NSSN-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of l -Lactide

Article abstract of DOI:10.1021/acs.inorgchem.1c01056

A new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands is reported. The ligands feature two amide functions coupled with two thioether groups linked by a central flexible ethane bridge and two lateral rigid phenylene bridges and differ for the substituents on the aniline nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN-iPr, NSSN-Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with dibromoethane to afford the NSSN ligands without substituents on the aniline nitrogen atoms, which were subsequently alkylated through a reductive amination of acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl bromide. The corresponding zirconium and hafnium complexes 1-5 were obtained through a transamination reaction between the neutral ligands and Zr(NMe2)4 or Hf(NMe2)4 [(NSSN-iPr)Zr(NMe2)2 (1), (NSSN-Cy)Zr(NMe2)2 (2), (NSSN-Mes)Zr(NMe2)2 (3), (NSSN-iPr)Hf(NMe2)2 (4), and (NSSN-Cy)Hf(NMe2)2 (5)]. They were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3). All complexes present an octahedral coordination geometry with a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands. The catalytic performances of 1-5 in the ring-opening polymerization of cyclic esters were investigated. Complex 1 was the most active: its polymerization activity was superior to those generally displayed by zirconium complexes featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution.

Full text of DOI:10.1021/acs.inorgchem.1c01056

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NSSN-Type Group 4 Metal Complexes in the Ring-Opening
Polymerization of L‑Lactide
Salvatore Impemba, Giuseppina Roviello, Stefano Milione,* and Carmine Capacchione
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ABSTRACT: A new class of zirconium and hafnium complexes coordinated by linear
dianonic tetradentate NSSN ligands is reported. The ligands feature two amide
functions coupled with two thioether groups linked by a central flexible ethane bridge
and two lateral rigid phenylene bridges and differ for the substituents on the aniline
nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN-iPr, NSSN-
Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with
dibromoethane to afford the NSSN ligands without substituents on the aniline
nitrogen atoms, which were subsequently alkylated through a reductive amination of
acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl
bromide. The corresponding zirconium and hafnium complexes 1−5 were obtained
through a transamination reaction between the neutral ligands and Zr(NMe2)4 or
Hf(NMe2)4 [(NSSN-iPr)Zr(NMe2)2 (1), (NSSN-Cy)Zr(NMe2)2 (2), (NSSN-Mes)-
Zr(NMe2)2 (3), (NSSN-iPr)Hf(NMe2)2 (4), and (NSSN-Cy)Hf(NMe2)2 (5)]. They
were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3). All complexes
present an octahedral coordination geometry with a fac−fac ligand wrapping and a cis relationship between the other two
monodentate ligands. The catalytic performances of 1−5 in the ring-opening polymerization of cyclic esters were investigated.
Complex 1 was the most active: its polymerization activity was superior to those generally displayed by zirconium complexes
featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution.
example of complexes with ONNO ligands is given in the
complexes reported by Jones et al. in which the zirconium or
hafnium atom is coordinated by a 2,2′-bipyrrolidine-derived
salan ligand.7 These complexes were able to afford highly
isotactic enriched polymers in the ROP of rac-lactide (LA).
The group 4 complexes with tetradentate (OSSO)-type
bis(phenolato) ligands were extensively studied by Buffet and
Okuda.8 These complexes promoted the controlled ROP of
meso-LA: using the titanium or zirconium complex, syndiotac-
tic or heterotactically enriched polylactide were respectively
produced. Another significant example is the hafnium complex
with the tetradentate dithiodiolate ligand reported by Kol et
al.;9 this complex was rated as one of the most active group 4
complexes, being able to convert and melt 300 equiv of
monomer after 1 min. The effectiveness of bridged bis-
(phenolato) ligands with mixed sulfur and nitrogen donor
atoms was recently explored by Kol and Okuda.10 They found
that {ONSO}-type complexes were able to polymerize rac-LA
INTRODUCTION
■
The sustainability issues of single-use plastics have raised
significant environmental concerns that could be loosened by a
more extensive use of biodegradable and renewable materials.1
Valid alternatives to the petroleum-derived plastics are
aliphatic polyesters;2 these materials have found wide
application, and polylactide, one of the most promising
polymers in the field, is now used in short-time application
as a commodity polymer3 and as an engineering material in
biomedical application, e.g., as scaffolds for tissue engineering,
as controlled-drug-release systems, or as bioresorbable
devices.4
An effective synthetic route to aliphatic polyesters is the
ring-opening polymerization (ROP) of lactides and lactones
mediated by coordination metal complexes.5 The employed
metals virtually spread all over the periodic table, but current
interest is direct toward those metals that allow the preparation
of robust, cheap, and, above all, nontoxic catalysts. These
characteristics are typically found in the group 4 metal
complexes.6
Received: April 6, 2021
Most of the group 4 catalytic systems are based on
octahedral complexes featuring tetradentate bis(phenolato)
ligands.6 In the ligands, the anionic oxygen atoms are usually
paired with neutral immine or amino groups (ONNO ligands),
with neutral thioether groups (OSSO ligands), or with a
combination of these (ONSO ligands). A representative
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to polylactide with different tacticities depending on the nature
of the substituents on the phenolate ring.
exhibit two amide functions coupled with two thioether
groups. Herein we present the synthesis and characterization of
a new class of complexes and the study of their activity in the
ROP of cyclic esters.
With the scope of exploring new coordinative environments,
we sought alternative donor groups that could match the hard
Lewis acidity of the metal center. In this search, our attention
focused on amide donor groups because amide donor ligands
have been proven to be well-suited ligands for early transition
metals.11 Moreover, the efficiency of group 4 complexes with
amide ligands in the ROP of a cyclic ester was barely
explored.12 The most significant reports are the following.
Mountford et al. developed group 4 initiators supported by
tetradentate bis(sulfonamide)amine ligands (Chart 1a), which
RESULTS AND DISCUSSION
■
Synthesis and Structure of Proligands. The linear
tetradentate NSSN ligands designed in this work feature a
central flexible ethane bridge flanked by two rigid phenylene
bridges. In order to introduce a steric hindrance around the
metal center, the aniline nitrogen atoms were designed with
isopropyl, cyclohexyl, or mesityl substituents. The synthesis of
the ligands was accomplished in two steps: first NSSN ligands
without substituents were prepared on the aniline nitrogen
atoms, and then the aniline nitrogen atoms were properly
alkylated with the desired alkyl groups. The ligand precursor
was obtained through the reaction of 2-aminothiophenol in a
1:2 ratio with dibromoethane in refluxing methanol according
to a literature procedure.16 Starting from this, preparation of
the isopropyl or cyclohexyl derivative was carried out via a
reductive amination of acetone or cyclohexanone using zinc/
acetic acid17 according to Scheme 1. Instead, introduction of
Chart 1
Scheme 1
were fairly active (95% of 100 equiv within 6 h in toluene at 70
°C) with good control over the polymerization process.12a,b
Dagorne et al. reported on zirconium complexes with
tetradentate dianionic cyclam ligands (Chart 1b), which, in
the polymerization of rac-LA, afforded polylactide with narrow
polydispersity indexes (PDIs) and predictable molecular
weights.12c Schaper et al. developed zirconium complexes
bearing cyclohexanediyl-bridged bis(diketiminate) ligands,
which resulted in the highest active group 4 metal complex
(Chart 1c):13 it was able to convert 300 equiv of rac-LA within
7 min in tetrahydrofuran (THF) at room temperature (even if
the polymerization was poorly controlled and broadly
dispersed). Recently, Otero et al. described a series of
thermally stable and robust zirconium complexes coordinated
by NNN-scorpionate ligands (Chart 1d).14 These complexes
were effective for ROP of unpurified rac-LA under industrially
relevant conditions (absence of solvent, elevated temperature,
and monomer not rigorously purified).
