Palladium-ADC complexes as efficient catalysts in copper-free and room temperature Sonogashira coupling

Article abstract of DOI:10.1016/j.molcata.2014.08.018

The metal-mediated coupling between cis-[PdCl₂(CNR ¹)₂] [R¹ = cyclohexyl (Cy) 1, t-Bu 2, 2,6-Me₂C₆H₃ (Xyl) 3, 2-Cl-6-MeC ₆H₃ 4] and hydrazones H₂

Full text of DOI:10.1016/j.molcata.2014.08.018

Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Palladium-ADC complexes as efficient catalysts in copper-free
and room temperature Sonogashira coupling
Elena A. Valishinaa,b, M. Fátima C. Guedes da Silvab,c, Mikhail A. Kinzhalovd,
Svetlana A. Timofeevad, Tatyana M. Buslaevaa, Matti Haukkae, Armando J.L. Pombeirob,∗∗
Vadim P. Boyarskiyd, Vadim Yu. Kukushkind, Konstantin V. Luzyanind,∗
,
a Moscow State University of Fine Chemical Technologies, Vernadskogo Pr. 86, 119571 Moscow, Russian Federation
b Centro de Química Estrutural, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1049–001 Lisbon, Portugal
c Universidade Lusófona de Humanidades e Tecnologias, ULHT Lisbon, Av. Campo Grande 376, 1749-024 Lisbon, Portugal
d Department of Chemistry, Saint Petersburg State University, Universitetsky pr. 26, 198504 Saint Petersburg, Russian Federation
e Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014, Finland
a r t i c l e i n f o
a b s t r a c t
The metal-mediated coupling between cis-[PdCl2(CNR1)2] [R1 = cyclohexyl (Cy) 1, t-Bu 2,
2,6-Me2C6H3 (Xyl) 3, 2-Cl-6-MeC6H3 4] and hydrazones H2NN CR2R3 [R2, R3 = Ph 5; R2,
R3 = C6H4(OMe-4) 6; R2/R3 = 9-fluorenyl 7; R2 = H, R3 = C6H4(OH-2) 8] provided carbene complexes
cis-[PdCl2{C(N(H)N CR2R3) N(H)R1}(CNR1)] (9–24) in good (80–85%) yields. Complexes 9–24 showed
high activity [yields up to 99%, and turnover numbers (TONs) up to 3.7 × 104] in the Sonogashira
coupling of various aryl iodides with a range of substituted aromatic alkynes without the need of copper
co-catalyst. The catalytic procedure runs at 80 ◦C for 1 h in EtOH using K2CO3 as a base. No formation of
homocoupling or acetylene decomposition products was observed. Designed copper-free Sonogashira
system can also run at room temperature giving target products with yields up to 87% and TONs up
to 87.
Article history:
Received 17 May 2014
Received in revised form 1 August 2014
Accepted 5 August 2014
Available online 15 August 2014
Keywords:
Acyclic diaminocarbenes
Palladium-(acyclic diminocarbene)
catalysts
Metal-mediated coupling
Isocyanides
© 2014 Elsevier B.V. All rights reserved.
Sonogashira cross-coupling
1. Introduction
[14–25], operate in toxic organic solvents (e.g. dimethylformamide
or dimethylacetamide) [14,15,18,19,21–23], frequently require
In the last decades, N-heterocyclic carbenes (NHCs) turned out
to be one of the most powerful classes of ancillary ligands [1,2–5],
and corresponding NHC-based metal complexes were successfully
employed as catalysts in a wide range of organic transforma-
tions [4,6–13]. In particular, outstanding accomplishments were
achieved in the field of cross-coupling, e.g. in Suzuki–Miyaura and
Heck reactions where palladium-NHC catalysts guaranteed some
of the highest catalytic efficiencies [6–8,10].
Compared to hundreds of reports describing the use of
palladium-NHCs in Suzuki–Miyaura and Heck reactions [7,8], the
application of NHCs for Sonogashira coupling is limited to just
a dozen of examples [7,8,14–25]. Nearly all known procedures
employ high catalyst loading (typically between 1 and 3 mol%)
copper co-catalysts [14,15,18,19,21,25], and demand further purifi-
cation of the target products due to occurrence of side-reactions [8].
Taking into account the constantly growing price of palladium and
tightened requirements for greener reaction conditions, the search
for alternative catalysts and catalytic systems is due.
In the past years, several research groups have demonstrated
that acyclic diaminocarbene ligands (ADCs, Scheme 1–A) are a
promising alternative to N-heterocyclic species. Indeed, some of the
reported ADC metal complexes catalyze cross-coupling processes
with activities comparable or even higher than those of NHCs (for
recent reviews and manuscripts on the topic see Refs. [26–32]). In
pursuit of our own research on chemistry of acyclic diaminocar-
benes [33–39], we found that palladium-ADC complexes prepared
via the metal-mediated coupling of N-phenylbenzamidine with
palladium-bound isocyanides (Scheme 1–B) work as efficient cata-
lysts for Sonogashira coupling [36]. Despite the fact that the scope
of that system is limited, it operates in EtOH (organic solvent
with the lowest environmental impact) and requires lower catalyst
loading (0.05 mol%) when compared to the conventional schemes
[36]. Following these initial reports, we decided to evaluate the
∗
Corresponding author. Tel.: +7 812 4286890.
Corresponding author. Tel.: +351 21 8419237.
∗∗
E-mail addresses: pombeiro@ist.utl.pt (A.J.L. Pombeiro), kluzyanin@gmail.com,
konstantin.luzyanin@chem.spbu.ru, konstantin.luzyanin@gmail.com
(K.V. Luzyanin).
http://dx.doi.org/10.1016/j.molcata.2014.08.018
1381-1169/© 2014 Elsevier B.V. All rights reserved.

E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
163
Scheme 1. Acyclic diaminocarbenes (A) and generation of palladium-ADC species via the addition of N-phenylbenzamidine to palladium-bound isocyanides (B).
catalytic properties of other ADC derivatives in Sonogashira reac-
tion. We focused our attention on previously described by some
of us palladium-ADC complexes formed via the addition of hydra-
zones to palladium-bound isocyanides (Scheme 2) [38]. These ADC
were evaluated previously as catalysts for Suzuki–Miyaura cou-
pling showing an excellent catalytic activity [38]. However, the use
of ADC complexes in Sonogashira catalysis is still rare and this field
remains almost unexplored [27,28] (see also below).
