New pentamethylene-bridged bis-imidazolium dication ligands and its palladium(II) complexes: Synthesis, characterization, and catalysis

Article abstract of DOI:10.1016/j.ica.2011.10.065

Two new pentamethylene-bridged bis-imidazolium dication ligands (L ¹ = 1,5-bis(1-(4-cyanophenyl)-imidazolium-1-yl)pentane dibromide) and L² = 1,5-bis(1-(4-methoxyphenyl)-imidazolium-1-yl)pentane dibromide) bearing CPI (1-(4-cyanophenyl)-imidazole) and MPI (1-(4-methoxyphenyl)-1H- imidazole)-functionality have been prepared via the reaction of 1,5-dibromopentane with a substituted imidazole derivative. The imidazolium ligand L¹ and L² on reaction with [PdBr₂(C ₆H₅CN)₂] led to the formation of neutral bis-imidazolium dication palladium(II) complexes [L¹] [PdBr ₄] [1] (L¹ = 1,5-bis(1-(4-cyanophenylimidazolium-1-yl) pentane) and [L²] [PdBr₄] [2] (L² = 1,5-bis(1-(4-methoxyphenyl) imidazolium-1-yl)pentane), respectively. The new ligands as well as their palladium(II) complexes, has been characterized by elemental analysis, electronic, IR, ¹H and ¹³C NMR, FAB-MS and ESI-MS spectroscopy. The molecular structure of the representative complex [L²] [PdBr₄] [2] have been determined by single crystal X-ray analysis. Complex 2 exhibits the strong intra- and inter-molecular C-H···X (X = Br, O, π) weak interactions, which plays an important role in stabilizing the crystal packing. Furthermore, the C-H···O interactions in 2 lead to a single-helical motif. The complexes 1 and 2 exhibited good activity in a model Suzuki-Miyaura coupling reaction.

Full text of DOI:10.1016/j.ica.2011.10.065

Inorganica Chimica Acta 383 (2012) 118–124
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Inorganica Chimica Acta
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New pentamethylene-bridged bis-imidazolium dication ligands
and its palladium(II) complexes: Synthesis, characterization, and catalysis
a
b,
c
d
Manoj Trivedi a, , R. Nagarajan , Nigam P. Rath , Abhinav Kumar , Kieran C. Molloy ,
⇑
⇑
Gabriele Kociok-Köhn d
a Department of Chemistry, University of Delhi, Delhi 110007, India
b Department of Chemistry and Biochemistry, Centre for Nanoscience, University of Missouri-St. Louis, One University Boulevard, St. Louis, MO 63121-4499, USA
c Department of Chemistry, Faculty of Science, Lucknow University, Lucknow 226007, India
d Department of Chemistry, University of Bath, Bath BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Two new pentamethylene-bridged bis-imidazolium dication ligands (L1 = 1,5-bis(1-(4-cyanophenyl)-
imidazolium-1-yl)pentane dibromide) and L2 = 1,5-bis(1-(4-methoxyphenyl)-imidazolium-1-yl)pentane
dibromide) bearing CPI (1-(4-cyanophenyl)-imidazole) and MPI (1-(4-methoxyphenyl)-1H-imidazole)-
functionality have been prepared via the reaction of 1,5-dibromopentane with a substituted imidazole
derivative. The imidazolium ligand L1 and L2 on reaction with [PdBr2(C6H5CN)2] led to the formation of
neutral bis-imidazolium dication palladium(II) complexes [L1] [PdBr4] [1] (L1 = 1,5-bis(1-(4-cyanopheny-
limidazolium-1-yl)pentane) and [L2] [PdBr4] [2] (L2 = 1,5-bis(1-(4-methoxyphenyl) imidazolium-
1-yl)pentane), respectively. The new ligands as well as their palladium(II) complexes, has been
characterized by elemental analysis, electronic, IR, 1H and 13C NMR, FAB–MS and ESI–MS spectroscopy.
