamantan-1-
yl)imidazol-2-ylidene as a stable, crystalline compound,8,9 the poor reputation about
nucleophilic carbenes as laboratory curiosities changed dramatically and nowadays
nucleophilic carbenes are recognized as useful substrates with diverse applications in
catalysis (metallic and organic),10-13 in material science14 and biology.15,16 In recent years, it
has been shown that the mechanosynthesis of ligands and corresponding NHC complexes
offers a very efficient approach to the target compounds (Scheme 1).17-30 Various types of
NHC ligands could be prepared or employed, together with a wide range of metals such as
Ag, Cu, Au, Pt, Ru, Pd, to prepare a new collection of organometallics. Of note, in general,
the process is faster, cleaner, and, even, in some cases, providing products difficult or

impossible to obtain via a solution synthesis. In recent years, numerous mechanosynthesized
NHC complexes found attractive applications in catalysis25,30,31 and medicinal chemistry.27
4-Hydroxyl-imidazole 3-oxides and imidazolium salts derived therefrom constitute a group of
heterocyclic compounds related to the NHCs since they can be considered as precursors of
their oxy-analogues (NOHCs). Efficient methods for their preparation have been reported in
recent decades,32-35 but so far, the number of applications is rather limited. One can cite
their usage as ionic liquids (in the form of corresponding imidazolium salt),36-42 for biological
applications,43 as ligands for complexation of metals,44-48 as additives in reactions49,50 or as
unique building blocks for the preparation of some co-crystals.51,52 In our continuing studies
on the preparation of this class of imidazole derivatives and owing to the success of the
mechanochemical strategy for the preparation of NHCs, we decided to explore the
mechanosynthesis of NOHCs and corresponding metal complexes, using a vibratory ball mill
(vbm) (Scheme 1). In addition, another goal of the present study was the evaluation of the
cytotoxicity of some of NHOHCs and their complexes on the colorectal HCT116 cell line.
Previous work
R2
N
R1
R1
R1
R1
O
O
(HCHO)n
vbm
2 NH2
+
R
X
N
R2/R4
N
M
N
R3/R4
R2
R4
R1
R1
M or MYn
vbm
R1
R1
R1
R1
R4X
vbm
N
N
N
X
N
R3
R3
R1
R1
This work
OR5
N
OR5
N
X
N
OR5
O
N
R1
R1
R1
R1
R1
O
R1
R1
R5X
vbm
O
HCHO
M or MYn
vbm
[M]
or
N
OR5
N
N
R1
HCl.NH2OH
vbm
O
OH
OH
1
2
Scheme 1: Mechanosynthesis of metalated NHCs and outline of the current work (vbm= vibratory ball mill)
2. Results and Discussion
2.1. Mechanosynthesis of NOHC ligands and metal-complexes
In a series of our recent papers, synthesis and reactivity of symmetric and non-symmetric
di(alkyloxy)imidazol-2-ylidines have been described.43,53,54 In solution, three efficient
methods are reported for the preparation of 1,3-dialkoxy imidazolium salts 2 which are
applied as precursors of NOHCs derived from imidazole.55 The first synthetic approach is

