Magnesium complexes containing biphenyl-based tridentate imino-phenolate ligands for ring-opening polymerization of rac-lactide and α- methyltrimethylene carbonate

Article abstract of DOI:10.1039/c3dt53513d

A series of racemic 2-[(2′-(dimethylamino)biphenyl-2-ylimino)methyl]- 4-R²-6-R¹-phenols (L¹H-L⁴H) were reacted with {Mg[N(SiMe₃)₂]₂}₂ to provide four heteroleptic magnesium complexes (L^1-4)MgN(SiMe ₃)₂·(THF)_n (1, R¹ = ^tBu, R² = Me, n = 1; 2, R¹ = R² = CMe₂Ph, n = 0; 3, R¹ = CPh₃, R² = ^tBu, n = 1; 4, R¹ = Br, R² = ^tBu, n = 0), which have been fully characterized. X-ray structural determination shows that complex 1 possesses a monomeric structure, but complex 4 is dimeric with C₂-symmetry where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. The heteroleptic complexes 1-4 efficiently initiate the ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding magnesium complex. Microstructure analyses of the resulting PLAs revealed that P_r values range from 0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC, detailed analyses using ¹H and ¹³C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen-acyl bond (X_reg = 0.65-0.86).

Full text of DOI:10.1039/c3dt53513d

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Magnesium complexes containing biphenyl-based
tridentate imino-phenolate ligands for ring-
opening polymerization of rac-lactide and
α-methyltrimethylene carbonate†
Cite this: Dalton Trans., 2014, 43,
5200
Wei Yi and Haiyan Ma*
A series of racemic 2-[(2’-(dimethylamino)biphenyl-2-ylimino)methyl]-4-R2-6-R1-phenols (L1H–L4H)
were reacted with {Mg[N(SiMe3)2]2}2 to provide four heteroleptic magnesium complexes (L1–4)MgN-
t
t
(SiMe3)2·(THF)n (1, R1 = Bu, R2 = Me, n = 1; 2, R1 = R2 = CMe2Ph, n = 0; 3, R1 = CPh3, R2 = Bu, n = 1; 4,
R1 = Br, R2 = Bu, n = 0), which have been fully characterized. X-ray structural determination shows that
t
complex 1 possesses a monomeric structure, but complex 4 is dimeric with C2-symmetry where the two
metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry
around the magnesium center in these complexes can be best described as a distorted tetrahedral geo-
metry. The heteroleptic complexes 1–4 efficiently initiate the ring-opening polymerization of rac-lactide
and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the
presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit
results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding
magnesium complex. Microstructure analyses of the resulting PLAs revealed that Pr values range from
0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC,
detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of
α-MeTMC at the most hindered oxygen–acyl bond (Xreg = 0.65–0.86).
Received 14th December 2013,
Accepted 15th January 2014
DOI: 10.1039/c3dt53513d
www.rsc.org/dalton
mechanical and physical properties of the polymeric materials,
the design and synthesis of well-defined metal catalysts to
Introduction
Biodegradable aliphatic polyesters and polycarbonates such as prepare PLA or P(α-MeTMC) of specific architectures have
poly(lactic acid) (PLA) and poly(α-methyltrimethylene carbon- become a major topic.5,6
ate) (P(α-MeTMC)) have received increased interest in recent
In the past two decades, a variety of well-characterized
years, which is largely due to the biodegradability and biocom- metal complexes capable of initiating the ROP of lactides have
patibility of the resulting materials as well as their versatile been reported.7–42 Among these numerous metal-based cata-
chain microstructures derived from the selective ring-opening lysts that have been disclosed for polymerization studies, com-
polymerization (ROP) process of related cyclic monomers in plexes of biocompatible metals such as Groups 1 15–18 and
racemic form.1–4 Since the microstructure of the monomeric 2 26–31 metals as well as zinc19–24 are preferable to be used. In
units in the polymer chain plays a decisive role in determining the complexes, the structure of the ancillary ligand proves to
play an important role in the stereo-chemistry of resulting
polymers. Discrete Mg and Zn complexes with β-diketiminate
Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of
ligands show high activity and heterotactic selectivity for the
ring-opening polymerization of rac-lactide.43–46 Lin and co-
Organometallic Chemistry, East China University of Science and Technology,
130 Meilong Road, Shanghai 200237, People’s Republic of China.
workers47 reported a series of zinc complexes bearing NNO-tri-
dentate iminophenolate ligands which initiate rac-lactide
E-mail: haiyanma@ecust.edu.cn; Fax: +86 21 64253519; Tel: +86 21 64253519
†Electronic supplementary information (ESI) available: A comparison of di-
methylamino proton chemical shifts of complexes and proligands, 1H NMR
trace spectra of the reaction between 2 and 2-propanol, 1H NMR spectrum of
PLA oligomer, homonuclear-decoupled 1H NMR spectra of PLAs, 1H NMR spec-
trum and ESI-TOF mass spectrum of poly(α-MeTMC) oligomer, DSC curves of
poly(α-MeTMC) and X-ray crystallographic data of complexes 1 and 4 in CIF
format. CCDC 973421 and 973422 for 1 and 4. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c3dt53513d
polymerization to afford heterotactic PLA (Pr = 0.59–0.74) at
25 °C. Darensbourg’s group48 introduced zinc silylamido com-
plexes supported by chiral NNO-tridentate iminophenolate
ligands for the polymerization of rac-lactide in dichloro-
methane to afford heterotactic predominant PLA (Pr = 0.68 to
0.89).
5200 | Dalton Trans., 2014, 43, 5200–5210
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Various types of metal-based catalysts49–55 are also known
to be efficient initiators for the ROP of trimethylene carbonate
(TMC). Unfortunately, polymerizations of similar chiral cyclic
Results and discussion
Synthesis and characterization of magnesium complexes
carbonates, for instance, α-methyltrimethylene carbonate Scheme 1 illustrates the synthetic strategies to prepare the
(α-MeTMC), have scarcely been explored. Rare earth alkoxides desired dimethylaminobiphenyl-based iminophenol pro-
(“Ln(OiPr)3”, Ln(OAr)3, Ar = 2,6-di-tert-butyl-4-methylphenolate; ligands. 2,2′-Dinitrobiphenyl was synthesized via an Ullmann
Ln = La, Dy, Y) were revisited recently for the polymerization of coupling reaction of 1-iodo-2-nitrobenzene.59 The selective
this monomer; polycarbonates of high molar mass (Mn up to reduction of one nitro group using an ethanol solution of NaSx
30 000 g mol−1) and relatively narrow molar mass distribution afforded 2-amine-2′-nitrobiphenyl in a moderate yield of
values (1.1 < Mw/Mn < 1.7) were obtained.56 Guillaume and co- 50%.60 2-Dimethylamino-2′-aminobiphenyl was then obtained
workers57 used [(BDIiPr)Zn(N(SiMe3)2)] and Al(OTf)3 to initiate in high yield via an N-methylation of 2-amine-2′-nitrobiphenyl
the ROP of α-MeTMC respectively. The β-diketiminate zinc followed by a reduction with tin as the reducing agent. Con-
catalyst can obtain 94% monomer conversion within 7 min in densation reactions of this amine with different salicylalde-
toluene at 60 °C, but the Al(OTf)3/BnOH system is only active hyde derivatives under reflux in ethanol yielded the target
at a high temperature of 110 °C. Meanwhile the zinc complex dimethylaminobiphenyl-based iminophenol proligands L1–4H.
is highly regioselective (Xreg > 0.98) and prefers to ring-open All the obtained iminophenols are yellow to orange crystalline
1
the more sterically hindered O–C(O) bond. Compared to the solids which have been well-characterized via H and 13C{1H}
extensive studies on rac-lactide polymerization, the catalyst NMR spectroscopy and HRMS.
systems involved in α-MeTMC polymerization are rather
The heteroleptic magnesium complexes 1–4 were prepared
limited.
in moderate yields from the reaction of {Mg[N(SiMe3)2]2}2 with
Very recently, we reported that a series of magnesium silyl- one equivalent of the corresponding proligand in toluene at
amido complexes supported by racemic methoxybiphenyl- room temperature, respectively (Scheme 2). Analytically pure
based iminophenolate ligands exhibit moderate activities and complexes 1 and 3 were successfully obtained via recrystalliza-
heterotactic selectivities for the polymerization of rac-lactide.58 tion with a THF–n-hexane mixture at −38 °C as yellow green
Possibly due to the rigidity of the biphenyl skeleton, the crystalline solids, where the coordination of one THF molecule
methoxy group is dissociated from the metal center with the to the magnesium center was characterized spectroscopically.
