The role of phosphine in cobalt-catalyzed carbonylative polymerization of N-alkylaziridine

Article abstract of DOI:10.1016/j.jorganchem.2005.03.050

A series of CH₃COCo(CO)₃L complexes (1, L = PCy ₃; 2, L = PMe₂Ph; 3, L = PPh₃; 4, L = P(para-F-Ph)₃; 5, L = P(meta-F-Ph)₃; and 6, L = P(ortho-tolyl)₃) were studied as precatalyst for the title polymerization. The Co-P bond length primarily responds to the cone angle of the phosphine ligand (6 > 1 > 2 ≈ 3 ≈ 4 ≈ 5), while the back-donation to the axial acetyl ligand and the equatorial CO ligand depends on the electron-donating ability of the phosphine and increases in the order 1 > 6 > 2 > 3 > 4 > 5. The equilibrium constant for CH ₃COCo(CO)₃L + CO ? CH₃COCo(CO)₄ + L depends on the electron-donating ability of the phosphine ligand except for 6 and follows the order 6 ?5 > 4 > 3 > 2 > 1. The catalytic activity follows the order 6 > 5 > 4 > 3 > 1 > 2. The activity difference cannot be explained solely by the above equilibrium and is consistent with the competition for the acyl site by the phosphine as nucleophile against aziridine. The production of the β-lactam byproduct is attributed to catalyst decomposition, which is accelerated to the basicity/nucleophilicity of the phosphine ligand.

Full text of DOI:10.1016/j.jorganchem.2005.03.050

Journal of Organometallic Chemistry 690 (2005) 5150–5158
www.elsevier.com/locate/jorganchem
The role of phosphine in cobalt-catalyzed
carbonylative polymerization of N-alkylaziridine
a
a
a,
b
b
Hongyu Xu , Nathalie LeGall , Li Jia *, William W. Brennessel , Benjamin E. Kucera
a
Department of Chemistry, Lehigh University, 6 E. Packer Avenue, Bethlehem, PA 18015, USA
b
Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA
Received 15 February 2005; received in revised form 25 March 2005; accepted 28 March 2005
Available online 23 May 2005
Abstract
A series of CH3COCo(CO)3L complexes (1, L = PCy3; 2, L = PMe2Ph; 3, L = PPh3; 4, L = P(para-F-Ph)3; 5, L = P(meta-F-Ph)3;
and 6, L = P(ortho-tolyl)3) were studied as precatalyst for the title polymerization. The Co–P bond length primarily responds to the
cone angle of the phosphine ligand (6 > 1 > 2 ꢀ 3 ꢀ 4 ꢀ 5), while the back-donation to the axial acetyl ligand and the equatorial CO
ligand depends on the electron-donating ability of the phosphine and increases in the order 1 > 6 > 2 > 3 > 4 > 5. The equilibrium
constant for CH3COCo(CO)3L + CO M CH3COCo(CO)4 + L depends on the electron-donating ability of the phosphine ligand
except for 6 and follows the order 6 ꢁ 5 > 4 > 3 > 2 > 1. The catalytic activity follows the order 6 > 5 > 4 > 3 > 1 > 2. The activity
difference cannot be explained solely by the above equilibrium and is consistent with the competition for the acyl site by the phos-
phine as nucleophile against aziridine. The production of the b-lactam byproduct is attributed to catalyst decomposition, which is
accelerated to the basicity/nucleophilicity of the phosphine ligand.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Catalysis; Carbonylative polymerization; Polyamide
1. Introduction
whelming focus on condensation polymerizations at
the time. In view of the current demand for functional
and environmentally benign polymeric materials [25–
27], the idea of obtaining polyesters and polyamides
through direct carbonylative polymerization of hetero-
atom-containing monomers (COPH) deserves re-
assessment. Particularly, the carbonylative approach
should serve as an alternative method for the synthesis
of these advanced materials complementary to the rel-
atively established methods of ring-opening polymeri-
zation of lactones and lactams [28–34]. In 1997, Sen
[35,36] reported an oxidative carbonylative polymeri-
zation producing polysuccinates. Further, in 1998,
Sen and Arndtsen independently proposed the CO–
imine copolymerization as a way of synthesizing
polypeptides [37–39]. Although the copolymerization
proposed by Sen and Arndtsen has yet to be realized,
they demonstrated the first example of stoichiometric
Metal-catalyzed carbonylation has been used in the
synthesis of a broad spectrum of organic carbonyl
compounds [1–7]. However, the use of carbonylation
in polymer synthesis has primarily been focused on
polyketone synthesis [8–17]. A few early reports and
patents in the 1960s disclosed that inorganic com-
pounds of group 8–10 metals, some times in the pres-
ence of main group organometallic compounds,
catalyzed copolymerizations of CO and oxygen con-
taining comonomers [18–23]. Radical initiated CO–azi-
ridine copolymerization was also reported in 1966 [24].