based on the initial preparation of 2-unsubstituted 1-alkoxy imidazole N-oxides, followed by
O-alkylation by treatment with benzyl bromide or benzyl chloride as typical alkylating
reagents.49 The second method is based on a three component reaction between glyoxal,
corresponding N-alkoxyamine, e.g. adamantyl-1-oxyamine, and formaldehyde in the
presence of a strong BrØnsted acid.56 Finally, the third strategy comprises double O-
alkylation of 1-hydroxyimidazole 3-oxides 1, prepared initially from a diketone,
hydroxylamine, and formaldehyde.55 This latter method (Scheme 1) was selected for the
development of the mechanosynthesis of 1,3-di(alkyloxy)imidazolium salts 2, and
subsequent preparation of corresponding 1,3-di(alkyloxy)imidazole-2-ylidenes metal
complexes, including metals, such as Ag, Cu and Au.
The synthesis of starting 1-hydroxyimidazole N-oxides 1a-c was carried out based on the
method presented in Scheme 1. Inspired by the synthesis performed in solution,49
corresponding diketone and hydroxylamine were mixed in a vibratory ball mill (vbm) for 1 h.
Paraformaldehyde and a small amount of 36% HClaq. used together as the acid catalyst and
additive for Liquid Assisted Grinding,57-59 were added to the milling jar and mixed for another
1h. Recovery of the reaction mixture and adjustment of the pH, as performed in classical
solution synthesis, proved to be inefficient for isolation of the expected product. However,
switching from 36% HClaq. to acetic acid gave much better results. It has to be noted that the
grinding could not be carried out in a stainless-steel jar because of its degradation resulting
from the acidity of the reaction medium. Instead, agate or zirconia-made jars and balls were
preferred to develop an efficient procedure. Next, the crude reaction mixture was treated
with aqueous ammonia and after separation of ammonium acetate, the products were
purified by column chromatography. Compared to the synthesis in solution, the reaction was
faster when performed in a ball mill (two times 1h vs 20h). Imidazole N-oxides 1a-c were
isolated as colorless solids and their structures were confirmed by spectroscopic analyses. In
the case of 1a, two 1H NMR signals found at 7.09 ppm and 8.30 ppm were attributed to two
equivalent H atoms, H(4) and H(5), and to H(2) atom, respectively (see Supporting
1
Information). The H NMR spectrum recorded for 1b showed two signals located at 2.10 (2
Me groups) and 8.18 (H(2)), respectively. Finally, the H(2) signal of 1c was found as a singlet
located at 8.01 ppm along with signals of aromatic C-H atoms found in the region 7.29-7.44
ppm.

Treatment of the starting N-oxides 1a-c with a variety of methyl-substituted benzyl bromides
in the presence of a base and MeOH as a liquid assistant to provide 1,3-
di(benzyloxy)imidazolium salts 2 was also performed in a vibratory ball mill. The use of a
base was necessary to remove the proton from the hydroxyl group located at N(1) of the
imidazole ring. Among diverse bases (Cs2CO3, K2CO3, Na2CO3, NaHCO3), which were tested
during the optimization process of this reaction, the best results were achieved using
Cs2CO3. In the case of 1a and 1b, benzylation led smoothly to the expected imidazole salts
2a-h (Scheme 2). However, attempted benzylations of N-oxides 1c was unsuccessful and, in
all cases, led to a complex mixture of unidentified products. Very likely, the unsatisfactory
course of this reaction resulted from the steric hindrance provided by the phenyl rings at
C(4) and C(5) positions. Products 2a-h were purified by filtration through a plug of celite.
Here again, the mechanochemical approach offered an advantageous solvent-free
procedure, with shorter reaction times (30 min) compared to experiments performed in
solution (24-48h).
Br
R8
R8
R9
O
R9
O
R6
R7
R7
R7
R6
R9
R6
Ag2O
75-91%
R8
O
N
R1
R1
R1
R1
Cs2CO3
R1
R1
N
N
Br
Ag-Br
R6
92-98%
N
N
N
O
OH
R6
O
3a-h
1a-c
2a-h
R7
R7
R9
R8
R9
R8
1
a
b
c
R1
H
Me
Ph
Yield (%)
2, 3 R1
R6
R7
R8
R9
Yield of 2 Yield of 3
(%)
92
98
94
97
93
(%)
76
91
84
75
86
69
89
56
a
b
c
H
H
H
H
H
H
Me
H
H
Me
H
H
H
H
H
Me
H
H
H
Me
H
H
H
d
e
H
Me

