Bimetallic anilido-aldimine zinc complexes for epoxide/CO2 copolymerization

Article abstract of DOI:10.1021/ja0435135

Acyclic o-phenylene-bridged bis(anilido-aldimine) compounds, o-C ₆H₄{C₆H₂R₂N=CH-C ₆H₄-(H)N(C₆H₃R′₂)} ₂ and related 30-membered macrocyclic compounds, o-C ₆H₄{C₆H₂R′₂N= CH-C₆H₄-(H)N-C₆H₂R₂} ₂ (o-C₆H₄) are prepared. Successive additions of Me₂Zn and SO₂ gas to the bis(anilido-aldimine) compounds afford quantitatively dinuclear μ-methylsulfinato zinc complexes, o-C₆H₄{(C₆H₂R₂N=CH-C ₆H₄-N(C₆H₃R′₂)- κ²-N,N)Zn(μ-OS(O)Me)}₂ (R = iPr and R′ = iPr, 29; R = Et and R′ = Et, 30; R = Me and R′ = Me, 31; R = Me and R′ = iPr, 32; R = Et and R′ = Me, 33; R = Et and R′ = iPr, 34; R = iPr and R′ = Et, 35) and o-C₆H₄{C ₆H₂R′₂N=CH-C₆H ₄-N-C₆H₂R₂-κ²-N,N) Zn(μ-OS(O)Me)}₂ (o-C₆H₄) (R = Et and R′ = Et, 36; R = Me and R′ = Me, 37; R = iPr and R′ = Me, 38; R = Et and R′ = Me, 39; R = Me and R′ = iPr, 40). Molecular structures of 34 and 40 are confirmed by X-ray crystallography. Complexes 30-35 show high activity for cyclohexene oxide/CO₂ copolymerization at low [Zn]/[monomer] ratio (1:5600), whereas the complex of mononucleating β-diketiminate {[(C₆H₃Et₂)N=C(Me)CH=C(Me) N(C₆H₃Et₂)]Zn(μ-OS(O)Et)}₂ shows negligible activity in the same condition. Activity is sensitive to the N-aryl ortho substituents and the highest activity is observed with 32. Turnover number up to 2980 and molecular weight (M_n) up to 284 000 are attained with 32 at such a highly diluted condition as [Zn]/[monomer] = 1:17 400. Macrocyclic complexes 36-40 show negligible activity for copolymerization.

Full text of DOI:10.1021/ja0435135

Published on Web 02/11/2005
Bimetallic Anilido-Aldimine Zinc Complexes for Epoxide/CO2
Copolymerization
Bun Yeoul Lee,*,† Heon Yong Kwon,† Su Yeon Lee,† Sung Jae Na,† Song-i Han,†
Hoseop Yun,† Hyosun Lee,‡ and Young-Whan Park‡
Contribution from the Department of Molecular Science and Technology, Ajou UniVersity,
Suwon 443-749 Korea, and LG Chem Ltd./Research Park, 104-1, Munji-dong, Yuseong-gu,
Daejon 305-380, Korea
Received October 26, 2004; E-mail: bunyeoul@ajou.ac.kr
Abstract: Acyclic o-phenylene-bridged bis(anilido-aldimine) compounds, o-C6H4{C6H2R2NdCH-C6H4-
(H)N(C6H3R′2)}2 and related 30-membered macrocyclic compounds, o-C6H4{C6H2R′2NdCH-C6H4-(H)N-
C6H2R2}2 (o-C6H4) are prepared. Successive additions of Me2Zn and SO2 gas to the bis(anilido-aldimine)
compounds afford quantitatively dinuclear µ-methylsulfinato zinc complexes, o-C6H4{(C6H2R2NdCH-C6H4-
N(C6H3R′2)-κ2-N,N)Zn(µ-OS(O)Me)}2 (R ) iPr and R′ ) iPr, 29; R ) Et and R′ ) Et, 30; R ) Me and R′
) Me, 31; R ) Me and R′ ) iPr, 32; R ) Et and R′ ) Me, 33; R ) Et and R′ ) iPr, 34; R ) iPr and R′ )
Et, 35) and o-C6H4{C6H2R′2NdCH-C6H4-N-C6H2R2-κ2-N,N)Zn(µ-OS(O)Me)}2 (o-C6H4) (R ) Et and R′
) Et, 36; R ) Me and R′ ) Me, 37; R ) iPr and R′ ) Me, 38; R ) Et and R′ ) Me, 39; R ) Me and R′
) iPr, 40). Molecular structures of 34 and 40 are confirmed by X-ray crystallography. Complexes 30-35
show high activity for cyclohexene oxide/CO2 copolymerization at low [Zn]/[monomer] ratio (1:5600), whereas
the complex of mononucleating â-diketiminate {[(C6H3Et2)NdC(Me)CHdC(Me)N(C6H3Et2)]Zn(µ-OS(O)Et)}2
shows negligible activity in the same condition. Activity is sensitive to the N-aryl ortho substituents and the
highest activity is observed with 32. Turnover number up to 2980 and molecular weight (Mn) up to 284 000
are attained with 32 at such a highly diluted condition as [Zn]/[monomer] ) 1:17 400. Macrocyclic complexes
36-40 show negligible activity for copolymerization.
