Comparative study of the coordination chemistry and lactide polymerization of alkoxide and amide complexes of zinc and magnesium with a β-diiminato ligand bearing ether substituents

Article abstract of DOI:10.1021/ic048363d

A series of β-diiminato complexes of the form (BDI-3)MX where (BDI-3) = [CH(CMeNC₆H₄-2-OMe)₂]; M = Zn, Mg; X = O ⁱPr, O^tBu, or N(SiMe₃)₂ has been synthesized. The (BDI-3) ligand

Full text of DOI:10.1021/ic048363d

Inorg. Chem. 2005, 44, 8004−8010
Comparative Study of the Coordination Chemistry and Lactide
Polymerization of Alkoxide and Amide Complexes of Zinc and
Magnesium with a -Diiminato Ligand Bearing Ether Substituents
â
Malcolm H. Chisholm,* Judith C. Gallucci, and Khamphee Phomphrai
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue,
Columbus, Ohio 43210-1185
Received November 18, 2004
A series of
â-diiminato complexes of the form (BDI-3)MX where (BDI-3) ) [CH(CMeNC6H4-2-OMe)2]; M ) Zn,
Mg; X
)
OiPr, OtBu, or N(SiMe3)2 has been synthesized. The (BDI-3) ligand is bidentate in (BDI-3)ZnN(SiMe3)2
and tetradentate in (BDI-3)MgN(SiMe3)2. The alkoxide complexes are shown to be active for lactide polymerization.
Polymerization of rac-lactide with (BDI-3)ZnOiPr gives a moderate preference for heterotactic PLA. Polymerization
of rac-lactide with [(BDI-3)MgOtBu]2 shows a slight preference for heterotactic PLA in CH2Cl2 but is highly
stereoselective in THF in the production of heterotactic PLA.
Introduction
but for M ) Mg and Zn, â-diiminate ligands have received
the most attention.7-10 First synthesized by Holm,11 bidentate
uninegatively charged â-diiminate ligands with a variety of
substituents have been attached to various metals.12
Polymerization of lactides has been studied extensively
in the literature using various single-site metal catalysts.1,2
The biocompatible nature of polylactide (PLA) has promoted
the use of biocompatible metals because trace amounts of
the catalyst may be tenaciously incorporated within the
polymer. Biocompatible metals such as zinc and magnesium
are of interest, though they exhibit rather different chemical
properties. Both are essential nutrients and minerals for plants
and humans3 and have similar ionic radii,4 but magnesium
(2+) is considered as a hard metal, while zinc (2+) is soft.5
The similarity in size of the M2+ ions4 allows a possible
comparison of their reactivity when an identical ligand
system is employed.6-8 Many ligand systems have been
employed giving different catalytic activity and selectivity,1
We previously reported that the â-diiminate magnesium
complex, (BDI-1)MgOtBu‚THF (Scheme 1), displayed high
solvent dependence for the polymerization of rac-lactide.8
The catalyst produces atactic PLA in CH2Cl2 and heterotactic
PLA in THF. The extensive coordination of THF to the Mg
core was proposed to cause such dramatic stereoselectivity.
Although THF can induce heterotactic selectivity, the use
of excess THF as a solvent decelerates the polymerization
because it competes with lactide for metal coordination. We
envisaged that the use of excess THF could be avoided if
the â-diiminate ligand possesses ether functionality that takes
over the THF’s role. By adding ether groups to the â-diimine
ligand, the effect of ligand variation from bulky bidentate
(BDI-1) to chelating tetradentate (BDI-3) ligand can be
investigated (Scheme 1). The potentially tridentate (BDI-
2)H ligand10 retains one of the bulky 2,6-diisopropylphenyl
groups of the (BDI-1)H ligand. The magnesium and zinc
complexes of the (BDI-2)H ligand have been synthesized
* To whom correspondence should be addressed. E-mail:
chisholm@chemistry.ohio-state.edu.
(1) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton
Trans. 2001, 2215-2224.
(2) Coates, G. W. J. Chem. Soc., Dalton Trans. 2002, 467-475.
(3) Campbell, N. A. Biology, 3rd ed.; Benjamin/Cummings Publishing
Company: Redwood City, CA, 1993; pp 718-722, 811-814.
(4) Cotton, F. A.; Wilkinson, E.; Murillo, C. A. AdVanced Inorganic
Chemistry, 6th ed.; Wiley and Sons: New York, 1999; pp 1302-
1304.
(5) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and ReactiVity, 4th ed.; Harper Collins College
Publishers: New York, 1993; pp 292, 344-348.
(8) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg. Chem. 2002, 41,
2785-2794.
