Palladium(II) Chloride Complexes of N,N′-Disubstituted Imidazole-2-thiones: Syntheses, Structures, and Catalytic Performances in Suzuki-Miyaura and Sonogashira Coupling Reactions

Article abstract of DOI:10.1021/acs.inorgchem.7b01616

Reactions of PdCl₂ with 2 equiv of N,N′-disubstituted-imidazole-2-thiones R¹R²C₃N₂S (R¹ = R² = Me (1a), ⁱPr (1b), Cy (1c), C₆Me₃H₂ (1d); R¹ = Me, R² = Ph (1e)) under the different conditions afford five mononuclear complexes trans-[(R¹R²C₃N₂S)₂PdCl₂] (R¹ = R² = Me (2a), ⁱPr (2b), Cy (2c), C₆Me₃H₂ (2d); R¹ = Me, R² = Ph (2e)) and five binuclear Pd(II) complexes [(PdCl₂){μ-(R¹R²C₃N₂S)}]₂ (R¹ = R² = Me (3a), ⁱPr (3b), Cy (3c), C₆Me₃H₂ (3d); R¹ = Me, R² = Ph (3e)), respectively. Complexes 2a-2e are easily converted into the corresponding 3a-3e by adding equimolar PdCl₂ in refluxing MeOH, while the reverse reaction is achieved at room temperature by addition of 2 equiv of 1a-1e. In 2b, 2d, and 2e, each Pd(II) holds a distorted square planar geometry completed by two trans Cl atoms and two trans S atoms. Complexes 3a-3e have a dimeric [Pd₂S₂] structure in which two {PdCl₂} units are interlinked by two N,N′-disubstituted-imidazole-2-thiones. Each Pd(II) adopts a distorted square planar geometry accomplished by two cis Cl atoms and two cis bridging S atoms. Among them, complex 3d has the two largest C₆Me₃H₂ groups on the 2 and 5 positions of imidazole-2-thione, the longest Pd-μ-S bond, the largest S-Pd-S angle, and displays the highest catalytic activity toward Suzuki-Miyaura and copper-free Sonogashira cross-coupling reactions, which are confirmed by density functional theory calculations. The results provide an interesting insight into the introduction of various substituent groups into the periphery ligands of coordination complex-based catalysts, which could tune their geometric structures to acquire the best catalytic activity toward organic reactions.

Full text of DOI:10.1021/acs.inorgchem.7b01616

Article
pubs.acs.org/IC
Palladium(II) Chloride Complexes of N,N′-Disubstituted Imidazole-2-
thiones: Syntheses, Structures, and Catalytic Performances in
Suzuki−Miyaura and Sonogashira Coupling Reactions
†
†
,†,‡
§
,†
Li-Ming Zhang, Hai-Yan Li, Hong-Xi Li,* David James Young, Yong Wang,*
,†,‡
and Jian-Ping Lang*
†State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical
Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu, People’s Republic of China
‡
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
§
Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Maroochydore DC, Queensland 4558
Australia
*
S Supporting Information
ABSTRACT: Reactions of PdCl with 2 equiv of N,N′-
2
1
2
1
2
disubstituted-imidazole-2-thiones R R C N S (R = R = Me
3
2
i
1
2
(
(
1a), Pr (1b), Cy (1c), C Me H (1d); R = Me, R = Ph
6 3 2
1e)) under the different conditions afford five mononuclear
complexes trans-[(R R C N S) PdCl ] (R = R = Me (2a),
Pr (2b), Cy (2c), C Me H (2d); R = Me, R = Ph (2e)) and
five binuclear Pd(II) complexes [(PdCl ){μ-(R R C N S)}]
1
2
1
2
3
2
2
2
i
1
2
6
3
2
1
2
2
3
2
2
1
2
i
1
(
R = R = Me (3a), Pr (3b), Cy (3c), C Me H (3d); R =
6 3 2
2
Me, R = Ph (3e)), respectively. Complexes 2a−2e are easily
converted into the corresponding 3a−3e by adding equimolar
PdCl2 in refluxing MeOH, while the reverse reaction is
achieved at room temperature by addition of 2 equiv of 1a−1e. In 2b, 2d, and 2e, each Pd(II) holds a distorted square planar
geometry completed by two trans Cl atoms and two trans S atoms. Complexes 3a−3e have a dimeric [Pd S ] structure in which
2
2
two {PdCl } units are interlinked by two N,N′-disubstituted-imidazole-2-thiones. Each Pd(II) adopts a distorted square planar
2
geometry accomplished by two cis Cl atoms and two cis bridging S atoms. Among them, complex 3d has the two largest
C Me H groups on the 2 and 5 positions of imidazole-2-thione, the longest Pd−μ-S bond, the largest S−Pd−S angle, and
6
3
2
displays the highest catalytic activity toward Suzuki−Miyaura and copper-free Sonogashira cross-coupling reactions, which are
confirmed by density functional theory calculations. The results provide an interesting insight into the introduction of various
substituent groups into the periphery ligands of coordination complex-based catalysts, which could tune their geometric
structures to acquire the best catalytic activity toward organic reactions.
INTRODUCTION
extensively used as versatile ligands for many Pd-catalyzed
■
37−48
organic reactions.
Imidazole-2-thiones have also attracted
The palladium-catalyzed cross-coupling reactions of aryl halides
with arylboronic acids (Suzuki−Miyaura reactions) or alkynes
Sonogashira reactions) have provided a powerful methodology
much attention, because their steric and electronic properties
can be tuned by varying groups on the imidazolyl ring.
Moreover, the CS double bond in these ligands displays high
thermal and chemical stability. A large number of Pd(II)
(
1
−6
7−9
for constructing C−C bonds.
Phosphine ligands
are
generally required to stabilize the active catalytic palladium
intermediates. However, such ligands suffer from disadvantages
of cost, toxicity, sensitivity to air/moisture, or difficulty of
removal from the organic products, which restricts their
4
9,50
complexes of unsubstituted imidazole-2-thiones,
N-
5
1−54
(
mono)substituted imidazole-2-thiones,
and tripodal
5
5−60
scorpionate tris(imidazole-2-thione) derivatives
have
1
0,11
been isolated. Currently, only a few reports deal with the
applications in synthesis.
Among the phosphine-free
12−16
synthesis of mono- and binuclear Pd(II) complexes with N,N′-
ligands, N-heterocyclic carbenes (NHCs)
and nitrogen-
6
0−62
1
7−21
22
disubstituted-imidazole-2-thiones
the C−C bond formation.
and their applications in
donors ligands
are arguably the most useful, but thiols
61,63
23 24 25,26 27 28,29
For example, one mononuclear
and thioether-based SS, NS, SCS,
NNS, NCS,
bidentate and tridentate ligands are also emerging as
3
0,31
complex [(phbtuimt)PdCl] (Hphbtuimt = 3,3′-(1,3-
Received: June 24, 2017
NSN
viable alternatives.
ligands, however, is that Pd catalysts can mediate C−S bond
3
2,33
One complication with sulfur-based
3
4−36
cleavage.
Nevertheless, thiourea derivatives have been
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
B
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Schematic Illustration of the Synthetic Process of Complexes 2a−2e and 3a−3e
Figure 1. View of the molecular structures of 2b (a), 3a (b), and 3d (c). All H atoms are omitted for clarity.
phenylene)bis(1-butyl-1,3-dihydro-2H-imidazole-2-thione)
shows moderate catalytic activity for Heck, Suzuki−Miyaura,
catalyzing Suzuki−Miyaura couplings in water, showing effects
of substituent groups on the catalytic activity. However, we
have no structural information for these catalysts and do not go
further into these catalytic reactions. In fact, many literature
reports also mention that catalytic activity can be tuned by
manipulating the substituted groups on the ancillary
61
and Sonogashira coupling reactions. The coupling reactions
of iodobenzene with phenyl boronic acid or p-tolyl boronic acid
provide the desired coupling products in moderate yields (67%
to ∼74%) in 48 h. Although it is known that the bimetallic
complexes can improve their catalytic activity and/or selectivity
84−87
ligands,
but the structures of the catalysts bearing these
64−78
due to the existence of cooperative effects,
there have been
ligands are unknown in most cases, which makes it hard to
correlate the relationship between structure and catalytic
activity. There always exists a strong demand to prepare a
series of ligands with tunable substituted groups and their
related metal coordination compounds for the systematic
studies on the aforementioned coupling reactions. To this end,
two dinuclear Pd(II) complexes of N,N′-disubstituted-
6
0
imidazole-2-thiones, namely, cis-[(PdCl ){μ-(Et C N S)}]
2
2
2
3
2
i
i
79
and [PdI( Pr C N S)(μ-I)(μ- Pr C N S)PdI ], and their
2
3
2
2
3
2
2
catalytic performance has not been explored yet.
