Synthesis, structure and catalytic activity of xylylene-bridged dipalladium complexes with triazolylidene ligands

Article abstract of DOI:10.1007/s11243-016-0117-5

A series of di-N-heterocyclic carbene (NHC) dipalladium complexes, [PdPyBr₂]₂(di-NHC) (2a–h), in which di-NHC represents a di-triazolylidene, featuring a 1,4-xylylene spacer between the carbene units, have been prepared from the reactions of the corresponding ditriazolium bromides with PdCl₂ and excess NaBr as an additive in the presence of K₂CO₃ in pyridine. All of the complexes were fully characterized by NMR spectra, IR and elemental analyses. The molecular structure of 2b was determined by X-ray diffraction study, showing the intermolecular hydrogen bonding of Br···H–C. The influences of size of ligands on the catalytic activity have been investigated in the Heck reaction of styrene with bromobenzene. The complex 2g showed the best catalytic activity, and it is active for various aryl bromides with different electronic and steric properties and styrenes with both electron-donating and electron-withdrawing groups.

Full text of DOI:10.1007/s11243-016-0117-5

Transit Met Chem (2017) 42:193–201
DOI 10.1007/s11243-016-0117-5
Synthesis, structure and catalytic activity of xylylene-bridged
dipalladium complexes with triazolylidene ligands
Yufeng Du1 Beibei Liang1 Fenglei Yang1 Yanhui Shi1 Xiuling Li1
•
•
•
•
•
Guangsheng Pang2 Changsheng Cao1
•
Received: 24 October 2016 / Accepted: 21 December 2016 / Published online: 14 February 2017
Ó Springer International Publishing Switzerland 2017
Abstract A series of di-N-heterocyclic carbene (NHC)
dipalladium complexes, [PdPyBr2]2(di-NHC) (2a–h), in
which di-NHC represents a di-triazolylidene, featuring a
1,4-xylylene spacer between the carbene units, have been
prepared from the reactions of the corresponding ditria-
zolium bromides with PdCl2 and excess NaBr as an addi-
tive in the presence of K2CO3 in pyridine. All of the
complexes were fully characterized by NMR spectra, IR
and elemental analyses. The molecular structure of 2b was
determined by X-ray diffraction study, showing the inter-
molecular hydrogen bonding of BrÁÁÁH–C. The influences
of size of ligands on the catalytic activity have been
investigated in the Heck reaction of styrene with bro-
mobenzene. The complex 2g showed the best catalytic
activity, and it is active for various aryl bromides with
different electronic and steric properties and styrenes with
both electron-donating and electron-withdrawing groups.
Introduction
The use of N-heterocyclic carbenes (NHCs) as a class of C
donor ligands in coordination chemistry has attracted
considerable attention since the report of the first N-hete-
rocyclic carbene (NHC) in 1991 [1]. The transition metal
NHC complexes as catalysts have been successfully
applied in many important reactions, such as Pd-catalyzed
cross-coupling reactions [2, 3] and ruthenium-mediated
olefin metathesis [4, 5]. Compared to widely used and
studied imidazolylidene-based NHC ligands, less studied
triazolylidene-based ligands have proved to be a class of
useful NHCs as well. For example, Enders and coworkers
reported a stable carbene based on a 1,2,4-triazol-5-ylidene
derivative in 1995, which has since proved to be extremely
useful in organocatalysis (Scheme 1) [6]. Since then, sev-
eral metal complexes using Enders carbene as a mon-
odentate ligand have been reported, e.g., ruthenium [7],
palladium [8], rhodium [9], iridium [10] or platinum [11]
complexes.
We have been interested in developing binuclear palla-
dium–NHC complexes and studying their catalytic prop-
erties, as introducing multiple catalytic sites into a catalyst
may induce cooperative effects resulting in improving the
activities and selectivities [12–15]. However, only NHCs
bearing an imidazolylidene backbone have been studied in
these di-NHC dipalladium complexes so far. In this paper,
we designed and prepared a series of binuclear Pd–NHC
complexes bearing triazolylidene ligands linked with a
xylylene spacer. In order to elucidate the influence of steric
effect of the ligands, we altered the size of the triazolyli-
dene by changing the wingtip substituent in position N(1)
of the triazole ring. The steric effects of the ligands on the
catalytic activity were studied in Heck reactions.
Yufeng Du and Beibei Liang have contributed equally to the work.
& Yanhui Shi
yhshi@jsnu.edu.cn
& Changsheng Cao
chshcao@jsnu.edu.cn
1
College of Chemistry and Chemical Engineering and Jiangsu
Key Laboratory of Green Synthetic Chemistry for Functional
Materials, Jiangsu Normal University,
Xuzhou 221116, Jiangsu, People’s Republic of China
2
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun 130012, Jilin,
People’s Republic of China
123

194
Transit Met Chem (2017) 42:193–201
2
N
1578(tC=N), 1527(tC=C), 1456, 1402, 1161, 1066, 981, 899,
1
N
R
769, 734, 673, 620, 503 cm-1 1H NMR (400 MHz,
.
