Coordination chemistry and reactivity of monomeric alkoxides and amides of magnesium and zinc supported by the diiminato ligand CH(CMeNC6H3-2,6-i Pr2)2. A comparative study

Article abstract of DOI:10.1021/ic020148e

The preparation and characterization of a series of closely related magnesium and zinc compounds are reported: LMg(NⁱPr₂)(THF), 1; LZn(NⁱPr₂), 2; LMg(O^tBu)(THF), 3; LZn(O^tBu), 4; and LZn(OSiPh₃)(THF), 6; where L = CH(CMeNC₆H₃-2,6-ⁱ Pr₂)₂. Their dynamic solution behavior has been examined by variable-temperature NMR studies and reveals that THF reversibly dissociates in toluene-d₈ or CD₂Cl₂ and that exchange with free THF occurs by a dissociative process. Compounds 1-4 and 6 all initiate and subsequently sustain ring-opening polymerization (ROP) of lactides. For a related series of compounds LMX(THF)_n, where n = 1 or 0, the rate of initial ring-opening follows the order M = Mg > Zn and X = O^tBu > NⁱPr₂ > NSi₂Me₆ > OSiPh₃. In THF at 25 °C, compounds 3 and 4 polymerize 100 equiv of rac-lactide to >95% conversion in 5 and 80 min for M = Mg and Zn, respectively, and yield ca. 90% heterotactic PLA, (isi + sis tetrads). The reactions proceed faster in methylene chloride, but for M = Mg, a Bernoulian distribution of tetrads is formed from rac-lactide (3iii:2isi:sii:sis:iis) prior to trans-esterification. Polymerization of L-LA in toluene-d₈ and THF-d₈ by 3 and 4 have been studied by VT ¹H NMR spectroscopy: the resting state for zinc is proposed to be a monomeric species akin to LZn(η²-OCHMeC(O)OMe), whereas the magnesium complex appears to be dimeric LMg(μ-OP)₂MgL. None of the compounds is capable of initiating homopolymerization of propylene oxide (PO) or cyclohexene oxide (CHO), although the magnesium amide 1 effects ring-opening by allylic proton abstraction and the dimeric compound [LMg(μ-OC₆H₉)]₂, 7, is formed. Reactions with carbon dioxide are also described, along with the characterization of LZnO₂CNⁱPr₂, 8, which is shown to be inert with respect to CHO and PO at room temperature. All the compounds are hydrolytically sensitive, and LZn(μOH)₂ZnL, 5, has been isolated from hydrolysis of compound 4. The crystal and molecular structures are reported for compounds 1-5, 7, and 8. These results are compared with those recently reported by Coates et al.

Full text of DOI:10.1021/ic020148e

Inorg. Chem. 2002, 41, 2785−2794
Coordination Chemistry and Reactivity of Monomeric Alkoxides and
Amides of Magnesium and Zinc Supported by the Diiminato Ligand
i
CH(CMeNC H -2,6- Pr2)2. A Comparative Study
6 3
Malcolm H. Chisholm,* Judith Gallucci, and Khamphee Phomphrai
Department of Chemistry, Newman & Wolfrom Laboratories, The Ohio State UniVersity,
100 W. 18th AVenue, Columbus, Ohio 43210-1185
Received February 25, 2002
The preparation and characterization of a series of closely related magnesium and zinc compounds are reported:
i
i
t
t
LMg(NPr2)(THF), 1; LZn(NPr2), 2; LMg(OBu)(THF), 3; LZn(OBu), 4; and LZn(OSiPh3)(THF), 6; where L ) CH-
i
(CMeNC H -2,6-Pr2)2. Their dynamic solution behavior has been examined by variable-temperature NMR studies
6
3
and reveals that THF reversibly dissociates in toluene-d8 or CD Cl2 and that exchange with free THF occurs by a
2
dissociative process. Compounds 1−4 and 6 all initiate and subsequently sustain ring-opening polymerization (ROP)
of lactides. For a related series of compounds LMX(THF)n, where n ) 1 or 0, the rate of initial ring-opening follows
t
i
the order M ) Mg > Zn and X ) OBu > NPr2 > NSi2Me6 > OSiPh3. In THF at 25 °C, compounds 3 and 4
polymerize 100 equiv of rac-lactide to >95% conversion in 5 and 80 min for M ) Mg and Zn, respectively, and
yield ca. 90% heterotactic PLA, (isi + sis tetrads). The reactions proceed faster in methylene chloride, but for M
) Mg, a Bernoulian distribution of tetrads is formed from rac-lactide (3iii:2isi:sii:sis:iis) prior to trans-esterification.
Polymerization of L-LA in toluene-d8 and THF-d8 by 3 and 4 have been studied by VT 1H NMR spectroscopy: the
resting state for zinc is proposed to be a monomeric species akin to LZn(η2-OCHMeC(O)OMe), whereas the
magnesium complex appears to be dimeric LMg(µ-OP)2MgL. None of the compounds is capable of initiating
homopolymerization of propylene oxide (PO) or cyclohexene oxide (CHO), although the magnesium amide 1 effects
ring-opening by allylic proton abstraction and the dimeric compound [LMg(µ-OC H )]2, 7, is formed. Reactions with
6
i
9
carbon dioxide are also described, along with the characterization of LZnO CNPr2, 8, which is shown to be inert
2
with respect to CHO and PO at room temperature. All the compounds are hydrolytically sensitive, and LZn(µ-
OH)2ZnL, 5, has been isolated from hydrolysis of compound 4. The crystal and molecular structures are reported
for compounds 1−5, 7, and 8. These results are compared with those recently reported by Coates et al. (J. Am.
Chem. Soc. 2001, 123, 3229−3238).
Introduction
predictable. Indeed, there are relatively few examples of
closely related reactions involving both metals wherein the
coordination sphere is identical.4 In this paper, we set out to
describe the chemistry of closely related compounds of
magnesium and zinc supported by the â-diiminato ligand
CH(CMeNC6H3-2,6-iPr2)2 ) L. These compounds have the
formula LMX(THF)n, where X ) NiPr2, OtBu and n ) 1 or
0. We examine these monomeric compounds in terms of their
solid-state molecular structures, their dynamic solution
behavior, and their reactivity toward lactides, epoxides, and
carbon dioxide. These studies complement the recent elegant
The elements magnesium and zinc share many similar
properties. They form M2+ salts and have very similar ionic
radii.1 They are both essential for life, yet nature employs
them in very different ways.2 Magnesium (2+) can be
considered a hard metal, while zinc (2+) is soft.3 However,
just how this translates into chemical reactivity is not always
(1) Cotton, F. A.; Wilkinson, E.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999;
pp 1302-1304.
(2) Campbell, N. A. Biology, 3rd ed.; Benjamin/Cummings Publishing
Co.: California, 1993; pp 718-722, 811-814.
(3) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
iPrnciples of Structure and ReactiVity, 4th ed.; Harper Collins College
Publishers: New York, 1993; pp 344-348.
