Mechanism studies of oxidation and hydrolysis of Cu(I)–NHC and Ag–NHC in solution under air

Article abstract of DOI:10.1016/j.jorganchem.2019.121025

The decomposition of copper(I)–NHC and silver-NHC complexes in solution under air was studied. The Cu(I)–NHCs were oxidized into urea derivatives and hydrolysed into imidazoliums or benzimidazoliums. The decomposition of Ag–NHC with a saturated backbone led to ring-opening product, while the Ag–NHC with an unsaturated backbone led to imidazolium and Ag-bisNHC complex. The effects of steric property, hydrophilicity, and binding energy of NHC to O₂ and H₂O on the decomposition of Cu(I)–NHC were studied using theoretical calculations. Steric hindrance played an important role on the stability of Cu(I)–NHC. Pathways for the decomposition of Cu(I)–NHC and Ag–NHC were proposed.

Full text of DOI:10.1016/j.jorganchem.2019.121025

Journal Pre-proof
Mechanism studies of oxidation and hydrolysis of Cu(I)–NHC and Ag–NHC in solution
under air
Dazhi Li, Thierry Ollevier
PII:
S0022-328X(19)30468-1
DOI:
https://doi.org/10.1016/j.jorganchem.2019.121025
JOM 121025
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 18 October 2019
Revised Date: 12 November 2019
Accepted Date: 13 November 2019
Please cite this article as: D. Li, T. Ollevier, Mechanism studies of oxidation and hydrolysis of Cu(I)–
NHC and Ag–NHC in solution under air, Journal of Organometallic Chemistry (2019), doi: https://
doi.org/10.1016/j.jorganchem.2019.121025.
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Author Name / Journal of Organometallic Chemistry
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Mechanism studies of oxidation and hydrolysis of Cu(I)-NHC and Ag-NHC in solution
under air
Dazhi Li a, Thierry Ollevier a,*
a Département de chimie, Université Laval, 1045 avenue de la Médecine, Québec, QC, G1V 0A6, Canada
ABSTRACT
ARTICLE INFO
Article history:
Received
The decomposition of copper(I)-NHC and silver-NHC complexes in solution under air was studied. The Cu(I)-
NHCs were oxidized into urea derivatives and hydrolyzed into imidazoliums or benzimidazoliums. The
decomposition of Ag-NHC with a saturated backbone led to ring-opening product, while the Ag-NHC with an
unsaturated backbone led to imidazolium and Ag-bisNHC complex. The effects of steric property, hydrophilicity,
and binding energy of NHC to O2 and H2O on the decomposition of Cu(I)-NHC were studied using theoretical
calculations. Steric hindrance played an important role on the stability of Cu(I)-NHC. Pathways for the
decomposition of Cu(I)-NHC and Ag-NHC were proposed.
Received in revised form
Accepted
Available online
Keywords:
N-Heterocyclic carbene
Copper
© 2019 Elsevier B.V. All rights reserved.
Silver
Decomposition
Air
1. Introduction
the decomposition of Ni(II)-NHCs through potential Ni(III) species
[22]. In 2010, Cazin found that the reaction between an NHC
The synthesis and the catalytic applications of copper(I)-N-
heterocyclic carbene complexes (Cu-NHC) have been studied
extensively over the past decades[1]. Different types of Cu-NHC
have been synthesized, and many reactions have been
successfully catalysed by Cu-NHCs, such as allylic arylation [2],
hydroboration of alkynes [3], and [3+2] cycloaddition of azides [4].
Silver(I)-N-heterocyclic carbene complexes (Ag-NHC) are usually
used as intermediates to synthesize other metal-NHC complexes
by transmetalation. Many Ag-NHCs have been easily obtained
through the reactions between Ag2O and imidazolium [5],
imidazolinium [6], or benzimidazolium [7]. Current research
mainly focuses on the synthesis and catalytic performance of Cu-
NHCs and Ag-NHCs, while their decomposition or deactivation
pathways received almost no attention.
In the recent decades, most metal-NHC complexes were
thought to be highly stable and resistant to decomposition [8, 9],
which is an advantage with NHC compared to easily oxidized
phosphine ligands [10]. Despite these literature precedents,
some reports revealed that metal-NHC complexes could undergo
decomposition under certain conditions. In 1960s, the reactions
of in-situ-prepared NHCs with several reagents, including O2 and
H2O, have been studied by Wanzlick [11-13]. In 1970s and 1980s,
isomerization reactions of Rh-NHC [14], Ru-NHC [15], Mo-NHC
[16] and Fe-NHC [17] were observed by Lappert, and the
electrochemical oxidation and isomerization of W-NHC, Cr-NHC,
and Mo-NHC studied by Öfele [18-21]. In 2005, Sigman reported
precursor and Cu2O gave a urea derivative instead of Cu-NHC
[6]. In 2017, Youn also isolated a urea derivative from an in-situ-
generated Rh-NHC [23]. In 2009, Collins reported the
decomposition of some Ru-NHCs, but the decomposition
products were not identified in his study [24]. In 2013, Bala tried
to synthesize Fe(II)-NHC from free carbene. However, only the
ring-opening product (formamide) was isolated, and FeCl2 was
found to be non-innocent in the decomposition process [25]. In
addition, Nechaev disclosed a case of Cu(II)-NHC hydrolysis
reaction in 2011 [26]. In 2015, Ananikov tried to synthesize Ni-
NHCs containing a triazolium backbone in water medium. The
studies revealed that their unsuccessful synthesis in water was
due to the facile hydrolysis of Ni-NHCs complexes [10]. Recently,
the same group disclosed the decomposition of Pd-NHC when
they investigated a catalytic Heck reaction. The formation of Pd
nanoparticles was confirmed, and the reaction was actually
catalysed by a NHC-free catalyst, while Pd-NHC has no catalytic
ability [27-29]. In 2015, Fairlamb disclosed the arylation of Cu-
NHC in the presence of phenyl iodide [30]. In that study, one
case of decomposition of Cu(I)-NHC in solution under air to
produce the benzimidazolium and urea was reported. The
question of whether or not the metals remain bound to NHC
ligands or if they remain homogeneous during catalysis was
raised [8,31]. According to the aforementioned elements, the
decomposition of metal-NHCs should be taken into consideration
when doing catalysis, and the catalytic pathway involving NHC
should be considered more carefully.
