Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity

Article abstract of DOI:10.1002/chem.202000600

The synthesis and full characterization of new air-stable Ag^I and Cu^I complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag₂

Full text of DOI:10.1002/chem.202000600

A Journal of
Accepted Article
Title: Sterically Demanding Ag(I) and Cu(I) N-Heterocyclic Carbene
Complexes: Synthesis, Structures, Steric Parameters and
Catalytic Activity
Authors: Alejandro Cervantes-Reyes, Frank Rominger, and A. Stephen
K. Hashmi
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To be cited as: Chem. Eur. J. 10.1002/chem.202000600
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Chemistry - A European Journal
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Sterically Demanding Ag(I) and Cu(I) N-Heterocyclic Carbene
Complexes: Synthesis, Structures, Steric Parameters and
Catalytic Activity
[
a]
[a][+]
and A. Stephen K. Hashmi*[a,b]
Alejandro Cervantes-Reyes, Frank Rominger,
Abstract: The synthesis and full characterization of new air-stable Ag(I) and
Cu(I) complexes bearing structurally bulky expanded-ring N-heterocyclic
carbene (erNHC) ligands is presented. The condensation of protonated NHC
or the backbone of the NHC skeleton,[13] a unique class of
structurally imposing metal complexes has arisen (%Vbur > 52).[14]
For several years, Ag(I) NHC complexes have been fruitfully
explored in medicinal chemistry as biologically active
2
salts with Ag O afforded a collection of Ag(I) complexes, and their first use as
ligand transfer reagents led to novel isostructural Cu(I) or Au(I) complexes. In
situ deprotonation of the NHC salts in the presence of a copper(I) source,
provides a library of new Cu(I) complexes. The solid state structures feature
large N-CNHC-N angles (118–128°) and almost identical angles between the aryl
groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene
organometallic complexes,[15] joining
a swift application in
catalysis.[16] Owing to the low thermodynamic stability of the
carbene carbon–silver bond, Ag(I) NHC complexes are also
useful synthetic reagents in ligand transfer reactions,[17] becoming
[
18]
a convenient entry point into other NHC-metal compounds. On
the other hand, many NHC-stabilized Cu(I) complexes are key
catalysts of synthetically relevant transformations,[ and during
(
i.e. torsion angles close to 0°). Among the steric parameters, the percent buried
19]
volume (%Vbur) values span easily in the 50–57% range, and that one of (9-
Dipp)CuBr complex (%Vbur = 57.5) overcomes to other known erNHC-metal
complexes reported to date. Preliminary catalytic experiments in the copper-
catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded
the last decade, became
photophysical properties.[
a “hot topic” thanks to their
20]
Although manifold examples of Ag(I) and Cu(I) complexes
bearing erNHC ligands have been developed,[ their structural
diversity is limited by a flexible alkyl-chain design, which lead to a
poor prediction of their structural features, as the ring geometry
affects the NHC complex sterics and electronics.[5i,9l,13a] The steric
bulkiness of several erNHC metal complexes generally
overcomes that of the imidazolylidene-containing ligands (i.e. IPr)
albeit rarely surpass the “latest generation” five-membered NHCs
such as IPr** or IPr*(2-Np) (Figure 1).
21]
7
6–93% product at the 0.5–2.5 mol% catalyst loading, proving the stability of
Cu(I) erNHC complexes at elevated temperatures (100°C).
Introduction
Pioneering work of by Wanzlick in the early 60’s inaugurated the
era of N-heterocyclic carbenes (NHCs) as ligands for metals.[1]
Thereafter, new breakthroughs achieved by Arduengo, Bertrand,
and Herrmann, led to the application of NHCs in organometallic
chemistry and catalysis.[2-4] Since then, NHC ligands are
IPr*(2-Np)-AgCl
57.4
IPr-CuCl
7.6
IPr*-AgCl
53.6
IPr**-AuCl
55.4
4
6-Dipp-AuCl
considered excellent candidates for the stabilization of a wide
range of metal complexes,[5] and particularly for the ubiquitous
N
N
50.9
7
-Dipp-AuCl
2.6
[
6]
IPr
coinage metals (Ag, Cu, Au), thanks to their strong σ-donating
ability and steric properties.[
5
8
-Xyl-AgBr
49.1
This work
7]
N
N
N-heterocyclic carbenes based on expanded-ring backbone[8]
have emerged as outstanding ligands for transition-metal
9-Dipp-AuCl
53.0
9-DippCu-Br
57.5
6-Dipp
[
9]
10-DippAg-Br
56.6
complexes, as a result of the enlarged N-CNHC-N angle that
NHC
ring-size
N
N
pushes the N-substituent closer to the carbene carbon providing
steric protection around the metal center and consequently,
enhancing the final catalytic activity of the complex.[10-11]
7-Dipp
O
N
N
Furthermore, the trend to increase the steric demand of the ligand,
has strongly influenced the field of catalyst design.[7,8,10] For
instance, by tuning the substituent on the N-atom in the tether[
N
N
N
N
8
-Xyl
%Vbur
9-Dipp
10-Dipp
12]
Figure 1. Selected complexes of Ag(I), Cu(I) and Au(I) complexes stabilized by
five- to ten-membered NHC ligands.
[a]
A. Cervantes-Reyes, Dr. F. Rominger,
Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
Heidelberg University
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: hashmi@hashmi.de
Results and Discussion
Protonated BF - (1a) and PF -
(1b) salts shown in Figure 2, served
4 6
[
[
b]
+]
Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science, King Abdulaziz
University, Jeddah 21589, Saudi Arabia
Crystallographic investigation
as ligand source for the synthesis of Ag(I) and Cu(I) erNHC
complexes. Ligands 9-Dipp, 9-Mes, 9-Dietph, 9-Xyl, 10-Dipp, 10-
Mes, 10-Dietph and 10-Xyl are relatively unexplored in metal
complex synthesis so far.[
14]
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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X
O
X
respectively) after 24 h. The total consumption of the substrate
required a longer reaction time (ca. 3 days) yielding <50% product.
3
The reaction also was effective in CH CN under heating
N
N
N
N
conditions (Entries 3 and 4), where the yields raised up to 54%.
By using THF as the solvent and higher temperature (68°C), the
yields enhanced notably (>70%, Entries 5 and 6) as a result of a
9
9
-Dipp HBF , 1aa
10-Dipp HBF , 1ae
4
4
-Dipp HPF , 1ba
10-Dipp HPF , 1be
6
6
2
better solubility of substrate and Ag O, leading to a full
consumption (revealed by TLC, CH Cl 100%). Interestingly, the
2
2
X
O
X
use of salt 1ba under same reaction conditions, diminished the
yield to a half, and most substrate remained unreacted (Entry 7).
N
N
N
N
9
9
-Mes HBF4, 1ab
-Mes HPF6, 1bb
10-Mes HBF , 1af
4
10-Mes HPF , 1bf
6
Table 1. Optimization of the synthesis of Ag(I) erNHC complexes.
X
O
X
N
N
N
N
9
9
-Dietph HBF , 1ac
10-Dietph HBF , 1ag
10-Dietph HPF
6
, 1bg
4
4
-Dietph HPF , 1bc
6
X
O
X
Entry
Substrate
1aa
Conditions[a]
Product
Yield (%)[b]
N
N
N
N
1
2
3
4
5
6
7
2
6
2
6
2
6
2
17
21
48
54
75
80
41
2 2
CH Cl , r.t.
9
9
-Xyl HBF , 1ad
10-Xyl HBF
10-Xyl HPF
4
, 1ah
, 1bh
6
4
1ae
-Xyl HPF , 1bd
6
1aa
Figure 2. Protonated erNHC salts employed in this study (Dipp = 2,6-
diisopropylphenyl, Mes = 2,4,6-trimethylphenyl, Dietph = 2,6-diethylphenyl and
Xyl = 2,6-dimethlyphenyl).
3
CH CN, 50°C
1ae
1aa
1ae
THF, 68°C
-
The BF
4
salts 1aa–1ah were easily accessible by our previously
[
14]
reported procedure.
The reaction of erNHC-HBr precursors
1ba
with NH PF in MeOH at room temperature, originated their
4
6
hexafluorophosphate analogues 1ba–1bh. Anion exchange was
[a] Reaction conditions: NHC-HBF
4
(0.1 mmol, 1.0 equiv), Ag
2
O (0.1 mmol,
confirmed via 1H, P and F NMR analysis. The amidinium
31
19
1.0 equiv), NaBr (0.6 mmol, 6.0 equiv), absolute solvent (3 mL), in dark,
4 h. [b] Isolated yields as an average of two runs.