N
Ph
R
N
3
N
4
DMSO-d6): 10.33 (s, 2H, H5-triazole), 9.35 (s, 2H, H3-triazole),
7.57 (s, 4H, Ph), 5.57 (s, 4H, CH2), 4.07 (s, 6H, CH3). 13
N
N
N
N
Ph
5
Ph
C
NMR (100 MHz, DMSO-d6): 144.9, 143.4, 135.0, 130.0,
50.4, 39.3. Anal. Calc. for C14H18Br2N6 (430.14 g/mol): C,
39.09; H, 4.22; N, 19.54, Found: C: 39.23; H: 4.29; N:
19.67%.
Scheme 1 Enders carbene (left) and triazolylidene (NHC) ligands
employed in this study (right). R = Me, Et, n-Pr, i-Pr, n-Bu, n-pentyl,
n-hexyl, cyclohexyl
Experimental
4,40-(1,4-phenylenebis(methylene))bis(1-ethyl-1,2,4-
triazol-4-ium) dibromide (1b)
Materials and methods
Yield: 93%. White solid. M.p.: 271–272 °C. IR (KBr):
3101(tC–H), 3027(tC–H), 2990(t=C–H), 1578(tC=N),
1527(tC=C), 1452, 1387, 1331, 1242, 1151, 993, 871, 779,
All manipulations were carried out under a nitrogen atmosphere
using standard Schlenk or glovebox techniques. Solvents were
purified and degassed by standard procedures. DMF and pyr-
idine were distilled over calcium hydride under argon atmo-
sphere prior to use. Potassium carbonate and tripotassium
phosphate were ground to a fine powder prior to use. All other
chemicals were obtained from common suppliers and used
without further purification. 1H and 13C spectra were recorded
on a Bruker AV 400-MHz spectrometer at room temperature
and referenced to the residual signals of the solvent. Elemental
analyses were obtained on a EuroVektor EuroEA-3000 ele-
mental analyzer. IR spectra were recorded on KBr pellets on a
FTIR Tensor 27 spectrometer. GC–MS was performed on an
Agilent 6890–5973-N system with electron ionization (EI)
mass spectrometry. X-ray crystallography was conducted on a
BRUKER smart CCD device with Mo Ka radiation
741, 623, 508 cm-1 1H NMR (400 MHz, DMSO-d6):
.
10.44 (s, 2H, H5-triazole), 9.39 (s, 2H, H3-triazole), 7.60 (s, 4H,
Ph), 5.57 (s, 4H, CH2), 4.40 (q, J = 7.2 Hz, 4H, CH2CH3),
1.47 (t, J = 7.2 Hz, 6H, CH2CH3). 13C NMR (100 MHz,
DMSO-d6): 145.0, 142.7, 134.9, 130.0, 50.4, 47.7, 14.0.
Anal. Calc. for C16H22Br2N6 (458.19 g/mol): C, 41.94; H,
4.84; N, 18.34. Found: C: 41.71; H: 4.77; N: 18.21%.
4,40-(1,4-phenylenebis(methylene))bis(1-propyl-1,2,4-
triazol-4-ium) dibromide(1c)
Yield: 95%. White solid. M.p. [300 °C. IR (KBr):
3103(tC–H), 2976(t=C–H), 2877(t=C–H), 1585(tC=N),
1518(tC=C), 1419, 1389, 1297, 1181, 1151, 998, 878, 783,
˚
(k = 0.71073 A). Absorption correction was performed using
the SADABS program. The structure was solved by directed
methods using the SHELXS-2016 program and refined by full-
matrix least square techniques on F2. The SQUEEZE program
was used to remove the contributions of disordered solvent.
761, 621, 520 cm-1 1H NMR (400 MHz, DMSO-d6):
.
10.49 (s, 2H, H5-triazole), 9.42 (s, 2H, H3-triazole), 7.60 (s, 4H,
Ph), 5.59 (s, 4H, CH2), 4.36 (t, J = 7.0 Hz, 4H, CH2CH2),
1.92–1.83 (m, 4H, CH2CH3), 0.89 (t, J = 7.4 Hz, 6H,
CH2CH3). 13C NMR (100 MHz, DMSO-d6): 145.1, 143.1,
134.9, 130.0, 53.6, 50.4, 22.0, 10.9. Anal. Calc. for C18-
H26Br2N6 (486.25 g/mol): C, 44.46; H, 5.39; N, 17.28.
Found: C: 44.31; H: 5.32; N: 17.41%.
Synthesis of triazolium bromides (1a–h)
In a typical run, a 10-mL thick wall pressure tube was
charged with 1,4-bis(bromomethyl)benzene (0.20 g,
0.76 mmol), the corresponding 1-alkyl-1H-1,2,4-triazoles
(1.60 mmol) and DCM (3 mL). The reaction mixture was
heated at 120 °C for 12–16 h. The reaction mixture was
cooled to room temperature and extracted with water
(3 9 5 mL). The collected aqueous phase was dried by
rotovap, and the crude product was recrystallized from
methanol/acetone (1:4) to give the product as a white solid.