However, we have involved in the syntheses and structures of
various metal thiolate complexes and their catalytic perform-
1
2
N,N′-disubstituted-imidazole-2-thione derivatives (R R C N S)
3
2
80−83
ances in the above-mentioned cross-coupling reactions.
For example, one tetranuclear Pd(II)/thiolate cluster
Pd (IMe) (Tab) ](OTf) (Cl) ] (IMe = 1,3-dimethylimidazo-
with tunable substituent groups are believed to be ideal
1
2
1
2
candidates. In this article, we prepared five R R C N S (R = R
3
2
i
1
2
[
= Me (1a), Pr (1b), Cy (1c), C Me H (1d); R = Me, R =
4
4
6
6
2
6 3 2
line-2-ylidene; Tab = 4-(trimethylammonio)benzenethiolate;
OTf = trifluoromethanesulfonate) exhibits good catalytic
activity toward Suzuki−Miyaura couplings in water, though it
Ph (1e)) ligands and performed their reactions with 1 or 2
equiv of PdCl2 under different reaction conditions. Five
1
2
1
2
mononuclear complexes [(R R C N S) PdCl ] (R = R =
3
2
2
2
81
i
1
2
could catalyze only a limited scope of the substrates. Another
Me (2a), Pr (2b), Cy (2c), C Me H (2d); R = Me, R = Ph
6 3 2
example is related to the catalytic systems of Pd(OAc) and
(2e)) and five binuclear complexes [(PdCl ){μ-
2
2
1
2
1
2
i
three thioether ligands with a 4-trimethylbenzeneammonium
(R R C N S)}] (R = R = Me (3a), Pr (3b), Cy (3c),
3 2 2
1
1
2
group at one side and different substituent groups (benzyl (L ),
C Me H (3d); R = Me, R = Ph (3e)) were isolated. Each
6 3 2
2
3
1
-phenylethyl (L ), and naphthalen-1-yl-methyl (L )) at the
mononuclear Pd(II) complex can be readily converted into its
corresponding binuclear Pd(II) one. For the Suzuki−Miyaura
8
2
1
other side. The Pd(OAc) /L goes most effective at
2
C
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 2b, 2d·2MeCN, 2e, 3a·H O, and 3b−3e
Article
a
2
2
b
2d·2MeCN
2e
3a·H O
3b
3c
3d
3e
2
Pd(1)−Cl(1)
Pd(1)−Cl(2)
Pd(2)−Cl(3)
Pd(2)−Cl(4)
Pd(1)−S(1)
Pd(1)−S(2)
Pd(1)−S(1A)
2.3145(7)
2.3060(11)
2.3198(17)
2.315(2)
2.3083(10)
2.3152(9)
2.3056(10)
2.3064(9)
2.2867(9)
2.2918(9)
2.3081(7)
2.3183(7)
2.3265(10)
2.3103(12)
2.3123(6)
2.3024(6)
2.305(2)
2.3066(18)
2.3219(18)
2.2876(19)
2.3049(18)
2.3352(7)
2.3318(11)
2.3194(18)
2.2784(7)
2.2892(7)
2.3196(10)
2.3428(11)
2.2898(5)
2.2995(5)
Pd(2)−S(1)
Pd(2)−S(2)
2.2880(18)
2.2977(18)
90.86(7)
2.2896(10)
2.2931(9)
169.92(4)
S(1)−Pd(1)−Cl(2)
S(1)−Pd(1)−Cl(1A)
S(1)−Pd(1)−S(2)
S(1)−Pd(1)−S(1A)
Cl(2)−Pd(1)−S(2)
Cl(2)−Pd(1)−S(1A)
S(1)−Pd(1)−Cl(1)
Cl(2)−Pd(1)−Cl(1)
Cl(1)−Pd(1)−Cl(1A)
Cl(1)−Pd(1)−S(2)
Cl(1)−Pd(1)−S(1A)
Cl(1A)−Pd(1)−S(1A)
S(1)−Pd(2)−S(2)
S(1)−Pd(2)−Cl(3)
S(2)−Pd(2)−Cl(3)
S(1)−Pd(2)−Cl(4)
S(2)−Pd(2)−Cl(4)
Cl(3)−Pd(2)−Cl(4)
90.98(2)
82.34(2)
89.41(3)
86.17(3)
90.05(2)
94.38(2)
180.0
83.23(4)
180.0
85.70(6)
180.0
81.63(6)
79.33(3)
92.03(3)
85.227(19)
172.49(7)
167.57(2)
169.18(2)
97.02(2)
173.43(3)
177.23(3)
90.80(4)
171.10(2)
174.59(2)
95.02(2)
85.62(2)
180.00
96.77(4)
180.0
94.30(6)
180.0
173.20(7)
94.24(8)
95.19(4)
93.22(4)
93.25(7)
174.13(4)
94.38(2)
85.62(2)
83.23(4)
96.77(4)
85.70(6)
94.30(6)
91.06(2)
93.38(3)
90.00(2)
81.78(6)
173.63(7)
91.85(7)
91.95(7)
172.59(7)
94.42(7)
79.25(3)
95.19(4)
174.36(4)
170.50(4)
91.74(3)
93.86(4)
a
Symmetry codes: (A) 1 − x, 2 − y, −z for 2b·2MeCN; 1 − x, 1 − y, 1 − z for 2d and 2e. (A) 1 − x, 1 − y, −z for 3c and 3d; −x, 2 − y, −z for 3e.
and copper-free Sonogashira cross-coupling reactions, com-
pound 3d exhibits the best catalytic performance, which
correlates well with its presence of the largest substituted
groups (C Me H ) in the imidazole ring and the longest Pd−μ-
as the eluent to provide the pure product 1e. Yield: 1.56 g (82%).
Anal. Calcd for C H N S: C, 63.13; H, 5.30; N, 14.72%. Found: C,
10 10
2
1
6
3.43; H, 5.34; N, 15.11%. H NMR (400 MHz, CDCl , ppm): δ 7.55
3
(
6
d, J = 7.7 Hz, 2H), 7.47 (t, J = 7.6 Hz, 2H), 7.38 (t, J = 7.3 Hz, 1H),
.82 (d, J = 14.3 Hz, 2H), 3.66 (s, 3H). C NMR (101 MHz, CDCl3,
6
3
2
13
S bond, the largest S−Pd−S angle, and the lowest activation
ppm): δ 163.4, 138.5, 129.2, 128.5, 126.1, 118.6, 117.9, 35.5. IR (KBr
barrier (11.6 kcal/mol) provided by density functional theory
−1
disk, selected peak, cm ): 1144 (C = S).
(
DFT) calculations. Described below are the syntheses of the
Preparation of 2a. To a solution of PdCl (0.0177 g, 0.10 mmol)
2
ligands 1a−1e, complexes 2a−2e and 3a−3e, their structural
in MeCN (15 mL) was added a solution of 1a (0.0256 g, 0.20 mmol)
characterization, and the catalytic properties of 2a−2e and 3a−
in CH Cl2 (4 mL). A large amount of brick-red precipitate was
2
3
e.
observed to form over several hours. The mixture was stirred at room
temperature for 12 h, and the solid was collected by filtration, washed
EXPERIMENTAL SECTION
with Et
2
O, and dried in vacuo. Yield: 0.0346 g (88%). Anal. Calcd for
■
C H Cl N PdS : C, 27.69; H, 3.72; N, 12.92%. Found: C, 27.86; H,
10
16
2
4
2
General Procedures. N-Methyl-N-phenylimidazolium iodide, 1,3-
dimethyl-1,3-dihydro-imidazole-2-thione (1a), 1,3-diisopropyl-1,3-di-
hydro-imidazole-2-thione (1b), 1,3-dicyclohexyl-1,3-dihydro-2H-imi-
dazole-2-thione (1c), and 1,3-bis(2,4,6-trimethyl-phenyl)-1,3-dihydro-
imidazole-2-thione (1d) were prepared according to the literature
−1
3
1
.86; N, 13.0%. IR (KBr disk, cm ): 3146 (w), 3118 (s), 2946 (w),
564 (s), 1490 (s), 1469 (w), 1441 (w), 1396 (m), 1346 (w), 1238
(s), 1179 (s), 1097 (w), 744 (s), 678 (s).