2
protons were evidenced in the 1H NMR spectra by the
31
characteristic singlet signal at δ 7.5–8.0 ppm, while the P NMR
displayed a septet signal at ca. δ -144 ppm, in the typical range
6
for hexafluorophosphate derivatives. The structure of PF salt 1ba
2
While attempting the synthesis of 4 from 1ac and Ag O in THF at
reflux for 3 days, organic material was isolated as the main
product and further identified as the ring opened NHC 1ac’
was unambiguously confirmed by a single crystal X-ray diffraction
analysis (Figure S2, see Supporting Information).[
22]
(
formed presumably as a result of hydrolysis with water catalyzed
Ag(I) and Cu(I) NHC complexes
by heating). A sample of this material crystallized from a toluene
solution after several days and the X-ray diffraction analysis of a
single crystal confirmed its composition and structure (1ac’,
Figure S3, see Supporting Information).[
2
The reaction of protonated NHC salts and Ag O to erNHC-AgX
complexes has been studied in great detail.[
17,23]
This reaction
22]
-
proceeds better with weakly coordinating anions (e.g. BF
4
or
PF -
) rather than halides (e.g. Br , Cl , I ), presumably due to the
-
-
-
6
halide counter anion is being able to form an hydrogen bond with
the C2-H amidinium proton.[5e,21c] Hence, the optimization
experiments employing the erNHC salts of bulky ligands 9-Dipp
and 10-Dipp were conducted (Table 1). Satisfyingly, the formation
of Ag(I) erNHC complexes was observed in all cases.
The screening in CH
2 2
Cl at room temperature (Entries 1 and 2)
led to the expected products in poor yields (17 and 21%,
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Spectroscopic characterization of Ag(I) erNHC complexes 2–7 by
13
1
C{ H} NMR reveals the typical carbene carbon (CNHC) signals of
two doublets displaced in high chemical shifts with coupling
N
N
N
N
N
N
constants of 1J
107Ag = 224–227 Hz, and 1J
109
Ag = 260–263 Hz, in
agreement with the spin-spin interaction between Ag (spin ½,
1.8%) and 109Ag (spin ½, 48.2%) isotopes with the singlet
C
C
Ag
Br
2
Ag
Br
3
Ag
Br
4
107
5
C
[
21c]
(
9-Dipp)AgBr
8%
(9-Mes)AgBr
51%
(9-Dietph)AgBr
64%
NHC
.
The ratio of these constants reflects approximately the
7
gyromagnetic ratio γ for the two silver nuclei involved that is
109
107
[24]
γ( Ag)/γ( Ag) = 1.15.
O
O
These observed couplings indicate a relatively strong CNHC–Ag
bond as a result of no or slow exchange of the erNHC ligands
between the Ag atoms at the NMR scale[13b] and confirms that
complexes 2–7 do not exhibit a fluxional behavior in the CNHC–Ag
bond, characterized by the lack of CNHC-Ag couplings.[17]
Mass spectrometry analyses (MALDI, ESI+) proved the [(erNHC-
Ag)
2 2 2
Br]+, [(erNHC) -Ag]+ and [(erNHC-Ag) ]+ fragments but
N
N
N
N
N
N
Ag
Br
Ag
Br
6
Ag
Br
5
7
(
9-Xyl)AgBr
5%
(10-Dipp)AgBr
80%
(10-Dietph)AgBr
62%
6
commonly, the most prominent peak in the spectra is that of the
+
Figure 3. Ag(I) complexes 2–7 bearing erNHC ligands.
[NHC-Ag] fragment that denotes the molecular structure of type
erNHC-AgX which could be further confirmed by X-ray diffraction
analysis of selected examples (Figures 6 and 9).
Finally, the reaction of various BF -
salts with Ag O and NaBr in
4 2
Next, the synthesis of Cu(I) erNHC complexes, following literature
protocols, was explored (Scheme 2). For example, by attempting
the condensation of erNHC salts and a Cu(I) source (i.e. Cu(0)/air,
THF under reflux for 24 h, provides a collection of Ag(I) erNHC
complexes in acceptable to good yields (51–80%, Figure 3).
Complexes 2–7 are air- and moisture-stable, colorless solids and
Cu
2
O, CuBr),[25] most starting material was recovered and only
after recrystallization from a CH
crystalline solids can be obtained.
Unfortunately, the isolation of Ag(I) complexes bearing 10-Mes or
2
Cl
2
:pentane mixture, high purity
traces of by-product were isolated, presumably formed upon
oxidation of the CNHC under heating in open-air conditions
(Pathway B). At the latter, the synthesis of Cu(I) erNHC
1
0-Xyl ligands was unsuccessful. Under these reaction conditions, complexes was accomplished by the free carbene strategy,
decomposition material was observed instead.
involving the in situ deprotonation of the erNHC salts (1a) in
presence of (SMe
)CuBr (Pathway C).[26]
2
X
R
R
N
N
X = BF or PF6
4
R
R
pathway A
pathway B
a)
b) - e)
2
Ag O, NaBr
THF, reflux
[
O]
R
R
N
R
R
N
N
N
R
R
N
N
Cu
Br
O
R
R
R
R
Ag
Br
R
R
R = iPr, N.D
R = Me, 21%
R
N
R
traces
N
CuBr(SMe2)
-AgBr
pathway C
Cu
R
R
a) CuBr, tBuOK, THF, -78°C to r.t
Br
0
b) Cu , NaBr, CH CN:H O, 80°C, 48 h.
3
2
c) Cu
d) CuBr, K
e) CuBr, NH3 (aq), MeOH, 60°C, 24 h.
2
O, NaBr, THF, reflux, 48 h.
R = iPr, 13 (43%)
R = Me, 16 (52%)
2
CO , acetone, 60C°, 24 h.
3
Scheme 2. Synthetic approaches to Cu(I) erNHC complexes.
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Gratifyingly, most of the erNHC salts underwent deprotonation
under these conditions and complexes 8–15 were successfully
obtained (Figure 4). All of the Cu(I) erNHC complexes adopt a
neutral, mononuclear erNHC-Cu-Br conformation as judged in the
The equilibrium is displaced towards the formation of neutral
mononuclear Cu(I) erNHC complex and to support this hypothesis,
the structure of 15 was unequivocally established by single-
crystal X-ray analysis (Figure 9, right). Although two molecules
were found in the unit cell, no evidence of a metallic Cu···Cu
interaction or a bromide-bridged species was observed.
1
H-NMR spectra which remained identical to those of Ag(I) erNHC
[
14]
complexes and the previously reported Au(I) erNHC ones.
The carbene carbon signals in the 13C{ H} NMR spectra show a
1
The carbene transfer ability of selected Ag(I) compounds in the
singlet at around
erNHC), and at around
10-erNHC). This implies that the CNHC in the 9-erNHC is
significantly less shielded (by ca. 9 ppm) than the respective one
δ
= 223 ppm for the nine-membered NHCs (9-
2 2
presence of (SMe )AuCl or (SMe )CuBr as metal sources was
δ
= 214 ppm for the ten-membered NHC
further investigated (Scheme 1). The reaction of 2 or 5 conducted
to novel erNHC-AuBr complexes 16 (31% yield) and 17 (53%
(
-
δ
yield), respectively. These complexes retained the Br instead of
-
of the 10-erNHC. In general, the carbenic carbon of Cu(I) erNHC
complexes is highfield shifted by ca. Δδ = 10 ppm than their Ag(I)
erNHC congeners. HRMS spectra (ESI+) of Cu(I) erNHC
the Cl , contrary to the stability of the insoluble halide salts which
increases in the order AgCl < AgBr < AgI and precipitation of AgBr
is more favorable than that of AgCl.[25] The structure of complex
(9-Xyl)AuBr 17 was confirmed by X-ray diffraction analysis of a
single crystal (Figure 5).[22]
+
complexes reflect the peak of the [NHC-Cu] fragment at the
+
highest intensity nevertheless, the [(NHC)
found in a minor ratio.
2
Cu] species was also
a)
(
2
SMe )AuCl
N
N
N
N
Ar
Ar
Ar
Ar
N
N
N
N
N
N
N
N
Ag
Br
Au
Br
Cu
Br
8
Cu
Br
9
Cu
Br
Cu
Br
Ar = Dipp,
Ar = Xyl,
2
5
16 (31%)
17 (53%) (X-ray)
10
11
(
9-Dipp)CuBr
(9-Mes)CuBr
(9-Dietph)CuBr
(9-Dietph)CuBr
b)
Y
Y
Y
(
SMe
2
)CuBr
2
(SMe )AuCl
N
N
N
N
N
N
O
O
O
O
Ag
Br
Cu
Br
Au
Br
N
N
N
N
N
N
N
N
Y = none, 2
Y = O,
8
(46%)
Cu
Br
Cu
Br
Cu
Br
Cu
Br
6
12 (40%)
1
2
13
14
15
(
10-Dipp)CuBr
(10-Mes)CuBr
(10-Dietph)CuBr
(10-Xyl)CuBr
Scheme 1. Ligand transfer from Ag(I) erNHC complexes to (a) Gold and (b)
Copper. Reaction conditions: Ag(I) complex (0.1 mmol), (SMe )AuCl or
SMe )CuBr (0.11 mmol, 1.1 equiv), CH Cl (10 mL), room temperature, 24 h,
light exclusion.