Introduction
erization reaction by dinuclear homogeneous Ziegler-Natta
catalysts3 are among the typical examples. To achieve the
Bimetallic catalysis has been commonly observed in metal-
loenzymes, and scientific activities have been devoted not only
to reveal its action but also to mimic its advantageous
characteristics by constructing bimetallic systems.1 Asymmetric
aldol condensation by bimetallic zinc complexes2 and polym-
cooperative action of the two metal centers that is exhibited by
natural metalloenzymes, the two metals should be suitably
arranged in space by the construction of a well-designed ligand
system. Recently, a large number of â-diketiminato complexes
have been reported in bioorganic, main-group, and transition-
metal chemistry.4 We envisaged synthesis of bis(diketimine)
compounds such as I and II. A macrocyclic compound of type
II with R ) H was recently reported but the metal complexes
derived from the compound either are not soluble or decompose
in common organic solvents, thus hampering further studies.5
Most of the recently reported â-diketiminato complexes are
those derived from ligands bearing N-aryl ortho substituents.
The substituents endow the complexes not only with enhanced
solubility but also with more stability. Furthermore, the structure
and the reactivity are highly sensitive to the N-aryl ortho
substituents. Synthesis and application of macrocyclic com-
pounds is also currently a hot research field in supramolecular
chemistry.6 In the dinuclear complexes derived from I and II,
the two metals are situated opposite each other at a distance of
† Ajou University.
‡ LG Chem Ltd.
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10.1021/ja0435135 CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 3031-3037
3031

A R T I C L E S
Lee et al.
Scheme 1 a
∼5 Å, and consequently cooperative actions of the two metal
centers are to be expected in some catalysis reactions.
a (i) n-BuLi; (ii) B(OiPr)3; (iii)1,2-dibromobenzene, Pd(PPh3)4 (3 mol
%), Na2CO3; (iv) HCl.
in reduced activity. Even though the â-diketiminato zinc
complexes have a strong tendency to form the associated
dinuclear species in solution, the probability for the complexes
to be present as the relatively less active monomeric species is
increased by reducing the concentration of the catalyst, that is,
by reducing the [Zn]/[monomer] ratio in neat polymerization
conditions. Dissociation to less active monomeric species can
be excluded for bimetallic zinc complexes derived from I and
II, and hence high activity is expected even at a low [Zn]/
[monomer] ratio. By realization of high activity at the low mole
ratio, increase of turnover number (TON) and concomitant
increase of molecular weight by its living polymerization
character are expected. The copolymer shows brittle character.10
Increasing the molecular weight may be one of the tools to
overcome the brittleness.
Coates et al.7-9 have shown that the â-diketiminato zinc
complexes can serve as catalysts for epoxide/CO2 copolymer-
ization. The activity is highly sensitive to the N-aryl ortho
substituents. They attributed this sensitivity to the formation of
an associated bimetallic active species and proposed bimetallic
catalytic action as III.8 If the substituents are too small, such
as methyl, the complex forms a tightly bound bimetallic
complex, which is not active. If the substituents are too large,
the complex cannot form the bimetallic species and this results
Results and Discussion
Synthesis and Characterization. Key building blocks for
constructing compounds I and II are the 4,4′′-diamino-o-
terphenyl derivatives bearing alkyl substituents at the 3, 5, 3′′,
and 5′′ positions, for which the synthetic route is shown in
Scheme 1. N-Diphenylmethylene-2,6-dialkyl-4-bromoanilines
1-3 are prepared in 50-g scale by the tetraethyl orthosilicate-
mediated Schiff base condensation of benzophenone with the
corresponding 2,6-dialkyl-4-bromoanilines.11 The diphenylketimine
functional group is stable to lithiation conditions [n-BuLi,
tetrahydrofuran (THF), -78 °C]12 and hence the bromo
compounds 1-3 can be converted to boronic acids 4-6 by the
conventional method, in good yields (82-87%). Suzuki coupling
reactions of the boronic acids with 1,2-dibromobenzene and
subsequent hydrolysis of the diphenylketimine group under
acidic conditions affords blue fluorescent diamino compounds
7-9 in 82-87% yields. The synthetic method is so straight-
forward that 7 can be synthesized in 7-g scale without any
chromatographic purification procedure. Since various Schiff
base metal complexes of bulky 2,6-diisopropylaniline have been
developed as precursors for olefin polymerization catalysts,13
compounds 7-9 may be used as building blocks to construct
such bimetallic complexes.
