N-heterocyclic carbene complexes of Zn(II): Synthesis, X-ray structures and reactivity

Article abstract of DOI:10.1016/j.jorganchem.2004.03.039

The synthesis of six novel zinc (II) mono(N-heterocyclic carbene) complexes is described. 1,3-Bis(mesityl)-imidazol-2-ylidene was reacted with the zinc salts ZnX₂ (X=Cl, CH₃COO, PhCOO, and PhCH₂COO) to yield the corresponding monomeric Zn-NHC complex ZnCl₂(NHC)(THF) (1) and dimeric [Zn(OOCCH₃)₂(NHC)]₂ (2), [Zn(OOCPh)₂(NHC)]₂ (3), [Zn(OOCCH₂Ph)₂(NHC)]₂ (4) (NHC=1,3-bis(mesityl)-imidazol-2-ylidene). Reaction of 1 with 2 equivalents of silver trifluoromethanesulfonate yielded monomeric Zn(O₃SCF₃)₂(NHC)(THF) (5), reaction of 1 with sodium {[R(+)-α-2-(1-phenyl-ethylimino)-methyl]- phenolate} yielded monomeric ZnCl(OC₆H₄- 2-CH=N(CHPhCH₃)(NHC) (6). Compounds 1, 4-6 were structurally characterized by X-ray analysis. Selected compounds were investigated for their activity in the copolymerization of carbon dioxide with cyclohexene oxide as well as in the ring-opening polymerization of cyclohexene oxide and E-caprolactone.

Full text of DOI:10.1016/j.jorganchem.2004.03.039

Journal of Organometallic Chemistry 689 (2004) 2123–2130
www.elsevier.com/locate/jorganchem
N-heterocyclic carbene complexes of Zn(II): synthesis,
X-ray structures and reactivity q
a
Dongren Wang , Klaus Wurst , Michael R. Buchmeiser
b
a,
*
a
Institut f€ur Analytische Chemie und Radiochemie, Leopold-Franzens-Universit€at Innsbruck, Innrain 52 a, A-6020 Innsbruck, Austria
b
Institut f€ur Allgemeine, Anorganische und Theoretische Chemie, Leopold-Franzens-Universit€at Innsbruck, Innrain 52 a, A-6020 Innsbruck, Austria
Received 9 December 2003; accepted 8 March 2004
Abstract
The synthesis of six novel zinc (II) mono(N-heterocyclic carbene) complexes is described. 1,3-Bis(mesityl)-imidazol-2-ylidene was
reacted with the zinc salts ZnX2 (X ¼ Cl, CH3COO, PhCOO, and PhCH2COO) to yield the corresponding monomeric Zn–NHC
complex ZnCl2(NHC)(THF) (1) and dimeric [Zn(OOCCH3)2(NHC)]2 (2), [Zn(OOCPh)2(NHC)]2 (3), [Zn(OOCCH2Ph)2(NHC)]2
(4) (NHC ¼ 1,3-bis(mesityl)-imidazol-2-ylidene). Reaction of 1 with 2 equivalents of silver trifluoromethanesulfonate yielded mo-
nomeric Zn(O3SCF3)2(NHC)(THF) (5), reaction of 1 with sodium {[R(+)-a-2-(1-phenyl-ethylimino)-methyl]-phenolate} yielded
monomeric ZnCl(OC6H4-2-CH@N(CHPhCH3)(NHC) (6). Compounds 1, 4–6 were structurally characterized by X-ray analysis.
Selected compounds were investigated for their activity in the copolymerization of carbon dioxide with cyclohexene oxide as well as
in the ring-opening polymerization of cyclohexene oxide and e-caprolactone.
Ó 2004 Published by Elsevier B.V.
Keywords: N-heterocyclic carbenes; Zinc; Copolymerization; Ring-opening polymerization
1. Introduction
37] and others [38] reported on Ru-NHC and Pd-NHC-
catalyzed polymerization reactions such as the cyclop-
olymerization of 1,6-heptadiynes and the oxidative
carbonylation reaction of bisphenol A, respectively.
The first NHC complexes of Zn(II), Zn(1,3-di(1-ad-
amantyl)imidazol-2-ylidene)Et2 and Zn(1,3-dimesityl-
imidazol-2-ylidene)Et2, were reported by Arduengo et
al. [39] In this contribution, we report on the synthesis of
various Zn(1,3-dimesitylimidazol-2-ylidene)-complexes
and their use in ring-opening polymerization and co-
polymerization, respectively.
