A cationic (N-heterocyclic carbene)silver complex as catalyst for bulk ring-opening polymerization of L-lactides

Article abstract of DOI:10.1002/ejic.200600209

Synthetic, theoretical, and catalysis studies of a cationic functionalized N-heterocyclic carbene complex of silver, namely [{1-isopropyl-3-(N- phenylacetamido)imidazol-2-ylidene}₂-Ag]⁺Cl^- (1b), is reported. Specifically,

Full text of DOI:10.1002/ejic.200600209

FULL PAPER
DOI: 10.1002/ejic.200600209
A Cationic (N-Heterocyclic carbene)silver Complex as Catalyst for Bulk
Ring-Opening Polymerization of -Lactides
L
Manoja K. Samantaray,[a] Vimal Katiyar,[b] Dipankar Roy,[a] Keliang Pang,[c]
Hemant Nanavati,[b] Raji Stephen,[a] Raghavan B. Sunoj,*[a] and Prasenjit Ghosh*[a]
Keywords: Carbenes / DFT studies / Functionalized NHC / Ring-opening polymerization (ROP) / Organometallic
catalysis / Silver
Synthetic, theoretical, and catalysis studies of a cationic func-
tionalized N-heterocyclic carbene complex of silver, namely
[{1-isopropyl-3-(N-phenylacetamido)imidazol-2-ylidene}2-
Ag]+Cl– (1b), is reported. Specifically, 1b was synthesized by
the reaction of 1-isopropyl-3-(N-phenylacetamido)imid-
azolium chloride (1a) with Ag2O in 64% yield; 1a was syn-
thesized by the alkylation reaction of 1-isopropylimidazole
with N-phenyl chloroacetamide in 90% yield. The molecular
structure of 1b was determined by X-ray diffraction studies
cule. Bonding in complex 2b has been probed with the help
of charge decomposition analysis (CDA), the atoms in mole-
cules (AIM) approach as well as natural bond orbital (NBO)
methods. The Ag–NHC bond has been found to be more co-
valent with NHC acting as an effective σ-donor. The π-back-
bonding from the metal atom to the ligand was found to be
negligible. It has been noticed that the imidazole rings re-
main nearly orthogonal with respect to each other, in contrast
to the experimental geometry. Intramolecular hydrogen
bonds as well as Ag···O interaction involving the carbonyl
oxygen atoms have been identified as additional stabilizing
factors contributing towards the lower energy conformer of
2b compared to the corresponding planar geometry.
and was found to be active for polymerization of L-lactide at
elevated temperatures under solvent-free melt conditions to
give polylactide of moderate molecular weight with narrow
molecular weight distribution. Density functional theory
studies of the cationic species 2b, derived from NHC silver
complex 1b, were employed to obtain an understanding of
the structure, bonding, and electronic features of the mole-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
the original phosphane-based ruthenium catalyst, (PCy3)2-
(Cl)2Ru=CHPh.[4–6] Not surprisingly, the trendy NHCs are
fast replacing the phosphanes and are gradually emerging
as the designer’s choice of ligand in organometallic cataly-
sis. Remarkably, the NHC transition metal complexes have
exhibited catalytic activities for a truly impressive range of
reactions covering the C–C coupling reaction,[7,8] olefin me-
tathesis,[9] hydrogenation,[10,11] hydroformylation,[12] hydro-
silylation,[13] CO/ethylene copolymerization,[14] and hydro-
boration[15] reactions to name a few. In addition to the
newly found successes in organometallic catalysis, the
NHCs have shown promise in organocatalysis as potent nu-
cleophilic organic catalysts and are increasingly gaining
popularity and acceptance in such roles. Recently, there
have been several reports of organocatalysis by NHCs par-
ticularly in transesterification reactions[16] and in ring-open-
ing lactide polymerization reactions.[17–19] Both the in-situ
generated carbenes as well as the isolable stable singlet N-
heterocyclic carbenes have been directly employed in these
organocatalysis reactions.
Introduction
Ever since the first successful isolation of a stable singlet
carbene by Arduengo,[1] there has been phenomenal growth
in the field of N-heterocyclic carbene (NHC) chemistry.[2]
As a consequence, the N-heterocyclic carbenes have found
wide-spread application in chemical catalysis with new cata-
lysts being discovered recently.[3] The growth in NHC chem-
istry has made a profound impact, particularly in the field
of organometallic catalysis, with the NHC-based catalysts
exhibiting superior performance over the traditional cata-
lysts in many important transformations. For example, it is
now a well-established fact that Grubbs’ second-generation
ROMP catalyst bearing NHC exhibits better efficiency over
[a] Department of Chemistry, Indian Institute of Technology
Bombay,
Powai, Mumbai 400076, India
Fax: +91-22-2572-3480
E-mail: pghosh@chem.iitb.ac.in
sunoj@chem.iitb.ac.in
[b] Department of Chemical Engineering, Indian Institute of
Technology Bombay,
The mainstay of N-heterocyclic carbene chemistry has
been its silver complexes, which played an important role
in the development of the field of NHCs.[20] The interest in
NHC silver complexes is primarily due to it being an effec-
Powai, Mumbai 400076, India
[c] Department of Chemistry, Columbia University,
3000 Broadway, New York, NY 10027, USA
Supporting information for this article is available on the
WWW under http://www.eurjic.org/ or from the author.
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R. B. Sunoj, P. Ghosh et al.
