Highly Active and Iso-Selective Catalysts for the Ring-Opening Polymerization of Cyclic Esters using Group 2 Metal Initiators

Article abstract of DOI:10.1002/chem.201700672

A series of alkali and alkaline earth (Ae) metal complexes bearing 1,2-phenylene(bis-diphenylphosphinothioic/selenoic amine) [{Ph₂P(E)NH}₂C₆H₄] (E=S (1-H2); Se (2-H2) ligands are reported. Alkali metal complexes [{Ph₂P(S)N}₂C₆H₄]Na(THF)₄ (3 a) [{Ph₂P(Se)N}₂C₆H₄]Na(THF)₄ (3 b), and [{Ph₂P(Se)N}₂C₆H₄]K(THF)₅ (4 b) were obtained in good yield by treating protic ligands 1-H2 or 2-H2 with metal hexamethyldisilazides [MN(SiMe₃)₂] (M=Na or K) at ambient temperature. The Ae metal complexes formulated as [{Ph₂P(E)N}₂C₆H₄]M(THF)₃ [E=S, M=Ca (5 a), Sr (6 a), Ba (7 a); E=Se, M=Ca (5 b), Sr (6 b), Ba (7 b)] can be synthesized by using two routes. The molecular structures of the free ligand 1-H2 and metal complexes 5 a,b–7 a,b in their solid states were established. Complexes 3 a and 3 b are isostructural; however, in complex 4 b, an attachment different from ligand 2 was observed. The complexes 5 a,b–7 a,b are isostructural and each metal ion exhibits a distorted pentagonal bipyramidal geometry around it. All Ae metal complexes 5 a,b–7 a,b were tested for the ring-opening polymerization (ROP) of racemic lactide (rac-LA) and ?-caprolactone (?-CL) at room temperature. Calcium complexes 5 a and 5 b show excellent iso-selectivity, with P_i values of 0.78–0.87 at 298 K and with a high degree of polymerization control, whereas the corresponding strontium complexes 6 a and 6 b exhibit moderate iso-selectivity, and barium complexes 7 a and 7 b yield only atactic polylactides (PLAs). In all cases, the catalyst initiates the ROP catalytic cycle in the absence of any external initiator. Kinetic studies of the polymerization reactions indicate the relative order of polymerization rate increases with increase in the size of the metal ion: Ba>Sr>Ca.

Full text of DOI:10.1002/chem.201700672

A Journal of
Accepted Article
Title: Highly Active and Iso-selective Catalysts for ROP of Cyclic Esters
Using Group 2 Metal Initiators
Authors: Tarun Kanti Panda, Jayeeta Bhattacharjee, A Harinath, Hari P
Nayek, and Alok Sarkar
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To be cited as: Chem. Eur. J. 10.1002/chem.201700672
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FULL PAPER
Highly Active and Iso-selective Catalysts for ROP of Cyclic Esters
Using Group 2 Metal Initiators
[
a]
[a]
[b]
[c]
[a]
Jayeeta Bhattacharjee, A. Harinath, Hari Pada Nayek, Alok Sarkar, and Tarun K. Panda*
site catalyst that can control the stereochemistry of polyesters,[4]
whose mechanical properties and degradation profile may be
fine-tuned, and thermal stability and crystallinity may be escalated,
Abstract. A series of alkali and alkaline earth (Ae) metal complexes
bearing 1,2 phenylene(bis-diphenylphosphinothioic/selenoic amine)
[
{Ph
2
P(E)NH}
metal
P(Se)N}
4b) were obtained in good yield by treating protic ligand 1-H2 or 2-
] (M
Na and K) at ambient temperature. The Ae metal complexes
formulated as [{Ph P(E)N} ]M(THF) [E = S, M = Ca (5a), Sr (6a),
2
C
6
H
4
] (E = S (1-H2); Se (2-H2) ligands are reported.
complexes [{Ph P(S)N} ]Na(THF) (3a)
]Na(THF) (3b), and [{Ph P(Se)N} ]K(THF)
Alkali
[
(
2
2
C
H
6 4
4
n
to produce a well-regulated molecular weight (M ) and low
{Ph
2
2
C
6
H
4
4
2
C H
2 6 4
5
[5]
polydispersity (PDI) polymer. The polymerization of racemic
lactide (rac-LA) may produce either (i) an isotactic or (ii) a
heterotactic PLA and atactic PLA, following a homochiral and
heterochiral preference, introducing stereo irregularity in PLA.[
Isotactic PLAs obtained through the ROP of the enantio-pure L-
3 2
H2 with corresponding metal hexamethyldisilazides [MN(SiMe )
=
4]
2
C H
2 6 4
3
Ba (7a); E = Se, M = Ca (5b), Sr (6b), Ba (7b)] can be synthesized
using two routes. The molecular structures of the free ligand 1-H2 and
metal complexes 5a,b-7a,b in their solid states were established.
Complexes 3a and 3b are isostructural; however, in complex 4b, an
attachment different from ligand 2 was observed. The complexes
lactide (L-LA) and D-LA have higher melting points (T
m
) and
modulus than atactic PLAs.[ The iso-selectivity gained from
using rac-LA is especially important and useful because the
products—stereo-block/complex PLAs—obtained from rac-LA
have superior thermal and mechanical properties when compared
to PLAs derived from pure L-LA.[4] For instance, a crystalline-
6]
5
a,b-7a,b are isostructural and each metal ion exhibits a distorted
pentagonal bipyramidal geometry around it. All Ae metal complexes
a,b-7a,b were tested for the ring-opening polymerization (ROP) of
5
ordered polymer with a high T
properties than an isotactic poly-L-lactide (PLLA). In some cases,
increase in T by as much as 50°C can improve the thermal
m
will have better mechanical
racemic lactide (rac-LA) and - caprolactone (-CL) at room
temperature. Calcium complexes 5a and 5b show excellent iso-
m
selectivity, with a P
i
value of 0.78–0.87 at 298 K with a high degree of
stability to such an extent that it can enhance the PLA’s industrial
application as an engineering polymer. Therefore, the stereo-
regulated ROP of rac-LA has always been a topic of interest in
the field of polymer chemistry. In recent years, the catalytic ROP
of cyclic esters such as lactides (LAs) and lactones has been
frequently explored by using organocatalysts,[8] as well as
polymerization control, whereas the corresponding strontium
complexes 6a and 6b exhibit moderate iso-selectivity, and barium
complexes 7a and 7b yield only atactic polylactides (PLAs). In all
cases catalyst initiates the ROP catalytic cycle in the absence of any
external initiator. Kinetics studies of the polymerization reactions
indicate the relative order of polymerization rates increases with
increase in the size of the metal ion: Ba>Sr>Ca.
[7]
discrete metal precursors such as complexes of tin,[
9]
aluminum,[ zinc,[11] magnesium, iron, titanium,[14] indium,
10]
[12]
[13]
[15]
yttrium,[16] rare-earth metals,[17] and organo-initiators. The main
challenge in choosing a catalyst is identifying one that can
produce high rates of polymerization with excellent stereo
regularity, ideally without the need for expensive chiral auxiliaries
or ligands.[19]
[18]
Introduction
Various research groups worldwide are working to develop a
substance that can efficiently catalyze the ring-opening
polymerization (ROP) of biodegradable polyesters such as
polylactide[1] (PLA) or polycaprolactone for use in medical
applications[ such as sutures, dental devices, orthopedic fixation
devices, drug delivery systems, and tissue engineering.[3] In
particular, the focus is on developing a distinct well-defined single-
Several highly active achiral catalysts are capable of producing
amorphous, heterotactic PLAs of low polydispersity values from
rac-LA.[ Generally, there is limited mention of iso-selective
achiral catalysts in literature,[21] and most iso-selective ROPs
known in literature are based on chiral catalysts.[22] Recently, Kol
et al. reported a number of chiral aluminum salen complexes as
active catalysts.[10] Despite high selectivity, chiral aluminum
catalysts suffer from low reactivity and require several hours
(sometimes days) at high temperature to achieve higher
conversions. Williams et al. reported highly active chiral yttrium
20]
2]
[
[
[
a]
b]
c]
J. Bhattacharjee, A. Harinath, Dr. T. K. Panda
Department of Chemistry, Indian Institute of Technology Hyderabad
Kandi – 502 285, Sangareddy, Telangana, India.