The scenario of our work was the following. Firstly, we pre-
pared a series of palladium-diaminocarbene complexes through
the addition of hydrazones to palladium-bound isocyanides. Some
of these species were previously reported but we substantially
extended the series by application of new hydrazones and new
isocyanides. Secondly, we employed the prepared palladium-ADCs
as catalysts in the Sonogashira cross-coupling and discovered that
they exhibit a remarkable catalytic activity without the need of
copper co-catalyst. In addition, we evaluated the efficacy of these
complexes prepared in this work at room temperature Sonogashira
coupling. Finally, we got some first evidence on the nature of the
catalytic species in this palladium-ADC-based system. Our results
are disclosed in the sections that follow.
similar conditions for the coupling of any palladium-isocyanide
complex 1–4 with any of hydrazones 5–8. Therefore, reflux of
the reaction mixture in chloroform for ca. 8 h [or 12 h for the
reactions of cis-[PdCl2(CNt-Bu)2] (2)] followed by the appropri-
ate workup, allowed the isolation of the corresponding carbene
complexes cis-[PdCl2{C(N(H)N CR2R3) N(H)R1}(CNR1)] (9–24) in
good (80–85%) yields (Table 1, Scheme 2).
The authenticity of the known species 9–11 was established
upon comparison of the recorded 1H nuclear magnetic resonance
spectra (1H NMR) and IR spectra with those previously reported
by some of us [38], while the new complexes 12–24 were charac-
terized using (C, H, N) analyses, ESI+-MS (electrospray ionization
mass-spectrometry), IR, 1D (1H, 13C{ H}) and 2D (1H,1H-COSY,
1
1H,13C-HMQC/1H,13C-HSQC, 1H,13C-HMBC) NMR spectroscopies.
In addition, the structures of 15, 19, and 24 were elucidated by
single crystal X-ray diffraction.
Compounds 12–24 gave satisfactory C, H, and N elemental
analyses, which are consistent with the proposed formulations
for cis-[PdCl2{C(N(H)N CR2
)
N(H)R1}(CNR1)]. The ESI+-MS mass
2
spectra display molecular ion peaks and/or a fragmentation corre-
sponding to the loss of Cl’s from the molecular ion, viz. [M − nCl]+,
with the characteristic isotopic distribution. The examination of the
IR spectra revealed the presence of one strong ꢀ(C N) stretching
vibration in the range between 2229 and 2198 cm−1, that belongs
to the unreacted isocyanide ligand. In the formed carbene moi-
2. Results and discussions
2.1. Synthesis and characterization of palladium-aminocarbene
complexes
ety, the ꢀ(N–H) bands emerge in the range of 3256–3210 cm−1
,
while the corresponding very strong bands due to ꢀ(Ccarbene–N)
stretches appear between 1570 and 1533 cm−1. The medium/weak
intensity bands in the range of 2980–2836 cm−1 are characteris-
tic of the ꢀs(C–H) and ꢀas(C–H) vibrations, while medium intensity
Previously, we reported [38] the coupling of cis-[PdCl2(CNR1)2]
[R1 = cyclohexyl (Cy) 1, t-Bu 2, 2,6-Me2C6H3 (Xyl) 3] [35,40–42],
and benzophenone hydrazone [IUPAC name: (diphenyl-
methylene)hydrazine] (5), leading to the corresponding
aminocarbene complexes 9–11 (Scheme 2). In the current
study, we extended the series by using the new isocyanide
species cis-[PdCl2{CN(2-Cl-6-MeC6H3)}2] [35] (4) and three
new hydrazones with different donor and sterical properties.
Thus, we used 4,4-dimethoxybenzophenone hydrazone [(bis(4-
methoxyphenyl)methylene)hydrazine] (6) containing donor
substituents in the aromatic ring [43,44], sterically hindered
9-fluorenone hydrazone [(9H-fluoren-9-ylidene)hydrazine] (7),
and salicylaldehyde hydrazone [2-(hydrazonomethyl)phenol] (8),
that is derived from the corresponding aldehyde (R2 = H).
bands due to ı(C–H from Ar) appear in the range of 698–783 cm−1
.
The 1H NMR spectra of carbene complexes 12–24 display a broad
peak in the range of ı 8.83–9.94 assigned to the Ccarbene–N(H)R pro-
ton, while the corresponding Ccarbene–NH 13C signals were found to
resonate in the range of 180–165 ppm, i.e. they are approximately
65–45 ppm downfield shifted in comparison with the 13C signal of
the isocyanide in the starting (isocyanide)PdII complexes 1–4 (e.g.
115 ppm for C N in cis-[PdCl2(CNCy)2]).
The crystallographic data and processing parameters for 15, 19,
and 24 are very similar and are listed in Table 1S of Supplementary
Data, while corresponding plots can be found in Figs. 1–3, respec-
tively. Bond lengths and angles are given in the legends to each
plot.
Previously, we reported [38] that the reaction between equimo-
lar amounts of 1–3 and 5 (in all combinations) proceeds in refluxing
CHCl3 for ca. 8 h. In the current study, we employed nearly
In 15, 19, and 24, the chloride ligands are mutually in cis posi-
tion [Cl1–Pd1–Cl2 in 15: 93.001(15), 19: 91.73(3), Cl1–Pd1–Cl2
in 24: 93.41(9)◦]. The slightly distorted square planar environ-
ment around the metal center is completed with one unreacted
isocyanide ligand, CNR1, and one newly formed carbene ligand,
C(N(H)N CR2
)
N(H)R1.
2
The Pd–Ccarbene distances [1.9805(15) in 15, 2.006(3)
˚
in 19, and 1.964(10) A in 24] are comparable to those
reported for the similar palladium carbene complexes
cis-[PdCl2{C(N(H)N CPh2) N(H)Xyl}(CNXyl)]
(Pd1–C11
˚
1.973(3) A).[38] In all complexes, the carbene moiety is roughly
planar and the angles around the carbene carbon atom range from
Scheme 2. Generation of aminocarbene complexes 9–24 through the reaction
between 1–4 and hydrazones 5–8.

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E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
Table 1
Numbering scheme for 9–24.
Starting cis-[PdCl2(CNR1)2]
1 (R1 = Cy)
9 [38]
10 [38]
11 [38]
12
13
14
15
16
17
18
19
20
21
22
23
24
2 (R1 = t-Bu)
3 (R1 = Xyl)
4 (R1 = 2-Cl-6-MeC6H3)
116.0(3) to 127.5(2)◦ therefore sustaining the sp2 hybridization
of this atom. Short and nearly equal (within 3ꢁ) N–Ccarbene bond
˚
lengths in 15, 19, and 24 (aver. 1.33 A) imply substantial pꢂ–pꢂ
interactions within the diaminocarbene fragments, as detected
in the related abovementioned carbene complexes [38]. In the
isocyanide ligand, the C N triple bond [1.1499(15) in 15, 1.149(4)
˚
in 19, 1.146(10) A in 24] is of a normal value for triple CN bonds
˚
(1.136–1.144 A) [45,46] and is coherent with those observed in
the related isocyanide palladium and platinum complexes cis-
1
1
˚
˚
[PdCl2(CNR )2] [R = Cy (1.128–1.142 A) [47], t-Bu (1.108–1.149 A)
˚
[48], Xyl (1.145–1.156 A)] [49].
