Magnesium and zinc complexes containing pendant pyrazolyl-phenolate ligands as catalysts for ring opening polymerisation of cyclic esters

Article abstract of DOI:10.1016/j.jorganchem.2013.04.004

Reactions of HOPh^tBu(CH₂)Pz {4,6-di-tert-butyl-2- (pyrazol-1-yl-methyl)phenol} or HOPh^tBu(CH₂)Pz ^Me {4,6-di-tert-butyl-2-(3,5-dimethylpyrazol-1-yl-methyl)phenol} with 1.1 equivalent of MgⁿBu₂ in toluene gave the binuclear compounds (ⁿBuMgOPh^tBu(CH₂)Pz)₂ (1) or (ⁿBuMgOPh^tBu(CH₂)Pz^Me) ₂ (2). Treatment of HOPh^tBu(CH₂)Pz or HOPh ^tBu(CH₂)Pz^Me with 1.1 equivalent of ZnEt ₂ in n-hexane yielded binuclear compounds (EtZnOPh ^tBu(CH₂)Pz)₂ (3) and (EtZnOPh ^tBu(CH₂)Pz^Me)₂ (4). Colourless crystals of (ClZnOPh^tBu(CH₂)Pz)₂ (5) were obtained via the recrystallisation of 3 from a concentrated dichloromethane solution at room temperature. Alternative route for the synthesis of 5 can be achieved via the reaction of HOPh^tBu(CH₂)Pz with 1.1 equivalent of n-butyllithium in diethylether followed by the treatment of 1.1 equivalent of ZnCl₂ in tetrahydrofuran gave compound 5 as binuclear chloride complex. The compounds 2 and 5 were crystallographically characterised. The catalytic activities of 1-5 towards the ring opening polymerisation reactions of cyclic esters in the presence of benzyl alcohol were under investigation.

Full text of DOI:10.1016/j.jorganchem.2013.04.004

Journal of Organometallic Chemistry 738 (2013) 1e9
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Magnesium and zinc complexes containing pendant pyrazolylephenolate ligands
as catalysts for ring opening polymerisation of cyclic esters
*
Chi-Tien Chen , Chia-Chi Hung, Yu-Jen Chang, Kuo-Fu Peng, Ming-Tsz Chen
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Reactions of HOPhtBu(CH2)Pz {4,6-di-tert-butyl-2-(pyrazol-1-yl-methyl)phenol} or HOPhtBu(CH2)PzMe
{4,6-di-tert-butyl-2-(3,5-dimethylpyrazol-1-yl-methyl)phenol} with 1.1 equivalent of MgnBu2 in toluene
Article history:
Received 15 January 2013
Received in revised form
11 March 2013
gave the binuclear compounds (nBuMgOPhtBu(CH2)Pz)2 (1) or (nBuMgOPhtBu(CH2)PzMe
) (2). Treatment
2
of HOPhtBu(CH2)Pz or HOPhtBu(CH2)PzMe with 1.1 equivalent of ZnEt2 in n-hexane yielded binuclear
compounds (EtZnOPhtBu(CH2)Pz)2 (3) and (EtZnOPhtBu(CH2)PzMe (4). Colourless crystals of (ClZnOPht-
Accepted 3 April 2013
)
2
Bu(CH2)Pz)2 (5) were obtained via the recrystallisation of 3 from a concentrated dichloromethane solu-
tion at room temperature. Alternative route for the synthesis of 5 can be achieved via the reaction of
HOPhtBu(CH2)Pz with 1.1 equivalent of n-butyllithium in diethylether followed by the treatment of 1.1
equivalent of ZnCl2 in tetrahydrofuran gave compound 5 as binuclear chloride complex. The compounds
2 and 5 were crystallographically characterised. The catalytic activities of 1e5 towards the ring opening
polymerisation reactions of cyclic esters in the presence of benzyl alcohol were under investigation.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Magnesium
Zinc
Pendant pyrazolylephenolate
Ring opening polymerisation
Biodegradable polymer
1. Introduction
unsuccessfully [39,41]. Therefore we report here the syntheses of
bulk phenolate ligand precursors with one pendant pyrazolyl
Aliphatic polyesters, prepared from lactones and/or lactides,
having good mechanical properties, hydrolyzability, and biocom-
patibility make them to be the leading candidates in biomedical
and pharmaceutical industries as a resorbable implant materials
and a vehicle for controlled drug delivery [1]. The major polymer-
isation method employed to synthesise these polymers has been
the ring opening polymerisation using well-defined metal com-
plexes with auxiliary ligands. To date, a large diversity of main
group and transition metal complexes has been reported and these
have been reviewed recently [2e9]. During the past decade, mag-
nesium and zinc complexes containing pendant functionalities
phenolate(s) ligands have been a focus of interest and used in ring
opening polymerisation of cyclic esters with great success [10e35].
Recently pyrazole-containing metal complexes have gained sub-
stantial attention since Chisholm reported tris(pyrazolyl)hydro-
borate metal complexes exhibited efficient activities and controlled
characters [36e51]. Several metal complexes bearing multi-dentate
phenolate ligands with pendant pyrazolyl functionalities have been
reported and applied in catalysing ring opening polymerisation of
cyclic esters [16,17,21,39]. Based on crystallographic studies, coor-
dination of all dative atoms to metal centres has been proved
functionality and their zinc and magnesium complexes. The
application of these complexes towards ring opening polymerisa-
tion of cyclic esters was also examined.
2. Results and discussion
2.1. Synthesis and characterisation
Preparation of ligand precursors, HOPhtBu(CH2)Pz and HOPht-
Bu(CH2)PzMe, was straightforward by using reactions of 2,4-di-tert-
butyl-6-(chloromethyl)phenol with pyrazole or 3,5-dimethylpy
razole in the presence of K2CO3 in refluxing acetonitrile to afford
the target compounds in high yield. Compounds, HOPhtBu(CH2)Pz
and HOPhtBu(CH2)PzMe, were characterised by NMR spectroscopy
as well as elemental analyses.
