Novel titanatranes with different ring sizes: Syntheses, structures, and lactide polymerization catalytic capabilities

Article abstract of DOI:10.1021/om0110686

The new titanatranes ArOTiO(o-Ar′CH₂)_3-xN(CH₂CH₂O) _x (Ar = 2,6-di-z-Pr-phenoxy; Ar′ = 2,4-di-MeC₆H₂; x = 0, 5; x = 1, 6; x = 2, 7; x = 3, 8) featuring three six-membered chelating rings (5) to three five-membered chelating rings (8) in a stepwise fashion through 6 and 7 were synthesized from the corresponding trihydroxy chelating ligands 1-4, respectively, using an equimolar mixture of Ti(O-i-Pr)₄ and 2,6-di-i-Pr-phenol. The molecular structures of 5 and 6, determined by X-ray means, revealed that in both of these complexes the transannular N-Ti bond lengths [2.305(2) ?, 5; 2.287(4) ?, 6] are at the short end of the range for titanatranes possessing three five-membered rings. These compounds show good catalytic activity for the bulk homopolymerization of l- and rac-lactide at 130 °C.

Full text of DOI:10.1021/om0110686

Organometallics 2002, 21, 2395-2399
2395
Novel Tita n a tr a n es w ith Differ en t Rin g Sizes:
Syn th eses, Str u ctu r es, a n d La ctid e P olym er iza tion
Ca ta lytic Ca p a bilities
Youngjo Kim and J ohn G. Verkade*
Department of Chemistry, Iowa State University, Ames, Iowa 50011
Received December 17, 2001
The new titanatranes ArOTiO(o-Ar′CH2)3-xN(CH2CH2O)x (Ar ) 2,6-di-i-Pr-phenoxy; Ar′
) 2,4-di-MeC6H2; x ) 0, 5; x ) 1, 6; x ) 2, 7; x ) 3, 8) featuring three six-membered chelating
rings (5) to three five-membered chelating rings (8) in a stepwise fashion through 6 and 7
were synthesized from the corresponding trihydroxy chelating ligands 1-4, respectively,
using an equimolar mixture of Ti(O-i-Pr)4 and 2,6-di-i-Pr-phenol. The molecular structures
of 5 and 6, determined by X-ray means, revealed that in both of these complexes the
transannular N-Ti bond lengths [2.305(2) Å, 5; 2.287(4) Å, 6] are at the short end of the
range for titanatranes possessing three five-membered rings. These compounds show good
catalytic activity for the bulk homopolymerization of l- and rac-lactide at 130 °C.
In tr od u ction
chelated by the trialkanolate ligand (-OCH2CH2)2NCH2-
CH2CH2O-.5 Examples of titanatranes containing two
different ring sizes have, to our knowledge, not been
reported.
Five-membered ring atranes synthesized from tri-
ethanolamine feature a wide variety of metallic and
nonmetallic central atoms.1 Many studies of these
compounds have focused on main group elements such
as silicon, phosphorus, aluminum, and tin.1,2 Among the
transition metallic atranes, relatively few reports have
appeared concerning titanatranes.3 Recently, Holmes et
al. reported the synthesis of various silatranes contain-
ing three six-membered rings using ligand precursor 1,4
whereas all previous work had centered on silatranes
possessing five-membered rings arising from the use of
the ligand precursor triethanolamine. The few examples
of atranes having different ring sizes that have been
reported in the literature contain a main group element
A wide variety of catalytic alkoxide systems based on
tin,6 aluminum,7 zinc,8 magnesium,8b-e iron,9 and lan-
thanide10 organometallic complexes have been reported
to function as catalysts for the polymerization of lactide
(LA). Despite the fact that excellent initiators have been
found among these systems for the polymerization of
LA, the search for new catalysts for generating well-
defined polylactide (PLA) still remains of interest. In
view of the well-known significant number of similari-
ties in the chemistries of tin and titanium, we were
(5) Silicon derivatives: (a) Liepins, E.; Birgele, I.; Solomennikova,
I. I.; Lapsina, A.; Zelcans, G.; Lukevics, E. Zh. Obshch. Khim. 1980,
50, 2462. (b) Mazeika, I.; Gaukhman, A. P.; J ankovska, I.; Zelcans,
G.; Solomennikova, I. I.; Lukevics, E. Zh. Obshch. Khim. 1978, 48,
2722. Germanium derivatives: (a) Mazeika, I.; Grinberga, S.; Gaukh-
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41. (b) Shatz, V. D.; Belikov, V. A.; Zelchan, G. I.; Solomennikova, I.
