Tetranuclear complexes of group 12 and 13 supported on a polynucleating ligand and activity studies in the ROP of rac-lactide

Article abstract of DOI:10.1016/j.ica.2019.02.007

A polynucleating ligand (L-H₄) containing four N,O bidentate sites was cleanly metallated by four equivalents of AlMe₃, GaMe₃ and ZnEt₂ resulting in tetranuclear complexes L(MMe₂)₄ (M = Al

Full text of DOI:10.1016/j.ica.2019.02.007

InorganicaChimicaActa489(2019)120–125
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Research paper
Tetranuclear complexes of group 12 and 13 supported on a polynucleating
ligand and activity studies in the ROP of rac-lactide
⁎
Dolores-Judith Caballero-Jiméneza, Omar-Jorge García-de-Jesúsa, Nazario Lopeza,
,
⁎
Yasmi-Graciela Reyes-Ortegab, Miguel-Ángel Muñoz-Hernándeza, ,1
a Centro de Investigaciones Químicas, IICBA, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, Cuernavaca, Morelos 62209, Mexico
b Centro de Química, Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla, CU, San Manuel, Puebla 72570, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
A polynucleating ligand (L-H4) containing four N,O bidentate sites was cleanly metallated by four equivalents of
AlMe3, GaMe3 and ZnEt2 resulting in tetranuclear complexes L(MMe2)4 (M = Al (1), Ga (2)) and L(ZnEt
Al, Ga, and Zn complexes
Polynucleating ligands
Polylactide
(CH3CN)0.5)4 (3). The Al complex shows the best catalytic activity for the ring-opening polymerization (ROP) of
rac-lactide (rac-LA) in the present family of tetranuclear compounds. Polymerization studies indicate the lack of
cooperativity between metal centers in the ROP of rac-LA, despite having a ligand platform which binds to four
metal centers.
rac-Lactide
Polymerization
1. Introduction
the ROP of rac-LA [11–15]. Recent studies with trinuclear salen-Al
complexes show very high activity and isoselectivity [16]. Additional
Polymerization of rac-lactide (rac-LA) has been extensively studied
due to its potential use as biodegradable, biocompatible, renewable
polymer [1]. Polylactide (PLA) can be used in sutures and packaging
materials, and eventually might replace the polymers obtained from
non-renewable fossil sources. PLA can be efficiently obtained by ring
opening polymerization (ROP) of rac-LA utilizing diverse catalysts,
many of them are based on Al(III) complexes of tetradentate Schiff base
ligands [2,3]. Particularly, salen-type Al complexes have been suc-
cessfully used as initiators to obtain isotactic PLA with narrow mole-
cular weight distributions [4–6]. Systematic investigations showed that
the role of the substituents in the phenoxide donor and the backbone
linker of the salen ligand have significant influence over the poly-
merization. Sterically demanding ortho-phenoxy substituents and larger
and flexible backbone linkers increase the isotacticity of the resulting
polymer, while electron-withdrawing groups increase the polymeriza-
tion rate [7,8]. Related Al complexes of salicylaldiminato bidentate N,O
ligands are also explored as alternatives to Al-salen complexes in the
ROP of rac-LA, as the former have less coordinatively saturated metal
centers [9]. Interestingly, salen-Al dinuclear complexes show co-
operative effects between the metallic centers leading to an increase in
the activity in comparison with the analogous mononuclear complex,
although in some cases lower Pm values are observed [10]. Several
other Al dinuclear complexes have also shown cooperativity effects in
studies include copolymerization of L-LA and ε-caprolactone by salen-
Al dinuclear complexes [17]. Several reports include Al4 aggregates
with some Al centers bound (Albound) to the polynucleating ligand while
the remaining interact with substituents from Albound forming adducts,
although the aggregates are structurally interesting, reactivity studies
on ROP of rac-LA were not performed [18–20]. To the best of our
knowledge, the formation of tetranuclear Al complexes on a single
polydentate ligand platform with each Al center coordinated to an in-
dividual bidentate site has not been investigated. Herein, we report on
related tetranuclear Al, Ga, and Zn complexes supported on a ligand
with four bidentate N,O sites and their catalytic activity in the ROP of
rac-LA.
