Synthesis and structural characterisation of zinc complexes bearing furanylmethyl and thiophenylmethyl derivatives of (R,R)-1,2-diaminocyclohexanes for stereoselective polymerisation of poly(rac-lactide)

Article abstract of DOI:10.1016/j.poly.2014.03.055

Novel dichloro zinc complexes based on enantiopure N,N-diamine ligands bearing furanylmethyl and thiophenylmethyl pendent groups were synthesised, and their crystal structures were determined using X-ray crystallography. The isopropoxide derivatives (gene

Full text of DOI:10.1016/j.poly.2014.03.055

Polyhedron 77 (2014) 32–38
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and structural characterisation of zinc complexes
bearing furanylmethyl and thiophenylmethyl derivatives
of (R,R)-1,2-diaminocyclohexanes for stereoselective polymerisation
of poly(rac-lactide)
Kyuong Seop Kwon a, Saira Nayab a,b, Hyosun Lee a, Jong Hwa Jeong a,
⇑
a Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 1370 Sankyuk-dong, Taegu 702-701, Republic of Korea
b Department of Chemistry, Bacha Khan University, Charsadda 24420, KPK, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel dichloro zinc complexes based on enantiopure N,N-diamine ligands bearing furanylmethyl and
thiophenylmethyl pendent groups were synthesised, and their crystal structures were determined using
X-ray crystallography. The isopropoxide derivatives (generated in situ) of these well-characterised com-
plexes efficiently catalysed the ring-opening polymerisation (ROP) of rac-lactide (rac-LA) at two different
temperatures under controlled conditions. Highly heterotactic polylactide (PLA) was obtained with
Pr = 0.80 at ꢀ25 °C in THF.
Received 18 January 2014
Accepted 27 March 2014
Available online 6 April 2014
Keywords:
Enantiopure ligand
Zinc complexes
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
X-ray crystallography
Ring opening polymerisation
Heterotactic polylactide
1. Introduction
shown to be active catalysts for ROP of LA. Choosing the appropri-
ate ancillary ligand is important to control the molecular weight
PLA has been commonly used as an eco-friendly commodity
polymer over the past decade due to its biodegradable and biocom-
patible nature [1,2], and is an attractive alternative to petrochem-
ical-derived polyolefins due to the fact that lactide feedstocks are
derived from renewable resources [3–5]. PLA is typically produced
by ring-opening polymerisation (ROP) of LA initiated by metal cat-
alysts. Among the metal-based initiators, complexes based on tita-
nium [6], zirconium [7], calcium [8], magnesium [9–11], zinc [12–
14] and aluminium [15] have attracted attention due to the broad
applicability of PLA in food packaging [16,17], biomedical and
pharmaceutical fields [18,19]. Zinc-based catalysts are common,
highly active and stereoselective metal-based catalysts used to
control LA polymerisation. For example, zinc complexes with anio-
nic N-donor ligands including b-diketiminates [20,21], phenoxy-
amines [22], trispyrazolylborates [23], Schiff bases [24],
aminophenolate- and pyrazole-based N,N,O-tridentate ligands
[25,26] were reported to polymerise rac-LA with effective stereose-
lectivity. Several other zinc complexes bearing neutral ligands such
as guanidines [27], carbenes [28], phosphinimines [29], tris-
pyrazolylmethanes [30] and substituted amines [31] were also
distribution and microstructure of PLA, which determine the poly-
mer’s mechanical and physical properties. Recently, chiral zinc cat-
alysts for rac-LA polymerisation [32], specifically those based on C2
symmetric chiral ligands bearing (R,R)-1,2-diaminocyclohexane
derivatives, have been used as auxiliaries to serve as powerful ste-
reo-regulating systems for ROP of rac-LA [33].
Our previous report described a zinc complex ligated to N,N0-
bis-(2,6-dichloro-benzyl)-(R,R)-1,2-diaminocyclohexane for stere-
oselective polymerisation of rac-LA [34]. In the current study, we
optimised the ligand scaffold by installing furanylmethyl and thio-
phenylmethyl pendent groups. Herein, we report the synthesis and
structural characterisation of dichloro zinc complexes containing
furanylmethyl and thiophenylmethyl derivatives of (R,R)-1,2-dia-
minocyclohexanes. The isopropoxide derivatives of these com-
plexes, generated in situ, were assessed for ROP of rac-LA.