f
g
h
Me
Me
Me Me
H
H
Me
H
H
H
Me
H
Me
H
H
98
97
97
89
85
77
Scheme 2. Synthesis of 1,3-(dibenzyloxy)imidazolium bromides 2 and NOHC silver complexes 3.
The imidazolium salts 2 were soluble in organic solvents such as MeOH, CH2Cl2, CHCl3, and
the NMR spectra run in CDCl3 solutions confirmed the symmetric structure of the studied
molecules. For example, in the case of 2a, a characteristic signal of C(2)H was down-field
shifted compared to 1a and appeared at 10.98 ppm and the signals of two equivalent CH2
groups were found as a singlet at 5.53 ppm. In the 13C NMR spectra, one signal located at
117.8 ppm was attributed to two equivalent C(4) and C(5) atoms of the imidazole ring. The
1
symmetric structure of all imidazolium salts 2b—h was confirmed by analogous H and 13C
NMR data.
The next step of this study consisted in the preparation of NOHC metal complexes. Since
silver complexes are recommended as entry materials to prepare other metalated products
via transmetalation protocol, the mechanosynthesis of Ag complexes of the studied NOHCs 2
was next performed. It is known that NHC-silver complexes can easily be prepared in
solution60 or alternatively in a ball mill18,19,21,27 by reaction of an imidazolium salt with Ag2O.
The previously optimized ball mill procedure was thus applied starting with a 1:0.5 mixture
of a corresponding bis-alkylated imidazolium oxide salt 2 and Ag2O, reacted neat in a milling
jar agitated at 25 Hz.
In all cases, the experiment was finished after 30 minutes, and the desired complexes were
isolated as pale yellow or colorless solids (Scheme 2). This procedure was applied
successfully to precursors 2a-h and actually, this is the first synthesis of monodentate 1,3-
di(benzyloxy)imidazol-2-ylidene silver complexes. In the representative case of symmetric
salt 2e, the isolated complex 3e was characterized spectroscopically, showing in the 1H NMR
spectrum two characteristic singlets located at 1.85 and 5.28 ppm corresponding to two
equivalent Me and two equivalent benzylic CH2 units, respectively. In the 13C NMR, the
corresponding signals were found at 7.7 and 82.2 ppm, respectively. In addition, the absence
of the C(2)H signal of the imidazole ring at ca. 11.72 ppm confirmed the elimination of the H-
atom and the formation of the expected carbenic center. Apparently, the observed down-
field shift of the signals attributed to the CH2 groups results from the presence of oxygen

atoms forming the N-O bond. The most characteristic C(2)-atom forming the carbenic center
absorbed at 166.5 ppm. Similarly, the signals of the C(2) atoms in other complexes 3 were
found in the region between 165.6-170.6 ppm. HRMS confirmed molecular formulas of
complexes 3 with peaks corresponding to the [M-Br-]+ ion.
Since, as reported earlier, NHC-homoleptic silver complexes were also synthesized
mechanochemically,19,27 starting with an imidazolium salt and Ag2O, we also decided to
apply this approach for the preparation of NOHCs silver analogues. The first step consisted in
introducing a less coordinating anion by ball milling 2f with KPF6, and the anion metathesis
provided the desired imidazolium 4 in 90% yield. Gratifyingly, grinding of imidazolium
hexafluorophosphate salt 4 with 2 equivalents of Ag2O furnished complex 5 in 87% yield
after 4 h (Scheme 3).
Me
Me
Me
Me
Me
Me
Me
Me
O
N
O
N
O
N
O
N
Me
Me
Me
Me
Me
Me
Me
Me
Ag2O
KPF6
Ag
PF6
Br
PF6
N
vbm
87%
N
N
O
vbm
90%
N
O
O
O
Me
Me
Me
Me
Me
5
2f
Me
4
Me
Me
1. KPF6
2. Ag2O
vbm
80%
Scheme 3. Synthesis of [Ag(NOHC)2]PF6-complex 5.
The same product was also obtained in a one-jar stepwise procedure which involves anion
-
exchange Br-→PF6 as an initial step, achieved by 30 min of grinding bromide 2f with an
equimolar amount of KPF6. Subsequent addition of Ag2O and further grinding for 1 h led to a
black powder. Filtration through a celite layer and evaporation of the solvent delivered the
desired complex 5 in 80% yield (Scheme 3). It is worth mentioning that only a single example
of such complexes, synthesized exclusively in solution, has yet been reported.47 However, a
longer reaction time (24h) was required to complete the complexation reaction.
1
In analogy to complexes 3f, signals of two equivalent Me groups appeared in H NMR as a
singlet found at 6.93 ppm along with four equivalent CH2 benzyl groups located at 5.06 ppm.

- +
The molecular mass M = 835.3352 [M-PF6 ] was confirmed by HRMS. Of note, all Ag-
complexes 3 and 5 were light-sensitive substances that underwent a slow decomposition
during the storage and for that reason, they had to be stored in the dark.
Anticipated structures of 2f and 5 were unambiguously confirmed by single crystal X-ray
diffraction analysis. The structure of 5 (Figure 1a) was compared to the structure of cationic
[Ag(2f)2]PF6 complex 6, reported in the literature47 (Figure 1b).
a)
b)