2. Experimental
2.1. General methods
All manipulations involving air-sensitive substances were carried
out in an inert-atmosphere glovebox or using standard Schlenk tech-
niques under argon or dinitrogen. All solvents were dried using an
MBraun-SPS system. Commercially available reagents were used
without further purification, 3,5-di-tert-butyl-2-hydroxybenzaldehyde
was synthesized according to the literature [21]. The 1H and 13C NMR
⁎ Corresponding authors.
E-mail addresses: nazario.lopez@uaem.mx (N. Lopez), mamund2@uaem.mx (M.-Á. Muñoz-Hernández).
1 Miguel‐Ángel Muñoz‐Hernández http://orcid.org/0000-0003-3030-6249.
https://doi.org/10.1016/j.ica.2019.02.007
Received 1 November 2018; Received in revised form 9 February 2019; Accepted 9 February 2019
Availableonline11February2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

D.-J. Caballero-Jiménez, et al.
InorganicaChimicaActa489(2019)120–125
spectra were recorded on a Varian Mercury-400 MHz (400 MHz for 1H
and 100.58 MHz for 13C) spectrometer in CDCl3 or C6D6 at room tem-
perature (293 K). 1H and 13C NMR chemical shifts were determined by
reference to residual solvent signals. Melting points were measured in a
2.3.2. L(AlMe2)4 (1)
A solution of Me3Al (27.4 mg, 0.38 mmol, 4.5 equiv.) in 20 mL of
hexane was added dropwise to a stirred solution of L-H4 (100 mg,
0.085 mmol, 1 equiv.) in 20 mL of hexane. The reaction mixture was
stirred for 6 h at room temperature followed by filtration, the volatiles
in the filtrate were removed under dynamic vacuum resulting in a
yellow powder, which was rinsed with diethyl ether (3 × 5 mL) and
decanted to remove any residue of free ligand, the powder was left
under dynamic vacuum for 4 h to remove the volatiles (100 mg,
0.071 mmol, yield = 84%, MW: 1406.01). m.p. 115–120 °C. 1H NMR
(400 MHz, 25 °C, CDCl3): δ 8.14 (s, 4H, H-C]N), 7.52 (d, 4H, Ph-H),
7.01 (d, 4H, Ph-H), 3.60 (t, 8H, ]N–CH2), 2.50 (q, 8H, –CH2–N), 2.46
(s, 4H, –N–CH2), 1.89 (q, 8H, ]N–CH2–CH2), 1.41 (s, 36H, –CH3 tBu),
1.37 (s, 4H, N–CH2–CH2), 1.29 (s, 36H, –CH3 tBu), −0.74 (s, 24H,
Al–(CH3)2). 13C{1H} NMR (100 MHz, 25 °C, CDCl3): δ 171.85 (C]N),
161.23 (C–O, Ph), 140.49 (C–C(CH3)3, Ph), 139.06 (C–C(CH3)3, Ph),
131.78 (C–H, Ph), 1258.54 (C–H, Ph), 118.32 (C–C]N, Ph), 57.68 (]
N–C), 54.22 (–N–C), 51.60 (–CH2–CH2–N–), 35.17 (–C(CH3)3 tBu),
34.25 (–C(CH3)3 tBu), 31.49 (–CH3 tBu), 29.44 (–CH3 tBu), 28.24
(–CH2–CH2–CH2–) 25.34 (–CH2, butyl), −9.24 (Al-(CH3)2). EI-MS: m/
z = 1391 [M+−CH3]. IR (KBr, cm−1): 2945 (st), 2857 (m), 1617 (st),
1543 (m), 1438 (st), 1440 (m), 1358 (s), 1255 (s), (m), 1173 (m), 850
(m), 670 (w).
Mel Temp II device using sealed capillaries. Mass spectra (EI or FAB+
-
MS) were obtained using a JEOL magnetic sector spectrometer JMS700.