addition of
a donating solvent or monomer, which is Complexes 2 and 4 were recrystallized with a toluene–n-hexane
suggested to weaken the chiral induction effect of the biphenyl mixture at −38 °C and isolated as yellow green crystalline
moiety and lead to a heteroselectivity instead of the desired solids. Although there are both axial chirality of the biphenyl
isoselectivity. To further understand the effect of the ligand moiety and a stereogenic metal center in these complexes, no
framework on the selective polymerization of rac-LA and diastereomer could be observed in the 1H NMR spectra,
α-MeTMC, we report herein a series of magnesium complexes suggesting that the axial chirality of biphenyl may have
supported by tridentate iminophenolate ligands based on the induced exclusively a certain configuration around the mag-
racemic dimethylaminobiphenyl framework. The catalytic per- nesium center. As indicated in Fig. 1 and 2, in the solid state,
formance of these silylamido complexes towards the ROP of complexes 1–3 are monomeric, while complex 4 possesses a
rac-lactide and α-MeTMC are studied in detail.
dimeric structure. These structural features are however
Scheme 1 Synthesis of proligand L1H–L4Ha. (a) −5 to 0 °C, HCl/NaNO2/KI. (b) Cu, 60 °C, under argon. (c) NaSx, ethanol, reflux 6 h. (d) 40% HCHO,
20% H2SO4, NaBH4, THF, −5 to 5 °C. (e) Sn/HCl, ethanol, reflux 5 h. (f) ethanol, reflux.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 5200–5210 | 5201

View Article Online
Paper
Dalton Transactions
Fig. 2 ORTEP diagram of complex 4 (thermal ellipsoids drawn at 30%
probability level). Selected bond lengths (Å) and angles (°): Mg1–O1
1.996(3), Mg1–O1A 2.026(3), Mg1–N3 2.004(4), Mg1–N1A 2.159(4),
Mg1⋯Mg1A 3.076(3), Mg1⋯Br1 3.803, O1–Mg1–N3 131.02(17), O1–
Mg1–O1A 80.22(13), O1–Mg1–N1A 109.66(14), N3–Mg1–O1A 126.28(16),
N3–Mg1–N1A 112.39(17).
Scheme 2 Synthesis of magnesium complexes 1–4.
2.22 ppm accounting for protons of the dimethylamino group
indicates that this group is dissociated from the metal center.
1
A similar phenomenon is also observed in the H NMR spec-
trum of 3 in benzene-d6 where the resonance of dimethyl-
amino protons appears at 2.23 ppm, which is close to that of
the free ligand L3H (2.17 ppm). As for complex 2, the relevant
resonance appears at 2.22 ppm, which is also close to the
corresponding signal of the free ligand L2H in benzene-d6, but
broadened reasonably. We suggest that it is most likely due to
the specific shielding effect of the aromatic ring of the cumyl
group and in complex 2 the dimethylamino group is still co-
ordinated to the metal center in solution. To prove this
assumption, 3 equiv. of THF was added to the solution of
1
complex 2 in benzene-d6 and the mixture was checked with H
NMR spectroscopy. A sharp signal assignable to dimethyl-
amino protons could be observed at 2.10 ppm which is slightly
upfield shifted, indicating that the addition of THF leads to
the dissociation of this dimethylamino group. Similar to our
previous work,58 two sets of signals accounting for the stoi-
Fig. 1 ORTEP diagram of complex 1 (thermal ellipsoids drawn at 30%
probability level). Selected bond lengths (Å) and angles (°): Mg1–O1
1.907(4), Mg1–O2 2.036(4), Mg1–N3 1.986(4), Mg1–N1 2.101(4), O1–
Mg1–N3 126.0(2), O1–Mg1–O2 95.29(17), N3–Mg1–O2 109.19(19), O1–
Mg1–N1 90.23(16), N3–Mg1–N1 117.44(17).
1
chiometric structure are displayed in the H NMR spectrum of
complex 4 in benzene-d6, represented by the dimethylamino
resonances at 2.55 and 2.36 ppm (with a ratio of 3 : 1).
similar to the previously reported magnesium complexes Although the X-ray diffraction determination indicates a
ligated by methoxybiphenyl-based iminophenolate ligands.58
dimeric structure of complex 4 in the solid state where the di-
The stoichiometric structures of complexes 1–4 were further methylamino group of the biphenyl moiety is not coordinated
1
confirmed on the basis of H and 13C{1H} NMR spectroscopy to the magnesium center, a significant downfield shift of the
as well as elemental analysis. The 1H NMR spectrum of 1 in dimethylamino resonance attributable to the major structure
benzene-d6 shows one set of resonances of THF protons at is also observed when compared to that of the free ligand. The
3.34 and 1.21 ppm, and one singlet assignable to N(SiMe3)2 relevant resonance of the minor structure however resembles
protons at 0.31 ppm as well as one set of signals for the multi- the one of the free ligand. In the presence of added THF
dentate iminophenolate ligand, consistent with the stoichio- (around 3 equiv.), these two signals coalesce to a singlet at
metric structure illustrated in Scheme 2. The sharp signal at 2.09 ppm (Table S1†). Obviously, the structure of 4 in solution
5202 | Dalton Trans., 2014, 43, 5200–5210
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
is mainly monomeric where the dimethylamino group in the C2-symmetry. Each magnesium center adopts a distorted tetra-
biphenyl fragment is coordinated to the metal center.58
hedral geometry. The dimethylamino groups of both imino-
phenolate ligands are not coordinated with the magnesium
centers. In the dimeric structure magnesium centers are
achiral and the biphenyl moieties of the two ligands possess
opposite configuration. The dihedral angle of biphenyl in 4 is
87.28°, which is significantly larger than that of complex 1.
Being different from our previous work where the
C1-symmetric magnesium silylamido dimer with ortho-bromo
substitution gives two different Mg⋯Br distances of 3.469 Å
and 3.871 Å,58 the distance between Mg1⋯Br1 (or
Mg1A⋯Br1A) in 4 is 3.803 Å, clearly indicating that there is no
clear interaction between the Mg center and the Br atom in
this work.
Molecular structures of magnesium complexes
Complexes 1 and 4 were further characterized by single crystal
X-ray diffraction. Complex 1 was obtained as needle crystals by
slightly cooling a saturated tetrahydrofuran–n-hexane mixture
and complex 4 was recrystallized as tabular crystals with a
toluene–n-hexane mixture. Crystallographic data and results of
the refinements are summarized in Table 1.
As shown in Fig. 1, complex 1 has a monomeric structure in
the solid state in which the magnesium atom is four-
coordinated by two heteroatom donors of the tridentate
ligand, one bis(trimethylsilyl)amido group and one THF mole-
cule adopting a distorted tetrahedral geometry. The molecule
shows C1-symmetry and both enantiomers are found in
the centrosymmetric crystal structure. Without exception, the
Ra-configuration of the biphenyl moiety of the iminophenolate
ligand leads to the R-configuration of the magnesium center,
and vice versa Sa leads to the S-configuration. The bond length
of magnesium to silylamido nitrogen atom (Mg1–N1) in
complex 1 is 1.986(4) Å, which is very similar to that we
reported before.58 The dihedral angle of the biphenyl moiety
being 71.34°, obviously also similar to that we reported pre-
viously, is likely due to the dissociation state of the dimethyl-
amino group from the magnesium center. The ORTEP drawing
of the molecular structure of 4 given in Fig. 2 indicates that in
the solid state, complex 4 possesses a dimeric structure with a
Mg1/O1/Mg1A/O1A planar core bridged by the two phenolato
oxygen atoms of the ligands and the whole molecule has
Ring-opening polymerization of rac-lactide by complexes 1–4
As can be seen from the data compiled in Table 2, all of these
magnesium complexes can effectively initiate the rac-lactide
polymerization in THF at room temperature or in toluene at
70 °C. The polymers produced in either solvent have high
molecular weights and relatively broad molecular weight distri-
butions (Mw/Mn = 1.12–1.66).
To examine the influence of this type of dimethylamino-
biphenyl-based iminophenolate ligand on the catalytic per-
formance of the corresponding magnesium complexes, the
polymerization behaviors of complexes 1–4 with different R1,
R2 groups for rac-lactide polymerization were examined in
detail. It is found that the presence of substituents, particu-
larly the one at the ortho-position of the phenoxide unit, plays
an important role in determining the polymerization activity.
