Three-coordinate zinc amide and phenoxide complexes supported by a schiff base ligand

Article abstract of DOI:10.1021/ic010560e

Three-coordinate zinc complexes are potential catalysts for the ring-opening of cyclic monomers such as lactide and oxiranes. By use of a bulky Schiff base ligand, monomeric zinc amide and phenoxide are prepared and characterized by X-ray crystallography.

Full text of DOI:10.1021/ic010560e

Inorg. Chem. 2001, 40, 5051-5054
5051
Coates group was that the isopropoxide species shown in B was
capable of polymerizing rac-lactide to give mostly heterotactic
polylactide, PLA. Given that the compound shown in B is not
chiral, this result was attributed to chain end-group control
wherein a lactide molecule with R,R-stereocenters preferentially
inserts into a Zn-(S,S) chain and vice versa. In the CHO-CO2
copolymerization, it was shown by polymer degradation that
ring-opening of CHO occurred with inversion of configuration,
i.e., by backside attack.
In this paper, we describe our preparation and characterization
of some new zinc Schiff-base complexes of the form LZnX,
where X ) an aryloxide or amide, and L2Zn, where L is one of
the Schiff-base ligands shown in C and D. The reader may note
that the ligand C is related to the BDI ligand employed by
Coates et al. in their studies of A and B.
Three-Coordinate Zinc Amide and Phenoxide
Complexes Supported by a Bulky Schiff Base
Ligand
Malcolm H. Chisholm,* Judith C. Gallucci, and
Hongshi Zhen
Department of Chemistry, The Ohio State University,
Newman & Wolfrom Laboratories, 100 West 18th Avenue,
Columbus, Ohio 43210-1185
John C. Huffman
Molecular Structure Center, Indiana University,
800 East Kirkwood Avenue,
Bloomington, Indiana 47405
ReceiVed May 29, 2001
Introduction
There is considerable current interest in the ring-opening
polymerization, ROP, of lactides and epoxides by discrete
coordination complexes. The Darensbourg1 group have had
considerable success in ROP of cyclohexene oxide and the
copolymerization of cyclohexene oxide, CHO, with CO2 by the
use of zinc phenoxide precursors of the form Zn(OAr′)2L2,
where OAr′ is a bulky phenoxide such as 2,6-tBu2C6H3O and
L is a labile ligand such as THF, Et2O, or a tertiary phosphine.
In their early work, it was not clear that mononuclear zinc
species were catalytically active, and from more recent work
from that laboratory where Ar′ is 2,6-X2C6H3- and X ) F, Cl,
and Br, it appears that dimeric species are the active species.
Bridged complexes of the type [L(Ar′O)Zn(µ-OAr′)]2 were
structurally characterized, and the polymerization process was
found to be dependent on the nature of OAr′, a fact which
implicated their continued presence at the active site. While one
OAr′ is used to initiate polymerization and becomes an end
group, the other is involved in bridging to a neighboring zinc
atom.
Experimental Section
General. All reactions, unless otherwise mentioned, were carried
out under an atmosphere of N2 purified over 4 Å molecular sieves.
Flamed-out glassware and standard vacuum line, Schlenk, and N2-
atmosphere drybox techniques were employed. Benzene, toluene, ether,
hexanes, and THF were distilled from purple solutions of sodium/
benzophenone. Propylene oxide was distilled over calcium hydride,
and carbon dioxide was purchased from BOC Gases. Bis(trimethyl-
silylamide) sodium salt and ZnCl2 were purchased from Aldrich and
used as received. Bis(trimethylsilylamide) zinc was prepared by reported
procedures.1 Lactide was sublimed twice prior to use. Elemental
analyses were performed by Atlantic Microlab Inc. Free Schiff-base
ligands were prepared by acid-catalyzed condensation following a
literature procedure.6
(LC)ZnN(SiMe3)2 (I). To a solution of 0.38 g (1.0 mmol) of Zn-
(NSi2Me6)2 in 20 mL of toluene at -78 °C was slowly added 0.39 g
(1.0 mmol) of free ligand A in 20 mL of toluene. The reaction mixture
was allowed to warm to 22 °C and stirred for 3 h. Volatile components
were evaporated under a dynamic vacuum, and the resulting yellow
solid was washed with 3 × 5 mL of cold hexanes. Yield: 0.50 g, 81%.
