Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for the ring-opening polymerization of epoxides

Article abstract of DOI:10.1016/j.jorganchem.2011.02.015

A set of multidentate ligands have been synthesized and used to stabilize the putative highly electrophilic zinc species initiating ring-opening polymerization (ROP) of cyclohexene oxide (CHO) and propylene oxide (PO). Reaction of the bidentate C₂

Full text of DOI:10.1016/j.jorganchem.2011.02.015

Journal of Organometallic Chemistry 696 (2011) 1691e1697
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for
the ring-opening polymerization of epoxides
*
Nicolas Merle, Karl W. Törnroos, Vidar R. Jensen, Erwan Le Roux
Department of Chemistry, University of Bergen, Allégaten 41, N 5007 Bergen, Norway
a r t i c l e i n f o
a b s t r a c t
Article history:
A set of multidentate ligands have been synthesized and used to stabilize the putative highly electro-
philic zinc species initiating ring-opening polymerization (ROP) of cyclohexene oxide (CHO) and
propylene oxide (PO). Reaction of the bidentate C2-chiral bis(oxazoline) ligand (R2,R3BOX: R2 ¼ (4S)-tBu,
R3 ¼ H (a); R2 ¼ (4S)-Ph, R3 ¼ H (b); R2 ¼ (4R)-Ph, R3 ¼ (5S)-Ph (c)) with Zn(R1)2 (R1 ¼ Et (1), Me (2)) led
to the heteroleptic three-coordinate complexes (R2,R3BOX)ZnR1, 1aec and 2a, which were isolated in 92
e96% yield. Next, two pyridinyl-functionalized N-heterocyclic carbene (NHC) ligands have been designed
and synthesized: the 1,3-bis(2-pyridylmethyl)imidazolinium salt (d) and the protected NHC adduct 2-
(2,3,4,5,6-pentafluorophenyl)-1,3-bis(2-pyridylmethyl)imidazolidine (e). The reaction of ligands d and e
with ZnEt2 led directly to the formation of (NHC)ZnEt(Cl) 3d complex with ethane elimination and the
adduct (NHCeC6F5(H))ZnEt2 4e, respectively, in high yield. In situ combinations of selected complexes
1aec, 3d and 4e with B(C6ꢀF51)3 (1 or 2 equivalents) give active systems for ROP, with high productivity
Received 2 November 2010
Received in revised form
13 January 2011
Accepted 10 February 2011
Keywords:
Zinc
Bis(Oxazoline)
N-Heterocyclic carbene
Ring-opening polymerization
Epoxides
(3.3e5.9 106 gpolym. molZn
h
ꢀ1) and high molecular weight (Mn up to 132 103 g molꢀ1) for CHO
polymerization. Although the in situ B(C6F5)3-activated zinc species were not isolated, the sterically
demanding BOX ligands (1c > 1b > 1a) and functionalized NHC ligands seem to enhance the stability of
highly electrophilic zinc complexes over ligand redistribution, allowing a better control of the cationic
ROP as reflected particularly for 3d and 4e complexes by their respective efficiency (42e88%).
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
ligands to enhance the stabilization of the highly reactive electro-
philic zinc species. Here we report the synthesis of zinc alkyl
The ion pairs formed by cationic zinc complexes have recently
attracted considerable interests due to their strong Lewis acidity
[1]. For example, cationic zinc complexes generated in situ or iso-
lated efficiently catalyze the intramolecular hydroamination/cycli-
zation reactions [2e11] and the ROP of propylene oxide (PO),
complexes with bis(oxazoline) and N-heterocyclic carbene with
pending hemilabile pyridine arms, their reactions with B(C6F5)3 as
cationizing reagents, and their reactivity in ring-opening poly-
merizations (ROP) of epoxides.
cycloxene oxide (CHO) and e-caprolactone [12e15]. Generally, the
2. Results and discussion
organometallic chemistry of transition metal alkyls in the presence
of cationizing reagents (e.g. B(C6F5)3 [16e19], [Ph3C][B(C6F5)4] [20],
and [HNRR02][B(C6F5)4] [21]) proceed preferentially by alkyl
abstraction [17,18,22e26], while main group metals such as zinc,
aluminum and gallium alkyls with B(C6F5)3 tend to provide alkyl/
C6F5 exchange [27e36]. This ligand exchange may be totally sup-
pressed, either by using neutral (or anionic) N,N0-chelating ligands
or in a coordinating solvent (Et2O), generating stable four- and even
three-coordinate zinc alkyl or silylamide cations [12e15,37e40].
Toward the generation of the intriguing cationic zinc complexes,
we were motivated to use bidentate and tridentate with N-donor
2.1. Synthesis of bidentate N,N0-chelating BOX-zinc complexes
We have recently reported the synthesis of alkyl zinc(II)
complexes (R2,R3BOX)ZnEt 1a, 1b and 1c, which are obtained
straightforwardly from (R2,R3BOX)H and ZnEt2 (Scheme 1) [41].
Complex 2a was obtained in similar manner in high yield (96%) by
slow addition of 1 equivalent of neutral ligand (tBu,HBOX)H to
ZnMe2 in hexane at room temperature (Scheme 1).
Complex 2a was characterized by DRIFT, 1H and 13C{1H} NMR
spectroscopies. TheNMR spectra showthe expected setof signals for
the BOX ligand and for the ZnMe group which is shifted in the high
field region (1H NMR:
d
ꢀ0.11 ppm and 13C{1H} NMR: ꢀ10.5 ppm) in
* Corresponding author. Tel.: þ47 555 89854; fax: þ47 555 89490.