(4) Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C.; Iyer, S. S.; Pacold,
M.; Phomphrai, K. J. Am. Chem. Soc. 2000, 122, 11845-11854.
10.1021/ic020148e CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/25/2002
Inorganic Chemistry, Vol. 41, No. 10, 2002 2785

Chisholm et al.
work described by Coates’ group, who have studied a family
of â-diiminato zinc isopropoxide complexes, [L′Zn(µ-OiPr)]2,
and shown these to be active in the ring-opening polymer-
ization (ROP) of lactides5 and the copolymerization of
cyclohexene oxide (CHO) and carbon dioxide.6 Brief mention
has also been made by both Coates and us7 of related
magnesium alkoxides in the ROP of lactides.
Results and Discussion
Syntheses. The synthesis of related magnesium and zinc
compounds by metathetic reactions would, at first, seem a
rather trivial affair. However, we have found this not to be
the case. Indeed, the reason for this could not always be
explained, and even when determined, it proved puzzling.
LMg(NiPr2)(THF), 1. The reaction between Mg(NiPr2)2
and the free â-diiminato ligand LH (1 equiv) in refluxing
THF proved successful in the preparation of compound 1.
In this reaction, proton transfer occurs with the elimination
of HNiPr2. This reaction does not occur in toluene or benzene
under reflux. The compound is very air- and moisture-
sensitive and forms light green crystals from cooled con-
centrated THF solutions.
LZn(NiPr2), 2. Our best preparative route to this com-
pound involved the direct reaction between LiNiPr2 (2 equiv)
and LH in THF, stirred for 30 min at room temperature,
followed by the addition of a solution of ZnCl2 in THF.
Subsequent extraction of the dried residue with hexane gave
compound 2, which gives light green crystals upon recrys-
tallization from hexane. This compound is also air- and
moisture sensitive.
Figure 1. ORTEP drawing of LMg(NiPr2)(THF) showing the pseudo-
tetrahedral geometry at the Mg center. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms are omitted for clarity. The disorder
of the methylene carbon atoms is shown by the presence of C37, C38 in
both a and b sites.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
LMg(NiPr2)(THF)
A
B
distance
A
B
C
angle
Mg
Mg
Mg
Mg
N1
N2
N3
O
2.091(2)
2.071(2)
1.968(2)
2.092(2)
N1
N1
N1
N2
N2
N3
Mg
Mg
Mg
Mg
Mg
Mg
N2
N3
O
N3
O
92.79(7)
131.28(9)
99.27(8)
120.04(9)
99.37(8)
108.49(8)
O
in THF solvent. This compound is a white, air-sensitive,
crystalline compound soluble in common hydrocarbon
solvents.
[LMg(µ-OC6H9)]2, 7. A benzene solution of 1 or LMgNSi2-
Me6 reacts with cyclohexene oxide to form compound 7 as
a colorless crystalline solid that is very sparingly soluble in
benzene or toluene. Compound 7 is formed by allylic proton
abstraction by the amide bound to magnesium.
LZn(O2CNiPr2), 8. The addition of CO2 to compound 2
in benzene at room temperature leads to the formation of
compound 8 in quantitative yield. Compound 8 is readily
soluble in hydrocarbon solvents and may be recrystallized
from toluene as a white, air-sensitive material.
Single Crystal and Molecular Structures. LMg(NiPr2)-
(THF), 1. An ORTEP drawing of the molecular structure
of 1 is given in Figure 1. The view of the molecule shows
the disorder of the THF ligand and shows how this ligand
fits within a pocket created by two of the isopropyl groups
of the â-diiminate ligand. The diisopropylamide ligand has
its NC2 plane perpendicular to the Mg-O(THF) axis and is
nearly contained in the N(1)-Mg-N(2) plane of the â-di-
iminate.
LMg(OtBu)(THF), 3, can be prepared by the addition of
tBuOH (1 equiv) to a hydrocarbon solution of 1 and may be
prepared in a one-pot procedure without the isolation of 1,
as described in the Experimental Section. Compound 3 is a
white hydrocarbon-soluble air-sensitive material.
LZn(OtBu), 4. The preparation of this compound proved
t
very problematic. Addition of BuOH to LZnNSi2Me6 does
not lead to any reaction under the conditions that Coates8
used to prepare the related isoproxide [LZn(µ-OiPr)]2 by the
addition of iPrOH. The more reactive LZn(NiPr2), compound
t
2, readily reacts with BuOH in solvents such as hexane,
toluene, and THF at room temperature, but no simple product
is obtained in these reactions. However, by combining two
toluene solutions cooled to -78 °C, one of compound 2 and
the other containing tBuOH (1 equiv), compound 4 is formed
in near quantitative yield. This compound can be recrystal-
lized from toluene. It is air- and moisture-sensitive and is
labile to formation of a µ-hydroxide, LZn(µ-OH)2ZnL, 5.
LZn(OSiPh3)(THF), 6, is readily prepared by the addition
of Ph3SiOH (1 equiv) to either LZnNSi2Me6 or LZn(NiPr2)
(5) 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.
(6) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.;
Lovkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738-
8749.
(7) Chisholm, M. H.; Huffman, J. C.; Phomphrai, K. J. Chem. Soc., Dalton
Trans. 2001, 222-224.
(8) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 1999, 121, 11583-11584.
Selected bond distances and angles are given in Table 1.
The Mg-NiPr2 bond distance, 1.97 Å, is 0.1 Å shorter than
the Mg-N distances to the â-diiminate, 2.08 Å (ave), which
are comparable to the Mg-O(THF) distance, 2.09 Å. The
N-Mg-N angle associated with L is 92°, while the other
N-Mg-N angles are 131° and 120°. The sum of the
N-Mg-N angles is 344°, which is between that expected
2786 Inorganic Chemistry, Vol. 41, No. 10, 2002

Monomeric Alkoxides and Amides of Mg and Zn
Figure 3. ORTEP drawing of LZn(NiPr2) indicating the trigonal planar
geometry at the Zn center. Thermal ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity.
Figure 2. ORTEP drawing of LMg(OtBu)(THF) with thermal ellipsoids
Table 3. Selected Bond Distances (Å) and Angles (deg) for LZnNiPr2
drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.
A
B
distance
A
B
C
angle
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Zn
Zn
Zn
N1
N1′
N2
1.963(1)
1.963(1)
1.852(2)
N1
N1
N1′
Zn
Zn
Zn
N1′
N2
N2
97.05(8)
131.17(4)
131.17(4)
LMg(OtBu)(THF)
A
B
distance
A
B
C
angle
Mg1
Mg1
Mg1
Mg1
C2
O33
O38
N2
N6
C3
1.844(2)
2.048(2)
2.054(2)
2.059(2)
1.325(5)
O33
O33
O33
O38
O38
N2
Mg1
Mg1
Mg1
Mg1
Mg1
Mg
O38
N2
N6
N2
N6
N6
98.77(9)
127.8(1)
126.1(1)
103.79(9)
105.15(9)
92.20(9)
plane of the â-diiminate is 43.5°. The disorder leads to 50:
50 left- and right-handed pitches of the NC2 plane. Clearly,
from purely steric considerations, we might have expected
the NC2 unit to be perpendicular to the ZnN3 plane. We
propose that the adoption of the propeller pitch of the NC2
unit arises from a compromise of steric and electronic factors.