The molecular structure of the representative complex [L2] [PdBr4] [2] have been determined by single
Article history:
Received 26 August 2011
Accepted 27 October 2011
Available online 12 November 2011
Keywords:
Imidazolium dication ligand
Palladium complex
Crystal structure
Weak interaction
Catalysis
crystal X-ray analysis. Complex 2 exhibits the strong intra- and inter-molecular C–HꢀꢀꢀX (X = Br, O,
p)
weak interactions, which plays an important role in stabilizing the crystal packing. Furthermore, the
C–HꢀꢀꢀO interactions in 2 lead to a single-helical motif. The complexes 1 and 2 exhibited good activity
in a model Suzuki–Miyaura coupling reaction.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
groups separated by various spacer such as pyridine, benzene,
amido, ether, and pentamethylene functionalities have been
N-heterocyclic carbenes (NHCs) are an extremely useful and
versatile class of ligands for catalyst design. After the pioneering
reports by Öfele and Wanzlick [1,2], and the early studies by
Lappert [3–8] on the coordination to late transition-metal com-
plexes, NHC chemistry remained quiescent for more than twenty
years, until Arduengo pointed out the idea that NHCs could be sta-
ble enough for crystallographic characterization [9]. In 1995,
Herrmann took the story a step forward, with the use of NHCs in
the preparation of the first NHC-based homogeneous catalysts
[10]. Since then, many research groups have provided a large num-
ber of NHC-based catalysts for a wide variety of reactions, and
many reviews covering aspects such as preparation [11–13],
stability [14], stereoelectronic properties [15–17], coordination
strategies [13,18,19], and catalytic applications [20–26] have been
entirely devoted to this type of ligand. The concept of two NHC
explored by several groups, yielding Pd(II) pincer complexes
containing various alkyl/aryl groups [27–35]. These ligands are
highly ‘‘tunable’’ and active precatalysts for C–C coupling reactions
and exhibit excellent thermal stability and resistance to degrada-
tion reactions [27,30,31,33,36,24]. Ag(I) (NHC) complexes have
been shown to be facile reagents for the transmetalation of a
variety of functionalized NHC ligands to Pd(II) [37–42]. They are
readily accessed through the reaction of an imidazolium salt with
Ag2O by the method of Wang and Lin [43]. In recent years the
number of crystallographically characterized Pd(II) (NHC) com-
plexes has increased considerably with a rich structural diversity
revealed, especially when halide ions are present in the compound
[44–46]. Keeping these facts in mind, herein, we present synthetic,
spectral characterization of two new pentamethylene-bridged
bis-imidazolium dication ligands (L1 = 1,5-bis(1-(4-cyanophenyl)-
imidazolium-1-yl)pentane dibromide) and L2 = 1,5-bis(1-(4-
methoxyphenyl)-imidazolium-1-yl)pentane dibromide) bearing CPI
(1-(4-cyanophenyl)-imidazole) and MPI (1-(4-methoxyphenyl)-
1H-imidazole)-functionality which are potential precursors to
⇑
Corresponding authors. Tel.: +91 (0) 9811730475 (M. Trivedi); +314 516 5333
(N.P. Rath).
E-mail addresses: manojtri@gmail.com (M. Trivedi), rathn@umsl.edu (N.P. Rath).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.10.065

M. Trivedi et al. / Inorganica Chimica Acta 383 (2012) 118–124
119
novel, bi-functional NHCs. We, also report their palladium(II) com-
plexes (1 and 2) and their comparative catalytic activities in the
Suzuki–Miyaura coupling reaction.
400 MHz, CDCl3, 298 K): 9.20 (s, 2H), 7.80 (d of t, 2H, 1.8 Hz),
7.73(d of t, 2H, 1.8 Hz) 7.38–7.43 (m, 8H, 7.1 Hz), 4.18(t, 4H,
7.2 Hz), 3.49(s, 6H), 1.82(quintet, 4H, 7.4 Hz), 1.21(br quintet, 2H,
7.0 Hz). 13C NMR (d ppm, 400 MHz, CDCl3, 298 K): 136.20,
132.39, 129.29, 127.46, 124.69, 123.92, 122.97, 54.26, 49.48,
35.14, 28.92, 22.58. FAB–MS m/z 578(577), [M](40%); 498(498),
[M]+-Br(90%); 418(418), [M]2+-Br2(20%).
2. Experimental
2.1. Materials and physical measurements
2.3. Bis-imidazolium dication palladium(II) complexes
All reactions were performed under an atmosphere of dry dini-
trogen or argon using standard Schlenk techniques. Solvents were
dried and distilled before use following the standard literature pro-
cedures. 1,5-dibromopentane (Aldrich), 1-(4-methoxyphenyl)-1H-
imidazole (MPI) (Aldrich), was used as received. All other reagents
were used as received. [PdBr2(C6H5CN)2] [47] and 1-(4-cyano-
phenyl)-imidazole) (CPI) were prepared and purified following
the literature procedure [48].
Elemental analyses were performed on a Carlo Erba Model EA-
1108 elemental analyzer and data of C, H and N is within 0.4% of
calculated values. IR(KBr) and electronic spectra were recorded
using Perkin–Elmer FT-IR spectrophotometer and Perkin Elmer
Lambda-35 spectrometer, respectively. FAB mass spectra were re-
corded on a JEOL SX 102/DA 6000 mass spectrometer using Xenon
(6 kV, 10 mA) as the FAB gas. The accelerating voltage was 10 kV
and the spectra were recorded at room temperature with m-nitro-
benzyl alcohol as the matrix. ESI–MS data were recorded using a
waters micromass LCT Mass Spectrometer/Data system. The 1H
and 13C NMR spectra were recorded on a JEOL DELTA2 spectrome-
ter at 400 MHz using TMS as an internal standard. The chemical
shift values are recorded on the d scale and the coupling constants
(J) are in Hz. GCMS studies were done with the Shimadzu-2010
instrument containing a DB-5/RtX-5MS-30Mt column of 0.25 mm
internal diameter.