Subsequent to the initial submission of this work, the Darensbourg1c
group also reported the preparation of this complex.
The Coates2 group has employed the BDI ligand (2-[(2,6-
diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-
2-ene) as a spectator group bonded to zinc, and the dimeric
acetate and isopropoxide shown in A and B have been used in
(LD)2Zn (II). To a solution of 0.39 g (1.0 mmol) of Zn(NSi2Me6)2
in 10 mL of toluene at -78 °C was slowly added 0.33 g (1.0 mmol)
of free ligand D in 15 mL of toluene solution. The reaction mixture
was allowed to warm to 22 °C and stirred for 3 h. Volatile components
were evaporated under vacuum, and a yellow solid was obtained.
Yield: 0.34 g, 94%. H NMR (benzene-d6, 400 MHz): δ 0.65 (bs,
12H, CH(CH3)2), 0.92 (d, J ) 6.0 Hz, 12H, CH(CH3)2), 2.86 (m, 4H,
1
the copolymerization of CHO and CO2 and in lactide polym-
erization, respectively. A particularly interesting finding of the
(1) (a) 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. (b) Darensbourg,
D. J.; Wildeson, J. R.; Yarbrough, J. C.; Reibenspies, J. H. J. Am.
Chem. Soc. 2000, 122, 12487. (c) Darensbourg, D. J.; Rainey, P.;
Yarbrough, J. Inorg. Chem. 2001, 40, 986-993.
(2) (a) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
1998, 120, 11018. (b) Cheng, M.; Attygalle, A. B.; Lobkovsky, E.
B.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 11583.
(3) Looney, A.; Han, R.; Gorrel, I. B.; Cornebise, M.; Yoon, K.; Parkin,
G.; Rheingold, A. L. Organometallics 1995, 14, 274 and references
therein.
(4) Chisholm, M. H.; Huffman, J. C.; Phomprai, K. J. Chem. Soc., Dalton
Trans. 2001, 222-224.
(5) Antelmann, B.; Chisholm, M. H.; Iyer, S. S.; Huffman, J. C.; Navarro,
D. L.; Pagel, M. Macromolecules 2001, 34 (10), 3159-3175.
(6) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.;
Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149.
10.1021/ic010560e CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/15/2001

5052 Inorganic Chemistry, Vol. 40, No. 19, 2001
Notes
CH(CH3)2), 6.58 (s, 1H), 6.60 (s, 1H, CH), 6.86 (s, 2H, CH), 6.88 (s,
2H, CH), 6.98 (m, 2H, CH), 7.24 (s, 2H, CH), 7.70 (d, J ) 2.8 Hz,
2H, CH), 7.86 (d, J ) 3.2 Hz, 2H, CH), 7.89 (d, J ) 3.2 Hz, 2H, CH).
13C{H} (benzene-d6, 100 MHz): δ 23.0, 24.6, 28.3, 116.1, 123.8, 124.5,
127.9, 128.5, 131.1, 134.1, 141.2, 145.4, 174.8, 176.4. Anal. Calcd for
C38H42N4O6Zn: C, 63.73; H, 5.91; N, 7.82. Found: C, 63.95; H, 6.09;
N, 7.63.
(LC)Zn(2,6-di-tbutylphenoxide) (III). To a mixture of I, 0.41 g (0.66
mmol), and 2,6-di-tbutylphenol, 0.14 g (0.66 mmol), was added 30 mL
of benzene. The mixture was stirred overnight at ambient temperature.