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2004, 23, 5382.
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Reaction of 7-9 with excess 2,4-pentanedione affords the
desired â-ketoamines, but various attempts to construct com-
9
3032 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005

Bimetallic Anilido-Aldimine Zinc Complexes
A R T I C L E S
of the fluoride with Li(H)NAr affords the desired bis(anilido-
aldimine) compounds 13-19 in excellent yields (86-95%). The
1H and 13C NMR spectra and the elemental analysis data are in
agreement with the structures, and the connectivity of 15 is
confirmed by X-ray crystallography (Figure 1).
Thirty-membered macrocyclic bis(anilido-aldimine) com-
pounds are also effectively synthesized as well. The synthetic
route used for constructing the acyclic bis(anilido-aldimine)
compounds 13-19 (Scheme 2) does not work in these cases
and another was devised (Scheme 3). An amination reaction of
7-9 with 1,3-dioxolane-protected 2-bromobenzadehyde and
subsequent deprotection of the dioxolane group furnishes 20-
22 in 73-87% yields.15 Cyclization is carried out by refluxing
the aldehydes with HCl salts of 7-9 in ethanol for several days.
HCl salts of the cyclized compounds are deposited from the
ethanol solution and neutralized with aqueous NaHCO3 solution.
The macrocyclic compounds 23-28 are routinely purified by
column chromatography on silica gel and the isolated yields
are exceptionally high (73-88%). The high yields may be
attributed to a proton-template effect or to rigidity of the
Figure 1. Thermal ellipsoid plot (30% probability level) of 15. Selected
bond distances (in angstroms) and angles (in degrees): N(1)-C(15),
1.266(5); C(15)-C(16), 1.456(6); C(16)-C(21), 1.404(6); N(2)-C(21),
1.379(6); N(3)-C(38), 1.275(5); C(38)-C(39), 1.459(6); C(39)-C(44),
1.421(6); N(4)-C(44), 1.354(6); C(15)-N(1)-C(10), 117.6(4); N(1)-
C(15)-C(16), 126.1(5); C(21)-N(2)-C(22), 123.8(5); C(38)-N(3)-C(33),
121.8(4); N(3)-C(38)-C(39), 124.8(5); C(44)-N(4)-C(45), 126.8(5).
1
macrocycles.16 The H and 13C NMR spectra and fast atom
bombardment (FAB) mass data are in agreement with the ring
structure.
pounds of types I or II from the â-ketoamines were not
successful (eq 1). Instead of constructing I and II, syntheses of
related anilido-aldimine compounds were pursued (Scheme 2).14
Reactions of 13-19 and 24-28 with excess Me2Zn furnish
quantitatively the desired dinuclear methylzinc complexes
1
(Scheme 4). In the H NMR spectra, the N-H proton signals
that are observed in the 10-11 ppm range for 13-19 and 23-
28 completely disappear and new Zn-CH3 proton signals
appears at -0.5 to -1.0 ppm as singlets. The H NMR study
Scheme 2 a
1
indicates that the corresponding reaction of the macrocyclic
compound 23, which bears isopropyl groups on both sides, gives
cleanly a monometalated complex. Even heating to 80 °C for
several days does not provide the desired dinuclear complex.
Because â-diketiminato alkoxy or acetoxy zinc complexes are
reported to be effective catalysts for epoxide/CO2 copolymer-
ization, transformation of the methylzinc complexes to the
corresponding alkoxy or acetoxy complexes by treatment of
methanol or acetic acid were attempted, but the trails were
unsuccessful. The demetalated ligands were the main products.
Because alkylsulfinato complexes likewise were reported to
be active initiators for the epoxide/CO2 copolymerization,
transformation of the methylzinc complexes to the corresponding
methylsulfinato complexes was pursued (Scheme 4).17 Addition
of anhydrous SO2 gas to the methylzinc complexes results in
a (i) 2-Fluorobenzaldehyde; (ii) Li(H)N-C6H3R′2.
Schiff bases 10-12 are prepared by the conventional method
in good yields (73-87%). Nucleophilic aromatic substitution
Scheme 3 a
a (i) Pd(OAc)2 (0.8 mol %), DPEphos (1.2 mol %), NatOBu; (ii) HCl; (iii) HCl salt of 7-9; (iv) aq NaHCO3.
J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005 3033
9

A R T I C L E S
Scheme 4 a
Lee et al.
a (i) Me2Zn; (ii) SO2.
