Four- and five-coordinate aluminum ketiminate complexes: Synthesis, characterization, and ring-opening polymerization

Article abstract of DOI:10.1021/ic025785j

A series of aluminum complexes featuring with the ketiminate ligand, OCMeCHCMeNHAr (Ar = 2,6-ⁱPr₂C₆H₃, 1), have been prepared and characterized spectroscopically and structurally. Reactions of 1 with trialkylaluminum in 1:1 or 1:2 molar ratio generate four- and five-coordinated aluminum complexes (OCMeCHCMeNAr)AlR₂ (R = Me (2); R = Et (3)) and (OCMeCHCMeNAr)₂AlR (R = Me (4); R = Et (5)) in high yields. Similarly, reaction of AlCl₃ with 1 or 2 equiv of the lithiated 1 in toluene afforded bis(ketiminate) aluminum chloride complex, (OCMeCHCMeNAr)₂AlCl (6) or (OCMeCHCMeNAr)AlCl₂ (7). Surprisingly, reacting 6 with 1 equiv of AgBF₄ in methylene/acetonitrile mixsolvents generates (OCMeCHCMeNAr)₂AlF (8) in moderate yield. The structures of complexes 2-6 and 8 have been determined by X-ray crystallography. Complexes 2 and 3 both exhibit tetrahedron structures with the aluminum atom surrounded by oxygen and nitrogen atoms of chelating ketiminate and two alkyl groups. The mono- and bis-ketiminate aluminum complexes 2 - 5 have shown moderate activity toward the ring-opening polymerization of ε-caprolactone.

Full text of DOI:10.1021/ic025785j

Inorg. Chem. 2002, 41, 6450−6455
Four- and Five-Coordinate Aluminum Ketiminate Complexes: Synthesis,
Characterization, and Ring-Opening Polymerization
,†
Ru-Ching Yu,† Chen-Hsiung Hung,† Jui-Hsien Huang,* Horng-Yi Lee,‡ and Jwu-Ting Chen‡
Department of Chemistry, National Changhua UniVersity of Education, Changhua, Taiwan 500,
and Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 106
Received June 10, 2002
i
A series of aluminum complexes featuring with the ketiminate ligand, OCMeCHCMeNHAr (Ar ) 2,6-Pr2C H , 1),
6
3
have been prepared and characterized spectroscopically and structurally. Reactions of 1 with trialkylaluminum in
1:1 or 1:2 molar ratio generate four- and five-coordinated aluminum complexes (OCMeCHCMeNAr)AlR (R ) Me
2
(2); R ) Et (3)) and (OCMeCHCMeNAr)2AlR (R ) Me (4); R ) Et (5)) in high yields. Similarly, reaction of AlCl3
with 1 or 2 equiv of the lithiated 1 in toluene afforded bis(ketiminate) aluminum chloride complex, (OCMeCHCMeNAr)2AlCl
(6) or (OCMeCHCMeNAr)AlCl2 (7). Surprisingly, reacting 6 with 1 equiv of AgBF in methylene/acetonitrile mix-
4
solvents generates (OCMeCHCMeNAr)2AlF (8) in moderate yield. The structures of complexes 2−6 and 8 have
been determined by X-ray crystallography. Complexes 2 and 3 both exhibit tetrahedron structures with the aluminum
atom surrounded by oxygen and nitrogen atoms of chelating ketiminate and two alkyl groups. The mono- and
bis-ketiminate aluminum complexes 2−5 have shown moderate activity toward the ring-opening polymerization of
ꢀ-caprolactone.
Chart 1
Introduction
The aluminum complexes draw considerable attention for
organic synthesis1 and polymerization2 due to their strong
Lewis acidity and inexpensiveness. In addition to the
aluminum atom itself, the surrounded coordinating ligands
affect the properties of the aluminum complexes sterically
and electronically. â-Diketiminate3 and Schiff base (Chart
1), monoanionic bidentate ligands, are used extensively in
synthesizing organometallic complexes.4 More recently, the
â-diketiminate and Schiff base, with bulky substitutents, have
raised much attention because they are able to stabilize metal
complexes5 and to polymerize cyclic monomer.
strained ligands leave the catalysts more open toward the
entering monomer. To obtain stable aluminum complexes
as well as good activity toward monomer, we have decided
to investigate the aluminum complexes containing ketiminate
ligand. Few studies have involved the aluminum complexes
with mono anionic bidentate ketiminate ligands (Chart 1).
In viewing the coordinating sphere of the metal complexes,
steric bulky ligands can stabilize catalysts; however, con-
* E-mail: juihuang@cc.ncue.edu.tw.
† National Changhua University of Education.
(4) Qian, B.; Scanlon, W. J., IV; Smith, M. R., III. Organometallics 1999,
18, 1693. (b) Qian, B.; Ward, D. L.; Smith, M. R., III. Organometallics
1998, 17, 3070. (c) Rahim, M.; Taylor, N. J.; Xin, S.; Collins, S.
Organometallics 1998, 17, 1315. (d) Kakaliou, L.; Scanlon, W. J.,
IV.; Qian, B.; Baek, S. W.; Smith, M. R., III. Inorg. Chem. 1999, 38,
5964. (e) Vollmerhaus, R.; Rahim, M.; Tomaszewski, R.; Xin, S.;
Taylor, N. J.; Collins, S. Organometallics 2000, 19, 2161. (f)
Hitchcock, P. B.; Lappert, M. F.; Liu, D.-S. J. Chem. Soc. Chem.
