A series of Bis(phosphinic)diamido yttrium complexes as initiators for lactide polymerization

Article abstract of DOI:10.1021/ic800419t

A series of new bis(phosphinic)diamido yttrium complexes have been synthesized and fully characterized. The complexes adopt dimeric structures, both in solution and in the solid state, where one phosphinic group bonds to one yttrium center and the other bonds to two yttrium centers. The complexes have all been tested as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is controlled by the diamine backbone substituent with the rate depending on the backbone flexibility. The order of decreasing rates were 2,2-dimethyl-1,3-propylene > trans-1,2-cyclohexylene > 1,2-ethylene ? 1,2-phenylene. The polymerization kinetics showed, in most cases, an initiation period, during which the percentage conversion and the rate of polymerization were much lower than during propagation. This was attributed to relatively slow initiation by the bulky amido group. The initiator structure was probed using ³¹P{ ¹H} NMR spectroscopy, which showed that the dimeric structure was maintained throughout the polymerization. The initiators give rise to controlled ring-opening polymerization as shown by the linear relationship between the percentage conversion and the number-average molecular weight.

Full text of DOI:10.1021/ic800419t

Inorg. Chem. 2008, 47, 6840-6849
A Series of Bis(phosphinic)diamido Yttrium Complexes As Initiators for
Lactide Polymerization
Rachel H. Platel, Andrew J. P. White, and Charlotte K. Williams*
Department of Chemistry, Imperial College London, London, SW7 2AZ, U.K.
Received March 10, 2008
A series of new bis(phosphinic)diamido yttrium complexes have been synthesized and fully characterized. The
complexes adopt dimeric structures, both in solution and in the solid state, where one phosphinic group bonds to
one yttrium center and the other bonds to two yttrium centers. The complexes have all been tested as initiators for
the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is controlled by the
diamine backbone substituent with the rate depending on the backbone flexibility. The order of decreasing rates
were 2,2-dimethyl-1,3-propylene > trans-1,2-cyclohexylene > 1,2-ethylene . 1,2-phenylene. The polymerization
kinetics showed, in most cases, an initiation period, during which the percentage conversion and the rate of
polymerization were much lower than during propagation. This was attributed to relatively slow initiation by the
bulky amido group. The initiator structure was probed using 31P{1H} NMR spectroscopy, which showed that the
dimeric structure was maintained throughout the polymerization. The initiators give rise to controlled ring-opening
polymerization as shown by the linear relationship between the percentage conversion and the number-average
molecular weight.
Introduction
product), good control and in some cases the ability to control
the stereochemistry.7–35
Polylactide is a commercially produced polyester derived
from renewable resources.1 It degrades in vivo, or in the
environment, to yield metabolites.2 It has long been used in
medicine, but recently applications in packaging and fiber
technology have widened its scope.3 It is prepared by the
ring-opening polymerization of lactide (Scheme 1); the
polymerization is initiated by a variety of species, including
metal complexes or organo-catalysts.4–6 Metal complexes are
applied industrially and are proposed to operate via a
coordination-insertion mechanism.4 Yttrium complexes are
particularly useful initiators because of their very high rates,
low toxicity (the initiator frequently contaminates the
The initiator should be a well-defined complex of the
general structure [LYX], where L is an ancillary ligand
and X is an amido/alkoxide group. The synthesis of such
heteroleptic yttrium complexes requires ancillary ligands
which are dianionic and sufficiently sterically protected
to prevent complex redistribution reactions.36 Bis(phos-
phinic)diamido ligands have previously been used to
prepare yttrium and zirconium complexes in situ; these
species have shown good activity and stereoselectivity for
hydroamination reactions.37,38 The requirements for the
hydroamination catalysts (high Lewis acidity but with a
labile metal-amido bond) parallel those of initiators for
lactide ring-opening polymerization. Therefore, we have
previously discovered that [{N,N′-1,3-bis(P,P′-di-isopropyl
thiophosphinic)-2,2-dimethylamido}{bis(trimethylsilyl)am-
ido}yttrium] was a highly active initiator for lactide ring-
opening polymerization.24 The oxo-analogue complex was
also an excellent initiator.27 This study builds upon these
results and investigates the relationship between the
ancillary ligand substituents and the polymerization rate
and control.
* To whom correspondence should be addressed. E-mail: c.k.williams@
imperial.ac.uk.
(1) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.;
Cairney, J.; Eckert, C. A., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.;
Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. Science
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11.
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6840 Inorganic Chemistry, Vol. 47, No. 15, 2008
10.1021/ic800419t CCC: $40.75  2008 American Chemical Society
Published on Web 06/25/2008

Bis(phosphinic)diamido Yttrium Complexes
Measurements. 1H and 13C{1H} NMR spectra were performed
on a Bruker Av400 instrument, unless otherwise stated. 31P{1H}
NMR experiments were performed on a Bruker Av500 spectrometer
equipped with a z-gradient bbo/5 mm tuneable probe and a BSMS
GAB 10 A gradient amplifier providing a maximum gradient output
of 5.35 G/cmA. To observe the complex P-P coupling patterns,
31P{1H} spectra were collected at a frequency of 202.47 MHz using
a 30 degree pulse, a spectral width of 8082 Hz (centered on 57.5
ppm), and 65536 data points with a relaxation delay of 2 s. The
spectra were zero filled (0.12 Hz/point resolution) and processed
with no apodization.
Elemental analyses were determined by Mr. Stephen Boyer at
London Metropolitan University, North Campus, Holloway Road,
London, N7. Mass spectra were performed on a Micromass
Autospec Premier machine, using LSIMS (FAB) ion sources. The
polymer’s molecular weight and polydispersity index were deter-
mined from gel permeation chromatography with multiangle laser
light scattering (GPC-MALLS). Two Polymer laboratories Mixed
D columns were used in series, with THF as the eluent, at a flow
rate of 1 mL min-1, on a Polymer laboratories PL GPC-50
instrument. The light scattering detector was a triple-angle detector
(Dawn 8+, Wyatt Technology), and the data were analyzed using
Scheme 1. Ring-Opening Polymerization of Lactide, Initiated by
Yttrium Complexes, Where X ) Alkoxide or Amido Group
Experimental Section
Materials and Methods. All reactions were conducted under a
nitrogen atmosphere, using either standard anaerobic techniques
or in a nitrogen filled glovebox. All solvents and reagents were
obtained from commercial sources (Aldrich and Merck) and dried.
Toluene, pentane, and hexane were distilled from sodium and stored
under nitrogen. Methylene chloride was distilled from CaH2 and
stored under nitrogen. Deuterated solvents were dried over either
CaH2 (methylene chloride-d2) or potassium (THF-d8, toluene-
d8, benzene-d6), performing three freeze-thaw cycles under
vacuum, refluxing them for 1-2 days, distilling them under
vacuum, and storing them under nitrogen. Rac-lactide was
generously donated by Purac Plc. and was recrystallized from
anhydrous ethyl acetate and sublimed three times under vacuum
prior to use. Tris{N,N′-bis(dimethylsilyl)amido}bis(tetrahydro-
furano)yttrium, [Y{N(SiHMe2)2}3(THF)2], was prepared accord-
ing to the literature procedure.39
Astra V version 5.3.1.4. The refractive angle increment for
40
polylactide in THF, was taken as 0.042 mL g-1
.
General Protocol for 1-5. Chloro di-isopropyl phosphine (2.67
mL, 16.80 mmol), was added dropwise, via a syringe, to a stirred
solution of the appropriate diamine (8 mmol) and triethylamine
(6.97 mL, 40 mmol) in CH2Cl2 (40 mL). The reaction mixture was
stirred for 12 h, after which time it was opened to the atmosphere
and cooled to 0 °C. H2O2 (30% aqueous soln., 4.10 mL, 32 mmol)
was added, cautiously. After the addition, the reaction mixture was
stirred for a further 10 min, at 0 °C, and at 25 °C, for 30 min,
before being quenched with aqueous Na2S2O3 (80 mL of a 1 M
solution, 80 mmol). The layers were separated, and the aqueous
layer extracted with CH2Cl2 (3 × 20 mL). The combined organic
extracts were dried (MgSO4) and the solvents removed under
reduced pressure to give a colorless residue.
