Rare-earth amidate complexes. Easily accessed initiators for ε-caprolactone ring-opening polymerization

Article abstract of DOI:10.1021/ic8010635

The direct synthesis of yttrium amidate complexes using the simple reaction of amide proligands and Y(N(SiMe₃)₂)₃ results in the high-yielding preparation and isolation of crystalline, monomeric materials. The complex, tris(N-2′,6′-diisopropylphenyl(naphthyl) amidate)yttrium mono(tetrahydrofuran) (4), was structurally determined to be a 7-coordinate C₁ symmetric structure, maintaining one bound tetrahydrofuran molecule. Compound 4 (C₁₂H₁₇[NCO]C ₁₀H₇)₃Y(C₄H₈O) crystallized in the monoclinic space group P2(1)/c with a = 13.7820(11) A, b = 33.598(3) A, c = 16.0575(12) A, α = 90°, β = 98.762(3)°, γ = 90°, Z = 4. Solution phase NMR spectroscopic characterization of this same complex showed a highly symmetric species, consistent with a fluxional coordination environment for these compounds. Preliminary studies into the initiation of ε-caprolactone ring-opening polymerization using these complexes indicate high activity, producing high molecular weight polymer.

Full text of DOI:10.1021/ic8010635

Inorg. Chem. 2008, 47, 8062-8068
Rare-Earth Amidate Complexes. Easily Accessed Initiators For
ε-Caprolactone Ring-Opening Polymerization
Louisa J. E. Stanlake, J. David Beard, and Laurel L. Schafer*
Department of Chemistry, UniVersity of British Columbia, 6174 UniVersity BouleVard, VancouVer,
British Columbia V6T 1Z3, Canada
Received March 8, 2008
The direct synthesis of yttrium amidate complexes using the simple reaction of amide proligands and Y(N(SiMe3)2)3
results in the high-yielding preparation and isolation of crystalline, monomeric materials. The complex, tris(N-2′,6′-
diisopropylphenyl(naphthyl)amidate)yttrium mono(tetrahydrofuran) (4), was structurally determined to be a 7-coordinate
C1 symmetric structure, maintaining one bound tetrahydrofuran molecule. Compound 4 (C12H17[NCO]C10H7)3Y(C4H8O)
crystallized in the monoclinic space group P2(1)/c with a ) 13.7820(11) Å, b ) 33.598(3) Å, c ) 16.0575(12) Å,
R ) 90°, ꢀ ) 98.762(3)°, γ ) 90°, Z ) 4. Solution phase NMR spectroscopic characterization of this same
complex showed a highly symmetric species, consistent with a fluxional coordination environment for these
compounds. Preliminary studies into the initiation of ε-caprolactone ring-opening polymerization using these complexes
indicate high activity, producing high molecular weight polymer.
Introduction
of catalytic activity for these compounds. To this end,
substantial research with group 3 and lanthanide complexes
containing ancillary nitrogen donor ligands such as ami-
do,1-3,14-23 amido pyridinate,24 amidinate,4,25-27 guanidi-
nate,5,6,28-37 and ꢀ-diketiminate7-12,38-43 ligand sets has
been conducted. However, the synthesis of discrete rare-earth
Rare-earth complexes are very attractive catalyst systems
because of their low toxicity, low cost, and high reactiv-
ity.1-13 Easily accessed and modular ligand sets suitable for
rare-earth complexation are ideally suited for the optimization
* To whom correspondence should be addressed. E-mail: schafer@
chem.ubc.ca.
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8062 Inorganic Chemistry, Vol. 47, No. 18, 2008
10.1021/ic8010635 CCC: $40.75  2008 American Chemical Society
Published on Web 08/19/2008

Direct Synthesis of Yttrium Amidate Complexes
Scheme 1. Synthesis of Tris(amidate) Complexes
complexes as catalyst precursors can be difficult because of
low energy ligand redistribution pathways. Furthermore,
because of the high Lewis acidity of these rare-earth metals,
in combination with their larger radii, synthetic approaches
using multidentate ligands often results in dimer formation
or even higher aggregates.44 Typically, the formation of
discrete species is most commonly controlled by the steric
environment of the ligand. Furthermore, facile modification
of steric and electronic properties of the ancillary ligand of
new complexes is desirable for catalyst optimization. To this
end, homoleptic amidinate complexes of the lanthanides and
group 3 metals have been extensively investigated and have
been modestly successful in ring-opening polymerization of
ε-caprolactone.45,46 However, the similar ancillary amidate
ligand, which replaces one nitrogen donor for an oxygen
donor, has been largely overlooked as a class of auxiliary
ligand capable of providing variable reactivity at the metal
center in the resultant complexes. Our research focuses on
new high-yielding preparative methods to access crystalline,
discrete rare-earth amidate complexes that display promising
catalytic activity. Here we report a direct route for the
synthesis, isolation, and characterization of a family of
crystalline, yttrium tris(amidate) complexes which rapidly
initiate the ring-opening polymerization of ε-caprolactone
to give high molecular weight, biodegradable, polyester
product.