If the NC2 unit is contained in the ZnN3 plane, maximum N
pπ to Zn pπ bonding is possible, but the bulky C6H3-2,6-iPr2
ligands impede this. The Zn-N distances are all roughly
0.1 Å shorter than their related distances in compound 1. In
part, this must be a reflection of the different coordination
numbers, 4 for Mg in 1 and 3 for Zn in 2. In the structurally
related LZnNSi2Me6 compound prepared and characterized
by Coates,6 which also contains three coordinate zinc (2+)
ions, the M-N distances differ by 0.044 Å to the amide
nitrogen with the Zn-NiPr2 having the shorter distance.
LZn(OtBu), 4. An ORTEP drawing of this three-
coordinate zinc(II) complex is given in Figure 4, and selected
bond distances are given in Table 4. The Zn-N distances
are notably shorter than in the amide compound 2, and the
Zn-O distance of 1.80 Å is 0.04 Å shorter than in LMg-
(OtBu)(THF). Again, in part, this can be attributed to the
differing coordination numbers of the metal ions. The Zn-
O-C angle is 138° and the orientation of the Zn-O-C plane
allows O pπ to Zn pπ bonding. Note this orientation is quite
different from that seen in the four-coordinate magnesium
compound 2.
for a tetrahedral MgN3O moiety (328.5°) and a trigonal N3-
Mg unit (360°). The O-Mg-N angles span the range of
99°-108°, with the latter including the NiPr2 nitrogen. The
orientation of the 2,6-iPr2C6H3 groups in this structure is as
seen in the others reported in this work and as previously
seen in the compounds LMNSi2Me6, where M ) Mg and
Zn, reported by Coates.6
LMg(OtBu)(THF), 3. An ORTEP drawing of compound
3 is given in Figure 2, and selected bond distances and bond
angles are given in Table 2. The view shown in Figure 2
emphasizes the similarity to the structure just described for
LMg(NiPr2)(THF), 1. The Mg-O distances are 1.84 and 2.05
Å to the tertiary butoxide and THF ligands, respectively. The
Mg-O(THF) distance is shorter in this compound than that
seen in the amide, compound 1. The Mg-N distances
associated with the â-diiminate ligand are also very slightly
shorter, 2.06 Å (ave) in 3 versus 2.08 Å (ave) in 1. The sum
of the N-Mg-N and N-Mg-O angles involving the OtBu
ligand is 346°, once again implying a significant distortion
from tetrahedral geometry. Indeed, the structures of 1 and 3
may be viewed as flattened tetrahedra formed by the uptake
of a THF ligand by a trigonal LMX molecule. The THF
ligand is oriented within the pocket formed by the 2,6-
iPr2C6H3 groups, as is the Mg-O-CMe3 group, which has
a Mg-O-C angle of 135°.
LZn(µ-OH)2ZnL, 5. The structure of this compound is
reported in the Supporting Information. It is very similar to
the molecular structure of L′Zn(µ-OH)2ZnL′ recently reported
by Coates,6 where L′ ) CH(CMeNC6H3-2,6-Et2)2.
[LMg(µ-OC6H9)]2, 7. An ORTEP view of the dimeric
alkoxide-bridged compound, 7, is shown in Figure 5, and
selected bond distances and angles are given in Table 5. The
molecule has a crystallographically imposed center of
inversion and a mirror plane containing the central Mg2O2
moiety. The oxygen atoms are not trigonal planar but are
LZn(NiPr2), 2. An ORTEP drawing of the three-
coordinate zinc amide is given in Figure 3, and selected bond
distances and angles are given in Table 3. This molecule
contains a planar ZnN3 moiety, and the NC2 unit of the NiPr2
ligand is disordered over two sites. The dihedral angle
between the NC2 plane of the NiPr2 ligand and the ZnN2
Inorganic Chemistry, Vol. 41, No. 10, 2002 2787

Chisholm et al.
Figure 4. ORTEP drawing of LZn(OtBu) with thermal ellipsoids drawn
at the 50% probability level. Hydrogen atoms are omitted for clarity.
Table 4. Selected Bond Distances (Å) and Angles (deg) for LZnOtBu
A
B
distance
A
B
C
angle
Zn1
Zn1
Zn1
O1a
N1a
N2a
1.800(5)
1.931(5)
1.917(5)
O1a
O1a
N1a
Zn1
Zn1
Zn1
N1a
N2a
N2a
117.8(2)
142.8(2)
99.1(3)
Figure 5. ORTEP drawing of [LMg(µ-OC6H9)]2 with thermal ellipsoids
drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.
pyramidal, and the C6 ring is disordered above and below
the Mg2O2 plane. Within the C6 ring, the C-C distances
allow the identification of the C-C double bond, C(2)-
C(3) ) 1.325(5) Å, and clearly identify this ligand as a cyclo-
hexenyl oxide formed by deprotonation of cyclohexene oxide
at the allylic position. This compound is superficially related
to the Coates5,8 compounds [LM(µ-OiPr)]2, and a direct com-
parison of M-N and M-O distances and angles is possible
for the central N4M2O2 skeletons. This is given is Table 6.
These data reveal the structural similarities in these three
molecules. Note that the Mg-O and Zn-O distances are
virtually identical, though the Mg-N distances are signifi-
cantly longer by 0.05 Å.
LZn(η2-O2CNiPr2), 8. An ORTEP drawing of this mon-
omeric four-coordinate zinc complex is given in Figure 6,
and selected bond distances and angles are given in Table
7. An important but not unexpected observation is the
planarity of the O2CNC2 unit of the carbamate ligand and
the small O-Zn-O angle. It is also particularly interesting
that this compound is monomeric when closely related zinc-
(II) complexes, LZnO2CMe9 and LZnO2COiPr,10 are dimeric
in the solid-state having µ-η1,η1-O2CX bridges.
Solution Behavior: NMR Studies. A trigonal planar
molecule of formula LMX and a four-coordinate one of
formula LMX2 contain a molecular plane of symmetry, and
by NMR spectroscopy, one observes two isopropyl methyl
doublets as a result of restricted rotation about the aryl-
carbon-to-nitrogen bond of the â-diiminato ligand. This
situation is seen for the trigonal planar zinc compounds 2
and 4 and parallels the previous observation of Coates for
compounds such as LZnNSi2Me6 and [LZn(µ-Cl)]2.6 How-
ever, for a four-coordinate compound of the type LM(X)-
(Y), such as in the THF adducts 1 and 3, there are now four
chemically inequivalent isopropyl methyl groups.