(9) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.; Williams,
D. J. Chem. Commun. 2001, 283.
(6) Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C.; Iyer, S. S.; Pacold,
M.; Phomphrai, K. J. Am. Chem. Soc. 2000, 122, 11845-11854.
(7) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229-
3238.
(10) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.; Williams,
D. J. J. Chem. Soc., Dalton Trans. 2004, 570-578.
(11) Parks, J. E.; Holm, R. H. Inorg. Chem. 1968, 7, 1408-1416.
(12) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002,
102, 3031-3065.
8004 Inorganic Chemistry, Vol. 44, No. 22, 2005
10.1021/ic048363d CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/24/2005

ComparatiWe Study of Alkoxide and Amide Complexes
Scheme 1
Figure 1. ORTEP drawing of the (BDI-3)ZnN(SiMe3)2, 1, molecule.
Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity.
by Gibson and co-workers and have been shown to be active
initiators for lactide polymerization.10 The (BDI-3)H ligand
has two ether functional groups that allow the ligand to
expand its chelating ability from two to three or four.13 We
hypothesized that the stereoselectivity for heterotactic bias
could be reproduced in the chelating (BDI-3) ligand system
where the ether groups could bind reversibly to the metal
center, creating a similar effect to THF. Herein, the syntheses
of the zinc and magnesium complexes of the type (BDI-3)-
MX, where X ) amide and alkoxide, are reported along with
their reactivities toward the ring-opening polymerization of
lactides.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
(BDI-3)ZnN(SiMe3)2, 1
Zn-N1
1.952(3)
1.881(3)
3.236(3)
Zn-N2
1.933(3)
4.212(3)
98.9(1)
Zn-N3
Zn-O1
Zn-O2
N1-Zn-N3
N1-Zn-N2
N2-Zn-N3
129.6(2)
131.2(2)
(Å) and angles (deg) in Table 1. Compound 1 has a
monomeric structure in the solid state where the Zn center
is an example of a neutral monometallic three-coordinate
zinc complex.10,15-20 Compound 1 has a structural similarity
18
to the related (BDI-1)ZnNR2 complexes where R ) SiMe3
and Pr.8 The zinc atom adopts a distorted trigonal planar
i
Results and Discussion
geometry with the sum of the angles around the ZnN3 core
of 359.7°. The coordination of the ether functional group to
zinc atom is not observed in the solid state (Zn-to-O distances
are 4.212(3) and 3.236(3) Å), presumably due to the soft
nature of zinc and a steric impediment of the bulky amide.
The two ether groups have a syn arrangement. The dihedral
angle between the Zn-N1-N2 plane and N3-Si1-Si2
plane is 64.9(4) Å. From purely electronic considerations,
the Zn-N1-N2 and N3-Si1-Si2 planes are expected to
be parallel to maximize the Npπ and Znpπ interactions.
Therefore, such adoption of the amide arrangement must be
a result of a compromise imposed by the steric impediment
of the bulky amide. The zinc amide nitrogen bond distance
in 1 is 1.881(3) Å, which is slightly shorter than in (BDI-
1)ZnN(SiMe3)2 and (BDI-2)ZnN(SiMe3)2,13,18 possibly due
to less steric hindrance of the (BDI-3) ligand compared to
(BDI-1) and (BDI-2) making the amide more accessible to
the zinc center. The zinc atom lies only 0.031(1) Å above
the plane defined by N1, C2, and N2 making the six-
membered ZnNCCCN ring very close to planar.
Syntheses. The (BDI-3)H ligand can be easily synthesized
using a published procedure.13 The reaction between an
equimolar amount of (BDI-3)H with Zn[N(SiMe3)2]2 in
benzene gave the light yellow compound (BDI-3)ZnN-
(SiMe3)2, 1, in quantitative yield. The reaction was complete
in 10 min at room temperature. For a bulkier ligand such as
(BDI-1)H, the reaction requires heating in toluene at 80 °C
for 6 days.14 The faster reaction is probably due to the less
sterically demanding OMe groups, and the oxygen may
participate in hydrogen bonding which may facilitate proton
transfer. Addition of 1 equiv of HOiPr into a solution of
compound 1 in benzene gave the light yellow compound
(BDI-3)ZnOiPr, 2, in high yield.
The reaction between 1 equiv of (BDI-3)H with 1 equiv
of Mg[N(SiMe3)2]2 at room temperature gave an orange
compound, (BDI-3)MgN(SiMe3)2, 3. When 2 equiv of (BDI-
3)H were used along with heating at 80 °C, bis(â-diiminato)
complex, (BDI-3)2Mg, was obtained in quantitative yield as
an orange solid. Addition of 1 equiv of HOtBu into a solution
of compound 3 produced the light yellow complex [(BDI-
3)MgOtBu]2, 4, in high yield. Compounds 1-4 are highly
air- and moisture-sensitive.
