Article
Organometallics, Vol. 29, No. 22, 2010 5789
The benzyloxy complex can survive for several hours in
solution and then decomposes gradually to unidentified
(2,6-(2,6-Me2C6H3NdCH)2C6H3)AlEt2 (2). Complex 2 was
synthesized in the same manner as 1 with 2,6-(2,6-Me2C6H3Nd
CH)2C6H3-1-Br (0.49 g, 1.17 mmol), BuLi (1.21 mmol), and
n
1
compounds. The H NMR spectrum of a PLLA oligomer
AlEt2Cl (0.14 g, 1.16 mmol) as starting materials or reagents.
Pure 2 was obtained as a yellow powder. Yield: 0.26 g (52%).
Anal. Calcd for C28H33AlN2 (%): C, 79.21; H, 7.83; N, 6.60.
Found: C, 79.34; H, 7.91; N, 6.49. 1H NMR (300 MHz, CDCl3,
25 °C): δ -0.02 (q, JH-H = 8.1 Hz, 4H, Al-CH2CH3), 0.75
(t, JH-H = 8.1 Hz, 6H, Al-CH2CH3), 2.20 (s, 12H, CH3), 7.00-
7.11 (m, 6H, N-C6H3), 7.50 (t, JH-H = 7.5 Hz, 1H, p-Al-C6H3),
7.73 (d, JH-H = 7.5 Hz, 2H, m-Al-C6H3), 8.35 (s, 2H, CHdN).
13C NMR (75 MHz, CDCl3, 25 °C): δ -0.88 (s, 2C, Al-CH2CH3),
9.47 (s, 2C, Al-CH2CH3), 19.34 (s, 4C, CH3), 124.95 (s, 2C, p-N-
C6H3), 128.01 (s, 1C, p-Al-C6H3), 128.19 (s, 4C, m-N-C6H3),
128.90 (s, 2C, CdN-C), 132.40 (s, 2C, m-Al-C6H3), 141.92
(s, 2C, o-Al-C6H3), 148.98 (s, 4C, o-N-C6H3), 169.68 (s, 2C,
CdN-C) ppm.
obtained from the 8/BnOH initiating system with a M/I/B
molar ratio of 25:1:1 shows that the polymer chains are
capped with a benzyl ester group, indicating that the alkyl
zinc complex has been converted to benzyloxy zinc species
before the polymerization begins and the real initiator is the
benzyloxy zinc species as shown in Scheme 3.
Conclusion
A series of aluminum and zinc complexes 1-8 chelated
by bis(imino)aryl NCN ligands have been synthesized and
structurally characterized. All aluminum complexes 1-4
were obtained in a monoligated form in which the Al atom
is chelated by a κ3N,C,N0-NCN pincer ligand. The zinc
complexes 5, 6, and 7 were proved to be bisligated with the
zinc being chelated by two κ2N,C-NCN ligands, while com-
plex 8 possesses a structure similar to those of complexes
1-4, with the central zinc atom being chelated by a κ3N,C,
N0-NCN pincer ligand. All these Al and Zn complexes are
efficient initiators for L-lactide ring-opening polymerization
in the presence of benzyl alcohol, and the polymerization
reaction takes place in an immortal manner. The productiv-
ity of the Zn complexes is generally higher than that of the
Al complexes under similar conditions.
(2,6-(2,6-Et2C6H3NdCH)2C6H3)AlEt2 (3). Complex 3 was
synthesized in the same manner as 1 with 2,6-(2,6-Et2C6H3Nd
CH)2C6H3-1-Br (0.54 g, 1.14 mmol), BuLi (1.20 mmol), and
n
AlEt2Cl (0.14 g, 1.20 mmol) as starting materials or reagents.
Pure 3 was obtained as a yellow powder. Yield: 0.33 g, (61%).