4,40-(1,4-phenylenebis(methylene))bis(1-isopropyl-1,2,4-
triazol-4-ium) dibromide (1d)
Yield: 92%. White solid. M.p.: 289–290 °C. IR (KBr):
3099(tC–H), 2976(t=C–H), 2806(t=C–H), 1585(tC=N),
1513(tC=C), 1421, 1388, 1355, 1296, 1181, 1151, 998, 878,
785, 739, 621, 530 cm-1. 1H NMR (400 MHz, DMSO-d6):
10.47 (s, 2H, H5-triazole), 9.37 (s, 2H, H3-triazole), 7.61 (s, 4H,
Ph), 5.55 (s, 4H, CH2), 4.86–4.75 (m, 2H, CH), 1.51 (d,
J = 6.8 Hz, 12H, CH3). 13C NMR (100 MHz, DMSO-d6):
145.0, 142.0, 135.0, 130.0, 55.7, 50.4, 21.6. Anal. Calc. for
C18H26Br2N6 (486.25 g/mol): C, 44.46; H, 5.39; N, 17.28.
Found: C: 44.22; H: 5.45; N: 17.36%.
4,40-(1,4-phenylenebis(methylene))bis(1-methyl-1,2,4-
triazol-4-ium) dibromide (1a)
Yield: 89%. White solid. M.p.: 276–277 °C. IR (KBr):
3103(tC–H), 3025(tC–H), 2988(t=C–H), 2813(t=C–H),
123

Transit Met Chem (2017) 42:193–201
195
4,40-(1,4-phenylenebis(methylene))bis(1-butyl-1,2,4-
2802(t=C–H), 1586(tC=N), 1515(tC=C), 1445, 1365, 1309,
1
1153, 993, 892, 759, 619, 527 cm-1. H NMR (400 MHz,
triazol-4-ium) dibromide (1e)
DMSO-d6): 10.45 (s, 2H, H5-triazole), 9.36 (s, 2H, H3-triazole),
Yield: 89%. White solid. M.p.: 278–279 °C. IR (KBr):
3102(tC–H), 3020(tC–H), 2955(t=C–H), 2871(t=C–H),
1580(tC=N), 1520(tC=C), 1417, 1391, 1332, 1303, 1245,
7.60 (s, 4H, Ph), 5.55 (s, 4H, CH2), 4.55–4.47 (m, 2H,
H
1-cyclohexane), 2.17–2.13 (m, 4H, Hcyclohexane), 1.85–1.81
(m, 4H, Hcyclohexane), 1.77–1.65 (m, 6H, Hcyclohexane),
1.47–1.37 (m, 4H, Hcyclohexane), 1.30–1.17 (m, 2H, Hcyclo-
hexane). 13C NMR (100 MHz, DMSO-d6): 144.9, 142.0,
135.0, 130.0, 61.6, 50.4, 31.7, 24.9, 24.4. Anal. Calc. for
C24H34Br2N6 (566.37 g/mol): C, 50.90; H, 6.05; N, 14.84.
Found: C: 50.78; H: 6.10; N: 14.92%.
1
1197, 1151, 996, 871, 783, 759, 622, 510 cm-1. H NMR
(400 MHz, DMSO-d6): 10.39 (s, 2H, H5-triazole), 9.36 (s,
2H, H3-triazole), 7.58 (s, 4H, Ph), 5.56 (s, 4H, CH2), 4.38 (t,
J = 7.2 Hz, 4H, H1-Bu), 1.87–1.79 (m, 4H, H2-Bu),
1.34–1.25 (m, 4H, H3-Bu), 0.90 (t, J = 7.4 Hz, 6H, CH3).
13C NMR (100 MHz, DMSO-d6): 145.1, 143.0, 135.0,
130.0, 51.9, 50.5, 30.4, 19.2, 13.7. Anal. Calc. for C20-
H30Br2N6 (514.30 g/mol): C, 46.71; H, 5.88; N, 16.34.
Found: C: 46.48; H: 5.94; N: 16.22%.
Synthesis of di-NHC dipalladium complexes (2a–h)
In a typical run, to a mixture of bis(triazolium) bromide
(1.0 mmol), PdCl2 (0.534 g, 2.0 mmol), NaBr (0.823 g,
8 mmol) and K2CO3 (2.764 g, 20 mmol) in a 50-mL
round-bottom flask was added 15 mL of pyridine. The
reaction mixture was heated at 85°C for 20 h, after which
time the mixture was filtered through Celite and washed
with DCM. The solvent was removed under vacuum, and
the crude product was washed with diethyl ether (15 mL).
The product was obtained as a yellow solid by recrystal-
lization from ether/DCM.