All other bond lengths in all three complexes are of normal
values and they agree with those reported [35–39] for related pal-
ladium(II) aminocarbene and isocyanide complexes.
2.2. Catalytic studies
Complexes bearing acyclic aminocarbenes were scarcely used in
Sonogashira catalysis [27,28]. Indeed, only few examples for both
copper-free and copper co-catalyzed protocols were reported up
to date. Thus, Dhudshia and Thadani [50] found that the optimized
copper-free protocol allowed the transformation of sterically
hindered aryl bromides and aromatic- or aliphatic alkynes. The
reaction proceeded in a toluene/tetrahydrofuran mixture at room
temperature (RT); it required 1.5 mol% of palladium-ADC catalyst
Fig. 1. View of 15 with the atomic numbering schemes. Thermal ellipsoids are
drawn with the 50% probability. Hydrogen labels are omitted for simplicity. Bond
lengths: Pd–C1 1.9805(15), Pd–C25 1.9170(16), Pd–Cl1 2.3543(4), Pd–Cl2 2.3282(4),
C25–N4 1.143(2), C1–N1 1.3201(19), C1–N2 1.3392(19), C2–N1 1.4440(19), C10–N3
1.2926(19), N2–N3 1.3799(17). Bond angles: C25–Pd1–C1 88.39(6), C25–Pd1–Cl2
173.33(5), C1–Pd1–Cl2 88.45(4), C25–Pd1–Cl1 90.28(5), C1–Pd1–Cl1 178.16(4),
Cl1–Pd1–Cl2 93.001(15), N4–C25–Pd1 174.14(14), Pd1–C1–N1 120.21(11),
Pd1–C1–N2 122.74(11), N1–C1–N2 117.04(13), N2–N3–C10 116.19(13), N3–N2–C1
120.80(13), C25–N4–C26 177.66(18), C1–N1–C2 125.01(13).
Fig. 2. View of 19 with the atomic numbering scheme. Thermal ellipsoids are
drawn with the 50% probability. Hydrogen labels are omitted for simplicity.
Crystallization chloroform molecule is omitted for clarity. Bond lengths: Pd–C1
1.937(3), Pd–C2 2.006(3), Pd–Cl1 2.3048(8), Pd–Cl2 2.3507(8), C1–N1 1.149(4),
C2–N2 1.315(4), C2–N3 1.343(4), C21–N2 1.434(4), C31–N31 1.290(4), N3–N31
1.381(4). Bond angles: C1–Pd1–C2 95.32(12), C1–Pd1–Cl1 174.73(9), C2–Pd1–Cl1
89.92(8), C1–Pd1–Cl2 83.05(9), C2–Pd1–Cl2 177.46(8), Cl1–Pd1–Cl2 91.73(3),
N1–C1–Pd1 168.6(3), Pd1–C2–N3 116.3(2), Pd1–C2–N2 127.5(2), N2–C2–N3
116.1(3), N3–N31–C31 118.2(3), N31–N3–C2 119.4(3), C1–N1–C11 173.3(3),
C2–N2–C21 125.2(3).
Fig. 3. View of 24 with the atomic numbering schemes. Thermal ellipsoids
are drawn with the 50% probability. Hydrogen labels are omitted for sim-
plicity. Bond lengths: Pd1–C1 1.923(9), Pd1–C2 1.964(10), Pd1–Cl1 2.311(2),
Pd1–Cl2 2.374(3), C1–N1 1.146(10), C2–N2 1.332(11), C2–N3 1.327(10), C21–N2
1.437(10), C31–N31 1.259(10), N3–N31 1.377(10). Bond angles: C1–Pd1–C2
88.2(4), C1–Pd1–Cl1 169.9(3), C2–Pd1–Cl1 87.0(2), C1–Pd1–Cl2 91.8(3), C2–Pd1–Cl2
177.6(3), Cl1–Pd1–Cl2 93.41(9), N1–C1–Pd1 172.3(8), Pd1–C2–N2 124.7(7),
Pd1–C2–N3 119.8(7), N2–C2–N3 115.6(9), N3–N31–C31 116.4(8), N31–N3–C2
120.1(8), C1–N1–C11 175.1(9), C2–N2–C21 124.2(8).

E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
165
Table 3
Screening of the solvent and temperature in the model Sonogashira system.
Entry
Solvent
Yield, % at the indicated temperature and with
the selected base
Scheme 3. Model reaction for Sonogashira coupling.
K2CO3
Na2CO3
Table 2
Screening of base in the model Sonogashira system.
20 ◦
C
40 ◦
C
80 ◦
C
20 ◦
C
40 ◦
C
80 ◦
C
1–6
7–12
13–18
19–24
25–30
31–36
MeOH
EtOH
n-PrOH
t-BuOH
Toluene
p-xylene
<5
30
14
<5
<5
<5
<5
61
45
<5
<5
<5
12a
92
62
6
<5
<5
14
7
<5
<5
<5
<5
67
12
<5
<5
<5
<5
77a
75
7
<5
<5
<5
Entry
Base
Yield, % at the indicated temperature
20 ◦
C
40 ◦
C
80 ◦
C
1–3
4–6
7–9
10–12
13–15
16–18
19–21
NEt3
t-BuOK
KOH
Na2CO3
K2CO3
Cs2CO3
K3PO4
<5
<5
<5
7
30
<5
6
<5
<5
<5
12
61
<5
10
<5
<5
<5
75
92
<5
24
Base (1.5 × 10−4 mol, 1.5 equiv), 4-iodoanisole (1.0 × 10−4 mol, 1.0 equiv), pheny-
lacetylene (1.0 × 10−4 mol, 1.0 equiv); catalyst 9 (1 × 10−7 mol, 0.1 mol%); solvent
(1 mL), reaction time: 24 h (20 ◦C), 14 h (40 ◦C), 4 h (80 ◦C).
Reaction was performed in refluxing MeOH (ca. 66 ◦C).
a
Base (1.5 × 10−4 mol, 1.5 equiv), 4-iodoanisole (1.0 × 10−4 mol, 1.0 equiv), pheny-
lacetylene (1.0 × 10−4 mol, 1.0 equiv); catalyst 9 (1 × 10−7 mol, 0.1 mol%); EtOH
(1 mL), reaction time: 24 h (20 ◦C), 14 h (40 ◦C), 4 h (80 ◦C).
Table 4
Screening of the catalyst loading in the model Sonogashira system.
Entry
Catalyst loading, mol (mol%)
Yield (%)
TON
accomplishing target products in 80–91% yields with the TONs
(turnover number = moles of product per mole of catalyst) up to
61. Vasilyev et al. reported [51] on the use of palladium-ADCs for
the transformation of aroyl chlorides. Catalytic system required
0.04 mol% catalyst loading (and 2 mol% of CuI as co-catalyst) and
proceeded in benzene or tetrahydrofuran at 60 ◦C giving target
products in 52–98% yields. Another copper co-catalyzed Sono-
gashira system reported recently by some of us [36] enabled the
coupling of 4-NO2C6H4I with oct-1-yne (EtOH, K2CO3, 60 ◦C) to
yield 1-nitro-2-(oct-1-ynyl)benzene. The palladium-ADC catalyst
loading was as low as 0.05 mol%, providing product yields of
up to 99%, and TONs up to 2000 [36]. Inspired by our previous
findings we evaluated catalytic properties of 9–24 in Sonogashira
cross-coupling.