and allenes.51 The amide proligands can be easily varied for
desired steric and electronic properties because of their facile
synthesis, from acid chlorides and primary amines. The
resultant organic amides can be used in the direct preparation
of homoleptic group 3 amidate complexes with a protonolysis
reaction of organic amides with commercially available
Y(N(SiMe3)2)3 (Scheme 1) to give easily purified crystalline
materials. This contrasts with previous examples of ami-
date complexes of the lanthanide and group 3 metals that
have been synthesized via isocyanate insertion into a reactive
M-C bond of lanthanide alkyl complexes.52-54 This pre-
parative approach typically yields amidate bridged bimetallic
complexes, which can be fully characterized.53 Moreover, a
decomposition product of an attempted isocyanate insertion
reaction led to the only previous report of a homoleptic
lanthanide amidate complex,53 a Ho[OC(Bun)NPh]3 complex,
which was characterized by elemental analysis and IR
spectroscopy but no structural information was provided.53
While the isocyanate insertion method is well established,
the requisite lanthanide alkyl starting materials are nontrivial
to synthesize and handle. Furthermore, this route has been
exploited for the exploration of the reactivity of M-C bonds,
rather than using the resultant amidate complexes as potential
tunable catalytic systems. Most importantly, our direct, high-
yielding route gives easily isolable monomeric, crystalline
amidate complexes that show promising reactivity in the ring-
opening polymerization of lactones.
Recently our research group has demonstrated that bis(a-
midate)bis(amido) complexes of group 4 metals are tunable
catalysts for the hydroamination of alkynes,47,48 alkenes,49,50
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Experimental Section
All operations were performed under an inert atmosphere of
nitrogen using standard Schlenk-line or glovebox techniques.
Tetrahydrofuran (THF), toluene, pentane, and hexanes were all
purified by passage through an alumina column and sparged with
nitrogen. ε-Caprolactone was dried by stirring over CaH2 for 4 days,
allowed to settle for 2 days, decanted to a new flask and then
distilled under reduced pressure, and stored over molecular sieves.
Y(N(SiMe3)2)3 was synthesized as described in literature.55 All other
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Wong, W.-T.; Yu, K.; Sun, J. J. Organomet. Chem. 2003, 678, 108.
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Inorganic Chemistry, Vol. 47, No. 18, 2008 8063

Stanlake et al.
chemicals were commercially available and used as received unless
otherwise stated. 1H and 13C NMR spectra were recorded on Bruker
AV300, AV400, and AV600 spectrometers. Molecular weights were
estimated by triple detection gel permeation chromatography (GPC
- LLS) using a Waters liquid chromatograph equipped with a Waters
515 HPLC pump, Waters 717 plus autosampler, Waters Styragel
columns (4.6 × 300 mm) HR5E, HR4 and HR2, Waters 2410
differential refractometer, Wyatt tristar miniDAWN (laser light
scattering detector), and a Wyatt ViscoStar viscometer. A flow rate
of 0.5 mL min-1 was used and samples were dissolved in THF
(ca. 4 mg mL-1). Molecular weights were determined by compari-
son to a polystyrene standard curve, and absolute molecular weights
56,57
were determined using a dn/dc of 0.079 mL g-1
.
Elemental
analyses and mass spectra were performed by the microanalytical
laboratory of the Department of Chemistry at the University of
British Columbia. X-ray crystallography was conducted at the
University of British Columbia by Dr. Brian Patrick. Structure 4
was modified using the squeeze function of the Platon software
package to remove disordered THF.58 Compounds 1 and 2 have
been previously reported, but complete characterization data was
not provided.59-62
Figure 1. Oak Ridge Thermal Ellipsoid Plot (ORTEP) diagram of the solid-
state molecular structure of [(THF)Y(Nap[O,N](i-Pr)2Ph)3]2 (4) with the
probability ellipsoids drawn at the 50% level. Naphthyl groups (except for
ipso-carbon) and the carbons of the THF groups omitted for clarity.65
(C6D6, 400 MHz, 293 K) δ 7.05 (m, 1H, phenyl-H), 6.95 (m, 2H,
phenyl-H), 6.17 (s, 1H, N-H), 2.09 (s, 6H, CH3), 1.08 (s, 9H,
C(CH3)3). 13C{1H} NMR (C6D6, 100 MHz, 293 K) δ 175.6 (CdO),
135.6, 135.1, 127.7, 126.6 (phenyl C), 38.7 (C(CH3) 3), 27.3
(C(CH3)3), 18.0 (CH3). IR data (KBr, cm-1): 3295 (br), 3021 (w),
2956 (s), 2921 (w), 1650 (s), 1593 (w), 1506 (s), 1278 (w), 1221
(m) 938 (w), 768 (s), 722 (w), 647 (w). EIMS( m/z): 205 [M]+.
Anal. Found (calcd for C13H19NO): C 76.12% (76.06%), N 7.00%
(6.82%), H 9.09% (9.33%).