I.; Yerchak, N. P.; Pudova, O. A.; Lukevics, E. J . Chromatogr. 1980,
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Macromolecules 1996, 29, 1375. (c) Kowalski, A.; Libiszowski, J .; Duda,
A.; Penczek, S. Macromolecules 2000, 33, 1964.
* Corresponding author. Fax: 515-294-0105. E-mail: jverkade@
iastate.edu.
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Macromolecules 1999, 32, 6412.
10.1021/om0110686 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/07/2002

2396 Organometallics, Vol. 21, No. 12, 2002
Kim and Verkade
Exp er im en ta l Section
somewhat surprised to find that no investigations of
titanium alkoxides as potential catalysts in this poly-
merization reaction have been reported, although they
are well-known to homogeneously catalyze olefin po-
lymerization,11 and some Ti complexes have been found
to polymerize ꢀ-caprolactone.12
Gen er a l P r oced u r es. All reactions were carried out under
an argon atmosphere using standard Schlenk and glovebox
techniques.14 All chemicals were purchased from Aldrich and
were used as supplied unless otherwise indicated. THF and
toluene (Fischer HPLC grade) were dried and purified under
a nitrogen atmosphere in a Grubbs-type nonhazardous two-
column solvent purification system15 (Innovative Technologies)
and were stored over activated 3 Å molecular sieves. All
deuterium solvents were dried over activated molecular sieves
(3 Å) and were used after vacuum transfer to a Schlenk tube
equipped with a J . Young valve.
Mea su r em en ts. 1H and 13C{1H} NMR spectra were re-
corded at ambient temperature on a Varian VXR-400, VXR-
300, or Bruker AC200 NMR spectrometer using standard
parameters. The chemical shifts are referenced to the residual
peaks of CDCl3 (7.24 ppm, 1H NMR; 77.0 ppm, 13C{1H} NMR)
and C6D6 (7.15 ppm, 1H NMR; 128 ppm, 13C{1H} NMR).
Elemental analyses were performed by Desert Analytics
Laboratory. Molecular weights of polymers were determined
by gel permeation chromatography (GPC), and the measure-
ments were carried out at room temperature with THF as the
eluent (1 mL/min) using a Waters 510 pump, a Waters 717
Plus Autosampler, four Polymer Laboratories PLgel columns
(100, 500, 104, 105 Å) in series, and a Wyatt Optilab DSP
interferometric refractometer as a detector. The columns were
calibrated with polystyrene standards.
Here we describe the synthesis, characterization, and
catalytic ability for the bulk polymerization of LA, for
four titanatranes possessing the ligands derived by
deprotonation of the OH groups in tris(2-hydroxy-3,5-
dimethylbenzyl)amine, 1,4a bis(2-hydroxy-3,5-dimeth-
ylbenzyl)ethanolamine, 2,13 (2-hydroxy-3,5-dimethyl-
benzyl)diethanolamine, 3,13 and triethanolamine, 4, in
reactions 1-4. The corresponding four titanatranes 5-8
all possess an axial anionic 2,6-di-i-Pr-phenolate ligand.
Syn th eses. Compounds 6-8 were made by a procedure
analogous to that given here for 5. To a THF solution composed
of 2,6-di-i-Pr-phenol (0.891 g, 5.00 mmol) in 10 mL of THF
was added dropwise at room temperature a solution of Ti(O-
i-Pr)4 (1.42 g, 5.00 mmol) in 10 mL of THF. After 1 h, a solution
of tris(2-hydroxy-3,5-dimethylbenzyl)amine (2.10 g, 5.00 mmol)
in 10 mL of THF was added dropwise to the reaction vessel.