For example, using complex 2 bearing an ortho-cumyl substitu-
ent as the initiator, a monomer conversion of 88% could be
obtained within 15 min at room temperature (Run 5). Complex
3 with a sterically bulky trityl group shows much higher activity
which gives 94% conversion within 2 min under otherwise the
same conditions (Run 9). Complex 1 with less sterically hin-
dered tert-butyl group is found to be less active than complexes
2 and 3. As observed for most of the systems reported,33,61 the
presence of a sterically bulky substituent at the ortho-position
of the anionic atom is beneficial to the catalytic activity, likely
due to the fact that the sterically bulky group might efficiently
prevent the active centre from aggregation.62
Besides, the introduction of an electron-withdrawing Br
group on the ortho-position of the phenoxide unit decreases
the activity of magnesium complex 4, which exhibits the lowest
catalytic activity for rac-lactide polymerization among these
complexes. A similar phenomenon was also observed by Lin’s
group61,63 that magnesium benzyl alkoxide complexes [LMg-
(µ-OBn)]2 (L = NNO-tridentate iminophenolate61 or β-ketimi-
nato ligands63) with an electron-donating group on the ligand
framework displayed high reactivity, while the reactivity
decreased substantially with the substitution of an electron-
withdrawing group. It is also in accord with the results of alu-
minium complexes reported by our group64 and Tolman’s
group.65
Table 1 Crystallographic data for complexes 1 and 4
1
4
Formula
Fw
C
72H110Mg2N6O4Si4 C62H88Br2Mg2N6O2Si4
1284.64
296(2)
0.35 × 0.20 × 0.04
Monoclinic
P2(1)/c
9.638(4)
23.829(10)
17.341(7)
90
101.146(7)
90
3908(3)
2
1.092
0.139
1392
1270.18
296(2)
0.25 × 0.15 × 0.12
Triclinic
ˉ
P1
11.442(7)
13.370(8)
14.072(8)
61.692(11)
87.288(12)
70.508(11)
1770.7(18)
1
Temp. (K)
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Dcalcd (mg m−3
)
1.191
1.272
668
Abs. coeff. (mm−1
F (000)
)
θ range (°)
Data collected (hkl)
1.47 to 25.05
11, 0 to 28, 0 to 20 −14 to 13, 16,
1.66 to 26.00
−17 to 16
12 709/6899
0.0346
0.8623 and 0.7416
6899/222/435
1.039
0.0659, 0.1978
0.1174, 0.2472
0.761 and −0.865
Reflns collected/unique 15 459/6976
Rint 0.0558
Max. and min. transmn 0.7456 and 0.4758
Data/restraints/para
Goodness-of-fit on F2
Final R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
6976/106/446
0.994
0.0813, 0.2012
0.1520, 0.2494
0.298 and −0.374
Δρmax, min/e Å−3
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 5200–5210 | 5203

View Article Online
Paper
Dalton Transactions
Table 2 Ring-opening polymerization of rac-lactide initiated by magnesium silylamido complexes 1–4a
[LA]0/[Mg]0/[iPrOH]0
Solvent
T (°C)
t (min)
Conv.b (%)
Mn,calcd c (×104)
Mn d (×104)
Mw/Mn
Pr
d
e
Run
Initiator
1
2
3
4
5
6
7
8
1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
THF
THF
Tol
25
25
70
70
25
25
70
70
25
−38
25
70
70
25
25
70
70
30
60
180
60
15
30
180
30
2
2d
30
120
20
30
180
480
300
90
96
94
98
88
96
94
98
94
73
98
93
98
88
80
94
95
2.59
2.77
2.71
2.82
2.54
2.77
2.71
2.82
2.71
2.10
2.82
2.68
2.82
2.54
2.30
2.71
2.74
3.49
2.65
3.67
2.73
3.82
2.53
3.70
3.57
13.7
1.56
2.95
3.64
2.82
4.75
2.44
3.44
2.36
1.53
1.27
1.47
1.42
1.63
1.28
1.48
1.39
1.51
1.55
1.31
1.40
1.66
1.66
1.12
1.46
1.48
0.69
0.67
0.49
0.49
0.72
0.70
0.51
0.49
0.77
0.81
0.71
0.48
0.46
0.71
0.69
0.50
0.50
Tol
2
3
THF
THF
Tol
Tol
9
THF
THF
THF
Tol
10
11
12
13
14
15
16
17
Tol
4
THF
THF
Tol
Tol
c
i
a [rac-LA]0 = 1.0 M. b Determined by 1H NMR spectroscopy. Mn,calcd = ([LA]0/[Mg]0) × 144.13 × conv.%; in the presence of PrOH, Mn,calcd
=
([LA]0/[iPrOH]0) × 144.13 × conv.% + 60. d Determined by GPC. e Pr is the probability of forming a new r-diad, determined by homonuclear
decoupled 1H NMR spectroscopy.
As shown in Table 2, the polymerization medium also plays 2-propanol significantly shortens the polymerization time to
an important role in influencing the polymerization, and THF 60 min accompanied by a conversion of 98% (Runs 3, 4). It is
is a better solvent than toluene. When complex 1 is used as an thus suggested that although an alkoxide group is superior to
initiator, a monomer conversion up to 90% is achieved within an amide group in initiation, a coordinative solvent may bring
30 min in THF at room temperature (Run 1), whereas the reac- a complicated effect during the polymerization by either facili-
tion takes 180 min in toluene at 70 °C (Run 3). A similar tating the dissociation of the in situ formed dimeric metal alk-
phenomenon was also observed by Lin and co-workers.61
In general, metal silylamido complexes have been reported coordination to the metal center.
oxide species or blocking the coordination site via competitive
to be inferior initiators for the ROP of rac-lactide and afforded
The initiation mechanism was elucidated by end-group ana-
broadly distributed polymers.62,66 Therefore, rac-lactide lysis of a rac-LA oligomer sample, which was prepared by the
polymerization initiated with complexes 1–4 in the presence of reaction of complex 2 with rac-LA in a 1 : 20 molar ratio. The
2-propanol was also investigated. Before conducting systematic existence of both terminal groups could be confirmed accord-
polymerization studies, the NMR tube reaction of complex 2 ing to the resonances at about 1.23, 5.03 ppm (for isopropoxy)
with 2-propanol was monitored. The 1 : 1 ratio reaction gene- and 1.48, 4.34 ppm (for HOCH(CH3)CO−) via 1H NMR spec-
rated the expected iminophenolato magnesium alkoxide troscopy (Fig. S2†). Meanwhile, no proton resonance of the
1
“[L2MgOiPr]” along with the release of free amine HN(SiMe3)2 ligand is observed in the H NMR spectrum of the oligomer,
(δ = 0.091 ppm, Fig. S1†), indicating adequate tolerance of the which reveals that the biphenyl-based tridentate iminopheno-
bonding between the tridentate ligand and the magnesium late group is not involved in the polymerization process. Thus,
center toward protonic sources. As shown in Table 2, the poly- the polymerization proceeds via a common “coordination–
merizations of rac-lactide initiated by complexes 1–4/2-propa- insertion” mechanism initiated by the in situ generated mag-
nol are well-controlled, giving polymers with relatively nesium isopropoxide “LMg(OiPr)”.
