Silver N-heterocyclic carbene complexes as initiators for bulk ring-opening polymerization (ROP) of l-lactides

Article abstract of DOI:10.1016/j.jorganchem.2006.12.022

Synthetic, structural and catalysis studies of two silver complexes namely, {[1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imidazol-2-ylidene]₂Ag}⁺Cl^- 1b, supported over an amido-functionalized N-heterocyclic carbene lig

Full text of DOI:10.1016/j.jorganchem.2006.12.022

Journal of Organometallic Chemistry 692 (2007) 1672–1682
www.elsevier.com/locate/jorganchem
Silver N-heterocyclic carbene complexes as initiators for bulk
ring-opening polymerization (ROP) of L-lactides
a
b
c
b
Manoja K. Samantaray , Vimal Katiyar , Keliang Pang , Hemant Nanavati ,
a,
*
Prasenjit Ghosh
a
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
b
c
Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027, USA
Received 7 May 2006; received in revised form 4 December 2006; accepted 19 December 2006
Available online 23 December 2006
Abstract
Synthetic, structural and catalysis studies of two silver complexes namely, {[1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imi-
dazol-2-ylidene]2Ag}+Clꢀ 1b, supported over an amido-functionalized N-heterocyclic carbene ligand, and [1-(i-propyl)-3-(benzyl)imi-
dazol-2-ylidene]AgCl 2b, supported over a non-functionalized N-heterocyclic carbene ligand, are reported. Specifically, 1b, a cationic
complex bearing 2:1 NHC ligand to metal ratio, was obtained from the reaction of 1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imi-
dazolium chloride 1a with Ag2O in 52% yield. The corresponding 1a was synthesized by the alkylation reaction of 1-(2,4,6-trimethylph-
enylimidazole) with N-phenyl chloroacetamide in 73% yield. The other silver complex 2b, a neutral complex bearing 1:1 NHC ligand to
metal ratio, was obtained from the reaction of 1-(i-propyl)-3-(benzyl)imidazolium chloride 2a with Ag2O in 42% yield. The 2a was syn-
thesized by the alkylation reaction of 1-(i-propylimidazole) with benzyl chloride in 45% yield. The molecular structures of the imidazo-
lium chloride, 1a, and the silver complexes, 1b and 2b, have been determined by X-ray diffraction studies. The silver complexes, 1b and
2b, successfully catalyze bulk ring-opening polymerization (ROP) of L-lactides at elevated temperatures under solvent-free melt condi-
tions producing moderate to low molecular weight polylactide polymers having narrow molecular weight distributions.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Carbenes; Functionalized NHC; Organometallic complex; Silver–NHC complex
1. Introduction
plexes have also been reported to exhibit catalytic activities
for several chemical transformations like, ethyl diazoace-
tate (EDA) assisted carbene transfer reactions [5], catalytic
preparation of 1,2-bis(boronate) esters, [6] in trans-esterifi-
cation reactions and in ring-opening lactide polymerization
reactions [7]. Though originally known for their transmet-
allation property, the new found applications in homoge-
neous catalysis as well as in medicine are redrawing the
role of these silver–NHC complexes and, thus, are opening
up new frontiers of research.
Another notable feature of the silver–NHC complexes is
its structural diversity in the solid state [1,2]. Depending
upon the type of the ligands used and the reaction condi-
tions employed, the silver–NHC complexes display a vari-
ety of motifs that range from monomeric to oligomeric to
There has been a recent surge of interest in silver N-het-
erocyclic carbene chemistry with the emergence of diverse
range of applications of these complexes that span from
biomedical applications to homogeneous catalysis [1,2].
For example, Youngs and coworkers have recently
reported antimicrobial activities of water soluble silver–
NHC complexes [3] that showed improved activity upon
encapsulation in polymer mats due to greater bio-availabil-
ity of active silver species in the polymer matrix [4]. Apart
from the biomedical applications, the silver–NHC com-
*
Corresponding author. Fax: +91 22 2572 3480.
E-mail address: pghosh@chem.iitb.ac.in (P. Ghosh).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.12.022

M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
1673
polymeric structures [8–11]. The aggregated structures are
-
+
Cl
particularly important with regard to designing supramo-
lecular architectures and in materials related applications.
Though, the reasons determining the structural intricacies
of these complexes are yet to be fully understood, the obvi-
ous factors like sterics, electronics and reactions conditions
play a significant role. For example, a NHC ligand with a
sterically demanding mesityl substituent gave a neutral
(NHC)AgCl type complex having 1:1 ligand to metal ratio,
whereas the less bulky variant of the same ligand bearing
methyl substituent gave a cationic (NHC)2Ag+ type com-
plex having 2:1 ligand to metal ratio [12]. Furthermore,
Ko¨hler and coworkers [13] have recently shown that the
formation of either type of the complexes, i.e. the cationic
2:1 (NHC to metal) complex or the neutral 1:1 (NHC to
metal) complex could be favored by simply changing the
reaction conditions.
N
N
N
N
N
Ag
Ag Cl
N
O
O
NH
NH
(1b)
(2b)
Fig. 1. The amido-functionalized, {[1-(2,4,6-trimethylphenyl)-3-(N-
phenylacetamido)imidazol-2-ylidene]2Ag}+Clꢀ 1b, and the non-function-
alized, [1-(i-propyl)-3-(benzyl)imidazol-2-ylidene]AgCl 2b, silver complexes
are shown.
We became interested in designing new silver–NHC com-
plexes, through functionalization of the N-heterocyclic car-
bene ligand, particularly, because of their potential
applications in homogeneous catalysis. Specifically, we were
interested in designing new N-heterocyclic carbene based
initiators for ring-opening polymerization (ROP) of
L-lactides. Ring-opening polymerization (ROP) of L-lactides
is important on account of being eco-friendly as not only the
polylactide polymer (PLA) is biodegradable but also the lac-
tide monomer can be generated from renewable resources
by corn fermentation process or from agricultural starch
wastes [14,15]. Moreover, the PLAs have been widely used
in medical and pharmaceutical applications [16,17] because
of their good mechanical properties and biocompatibility.