E-mail address: Erwan.LeRoux@kj.uib.no (E. Le Roux).
comparison with the starting material ZnMe2 (1H NMR:
d 0.51 ppm
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.02.015

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N. Merle et al. / Journal of Organometallic Chemistry 696 (2011) 1691e1697
R1
R2
R3
H
O
O
O
O
1a
1b
1c
2a
Et (4S)-tBu
Et (4S)-Ph
3 h/rt
R3
R3
R3
R3
+
Zn(R1)2
N
N
N
N
H
toluene
- R1-H
Zn
R1
R2
R2,R3BOX)H:
R2
R2
R2
Et (4R)-Ph (5S)-Ph
Me (4S)-tBu
H
(
R2 = (4S)-tBu, R3 = H (a)
R
R
2 = (4S)-Ph, R3 = H (b)
2 = (4R)-Ph, R3 = (5S)-Ph (c)
Scheme 1. Synthesis of bis(oxazoline) alkyl zinc complexes 1aec and 2a.
Systems based on cationic Zn(II) are well-known catalysts for the
ROP of epoxides. In a first attempt, the complexes 1aec and 2a were
used and activated by 1 equivalent of B(C6F5)3, increasing the Lewis
acidityofthezinccenter, undermildconditionsforthepolymerization
of CHO. All complexes were found to be highly active with nearly
quantitative yields after 1 min of reaction time (Table 1). These poly-
merization reactions were extremely exothermic with temperatures
rising rapidly above 100 ꢁC. For all cationic zinc complexes generated
in situ, productivities reaching 5500e5900 kgpolym. molZnꢀ1 hꢀ1 and
turn-over frequencies (TOFs) > 56,000 hꢀ1 were observed. The poly-
mers produced by the putative BOX divalent cationic zinc complexes
were of reasonably high weight-average molecular weight
(86 < Mw < 108 ꢂ 103 g molꢀ1), with a broad polydispersity
(PDI
¼
2.9e5.7). The low number-average molecular weights
measured (Mn < 37,000 g molꢀ1) in comparison with the calculated
Mn (98,140 g molꢀ1) suggest significant ligand redistribution (assim-
ilated to chain transfer here), most likely leading to the homoleptic
BOXeZn complexes and Zn(OPþ)2 species which could furthermore
decompose into [Zn(OPþ)2]n polymeric aggregates as depicted in
Scheme 2. This ligand redistribution in BOXeZn systems, which has
been previously demonstrated in the presence of donor ligands or
solvents [41], seems to decrease with the bulkiness of BOX ligands.
This observed trend (1c > 1b > 1a) indicates that the bulkier BOX
ligands are better at “stabilizing” in situ the highly electrophilic zinc
complexes generated and thus also at enhancing the cationic poly-
Fig. 1. Molecular structure of (tBu,HBOX)ZnMe (2a). Selected bond lengths (Å) and
angles (ꢁ) for 2a: Zn(1)eC(8) 1.9543(11), Zn(1)eN(1) 1.9676(9), Zn(1)eN(2) 1.9628(9),
N(1)eC(3) 1.3206(14), N(2)eC(1) 1.3248(14), C(1)eC(2) 1.3889(16), C(3)eC(2) 1.3923
(16), N(1)eZn(1)eN(2) 93.13(4), N(1)eZn(1)eC(8) 131.49(5), N(2)eZn(1)eC(8) 132.34
(5), C(1)eC(2)eC(3) 122.88(10), C(2)eZn(1)eC(8) 177.74(5).
and 13C{1H} NMR: ꢀ4.2 ppm) [42]. The structure of 2a was
confirmed by single-crystal X-ray diffraction and selected bond
lengths and angles are given in Fig. 1. Compound 2a crystallizes in
the chiral tetragonal space group P41212. The structure determined
for 2a shows that the zinc metal center adopts a slightly distorted
trigonal geometry with a six-membered ring almost coplanar with
the Zn(1) atom which is only displaced a) by 0.022(2) Å from the
plane defined by N(1), N(2) and C(2) with a C(2)eZneC(8) angle
close to 180ꢁ (177.74(5)ꢁ), and b) with the sum of angles of the
ZneN2C core atoms close to 360ꢁ (sum 359.96(5)ꢁ). Moreover, both
oxazolinyl rings are slightly twisted as seen from the torsion angles
of :C(3)eN(1)eC(5)eC(4) ¼ 17.43(11)ꢁ and :C(1)eN(2)eC(7)eC
(6) ¼ 17.52(11)ꢁ. The ZneC distance and ZneN distances are in the
expected range of anionic bidentate N,N0-chelating zinc methyl
merization control (1c: Mn
¼
37,000
g
molꢀ1 and 1a:
Mn ¼ 15,000 g molꢀ1). Note that the ligand redistribution itself might
not be thewhole reason explaining the low molecular weightand that
sidereactions suchasdeprotonation/chaintransferand/orback-biting
(giving macrocyclic ethers) phenomena can also occur during the
polymerization, albeit rather limited considering the high reactivity of
the CHO monomerdue to its strained 3-membered ring. Nevertheless,
the observed apparent ligands dependency on the productivity could
therefore also arise from the more or less efficient reaction of B(C6F5)3
with the Zn-alkyl, or the formation of tight ionpairs, leading to a lower
concentration of highly electrophilic zinc initiator in solution.
compounds and comparable tob-diketoiminate, anilido-aldimine or
Preliminary investigations into the polymerization of propylene
oxide catalyzed by the B(C6F5)3-activated complexes 1aec revealed
good productivity under mild conditions (Table 1). The same trend is
aminotroponiminate ZnMe compounds with a trigonal geometry:
(Zn(1)eC(8) 1.9543(11) Å and (Zn(1)eN(1) 1.9676(9) Å and Zn(1)eN
(2) 1.9628(9) Å) [2e5,9,43,44].
Table 1
Results from polymerization reactions using 1aec and 2a complexes.