2
(
2
2
2
Figure 4. Synthesized Cu(I) complexes 8–15 bearing erNHC ligands.
Interestingly, the mass spectrometry analyses of complexes 11
and 15, bearing less bulky ligands, namely 9-Xyl and 10-Xyl,
exhibit peaks of the [(erNHC-Cu) Br] and [(erNHC-Cu) Br]
2 2
fragments, which might suggest that the linear erNHC-Cu-Br
complex is in equilibrium with the bromide-bridged
By employing the complexes 2 and 6 bearing Dipp ligands, in the
presence of (SMe )CuBr, the expected Cu(I) compounds 8 and
2 were obtained in acceptable yields (40–46%).
2
+
+
1
2
{[(erNHC) Cu(µ-Br)]}[Br] species and the neutral tri-coordinate
(
erNHC) CuBr (Scheme 3).
2
Y
Br
Y
N
N
N
N
N
N
Cu
N
N
CuBr
2
Y
CuBr
Br
Cu Br
N
Cu
N
Y
Y
{
[(erNHC)Cu]
2
(µ-Br)]}[Br]
2
(erNHC) CuBr
Y = none, 11
Y = O,
15 (X-ray)
Y = none, m/z not observed
Y = O, m/z = 1099 [M-Br]
Y = none, m/z = 1023 [M+Na]
Y = O, m/z = 1055 [M+Na]
3
Figure 5. Molecular structure of (9-Xyl)AuBr·CHCl 17 in the solid state.
Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are
omitted for clarity.
Scheme 3. Postulated equilibrium in solution of Cu(I) complexes stabilized by
-Xyl and 10-Xyl ligands.
9
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These results underline the carbene transfer ability of the bulky
Ag(I) erNHC complexes, in contrast to other sterically demanding
NHC systems which were unsuccessful in the transfer of ligand to
other metals.[27]
Comparatively, the Au–Br bond length (2.4036(5) Å) is longer
than that of Au–Cl found in 9-NHC-Au(I) complexes (2.29–2.35
Å).[ The CNHC–Au length is 2.006(4) Å and the N-CNHC-N is
119.6(4)°
14]
Finally, the Cu(I) erNHC complexes 8 and 12 were mixed with
(
SMe
2
)AuCl in the typical conditions for the transfer of ligand
, r.t, light exclusion)[ nonetheless, the reaction failed to
28]
(CH Cl
2
2
afford the desired Au(I) erNHC complexes and led to the
deposition of a gold mirror.
Structural Studies and Steric Parameters
A
single crystal of complex 17[22] was grown from
CHCl /pentane mixture. This complex crystallized with a CHCl
molecule from the solvent (Figure 5).
a
3
3
Figure 7. Solid state molecular structure of (9-Dipp)CuBr (8). Thermal ellipsoids
are drawn at 50% probability level. Hydrogen atoms and one CH
2 2
Cl solvent
molecule are omitted for clarity.
X-ray quality single crystals of complexes 2, 4, 6, 7, 8, 13 and
5,[22] were grown by slow diffusion of pentane into a saturated
solution of the complex in CH Cl or CHCl followed by
1
2
2
3
evaporation of solvents at room temperature. Table 2 summarizes
relevant geometrical parameters of Ag(I) erNHC complexes 2, 4,
6
and 7, and other related examples selected from literature.
Table 2. Selected geometrical parameters (Å and °) and %Vbur[29] of some
NHC-Ag(I) complexes
Complex
CNHC–Ag
Ag–X
∠N-CNHC-N
%Vbur
(
(
(
(
(
(
(
(
(
(
(
(
(
6-Mes)AgCl[18a]
6-Dipp)AgBr[18b,5e]
7-Mes)AgBr[5e]
7-Xyl)AgI[5e]
2.095(3)
2.104(6)
2.097(6)
2.114(6)
2.067(1)
2.132(4)
2.123(3)
2.135(7)
2.110(4)
2.119(3)
2.081(2)
2.137(3)
2.068(2)
2.3213(1)
2.4254(9)
2.3792(2)
2.5659(7)
2.111(2)
118.3(3)
118.8(5)
118.8(6)
121.2(5)
103.9(6)
123.2(3)
123.3(3)
126.6(7)
118.1(3)
119.1(3)
104.2(2)
126.5(3)
104.2(1)
44.0
49.5
44.7
43.6
48.6
48.7
49.1
50.3
51.9
53.3
53.5
56.6
57.4
IPent)AgOAc[16d]
8-Mes)AgBr[8a]
8-Xyl)AgBr[8a]
2.4553(5)
2.4370(5)
2.4285(10)
2.4314(5)
2.4399(5)
2.3189(9)
2.4449(4)
2.317(14)
10-Dietph)AgBr 7
9-Dipp)AgBr 2
9-Dietph)AgBr 4
IPr*)AgCl[27a]
10-Dipp)AgBr 6
IPr*(2-Np))AgCl[12h]
Figure 6. Molecular structures of (9-Dipp)AgBr (2, top) and (9-Dietph)AgBr (4,
bottom) in the solid state. Thermal ellipsoids are drawn at 50% probability level.
Hydrogen atoms are omitted for clarity. For 4 only one of the two symmetry-
independent molecules is shown.
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Focusing on the 9-NHCs 2 and 4, the CNHC–Ag bond lengths are
complexes 6, 7 and 15 (around 126°). To the best of our
knowledge these are the widest N-CNHC-N angles of any erNHC-
metal complex in current literature. CNHC–Ag bond lengths of 6
and 7 lie around 2.14 Å and are marginally longer than those of
six- to eight-membered NHCs, while the Ag–X bond distances are
within the same range 2.42–2.44 Å (Table 2).
2.110(4) Å and 2.119(3) Å, respectively. Bond lengths Ag–Br lie
between 2.43–2.44 Å and are within the range of the previously
reported Ag(I) NHC complexes of small ring size (2.38–2.57 Å)
but slightly larger than those of the corresponding five-membered
compounds (2.11–2.32 Å). The N-CNHC-N angles (118.2–119.1°)
are smaller than those of the eight-membered NHCs (123.2–
1
23.3°), and similar to the six- and seven-membered NHCs (6-
Mes)AgCl, (6-Dipp)AgBr and (7-Mes)AgBr (118.3–118.9°). CNHC
Cu and Cu–Br bond lengths in Cu(I) complex 8 are 1.904(4) Å and
Y
Y
–
Y
b
N
N
N
N
N
N
2.2300(8) Å, respectively (Figure 7). The N-CNHC-N angle found in
M
X
M
X
M
X
8
is 119.0°. Complexes 2 and 4 adopted nearly linear erNHC-Ag-
Br geometry with CNHC-Ag-Br angles around 176° while complex
appeared considerably out of linearity (∠CNHC-Ag-Br = 174.7°).
a)
b)
c)
8
The torsion angles (α°) found in these complexes are 48.4°, 37.9°
and 28.4°, respectively.[30] The latter values confirm the twisting of
the aryls, respect to the NHC plane, observed in the solid state
structures of the complexes (see (a) in Figure 8). While a sharp
twisting is perceived in Ag(I) complex 2 (top, Figure 6; α = 48.4°),
a significatively smaller twisting is observed in the isostructural
Cu(I) congener 8 (Figure 7, α = 28.4°).
Moving to the X-ray structures of the 10-erNHC complexes of
Ag(I) and Cu(I) from a front view (top, Figure 9), is easy to disclose
their wide N-CNHC-N angles (126–128°). Cu(I) complex 13 exhibit
the widest N-CNHC-N angle of this series (128.6°) followed by
Figure 8. Didactic illustration of (a) the twisting of the N-Aryls around the N···N
co-planarity, (b) the folding of the backbone ring limiting the twisting of the ortho-
substituents at the N-Aryls, and (c) the angles b defined by N-CH -CAr
2
The CNHC–Cu bond distances found in Cu(I) complexes 13 and 15
are 1.924(12) Å and 1.915(4) Å, respectively. To our surprise,
torsion angles of these 10-erNHC complexes lie between 0° and
4.5°, which implies that the two aryl planes are essentially in
coplanarity with the NHC plane.
Figure 9. Top (left to right): Solid state molecular structures of (10-Dipp)AgBr (6), (10-Mes)CuBr (13), (10-Dietph)AgBr (7), and (10-Xyl)CuBr (15) in the solid state.