Unfortunately, these reports attracted little attention
or further research activity perhaps owing to the over-
*
Corresponding author. Tel.: +1 6107585715; fax: +1 6107586536.
E-mail address: lij4@lehigh.edu (L. Jia).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.03.050

H. Xu et al. / Journal of Organometallic Chemistry 690 (2005) 5150–5158
5151
imine insertion into metal–acyl bonds, a necessary
reaction for the copolymerization to occur. Several
groups have noted that the Co2(CO)8/3-hydroxypyri-
dine catalyst system produces polyester as the major
product in contrary to the patent report that b-lac-
tones were the major products [7] and have subse-
quently studied the binary catalyst systems based on
Co2(CO)8 [40–45]. We have been interested in the de-
sign and development of carbonylative polymerization
of COPH using the de novo approach based on well-
defined single-site molecular catalysts. We reported in
2001 the first example of metal-catalyzed carbonylative
polymerization of aziridine under the hypothesis that
aziridine enchainment would occur via sequential
nucleophilic reactions as shown in Scheme 1 [46]. In
the past several years, we investigated the hetero-
atom-containing substrates including N-alkylaziridnes,
2-oxazolines, epoxides, N-alkylazetidines, and THF
[47–52]. DarensbourgÕs group directly investigated the
kinetics and mechanism of the N-alkylaziridine car-
bonylative polymerization in a joint effort with us
[53]. The study substantiated the mechanism that we
proposed for the polymerization and yielded an
important proposition that the phosphine ligand sup-
presses the polymerization rate by competing against
the aziridine monomer for the acyl-site as shown in
Scheme 2. Further, a back-biting depolymerization
mechanism was proposed for the formation of b-lac-
tam byproduct when the PPh3 ligand is present in
the polymerization system.
zation rate, we propose here an alternative explanation
for the formation of b-lactam byproduct.
2. Experimental
All manipulations were performed in a Vacuum
Atmosphere DRI-LAB-08/85 dry box under a nitrogen
atmosphere or using standard Schlenk line techniques.
Diethyl ether, n-hexane and tetrahydrofuran (THF)
were dried by refluxing over sodium/benzophenone un-
der nitrogen. Toluene was dried by refluxing over so-
dium under nitrogen. Benzene-d6 was dried over Na/K
alloy, freeze–pump–thaw degassed, and kept over Na/
K alloy. NaCo(CO)4 was synthesized according to the
published procedure [54]. N-butylaziridine was synthe-
sized using a modified method from literature [55],
dried by Na/K alloy, and kept over Na/K alloy. The
purity of the monomers is crucial for maximizing the
catalyst turnover number. The complex CH3C(O)Co
(CO)3(PPh3) were prepared by a modified literature
procedures [56].