narrower molecular weight distributions. The addition of
Microstructure analyses of PLAs were achieved through
1
2-propanol has different influence on the polymerization con- inspecting the methine region of homonuclear decoupled H
ducted in THF or in toluene which is different from most pre- NMR spectra of the resulting polymers. This series of silyla-
vious results, but consistent with the regularity we reported mido magnesium complexes give moderate heterotactic
previously.58 In THF, the polymerizations of rac-lactide selectivity (Pr = 0.67–0.77) in THF, whereas mostly atactic PLAs
initiated by complexes 1–4 in the presence of 2-propanol are (Pr = 0.50–0.46) are obtained in toluene. A similar trend of
unusually slow when compared to those without the addition solvent effect on the selectivity was often reported in the litera-
of 2-propanol, while the order in toluene is still consistent ture. For instance, Chisholm and co-workers67 recently
with the literature reports.20,21 Using complex 1 as the reported magnesium complexes L′MgnBu(THF) supported by
initiator, the polymerization can reach 90% conversion within β-diimine ligands which showed high heteroselectivity (Pr
=
30 min in THF (Run 1), whereas the yield is only 96% after 0.96) in THF. When the solvent was changed to toluene/
60 min (Run 2) when initiated by complex 1/2-propanol under dichloromethane, the selectivity decreased considerably to
otherwise the same conditions. In toluene, a monomer conver- 0.87. Carpentier and co-workers68 also reported dianionic
sion of 94% can be achieved by 1 in 180 min; the addition of alkoxy-amino-bisphenolate yttrium complexes as initiators in
5204 | Dalton Trans., 2014, 43, 5200–5210
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Table 3 Ring-opening polymerization of α-MeTMC initiated by magnesium silylamido complexes 1–4a
d
e
Run
Initiator
[α-MeTMC]0/[Mg]0/[iPrOH]0
t (min)
Conv.b (%)
Mn,calcd c (×104)
Mn d (×104)
Mw/Mn
Xreg
Tg f (°C)
4.32
1
2
3
4
5
6
7
8
1
2
3
4
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
200 : 1 : 0
200 : 1 : 1
180
120
120
60
60
30
90
95
90
90
92
93
88
89
2.09
2.20
2.09
2.09
2.13
2.16
2.04
2.06
4.56
2.34
6.09
3.07
10.06
3.07
7.26
3.73
1.42
1.37
1.61
1.44
1.48
1.44
1.50
1.61
0.74
0.85
0.86
0.65
4.01
−7.58
1.43
180
120
c
a [α-MeTMC]0 = 1.0 M; in toluene, at 70 °C. b Determined by 1H NMR spectroscopy. Mn,calcd = ([α-MeTMC]0/[Mg]0) × 116.05 × conv.%; in the
i
e
presence of PrOH, Mn,calcd = ([α-MeTMC]0/[iPrOH]0) × 116.05 × conv.% + 60. d Determined by GPC. Xreg is the percentage of head-to-tail/tail-to-
head linkages (A and D) in the polymer chain, determined by 13C NMR spectroscopy. f Determined by DSC.
the ROP of rac-lactide which gave high heteroselectivity in THF lactide polymerization in toluene, complex 4 with an ortho-
whereas it showed no selectivity in toluene.
bromo group exhibits a similar activity as complex 1, although
In comparison with the crucial effect of a solvent, the it is still less active (Runs 1, 2 vs. 7, 8).
nature of ligand substituents has a slight impact on the stereo-
To understand the difference between amide and alkoxide
selectivity. Complex 3 with a sterically bulky trityl group at the systems in initiation, α-MeTMC was polymerized with com-
ortho-position of the phenoxide unit displays the highest pre- plexes 1–4 in the presence of 2-propanol. Similar to rac-LA
ference for heterotactic diad enchainment during the polymer- polymerization carried out in toluene, the polymerizations by
ization of rac-lactide (Pr = 0.77, in THF, Fig. S3†), while the 1–4/2-propanol are much faster and better controlled than
other complexes including complex 4 with an ortho-bromo those without 2-propanol. For example, using 1/2-propanol as
substitution exhibit quite similar heterotactic selectivities (Pr = the initiator, the polymerization can reach 95% monomer con-
0.69–0.72, in THF). Furthermore, the Pr values of the resulting version in 120 min and produce poly(α-MeTMC) with an
polymer samples vary slightly upon changing the temperature. average number molecular weight of 2.34 × 104 g mol−1, which
Using complex 3 as the initiator, the Pr value increases to 0.81 is very close to the theoretical value (2.20 × 104 g mol−1, Mw/Mn
as the polymerization temperature decreases to −38 °C (Run = 1.37, Run 2).
10; Fig. S4†).
To gain some insights into the polymerization of α-MeTMC
Generally, this series of magnesium complexes exhibits with complexes 1–4/2-propanol system, an NMR scale polymer-
similar catalytic activity and heteroselectivity for the ROP of ization of α-MeTMC by complex 2 in the presence of 2-propa-
rac-lactide as those we reported previously,58 implying that the nol was carried out. Treatment of complex 2 with one equiv. of
replacement of the methoxy group with a dimethylamino 2-propanol, followed by addition of 20 equiv. of α-MeTMC in
group in the biphenyl moiety has not brought a noticeable C6D6 at 30 °C revealed that, after 30 min, the in situ generated
influence on the coordination mode of the ligand as well as magnesium isopropoxide species initiate the ROP of α-MeTMC
1
their steric and electronic features.
to give oligomers with an OiPr end group. The H NMR spec-
trum of the purified oligomer clearly shows a set of resonances
at δ 1.22 and 4.98 ppm assignable to the –OCH(CH3)2 end
group (Fig. 3). The three signals at δ 4.36, 3.68, 1.28 ppm
(labelled as a′, c′, and d′, respectively) can be assigned
Ring-opening polymerization of α-MeTMC by complexes 1–4
The catalytic behavior of complexes 1–4 for the ROP of racemic
α-MeTMC was examined and the results are summarized in
Table 3. It can be found that all of these magnesium silyl-
amido complexes can initiate effectively α-MeTMC polymeriz-
ation in toluene at 70 °C, giving poly(α-MeTMC) with high
molecular weights and relatively broad molecular weight distri-
butions (Mw/Mn = 1.37–1.61).
The structure of the ancillary ligand also has a significant
influence on the polymerization activity. Upon increasing the
steric bulkiness of the ligand ortho-substituent, the catalytic
activity of the corresponding magnesium complex apparently
increases. Using complex 3 with an ortho-trityl group on the
phenolate ring as the initiator, the monomer conversion can
reach 92% within 60 min when adopting a monomer-to-
initiator molar ratio of 200 (Run 5), whereas it is 90% when
using complex 1 with an ortho-tert-butyl group after 180 min
Fig. 3 1H NMR spectrum of α-MeTMC oligomers initiated by complex
(Run 1). In contrast to the low efficiency displayed in rac- 2/2-propanol ([α-MeTMC]0 : [2]0 : [iPrOH]0 = 20 : 1 : 1, 30 °C, CDCl3).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 5200–5210 | 5205

View Article Online
Paper
Dalton Transactions
Fig. 4 ESI-TOF mass spectrum of α-MeTMC oligomers initiated by
complex 2/2-propanol ([α-MeTMC]0 : [2]0 : [iPrOH]0 = 20 : 1 : 1, 30 °C).
Scheme 3 Possible regioselective enchainments in the ROP of
α-MeTMC.
result from the regioirregular enchainment of the monomer
units, that is, from the alternated cleavage of different O–C(O)
bonds in two sequential α-MeTMC molecules, denoted by
diads B and C. By integrating these resonances in the carbonyl
region of the 13C{1H} NMR spectra of poly(α-MeTMC) samples,
it is clear that complexes 1–3 with a sterically bulky ortho-sub-
stituent on the phenolate ring are more regioselective in the
polymerization (Xreg = 0.74 to 0.86) than complex 4 with an
ortho-Br group, which displays a significantly lower regioselec-
tivity of Xreg = 0.65. Furthermore, poly(α-MeTMC)s resulted by
these complexes exhibit different thermal behaviour. The
polymer sample obtained by complex 3 showed the character-
istic thermal behavior of a Tg at −7.58 °C (Table 3, run 5)
which is slightly higher than those reported in the literature
(−18 to −10 °C).56,57 The Tg values of other poly(α-MeTMC)s
range around 4.32/4.01/1.43 °C (Table 3, runs 1, 3 and 7,
respectively).
Fig. 5 Details of the carbonyl region of the 13C{1H} NMR spectra
(100 MHz, CDCl3, 25 °C) of poly(α-MeTMC): 1 (Table 3, Run 1) (Xreg = ca.
0.74), 2 (Table 3, Run 3) (Xreg = ca. 0.85), 3 (Table 3, Run 5) (Xreg = ca.
0.86), 4 (Table 3, run 7) (Xreg = ca. 0.65).
respectively to the methine, methylene, and methyl groups of
the other chain terminus.57 Based on these features, the pre-
ferential ring-opening of α-MeTMC at the most hindered
oxygen–acyl O–C(O) bond is therefore suggested. The ESI-TOF
mass spectrum of the oligomer depicted in Fig. 4 also
undoubtedly features one major distribution of peaks assigned
to Na+ ion cationized α-MeTMC oligomers terminated with
hydroxyl and isopropoxy groups and with a repeat unit of
116 g mol−1 (i.e., the molar mass of α-MeTMC).