During the synthesis of some Cu(I)-NHCs and Ag-NHCs in our
group, repeated attempts to isolate them in air were not
Corresponding author. Tel.: +1-418-656-5034;
e-mail: thierry.ollevier@chm.ulaval.ca

2
Author Name / Journal of Organometallic Chemistry
successful. Herein, the decomposition of Cu(I)-NHC and Ag-NHC
complexes under air is discussed, using quantifiable parameters,
such as half reaction times and percent buried volume (%VBur), to
provide more insights into the deactivation of metal-NHCs, which
has not been discussed before.
Table 1
Decomposition of Cu(I)-NHC 1a–j under air.a
2. Results and discussion
2.1. Decomposition of Cu(I)-NHC under air
Entry
Cu-NHC
Temp (°C)
t (h)
Yield 2 (%)
Yield 3 (%)
In our previous study, the decomposition of several Cu(I)-
NHCs in solution under air has been established as a new
method to obtain urea derivatives [32]. Decomposition of the
Cu(I)-NHCs shown in Scheme 1 has also been monitored in
CDCl3 under air [32]. The rate constant was determined as
pseudo-first order where the natural logarithm of Cu(I)-NHC
concentration (ln[Cu-NHC]) against time (t) was linearly
correlated (see Supplementary data). It was found that the
reaction mainly occurred at the interface between air and solution,
where copper precipitation was first observed (Fig. 1). Cu(I)-
1
2
3
1a
1b
1c
25
25
25
30
75
63
24
36
95
74
60
<5
4
1d
1e
1f
25
109
109
286
666
235
136
155
75
56
85
86
49
14
16
20
37
13
10
47
83
81
5
25
6
25
7
1g
1h
1i
25
8 b
9 b
10 b
100
150
150
NHCs 1a–j were oxidised and hydrolysed under air to afford urea
derivatives and NHC precursors (Table 1), which were confirmed
by 1H, 13C NMR, and HRMS. In general, the more sterically
hindered Cu(I)-NHC resulted in longer reaction time. The NHC
backbone has an important impact on the reaction time and the
ratio of products as well. The aromatic backbone led to the longer
reaction time, compared to unsaturated and saturated backbones
1j
a Reaction conditions: 1 (0.04 mmol) in CDCl3 (0.4 mL) under air (relative humidity =
80%); Yields determined by 1H NMR. b (CD3)2SO was used as solvent.
saturated backbones might probably have more affinity towards O2
than that bearing aromatic and unsaturated backbones.
with the same N-substituents (1d
> 1b > 1c), due to the
stabilizing aromaticity. The trend of yields of urea derivatives with
various NHC backbone types are as follows: saturated backbone (1c,
1f, and 1g) > aromatic backbone (1d and 1e) > unsaturated backbone
(1a and 1b). i.e., the Cu(I)-NHCs bearing
From the fast decomposition of 1a (30 h) to the slow decomposition
of 1g (28 days), the decomposition of 1a−g occurred smoothly. In
contrast, the combination of bulky N-substituents with an
unsaturated backbone provided 1h with high stability where the
decomposition of 1h was not even observed at 65 °C in CDCl3
after 116 h. In order to observe the decomposition of 1h, 100 °C
in DMSO was used as the model condition (Table 1, entry 8).
When 2,6-diisopropylphenyl was used as the N-substituent, the
large steric hindrance became the dominant factor for the
stabilization. Only 5% of 1i and 1j decomposed at 100 °C after 21
h, and 150 °C was used to observe significant decomposition of
1i and 1j (Table 1, entries 9 and 10). Due to the low content of
O2 at high temperature, and the hygroscopic nature of DMSO,
low yields of ureas and high yields of hydrolysis products were
obtained for 1i and 1j
.
In addition to CDCl3, various solvents, such as CH2Cl2, CHCl3,
THF, EtOH, MeOH, acetone, and CH3CN in the decomposition of
Cu-NHC into urea under air have been studied [32]. The ratio of
oxidation/hydrolysis product was mainly affected by the O2 and
H2O content in solvent and air. When dried air was used, urea
derivative was obtained from 1b in almost quantitative yield in
CDCl3 solution. When H2O was added directly into the Cu-NHC
solution, the hydrolysis product was obtained. These two cases
confirmed that the oxygen atom of the urea originated from O2
instead of H2O. Also, various substrates containing different
halogen ligands, such as Cl, Br, and I have been studied.
Decomposition of Cu-NHC into urea has proved not to depend on
the nature of the halide ligands. However, when replacing the Cl–
counterion in 1b with PF6–, only hydrolysis product was obtained
under air, due to the strong hygroscopic nature of the Cu-NHC
containing PF6–. In the presence of excess Cu (0.1 equiv), tBuOK
Scheme 1. Structure of Cu(I)-NHC.
(0.1
equiv),
Ag2O
(0.1–
0.5
equiv)
or
1,4-
diazabicyclo[2.2.2]octane (DABCO, 0.55 equiv), the oxidation
and hydrolysis of Cu-NHC were still observed in air. This
indicated that the decomposition was not catalysed by the
potential trace of acid in chlorinated solvents. The presence of
DABCO (an efficient singlet O2 quencher) [33] led to a slower
oxidation of 1c [32]. Thus, singlet O2 was considered being
involved in the oxidation process.