1
The H NMR spectra were recorded with an AMX
360 MHz or a DRX 500 MHz NMR spectrometer.
1
The H and 13C chemical shifts were measured using
the solvent resonances as internal references. The 1H
and 13C NMR spectra of the polymer in d2-1,1, 2,2-tet-
rachloroethane are consistent with its proposed struc-
ture. GC/MS were obtained with a Hewlett–Packard
5890 series instrument. The elemental analyses were per-
formed by Micro-analysis Inc., DE. Infrared spectra
were recorded on an ASI ReactIR 4000 system equipped
with a MCT detector. The resolution of all collected
The goal of this work is to investigate further the
effect of the phosphine on the polymerization rate and
the formation of the b-lactam byproduct. A series of
complexes bearing phosphine with varied electronic
and steric characteristics was synthesized and studied
as precatalysts for the carbonylative polymerization of
N-butylaziridine. While all evidences support the mech-
anism in Scheme 2 for the suppression of the polymeri-
data was set to be 4 cmꢂ1
.
GPC analyses of the molecular weight of the polymer
products were performed using chloroform as the sol-
vent, with a Waters 1515 isocratic HPLC pump and
Waters 2414 refractive index detectors at 35 ꢁC. The col-
umn set was composed of one styragel HR 5E column
and two HR2 columns, targeting the molecular weight
region from 1000 to 10,000 but also covering molecular
weight up to 100,000.
R
cobaltate
nucleophilic
addition
O
N
nucleophilic
attack
O
O
+
[Co(CO)3L]-
N
Co(CO)3L
R
2.1. Synthesis of CH3C(O)Co(CO)3PCy3 (1)
O
repeat previous
steps
O
Co(CO)3L
CO
N
Co(CO)3L
N
NaCo(CO)4 (0.97 g, 5 mmol) and equimolar tricy-
clohexylphosphine were loaded into a 100 mL flask in
the glove box. The flask was connected to the Schlenk
line, evacuated and back-filled with an atmosphere of
CO. All the operations were then carried out under a
CO atmosphere. Diethyl ether (40 mL) was added to
the flask at 0 ꢁC, under stirring to ensure the dissolution
of both reactants. After 5 min, an equimolar amount of
iodomethane (5 mmol; 0.32 mL) was added via a syr-
inge. After the mixture was stirred at 0 ꢁC for 1 h and
at room temperature for 4 additional hours, the solvent
R
R
Scheme 1. Mechanism for carbonylative polymerization of aziridines.
O
R
O
N
polymer
Co(CO)4
+
N
polymer
Co(CO)4
PT3
+
PR3
R
Scheme 2. Competition between phosphine and aziridne for the acyl
site.

5152
H. Xu et al. / Journal of Organometallic Chemistry 690 (2005) 5150–5158
was removed in vacuum at room temperature. The res-
idue was extracted with toluene (2 · 20 mL), and
40 mL of n-hexane was added to induce crystallization
of the product. Isolated yield: 1.61 g (70%). Suitable sin-
gle-crystals for X-ray crystallography studies were ob-
tained from the solution of 1:1 mixture of toluene and
n-hexane at ꢂ30 ꢁC. Anal. Calc. for C23H36CoO4P: C,
59.23; H, 7.78. Found: C, 59.02; H, 7.68. 1H NMR
(20 ꢁC, C6D6): d 0.97–1.88 (m, 33H, P(C6H11)3), 2.77
(s, 3H, C(O)CH3), IR (20 ꢁC, THF, cmꢂ1): t CO 2038
(w), 1968 (s), 1945 (s); t C(O)Me, 1675 (m).
Calc. for C23H15CoF3O4P: C, 55.00; H, 3.01. Found:
C, 54.72; H, 3.13. 1H NMR (20 ꢁC, C6D6): d ꢂ2.55
(s, 3H, C(O)CH3), 6.55–6.57 (m, 1H, m-C6H4F)3,
6.66–6.68 (m, 1H, m-C6H4F), 7.06–7.08 (m, 1H, m-
C6H4F), 7.16–7.17 (m, 1H, m-C6H4F). IR (25 ꢁC,
THF, cmꢂ1): t CO 2052 (w), 1984 (s), 1964 (s); t
C(O)Me 1686 (m).
