Group 4 salalen complexes and their application for the ring-opening polymerization of rac -lactide

Article abstract of DOI:10.1021/ic1010488

In this paper, we report the preparation and characterization of a series of unsymmetrical group 4 metal complexes based on ONNO salalen-type ligands. In these examples, the ligand is unsymmetrical with an amine and an imine nitrogen center. All complexes {except Hf(3)(OⁱPr)₂} adopt a β-cis configuration in the solid state, which is also in agreement with solution-state NMR spectroscopic measurements. The complexes have been tested for the ROP of rac-lactide (both in solution and under melt conditions). Narrow polydispersities have been seen (1.07-1.82). The Ti(IV) complexes produce atactic PLA, whereas the Zr(IV) complexes form PLA with a heterotactic bias. Hf(IV) complexes have been shown to produce isotactic PLA with a P_m of 0.75. The kinetics for all initiators have been investigated and are discussed.

Full text of DOI:10.1021/ic1010488

7176 Inorg. Chem. 2010, 49, 7176–7181
DOI: 10.1021/ic1010488
Group 4 Salalen Complexes and Their Application for the Ring-Opening
Polymerization of rac-Lactide
Emma L. Whitelaw, Matthew D. Jones,* and Mary F. Mahon
Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY United Kingdom
Received May 24, 2010
In this paper, we report the preparation and characterization of a series of unsymmetrical group 4 metal complexes
based on ONNO salalen-type ligands. In these examples, the ligand is unsymmetrical with an amine and an imine
nitrogen center. All complexes {except Hf(3)(OiPr)2} adopt a β-cis configuration in the solid state, which is also in
agreement with solution-state NMR spectroscopic measurements. The complexes have been tested for the ROP of
rac-lactide (both in solution and under melt conditions). Narrow polydispersities have been seen (1.07-1.82). The
Ti(IV) complexes produce atactic PLA, whereas the Zr(IV) complexes form PLA with a heterotactic bias. Hf(IV)
complexes have been shown to produce isotactic PLA with a Pm of 0.75. The kinetics for all initiators have been
investigated and are discussed.
Introduction
(although organocatalyzed processes are known)7-9 such as
group 3 elements,10-14 group 4 elements,15-20 Zn(II),21-25
Due to the ever increasing demands on the world’s natural
resources, now is the time to develop new products from
sustainable resources. One such example of this is the produc-
tion of polylactide (PLA) via the ring-opening polymerization
(ROP) of the cyclic ester monomer lactide (LA). The mono-
mer can be prepared from lactic acid which is derived from
corn starch. The polymer itself is utilized in many applications
from drug delivery systems, surgical sutures, food packaging,
to clothing products.1,2 These polymers have also been
extensively utilized as scaffolds for tissue engineering
applications.3 Therefore, the development of novel and con-
trollable initiators is of vital importance, as polymers
with different properties are required for each application.
Initiators are often based on Lewis acidic metal centers4-6
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*To whom correspondence should be addressed. E-mail: mj205@
bath.ac.uk.
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pubs.acs.org/IC
Published on Web 07/01/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 7177
lanthanides,26,27 Al(III),28-34 Ca(II),35 Mg(II),21,22 and Ba(II).36
Most pertinent to this study are those produced with group 4
metals.15-20,37-45 It has been observed that it is possible to
produce highly heterotactic PLA from Zr(IV) and Hf(IV) C3
symmetric amine tris(phenolate) complexes under the indust-
rially preferred melt conditions.16 However, we have recently
shown that subtle changes to the ligand mean that atactic PLA
is produced.46 Recently, Kol and co-workers observed that
amine bis(phenolate) Ti(IV) and Zr(IV) complexes are ex-
tremely active for the polymerization of rac-lactide and
L-lactide.47 In the case of rac-lactide, Kol’s catalysts produced
heterotactic PLA.47 One of us has demonstrated that Zr(IV)
and Hf(IV) amine bis(phenolate) complexes can also afford
isotactic PLA under melt conditions.17
In this paper, we report the preparation and characteriza-
tion of a series of unsymmetrical amine/imine bis(phenolate)
(salalen) complexes and their exploitation for the ROP ofrac-
lactide. To the best of our knowledge, this is the first example
of the use of salalen ligands for the ROP of rac-lactide.