TOF (hꢀ1
)
Productivity (kgpolym. molZn hꢀ1
)
Mw (ꢂ103)
Mnc (ꢂ103)
PDId
ꢀ1
c
Initiator
Monomer
M/Cat ratio
Yield (%)
a
a
1a/B(C6F5)3
1b/B(C6F5)3
1c/B(C6F5)3
2a/B(C6F5)3
1a/B(C6F5)3
CHO
CHO
CHO
CHO
PO
1000
1000
1000
1000
1000
1000
1000
94
100
100
96
7
56,312
60,121
60,121
57,520
86
5526
5900
5900
5645
8.4
86
104
108
86
16.4
19.9
18.2
15
28
37
15
2.0
2.6
5.7
5.5
3.6
2.9
5.7
8.2
7.7
3.2
a
a
b
b
1b/B(C6F5)3
1c/B(C6F5)3
PO
PO
9
25
103
300
10.1
29.4
b
a
Polymerization procedure: 21.2
Polymerization procedure: 21.2
m
m
mol pre-catalyst, 5 mL toluene, 21.2
mol pre-catalyst, 5 mL toluene, 21.2
m
m
mol B(C6F5)3, 21.2 mmol CHO; 1 min at T0 ¼ 30 ꢁC.
mol B(C6F5)3, 21.2 mmol PO; 30 min at T0 ¼ 30 ꢁC.
b
c
Determined by means of size exclusion chromatography (SEC). Mw ¼ weight-average molecular weight; Mn ¼ number-average molecular weight in g molꢀ1
Polydispersity index ¼ Mw/Mn.
.
d

N. Merle et al. / Journal of Organometallic Chemistry 696 (2011) 1691e1697
1693
R3
R3
R3
R2
R2
O
O
O
R2
Zn
R2
O
O
P+
P+
P+
N
Zn
N
N
N
N
Zn
O
+
Zn O
2
O
2
2
N
n
O
R2
R2
R3
R3
R3
Scheme 2. Illustration of ligand redistribution with BOXeZn systems during the ROP.
N
N
Cl
N
N
N
HC(OEt)3
NH4Cl
ZnEt2
- Et-H
N
N
Zn
reflux
N
Cl
d
3d
NH
HN
N
N
F
F
F
O
N
N
N
N
N
N
Δ
X
F
ZnEt2
F
H
C6F5
N
N
H
C6F5
N
N
Zn
reflux
- H-C6F5
Zn
4e
Scheme 3. Synthesis of ligands d and e, and their reactivity with ZnEt2.
N
N
e
observed (1c > 1b > 1a) for the polymerization of PO as was
previously noted for the polymerization of CHO. The GPC data of the
poly(propylene oxide) produced indicate an important amount of
oligomers along with low molecular weight polymers, suggesting
a substantial degradation of the cationic zinc species over time along
with presumably side reactions such as deprotonation, inter- and
intra-molecular (back-biting) chain transfer. Note that the observed
weight-averagemolecular weights (Mw ¼ 15e20,000 g molꢀ1) show
a higher formation of polymers than those reported for the same
polymerization reaction initiated by cationic zinc and aluminum
complexes (<3000 g molꢀ1) [12e15,45,46].
functionalized by two pyridines. The latter are known to be poor
Lewis bases and thus may be presumed to readily generate a vacant
site.
So far, 1,3-bis(2-pyridylmethyl)imidazolidinylidene transition
metal complexes (with W, Pd, Au, Ru) have been synthesized only
by post-functionalization or indirect methods, and the synthesis
and characterization of isolated 1,3-bis(2-pyridylmethyl)imidazo-
linium salts have so far not been reported [50,51]. Ligand d was
prepared in high yield by a condensation reaction of N,N0-bis(2-
pyridylmethyl)-ethane-1,2-diamine with an excess of triethyl
orthoformate in the presence of ammonium chloride (Scheme 3).
The 1H and 13C{1H} NMR spectra of d show characteristic peaks of
the imidazolinium methyne group at 9.76 and 160.0 ppm, respec-
tively, confirming the cyclization. The proton resonance of methyl
pyridyl and the bridging methylic groups, appearing each as
All attempts to synthesize and isolate the BOX cationic zinc
complexes failed and NMR-scale reactions with 1c and 1 equivalent
of B(C6F5)3 in benzene-d6 led to the formation of indistinct
complexes. The 19F NMR spectrum corroborates the presence of
several cationic and neutral zinc complexes in solution, with ion
pair [(R2,R3BOX)Zn]þ[EtB(C6F5)3]ꢀ and
(
R2,R3BOX)Zn(C6F5)/EtB
(C6F5)2 as major species [31]. The lack of stabilization by the BOX
ligands in these highly electrophilic zinc systems is presumably
responsible for the presence of several zinc species in solution
which also seems to be a major drawback regarding the control of
the polymerization.
2.2. Synthesis of tridentate N,C,N0 chelating NHC zinc complexes
In order to stabilize the active cationic species the use of a strong
s
-donor with potentially labile pending arms was investigated. The
ability of N-heterocyclic carbenes to stabilize a wide range of metal
complexes combined with the fact that the number of examples of
functionalized NHC derivatives is rapidly growing renders this class
of ligands highly interesting for the present study [47e49]. Herein,
Fig. 2. Molecular structure of the 1,3-bis(2-pyridylmethyl)imidazolium chloride salt
(d). Selected bond lengths (Å) and angles (ꢁ) for d: N(1)eC(1) 1.3131(15), N(2)eC(1)
1.3114(16), N(1)eC(2) 1.4733(15), N(1)eC(4) 1.4563(15), N(2)eC(3) 1.4815(16), N(2)eC
(10) 1.4507(16), N(1)eC(1)eN(2) 113.27(11), C(1)eN(1)eC(4) 125.15(10), C(1)eN(2)eC
(10) 125.30(11), C(1)eN(1)eC(2) 110.29(10), C(1)eN(2)eC(3) 110.02(10).
we report the synthesis and coordination of
a NHC ligand

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N. Merle et al. / Journal of Organometallic Chemistry 696 (2011) 1691e1697
Table 2
Results from polymerization reactions using 3d and 4e complexes.