1
2
3
4
5
–
<5
92
89
6
–
10−7 (0.1)
10−8 (0.01)
10−9 (0.001)
10−10 (0.0001)
9.2 × 102
8.9 × 103
6.0 × 103
–
<5
K2CO3 (1.5 × 10−4 mol, 1.5 equiv), 4-iodoanisole (1.0 × 10−4 mol, 1.0 equiv), pheny-
lacetylene (1.0 × 10−4 mol, 1.0 equiv); catalyst 9; EtOH (1 mL), 80 ◦C, 4 h.
(entries 7–9), Cs2CO3 (entries 16–18), or K3PO4 (entries 19–21),
were less active or inactive under studied conditions. Moreover, no
increase in the product yield was observed when the reaction was
performed for longer time at any of the studied temperature (e.g.
for 14 h instead of 4 h at 80 ◦C). As a result, K2CO3 and Na2CO3 were
selected as suitable bases.
2.2.1.2. Effect of solvent and temperature. As the next step, we ver-
ified the effect of solvent at different temperatures (20, 40, and
80 ◦C) using K2CO3 and Na2CO3 as bases (Table 3). As can be inferred
from inspection of the obtained results, K2CO3 in EtOH (entries
7–12, Table 3) and Na2CO3 in MeOH (entries 1–6) were the most
suitable combinations. When K2CO3 or Na2CO3 were used in n-
PrOH (entries 13–18), the product yield was significantly lower.
Other evaluated solvents, i.e. t-BuOH, toluene, or p-xylene were
not efficient with either K2CO3 or Na2CO3 at 20–80 ◦C range (entries
19–36). For further studies, EtOH (with K2CO3 as a base) and MeOH
(with Na2CO3) were chosen. Furthermore, the best product yields
were achieved at 80 ◦C.
2.2.1. Optimization of the reaction conditions for Sonogashira
coupling
As a model system for all optimizations, we have chosen the
reaction of 4-methoxy-iodobenzene (4-iodoanisole) with pheny-
lacetylene accomplishing 1-methoxy-4-(phenylethynyl)benzene
(Scheme 3).
2.2.1.1. Effect of base. It is known that the Sonogashira reaction is
strongly affected by the nature of the base employed. Therefore, we
performed a wide base screening in the model coupling (Table 2).
Based on our previous studies [36], ethanol was employed as the
reaction medium for the base optimization.
Among bases studied, K2CO3 was the most effective one (entries
13–15, Table 2) at all temperatures; closely related Na2CO3 (entries
10–12) was substantially less effective, in particular, at 20–40 ◦C.
Other bases such as NEt3 (entries 1–3), t-BuOK (entries 4–6), KOH
2.2.1.3. Effect of catalyst loading. In order to achieve maximum
TON, we verified the yield of target product with different cata-
lyst loadings (Table 4). A high yield (89%) was obtained with as low
Table 5
Screening of the catalyst loading in the model Sonogashira system.
Entry
Catalyst
Yield, %
TON
Entry
Catalyst
Yield, %
TON
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
89
68
89
58
8.9 × 103
6.8 × 103
8.9 × 103
5.8 × 103
9.5 × 103b
8.7 × 103
9.4 × 103b
8.6 × 103
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
88
83
86
49
48
73
8.8 × 103
8.3 × 103
8.6 × 103
4.9 × 103
4.8 × 103
7.3 × 103
4.6 × 103
9.7 × 103b
95(5)a
87
94(5)a
86
46
97(37)a
K2CO3 (1.5 × 10−4 mol, 1.5 equiv), 4-iodoanisole (1.0 × 10−4 mol, 1.0 equiv), phenylacetylene (1.0 × 10−4 mol, 1.0 equiv); selected catalyst (1 × 10−8 mol, 0.01 mol%); EtOH
(1 mL), 80 ◦C, 4 h.
a
Values in parentheses are obtained with the 1 × 10−9 mol catalyst loading;
b
TON is calculated with the 1 × 10−8 mol of catalyst.

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E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
Table 6
Evaluation of the scope of the catalytic system with the selected catalysts.
R5
X
catalyst
R4
R5
EtOH, K2CO3, 80 oC
R4
Entry
Alkyne
Aryl halide
Yield, % with selected catalyst
13
15
24
1–3
4–6
7–9
10–12
13–15
16–18
PhC≡CH
4-MeOC6H4I
2-MeC6H4I
4-MeC6H4I
4-NO2C6H4I
3-I-4,5-(MeO)2-C6H2C( O)H
1,4-I2C6H4
95 (91)a
90 (90)
99 (95)
51 (49)
70 (68)
51b (47)
94 (88)
92 (88)
99 (95)
46 (39)
70 (64)
48b (45)
97 (89)
89 (85)
99 (93)
44 (37)
72 (68)
54b (52)
19–21
22–24
2-MeC6H4C≡CH
4-MeC6H4C≡CH
2-FC6H4C≡CH
4-MeOC6H4I
2-MeC6H4I
43 (43)
60 (56)
44 (42)
56 (52)
46 (41)
62 (56)
25–27
28–30
4-MeOC6H4I
4-NO2C6H4I
87 (82)
51 (53)
84 (81)
42 (41)
91 (88)
39 (38)
31–33
34–36
4-MeOC6H4I
2-MeC6H4I
48 (46)
90 (84)
46 (42)
91 (82)
48 (46)
88 (72)
37–39
40–42
PhC≡CH
PhC≡CH
4-MeOC6H4Br
4-MeOC6H4Cl
<5 (<5)
<5 (<5)
<5 (<5)
<5 (<5)
<5 (<5)
<5 (<5)
K2CO3 (1.5 × 10−4 mol, 1.5 equiv), aryl halide (1.0 × 10−4 mol, 1.0 equiv), phenylacetylene (1.0 × 10−4 mol, 1.0 equiv); selected catalyst (1 × 10−8 mol, 0.01 mol%); EtOH (1 mL),
80 ◦C, 4 h.
a
Yield of corresponding products indicated in parentheses is achieved within 1 h of reaction with 1.5 equivs of phenylacetylene (or three equivs for entries 16–18).