Figure 1. ORTEPs (at 50% probability level) of compounds (a) 5, (b) 6 (from ref47). Hydrogen atoms are omitted
for clarity.
Firstly, compared to imidazolium salt 2f, the N-C-N angles in silver carbene 5 are smaller
(104.4° in 2a vs 99.8°/100.5° in 5).This observation is in agreement with reports and reviews
from the literature.8,9,61 Interestingly, silver complex 6 exhibits a linear arrangement of the
two carbene ligands and the silver atom (angle C–Ag–C: 180°), while in complex 5 the C–Ag–
C arrangement presents a distortion with an angle of 168°. Furthermore, the two molecules
of NOHCs in complex 6 lie essentially in the same plane (angle of 5.4°). In sharp contrast, a
twist of 31.5° is observed between the NOHCs molecules in 5. Finally, the torsion angles
between the NOHCs and the four benzyl substituents (angle N-O-CH2-aryl) are quite
different in each molecule: 159.1°, 159.1°, 178.5°, 178.5° in 7, confirming that the benzyl
substituents are pointing away from the center of the complex. In 5, among these angles
(170.5°, 171.9°, 64.2°, 67.1°) two of them have similar values as the ones in 6 (benzyl
pointing away) while two are much smaller with benzyl substituent pointing more towards
the C–Ag–C bonds. Most probably, methyl groups present on the NOHC frameworks and on
the benzyl substituents in 5 exert steric effects, inducing the various distortions observed in
the structure compared to 6.
The mechanoconversion of silver complexes into corresponding gold or copper complexes
was tested using complex 3f. Transmetalation of 3f into a gold(I) complex was also
performed in a ball mill by grinding a mixture of 3f and AuCl·SMe2 complex without any
solvent (Scheme 4). The reaction was complete after 1 h and after typical work-up, the
gold(I) complex 7 was obtained in 87% yield. The postulated structure was confirmed

spectroscopically. Compared with its Ag precursor 3f, the diagnostic C(2) signal of the
carbenic center was high-field shifted and appeared at 157.9 ppm.
Analogously, Ag complex 3f was converted into the copper complex 8 by grinding with CuCl.
In this case, the transmetalation was completed after 1 h and the pure product was isolated
in 92% yield after filtration through a pad of celite layer (Scheme 4).
Me
Me
Me
Me
Me
Me
O
N
O
N
O
N
Me
Me
Me
Me
Me
Me
AuCl·SMe2
CuCl
Cu Cl
Ag Br
Au Cl
vbm
87%
N
O
N
O
N
O
vbm
92%
Me
Me
Me
3f
7
8
Me
Me
Me
Scheme 4. Synthesis of NOHC-Au(I) complex 7 and NOHC-Cu(I) complex 8.
2.2. Biological testing
NHC metal complexes, including mechanosynthesized compounds,27 have been tested in
recent years on cancer cell lines and promising results were reported in some cases.15,16,56,62
Because of their close structural similarity to NHCs, we investigated the possibility of the
newly synthesized NOHC-stabilized metal complexes to display biological activities, all the
more that, cytotoxic activities of alkylated imidazole N-oxides were already reported.43 This
represents the first report of a cytotoxicity test performed on this family of potential
metallodrugs.
The NOHC-metal complexes were tested against HCT116 colorectal cancer cell lines and the
IC50 (half growth inhibition concentration) for the compounds that displayed the best
activities in preliminary testing (see Supporting Information) was determined. Doxorubicin
was used as a positive control and displayed an IC50 value of 0.81 μM. First, we focused on

complexes derived from imidazolium 2f since corresponding silver complexes 3f and 5, gold
complex 6 and copper complex 7 were available for comparison of their activities.
Imidazolium 2f displayed an activity of 7.5 μM. The corresponding silver complex 3f did not
show much better activity (7.1 μM). To our disappointment, switching from silver to gold 7
or copper 8 did not result in a remarkable increase in the cytotoxicity of the tested
compounds (8.70 and 7.8 μM, respectively). Subsequently, silver complexes 3b-e and 3g
were tested and it turned out that 3c, 3d and 3g displayed such a low activity in preliminary
cytotoxicity tests that further investigation of these compounds was not pursued.
Interestingly, absence of methyl groups on the benzyloxy and keeping their presence on the
NOHC backbone (complex 3e) resulted in decreased activity, compared to 3f, with an IC50
value down to 8.65 μM. On the other hand, removing the methyl groups on the backbone
and keeping one methyl at ortho position of the benzyloxy unit (complex 3h) or two Me
groups at meta position (complex 3b) led to a slight increase of activity (5.6 and 4.68 μM,
respectively). Finally, the cationic bis-NOHC silver complex 5 gave the best result with a
cytotoxicity of 0.75 μM, similar to doxorubicin (0.81 μM), and about ten times more potent
than corresponding ligand 2f or mono-NOHC-silver complex 3f. The observed improvement
of activity could be a consequence of an increased stability of 3f, leading to a slow release of
the silver ion.56