Commun. 1994, 1699. (g) Hitchcock, P. B.; Lappert, M. F.; Liu, D.-
S. J. Chem. Soc. Chem. Commun. 1994, 2637. (h) Budzelaar, P. H.
M.; Moonen, N. N. P.; de Gelder, R.; Smits, J. M. M.; Gal, A. W.
Eur. J. Inorg. Chem. 2000, 753.
‡ National Taiwan University.
(1) Yamamoto, H., Ed. Lewis Acids in Organic Synthesis; Wiley-VCH:
New York, 2000.
(2) Bochmann, M.; Dawson, D. M. Angew. Chem., Int. Ed. Eng. 1996,
35, 2226. (b) Atwood, D. A.; Jegier, J. A.; Rutherford, D. J. Am. Chem.
Soc. 1995, 117, 6779. (c) Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 1997, 119, 8125.
(3) Parks, J. E.; Holm, R. H. Inorg. Chem. 1968, 7, 1408. (b) Hitchcock,
P. B.; Lappert, M. F.; Layh, M. Chem. Commun. 1998, 201. (c) Clegg,
W.; Cope, E. K.; Edwards, A. J.; Mair, F. S. Inorg. Chem. 1998, 37,
2317.
6450 Inorganic Chemistry, Vol. 41, No. 24, 2002
10.1021/ic025785j CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/02/2002

Aluminum Ketiminate Complexes
Scheme 1
Scheme 2
in 1:2 ratio, bis(ketiminate) aluminum complexes, (OCMeCH-
CMeNAr)2AlR (R ) Me (4); R ) Et (5)), were obtained.
However, complexes 4 and 5 are more sterically hindered
and hence more strict conditions, i.e., refluxing for 3 days
in toluene, are needed in order to obtain reasonable yield.
All the compounds have been characterized by 1H and 13
C
NMR spectra, where the proton and carbon of the methine
groups of the backbone serve as excellent indicators for
evaluating the purity of the metal complexes. All the
complexes 2-5 show one resonance for the methine of the
backbone at ca. δ 5.0 and ca. δ 100.0 for 1H and 13C NMR
spectroscopy, respectively. For the mono-ketiminate com-
pounds 2 and 3, one isopropyl methine resonance and two
isopropyl methyl resonances were observed. This is consis-
tent with fast ring inversion, as shown in Scheme 2, and
slow aryl rotation. For the bis-ketiminate complexes 4-6,
the two ketiminate ligands are equivalent but the two
isopropyl groups on a given ketiminate are inequivalent and
the two methyl groups within each isopropyl group are
inequivalent. Therefore four methyl and two methine reso-
nances for the isopropyl groups are expected for the slow
ring inversion and slow aryl rotation. However, the 1H NMR
spectra of 4-6 are all shown one methyl resonance for the
isopropyl groups; that is no surprise due to the overlapping
of the methyl groups. By carefully reviewing the 13C{1H}
NMR data, all the methyl resonances of the isopropyl groups
in complexes 4-6 can be resolved.
Reactions of AlCl3 with 1 or 2 equiv of the lithiated
ketimine 1, obtained from the reaction of ketimine 1 with
2.5 M n-BuLi in toluene, afforded bis(ketiminate) aluminum
chloride complex, (OCMeCHCMeNAr)2AlCl (6) or (OC-
MeCHCMeNAr)AlCl2 (7) in high yield (Scheme 1). The
methine proton of the backbone of complexes 6 and 7
exhibits one resonance at ca. δ 5.2 for the 1H NMR
spectroscopy. Again, the methine protons of the isopropyl
groups of the bis-ketiminate complex 6 show two resonances
at δ 3.30 and 2.83, while that of the mono-ketiminate
complex 7 shows only one resonance at δ 3.0.
Bidentate monoanionic ketiminate ligands bind to metals that
can result in a six-membered ring and cause the metal atoms
surrounded by bulky substitutents on one side but left the
other side open. Here we report the syntheses and charac-
terization of aluminum complexes and their reactivity toward
ꢀ-caprolactone.
Results and Discussion
Synthesis and Characterization. Ketimine (1) can be
prepared easily according to the published procedure3a by
reacting a 1:1 ratio of 2,4-pentandione and 2,6-diisopropyl-
aniline in methanol with a small amount of formic acid as
catalysts. Reactions of ketimine 1 (Ar ) 2,6-iPr2C6H3) with
trialkylaluminum in 1:1 molar ratio generate four-coordinated
aluminum complexes (OCMeCHCMeNAr)AlR2 (R ) Me
(2); R ) Et (3)) in high yield (Scheme 1). The reactions
proceed along with the elimination of 1 equiv of methane
or ethane. Similarly, while reacting 1 and trialkylaluminum
(5) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. O. Organometallics 1997, 16, 1514. (b)
Hardman, N. J.; Power, P. P. Inorg. Chem. 2001, 40, 2474. (c) Stender,
M.; Eichler, B. E.; Hardman, N. J.; Power, P. P.; Prust, J.; Noltemeyer,
M.; Roesky, H. W. Inorg. Chem. 2001, 40, 2794. (d) Radzewich, C.