Volatile components were evaporated under vacuum, and the resulting
yellow solid was washed with 3 × 5 mL of cold hexanes. Yield: 0.37
g, 84%. 1H NMR (benzene-d6, 400 MHz): δ 0.91 (d, J ) 6.9 Hz, 6H,
CH(CH3)2), 1.19 (d, J ) 6.9 Hz, 6H, CH(CH3)2), 1.25 (s, 9H, C(CH3)3),
1.46 (s, 9H, C(CH3)3), 1.64 (s, 18H, C(CH3)3), 2.99 (m, 2H, CH(CH3)2),
6.77(s, 1H), 6.90 (m, 1H) 7.02-7.15 (m, 3H), 7.34 (s, 1H), 7.36 (s,
1H), 7.69 (s, 1H), 7.88 (s, 1H). 13C{H} (benzene-d6, 100 MHz): δ
23.5, 25.0, 28.9, 29.6, 31.3, 31.6, 34.1, 35.4, 35.7, 116.9, 117.4, 124.4,
125.2, 128.5, 130.2, 132.7, 138.3, 139.5, 141.5, 143.1, 145.1, 164.6,
170.2, 176.5. Anal. Calcd for C41H59NO2Zn: C, 74.24; H, 8.96; N,
2.11. Found: C, 73.84; H, 8.96; N, 2.08.
Table 1. Selected Crystallographic Data for I-III
II
C33H56N2OSi2Zn C38H42N4O6Zn C41H59NO2Zn
I
III
empirical
formula
fw
618.38
triclinic
P1h
716.14
monoclinic
P21/c
14.501(4)
14.379(4)
20.191(4)
90
107.00(1)
90
4031.20
4
663.26
orthorhombic
P212121
10.8679(1)
18.0093(1)
19.5705(1)
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
vol (Å3)
Z
10.7872(3)
10.8787(3)
17.4470(5)
78.847(1)
88.358(1)
64.149(1)
1804.14
2
3830.40
4
calcd density 1.138
1.309
1.150
(g/cm3)
temp (K) of
collection
indep reflns
R
113
115
200
8286
9259
8782
0.0280a
0.0321a
0.0277a
0.0254a
0.0405b
0.0807b
-0.014(7)
Rw
(LC)2Zn (IV). I (0.33 g, 0.53 mmol) in a Schlenk flask was heated
overnight at 150 °C under vacuum, and Zn(NSi2Me6)2 was distilled
out and trapped. A bright yellow solid was obtained. Yield: 0.21 g,
Flack param
a Refinement on F with I >2.33σ(I) and R(F) ) ∑||Fo| - |Fc||/
2
2
∑|Fo|. b Refinement on F2 with all data and Rw ) [∑w(|Fo| - |Fc| )2/
1
93%. H NMR (benzene-d6, 250 MHz): δ 0.28 (d, 6H, J ) 6.8 Hz,
∑w(Fo )2]1/2
.
2
CH(CH3)2), 0.90 (d, 6H, J ) 6.8 Hz, CH(CH3)2), 1.20 (s, 18H, CH-
(CH3)2), 1.22 (d, J ) 6.8 Hz, 6H, CH(CH3)2), 1.23 (d, J ) 6.8 Hz, 6H,
CH(CH3)3), 1.62 (s, 18H, C(CH3)3), 2.65 (m, 2H, CH(CH3)2), 3.94 (m,
2H, CH(CH3)2), 6.74-7.86 (m, 12H, CH). 13C{H} (benzene-d6, 62.9
MHz): δ 22.7, 23.7, 25.0, 25.1, 28.1, 28.5, 30.1, 31.4, 33.9, 35.8, 117.2,
124.4, 124.6, 127.0, 130.3, 131.6, 135.7, 142.0, 142.1, 142.2, 147.6,
171.4, 176.2. Anal. Calcd for C54H76N2O2Zn: C, 76.25; H, 9.01; N,
3.29. Found: C, 75.34; H, 9.01; N, 3.27.
using the Bruker SAINT software as well as utility programs from the
XTEL library. An absorption correction was performed using the
SADABS program supplied by Bruker AXS. The structures were solved
using SHELXTL and Fourier techniques. All hydrogen atoms were
clearly visible and were included as isotropic contributors in the final
cycles of refinement. The data collection of III was done on a Nonius
Kappa CCD diffractometer using 100 s frames with ω scans of 1.0°.
Data integration was done with Denzo,9 and scaling and merging of
the data were done with Scalepack.10 The structure was solved by the
Patterson method in SHELXS-86.11 Full-matrix least-squares refine-
ments based on F2 were performed in SHELXL-93.12 The hydrogen
atoms were included in the model at calculated positions using a riding
model. Refinement of the Flack parameter13 verified that this choice
of enantiomer is correct. A summary of crystal data is given in Table
1.