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1.27 Recrystallized from ethyl acetate to give colorless needles
(1.47 g, 50%). 1H NMR (400 MHz, CDCl3,) δ ppm: 3.20 (m, 2H,
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3
2
NH), 2.81 (t, 4H, JHH ) 8.0 Hz, CH2), 1.96 (d sept, 4H, JPH
)
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3
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Hz,3JHH ) 7.2 Hz, CH(CH3)2), 1.14 (dd, 12H, 3JPH ) 5.6 Hz, 3JHH
) 7.2 Hz, CH(CH3)2), 0.85 (s, 6H, CH3). 13C{1H} NMR (100 MHz,
1
CDCl3,) δ ppm: 46.2 (s, CH2), 37.0 (s, C(CH3)2), 26.5 (d, JPC
)
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2
81.0 Hz, CH(CH3)2), 23.3 (s, CH3), 16.2 (d, JPC ) 2.1 Hz,
CH(CH3)2), 15.9 (d, 2JPC ) 2.3 Hz, CH(CH3)2). 31P{1H} NMR (162
MHz, CDCl3) δ ppm: 54.0 (s). IR (nujol mull, NaCl) ν cm-1: 3195
(m, N-H), 1294 (w), 1273 (m, P ) O), 1180 (m), 1146 (s). m/z
(FAB+): 367 [M + H]+. Anal. Calcd for C17H40N2O2P2: C, 55.72;
H, 11.00; N, 7.64%. Found: C, 55.66; H, 11.07; N, 7.56%.
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Inorganic Chemistry, Vol. 47, No. 15, 2008 6841

Platel et al.
2. Precipitated from ethyl acetate, filtered and washed with
The combined organic extracts were dried (MgSO4), and the
solvents removed under reduced pressure to give a colorless residue.
The product was precipitated by addition of ethyl acetate and
filtered.
1
hexane (1.06 g, 41%). H NMR (400 MHz, CDCl3) δ ppm: 3.33
3
(m, 2 H, NH), 2.91 (m, 4 H, CH2), 1.78 (d sept, 4 H, JHH ) 7.2
2
3
Hz, JPH ) 11.2 Hz, CH(CH3)2), 0.99 (dd, 12 H, JHH ) 6.8 Hz,
6.37 Colorless solid (2.33 g, 58%). 1H NMR (400 MHz, CDCl3)
δ ppm: 7.82 (m, 8 H, ArH), 7.49-7.37 (m, 12 H, ArH), 4.08 (br
3JPH ) 13.6 Hz, CH(CH3)2), 0.95 (dd, 12 H, JHH ) 6.8 Hz, JPH
3
3
) 13.6 Hz, CH(CH3)2). 13C{1H} NMR (100 MHz, CDCl3) δ ppm:
2
2
1
s, 2 H, NH), 2.83 (dd, JHH ) 7.0 Hz, JHP ) 9.5 Hz, 4 H, CH2),
0.86 (s, 6 H, CH3). 31P{1H} NMR (162 MHz, CDCl3) δ ppm: 25.9
(s). Anal. Calcd for C29H32N2O2P2: C, 69.31; H, 6.42; N, 5.57%.
Found: C, 69.40; H, 6.38; N, 5.49%.
43.0 (s, CH2), 25.7 (d, JPC ) 82.9 Hz, CH(CH3)2), 15.9 (s,
CH(CH3)), 15.7 (s, CH(CH3)). 31P{1H} NMR (162 MHz, CDCl3)
δ ppm: 52.9 (s). m/z (FAB+): 325 [M + H]+. IR (nujol mull, NaCl)
ν cm-1: 3172 (m, N-H), 1209 (w), 1168 (m, P ) O), 1151 (m, P
) O), 1117 (m, P ) O), 1078 (w), 887 (m).
7. 41,42 Colorless solid (3.17 g, 86%). 1H NMR (400 MHz, CDCl3)
δ ppm: 7.75-7.71 (m, 8 H, ArH), 7.38-7.27 (m, 12 H, ArH), 4.24,
(br s, 2 H, NH), 2.98 (m, 4 H, CH2). 31P{1H} NMR (162 MHz,
CDCl3) δ ppm: 25.0 (s). Anal. Calcd for C26H26N2O2P2: C, 67.82;
H, 5.69; N, 6.08%. Found: C, 67.81; H, 5.80; N, 6.01%.
3. Recrystallized from ethyl acetate to give colorless crystals
(1.80 g, 59%). 1H NMR (400 MHz, CDCl3) δ ppm: 3.26 (m, 2 H,
NH), 2.87 (br s, 2 H, NCH), 2.15 (m, 4 H, CH2), 2.00 (d sept, 3JHH
3
) 7.2 Hz, JPH ) 2.8 Hz, 4 H, CH(CH3)2), 1.66 (m, 2 H, CH2),
8. Colorless solid (3.67 g, 89%). 1H NMR (400 MHz, CDCl3) δ
ppm: 7.85 (m, 4 H, ArH), 7.65 (m, 4 H, ArH), 7.31 (m, 8 H, ArH),
7.18 (m, 4 H, ArH), 4.16 (m, 2 H, NH), 2.82 (m, 2 H, NCH), 1.87
(m, 2 H, CH2), 1.38 (m, 2 H, CH2), 1.15 (m, 2 H, CH2), 0.92 (m,
2 H, CH2) ppm. 13C{1H} NMR (100 MHz, CDCl3) δ ppm: 133.7
(ArC), 133.6 (ArC), 132.5 (ArC), 132.4 (ArC), 131.5 (ArC), 131.4
(ArC), 131.3 (ArC), 128.3 (ArC), 128.1 (ArC), 55.7 (CN), 45.5
(CN), 35.8 (CH2), 35.7 (CH2), 24.9 (CH2), 8.4 (CH2). 31P{1H} NMR
3
3
1.25 (m, 4 H, CH2), 1.21 (dd, JHH ) 7.2 Hz, JPH ) 14.4 Hz, 12
3
3
H, CH(CH3)2), 1.17 (dd, JHH ) 7.2 Hz, JPH ) 14.4 Hz, 12 H,
CH(CH3)2). 13C{1H} NMR (100 MHz, CDCl3) δ ppm: 56.0 (s,
NCH), 36.3 (s, CH2) 27.0 (d, 1JPC ) 85.5 Hz, CH(CH3)2), 25.9 (d,
1JPC ) 79.9, CH(CH3)2), 25.1 (s, CH2), 16.5 (s, CH(CH3)), 16.4 (s,
CH(CH3)), 16.3 (s, CH(CH3)), 16.2 (s, CH(CH3)). 31P{1H} NMR
(162 MHz, CDCl3) δ ppm: 52.4. m/z (FAB+): 379 [M + H]+. IR
(nujol mull, NaCl) ν cm-1: 3501 (m, N-H), 3456 (m, N-H), 1270
(s), 1171 (s, P ) O), 1152 (s, P ) O), 1101 (s), 1074 (s), 901 (s),
884 (s). Anal. Calcd for C18H40N2O2P2: C, 57.12; H, 10.65; N,
7.40%. Found: C, 57.01, H, 10.60, N, 7.31%.
(162 MHz, CDCl3) δ ppm: 25.3 (s). IR (nujol mull, NaCl) ν cm-1
:
3183 (s, N-H), 1309 (w), 1283 (w), 1237 (w), 1189 (s, P ) O),
1122 (s), 1110 (s), 1068 (m), 976 (m), 904 (m). Anal. Calcd for
C30H32N2O2P2: C, 70.03; H, 6.27; N, 5.44%. Found: C, 69.92; H,
6.38; N, 5.34%.
4. Recrystallized from ethyl acetate to give colorless crystals
(1.51 g, 51%). 1H NMR (400 MHz, CDCl3) δ ppm: 3.26 (m, 2 H,
NH), 2.87 (br s, 2 H, NCH), 2.15 (m, 4 H, CH2), 2.00 (d sept, 3JHH
9.38 Colorless solid (3.15 g, 77%). 1H NMR (400 MHz, CDCl3)
δ ppm: 7.85 (m, 4 H, ArH), 7.65 (m, 4 H, ArH), 7.31 (m, 8 H,
ArH), 7.18 (m, 4 H, ArH), 4.16 (m, 2 H, NH), 2.82 (m, 2 H, NCH),
1.87 (m, 2 H, CH2), 1.38 (m, 2 H, CH2), 1.15 (m, 2 H, CH2), 0.92
(m, 2 H, CH2). 31P{1H} NMR (162 MHz, CDCl3) δ ppm: 25.3 (s).
Anal. Calcd for C30H32N2O2P2: C, 70.03; H, 6.27; N, 5.44%. Found:
C, 69.97; H, 6.35; N, 5.38%.