Microstructure analyses of poly(α-MeTMC)s were further
achieved through inspecting the carbonyl region of 13C{1H}
NMR spectra of the resulting polymers (Fig. 5), since the reson-
ances in this region are diagnostics of the diad sequences.69
According to a literature report,57 the biggest resonance at
154.4 ppm in the carbonyl region of the 13C{1H} NMR spec-
trum of poly(α-MeTMC) could be assigned to the indistin-
guishable diads A and D that are formed from the regioregular
cleavage of either of the O–C(O) bonds in α-MeTMC (as
depicted in Scheme 3). The other minor resonances at higher
and lower fields (154.88, 154.01, 153.95 ppm) are representa-
tives of those magnetically inequivalent carbonyl groups that
Conclusions
A series of racemic [ONN]-type iminophenols (L1H–L4H) based
on the N-dimethylaminobiphenyl skeleton and their mag-
nesium silylamido complexes have been synthesized and struc-
turally characterized. X-Ray diffraction studies of typical
complexes reveal that complex 1 is monomeric, but complex 4
with an ortho-bromo group on the phenoxide unit possesses a
dimeric structure in the solid state. These magnesium silyl-
amido complexes are efficient initiators for the ROP of rac-LA
and α-MeTMC. The substituents of the ancillary ligand,
especially the one at the ortho-position of the phenoxide ring
of the ligand, have a profound influence on the catalytic
activity and stereo-/region-selectivity. The most sterically hin-
dered initiator 3 exhibits the highest activity and stereo-
selectivity from rac-LA polymerization (Pr = 0.81, −38 °C);
meanwhile the same complex also shows the highest activity
for ROP of α-MeTMC to afford highly regioregular polymers
(Xreg = 0.86).
5206 | Dalton Trans., 2014, 43, 5200–5210
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.6,
158.4, 151.4, 147.0, 137.2, 136.8, 132.1, 131.8, 131.0, 130.9,
130.0, 128.3, 128.0, 126.5, 126.4, 120.8, 118.8, 118.7, 117.4 (all
Experimental section
General considerations
All manipulations were carried out under a dry argon atmos- Ar–C, N–CH–Ar), 43.1 ((CH3)2N), 34.7 (C(CH3)3), 29.2 (C(CH3)3),
phere using standard Schlenk-line or glove-box techniques. 20.6 (CH3–Ar).
Toluene and n-hexane were refluxed over sodium benzophe-
2-((2′-(Dimethylamino)biphenyl-2-ylimino)methyl)-4,6-dicumyl-
none ketyl prior to use. Benzene-d6, chloroform-d and other phenol (L2H). The procedure was the same as that of L1H,
reagents were carefully dried and stored in the glove-box. except that 2-hydroxy-3,5-dicumylbenzaldehyde (1.792 g,
{Mg[N(SiMe3)2]2}2,70 3-tert-butyl-2-hydroxy-5-methylbenzalde- 5.000 mmol) and amino-2′-(dimethylamino)biphenyl (1.062 g,
hyde,71 2-hydroxy-3,5-dicumylbenzaldehyde,71 5-tert-butyl- 5.000 mmol) were used to afford L2H as yellow crystalline
2-hydroxy-3-tritylbenzaldehyde,72 and 5-tert-butyl-3-bromo- solids (2.073 g, 75%). Found: 552.3143. HRMS Calcd for
2-hydroxybenzaldehyde73 were synthesized according to litera- C39H40N2O: 552.3141; 1H NMR (CDCl3, 298 K, 400 MHz):
ture methods. Racemic-4-methyl-1,3-dioxan-2-one (α-MeTMC) δ 12.91 (s, 1H, OH), 8.29 (s, 1H, N–CH–Ar), 7.37 (dd, 1H, J =
was synthesized following literature procedures.74 rac-Lactide 7.2 Hz, J = 1.7 Hz, ArH), 7.32–7.27 (m, 6H, ArH), 7.19 (m, 3H,
(Aldrich) was recrystallized with dry toluene and then sub- ArH), 7.17–7.08 (m, 6H, ArH), 7.03 (dd, 1H, J = 7.4 Hz, J =
limed twice under vacuum at 80 °C. 2-Propanol was dried over 1.7 Hz, ArH), 6.99 (d, 1H, J = 2.3 Hz, ArH), 6.85 (t, 1H, J = 7.4
calcium hydride prior to distillation. All other chemicals were Hz, ArH), 6.66 (d, 1H, J = 8.0 Hz, ArH), 2.25 (s, 6H, (CH3)2N),
commercially available and used after appropriate purification. 1.69 (s, 6H, C(CH3)2Ph), 1.59 (s, 6H, C(CH3)2Ph). 13C{1H} NMR
Glassware and vials used in the polymerization were dried in (CDCl3, 298 K, 100 MHz): δ 162.2, 157.6, 151.1, 150.8, 150. 6,
an oven at 120 °C overnight and exposed to vacuum–argon 146.7, 139.3, 137.1, 136.3, 131.9, 130.7, 129.2, 128.2, 128.1,
cycle three times.
128.0, 127.8, 127.6, 126.7, 126.5, 125.7, 125.6, 124.8, 120.4,
NMR spectra were recorded on a Bruker AVANCE-400 118.4, 117.3 (all Ar–C, N–CH–Ar), 42.8 ((CH3)2N), 42.4
spectrometer at 25 °C (1H: 400 MHz; 13C: 100 MHz) unless (C(CH3)2Ph), 41.9 (C(CH3)2Ph), 30.89 (C(CH3)2Ph), 30.81
1
otherwise stated. Chemical shifts for H and 13C NMR spectra (C(CH3)2Ph).
were referenced internally using the residual solvent reson-
2-((2′-(Dimethylamino)biphenyl-2-ylimino)methyl)-4-tert-butyl-
ances and reported relative to tetramethylsilane (TMS). 6-tritylphenol (L3H). The procedure was the same as that of
Elemental analyses were performed on an EA-1106 instrument. L1H, except that 5-tert-butyl-2-hydroxy-3-tritylbenzaldehyde
Spectroscopic analyses of polymers were performed in CDCl3. (2.101 g, 5.000 mmol) and amino-2′-(dimethylamino)biphenyl
Gel permeation chromatography (GPC) analyses were carried (1.062 g, 5.000 mmol) were used to afford L3H as yellow crystal-
out on an Agilent instrument (L1200 pump, Optilab Rex injec- line solids (2.580 g, 84%). Found: 614.3304. HRMS Calcd for
tor) in THF at 25 °C at a flow rate of 1 mL min−1. Calibration C44H42N2O: 614.3297; 1H NMR (CDCl3, 400 MHz, 298 K):
standards were commercially available narrowly distributed δ 8.34 (s, 1H, N–CH–Ar), 7.37 (m, 2H, ArH), 7.33 (dd, 1H, J =
linear polystyrene samples that cover a broad range of molar 7.6 Hz, J = 1.6 Hz, ArH), 7.29 (dd, 1H, J = 7.3 Hz, J = 1.4 Hz,
masses (103 < Mn < 2 × 106 g mol−1). Differential scanning ArH), 7.19 (br, 17H, ArH), 7.09 (dd, 1H, J = 7.6 Hz, J = 1.6 Hz,
calorimetric (DSC) curves were taken on a Perkin–Elmer Pyris ArH), 7.02 (dd, 1H, J = 7.3 Hz, J = 1.4 Hz, ArH), 7.09 (t, 1H, J =
1 instrument. All samples were cooled to −50 °C and heated to 7.4 Hz, ArH), 6.64 (d, 1H, J = 8.2 Hz, ArH), 2.19 (s, 6H,
100 °C for the first scan. After being kept for 1 min, they were (CH3)2N), 1.18 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3,
again cooled to −50 °C and heated to 100 °C and then cooled 100 MHz, 298 K): δ 162.6, 157.6, 150.9, 147.1, 145.5, 144.9,
to −50 °C for the second cycle. The heating rate was 10 °C 139.6, 136.8, 134.0, 132.1, 131.8, 131.4, 131.0, 130.9, 130.6,
min−1
.
128.2, 127.8, 127.3, 127.2, 127.0, 126.4, 125.2, 120.5, 118.8,
118.5, 117.5 (all Ar–C, N–CH–Ar), 63.4 (Ar–CPh3), 42.9
((CH3)2N), 34.0 (C(CH3)3), 31.3 (C(CH3)3).