Examples of X-ray structures of Hf(IV)-salalen complexes
are also presented. To date, the vast majority of ONNO-type
ligands in the literature are based on symmetrical amine
bis(phenolate), salan, or salen-type systems.4,6,28,34,48,49
Figure 1. Solid-state structure of Zr(2)(OiPr)2. The methyl groups of the
tBu moieties have been removed for clarity, as have all hydrogen atoms.
The ellipsoids are shown at the 30% probability level.
Scheme 1. Ligands and Complexes Prepared in This Study
Results and Discussion
Synthesis and Characterization of Ligands and Complexes.
The ligands prepared in this study are shown in Scheme 1,
and adapted literature procedures were followed for their
preparation.15,46 The complexes were prepared from a 1:1
reaction of the appropriate ligand with the group 4 alkoxide,
as shown in Scheme 1.46 All complexes were characterized
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M. F.; Kociok-Kohn, G. Inorg. Chem. 2007, 46, 7686.
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Bochmann, M. Dalton Trans. 2006, 340.
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48, 10442.
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49, 665.
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49, 3977.
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J. Inorg. Chem. 2006, 2306.
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Dalton Trans. 2010, 39, 4440.
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2008, 2620.
by 1H and 13C{1H} NMR spectroscopy and single-crystal
X-ray diffraction, Figure 1. In the solid state, the metal
center is seen to adopt a pseudo-octahedral geometry, Table 1.
This is exemplified by a O(1)-M-N(1) bond angle of
approximately 170° in all cases. As expected, the M-N-
(imine) distance is considerably shorter than the M-N-
(amine). The bond distances and angles are in agreement
with those in the literature for similar species.17,18,20,37,46,47
For such salalen ligands, it is possible to produce fac-fac,
mer-mer, fac-mer, or mer-fac isomers.50 In these exam-
ples, a fac-mer geometry is observed in the solid state, with
the salan fragment fac and the salen mer.50 The two
isopropoxides are cis-oriented, one being trans to the imine
with the other trans to a phenoxy oxygen, and overall the
complexes have C1 symmetry. The complexes are not
fluxional on the NMR time-scale, and the 1H NMR spectra
for the compounds {except Hf(3)(OiPr)2}, in CDCl3, con-
tain only one imine resonance and two septet isopropoxide
(45) Romain, C.; Brelot, L.; Bellemin-Laponnaz, S.; Dagorne, S. Orga-
nometallics 2010, 29, 1191.
(46) Whitelaw, E. L.; Jones, M. D.; Mahon, M. F.; Kociok-Kohn, G.
Dalton Trans. 2009, 9020.
(47) Zelikoff, A. L.; Kopilov, J.; Goldberg, I.; Coates, G. W.; Kol, M.
Chem. Commun. 2009, 6804.
(48) Chmura, A. J.; Davidson, M. G.; Jones, M. D.; Lunn, M. D.; Mahon,
M. F. Dalton Trans. 2006, 887.
(49) Kleij, A. W. Eur. J. Inorg. Chem. 2009, 193.
(50) Yeori, A.; Gendler, S.; Groysman, S.; Goldberg, I.; Kol, M. Inorg.
Chem. Commun. 2004, 7, 280.

7178 Inorganic Chemistry, Vol. 49, No. 15, 2010
Whitelaw et al.