2.3.1. [L1] [PdBr4] (1)
L1(0.568 g, 1 mmol) was dissolved in 1:1 DCM–MeOH (30 mL)
and [PdBr2(C6H5CN)2] (0.472 g, 1 mmol) added. The resulting yel-
low suspension was refluxed with stirring at room temperature
for 24 h. The solvent was removed in vacuo and the residue
washed with Et2O (5 ml) giving 1 as a pale yellow solid. Yield:
(0.603 g, 80%). Anal. Calc. for C25H24N6Br4Pd: C, 39.78; H, 3.18; N,
11.14. Found: C, 40.08; H, 3.45; N, 11.38%. IR(cmꢁ1, nujol):
m
= 3420, 3090, 2924, 2230, 1605, 1546, 1511, 1458, 1251, 1183,
1117, 1066, 1025, 833, 732, 624, 554. 1H NMR (d ppm, 400 MHz,
CDCl3, 298 K): 9.10 (s, 2H), 8.40 (s, 1H), 7.85 (d, 2H, 6.8 Hz), 7.81
(d of t, 2H, 3.6 Hz), 7.76 (d, 1H, 6.0 Hz), 7.70(d of t, 2H, 5.4 Hz)
7.47(d, 2H, 6.9 Hz), 7.40(d, 2H, 6.4 Hz), 4.23(t, 4H, 8.0 Hz),
1.81(quintet, 4H, 8.4 Hz), 1.25(br quintet, 2H, 7.5 Hz). 13C NMR (d
ppm, 400 MHz, CDCl3, 298 K): 136.20, 132.00, 129.50, 127.60,
124.10, 124.40, 123.60, 122.80, 113.11, 49.20, 35.40, 28.75, 22.60.
UV–VIS {DMSO, kmax nm (e
/Mꢁ1 cmꢁ1)}: 261(4725), 280(2421),
354(5346). ESI–MS (m/z): 408.9 (M+).
2.3.2. [L2] [PdBr4] (2)
L2(0.578 g, 1 mmol) was dissolved in 1:1 DCM–MeOH (30 mL)
and [PdBr2(C6H5CN)2] (0.472 g, 1 mmol) added. The resulting red
suspension was refluxed with stirring at room temperature for
24 h. The solvent was removed in vacuo and the residue washed
with Et2O (5 ml) giving 2 as a orange red solid. The solid mass thus
obtained was extracted with MeCN and filtered, and the filtrate
was layered with diethyl ether and left for slow crystallization.
After a couple of days a brown block crystals were obtained. It
was filtered out, washed several times with diethyl ether, and
dried under vacuo. Yield: (0.633 g, 75%). Anal. Calc. for
2.2. Pentamethylene-functionalised bis-imidazolium salts
2.2.1. 1,5-Bis(1-(4-cyanophenyl)-imidazolium-1-yl)pentane
dibromide (L1)
1-(4-Cyanophenyl)-imidazole) (5.306 g, 31.4 mmol) was added
to 1,5-dibromopentane (2.0 mL, 14.7 mmol) in THF (60 mL) and re-
fluxed for 24 h. The resultant white solid was filtered on a glass frit,
washed with Et2O (20 mL), and dried in vacuo, yielding L1 (14.26 g,
80%) as a white powder that was handled and stored in a vacuum
dessicator. Anal. Calc. For C25H24N6Br2: C, 52.82; H, 4.23; N, 14.79.
C
25H30N4O2Br4Pd: C, 35.54; H, 3.55; N, 6.64. Found: C, 35.78; H,
3.65; N, 6.78%. IR(cmꢁ1, nujol):
m
= 3434, 3047, 2921, 2852, 1604,
1583, 1477, 1432, 1309, 1251, 1180, 1094, 1024, 995, 744, 500,
429. 1H NMR (d ppm, 400 MHz, CDCl3, 298 K): 9.30 (s, 2H), 7.77
(d of t, 2H, 6.0 Hz), 7.72(d of t, 2H, 3.6 Hz) 7.39–7.45 (m, 8H,
7.5 Hz), 4.20(t, 4H, 8.0 Hz), 3.48(s, 6H), 1.81(quintet, 4H, 6.4 Hz),
1.20(br quintet, 2H, 7.5 Hz). 13C NMR (d ppm, 400 MHz, CDCl3,
298 K): 136.30, 132.40, 129.30, 127.45, 124.68, 123.90, 123.10,
Found: C, 53.10; H, 4.36; N, 14.94%. IR(cmꢁ1, nujol):
m = 3320, 2927,
2230, 1606, 1508, 1402, 1247, 1016, 848, 749, 568. 1H NMR (d ppm,
400 MHz, CDCl3, 298 K): 9.30 (s, 2H), 8.50 (s, 1H), 7.90 (d, 2H,
6.9 Hz), 7.80 (d of t, 2H, 2.4 Hz), 7.77 (d, 1H, 9.0 Hz), 7.71(d of t,
2H, 2.4 Hz) 7.48(d, 2H, 6.0 Hz), 7.41(d, 2H, 7.5 Hz), 4.20(t, 4H,
7.5 Hz), 1.80(quintet, 4H, 7.4 Hz), 1.20(br quintet, 2H, 6.0 Hz). 13C
NMR (d ppm, 400 MHz, CDCl3, 298 K): 136.16, 132.05, 129.30,
127.46, 124.00, 124.30, 123.61, 122.56, 115.64, 49.4, 35.21, 28.71,
22.61. FAB–MS m/z 568 (569), [M] (20%); 488 (488), [M]+-Br
(50%); 408 (407), [M]2+-Br2 (20%).
54.30, 49.20, 35.18, 28.98, 22.62. UV–VIS {DMSO, kmax nm (e/
Mꢁ1 cmꢁ1)}: 261(8020), 271(4981), 355(7099). ESI–MS (m/z):
418.8 (M+).