Synthesis of N-(Diisopropylphenyl)naphthyl Amide (1). To a
250 mL round-bottom flask was added 2,6-diisopropylaniline (4.95
mL, 26.2 mmol) dissolved in 125 mL of dichloromethane. The
reaction mixture was cooled to 0 °C, and triethylamine (8 mL) was
added dropwise by syringe. The resulting solution was stirred for
5 min with subsequent dropwise addition of 1-naphthoyl chloride
(3.95 mL, 26.2 mmol). The reaction was stirred overnight and then
was washed with 1 M HCl (3 × 50 mL), followed by 1 M NaOH
(30 mL), and brine (30 mL). The organic layer was dried over
MgSO4, filtered, then concentrated under reduced pressure to obtain
a beige solid. The solid was recrystallized twice from hot toluene
Synthesis of N-(2,6-Diisopropylphenyl)p-(trifluoromethyl)phe-
nyl Amide (3). The experimental method described for 1 was used
in the preparation of 3 using 2,6-diisopropylaniline (6.30 mL, 33.4
mmol) and p-(trifluoromethyl)benzoyl chloride (5.00 mL, 33.6
1
to obtain a white powder. Yield: 7.94 g, 91%. H NMR (CDCl3,
3
1
300 MHz, 293 K) δ 8.46 (d, J ) 6 Hz, 1H, 9-naphthyl-H), 7.98
mmol). Yield: 9.24 g, 79%. H NMR (C6D6, 600 MHz, 293 K) δ
(d, 3J ) 6 Hz, 1H, 6-naphthyl-H), 7.90 (d, 3J ) 6 Hz, 1H,
3
7.47 (d, J ) 12 Hz, 2H, aryl-H), 7.25 (m, 2H, aryl-H), 7.14 (m,
3
3H, aryl-H), 6.68 (s, 1H, N-H), 3.08 (septet, 3J ) 6 Hz, 2H,
4-naphthyl-H), 7.83 (d, J ) 5 Hz, 1H, 2-naphthyl-H), 7.57 (m,
3H, 8,7,3-naphthyl-H), 7.36 (t, 3J ) 6 Hz, 1H, 4-diisopropylphenyl-
H), 7.25 (d, 3J ) 6 Hz, 2H, 3,5-diisopropylphenyl-H), 7.21 (s, 1H,
N-H), 3.30 (septet, 3J ) 5 Hz, 2H, CH(CH3)2), 1.28 (d, 3J ) 5 Hz,
12H, CH(CH3)2). 13C{1H} NMR (CDCl3, 75.5 MHz, 293K) δ 170.4
(CdO), 147.8, 135.6, 135.3, 132.5, 132.2, 131.9, 130.1, 129.8,
128.8, 128.0, 126.8, 126.2, 126.1, 125.0 (aryl C), 30.4 (CH(CH3)2),
25.2(CH(CH3)2). IR data (KBr, cm-1): 3253 (br), 3047 (w), 2961
(s), 2865 (w), 1643 (s), 1591 (w), 1517 (s), 1297 (w), 913 (w),
785 (w), 734 (w), 654 (w). EIMS(m/z): 331 [M]+. Anal. Found
(calcd for C23H25NO): C 83.74% (83.34%), N 4.55% (4.23%), H
7.54% (7.60%).
3
CH(CH3)2), 1.21 (d, J ) 6 Hz, 12H, CH(CH3)2). 13C{1H} NMR
(C6D6, 375 MHz, 293K) δ 164.1 (CdO), 146.10, 137.2, 132.5,
130.8, 128.3, 125.1, 123.2 (aryl C), 28.6 (CH(CH3)2), 23.2 (CH-
(CH3)2). IR data (KBr, cm-1): 3300 (br), 2966 (s), 2929 (s) 1648
(s), 1580 (w), 1530 (s), 1500 (s), 1330 (m), 1317 (w), 1158 (w),
1116 (w), 862 (w), 801 (w). EIMS(m/z): 349 [M]+. Anal. found
(calcd for C20H22F3NO): C 68.75% (68.75%), N 4.30% (4.01%),
H 6.64% (6.35%).
Synthesis of Tris(N-2′, 6′-diisopropylphenyl(naphthyl)ami-
date)yttrium Mono(tetrahydrofuran) (4). Inside a nitrogen filled
glovebox, a vial was charged with amide 1 (0.205 g, 0.618 mmol),
yttrium tris(bis(trimethylsilyl)amide) (0.118 g, 0.206 mmol), and
a stirbar. To this, 5 mL of tetrahydrofuran was transferred to the
reaction vessel at room temperature. The solution was stirred within
the glovebox for 2 h at 60 °C, and then filtered through Celite and
concentrated under reduced pressure to a pale yellow powder. The
product was recrystallized by dissolving in a minimum amount of
hexanes and then left at -30 °C to yield colorless crystals. Yield:
0.223 g, 94%. X-ray quality crystals were grown from cold hexanes.
Synthesis of N-(2,6-Dimethylphenyl)t-butyl Amide (2). The
experimental method described for 1 was used in the preparation
of 2 using 2,6-dimethylaniline (5.10 mL, 0.041 mol) and trimethy-
lacetyl chloride (5.05 mL, 0.041 mol). Yield: 7.07 g, 84%. 1H NMR
(56) Knecht, M. R.; Elias, H.-G. Die Makro. Chem. 1972, 157, 1.
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Sci. 1998, 68, 1917.
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1
Refer to Figure 1, and Table 1 for crystallographic data. H NMR
(300 MHz, C6D6) δ 9.43 (d, J ) 9 Hz, 3H, aryl-H), 7.70 (d, J )
8 Hz, 3H, aryl-H), 7.66-7.60 (m, 10H, aryl-H), 7.55 (d, J ) 8 Hz,
3H, aryl-H), 7.43 (t, J ) 8 Hz, 3H, aryl-H), 7.33 (s, 5H, aryl-H),
7.02 (t, J ) 7 Hz, 3H, aryl-H), 4.25 (broad m, 4H, O-CH2), 3.90
(septet, J ) 7 Hz, 6H, CH(CH3)2), 1.58 (m, 4H, O-CH2CH2), 1.21
(d, J ) 7 Hz, 18H, CH(CH3)2), 0.94 (d, J ) 7 Hz, 18H, CH(CH3)2).
13C NMR (100.6 MHz, C6D6) δ 180.1 (CdO), 142.3, 141.6, 134.3,
(60) Joergensen, K. A.; Ghattas, A. B.; Lawesson, S. O. Bull. Chem. Soc.
Chim. Fra. 1984, 204.
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Biochem. 1984, B38, 397.
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7512.