The PLA synthesis is generally carried out by solution poly-
merization [18] and by bulk polymerization [19]. Owing to
its high reactivity, the solution polymerization is susceptible
to unwanted reactions like, racemization, trans-esterifica-
tions, especially to the impurity levels. Thus, for the large-
scale production of PLA for commercial purposes, the bulk
melt polymerization is preferred as it does not suffer from
the limitations faced by the solution polymerization [20].
Because of the aforementioned reasons we became inter-
ested in designing initiators for bulk polymerizations of
L-lactides. In this regard, we have recently reported gold
and silver–NHC complexes as initiators for ring-opening
polymerization (ROP) of L-lactide [21]. We rationalized that
the functionalization of N-heterocyclic carbene ligand
would provide extra stability to these metal catalysts
through chelation of the functionalized side arm to the metal
center. For example, in case of the phosphine catalysts, the
chelated ones have been reported to possess remarkably
high thermal stabilities [22]. For comparison, the syntheses
of non-functionalized N-heterocyclic carbene based initia-
tors were also undertaken (Fig. 1).
respectively over an amido-functionalized NHC ligand,
and an non-functionalized NHC ligand. Furthermore, in
this contribution we disclose that both the silver complexes,
1b and 2b, effectively catalyze ring-opening polymerization
(ROP) of L-lactides at elevated temperatures under solvent-
free melt conditions to give polylactide polymers of moder-
ate to low molecular weights with narrow molecular weight
distributions (Eq. 1).
O
O
1b or 2b
O
O
O
n
O
O
O
L-lactide
polylactide
Eq. 1. Ring-opening polymerization (ROP) of L-lactide by 1b and 2b.
2. Experimental
2.1. General procedures
All manipulations were carried out using a combination
of a glovebox and standard Schlenk techniques. Solvents
were purified and degassed by standard procedures. L-Lac-
tide was purchased from Sigma Aldrich, Germany and was
subjected to polymerization without further purification.
Ag2O was purchased from SD-fine chemicals (India) and
used without any further purification. N-Phenylchloroace-
tamide [23], 1-i-propylimidazole [24] and 2,4,6-trimethylph-
enylimidazole [25] were synthesized according to literature
1
procedures. H and 13C {1H} NMR spectra were recorded
1
in CDCl3 on a Varian 400 MHz NMR spectrometer. H
NMR peaks are labeled as singlet (s), doublet (d), triplet
(t), and septet (sept). Infrared spectra were recorded on a
Perkin Elmer Spectrum One FT-IR spectrometer. Mass
spectrometry measurements were done on a Micromass
Q-Tof spectrometer. Thermal-gravimeteric analysis of cat-
alyst (1b and 2b) were carried out using NETZSCH STA
409PC Luxx Differential Scanning Calorimeter in the
Here in this contribution, we report the synthesis and
structural characterizations of two such new silver
complexes namely, [(1-(2,4,6-trimethylphenyl)-3-{N-pheny-
lacetamido}imidazol-2-ylidene)2Ag]+Clꢀ 1b and [1-(i-pro-
pyl)-3-(benzyl)imidazol-2-ylidene]AgCl
2b
supported

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M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
temperature range of 25–700 ꢁC at the rate of 10 ꢁC/min
under nitrogen flow (60 mL/min). Molecular weights of
the polymers were determined using a Waters GPC
(Waters 2414 RI Detector) with PL-gel, 5 lm Mixed-D
(2 · 300 mm) Column, with polystyrene standards in chlo-
roform that covered a molecular weight range of l60 to
4 · 105. MALDI-TOF MS measurements have been per-
formed with a AXIMA CFR KRATOS Analytical mass
spectrometer, employing a 19 kV accelerating voltage with
pulsed ion extraction (PIE). The positive ions are detected
via ionization mode (20 kV). Laser desorption is achieved
by a nitrogen laser (337 nm, 1 ns pulse width, operating
at 4 Hz), and each spectrum scans 500–1000 shots. The
instrument has been linearly calibrated with three stan-
dards Insulin, Insulin B chain, Bradykinn, Angiotensin-1
and ACTH. The sample is prepared with a a-cyano-4-
hydroxy cinnamic acid (CHC) matrix (10 mg/mL). One
microliter of analyte solution (10 mg/mL) is deposited onto
the stainless steel sample plate, and allowed to air-dry. Sub-
sequently, a 1 lL matrix solution (30:70 v/v CHC with
0.1% TFA: acetonitrile) is added into the analyte. The dif-
ferences between the measured and the calculated masses
of peaks are within 0.47–3.91 Da, corresponding to poly-
mer chains bearing NHC–Ag and NHC fragments of 1b
as end groups.
Ag2O (0.479 g, 2.07 mmol) in dichloromethane (ca.
20 mL) was stirred at room temperature for 4 h. The reac-
tion mixture was filtered and the solvent was removed
under vacuum to give the product as a light yellow solid
(0.813 g, 52%). 1H NMR (CDCl3, 400 MHz, 25 ꢁC), d
3
11.0 (br, 1H, NH); 7.75 (d, 2H, JHH = 8 Hz, o-C6H5);
3
7.34 (s, 1H, NCHCHN); 7.16 (t, 2H, JHH = 8 Hz, m-
3
C6H5); 6.97 (t, 1H, JHH = 7 Hz, p-C6H5); 6.73 (s, 1H,
NCHCHN); 6.72 (s, 2H, m-C6H2{2,4,6-Me3}); 5.35
(s, 2H, CH2); 2.32 (s, 3H, p-C6H2{2,4,6-Me3}); 1.60 (s,
6H, o-C6H2{2,4,6-Me3}). 13C{1H} NMR (CDCl3,
100 MHz, 25 ꢁC), d 182.8 (broad, NCN); 165.0 (CO);
138.5 (ipso-C6H5); 138.2 (ipso-C6H2{2,4,6-Me3}); 135.0
(o-C6H2{2,4,6-Me3}); 134.5 (p-C6H2{2,4,6-Me3}); 128.8
(m-C6H2{2,4,6-Me3}); 128.5 (o-C6H5); 123.8 (NCHCHN);
123.2 (NCHCHN); 121.7 (p-C6H5); 119.7 (m-C6H5); 54.3
(CH2); 21.0 (p-C6H2{2,4,6-Me3}); 17.2 (o-C6H2{2,4,6-
Me3}). IR data cmꢀ1 1698 (s) (mco). HRMS (ES): m/z
745.2410 [(NHC)2Ag]+ calculated 745.2420.