(15) Chinn, M. S.; Chen, J. Inorg. Chem. 1995, 34, 6080-6084.
(16) Ellison, J. J.; Ruhlandt-Senge, K.; Hope, H. H.; Power, P. P. Inorg.
Chem. 1995, 34, 49-54.
(17) Darensbourg, D. J.; Zimmer, M. S.; Rainey, P.; Larkins, D. L. Inorg.
Chem. 1998, 37, 2852-2853.
Single Crystal and Molecular Structures. (BDI-3)ZnN-
(SiMe3)2, 1. An ORTEP drawing of the molecular structure
of 1 is given in Figure 1 along with selected bond distances
(18) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738-
8749.
(19) Prust, J.; Stasch, A.; Zheng, W.; Roesky, H. W.; Alexopoulos, E.;
Uso´n, I.; Bo¨hler, D.; Schuchardt, T. Organometallics 2001, 20, 3825-
3828.
(20) Prust, J.; Hohmeister, H.; Stasch, A.; Roesky, H. W.; Magull, J.;
Alexopoulos, E.; Uso´n, I.; Schmidt, H.-G.; Noltemeyer, M. Eur. J.
Inorg. Chem. 2002, 8, 2156-2162.
(13) Carey, D. T.; Cope-Eatough, E. K.; Vilaplana-Mafe´, E.; Mair, F. S.;
Pritchard, R. G.; Warren, J. E.; Woods, R. J. J. Chem. Soc., Dalton
Trans. 2003, 1083-1093.
(14) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 1999, 121, 11583-11584.
Inorganic Chemistry, Vol. 44, No. 22, 2005 8005

Chisholm et al.
Figure 2. ORTEP drawing of the (BDI-3)MgN(SiMe3)2, 3, molecule.
Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity.
Figure 3. ORTEP drawing of the [(BDI-3)MgOtBu]2, 4, molecule. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
(BDI-3)MgN(SiMe3)2, 3
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[(BDI-3)MgOtBu]2, 4
Mg-N1
2.023(1)
2.083(1)
2.144(1)
133.04(6)
74.87(5)
91.42(5)
Mg-N2
2.094(1)
2.129(1)
114.68(6)
83.98(5)
74.71(5)
Mg-N3
Mg-O1
Mg-N1
2.050(2)
1.944(2)
4.221(2)
2.918(2)
124.90(7)
121.73(7)
83.41(6)
Mg-N2
2.048(2)
1.965(2)
4.054(2)
91.29(8)
118.17(7)
120.91(7)
96.58(6)
Mg-O2
N1-Mg-N2
N2-Mg-N3
N3-Mg-O2
Mg-O3
Mg-O3′
N1-Mg-N3
N2-Mg-O1
O1-Mg-O2
Mg-O1
Mg-Mg′
N1-Mg-O3
N2-Mg-O3
O3-Mg-O3′
Mg-O2
N1-Mg-N2
N1-Mg-O3′
N2-Mg-O3′
Mg-O3-Mg′
(BDI-3)MgN(SiMe3)2, 3. An ORTEP drawing of the
molecular structure of 3 is given in Figure 2 along with
selected bond distances (Å) and angles (deg) in Table 2.
Compound 3 has a monomeric structure and the five-
coordinate Mg atom adopts a distorted square pyramidal
geometry. The Mg atom sits slightly above the square
pyramidal base defined by the O1, O2, N2, and N3 atoms.
Both ether oxygen atoms coordinate to the Mg atom giving
Mg-O bond distances of 2.129(1) and 2.144(1) Å. The
coordination of ether groups causes the phenyl rings to be
almost parallel to the plane defined by N2, C1, C2, C3, and
N3. Compound 3 is a rare example of structurally character-
ized tetradentate acyclic â-diiminate ligand complexes. The
first three examples are [CH(CMeN(CH2CH2NEt2))2]ScCl2,21
(BDI-3)Li,13 and (BDI-3) 2Ca.22
[(BDI-3)MgOtBu]2, 4. An ORTEP drawing of the mo-
lecular structure of 4 is given in Figure 3, and selected bond
distances (Å) and angles (deg) are listed in Table 3.
Compound 4 has a dimeric structure where the four-
coordinate Mg core is distorted tetrahedral. The tert-
butoxides are bridging between the two Mg cores. The Mg-
to-Mg distance in 4 is 2.918(2) Å, significantly shorter than
in the related compound [(BDI-1)MgOiPr]2 (3.01 Å)7 pos-
sibly due to a less-steric repulsion between the two (BDI-3)
ligands. The two ether groups are in a syn arrangement, and
they do not coordinate to the Mg atom (Mg-to-Oether distances
are 4.221(2) and 4.054(2) Å).
conformation exists in C6D6 in agreement with the crystal
structure. From the spectrum, however, one cannot tell if
there is a rapid interconversion between syn and anti
arrangement of the ether groups. The syn and anti arrange-
ments are expected to have different chemical shifts but
otherwise very similar NMR spectra. Cooling compound 1
to -80 °C in toluene-d8 only broadens the peaks slightly.