2. Experimental
2.1. General considerations and materials
All manipulations involved in the synthesis of enantiopure N,N0-
diamine ligands and their corresponding dichloro zinc complexes
were performed using bench-top techniques in the air, unless
⇑
Corresponding author. Tel.: +82 53 950 6343; fax: +82 53 950 6330.
E-mail address: jeongjh@knu.ac.kr (J.H. Jeong).
http://dx.doi.org/10.1016/j.poly.2014.03.055
0277-5387/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.

K.S. Kwon et al. / Polyhedron 77 (2014) 32–38
33
otherwise specified. All ROP reactions were performed using the
standard Schlenk techniques, high vacuum, and in a glove box
under argon. THF was dried over Na/benzophenone ketyl. EtOH,
MeOH, hexane and Et2O were purchased from high-grade commer-
cial suppliers and used as received. Starting materials, ( )-trans-
1,2-diaminocyclohexane, L-(+)-tartaric acid, NaBH4, 5-methyl-2-
thiophenecarboxaldehyde, 5-methyl-2-furaldehyde, ZnCl2 and
Me2CHOLi (2.0 M in THF) were purchased from Aldrich. NMR sol-
vents were purchased from Sigma Aldrich and stored over 3-Å
molecular sieves. The 3,6-dimethyl-1-dioxane-2,5-dione (rac-LA)
was purchased from Aldrich and stored in a glove box. L1 and L3
were synthesised as described previously [35,36].
2.2.3. Synthesis of L1ZnCl2
The EtOH (10 mL) solution of the L1 (1.04 g, 3.80 mmol) was
added dropwise to EtOH (7 mL) solution of ZnCl2 (0.52 g,
3.81 mmol). The precipitated solid resulted after stirring for 12 h
was filtered. Subsequent washing of solid with cold EtOH and
Et2O followed by drying in vacuo for overnight resulted in white
product (1.35 g, 87% yield). Anal. Calc. for C16H22Cl2N2O2Zn: C,
46.80; H, 5.40; N, 6.82. Found: C, 46.71; H, 5.36; N, 6.79%. 1H
NMR (400 MHz, CDCl3): d 7.34 (dd, J = 2.52 Hz, J = 0.75 Hz, 2H,
ArH), 6.50 (d, J = 3.28 Hz, 2H, ArH), 6.28 (dd, J = 3.28 Hz,
J = 1.76 Hz, 2H, ArH), 4.05–3.90 (m, 4H, (CHAHB)2), 2.50–2.40 (m,
2H, CyH), 2.32–2.19 (m, 2H, CyH), 1.85–1.72 (m, 2H, CyH), 1.69
(br s, 2H, (NH)2), 1.31–1.13 (m, 2H, CyH), 1.12–1.00 (m, 2H, CyH).
IR (solid neat; cmꢀ1): 3188 (m), 2933 (w), 2865 (w), 1504 (w),
1451 (m), 1350 (w), 1149 (m), 1104 (m), 1065 (m), 1009 (s), 987
(m), 917 (m), 951 (m), 857 (m), 807 (m), 733 (s).
2.2. Instrumentation
1H NMR spectra were recorded on a Bruker Advance Digital
400-NMR spectrometer and chemical shifts were recorded in
ppm units using SiMe4 as an internal standard. Coupling constants
were reported in Hertz (Hz). Infrared (IR) spectra were recorded on
Bruker FT/IR-Alpha (neat) and the data were reported in reciprocal
centimetres (cmꢀ1). Elemental analysis was performed using an EA
1108-Elemental Analyzer, and gel-permeation chromatography
(GPC) was conducted on a Waters Alliance GPCV2000, equipped
with differential refractive index detectors at the Chemical Analy-
sis Laboratory of the Center for Scientific Instruments of Kyung-
pook National University. The GPC columns were eluted using
THF at 1 ml/min at 25 °C, and were calibrated with monodisperse
polystyrene standards.
