Synthesis and structural characterization of metal complexes based on pyrazole/imidazolium chlorides

Article abstract of DOI:10.1016/j.jorganchem.2004.09.053

A series of imidzoalium salt, L ? HCl, for the potentially bidentate pyrazole/N-heterocyclic carbene was synthesized. Reactions of a 2:1 mixture between L ? HCl bearing bulky N-substitution and Ag₂O produced Ag(L)Cl, whereas a novel compound with unique stoichiometry AgL ₂(AgCl)_0.5Cl was produced from L ? HCl bearing N-methyl group under identical condition. Reactions of L ? HCl with PdCl₂ produced zwitterionic Pd^IICl₃L ? H. Selected structural determinations on L ? HCl, Ag(L)Cl, AgL ₂(AgCl)_0.5Cl, and Pd^IICl₃L ? H revealed intriguing crystal chemistry in which the less-stable gauche rotamers were obtained exclusively. A preliminary application of the zwitterionic complexes, Pd^IICl₃L ? H, in Heck coupling reaction of aryl bromide with n-butyl acrylate shows effective activity.

Full text of DOI:10.1016/j.jorganchem.2004.09.053

Journal of Organometallic Chemistry 690 (2005) 403–414
www.elsevier.com/locate/jorganchem
Synthesis and structural characterization of metal complexes based on
pyrazole/imidazolium chlorides
*
Hon Man Lee , Pei Ling Chiu, Ching-Han Hu, Chun-Liang Lai, Yi-Chun Chou
Department of Chemistry, National Changhua University of Education, Changhua, 50058, Taiwan, ROC
Received 21 June 2004; accepted 22 September 2004
Abstract
A series of imidzoalium salt, L Æ HCl, for the potentially bidentate pyrazole/N-heterocyclic carbene was synthesized. Reactions of
a 2:1 mixture between L Æ HCl bearing bulky N-substitution and Ag2O produced Ag(L)Cl, whereas a novel compound with unique
stoichiometry AgL2(AgCl)0.5Cl was produced from L Æ HCl bearing N-methyl group under identical condition. Reactions of L Æ HCl
with PdCl2 produced zwitterionic PdIICl3L Æ H. Selected structural determinations on L Æ HCl, Ag(L)Cl, AgL2(AgCl)0.5Cl, and
PdIICl3L Æ H revealed intriguing crystal chemistry in which the less-stable gauche rotamers were obtained exclusively. A preliminary
application of the zwitterionic complexes, PdIICl3L Æ H, in Heck coupling reaction of aryl bromide with n-butyl acrylate shows effec-
tive activity.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Pyrazole; Imidazolium chlorides; Gauche preference; Palladium complex; X-ray determination; Heck reaction
1. Introduction
In this paper, we report the synthesis of imidazolium
chloride L Æ HCl with pyrazole/NHC precursor func-
tionality and its metal complexes. Interesting, in the
course of our investigation, we structurally character-
ized selected examples of the ligand precursor L Æ HCl,
their silver NHC complexes AgL, and a zwitterionic pal-
ladium compound PdIICl3L Æ H, which display intrigu-
ing crystal chemistry. Most importantly, the ethylene
spacer in L has a high preference to adopt the less stable
gauche conformation as revealed by X-ray structural
analysis. The anomalous gauche effect in 1,2-difluoro-
ethane, with the gauche rotamer being more stable than
its anti rotamer, has been widely reported and can be
attributed to a stereo-electronic nature [5]. Alternatively,
the energetic gauche rotamer can be stabilized by inter-
molecular forces such as hydrogen bonds in the crystal
lattice [6]. Although, a DFT theoretical computation
on the silver and palladium complexes indicates that
the anti conformations are more stable in the gas phase,
the structural analysis of a variety of compounds
strongly indicate a gauche preference in the solid state.
In the past 10 years, N-heterocyclic carbenes (NHC)
are receiving much attention for their wide applicability
in coordination chemistry and catalysis [1]. They are
generally accessible via deprotonation of imidazolium
salts. Recently, research efforts have also been devoted
for the synthesis of functionalized N-heterocyclic car-
bene ligands [2,3]. Incorporation of the carbene func-
tionality into ligand systems containing other ꢀclassicalꢁ
donor groups offers vast opportunities for ligand design
which may lead to the discovery of new efficient cata-
lysts. Along this direction, we started out a project to
explore if the new seven-member palladacycle A with a
pyrazole-funtionalized NHC carbene L is a more effec-
tive catalyst than the structurally similar palladium
bis(NHC) compound B (Scheme 1) [4].
*
Corresponding author. Tel.: +886 4 7232105x3523; fax: +886 4
7211190.
E-mail address: leehm@cc.ncue.edu.tw (H.M. Lee).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.09.053