On the basis of the NMR spectrum and the syn conformation
in the crystal structure of 1, we conclude that, in solution,
complex 1 exists as a syn conformation, as seen in the crystal
structure.
The five-coordinate complex (BDI-3)MgN(SiMe3)2, 3, also
has an expected NMR spectrum for a syn arrangement. The
1H NMR spectra of 3 do not change upon cooling to -90
°C in toluene-d8, consistent with a five-coordinate complex
at low temperature. The addition of THF to 3 does not form
(BDI-3)MgN(SiMe3)2‚THF after any volatile components are
removed in vacuo. The surprising behavior was observed in
the variable-temperature 1H NMR spectra after the addition
of THF, as shown in Figure 4. The important features are
the splitting of signals arising from N(SiMe3)2, OMe, and
γ-CH3 from one singlet into two broad singlets upon cooling
to -90 °C while the â-CH proton remains unchanged. This
implies that THF coordinates to the magnesium atom in a
way that removes the mirror plane of symmetry. A possible
mechanism involving the coordination of THF is proposed
in Scheme 2. At low temperature, a THF molecule displaces
one of the ether groups. The N, N, O, and O atoms are no
longer in the same plane causing nonequivalent SiMe3, OMe,
and γ-CH3 groups. The fact that THF molecule can displace
an OMe group suggests that the ether groups of (BDI-3)
ligand bind to Mg atom reversibly. Thus, the OMe groups
A summary of crystallographic data for compounds 1, 3,
and 4 is given in Table 4.
1
Solution Behavior: NMR Studies. The H NMR spec-
trum of (BDI-3)ZnN(SiMe3)2, 1, indicates that only one
(21) Neculai, A. M.; Roesky, H. W.; Neculai, D.; Magull, J. Organome-
tallics 2001, 20, 5501-5503.
(22) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem. 2004,
43, 6717-6725.
8006 Inorganic Chemistry, Vol. 44, No. 22, 2005

ComparatiWe Study of Alkoxide and Amide Complexes
Table 4. Summary of Crystallographic Data of Compounds 1, 3, and 4
compound
1
3
4
empirical formula
fw
cryst syst
space group
temp (K)
C25H39N3O2Si2Zn
C25H39MgN3O2Si2
494.08
monoclinic
P21/c
C23H30MgN2O3
406.80
monoclinic
P21/c
535.14
triclinic
P1h
200
200
200
cell dimensions
a(Å)
b(Å)
c(Å)
8.139(2)
12.057(4)
15.201(5)
95.69(1)
105.07(1)
94.69(2)
2
1424.3(8)
1.248
2.07-24.99
22 395
10.507(2)
23.713(2)
11.334(1)
11.219(1)
19.481(3)
11.319(1)
R (°)
â (°)
90.19(1)
109.44(1)
γ (°)
Z
4
4
V (Å3)
2824.02(4)
1.162
2.12-27.47
45 638
6475
0.0413
0.0656
0.1207
1.033
2332.8(5)
1.158
2.09-25.02
34 894
4118
0.0513
0.0826
0.1457
1.020
densitycalc (g/cm3)
θ range (°)
reflns collected
independent reflns
R1(F)a [I > 2σ(I)]
R1(F)a (all data)
wR2(F2)a (all data)
GOF
4994
0.0595
0.0788
0.1571
1.047
a R1(F) ) ∑||Fo| -|Fc||/∑|Fo|; wR2(F2) ) {∑w(Fo - Fc )2/∑w(Fo )2}1/2; W ) 1/[σ2(Fo ) + (xP)2 + yP]; P ) (Fo + 2Fc )/3 where x ) 0.0691, y )
2
2
2
2
2
2
2.5762 for 1; x ) 0.0670, y ) 0.3638 for 3; x ) 0.0730, y ) 1.0001 for 4.
1
Figure 4. Variable-temperature H NMR (400 MHz, toluene-d8) of (BDI-3)MgN(SiMe3)2, 3, after the addition of THF.
t
1
can dissociate to some extent from the Mg atom, making
room for lactide association during the polymerization.
The Bu peak in the H NMR spectrum of the dimeric
[(BDI-3)MgOtBu]2, 4, is broad at room temperature, sug-
Inorganic Chemistry, Vol. 44, No. 22, 2005 8007

Chisholm et al.