3
) 7.2 Hz, JPH ) 2.8 Hz, 4 H, CH(CH3)2), 1.66 (m, 2 H, CH2),
3
3
1.25 (m, 4 H, CH2), 1.21 (dd, JHH ) 7.2 Hz, JPH ) 14.4 Hz, 12
3
3
H, CH(CH3)2), 1.17 (dd, JHH ) 7.2 Hz, JPH ) 14.4 Hz, 12 H,
CH(CH3)2). 13C{1H} NMR (100 MHz, CDCl3) δ ppm: 56.0 (s,
NCH), 36.3 (s, CH2), 27.0 (d, 1JPC ) 85.5 Hz, CH(CH3)2), 25.9 (d,
1JP-C ) 79.9 Hz, CH(CH3)2), 25.1 (s, CH2), 16.5 (s, CH(CH3)),
16.4 (s, CH(CH3)), 16.3 (s, CH(CH3)), 16.2 (s, CH(CH3)). 31P{1H}
NMR (162 MHz, CDCl3) δ ppm: 52.4 (s). m/z (FAB+): 379 [M +
H]+. IR (nujol mull, NaCl) ν cm-1: 3195 (m, N-H), 1269 (m),
1170 (m), 1156 (s, P ) O), 1156 (s, P ) O), 1068 (m), 887 (m).
Anal. Calcd for C18H40N2O2P2: C, 57.12; H, 10.65; N, 7.40%.
Found: C, 56.97, H, 10.68, N, 7.32%.
10.27 To a solution of [Y{N(SiHMe2)2}3(THF)2] (0.13 g, 0.20
mmol) in toluene (2 mL) was added 1 (0.07 g, 0.20 mmol) in
toluene (3 mL). The solution was stirred for 20 h, before removing
the solvents in vacuo to leave a white solid (0.09 g, 74%). 1H NMR
2
(400 MHz, C6D6) δ ppm: 5.19 (m, 2H, SiH), 3.42 (dd, JHH
)
11.8 Hz, 3JHP ) 4.0 Hz, 1H, CH2), 3.31 (dd, 2JHH ) 12.0 Hz, 3JHP
) 4.0 Hz, 1H, CH2), 2.94 (m, 3H, CH2 and CH(CH3)2), 2.32 (sept,
5. Recrystallized from ethyl acetate to give colorless crystals
3
1
3JHH ) 7.2 Hz, 1H, CH(CH3)2), 2.24 (sept, JHH ) 6.8 Hz, 1H,
(1.49 g, 50%). H NMR (400 MHz, CDCl3) δ ppm: 7.34-7.31
3
2
CH(CH3)2), 1.78 (sept, JHH ) 6.4 Hz, 1H, CH(CH3)2), 1.50 (dd,
(m, 2 H, ArH), 6.85-6.82 (m, 2 H, ArH), 6.45 (d, 2 H, JPH
)
3JHH ) 7.2 Hz, 3JHP ) 15.2 Hz, 3H, CHCH3), 1.41 (dd, 3JHH ) 7.2
2
3
11.6 Hz, NH), 2.13 (d sept, 4 H, JPH ) 11.6 Hz, JHH ) 7.2 Hz,
CH(CH3)2), 1.22 (dd, 12 H, JHH ) 7.2 Hz, JHP ) 23.4 Hz
Hz, 3JHP ) 15.8 Hz, 3H, CHCH3), 1.23 (m, 9H, CH3), 1.16 (s, 3H,
3
3
3
CH3), 1.05 (m, 9H, CHCH3), 0.92 (s, 3H, CH3), 0.49 (d, JHH
)
3
3
CH(CH3)2), 1.14 (dd, 12 H, JHH ) 7.2 Hz, JHP ) 23.4 Hz
3
2.8 Hz, 6H, SiHCH3), 0.47 (d, JHH ) 2.8 Hz, 6H, SiHCH3).
13C{1H} NMR (100 MHz, C6D6) δ ppm: 57.0 (s, CH2), 56.4 (s,
CH2), 37.5 (s, C(CH3)2), 31.5 (d, 1JCP ) 69.0 Hz, CH(CH3)2), 29.7
(d, 1JCP ) 33.6 Hz, CH(CH3)2), 29.0 (d, 1JCP ) 37.2 Hz, CH(CH3)2),
27.1 (s, CCH3), 26.6 (d, 1JCP ) 66.5 Hz, CH(CH3)2), 25.4 (s, CCH3),
18.6 (s, CHCH3), 17.7 (s, CHCH3), 17.4 (s, CHCH3), 17.2 (s,
CHCH3), 17.0 (s, CHCH3), 16.4 (s, CHCH3), 4.8 (s, SiCH3), 4.4
(s, SiCH3). 31P{1H} NMR (162 MHz, C6D6) δ ppm: 69.0 (m, 1P),
67.1 (ddd, 2JPY ) 5.10, 3.97 Hz; 4JPP ) 1.78 Hz, 13P), 56.9 (d, JPY
) 4 Hz, 1H,), 55.1 (dd, 2JPY ) 4.65 Hz; 4JPP ) 1.76 Hz, 13P). IR
(nujol mull, NaCl) ν cm-1: 2089 (s, Si-H), 2044 (m, Si-H), 1599
(w), 1288 (m, Si-CH3 deformation), 1263 (m, Si-CH3 deformation),
CH(CH3)2). 13C{1H} NMR (100 MHz, CDCl3) δ ppm: 134.0 (s,
1
ArC), 124.0 (s, ArC), 123.4 (s, ArC), 26.1 (d, JPC ) 101.4 Hz,
CH), 15.9 (d, JPC ) 28.8 Hz, CH3). 31P{1H} NMR (162 MHz,
2
CDCl3) δ ppm: 52.5 (s). m/z (FAB+): 379 [M + H]+. IR (nujol
mull, NaCl) ν cm-1: 3377 (m, N-H), 1667 (w), 1594 (m, Ar CdC
stretch), 1296 (s), 1208 (s, P ) O), 1157 (s, P ) O), 1027 (s), 951
(m), 887 (m). Anal. Calcd for C18H34N2O2P2: C, 58.05; H, 9.20;
N, 7.52%. Found: C, 57.90; H, 9.31; N, 7.62%.
General Procedure for 6-9.38 Diphenyl phosphinic chloride
(3.05 mL, 16 mmol) was added dropwise, via a syringe, to a stirred
solution of the appropriate diamine (8 mmol) and triethylamine
(6.97 mL, 40 mmol) in CH2Cl2 (40 mL) at 0 °C. The reaction
mixture was brought to room temperature and stirred for 2 h, after
which time the reaction mixture was washed with saturated NaHCO3
solution (20 mL). The aqueous and organic layers were separated,
and the aqueous layer extracted with dichloromethane (3 × 20 mL).
¨
¨
(41) Gu¨mgu¨m, B.; Akba, O.; Durap, F.; Yildirim, L. T.; Ulku¨, D.; Ozkar,
S. Polyhedron 2006, 25, 3133.
(42) Dolinsky, M. C. B.; Lin, W. O.; Dias, M. L. J. Mol. Catal. A: Chem.
2006, 258, 267.
6842 Inorganic Chemistry, Vol. 47, No. 15, 2008

Bis(phosphinic)diamido Yttrium Complexes
3
3
1240 (s, P ) O), 1223 (s, P ) O), 1184 (m), 1165 (m), 1061 (s),
1012 (s). Anal. Calcd for C18H46N3O2P2Si2Y: C, 39.77; H, 8.53;
N, 7.73%. Found: C, 39.82; H, 8.45; N, 7.88%.
6.70 (d, 1H, JHH ) 6.8 Hz, ArH), 6.63 (d, 1H, JHH ) 6.8 Hz,
ArH), 5.18 (m, 2H, SiH), 2.99 (sept, 1H, 3JHH ) 7.2 Hz, CH(CH3)2),
2.53 (sept, 1H, 3JHH ) 7.2 Hz, CH(CH3)2), 2.18 (sept, 1H, 3JHH
)
3
11. To a solution of [Y{N(SiHMe2)2}3(THF)2] (0.220 g, 0.349
mmol) in toluene (3 mL), was added 2 (0.113 g, 0.349 mmol) in
toluene (3 mL). The solution was stirred for 24 h, before removing
the solvents in vacuo to leave a white residue. This was dissolved
in pentane, and the solvents removed in vacuo to give a white solid.