2.4. Synthesis of 1-(i-propyl)-3-(benzyl)imidazolium
chloride 2a
Benzyl chloride (3.10 g, 24.6 mmol) and 1-i-propyl imid-
azole (2.71 g, 24.6 mmol) were taken in toluene (ca. 10 mL)
and heated at 140 ꢁC for 14 h during which a brown precip-
itate was formed. The precipitate was collected by filtration
and was washed with hot hexane (ca. 15 mL) and dried
under vacuum to give the product as brown sticky solid
2.2. Synthesis of 1-(2,4,6-trimethylphenyl)-3-
(N-phenylacetamido)imidazolium chloride 1a
1
N-Phenylchloroacetamide (0.659 g, 3.87 mmol) and
2,4,6-trimethylphenylimidazole (0.721 g, 3.87 mmol) were
taken in toluene (ca. 10 mL) and heated at 140 ꢁC for
15 h during which a white precipitate was formed. The pre-
cipitate was collected by filtration and was washed with hot
hexane (ca. 15 mL) and dried under vacuum to give the
(2.61 g, 45%). H NMR (CDCl3, 400 MHz, 25 ꢁC), d 10.6
(s, 1H, NCHN); 7.62 (s, 1H, NCHCHN); 7.52 (d, 2H,
3JHH = 7 Hz, o-C6H5); 7.49 (s, 1H, NCHCHN); 7.25
3
3
(t, 2H, JHH = 7 Hz, m-C6H5); 7.17 (t, 1H, JHH = 8 Hz,
3
p-C6H5); 5.59 (s, 2H, CH2); 4.78 (sept, 1H, JHH = 7 Hz,
3
CH(CH3)2); 1.56 (d, 6H, JHH = 7 Hz, CH(CH3)2).
1
product as white crystalline solid (1.00 g, 73%). H NMR
13C{1H} NMR (CDCl3, 100 MHz, 25 ꢁC),
d 135.0
(CDCl3, 400 MHz, 25 ꢁC), d 11.3 (s, 1H, NH); 9.54
(s, 1H, NCHN); 7.78 (s, 1H, NCHCHN); 7.64 (d, 2H,
(NCHN); 133.2 (ipso-C6H5); 128.7 (o-C6H5); 128.6
(p-C6H5); 128.5 (m-C6H5); 121.7 (NCHCHN); 120.0
(NCHCHN); 52.6 (C(CH3)2); 52.4 (CH2); 22.5 (C(CH3)2).
HRMS (ES): m/z 201.1385 (NHC-ligand)+ calculated
201.1392.
3
3JHH = 8 Hz, o-C6H5); 7.13 (t, 2H, JHH = 8 Hz, m-
3
C6H5); 7.00 (s, 1H, NCHCHN); 6.95 (t, 1H, JHH = 7 Hz,
p-C6H5); 6.91 (s, 2H, m-C6H2{2,4,6-Me3}); 5.80 (s, 2H,
CH2); 2.26 (s, 3H, p-C6H2{2,4,6-Me3}); 1.97 (s, 6H,
o-C6H2{2,4,6-Me3}). 13C{1H} NMR (CDCl3, 100 MHz,
2.5. Synthesis of [1-(i-propyl)-3-(benzyl)imidazol-2-
ylidene]AgCl 2b
25 ꢁC),
d
162.9 (CO); 141.2 (ipso-C6H5); 138.5
(ipso-C6H2{2,4,6-Me3}); 137.9 (NCHN); 134.2 (o-C6H2-
{2,4,6-Me3}); 130.5 (p-C6H2{2,4,6-Me3}); 129.7 (m-
C6H2{2,4,6-Me3}); 128.5 (o-C6H5); 124.3 (NCHCHN);
124.1 (NCHCHN); 122.2 (p-C6H5); 120.0 (m-C6H5); 52.7
(CH2); 20.9 (p-C6H2{2,4,6-Me3}); 17.4 (o-C6H2{2,4,6-
Me3}). IR data cmꢀ1 1697 (s) (mco). HRMS (ES): m/z
320.1767 (NHC-ligand)+ calculated 320.1763.
A mixture of 1-(i-propyl)-3-(benzyl)imidazolium chlo-
ride 2a (1.80 g, 7.61 mmol) and Ag2O (0.879 g, 3.80 mmol)
in dichloromethane (ca. 20 mL) was stirred for 4 h at room
temperature. The solution was filtered and the solvent was
removed under vacuum to give the product as an light
brown sticky solid (1.10 g, 42%). 1H NMR (CDCl3,
3
400 MHz, 25 ꢁC), d 7.36 (t, 3H, JHH = 8 Hz, m/p-C6H5);
3
2.3. Synthesis of {[1-(2,4,6-trimethylphenyl)-3-
(N-phenylacetamido)imidazol-2-ylidene]2Ag}+Clꢀ 1b
7.24 (d, 2H, JHH = 6 Hz, o-C6H5); 7.05 (s, 1H,
NCHCHN); 6.96 (s, 1H, NCHCHN); 5.26 (s, 2H, CH2);
3
4.75 (sept, 1H, JHH = 7 Hz, CH(CH3)2); 1.48 (d, 6H,
A mixture of 1-(2,4,6-trimethylphenyl)-3-(N-phenylace-
tamido)imidazolium chloride 1a (1.46 g, 4.10 mmol) and
3JHH = 7 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3,
100 MHz, 25 ꢁC), d 177.0 (NCN); 135.1 (ipso-C6H5);

M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
1675
127.9 (o-C6H5); 127.1 (p-C6H5); 126.8 (m-C6H5); 120.8
(NCHCHN); 117.3 (NCHCHN); 54.4 (C(CH3)2); 53.1
(CH2); 22.8 (C(CH3)2). HRMS (ES): m/z 307.0367
[(NHC)Ag]+ calculated 307.0364. Anal. Calc. for C13H16-
AgClN2: C 45.44%; H 4.69%, N 8.15%. Found: C
46.67%; H 4.40%, N 7.88%.
