A facile route to lithium complexes supported by β-ketoiminate ligands and their reactivity

Article abstract of DOI:10.1016/j.jorganchem.2013.09.027

Several β-ketoiminate lithium complexes Li₂(L) ₂·2(THF) (6-10) were conveniently synthesized by using 1 equiv. of n-BuLi with 1 equiv. of β-ketoiminate ligands (1-5) in THF-hexane at 60 C. All the new complexes have been characterize

Full text of DOI:10.1016/j.jorganchem.2013.09.027

Journal of Organometallic Chemistry 749 (2014) 7e12
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A facile route to lithium complexes supported by
ligands and their reactivity
b-ketoiminate
,
b
,
b
b
Zhen Liu a, Hong-Xia Chen b, Dan Huang a , Yong Zhang , Ying-Ming Yao
*
a Key Laboratory of Eco-Textile (Jiangnan University) of Ministry of Education, 214122 Wuxi, China
b Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123
Suzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several
b
-ketoiminate lithium complexes Li2(L)2$2(THF) (6e10) were conveniently synthesized by using
1 equiv. of n-BuLi with 1 equiv. of
complexes have been characterized by NMR and IR spectroscopy. Complexes (8e10) were determined by
single crystal X-ray diffraction. All the lithium complexes showed high activity for the ring-opening
polymerization (ROP) of L-lactide to give the polyesters with relative narrow molecular weight distri-
b
-ketoiminate ligands (1e5) in THF-hexane at 60 ꢀC. All the new
Received 5 August 2013
Received in revised form
17 September 2013
Accepted 18 September 2013
butions (Mw/Mn ¼ 1.42e1.87). Complex 10 showed the highest activity toward ROP of L-LA. Moreover,
the results proved that the polymerization initiated by complex 10 proceeded in a controllable fashion.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
b
-Ketoiminate
Lithium complex
Polymerizations
L-Lactide
1. Introduction
Various chelating N or O containing moieties have been
[23,26e29], but no papers have been found to concern the
application of the first group metals compounds in homogeneous
catalysis for the ROP of L-lactide. However, the preparation and
explored, such as amidinates [1e5], guanidinates [1,4,6e9],
diketiminates [10e12], and bridged bisphenolate [13e15], etc.
b
b
-
-
application of lithium catalysts/initiators such as, alkyl lithium,
alkoxy lithium and bimetallic lithium compounds for the ROP of
cyclic esters have developed [30,31]. Besides, the bimetallic
complexes containing alkali metals have recently been found to
be more active than the corresponding mononuclear partner,
which was the cooperation between metals existed in the re-
actions [32e35]. In this connection we start the project on the
ketoiminates, as one of the members, have attracted increasing
attention in organometallic chemistry, since their steric and elec-
tronic properties can be easily tuned by an appropriate choice of
starting materials used in their synthesis. A variety of complexes of
main and d-block metals with these ligands have been found wide
applications in homogeneous catalysis including organic synthesis
and polymerization of non-polar and polar monomers [16e24]. The
lanthanide alkoxide complexes stabilized by monoanionic N-ary-
synthesis of the first group metals stabilized by monoanionic
b-
ketoiminate ligands and their catalytic activity in hopes to assess
the potential applications of
organometallic catalysts.
b-ketoiminate ligands in design of
loxo-functionalized b-ketoiminates ligands are well known to be
active catalysts in various transformations, such as the ring-
opening polymerization of lactones and lactides [25] and organic
reactions [26].
It was found that the lithium complexes could be conveniently
synthesized by the reaction of n-BuLi with -ketoiminates and
b
these complexes were found to be active catalysts for the ROP of
lactide.
L-
The synthesis of the lanthanide aryloxide complexes sup-
ported by a dianionic N-aryloxo-functionalized
and their activity for ring-opening polymerization of
have been reported [25]. Several main group derivatives sup-
ported by -ketoiminate ligands have also been synthesized
b
-ketoiminate
L
-lactide
2. Experimental section
b
2.1. General remarks
All manipulations and reactions were performed under a
purified argon atmosphere using standard Schlenk techniques.
Solvents were degassed and distilled from sodium benzophenone
* Corresponding author. Key Laboratory of Eco-Textile (Jiangnan University) of
Ministry of Education, 214122 Wuxi, China. Tel./fax: þ86 51085705599.
E-mail addresses: huangdan6@163.com, huangdan@jiangnan.edu.cn (D. Huang).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.09.027

8
Z. Liu et al. / Journal of Organometallic Chemistry 749 (2014) 7e12
ketyl prior to use.
L
-lactide was recrystallized twice.
