Dinuclear gold(I)-N-heterocyclic carbene complexes: Synthesis, characterization, and catalytic application for hydrohydrazidation of terminal alkynes

Article abstract of DOI:10.1002/aoc.5942

Dinuclear gold(I)-N-heterocyclic carbene complexes were developed for the hydrohydrazidation of terminal alkynes. The gold(I)-N-heterocyclic carbene complexes 2a-2b were synthesized in good yields from silver complexes synthesized in situ, which in turn were obtained from the corresponding imidazolium salts with Ag₂O in dichloromethane as a solvent. The new air-stable gold(I)-NHC complexes, 2a-2b, were characterized using NMR spectroscopy, elemental analysis, infrared, and mass spectroscopy studies. The gold(I) complex 2a was characterized using X-ray crystallography. Bis-N-heterocyclic carbene–based gold(I) complexes 2a-2b exhibited excellent catalytic activities for hydrohydrazidation of terminal alkynes yielding acylhydrazone derivatives. The working catalytic system can be used in gram-scale synthesis. In addition, the catalytic reaction mechanism of the hydrohydrazidation of terminal alkynes by gold(I)-NHC complex was studied in detail using density functional theory.

Full text of DOI:10.1002/aoc.5942

Received: 15 May 2020
DOI: 10.1002/aoc.5942
Revised: 2 July 2020
Accepted: 5 July 2020
F U L L P A P E R
Dinuclear gold(I)-N-heterocyclic carbene complexes:
Synthesis, characterization, and catalytic application
for hydrohydrazidation of terminal alkynes
Seema Yadav1 | Sriparna Ray2 | Ajeet Singh3 | Shaikh M. Mobin3
Tapta Kanchan Roy4 | Chandrakanta Dash1
|
1Department of Chemistry, School of
Dinuclear gold(I)-N-heterocyclic carbene complexes were developed for the
hydrohydrazidation of terminal alkynes. The gold(I)-N-heterocyclic carbene
complexes 2a-2b were synthesized in good yields from silver complexes
synthesized in situ, which in turn were obtained from the corresponding
imidazolium salts with Ag2O in dichloromethane as a solvent. The new air-
stable gold(I)-NHC complexes, 2a-2b, were characterized using NMR spectros-
copy, elemental analysis, infrared, and mass spectroscopy studies. The gold(I)
complex 2a was characterized using X-ray crystallography. Bis-N-heterocyclic
carbene–based gold(I) complexes 2a-2b exhibited excellent catalytic activities
for hydrohydrazidation of terminal alkynes yielding acylhydrazone derivatives.
The working catalytic system can be used in gram-scale synthesis. In
addition, the catalytic reaction mechanism of the hydrohydrazidation of
terminal alkynes by gold(I)-NHC complex was studied in detail using density
functional theory.
Chemical Sciences and Pharmacy, Central
University of Rajasthan, Bandar Sindri,
Ajmer, Rajasthan, 305817, India
2Department of Chemistry, School of
Basic Sciences, Manipal University Jaipur,
Jaipur, Rajasthan, 303007, India
3Department of Chemistry, Indian
Institute of Technology (IIT) Indore,
Simrol, Khandwa Road, Indore, 453552,
India
4Department of Chemistry and Chemical
Sciences, Central University of Jammu,
Jammu, 180001, India
Correspondence
Tapta Kanchan Roy, Department of
Chemistry and Chemical Sciences,
Central University of Jammu, Jammu
180001, India.
K E Y W O R D S
Email: tapta.che@cujammu.ac.in
DFT study, gold, hydrohydrazidation, mechanism, N-heterocyclic carbene
Chandrakanta Dash, Department of
Chemistry, School of Chemical Sciences
and Pharmacy, Central University of
Rajasthan, Bandar Sindri, Ajmer 305817,
Rajasthan, India.
Email: ckdash@curaj.ac.in
Funding information
DST-FIST, Grant/Award Number:
SR/FST/CSI-257/2014(c); DST-SERB, New
Delhi, Grant/Award Number:
YSS/2014/000054
1 | INTRODUCTION
molecules using gold catalysts supported by ancillary
ligands such as phosphine and N-heterocyclic carbenes
Gold-catalyzed organic syntheses have attracted much
attention as it has wide applications in designing novel
building blocks for many biologically active compounds
and pharmaceuticals.[1] The synthesis of organic
(NHCs) can be performed under mild reaction condi-
tions. The addition of nucleophiles such as amine, hydra-
zine, hydrazide, and semicarbazide to alkynes is
considered as 100% atom economical process for the
Appl Organomet Chem. 2020;e5942.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 14
https://doi.org/10.1002/aoc.5942