2.4. X-ray crystallographic study
2.2.2. 1,5-Bis(1-(4-methoxyphenyl)-imidazolium-1-yl)pentane
dibromide (L2)
X-ray data for 2 was collected on a Nonius Kappa CCD diffrac-
tometer at 150(2) K using Mo K
a radiation (k = 0.71073 Å). Struc-
1-(4-Methoxyphenyl)-1H-imidazole (5.469 g, 31.4 mmol) was
added to 1,5-dibromopentane (2.0 mL, 14.7 mmol) in THF
(60 mL) and refluxed for 24 h. The resultant white solid was fil-
tered on a glass frit, washed with Et2O (20 mL), and dried in vacuo,
yielding L2 (10.88 g, 60%) as a white powder that was handled and
stored in a vacuum dessicator. Anal. Calc. For C25H30N4O2Br2: C,
ture solution was followed by full-matrix least squares
refinement and was performed using the WinGX-1.70 suite of pro-
grams [49]. Denzo-Scalepack software packages were used for data
collection and data integration for 2. Structure solution and refine-
ment were carried out using the SHELXL97-Program [50]. The non-
hydrogen atoms were refined with anisotropy thermal parameters.
All the hydrogen atoms were treated using appropriate riding
models. The computer programme PLATON was used for analyzing
the interaction and stacking distances [51,52].
51.90; H, 5.19; N, 9.69. Found: C, 52.20; H, 5.41; N, 9.85%. IR(cmꢁ1
nujol): = 3419, 3054, 2924, 1608, 1518, 1481, 1435, 1308, 1250,
1185, 1091, 1024, 841, 748, 722, 696, 541, 517. 1H NMR (d ppm,
,
m

120
M. Trivedi et al. / Inorganica Chimica Acta 383 (2012) 118–124
Scheme 1. Synthesis of L1 and L2.
Scheme 2. Synthesis of 1 and 2.
2.5. Catalysis
chromatography to give the coupling product. Product identities
were confirmed by 1H and 13C NMR and GC–MS.
K2CO3 (0.553 g, 4.0 mmol) was added to a 100 ml three-necked
flask with a stirring bar and the flask was dried under vacuum and
then filled with nitrogen, aryl halide or 4-halotolune (0.342 g,
2.0 mmol) and phenylboronic acid (0.244 g, 2.0 mmol) in DMF
(10 mL). Then 0.1 mol% palladium complex in DMF was added via
a syringe under nitrogen atmosphere. The mixture was stirred at
120 °C for the indicated reaction time. The mixture was cooled
and the precipitate was removed by filtration and the product
was extracted from the filtrate with diethyl ether. The combined
organic layer was dried over anhydrous Na2SO4 and filtered. After
evaporation, the obtained residue was purified by silica-gel column
3. Results and discussion
3.1. Synthesis
3.1.1. Pentamethylene-bridged bis-imidazolium salts
Methodology similar to that used for the preparation of the
ether-functionalised bis-imidazolium salt analogues [34] was
employed to prepare pentamethylene-bridged- functionalised
bis-imidazolium salts. Reaction of 1,5-dibromopentane with
Fig. 1. Projection view of 2 shown with 50% ellipsoids; hydrogen atoms are omitted for clarity.

M. Trivedi et al. / Inorganica Chimica Acta 383 (2012) 118–124
121
Table 1
Table 2
Crystallographic data for 2 at 150(2) K.
Selected non-hydrogen atom geometries for 2 at 150(2) K.
1
1
Empirical formula
Formula weight
Colour and habit
Crystal size/mm
Crystal system, space group
a (Å)
b (Å)
c (Å)
b (°)
C
25H30Br4N4O2Pd
Distances (Å)
Pd–Br(3)
Pd–Br(4)
Pd–Br(2)
Pd–Br(1)
844.57
2.4100(10)
2.4144(9)
2.4176(9)
2.4445(9)
1.452(9)
1.383(9)
1.334(9)
1.394(9)
1.380(9)
1.328(9)
1.464(9)
1.464(9)
1.488(12)
1.511(12)
1.544(12)
1.505(11)
1.394(9)
1.326(9)
1.326(9)
1.384(9)
1.366(9)
1.408(10)
Brown, Block
0.30 ꢃ 0.30 ꢃ 0.18 mm
monoclinic, P 21/c
11.0445(3)
10.1972(2)
25.9445(7)
101.428(1)
90
2864.02(12)
4, 1.959
6.258
150(2) K
0.71073
O(1)–C(1)
O(1)–C(2)
N(1)–C(10)
N(1)–C(8)
N(2)–C(9)
N(2)–C(10)
N(2)–C(11)
N(3)–C(15)
C(11)–C(12)
C(12)–C(13)
C(13)–C(14)
C(14)–C(15)
N(3)–C(16)
N(3)–C(18)
N(4)–C(18)
N(4)–C(17)
O(2)–C(22)
O(2)–C(25)
c
(°)
V (Å3)
Z, Dc (mg mꢁ3
l
)
(mmꢁ1
)
T/K
k (Mo K
a) (Å)
Number of reflections/unique
Number of refined parameter
40937/6510
327
0.0547
0.1390
0.1083
R factor [I > 2
r(I)]
wR2 [I > 2 (I)]
r
R factor (all data)
wR2 (all data)
Goodness-of-fit (GoF)
0.1619
1.043
Angles (°)
Br(3)–Pd–Br(2)
Br(4)–Pd–Br(1)
175.29(4)
176.82(4)
111.2(7)
109.7(7)
111.6(7)
112.7(7)
113.8(8)
128.3(6)
125.4(6)
123.5(6)
125.5(7)
108.1(6)
109.0(6)
109.5(6)
109.4(6)
107.5(6)
107.8(6)
125.9(6)
126.4(6)
126.6(6)
125.9(6)
108.4(7)
107.2(6)
106.7(6)
106.5(7)
1-(4-cyanophenyl)-imidazole)/1-(4-methoxyphenyl)-1H-imidaz-
ole in refluxing THF gave bis-imidazolium salts L1 and L2 as white
powder in good yields, as shown in Scheme 1.