N-Heterocyclic carbenes (NHCs) [1–6] are currently
among the most intensively investigated compounds
since the corresponding transition metal complexes were
found to possess good stabilities and high activities in
various catalytic processes [7–14]. In this context, Ti-
[15], Cr- [16,17], and Ni-NHC compounds [18] have
been reported to be active in ethylene polymerization
and oligomerization, respectively. Pd- and Ru-NHC
complexes belong to the most active catalysts for C–C
coupling reactions such as Heck couplings [19–21] and
metathesis-based reactions [22–30] and may also be used
for catalytic hydrogenation [31]. Important enough,
some of these reactions can be carried out in an enan-
tioselective way [32–34]. Quite recently, our group [35–
2. Results and discussion
2.1. Synthesis and X-ray structures of compounds 1–6
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.jorganchem.2004.03.039.
* Corresponding author. Tel.: +43-512-507-5184; fax: +43-512-507-
2677.
1,3-Dimesitylimidazol-2-ylidene was prepared from
1,3-dimesitylimidazolium chloride and potassium t-
butanolate in THF [40]. Reaction with anhydrous ZnCl2
in THF afforded ZnCl2(NHC)(THF) (1) (NHC ¼ 1,3-
bis(mesityl)-imidazol-2-ylidene) in 80% yield (Scheme 1).
E-mail address: michael.r.buchmeiser@uibk.ac.at (M.R. Buchme-
iser).
0022-328X/$ - see front matter Ó 2004 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2004.03.039

2124
D. Wang et al. / Journal of Organometallic Chemistry 689 (2004) 2123–2130
1/2
N
N
Zn
O
O
O
O
Ph
Ph
Ph
Ph
O
O
N
O
N
O
O
Zn
Zn
N
Cl
Ph
N
N
3
6
ONa
N
Zn(OOCPh)2
Ph
6a
ZnCl2
THF
N
N
N
N
Cl Zn
Cl
O
1
Zn(OOCCH3)2
2 CF3SO3Ag
THF
N
1/2
N
N
Zn
N
Zn
O
O
O
CH3
CH3
O
H3C
F3CSO3
O3SCF3
H3C
O
O
O
O
5
O
Zn
N
N
2
1/2
N
N
Zn
O
O
O
Ph
Ph
Ph
Ph
Zn(PhCH2COO)2
O
O
O
O
O
Zn
N
N
4
Scheme 1. Synthesis of Zn-complexes 1–6.
1 (Fig. 1) crystallizes in the monoclinic space group.
Relevant structural data, bond angles, and distances are
summarized in Tables 1 and 2.
The distance Zn(1)–C(1) is 203.6(2) pm, the distance
Zn(1)–O(1) is 210.60(15) pm, indicative for a compara-
bly weak coordination of THF. In view of the
rather poor propensity of chloride to form bridged
species with Zn, a monomeric structure was obtained
(see Scheme 1).
Reaction of 1,3-dimesitylimidazol-2-ylidene with an-
hydrous Zn(II) acetate and Zn(II) benzoate, prepared by
the reaction of ZnEt2 and PhCOOH in THF and used
without further purification, resulted in the formation
of both [Zn(OOCCH3)2(NHC)]2 (2) and [Zn(OOCPh)2-
(NHC)]2 (3) in 60% yield. In analogy, [Zn(OOCCH2-
Ph)2(NHC)]2 (4) (Fig. 2) was prepared via reaction of
1,3-dimesitylimidazol-2-ylidene with Zn(II) phenylace-
tate in 84% yield. Mass-spectroscopic investigations

D. Wang et al. / Journal of Organometallic Chemistry 689 (2004) 2123–2130
2125
Table 2
Bond lengths (pm) and angles (°) for 1
Zn(1)–C(1)
Zn(1)–O(1)
Zn(1)–Cl(2)
Zn(1)–Cl(1)
203.6(2)
210.60(15)
223.38(6)
223.44(7)
C(1)–Zn(1)–O(1)
C(1)–Zn(1)–Cl(2)
O(1)–Zn(1)–Cl(2)
C(1)–Zn(1)–Cl(1)
O(1)–Zn(1)–Cl(1)
Cl(2)–Zn(1)–Cl(1)
108.84(7)
115.13(6)
99.26(5)
115.32(6)
100.27(5)
115.21(3)
Fig. 1. X-ray structure of compound 1.
Table 1
Crystal data and structure refinement for 1 and 4
1
4
Molecular formula
C25H32Cl2N2OZn ꢀ C74H76N4O8Zn2 ꢀ
0.5 THF
548.85
1.333 THF
1376.27
Cubic
Fig. 2. X-ray structure of compound 4.