This was recrystallized from hexane to give the title compound as
a white, crystalline solid (0.130 g, 69%). 1H NMR (400 MHz, C6D6)
δ ppm: 5.28 (m, 2H, SiH), 3.72 (m, 1H, CH2), 3.40 (m, 1H, CH2),
3.27 (m, 1H, CH2), 3.11 (m, 1H, CH2), 2.30 (m, 2H, CH(CH3)2),
1.95 (m, 1H, CH(CH3)2), 1.78 (m, 2H, CH(CH3)2), 1.42-1.03 (m,
24H, CH3), 0.52 (m, 12H, Si(CH3)2);13C{1H} NMR (100 MHz,
7.2 Hz, CH(CH3)2), 1.89 (sept, 1H, JHH ) 7.2 Hz, CH(CH3)2),
3
3
1.55 (dd, 3H, JHH ) 7.2 Hz, JHP ) 16.2 Hz, CHCH3), 1.51 (dd,
3
3
3
3H, JHH ) 7.2 Hz, JHP ) 15.8 Hz, CHCH3), 1.30 (dd, 3H, JHH
) 7.2 Hz, 3JHP ) 15.8 Hz, CHCH3), 1.24 (dd, 3H, 3JHH ) 7.2 Hz,
3JHP ) 16.4 Hz, CHCH3), 1.13 (dd, 3H, JHH ) 7.2 Hz, JHP
)
3
3
3
3
16.8 Hz, CHCH3), 1.04 (dd, 3H, JHH ) 7.2 Hz, JHP ) 15.0 Hz,
CHCH3), 0.91 (dd, 3H, JHH ) 7.2 Hz, JHP ) 14.6 Hz, CHCH3),
0.82 (dd, 3H, JHH ) 7.2 Hz, JHP ) 15.2 Hz, CHCH3), 0.42 (d,
3
3
3
3
3
3
6H, JHH ) 3.2 Hz, SiH(CH3)2), 0.40 (d, 6H, JHH ) 2.8 Hz,
SiH(CH3)2). 13C{1H} NMR (100 MHz, C6D6) δ ppm: 143.3 (d,
2JCP ) 15 Hz, ArCN), 142.1 (d, JCP ) 13 Hz, ArCN), 120.2 (s,
2
1
C6D6) δ ppm: 48.8 (s, CH2), 48.7 (s, CH2), 28.6 (d, JP-C ) 76.8
ArC), 119.3 (s, ArC), 118.7 (s, ArC), 118.1 (s, ArC), 30.3 (d, 1JCP
) 44.6 Hz, CH(CH3)2), 29.4 (d, 1JCP ) 44.6 Hz, CH(CH3)2), 26.5
(d, 1JCP ) 44.6 Hz, CH(CH3)2), 25.5 (d, 1JCP ) 44.6 Hz, CH(CH3)2),
17.6 (s, CHCH3), 17.4 (s, CHCH3), 17.2 (s, CHCH3), 16.7 (s,
CHCH3), 16.3 (s, CHCH3), 16.2 (s, CHCH3), 16.1 (s, CHCH3),
16.0 (s, CHCH3), 15.9 (s, CHCH3), 15.8 (s, CHCH3), 4.1 (s,
SiH(CH3)2) ppm. 31P{1H} NMR (162 MHz, C6D6) δ ppm: 67.0
Hz, CH(CH3)2), 27.0 (d, 1JPC ) 107 Hz, CH(CH3)2), 24.9 (d, 1JP-C
1
) 163 Hz, CH(CH3)2), 22.9 (d, JPC ) 76.8 Hz, CH(CH3)2), 16.6
(s, CH3), 16.4 (s, CH3), 16.3 (s, CH3), 16.0 (s, CH3), 15.7 (s, CH3),
4.3 (s, SiCH3), 3.9 (s, SiCH3). 31P{1H} NMR (162 MHz, C6D6) δ
2
2
ppm: 61.0 (1P, d, JPY ) 3.79 Hz,), 53.2 (1P, d, JPY ) 9.12 Hz).
IR (nujol mull, NaCl) ν cm-1: 2101 (m, Si-H), 2057 (m, Si-H),
1340 (m), 1272 (s), 1240 (s, P ) O), 1123 (m, P ) O), 1084 (m),
1052 (m), 1052 (m), 1022 (m). Anal: Calcd for C21H52N3O2P2Si2Y:
C, 43.07; H, 8.95; N, 7.17%; Found: C, 43.09; H, 9.05; N, 7.08%.
12. To a solution of [Y{N(SiHMe2)2}3(THF)2] (0.126 g, 0.200
mmol) in toluene (3 mL) was added 4 (0.076 g, 0.200 mmol) in
toluene (3 mL). The solution was stirred at 70 °C for 48 h, before
removing the solvents in vacuo to leave a white residue. Pentane
was added, and the solvents removed in vacuo to give the title
2
2
(dd, JPY ) 5.15, 3.24 Hz, 1 P), 54.5 (d, JPY ) 4.24 Hz, 1 P). IR
(nujol mull, NaCl) ν cm-1: 2360 (m), 2341 (m), 2040 (w, Si-H
stretch), 1311 (m), 1274 (m), 1154 (m), 1116 (w), 1060 (m). Anal.
Calcd. for C22H46N3O2P2Si2Y: C, 44.66; H, 7.84; N, 7.10%. Found:
C, 44.55; H, 6.80; N, 7.00%.
14. To
a mixture of 6 (0.251 g, 0.500 mmol) and
[Y{N(SiHMe2)2}3(THF)2] (0.315 g, 0.500 mmol) was added toluene
(5 mL). The resulting colorless solution was heated to 70 °C, with
stirring for 24 h. The pale yellow residue was washed with hexane
(7 mL) and recrystallized from the minimum volume of toluene to
give the title compound as a white, crystalline solid (0.108 g, 30%).
1H NMR (400 MHz, C7D8) δ ppm: 8.30 (br s, 4H, ArH), 7.61 (br
s, 4H, ArH), 7.37-7.30 (m, 6H, ArH), 7.13-7.05 (m, 6H, ArH),
1
compound as an off-white solid (0.065 g, 54%). H NMR (400
MHz, C6D6) δ ppm: 5.32 (m, 1H, SiH(CH3)2), 5.28 (m, 1H,
SiH(CH3)2), 3.61 (m, 1H, CHN), 3.06 (m, 1H, CHN), 2.88 (m, 2
H, CH2), 2.16 (m, 2H, CH(CH3)2), 1.95 (m, 1H, CH(CH3)2), 1.83
(m, 1H, CH(CH3)2), 1.75 (m, 2H, CH2), 1.61 (m, 4H, CH2),
3
2
3
1.52-1.05 (m, 24H, CH(CH3)2), 0.54 (d, JHH ) 2.8 Hz, 3H,
5.32 (m, 2H, SiH), 2.99 (dd, 2H, JHH ) 12 Hz, JHP ) 20 Hz,
SiH(CH3)2), 0.50 (d, 3JHH ) 2.8 Hz, 6H, SiH(CH3)2), 0.49 (d, 3JHH
) 2.8 Hz, 3H, SiH(CH3)2). 13C{1H} NMR (100 MHz, C6D6) δ ppm:
66.1 (d, 2JCP ) 9.6 Hz, CHN), 65.2 (d, 2JCP ) 9.2 Hz, CHN), 63.7
CH2), 1.53 (dd, 2H, JHH ) 12 Hz, JHP ) 20 Hz, CH2), 0.86 (s,
2
3
3H, CH3), 0.66 (s, 3H, CH3), 0.31 (d, 12H, 3JHH ) 2.8 Hz, Si(CH3)2)
ppm. 13C{1H} NMR (100 MHz, C7D8) δ ppm: 135.0 (d, JCP
)
1
(d, 2JCP ) 16.3 Hz, CHN), 63.1 (d, 2JCP ) 16.7 Hz, CHN), 39.1 (s,
106 Hz, ArC), 133.1 (s, ArC), 132.9 (s, ArC), 131.4 (s, ArC), 131.1
(s, ArC), 131.0 (s, ArC), 56.2 (s, CH2), 37.0 (s, C(CH3)2), 26.7 (s,
CH3), 22.8 (s, CH3), 3.6 (s, SiCH3).31P{1H} NMR (162 MHz, C7D8)
δ ppm: 293 K, 38.8 (br s), 383 K, 38.3 (s). IR (nujol mull, NaCl)
ν cm-1: 2187 (m, Si-H), 2044 (m, Si-H), 1305 (w), 1240 (m),
1221 (m), 1175 (m), 1123 (m), 1056 (w). Anal. Calcd for
C33H44N3O2P2Si2Y: C, 54.92; H, 6.14; N, 5.82%. Found: C, 55.00;
H, 6.03; N, 5.71%.