Polymerization of L-Lactide with I. To a mixture of 12 mg of I
and 60 mg of L-lactide (ca. 20 equiv) in a NMR tube was added 0.5
mL of benzene-d6. The reaction was then monitored by 1H NMR
spectroscopy. Over 80% of the monomer was converted to PLA in 3
h. 1H and 13C NMR data were consistent with the formation of isotactic
PLA.7
Polymerization of rac-Lactide with I. To a mixture of 12 mg of I
and 60 mg of rac-lactide (ca. 20 equiv) in a NMR tube was added 0.5
mL of benzene-d6. The polymerization reaction was significantly slower
than that with L-lactide. The solvent was then removed under vacuum,
Results and Discussion
1
and the polymer was redissolved in CDCl3. The proton-decoupled H
NMR and the 13C{1H} NMR spectra of the polymer showed that there
was no stereoselectivity for the polymerizarion of rac-lactide with
compound I.
Synthesis of Zinc Complexes. Preparation of the new
compounds was carried out in toluene employing Zn(NSi2Me6)2
and 1 equiv of the Schiff base. In the case of the protonated
ligand shown in C, hereafter referred to as LCH, the reaction
proceeded to give (LC)Zn(NSi2Me6) (I) and free HNSi2Me6.
However, for the Schiff base shown in D, LDH, the reaction
did not stop at the monosubstituted product, but rather the bis-
Schiff base complex (LD)2Zn (II) and the starting material amide
were obtained. The differing reactivity is attributed to steric
factors, and even compound I undergoes disproportionation upon
heating to 150 °C under vacuum. Under these conditions,
Reaction of I with Propylene Oxide. PO (4.0 mL) was added to a
Schlenk flask equipped with 47 mg of I and a stir bar. The mixture
was then stirred for 3 days at ambient temperature. No PPO was
obtained.
Reaction of I with CO2. Carbon dioxide was introduced into a
solution of 0.11 g (0.18 mmol) of I in 10 mL of benzene. The reaction
mixture was stirred overnight at ambient temperature, during which a
yellow precipitate formed. Volatile components were removed under
vacuum and a yellow solid was obtained. Its IR spectrum did not show
a characteristic CdO stretching frequency. This material had low
solubility in common solvents such as benzene, THF, and methanol.
Attempts to obtain crystals suitable for X-ray crystal analysis were not
successful.
Copolymerization of Propylene Oxide and CO2 with I. A stainless
steel reactor was charged with a stir bar and 0.15 g (0.24 mmol) of I.
After 8.5 mL (121.5 mmol) of PO was added, carbon dioxide was then
introduced. The mixture was stirred overnight at 22 °C under a CO2
pressure of 500 psi. No copolymer was obtained after workup of the
reaction mixture.
X-ray Crystallography.14 For crystals I and II, the data were
collected using 5 s frames with an ω scan of 0.30°. Data were corrected
for Lorentz and polarization effects and equivalent reflections averaged
(8) Chisholm, M. H.; Iyer, S. S.; McCollum, D. G.; Pagel, M.; Werner-
Zwanziger, U. Macromolecules 1999, 32, 963.
(9) DENZO: Otwinowski, Z.; Minor, W. Macromolecular Crystal-
lography; Carter, C. W., Jr., Sweet, R. M., Eds.; Methods in
Enzymology 276; Academic Press: New York, 1967; Part A, pp 307-
326.
(10) teXsan: Crystal Structure Analysis Package, version 1.7-2; Molecular
Structure Corporation: The Woodlands, TX, 1995.
(11) SHELXS-86: Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-
473.
(12) SHELXS-93: Sheldrick, G. M., Universita¨t Go¨ttingen, Germany, 1993.
(13) International Tables for Crystallography; Kluwer Academic Publish-
ers: Dordrecht, 1992; Vol. C.
(14) The supplementary crystallographic data were passed to the Cambridge
Crystallographic Data Center (CCDC). The CCDC Deposition Num-
bers are 159076-159078 (deposit@ccdc.cam.ac.uk).
(7) Chisholm, M. H.; Eilerts, N. W. Chem. Commun. 1996, 853.