Polymerizations were carried out in the temperature range
100–180 ꢁC. After a predetermined time (0.5–8 h) the glass
ampoule was removed and subsequently, the molten reac-
tive polymer mixture was cooled while immersing sealed
ampoule in liquid nitrogen to stop the polymerization
and thereafter samples were removed for analysis. The
analyses were performed on the crude reaction mixture,
no precipitation was executed to avoid fractionation of
the sample in order to not to influence the results.
2.6. X-ray crystallography
Single crystals of 1a, 1b, and 2b suitable for X-ray dif-
fraction, were grown from acetonitrile at 25 ꢁC. X-ray dif-
fraction data were collected on a Bruker P4 diffractometer
equipped with a SMART CCD detector, and crystal data col-
lection and refinement parameters are summarized in Table
1. The structures were solved using direct methods and
standard difference map techniques, and were refined by
full-matrix least-squares procedures on F2 with SHELXTL
(Version 6.10). For all structures the hydrogen atoms on
nitrogen or oxygen (N–H, O–H) were located from the dif-
ference map and refined, while the hydrogen atoms
attached to carbon (C–H) were geometrically fixed and
subsequently refined using a riding model.
3. Results and discussion
3.1. Amido-functionalized N-heterocyclic carbene complex of
silver(I)
The 1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)-
imidazolium chloride 1a was synthesized by the alkylation
reaction of 2,4,6-trimethylphenylimidazole with N-phenyl-
1
chloroacetamide in 73% yield (Scheme 1). The H NMR
spectrum of 1a in CDCl3 showed the characteristic imi-
dazolium proton peak (NCHN) at 9.54 ppm while the cor-
responding carbon resonance (NCHN) appeared at
137.9 ppm in CDCl3 in the 13C NMR spectrum. The amido
(–CONH–) and the bridging methylene (–CH2–) reso-
nances appeared at 11.30 ppm and at 5.80 ppm respectively
2.7. Polymerization experiments
1
Bulk polymerizations of L-lactide were carried out in
vacuum-sealed glass ampoules. Firstly, the glass ampoule
was charged with monomer (L-lactide) and dried for a per-
iod of 2 h under high vacuum at 50 ꢁC. Subsequently, the
catalyst (1b or 2b) was added keeping with the monomer
to catalyst ratio ranging from 50 to 300. The ampoule
was sealed under high vacuum and immersed in an oil bath.
in the H NMR spectrum and at 162.9 ppm (–CONH–)
and 52.7 ppm (–CH2–) respectively in the 13C NMR spec-
trum. The infrared spectrum showed the carbonyl reso-
nance of the amido group (–CONH–) at 1697 cmꢀ1. The
electrospray mass analysis gave a 100% abundance peak
at 320 m/z corresponding to the cationic [1-(2,4,6-trimeth-
ylphenyl)-3-(N-phenylacetamido)imidazolium]+ species.
The molecular structure of 1-(2,4,6-trimethylphenyl)-
3-(N-phenylacetamido)imidazolium chloride 1a was
determined by X-ray diffraction (Fig. 2). Two crystallo-
Table 1
X-ray crystallographic data for 1a, 1b and 2b
Compound
1a
1b
2b
O
Lattice
Formula
Formula weight 373.87
Orthorhombic Triclinic
C20H24ClN3O2 C43.5H46AgClN6O2 C13H16AgClN2
Monoclinic
Cl
R
R
R
Cl
N
H
N
N
+
N
+
828.18
343.60
P2(1)/c
10.4891(9)
9.7563(9)
13.5900(13)
90.00
97.884(2)
90.00
2377.6(2)
4
ꢀ
Space group
Pca2(1)
16.9660(10)
7.7816(4)
30.4169(18)
90.00
90.00
90.00
4015.7(4)
8
P1
Cl-
N
N
Cl-
N
˚
a (A)
10.3763(7)
15.0117(10)
15.4734(9)
80.5310(10)
83.6870(10)
82.2810(10)
2346.3(3)
2
˚
b (A)
O
˚
c (A)
NH
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Ag2O
V (A )
Ag2O
Z
R
R
Temperature (K) 243(2)
243(2)
0.71073
1.303
243(2)
0.71073
1.657
Cl-
+
R
˚
N
N
Radiation (k, A) 0.71073
N
q(calcd.), g cmꢀ3 1.237
Ag
Ag Cl
N
N
N
l(Mo Ka),
0.209
0.531
1.637
mmꢀ1
O
O
NH
hmax (ꢁ)
No. of data
No. of
parameters
R1
28.28
26,227
487
28.34
16,222
591
28.30
9309
155
NH
2b
(
)
1b
(
)
0.0410
0.0900
1.002
0.0426
0.0950
1.007
0.0283
0.0651
1.011
R=
i
-Pr
R= Mesityl
wR2
GOF
Scheme 1.