Two equivs of phenylacetylene were used; reaction proceeds with replacement of both iodine atoms.
b
Table 7
as 0.01 mol% of catalyst (entry 3, Table 4). The yield of product dra-
Screening of the catalyst activity in the model Sonogashira system at RT.
matically dropped with a lower catalyst amount (0.001 mol%, entry
4), and no formation of the product was observed without catalyst
(entry 1) and with the loading of 0.0001 mol% (entry 5). The maxi-
mum TON for the system (8.9 × 103) was achieved with the catalyst
loading of 0.01 mol% (entry 3). Consequently, the catalyst loading
of 0.01 mol% was used for further studies.
Entry
Catalyst
Yield, %
TON
1
2
3
4
–
<5
82
56
87
–
13
15
24
82
56
87
K3PO4 (2.5 × 10−4 mol, 2.5 equivs), 4-iodoanisole (1.0 × 10−4 mol, 1.0 equiv), pheny-
lacetylene (1.5 × 10−4 mol, 1.5 equivs); catalyst 24 (1 × 10−6 mol, 1.0 mol%); EtOH
(1 mL), RT, 24 h.
2.2.2. Screening of the catalytic activity of 9–24
We examined the efficiency of the 9–24 complexes in the model
Sonogashira system and the results obtained are summarized in
Table 5. Among the palladium aminocarbenes 9–24, complexes 13,
15, and 24 exhibited the highest efficiencies furnishing the cou-
pling product in 94–97% yield with 0.01 mol% (1 × 10−8 mol) of the
catalyst (entries 5, 7, and 16, correspondingly). Catalysts 9 (entry 1,
product yield 89%), 11 (entry 3, 89%), 14 (entry 6, 87%), 16 (entry 8,
86%), 17 (entry 9, 88%), 18 (entry 10, 83%), and 19 (entry 11, 86%)
were slightly less active. All other tested aminocarbene complexes
were significantly less active. The maximum TON of 3.7 × 104
was accomplished with the catalyst 24 (loading: 0.001 mol%, or
1 × 10−9 mol), however, the yield of the product was only 37%. With
higher catalyst loading of 24 (0.01 mol%, 1 × 10−8 mol), the TON was
of 9.7 × 103.
Unfortunately, the attempted coupling between 4-bromoanisol
or 4-chloroanisol (entries 37–39 and 40–42, correspondingly,
Table 4) and phenylacetylene was unsuccessful. It is noteworthy
that the reaction of 4-bromoanisol was extensively tested under
a wide range of different conditions (Table 2S of Supplementary
data). At elevated temperatures (110–130 ◦C) and in the presence
of strong bases (e.g. t-BuOK, KOH, or Cs2CO3), the formation of
cross-coupling product was observed, however, it was strongly
contaminated with 4,4ꢀ-dimethoxybiphenyl derived from the
homonuclear coupling of 4-bromoanisole. At lower temperatures
and with milder bases only trace amounts of the desired product
were detected.
2.2.3. Sonogashira coupling at room temperature using 9–24 as
catalysts
With three most active catalysts in our hands (13, 15, and 24), we
tried to shorten the reaction time for our catalytic system by using
an excess of phenylacetylene (1.5 equivs) instead of stoichiometric
amounts employed beforehand. We found out that nearly identical
product yields were obtained within 1 h of the reaction (yields were
91% for 13, 88% for 15 or 89% for 24).
We attempted to run our system at room temperature
(Table 7). It was found out that under modified conditions
[K3PO4 (2.5 × 10−4 mol, 2.5 equiv), 4-iodoanisole (1.0 × 10−4 mol,
1.0 equiv), phenylacetylene (1.5 × 10−4 mol, 1.5 equiv), selected
catalyst (1 × 10−6 mol, 1.0 mol%)], the product is formed in 82–87%
yield after 24 h. Although, as expected, much lower TONs were
achieved compared to the trials at higher temperature (80 ◦C).
As the next step, we examined the scope of our catalytic system
for various organohalides and found out that several phenyl iodides
containing either electron-withdrawing or electron-donating sub-
stituents can react with various substituted aromatic alkynes giving
the target products in excellent to moderate yields (Table 6). No by-
products due to acetylene decomposition or homocoupling were
detected in any of these trials.
2.2.4. Attempts of identification of catalytic species
Although recent studies aimed to shed light on mechanistic
aspects of both copper-mediated and copper-free versions of

E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
167
that these species are rapidly consumed throughout the catalytic
cycle.
In the end of the catalytic cycle, along with the disappearance
of the ([M(carbene) + H]+) species, accumulation of the palladium
clusters containing from 2 to 5 palladium atoms, is observed
(Figure 2S). Despite the previous reports on catalytic activity of
small palladium clusters (see Ref. [54] and references therein), in
our case these compounds are not catalytically competent, indeed,
no catalysis was observed upon addition of the fresh substrates and
the base to the system. It is worthwhile noticing that the formation
of the catalytically active metal clusters from the corresponding
complexes is known to be inhibited in the presence of the carbene
ligands presumably due to their strong binding to the metal center
[55].
3. Final remarks
Scheme 4. Proposed mechanism for the copper-free Sonogashira coupling.
In conclusion, we developed an efficient copper-free Sono-
gashira protocol that employs palladium complexes with acyclic
aminocarbene ligands derived from the metal-mediated coupling
of hydrazones and isocyanides. The reported catalytic procedure
runs at 80 ◦C for 1 h, and employs K2CO3 as cheap inorganic base
and EtOH as environmentally benign solvent. It allows the trans-
formation of a range of aryl iodides into the target internal alkynes
with the highest obtained yields of 99%, and the maximum TON
of 3.7 × 104. Comparison of our results with systems based on
palladium-ADC catalysts (Dhudshia and Thadani [50], 1.5 mol%
loading; or Kukushkin et al. [36], 0.05 mol%) or with those running
under comparable conditions (Li et al. [56]; based on nanosized
palladium, 1 mol%) clearly indicates that our protocol is substan-
tially more efficient than the previous ones. We also found that
our copper-free Sonogashira system can run at room temperature
although with a higher catalyst loading (1% mol); K3PO4 and EtOH
were employed as base and solvent, respectively. For the latter, the
maximum TON was 87, and the achieved product yield was 87%.
Finally, we got some first evidence on the nature of the cat-
alytic species formed from the palladium-ADC precatalysts under
action of the base and we believe that the [M(carbene)] fragment is
responsible for the homogeneous catalysis. Mechanism is expected
to be monomolecular and the deactivation of the catalyst proceeds
with the formation of the catalytically inactive Pd2–5 clusters.
Further studies aiming to establish the full mechanism of the
palladium-ADC catalyzed cross-coupling reactions are currently
underway in our group.
Sonogashira cross-coupling, their mechanisms are still unclear
[52]. With respect to the copper-free process, most of the involved
researchers support the hypothesis that the mechanism is based
on ␲-coordination of an alkyne followed by proton removal from
alkyne upon action of base that occurs directly at the catalytic
center (Scheme 4) [52]. However, several studies demonstrated
that other components of the catalytic system, e.g. amine (if used
as a base), solvent or alkyne can replace the original supporting
ligands L (e.g. phosphines) at the catalyst core resulting in different
intermediates, and, consequently altering the mechanism of the
process. It is worthwhile mentioning that even in the case of
extremely popular palladium-NHC species, whose activity in
copper-free Sonogashira reaction was documented [7,8,52], only
limited evidence on the nature of the active catalytic species was
gathered and all proposed mechanisms remain hypothetical.