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Table 1. Selected Bond Lengths (A) and Angles (deg) for Complexes Ti(1)(O Pr)2, Ti(2)(O Pr)2, Zr(1)(O Pr)2, Zr(2)(O Pr)2, Hf(1)(O Pr)2, Hf(2)(O Pr)2, and Hf(3)(O Pr)2
Ti(1)(OiPr)2
Ti(2)(OiPr)2
Zr(1)(OiPr)2
Zr(2)(OiPr)2
Hf(1)(OiPr)2
Hf(2)(OiPr)2
Hf(3)(OiPr)2
M-O1
M-O2
1.8410(13)
1.8274(13)
1.9032(12)
1.9405(13)
2.1972(16)
2.3053(16)
168.43(6)
1.8269(10)
1.8284(10)
1.8952(10)
1.9369(10)
2.1966(12)
2.3932(12)
170.62(5)
1.9788(12)
1.9567(12)
2.0268(11)
2.0534(11)
2.3385(14)
2.4335(14)
168.59(5)
1.963(2)
1.949(2)
2.016(2)
2.062(2)
2.340(2)
2.487(3)
171.71(10)
72.74(9)
1.9739(16)
1.9521(17)
2.0168(16)
2.0471(16)
2.3087(19)
2.399(2)
1.962(2)
1.944(2)
2.007(2)
2.048(2)
2.308(3)
2.452(3)
171.58(10)
73.45(8)
1.964(2)
1.954(2)
2.008(2)
2.041(2)
2.327(3)
2.379(3)
176.44(10)
74.19(10)
M-O3
M-O4
M-N1
M-N2
O1-M-N1
N1-M-N2
169.24(7)
72.74(7)
74.87(6)
75.39(4)
71.94(5)
Table 2. Solution Polymerization of rac-Lactidea
resonances and are consistent with the solid state structures
being maintained in solution. In the 13C{1H} NMR spectra,
three CH2 peaks are observed, and four resonances for the
methyl groups of the tBu are seen, again indicating that the
solid-state structure is maintained in solution. When ligand
3H2 was reacted with Hf(OiPr)4(OHiPr), the desired 1:1
complex was isolated, as indicated by the single-crystal
X-ray diffraction, which again showed the fac-mer isomer
was present in the solid state. However, the 1H and 13C{1H}
NMR spectra indicate that there are two structural isomers
present in solution, in equal proportions; this is exemplified
by the fact there are two imine resonances at 7.92 and 6.87
ppm in the 1H spectrum and consequently two imine
resonances in the 13C{1H} NMR spectrum together with
24 aromatic resonances. This is presumably facilitated by
the decrease in steric bulk of the ligand. A low-temperature
NMR spectrum (CDCl3 243 K) was equivalent to that at
room temperature.
conv/%b
Mn
PDIc
Prd
c
initiator
time/h
Ti(1)(OiPr)2
Ti(1)(OiPr)2
Ti(2)(OiPr)2
Ti(2)(OiPr)2
Zr(1)(OiPr)2
Zr(1)(OiPr)2
Zr(2)(OiPr)2
Hf(1)(OiPr)2
Hf(2)(OiPr)2
Hf(3)(OiPr)2e
Hf(3)(OiPr)2
2
24
2
24
2
24
24
24
24
24
6
98
98
69
97
91
99
96
98
96
99
99
11600
9900
8500
10100
10500
15800
10500
8100
1.45
1.62
1.18
1.82
1.13
1.44
1.09
1.08
1.07
1.43
1.25
0.5
0.5
0.55
0.5
0.6
0.56
0.57
0.30
0.5
0.30
0.25
14300
1000
8250
a In all cases, a 100:1 monomer-to-initiator ratio was employed; 0.72 g
of monomer was used in all cases, with toluene (10 mL) as the solvent and
at 80 °C. b Conversion as determined from 1H NMR spectroscopy.
c Determined from GPC analysis using THF as the solvent. The theore-
tical molecular weight = (conversion/100) ꢀ 100 ꢀ 144 þ 60. d Deter-
mined from 1H NMR homonuclear decoupled NMR spectroscopy.
Nota bene, Zr(2)(OiPr)2, Hf(1)(OiPr)2, and Hf(2)(OiPr)2 were tested
for 2 h but failed to produced any polymer. e In this case, the solvent was
CH2Cl2 and the temperature = 20 °C.
Polymerization of rac-Lactide. All complexes prepared
were trialed for the polymerization of rac-LA, both under
solution for 2 and 24 h and under the industrially pre-
ferred melt conditions at 130 °C in the absence of a solvent
(Tables 2 and 3). All initiators were active for the ROP of
rac-LA in solution at 80 °C for 24 h.