2.2.4. Synthesis of L2ZnCl2
The analogues method to that of L1ZnCl2 was applied to L2ZnCl2
except that L3 (1.06 g, 3.46 mmol) and ZnCl2 (0.48 g, 3.52 mmol)
were used. The product was obtained as white solid (1.36 g, 90%
yield). Anal. Calc. for C18H26Cl2N2O2Zn: C, 49.28; H, 5.97; N, 6.39.
Found: C, 49.21; H, 5.89; N, 6.36%. 1H NMR (400 MHz, CDCl3): d
6.31 (d, J = 3.03 Hz, 2H, ArH), 5.85 (m, 2H, ArH), 3.98–3.81 (m,
4H, (CHAHB)2), 2.50–2.37 (m, 2H, (NH)2), 2.30–2.14 (m, 2H, CyH),
2.20 (s, 6H, (Ar-CH3)2), 1.82–1.71 (m, 2H, CyH), 1.26–1.16 (m, 2H,
CyH), 1.26–1.16 (m, 2H, CyH), 1.12–0.98 (m, 2H, CyH). IR (solid
neat; cmꢀ1): 3182 (w), 2934 (w), 2863 (w), 1560 (w), 1447 (m),
1365 (w), 1286 (w), 1218 (m), 1100 (w), 1010 (m), 949 (m), 805 (s).
2.2.1. Synthesis of L2
2.2.5. Synthesis of L3ZnCl2
The analogues method to that of L1ZnCl2 was applied to L3ZnCl2
except that L3 (1.05 g, 3.47 mmol) and ZnCl2 (0.48 g, 3.52 mmol)
were used. The product was obtained as white solid (1.30 g, 85%
yield). Anal. Calc. for C16H22Cl2N2S2Zn: C, 43.40; H, 5.01; N, 6.33.
Found: C, 43.31; H, 4.97; N, 6.28%. 1H NMR (400 MHz, DMSO): d
7.48–7.42 (m, 2H, ArH), 7.30–7.20 (m, 2H, ArH), 7.02–6.97 (m,
2H, ArH), 4.18–3.90 (m, 4H, (CHAHB)2), 2.40 (br s, 2H, (NH)2),
2.27–2.14 (m, 2H, CyH), 1.93–1.83 (m, 2H, CyH), 1.73–1.57 (m,
2H, CyH), 1.40–1.20 (m, 2H, CyH), 1.17–1.02 (m, 2H, CyH). IR (solid
neat; cmꢀ1): 3197 (w), 2933 (w), 2863 (w), 1434 (m), 1350 (w),
1219 (w), 1100 (w), 1031 (m), 1004 (m), 843 (m), 715 (s).
(1R,2R)-(+)-1,2-diaminocyclohexane L-tartrate salt (3.54 g,
13.00 mmol) was dissolved in 2 N NaOH aqueous followed by addi-
tion of CH2Cl2 (40 mL) solution of 5-methyl-2-furylcarbaldehyde
(2.95 g, 26.00 mmol). The organic layer was separated after stirring
at RT for 4 days, dried over MgSO4 and concentrated in vacuo giving
a crude diimine as a waxy solid). MeOH (40 mL) solution of the
crude diimine (4.07 g, 15.00 mmol) was treated with NaBH4
(1.26 g, 33.00 mmol) at 0 °C. Solvent was removed in vacuo after
stirring for 24 h and the resultant residue was treated with water
(40 mL) and CH2Cl2 (40 mL). Combined organic phase was sepa-
rated, dried over MgSO4 and concentrated in vacuo to yield light
yellow oil as final product (3.76 g, 83% yield). Anal. Calc. for
2.2.6. Synthesis of L4ZnCl2
C
18H26N2O2: C, 71.49; H, 8.67; N, 9.26. Found: C, 71.32; H, 8.69;
N, 9.29%. 1H NMR (400 MHz, CDCl3): d5.95 (d, J = 3.03 Hz, 2H,
ArH), 5.79–5.77 (m, 2H, ArH), 3.71 (d, J = 14.1 Hz, 2H, CHAHB),
3.54 (d, J = 14.4 Hz, 2H, CHAHB), 2.17 (s, 6H, (Ar-CH3)2), 2.20–2.10
(m, 2H, CyH), 2.01–1.92 (m, 2H, CyH), 1.85 (br s, 2H, (NH)2),
1.67–1.56 (m, 2H, CyH), 2.20–1.07 (m, 2H, CyH), 1.01–0.90 (m,
2H, CyH). IR (solid neat; cmꢀ1): 3302 (w), 2854 (m), 1567 (m),
1450 (s), 1348 (br, m), 1216 (s), 1112 (m), 1058 (s), 1017 (w),
945 (m), 1851 (w), 770 (s).
