Synthesis of cyclic polylactide catalysed by bis(salicylaldiminato)tin(ii) complexes

Article abstract of DOI:10.1039/c2dt31678a

Eight bis(salicylaldiminato)tin(ii) complexes have been synthesized from the reaction of Sn[N(SiMe₃)₂]₂ and 2 equiv of the corresponding ligands at room temperature. The ligands, synthesized from salicylaldehyde and amines, were designed to have different electronic and steric properties using different amines to synthesize the tin(ii) complexes as aniline (2a), 2,6-dimethylaniline (2b), 2,6-diisopropylaniline (2c), 4-methoxyaniline (2d), 4-trifluoromethylaniline (2e), methylamine (2g), and tert-butylamine (2h). Ligand variation at the salicyl group synthesized from 4-bromosalicylaldehyde and 2,6-diisopropylaniline was used to form complex 2f. Complex 2c was characterized crystallographically. All catalysts were active for the neat polymerization of l-lactide at 115 °C. At a lactide:Sn molar ratio of 10:1, cyclic polylactide (PLA) was obtained as demonstrated by ¹H NMR and mass spectrometry. Addition of 1 equiv of benzyl alcohol in the polymerization produced linear PLA. At a higher lactide:Sn molar ratio of 200:1, high molecular weight PLAs with M_n up to 132200 Daltons were obtained. Results from GPC coupled with light scattering detector and viscometer suggested that they are cyclic PLA. The order of reactivity based on conversion was determined to be 2c < 2b < 2a in accordance with lower steric hindrance. For electronic contribution, the order of 2e < 2a < 2d was observed in agreement with the increasing electron donation of the ligands. Complex 2g having the smallest substituents was found to be the most active catalyst.

Full text of DOI:10.1039/c2dt31678a

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PAPER
Synthesis of cyclic polylactide catalysed by bis(salicylaldiminato)tin(II)
complexes†
Parichat Piromjitpong, Passachon Ratanapanee, Wipavee Thumrongpatanaraks, Palangpon Kongsaeree and
Khamphee Phomphrai*
Received 25th July 2012, Accepted 24th August 2012
DOI: 10.1039/c2dt31678a
Eight bis(salicylaldiminato)tin(II) complexes have been synthesized from the reaction of Sn[N(SiMe3)2]2
and 2 equiv of the corresponding ligands at room temperature. The ligands, synthesized from
salicylaldehyde and amines, were designed to have different electronic and steric properties using
different amines to synthesize the tin(II) complexes as aniline (2a), 2,6-dimethylaniline (2b), 2,6-
diisopropylaniline (2c), 4-methoxyaniline (2d), 4-trifluoromethylaniline (2e), methylamine (2g), and
tert-butylamine (2h). Ligand variation at the salicyl group synthesized from 4-bromosalicylaldehyde and
2,6-diisopropylaniline was used to form complex 2f. Complex 2c was characterized crystallographically.
All catalysts were active for the neat polymerization of L-lactide at 115 °C. At a lactide : Sn molar ratio
of 10 : 1, cyclic polylactide (PLA) was obtained as demonstrated by 1H NMR and mass spectrometry.
Addition of 1 equiv of benzyl alcohol in the polymerization produced linear PLA. At a higher lactide : Sn
molar ratio of 200 : 1, high molecular weight PLAs with Mn up to 132 200 Daltons were obtained. Results
from GPC coupled with light scattering detector and viscometer suggested that they are cyclic PLA.
