Synthesis and structures of copper(II) complexes containing N,N-bidentate N-substituted phenylethanamine derivatives as pre-catalysts for heterotactic-enriched polylactide

Article abstract of DOI:10.1016/j.poly.2019.02.014

A series of Cu(II) complexes, [LⁿCuCl₂]₂ (Lⁿ = (S)-N-((1H-pyrazol-1-yl)methyl)-1-phenylethanamine (L¹), (S)-N-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-1-phenylethanamine (L²) and (S)-1-phenyl

Full text of DOI:10.1016/j.poly.2019.02.014

Accepted Manuscript
Synthesis and structures of copper(II) complexes containing N,N-bidentate N-
substituted phenylethanamine derivatives as pre-catalysts for heterotactic-en-
riched polylactide
Juhyun Cho, Min Kyung Chun, Saira Nayab, Jong Hwa Jeong
PII:
S0277-5387(19)30107-X
https://doi.org/10.1016/j.poly.2019.02.014
POLY 13758
DOI:
Reference:
Polyhedron
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Revised Date:
Accepted Date:
10 December 2018
7 February 2019
8 February 2019
Please cite this article as: J. Cho, M. Kyung Chun, S. Nayab, J. Hwa Jeong, Synthesis and structures of copper(II)
complexes containing N,N-bidentate N-substituted phenylethanamine derivatives as pre-catalysts for heterotactic-
enriched polylactide, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.02.014
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Revised
Synthesis and structures of copper(II) complexes containing N,N-bidentate N-
substituted phenylethanamine derivatives as pre-catalysts for heterotactic-
enriched polylactide
Juhyun Cho, Min Kyung Chun, Saira Nayaba, Jong Hwa Jeong*
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, Shaheed Benazir Bhutto University, Sheringal, Dir (Upper), Khyber
Pakhtunkhwa, Islamic Republic of Pakistan.
----------------------------------
*Corresponding author. Tel: +82-53-950-6343; fax: +82-53-950-6330
E-mail address: jeongjh@knu.ac.kr
Synthesis and structures of copper(II) complexes containing N,N-bidentate N-
substituted phenylethanamine derivatives as pre-catalysts for heterotactic-
enriched polylactide
Juhyun Cho, Min Kyung Chun, Saira Nayaba, Jong Hwa Jeong*
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, Shaheed Benazir Bhutto University, Sheringal, Dir (Upper), Khyber
Pakhtunkhwa, Islamic Republic of Pakistan.
Abstract
A series of Cu(II) complexes, [LnCuCl2]2 (Ln = (S)-N-((1H-pyrazol-1-yl)methyl)-1-
phenylethanamine (L1), (S)-N-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-1-phenylethanamine (L2)
and (S)-1-phenyl-N-((pyridin-2-yl)methyl)ethanamine (L3)) were synthesised and characterised.
X-ray diffraction analysis revealed that the molecular structures of these distorted square planar
Cu(II) complexes were dimeric with two asymmetric units crystallised in the (SC, SN) and (SC,
RN) configurations. These are rare examples of dimeric Cu(II) complexes. The alkoxide
derivatives, generated in situ, catalysed the ring-opening polymerisation of rac-lactide in a
controlled fashion and displayed high activities (2 or 3 min for complete conversion).
1

Heterotactic-enriched polylactide (with Pr up to 0.86) was formed using these initiators at room
temperature in CH2Cl2.
Keywords: Copper complexes, Heterotactic-enriched polylactide, Ring-opening polymerisation,
Single-site catalysts, X-ray structure
1. Introduction
Depleting petroleum resources and the accumulation of plastics in the environment favours the
use of bio-renewable and biodegradable polymers. Polylactide (PLA), a biodegradable and
biocompatible polymer, has attracted interest because lactide (monomer) can be obtained from
natural resources and does not depend directly on fossil fuels [1-4]. Using organometallic
complexes as initiators via the ″coordination-insertion mechanism″ is the preferred route to
PLAs because the products have high number average molecular weights (Mn) and low
polydispersity indices (PDIs) [5-9]. Polylactide has applications in biomedical, packaging, tissue
engineering, and pharmaceutical fields [9-12]. Therefore, using initiators based on complexes
containing biologically benign metals are important. In this regard, initiators based on alkali
metals [13], magnesium [14,15], calcium [16,17], zinc [18–20], iron [21], and aluminium [22,23]
have been used for the ring-opening polymerisation (ROP) of LA, with promising results.
However, there have been few studies regarding the use of Cu(II)-based initiators [24-30], and
those reported polymerised LA with low activities and stereoselectivities and required high
temperatures. Recently, (nacnacBnCuOiPr)2 showed unusually high activity with low
heteroselectivity (Prꢀ=ꢀ0.56) for the ROP of LA [31]. More recently, diimino pyrrolide copper
alkoxide yielded isotactic PLA with high Mn (44–45 kg/mol) and narrow PDIs (1.0–1.2) [32].
Similarly, our group demonstrated that Cu(II) initiators bearing ligands such as N1,N1-dimethyl-
N2-[(1R)-myrtenylmethyl]ethane-1,2-diamine [33], N,N-dimethylethylenamine-camphorylimine
[34], N-(pyridin-2-ylmethyl)amine [35] and (R,R)-1,2-diaminocyclohexane derivatives
effectively catalysed the ROP of rac-lactide (rac-LA) in a controlled manner and with high
activities and stereoselectivities [36,37].
In continuation of our previous work toward the development of effective and stereoselective
Cu(II) catalysts for the ROP of rac-LA, we describe herein the synthesis and structural
characterisation of Cu(II) complexes bearing N-substituted (S)-phenylethanamine derivatives and
their suitability as catalytic precursors for the polymerisation of rac-LA. Furthermore, we sought
to investigate the effects of ligand architecture on the catalytic efficacy and stereoselectivity of
the synthesised complexes toward the ROP of rac-LA.
2. Experimental
2.1. General considerations and materials
All manipulations involved in the synthesis of ligands (L1 – L3) and their corresponding
Cu(II) complexes, [LnCuCl2]2 (Ln = L1 – L3), were performed using bench-top techniques in the
air, unless otherwise specified. The synthesis of alkoxide derivatives of the Cu(II) complexes and
ROP reactions was performed using standard Schlenk techniques, high vacuum, and glove box
under argon. Tetrahydrofuran (THF) was dried over Na/benzophenone ketyl, while CH2Cl2 was
dried over CaH2; these solvents were subsequently distilled from these reagents under argon
prior to use. The rac-lactide (rac-LA) was purchased from Aldrich and was sublimed prior to
use. CuCl2.2H2O, 2-pyridinecarboxyaldehyde, pyrazole, 3,5-dimethylpyrazole, (S)-
phenylethanamine and LiOCHMe2 (2.0 M in THF) were obtained from Aldrich chemical
company. NMR solvents were purchased from Sigma Aldrich and stored over 3 Å molecular
2