Table 5. Selected Bond Distances (Å) and Angles (deg) for
[LMg(µ-OC6H9)]2
A
B
distance
A
B
C
angle
Mg
Mg
Mg
O
O′
N1
1.970(2)
1.994(2)
2.110(1)
O
O
O
O′
O′
N1
Mg
Mg
Mg
Mg
Mg
Mg
Mg
O
O′
N1
81.44(6)
125.10(4)
125.09(4)
118.28(5)
118.28(5)
91.76(7)
N1′
N1
N1′
N1′
Mg′
98.56(6)
Table 6. Bond Distances (Å) and Angles (deg) around N4M2O2
Skeletons of Dimeric [LZn(µ-OiPr)]2, [LMg(µ-OiPr)]2, and
[LMg(µ-OC6H9)]2 in a Form of (N1)(N2)(M1)[µ-O1-µ-O2]
(M2)(N3)(N4)
a
a
b
[LZn(µ-OiPr)]2 [LMg(µ-OiPr)]2 [LMg(µ-OC6H9)]2
Bonds
M1-N1
M1-N2
M1-O1
M1-O2
2.074(4)
2.054(4)
1.983(3)
1.983(3)
2.123(3)
2.114(3)
1.987(2)
1.978(2)
2.110(1)
2.110(1)
1.994(2)
1.970(2)
Angles
N1-M1-N2
N1-M1-O1
N2-M1-O1
O1-M1-O2
M1-O1-M2
94.8(2)
123.0(2)
126.1(1)
78.5(1)
91.3(1)
123.4(1)
126.2(1)
80.95(9)
99.05(9)
91.76(7)
125.09(4)
125.10(4)
81.44(6)
98.56(6)
101.7(1)
a Data taken from refs 5 and 8. b This work.
The magnesium compounds 1 and 3 show variable-
temperature NMR behavior consistent with the generalized
equilibrium reaction shown in eq 1,
LMgX‚THF a LMgX + THF
(1)
where X ) OtBu and NiPr2. The spectral behavior is similar
in both CD2Cl2 and toluene-d8, although compound 1 reacts
1
with CD2Cl2. The VT H NMR spectra obtained for 3 in
toluene-d8 are shown in Figure 7. By -10 °C, the equilibrium
is slow on the NMR time scale and lies to the left, in favor
of the THF adduct. Under similar conditions of concentration,
the amide complex 1 does not show static THF complexation
(9) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998,
120, 11018-11019.
(10) Moore, D. R.; Coates, G. W. The 221st ACS National Meeting, San
Diego, CA; Inorganic Division-232, April 2001.
2788 Inorganic Chemistry, Vol. 41, No. 10, 2002

Monomeric Alkoxides and Amides of Mg and Zn
Figure 6. ORTEP drawing of LZn(η2-O2CNiPr2) with thermal ellipsoids
drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.
Table 7. Selected Bond Distances (Å) and Angles (deg) for
LZn(η2-O2CNiPr2)
A
B
distance
A
B
C
angle
Zn
Zn
Zn
Zn
N3
N1
N2
O1
O2
C6
1.935(1)
1.952(1)
2.028(2)
2.041(1)
1.349(2)
N1
N1
N2
N1
N2
Zn
Zn
Zn
Zn
Zn
N2
O1
O1
O2
O2
100.47(5)
127.57(4)
114.76(4)
129.43(4)
117.69(4)
Figure 7. VT 1H NMR spectra (toluene-d8, 400 MHz) of LMg(OtBu)-
(THF), 3, where $A and $B are CHMeMe′ and CH′Me′Me′′′ protons, and
@A, @B, @C, and @D are nonequivalent CHMeMe′, CH′Me′′Me′′′,
CH′Me′′Me′′′, and CHMeMe′ protons.
until ca. -70 °C. At -80 °C, we observe restricted rotation
about the Mg-O THF bond. From this, we know that the
equilibria are similar for both complexes but that THF
complexation is more favored for the tert-butoxide compound
3. This is understandable on the basis of both steric and
electronic grounds.
In contrast to the coordination of THF to Mg2+ in
compounds 1 and 3, the LZn(OSiPh3)‚(THF) complex readily
dissociates its THF in solution and exchange is rapid on the
1H NMR time scale, even at -80 °C in toluene-d8. The
isolation of the Zn2+ 3-coordinate complex 2, which is
prepared in THF, clearly indicates that the equilibrium
reaction that is related to 1 must lie to the right for zinc in
related pairs of compounds.
NMR data for the above and other compounds described
in this work are given in the Experimental Section.
Reactivity Studies. Ring-Opening Polymerizations of
Lactides. All the compounds reported here will initiate and
then sustain ring-opening polymerization of lactides (L-, rac-
and meso-). The zinc (2+) compounds ultimately generate
Coates’ catalyst system8 as previously described from
reactions employing [LZn(µ-OiPr)]2. From this work, we can
obtain a clear dependence of the rate of initiation, which
follows the order Mg > Zn and OtBu > NiPr2 > NSi2Me6
> OSiPh3. We feel that this order reflects both electronic
and steric factors, so while NiPr2 is clearly the most basic
ligand, its lone pair is sterically less accessible than that of
OtBu.
In the presence of added THF, one can observe the
exchange between free and coordinated THF such that, at
room temperature, only one set of THF signals is seen, but
at low temperatures, one sees signals for free and coordinated
THF. See the Supporting Information. Most significantly,
the coalescence temperature for the iPr signals is not affected
by the added THF. Indeed, even when spectra are recorded
in neat THF-d8, the coalescence behavior is remarkably
similar, as shown in Figure 8 for the diisopropylamide
complex 1. The spectrum recorded at -60 °C in THF-d8
corresponds to that expected for the complexed molecule.
This indicates that, even in neat THF, the exchange mech-
anism is a dissociative interchange one based on the
equilibrium 1. Inspection of Figures 1 and 2 might have led
one to anticipate a bimolecular THF exchange mechanism
wherein as one Mg‚‚‚O(THF) bond breaks, another forms
at the opposite site. If an associative interchange mechanism
were operative, this would generate a plane of symmetry as
shown in I below. This would give rise to an NMR spectrum
equivalent to that of a three-coordinate LMgX compound.
The relative order of the reactivities of the metals Mg >
Zn was seen in a competition experiment wherein 1 equiv
of lactide was allowed to react with an equimolar solution
of LMNSi2Me6. Here, close to 100% (within NMR detection
limits) of the LMgNSi2Me6 reacted, while the zinc analogue
remained.
Coates and co-workers previously noted that [LZn(µ-
OiPr)]2 reacted with meso-LA to give syndiotactic PLA5 (sss
tetrads) and with rac-lactide to give heterotactic PLA8 (∼90%
isi + sis tetrads). It is therefore not surprising that we observe
Inorganic Chemistry, Vol. 41, No. 10, 2002 2789

Chisholm et al.