ꢀ1
c
Initiator
Monomer
Yield (%)
TOF (hꢀ1
)
Productivity (kgpolym. molZn hꢀ1
)
Mwc (ꢂ103)
Mn (ꢂ103)
PDId
a
3d/B(C6F5)3
4e/B(C6F5)3
4e/2 B(C6F5)3
3d/B(C6F5)3
CHO
CHO
CHO
PO
57
74
100
43
34,038
44,143
60,121
345
3340
4332
5690
33.8
387
176
77
132
82
9
2.9
2.1
8.3
ee
a
a
ee
ee
b
a
Polymerization procedure: 21.2
m
mol pre-catalyst, 5 mL toluene, 21.2
m
mol or 42.4
m
mol B(C6F5)3 (1 or 2 equiv), 21.2 mmol CHO ([CHO]0/[cat]0 ¼ 1000:1); 1 min at
T0 ¼ 30 ꢁC.
b
c
Polymerization procedure: 21.2
m
mol pre-catalyst, 5 mL toluene, 21.2
m
mol B(C6F5)3, 14.3 mmol PO ([PO]0/[cat]0 ¼ 675:1); 30 min at T0 ¼ 30 ꢁC.
Determined by means of size exclusion chromatography (SEC). Mw ¼ weight-average molecular weight; Mn ¼ number-average molecular weight in g molꢀ1
.
d
e
Polydispersity index Mw/Mn.
Not determined.
a singlet at 4.99 and 4.01 ppm respectively, highlights the C2v
geometry. Ligand d was further characterized by X-ray crystallog-
raphy and crystallizes as a well-separated ion pair as found in its
reported unsaturated imidazolium analogs (Fig. 2) [52]. The shorter
distances between N(1)eC(1) 1.3131(15) and C(1)eN(2) 1.3114
(16) Å in comparison to N(1)eC(2) 1.4733(15) and N(2)eC(3) 1.4815
(16) Å indicate a delocalization between N1eC1eN2 atoms.
With the aim to synthesize a (NHC)ZnEt2 complex, repeated
efforts to generate the free carbene prior to complexation using
various bases such as KN(SiMe3)2, KOtBu, nBuLi at low temperature
and short reaction time led to a complicated mixture. Such reac-
tivity is not surprising for this imidazolinium salt with its electro-
philic pyridinylmethyl moieties in the presence of the nucleophilic
carbene which tends to initiate the 1,2 migration rearrangement or
enetetramines formation [53e58]. Attempts to generate a silver
complex (from Ag2O, Ag2CO3) were also thwarted by the presence
of pyridinyl groups which can lead to a mixture of mono-, di- and
polymeric compounds in solution as reported for similar bi- and tri-
dentate NHC ligands [59e66]. In contrast, the imidazolinium salt
reacts readily with ZnEt2 in toluene to give (NHC)ZnEt(Cl) 3d
complex with ethane elimination (Scheme 3). The disappearance of
the imidazolinium CH resonance in the 1H NMR spectrum with
a concomitant downfield shift to 206.7 ppm in the 13C{1H} NMR
spectrum reveals the coordination of the carbene to the metal.
Surprisingly, both pyridinylmethyl and methylenic signals of the
NHC ligand appear as a singlet (at 4.58 and 2.75 ppm, respectively),
suggesting a highly fluxional process arising from the inherent
weak interactions between the pyridine groups and zinc center
[67]. In addition, the 1H and 13C{1H} NMR resonances of the Cipso
from the pyridine ring (8.65 and 155.8 ppm, respectively) are only
slightly shifted downfield, corroborating a weak coordination of the
pyridine groups.
ZnEt2 and starting from complex 4e, failed and led to unidentified
mixtures of products.
Complexes 3d and 4e activated with B(C6F5)3 (ratio 1:1) effi-
ciently catalyze the polymerization of CHO under the aforemen-
tioned conditions (Table 2). The polymerization of CHO proceeds
more slowly (polymer yield with 3d: 57%; 4e: 74% after 1 min)
than with the cationized BOX-zinc systems, but the prodꢀu1ctivity
remains consistently high (3.3 and 4.3 ꢂ 106 gpolym. molZn
h
ꢀ1).
The polymers produced by 3d and 4e possess high molecular
weights (>175 000 g molꢀ1) with a moderately narrow poly-
dispersity (Mw/Mn > 2.9). Interestingly, the multidentate N-donor
ligands of the B(C6F5)3-activated complexes 3d and 4e enhance the
efficiency (I*) of the polymerization of CHO, with an I* corre-
sponding to 42% and 88%, respectively (I* ¼ Mn(calculated)/
Mn(measured) ꢂ 100), albeit to some extent at the expense of
polymer yields. These results suggest that the “NHC” with pending
hemilabile pyridine arms around the metal centers are more prone
to stabilize the activated zinc species during the polymerization.
Even though related cationic zinc species have previously been
reported such as [(DAD)ZnMe]þ[B(C6F5)4]ꢀ (DAD ¼ diazadiene,
((2,6-iPr2Ar)N]C(Me))2), or to a lesser extent [(Et2O)3ZnX]þ[B
(C6F5)4]ꢀ (X ¼ Et, N(SiMe3)2) ion pairs [12,14] present nearly the
same analogy with a common putative initiator as 4e/B(C6F5)3, it
seems that the hemilabile pyridine groups in 4e system suppress
partially the ligand redistribution. However, when 2 equivalents of
B(C6F5)3 are used to activate the 4e complex, the gain in produc-
tivity is enhanced (ꢂ1.4), but the control over ligand exchange is
lost as previously observed with the BOX-systems or in these Cp-
systems: [CpmesZn(TMEDA)]þ[EtB(C6F5)3]ꢀ and CppyrZn(TMEDA)
Et/B(C6F5)3 (Cpmes
C4H4NSiMe2C5H5, TMEDA
amine) [13].