Notes
Inorganic Chemistry, Vol. 40, No. 19, 2001 5053
Table 2. Selected Bond Distances (Å)and Angles (deg) for
Complexes I-III
Complex I
Zn1-O2
Zn1-N10
Zn1-N31
1.8859(19)
1.9805(23)
1.8689(24)
O2-Zn1-N10
O2-Zn1-N31
N10-Zn1-N31
95.99(9)
127.26(9)
136.69(10)
Complex II
Zn1-O22
Zn1-O46
Zn1-N2
1.9413(22)
1.9373(22)
1.9952(26)
1.9994(25)
O22-Zn1-O46
O22-Zn1-N2
O22-Zn1-N26
O46-Zn1-N26
O46-Zn1-N2
N2-Zn1-N26
106.33(10)
95.30(10)
113.63(10)
94.91(10)
115.43(10)
130.26(11)
Zn1-N26
Figure 1. Molecular structure of compound I, (LC)ZnN(SiMe3)2.
Complex III
Zn1-O1
Zn1-O2
Zn1-N1
1.872(1)
1.818(1)
1.962(2)
O1-Zn1-N1
O2-Zn1-O1
O2-Zn1-N1
99.06(6)
128.75(6)
131.94(6)
base ligands are less different though the Zn-O bond to the
phenoxide is still shorter. The complexes are trigonal planar
with the O-Zn-N angles of the Schiff base being less than
100°. In complex I, the O-Zn-N angle of 96° is very similar
to that seen for the N-Zn-N angle of the BDI ligand in (BDI)-
Zn(NSi2Me6). In compound II, the four-coordinate and pseudo-
tetrahedral zinc(II) ion has somewhat longer Zn-O and Zn-N
distances as expected for a metal ion with a higher coordination
number.
Solution 1H NMR Spectra. The 1H NMR spectra for
compounds I and III are particularly interesting in allowing
for a comparison with related BDI complexes. The isopropyl
methyl groups of the Schiff base ligands shown in C and D are
equivalent as long as there is rapid rotation on the NMR time
Figure 2. Molecular structure of compound II, (LD)2Zn.
1
scale about the N-C(phenyl) bond. The H NMR spectra of
the free ligands, C and D, show just one isopropyl methyl
doublet. However, when complexed to the zinc ion in com-
pounds I and III, the ligand LC displays two sets of isopropyl
doublets but only one isopropyl methine septet, indicating that
upon complexation the rotation about the N-C(phenyl) bond
is slow or frozen out on the NMR time scale. This is similar to
that seen for BDI isopropyl methyl groups in the complexes
shown in A and B.4 However, even the free ligand BDI shows
two isopropyl methyl doublets, and this is presumably because
the methyl groups of the â-diiminato backbone CH(C(Me)N-
2,6-iPr2C6H3)2 hinder rotation in relation to the less sterically
demanding CH group of the Schiff bases C and D.
Complexes II and IV, which contain four-coordinate zinc-
(II) ions, are chiral. In these instances there are four sets of
isopropyl doublets in the low-temperature spectra of II and for
the room-temperature spectrum of IV. Since the molecules have
only C2 symmetry, the isopropyl methyl groups are diaste-
reotopic and we therefore should expect to see two doublets
for the isopropyl methyl group even when rotation about the
N-C(phenyl) bond is rapid. If this is frozen out, we may see
four sets of doublets.
Figure 3. Molecular structure of compound III (LC)Zn(2,6-
di-tbutylphenoxide).
Zn(NSi2Me6)2 distills, leading to Zn(LC)2 (IV). Compound I
and the bulky phenol 2,6-tBu2C6H3OH react in benzene to give
(LC)Zn(OC6H3-2,6-tBu2) (III).
The new compounds are air-sensitive, crystalline materials
and are soluble in aromatic hydrocarbon solvents. Compounds
I, II, III, and IV are all yellow. It may be noted that the carbon
content analysis of IV was low, although NMR spectra indicated
that it was pure.
Solid-State and Molecular Structures. A summary of
crystallographic data is given in Table 1, and ORTEP drawings
of the molecular structures of compounds I, II, and III are given
in Figures 1, 2, and 3, respectively. The compounds (LC)Zn(NSi2-
Me6) and (LC)Zn(OC6H3-2,6-tBu2) provide relatively rare ex-
amples of three-coordinate monomeric planar zinc(II) com-
plexes.3 A summary of selected bond distances and angles is
given in Table 2. In compound I, the Zn-N distance to the
silylamide is notably shorter than to the Schiff base nitrogen.