Figure 8. VT 1H NMR spectra (THF-d8, 400 MHz) of LMg(NiPr2)(THF), 1, where $A and $B are CHMeMe′ and CH′Me′′Me′′′ protons, and @A, @B, @C
and @D are nonequivalent CHMeMe′, CH′Me′′Me′′′, CH′Me′′Me′′′, and CHMeMe′ protons.
that all the zinc compounds will react with rac-LA in CH2-
Cl2 to give similar stereoselectivity. However, the magnesium
compounds 1 and 3 do not show this stereoselectivity in CH2-
Cl2 or benzene. (Compound 1 reacts with methylene chloride,
presumably to abstract Cl and form LMgCl, and no poly-
merization of LA is observed.) The reaction between 3 and
100 equiv of LA is very rapid at room temperature in CH2-
Cl2, giving ca. 98% conversion within 2 min. The related
zinc tertiary butoxide compound 4 gives ca. 95% conversion
in 10 min under otherwise identical conditions. The initially
formed PLA in reactions employing 1 and 3 and rac-LA
conforms to a Bernoulian distribution of tetrads 3iii:2isi:sis:
iis:sii.11 With time, the magnesium catalyst system shows
the formation of the other tetrads (sss, ssi, and iss) arising
from transesterification. However, it is clear that the rate of
ROP of LA is faster than the rate of transesterification.
We have also carried out the polymerization in THF
solutions with rather interesting observations. First, the rates
of polymerization are slower, but still rapid by most catalyst
standards.12 The related magnesium and zinc tert-butoxides,
2 and 4, polymerized 100 equiv of rac-lactide to >95%
conversion in 5 and 80 min, respectively. The somewhat
slower rate of ROP is understandable in terms of competition
for LA coordination to the metal center in the presence of
the donor solvent, THF. Most interestingly, we observed that
the magnesium catalyst system showed stereoselectivity in
THF similar to that found for zinc. See Figure 9. The relative
stereoselectivity shown by the zinc catalyst system was,
however, not changed in THF relative to CH2Cl2.
A summary of the ring-opening polymerization of rac-
lactide by the various magnesium and zinc compounds
reported in this study is given in Table 8. The time to ca.
95% conversion is clearly seen to be dependent on the metal,
the solvent, and the initiating group bound to the metal (this
becomes the end-group of the growing polymer chain). With
a slow initiation rate, relative to propagation, we observed
large PDI values and higher molecular weight polymer as
judged by the Mn values.
NMR Studies Aimed at Probing the Resting State of
the Metal Centers. We allowed samples of the related
magnesium and zinc tert-butoxide complexes 3 and 4 to react
with L-LA (ca. 5 equiv.) in toluene-d8 and THF-d8 and then
1
monitored the H NMR spectrum as a function of temper-
ature. As we have shown from studies of the binding of THF
to the magnesium center in 3, a four-coordinate metal center
having N2MO(O′) coordination, where O ) an alkoxide and
O′ ) an O-donor such as THF or a ketonic group, will lead
to the appearance of four isopropyl doublets. This is indeed
seen for the zinc complex, as is shown in Figure 10. This is
(11) Kricheldorf, H. R.; Boettcher, C.; Tonnes, K. U. Polymer 1992, 33,
2817-2824.
(12) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton
Trans. 2001, 15, 2215-2224.
2790 Inorganic Chemistry, Vol. 41, No. 10, 2002

Monomeric Alkoxides and Amides of Mg and Zn
Figure 9. 1H NMR spectra (CDCl3, 400 MHz, 27 °C) of the homodecoupled CH resonance of poly(rac-lactide) prepared in CH2Cl2 and THF using (a)
LMg(OtBu)(THF) and (b) LZn(OtBu) as initiators.
Table 8. Polymerization of rac-LA (100:1 [LA]:[catalyst]) in CH2Cl2 and THF at 20 ˚C Using the Magnesium and Zinc Complexes
convn
(%)
Mn × 103
entry
catalyst
solvent
time
(Da)
Mw/Mn
1
2
3
4
5
6
7
8
9
LMg(OtBu)‚(THF)
LMg(OtBu)‚(THF)
LMg(NiPr2)‚(THF)
LZn(OtBu)
CH2Cl2
THF
THF
CH2Cl2
THF
CH2Cl2
THF
2 min
5 min
5 min
10 min
50 min
40 min
100 min
3 h
97
95
94
95
93
94
93
97
91
19.8
14.6
13.3
16.0
17.4
18.5
19.6
18.4
22.8
1.49
1.47
1.60
1.15
1.22
1.45
1.37
1.55
1.45
LZn(OtBu)
LZn(NiPr2)
LZn(NiPr2)
LZnN(SiMe3)2
LZn(OSiPh3)‚(THF)
CH2Cl2
CH2Cl2
70 h
consistent with the view that the growing polymer chain
binds to the metal center, as was seen, for example, in the
model compound LZn(OCHMeC(O)OMe).5 The resting state
of LZn(OP) is illustrated in II.
In contrast, the magnesium system shows only two
isopropyl doublets for the â-diiminate ligand, even at -80
°C. See Figure 10. Given that we have established that
magnesium binds THF more aggressively than zinc in the
related alkoxide and amide complexes, the appearance of
only two isopropyl doublets is at first surprising. We propose
that this arises because the magnesium compound exists in
solution as a dimeric molecule wherein the two magnesium
atoms are united by a pair of alkoxide ligands from the
growing polymer chain. This effectively creates a mirror
Figure 10. 1H NMR spectra (toluene-d8, 400 MHz, 27 °C) of isopropyl
protons from the â-diiminate ligands when 5 equiv of L-LA was reacted
with (a) LZnOtBu showing @ as four isopropyl doublets and (b) LMg(Ot-
Bu)(THF) showing $ as two isopropyl doublets.
plane of symmetry, as seen in the molecular structure of 7.
Inorganic Chemistry, Vol. 41, No. 10, 2002 2791

Chisholm et al.
The proposed resting state of LMg(OP) is depicted in III.
catalyst [LZn(µ-OiPr)]2, which is believed to act as a single-
site catalyst.
Concluding Remarks
In the present study, we have prepared and characterized
closely related monomeric magnesium and zinc alkoxide and
amide complexes. This has allowed a detailed comparison
of the coordination properties and reactivities of pairs of
magnesium and zinc compounds. Magnesium shows a higher
binding affinity toward donor ligands and a higher reactivity
for its M-N and M-O bonds. The latter we can reasonably
attribute to the greater polarity of M-X bonds for M ) Mg
relative to M ) Zn, where X ) OR or NR2. Polymerization
of lactide is notably faster for magnesium than for zinc, and
rac-LA shows a remarkable solvent dependence in giving
atactic PLA in CH2Cl2 versus heterotactic PLA in THF. Our
1H NMR studies suggest that the resting state of the zinc
complex during polymerization of lactide is a monomeric
four-coordinate center, where the growing chain chelates to
the metal center akin to that in LZn(η2-OCHMeC(O)OMe).5
For magnesium, the major species present in solution is
consistent with a dimer LMg(µ-OP)2MgL. These results
underscore the intricacies of the fundamental steps in the
reactions of LA and CHO/CO2 recently reported by Coates
and co-workers.5,6 It is, for example, fascinating to note that
LZnOtBu reacts with PO and CO2 and CHO and CO2 to give,
respectively, propylene carbonate and the alternating co-
polymer of CO2 and CHO, whereas the compound LZnNiPr2
reacts with CO2 in the presence of either PO or CHO to
give only LZn(η2-O2CNiPr2).