404
H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
45 °C for 3.5 h. The solution was filtered, washed
N
N
N
N
with water, and dried with anhydrous MgSO4. Re-
moval of the solvent under vacuum gave a yellow liq-
uid as pure product. Yield: 4.7 g (71%). 1H NMR
(CDCl3): d 2.11 (s, 3H, CH3), 2.15 (s, 3H, CH3),
3.74 (t, 3JHH = 6.3 Hz, 2H, CH2), 4.13 (t,
3JHH = 6.3 Hz, 2H, CH2), 5.69 (s, 1H, CH).
13C{1H} NMR (CDCl3): d 10.7 (C3), 13.2 (C3), 42.7
(C2), 49.4 (C2), 104.8 (py-CH), 139.4 (quaternary C),
147.9 (quaternary C).
N
N
R
N
N
R
N
N
N
Pd
Pd
R
R
N
Cl
Cl
Cl
Cl
A
L
B
Scheme 1.
This gauche preference provides a potential utility of
compounds based on L Æ HCl in the construction of no-
vel supramolecular solids for materials and bio-organic
applications.
2.3. General procedure for the preparation of 1a–1d
Neutral and cationic palladium(II) complexes are
widely used as catalyst precursors in Csp2–Csp2 cou-
pling reactions [7]. However, zwitterionic palladium
complexes with different electrophilicities and donor
ligand lability in comparison with the neutral and cat-
ionic complexes have not previously been employed.
To understand the utility of the zwitterionic system
in C–C coupling reactions, a preliminary application
of PdIICl3L Æ H in Heck coupling reaction of aryl bro-
mide with n-butyl acrylate is also reported, which
shows effective activity.
A mixture of 1-(2-chloroethyl)-3,5-dimethyl-1H-pyr-
azole and an appropriate N-substituted imidazole with
mole ratio of ca. 1:2–4 in 10–30 mL of DMF was heated
at 120 °C for 1 day. The solvent was allowed to remove
completely under vacuum. Upon addition of diethyl-
ether, the solid formed was filtered, washed with more
diethylether, and dried under vacuum.
2.4. 1-[2-(-3,5-Dimethylpyrazol-1-yl)ethyl]-3-(4-fluoro-
benzyl)-3H-imidazol-1-ium chloride (1a)
1-(2-Chloroethyl)-3,5-dimethyl-1H-pyrazole (1.00 g,
6.30 mmol) and 1-(4-fluorobenzyl)-1H-imidazole (4.44
g, 25.22 mmol) were used. A cream-white solid was ob-
tained. Yield: 2.09 g (96%). Anal. Calc. for C17H20-
ClFN4: C, 60.98; H, 6.02; N, 16.73. Found: C, 60.87;
H, 6.10; N, 16.71%. m.p. 180–185 °C. 1H NMR
(CDCl3): d 1.90 (s, 3H, CH3), 2.08 (s, 3H, CH3), 4.42
2. Experimental
2.1. General procedure
All reactions were performed under a dry nitrogen
atmosphere using
3
3
(d, JHH = 5.4 Hz, 2H, CH2), 4.86 (d, JHH = 5.4 Hz,
2H, CH2), 5.48 (s, 2H, CH2), 5.62 (s, 1H, CH), 6.88 (s,
a Schlenk line with standard
Schlenk technique. All solvents were distilled under
nitrogen from the appropriate drying agent and stored
3
1H, imi-H), 7.00 (t, JHH = 3JHF = 8.7 Hz, 2H, Ar-H),
3
7.24 (s, 1H, imi-H), 7.43 (dd, JHH = 8.7 Hz,
˚
in solvent reservoirs, which contained 4 A molecular
4JHH = 5.1 Hz, 2H, Ar-H), 10.30 (s, 1H, NCHN).
13C{1H} NMR (CDCl3): d 10.6 (C3), 13.3 (C3), 47.8
(C2), 49.2 (C2), 52.6 (C2), 105.6 (py-CH), 116.4 (d,
2JCF = 21.8 Hz, Cmeta), 121.2 (imi-CH), 122.9 (imi-
sieves, and purged with nitrogen. 1H and 13C{1H}
NMR spectra were recorded at 300.13 and 75.48
MHz, respectively, on a Bruker AV-300 spectrometer.
Chemical shifts for H and 13C spectra were recorded
1
3
CH), 129.0 (Cipso), 135.7 (d, JCF = 8.4 Hz, Cortho),
137.6 (py-C), 140.4 (py-C), 148.7 (NCHN), 163.1 (d,
1JHF = 250.0 Hz, C-F).
in ppm relative to residual proton of CDCl3 (1H: d
7.24; 13C: d 77.0) and DMSO-d6 (1H: d 2.50; 13C: d
39.5). Elemental analyses were performed on a Her-
aeus CHN-OS Rapid Elemental Analyzer at Instru-
ment Center, National Chung Hsing University,
Taiwan. 1-Naphthalen-1-ylmethyl-1H-imidazole [4],
1-(4-fluorobenzyl)-1H-imidazole [4], and 1-(2,4,6-tri-
methylphenyl)-1H-imidazole [8] were synthesized
according to the literature procedures.
2.5. 1-[2-(3,5-Dimethylpyrazol-1-yl)ethyl]-3-(2,4,6-
trimethylbenzyl)-3H-imidazol-1-ium chloride (1b)
1-(2-Chloroethyl)-3,5-dimethyl-1H-pyrazole (0.54 g,
3.39 mmol) and 1-(2,4,6-trimethylphenyl)-1H-imidazole
(1.26 g, 6.78 mmol) were used. An off-white solid was
obtained. Yield: 0.68
C19H25ClN4: C, 50.77; H, 4.14; N, 4.93. Found: C,
50.67; H, 4.13; N, 4.91%. m.p. 203–205 °C. H NMR
g (58%). Anal. Calc. for
2.2. 1-(2-Chloroethyl)-3,5-dimethyl-1H-pyrazole
1
A 50 mL 1,2-dichloroethane solution of KOH
(15.57 g, 0.28 mol), K2CO3 (12.16, 0.09 mol), tetrabu-
tylammonium chloride (0.24 g, 0.88 mmol), and 3,5-
dimethylpyrazole (4.00 g, 0.04 mmol) was heated at
(CDCl3): d 1.93 (s, 6H, CH3), 2.08 (s, 6H, CH3), 2.26
(s, 3H, CH3), 4.57 (m, 2H, CH2), 5.18 (m, 2H, CH2),
5.68 (s, 1H, CH), 6.91 (s, 2H, Ar-H), 7.04 (s, 1H, imi-
H), 7.51 (s, 1H, imi-H), 10.00 (s, 1H, NCHN).