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M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
reaction of 1-(2,4,6-trimethylphenyl)-3-(N-phenylacetam-
ido)imidazolium chloride 1a with Ag2O in 52% yield fol-
lowing a methodology reported by Lin and coworkers
[35]. Lin’s versatile Ag2O methodology for synthesizing
the Ag(I)–NHC complexes is particularly useful in cases
when the generation of free carbenes from the imidazolium
salts becomes difficult. Especially for imidazolium salts
having base sensitive functional groups, the generations
of free carbenes are often found to be challenging. It is
noteworthy that with Ag2O, the deprotonation of the
acidic proton at the imidazolium 2-position (NCHN) was
observed instead of the amide proton (–CONH–) of the
functionalized side arm. This observation is in contrary
to that observed for the hydrogen bonding interaction in
the crystal structure of 1a, where the Clꢀ anion preferred
to interact with the amide proton (–CONH–) instead of
the imidazolium 2-position (NCHN) proton. The charac-
teristic imidazolium proton peak (NCHN) at 9.54 ppm
O2
C203
N23
N21
N22
Cl1
Fig. 2. ORTEP of 1a with thermal ellipsoids drawn at 50% probability
level. Two crystallographically unique molecules are present in the unit
cell and only one of them is shown. Hydrogen atoms on carbon are
˚
omitted for clarity. Selected bond lengths (A) and angles (ꢁ): N22–C203
1.324(3), N23–C203 1.322(3), N22–C203–N23 108.7(2). H-bond angles
N11–H11Aꢁ ꢁ ꢁCl2 173ꢁ, N21–H21Aꢁ ꢁ ꢁCl1 174ꢁ, O3–H301ꢁ ꢁ ꢁCl2 173ꢁ, O3–
H302ꢁ ꢁ ꢁCl1 169ꢁ, O4–H401ꢁ ꢁ ꢁCl2 173ꢁ, O4–H402ꢁ ꢁ ꢁCl1 160ꢁ.
1
graphically unique molecules were present in the unit cell. It
is interesting to note that both the mesityl [26] and the
N-phenylacetamido (–CH2CONHPh) [27] substituents lie
upright to the plane of the imidazole ring presumably due
to the steric reasons. The two equivalent C–N bond dis-
tances [N22–C203 = 1.324(3) A, N23–C203 = 1.322(3) A,
N13–C103 = 1.331(3) A, N12–C103 = 1.312(3) A], which
are shorter than the sum (1.472 A) of the individual single
bond covalent radii of C (0.772 A) and N (0.70 A) [28], sug-
gest partial double bond character of the C–N bonds due to
the p-electron delocalization in the imidazole ring. The imi-
dazolium C–N bond distances [N22–C203 = 1.324(3) A,
N23–C203 = 1.322(3) A, N13–C103 = 1.331(3) A, N12–
was conspicuously absent in the product H NMR spec-
trum in CDCl3 owing to deprotonation by the weakly basic
Ag2O. Consistent with the formation of the singlet carbene,
an additional broad peak corresponding to the metal coor-
dinated carbene (NCN) was seen at 182.8 ppm in the
13C NMR spectrum in CDCl3. The amide carbonyl
(–CONH–) peak appeared at 165.0 ppm in the 13C NMR
spectrum and at 1698 cmꢀ1 in the infrared spectrum. The
˚
˚
˚
˚
˚
˚
˚
electrospray mass spectrometry showed a molecular
ion peak at 745 m/z corresponding to the cationic {[1-
(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imidazol-2-
ylidene]2Ag}+ species.
˚
˚
˚
The definitive proof of the {[1-(2,4,6-trimethylphenyl)-
3-(N-phenylacetamido)imidazol-2-ylidene]2Ag}+Clꢀ
1b
˚
C103 = 1.312(3) A] in 1a are consistent with that observed
for related compounds such as, 1-(ethyl)-3-(methyl)imi-
˚
˚
dazolium bromide [1.327(5) A and 1.323(6) A] [29] and
1-(cyano methylene)-3-(methyl)imidazolium chloride
Cl
˚
˚
[1.348(3) A and 1.336(3) A] [30]. It is interesting to note that
weak hydrogen bonding interaction between the Clꢀ anion
with the amide proton (–CONH–) has been found in the 1a
N2
N3
˚
N1
structure. For example, the Cllꢁ ꢁ ꢁN21 (3.244 A) ꢀand the
˚
Cl2ꢁ ꢁ ꢁN11 (3.236 A) distances between the Cl anion
C11
and the amide nitrogen (–CONH–) are slightly shorter than
the sum of the van der Waals radii of Cl and N atoms
O1
Ag
˚
(3.31 A) [31]. Another interesting observation that emerges
out is that the Clꢀ anion prefers to interact with the amide
proton (–CONH–) instead of hydrogen bonding to the
acidic proton at the 2-imidazolium position (NCHN). For
example, in case of a non-functionalized imidazolium halide
like, 1-(ethyl)-3-(methyl)imidazolium bromide, [29] which is
bereft of any NH or OH protons, show extensive hydrogen
bonding of the Brꢀ anion not only with the acidic proton of
the 2-position (NCHN) but also with that of the olefinic
protons of the 4- and 5-positions of the imidazole ring.
Hydrogen bondings in imidazolium halide salts are quite
common and have been extensively studied by NMR
[32,33], and X-ray diffraction [34,29,30] techniques.
C41
N5
N4
N6
O2
Fig. 3. ORTEP of 1b with thermal ellipsoids drawn at 50% probability
level. Solvent molecules present in the unit cell are not shown. Hydrogen
atoms on carbon are omitted for clarity. Selected bond lengths (A) and
angles (ꢁ): Ag–C11 2.076(3), Ag–C41 2.071(3), N1–C11 1.353(3), N2–C11
1.344(3), N4–C41 1.354(3), N5–C41 1.351(3), C11–Ag–C41 174.49(10),
N2–C11–N1 104.5(2), N4–C41–N5 104.2(2). H-bond angles N3–
H3Aꢁ ꢁ ꢁCl 175ꢁ, N6–H6Aꢁ ꢁ ꢁCl 171ꢁ, C57–H57Cꢁ ꢁ ꢁO2 161ꢁ.
˚
The {[1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)-
imidazol-2-ylidene]2Ag}+Clꢀ 1b was synthesized by the

M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
1677
0
0
˚
structure came from X-ray diffraction studies (Fig. 3). The
molecular structure showed that two amido-functionalized
ligands were bound to the center silver atom resulting in a
cationic 2:1 complex. The geometry around silver is almost
linear [C11–Ag–C41 174.49(10)ꢁ] and is consistent with the
d10 configuration of silver(I) ion. The Ag–Ccarb bond dis-
3.325(9) A in (1)2[Cl Æ H2O] (1 = 3-methylamido-3 ,4 -eth-
ylenedithiotetrathiafulvalene) [43].