E.; Guzei, I. A.; Jordan, R. F. J. Am. Chem. Soc. 1999, 121, 8673. (e)
Gibson, V. C.; Segal, J. A.; White, A. J. P.; Williams, D. J. J. Am.
Chem. Soc. 2000, 122, 7120. (f) Radzewich, C. E.; Coles, M. P.;
Jordan, R. F. J. Am. Chem. Soc. 1998, 120, 9384. (g) Cui, C.; Roesky,
H. W.; Hao, H.; Schmidt, H.-G.; Noltemeyer, M. Angew. Chem., Int.
Ed. 2000, 39, 1815. (h) Cui, C.; Roesky, H. W.; Schmidt, H.-G.;
Noltemeyer, M.; Hao, H.; Cimpoesu, F. Angew. Chem., Int. Ed. 2000,
39, 4274. (i) Gibson, V. C.; Maddox, P. J.; Newton, C.; Redshaw, C.;
Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998,
1651. (j) Lee, L. W. M.; Piers, W. E.; Elsegood, M. R. J.; Clegg, W.;
Parvez, M. Organometallics 1998, 18, 2947. (k) Cheng, M.; Lobk-
ovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998, 120, 11018. (l)
Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 1999, 121, 11583.
Surprisingly, attempts to synthesize cationic aluminum
complex by reacting 6 with 1 equiv of AgBF4 in methylene
chloride/acetonitrile mix-solvent generates (OCMeCH-
CMeNAr)2AlF (8) in moderate yield (Scheme 3). However,
small amounts of unidentified compounds were presented
in the product, even after repeated recrystallization. This
prevents us from obtaining satisfactory elemental analysis.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6451

Yu et al.
Table 1. Crystallographic Data and Structure Refinement for Complexes 2-6 and 8
2
3
4
5
6
8
empirical formula
fw
temp, K
wavelength, Å
cryst syst
space group
unit cell dimens
a, Å
C18H30AlNO
315.42
293(2)
0.71073
triclinic
P1h
C21H34AlNO
343.47
293(2)
0.71073
monclinic
P21/n
C35H51AlN2O 2
558.76
293(2)
0.71073
triclinic
P1h
C36H53AlN2O 2
572.78
293(2)
0.71073
triclinic
P1h
C34H48AlClN2 O2 C34H48AlFN2 O2
579.17
293(2)
0.71073
triclinic
P1
562.72
293(2)
0.71073
monclinic
C2/c
8.9531(12)
13.5605(17)
33.193(4)
89.897(3)
89.967(3)
89.915(2)
4029.9(9)/2
1.040
10.188(5)
20.722(10)
10.400(5)
90
95.002(9)
90
2187.2(18)/4
1.043
0.099
10.4201(12)
12.3385(15)
14.6368(18)
70.265(2)
87.053(2)
72.735(2)
1689.0(4)/2
1.099
10.7606(7)
12.3944(8)
14.6146(10)
70.1030(10)
85.324(2)
71.6600(10)
1739.0(2)/2
1.094
10.416(3)
12.683(4)
14.364(4)
69.148(6)
87.452(7)
71.598(5)
1677.6(9)/1
0.573
24.1082(17)
11.4039(8)
12.8490(9)
90
110.0220(10)
90
b, Å
c, Å
R, deg
â, deg
γ, deg
vol, Å3/Z
calcd density, Mg/m3
abs coeff, mm-1
F(000)
3319.0(4)/4
1.126
0.103
1376
0.091
608
0.090
624
0.085
0.097
752
312
1216
cryst size, mm
θ for data collection, deg 1.23-27.55
limiting indices
0.37 · 0.32 · 0.29
0.33 · 0.25 · 0.21 0.38 · 0.35 · 0.29 0.33 · 0.31 · 0.25 0.34 · 0.33 · 0.29
0.32 · 0.28 · 0.23
1.80-27.57
-30 e h e +30
-14 e k e +14
-12 e l e +16
10 345
1.97-27.61
1.48-27.53
1.48-27.56
1.52-27.50
-11 e h e +11
-13 e h e +12 -12 e h e +13 -14 e h e +7
-13 e h e +11
-17 e k e +15
-43 e l e +37
25 685
-26 e k e +20 -16 e k e +14 -16 e k e +16 -16 e k e +14
-13 e l e +13 -19 e l e +19 -18 e l e +18 -18 e l e +13
no. of reflns collected
indep reflns
13 657
10 761
22 378
10 686
17 816
4 997
7 475
7 691
8 836
3 796
(Rint ) 0.0281)
empirical used SADABS
full-matrix least-squares on F2
17816/0/825
(Rint ) 0.0997)
(Rint ) 0.0267)
(Rint ) 0.0216)
(Rint ) 0.0367)
(Rint ) 0.0259)
abs corr
refinement method
data/restraints /params
GOF on F2
4997/0/225
0.710
7475/0/374
0.593
7691/0/383
0.885
8836/3/745
0.511
3796/0/184
0.781
0.544
final R indices
[I > 2σ(I)]
R indices (all data)
R1 ) 0.0365
R1 ) 0.0589
wR2 ) 0.1148
R1 ) 0.1532
wR2 ) 0.1373
R1 ) 0.0371
wR2 ) 0.0637
R1 ) 0.1521
wR2 ) 0.0753
R1 ) 0.0450
wR2 ) 0.1050
R1 ) 0.1138
wR2 ) 0.1174
R1 ) 0.0338
wR2 ) 0.0565
R1 ) 0.1654
wR2 ) 0.0752
R1 ) 0.0373
wR2 ) 0.0918
R1 ) 0.0789
wR2 ) 0.1004
wR2 ) 0.0622
R1 ) 0.2101
wR2 ) 0.0800
Scheme 3
The 1H and 13C NMR spectra of 8 are very similar to that of
6. Structurally characterized aluminum fluoride complexes
have been seen;6 however, no example of halogen exchange
between aluminum chloride and fluoroborate was observed.