The order of reactivity based on conversion was determined to be 2c < 2b < 2a in accordance with lower
steric hindrance. For electronic contribution, the order of 2e < 2a < 2d was observed in agreement with
the increasing electron donation of the ligands. Complex 2g having the smallest substituents was found
to be the most active catalyst.
temperatures (Tg), melting temperatures (Tm), morphologies,
melt viscosities, thermostabilities, compatibilities, hydrodynamic
Introduction
Biodegradable polyesters such as polylactide (PLA) and poly-
lactones have been extensively explored in both academic
and industrial research due to several appealing properties such
as biodegradability and biocompatibility.1–3 The polymers have
found numerous applications for instance in drug delivery,4 scaf-
folds,5,6 and food packaging.7 The physical properties of the
polymers can be fine-tuned by copolymerization with other
monomers giving polymers with different regio- and stereo-
chemistry resulting in specific properties or functions.8–11 In
addition to copolymerization, the physical properties of the poly-
mers can be adjusted by changing the topologies of the polymers
such as star, graft, cross-linked, and hyperbranched struc-
tures.12–15 Among these elaborate polymer topologies, cyclic
polymers have received significant attention.16–18 Cyclic struc-
tures have several physical properties such as glass transition
volume, and intrinsic viscosity different from their linear
counterparts of the same molecular weight.17 There are two
general methods to synthesize cyclic polymers: ring-closure and
ring-expansion techniques.19 The ring-closure techniques usually
suffer from highly diluted conditions and the need to purify
cyclic structures from the linear impurities. However, high
dilution is not required for the ring-expansion techniques making
large-scale synthesis of cyclic polymers possible.
Although several metal alkoxide complexes have been
reported in the polymerizations of lactide or lactones, they only
produced linear polymers or cyclic impurities.20 One of the most
successful syntheses16,17 of cyclic polylactide or polylactones
using a ring-expansion technique was reported by Waymouth
and co-workers.21–23 Zwitterionic polymerization of lactides by
N-heterocyclic carbenes (NHCs) has led to cyclic polylactides in
high yield. Interestingly, polymerization of lactides using NHCs
in the presence of alcohol initiators gave linear polylactide.24
Furthermore, NHCs can be used in the synthesis of linear25 and
cyclic26 poly(ε-caprolactone) (PCL) in a similar concept. Fol-
lowing the success of carbene initiators, we hypothesize that
stannylenes,27 being three rows below carbon in the periodic
table, could be active catalysts for the polymerization of cyclic
esters leading to cyclic polymers similar to carbenes. We recently
reported that bis(amidinate)tin(II) complexes were active for the
Center for Catalysis, Department of Chemistry and Center of Excellence
for Innovation in Chemistry, Faculty of Science, Mahidol University,
Rama 6 Road, Bangkok 10400, Thailand.
E-mail: khamphee.pho@mahidol.ac.th; Fax: +66-2354-7151;
Tel: +66-2201-5146
†Electronic supplementary information (ESI) available: 1H and 13C{1H}
NMR spectra of complexes 2b–h. CCDC 893746. For ESI and crys-
tallographic data in CIF or other electronic format see DOI:
10.1039/c2dt31678a
12704 | Dalton Trans., 2012, 41, 12704–12710
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polymerization of ε-caprolactone.28 At a low monomer : catalyst
ratio, cyclic PCL was obtained as confirmed by NMR and mass
spectrometry. Thus, tin(II) complexes under selected ligand sets
can function similarly to carbenes in the polymerization. This
work was the only example that attempted to use the lone-pair
electrons on tin atom as initiator in the polymerization as
opposed to using tin alkoxides as an initiator.20 Herein, we
demonstrate another example using this concept in the polymer-
ization of lactide by bis(salicylaldiminato)tin(II) complexes.
substituents at the para positions is 2e (CF3) < 2a (H) < 2d
(OMe). In addition, catalysts containing smaller salicylaldimi-
nato ligands that do not contain N-phenyl rings were also investi-
gated as in complexes 2g (Me) and 2h (tBu). The electronic
effect of bromine at the para position of the salicyl group was
also investigated as in complex 2f compared to 2c.