This, of course, does not mean that the active form of the
magnesium catalyst system is not monomeric in THF, and
this could account for the different stereoselectivity of
polymerizations in THF versus toluene and methylene
chloride.
Reactions with Propylene Oxide (PO) and Cyclohexene
Oxide (CHO). The zinc compounds 2 and 4 in benzene
showed no reactivity toward PO and CHO (1 and 5 equiv).
Nor was there any reaction in the neat epoxide as solvent.
Clearly these monomeric zinc compounds are not effective
as homopolymerization catalyst precursors.
In contrast, the magnesium compounds 1 and 3 react in
benzene with PO and CHO at room temperature. No isolable
product has been obtained from the reactions involving PO,
although we can state that PPO is not formed. In the case of
CHO, compound 7 is formed in near quantitative yield and
is obtained as colorless crystals from benzene. As shown by
the crystallographic study, this compound results from ring-
opening of CHO by allylic proton abstraction. Collectively,
these findings further underscore the fact that ring-opening
of epoxides by alkoxide ligands does not operate by a cis-
migratory mechanism.13 In this work, we see that the more
basic alkoxide and amide ligands bound to magnesium may
effect allylic proton abstraction. The inability to achieve
homopolymerization of PO or CHO presumably arises
because a bimolecular mechanism involving a reaction
between an epoxide coordinated to one metal and an alkoxide
ligand bound to another is not favorable.
Experimental Section
General Considerations. The manipulation of air-sensitive
compounds involved the use of anhydrous solvents and dry and
oxygen-free nitrogen employing standard Schlenk line and drybox
techniques. rac-Lactide was purchased from Aldrich and was
sublimed three times prior to use. Tetrahydrofuran, dichloromethane,
hexane, and toluene were distilled under nitrogen from sodium/
benzophenone, calcium hydride, potassium metal, and sodium
metal, respectively. The â-diiminato ligand CH(CMeNC6H3-2,6-
Reactions with Carbon Dioxide. The magnesium com-
pounds 1 and 3 react rapidly with CO2 in hydrocarbon
solutions, even at low temperatures. The materials produced
1
are hydrocarbon-soluble but yield extremely complex H
15
iPr2)2,14 LZnN(SiMe3)2,6 LMgN(SiMe3)2,5 and Mg(NiPr2)2 were
NMR spectra. It seems that more than one compound is being
formed and that we are quite probably getting insertion of
CO2 into a â-diiminato nitrogen bond as well as the NiPr2
or OtBu group.
The reaction between CO2 and hydrocarbon solutions of
compound 2 react cleanly to form the carbamate LZn(η2-
O2CNiPr2), 8, as the single detectable and isolable product.
This insertion appears irreversible. Compound 8 does not
react with either PO or CHO.
prepared according to literature procedures. LMg(OtBu)(THF) may
be synthesized according to our previous procedure,7 but we prefer
the one-pot synthesis described below. Hydrous ZnCl2 was dried
using chlorotrimethylsilane.16 LiNiPr2 was prepared from the
reaction of BuLi and HNiPr2. CO2 gas was purchased from The
t
BOC Group, Inc. and used as received. Anhydrous BuOH was
purchased from Aldrich and used as received. Cyclohexene oxide
(CHO) and propylene oxide (PO) were distilled from calcium
hydride under vacuum.
Measurements. 1H and 13C{1H} spectra were recorded in C6D6,
THF-d8, CD2Cl2, and toluene-d8 on Bruker DPX-400 NMR
spectrometers and were referenced to the residual protio impurity
peak (C6D6, δ 7.15; THF-d8, δ 3.58; CD2Cl2, δ 5.32; toluene-d8,
The reaction between CO2 and compound 4 appears more
complex, and no simple compound such as LZnO2COtBu
has been isolated. However, compound 4 does react with
PO and CO2 to give propylene carbonate and with CHO and
CO2 to give the alternating copolymer of CHO-CO2. In this
regard, LZnOtBu shows similar reactivity to the Coates’
(14) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514-1516.
(15) Ashby, E. C.; Lin, J. J.; Goel, A. B. J. Org. Chem. 1978, 43, 1564-
1566.
(16) Boudjouk, P.; So, J. H.; Ackermann, M. N.; Hawley, S. E.; Turk, B.
E. Inorg. Synth. 1992, 29, 108-111.
(13) Antelmann, B.; Chisholm, M. H.; Huffman, J. C.; Iyer, S. S.; Navarro-
Llobet, D.; Pagel, M.; Simonsick, W. J.; Zhong, W. Macromolecules
2001, 34, 3159-3175.
2792 Inorganic Chemistry, Vol. 41, No. 10, 2002

Monomeric Alkoxides and Amides of Mg and Zn
2.09, for 1H; and C6D6, δ 128.0; THF-d8, δ 67.57; CD2Cl2, δ 53.8;
toluene-d8, 20.4, for 13C{1H}). Elemental analyses were done by
Atlantic Microlab, Inc. Gel permeation chromatography measure-
ments were carried out using a Waters 1525 binary HPLC pump
and Waters 410 differential refractometer equipped with styragel
HR 2&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 standard. Mass spectrometry was done by electron
impact ionization at 60 eV using a Kratos MS890 double-focusing
magnetic-sector instrument at 6000 V ion-acceleration energy in
the extended mass-range mode of the magnet.
LZnOtBu, 4. A solution of LZn(NiPr2) (0.500 g, 0.858 mmol)
in 15 mL of toluene was cooled to -78 °C. A precooled (-78 °C)
solution of BuOH (82 µL, 0.86 mmol) in 10 mL of toluene was
t
then added dropwise to the zinc complex solution. The mixture
was allowed to warm slowly to room temperature and stirred for
15 min. The volatile liquid was removed under dynamic vacuum,
giving white solid (0.44 g, 92%). X-ray-suitable crystals were grown
by placing a concentrated toluene solution in a freezer overnight.
MS (EI): m/z ) 554.3 (M+). Anal. Calcd for C33H50N2OZn: C,
1
70.29; H, 8.87; N, 4.97. Found: C, 70.31; H, 8.92; N, 5.08. H
NMR (C6D6): 7.11 (m, 6H, ArH), 4.91 (s, 1H, â-CH), 3.16 (sept,
4H, J ) 6.9 Hz, CHMe2), 1.66 (s, 6H, R-Me), 1.40 (d, 12H, J )
6.9 Hz, CHMe2), 1.15 (d, 12H, J ) 6.9 Hz, CHMe2), 0.96 (s, 9H,
OtBu). 13C{1H} NMR (C6D6): 169.48 (CdN), 143.89 (ipso-Ar),
141.77 (o-Ar), 126.29 (p-Ar), 123.89 (m-Ar), 94.87 (â-C), 68.66
(OCMe3), 35.41 (OCMe3), 28.62 (CHMe2), 24.36 (CHMe2), 23.71
(CHMe2), 23.64 (R-Me).