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YADAV ET AL.
synthesis of C–N bonded framework.[2] Various research
groups have already described the importance of gold(I)-
NHC catalysts for these processes.[1k,m,2d,e,g,3] The cata-
lytic activities of various dinuclear gold complexes have
also been investigated, and these complexes have shown
excellent catalytic activity compared to mononuclear
complexes due to its cooperativity effects.[4] As a part of
our study on designing new transition metal–based
catalysts for C–C and C-heteroatom bond-forming
reactions,[5] we have reported a series of well-defined pal-
ladium catalysts supported by bis-N-heterocyclic carbene
ligands for C–H bond arylation of benzothiazole and
domino Sonogahsira coupling/cyclization reaction.[6]
Based on an earlier work on catalytic performances of
dinuclear complexes, analogs of gold(I) complexes of the
bis-N-heterocyclic carbene ligands have been synthesized,
and their catalytic activity in C-heteroatom bond-forming
reactions has been studied.
efficiently catalyze the synthesis of azylhydrazone deriva-
tives. We carried out the DFT study to understand the
catalytic reaction mechanism of the hydrohydrazidation
of alkynes by [(NHC)Au]+.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis and characterization
NHC-based gold(I) complexes 2a and 2b were synthe-
sized in 79% and 85% yields, respectively, from the in
situ-generated silver complexes (reaction of imidazolium
salts with Ag2O) followed by transmetallation using
gold(I) precursor (SMe2)AuCl (Scheme 1). The gold(I)
complexes 2a and 2b are stable in air and moisture and
could be stored for several months without undergoing
any decomposition.
Keto-N-acylhydrazones are used as synthetic interme-
diates for a variety of medicinal compounds.[7] Due to its
significance in therapeutic uses, the development of highly
efficient methods remains a crucial challenge in the area.
Lewis acid-mediated processes, particularly gold-based
catalysis involving the activation of alkynes followed by
the addition of nucleophiles, are becoming increasingly
popular. Studies on the gold(I)-catalyzed addition of
hydrazides to alkynes for the synthesis of acylhydrazone
are limited. Recently, Kukushkin and coworkers
developed a phosphine-based gold(I) catalyst for hydro-
hydrazidation of terminal alkynes.[8] The catalyst showed
good catalytic activity with 6 mol% catalyst loading under
mild conditions. Phosphine ligands were replaced by
N-heterocyclic carbenes in many transition metal–
catalyzed processes (due to their air and moisture stability
and high thermal stability),[9] and thus we became inter-
ested in exploring the catalytic activity of our synthesized
dinuclear gold(I)-N-heterocyclic carbene (NHC) com-
plexes for hydrohydrazidation of terminal alkynes.
The gold(I) complexes 2a and 2b were characterized
usingvariousspectroscopictechniquessuchas1HNMR,13C
NMR,high-resolutionmassspectrometry(HRMS),infrared
spectroscopy, and elemental analysis studies. The 13C{1H}
NMR spectra of 2a and 2b showed a peak at around
171.4 ppm (2a) and 172.6 (2b), which corresponded to the
Au–Ccarbene peak, similar to the corresponding signal of
[1-(benzyl)-3-(2,4,6-trimethylphenyl)imidazol-2-ylidene]
AuCl[10] (δ 172.1 ppm). The gold(I) complex 2a was
structurally characterized using X-ray crystallography
(Figure 2). The complex 2a showed linear geometry with
two coordinated gold(I) atoms having a bond angle
ꢀ180o. The average distances of Au–Ccarbene and Au–Cl
bonds were 1.964 and 2.281 Å, respectively. The distance
between two gold(I) atoms was 7.226 Å. Despite all our
efforts to grow the crystals in various solvents, we could
not obtain suitable crystals of 2b for X-ray crystallo-
graphic study.
Herein, we report the synthesis, characterization, and
catalytic activity of bimetallic gold(I)-NHC complexes 2a
and 2b (Figure 1). Our dinuclear gold(I)-NHC catalysts
2.2 | Hydrohydrazidation of terminal
alkynes
Gold(I) catalysts 2a and 2b were used for the synthesis of
acylhydrazone derivatives, which are essential building
FIGURE 1 Dinuclear gold(I) complexes of N-heterocyclic
SCHEME 1 Synthesis of dinuclear gold(I)-N-heterocyclic
carbene ligands
carbene complexes

YADAV ET AL.
3 of 14
blocks of many biologically active compounds. Initially,
the substrates benzohydrazide and phenylacetylene were
used to evaluate the optimum reaction conditions for
gold(I)-NHC-catalyzed hydrohydrazidation of terminal
alkynes. We first screened with various solvents,
temperature conditions, and reaction time using 2a as a
catalyst in addition to the control experiments. Common
solvents such as CHCl3, DCE, CH3CN, EtOH, DMF,
1,4-dioxane, and PhCl were used (Table 1, entries 1–7)
for the hydrohydrazidation reaction. The results showed
that chlorobenzene is the best solvent and produced the
highest product yield, owing to its solubility, the stabili-
zation of the catalytic intermediate, and so on. The effect
of phosphine-based catalyst was also studied. Phosphine-
based gold(I) precatalyst (PPh3)AuCl did not show higher
catalytic activity compared to gold(I)-NHC complexes 2a
and 2b either in the same reaction conditions or at low
temperature (Table 1, entries 13–14).
FIGURE 2 Molecular structure of 2a. Ellipsoids are shown at
30% probability level. Selected bond lengths (Å) and angles (deg):
Au1–C1 1.963(9), Au1–Cl1 2.278(3), Au2–C2 1.965(9), Au2–Cl2
2.283(3); C1–Au1–Cl1 178.9(3), N2–C1–N3 103.3(8), C2–Au2–Cl2
178.4(3), N4–Au2–N5 103.7(8)
The reaction of benzohydrazide with phenylacetylene
ꢁ
in chlorobenzene at 95 C in the presence of 2 mol% of
TABLE 1 Optimization of reaction conditions for hydrohydrazidation of terminal alkynesa
Entry
1.
Catalyst loading (mol%)
2a (2.0)
Additive (mol%)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (2.0)
AgSbF6 (2.0)
AgSbF6 (4.0)
AgSbF6 (2.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
AgSbF6 (4.0)
–
Solvent
CHCl3
DCE
Yield (%) b
Trace
26
2.
2a (2.0)
3.
2a (2.0)
CH3CN
EtOH
DMF
13
4.
2a (2.0)
37
5.
2a (2.0)
Trace
80
6.
2a (2.0)
1,4-Dioxane
PhCl
7.
2a (2.0)
>99
34
8.
2a (1.0)
PhCl
9.
SIPrAuCl (2.0)
SIPrAuCl (4.0)
IMeAuCl (2.0)
IMeAuCl (4.0)
Ph3PAuCl (4.0)
Ph3PAuCl (4.0)
2a (2.0)
PhCl
40
10.
11.
12.
13.
14.
15.
16.
17.
PhCl
90
PhCl
38
PhCl
80
PhCl
14
PhCl
22[c]
32[c]
trace
trace
PhCl
2a (2.0)
PhCl
–
AgSbF6 (4.0)
PhCl
aReaction conditions: benzohydrazide (1.00 mmol), phenylacetylene (1.20 mmol), and solvent (2 mL).
bThe yields (%) were determined by gas chromatography using 1,3,5-trimethoxybenzene as an internal standard.
cReactions were performed at 60 ꢁC.