The reaction mixture was stirred at room temperature over-
night, and then the volatiles were evaporated under vacuum,
leaving an orange-yellow solid, to which was added 15 mL of
toluene. The orange solution was filtered, and the desired
product 5 was isolated as orange-yellow crystals after the
solution remained at -15 °C in a refrigerator for a few days
(2.25 g, 70%). 1H NMR (CDCl3, 400.147 MHz): δ 7.18-6.73
(m, 9H, aryl-H), 4.11 (m, 5H, overlap of CHMe2 with NCH2-
aryl), 2.97 (d, J ) 13.4 Hz, 3H, NCH2-aryl), 2.23 (d, J ) 3.0
Hz, 9H, aryl-Me), 2.06 (d, J ) 4.0 Hz, 9H, aryl-Me), 1.24 (s,
1
12H, CHMe2). H NMR (C6D6, 400.147 MHz): δ 7.28 (d, J )
7.6 Hz, 2H, aryl-H), 7.09 (m, 1H, aryl-H), 6.74 (s, 3H, aryl-H),
6.40 (s, 3H, aryl-H), 4.48 (m, 2H, CHMe2), 3.95 (d, J ) 13.8
Hz, 3H, NCH2-aryl), 2.45 (d, J ) 13.8 Hz, 3H, NCH2-aryl), 2.20
(s, 9H, aryl-Me), 2.14 (s, 9H, aryl-Me), 1.45 (t, J ) 7.2 Hz, 12H,
overlapping pair of doublets for CHMe2). 13C{1H} NMR (CDCl3,
100.626 MHz): δ 162.5, 159.7, 137.6, 130.8, 129.9, 127.3, 124.1,
123.3, 122.5, 121.3 (aryl), 58.56 (NCH2), 26.67 (CHMe2), 23.93
(CHMe2), 23.73 (CHMe2), 20.59 (aryl-Me), 15.85 (aryl-Me).
Anal. Calcd for C39H47NO4Ti‚1/3 toluene: C, 73.83; H, 7.44;
N, 2.08. Found: C, 74.10; H, 7.59; N, 2.16.
6: yield 62%. 1H NMR (CDCl3, 400.147 MHz): δ 7.27-6.80
(m, 7H, aryl-H), 4.53 (t, J ) 5.6 Hz, 2H, CH2O), 4.00 (m, 2H,
CHMe2), 3.91 (d, J ) 13.4 Hz, 2H, NCH2-aryl), 3.63 (d, J )
13.4 Hz, 2H, NCH2-aryl), 3.00 (br s, 2H, NCH2CH2), 2.25 (s,
6H, aryl-Me), 2.15 (s, 6H, aryl-Me), 1.29 (d, J ) 6.9 Hz, 12H,
CHMe2). 13C{1H} NMR (CDCl3, 100.626 MHz): δ 161.4, 159.1,
138.0, 131.1, 129.4, 127.6, 124.8, 122.9, 122.6, 121.0 (aryl),
72.00 (CH2O), 57.18 (NCH2-aryl), 56.85 (NCH2CH2), 26.76
(CHMe2), 23.65 (CHMe2), 20.55 (aryl-Me), 16.11 (aryl-Me).
Anal. Calcd for C32H41NO4Ti‚1/3 toluene: C, 70.82; H, 7.56;
N, 2.41. Found: C, 70.96; H, 7.83; N, 2.53.
(10) (a) Stevels, W. M.; Ankone, M. J . K.; Dijkstra, P. J .; Feijen, J .
Macromolecules 1996, 29, 3332. (b) Stevels, W. M.; Ankone, M. J . K.;
Dijkstra, P. J .; Feijen, J . Macromolecules 1996, 29, 6132. (c) Simic, V.
Spassky, N.; Hubert-Pfalzgraf, L. G. Macromolecules 1997, 30, 7338.