Synthesis of 2b. The brick-red solid 2b was prepared in a similar
manner to that used for 2a but using PdCl (0.0178 g, 0.10 mmol) and
88−90
2
methods.
All commercial reagents were used without further
1
b (0.0371 g, 0.20 mmol) as starting materials. Yield: 0.0447 g (82%).
purification. Column chromatography was performed on silica gel. The
1
13
Anal. Calcd for C H Cl N PdS : C, 39.60; H, 5.91; N, 10.26%.
H NMR and C NMR spectra were recorded at ambient temperature
18 32
2
4
2
−1
1
Found: C, 39.94; H, 6.00; N, 10.42%. IR (KBr disk, cm ): 3169 (m),
144 (w), 2976 (m), 2933 (w), 2876 (w), 1684 (w), 1628 (w), 1577
w), 1559 (s), 1439 (s), 1415 (m), 1400 (w), 1371 (m), 1337 (m),
1205 (s), 1175 (w), 1136 (m), 1122 (m), 1067 (w), 1052 (m), 1001
on a Varian UNITY plus-400 spectrometer. The H chemical shifts
3
(
were referenced to Me Si (δ 0.0 ppm) and residual protons in CDCl3
4
(
δ 7.26 ppm), deuterated dimethyl sulfoxide (DMSO-d ) (δ 2.50
6
13
ppm), CD CN (δ 1.94 ppm). The C NMR spectra were reported
3
(w), 880 (w), 736 (s), 693 (s), 676 (w), 643 (w), 582 (w).
relative to CDCl (δ 77.16 ppm) and DMSO-d (δ 39.5 ppm).
3
6
Synthesis of 2c. The brick-red solid 2c was prepared in an
analogous method to that used for 2a but using PdCl (0.0176 g, 0.10
mmol) and 1c (0.0534 g, 0.20 mmol) as starting materials. Yield:
0.0571 g (81%). Anal. Calcd for C30H48Cl N PdS : C, 51.03; H, 5.85;
N, 7.93%. Found: C, 49.85; H, 6.22; N, 7.52%. IR (KBr disk, cm ):
3119 (m), 3082 (w), 2933 (s), 2855 (s), 1556 (s), 1495 (m), 1447 (s),
1415 (m), 1396 (m), 1365 (w), 1323 (m), 1289 (m), 1267 (w), 1246
(s), 1204 (s), 1151 (m), 1129 (w), 1058 (w), 1030 (w), 1001 (m),
Synthesis of 1e. N-Methyl-N-phenylimidazolium iodide (2.87 g,
0 mmol) in MeOH (30 mL) was added to a 100 mL round-bottomed
flask fitted with a reflux condenser. Sulfur powder (0.32 g, 10 mmol)
1
2
and K CO (1.38 g, 10 mmol) were further added, and the mixture
2 4 2
2
3
−1
was stirred at 40 °C for 6 h. After that the methanol was removed, and
the remaining solid was extracted with 2 × 30 mL of CH Cl , which
was then filtered and evaporated. The crude product was purified by
flash column chromatography using petroleum ether and ethyl acetate
2
2
D
DOI: 10.1021/acs.inorgchem.7b01616
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Inorganic Chemistry
Article
−
1
2
1
(
H), 3.76 (s, 6H). IR (KBr disk, cm ): 3146 (m), 3118 (s), 2946 (w),
625 (w), 1585 (w), 1564 (s), 1490 (s), 1470 (m), 1441 (m), 1397
Table 3. Optimizing the Reaction Conditions for Suzuki−
Miyaura Cross-Coupling of 4a with 5a
s), 1238 (s), 1179 (s), 1133 (w), 1097 (w), 745 (s), 678 (s), 665 (w),
6
37 (w), 617 (w), 480 (w).
Synthesis of 3b. The red solid 3b was prepared in a similar way to
that used for 3a but using PdCl (0.0179 g, 0.10 mmol) and 1b
2
(
0.0185 g, 0.10 mmol) as starting materials. Yield: 0.0307 g (85%).
cat loading
(mol %)
temp
a
b
Anal. Calcd for C H Cl N Pd S : C, 29.89; H, 4.46; N, 7.75%.
Found: C, 29.52; H, 4.77; N, 7.76%. H NMR (400 MHz, CD CN,
ppm): δ 7.35 (s, 2H), 5.21 (m, J = 6.7 Hz, 2H), 1.47 (d, J = 6.8 Hz,
12H). IR (KBr disk, cm ): 3138 (w), 3111 (w), 3081 (m), 2977 (m),
2935 (w), 2876 (w), 1624 (m), 1556 (s), 1458 (s), 1406 (s), 1385 (s),
1375 (m), 1213 (s), 1180 (m), 1137 (m), 1083 (w), 1058 (w), 883
(w), 760 (m), 689 (m), 664 (w), 618 (w), 582 (w), 528 (w).
Synthesis of 3c. The red solid 3c was prepared in a similar
18
32
4
4
2 2
entry
cat
solvent
base
(°C)
yield
1
3
1
2
3
4
5
6
7
8
9
3d
3d
3d
3d
3d
3d
2a
2b
2c
2d
2e
3a
3b
3c
3e
3d
0.25
0.25
0.25
0.25
0.25
0.25
0.5
CH CH OH K CO
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
82%
62%
43%
86%
63%
3
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
−
1
CH CN
K CO
3
2
dioxane
DMF
K CO
2
K CO
2
toluene
K CO
2
H O
K CO
>99%
5%
2
2
approach to that used for 3a but using PdCl (0.0178 g, 0.10 mmol)
and 1c (0.0264 g, 0.10 mmol) as starting materials. Yield: 0.0343 g
H O
K CO
2
2
2
0.5
H O
K CO
15%
74%
87%
84%
85%
87%
95%
72%
>99%
2
2
(78%). Anal. Calcd for C H Cl N Pd S : C, 40.78; H, 5.48; N,
30 48 4 4 2 2
0.5
H O
K CO
1
2
2
6
.34%. Found: C, 39.94; H, 6.00; N, 10.42%. H NMR (400 MHz,
1
1
1
1
1
1
1
0
1
2
3
4
5
6
0.5
H O
K CO
2
2
CD CN, ppm): δ 7.32 (s, 2H), 4.86 (m, 2H), 1.77 (d, J = 10.9 Hz,
3H), 1.58 (m, 8H), 1.28 (m, 2H). Eight hydrogen signals got lost due
to peak overlap. IR (KBr disk, cm ): 3169 (m), 3144 (w), 2976 (m),
2933 (w), 2876 (w), 1684 (w), 1628 (w), 1577 (w), 1559 (s), 1439
(s), 1415 (m), 1400 (w), 1371 (m), 1337 (m), 1205 (s), 1175 (w),
1136 (m), 1122 (m), 1067 (w), 1052 (m), 1001 (w), 736 (s), 693 (s),
3
0.5
H O
K CO
2
2
0.25
0.25
0.25
0.25
0.25
H O
K CO
−1
2
2
H O
K CO
2
2
H O
K CO
2
2
H O
K CO
2
2
6
76 (w), 643 (w), 582 (w).
Synthesis of 3d. The red solid 3d was prepared in an analogous
H O
K PO ·
2
3
3
4
H O
2
17
18
19
20
21
22
23
24
25
3d
3d
3d
3d
3d
3d
3d
3d
3d
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.05
0.025
H O
NaOH
NaAc
60
60
60
60
60
60
40
60
60
94%
93%
97%
96%
94%
60%
97%
>99%
86%
way to that used for 3a but using PdCl (0.0176 g, 0.10 mmol) and 1d
2
2
(
0.0336 g, 0.10 mmol) as starting materials. Yield: 0.0380 g (74%).
H O
2
Anal. Calcd for C H Cl N Pd S : C, 49.09; H, 4.71; N, 5.45%.