Table 3. Melt Polymerization of rac-Lactidea
c
initiator
time/h
conv/%b
Mn
PDI
Prd
Ti(1)(OiPr)2
Ti(2)(OiPr)2
Zr(1)(OiPr)2
Zr(2)(OiPr)2
Hf(1)(OiPr)2
Hf(2)(OiPr)2
Hf(3)(OiPr)2
0.25
0.25
0.25
0.25
48
89
95
98
52
75
96
99
38000
43200
42400
8700
24400
46600
7700
1.44
1.51
1.19
1.07
1.32
1.49
1.45
0.5
0.5
0.57
0.56
0.30
0.5
In general, the Zr(IV) and Hf(IV) complexes appear to
offer a greater degree of control, as observed by the lower
polydispersity indexes of the resulting polymers, Table 2.
Although Hf(1)(OiPr)2 as an initiator was unsuccessful
for the production of polymeric material after 2 h, an
isotactic oligomeric product was obtained (Pr = 0.2 via
1H homonuclear decoupled NMR, which showed an
increase in the intensity of the iii tetrad). When Hf(1)-
(OiPr)2 was used for the same reaction over a longer
period of time, Mn increased, but the isotactic content
had decreased and a Pr value of 0.3 was obtained. (See the
Supporting Information for the spectra of the methine
24
0.25
0.30
a In all cases, a 300:1 monomer-to-initiator ratio was employed; 2.0 g
of monomer was used, with the temperature = 130 °C. b Conversion as
determined from 1H NMR spectroscopy. c Determined from GPC
analysis using THF as the solvent. The theoretical molecular weight =
(conversion/100) ꢀ 300 ꢀ 144 þ 60. d Determined from 1H NMR homo-
nuclear decoupled NMR spectroscopy.
be fast with almost a quantitative yield after 15 min for
Zr(1)(OiPr)2. Hf(1)(OiPr)2 was much slower than either
the Ti(IV) or Zr(IV) analogue but, interestingly, showed a
preference for isotactic PLA. It has been shown for a
series of amine bis(phenolate) ligands that reduction in
the steric bulk of the ortho substituent from a tBu to a Me
changed the selectivity of the polymerization from atactic
to isotactic.17 Therefore, ligand 3H2 was prepared and
complexed to Hf(IV) in an attempt to increase the iso-
tactic enrichment in the polymer shown by Hf(1)(OiPr)2.
Unfortunately, there was no significant increase in the
isotacticity of the resulting polymer under melt condi-
tions; however, the rate of the reaction was significantly
enhanced with near quantitative conversion in the melt
after only 15 min, c.f. 48 h for Hf(1)(OiPr)2. Presumably,
the reduction in steric bulk facilitates the coordination of
the lactide monomer to the metal center. For Hf(3)(OiPr)2,
1
region of the H homonuclear decoupled NMR and the
13C{1H} NMR spectra, which confirm the presence of
isotactic PLA.) Interestingly, with Hf(2)(OiPr)2, atactic
PLA was produced under solution conditions. The Zr(IV)
complexes produced PLA with a slight heterotactic bias.
This illustrates the subtle effect that the ligand-metal
combination has on the stereoselectivity for the poly-
merization of rac-lactide. Analysis of the polymers pro-
duced with Ti(1)(OiPr)2, Zr(1)(OiPr)2, and Hf(1)(OiPr)2
at 80 °C in solution via MALDI-TOF mass spectrometry
indicated the presence of the isopropoxide end group,
which suggests that the expected coordination-insertion
mechanism is in operation. The Ti(IV) and Zr(IV) com-
plexes formed with 1H2 were shown to be more active
than those from 2H2, as higher conversions were
achieved. In the melt, the Zr(IV) complex was shown to

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 7179
Supporting Information for data for Ti/Zr/Hf(2)(OiPr)2
and Hf(3)(OiPr)2. The apparent rate of propagation
(kapp) can be determined by analysis of a semilogarithmic
plot of ln{[LA]o/[LA]t} vs time, where [LA]o = 0.578 mol
dm-3 with a monomer/initiator ratio of 100:1. The pseu-
do-first-order rate constant for Hf(1)(OiPr)2 was 3.0 ꢀ