Formula weight
Crystal system
Space group
a (pm)
b (pm)
c (pm)
Monoclinic
C2=c (no. 15)
3002.09(2)
1250.77(2)
1542.35(3)
90
ꢀ
Pn3 (no. 201)
ꢀ
THF. 4 crystallizes in the cubic space group Pn3.
Compound 4 is a dimeric Zn complex with four l2-
bridging phenylacetates. This results in a square-pyra-
midal arrangement of the ligands around the 5-fold
coordinated zinc. As a consequence of the reduced trans-
effect of the carboxylates, the distance Zn(1)C(1) is
slightly enlarged to 207.5(6) pm. Not unexpected, the
Zn(1)–O distances to all four oxygen atoms are almost
identical, i.e. 206.1(3) and 207(5) pm, respectively.
Further relevant structural data, bond angles, and dis-
tances are summarized in Tables 1 and 3.
2229.79(5)
2229.79(7)
2229.79(7)
90
a (°)
b (°)
c (°)
98.982(1)
90
90
90
Volume (nm3)
Z
5.72039(15)
8
233(2)
11.0864(6)
6
Temperature (K)
Dcalc (Mg/m3)
Absorption
coefficient (mmꢁ1
Color, habit
233(2)
1.237
0.707
1.275
1.068
)
Colorless prism
Colorless prism
1980
Reaction of 1 with 2 equivalents of CF3SO3Ag in
THF afforded monomeric Zn(O3SCF3)2(NHC)(THF)
(5) (Fig. 3) in 83% yield. 5 crystallizes in the triclinic
Number of reflections 4735
with with I > 2rðIÞ
Goodness-of-fit on F 2 1.045
1.220
R1 ¼ 0:0552
xR2 ¼ 0:1335
R indices I > 2rðIÞ
R1 ¼ 0:0354
ꢀ
space group P1. Not unexpected, 5 shows the strongest
xR2 ¼ 0:0955
trans-effect induced by the strongly electron-withdraw-
ing trifluorosulfonates, resulting in a Zn(1)–C(1) dis-
tance of 199.6(3) pm. A strong binding was also
observed for THF, expressed by a comparably short
Zn(1)–O(7) distance of 199.83(18) pm. Further relevant
structural data, bond angles, and distances are summa-
rized in Tables 4 and 5.
confirmed the dimeric structure of these three com-
pounds. While in the case of 4 the ion peak of the dimer
minus an acetate group can be clearly identified, com-
pounds 2 and 3 are analyzed as the corresponding di-
mers with one carboxylate missing and one carboxylate
substituted by the matrix, i.e. 3-nitrobenzylic alcohol.
Crystals suitable for X-ray analysis were obtained from
Finally, ZnCl(OC6H4-2-CH@N(CHPhCH3)(NHC)
(6) (Fig. 4) was accessible by reaction of 1 with sodium

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D. Wang et al. / Journal of Organometallic Chemistry 689 (2004) 2123–2130
Table 3
Bond lengths (pm) and angles (°) for 4
Table 4
Crystal data and structure refinement for 5 and 6
Zn(1)–O(2)#1
Zn(1)–O(2)#2
Zn(1)–C(1)
206.1(3)
206.1(3)
207.5(6)
207.6(3)
207.6(3)
5
6
Molecular formula
C27H32F6N2O7S2Zn ꢀ C36H38ClN3OZn ꢀ
0.5 THF
776.11
Triclinic
CH2Cl2
714.44
Zn(1)–O(1)
Zn(1)–O(1)#3
Formula weight
Crystal system
Space group
a (pm)
b (pm)
c (pm)
Orthorhombic
P212121 (no. 19)
1404.18(2)
1577.88(4)
1586.03(4)
90
ꢀ
P1 (no. 2)
O(2)#1–Zn(1)–O(2)#2
O(2)#1–Zn(1)–C(1)
O(2)#2–Zn(1)–C(1)
O(2)#1–Zn(1)–O(1)
O(2)#2–Zn(1)–O(1)
C(1)–Zn(1)–O(1)
154.46(18)
102.77(9)
102.77(9)
86.66(14)
86.07(14)
106.67(9)
86.07(14)
86.66(14)
106.67(9)
146.66(17)
1003.5(2)
1345.97(3)
1422.84(3)
100.200(1)
94.435(1)
97.774(1)
1.8640(4)
2
a (°)
b (°)
c (°)
90
90
O(2)#1–Zn(1)–O(1)#3
O(2)#2–Zn(1)–O(1)#3
C(1)–Zn(1)–O(1)#3
O(1)–Zn(1)–O(1)#3
Volume (nm3)
Z
3.51405(10)
4
233(2)
Temperature (K)
Dcalc (Mg/m3)
Absorption
coefficient (mmꢁ1
Color, habit
Number of
reflections with
I > 2rðIÞ
233(2)
1.447
0.849
1.350
0.960
)
Colorless prism
5096
Colorless prism
3626
Goodness-of-fit on
F 2
1.053
1.060
R indices I > 2rðIÞ