H O
2
Na CO
2
42 48
4
4
2 2
3
−
1
Found: C, 48.89; H, 4.88; N, 5.20%. IR (KBr disk, cm ): 3160 (m),
102 (s), 3066 (s), 2962 (w), 2918 (m), 2858 (w), 1607 (m), 1550
s), 1480 (s), 1443 (m), 1382 (m), 1325 (w), 1305 (w), 1225 (m),
169 (w), 1137 (m), 1081 (w), 1100 (w), 1035 (w), 927 (w), 862
H O
2
KOH
3
(
1
H O
2
Et N
3
H O
2
NaHCO3
K CO
H O
2
2
3
3
3
(
m), 850 (m), 774 (m), 733 (w), 723 (w), 703 (m), 599 (w), 580 (w).
Synthesis of 3e. The brick-red solid 3e was prepared in a similar
H O
2
K CO
2
H O
2
K CO
2
method to that used for 3a but using PdCl (0.0177 g, 0.10 mmol) and
2
a
1
e (0.0190 g, 0.10 mmol) as starting materials. Yield: 0.0305 g (83%).
Reaction conditions: 4a (1.0 mmol), 5a (1.5 mmol), base (2 mmol),
b
Anal. Calcd for C H Cl N Pd S : C, 32.68; H, 2.74; N, 7.62%.
Found: C, 34.44; H, 3.18; N, 7.99%. H NMR (400 MHz, CD CN,
ppm): δ 7.66 (m, 2H), 7.63 (m, 3H), 7.38 (m, 2H), 3.90 (s, 3H). IR
cat, solvent (3 mL), for 5 h in N2. Determined by gas
chromatography.
20 20
4
4
2 2
1
3
−1
(KBr disk, cm ): 3149 (m), 3109 (s), 3060 (w), 1619 (w), 1593 (w),
86 (m), 895 (s), 820 (m), 756 (m), 735 (s), 726 (s), 687 (m), 572
1498 (s), 1479 (w), 1457 (w), 1416 (w), 1384 (w), 1330 (w), 1239
(s), 1172 (w), 1126 (w), 1075 (w), 1023 (w), 1004 (w), 764 (s), 726
(w), 693 (s), 665 (w), 629 (w), 558 (m).
Synthesis of 2d. The red solid 2d was prepared in a similar way to
that used for 2a but using PdCl (0.0089 g, 0.05 mmol) and 1d
General Procedure for the Suzuki−Miyaura Reaction. A
mixture of aryl bromide (1.0 mmol), arylboronic acid (1.5 mmol), 3d
(0.05 mol %), K CO (2 mmol), and deoxygenated H O (3 mL) was
2
(
0.0336 g, 0.10 mmol) as starting materials. Yield: 0.0425 g (75%).
Anal. Calcd for C H Cl N PdS : C, 59.33; H, 5.69; N, 6.59%.
42
48
2
4
2
2
3
2
−1
Found: C, 58.75; H, 5.90; N, 8.67%. IR (KBr disk, cm ): 3160 (m),
102 (s), 3066 (s), 2963 (w), 2918 (m), 2855 (w), 2741 (w), 1762
w), 1608 (s), 1550 (s), 1482 (s), 1442 (m), 1415 (w), 1364 (m),
added to a 25 mL round-bottom flask. The mixture was stirred at 60
3
(
°C under N and cooled to room temperature. After that it was
2
extracted three times with diethyl ether (3 × 5 mL), and the combined
organic layer was washed with brine (20 mL), dried over anhydrous
Na SO , and concentrated to dryness under reduced pressure. The
1
8
226 (s), 1137 (m), 1081 (w), 1035 (m), 982 (w), 941 (w), 927 (m),
61 (s), 850 (s), 774 (m), 733 (w), 703 (m), 599 (w), 579 (w).
Synthesis of 2e. The brick-red solid 2e was prepared in a similar
2
4
crude product was purified by flash column chromatography using
petroleum ether and ethyl acetate as the eluent to provide the
corresponding pure product.
route to that used for 2a but using PdCl (0.0179 g, 0.10 mmol) and
2
1
e (0.0380 g, 0.20 mmol) as starting materials. Yield: 0.0557 g (83%).
Anal. Calcd for C H Cl N PdS : C, 43.06; H, 3.61; N, 10.04%.
General Procedure for the Sonogashira Cross-Coupling
20
20
2
4
2
−1
Found: C, 43.19; H, 3.82; N, 10.25%. IR (KBr disk, cm ): 3156 (w),
123 (w), 3089 (w), 3048 (w), 1595 (m), 1559 (m), 1498 (s), 1464
m), 1411 (w), 1382 (m), 1239 (s), 1154 (s), 1073 (w), 1025 (w),
Reaction. A mixture of aryl iodide (0.5 mmol), terminal alkyne (1.0
3
mmol), Et
loaded into a 25 mL round-bottom flask. The mixture was stirred at 60
°C for 24 h under N and cooled to ambient temperature. After that it
3
N (1.5 mmol), 3d (0.5 mol %), and MeCN (3 mL) was
(
9
61 (w), 780 (m), 747 (m), 730 (m), 696 (s), 684 (m), 564 (m).
2
Synthesis of 3a. Ligand 1a (0.0128 g, 0.10 mmol) was loaded into
was partitioned between water and ethyl acetate, and the organic layer
was separated, while the aqueous layer was extracted with ethyl acetate
(3 × 5 mL). A similar workup to that described above yielded the pure
desired product.
a MeOH (10 mL) solution of PdCl (0.0177 g, 0.10 mmol). The
2
resulting mixture was refluxed for 12 h to yield a red precipitate, which
was washed sequentially with MeOH and Et O and dried in vacuo.
2
Red blocks of 3a suitable for X-ray diffraction were obtained after a few
days by slow evaporation of solvents from the filtrate at room
temperature. Yield: 0.0271 g (89%). Anal. Calcd for
C H Cl N Pd S : C, 19.66; H, 2.64; N, 9.17%. Found: C, 19.19;
Single-Crystal X-ray Crystallography. Single crystals of 2b, 2d·
2MeCN, or 2e were obtained by slow diffusion of a MeCN solution of
PdCl into a CH Cl solution of 1b, 1d, or 1e, while those of 3a·H O
and 3b−3e were obtained directly from the above preparations. X-ray
diffraction data collection was performed on an Xcalibur, Atlas, Gemini
2
2
2
2
1
0
16
4
4
2 2
1
H, 3.10; N, 9.65%. H NMR (400 MHz, CD CN, ppm): δ 7.21 (S,
3
E
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 4. Suzuki−Miyaura Cross-Coupling Reaction of Aryl Bromides with Aryl Boronic Acids Catalyzed by 3d
a
All reactions were performed under N in the presence of 1.0 mmol of aryl bromide, 1.5 mmol of arylboronic acid, 0.05 mol % of 3d, and 2 mmol of
2
b
c
K CO in 3 mL of H O, for 5 h at 60 °C. Isolated yield. 24 h.
2
3
2
(
2b) or a Bruker APEX-II CCD (2d·2MeCN, 2e, 3a·H O, 3b−3e) X-
dazol-3-ium, 1,3-dicyclohexyl-1H-imidazol-3-ium, 1,3-dimesityl-1H-
imidazol-3-ium, or 1-methyl-3-phenyl-1H-imidazol-3-ium with ele-
mental sulfur and K CO in MeOH at 40 °C afforded 1a, 1b, 1c,
2
ray diffractometer using graphite monochromated Mo Kα (λ =
.710 73 Å). Each single crystal was mounted on a glass fiber with
grease and cooled in a liquid nitrogen stream at 223 (2b, 3d), 193 (2d·
MeCN and 2e), 173 (3a·H O, 3b, and 3e), or 150 (3c) K. The
0
2
3
8
8−90
1
1d,
and 1e, respectively. In the H NMR spectrum of 1e, the
2
singlet at δ = 3.66 ppm belongs to the three protons of methyl group.