¼
3,5-Me2C6H3CH2CMe2C5H5, Cppyr ¼ cyclo-
¼
N,N,N0,N0-tetramethylethylenedi-
In order to form the free carbene in situ, we adapted the method
developed by Waymouth and Hedrik, which does not need the
presence of a base but in which the free carbene can be generated
by thermal deprotection of pentafluorobenzene NHC adducts [68].
Thus, the protected adduct of 2-(2,3,4,5,6-pentafluorophenyl)-1,3-
bis(2-pyridylmethyl)imidazolidine (e) was synthesized in high
yield by condensation of N,N0-bis(2-pyridylmethyl)-ethane-1,2-
diamine with pentafluoro-benzaldehyde catalyzed by acetic acid.
The methyl pyridyl protons now appear as two sets of doublets
centered at 3.87/3.81 ppm and the methylenic bridging protons
resonate as a double doublet of doublets centered at 3.32/2.82 ppm,
revealing the loss of C2v symmetry. The complex (NHCeC6F5(H))
ZnEt2 4e is formed in high yield upon contacting the NHC adduct e
with ZnEt2 in toluene at room temperature (Scheme 3). Except for
the appearance of the ethyl peaks at 1.57 and 0.39 ppm, no major
changes in the 1H and 13C{1H} spectra were observed, demon-
strating the integrity of the ligand. Unfortunately, the thermal
activation of the pentafluorobenzene NHC adduct e, involving
temperatures ranging from 30 to 80 ꢁC, both prior to reaction with
The formation of poly(propylene oxide) by complex 4e upon
activation with 1 or 2 equivalents of B(C6F5)3 was found to be
negligible (ꢃ5% yield) at room temperature. However, polymeri-
zation of PO could be achieved in moderate yield (43%) and
productivity after 30 min by 3d activated with 1 equivalent of B
(C6F5)3 at room temperature.
3. Conclusions
In summary, we have successfully prepared and structurally
characterized neutral heteroleptic zinc alkyls complexes containing
either bis(oxazoline), pyridinyl-functionalized NHC and its penta-
fluorobenzene adduct. The stabilization effect on the B(C6F5)3-
activated zinc species supported by the multidentate N-donor
ligands operates most likely more efficiently with bulkier BOX and
pyridinyl-functionalized NHC ligands, albeit not isolatable.
Although the well-establish ring-opening and propagating cationic
species are responsible for chain growth using the putative highly
electrophilic zinc species as initiator, it seems that ligands with

N. Merle et al. / Journal of Organometallic Chemistry 696 (2011) 1691e1697
1695
hemilabile N-donors pendant arms are less prone to ligand redis-
tribution and allow a better control over the molecular weights
during cationic polymerization.
conventional calibration and was dissolved in THF (1.153 mg mLꢀ1
)
and PS-280K was used as control experiment (2.458 mg mLꢀ1).
Sample solutions (z1.0 mg mLꢀ1 in THF) were filtered through
a syringe filter WhatmannÒ (0.45
mm pore size) prior to injection.
4. Experimental
Chromatographic separation of polyisoprene samples was per-
formed at a column temperature of 40 ꢁC with a flow-rate of
1 mL minꢀ1 as well as the measurement of the RI-, UV-, LALS- and
RALLS-responses. Rinsing of the sample syringe before and
4.1. General procedures
All operations were performed with rigorous exclusion of air and
water, using standard Schlenk, high-vacuum and glovebox tech-
niques (MB Braun MB200B-G; <1 ppm O2, <1 ppm H2O).
Dichloromethane, hexane and toluene were purified by using Grubbs
columns (MB Braun Solvent Purification System 800). Benzene-d6,
toluene-d8, and chloroform-d (99.96% atom% D) were obtained from
Aldrich, dried over sodium or CaH2, vacuum transferred, degassed
and filtered prior to use. 2,20-Methylenebis[(4S)-4-tert-butyl-
2-oxazoline] (a), 2,20-methylenebis[(4S)-4-phenyl-2-oxazoline] (b),
2,20-methylenebis[(4R,5S)-4,5-diphenyl-2-oxazoline] (c), ethane-
1,2-diamine (99%), 2-pyridinecarboxaldehyde (99%), NaBH4 (98.5%),
pentafluoro-benzaldehyde (98%), triethyl orthoformate (98%), NH4Cl
(99%), acetic acid (ACS grade), anhydrous methanol (99.8%), ZnMe2
(1 M solution in heptane) and ZnEt2 (1 M in hexane) were purchased
from Aldrich and used as received. Tris(pentafluorophenyl)boron was
purchased from Boulder Scientific Company and used as received.
N,N0-Bis(2-pyridylmethyl)-ethane-1,2-diamine [69] and complexes
1aec [41] were synthesized according to the literature. Cyclohexene
oxide (CHO) and propylene oxide (PO) were purchased from Aldrich
and were distilled from CaH2 under vacuum following three
freezeepumpethaw cycles and stored at ꢀ37 ꢁC in a glovebox.
The NMR spectra of air and moisture sensitive compounds were
recorded using J. Young valve NMR tubes at 298 K on a Bruker-
AVANCE-DMX400 spectrometer (5 mm BB, 1H: 400.13 MHz; 13C:
100.62 MHz) and a Bruker-BIOSPIN-AV500 and AV600 (5 mm BBO,
1H: 500.13 MHz; 13C: 125.77 MHz and 5 mm triple resonance
inverse CryoProbe, 1H: 600.13 MHz; 13C: 150.91 MHz, respectively).