R1 ¼ 0:0393
R1 ¼ 0:0558
xR2 ¼ 0:1022
xR2 ¼ 0:1508
Table 5
Bond lengths (pm) and angles (°) for 5
Zn(1)–O(1)
Zn(1)–O(4)
Zn(1)–C(1)
Zn(1)–O(7)
196.6(2)
197.3(2)
199.6(3)
199.83(18)
O(1)–Zn(1)–O(4)
O(1)–Zn(1)–C(1)
O(4)-Zn(1)–C(1)
O(1)–Zn(1)–O(7)
O(4)-Zn(1)–O(7)
C(1)–Zn(1)–O(7)
102.39(10)
110.95(11)
117.18(9)
103.64(9)
101.07(9)
119.46(10)
Fig. 3. X-ray structure of compound 5.
ization (ROP) of oxiranes and for the copolymerization
of oxiranes with CO2, respectively [41–57]. In a first
attempt to use the new complexes for polymerization,
compounds 1, 2, and 5 were used in the ROP of pro-
pylene oxide. No catalytic activity was observed at all.
With e-caprolactone, 1 was found to possess extremely
low activity, the turn-over number (TON) was as low as
4. An interesting finding was that of all new compounds,
5 was the only one which permitted the ROP of cyclo-
{[R(+)-a-2-(1-phenyl-ethylimino)-methyl]-phenolate} (6a)
in 70% yield. Again, a monomeric rather than a dimeric
1
product was obtained. According to 13C and H NMR,
6 exists in two isomeric forms (73:27), of which one
crystallizes in the orthorhombic space group P212121. A
comparable large Zn(1)–C(1) distance of 204.8(7) pm
was found, indicative for the electron-donating charac-
ter of the ligands. The Zn(1)–Cl(1) distance is almost
identical to the one found in compound 1 (223.7(2)°
versus 223.38(6)° and 223.44(7)°, respectively). Further
relevant structural data, bond angles, and distances are
summarized in Tables 4 and 6.
hexene oxide in virtually quantitative yields.
A
TON>500 was observed. Nevertheless, the molecular
weight (Mn) of poly(cyclohexene oxide) was 3300 g/mol
with a polydispersity (PDI) of 1.72. These data suggest
that there was some significant chain transfer during
polymerization. However, these findings indicate that 5
containing the strongest electron-withdrawing substitu-
ents (trifluorosulfonates) had the strongest Lewis acidity
and therefore best bonding properties for the monomer.
2.2. Catalytic activity of compounds 1,2, 4–6
Binary and ternary catalytic systems based on Zn(II)
are well-known catalysts for the ring-opening polymer-

D. Wang et al. / Journal of Organometallic Chemistry 689 (2004) 2123–2130
2127
chlorine or better various carboxylate groups as realized
in compounds 1, 2, 4, and 6, was believed to provide
better conditions. Consequently, these compounds were
used as catalysts in the copolymerization of cyclohexene
oxide with CO2 applying a pressure of 65–68 bar.
Polymers consisting of 100% poly(carbonate) were ob-
tained with compounds 1, 2, and 6, while compound 4
was virtually inactive. With the exception of poly(car-
bonate) prepared by the action of 1 (Mn ¼ 13900,
PDI ¼ 3.49), molecular weights (Mn) were in the range of
1800–2400 g/mol, with PDIs of 1.08 and 1.6, respec-
tively. In view of the dimeric structure of 4, its inactivity
was not surprising at all, though the dimeric compound
2 showed some (low) polymerization activity indicating
the presence of small amounts of monomeric species. In
summary, it should be emphasized that activities of
those complexes that allowed the copolymerization of
oxiran with CO2 were generally low, making this class
of compounds less attractive for these purposes. Thus,
typical TONs were <25, thus requiring at least 4 mol%
of catalyst in order to reach full conversion.
Fig. 4. X-ray structure of compound 6.