(d) Chamberlain, B. M.; Sun, Y.; Hagadorn, J . R.; Hemmesch, E. W.;
Young, V. G., J r.; Pink, M.; Hillmyer, M. A.; Tolman, W. B. Macro-
molecules 1999, 32, 2400. (e) Chamberlain, B. M.; J azdzewski, B. A.;
Pink, M.; Hillmyer, M. A.; Tolman, W. B. Macromolecules 2000, 33,
3970. (f) Giesbercht, G. R.; Whitener, G. D.; Arnold, J . J . Chem. Soc.,
Dalton Trans. 2001, 923.
(11) Gladysz, J . A., Ed. Chem. Rev. 2000, 100, 1167-1682.
(12) Takeuchi, D.; Nakamura, T.; Aida, T. Macromolecules 2000, 33,
725.
(13) The syntheses of 2 and 3 were reported in the patent literature
(Siegl, W. O.; Chattha, M. S. Eur. Pat. Appl. 276073, 1988), but no
spectroscopic data were provided. 2: 1H NMR (CDCl3, 400.147 MHz):
δ 6.83 (s, 2H, aryl-H), 6.67 (s, 2H, aryl-H), 3.85 (t, J ) 5.2 Hz, 2H,
CH2O), 3.71 (s, 4H, NCH2-aryl), 2.67 (t, J ) 5.2 Hz, 2H, NCH2CH2),
2.18 (d, J ) 8.9 Hz, 12H, aryl-Me). 13C{1H} NMR (CDCl3, 100.626
MHz): δ 152.2, 131.4, 128.7, 128.4, 125.1, 121.7 (aryl), 60.92 (CH2-
OH), 57.07 (NCH2-aryl), 53.58 (NCH2CH2), 20.61, 16.19 (aryl-Me).
HRMS (EI) m/z calcd: 239.1521. Found: 239.1524. 3: 1H NMR (CDCl3,
400.147 MHz): δ 6.83 (s, 1H, aryl-H), 6.61 (s, 1H, aryl-H), 3.77 (s, 2H,
NCH2-aryl), 3.73 (t, J ) 6.8 Hz, 4H, CH2O), 2.74 (t, J ) 6.8 Hz, 4H,
NCH2CH2), 2.19 (d, J ) 8.5 Hz, 6H, aryl-Me). 13C{1H} NMR (CDCl3,
100.626 MHz): δ 152.9, 130.7, 127.9, 126.8, 124.8, 121.3 (aryl), 60.25
(CH2OH), 59.45 (NCH2CH2), 56.10 (NCH2-aryl), 20.38, 15.66 (aryl-Me).
HRMS (EI) m/z calcd: 329.1991. Found: 329.1996.
(14) Shriver, D. F. The Manipulation of Air-Sensitive Compounds;
McGraw-Hill: New York, 1969.
(15) Pangborn A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.

Novel Titanatranes with Different Ring Sizes
Organometallics, Vol. 21, No. 12, 2002 2397
Ta ble 1. Cr ysta llogr a p h ic Da ta a n d P a r a m eter s
for 5 a n d 6
5
6
formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C
41H51NO4.5Ti
C34H45NO4.5Ti
585.59
triclinic
P1h
10.2280(19)
11.448(2)
15.344(3)
70.710(3)
85.085(4)
80.038(3)
1669.4(5)
2
677.73
monoclinic
P2/n
14.890(4)
11.530(3)
22.378(6)
90
105.533(4)
90
3701.6(17)
4
Z
dc (g/cm3)
F(000)
1.216
1448
1.165
624
T (K)
173(2)
0.273
1.89; 26.38
29 943
7539
293(2)
0.293
1.91; 20.82
8305
3489
abs coeff (mm-1
θ range (deg)
)
1
no. of reflns collected
no. of ind reflns
F igu r e 1. Variable-temperature H NMR spectra of 5.
no. of params refined
429
388
0.0562
0.1538
1.017
amount of catalyst precursor were charged to a 25 mL Schlenk
flask. The flask was then immersed in an oil bath at 130 °C.