4 of 14
YADAV ET AL.
TABLE 2 Hydrohydrazidation of terminal alkynes by 2a and 2ba
Yield (%)b
S.No.
Alkyne
Benzohydrazide
Product
3aa
2a
2b
1.
97
96
2.
3.
4.
5.
3ab
3 ac
3ad
3ae
97
76
76
73
99
68
87
69
6.
7.
8.
3af
67
76
70
62
66
71
3ag
3ah

YADAV ET AL.
5 of 14
TABLE 2 (Continued)
Yield (%)b
2a
S.No.
Alkyne
Benzohydrazide
Product
2b
9.
3ai
3aj
67
69
59
64
10.
11.
12.
13.
14.
3ak
3al
68
61
66
37
63
63
65
31
3am
3an
aReaction conditions: benzohydrazide (1.00 mmol), alkyne (1.20 mmol), 2.0 mol% of 2a or 2b, AgSbF6 (4.0 mol%), chlorobenzene (2 ml),
95 ꢁC, 6 h.
bYields refer to isolated products.

6 of 14
YADAV ET AL.
gold(I)-NHC/4 mol% AgSbF6 resulted in a complete con-
version in 6 h in the air (Table 1, entry 7). No
acylhydrazone product was observed when the 2a pre-
catalyst was used without the addition of AgSbF6
(Table 1, entry 16).
9–12). We observed that the reaction yield obtained from
the 2 mol% of bimetallic catalyst loading of 2a matched
up with the double catalyst loading (4 mol%) of monome-
tallic complex (SIPr)AuCl. This observation concludes
that the monometallic and bimetallic gold(I) complexes
are equally active, which ruled out the possibility of coop-
erativity effect of bimetallic catalysts 2a and 2b. The two
sites of bimetallic gold(I) complexes performed indepen-
dently as two separate catalytic cycles in hydro-
hydrazidation reactions.
By using the optimized condition, we explored
the various alkyne and benzohydrazide substartes.
Benzohydrazide substrates, including the electron-
donating (4-OMeC6H4) and electron-withdrawing group
(4-NO2C6H4), were tested with several terminal alkynes
and resulted in moderate to excellent yields (isolated
yield: 59–99%) of acylhydrazone derivatives (Table 2).
The functional groups –OMe, –NO2, and –OH were
tolerated on both alkynes and hydrazides under the cat-
alytic conditions. The reaction of propargyl alcohol with
benzohydrazide synthesized the corresponding product
in moderate yield (31–37%), whereas the internal
alkyne such as diphenylacetylene showed less product
conversion up to 12%. Other aliphatic alkynes such as
ethylpropiolate and 1-hexyne yielded hydrazide
products in traces. The acylhydrazone product 3aa was
also synthesized in gram-scale in excellent yield, which
shows that this catalytic system can be used in
large-scale syntheses. It is important to note that in
most of the cases, acylhydrazones were obtained as
(E)-isomers.
2.3 | Mechanistic study
The reaction mechanism of gold(I)-catalyzed nucleophilic
addition to alkynes has been studied extensively by
several groups.[2e,12] The detailed reaction mechanism of
gold(I)-catalyzed reaction of hydrazides with alkynes was
not reported so far. To gain insight into the hydro-
hydrazidation of terminal alkynes by gold(I)-NHC cata-
lyst, calculations based on density functional theory
(DFT) were performed to evaluate the catalyst mode of
action during the process. We have already shown experi-
mentally that the two sites of bimetallic gold(I) com-
plexes 2a and 2b performed independently as two
separate catalytic cycles. Therefore, we used the gold(I)-
NHC complex, [1,3-dimethylimidazol-2-ylidene]AuCl, as
a precatalyst, phenylacetylene, and benzohydrazide fol-
lowing the original reaction. Based on the earlier
reports,[2e,j,3a,12g,13] we proposed a mechanism of hydro-
hydrazidation of terminal alkynes, which involves the
formation of gold(I)-activated alkyne complex (A) from
the reaction of NHCAuCl, AgSbF6, and phenylacetylene
(Scheme 2). Complex A further reacts with benzohy-
drazide (A') to form the intermediate B, which undergoes
a nucleophilic attack of hydrazide nitrogen atom at the
alkyne carbon, resulting in C. The intermediate C further
undergoes a proton transfer process to form a gold(I)-
bound alkene species D. The species D yields the
enamine E along with the active species A. Finally, the
tautomerization of E results in the desired product
F (Scheme 2).
The gold(I) complexes 2a and 2b are almost equally
active in hydrohydrazidation reactions. The thermal sta-
bility of gold(I)-NHC precatalysts (2a and 2b) was
examined using thermo-gravimetric analysis (TGA). TGA
data suggested the gold(I)-NHC catalysts are stable in the
ꢁ
catalysis temperature at 95 C (Supporting Information
Figure S11).
The synthesis of acylhydrazone derivatives by the
Ph3PAuNTf2 catalyst was recently reported by Kukushkin
et al. with catalyst loaꢁding of 6 mol%.[8] The reactions
were performed at 60 C, but due to its instability, the
catalyst was inactive at higher temperatures as well as
during longer reaction time periods. The catalysts 2a and
2b are highly active and have broader substrate scope at
2 mol% of catalyst loading in 6-h reaction time compared
to Ph3PAuNTf2. Further, a mercury poisoning experi-
ment was performed to investigate the homogeneous
nature of the gold(I) catalyst.[11] The metallic mercury
did not suppress the catalytic activity, which indicates
homogeneous catalysis.
To study the cooperativity effect of bimetallic
gold(I)-NHC complexes 2a and 2b, we used the
monometallic gold(I)-NHC complexes, that is (SIPr)AuCl
(SIPr = 1,3-bis(2,6-di-iso-propylphenyl)-4,5-dihydroimi-
dazol-2-ylidene) and (IMe)AuCl (IMe =1,3-dimethy-
limidazol-2-ylidene) as catalysts for hydrohydrazidation
reactions under the same reaction conditions (entries
Initially, a systematic conformational search was per-
formed for all the reactants, reaction intermediates, tran-
sition states, and products using DFT-based hybrid
B3LYP functional[14] with 6-31G(d) basis sets except for
Au for which LanL2DZ basis with pseudo potential was
used.[15] The lowest energy minima obtained from con-
formational search and transition states for each struc-
ture were further optimized at higher 6-31G(d,p) basis
sets (for Au LanL2DZ with pseudo potential was used)
with tight convergence criteria and “ultrafine” numerical
grids followed by harmonic frequency calculations to