Ellipsoids drawn at 50% probability level. Solvent molecules and hydrogen atoms have been omitted for clarity. Bottom (left to right): Steric maps of (10-Dipp)AgBr
(
6), (10-Mes)CuBr (13), (10-Dietph)AgBr (7), and (10-Xyl)CuBr (15). Values in the four corners of the maps are the %Vbur of the erNHC ligand in the corresponding
quadrant. The positive values (in Å) refer to the upper hemisphere, which is the hemisphere where the NHC ligand dwells.
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This implies that the geometry of the diphenylether moiety exerts
a domino effect in the overall arrangement of the erNHC complex,
which ends up in diminished torsions of the N-Aryls. The folding
(
see (b) in Figure 8) of the diphenylether moiety influences the
arrangement of the methylene groups, which adopt a nearly
tetrahedral arrangement with N-CH -CAr angles b = 112.7–118.9°
see (c) in Figure 8). Along with the non-trivial presence of
2
(
hydrogen atoms in these methylene groups, the folding limit the
twisting of the N-Aryls as their ortho-substituents are equally
pushed down and consequently, the N-Aryl planes reach the more
stable geometry (the coplanarity relative to the NHC plane). The
molecular structures from side-views presented in Figure S1
showcase the folding of the backbone ring and the twisting of the
N-aryls (see Supporting Information).
For the quantification of the steric profiles of Ag(I) and Cu(I)
erNHC complexes, the buried volume values (%Vbur)[29] and
topographic steric maps[ were derived using SambVca 2.0
package.[32] The typical settings for the calculation (r = 3.5 Å
31]
(radius of the sphere around the metal center), d = 2.0 Å (bond
length CNHC–metal), Bondi radii scaled by 1.17 and a mesh
spacing of 0.10 Å) were used to scan the sphere of the buried
voxels.[33] The steric distribution around the first coordination
sphere of the metal (upper hemisphere), is represented by iso-
contour lines visualized from a bottom point. In addition, the colour
scale help to identify the less-to-more buried (occupied) zones.
Figure 10, features illustrations that resemble the coordination
sphere in whose center the metal lies (Ag or Cu) and the
reference axes (X, Y) partition the hemisphere, giving the steric
component for each quadrant.
Figure 10. Visualizing of the steric impact of the top hemisphere of 2 (a) and 8
(b) indicating the buried volume for each quadrant.
The %Vbur values of Ag(I) erNHC complexes given in the Table 2,
are in the range 50.3–56.6% and are comparatively higher than
those of the six- to eight-membered NHCs counterparts (%Vbur up
to 49.5). Noteworthy, Ag(I) complex 6 surpass the %Vbur value of
the extremely hindered (IPr*)AgCl complex reported by Markó et.
Since
a direct comparison between two NHC complexes
[
7e]
possessing different metals might be meaningless,
a few
interesting observations, regarding the steric impact of 9-Dipp
ligand on these erNHC complexes can be pointed out. The steric
maps of Ag(I) complex 2 and Cu(I) complex 8, are shown in Figure
10. The steric contribution of the 9-Dipp ligand in Cu(I) complex 8
(bottom) is notably higher than that of its Ag(I) congener 2 (top).
The top-left quadrant of 2 (45.3%) is less hindered than the
analogous quadrant of 8 (55.4%) and a similar asymmetry can be
appreciated in the corresponding right tail quadrants (51.1% and
59.1%, respectively). The overall buried volume of complex 2
(51.9%) is still outstanding within the family of Ag(I) erNHC
complexes, while complex 8, possess the most sterically
encumbered structure within the Cu(I) erNHC family, displaying
an impressive %Vbur = 57.5%.
[
27a]
(2-Np)
al, (%Vbur = 53.5)
5
and beside to (IPr*
)AgCl (%Vbur =
7.4),[12h] these represent the structurally more imposing NHC-
Ag(I) complexes in current literature.
The steric maps of the 10-erNHC complexes are presented in
Figure 9 (bottom). In complex 6 (%Vbur = 56.6), the ortho-isopropyl
substituents of the N2- and N5-Aryl substituents prevent the
rotation around the N–Aryl bond effectively, by limiting the twisting
of the Dipp substituents, which results in a homogeneous steric
delivery. Although the top-left quadrant is less bulky (46.4%), the
steric impact of the ligand lies on the three bulkier quadrants (59–
6
3% buried volume). Similarly, complex 7 (%Vbur = 50.3) two
quadrants exert lower steric impact (42–45%) than the top-left
quadrant which overcomes 60% buried volume.
The size of the metal ion and somehow, the halogen size, are also
factors governing the spatial arrangement of the ligand around the
metal itself. Given that the ionic radius of Cu is just much smaller
than that of Ag , the large silver ion may affect the steric
distribution by pushing the substituents away from the first
The %Vbur value calculated for Cu(I) complex 13 (52.1%), is
marginally larger than that of 15 (51.9%) as their structures differ
only in the presence of para-methyl groups at the N-Aryls. The
torsion angles (0° and 3.3°, respectively), best describe a slight,
but still relevant influence of these para-methyl substituents, into
the steric distribution of the complex, through the stabilization of
the NHC skeleton. We might better illustrate this with the imprint
of one para-methyl substituent visible in the steric map of 13 (see
the blue area), whereas in analogous zone of 15, there is no trace
of covered field as its para position is vacant (Figure 9).
+
+
coordination sphere, causing a diminished %Vbur value.
Catalytic applications of NHC-MX complexes
Our previous study on Au(I) erNHC complexes, shed light upon
the strong effect that the steric distribution of the erNHC complex
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Chemistry - A European Journal
10.1002/chem.202000600
FULL PAPER
exerts on the catalyst performance.[14] For example, we found that
catalytic efficiency (up to 93% yield, 2.5 mol% loading).[34] The
nature of the used NHC ligand is also critical. The replacement of
the semi rigid biphenyl moiety in (9-Dipp) ligand by a yet flexible
diphenyl ether moiety in (10-Dipp) ligand led to a diminished
performance at low loadings (0.50 mol%): the coupling catalyzed
by 12 gave only 34% product after 2 days.
(
9-Mes)AuCl (%Vbur = 43.6, α = 39.3°), promoted the cyclization
of a propargylamide in 60% yield (0.025 mol% catalyst loading,
0 °C, 24 h), while (9-Dipp)AuCl (%Vbur = 53.0, α = 48.2°) was not
5
that effective (6% yield) under same reaction conditions. We
chose, namely (9-Dipp)CuBr 8 (%Vbur = 57.5), because it is the
bulkiest NHC Cu(I) complex, in combination with its well-
distributed steric demand (α = 28.4°) as discussed above. To
further compliment this preliminary catalytic study, we selected
the 10-erNHC complex 12 as it also bears the bulky 9-Dipp ligand.
The coupling between N-tosylhydrazones and terminal alkynes
for the synthesis of Benzofuranswas investigated as test reaction
We next set about assessing the reach of the catalytic activity of
novel erNHC-AuBr complex 16 in the cycloisomerization of N-
propargylamide I (Scheme 4, top).[ The reaction was promoted
35]
2
by the gold pre-catalyst (2 mol% loading), and AgNTf as bromide
scavenger. Oxazoline II was obtained (93% yield) as the only
product after 8 h, conducting the reaction in chloroform at room
temperature. A brief survey on the existing literature reveals only
a few examples of gold(I) bromide complexes that spans from six-
to eight-membered NHC ligands, including their structural and
catalytic features.[8d,36]
[
34]
(
Table 3).
Table 3. Synthesis of Benzofuran 3a by coupling between 2-Hydroxy-N-
[
a]
Tosylhydrazone and Phenylacetylene using 8 and 12 as catalysts
Au catalysis
16 (2 mol%)
93%
AgNTf
CHCl , r.t, 8 h.
2
(2 mol%)
O
O
3
N
N
H
8
(5 mol%)
I
71%
II
Entry
[Cu] cat.
[Cu] mol%[b]
Time(h)
Yield(%)[c]
AgBF
4
(5 mol%)
1
2
3
4
5
6
7
8
8
12
8
2.5
12
12
12
12
24
24
24
48
93[d]
88[d]
68[d]
41
CHCl , 50°C, 12 h.
3
2.5
Cu catalysis
1.25
0.50
0.25
0.05
1.25
0.50
8
Scheme 4. Gold(I)- or copper(I)-catalyzed synthesis of oxazoline II. Isolated
yields are presented.