Fig. 1. Graphical representation of Cu-NHC solution and appearance of 1b
solution under air.

Author Name / Journal of Organometallic Chemistry
2.2. Steric effect on Cu(I)-NHC decomposition
t50 (h)
160
As discussed before, the steric properties of Cu(I)-NHC
indeed correlate with the decomposition rates. The less sterically
hindered the Cu-NHC was, the faster it decomposed. In order to
quantify the steric properties of NHC, crystallographic data of
Cu(I)-NHC was used to calculate the percent buried volume
(%VBur) of the NHC ligand. The %VBur value allows to evaluate
the space occupied by the ligand in the sphere of a certain radius
(3.5 Å was used in this study) around the metal center [34-37].
This parameter has been used to study the relation between the
steric properties and the catalytic performance of some metal-
NHCs [38-41]. When the crystallographic data was not available,
density functional theory (DFT) calculations was applied to
optimize the structures of Cu(I)-NHCs. According to the literature,
the complexes in this study mainly adopted two-coordinate linear
pattern [42-46], although other non-linear three- coordinate
reaction at 25 °C
reaction at 100 °C
reaction at 150 °C
140
120
100
80
y = (1.69×10-9)x6.73
R² = 0.84
60
40
20
0
%VBur
55
25
30
35
40
45
50
Fig. 2. Plot of half-reaction times vs %VBur . (Data at 25 oC was used for the line
fitting.)
backbone. 1i and 1j possessed %VBur around 50%, showing the
large steric hindrance to prevent the approach of O2 and H2O
from air.
To better evaluate the diversely distributed steric properties of
the Cu(I)-NHCs, their steric maps (contours) calculated from
1
complexes could be envisioned. According to H NMR, only one
type of Cu-NHC was found in CDCl3 solution in each case.
At first, various DFT methods have been examined to optimize
the structures of several types of Cu(I)-NHCs. 9 methods out of
11 gave the results of %VBur with an accuracy > 95% (See
supplementary data). PBE/LanL2DZ method was then chosen to
calculate the buried volume in this study [47]. The results of
%VBur, decomposition rate constants (k), and half-reaction times
optimized structures were compared (Fig. 3). 1a–c possessed
large open area at the top and the bottom of the circle. The steric
maps of 1d and 1e possessed two open quadrants at the bottom
of the circle and two filled quadrants at the top with notches of
similar widths (1.46 and 1.47 Å, respectively). With the smallest
methyl groups, 1a has four open quadrants, which indicated that
1a was attacked by H2O and O2 from almost every direction. Due
to free rotation of benzyl groups in solution, the approach of H2O
(t50) of the decomposition of 1a-j are summarized in Table 2. The
general trend of half-reaction times versus %VBur is shown in Fig.
2. With the increase of %VBur, the half-reaction times increase
exponentially.
Looking at each case, 1a possessed the shortest t50 since it
had the lowest %VBur. 1b and 1c possessed close %VBur, but the
and O2 to the carbene and Cu(I) centre in 1b
accessible, but apparently more difficult than the approach to 1a
In the previous discussion, the %VBur of 1f was found to be larger
than that of 1g, while 1f actually decomposed faster than 1g
which was contradictory to the prediction by the comparison of
%VBur. A straightforward answer was provided by the steric
maps: 1f had more vacant quadrants (1.66 Å of notch width) than
1g (0.97 Å of notch width, 42% smaller than 1f), thus 1f has a
higher probability of being exposed to H2O and O2. Similarly, the
%VBur of 1g (39.1%) is larger than 1h (37.4%), which leads to the
prediction of 1g being more stable than 1h in terms of %VBur. But
actually, 1g decomposed slowly, while 1h did not decompose at
25 °C over 30 days. Compared to the steric map
–e was considered
.
decomposition of 1b was slower than 1c due to the unsaturated
backbone. 1d and 1e had a %VBur around 33% close to 1b, but
their t50 are approximately 30% longer than that of 1b due to the
delocalized aromatic backbones of benzimidazole. The
decomposition of 1f (t50 = 83.7 h) was faster than 1g (t50 = 154 h),
and there are two methyl groups less in 1f than in 1g on the N-
aryl substituents. For this reason, the buried volume of 1f was
intuitively expected to be smaller than that of 1g. However, the
calculated %VBur of 1f (41.1%) was larger than the %VBur of 1g
(39.1%) by 2.0%. It seemed that the %VBur alone was not
sufficient to evaluate the steric properties of Cu(I)-NHC. 1h
possessed a lower %VBur than 1g by 1.7%, but 1h did not
decompose at room temperature, revealing the important
stabilization from the unsaturated
,
Table 2
%VBur, decomposition rate constants (k), and half-reaction times (t50) of 1a– .
j a
b
3
1
Cu-NHC
%VBur
27.5
k (×10– h–
)
t50 (h)
7.4 1.0
1a
1b
1c
120.8 16.3
±
±
35.3 (35.4) [42]
33.2
34.1
41.0
±
±
3.9
5.0
30.5
22.8
±
±
3.5
2.8
5.1
4.3
8.3
1d
1e
1f
33.1 (33.0) [43]
33.0
21.7
21.6
12.5
±
±
±
2.9
2.3
1.2
37.6
40.8
83.7
±
±
±
41.1
1g
1h
1i
39.1
4.3
±
0.5
154.0
±
17.1
37.4 (37.7) [44]
48.7 (50.0) [45]
47.9 (48.5) [46]
12.0
24.5
24.2
±
±
±
2.3 c
29.5
39.5
45.0
±
±
±
5.7 b
4.1 c
4.0 c
2.5 d
2.2 d
1j
a Determined in CDCl3 at 25 °C. b %VBur calculated from crystallographic data are
in parenthesis. The optimized or crystallographically-determined bond length was
scaled by 1.17, with sphere radius 3.5 Å centering at the Cu atom, and mesh spacing
0.1 Å. The hydrogen atoms were excluded from the %Vbur calculation. c Determined in
(CD3)2SO at 100 °C. d Determined in (CD3)2SO at 150 °C.