Complexes 1e4 were synthesised by alkane elimination reaction
in moderate to high yields. Treatment of ligand precursors,
HOPhtBu(CH2)Pz or HOPhtBu(CH2)PzMe, with one equivalent of
MgnBu2 or ZnEt2 in toluene or n-hexane yields the desired mag-
nesium n-butyl complexes 1e2 or zinc ethyl complexes 3e4. The
disappearance of the OeH signal (around 10e11 ppm) of the ligand
precursors and the appearance of the resonances for protons of n-
butyl or ethyl groups in high field region are consistent with the
structures proposed in (Scheme 1). Two sets of diastereotopic sig-
nals corresponding to the methylene protons of NPzeCH2earyl are
* Corresponding author. Tel.: þ886 4 22840412; fax: þ886 4 22862547.
E-mail address: ctchen@dragon.nchu.edu.tw (C.-T. Chen).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.04.004

2
C.-T. Chen et al. / Journal of Organometallic Chemistry 738 (2013) 1e9
Scheme 1. Preparation of complexes 1e5.
found on the 1H NMR spectra of 1e4 (4.47 and 6.25 ppm on 1H NMR
spectrum correlate to 54.4 ppm on 13C{1H} NMR spectrum for 1;
4.92 and 6.39 ppm on 1H NMR spectrum correlate to 49.9 ppm on
13C{1H} NMR spectrum for 2; 4.86 and 6.41 ppm on 1H NMR
spectrum correlate to 54.0 ppm on 13C{1H} NMR spectrum for 3;
4.87 and 6.36 ppm on 1H NMR spectrum correlate to 49.1 ppm on
13C{1H} NMR spectrum for 4). During the recrystallisation of com-
pound 3 in dichloromethane, compound 5 was obtained as col-
ourless crystals which were identified as binuclear chloride
complex after refinement of molecular structure [52,53]. It can be
synthesised alternatively by the reaction of ZnCl2 with lithium salt
of HOPhtBuPz in THF solution to afford a white powder. Complexes
1e5 were all characterised by NMR spectroscopy and elemental
analyses.
Suitable crystals for structural determination of 2 or 5 were
obtained from concentrated toluene (for 2) or dichloromethane (for
5) solutions. The molecular structures are depicted in Figs. 1 and 2.
Both compounds are found to be binuclear species bridged via two
oxygen atoms of two phenoxy ligands coming from separate pyr-
azolyl phenolate ligands. Each metal centre can be described as a
distorted tetrahedral coordination geometry surrounded by two
bridged oxygen atoms from two different ligands, one nitrogen
atom from a pyrazolyl group, and one carbon atom from an alkyl
group (for 2) or a chloride ligand (for 5). For 2, the MgeOPhenolate
ꢀ
ꢀ
bond distances (2.0231(15) A and 1.9922(15) A) are longer than
ꢀ
those (1.892(3)e1.979(3) A) found in magnesium phenolate com-
ꢀ
ꢀ
Fig. 1. Molecular structure of 2. Selected bond lengths [A] and bond angles [ ]: MgeO,
2.0231(15); MgeO(A), 1.9922(15); MgeC(21), 2.136(2); MgeN(2A), 2.1365(19); Mg(A)e
N(2), 2.1365(19); O(A)eMgeO, 82.33(6); O(A)eMgeC(21), 124.76(8); OeMgeC(21),
116.00(8); O(A)eMgeN(2A), 99.62(7); OeMgeN(2A), 115.77(7); C(21)eMgeN(2A),
114.22(9). Hydrogen atoms on carbon atoms omitted for clarity. The symmetry oper-
ation implied by the additional ‘A’ letters in the atom labels is (ꢁx, ꢁy þ 1, ꢁz).
plexes [12,17,21,24,30,32]. The MgeNPz bond distances (2.1365(19)
ꢀ
ꢀ
A) are comparable with those (2.100(3)e2.315(4) A) found in
magnesium pyrazolyl complexes [21,40]. The MgeC bond distances
ꢀ
ꢀ
(2.136(2) A) are similar to those (2.124(3)e2.133(4) A) found in
magnesium alkyl complexes [21,40]. For 5, the ZneOPhenolate bond

C.-T. Chen et al. / Journal of Organometallic Chemistry 738 (2013) 1e9
3
ꢀ
ꢀ
(2.1761(6) A) is close to those (2.162(5)e2.2430(11) A) found in zinc
chloride complexes [21,52,53].
2.2. Polymerisation studies
Several magnesium or zinc complexes bearing pendant func-
tionalised phenolate ligands are known as initiators/catalysts in the
ring opening polymerisation (ROP) [10e35]. Attempts to synthesise
a related magnesium or zinc complexes containing alkoxide group
proved unsuccessful. Therefore magnesium and zinc complexes 1e
5 were expected to work as catalysts in the presence of benzyl
alcohol towards the ring opening polymerisation of cyclic esters.
The ring opening polymerisation of ε-caprolactone employing
1e5 as catalysts is examined under a dry nitrogen atmosphere.
Surprisingly magnesium alkyl 2 demonstrates rapid polymerisation
within 2 min in toluene at 25 ꢀC with poor PDI value, whereas zinc
alkyl 3 exhibits no activity towards ring opening polymerisation
(entries 1 and 3). Compound 2 shows comparable activity to the
previously magnesium alkyl complexes [40,49]. The polymerisation
might follow a nucleophilic addition of alkyl ligand to monomer
with the formation of a metal alkoxide species for propagating step
[40,44,49]. Representative results are collected in Table 1. Opti-
mised conditions were found to be 15 mL toluene at 25 ꢀC in the
presence of benzyl alcohol after several trials on running poly-
merisation with CH2Cl2, THF and toluene using 2 or 3 as catalysts
(entries 1e6). The same conditions were applied to examine the
catalytic activities of 1 and 4 (entries 7e10). Experimental results
show, for the same ligand system, magnesium complexes seem to
be more efficient than zinc complexes (1 > 3 and 2 > 4), however,
for the same metallic system, the order should be 2 > 1 and 3 > 4 in
catalysing the polymerisation of ε-caprolactone. Therefore the
controlled behaviours are demonstrated by complexes 2 and 3. The
linear relationship between the number-average molecular weight
ꢀ
ꢀ
Fig. 2. Molecular structure of 5. Selected bond lengths [A] and bond angles [ ]: Zne
N(2), 1.9928(17); ZneO, 1.9952(14); ZneO(0A), 1.9959(13); ZneCl(1), 2.1761(6); N(2)e
ZneO, 104.34(6); N(2)eZneO(0A), 114.88(6); OeZneO(0A), 83.12(6); N(2)eZneCl(1),
110.02(5); OeZneCl(1), 126.27(4); O(0A)eZneCl(1), 115.95(4). Hydrogen atoms on
carbon atoms omitted for clarity. The symmetry operation implied by the additional ‘A’
letters in the atom labels is (ꢁx, ꢁy þ 1, ꢁz þ 2).