1
CH2), 38.6 (s, CH2), 37.6 (s, CH2), 37.2 (s, CH2), 30.7 (d, JCP
)
37.2 Hz, CH2), 29.9 (d, 1JCP ) 71.8 Hz, CH2), 29.7 (s, CH2), 29.8
1
1
(d, JCP ) 70.2 Hz, CH2), 28.8 (d, JCP ) 12.7 Hz, CH2), 27.9 (d,
1JCP ) 59.7 Hz, CH2), 26.1 (s, CH2), 26.0 (s, CH2), 25.8 (s, CH2),
1
25.4 (d, JCP ) 31.7 Hz, CH2), 18.3 (s, CH(CH3)2), 17.9 (s,
CH(CH3)2), 17.6 (s, CH(CH3)2), 17.5 (s, CH(CH3)2), 17.3
(s, CH(CH3)2), 17.2 (s, CH(CH3)2), 17.0 (s, CH(CH3)2), 16.9 (s,
CH(CH3)2), 16.5 (s, CH(CH3)2), 16.3 (s, CH(CH3)2), 15.8 (s,
CH(CH3)2), 4.7 (s, SiH(CH3)2), 4.6 (s, SiH(CH3)2), 4.5 (s, Si-
H(CH3)2). 31P{1H} NMR (162 MHz, C6D6) δ ppm: 66.7 (dd,
15. To a solution of [Y{N(SiHMe2)2}3(THF)2] (0.315 g, 0.500
mmol) in toluene (3 mL), was added 7 (0.243 g, 0.500 mmol) in
toluene (3 mL). The solution was stirred for 24 h, before removing
the solvents in vacuo to leave a white residue. This was recrystal-
lized from toluene at -35 °C to give the title compound as a white,
2
unresolved, 1H), 62.1 ppm (dd, JPY ) 3.23, 4.55 Hz, 1H), 54.0
ppm (br s, 1H), 49.2 ppm (d, 2JPY ) 4.65 Hz, 1H). IR (nujol mull,
NaCl) ν cm-1: 2123 (m, Si-H), 2048 (m, Si-H), 1239 (w), 1240
(s, P ) O), 1231 (s, P ) O), 1195 (w), 1120 (w), 1041 (s), 1025
(s). Anal. Calcd. for C22H52N3O2P2Si2Y: C, 44.21; H, 8.77; N,
7.03%. Found: C, 44.25; H, 8.91; N, 6.91%.
1
crystalline solid (0.226 g, 67%). H NMR (500 MHz, THF-d8) δ
ppm: 8.50-6.49 (m, 20H, ArH), 5.05 (m, 2H, SiH), 3.75-1.73
(m, 4H, CH2), 0.31 (m, 12H, SiH(CH3)2. 13C{1H} NMR (125 MHz,
CD2Cl2) δ ppm: 132.7 (br m, ArC), 128.7 (br m, ArC), 3.66
(SiH(CH3)2). 31P{1H} NMR (162 MHz, THF-d8) δ ppm: 33.8 (m).
IR (nujol mull, NaCl) ν cm-1: 2039 (m, Si-H), 1591 (w), 1305
(m), 1242 (m), 1122 (s, P ) O), 1067 (s), 1028 (m). Anal. Calcd.
for C30H38N3O2P2Si2Y: C, 53.01; H, 5.64; N, 6.18%. Found: C,
53.09; H, 5.72; N, 6.23%.
13. To
a mixture of 5 (0.186 g, 0.500 mmol) and
[Y{N(SiHMe2)2}3(THF)2](0.315 g, 0.500 mmol) was added toluene
(5 mL). The resulting colorless solution was heated to 70 °C, with
stirring for 24 h. The solvents were removed in vacuo to leave a
pale brown residue. To this was added hexane (7 mL). The resulting
white precipitate was filtered, washed with cold hexane (5 mL)
and dried in vacuo to give a white powder (0.130 g, 42%). 1H NMR
(400 MHz, C6D6) δ ppm: 6.88 (m, 1H, ArH), 6.80 (m, 1H, ArH),
16. To a mixture of 8 (0.257 g, 0.500 mmol) and [Y{N(Si-
HMe2)2}3(THF)2] (0.315 g, 0.500 mmol) was added toluene (20
mL). The resulting colorless solution was stirred at room temper-
Inorganic Chemistry, Vol. 47, No. 15, 2008 6843

Platel et al.
Table 1. Crystallographic Data for Compounds 13, 14, and 16
data 13 14 16
chemical C44H92N6O4P4Si4Y2 C66H88N6O4P4Si4Y2 C68H88N6O4P4Si4Y2
ature for 20 h. The solvents were removed in vacuo to leave a
white residue. To this was added pentane (20 mL), which was then
removed in vacuo to leave the title product (0.148 g, 40%) as a
fine white powder. 1H NMR (500 MHz, C7D8) δ ppm: 8.10-8.02
(m, 4H, ArH), 7.53-7.47 (m, 4H, ArH), 7.21-7.16 (m, 4H, ArH),
7.13-7.09 (m, 4H, ArH), 6.98-6.96 (m, 4H, ArH), 6.86-6.82 (m,
4H, ArH), 5.45 (m, 0.5H, SiH(CH3)2), 4.92 (m, 1.5H, SiH(CH3)2),
3.69 (m, 1H, NCH), 2.53 (m, 1H, NCH), 1.84 (m, 1H CH2), 1.74
(m, 1H CH2), 1.65 (m, 1H CH2), 1.31 (m, 2H CH2), 1.10 (m, 1H
CH2), 0.96 (m, 1H CH2), 0.70 (m, 1H CH2), 0.60 (d, 3H, SiH(CH3),
0.20 (d, 6H, SiH(CH3), 0.12 (d, 3H, SiH(CH3). 13C{1H} NMR (125
MHz, C7D8) δ ppm: 138.2 (ArC), 136.9 (ArC), 135.9 (ArC), 135.5
(ArC), 134.5 (ArC), 134.1 (ArC), 133.1 (ArC), 131.9 (ArC), 131.3
(ArC), 130.7 (ArC), 65.9 (CHN), 64.4 (CHN), 38.2 (CH2), 35.7
(CH2), 26.1 (CH2), 25.6 (CH2), 4.2 (SiH(CH3)), 3.6 (SiH(CH3)),
3.3 (SiH(CH3)).31P{1H} NMR (162 MHz, C6D6) δ ppm: 293 K,
solvent
fw
0.5C7H8
1229.36
-100
I41/a (No. 88)
2C7H8
1627.75
-100
P21/n (No. 14)
1467.50
T (°C)
space
group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
-100
j
P1 (No. 2)
17.77032(11)
40.3177(4)
14.23934(5)
18.35109(5)
16.82037(6)
12.5447(3)
13.3084(4)
13.3297(4)
110.425(3)
94.826(2)
117.684(3)
1763.55(9)
1b
109.9403(4)
12731.67(16)
4131.78(3)
2b
8a
Fcalcd
1.283
1.308
1.382
(g cm-3
λ (Å)
)
0.71073
2.032
0.035
0.08
1.54184
3.558
0.024
0.71073
1.847
0.040
34.1 (br s, 1H), 27.9 (br s, 1H). IR (nujol mull, NaCl) ν cm-1
:
µ (mm-1
)
c
R1
2047 (s, Si-H), 1591 (m), 1331 (w), 1236 (s), 1182 (m), 1122 (s),
1045 (s). Anal. Calcd. for C34H44N3O2P2Si2Y: C, 55.65; H, 6.04;
N, 5.73%. Found: C, 55.71; H, 5.99; N, 5.82%.
d
wR2
0.066
0.096
a The molecule has crystallographic C2 symmetry. b The molecule has
crystallographic Ci symmetry. c R1 ) ∑||Fo| - |Fc||/∑|Fo|. d wR2 ) {Σ[w(Fo
2
- Fc )2]/Σ[w(Fo )2]}1/2; w-1 ) σ2(Fo ) + (aP)2 +bP.
17. To a mixture of 9 (0.257 g, 0.500 mmol) and [Y{N(Si-
HMe2)2}3(THF)2] (0.315 g, 0.500 mmol) was added toluene (5 mL).