Important is the comparison of the structures of
1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imidazo-
lium chloride 1a with {[1-(2,4,6-trimethylphenyl)-3-(N-
phenylacetamido)imidazol-2-ylidene]2Ag}+Clꢀ 1b. Quite
noticeably, marked decrease in the N–C–N bond angle
was observed on going from that in 1a [108.7(2)ꢁ,
109.4(2)ꢁ] to 1b [104.5(2)ꢁ, 104.2(2)ꢁ]. This was further
accompanied by the increase in the average C–N bond dis-
tance on going from 1a [1.321(3) A] to 1b [1.351(7) A].
Shorter C–N bond distance along with the N–C–N angle,
being closer to 120ꢁ, in 1a suggests that the imidazolium
ring in 1a is relatively more aromatic compared to that in
1b. As was observed in 1a, both the mesityl and the N-
phenylacetamido (–CH2CONHPh) groups in 1b were
found to be perpendicular to that of the imidazole ring [44].
˚
˚
tances [Ag–C11 = 2.076(3) A and Ag–C41 = 2.071(3) A]
are comparable to the sum of the individual covalent radii
˚
of Ag and C (2.111 A) [28]. Quite interestingly, the amido
side arm of the complex 1b was found to be disposed away
form the silver instead of chelating to the metal center as
was expected. Similar non-chelation of the functionalized
side arm to the metal center has been reported for groups
like 2-pyridyl and 2-pyridinylmethyl moieties in case of sil-
ver–NHC complexes [9,36]. It is worth noting that in a
functionalized Pd–NHC complex reported by Waymouth
[37] and coworkers the chelation to the metal center
through the enolate-O of a 2-oxo-2-phenylethyl functional-
ized side arm was however observed.
˚
˚
3.2. Non-Functionalized N-heterocyclic carbene complex of
silver(I)
Another notable feature of the 1b structure is that the
two imidazole rings were found to be non-coplanar having
a N1–C11–C41–N4 dihedral angle of 57.7ꢁ. At this junc-
ture it is worth mentioning that the analogous cationic
2:1 (NHC:silver) complexes have been found to exhibit
both coplanar as well as non-coplanar orientations of the
imidazole rings. For example, the non-coplanar structures
have been reported for f½1; 3-dið2-pyridylÞimidazol-
2-ylideneꢂ2AggþBF4ꢀ [38] (dihedral angle N4–C14–C1–N2
= 41.9ꢁ) and for [1-ð2; 6-diisopropylphenylÞ-3- ð2-pyridyl-
methylÞimidazol-2-ylideneꢂAgþAgBr2ꢀ [9] (dihedral angle
N6–C47–C20–N8 = 32.3ꢁ) whereas the coplanar structures
with dihedral angles almost close to 0ꢁ haveþbeen reported
for f½1; 3-diðmethylÞimidazol-2-ylideneꢂ2Agg - AgClꢀ2 [10]
and f1-½2-ð3; 5-dimethylpyrazol-1-ylÞethylꢂ- 3-ðmethylÞ-
imidazol-2-ylideneꢂAgþAgClꢀ2 [12]. Theoretical study
recently reported by Frenking and coworkers [39] suggests
that both the coplanar and the non-coplanar perpendicular
orientations of the imidazole rings are very close in energy.
Quite interestingly, the Clꢀ anion was found to be in
the hydrogen bonding (Clꢁ ꢁ ꢁH–N) distance with one of
the amido proton (–CONH–) of the side-arm substituent.
A non-functionalized imidazolium chloride salt, 1-(i-pro-
pyl)-3-(benzyl)imidazolium chloride 2a, was synthesized
analogously by the reaction of i-propylimidazole with
benzyl chloride in 45% yield (Scheme 1). The characteristic
imidazolium peak (NCHN) and the bridging methylene
(–CH2–) peak appeared each as singlets at 10.6 ppm and
at 5.59 ppm respectively in the 1H NMR spectrum in CDCl3
and the corresponding resonances appeared at 135.0 ppm
(NCHN) and at 52.4 ppm (–CH2–) in the 13C NMR
spectrum in CDCl3. Consistent with the formation of 2a,
Electrospray Mass analysis showed a cationic [1-(i-pro-
pyl)-3-(benzyl)imidazolium]+ species at 201 m/z and was
further confirmed by HRMS results.
The silver complex, [1-(i-propyl)-3-(benzyl)imidazol-
2-ylidene]AgCl 2b, was obtained from the reaction of
1-(i-propyl)-3-(benzyl)imidazolium chloride 2a with Ag2O
in 42% yield. Characteristic to the product formation, the
silver coordinated imidazolium carbene resonance (NCN)
appeared as a sharp peak at 177.0 ppm in the 13C NMR
spectrum in CDCl3. The bridging methylene peak (–CH2–)
1
˚
The Clꢁ ꢁ ꢁN3 distance of 3.189 A is shorter than the sum
appeared as singlet at 5.26 ppm in the H NMR spectrum
of the individual van der Waals radii (3.31 A) and, thus,
in CDCl3 and at 53.1 ppm in the 13C NMR spectrum in
CDCl3.
˚
is consistent with a hydrogen bonding interaction [31].
˚
Notably, the Clꢁ ꢁ ꢁN3 distance (3.189 A) in 1b is even
The molecular structure of [1-(i-propyl)-3-(benzyl)imi-
dazol-2-ylidene]AgCl 2b has been determined by X-ray dif-
fraction (Fig. 4). A neutral monomeric 1:1 complex
containing one NHC ligand and a silver ion obtained in
2b is in sharp contrast to the cationic 2:1 complex observed
in case of the amido-functionalized NHC ligand. The Ag–
shorter than that observed in case of the similar
˚
interaction in 1a (Cllꢁ ꢁ ꢁN21 = 3.244 A and Cl2ꢁ ꢁ ꢁN11 =
˚
3.236 A), suggesting a stronger hydrogen bond in the
former. Similar Clꢁ ꢁ ꢁN distances have been observed in
other compounds displaying Clꢁ ꢁ ꢁH–N (amido) hydrogen
˚
˚
bonding interactions. For example, 3.2648(16) A in
Cl bond distance of 2.3435(7) A (Ag–Cl) compares well
[N-(2,6-diisopropylphenyl)-3-[bis(2-pyridyl-methyl)amino]-
with the sum of the individual covalent radii of Ag and
Cl (2.329 A) [28]. The angle at silver is slightly bent from
the linearity with the C1–Ag–Cl angle being 166.08(7)ꢁ.