H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
405
13C{1H} NMR (CDCl3): d 11.1 (C3), 13.4 (C3), 17.3
(C3), 21.0 (C3), 48.0 (C2), 49.5 (C2), 105.6 (py-CH),
122.5 (imi-CH), 123.7 (imi-CH), 129.8 (Ar-CH), 130.5
(quaternary C), 134.0 (quaternary C), 138.3 (quaternary
C), 140.5 (quaternary C), 141.3 (quaternary C), 148.6
(NCHN).
Yield: 0.83 g (76%). Anal. Calc. for C17H19AgClF1N4:
C, 46.23; H, 4.34; N, 12.68. Found: C, 46.20; H, 4.31;
1
N, 12.65%. m.p. 60–65 °C. H NMR (CDCl3): d 1.80
(s, 3H, CH3), 2.21 (s, 3H, CH3), 4.32 (t, JHH = 5.4
3
3
Hz, 4H, CH2), 4.59 (t, JHH = 5.4 Hz, 4H, CH2), 5.26
(s, 2H, CH2), 5.70 (s, 1H, CH), 6.46 (s, 2H, imi-H),
3
6.84 (s, 2H, imi-H), 7.03 (t, JHH = 3JHF = 8.3 Hz, 2H,
3
4
2.6. 1-[2-(3,5-Dimethylpyrazol-1-yl)ethyl]-3-napthalen-
1-ylmethyl-3H- imidazol-1-ium chloride (1c)
Ar-H), 7.25 (dd, JHH = 8.3 Hz, JHH = 5.4 Hz, 2H,
Ar-H). 13C{1H} NMR (CDCl3): d 15.1 (C3), 18.7 (C3),
53.6 (C2), 56.4 (C2), 58.5 (C2), 109.7 (py-CH), 120.6
2
1-(2-Chloroethyl)-3,5-dimethyl-1H-pyrazole (2.35 g,
14.80 mmol) and 1-naphthalen-1-ylmethyl-1H-imidaz-
ole (9.16 g, 43.90 mmol) were used. A white solid was
(d, JCF = 21.6 Hz, Cmeta), 127.4 (imi-CH), 127.5 (imi-
3
CH), 134.7 (d, JCF = 8.4 Hz, Cortho), 138.5 (Cipso),
1
144.6 (py-C), 152.2 (py-C), 166.9 (d, JHC = 244.1 Hz,
CF), 184.5 (Ag-C).
obtained. Yield: 4.09
C21H23ClN4: C, 68.74; H, 6.32; N, 15.27. Found: C,
g
(75%). Anal. Calc. for
1
68.69; H, 6.23; N, 15.18%. m.p. 115–120 °C. H NMR
2.10. Preparation of 2b
(CDCl3): d 1.85 (s, 3H, CH3), 2.06 (s, 3H, CH3), 4.45
3
3
(t, JHH = 5.4 Hz, 2H, CH2), 4.95 (t, JHH = 5.4 Hz,
2H, CH2), 5.50 (s, 1H, CH), 5.93 (s, 2H, CH2), 6.94 (s,
1H, imi-H), 7.14 (s, 1H, imi-H), 7.47–7.62 (m, 4H, Ar-
H), 7.85–7.98 (m, 3H, Ar-H), 10.48 (s, 1H, NCHN).
13C{1H} NMR (CDCl3): d 10.6 (C3), 13.3 (C3), 47.8
(C2), 49.2 (C2), 51.4 (C2), 105.4 (py-CH); 121.3, 122.5,
122.8, 125.4, 126.7, 127.9, 129.1, 130.8, 133.4, 137.5,
140.5, (py-C, Ar-C, Ar-CH, imi-CH); 148.7 (NCHN).
Complex 1b (0.19 g, 0.55 mmol) and Ag2O (63 mg,
0.27 mmol) were used. A grey-white solid was obtained.
Yield: 0.19 g (75%). Anal. Calc. for C19H24AgClN4: C,
50.52; H, 5.36; N, 12.40. Found: C, 50.48; H, 5.30; N,
1
12.51%. m.p. 175–180 °C. H NMR (CDCl3): d 1.93
(s, 6H, CH3), 2.02 (s, 3H, CH3), 2.23 (s, 3H, CH3),
3
2.32 (s, 3H, CH3), 4.36 (t, JHH = 5.4 Hz, 2H, CH2),
3
4.69 (t, JHH = 4.8 Hz, 2H, CH2), 5.77 (s, 1H, CH),
6.64 (s, 1H, imi-H), 6.81 (s, 1H, imi-H), 6.94, (s, 2H,
Ar-H). 13C{1H} NMR (CDCl3): d 10.9 (C3), 13.5 (C3),
17.6 (C3), 21.0 (C3), 49.0 (C2), 51.6 (C2), 105.5 (py-
CH), 121.6 (imi-C), 122.6 (imi-C), 129.5 (Ar-CH),
134.4 (quaternary C), 135.1 (quaternary C), 139.6 (qua-
ternary C), 139.8 (quaternary C), 148.7 (quaternary C),
signal for Ag-C was not observed.
2.7. 1-[2-(3,5-Dimethylpyrazol-1-yl)ethyl]-3-methyl-3H-
imidazol-1-ium chloride (1d)
1-(2-Chloroethyl)-3,5-dimethyl-1H-pyrazole (2.01 g,
12.60 mmol) and 1-methylimidazole (4.0 mL, 50.40
mmol) were used. A white solid was obtained. Yield:
2.40 g (80%). Anal. Calc. for C11H17ClN4: C, 54.88;
H, 7.12; N, 23.27. Found: C, 54.77; H, 7.13; N,
2.11. Preparation of 2c
1
23.25%. m.p. 198–202 °C. H NMR (CDCl3): d 1.93
(s, 3H, CH3), 2.06 (s, 3H, CH3), 3.94 (s, 3H, NCH3),
Complex 1c (0.49 g, 1.33 mmol) and Ag2O (0.16 g,
0.66 mmol) were used. A white solid was obtained.
Yield: 0.45 g (72%). Anal. Calc. for C21H22AgClN4: C,
53.24; H, 4.68; N, 11.83. Found: C, 53.29; H, 4.62; N,
3
3
4.36 (t, JHH = 5.4 Hz, 2H, CH2), 4.84 (t, JHH = 5.4
Hz, 2H, CH2), 5.64 (s, 1H, CH), 6.79 (s, 1H, imi-H),
7.43 (s, 1H, imi-H), 10.22 (s, 1H, NCHN). 13C{1H}
NMR (CDCl3): d 10.8 (C3), 13.4 (C3), 36.6 (C3), 47.9
(C2), 48.9 (C2), 105.7 (py-CH), 123.0 (imi-CH), 122.7
(imi-CH), 138.0 (py-C), 140.4 (py-C), 148.7 (NCHN).
1
11.83%. m.p. 90–92 °C. H NMR (CDCl3): d 1.75 (s,
3H, CH3), 2.20 (s, 3H, CH3), 4.29 (t, JHH = 5.4 Hz,
3
3
2H, CH2), 4.58 (t, JHH = 5.4 Hz, 2H, CH2), 5.66 (s,
1H, CH), 5.70 (s, 2H, CH2), 6.31 (s, 1H, imi-H), 6.68
(s, 1H, imi-H), 7.32–7.54 (m, 4H, Ar-H), 7.85–7.90 (m,
3H, Ar-H). 13C{1H} NMR (CDCl3): d 10.6 (C3), 13.5
(C3), 49.1 (C2), 51.8 (C2), 53.7 (C2), 105.2 (py-CH),
120.8, 121.8, 122.7, 125.3, 126.3, 127.1, 127.5, 127.9,
129.0, 130.3, 130.8, 133.8, 140.0, 148.7 (py-C, Ar-C,
Ar-CH, imi-CH), 180.6 (Ag-C).
2.8. General procedure for the preparation of 2a–2d
A mixture of 1 and Ag2O (2:1) was stirred in ca. 20
mL of dichloromethane in the dark overnight. The solu-
tion was then filtered through a small pad of Celite. The
solvent was removed completely under vacuum. Upon
addition of diethylether, the solid formed was filtered,
washed with more diethylether and dried under vacuum.
2.12. Preparation of 2d
2.9. Preparation of 2a
Complex 1d (0.33 g, 1.38 mmol) and Ag2O (0.16 g,
0.69 mmol) were used. A white solid was obtained.
Yield: 0.42 g (88%). Anal. Calc. for C22H32Ag1.5Cl1.5N8:
C, 42.38; H, 5.17; N, 17.97. Found: C, 42.41; H, 5.13; N,
Complex 1a (0.83 g, 2.47 mmol) and Ag2O (0.29 g,
1.24 mmol) were used. A yellow solid was obtained.