Fig. 3. Steric maps of Cu(I)-NHCs.

4
Author Name / Journal of Organometallic Chemistry
of 1g, the quadrants of 1h were filled up more evenly with smaller
vacant areas (0.84 Å of notch width), reducing the attack of H2O
and O2 onto the NHC. For 1i and 1j, there are not many vacant
areas left, indicating that the NHCs were well protected in an
enclosed space. Thus, 1i and 1j needed high temperature to
observe significant decomposition.
In fact, large N-substituents leading to high stabilization of M-
NHC have been studied by Nolan [48, 49]. Due to the high
stabilization by 2,6-diisopropylphenyl, a widespread use of this
group in NHC has been met in the literature[50, 51]. When the
synthesis of Cu-NHC was carried out in air, the substrates
bearing large substituents usually led to better yields than those
bearing small groups[52-54].
hydrolysis product (20–74%) than 1f–j (10–13% for 1f–g; 1h–j
was not hydrolysed at 25 °C) supported the trend predicted by
the dipole moments.
To further evaluate the affinity of Cu(I)-NHC with H2O, the log
P values were calculated and summarized in Table 3. Although
different online servers gave different calculated log P values, the
trend was the same (larger molecules gave larger log P values).
1a has a negative log P value, implying that it was soluble in
water; this is in agreement with the hygroscopic nature of 1a
.
Thus, the decomposition of 1a under air gave the highest yield of
imidazolium (74%). In contrast, 1j has the highest log P value,
which indicated the most hydrophobic character, and it was not
hydrolysed at room temperature. Although 1e has the greatest
dipole moment, its log P value (4.51) indicates that 1e has the
same H2O affinity as 1d has. The dipole moments and log P
values showed a rough trend to predict the yields of hydrolysis
products. But for compound 1c, it is still difficult to explain why 1c
gave very a low yield of the hydrolysis product; even its log P
value indicated the second highest hydrophilic character among
the ten Cu-NHCs. On the other hand, there is no available
parameter to evaluate the affinity toward O2 (like log P for H2O),
which makes this analysis difficult to establish a comparison of
the affinity of Cu(I)-NHC toward O2 and H2O.
2.3. Effect of properties on hydrolysis and oxidation ratio
Although the %VBur and the steric maps explained the overall
decomposition rate of Cu-NHC, the decomposition phenomena of
Cu(I)-NHCs with different types of backbones giving different
main products was not well understood. Thus, other properties of
the Cu(I)-NHC, such as the dipole moments, the logarithm of the
partition coefficient in 1-octanol and water (log P), and binding
energies were calculated in an effort to get more clues on the
driving factor (Table 3). It is important to note that the objective of
calculating dipole moments, log P values, and energies is not to
provide exact values, but only to give trends.
Thus, the binding energies (
form the imidazolium hydroxide), and O2 were calculated and the
negative values followed the order: O2 < CuCl < H2O. The G of
∆G) of NHC with CuCl, H2O (to
∆
The dipole moment of a molecule evaluates the extent of the
uneven distribution of positive and negative charges on the
compound. As a polar molecule, H2O could interact with Cu-NHC
the reaction between Cu(I)-NHC and H2O or O2 were not
calculated due to the uncertainty of the products of Cu species
(decomposition of different Cu(I)-NHCs were known to yield
different copper precipitates) [32]. Thus, only the binding
energies of NHC with CuCl, H2O, and O2, respectively, were
compared. The overall binding energy of NHC to O2 is lower than
the energy to CuCl by 0.66 eV or H2O by 3.00 eV. This implied
that, in accordance with the HOMO-LUMO energy gap analysis
(See supplementary data), the reactions of Cu(I)-NHCs with O2
are more favourable than that with H2O, from the aspect of
thermodynamics.
through dipole-dipole interaction. The dipole moments of 1a–j
were calculated accordingly. The calculated dipole moments of
Cu(I)-NHC were in the range of 6 to 10, and the calculated dipole
moment of H2O was 1.931, which was close to the experimental
value (1.855). 1a
larger overall dipole moments than 1f
groups, indicating that 1a have stronger dipole-dipole
. The fact that 1a and 1d
–
e
(group 1) bearing small substituents have
–
j
(group 2) bearing N-aryl
–
e
interaction with water than 1f
–
j
–
b
–e
decomposed to produce more
It was found that different types of NHCs have different levels
of binding energy gaps between H2O and O2 (binding energy gap
Table 3
= [binding energy of NHC to H2O]
– [binding energy of NHC to
Dipole moments (
µ
), log P and binding energies of 1a
–j.
O2]). The levels of these binding energy gaps roughly correlated
with the proportion of the resulting imidazolium and urea products
(Fig. 4). If the binding energy gap is larger than 3.00 eV, the
Cu-
µ (D)
log P a NHC binding energy (eV) b
NHC
CuCl
H2O
0.309
O2
3.231
decomposition gave > 50% yield of urea products (1c
saturated or aromatic backbones; 1i was
–g with
1a
1b
1c
1d
1e
1f
8.892
−
0.21 (
−
0.11)
−
−
−
−
−
−
−
−
−
−
−
−
2.645
2.638
2.645
2.601
2.617
2.509
2.658
2.581
2.591
2.681
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
8.017
2.98 (2.87)
2.31
0.317
0.305
0.304
0.296
0.200
0.303
0.176
0.205
0.296
3.218
8.324
3.373
3.369
3.387
3.263
3.326
3.083
3.279
3.222
8.658
4.48 (5.29)
4.51 (5.45)
3.71
10.14
7.132
1g
1h
1i
7.644
4.51
7.551
4.79 (3.59)
6.88
6.528
1j
6.447
7.15 (6.22)
H2O
O2
1.855 [55]
0.065 [56]
−
−
0.29
a
Calculated by Molinspiration software [57] (values calculated by the Virtual
b
Computational Chemistry Laboratory [58,59] online server is in parenthesis).