the mesityl group was achieved by reacting the ligand
precursor with mesityl bromide through a palladium-catalyzed
cross-coupling reaction (Scheme 1).18 All of the ligands were
prepared in high yields, purified by recrystallization in hexane,
1
and fully characterized by H NMR spectroscopy. The NMR
patterns of the signals were consistent with the expected
symmetry, and the chemical shifts were observed in the typical
ranges for this class of compounds (see the Supporting
Information).
X-ray-quality crystals of NSSN-iPr and NSSN-Cy were
grown from a saturated hexane solution upon slow evaporation
of the solvent. The molecular structures are reported in Figure
1. NSSN-iPr crystallizes in a triclinic unit cell according to the
In the course of our studies, we have been interested in
developing ligands with a combination of hard and soft donor
groups because this combination was proven to be effective in
obtaining catalytic systems with good performances in the
polymerization of cyclic esters.15 As a further contribution, we
decided to explore the catalytic performances of group 4 metal
complexes coordinated by linear tetradentate ligands that
space group P1
̅
. In the unit cell, the asymmetric unit is
B
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Scheme 2
indicates that, at room temperature, the complexes are present
in solution as a single isomer. The 1H NMR resonances for the
methylene protons of the ethylene bridge appeared as a four-
line AB pattern, suggesting C2 symmetry for the isomer and,
consequently, a wrapping of the ligand in fac−fac coordination
mode. The variable-temperature NMR analysis in the range
25−100 °C revealed no broadening or coalescence of
resonances, indicating that the structure is rigid on the NMR
time scale (Figures S19 and S20).
Figure 1. Crystal structures of NSSN-iPr (a) and NSSN-Cy (b)
molecules. Thermal ellipsoids are shown at the 30% probability level.
(a) Selected geometrical parameters for NSSN-iPr: S1−C1 =
1.8231(15) Å, S1−C2 = 1.7693(15) Å, C1−C1_i = 1.508(3) Å,
C1−S1−C2 = 99.57(7)°, S1−C1−C1_i = 111.85(13)°, and S1−C1−
C1_i−S1_i = 180°. (b) Selected geometrical parameters for NSSN-
Cy: S1−C1 = 1.8195(14) Å, S1−C2 = 1.7650(13) Å, C1−C1_i =
1.507(2) Å, C1−S1−C2 = 101.76(6)°, S1−C1−C1_i = 112.61(12)°,
and S1−C1−C1_i−S1_i = 180°.
The two nonequivalent methyl groups of −NMe2 moieties
bound to metal center appear as sharp singlets in the cases of
the complexes with isopropyl or cyclohexyl substituents,
whereas they appear as broad singlets in the case of the
complex with mesityl substituents. The barrier to rotation of
the methyl amide groups around the Mt−NMe2 bond was
evaluated by theoretical calculations. The energy profile
determined by plotting the total energy as a function of the
torsion angle revealed an activation barrier of 3.0, 3.1, and 8.4
kcal/mol for 1−3, respectively. These results are in accordance
with the free rotation observed for 1 or 2 and a restricted
rotation for 3 on the NMR time scale and can be addressed to
the higher steric encumbering determined by the mesityl
substituent in 3, compared with the isopropyl or cyclohexyl
substituents in 1 or 2. To visualize steric hindrance of the
substituents around the metal center, we made use of
topographic steric maps, in which altimetry isocontour lines
circumscribes the hampering areas of the ligand. In these maps,
the metal atom is placed in the center, the nitrogen atoms
occupy the apical positions, and the sulfur atoms lie in the
equatorial belt. Inspection of the maps reported in Figure 2
revealed that coordination of the ligand produces a horizontal
groove in which the metal and amide ligands can be allocated.
The substituents on the amide atoms of the ligand cause an
encumber that is visible by the two bulges in the north and
south positions of the map. The size of this protuberance
increases from the ligand with isopropyl or cyclohexyl
substituents to the ligand with a mesityl substituent. Anyway,
the coordination environment around the metal center is
rather open, and the percentages of the total volume occupied
by the ligand of the sphere with a radius of 4.5 Å and the metal
at the core (% Vbur) are 54.2%, 54.4%, and 62.0% for
complexes 1−3.
represented by two independent half-molecules, generating, by
symmetry, two independent molecules characterized by a
similar trans-planar conformation and analogous geometrical
parameters. In Figure 1a, only one complete molecule,
generated by the inversion −x + 1, −y + 2, −z, is shown
(see the Supporting Information for complete data). The
aromatic rings of each molecule are parallel to each other. A
NSSN-Cy molecule (monoclinic, P21/c) sits on a crystallo-
graphic inversion center. Also, in this case, because of
conditions imposed by the crystallographic elements, the
molecule assumes a trans-planar conformation, with the
aromatic moieties pointing to opposite sides. The phenyl
rings are parallel to each other. Cyclohexyl substituents show a
chair conformation.
Synthesis and Characterization of Complexes 1−3.
The reaction of NSSN-iPr, NSSN-Cy, or NSSN-Mes with
Zr(NMe2)4 or Hf(NMe2)4 in refluxing toluene clearly afforded
the zirconium or hafnium complexes (NSSN-iPr)Zr(NMe2)2
(1), (NSSN-Cy)Zr(NMe2)2 (2), (NSSN-Mes)Zr(NMe2)2 (3),
(NSSN-iPr)Hf(NMe2)2 (4), and (NSSN-Cy)Hf(NMe2)2 (5)
in good yields (54−85%; Scheme 2). All complexes were
purified by recrystallization from cold hexane and isolated as
yellow powders.
With the aim of preparing the entire series of group 4 metal
complexes, synthesis of the titanium amide derivative was
attempted, but the treatment of tetrakis(dimethylamido)
titanium with NSSN-iPr-H2 resulted in no reaction, even
after prolonged heating.
Low-temperature NMR experiments were carried out in
order to explore the presence of other isomers in solution.
Upon cooling of a methylene chloride-d2 solution of 1, the
NMR signals broadened and split into different sets of
resonances (Figure S21). For example, the methyl groups of
−NMe2 functions that appear as a sharp singlet at 2.90 ppm at
1
The complexes were characterized by H and 13C NMR
spectroscopy. In all cases, one set of resonances was found; this
C
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As matter of fact, the energy costs to distort the structure in
order for the ligand to adopt the conformations for the fac−fac,
fac−mer, and mer−mer wrappings around the metal center
were 12.8, 18.7, and 20.3 kcal/mol, respectively.
