A R T I C L E S
O’Keefe et al.
NY. IR spectra for characterization were recorded on a Nicolet Avatar
320 FT-IR spectrometer, as Nujol films between NaCl plates. UV-
vis specta were recorded on a Hewlett-Packard HP8452A diode array
spectrophotometer (190-820 nm scan range). Molecular weights (Mn
and Mw) and polydispersities (Mw/Mn) were determined by size
exclusion chromatography with respect to polystyrene standards.
Samples were analyzed at 40 °C (THF eluent) or 30 °C (CHCl3 or
CH2Cl2 eluent) with a Waters high performance liquid chromatograph
connected to three Jordi Gel DVB columns with pore sizes of 104,
103, and 500 Å and a Waters 2410 refractive index detector. The
solvents were eluted at a flow rate of 1.0 mL/min.
invoking steric inhibition of monomer binding and/or ring
opening by the bulky supporting amidinate ligands. Accordingly,
the absence of such ligands in Fe2(OCHPh2)6 would rationalize
its faster propagation rate.
Conclusion
We have prepared and characterized several new iron(III)-
alkoxide complexes, including amidinate-supported single site
and homoleptic dinuclear complexes that incorporate the same
alkoxide (Ph2HCO-), and have compared their reactivity in
cyclic ester polymerizations. While both complexes comprising
Ph2HCO- are effective CL and LA polymerization catalysts,
the mononuclear complex shows poorer polymerization behav-
ior. Kinetic investigations revealed that for the polymerization
of CL by the dinuclear complex the reaction is first order in
both monomer and the diiron complex when taking into account
a low-level impurity that deactivates the catalyst. Polymerization
of CL by the mononuclear complex displayed kinetics with a
similar first-order dependence on [CL], but a dependence on
the [L2FeOCHPh2] of approximately one-half. This fractional
dependence was interpreted by using a model of active chain
aggregation, enabling a comparison of kprop values for the mono-
and dinuclear systems that revealed ∼50-fold propagation rate
enhancement for the latter.
The results reported herein thus present a conundrum. To
access single site catalysts that would be expected to exhibit
simpler kinetic behavior in cyclic ester polymerizations, steri-
cally hindered ligands that inhibit formation of dimeric (or
higher nuclearity) species are required. However, such support-
ing ligands can drastically slow polymer propagation, and in a
counterintuitive fashion, may actually induce aggregation of
growing polymer chains, thus complicating the polymerization
kinetics and potentially affecting polymer structural features.
A more complete understanding of the nature of the reactive
species in cyclic ester polymerizations is needed in order to
understand these intriguing structure/reactivity relationships and
to implement new strategies for catalyst design.
[Tl(OCHPh2)]n. A solution of TlOEt (2.34 g, 9.38 mmol) in pentane
(2 mL) was added to a solution of benzhydrol (1.73 g, 9.39 mmol) in
pentane/THF (8 mL/3 mL). A white precipitate formed within 5 min.
The reaction mixture was stirred for 16 h, then was filtered off to give
[Tl(OCHPh2)]n as a colorless crystalline solid (3.30 g, 91%). IR (Nujol,
cm-1) 1590, 1487, 1341, 1177, 1079, 1033, 1017, 991, 767, 738, 710,
1
704, 661, 618, 606; H NMR (500 MHz, d8-THF) δ 6.44 (d,br, 1H,
CH), 7.14 (t, 2H, p-CH), 7.23 (t, 4H, m-CH), 7.29 (m, 4H, o-CH). 13
C
NMR (300 MHz, d8-THF) δ 81.24 (Ph2CHO, br), 127.41 (o-CH),
128.34 (p-CH, br), 129.32 (m-CH), 150.28 (i-C).
Fe2(OCHPh2)6. A solution of [Tl(OCHPh2)]n (993 mg, 2.56 mmol)
in THF (5 mL) was added to a solution of anhydrous FeCl3 (144 mg,
0.891 mmol) in THF (6 mL). A white precipitate formed immediately.
The mixture was stirred for 16 h, then filtered through a plug of Celite
to remove TlCl, and the resulting yellow solution was reduced to
dryness in vacuo. The yellow residue was redissolved in toluene, filtered
through Celite, concentrated and stored overnight at -35 °C to give
Fe2(OCHPh2)6 as a yellow crystalline solid (288 mg, 53%). IR (Nujol,
cm-1) 1490, 1332, 1300, 1282, 1260, 1183, 1150, 1082, 1044, 1022,
988, 770, 742, 698, 667, 629. Anal. Calcd for C78H66O6Fe2: C, 77.36;
H, 5.49. Found: C, 77.41; H, 5.38. Crystals suitable for analysis by
X-ray diffraction were grown at -35 °C from toluene.