The doublet at δ = 6.8 ppm is ascribed to the two protons of imidazole
ring. The doublets/multiplets at δ = 7.56, 7.47, 7.38 ppm are assigned
as the five protons of phenyl ring (Figure S1). The IR spectrum of 1e
2
crystal structures of 2b, 2d·2MeCN, 2e, 3a·H O, and 3b−3e were
solved by direct methods using the SHELXS-97 (2b, 2d·2MeCN, 2e,
3
matrix least-squares on F . All of the non-H atoms were refined on
F anisotropically by the full-matrix least-squares method. All H atoms
were introduced at the calculated positions and included in the
structure-factor calculations. A summary of the pertinent crystal data
and structure refinement parameters for 2b, 2d·2MeCN, 2e, 3a·H O,
2
a·H O, 3b, 3c, 3e) or SHELXL-2014/7 (3d) and refined by full
2
91
2
−1
exhibits strong absorption band at λ = 1144 cm attributable to the
2
CS bond. Notably, the reactions of PdCl
stoichiometry-dependent. Treatment of PdCl with 2 equiv of 1a−
1e in MeCN and CH Cl gave rise to five mononuclear Pd(II)
with 1a−1e are
2
2
2
2
1
2
1
2
i
complexes trans-[(R R C N S) PdCl ] (R = R = Me (2a), Pr (2b),
2
3 2 2 2
1
2
and 3b−3e is in Table 1.
Cy (2c), C
to ∼88%; Scheme 1). The corresponding reactions of PdCl
equimolar 1a−1e in refluxing MeOH produced five binuclear Pd(II)
6
Me
3
H
2
(2d); R = Me, R = Ph (2e)) in good yields (75%
Results and Discussion. Synthesis of N,N′-Disubstituted
Imidazole-2-thiones and Complexes 2a−2e and 3a−3e.
Reaction of 1,3-dimethyl-1H-imidazol-3-ium, 1,3-diisopropyl-1H-imi-
2
with
1
2
1
2
i
complexes [(PdCl ){μ-(R R C N S)}] (R = R = Me (3a), Pr (3b),
2
3
2
2
F
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 5. Comparison of the Catalytic Performances of 3d and Other Known Catalysts in the Suzuki−Miyaura Reactions of
Arylboronic Acids with Arylbromides or Aryliodides
entry
cat
mol % Pd
R1, X
R2
solvent
base
T (°C)
time
yield
ref
1
2
3
4
5
6
8
9
dtobimth/PdI2
1
OMe, Br
COMe, Br
I
H
i-PrOH/H O
K CO3
80
100
80
5 h
98%
99%
74%
67%
33%
93%
18%
95%
33%
77%
90%
99%
95%
96%
99%
95%
60
37
38
61
61
40
24
23
23
23
23
22
22
22
22
27
28
31
31
2
2
1,3-bdtbbphimth/Pd(OAc)2
[(phbtuimt)PdCl2]
1.5
2
OMe
H
i-PrOH/H O
K CO3
2
24 h
48 h
48 h
10 h
5 h
2
dioxane
dioxane
Cs CO3
2
2
I
Me
H
Cs CO3
2
80
3,3′-Hphbdbphimth/Pd(dba)2
[(pybethim)PdCl(PPh )]
0.5
0.1
0.1
1
COMe, Br
OMe, Br
Me, Br
H O/NMP
2
K CO3
2
120
80
Me
H
toluene
K CO3
2
3
[(PhSCH CH SPh)PdCl ]
DMF/H O
K CO3
2
100
100
100
100
100
100
100
100
75
12 h
4 h
2
2
2
2
Me, Br
H
DMF/H O
K CO3
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
[(PhSCH CH SePh)PdCl ]
0.1
1
Me, Br
H
DMF/H O
K CO3
2
12 h
12 h
10 h
10 h
10 h
10 h
13 h
15 h
12
2
2
2
2
Me, Br
H
DMF/H O
K CO3
2
2
[Pd (dppe) (S(C H )S)] (OTf)
0.04
0.04
0.0004
0.4
0.01
0.1
2
Me, Br
H
H O/1,4-dioxane
2
K CO3
2
2
2
6
4
2
4
COMe, Br
COMe, Br
Me, Br
H
H O/1,4-dioxane
2
K CO3
2
H
H O/1,4-dioxane
2
K CO3
2
[Pd (dppe) (S(C H )S)] (OTf)
H
H O/1,4-dioxane
2
K CO3
2
2
2
12
8
2
4
[(phpymhcth)Pd(PPh )]Cl
COMe, Br
Me, Br
OMe
H
EtOH
Na CO3
2
3
[(phphthprammph)PdCl]
DMF/H O
K CO3
2
100
100
100
60
2
{[(PhSCH CH ) NH]PdCl}Cl
COMe, Br
COMe, Br
COMe, Br
Me, Br
H
H O
2
K CO
2
2
2
2
{[(PhSeCH CH ) NH]PdCl}Cl
2
H
H O
2
K CO
2
12
83
2
2
2
3d
3d
3d
0.1
0.1
0.1
H
H O
2
K CO3
2
5 h
99%
95%
92%
this work
this work
this work
OMe
OMe
H O
2
K CO3
2
60
5 h
OMe, Br
H O
2
K CO3
2
60
5 h
1
2
Cy (3c), C Me H (3d); R = Me, R = Ph (3e)) in reasonable yields
structures of 2b, 2d·2MeCN, 2e, 3a·H O, and 3b−3e are determined
6
3
2
2
(
74% to ∼89%), while the reaction of PdCl with 2 equiv of Et C N S
in EtOH/H O was reported to afford cis-[(Et C N S) PdCl ].
Interestingly, these mono- and binuclear complexes could be readily
converted into each other. Mononuclear complexes 2a−2e could also
be formed in relatively high yields subject to the addition of 2 equiv of
imidazole-2-thione into the solution of the corresponding binuclear
complexes 3a−3e. Heating each of 2a−2e with equimolar PdCl in
MeOH at 90 °C for 12 h afforded the corresponding binuclear
complexes 3a−3e.
by single-crystal X-ray crystallography.
2
2
3
2
6
0
Crystal Structures of 2b, 2d·2MeCN, 2e, 3a·H
Complexes 2b, 2d·2MeCN, and 2e crystallize in the monoclinic space
groups P2 /n, P2 /c, and C2/c, respectively. In the structures of 2b,
O, and 3b−3e.
2
2
3
2
2
2
2
1
1
2d, and 2e, each Pd(II) takes a square-planar coordination geometry,
coordinated by two trans S atoms from N,N′-disubstituted imidazole-
2
-thiones and two trans Cl atoms (Figures 1a and S7). However, each
2
Pd(II) center in cis-[(Et C N S) PdCl ] is coordinated by two cis Cl
2
3
2
2
2
6
0
atoms and two cis S atoms from two Et C N S ligands. The Pd−Cl
2
3
2
bond lengths (2.060(1) to ∼2.3198(17) Å; Table 2) are close to those
found in [PdCl (L) ] (2.318(4) to ∼2.362(4) Å; L = 1,3-
Complexes 2a−2e and 3a−3e are all stable toward oxygen and
moisture. Complexes 2a−2e and 3d are sparingly soluble in common
organic solvents, while 3a−3c and 3e are insoluble in toluene, hexane,
2
2
4
4
dimesitylimidazolidine-2-thione).
The Pd−S bond lengths
(
2.3194(18) to ∼2.3352(7) Å) in 2b, 2d·2MeCN, and 2e are longer
than those observed in [PdI( Pr C N S)(μ-I)(μ- Pr C N S)PdI ]
2.318(2) Å) and [PdCl (L)] (2.271(3) to ∼2.268(1) Å; L = 1-
1-(4-isopropyl-4,5-dihydrooxazol-2-yl)-1-methylethyl]-3-(2-isopro-
and Et O but slightly soluble in MeCN, CH Cl , DMSO, and
2
2
2
i
i
2
3
2
2
3
2
2
dimethylformamide (DMF). The elemental analyses of these Pd(II)
complexes are consistent with their chemical formulas. In the IR
spectra of 2a−2e and 3a−3e, the peaks at 1144 (2a), 1136 (2b), 1151
7
9
(
[
2
9
2
pylphenyl)-1,3-dihydrobenzoimidazole-2-thione).