2.2.5. p-MeO-C6H5NHC(CH3)CHC(Ph)O (Lp-MeOH) (5)
(C6H5NC(CH3)CHC(Ph)O) (1) (LH), 2,6-Me-C6H5NC(CH3)CHC(Ph)O
(2) (L2,6-MeH), 2,6-iPr-C6H5NC(CH3)CHC(Ph)O (3) (L2,6-iPrH), p-Cl-
C6H5NC(CH3)CHC(Ph)O (4) (Lp-ClH) and p-CH3OeC6H5NC(CH3)
CHC(Ph)O (5) (Lp-MeOH) were prepared according to the pub-
lished procedures [36e38].
Carbon, hydrogen, and nitrogen analyses were performed by
direct combustion with a Carlo-Erba EA-1110 instrument. The IR
spectra were recorded with a Nicolet-550 FTIR spectrometer as KBr
pellets. 1H NMR and 13C NMR spectra of complexes 1e5 were
recorded on a 300 MHz instrument in a CDCl3 solution, while
complexes 6e10 were recorded on a 400 MHz instrument in a THF-
d8 solution. Melting points were determined in sealed Ar-filled
capillary tubes, and are not corrected. Molecular weight and mo-
lecular weight distribution (PDI) were determined against a poly-
styrene standard by gel permeation chromatography (GPC) on a PL-
GPC 50 apparatus, and THF was used as an eluent at a flow rate of
1.0 mL/min at 40 ꢀC.
Yield ¼ 60%. 1H NMR (300 MHz, CDCl3, 25 ꢀC,
d [ppm]): 12.80(s,
1H), 7.92 (d, J ¼ 2.3 Hz, 2H), 7.44 (s, 3H), 7.22e7.03 (m, 2H), 7.00e
6.80 (m, 2H), 5.87 (s, 1H), 3.81 (s, 3H), 2.06 (s, 3H). 13C NMR
(75.5 MHz, CDCl3, 25 ꢀC,
d [ppm]): 188.95 (C]O), 162.81 (CeN),
111.73e153.38 (Carom), 94.91 (CHCO(Ph)), 56.20 (2-OCH3-C6H4),
20.95 (CH3eCN).
2.3. Synthesis of complexes 6e10
In an inert atmosphere, b-ketoiminate ligands (1 equiv.) were
dissolved in anhydrous THF at room temperature. n-BuLi (1 equiv.)
was added to the rapidly stirring solution in ice bath. Raising the
temperature to room temperature, the complexes were isolated as
yellow crystals in good yield after stirring for 4 h and workup from
the concentrated THF solution.
2.3.1. Li2(L)2$2(THF) (6)
Following the general procedure, from LH (237 mg,
2.2. Syntheses of
b
-ketoiminate ligands
1.0 mmol) and n-BuLi (2.6 M in hexane, 385 mL, 1.0 mmol) was
obtained 478.3 mg of 6 as yellow crystals from concentrated
tetrahydrofuran-cyclohexane solution at room temperature for two
days (66%). m.p.: 254.5e255.0 ꢀC (dec.). (Found: C, 77.82; H, 7.89; N,
6.40; C48H58N2Li2O4 (740.96) requires C, 77.74; H, 7.83; N, 6.48. IR
(KBr,cmꢁ1): 3057 (w, ṽC-H), 1618 (s, ṽC¼O), 1549 (m), 1520 (m, ṽC¼N),
1434 (s), 1380 (w, dCeH), 1327 (s), 1158 (s), 1093 (w), 1068 (m), 1026
To a stirred solution of 1-phenylbutane-1, 3-dione (20 mmol) in
dried toluene were added the aniline (20 mmol) and p-toluene-
sulfonic acid (ca. 20 mg) as a catalyst at room temperature. The
mixture was heated and refluxed, and the water formed was
removed azotropically using a DeaneStark apparatus for 24 h.
Evaporation of toluene gave yellow solid or oil. The crude product
was purified by recrystallization from hexane. Light yellow or yel-
low solids were obtained, and they were characterized.
(m). 1H NMR (400 MHz, THF-d8, 25 ꢀC,
d
[ppm]): 7.71 (d, J ¼ 7.2 Hz,
2H), 7.22 (t, J ¼ 7.5 Hz, 2H), 7.12 (dd, J ¼ 14.2 Hz, 7.1 Hz, 3H), 6.95 (s,
1H), 6.80 (d, J ¼ 7.5 Hz, 2H), 5.49 (s, 1H), 1.82 (s, 3H). 13C NMR
(100.6 MHz, THF-d8, 25 ꢀC,
d [ppm]): 174.59, 167.91, 154.49, 144.24,
2.2.1. (C6H5NHC(CH3)CHCO(Ph) (1)
129.42, 128.58, 128.26, 127.19, 123.42, 122.83, 95.34, 22.66.