4,40-(1,4-phenylenebis(methylene))bis(1-pentyl-1,2,4-
triazol-4-ium) dibromide (1f)
Yield: 83%. White solid. M.p.: 285–286 °C. IR (KBr):
3102(tC–H), 3030(tC–H), 2959(t=C–H), 2869(t=C–H),
2853(t=C–H), 1580(tC=N), 1518(tC=C), 1463, 1390, 1334,
1
1245, 1151, 1068, 998, 871, 783, 759, 623, 511 cm-1. H
NMR (400 MHz, DMSO-d6): 10.35 (s, 2H, H5-triazole), 9.34
(s, 2H, H3-triazole), 7.58 (s, 4H, Ph), 5.56 (s, 4H, CH2), 4.37
(t, J = 7.0 Hz, 4H, H1-pentyl), 1.89–1.82 (m, 4H, H2-pentyl),
1.34–1.24 (m, 8H, H3-pentyl, H4-pentyl), 0.87 (t, J = 7 Hz,
6H, CH3). 13C NMR (100 MHz, DMSO-d6): 145.1, 143.0,
134.9, 129.9, 52.2, 50.5, 28.1, 28.0, 21.9, 14.2. Anal. Calc.
for C22H34Br2N6 (542.35 g/mol): C, 48.72; H, 6.32; N,
15.50. Found: C: 48.58; H: 6.26; N: 15.62%.
Complex 2a
Yield: 83%. Light yellow solid. IR (KBr): 3134(tC–H),
3064(tC–H), 3002(tC–H), 2942(t=C–H), 1604 (tC=N),
1538(tC=C), 1485, 1447, 1410, 1358, 1332, 1231, 1171,
1
1117, 1073, 978, 863, 791, 762, 693, 645 cm-1. H NMR
(400 MHz, CDCl3): 8.95 (d, J = 4.8 Hz, 4H, Ha-Py), 7.85
(s, 2H, Htriazole), 7.78 (t, J = 7.6 Hz, 2H, Hc-Py), 7.61 (s,
4H, Ph), 7.36 (t, J = 7.0 Hz, 4H, Hb-Py), 5.75 (s, 4H, CH2),
4.27 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3): 155.9,
152.6, 142.5, 138.1, 134.9, 130.3, 124.8, 52.5, 40.5. Anal.
Calc. for C24H26Br4N8Pd2 (958.98 g/mol): C, 30.06; H,
2.73; N, 11.68. Found: C: 30.31; H: 2.71; N: 11.82%.
4,40-(1,4-phenylenebis(methylene))bis(1-hexyl-1,2,4-
triazol-4-ium) dibromide (1g)
Yield: 96%. White solid. M.p. [300 °C. IR (KBr):
3101(tC–H), 3031(tC–H), 2952(t=C–H), 2867(t=C–H),
1578(tC=N), 1517(tC=C), 1464, 1389, 1332, 1245, 1151,
1
1068, 890, 871, 760, 726, 703, 623, 511 cm-1. H NMR
(400 MHz, DMSO-d6): 10.30 (s, 2H, H5-triazole), 9.32 (s, 2H,
Complex 2b
H
3-triazole), 7.60 (s, 4H, Ph), 5.55 (s, 4H, CH2), 4.37 (t,
J = 7.0 Hz, 4H, H1-hexyl), 1.88–1.81 (m, 4H, H2-hexyl),
1.33–1.78 (m, 12H, H3-hexyl, H4-hexyl, H5-hexyl), 0.86 (t,
J = 6.4 Hz, 6H, CH3). 13C NMR (100 MHz, DMSO-d6):
145.1, 143.0, 135.0, 130.0, 52.2, 50.5, 31.0, 28.5, 25.5, 22.3,
14.3. Anal. Calc. for C24H38Br2N6 (570.41 g/mol): C, 50.54;
H, 6.71; N, 14.73. Found: C: 50.35; H: 6.64; N: 14.84%.
Yield: 85%. Light yellow solid. IR (KBr): 3136(tC–H),
3053(tC–H), 2986(t=C–H), 1604(tC=N), 1542(tC=C), 1479,
1447, 1369, 1352, 1333, 1245, 1217, 1073, 965, 757, 737,
695, 663 cm-1 1H NMR (400 MHz, CDCl3): 8.96 (d,
.