10-4 min-1, and Zr(1)(OiPr)2 was found to be 10 times
faster with kapp = 3.2 ꢀ 10-3 min-1 20
,
the latter being
similar in magnitude to Ti(salen)(OiPr)2 complexes pre-
pared by Gibson et al.,20 whose fastest initiator afforded a
kapp of 3.7 ꢀ 10-3 min-1, albeit at 70 °C. Interestingly, the
Ti(1)(OiPr)2 initiated system revealed two reaction re-
gimes. Initially, the polymerization proceeds with a kapp
of 6.4 ꢀ 10-3 min-1, which is significantly faster than
literature values. However, after ca. 3 h, the polymeriza-
tion slows significantly, to afford a kapp of 2.2 ꢀ 10-3
min-1. This could be due to the mass transport limitations
in the NMR tube as the viscosity of the polymerization
increases, or alternatively the polymer could wrap around
the metal center, hindering the approach of the monomer
to the active center. A similar trend was observed with
complexes Ti(2)(OiPr)2 (5.5 ꢀ 10-3 min-1 initial and 3.0 ꢀ
10-3 min-1 in the second regime), Zr(2)(OiPr)2 (2.6 ꢀ 10-3
min-1), and Hf(2)(OiPr)2 (0.7 ꢀ 10-3 min-1). When the
kinetics were investigated for Hf(3)(OiPr)2 at 80 °C in C6D5-
CD3, a rate of 1.2 ꢀ 10-2 min-1 was observed; however, it
was clear under these conditions that the catalysis was
proceeding too fast for this to be a reliable method to
determine kapp, as a conversion of 60% was observed after
the first 15 min. It was decided to perform the same
experiment at room temperature (for lactide solubility
reasons, it was necessary to perform this in CDCl3), and
in this experiment, a kapp of 4.6ꢀ 10-3 min-1 was obtained
(see the Supporting Information). For L-lactide, Hf(3)-
(OiPr)2 affords a kapp of 7.5 ꢀ 10-3 min-1. For comparison,
the fastest titanium complex {Ti(1)(OiPr)2} was tested in
CDCl3 at room temperature, and in this case, no polymer
was detected. Hf(3)(OiPr)2 is complementary to the pre-
viously reported Zr(IV) amine tris(phenolate)16 complex,
which has a kapp of 4.2ꢀ 10-3 min-1 for rac-lactide, but in
this case, isotactic PLA is formed.
Figure 2. (Top) Kinetic measurements for the solution polymerization
of rac-LA with Ti(1)(OiPr)2, Zr(1)(OiPr)2, and Hf(1)(OiPr)2 at 80 °C. The
equations of the lines are Ti(1)(OiPr)2 initial y= 0.0064xþ 0.6117, R2 =0.99;
Ti(1)(OiPr)2 y=0.0022xþ 1.4024, R2 = 0.9906; Zr(1)(OiPr)2 y=0.0031xþ
0.0101, R2 = 0.9978; Hf(1)(OiPr)2 y = 0.0003x þ 0.0391, R2 = 0.9969.
(Bottom) A plot of Mn (squares) and Mw/Mn (circles) versus time for
Zr(1)(OiPr)2.
the molecular weights of the polymers (both in solution
and under melt conditions) were lower than expected for
one polymer chain growing per metal center. In this case,
due to the reduction in steric bulk of the ligand, the lower
molecular weights may be due to more than one polymer
chain growing per metal center. Due to the high acti-
vity of Hf(3)(OiPr)2, transesterification processes maybe
prevalent, reducing the control the initiator provides for
the molecular weight.47 To achieve a controlled molecular
weight, the polymerization was carried out at room tem-
perature for 6 h in CH2Cl2. In this case, a Mn of 8250 was
observed, and a polymer had a Pr = 0.25.
Conclusion
In conclusion, seven new group 4 metal complexes have
been prepared with salalen ligands, and all complexes have
been characterized via single-crystal X-ray diffraction. These
complexes were shown to initiate the ring-opening polymeri-
zation of rac-lactide in a controlled manner, and several
systems were also able to control the tacticity of the resulting
polymer. Further experiments are on-going to probe the
kinetics of the polymerizations.