The crystal structures of 1 and 4 are shown in Figure 3.
Figure 4 shows the molecular structures of 2 and 5. Because of
the chiral nature of the complexes and their crystallization in
centrosymmetric space groups (see the Supporting Informa-
tion), the crystals contain the racemic forms of the Λ and Δ
enantiomers. Only in the case of the Hf-NSSN-iPr complex
does the asymmetric unit contain both enantiomers (see Table
1 and the Supporting Information for geometrical parameters).
In all cases, a distorted octahedral coordination geometry
around the metal atoms is observed. All of the complexes
adopt a pseudo-C2-symmetrical configuration with two cis-
arranged dimethylamine groups (N3−M−N4 between 98.9°
and 103.9°). As shown in Table 1, the octahedral coordination
geometry is distorted because of the restrictions imposed by
the tetradentate ligand. Major deviations from ideal octahedral
coordination are observed in both complexes containing the
cyclohexyl group. Probably, the steric hindrance of the
cyclohexyl group induces a significant deformation in the
coordination area (see Table 1 and the Supporting
Information). In all complexes, the two sulfur atoms of the
NSSN ligand show a gauche conformation (see the dihedral
angle SCCS in Table 1), which induces the trans coordination
of the nitrogen atoms of the tetradentate ligand. As can be seen
from Table 1, there are notable similarities in the geometric
parameters of the coordination environment within each of the
pairs 1/4 and 2/5. The M−N and M−S distances are in
agreement with those reported in the literature for analogous
complexes.19,17b It is worth pointing out that a slight
elongation of the N−M bond of the NSSN ligand with
respect to that of the dimethylamido group (see Table 1) is
observed.19a,b,17b Finally, it is worth noting that structures of 2
and 5 are isomorphic. In fact, both complexes crystallize in an
orthorhombic lattice, space group Pbca, showing the same cell
parameters (see the Supporting Information).
Figure 2. Steric maps of complexes 1−3. The isocontour curves are
given in angstroms.
room temperature gave rise at −60 °C to three singlets at 2.63,
2.80, and 2.93 ppm in a 1:2:1 intensity ratio (Figure S22). The
signals for methylene protons of the ethylene bridge gave rise
to two sets of resonances: one displayed the same AB pattern
observed at room temperature and the other consisted of a
complicated non-first-order pattern of signals composed by
three doublets at 2.28, 3.06, and 3.39 ppm and a multiplet at
2.40 ppm. This picture is consistent with the presence of two
distinct stereoisomers in solution in a 2:1 molar ratio. The
more abundant stereoisomer is the isomer with C2 symmetry
already observed at room temperature, whereas the other one
is the stereoisomer with C1 symmetry that originated by the
fac−mer wrapping of the ligand around the metal center. To
gain insight into the equilibrium between the different
stereoisomers of 1, density functional theory (DFT)
calculations were carried out. The three possible stereoisomers
expected for 1 are sketched in Scheme 3, and the
Scheme 3
Polymerization Studies. Complexes 1−3 were assessed as
initiators for the ROP of L- and rac-LA. Polymerization
experiments were carried out in a toluene solution at 80 °C,
and representative results are reported in Table 2.
Complex 1 was revealed to be an efficient initiator: after 1 h,
69% monomer conversion was achieved, while almost
quantitative conversion was obtained in 2 h. The turnover
frequency (TOF) was 42 h−1, which is superior to those
generally displayed by zirconium complexes featuring OSSO
ligands and compares well with those of the most active group
4 complexes operating in a toluene solution.6,8,9,20 To achieve
more insights in the polymerization process, the reaction was
sampled by taking aliquots of the product mixture at the
1
prescribed reaction times and analyzing them by H NMR
spectroscopy and gel permeation chromatography (GPC)
measurements. It resulted that propagation follows a first-order
kinetic law with respect to the monomer concentration, as
shown by the linearity of the plot reporting ln([L-LA]0/[L-
LA]t) versus time. The apparent propagation rate constant,
corresponding minimum-energy structures were successfully
located and are reported in Figure S23. The differences in the
gas-phase energies are in the range 0.0−23.7 kcal/mol. The
most stable stereoisomer is that with the C2-symmetric
structure, and the energies of the C1- and Cv-symmetric
structures are higher by 10.3 and 23.7 kcal/mol, respectively.
These results justify the nonobservation of the Cv-symmetric
stereoisomer in solution. The differences in energies between
the three isomers have a significant contribution from the
energy required for the dianonic ligand to adopt the
conformation necessary for coordination to the metal center.
k
app, was 1.33
0.03 h−1 (Figure 5). GPC analysis of the
isolated polymer revealed that the number-average molecular
weights (Mn(expt)) increase linearly with monomer conversion,
with the PDIs relatively narrow and constant during the
polymerization process (Figure 6). The slope of the least-
squares best fit of the number-average molecular weights is
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Figure 3. ORTEP-3 views of 1 (a) and 4 (b). Thermal ellipsoids are shown at the 30% probability level. Selected bond distances and angles are
reported in Table 1. Hydrogen atoms are omitted for clarity.
Figure 4. ORTEP-3 views of 2 (a) and 5 (b). Thermal ellipsoids are shown at the 30% probability level. Selected bond distances and angles are
reported in Table 1. Hydrogen atoms are omitted for clarity.
12.1 kDa, and it is lower compared with that expected (14.4
kDa) for the growth of one chain per metal center. The
mismatch may be due to inter- and intramolecular trans-
esterification processes that affect the polymerization reaction.
The polymers were subjected to matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) analysis.
In the mass spectrometry (MS) spectrum, the peaks were
spaced by 72 Da, this distribution confirmed that side reactions
are active during the propagation step.
same metal center (Figure S36). As would be expected,
analysis of the heteronuclear single quantum coherence
(HSQC) spectrum showed that these resonances correlate
with two 13C NMR signals at 31.7 and 22.8 ppm through
positively phased cross-peaks. These data suggest that the
polymerization proceeds via a coordination−insertion mecha-
nism.
To probe the robustness of the catalytic system, the amount
of catalyst was reduced. With a [L-LA]/[1] ratio of 1000 or
3000, the catalyst still preserved a good activity and reached a
TOF of 59 or 139 h−1 in 16 h, respectively. The kinetic plots
are provided in Figure S42. The molecular weights increased
with the [L-LA]/[1] ratio but still remained lower that than
expected.