(
(
2c), 1137 (2d), 1154 (2e), 1133 (3a), 1137 (3b), 1136 (3c), 1137
−1
Compound 3a crystallizes in the triclinic space group P1
asymmetric unit contains one discrete [(PdCl ){μ-(Me C N S)}]
2
̅
, and its
3d), and 1172 (3e) cm are assigned to be the v(CS) vibration of
1
2
2
3
2
imidazole-2-thione ligands. The H NMR spectrum of 3a shows a
singlet at δ = 3.76, which is assignable to the protons of the two methyl
molecule and one H O lattice molecule. Compounds 3b−3e
2
crystallize in the monoclinic space groups P2 /c (3b) and P2 /n
1
1
1
groups (Figure S2). In the H NMR spectrum of 3b, the doublet at δ =
(
3c−3e), and their asymmetric units contain one independent
1
.47 ppm and the multiplet at δ = 5.21 ppm belong to the protons of
isopropyl groups (Figure S3). The signals at δ = 1.3, 1.57, and 1.75
ppm in the H NMR spectrum of 3c are ascribed to the protons of
cyclohexyl group (Figure S4). One singlet at δ = 3.90 ppm and
multiplets at δ = 7.66 and 7.62 pmm in the H NMR spectrum of 3e
can be assigned as the protons of the methyl group and the protons of
phenyl rings, respectively (Figure S5). The H NMR spectra of 3a−3c
and 3e exhibit the singlet at 7.21 ppm (3a), δ = 7.35 ppm (3b), δ =
i
molecule [(PdCl ){μ-L}] (L = Pr C N S (3b), Cy C N S (3c),
2
2
2
3
2
2
3
2
(
C Me H ) C N S (3d), or MePhC N S (3e)). These complexes
1
6 3 2 2 3 2 3 2
contain two [PdCl ] moieties linked by two μ-S atoms from two
2
imidazole-2-thiones to form a dimeric [Pd(μ-S) Pd] unit (Figures 1b,c
2
1
and S8). Each Pd(II) in 3a−3e has a distorted square-planar
coordination geometry completed by two cis bridging μ-S atoms and
1
two cis terminal Cl atoms. However, the [Pd(μ-S) Pd] structures of 3a
2
and 3b are somewhat different from those of 3c−3e. Two Pd atoms
and two S atoms deviate from the same planarity with a dihedral angle
7
.32 ppm (3c), or δ = 7.39/7.37 ppm (3e), which belongs to the two
protons of the imidazole ring. As a representative example, we examine
the solution behavior of 3d in MeCN by measuring its positive-ion
electrospray ionization mass (ESI-MS) spectrum. One peak at m/z =
between two “PdS ” planes of 38.1° (3a) and 41.8° (3b), forming a
2
bent [Pd(μ-S) Pd] rhomb, while those in 3c−3e are located in the
2
same plane, forming a normal [Pd(μ-S) Pd] rhomb. Such dihedral
2
6
0
9
[
91.0 can be assigned to be one dinuclear cation
angles in 3a, [(PdCl ){μ-(Me C N S)}] (40.02°) and 3b increase
2 2 3 2 2
i
+
{(C Me H ) C N S} Pd Cl ] , which matches well with the
along with the size of substituents in the order of Me < Et < Pr. The
two imidazole rings in 3a and 3b are situated on the same side of the
bent rhomb, while those in 3c−3e are centrosymmetric with a
6
3
2
2
3
2
2
2
3
simulated isotopic patterns (Figure S6). Thus, we assume that 3d
should keep its dinuclear structure in the solution. Finally the crystal
G
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
complete within 5 h at 60 °C (entry 6). The catalytic performances of
the mononuclear precatalysts 2a−2e were also evaluated using
Suzuki−Miyaura coupling reactions in water under similar reaction
conditions. The desired product 6a was isolated in 5%, 15%, 74%,
Table 6. Optimizing Reaction Conditions for Sonogashira
Cross-Coupling of 7a with 8a
8
7%, and 84% yields, respectively (entries 7−11). By comparison,
reactions catalyzed by 3a−3e performed much better (entries 6 and
12−15) than those of the corresponding mononuclear complexes,
which may be due to the cooperative effect of dinuclear unit in 3a−3e.
In addition, the two trans thione ligands in 2a−2e depressed the ability
of Pd center to oxidatively add the aryl bromide substrate. Complex 3d
exhibited the highest catalytic activity of the five binuclear complexes,
indicating that introducing two bulky C Me H groups on the 2 and 5
a
b
entry
cat
solvent
base
temp (°C) time (h)
yield
51%
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
MeCN
MeCN
MeCN
MeCN
MeCN
Et N
3
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
40
60
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
18
24
12
Et N
3
58%
62%
>99%
30%
60%
10%
60%
30%
88%
59%
43%
70%
4%
Et N
3
Et N
3
6
3
2
Et N
3
positions of imidazole-2-thione did improve the catalytic performance.
As discussed above, the longest Pd−μ-S bond and the enlarged S−
Pd−S angle in 3d may favor the reductive elimination and thus
H O
2
Et N
3
toluene
MeOH
THF
Et N
3
increase the stability of coordinatively unsaturated active catalytic
Et N
3
93,94
species.
Such a structure-to-activity relationship is in good
Et N
3
95,96
agreement with previous observations
and is confirmed by our
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
DMF
Et N
3
theoretical calculations described below. Complexes 3d, 4a, and 5a
were thus chosen as a model system to optimize reaction conditions.
The effect of base (K PO , NaOH, NaOAc, Na CO , KOH, NaHCO ,
acetone
EtOH
Et N
3
Et N
3
3
4
2
3
3
Et N
3
and Et N) was examined (entries 16−22). It was found that either
3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Na CO
2
K PO or K CO was the most suitable base. When the reaction
3
3
4
2
3
K CO
47%
89%
<2%
65%
2%
temperature (60 °C) was decreased to 40 °C, the yield of 6a got
dropped almost imperceptibly from greater than 99% to 97% (entry
2
3
NaOAc
NaOH
KOH
2
3). The catalyst loading could be decreased to 0.05 mol %, which still
afforded 6a in a quantitative yield (entry 25). Therefore, the optimized
reaction conditions were determined to be 0.05 mol % of 3d, with
K CO (as a base) for 5 h at 60 °C in H O.
NaOEt
NMI
2
3
2
<2%
70%
78%
95%
With suitable reaction conditions established, we next investigated
K PO
3
4
the scope of the substrates with a series of aryl bromides and
arylboronic acids (Table 4). This study revealed that the electronic
nature of the substituents on the aryl bromides had some effects on the
yields of the coupling products. Reaction of (4-methoxyphenyl)-
boronic acid (5b) with phenyl bromide bearing an electron-
withdrawing substituent group (carbonyl or nitro) or electron-
donating substituent group (methoxy or methyl) could afford the
corresponding product in almost quantitative yields (entries 2−5).
The reactions of 3- or 4-bromoacetophenone with 5b provided the
corresponding products 1-(4′-methoxy-[1,1′-biphenyl]-4-yl)ethan-1-
one (6b, 99% yield; entry 2) and 1-(4′-methoxy-[1,1′-biphenyl]-3-
yl)ethan-1-one (6f, 92% yield; entry 6) in excellent yields. Reaction of
1-(2-bromophenyl)ethan-1-one with 5b, however, produced 1-(4′-
methoxy-biphenyl-2-yl)ethanone in only 56% yield (entry 7),
presumably due to the steric hindrance by the ortho substituent.
When the reaction time was extended to 24 h, the same substrates
afforded 6g in a significantly higher yield (88%). Electron-rich and
-neutral arylboronic acids reacted smoothly to give the corresponding
products in 92%−99% yields (entries 1, 2, 8, 9, 11). The reaction also
proceeded well for a para-substituted electron-deficient arylboronic
acid (entry 10). Notably, the steric hindrance of an ortho substitutent
on the arylboronic acids did not significantly affect the yield (entry 8).