Synthesis of the proligands and complexes
2-((2′-(Dimethylamino)biphenyl-2-ylimino)methyl)-4-methyl-
2-((2′-(Dimethylamino)biphenyl-2-ylimino)methyl)-4-tert-butyl-
6-tert-butylphenol (L1H). 3-tert-Butyl-2-hydroxy-5-methylben- 6-bromophenol (L4H). The procedure was the same as that of
zaldehyde (0.961 g, 5.00 mmol) was mixed with 2-amino-2′- L1H, except that 5-tert-butyl-3-bromo-2-hydroxybenzaldehyde
(dimethylamino)biphenyl (1.062 g, 5.000 mmol) in ethanol (1.286 g, 5.000 mmol) and amino-2′-(dimethylamino)biphenyl
(50 mL). The mixture was refluxed at 80 °C and stirred for 5 h (1.062 g, 5.000 mmol) were used to afford L4H as orange crys-
at this temperature. After being cooled to r.t., the solution was talline solids (2.009 g, 89%). Found: 450.1309. HRMS Calcd for
concentrated to about 20 mL and kept at −20 °C to afford C25H27BrN2O: 450.1307; 1H NMR (CDCl3, 400 MHz, 298 K):
yellow crystalline solids (1.546 g, 80%). Found: 386.2357. δ 13.31 (s, 1H, OH), 8.41 (s, 1H, N–CH–Ar), 7.57 (d, 1H, J =
HRMS Calcd for C26H30N2O: 386.2358; 1H NMR (CDCl3, 2.4 Hz, ArH), 7.45 (dd, 1H, J = 7.3 Hz, J = 1.6 Hz, ArH),
400 MHz, 298 K): δ 13.22 (s, 1H, OH), 8.36 (s, 1H, N–CH–Ar), 7.40–7.34 (m, 2H, ArH), 7.27 (m, 1H, ArH), 7.22–7.13 (m, 3H,
7.43 (d, 1H, J = 7.4 Hz, ArH), 7.37 (t, 1H, J = 7.4 Hz, ArH), 7.31 ArH), 6.99 (m, 2H, ArH), 2.47 (s, 6H, (CH3)2N), 1.29 (s, 9H,
(t, 1H, J = 7.3 Hz, ArH), 7.24 (m, 1H, ArH), 7.16 (t, 2H, J = 7.4 C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 161.8,
Hz, ArH), 7.09 (s, 1H, ArH), 6.95 (m, 2H, ArH), 6.89 (s, 1H, 155.4, 151.3, 146.3, 142.4, 136.6, 133.3, 132.1, 131.4, 131.1,
ArH), 2.48 (s, 6H, (CH3)2N), 2.25 (s, 3H, CH3–Ar), 1.35 (s, 9H, 128.5, 128.0, 127.8, 126.9, 121.0, 119.4, 119.0, 117.6, 110.5
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 5200–5210 | 5207

View Article Online
Paper
Dalton Transactions
(all Ar–C, N–CH–Ar), 43.1 ((CH3)2N), 34.0 (C(CH3 )3), 31.3 (s, 6H, (CH3)2N), 1.17 (s, 9H, C(CH3)3), 1.15 (m, 4H THF), 0.09
(C(CH3)3).
(s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K):
[(L1)MgN(SiMe3)2·THF] (1). The proligand L1H (0.386 g, δ 174.4, 168.1, 151.6, 149.7, 147.1, 138.6, 135.9, 135.0, 134.4,
1.00 mmol) was added slowly to a solution of {Mg[N- 133.6, 132.9, 132.3, 131.9, 131.7, 128.8, 128.7, 127.4, 126.4,
(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL). The 126.0, 125.5, 122.4, 120.1, 118.4 (all Ar–C, N–CH–Ar), 68.4
solution was stirred for 24 h at r.t. All the volatiles were ((CH2CH2)2O), 64.1 (Ar–CPh3), 44.2 (CH3)2N), 33.9 (C(CH3)3),
removed under vacuum. The resulting yellow solids were 31.5 (C(CH3)3), 24.9 ((CH2CH2)2O), 6.1 (N(Si(CH3)3)2).
recrystallized with a mixture of THF and n-hexane at −38 °C to
[(L4)MgN(SiMe3)2]2 (4). Following a procedure similar to
afford yellow crystals (321 mg, 50%). Found: C, 66.89; H, 8.46; that described for 1, L4H (0.451 g, 1.00 mmol) was treated with
N, 6.47. Anal. Calcd for C36H54MgN3O2Si2: C, 67.42; H, 8.49; N, {Mg[N(SiMe3)2]2}2 (0.345 g, 1.00 mmol) in toluene (20 mL) at
1
6.55%; H NMR (C6D6, 400 MHz, 298 K): δ 8.10 (s, 1H, N–CH– r.t. to give yellow solids. Recrystallization with a mixture of
Ar), 7.30 (d, 1H, J = 2.4 Hz ArH), 7.18 (m, 2H, ArH), 7.12 (m, toluene and hexane at −38 °C afforded yellow crystals (324 mg,
2H, ArH), 7.05 (td, 1H, J = 7.2 Hz, J = 1.4 Hz, ArH), 6.93 (td, 1H, 51%). Found: C, 58.19; H, 6.92; N, 6.27. Anal. Calcd for
1
J = 8.2 Hz, J = 1.4 Hz, ArH), 6.78 (t, 1H, J = 7.2 Hz, ArH), 6.61 C62H88Br2Mg2N6O2Si4: C, 58.63; H, 6.98; N, 6.62%; H NMR of
(m, 2H, ArH), 3.34 (m, 4H, THF), 2.22 (s, 6H, (CH3)2N), 2.16 (s, the major isomer (C6D6, 400 MHz, 298 K): δ 7.76 (d, 1H, J =
3H, CH3–Ar), 1.60 (s, 9H, C(CH3)3), 1.20 (m, 4H, THF), 0.31 (s, 2.6 Hz, N–CH–Ar), 7.45 (s, 1H, ArH), 7.08 (m, 1H, ArH), 7.01
18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ (m, 2H, ArH), 6.70 (m, 4H, ArH), 6.59 (d, 1H, J = 2.6 Hz, ArH),
175.0, 168.6, 151.8, 150.4, 141.3, 135.4, 134.6, 134.3, 133.6, 6.54 (d, 1H, J = 7.9 Hz, ArH), 2.55 (s, 6H, (CH3)2N), 0.98 (s, 9H,
133.0, 132.0, 128.84, 128.80, 126.6, 125.5, 122.2, 121.2, 120.0, Ar–C(CH3)3), 0.41 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR of the
118.3 (all Ar–C, N–CH–Ar), 68.8 (THF), 44.0 (CH3)2N), 35.4 major isomer (C6D6, 100 MHz, 298 K): δ 172.3, 164.4, 148.6,
(C(CH3)3), 30.1 (C(CH3)3), 25.2 (THF), 20.8 (CH3–Ar), 6.2 (N(Si- 147.5, 137.24, 137.21, 135.8, 134.4, 133.1, 130.9, 130.4, 129.4,
(CH3)3)2).
127.5, 127.2, 125.5, 123.7, 119.4, 118.7, 118.5 (all Ar–C, N–CH–
procedure Ar), 44.0 (CH3)2N), 33.9 (C(CH3)3), 31.1 (C(CH3)3), 6.9 (N(Si-
[(L2)MgN(SiMe3)2·toluene] (2). Following
a
similar to that described for 1, L2H (0.553 g, 1.00 mmol) was (CH3)3)2).
treated with {Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in
toluene (20 mL) at r.t. to give yellow solids. Recrystallization
with a mixture of toluene and hexane at −38 °C afforded
yellow crystalline solids (390 mg, 53%). Found: C, 75.21; H,
7.91; N, 5.06. Anal. Calcd for C45H57MgN3OSi2·C7H8: C, 75.38;
Typical procedure for the polymerization reaction
In a Braun Labstar glove-box, an initiator solution from a stock
solution in THF or toluene was injected sequentially to a series
of 10 mL vials loaded with rac-lactide or α-MeTMC and suit-
able amounts of dry solvent. After specified time intervals,
each vial was taken out of the glove-box; an aliquot was with-
drawn and quenched quickly with wet light petroleum ether,
and the reaction mixture was quenched at the same time by
adding an excess amount of light petroleum ether and one
drop of water. All the volatiles in the aliquots were removed
and the residue was subjected to monomer conversion deter-
mination which was monitored by integration of monomer vs.
polymer methine or methyl resonances in 1H NMR (CDCl3,
400 MHz, 298 K). The precipitates collected from the bulk
mixture were dried in air, dissolved with dichloromethane and
sequentially precipitated into methanol. The obtained polymer
was further dried in a vacuum oven at 50 °C for 16 h. Each
reaction was used as one data point. In the cases where 2-pro-
panol was used, the solution of the initiator was injected into
the solution of the monomer in toluene to which 2-propanol
was added. Otherwise the procedures were the same.