1
several sets of rather complex H and 13C NMR signals. The
Zn-CH3 signal is completely removed but several new singlet
signals, which can be assigned to S-CH3, are observed in the
are fully observed in the 13C NMR spectra. Only a set of signals
is observed for 31 and a set of signals is dominant with minimal
amounts of minor isomers (less than 10%) for 32-34. When
1
1
1-2 ppm range in the H NMR spectra (C6D6, 25 °C). Three
the temperature is increased to 100 °C in the H NMR study
NdCH signals are observed at 8.10, 8.07, and 8.04 ppm in 0.43:
0.31:0.26 ratio as singlets in the H NMR spectrum (C6D6, 25
(toluene-d8), several sets of signals collapse to a set of signals
for all complexes. The signals for the bis(anilido-imine) ligand
frame are sharp although the S-CH3 signals are very broad at
1.5-2.0 ppm. Similarly, several sets of signals (two sets for
36-37 and 39-40 and three sets for 38) are observed for the
macrocyclic complexes in the room-temperature 1H NMR
spectra, but in these cases too, the sets of signals collapse to a
set on heating to 100 °C in toluene-d8. Contrasting with the
observation of broad S-CH3 signals for 29-35, sharp S-CH3
1
°C) of 29, and four S-CH3 signals are observed as singlets at
1.93, 1.83, 1.65, and 1.53 ppm in 0.15:0.26:0.43:0.15 ratio.
Three S-C signals are also observed at 48.01 (weak), 46.89
(strong), and 48.06 (weak) ppm in the 13C NMR spectrum. These
observations may be explained by isomerism that arises from
the tetrahedral nature of the sulfur atom and the dinuclear
µ-methylsulfinate structure shown in Scheme 4. The similar
µ-ethylsulfinate-bridged structure was also observed for mono-
nucleating â-diketiminato complexes.17 For the µ-methylsulfi-
nate structure, three isomers are possible from the relative
configurations of the methyls at the tetrahedral sulfur centers,
as shown in Figure 2. For isomer A, the two S-CH3 methyls
1
signals are observed in the high-temperature H NMR spectra
of the macrocyclic complexes 36-40. Single crystals suitable
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Figure 2. Possible isomers for 29-35.
are not equivalent and observation of two separate signals is
1
expected in the H and 13C NMR spectra. For the B and C
(11) Love, B. E.; Boston, T. S.; Nguyen, B. T.; Rorer, J. R. Org. Prep. Proc.
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isomers, the two methyls are equivalent and hence a single signal
is expected. Observation of three NdCH signals in 0.43:0.31:
0.26 ratio and four S-CH3 signals in 0.15:0.26:0.43:0.15 ratio
can be well interpreted as the presence of the three isomers.
Observation of three S-CH3 signals instead of four in the 13C
NMR is probably due to collapse of two signals to a single
one. The same three sets of signals are observed in the 1H NMR
spectra of 30 and 35. In these cases, four S-CH3 carbon signals
(12) (a) Wolfe, J. P.; Åhman, J.; Sadighi, J. P.; Singer, R. A.; Buchwald, S. L.
Tetrahedron Lett. 1997, 38, 6367. (b) Singer, R. A.; Sadighi, J. P.;
Buchwald, S. L. J. Am. Chem. Soc. 1998, 120, 213.
(13) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283. (b)
Mecking, S.; Held, A.; Bauers, F. M. Angew. Chem., Int. Ed. 2002, 41,
545. (c) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169. (d) Mecking, S. Coord. Chem. ReV. 2000, 203, 325. (d) Johnson, L.
K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (e)
Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.;
McTavish, S. J.; Solan, G. A.; Stromberg, S.; White, A. J. P.; Williams,
D. J. Chem. Commun. 1998, 849 (f) Younkin T. R.; Connor, E. F.;
Henderson, J. I.; Friedrich S. K.; Grubbs, R. H.; Banselben, D. A. Science
2000, 287, 460.
(7) (a) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43, 6618.
(b) Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2002, 124, 14284. (c) Cheng, M.; Moore, D. R.; Reczek, J. J.;
Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
2001, 123, 8738. (d) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates,
G. W. Angew. Chem., Int. Ed. 2002, 41, 2599. (e) Byrne, C. M.; Allen, S.
D.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2004, 126, 11404.
(8) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem.
Soc. 2003, 125, 11911.
(14) Hayes, P. G.; Welch, G. C.; Emslie, D. J. H.; Noack, C. L.; Piers, W. E.;
Parvez, M. Organometallics 2003, 22, 1577.
(15) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem.
Res. 1998, 31, 805. (b) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852. (c)
Liang, L.-C.; Lee, W.-Y.; Hung, C.-H. Inorg. Chem. 2003, 42, 5471.
(16) Dutta, B.; Bag, P.; Adhikary, B.; Flo¨rke, U.; Nag, K. J. Org. Chem. 2004,
69, 5419.