2.5. X-ray structure determination
Crystals of 1, 2, 4, and 5 were placed on the tip of a
0.1 mm diameter glass capillary and mounted on a Sie-
mens or Bruker SMART Platform CCD diffractometer
for data collection at 173(2) K. A preliminary set of cell
constants was calculated from reflections harvested from
three sets of 20 frames. These initial sets of frames were
oriented such that orthogonal wedges of reciprocal space
were surveyed. The data collection was carried out using
Mo Ka radiation (graphite monochromator). A ran-
domly oriented region of reciprocal space was surveyed
to the extent of one sphere and to a resolution of
2.2. Synthesis of CH3C(O)Co(CO)3PMe2Ph (2)
Complex 2 was synthesized following a procedure
similar to that used for 1 except that dimethylphenyl-
phosphine was used as the starting material. The prod-
uct was extracted with 40 mL of hexane. The hexane
solution was stored at ꢂ30 ꢁC for a week to give rise
to yellow crystals. Isolated yield: 0.83 g (50%). Suitable
single-crystals for X-ray crystallography studies were
obtained from the n-hexane solution at ꢂ30 ꢁC. Anal.
Calc. for C13H14CoO4P: C, 48.17; H, 4.35. Found: C,
47.90; H, 4.39. 1H NMR (25 ꢁC, C6D6): d ꢂ1.00 (d,
J = 8.6 Hz, 6H, P(CH3)2Ph), 2.62 (s, 3H, C(O)CH3),
6.95–6.97 (m, 3H, C6H5), 7.17–7.20 (m, 2H, C6H5). IR
(25 ꢁC, THF, cmꢂ1): t CO 2046 (w), 1976 (s), 1957 (s);
t C(O)Me 1678.
˚
0.77 A. Four major sections of frames were collected
with 0.30ꢁ steps in x at four different u settings and a
detector position of ꢂ28ꢁ in 2h. The intensity data were
corrected for absorption, and decay (SADABS) [57]. Fi-
nal cell constants were calculated from the xyz centroids
of 3421, 2750, 1673, and 2684 strong reflections, respec-
tively, from the actual data collection after integration
(SAINT) [58].
2.3. Synthesis of CH3C(O)Co(CO)3P(p-C6H4F) (4)
The structures were solved by direct methods using
SIR2002 [59] for 4 and using SHELXS-97 [60] for 1 and
2 and 5. The space groups were determined based on
systematic absences and intensity statistics. A direct-
methods solution was calculated which provided most
non-hydrogen atoms from the E-map. Full-matrix least
squares/difference Fourier cycles were performed which
located the remaining non-hydrogen atoms. All non-
hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were placed in
ideal positions and refined as riding atoms with relative
isotropic displacement parameters.
For complex 5, one fluorine is disordered on a –
C6H4F group in a 0.776:0.224 ratio. The acetyl group
is disordered in a 0.646 : 0.354 ratio over two positions
rotated by 88ꢁ. Restraints were required to force the dis-
ordered acetyl groups to be planar with the Co atom,
respectively. It was also necessary to tie the anisotropic
displacement parameters of C40 to O40, since this gave a
cigar shaped ellipsoid.
Complex 4 was synthesized following a procedure
similar to that used for 1 except that tris(p-fluorophenyl)
phosphine was used as the starting material. The prod-
uct was extracted with diethyl ether (2 · 20 mL) and
20 mL of n-hexane was added to induce crystallization
of the product. Isolated yield: 1.50 g (61%). Suitable sin-
gle-crystals for X-ray crystallographic studies were ob-
tained from the hexane solution at ꢂ30 ꢁC. Anal.
Calc. for C23H15CoF3O4P: C, 55.00; H, 3.01. Found:
C, 54.70; H, 3.13. 1H NMR (20 ꢁC, C6D6): d ꢂ2.68 (s,
3H, C(O)CH3), 6.60 (t, J = 8.5 Hz, 2H, p-C6H4F)3,
7.15 (t, J = 8.5 Hz 2H, p-C6H4F). IR (20 ꢁC, THF,
cmꢂ1): t CO 2050 (w), 1980 (s), 1962 (s). t C(O)Me
1684 (m).