We also examined the cross-coupling reactions of heteroaryl boronic
acids with 4a. Good or excellent yields were observed under the
optimized conditions (entries 12−14). Table 5 lists comparison on the
catalytic performances in the Suzuki−Miyaura reactions of arylboronic
acids with aryl bromides or aryl iodides using 3d and other known
catalysts. Complex 3d possesses higher catalytic activity than some
DEA
Et N
3
Et N
3
50%
c
d
Et N
3
83% /3%
a
Reaction conditions: 7a (0.5 mmol), 8a (1.0 mmol), cat (0.0025
mmol), and base (1.5 mmol, 3 equiv), under N . NMI = N-
methylimidazole, DEA = diethylamine. Determined by gas
chromatography. With CuI (0.01 mmol) as a cocatalyst. The yield
2
b
c
d
of 1,4-diphenylbuta-1,3-diyne.
crystallographic inversion center located at the midpoint of the two Pd
centers. These differences result from the different steric constraints
imposed by the substituted groups on the imidazole ring. The two
ligands with larger substituted groups such as Cy (3c), C Me H (3d),
and Me/Ph (3e) tend to bind Pd(II) centers in an opposite direction
to minimize the steric hindrance caused by the bulky groups. Among
6
3
2
3
a−3e, compound 3d has the two largest C Me H groups on the 2
6
3
2
and 5 positions of imidazole-2-thione and thus holds the longest
Pd−μ-S (2.3428(11) Å) bond length (Table 2). In addition, its S−
Pd−S angle of 86.17(3)° is larger than those in 3a (81.63(6)° and
8
(
1.78(6)°), 3b (79.33(3)° and 79.25(3)°), 3c (82.34(2)°), and 3e
85.227(19)°). The average Pd−μ-S bond distance in 3d (2.3312(10)
Å) is longer than those of 3a (2.2945(18) Å), 3b (2.2903(9) Å), 3c
(
2.2838(7) Å), and 3e (2.2947(5) Å), while the mean terminal Pd−Cl
bond lengths (2.3074(6) to ∼2.3184(2) Å) in 3a−3e are comparable
to those in 2b, 2c, and 2d.
38
Suzuki−Miyaura Cross-Coupling Reactions. The catalytic
performance of precatalyst 3d was first evaluated in the Suzuki−
Miyaura reaction. We performed the cross-coupling reaction with 0.25
mol % of 3d, 1-(4-bromophenyl)ethanone (4a, 1.0 mmol), phenyl-
boronic acid (5a, 1.5 mmol), and K CO (2 mmol) in 3 mL of
CH CH OH at 60 °C for 5 h. A standard workup produced the
desired product 1-biphenyl-4-yl-ethanone (6a) in 82% (Table 3, entry
). This preliminary result implied that these Pd(II) complexes of
reported catalytic systems such as 1,3-bdtbbphimth/Pd(OAc)
2
(1,3-
bdtbbphimth = 1,3-bis(4,4′-di-tert-butyl-[1,1′-biphenyl]-2-yl)-
6
1
imidazolidine-2-thione), [(phbtuimt)PdCl
], 3,3′-Hphbdbphimth/
(3,3′-Hphbdbphimth = 3,3′-(1,3-phenylene)bis(1-(2,5-
ditert-butylphenyl)imidazolidine-2-thione); dba = dibenzylideneace-
2
4
0
Pd(dba)
2
2
3
2
7
tone), [(phpymhcth)Pd(PPh )]Cl (Hphpymhcth = 2-(phenyl-
3
2
3
(pyridin-2-yl)methylene)hydrazine-1-carbothioamide),
2
8
1
[(phphthprammph)PdCl] (Hphphthprammph = 2-(phenyl((3-
(phenylthio)propyl)amino)methyl)phenol), [(PhSCH CH SPh)-
imidazole-2-thiones could initiate the cross-coupling reactions of aryl
bromides with arylboronic acids with high catalytic activity. Some
other common solvents (toluene, dioxane, DMF, CH CN, and H O)
2
2
2
3
23
PdCl2], and [(PhSCH CH SePh)PdCl ]. When the crossing-
2
2
2
coupling reactions are performed in anisole or i-PrOH/H O at higher
3
2
2
37
were also assessed. The conversion of 4a to 6a in H O was found to be
temperature (80 °C), some Pd/S catalysts such as [dtobimth/PdI2]
2
H
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 7. Sonogashira Cross-Coupling Reactions of Aryl iodides with Alkynes Catalyzed by 3d
a
Reaction conditions: 7 (0.5 mmol), 8 (1.0 mmol), 3d (0.0025 mmol), and Et N (1.5 mmol) in 3 mL of MeCN, 60 °C under N atmosphere.
Isolated yield.
3
2
b
Figure 2. Ten possible transition states of Suzuki−Miyaura reactions of catalyzed by mono- and binuclear Pd(II) complexes (free energies in kcal/
mol) at the B3LYP + SMD (H O)/6-31G(d) level.
2
I
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
dtobimth = (3aR,7aR)-1,3-di-o-tolyloctahydro-2H-benzo[d]-
transmetalation was considered as the rate-determining step for
24
97,98
imidazole-2-thione) and [(pybethim)PdCl(PPh )] (Hpybethim =
N-(pyridin-4-yl)benzothioamide) show comparable activity to that of
Suzuki−Miyaura cross-coupling reactions.
Thus, we only calcu-
3
lated the Gibbs free energy for possible diaryl-palladium(II)
3
d. In water, the catalytic activity of 3d is much higher than those of
intermediates. Geometry and frequency calculations were performed
with the B3LYP functional using the Gaussian 09 suite of program.
The LANL2DZ basis set was employed for Pd, while the 6-31G(d)
basis set was used for other atoms. Frequency analysis at the same level
was performed to ensure the stationary point as minimum or transition
state. Single-point calculations on the gas-phase optimized geometry
99
{
[(PhSCH CH ) NH]PdCl}Cl and {[(PhSeCH CH ) NH]PdCl}-
2
2
2
2
2 2
3
1
Cl and comparable to that of [Pd (dppe) (S(C H )S)] (OTf)
2
2
6
4
2
4
(
dppe = 1,2-bis(diphenylphosphino)ethane; OTf = trifluoromethane-
22
sulfonate) at 100 °C in H O/1,4-dioxane.
2
Sonogashira Cross-Coupling Reactions. To explore the utility
of the precatalysts 3a−3e, the catalytic performances of these
complexes were investigated in Sonogashira cross-coupling reactions
of aryl iodines with terminal alkynes. We performed the reaction of
with the SMD solvation model (solvent = H O for mono- and
2
binuclear Pd(II) catalysts, respectively) were applied. The Gibbs free
energy was calculated by adding the gas-phase Gibbs free energy
correction with the solution-phase single-point energy. The structures
of the intermediates (Int) and the transition states (TS) for 2a−2e
and 3a−3e are shown in Figure S9. Cartesian coordinates of all
optimized intermediates (Int) and transition states (TS) are listed in
Table S1. As discussed above, 3d displays the highest activity among
the mononuclear complexes (2a−2e) and binuclear complexes (3a−
3e). Figure 2 shows that the activation barrier of 3d is 11.6 kcal/mol,
which is the lowest among those of 2a−2e and 3a−3e. The
favorability (TS3d: 11.6 kcal/mol) of Suzuki−Miyaura reactions
catalyzed by binuclear Pd(II) complex 3d is in excellent agreement
with those obtained experimentally.
iodobenzene (7a) with 2 equiv of phenylacetylene (8a) in CH CN at
3
6
0 °C with 0.5 mol % of 3a as catalyst and 3 equiv of Et N as base
3
(Table 6, entry 1). The standard workup produced the desired product
1
,2-diphenylethyne (9a) in 51% yield. Complexes 3b−3e were also
examined (entries 2−5), and 3d again exhibited the highest catalytic
activity (entry 4). 3a, 3b, 3c, and 3e provided 30−62% yields of the
desired product 9a, whereas 3d gave 9a in an almost quantitative yield.
Therefore, we selected 3d, 7a, and 8a as a model system to optimize
the reaction conditions. Acetonitrile proved to be the best solvent
(
entry 4). The yield of the cross-coupling product got decreased
significantly when the reaction was performed in toluene, tetrahy-
drofuran (THF), or CH CH OH. Subsequently, we examined the
3
2
influence of organic bases (Et N, N-methylimidazole (NMI), diethyl-
3
CONCLUSIONS
■
amine (DEA)) and inorganic bases including Na CO , K CO ,
2
3
2
3
In this work, we demonstrated that 1:2 or 1:1 reaction of PdCl
NaOAc, NaOH, KOH, NaOEt, and K PO (entries 14−22). Et N
2
3
4
3
in CH CN gave the highest yield under the same conditions. At lower
with N,N′-disubstituted imidazole-2-thiones under different
reaction conditions produced a series of mononuclear (2a−2e)
and binuclear (3a−3e) Pd(II) complexes. Complexes 2a−2e
and 3a−3e could readily be interconverted subject to changing
3
temperature (40 °C), the reaction gave the product in a
commensurately lower yield (entry 24). When CuI was used as the
cocatalyst, the reaction in 12 h produced 9a in 83% yield coupled with
1
,4-diphenylbuta-1,3-diyne in 3% yield (entry 25). The results outlined
the molar ratios of PdCl and N,N′-disubstituted imidazole-2-
2
in Table 6 revealed that the optimal conditions for such a Cu-free
thione and controlling the reaction temperatures. According to
the X-ray analysis, compound 3d has the longest Pd−μ-S bond
and the largest S−Pd−S angle among all the title complexes.