E-mail: tpanda@iith.ac.in
Dr. H. P. Nayek
Department of Applied Chemistry,
phosphasalen alkoxide complexes with the best iso-selectivity of
P
i
= 0.84.[ Chiral zinc amidooxazolinate complexes yielding a
23]
high iso-selectivity (P
i
= 0.91) were reported by Du’s group
Indian Institute of Technology (ISM), Dhanbad,
[24]
recently
.
Some achiral hetero-scorpionate zwitterionic zinc
8
26004, Jharkhand, India.
complexes as catalysts have also been reported (P
i
= 0.85).[
11b]
Dr. A. Sarkar
However, it is highly desirable to develop new complexes that can
broaden the range of catalysts for alkali and alkaline earth (Ae)
metals for the iso-selective ROP of rac-LA. To overcome
hardships, an alternative strategy is proposed. Using this
approach, an easily accessible achiral catalyst can promote
Momentive Performance Materials Pvt. Ltd.
Survey No. 09, Hosur Road, Electronic City (west)
Bangalore-560100, India
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Chemistry - A European Journal
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stereoselective transformations by adopting chirality in the
propagating step by chain-end bound to the catalyst in a chain-
end controlled mechanism. This strategy is advantageous, as it
averts the need for regenerated chiral precursors and rationalizes
catalyst modifications to influence stereochemical outcomes.[25]
resonance signals at 8.10–7.10 ppm. The sharp singlet at 5.84
ppm can be assigned to both the -NH protons attached to the
phosphorus atoms (Figure FS9 in ESI). In the 31P{ H} NMR
spectra of 1-H2, one sharp singlet was observed at  56.8 ppm
along with two satellite peaks, indicating the coupling of P-Se with
the adjacent selenium atom (Figure FS10 in ESI). The molecular
structure of compound 1-H2 in the solid state was confirmed by
single-crystal x-ray diffraction analysis and given in Figure FS1 in
ESI.
1
Scheme 1. Syntheses of N,N'-(1,2-phenylene)bis(P,P-diphenylphosphino-
chalcogen amide) ligands (1-H2 and 2-H2).
Figure 1. Various achiral catalysts to synthesize stereo regular PLA.
Although some complexes of Mg and Ca were recently reported
as catalyst precursors for lactide polymerization,[26] till date, other
group 2 metals have not been exploited in the ROP of cyclic
esters.[27] The infrequency of synthesis, stability, and reactivity of
such complexes in literature is often attributed to the kinetic lability
with Schlenk-type equilibria in solutions of heavier group 2
complexes, which makes it challenging to synthesize these
compounds.[ As the metal residues are difficult to remove from
the synthesized polymers, alternative nontoxic metal complexes
are encouraged to be active catalysts for the ROP of rac-LA.
Calcium and strontium are harmless, abundant elements in the
human body, and suitable for the catalytic synthesis of PLAs for
use in the medical field. Therefore, exploring the possibility of
using Ae metal complexes as catalysts for the ROP of lactides is
very valuable.[29]
Alkali metal complexes. Sodium and potassium complexes with
molecular
formulae
[{Ph P(S)N} C H ]Na(THF) ]
2
2
6
4
5
(3a),
[{Ph P(Se)N} C H ]-Na(THF) (3b), and [{Ph P(Se)N} C H ]-
2
2
6
4
4
2
2
6
4
K(THF) (4b) were prepared through the reaction between 1-H2
5
or 2-H2 and [MN(SiMe ) ] (M = Na, K) in THF (Scheme 2). All
3 2
three complexes were characterized using spectroscopic and
analytical techniques, and the solid-state structures of complexes
3a, 3b, and 4b were confirmed by single-crystal X-ray diffraction
analysis.
28]
Recently, we have introduced bis(phosphinoselenoic amine)
ligand [{Ph P(Se)NH} C H in the Ae earth metal coordination
2 2 2 4
sphere to study their structural aspects and catalytic potential in
ring opening polymerization of -caprolactone.[30] In this paper, we
report the synthesis of alkali and Ae metal complexes bearing 1,2
phenylene(bis-diphenylphosphino-chalcogen amine) ligand and
their structural aspects. We also demonstrate that the Ae metal
complexes show an unparalleled proclivity with site selectivity,
high activity, and controlled polymerization at high molecular
weights for the ROP of rac-LA and -caprolactone (-CLto form
stereo-block copolymers.
Scheme 2. Synthesis of alkali metal complexes 3a, 3b, and 4b.
Strong absorption bands for P=S and P=Se bonds appear at 550
cm-1 (for complex 3a) and 552 cm-1 and 585 cm-1 (for complexes
Results and Discussion
3
b and 4b respectively) in the FT-IR spectra, which are in
Ligand synthesis. Ligands N,N'-(1,2-phenylene)bis(P,P-
[30,32]
agreement with reported values.
show a sharp signal in the 31P{ H} NMR spectra,  52.4 ppm (3b)
Both complexes 3b and 4b
diphenylphosphino-chalcogen amine) [{Ph
2
P(E)NH}
2
C
6
H
4
] (1-H2
1
and 2-H2) were readily prepared according to reported procedure
by the reaction between N,N’-bis(diphenylphosphino)-benzene-
and 52.4 ppm (4b), which are in the same region as that of the
free ligand 2-H2 ( 53.8 ppm) (Figures FS18, FS21, FS13 in ESI).
In the case of complex 3a, the chemical shift of  60.4 ppm
indicates a significant low-field shift compared to that of the free
ligand 1-H2 (56.8 ppm) (Figures FS15 and FS10 in ESI). The
single resonance peak in complexes 3a, 3b, and 4b confirms the
chemical equivalence of both phosphorous atoms in solution. All
1
,2-diamine [Ph
2 6 4 2
PNHC H NH-PPh ] and elemental sulfur or
[
31]
selenium in a 1:2 molar ratio in toluene at 90ºC (Scheme 1).
The spectroscopic data for compound 2-H2 are in full agreement
with reported values and are given in the Supporting
Information.[ However, in the H NMR spectra of 1-H2, the 24
31]
1
phenyl protons attached to the ligand backbone exhibit multiplet
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the alkali metal complexes are coordinated to the THF molecules,
as evident from the multiplet signals at 3.59–3.56 ppm and 1.43–
1
.40 ppm (for 3a and 3b respectively) and triplet signals centered
1
at 3.50 ppm and 1.36 ppm (for 4b) observed in the H NMR
spectra (Figures FS14, FS17, and FS20 in ESI).
Single crystals of complexes 3a, 3b, and 4b were obtained from
a mixture of each corresponding complex with THF and pentane
solution. The molecular structures of 3b and 4b confirm the
attachment of ligand 2 to sodium and potassium ions respectively.
In complex 3a, two sodium ions are asymmetrically attached to
ligand 1. The details of the structural parameters are given in
Table TS1 in the ESI. The solid-state structures of complexes 3a,
3b, and 4b are shown in Figures FS2-3, and FS4 in ESI
respectively.
Figure 2. Solid-state structure of compound 5b. Hydrogen atoms are omitted
for clarity. Selected bond lengths [Å] and bond angles [°]: Ca1–N1 2.421(17),
Ca1–Se1 3.1274(5), Ca1–P1 3.3538(7), Ca1–N2 2.4298(18), Ca1–Se2
Ae metal complexes. Ae metal complexes with the general
molecular formula [{Ph P(E)N} C H ]M(THF) [E = S, M = Ca (5a),
2 2 6 4 3
Sr (6a), Ba (7a); E = Se, M = Ca (5b), Sr (6b), Ba (7b)] can be
readily obtained by two different synthetic approaches (Scheme
3.1179(5), Ca1–P2 3.3500(7), Ca1–O1 2.3815(16), Ca1–O2 2.4915(16), Ca1–
O3 2.4008(17), N1–Ca1–Se1 58.71(12), N1–Ca1–N2 63.45(17), Se1–Ca1–
Se2 175.44(2), O1–Ca1–O2 74.26(18), O1–Ca1–O3 155.90(19), O2–Ca1–O3
85.91(18), P1–Se1–Ca1 75.90(5), P2–Se2–Ca1 76.91(5).