As demonstrated in bis(amidinate)tin(II) complexes,28 we
believe that the lone-pair electrons of tin behave similarly to the
NHCs in the polymerization of LA giving cyclic PLA. To test
this hypothesis, complex 2c was reacted with 10 equiv of L-LA
at 115 °C for 10 min in the absence of solvent (melt polymeriz-
ation). For comparison, the polymerization was conducted with
and without the addition of benzyl alcohol in order to observe
the chain end for linear/cyclic analysis. Great care was taken to
exclude moisture or impurities from the reaction. 1H NMR
spectra of the crude polymers are shown in Fig. 2. Clearly, in
addition to the signals of the ligand at 1.2, 3, 7–7.5 ppm, the
linear polymer synthesized with addition of 1 equiv of benzyl
alcohol reveals the chemical shift of the HOCHMe– end group
at 4.3 ppm (Fig. 2a). This is in agreement with the result from
ESI mass spectrometry as shown in Fig. 3 (bottom) where
the repeating mass of 72.02n + 108.1 + 23 was assigned to
Results and discussion
Syntheses and characterizations of tin(II) complexes
Salicylaldimine ligands were chosen in this study due to the
simplicity of ligand preparation and modification. Both steric
and electronic contributions of the ligands can be tailored sys-
tematically and independently. This ligand set was extensively
used in catalyst development such as in olefin polymerization29
and in the polymerization of cyclic esters.30 Ligands 1a–h
were prepared by simple condensation reaction between salicyl-
aldehyde and the corresponding amines. Reactions between
2 equiv of ligands 1a–h with Sn[N(SiMe3)2]2 gave the corre-
sponding bis(salicylaldiminato)tin(II) complexes 2a–h as shown
in Scheme 1. Complex 2c was characterized crystallographically
and shown in Fig. 1 indicating a four-coordinate tin center
having distorted seesaw geometry. The crystal structure clearly
reveals the position of the lone-pair electrons on tin atom often
seen in tin(II) complexes.28,30g,h,31,32 The NMR spectrum (see
ESI†) of complexes 2b, 2c, and 2f at room temperature revealed
several broad peaks as a result of a fluxional process. These
peaks become sharper and coalescent at 50 °C. This result
indicated a slow rotation of N–CAr bonds (e.g. N1–C8 bond in
Fig. 1) compared to NMR time-scale.
Polymerizations of lactide
The catalysts were designed to have different steric contributions
experienced by the metal center where the order of increasing
steric hindrance of the substituents at the ortho positions is 2a
(H) < 2b (Me) < 2c (iPr). The catalysts were also designed to
have different electronic contributions experienced by the metal
center where the order of increasing electron donation of the
Fig. 1 ORTEP drawing of complex 2c with thermal ellipsoids drawn
at 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (°): Sn1–N1 2.477(2), Sn1–O1
2.052(2), O1–C1 1.323(3), N1–C7 1.276(3), N1–C8 1.438(3); O1–Sn1–
O1a 95.0(1), O1–Sn1–N1 78.35(8), N1–Sn1–N1a 148.5(1).
Fig. 2 1H NMR spectra of PLA synthesized from LA : 2c molar ratio
Scheme 1 Synthesis of bis(salicylaldiminato)tin(II) complexes.
= 10 : 1 (a) with and (b) without addition of 1 equiv of BnOH.
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Fig. 3 ESI mass spectra of PLA synthesized from LA : 2c molar ratio
= 10 : 1 with (bottom) and without (top) addition of 1 equiv of BnOH.