Complexes 3a−3e possess much better catalytic performance
than 2a−2e in Suzuki−Miyaura and Sonogashira cross-coupling
reactions. We propose that the two cis thione ligands in 3a−3e
promote oxidative addition of aryl halide to the Pd(II) center
and the cooperative effect of two Pd(II) centers. The
substituted groups on ligands do exert a significant influence
on catalytic performance. Complex 3d displays the highest
catalytic activity toward Suzuki−Miyaura cross-coupling re-
actions of aryl bromides with aryl boronic acids in water and
Cu-free Sonogashira cross-coupling reactions of aryl iodides
and alkynes in MeCN under mild conditions. DFT calculations
and the experimental observations support that the bimetallic
structure with the bulkier ligands are beneficial to the above
two catalysis reactions. Our current work represents a good
example in which the introduction of the suitable substituted
groups on the auxiliary ligands could finely adjust the geometric
structures of the resulting Pd(II) coordination compounds,
which, in turn, greatly enhances the catalytic activity toward
Suzuki−Miyaura and Sonogashira cross-coupling reactions.
Sonogashira reaction could be fixed as MeCN as solvent, Et N as base
3
at 60 °C with 0.5 mol % of catalyst (3d). The turnover frequency
−
1
(
3
TOF) of 6.9 h (18 h) was slightly lower than that observed in the
−1
d/CuI-catalyzed reaction (8.3 h ).
Again, we examined the scope of the substrates that could be
employed for this catalytic system (Table 7). All of the substrates
investigated gave the desired 1,2-diarylethyne products in excellent
yields. Neither o- nor m-substituted group on the phenyl rings of the
aryl iodide or phenylacetylene hampered the coupling reaction. The
2
′-, 3′-, and 4′-methylphenylacetylenes were converted smoothly to
their corresponding products (9c−9e) in 89%, 92%, and 98% yields,
respectively (entries 3−5). Reactions of ethynylbenzene with 2-, 3-,
and 4-iodotoluenes resulted in the formation of products in high yields
(
89%, 93%, and 96%, entries 6−8). Neither electron-donating nor
electron-withdrawing groups on either aryl iodides or alkynes
significantly affected the yields. Terminal alkynes and phenyl iodides
with electron-donating substituents like −Me and −OMe reacted in
excellent yields (entries 6 and 10). The presence of electron-
withdrawing groups at the para position of aryl iodide or alkyne
furnished the products in good yields (88% and 89%; entries 2 and 9).
Notably, most reports on the Sonogashira coupling involve phosphine-
3
−5
based catalysts,
while examples of S-based stabilizing ligands are
surprisingly rare. Comparative run with (phbtuimt)PdCl /CuI
2
61
(
(
76%) indicated that 3d exhibits better catalytic performance
91%) for Sonogashira cross-coupling reaction of 7a and 8a. In the
presence of CuI, [(phpymhcth)Pd(PPh )]Cl at higher temperature
27
ASSOCIATED CONTENT
3
■
(
110 °C) in EtOH and toluene (>99% yield) shows higher activity
*
S
Supporting Information
than that of 3d.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01616.
Computational Studies. The palladium-catalyzed Suzuki−
Miyaura cross-coupling reactions may involve three steps: the
oxidative addition of aryl halides to palladium, the transmetalation of
the resulting aryl-halo-Pd(II) complexes with boronate complexes into
diaryl-palladium(II) species, and the reductive elimination to yield
Additional information regarding computational detail,
the ESI-MS observed peaks and isotopic patterns of
8
,14,17
biaryl products.
To better understand their catalytic activity, the
+
1
13
[
{(C Me H ) C N S} Pd Cl ] , and the H and
C
Suzuki−Miyaura cross-coupling reaction of bromobenzene with
phenylboronic acid catalyzed by 2a−2e and 3a−3e were investigated
through density functional theory (DFT) calculations. The alkylboron
6
3
2 2
3
2
2
2
3
NMR spectra for 1-methyl-3-phenyl-1,3-dihydro-imida-
zole-2-thione and cross-coupling products (PDF)
J
DOI: 10.1021/acs.inorgchem.7b01616
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Accession Codes
CCDC 1477939, 1477940, 1477941, 1494637, 1494638,
(11) Laughlin, S. T.; Bertozzi, C. R. Metabolic Labeling of Glycans
with Azido Sugars and Subsequent Glycan-profiling and Visualization
via Staudinger Ligation. Nat. Protoc. 2007, 2, 2930−2944.
1494639, 1539252 and 1494640 contain the supplementary
(
12) Bernhammer, J. C.; Huynh, H. V. Benzimidazolin-2-ylidene
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Complexes of Palladium(II) Featuring a Thioether Moiety: Synthesis,
Characterization, Molecular Dynamics, and Catalytic Activities.
Organometallics 2014, 33, 1266−1275.
(13) Mitsui, T.; Sugihara, M.; Tokoro, Y.; Fukuzawa, S. Synthesis of
Adamantyl Substituted 1,2,3-Triazol-5-ylidene Ligands and Their
AUTHOR INFORMATION
PEPPSI-type Palladium Complexes. Tetrahedron 2015, 71, 1509−
■
1
514.
Corresponding Authors
(14) Kim, J. H.; Kim, J. W.; Shokouhimehr, M.; Lee, Y. S. Polymer-
*
E-mail: lihx@suda.edu.cn. Phone: 86-512-65883569. (H.-
X.L.)
E-mail: yowang@suda.edu.cn. Phone: 86-512-65884475.
Y.W.)
E-mail: jplang@suda.edu.cn. Fax: 86-512-65880328. Phone:
6-512-65882865. (J.-P.L.)
Supported N-Heterocyclic Carbene-Palladium Complex for Hetero-
geneous Suzuki Cross-Coupling Reaction. J. Org. Chem. 2005, 70,
*
(
*
8
6
714−6720.
(15) Kılınca
Çetinkaya, B. New Palladium(II)-N-heterocyclic Carbene Complexes
̈
̧ rslan, R.; Gunay, M. E.; Fırıncı, R.; Denizaltı, S.;
Containing Benzimidazole-2-ylidene Ligand Derived from Menthol:
Synthesis, Characterization and Catalytic Activities. Appl. Organomet.
Chem. 2016, 30, 268−272.
ORCID
Jian-Ping Lang: 0000-0003-2942-7385
(16) Karimi, B.; Akhavan, P. F. Main-chain NHC-palladium Polymer
Notes
as a Recyclable Self-supported Catalyst in the Suzuki-Miyaura
Coupling of Aryl Chlorides in Water. Chem. Commun. 2009, 3750−
The authors declare no competing financial interest.
3
752.
(
17) Grasa, G. A.; Hillier, A. C.; Nolan, S. P. Convenient and
ACKNOWLEDGMENTS
■
Efficient Suzuki-Miyaura Cross-Coupling Catalyzed by a Palladium/
Diazabutadiene System. Org. Lett. 2001, 3, 1077−1080.
(18) Li, J. H.; Liu, W. J. Dabco as an Inexpensive and Highly Efficient
Ligand for Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling
Reaction. Org. Lett. 2004, 6, 2809−2811.
The authors thank the National Natural Science Foundation of
China (21471108, 21373142, and 21531006), the Natural
Science Foundation of Jiangsu Province (BK20161276), the
State Key Laboratory of Organometallic Chemistry of Shanghai
Institute of Organic Chemistry (2015kf-07), and the “Priority
Academic Program Development” of Jiangsu Higher Education
Institutions. The authors greatly thank the editor and the
reviewers for their helpful comments.
(19) Mino, T.; Shirae, Y.; Sakamoto, M.; Fujita, T. Phosphine-Free
Hydrazone-Pd Complex as the Catalyst Precursor for a Suzuki-
Miyaura Reaction under Mild Aerobic Conditions. J. Org. Chem. 2005,
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Complexes: New and Efficient Catalysts for Suzuki, Heck and
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