Yield ¼ 83%. 1H NMR (300 MHz, CDCl3, 25 ꢀC,
d [ppm]): 13.09 (s,
1H), 7.91 (d, J ¼ 1.5 Hz, 2H), 7.40 (dd, J ¼ 23.0 Hz, 6.9 Hz, 6H), 7.19 (d,
2.3.2. Li2(L2,6-Me)2$2(THF) (7)
J ¼ 7.1 Hz, 2H), 5.90 (s, 1H), 2.14 (d, J ¼ 5.7 Hz, 3H); 13C NMR
The synthesis of complex 7 was carried out in the same way as
that described for complex 6, but L2,6-MeH (265 mg, 1.0 mmol) was
used instead of LH. Light-yellow crystals were obtained from a
mixture of THF/hexane solution (15 mL/1 mL) at room temperature
over a few days (0.44 g, 57%). m.p.: 310.3e312.0 ꢀC (dec.); (Found: C,
77.91; H, 8.36; N, 3.55; C50H64N2Li2O4 (770.96) requires C, 77.83; H,
8.30; N, 3.63. IR (KBr, cmꢁ1): 3066 (w, ṽCeH), 1600 (s, ṽC¼O), 1589 (s),
1504 (s, ṽC¼N), 1426 (w),1383 (w, dCeH), 1319 (s), 1281 (m),1088 (m),
(75.5 MHz, CDCl3, 25 ꢀC,
d [ppm]): 188.76 (C]O), 162.99 (CeN),
139.83 (Carom), 138.45 (Carom), 130.71 (Carom), 128.97 (Carom),
128.10 (Carom), 126.89 (Carom), 125.55 (Carom), 124.49 (Ph-C),
94.60 (CHCO(Ph)), 20.21 (CH3eCN).
2.2.2. 2,6-Me-C6H5NHC(CH3)CHCO(Ph) (L2,6-MeH) (2)
Yield ¼ 72%. 1H NMR (300 MHz, CDCl3, 25 ꢀC,
d [ppm]): 12.53 (s,
1H), 7.94 (s, 2H), 7.45 (s, 3H), 7.13 (s, 3H), 5.93 (s, 1H), 3.48 (s, 3H),
1027 (m). 1H NMR (400 MHz, THF-d8, 25 ꢀC,
d [ppm]): 7.58 (d,
2.25 (s, 6H); 13C NMR (75.5 MHz, CDCl3, 25 ꢀC,
d
[ppm]): 188.76 (C]
J ¼ 7.1 Hz, 2H), 7.12e7.00 (m, 3H), 7.00e6.81 (m, 3H), 5.65 (s, 1H),
O), 165.06 (CeN), 163.92 (Carom), 136.70 (Carom), 136.17 (Carom),
130.98 (Carom), 128.45 (Carom), 128.02 (Carom), 127.72 (Carom),
127.27 (Ph-C), 92.47 (CHCO(Ph)), 24.45 (CH3eCN), 19.70 (2,6-
(CH3)2eC6H3), 18.52 (2,6-(CH3)2eC6H2).
2.10 (s, 6H), 1.65 (s, 3H). 13C NMR (100.6 MHz, THF-d8, 25 ꢀC,
d
[ppm]): 173.52, 168.34, 151.88, 143.65, 129.78, 128.78, 128.65,
128.35, 127.03, 123.16, 94.58, 22.63, 18.82.
2.3.3. Li2(L2,6-ipr)2$2(THF) (8)
2.2.3. 2,6-iPr-C6H5NHC(CH3)CHCO(Ph) (L2,6-iPrH) (3)
Complex 7 was synthesized following a similar procedure in 6e7:
except [(PhCO)CH(CH3CN-(H)(2,6-i-Pr2C6H5)] (L2,6-iprH) (322 mg,
1 mmol) was used. Yellow crystals of complex 8 were obtained from
concentrated tetrahydrofuran-cyclohexane solution at room tem-
perature for one day (55%). m.p.: 178.5e179.0 ꢀC (dec.). Anal. Calc. For
Yield ¼ 86%. 1H NMR (300 MHz, CDCl3, 25 ꢀC,
d [ppm]): 12.66 (s,
1H), 7.96 (d, J ¼ 0.5 Hz, 2H), 7.56e6.99 (m, 6H), 5.91 (s, 1H), 3.19e
2.83 (m, 2H), 1.79 (s, 3H), 1.20 (dd, J ¼ 17.2 Hz, 6.7 Hz, 12H). 13C NMR
(75.5 MHz, CDCl3, 25 ꢀC,
d [ppm]): 188.46 (C]O), 165.23 (CeN),
146.02 (Carom), 139.93 (Carom), 133.40 (Carom), 130.61 (Carom),
128.27 (Carom), 128.09 (Carom), 126.97 (Carom), 123.48 (Ph-C),
92.39 (CHCO(Ph)), 28.59 (CH-iPr), 24.69 (CH3-iPr), 22.77 (CH3-iPr),
19.93 (CH3eCN).