With the aim of getting insight into the initiation process, a
polymerization run at low LA to the initiator molar ratio ([L-
1
LA]0/[I]0 = 10) was performed. H NMR analysis of the
isolated polymer revealed, among the other signals, two
singlets at 2.95 and 3.03 ppm attributable to two methyl
groups of the −C(O)NMe2 end group that originates from
the nucleophilic attack of the amide NMe2 bound to the metal
center to the carboxylic carbon of the LA coordinated to the
In order to assess the role of the substituents on the aniline
nitrogen atoms, the polymerization studies were extended to
complexes 2 and 3. Complex 2 featuring cyclohexyl
E
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1, 2, 4, and 5
a
M = Zr, Hf
1
2
4
5
M1−S1
M1−S2
M1−N4
M1−N3
M1−N2
M1−N1
N3−M1−S1
N4−M1−S2
N3−M1−N2
N4−M1−N3
N3−M1−N1
N3−M1−S2
S−C−C−S
2.7382(5)
2.8113(5)
2.057(1)
2.059(1)
2.154(1)
2.212(1)
173.02(4)
155.05(4)
97.90(5)
103.73(5)
98.90(5)
100.03(4)
−60.57(15)
2.724(1) [2.728(1)]
2.778(1) [2.758(1)]
2.030(5) [2.025(5)]
2.041(5) [2.047(5)]
2.147(4) [2.173(4)]
2.168(4) [2.179(4)]
172.59(15) [170.7(1)]
156.87(16) [169.93(14)]
97.88(18) [107.00(19)]
103.9(2) [97.7(2)]
2.780(1)
2.779(1)
2.061(2)
2.055(2)
2.194(2)
2.187(2)
168.94(6)
167.89(6)
94.26(8)
99.77(8)
110.01(7)
92.03(6)
−65.2(2)
2.753(1)
2.754(1)
2.041(4)
2.045(3)
2.177(3)
2.180(3)
168.97(10)
169.53(11)
94.11(13)
98.99(14)
108.15(13)
91.18(10)
−65.6(4)
98.86(18) [96.64(18)]
98.82(16) [92.37(16)]
−59.9(5) [62.9(5)]
a
2 crystallizes as a racemic mixture in the unit cell. In the square brackets are reported the geometrical parameters of the enantiomer not shown in
Figure 4 but reported in the Supporting Information.
Table 2. ROP of Cyclic Esters in the Presence of 1
a
b
c
d
d
entry
complex
monomer
t/h
conv/%
TOF
Mn(th)
Mn(expt)
PDI
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
2
3
3
4
4
5
5
1
1
1
L-LA
100
100
1000
3000
100
100
100
100
100
100
100
100
100
100
200
500
100
1
2
16
16
2
16
1
2
1
2
1
2
69
90
95
74
45
97
55
15
25
14
47
69
46
78
96
75
11
69
42
59
139
22
6
55
8
25
7
47
35
46
39
192
376
0.7
9.9
13.0
137.1
320.8
8.7
1.20
1.15
1.18
1.19
n.d.
1.12
1.12
n.d.
1.27
1.16
1.11
1.10
1.19
1.17
1.24
1.18
n.d.
L-LA
11.4
60.2
66.5
n.d.
4.0
6.5
n.d.
2.6
1.8
5.8
9.1
5.8
L-LA
L-LA
rac-LA
rac-LA
L-LA
14.0
7.9
rac-LA
L-LA
9
3.5
2.0
6.8
10.0
6.7
11.2
21.9
42.9
10
11
12
13
14
15
16
18
rac-LA
L-LA
L-LA
L-LA
1
2
1
1
L-LA
9.8
ε-CL
ε-CL
β-BL
33.5
20.6
n.d.
16
a
b
All reactions were carried out in 2.0 mL of toluene, 80 °C, [I]0 = 5.0 mM, [Monomer] = 1.4 M, and [Monomer]0/[I]0 = 100. Molecular
c
1
conversion determined by H NMR spectroscopy (CDCl3, 298 K). Calculated molecular weight using Mn(th) (kg/mol) = (PMMonomer × 100 ×
conversion)/1000. Experimental molecular weight Mn(expt) (kg/mol) and polydispersity index (PDI) determined by GPC in THF using
d
polystyrene standards and corrected using the factor 0.58 for PLA, 0.56 for PCL, and 0.54 for PBL.
substituents showed an activity very similar to that observed
for 1, whereas complex 3 with mesityl substituents was much
less active: in 1 h at 80 °C, it only allowed 25 mol %
conversion for 100 equiv of L-LA. For a better evaluation of the
polymerization rates, the semilogarithmic plots of ln([L-LA]0/
[L-LA]t) versus time were determined for 2 and 3. The
apparent propagation rate constant for 2 was very similar to
that found for 1, kapp(2) = 0.90 0.02 h−1, while the apparent
propagation rate constant for 3 was about 4 times lower than
that found for 1, kapp(3) = 0.27 0.02 h−1 (Figure 5). The
nonzero intercepts of the kinetic plots are due to the
experimental conditions used to carry out the polymerization
experiments; in particular, it is due to the time required for the
solid LA to dissolve. See the Supporting Information for
further details.
number-average molecular weights decreases with an increase
of the steric encumbering determined by the substituents on
the aniline nitrogen atoms. It passes from 12.6 kDa for 1 to
11.3 and 9.5 kDa for 2 and 3, respectively (Figures S43 and
S44).
The increase of the steric size of the substituents probably
makes access to the catalytic site more difficult for the
monomer, thus causing a decrease in the propagation rate. At
the same time, the transesterification reactions seem to be
unaffected and become progressively more significant up
moving from complex 1 to complex 3.
From zirconium to hafnium, the polymerization activity
decreased: when polymerization was promoted by 4 or 5, 47 or
46 mol % monomer conversion was obtained in 1 h. The
corresponding rate constant of 4 (0.56 0.01 h−1; Figure 5)
was about half that observed for 1. Also, in this case, the
molecular weights of the obtained polymer were slightly lower
than those expected but comparable with those obtained with
1. The slope of the least-squares best fit of the number-average
molecular weights was 12.5 kDa (Figure S45).
As was already observed in the case of the polymerization
promoted by 1, the molecular weights of the polymer
produced in the polymerizations promoted by complexes 2
and 3 were lower than those theoretically expected. Moreover,
it was observed that the slope of the least-squares best fit of the
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heterotactic microstructure. This may be a consequence of the
redistribution of stereosequences because of the transester-
ification reactions that affect the polymerization process.
Complex 1 was also evaluated as the catalyst for the ROPs of
ε-caprolactone. Polymerization experiments were carried out in
a toluene solution at 80 °C, and representative results are
reported in Table 2. In these experimental conditions, complex
1 was able to convert 100 equiv of monomer within 1 h. The
TOF of 96 h−1 well compares with those generally obtained by
zirconium complexes featuring OSSO ligands.
The polymerization process was investigated by analyzing
portions of the reaction mixture at prescribed times. As
expected, consumption of the monomer follows a first-order
kinetic law, and the obtained kinetic constant was 0.069
0.003 min−1. The experimental number-average molecular
weights of the polymers increase as the reaction proceeds, but
as was already observed in the polymerization of LA, the values
are lower than those expected supposing the growth of one
polymer chain per metal center. The distributions of the
molecular weights were monomodal and narrow with PDIs in
the range of 1.19−1.53.
Figure 5. Pseudo-first-order kinetic plots for ROP of L-LA by 1 (kapp
−1
2
= 1.33 0.03 h , R = 0.997, ), 2 (kapp = 0.90 0.02 h−1, R2 =
■
−1
2
●
▲
0.997, ), 3 (kapp = 0.27 0.01 h , R = 0.976, ), and 4 (kapp
=
0.56 0.01 h−1, R2 = 0.998, ×). Conditions: [L-LA]0 = 1.4 M, [L-
LA]0/[I]0 = 100, T = 80 °C, and toluene as the solvent.