YADAV ET AL.
7 of 14
SCHEME 2 Proposed mechanism for the
hydrohydrazidation of terminal alkynes
FIGURE 3 Density functional theory computed energy (kcal mol−1) profile for hydrohydrazidation of phenylacetylene by a gold(I)-N
heterocyclic carbene catalyst

8 of 14
YADAV ET AL.
assign each structure as a true minimum or maximum.
The transition state was determined with an imaginary
vibrational frequency corresponding to the reaction
coordinate. To calculate the energy profile, vibrational
zero-point energy was considered for all the cases. All the
theoretical calculations were performed using the Gauss-
ian16 package.[16] All the coordinates are given in
Supporting Information Table S4-S15.
exothermic by ꢀ30.32 kcal mol−1. Note that the net
energy lost due to this TS from the reaction intermedi-
ate B is only ꢀ16.56 kcal mol−1. This, along with the
high exothermic nature of this step, supports the forma-
tion of D' in the given experimental conditions. The
characteristic imaginary frequency of i525 cm−1 repre-
sents an excellent displacement vector in the direction
of hydrogen atom transfer from hydrazide nitrogen to
alkyne carbon via a water molecule. If this alternate
low-energy pathway had not been explored, the high
energy barrier without the water molecule would have
made the proton transfer step not viable considering
the reaction conditions. As the reaction barrier for a
four-membered ring is prohibitively high without water
and following the actual experimental conditions, the
investigation of the reaction path with an explicit water
molecule is therefore rationalized. The use of explicit
solvent reduced the barrier height significantly due to
the formation of a six-membered ring in place of the
four-membered ring, that reduced the ring strain and
assisted the smooth hydrogen shuttle motion following
As shown in Scheme 2, our proposed mechanism
starts with the reaction of cationic gold(I)-NHC-
phenylacetylene complex (A) with benzohydrazide (A')
to form the alkyne-benzohydrazide-bound cationic
intermediate species B. Due to the interaction of
A with benzohydrazide, the reactive intermediate is sta-
bilized by 3.7 kcal mol−1 (Figure 3). Next, the hydrazide
and alkyne-bound cationic intermediate species
B undergo a nucleophilic attack of the coordinated
hydrazide nitrogen at the alkyne carbon, resulting in
the C−N bond formation. During the course of reac-
tion, subsequent changes in the Au–C bond distances
with two different carbons of alkynes are observed. It
has been found that the attacking carbon loses its inter-
actions with the Au by increasing the bond length from
2.62, 2.86, and 3.08 Å, respectively, with a subsequent
decrease in the C–Au bond length of the other carbon
as 2.18, 2.11, and 2.04 Å, respectively, from the reactant
via the transition state (TS) to product. The energy
barrier for this step displays a low activation barrier of
6.85 kcal mol–1, and the TS is characterized by an imag-
inary frequency of i53 cm−1 along the displacement vec-
tor in the direction of C−N bond formation. Such a low
energy barrier indicates that this step occurs rapidly.
The overall reaction is exothermic in nature and there-
a
six-membered ring and were known in the
literature.[14a,17] This observation is thus emphasized
the importance of explicit solvation in understanding
the mechanism of such reactions. In the next step of
the catalytic cycle, gold(I)-NHC catalyst disassociates
gradually from D" by breaking a strong bond with bond
length 2.19 Å between Au and terminal alkene, thus
forming a weak bond with the phenyl ring (bond length
2.40 Å) with an energy loss of 17.25 kcal mol−1. This
weak bond further breaks to form E along with a free
gold(I)-NHC precatalyst, which has higher energy
(28.18 kcal mol−1) overall. Due to the cleavage of strong
Au–C bond in this step, the energy loss becomes appar-
ent, which is immediately compensated by the forma-
tion of another NHC-gold(I)-phenylacetylene complex
A as the starting point of the catalytic reaction cycle.
The final step of the catalytic cycle involves the
tautomerization of E to form the final product F, with
an energy gain of 1.54 kcal mol−1 (Figure 3).
fore thermodynamically favorable by ꢀ5.92 kcal mol−1
.
In the next step, this species undergoes a proton trans-
fer process between hydrazide and alkyne carbon to
yield D via a four-membered TS. Before the TS, a rota-
tion between N–N bond takes place to attain the favor-
able conformation where both the structures, before
and after rotation, are found to have similar energies.
The activation energy barrier for the proton transfer
from its nearest reactant species is ꢀ43.1 kcal mol–1,
which should be high and motivated us to find an alter-
native lower energy path for this step. That indicates
the possibility of the presence of any other adventitious
molecule such as a solvent (i.e., H2O), the prospects of
which can reduce the energy barrier. We found that a
water-mediated proton relay process drops down the
activation barrier by half (ꢀ22.48 kcal mol−1) where
the TS is a six-membered ring for the proton shuttle
step (Supporting Information, Figure S40). This energy
barrier is followed by a large gain in the stabilization of
the species D', where the overall reaction is highly
3 | CONCLUSIONS
In summary, we synthesized the dinuclear gold(I)-N-
heterocyclic carbene complexes and studied their cata-
lytic activity in hydrohydrazidation of terminal alkynes.
The two gold(I)-NHC complexes were fully character-
ized using NMR, IR, HRMS, and elemental analysis
studies. One of the complexes was structurally charac-
terized using X-ray crystallography. The gold(I)-NHC
complexes efficiently (in terms of catalyst loading and
regioselectivity) catalyze the hydrohydrazidation of