8064 Inorganic Chemistry, Vol. 47, No. 18, 2008

Direct Synthesis of Yttrium Amidate Complexes
Table 1. Crystallographic Data and Structure Refinement Details for 4
(100.6 MHz, C6D6) δ 175.7, 142.5, 141.2, 137.6, 132.4 (q, J ) 32
Hz, CF3), 130.5, 125.4, 124.7, 124.3 (aryl-C), 71.5 (O-CH2), 28.5
(CH(CH3)2), 25.4 (O-CH2CH2), 24.6 (CH(CH3)2), 24.0 (CH(CH3)2).
IR (KBr, cm-1): 2964 (w), 1617 (s), 1528 (s), 1503 (s), 1409 (s),
1325 (s), 1167 (w), 1129 (s), 1066 (s), 925 (w), 858 (s), 764 (w),
697 (w) cm-1. MS (EI): 1133 (M+), 785 (M+ - p-CF3phenyl-
[O,N]Dipp), 349 (p-CF3phenyl[O,N]Dipp). Anal. found (calcd for
C64H71F9N3O4Y): C 62.36% (63.73%), N 3.43% (3.48%), H 6.16%
(5.93%).
empirical formula
fw
C73H80N3O4Y
1152.31
temperature (K)
wavelength (Å)
cryst. syst., space group
unit cell dimensions
a, b, c (Å)
173(2)
0.71073
monoclinic, P2(1)/c
13.7820(11), 33.598(3), 16.0575(12)
90, 98.762(3), 90
7348.7(10), 4
1.042
0.837
R, ꢀ, γ (deg)
volume (Å3), Z
calcd. density (g/cm3)
abs. coeff (mm-1
F(000)
)
General Procedure for ε-Caprolactone Ring-Opening Poly-
merization (225:1 Monomer to Initiator Ratio). Inside a nitrogen
filled glovebox, complex 4 (13.5 mg, 0.0117 mmol) was dissolved
in 5 mL of toluene. The colorless solution was transferred to a 10
mL vial, equipped with a stir bar. ε-Caprolactone (0.3008 g, 2.64
mmol) was dissolved in 5 mL of toluene and pipetted directly into
the vigorously stirring solution of complex 4. The reaction was
stirred for 15 min within the glovebox and then exposed to air. A
couple drops of 1 M HCl solution was added to fully quench. The
polymer was precipitated from cold petroleum ether. The polymer
was isolated by vacuum filtration, and then dried overnight in vacuo.
Yield: 0.270 g, 90%.
2440
cryst. size (mm)
data collection θ range (deg)
limiting indices
0.25 × 0.20 × 0.20
1.77-22.77
-12 e h e 14, -36 e k e 36,
-17 e l e16
43713
reflections collected
indep. reflections
completeness to θ ) 22.77° (%) 98.8
abs. correction
9816 [R(int) ) 0.0735]
semiempirical from equivalents
max and min transmn
refinement method
data/restraints/param
GOF on F2
0.846 and 0.515
Full-matrix least-squares on F2
9816/0/744
0.976
final R indices [I > 2σ(I)]
R1 ) 0.0614, wR2 ) 0.1492
R indices (all data)
R1 ) 0.1040, wR2 ) 0.1649
largest diff. peak and hole (e/Å3) 0.590 and -1.863
Results and Discussion
Homoleptic yttrium complexes can be made from proli-
gands 1 (N-(diisopropylphenyl) naphthyl amide), 2 (N-
(dimethylphenyl) t-butyl amide) and 3 (N-(diisopropylphenyl)
p-(trifluoromethyl)phenyl amide) using a simple protonolysis
reaction. The complexes formed (4, 5, and 6) were recrystal-
lized from hexanes to give colorless crystals in high yield
(94% for 4, 88% for 5, and 79% for 6). These air and
moisture sensitive complexes were fully characterized and
are soluble in all common hydrocarbon solvents.
132.1, 132.0, 130.1, 129.4, 128.3, 127.8, 127.5, 127.4, 127.3, 126.3,
125.4, 124.3, 123.9, 123.6, 123.5 (aryl-C), 69.6 (O-CH2), 28.1 (O-
CH2CH2), 25.0 (CH(CH3) ), 23.5 (CH(CH3)2). IR (KBr, cm-1):
2
3052 (w), 2960 (s), 2860 (w), 1573 (w), 1513 (vs), 1406 (vs), 1382
(vs), 1318 (w), 1251 (s), 1191 (s), 1024 (s), 915 (s), 840 (w), 800
(w), 779 (s), 761 (w), 657 (w), 621 (w), 564 (w), 506 (w), 459 (w)
cm-1. MS (EI): 1079 (M+), 952 (M+ - Nap), 749 (M+
-
Nap[O,N]Dipp), 331 (M+ - Y(Nap[O,N]Dipp)2). Anal. found
(calcd. for C73H80N3O4Y): C 75.70% (76.09%), H 6.61% (7.00%),
N 3.68% (3.65%).
The 1H NMR spectra of complexes 4, 5, and 6 have very
similar characteristics, and all spectra show C3 symmetric
structures in the solution phase and contain 1 equiv of THF.
In complexes 4 and 5, the signals observed for the THF
methylene protons are shifted (δ 4.25 ppm and 1.58 ppm
for 4, δ 3.89 ppm and 1.42 ppm for 5) and broadened from
typical residual THF solvent signals (δ 3.57 ppm and 1.40
ppm).63 This data is consistent with labile, coordinated THF
that is rapidly exchanging on the NMR time scale. Variable
temperature NMR spectroscopic experiments on complex 4
failed to yield any energetic parameters for this exchange,
as no line-broadening was observed down to -45 °C.