The analogues method to that of L1ZnCl2 was applied to L4ZnCl2
except that L4 (1.02 g, 3.05 mmol) and ZnCl2 (0.42 g, 3.08 mmol)
were used. The product was obtained as white solid (1.28 g, 89%
yield). Anal. Calc. for C18H26Cl2N2S2Zn: C, 45.92; H, 5.57; N, 5.95.
Found: C, 45.89; H, 5.59; N, 5.90%. 1H NMR (400 MHz, DMSO): d
7.08–6.97 (m, 2H, ArH), 6.69–6.63 (m, 2H, ArH), 4.03–3.81 (m,
4H, (CHAHB)2), 2.40 (s, 6H, (Ar–CH3)2), 2.40 (br s, 2H, (NH)2),
2.25–2.13 (m, 2H, CyH), 1.92–1.84 (m, 2H, CyH), 1.71–1.57 (m,
2H, CyH), 1.37–1.19 (m, 2H, CyH), 1.17–1.02 (m, 2H, CyH). IR (solid
neat; cmꢀ1): 3243 (w), 3197 (w), 2933 (w), 2863 (w), 1550 (w),
1434 (m), 1350 (w), 1219 (w), 1100 (m), 1031 (m), 904 (w), 843
(m), 715 (s).
2.2.2. Synthesis of L4
The synthesis of L4 was carried out with analogous procedure to
that of L2 except that (3.40 g, 26.00 mmol) of 5-methyl-2-thio-
phenecarboxaldehyde was used. The product was obtained as light
yellow oil (4.03 g, 93% yield). Anal. Calc. for C18H26N2S2: C, 64.62; H,
7.83; N, 8.37. Found: C, 64.51; H, 7.79; N, 8.29%. 1H NMR (400 MHz,
CDCl3): d 6.60 (d, J = 3.28 Hz, 2H, ArH), 6.49–6.47 (m, 2H, ArH), 3.93
(d, J = 14.1 Hz, 2H, CHAHB), 3.72 (d, J = 14.1 Hz, 2H, CHAHB), 2.36 (s,
6H, (Ar–CH3)2), 2.22–2.25 (m, 2H, CyHN), 2.07–2.00 (m, 2H, CyH),
1.84 (br s, 2H, (NH)2), 1.67–1.58 (m, 2H, CyH), 1.20–1.07 (m, 2H,
CyH), 1.00–0.85 (m, 2H, CyH). IR (solid neat; cmꢀ1): 3300 (w),
2922 (s), 2854 (m), 1448 (s), 1352 (br, m), 1216 (w), 1155 (m),
1107 (s), 1038 (w), 963 (w), 855 (w), 702 (s), 677 (w), 601 (w).
2.3. General procedure for ROP of rac-LA
The active isopropoxide derivative initiators [(L1–L4)
ClZnOCHMe2] were prepared in situ by treating (L1–L4)ZnCl2 with
1 equiv. of Me2CHOLi in THF. The synthesis of L1ClZnOCHMe2 was
carried out as follows. L1ZnCl2 (0.205 g, 0.50 mmol) was added to
a 100 mL flame dried Schlenk flask and thoroughly vacuumed for
30 min followed by the addition of dried THF (7.75 mL) to make
a homogenous solution, under argon. Me2CHOLi (0.25 mL of

34
K.S. Kwon et al. / Polyhedron 77 (2014) 32–38
2.0 M solution in THF, 0.50 mmol) was added dropwise via gas
tight syringe at (78 °C) to the above mentioned solution. After
being stirred for 2 h at room temperature, the solvent from ca.