8
22
8
5
12
12
76[d]
34[d]
In addition to employing gold pre-catalyst 16, the use of complex
8, as a source of the cationic Cu(I) species, was also investigated
Scheme 4, bottom).[37] By using 5 mol% of complex 8 and AgBF
as bromide abstractor, the reaction afforded 71% of the expected
product II after 12 h under heating conditions. A summary of
screening experiments for this reaction can be found in Table S1
(
4
[
(
a] Reaction conditions: N-tosylhydrazone (0.10 mmol), phenylacetylene
0.125 mmol), Cs CO (0.3 mmol), CH CN (2.0 mL), 100°C, sealed tube. [b]
Based on Cu content and respect to the alkyne. [c] Yields determined by
GC-MS using dodecane as internal standard. [d] Isolated yield after column
chromatography.
2
3
3
(
4
see Supporting Information). The use 8 and AgBF separately,
yielded no product under similar conditions, a fact that we
+
interpret as evidence of the cationic moiety (9-Dipp)Cu , either
-
solvated or in contact to BF
4
, as the active catalytic species.
At 2.5 mol% catalyst loading, both complexes 8 (93% yield) and
2 (88% yield), proved capable of bringing about the coupling
Arguably, activity of Cu(I) complex 8, is likely due to the large
1
steric environment imparted by the 9-Dipp ligand around the metal
center, that is able to stabilize the erNHC-Cu species upon halide
between a N-tosylhydrazone and phenylacetylene to form
benzofuran 3a after 12 h (Table 3, Entries 1 and 2). Lowering
+
abstraction, so that the cycloisomerization of I is catalyzed by a π-
(1.25 mol%) the catalyst loading, complex 8 afforded 68% yield
acidic copper cation and favored by i) the presence of a weakly
(
Entry 3) and an acceptable 41% yield when a 0.5 mol% loading
-
(
BF
4
) coordinated anion and ii) the steric stabilization of a erNHC
was employed (Entry 4). At 0.25–0.05 mol% loadings (Entries 5
and 6), we observed a poor advance in the reaction (5–22% yield)
after 24 h. From the outset, Cu(I) catalyst 8 showed good result
at 0.25 mol% loading, forging the coupling (22% yield, Entry 5) in
spite of longer reaction time (24 h). In general, the contribution of
ligand with bulky (Dipp) substituents (complex 8 has the largest
calculated %Vbur (57.5%) of any NHC-Cu(I) reported to date).
the erNHC ligand to the stabilization of the active Cu(I) species is Conclusions
reflected by the ability of the catalysts to even operate at elevated
temperature (100°C), then complexes 8 and 12 proved their
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Chemistry - A European Journal
10.1002/chem.202000600
FULL PAPER
In summary, we succeeded in the synthesis and full
characterization of new Ag(I) and Cu(I) complexes with sterically
demanding expanded-ring NHC ligands. The steric profiling was
made in terms of two fundamental descriptors, the %Vbur which
revealed the highest value for any erNHC-stabilized complex
reported to date (8, Vbur = 57.5%), and the topographic steric
maps, that illuminated the real impact of the erNHC ligands in the
first coordination sphere of the metal center where the catalysis
take place. In contrast to the alky-chain-based erNHC ligands, our
analysis indicates that a limited twisting of the N-Aryls, is related
to the folded backbone ring and contributes to an improved spatial
arrangement of the NHC skeleton by pushing the ortho-
substituents of the N-Aryls into the metal’s coordination sphere.
Hence, the effective stabilization of the current Ag(I) and Cu(I)
erNHC complexes benefits from the joint effect of the steric
demanding nature of the bulky Dipp substituents and the semi-
rigid nature of the erNHC backbone.
[6]
Selected literature about NHC complexes of Ag(I), Cu(I) and Au(I) see:
a) M. M. Díaz-Requejo, P. J. Pérez, J. Organomet. Chem. 2005, 690,
5441–5450; b) I. J. B. Lin, C. S. Vasam, Coord. Chem. Rev. 2007, 251,
642–670; c) J. C. Y. Lin, R. T. W. Huang, C. S. Lee, A. Bhattacharyya,
W. S. Hwang, I. J. B. Lin, Chem. Rev. 2009, 109, 3561–3598; d) S. P.
Nolan, Acc. Chem. Res. 2011, 44, 91–100; e) S. Gaillard, C. S. J. Cazin,
S. P. Nolan, Acc. Chem. Res. 2012, 45, 778–787; f) L. Batiste, P. Chen,
J. Am. Chem. Soc. 2014, 136, 26, 9296–9307; g) A. Doddi, M. Peters, M.
Tamm, Chem. Rev. 2019, 119, 6994–7112; h) N. Marion, NHC-Copper,
Silver and Gold Complexes in Catalysis, in N-Heterocyclic Carbenes:
From Laboratory Curiosities to Efficient Synthetic Tools, (Ed.: S. Diez–
Gonzalez), RSC Publishing, Cambridge, 2010, ch. 11, pp. 317–336; i) T.
Wurm, A. M. Asiri, A. S. K. Hashmi, NHC-Au(I) Complexes: Synthesis,
Activation, and Application, in N-Heterocyclic Carbenes: Effective Tools
for Organometallic Synthesis, (Ed.: S. P. Nolan), Wiley-VCH, Weinheim,
2014, ch. 9, pp. 243–267; j) F. Lazreg, F. Nahra, C. S. J. Cazin, Coord.
Chem. Rev. 2015, 48, 293–294; k) A. S. K. Hashmi, C. Lothschütz, C.
Böhling, T. Hengst, C. Hubbert, F. Rominger, Adv. Synth. Catal. 2010,
352, 3001–3012; l) D. Riedel, T. Wurm, K. Graf, M. Rudolph, F. Rominger,
A. S. K. Hashmi, Adv. Synth. Catal. 2015, 357, 1515–1523; m) A. S. K.
Hashmi, C. Lothschütz, K. Graf, T. Häffner, A. Schuster, F. Rominger,
Adv. Synth. Catal. 2011, 353, 1407–1412; n) C. Lothschütz, T. Wurm, A.
Zeiler, A. Freiherr v. Falkenhausen, M. Rudolph, F. Rominger, A. S. K.
Hashmi, Chem. Asian J. 2016, 11, 342–346; o) A. S. K. Hashmi, T.
Hengst, C. Lothschütz, F. Rominger, Adv. Synth. Catal. 2010, 352,
The present work spotlights the first studies on Cu(I) erNHC
complexes as catalysts in coupling and cycloisomerization
reactions proving their high efficiency and resilience to heating.
The positive outcomes disclose their potential use in π-acid
catalysis.
1315–1337; p) A. S. K. Hashmi, D. Riedel, M. Rudolph, F. Rominger, T.
Oeser, Chem. Eur. J. 2012, 18, 3827–3830.
[
7]
a) L. Cavallo, A. Correa, C. Costabile. H. Jacobsen, J. Organomet. Chem.
Acknowledgements
2
005, 690, 5407–5413; b) S. Dıe
Rev. 2007, 251, 874–883; c) S. Würtz, F. Glorius, Acc. Chem. Res. 2008,
1, 1523–1533; d) H. Clavier, S. P. Nolan, Chem. Commun. 2010, 46,
́
z-González, S. P. Nolan, Coord. Chem.
A. C. R. is grateful for a Ph.D. fellowship from Consejo Nacional
de Ciencia y Tecnología (México).
4
841–861; e) A. Gómez-Suárez, D. J. Nelson, S. P. Nolan, Chem.
Commun. 2017, 53, 2650–2660; f) M. C. Jahnke, F. E. Hahn, Introduction
to N-Heterocyclic Carbenes: Synthesis and Stereoelectronic Parameters,
in N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient
Synthetic Tools, (Ed.: S. Diez-Gonzalez), RSC Publishing, Cambridge,
Keywords: silver • copper • carbene ligands • expanded-ring N-
heterocyclic carbenes • buried volume
2
010, ch. 1, pp. 1–41;
[
1]
2]
a) H. W. Wanzlick, Angew. Chem. Int. Ed. 1962, 1, 75–80; b) H. J,
Schönherr, H. W. Wanzlick, Angew. Chem. Int. Ed. 1968, 7, 141–142.
a) A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113,
[
[
8]
9]
a) W. Y. Lu, K. J. Cavell, J. S. Wixey, B. Kariuki, Organometallics 2011,
30, 5649–5655; b) J. Li, W.X. Shen, X.R. Li, Curr. Org. Chem. 2012, 16,
[
2879–2891; c) M. J. Page, W. Y. Lu, R. C. Poulten, E. Carter, A. G.
361–363; b) A. J. Arduengo, J. R. Goerlich, W. J. A. Marshall, J. Am.
Algarra, B. M. Kariuki, S. A. Macgregor, M. F. Mahon, K. J. Cavell, D. M.
Murphy, M. K. Whittlesey, Chem. Eur. J. 2013, 19, 2158–2167; d) A.