(17) Eberhardt, R.; Allmendinger, M.; Luinstra, G. A.; Rieger, B. Organome-
tallics 2003, 22, 211.
9
3034 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005

Bimetallic Anilido-Aldimine Zinc Complexes
A R T I C L E S
Figure 3. Thermal ellipsoid plot (30% probability level) of 34. Selected
bond distances (in angstroms) and angles (in degrees): N(75)-Zn(78),
1.984(4); N(73)-Zn(77), 2.000(4); N(76)-Zn(78), 1.938(4); N(74)-Zn-
(77), 1.938(4); O(71)-Zn(78), 1.963(4); O(72)-Zn(78), 1.969(3); O(69)-
Zn(77), 1.943(3); O(70)-Zn(77), 1.990(3); C(65)-N(75), 1.304(5); C(66)-
N(73), 1.290(5); C(7)-N(76), 1.358(6); C(36)-N(74), 1.360(5); C(65)-
N(75)-C(13), 116.2(4); C(66)-N(73)-C(28), 116.9(4); C(7)-N(76)-C(1),
120.1(4); C(36)-N(74)-C(37), 117.4(4); N(76)-Zn(78)-N(75), 97.35(15);
N(74)-Zn(77)-N(73), 96.15(15); O(71)-Zn(78)-O(72), 103.61(15); O(69)-
Zn(77)-O(70), 103.96(14); O(70)-S(68)-O(72), 109.2(2); O(70)-S(68)-
C(63), 99.4(2); O(72)-S(68)-C(63), 99.0(3); Zn(77)-Zn(78), 4.867.
Figure 4. Thermal ellipsoid plot (30% probability level) of 40. Selected
bond distances (in angstroms) and angles (in degrees): Zn(1)-N(3), 1.978-
(5); Zn(2)-N(4), 1.978(5); Zn(1)-N(1), 1.928(5); Zn(2)-N(2), 1.939(5);
Zn(1)-O(2), 1.943(4); Zn(1)-O(3), 1.946(4); Zn(2)-O(1), 1.947(4);
Zn(2)-O(4), 1.955(4); C(13)-N(3), 1.307(8); C(44)-N(4), 1.294(7);
C(15)-N(1), 1.350(7); C(38)-N(2), 1.352(7); C(13)-N(3)-C(10), 122.9(5);
C(44)-N(4)-C(45), 121.8(5); C(15)-N(1)-C(20), 122.0(5); C(38)-N(2)-
C(35), 123.0(5); N(1)-Zn(1)-N(3), 97.5(2); N(2)-Zn(2)-N(4), 98.1(2);
O(2)-Zn(1)-O(3), 110.93(19); O(1)-Zn(2)-O(4), 109.74(19); Zn(1)-
Zn(2), 4.690.
and 8.987 Å for 34 and 15, respectively). The N(anilido)-
N(anilido) separation (7.988 Å) is significantly longer than the
N(imine)-N(imine) separation (6.829 Å).
for X-ray crystallography are obtained for complexes 34 and
40, where one isomer is predominant at room temperature, and
their molecular structures have been confirmed by X-ray
crystallography (Table 2, Figures 3 and 4).
Solid structure of 40 is shown in Figure 4 with selected bond
distances and angles. One of the two CH3-S fragments is
disordered. By forming a macrocyclic ligand framework, both
the Zn-Zn separation (4.690 and 4.867 Å for 40 and 34,
respectively) and the N(anilido)-N(anilido) separation (7.365
and 7.988 Å for 40 and 34, respectively) are contracted while
the N(imine)-N(imine) separation (7.139 and 6.829 Å for 40
and 34, respectively) is elongated. The reduced Zn-Zn separa-
tion induces not only reduction of the average Zn-O distance
(1.948 and 1.966 Å for 40 and 34, respectively) but also increase
of the O-Zn-O angle (average 110.34° and 103.79° for 40
and 34, respectively). Both the reduced Zn-O distance and the
shift of the O-Zn-O angles close to the ideal tetrahedral value
may imply that the methylsulfinate ligand is more tightly bound
in the macrocyclic complex 40. The N(imine)-ring A(cen-
troid)-N(imine) and the N(anilido)-ring B(centroid)-N(a-
nilido) angles are close to the ideal value of 60° (60.46° and
62.37°, respectively). Each zinc atom is situated in a plane
formed by the chelated ligand and the two chelated six-
membered rings are situated almost in a plane [angle between
the planes, 2.91(23)°].
Polymerization Studies. The main expected advantage of
the complexes 29-40 for epoxide/CO2 copolymerization is that
they can form a dinuclear active species, as proposed by Coates
and co-workers,8 even at a low [Zn]/[monomer] ratio. They
usually carried out the copolymerization reactions with the
â-diketiminato zinc complexes in a neat monomer with [Zn]/
[monomer] ) 1:1000.7,8 In that situation, the attainable maxi-
mum TON is limited to 1000 and actually it is further limited
to ∼500 by the limited conversion (∼50%) caused by viscosity.