Structure of Ketiminate Aluminum Complexes. The
structures of complexes 2-6 and 8 are determined by X-ray
single-crystal analyses. The crystal data and structure refine-
ments of all complexes are summarized in Table 1. Selected
bond lengths and angles are listed in Table 2.
Figure 1. ORTEP drawing of complex 2 with the ellipsoids drawn at
50% probability. The hydrogen atoms are omitted for clarity.
is shown in Figure 2. Complexes 2 and 3 both exist as
tetrahedron structures with the aluminum atom surrounded
by the oxygen and nitrogen atoms of chelating ketiminate
and two alkyl groups. The biting angles of ketiminate ligands,
N(1)-Al(1)-O(1) (which are 93.9(3) and 94.56(9)° for 2
and 3, respectively), are smaller than the regular tetrahedral
bond angle of 109.28°. The backbone of ketiminate, O(1)-
C(1)-C(2)-C(3)-N(1), forms a plane, and the aluminum
atom is deviated from the plane at 0.6828 and 0.5035 Å for
complexes 2 and 3, respectively. The bond lengths between
atoms of the ketiminate backbone of complexes 2 and 3 are
all in the same ranges, which are comparable to those of
similar Zr and Ti complexes reported by Smith.4d
Only one of the four independent molecules existing in
an unit cell of complex 2 is shown, and its ORTEP drawing
is displayed in Figure 1. The ORTEP drawing of complex 3
(6) Rennekamp, C.; Wessel, H.; Roesky, H. W.; Muller, P.; Schmidt, H.-
G.; Noltemeyer, M.; Uson, I.; Barron, A. R. Inorg. Chem. 1999, 38,
5235. (b) Roesky, H. W.; Stasch, A.; Hatop, H.; Rennekamp, C.;
Hamilton, D. H.; Noltemeyer, M.; Schmidt, H.-G. Angew. Chem., Int.
Ed. 2000, 39, 171. (c) Hatop, H.; Roesky, H. W.; Labahn, T.; Ropken,
C.; Sheldrick, G. M.; Bhattachatjee, M. Organometallics 1998, 17,
4326. (d) Waezsada, S. D.; Feng-Quan Liu.; Barnes, C. E.; Roesky,
H. W.; Montero, M. L.; Uson, I. Angew. Chem., Int. Ed. 1997, 36,
2625. (e) Schnitter, C.; Klimek, K.; Roesky, H. W.; Albers, T.;
Schmidt, H.-G.; Ro¨pken, C.; Parisini, E. Organometallics 1998, 17,
2249.
Complexes 4-6 and 8 exist as five-coordinated trigonal
bipyramidal structures, with the two oxygen atoms in the
axial positions. There are two independent molecules in a
6452 Inorganic Chemistry, Vol. 41, No. 24, 2002

Aluminum Ketiminate Complexes
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes
2-6 and 8
Compound 2
Al(1)-O(1)
Al(1)-C(18)
N(1)-C(3)
C(2)-C(1)
1.795(5)
1.955(7)
1.304(8)
1.364(10)
Al(1)-C(19)
Al(1)-N(1)
C(2)-C(3)
C(1)-O(1)
1.7949(6)
1.954(6)
1.419(9)
1.302(9)
O(1)-Al(1)-C(18)
C(19)-Al(1)-C(18)
C(19)-Al(1)-N(1)
107.3(3)
118.6(4)
111.3(3)
O(1)-Al(1)-C(19)
O(1)-Al(1)-N(1)
C(18)-Al(1)-N(1)
111.6(3)
93.9(3)
111.3(3)
Compound 3
Al(1)-O(1)
Al(1)-C(20)
N(1)-C(3)
C(2)-C(1)
1.788(2) Al(1)-C(18)
1.950(3) Al(1)-N(1)
1.314(3) C(2)-C(3)
1.365(3) C(1)-O(1)
1.942(3)
1.958(2)
1.423(3)
1.305(3)
Figure 2. ORTEP drawing of complex 3 with the ellipsoids drawn at
50% probability. The hydrogen atoms are omitted for clarity.
O(1)-Al(1)-C(18)
113.27(12) O(1)-Al(1)-C(20) 105.35(13)
94.56(9)
111.17(11) C(20)-Al(1)-N(1) 114.19(12)
C(18)-Al(1)-C(20) 116.15(15) O(1)-Al(1)-N(1)
C(18)-Al(1)-N(1)
Compound 4
Al(1)-O(1)
Al(1)-C(35)
Al(1)-N(2)
C(2)-C(3)
C(1)-O(1)
C(8)-C(7)
C(6)-O(2)
1.8202(16)
1.975(2)
2.0518(18)
1.416(3)
1.284(2)
1.407(3)
1.288(2)
Al(1)-O(2)
Al(1)-N(1)
N(1)-C(3)
C(2)-C(1)
N(2)-C(8)
C(7)-C(6)
1.8076(16)
2.0444(17)
1.318(2)
1.363(3)
1.316(2)
1.355(3)
N(2)-Al(1)-N(1)
O(2)-Al(1)-O(1)
O(1)-Al(1)-N(1)
133.61(7)
160.32(8)
87.75(7)
C(35)-Al(1)-N(2)
C(35)-Al(1)-N(1)
O(2)-Al(1)-N(2)
116.31(8)
110.05(8)
87.41(7)
Compound 5
Figure 3. ORTEP drawing of complex 4 with the ellipsoids drawn at
50% probability. The hydrogen atoms are omitted for clarity.