2.4. Synthesis of CH3C(O)Co(CO)3P(m-C6H4F) (5)
Complex 5 was synthesized following a procedure
similar to that used for 1 except that tri(m-fluorophe-
nyl) phosphine was used as the starting material. The
residue was extracted with toluene (2 · 20 mL) and
20 mL of n-hexane was added to induce crystallization
of the product. Isolated yield: 1.55 g (63%). Suitable
single-crystals for X-ray crystallographic studies were
obtained from the hexane solution at ꢂ30 ꢁC. Anal.
Carystallographic data for the structural analysis
have been deposited with the Cambridge Crystallo-
graphic Data Center, CCDC No. 266914 for 4,
266915 for 1, 266916 for 5, and 266917 for 3. Copies
of this information may be obtained free of charge
from The Director, CCDC, 12 Union Road, Cam-
bridge, CB2 1EZ UK (fax: 44(1223)336-033; or e-mail:

H. Xu et al. / Journal of Organometallic Chemistry 690 (2005) 5150–5158
5153
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.
cam.ac.uk).
Section. Crystallization of 2 usually requires keeping
the orange oil that precipitates out easily upon cooling
at ꢂ30 ꢁC for a prolonged period. The preparation pro-
cedure for 6 was previously reported [53].
2.6. Polymerization procedure
The crystal structures of 1, 2, 4, and 5 were determined
by X-ray diffraction as shown in Fig. 1. Like the previ-
ously reported structures of 3 and 6, all structures adopt
the trigonal bipyramidal conformation, with the CO li-
gands occupying the equatorial sites and the phosphine
and acetyl ligands occupying the axial sites. The bond
distances and angles in the coordination core are summa-
rized in Table 1. The acetyl group in 5 is disordered in a
0.646:0.354 ratio over two positions, and only the bond
distances in the more abundant component are included
in Table 1. One of the fluorophenyl groups in 5 is also
disordered over two positions. Only the major occupan-
cies of the acetyl and the fluorophenyl is shown in Fig.
1(d). In the series 1–6, the Co–P distance (2.250(4) and
A 125 mL or 300 mL Parr bomb reactor was evacu-
ated on a Schlenk line and backfilled with CO (1 atm).
The aziridine, the catalyst solution, and additional sol-
vent were introduced into the autoclave with syringes
under a gentle flow of CO. The reactor was immediately
closed, and the pressure of CO was raised to 1000 psi.
The reactor was heated in a 60 ꢁC oil bath while being
magnetically stirred. After a certain reaction time speci-
fied in Table 5, the reactor was allowed to cool to the
ambient temperature. The CO pressure was slowly re-
leased in a fume hood. The reactor was opened. The
solution was transferred into a flask. The crude product
was obtained after removal of solvent. Diethyl ether
(50 mL) was used to wash away free phosphine and lac-
tam, if any was formed, from the crude product.
˚
2.3139(6) A in 3 and 6, respectively) apparently changes
in response to the cone angle of the phosphine ligand
[61]. All other bond distances and bond angles are rather
insensitive to the variation of the electronic and steric
properties of the phosphine ligand.
2.7. Polymerization monitored by IR spectroscopy
The reactor is modified with a 30 bounce SiCOMP
window to allow the use of an ASI ReactIR 4000 system
equipped with a MCT detector. In this manner, a single
256-scan background spectrum was collected. The infra-
red spectrometer was set to collect one spectrum every
2 min over a period up to 48 hours. In a typical experi-
ment, 80 mL of distilled THF was delivered via the
injection port into a 300-mL stainless steel Autoclave
Engineers autoclave reactor kept under dynamic vac-
uum overnight at room temperature. After 10 spectra
of THF solvent were collected, in the intervals between
10th and 11th spectra, a solution of the cobalt catalyst
(15 mmol in 4 mL THF) was injected into the reactor.