Because the catalyst shows living character, the molecular
X-ray Crystallographic Studies. Figure 1 shows an Ortep
drawing of 15 with selected bond distances and angles. The
metal binding sites are not situated opposite each other as in
the macrocyclic compounds 23-28. Interestingly, the C benzene
ring is situated slightly bent away from the B ring, which is
measured by deviation of the C6-C30-C33 angle form the
normal 180° to 169.9°. The same directional bending of the
N3 atom from its normal position is also observed (C30-C33-
N3 angle, 169.6°). These deviations are probably due to the
repulsion between the π-electrons on the B and C rings.
Figure 3shows the µ-methylsulfinate structure of 34. It reveals
an eight-membered jagged metallacycle with a Zn-Zn separa-
tion of 4.867 Å, a distance slightly shorter than that observed
for the associated dimeric â-diketiminato complex{[(C6H3iPr2)Nd
C(Me)CHdC(Me)N(C6H3iPr2)]Zn(µ-EtSO2)}2 (4.98 Å).17 Each
zinc atom is situated on a plane formed by the chelated ligand
(dihedral angles of C31-C66-N73-Zn77 and C12-C65-
N75-Zn78, -2.2° and 0.1°, respectively), which contrasts with
the observation of a slightly puckered structure found
in {[(C6H3iPr2)NdC(Me)CHdC(Me)N(C6H3iPr2)]Zn(µ-EtSO2)}2
(the corresponding dihedral angle, 7.2°).17 Almost perpendicular
arrangements between the chelated six-membered rings and the
benzene rings attached to them are observed. The CdN(imine)
distances [1.304(5) and 1.290(5) Å] are increased by the
coordination of zinc [the corresponding distances in 15, 1.266-
(5) and 1.275(5) Å] while the C-N(anilido) distances are not
altered severely by the coordination. The N(imine)-ring A(cen-
troid)-N(imine) angle (57.41°) is substantially reduced by the
coordination (the corresponding angle for 15, 78.95°), which
triggers a reduction of the N(imine)-N(imine) separation (6.829
9
J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005 3035

A R T I C L E S
Lee et al.
Table 1. Cyclohexene Oxide (CHO)/CO2 Copolymerization
34, bearing ethyl and isopropyl groups, are so highly active that
the limited TONs (1560 and 1450, respectively) are almost
achieved in 5 h (entries 4 and 8). The TOFs are calculated to
be 312 and 290, respectively, and these values are substantially
higher when compared with that observed for {[(C6H3Et2)Nd
C(Me)CHdC(Me)N(C6H3Et2)]Zn(µ-OS(O)Et)}2 at high [Zn]/
[CHO] ratio of 1:1000 (TOF 164).17 Coates and co-workers8
reported that the zinc â-diketiminato complex, where both aryls
are 2,6-dimethylphenyl, is totally inactive because of formation
of tightly bound associated dimeric species, but the bis(anilido-
imine) complex 31, where both aryls bear methyl substituents,
shows fairly good activity (entry 3). Noteworthy is that 35,
which bears the isopropyl group in its imine moiety and ethyl
groups in the anilido moiety, shows low activity (TON 670,
TOF 67) while 32, which bears the same substituents but with
their positions exchanged, is highly active (TON 1450, TOF
290). The TON of 32 can be almost doubled (TON 2720) by
reducing the [Zn]/[CHO] ratio by 2-fold (1:11 200) and
simultaneously increasing the reaction time by 2-fold (entries
4 and 5). When the mole ratio is reduced 3-fold (to 1:16 800)
and the reaction time is tripled (to 15 h), the complex shows
still a higher TON (2980) but it is not tripled (entry 6). Some
deactivation by protic impurities in the monomer may be
inevitable in that highly diluted condition, thus not giving the
desired tripled TON.
Resultsa
e
e
entry
catalyst
time (h)
TONb
TOFc
% carbonated
Mn
Mw/Mn
f
1
2
3
29
30
31
32
32
32
33
34
10
10
5
10
15
10
5
1680 168
1060 106
1560 312
2720 272
2980 200
96
93
94
91
91
93
93
85
129 000
80 000
225 000
261 000
284 000
77 000
1.3
1.3
1.7
1.6
1.7
1.3
1.6
1.3
4
5g
6h
7
670
1450 290
67
8
205 000
88 000
9
10
11
35
10
10
10
670
f
56
67
5.6
36-40
BDIi
74
27 000
1.8
a Polymerization conditions: neat CHO (8.0 mL, 79 mmol), [Zn]/[CHO]
) 1:5600, 80 °C, 12 bar of CO2 (initial pressure). b Turnover number in
moles of CHO consumed per mole of Zn. c Turnover frequency in moles
of CHO consumed per mole of Zn per hour. d Estimated by 1H NMR
spectroscopy. e Determined by GPC, calibrated with polystyrene standard
in THF. f Negligible. g Neat CHO (16 mL), [Zn]/[CHO] ) 1:11 200. h Neat
i
CHO (24 mL), [Zn]/[CHO] ) 1:16 800. BDI ) {[(C6H3Et2)NdC(Me)CHd-
C(Me)N(C6H3Et2)]Zn(µ-OS(O)Et)}2.