Al(1)-O(1)
Al(1)-C(35)
Al(1)-N(2)
C(2)-C(3)
C(1)-O(1)
C(18)-C(19)
N(2)-C(20)
1.8098(13)
1.957(2)
2.0678(17)
1.412(3)
1.287(3)
1.351(3)
1.326(2)
Al(1)-O(2)
Al(1)-N(1)
N(1)-C(3)
C(2)-C(1)
O(2)-C(18)
C(19)-C(20)
1.8181(14)
2.0616(18)
1.325(2)
1.352(3)
1.290 (2)
1.414(2)
N(2)-Al(1)-N(1)
O(2)-Al(1)-O(1)
O(1)-Al(1)-N(1)
138.25(7)
155.97(8)
87.35(7)
C(35)-Al(1)-N(2)
C(35)-Al(1)-N(1)
O(2)-Al(1)-N(2)
111.24(9)
110.51(9)
87.52(7)
Compound 6
Al(1)-O(1)
Al(1)-Cl(1)
Al(1)-N(2)
C(14)-C(15)
C(16)-O(1)
C(33)-C(32)
N(2)-C(31)
1.805(9)
Al(1)-O(2)
Al(1)-N(1)
N(1)-C(14)
C(15)-C(16)
O(2)-C(33)
C(31)-C(32)
1.799(9)
2.021(12)
1.346(14)
1.32(2)
1.290 (14)
1.448(15)
2.170(6)
2.035(12)
1.374(18)
1.281(15)
1.375(18)
1.358(13)
Figure 4. ORTEP drawing of complex 5 with the ellipsoids drawn at
50% probability. The hydrogen atoms are omitted for clarity.
N(2)-Al(1)-N(1)
O(2)-Al(1)-O(1)
O(1)-Al(1)-N(1)
134.6(4)
168.8(5)
88.6(5)
Cl(1)-Al(1)-N(2)
Cl(1)-Al(1)-N(1)
O(2)-Al(1)-N(2)
108.2(4)
117.2(3)
91.2(5)
Compound 8
Al(1)-O(1)
Al(1)-F(1)
Al(1)-N(1A)
N(1)-C(3)
O(1) -C(1)
C(5)-C(1A)
1.804(1)
1.6775(14)
1.9984(12)
1.329(2)
1.287(2)
1.357(2)
Al(1)-O(1A)
Al(1)-N(1)
N(1)-C(3)
C(1)-C(5A)
C(3)-C(5)
1.804(1)
1.9984(12)
1.329(2)
1.357(2)
1.410 (2)
N(1) -Al(1)-N(1A)
O(1) -Al(1)-O(1A)
O(1)-Al(1)-N(1A)
134.49(7)
169.64(8)
89.22(5)
F(1) -Al(1)-N(1)
F(1) -Al(1)-N(1A)
O(1A)-Al(1)-N(1)
112.76(4)
112.76(4)
89.22(5)
Figure 5. ORTEP drawing of complex 6 with the ellipsoids drawn at
50% probability. The hydrogen atoms are omitted for clarity.
unit cell for complex 6; however, only one is described due
to their similarity. The ORTEP drawings of complex 4-6
and 8 are shown in Figures 3-6. The bond lengths of
aluminum atom to oxygen and nitrogen atoms in complexes
4-6 and 8 are all in the ranges of 1.80 and 2.00 Å,
respectively. Moreover, the total angles of the trigonal plane
for all the complexes are close to 360°. However, in
comparing the angles of axial atoms, the more sterically
crowded ethyl group bends the angle of O(2)-Al(1)-O(1)
of complex 5 away from linear to 155.97(8)°, while the less
hindered fluorine atoms keeps the angle of O(1A)-Al(1)-
O(1) of complex 8 closer to linearity at 169.64(5)°. Again,
Inorganic Chemistry, Vol. 41, No. 24, 2002 6453

Yu et al.
Scheme 4
Figure 6. ORTEP drawing of complex 8 with the ellipsoids drawn at
50% probability. The hydrogen atoms are omitted for clarity.
results also indicate that the catalysts serve as single site
catalysts for the ring-opening polymerization of ꢀ-caprolac-
tone. However, by conversion of complexes 2-6 to alumi-
num alkoxide complexes may be a trend for obtaining a
polymer with narrower PDI.