C
52H68Li2N2O4: C, 78.10; H, 8.51; N, 3.50. Found: C, 78.15; H, 8.60; N,
3.45. IR (KBr, cmꢁ1): 3088 (w, ṽC-H), 1598 (s, ṽC¼O), 1576 (s), 1515 (w,
C¼N), 1456 (w), 1383 (w, dC-H), 1316 (s), 1239 (w), 1177 (m). 1H NMR
(400 MHz, THF-d8, 25 ꢀC,
[ppm]): 7.62 (s, 2H), 7.15e7.08 (m, 3H),
ṽ
d
6.94 (d, J ¼ 7.5 Hz, 3H), 5.64 (s, 1H), 3.20e3.17 (m, 2H), 1.27 (s, 3H),
2.2.4. p-Cl-C6H5NHC(CH3)CHC(Ph)O (Lp-ClH) (4)
1.09 (dd, J ¼ 29.4 Hz, 3.4 Hz, 12H). 13C NMR (100.6 MHz, THF-d8,
Yield ¼ 65%. 1H NMR (300 MHz, CDCl3, 25 ꢀC,
d
[ppm]): 13.08 (s,
25 ꢀC,
d [ppm]): 167.59, 141.68, 140.55, 138.58, 138.25, 128.27, 127.03,
125.34, 123.90, 110.41, 94.10, 28.41, 24.75, 24.52, 23.83.
1H), 8.00e7.74 (m, 2H), 7.41 (dd, J ¼ 31.2 Hz, 6.9 Hz, 5H), 7.13 (d,
J ¼ 7.9 Hz, 2H), 5.93 (s, 1H), 2.15 (s, 3H). 13C NMR (75.5 MHz, CDCl3,
25 ꢀC,
d
[ppm]): 188.95 (C]O),161.69 (CeN),130.09 (Carom),131.06
2.3.4. Li2(Lp-Cl)2$2(THF) (9)
(Carom), 129.29 (Carom), 128.30 (Carom), 127.06 (Carom), 125.89
(Ph-C), 94.65 (CHCO(Ph)), 20.38 (CH3eCN).
The synthesis of complex 9 was carried out in the same way as
that described for complex 6, but [(phCO)CH(CH3CN-(H)(p-Cl-

Z. Liu et al. / Journal of Organometallic Chemistry 749 (2014) 7e12
9
Table 1
Ar
Crystallographic data for complexes 8e10.
N
Complex
8
9
10
THF
Formula
Fw
T(K)
C52H68Li2N2O4 C40H42Cl2Li2N2O4 C42H48Li2N2O6
O
HN
Li
Li
O
Ar
798.96
293(2)
Tetragonal
I 41/a
25.8829(12)
25.8829(12)
14.2083(19)
90
699.54
293(2)
Triclinic
P ꢁ1
690.70
293(2)
Monoclinic
P 21/c
11.6613(9)
8.4971(8)
19.2270(11)
90
91.384(7)
90
1904.6(3)
2
n-BuLi
o
O
THF
Crystal system
Space group
THF
/0 C
N
Ar
ꢀ
a (A)
10.7407(8)
12.0863(7)
15.3976(10)
92.443(5)
97.514(6)
107.357(6)
1884.4(2)
2
ꢀ
b (A)
6. Ar = C6H5
1. Ar = C6H5
ꢀ
c (A)
7. Ar = 2, 6-(CH3)2-C6H3
8. Ar = 2, 6-(iPr)2-C6H3
9. Ar = p-Cl-C6H3
2. Ar = 2, 6-(CH3)2-C6H3
3. Ar = 2, 6-(iPr)2-C6H3
4. Ar = p-Cl-C6H3
a
b
g
(deg)
(deg)
(deg)
90
90
3
10.Ar = p-CH3O-C6H3
ꢀ
5.Ar = p-CH3O-C6H3
V (A )
Z
9518.5(14)
8
Scheme 1. Synthesis of complexes 6e10.