Kumar, D. Yuan, H. V. Huynh, Inorg. Chem. 2019, 58, 7545–7553.
a) J. J. Dunsford, K. J. Cavell, Dalton Trans. 2011, 40, 9131–9135; b) J.
J. Dunsford, D. S. Tromp, C. J. Elsevier, K. J. Cavell, B. M. Kariuki,
Dalton Trans. 2013, 42, 7318–7329; c) L. R. Collins, T. M. Rookes, M. F.
Mahon, I. M. Riddlestone, M. K. Whittlesey, Organometallics 2014, 33,
Chem. Soc. 1995, 117, 11027.
[3]
a) A. Igau, H. Grutzmacher, A. Baceiredo, G. Bertrand, J. Am. Chem.
Soc. 1988, 110, 6463–6466; b) A. Igau, A. Baceiredo, A. Trinquier, G.
Bertrand, Angew. Chem. Int. Ed. 1989, 28, 621–622; c) G. Gillette, A.
Baceiredo, G. Bertrand, Angew. Chem. Int. Ed. 1990, 29, 1429–1431. d)
R. Jazzar, H. Liang, B. Donnadieu, G. Bertrand, J. Organomet. Chem.
2006, 691, 3201–3205.
5882–5887; d) J. J. Dunsford, K. J. Cavell, Organometallics, 2014, 33,
[
[
4]
5]
a) W. A. Herrmann, M. Elison, J. Fisher, C. Köcher, G. R. Artus, Angew.
Chem. Int. Ed. 1995, 34, 2371–2374; b) W. A. Herrmann, Angew. Chem.
Int. Ed. 2002, 41, 1290–1309.
2
902–2905; e) N. Phillips, T. Dodson, R. Tirfoin, J. I. Bates, S. Aldrige.
Chem. Eur. J. 2014, 20, 16721–16731; f) A. R. Leverett, A. I. McKay, M.
L. Cole, Dalton Trans. 2015, 44, 498–500; g) J. J. Dunsford, D. J. Evans,
T. Pugh, S. N. Shah, N. F. Chilton, M. J. Ingleson, Organometallics 2016,
a) P. L. Arnold, Heteroatom Chem. 2002, 13, 534–539; b) V. César, S.
Bellemin-Laponnaz, L. H. Gade, Chem. Soc. Rev. 2004, 33, 619–636; c)
C. M. Crudden, D. P. Allen, Coord. Chem. Rev. 2004, 248, 2247–2273;
d) N. M. Scott, S. P. Nolan, Eur. J. Inorg. Chem. 2005, 1815–1828; e) M.
Iglesias, D. J. Beetstra, J. C. Knight, L. L. Ooi, A. Stasch, S. Coles, L.
Male, M. B. Hursthouse, K. J. Cavell, A. Dervisi, I. A. Fallis,
Organometallics 2008, 27, 3279–3289; f) S. Díez-González, N. Marion,
S. P. Nolan, Chem. Rev. 2009, 109, 3612–3676; g) D. J. Nelson, S. P.
Nolan, N-heterocyclic Carbenes in N-heterocyclic Carbenes: Effective
Tools for Organometallic Synthesis (Ed.: S. P. Nolan), Wiley-VCH,
Weinheim, 2014, ch 8, pp. 199–242. h) H. Yoshida, ACS Catal. 2016, 6,
35, 1098–1106; h) L. Banach, P. A. Guńkaa, W. Buchowicz, Dalton Trans.
2016, 45, 8688–8692; i) T. Wurm, F. Mulks, C. R. N. Böhling, D. Riedel,
P. Zargaran, M. Rudolph, F. Rominger, A. S. K. Hashmi, Organometallics
016, 35, 1070–1078; j) E. Ö. Karaca, M. Akkoç, M. N. Tahir, C. Arıcı, F.
İmik, N. Gürbüz, S. Yaşar, I. Özdemir, Tetttrahedron Lett. 2017, 58,
529–3532; k) E. Ö. Karaca, M. Akkoç, E. Öz, S. Altin, V. Dorcet, T.
2
3
Roisnel, N. Gürbüz, Ö. Çelik, A. Bayri, C. Bruneau, S. Yaşar, I. Özdemir,
J. Coord. Chem. 2017, 70, 1270–1284; l) K. R. Sampford, J. L. Carden,
E. B. Kidner, A. Berry, K. J. Cavell, D. M. Murphy, B. M. Kariuki, P. D.
Newman, Dalton Trans. 2019, 48, 1850–1858.
1799–1811: i) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius,
[10] a) M. Iglesias, D. J. Beetstra, A. Stasch, P. N. Horton, M. B. Hursthouse,
Nature 2014, 510, 485–496.
S. J. Coles, K. J. Cavell, A. Dervisi, I. A. Fallis, Organometallics 2007, 26,
4800–4809; b) C. J. E. Davies, M. J. Page, C. E. Ellul, M. F. Mahona, M.
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000600
FULL PAPER
K. Whittlesey, Chem. Commun. 2010, 46, 5151–5153; c) P. Hauwert, J.
J. Dunsford, D. S. Tromp, J. J. Weigand, M. Lutz, K. J. Cavell, C. J.
Elsevier, Organometallics 2013, 32, 131–140; d) Q. Teng, W. Wu, H. A.
Duong, H. V. Huynh. Chem. Commun. 2018, 54, 6044–6047.
Hermsen, J. Schießl, V. Mormul, A. S. K. Hashmi, T. Schaub, Org. Lett.
2019, 21, 1422–1425; f) G. Fang, X. Bi, Silver Complexes in Organic
Transformations, in Silver Catalysis in Organic Synthesis, (Eds.: C.-J. Li
and X. Bi), Wiley-VCH, Weinheim, 2018, ch. 11, pp. 661–722;
[
11] a) A. Binobaid, M. Iglesias, D. J. Beetstra, B. Kariuki, A. Dervisi, I. A.
Fallis, K. J. Cavell, Dalton Trans. 2009, 7099–7112; b) J. J. Dunsford, K.
J. Cavell, B. Kariuki, J. Organomet. Chem. 2011, 696, 188–194; c) E. L.
Kolychev, A. F. Asachenko, P. B. Dzhevakov, A. A. Bush, V. V. Shuntikov,
V. N. Khrustalevc, M. S. Nechaev, Dalton Trans. 2013, 42, 6859–6866;
d) O. S. Morozov, A. V. Lunchev, A. A. Bush, A. A. Tukov, A. F.
Asachenko, V. N. Khrustalev, S. S. Zalesskiy, V. P. Ananikov, M. S.
Nechaev, Chem. Eur. J. 2014, 20, 6162–6170; e) M. A. Topchiy, P. B.
Dzhevakov, M. S. Rubina, O. S. Morozov, A. F. Asachenko, M. S.
Nechaev, Eur. J. Org. Chem. 2016, 1908–1914.
[17] a) H. M. J. Wang, I. J. B. Lin, Organometallics 1998, 17, 972–975; b) U.
Hintermair, U. Englert, W. Leitner, Organometallics 2011, 30, 3726–
3731; c) H.-L. Su, L. M. Pérez, L. S.-J. Lee, J. H. Reibenspies, H. S.
Bazzi, D. E. Bergbreiter, Organometallics 2012, 31, 4063–4071; d) E.
Caytan, S. Roland, Organometallics 2014, 33, 2115–2118.
[18] a) W. A. Herrmann, S. K. Schneider, K. Ofele, M. Sakamoto, E.
Herdtweck. J. Organometallic Chem. 2004, 689, 2441–2449; b) E. L.
Kolychev, I. A. Portnyagin, V. V. Shuntikov, V. N. Khrustalev, M. S.
Nechaev. J. Organometallic Chem. 2009, 694, 2454–2462; c) A. Rit, T.
Pape, F.E. Hahn. J. Am. Chem. Soc. 2010, 132, 4572–4573; d) C. Topf,
C. Hirtenlehner, U. Monkowius. J. Organometallic Chem. 2011, 696,
3274–3278; e) A. Rit, T. Pape, A. Hepp, F. E. Hahn, Organometallics
2011, 30, 334–347.
[
12] a) L. Jafarpour, E. D. Stevens, S. P. Nolan, J. Organomet. Chem. 2000,
606, 49–54; b) G. Altenhoff, R. Goddard, C. W. Lehmann, F. Glorius, J.
Am. Chem. Soc. 2004, 126, 15195–15201; c) F. Lavallo, G. D. Frey, B.
Donnadieu, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2008,
[19] a) H. Kaur, F. K. Zinn, E. D. Stevens, S. P. Nolan, Organometallics 2004,
23, 1157–1160; b) S. Díez-González, A. Correa, L. Cavallo, S. P. Nolan,
Chem. Eur. J. 2006, 12, 7558–7564; c) S. Díez-González, E. D. Stevens,
N. M. Scott, J. L. Petersen, S. P. Nolan, Chem. Eur. J. 2008, 14, 158–
168; d) S. Díez-González, S. P. Nolan, Acc. Chem. Res. 2008, 41, 349–
358; e) O. Songis, P. Boulens, C. G. M. Benson, C. S. J. Cazin, RSC
Adv. 2012, 2, 11675–11677; f) B. R. M. Lake, C. E. Willans, Chem. Eur.