1 ml of the resultant isopropoxide derivative solution was removed
in vacuo to prepare NMR sample for L1ClZnOCHMe2. 1H NMR
(400 MHz, CDCl3): d 7.44–7.31 (m, 2H, ArH), 6.38–6.31 (m, 2H,
ArH), 6.31–6.20 (m, 2H, ArH), 4.17–3.73(m, 4H, CHAHB), 3.97 (br,
1H, CHMe2), 2.53 (br s, 2H, (NH)2), 2.40–2.17 (m, 2H, CyH), 2.14–
1.96 (m, 2H, CyH), 1.82–1.51 (m, 2H, CyH), 1.33–1.13 (m, 2H,
CyH), 1.13–0.95 (m, 2H, CyH), 1.16 (br, 6H, CHMe2).
For the polymerisation reaction 100 mL of Schlenk flask was
charged with rac-LA (0.90 g, 6.25 mmol) in the glove box. Dried
THF (5 mL) was transferred to the reaction vessel via gas tight syr-
inge and stirred to make a homogenous solution. The reaction was
initiated by adding the THF solution of catalyst (2.00 mL,
0.0625 mmol) via gas tight syringe under argon at 25 °C. The reac-
tion mixture was stirred at 25 °C or 25 °C for specified time. The
polymerisation reaction was quenched after prescribed duration
using H2O (3 mL) and sequentially precipitated by adding hexane
(20 mL). The resultant sticky polymeric material was dried com-
pletely in vacuo for 12 h to get white solid as final polymeric mate-
rial. 1H NMR (400 MHz, CDCl3): d 5.14–5.25 (m, 1H, –OCH–Me–
C(@O)), 1.54–1.63 (m, 3H, –OCH–Me–C(@O)).
HN
NH
Ar
Ar
L1: Ar = 2-Furaldehyde (C5H4O2)
L2: Ar = 5-Methyl-2-furaldehyde (C6H6O2)
L3: Ar = 2-Thiophenecarbaldehyde (C5H4OS)
L4: Ar = 5-Methyl-2-thiophenecarbaldehyde (C6H6OS)
Chart 1. List of N,N-diamine ligands used for syntheses of dichloro zinc complexes.
R
R
X
X
NH
NH
ZnCl2
Cl
Zn
Cl
2.4. X-ray crystallography
EtOH, 12 h, RT
NH
NH
An X-ray quality single crystal was mounted in a thin-walled
glass capillary on an Enraf-Noius CAD-4 diffractometer with Mo
X
Ka radiation (k = 0.71073 Å). Unit cell parameters were deter-
X
mined by least-squares analysis of 25 reflections (10° < h < 13°).
Intensity data were collected with h range of 1.59°–25.47° in
R
R
X = O, S
x
/2h scan mode. Three standard reflections were monitored every
X = O, S
R = H, CH3
1 h during data collection. The data was corrected for Lorentz-
polarization effects and decay. Empirical absorption corrections
R = H, CH3
with
v-scans were applied to the data. The structure was solved
Scheme 1. Synthesis of dichloro zinc complexes bearing furanylmethyl and
thiophenylmethyl derivatives of (R,R)-1,2-diaminocyclohexanes.
by using Patterson method and refined by full-matrix least-squares
techniques on F using SHELXL-97 and SHELXS-97 program packages
[37]. All non hydrogen atoms were refined positioned geometri-
cally using riding model with fixed isotropic thermal factors except
the disordered oxygen atoms at furanyl rings of L1ZnCl2 which
were refined isotropically.
Table 1
Crystal data and structure refinement for L1ZnCl2, L2ZnCl2, and L4ZnCl2.
L1ZnCl2
L2ZnCl2
L4ZnCl2
Empirical formula
C16 H22 Cl2 N2 O2 C18 H26 Cl2 N2 O2 C18 H26 Cl2 N2 S2
3. Results and discussion
Zn
Zn
Zn
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
410.65
monoclinic
C2/c
438.70
orthorhombic
P212121
470.80
orthorhombic
P212121
3.1. Synthesis and characterisation
The synthesis of two new C2 symmetric enantiopure N,N0-dia-
mine ligands with furanylmethyl and thiophenylmethyl pendent
groups, L2 and L4, was performed as described previously [34,38].