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tive transmetallating agent, a property which is being exten- (PLAs), not only are biodegradable but they can also be
sively used for synthesizing other desired transition metal generated from renewable resources (i.e. the - and -lactide
complexes. Lin[21] provided a commonly employed method- monomers) by a corn fermentation process or from agricul-
ology for synthesizing the NHC silver complexes using tural starch wastes.[41] Because of their good mechanical
Ag2O, which represents by far the most popular route to properties and biocompatibility, the PLAs have found wide
these complexes. As an outcome of all the efforts toward utility in medical and pharmaceutical applications.[42,43] As
designing new N-heterocyclic carbene systems, many silver the sequence of stereocenters in the polymer chain has a
complexes were synthesized and structurally charac- direct bearing on its mechanical properties, the ring-open-
terized.[22–25] A notable feature of the silver complexes is ing polymerization of cyclic lactides thus enjoys inherent
their structural diversity in the solid state. For example, an advantage over α-olefin polymerizations that need intricate
NHC ligand with a sterically demanding mesityl substituent ligand design for control of polymer tacticity.
gave a neutral (NHC)AgCl-type complex with a 1:1 ligand/
Herein, we report the synthesis and structural characteri-
metal ratio, whereas the less bulky variant of the same li- zation of a silver complex, namely [{1-isopropyl-3-(N-phen-
gand bearing a methyl substituent gave a cationic (NHC)2- ylacetamido)imidazol-2-ylidene}2Ag]+Cl– (1b), of a new
Ag+-type complex with a 2:1 ligand/metal ratio.[26]
functionalized N-heterocyclic carbene ligand along with a
Interestingly, apart from its exclusive use in transmetalla- theoretical study. Furthermore, in this contribution we dis-
tion reactions, a few reports of the utility of NHC silver close that 1b effectively catalyzes ring-opening polymeriza-
complexes in chemical catalysis have appeared lately. For tion of -lactides (Scheme 1) at elevated temperatures under
instance, there have been reports of the NHC silver com- solvent-free melt conditions to give a polylactide polymer
plexes exhibiting catalytic activity in ethyl diazoacetate of moderate molecular weight with narrow molecular
(EDA) assisted carbene transfer reactions,[27] in transesteri- weight distribution.
fication reactions, and in ring-opening lactide polymeriza-
tion reactions.[28] Thus, for the NHC silver complexes, the
focus of research is gradually shifting from being solely
used in transmetallation reactions to its applications in
chemical catalysis, which has opened up a new frontier of
research. As the catalytic applications of NHC silver com-
plexes are relatively less explored, we became interested in
investigating the catalytic properties of these complexes.
NHCs are often compared with the tertiary phosphane
ligands because of their good σ-donating ability.[29] How-
ever, a closer scrutiny based on both theoretical[30–34] and
experimental[35,36] results revealed that the NHCs are more
strongly σ-donating than the phosphanes.[37] Another no-
table difference between the two is that the NHCs are gen-
erally easier to synthesize but harder to metallate while the
reverse is true for phosphanes. In general, the success of
NHCs can be attributed partly to their ease of synthetic
accessibility and partly to the observed improved air, moist-
ure,[38] and thermal stabilities[39] of the so-called NHC cata-
lysts. The issue of improved stabilities of NHC metal com-
plexes may be related to the nature of the metal–carbene
(NHC) bond in these complexes and this requires a detailed
Scheme 1.
theoretical study to gain a better understanding of these
complexes. Alongside our main aim of designing new NHC
Results and Discussion
silver complexes for applications in chemical catalysis, we
Despite the phenomenal successes of NHCs and NHC
became interested in carrying out a detailed theoretical metal complexes in chemical catalysis, the NHC silver com-
analysis of our NHC silver complexes in order to gain in- plexes have so far shown only limited utility as catalysts.
sight into their reactivity pattern and catalytic potential. In Recently, there was a report of NHC silver complexes cata-
this regard, we have recently communicated a detailed syn- lyzing a carbene transfer reaction from ethyl diazoacetate
thetic and theoretical study of a newly designed neutral mo- to cyclohexane.[27] On another occasion, Waymouth and
nomeric NHC silver complex, [1-isopropyl-3-(2-tert-butyl- Hedrick reported NHC silver complexes as novel carbene
2-oxoethyl)imidazol-2-ylidene]AgCl.[40] In addition to the delivery agents for ring-opening polymerization of -lac-
theoretical understanding, our interest is in the testing of tides and for transesterification reactions.[28] These promis-
these new NHC silver complexes for catalysis, particularly ing applications of NHC silver complexes in chemical catal-
with regard to ring-opening polymerization (ROP) of -lac- ysis have opened up a new frontier of research. As this area
tides. Because of ecological reasons, the ROP of -lactides still remains largely unexplored, we became interested in
is of current research interest. The polylactide polymers designing novel N-heterocyclic carbene silver complexes
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A Cationic (N-Heterocyclic carbene)silver Complex
FULL PAPER
and testing their catalytic utility in ring-opening polymeri- ligands are found to be bound to the metal center. It is
zation of -lactides.
noteworthy that for a related NHC ligand, 1-isopropyl-3-
(2-tert-butyl-2-oxoethyl)imidazolin-2-ylidene, a neutral mo-
nomeric complex [1-isopropyl-3-(2-tert-butyl-2-oxoethyl)-
imidazolin-2-ylidene]AgCl[40] bearing a 1:1 ligand/metal ra-
tio was obtained under analogous reaction conditions un-
like the cationic 2:1 ligand/metal ratio observed in the pres-
ent case. Köhler et al.[45] have recently demonstrated that
for the same ligand both the cationic 2:1 NHC/metal com-
plex and the neutral 1:1 NHC/metal complex could be iso-
lated upon changing the polarity of the reaction medium.