Dcalc
m
1.115
0.068
1.233
0.214
1.204
0.079
(mmꢁ1
)
F (000)
Qmax(ꢀ)
3456
25.50
736
25.50
736
25.49
2.5. X-ray crystallography
Reflections collected
11370
14238
8409
Independent reflections 4416
7003
4016
469
0.0690
0.2215
1.022
3539
2777
246
0.0488
0.1310
1.034
Suitable single crystals of complexes 8, 9 and 10 were sealed in a
thin-walled glass capillary for determination of the single-crystal
structures. Intensity data were collected with a Rigaku Mercury
Observed reflections
Parameters refined
3036
273
R(I > 2
wR2 (all data)
GOF on F2
s
(I))
0.0648
0.1796
1.067
CCD area detector in the
u scan mode using MoKa radiation
ꢀ
(l
¼ 0.71070 A). The diffracted intensities were corrected for Lor-
entz/polarization effects and empirical absorption corrections.
Details of the intensity data collection and crystal data are given in
Table 1.
C6H5)] (Lp-ClH) (272 mg, 1 mmol) was used. Yellow crystals of
compound 9 were obtained from concentrated tetrahydrofuran-
cyclohexane solution at room temperature for one day (60%).
m.p.: 275.2e276.5 ꢀC (dec.). Anal. Calc. For C40H42Cl2Li2N2O4: C,
68.62; H, 6.00; N, 4.00. Found: C, 68.70; H, 6.09; N, 3.96. IR (KBr,
cmꢁ1): 3080 (w, ṽC-H),1623 (m, ṽC¼O),1520 (m, ṽC¼N),1326 (m, dC-H),
The structures were solved by direct methods and refined by
full-matrix least-squares procedures based on jFj2. All of the non-
hydrogen atoms were refined anisotropically. The hydrogen
atoms in these complexes were all generated geometrically,
assigned appropriate isotropic thermal parameters, and allowed to
ride on their parent carbon atoms. All of the hydrogen atoms were
held stationary and included in the structure factor calculation in
the final stage of full-matrix least-squares refinement. The struc-
tures were solved and refined using the SHELEXL-97 programs [39].
1289 (w), 1157 (m). 1H NMR (400 MHz, THF-d8, 25 ꢀC,
d
[ppm]): 7.72
(s, 2H), 7.37e7.00 (m, 5H), 6.65 (d, J ¼ 6.6 Hz, 2H), 5.31 (s,1H),1.76(s,
3H). 13C NMR (100.6 MHz, THF-d8, 25 ꢀC,
[ppm]): 163.73, 153.22,
d
144.33, 144.24, 129.40, 128.91, 128.27, 127.24, 125.00, 95.49, 22.58.
2.3.5. Li2(Lp-MeO)2$2(THF) (10)
3. Results and discussion
The synthesis of complex 10 was carried out in the same way as
that described for complex 6, but [(phCO)CH(CH3CN-(H)(p-CH3Oe
C6H5)] (Lp-MeOH) (267 mg, 1 mmol) was used. Yellow crystals of
compound 10 were obtained from concentrated tetrahydrofuran-
cyclohexane solution at room temperature for one day (52%).
m.p.: 194.0e195.2 ꢀC (dec.). Anal. Calc. For C42H48Li2N2O6: C, 76.38;
H, 6.41; N, 5.24. Found: C, 76.40; H, 6.35; N, 5.20. IR (KBr, cmꢁ1):
3062 (w, ṽC-H),1609 (s, ṽC¼O),1576 (m),1507 (s, ṽC¼N),1383 (w, dC-H),
1249 (s), 1157 (w), 1070 (m). 1H NMR (400 MHz, THF-d8, 25 ꢀC,
3.1. Syntheses of complexes 6e10
The b
-ketoiminate ligands, LH (1), L2,6-MeH (2), L2,6-iPrH (3), Lp-
ClH (4) and Lp-MeOH (5) were synthesized by the published method
[38].
The reaction of 1 equiv. of n-BuLi with 1 equiv. of b-ketoiminate
ligands were tried at 0 ꢀC and then raising the temperature to room
temperature led in the isolation of the desired lithium complexes
6e10 (Scheme 1).
d
[ppm]): 7.71 (d, J ¼ 6.5 Hz, 2H), 7.16e7.14 (m, 3H), 6.81 (d,
J ¼ 9.2 Hz, 2H), 6.73 (d, J ¼ 6.8 Hz, 2H), 5.47 (s, 1H), 3.78 (s, 3H
OCH3), 2.06 (s, 3H). 13C NMR (100.6 MHz, THF-d8, 25 ꢀC,
[ppm]):
Complexes 6e10 were characterized by standard analytical/
spectroscopic techniques. The IR spectra of 6e10 exhibited strong
absorptions near 1550 and 1510 cmꢁ1, indicative of the partial C]N
d
163.40, 132.85, 131.42, 128.97, 128.32, 128.00, 127.38, 124.11, 115.10,
114.80, 93.75, 55.77, 22.64.