H[LA/2]nOBn + Na+ indicating linear PLA with HOCHMe– and
–C(O)OBn end groups. For the polymerization without the
addition of benzyl alcohol, the chemical shift of the HOCHMe–
end group at 4.3 ppm is not observed as shown in Fig. 2b. This
result indicates the cyclic structure of PLA. The cyclic structure
is also confirmed by ESI mass spectrometry as shown in Fig. 3
(top). The repeating mass (72.02n + 23) was assigned to cyclic
[LA/2]n + Na+ where the mass of the end group was not
detected. Two minor repeating masses were also observed in
Fig. 3 (top). The taller series are 72.02n + 39 assignable to
cyclic [LA/2]n + K+ and the shorter series are 72.02n + 32 + 23
assignable to linear H[LA/2]nOCH3 + Na+. The shorter series
having methoxy end group were possibly generated during the
polymer precipitation where methanol was used. Interestingly,
the repeating mass having ligand 1c as an end group was not
observed indicating that the ligand did not participate in the
ring-opening/closing steps. We also found that the cyclic and
linear structures of PLA were retained at a higher LA : 2c molar
ratio of 100 : 1 under the same polymerization condition where,
Fig. 4 (a) Plot of logarithm of molecular weight vs. elution time and
(b) double logarithmic plots of intrinsic viscosity (IV) vs. molecular
weight.
complex 2c in the absence of BnOH eluted slower than linear
PLA prepared in the presence of BnOH as demonstrated in
Fig. 4a. This is a result of smaller hydrodynamic volume of the
cyclic structure compared to the linear structure. Results from
the viscometer also confirmed the cyclic structure of PLA syn-
thesized in the absence of alcohol. A plot of log IV vs. log Mw is
shown in Fig. 4b. The cyclic structure has less intrinsic viscosity
compared to the linear structure of the same molecular
weight.21,33,34
Melt polymerizations of L-LA were performed at 115 °C using
complexes 2a–h as catalyst. The melt polymerization results
using a LA : Sn molar ratio of 200 : 1 and a fixed time of 25 min
are summarized in Table 1, entries 1–8. All catalysts were active
for the melt polymerization of L-LA. In general, the molecular
weights of PLA are much higher than the expected values and
the PDIs are rather broad. This is indicative of a slow initiation
step compared to the propagation step. In addition, the broad
PDI could be a result of mass transportation problems due to
high viscosity. We have designed the substituents of the ligands
in order to tailor the effect of the electronic and steric influences.
Based on these considerations, it is more informative to split the
catalysts into 3 groups having different steric (2a–c) and elec-
tronic (2a, 2d–f) contributions of the ligands derived from sub-
stituted anilines. The third group contains ligands derived from
alkyl amines (2g, 2h).
1
from H NMR spectra, the chemical shift of the HOCHMe– end
group at 4.3 ppm appeared in linear PLA but disappeared in
cyclic PLA.
To confirm the cyclic structure of PLA at higher molecular
weight, gel permeation chromatography (GPC) coupled with a
light-scattering detector and viscometer was used. In order to
compare between linear and cyclic structures, both polymers
must have enough molecular weight overlap in GPC. We found
that, in melt polymerization, the molecular weight of PLA
cannot be correctly predicted by the monomer : catalyst molar
ratio as in the NHC system. Thus, several batches of L-lactide
polymerization using complex 2c with (LA : 2c : BnOH =
500 : 1 : 1) and without (LA : 2c = 200 : 1) addition of BnOH
were carried out at 115 °C and quenched at different times.
Linear PLA having Mn = 43 000 Daltons (PDI = 1.66) and
cyclic PLA having Mn = 71 200 Daltons (PDI = 1.74) were
selected and the GPC results are shown in Fig. 4. For polymers
having the same molecular weight, cyclic PLA prepared by
12706 | Dalton Trans., 2012, 41, 12704–12710
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Table 1 Melt polymerization of L-LA using complexes 2a–h at 115 °C
b
Entry Catalyst [LA] : [Sn] Time/min Con.a (%) Mn
PDIb
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
200
200
200
200
200
200
200
200
25
25
25
25
25
25
25
25
>99
85
77
>99
89
63
109 700 1.90
96 800 1.96
113 500 1.72
88 500 1.59
132 200 1.85
71 400 1.60
92 000 1.90
13 600 1.39
2g
2h
>99
54
a Conversion determined by 1H NMR spectroscopy. b Determined by
GPC, calibrated using polystyrene standards.