For example, the reaction in THF gave a neutral 1:1 NHC/
metal complex of the type [(saturated NHC)AgCl] while the
same in highly polar water gave a 2:1 NHC/metal complex
Synthesis of [{1-Isopropyl-3-(N-phenylacetamido)imidazol-2-
ylidene}2Ag]+Cl– (1b)
The N-heterocyclic carbene ligand precursor, 1-isopro-
pyl-3-(N-phenylacetamido)imidazolium chloride (1a), was
synthesized by the alkylation reaction of 1-isopropylimid-
azole with N-phenyl chloroacetamide in refluxing toluene
in 90% yield (Scheme 2). Recently, Burgess[44] synthesized
similar N-acetamidoimidazolium chloride derivatives, syn-
thesized by analogous alkylation procedures, for preparing
theNHC silver and palladium complexes. The NMR analy-
sis of 1-isopropyl-3-(N-phenylacetamido)imidazolium chlo-
ride (1a) showed that the diagnostic imidazolium (NCHN)
resonance appeared downfield-shifted at δ = 9.97 ppm in
–
of the type [(saturated NHC)2Ag]+BF4 .
1
the H NMR spectrum and at δ = 137.7 ppm in the 13C
NMR spectrum. As expected, the amide proton (CONH)
resonance appeared downfield-shifted at δ = 11.37 ppm in
1
the H NMR spectrum, while the amide carbon (CONH)
resonance appeared at δ = 163.0 ppm in the 13C NMR spec-
trum. The C–O stretching frequency of the amide group
(CONH) appeared at 1697 cm–1 in the IR spectrum.
The silver complex, [{1-isopropyl-3-(N-phenylacetamido)-
imidazol-2-ylidene}2Ag]+Cl– (1b), was prepared by the
reaction of 1-isopropyl-3-(N-phenylacetamido)imidazolium
chloride (1a) with Ag2O in CH2Cl2 at room temperature in
64% yield (Scheme 2). Consistent with the formation of the
silver complex, the (NCHN) resonance of the starting mate-
1
rial was conspicuously absent at δ = 9.97 ppm in the H
NMR spectrum, while a new peak at δ = 179.5 ppm, corre-
sponding to the silver-bound carbene (NCN) resonance,
was observed in the 13C NMR spectrum. The (CONH) res-
onance appeared at δ = 165.0 ppm in the 13C NMR spec-
trum. The carbonyl peak (νCO) of the amide moiety
(CONH) appeared at 1694 cm–1 in the IR spectrum. The
electrospray mass analysis gave a 100% abundance peak at
Figure 1. ORTEP of 1b. The Cl– ion is disordered with Cl1 having
1/3 occupancy and Cl2 having 2/3 occupancy. Selected bond lengths
[Å] and angles [°]: Ag–C(1) 2.085(3), N(1)–C(1) 1.350(4), N(1)–
C(2) 1.379(4), C(1)–Ag–C(1) 180.000(1), N(2)–C(1)–N(1) 104.0(2),
N(2)–C(1)–Ag 124.7(2), N(1)–C(1)–Ag 131.3(2).
Another notable feature of the 1b structure is that the
m/z = 593 corresponding to the cationic [{1-isopropyl-3-(N- two imidazolyl rings surrounding the silver atom are copla-
phenylacetamido)imidazol-2-ylidene}2Ag]+ fragment.
nar. The isopropyl and the N-phenylacetamide substituents
The molecular structure of [{1-isopropyl-3-(N-phenyl- on the imidazole rings are at a trans disposition to each
acetamido)imidazol-2-ylidene}2Ag]+Cl– (1b) was deter- other presumably for steric reasons. The coordination ge-
mined by X-ray diffraction (Figure 1). In the complex, two ometry around the silver atom is linear [ЄCcarb–Ag–Ccarb
[{1-isopropyl-3-(N-phenylacetamido)imidazol-2-ylidene}] angle is 180.0(1)°] and is consistent with the preferred linear
Scheme 2.
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R. B. Sunoj, P. Ghosh et al.
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geometry exhibited by two-coordinate AgI complexes hav- Density Funtional Theory Studies
ing d10 configuration.[46] The chloride anion is disordered
in the structure with Cl2 having 2/3 occupancy while Cl1
Electronic structure calculations using the density func-
has 1/3 occupancy. The Ag–Ccarb bond length of 2.085(3) Å tional theory were performed on the [{1-isopropyl-3-(N-
more closely resembles a single bond, being slightly shorter phenylacetamido)imidazol-2-ylidene}2Ag]+ cationic com-
than the sum of the covalent radii of silver and carbon plex (2b) in order to gain further insights into structure and
(2.111 Å)[47] and also falls well within the observed range bonding. We used the B3LYP/SDD,6-31G* level of theory.
of Ag–Ccarb bond lengths of other analogous complexes Two major conformers differing in energy by about
(Table 1). Furthermore, the Є(N–Ccarb–N) angle of 2.66 kcal/mol were identified.[48] In the lower energy con-
104.0(2)° in 1b is consistent with that observed for other former the imidazole rings were found to remain nearly or-
analogous complexes (Table 1).