In the 1H and 13C NMR spectra of the obtained
polycaprolactones, the resonances due to the terminal groups
were clearly detectable. In particular, in the 1H NMR spectrum,
the −C(O)NMe2 end group was recognized by the presence
of two singlets at 2.94 and 3.03 ppm, and the hydroxyl end
group CH2CH2OH was identified by the presence of a
multiplet at 3.61 ppm. These end groups confirm that the
reaction follows a coordination−insertion mechanism.
Molecular Modeling Studies. In order to better assess
the catalytic performances of the catalysts developed in this
work and put them in a comparative prospective with respect
to other group 4 catalysts, DFT calculations were undertaken.
Our calculations were focused on complex 1 with a C2-
symmetric structure because this isomer is the most stable
isomer found in solution. We decided to compare 1 with the
group 4 complexes reported in Chart 2: c and d are among the
most active complexes in this group,13,22 and e and f are
structurally similar to the complexes reported in this work.8
Complex f bears tert-butyl substituents on the aromatic rings,
and it is considered a prototype of the OSSO complexes, e,
bearing bromine atoms on the aromatic rings, resultin in them
being particularly active.20
Lewis acidity and steric hindrance around the metal center
are generally considered two important parameters that
influence the catalytic activity, and both are determined by
the coordinated ligand. The Lewis acidity was estimated with
the partial charge of the zirconium atom obtained through
natural population analysis (NPA). The results are reported in
Table 3. The partial charges at the zirconium atom range
between 1.28 and 1.72 au. The highest values were found for
complexes c and d, intermediate values were found for e and f,
and the lowest value was obtained for complex 1. From these
data, it results that the NSSN ligand transfers a higher charge
density to the metal atom compared with those transferred
from the bis(phenolate) or diketiminate ligands. Complex 1 is
the least acidic complex in the series of complexes examined.
The steric hindrance due to the coordinated ligands for c−f
was determined from the percentage of the total volume
occupied by the ligand of the sphere with a radius of 4.5 Å and
the metal at the core. The percent buried volume (% Vbur) is
reported in Table 3. The complexes with the more open
catalytic pockets are those bearing bis(phenolate) ligands, and
Figure 6. Plot of the number-average molecular weights Mn(expt)
(dots) versus monomer conversion with theoretical Mn(th) (dashed
line) using 1 as the initiator. Conditions: [L-LA]0 = 1.4 M, [L-LA]0/
[I]0 = 100, T = 80 °C, and toluene as the solvent.
The ROP of rac-LA was significantly slower. When the
reaction was promoted by complex 1, only 44 mol %
conversion was achieved after 2 h, and quantitative conversion
was reached in 16 h. The apparent propagation rate constant
was about one-third of that observed in the polymerization of
L-LA (kapp = 0.31
0.02 h−1). A decrease in the
polymerization rate of rac-LA was also observed when the
reaction was promoted by 2 and 3. The apparent propagation
rate constants were 0.12 0.03 and 0.11 0.01 h−1 for 2 and
3, respectively.
The molecular weights of the obtained polymer increase
with progress of the reaction, but the values are approx-
imatively half of those expected for the growth of one chain per
metal center. The PDIs are in the range 1.12−1.17. The
tacticity of the obtained polymers was investigated by the
21
1
homonuclear-decoupled H NMR spectrum. The inspection
of the methine regions for the poly-rac-LAs obtained by 1 and
1
2 revealed that the microstructure is atactic. In the H NMR
spectrum of poly-rac-LAs obtained by 3, the signal due to the
mrm tetrad was the most intense, but it was not paired with a
similar signal for the rmr tetrad (rmr:mrm = 1:3.7). The
distribution of tetrads and the integration ratio of the peaks
were inconsistent with either an atactic microstructure or an
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adduct has been observed in other DFT calculations on the
ROP of cyclic esters.20,23 In the case of c/LA, d/LA, and e/LA,
the LA monomer is located from the metal center at 2.57, 2.51,
and 2.49 Å for c−e, respectively. Therefore, formation of the
coordination adduct is not hindered by the steric encumber,
but it is determined by the partial charge present on the metal
atom. In the cases explored in this study, formation of the
coordination adduct occurs in all cases in which the charge of
the metal center is greater than +1.50 au, regardless of the
steric hindrance of the coordination sphere. The coordination
of LA is endothermic in all cases. From the values reported in
Table 3, it is possible to observe a linear relationship between
the binding energy and buried volume. We thought that the
energy cost due to deformation of the complex to
accommodate the LA in the coordination sphere of the
metal should play a crucial role. So, we determined the
framework distortion energy (FDE). FDE was calculated as the
difference between the energy of the complex obtained by
removing the LA from the coordination sphere of the adduct
and that of the starting complex. The FDEs for c−e are 16.7,
12.3, and 2.5 kcal/mol, respectively: as expected, it gradually
decreases with a decrease of the buried volume. To further
characterize the coordination step, we performed a scan of the
energy surface corresponding to the approach of the LA to the
metal center for c−f and 1. The curves reported in Figure 7
show that the energy is quite flat in the region between 2.2 and
3.2 Å and that all of the coordination adducts lie in shallow
potential wells.
Chart 2
Table 3. Summary of DFT Results
a
b
c
d
e
complex
Zr charge
% Vbur
ΔEbind
FDE
ΔGTS1
c
d
e
f
1.68
1.72
1.52
1.48
1.28
66.6
50.1
42.5
46.9
54.2
16.7
12.3
2.5
18.5
12.3
9.6
4.1
7.9
8.9
13.3
14.5
1
a
Charge of the zirconium atom determined through NPA (in au).
b
c
Percent buried volume. Energy variation due to the coordination of
d
LA to the complex (in kcal/mol). Framework distortion energy
relative to coordination of LA to the complex (in kcal/mol). Barrier
e
Figure 7. Energy profiles for the approach of L-LA to the metal center
of c−e and 1. The energies are relative to the separated L-LA and
initiators.
for the nucleophilic attack of the growing alkoxide polymer chain (in
kcal/mol).
Last, we computed the energies required to the overcome
the barrier for the nucleophilic attack of the growing alkoxide
polymer chain to the carboxylic carbon of the monomer, i.e.,
the barrier for first step of the propagation reaction. The values
of the activation barriers (ΔGTS1) are shown in Table 3.
Complex c, the most active in the group, has the lowest barrier,
4.1 kcal/mol, while complex 1 has the highest barrier, 14.5
kcal/mol. The ΔGTS1 increases as the charge at the metal
center decreases.
the most open one is complex e with a % Vbur of 42.5. Complex
c, bearing the diketiminate ligand, has the most closed catalytic
pocket, and the value of % Vbur is 66.6. So, the steric bulk of the
NSSN ligand is intermediate between those of the bis-
(phenolate) and diketiminate ligands.