Binding energies are presented as
H2O and O2 to yield Cu(I)-NHC, imidazolium hydroxide and urea, respectively, at
298 K for 1a , 373 K for 1h, 423 K for 1i
∆
G of the reaction between NHC with CuCl,
Fig. 4. Plot of the urea and imidazolium yields versus the NHC binding energy
gaps.
–
g
–j.

Author Name / Journal of Organometallic Chemistry
an exception due to the reaction at 150 °C). If the binding energy
gap is smaller than 3.00 eV, the decomposition gave 50% yield
of urea products (1a 1h and 1j with unsaturated backbones).
decomposition than the corresponding Cu-NHCs with
unsaturated and saturated backbones. On the other hand, the C
≤
–
–b,
Cu bond orders increase with the decreasing buried volume (Fig.
Thus, it was postulated that, with saturated or aromatic
backbones, the reactions of Cu(I)-NHCs towards O2 outweighed
the reactions towards H2O, due to the high level of
thermodynamic favour of O2 (binding energy gap (∆∆G) > 3.00
eV); with unsaturated backbones, the trend was reversed ((∆∆G)
< 3.00 eV).
5), which is due to the smaller N-substituents exert less steric
repulsion on the C–CuCl portion which helps to develop a
stronger C Cu bond. However, the large steric hindrance was
–
considered to be the most important stabilizing factor, since the
role of steric hindrance to protect the carbene centre outweighed
In addition, the 13C NMR chemical shift of NHC (δNHC) was
its role of weakening the C–Cu bond, such as 1f–j vs 1a–e.
determined and C
electronic properties of NHC ligands (Table 4). The Cu(I)-NHCs
with unsaturated (1a 1h and 1j), aromatic (1d ), and
saturated (1c 1f , and 1i) backbones possessed δNHC in the
range of 176 183 ppm, 185 189 ppm, and 199 203 ppm,
–Cu bond order was calculated to evaluate the
2.4. Postulated decomposition pathway of Cu(I)-NHC under air
−b,
–e
According to the study by Willans [60], some Cu(II)-NHCs
could be isolated after the corresponding Cu(I)-NHCs were
exposed to low oxygen levels. Those rare Cu(II)-NHCs contained
pyridyl as ancillary coordinating site providing additional stability.
In our study, the Cu(II)-NHCs binding with an O atom were also
observed by HRMS after the Cu(I)-NHCs have been exposed to
,
–g
–
–
–
respectively. The 13C NMR chemical shifts implied that the
saturated NHCs possess lower electron density (downfield
shifted) than unsaturated and aromatic NHCs having the same
N-substituents (1c vs 1b; 1g vs 1h; 1i vs 1j). Charge analysis
air (Fig. 6 and see Fig. S12−S20 in supplementary data for more
showed that the NHC of 1c occupied low electron density among
the ten Cu-NHCs, indicating the weak ability to be protonated by
H2O. This could explain why the sterically unhindered 1c
produced the low yield of hydrolysis product.
examples). This observation is in line with the studies where
Cu(II) peroxide [61-69] or Cu(III) oxide [70-73] complexes have
been obtained after Cu(I) complexes were exposed to O2. The
synthesis of NHC-Cu(II)-O2 adduct was attempted using liquid O2
The C
whole molecule of Cu(I)-NHC, and the data of bond orders was
summarized in Table 4. The C Cu bond orders in Cu-NHCs with
–Cu bonds were found as the weakest bonds in the
and a solution of 1j in CH2Cl2 at –190 °C. However, the adduct
was not isolated, probably due to thermodynamic instability
[69,72,73]. Accordingly, one of the possible decomposition
pathways of Cu(I)-NHC is considered to go through Cu(II)-NHC
[74]. First, due to the oxophilic nature of Cu [75, 76], Cu(I)-NHC
could bind with O2 to form NHC-Cu(II)-oxygen adduct. The
formation of Cu(II)-O2 adduct complexes could be reversible to
give Cu(I) and O2 [67]. The copper species thus could serve as a
catalyst to activate the O2 molecule [66,77-79], accelerating its
decomposition. On the other hand, the resulting Cu(II)-NHC was
more hydrophilic than Cu(I)-NHC. It is known that most Cu(II)-
NHCs are sensitive to air, and quickly decompose, even when
they possess large N-substituents [26,60]. The formed Cu(II)-
–
the same N-substituents follow the increasing order: saturated
backbone < unsaturated backbone < aromatic backbone (such
as 1c
< 1b < 1d). The larger the bond orders, the stronger the C–
Cu bonds are. This explains why Cu-NHCs with aromatic
backbones possess a greater resistance to
Table 4
Carbene 13C NMR (δNHC) and C
–
Cu Bond Order of 1a
–j.
a
Cu-NHC
1a
δNHC
C–
Cu bond order b
NHC leads to a lengthened C–Cu bond [60], which would result
183.09
175.78
199.08
188.21
185.59
201.86
202.50
178.87
202.97
180.73
0.595
0.589
0.583
0.600
0.598
0.574
0.556
0.565
0.519
0.529
in easier decomposition than the Cu(I)-NHC analogue. After the
oxygen insertion or protonation to NHC, the urea or imidazolium
products were generated upon the release of different Cu
species, such as CuO CuCl2, Cu(OH)2, and CuCl2. Finally, all Cu
1b
1c
1d
species were converted into
a
green co-precipitate as
1e
xCuCl2 yCu(OH)2 zH2O under humid air [32].