Furthermore, the overall C3 symmetry of this complex was
maintained at these lower temperatures, also with no observ-
able line broadening, consistent with either a highly fluxional
complex or a static structure. 1H NMR spectrum of complex
5 shows THF methylene proton signals even after exposure
of the complex to vacuum overnight at 25 °C, but after
exhaustive pumping (more than 3 days) the THF can be
partially removed. However, this is not the case for com-
plexes 4 and 6, as exhaustive vacuum does not remove the
THF molecule.
Synthesis of Tris(N-2′, 6′-dimethylphenyl(tert-butyl)ami-
date)yttrium Mono(tetrahydrofuran) (5). The experimental method
described for 4 was used in the preparation of 5 using 2 (0.201 g,
0.981 mmol) and yttrium tris(bis(trimethylsilyl)amide) (0.187 g,
0.327 mmol) to give a yellow oil which solidified over time. The
product was recrystallized by dissolving in a minimum amount of
hexanes and then left at -30 °C to give a white crystalline solid.
1
Yield: 0.223 g, 88%. H NMR (300 MHz, C6D6) δ 6.88 (m, 9H,
aryl-H), 3.67 (m, 4H, O-CH2), 2.24 (s, 18H, CH3), 1.28 (m, 4H,
O-CH2CH2), 1.08 (s, 27H, C(CH3)3). 13C NMR (100.6 MHz, C6D6)
δ 185.2 (CdO), 145.8, 130.8, 127.4, 122.7 (aryl-C), 68.7 (O-CH2),
41.2 (O-CH2CH2), 28.0 (C(CH3)3), 25.0 (CH3), 19.2 (CH3). IR (KBr,
cm-1): 2986 (w), 2951 (s), 2904 (w), 1541 (s), 1522 (s), 1477 (s),
1398 (s), 1352 (w), 1219 (s), 1183 (s), 927 (s), 759 (s), 605 (w)
cm-1. MS (EI): 701 (M+), 686 (M+ - CH3), 644 (M+ - tBu), 497
(M+ - (tBu[O,N]Dmp). Anal. found (calcd for C43H62N3O4Y): C
66.70% (66.74%), N 6.25% (5.43%), H 7.89% (8.08%).
Synthesis of Tris(N-2′, 6′-diisopropylphenyl(p-(trifluorom-
ethylphenyl)amidate)yttrium Mono(tetrahydrofuran) (6). The
experimental method described for 4 was used in the preparation
of 6 with amide 3 (0.401 g, 1.15 mmol) and yttrium tris(bis(trim-
ethylsilyl)amide) (0.220 g, 0.386 mmol) to give a yellow oil which
solidified over time. The product was recrystallized by dissolving
in a minimum amount of hexanes and then left at -30 °C to give
a white crystalline solid. Yield: 0.365 g, 79%. 1H NMR (400 MHz,
C6D6) δ 7.65 (d, J ) 8 Hz, 6H, aryl-H), 7.20 (m, 6H, aryl-H), 7.12
(m, 9H, aryl-H), 3.89 (m, 4H, O-CH2), 3.46 (septet, 6H, J ) 7
Hz, CH(CH3)2), 1.41 (m, 4H, O-CH2CH2), 1.12 (d, 18H, J ) 7
Hz, CH(CH3)2), 0.89 (d, 18H, J ) 7 Hz, CH(CH3)2). 13C NMR
IR spectroscopy is very diagnostic for the formation of
these complexes as the disappearance of the N-H stretch
from proligands and a shift of the CdO stretches are
consistently observed. For example, from proligand 1 to the
formation of complex 4, the disappearance of the N-H
Inorganic Chemistry, Vol. 47, No. 18, 2008 8065

Stanlake et al.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for
Complex 4
bond length (Å)
bond length (Å)
or bond angle (deg)
or bond angle (deg)
Y1-O1
Y1-N1
O1-C1
N1-C1
Y1-O2
Y1-N2
O2-C24
N2-C24
Y1-O3
Y1-N3
O3-C47
N3-C47
Y1-O4
2.334(3)
2.386(4)
1.292(5)
1.304(5)
2.307(3)
2.369(3)
1.272(5)
1.318(5)
2.268(3)
2.479(4)
1.293(6)
1.304(6)
2.332(3)
Y1-O1-C1
O1-C1-N1
C1-N1-Y1
N1-Y1-O1
Y1-O2-C24
O2-C24-N2
C24-N2-Y1
N2-Y1-O2
Y1-O3-C47
O3-C47-N3
C47-N3-Y1
N3-Y1-O3
95.8(3)
115.5(4)
93.0(3)
55.44(12)
94.8(3)
117.0(4)
90.8(3)
56.38(11)
97.6(3)
118.5(4)
87.8(3)
55.87(12)
Figure 2. ORTEP of [Y(tBu[O,N](CH3)2Ph)3]2 (7) with the probability
ellipsoids drawn at the 50% level. tBu groups omitted for clarity.66 Selected
bond lengths (Å) and bond angles (deg): Y1-O1 2.260(4), Y1-N1 2.449(5),
O1-C1 1.286(8), N1-C1 1.299(9), Y1-O2 2.270(4), Y1-N2 2.492(5),
O2-C14 1.286(8), N2-C14 1.309(9), Y1-O3 2.384(4), Y1-N3 2.483(5),
O3-C27 1.327(8), N3-C27 1.302(9), Y1-O1-C1 99.6(4), O1-C1-N1
115.0(6), C1-N1-Y1 90.3(4), N1-Y1-O1 54.99(17), Y1-O2-C14
99.8(4), O2-C14-N2 116.3(6), C14-N2-Y1 88.9(4), N2-Y1-O2
54.90(18), Y1-O3-C27 97.2(4), O3-C27-N3 114.8(6), C27-N3-Y1
93.4(4), N3-Y1-O3 54.08(16).