Between the interval of the 14th and 15th spectra, the
reactor was pressurized to 1000 psi and the temperature
was raised to 60 ꢁC. After about 2 h under these condi-
tions, the monomer was added through an addition tube
at higher pressure. The polymer growth is depicted by
the absorbance profile of the amide carbonyl observed
The solution IR data are much more informative
and perhaps more relevant than the X-ray data. The
wave numbers of the stretching vibration of the car-
bonyl ligands and the C@O bond in the acetyl ligand
are summarized in Table 2 with the phosphine-free
CH3Co (CO)4 (7) also included. The electron-donating
ability of the axial phosphine ligand in the trigonal
bipyramidal complexes clearly exerts a consistent effect
on both the stretching vibration of the equatorial CO
and the stretching vibration of the C@O bond of the
acetyl ligand. The latter event has important bearings
to the carbonylative polymerization because an in-
crease of back-donation from the Co dxz and dyz orbi-
tals to the p* orbital of the acetyl C@O double bond
not only weakens the C=O bond, but also strengths
the acetyl-Co bond and thus retards its reactivity
toward nucleophiles [62].
3.2. The effect of phosphine on the polymerization
at 1644 cmꢂ1
.
With the series 1–6, we are now able to study system-
atically the phosphine effect on the polymerization using
in situ ATR-IR as the monitoring tool. All of the exper-
iments for elucidating the phosphine effect were carried
out under a set of uniform conditions (60 ꢁC, 1000 psi
CO, in THF). The first step that we took before per-
forming the polymerization was to determine the equi-
librium position between 1–6 and the phosphine free 7
at the polymerization temperature and pressure (Eq.
(1)). The processes involving the establishment
3. Results and discussion
3.1. Synthesis and characterization of catalysts
Synthesis of compounds 1, 2, 4, and 5 (CH3C(O)Co
(CO)3L: 1, L = PCy3; 2, L = PMe2Ph; 4, L = P(p-F-
Ph)3; 5, L = P(m-F-Ph)3) was straightforward, following
modified procedures similar to the one for CH3C(O)-
Co(CO)3(PPh3) (3) reported by Heck [56]. All com-
pounds were purified by recrystallization using
appropriate solvents as specified in the Experimental
CH3CðOÞCoðCOÞ L þ CO ꢀ CH3CðOÞCoðCOÞ þ L
3
4
ð1Þ

5154
H. Xu et al. / Journal of Organometallic Chemistry 690 (2005) 5150–5158
(a)
(c)
(d)
(b)
Fig. 1. ORTEP drawings with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted. (a) 1, (b) 2, (c) 4, and (d) 5.
Table 1
complete conversion to 7 was observed instantaneously.
Selected bond lengths and bond angles in 1, 2, 4, and 5
For 1–5, the IR band of the lowest wave number CO
stretching vibration in each case is completely or nearly
completely separated from the IR absorptions of 7
(Table 2). Monitoring the decrease of the lowest-fre-
quency CO absorbance over time allows the observation
of the equilibrium, which takes about 10–15 min to
establish. Two examples of the in situ IR plots are
shown in Figs. 2 and 3. Under the reasonable assump-
tion that the conversion to 7 accounts for the entire loss
of 1–6, the equilibrium constants and concentrations of
7 at equilibrium can be obtained (Table 3). With the
exception of 6, the phosphine electron-donating ability
apparently governs the equilibrium position. The equi-
librium position does not follow the same trend as the
Co–P bond distance in the solid state.