3
). However, the most convenient method for obtaining all these
complexes is a one-pot reaction, in which ligands 1-H2 and 2-H2
are first reacted with anhydrous [KN(SiMe ] in a 1:2 molar ratio
in THF to generate in situ potassium salts 4a and 4b, followed by
the addition of anhydrous metal diiodide AeI (Ae = Ca, Sr, and
Ba) to the reaction mixture (Scheme 3). The new Ae complexes
a,b-7a,b were characterized using multinuclear NMR
Crystals of compounds 5-7a, were isolated from a mixture of THF
and pentane solution at –35°C, and crystals of compounds 5-7b,
were isolated from hot THF solution. The single-crystal x-ray data
obtained for complex 7a was not complete, and therefore poor. A
representation of the molecular structure for complex 7a is
available in the ESI. The bond lengths and bond angles of
complex 7a are used for comparison with other complexes only.
The calcium, strontium, and barium complexes 5a, 5b, 6a, 6b,
and 7b crystallize in the triclinic space group P-1, with two
molecules of compounds 5, 6, and 7 in their respective unit cells
along with half THF as solvate molecule. The details of the
structural and refinement parameters are given in Table TS1 in
ESI. The molecular structures of complexes 5b and 7b in the solid
3 2
)
2
5
spectroscopic and combustion analysis techniques. The solid-
state structures of complexes 5a,b-7a,b were confirmed by single
crystal X-ray diffraction analysis.
state are provided in Figures
2 and 3 respectively as
representative examples of all Ae complexes, while other
structures are given in ESI (Figures FS5-8). Complexes 5a,b-7a,b
are isostructural to each other due to the similar ionic radii of the
metal ions (1.00 Å, 1.18 Å, and 1.35 Å respectively) for a
Scheme 3. Synthesis of Ae metal complexes 5a,b-7a,b.
1
In the H NMR spectra of the complexes, the disappearance of
[33]
coordination number of six.
signals at  5.84 ppm for complexes 5a, 6a, and 7a, as well as the
disappearance of the signals at  5.92 ppm for complexes 5b, 6b,
and 7b assigned for -NH protons indicate the formation of
dianionic ligand fragments 1 and 2 in each complex (Figures
FS23, FS26, FS29, FS32, FS35, FS38 in ESI). In the 31P{ H} NMR
spectra, each complex shows a sharp signal at  66.7 ppm (for
complex 5b), 66.7 ppm (for complex 6b), and 66.6 ppm (for
complex 7b) and the values are significantly low-field shifted in
comparison to that of free ligand 2-H2 ( 53.8 ppm) (Figures
FS33, FS36, FS39, FS13 in ESI). Similar sharp signals at δ 69.4
ppm (for complex 5a), 69.6 ppm (for complex 6a), and 66.6 ppm
1
(
for complex 7a) are also observed and the values are
significantly low-field shifted when compared to that of free ligand
-H2 ( 56.8 ppm) (Figures FS24, FS27, FS30, FS10 in ESI).The
1
appearance of only one resonance signal in each complex
indicates the chemical equivalence of both phosphorous atoms
present in the ligand moiety 1 and 2. In addition, two typical
multiplet signals, at  3.65–3.55 ppm and 1.35–1.33 ppm, indicate
the resonance of protons attached to the coordinated THF
molecules.
Figure 3. Solid-state structure of compound 7b. Hydrogen atoms are omitted
for clarity. Selected bond lengths [Å] and bond angles [°]: Ba1–N1 2.706(4),
Ba1–Se1 3.3059(6), Ba1–P1 3.5764(13), Ba1–N2 2.708(4), Ba1–Se2
3
2
.2982(7), Ba1–P2 3.5818(13), Ba1–O1 2.697(4), Ba1–O2 2.774(4), Ba1–O3
.756(5), N1–Ba1–Se1 60.70(8), N1-Ba-Se2 121.15(9), N1–Ba1–N2 60.53(12),
Se1–Ba1–Se2 177.086(17), O1–Ba1–O2 82.86(13), O1–Ba1–O3 158.55(15),
O2–Ba1–O3 76.75(15), P1–Se1–Ba1 78.77(4), P2–Se2–Ba1 79.06(4).
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Ligands 1 and 2 are bonded to the Ae metal ion through the
chelation of two amido nitrogen atoms and two sulfur atoms
2
.4
.2
8
0000
0000
(
complexes 5a, 6a, and 7a) or two selenium atoms (complexes
b, 6b, and 7b) attached to the phosphorus atoms. The
phosphorus–metal distances (3.365 Å and 3.359 Å for complexes
a and 5b respectively, 3.452 Å and 3.446 Å for complexes 6a
2
5
6
2.0
1.8
5
and 6b respectively, and 3.593 Å and 3.613 Å for complexes 7a
and 7b respectively) are significantly longer than the sum of the
covalent radii of the corresponding metal ion and phosphorus
atom, which indicates that there is no interaction between them.
Thus, in each case, the central Ae metal ion adopts a distorted
pentagonal bi-pyramidal geometry around it, with two selenium
atoms, two nitrogen atoms of ligand 1, along with one oxygen
atom from the THF molecule, which are in the basal plane,
whereas the two remaining THF molecules occupy the apical
positions. The M–N distances, 2.386(8) Å and 2.418(8) Å
complexes 5a and 5b respectively, 2.517(5) Å and 2.540(5) Å for
complexes 6a and 6b respectively, and 2.657(5) Å and 2.654(6)
Å for complexes 7a and 7b respectively, and M–Se distances,
40000
1
1
1
.6
.4
.2
20000
0
1.0
0
100
200
300
400
500
rac-LA / 5b
Figure 4. Plot of observed (M
functions of added rac-LA with respect to catalyst 5b (M
molecular weight, PDI = polydispersity index). The line indicates calculated M
values based on the LA: initiator ratio. All reactions were carried out at room
temperature in toluene, and conversion to polymer samples was >90%.
n
) and molecular weight distribution of PLA as
n
= number averaged
n
3.252(2) Å and 3.300(2) Å for complexes 5a and 5b respectively,
.2788(1) and 3.3259(1) for complexes 6a and 6b
3
Å
Å
respectively, and 3.4706(9) Å and 3.4071(9) Å for complexes 7a
and 7b respectively, indicate a slight asymmetrical attachment of
the tetra-dentate ligand 1 to the Ae metal ion. Nevertheless,
similar M–N distances and M–Se distances were observed in the
findings we previously reported for complexes [(THF)
2
M{Ph
2
P-
[
31]
(
Se)N(CHPh
2 2
)} ] (M = Ca, Sr, Ba)
and Ae metal complexes
[
34]
reported by other groups.
Polymerization of rac-LA. Complexes 5a,b-7a,b were tested as
initiators for the ROP of rac-LA (Scheme 4). Table 1 summarizes
the polymerization result of rac-LA associated with initiators 5b,
Scheme 4. ROP of rac-LA catalyzed by Ae metal complexes 5a, 5b, 6a, 6b, 7a,
and 7b.
6
b, and 7b, whereas all the polymerization results initiated by
complexes 5a, 6a, and 7a are given in the supporting information
Table TS37 in ESI). All polymerizations were carried out in
Despite the high overall rates of polymerization, the
polymerizations time required to accomplish the complete
conversion of rac-LA into PLA, the order of activity decreased as
Ba>Sr>Ca. In addition, they followed very well controlled manner.
In general, there was close agreement between the theoretical
and observed molecular weights, as well as the narrow
polydispersity index (PDI) (PDI<1.6) (Figure 4). It is noteworthy
that the polymerization of rac-LA in the presence of the catalysts
(
toluene at room temperature, and the percentage of conversion
was monitored through H NMR spectroscopy by taking samples
1
from the reaction mixtures at certain time intervals. Reactions
conducted using catalysts 5a,b-6a,b (1 mM) with 100 equivalents
of rac-LA at 25ºC in toluene resulted in 99% conversion in one
hour. However, the reaction was very fast and 99% conversion
was achieved in only 10 minutes when the corresponding barium
complexes 7a and 7b were used as initiators (Table 1 and Table
TS37 in ESI). Polymerization up to 1000 equivalents of rac-LA
with 7b was completed in less than 10 minutes in toluene (Table
(
5a,b-7a,b) was a living process, with a linear increase of M
n
values and low molecular weight distributions for monomer:
initiator (M/I) ratios up to 500 (Tables 1 and Table TS37 in ESI
and Figure 4 and Figures FS103–FS105 in ESI). Depending upon
the Ae metal coordination is changed from sulfur (5-7a) to
selenium (5-7b), we also noticed a significant enhancement in the
order of activity. To quantify these observations, the
polymerization kinetics was thoroughly monitored and the data
are given in ESI (Table 2 and Tables TS2–TS11, Tables TS19–
TS25).
1
and Table TS37 in ESI).