thogonal to each other. In an earlier study by Frenking and
co-workers on a series of unfunctionalized (NHC)2Ag+
complexes, the energy difference between such conformers
was reported to be of the order of 5 kcal/mol with only a
negligible barrier to interconversion.[33] A close inspection
of the optimized structure with that obtained through X-
ray crystallography revealed interesting facts. In the experi-
mental structure, where the imidazole rings are coplanar, it
appears that there is an obvious intermolecular hydrogen
bonding interaction involving olefinic hydrogen atoms (C8–
C10) from the imidazole unit with that of the carbonyl O
atom from the functional side-arm of the neighboring NHC
silver molecule.[49] Furthermore, additional intramolecular
stabilization interactions were noticed in the nonplanar
conformer rather than in the planar one (vide infra).
In general, agreement between computed and experimen-
tal geometrical parameters was found to be quite good
(Table 2). Calculated structural parameters for the unbound
carbene, in its singlet ground state, are indicated in Fig-
ure 2. It can be seen that the bond lengths in the imidazole
ring do not undergo any major change, except for the
Є(N–Ccarb–N) angle, which shows a modest widening by
about 1.8°. Since NHCs are known to act as efficient σ-
donors through the carbenic carbon atom, one can expect
only a minimal geometric distortion as predicted by the
computed geometries in Figure 2.
Table 1. Selected metrical data for some [(N-R1-NЈ-R2-imidazol-2-
ylidene)2Ag]+-type complexes.
The σ-donation increases the electron deficiency on the
Ccarb atom and thus facilitates a better NǞCcarb π-do-
nation. We have found an effective delocalization of the ni-
trogen lone pair electrons into the adjacent C–N bond with
the help of natural bond orbital (NBO) analysis. Such delo-
calizations are responsible for shorter C–N distances in
complex 2b compared to that of the unbound carbene.[50]
The computed natural population on C7 (and C42) re-
–
–
[a] Counter anion is AgCl2 . [b] Counter anion is AgBr2 .
Important is the comparison of the structure of 1b with vealed that the pπ atomic orbital is substantially more pop-
a related [(saturated NHC)2Ag]+ counterpart. For example, ulated (1.14 e) in the complex 2b than in the unbound sing-
in an analogous saturated NHC silver complex, bis[(4R,5S)- let ground state (0.83 e).[51] The NPA charges on the imid-
4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H- azole nitrogen atoms were found to be increasingly positive
imidazolin-2-ylidene]silver(I)
tetrafluoroborate,[45]
the upon complexation while the charges on the Ccarb atoms
Є(N–Ccarb–N) angle of 109.0(4)° is larger than the corre- remain unchanged. Similar observations on related systems
sponding value of 104.0(2)° observed in the case of 1b and have been made earlier and independently by Frenking[31]
this is presumably due to the differences in their respective and Scherer.[32]
N-heterocyclic ring geometries. The Ag–Ccarb bond lengths
In order to gain better insight into the nature of the
[2.082(4) and 2.087(4) Å] in bis[(4R,5S)-4,5-diallyl-1,3- chemical bonding we carried out charge decomposition
bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazolin-2- analysis (CDA) using the B3LYP wave function. CDA has
ylidene]silver(I) tetrafluoroborate, however, compare well been known to be a useful tool in analyzing the extent of
with the corresponding values of 2.085(3) Å observed in back-bonding in organometallic systems.[52] The CDA re-
complex 1b.
sults in 2b show a significant CcarbǞAg σ-donation (d =
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A Cationic (N-Heterocyclic carbene)silver Complex
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Table 2. Comparison between selected distances [Å] and angles [°]
in 1b (X-ray structure) and 2b (computed structure).
Analysis of the topological features of electron density
offers valuable information pertaining to the nature of
chemical bonding in molecules. Bader’s theory of “Atoms
In Molecules” (AIM) has widely been applied to examine
interesting bonding situations.[53] Topological properties
such as the density at the bond critical point (ρbc), Lapla-
cian of electron density (ٌ2ρbc) as well as the total energy
density (H) are analyzed. A covalent interaction is charac-
terized by a negative value for ٌ2ρbc as well as H Ͻ 0. The
value of H is widely accepted as a better descriptor in de-
termining the nature of interaction than the ٌ2ρbc value.[54]
Inspection of computed topological features in the present
system, as given in Table 3, reveals that the Ccarb–Ag bond
is covalent. For 2b, values of Laplacian ٌ2ρbc as well as
total energy density Hbc for the Ccarb–Ag bonds were found
to be –0.07 and –4.5 au, respectively. These values suggest
that the Ccarb–Ag bond is more covalent than ionic. Fur-
ther, the computed NPA charges on Ag (+0.49) and Ccarb
(+0.08) are consistent with this observation (Figure 2). All
other bonds such as C–N and C=C of the imidazole ring
were found to be more covalent, as expected. Another no-
ticeable feature emerging from the AIM analysis pertains
to the π-bonds in the imidazole rings. The characteristic
ellipticity associated with π-bonds is clearly reflected in the
computed ε values that are found to be as high as 0.4 for
the C=C bonds. The Ccarb–Ag bond exhibits the lowest
value for ellipticity, again indicating lack of π-bonding in-
teractions. Interestingly, the ε values for the N3–C8 and
N4–C10 (also N38–C43 and N39–C45) bonds suggest a
moderate π-character, presumably arising due to an
NǞCcarb π-donation into the carbene π-type orbital.