sieves. Various solvents such as EtOH, Et2O, n-hexane and MeOH were purchased from high
grade commercial suppliers.
2.2. Instrumentation
1H (operating at 500 MHz) and 13C (operating at 125 MHz) NMR spectra were recorded
on a Bruker Avance Digital 500-NMR spectrometer (Bruker, Billerica, MA), and chemical
shifts were recorded in ppm units () relative to residual protium in the deuterated solvents
(CDCl3,  = 7.26). Coupling constants were reported in Hertz (Hz). Data was recorded as m =
multiplet, br = broad, s = singlet, d = doublet, t = triplet and q = quartet. For the homonuclear
decoupling NMR spectroscopy, Bruker Avance digital 600-NMR spectrometer was used.
Infrared spectra (IR) (neat) were recorded on Bruker FT/IR-Alpha and the data were reported in
cm-1. Elemental analyses were determined using the EA 1108-Elemental Analyzer at the
Chemical Analysis Laboratory of the Centre for Scientific Instruments of Kyungpook National
University. Gel permeation chromatography (GPC) analyses were carried out on a Waters
Alliance GPCV2000, equipped with differential refractive index detectors. The GPC columns
were eluted with tetrahydrofuran (THF) with 1 mL/min rate at 25 C and were calibrated with
monodisperse polystyrene standards. The 1H NMR spectra for examining PLA microstructures
in terms of tetrad distributions were recorded in CDCl3 and analysed according to previous
studies [38-41].
2.3. Synthesis of ligands
2.3.1. Synthesis of L1
A CH2Cl2 (10 mL) solution of (S)-(-)-methylbenzylamine (2.00 g, 16.50 mmol) was
treated with a CH2Cl2 (10 mL) solution of (1H-pyrazol-1-yl)methanol (1.62 g, 16.50 mmol) at
0 oC for 12 h. The reaction mixture was dried over MgSO4 and the solvent was removed in
vacuo to yield colorless oil as a final product (2.86 g, 86.1%). Anal. Calcd. for C12H15N3: C
71.61; H 7.51; N 20.88, Found: C 71.78, H 7.60, N 21.06 %. 1H NMR (500 MHz, CDCl3, 298
K) δ = 7.50 (d, J = 1.78 Hz, 1H, pzH), 7.27-7.26 (m, 1H, pzH), 7.25-7.16 (m, 5H, phH), 6.14
(t, J = 2.02 Hz, 1H, pzH), 4.92 (d, J = 13.64 Hz, 1H, CHaHb), 4.54 (d, J = 13.64 Hz, 1H,
CHaHb), 3.45 (q, 1H, J = 6.31 Hz, ph-CH-NH), 2.26 (br, 1H, NH), 1.20 (d, J = 6.31 Hz , 3H,
CH3). 13C NMR (500 MHz, CDCl3): δ =144.11, 140.08, 129.51, 128.64 (2C, phC), 127.36,
127.08 (2C, phC), 104.92, 63.23, 53.58, 24.46. IR (Neat; oily liquid; cm-1): 3298 (w), 2965
(m), 2867 (w), 1510 (m), 1490 (m), 1451 (s), 1393 (m) 1266 (s), 1226 (w), 1083 (s), 749 (s),
701 (s).
2.3.2. Synthesis of L2
An analogous method to that of L1 was utilized except that CH2Cl2 (10 mL) solution of
(S)-(-)-methylbenzylamine (2.00 g, 16.50 mmol) was treated with CH2Cl2 (10 mL) solution of
3,5-dimethyl-1H-pyrazol-1-yl)methanol (2.08 g, 16.50 mmol) at 0 oC to yield colorless oil
(3.22 g, 84.9%). Anal. Calcd. for C14H19N3: C 73.33; H 8.35; N 18.32, Found: C 73.39, H 8.37,
N 18.37 %. 1H NMR (500 MHz, CCDl3, 298 K) δ = 7.29-7.16 (m, 5H, phH), 5.67 (s, 1H, pzH),
4.70 (d, J = 13.64 Hz, 1H, CHaHb), 4.49 (d, J = 13.64 Hz, 1H, CHaHb), 3.54 (q, 1H, J = 6.56
Hz, ph-CH-NH), 2.43 (br, 1H, NH), 2.17 (s, 3H, pz-CH3), 1.85 (s, 3H, pz-CH3), 1.23 (d, J =
6.56 Hz , 3H, CH3). 13C NMR (500 MHz, CDCl3): δ = 148.01, 144.57, 139.39, 128.49(2C,
phC), 127.11, 126.82 (2C, phC), 104.70, 59.63, 53.59, 24.89, 13.55, 10.51. IR (Neat; oily
3