J = 4.8 Hz, 4H, Ha-Py), 7.86 (s, 2H, Htriazole), 7.77 (t,
J = 7.6 Hz, 2H, Hc-Py), 7.62 (s, 4H, Ph), 7.36 (t,
J = 7.0 Hz, 4H, Hb-Py), 5.75 (s, 4H, CH2), 4.73 (q,
J = 7.4 Hz, 4H, CH2CH3), 1.65 (t, J = 7.4 Hz, 6H, CH2-
CH3). 13C NMR (100 MHz, CDCl3): 155.1, 152.7, 142.7,
138.2, 135.0, 130.5, 124.9, 52.7, 49.0, 14.9. Anal. Calc. for
C26H30Br4N8Pd2 (987.03 g/mol): C, 31.64; H, 3.06; N,
11.35. Found: C: 31.50; H: 3.11; N: 11.42%.
4,40-(1,4-phenylenebis(methylene))bis(1-cyclohexyl-1,2,4-
triazol-4-ium) dibromide (1 h)
Yield: 83%. White solid. M.p.: 280–281 °C. IR (KBr):
3099(tC–H), 3024(tC–H), 2932(t=C–H), 2852(t=C–H),
123

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Transit Met Chem (2017) 42:193–201
Complex 2c
J = 7.6 Hz, 2H, Hc-Py), 7.64 (s, 4H, Ph), 7.38 (t, J = 7.0 Hz,
4H, Hb-Py), 5.78 (s, 4H, CH2), 4.66 (t, J = 7.6 Hz, 4H,
Yield: 81%. Light yellow solid. IR (KBr): 3112(tC–H),
3043(tC–H), 2963(t=C–H), 2933(t=C–H), 2873(t=C–H),
1604(tC=N), 1536(tC=C), 1477, 1447, 1380, 1218, 1168,
H
1-pentyl), 2.18–2.13 (m, 4H, H2-pentyl), 1.50–1.46 (m, 8H, H3-
pentyl, H4-pentyl), 0.97 (t, J = 7.0 Hz, 6H, CH3). 13C NMR
(100 MHz, CDCl3): 155.1, 152.6, 142.4, 138.1, 134.9, 130.3,
124.7, 52.6, 30.9, 29.0, 28.7, 22.3, 13.9. Anal. Calc. for
C32H42Br4N8Pd2 (1071.20 g/mol): C, 35.88; H, 3.95; N,
10.46. Found: C: 36.01; H: 3.91; N: 10.40%.
1
1074, 1017, 993, 857, 802, 758, 693, 646 cm-1. H NMR
(400 MHz, CDCl3): 8.98 (d, J = 4.8 Hz, 4H, Ha-Py), 7.88
(s, 2H, Htriazole), 7.79 (t, J = 7.6 Hz, 2H, Hc-Py), 7.64 (s,
4H, Ph), 7.38 (t, J = 7.0 Hz, 4H, Hb-Py), 5.79 (s, 4H, CH2),
4.64 (t, J = 7.6 Hz, 4H, CH2CH2), 2.24–2.14 (m, 4H,
CH2CH3), 1.10 (t, J = 7.4 Hz, 6H, CH2CH3). 13C NMR
(100 MHz, CDCl3): 155.1, 152.6, 142.4, 138.1, 134.9,
130.3, 124.7, 55.0, 52.6, 22.8, 11.3. Anal. Calc. for C28-
H34Br4N8Pd2 (1015.08 g/mol): C, 33.13; H, 3.38; N, 11.04.
Found: C: 33.00; H: 3.34; N: 11.11%.
Complex 2g
Yield: 88%. Light yellow solid. IR (KBr): 3114(tC–H),
3042(tC–H), 2950(t=C–H), 2857(t=C–H), 1602(tC=N),
1534(tC=C), 1473, 1447, 1389, 1215, 1198, 1068, 1016,
979, 783, 762, 701, 667, 644 cm-1. H NMR (400 MHz,
1
CDCl3): 8.97 (d, J = 4.9 Hz, 4H, Ha-Py), 7.87 (s, 2H,
Complex 2d
H
triazole), 7.79 (t, J = 7.6 Hz, 2H, Hc-Py), 7.64 (s, 4H, Ph),
7.38 (t, J = 7.2 Hz, 4H, Hb-Py), 5.78 (s, 4H, CH2), 4.66
(t, J = 7.4 Hz, 4H, H1-hexyl), 2.19–2.12 (m, 4H, H2-hexyl),
1.53–1.35 (m, 12H, H3-hexyl, H4-hexyl, H5-hexyl), 0.92 (t,
J = 7.0 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3):
155.1, 152.6, 142.4, 138.1, 134.9, 130.3, 124.7, 53.4, 52.6,
31.3, 29.2, 26.3, 22.5, 14.1. Anal. Calc. for C34H46Br4N8-
Pd2 (1099.24 g/mol): C, 37.15; H, 4.22; N, 10.19. Found:
C: 36.98; H: 4.29; N: 10.27%.
Yield: 83%. Light yellow solid. IR (KBr): 3115(tC–H),
2981(t=C–H), 2934(t=C–H), 1605(tC=N), 1538(tC=C), 1471,
1447, 1368, 1216, 1179, 1072, 1047, 992, 857, 753, 696,
659, 645 cm-1 1H NMR (400 MHz, CDCl3): 8.98 (d,
.
J = 5.2 Hz, 4H, Ha-Py), 7.88 (s, 2H, Htriazole), 7.79 (t,
J = 7.6 Hz, 2H, Hc-Py), 7.64 (s, 4H, Ph), 7.37 (t,
J = 7.0 Hz, 4H, Hb-Py), 5.79–5.72 (m, 6H, CH2, CH), 1.61
(d, J = 6.8 Hz, 12H, CH3). 13C NMR (100 MHz, CDCl3):
153.7, 152.6, 142.6, 138.1, 134.8, 130.3, 124.7, 55.6, 52.5,
21.8. Anal. Calc. for C28H34Br4N8Pd2 (1013.77 g/mol): C:
33.01; H: 3.32; N: 11.13%.
Complex 2h
Yield: 69%. Light yellow solid. IR (KBr): 3113(tC–H),
3041(tC–H), 2939(t=C–H), 2856(t=C–H), 1604(tC=N),
1539(tC=C), 1469, 1447, 1378, 1217, 1158, 1071, 1017,
Complex 2e
995, 895, 821, 761, 696, 659, 644 cm-1 1H NMR
.