J. 2013, 19, 16780–16790; g) J. D. Egbert, C. S. J. Cazin, S. P. Nolan,
Catal. Sci. Technol. 2013, 3, 912–926; h) Y. D. Bidal, F. Lazreg, C. S. J.
Cazin, ACS Catal. 2014, 45, 1564–1569; i) M. Trose, F. Lazreg, M.
Lesieur, C. S. J. Cazin, J. Org. Chem. 2015, 80, 9910–9914; j) W. Xie, J.
H. Yoon, S. Chang, J. Am. Chem. Soc. 2016, 138, 12605−12614; k) J. I.
Urzua, R. Contreras, C. O. Salas, R. A. Tapia, RSC Adv. 2016, 6, 82401–
82408; l) H. Xu, Z. Sun, Adv. Synth. Catal. 2016, 358, 1736–1740; m) T.
Wakamatsu, K Nagao, H. Ohmiya, M. Sawamura, Organometallics 2016,
35, 1354–1357; n) M. Trose, F. Lazreg, T. Chang, F. Nahra, D. B. Cordes,
A. M. Z. Slawin, C. S. J. Cazin, ACS Catal. 2017, 7, 238–242; o) M. Trose,
F. Nahra, A. Poater, D. B. Cordes, A. M. Z. Slawin, L. Cavallo, C. S. J.
Cazin, ACS Catal. 2017, 7, 8176–8183; p) L.-J. Cheng, N. P. Mankad,
Angew. Chem. Int. Ed. 2018, 57, 10328–10332; q) L. Kuehn, M. Huang,
U. Radius, T. B. Marder, Org. Biomol. Chem. 2019, 17, 6601–6606.
[20] a) K. Matsumoto, N. Matsumoto, A. Ishii, T. Tsukuda, M. Hasegawab, T.
Tsubomura, Dalton Trans. 2009, 6795–6801; b) R. Marion, F. Sguerra,
F. Di-Meo, E. Sauvageot, J.-F. Lohier, R. Daniellou, J.-L. Renaud, M.
Linares, M. Hamel, S. Gaillard, Inorg. Chem. 2014, 53, 9181–9191; c) R.
Visbala, M. C. Gimeno, Chem. Soc. Rev. 2014, 43, 3551–3574; d) M.
Elie, F. Sguerra, F. Di-Meo, M. D. Weber, R. Marion, A. Grimault, J.-F.
Lohier, A. Stallivieri, A. Brosseau, R. B. Pansu, J.-L. Renaud, M, Linares,
M. Hamel, R. D. Costa, S. Gaillard, ACS Appl. Mater. Interfaces 2016, 8,
14678–14691; e) V. A. Krylova, P. I. Djurovich, M. T. Whited, M. E.
Thompson, Chem. Commun. 2010, 46, 6696–6698; f) F. Lazreg, C. S. J.
Cazin, NHC-Copper Complexes and their Applications in N-heterocyclic
Carbenes: Effective Tools for Organometallic Synthesis (Ed.: S. P.
Nolan), Wiley-VCH, Weinheim, 2014, ch 8, pp. 199–242.
4
7, 5224–5228; d) S. Würtz, C. Lohre, R. Fröhlich, K. Bergander, F.
Glorius, J. Am. Chem. Soc. 2009, 131, 8344–8345; e) S. G. Alexander,
M. L. Cole, J. C. Morris, New J. Chem. 2009, 33, 720–724; f) T. Dröge,
F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 6940–6952; g) S. G. Weber,
C. Loos, F. Rominger, B. F. Straub, Archivoc 2012, 3, 226–242; h) S.
Dierick, D. F. Dewez, I. E. Markó, Organometallics 2014, 33, 677–683; i)
M. W. Hussong, W. T. Hoffmeister, F. Rominger, B. F. Straub, Angew.
Chem. Int. Ed. 2015, 54, 10331–10335; j) P. Shaw, Alan R. Kennedya,
D. J. Nelson, Dalton Trans. 2016, 45, 11772–11780; k) Y. Tang, I.
Benaissa, M. Huynh, L. Vendier, N. Lugan, S. Bastin, P. Belmont, V.
César, V. Michelet, Angew. Chem. Int. Ed. 2019, 58, 7977–7981.
[
13] a) P. Bazinet, G. P. A. Yap, D. S. Richeson, J. Am. Chem. Soc. 2003,
125, 13314–13315; b) M. Mayr, K. Wurst, K. Ongania, M. R. Buchmeiser,
Chem. Eur. J. 2004, 10, 1256–1266; c) W. A. Herrmann, S. K. Schneider,
K. Öfele, M. Sakamoto, E. J. Herdtweck, J. Organomet. Chem. 2004, 689,
2
441–2449; d) C. C. Scarborough, M. J. W. Grady, I. A. Guzei, B. A.
Gandhi, E. E. Bunel, S. S. Stahl, Angew. Chem. Int. Ed. 2005, 44, 5269–
272; e) C. C. Scarborough, B. V. Popp, I. A. Guzei, S. S. Stahl, J.
5
Organomet. Chem. 2005, 690, 6143–6145; f) R. Jazzar, H. Liang, B.
Donnadieu, G. Bertrand, J. Organomet. Chem. 2006, 691, 3201–3205;
g) I. Özdemir, N. Gürbüz, Y. Gök, B. Çetinkaya, Heteroatom Chem. 2008,
19, 82–86; h) M. Iglesias, D. J. Beetstra, B. Kariuki, K. J. Cavell, A.
Dervisi, I. A. Fallis, Eur. J. Inorg. Chem. 2009, 1913–1919; i) J. J.
Dunsford, B. M. Kariuki, K. J. Cavell, Organometallics 2012, 31, 4118–
4121; j) L. Huang, Y. Cao, M. Zhao, Z. Tangb, Z. Sun, Org. Biomol. Chem.
2014, 12, 6554–6556; k) B.-M. Yang, K. Xiang, Y.-Q. Tu, S.-H. Zhang,
D.-T. Yang, S.-H. Wanga, F.-M. Zhanga, Chem. Commun. 2014, 50,
163–7165; l) S. Pelties, E. Carter, A. Folli, M. F. Mahon, D. M. Murphy,
7
M. K. Whittlesey, R. Wolf, Inorg. Chem. 2016, 55, 11006–11017; m) E.
Ö. Karaca, M. Akkoç, M. N. Tahir, C. Arıcı, F. İmik, N. Gürbüz, S. Yaşar,
İ. Özdemir, Tetrahedron Lett. 2017, 58, 3529–3532.
[
14] A. Cervantes-Reyes, F. Rominger, M. Rudolph, A. S. K. Hashmi, Chem.
Eur. J. 2019, 25, 11745–11757.
[
15] a) M. L. Teyssot, A. S. Jarrousse, M. Manin, A. Chevry, S. Roche, F.
Norre, C. Beaudoin, L. Morel, D. Boyer, R. Mahiou, A. Gautier, Dalton
Trans. 2009, 6894–6902; b) L. Mercs, M. Albrecht, Chem. Soc. Rev.
[21] a) E. L. Kolychev I. A. Portnyagin V. V.Shuntikov V. N. Khrustalev, M. S.
Nechaev, J. Organomet. Chem. 2009, 694, 2454–2462; b) A. J. Jordan,
C. M. Wyss, J. Bacsa, J. P. Sadighi, Organometallics 2016, 35, 613–616;
c) G. A. Chesnokov, M. A. Topchiy, P. B. Dzhevakov, P. S. Gribanov, A.
A. Tukov, V. N. Khrustalev, A. F. Asachenkob, N. S. Nechaev. Dalton
Trans. 2017, 46, 4331–4345; d) J. W. Hall, D. M. L. Unson, P. Brunel, L.
R. Collins, M. K. Cybulski, M. F. Mahon, M. K. Whittlesey,
Organometallics 2018, 37, 3102–3110; e) F. Sebest, J. J. Dunsford, M.
Adams, J. Pivot, P. D. Newman, S. Diez-Gonzalez, ChemCatChem 2018,
10, 2041–2045; f) M. A. Topchiy, A. A. Ageshina, P. S. Gribanov, S. M.
Masoud, T. R. Akmalov, S. E. Nefedov, S. N. Osipov, M. S. Nechaev, A.