Figure 2: IC50 (μM) values against HCT116 carcinoma cells.
3. Experimental section
Synthesis of Imidazolium Bromides 2.
Imidazole N-oxide 1 (1.00 eq), benzyl bromide or its methyl-substituted derivative (2.2 eq),
cesium carbonate (0.5 eq) and MeOH (in the case of 2b, 2c, 2f, 2g : 0.5 μL/mg) were placed
in a 10 mL stainless steel grinding jar with one stainless steel ball (1 cm diameter). The jar
was closed and subjected to grinding for 30 min in the vibratory ball mill operated at 25 Hz.
After that time, CHCl3 was added to the reaction mixture and the resulting solution was
filtrated through a pad of celite. The solvent was evaporated and the precipitate was washed
several times with Et2O to afford pure imidazolium bromide.
Synthesis of NOHC-Ag(I)-complexes 3.
Imidazolium bromide 2 (1.00 eq) and silver oxide(I) (0.5 eq) were placed in a 10 mL stainless
steel grinding jar with one stainless steel ball (1 cm diameter). The jar was closed and
subjected to grinding for 30 min in the vibratory ball mill operating at 25 Hz. After that time,
CHCl3 was added to the reaction mixture and the resulting suspension was filtrated through
a pad of celite. The solvent was evaporated and the obtained solid was washed several times
with Et2O to afford pure NOHC-Ag(I)-complex.
Synthesis of imidazolium hexafluorophosphate 4.
Imidazolium bromide 2f (69.7 mg, 0.156 mmol, 1.00 eq) and potassium
hexafluorophosphate (30.3 mg, 0.165 mmol, 1.05 eq) were placed in a 10 mL stainless steel
grinding jar with one stainless steel ball (1 cm diameter). The total mass of the reagents has
been calculated so that the miling load equals 10.55 mg/mL. The jar was closed and
subjected to grinding for 30 min in the vibratory ball mill operated at 25 Hz. After that time,
CHCl3 was added to the reaction mixture and the resulting solution was filtrated through a
pad of celite. The solvent was evaporated and the precipitate was washed several times with
Et2O to afford pure 1,3-di[(3,5-dimethylbenzyl)oxy]-4,5-dimethyl-1H-imidazol-3-ium
hexafluorophosphate 4 (71.8 mg, 0.141 mmol, 90%) as colourless crystals. Mp. = 147—148
⁰C.