To explore the reaction order with respect to the monomer and
catalyst, a typical kinetics study was conducted.[35] For this study,
rac-LA (0.228 g, 2.0 mmol) was added to a solution of catalysts
(
(
5a,b-7a,b) (0.01, 0.02, 0.03, 0.04, 0.06 M respectively) in CDCl
3
1 mL), at room temperature. The solution was placed in the NMR
o
tube at 25 C and at the indicated time intervals, the tube was
1
analyzed by H NMR. As expected, plots of [LA]
0
/[LA] versus time
Ca(THF) ] are linear,
indicating the usual first order dependence on monomer
for a wide range of [{Ph
2
P(Se)N}
2
C
6
H
4
3
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Chemistry - A European Journal
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FULL PAPER
concentration (Figure 5).Thus, the rate expression can be
1
Table 1. Polymerization of rac-LA in the presence of Ae metal complexes
bearing the bis-phosphinamine selenoid ligand.
written as follows: –d[LA]/dt
=
K
app[LA] [{Ph
2
P(Se)-
]x
1
N}
2
C
6
H
4
Ca(THF)
P(Se)N}
P(Se)N}
the order of [{Ph
observation was also made in the case of other catalysts 6b and
b and 5a, 6a, and 7a (Figures FS44–FS50 and Figures FS66–
3
=
K
obs[LA] ,
where
K
obs
=
x
C
Pmd
K
ln[{Ph
app[{Ph
2
2
C
6
H
4
]Ca(THF)
]Ca(THF)
P(Se)N}
3
] . A plot of ln(kobs) versus
] (Figure 5) is linear, indicating
]Ca(THF) ] is x = 0.999. A similar
M:1
Ca
tal
yst
Time
(h:m)
conv
ersio
n
M
(KDa)
ntheo
M
GPC
nexp
PDI
En
try
2
2
C
6
H
4
3
b
(KDa)
2
2
C
6
H
4
3
1
2
3
4
5
6
7
8
9
100
200
300
400
500
100
200
300
400
500
100
200
300
400
500
5b
5b
5b
5b
5b
6b
6b
6b
6b
6b
7b
7b
7b
7b
7b
00.30
01.00
01.30
01.30
02.00
00.30
00.30
01.00
01.00
01.30
00:03
00:05
00:08
00:10
00:15
99
99
95
95
99
99
99
95
95
97
99
99
99
99
14.3
28.5
41
51.8
67
14.3
28.5
41
54.7
69.8
14.3
28.5
42.8
57.1
71.3
16.7
29
1.53
1.50
1.47
1.44
1.34
1.35
1.37
1.38
1.50
1.52
1.21
1.41
1.40
1.42
1.51
0.83
0.80
0.79
0.78
0.81
0.75
0.69
0.72
0.71
0.68
0.52
0.48
0.41
0.42
0.47
7
FS74 in ESI). The polymerizations with respect to all catalysts
followed first-order kinetics, indicating that there is only one
initiating species for all complexes, and gives an overall second-
order rate law. Therefore, the polymerization of rac-LA in the
presence of all catalysts followed the following overall rate
equation:
40.3
50.5
70.8
22.1
33.2
40.6
56.5
68.2
15.9
28.8
42.3
54.6
78
1
1
p
rate = -d[LA]/dt = k [catalyst] [LA] .
1
1
0
1
First order kinetics plots rac-LA Polymerizations vs time
12
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
13
14
15
rac LA : 5b
100:0.5
99
b
-
1
1
-------- Kobs= 0.8609 h
R² = 0.9993
In toluene at 25ºC, [Catalyst] = 1 mM. Conversions were determined by H NMR
1
1
1
1
00:1
-1
spectroscopy. M theo = molecular weight of chain-end + 144 gmol × (M:1) ×
n
00:1.5
00:2
-1
Kobs= 0.5937 h
R² = 0.9961
c
In THF (2 mg mL-1) and molecular weights were determined by
conversion.
-1
00:3
GPC-LLS (flow rate ¼ 0.5 mL min ). Universal calibration was carried out with
-1
Kobs= 0.4386 h
R² = 0.9965
polystyrene standards, laser light scattering detector data, and concentration
detector. Each experiment was duplicated to ensure precision. dCalculated
according to using the relative integrals of rmr and rmm resonance.
-
1
-
----- Kobs= 0.2954 h
R² = 0.9974
-1
Kobs= 0.1462 h
R² = 0.9977
ESI). On the basis of our study, the enhanced Kobs of the barium
complex can be attributed to the larger size of the barium ion
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[36]
compared to the other Ae metal ions. Due to greater size of
TIME(h)
barium ion, the availability of free space in the complex increases,
favoring initiator−LA interactions, which eventually enhances the
ring-opening of LA and drives the propagation step. Notably, the
calcium (5a and 5b) and strontium (6a and 6b) complexes have
nearly identical rates, within error, due to the smaller difference of
ion size between them. Additionally, the order of rates correlates
with the metal–selenide bond lengths, as determined by single-
crystal X-ray diffraction analysis experiments. The barium–
selenide bond is significantly longer than the calcium–selenide
and strontium–selenide bonds [M–Se: 2.196(4) Å, 2.060(3) Å, and
Figure 5. First-order kinetics plots for rac-LA polymerizations (with time) in
CDCl (1 mL) with different concentrations of complex 5b.
3
0.0
Linear plot of lnK
vs ln[5b]
obs
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
y = 0.9962x + 2.6677
R² = 0.9996
2.069(6) Å in complex 7 (Ba), complex 6 (Sr), and complex 5 (Ca)
respectively], which also determines the requirements for efficient
polymerization, that is, the need for sufficient Lewis acidity to
ensure rapid coordination.
In addition, for a given Ae metal, the rate of polymerization
increases when the donor atom of the ligand changes from sulfur
-4.8 -4.6 -4.4 -4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6
to selenium (Kobs = 0.906 M h for 7b, 0.839 M-1h-1 for 7a) (Table
-1 -1
ln[Ca-Se comp](M)
2
and Table TS25 in ESI). This clearly indicates that not only the
Figure 6. Kinetics plots of Kobs versus in [{Ph
polymerization of rac-LA with [LA] = 2.0 M in CDCl
2
P(Se)N}
2
C
6
H
4
]Ca(THF)
3
] for the
metal ion, but also the nature of the ligand plays an important role
in the ROP of rac-LA. The increased size of selenium compared
to sulfur presumably aids in providing an additional free space
around the metal ion, causing the metal chalcogen bond to be
more labile in the initiation step.
o
3
(1 mL) at 25 C.
Table 2 summarizes the rate constants for the concentration of
catalyst versus time for the ROP of rac-LA catalyzed by
complexes (5-7b). This allows us to directly compare Kobs values
for all the six catalysts used in the ROP of rac-LA. The Kobs values
-1 -1
for the barium complexes (Kobs = 0.839 M h for complex 7a,
-1 -1
0
.906 M h for complex 7b) are relatively higher than those for
-1 -1
the calcium and strontium complexes (Kobs= 0.137 M h for
complex 5a, 0.146 M h for complex 5b, 0.149 M-1h-1 for complex
-1 -1
-1 -1
6
a, and 0.147 M h for complex 6b) (Table 2 and Table TS25 in
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Chemistry - A European Journal
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Table 2. Rate constants for polymerization of rac-LA with various
concentrations of complexes 2, 3, and 4 as a catalyst.
The tacticity of the PLA obtained from catalysts (5a,b-7a,b) was
determined with the help of a homonuclear-decoupled H NMR
1
spectrum of the methine region (Figures FS97–FS102 in ESI).[38]
It was observed that the PLA obtained from catalysts 5a, 5b, 6a,
and 6b at 298 K were highly isotactic in nature, whereas the PLAs
formed by catalysts 7a and 7b at 298 K were atactic in nature.
The significant steric bulk of the phosphimino ligands (1 and 2),
along with the achiral nature of the Ae metal complexes induce
stereo selectivity in the PLA through a chain-end control
Catalyst
rac-lac:
catalyst
100:0.5
Kobs in
( M h )
0.1462
Entry
1
-
1 -1
2
[{Ph P(Se)N}
2
C
6
H
4
]Ca(THF)
3
2
3
4
5
6
7
8
9
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
[{Ph
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
P(Se)N}
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
2
C
6
H
6
H
6
H
6
H
6
H
6
H
6
H
6
H
6
H
6
H
6
H
6
H
6
H
6
H
4
4
4
4
4
4
4
4
4
4
4
4
4
4
]Sr(THF)
3
100:0.5
100:0.5
100:1
0.1467
0.906
]Ba(THF)
3
]Ca(THF)
3
0.2954
0.2291
1.362
i
mechanism. The degree of isotacticity (P) values were
]Sr(THF)
3
100:1
determined by the (normalized) integration of the homonuclear-
decoupled NMR spectra and comparison with the values
predicted by Bernoullian statistics (Figures FS97-FS102 in ESI).