In order to make an approach to mechanism of our ADC-based
system, that is structurally relevant to that with palladium-NHC
catalysts, we attempted to identify the real catalytic species. In the
first set of experiment, we performed several catalytic runs with
substrates from Table 6 in the presence of metallic Hg (the mer-
cury drop test [53]). We observed similar reaction rates for the
corresponding processes both in the absence and in the presence of
metallic Hg for any combination of substrates. These data support
an idea that no formation of palladium nanoparticles occurs dur-
ing the reaction and the catalytic system operates under typically
homogeneous conditions. Furthermore, we observed no induction
period in the system, and the accumulation of the product in the
initial period follows nearly linear time-dependence.
4. Experimental part
In the second set of experiments, we performed ESI-MS mon-
itoring of the reaction mixture for the model catalytic system
with 13 [cis-[PdCl2{C(N(H)N C(C6H4OMe-4)2) N(H)Cy}(CNCy)],
C29H38N4Cl2O2Pd, Mcat = 651.96] as precatalyst (see Figures 1S and
2S, Supplementary data). Before addition of the base to the reac-
tion, only signal corresponding to the [Mcat + Na + H]+ [m/z: 674.34
(found), 674.14 (calcd)], was detected in the ESI+-MS spectra. Upon
addition of the base, this signal rapidly vanishes giving rise to those
with m/z of 615.30 [Mcat–Cl2 + 2H2O + H]+ (m/z: 615.22, calcd.) and
with the m/z of 472.26 (Figure 1S). The latter corresponds to the
protonated pure metal-carbene species ([M(carbene) + H]+) (m/z:
472.12 calcd.) generated via the subsequent elimination of the
chloride and the isocyanide ligands and reduction of precatalyst 13.
This signal remains dominant in the spectra and gradually disap-
pears in the end of the catalytic trial. We believe it is characteristic
of the real catalytically active species and being the catalyst resting
state. Unfortunately, we were unable to identify by ESI-MS other
intermediates of the catalytic cycle (e.g. products of the oxidative
addition or alkyne binding to the palladium center) using either
ESI-MS or NMR spectroscopy in different solvents that can indicate
4.1. Materials and instrumentation
Solvents, PdCl2, all isocyanides, and hydrazones 5–7 were
obtained from commercial sources and used as received. (Bis(4-
methoxyphenyl)methylene)hydrazine (8) was synthesized via a
known procedure starting from 4,4ꢀ-dimethoxybenzophenone
[43,44]. The complexes cis-[PdCl2(R1NC)2] (R1 = Cy 1, t-Bu 2,
Xyl 3, 2-Cl-6-MeC6H3 4) were prepared as previously reported
[35,40–42]. C, H, and N elemental analyses were carried out by
the Microanalytical Service of the Instituto Superior Técnico. ESI+
mass-spectra were obtained on a VARIAN 500-MS LC ion trap
mass-spectrometer in MeOH (or in MeCN) (ion spray voltage:
+5 kV, capillary voltage: 30 V, RF loading: 100%). EI mass-spectra
of organic products from the catalytic studies were acquired on
a Shimadzu GCMS-QP2010 SE. Infrared spectra (4000–400 cm−1
)
were recorded on a BIO-RAD FTS 3000MX instrument in KBr pel-
1
lets. 1H, 13C{ H} and 19F NMR spectra were recorded on Bruker
Avance II+ 400 MHz (UltraShieldTM Magnet) and Bruker Avance II+
500 MHz (UltraShieldTM Plus Magnet) spectrometers at ambient
temperature.

168
E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
4.2. X-ray studies
ꢀ(C–H), 2198 s ꢀ(C N), 1595 s ꢀ(N C), 1536 s ꢀ(N–Ccarbene), 774 m
ı(C–H from Ar). 1H NMR (CDCl3, ı): 9.81 (s, 1H) and 9.40 (s,
Crystals were immersed in cryo-oil, mounted in a Nylon loop
and measured at a temperature of 100 K (for 15) and 150 K (19 and
24). Intensity data were collected using a Bruker Kappa Apex II Duo
(for 15) or Bruker Kappa Apex II (for 19 and 24) diffractometers
with graphite monochromatic Mo K␣ (ꢃ = 0.71073) radiation.
Crystallographic details for 15, 19 and 24 are listed in Table S1.
1H, Ccarbene–(NH)NH), 7.69–7.30 and 7.19–7.08 (m, 16H, aryls),
1
2.83 (s, 3H) and 2.23 (s, 3H, Me). 13C{ H} NMR (CDCl3, ı): 180.3
(Ccarbene–NH), 158.9 (C N), 139.3–126.8 (aryls), 20.5 and 19.1 (Me).
4.2.1. For 15
The APEX2 [57] program package was used for cell refinements
and data reductions. The structure was solved by direct methods
using the SHELXS-97 [58] program with the WinGX [59] graph-
ical user interface. A multi-scan absorption correction based on
equivalent reflections (SADABS) [60] was applied to the data. Struc-
tural refinement was carried out using SHELXL-97 [58]. The NH
hydrogen atoms were located from the difference Fourier map but
constrained to ride on their parent atom, with Uiso = 1.5Ueq(parent
atom). Other hydrogen atoms were positioned geometrically and
(13).
Anal.
Calcd
for
C
29H38N4Cl2O2Pd: C, 53.42; H, 5.87; N, 8.59. Found: C, 53.14;
H, 5.91; N, 8.39. ESI+-MS, m/z: 617 [M − Cl]+. IR (KBr, selected
bands, cm−1): 2931–2855 m ꢀ(C–H), 2226
s ꢀ(C N), 1567s
ꢀ(N C + N–Ccarbene), 848 m ı(C–H from Ar). 1H NMR (CDCl3, ı):
8.72 (s, 1H, Ccarbene–NH), 7.67–7.44 and 7.23–6.80 (m, 8H, aryls),
4.43 (s, br, 1H, CH), 4.45 (s, br, 1H, CH), 3.93 (s) and 3.84 (s,
˚
constrained to ride on their parent atoms with C–H = 0.95–0.98 A
and Uiso = 1.2–1.5Ueq(parent atom).
6H, OMe), 2.19–1.12 (m, 20H, CH2). 13C{ H} (CDCl3, ı): 176.2
1
4.2.2. For 19 and 24
(Ccarbene–NH), 162.1 (C–O), 156.6 (C N), 136.7–127.1 (aryls), 58.6
(CH), 55.7 (OMe), 33.9–22.7 (CH2).
Cell parameters were retrieved using Bruker SMART software
and refined using Bruker SAINT [61] on all the observed reflections.