1H NMR (500 MHz, CDCl3) δ 8.84 (s, 1H), 6.99 (s, 2H), 6.96 (s, 4H), 5.17 (s, 4H), 2.26 (s, 12H),
2.14 (s, 6H) ppm. 13C NMR (126 MHz, CDCl3) δ 138.8, 132.0, 131.3, 128.3, 126.2, 123.8, 85.0,
21.2, 7.2 ppm. 31P NMR (202 MHz, CDCl3) δ -143.97 (sept, J = 714.9 Hz) ppm. 19F NMR (470
MHz, CDCl3) δ -72.31 (d, J = 714.2 Hz) ppm.
- +
HRMS (ESI/TOF) m/z: calcd for C23H29N2O2: 365.2229 [M-PF6 ] ; found: 365.2225.
IR (cm-1): 3138m, 2959m, 1743s, 1606m, 1445m, 1378m, 1211m, 1077w, 896m, 826s, 724m,
610m, 557s, 522m.
Anal. Calcd for C23H29N2O2F6P: C, 54.12; H, 5.73; N, 5.49. Found: C, 54.12; H, 5.65; N, 5.46.
Synthesis of [Ag(NOHC)2]PF6-complex 5.
Method
A:
1,3-Di[(3,5-dimethylbenzyl)oxy]-4,5-dimethyl-1H-imidazol-3-ium
hexafluorophosphate 4 (51.2 mg, 0.100 mmol, 1.00 eq) and silver oxide (48.8 mg, 0.211
mmol, 2.10 eq) were placed in a 10 mL stainless steel grinding jar with one stainless steel
ball (10 mm diameter). The total mass of the reagents was calculated so that the milling load
equals 10.55 mg/mL. The jar was closed and subjected to grinding for 4 hours in the
vibratory ball mill operated at 25 Hz. The black powder was recovered with dichloromethane
and the suspension was filtrated through a pad of celite. The filtrate was concentrated under
vacuum to afford 1,3-di{[(3,5-dimethylbenzyl)oxy]-4,5-dimethylimidazolyl-2-ene]silver
hexafluorophosphate 5 (42.8 mg, 0.043 mmol, 87%) as a white solid.
Method B: 1,3-Di[(3,5-dimethylbenzyl)oxy]-4,5-dimethyl-1H-imidazol-3-ium bromide 2f (39.9
mg, 0.090 mmol, 1.00 eq) and potassium hexafluorophosphate (16.5 mg, 0.090 mmol, 1.00
eq) were placed in a 10 mL stainless steel grinding jar with one stainless steel ball (10 mm
diameter). The jar was closed and subjected to grinding for 30 minutes in the vibratory ball
mill operated at 25 Hz. Afterwards, silver oxide (43.6 mg, 0.188 mmol, 2.10 eq) was added.
The total mass of the reagents was calculated so that the milling load equals 19.2 mg/mL.
The jar was closed and subjected to grinding for 1 hour. The black powder was recovered
with dichloromethane and filtrated through a pad of celite. The filtrate was concentrated
under vacuum to afford 1,3-di[(3,5-dimethylbenzyl)oxy]-4,5-dimethylimidazolyl-2-ene]silver
hexafluorophosphate 5 (35.1 mg, 0.0358 mmol, 80%) as a white solid. Mp. = 197—198 ⁰C.

1H NMR (500 MHz, CDCl3) δ 6.93 (s, 12H), 5.06 (s, 8H), 2.20, 2.15 (2xs, 36H) ppm. 13C NMR
(126 MHz, CDCl3) δ 138.7, 132.6, 131.5, 128.0, 121.7, 82.9, 21.2, 7.8 ppm. 31P NMR (202
MHz, CDCl3) δ -144.08 (sept, J = 707.9 Hz) ppm. 19F NMR (470 MHz, CDCl3) δ -73.70 (d, J =
718.7 Hz) ppm.
- +
HRMS (ESI/TOF) m/z: calcd for C46H56N4O4Ag: 835.3352 [M-PF6 ] ; found: 835.3366.
IR (cm-1): 3013m, 2966m, 1737s, 1605w, 1442m, 1366s, 1229m, 1214w, 1200m, 1074w,
945m, 835s, 715m, 680m, 554m, 522m.
Anal. Calcd for C46H56AgN4O4F6P: C, 56.27; H, 5.75; N, 5.71. Found: C, 56.25; H, 5.83; N, 5.95.
Synthesis of NOHC-Au(I)-complex 7.
Imidazolium silver bromide 3f (61.0 mg, 0.110 mmol, 1.00 eq) and
chloro(dimethylsulfide)gold(I) (39.0 mg, 0.133 mmol, 1.20 eq) were placed in a 10 mL
stainless steel grinding jar with one stainless steel ball (1 cm diameter). The total mass of the
reagents has been calculated so that the miling load equals 10.55 mg/mL. The jar was closed
and subjected to grinding for 1 h in the vibratory ball mill operated at 25 Hz. After that time,
CHCl3 was added to the reaction mixture and the resulting solution was filtrated through a
pad of celite. The solvent was evaporated, and the precipitate was washed several times
with Et2O to afford pure 1,3-di{[(3,5-dimethylbenzyl)oxy]-4,5-dimethylimidazol-2-
ylidene}gold chloride 7 (57.2 mg, 0.096 mmol, 87%) as colourless crystals. Mp. = 217-218 ⁰C.
1H NMR (500 MHz, CDCl3) δ 7.08-7.01 (m, 6H), 5.29 (s, 4H), 2.32 (s, 12H), 1.92 (s, 6H) ppm.
13C NMR (126 MHz, CDCl3) δ 157.9, 138.6, 132.5, 131.6, 128.2, 120.8, 82.7, 21.3, 7.8 ppm.
HRMS (ESI/TOF): m/z calcd for C23H28N2O2Au1Cl1Na1: 619.1406 [M+Na+]; found: 619.1403,
m/z calcd for C23H28N2O2Au1: 561.1816 [M-Cl]+; found: 561.1818, m/z calcd for
C46H56N4O4Au1: 925.3967 [2M-AuCl2]+; found: 952.3972.
IR (cm-1): 2919m, 1769m, 1608m, 1468m, 1386m, 1240m, 1167m, 1077m, 943s, 919m, 849s,
709m, 674s.
Anal. Calcd for C23H28AuClN2O2•0.2CH2Cl2: C, 45.39; H, 4.66; N, 4.56. Found: C, 45.17; H,
4.94; N, 4.61.
Synthesis of NOHC-Cu(I)-complex 8