1
H, 7.91;N, 5.07%; H NMR (C6D6, 400 MHz, 298 K): δ 7.57 (s,
1H, N–CH–Ar), 7.49 (d, 1H, J = 2.7 Hz, ArH), 7.25 (d, 2H, J =
7.2 Hz, ArH), 7.19 (m, 5H, ArH), 7.12–7.04 (m, 7H, ArH), 7.00
(m, 4H, ArH), 6.84 (m, 2H, ArH), 6.75 (d, 1H, J = 2.7 Hz, ArH),
6.60 (m, 2H, ArH), 6.48 (d, 1H, J = 8.2 Hz, ArH), 2.22 (s, 6H,
(CH3)2N), 2.10 (s, 3H, CH3 of toluene), 1.98 (s, 3H, C(CH3)2Ph),
1.54 (s, 6H, C(CH3)2Ph), 1.42 (s, 3H, C(CH3)2Ph), 0.26 (s, 18H,
N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 172.5,
167.9, 152.5, 151.1, 149.1, 147.5, 141.0, 137.9, 136.1, 135.1,
134.3, 133.5, 13217, 131.9, 130.1, 129.5, 129.3, 128.5, 127.3,
127.1, 126.2, 125.8, 125.7, 125.4, 124.3, 123.9, 119.6, 119.3
(all Ar–C, N–CH–Ar), 46.2 (br, CH3)2N), 42.9 (C(CH3)2Ph), 42.4
(C(CH3)2Ph), 34.4 (C(CH3)2Ph), 31.1 (C(CH3)2Ph), 30.9
(C(CH3)2Ph), 26.5 (C(CH3)2Ph), 21.4 (CH3–toluene), 7.4 (N(Si-
(CH3)3)2).
[(L3)MgN(SiMe3)2·THF] (3). Following a procedure similar to
that described for 1, L3H (0.615 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
r.t. to give yellow solids. Recrystallization with a mixture of
THF and hexane at −38 °C afforded yellow crystalline solids
Oligomer preparation
(444 mg, 51%). Found: C, 74.50; H, 7.84; N, 4.92. Anal. Calcd Oligomerizations of rac-LA and α-MeTMC were carried out
for C54H67MgN3O2Si2: C, 74.50; H, 7.76; N, 4.83%; 1H NMR with complex 2/2-propanol as the initiator in toluene, respect-
(C6D6, 400 MHz, 298 K): δ 8.19 (s, 1H, N–CH–Ar), 7.63 (d, 1H, ively, at 30 °C under the condition of a molar ratio of
J = 2.6 Hz, ArH), 7.43 (m, 6H, ArH), 7.36 (d, 1H, J = 7.8 Hz, [Monomer]0 : [2]0 : [iPrOH]0 = 20 : 1 : 1. The reaction was stirred
ArH), 7.21 (t, 2H, J = 7.6 Hz, ArH), 7.09 (br, 9H, ArH), 6.98 (m, for 0.5 h and then quenched by adding wet hexane. The preci-
3H, ArH), 6.93 (d, 1H, J = 2.6 Hz, ArH), 6.80 (t, 1H, J = 7.3 Hz, pitated oligomers were collected, dried under vacuum, and
ArH), 6.70 (d, 1H, J = 8.1 Hz, ArH), 3.12 (m, 4H, THF), 2.23 used for 1H NMR measurement or ESI-TOF.
5208 | Dalton Trans., 2014, 43, 5200–5210
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
X-ray crystallographic study
12 M. Normand, V. Dorcet, E. Kirillov and J.-F. Carpentier,
Organometallics, 2013, 32, 1694–1709.
Suitable crystals of complexes 1 and 4 for X-ray analysis were
obtained from a saturated solution of a tetrahydrofuran–
pentane mixture or a toluene–pentane mixture, respectively, at
−38 °C. Diffraction data were collected on a Bruker SMART
APEX II diffractometer for complexes 1 and 4 with graphite-
monochromated Mo Kα (λ = 0.71073 Å) radiation. All data were
collected at 20 °C using the ϕ- and ω-scan techniques. All
structures were solved by direct methods and refined using
Fourier techniques. An absorption correction based on
SADABS was applied.75 All non-hydrogen atoms were refined by
full-matrix least squares on F2 using the SHELXTL76 program
package. Hydrogen atoms were located and refined by the geo-
metry method. The cell refinement, data collection, and
reduction were done using Bruker SAINT.77 The structure solu-
tion and refinement were performed using SHELXS-9778 and
SHELXL-2013 respectively. Molecular structures were generated
using ORTEP program.79
13 A. Pietrangelo, S. C. Knight, A. K. Gupta, L. J. Yao,
M. C. Hillmyer and W. B. Tolman, J. Am. Chem. Soc., 2010,
132, 11649–11657.
14 D. C. Aluthge, B. O. Patrick and P. Mehrkhodavandi, Chem.
Commun., 2012, 31, 3499–3511.
15 L. B. Calvo, M. G. Davidson and D. García-Vivó, Inorg.
Chem., 2011, 50, 3589–3595.
16 W.-Y. Lu, M.-W. Hsiao, S. C. N. Hsu, W.-T. Peng,
Y.-J. Chang, Y.-C. Tsou, T.-Y. Wu, Y.-C. Lai, Y. Chen and
H.-Y. Chen, Dalton Trans., 2012, 41, 3659–3667.
17 S. C. Roşca, D. A. Roşca, V. Dorcet, C. M. Kozak, F. Kerton,
J.-F. Carpentier and Y. Sarazin, Dalton Trans., 2013, 42,
9361–9375.
18 R. K. Dean, A. M. Reckling, H. Chen, L. N. Dawe,
C. M. Schneider and C. M. Kozak, Dalton Trans., 2013, 42,
3504–3520.
19 Y. Sarazin, B. Liu, T. Roisnel, L. Maron and J.-F. Carpentier,
J. Am. Chem. Soc., 2011, 133, 9069–9087.
20 L. Wang and H. Ma, Dalton Trans., 2010, 39, 7897–7910.
21 S. Song, X. Zhang, H. Ma and Y. Yang, Dalton Trans., 2012,
41, 3266–3277.
22 I. D’Auria, M. Lamberti, M. Mazzeo, S. Milione, G. Roviello
and C. Pellecchia, Chem.–Eur. J., 2012, 18, 2349–2360.
23 H. Wang and H. Ma, Chem. Commun., 2013, 49, 8686–
8688.
24 S. Song, H. Ma and Y. Yang, Dalton Trans., 2013, 42, 14200–
14211.
25 A. Garcés, L. F. Sánchez-Barba, C. Alonso-Moreno,
M. Fajardo, J. Fernández-Baeza, A. Otero, A. Lara-Sánchez,
I. López-Solera and A. M. Rodríguez, Inorg. Chem., 2010, 49,
2859–2871.
Acknowledgements
This work was subsidized by the National Natural Science
Foundation of China (NNSFC, 20604009, 21074032), the
Program for New Century Excellent Talents in University (for
H. Ma, NCET-06-0413) and the Fundamental Research Funds
for the Central Universities (WD1113011, WK1214048). All the
financial supports are gratefully acknowledged. H. Ma also
acknowledges the very kind donation of a Braun glove-box by
AvH Foundation.
Notes and references
26 A. Grala, J. Ejfler, L. B. Jerzykiewicz and P. Sobota, Dalton
Trans., 2011, 40, 4042–4044.
1 A. J. Ragauskas, C. K. Williams, B. H. Davison and
T. Tschaplinski, Science, 2006, 31, 484–489.
27 L. Wang and H. Ma, Macromolecules, 2010, 43, 6535–6537.
2 C. K. Williams and M. A. Hillmyer, Polym. Rev., 2008, 48, 28 H.-Y. Chen, L. Mialon, K. A. Abboud and S. A. Miller,
1–10. Organometallics, 2012, 31, 5252–5261.
3 S. Inkinen, M. Hakkarainen, A. C. Albertsson and 29 Y. Wang, W. Zhao, D. Liu, S. Li, X. Liu, D. Cui and X. Chen,
A. Sodergard, Biomacromolecules, 2011, 12, 523–532. Organometallics, 2012, 31, 4182–4190.
4 O. Dechy, B. Martin and D. Bourissou, Chem. Rev., 2004, 30 J. P. Davin, J.-C. Buffet, T. P. Spaniol and J. Okuda, Dalton
104, 6147–6176.