character of the b
-ketoiminate ligands. 1H NMR spectrum of 6e10
revealed signals assigned to the corresponding ligand. The forma-
tions of 8e10 were further confirmed by single-crystal structure
analysis. An attempt to determine the molecular structures of 6 and
7 were unsuccessful, due to the poor quality and weak diffraction of
the crystals. Complexes 6e10 are sensitive to air and moisture. They
have good solubility in THF except complex 8.
2.4. General procedure for the polymerization reaction
The ring-opening polymerization of L-lactide initiated by com-
plexes 6e10, and a typical polymerization procedure is as follows: A
50 mL Schlenk flask, equipped with a magnetic stirring bar, was
charged with the desired amount of
of the flask were then stirred until
L
-lactide and THF. The contents
-lactide was dissolved, then a
L
solution of the initiator in THF was added using a syringe. The
mixture was stirred vigorously for the desired time, during which
an increase in the viscosity was observed. The reaction mixture was
quenched by the addition of ethanol and then poured into ethanol
to precipitate the polymer. The polymer was dried under a vacuum
and weighed.
3.2. Molecular structures of complexes 8e10
To provide complete structural information for these new
lithium complexes, single-crystal X-ray structural analyses were
carried out for complexes 6e10. Crystals suitable for an X-ray
structure determination of complexes 8e10 were obtained from a

10
Z. Liu et al. / Journal of Organometallic Chemistry 749 (2014) 7e12
ꢀ
LieO distance of 1.912 (5) A for 8, 1.902 (6) for 9 are shorter than in
[{PriNCMeCHCMeOLi$OP(NMe2)32
] (1.918 (5) A) [29], but the
ꢀ
average LieO distance of 1.919 (3) for 10 is similar with the complex
[{PriNCMeCHCMeOLi$OP(NMe2)32] (1.918 (5) A) [29]. The average
ꢀ
ꢀ
ꢀ
LieN distance of 8 (2.020 Å), 9 (2.020 A) and 1.989 A are longer than
i
ꢀ
the complex [(Pr NCMeCHCMeOLi)4] (1.977 (8) A) [29]. This may be
also attributed to the steric demand resulted from the different
bond length between these complexes, which are in accordance
with the corresponding bond lengths in the Schiff base complex of
lithium [31], and the other lithium
[31,40e43], when the differences in the steric demand among these
complexes are considered. The bond parameters revealed a
electron delocalization within the -ketoiminate moieties.
b-ketoiminate complexes
p
-
b
3.3. Ring-opening polymerization of L-lactide (L-LA) by complexes
6e10
To further explore the relationship between the structures of the
catalysts and their catalytic properties and behaviors, complexes
Fig. 1. ORTEP diagram of complex Li2(L2,6-ipr)2$2(THF) 8 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level and hydrogen atoms
are omitted for clarity (#1 ꢁx þ 1, ꢁy þ 1, ꢁz þ 2).
6e10 were examined in the ROP of
ketoiminate lithium complexes were efficient initiators in the ROP
of -lactide. The representative polymerization results are shown in
Table 3. It is clear that -lactide polymerization can be initiated by
all of the -ketoiminate lithium complexes. Among the five com-
L-lactide. As expected, the b-
L
L
b
THF solution at room temperature. X-ray diffraction analyses
showed that complexes 8e10 are dimeric structure and all crys-
tallizes with two THF molecules in the unit cell. The molecular
structure of complexes 8e10 was shown in Figs. 1e3, their selected
plexes, 10 showed the highest activity toward ROP of L-LA (Table 3,
entries 1e27). To compare with lithium complexes supported by
bridged bis-phenolate ligands reported [44,45], complexes 6e10
show higher reactivity but less controllability in the ROP of
lactide.