These ligands were isolated in high yields (92%). Reactions of enan-
tiopure N,N0-bidentate ligands (Chart 1) with ZnCl2 in a 1:1 M ratio
in absolute EtOH at ambient temperature yielded L1ZnCl2, L2ZnCl2,
L3ZnCl2, and L4ZnCl2 as white solids with high yields (88–90%)
(Scheme 1). These complexes were characterised using 1H NMR,
IR and CHN elemental analysis. All resonance peaks and the inte-
gration ratios of 1H NMR spectra were completely consistent with
the formulation of the complexes, indicating that the compounds
seen in the solid state by X-ray diffraction was preserved. The
structure of L3ZnCl2 was confirmed by the NMR spectrum of the
complex compared with that of L4ZnCl2 which established by X-
ray structure.
12.6721(7)
13.2123(11)
11.2029(9)
108.795(5)
1775.7(2)
4
10.0073(11)
11.1202(6)
18.4284(11)
10.2683(10)
11.1405(12)
18.7145(15)
b (Å)
c (Å)
b (°)
V (Å3)
2050.8(3)
4
2140.8(4)
4
Z
Dcalc (Mg/m3)
F(0 0 0)
1.536
848
1.421
912
1.461
976
Reflections collected 1774
4494
3797
4712
3984
Independent
reflections
Reflections obsd
(>2d)
Data Completeness 0.979
Data/parameters
1650
1374
3132
3037
1.000
1.000
1617/104
3797/228
1.063
3984/228
1.030
Goodness-of-fit on 1.027
F2
Final R [I > 2d(I)]
R1 = 0.0268
wR2 = 0.0734
R1 = 0.0364
R1 = 0.0280
wR2 = 0.0705
R1 = 0.0424
wR2 = 0.0736
R1 = 0.0338
wR2 = 0.0892
R1 = 0.0586
wR2 = 0.0950
A single X-ray quality crystal was obtained from slow evapora-
tion of hot EtOH solutions. The crystal structures of L1ZnCl2,
L2ZnCl2 and L4ZnCl2 were determined using single crystal X-ray
diffraction. Some details on data collection and refinement are
R (all data)
wR2 = 0.0757
D
qmax,
(e Åꢀ3
)
0.346 and ꢀ0.348 0.371 and ꢀ0.384 0.324 and ꢀ0.392
min

K.S. Kwon et al. / Polyhedron 77 (2014) 32–38
35
given in Table 1. An ORTEP drawings of L1ZnCl2, L2ZnCl2 and
L4ZnCl2 with atomic labelling are shown in Figs. 1-3, respectively,
along with their selected bond lengths (Å) and angles (°). These
complexes were found to be solvent-free and existed in a four-
coordinated monomeric form. The central zinc atom of each com-
plex was four-coordinated and adopted a somewhat distorted tet-
rahedral geometry by coordinating to the two N atoms of the
enantiopure N,N-diamine ligands and two chloro ligands. The dif-
ference between the angles (°) N1–Zn1–N#1 and N1–Zn1–N2
[86.3(1) and 84.15(8), 86.7(1)] and Cl1–Zn1–Cl1#1 and Cl1–Zn1–
Cl2 [118.49(4) and 122.30(3), 122.98(6)] for L1ZnCl2, L2ZnCl2 and
L4ZnCl2, respectively, are indicative of distortion from ideal tetra-
hedral geometry. A similar distorted tetrahedral coordination
sphere with smaller N–Zn–N angles has been reported in com-
pounds containing diamine ligands [30d,39]. Furthermore, the
Zn–N and Zn–Cl bond lengths of the complexes L1ZnCl2, L2ZnCl2,
and L4ZnCl2 showed only minor variations [2.079(2)–2.103(2)
and 2.198(1)–2.2241(7)], and these metric parameters were simi-
lar to complexes reported previously [34,38,40]. Distances
between Zn and O or S at L2ZnCl2, and L4ZnCl2 were 3.118(2) &
3.332(2) Å and 3.498(1) & 3.500(1) Å, respectively, which are indic-
ative of weak interactions between Zn with O or S atoms. More-
0
over, two chiral centres RC and RC were confirmed based on the
crystal structures of these dichloro-zinc complexes, which are
derived from the (R,R)-1,2-diaminocyclohexane fragment, while
the hydrogen atoms of the chiral carbon and nitrogen were found
to be in a head-to-tail conformation. Coordination of the enantio-
pure ligand with a divalent zinc centre resulted in five-membered
metallaheterocyclic rings that induced chirality in two nitrogen
atoms, i.e. RN and RN, of diaminocyclohexane moieties.