Table 6
Bond lengths (pm) and angles (°) for 6
3. Experimental
Zn(1)–O(1)
Zn(1)-N(3)
Zn(1)–C(1)
Zn(1)–Cl(1)
196.3(5)
204.1(5)
204.8(7)
223.7(2)
All manipulations were performed under a nitrogen
atmosphere in a glove box (MBraun LabMaster 130) or
by standard Schlenk techniques. Purchased starting
materials were used without any further purification.
Pentane and tetrahydrofurane (THF) were distilled from
sodium benzophenone ketyl under nitrogen. Methylene
chloride was distilled from calcium hydride.
O(1)–Zn(1)-N(3)
O(1)–Zn(1)–C(1)
N(3)-Zn(1)–C(1)
O(1)–Zn(1)–Cl(1)
N(3)–Zn(1)–Cl(1)
C(1)–Zn(1)–Cl(1)
94.7(2)
108.0(3)
115.9(2)
110.8(2)
110.64(17)
114.9(2)
NMR data were obtained at 300.13 MHz for proton
and 75.74 MHz for carbon in the indicated solvent at
25 °C on a Bruker Spectrospin 300 and are listed in parts
per million downfield from tetramethylsilane for proton
and carbon. Coupling constants are listed in Hz. IR
spectra were recorded on a Bruker Vector 22 using ATR
technology. Elemental analyses were carried out at the
Institute of Physical Chemistry, University of Vienna
and at the Mikroanalytisches Labor, Anorganisch-
In addtion, a monomeric structure appears favorable.
When subject to the copolymerization of cyclohexene
oxide with CO2, a polymer (Mn ¼ 2700, PDI ¼ 1.19) was
formed in quantitative yield and with high TONs ¼ 450.
Unfortunately, it consisted of only 30% of the desired
copolymer and 70% of pure poly(cyclohexene oxide)
(Table 7). Therefore, coordination/insertion of the ep-
oxide into the growing polyether chain must be favored
over CO2 insertion by roughly a factor of 3.
€
Chemisches Institut, TU Munchen, Germany. MS
spectra (FAB) were recorded on a Finnigan MAT 95S
using FAB ionization (Cs-gun: 20 kV, 3 lA, matrix: m-
nitrobenzylic alcohol).
Consequently, a more balanced reactivity was re-
quired. Substitution of both triflate groups in 5 by
Table 7
Summary of polymerization results for the copolymerization of CO2 with cyclohexene oxide (CHO)
Category
(mg)
CHO
(ml)
CO2
(bar)
Temperature
(°C)
Time
(h)
Mn
PDI
Poly(carbonate)
(%)
Yield
(mg)
(g/mol)
5 (76)
5 (76)
1 (45)
2 (36)
4 (65)
6 (64)
5
5
5
5
5
5
65
55
65
68
65
65
80
r.t.
80
80
80
80
24
6
2700
–
1.19
–
30
0
6000
0
24
24
24
24
13 900
1800
–
3.49
1.08
–
100
100
0
100
200
0
2400
1.60
100
100

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D. Wang et al. / Journal of Organometallic Chemistry 689 (2004) 2123–2130
3.1. ZnCl2(NHC) (1)
J ¼ 7:60), 7.17 (2H, s), 6.65 (4H, s), 2.11 (12H, s), 1.79
(6H, s); 13C NMR (CDCl3) d 177.07, 173.55, 139.85,
135.04, 133.57, 130.71, 129.06, 127.15, 123.10, 20.61,
17.40. Anal. Calc. for C35H34N2O4Zn: C, 68.68; H, 5.60;
N, 4.58. Found: C, 68.69; H, 5.96; N, 4.33%. MS (FAB):
m=z calc. for C35H34N2O4Zn: 610.18, found
m=z ¼ 1130:8 (C63H64N5O7Zn2, 20%).