The resulting colorless solution was stirred at room temperature
for 20 h. The solvents were removed in vacuo to leave a white
residue. To this was added pentane (20 mL), which was then
removed in vacuo to leave the title product (0.132 g, 34%) as a
fine white powder. 1H NMR (500 MHz, C7D8) δ ppm: 8.10-8.02
(m, 4H, ArH), 7.53-7.47 (m, 4H, ArH), 7.21-7.16 (m, 4H, ArH),
7.13-7.09 (m, 4H, ArH), 6.98-6.96 (m, 4H, ArH), 6.86-6.82 (m,
4H, ArH), 5.45 (m, 0.5H, SiH(CH3)2), 4.92 (m, 1.5H, SiH(CH3)2),
3.69 (m, 1H, NCH), 2.53 (m, 1H, NCH), 1.84 (m, 1H CH2), 1.74
(m, 1H CH2), 1.65 (m, 1H CH2), 1.31 (m, 2H CH2), 1.10 (m, 1H
CH2), 0.96 (m, 1H CH2), 0.70 (m, 1H CH2), 0.60 (d, 3H, SiH(CH3),
0.20 (d, 6H, SiH(CH3), 0.12 (d, 3H, SiH(CH3). 13C{1H} NMR (125
MHz, C7D8) δ ppm: 138.2 (ArC), 136.9 (ArC), 135.9 (ArC), 135.5
(ArC), 134.5 (ArC), 134.1 (ArC), 133.1 (ArC), 131.9 (ArC), 131.3
(ArC), 130.7 (ArC), 65.9 (CHN), 64.4 (CHN), 38.2 (CH2), 35.7
(CH2), 26.1 (CH2), 25.6 (CH2), 4.2 (SiH(CH3)), 3.6 (SiH(CH3)),
3.3 (SiH(CH3)). 31P{1H} NMR (162 MHz, C6D6) δ ppm: 293 K,
34.1 (br s, 1P), 27.9 (br s, 1P). IR (nujol mull, NaCl) ν cm-1: 2049
(Si-H stretch), 1237, 1122. Anal. Calcd for C34H44N3O2P2Si2Y:
C, 55.65; H, 6.04; N, 5.73%. Found: C, 55.55; H, 5.91; N, 5.96%.
In Situ Generation of Isopropoxide Complex. To a solution
of 13 (0.059 g, 0.1 mmol) in C6D6 (0.25 mL) in a Young’s tap
NMR tube was added isopropanol (7.7 µL, 0.2 mmol) via a
microsyringe. The mixture was shaken vigorously. After 10 min,
NMR spectra confirmed the reaction was complete. 1H NMR (500
MHz, C6D6) δ ppm: 6.85 (m, 2H, ArH), 6.65 (m, 2H, ArH), 4.39
2
2
2
Results and Discussion
Synthesis. Previously, we reported the preparation of three
(bis(di-isopropyl-phosphinic)-2,2-dimethylpropyldiamido)(a-
mido) yttrium complexes where the amido group was
trimethylsilyl, dimethylsilyl (10) and di-isopropyl.27 The
complexes were very rapid initiators for lactide ring-opening
polymerization. The goal of the current study is to establish
the influence, if any, of the ancillary ligand. A series of
proligands were prepared, with two substitution sites being
varied: the substituents on the phosphorus atom were
isopropyl or phenyl and the linkage between the diamido
groups were 2,2-dimethyl-1,3-propylene, 1,2-ethylene, rac-
trans-cyclohexylene, RR-trans-cyclohexylene, and 1,2-phe-
nylene (Table 2). They were synthesized by literature
methods (1, 6, 7, 9)27,37,38,41,42 or by an adaptation of the
literature methods27 and were isolated in good yields
(41-89%). The yttrium initiators were prepared by an
extended silylamide route,43 where the diamine proligands
were reacted with [Y{N(SiHMe2)2}3(THF)2], in toluene for
20 h (Scheme 2, Table 2). The complexation occurred at
room temperature using ligands 1 and 2, but required elevated
temperature for ligands 4-9, presumably because of the
steric protection afforded by phenyl substituents and/or cyclic
diamine groups.
The reactions were monitored by 31P{1H} NMR spectros-
copy which showed the disappearance of the singlet due to
the diamine ligand (1-9) and the growth of multiplets due to
the coordination complex (10-17). The complexes were
isolated in reasonable yields (30-60%) and their purity was
established by elemental analysis.
X-ray Crystal Structures. Crystals suitable for X-ray
diffraction were grown from solutions of 13, 14, and 16
(Table 1) and their molecular structures are represented in
Figures 1–3 (Tables 3–6).
3
(sept. 1H, JHH ) 6.6 Hz, OCH(CH3)2), 2.19-2.03 (m, 4H,
3
CH(CH3)2), 1.30-1.17 (m, 24H, CH(CH3)2), 1.09 (d, 3H, JHH
)
3
6.6 Hz, OCH(CH3)2), 1.05 (d, 3H, JHH ) 6.6 Hz, OCH(CH3)2).
31P{1H} NMR (162 MHz, C6D6) δ ppm: 58.8 ppm.
Typical Polymerization Procedure. To a rapidly stirred solution
of rac-lactide (0.216 g, 1.5 mmol) in dichloromethane (1.12 mL),
in a vial in the glovebox, was added the appropriate initiator, 10-17
(0.38 mL of a 0.02 M solution in dichloromethane) via a syringe.
Aliquots of the reaction mixture were taken at various times and
quenched in hexanes. Solvents were removed in vacuo to leave
the samples of lactide/polylactide as colorless residues.
X-ray Crystallography. Data were collected using Oxford
Diffraction Xcalibur 3 (13 and 16) and Xcalibur PX Ultra (14)
diffractometers. CCDC 676795 to 676797 respectively (Table 1).
(43) Runte, O.; Priermeier, T.; Anwander, R. Chem. Commun. 1996, 1385.
6844 Inorganic Chemistry, Vol. 47, No. 15, 2008

Bis(phosphinic)diamido Yttrium Complexes
Table 2. Key Showing the Compound Numbering and Substituents at
Positions X and R
Figure 2. Molecular structure of the Ci symmetric complex 14 (phenyl
rings replaced with Ph for clarity). The H · · ·Y contact (a) has H · · ·Y 2.58
Å and Si-H· · ·Y 154° (Si-H distance fixed at 1.45 Å), and the
H· · ·Y-N(40) angle is about 151°.
Scheme 2. Synthesis of Complexes 10-17a
a
X and R were systematically varied (see Table 2 for the key) and R′
) SiMe2H. Reagents and conditions: (i) [Y{N(SiHMe2)2}3(THF)2], toluene,
20 h, 298 K (10, 11); (ii) [Y{N(SiHMe2)2}3(THF)2], toluene, 20 h, 373 K
(12-17).
Figure 3. Molecular structure of the Ci symmetric complex 16 (phenyl
rings replaced with Ph for clarity). The H · · ·Y contact (a) has H · · ·Y 3.01
Å and Si-H· · ·Y 154° (Si-H distance fixed at 1.45 Å), and the
H· · ·Y-N(40) angle is about 156°.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 13
Y-O(1)
2.4720(13)
2.3348(17)
2.2406(17)
3.9234(4)
60.52(5)
136.23(5)
69.25(6)
123.98(6)
126.50(5)
99.96(7)
Y-N(3)
Y-O(8)
Y-O(1′)
2.3560(17)
2.3409(15)
2.3121(13)
Y-N(6)
Y-N(30)
Y· · ·Y
O(1)-Y-N(3)
O(1)-Y-O(8)
O(1)-Y-O(1′)
N(3)-Y-O(8)
N(3)-Y-O(1′)
N(6)-Y-N(30)
O(8)-Y-N(30)
N(30)-Y-O(1′)
O(1)-Y-N(6)
O(1)-Y-N(30)
N(3)-Y-N(6)
N(3)-Y-N(30)
N(6)-Y-O(8)
N(6)-Y-O(1′)
O(8)-Y-O(1′)
Y-O(1)-Y′
122.42(6)
111.26(6)
67.15(6)
100.61(6)
62.27(6)
137.13(5)
81.38(5)
110.14(5)
110.08(6)
114.07(6)
The geometries at the yttrium centers are best described as
pentagonal pyramidal with the N(SiMe2H)2 groups occupying
the apical position. The bond lengths for the Y-O and Y-N
ligand substituents for all three complexes are similar, and
also compare closely with those previously reported for 10.27
There is a correlation between the phosphorus substituents
and the syn or anti disposition of the N(SiMe2H)2 ligands.
Thus, 1027 and 13 (with iso-propyl substituents) have C2
Figure 1. Molecular structure of the C2 symmetric complex 13.