Scheme 2. Proposed Reversible Process When THF Was Added to
(BDI-3)MgN(SiMe3)2, 3
bias due to a less-bulky ancillary ligand was also observed
in the (BDI-2)Zn system.10
The magnesium alkoxide complex 4 is also active for ROP
of lactides (Table 5, entry 3). The polymerization proceeds
to 91% conversion in 20 min. In contrast to the reactivity
order observed in the zinc complexes, the less-hindered
complex 4 is less reactive than the bulky (BDI-1)Mg
complexes.7,8 The order of reactivity decreases with increas-
ing number of ether groups: (BDI-1) > (BDI-2) > (BDI-
3).7,8,10 Clearly, the effect of chelating OMe groups overrides
the steric influence. The (BDI-2) and (BDI-3) ligands contain
ether groups that can effectively compete with lactide for
metal coordination. The chelating effect significantly influ-
ences the rate of polymerization in the hard magnesium
complex but is negligible for a softer zinc analogue. The
molecular weight of the polymer obtained from 4 (Table 5,
entry 3) is 96.5 kDa, which is significantly greater than the
theoretical value of 13.1 kDa for a living polymerization with
kinit g kprop. The polydispersity index of 2.23 is quite high.
We tentatively attribute the high Mn and broad PDI of the
polymer to a slow initiation possibly as a result of the dimeric
structure of 4 and transesterification.7 The polymer shows
ca. 50% heterotactic junctions (isi + sis). In contrast to the
decrease of heterotactic bias from (BDI-1) to (BDI-3) in the
zinc complexes, complex 4 exhibits a higher preference for
heterotacticity compared to the (BDI-1)Mg complex, which
produces atactic PLA (37.5% heterotactic bias).7,8 We have
reported that (BDI-1)MgOtBu‚THF showed no stereocontrol
for polymerization of rac-lactide in dichloromethane giving
atactic poly(rac-lactide).8 However, when the polymerization
was done in THF, the catalyst system exhibited a significant
preference for heterotacticity presumably as a result of THF
coordination to the magnesium atom making the magnesium
core more coordinately hindered. A similar scenario is
proposed to take place for 4 where the OMe groups reversibly
coordinate to the Mg(II) center causing complex 4 to be
coordinately hindered and more discriminating to the incom-
ing lactide. The effect of ether groups becomes important in
a hard Mg metal but much less pronounced in a softer Zn
metal. As a result, a higher stereoselectivity is observed in
4 although the (BDI-3) ligand is less sterically demanding
compared to (BDI-1) ligand. A comparison of the stereose-
lectivity of the zinc and magnesium complexes is illustrated
in Figure 5.
Table 5. Polymerization of rac-Lactide (100:1 [lactide]/[catalyst]) in
CH2Cl2 and THF at 20 °C
time conversion
entry catalyst solvent (min)
(%)
Mn
Mw/Mn %heta
1
2
3
4
2
2
4
4
CH2Cl2
THF
CH2Cl2
THF
3
10
20
90
95
91
91
90
16 800
18 100
96 500
40 200
1.19
1.19
2.23
1.70
59
67
49
85
a % heterotactic junction is defined by (sis + isi)/(sis + isi + iis + sii
+ iii) × 100%.
gesting some dynamic process, but attempts to freeze this
process failed as low-temperature spectra were not resolved.
On the basis of the crystal structure of 4, we propose that 4
t
remains dimeric in solution and the broadening of the Bu
peak in 1H NMR spectrum of 4 may arise from a reversible
coordination of the ether group to the magnesium (2+)
center.
Ring-Opening Polymerization of Lactides. The amide
and alkoxide complexes of zinc and magnesium are capable
of initiating the ring-opening polymerization (ROP) of rac-
lactides (100 equiv). For the zinc amide and alkoxide
complexes 1 and 2, the rate of initiation follows the order
OiPr > N(SiMe3)2, reflecting both steric and electronic
factors. The N(SiMe3)2 group is a more basic ligand than
the OiPr group, but its lone pair is more sterically protected.