The PLA obtained from calcium complex 5a showed excellent iso-
selectivity (P = 0.87–0.78) at 298 K, which is the highest value
i
obtained so far by an Ae initiator. In addition, when the strontium
complex 6a was substituted for the calcium complex 5a, the
i
degree of iso-selectivity dropped (P = 0.75–0.63) (Figures FS97-
FS102 in ESI). It is surprising to note that the PLA obtained
through barium catalysts (7a and 7b) exhibited atacticity (Pi <
]Ba(THF)
3
100:1
]Ca(THF)
3
100:1.5
100:1.5
100:1.5
100:2
0.4386
0.4418
1.758
]Sr(THF)
3
]Ba(THF)
3
10
11
12
13
14
15
]Ca(THF)
3
0.5937
0.5958
2.262
]Sr(THF)
]Ba(THF)
3
100:2
3
100:2
]Ca(THF)
3
100:3
0.8609
0.8673
2.67
0.50) (Figure FS84 in ESI). It can be rationalized that atacticity is
]Sr(THF)
3
100:3
observed since the larger metal center (Ba) provides a more open
coordination geometry on the metal ion, whereas iso-selectivity is
induced due to the steric congestion around the metal center as
we move towards the smaller Ae metals. This is expected, as a
gradual decrease in stereo control on decreasing steric shielding
is usual and well-reported in literature.[23] However, when we
change the ligand from sulfur to selenium, the degree of iso-
]Ba(THF)
3
100:3
Activation parameters for the ROP of rac-LA catalyzed by Ae
were found to be ΔH 22.27 (3) kJ/mol, ΔS⧧
⧧
complexes in CDCl
3
⧧
⧧
-
172.76 (7) J/mol K, ΔH 13.23 (4) kJ/mol, ΔS -200.65 (3) J/mol
K, ΔH 17.58 (1) kJ/mol, and ΔS -182.07 (4) J/mol K for (5-7b)
Figures FS51-FS65 and Table TS12-TS18 in ESI) whereas in
⧧
⧧
(
i i
selectivity dropped (P = 0.87 complex 5a, versus P = 0.82 for
⧧
⧧
case of (5-7a) values are ΔH 23.76 (4) kJ/mol, ΔS -169.10 (7)
complex 5b), presumably because of some scrambling of
stereochemistry during chain transfer. A similar observation was
made in case of strontium complexes where PLA exhibits
⧧
⧧
⧧
J/mol K, ΔH 15.22 (3) kJ/mol, ΔS -195.46 (5) J/mol K, ΔH 20.57
5) kJ/mol, and ΔS⧧ -173.34 (8) J/mol K respectively. (Figures
(
FS75-FS82 and Tables TS26-TS29 in ESI).These values were
calculated from the temperature-dependent second-order rate
constants determined from Kobs divided by temperature values as
provided in Table TS30 in ESI. The Eyring plot (Figure 7) indicates
similar ordered transition states in a coordination insertion
mechanism.[37] ΔG values were calculated for the ROP of rac-LA
isotacticity but the degree of iso-selectivity of complex 6b (P
0.71) was lower, compared to complex 6a (P = 0.75). In contrast,
the barium complex of bisphosphinoamide selenoid ligand
produced an atactic PLA (P < 0.50).
i
=
i
i
All the catalysts, (5a,b-7a,b), showed excellent catalytic activities.
Control over polymerization and degree of stereo control were
found to be inversely proportional to each other during PLA
formation. For instance, larger Ae metal ions resulted in a higher
observed rate of polymerization (Ba>Sr>Ca), whereas for the
same set of metal complexes the highest stereo selectivity was
achieved with the smaller Ae ions (Ca>Sr>Ba). The smaller metal
ion of calcium gave the PLA with the highest iso-selectivity, while
barium led to the formation of PLA with atacticity. Similar
observations were noted while switching from a sulfur-based to a
selenium-based ligand. In the case of the complexes based on
selenium ligand (5b, 6b, and 7b), an increased rate of
polymerization was seen, when compared to the corresponding
complexes of sulfur-based ligand (5a, 6a, and 7a). However, as
far as stereo selectivity is concerned, a different mode with higher
stereo selectivity was achieved just by switching from a selenium-
based (5b, 6b, and 7b), to a sulfur-based ligand (5a, 6a, and 7a).
Thus, from a single catalytic system it is possible to achieve two
different modes of stereo control, either by changing the metal or
nature of the ligand system.
⧧
o
catalyzed at 25 C. (Table TS30 in ESI).
-5.0
-5.2
-5.4
-5.6
-5.8
-6.0
-6.2
Eyring plots for 5b
y = -2679.5x + 2.9783
R² = 0.9949
0.0030
0.0031
0.0032
0.0033
0.0034
-1
(1/T)(K
)
Figure
ln[{Ph
with [LA] = 2.0 M in CDCl
7.
Eyring
plots
of
ln(Kobs/T)
versus
(1/T)
for
2
P(Se)N}
2
C
6
H
4
Ca(THF)
3
]catalyst for the polymerization of rac-LA (0.01M)
(1 mL). ΔH‡ = 22(3) kJ mol ΔS = –172(7) J mol–
–1
‡
3
1
–1
K
3
(CDCl ).
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Chemistry - A European Journal
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of concentrations of benzyl alcohol (0.02-0.2 M) with constant
concentration of 5b (0.02 M) and rac-LA (0.228 g, 2.0 mmol),
however Kobs remained unchanged. Thus the lack of dependence
of benzyl alcohol concentration confirms its zero-order
contribution to the rate law (Figure FS87/88). So kinetics study
proved that the polymerization reaction does not depend on
external initiator and the Ae catalysts 5a,b-7a,b themselves act
as initiator for ROP of rac-LA. In addition to investigate the
mechanism of initiation we have carried out reactions of 5:1 and
100
80
60
40
20
0
1
0:1 loadings of monomer: catalyst, and analyzed the reaction
mixture by H NMR, 31P and 13C spectra. The H NMR spectra
using 10:1 (as well as 5:1) loadings of monomer:catalyst showed
that the catalyst is present in the (expected) ratio with the PLA
1
1
(
Figures FS91 - 96 in ESI). Moreover, we have also observed two
sharp resonance peaks in the 31P{ H} NMR spectra, at  61.5 and
1
0
100
200
300
400
500
600
56.5 ppm for 7a (FS 92 in ESI) at  60.2 and 52.6 ppm for 5b (FS
95 in ESI). The resonance peaks at  61.5 (for 7a) and 60.2 (for
5b) indicated the chemically non-equivalence nature of two
o
Temperature ( C)
phosphorus atoms during the polymerization process. Based on
these study, a mechanism of stereo-controlled polymerization of
rac-LA is proposed in Scheme 5. In the initiation step of the
polymerization, in the presence of the lactide molecule, the
phosphorus–chalcogen (S, Se) double bond of the complex
Figure 8. Thermogravimetric analysis (TGA) curves for the PLA catalyzed by
complex 5a.
Polymer melting point often serves as strong evidence to
determine the degree of stereo selectivity in PLAs. In our study,
the PLAs obtained from rac-LA catalyzed by barium complexes
[
2 2 6 4 3
{Ph P(Se)N} C H M(THF) ] could be converted into a single
o
were amorphous in nature with a Tg value <55 C (Figure FS110
bond to form a zwitterionic species (Scheme 5). The equilibrium
between the first and second (I and II) forms of the catalysts could
be dictated by the steric bulk around the metal centers and is
directly related to the size of the metal ion. The larger the metal
in ESI), indicating its atacticity. However, the PLAs obtained from
the calcium and strontium complexes were found to be highly
o
crystalline in nature, with the T
m
values being >170 C, which is
indicative of its high iso-selectivity, similar to the optically pure L-
LA (based on the DSC curve in Figure 9).[39] All the polymers
size, greater the possibility of the metal-lactide interaction
[37]
period.