weight of the obtained polymer is governed by the TON. To
achieve high TON, and consequently to obtain high molecular
weight polymer, the [Zn]/[monomer] ratio should be reduced.
The zinc complexes constructed from the mononucleating
â-diketiminate ligands exist in an equilibrium involving mon-
omeric and associated dimeric species. Even though some
complexes strongly favor the active dimeric species, the
probability for the less active monomeric species increases by
reducing the mole ratio, which might result in low activity under
low mole ratio conditions. Recently, [ArNdC(Me)CHdC(Me)-
NAr]ZnN(SiMe3)2 (Ar ) 2,6-diethylphenyl or 2,6-diisopropy-
lphenyl), which showed high activity at the relatively high [Zn]/
[cyclohexene oxide] ratios of 1:1700 and 1:1000, was reported
to show negligible activity (TOF 32) at the low mole ratio of
[Zn]/[cyclohexene oxide] ) 1:3300.18
The macrocyclic complexes 36-40 show negligible activity
in the standard polymerization condition. This negligible activity
is presumably attributed to the tight binding of the methylsul-
finate ligand, which hampers the initiation reaction. Observation
1
of the sharp S-CH3 signals in the H NMR spectra (100 °C,
toluene-d8) of 36-40 and observation of the relatively short
Zn-O distances and the ideal tetrahedral O-Zn-O angles in
the X-ray structure of 40 support that the methylsulfinate is
strongly bound in the preorganized macrocyclic rings, thus not
permitting initiation.
The standard polymerization condition is set at a more dilute
[Zn]/[cyclohexene oxide] ) 1:5800 to ascertain if 29-40 show
high activity even at the low mole ratio. Dissociation to the
less active monomeric species can be excluded for 29-40. To
realize the polymerization activity at such a low concentration
of catalysts, the cyclohexene oxide (CHO) and CO2 should be
rigorously purified (see Experimental Section in the Supporting
Information). Under standard conditions, a negligible amount
of polymer (TON 56, TOF 5.6; entry 11 in Table 1) is obtained
with {[(C6H3Et2)NdC(Me)CHdC(Me)N(C6H3Et2)]Zn(µ-OS-
(O)Et)}2, which was reported to show high activity at high [Zn]/
[CHO] ratio of 1:1000 (TON 328, TOF 164).17 The complex is
chosen as a comparison catalyst because the corresponding
acetoxy derivative was reported to have the strongest tendency
to form the associated dimeric species, even at high tempera-
ture.8 As expected, all acyclic bis(anilido-imine) complexes
except 29 show high activity under standard conditions. The
turn of frequency (TOF) is still sensitive to the N-aryl ortho
substituents, as is observed for the zinc complexes of the
mononucleating â-diketiminates. In the standard polymerization,
the solution is stirred by using a magnetic stirring bar, and in
that case, maximum conversion was limited to ∼30% by
viscosity, which limited the attainable TON to ∼1700. The
complex 32, bearing methyl and isopropyl groups, and complex
As expected, molecular weights of the polymer can be
increased by the increase of TONs and number-average mo-
lecular weight (Mn) up to 284 000 is attained (entry 6). The
reported molecular weights (Mn) of the polymers obtained by
the conventional zinc complexes of the mononucleating â-diketim-
inate ligands have not exceeded 50 000. Observation of rather
broad molecular weight distributions (Mw/Mn 1.3-1.7) may be
explained by some kind of chain transfer reaction at the high
polymerization temperature (85-75 °C). Coates and co-work-
ers7,8 observed narrow molecular weight distributions at
relatively lower polymerization temperature (50 °C), and Rieger
and co-workers17 also observed broadening of the molecular
weight distribution by increasing the polymerization temperature
to 70 °C. Molecular weights of polymers obtained in 5 h
polymerization time are in good agreement with those calculated
from the TONs (entries 4 and 8) but those of the polymers
obtained in 10 or 15 h are rather lower than the calculated
values, which implies that some kind of chain transfer reaction
occurs during the long polymerization times. The main drawback
of the CO2/CHO copolymer compared with the conventional
bisphenol A-based polycarbonate is the brittleness.10 The chain
entanglement molar mass, Me, was reported to be about 15 000,
which is comparable to that of brittle polystyrene but substan-
tially higher than that of bisphenol A-based polycarbonate
(1800). The increased molecular weight may induce to more
(18) van Meerendonk, W. J.; Duchateau, R.; Koning, C. E.; Gruter, G.-J. M.