L-
bond lengths and bond angles are listed in Table 2. Each b-ketoi-
minate ligand binds to a Li atom in the typical chelating fashion,
with the OeLieN angles of 131.6 (3), 112.8 (2) and 95.94 (18)o for 8,
95.2 (2), 106.1 (3), 140.0 (3) for 9 and 128.22 (16), 94.06 (13), 116.69
(15) for 10 (Table 2). In the unit cell of complexes 8, 9 and 10
crystallize with two THF molecules, the Li atom is four-coordinated
As shown in Table 3, the polymerization medium also has a
profound effect on the catalytic activity and the molecular weight
by two oxygen atoms from two different
b-ketoiminate ligands, one
nitrogen atom from one -ketoiminate ligand and one oxygen atom
b
from one solvated THF molecule. The coordination geometry
around the Li metal can be best described as a distorted pseudo-
tetrahedron, which is different from those for cubane Li4O4
b-
ketoiminate complexes [29]. The cubane Li4O4 structures of re-
ported complexes are comprised of four lithium atoms and four
ligands, e.g. [{PriNCMeCHCMeOLi$OP(NMe2)3}2] and [(PriNC-
MeCHCMeOLi)4] [29]. The O (1), O (2), O (4), O (5) and N (2) atoms
can be considered to occupy equatorial positions, O (3) and N (1)
atoms occupy axial positions with the angle of O(3)eLieN(1) being
distorted away from the idealized 180ꢀ to 167.78 (8)o for 8,148.8 (3)
o to 95.1 (2)o for 9 and 128.2 (16)o to 94.1 (3)o for 10. The average
Fig. 2. ORTEP diagram of complex Li2(Lp2-Cl)2$2(THF) 9 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level and hydrogen atoms
are omitted for clarity.
Fig. 3. ORTEP diagram of complex Li2(Lp2-CH3O)2$2(THF) 10 showing an atom
numbering scheme. Thermal ellipsoids are drawn at the 30% probability level and
hydrogen atoms are omitted for clarity (#1 ꢁx, ꢁy þ 1, ꢁz).

Z. Liu et al. / Journal of Organometallic Chemistry 749 (2014) 7e12
11
Table 2
Table 3
Polymerization of L-LA Initiated by complexes 6e10.a
ꢀ
ꢀ
Selected bond lengths (A) and bond angles ( ) of complexes 8e10.
Complex 8
Entry Init [M]0/[I]0 Temp (ꢀC) Conv (%)b Mnn(Calcd)c Mnn(104)d PDI
(104)
O(1)eLi(22)#1
O(2)eLi(22)
1.879(4)
1.965(5)
O(1)-Li(22)
N(2)-Li(22)#1
1.892(4)
2.020(4)
124.6(3)
47.18(14)
112.8(2)
95.94(18)
109.5(2)
1
2
3
4
5
6
7
8
6
6
6
6
6
7
7
7
7
7
8
8
8
8
8
9
9
9
9
9
100:1
200:1
300:1
400:1
500:1
100:1
200:1
300:1
400:1
500:1
100:1
200:1
300:1
400:1
500:1
200:1
300:1
400:1
500:1
800:1
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
30
60
90
120
20
20
20
96
91
83
75
72
88
94
91
87
78
91
92
80
78
65
85
80
60
60
41
93
92
90
82
78
56
98
94
81
74
81
76
71
1.38
2.62
3.59
4.32
5.20
1.27
2.71
3.94
5.02
5.62
1.31
2.65
3.46
4.50
4.68
2.45
3.46
3.46
4.32
4.73
1.34
2.65
3.89
4.72
5.62
6.46
5.65
5.42
4.67
4.27
3.50
4.38
5.12
9.28
6.44
5.70
5.00
5.11
1.65
3.34
5.81
6.17
5.01
3.82
5.14
5.26
4.81
1.93
3.91
3.09
2.59
2.44
1.75
8.94
8.14
8.60
7.29
6.31
4.98
7.61
6.17
5.17
5.14
4.41
3.97
3.54
1.54
Li(22)#1eO(1)eLi(22)
O(2)eLi(22)eLi(22)#1
O(1)#1eLi(22)eLi(22)#1
O(1)eLi(22)eN(2)#1
O(1)eLi(22)eO(2)
Complex 9
85.22(19) N(2)#1-Li(22)-Li(22)#1
118.0(3) O(1)-Li(22)-Li(22)#1
47.60(14) O(2)-Li(22)-N(2)#1
131.6(3)
107.5(2)
1.58
1.56
1.55
1.53
1.49
1.53
1.51
1.42
1.48
1.50
1.48
1.44
1.43
1.42
1.48
1.65
1.60
1.52
1.53
1.62
1.53
1.50
1.50
1.50
1.45
1.46
1.51
1.51
1.52
1.51
1.53
1.57
O(1)#1-Li(22)-N(2)#1
O(1)#1eLi(22)eO(2)
O(1)eLi(2)
O(1)eLi(1)
1.911(6)
1.917(6)
2.009(6)
87.9(2)
9
1.891(6)
1.891(6)