F. Asachenko, Eur. J. Org. Chem. 2019, 1016–1020; g) J. W. Hall, F.
Seeberger, M. F. Mahon, M. K. Whittlesey, Organometallics 2020, 39, 1,
227–233
2010, 39, 1903–1912; c) S. Roland, C. Jolivalt, T. Cresteil, L . Eloy, P.
Bouhours, A. Hequet, V. Mansuy, C. Vanucci, J.-M. Paris, Chem. Eur. J.
2
011, 17, 1442–1446; d) L. Oehninger, R. Rubbiani, I. Ott, Dalton Trans.
013, 42, 3269–3284; e) R. Visbal, V. Fernández-Moreira, I. Marzo, A.
2
Laguna, M. C. Gimeno, Dalton Trans. 2015, 44, 11911–11918.
[
16] a) Y. B. Li, X. F. Chen, Y. Song, L. Fang, G. Zou, Dalton Trans. 2011, 40,
2046–2052; b) Z. Wang, J. Wen, Q. W. Bi, X. Q. Xu, Z. Q. Shen, X. X. Li,
Z. L. Chen, Tetrahedron Lett. 2014, 55, 2969–2972; c) K. Yamashita, S.
Hase, Y. Kayaki, T. Ikariya, Org. Lett. 2015, 17, 2334–2337; d) V. H. L.
Wong, S. V. C. Vummaleti, L. Cavallo, A. J. P. White, S. P. Nolan, K. K.
Hii, Chem. Eur. J. 2016, 22, 13320; e) S. Dabral, B. Bayarmagnai, M.
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[
22] Single-crystal X-ray structure analysis: CCDC 1980967 (1ba), 1980968
2), 1980969 (4), 1980970 (4a), 1980971 (6), 1980972 (7), 1980973 (8),
980974 (13), 1980975 (15), 1980976 (17) contain the supplementary
(
1
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
[31] A. Poater,F. Ragone, R. Mariz, R. Dorta, L. Cavallo, Chem. Eur. J. 2010,
16, 14348–14353.
[
23] a)J. C. Garrison, W. J. Youngs, Chem. Rev. 2005, 105, 3978–4008; b)
P. de Frémont, N. M. Scott, E. D. Stevens, T. Ramnial, O. C. Lightbody,
C. L. B. Macdonald, J. A. C. Clyburne, C. D. Abernethy, S. P. Nolan,
Organometallics 2005, 24, 6301–6309; c) L. Benhamou, E. Chardon, G.
Lavigne, S. Bellemin-Laponnaz, V. César, Chem. Rev. 2011, 111, 2705–
[32] a) A. Poater, B. Cosenza, A. Correa, S. Giudice, F. Ragone, V. Scarano,
L. Cavallo. Eur. J. Inor. Chem. 2009, 1759–1766; b) L. Falivene, R.
Credendino, A. Poater, A. Petta, L. Serra, R. Oliva, V. Scarano, L.
Cavallo, Organometallics 2016, 35, 2286–2293.
[33] a) F. Ragone, A. Poater, L. Cavallo, J. Am. Chem. Soc. 2010, 132, 4249–
4258; b) Hydrogen atoms were omitted for the calculation. For direct
comparison, listed parameters are identical to those of the NHC
examples reported in literature.
2733.
[24] H. V. Huynh in The Organometallic Chemistry of N-heterocyclic
Carbenes, 1st edition, John Wiley & Sons Ltd, Weinheim, 2017, chapters
3
and 11, pp. 52–90, and 171–218.
[34] L. Zhou, Y. Shi, Q. Xiao, Y. Liu, F. Ye, Y. Zhang, J. Wang, Org. Lett.
2011, 13, 968–971.
[
25] a) C. A. Citadelle, E. Le-Nouy, F. Bisaro, A. M. Z. Slawin, C. S. J. Cazin,
Dalton Trans. 2010, 39, 4489–4491; b) J. Chun, H. Sol-Lee, I. Gu-Jung,
S. Won-Lee, H. Jin-Kim, S. Uk-Son, Organometallics 2010, 29, 1518–
[35] a) J. P. Weyrauch, A. S. K. Hashmi, A. Schuster, T. Hengst, S. Schetter,
A. Littmann, M. Rudolph, M. Hamzic, J. Visus, F. Rominger, W. Frey, J.
W. Bats, Chem. Eur. J. 2010, 16, 956--963; compare also: b) A. S. K.
Hashmi, M. C. Blanco Jaimes, A. M. Schuster, F. Rominger, J. Org.
Chem. 2012, 77, 6394--6408; c) A. S. K. Hashmi, A. Littmann, Chem.
Asian J. 2012, 7, 1435--1442; d) A. S. K. Hashmi, M. Rudolph, S.
Schymura, J. Visus, W. Frey, Eur. J. Org. Chem. 2006, 4905--4909.
[36] A. Kumar, C. Singh, H. Tinnermann, H. V. Huynh, Organometallics 2020,
39, 172−181.
1521; c) O. Santoro, A. Collado, A. M. Z. Slawin, S. P. Nolan, C. S. J.
Cazin, Chem. Commun. 2013, 49, 10483–10485; d) C. Gibard, H.
Ibrahim, A. Gautier, F. Cisnetti, Organometallics 2013, 32, 4279–4283;
e) B. Liu, X. Ma, F. Wua, W. Chen, Dalton Trans. 2015, 44, 1836–1844;
f) B. Liu, Q. Xia, W. Chen, Angew. Chem. Int. Ed. 2009, 48, 5513–5516.
26] Commercially available CuBr was initially employed nonetheless, due to
its rapid oxidation in air, a low purity greenish solid prevailed, which
resulted in poor yields. To overcome this, we purify CuBr by preparing
[
[37] For cationic NHC-Cu(I) complexes as catalysts see: a) M. R. Fructos, P.
de Frémont, S. P. Nolan, M. M. Díaz-Requejo, P. J. Pérez,
Organometallics 2006, 25, 2237–2241; b) S. Díez-González, N. M. Scott,
S. P. Nolan, Organometallics 2006, 25, 2355–2358; c) J. Liu, R. Zhang,
S. Wang, W. Sun, C. Xia, Org. Lett. 2009, 11, 1321–1324; d) L.-J. Cheng,
C. J. Cordier, Angew. Chem. Int. Ed. 2015, 54, 13734–13738; e) I.
Nakamura, T. Jo, Y. Ishida, H. Tashiro, M. Terada, Org. Lett. 2017, 19,
3059–3062; f) Y. Ishida, I. Nakamura, M. Terada, J. Am. Chem. Soc.
2018, 140, 8629–8633.
2
the (SMe )CuBr complex and employed it as freshly crystallized material.
[
[
27] a) G. Berthon-Gelloz, M. A. Siegler, A. L. Spek, B. Tinant, J. N. H. Reekc,
I. E. Markó. Dalton Trans. 2010, 39, 1444–1446; b) A. Gómez-Suárez,
R. S. Ramón, O. Songis, A. M. Z. Slawin, C. S. J. Cazin, S. P. Nolan,
Organometallics 2011, 30, 5463–5470.
28] For the use of NHC-Cu(I) as ligand transfer reagents see: a) G.
Venkatachalam, M. Heckenroth, A. Neels, M. Albrecht, Helv. Chim. Acta
2
009, 92, 1034–1045; b) M. R. L. Furst, C. S. J. Cazin, Chem. Commun.
010, 46, 6924–6925; c) F. Nahra, A. Gómez-Herrera, C. S. J. Cazin,
2
Dalton Trans. 2017, 46, 628–631.
[
29] a) A. Poater, F. Ragone, S. Giudice, C. Costabile, R. Dorta, S. P. Nolan,
L. Cavallo, Organometallics 2008, 27, 2679–2681; b) H. Clavier, S. P.
Nolan. Chem. Commun. 2010, 46, 841–861.
[
30] The torsion angle (
atoms (vide infra), is a measurement of the spatial twist and the relative
position of
and R1 substituents, which point into the metal’s
α
°) between the planes, defined by the R–N···N–R1
R
coordination sphere. The narrower the angle, the closer are the
substituents that point into the metal center. See Refs. [5e, 10a, 14].
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Chemistry - A European Journal
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FULL PAPER
A. Cervantes-Reyes, F. Rominger,
A. S. K. Hashmi*
A unique family of Ag(I) and Cu(I)
complexes
possessing
sterically
demanding N-heterocyclic carbene
ligands is described. Their structural
and steric profiling reveals the highest
buried volumes (%Vbur up to 57.5) for
any expanded-ring backbone NHC
complex. For the first time, the Ag(I)
complexes have been employed as
ligand transfer reagents while the Cu(I)
complexes proved to be efficient
candidates for catalysis.
Page No. – Page No.
Highly Hindered Ag(I) and Cu(I) N-
Heterocyclic Carbene Complexes:
Synthesis, Structures and Steric
Parameters
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