After 24 h, the reaction was terminated by the addition of 5
mL of methanol. The polymers so obtained as precipitates were
dissolved in a minimum amount of methylene chloride, and
then excess methanol was added. The resulting reprecipitated
polymers were collected, washed with 3 × 50 mL of methanol,
and dried in vacuo at 50 °C for 12 h.
a
R1
wR2
0.0586
0.1772
1.089
a
GOF
min. and max. dens (e Å-3
)
1.156, -0.426
0.476, -0.264
R1 ) ∑|Fo| - |Fc|/∑|Fo| and wR2 ) {∑[w(Fo - Fc2)2]/
a
2
∑[w(Fo2)2]}1/2
.
7: yield 67%. 1H NMR (CDCl3, 400.147 MHz): δ 7.07-6.81
(m, 5H, aryl-H), 4.60 (t, J ) 5.8 Hz, 2H, CH2O), 4.45 (t, J )
5.6 Hz, 2H, CH2O), 3.96 (s, 2H, NCH2-aryl), 3.80 (t, J ) 6.4
Hz, 2H, CHMe2), 3.20 (t, J ) 5.6 Hz, 2H, NCH2CH2), 3.13(t, J
) 5.8 Hz, 2H, NCH2CH2), 2.25 (s, 3H, aryl-Me), 2.16 (s, 3H,
aryl-Me), 1.27 (d, J ) 6.7 Hz, 12H, CHMe2). 13C{1H} NMR
(CDCl3, 100.626 MHz): δ 160.7, 158.9, 138.1, 133.5, 131.3,
128.9, 127.9, 125.4, 123.4, 122.6, 120.7, 120.5 (aryl), 71.56
(CH2O), 56.34 (NCH2-aryl), 26.80 (CHMe2), 23.50 (CHMe2),
22.75 (CHMe2), 20.50 (aryl-Me), 16.67 (aryl-Me). Anal. Calcd
for C25H35NO4Ti: C, 65.08; H, 7.65; N, 3.04. Found: C, 64.60;
H, 7.94; N, 3.11.
Resu lts a n d Discu ssion
The treatment of Ti(O-i-Pr)4 with 1 equiv of 2,6-di-i-
Pr-phenol and 1 equiv of the ligand precursors 1-4 in
THF gave, after workup, the novel titanatranes 5-8 as
orange-yellow crystals in 62-76% isolated yield. These
four products in the solid state were stable in air for a
few weeks, and according to 1H NMR spectroscopy, they
decomposed slightly after a few days at room temper-
ature in CDCl3 solutions contained in capped NMR
tubes. They are soluble in polar organic solvents and
in toluene, but are insoluble in alkanes such as n-
hexane.
1
8: yield 76%. H NMR (CDCl3, 400.147 MHz): δ 7.02 (d, J
) 7.6 Hz, 2H, aryl-H), 6.84 (t, J ) 7.6 Hz, 1H, aryl-H), 4.54 (t,
J ) 5.6 Hz, 6H, CH2O), 3.62 (m, 2H, CHMe2), 3.29 (t, J ) 5.5
Hz, 6H, NCH2), 1.25 (d, J ) 6.9 Hz, 12H, CHMe2). 13C{1H}
NMR (CDCl3, 100.626 MHz): δ 160.0, 137.9, 122.4, 120.4
(aryl), 71.17 (CH2O), 56.78 (NCH2), 26.84 (CHMe2), 23.33
(CHMe2). Anal. Calcd for C18H29NO4Ti: C, 58.23; H, 7.87; N,
3.77. Found: C, 58.35; H, 8.12; N, 3.79.
The 1H NMR spectra of 5-8 display well-defined
resonances with their expected integrations. At 325K
in toluene-d8 solution, the two resonances for aryl
methylene protons in 5 coalesce to a single resonance
(Figure 1). On the NMR time scale, 5 should have
pseudo-C3 symmetry, and therefore the benzylic CH2
protons should not be equivalent. Indeed, the proximity
of these protons to aromatic rings may explain the ca.
1.4 ppm chemical shift difference displayed in the room-
temperature spectrum shown in Figure 1. Upon heating,
compound 5 assumes pseudo-C3v symmetry on the NMR
time scale and all six CH2 protons become equivalent.