1H and 13C shifts are referenced to internal solvent resonances and
reported in parts per million relative to TMS. IR spectra were
recorded on a Nicolet FT-IR Protégé 460 spectrometer with a DRIFT
collector (Spectra-TechÔ Collector IIÔ from Thermo Scientific), and
the samples were prepared in a glovebox and mixed in a KBr
powder matrix before loading to the micro-cup. This cup is
mounted into the well-sealed DRIFT dome (equipped with KBr
windows). The spectra were averaged over 256 scans; the resolu-
tion was ꢄ4 cmꢀ1. Elemental analyses of C, H and N were performed
on an Elementar Vario EL III.
after injection of every sample performed with 300
phase.
ml of mobile
4.2. Ligand syntheses
4.2.1. 1,3-Bis(2-pyridylmethyl)imidazolium chloride salt (d)
In a distillation apparatus N,N0-bis(2-pyridylmethyl)-ethane-
1,2-diamine (8.44 g, 35.1 mmol) and NH4Cl (2.00 g, 37.4 mmol)
were dissolved in triethyl orthoformate (15.00 mL, 90.2 mmol) and
the resulting solution stirred and heated to 110 ꢁC until the
evolution of EtOH ceased. After this time, the solvent was removed
in vacuo to leave a brown solid, which was dissolved in CH2Cl2 and
filtered through celite. The remaining solution was concentrated
and layered with Et2O to give colorless crystals of d. Yield: 8.05 g
(79%). 1H NMR (400.13 MHz, CDCl3, 25 ꢁC):
d 9.76 (s, 1H, NeCH]N);
8.57 (dd, 2H, J ¼ 5.0 Hz, 1.5 Hz, C5H py); 7.74 (ddd, 2H, J ¼ 7.8, 7.7,
1.5 Hz, C3H py); 7.55 (dd, 2H, J ¼ 7.7, 0.7 Hz, C2H py); 7.30 (ddd, 2H,
J ¼ 7.8, 5.0, 0.7 Hz, C4H py); 4.99 (s, 4H, NeCH2-py); 4.01 (s, 4H,
NeCH2eCH2eN). 13C{1H} NMR (100.62 MHz, CDCl3, 25 ꢁC):
d 160.0
(s, NeCH]N); 152.4 (C2 py); 149.5 (C6 py); 137.2 (C4 py); 123.3,
123.0 (C3 py, C5 py); 52.6 (s, NeCH2-py), 48.6 (s, NeCH2eCH2eN).
Anal. Calcd for C15H17ClN4 (288.78): C, 62.39; H, 5.93; N, 19.40%.
Found: C, 61.17; H, 3.95; N, 12.57%.
4.2.2. 2-(2,3,4,5,6-Pentafluorophenyl)-1,3-bis(2-pyridylmethyl)
imidazolidine (e)
N,N0-Bis(2-pyridylmethyl)-ethane-1,2-diamine (1.50 g, 6.28 mmol)
and pentafluoro-benzaldehyde (1.23 g, 6.28 mmol) were dissolved in
2 mL of acetic acid and stirred at room temperature for 4 h. The solvent
was removed in vacuo and the remaining solid was neutralized with
an aqueous solution of Na2CO3. The aqueous solution was then
extracted with CH2Cl2 and dried over MgSO4. The volatiles were
removed in vacuo to leave a brown oil. Yield: 1.72 g (65%). 1H NMR
(400.13 MHz, CDCl3, 25 ꢁC):
d
8.41 (d, 2H, J ¼ 5.0 Hz, C6H py); 7.54 (dt,
2H, J ¼ 7.6, 1.5, C3H py); 7.29 (d, 2H, J ¼ 7.6 Hz, C4H py); 7.06 (dd, 2H,
J ¼ 7.6, 5.0 Hz, C5H py); 4.81 (s, 1H, CH(C6F5)); 3.87 (d, 2H, J ¼ 14.0 Hz,
NeCHaHb-py); 3.81 (d, 2H, J ¼ 14.0 Hz, NeCHaHb-py); 3.33 (ddd, 2H,
The molecular weights (Mn and Mw) and PDIs were determined
by GPC-SEC. The chromatographic system consisted of a model
GPCmax apparatus comprising isocratic pump, autosampler and
degassing unit in a common housing, a model 302 TDA triple
detector array detector comprising refractive index (RI), differential
pressure (DP), low angle (7ꢁ) light scattering (LALS) and right angle
(90ꢁ) laser light scattering (RALLS) detection with an integrated
column oven all housed together and a model K-2501 UV-detector,
all obtained from Viscotek (Houston, TX, USA). The sample loop
J ¼ 14.8 Hz, J ¼ 9.6 Hz, J ¼ 9.4 Hz, NeCHa1
H
b1eCHa2
b1eCHa2
158.8 (C2 py); 149.1 (C6 py);
136.5 (C4 py); 122.6, 122.2 (C3 py, C5 py); 79.1 (CHeC6F5); 59.6
(NeCH2-py); 52.0 (NeCH2eCH2eN) ppm. IR (KBr,
/cmꢀ1): 3083vw,
Hb2eN); 2.80 (ddd,
2H, J ¼ 14.8 Hz, J ¼ 9.6 Hz, J ¼ 9.4 Hz, NeCHa1
H
Hb2eN) ppm.
13C{1H} NMR (100.62 MHz, CDCl3, 25 ꢁC):
d
n
3062w, 3010w, 2932w, 2880w, 2824m, 2775vw,1676m,1651m,1590s,
1570m, 1520vs, 1504vs, 1475s, 1433s, 1387m, 1337m, 1303m, 1252w,
1219w, 1148m, 1129m, 1046w, 997s, 960m, 944m, 893w, 880w,
836vw, 762s, 683w, 646vw, 623vw, 606vw, 577vw, 489vw, 466vw,
409w. Anal. Calcd for C21H17F5N4 (420.38): C, 60.00; H, 4.08; F, 22.60;
N, 13.33%. Found: C, 61.17; H, 3.95; N, 12.57%.