Macromol. Rapid Commun. 2004, 25, 382.
9
3036 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005

Bimetallic Anilido-Aldimine Zinc Complexes
A R T I C L E S
Table 2. Crystallographic Parameters of 15, 34, and 40
15
34
40
formula
fw
T, K
a, Å
b, Å
C52H50N4
730.96
293(2)
8.1137(5)
13.1016(7)
39.0663(14)
90
92.2016(3)
90
4149.8(4)
monoclinic
P21/N
1.170
4
0.068
12 040
3548
C66H78N4O4S2Zn2‚(C2H5O0.5
)
C68H72N4O4S2Zn2
1204.26
150(2)
41.0701(2)
15.8235(5)
22.6557(8)
90
101.666(2)
90
14 419.2(7)
monoclinic
C2/c
1.109
8
0.767
52 341
15 374
812
0.0864
0.2650
1223.34
150(2)
24.4623(10)
12.9181(3)
24.3219(10)
90
118.4620(10)
90
6756.9(4)
monoclinic
P2/a
1.203
4
0.819
65 938
15 472
810
0.0717
0.2156
1.057
c, Å
R, deg
â, deg
γ, deg
V, Å3
crystal system
space group
D(calc), g‚cm-1
Z
µ, mm-1
no. of data collected
no. of unique data
no. of variables
R (%)
521
0.0528
0.1026
1.006
Rw (%)
goodness of fit
1.019
2
a Data collected with Mo KR radiation [λ(KR) ) 0.7107 Å]; R(F) ) ∑||Fo| - |Fc||/∑|Fo| with Fo > 2.0σ(I); Rw ) [∑[w(Fo2 - Fc )2]/∑[w(Fo)2]2]1/2 with
Fo > 2.0σ(I).
efficient chain entanglements, consequently resulting in in-
creased toughness.
Activity is sensitive to the N-aryl ortho substituents and the
highest activity is observed with 32, bearing methyl and
isopropyl substituents. The complex shows high activity at
extremely dilute conditions such as [Zn]/[monomer] ) 1:16 800
and TON up to 2980 is attained. High molecular-weight
polymers (Mn up to 284 000) are obtained by the increase of
TONs. The macrocyclic complexes 36-40 show negligible
activity. While this work strongly supports the bimetallic
mechanism proposed by Coates and co-workers8 for the
copolymerization of CHO/CO2, others such as Darensbourg and
co-workers with (Salen)Cr(III) and Chisholm and co-workers
with (porphyrin)Al(III) have observed a reaction pathway that
involves only one metal center.9 Different metals with different
ligand sets may well react via different pathways.
The carbonate linkage is more than 90% for all polymers
obtained by the catalysts showing high activity (entries 2-8).
Only the polymer obtained by 35, which shows relatively lower
activity, has rather low carbonate linkage (85%). The copolymer
obtained by the comparison catalyst under standard conditions
contains a substantial amount of ether linkage (carbonate
1
linkage, 74%). The H NMR spectra of the polymerization
solutions indicate that only a few percent (3-4%) of cyclic
carbonate compound is formed during the polymerization.
Conclusion
Acyclic bis(anilido-aldimine) compounds (13-19 in Scheme
2) and 30-membered cyclic bis(anilido-aldimine) compounds
(23-28 in Scheme 3) have been prepared. Yields in the
macrocyclic forming reaction are exceptionally high (73-88%).
Dinuclear µ-methylsulfinato zinc complexes (29-40 in Scheme
4) have been prepared from the bis(anilido-aldimine) compounds
by successive addition of Me2Zn and SO2 gas. Solid structures
of 34 and 40 have been determined by X-ray crystallography.
The acyclic complexes 30-35 show high activity for cyclo-
hexene oxide/CO2 copolymerization even at low [Zn]/[mono-
mer] ratio, typically 1:5600, whereas the â-dikeminate com-
plex {[(C6H3Et2)NdC(Me)CHdC(Me)N(C6H3Et2)]Zn(µ-OS-
(O)Et)}2 shows negligible activity at the same condition.
Acknowledgment. This work was supported by the Ministry
of Science and Technology of Korea through Research Center
for Nanocatalysis, one of the National Science Programs for
Key Nanotechnology. We also thank LG Chem Ltd. for financial
support.
Supporting Information Available: CIF files for 15, 34, and
40 and Experimental Section (PDF, CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0435135
9
J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005 3037