Absorption corrections were applied using SADABS [60]. Structures
were solved by direct methods by using the SHELXS–97 package
[58] and refined with SHELXL–97 [58]. Calculations were per-
formed using the WinGX System–Version 1.80.03 [59]. Hydrogen
atoms were inserted in calculated positions. For 24, the quality of
the data was poor due to the poor crystal quality and weak diffrac-
tion. Least square refinements with anisotropic thermal motion
parameters for all the non-hydrogen atoms and isotropic for the
remaining atoms, were employed.
(14).
Anal.
Calcd
for
4.3. Synthetic work
C25H34N4Cl2O2Pd: C, 53.20; H, 6.07; N, 9.93. Found: C, 53.24;
H, 5.95; N, 9.92. ESI+-MS, m/z: 565 [M − Cl]+. IR (KBr, selected
bands, cm−1): 2966–2845 m ꢀ(C–H), 2218 s ꢀ(C N), 1603 s
ꢀ(N C + N–Ccarbene), 838 m ı(C–H from Ar). 1H NMR (CD3OD, ı):
8.95 (s, 1H) and 8.52 (s, 1H, Ccarbene–(NH)NH), 7.77–7.75 and
7.06–7.04 (m, 8H, aryls), 3.90 (s, 6H, OMe), 1.59–1.38 (m, 18H,
4.3.1. Synthesis of the aminocarbene complexes 9–24
A solution of any of the hydrazones (5–8; 0.20 mmol) in CHCl3
(2 mL) was added to a solution of cis-[PdCl2(R1NC)2] (0.20 mmol) in
CHCl3 (5 mL) and the reaction mixture was refluxed for 8 h (R1 = Cy
1, Xyl 3, 2-Cl-6-Me-C6H3 4) or 12 h (R1 = t-Bu 2). During this period,
the color of the mixture gradually turned from light orange to bright
yellow. The reaction mixture was further evaporated to dryness at
20–25 ◦C under a stream of dinitrogen, and the solid residue was
extracted with two 5-mL portions of CHCl3. The bright yellow solu-
tion was filtered off to remove some insoluble material, the filtrate
was evaporated at RT to dryness under a stream of N2, and washed
with two 5-mL portions of i-Pr2O, one 5-mL portion of cold (5 ◦C)
Et2O, and again with two 5-mL portions of i-Pr2O, and then dried
in vacuo at 20–25 ◦C. Yields of 9–24 were 80–85%, based on the
metal. The authenticity of the known species 9–11 was established
upon comparison of the recorded 1H NMR and IR spectra with those
previously reported by some of us [38].
1
Me). 13C{ H} (CD3OD, ı): 195.4 (Ccarbene–NH), 163.4 (C–O), 156.6
(C N), 131.9–130.2 (aryls), 113.3 (C N), 54.6 (CMe3), 54.6 (OMe),
28.4 (CMe3).
(15).
Anal.
Calcd
for
C33H34N4Cl2O2Pd: C, 56.95; H, 4.92; N, 8.05. Found: C, 56.92;
H, 4.91; N, 8.13. ESI+-MS, m/z: 695 [M + H]+, 661 [M − Cl]+. IR (KBr,
selected bands, cm−1): 2930–2922 m ꢀ(C–H), 2198 s ꢀ(C N), 1536
s ꢀ(N C + N–Ccarbene), 752 m ı(C–H from Ar). 1H NMR (CDCl3,
ı): 9.75 (s, 1H) and 9.21 (s, 1H, Ccarbene–(NH)NH), 7.82–7.41 and
7.15–6.88 (m, 14H, aryls), 3.91 (s, 3H) and 3.84 (s, 3H, OMe),
1
2.50 (s), 2.47 (s), 2.37 (s), and 2.15 (s, 12H, Me). 13C{ H} (CDCl3,
(12). Anal. Calcd for C29H24N4Cl4Pd: C,
51.47; H, 3.57; N, 8.28. Found: C, 51.92; H, 3.52; N, 8.12. ESI+-MS,
m/z: 641 [M − Cl]+. IR (KBr, selected bands, cm−1): 2967–2944 m
ı): 178.5 (Ccarbene–NH), 162.2 and 161.6 (C–O), 158.2 (C N),
135.6–127.9 (aryls), 115.4 (C N), 55.5 (OMe), 19.5 and 18.7
(Me).

E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
169
ı(C–H from Ar). 1H (CDCl3, ı): 9.47 (s, 1H) and 8.42 (s, 1H,
C
carbene–(NH)NH), 8.15–7.28 and 7.24–6.95 (m, 14H, aryls), 2.49 (s)
1
and 2.45 (s, 12H, Me). 13C{ H} NMR (CDCl3, ı): 180.6 (Ccarbene–NH),
154.8 (C N), 142.9–120.1 (aryls), 19.5, 18.7, and 18.6 (Me).
(16).
Anal.
Calcd
for
C31
H28N4Cl4O2Pd: C, 50.53; H, 3.83; N, 7.60. Found: C, 50.84;
H, 3.78; N, 7.38. ESI+-MS, m/z: 701 [M − Cl]+. IR (KBr, selected
bands, cm−1): 2968–2922 m ꢀ(C–H), 2198 s ꢀ(C N), 1535 s
ꢀ(N C + N–Ccarbene), 838 m ı(C–H from Ar). 1H NMR (CDCl3, ı):
9.75 (s, 1H) and 9.30 (s, 1H, Ccarbene–(NH)NH), 7.50–7.27 and
7.15–6.89 (m, 14H, aryls), 3.91 (s, 3H) and 3.84 (s, 3H, OMe),
(20). Anal. Calcd for C29H22N4Cl4Pd: C,
51.62; H, 3.29; N, 8.30. Found: C, 51.82; H, 3.32; N, 8.30. ESI+-MS,
m/z: 638 [M − Cl]+. IR (KBr, selected bands, cm−1): 2988–2836 m
ꢀ(C–H), 2201 s ꢀ(C N), 1595 s ꢀ(N C), 1533 s ꢀ(N–Ccarbene), 731 m
ı(C–H from Ar). 1H (CDCl3, ı): 9.57 (s, 1H) and 8.40 (s, 1H,
1
2.79 (s, 3H) and 2.20 (s, 3H, Me). 13C{ H} NMR (CDCl3, ı): 178.8
C
carbene–(NH)NH), 8.15–7.38 (m) and 7.15–7.00 (m, 14H, aryls), 2.46
(Ccarbene–NH), 162.4 (C–O), 158.2 (C N), 130.9–127.5 (aryls), 55.7
(OMe), 20.7 and 19.3 (Me).
1
and 2.23 (s, 6H, Me). 13C{ H} NMR (CDCl3, ı): 181.3 (Ccarbene–NH),
154.2 (C N), 142.3–122.8 (aryls), 120.1 (C N), 22.9 and 18.9 (Me).