Parameter[a]
Exp.
Calcd. Parameter
Exp.
Calcd.
Ag–C1
Ag–C1
N1–C1
N1–C2
N1–C11
N2–C1
N2–C3
N2–C4
N3–C5
N3–C21
C5–O
2.085(3) 2.109 C2–C3
2.085(3) 2.109 C4–C5
1.350(4) 1.356 C11–C12
1.379(4) 1.388 C11–C13
1.502(5) 1.484 C21–C22
1.350(4) 1.363 C21–C26
1.384(4) 1.388 C22–C23
1.445(4) 1.453 C23–C24
1.365(4) 1.361 C24–C25
1.427(4) 1.422 C25–C26
1.206(3) 1.227 Ag–O
Ag–O
1.328(5) 1.357
1.507(4) 1.546
1.473(10) 1.532
1.526(12) 1.532
1.378(6) 1.401
1.388(6) 1.402
1.390(7) 1.396
1.369(12) 1.395
1.326(11) 1.396
1.383(7) 1.392
3.980
3.980
3.262
3.220
C1–Ag–C1
C1–N1–C2
C1–N1–C11
C2–N1–C11
C1–N2–C3
C1–N2–C4
C3–N2–C4
C5–N3–C21
N2–C1–N1
N2–C1–Ag
N1–C1–Ag
C3–C2–N1
C2–C3–N2
N2–C4–C5
180.0(1) 172.5 O–C5–N3
110.9(3) 111.1 O–C5–C4
124.5(3) 124.3 N3–C5–C4
124.4(3) 124.6 C12–C11–N1
111.5(2) 111.1 C12–C11–C13 113.6(6) 113.3
123.3(2) 124.3 N1–C11–C13 106.8(6) 110.4
125.1(2) 124.5 C22–C21–C26 120.9(4) 120.0
125.1(3) 125.6
122.7(3) 120.8
112.2(3) 113.6
111.5(5) 110.7
127.5(3) 129.2 C22–C21–N3
104.0(2) 104.6 C26–C21–N3
122.9(4) 123.1
116.1(4) 116.8
124.7(2) 125.5 C21–C22–C23 117.6(6) 119.1
131.3(2) 128.4 C24–C23–C22 120.7(7) 121.2
107.5(3) 106.9 C25–C24–C23 121.3(6) 119.4
106.2(3) 106.4 C24–C25–C26 120.4(7) 120.2
112.3(2) 112.4 C25–C26–C21 119.0(5) 120.2
N3–C7–C42–N38
0.85
77.94
[a] The calculated structure does not include chloride ions and thus
refers to the cationic complex 2b. The atom numbering is therefore
different from that in the cif file. For optimized coordinates, please
refer to the Supporting Information.
Table 3. Summary of the AIM analysis performed on 2b at the
AIM//B3LYP/SDD,6-31G* level of theory.[a]
Bond
ρbc [e·Å–3]
ٌ2ρbc [e·Å–5]
ε
Hbc
Ag1–C7
C7–N3
C7–N4
0.102
0.320
0.314
0.301
0.299
0.337
0.102
0.320
0.315
0.301
0.300
0.337
0.008
0.003
0.009
0.010
–0.071
0.158
0.150
0.187
0.173
0.243
–0.071
0.158
0.154
0.186
0.177
0.243
–0.007
–0.003
–0.006
–0.007
0.04
0.09
0.06
0.16
0.17
0.39
0.04
0.08
0.04
0.17
0.17
0.39
0.13
0.13
0.50
0.61
–4.49
–14.68
–14.41
–12.52
–12.47
–13.19
–4.50
–14.53
–14.56
–12.28
–12.61
–13.09
–0.30
N3–C8
N4–C10
C8–C10
Ag1–C42
C42–N38
C42–N39
N38–C43
N39–C45
C45–C43
O2···H49
O37···H21
Ag1···O2
Ag1···O37
–0.12
–0.38
–0.40
Figure 2. Calculated key structural parameters of free carbene 2a
and the complex 2b. NPA charges are given in parentheses.
[a] See Figure 3 for atom numbers.
0.656) and a very negligible CcarbǟAg back-donation (b =
As mentioned earlier, the geometry of the lowest energy
0.053). A d/b ratio of 11.4 and 12.4 between Ag and the conformer has a nearly orthogonal disposition of the imid-
two NHC ligands evidently underscores the ability of NHC azole rings. In the optimized geometry, there are two inter-
to function as an effective σ-donor ligand (see Table S3, esting intramolecular hydrogen bonds, rendering additional
Supporting Information). The structural parameters as well stabilization as shown in Figure 3. The first one is between
as population analysis presented in the previous sections the hydrogen atom of the tethered isopropyl group of one
are in concurrence with the CDA result that the back-bond- imidazole ring with the carbonyl group of the other imid-
ing in 2b is not very significant.
azole ring (2.91 Å, ρ = 0.003). The second hydrogen bond-
Eur. J. Inorg. Chem. 2006, 2975–2984
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R. B. Sunoj, P. Ghosh et al.