The steric crowding enhances the initiation of the
obtained from catalysts (5a,b-7a,b), are stable up to more than
selenoid to monomer by releasing the unstable energy of
repulsion.
°
2
80 C (based on the TGA curve in Figure 8 and Figures FS106–
108 in ESI), illustrating very high level of polymer purity.
Scheme 5. Proposed mechanism of lactide polymerization initiated by
complexes 5a,b–7a,b.
Figure 9. DSC curves for the PLA catalyzed by complexes 5b and 6b.
End-group analysis of the polymer was performed by 1H and 13C
NMR quenched by benzyl alcohol (Figures FS90 and FS111 in
ESI). It was revealed that the presence of two peaks at δ 5.20
ppm (benzyl protons on the benzyl ester end-group) and δ 4.34
ppm (methine proton connected to the hydroxyl end-group) in the
A typical kinetics study was conducted to establish that the ROP
of rac-LA occurs without the presence of any external initiator and
the phosphorus chalcogen bond of catalysts 5a,b-7a,b acts as an
initiator for the reaction. From the kinetics study it is observed that
the rate of polymerization catalyzed by 5b remain same even in
presence of external initiator such as benzyl alcohol. (Figures
FS83 - 85 in ESI). Several reactions were attempted with a range
1
H NMR of the proposed propagation species (Figures FS90 and
FS111 in ESI). These results are indicative of the fact that the
aggregating chains were end-capped by benzyl ester and
hydroxyl groups (Figures FS90 and FS111 in ESI).
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Chemistry - A European Journal
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39 in ESI). Therefore, no further kinetics studies were undertaken
for these polymerization reactions of -CL.
Conclusions
To conclude, we have demonstrated the synthesis and catalytic
applications of a series of alkali and Ae metal complexes of bis-
phosphinimino-chalcogen amide ligand. All the Ae metal
complexes (5a,b-7a,b) are found to serve as superior initiators of
the ROP of rac-LA. The relative order of the rate of polymerization
increases with an increase in the ionic radius of the metal center
(
Ba>Sr>Ca) and size of the chalcogen atom in the ligand
1
Figure 10. H NMR spectrum of PLA synthesized by the complex 5a.
moieties. Calcium complexes 5a and 5b proved to be highly iso-
selective initiators of the ROP of rac-LA. The sulfide complex 5a
shows a Pi value of 0.87, and the selenide complex exhibited a Pi
value of 0.81 at 298 K, The strontium complexes exhibit moderate
iso-selectivity, whereas the barium complex induces only
actacticity in the PLA stereochemistry. Thus, the smaller metal
center of calcium promotes a high degree of iso-selectivity, while
the more open-coordination geometry of the relatively larger
barium ions leads to atacticity. Thus, the choice of Ae metal center
and the chalcogen atom in the ligand backbone provides an
attractive route to obtain a high yield of PLA with excellent control
over iso-selectivity.
End-group analysis was also extended to understand the
polymerization reaction of the lactide, initiated by the present
catalytic system. Therefore,
a controlled experiment was
performed where the polymerization of rac-LA in the presence of
catalyst 7b was quenched by isopropanol and the product
obtained was characterized using 1H NMR (Figure 10). The
presence of the two peaks at δ 1.34 ppm (methyl protons on the
isopropoxy end-group) and
δ 4.34 ppm (methine proton
connected to the hydroxyl end-group) is clear evidence that the
aggregating chains were end-capped by an isopropyl ester and a
hydroxyl group.
Polymerization of -CL. Compounds (5a,b-7a,b) were tested as
initiators for the ROP of -CL. Tables TS38 and TS39 in ESI
summarize the polymerization results of -CL using the initiators
Experimental Section
General considerations
(
5a,b-7a,b). Polymerization studies were typically conducted in
toluene, with various monomer/catalyst ratios at 25°C. The
reaction between catalysts (5a,b-6a,b) (1 mM) with 100
equivalents of -CL at 25ºC in toluene resulted in 99% conversion
in five minutes, whereas when barium complexes 7a and 7b were
used as initiators the reaction was very fast and 99% conversion
was achieved in only one minute (Table TS38-TS39 in
ESI).Polymerization up to 1000 equivalents of -CL with
complexes 7a and 7b was completed within one minute in toluene
All manipulations involving air- and moisture-sensitive organometallic
compounds were carried out under argon using the standard Schlenk
technique or argon-filled glove box. Hydrocarbon solvents (n-pentane,
toluene) were distilled under nitrogen from LiAlH
box. CH Cl was dried over P and distilled under reflux condition. THF
was dried and deoxygenated by distillation over sodium benzophenone
ketyl under argon and then distilled and dried over CaH prior to being
was dried over Na/K, distilled, and stored in
4
and stored in the glove
2
2
2 5
O
2
stored in the glove box. C
the glove box. H NMR (400 MHz), C{ H} NMR (100 MHz), and B{ H}
6
D
6
1
13
1
11
1
(
Tables TS38 and TS39 in ESI). Given the large ionic radius of
(128.3 MHz) spectra were measured on a BRUKER AVANCE III-400
the barium atom, it was expected that complexes 7a and 7b would
spectrometer. Elemental analyses were performed on a BRUKER EURO
EA at the Indian Institute of Technology Hyderabad. All the polymer
samples were analyzed using Shimadzu LC solution GPC at Osaka
University. Ligand 2-H2 was prepared according to procedures prescribed
in literature[ and ligand 1-H2 was synthesized in a modified protocol
show the highest reactivity towards the ROP of -CL, similar to
what was observed in some previously reported studies using
other barium complexes.[31,40] The overall control over the ROP
process was good, affording PCLs, featuring a considerable
match between the observed (as determined by GPC) and
calculated molar mass values, while maintaining narrow
polydispersity with a PDI up to 1.18 (Table TS38-TS39 in ESI).
29]
described in literature.[29] [Li{N(SiMe
MI were purchased from Alfa aesar India. [M{N(SiMe
Sr, Ba) were prepared according to the procedures given in literature, by
using the respective metal diiodides and [KN(SiMe
].[41]
], [Na{N(SiMe
)
], [K{N(SiMe
(THF) ] (M = Ca,
3
)
2
3
2
3 2
) ],
2
3
)
2
}
2
2
3 2
)
2 2 6 4
Synthesis of [{Ph P(S)NH} C H
] (1-H2) [29]
In a 100 mL Schlenk flask, one equivalent (616 mg, 5.57 mmol) of 1,2-
phenylenediamine, two equivalents of triethylamine (1.56 ml, 11.4 mmol),
and two equivalents of chlorodiphenylphosphine (2 ml, 11.4 mmol) were
Scheme 6. ROP of ε-caprolactone in toluene with barium complexes (5a,b-
2 2
mixed together with 30 mL of CH Cl and THF mixture (2:1) and the
7a,b).
reaction mixture was stirred for 12 hours. After that, two equivalents of
elemental sulfur powder (365 mg, 11.4 mmol) were added and the mixture
The Ae metal complexes (5a,b-7a,b) as pre- catalysts were
o
was stirred for another 20 hours at 90 C. The solution was then evaporated
extremely fast initiators, enabling almost complete conversion of
under reduced pressure and THF (10 mL) was added to the solution.
Unreacted excess sulfur was filtered through a G4 frit and the filtrate was
1000 equivalents of -CL in less than 30 seconds (Table 38 and
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Chemistry - A European Journal
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collected. After evaporation of the solvent from filtrate in vacuo, a light
yellow solid residue was obtained. The colorless residue was further
purified by washing with n-pentane or n-hexane.
THF. After six hours of stirring at room temperature, another 2 mL of THF
and n-pentane (2 mL) were added and the resulting mixture was kept at
−35 °C. Light yellow-colored crystals were obtained after one day.
1
Yield: 2.76 g (92%). H NMR (400 MHz, CDCl
3
): δ 7.99–7.94 (m, 8H, ArH),
Route 2. In a 25 mL pre-dried Schlenk flask, potassium salt of 1-H2 (100
mg, 0.177 mmol) (which was prepared like 4a) was mixed with
corresponding metal diiodides (0.177 mmol) in 10 mL of THF solvent at
ambient temperature and the reaction mixture was stirred for 12 hours.
The white precipitate of KI was filtered and the filtrate was evaporated
under reduced pressure. The resulting white compound was further
purified by washing with pentane and was recrystallized from THF–
pentane (1:2 ratio) mixture at −35 °C.
7
.52-7.43 (m, 12H, ArH), 6.80-6.52 (m, 4H, ArH), 5.84 (bs, 2H, NH) ppm.