406
H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
1
17.90%. m.p. 98–100 °C. H NMR (CDCl3): d 1.81 (s,
6H, CH3), 2.15 (s, 6H, CH3), 3.76 (s, 3H, CH3), 4.23
2.16. Preparation of 3c
3
3
(t, JHH = 5.4 Hz, 4H, CH2), 4.49 (t, JHH = 5.4 Hz,
4H, CH2), 5.66 (s, 2H, CH), 6.34 (s, 2H, imi-H), 6.80
(s, 2H, imi-H). 13C{1H} NMR (CDCl3): d 10.6 (C3),
13.5 (C3), 38.8 (C3), 49.2 (C2), 51.4 (C2), 105.2 (py-
CH), 121.8 (imi-CH), 121.9 (imi-CH), 140.0 (py-C),
148.6 (py-C), 180.5 (Ag-C).
Complex 1c (0.30 g, 0.82 mmol) and PdCl2 (73 mg,
0.41 mmol) were used. An orange solid was obtained.
Yield: 0.18 g (82%). Anal. Calc. for C21H23Cl3N4 Pd:
C, 46.35; H, 4.26; N, 10.29. Found: C, 46.68; H, 4.64;
1
N, 10.25%. m.p. 155–160 °C. H NMR (DMSO-d6): d
1.85 (s, 3H, CH3), 1.93 (s, 3H, CH3), 4.32 (t, 2H,
3
3JHH = 5.1 Hz, 2H, CH2), 4.56 (t, 2H, JHH = 5.1 Hz,
CH2), 5.54 (s, 1H, CH), 5.89 (s, 2H, CH2), 7.38–7.63
(m, 5H, imi-H, Ar-H), 7.81 (s, 1H, imi-H), 8.00–8.10
(m, 3H, Ar-H), 8.81 (s, 1H, NCHN). 13C{1H} NMR
(DMSO-d6): d 10.5 (C3), 13.6 (C3), 47.5 (C2), 49.3
(C2), 50.2 (C2), 105.2 (py-CH), 123.3, 126.0, 126.9,
127.6, 127.8, 129.3, 130.0, 130.5, 130.8, 133.8, 137.1,
139.7, 140.0 (py-C, Ar-C, Ar-CH, imi-CH), 147.3
(NCHN).
2.13. General procedure for the preparation of 3a–3d
A mixture of 1 and PdCl2 (2:1) was stirred in ca. 10
mL of DMF at room temperature for 4 h. The solvent
was removed completely under vacuum. The solid,
formed upon addition of THF, was filtered, washed with
more THF, and dried under vacuum.
2.14. Preparation of 3a
2.17. Preparation of 3d
Complex 1a (0.27 g, 0.78 mmol) and PdCl2 (69
mg, 0.39 mmol) were used. An orange solid was ob-
Complex 1d (0.50 g, 2.08 mmol) and PdCl2 (0.18 g,
1.04 mmol) were used. An orange solid was obtained.
Yield: 0.38 g (86%). Anal. Calc. for C11H17Cl3N4 Pd:
C, 31.60; H, 4.10; N, 13.40. Found: C, 31.55; H, 4.08;
tained. Yield: 0.20
g
(100%). Anal. Calc. for
C17H20Cl3 FN4 Pd: C, 39.87; H, 3.94; N, 10.94.
Found: C, 39.98; H, 3.83; N, 10.91%. m.p. 201 °C.
1H NMR (DMSO-d6): d 1.95 (s, 3H, CH3), 2.00
1
N, 13.37%. m.p. 258–260 °C. H NMR (DMSO-d6): d
2.05 (s, 6H, CH3), 2.07 (s, 6H, CH3), 3.83 (s, 3H,
3
(s, 3H, CH3), 4.38 (t, JHH = 5.7 Hz, 2H, CH2),
3
4.56 (t, JHH = 5.7 Hz, 2H, CH2), 5.38 (s, 2H,
3
CH3), 4.39 (t, JHH = 5.7 Hz, 2H, CH2), 4.55 (t,
3JHH = 5.7 Hz, 2H, CH2), 5.79 (s, 1H, CH), 7.53 (s,
1H, imi-H), 7.69 (s, 1H, imi-H), 9.02 (s, 1H, NCH N).
13C{1H} NMR (DMSO-d6): d 10.7 (C3), 13.7 (C3),
36.2 (C2), 47.6 (C2), 49.3 (C2), 105.5 (py-CH), 123.0
(imi-C), 123.9 (imi-C), 137.4 (py-C), 139.8 (py-C),
147.2 (NCHN).
3
CH2), 5.74 (s, 1H, CH), 7.27 (t, JHH = 3JHF = 8.4
3
Hz, 2H, Ar-H), 7.41 (virtual t, JHH = 4JHH = 5.4
Hz, 2H, Ar-H), 7.62 (s, 2H, imi-H), 7.78 (s, 2H,
imi-H), 8.94 (s, 1H, NCH N). 13C{1H} NMR
(DMSO-d6): d 15.3 (C3), 18.4 (C3), 52.3 (C2), 54.0
2
(C2), 56.1 (C2), 110.1 (py-CH), 120.6 (d, JCF = 21.8
Hz, Cmeta), 127.5 (imi-CH), 128.2 (imi-CH), 135.7
2.18. General procedure for the Heck coupling reactions
3
(d, JCF = 8.7 Hz, Cortho), 136.3 (Cipso py-C), 141.7
1
(py-C), 143.0 (d, JHC = 210.4 Hz, CF), 152.0
(NCHN).
In a typical run, a 50 mL two-neck flask equipped
with a reflux condenser was charged with 4-bromoaceto-
phenone (1.0 mmol), n-butyl acrylate (1.4 mmol), anhy-
drous sodium acetate (1.1 mmol) and 0.5 mol% of
catalyst. The flask was thoroughly degassed, added with
5 mL of N,N-dimethylacetamide via a syringe, and then
placed in a preheated oil bath at 170–175 °C. After 6 h,
10 mL of dichloromethane was added to the reaction
mixture and the organic layer was washed with five
times of water and dried with anhydrous MgSO4. The
solution was then filtered. The solvent and any volatiles
were removed completely under high vacuum to give the
isolated product. The residue was dissolved in CDCl3
2.15. Preparation of 3b
Complex 1b (81 mg, 0.23 mmol) and PdCl2 (21 mg,
0.12 mmol) were used. A dark brown solid was ob-
tained. Yield: 42 mg (70%). Anal. Calc. for C19H25Cl3N4
Pd: C, 43.70; H, 4.83; N, 10.73. Found: C, 43.38; H,
1
4.92; N, 10.63%. m.p. >280 °C. H NMR (DMSO-d6):
d 1.95 (s, 6H, CH3), 2.00 (s, 3H, CH3), 2.17 (s, 3H,
CH3), 2.32 (s, 3H, CH3), 4.51 (br s, 2H, CH2), 4.67 (br
s, 2H, CH2), 5.79 (s, 1H, CH), 7.13, (s, 2H, Ar-H),
7.97 (s, 1H, imi-H), 7.87 (s, 1H, imi-H), 9.23 (s, 1H,
NCHN). 13C{1H} NMR (DMSO-d6): d 10.8 (C3), 13.7
(C3), 17.2 (C3), 21.0 (C3), 47.5 (C2), 49.6 (C2), 105.5
(py-CH), 123.7 (imi-CH), 124.3 (imi-CH), 129.6 (Ar-
CH), 131.4 (quaternary C), 134.6 (quaternary C),
138.1 (quaternary C), 139.7 (quaternary C), 140.7 (qua-
ternary C), 147.2 (NCHN).
1
and analyzed by H NMR.
2.19. X-ray data collection
Crystal of 1a, 2b, 2d, and 3a were obtained by vapor
diffusion of diethylether into its corresponding acetonit-
rile solution. Typically, the crystal was removed from

H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
407
the vial with a small amount of mother liquor and
immediately coated with silicon grease on a weighting
paper. A suitable crystal was mounted on a glass fiber
with silicon grease and placed in the cold stream of a
Bruker SMART CCD with graphite monochromated
were needed to speed up the reaction and afford better
yields. Subsequent removal of solvent and precipitation
with diethylether produced 1a–1d as white solids in good
yields (58–96%). All the new imidazolium chlorides ob-
tained dissolve readily in dichloromethane and chloro-
form. They were characterized by 1H and 13C{1H}
NMR spectroscopes as well as elemental analyses. The
carbenic hydrogen in 1a–1d observed at d 10.00–10.48
is in the usual range of related imidazolium halides (d
9.0–12.0) [4].
˚
Mo Ka radiation (k = 0.71073 A) at 273(2) K. No decay
was observed in 50 duplicate frames at the end of the
data collection. Crystallographic data of 1a, 2b, 2d,
and 3a are listed in Table 1.
2.20. Solution and structure refinements
For a representative example of 1, an X-ray structural
determination on a crystal of 1a, obtained by vapor dif-
fusion of diethylether into an acetonitrile solution, was
performed. The molecular structure and its selected bond
distances and angles are shown in Fig. 1. The structure of
1a includes a water molecule in the asymmetric unit, con-
ceivably coming from the wet re-crystallization solvents
used. Interestingly, 1a adopts a less stable gauche form
instead of its more stable staggered conformation, with
a N(2)–C(4)–C(5)–N(3) dihedral angle of 56.71°. Also,
the 4-fluorobenzyl group is disposed on the same side
of the pyrazolylethyl group across the imidazole ring.
To obtain a rationale for this apparent unstable confor-
mation, the packing of the structure was examined care-
fully. In fact, the solid of 1a Æ H2O forms an intriguing
layer structure. In the crystal lattice, each water molecule
forms O–Hꢀ ꢀ ꢀCl hydrogen bridges with two chloride ani-
Calculations for the structures were performed using
SHELXS-97 and SHELXL-97. Tables of neutral atom scat-
tering factors, f0 and f00, and absorption coefficient are
from a standard source [9]. All atoms except hydrogen
atoms were refined anisotropically. All hydrogen atoms
were located in difference Fourier maps and included
through the use of a riding model.
3. Results
3.1. Synthesis of ligand precursors 1a–1d
The key intermediate 1-(2-chloroethyl)-3,5-dimethyl-
1H-pyrazole was first prepared following a published
procedure of phase-transfer catalytic approach in 71%
yield [10]. The pyrazole/imidazolium chlorides 1a–1d
were then obtained by quaternization reaction of an
appropriate N-substituted imidazole with the pyrazole
derivative in DMF at 120 °C for 1 day (Scheme 2).
Excess amount of N-substituted imidazoles (2–4 times)
˚
˚
ons (d(Hꢀ ꢀ ꢀCl) = 2.556 A, d(Oꢀ ꢀ ꢀCl) = 3.236 A, and
˚
\(OHCl) = 170.16°; d(Hꢀ ꢀ ꢀCl) = 2.506 A, d(Oꢀ ꢀ ꢀCl) =
˚
3.196 A, and \(OHCl) = 151.92°) linking an inversion
related water molecule together into a cyclic network
(Fig. 2). This cyclic network further links up two inver-
sion-related molecules of 1a through an unconventional
Table 1
Crystallographic data of 1a, 2b, 2d, and 3a
1a Æ H2O
17H20ClFN4 Æ H2O
352.84
Colorless prism
Monoclinic
P21/c
2b
2d Æ H2O
3a
Empirical formula
Formula weight
Color and Habit
Crystal system
Space group
C
C19H24AgClN4
451.74
C22H32Ag1.5Cl1.5N8 Æ H2O
641.55
Colorless prism
C17H20Cl3F4N4 Pd
512.12
Orange prism
Monoclinic
Cc
Colorless prism
Monoclinic
P21/c
Triclinic
ꢀ
P1
˚
a (A)
15.192(9)
14.130(9)
8.558(5)
90
7.522(2)
18.098(5)
15.066(5)
90
10.707(3)
10.813(3)
14.121(4)
72.255(10)
89.616(12)
64.814(13)
1394.7(7)
273(2)
19.73(5)
9.76(2)
13.19(3)
90
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
93.94(3)
90
101.94(3)
90
127.20(5)
90
3
˚
V (A )
1832.7(19)
273(2)
4
2006.6(11)
273(2)
2024(8)
273(2)
T (K)
Z
4
1.495
1.146
2
1.528
1.236
4
1.680
1.330
Dcalcd (Mg/m3)
1.279
0.229
l (mmꢁ1
)
Range of transmission factor
Number of unique data
Number of parameters refined
R1[I > 2rI]
0.96–0.47
4310
218
0.83–0.76
4727
226
0.81–0.78
5400
316
0.90–0.87
2851
224
0.0547
0.1822
0.0410
0.1174
0.0865
0.2509
0.0930
0.2558
WR2 (all data)

408
H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
N
R
N
N
Cl
N
N
N
Cl-
N
N
R
1a-1d
PdCl2
Ag2O
Ag2O
R
N
N
N
R
N
N
N
Cl
(
N
N
Ag
N
N
Ag
Ag
Cl
N
N
N
N
Cl (
Cl
0.5
Pd
N
N
Cl
Cl
2a-2c
2d
3a-3d
R =
Me
F
c
d
a
b
Scheme 2.
hydrogen contact between the oxygen and hydrogen
˚
atom upon C(7) (d(Hꢀ ꢀ ꢀO) = 2.395 A, d(Oꢀ ꢀ ꢀC) = 3.174
˚
A, and \(OHC) = 141.2°). This supramolecular hydro-
gen network together with p–p stacking of imidazole
and phenyl rings generate a layer structure in which the
chloride anions and water molecules are concentrated
in the central hydrophilic portions of each layer. The
organic portions with all the fluorine atoms pointing
away from the central portions form the fluorous layers
that sandwich the ionic portions. It is reasonable to sug-
gest that the more energetic gauche conformation in 1a is
stabilized by the formation of such layer packing
structure.
O1
Cl1
C2
C3
N2
C4
C1
N1
C5
C11
N3
3.2. Synthesis of silver complexes 2a–2c
C8
N4
By stirring a dichloromethane solution of the ligand
precursors 1a–1c and Ag2O in a molar ratio of 2:1 over-
night, white solids of 2a–2c were formed after simple
work-up procedure. The compounds are air-stable and
soluble readily in dichloromethane and chloroform.
C7
F1
C6
1
The H NMR spectra of 2a–2c clearly show that the
carbenic protons of 1a–1c were absent, indicating the
successful deprotonation and formation of Ag(NHC)
moiety. It has been shown that reactions of imidazolium
salts with Ag2O can afford silver NHC compounds with
˚
Fig. 1. Molecular structure of 1a Æ H2O. Selected bond distances (A):
N(1)–C(1), 1.327(3); N(2)–C(1), 1.320(3); N(3)–N(4), 1.356(3); Selected
bond angles (°): C(1)–N(1)–C(3), 108.1(2); C(1)–N(2)–C(2), 108.2(2);
N(1)–C(1)–N(2), 109.1(2); C(6)–N(4)–N(3), 104.9(2).

H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
409
F1C
F1B
O1AA
Cl1B
O1A
Cl1
Cl1A
Cl1C
C7D
O1
O1B
F1A
F1
F1D
Fig. 2. A view of the layer structure in 1a along a-axis showing the hydrogen contacts between the water molecules, chloride anions, and organic
cations.
a wide range of stoichiometry, including 1:1 or 2:1 Ag/
NHC adducts [11]. In order to confirm the stoichiometry
of the white products formed, an X-ray structural anal-
ysis on a crystal of 2b was performed. The molecular
structure and its selected bond distances and angles
are shown in Fig. 3. The structure confirms that 2b
was Ag(L)Cl with 1:1 L:Ag(I) stoichiometry. The
Ag(I) center adopts a distorted linear coordination
geometry with a bond angle of 171.71(11)°. The
Ag(1)–C(1) distance of 2.083(4) A and Ag(1)–Cl(1) dis-
˚
˚
tance of 2.436(8) A are within expected values.
Our previous structural studies on several bis(imida-
zolium) salts indicated that different N-substitutions
have little effect on structural dimensions of the imidaz-
ole rings [4,12]. Therefore, even though crystal struc-
tures of 1b and 2a were not obtained, it would be still
of significant interest to compare the structures of 1a
and 2b for the dimensional change of the imidazole ring
upon deprotonation and coordination to the silver(I)
center. The structural change of the imidazole ring is
best visualized by turning to Fig. 4, which compares
the bond lengths and angles in 1a and 2b. It clearly
C3
C2
N2
N1
C4
C1
C5
Ag1
+0.001 Å
N3
C6
C
C
C7
N4
˚
˚
0.5 -1.6
Cl1
+0.001
Å
+0.019
N
Å
C8
˚
2.8
˚
3.0
N
-4.8
˚
+0.028
+0.032
Å
Å
C
Fig. 3. Thermal ellipsoid plot of 2b at the 30% probability level.
˚
Selected bond distances (A): Ag(1)–Cl(1), 2.083(4); Ag(1)–C(1),
2.3375(12); N(1)–C(1), 1.355(5); N(2)–C(1), 1.352(5); N(3)–N(4),
1.364(5); Selected bond angles (°): C(1)–Ag(1)–Cl(1), 171.71(11);
N(1)–C(1)–Ag(1), 128.1(3); N(2)–C(1)–Ag(1), 127.2(3); C(1)–N(1)–
C(3), 110.9(3); C(1)–N(2)–C(2), 111.2(3); N(1)–C(1)–N(2), 104.3(3);
C(8)–N(4)–N(3), 105.1(4).
Ag
Fig. 4. A schematic diagram showing the dimensional change of the
imidazoleringupondeprotonationandcoordinationofAg(I)centerin2b.

410
H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
shows that the two C–N bonds at the carbenic carbon
C(1) were lengthened resulting in a significant decrease
of 4.8° for the N(1)–C(1)–N(2) bond angle.
C14
N6
C13
C22
C17
C16
Remarkably, like 1a, 2b also adopts a gauche confor-
mation such that the pyrazolylethyl group is folded and
disposed above the metal center. The N(1)–C(4)–C(5)–
N(3) dihedral angle is 52.81°. However, examination
of the packing in 2b did not reveal any significant intra-
and intermolecular forces in this case. Because of the
similar bulky N-substitution on the imidazole rings, 2a
and 2c presumably have the same molecular structure.
Satisfactory elemental analyses for 2a–2c were obtained
and their yields were in the range 72–76%.
N5
C15
C12
C20
C19
Cl2
N8
C11
N7
Ag2
C9
C18
C8
C7
Ag1
N4
Cl1
C21
N3
C1
C4
N1
N2
C6
C5
3.3. Synthesis of silver complexes 2d
C10
In remarkable contrast to 2a–2c, reactions of 1d with
Ag2O under identical condition produced instead of 1:1
NHC/Ag complex like 2a–2c, a 2:1 complex 2d as re-
vealed by the X-ray structural determination. The white
solid obtained is also air-stable and dissolve readily in
C2
C3
Fig. 5. Thermal ellipsoid plot of 2d at the 30% probability level. The
fragment Ag(2)–Cl(2) is of 50% occupancy. Selected bond distances
˚
(A): N(1)–C(1), 1.338(12); N(2)–C(1), 1.346(13); N(3)–N(4), 1.373(12);
1
chlorinated solvent. Likewise, in its H NMR spectrum,
N(6)–C(12), 1.333(13); N(5)–C(12), 1.352(12); N(7)–N(8), 1.419(13);
Ag(1)–C(1), 2.088(11); Ag(1)–C(12), 2.092(10); Ag(2)–Cl(1), 2.630(4);
Ag(2)–Cl(2), 2.436(8); Ag(2)–N(4), 2.354(9). Selected bond angles (°):
Ag(1)–C(1)–N(1), 128.5(8); Ag(1)–C(1)–N(2), 127.6(7); C(1)–N(1)–
C(2), 112.3(9); C(1)–N(2)–C(3), 110.4(9); N(1)–C(1)–N(2), 103.9(9);
C(12)–N(6)–C(14), 111.1(9); C(12)–N(5)–C(13), 110.2(9); N(6)–C(12)–
N(5), 105.0(9); Ag(1)–C(12)–N(5), 126.6(8); Ag(1)–C(12)–N(6),
128.5(8); C(9)–N(4)–N(3), 105.4(9); N(3)–N(4)–Ag(2), 119.9(7);
N(4)–Ag(2)–Cl(1), 113.4(3); N(4)–Ag(2)–Cl(2), 106.4(3); Cl(1)–Ag(2)–
Cl(2), 139.8(2).
the absence of downfield resonance at d 10.22 suggested
the successful formation of the Ag(NHC) moiety.
Crystals of 2d were grown by vapor diffusion of dieth-
ylether into an acetonitrile solution. Complex 2d also
incorporates a water molecule its asymmetric unit of
ꢀ
the triclinic space group P1. Surprisingly, one of the pyr-
azole rings was coordinated to a AgCl2 unit, which was
formed from the associated counter chloride anion and
an additional AgCl fragment refined best to have a
50% site of occupancy. This occupancy factor appears
to be reliable as the equivalent isotropic thermal dis-
placement parameters of the two independent silver
atoms in 2d were almost equal. Therefore, the overall
stoichiometry of the reaction between a 2:1 mixture of
1d and Ag2O is the formation of AgL2(AgCl)0.5Cl Æ H2O
and 0.5 equiv of AgCl, which was filtered during the
work-up procedure. This composition of 2d was con-
firmed by the elemental analysis of the bulk sample
and the X-ray structural analysis of a crystal from an
independent preparation. An unusual stoichiometry
was also proposed in a closely related Ag(NHC)2I Æ xA-
gI compound, in which the value x = 0.3 to 0.4 was esti-
mated based on elemental analysis [13]. The
[Ag(NHC)2][AgBr2] reported is another closely related
compound [11].
˚
A). Similar to 2b, upon deprotonation and coordination
to the silver center, a significant reduction of bond an-
gles at the carbenic carbons C(1) (ꢁ5.2°) and C(12)
(ꢁ4.1°) were observed. Most remarkably, the two or-
ganic ancillaries also adopt a gauche conformation with
two dihedral angles of 66.81° (N(6)–C(16)–C(17)–N(8))
and 63.13° (N(2)–C(5)–C(6)–N(3)), respectively. Again,
we examined the possible intermolecular forces that
can stabilize this less stable conformation in the crystal
lattice. As seen in Fig. 6, two molecules of 2d indeed
pack about a center of symmetry making face-to-face
˚
contact with a mean plane separation of 3.407 A into
a supramolecular pair. The non-bonded Agꢀ ꢀ ꢀAg inter-
˚
action is 3.798 A. The Ag centers also interact with
N(2) atoms forming a pair-wise centrosymmetric non-
˚
bonded interactions of 3.739 A. Within each pair, the
As seen in Fig. 5, the less bulky N-methyl group
favors the formation of a coplanar Ag(I) bis(NHC)
complex with a dihedral angle of 2.1° between the imi-
dazole rings. The Ag(1) center adopts a linear coordina-
tion geometry (\ at Ag(1) = 179.5 (4)°) with two Ag–C
3,5-dimethylpyrazolylethyl groups on each imidazole ring
are directed away from the adjacent molecule. The incor-
porated water molecule also plays a significant role in
pairing 2d by forming hydrogen bonds via the hydrogen
˚
˚
atoms on O(1) with N(7) (d(Hꢀ ꢀ ꢀN) = 1.941 A,
bonds (2.088(11) and 2.092(10) A) similar to that in 2b.
\(OHN) = 169.25°), and with Cl(1) (d(Hꢀ ꢀ ꢀCl) = 2.371
The second Ag center adopts a trigonal planar coordina-
tion geometry (\ at Ag(2) = 359.6 °) with two signifi-
cantly unequal Ag–Cl distances (2.630(4) and 2.354(9)
˚
A, \(OHCl) = 154.09°), respectively. Further non-con-
ventional hydrogen contacts of oxygen atoms link up

H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
411
Cl2
N7
Ag2
O1
Ag1
N2
Cl1
Cl1A
Ag1A
N2A
O1A
Ag2A
N7A
Cl2A
Fig. 6. A view of the face-to-face contact between a pair of 2d molecules. Hydrogen atoms are omitted for clarity.
pairs of 2d into a network structure (see supplementary
material). It is obvious that the gauche conformation in
2d further enhances such supramolecular interactions by
folding the dangling organic fragments inwards (see Fig.
5) such that the occupied volume and intermolecular
repulsions among pairs in the crystal lattice are mini-
mized. Notably, the present model for 2d Æ H2O implies
that there is a full AgCl portion scrambled within each
pair.
reduction of charge density on the imidazole rings due
to the zwitterionic nature. For example, the carbenic
proton in 1d observed at d 10.0 (in CDCl3) was upfield
shifted in 3d to d 9.00 in (DMSO-d6).
For a representative example, crystals of 3a suitable
for X-ray structural determination were grown by vapor
diffusion of diethylether in an acetonitrile solution.
Complex 3a was successfully solved in the non-centro-
symmetric space group Cc, despite the moderate quality
of the crystal. Unlike 1a, 2b, and 2d which crystallized
with pairs of enantiomers in the crystal lattices, 3a crys-
tallized with only a single enantiomer in a chiral pseudo-
helical conformation (C1 symmetry). Fig. 7 shows that
the palladium center adopts a square planar coordina-
tion environment with a dihedral angle between the pyr-
azole ring and metal coordination plane being 72.3°.
Surprisingly, the dangling imidazolium group is again
locked in a gauche conformation with a N(2)–C(4)–
C(5)–N(3) dihedral angle of 59.88°. No significant short
intra- or intermolecular interactions were observed in
this case.
3.4. Synthesis of zwitterionic palladium complexes 3a–3d
The silver carbene transfer reaction is a general pro-
tocol for the preparation of palladium NHC compounds
[14]. We anticipated that reactions between the silver
carbene complexes 2a–2d and PdCl2 would afford palla-
dium complexes of chelating pyrazole/NHC ligand.
However, to our surprise, the reactions of 2a–2d with
PdCl2 did not afford the anticipated palladium com-
plexes of pyrazole/NHC ligand. This is rather unex-
pected, as reactions of excess 1a–1d with PdCl2 cleanly
produced the zwitterionic palladium pyrazole complexes
3a–3d, PdIICl3L Æ H in excellent yields. The yellow-or-
ange compounds are stable in the solid state. They are
insoluble in THF and partly soluble in dichloromethane
and DMF. The carbenic proton in L Æ HCl is upfield
shifted upon coordination with PdCl2, reflecting the
3.5. Theoretical computation on 2b and 3a
In order to explore if the gauche conformations in 2b
and 3a are stereo-electronic in nature, we perform theo-
retical computation on these two compounds. The three-

412
H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
tively in 6 h (entries 1–4). To gauge the effectiveness of
3a–3d in comparison with simple phosphine-based cata-
lytic systems of Pd(OAc)2/2PPh3 and Pd(PPh3)4, we car-
ried out the standard catalytic reaction with the latter
two systems under identical conditions (entries 5–6).
The results clearly indicate that 3a–3d and the phos-
phine-based systems are equally effective. In order to dif-
ferentiate their catalytic performance, we lowered the
reaction temperature to 120 °C in entries 7–8. Conse-
quently, 3a became less effective than Pd(OAc)2/2PPh3.
The catalytic performance of 3a–3d is also probed by
using the much less reactive 4-chloroacetophenone as
substrate, entry 9 shows that 3a can produce a 37% of
the coupled product with a prolonged reaction time of
24 h at 173 °C, whereas the simple phosphine systems
are totally ineffective under the same condition (entries
11–12). Increasing the catalyst loading of 3a to 3
mol% does not improve the yield, however (entry 10).
F1
C11
C3
C2
N1
C1
C7
N2
C8
N3
C6
C4
C5
N4
Cl1
4. Discussion
Pd1
A series of novel imidazolium salts (L Æ HCl) for the
potentially bidentate ligand based on pyrazole/N-hetero-
cyclic carbene (L) was synthesized by reacting 1-(2-chlo-
roethyl)-3,5-dimethyl-1H-pyrazole with an appropriate
N-substituted imidazole. The X-ray structural analysis
of the fluorine-containing 1a reveals the formation of
an intriguing layer structure in which the hydrophilic
layer is sandwiched between the fluorous hydrophobic
layers. The hydrogen bonding and p–p stacking play a
crucial role in this overall layer structure.
Cl2
Cl3
Fig. 7. Thermal ellipsoid plot of 3a at the 30% probability level.
˚
Selected bond distances (A): Pd(1)–N(4), 1.97(2); Pd(1)–Cl(1),
2.288(9); Pd(1)–Cl(2), 2.294(8); Pd(1)–Cl(3), 2.301(9); N(1)–C(1),
1.33(3); N(2)–C(1), 1.32(3); N(3)–N(4), 1.41(3); Selected bond angles
(°): C(1)–N(1)–C(3), 106.9(19); C(1)–N(2)–C(2), 109(2); N(1)–C(1)–
N(2), 108(2); C(6)–N(4)–N(3), 102(2); N(4)–Pd(1)–Cl(1), 86.2(7);
N(4)–Pd(1)–Cl(2), 90.2(7); Cl(1)–Pd(1)–Cl(3), 176.3(3); Cl(2)–Pd(1)–
Cl(3), 91.1(4).
Silver(I) NHC complexes are generally accessible by
reacting the basic Ag2O with the imidazolium salts in
the dark [14]. Silver complexes 2a–2c of 1:1 Ag:L stoi-
chiometry were produced by reactions of L Æ HCl with
Ag2O. However, reactions of 1d with Ag2O under iden-
tical synthetic conditions produced a novel solid 2d
which identified to be AgL2(AgCl)0.5Cl Æ H2O by X-ray
structural analysis. Molecules of 2d are aligned in supra-
molecular pairs via face-to-face contacts in the crystal
lattice. A silver center is coordinated to two L via the
NHC functionality with an additional pyrazole-bound
AgCl fragment scramble among the pair. It is obvious
that the bulkiness of the N-substitution on the imidazole
ring has a profound effect on the formation of different
adducts with the smaller N-methyl group favors the for-
mation of a 1:2 Ag:NHC compound, whereas bulky
parameter hybrid of exact exchange and Beckeꢁs
exchange energy [15], and of Lee, Yang, and Parrꢁs non-
local correlation energy [16] (B3LYP) has been applied.
We used the 6-31G(d) basis sets for H, C, N, Cl and the
LANL2DZ effective core potential plus basis functions
for Pd and Ag [17]. The GAUSSIAN03 suite of programs
has been used in these computations [18]. The energetic
difference between gauche/anti conformations in 2b and
3a were estimated to be 1.9 and 0.9 kcal molꢁ1, respec-
tively, with the anti conformation being more stable in
both cases.
3.6. Heck coupling
Even though, chelating palladium complexes A have
not been prepared yet, we were interest to see if zwitter-
ionic Pd(II) complexes 3a–3d are effective catalysts in
Heck reaction. We employ the reaction between 4-brom-
oacetophenone and n-butyl acrylate with sodium acetate
as base at 173 °C as a standard test (Table 2). Our pre-
liminarily investigation shows that catalysts 3a–3d, in
regardless of the N-substitutions, are equally effective
in the production of trans n-butyl cinnamate quantita-
N-substitution leads to
compound.
a
neutral 1:1 Ag:NHC
Preparation of Pd(II) NHC complexes via the silver
carbene transfer reactions is becoming a general proto-
col [14]. It is rather surprising that anticipated reactions
between 2a–2d and PdCl2 failed to produce the chelating
palladium complexes, PdIILCl2. Indeed, reactions of
L Æ HCl with PdCl2 produce readily the zwitterionic

H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
413
Table 2
Catalytic Heck reactionsa
R'
X
cat./NaOAc
+
R'
R
R = COMe
R
R' = n-BuO-CO
trans
Entry
Catalyst
X
Temperature (°C)
Time
Yieldb (%)
1
2
3a
3b
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
173
173
173
173
173
173
120
120
173
173
173
173
6
6
100
100
100
100
100
100
69
3
3c
3d
6
4
6
5
Pd(OAc)2/2PPh3
Pd(PPh3)4
3a
6
6
6
7
6
8
Pd(OAc)2/2PPh3
3a
3a
6
100
37
36c
0
9
24
24
24
24
10
11
12
a
Pd(OAc)2/2PPh3
Pd(PPh3)4
0
Reaction condition: 1 mmol of aryl bromide, 1.4 mmol of n-butyl acrylate, 1.1 mmol of NaOAc, 0.5 mol% of Pd catalyst, 5 mL of DMA, an
average of two runs.
Determined by 1H NMR.
3 mol% of Pd catalyst.
b
c
PdIICl3L Æ H which, in 3a crystallizes into a chiral atrop-
state. In 1a and 2d, extra stabilization of gauche rotam-
ers by intermolecular forces exists. It seems likely that
compounds based on pyrazole/imidazolium chloride
L Æ HCl have a general preference for the less-stable
gauche forms in solid states. This unique gauche prefer-
ence in L Æ HCl may provide potential applicability in
crystal engineering for the construction of novel supra-
molecular molecules for materials and bio-organic
applications.
isomer with pseudo-helical conformation. We sought to
prepare the target compound A by the routine protocol
of trapping the free carbene, generated in situ by depro-
tonation of L Æ HCl, with PdCl2 as well as direct reac-
tions of PdIICl3L Æ H with a suitable base. However,
both of these procedures resulted in decomposition
products only. Notably, a similar silver complex of pyri-
dine/NHC ligand with an ethylene spacer also failed to
produce a chelating palladium complex [13]. These re-
sults suggest that seven-member palladacycles, such as
A, containing NHC and nitrogen donor ligands may
be intriscially unstable in comparison with the seven-
member palladacycles of bis(NHC) B [4].
Most interestingly, in each of the four X-ray struc-
tures of compounds based on L Æ HCl determined, the
ethylene spacer in L is in the less stable gauche confor-
mation. In case of 1a and 2d, close examination of the
crystal packing revealed the involvements of intermo-
lecular hydrogen bonds and face-to-face contacts which
provide the stabilization forces in the crystal lattices.
However, in 2b and 3a, the apparent lack of such sta-
bilizing intermolecular forces makes the gauche confor-
mations very peculiar. In fact, the energy difference in
the gas phase between the staggered and the gauche
conformations in 2b and 3a was estimated by DFT
to be as high as 1.9 and 0.9 kcal molꢁ1, respectively,
with the anti conformation being more stable in both
cases. This theoretical computation suggested that the
gauche rotamers in 2b and 3a are not stereo-electronic
in nature as in 1,2-difluoroethane [5] and the prevailing
gauche rotamers in crystals of 2b and 3a can only be
attributed to the crystal packing effect in the solid
Our preliminarily application of 3a–3d in the Heck
reaction of 4-bromoacetophenone and n-butyl acrylate
shows that the zwitterionic 3a–3d are also active cata-
lysts. In comparison with simple phosphine-based Pd
catalytic systems, 3a–3d are equally efficient at high
temperature. However, at a milder reaction tempera-
ture, 3a is less efficient than Pd(OAc)2/2PPh3. Interest-
ingly, 3a is able to mediate the much less reactive
4-chloroacetophenone as substrate, whereas the phos-
phine-based systems are totally ineffective. This differ-
ence in reactivity can be attributed to the better
thermal stability of 3a supported by L Æ HCl than
Pd(OAc)2/2PPh3 under harsh reaction conditions. It
should be noted that the active catalyst of 3a–3d might
be a monodentate Pd(II) species bearing a pyrazolyl lig-
and or an effective amount of species A produced under
catalytic conditions. Recently, the use of ionic liquid in
organometallic catalysis is receiving much attention
[19]. 3a–3d with the pendent imidazolium groups con-
tain the favorable structural element for their immobi-
lization in ionic liquids of 1,3-dialkylimidazolium salts.
The catalytic application of 3a–3d in C–C coupling
reactions with ionic liquid as solvent is currently under
investigation.

414
H.M. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 403–414
[4] H.M. Lee, C.Y. Lu, C.Y. Chen, W.L. Chen, H.C. Lin, P.L. Chiu,
P.Y. Cheng, Tetrahedron 60 (2004) 5807.
Acknowledgement
[5] (a) N.C. Craig, A. Chen, K.H. Suh, S. Klee, G.C. Mellau, B.P.
Winniewisser, M. Winnewisser, J. Am. Chem. Soc. 119 (1997)
4789;
We are grateful to the National Science Council of
Taiwan for financial support of this work (Grant NSC
93-2113-M-018-004).
(b) R.D. Amos, N.C. Handy, P.G. Jones, A.J. Kirby, J.K.
Parker, J.M. Percy, M.D. Su, J. Chem. Soc., Perkin Trans. 2
(1992) 549;
(c) K.B. Wiberg, Acc. Chem. Res. 29 (1996) 229.
[6] R. Mondal, J.A.K. Howard, R. Banerjee, G.R. Desiraju, Chem.
Commun. (2004) 644.
Appendix A. Supplementary data
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication numbers CCDC
238528-238531. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK [fax: +44(0)-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk]. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2004.09.053.
[7] For a recent review, see: S. Brase, A. de Meijere, in: F. Diederich,
P.J. Stang (Eds.), Metal-catalyzed Cross-coupling Reactions,
Wiley-VCH, Weinheim, 1998.
[8] M.G. Gardiner, W.A. Herrmann, C.-P. Reisinger, J. Schwarz, M.
Spiegler, J. Organomet. Chem. 572 (1999) 239.
[9] L.E. Sutton, Tables of interatomic distances and configurations in
molecules and iochemical society publications, Chemical Society
Publications, UK, 1965.
[10] D. Bogdal, K. Jaskot, Synth. Commun. 30 (2000) 3341.
[11] A.A.D. Tulloch, A.A. Danopoulos, S. Winston, S. Kleinhenz, G.
Eastham, J. Chem. Soc., Dalton Trans. (2000) 4499.
[12] H.M. Lee, P.L. Chiu, Acta Crystallogr., Sect. E 60 (2004) ol384–
ol385.
[13] 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.
[14] (a) H.M. Wang, I.J.B. Lin, Organometallics 17 (1998) 972;
(b) B. Bildstein, M. Malaun, H. Kopacka, K. Wurst, M.
Mitterbo¨ck, K. Ongania, O. Giuliana, P. Zanello, Organometal-
lics 18 (1999) 4325.
References
[1] (a) For a review, see for example: W.A. Herrmann, Angew. Chem.
Int. Ed. 41 (2002) 1290;
(b) D. Bourisou, O. Guerret, F.P. Gabba¨ı, G. Bertrand, Chem.
Rev. 100 (2000) 39;
[15] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[16] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[17] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270.
[18] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.
Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N.
Kudin, J.C. Burant, J.M. Millam, S.S. Lyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.
Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador,
J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels,
M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul,
S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challaco-
mbe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C.
Gonzalez, J.A. Pople, Revision B.05, Gaussian, Inc., Pittsburgh,
PA, 2003.
(c) A.C. Hillier, G.A. Grasa, M.S. Viciu, H.M. Lee, C. Yang,
S.P. Nolan, J. Organomet. Chem. 653 (2002) 69.
[2] (a) L.G. Bonnet, R.E. Douthwaite, Organometallics 22 (2003) 4187;
(b) R.S. Simons, P. Custer, C.A. Tessier, W.J. Youngs, Organo-
metallics 22 (2003) 1979;
(c) A.A. Danopuolos, S. Winston, T. Gelbrich, M.B. Hursthouse,
R.P. Tooze, Chem. Commun. (2002) 482;
(d) A.A.D. Tulloch, A.A. Danopoulos, R.P. Tooze, S.M. Caffer-
key, S. Kleinhenz, M.B. Hursthouse, Chem. Commun. (2000) 1247;
(e) J.C.C. Chen, I.J.B. Lin, Organometallics 19 (2000) 5113;
(f) D.S. McGuiness, K.S. Cavell, Organometallics 19 (2000) 741;
(g) E. Peris, J.A. Loch, J. Mata, R.H. Crabtree, Chem. Commun.
(2001) 201;
(h) P.L. Arnold, A.C. Scarisbrick, A.J. Blake, C. Wilson, Chem.
Commun. (2001) 2340.
[3] (a) A. Furstner, H. Krause, L. Ackermann, C.W. Lehmann,
¨
Chem. Commun. (2001) 2240;
(b) W.A. Herrmann, C. Kocher, L.J. Goossen, G.R.J. Artus,
Chem. Eur. J. 2 (1996) 1627;
(c) 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;
[19] J. Dupont, R.F. de Souza, P.A.Z. Suarez, Chem. Rev. 102 (2002)
3667.
(d) M.F. Lappert, J. Organomet. Chem. 451 (1993) 389.