liquid; cm-1): 3287 (w), 3028 (w), 2963 (m), 2868 (w), 1551 (s), 1452 (s), 1269 (s), 1029 (s),
762 (s), 701 (s).
2.3.1. Synthesis of L3
A CH2Cl2 solution (20 mL) of (S)-methylbenzylamine (3.00 g, 24.75 mmol) and 2-
pyridinecarboxaldehyde (3.12 g, 24.75 mmol) was stirred at ambient temperature for 2 days.
The reaction mixture was dried over MgSO4 and the solvent was removed in vacuo. The imine
compound was isolated as yellow sticky oil. NaBH4 (1.30 g, 3.43 mmol) was added to the
MeOH (50 mL) solution of imine compound at 0 oC. The solvent was removed in vacuo after
stirring for one day and the resultant residue was treated with water (40 mL) and CH2Cl2 (40
mL). Combined organic phase was separated, dried over MgSO4. The solvent was removed in
vacuo to yield light yellow oil as a final product (4.36 g, 90 %). Anal. Calcd. for C14H16N2: C
79.21; H 7.60; N 13.20, Found: C 79.30, H 7.65, N 13.24 %. 1H NMR (500 MHz, CDCl3, 298
K) δ = 8.50 (d, J = 4.80 Hz, 1H, pyH), 7.64 (t, J = 7.83 Hz, 1H, pyH), 7.48 (d, J = 7.83 Hz, 1H,
pyH), 7.42-7.40 (m, 2H, phH), 7.34-7.31 (m, 2H, phH), 7.25-7.20 (m, 1H, phH), 7.13 (t, J =
6.06 Hz, 1H, pyH), 3.73 (d, J = 14.40 Hz, 1H, CHaHb), 3.70 (q, J = 6.56 Hz, 1H, ph-CH-NH),
3.53 (d, J = 14.40 Hz, 1H, CHaHb), 1.78 (br, 1H, NH), 1.44 (d, J = 6.56 Hz, 3H, CH3). 13C
NMR (500 MHz, CDCl3): δ = 159.88, 149.32, 145.44, 136.32, 128.48, 126.97, 126.80, 122.43,
121.85, 58.06, 53.13, 24.47. IR (Neat; oily liquid; cm-1): 3315 (w), 3060 (w), 2965 (w), 1590
(m), 1568 (m), 1491(m), 1473 (m), 1450 (m), 1432 (m), 1369 (m), 1128 (m), 756 (s), 700 (s).
2.4. Synthesis of Cu(II) complexes
2.4.1. Synthesis of [L1CuCl2]
A CH2Cl2 solution (10 mL) of L1 (0.56 g, 2.78 mmol) and CuCl22H2O (0.47 g, 2.78
mmol) was stirred at ambient temperature for 12 h. The solution of reaction mixture was dried
over anhydrous MgSO4 and the organic solvent was reduced under reduced pressure. Washing
with Et2O afforded green solid. The green solid was dried under vacuum at 60 oC to get final
product (0.80 g, 88.2%). Anal. Calcd. for C24H30Cl4Cu2N6: C 42.93; H 4.50; N 12.52, Found: C
43.00, H 4.54, N 12.56%. IR (solid neat; cm-1): 3223 (w), 3184 (m), 3106 (w), 2965 (w), 1518
(m), 1499 (m), 1452 (s), 1390 (m), 1269 (s), 1196 (m), 1065 (s), 782 (s), 755 (s), 732 (s), 701
(s).
2.4.2. Synthesis of [L2CuCl2]
An analogous method to that of [L1CuCl2] was utilize except that L2 (0.52 g, 2.26 mmol) was
treated with CH2Cl2 (30 mL) solution of CuCl22H2O (0.38 g, 2.26 mmol) to afforded green
solid as a final product (0.70 g, 87.4%). Anal. Calcd. for C28H38Cl4Cu2N6: C 46.22; H 5.26; N
11.55, Found: C 46.27, H 5.24, N 12.00%. IR (solid neat; cm-1): 3162 (w), 2975 (w), 2928 (w),
1552 (s), 1468 (s), 1385 (s), 1290 (m), 1265 (m), 1017 (s), 785 (s), 739 (s), 703 (s).
2.4.3. Synthesis of [L3CuCl2]
An analogous method to that of [L1CuCl2] was utilize except that L3 (2.58 g, 12.15 mmol) was
treated with CH2Cl2 (30 mL) solution of CuCl22H2O (2.07 g, 12.15 mmol) CuCl22H2O to
afford green solid as a final product (3.81 g, 91 %). Anal. Calcd. for C28H32Cl4Cu2N4: C 48.49;
H 4.65; N 8.08, Found: C 49.05, H 4.69, N 8.12%. IR (solid neat; cm-1): 3202 (m), 3171 (m),
3070 (w) 2943 (w), 1606 (m), 1571 (m), 1482 (m), 1444 (m), 1420 (m), 1379 (m), 1289 (m),
1104 (m), 1053 (m), 1027 (s), 767 (s), 695 (s).
4