C-{ H} NMR (100 MHz, CDCl ): δ 133.8 (P attached ArC), 132.4 (P
3
attached o-ArC), 131.9 (P attached o-ArC), 131.7 (P attached p-ArC),
28.5 (P attached m-ArC), 127.3 (P attached m-ArC) ppm. 31P{ H} NMR
): δ 56.8 ppm. (C30
.18; found C 66.33 H 4.61, N 4.88.
1
3
1
1
1
(161.9 MHz, CDCl
3
26 2 2 2
H N P S ) Calc. C 66.65, H 4.85, N
5
2 2 6 4 2
Synthesis of [{Ph P(S)N} C H ]Na(THF) (3a)
1
M = Ca (5a). Yield: 140 mg (91%). H NMR (400 MHz, C
6 6
D ): δ 8.23–8.18
In a 10 mL sample vial, one equivalent (50 mg, 0.093 mmol) of 1-H2 and
two equivalents of [NaN(SiMe ] (34 mg, 0.185 mmol) were mixed
(m, 8H, ArH), 7.10–6.94 (m, 12H, ArH), 6.61-6.59 (d, 4H, ArH) 3.72 (bs,
THF), 1.36 (bs, THF) ppm. C-{ H} NMR (100 MHz, C D ): δ 132.3 (P
6 6
13
1
3
)
2
together with 5 mL of THF. After six hours, a small amount of THF (2 mL)
and n-pentane (2 mL) were added and kept at −35ºC. Colorless crystals
of 3a were obtained after one day.
attached ArC), 132.1 (P attached o-ArC), 132.0 (P attached o-ArC), 129.9
(P attached p-ArC), 128.2 (P attached p-ArC), 127.9 (P attached m-ArC),
127.7 (P attached m-ArC), 68.6 (THF), 25.4 (THF) ppm. 31P{ H} NMR
1
(
6 6 2 3.50 2 2
161.9 MHz, C D ): δ 69.4 ppm. (C44H52CaN O P S ) (831, 5a. 1/2THF).
1
Yield: 80 mg (91%). H NMR (400 MHz, C
6 6
D ): δ 8.25–8.20 (m, 8H, ArH),
Calc. C 63.59, H 6.61, N 3.37; found C 62.98 H 6.37, N 3.22.
7
1
.06–7.92 (m, 12H, ArH), 6.38–6.36 (m, 4H, ArH), 3.43–3.40 (m, THF),
.30-1.23 (m, THF) ppm. 13C{ H} NMR (100 MHz, C
1
M = Sr (6a). Yield: 150 mg (92%). H NMR (400 MHz, C
1
6 6
D ): δ 131.8 (P
6 6
D ): δ 8.14–8.12
attached ArC), 131.7 (P attached o-ArC), 128.4 (P attached o-ArC), 128.3
(m, 8H, ArH), 7.02 (bs, 12H, ArH), 6.56–6.53 (m, 4H, ArH), 3.64 (bs, THF),
1.37 (bs, THF) ppm. 13C-{ H} NMR (100 MHz, C
1
(
6
P attached p-ArC), 128.2 (P attached m-ArC), 127.9 (P attached m-ArC),
6 6
D ): δ 132.3 (P attached
7.8 (THF), 25.6 (THF) ppm. 31P{ H} NMR (161.9 MHz, C
1
ArC), 132.1 (P attached o-ArC), 128.3 (P attached o-ArC), 128.1 (P
attached p-ArC), 128.0 (P attached p-ArC), 127.9 (P attached m-ArC),
127.8 (P attached m-ArC), 68.2 (THF), 25.7 (THF). 31P{ H} NMR (161.9
6 6
D ): δ 60.4 ppm.
(
50 64 2 2 5 2 2
C H N Na O P S ) (945.07) Calc. C 63.54, H 6.83, N 2.96; found C
1
63.19 H 6.47, N 2.58.
6 6 52 2 3.50 2 2
MHz, C D ): δ 69.6 ppm. (C44H N O P S Sr) (878.5, 6a.1/2THF). Calc.
2
Synthesis of [{Ph P(Se)N}
2 6 4
C H ]Na(THF)
4
(3b)
C 60.15, H 5.97, N 3.19; found C 59.73 H 5.49, N 2.97.
1
In a 10 mL sample vial, one equivalent (50 mg, 0.078 mmol) of 2-H2 and
two equivalents of [NaN(SiMe ] amide (27 mg, 0.157 mmol) were mixed
together with 5 mL of THF. After six hours, a small amount of THF (2 mL)
and n-pentane (2 mL) were added and kept at −35ºC. Colorless crystals
of 3b were obtained after one day.
M = Ba (7a). Yield: 165 mg (96%). H NMR (400 MHz, C
6
D
6
): δ 8.24–8.04
3
)
2
(d, 8H, ArH), 7.16–6.96 (m, 12H, ArH), 6.56–6.54 (m, 4H, ArH), 3.56 (bs,
THF) 1.40 (bs, THF) ppm. 13C-{ H} NMR (100 MHz, C
attached ArC), 132.1 (P attached o-ArC), 128.3 (P attached o-ArC), 128.1
(P attached p-ArC), 128.0 (P attached p-ArC), 127.9 (P attached m-ArC),
27.8 (P attached m-ArC), 68.2 (THF), 25.7 (THF). 31P{ H} NMR (161.9
1
6
D
6
): δ 132.3 (P
1
1
1
Yield: 75 mg (92%). H NMR (400 MHz, C
6 6
D ): δ 8.39–8.32 (m, 8H, ArH),
MHz, C
6 6 2 3.50 2 2
D ): δ 66.6 ppm. (C44H52BaN O P S ) (928.3, 7a.1/2THF). Calc.
7
.05–7.00 (m, 16H, ArH), 3.53-3.52 (m, THF), 1.51–1.37 (m, THF) ppm.
C 56.93, H 5.65, N 3.02; found C 56.59 H 5.43, N 2.86.
13
1
C-{ H} NMR (100 MHz, C
attached o-ArC), 128.4 (P attached o-ArC), 127.9 (P attached p-ArC),
27.6 (P attached m-ArC), 127.1 (P attached m-ArC), 67.7 (THF), 25.6
6 6
D ): δ 131.8 (P attached ArC), 130.8 (P
2 2 6 4 3
General preparation of [{Ph P(Se)N} C H ]M(THF) [M = Ca (5b), Sr
1
(6b), Ba (7b)]
31
1
(
(
6
THF) ppm.
Na
7.38 H 5.93, N 2.50.
P{ H} NMR (161.9 MHz,
C
6 6
D ):
δ
52.4 ppm.
Route 1. In a 10 ml sample vial, one equivalent (100 mg, 0.158 mmol) of
2-H2 and one equivalent of the corresponding metal precursor
[M{N(SiMe ) } (THF) ] (M = Ca, Sr, Ba) were mixed together with 5 ml of
3 2 2 2
C
H N
50 64 2
2 5
O
P
2
Se ) (1038.87). Calc. C 57.80, H 6.21, N 2.70; found C
2
THF. After six hours of stirring at room temperature another 2 ml of THF
2
Synthesis of [{Ph P(Se)N}
2 6 4
C H ]K(THF)
5
(4b)
and n-pentane (2 ml) were added and the resulting mixture was kept at
−
35 °C. Colorless crystals were obtained after one day.
In a 10 mL sample vial, one equivalent (50 mg, 0.078 mmol) of 1-H2 and
two equivalents of [K(N(SiMe ] (32 mg, 0.157 mmol) were mixed together
with 5 mL of THF. After six hours, a small amount of THF (2 mL) and n-
pentane (2 mL) were added and kept at −35 ºC. Colorless crystals of 4b
were obtained after one day.
3
)
2
Route 2. In a 25 ml pre-dried Schlenk flask, potassium salt 4a (100 mg,
0.156 mmol) was mixed with corresponding metal diiodides (0.156 mmol)
in 10 ml of THF solvent at ambient temperature and the reaction mixture
was stirred for 12 hours. The white precipitate of KI was filtered and the
filtrate was evaporated under reduced pressure. The resulting white
compound was further purified by washing with pentane and was
recrystallized from the THF-pentane (1:2 ratio) mixture at −35 °C. Yield:
125 mg (90%).