YADAV ET AL.
9 of 14
terminal alkynes. Also, this catalytic process can be
even used for gram-scale synthesis. DFT calculations
explored the details of the reaction mechanism and
corresponding structural features such as the interac-
tions of gold with other atoms and the nature of bond-
breaking and bond-making processes during the course
of the reaction. The energy data reveal that the barrier
for the nucleophilic attack on the hydrazide and
alkyne-bound cationic intermediate was very low, and
the proton transfer process between hydrazide and
alkyne carbon was found highly adventitious with pro-
ton relay process using an explicit solvent like water.
Both the steps were found thermodynamically stable,
and the hydrogen transfer process was, in particular,
highly exothermic in nature. This work will further
illustrate the application of gold(I)-NHC catalysts in
organic synthesis, in particular for the synthesis of
acylhydrazone derivatives.
procedures against F2 utilizing SHELXTL (Version 6.10)
software.[19] Olex2 was also used for labeling and refin-
ing of crystal 2a. CCDC-1961139 (2a) contains the sup-
plementary crystallographic data for this paper. The data
can be obtained freely at www.ccdc.cam.ac.uk/conts/
retrieving.html.
4.2 | Materials
NHC ligand precursors,[20] (Ph3P)AuCl,[21] (SIPr)AuCl,[22]
(IMe)AuCl,[23] and Au (SMe2)Cl[24] were synthesized
using literature procedures. AgSbF6, phenylacetylene,
4-ethynyltoluene,
4-ethylphenylacetylene,
4-tert-but-
ylphenylacetylene, 4-ethynylanisole, propargyl alcohol,
diphenylacetyelene, benzohydrazide, 4-nitrobenzohydrazide,
and 4-methoxybenzohydrazide were purchased from com-
mercial suppliers and used as received.
-
4 | EXPERIMENTAL
4.1 | General procedures
4.3 | Syntheses
All reactions were carried out under an atmosphere of
nitrogen or in air. Solvents were purchased from com-
mercial sources. NMR spectra were recorded at 298 K on
a Bruker 500 MHz and JEOL 400 MHz NMR spectrome-
ter. The chemical shifts of proton and carbon are
reported in ppm and referenced using residual proton
(7.26 ppm) and carbon signals (77.16 ppm) of CDCl3 and
4.3.1 | Synthesis of [(bis-NHCMes)Au2Cl2]
complex (2a)
A mixture of imidazolium salt 1a (0.350 g, 0.578 mmol)
and Ag2O (0.161 g, 0.694 mmol) in dichloromethane
(30 mL) was stirred at room temperature for overnight.
The reaction mixture was filtered through Celite,
and Au (SMe2)Cl (0.341 g, 1.16 mmol) was added to it
under stirring. The reaction mixture was further stirred
for 10 h and filtered through Celite pad and washed
with dichloromethane (2 × 5 ml). The solvent was con-
centrated, and hexane was added. A white precipitate
obtained was washed with hexane. The residue was
residual proton (2.50 ppm) and carbon signals
[18]
(39.5 ppm) of DMSO-d6.
NMR annotations used:
br. = broad, d = doublet, m = multiplet, s = singlet,
t = triplet, sept = septet. Infrared spectra were recorded
on a Perkin Elmer, Spectrum 2 spectrometer operating
at 2 cm−1 spectral resolution. IR spectroscopic data were
collected using KBr pellets. Elemental analyses were per-
formed using a Thermo Quest FLASH 2000 SERIES
(CHNS) elemental analyzer. Mass data were collected
using the Micromass Q-TOF spectrometer. All catalytic
reactions were monitored on a Thermo Fisher Trace
1300 series GC system by Gas Chromatography with
Flame Ionization Detector (GC-FID) with a TG-IMS
column of 30 m length, 0.25 mm diameter, and 0.25 μm
film thickness. X-ray diffraction data for compounds 2a
were collected on a dual core Agilent Technologies
(Oxford Diffraction) Super Nova CCD system. Data were
collected at 293 K using graphite-monochromated CuKα
radiation (λ = 1.54184 Å). The data collection strategy
was interpreted by employing the CrysAlisPro software.
The structure was solved using intrinsic phasing
(SHELXT) and was refined by full-matrix least-squares
dried under vacuum to obtain
a white solid as
product 2a (0.492 g, 85% yield). 1H NMR (500 MHz,
CDCl3): δ 7.34 (s, 2H), 7.28–7.23 (m, 5H), 6.96 (s, 4H),
6.85 (s, 2H), 4.04 (t, 4H, JHH = 6.7 Hz, CH2), 3.88
(s, 2H, CH2Ph), 3.13 (t, 4H, JHH = 6.7 Hz, CH2), 2.33
(s, 6H, p-CH3C6H2), 2.02 (s, 12H, o-CH3C6H2). 13C{1H}
NMR (125 MHz, CDCl3): δ 171.4 (Au–Ccarbene), 139.6,
138.1, 134.7, 129.4, 129.0, 128.5, 127.5, 122.2, 121.4,
59.6, 54.4, 49.3, 21.1, 17.9 ppm. IR (KBr, pellet): ʋmax
3,121(m), 3,026(w), 2,920(s), 2,852(m), 1,607(w),
1,561(w), 1,488(s), 1,451(s), 1,418(s), 1,378(w), 1,240(m),
1,143(w), 1,029(w), 854(w), 738(s), 696(m), 584(w) cm−1
.
HRMS (ESI) calcd. For C35H41Au2ClN5 [M-Cl]+ m/z
+
960.2381;
found
960.2376.
Anal.
Calcd
for
C35H41Au2Cl2N5•0.5CH2Cl2: C, 41.04; H, 4.07; N, 6.74;
found: C, 41.16; H, 4.36; N, 5.59.