Fig. 5 Homonuclear decoupled 1H NMR spectrum of the methine
proton of poly(rac-lactide) prepared using complex 2c.
According to the percent conversions (Table 1, entries 1–3),
the order of reactivity of the first group having different steric
hindrance is 2c (iPr, 77%) < 2b (Me, 85%) < 2a (H, >99%).
This order is in agreement with decreasing steric hindrance of
the ortho-alkyl groups (iPr, Me, and H) where the crowded
ligands should suppress the polymerization rates. The order of
reactivity of the second group (Table 1, entries 1, 4–5) having
different electronic contribution is 2e (CF3, 89%) < 2a (H,
>99%), 2d (OMe, >99%). Because the polymerizations using
complexes 2a and 2d were greater than 99% in 25 min, another
set of polymerizations having shorter polymerization time was
used for complexes 2a and 2d. The conversions of L-LA
polymerization at 115 °C and 5 min were 15 and 42% for com-
plexes 2a and 2d, respectively. Thus, the order of reactivity for
the second group is 2e (CF3) < 2a (H) < 2d (OMe) in agreement
with the increasing electron donation of the ligands where OMe
group is more electron-donating than the CF3 group. The elec-
tron donation from the ligand makes the tin complex more
nucleophilic28 and susceptible to attack the ester group of LA in
agreement with the polymerization mechanism using NHCs pro-
posed by Waymouth.21 This explanation is also in agreement
with the result in entry 6 when complex 2c is compared with
complex 2f where the Br atom pulls electrons from the metal
making complex 2f less active. For the third group (Table 1,
entries 7–8), the order of reactivity is 2h (tBu, 54%) < 2g (Me,
>99%) in agreement with decreasing steric hindrance (tBu > Me)
of the ligands. When complex 2g was used to polymerize L-LA
at 115 °C and 5 min, the conversion was 78%. Thus, the reactiv-
ity of complex 2g derived from methyl amine is higher than
those of complexes 2a and 2d derived from substituted anilines
possibly because of the much less crowded environment around
the metal center in 2g. Note that racemization of the stereocenter
of poly(L-lactide) was not observed for all catalysts. All com-
plexes were active in the melt polymerization of rac-lactide
(ε-caprolactone) by Okuda,36 Duchateau and Mountford,37 and
Trifonov and Kerton38 where the multidentate di-anionic ligands
participated in the ring-opening polymerization. In our case, if
the mono-anionic salicylaldiminato ligand participated in the
ring-opening polymerization, the ligand should come off the
metal and become the chain end because there is nothing else to
hold the ligand firmly to the metal as in the di-anionic ligands
giving linear PLA as a product. Because linear PLA was not
observed in our system, ligand participation can be excluded.
This is because the negative charge on oxygen atom delocalized
into the π-system reducing its nucleophilicity to attack the
monomer. It is interesting to note that both odd and even
numbers of n are observed in the ESI mass spectrum (Fig. 3).
This information implies that, during the ring-closing step, the
alkoxy group of the growing polymer chain can attack any ester
group along the macrocycles.
Conclusions
We have demonstrated a simple and convenient preparation of
bis(salicylaldiminato)tin(II) complexes that are active catalysts
for the polymerization of L-LA leading to cyclic PLA. The cata-
lyst activities are greatly affected by steric and electronic contri-
butions of the ligands where steric hindrance suppresses the
polymerization rates and electron-donating groups enhances the
polymerization rates. The electronic effect in this catalyst system
is in agreement with our recent findings in the polymerization of
ε-caprolactone leading to PCL using bis(amidinate)tin(II) com-
plexes.28 In addition, the melt polymerization does not need
solvent, thus, making the process more environmentally friendly
and suitable for large-scale synthesis. Since the ligand libraries
for tin(II) are endless, the improved catalysts can be easily
and systematically fine-tuned in both electronic and steric
contributions.