ꢀ
distances (1.9952(14) and 1.9959(13) A) are longer than those
ꢀ
(1.901(10)e1.9728(16) A) found in zinc phenolate complexes
ꢀ
[16,17,19,21,52]. The ZneNPz bond distances (1.9928(17) A) are
ꢀ
comparable with those (2.030(8)e2.105(2) A) found in zinc pyr-
azolyl complexes [16,17,21,52]. The bond length of ZneCl
Table 1
Polymerisation of ε-caprolactone using compounds 1e5 as catalysts in toluene at 25 ꢀC if not otherwise stated.a
Mn [obsd]b
Mn [calcd]c
Conv. [%]d
Yield [%]e
Mw/Mn
b
Entry
Catalyst
{[M]0:[Met.]0}:[BnOH]
T [min]
1
2
3
2
2
3
3
2
2
1
1
4
4
2
2
2
2
2
2
2
3
3
3
3
3
3
3
5
50:0
50:1
50:0
50:1
50:1
50:1
50:1
50:1
2
8450
5800
e
5130
5470
e
90
94
Trace
96
32
32
17
93
53
90
97
96
91
99 (98)
91
99
98
95
56
72
e
1.51
1.10
e
1.5
120
50
1.5
1.5
1.5
8
50
120
4
4
6610
e
5580
e
71
e
1.07
e
5f
6g
e
e
e
e
7
8
9
e
e
e
e
4580
e
5400
e
69
e
1.09
e
50:1
50:1
10
11
12
13
14
15
16
17
18h
19h
20h
21h
22h
23h
24h
25
5240
16,000
26,220
29,180
13,240
7830
6160
5790
7150
16,270
27,000
31,890
7090
5810
5650
e
5400
11,170
16,520
20,860
11,280
5300
5760
5700
5520
10,480
16,520
21,540
5300
5760
5760
e
68
90
97
80
76
76
90
93
75
92
97
88
76
94
91
e
1.14
1.20
1.21
1.25
1.18
1.09
1.22
1.12
1.06
1.07
1.09
1.10
1.06
1.12
1.10
e
100:1
150:1
200:1
50(50):1
100:2
150:3
200:4
50:1
100:1
150:1
200:1
100:2
150:3
200:4
50:1
5
6
1.5 (1.5)
2.5
14
20
10
13
16
20
10
14
16
50
91
96
94
91
99
99
Trace
a
In 15 mL toluene.
Obtained from GPC analysis times 0.56.
b
c
Calculated from [M(monomer) ꢂ [M]0/[Metal]0 ꢂ conversion yield/([BnOH]eq)] þ M(BnOH).
d
e
f
Obtained from 1H NMR analysis.
Isolated yield.
In 15 mL CH2Cl2.
In 15 mL THF.
50 ꢀC.
g
h

4
C.-T. Chen et al. / Journal of Organometallic Chemistry 738 (2013) 1e9
(Mn) and the monomer-to-initiator ratio ([M]0/[I]0) exhibited by 2
and benzyl alcohol (entries 2, 11e13) implies the “living” character
of the polymerisation process. Representative results initiated by 2
are demonstrated in Fig. 3. This controlled behaviour is further
confirmed by the resumption experiment (entry 14). The ‘immortal’
character were examined using two, three or four equivalent ratios
(on [M]0/[Mg]0) of benzyl alcohol as the chain transfer agent (en-
tries 15e17). The Mns of the products created from these poly-
merisation reactions became half of those found in the reactions
with addition of one equivalent ratios of benzyl alcohol. Compound
3 also demonstrates ‘living’ (entries 18e21) and ‘immortal’ (entries
22e24) characters with controlled behaviour at elevated temper-
ature. The conversions reach 90% within 20 min at 50 ꢀC with
narrow PDI values. However, compound 5 exhibited poor conver-
sion under optimised condition (entry 25). The end group analysis
is demonstrated by the 1H NMR spectrum of the polymer produced
from ε-caprolactone and 2 ([M]0/[Mg]0 ¼ 50) in the presence of
benzyl alcohol. Peaks were assignable to the corresponding protons
in the proposed structure with capped benzyl alkoxyl group, as
shown in Fig. 4. Isolation of metal alkoxide complexes has been
proved unsuccessful after several trials. Based on the end group
analysis, the mechanism might proceed either via coordinatione
insertion mechanism or undergo ‘activated monomer’ pathways in
which the monomer or benzyl alcohol are activated by 2 and benzyl
alcohol acting as a nucleophile [54,55].
polymerisation of ε-caprolactone, compound 5 demonstrated poor
activity under optimised condition (entry 19). The Mns of the
polymers created from these polymerisation reactions became half
of those found in the reactions with addition of one equivalent
ratios of benzyl alcohol. The end group analysis is demonstrated by
the 1H NMR spectrum of the polymer produced from
-lactide and 3
L
([M]0/[Zn]0 ¼ 50). Peaks were assignable to the corresponding
protons in the proposed structure with capped benzyl alkoxyl
group, as shown in Fig. 6. The ring opening polymerisation of rac-
lactide using 3 as a catalyst under the optimised conditions pro-
duces atactic PLA (entry 20, Pr ¼ 0.52). Experimental results show
that compounds 1e2 have higher activities in catalysing ring
opening polymerisation of ε-caprolactone, whereas compound 3
demonstrates higher catalytic activity in catalysing ring opening
polymerisation of L-lactide.