Table 3. Ring-Opening Polymerization of ꢀ-Caprolactone Catalyzed by
Aluminum Ketiminate Complexesa
entry catalyst monomer (g) time(h) temp (°C)
Mn
PDI yield
A possible polymerization mechanism for the ring-opening
polymerization is shown in Scheme 4a. An anionic coordina-
tion mechanism for the ring-opening polymerization of
ꢀ-caprolactone can be observed from the reactions of entries
1-8 and entries 9-10. While the more reactive Al-alkyl
complexes were used as catalysts, the reactions were faster
and higher molecular weight polymers resulted. On the other
hand, while the Al-chloride complex was used as catalyst,
a low molecular weight oligomer was obtained, indicating
the inert Al-chloride bond. However, we are not able to
eliminate the possibility of initiation occurring at the Al-
ketiminate bond, as shown in Scheme 4b. A similar polym-
erization mechanism has been proposed by Chen.7g That is
observed from the interesting results in which the polymer
molecular weight increased sharply from the methyl to ethyl
derivatives (entry 3 vs 1; entry 7 vs 5). One possible reason
is the bulkier ethyl group protecting the active species from
aggregation, which results in a higher polymer. A more
detailed mechanism is under investigation.
1
2
2
2
3
3
4
4
5
5
6
6
3.26
3.26
2.99
2.99
1.84
1.84
1.80
1.80
1.77
1.77
3.5
2.4
3.5
2.4
3.5
2.4
3.5
2.4
3.5
2.4
60
25
60
25
60
25
60
25
60
25
34 997 1.99 93.7
25 210 1.64 93.7
10 0362 1.75 92.1
26 560 2.80 91.7
44 009 2.35 94.3
3 717 4.65 90.8
77 597 2.06 96.1
37 210 1.79 95.8
778 1.34 95.6
3
4
5
6
7
8
9
10
746 1.29 91.5
a Reaction conditions: 2 mL of toluene was used as solvent; 0.03 g of
catalyst was used for the polymerization; 300 equiv of monomer was used.
the bond lengths of ketiminate backbone of complexes 4-6
and 8 are comparable with those in complexes 2 and 3.
Ring Opening Polymerization of E-Caprolactone. Ring-
opening polymerizations of ꢀ-caprolactone by aluminum7 and
other metal complexes8 have been seen. The ring-opening
polymerization of ꢀ-caprolactone catalyzed by complexes
2-6 have been examined. In general, the ꢀ-caprolactone and
catalysts were placed in Schlenk flask separately and toluene
(2 mL) was filled. The monomer was added to the catalyst/
toluene solution and stirred at room temperature or 60 °C.
The results are listed in Table 3. Mono- and bis-ketiminate
aluminum alkyl complexes all show activities toward the
polymerization of ꢀ-caprolactone. However, the correspond-
ing aluminum chloride complex only results in oligomer. In
general, the higher temperature results in higher poly-
(caprolactone) for all catalysts (entries 1-8) excepting 6
(entries 9-10). In the case of entry 3, Mn reaches to 105.
The PDI values are generally low, which are comparable
with the diketiminate aluminum system by Chen7g but are
larger than aluminum7f and yittrium8a alkoxide systems. The
Concluding Remarks
We have been successful in the synthesis of a series of
aluminum complexes containing ketiminate ligands. All the
complexes are found to be active toward the ring-opening
polymerization of ꢀ-caprolactone. The aluminum alkyl
complexes are generally causing a higher molecular weight
polymer, while the aluminum halide complex only results
in low molecular weight oligomer. A mechanistic study of
the ring-opening polymerization as well as the steric effects
of the substitutents of the aluminum alkyl groups is our
current investigation.
(7) Huang, C.-H.; Wang, F.-C.; Ko, B.-T.; Yu, T.-L.; Lin, C.-C.
Macromolecules 2001, 34, 356. (b) Wu, B.; Lenz, R. W.; Hazer, B.
Macromolecules 1999, 32, 6856. (c) Tian, D.; Dubois, P.; Je´roˆme, R.
Macromolecules 1997, 30, 2575. (d) Duda, A. Macromolecules 1996,
29, 1399. (e) Chen, H.-L.; Ko, B.-T.; Huang, B.-H.; Lin, C.-C.
Organometallics 2001, 20, 5076. (f) Liu, Y.-C.; Ko, B.-T.; Lin, C.-C.
Macromolecules 2001, 34, 6196. (g) Chakraborty, D.; Chen, E. Y.-X.
Organometallics 2002, 21, 1438.
(8) Chamberlain, B. M.; Jazdzewski, B.; Pink, M.; Hillmyer, M. A.;
Tolman, W. B. Macromolecules 2000, 33, 3970. (b) Ravi, P.; Gro¨b,
T.; Dehnicke, K.; Greineer, A. Macromolecules 2001, 34, 8649. (c)
Martin, E.; Dubois, P.; Je´roˆme, R. Macromolecules 2000, 33, 1530.
(d) Takeuchi, D.; Nakamura, T.; Aida, T. Macromolecules 2000, 33,
725. (e) Stevels, W. M.; Ankone´, M. J. K.; Dijkstra, P.; Feijen, J.
Macromolecules 1996, 29, 8296.
Experimental Section
General Procedures. All reactions were performed under a dry
nitrogen atmosphere using standard Schlenk line or glovebox
technique. Toluene and diethyl ether were dried by refluxing over
sodium benzophenone ketyl. CH2Cl2 and acetonitrile were dried
over P2O5 and CaH2, respectively. All solvents were distilled and
stored in solvent reservoirs, which contained 4 Å molecular sieves,
and purged with nitrogen. 1H and 13C NMR spectra were recorded
on a Bruker AC200 spectrometer. Chemical shifts for H and 13C
spectra were recorded in parts per million relative to the residual
proton of CDCl3 (δ 7.24). Elemental analyses were performed on
1
6454 Inorganic Chemistry, Vol. 41, No. 24, 2002

Aluminum Ketiminate Complexes
a Heraeus CHN-OS Rapid Elemental Analyzer at Instrument
Center, NCHU. Ketimine (1), OCMeCHCMeNHAr where Ar )
2,6-diisopropylphenyl, was prepared according to previously re-
ported procedure.3a AlCl3 (Strem), MeLi, and EtLi (Aldrich) were
used as received.