Experimental Section
To study the polymerization further, Zr(1)(OiPr)2 was
employed as an initiator in a time study to investigate the
living character of the polymerization, Figure 2. It can be
seen that as time increased, Mw/Mn remained constant
while Mn increased linearly, supporting the idea that the
polymerization is well-controlled. The kinetics for the
polymerization using Ti(1)(OiPr)2, Zr(1)(OiPr)2, and Hf-
(1)(OiPr)2 as the initiators were investigated at 80 °C by
1H NMR spectroscopy in C6D5CD3, Figure 2; see the
General Procedures. For the preparation and characteriza-
tion of metal complexes, all reactions and manipulations were
performed under an inert atmosphere of argon using standard
Schlenk or glovebox techniques. Ti(OiPr)4 (97% Aldrich) was
i
purified by vacuum distillation prior to use; Zr(OiPr)4 PrOH
(99.9%, Aldrich) and Hf(OiPr)4 PrOH (99.9%, Strem) were
i
used without further purification. rac-LA (Aldrich) was recrys-
tallized from toluene and sublimed twice prior to use. All other
chemicals were purchased from Aldrich. All solvents used in the
preparation of metal complexes and polymerization reactions

7180 Inorganic Chemistry, Vol. 49, No. 15, 2010
Whitelaw et al.
Table 4. Crystallographic Details for Ti(1)(OiPr)2, Ti(2)(OiPr)2, Zr(1)(OiPr)2, Zr(2)(OiPr)2, Hf(1)(OiPr)2, Hf(2)(OiPr)2, and Hf(3)(OiPr)2
complex
Ti(1)(OiPr)2
Ti(2)(OiPr)2
Zr(1)(OiPr)2
Zr(2)(OiPr)2
empirical formula
formula weight
crystal system
space group
C39H64N2O4Ti1
672.82
monoclinic
P21/c
14.5370(3)
20.9780(4)
14.0970(2)
90
114.437(1)
90
3913.86(12)
4
1.142
0.257
51088
3.72-26.01
7670, 0.0767
1.000
C44H66N2O4Ti1
734.89
monoclinic
C2/c
24.0590(3)
18.0360(3)
20.0870(3)
90
102.740(1)
90
8501.7(2)
8
1.148
0.242
18758
3.85-27.47
9698, 0.0265
1.024
C39H64N2O4Zr1
716.14
monoclinic
P21/c
14.6670(2
21.1850(4)
14.0480(3)
90
114.037(1)
90
3986.5(1)
4
1.193
0.314
84036
3.52-27.50
9117, 0.0638
1.051
C44H66N2O2Zr1
778.21
monoclinic
C2/c
24.3950(3)
17.9500(3)
20.0070(3)
90
102.602(1)
90
8549.8(2)
8
1.209
0.298
82109
3.61-27.48
9779, 0.1021
1.131
˚
a/A
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
3
˚
volume/A
Z
Dcalc/gcm-3
μ/mm-1
reflns collected
θ range/deg
indep. reflns (Rint
)
goodness-of-fit
R1, wR2 [I > 2σ(I )]
R1, wR2 [all data]
max, min difference/e A
0.0408, 0.0960
0.0651, 0.1105
0.237, -0.379
0.0398, 0.0930
0.0587, 0.1030
0.294, -0.395
0.0333, 0.0767
0.0474, 0.0829
0.593, -0.599
0.0599, 0.1115
0.0953, 0.1229
0.832, -0.835
-3
˚
complex
Hf(1)(OiPr)2
Hf(2)(OiPr)2
Hf(3)(OiPr)2
empirical formula
formula weight
crystal system
space group
C39H64N2O4Hf1
803.41
monoclinic
P21/a
14.0610(1)
21.1280(2)
14.6510(1)
90
114.080(2)
90
3973.8(5)
4
1.342
2.662
73379
3.52-27.48
9085, 0.0462
1.073
C44H66N2O4Hf1
865.48
monoclinic
C2/c
24.3640(2)
17.9370(2)
20.0680(2)
90
102.801(1)
90
8552.1(2)
8
1.344
2.480
65155
3.86-27.49
9780, 0.0730
1.076
C38H64N2O4Hf1
791.40
monoclinic
P21/c
11.0520(1)
24.7750(3)
14.5020(2)
90
97.668(1)
90
3935.33(8)
4
1.336
2.688
56183
2.91-27.48
8956, 0.0972
1.063
˚
a/A
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
3
˚
volume/A
Z
Dcalc/gcm-3
μ/mm-1
reflns collected
θ range/deg
indep. reflns (Rint
)
Goodness-of-fit
R1, wR2 [I > 2σ(I )]
R1, wR2 [all data]
max, min difference/e A
0.0246, 0.0563
0.0312, 0.0592
1.504, -0.947
0.0327, 0.0715
0.0444, 0.0798
0.973, -1.762
0.0347, 0.0679
0.0544, 0.0743
0.955, -1.466
-3
˚
were dry and obtained via SPS (solvent purification system).