ZnCl2 (136.3 mg, 1.000 mmol) was dissolved in 15 ml
of THF and a solution of 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene (303.4 mg, 1.000 mmol) in 3 ml of
THF was added slowly. The mixture was stirred at room
temperature for 12 h. The resulting solution was then
concentrated to ꢂ5 ml. After the addition of 20 ml of
pentane, a white solid precipitated, which was filtered off
and washed three times with pentane, and dried in va-
cuo. Yield: 350.2 mg (80%). A crystal of 1 suitable for X-
ray analysis was obtained by cooling a concentrated
THF solution to )40 °C for several days. FT-IR (ATR-
3.4. [Zn(OCOCH2Ph)2(NHC)]2 (4)
Zn(OCOCH2Ph)2 (100.0 mg, 0.298 mmol, prepared
by the reaction of ZnEt2 and PhCH2COOH in THF and
used without further purification) was suspended in 10
ml of THF, then a solution of 1,3-bis(2,4,6-trimethyl-
phenyl)-imidazol-2-ylidene (90.5 mg, 0.299 mmol) in 2
ml of THF was added. After 20 min of stirring, the
precipitate had dissolved. The mixture was stirred
overnight, and was then concentrated to 5 ml. Ten
milliliters of pentane was added, resulting in the for-
mation of a white solid, which was filtered off, washed
with pentane, and dried in vacuo. Yield: 160 mg (84%).
Crystals of 4 were obtained by slowly cooling a con-
centrated THF solution of the complex. FT-IR (ATR-
mode, cmꢁ1): 3025m, 2916m, 1586s, 1487m, 1366w,
850m; 1H NMR (CDCl3) d 7.32–6.63 (16H, m), 3.15
(4H, s), 2.35–1.74 (18H, m); 13C NMR (THF-d8) d
182.51, 143.83, 141.03, 140.38, 140.32, 134.89, 134.62,
134.06, 132.68, 130.81, 130.38, 128.66, 48.34, 25.78,
22.25; MS calc. for C74H76N4O8Zn2 1276.42, found
1162.56 ([(M+H)-PhCH2CO]þ), 1145.57 ([(M+H)-
PhCH2COOH]þ).
mode, cmꢁ1): 3030m, 2920m, 1600m, 1550s, 1482w; H
1
NMR (CDCl3) d 7.02 (2H, s), 6.89 (4H, s), 2.30 (6H, s),
2.03 (12H, s). 13C NMR (CDCl3) d 172.98, 139.22,
135.45, 133.79, 129.06, 122.90, 21.11, 17.76. Anal. Calc.
for C21H24N2Cl2Zn: C, 57.23; H, 5.49; N, 6.39. Found:
C, 56.99; H, 5.39; N, 6.25%.
3.2. [Zn(OAc)2(NHC)]2 (2)
Zn(OAc)2 (183.5 mg, 1.000 mmol) was suspended in
15 ml of THF and a solution of 1,3-bis(2,4,6-trimeth-
ylphenyl)-imidazol-2-ylidene (304.4 mg, 1.000 mmol) in
3 ml THF was added dropwise. Then the mixture was
stirred overnight at room temperature. The solution was
then concentrated to ꢂ5 ml, ca. 10 ml of pentane were
added resulting in the formation of a white precipitate.
It was filtered off, washed three times with pentane, and
dried in vacuo. Yield: 292 mg (60%). FT-IR (ATR-
mode, cmꢁ1): 3050m, 2920m, 1580s, 1486s; H NMR
1
(CDCl3) d 7.19 (2H, s), 7.00 (4H, s), 2.35 (6H, s), 2.10
(12H, s), 1.72 (6H, br); 13C NMR (CDCl3) d 179.15,
173.46, 139.66, 135.25, 134.06, 129.02, 123.17, 22.65,
21.02, 17.24. Anal. Calc. for C25H30N2O4Zn: C, 61.54;
H, 6.20; N, 5.74. Found: C, 61.32; H, 6.01; N, 5.73%.
MS (FAB): m=z calc. for C25H30N2O4Zn: 486.15, found
m=z ¼ 1007:06 (C53H60N5O7Zn2, 73.4%).
3.5. Zn(OSO2CF3)(NHC) (5)
A solution of AgOSO2CF3 (358.3 mg, 1.394 mmol) in
3 ml of THF was added slowly to a solution of 1 (309.4
mg, 0.6973 mmol) in 7 ml of THF. A white solid pre-
cipitated immediately. The mixture was stirred for 2 h at
room temperature, then it was filtered. The filtrate was
concentrated to ꢂ5 ml, after the addition of 20 ml
pentane, a white solid precipitated, which was filtered
off, washed three times with pentane and dried in vacuo.