2.5. X-ray Crystallography
An X-ray quality single crystals of [LnCuCl2] (Ln = L1 - L3) were mounted in a thin-walled
glass capillary on an Enraf-Noius CAD-4 diffractometer with Mo Kα radiation (λ = 0.71073 Å).
Unit cell parameters were determined by least-squares analysis of 25 reflections (11° < θ < 14°).
Intensity data were collected with θ range of 1.56° – 25.47° in ω/2θ scan mode. Three standard
reflections were monitored every 1 h during data collection. The data was corrected for Lorentz-
polarization effects and decay. Empirical absorption corrections with ψ-scans were applied to the
data. The structure was solved by using Patterson method and refined by full-matrix least-squares
techniques on F2 using SHELXL-97 [42] and SHELXSL program packages [43]. All non-hydrogen
atoms were refined anisotropically and all hydrogen atoms were positioned geometrically using a
riding model with fixed isotropic thermal factors. Structural refinement and crystallographic data
for [LnCuCl2]2 (Ln = L1 – L3) is presented in Table 1.
2.6. Polymerisation studies
Alkoxide derivatives were prepared in situ by adding a THF solution of LiOCHMe2 (0.50 mL of
2.0 M solution of in THF, 1.00 mmol) dropwise to a THF (7.50 mL) solution of Cu(II) complex
(0.50 mmol) at -78 °C. After being stirred for 2 h at room temperature, the resulting THF
solution of alkoxide derivatives was used as a catalyst for polymerisation reaction. The general
procedure for the ROP of rac-LA was as follows. A 100 mL of Schlenk flask was charged with
rac-LA (0.90 g, 6.25 mmol) in the glove box. Dried CH2Cl2 (5 mL) was transferred to the
reaction vessel via syringe and stirred to make a clear solution. The reaction was initiated by
quickly adding the catalyst solution (1.00 mL, 0.0625 mmol) via a gas tight syringe under argon
in the monomer solution at 25 °C. Then the reaction mixture was stirred at 25 °C for specified
time. The polymerisation reaction was quenched after 3 min using H2O (3 mL) and sequentially
precipitated by adding hexane (20 mL). The resultant sticky polymeric material was dried in
vacuo and the obtained fluffy solid was subjected to 1H NMR analysis. 1H NMR (400 MHz,
CDCl3, 298 K): δ 5.145.25 (m, 1H), 1.541.63 (m, 3H). Conversion yield was determined by
observing the methine resonance integration of monomer vs. polymer methyl resonances in the
1H NMR spectra.
3. Results and Discussion
3.1. Synthesis
The ligands (S)-N-((1H-pyrazol-1-yl)methyl)-1-phenylethanamine (L1) and (S)-N-((3,5-
dimethyl-1H-pyrazol-1-yl)methyl)-1-phenylethanamine (L2) were obtained via a single step
condensation reaction of (S)-phenylethanamine with (1H-pyrazol-1-yl)methanol and (3,5-
dimethyl-1H-pyrazol-1-yl)methanol, respectively, in CH2Cl2. However, for (S)-1-phenyl-N-
5

((pyridin-2-yl)methyl)ethanamine (L3), the condensation reactions of (S)-phenylethanamine with
corresponding 2-pyridinecarboxyaldehyde yielded the imine intermediate, which was
sequentially reduced to the amine using a mild reducing agent, i.e. NaBH4 (Scheme 1). The
structure and identity of the ligands were characterised using 1H nuclear magnetic resonance (1H
13
NMR) and C NMR spectroscopies. The methine protons Ph–CH–CH3 appeared at δ = 3.45,
3.54, and 3.70 ppm in L1, L2, and L3, respectively. Similarly, the CH3 protons appeared at 1.20
for L1, 1.23 for L2, and 1.44 ppm for L3. The corresponding [LnCuCl2]2 (Ln = L1–L3) complexes
were obtained (88–91% yields) by direct ligation of the metal-containing starting materials with
the ligands in the 1:1 ratio in dry CH2Cl2 at ambient temperature (Scheme 1). Owing to the
paramagnetic nature of the Cu(II) complexes, we were unable to structurally characterise the
[LnCuCl2]2 (Ln = L1–L3) via NMR spectroscopy. However, the infrared spectra of the ligands in
the N–H stretching region were compared with those of the complexes. Characteristic N–H
peaks for L1, L2, and L3 were observed at 3298, 3287, and 3315 cm-1, whereas the N–H
stretching bands appeared at 3223, 3162, and 3203 cm–1 in the corresponding Cu(II) complexes.
Elemental analyses of the synthesised complexes were consistent with the proposed structures
shown in Scheme 1 and confirmed the purity of the isolated complexes [LnCuCl2]2 (Ln = L1–L3).
All of the synthesised complexes were stable toward oxygen and moisture.
3.2. Molecular structure
X-ray quality single crystals of [LnCuCl2]2 (Ln = L1–L3) were obtained by layering n-
hexane on CH2Cl2 solutions. Their molecular structures at the 30% probability level are
illustrated in Figs. 1–3, respectively. The representative bond lengths and angles are listed in
Table 2. The synthesised Cu(II) complex [L1CuCl2]2 crystallised in the orthorhombic system
with space group P212121, whereas [L2CuCl2]2·2CH2Cl2 and [L3CuCl2]2 crystallised in the
monoclinic system with space group P21 .
It is evident from the molecular structures that the Cu atom in each independent unit
adopted a distorted square planar geometry by coordinating with two nitrogen atoms of the
6