Compared with other structurally related zinc complexes, the
catalytic activities of 3 in the presence of benzyl alcohol seem to be
more active than other zinc pyrazolyl-phenolate complexes at 25 ꢀC
within 50 min for ROP of ε-CL [16]. The P. Mountford’s group also
reported the synthesis of zinc and magnesium heteroscorpionate
complexes and show excellent polymerisation rates towards the
cyclic ester polymerisation. However, the molecular weight deter-
mined by GPC deviated largely from the theoretical value, indi-
cating the amide group is not a good initiator for the propagation
step [21]. Furthermore, the catalytic activities of 2 in the presence of
benzyl alcohol exhibit even better activities than that demon-
strated by the [Mg(CH2SiMe3)tbpamd] system [40]. Therefore it is
worth noting that introduction of pyrazolylephenolate ligands
could efficiently enhance the catalytic activities and demonstrate a
narrow weight distribution of isolated cyclic esters.
Complexes 1e5 were also investigated with respect to their
catalytic behaviour in the ring opening polymerisation of
Both compounds 1 and 3 show no activities in catalysing ring
opening polymerisation of
-lactide at 50 ꢀC (entries 1 and 3).
L-lactide.
L
Representative results are collected in Table 2. Optimised condi-
tions were found to be 10 mL toluene at 50 ꢀC in the presence of
benzyl alcohol after several trials on running polymerisation with
THF and toluene using 1 or 3 as catalysts (entries 2 and 4e6). The
same conditions were applied to examine the catalytic activities of
2 and 4 with conversions up to 90% within 150 min (entries 7e10).
Better activities were shown using compounds 1 and 3 as catalysts
at elevated temperatures. Short period of time is required for
running polymerisation at 80 ꢀC (entries 11e12). The linear rela-
tionship between the number-average molecular weight (Mn) and
the monomer-to-initiator ratio ([M]0/[I]0) exhibited by 3 (entries 4,
13e15) implies the “living” character of the polymerisation process.
Representative results initiated by 3 are demonstrated in Fig. 5. The
‘immortal’ character were examined using two, three or four
equivalent ratios (on [M]0/[Zn]0) of benzyl alcohol as the chain
transfer agent (entries 16e18). Similar to ring opening
3. Conclusion
A family of magnesium and zinc complexes containing pendant
pyrazolylephenolate ligands has been prepared and fully charac-
terised. Four metal alkyl complexes 1e4 were employed as catalysts
for the ring opening polymerisation of ε-caprolactone or
the presence of benzyl alcohol. They all demonstrate efficient ac-
tivities for the controlled polymerisation of ε-caprolactone or
L-lactide in
L
-
lactide with both living and immortal characters. However, com-
pound 5 demonstrated poor activities in catalysing ring opening
polymerisation of both monomers. The magnesium alkyl complex 2
shows comparable activity to the previously described magnesium
alkyl complexes in catalysing ring opening polymerisation of
ε-caprolactone. Based on the substituents on the pyrazolyl group,
steric demanding of ligands could enhance the catalytic activities
for magnesium complexes in catalysing ring opening polymerisa-
tion of ε-caprolactone whereas zinc complexes seem not in this
system. However, less steric demanding zinc complex 3 shows
better catalytic activities in catalysing ring opening polymerisation
of L-lactide. Preliminary studies on fine-tuning of ligand precursors
and further application of metal complexes to the catalytic re-
actions are currently underway.
4. Experimental
4.1. General
All manipulations were carried out under an atmosphere of
dinitrogen using standard Schlenk-line or drybox techniques. Sol-
vents (tetrahydrofuran, dichloromethane, hexane, and toluene) for
air-sensitive compounds were refluxed over the appropriate drying
agent and distilled prior to use. Deuterated solvents were dried
over molecular sieves.
Fig. 3. Polymerisation of ε-caprolactone catalysed by 2 and BnOH in toluene at 25 ꢀC
(Table 1, entries 2, 9e11).

C.-T. Chen et al. / Journal of Organometallic Chemistry 738 (2013) 1e9
5
1
Fig. 4. H NMR spectrum of PCL-50 catalysed by 2 and BnOH in toluene at 25 ꢀC (Table 1, entry 2).
1H and 13C{1H} NMR spectra were recorded either on Varian
Mercury-400 (400 MHz) or Varian Inova-600 (600 MHz) spec-
trometers in chloroform-d at ambient temperature unless stated
otherwise and referenced internally to the residual solvent peak
and reported as parts per million relative to tetramethylsilane.
Elemental analyses were performed by Elementar Vario ELIV in-
strument. The GPC measurements were performed in THF at 35 ꢀC
with a Waters 1515 isocratic HPLC pump, a Waters 2414 refractive
index detector, and Waters styragel column (HR4E). Molecular
weights and molecular weight distributions were calculated using
polystyrene as standard.
Pyrazole (Acros), 3,5-dimethylpyrazole (TCI), 2,4-di-tert-butyl-
phenol (Acros, 97%), di-n-butylmagnesium (Aldrich, 1.0 M in
n-heptane), ZnEt2 (Aldrich, 1.0 M in n-hexane), n-BuLi (Chemetall,
2.5
M in n-hexane), ZnCl2 (Riedel-deHaen), thionyl chloride
(SeedChem, 99%), K2CO3 (Union), CH3CN (J. T. Baker) were used as
Table 2
Polymerisation of L
-lactide using compounds 1e5 as catalysts in toluene at 50 ꢀC if not otherwise stated.a
Mn [obsd]b
Mn [calcd]c
Conv. [%]d
Yield [%]e
Mw/Mn
b
Entry
Catalyst
{[M]0:[Met.]0}:[BnOH]
T [min]
1
2
3
1
1
3
3
1
3
2
2
4
4
1
3
3
3
3
3
3
3
5
3
50:0
50:1
50:0
50:1
50:1
50:1
50:1
50:1
50:1
50:1
50:1
50:1
100:1
150:1
200:1
100:2
150:3
200:4
50:1
50:1
120
120
70
70
120
70
120
150
70
150
40
30
80
90
100
70
120
180
70
e
e
Trace
95
Trace
93
78
13
61
90
32
91
91
90
90
95
93
97
90
92
e
e
5630
e
6950
e
86
e
1.11
e
4
9830
e
6800
e
75
e
1.06
e
5f
6f
e
e
e
e
7
8
9
e
e
e
e
9050
e
6590
e
83
e
1.12
e
10
11g
12g
13
14
15
16
17
18
19
20h
11,570
6690
9840
20,700
33,000
41,400
8230
5700
6230
e
6660
6660
6590
13,070
20,630
26,890
7100
6580
6730
e
78
88
83
74
94
95
99
81
84
e
1.05
1.22
1.17
1.05
1.05
1.05
1.05
1.05
1.07
e
Trace
90
70
4510
6580
78
1.16
a
In 10 mL toluene.