Trans., 2012, 41, 12612–12618.
5 C. M. Thomas, Chem. Soc. Rev., 2010, 39, 165–173.
6 M. J. Stanford and A. P. Dove, Chem. Soc. Rev., 2010, 39,
486–494.
7 F. Qian, K. Liu and H. Ma, Dalton Trans., 2010, 39, 8071–
8083.
8 A. D. Schwarz, Z. Chu and P. Mountford, Organometallics,
2010, 29, 1246–1260.
9 C. Bakewell, R. H. Platel, S. K. Cary, S. M. Hubbard,
31 B. Liu, T. Roisnel, J.-P. Guégan, J.-F. Carpentier and
Y. Sarazin, Chem.–Eur. J., 2012, 18, 6289–6301.
32 E. L. Whitelaw, M. D. Davidson and M. D. Jones, Chem.
Commun., 2011, 47, 10004–10006.
33 A. Sauer, J.-C. Buffet, T. P. Spaniol, H. Nagae, K. Mashima
and J. Okuda, Inorg. Chem., 2012, 51, 5764–5770.
34 A. Stopper, J. Okuda and M. Kol, Macromolecules, 2012, 45,
698–704.
J. M. Roaf, A. C. Levine, A. J. P. White, N. J. Long, M. Haaf 35 C.-Y. Li, C.-J. Yu and B.-T. Ko, Organometallics, 2013, 32,
and C. K. Williams, Organometallics, 2012, 31, 4729–4736. 172–180.
10 M. Lamberti, I. D’Auria, M. Mazzeo, S. Milione, V. Bertolasi 36 H. Ma, T. P. Spaniol and J. Okuda, Dalton Trans., 2003,
and D. Pappalardo, Organometallics, 2012, 31, 5551–5560. 4770–4780.
11 G. Li, M. Lamberti, M. Mazzeo, D. Pappalardo, G. Roviello 37 S. Yang, D. Zhu, Y. Zhang and Q. Shen, Chem. Commun.,
and C. Pellecchia, Organometallics, 2012, 31, 1180–1188.
2012, 48, 9780–9782.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 5200–5210 | 5209

View Article Online
Paper
Dalton Transactions
38 C. Bakewell, T.-P.-A. Cao, N. Long, X. F. L. Goff, A. Auffrant 57 P. Brignou, J.-F. Carpentier and S. M. Guillaume, Macromol-
and C. K. Williams, J. Am. Chem. Soc., 2012, 134, 20577–
ecules, 2011, 44, 5127–5135.
20580.
58 W. Yi and H. Ma, Inorg. Chem., 2013, 52, 11821–11835.
39 J. Kratsch, M. Kuzdrowska, M. Schmid, N. Kazeminejad, 59 W. L. Mosby, J. Org. Chem., 1957, 22, 671–673.
C. Kaub, P. Oña-Burgos, S. M. Guillaume and P. W. Roesky, 60 D. Purdie, J. Am. Chem. Soc., 1941, 63, 2276.
Organometallics, 2013, 32, 1230–1238.
40 K. Nie, W. Gu, Y. Yao, Y. Zhang and Q. Shen, Organometal-
lics, 2013, 32, 2608–2617.
61 H.-Y. Tang, H.-Y. Chen, J.-H. Huang and C.-C. Lin, Macro-
molecules, 2007, 40, 8855–8860.
62 H. Ma and J. Okuda, Macromolecules, 2005, 38, 2665–2673.
41 B. Calvo, M. G. Davidson and D. García-Vivó, Inorg. Chem., 63 W.-C. Hung and C.-C. Lin, Inorg. Chem., 2009, 48, 728–734.
2011, 50, 3589–3595. 64 Y. Wang and H. Ma, Chem. Commun., 2012, 48, 6729–6731.
42 W.-Y. Lu, M.-W. Hsiao, S. C. N. Hsu, W.-T. Peng, 65 L. M. Alcazar-Roman, B. J. O’Keefe, M. A. Hillmyer and
Y.-J. Chang, Y.-C. Tsou, T.-Y. Wu, Y.-C. Lai, Y. Chen and
H.-Y. Chen, Dalton Trans., 2012, 41, 3659–3667.
43 B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt,
W. B. Tolman, Dalton Trans., 2003, 3082–3087.
66 H. Ma, T. P. Spaniol and J. Okuda, Angew. Chem., Int. Ed.,
2006, 45, 7818–7821.
E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2001, 67 M. H. Chisholm, K. Choojun, J. C. Gallucci and
123, 3229–3238. P. M. Wambua, Chem. Sci., 2012, 3, 3445–3457.
44 M. Chisholm, J. Huffman and K. Phomphrai, J. Chem. Soc., 68 A. Amgoune, C. M. Thomas, T. Roisnel and J.-F. Carpentier,
Dalton Trans., 2001, 222–224. Chem.–Eur. J., 2006, 12, 169–179.
45 M. Chisholm and K. Phomphrai, Inorg. Chim. Acta, 2003, 69 P. Brignou, G. M. Priebe, O. Casagrande, J.-F. Carpentier
350, 121–125. and S. M. Guillaume, Macromolecules, 2010, 43, 8007–8017.
46 A. Dove, V. Gibson, E. Marshall, A. White and D. Williams, 70 M. Westerhausen, Inorg. Chem., 1991, 30, 96–101.
Dalton Trans., 2004, 570–578.
47 H.-Y. Chen, H.-Y. Tang and C.-C. Lin, Macromolecules, 2006,
39, 3745–3752.
48 D. J. Darensbourg and O. Karroonnirun, Inorg. Chem.,
2010, 49, 2360–2371.
71 G. Casiraghi, G. Casnati, G. Puglia, G. Sartori and
G. Terenghi, J. Chem. Soc., Perkin Trans. 1, 1980, 1862–1865.
72 A. I. Kochnev, I. I. Oleynik, I. V. Oleynik, S. S. Ivanchev and
G. A. Tolstikov, Russ. Chem. Bull., Int. Ed., 2007, 56, 1125–
1129.
49 K. Makiguchi, Y. Ogasawara, S. Kikuchi, T. Satoh and 73 F. Lam, J. Xu and K. Chan, J. Org. Chem., 1996, 61, 8414–
T. Kakuchi, Macromolecules, 2013, 46, 1772–1782. 8418.
50 B. Liu, T. Roisnel, L. Maron, J.-F. Carpentier and Y. Sarazin, 74 H. Yasuda, M.-S. Aludin, N. Kitamura, M. Tanabe and
Chem.–Eur. J., 2013, 19, 3986–3994. H. Sirahama, Macromolecules, 1999, 32, 6047–6057.
51 F. Hild, N. Neehaul, F. Bier, M. Wirsum, C. Gourlaouen 75 SADABS, Bruker Nonius area detector scaling and absorption
and S. Dagorne, Organometallics, 2013, 32, 587–598.
52 H. Sheng, J. Shi, Y. Feng, H. Wang, Y. Jiao, H. Sheng,
Y. Zhang and Q. Shen, Dalton Trans., 2012, 41, 9232–9240.
53 P. Brignou, S. M. Guillaume, T. Roisnel, D. Bourissou and
J.-F. Carpentier, Chem.–Eur. J., 2012, 18, 9360–9370.
correction, version 2.05, Bruker AXS Inc., Madison, WI,
1996.
76 G. M. Sheldrick, SHELXTL 5.10 for windows NT, Structure
Determination Software Programs, Bruker AXS Inc.,
Madison, WI, 1997.
54 V. Poirier, T. Roisnel, S. Sinbandhit, M. Bochmann, 77 SAINT, version 6.02, Bruker AXS Inc., Madison, WI, 1999,
J.-F. Carpentier and Y. Sarazin, Chem.–Eur. J., 2012, 18,
2998–3013.
55 D. J. Darensbourg, O. Karroonnirun and S. J. Wilson, Inorg.
Chem., 2011, 50(14), 6775–6787.
pp. 53711–55373.
78 G. M. Sheldrick, SHELXS-97, Program for the Solution of
Crystal Structures, University of Göttingen, Göttingen,
Germany, 1990.
56 J. Ling, Y. Dai, Y. Zhu, W. Sun and Z. Shen, J. Polym. Sci., 79 L. J. Farrugia, ORTEP-III for Windows, version 2.0, J. Appl.
Part A: Polym. Chem., 2010, 48, 3807–3815. Crystallogr., 1997, 30, 565.
5210 | Dalton Trans., 2014, 43, 5200–5210
This journal is © The Royal Society of Chemistry 2014