bidentate ligand in a chelating manner and with one terminal and one bridging chloro ligand. The
Cu–Namine bond distances varied from 2.086(3) to 2.0793(3) Å in [L1CuCl2]2, 2.072(3) to
2.104(3) Å in [L2CuCl2]2 and 2.022(3) to 1.999(2) Å in [L3CuCl2]2. Similarly, the Cu–Npyrazole
bond lengths were 1.961(4) and 1.963(4) Å for [L1CuCl2]2 and 1.977(4) and 1.984(4) Å for
[L2CuCl2]2, whereas the Cu–Npyridine lengths were 2.003(2) and 1.999(2) Å for [L3CuCl2]2. These
Cu–Namine and Cu–Npyrazole/pyridine bond lengths are in the expected range for complexes with
similar ligands [35,44]. It is evident that the pyrazole substituents influenced the Cu–Npyrazole
bond lengths. Ligands with the bulkier pyrazole moiety in [L2CuCl2]2 led to longer bond lengths
compared with the [L1CuCl2]2 analogue having an unsubstituted pyrazole moiety. The Cu–
Clterminal bond lengths ranged from 2.231(9) to 2.275(12) Å in [LnCuCl2]2 (Ln = L1–L3); they were
shorter than the Cu1–Cl1 and Cu2–Cl4 bond lengths, which ranged from 2.2620(11) to
2.2702(19) Å. However, the Cu–Clterminal lengths lay in the range reported for related complexes
[35,45]. The Cu1∙∙∙Cl4 and Cu2∙∙∙Cl2 bond lengths, ranging from 2.745(2) to 2.818 Å in the
[LnCuCl2]2 (Ln = L1–L3) complexes, are consistent with weak interactions; the lengths were
slightly affected by the substituents attached to the pyrazole moiety (Table 2). All of the
synthesised complexes had a Cu∙∙∙Cu separation averaging 3.435(7) Å. This length is shorter than
that previously reported for complexes having the same ligand framework [35, 46].
The average Namine–Cu–Npyrazole/pyridine bond angles in [L1CuCl2]2, [L2CuCl2]2, and
[L3CuCl2]2 ranged from 80.37(13) to 82.49(10), revealing the distortion from ideal square
planar geometry. Distortion from the ideal geometry is also evident from the dihedral angle
between the Cl2–Cu–Cl1 and Npyrazole/pyridine–Cu–Namine planes, which ranged from 11.8(2) to
19.8(2). Similarly, the Npyrazole–Cu–Clterminal angles are 94.7(1) and 94.8(1) in [L1CuCl2]2,
98.3(1) and 97.5(1) in [L2CuCl2]2, 96.05(8) and 95.67(8) in [L3CuCl2]2 and are found to be
are smaller than those of our previously reported complexes (Table 3) [35,45]. The Npyrazole–Cu–
Clterminal angles were slightly affected by pyrazole/pyridine moieties. The Cl1–Cu1–Cl2 bite
angles were 94.04(5), 93.92(4) and 92.46(4), whereas the Cl3–Cu2–Cl4 angles were 94.11(5),
92.46(4) and 93.21(3) in [L1CuCl2]2, [L2CuCl2]2 and [L3CuCl2]2, respectively.
The formation of Cu(II) complexes by ligation of N-substituted phenylethanamine ligands
containing a stereogenic carbon centre (derived from (S)-methylbenzylamine) induced chirality
in the nitrogen atoms of the pyrazole and pyridine moieties by restricting free motion [47-49]. It
is evident from the molecular structures that the induced chirality of the nitrogen atom in the
same crystal structure is different. For example, [LnCuCl2]2 exists as a dimer with two molecules
in asymmetric units in the (SC, SN) and (SC, RN) configuration (Figs. 1–3). Wang et al. reported a
compound containing camphor-based ligands in which the asymmetric unit of the compound
comprised two diastereomers [50]. These compound represents a very rarely seen example of a
four-coordinate dimeric Cu(II) complex having a diastereomeric asymmetric unit.
7

3.3. rac-LA polymerisation
The alkoxide derivatives of the synthesised Cu(II) complexes [LnCu(OiPr)2] (Ln = L1–L3)
were investigated for the ROP of rac-LA. The polymerisation was carried out at room
temperature in CH2Cl2. The polymerisation data are listed in Table 4. It is evident that the ROP
of rac-LA was effectively initiated by the alkoxide derivatives, i.e. [LnCu(OiPr)2] (Ln = L1–L3),
yielding PLAs with high Mn and narrower PDIs. Complete conversion of rac-LA to PLA was
confirmed by the absence of monomer (rac-LA) signals in the 1H NMR spectra of the resultant
PLAs. The polymerisation data in Table 4 establish that the conversion of monomer to polymer
was complete within 2 min using the [LnCu(OiPr)2] (Ln = L1 and L3) initiators, whereas the
[L2Cu(OiPr)2] initiator effected complete polymerisation within 3 min (Table 4).
The Mn of the PLAs made using [LnCu(OiPr)2] (Ln = L1–L3) were determined based on using 1H
NMR end-group analysis and by GPC in THF relative to polystyrene standards [51]. The Mn
determined from the 1H NMR spectra of the obtained polymer and those determined with GPC
for the alkoxide derivatives of the Cu(II) initiators were almost identical to the Mn (corrected
using the Mark–Howink factor of 0.58) [52] value calculated from the monomer/initiator molar
ratio. The Mn of PLA determined by GPC are typically larger than the actual molecular weights,
when using polystyrene standards for GPC analysis of PLA. In order to compensate such
discrepancies, multiplication of the GPC data by a factor of 0.58 has been recommended to attain
more accurate Mn [53-55]. The experimentally determine Mn values were in good agreement with
theoretical Mn values, calculated for one growing polymer chain per initiator molecule. In
addition, the narrower PDIs (1.20–1.30) revealed that the polymerisation was well-controlled.
Notably, the activities of the Cu(II) initiators in the current study decreased with increasing size
of the substituent on the amine moiety. Polymerisation using [L2Cu(OiPr)2], bearing dimethyl
substituents at the 3 and 5 positions of the pyrazole moiety, was slower than with [L1Cu(OiPr)2],
bearing the unsubstituted pyrazole moiety (Table 4). That complexes with bulkier ligands were
less active toward LA polymerisation is attributed to bulky ligands tending to hamper the
coordination/insertion of an incoming monomer, which is detrimental to the catalytic activity
[56].
End-group analysis by 1H NMR spectroscopy revealed the hydroxyl group [–CHMe–
(OH)] signal at  2.90 ppm; the signals for the isopropoxide [–C(O)CH(CH3)2] group at the other
terminus overlapped with those of the PLA backbones. The ROP of rac-LA using these initiators
favours a coordination-insertion mechanism [57,58]. In this mechanism, initial coordination of
rac-LA to the central metal, yielding a penta-coordinated intermediate, is followed by cleavage
of the acyl-oxygen bond to open the LA ring. Another molecule of rac-LA is opened by attach-
ment to the metal centre in an identical manner. Subsequent addition of rac-LA produces hetero-
enriched PLA.
Polymerisation with Me2CHOLi alone was also attempted; the polymeric product was
obtained slowly with insignificant stereoselectivity (Table 4). The polymerisation data reveal the
role of the Lewis acidic metal centre and the influence of the ligand architecture that provided
better control over polymerisation and influenced the microstructures of the resultant PLAs. Sim-
ilarly, the dichloro complexes [LnCuCl2]2 (Ln = L1–L3) showed no activity toward ROP of rac-
LA under similar experimental conditions.
Microstructural analysis of the obtained PLAs was performed by inspecting the methine
proton region of the homodecoupled 1H NMR spectra and Pr values were calculated with
8