1
Yield: 78 mg (93%). H NMR (400 MHz, C
6 6
D ): δ 8.37–8.20 (d, 8H, ArH),
7
.16–6.92 (m, 16H, ArH), 3.51–3.50 (m, THF), 1.51–1.36 (m, THF) ppm.
13
1
C-{ H} NMR (100 MHz, C
attached o-ArC), 128.5 (P attached o-ArC), 128.2 (P attached p-ArC),
27.9 (P attached m-ArC), 127.6 (P attached m-ArC), 67.8 (THF), 25.5
6 6
D ): δ 131.8 (P attached ArC), 130.8 (P
1
1
M = Ca (5b). Yield: 140 mg (96%). H NMR (400 MHz, C
6 6
D ): δ 8.29-8.18
31
1
(
(
5
THF) ppm.
C
P{ H} NMR (161.9 MHz,
Se ) (1071.1). Calc. C 56.07, H 6.02, N 2.62; found C
5.81 H 5.78, N 2.44.
C
6 6
D ):
δ
52.4 ppm.
(m, 8H, ArH), 7.20-7.07 (m, 14H, ArH), 6.66-6.64 (m, 2H, ArH), 3.80-3.70
1
3
1
H K N O P
50 64 2 2 5 2
2
(m, THF), 1.45 (bs, THF) ppm. C-{ H} NMR (100 MHz, C
attached ArC), 132.4 (P attached o-ArC), 131.8 (P attached o-ArC), 128.7
P attached p-ArC), 128.5 (P attached p-ArC), 128.2 (P attached m-ArC),
6 6
D ): δ 132.5 (P
(
31
1
2 2 6 4
General preparation of [{Ph P(S)N} C H ]M(THF)
Ba (7a)]
3
[M = Ca (5a), Sr (6a),
125.3 (P attached m-ArC), 67.9 (THF), 25.7 (THF) ppm. P{ H} NMR
(161.9 MHz, ): 66.7 ppm. (C44 Se (924.8,
C D
6 6
δ
H52CaN O P
2 3.50 2
2
)
Route 1. In a 10 mL sample vial, one equivalent (100 mg, 0.185 mmol) of
-H2 and one equivalent of the corresponding metal precursor
M{N(SiMe (THF) ] (M = Ca, Sr, Ba) were mixed together with 5 mL of
5b.1/2THF). Calc. C 57.14, H 5.67, N 3.03; found C 56.78 H 5.46, N 2.75.
1
[
1
3
)
}
2 2
2
M = Sr (6b). Yield: 145 mg (95%). H NMR (400 MHz, C
6 6
D ): δ 8.24-8.04
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201700672
FULL PAPER
THF), 1.40 (bs, THF) ppm. 13C-{ H} NMR (100 MHz, C
(
m, 8H, ArH), 7.05-6.96 (m, 14H, ArH), 6.56-6.54 (m, 2H, ArH), 3.55 (bs,
Bourissou, Chem. Rev. 2004, 104, 6147-6176; c) N. Ajellal, J. Carpentier,
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Trifonov, Dalton Trans. 2010, 39, 8363-8376; d) C. M. Thomas, J. F. Lutz,
Angew. Chem. Int. Ed 2011, 123, 9412 –9414; Angew. Chem. Int. Ed.
2011, 50, 9244 – 9246.
1
6 6
D ): δ 132.5 (P
attached ArC), 132.4 (P attached o-ArC), 131.9 (P attached o-ArC), 128.3
(
1
P attached p-ArC), 128.1 (P attached p-ArC), 127.9 (P attached m-ArC),
27.8 (P attached m-ArC), 68.2 (THF), 25.7 (THF) ppm. 31P{ H} NMR
1
[
[
2] a) I. Blanco, Chinese J. Polym. Sci. 2014, 32, 681-689; b) A.
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6 6 52 2 3.50 2 2
(161.9 MHz, C D ): δ 66.6 ppm. (C44H N O P Se Sr) (972.3,
6b.1/2THF). Calc. C 54.35, H 5.39, N 2.88; found C 53.87 H 5.11, N 2.72.
4
3
4
36-447; b) M. Labet, W. Thielemans, Chem. Soc. Rev. 2009, 38, 3484-
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1
M = Ba (7b). Yield: 150 mg (93%). H NMR (400 MHz, C
6 6
D
): δ 8.22-8.17
(
m, 8H, ArH), 7.00-6.96 (m, 14H, ArH), 6.45 (m, 2H, ArH), 3.48-3.45 (m,
THF), 1.31-1.28 (m, THF) ppm. 13C-{ H} NMR (100 MHz, C
1
D
6
): 13C-{ H}
1
6
Petillo, G. Peluso, Biomacromolecules 2008, 9, 1527-1534.
NMR (100 MHz, C ): δ 132.5 (P attached ArC), 132.4 (P attached o-
ArC), 131.9 (P attached o-ArC), 128.3 (P attached p-ArC), 128.1 (P
6
D
6
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1
6
7.9 (THF), 25.7 (THF) ppm. 31P{ H} NMR (161.9 MHz, C
6 6
D ): δ 66.6 ppm.
(
44 2 3.50 2 2
C H52BaN O P Se ) (1022.1, 7b.1/2THF) Calc. C 51.70, H 5.13, N
[
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X-ray crystallographic studies of complexes 1-H2, 3a,b, 4b, 5a and 5b,
a and 6b and 7a and 7b
6
X-ray crystallographic analyses. Single crystals of complexes 1-H2, 3a,
b, 4b, 5a, 6a, and 7a were obtained from a mixture of THF and n-pentane
3
solutions under argon atmosphere at a temperature of –35 °C. However,
single crystals of complexes 5b, 6b, and 7b were isolated from hot THF
solution. In each case, a crystal of suitable dimensions was mounted on a
CryoLoop (Hampton Research Corp.) with a layer of light mineral oil and
placed in a nitrogen stream at 150(2) K. All measurements were recorded
on an Agilent Supernova X-calibur Eos CCD detector with either graphite-
monochromatic Cu-Kα (1.54184 Å for complexes 1-H2, 3a, 3b, 4b, 5b, 6b,
and 7b) or Mo-Kα (0.71073 Å for 5a and 6a) radiation. Crystal data and
structure refinement parameters are summarized in Table TS1 in ESI. The
structures were solved by direct methods (SIR2004)[42] and refined on F2
using the full-matrix least-squares method, using SHELXL-97.[43] Non-
hydrogen atoms were anisotropically refined. H-atoms were included in the
refinement on calculated positions riding on their carrier atoms. The
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2
2
2
2
2
2
function minimized was [w(Fo - Fc ) ] (w = 1 / [ (Fo ) + (aP) + bP]),
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2
2
where P = (Max(Fo ,0) + 2Fc ) / 3 with  (Fo ) from counting statistics.
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(wFo )] respectively. The ORTEP-3 program was used to draw the
2
2 2
4
1/2

4
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Crystallographic Data Centre as supplementary publication nos. CCDC
1
Hedrick, C. G. Wade, Macromolecules 2007, 40, 4154-4158; l) D. A.
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527228-1527236. Copies of the data can be obtained free of charge on
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Acknowledgments
This work was supported by the Science and Engineering
Research Board (SERB), Department of Science and Technology
(
DST), India, under project no. (SB/S1/IC/045/2013). J.B. and A.H.
8176; e) N. Nimitsiriwat, E. L. Marshall, V. C. Gibson, M. R. Elsegood,
thank UGC and CSIR India respectively for their PhD fellowships.
We thank Prof. Kazushi Mashima and Dr. Hayato Tsurugi for their
generous support. We also thank reviewers for their valuable
comments to improve the manuscript.
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Keywords: Bis-diphenylphosphino-chalcogen amide • Alkali
metal • Alkaline earth metal • Lactide polymerization • isotactic
PLA
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FULL PAPER
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Entry for the Table of Contents
FULL PAPER
Ring-opening polymerization of rac-
lactide initiated by novel achiral Ca
Jayeeta Bhattacharjee,[ A. Harinath,[a]
Hari Pada Nayek,[ Alok Sarkar,[c] and
Tarun K. Panda*[
a]
b]
complexes supported by [{Ph
N} ] moiety is exhibited to yield
excellent iso-selectivity, with a P value
2
P(E)-
a]
2
6 4
C H
i
Page No. – Page No.
of 0.87 at 298 K with a high degree of
polymerization control. Sr and Ba
complexes showed moderate to lower
selectivity.
Highly Active and Iso-selective
Catalysts for ROP of Cyclic Esters
Using Group 2 Metal Initiators
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