Important is the structural comparison of 1b and 2b,
which reveals that amido-functionalization of the side-
arm substituents has very little effect on the bonding of
˚
˚
propanamide]copper(I) chloride [40], 3.341(3) A and
3.325(3) A in (Et4N)2[CuH21Me(Cl) Cl] (H21Me = 2,6-
˚
bis[N,N0-(2-acetamidophenyl)carbamoyl]pyridine
[41],
t
3.2127(19) A in [(L0)Zn(Cl)](Cl) [L0 = (6-NHCOBu -2-
˚
pyridylmethyl)-bis-(2-pyridylmethyl)amine]
[42]
and

1678
M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
Table 2
Melt polymerization of L-lactide by 1b
Entry L-Lactide/ Temperature Time Mn
Mw/ Conversion
Mn (%)
C1
(1b) ratio
(ꢁC)
(h)
N2
5.3 · 103 1.28 90
5.3 · 103 1.29 92
5.3 · 103 1.29 92
6.2 · 103 1.19 81
6.3 · 103 1.23 77
6.1 · 103 1.20 75
3.2 · 103 1.13 55
5.2 · 103 1.09 76
5.0 · 103 1.15 67
7.2 · 103 1.34 99
3.0 · 103 1.05 21
3.5 · 103 1.15 36
4.6 · 103 1.11 51
5.3 · 103 1.30 52
7.0 · 103 1.35 91
7.6 · 103 1.30 98
N1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
50
100
150
200
250
300
250
250
250
250
250
250
250
250
250
250
160
160
160
160
160
160
100
120
140
180
160
160
160
160
160
160
4
4
4
4
4
4
4
4
4
4
0.5
1
2
3
6
8
Ag
Cl
Fig. 4. ORTEP of 2b with thermal ellipsoids drawn at 50% probability
level. Hydrogen atoms on carbon are omitted for clarity. Selected bond
lengths (A) and angles (ꢁ): Ag–C1 2.077(2), Ag–Cl 2.3435(7), N1–C1
1.358(3), N2–C1 1.350(3), N1–C1–N2 104.8(2), C1–Ag–Cl 166.08(7).
˚
the N-heterocyclic carbene moiety to the silver. For exam-
˚
ple, the Ag–Ccarb bond distance of 2.077(2) A (Ag–C1) in
Table 3
2b compare well with the Ag–Ccarb bond distances [Ag–
Melt polymerization of L-lactide by 2b
˚
˚
C11 = 2.076(3) A and Ag–C41 = 2.071(3) A] in 1b. Simi-
larly, the NCN bond angles in 1b [N1–C11–N2 = 104.5
(2)ꢁ and N4–C41–N5 = 104.2(2)ꢁ] and in 2b [N1–C1–
N2 = 104.8(2)ꢁ] along with the C–N bond distances in 1b
Entry L-lactide/
Temperature Time Mn
Mw/ Conversion
Mn (%)
(2b) ratio
(ꢁC)
(h)
1
2
3
4
5
6
7
8
9
10
11
11
12
13
14
15
50
100
150
200
250
300
150
150
150
150
150
150
150
150
150
150
160
160
160
160
160
160
100
120
140
180
160
160
160
160
160
160
4
4
4
4
4
4
4
4
4
4
0.5
1
2
3
6
8
5.2 · 103 1.42 98
5.6 · 103 1.36 99
9.0 · 103 1.23 96
7.5 · 103 1.23 86
6.8 · 103 1.25 88
6.9 · 103 1.21 69
3.9 · 103 1.20 47
4.4 · 103 1.16 44
6.9 · 103 1.09 74
6.9 · 103 1.35 99
2.8 · 103 1.06 21
4.1 · 103 1.19 58
5.4 · 103 1.29 86
7.1 · 103 1.28 92
9.4 · 103 1.26 99
9.6 · 103 1.36 99
˚
˚
[N1–C11 = 1.353(3) A, N2–C11 = 1.344(3) A and N4–C41
˚
˚
= 1.354(3) A, N5–C41 = 1.351(3) A] and in 2b [N1–
˚
˚
C1 = 1.358(3) A, N2–C1 = 1.350(3) A] are also compara-
ble. The coordination geometry of silver is linear in 1b
(C11–Ag–C41 = 174.49(10)ꢁ) whereas it is slightly dis-
torted from linearity in 2b (C1–Ag–Cl = 166.08(7)ꢁ).
The two silver complexes 1b and 2b, efficiently catalyze
the ring-opening polymerization (ROP) of L-lactides under
solvent-free melt conditions at elevated temperatures. Spe-
cifically,
a typical polymerization experiment would
involve heating L-lactide and the catalyst, 1b or 2b, for a
given monomer to catalyst ratio in a sealed vessel under
vacuum at a designated temperature for a specific period
of time. Under these conditions the reaction mixture would
form a monomer melt in which the polymerization would
occur. The variation of the [M]:[C] ratio (M = monomer,
C = catalyst) polymerization run showed that maximum
molecular weight (entry 5: Mw = 6.3 · 103, Table 2) was
obtained at [M]:[C] ratio 250:1 in case of 1b whereas the
same (entry 3: Mw = 9.0 · 103, Table 3) was obtained at
[M]:[C] ratio 150:1 in case of 2b for a 4 h run at 160 ꢁC.