Infrared spectra were recorded in KBr pellets using a Nicolet 6700
Analytical FT-IR spectrometer. Molecular weight and molecular weight
distributions of PLA polymers were determined by GPC chromato-
graphy on a Waters 2695 ALLIANCE Separation Module equipped with
two HSP gel columns (HR 4E molecular weight range from 50 to
1 × 105 and HR 5E from 2 × 103 to 4 × 106) in series and a RI Waters
2414 detector. THF was used as eluent at 35 °C with a flow rate of
1.0 mL/min. Ten linear polystyrene standards of narrow dispersity were
used for GPC calibration curves.
2.2. X-ray diffraction
Yellow rhombic crystals of Zn complex 3 suitable for X-ray dif-
fraction measurements were taken directly from solution under argon
atmosphere, coated with mineral oil, mounted on a glass fiber and
measured at 100 K. X-ray intensity data were collected using the soft-
ware CrysAlisPro [22] on a four-circle SuperNova, Dual EosS2 CCD
diffractometer with monochromatic Cu-Kα radiation (λ = 1.54184 Å).
Cell refinement, data reduction, incident beam, decay and absorption
corrections were carried out with the use of the program CrysAlisPro
[22]. Using Olex 2 [23], the structure was solved by direct methods
with the program SHELXT and refined by full-matrix least-squares
techniques with SHELXL [24–26]. All hydrogen atoms were generated
in calculated positions and constrained with the use of a riding model.
The final model involved anisotropic displacement parameters for all
non-hydrogen atoms. CCDC 1837655 contain the supplementary crys-
tallographic data for compound 3. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.uk.
2.3.3. L(GaMe2)4 (2)
A solution of Me3Ga (39.0 mg, 0.34 mmol, 4 equiv.) in 20 mL of
hexane was added dropwise to a stirred solution of L-H4 (100 mg,
0.085 mmol, 1 equiv.) in 20 mL of hexane. The reaction mixture was
stirred for 6 h at room temperature followed by filtration, the volatiles
in the filtrate were removed under dynamic vacuum resulting in a
yellow powder, which was rinsed with diethyl ether (3 × 5 mL) and
decanted to remove any residue of free ligand, the powder was left
under dynamic vacuum for 4 h to remove the volatiles (120 mg,
0.076 mmol, yield = 90%, MW: 1576.97). m.p. 76 °C. 1H NMR
(400 MHz, 25 °C, CDCl3): δ 8.00 (s, 4H, H-C]N), 7.45 (d, J = 2.6 Hz,
4H, Ph-H), 6.88 (d, J = 2.6 Hz, 4H, Ph-H), 3.55 (t, 8H, ]N–CH2), 2.47
(q, 8H, –CH2–N), 2.44 (s, br, 4H, –N–CH2), 1.81 (q, 8H, ]N–CH2–CH2),
1.42 (s, 36H, –CH3 tBu), 1.40 (s, 4H, N–CH2–CH2), 1.28 (s, 36H, –CH3
tBu), −0.30 (s, 24H, Ga–(CH3)2). 13C{1H} NMR (100 MHz, 25 °C,
CDCl3): δ 169.88 (C]N), 164.35 (C–O, Ph), 141.15 (C–C(CH3)3, Ph),
137.30 (C–C(CH3)3, Ph), 131.06 (C–H, Ph), 128.76 (C–H, Ph), 117.32
(C–C]N, Ph), 57.28 (]N–C), 53.88 (–N–C), 51.29 (–CH2–CH2–N–),
35.51 (–C(CH3)3 tBu), 34.04 (–C(CH3)3 tBu), 31.52 (–CH3 tBu), 29.48
(–CH3 tBu), 27.92 (–CH2–CH2–CH2–), 25.09 (–CH2, butyl), −6.62 (Ga-
(CH3)2). FAB+-MS: m/z = 1561 [M+−CH3]. IR (KBr, cm−1): 2948 (m),
2899 (w), 2860 (w), 1619 (st), 1536 (m), 1440 (m), 1434 (m), 1411 (m),
1384 (m), 1360 (m), 1359 (w), 1314 (w), 1255 (m), 1176 (w), 1170 (m),
1067 (w), 1014 (w), 873 (m), 839 (w), 784 (m), 743 (m), 682 (w).
2.3. Synthesis
2.3.1. L-H4
L-H4 was obtained via a Schiff base condensation reaction [21].