1H and 13C{1H} NMR spectra were recorded on a Bruker
250, 300, or 400 MHz instrument and referenced to residual
solvent peaks. Coupling constants are given in hertz. Elemental
analyses were performed by Mr. A. K. Carver at the Depart-
ment of Chemistry, University of Bath. The ligands were
prepared according to standard literature procedures and the
purity confirmed via 1H/13C{1H} NMR and HR-MS prior
to use.
CH2), 3.76 (2H, s, CH2), 3.78 (2H, t J = 6.5 Hz, CH2), 6.84 (1H,
d J = 2.5 Hz, Ar-H), 7.09 (1H, d J = 2.5 Hz, Ar-H), 7.22 (1H,
d J = 2.5 Hz, Ar-H), 7.40 (1H, d J = 2.5 Hz, Ar-H), 8.39 (1H,
s, CH). m/z calcd for C33H53N2O2 = 509.4107. Found: 509.4145
Synthesis and Characterization of the Complexes. A typical
example for the preparation of a complex is as follows. Zr(2)-
i
(OiPr)2: Zr(OiPr)4 PrOH (0.5 g, 1.28 mmol) was dissolved in
toluene (20 cm3), to which 2H2 (0.731 g, 1.28 mmol) was added.
This was stirred for 2 h, after which time the solvent was
removed in vacuo, and the product was recrystallized in hexane.
After 4 days at -20 °C, a crop of crystals was obtained, which
were filtered and dried. 1H (CDCl3): 0.43 (3H, d J = 5 Hz, CH3
isopropoxide), 0.51 (3H, d J = 5 Hz, CH3 isopropoxide), 1.19
(9H, s, C(CH3)3), 1.25-1.30 (18H, m, C(CH3)3), 1.30-1.35 (6H,
m, CH3 isopropoxide), 1.53 (9H, s, C(CH3)3), 3.26 (1H, sept J =
5.5 Hz, CH isopropoxide), 3.38 (1H, t J = 11 Hz, CH2), 3.44
(1H, m, CH2), 3.64 (1H, d J = 13 Hz, CH2), 3.79 (1H, t J = 11
Hz, CH2), 4.60 (1H, sept J = 5.5 Hz, CH isopropoxide), 5.09
(1H, d J = 13 Hz, CH2), 6.83 (1H, s, Ar-H), 6.95 (1H, s, Ar-H),
7.19-7.24 (2H, m, Ar-H), 7.40-7.49 (3H, m, Ar-H), 7.72 (2H,
d J = 8 Hz, Ar-H), 7.97 (1H, s, CH). 13C{1H} (CDCl3): 26.3,
27.3 (CH3 isopropoxide), 29.3, 29.6, 31.5, 31.8 (C(CH3)3), 33.9,
34.0, 34.8, 35.3 (C(CH3)3), 52.2, 58.2, 68.6 (CH2), 69.6, 70.8 (CH
isopropoxide), 121.5 (Ar-C), 122.0 (Ar-CH), 123.1 (Ar-C),
123.9, 124.0, 125.2, 127.8, 128.9, 129.2 (Ar-CH), 136.7, 137.6,
Synthesis of Ligands. A typical ligand synthesis is as follows.