(21).
Anal.
Calcd
for
C
21H30N4Cl2OPd: C, 47.27; H, 5.69; N, 10.53. Found: C, 46.95;
(17). Anal. Calcd for C27H32N4Cl2Pd:
C, 54.97; H, 5.47 N, 9.49. Found: C, 54.44; H, 5.48; N, 9.69%.
ESI+-MS, m/z: 553 [M − Cl]+, 517 [M − 2Cl −H]+. IR (KBr, selected
bands, cm−1): 2933–2856 m ꢀ(C–H), 2229 s ꢀ(C N), 1575 s
ꢀ(N C + N–Ccarbene), 732 m ı(C–H from Ar). 1H NMR (C DCl3, ı):
10.02 (s, 1H, Ccarbene–NH), 8.15–7.41 (m, 9H, Ccarbene–NH, aryls),
4.45 (s, br, 1H, CH), 4.46 (s, br, 1H, CH), 1.95–1.78 (m, 20H, CH2).
H, 5.77; N, 10.54%. ESI+-MS, m/z: 497 [M − Cl]+. IR (KBr, selected
bands, cm−1): 2932–2856 m ꢀ(C–H), 2228 s ꢀ(C N), 1619 s ꢀ(N C),
1584 s ꢀ(N–Ccarbene), 753 m ı(C–H from Ar). 1H NMR (CDCl3, ı):
9.64 (s, 1H) and 8.90 (s, 1H, Ccarbene–(NH)NH), 8.71 (s, 1H, OH),
7.40–7.31 (m) and 7.05–6.59 (m, 4H, aryls), 4.69 (s, br) and 4.01
(s, br, 2H, CH), 1.82–1.37 (m, 20H, CH2). 13C{ H} NMR (CDCl3,
1
1
13C{ H} (CDCl3, ı): 175.0 (Ccarbene–NH), 155.5 (C N), 132.9–120.4
ı): 172.6 (Ccarbene–NH), 166.7 (C N), 153.4 (C–OH), 133.6–132.7
(aryls), 117.2 (C N), 58.7 and 55.5 (CH), 33.8–22.7 (CH2).
(aryls), 68.4 (CH), 68.6 (CH), 35.4–22.3 (CH2).
t-Bu
H
N
N
Cl
C
N
H
Pd
Cl
C
(22).
Anal.
Calcd
for
N
C17H26N4Cl2OPd: C, 42.56; H, 5.46; N, 11.68. Found: C, 42.64;
H, 5.51; N, 11.50%. ESI+-MS, m/z: 407 [M − 2Cl − H]+. IR (KBr,
selected bands, cm−1): 2981–2935 m ꢀ(C–H), 2228 s ꢀ(C N), 1621
s ꢀ(N C), 1570 s ꢀ(N–Ccarbene), 749 m ı(C–H from Ar). 1H NMR
(CDCl3, ı): 9.63 (s, 1H), 9.04 (s, 1H, Ccarbene–(NH)NH), 8.74 (s, 1H,
OH), 7.39–6.98 (m, 4H, aryls), 3.49 (s, 1H, CH), 1.80–1.60 (m, 18H,
t-Bu
(18). Anal. Calcd for C23H28N4Cl2Pd: C,
51.36; H, 5.26; N, 10.42. Found: C, 51.67; H, 5.27; N, 10.53%. ESI+-
MS, m/z: 539 [M + H]+. IR (KBr, selected bands, cm−1): 2982–2931 m
ꢀ(C–H), 2228 s ꢀ(C N), 1609 s ꢀ(N C), 1569 s ꢀ(N–Ccarbene),
731 m ı(C–H from Ar). 1H NMR (CDCl3, ı): 9.86 (s, 1H) and 9.11
(s, 1H, Cca1rbene–(NH)NH), 8.14–7.38 (m, 8H, aryls), 1.80 (s, 18H,
Me). 13C{ H} NMR (CDCl3, ı): 207.1 (Ccarbene–NH), 154.9 (C N),
142.4–120.2 (aryls), 115.8 (C N), 59.2 (CMe3), 30.0 (CMe3).
1
Me). 13C{ H} NMR (CDCl3, ı): 164.8 (Ccarbene–NH and C N), 159.9
(C–OH), 133.6–132.7 (aryls), 117.3 (C N), 55.3 (CMe3), 31.7 and
30.1 (CMe3).
(23).
Anal.
Calcd
for
C25H26N4Cl2OPd: C, 52.15; H, 4.55; N, 9.73. Found: C, 52.15;
H, 4.63; N, 9.78. ESI+-MS, m/z: 541 [M − Cl]+. IR (KBr, selected
bands, cm−1): 2967–2896 m ꢀ(C–H), 2207 s ꢀ(C N), 1620 s
ꢀ(N C), 1568 s ꢀ(N–Ccarbene), 771 m ı(C–H from Ar). 1H NMR
(CDCl3, ı): 9.21 (s, 1H) and 8.82 (s, 1H, Ccarbene–(NH)NH), 8.14
(19). Anal. Calcd for C31H28N4Cl2Pd: C,
58.74; H, 4.45; N, 8.84. Found: C, 58.60; H, 4.65; N, 8.80 ESI+-MS,
m/z: 599 [M − Cl]+. IR (KBr, selected bands, cm−1): 2965–2921 m
ꢀ(C–H), 2199 s ꢀ(C N), 1597 s ꢀ(N C), 1528 s ꢀ(N–Ccarbene), 783 m

170
E.A. Valishina et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 162–171
(s, 1H, OH), 7.40–7.28 and 7.22–6.76 (m, 10H, aryls), 3.54 (s, 1H,
Appendix A. Supplementary data
1
CH), 2.49 (s) and 2.41 (s, 12H, Me). 13C{ H} NMR (CDCl3, ı): 164.9
(Ccarbene–NH + C N), 160.0 (C–OH), 136.8 (C N), 136.5–128.3
(aryls), 119.9 (C N), 23.0, 18.8, 18.7, and 15.4 (Me).
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.08.018.
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Acknowledgements
This work has been partially supported by the Fundac¸ ão
para a Ciência e a Tecnologia (FCT), Portugal, its PPCDT program
(FEDER funded), and through the research projects PTDC/QUI-
QUI/109846/2009 and Pest-OE/QUI/UI0100/2013. The authors
are grateful to Saint Petersburg State University for the research
grant (Laboratory of Cluster Catalysis), RAS Presidium Subprogram
coordinated by acad. N. T. Kuznetsov, and Russian Fund for Basic
Research (grants 12-03-00076, 14-03-00297, 14-03-31204, and
14-03-01005). SAT is grateful to Saint Petersburg State University
for the postdoctoral fellowship. Authors thank the Center for
Magnetic Resonance, Center for X-ray Diffraction Studies, and
Center for Chemical Analysis and Materials Research of Saint
Petersburg State University, and Portuguese NMR (IST-UTL Center)
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