10 of 14
YADAV ET AL.
4.3.2 | Synthesis of [(bis-NHCDipp)Au2Cl2]
complex (2b)
Colorless solid, m.p. = 168–169.1 ꢁC. 1H NMR
(500 MHz, DMSO-d6): δ 10.76 (s, 1H, NH), 7.88 (br. s,
4H), 7.57–7.56 (m, 1H), 7.52–7.49 (m, 2H), 7.42 (s, 3H),
2.36 (s, 3H, CH3). 13C{1H} NMR (125 MHz, DMSO-d6): δ
164.0, 155.6, 138.1, 134.1, 131.5, 129.4, 128.3, 127.9, 126.4,
14.5 ppm.
A mixture of imidazolium salt 1b (0.100 g, 0.145 mmol)
and Ag2O (0.041 g, 0.176 mmol) in dichloromethane
(30 mL) was stirred at room temperature for overnight.
The reaction mixture was filtered through Celite, and Au
(SMe2)Cl (0.085 g, 0.290 mmol) was added to it. The reac-
tion mixture was further stirred for 10 h and filtered
through Celite pad and washed with dichloromethane (ꢀ2
× 5 mL). The solvent was concentrated to 3 ml, and hexane
was added to yield a white precipitate. The white solid was
further washed with hexane (ꢀ30 ml) and dried under
vacuum. The product 2b was obtained as a white solid
(0.123 g, 79% yield). 1H NMR (500 MHz, CDCl3): δ
7.48–7.47 (m, 2H), 7.35 (s, 2H), 7.27–7.21 (m, 9H), 6.88 (s,
2H), 4.42 (t, 4H, JHH = 6.7 Hz, CH2), 3.89 (s, 2H, CH2Ph),
3.13 (t, 4H, JHH = 6.7 Hz, CH2), 2.40 (sept, 4H, JHH = 7.0 Hz,
CH (CH3)2), 1.29 (d, 12H, JHH = 7.0 Hz, CH (CH3)2), 1.13
(d, 12H, JHH = 7.0 Hz, CH (CH3)2). 13C{1H} NMR
(125 MHz, CDCl3): δ 172.6 (Au–Ccarbene), 145.7, 138.1,
134.0, 130.6, 129.0, 128.4, 127.4, 124.2, 123.4, 121.1, 59.6,
54.3, 49.1, 28.4, 24.4, 24.3 ppm. IR (KBr, pellet): ʋmax
3,123(w), 2,961(s), 2,867(m), 1723(m), 1,663(m), 1,460(s),
4.4.2 | (E)-N0-(1-(p-Tolyl)ethylidene)
benzohydrazide (3ab)[8]
Colorless solid, m.p. = 168.3–169.0 ꢁC. 1H NMR
(500 MHz, DMSO-d6): δ 10.73 (s, 1H, NH), 7.87 (s, 2H),
7.75 (s, 2H), 7.57–7.50 (m, 3H), 7.23 (s, 2H), 2.33 (s, 6H).
13C{1H} NMR (125 MHz, DMSO-d6): δ 163.9, 155.8, 139.2,
135.3, 134.1, 129.0, 128.6, 128.3, 127.8, 126.4, 20.9,
14.5 ppm.
4.4.3 | (E)-N0-(1-(4-Ethylphenyl)
ethylidene)benzohydrazide (3ac)
1,361(w), 1,062(m), 804(w), 753(m), 698(w), 471(w) cm−1
.
HRMS (ESI) calcd. For C41H53Au2ClN5 [M-Cl]+ m/z
1044.3315; found 1044.3305. Anal. Calcd. for
C41H53Au2Cl2N5•0.3CH2Cl2: C, 44.76; H, 4.88; N, 6.31;
found: C, 44.38; H, 4.35; N, 6.73.
+
Colorless solid, m.p. = 163.0–164.6 ꢁC. 1H NMR
(500 MHz, DMSO-d6): δ 10.73 (s, 1H, NH), 7.87–7.77 (m,
4H), 7.57–7.50 (m, 3H), 7.27 (s, 2H), 2.63–2.62 (m, 2H),
2.33 (s, 3H), 1.21–1.17 (m, 3H) ppm. 13C{1H} NMR
(100 MHz, DMSO-d6): δ 164.7, 156.9, 146.1, 135.8, 134.1,
132.1, 128.9, 128.2, 127.9, 127.0, 28.4, 15.8, 15.0 ppm.
HRMS (ESI), m/z: [M + H]+ calcd. For C17H19N2O+:
267.1497; found 267.1495.
4.4 | General procedure for
hydrohydrazidation of alkynes
Gold(I) complex (2a or 2b, 2 mol%) and AgSbF6
(4 mol%) were added to a solution of benzohydrazide
(1.00 mmol) and alkynes (1.20 mmol) in 2 ml of chloro-
benzene in air. The reaction mixture was heated at
95 ꢁC for 6 h. After cooling the reaction mixture at
room temperature, chlorobenzene was removed under
vacuum distillation, and the crude mixture was purified
by column chromatography (Eluent EtOAc: petroleum
ether = 40:60) to obtain the keto-N-acylhydrazones
(3aa-3an).
4.4.4 | (E)-N0-(1-(4-Methoxyphenyl)
ethylidene)benzohydrazide (3ad)[8]
4.4.1 | (E)-N0-(1-Phenylethylidene)
Colorless solid, m.p. = 167.8–169.0 ꢁC. 1H NMR
(500 MHz, DMSO-d6): δ 10.6 (s, 1H, NH), 7.87–7.81 (m,
4H), 7.57–7.50 (m, 3H), 6.99–6.97 (m, 2H), 3.79 (s, 3H),
2.32 (s, 3H) ppm. 13C{1H} NMR (100 MHz, DMSO-d6): δ
163.8, 160.5, 156.0, 134.2, 132.6, 132.0, 131.5, 130.5, 128.4,
128.1, 127.8, 113.8, 55.3, 14.5 ppm.
benzohydrazide (3aa)[8]