A similar trend has been observed in the related bulky (BDI-
1)Zn alkoxide and amide complexes.7,8 Compound 2 is more
reactive than the related [(BDI-1)ZnOiPr]2 as a result of a
less-hindered ligand in 2 making the zinc atom more
accessible to lactides.10 The polymerization of rac-lactide
with 2 in CH2Cl2 is shown in Table 5, entry 1. By comparison
to the earlier work on (BDI-1) and (BDI-2) ligands, the order
of reactivity of â-diiminate zinc complexes containing (BDI-
1), (BDI-2), and (BDI-3) can be approximated. The reactivity
increases in the order (BDI-1) < (BDI-2) < (BDI-3),
reflecting the decrease of steric encumbrance from (BDI-1)
to (BDI-3).7,8,10 Although the ether groups could bind to the
Zn atom decelerating the polymerization, this chelating effect
is not predominant in a soft metal such as Zn. The molecular
weight of the polymer is 16.8 kDa, which is close to the
expected value of 13.7 kDa. The polymer has a narrow
polydispersity index (Mw/Mn) of 1.19. The polymer shows a
59% heterotactic junctions (isi + sis) as a result of a chain-
end control. The preference for heterotacticity in 2 is
significantly less than that (90%) of the related [(BDI-1)-
ZnOiPr]2.7 Because the (BDI-3) ligand is less sterically
demanding, the catalyst 2 is less discriminating to the
incoming lactide compared to [(BDI-1)ZnOiPr]2, causing a
less-profound stereoselectivity. Similar loss of heterotactic
The influence of solvent on the polymerization of rac-
lactide was also investigated. The polymerization of rac-
lactide in THF is generally slower than in dichloromethane
as a result of a competitive coordination between THF and
lactide to the catalysts (Table 5, entries 2 and 4). The
molecular weight of the polymer and the stereoselectivity
of catalyst 2 only change slightly in THF. The influence of
THF is, on the other hand, more profound for complex 4.
The rate of ROP is significantly slower. Nonetheless, the
polymerization is more controllable than in CH2Cl2 and PLA
with lower Mn and narrower PDI is produced. Furthermore,
the polymerization of rac-lactide using 4 in THF exhibits a
significant increase in stereoselectivity over CH2Cl2. The
extensive coordination of the ether groups and THF to the
8008 Inorganic Chemistry, Vol. 44, No. 22, 2005

ComparatiWe Study of Alkoxide and Amide Complexes
128.0; toluene-d8, 20.4 for 13C{1H}). Elemental analyses were done
by Atlantic Microlab, Inc. Gel permeation chromatography (GPC)
measurements were carried out using a Waters 1525 binary HPLC
pump and Waters 410 differential refractometer equipped with
styragel HR 2 and 4 columns (100 and 10 000 Å). The GPC was
eluted with THF at 35 °C running at 1 mL/min and was calibrated
using polystyrene standards.
(BDI-3)ZnN(SiMe3)2, 1. A solution of (BDI-3)H (0.500 g, 1.61
mmol) in 10 mL of benzene was added dropwise to a solution of
Zn[N(SiMe3)2]2 (0.622 g, 1.61 mmol) in 5 mL of benzene at room
temperature. The reaction was stirred for 10 min, and any volatile
components were removed under a dynamic vacuum giving a light
yellow solid in quantitative yield. Crystals suitable for single-crystal
X-ray crystallography were obtained by placing a concentrated
hexanes solution in a freezer. CCDC 217760. Anal. Calcd for
C25H39N3O2Si2Zn: C, 56.14; H, 7.29; N, 7.86. Found: C, 56.15;
H, 7.15; N, 7.55. 1H NMR (C6D6): 6.96-7.01 (m, 4H, ArH), 6.85
(t, 2H, J ) 7.6 Hz, ArH), 6.59 (d, 2H, J ) 8.4 Hz, ArH), 4.88 (s,
1H, â-CH), 3.34 (s, 6H, OMe), 1.76 (s, 6H, R-Me), 0.04 (s, 18H,
SiMe3). 13C{1H} NMR (C6D6): 168.81 (CdN); 153.17, 138.04,
126.74, 126.11, 120.82, 111.36 (Ar-C); 96.32 (â-C), 54.80 (OCH3),
23.35 (R-CH3), 4.78 (Si(CH3)3).
Figure 5. 1H NMR spectra (CDCl3, 400 MHz) of the homodecoupled
CH resonance of poly(rac-lactide) prepared in CH2Cl2 and THF using (a)
(BDI-3)ZnOiPr, 2, and (b) [(BDI-3)MgOtBu]2, 4, as initiators.
magnesium core is proposed to be responsible for the higher
stereoselectivity, as was seen for the (BDI-1)MgOtBu‚THF
complex.8
(BDI-3)ZnOiPr, 2. HOiPr (72 µL, 0.94 mmol) was added
dropwise to a solution of (BDI-3)ZnN(SiMe3)2 (0.500 g, 0.934
mmol) in 15 mL of benzene and stirred for 30 min. The reaction
mixture was dried in vacuo giving a light yellow solid (0.361 g,
89%). Anal. Calcd for C22H28N2O3Zn: C, 60.93; H, 6.46; N, 6.46.