permitted injection of fixed sample amounts and volumes of 100 mL
of samples were injected onto the two SEC columns. For UV-
detection a wavelength of 254 nm was chosen, whereas the laser
wavelength for LALS- and RALS-detection was 670 nm. The
4.3. Complexes syntheses
volumes of the RI-, and LALS/RALLS measuring cells were 12
ml and
10 l, respectively. The stationary phase system consisted of two
m
4.3.1. (tBuBOX)ZnMe (2a)
ViscoGelÔ GMHHR-H columns (300 (L) ꢂ 7.8 mm (I.D.), 10
m
m
To a solution of ZnMe2 (0.38 mL of a 1 M solution in heptane,
0.38 mmol) in hexane (5 mL) was added a solution of (tBu,HBOX)H
(100 mg, 0.38 mmol) in hexane (10 mL). The colorless solution
was stirred at ambient temperature for 5 h. The reaction mixture
was dried in vacuo producing a white solid. The white solid was
particle size, 100 Å pore size). Polystyrene PS-99K (Mw ¼ 98,946,
IV ¼ 0.477) and PS-280K (Mw ¼ 268,726, IV ¼ 0.894) calibration
standards for molecular weight calibration were obtained from
ViscotekÔ (PolyCAL TDS-PS standards). The PS-99K was used for

1696
N. Merle et al. / Journal of Organometallic Chemistry 696 (2011) 1691e1697
solubilized in 5 mL toluene, filtered and concentrated to ca. 1 mL.
Colorless crystals of 2a were isolated at ꢀ37 ꢁC after 3 days. Yield:
4.4. Representative procedure for the ring-opening polymerization
of epoxides
124.6 mg (96%). 1H NMR (400.13 MHz, C6D6, 25 ꢁC):
d 4.74 (s, 1H,
HC1); 3.87 (dd, 2H, J ¼ 8.5, 3.3 Hz, HaC5); 3.64 (t, 2H, J ¼ 8.5 Hz, HC4);
An oven-dried glass vial was loaded with a solution of precursor
3.38 (dd, 2H, J ¼ 8.5, 3.3 Hz, HbC5); 0.72 (s, 18 H, Ce(CH3)3); ꢀ0.11 (s,
(21.2
mmol) in toluene (5 mL) followed by the addition of a solution
3H, ZneCH3) ppm. 13C{1H} NMR (100.62 MHz, C6D6, 25 ꢁC):
d
174.0
of B(C6F5)3 (1 equivalent, 21.2 mmol) in toluene (1 mL). This was
(C2); 72.3 (C4); 68.9 (C5); 57.2 (C1); 35.1 (Ce(CH3)3), 26.4 (Ce
allowed to stir for 15 min at room temperature. 2.14 mL of CHO or
1.48 mL of PO (21.2 mmol) was added to the freshly-made active
catalyst and stirred at 30 ꢁC, for the time indicated in Tables 1 and 2.
Caution: the reaction is extremely exothermic when CHO is added
(>100 ꢁC). The reaction mixture was quenched with 1 mL of
anhydrous methanol in a glovebox followed by an extra addition
of methanol (20 mL) outside the glovebox to precipitate the
polymer. The product was then dried in vacuo to a constant
weight. The polymer yield was determined gravimetrically. All
(CH3)3), ꢀ10.5 (ZneCH3) ppm. IR (KBr,
n
/cmꢀ1): 3139vw, 3002w,
2952s, 2905m, 2886m, 2871m, 2844w, 1602s, 1552vs, 1469m,
1452m, 1394w, 1364w, 1346w, 1326w, 1329w, 1269w, 1233m,
1212w, 1197vw, 1155w, 1089s, 1054m, 1024m, 995m, 980w, 952vw,
932vw, 851vw, 825vw, 779w, 759w, 745m, 726vw, 713vw, 657m,
549m, 518vw, 479vw, 404vw. Anal. Calcd for C16H28N2O2Zn (345.8):
C, 55.57; H, 8.16; N, 8.10; O, 9.25; Zn, 18.91. Found: C, 55.30; H, 7.49;
N, 7.96.
polymers showed an atatic microstructure determined by 1H/13
NMR [70].
C
4.3.2. (NHC)ZnEt(Cl) (3d)
To a solution of 1,3-bis(2-pyridylmethyl)imidazolinium chloride
(d) (100 mg, 0.34 mmol) in toluene (10 mL) was added a solution of
ZnEt2 (0.34 mL of a 1 M solution in hexane, 0.34 mmol). The orange
dark solution was stirred at ambient temperature overnight. The
reaction mixture was filtered and dried in vacuo, leading to a yellow-
orange powder. Yield: 80 mg (60%). 1H NMR (400.13 MHz, C6D6,
4.5. X-ray crystallography and crystal structure determination
A crystal was selected in a glovebox and mounted inside a nylon
loop containing Paratone-N cryoprotectant oil (Hampton Research).
Raw data were absorption corrected using the SADABS multiscan
method. Structure solution was performed by direct methods and
model refinement using programs contained in the Bruker AXS
APEX2 package [71]. Crystal data: for 2a: from toluene.
C16H28N2O2Zn, M ¼ 345.77, tetragonal, space group P41212 (No.92),
a ¼ b ¼ 12.1279(5), c ¼ 23.4565(10) Å, V ¼ 3450.1(2) Å3, Z ¼ 8,
25 ꢁC):
d
8.65 (d, 2H, J ¼ 4.6 Hz, C6H py); 7.16 (d, 2H, J ¼ 7.7 Hz, C4H
py); 7.04 (dt, 2H, J ¼ 7.7, 0.8 Hz, C3H py); 6.59 (dd, 2H, J ¼ 7.7, 6.2 Hz,
C5H py); 4.48 (s, 4H, NeCH2-py); 2.75 (s, 4H, NeCH2eCH2eN); 1.85
(sb, 3H, ZneCH2CH3); 0.79 (sb, 2H, ZneCH2CH3) ppm. 13C{1H} NMR
(150.91 MHz, C6D6, 25 ꢁC):
d 206.7 (NeCeN); 155.8 (C2 py); 149.9 (C6
rcalc ¼ 1.331 g cmꢀ3, F(000) ¼ 1472,
m
(Mo-Ka) ¼ 1.430 mmꢀ1
,
py); 137.8 (C4 py); 123.6, 123.3 (C3 py, C5 py); 53.3 (NeCH2-py);
50.2 (NeCH2eCH2eN); 14.6 (ZneCH2CH3), 0.9 (ZneCH2CH3) ppm.