1
under the same conditions. Homonuclear decoupled H NMR of
the methine proton of poly(rac-lactide) obtained from all cata-
lysts showed the same pattern as seen in Fig. 5 indicating atactic
PLAwith slight heterotactic enhancement.35
Experimental
At this point we believe that the mechanism of LA polymeri-
zation using bis(salicylaldiminato)tin(II) complexes is similar to
that using NHCs.21 Thus, the polymerization is believed to occur
using the lone-pair electrons on tin(II) to attack the ester group of
lactide. Cyclic PLA is formed after the intramolecular ring-
closing step. This is different from the synthesis of cyclic poly-
General details
All operations were carried out under dry argon atmosphere
using standard Schlenk techniques. Benzene, hexanes, and
toluene were dried using a PURE SOLV MD-5 solvent purifi-
cation system from Innovative Technology Inc. Ligands 1a–h
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were synthesized according to the literature with minor modifi-
4.22, 3.13 (broad s, 4H, CH(CH3)2), 1.60–0.80 (broad m, 24H,
CH(CH3)2). 13C{1H} NMR (75 MHz, C6D6, 25 °C): δ 167.57
(CHvN), 165.67 (C–O–Sn), 146.81 (CAr–N–Sn), 141.73,
135.34, 135.29, 128.71, 127.21, 123.54, 121.85, 117.03 (CAr),
28.89 (CHMe2) 25.07 (CHMe2). Anal. Calcd for C38H44N2-
O2Sn: C, 67.17; H, 6.53; N, 4.12. Found: C, 66.89; H, 6.40;
N, 3.84.
44
cation.39–42 The syntheses of complex 2a43 and Sn[N(SiMe3)2]2
were reported in the literature. Lactides were purchased from
Aldrich and sublimed three times before use. All compounds
used in the synthesis of ligands were purchased from Aldrich or
Acros and used as received.
Measurements
Crystal data for complex 2c. C38H44N2O2Sn, M = 679.47,
monoclinic, space group C2/c, a = 8.8207(4) Å, b = 17.2625(5) Å,
c = 23.179(1) Å, α = 90°, β = 90.977(2)°, γ = 90°, V = 3528.9(2)
Å3, Z = 4, λ = 0.71073 Å, μ = 0.757 mm−1, T = 150 K, 7069
reflections measured, 4373 unique, Rint = 0.0610, R = 0.0405
(obs. data), wR = 0.0842 (obs. data), GOF = 1.029.
1H and 13C NMR spectra were recorded on a Bruker DPX-300
or AVANCE 500 spectrometer and referenced to protio impurities
of commercial chloroform-d (CDCl3, δ 7.26 ppm) or benzene-d6
(C6D6, δ 7.16 ppm) as internal standards. The X-ray crystallogra-
phy data were collected at 293 K on a Bruker SMART CCD
area-detector diffractometer using graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å). Mass spectrometry was obtained
by micro-TOF-LC Bruker mass spectrometer. Mass spectrometry
was carried out using a Bruker Data Analysis Esquire-LC mass
spectrometer, ESI mode. Gel permeation chromatography (GPC)
analyses were carried out on a Waters e2695 instrument
equipped with Model 3580 refractive index detectors (Viscotek),
Model 270 Differential Viscometer/Light Scattering Dual Detec-
tor and two 10 μm PL Gel columns. The GPC columns were
eluted using tetrahydrofuran with a flow rate of 1.0 mL min−1 at
35 °C. Molecular weights and molecular weight distributions
were calibrated with polystyrene standards ranging from 500 to
10 000 000 amu. Elemental analyses were performed on a
Perkin-Elmer series II CHNS/O Analyzer 2400.