1H2: A solution of 3,5-di-tert-butyl-2-hydroxybenzaldehyde
(3.23 g, 0.01 mol) and N-methyl-ethylenediamine (1.02 g, 1.2
mL, 0.01 mol) in methanol (20 mL) was prepared. The solution
was stirred until a clear solution was observed and left to stand
for 24 h to yield a yellow oil, which was collected and dried (2.68
g, 66%). The oil (1 g, 3.44 mmol) was dissolved in THF (20 cm3),
to which a solution of 3,5-di-tert-2-hydroxybenzyl bromide
(1.03 g, 3.44 mmol) in THF (20 cm3) was added. Triethylamine
(0.35 g, 0.48 mL, 3.44 mmol) was added and the mixture stirred
at 80 °C for 3 h. The white precipitate was filtered and the
solvent removed under reduced pressure. The product was
isolated via flash chromatography (CH2Cl2) to obtain the
product (1.33 g, 76%). 1H NMR (CDCl3): 1.29 (9H, s, C-
(CH3)3), 1.32 (9H, s, C(CH3)3), 1.39 (9H, s, C(CH3)3), 1.46
(9H, s, C(CH3)3), 2.40 (3H, s, CH3), 2.85 (2H, t J = 6.5 Hz,

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 7181
138.4, 138.6, 152.3 (Ar-C), 159.4, 159.8 (Ar-O), 165.2 (CH).
Anal Calcd for C44H66N2O4Zr: C, 67.90; H, 8.55; N, 3.60.
Found: C, 67.1; H, 8.57; N, 3.56.
reaction, and the resulting solid was dissolved in dichloro-
methane. The solvents were removed in vacuo, and the resulting
solid was washed with copious amounts of methanol to remove
1
any unreacted monomer. H NMR spectroscopy (CDCl3) and
X-Ray Crystallography. Crystallographic data are summari-
zed in Table 4. All data were collected on a Nonius Kappa CCD
area detector diffractometer using Mo KR radiation (λ =
GPC (THF) were used to determine tacticity and molecular
weights (Mn and Mw) of the polymers produced; Pr values (the
probability of heterotactic linkages) were determined by analysis
˚
0.71073 A) at a temperature of 150(2) K, and all structures were
1
solved by direct methods and refined on all F2 data using the
SHELXL-97 suite of programs.51 Refinement was straightfor-
ward with the following noteworthy points: For Ti(1)(OiPr)2,
two tBu groups showed disorder over two positions in an 70:30
ratio, and in Ti(2)(OiPr)2, a tBu group showed disorder over two
positions in a 60:40 ratio. For Zr(1)(OiPr)2, tBu groups showed
disorder over two positions in a 70:30 ratio, and for Zr(2)-
of the methine region of the homonuclear decoupled H NMR
spectra. The equations used to calculate Pr and Pm are given by
Coates et al.21 1H homonuclear decoupled spectra are given in the
Supporting Information. Gel permeation chromatography (GPC)
analyses were performed on a Polymer Laboratories PL-GPC 50
integrated system using a PLgel 5 μm MIXED-D 300 ꢀ 7.5 mm
column at 35 °C, with THF solvent (flow rate, 1.0 mL/min). The
polydispersity index (PDI) was determined from Mw/Mn, where
Mn is the number average molecular weight and Mw the weight
average molecular weight. The polymers were referenced to 11
narrow molecular weight polystyrene standards with a range of
Mw 615-568 ,000 Da.
t
(OiPr)2, a Bu group showed disorder over two positions in a
50:50 ratio. For Hf(1)(OiPr)2, a tBu group showed disorder over
two positions in a 65:35 ratio. For Hf(2)(OiPr)2, two tBu groups
were disordered over two positions in a 60:40 ratio. Hydrogen
atoms were included in idealized positions and refined using the
riding model. Data can be obtained free of charge from the
Cambridge Crystallographic Data Center via www.ccdc.cam.
ac.uk/data_request/cif.
Acknowledgment. We gratefully acknowledge the EPSRC
(National mass spectrometry service center Swansea), the University
of Bath, and Johnson Matthey for funding.
Polymerization Procedure. For solvent-free polymerizations,
the monomer/initiator ratio employed was 300:1 at a tempera-
ture of 130 °C. In all cases, 2 g of rac-lactide were used. After the
reaction time, methanol (20 mL) was added to quench the
1
Supporting Information Available: H homonuclear decoupled
NMR spectra, crystal data in .cif format, full experimental
procedures, and further kinetic details. This material is available
free of charge via the Internet at http://pubs.acs.org.
(51) Sheldrick, G. M. SHELXL-97; Institute for Anorganische University of
€
€
Gottingen: Gottingen, Germany, 1998.