Found: C, 60.46; H, 6.45; N, 6.38. 1H NMR (C6D6): 6.99 (m, 4H,
ArH), 6.79 (t, 2H, J ) 7.5 Hz, ArH), 6.63 (d, 2H, J ) 8.6 Hz,
ArH), 4.85 (s, 1H, â-CH), 3.97 (br, 1H, CHMe2), 3.34 (s, 6H, OMe),
1.74 (s, 6H, R-Me), 1.16 (br, 6H, CHMe2). 13C{1H} NMR (C6D6):
168.50 (CdN); 153.45, 140.45, 126.77, 124.72, 121.76, 112.31
(Ar-C); 95.15 (â-C), 65.90 (OCHMe2), 55.12 (OMe), 27.92
(OCHMe2), 23.48 (R-CH3).
(BDI-3)MgN(SiMe3)3, 3. A solution of (BDI-3)H (0.500 g, 1.61
mmol) in 10 mL of benzene was added dropwise to a solution of
Mg[N(SiMe3)2]2 (0.556 g, 1.61 mmol) in 5 mL of benzene. The
solution was stirred at room temperature for 10 min and dried in
vacuo giving an orange solid (0.602 g, 92%). Crystals suitable for
single-crystal X-ray crystallography were obtained by placing a
concentrated hexanes solution in a freezer. CCDC 217758. Anal.
Calcd for C25H39N3O2Si2Mg: C, 60.81; H, 7.90; N, 8.51. Found:
C, 60.55; H, 7.65; N, 8.47. 1H NMR (C6D6): 6.99 (d, 2H, J ) 8.0
Hz, ArH), 6.87 (t, 2H, J ) 8.0 Hz, ArH), 6.75 (t, 2H, J ) 7.8 Hz,
ArH), 6.54 (d, 2H, J ) 8.1 Hz, ArH), 5.08 (s, 1H, â-CH), 3.42 (s,
6H, OMe), 2.06 (s, 6H, R-Me), 0.21 (SiMe3). 13C{1H} NMR
(C6D6): 164.68 (CdN); 151.49, 140.36, 123.88, 121.41, 120.64,
109.38 (Ar-C); 100.28 (â-C), 54.48 (OCH3), 23.10 (R-CH3), 4.38
(Si(CH3)3).
Conclusions
This work demonstrates the application of the chelating
â-diimine (BDI-3) ligand in lactide polymerization. The
potential advantage of the (BDI-3) ligand lies in its ability
to expand its coordination from two to three and four. The
influence of the ether groups is less pronounced with the
soft-metal zinc where ether coordination to the metal is less
significant. On the other hand, their coordination to the hard-
metal magnesium is more aggressive, affecting the solid-
state structures and the catalytic activity and stereoselectivity.
By comparison to the related bidentate â-diiminate ligands,
the ether groups in (BDI-3) ligand in conjunction with Mg
atom are believed to promote the formation of heterotactic
bias in the polymerization of rac-lactide. We propose that
this is a result of a hemi-labile ether-donor binding in a
manner akin to that previously seen in THF solution for
(BDI-1)MgOR initiators.
Experimental Section
General Considerations. The manipulation of air-sensitive
compounds involved the use of anhydrous solvents and dry and
oxygen-free nitrogen with standard Schlenk line and drybox
techniques. rac-Lactide was purchased from Aldrich and was
sublimed three times prior to use. Tetrahydrofuran, dichloromethane,
hexanes, and toluene were distilled under nitrogen from sodium/
benzophenone, calcium hydride, potassium metal, and sodium
metal, respectively. The (BDI-3)H ligand,13 Zn[N(SiMe3)2]2,23 and
Mg[N(SiMe3)2]224 were prepared according to literature procedures.
(BDI-3)2Mg. A solution of (BDI-3)H (0.500 g, 1.61 mmol) and
Mg[N(SiMe3)2]2 (0.278 g, 0.805 mmol) in 15 mL of benzene was
refluxed for 20 min. The solution was dried in vacuo giving an
orange solid in quantitative yield. Anal. Calcd for C38H42N4O4Mg:
C, 71.01; H, 6.53; N, 8.72. Found: C, 70.79; H, 6.48; N, 8.66. 1H
NMR (C6D6): 6.96 (d, 2H, J ) 7.6 Hz, ArH), 6.70-6.79 (m, 4H,
ArH), 6.33 (d, 2H, J ) 7.8 Hz, ArH), 4.73 (s, 1H, â-CH), 3.29 (s,
6H, OMe), 1.89 (s, 6H, R-Me). 13C{1H} NMR (C6D6): 164.89 (Cd
N); 151.69, 140.57, 124.08, 121.62, 120.85, 109.59 (Ar-C); 100.48
(â-C), 54.69 (OMe), 23.30 (R-CH3).
t
i
Anhydrous BuOH and PrOH were purchased from Aldrich and
used as received.