(s, 6H, CMe), 2.82 (m, 2H, CHMe2), 3.30 (m, 2H, CHMe2), 5.22
(s, 2H, CMeCHCMe), 7.15 (m, 6H, Ph). 13C NMR (CDCl3): δ
23.5, 23.6, 24.4, 24.8 (br, two peaks overlapping), 25.8, 27.6, 28.4,
100.3, 122.8, 124.0, 125.3, 141.0, 142.8, 145.2, 176.6, 178.5. Anal.
Calcd for C34H48N2O2ClAl: C, 70.51; H, 8.35; N, 4.84. Found:
C, 69.93; H, 8.19; N, 5.67.
(OCMeCHCMeNAr)AlMe2 (2). AlMe3 (1.9 mL, 2 M, 3.80
mmol) was added to a stirred solution of 1 (1.0 g, 3.86 mmol) in
20 mL of toluene at 0 °C. The mixture was then allowed to warm
to room temperature and stirred for additional 3 h. The mixture
was vacuum-dried to yield a pale yellow oil, which was solidified
after being immersed in liquid nitrogen and warmed to room
temperature. The solid was recrystallized from either toluene or
heptane at -20 °C to yield 1.10 g colorless product (92% yield).
1H NMR (CDCl3): δ -0.93 (s, 6H, AlMe2), 1.08 (d, 6H, CHMe2),
1.19 (d, 6H, CHMe2), 1.74 (s, 3H, CMe), 2.07 (s, 3H, CMe), 2.92
(m, 2H, CHMe2), 5.34 (s, 1H, CMeCHCMe), 7.20 (m, 3H, Ph).
13C NMR (CDCl3): δ -11.0, 23.2, 24.4, 24.6, 25.8, 27.9, 100.4,
124.2, 127.2, 139.0, 142.9, 176.1, 181.0. Anal. Calcd for C19H30-
NOAl: C, 72.35; H, 9.59. Found: C, 72.31; H, 9.82.
(OCMeCHCMeNAr) AlCl2 (7). A similar manner as that for
preparing complex 6 was taken. Complex 7 was prepared from
AlCl3 (0.5 g, 3.76 mmol) and lithiated 1 (1.0 g, 3.77 mmol), and
0.78 g (58%) of the desired product was obtained with a small
1
amount of impurity presented. H NMR (CDCl3): δ 1.08-1.29
(m, 12H, CHMe2), 1.88 (s, 3H, CMe), 2.18 (s, 3H, CMe), 2.94 (m,
2H, CHMe2), 5.61 (s, 1H, CMeCHCMe), 7.27 (m, 6H, Ph). 13C
NMR (CDCl3): δ 23.8, 24.4, 24.9, 25.5, 28.3, 102.0, 124.8, 128.5,
136.0, 143.5, 180.8, 182.3.
(OCMeCHCMeNAr) 2AlF (8). A Schlenk flask was charged
6 (0.30 g, 0.52 mmol) and AgBF4 (0.10 g, 0.51 mmol) in a
glovebox. Methylene chloride (10 mL) and acetonitrile (10 mL)
were added to 6 and AgBF4, respectively. The AgBF4 solution was
added dropwise to the methylene chloride solution at 0 °C and
stirred for another hour at room temperature. The solvent was
removed, and the residual was extracted with methylene chloride
and filtered through Celite. The filtrate was concentrated to a small
amount of volume. Colorless crystals were obtained from the
concentrated methylene chloride solution at -20 °C. A 0.29 g
amount of solid was obtained (76% yield). Small amounts of
unidentified compounds were present in the product, even after
repeated recrystallization. 1H NMR (CDCl3): δ 1.04-1.25 (m, 24H,
CHMe2), 1.21 (s, 6H, CMe), 1.63 (s, 6H, CMe), 2.77 (m, 2H,
CHMe2), 3.18 (m, 2H, CHMe2), 5.15 (s, 2H, CMeCHCMe), 7.15
(m, 6H, Ph).
(OCMeCHCMeNAr)AlEt2 (3). A similar manner was taken
as that for preparing complex 2. Complex 3 was prepared from
AlEt3 (5.1 mL, 1.9 M, 9.69 mmol) and 1 (2.5 g, 9.65 mmol) in
98% yield as a pale yellow oil, which partially solidified on standing
1
over a period of time in a glovebox. H NMR (CDCl3): δ -0.22
(m, 4H, AlCH2CH3), 0.86 (t, 6H, AlCH2CH3), 1.10 (d, 6H, CHMe2),
1.22 (d, 6H, CHMe2), 1.75 (s, 3H, CMe), 2.10 (s, 3H, CMe), 2.93
(m, 2H, CHMe2), 5.34 (s, 1H, CMeCHCMe), 7.20 (m, 3H, Ph).
13C NMR (CDCl3): δ - 0.9, 8.9, 23.1, 24.3, 24.7, 25.7, 27.9, 100.6,
124.2, 127.2, 139.3, 142.9, 176.3, 181.4.