To estimate the barrier to inversion in 5, ∆Gq (J /mol)
) 19.14Tc[9.97 + log(Tc/δν)] was calculated from the
coalescence temperature of the NCH2C6H2 methylene
X-r a y Cr ysta llogr a p h y for 5 a n d 6. The crystallographic
measurements were performed at 173 K for 5 or 293 K for 6
using a Bruker CCD-1000 diffractometer with Mo KR (λ )
0.71073 Å) radiation and a detector-to-crystal distance of 5.03
cm. Specimens of suitable quality and size (0.2 × 0.2 × 0.2
mm3) were selected and mounted onto glass fibers with silicon
grease or epoxy glue. The initial cell constants were obtained
from three series of ω scans at different starting angles. Each
series consisted of 30 frames collected at intervals of 0.3° in a
10° range about ω with the exposure time of 30 s per frame
for 5 and 20 s per frame for 6. All the intensity data were
corrected for Lorentz and polarization effects. The structures
were solved by the Patterson method and the direct method
and were refined by full-matrix anisotropic approximation. All
hydrogen atoms were placed at idealized positions in the
structure factor calculation and were allowed to ride on the
neighboring atoms with relative isotropic displacement coef-
ficients. Final refinement based on the reflections (I > 2.0σ(I))
converged at R1 ) 0.0586, wR2 ) 0.1772, and GOF ) 1.089
for 5 and at R1 ) 0.0562, wR2 ) 0.1538, and GOF ) 1.017 for
6. Further details are listed in Table 1.
1
protons in the H NMR spectrum.16 ∆Gq for 5 (at Tc )
325 K with δν ) 292.66 Hz) is 62.3 kJ /mol. By contrast,
the analogous benzylic protons of 6 cannot become
equivalent upon ring inversion because the cage moiety
of 6 is pseudo-Cs symmetric. The C1-symmetric twisted
solid-state structure for 6 suggested that four chemical
shifts should be observed in the 1H NMR spectrum, but
P olym er iza tion P r oced u r e. LA polymerizations were
carried out as follows: 2.00 g of LA and then the appropriate
(16) Gu¨nther, H. NMR Spectroscopy-an Introduction; Wiley: New
York, 1980.

2398 Organometallics, Vol. 21, No. 12, 2002
Kim and Verkade
1
Ta ble 2. Com p a r ison of H Ch em ica l Sh ifts for Com p ou n d s 1-8 Dissolved in CDCl3
peak assignment
compound
aryl-H
CH2O
3.85
3.73
3.69
CHMe2
NCH2-aryl
NCH2CH2
aryl-Me
CHMe2
1
2
3
4
5
6
7
8
6.83, 6.71
6.83, 6.67
6.83, 6.61
3.61
3.71
3.77
2.19
2.18
2.19
2.67
2.74
2.43
7.18-6.73
7.27-6.80
7.07-6.81
7.02, 6.84
4.11a
4.00
3.80
3.62
4.11,a 2.97
3.91, 3.63
3.96
2.23, 2.06
2.25, 2.15
2.25, 2.16
1.24
1.29
1.27
1.25
4.53
4.60, 4.45
4.54
3.00
3.20, 3.13
3.29
a
The CHMe2 and NCH2-aryl proton resonances overlap.
F igu r e 2. ORTEP drawing of 5 showing 50% probability
thermal ellipsoids with H atoms and solvent omitted for
clarity. Selected bond distances (Å): Ti1-O1 ) 1.836(2),
Ti1-O2 ) 1.822(2), Ti1-O3 ) 1.831(2), Ti1-O4 ) 1.834-
(2), Ti1-N1 ) 2.306(2). Selected bond angles (deg): C28-
O4-Ti1 ) 138.52(17), O4-Ti1-N1 ) 174.89(8), O1-Ti1-
O4 ) 100.05(9), O2-Ti1-O4 ) 97.96(9), O3-Ti1-O4 )
92.93(9).