FULL PAPER
ing interaction is between the methylene hydrogen atom and 10–13, Table 4) showed that the molecular weight of the
the carbonyl group attached to another imidazole ring polymer decreased with an increase in temperature, with the
(2.49 Å, ρ = 0.008). These intramolecular interactions were highest molecular weight observed at 100 °C (13.5·103, En-
identified correctly by the AIM calculations, where the try 10, Table 4). The decrease in molecular weight with the
densities at the bond critical points (ρbc) were found to fall increase in temperature may be caused by thermal depoly-
within the hydrogen bonding range.[55] There are also ad- merization as well as an enhanced transesterification reac-
ditional interactions between the carbonyl oxygen atoms tion occurring at elevated temperatures. In this regard, Liao
from both imidazole rings and the silver atom (3.26 Å, ρ = et al.[56a] and Albertsson and Varma[42] have recently re-
0.01). These interactions together could be responsible for ported a similar decrease in polymer molecular weight with
the near orthogonal orientations of the imidazole rings in temperature, arising from thermal depolymerization and ac-
the gas-phase-optimized geometry.
celerated transesterification reactions.
Table 4. Melt polymerization of -lactide by 1b.
Entry -Lactide/1b Temp. [°C] Time [h] Mw Mw/Mn Conv. [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
50
100
250
500
750
1000
100
100
100
100
100
100
100
160
160
160
160
160
160
160
160
160
100
120
140
180
4
4
4
4
4
4
3
1.5
0.5
4
8.1·103 1.37
12.2·103 1.40
9.2·103 1.41
6.6·103 1.28
5.3·103 1.23
4.9·103 1.22
11.0·103 1.41
11.4·103 1.47
10.6·103 1.29
13.5·103 1.27
11.9·103 1.46
11.2·103 1.36
8.3·103 1.35
90
98
78
46
35
36
79
78
65
84
81
91
94
4
4
4
In order to gain insight into the nature of the active spe-
cies in the catalytic cycle, the thermal stability of [{1-isopro-
pyl-3-(N-phenylacetamido)imidazol-2-ylidene}2Ag]+Cl– (1b)
was studied by a thermogravimetric analysis (TGA) experi-
ment (Figure 4). Compound 1b was found to be stable up
to 180 °C above which drastic mass loss started to occur
presumably caused by the generation of the carbene. Con-
sistent with the TGA results, the attempts to trap the in-
situ generated carbene from the thermal decomposition of
1b with CS2, carried out at a much lower temperature
(60 °C), proved to be unsuccessful.[58]
Figure 3. The B3LYP/SDD,6-31G* optimized geometry of 2b
showing weak intramolecular interactions and the corresponding
densities at the bond critical points (ρbc) computed using the AIM
theory (only selected hydrogen atoms are shown for clarity).
Polymerization Studies
The silver complex [{1-isopropyl-3-(N-phenylacetamido)-
In this regard, it is worth mentioning that Waymouth
imidazol-2-ylidene}2Ag]+Cl– (1b) was found to be an active and Hedrick[28] have recently proposed a mechanism invok-
catalyst for ring-opening polymerization of -lactides car- ing in-situ generation of carbene from similar NHC silver
ried out at elevated temperatures under melt conditions in complexes for the ring-opening polymerization of -lac-
the absence of any solvent.[56] For example, a typical run tides. However, for sterically demanding NHC silver precat-
involved heating of -lactide and [{1-isopropyl-3-(N-phenyl- alysts that were thermally sufficiently stable and did not
acetamido)imidazol-2-ylidene}2Ag]+Cl– (1b) in a sealed ves- dissociate to generate carbenes at the polymerization tem-
sel under vacuum at designated temperatures for stipulated perature, the possibility of the silver complexes directly me-
periods of time. Under these conditions the reaction mix- diating the polymerization was also suggested to exist. A
ture would form a monomer melt in which the polymeriza- similar possibility of silver directly participating in the poly-
tion would occur. The variation of the [M]/[C] ratio (M = merization also exists in the case of 1b and is in agreement
monomer, C = catalyst) at 160 °C showed that the maxi- with the TGA experiment that showed the complex did not
mum molecular weight (Mw = 12.2·103, Entry 2, Table 4) undergo significant mass loss below 180 °C while the poly-
was obtained at a 100:1 monomer/catalyst ratio. The ob- merizations were carried out at a lower temperature range
served polydispersity index (PDI) was in the range 1.22– (100–180 °C). Such direct metal-mediated polymerization
1.47. Consistent with the observed molecular weight distri- would lead to the capping of polymer chain ends with NHC
butions, the time dependence studies (Entries 2 and 7–9, fragments (Scheme 3) and indeed, the characteristic reso-
1
Table 4) showed no correlation of the observed molecular nances of the NHC moieties could be seen in both the H
weight with time thereby ruling out the possibility of a true NMR and 13C NMR spectra of the polymer.[59] The
living polymerization process. The temperature dependence MALDI spectrometric analysis of the polymer also con-
study carried out in the range 100–180 °C[57] (Entries 2 and firms the presence of NHC chain end groups.[60] In this re-
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Eur. J. Inorg. Chem. 2006, 2975–2984

A Cationic (N-Heterocyclic carbene)silver Complex
FULL PAPER
Figure 4. Thermogravimetric analysis of 1b as a function of temperature.
gard it is worth mentioning that a (NHC)YIII complex has
Conclusions
been recently reported to exhibit metal-mediated coordina-
tion insertion polymerization of lactides.[61] However, the
other possibility that minute amounts of carbene, generated
during the thermal decomposition of 1b, are responsible for
the polymerization also cannot be ruled out completely.