N,N,N′,N′-Tetrakis(3-aminopropyl)-1,4-butadiene (DAB-Am4, 1.6 mL,
0.005 mol) was dissolved in 20 mL of ethanol and added dropwise to a
suspension of 3,5-di-tert-butyl-2-hydroxybenzaldehyde (4.6 g, 0.02 mol)
in 30 mL of ethanol. The reaction mixture was stirred at room tem-
perature for 4 h, then filtered, the cake was rinsed with 30 mL of water
and the filtrate was extracted with 40 mL of CH2Cl2 three times. The
organic phases were combined and dried over Na2SO4 for 1 h and after
filtration the volatiles were removed under dynamic vacuum resulting
in a yellow solid (5.5 g, yield = 93%, MW: 1181.83). m.p.: 54 °C. 1H
NMR (400 MHz, 25 °C, CDCl3): δ 13.95 (b, 4H, –OH), 8.35 (s, 4H, CH]
N), 7.37 (d, 4H, Ph-H), 7.07 (d, 4H, Ph-H), 3.60 (t, 8H, ]N–CH2–), 2.53
(t, 8H, –CH2–N), 2.43 (s, 4H, N–CH2–), 1.83 (t, 8H, –CH2–CH2-CH2–),
1.45 (s, 36H, –C(CH3)3), 1.43 (s, 4H, –CH2–CH2–CH2), 1.30 (s, 36H,
–C(CH3)3). 13C{1H} NMR (100 MHz, 25 °C, CDCl3): δ 165.94 (C]N),
158.35 (C–OH, Ph), 139.97 (C–C(CH3)3, Ph), 136.77 (C–C(CH3)3, Ph),
126.79 (C–H, Ph), 125.81 (C–H, Ph), 118.01 (C–C]N, Ph), 57.68 (]
N–C), 54.22 (–N–C), 51.60 (–CH2–CH2–N–), 35.17 (–C(CH3)3 tBu),
34.25 (–C(CH3)3 tBu), 31.67 (–CH3 tBu), 29.59 (–CH3 tBu), 28.74
(–CH2–CH2–CH2–) 25.34 (–CH2, butyl). FAB+-MS: m/z = 1182 [M+].
IR (KBr, cm−1): 3355 (br), 2947 (st), 2860 (m), 1629 (st), 1444 (st),
1438 (st), 1359 (m), 1250 (m), 1230 (m), 1176 (m), 1172 (m), 1022 (w),
970 (s), 874 (s), 826 (s), 824 (s), 771 (m), 1052.0 (w), 727 (s), 694 (s),
643(m).
2.3.4. L(ZnEt(CH3CN)0.5
) (3)
4
A solution of Et2Zn (41.99 mg, 0.34 mmol) in 20 mL of a 1:1 toluene
and acetonitrile mixture was added dropwise to a stirred solution of L-
H4 (100 mg, 0.085 mmol) in 20 mL of toluene. The reaction mixture
was stirred for 6 h at room temperature, filtered, concentrated and
stored at −25 °C. After two days transparent yellow crystals were ob-
tained from the solution, the crystals were collected on a frit, rinsed
with cold toluene (3 × 3 mL), and the volatiles removed under dynamic
vacuum (104 mg, 0.064 mmol, 75%, MW: 1637.68): m.p. 115–120 °C.
1H NMR (400 MHz, 25 °C, CDCl3): δ 8.06 (s, 4H, H-C]N), 7.30 (m, 4H,
Ph-H), 6.87 (s, 4H, Ph-H), 4.47 (m, 4H, ]N–CH2), 3.48–3.14 (m, 4H, ]
N–CH2), 2.86–2.51 (m, 12H, –CH2–N, –N–CH2–CH2), 2.36 (s, 3H,
CH3CN), 1.97–1.63 (m, 8H, ]N–CH2–CH2), 1.48–1.34 (m, 44H, –CH3
tBu, –CH2CH3), 1.32–1.16 (m, 52H, –CH3 tBu, –CH2CH3). 13C{1H} NMR
(100 MHz, 25 °C, C6D6): δ 169.92 (C]N), 168.85 (C–OH, Ph), 141.91
(C–C(CH3)3, Ph), 135.72 (C–C(CH3)3, Ph), 130.08 (C–H, Ph), 129.69
(C–H, Ph), 118.43 (C–C]N, Ph), 60.09 (]N–C), 50.98
121

D.-J. Caballero-Jiménez, et al.