Obtained from GPC analysis times 0.58.
b
c
Calculated from [M(monomer) ꢂ [M]0/[Metal]0 ꢂ conversion yield/([BnOH]eq)] þ M(BnOH).
d
e
f
Obtained from 1H NMR analysis.
Isolated yield.
In 15 mL THF.
80 ꢀC.
Using rac-lactide.
g
h

6
C.-T. Chen et al. / Journal of Organometallic Chemistry 738 (2013) 1e9
washed with 5 mL CH2Cl2. The combined filtrate was pumped off to
afford brown oil. The oily crude product was solidified by cooling in
the fridge to afford yellow solid (4.23 g, 98%).
Anal. Calc. for C18H26N2O: C, 75.48; H, 9.15; N, 9.78. Found: C,
75.50; H, 9.36; N,10.05. 1H NMR (400 MHz, CDCl3):
d (ppm) 10.24 (s,
1H, OH), 7.51 (d, J ¼ 2.0 Hz, 1H, CHePz), 7.50 (d, J ¼ 2.4 Hz, 1H, CHe
Pz), 7.32 (d, J ¼ 2.4 Hz, 1H, CHePh), 7.05 (d, J ¼ 2.4 Hz, 1H, CHePh),
6.23 (m, 1H, CHePz), 5.20 (s, 2H, CH2), 1.44 (s, 9H, C(CH3)3), 1.29 (s,
9H, C(CH3)3). 13C NMR (100 MHz, CDCl3):
d (ppm) 153.1, 141.8, 138.6,
123.5 (tert-C), 138.9, 129.3, 124.9, 124.7, 105.4 (CHePz or CHePh),
54.1 (CH2), 35.2 (C(CH3)3), 34.2 (C(CH3)3), 31.6 (s, C(CH3)3), 29.8
(C(CH3)3).
4.3. Synthesis of HOPhtBu(CH2)PzMe
Fig. 5. Polymerisation of L-lactide catalysed by 3 and BnOH in toluene at 50 ꢀC (Table 2,
entries 4, 11e13).
To a flask containing 3,5-dimethylpyrazole (1.44 g,15 mmol) and
K2CO3 (4.15 g, 30 mmol) in 40 mL CH3CN, 2,4-di-tert-butyl-6-
(chloromethyl)phenol (3.8 g, 15 mmol) in 5 mL CH3CN was added.
The reaction mixture was refluxed for 12 h and cooled to room
temperature. The resulting mixture was filtered and the residue
was washed with 5 mL CH2Cl2. The combined filtrate was pumped
off to afford brown oil. The oily crude product was triturated with
5 mL n-hexane and washed with 5 mL ethanol to afford yellow
powder (3.40 g, 73%). Anal. Calc. for C20H30N2O: C, 76.38; H, 9.62; N,
8.91. Found: C, 76.72; H, 9.69; N, 8.56. 1H NMR (400 MHz, CDCl3):
supplied. 2,4-di-tert-butyl-6-(hydroxymethyl)phenol [56] and 2,4-
di-tert-butyl-6-(chloromethyl)phenol [57] are prepared by the lit-
erature’s methods. Benzyl alcohol was dried over magnesium sul-
phate and distilled before use. ε-Caprolactone was dried over
magnesium sulphate and distilled under reduced pressure. L- or
rac-Lactide was recrystallised from toluene prior to use.
d
(ppm) 10.62 (s, br, OH), 7.30 (d, J ¼ 2.4 Hz, 1H, CHePh), 7.01 (d,
4.2. Synthesis of HOPhtBu(CH2)Pz
J ¼ 2.0 Hz, 1H, CHePh), 5.75 (s, 1H, CHePz), 5.04 (s, 2H, CH2), 2.33 (s,
3H, CH3), 2.17 (s, 3H, CH3), 1.43 (s, 9H, C(CH3)3), 1.28 (s, 9H, C(CH3)3).
To
a flask containing 2,4-di-tert-butyl-6-(hydroxymethyl)
13C NMR (100 MHz, CDCl3):
d (ppm) 153.6, 147.0, 141.4, 138.8, 138.3,
phenol (3.55 g, 15 mmol) in 20 mL toluene, thionyl chloride
(1.35 mL, 18 mmol) were added at 0 ꢀC. The reaction mixture was
warmed up to room temperature and stirred for 1 h. All the vola-
tiles were pumped off to afford 2,4-di-tert-butyl-6-(chloromethyl)
phenol as pale-yellow oil. This pale-yellow oil was dissolved in 5 mL
CH3CN and transferred to a flask containing pyrazole (1.02 g,
15 mmol) and K2CO3 (4.15 g, 30 mmol) in 40 mL CH3CN. The re-
action mixture was refluxed for 12 h and cooled to room temper-
ature. The resulting mixture was filtered and the residue was
123.5, (tert-C), 124.6, 124.5, 105.1 (CHePz or CHePh), 50.3, (CH2),
35.2 (C(CH3)3), 34.1 (C(CH3)3), 31.6 (C(CH3)3), 29.7 (C(CH3)3), 13.2
(CH3), 11.1 (CH3).