Bis[(N-salicylidene)-4-methoxyanilinato]tin(II), 2d. Green powder
(0.46 g, 84%). 1H NMR (500 MHz, CDCl3, 25 °C): δ 8.21
(s, 2H, CHvN), 7.41 (d, 4H, ArH), 7.23–7.17 (m, 4H, ArH),
7.00 (d, 4H, ArH), 6.64 (m, 4H, ArH), 3.85 (s, 6H, OCH3). 13C
{1H} NMR (125 MHz, CDCl3, 25 °C): δ 164.85 (C–O–Sn),
163.66 (CHvN), 158.68 (C–OMe), 143.50 (CAr–N–Sn),
135.19, 134.61, 123.39, 123.11, 121.43, 116.60, 114.96 (CAr),
55.78 (OCH3). Anal. Calcd for C28H24N2O2Sn: C, 58.87; H,
4.23; N, 4.90. Found: C, 59.25; H, 4.16; N, 4.94.
Bis[(N-salicylidene)-4-trifluoromethylanilinato]tin(II), 2e. Yellow
powder (0.44 g, 37%). 1H NMR (300 MHz, C6D6, 25 °C):
δ 7.62 (s, 2H, CHvN), 7.40 (d, 4H, ArH), 7.17–7.10 (m, 6H,
ArH), 6.97 (d, 4H, ArH), 6.55 (t, 2H, ArH). 13C{1H} NMR
(125 MHz, C6D6, 25 °C): δ 166.22 (CHvN), 166.03 (C–O–Sn),
153.58 (CAr–N–Sn), 136.38, 136.20, 129.03, 127.25 (CAr),
125.38 (CF3), 123.74, 123.28, 121.44, 117.29 (CAr). Anal.
Calcd for C28H18F6N2O2Sn: C, 51.97; H, 2.80; N, 4.33. Found:
C, 51.71; H, 2.70; N, 4.36.
The synthesis of some bis(salicylaldiminato)tin(II) complexes
was reported earlier starting from SnCl2 and sodium salt of the
corresponding ligands.43 However, a preparation using 2 equiv
of ligands and Sn[N(SiMe3)2]2 is preferred.
Bis[(N-(4-bromosalicylidene))-2,6-diisopropylanilinato] tin(II),
2f. Yellow powder (0.68 g, 86%). H NMR (500 MHz, CDCl3,
General synthesis for complexes 2b–h
1
Benzene (20 mL) was added to a mixture of Sn[N(SiMe3)2]2
(0.420 g, 0.956 mmol) and the corresponding salicylaldimine
ligand (1.91 mmol) in a Schlenk flask. The mixture was stirred
at room temperature for 5 h. The volatile components were sub-
sequently removed under vacuum. The crude product was
washed with hexanes giving a yellow or light green powder in
moderate to high yield.
25 °C): δ 7.82 (s, 2H, CHvN), 7.18–7.07 (m, 10H, ArH), 6.23
(d, 2H, ArH), 3.64, 2.85 (br, 4H, CHMe2), 1.50–0.80 (br m, 24
H, CHMe2). H NMR (500 MHz, CDCl3, 50 °C): δ 7.82 (s, 2H,
1
CHvN), 7.16–7.09 (m, 10H, ArH), 6.24 (d, 2H, ArH), 3.26 (br,
4H, CHMe2), 1.16 (br, 24 H, CHMe2). 13C{1H} NMR
(125 MHz, CDCl3, 50 °C): δ 165.72 (CHvN), 163.78 (C–O–
Sn), 145.68 (CAr–N–Sn), 141.13 (CAr–iPr), 137.33, 136.15,
128.58, 126.89, 125.00, 124.16, 122.79 (CAr), 107.76 (CAr–Br),
28.40 (CHMe2), 24.75 (CHMe2). Anal. Calcd for
C38H42Br2N2O2Sn: C, 54.51; H, 5.06; N, 3.35. Found: C, 54.74;
H, 5.38; N, 3.18.