InorganicaChimicaActa489(2019)120–125
(–CH2–CH2–N–), 36.32 (–C(CH3)3 tBu), 34.44 (–C(CH3)3 tBu), 32.14
(–CH3 tBu), 30.40 (–CH3 tBu), 26.36 (–CH2–CH2–CH2–), 11.85
(Zn–(CH2CH3), 4.41 (Zn–(CH2CH3). FAB+-MS: m/z = 1525 [M+−Et,
–2CH3CN]. IR (KBr, cm−1): 2947 (m), 2899 (w), 2858 (w), 1616 (st),
1528 (m), 1435 (st), 1410 (m), 1382 (m), 1358 (m), 1324 (w), 1254 (m),
1231 (w), 1197 (w), 1162 (m), 1086 (m), 911 (w), 871 (w), 834 (m), 789
(m), 741 (m), 697 (w).
spectrum provides further evidence of a clean metallation due to the
disappearance of the OeH signal at about 14 ppm of the proligand L-H4
and the appearance of a single set of signals for the complexes in 1H and
13C{1H} NMR (Figs. 1 and S2–3), which is indicative of a symmetric
product, proposed as the tetranuclear complex. 1H NMR signals of Me
substituents on the metal are singlets at −0.74 and −0.30 ppm for 1
and 2 respectively, the Et substituent on Zn, complex 3, overlaps with
ligand signals at 1.38 and 1.28 ppm. The disappearance of the IR ab-
sorption band, in all cases, around 3355 cm−1 is an additional indica-
tion that metallation took place (Fig. S4–7), since such band is assigned
to the stretching of the O–H bond of the proligand. Crystals of 3 suitable
for Single Crystal X-ray diffraction studies were obtained from saturated
1:1 (vol) acetonitrile:toluene solution (Table S1).
2.4. Ring-opening polymerization studies
The catalytic activity was studied in toluene in Ace pressure reactors
under argon atmosphere. The monomer, initiator, solvent, and benzyl
alcohol were loaded into the Ace reactor in an MBraun glovebox, the
reactor was taken out of the glovebox and immersed in an oil bath at
70 °C. After the selected stirring time, the reactor was let to cool down
to room temperature and opened to quench the reaction by addition of
a few drops of diluted HCl. An aliquot was taken, and the volatiles were
removed under dynamic vacuum, the residue was dissolved in CDCl3
and analyzed by 1H NMR spectroscopy to determine the percentage of
conversion. The PLA was precipitated in the remaining solution by
addition of cold methanol, the polymer was collected by filtration and
the volatiles were removed under dynamic vacuum, samples from the
residue were taken for additional characterization by gel permeation
chromatography (GPC).
The asymmetric unit of 3 consists of a tetranuclear complex (Fig. 2)
with Zn ions in distorted tetrahedral coordination environments with
angles in the range of 90.59–126.42° and 87.03–135.59° for Zn1 and
Zn2, respectively. The ligand wraps around Zn1 in a tridentate fashion
with O,N,N donor atoms from phenolate, imine and amine groups, with
the remaining coordination site occupied by Et; while binding in a bi-
dentate fashion to Zn2 having O,N as donor atoms from a phenolate and
imine groups, with the remaining sites occupied by an ethyl group and
acetonitrile. Similar structures are expected for the additional tetra-
nuclear complexes 1 and 2, but having all the metal centers with the
same coordination environment with two methyl groups and the N,O
chelation from imine-phenoxy moiety as shown in Scheme 1. The
proposed structure for 1 and 2 correlates with the 1H NMR data which
shows a single set of signals, indicative of equivalent coordination en-
vironment for all the metal centers.