equation Pr = 2I1/(I1 + I2), where I1 = (sis + sii) and I2 = (iis + iii + isi), as reported previously
[41,59,60]. A representative homodecoupled 1H NMR spectrum of methine region of obtained
PLA has been shown in Fig. 4. It is evident from the polymerisation data that all of the
complexes gave heterotactic PLAs (Table 4). Unlike the catalytic activities, the heterotactic
enchainment was not significantly affected by the steric bulk of the ligands around the metal
centre. However, it was observed that 3,5-dimethyl-substituted pyrazole-bearing ligands attached
to the metal centre exhibited slightly higher heterotactic bias, indicating that large steric
hindrance might assist the regularity of monomer insertion.
Compared with our previously reported dimeric Cu(II) complexes, the current system is superior
in terms of activity as well heterotacticity [35]. Similarly, unsymmetrical formamidine dimeric
Cu(II) complexes exhibited lower activities toward ROP of rac-LA compared with our current
system, resulting in PLAs with broad PDIs (1.78–1.87) [61]. Imino phenoxide-based copper
complexes reported by Mandal et al. for the ROP of LA yielded PLAs with moderate molecular
weights with low stereoselectivity at a conversion rate of approximately 90% [62]. Recently
reported NacnacBnCuOiPr, proved to be highly active catalyst for ROP of LA but essentially
produced atactic PLA [31]. The ligand architecture and pendent groups strongly influence the
reactivity and stereo-chemical outcomes in ROP of rac-LA. As evident from our previous reports
where CuII) ligated to N,N-dimethylethylenamine-camphorylimine [(CDM)Cu(OiPr2)2] [34] and
N1,N1-dimethyl-N2-[(1R)-myrtenylmethyl]ethane-1,2-diamine [(L)Cu(OiPr2)2] [35] ligands have
almost identical backbone and methyl substituents with the only difference of R-myrentyl and R-
camphor pendent groups, exhibited variable activities and stereoslectivities. Thus, the activity as
well stereoselectivity for ROP of rac-LA can be control by the proper complexity of the
substituents at the donor nitrogen atoms of N,N-bidentate ligands.
The polymerisation results of our current catalytic system show that the activities of
these initiators were affected by the steric hindrance provided by the ligand framework around
the metal centre. Increased steric bulk around the metal centre negatively affected the activities,
whereas the stereoslectivities toward ROP of rac-LA remained unaffected. Our results are con-
sistent with polymerisation progressing through a chain-end mechanism, as reported previously
[63].
4. Conclusion
We investigated the synthesis and X-ray crystallographic structures of [LnCuCl2]2 (Ln = L1–L3)
comprising chiral N-substituted phenylethanamine derivatives. The molecular structures revealed
that the Cu(II) complexes favoured a distorted square planar geometry and existed as dimers in
the solid state. The two asymmetric units were diastereomeric with (SC, SN) and (SC, RN)
configurations. The alkoxide derivatives, generated in situ, of the Cu(II) complexes exhibited
higher activities toward ROP of rac-LA and afforded heterotactic PLAs. Steric bulk around the
9

metal centre negatively affected the polymerisation activity, whereas the resultant
stereoselectivity remained unchanged.
Supplementary materials
CCDC 1883665-1883667 contains the supplementary crystallographic data for complexes
[L1CuCl2]2, [L1CuCl2]2 and [L1CuCl2]2, respectively. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research
Foundation of Korea (NRF-2017R1D1A3B03030670).
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13

OH
Ph
Cl
N
NH
N
N
N
Cl
N
Cu
Cu
N
N
CH2Cl2/ 0 oC/ 12 h
N
H
Cl
Cl
HN
Ph
N
OH
L1
[L1CuCl2]2
Ph
N
N
Ph
Cl
NH
N
N
Cl
CuCl2. 2H2O
Cu
N
H2N
Cu
N
N
N
H
CH2Cl2/ 0 oC/ 12 h
CH2Cl2/ RT/ 12 h
Cl
Ph
Ph
Cl
N
HN
L2
Ph
[L2CuCl2]2
H
Ph
N
Cl
NH
N
N
H
O
N
Cl
Ph
Cu
(i) = CH2Cl2/ RT/ 48 h
(ii) = MeOH/ NaBH4/RT/12 h
N
Cu
L3
Cl
Cl
HN
Ph
[L3CuCl2]2
Scheme 1. Synthetic route of chiral N-substituted phenylethanamine derivatives and corresponding Cu(II) complexes.
Table 1. Crystal data and structure refinement for [LnCuCl2]2 (Ln = L1 – L3) complexes.
14