YADAV ET AL.
11 of 14
4.4.5 | (E/Z)-4-Methoxy-N0-
(1-phenylethylidene)benzohydrazide
(3ae)[8,25]
6.80; N, 9.45; found: C, 72.76; H, 6.14; N, 9.82. HRMS
(ESI), m/z: [M + H]+ calcd. For C18H21N2O2+: 297.1603;
found 297.1603.
4.4.8 | (E)-N0-(1-(4-(t-butyl)phenyl)
ethylidene)-4-methoxybenzohydrazide
(3ah)
Colorless solid, m.p. = 169.0–170.9 ꢁC. 1H NMR
(500 MHz, DMSO-d6): described as a mixture of (E/Z)
isomers, a molar ratio ꢀ4:1, signals of minor isomer
marked with asterisk, δ 10.6 (s, 1H), 10.2* (s, 0.24 H),
7.90–7.87 (m, 2H), 7.82 (br.s, 2H), 7.41 (s, 3H),
7.04–7.03 (m, 2H), 3.82 (s, 3H), 2.35 (s, 3H) ppm.
13C{1H} NMR (125 MHz, DMSO-d6): δ 165.5*, 162.0,
138.2, 130.0, 129.4, 128.8, 128.4, 128.2, 126.4, 126.1,
113.8, 113.6, 55.5, 14.5 ppm.
Colorless solid, m.p. = 182.3–183.1 ꢁC. 1H NMR
(500 MHz, DMSO-d6):10.5 (s, 1H), 7.87 (d, 2H,
JHH = 8.5 Hz), 7.74 (br. s, 2H), 7.43 (d, 2H, JHH = 8.5 Hz),
7.03 (d, 2H, JHH = 8.5 Hz), 3.82 (s, 3H), 2.32 (s, 3H),
1.28(s, 3H) ppm. 13C{1H} NMR (125 MHz, DMSO-d6): δ
161.9, 152.1, 135.5, 129.8, 126.2, 125.2, 113.9, 113.6, 55.5,
34.5, 31.09, 14.4 ppm. HRMS (ESI), m/z: [M + H]+ calcd.
For C20H25N2O2+: 325.1916; found 325.1915.
4.4.6 | (E)-4-Methoxy-N0-(1-(p-tolyl)
ethylidene)benzohydrazide (3af)[26]
4.4.9 | (E/Z)-4-Methoxy-N0-
(1-(4-methoxyphenyl)ethylidene)
benzohydrazide (3ai)[26]
Colorless solid, m.p. = 171.6–172.2 ꢁC. 1H NMR
(500 MHz, DMSO-d6): δ 10.57 (s, 1H, NH), 7.88 (d, 2H,
JHH = 8.5 Hz), 7.73 (br. s, 2H), 7.23–7.22 (m, 2H),
7.04–7.02 (m, 2H), 3.82 (s, 3H), 2.32 (s, 6H). 13C{1H} NMR
(125 MHz, DMSO-d6): δ 161.8, 147.1, 139.0, 135.4, 131.3,
129.3, 128.9, 126.3, 126.1, 113.5, 55.4, 20.8, 14.3 ppm.
Anal. Calcd for C17H18N2O2: C, 72.32; H, 6.43; N, 9.92;
found: C, 71.97; H, 6.52; N, 8.27. HRMS (ESI), m/z:
[M + H]+ calcd. For C17H19N2O2+: 283.1447; found
283.1446.
Colorless solid, m.p. = 172.3–173.1 ꢁC. 1H NMR
(500 MHz, DMSO-d6): described as a mixture of (E/Z) iso-
mers, a molar ratio ꢀ3.7:1, signals of minor isomer mar-
ked with asterisk, δ 10.5 (s, 1H), 10.2* (s, 0.27 H),
7.91–7.86 (m, 2H), 7.79 (br.s, 2H), 7.04–7.02 (m, 2H),
6.98–6.96 (m, 2H), 3.82 (s, 3H), 3.79* (s, 2.3H), 2.31 (s,
3H) ppm. 13C{1H} NMR (100 MHz, DMSO-d6): δ 165.5,
162.0, 161.8, 160.4, 130.6, 129.7, 129.4, 127.9, 126.2, 124.8,
113.7, 55.4, 55.3, 14.4 ppm. HRMS (ESI), m/z: [M + H]+
calcd. For C17H19N2O3+: 299.1396; found 299.1399.
4.4.7 | (E)-N0-(1-(4-Ethylphenyl)
ethylidene)-4-methoxybenzohydrazide
(3ag)
4.4.10 | (E)-4-Nitro-N0-
(1-phenylethylidene)benzohydrazide
(3aj)[8]
Colorless solid, m.p. = 169.4–170.1 ꢁC. 1H NMR
(500 MHz, DMSO-d6): δ 10.5 (s, 1H, NH), 7.88 (d, 2H,
JHH = 8.5 Hz), 7.75 (br. s, 2H), 7.26–7.24 (m, 2H), 7.03 (d,
2H, JHH = 8.5 Hz), 3.82 (s, 3H), 2.62 (q, 2H, JHH = 7.5 Hz),
2.33 (s, 3H), 1.18 (t, 3H, JHH = 7.5 Hz) ppm. 13C{1H}
NMR (125 MHz, DMSO-d6): δ 161.8, 145.2, 135.7, 131.3,
129.8, 127.7, 126.4, 126.1, 113.8, 113.5, 55.4, 27.9, 15.4,
14.4 ppm. Anal. Calcd for C18H20N2O2: C, 72.95; H,
1
Pale-yellow solid, m.p. = 197.6–198.0 ꢁC. H NMR
(500 MHz, DMSO-d6): δ 11.0 (s, 1H), 8.35–8.33 (m, 2H),