3. Results and discussion
The reaction of polydentate ligand L-H4 with four equivalents of
MMe3 or ZnEt2 resulted in tetranuclear complexes L(MMe2)4 (M = Al
The catalytic activity was studied in toluene. The typical ratio of
monomer to initiator used was M/I = 100/1 or 400/1, addition of
benzyl alcohol was necessary because the alkyl complexes resulted in-
active in the ROP. The in-situ generation of main group alkoxide com-
plexes is a common strategy to obtain more active initiators due to
increased nucleophilicity of the OR group which more easily migrates
to the carbonyl carbon of the monomer in comparison with the R alkyl
group.[27] The conversion was followed by 1H NMR analysis of the
methine proton for rac-LA and PLA (Figs. 3 and S8, S9), which have
distinctive chemical shifts of 5.02 and 5.15–5.25 ppm, respectively.
Complex 1 was the most active catalyst of the series, the polymerization
results are included in Table 1, complex 2 and 3 showed low activity
(1), Ga (2)), and L(ZnEt(CH3CN)0.5)4 (3), (Scheme 1) as corroborated
by mass spectrometry (MS), infrared and 1H, 13C nuclear magnetic re-
sonance spectroscopy as well as X-ray crystallographic analysis for 3.
The Zn(II) tetranuclear complex was synthesized in an attempt to ob-
tain crystals suitable for Single Crystal X-ray diffraction studies, since
the analogous tetranuclear complexes of Al and Ga did not resulted on
good quality crystals. The MS studies indicate the presence of tetra-
nuclear complexes with m/z values of 1391 and 1561 [M+−CH3]
corresponding to the loss of a Me group from MMe2 organometallic
fragment in 1 and 2 respectively, and 1525 [M+−Et, –2CH3CN] from
loss of two acetonitrile and one Et group for 3 (Fig. S1). The 1H NMR
Scheme 1. Synthetic route for tetranuclear complexes.
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InorganicaChimicaActa489(2019)120–125
Fig. 1. 1H NMR spectra for 1 (400 MHz, CDCl3, 25 °C).
(Đ = 1.66), the tacticity decreased and became close to atactic (Entry 3,
Table 1).
The low activity of present complexes can, to some extend, be
correlated to the solid-state structure. In the case of 3, the Zn centers
are far away at a minimum of 7.01 Å, which might be too long for the
metal centers to act cooperatively on ROP of rac-LA. There are also
ligand segments between Zn ions that might block the alkoxide mi-
gration, necessary to observe cooperativity. The tertiary nitrogen site
coordinates to Zn center in some cases as observed in the solid-state
structure. This asymmetry is also observed in solution by 1H NMR
studies. In contrast, the tertiary nitrogen most likely does not co-
ordinate to the metal center in compounds 1 and 2, therefore resulting
in a different structure than compound 3. The low ROP activity of 1 and
2 suggests the absence of cooperativity effects, probably due to the
reasons pointed out for 3. Alkoxide migration is very important for
accelerating ROP, it was proposed in previous studies that the RO-
group bound to an aluminum center migrates an attacks lactide co-
ordinated to the adjacent Al resulting in cooperativity which accel-
erates ring opening of lactide in a back and forth process [11,13,14,16].
Attempts to obtain monometallated complex, L-H3Al-Me2, for eva-
luation of its activity in ROP of rac-LA, resulted in a mixture of me-
tallated species that cannot be isolated by fractional crystallization
techniques (Fig. S11). Thus, the control experiment for ROP of rac-LA
with monometallated complex was not performed, and a comparison
with catalysts having similar coordination sphere is presented instead.
The activity of 1 is similar to related dialkylaluminum mononuclear
complexes with bidentate phenoxide-imine N,O ligands [13]. It can
then be inferred that the low activity of complex L(AlMe2)4 (1) most
likely is due to having ROP independently on each metal site, as the
activity is at least one order of magnitude higher for bimetallic alu-
minum complexes that show cooperativity.
Fig. 2. Solid-state structure for 3, with ellipsoids at 50% probability level.
Hydrogen atoms are omitted for clarity. Color code = Zn: pink, N: blue, O: red,
C: grey. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
after 24 h even in the presence of benzyl alcohol.
The tacticity of polymers was determined using homonuclear de-
coupled 1H NMR by analyzing the methine region following the Zelĺs
method. The value of the integral corresponding to rmr was taken for
five tetrad populations and Pr was calculated (Pr = (2 × rmr)0.5
(Pr + Pm = 1), see Figs. 4 and S10.