1f
1g
1h
1i
1j
a CDCl3 as reference at 77.16 ppm. b Calculated at PBE/LanL2DZ level.
%VBur
50
y = -219.89x + 163.16
R² = 0.86
45
40
35
30
25
Fig. 6. HRMS of Cu(II)-NHC after Cu(I)-NHC being exposed to air.
0.5
0.52
0.54
0.56
0.58
0.6
0.62
bond order
Fig. 5. Plot of %VBur against C
–
Cu Bond Order.

6
Author Name / Journal of Organometallic Chemistry
R
R
R
almost unchanged within 24 hours. But in solution, Ag-NHC 4f
O
O
O
II
N
N
N
O2
almost completely hydrolysed to generate 5f within 18 hours
under air at room temperature (see supplementary data), and an
off-white precipitate (AgCl) was observed. Subsequently,
hydrolysis of Ag-NHC 4g (Scheme 3(c)) was tested. It was found
that 4g was not hydrolysed at 25 oC in organic solvents, such as
CuI
or
O2, H2O
Cl
X
CuII
Cu
Cl
N
R
N
R
N
R
Cl
R
R
R
N+
N
N
X-
H
O2, H2O
o
CuII
CH2Cl2 and THF under air, neither in aqueous solution at 25 C
+
+
O
Cu salts
X
N
due to steric hindrance. When heating 4g in H2O at 70 oC,
hydrolysis was observed affording 5g in 95% yield. When using a
free NHC with H2O, the reaction led to the formamide product in
quantitative yield within short time. The decomposition pathway
was postulated in Scheme 4. At first, the imidazolinium halide
reacts with Ag2O to generate Ag-NHC and H2O. Through
N
N
R
R
R
X = Cl or OH
Scheme 2. Decomposition of Cu(I)-NHC under air.
It was not clear yet how oxidation and protonation of NHC
affected each other under air. The exact role of the Cu species in
the solution also remains complicated. The actual Cu(I)/Cu(II)-
NHC decomposition could be far more complex. Thus, the
proposed pathways should be regarded as a simplified model
dissociation-association of the C–AgX bond, the Ag-mono(NHC)
is in equilibrium with Ag-bis(NHC), free NHC, and AgX (the
reason for easy transmetalation) [81,82,85]. The free NHC
subsequently inserts into H2O to form imidazolidinol as observed
by Fogg [86]. The imidazolidinol quickly undergoes
rearrangement to produce a formamide irreversibly [87]. To
prevent the hydrolysis of Ag-NHC bearing saturated backbone,
the increase of steric bulkiness of N-substituents is an efficient
method. By changing the 2-methylphenyl in 4f (Scheme 3(b)) to
the 2,6-dimethylphenyl in 4g (Scheme 3(c)), pure 4g indeed
could be obtained using Ag2O [6].
but
remained unexplained so far. Further studies are therefore
necessary for better understanding of the decom-
shedding
light
on
various
precedents
that
a
position mechanism of Cu-NHC.
2.5. Decomposition of Ag-NHC under air
The Ag-NHC complexes usually served as transmetalation
agents to obtain other metal-NHCs. Most of Ag-NHCs were
considered tolerant toward H2O since their usual synthesis
generates stoichiometric amounts of H2O in the reaction mixture
[5,80,81]. However, according to some reports, the Ag-NHCs
underwent hydrolysis under certain conditions [82,83]. For
example, de Jesús reported the hydrolysis of water soluble Ag-
NHC bearing sulfonate groups to regenerate the NHC precursors
in 2013 [82]. In 2017, instead of obtaining Ag-NHC, Liu observed
a formamide ring-opening product by mixing NHC precursor with
Ag2O [83]. In 2017, Flores-Parra observed the hydrolysis of a Ag-
NHC into imidazolium [84].
The formamide products resulting from the hydrolysis of in situ
Ag-NHCs were also obtained in our group (Scheme 3(a) and
3(b)). Sterically unhindered Ag-NHC bearing saturated backbone
was found to be easily hydrolysed under air in different solvents,
such as THF, CH2Cl2, CHCl3, and acetone. The reaction of 3f
(thoroughly dried under vacuum) with Ag2O led
On the other hand, Ag-NHCs bearing unsaturated and
aromatic backbones are more resistant to decomposition, given
the fact that those Ag-NHCs are isolable in air without
decomposition [88], such as 4b [5] (Fig. 7) The decomposition of
4b in aqueous solution (CH3CN/H2O = 2:1) under heating was
studied. After 60 h, Ag-mono(NHC) 4b decomposed to give an
equilibrium mixture containing 56% of imidazolium and 44% of
Ag-bis(NHC) complex (Fig. 7(a)), monitoring by H NMR (See
supplementary data) and HRMS. The formation of imidazolium
and Ag-bis(NHC) complex from the decompo-
1
R
N
R
R
R
R
H
-AgX
N
N
N
N
H2O
OH
H
Ag2O
-H2O
AgX
O
N+ X-
R
N
R
N
R
N
+AgX
NH
R
R
R = non-bulky group
X = Cl, Br, I
R
N
R
N
AgX
N
R
N
R
Ag2O
NH
H
Scheme 4. Proposed decomposition pathway of saturated Ag-NHC.
N
N
N
N
H2O
4Å MS
O
Br-
(a)
+
AgBr
O
N+
CH2Cl2
25 oC, 24 h
N
H
NH
detected by HRMS
Ag2O, 4Å MS
mixture of four rotamers:
8% : 12% : 61% : 19%
H
Cl-
N+
AgCl
N
O
HN
(b)
+
N
N
N
CH2Cl2
25 oC, 16 h
3f
5f
, 45%
4f
, 55%
(not hydrolysed in H2O)
(further hydrolysed after exposure to air)
H
AgCl
O
HN
H2O (50 equiv.)