F igu r e 3. ORTEP drawing of 6 showing 50% probability
thermal ellipsoids with H atoms and solvent omitted for
clarity. Selected bond distances (Å): Ti1-O1 ) 1.828(3),
Ti1-O2 ) 1.825(3), Ti1-O3 ) 1.807(3), Ti1-O4 ) 1.829-
(3), Ti1-N1 ) 2.287. Selected bond angles (deg): C10-
O2-Ti1 ) 145.7(3), O1-Ti1-O2 ) 102.43(13), O2-Ti1-
O3 ) 93.68(14), O2-Ti1-O4 ) 99.66(14), O2-Ti1-N1 )
170.38(13).
X-ray analyses reveal that 5 and 6 have similar solid-
state structures, and both possess 0.5 THF molecule of
solvation. In contrast to oxygen-bridged dimeric struc-
tures frequently observed for titanatranes,3d,f the mo-
lecular structures of 5 and 6 are monomeric (which is
consistent with their NMR spectra) presumably because
of the steric bulk of the axially located di-i-Pr-phenolate
ligand.
The tricyclic cage moiety of 5 resembles a three-bladed
turbine of C3 symmetry, while that in 6 is similar (with
an ethylene bridge replacing an aryl methylene group)
resulting in Cs symmetry. In addition to the three
anionic oxygens, the titanium atom in 5 and 6 is ligated
via a transannular interaction stemming from the
bridgehead amino nitrogen, giving a slightly distorted
trigonal bipyramidal local geometry around the metal.
The sum of the angles around the equatorial oxygens
is 355.68(10)° and 353.54(15)° in 5 and 6, respectively.
As a result, the acute Oeq-Ti-N angles [av ) 83.07(8)°
for 5 and 81.59(14)° for 6] and the obtuse Oeq-Ti-Oax
angles [av ) 96.98(9)° for 5 and 98.59(14)° for 6] reflect
a displacement of the titanium atom toward the axial
only two were seen (Table 2). Similar considerations
hold for the NCH2 groups of 7, for which two peaks were
1
also observed in the H NMR spectrum. (See Table 2)
As observed in other five-membered titanatranes,3 the
protons for both sets of methylene groups of 8 should
be equivalent owing to the rate of ring inversion, which
is rapid on the NMR time scale. Similarly, the 13C{1H}
NMR spectrum of 5 exhibits resonances corresponding
to nonequivalent i-Pr groups even at room temperature,
owing to its pseudo-Cs symmetry. In the 1H NMR
spectrum of 5, the two i-Pr methyl groups displayed a
somewhat broadened singlet, a doublet, and a triplet
at room temperature in chloroform-d1, toluene-d8, and
benzene-d6, respectively. However, as the temperature
of the benzene-d6 solution decreased, the triplet gradu-
ally became two doublets, which is expected for two
nonequivalent i-Pr groups whose CH3 protons are
equivalent owing to rapid rotation.
To elucidate the nature of the metal-ligand bonding
in these titanatranes, we carried out single-crystal X-ray
diffraction studies on 5 and 6. ORTEP depictions,17
selected bond distances, and selected bond angles for 5
and 6 are shown in Figures 2 and 3, respectively. The
oxygen, which is larger for 6. Furthermore, the Nax
-
Ti-Oax angle deviates from linearity by 5.11(8)° in 5
and by 9.62(13)° in 6. These deviations are large
compared with deviations of 0.20(8)-1.46(3)° reported
(17) J ohnson, C. K. ORTEP, Report ORNL-5138; Oak Ridge Na-
tional Laboratory: Oak Ridge, TN, 1976.

Novel Titanatranes with Different Ring Sizes
Organometallics, Vol. 21, No. 12, 2002 2399
Ta ble 3. Da ta for l- a n d r a c-La ctid e Bu lk
P olym er iza tion s Ca ta lyzed by 5-8a
somewhat large polydispersities and low molecular
weights. This may be attributed to a rate of initiation
that is slower than the rate of polymer propagation, thus
allowing more time for the occurrence of transesterifi-
cation reactions during propagation. As a consequence,
bimodal and unimodal molecular weight distributions
exhibiting a side tail or shoulder can be encountered,
as was observed in the GPC trace of several of the
polymers. However, despite this problem and the fact
that the polymerizations were carried out at an elevated
temperature, the PLA polydispersity indices are in an
acceptable range (1.42-1.97 in Table 2).