All three complexes are dimeric in the solid state, with
the each ligand having one phosphinic group bridging
between two yttrium centers and the other coordinating a
single yttrium center. Such a coordination mode has previ-
ously been observed in the X-ray crystal structure of 10.27
Inorganic Chemistry, Vol. 47, No. 15, 2008 6845

Platel et al.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 14
Y-O(1)
2.4621(10)
2.3681(12)
2.2848(13)
3.7938(2)
61.34(4)
Y-N(3)
Y-O(9)
Y-O(1′)
2.3542(12)
2.3548(10)
2.3085(10)
Y-N(7)
Y-N(40)
Y· · ·Y
O(1)-Y-N(3)
O(1)-Y-O(9)
O(1)-Y-O(1′)
N(3)-Y-O(9)
N(3)-Y-O(1′)
N(7)-Y-N(40)
O(9)-Y-N(40)
N(40)-Y-O(1′)
O(1)-Y-N(7)
O(1)-Y-N(40)
N(3)-Y-N(7)
N(3)-Y-N(40)
N(7)-Y-O(9)
N(7)-Y-O(1′)
O(9)-Y-O(1′)
Y-O(1)-Y′
129.52(4)
86.30(4)
75.04(4)
98.37(4)
62.72(4)
133.52(4)
81.65(4)
105.31(4)
154.12(4)
74.69(4)
137.67(4)
134.54(4)
126.02(5)
103.96(4)
89.32(4)
Table 5. Selected Bond Lengths (Å) and Angles (deg) for 16
Y-O(1)
2.4444(16)
2.336(2)
2.236(2)
3.7669(5)
61.46(6)
152.06(6)
74.74(6)
128.34(7)
131.26(7)
108.62(8)
104.82(7)
100.48(7)
Y-N(3)
Y-O(8)
Y-O(1′)
2.395(2)
2.3336(17)
2.2942(16)
Y-N(6)
Y-N(40)
Y· · ·Y′
Figure 4. 31P{1H} NMR spectrum of complex 13 in benzene-d6. Main
O(1)-Y-N(3)
O(1)-Y-O(8)
O(1)-Y-O(1′)
N(3)-Y-O(8)
N(3)-Y-O(1′)
N(6)-Y-N(40)
O(8)-Y-N(40)
N(40)-Y-O(1′)
O(1)-Y-N(6)
O(1)-Y-N(40)
N(3)-Y-N(6)
N(3)-Y-N(40)
N(6)-Y-O(8)
N(6)-Y-O(1′)
O(8)-Y-O(1′)
Y-O(1)-Y′
127.29(7)
96.01(7)
67.53(7)
104.15(8)
63.06(6)
139.70(7)
83.20(6)
105.26(6)
spectrum: the whole spectrum containing two peaks (67.0 and 54.5 ppm)
2
in a 1:1 ratio. Inset a: The peak at 67.0 ppm (dd, JPY ) 5.15, 3.24 Hz).
2
Inset b: The peak at 54.5 ppm (d, JPY ) 4.24 Hz).
erization mechanism. The similarity of the bond lengths
suggests that this bond strength does not vary greatly and is
minimally influenced by the ancillary ligand environment.
Some caution should be applied to such analysis as the bond
length/strength could vary on monomer coordination, but the
differences in rate observed with the different initiators (see
polymerization kinetics) are tentatively assigned to variations
in monomer binding and/or steric hindrance of the active
site.
Table 6. Comparative Selected Bond Lengths (Å) and Angles (deg) for
10,27 13, 14, and 16
syn
anti
1027
13
14
16
Y· · ·Y
4.0472(17)
2.691(6)
67.46(15)
66.48(15)
113.74(18)
111.36(17)
3.9235(4)
2.722(3) 2.8964(19)
69.25(6)
3.7938(2) 3.7669(5)
O· · ·O
2.879(3)
74.74(6)
Solution Structures. As the polymerization is conducted
in solution, it is useful to establish the solution structures of
O-Y-O
74.69(4)
Y-O-Y
110.14(5)
105.31(4) 105.26(6)
1
the novel yttrium complexes. The H NMR spectra show
the disappearance of the NH resonances, the splitting into
doublets of the resonances due to the methylene protons
(10-12, 14-17) and the shifting to lower field of the Si-H
groups. The IR spectra show the appearance of weak Si-H
absorptions at about 2040 cm-1. These changes are all
consistent with complexation, but it is the 31P{1H} NMR
spectra which are the most useful for elucidating the solution
structures. The 31P{1H}NMR spectra for the isopropyl
substituted complexes (10, 11, 13) show two multiplets,
consistent with the dimeric structure being maintained in
solution. The spectrum of 13 (Figure 4) shows a doublet of
doublets at 67.0 ppm, assigned to the bridging phosphinic
group, and a doublet at 54.5 ppm, assigned to the terminally
bound phosphinic group. The bridging group is asymmetri-
cally placed between the two yttrium centers in the solid
state (for 13: Y-O(1A) ) 2.31, Y-O(1) ) 2.47 Å). This
asymmetry is maintained in solution as the magnitude of the
coupling constants differs, indicating that the phosphorus is
closer to one yttrium center than the other (for 13: the doublet
Y-N(SiMe2H)2
2.216(6) 2.2406(17) 2.2848(13)
2.253(7)
2.236(2)
symmetry, and a syn arrangement for the two N(SiMe2H)2
ligands, while 14 and 16 (with phenyl substituents) have Ci
symmetry and an anti arrangement for the two N(SiMe2H)2
ligands. The orientation of each N(SiMe2H)2 ligand with
respect to the Y · · · Y vector also correlates with the
phosphorus substituents. In 14 and 16 the N-Si vectors are
almost perfectly aligned with the Y · · · Y vector [the
Y′ · · · Y-N(40)-Si(44) dihedral angles are about 2° in each
case] while in 10 and 13 the N-Si vectors are significantly
skewed with respect to the Y · · · Y vector [the equivalent
dihedral angles being about 43° in 13, and about 64 and 77°
in 10]. The reason for both these differences is the presence
in both 14 and 16 of four C-H · · · π interactions between
the methyl groups of one SiMe2H unit [based on Si(44)] and
their proximal phenyl rings on P(2) and its symmetry related
counterpart (see interactions b and c in Supporting Informa-
tion, Figures S3 and S5). These interactions also bring the
Si-H proton on Si(44) close to the other yttrium center
(interaction a in Figures 2 and 3), approaching the pentagonal
plane approximately opposite to the apical ligand [the
H · · · Y-N(40) angles are ca. 151 and 156° in 14 and 16
respectively].
2
of doublets at 67.0 has JPY ) 5.15, 3.24 Hz).
Complex 12 has an R,R-cyclohexylene backbone group
and therefore all the phosphorus atoms are inequivalent. The
31P{1H} NMR spectrum shows four multiplets; two doublets
of doublets, due to the bridging groups, and upfield two
doublets, due to the terminally bound groups. The isopropyl
substituted complexes show NMR spectra which do not
change on heating to 363 K, consistent with the dimeric
structures being maintained over the temperature range. The
All four complexes have the same Y-N bond lengths to
the apical N(SiMe2H)2 ligands. The length of the
Y-N(SiMe2H)2 bond is significant because it must be
broken/inserted into during the initiation step in the polym-
6846 Inorganic Chemistry, Vol. 47, No. 15, 2008

Bis(phosphinic)diamido Yttrium Complexes
It was proposed that k1 represents an initiation process(es),
while k2, which is significantly (∼8 ×) larger, represents
propagation. To investigate any changes in coordination
chemistry of the yttrium intermediate, the slowest polym-
erization (using initiator 13) was monitored by spectroscopy.
The 31P{1H} NMR spectra were measured in both the
initiation and propagation regimes (Figure 7).
The dimeric structure was clearly maintained both during
the initiation and propagation phases, so cleavage of the
dimer was not occurring during the initiation period. Instead,
it is proposed that the hindered bis(dimethylsilyl)amido group
initiates the lactide binding/ring-opening relatively slowly
(this is responsible for k1 in Figure 6). Once the insertion
has occurred, then the putative yttrium-alkoxide propagating
species is less hindered and therefore polymerizes lactide
more rapidly (responsible for k2 in Figure 6). Initiators 11
and 15 (both have ethylene backbones) do not show the
initiation period, that is, they show initiation rates which
exceed the propagation rate. The behavior of these ethylene
bridged complexes is under further study. Therefore, the
different initiators are best compared using k2 values (Table
7). The k2 values compare with the most active initiators for
this polymerization.5 In the series of complexes, the rates
depend on the diamine backbone substituents rather than on
the phosphorus substituents. In general, the rates decrease
in the order 2,2-dimethyl-1,3-propylene > trans-1,2-cyclo-
hexylene > 1,2-ethylene . 1,2-phenylene. It is proposed
that the C-2 backbones provide a more sterically congested
active site and thus slow the binding and/or ring-opening of
the lactide and decrease the rate. The large difference in rate
between complex 13 and the other complexes indicates that
the electronic nature of the ligand also affects the rate.
Other groups increased the polymerization rate and
improved the control by addition of alcohol to yttrium amido
initiators.19,34,35 Therefore, complex 13, which showed the
least control, was reacted with 2 equiv of iso-propanol and
Figure 5. 31P{1H} NMR spectra of 16 from 293-363 K.
phenyl substituted complexes (14-17) have broadened
spectra, at room temperature, indicating fluxionality. Com-
plex 14 shows an averaged broad signal at room temperature
which sharpens on heating to 383 K. However, complex 15
shows a very broad resonance which does not sharpen on
heating. As expected, complexes 16 and 17 show identical
spectra (Figure 5). At 293 K, two broadened resonances are
observed which resolve to four resonances, on heating to
363 K. These four resonances are assigned to a dimeric
solution structure where all the phosphorus atoms are
inequivalent because of the chirality of the diamine backbone.