C(12)–C(13)–C(14)
C(15)–C(14)–C(13)
N(3)–C(15)–C(14)
N(2)–C(11)–C(12)
C(11)–C(12)–C(13)
C(18)–N(3)–C(15)
C(10)–N(2)–C(11)
C(16)–N(3)–C(15)
C(9)–N(2)–C(11)
C(18)–N(3)–C(16)
C(10)–N(2)–C(9)
N(3)–C(18)–N(4)
N(2)–C(10)–N(1)
C(18)–N(4)–C(17)
C(10)–N(1)–C(8)
C(17)–N(4)–C(19)
C(10)–N(1)–C(5)
C(18)–N(4)–C(19)
C(8)–N(1)–C(5)
Bis-imidazolium salts L1 and L2 were characterized by elemen-
tal analysis, electronic, IR, 1H and 13C NMR and FAB–MS spectros-
copy. Analytical data of L1, and L2 corroborated well with their
respective formulations (see F-1 to F-4, Supporting material). A
comparison of the 1H and 13C NMR spectra of the CPI and MPI-
functionalised bis-imidazolium salts shows that variation of the
functional group has a negligible effect on the chemical shifts of
the imidazolium ring protons.
3.1.2. Bis-imidazolium dication Pd(II) complexes
Bis-imidazolium salts L1 and L2 were reacted with
[PdBr2(C6H5CN)2] in 1:1 stoichiometric ratio in a mixture of meth-
anol and dichloromethane (1:1 v/v) under refluxing followed by
stirring at RT for 3 h, which afforded neutral, bis-imidazolium dica-
tion palladium complexes [L1] [PdBr4] [1] (L1 = 1,5-bis(1-(4-cyan-
ophenylimidazolium-1-yl)pentane) and [L2] [PdBr4] [2] (L2 = 1,5-
bis(1-(4-methoxyphenyl) imidazolium-1-yl)pentane) in reason-
able yields (75, 80%) as shown in Scheme 2.
C(16)–C(17)–N(4)
C(9)–C(8)–N(1)
C(8)–C(9)–N(2)
C(17)–C(16)–N(3)
Torsion angles (°)
C(12)–C(13)–C(14)–C(15)
C(13)–C(14)–C(15)–N3
C(13)–C(12)–C(11)–N2
C(14)–C(15)–N3–C(18)
C(12)–C(11)–N2–C(10)
ꢁ175.91
ꢁ83.46
59.82
109.30
80.06
1 and 2 were isolated as air-stable, non-hygroscopic solids and
soluble in dichloromethane, chloroform, methanol, dimethylform-
amide, and dimethylsulphoxide but insoluble in petroleum ether
and diethyl ether. Analytical data of 1 and 2 corroborated well with
their respective formulations. Information about composition of
the complexes was also obtained from ESI–MS spectra of the com-
plexes (recorded in the Section 2). The position of various peaks
and overall fragmentation pattern in the ESI–MS spectra of the
respective complexes conformed well to their respective formula-
tions. More information about the structure and bonding in the
complexes has been deduced from the spectral studies.
0
C3N2/C3N2 interplanar dihedral (h, °)
h
48.94
which integrate well to the corresponding hydrogens. Further, it
was observed that the signals associated with various protons
and carbons of the complexes displayed little change on coordina-
tion in comparison to the bis-imidazolium salts.
The electronic absorption spectra of the complexes 1 and 2 dis-
play bands at 260–280 and 354–355 nm. The higher energy band
corresponds to the intra-ligand transition, whilst the lower energy
band can be assigned to MLCT transition.
Infrared spectrum of 1 in nujol displayed sharp and intense
bands around 2230 cmꢁ1 corresponding to
m(C„N). Two charac-
teristic frequencies observed at 1251 cmꢁ1 and 1024 cmꢁ1 in 2 cor-
responds to the asymmetric and symmetric stretching frequencies
of C–O–C of MPI. The vibrations corresponding to m(C@N) in 1 and
2 which appeared at ꢂ1600 cmꢁ1 are because of the imidazole
moiety.
3.2. Molecular structure determination
The purity and composition of both the ligands L1 and L2, and
their complexes have been checked by NMR spectroscopy. Both
the ligands and their complexes display sharp 1H NMR signals
The molecular structure for 2 with atom-labels is shown in
Fig. 1. Details concerning the data collection, solution and refine-

122
M. Trivedi et al. / Inorganica Chimica Acta 383 (2012) 118–124
Table 3
with pentamethylene linkage. One phenyl group of the 4-meth-
Hydrogen bond parameters for 2 at 150(2) K.
oxyphenylimidazole is not coplanar with imidazole ring and is
tilted with respect to the imidazole ring plane at an angle of
30.36° in 2, while the other ring is remains essentially coplanar (tilt
angle 2.65°); crystal packing, rather than electronic factors, are
presumably the origin of this difference.