In III, the Zn-O distances to the 2,6-tBu2C6H3O- and Schiff
1
When the temperature is raised, the H NMR spectra of II
show that there is a pairwise coalescence of the isopropyl
methine and methyl signals giving rise to one septet and two
doublets at 50 °C. With further heating the latter start to broaden
and approach coalescence at 100 °C. These changes are shown
in Figure 4. At high temperatures we propose that we are
observing the reversible dechelation of the Schiff base ligand
as represented by eq 1. The dissociation of the Zn-N bond
will generate a three-coordinate trigonal zinc(II) species which
will be akin to complexes I and III. Thus, with free rotation
about the N-C(phenyl) bond, we expect to see just one
isopropyl doublet. This situation parallels that recently described

5054 Inorganic Chemistry, Vol. 40, No. 19, 2001
Notes
rac-lactide. In our studies, the ratio of [lactide] to [Zn] was ca.
20:1 and polymerization occurred slowly at 25 °C proceeding
to 90% conversion in ca. 3 h for I and ca. 72 h for III. The
difference in reactivity between those two compounds can be
traced to the rate of initiation which is notably slower for the
bulky 2,6-tert-butylphenoxide. The lack of stereoselectivity in
the polymerization of rac-lactide by I and III when compared
to the polymerization induced by [(BDI)Zn(OiPr)]2b presumably
reflects the less sterically hindered metal center when coordi-
nated by the Schiff base ligand LC relative to its â-diiminato
counterpart, BDI.
Compounds I and III showed no reactivity toward ring-
opening of propylene oxide at room temperature, nor did we
observe any copolymerization in attempted reactions employing
PO and CO2 under ambient conditions. Upon heating com-
pounds I and III in solution to 50 °C and above, ligand
scrambling was observed yielding (LC)2Zn (IV) along with either
Zn(NSi2Me6)2 or Zn(OC6H3-2,6-tBu2)2. The occurrence of this
Schlenk equilibrium precludes the use of I and III as single-
site catalysts for ROP of epoxides and copolymerization of
epoxides with CO2. The formation of Zn(OAr′)2 effectively
yields a catalyst system analogous to that employed by
Darensbourg et al.1 The bis-Schiff base complexes, II and IV,
are inactive for ROP of PO and lactides, at least under mild
conditions.
Concluding Remarks
Figure 4. Variable-temperature 1H NMR spectra of the methyl groups
in compound II (in toluene-d8).
The use of the sterically demanding Schiff base ligand shown
in C has allowed the preparation of the three-coordinate
monomeric zinc amide and phenoxide complexes I and III.
These compounds are, however, thermally sensitive to ligand
scrambling upon heating both in the solid state and in solution.
While lactide polymerization can be initiated, the ring-opening
of PO is not seen. There is now increasing evidence to support
the view that ROP of lactides may be achieved at a single metal
center, but ROP of PO has not been documented to occur in a
similar manner. Only strong Lewis acidic metal centers are
known to initiate cationic type polymerization of PO, which
gives regioirregular PPO, unlike base catalysis, which generates
regioregular (HTHTHT) triads.5 The present work is, however,
of interest in comparison to the (BDI)Zn systems developed by
Coates and co-workers2 in revealing the importance of the
greater steric demands of the BDI ligands.
for the BDI complex (BDI)Mg(OtBu)(THF) which reversibly
dissociates THF in solution according to eq 2.4 At low
temperatures, the BDI ligand shows four isopropyl methyl
groups, but, when dissociation of THF occurs on increasing the
temperature and the equilibrium shown in eq 2 is driven to the
right, there are two isopropyl methyl groups. Again the
difference between the Schiff base ligands and the BDI ligand
can be attributed to the lack of rotation about the N-C(phenyl)
bond in the latter, even at high temperatures.
Polymerization Studies. The focus of our attention has been
the reactivity of the monomeric three-coordinate zinc amide and
phenoxide compounds, I and III, respectively, as precursors
for lactide and propylene oxide polymerization.
Acknowledgment. We thank Dr. Karl Vermillion for his
help with the variable-temperature NMR experiments.
Supporting Information Available: Crystallographic data in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
Both I and III were found to ring-open L-lactide to produce
isotactic PLA in benzene at room temperature. Similarly, I and
III catalyzed polymerization of rac-lactide to give atactic poly-
IC010560E