At intermediate temperatures (313 and 333 K), there are
minor peaks integrating to <10% of the total, which are
attributed to other conformations.
Finally, all attempts to disrupt the dimeric solution
structures by the addition of coordinating solvents (e.g.,
THF), did not change the NMR spectra.
Lactide Ring-Opening Polymerization. Lactide ring-
opening polymerization was undertaken using initiators
10-17 (Table 7) under mild conditions (CH2Cl2, 298 K).
All the polymerizations occurred rapidly. The conversion was
monitored by 1H NMR and the Mn was determined by SEC
(with light scattering detection). The time required for
complete conversion of 200 equiv of lactide for all the
initiators, except 13 and 15, was less than 10 min. These
times compare with the fastest yttrium initia-
tors.7,10,14,18,19,22,24 However, caution should be applied on
analyzing such rapid polymerizations using single point
kinetic data as the precise end-point is difficult to determine.
Polymerization kinetics. The pseudo first order rate
constants, k1 and k2, were determined from the plots of ln
{[LA]0/[LA]t} versus time (Figure 6 shows the plot for 13).
These plots showed two stages to the polymerizations: an
initial region with slow polymerization occurring (k1), but
where the total conversion remains low (>15%), and then a
second stage in which very rapid polymerization occurs (k2)
and the conversion reaches completion (>99%).
1
cleanly formed an alkoxide complex, in situ. The H NMR
spectrum showed the liberation of two equivalents of
bis(dimethylsilyl)amine and new peaks at 4.39, 1.09, and
1.05 ppm due to the isopropoxide groups. The 31P{1H} NMR
spectrum showed a single resonance at 58.8 ppm. The
alkoxide was a controlled initiator; the ln {[LA]0/[LA]t}
versus time plot was linear (Supporting Information, Figure
S7), showed no initiation period, and a high k2 value (17.3
× 10-3 s-1). The k2 value far exceeds that for complex 13.
This indicates that although propagation reactions dominate
the k2 regime there can still be some initiation occurring and
this decreases the overall rate.
Polymerization Control. Although there was a linear
increase in Mn with % conversion for all the initiators
(illustrated in Figure 8 for 17), the Mn obtained exceed those
predicted on the basis of initiator concentration (Mn predicted
) 28000). In fact, the Mn obtained for most initiators were
approximately double the predicted values (Figure 8, Table
7). The polymerizations were all conducted in an anaerobic
environment (glovebox) and the monomer purity was
established by elemental analysis. The initiator purity was
established by elemental analysis and by in situ 31P{1H}
(44) Kowalski, A.; Duda, A.; Penczek, S. Macromolecules 1998, 31, 2114.
(45) At t0 there should be no LA conversion, the appearance of a positive
y-intercept is due to the low conversions from 0-500s (4-6%) and
the low signal-to-noise ratio of NMR; thus there are significant errors
associated with these initial conversions.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6847

Platel et al.
Table 7. Polymerization Data for Initiators 10-17a
c
c
d
complex #
% conversionb
time/s
k1 × 10-3/s-1
k2 × 10-3 /s-1
Mn
PDId
10
11
12
13
13f
14
15
16
17
99
97
99
99
98
99
80
97
98
221
540
400
8515
390
360
1200
360
320
3.69
n/a
2.12
0.0365 ( 0.0004
n/a
1.03 ( 0.2
n/a
3.46 ( 0.82
1.41
24.8
13.8 ( 4.1
15.5
0.252 ( 0.05
17.3
32.0 ( 4.7
7.60
66409e
48500
41120
147900
29060
56900
47500
46900
41200
1.66
1.29
1.71
1.66
1.15
1.39
1.80
1.47
1.28
14.2 ( 0.2
14.3
a Reaction conditions: [LA]0 ) 1.0 M in CH2Cl2, [LA]0/[N(SiHMe2)2] ) 200. Determined from the H NMR spectra, by integration of the methyne
resonances for lactide and polylactide. c Determined from the gradients of the ln {[LA]t/[LA]0} versus time plots. d Determined by SEC, in THF, using
multiangle laser light scattering (GPC-MALLS) to obtain the absolute Mn. e Determined by SEC, in THF, using polystyrene standards and a correction factor
of 0.58.44 f [LA]0 ) 0.5 M. [LA]0/[iPrOH]/[N(SiHMe2)2] ) 200:1:1.
b
1
Figure 6. Plot showing ln {[LA]0/[LA]t} versus time (s) for initiator 13.45
The polymerization conditions: [LA]0 )1 M, [N(SiHMe2)2]0 ) 0.01 M,
CH2Cl2, 289 K.
NMR monitoring (Figure 7). So, the deactivation of ap-
proximately half the initiator (e.g., by reaction with water)
can be ruled out as an explanation for the high Mn. Instead,
we propose that the anomalous Mn data likely arise due to
only a fraction (in most cases, one-half) of the amido groups
initiating the polymerization. This is also consistent with the
slow initiation periods observed in the reaction kinetics and
the solution studies which indicate that the initiators remain
as dimers throughout the polymerization. In contrast, the in
situ generated alkoxide complex showed much better control
with a Mn of 29000 and a polydispersity index (PDI) of 1.15.
The PDIs are broad throughout the polymerization (Table
7), rationalized by both the slow initiation and transesteri-
fication side reactions occurring. This is supported by
MALDI-TOF spectra (Supporting Information, Figure S8),
at low conversions, showing a series of peaks, capped by
-N(SiHMe2)2 groups, separated by 72 amu.
Figure 7. Conversion versus time plot for initiator 13.45 (a) 31P{1H} NMR
spectrum of the species during the initiation region. (b) 31P{1H} NMR
spectrum of the species during the propagation region.
characterized, including by X-ray crystallography. In the solid
state, the complexes have dimeric structures where one
phosphinic group bridged two yttrium centers and the other
bonds to only one yttrium center. These dimeric structures
were maintained in solution, as shown by 31P{1H} NMR
spectroscopy where multiplets consistent with bridging and
terminally bound phosphinic groups are observed. All the
new complexes were tested as initiators for lactide ring-
opening polymerization. They showed very high activity and
had rates comparable with the best literature initiators. Most
of the initiators displayed kinetic plots showing two distinct
regions: an initiation period and a propagation period. The
polymerization was monitored by 31P{1H} NMR spectros-
Complex 10 gave slightly heterotactic PLA (Pr ) 0.62)
as discussed previously;27 changes in ligand structure did
not affect the stereochemistry.
Conclusions
A series of novel bis(phosphinic)diamido yttrium com-
plexes have been synthesized, using a straightforward
extended silylamide method. The new complexes were fully
6848 Inorganic Chemistry, Vol. 47, No. 15, 2008

Bis(phosphinic)diamido Yttrium Complexes
> trans-1,2-cyclohexylene > 1,2-ethylene . 1,2-phenylene.
The initiators all gave rise to linear increases in Mn with
percentage conversion; however, the values of Mn obtained
were approximately double the value expected, on the basis
of the initiator stoichiometry, because of only a fraction of
the amide groups initiating the polymerization reaction.
In conclusion, we report the synthesis, characterization,
and polymerization studies of a series of new yttrium
complexes. These are highly active species for lactide ring-
opening polymerization. This study has uncovered the
influence of ligand substitution on rate. In the future, it will
be possible to further modify the ligand structure leading to
improvements in both the polymerization rate and control.
Acknowledgment. The EPSRC are acknowledged for
funding this research (EP/C544846/1 and EP/C544838/1).
Rac-Lactide was donated by Purac Plc. The EPSRC National
Mass Spectrometry Service (Swansea) are acknowledged for
the mass spectrometry. Mr. Peter Haycock, Imperial College
London, is thanked for his help with the 31P NMR experi-
ments.
Figure 8. Plot showing Mn vs % conversion for 17. Polymerization
conditions: [LA]0 ) 1.0 M, [N(SiHMe2)2]0 ) 5 mM in CH2Cl2 at 298 K.
Mn calculated ) (144 × [LA]0/[N(SiHMe2)2]0 × % conversion/100).
copy which showed that the dimeric structure was maintained
throughout the polymerization. The bulky amido groups are
responsible for the slow initiation from these complexes. The
propagation rate constants were compared; the backbone
diamine linker groups exerted the most influence, with
the order of decreasing rate being 2,2-dimethyl-1,3-propylene
Supporting Information Available: X-ray crystallographic file
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC800419T
Inorganic Chemistry, Vol. 47, No. 15, 2008 6849