(c)
(d)
N
+ AgCl
N
N
N
N
THF
70 oC, 24 h
5g
H
, 95%
4g
O
HN
H2O (10 equiv.)
C
N
THF
25 oC, 0.5 h
5g
, 99%
(in situ)
Fig. 7. Decomposition of Ag-NHC 4b, and the change of solution appearance.
sition of Ag-mono(NHC) in this study was in agreement with
previous report [82]. The decomposition of Ag-NHCs bearing
unsaturated and aromatic backbones tends to regenerate the
Scheme 3. Hydrolysis of Ag-NHCs.
to a mixture of Ag-NHC 4f and a formamide 5f in a 55:45 ratio
(Scheme 3(b)). The mixture of 4f and 5f as a solid was found

Author Name / Journal of Organometallic Chemistry
cationic NHC precursors [82] for maintaining the aromatic
in the dark for 16 h, diluted with dried CH2Cl2 (1 mL), and filtered under
argon. Removal of solvent in vacuo gave a mixture of 5f (55% yield,
estimated by 1HNMR) and 4f (45% yield, estimated by 1HNMR) with
total quantity of 0.12 g as an off-white solid. Characterization data for
5f: 1H NMR (400 MHz, CDCl3) δ 7.34 – 7.27 (m, 8H), 4.12 (s, 4H),
2.39 (s, 6H) ppm. HRMS (ESI-TOF): m/z [2M-AgCl2]+ calcd for
C34H36107AgN4, 607.1985; found, 607.1965; C34H36109AgN4, 609.1982;
found, 609.1976. Due to decomposition, 13C NMR of 5f could not be
recorded. Characterization data for 4f: 1H NMR (400 MHz, CDCl3) δ
8.20 (s, 1H), 7.34–7.29 (m, 2H), 7.26–7.20 (m, 1H), 7.16–7.01 (m, 3H),
6.65 (t, J = 7.4 Hz, 1H), 6.49 (d, J = 8.0 Hz, 1H), 4.19 (s, 1H), 4.02
(pseudo t, J = 5.6 Hz, 2H), 3.31 (dd, J = 10.5, 5.5 Hz, 2H), 2.26 (s, 3H),
2.12 (s, 3H) ppm. 13C NMR (75 MHz, CDCl3) δ 164.31, 146.01, 139.18,
136.07, 131.76, 130.16, 129.06, 128.92, 127.38, 127.16, 122.17,
116.94, 108.88, 45.44, 42.57, 17.80, 17.60 ppm. HRMS (ESI-TOF):
m/z [M+H]+ calcd for C17H21N2O, 269.1648; found, 269.1649.
character. The clear solution of 4b at the beginning (Fig. 7(b)),
gradually turned to a reddish-brown mixture with a grey
precipitate (Fig. 7(c)), which was a mixture of AgBr and Ag2O.
3. Conclusions
The decomposition pathways of different types of Cu(I)-NHCs
and Ag-NHCs in solutions under humid air were studied. Cu(I)-
NHCs underwent hydrolysis and oxidation under humid air to
generate imidazoliums (or imidazoliniums, benzimidazoliums)
and urea derivatives, respectively. The decomposition of Cu(I)-
NHC was found as pseudo-first order reaction. The
decomposition rate constants, percent buried volumes (%VBur),
and steric maps of the Cu(I)-NHCs were determined. The more
sterically hindered Cu-NHCs led to slower decomposition. The
types of Cu-NHC backbones affected the ratio of the
decomposition products, due to different levels of affinities
towards O2 and H2O: saturated and aromatic backbones
generated more urea products, but unsaturated backbones
produced more hydrolysis products. When designing an air-
stable Cu(I)-NHC in solution, a %VBur larger than 40% for each
NHC unit was suggested. The Ag-NHCs underwent hydrolysis to
yield ring-opening formamides or imidazoliums in solution under
humid air. It was found that the Ag-NHCs bearing saturated
backbones with non-bulky N-substituents hydrolysed easily. The
Ag-NHCs with unsaturated and aromatic backbones were more
resistant towards hydrolysis than the saturated ones. The current
study indicates that the metal leaching is possible when Cu-
NHCs and Ag-NHCs are used in catalysis. This study provides a
method to observe the decomposition or deactivation of other
metal-NHC complexes under air.
4.4. Hydrolysis of (1,3-bis(2,6-dimethylphenyl)imidazolidin-2-ylidene)-
silver(I) chloride (4g)
Ag-NHC 4g (50.0 mg, 0.12 mmol, 1.0 equiv.) and H2O (0.107 mL,
5.93 mmol, 50 equiv.) in THF (0.2 mL) was stirred at 70 oC for 24
hours. The mixture was diluted with Et2O (5 mL), dried over Na2SO4,
and filtered. The solution was concentrated to afford a yellow residue
which was purified on silica gel column to yield formamide 5g (33.5 mg,
0.113 mmol, 95%) as white solid. 1H NMR (300 MHz, CDCl3, major
isomer ) δ 8.10 (s, 1H), 7.24 –7.12 (m, 3H), 6.98 (d, J = 7.4 Hz, 2H),
6.80 (t, J = 7.4 Hz, 1H), 3.86 (t, J = 7.0 Hz, 2H), 3.47 (s, 1H), 3.25 (t, J
= 7.0 Hz, 2H), 2.29 (s, 6H), 2.25 (s, 6H) ppm. 13C NMR (75 MHz,
CDCl3, major isomer) δ 164.20, 145.99, 138.51, 136.94, 129.20,
129.02. 128.75, 128.72, 121.63, 47.22, 46.58, 18.90, 18.52 ppm.