[L1CuCl2]2
[L3CuCl2]2
[L2CuCl2]22CH2Cl2
Empirical formula
Formula weight (g/mol)
C24H30Cl4Cu2N6
671.42
C28H32Cl4Cu2N2
693.46
C28H38Cl4Cu2N62CH2Cl2
897.37
Wavelength (Å)
0.71069
0.71069
0.71306
Crystal system, Space group
Orthorhombic, P212121
Monoclinic, P21
Monoclinic, P21
Unit cell dimensions
a (Å)
b (Å)
c (Å)
12.0544(1)
13.3826(1)
17.5951(9)
11.481(8)
9.941(7)
17.913(6)
10.0923(7)
13.1533(8)
11.6087(7)
90
102.367(5)
1997(2), 2
1.492
98.109(7)
1525.61(17), 2
1.510
 ()
V (Å3) , Z
2737.9(4), 2
1.571
Dcalc (Mg/m3)
Absorption coefficient (mm-1)
F(0 0 0)
1.900
1.603
1.769
1368
916
708
θ range for data collection ( °)
Crystal size (mm)
Index ranges
1.91 - 25.47
0.50 x 0.50 x 0.45
1.16 - 25.46
0.50 x 0.45 x 0.40
1.77- 25.47
0.50 x 0.46 x 0.35
-14 ≤ h ≤ 124, -6 ≤ k ≤ 16,
-21 ≤ l ≤21
-13 ≤ h ≤ 13, -12 ≤ k ≤ 12,
-21 ≤l ≤ 21
-12 ≤ h≤ 12, -15 ≤ k ≤ 15,
-14 ≤ l ≤ 14
Reflections collected
6085
8397
6301
Independent reflections
5271 (Rint = 0.0229 )
7358 (Rint = 0.0126)
5639 (Rint t = 0.0259)
Reflections observed (>2σ)
4860
5962
3776
Data Completeness
0.999
0.994
0.999
Refinement method
Full-matrix least-squares on
F2
Full-matrix least-squares on
F2
Full-matrix least-squares on
F2
Data / restraints / parameters
Goodness-of-fit (GOF) on F2
Final R indices [I >2δ (I)]
R indices (all data)
5271 / 0 / 327
1.266
R1 = 0.0234 wR2 = 0.0706
R1 = 0.0362 wR2 = 0.1095
7358 / 1 / 421
1.069
R1 = 0.0502 wR2 = 0.0938
R1 = 0.0502 wR2 = 0.1016
5639 / 1 / 345
1.071
R1 = 0.0300 wR2 = 0.0769
R1 = 0.0356 wR2 = 0.0793
Absolute structure parameter
0.007(15)
0.002(11)
0.000(10)
Largest diff. peak & hole (e. Å-3)
0.249 and -0.293
0.521 and -0.398
0.641 and -0.539
15

Table 2. Selected bond lengths (Å) and bond angles (º) of synthesised [LnCuCl2]2 (Ln = L1 – L3)
complexes.
[L1CuCl2]2
[L2CuCl2]2
[L3CuCl2]2
Bond Lengths (Å)
1.961(4)
1.977(4)
N(1)-Cu(1)
N(3)-Cu(1)
Cu(1)-Cl(1)
Cu(1)-Cl(2)
Cu(1)-Cl(4)
N(4)-Cu(2)
N(6)-Cu(2)
Cu(2)-Cl(4)
Cl(2)-Cu(2)
Cu(2)-Cl(3)
N(1)-Cu(1)
N(2)-Cu(1)
Cu(1)-Cl(1)
Cu(1)-Cl(2)
Cu(1)-Cl(4)
N(4)-Cu(2)
N(3)-Cu(2)
Cu(2)-Cl(4)
Cl(2)-Cu(2)
Cu(2)-Cl(3)
2.003(2)
2.086(3)
2.257(1)
2.262(1)
2.765(1)
1.963(4)
2.079(3)
2.264(1)
2.745(1)
2.231(1)
2.072(3)
2.275(1)
2.270(2)
2.818(2)
1.984(4)
2.104(3)
2.269(2)
2.876(2)
2.276(1)
2.023(3)
2.2385(9)
2.2887(8)
2.811(1)
2.057(3)
1.999(2)
2.2691
2.7698(9)
2.264(1)
Bond Angles ()
N(1)-Cu(1)-N(3)
N(4)-Cu(2)-N(6)
N(1)-Cu(1)-Cl(1)
N(6)-Cu(2)-Cl(4)
Cl(1)-Cu(1)-Cl(2)
Cl(3)-Cu(2)-Cl(4)
N(1)-Cu(1)-Cl(2)
N(4)-Cu(2)-Cl(4)
81.6(2)
81.0(2)
94.7(1)
88.7(1)
94.04(5)
94.11(5)
161.9(1)
167.8(1)
80.9(2)
N(1)-Cu(1)-N(2)
N(3)-Cu(2)-N(4)
N(1)-Cu(1)-Cl(1)
N(4)-Cu(2)-Cl(4)
Cl(1)-Cu(1)-Cl(2)
Cl(3)-Cu(2)-Cl(4)
N(1)-Cu(1)-Cl(2)
N(3)-Cu(2)-Cl(4)
82.5(1)
80.4(1)
81.9(1)
98.4(1)
96.05(8)
90.07(7)
92.92(3)
93.21(3)
164.15(7)
161.51(8)
91.31(9)
93.94(4)
92.46(4)
159.3(1)
164.4(1)
16