(OCMeCHCMeNAr) 2AlMe (4). A similar manner was taken
as that for preparing complex 2; however, longer reaction time was
needed. Complex 4 was prepared by stirring AlMe3 (4.8 mL, 2.0
M, 9.6 mmol) and 1 (5.0 g, 19.31 mmol) in toluene at 100 °C for
3 days. Colorless crystals were obtained in 78% yield after
recrystallization from toluene at -20 °C (4.18 g). 1H NMR
(CDCl3): δ -0.97 (s, 3H, AlCH3), 1.12 (m, 24H, CHMe2), 1.17
(s, 6H, CMe), 1.61 (s, 6H, CMe), 2.93 (m, 2H, CHMe2), 3.06 (m,
2H, CHMe2), 5.06 (s, 2H, CMeCHCMe), 7.10 (m, 6H, Ph). 13C
NMR (CDCl3): δ - 10.5, 23.6, 23.7, 24.3 (br, two peaks
overlapping), 24.9, 25.4, 27.5, 28.3, 99.2, 122.9, 123.3, 124.7, 141.7,
146.1, 174.1, 177.5. Anal. Calcd for C35H51N2O2Al: C, 75.23; H,
9.20; N, 5.01. Found: C,75.75; H, 9.39; N, 5.69.
(OCMeCHCMeNAr) 2AlEt (5). A similar manner as that for
preparing complex 2 was adopted. However, longer reaction time
was needed. Complex 5 was prepared in 73% yield by stirring AlEt3
(5.1 mL, 1.9 M, 9.69 mmol) and 1 (5.0 g, 19.31 mmol) in toluene
at 100 °C for 3 days. Colorless crystals were obtained after
recrystallization from toluene at -20 °C (4.01 g). 1H NMR
(CDCl3): δ -0.27 (m, 1H, AlCHaHbCH3), -0.08 (m, 1H, AlCHaHb-
CH3), 0.89 (t, 3H, AlCH2CH3), 1.06-1.28 (m, 24H, CHMe2), 1.17
(s, 6H, CMe), 1.62 (s, 6H, CMe), 2.94 (m, 2H, CHMe2), 3.19 (m,
2H, CHMe2), 5.00 (s, 2H, CMeCHCMe), 7.12 (m, 6H, Ph). 13C
NMR (CDCl3): δ 3.4, 10.2, 23.7, 23.8, 24.5, 24.8, 25.0, 27.6, 28.1,
28.5, 98.9, 122.9, 123.3, 124.8, 141.3, 142.2, 146.4, 175.2, 178.6.
Anal. Calcd for C36H53N2O2Al: C, 75.49; H, 9.33; N, 4.89.
Found: C, 75.25; H, 9.93; N, 5.46.
E-Caprolactone Polymerization. A general procedure for ꢀ-ca-
prolactone polymerization catalyzed by aluminum complexes was
described as follow. In a glovebox, ꢀ-caprolactone and catalyst were
placed in separate Schlenk flasks and moved out of the glovebox.
Toluene (2 mL) was added with a syringe to the catalyst and heated
to the desired temperature followed by the addition of ꢀ-caprolac-
tone. The reactions were proceeded in desired condition and
quenched with water. Solid was obtained by filtration, washed with
methanol to remove excess metal catalysts, and dried under vacuum.
X-ray Structure Determination of Complexes 2-6 and 8.
Crystals of complex 2 were obtained from a concentrated heptane
solution of 1 at -20 °C. Crystals of 3 were obtained directly from
the sublimation of viscous complex 3 by storing it in a glovebox.
Crystals of 4-6 were obtained from saturated toluene solution at
-20 °C, and crystals of 8 were obtained from a concentrated
methylene chloride solution of 8. All of the crystals were sealed in
a glass capillary and mounted on a goniostat. Data collections were
proceeded at 293(2) K for all complexes. Data were collected on
a Bruker SMART CCD diffractometer with graphite-monochro-
mated Mo KR radiation. Structural determinations were made using
the SHELXTL package of programs. All refinements were carried
out by full-matrix least squares using anisotropic displacement
parameters for all non-hydrogen atoms. All the hydrogen atoms
are calculated. The crystal data are summarized in Table 1.
(OCMeCHCMeNAr) 2AlCl (6). A toluene solution (30 mL)
of lithiated 1 (8.0 g, 30 mmol) was added to a AlCl3 (2.0 g, 15
mmol) and toluene (20 mL) suspension at 0 °C with stirring. The
suspension was warmed to room temperature and stirred for an
additional 3 h. The mixture was filtered through Celite to remove
LiCl, and the filtrate was dried under vacuum to yield an off-white
solid, which was recrystallized from toluene at -20 °C to generate
8.7 g (78% yield) of colorless crystalline product. 1H NMR
(CDCl3): δ 1.04-1.25 (m, 24H, CHMe2), 1.21 (s, 6H, CMe), 1.68
Acknowledgment. We thank the National Science Coun-
cil of Taiwan for financial support and the National Center
for High Performance Computing for databank search.
Supporting Information Available: Single-crystal X-ray struc-
ture data of complexes 2-6 and 8 (CIF format). This material is
available free of charge via the Internet at http://pubs.acs.org.
IC025785J
Inorganic Chemistry, Vol. 41, No. 24, 2002 6455