stretch (3253 cm-1 for 1) and the shifting of the CdO stretch
from 1643 cm-1 to 1513 cm-1 is observed. This weakening
of the CdO stretch is consistent with both amidate delocal-
ization and metal complexation. Furthermore, the appearance
of a weak CdN stretch at 1406 cm-1 supports the formation
of the desired complex with the delocalized monoanionic
ligand. These IR trends are mirrored in complexes 5 and 6
(CdO stretch: 1650 cm-1 to 1541 cm-1 for 2 to 5 and 1648
cm-1 to 1528 cm-1 for 3 to 6). Finally, mass spectrometry
of 4, 5, and 6 gives molecular ion peaks corresponding to
the complex without THF in all cases, and the fragmentation
pattern for these complexes show signals for the loss of one
ligand and for the free ligand itself.
X-ray quality crystals of complex 4 can be grown from
hexanes at -35 °C, and the solid-state molecular structure
is shown in Figure 1. The 7-coordinate C1 symmetric
molecular structure of 4 confirms electron delocalization
through the κ2-amidate backbone as indicated by the C-O
and C-N bond lengths (average C-O is 1.290 Å and
average C-N is 1.310 Å). The average sum total of the four
angles of each amidate metallacycle is 359.5°, indicating no
significant deviation from planarity (see Table 2). This is
similar to a previously reported product from an isocyanate
insertion reaction, in which a related monoamidate yttrium
complex, (MeC5H4)2Y(THF)(N(iPr)2[O,N]Ph), has a similar
Y-O(amidate) bond at 2.285(2) Å.64
In contrast, initial preparative efforts toward yttrium
amidate complexes using salt metathesis illustrated the
propensity for amidate ligands to promote the formation of
bridged dimeric or ill-defined multimetallic species. The use
of traditional salt metathesis routes typically resulted in the
formation of insoluble, ill-defined “ate” complexes. However,
the amidate salt of N-2′,6′-dimethylphenyl(t-butyl)amide (2)
could be formed in situ by addition of an equivalent of
sodium bis(trimethylsilyl)amide suspended in THF, and after
solvent removal, the amidate salt was reacted directly with
yttrium trichloride, in a 3:1 molar ratio. The product was
recrystallized from warm toluene to give dinuclear complex
7 in low yield. Unfortunately full characterization was not
possible as this low yielding crystalline sample was not
representative of the bulk material which had complex NMR
spectra consistent with a highly fluxional species of ill-
defined structure in the solution phase. Notably, the use of
the large sodium counterion and bulky base were found to
be necessary as deprotonation of 2 with n-butyllithium and
subsequent reaction with yttrium trichloride resulted in the
formation of completely insoluble materials.
Figure 2 shows the solid-state molecular structure of 7,
which is a centrosymmetric dimer, with each yttrium atom
having three amidate ligands. One amidate ligand is bridging
to another yttrium atom through the amidate oxygen. Side-
on bonding of the amidate ligand, resulting in a η3 hapticity,
has been suggested for complexes resulting from isocyanate
insertions into Ln-C bonds.52 In this case, the amidate
scaffold is coplanar with the yttrium atom and ligand
denticity is best described as κ2.
In an attempt to prepare a 6-coordinate, monomeric,
tris(amidate) complex, 3 equiv of proligand 1 were reacted
with Y(N(SiMe3)2)3 in a non-coordinating solvent at 60 °C.
The resultant air and moisture sensitive microcrystalline
product (8) was isolated from warm toluene in poor yield
(12%). The residual material from the reaction was insoluble
in common organic solvents, consistent with the formation
of aggregate species. Complex 8 could not be fully charac-
terized, although the data from the X-ray crystallographic
studies of a low quality crystal were satisfactory for
establishing connectivity (Scheme 2, Supporting Information,
Table S1). Two of the ligands are bound as bidentate
amidates, as seen previously, while one monodentate amidate
ligand, bound through the oxygen only, displays the hemi-
labile coordination mode that can be adopted by this ligand
framework. Finally, the fourth ligand is a neutral amide, also
bound through the oxygen. It is evident by 1H NMR
spectroscopy that one neutral amide is incorporated as a
donor ligand, as the amide N-H signal can be seen as a
(65) Data was processed using the SQUEEZE function of the PLATON
software to remove disordered THF. Speck, A. L. J. Appl. Crystallogr.
2003, 36, 7.
(66) Data was processed using the SQUEEZE function of the PLATON
software to remove disordered hexanes. Speck, A. L. J. Appl.
Crystallogr. 2003, 36, 7.
(64) Mao, L.; Shen, Q.; Xue, M. Organometallics 1997, 16, 3711.
8066 Inorganic Chemistry, Vol. 47, No. 18, 2008

Direct Synthesis of Yttrium Amidate Complexes
Scheme 2. Formation of Complex 8
Table 3. Summary of Ring-Opening Polymerization of ε-Caprolactone
d
e
Mw (× 104)
Mw (× 104)
entry
I
[M]/[I] yield (%)c
g mol-1
PDId
g mol-1
PDIe
1
2
3
4
4a
50
95
96
91
63
80
56
100
65
24.7
27.8
32.5
31.7
41.7
17.8
7.98
151
2.56
2.40
2.12
2.20
2.29
2.47
1.81
2.90
11.8
10.2
10.7
10.7
15.8
5.57
1.62
1.38
1.28
1.28
1.43
1.41
4a 100
4a 225
4a 500
5a 225
6a 225
9a 500
5
6
745
highly deshielded broad singlet at approximately δ 11. Also
in the 1H NMR spectrum, the methyl groups and the methine
protons for all diisopropylphenyl substituents are seen as
broad singlets or multiplets, respectively, indicating the rapid
exchange of proton and coordination modes between the
ligands on the NMR time scale.