Single crystals for X-ray diffraction analysis were obtained from
hexane at -30 °C. Anal. Calcd for C32H41AlN2 (%): C, 79.96;
1
H, 8.60; N, 5.83. Found: C, 79.52; H, 8.47; N, 5.69. H NMR
(300 MHz, CDCl3, 25 °C): δ -0.06 (q, JH-H = 8.1 Hz, 4H,
Al-CH2CH3), 0.73 (t, JH-H = 8.1 Hz, 6H, Al-CH2CH3), 1.16
(t, JH-H = 7.5 Hz, 4H, CH2CH3), 2.55 (q, 4H, JH-H = 7.5 Hz,
CH2CH3), 7.16 (b, 6H, N-C6H3), 7.51 (t, JH-H = 7.5 Hz, 1H,
p-Al-C6H3), 7.76 (d, JH-H = 7.5 Hz, 2H, m-Al-C6H3) 8.35
(s, 2H, CHdN). 13C NMR (75 MHz, CDCl3, 25 °C): δ -1.15 (s,
2C, Al-CH2CH3), 9.40 (s, 2C, Al-CH2CH3), 15.20 (s, 4C,
CH2CH3), 24.10 (s, 4C, CH2CH3), 125.41 (s, 2C, p-N-C6H3),
126.11 (s, 4C, m-N-C6H3), 128.08 (s, 1C, p-Al-C6H3), 132.50 (s,
2C, m-Al-C6H3), 135.01 (s, 4C, o-N-C6H3), 141.97 (s, 2C, o-Al-
C6H3), 147.73 (s, 2C, CdN-C), 169.52 (s, 2C, CHdN-C) ppm.
(2,6-(2,6-iPr2C6H3NdCH)2C6H3)AlEt2 (4). Complex 4 was
synthesized in the same manner as 1 with 2,6-(2,6-iPr2C6H3Nd
Experimental Section
General Considerations. All manipulations involving air- and
moisture-sensitive compounds were carried out under an atmo-
sphere of dried and purified nitrogen using standard Schlenk
or drybox techniques. Toluene and hexane were dried over
sodium/benzophenone and distilled under nitrogen prior to
use. Elemental analyses were performed on a Varian EL micro-
analyzer. NMR spectra were recorded on a Varian Mercury-300
NMR spectrometer at room temperature in CDCl3 or C6D6.
The molecular weight and molecular weight distribution of the
polymers were measured on a TOSOH HLC 8220 GPC at 40 °C
using THF as eluent against polystyrene standards. ZnEtCl,33
2-bromoisophthalaldehyde, and the ligands19 were synthesized
according to literature procedures. 2,6-Dimethylaniline, 2,6-
diethylaniline, 2,6-diisopropylaniline, and AlEt2Cl were pur-
chased from Aldrich.
n
CH)2C6H3-1-Br (0.57 g, 1.07 mmol), BuLi (1.12 mmol), and
AlEt2Cl (0.14 g, 1.16 mmol) as starting materials or reagents.
Pure 4 was obtained as a yellow powder (0.32 g, 55%). Single
crystals for X-ray diffraction analysis were obtained from
hexane at -30 °C. Anal. Calcd for C36H49AlN2 (%): C, 80.55;
1
H, 9.20; N, 5.22. Found: C, 80.10; H, 9.37; N, 4.99. H NMR
(300 MHz, CDCl3, 25 °C): δ -0.05 (q, JH-H = 8.1 Hz, 4H, Al-
CH2CH3), 0.75 (t, JH-H = 8.1 Hz, 6H, Al-CH2CH3), 1.20 (d,
24H, JH-H = 6.6 Hz, CH(CH3)2), 3.04 (hept, JH-H = 6.6Hz,
4H, CH(CH3)2), 7.22 (m, 6H, N-C6H3), 7.53 (t, JH-H = 7.5 Hz,
1H, p-Al-C6H3), 7.77 (d, JH-H = 7.5 Hz, 2H, m-Al-C6H3) 8.36
(s, 2H, CHdN). 13C NMR (75 MHz, CDCl3, 25 °C): δ -1.85 (s,
2C, Al-CH2CH3), 9.53 (s, 2C, Al-CH2CH3), 24.20 (s, 8C, CH-
(CH3)2) 27.69 (s, 4C, CH(CH3)2), 123.40 (s, 2C, p-N-C6H3),
125.74 (s, 4C, m-N-C6H3), 128.11 (s, 1C, p-Al-C6H3), 132.53
(s, 2C, m-Al-C6H3), 139.98 (s, 4C, o-N-C6H3), 142.02 (s, 2C,
o-Al-C6H3), 146.27 (s, 2C, CdN-C), 169.04 (s, 2C, CdN-C).