Afterward, we turned our attention to the coordination step
in order to verify how these two parameters influence the
process. The coordination adducts between LA and the metal
complexes were successively localized only for complexes c−e,
while despite several attempts, no coordination adduct was
found for complexes 1 and e. In the structures optimized for 1/
LA and f/LA, the distance between the oxygen atom of the
carbonyl group and the metal atom is not compatible with a
coordination bond distance (4.78 Å for 1/LA and 4.38 Å for f/
LA); rather LA interacts with the complex through a series of
van der Waals interactions. The absence of a coordination
CONCLUSIONS
■
In this work, we have reported a series of group 4 metal
complexes supported by a new class of linear tetradentate
dianionic ligands featuring an unexplored combination of hard
and soft donor groups. In particular, on the ligand skeleton,
two amide functions are interspersed with two thioether
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(13.46 g, 0.21 mol) were added to a 250 mL round-bottom one-
necked flask equipped with a condenser and a Teflon-sealed stirbar.
The mixture was heated to room temperature for 48 h. After it was
cooled to room temperature, the mixture was quenched with a 30%
NH3 aqueous solution (150 mL) and diethyl ether (200 mL). The
organic layer was dried with anhydrous MgSO4, and a whitish solid
groups. The donor functions are linked by a central flexible
ethane bridge and two lateral rigid phenylene bridges, and once
coordinated to the metal center, a 5−5−5 array of chelating
rings is obtained.
The target NSSN ligands were synthesized through the
reaction of 2-aminothiophenol with dibromoethane, and this
afforded the NSSN skeleton that was subsequently alkylated on
the aniline nitrogen atoms through a reductive amination of
acetone or cyclohexanone or a palladium-catalyzed cross-
coupling reaction with mesityl bromide. The corresponding
zirconium and hafnium complexes were obtained through a
transamination reaction between the neutral ligands and
Zr(NMe2)4 or Hf(NMe2)4. As elucidated by X-ray diffraction
studies, the complexes present a distorted octahedral
coordination geometry with a fac−fac ligand wrapping and a
cis relationship between the other two monodentate ligands in
a pseudo-C2-symmetrical configuration. This structure was
retained in solution, as evidenced by NMR solution studies.
Although the ligands were targeted to provide a steric
hindrance around the metal center, the percentage of the
volume occupied in the coordination sphere was in the range
54.2−62.0%.
All of the complexes were able to promote the ROP of cyclic
esters. The most active was the zirconium complex featuring
the ligand provided with isopropyl groups on the amide
functions: its polymerization activity was superior to those
generally displayed by zirconium complexes featuring OSSO
ligands and compare well with that of the most active group 4
complexes operating in a toluene solution.
The DFT investigation revealed that the NSSN ligand
transfers a higher charge density to the metal atom compared
with those transferred from other ligands such as bis-
(phenolate) or diketiminate ligands. The NSSN complexes
show a low charge at the metal center, and it is expected that
an increase of the Lewis acidity of the metal center should lead
to a more active catalyst.
1
product was obtained upon removal of the solvent (6.7 g, 88%). H
NMR (400.13 MHz, CDCl3, 25 °C); δ 6.54−7.35 (m, 8H, ArH), 4.93
(d, 2H, NH), 3.64 (m, 2H, CH), 2.77 (s, 4H, S−CH2), 1.22 (d, J =
6.3 Hz, 12H, CH3). 13C NMR (100.62 MHz, CDCl3, 25 °C): δ 23.05,
34.68, 44.12, 110.78, 116.19, 116.26, 130.47, 136.82, 148.78. HRMS
(MALDI). Calcd for C20H29N2S2 ([M + H]+): m/z 361.1772. Found:
m/z 361.1785.
N,N′-[2,2′-[Ethane-1,2-diylbis(sulfanediyl)]bis(2,1-phenylene)]-
bis(methylene)dicyclohexanamine, [NSSN-Cy]. [NSSN] (6.67 g, 24
mmol), zinc (15.8 g, 0.24 mol), acetic acid (100 mL), and
cyclohexanone (9.48 g, 48 mmol) were added to a 250 mL round-
bottom one-necked flask equipped with a condenser and a Teflon-
sealed stirbar. The mixture was heated to 65 °C for 48 h. After it was
cooled to room temperature, the mixture was quenched with a 30%
NH3 aqueous solution (200 mL) and diethyl ether (300 mL). The
organic layer was dried with anhydrous Na2SO4, and a whitish solid
product was obtained upon removal of the solvent (10.3 g, 97%). 1H
NMR (400.13 MHz, CDCl3, 25 °C): δ 6.52−7.35 (m, 8H, ArH), 5.01
(s, 2H, NH), 3.27 (m, 2H, Cy), 2.77 (s, 4H, S−CH2), 1.42−2.02 (m,
20H, Cy). 13C NMR (100.62 MHz, CDCl3, 25 °C): δ 25.04, 26.06,
33.31, 34.80, 51.43, 110.71, 110.73, 116.14, 130.43, 136.89, 148.69.
HRMS (MALDI). Calcd for C26H37N2S2 ([M + H]+): m/z 441.2398.
Found: m/z 441.2392.
N,N′-[2,2′-[Ethane-1,2-diylbis(sulfanediyl)]bis(2,1-phenylene)]-
bis(methylene)bis(2,4,6-trimethylaniline), [NSSN-Mes]. A Schlenk
flask was charged with [NSSN] (0.6 g, 2.2 mmol), mesityl bromide
(0.8 g, 4.0 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.078
g, 0.085 mmol), rac-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl
(0.13 g, 0.21 mmol), sodium tert-butoxide (0.63 g, 6.56 mmol),
and toluene (15 mL). The reaction mixture was stirred and heated to
110 °C under a stream of N2 for 72 h. After it was cooled to room
temperature, the mixture was quenched with a saturated NH4Cl
aqueous solution and extracted with methylene chloride. The organic
layer was dried with anhydrous MgSO4 and concentrated to dryness
under reduced pressure to afford a brown oil. This product was
purified via flash column chromatography (SiO2, 230−400 mesh, 4:1
n-hexane/CH2Cl2 as the eluent) to give a white solid (0.5 g, 45%). 1H
NMR (400.13 MHz, CDCl3, 25 °C): δ 7.40 (d, 2H, ArH), 7.04 (t,
4H, ArH), 6.60 (t, 4H, ArH), 6.37 (s, 2H, NH), 6.09 (d, 2H, ArH),
The results achieved in this work demonstrate that ligands
with a combination of amide and thioether functions as donor
groups are a good platform for the development of new
catalysts for the polymerization of cyclic esters. We think that
our study could contribute to a deepening of the structure−
activity relationships affecting the reactivity of group 4
complexes and to the obtainment of more insights into the
parameters that control their activity.