Yield: 77%. Light yellow solid. IR (KBr): 3117(tC–H),
3063(tC–H), 2953(t=C–H), 2869(t=C–H), 1604(tC=N),
1541(tC=C), 1475, 1447, 1366, 1239, 1216, 1165, 1073,
(400 MHz, CDCl3): 9.00 (d, J = 4.9 Hz, 4H, Ha-Py), 7.83
(s, 2H, Htriazole), 7.77 (t, J = 7.8 Hz, 2H, Hc-Py), 7.61
(s, 4H, Ph), 7.35 (t, J = 7.0 Hz, 4H, Hb-Py), 5.75 (s, 4H,
CH2), 5.32–5.25 (m, 2H, H1-cyclohexyl), 2.28–2.25 (m, 4H,
1018, 1002, 951, 859, 762, 698, 646 cm-1 1H NMR
.
(400 MHz, CDCl3): 8.97 (d, J = 4.9 Hz, 4H, Ha-Py), 7.88
(s, 2H, Htriazole), 7.79 (t, J = 7.8 Hz, 2H, Hc-Py), 7.64 (s,
4H, Ph), 7.38 (t, J = 7.0 Hz, 4H, Hb-Py), 5.78 (s, 4H, CH2),
4.67 (t, J = 7.4 Hz, 4H, H1-Bu), 2.16–2.10 (m, 4H, H2-Bu),
1.57- 1.47 (m, 4H, H3-Bu), 1.05 (t, J = 7.4 Hz, 6H, CH3).
13C NMR (100 MHz, CDCl3): 155.1, 152.6, 142.4, 138.1,
134.9, 130.3, 124.7, 52.6, 31.2, 29.7, 19.8, 13.7. Anal.
Calc. for C30H38Br4N8Pd2 (1043.13 g/mol): C, 34.54; H,
3.67; N, 10.74. Found: C: 34.37; H: 3.62; N: 10.82%.
H
cyclohexyl), 1.94–1.74 (m, 10H, Hcyclohexyl), 1.60–1.49 (m,
4H, Hcyclohexyl), 1.32–1.26 (m, 2H, Hcyclohexyl). 13C NMR
(100 MHz, CDCl3): 153.7, 152.7, 142.4, 138.1, 134.8,
130.3, 124.7, 62.5, 52.6, 31.9, 25.3, 25.1. Anal. Calc. for
C34H42Br4N8Pd2 (1095.22 g/mol): C, 37.29; H, 3.87; N,
10.23. Found: C: 39.05; H: 3.90; N: 10.44%.
Procedure for the catalytic Heck coupling reaction
In a typical run, a 5-mL vial equipped with a magnetic
stirrer was charged with a mixture of phenyl bromide
(78.5 mg, 0.5 mmol), styrene (62.5 mg, 0.6 mmol), Pd
catalyst (0.0025 mmol), K3PO4 (212 mg, 1.0 mmol) and
DMF (1 mL) under nitrogen. The mixture was heated at
100 °C for 4 h and then cooled to room temperature. Brine
was added, and the resulting mixture was extracted with
ethyl acetate (3 9 5 mL). The GC–MS samples were
Complex 2f
Yield: 79%. Light yellow solid. IR (KBr): 3116(tC–H),
3064(tC–H), 2953(t=C–H), 2869(t=C–H),
1539(tC=C), 1475, 1447, 1369, 1217, 1071, 1016, 976, 862,
758, 699, 664, 645 cm-1. 1H NMR (400 MHz, CDCl3): 8.98
(d, J = 4.9 Hz, 4H, Ha-Py), 7.88 (s, 2H, Htriazole), 7.79 (t,
1603(tC=N),
123

Transit Met Chem (2017) 42:193–201
197
prepared by diluting 10 lL of ethyl acetate solution to
1 mL.
The bis(triazolium) bromides and palladium–NHC
1
complexes were fully characterized by H and 13C NMR
spectra, IR spectra and elemental analyses. For example, in
1
the H NMR spectrum of imidazolium bromide 1b, two
Results and discussion
Synthesis
singlets at 10.44 and 9.39 ppm correspond to two triazole
protons; one singlet at 7.60 ppm corresponds to phenyl
protons of xylene; one singlet at 5.57 ppm corresponds to
methylene proton of xylene and a quartet at 4.40 and a
triplet at 1.47 ppm correspond to ethyl protons. In the 13C
NMR spectrum of 1b, four signals (145.0, 142.7, 134.9,
130.0 ppm) corresponding to triazole and phenyl moieties
are observed in the aromatic region and three signals (50.4,
47.7, 14.0 ppm) corresponding to methylene and ethyl
The 1-alkyl-1H-1,2,4-triazoles were prepared from the
nucleophilic substitution reaction of the corresponding
alkyl bromide or iodide with 1H-1,2,4-triazole according to
the literature procedure [16]. The bis(triazolium) bromides
1a–h were prepared by direct alkylation of triazoles with
1,4-xylylene dibromide in CH2Cl2 in good yields. The
synthesis of di-NHC dipalladium complexes (2a–h) was
achieved by the reactions of the corresponding bis(tria-
zolium) bromides with PdCl2 in the presence of K2CO3
with NaBr as additive in pyridine at 85 °C for 20 h
(Scheme 2). All the complexes were isolated in good yields
of 69–88%.