type of
yield
(%)
catalyst lactide g polymer
Mw
Mn
PDIb
b
b
5
6
7
8
l-LA
rac-LA
l-LA
rac-LA
l-LA
rac-LA
l-LA
1.38
1.35
1.50
1.47
1.82
1.79
1.98
1.92
69
68
75
74
91
90
99
96
29 300 19 400 1.51
23 000 16 000 1.43
30 700 18 700 1.64
24 700 16 500 1.50
30 800 20 000 1.56
32 900 23 100 1.42
44 500 25 400 1.75
66 100 33 600 1.97
A determination of the stereochemical microstructure
of PLA can be achieved upon inspection of the methine
region of homonuclear decoupled 1H NMR spectra of
PLA solutions.18 Such spectra of PLA derived from rac-
LA display the typical five resonances predicted from a
Bernouillian analysis of totally random (atactic) PLA,
whereas spectra of PLA derived from l-LA exhibit only
one peak, corresponding to the mmm tetrad for isotactic
rac-LA
a
LA (2 g) LA/Ti ) 300, polymerization temperature ) 130 °C,
b
polymerization time ) 24 h. The weight average molecular
weight (Mw), the number average molecular weight (Mn), and the
polydispersity index (PDI ) Mw/Mn) were determined by GPC.
1
PLA. H NMR spectra corresponding to these descrip-
tions were also observed for our correspondingly derived
polymers.
for other mononuclear titanatranes,3g-i,k,l although val-
ues of 15.5(1)-51.05(8)° for this angle have been de-
scribed for di- or multinuclear oxo-bridged titana-
tranes.3b,d-f,j The average Ti-O bond distance for all
four oxygens in each of 5 [1.831(2) Å] and 6 [1.822(3)
Å] is similar to the average of this distance observed
for other structurally characterized titanatranes.3 In-
terestingly, the transannular Ti-N bond distance in 5
[2.305(2) Å] and 6 [2.287(4) Å] falls near the short end
of the range 2.264(3)-2.400(3) Å found in previously
structurally characterized titanium trialkanolamine
derivatives.3
In summary, we have synthesized a novel series of
four titanatranes featuring a stepwise change in ring
size from five- to six-membered rings. These complexes
function, with a trend in efficiency roughly paralleling
the number of five-membered rings they possess, as
single-site initiators for the polymerization of l-LA to
isotactic PLA and rac-LA to atactic PLA.
Ack n ow led gm en t. This work was generously sup-
ported by the National Science Foundation through a
grant. We thank Dr. G. K. J naneshwara for aid in
synthesizing compounds 2 and 3, Dr. Arkady Ellern for
carrying out the X-ray structure determinations of
compounds 5 and 6, and Mr. Erik C. Hagberg in Prof.
Valerie Sheares’ group in this department for obtaining
the GPC data.
Our preliminary results on the use of titanium
alkoxide catalysts for the bulk polymerization of l-LA
and rac-LA are summarized for 5-8 in Table 3. It
appears that the initiating group is the highly bulky
1
di-i-Pr-phenolate group, which was shown by H NMR
spectroscopy to be present in solutions of the isolated
PLA samples. It is seen from Table 2 that the nature of
the chelating tetradentate ligand significantly affects
the molecular weight and the polydispersity indices
(measured by GPC) and also the yield of the polymer.
As the number of five-membered rings in the tetraden-
tate ligand increases, there is a rough trend toward
increasing polymerization activity and polydispersity
index. Interestingly, catalysts 5-8 yield polymers with
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, atomic coordinates, thermal parameters, bond distances,
and bond angles for compounds 5 and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0110686
(18) (a) Thakur, K. A. M.; Kean, R. T.; Zell, M. T.; Padden, B. E.;
Munson, E. J . Chem. Commun. 1998, 1913. (b) Chisholm, M. H.; Iyer,
S. S.; Matison, M. E.; McCollum, D. G.; Pagel, M. Chem. Commun.
1997, 1999.