867 10b 224
a General polymerization conditions: in toluene, 15 min of stirring, 25
°C. b Toluene, 1.5 h, 25 °C. c Yield ) weight of polymer obtained/weight
of monomer used. d Measured by GPC calibrated with standard polystyrene
samples. e Measured by GPC (triple detection) equipped with differential
refractometer (Waters), viscometer, and laser-light scattering detectors
(Wyatt).
In non-coordinating solvent, even strict control of the
reaction stoichiometry (3 proligands to 1 Y) consistently gave
ring-opening polymerization of lactones with metal-alkox-
ides, including lanthanide and group 3 complexes, results in
the efficient preparation of these desirable polymeric materi-
als.68-72 Here, complexes 4, 5, and 6 can be used as initiators
for this process (Table 3, Entries 1-6).73 ε-Caprolactone is
added directly to a stirring solution of a 1.2 mM (for [M]/[I]
) 225) toluene solution of initiator. The reactions are stirred
vigorously for 15 min under inert atmosphere, exposed to
air, and then quenched with 1 M HCl. The polymer can be
isolated by precipitation from cold petroleum ether. The
molecular weights obtained in entries 1-4, are among the
highest molecular weights observed for rare-earth metal
initiated ε-caprolactone polymerization.5,7,8,29,74 However,
when comparing entries 1-4, the yield of the polymer is
decreased as the monomer to initiator ratio ([M]/[I]) is
increased. This may be due to bulk transfer properties,
resulting in an increase in chain termination mechanisms as
the concentration of monomer is increased.75,76 From these
results the [M]/[I] ratio of 225 was regarded as the optimized
ratio (as shown in entry 3 where polymer yield and molecular
weight were maximized for compound 4) and was used for
studies with complexes 5 and 6 as initiators (Table 3, entries
5-6). The theoretical Mw value for a [M]/[I] of 225:1 is
calculated to be 2.57 × 104 g mol-1. The Mw obtained by
complexes 4, 5, and 6 are much larger than calculated,
suggesting slow initiation followed by rapid polymerization
1
evidence (signal at δ 11 in the H NMR spectrum) for
formation of product 8 in poor yield. Interestingly, product
8 can also be prepared in situ on NMR tube scale by addition
of 1 equiv further of proligand 1 to complex 4, via THF
displacement by the neutral amide group. Furthermore,
pyridine has also been shown to add to complex 4, also
resulting in displacement of coordinated THF.
As demonstrated by NMR spectroscopic investigations,
the ancillary amidate ligands in complex 4 are highly
fluxional and additional donors can be easily introduced to
the metal center. Thus, when complex 4 was reacted with
complex 5, a scrambling of the ligands occurred, as seen by
1H NMR spectroscopy. The signals for the methylene protons
of the THF molecules shifted from δ 4.25 ppm to δ 3.84
ppm, indicating loss of bound THF, and multiple signals were
observed for the diisopropyl substituents, as well as naphthyl
aryl signals. This reaction occurred immediately at room
temperature, and no further change in the 1H NMR spectrum
was seen when the solution was heated to 110 °C. Attempts
to isolate a mixed amidate yttrium complex have been
unsuccessful. In solution phase, it is apparent that the amidate
ligands are rapidly exchanging and are thought to exchange
through a mechanism that takes advantage of the potential
bridging mode of the ligand as seen in complex 7 and ligand
hemilability as observed in complex 8.
The hemilability of the amidate scaffold, and the ease with
which the donor ligand can be displaced led us to consider
yttrium complexes 4, 5, and 6 as being sterically accessible
Lewis acids for a range of potential Lewis basic donors.
Consequently, investigations using complexes 4, 5, and 6
as initiators in ε-caprolactone ring-opening polymerization
have been performed and compared with other known
yttrium ε-caprolactone polymerization initiators. These pre-
liminary results show that these new homoleptic amidate
complexes are very effective initiators for the preparation
of high-molecular weight polycaprolactone, a biodegradable
polyester.
(67) Hultzsch, K. C.; Spaniol, T. P.; Okuda, J. Organometallics 1997, 16,
4845.
(68) Xu, X.; Yao, Y.; Hu, M.; Zhang, Y.; Shen, Q. J. Polym. Sci. Part A:
Polym. Sci. 2006, 44, 4409.
(69) Annunziata, L.; Pappalardo, D.; Tedesco, C.; Antinucci, S.; Pellecchia,
C. J. Organomet. Chem. 2006, 691, 1505.
(70) Qi, C. Y.; Wang, Z.-X. J. Polym. Sci. Part A: Polym. Sci. 2006, 44,
4621.
(71) Sun, H.; Chen, S.; Yao, Y.; Shen, Q.; Yu, K. Appl. Organomet. Chem.
2006, 20, 310.
(72) Sheng, H.; Xu, F.; Yao, Y.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007,
46, 7722.
(73) It should be noted that each entry has been completed in triplicate to
ensure validity of values for yield, molecular weight, and PDI. Two
different methods of GPC results are reported for comparative reasons.