1
moieties are observed in the alkyl region. In the H NMR
spectra of palladium complexes 2a–h, the proton signals of
NCHN (ca. 10.49–10.30 ppm) in the starting ligand pre-
cursors were absent after metalation, due to the formation
of Pd–Ccarbene bonds. The upfield shifts of the signals rel-
ative to the triazolidene proton are observed, and the proton
signals relative to pyridine are observed. In the 13C NMR
Scheme 2 Synthesis of di-
NHC dipalladium complexes
2a–h
1
Table 1 Selected H and 13C NMR data of bis(triazolium) salt 1a–h, and bis(NHC) palladium complexes 2a–h
Bis(triazolium) 1H (triazole)a 13C (5-triazole) Pd complex 1H (triazole)b 1H (pyridine)b
bromide
1H (methylene 13C (carbene–Pd)b
of xylene)b
1a
1b
1c
1d
1e
1f
10.33, 9.35
10.44, 9.39
10.49, 9.42
10.47, 9.37
10.39, 9.36
10.35, 9.34
10.30, 9.32
10.45, 9.36
143.4
145.0
145.1
145.0
145.1
145.1
145.1
144.9
2a
2b
2c
2d
2e
2f
7.85
7.86
7.88
7.88
7.88
7.88
7.87
7.83
8.95, 7.78, 7.36 5.75
155.9
155.1
155.1
153.7
155.1
155.1
155.1
153.7
8.96, 7.77, 7.36 5.75
8.98, 7.79, 7.38 5.79
8.98, 7.79, 7.37 5.74
8.97, 7.79, 7.38 5.78
8.98, 7.79, 7.38 5.78
8.97, 7.79, 7.38 5.78
9.00, 7.77, 7.35 5.75
1 g
1 h
a
2g
2h
In ppm in DMSO-d6
b
In ppm in CDCl3
123

198
Transit Met Chem (2017) 42:193–201
spectra of palladium complexes 2a–h, the signal at ca.
155 ppm which is assigned to the Ccarbene–Pd resonance
shifted downfield relative to that of its precursor (ca.
145 ppm) providing direct evidence of the metalation of
palladium centers bound by a xylylene-bridged bis(tria-
zolylidene) ligand. Each palladium atom completes its
coordination sphere with two bromides in a trans confor-
mation and a pyridine. The geometry around the palladium
center is slightly distorted from square planar. The bond
angles around palladium are a little off from 180° or 90°
[angle of Br–Pd–Br: 174.089(17)°; angle of Ccarbene–Pd–
1
the ligand. The selected H and 13C NMR data of bis(tri-
azolium) bromides and their corresponding palladium
complexes are listed in Table 1. The infrared spectra of
1a–h and 2a–h showed the absorption bands corresponding
to the stretching vibrations of C–H of saturated alkanes and
unsaturated alkenes, C=C and C=N double bonds. The
absorption bands shift to higher frequencies upon
metalation.
The palladium complexes 2a–h are air- and moisture-
stable in the solid state and even in common organic
solvents.
Description of the structures
Suitable crystals of complex 2b for X-ray diffraction
analysis were obtained by slow diffusion of methanol into a
dichloromethane-saturated solution at room temperature.
The selected crystallographic data and refinement param-
eters are summarized in Table 2. The molecular structure
of complex 2b is unambiguously confirmed by means of
X-ray diffraction studies, and Fig. 1 shows the molecular
diagram of the complex. The molecule consists of two
Fig. 1 Molecular structure of complex 2b. Hydrogen atoms are
omitted for clarity. Ellipsoids are at the 50% probability level
Table 2 Crystallographic
Formula
C26H30Br4N8Pd2
parameters of 2b
FW
987.02
Crystal system
Space group
Crystal size/(mm)
Radiation
Monoclinic
C2/c
0.21 9 0.20 9 0.19
Mo Ka (k = 0.71073)
˚
a (A)
12.2695(5)
˚
b (A)
11.4067(5)
˚
c (A)
26.5584(12)
a (°)
b (°)
c (°)
90
103.3557(5)
90
3
˚
V (A )
3616.4(3)
Z
4
D
calcd.(mg cm-3
)
1.813
Absorption coefficient (mm-1
)
5.441
F(000)
1896
h range (°)
Index ranges
2.71–28.71
-16 B h B 16, -11 B k B 14, -35 B l B 35
24252/4593
Reflections collected/unique
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I [ 2r(I)]
R indices (all data)
4593/0/183
1.003
R1 = 0.0447, wR2 = 0.0914
R1 = 0.0548, wR2 = 0.0938
123

Transit Met Chem (2017) 42:193–201
199
Br(1): 86.88(8) °]. The two palladium fragments are loca-
ted on one side of the plane defined by the xylylene bridge
showing an anti-orientation with respect to the bis-NHC
˚
ligand. This conformation leads to a separation of 7.454 A
˚
between two Pd. The Pd–Ccarbene distance is 1.956(3) A
(Table 4). The results revealed that the complex 2e with n-
butyl and 2g with n-hexyl substitution on 1-triazolylidene
showed good performance in the coupling reaction giving
good yield and regioselectivity (96% yield with 27:1 of
E:Z ratio for 2e and 98% yield of 26:1 of E:Z ratio for 2g).