10
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
O(2)eLi(1)
O(2)eLi(2)
N(1)eLi(2)
N(2)eLi(1)
2.036(6)
88.1(2)
Li(2)eO(1)eLi(1)
O(2)eLi(1)eO(1)
O(1)eLi(1)eO(4)
O(1)eLi(1)eN(2)
O(2)eLi(1)eLi(2)
O(4)eLi(1)eLi(2)
O(1)eLi(2)eO(2)
O(2)eLi(2)eO(3)
O(2)eLi(2)eN(1)
O(1)eLi(2)eLi(1)
O(3)eLi(2)eLi(1)
Li(1)eO(2)eLi(2)
O(2)eLi(1)eO(4)
O(2)eLi(1)eN(2)
O(4)eLi(1)eN(2)
O(1)eLi(1)eLi(2)
N(2)eLi(1)eLi(2)
O(1)eLi(2)eO(3)
O(1)eLi(2)eN(1)
O(3)eLi(2)eN(1)
O(2)eLi(2)eLi(1)
N(1)eLi(2)eLi(1)
112.0(3)
95.0(2)
92.0(3)
110.1(3)
140.1(3)
46.44(18)
122.4(3)
91.9(3)
103.3(3)
45.63(18)
127.2(3)
114.0(3)
94.9(2)
10 100:1
10 200:1
10 300:1
10 400:1
10 500:1
10 800:1
10 400:1
10 400:1
10 400:1
10 400:1
10e 300:1
10e 400:1
10e 500:1
106.2(3)
140.8(3)
46.27(18)
118.3(3)
106.1(3)
45.64(18)
129.4(3)
a
General polymerization conditions:THF solution, [V]sol/[V]mono
t ¼ 20 min for L-LA.
¼
10: 1,
b
Complex 10
Conv: weight of polymer obtained/weight of monomer used.
c
Mnn (Calcd) ¼ Mmono ꢂ [M]0/[I]0 ꢂ Conv.
O(1)eLi(1)#1
O(1)eLi(1)
1.902(3)
1.987(3)
d
Measured by GPC relative to polystyrene standards.
1.892(3)
e
Solvent: toluene.
O(3)eLi(1)
N(3)eLi(1)
1.963(3)
Li(1)#1eO(1)eLi(1)
O(1)#1eLi(1)eO(3)
O(1)#1eLi(1)eN(3)
O(3)eLi(1)eN(3)
O(1)eLi(1)eLi(1)#1
N(3)eLi(1)eLi(1)#1
O(1)#1eLi(1)eO(1)
O(1)eLi(1)eO(3)
O(1)eLi(1)eN(3)
O(1)#1eLi(1)eLi(1)#1
O(3)eLi(1)eLi(1)#1
96.42(13)
104.87(13)
94.06(13)
48.34(10)
116.02(18)
83.65(13)
109.14(14)
128.22(16)
116.69(15)
48.01(10)
121.06(19)
4. Conclusion
Five
b-ketoiminate ligands (1e5) were synthesized and their
lithium complexes (6e10) were convenient prepared by reaction
with n-BuLi, respectively. All complexes were found to be active
catalysts for the ROP of L-LA to give the polyesters with high mo-
lecular weights and relative narrow molecular weight distributions.
Among the five lithium complexes, complex 10 showed the highest
activity toward ROP of L-LA. Moreover, the results proved that the
polymerization initiated by 10 proceeded in a controllable fashion.
Symmetry transformations used to generate equivalent atoms for 8:
#1 ꢁx þ 1, ꢁy þ 1, z þ 2; 10: #1 ꢁx, ꢁy þ 1, ꢁz.
The study on the sign of main group catalysts with
ligands is going on in our laboratory.
b-ketoiminate
of the resultant polymers. It was found that complex 10 showed
greater activity in THF than in toluene (Table 3, entries 24e26 and
52e34). The reaction time played an important role in the ROP of
L-LA. For example, when complex 10 was used, the yield increased
with the prolonger of the reaction time, and decreased when
prolonging the reaction time over 30 min (Table 3, entries 25 and
28e31). The reason may be that the monomer had been converted
completely after the transesterification reactions for 30 min
(Table 3, entries 28e31). The molecular weights obtained by GPC
followed the trend of the yield when the reaction time prolonged,
while the molecular weight distributions remained almost
unchanged (1.46e1.52), indicating a controllable polymerization
of 10.
Acknowledgments
We are grateful for the support from the Open Project Program
(KJS1007) of Key Lab of Organic Synthesis (Soochow University) of
Jiangsu Province.
Appendix A. Supplementary data
CCDC 903670, 884416 and 903669 contain the supplementary
crystallographic data for this paper. These data can be obtained free

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