Yield: 430 mg (83%). Crystals of 5 were obtained by
slowly diffusion of pentane to a concentrated THF so-
lution of the complex. IR (ATR-mode, cmꢁ1): 3121m,
3.3. [Zn(OCOPh)2(NHC)]2 (3)
Zn(OCOPh)2 (44.0 mg, 0.298 mmol, prepared by the
reaction of ZnEt2 and PhCOOH in THF and used
without further purification) was suspended in 10 ml of
THF, then a solution of 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene (43.5 mg, 0.299 mmol) in 2 ml of
THF was added. After 20 min of stirring, the precipitate
had dissolved. The mixture was stirred overnight, and
was then concentrated to ca. 2 ml. Ten milliliters of
pentane was added and the mixture was cooled to )40
°C. A white solid formed after 2 days, which was filtered
off, washed with pentane, and dried in vacuo. Yield: 52.5
mg (60%). FT-IR (ATR-mode, cmꢁ1): 3025m, 2914m,
1636s, 1585w, 1367s, 850s. 1H NMR (CDCl3) d 7.77
(4H, t, J ¼ 6:93), 7.37 (2H, t, J ¼ 6:93), 7.26 (4H, t,
1
2921m, 1610m, 1486m, 1320s, 1201w, 1009s; H NMR
(CDCl3) d 7.24 (2H, s), 7.03 (4H, s), 3.63 (4H, t), 2.33
(6H, s), 2.09 (12H, s), 1.75 (4H, m); 13C NMR (CDCl3) d
166.85, 140.86, 134.81, 133.00, 130.0, 129.66, 125.38,
121.16, 116.95, 112.74, 69.64, 25.19, 21.03, 17.22.
3.6. {[R(+)-a-2-(1-phenylethylimino)-methyl]-phenol}
(6a)
Salicyaldehyde (3.05 g, 25.0 mmol) and R(+)-a-
methylbenzylamine (3.03 g, 25.0 mmol) were dissolved

D. Wang et al. / Journal of Organometallic Chemistry 689 (2004) 2123–2130
2129
in a mixture of ethanol and toluene (6:4). The solution
3.9. Ring-opening polymerization of e-caprolactone
was refluxed for 2 h. Upon cooling to 0 °C, a yellow
solid precipitated from the reaction mixture. The solid
was filtered, washed with cold pentane and then dried in
vacuo. Yield: 5.00 g (90%). FT-IR (ATR-mode, cmꢁ1):
1 (30 mg, 58 lmol) was dissolved in 5 ml of toluene and
e-caprolactone (1.1 ml, 9.6 mmol) was added. The result-
ingsolutionwasplacedinsideaSchlenktube andheatedto
50 °C for 24 h. After reaction, the mixture was cooled to
room temperature and added to acidic methanol. Twenty
five milligrams of product (2.2 %) was obtained.
1
2913m, 1620s, 1493w, 1211s, 802s; H NMR (CDCl3) d
13.53 (1H, s), 8.40 (1H, s), 7.40–7.2 (7H), 6.94 (1H, d,
J ¼ 8:22), 6.85 (1H, t, J ¼ 7:31), 4.54 (1H, q, J ¼ 6:85),
1.62 (3H, d, J ¼ 6:86); 13C NMR (CDCl3) d 163.33,
160.95, 143.70, 132.15, 131.29, 128.54, 127.12, 126.25,
118.70, 118.50, 116.82, 68.36, 24.84.
3.10. ROP of cyclohexene oxide
Seventy six milligrams of 5 and 5 ml of CHO were
dissolved in 5 ml of toluene. The resulting solution was
added to Schlenk and heated to 80 °C for 6 h. Then the
mixture was cooled to room temperature and added to
acidic methanol; the solid that precipitated was filtered
off and dried in vacuo. Yield: 5.2 g (100%), Mn, 3300,
PDI, 1.72. 1H NMR (CDCl3) d 4.66 (br), 3.16 (br),
2.14ꢂ1.39 (m); 13C NMR (CDCl3) d 79.0–77.9 (m),
76.0–74.6 (m), 33.4–18.5 (m).
3.7. ZnCl(NHC){[R(+)-a-2-(1-phenylethylimino)-me-
thyl]-phenolate} (6)
A solution of sodium {[R(+)-a-2-(1-phenylethyli-
mino)-methyl]-phenolate} (108 mg, 437 mmol, obtained
by the reaction of 6a and NaH) in 3 ml of THF was added
to a solution of 1 (192mg, 437mmol) in 10 ml of THF.