(2,6-(C6H5NdCH)2C6H3)2Zn (5). A nBuLi solution in hexane
(0.60 mL, 0.83 mmol) was added dropwise to a THF (20 mL)
solution of 2,6-(C6H5NdCH)2C6H3-1-Br (0.29 g, 0.80 mmol)
at -78 °C. The reaction mixture was stirred for 4 h before the
addition of ZnEtCl (0.11 g, 0.85 mmol). The mixture was
allowed to warm to room temperature gradually and stirred
overnight. The solvents were removed under reduced pressure,
and the residue was extracted with hexane. The volume of the
solvent was reduced to precipitate the product as an orange
powder. Yield: 0.11 g, (43%). Anal. Calcd for C40H30N4Zn (%):
C, 76.01; H, 4.78; N, 8.86. Found: C, 75.89; H, 4.88; N, 8.81. 1H
NMR (300 MHz, CDCl3, 25 °C): δ 7.04 (m, 20H, N-C6H5), 7.41
(t, JH-H = 7.5 Hz, 2H, p-Zn-C6H3), 7.83 (d, JH-H = 7.5 Hz, 4H,
m-Zn-C6H3), 8.53 (s, 4H, CHdN). 13C NMR (75 MHz, CDCl3,
25 °C): δ 121.10 (s, 4C, p-N-C6H3), 125.85 (s, 2C, p-Zn-C6H3),
(2,6-(C6H5NdCH)2C6H3)AlEt2 (1). A nBuLi solution in hex-
ane (0.98 mL, 1.35 mmol) was added dropwise to a THF (20 mL)
solution of 2,6-(C6H5NdCH)2C6H3-1-Br (0.47 g, 1.29 mmol)
at -78 °C. The reaction mixture was stirred for 1 h before the
addition of AlEt2Cl (0.16g, 1.35 mmol). The mixture was
allowed to warm to room temperature gradually and stirred
overnight. The solvents were removed under reduced pressure,
and the residue was extracted with hexane. The volume of the
solvent was reduced to precipitate the product as a yellow
powder. Yield: 0.22 g (46%). Anal. Calcd for C24H25AlN2
(%): C, 78.23; H, 6.84; N, 7.60. Found: C, 78.34; H, 6.88;
N, 7.53. 1H NMR (300 MHz, CDCl3, 25 °C): δ -0.01 (q,
JH-H = 8.1 Hz, 4H, Al-CH2CH3), 0.70 (t, JH-H = 8.1 Hz,
6H, Al-CH2CH3), 7.31-7.47 ( m, 11H, N-C6H3, p-Al-C6H3),
7.65 (d, JH-H = 7.5 Hz, 2H, m-Al-C6H3), 8.62 (s, 2H, CHdN).
13C NMR (75 MHz, CDCl3, 25 °C): δ -0.81 (s, 2C, Al-CH2CH3),
9.89 (s, 2C, Al-CH2CH3), 121.65 (s, 2C, p-N-C6H3), 126.96 (s, 1C,
p-Al-C6H3), 128.13 (s, 4C, m-N-C6H3), 129.27 (s, 2C, Cd
N-C), 131.38 (s, 2C, m-Al-C6H3), 141.95 (s, 2C, o-Al-C6H3),
148.52 (s, 4C, o-N-C6H3), 164.22 (s, 2C, CdN-C) ppm.
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