Complex 2 h with cyclohexyl substitution led to a lower
yield (69%) and poor selectivity (13:1 of E:Z ratio).
(Table 3), similar to the values for other palladium-related
species [14]. The crystal packing of the molecule reveals
the intermolecular hydrogen bonding of BrÁÁÁH-C with
˚
BrÁÁÁH distance of 2.997(5) A (Fig. 2).
Table 4 Arylation of styrene with bromobenzene catalyzed by 2a–h
Catalytic studies
Br
0.5
mol%
Pd-NHC,
K
3PO4
To elucidate the influence of steric effects of the ligands,
the catalytic activity of complexes 2a–h with different
alkyl substitution in N(1) position of the triazole ring was
investigated in the arylation of styrene with bromobenzene
4h
DMF,
110 oC
Entrya
Catalyst
Yield (%)a
E/Z ratiob
1
2
3
4
5
6
7
8
a
2a
2b
2c
2d
2e
2f
83
87
90
92
96
95
98
69
23:1
20:1
23:1
21:1
27:1
22:1
26:1
13:1
˚
Table 3 Selected bond lengths (A) and angles (8) for 2b
˚
Distance (A)
Angle (°)
Pd(1)–C(6)
Pd(1)–N(1)
Pd(1)–Br(1)
Pd(1)–Br(2)
N(2)–C(6)
N(2)–N(3)
N(3)–C(9)
N(4)–C(6)
N(4)–C(9)
1.956(3)
2.099(3)
2.4296(4)
2.4460(4)
1.325(4)
1.376(3)
1.296(4)
1.355(4)
1.356(4)
C(6)–Pd(1)–N(1)
C(6)–Pd(1)–Br(1)
N(1)–Pd(1)–Br(1)
C(6)–Pd(1)–Br(2)
N(1)–Pd(1)–Br(2)
Br(1)–Pd(1)–Br(2)
N(2)–C(6)–N(4)
176.78(11)
86.88(8)
91.45(8)
2g
2h
89.83(8)
92.07(8)
174.089(17)
104.0(2)
Reaction conditions: phenylbromide (0.5 mmol, 52 lL), styrene
(0.6 mmol, 63 lL), Pd–NHC complex (0.0025 mmol) and K3PO4
(1 mmol, 0.23 g) in DMF (2 mL)
b
Determined by GC using dodecane as internal standard
Fig. 2 One section of the
crystal packing of complex 2b,
showing the hydrogen bonding
of BrÁÁÁH–C. Other hydrogen
atoms are omitted for clarity
123

200
Transit Met Chem (2017) 42:193–201
1-bromo-2-methoxybenzene
and
with
1-bromo-4-
Table 5 Arylation of styrene with aryl bromide catalyzed by 2g
methoxybenzene (Table 5, entries 2 and 3).
Br
0.5
K PO
3
4
mol%
,
2g
+
R1
R2
100 o 4 h
R1
Conclusion
DMF,
C,
R2
We have synthesized a series of dipalladium complexes
2a–h having di-triazolylidene ligands linked by xylylene
spacers. X-ray study showed that both palladium centers
are coordinated by the triazolylidene, two bromides in a
trans configuration and a pyridine ligand with slightly
distorted square-planar geometry. Two pseudo-square-pla-
nar palladium fragments are in an anti-orientation. The
crystal packing of the molecule reveals the intermolecular
hydrogen bonding of BrÁÁÁH–C. Catalytic results of com-
plexes 2a–h in the arylation of styrene with phenyl bro-
mide show that suitable size of the substituted ligand is
important for the good catalytic results. The catalyst 2g is
active for various aryl bromides with different electronic
and steric properties and also active for styrenes with both
electron-donating and electron-withdrawing groups.
Entrya
R1
H
R2
Yield (%)b
1
H
86
91
93
82
82
77
96
84
85
93
83
86
73
90
77
81
82
73
75
80
86
2
o-OMe
p-OMe
o-F
H
3
H
4
H
5
m-F
H
6
p-F
H
7
2,3-benzo
H
H
8
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
p-Cl
p-Cl
p-Cl
p-Cl
p-Cl
p-Cl
p-Cl
9
o-OMe
p-OMe
o-F
10
11
12
13
14
15
16
17
18
19
20
21
a
m-F
p-F
2,3-benzo
H
Supplementary material
o-OMe
p-OMe
o-F
CCDC 1511565 (2b) contains the supplementary crystal-
lographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
m-F
p-F
2,3-benzo
Acknowledgement We are grateful to National Natural Science
Foundation ofChina (NSFC 21571087, 21172188and 21271091), Major
Projects of Natural Science Research in Jiangsu Province
(15KJA150004), PAPD and TAPP of Jiangsu Higher Education Insti-
tutions, and the State Key Laboratory of Inorganic Synthesis and
Preparative Chemistry at Jilin University (2016-01) for financial support.
Reaction conditions: phenylbromide (0.5 mmol, 52 lL), styrene
(0.6 mmol, 63 lL), complex 2g (0.0025 mmol) and K3PO4 (1 mmol,
0.23 g) in DMF (2 mL)
b
Isolated yield
Therefore, suitable size of the substituted ligand, neither
too large nor too small, appears to be necessary for good
catalytic results.
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