HRMS (ESI-TOF): m/z [M+H]+ calcd for C19H25N2O, 297.1961; found,
297.1964.
4. Experimental
4.5. Computational details
4.1. General information
The DFT calculations were performed using GAMESS 2013 (R1)
program package [100,101]. The Perdew-Burke-Ernzerhof (PBE)
exchange-correlation function[102] was used to optimize the ground
state geometries in gas phase, starting from the corresponding crystal
structures when they are available. The default settings were used for
the optimization of ground state geometries with unconstrained
symmetry (convergence criterion 0.0005 was used). The effective-core
potential (ECP) of Hay and Wadt with a double-ξ valence basis set
(LanL2DZ) [103-106] was chosen to describe Cl and Cu atoms.
Dunning-Huzinaga full double-zeta basis set was applied to H, C, N,
and O atoms [107]. The basis set description was obtained directly
from the EMSL Basis Set Library [108-110]. Frequency calculations
were carried out at the corresponding level to check the ground
geometries (frequencies found to be positive). The percent buried
volume (%VBur) was calculated by SambVca tool (Salerno molecular
buried volume calculation) [37], using optimized structure or
crystallographic data. Typically, the optimized or crystallographically-
determined bond length was scaled by 1.17, with sphere radius 3.5 Å
centering at the Cu atom, and mesh spacing 0.1 Å. The hydrogen
atoms were excluded from the %Vbur calculation, since the positions of
light hydrogen atoms could not be determined precisely by X-ray.
About the steric maps (contours), red color (positive value) represents
areas of large steric hindrance (into the sphere), and blue color
(negative value) represents distance away from the coordinate center.
The dipole moments were calculated using Avogadro software
[111,112], and the visualization of HOMO and LUMO orbitals also
used the same software. The logarithm of the partition coefficient in 1-
octanol and water (log P) was calculated by Molinspiration software
[57], or by the Virtual Computational Chemistry Laboratory (online
server) [58, 59].
1H and 13C NMR spectra were measured on a Bruker AC 300 MHz,
or Varian Inova 400 MHz spectrometers. Chemical shifts are given in
ppm and residual solvent peaks were used as reference. The coupling
constants were reported in hertz. High-resolution mass spectra
(HRMS) were measured on an Agilent 6210 mass spectrometer using
electrospray ionization (ESI-TOF). The synthesis of NHC precursors
[32,89-95], Cu(I)-NHCs[32,53,93,96-99] and Ag-NHCs 4b [5] and 4g
[6,93] was performed according to the literature methods.
4.2. Decomposition of Cu(I)-NHC in solution under air
Cu(I)-NHC 1a−g (0.04 mmol, 1.0 equiv) and CDCl3 (0.4 mL,
degassed by bubbling argon for 30 min) were added into a flame-dried
NMR tube. (CD3)2SO (0.4 mL) was used for 1h (0.04 mmol), 1i (0.02
mmol), and 1j (0.02 mmol). The NMR tube was closed with a septum
and equipped with an air balloon (approximate 500 mL) containing
approximately 100 mL of O2 (4.5 mmol, 112 equiv.) and approximately
12.6 mL of H2O (gas, 0.56 mmol, 14 equiv., air relative humidity =
75%). The solution (not agitated) was placed at room temperature and
was monitored by 1H NMR. 100 oC was used for the decomposition of
1h, and 150 oC was used for the decomposition of 1i and 1j. The
precipitate in the NMR tube was removed by quick filtration using a
1
membrane filter before each H NMR measurement. The ratio of Cu-
NHC, urea, and imidazolium were calculated through the integration of
1H NMR, using the normalization method. The characterization of
products could be found in the previous study [32].
4.3. Preparation of (1,3-di-o-tolylimidazolidin-2-ylidene)-silver(I)
chloride (4f) and N-o-tolyl-N-(2-(o-tolylamino) ethyl)formamide (5f) as
mixture
Into a flame-dried tube, 3f (0.100 g, 0.35 mmol, 1.0 equiv; dried
under vacuum at 110 °C for 2 h before use), Ag2O (48.5 mg, 0.21
mmol, 0.6 equiv) and 4 Å molecular sieves (0.100 g), were added,
followed by the freshly-distilled CH2Cl2 (2 mL). The mixture was stirred

8
Author Name / Journal of Organometallic Chemistry
[22] B.R. Dible, M.S. Sigman, A.M. Arif, Oxygen-induced ligand dehydrogenation of a
Acknowledgments
planar bis-µ-chloronickel(I) dimer featuring an NHC ligand, Inorg. Chem. 44
(2005) 3774−3776.
This work was supported by the Natural Sciences and Engineering
Research Council of Canada (NSERC) Discovery Grant RGPIN-2017-
04272, the FRQNT Centre in Green Chemistry and Catalysis (CGCC)
Strategic Cluster FRQNT-2020-RS4-265155-CCVC, and Université
Laval. D. L. thanks China Scholarship Council (CSC) for a doctoral
scholarship. The authors thank Nour Tanbouza (Département de
chimie, Université Laval) for proofreading the manuscript.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.xxxx.xx.xxx.
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10
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below.
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Mechanism studies of oxidation and hydrolysis
of Cu(I)-NHC and Ag-NHC in solution under air
Leave this area blank for abstract info.
Dazhi Li, Thierry Ollevier*

Highlights
•
Decomposition of Cu(I)-NHC and Ag-NHC under air was
studied.
•
•
•
•
Decomposition of Cu(I)-NHC led to urea and hydrolysis
products.
Theoretical calculations were used to study the decomposition
of Cu(I)-NHC.
Decomposition of Ag-NHC led to ring-opening product or
regenerated NHC precursor.
This article provides a method to study the decomposition of
metal-NHC under air.

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.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: