Non-symmetrically substituted di-N-heterocyclic carbene palladium(II) complexes: Synthesis, structure and catalytic activity

Article abstract of DOI:10.1016/j.jorganchem.2008.07.040

The synthesis and characterization of non-symmetric di-N-heterocyclic carbene (diNHC) silver(I) and palladium(II) complexes are described. The activity of the Pd(II) complexes in the Suzuki-Miyaura coupling reaction of bulky substrates was investigated. This synthetic route represents a possible general pathway into a wide variety of non-symmetrically substituted diNHC ligands.

Full text of DOI:10.1016/j.jorganchem.2008.07.040

Journal of Organometallic Chemistry 693 (2008) 3405–3410
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Non-symmetrically substituted di-N-heterocyclic carbene palladium(II)
complexes: Synthesis, structure and catalytic activity
Tressia A.P. Paulose a, Jeremy A. Olson a, J. Wilson Quail b, Stephen R. Foley a,
*
a Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, Canada S7N 5C9
b Saskatchewan Structural Sciences Centre, 110 Science Place, Saskatoon, Saskatchewan, Canada S7N 5C9
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2008
Received in revised form 30 July 2008
Accepted 30 July 2008
Available online 6 August 2008
The synthesis and characterization of non-symmetric di-N-heterocyclic carbene (diNHC) silver(I) and pal-
ladium(II) complexes are described. The activity of the Pd(II) complexes in the Suzuki–Miyaura coupling
reaction of bulky substrates was investigated. This synthetic route represents a possible general pathway
into a wide variety of non-symmetrically substituted diNHC ligands.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Palladium
Catalysis
Suzuki–Miyaura reaction
N-Heterocyclic carbene
1. Introduction
2. Results and discussion
N-Heterocyclic carbenes (NHCs) have received considerable
attention as ligands in transition metal coordination chemistry
which has resulted in the synthesis of a wide variety of stable me-
tal–carbene complexes with numerous applications as catalysts for
organic transformations [1]. Chelating di-N-heterocyclic carbene
ligands (diNHCs) in particular are expected to provide increased
entropic stability to a metal center and consequently many reports
have emerged in recent years concerning cis-chelating bidentate
alkane-bridged diNHCs [2].
While the preparation of diverse symmetrically substituted
diNHC ligands have been reported, there are no examples of
non-symmetrically substituted diNHC ligands in the literature
where R ¼ R0 (Fig. 1) [3]. There is, however, one example of a
non-symmetric diNHC ligand employing variations of Crabtree’s
abnormal NHC ligand for applications in iridium chemistry [4].
Here, we report the synthesis of non-symmetrically substituted
ethylene-bridged diimidazolium salts and their corresponding
diNHC palladium(II) complexes. Given the effectiveness of re-
ported (diNHC)Pd(II) complexes in C–C coupling reactions [1–3],
preliminary investigations into the activity of these complexes
in the Suzuki–Miyaura coupling reaction of bulky substrates were
performed.
2.1. Synthesis of ligands
The key step in the synthesis of non-symmetric imidazolium
salts 3a,b is isolation of the bromoethylimidazolium bromide pre-
cursor 2a,b which is easily synthesized from an N-substituted
imidazole and excess 1,2-dibromoethane (Scheme 1). The synthe-
sis of 2a has been previously reported [5]. The second step simply
involves reaction of 2a,b with another equivalent of a N-substi-
tuted imidazole in toluene resulting in the non-symmetrically
substituted diimidazolium salts 3a,b in 90% yield. Preliminary
results suggest this is a general route to non-symmetric diNHCs
provided the bromoethylimidazolium bromide precursors can be
isolated. The 1H NMR spectra of 3 in DMSO-d6 exhibit two distinct
resonances at 9.32 and 9.17 ppm for 3a and at 9.25 and 9.12 ppm
for 3b for the two NCHN protons on the two ethylene-bridged
imidazolium rings. There are also four distinct resonances for the
protons on the remaining four carbons of the two imidazolium
rings. Single crystals of 3a were obtained by slow diffusion of ether
into a concentrated solution in DMF. The crystal structure of 3a
shows an anti conformation of the imidazolium units about
the C2 bridge with the imidazolium units oriented in opposite
directions (Fig. 2 and Table 1).
2.2. Synthesis of (diNHC)Ag(I) and (diNHC)Pd(II) complexes
Complexes 5a,b were synthesized via a transmetallation route
using (diNHC)Ag(I) complexes (Scheme 1). The advantage of using
* Corresponding author. Tel.: +1 306 966 2960; fax: +1 306 966 4730.
E-mail address: stephen.foley@usask.ca (S.R. Foley).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.07.040

3406
T.A.P. Paulose et al. / Journal of Organometallic Chemistry 693 (2008) 3405–3410
non-symmetric carbenic (NCN) carbons. Elemental analysis is con-
sistent with formation of (diNHC)Ag2Br2.
n N
N
N
Subsequent reaction of the silver complexes 4a,b with one
equiv. PdCl2 in DMF at 90 °C for 16 h afforded the desired com-
plexes 5a,b in 50–60% yield. The 13C NMR spectra of 5 show the
appearance of two resonances at 156 and 154 ppm for 5a and at
155 and 154 ppm for 5b which is again characteristic of the two
non-symmetric carbenic carbons. The ethylene bridge and methy-
lene protons of the benzyl moiety exhibit broad resonances in the
1H NMR spectra which is likely due to the inherent fluxionality of
the seven-membered palladacycle. This phenomenon has been
previously observed in reported (diNHC)Pd(II) complexes [3b].
Complexes 5a,b are air and moisture stable and can be crystallized
by slow diffusion of ether into a concentrated solution of the
sample in DMF. The solid state structures of complexes 5a,b were
determined by single-crystal X-ray diffraction, which showed the
palladium in a distorted square planar geometry (Figs. 3 and 4
and Table 1). Complex 5b co-crystallized with one molecule of
DMF. In both structures the palladacycles adopt boat-like confor-
N
R = R'
Pd
R
R'
Cl
Cl
Fig. 1. Non-symmetrically substituted (diNHC)PdCl2.
NR
+
+
N
NR'
NR
-
+
N
N
Br
-
N
Br
Br
Br
RN
N
-
Br
ii
i
1a,b
2a,b
Br
NR'
3a,b
a: R = Me, R' = Bz
b: R =
-tBu(C6H4)CH2, R' = Me
p
Ag2O
iii
N R
Cl
N
N
PdCl2
iv
Pd
(diNHC)Ag2Br2
Cl
N R'
4a,b
5a,b
Scheme 1. Synthesis of non-symmetric (diNHC)PdCl2 complexes. Conditions: (i)
neat, 2 d, 80 °C; (ii) toluene, 3 d, 80–150 °C; (iii) DCM, 2 h, rt and (iv) DMF, 24 h,
90 °C.
Table 1
Crystal data and refinement parameters for compounds 3a, 5a and 5b-DMF
3a
5a
5b-DMF
Formula
Formula weight
Color
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b (°)
C
16H20Br2N4
C16H18Cl2N4Pd
443.64
Pale yellow
Monoclinic
P21/c
8.3715(2)
17.8428(4)
14.3790(4)
124.492(2)
4
C23H33Cl2N5OPd
572.84
Colorless
Monoclinic
P21/c
13.6740(9)
14.0256(9)
13.3806(11)
94.680(4)
4
428.18
Colorless
Monoclinic
P21/c
27.1963(15)
5.1619(3)
12.5926(5)
91.922(3)
4
Fig. 2. ORTEP plot of non-symmetrically substituted diimidazolium salt 3a at the 50%
probability level. The hydrogen atoms and bromide anions are omitted for clarity.
Selected bond angles (°): N(2)–C(10)–N(1) = 108.4(12), N(4)–C(15)–N(3) =
108.5(16).
Z
qcalc (mg mꢀ3
)
1.610
1.665
1.488
Temperature, K
173(2)
173(2)
173(2)
F(000)
856
888
1176
h Range (°)
Reflections
collected/unique
Rint
3.00–23.25
12024/2398
2.69–27.46
25351/4040
2.99–25.35
19094/4659
0.1221
0.0772
0.1088
Final R1 (I > 2
r
I)
R1 = 0.0943,
wR2 = 0.1814
R1 = 0.1219,
wR2 = 0.1955
R1 = 0.0552,
wR2 = 0.1412
R1 = 0.0727,
wR2 = 0.1550
R1 = 0.0600,
wR2 = 0.1333
R1 = 0.0848,
wR2 = 0.1473
R1 (all data)
(diNHC)Ag(I) complexes as carbene transfer reagents include sim-
ple preparation directly from the imidazolium salt and ease of han-
dling due to their stability in air [6]. (DiNHC)Ag(I) complexes 4a,b
were synthesized by the reaction of the diimidazolium salt precur-
sors 3a,b with an equivalent amount of Ag2O according to estab-
lished procedures [3c]. The 1H NMR spectra of 4a,b (DMSO-d6)
show the absence of any signal in the 8–10 ppm region indicating
the successful deprotonation of the carbenic protons in 3a,b. The
bridging ethylene protons in 4a,b now appear as two multiplets
each integrating for two protons compared to the singlet obtained
for 3a,b. The 13C NMR spectra of 4a,b show the appearance of two
resonances at 181 and 180 ppm which is characteristic of the two
Fig. 3. ORTEP plot of 5a at the 50% probability level. Hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (°): Pd(1)–C(15) = 1.967(5),
Pd(1)–C(10) = 1.982(6), C(15)ꢀPd(1)–C(10) = 86.3(2), N(1)–C(10)–N(2) = 104.9(5),
N(4)–C(15)–N(3) = 106.0(5).

T.A.P. Paulose et al. / Journal of Organometallic Chemistry 693 (2008) 3405–3410
3407
Table 3
Suzuki–Miyaura cross-coupling of bulky aryl halides and aryl boronic acids by
complex 5aa
Entry
1
ArBr
Ar0B(OH)2
Product
Conversion (%)b
92
(HO)2B
Br
Br
Ph
Ph
2
3
66
0
(HO)2B
(HO)2B
No reaction
Br
Br
4
5
80
(HO)2B
(HO)2B
13
17
Br
Ph
Fig. 4. ORTEP plot of 5b-DMF at the 50% probability level. Hydrogen atoms and the
solvent molecule have been omitted for clarity. Selected bond lengths (Å) and
angles (°): Pd(1)–C(15) = 1.953(6), Pd(1)–C(10) = 1.985(6), C(15)–Pd(1)–C(10) =
85.9(2), N(2)–C(10)–N(1) = 104.5(5), N(3)–C(15)–N(4) = 104.8(5).
a
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol),
base (2.0 mmol), precatalyst 5a, dioxane (5 mL), 80 °C, 24 h.
b
Determined by 1H NMR based on internal standard dodecahydrotriphenylene.
mations where the benzyl moiety of the chelating diNHC ligand is
oriented towards the axial face of the palladium center.
sults are in contrast to trans-(NHC)2PdCl2 complexes which show
high activity for deactivated aryl chlorides under similar conditions
[9]. Current state of the art for Suzuki–Miyaura reactions employ-
ing NHC complexes of palladium is 0.1 mol% catalyst loading at
ambient temperature for deactivated aryl chlorides [10].
Given the recent interest in catalysts that can couple sterically
hindered substrates [8], the activity of precatalyst 5a for the cou-
pling of bulky substrates was investigated (Table 3). High to mod-
erate yields were obtained for mono and di ortho-substituted
biaryls provided the steric bulk was incorporated on only one of
the coupling partners. No coupled product was obtained when
both partners were sterically hindered. Attempts to couple 2,4,6-
trimethylphenylboronic acid yielded mesitylene as the major
product.
2.3. Catalytic activity
The catalytic activity of complex 5a was investigated in the Su-
zuki–Miyaura coupling reaction of bulky and non-bulky substrates.
In recent years, several powerful systems have been developed for
the activation of not only para-substituted aryl chlorides [7] but
hindered substrates such as ortho-substituted aryl halides as
shown by Buchwald’s group [8]. In our case, excellent yields were
obtained in the standard coupling reaction of para-substituted aryl
bromides with phenyl boronic acid when both activated and deac-
tivated aryl bromides were employed even at very low catalyst
loading (Table 2). However, activity significantly decreased when
aryl chlorides were employed with no coupling product observed
in the presence of deactivated aryl chlorides (entry 5). These re-
3. Conclusion
In conclusion, we have synthesized stable non-symmetrically
substituted diNHC palladium(II) complexes and have investigated
their catalytic activity in the Suzuki–Miyaura coupling reaction of
bulky substrates. This synthetic route represents a possible general
pathway into a wide variety of non-symmetrically substituted diN-
HC ligands. These examples of non-symmetrically substituted che-
lating diNHC ligands can serve in designing NHC ligands with fine-
tuned steric and electronic environments around the metal centre.
Table 2
Suzuki–Miyaura cross-coupling of aryl halides by complex 5aa
X
B(OH)2
5a
+
Cs2CO3, dioxane
80oC
R
R
Entry
R
X
Time (h)
Catalyst (mol%)
Yield (%)b
TONc
4. Experimental
1
2
3
4
5
COMe
OMe
COMe
H
Br
Br
Cl
Cl
Cl
1
6
24
24
24
0.01
0.1
1
1
1
96
95
37
20
0
9600
950
37
20
0
4.1. General
Unless otherwise stated, all reactions were performed under N2
using standard Schlenk techniques or in a N2-filled drybox. Dichlo-
romethane and THF were dried using a MBraun Solvent Purifica-
tion System and were stored in
temperatures for catalytic reactions refer to the temperature of
OMe
a
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol),
base (2.0 mmol), precatalyst 5a, dioxane (5 mL), 80 °C.
b
Determined by 1H NMR based on internal standard dodecahydrotriphenylene.
TON = moles of product per mol of catalyst.
a glovebox. All reaction
c

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T.A.P. Paulose et al. / Journal of Organometallic Chemistry 693 (2008) 3405–3410
pre-equilibrated sand baths. 1H and 13C {1H} NMR spectra were
recorded on a Bruker 500 MHz Avance spectrometer. Chemical
shifts for 1H and 13C NMR are reported in ppm in reference to
the residual 1H and 13C resonances of CDCl3 (1H: d 7.24; 13C: d
77.23) and DMSO-d6 (1H: d 2.50; 13C: d 39.51). Coupling constants
are given in Hz. Elemental analyses were performed on a Perkin–
Elmer 2400 CHN elemental analyzer. 1,2-dibromoethane and
Ag2O were purchased from Alfa-Aesar Chemical Company and
used as received. Imidazole, N-methylimidazole, benzyl bromide,
tert-butylbenzyl bromide, anhydrous DMF, anhydrous dioxane
and NaH were purchased from Sigma–Aldrich Chemical Company
and used as received. PdCl2 was obtained from PMO Pty Ltd., Aus-
tralia. N-benzylimidazole [3b] and 1-(2-bromoethyl)-3-methylimi-
dazolium bromide (2a) [5] were synthesized according to
literature procedures.
136.72 (NCHN), 134.57 (CAr(ipso)), 128.94 (CHAr), 128.73 (CHAr),
128.27 (CHAr), 123.82 (CHimid), 122.88 (2C, CHimid), 122.27 (CHimid),
51.98 (ArCH2N), 48.55 (NCH2CH2N), 48.19 (NCH2CH2N), 35.98
(CH3). ESI-MS (m/z): Calc. for C16H20Br2N4 [MꢀBr]+: 347. Found: 347.
4.5. Synthesis of 1-(4-tert-butylbenzyl)-10-methyl-3,30-
ethylenediimidazolium dibromide (3b)
A Schlenk flask was charged with a mixture of 2b (0.85 g,
2.11 mmol), N-methylimidazole (0.179 mL, 2.26 mmol) and tolu-
ene (20 mL) and was stirred at 150 °C for 3 d under nitrogen atmo-
sphere. A white precipitate was isolated, washed with toluene
(3 ꢁ 15 mL) and dried under vacuum to yield 3b as a white pow-
der. (0.96 g, 94%). 1H NMR (DMSO-d6): d 9.25 (s, 1H, NCHN), 9.12
(s, 1H, NCHN), 7.85 (s, 1H, CHimid), 7.73 (s, 2H, CHimid), 7.63 (s,
1H, CHimid), 7.45 (d, J = 8.2, 2H, CHAr), 7.32 (d, J = 8.2, 2H, CHAr),
5.38 (s, 2H, ArCH2N), 4.71 (s, 4H, NCH2CH2N), 3.84 (s, 3H, CH3),
1.27 (s, 9H, C(CH3)3). 13C NMR (DMSO-d6): d 151.32 (CAr(para)),
137.11 (NCHN), 136.63 (NCHN), 131.66 (CAr(ipso)), 128.10 (CHAr),
125.74 (CHAr), 123.89 (CHimid), 122.93 (CHimid), 122.87 (CHimid),
122.32 (CHimid), 51.73 (ArCH2N), 48.60 (NCH2CH2N), 48.24
(NCH2CH2N), 35.99 (CH3), 34.37 (C(CH3)3), 31.02 (C(CH3)3). ESI-
MS (m/z): Calc. for C20H28Br2N4 [MꢀBr]+: 403. Found: 403.
4.2. Synthesis of 1-(4-tert-butylbenzyl)imidazole (1b)
A Schlenk flask was charged with imidazole (1.85 g, 0.0272 mol)
and NaH (2.57 g, 0.107 mol) in THF (50 mL) under dinitrogen at
ambient temperature. After 5 min, 4-(tert-butyl)benzyl bromide
(5.0 mL, 0.0272 mol) was added. The mixture was stirred for 12 h
following which the solvent was removed under vacuum. Water
(20 mL) and dichloromethane (30 mL) were added to the reaction
products and the organic layer was separated. The organic layer
was further extracted with water (2 ꢁ 20 mL) and the solvent
was removed under vacuum to yield 1b as a pale yellow solid
which can be used without further purification. (4.93 g, 85%). 1H
NMR (CDCl3): d 7.52 (s, 1H, NCHN), 7.35 (d, J = 8.1, 2H, CHAr),
7.07 (d, J = 8.1, 2H, CHAr), 7.06 (s, 1H, CHimid), 6.89 (s, 1H, CHimid),
5.06 (s, 2H, ArCH2N), 1.29 (s, 9H, CH3). 13C NMR (CDCl3): d
151.59 (CAr(para)), 137.66 (NCHN), 133.44 (CAr(ipso)), 130.00 (CHimid),
127.26 (CHAr), 126.11 (CHAr), 119.50 (CHimid), 50.71 (ArCH2N),
34.82 (C(CH3)3), 31.51 (C(CH3)3). EI-MS (m/z): Calc. for C14H18N2
[M]+: 214. Found: 214.
4.6. Synthesis of dibromo(1-benzyl-10-methyl-3,30-
ethylenediimidazolin-2,20-diylidene)disilver(I) (4a)
A suspension of 3a (1.0 g, 2.34 mmol) and one equiv of Ag2O
(0.541 g, 2.34 mmol) in dichloromethane (25 mL) was stirred at
room temperature for 2 h under dinitrogen in the absence of light.
The color of the suspension gradually changed from black to white.
The precipitate was then isolated, washed with dichloromethane
(3 ꢁ 10 mL) and dried under vacuum to yield 4a as a white powder
(1.42 g, 95%). 1H NMR (DMSO-d6): d 7.49 (s, 1H, CHimid), 7.45 (s, 1H,
CHimid), 7.35 (t, J = 7.3, 2H, CHAr(meta)), 7.29 (t, J = 7.3, 1H, CHAr(para)),
7.27 (s, 1H, CHimid), 7.16 (s, 1H, CHimid), 7.09 (d, J = 7.3, 2H,
CHAr(ortho)), 5.25 (s, 2H, ArCH2N), 4.67-4.61 (m, 2H, NCH2CH2N),
4.58–4.51 (m, 2H, NCH2CH2N), 3.56 (s, 3H, CH3). 13C NMR
4.3. Synthesis of 1-(4-tert-butylbenzyl)-3-(2-bromoethyl)imidazolium
bromide (2b)
(DMSO-d6):
d 180.50 (NCN), 179.78 (NCN), 137.32 (CAr(ipso)),
A Schlenk flask was charged with 1b (0.213 g, 0.994 mmol) and
1,2-dibromoethane (10 mL) under dinitrogen. After stirring for 2 d
at 80 °C, the resulting white precipitate was isolated and washed
with diethyl ether (3 ꢁ 10 mL) and dried under vacuum to yield
2b as a white powder. (0.236 g, 59%). 1H NMR (DMSO-d6): d 9.39
(s, 1H, NCHN), 7.87 (s, 1H, CHimid), 7.76 (s, 1H, CHimid), 7.44 (d,
J = 8.3, 2H, CHAr), 7.33 (d, J = 8.3, 2H, CHAr), 5.43 (s, 2H, ArCH2N),
4.63 (t, J = 5.8, 2H, BrCH2CH2N), 3.96 (t, J = 5.8, 2H, BrCH2CH2N),
1.26 (s, 9H, C(CH3)3). 13C NMR (DMSO-d6): d 151.29 (CAr(para)),
136.61 (NCHN), 131.81 (CAr(ipso)), 127.95 (CHAr), 125.75 (CHAr),
122.79 (CHimid), 122.72 (CHimid), 54.89 (ArCH2N), 51.68
(BrCH2CH2N), 50.20 (BrCH2CH2N), 34.34 (C(CH3)3), 30.99
(C(CH3)3). ESI-MS (m/z): Calc. for C16H22Br2N2 [MꢀBr]+: 321.
Found: 321.
128.67 (CHAr), 127.81 (CHAr(para)), 127.12 (CHAr), 122.88 (CHimid),
122.74 (CHimid), 122.57 (CHimid), 121.93 (CHimid), 53.93 (ArCH2N),
51.47 (NCH2CH2N), 50.64 (NCH2CH2N), 38.25 (CH3). Anal. Calc.
for C16H18Br2N4Ag2: C, 29.94; H, 2.83; N, 8.73. Found: C, 29.78;
H, 2.53; N, 8.59%.
4.7. Synthesis of dibromo[1-(4-tert-butylbenzyl)-10-methyl-3,30-
ethylenediimidazolin-2,20-diylidene]disilver(I) (4b)
A suspension of 3b (0.403 g, 0.832 mmol) and one equiv of Ag2O
(0.193 g, 0.832 mmol) in dichloromethane (15 mL) was stirred at
room temperature for 4 h under dinitrogen in the absence of light.
The color of the suspension gradually changed from black to white.
The precipitate was isolated, washed with dichloromethane
(3 ꢁ 10 mL) and dried under vacuum to yield 4b as a white powder
(0.41 g, 70%). 1H NMR (DMSO-d6): d 7.50 (s, 1H, CHimid), 7.47 (s, 1H,
CHimid), 7.34 (d, J = 8.1, 2H, CHAr), 7.32 (s, 1H, CHimid), 7.16 (s, 1H,
CHimid), 6.96 (d, J = 7.4, 2H, CHAr), 5.21 (s, 2H, ArCH2N), 4.71–4.61
(m, 2H, NCH2CH2N), 4.58–4.50 (m, 2H, NCH2CH2N), 3.44 (s, 3H,
CH3), 1.23 (s, 9H, C(CH3)3). 13C NMR (DMSO-d6): d 180.58 (NCN),
179.83 (NCN), 150.22 (CAr(para)), 134.44 (CAr(ipso)), 126.75 (CHAr),
125.36 (CHAr), 122.98 (CHimid), 122.65 (2C, CHimid), 121.69 (CHimid),
53.55 (ArCH2N), 51.50 (NCH2CH2N), 50.50 (NCH2CH2N), 38.14
(CH3), 34.22 (C(CH3)3), 31.04 (C(CH3)3). Anal. Calc. for
4.4. Synthesis of 1-benzyl-10-methyl-3,30-ethylenediimidazolium
dibromide (3a)
A Schlenk flask was charged with a mixture of 2a (1.0 g,
3.70 mmol), N-benzylimidazole (0.586 g, 3.70 mmol) and toluene
(20 mL) and was stirred at 80 °C for 3 d under dinitrogen. A white
precipitate was isolated, washed with THF (3 ꢁ 15 mL) and dried
under vacuum to yield 3a as a white powder. (1.43 g, 90%). 1H
NMR (DMSO-d6): d 9.32 (s, 1H, NCHN), 9.17 (s, 1H, NCHN), 7.86 (s,
1H, CHimid), 7.77 (s, 1H, CHimid), 7.72 (s, 1H, CHimid), 7.64 (s, 1H, CHi-
mid), 7.48–7.36 (m, 5H, CHAr), 5.45 (s, 2H, ArCH2N), 4.74 (s, 4H,
NCH2CH2N), 3.84 (s, 3H, CH3). 13C NMR (DMSO-d6): d 137.11 (NCHN),
C20H26Br2N4Ag2: C, 34.41; H, 3.75; N, 8.03. Found: C, 34.51; H,
3.42; N, 7.84%.

T.A.P. Paulose et al. / Journal of Organometallic Chemistry 693 (2008) 3405–3410
3409
4.8. Synthesis of dichloro(1-benzyl-10-methyl-3,30-
reductions used the programs DENZO and SCALEPACK [12]. SIR97
[13] was used to solve the structures and SHELXL97 [14] was used
to refine the structures. ORTEP-3 for Windows [15] was used for
molecular graphics and PLATON [16] was used to prepare material
for publication. H atoms were placed in calculated positions with
Uiso constrained to be 1.5 times Ueq of the carrier atom for methyl
protons and 1.2 times Ueq of the carrier atom for all other hydrogen
atoms.
ethylenediimidazolin-2,20-diylidene)palladium(II) (5a)
A suspension of 4a (0.4 g, 0.62 mmol) and PdCl2 (110 mg,
0.62 mmol) in DMF (10 mL) was stirred at 90 °C for 24 h under
dinitrogen. The color of the suspension gradually changed from
white to grey. The solution was then filtered to remove the AgBr
precipitate. The filtrate was concentrated under vacuum. Upon
addition of diethyl ether a white precipitate was obtained which
was filtered and dried under vacuum to yield 5a as a white powder.
Crystals suitable for X-ray diffraction were grown by slow diffusion
of ether into a concentrated solution of the sample in DMF (0.14 g,
52%). 1H NMR (DMSO-d6): d 7.45 (br s, 2H, CHimid), 7.36 (br s, 1H,
Acknowledgements
We gratefully thank the Natural Sciences and Engineering Re-
search Council of Canada (NSERC) for financial support.
CHimid), 7.32 (br s, 3H, CHAr(meta
para)), 7.26 (br s, 1H, CHimid),
and
7.10 (br s, 2H, CHAr(ortho)), 6.08 (m, 1H, ArCHaHbN)), 5.41–5.22 (m,
2H, NCH2CH2N), 5.16 (m, 1H, ArCHaHbN), 4.69–4.40 (m, 2H,
NCH2CH2N), 3.37 (s, 3H, CH3). 13C NMR (DMSO-d6): d 156.68
(NCN), 154.87 (NCN), 137.52 (CAr(ipso)), 128.62 (CHAr), 127.67
Appendix A. Supplementary material
CCDC 690083, 687292 and 687293 contain the supplementary
crystallographic data for 3a, 5a and 5b-DMF. The data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336 033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2008.07.040.
(CHAr(para), 127.06 (CH ), 123.49 (CHimid), 123.27 (CHimid), 122.79
Ar
(CHimid), 122.64 (CHimid), 52.71 (ArCH2N), 46.94 (NCH2CH2N),
46.94 (NCH2CH2N), 37.26 (CH3). ESI-MS (m/z):
C
Calc. for
16H18Cl2N4Pd2 [MꢀCl]+: 407. Found: 407.
4.9. Synthesis of dichloro[1-(4-tert-butylbenzyl)-10-methyl-3,30-
ethylenediimidazolin-2,20-diylidene]palladium(II) (5b)
References
A suspension of 4b (0.203 g, 0.291 mmol) and PdCl2 (50 mg,
0.282 mmol) in DMF (10 mL) was stirred at 90 °C for 24 h under
dinitrogen. The color of the suspension gradually changed from
white to grey. The solution was then filtered to remove the AgBr
formed. The filtrate was concentrated under vacuum. Upon addi-
tion of diethyl ether a white precipitate was obtained which was
filtered and dried under vacuum to yield 5b as a white powder.
Crystals suitable for X-ray diffraction were grown by slow diffusion
of ether into a concentrated solution of the sample in DMF (84 mg,
60%). 1H NMR (DMSO-d6): d 7.52–7.40 (br s, 2H, CHAr), 7.39–7.30
(br s, 3H, CHimid), 7.26 (br s, 1H, CHimid), 7.13–6.98 (br s, 2H, CHAr),
6.18–5.90 (m, 1H, ArCHaHbN), 5.32–5.11 (m, 3H, ArCHaHbN and
NCH2CH2N), 4.68–4.35 (m, 2H, NCH2CH2N), 2.50 (s, 3H, CH3), 1.27
(s, 9H, C(CH3)3). 13C NMR (DMSO-d6): d 155.05 (NCN), 154.02
(NCN), 150.12 (CAr(para)), 134.59 (CAr(ipso)), 126.91 (CHAr), 125.34
(CHAr), 123.46 (CHimid), 123.32 (CHimid), 122.65 (CHimid), 122.50
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37.15 (CH3), 34.23 (C(CH3)3), 31.07 (C(CH3)3). ESI-MS (m/z): Calc.
for C20H26Cl2N4Pd2 [MꢀCl]+: 463. Found: 463.
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