4.4. Synthesis of (nBuMgOPhtBu(CH2)Pz)2 (1)
To a flask containing HOPhtBu(CH2)Pz (0.86 g, 3 mmol) and 15 mL
toluene, di-n-butylmagnesium (3.3 mL,1.0M inn-heptane, 3.3 mmol)
1
Fig. 6. H NMR spectrum of PLA-50 catalysed by 3 and BnOH in toluene at 50 ꢀC (Table 2, entry 4).

C.-T. Chen et al. / Journal of Organometallic Chemistry 738 (2013) 1e9
7
was added at 0 ꢀC. The colour of this yellow solution became deeper
and precipitates appeared gradually. The suspension was allowed to
warm to room temperature and reacted at room temperature for
further 3 h. The reaction mixture was filtered to afford yellow solid
(0.70 g, 64%). Anal. Calc. for C44H68N4O2Mg2: C, 72.03; H, 9.34; N, 7.64.
was added at 0 ꢀC. The colour of this yellow solution became
deeper and precipitates appeared gradually. The suspension was
allowed to warm to room temperature and reacted at room tem-
perature for further 1 h. The reaction mixture was filtered and
washed with 5 mL n-hexane to afford white solid (0.67 g, 82%).
Anal. Calc. for C44H68N4O2Zn2: C, 64.77; H, 8.40; N, 6.87. Found: C,
Found: C, 71.98; H, 9.68; N, 8.09. 1H NMR (600 MHz, C6D6):
d (ppm)
7.49 (d, J ¼ 2.4 Hz, 2H, CHePh or CHePz), 7.42 (d, J ¼ 2.4 Hz, 2H, CHe
Ph or CHePz), 7.03 (d, J ¼ 2.4 Hz, 2H, CHePh or CHePz), 6.70 (d,
J ¼ 2.4 Hz, 2H, CHePh or CHePz), 6.25 (d, J ¼ 15 Hz, 2H, CH2), 5.58 (t,
J ¼ 1.8 Hz, 2H, CHePz), 4.47 (d, J ¼ 14.4 Hz, 2H, CH2),1.72,1.78 (m, 4H,
Mg(CH2)3CH3), 1.48 (s, 18H, C(CH3)3),1.41 (m, 4H, Mg(CH2)3CH3),1.29
(s, 18H, C(CH3)3), 1.01 (t, J ¼ 7.2 Hz, 6H, Mg(CH2)3CH3), 0.06, 0.09 (m,
65.07; H, 7.90; N, 6.46. 1H NMR (600 MHz, CDCl3):
d (ppm) 7.21 (d,
J ¼ 2.4 Hz, 2H, CHePh), 7.04 (d, J ¼ 2.4 Hz, 2H, CHePh), 6.36 (m,
2H, CH2), 5.80 (s, 2H, CHePz), 4.87 (d, J ¼ 14.4 Hz, 2H, CH2), 2.52 (s,
6H, CH3), 2.14 (s, 6H, CH3), 1.31 (s, 18H, C(CH3)3), 1.26 (s, 18H,
C(CH3)3), 0.80 (t, J ¼ 7.8 Hz, 6H, Zn(CH2CH3)), 0.12 (br, 4H,
Zn(CH2CH3)). 13C NMR (150 MHz, CDCl3):
d (ppm) 159.8, 149.4,
4H, Mg(CH2)3CH3). 13C NMR (150 MHz, C6D6):
d
(ppm) 156.5, 140.8,
139.9, 139.7, 139.1, 126.9 (tert-C), 124.9, 124.4, 106.2 (CHePh or
CHePz), 49.1 (CH2), 35.1 (C(CH3)3), 34.0 (C(CH3)3), 31.7 (C(CH3)3),
30.2 (C(CH3)3), 13.0 (PzeCH3), 11.9 (PzeCH3), 11.6 (Zn(CH2CH3)),
2.6 (Zn(CH2CH3)).
139.9, 126.5 (tert-C), 142.0, 131.4, 125.8, 125.4, 106.0 (CHePh or CHe
Pz), 54.4 (CH2), 35.4 (C(CH3)3), 34.2 (C(CH3)3), 33.2 (Mg(CH2)3CH3),
32.1 (Mg(CH2)3CH3), 31.7 (C(CH3)2), 31.2 (C(CH3)2), 14.6
(Mg(CH2)3CH3), 7.7 (Mg(CH2)3CH3).