The polydispersity indexes are almost similar for both these
catalysts 1b (PDI = 1.05 ꢀ 1.35) and 2b (PDI = 1.06 ꢀ
1.42). It is interesting to note that the polymer molecular
weights obtained in case of a 1b and 2b is slightly shorter
than that of a related cationic 2:1 (NHC ligand:metal) sil-
ver complex (maximum Mw = 12.2 · 103) we recently
reported for bulk polymerization of L-lactide under analo-
gous melt conditions [21b].
reached toward saturation whereas the molecular weight
distribution (PDI) remained constant all throughout the
course of the polymerization (Fig. 5). Such an observation
is consistent with a pseudo-living polymerization process
[45]. The temperature dependence study carried out in the
range (100–180) ꢁC showed that in case of 1b molecular
weight (Mn) increased steadily with temperature whereas
for 2b the Mn reached a maximum at 160 ꢁC after which
it started to decrease. The decrease in molecular weight
may be attributed due to depolymerization taking place
at higher temperatures. Similar decrease in molecular
weight at higher temperatures has been reported by Liao
[19a] and Albertson and Varma [16]. Interestingly, compar-
ison between the silver complexes revealed that slightly
higher molecular weight polymer is obtained in case of 2b
than in 1b.
The time dependence study showed that for both 1b and
2b, the number average molecular weight (Mn) of the poly-
mer increased with time for the 4 h after which it gradually
The stability of the catalysts, 1b and 2b, were assessed
by the thermogravimetric analysis (TGA) which showed

M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
1679
polymerization of lactide has recently been reported by
us [21b], for a silver–NHC complex, and by Arnold
[46], for an Y(III)–NHC complex. A consequence of
metal mediated mechanism is the capping of the polymer
chain-ends by NHC fragments. Indeed, the MALDI
spectrometric analysis of polymer confirms the presence
of NHC end groups (Fig. 7) [47]. It is worthy of mention
that for the relatively less thermally stable neutral silver–
NHC complex 2b, the other possibility of carbene medi-
ated mechanism cannot be ruled out entirely. Detailed
mechanistic studies to establish the nature of the active
species responsible for catalysis by 2b are underway.
1b [L-lactide]/[1b] = 250
2b [L-lactide]/[2b] = 150
10000
8000
6000
4000
2000
0
5
4
3
2
1
0
4. Conclusion
1b (PDI)
2b (PDI)
In summary, two new silver complexes, 1b and 2b,
supported respectively over an amido-functionalized and
a non-functionalized N-heterocyclic carbene ligands have
been synthesized. The silver complexes, 1b and 2b, along
with the amido-functionlized N-heterocyclic carbene
ligand precursor, 1a, have been structurally characterized
by X-ray diffraction studies. A cationic 2:1 (NHC
ligand:metal) complex obtained in case of the amido-
functionalized ligand is in sharp contrast to the neutral
1:1 (NHC ligand:metal) complex obtained in case of
the non-functionalized ligand. Structural comparison of
1b with 2b reveal that amido-functionalization of the side
arm had little bearing on its structures. The silver com-
plexes, 1b and 2b, effectively catalyze the ring-opening
polymerization (ROP) of L-lactide under solvent-free melt
conditions producing polylactide polymer of moderate to
low molecular weight with narrow molecular weight
distribution.
0
2
4
6
8
10
Time (hour)
Fig. 5. Plot of Mn and PDI vs. time for the ring-opening polymerization
(ROP) of 1b (monomer to catalyst ratio = 250) and 2b (monomer to
catalyst ratio = 150) at 160 ꢁC.
that compound 1b is stable upto 300 ꢁC whereas slow
mass loss was observed after 95 ꢁC for compound 2b
(Fig. 6). It is noteworthy that remarkable stability of 1b
is similar to that observed for a analogous cationic 2:1
(NHC ligand:metal) silver complex namely, [(1-i-propyl-
3-{N-phenylacetamido}imidazol-2-ylidene)2Ag]+Clꢀ [21b],
which too was stable up to 180 ꢁC. The extreme stability
of cationic 2:1 (NHC ligand:metal) silver complexes, like
that of 1b, point toward a metal mediated polymeriza-
tion (Scheme 2). It is worth noting that metal mediated
1b
-
+
Cl
2b
N
N
N
Ag
N
O
O
NH
NH
N
Ag Cl
N
1b
2b
Fig. 6. Thermogravimetric analysis of 1b and 2b as a function of temperature.

1680
M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
O
O
O
O
R
R
O
O
+ Cl-
+ Cl-
R
R
N
N
N
N
O
N
N
Ag
Ag
N
N
O
O
O
O
O
NH
NH
NH
NH
1b
(
)
R=Mesityl
O
O
HN
HN
n
O
R
R
O
O
O
O
O
N
N
+
N
N
N
+
O
O
O
Ag
Ag
O
O
N
N
N
2n+1
Cl-
O
O
Cl-
O
R
R
NH
NH
Scheme 2. Proposed scheme for ring-opening polymerization of lactide by 1b.
Fig. 7. A zoomed MALDI-TOF MS spectrum of the polylactide polymer.
Acknowledgements
Appendix A. Supplementary data
We thank Council of Scientific and Industrial Research
(CSIR), New Delhi (Grant No: 01(1901)/03/EMR-II) for
financial support of this research. M.K.S. thanks CSIR,
New Delhi, for research fellowships.
CCDC 604566, 604565, and 604564 contain the sup-
plementary crystallographic data for 1-(2,4,6-trimethyl-
phenyl)-3-(N-phenylacetamido)imidazolium chloride 1a,
{[1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imidazol-

M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
1681
2-ylidene]2Ag}+Clꢀ 1b, and [1-(i-propyl)-3-(benzyl)imi-
dazol-2-ylidene]AgCl 2b. These 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. Sup-
plementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2006.12.022.
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´
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´ ˆ
´
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1682
M.K. Samantaray et al. / Journal of Organometallic Chemistry 692 (2007) 1672–1682
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[47] Specifically, a series of sodium (23 Da) cationized peaks of the
polymers bearing NHC end groups can be recognized in the MALDI
spectrum. The mass (MC) of the sodium cationized peak of the
polymer bearing the NHC–Ag (426 Da) and NHC (319 Da) end
groups is given by, MC = 72x + 426 + 319 + 23 where x = number of
repeat unit (72 Da).