D–HꢀꢀꢀA–X
Complex 2
d HꢀꢀꢀA Å
D DꢀꢀꢀA Å
h D–HꢀꢀꢀA°
C(9)–H(9)ꢀꢀꢀBr(4)a
C(10)–H(10)ꢀꢀꢀBr(1)b
C(11)–H(11A)ꢀꢀꢀBr(2)b
C(16)–H(16)ꢀꢀꢀBr(1)c
C(16)–H(16)ꢀꢀꢀBr(2)c
C(17)–H(17)ꢀꢀꢀBr(4)d
C(18)–H(18)ꢀꢀꢀBr(1)a
C(18)–H(18)ꢀꢀꢀBr(3)a
C(21)–H(21)ꢀꢀꢀBr(3)e
2.87
2.79
2.89
2.84
2.81
2.81
2.88
2.68
2.74
3.7893
3.6295
3.7669
3.6617
3.5248
3.4782
3.5318
3.5789
3.6806
162
149
148
146
133
128
127
157
168
Crystal packing in 2 is stabilised by inter- and intra-molecular
C–HꢀꢀꢀX (X = Br, O,
p) weak interactions. An interesting feature of
the crystal packing in 1 is a single helical motif (Fig. 2) resulting
from C–HꢀꢀꢀO interactions having contact distances of 2.61 Å [55].
The C–HꢀꢀꢀBr contact distances in the crystal packing are in the
range of 2.68–2.88 Å and the associated angles having range from
127 to 168° (see F-5, Supporting material). These are within the
range for analogous distances reported in the literature [56]. Addi-
Symmetry equivalents.
a
1 ꢁ x, 1/2 + y, 1/2 ꢁ z.
b
1 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z.
tionally, the contact distances for intra-molecular C–Hꢀꢀꢀ
p interac-
c
1 + x, y, z.
tions are in the range of 2.64–2.89 Å (see F-6, Supporting material)
[57]. The crystal packing of 2 closely resembles that of related tet-
rahalometallate derivatives of simple pyridinium cations [53,58–
60]. The distinctive feature is therefore the chains of the cations
building up a lamellar structure with single-layer type of organiza-
tion (Fig. 3). This results in the propagation of alternate cation and
anion layers along the [010] direction.
d
2 ꢁ x, 1/2 + y, 1/2 ꢁ z.
e
1 + x, 1/2 ꢁ y, ꢁ1/2 + z.
ment are enlisted in Table 1 and non-hydrogen atom geometries
and hydrogen bond parameters are tabulated in Tables 2 and 3,
respectively. The asymmetric unit contains one 1,5-bis(1-(4-
methoxyphenyl)imidazolium-1-yl) pentane dication and one
[PdBr4]2ꢁ anion, with the palladium atom on a center of symmetry.
The molecular structure shows that the imidazolium unit is not di-
rectly attached to the metal. The square planar environment of Pd
is completed by four Br atoms. The Pd–Br bond distances are
2.4100(10) to 2.4445(9) Å which are comparable with those found
for other [PdBr4]2ꢁ containing compounds [53,54]. The angles
around Pd are close to 90° and suggest that the hydrogen-bonding
interaction with the 1,5-bis(1-(4-methoxyphenyl)imidazolium-1-
yl) pentane dication causes little distortion on the [PdBr4]2ꢁ anion.
The bond lengths and angles within the C3N2 rings of 2 are compa-
rable to those found in the complex [Ag2(MeC(CH2)5C)2][AgBr2]2
3.3. Catalytic tests
The complexes 1 and 2 were tested as catalysts in the model
Suzuki–Miyaura reaction of selected aryl halides with phenylbo-
ronic acid (Table 4). Complex 2 exhibited higher catalytic activity
than the complex 1; that is, the reaction of aryl halides with phen-
ylboronic acid in the presence of 0.1 mol% of Pd complex (1, and 2)
in DMF at 120 °C for 12 h gave 65% to 100% yield of the Suzuki–
Miyaura coupling product, corresponding to TON of 100–169.
The results presented in Table 4 are in agreement with the earlier
observation by Trzeciak et al. on Pd(II) anionic, square planar com-
plexes of the type [IL]2[PdX4] (where IL = imidazolium cations,
X = Cl, Br) in the Suzuki–Miyaura reaction in alcohols at 40 °C
[61]. The higher product yield obtained with palladium precursor
2 containing the methoxy functionalised imidazolium cation may
be explained by the stabilizing effect of methoxy moiety on the
palladium active form. In all the Suzuki–Miyaura reactions under
study, with Pd-catalyst precursors 1 and 2, colloidal nanoparticles
(
MeC(CH2)5C) = C,C0-1,5-bis(3-methylimidazolin-2-yliden-1-yl) pen-
tane) [34] and in other related complexes [27–35]. The two C3N2
rings in 2 differ significantly only in the C(12)–C(11)–N(2)–C(10)
and C(14)–C(15)–N(3)–C(18) torsions for each section, being rela-
tively rotated about the C(11)–N(2) and C(15)–N(3) bond and dis-
persed to either side of the alkyl linkage. The C(sp3)–O bond
distances in 2 are 1.408(10)–1.452(9) Å, respectively. The ligand
4-methoxyphenylimidazole has lost planarity upon coordination
Fig. 2. Single helical structure in 2 resulting from C–HꢀꢀꢀO interaction: (a) wire-frame model, and (b) space-filled model.