These different conditions are known to give variable results for this
class of polymers. Refer to (a) Huang, Y. Int. J. Polym. Anal. Charact.
2003, 8, 383.
(74) Bonnet, F.; Cowley, A. R.; Mountford, P. Inorg. Chem. 2005, 44,
9046.
(75) Penczek, S.; Szymanski, R.; Duda, A.; Baran, J. Macromol. Symp.
2003, 201, 261.
(76) Stridsberg, K. M.; Ryner, M.; Albertsson, A.-C. AdV. Polym. Sci. 2002,
157, 41.
The commercial synthesis of biodegradable polymers by
Inorganic Chemistry, Vol. 47, No. 18, 2008 8067

Stanlake et al.
tion environment (resulting from a larger metal radius)
facilitates monomer coordination and subsequent inser-
tion.1,77 This suggests that neodymium compounds should
be more reactive than yttrium (since Nd is larger than Y).
The tris(amidate) complex 4 compares favorably to the
neodymium complex, indicating the amidate scaffold may
enhance reactivity over the amidinate ligand set. Interestingly,
in slightly different conditions (1.5 h instead of 15 min.) the
starting material to form 4, 5, and 6, Y(N(SiMe3)2)3 (10)
(Table 3, Entry 8) can form very large molecular weight
polymers, with a broad PDI.67 This shows that the amidate
complexes can yield polymers of higher molecular weights
than similar amidinate complexes, as well as improved PDI
values over Y(N(SiMe3)2)3, and suggests that this easily
accessed and tunable family of initiators should be investi-
gated further for optimized reactivity in the ring-opening
polymerization of lactones.
Figure 3. 1H NMR (600 MHz) spectrum (in CDCl3) of poly(ε-caprolactone)
using initiator 4.
by small quantities of catalytically active species. The
polymer produced using initator 4 and a 10:1 monomer to
initiator ratio was evaluated using end group analysis by 1H
NMR spectroscopy, as shown in Figure 3. The incorporation
of one ligand in the polymer chain is observed, as evidenced
by the signal at δ 3.66, (for the OH terminus of the polymer)
that integrates for 2 protons, as does the multiplet at δ 3.33
for the methine protons of the ligand isopropyl substituents
(also 2 protons). This is consistent with the mechanism of
polymerization proceeding through ligand promoted initiation
to produce a polymer with amide 1 as the terminus. The
efficiency of this step continues to be an area of investigation
within the group and is a current focus in the development
of new initiators with improved reactivity patterns.
The PDI values are typical for rare-earth initiated ring-
opening polymerization of ε-caprolactone. The more steri-
cally accessible complex 5 gave a reduced yield in com-
parison to 4, but also a higher molecular weight and slightly
broader PDI. The impact of the efficiency of polymerization
initiation is also apparent in the changes in PDI between
complexes 4, 5, and 6. By placing the electron-withdrawing
group CF3 in the amidate scaffold, as in complex 6, the
efficiency of initiating the ring-opening polymerization
diminished (as detected by the lower polymer yield, lower
molecular weight polymer, and higher PDI). These data
indicate that complexes 4, 5, and 6 are already good initiators
of ring-opening ε-caprolactone and that the amidate scaffold
provides an easily varied ligand set that can be exploited
for enhancing reactivity.
Summary and Conclusions
In summary, a reliable synthetic route has been developed
for the first, fully characterized examples of monomeric
tris(amidate) complexes of yttrium. The characterized ho-
moleptic tris(amidate) complexes of yttrium (4, 5, and 6)
were synthesized directly from sterically bulky amide pro-
ligands and Y(N(SiMe3)2)3 in high yields. Spectroscopic
studies of these 7-coordinate compounds indicate that the
amidate ligands are very fluxional, and neutral donor ligands
are readily exchanged, making them interesting initiators for
the ring-opening polymerization of lactones. These com-
plexes have high activity as ε-caprolactone polymerization
initiators, producing large molecular weight biodegradable
polymers, and the modest variations in ligand structure
included here have resulted in notable changes in polymer-
ization initiation activity. On-going research will take
advantage of the tunable amidate ligand set to generate new
rare-earth amidate complexes as optimized initiators for the
ring-opening polymerization of a variety of cyclic lactones
including ε-caprolactone and lactide.
Acknowledgment. The authors thank UBC, CFI, BCKDF,
and DuPont for financial support of this work. L.J.E.S. thanks
UBC and MEC for scholarships, as well as the Gates group
at the University of British Columbia, and Alissa Han of
the Holdcroft group at Simon Fraser University for technical
assistance with GPC measurements.
Similar rare-earth complexes to that of the amidates, such
as the amidinates, have been found to be active for the
initiation of the ring-opening polymerization of ε-caprolac-
tone. For example, the homoleptic neodymium amidinate
complex, [CyNC(Me)NCy]3Nd (9) (Table 3, Entry 7), forms
polymer of a moderate molecular weight (7.98 × 104 g
mol-1) with a slightly narrower PDI value of 1.81. Although
the analogous yttrium amidinate complex was prepared, no
polymerization results were reported.45 Polymerization re-
activity of some rare-earth complexes has been found to
increase with increasing radii, since a less crowded coordina-
Supporting Information Available: Crystallographic informa-
tion files (CIF) of complexes 4, and 7, as well as solid-state
molecular structure data for complex 8. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC8010635
(77) Wang, J.; Sun, H.; Yao, Y.; Zhang, Y.; Shen, Q. Polyhedron 2008,
27, 1977.
8068 Inorganic Chemistry, Vol. 47, No. 18, 2008