l
¼ 0.71073 Å, T ¼ 101(2) K. The 57,620 reflections measured on
IR (KBr, n
/cmꢀ1): 3060w, 3024w, 3008w, 2922s, 2879s, 2844s,
a Bruker AXS APEXII Ultra CCD area detector system yielded 5072
unique data (qmax ¼ 30.08ꢁ, Rint ¼ 0.0347) [4963 observed reflec-
2802m, 2709vw, 2278vw, 1671w, 1600s, 1571s, 1511vs, 1476s,
1461m, 1438s, 1359m, 1328vw, 1265m, 1224m, 1179w, 1154w,
1132vw, 1098vw, 1052m, 1017m, 994m, 958vw, 906w, 839vw,
811vw, 761m, 695vw, 660w, 640vw, 601m, 542vw, 502m, 466vw,
427w, 418w, 402w. Anal. Calcd for C17H22ClN4Zn (383.23): C, 53.28;
H, 5.79; Cl, 9.25; N, 14.62; Zn, 17.06%. Found: C, 53.58; H, 5.8; N,
14.23%.
tions I > 2
s
(I)]. R1 ¼ 0.0172, wR2 ¼ 0.0483. Crystal data: for d: from
CH2Cl2/Et2O layer. C15H17ClN4, M ¼ 288.78, monoclinic, space group
P21/n (No. 14), a ¼ 8.3324(3), b ¼ 19.0521(6), c ¼ 9.8245(3) Å,
b
¼ 112.87ꢁ, V ¼ 1437.00(8) Å3, Z ¼ 4, rcalc ¼ 1.335 g cmꢀ3, F
(000) ¼ 608,
m
(Mo-Ka) ¼ 0.262 mmꢀ1
,
l
¼ 0.71073 Å, T ¼ 123(2) K.
The 24,233 reflections measured on a Bruker AXS APEXII Ultra CCD
area detector system yielded 4377 unique data (qmax ¼ 30.50ꢁ,
4.3.3. (NHCeC6F5(H))ZnEt2 (4e)
To a solution of ZnEt2 (0.59 mL of a 1 M solution in hexane,
0.59 mmol) in toluene (5 mL) was added a solution of 2-(2,3,4,5,6-
Rint ¼ 0.0327) [4111 observed reflections I > 2
s
(I)]. R1 ¼ 0.0420,
wR2 ¼ 0.1158.
pentafluorophenyl)-1,3-bis(2-pyridylmethyl)imidazolidine
(e)
Acknowledgments
(250 mg, 0.59 mmol) in toluene (10 mL). The dark orange solution
was stirred at ambient temperature overnight. The reaction
mixture was dried in vacuo, yielding a brown oil. Yield: 320 mg
Financial supports from the Norwegian Research Council
(GASSMAKS program, project No. 182536) and the University of
Bergen (program Nanoscience@UiB) are gratefully acknowledged.
(91%). 1H NMR (400.13 MHz, C6D6, 25 ꢁC):
d
8.21 (d, 2H, J ¼ 4.7 Hz,
C6H py); 6.86 (dt, 2H, J ¼ 7.8, 1.5, C3H py); 6.68 (d, 2H, J ¼ 7.8 Hz, C4H
py); 6.46 (dd, 2H, J ¼ 7.8, 4.7 Hz, C5H py); 5.14 (s, 1H, CH(C6F5)); 3.81
(d, 2H, J ¼ 14.4 Hz, NeCHaHb-py); 3.57 (d, 2H, J ¼ 14.4 Hz,
NeCHaHb-py); 3.00 (ddd, 2H, J ¼ 14.4 Hz, J ¼ 9.2 Hz, J ¼ 8.4 Hz,
Appendix A. Supplementary material
CCDC 799049 (2a) and CCDC 799050 (d) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
or by emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 1EZ, UK; fax: þ44 1223 336033.
NeCHa1
H
b1eCHa2
H
b2eN); 2.67 (ddd, 2H, J ¼ 14.4 Hz, J ¼ 9.2 Hz,
J ¼ 8.4 Hz, NeCHa1
H
b1eCHa2
H
b2eN), 1.57 (t, 3H, J ¼ 7.9 Hz,
ZneCH2CH3); 0.39 (q, 2H, J ¼ 7.9 Hz, ZneCH2CH3) ppm. 13C{1H}
NMR (100.62 MHz, C6D6, 25 ꢁC):
d 158.0 (C2 py); 149.3 (C6 py); 136.9
(C4 py); 122.9, 122.8 (C3 py, C5 py); 78.2 (CHeC6F5); 59.2 (NeCH2-
py); 52.5 (NeCH2eCH2eN); 13.9 (ZneCH2CH3), 3.5 (ZneCH2CH3)
ppm. IR (KBr, n
/cmꢀ1): 3081vw, 3061w, 3008w, 2961w, 2927w,
References
2879w, 2823w, 2719vw, 1676w, 1651w, 1587s, 1569m, 1520s,
1502vs, 1475s, 1433s, 1392m, 1367m, 1347m, 1303w, 1288w,
1262vw, 1212vw, 1140w, 1134w, 1093vw, 1046vw, 1026w, 1000s,
946w, 893vw, 880vw, 841vw, 802vw, 795vw, 759m, 690vw, 659vw,
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55.21; H, 5.00; F, 17.47; N, 10.30; Zn, 12.02%. Found: C, 55.58; H, 6.8;
N, 10.23%.
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