)
Values for Pr and Pm indicate the tacticity of polymers (atactic: Pr or
m = 0.5; heterotactic: Pr = 1.0, Pm = 0; isotactic: Pr = 0, Pm = 1.0).
P
The PLA polymer obtained was predominantly isotactic for M/I ratios of
100/1 (Entries 1 and 2, Table 1). Compound 1 shows moderate activity
for the ROP of rac-LA in experimental conditions shown in entry 1
(Table 1), with calculated Mn close to the experimental value, and good
Đ = 1.16 and Pm = 0.60. Increasing the amount of benzyl alcohol
maintained isotacticity about the same (Pm = 0.67) but caused wider
molecular weight distribution (Đ = 1.60, Entry 2, Table 1), indicating
that chain transfer is a significant degradation pathway. The M/I ratio
was increased to probe the catalysts closer to actual polymerizations
conditions, but a wide molecular weight distribution was obtained
In the case of 3, thus, in analogous complexes 1 and 2 the ROP most
likely proceeds by individual metal sites without cooperativity.
4. Conclusions
Three new tetranuclear Al, Ga, Zn complexes of polynucleating li-
gand with four N,O bidentate sites have been prepared and investigated
as precatalysts in the presence of benzyl alcohol for the ring-opening
polymerization of rac-lactide. Complex 1 was the best catalyst in this
123

D.-J. Caballero-Jiménez, et al.
InorganicaChimicaActa489(2019)120–125
Fig. 3. 1H NMR spectrum for PLA obtained with catalyst 1 (Table 1, entry 1, 400 MHz, CDCl3, 25 °C).
Table 1
rac-LA polymerization with aluminum complex 1.
Cata
[LA]0/[cat.]0/[BnOH]0
t (h)
Convb (%)
Mn(calcd)c (g/mol)
Mn(obsd)d (g/mol)
Đd
Pm
e
Entry
1
2
3
4
5
1
1
1
2
3
100:1:4
100:1:8
400:1:4
100:1:4
100:1:4
24
26
24
48
48
95
91
98
93
14
3531
1802
14,233
3450
3698
1184
10,919
–
1.165
1.603
1.659
–
0.60
0.67
0.43
0.56
–
f
f
–
–
–
a
Reactions were performed in 5 mL of toluene, [cat.]0 = 0.0173 mM, at 70 °C.
Determined by 1H NMR spectroscopy.
b
c
d
e
f
Calculated by Mrac-LA × ([LA]0/[BnOH]0) × conversion + MBnOH
.
Measured by GPC in THF calibrated with polystyrene standards.
Determined by analysis of all the tetrad signals in the methine region of the homonuclear-decoupled 1H NMR spectrum.
Polymer could not be isolated.
Fig. 4. Deconvolution of the methine region of the homonuclear-decoupled 1H NMR spectrum for PLA obtained with catalyst 1 (Table 1, entry 1).
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InorganicaChimicaActa489(2019)120–125
family of complexes, the better performance is due to the Al-OR site in 1
being more active than in the Ga complex 2. For compound 1, in-
creasing the amount of benzyl alcohol or rac-LA, independently, caused
high Đ. The increase in rac-LA also caused the formation of polymer
close to atactic. Zn complex 3 might not be stable as alkoxide Zn-OR,
thus resulting in low activity in the ROP of rac-LA. Cooperativity be-
tween metal centers in the ROP of rac-LA was not observed in the tet-
ranuclear complexes due to having long distance between metal centers
and also having ligand segments between metal centers which impede
direct interaction of the growing chain with at least two metals si-
multaneously. Ongoing studies include ligand modifications to enhance
metal cooperativity in the ROP of rac-LA.
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The authors are grateful for financial support from CONACyT (grant
25352). N.L. is grateful for financial support from the Secretariat of
Public Education (grant UAEMOR-PTC-388) and CONACyT (grant PCI-
063-11-14-242121). We thank Salvador López Morales from Instituto
de Investigaciones en Materiales UNAM for assistance with GPC mea-
surements. We also thank LANEM (Laboratorio Nacional de Estructura
de Macromoléculas) CONACyT (grant 279905) for spectroscopic ana-
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Appendix A. Supplementary data
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