4.8. Synthesis of (ClZnOPhtBu(CH2)Pz)2 (5)
4.5. Synthesis of (nBuMgOPhtBu(CH2)PzMe
) (2)
2
To a flask containing HOPhtBu(CH2)Pz (0.572 g, 2 mmol) and
10 mL THF, n-BuLi (0.8 mL, 2.5 M in n-hexane, 2.0 mmol) was added
at 0 ꢀC. The reaction mixture was allowed to warm to room tem-
perature and reacted at room temperature for further 1 h. This pale-
green solution was then transferred via cannulae into a flask con-
taining ZnCl2 (0.272 g, 2 mmol) and 10 mL THF at room tempera-
ture. After 12 h of stirring, the suspension was filtered and all the
volatiles were removed under vacuum to afford white solid. The
white solid was washed with 15 mL CH2Cl2 followed by 15 mL n-
hexane to afford white powder (0.58 g, 75%). Anal. Calc. for
To a flask containing HOPhtBu(CH2)PzMe (0.94 g, 3 mmol) and
15 mL toluene, di-n-butylmagnesium (3.3 mL, 1.0 M in n-heptane,
3.3 mmol) was added at 0 ꢀC. The colour of this yellow solution
became deeper and precipitates appeared gradually. The sus-
pension was allowed to warm to room temperature and reacted
at room temperature for further 3 h. The reaction mixture was
filtered to afford yellow solid (0.77 g, 65%). Suitable crystals of 4
for structural determination were recrystallised from concen-
trated toluene solution. Anal. Calc. for C48H76N4O2Mg2: C, 73.00;
H, 9.70; N, 7.09. Found: C, 73.33; H, 9.90; N, 7.08. 1H NMR
C
36H50Cl2N4O2Zn2: C, 55.97; H, 6.52; N, 7.25. Found: C, 55.47; H,
7.02; N, 7.30. 1H NMR (600 MHz, CDCl3):
d
(ppm) 7.79 (d, J ¼ 2.4 Hz,
(600 MHz, C6D6):
d
(ppm) 7.53 (d, J ¼ 2.4 Hz, 2H, CHePh), 7.16 (d,
J ¼ 3.0 Hz, 2H, CHePh), 6.39 (d, J ¼ 15 Hz, 2H, CH2), 5.23 (s, 2H,
CHePz), 4.92 (d, J ¼ 14.4 Hz, 2H, CH2), 2.10 (s, 6H, CH3), 1.81 (s,
6H, CH3), 1.70 (m, 4H, Mg(CH2)3CH3), 1.60 (s, 18H, C(CH3)3), 1.57
(m, 4H, Mg(CH2)3CH3), 1.29 (s, 18H, C(CH3)3), 1.08 (t, J ¼ 7.2 Hz,
6H, Mg(CH2)3CH3), 0.04, 0.12 (m, 4H, Mg(CH2)3CH3). 13C NMR
2H, CHePh or CHePz), 7.69 (d, J ¼ 2.4 Hz, 2H, CHePh or CHePz),
7.30 (d, J ¼ 2.4 Hz, 2H, CHePh or CHePz), 7.06 (d, J ¼ 2.4 Hz, 2H,
CHePh or CHePz), 6.38 (m, 2H, CHePz), 6.33 (d, J ¼ 15 Hz, 2H, CH2),
5.03 (d, J ¼ 15 Hz, 2H, CH2), 1.29 (s, 18H, C(CH3)3), 1.26 (s, 18H,
C(CH3)3). 13C NMR (150 MHz, CDCl3):
d (ppm) 155.8, 142.4, 140.5,
(150 MHz, C6D6):
d (ppm) 157.0, 150.8, 140.9, 140.7, 140.0, 127.4
125.5 (tert-C), 141.8, 132.4, 126.0, 125.8, 106.7 (CHePh or CHePz),
54.2 (CH2), 35.1 (C(CH3)3), 34.2 (C(CH3)3), 31.5 (C(CH3)2), 30.7
(C(CH3)2).
(tert-C), 125.36, 125.33, 107.7 (CHePh or CHePz), 49.9 (CH2), 35.4
(C(CH3)3), 34.1 (C(CH3)3), 32.6 (Mg(CH2)3CH3), 32.4 (Mg(CH2)3
CH3), 31.7 (C(CH3)2), 31.2 (C(CH3)2), 14.5 (Mg(CH2)3CH3), 13.1
(PzeCH3), 11.1 (PzeCH3), 10.6 (Mg(CH2)3CH3).
4.9. General procedure for ε-caprolactone polymerisation
4.6. Synthesis of (EtZnOPhtBu(CH2)Pz)2 (3)
Typically, to a flask containing catalyst (0.0625 mmol) was
added 14.5 mL toluene followed by the addition of 0.5 mL benzyl
alcohol solution (0.25 mmol benzyl alcohol/1 mL toluene). To this
solution was added prescribed amount of ε-caprolactone after
stirring for 5 min. The reaction mixture was stirred at prescribed
temperature for prescribed time. After the reaction was quenched
by the addition of 5 mL acetic acid solution (0.35 N), the resulting
mixture was poured into 25 mL n-heptane to precipitate polymers.
Crude products were recrystallised from THF/hexane and dried in
vacuo up to a constant weight.
To a flask containing HOPhtBu(CH2)Pz (0.86 g, 3 mmol) and 20 mL
n-hexane, diethylzinc (3.3 mL, 1.0 M in n-hexane, 3.3 mmol) was
added at 0 ꢀC. The white precipitates appeared gradually. The sus-
pension was allowed to warm to room temperature and reacted at
room temperature for further 30 min. The reaction mixture was
filtered to afford white solid (0.97 g, 85%). Anal. Calc. for
C
40H60N4O2Zn2: C, 63.24; H, 7.96; N, 7.37. Found: C, 63.62; H, 8.34; N,
7.64. 1H NMR (600 MHz, CDCl3):
d
(ppm) 7.65 (d, J ¼ 1.8 Hz, 2H, CHe
Ph or CHePz), 7.46 (s, 2H, CHePh or CHePz), 7.23 (d, J ¼ 1.8 Hz, 2H,
CHePh or CHePz), 7.03 (d, J ¼ 2.4 Hz, 2H, CHePh or CHePz), 6.41 (d,
J ¼ 14.4 Hz, 2H, CH2), 6.26 (s, 2H, CHePz), 4.86 (d, J ¼ 15 Hz, 2H, CH2),
1.26 (s, 18H, C(CH3)3), 1.25 (s, 18H, C(CH3)3), 0.91 (t, J ¼ 7.8 Hz, 6H,
Zn(CH2CH3)), 0.06 (m, 4H, Zn(CH2CH3)). 13C NMR (150 MHz, CDCl3):
4.10. General procedure for L- or rac-lactide polymerisation
Typically, to a flask containing catalyst (0.025 mmol) and pre-
scribed amount of - or rac-lactide was added 9.8 mL toluene fol-
d
(ppm) 159.0, 140.0, 139.6, 126.2 (tert-C), 141.0, 130.6, 125.1, 124.9,
L
105.6 (CHePh or CHePz), 54.0 (CH2), 35.1 (C(CH3)3), 34.1 (C(CH3)3),
31.7 (C(CH3)2), 30.2 (C(CH3)2),12.9 (Zn(CH2CH3)), ꢁ0.8 (Zn(CH2CH3)).
lowed by the addition of 0.2 mL benzyl alcohol solution (0.25 mmol
benzyl alcohol/1 mL toluene). The reaction mixture was stirred at
prescribed temperature for prescribed time. After the reaction was
quenched by the addition of 10 mL acetic acid solution (0.35 N), the
resulting mixture was poured into 50 mL n-heptane to precipitate
polymers. Crude products were recrystallised from THF/n-hexane
and dried in vacuo up to a constant weight.