Measurements. 1H (400.1 MHz) and 13C{1H} (100.6 MHz)
spectra were recorded in C6D6 and toluene-d8 on Bruker DPX-400
NMR spectrometers and were referenced to the residual protio
impurity peak (C6D6, δ 7.15; toluene-d8, 2.09 for 1H and, C6D6, δ
[(BDI-3)MgOtBu]2, 4. HOtBu (97 µL, 1.0 mmol) was added
dropwise to a solution of (BDI-3)MgN(SiMe3)2 (0.500 g, 1.01
mmol) in 15 mL of benzene and stirred for 30 min. The reaction
mixture was dried in vacuo giving a yellow solid (0.349 g, 85%).
(23) Darensbourg, D. J.; Holtcamp, M. W.; Struck, G. E.; Zimmer, M. S.;
Niezgoda, S. A.; Rainey, P.; Robertson, J. B.; Draper, J. D.;
Reibenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107-116.
(24) Kennedy, A. R.; Mulvey, R. E.; Rowlings, R. B. J. Am. Chem. Soc.
1998, 120, 7816-7824.
Inorganic Chemistry, Vol. 44, No. 22, 2005 8009

Chisholm et al.
Crystals suitable for single-crystal X-ray crystallography were
obtained by placing a concentrated THF solution in a freezer. CCDC
217759. Anal. Calcd for C23H30N2O3Mg: C, 67.95; H, 7.38; N,
6.89. Found: C, 67.34; H, 7.26; N, 6.84. 1H NMR (C6D6): 6.90-
7.10 (m, 6H, ArH), 6.70 (d, 2H, J ) 7.9 Hz, ArH), 4.99 (s, 1H,
â-CH), 3.36 (s, 6H, OMe), 1.83 (s, 6H, R-Me), 1.05 (br, 9H, tBu).
13C{1H} NMR (C6D6): 169.08 (CdN); 153.38, 140.92, 126.48,
124.34, 121.78, 111.81 (Ar-C); 95.89 (â-C), 67.24 (OCMe3), 55.16
(OCH3), 33.30 (OCMe3), 23.61 (R-CH3).
General Polymerization Procedure. rac-Lactide (0.500 g, 3.47
mmol) was dissolved in 6.00 mL of CH2Cl2 or THF. A solution of
the corresponding catalyst precursor (3.47 × 10-5 mol) in 1.50 mL
of CH2Cl2 or THF was then added to the lactide solution (100:1
[lactide]/[catalyst]). The reaction was stirred at room temperature
for the desired time, after which small aliquots were taken to
monitor the conversion. When the conversion was greater than 90%,
the polymerization was quenched with excess methanol. The
polymer precipitate was then filtered and dried under vacuum to
constant weight.
X-Ray Crystallography. Single-crystal X-ray diffraction data
were collected on a Nonius Kappa CCD diffractometer at low
temperature using an Oxford Cryosystems Cryostream Cooler.
Crystals were coated with oil prior to being placed in the nitrogen
gas stream. The data collection strategy was set up to measure a
hemisphere of reciprocal space for compounds 1, a quadrant for 3,
and 4 with a redundancy factor of 2.3, 3.9, and 3.6, which means
that 90% of the reflections were measured at least 2.3, 3.9, and 3.6
times, respectively. A combination of φ and ω scans with a frame
width of 1.0° was used. Data integration was done with Denzo.25
Scaling and merging of the data were done with Scalepack.25 The
structures were solved by either the Patterson method or direct
methods in SHELXS-86.26 Full-matrix least-squares refinements
based on F2 were performed SHELXL-93.27 The methyl-group
hydrogen atoms were added at calculated positions using a riding
model with U(H) ) 1.5 × Ueq (bonded C atom). For each methyl
group, the torsion angle which defines its orientation about the C-C
bond was refined. The other hydrogen atoms were included in the
model at calculated positions using a riding model with U(H) )
1.2 × Ueq (bonded C atom). Neutral-atom scattering factors were
used and include terms for anomalous dispersion.28
Acknowledgment. We thank the Department of Energy,
Office of Basic Sciences, Chemical Sciences Division for
financial support of this work. K.P. acknowledges the
Institute for the Promotion of Teaching Science and Tech-
nology (IPST), Thailand, for an opportunity to work on this
project.
Supporting Information Available: Crystallographic data for
compounds 1, 3, and 4 in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC048363D
(25) Otwinowski, Z.; Minor, W. Methods in Enzymology, Macromolecular
Crystallography, part A; Carter, C. W., Jr., Sweet, R. M., Eds.;
Academic Press: New York, 1997; Vol. 276, pp 307-326.
(26) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473.
(27) Sheldrick, G. M. Universitat Gottingen, Germany 1993.
(28) International Tables for Crystallography; Kluwer Academic Publish-
ers: Dordrecht, 1992; Vol. C.
8010 Inorganic Chemistry, Vol. 44, No. 22, 2005