M. Trivedi et al. / Inorganica Chimica Acta 383 (2012) 118–124
123
Fig. 3. Polyhedral representation of 2 viewed down the [010] direction.
Table 4
Results of the Suzuki–Miyaura cross-coupling reactions of aryl halides with phenylboronic acid using complex 1and 2.a
Entry
R1
X
Yield (%)b
TONc
1
2
1
2
a
b
c
CH3
H
H
I
Br
I
91
65
74
81
100
78
83
153
100
113
137
169
120
127
159
d
CH3
Br
94
a
b
c
All reactions were carried out using 2.0 mmol aryl halide, 2.0 mmol phenylboronic acid, 4.0 mmol K2CO3, 0.1 mol% Pd complex and 10 ml DMF at 120 °C for 12 h.
Isolated yield.
Mol(product)/mol(Pd).
of Pd(0) were formed. Their formation was confirmed by the
change of the colour of Pd(II) solution to almost black at the end
of reaction.
Appendix A. Supplementary material
CCDC 795256 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2011.
10.065.
4. Conclusion
In this work, we have presented two new pentamethylene-
bridged bis imidazolium dication ligands and their Pd(II) com-
plexes. We have also demonstrated that both 1 and 2 exhibit good
catalytic activity in the Suzuki–Miyaura reaction at 120 °C. From
the catalytic studies it is evident that by judicious selection of
the functional groups especially the +R groups at the para position
of the benzene ring of 1-Phenyl-1H-imidazole, one can tune the
catalytic responses of these compounds and their coordination
complexes. Also, the ligands under investigation can be a useful
source for the preparation of N-heterocyclic carbenes complexes.
The utilization of these ligands as potential NHCs will be the target
of our future investigations.
References
[1] K. Ofele, J. Organomet. Chem. 12 (1968) P42.
[2] H.-W. Wanzlick, Angew. Chem., Int. Ed. Engl. 7 (1968) 141.
[3] B. Cetinkaya, P. Dixneuf, M.F. Lappert, J. Chem. Soc., Chem. Commun. (1973)
206.
[4] B. Cetinkaya, P. Dixneuf, M.F. Lappert, J. Chem. Soc., Dalton Trans. (1974) 1827.
[5] P.B. Hitchcock, M.F. Lappert, P. Terreros, K.P. Wainwright, J. Chem. Soc., Chem.
Commun. (1980) 1180.
[6] P.B. Hitchcock, M.F. Lappert, P. Terreros, J. Organomet. Chem. 239 (1982)
C26.
[7] M.J. Doyle, M.F. Lappert, P.L. Pye, P. Terreros, J. Chem. Soc., Dalton Trans. (1984)
2355.
[8] M.F. Lappert, J. Organomet. Chem. 690 (2005) 5467.
[9] A.J. Arduengo, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361.
[10] W.A. Herrmann, M. Elison, J. Fischer, C. Kocher, G.R.J. Artus, Angew. Chem., Int.
Ed. Engl. 34 (1995) 2371.
[11] F.E. Hahn, Angew. Chem., Int. Ed. 45 (2006) 1348.
[12] W.A. Herrmann, T. Weskamp, V.P.W. Bohm, Adv. Organomet. Chem. 48 (2001)
1.
[13] F.E. Hahn, M.C. Jahnke, Angew. Chem., Int. Ed. 47 (2008) 3122.
[14] D. Bourissou, O. Guerret, F.P. Gabbai, G. Bertrand, Chem. Rev. 100 (2000) 39.
[15] N.M. Scott, S.P. Nolan, Eur. J. Inorg. Chem. (2005) 1815.
[16] R. Dorta, E.D. Stevens, N.M. Scott, C. Costabile, L. Cavallo, C.D. Hoff, S.P. Nolan, J.
Am. Chem. Soc. 127 (2005) 2485.
Acknowledgements
M.T. gratefully acknowledges financial support from University
Grants Commission, New Delhi (Grant No. F.4–2/2006(BSR)/13–76/
2008(BSR)). We also thank the Director, USIC, University of Delhi,
Delhi and AIRF center of Jawaharlal Nehru University, New Delhi
for analytical and spectral and GC–MS facilities. Special thanks
are due to the Head, Department of Chemistry, University of Delhi,
and Dr. S. Uma for their kind encouragement.

124
M. Trivedi et al. / Inorganica Chimica Acta 383 (2012) 118–124
[17] S. Diez-Gonzalez, S.P. Nolan, Coord. Chem. Rev. 251 (2007) 874.
[18] K. Ofele, W.A. Herrmann, D. Mihalios, M. Elison, E. Herdtweck, T. Priermeier, P.
Kiprof, J. Organomet. Chem. 498 (1995) 1.
[19] E. Peris, Top. Organomet. Chem. 21 (2007) 83.
[20] W.A. Herrmann, Angew. Chem., Int. Ed. 41 (2002) 1290.
[21] L.H. Gade, S. Bellemin-Laponnaz, Coord. Chem. Rev. 251 (2007) 718.
[22] V. César, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004)
619.