Bis[N-(salicylidene)-2,6-dimethylanilinato]tin(II),
2b. Light
green powder (0.39 g, 72%). 1H NMR (300 MHz, C6D6, 50 °C):
δ 7.59 (s, 2H, CHvN–), 7.03–6.94 (m, 8H, ArH), 6.82 (d, 2H,
JHH = 6.0 Hz, ArH), 6.77 (d, 2H, JHH = 8.3 Hz, ArH), 6.49 (t,
2H, ArH), 2.31 (s, 12H, CH3). 13C{1H} NMR (125 MHz, C6D6,
50 °C): δ 167.74 (CHvN), 165.94 (C–O–Sn), 149.22 (CAr–N–
Sn), 135.47, 135.18, 130.82, 129.20, 128.68, 126.26, 123.67,
121.81, 116.76 (CAr), 19.46 (CH3). Anal. Calcd for
C30H28N2O2Sn: C, 63.52; H, 4.98; N, 4.94. Found: C, 63.84; H,
4.62; N, 5.10.
Bis[(N-salicylidene)methyliminato]tin(II), 2g. Yellow powder
1
(0.35 g, 94%). H NMR (300 MHz, CDCl3, 25 °C): δ 7.96 (s,
2H, CHvN), 7.13 (t, 2H, JHH = 7 Hz, ArH), 6.99 (d, 2H, JHH
7 Hz, ArH), 6.64 (d, 2H, JHH = 7 Hz, ArH), 6.53 (t, 2H, JHH
=
=
7 Hz, ArH), 3.49 (s, 6H, CH3). 13C{1H} NMR (75 MHz,
CDCl3, 25 °C): δ 165.53 (CHvN), 164.79 (C–O–Sn), 134.07,
133.79, 122.69, 120.97, 116.24 (CAr), 45.62 (CH3). Anal. Calcd
for C16H16N2O2Sn: C, 49.65; H, 4.17; N, 7.24. Found: C, 49.76;
H, 3.92; N, 7.17.
Bis[N-(salicylidene)-2,6-diisopropylanilinato]tin(II), 2c. Light
green powder (0.43 g, 67%). X-ray suitable crystals were grown
1
from concentrated benzene solution. H NMR (300 MHz, C6D6,
25 °C): δ 7.91 (s, 2H, CHvN), 7.10 (m, 6H, ArH), 7.01 (t, 2H,
ArH), 6.88 (d, 2H, ArH), 6.76 (d, 2H, ArH), 6.47 (t, 2H, ArH),
Bis[(N-salicylidene)-tert-butyliminato]tin(II), 2h. Yellow oil
purified by vacuum distillation (0.31 g, 68%). 1H NMR
12708 | Dalton Trans., 2012, 41, 12704–12710
This journal is © The Royal Society of Chemistry 2012

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(300 MHz, C6D6, 25 °C): δ 8.05 (s, 2H, CHvN), 7.17 (t, 2H,
ArH), 7.00 (d, 2H, ArH), 6.92 (d, 2H, ArH), 6.65 (t, 2H, ArH),
1.46 (s, 18H, CMe3). 13C{1H} NMR (75 MHz, CDCl3, 25 °C):
δ 163.21 (C–O–Sn), 160.18 (CHvN), 134.55, 133.13, 122.62,
121.85, 116.37 (CAr), 60.00 (CMe3), 30.61 (CMe3). Anal. Calcd
for C22H28N2O2Sn: C, 56.08; H, 5.99; N, 5.95. Found: C, 55.85;
H, 5.79; N, 5.88.
of the Higher Education Commission and Mahidol University
under the National Research University Initiative are acknowl-
edged. This work is also supported by Center for Catalysis,
Faculty of Science, Mahidol University. We thank Prof. Tae-Lim
Choi for help with linear/cyclic structural determination of PLA
and Pailin Srisuratsiri for the help with X-ray crystallography.
Notes and references
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Royal Golden Jubilee Ph.D. Program (PHD/0039/2553), The
Thailand Research Fund, and Mahidol University. Financial and
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