3.01 (s, 4H, S−CH2), 2.30 (s, 6H, p-CH3), 2.08 (s, 12H, o-CH3). 13
C
NMR (100.62 MHz, CDCl3, 25 °C): δ 18.33, 21.08, 34.51, 111.29,
115.91, 117.54, 129.34, 130.44, 135.28, 135.96, 136.42, 136.59,
147.98. HRMS (MALDI). Calcd for C32H36N2S2 ([M]+): m/z
512.2320. Found: m/z 512.2312.
EXPERIMENTAL PART
Synthesis of the Complexes. Synthesis of [NSSN-iPr]Zr(NMe2)2
(1). A solution of NSSN-iPr (0.53 g, 1.5 mmol) in benzene (10 mL)
was added to a stirred solution of Zr(NMe2)4 (0.40 g, 1.5 mmol) in
benzene (20 mL) at room temperature. The resulting pale-yellow
solution was refluxed for 24 h, after which the volatiles were removed
under vacuum. The crude product was washed with pentane (5 mL)
to give 1 as an orange solid (0.68 g, 85%). Suitable crystals for X-ray
diffraction analysis were obtained by cooling a pentane solution at
■
General Procedures. The description of materials, methods,
instruments, and measurements is provided in the Supporting
Information.
Synthesis of the Ligands. 1,2-Bis(aminophenylthio)ethane,
[NSSN]. 1,2-Dibromoethane (7.3 g, 39 mmol) was added dropwise to
a refluxing solution of 2-aminothiophenol (9.8 g, 78 mmol) and
sodium hydroxide (3.1 g, 78 mmol) in methanol (100 mL). The
refluxing reaction mixture was stirred for 16 h. Upon completion of
the reaction, the solvent was removed in vacuo and the reaction
mixture was cooled to room temperature. Water (50 mL) was added,
and the reaction mixture was extracted with diethyl ether (4 × 50
mL). The organic layer was dried with Na2SO4 and evaporated to
dryness in vacuo to get a yellow solid product (10.7 g, 99%). 1H NMR
(400.13 MHz, CDCl3, 25 °C): δ 6.64−7.31 (m, 8H, ArH), 4.35 (s,
4H, NH2), 2.86 (s, 4H, S−CH2). 13C NMR (100.62 MHz, CDCl3, 25
°C); δ 34.54, 115.09, 116.79, 118.65, 130.09, 136.23, 148.61.
N,N′-[2,2′-[Ethane-1,2-diylbis(sulfanediyl)]bis(2,1-phenylene)]-
bis(methylene)dipropan-2-amine, [NSSN-iPr]. [NSSN] (5.79 g, 21
mmol), zinc (13.73 g, 0.21 mol), acetic acid (60 mL), and acetone
1
−20 °C overnight. H NMR (400.13 MHz, CDCl3, 25 °C): δ 7.19
(dd, J1 = 7.5 Hz, J2 = 1.4 Hz, 2H, ArH), 7.10 (t, J = 7.7 Hz, 2H, ArH),
6.73 (d, J = 8.3 Hz, 2H, ArH), 6.40 (t, J = 7.6 Hz, 2H, ArH), 4.17 (m,
2H, CH), 3.12 (d, J = 10.2 Hz, 2H, S-CH2), 2.90 (s, 12H, N(CH3)2),
2.70 (d, J = 10.2 Hz, 2H, S-CH2), 1.45 (d, J = 6.7 Hz, 12H, CH3). 13
C
NMR (100.62 MHz, CDCl3, 25 °C): δ 161.03, 135.33, 130.73,
115.55, 114.48, 114.26, 48.26, 44.51, 39.67, 22.23, 21.81. Anal. Calcd
for C24H38N4S2Zr: C, 53.59; H, 7.12; S, 11.92. Found: C, 53.84; H,
7.23; S, 11.87.
Synthesis of [NSSN-Cy]Zr(NMe2)2 (2). Complex 2 was prepared
from [NSSN-Cy] (0.70 g, 1.6 mmol) and Zr(NMe2)4 (0.42 g, 1.6
mmol), as described above for 1. Yield: 0.72 g (73%). 1H NMR
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(400.13 MHz, CDCl3, 25 °C): δ 7.15 (m, J = 7.6 Hz, 2H, ArH), 7.07
(m, J = 7.4 Hz, 2H, ArH), 6.82 (m, J = 8.4 Hz, 2H, ArH), 6.37 (m, J =
7.2 Hz, 2H, ArH), 3.61 (m, 2H, CH), 3.11 (d, J = 10.2 Hz, 2H, S-
CH2), 2.93 (s, 12H, N(CH3)2), 2.77 (d, J = 10.2 Hz, 2H, S-CH2),
2.24−1.17 (m, 22H, Cy). 13C NMR (100.62 MHz, CDCl3, 25 °C): δ
161.69, 135.28, 130.82, 115.31, 114.09, 114.03, 58.74, 44.78, 39.99,
32.01, 31.48, 27.06, 26.40. Anal. Calcd for C30H46N4S2Zr: C, 58.30;
H, 7.50; S, 10.38. Found: C, 58.61; H, 7.84; S, 10.57.
Synthesis of [NSSN-Mes]Zr(NMe2)2 (3). Complex 3 was prepared
from [NSSN-Mes] (0.52 g, 1.0 mmol) and Zr(NMe2)4 (0.27 g, 1.0
mmol), as described above for 1. Yield: 0.41 g (60%). 1H NMR
(400.13 MHz, CDCl3, 25 °C): δ 7.26 (m, J = 7.5 Hz, 2H, ArH), 6.93
(m, 4H, ArH + H-Mes), 6.45 (m, J = 7.4 Hz, 2H, ArH), 5.72 (m, J =
8.4 Hz, 2H, H-Mes), 3.25 (d, J = 9.9 Hz, 2H, S-CH2), 3.04 (d, J = 9.9
Hz, 2H, S-CH2), 2.30 (brs, 12H, N(CH3)2), 2.26 (s, 6H, CH3-Mes),
2.18 (s, 6H, CH3-Mes), 2.00 (s, 6H, CH3-Mes). 13C NMR (100.62
MHz, CDCl3, 25 °C): δ 161.15, 145.90, 135.73, 135.48, 134.34,
132.85, 131.84, 129.71, 129.48, 115.18, 113.70, 111.51, 39.44, 20.90,
19.80, 18.21. Anal. Calcd for C36H46N4S2Zr: C, 62.65; H, 6.72; S,
9.29. Found: C, 62.89; H, 6.91; S, 9.50.
Synthesis of [NSSN-iPr]Hf(NMe2)2 (4). A solution of NSSN-iPr
(1.09 g, 3.0 mmol) in toluene (10 mL) was added to a stirred solution
of Hf(NMe2)4 (1.07 g, 3.0 mmol) in toluene (20 mL) at room
temperature. The resulting pale-yellow solution was refluxed for 24 h,
after which the volatiles were removed under vacuum. The crude
product was washed with pentane (10 mL) to give 4 as a yellow solid
(1.01 g, 54%). The suitable crystals for X-ray diffraction analysis were
obtained by cooling a pentane solution at −20 °C overnight. 1H
NMR (400.13 MHz, CDCl3, 25 °C): δ 7.18 (dd, J1 = 7.5 Hz, J2 = 1.4
Hz, 2H, ArH), 7.10 (t, J = 7.2 Hz, 2H, ArH), 6.76 (d, J = 8.5 Hz, 2H,
ArH), 6.39 (t, J = 7.3 Hz, 2H, ArH), 4.25 (m, 2H, CH), 3.17 (d, J =
9.9 Hz, 2H, S-CH2), 2.96 (s, 12H, N(CH3)2), 2.71 (d, J = 9.9 Hz, 2H,
S-CH2), 1.47 (m, 12H, CH3). 13C NMR (100.62 MHz, CDCl3, 25
°C): δ 161.84, 135.31, 130.64, 116.57, 114.09, 113.72, 47.85, 43.95,
39.64, 22.03, 21.32. Anal. Calcd for C24H38N4S2Hf: C, 46.11; H, 6.13;
S, 10.26. Found: C, 46.27; H, 6.32; S, 10.32.
basis set employed was LANL2DZ26 with associated effective-core
potentials for the Zr atom and 6-31G(d) for all other atoms.