12 of 14
YADAV ET AL.
8.13–8.12 (m, 2H), 7.87 (s, 2H), 7.44 (s, 3H), 2.39 (s, 3H)
ppm. 13C{1H} NMR (125 MHz, DMSO-d6): δ 162.5, 156.9,
149.1, 139.8, 137.8, 129.7, 129.4, 128.4, 126.5, 123.4,
14.8 ppm. HRMS (ESI), m/z: [M + H]+ calcd. For
C15H14N3O3+: 284.1035; found 284.1034.
8.16–8.10(m,2H),7.84–7.83(m,2H),7.01–6.99(m,2H),3.80
(s, 3H), 2.34 (s, 3H) ppm. 13C{1H} NMR (100 MHz, DMSO-
d6): δ 164.3, 160.7, 157.2, 149.5, 129.4, 129.1, 128.2, 123.9,
123.5, 113.8, 55.3, 14.7 ppm. HRMS (ESI), m/z: [M + H]+
calcd.ForC16H16N3O4+:314.1141;found314.1144.
4.4.11 | (E)-4-Nitro-N0-(1-(p-tolyl)
4.4.14 | (E)-N0-(1-Hydroxypropan-
2-ylidene)benzohydrazide (3an)
ethylidene)benzohydrazide (3ak)[27]
1
Pale-yellow solid, m.p. = 214.2–215.6 ꢁC. H NMR
Colorless solid, m.p. = 126.0–128.3 ꢁC. 1H NMR
(500 MHz, DMSO-d6): δ 10.51 (s, 1H, NH), 7.92 (s, 1H),
7.92–7.91 (m, 2H), 7.61–7.58 (m, 1H), 7.54–7.51 (m, 2H),
1.29 (s, 2H), 1.25 (s, 3H) ppm. 13C {1H} NMR (125 MHz,
CDCl3): δ 166.3, 133.0, 132.3, 129.0, 127.9, 31.6, 22.5 ppm.
(500 MHz, DMSO-d6):11.05 (s, 1H), 8.34–8.33 (m, 2H),
8.12–8.10 (m, 2H), 7.77–7.76 (m, 2H), 7.25–7.24 (m, 2H),
2.35–2.34 (m, 6H) ppm. 13C{1H} NMR (125 MHz, DMSO-
d6): δ 162.4, 157.1, 149.1, 139.9, 139.5, 135.1, 129.4, 129.0,
126.5, 123.4, 20.8, 14.8 ppm. HRMS (ESI), m/z: [M + H]+
calcd. For C16H16N3O3+: 298.1192; found 298.1190.
4.5 | Mercury poisoning experiment
4.4.12 | (E/Z)-N0-(1-(4-Ethylphenyl)
ethylidene)-4-nitrobenzohydrazide (3al)
Gold(I) complex (2a, 2 mol%) and AgSbF6 (4 mol%) were
added to a solution of benzohydrazide (1.00 mmol) and
phenylacetylene (1.20 mmol) in 2 ml of chlorobenzene.
Excess Hg(0) (500 times with respect to catalyst loading)
ꢁ
was added. The reaction mixture was heated at 95 C for
6 h and was cooled to room temperature. DMSO (2 ml)
was added to make a clear solution. The product was
analyzed using GC, and yield was determined based on
1,3,5-trimethoxybenzene as an internal standard. The
result showed no significant decrease in the product yield
under the same conditions.
1
Pale-yellow solid, m.p. = 220.3–221.8 ꢁC. H NMR
(500 MHz, DMSO-d6): described as a mixture of (E/Z) iso-
mers, a molar ratio ꢀ2.9:1, signals of minor isomer mar-
ked with asterisk, δ 11.04 (s, 1H), 11.0* (s, 0.34H),
8.38–8.37 (m, 1H), 8.34–8.32 (m, 2H), 8.16–8.15 (m, 1H),
8.12–8.11 (m, 2H), 7.79–7.78 (m, 2H), 7.28–7.27 (m, 2H),
2.64–2.63 (m, 3H), 2.36 (s, 3H), 1.21–1.18 (m, 5H) ppm.
13C{1H} NMR (100 MHz, DMSO-d6): δ162.4, 157.1, 149.1,
145.8, 135.3, 129.4, 129.1, 127.8, 126.7, 123.9, 123.5, 28.0,
15.4, 14.8 ppm. HRMS (ESI), m/z: [M + H]+ calcd. For
C17H18N3O3+: 312.1348; found 312.1345.
4.6 | Gram-scale synthesis of (E)-N0-
(1-phenylethylidene)benzohydrazide (3aa)
Gold(I) complex (2a, 2 mol%) and AgSbF6 (4 mol%) were
added to a solution of benzohydrazide (1.00 g, 7.34 mmol)
and phenylacetylene (0.899 g, 8.81 mmol) in 15 ml of chlo-
ꢁ
robenzene. The reaction mixture was heated at 95 C for
4.4.13 | (E)-N0-(1-(4-Methoxyphenyl)
ethylidene)-4-nitrobenzohydrazide
(3am)[27]
6 h and was cooled to room temperature. The solvent was
removed under vacuum distillation, and the crude mixture
was purified using column chromatography using ethyl
acetate/petroleum ether solvent system to obtain the
desired product 3aa as a colorless solid (1.57 g, 90% yield).
ACKNOWLEDGEMENTS
This work was supported by the DST-SERB, New Delhi
(Grant Number: YSS/2014/000054). S.Y. thanks Central
University of Rajasthan for the research fellowship. The
1
Pale-yellow solid, m.p. = 216.6–217.0 ꢁC. H NMR
(500 MHz, DMSO-d6):11.00 (s, 1H), 8.39–8.33 (m, 2H),

YADAV ET AL.
13 of 14
J. N. H. Reek, M. A. Siegler, J. I. van der Vlugt, Angew. Chem.
Int. Ed. 2016, 55, 10042; c) M. D. Levin, F. D. Toste, Angew.
Chem. Int. Ed. 2014, 53, 6211.
authors thank DST-FIST (Grant Number: SR/FST/CSI-
257/2014(c)) for providing instrumental facilities. The
authors acknowledge the IIT Bombay and Materials
Research Centre, MNIT, Jaipur for collecting IR and
HRMS data.
[5] a) S. Yadav, A. Singh, N. Rashid, M. Ghotia, T. K. Roy,
P. P. Ingole, S. Ray, S. M. Mobin, C. Dash, ChemistrySelect
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A. M. Z. Slawin, N. Shivran, New J. Chem. 2019, 43, 2381.
[6] S. Yadav, A. Singh, I. Mishra, S. Ray, S. M. Mobin, C. Dash,
Appl. Organomet. Chem. 2019, 33, e4936.
CONFLICTS OF INTEREST
The authors declare no competing financial interest.
[7] a) N. Gupta, M. Badeaux, Y. Liu, K. Naxerova, D. Sgroi,
L. L. Munn, R. K. Jain, I. Garkavtsev, Proc. Natl. Acad. Sci.
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ORCID
Shaikh M. Mobin https://orcid.org/0000-0002-2665-
5123
Chandrakanta Dash https://orcid.org/0000-0002-6945-
1366
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How to cite this article: Yadav S, Ray S, Singh A,
Mobin SM, Roy TK, Dash C. Dinuclear gold(I)-
N-heterocyclic carbene complexes: Synthesis,
characterization, and catalytic application for
hydrohydrazidation of terminal alkynes. Appl
Organomet Chem. 2020;e5942. https://doi.org/10.
1002/aoc.5942