4.7. Synthesis of (EtZnOPhtBu(CH2)PzMe
) (4)
2
To a flask containing HOPhtBu(CH2)PzMe (0.63 g, 2 mmol) and
15 mL n-hexane, diethylzinc (2.2 mL, 1.0 M in n-hexane, 2.2 mmol)

8
C.-T. Chen et al. / Journal of Organometallic Chemistry 738 (2013) 1e9
[7] J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev. 250 (2006)
Table 3
602e626.
Summary of crystal data for compounds 2 and 5.
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[9] A.K. Sutar, T. Maharana, S. Dutta, C.-T. Chen, C.-C. Lin, Chem. Soc. Rev. 39
(2010) 1724e1746.
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(2002) 2132e2133.
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W.B. Tolman, J. Am. Chem. Soc. 125 (2003) 11350e11359.
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2050.
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Dalton Trans. (2006) 928e936.
2
5$2CH2Cl2
Formula
Fw
T [K]
C48H76Mg2N4O2
789.75
100(2)
Monoclinic
P21/c
13.5027(3)
19.1081(5)
9.5821(3)
90
106.152(3)
90
2374.70(11)
2
C38H54Cl6N4O2Zn2
942.29
100(2)
Monoclinic
P21/c
9.3207(2)
21.3192(4)
11.6070(2)
90
106.583(2)
90
2210.49(7)
2
1.416
1.484
12,405
235
Crystal system
Space group
ꢀ
a [A]
ꢀ
b [A]
ꢀ
c [A]
a
b
g
[ꢀ]
[ꢀ]
[ꢀ]
[15] P.D. Knight, A.J.P. White, C.K. Williams, Inorg. Chem. 47 (2008) 11711e
11719.
3
ꢀ
[16] C. Zhang, Z.-X. Wang, J. Organomet. Chem. 693 (2008) 3151e3158.
[17] Z. Zheng, G. Zhao, R. Fablet, M. Bouyahyi, C.M. Thomas, T. Roisnel,
O. Casagrande Jr., J.-F. Carpentier, New J. Chem. 32 (2008) 2279e2291.
[18] D.J. Darensbourg, W. Choi, O. Karroonnirun, N. Bhuvanesh, Macromolecules 41
(2008) 3493e3502.
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(2008) 6556e6562.
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6466e6476.
[21] A.D. Schofield, M.L. Barros, M.G. Cushion, A.D. Schwarz, P. Mountford, Dalton
Trans. (2009) 85e96.
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9827.
V [A ]
Z
rcalc [Mg/m3]
1.104
0.090
m
(Mo Ka) [mmꢁ1
]
Reflections collected
13,154
268
0.0617
0.1869
1.008
No. of parameters
R1a
0.0336
0.0891
1.005
wR2a
GoFb
P
P
P
P
R1 ¼ ½ jF0j ꢁ jFcj= jF0jꢃ; wR2 ¼ ½ wðF02 ꢁ Fc2Þ2= wðF02Þ2ꢃ1=2W ¼ 0:10:
a
GoF ¼ ½SwðF02 ꢁ Fc2Þ2=ðNrflns ꢁ NparamsÞꢃ1=2
:
b
[23] M.-Y. Shen, Y.-L. Peng, W.-C. Hung, C.-C. Lin, Dalton Trans. (2009) 9906e9913.
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(2010) 3564e3572.
4.11. X-ray crystallography
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3423e3428.
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Chem. (2010) 3602e3609.
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523e534.
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4044.
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(2011) 9601e9607.
[34] G. Labourdette, D.J. Lee, B.O. Patrick, M.B. Ezhova, P. Mahrkhodavandi, Or-
ganometallics 28 (2009) 1309e1319.
[35] M.D. Jones, M.G. Davidson, C.G. Keir, L.M. Hughes, M.F. Mahon, D.C. Apperley,
Eur. J. Inorg. Chem. (2009) 635e642.
[36] M.H. Chisholm, N.W. Eilerts, J.C. Huffman, S.S. Iyer, M. Pacold, K. Phomphrai,
J. Am. Chem. Soc. 122 (2000) 11845e11854.
[37] M.H. Chisholm, J. Gallucci, K. Phomphrai, Chem. Commun. (2003) 48e49.
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6725.
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Baeza, A. Otero, A. Antiñolo, J. Tejeda, A. Lara-Sánchez, M.I. López-Solera,
Organometallics 26 (2007) 6403e6411.
Crystals were grown from concentrated toluene (for 2) or
concentrated dichloromethane solution (for 5), and isolated by
filtration. Suitable crystals of 2 or 5 were mounted onto a glass fibre
using perfluoropolyether oil and cooled rapidly in a stream of cold
nitrogen gas using an Oxford Cryosystems Cryostream unit.
Diffraction data were collected at 100 K using Oxford Gemini S
diffractometer. The absorption correction was based on the sym-
metry equivalent reflections using the SADABS program [58]. The
space group determination was based on a check of the Laue
symmetry and systematic absences and was confirmed using the
structure solution. The structure was solved by direct methods
using a SHELXTL package [59]. All non-H atoms were located from
successive Fourier maps, and hydrogen atoms were refined using a
riding model. Anisotropic thermal parameters were used for all
non-H atoms, and fixed isotropic parameters were used for H
atoms. Some details of the data collection and refinement are given
in Table 3.
Acknowledgements
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F. Carpentier, Inorg. Chem. 46 (2007) 328e340.
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ganometallics 27 (2008) 976e983.
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A.M. Rodríguez, Organometallics 27 (2008) 1310e1321.
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Segovia, L.F. Sánchez-Barba, A.M. Rodríguez, Angew. Chem. Int. Ed. 48 (2009)
2176e2179.
We would like to thank the National Science Council of the
Republic of China for financial support (grant number NSC 98-2113-
M-005-002-MY3 and NSC 101-2113-M-005-012-MY3).
Appendix A. Supplementary material
CCDC-853413 (for 2) and -853414 (for 5) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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