LMg(NiPr2)(THF), 1. THF (15 mL) was added to a mixture of
LH (0.400 g, 0.956 mmol) and Mg(NiPr2)2 (0.215 g, 0.957 mmol).
The solution was then refluxed for 3 h and cooled to room
temperature. The volatile components were removed under a
dynamic vacuum pump, giving a light green solid. Fine crystals
were obtained by recrystallization in THF (0.36 g, 61%). MS (EI):
(BDI)ZnOSiPh3.(THF), 6. A solution of Ph3SiOH (0.215 g,
0.778 mmol) in 10 mL of THF was added slowly to a solution of
(BDI)ZnN(SiMe3)2 (0.500 g, 0.778 mmol) in 5 mL of THF. The
resulting clear solution was stirred overnight and the solvent was
1
m/z ) 541.4 (M - THF)+. H NMR (C6D6): 7.16-7.20 (m, 6H,
ArH), 4.77 (s, 1H, â-CH), 3.85 (m, 4H, O(CH2CH2)2), 3.25 (sept,
4H, J ) 6.9 Hz, CHMe2), 2.96 (sept, 2H, J ) 6.3 Hz, NCHMe2),
1.64 (s, 6H, R-Me), 1.40 (m, 4H, O(CH2CH2)2), 1.37 (d, 12H, J )
6.9 Hz, CHMe2), 1.21 (d, 12H, J ) 6.9 Hz, CHMe2), 0.87 (d, 12H,
J ) 6.3 Hz, NCHMe2). 13C{1H} NMR (C6D6): 168.46 (CdN),
147.07 (ipso-Ar), 142.30 (o-Ar), 125.34 (p-Ar), 124.02 (m-Ar),
94.82 (â-C), 70.09 (OCH2), 47.79 (NCHMe2), 28.45 (NCHMe2),
28.34 (CHMe2), 25.40 (O(CH2CH2)2), 24.93 (CHMe2), 24.87 (R-
Me), 24.58 (CHMe2).
1
removed, giving a white powder (0.613 g, 95%). H NMR (CD2-
Cl2, δ): 7.30 (t, 2H, J ) 7.8 Hz, p-iPr2ArH), 7.18 (d, 4H, J ) 7.8
Hz, m-iPr2ArH), 7.13 (m, 3H, p-SiArH), 7.01 (t, 6H, J ) 6.7 Hz,
m-SiArH), 6.92 (m, 6H, o-SiArH), 5.09 (s, 1H, â-CH), 3.68 (m,
4H, O(CH2CH2)2), 3.00 (heptet, 4H, J ) 7.2 Hz, CHMeMe′), 1.82
(m, 4H, O(CH2CH2)2), 1.79 (s, 6H, R-Me), 1.20 (d, 12H, J ) 6.9
Hz, CHMeMe′), 0.90 (d, 12H, J ) 6.9 Hz, CHMeMe′).
[LMg(µ-OC6H9)]2, 7. Cyclohexene oxide (15 µL, 0.015 mmol)
was added to a solution of LMgN(SiMe3)2 (0.049 g, 0.081 mmol)
or LMg(NiPr2)(THF) (0.050 g, 0.081 mmol) in 5 mL of benzene.
The mixture was stirred for 15 s and then left without stirring
overnight. X-ray-suitable colorless crystals were formed and
separated by filtration (0.080 g, 95%). MS (EI): m/z ) 1077.8
(M+).
LZn(NiPr2), 2. THF (15 mL) was added to a mixture of LH
(0.500 g, 1.2 mmol) and LiNiPr2 (0.260 g, 2.43 mmol). The mixture
was stirred for 30 min and then added to a solution of ZnCl2 (0.163
g, 1.2 mmol) in 10 mL of THF dropwise. The solution was then
stirred for 1 h and the solvent was removed under dynamic vacuum.
The product was extracted with 20 mL of hexane, giving a light
green solid (0.53 g, 76%). X-ray-suitable single crystals were
obtained by placing a concentrated hexane solution in a freezer.
MS (EI): m/z ) 581.4 (M+). 1H NMR (C6D6): 7.12 (m, 6H, ArH),
4.94 (s, 1H, â-CH), 3.24 (sept, 4H, J ) 6.8 Hz, CHMe2), 2.87
(sept, 2H, J ) 6.4 Hz, NCHMe2), 1.68 (s, 6H, R-Me), 1.38 (d,
12H, J ) 6.8 Hz, CHMe2), 1.14 (d, 12H, J ) 6.8 Hz, CHMe2),
0.80 (d, 12H, J ) 6.4 Hz, NCHMe2). 13C{1H} NMR (C6D6): 168.94
(CdN), 145.54 (ipso-Ar), 141.94 (o-Ar), 126.13 (p-Ar), 124.21 (m-
Ar), 94.98 (â-C), 48.81 (NCHMe2), 28.74 (CHMe2), 27.60
(NCHMe2), 24.32 (CHMe2), 24.25 (R-Me), 24.21 (CHMe2).
LZn(η2-O2CNiPr2), 8. LZn(NiPr2) (0.500 g, 0.858 mmol) was
dissolved in 15 mL of benzene. CO2 gas was bubbled through the
solution for 10 min and solvent was removed under dynamic
vacuum, giving a white solid in quantitative yield. X-ray-suitable
crystals were obtained by placing the concentrated THF solution
in a freezer. Anal. Calcd for C36H55N3O2Zn: C, 68.92; H, 8.85; N,
6.70. Found: C, 68.97; H, 8.76; N, 6.59. 1H NMR (C6D6): 7.00-
7.12 (m, 6H,ArH), 4.94 (s, 1H, â-CH), 3.55 (sept, 2H, J ) 6.8 Hz,
NCHMe2), 3.37 (sept, 4H, J ) 6.9 Hz, CHMe2), 1.72 (s, 6H, R-Me),
1.48 (d, 12H, J ) 6.9 Hz, CHMe2), 1.18 (d, 12H, J ) 6.9 Hz,
CHMe2), 0.83 (d, 12H, J ) 6.8 Hz, NCHMe2). 13C{1H} NMR
(C6D6): 169.29 (CdN), 166.49 (O2CN), 143.51 (ipso-Ar), 142.42
(o-Ar), 126.04 (p-Ar), 123.76 (m-Ar), 94.19 (â-C), 46.12 (NCHMe2),
28.45 (CHMe2), 24.32 (CHMe2), 24.17 (CHMe2), 23.50 (R-Me),
20.74 (NCHMe2).
General Polymerization Procedure. rac-Lactide (0.500 g, 3.47
mmol) was dissolved in 6.0 mL of CH2Cl2 or THF. A solution of
the corresponding catalyst (0.0347 mmol) in 1.5 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 period, at which time small aliquots were taken to monitor
the conversion. When the conversion is greater than 90%, the
polymerization was quenched with excess methanol. The polymer
precipitate was then filtered and dried under vacuum to constant
weight.
LMg(OBut)(THF), 3. THF (15 mL) was added to a mixture of
LH (0.400 g, 0.956 mmol) and Mg(NiPr2)2 (0.215 g, 0.957 mmol).