Imidazolium silver bromide 3f (55.2 mg, 0.100 mmol, 1.00 eq) and copper(I) chloride (14.8
mg, 0.150 mmol, 1.50 eq) were placed in a 10 mL stainless steel grinding jar equipped with
one stainless steel ball (1 cm diameter). The total mass of the reagents has been calculated
so that the miling load equals 7.39 mg/mL. The jar was closed and subjected to grinding for 1
h in the vibratory ball mill operated at 25 Hz. After that time, CHCl3 was added to the
reaction mixture and the resulting solution was filtrated through a pad of celite. The solvent
was evaporated, and the obtained solid was washed several times with Et2O to afford pure
1,3-di{[(3,5-dimethylbenzyl)oxy]-4,5-dimethylimidazol-2-ylidene}copper chloride 8 (42.6 mg,
0.092 mmol, 92%) as a pale yellow powder. Mp. = 175-176 ⁰C.
1H NMR (500 MHz, CDCl3) δ 7.02 (s, 6H), 5.18 (s, 4H), 2.32 (s, 12H), 1.92 (s, 6H) ppm. 13C NMR
(126 MHz, CDCl3) δ 138.7, 132.6, 131.5, 128.1, 120.7, 82.9, 21.3, 7.6 ppm.
HRMS (ESI/TOF): m/z calcd for C23H28N2O2Cu1Cl1Na1: 485.1033 [M+Na+]; found: 485.1035,
m/z calcd for C23H28N2O2Cu1: 427.1447 [M-Cl]+; found: 427.1447, m/z calcd for
C46H56N4O4Cu1: 791.3598 [2M-CuCl2]+; found: 791.3605.
IR (cm-1): 2916m, 1757m, 1643m, 1608m, 1456m, 1380m, 1167m, 1077m, 1036m, 942m,
893m, 849s, 683s, 665m.
Anal. Calcd for C23H28ClCuN2O2•0.2CH2Cl2: C, 58.26; H, 5.98; N, 5.87. Found: C, 58.26; H,
5.99; N, 6.00.

4. Conclusions
A multi-step mechanosynthesis of 1,3-di(benzyloxy)imidazol-2-ylidene metal complexes is
described. Starting N-hydroxy imidazole N-oxides were prepared from respective substrates
by reaction in a vibratory ball-mill. This initial step was followed by O-benzylation, and
subsequent metalation which were also performed in a vbm. In these three steps,
mechanical milling provided an essentially solvent-free procedure with considerable
reduction of the reaction times. These protocols opened the route to an efficient
preparation of new 1,3-di(benzyloxy)imidazol-2-ylidene silver, and gold and copper
complexes. Two latter representatives were obtained via mechanotransmetalation
approach. All new complexes were characterized spectroscopically and two of them were
subjected to single crystal X-ray analysis. In extension of the study, new NHOCs complexes
were evaluated for their cytotoxic activity against the HCT116 colorectal cancer cell line.
Homoleptic silver complex 5 displayed remarkably high activity, similar to the known
doxorubicin. These reported results demonstrate once more that mechanochemistry can be
exploited as a well-adapted method for the preparation of NOHCs and corresponding
metallic complexes. Furthermore, this work supports the validity of a strategical approach to
discover new metallodrugs based on of principles of medicinal mechanochemistry.27,63-65
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank for financial support the National Science Center (Cracow, Poland) within
the Projects: OPUS-13 Grant no. 2017/25/B/ST4/02684 (A.W.) and BEETHOVEN-2 Grant no.
2016/23/G/ST5/04115 (G.M.), the University of Montpellier and the CNRS. G.M. thanks
Professor Peter R. Schreiner (Giessen) for valuable discussion on the chemistry of
nucleophilic heterocyclic carbenes (NOHC type).

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