3.2. Ring-opening polymerisation of rac-LA using Me2CHOZnCl(L1–L4)
complexes
The catalytic capabilities of isopropoxide derivatives from zinc
complexes, generated in situ by treating dichloro analogues with
one equivalent of Me2CHOLi in THF, were assessed for ROP of
rac-LA. The polymerisation was conducted at two different temper-
atures; i.e. 25 and 25 °C. The isopropoxide complexes acted as
highly effective single component living initiators for ROP of rac-
LA at both temperatures. The ROP for 50 equivalent of rac-LA
was completed almost within 30 min. The in situ formation of
active catalytic species was confirmed using 1H NMR analysis.
The Mn values of the PLA were determined based on end-group
analysis with NMR spectra and by GPC in THF relative to the poly-
styrene standard. The polymerisation results are summarised in
Table 2. The experimentally determined Mn values of the PLAs
obtained from NMR and GPC were similar to the theoretical values.
In addition, relatively narrow PDIs of 1.26–1.30 were indicative of
well-controlled living polymerisation mechanisms with a single
reaction site provided by these isopropoxide zinc complexes
(Table 2). However, Mn values determined by GPC were lower than
the predicted/theoretical values based on (%) conversion. The 1H
NMR spectra of the resultant PLAs indicated that the polymer chain
was capped with an ester [–C(@O)OCHMe2] group at one end and a
hydroxyl group [–CH(Me)–OH] at the other end. These results sug-
gested that polymerisation mediated by these isopropoxide zinc
complexes may be initiated by the transfer of an alkoxy (–OCH-
Me2) group to the monomer, with the formation of metal alkoxide
species for a propagating step [42,43].
C3
C2
C1
C4
N1
C5
Zn1
O1A
C7
C8
C6
Cl1
Fig. 1. An ORTEP drawing of L1ZnCl2 with the numbering scheme at 40% probability
level. Zn–N1, 2.079(2); Zn–Cl1, 2.2218(6). N1#1–Zn–N1, 86.3(1); N1#1–Zn–Cl1#1
,
114.74(5); N1–Zn–Cl1#1
,
109.13(5); N1#1–Zn–Cl1, 109.13(5); N1–Zn–Cl1,
Microstructure analysis of the PLAs at two different tempera-
tures were assigned using the peak area of the methine proton
114.74(5); Cl1#1–Zn–Cl1, 118.49(4). #1 Symmetry transformations used to generate
equivalent atoms: ꢀx, y, ꢀz + 1/2.
C4
C3
C2
C5
C1
C6
C13
C7
O1
N2
C15
C14
N1
C8
Zn1
C9
C16
O2
C17
Cl1
Cl2
C10
C11
C18
C12
Fig. 2. An ORTEP drawing of L2ZnCl2 with the numbering scheme at 40% probability. Selected bond distances (Aꢀ) and angles (°): Zn–N1, 2.103(2); Zn–N2, 2.110(2); Zn–Cl1,
2.2047(8); Zn–Cl2, 2.2241(7). N1–Zn–N2, 84.15(8); N1–Zn–Cl1, 113.65(6); N1–Zn–Cl2, 108.95(6); N2–Zn–Cl1, 110.61(6); N2–Zn–Cl2, 110.73(6); Cl1–Zn–Cl2, 122.30(3).

36
K.S. Kwon et al. / Polyhedron 77 (2014) 32–38
C3
C4
C2
C5
C1
N1
C6
N2
C7
C13
S2
C9
C14
C8
Zn1
C15
C10
Cl2
C16
C11
S1
Cl1
C17
C12
C18
Fig. 3. An ORTEP drawing of L4ZnCl2 with the numbering scheme at 40% probability. Selected bond distances (Aꢀ) and angles (°): Zn–N1, 2.086(3); Zn–N2, 2.081(3); Zn–Cl1,
2.216(1); Zn–Cl2, 2.198(1). N1–Zn–N2, 86.7(1), N2–Zn–Cl2, 114.84(9); N1–Zn–Cl1, 110.05(9), N1–Zn–Cl2, 109.0(1), N2–Zn–Cl2, 114.84(9); N2–Zn–Cl1, 107.31(9); Cl2–Zn–
Cl1, 122.98(6).