The mixture was stirred at room temperature overnight
and the precipitate that formed was removed by filtration
through celite. The filtrate was concentrated to 5 ml, then
pentane (20 ml) was added. A white solid precipitated, it
was filtered off, washed three times with pentane and
dried in vacuo. Yield: 170 mg (70%). Crystals of 6 were
obtained by slowly diffusion of pentane to a concentrated
CH2Cl2 solution of the complex. FT-IR (ATR-mode,
3.11. X-ray measurement and structure determination of
1, 4, 5 and 6
Data collection was performed on a Nonius Kappa
CCD equipped with graphite-monochromatized Mo
ꢁ
Ka-radiation (k ¼ 0:71073 A) and a nominal crystal to
1
cmꢁ1): 3050m, 2920m, 1605s, 1550w, 1483s; H NMR
area detector distance of 36 mm. Intensities were inte-
grated using DENZO and scaled with SCALEPACK
[58]. Several scans in / and x direction were made to
increase the number of redundant reflections, which
were averaged in the refinement cycles. This procedure
replaces in a good approximation an empirical absorp-
tion correction. The structures were solved with direct
methods SHELXS86 and refined against F 2 SHELX97
(CDCl3) d 7.38 (H, s), 7.35–6.02 (15H, m), 3.87 (H, q,
J ¼ 7:31), 2.29 (6H, d, J ¼ 3:65), 2.15 (6H, d, J ¼ 5:48),
2.06 (6H, d, J ¼ 3:2), 1.55 (3H, d, J ¼ 6:85); 13C NMR
(CDCl3) d 176.5, 170.8, 170.3, 168.0, 143.5, 141.1, 139.5,
139.4, 135.6 135.2, 135.0, 134.5, 133.7, 133.4, 129.4,
129.3, 129.1, 129.0, 128.6, 128.3, 128.0, 127.6, 127.3,
123.4, 123.3, 123.3, 118.5, 118.3, 112.5, 112.4, 68.0, 65.5,
63.0, 25.6, 24.4, 22.1, 21.2, 17.9, 17.8, 17.5; Anal. Calc.
for C36H38N3OClZn: C, 68.68; H, 6.08; N, 6.67; Cl, 5.63.
Found: C, 68.57; H, 6.27; N, 6.31; Cl, 5.37%.
P
2
[59]. The function minimized was ½wðFo2 ꢁ Fc2Þ ] with
2
the weight defined as wꢁ1 ¼ ½r2ðFo2Þ þ ðxPÞ þ yP] and
P ¼ ðFo2 þ 2Fc2Þ=3. All non-hydrogen atoms were refined
with anisotropic displacement parameters. In the struc-
ture of 4, there was a 1:1 positional disorder of the
C6H5-moiety in the benzyl group with one overlying C-
atom (C16). The solvent THF is positioned near a 3-fold
axis of rotation. In the structure of 5, there was a 1:1
disorder of THF into 2 nearly overlaying positions.
3.8. Copolymerization of cyclohexene oxide with CO2
A 0.10 mmol amount of active catalyst was dissolved
in 5 ml (51.0 mmol) of cyclohexene oxide. The resulting
solution was added through the injection port to a pre-
dried autoclave and the reactor was pressurized to 50
bar with CO2. The reactor was then heated to 80 °C,
raising the pressure to ca. 65 bar, for the time indicated
in Table 1. After cooling and release of the pressure, the
reaction mixture was diluted with CH2Cl2 and washed
with 0.1 N HCl. The organic phase was separated, dried
over Na2SO4, and then concentrated to ꢂ3 ml. Finally,
20 ml of methanol were added to the solution to induce
4. Supporting information
The crystallographic data for 1, 4, 5 and 6 have been
deposited with the CCDC Nos. 225967–225970, and on
the Cambridge Crystallographic Data Centre. The co-
ordinates can be obtained, on request, from the Direc-
tor, Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-
336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
1
precipitation. H NMR (CDCl3) d 4.57 (br, H–CO2–),
3.31 (br, H–CO–), 2.04–1.17 (m, –CH2–); 13C NMR
(CDCl3) d 153.69, 153.23, 153.06, 152.76, 79.22–78.15
(m), 76.13–74.85 (m), 33.08–27.30 (m), 24.38–20.76 (m).

2130
D. Wang et al. / Journal of Organometallic Chemistry 689 (2004) 2123–2130
[26] M. Scholl, S. Ding, C.W. Lee, R.H. Grubbs, Org. Lett. 1 (1999)
953.
5. Summary
[27] M. Scholl, T.M. Trnka, J.P. Morgan, R.H. Grubbs, Tetrahedron
Lett. 40 (1999) 2247.
A series of new NHC complexes of Zn(II) have been
prepared. These compounds are accessible via straight-
forward synthetic routes and possess low activity yet
high selectivity in the copolymerization of cyclohexene
oxide with CO2.
[28] C.W. Bielawski, R.H. Grubbs, Angew. Chem. 112 (2000) 3025.
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