The solution was then refluxed for 3 h and cooled to room
temperature. To this solution was added tBuOH (95 µL, 0.99 mmol)
via a microsyringe and the mixture stirred for 10 min. The volatile
components were subsequently removed under dynamic vacuum,
giving a white power (0.52 g, 93%). X-ray-suitable crystals were
obtained by placing a concentrated THF solution in a freezer. MS
(EI): m/z ) 514.4 (M - THF)+. 1H NMR (CD2Cl2, -30 °C): 7.15
(m, 6H, ArH), 4.78 (s, 1H, â-CH), 4.02 (br, 4H, O(CH2CH2)2),
3.16 (sept, 2H, J ) 7.1 Hz, CHMeMe′), 2.95 (sept, 2H, J ) 7.1
Hz, CH′Me′′Me′′′), 1.96 (br, 4H, O(CH2CH2)2), 1.60 (s, 6H, R-Me),
1.22 (d, 6H, J ) 7.1 Hz, CHMeMe′), 1.19 (d, 6H, J ) 7.2 Hz,
CH′Me′′Me′′′), 1.13 (d, 6H, J ) 6.7 Hz, CH′Me′′Me′′′), 1.05 (d,
6H, J ) 6.7 Hz, CHMeMe′), 0.57 (s, 9H, OtBu).
General Epoxides/CO2 Copolymerization Procedure. Ep-
oxides (34.6 mmol) and the corresponding catalyst (0.0346 mmol)
(1000:1 [epoxide]:[catalyst]) were placed in a 40-mL stainless steel
Inorganic Chemistry, Vol. 41, No. 10, 2002 2793

Chisholm et al.
Table 9. Summary of Crystallographic Data
compound
1
2
3
4
7
8
empirical formula
C39H63MgN3O
C35H55N3Zn
C37H58MgN2O2
C33H50N2OZn
C70H100Mg2N4O2 + C36H55N3O2Zn
C6D6
formula weight
color
614.23
pale yellow
583.19
colorless
587.18
colorless
556.12
colorless
1162.31
colorless
627.20
colorless
crystal size (mm)
crystal system
space group
cell dimensions
temp (K)
a (Å)
0.23 × 0.38 × 0.38 0.08 × 0.35 × 0.42 0.4 × 0.4 × 0.15 0.23 × 0.46 × 0.46 0.15 × 0.23 × 0.35
0.27 × 0.27 × 0.38
monoclinic
P21/n
orthorhombic
Pbca
monoclinic
monoclinic
orthorhombic
monoclinic
P21/m
P21
Pca21
C2/m
200
200
111
150
150
150
18.273(1)
18.636(1)
22.520(2)
8.911(1)
20.807(2)
10.058(1)
113.585(1)
2
9.7999(3)
16.6069(5)
11.5815(3)
109.345(1)
2
17.3061(2)
21.2069(3)
17.8377(2)
17.9210(2)
19.7085(3)
11.5343(2)
123.072(1)
2
12.0085(1)
20.5887(1)
14.7710(1)
104.378(1)
4
b (Å)
c (Å)
â (deg)
Z
8
8
volume (Å3)
7669.05(9)
1.064
1709.13(3)
1.133
1778.43
1.097
6546.6(1)
1.128
3413.84(9)
1.131
3537.59(4)
1.178
d
calc (g/cm3)
λ (Å)
0.71073
0.078
2.12-25.01
121866
6743
0.71073
0.744
2.42-27.46
29064
0.71073
0.082
0-30
27281
0.71073
0.775
0.71073
0.083
2.06-27.47
31417
0.71073
0.727
2.20-27.49
62379
8113
0.0302
0.0400
0.0809
abs coeff (cm-1
θ range (deg)
reflns collect.
indpndnt reflns
)
2.25-27.50
88477
14581
0.0541
0.112
4013
8177
4027
R1(F)a [I > 2σ(I)]
R1(F)a (all data)
0.0632
0.0858
0.0355
0.0454
0.0924
1.042
R(F)b ) 0.0327
0.0509
0.0796
0.141
wR2(F2)a (all data) 0.195
0.148
1.010
goodness of fit
1.024
0.633
1.047
1.049
2
2
2
2
2
2
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 ) 0.1005, y )
4.5413 for 1; x ) 0.0434, y ) 0.6374 for 2; x ) 0.0570, y ) 6.3321 for 4; x ) 0.0680, y ) 1.8580 for 7; x ) 0.0416, y ) 0.9763 for 8. b R(F) ) ∑||Fo|
- |Fc||/∑|Fo|; Rw(F) ) {∑w(|Fo| - |Fc|)2/∑w|Fo| }1/2, where w ) 1/[|σ2||Fo|].
2
in SHELXL-93.19 The methyl group hydrogen atoms were added
reactor equipped with a stirring bar. CO2 (100 psi) was then charged
at calculated positions using a riding model with U(H) ) 1.5Ueq-
(bonded C atom). For each methyl group, the torsion angle that
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) ) 1.2Ueq(bonded C atom). Neutral
atom scattering factors were used and include terms for anomalous
dispersion.20
to the reactor. The reaction was stirred at the desired temperature
and time. After CO2 gas was released, a small aliquot was taken to
determine the conversion. The polymerization was then quenched
with excess methanol. The polymer was subsequently 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
(Table 9). Crystals were coated with oil prior to being placed in
the nitrogen gas stream. The data collection strategy was set up to
measure a quadrant of reciprocal space for compounds 7 and 8, an
octant for 1 and 4, a hemisphere for 2 and 5, with a redundancy
factor of 1.9, 2.7, or 4, which means that 90% of the reflections
were measured at least 1.9, 2.7, or 4 times, respectively. A
combination of ψ and ω scans with a frame width of 1.0° was
used. Data integration was done with Denzo.17 Scaling and merging
of the data was done with Scalepack.17 The structures were solved
by either the Patterson method or direct methods in SHELXS-86.18
Full-matrix least-squares refinements based on F2 were performed
Acknowledgment. We thank the Department of Energy,
Office of Basic Sciences, Chemistry Division for financial
support of this work. K.P. acknowledges the Institute for
the Promotion of Teaching Science and Technology (IPST),
Thailand, for an opportunity to work on this project.
Supporting Information Available: Crystallographic data for
1
compounds 1-5, 7, and 8; VT H NMR spectra of LMg(OtBu)-
(THF). This material is available free of charge via the Internet at
http://pubs.acs.org.
IC020148E
(18) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473.
(19) Sheldrick, G. M. Universitat Gottingen, Germany, 1993.
(20) International Tables for Crystallography; Kluwer Academic Publish-
ers: Dordrecht, 1992; Volume C.
(17) Otwinowski, Z.; Minor, W. Methods in Enzymology, Vol. 276:
Macromolecular Crystallography, Carter, C. W., Jr., Sweet, R. M.,
Eds.; Academic Press: New York, 1997; Part A, pp 307-326.
2794 Inorganic Chemistry, Vol. 41, No. 10, 2002