L2FeCl. This complex has previously been reported in the literature,
but was prepared by a different method.15 A solution of L‚Li(TMEDA)
(3.24 g, 8.38 mmol) in THF (15 mL) was added to a solution of FeCl3
(0.68 g, 4.21 mmol) in THF (15 mL). The purple solution was stirred
at room temperature overnight, and then the solvent was removed in
vacuo. Pentane was added and the LiCl was removed by filtration. The
filtrate was concentrated in vacuo to about 4 mL and the solution was
cooled to -35 °C to produce X-ray quality, dark purple crystals of
L2FeCl (1.61 g, 62%). A unit cell determination by X-ray crystal-
lography of a single crystal of L2FeCl gave the same unit cell
dimensions as those reported previously (a ) 12.097(7) Å, b ) 12.776-
(6) Å, c ) 13.174(7) Å; R ) 67.35(3)°, â ) 75.56(3)°, γ ) 66.84(3)°;
V ) 1716(1) Å3).15 IR (Nujol, cm-1) 2360, 1244, 983, 837, 760, 702.
Anal. Calcd for C26H46N4Si4ClFe: C, 50.50; H, 7.50; N, 9.06. Found:
C, 50.48; H, 7.33; N, 9.10.
L2FeOEt. A solution of L2FeCl (1.19 g, 1.92 mmol) in THF (15
mL) was added to a solution of TlOEt (0.48 g, 1.92 mmol) in THF (15
mL). The resulting terracotta solution was stirred at room temperature
overnight. The insoluble TlCl was removed by filtration through Celite
and the filtrate was dried in vacuo. The residue was dissolved in pentane
and filtered through Celite to remove the remaining insolubles. The
filtrate was dried in vacuo, dissolved in 4 mL of a 50/50 HMDSO/
pentane mixture, and cooled to -35 °C to produce X-ray quality, dark
red crystals of L2FeOEt (0.69 g, 57%). IR (Nujol, cm-1) 1666, 1260,
1096, 1019, 843, 796; UV-vis (toluene) (λmax, nm (ꢀ, M-1 cm-1)) 395
(2480). Anal. Calcd for C28H51N4OSi4Fe: C, 53.56; H, 8.19; N, 8.92.
Found: C, 53.38; H, 8.02; N, 8.86. Evans method (C6D6): µeff ) 5.9
µB.
Experimental Section
General Procedures. All air-sensitive reactions were performed in
a MBraun glovebox under a N2 atmosphere or with standard Schlenk
techniques. N,N,N′N′-Tetramethylethylenediamine (TMEDA), tetra-
hydrofuran (THF), pentane, and hexamethyldisiloxane (HMDSO) were
distilled from Na/benzophenone. Toluene was distilled from sodium.
Benzonitrile and NH(SiMe3)2 were distilled from 3 Å molecular sieves.
D,L-Lactide (Aldrich) was recrystallized from dry toluene, sublimed,
dissolved in THF, passed through a plug of activated neutral alumina,
and pumped to dryness. ꢀ-Caprolactone (Aldrich) was dried over CaH2
and distilled under vacuum. n-BuLi (Acros) was titrated with Ph2-
CHCO2H prior to use.39 Anhydrous FeCl3 was prepared from FeCl3‚
6H2O by the method of So and Boudjouk.40 PhC(NSiMe3)2Li(TMEDA)
(LLi(TMEDA)) was prepared according to published procedures and
1
the structure was confirmed by H NMR spectroscopy.41 Benzhydrol
(Aldrich) was recrystallized from dry diethyl ether prior to use. TlOEt
(Aldrich) was filtered through a plug of Celite prior to use.
Physical Methods. NMR spectra were collected with a Varian VI-
300, Varian VXR-300, or Varian VXR-500 spectrometer. Elemental
analyses were determined by Oneida Research Services, Whitesboro,
L2FeOCHPh2. A solution of L2FeCl (0.72 g, 1.16 mmol) in THF
(10 mL) was added to a solution of [TlOCHPh2]n (0.45 g, 1.12 mmol)
in THF (10 mL). The resulting terracotta solution was stirred at room
temperature overnight. The insoluble TlCl was removed by filtration
through Celite and the filtrate was dried in vacuo. The residue was
(39) Kofron, W. G.; Baclawski, L. M. J. Org. Chem. 1976, 41, 1879-1880.
(40) So, J.-H.; Boudjouk, P. Inorg. Chem. 1990, 29, 1592-1593.
(41) Dick, D. G.; Duchateau, R.; Edema, J. J. H.; Gambarotta, S. Inorg. Chem.
1993, 32, 1959-1962.
9
4392 J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002