Geometry optimizations were performed without symmetry con-
straints. Stationary point geometries were characterized as local
minima on the potential energy surfaces. The absence of an imaginary
frequency verified that structures were true minima at their respective
levels of theory. The structures of the transition states were located by
applying Schlegel’s synchronous-transit-guided quasi-Newton
(QST2) method, as implemented in Gaussian 09. The transition
states were verified with frequency calculations to ensure that they
were first-order saddle points with only one negative eigenvalue.
Cartesian coordinates of all DFT-optimized structures are available
upon request.
Single-Crystal X-ray Crystallography. Single crystals of the
complexes suitable for X-ray analysis were obtained by slow
evaporation of a saturated hexane solution at room temperature,
while single crystals of the ligand molecules were obtained from
pentane (PrPr) and diethyl ether (Cy) solutions.
Data collection was performed in flowing N2 at 173 K on a Bruker-
Nonius Kappa CCD diffractometer equipped with an Oxford
Cryostream apparatus (Mo Kα radiation, CCD rotation images,
thick slices, ϕ scans + ω scans to fill the asymmetric unit).
Semiempirical absorption corrections (multiscan SADABS)27 were
applied. The structure was solved by direct methods (SIR 97
package)28 and refined by the full-matrix least-squares method
(SHELXL program of SHELXL-2018/3)29 on F2 against all
independent measured reflections, using anisotropic thermal param-
eters for all non-hydrogen atoms. Hydrogen atoms were placed in
calculated positions with Ueq equal to those of the carrier atom and
refined by the riding method. Only the N−H atoms of the two ligand
molecules (iPr and Cy) were found in difference Fourier maps, and
their coordinates were refined without restraints. Crystal data and
structure refinement details are reported in the Supporting
Information. The figures were generated using the ORTEP-330
program.
Synthesis of [NSSN-Cy]Hf(NMe2)2 (5). Complex 5 was prepared
from [NSSN-Cy] (1.02 g, 2.3 mmol) and Hf(NMe2)4 (0.82 g, 2.3
mmol), as described above for 4. The complex was obtained as a
yellow solid (1.35 g, yield = 83%). 1H NMR (400.13 MHz, CDCl3, 25
°C): δ 7.15 (m, J = 7.4 Hz, 2H, ArH), 7.08 (m, J = 7.7 Hz, 2H, ArH),
6.87 (m, J = 8.3 Hz, 2H, ArH), 6.37 (m, J = 7.5 Hz, 2H, ArH), 3.68
(m, 2H, CH), 3.18 (d, J = 9.4 Hz, 2H, S-CH2), 2.98 (s, 12H,
N(CH3)2), 2.76 (d, J = 9.4 Hz, 2H, S-CH2), 2.22−2.18 (m, 22H, Cy).
13C NMR (100.62 MHz, CDCl3, 25 °C): δ 162.67, 135.40, 130.85,
116.54, 114.14, 113.52, 58.48, 44.41, 40.07, 31.80, 31.15, 27.09, 27.01,
26.38. Anal. Calcd for C30H46N4S2Hf: C, 51.09; H, 6.57; S, 9.09.
Found: C, 51.55; H, 6.83; S, 9.23.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01056.
■
sı
Materials and methods, experimental and computational
details, NMR and high-resolution MALDI MS spectra,
ORTEP views, crystallographic data, bond lengths and
angles, HSQC, DFT-optimized structures, pseudo-first-
order kinetic plots, plots of Mn(expt) versus Mn(th), and
SEC (PDF)
LA Polymerization. In a typical polymerization, in a glovebox, a
Schlenk flask (10 cm3) was charged sequentially with LA (L- or rac-
LA; 0.404 g, 2.80 mmol), and the precatalyst (28.0 μmol) was
dissolved in 2.0 mL of dry toluene. The mixture was thermostated at
the required temperature. At specified time intervals, a small amount
of the polymerization mixture was sampled by a pipet and quenched
in wet CDCl3 to evaluate the yields. This fraction was subjected to
monomer conversion determination, which was monitored by
integration of the monomer versus polymer methine resonances in
Accession Codes
CCDC 2048897−2048902 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
the H NMR spectrum (CDCl3). After the required polymerization
AUTHOR INFORMATION
time, the reaction mixture was quenched with wet n-hexane. The
precipitates collected from the bulk mixture were dried in air,
dissolved with dichloromethane, and sequentially precipitated into
methanol. The obtained polymer was collected by filtration and
further dried in a vacuum oven at 40 °C for 16 h.
ε-Caprolactone, β-Butyrolactone, or γ-Valerolactone Poly-
merization. Polymerization tests for ε-caprolactone, β-butyrolac-
tone, and γ-valerolactone were performed as for LA polymerization.
Computational Details. DFT calculations were performed with
the program suite Gaussian 09.24 All geometries were optimized
without constraints at the BP86 level, i.e., employing the exchange
and correlation functionals of Becke and Perdew,25 respectively. The
■
Corresponding Author
Stefano Milione − Department of Chemistry and Biology,
University of Salerno, Fisciano, Salerno I-84084, Italy;
Interuniversity Consortium Chemical Reactivity and
Catalysis, Bari 70126, Italy; orcid.org/0000-0002-3473-
1480; Email: smilione@unisa.it
Authors
Salvatore Impemba − Department of Chemistry and Biology,
University of Salerno, Fisciano, Salerno I-84084, Italy;
J
https://doi.org/10.1021/acs.inorgchem.1c01056
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
(9) Sergeeva, E.; Kopilov, J.; Goldberg, I.; Kol, M. Dithiodiolate
Ligands: Group 4 Complexes and Application in Lactide Polymer-
ization. Inorg. Chem. 2010, 49, 3977−3979.
Interuniversity Consortium Chemical Reactivity and
Catalysis, Bari 70126, Italy
Giuseppina Roviello − Department of Engineering, University
of Naples Parthenope, Naples 80143, Italy
Carmine Capacchione − Department of Chemistry and
Biology, University of Salerno, Fisciano, Salerno I-84084,
Italy; Interuniversity Consortium Chemical Reactivity and
Catalysis, Bari 70126, Italy; orcid.org/0000-0001-7254-
8620
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01056
Notes
The authors declare no competing financial interest.
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