Table 3. The comparison of geometric parameters of synthesized dimeric Cu(II) complexes
[LnCuCl2]2 (Ln = L1 – L3), with related complexes from literature.
Bond Lengths (Å)
Cu-Npyrdine
Cu-Pyrazole
Cu–Namine
2.086(3)
2.072(3)
2.023(3)
2.038(4)
2.069(1)
-
Cu-Clterminal
2.257(1)
2.275(1)
2.2385(9)
2.239(2)
2.236(9)
2.294(1)
Complexes
[L1CuCl2]2
References
This work
This work
This work
[45]
-
1.961(4)
-
1.977(4)
[L2CuCl2]2
2.003(2)
2.012(7)
2.000(1)
-
-
[L3CuCl2]2
-
[(npmb)Cu(μ-Cl)Cl]2
[LBCu(μ-Cl)Cl]2
[Cu2Cl4(C6H10N2)4]
[LCuCl2]2 (5)
-
[35]
2.026(3)
2.089(1)
[46]
2.092(0)
-
[44]
Bond Lengths ()
Npyrazole/pyridine–Cu–Namine
81.6(2)
Npyrazole–Cu–Clterminal
94.7(1), 94.8(1)
98.4(1), 97.5(1)
96.05(8), 95.67(8)
97.64(4)
Complexes
[L1CuCl2]2
References
This work
This work
This work
[35]
80.9(2)
[L2CuCl2]2
82.5(1)
[L3CuCl2]2
83.00(5)
[LBCu(μ-Cl)Cl]2
[(npmb)Cu(μ-Cl)Cl]2
80.40(3)
94.80(2)
[45]
17

Table 4. Polymerisation of rac-LA with in situ generated [LnCu(OiPr)2] (Ln = L1 – L3).
O
O
O
O
O
(S)
O
(R)
[LnCu(OiPr)2]
CH2Cl2,RT
O
O
*
(S)
(S)
(R)
(S)
(R)
(R)
O
O
O
O

n
O
O
O
O
Heterotatic PLA
rac-lactide
c
e
Time
(min)
Conv.b (%)
Mn (g/mol)
Mnd (g/mol)
Mn (g/mol)
f
Runa
Catalyst
PDI
Pr
(calcd)
(NMR)
(GPC)
g
1
2
LiOCHMe2
10
2
100
100
14,400
14,400
15,090
14,480
14,030
14,110
1.43
1.28
0.54
0.85
[L1Cu(OiPr)2]
3
4
[L2Cu(OiPr)2]
[L3Cu(OiPr)2]
3
2
100
100
14,400
14,400
14,410
14,480
14,250
14,750
1.25
1.30
0.86
0.85
aMonomer/catalyst ratio = 100, Solvent CH2Cl2 (5.0 mL), room temperature. b Monomer conversion was determined by 1H NMR. cMn x103(calcd.)
d
was calculated from [molecular weight of rac-LA]×[rac-LA]/[initiator] ×conversion%. Experimental molecular weight determined by the
1
relative intensities of the main chain and terminal resonances from H-NMR spectra. e Experimental values were determined by gel permeation
chromatography (GPC) in THF relative to 10 polystyrene standards (corrected using the Mark–Houwink factor of 0.58) [52]. f Probability of
heterotactic enchainment (Pr) were calculated on the basis of 1H NMR spectra according to literature [41,59,60]. g Blank polymerisation solely by
LiOCHMe2, monomer/catalyst ratio was kept constant at 100 in 5.0 mL of CH2Cl2.
18

C14
C6
C13
C12
C11
C7
Cl3
N4
Cl2
C5
C4
C15
N3
C10
C8
N5
C9
C16
Cu1
Cu2
N2
C3
N6
C17
C18
Cl4
N1
Cl1
C19
C1
C24
C2
C20
C23
C21
C22
Fig. 1. An ORTEP drawing of [L1CuCl2]2 with the numbering scheme at 30% probability level.
19

C8
C16
C15
C13
C9
Cl3
C14
C17
C18
C7
N3
N4
Cl2
C10
C11
C6
Cu2
C12
C19
N5
C20
N2
C5
C26
C25
Cu1
C24
C4
Cl4
N1
N6
C27
C1
C28
C3
C23
Cl1
C21
C2
C22
Fig. 2. An ORTEP drawing of [L2CuCl2]2 with the numbering scheme at 30% probability level.
20

C16
C15
C17
C8
Cl3
N3
Cl2
C18
C9
C14
C19
N2
C13
C12
C7
C20
C6
Cu1
C10
Cu2
C11
C5
N4
C21
C22
C4
Cl4
C23
C28
N1
Cl1
C3
C1
C24
C2
C27
C26
C25
Fig. 3. An ORTEP drawing of [L3CuCl2]2 with the numbering scheme at 30% probability level.
21

Fig. 4. Homonuclear decoupled 1H NMR spectrum of methine region of heterotactic PLA
obtained with [L2CuOiPr2] in CH2Cl2.
Graphical Abstract (Pictorial)
R
N
N
H
N
R
N
H
N
Cu
Ph
Cu
iPrO
OiPr
O
Ph
iPrO
OiPr
R = -H, -CH3
CH3
O
O
O
O
(S)
(R)
O
H
O
(S)
H3C
O
O
D,L-Lactide
CH2Cl2/ RT
Heterotatic PLA
22

Graphical Abstract (Synopsis)
A series of Cu(II) complexes supported by bearing N-substituted phenylethanamine derivatives
have been synthesised and characterised by X-ray diffraction. The distorted square planner
Cu(II) complexes crystallized in dimeric form with two diastereomeric asymmetric units, i.e.
(SC,SN and SCRN). Alkoxy derivatives, generated in situ, effectively polymerised rac-LA in
controlled fashion yielding hetero-enriched PLA (Pr = 0.86). The ROP activity of the current
system largely rely on steric hindrance imposed by the substituents attached to ligand
architecture.
23