Currently, detailed mechanistic studies are underway to es-
tablish the identity of the active species responsible for the
catalysis.
In summary, a new functionalized N-heterocyclic car-
bene ligand precursor, 1-isopropyl-3-(N-phenylacetamido)-
imidazolium chloride (1a), was synthesized by the alky-
lation of 1-iso-proplyimidazole with N-phenyl chloroacet-
amide. The silver complex, [{1-isopropyl-3-(N-phenylacet-
amido)imidazol-2-ylidene}2Ag]+Cl– (1b), was synthesized
by treatment of 1a with Ag2O. Complex 1b has been struc-
turally characterized by X-ray diffraction studies and was
found to effectively initiate polymerization of -lactide at
higher temperatures under solvent-free melt conditions pro-
ducing polylactide polymers with a narrow molecular
weight distribution. Density functional theory studies and
post-wave function analysis were able to establish that the
functionalized NHC was acting as an effective σ-donor.
Weak intramolecular stabilizing interactions were identified
as responsible factors in contributing to the perpendicular
dispositions of the imidazole rings, compared to the copla-
nar arrangement as revealed by the X-ray crystallographic
study.
Experimental Section
General Procedures: All manipulations were carried out using a
combination of a glovebox and standard Schlenk techniques. Sol-
vents were purified and degassed by standard procedures. Ag2O
was purchased from SD-Fine Chemicals (India) and used without
any further purification. -Lactide was purchased from Sigma Ald-
rich (Germany) and was subjected to polymerization without fur-
ther purification. -Lactide was stored at 4 °C as supplied, to avoid
the formation of lactyl lactate. N-Phenyl chloroacetamide[62] and 1-
Scheme 3.
Eur. J. Inorg. Chem. 2006, 2975–2984
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2981

R. B. Sunoj, P. Ghosh et al.
FULL PAPER
isopropylimidazole[63] were synthesized according to literature pro-
washed with hot hexane (ca. 8 mL) and dried under vacuum to give
the product as a brown crystalline solid (2.98 g, 90%). 1H NMR
(400 MHz, CDCl3, 25 °C, TMS): δ = 11.37 (s, 1 H, NH), 9.97 (s, 1
1
cedures. H and 13C{1H} NMR spectra were recorded with a Var-
1
ian 400 MHz NMR spectrometer. H NMR peaks were labeled as
3
singlet (s), doublet (d), triplet (t), and septet (sept). Infrared spectra
were recorded with a Perkin–Elmer Spectrum One FT-IR spec-
trometer. Mass spectrometry measurements were carried out with
a Micromass Q-Tof spectrometer. The thermal analysis was per-
H, NCHN), 7.71 (d, JH,H = 7 Hz, 2 H, o-C6H5), 7.59 (s, 1 H,
3
NCHCHN), 7.28 (s, 1 H, NCHCHN), 7.22 (t, JH,H = 7 Hz, 2 H,
3
m-C6H5), 7.03 (t, JH,H = 7 Hz, 1 H, p-C6H5), 5.60 (s, 2 H, CH2),
3
3
4.63 [sept, JH,H = 6 Hz, 1 H, CH(CH3)2], 1.52 [d, JH,H = 7 Hz, 6
formed in N2 (flow rate: 10 mL/min) with a heating rate of 10 °C/ H, CH(CH3)2] ppm. 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ
min using a Perkin–Elmer Pyris thermal analyzer. X-ray diffraction
data were collected with a Bruker P4 diffractometer equipped with
a SMART CCD detector, and crystal data collection and refine-
ment parameters are summarized in Table 5. The structures were
solved by direct methods and standard difference map techniques,
and were refined by full-matrix least-squares procedures on F2 with
SHELXTL (Version 6.10). CCDC-297479 (for compound 1b) con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The
polymer molecular weights were determined using a Waters GPC
(Waters 2414 RI Detector) with PL-gel, a 5 µ-mixed-D (2×300 mm)
column, with polystyrene standards in chloroform, and a covered
molecular weight range of 140 to 4·105. The MALDI-TOF MS
measurements were performed with a Voyager-DE STR Biospec-
trometry Workstation mass spectrometer. A 19 kV accelerating
voltage was used with pulsed ion extraction (PIE). The positive
ions were detected using the reflection mode (20 kV). A nitrogen
laser (337 nm, 1 ns pulse width operating at 4 Hz) was used to pro-
duce laser desorption and 50 shots were scanned per spectra. The
instrument was calibrated with four standards [Des-Arg 1,
Bradykinn and Angiotensin-1, and ACTH (1-17)] by means of lin-
ear calibration. The sample was prepared with an α-cyanohydroxy
cinnamic acid (CHC) matrix (10 mg /mL). A 1 µL analyte solution
(10 mg/mL) was deposited onto the sample plate (stainless steel)
and allowed to air-dry. Subsequently, a 1 µL matrix solution (50:50
v/v, CHC/acetonitrile) was added to the analyte.
= 163.0 (CO), 137.7 (NCN), 135.8 (ipso-C6H5), 128.4 (o-C6H5),
124.0 (NCHCHN), 123.7 (NCHCHN), 119.8 (p-C6H5), 118.9 (m-
C6H5), 52.9 (CH2), 51.9 [C(CH3)2], 22.6 [C(CH3)2] ppm. IR (KBr
pellet): ν = 1697 (s) (νCO) cm–1. LRMS (ES): m/z (%) = 244 (100)
˜
[NHC ligand]+. HRMS (ES): calcd. for [NHC ligand]+ 244.1450,
found 244.1455.