[38] A.A.D. Tulloch, S. Winston, A.A. Danopoulos, G. Eastham, M.B. Hursthouse,
Dalton Trans. (2003) 699.
[39] M. Froseth, A. Dhindsa, H. Roise, M. Tilset, Dalton Trans. (2003) 4516.
[40] V. César, S. Bellemin-Laponnaz, L.H. Gade, Organometallics 21 (2002) 5204.
[41] A.A.D. Tulloch, A.A. Danopoulos, G.J. Tizzard, S.J. Coles, M.B. Hursthouse, R.S.
Hay-Motherwell, W.B. Motherwell, Chem. Commun. (2001) 1270.
[42] R.S. Simons, P. Custer, C.A. Tessier, W.J. Youngs, Organometallics 22 (2003)
1979.
[23] S. Diez-Gonzalez, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612.
[24] E. Peris, R.H. Crabtree, Coord. Chem. Rev. 248 (2004) 2239.
[25] J.A. Mata, M. Poyatos, E. Peris, Coord. Chem. Rev. 251 (2007) 841.
[26] A.T. Normand, K.J. Cavell, Eur. J. Inorg. Chem. (2008) 2781.
[27] D.J. Nielsen, K.J. Cavell, B.W. Skelton, A.H. White, Inorg. Chim. Acta 327 (2002)
116.
[43] H.M.J. Wang, I.J.B. Lin, Organometallics 17 (1998) 972.
[44] K.M. Lee, H.M.J. Wang, I.J.B. Lin, J. Chem. Soc., Dalton Trans. (2002) 2852.
[45] P.L. Arnold, Heteroat. Chem. 13 (2002) 534.
[46] I.J.B. Lin, C.S. Vasam, Comments Inorg. Chem. 25 (2004) 75.
[47] M.A. Kharash, R.C. Seylera, F.R. Mayor, J. Am. Chem. Soc. 60 (1938) 882.
[48] A. Hatzidimitriou, A. Gourdon, J. Devillers, J.-P. Launay, E. Mena, E. Amouyal,
Inorg. Chem. 35 (1996) 2212.
[28] S. Gründemann, M. Albrecht, J.A. Loch, J.W. Faller, R.H. Crabtree,
Organometallics 20 (2001) 5485.
[49] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[29] A.A. Danopoulos, A.A.D. Tulloch, S. Winston, G. Eastham, M.B. Hursthouse,
Dalton Trans. (2003) 1009.
[30] D.J. Nielsen, K.J. Cavell, B.W. Skelton, A.H. White, Inorg. Chim. Acta 359 (2006)
1855.
[50] G.M. Sheldrick, Bruker Analytical X-ray Division, Madison, WI, 1998.
[51] G.M. Sheldrick, SHELX-97; Programme for Refinement of Crystal Structures,
University of Gottingen, Gottingen, Germany, 1997.
[52] PLATON, A.L. Spek, Acta Cryst. 46A (1990) C34.
[31] E. Peris, J.A. Loch, J. Mata, R.H. Crabtree, Chem. Commun. (2001) 201.
[32] J.A. Loch, M. Albrecht, E. Peris, J. Mata, J.W. Faller, R.H. Crabtree,
Organometallics 21 (2002) 700.
[53] F. Neve, A. Crispini, Cryst. Eng. Comm. 9 (2007) 698.
[54] C.-M. Zhong, Y.-J. Zuo, H.-S. Jin, T.-C. Wanga, S.-Q. Liua, Acta Cryst. E 62 (2006)
m2281.
[33] D.J. Nielsen, A.M. Magill, B.F. Yates, K.J. Cavell, B.W. Skelton, A.H. White, Chem.
Commun. (2002) 2500.
[34] D.J. Nielsen, K.J. Cavell, B.W. Skelton, A.H. White, Organometallics 25 (2006)
4850.
[35] D.J. Nielsen, K.J. Cavell, B.W. Skelton, A.H. White, Inorg. Chim. Acta 352 (2003)
143.
[55] D. Braga, F. Grepioni, G.R. Desiraju, J. Organomet. Chem. 548 (1997) 33.
[56] G.R. Desiraju, T. Steiner, The Weak Hydrogen Bond in Structural Chemistry and
Biology, Oxford University Press, Oxford, 1999.
[57] D. Braga, F. Grepioni, E. Tedesco, Organometallics 17 (1998) 2669.
[58] F. Neve, A. Crispini, S. Armentano, O. Francescangeli, Chem. Mater. 10 (1998)
1904.
[36] D.S. McGuinness, N. Saendig, B.F. Yates, K.J. Cavell, J. Am. Chem. Soc. 123
(2001) 4029.
[37] A.M. Magill, D.S. McGuinness, K.J. Cavell, G.J.P. Britovsek, V.C. Gibson, A.J.P.
White, D.J. Williams, A.H. White, B.W. Skelton, J. Organomet. Chem. 617–618
(2001) 546.
[59] F. Neve, O. Francescangeli, A. Crispini, Inorg. Chim. Acta 338 (2002) 51.
[60] F. Neve, O. Francescangeli, A. Crispini, J. Charmant, Chem. Mater. 13 (2001)
2032.
[61] W. Zawartka, A. Gnieweka, A.M. Trzeciak, J.J. Ziłówski, J. Pernak, J. Mol. Catal. A:
Chem. 304 (2009) 8.