Table 2
Polymerisation of rac-LA with in situ generated (L1–L4)ClZnOCHMe2 initiators.
O
O
O
O
H
R
S
(L1-L4)ClZnOCHMe2
R
S
O
O
n
O
+ n
O
O
n
O
O
Runa
Catalyst
T (°C/min.)
Conv. %b
Mn (g/mol)c x103 (calcd.)
Mn (NMR) (g/mol)d ꢁ103
Mn (g/mol)e ꢁ103
PDId
Prf
1
2
3
4
5
6
7
8
9
10
Me2CHOLi
Me2CHOLi
25/10
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
7.2
7.2
7.2
7.2
7.2
7.2
7.2
7.2
7.2
7.2
8.51
8.61
6.11
6.31
6.12
6.35
7.15
6.75
7.17
7.62
7.93
8.03
5.14
5.23
5.12
5.24
5.27
5.59
5.79
5.54
1.30
1.30
1.30
1.30
1.31
1.29
1.30
1.25
1.30
1.31
0.50
0.54
0.71
0.74
0.74
0.74
0.71
0.77
0.77
0.80
ꢀ25/20
(L1)ClZnOCHMe2
(L1)ClZnOCHMe2
(L2)ClZnOCHMe2
(L2)ClZnOCHMe2
(L3)ClZnOCHMe2
(L3)ClZnOCHMe2
(L4)ClZnOCHMe2
(L4)ClZnOCHMe2
25/5
ꢀ25/10
25/5
ꢀ25/10
25/15
ꢀ25/30
25/15
ꢀ25/30
a
b
c
Conditions: [Initiator] = 0.0625 mmol, [rac-LA]/[Initiator] = 50, 5 mL of THF, polymerization time = 10 ꢂ 30 min.
Conversion (%) determined by 1H NMR spectroscopy.
Calculated from [molecular weight of rac-LA] ꢁ [rac-LA]/[initiator] ꢁ conversion%.
d
e
Experimental molecular weight determined by 1H NMR from the relative intensities of the main chain and terminal resonances.
Experimental values (corrected using the Mark-Houwink factor of 0.58) [41] were determined by gel permeation chromatography (GPC) in THF relative to polystyrene
standard.
f
Probability of heterotactic enchainment (Pr) were calculated on the basis of homonuclear decoupled 1H NMR spectra using equation Pr = 2I1/(I1 + I2), where I1 = (sis + sii),
and I2 = (iis + iii + isi) [12b,44,46].
region of the homodecoupled 1H NMR spectra [12b,44–46]. Analy-
sis of the polymer microstructures indicated that all complexes
showed a strong preference for alternating insertion of RR- and
SS-enantiomers, leading to heterotactic PLA (Pr = 0.80) at 25 °C,
where Pr is the probability of rac-dyad by insertion [12b,44–48].
However, PLAs obtained using Me2CHOLi as an initiator were
almost atactic. The homodecoupled 1H NMR spectrum obtained
with (L4)ClZnOCHMe2 (at 25 °C in THF) is shown in Fig. 4, and cal-
culated heterotacticities of obtained PLAs are listed in Table 2.
Polymerisation data shown in Table 2 clearly indicated that
temperature significantly influenced microstructures of the
obtained PLAs. Decreases in the polymerisation temperature from
25 to 25 °C resulted in a minor increase of heterotactic bias in all
cases. Methyl groups of furanylmethyl and thiophenylmethyl moi-
eties of the (R,R)-1,2-diaminocyclohexane fragment would impart
steric hindrance around the metal centre and then improve slightly
heterotacticities comparing L1 to L2 and L3 to L4. As reported by
other researchers, more flexible ligands can increase the stereopre-
ference in ROP of rac-LA [33b,49]. Unexpectedly, the heterotactic
bias was not affected by methyl substituents in pendent groups
at both temperatures in our current study. The 99% conversion
periods of LA by using L1 or L2 were shorter than those by using
L3 or L4 at 25 and 25 °C. The initiators (L1–L4)ClZnOCHMe2 may
generate heterotactic-enriched PLA by a chain-end-control mecha-
nism with unclear assist of the stereogenic centers, which has been
observed in previous studies [50,51], which is similar to findings at

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Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (NRF-2011-
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