Synthesis of [{1-Isopropyl-3-(N-phenylacetamido)imidazol-2-yl-
idene}2Ag]+Cl– (1b): The reaction was carried out under exclusion
of light. A mixture of 1a (1.81 g, 6.47 mmol) and Ag2O (0.748 g,
3.23 mmol) in dichloromethane (ca. 25 mL) was stirred at room
temperature for 4 h. The reaction mixture was filtered and the sol-
vent was removed under vacuum to give the product as a light
yellow solid (1.30 g, 64%). 1H NMR (400 MHz, CDCl3, 25 °C,
TMS): δ = 11 (br., 1 H, NH), 7.78 (d, 3JH,H = 8 Hz, 2 H, o-C6H5),
7.29 (s, 1 H, NCHCHN), 7.20 (t, 3JH,H = 8 Hz, 2 H, m-C6H5), 7.02
3
(t, JH,H = 8 Hz, 1 H, p-C6H5), 6.99 (s, 1 H, NCHCHN), 5.27 (s,
2 H, CH2), 4.60 [sept, 3JH,H = 7 Hz, 1 H, CH(CH3)2], 1.42 [d, 3JH,H
= 7 Hz, 6 H, CH(CH3)2] ppm. 13C{1H} NMR (CDCl3, 100 MHz,
25 °C): δ = 179.5 (NCN), 165.0 (CO), 138.0 (ipso-C6H5), 128.1 (o-
C6H5), 123.5 (NCHCHN), 122.6 (NCHCHN), 119.5 (p-C6H5),
116.4 (m-C6H5), 54.4 [C(CH3)2], 53.5 (CH2), 23.2 [C(CH3)2] ppm.
IR (KBr pellet): ν = 1694 (s) (νCO) cm–1. LRMS (ES): m/z (%) =
˜
593 (100) [(NHC ligand)2Ag]+.
Computational Methods: The density functional theory calculations
were performed on the [{1-isopropyl-3-(N-phenylacetamido)imid-
azol-2-ylidene}2Ag]+ cationic complex (2b) using the Gaussian98
suite of quantum chemical programs.[64] The Becke three-parameter
exchange functional in conjunction with the Lee–Yang–Parr corre-
lation functional (B3LYP) was employed in this study.[65,66] The
Stuttgart-Dresden effective core potential (ECP), representing 19
core electrons, along with valence basis sets (SDD) was used for
the silver atom.[67] All other atoms were treated with the 6-31G(d)
basis set.[68] We will be designating the level of theory hereafter as
B3LYP/SDD,6-31G*. Inspection of the metal–ligand donor–ac-
ceptor interactions was carried out using charge decomposition
analysis (CDA).[69] CDA is a valuable tool for analyzing the inter-
actions between molecular fragments in a quantitative fashion,
with an emphasis on the electron donation.[70] The CDA calcula-
tions were performed with the program AOMIX using the B3LYP/
SDD,6-31G* wave function.[71] Natural bond orbital analysis was
performed using the NBO3.1 program as implemented in the
Gaussian98 package.[72] Second-order perturbation energy analysis,
natural charges,[73] and bond orders[74] for 2b were calculated using
the B3LYP wave function. The nature of the chemical bonding was
further explored with Bader’s “Atoms in Molecules” approach[75]
using the AIM2000 program.[76]
Table 5. X-ray crystallographic data for 1b.
Lattice
rhombohedral
C28H34AgClN6O2
629.93
Empirical formula
Formula mass
Space group
a [Å]
b [Å]
c [Å]
¯
R3
18.2066(8)
18.2066(8)
28.102(2)
90
α [°]
β [°]
90
120
8067.3(8)
9
γ [°]
V [Å3]
Z
Temperature [K]
Radiation, λ [Å]
ρ(calcd.) [g·cm–3]
µ(Mo-Kα) [mm–1]
θmax [°]
No. of data
No. of parameters
R1
243(2)
0.71073
1.167
0.665
28.30
4167
175
0.0461
0.1326
1.030
wR2
GOF
Polymerization Experiments: Typical polymerization experiment: -
Lactide (1.01 g, 7.01 mmol) and 1b (0.044 g, 0.070 mmol) were
charged in an ampoule inside a glove box. The ampoule was taken
out and put under vacuum at 50 °C for 15 min, after which the
ampoule was sealed under vacuum and heated at 160 °C for 4 h.
Under these conditions, the reaction mixture turned into a mono-
mer melt in which polymerization occurred. Subsequently, the
molten reactive polymer mixture was cooled by submerging the se-
Synthesis of 1-Isopropyl-3-(N-phenylacetamido)imidazolium Chlo-
ride (1a): N-Phenyl chloroacetamide (2.01 g, 11.8 mmol) and 1-iso-
propylimidazole (1.30 g, 11.8 mmol) were added to toluene (ca.
10 mL) and heated at 140 °C for 12 h during which a brown pre-
cipitate was formed. The precipitate was collected by filtration and
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Eur. J. Inorg. Chem. 2006, 2975–2984

A Cationic (N-Heterocyclic carbene)silver Complex
FULL PAPER
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Received: March 9, 2006
Published Online: June 12, 2006
2984
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2975–2984