1
2
4
5
˚
Bond length (A)
Co(1)–C(1)
Co(1)–C(2)
Co(1)–C(3)
Co(1)–C(4)
Co(1)–P(1)
C(4)–O(4)
1.8029(14)
1.7740(14)
1.7777(13)
2.0105(13)
2.2776(3)
1.7809(17)
1.7757(19)
1.7879(18)
2.0133(16)
2.2414(5)
1.204(2)
1.771(2)
1.791(2)
1.792(2)
2.007(2)
2.2485(6)
1.199(3)
1.786(2)
1.789(2)
1.779(2)
2.019(3)
2.2472(5)
1.212(5)
1.1949(18)
Bond angle (ꢁ)
C(1)–Co(1)–C(2)
C(1)–Co(1)–C(3)
C(2)–Co(1)–C(3)
C(4)–Co(1)–P(1)
O(4)–C(4)–Co(1)
120.89(6)
110.89(6)
127.49(6)
173.06(4)
123.44(11)
119.97(8)
118.71(8)
121.09(8)
174.83(5)
122.06(12)
120.91(10)
122.71(10)
115.44(10)
175.55(6)
116.08(10)
117.43(10)
125.93(10)
170.83(10)
120.6(3)
122.04(17)
Table 2
IR data (hexane, RT) of the CH3Co(CO)3PR3 complexes
After the equilibrium was established, N-butylaziri-
dine was injected into the autoclave under high pressure
via a slightly overpressurized addition tube (Figs. 2 and
3). Polymerization immediately started as evidenced by
the growth of the absorbance at 1644 cmꢂ1. A new IR
CH3C(O)
Co(CO)3L
L
mC„O (cmꢂ1
)
m(C@O)Me
(cmꢂ1
)
1
2
3
4
5
6
7
PCy3
PMe2Ph
PPh3
2041(w), 1970(s), 1949(s)
2048(w), 1979(s), 1961(s)
2051(w), 1983(s), 1963(s)
2052(w), 1985(s), 1965(s)
2054(w), 1987(s), 1969(s)
2047(w), 1980(s), 1959(s)
2107(w), 2025(s), 2010(s)
1685(m)
1689(m)
1691(m)
1692(m)
1698(m)
1686(m)
1722(m)
band at 1890 cmꢂ1 attributable to CoðCOÞꢂ also ap-
4
P(p-C6H4F)
P(m-C6H4F)
P(o-Tol)
CO
peared immediately after the reaction was started in all
cases with 1–6 as the precatalyst. The lowest frequency
CO absorption from acyl-C(O)Co(CO)3L does not
noticeably shift during the entire reaction, but its absor-
bance dwindles somewhat as the polymerization pro-
ceeds. Assuming that the molar absorbability is not
affected by the chain growth, the above observation
of the equilibrium and the subsequent polymerization
were monitored by in situ ATR IR. Upon subjecting 6
to the above specified temperature and pressure,

H. Xu et al. / Journal of Organometallic Chemistry 690 (2005) 5150–5158
5155
indicates that the concentration of acyl-C(O)Co(CO)3L
is the highest at the beginning of the polymerization
and decreases somewhat as the polymerization proceeds.
A direct comparison of the rate constants was impossible
because the order of rate dependence on aziridine con-
centration appeared to be different when different cata-
lysts were used. Two forms of kinetic profiles for the
polymer growth were observed as exemplified by Figs.
2 and 3. For precatalysts 5 and 6, the polymer growth
profile appeared to be independent of the aziridine con-
centration. For precatalysts 1–4, the rate dependence
deviates more and more from the zero-order dependence
to a positive-order dependence on aziridine concentra-
tion. We therefore opted to use the half-life of reaction
and the initial rate for the rate comparison. The half-life
and the initial rate of the polymerization are significantly
affected by the electronic and steric properties of the var-
ious phosphines (Table 4). It is tempting to explain the
rate difference based on the equilibrium involving
the phosphine-ligated RC(O)Co(CO)3L species and the
phosphine-free RC(O)Co(CO)4 species (Eq. (1)) during
the polymerization because it is not unreasonable to
anticipate that the former are inactive or much less active
than the latter since the electron-donating phosphine li-
gand inhibits the reactivity of the acyl-Co bond toward
nucleophiles (vide supra). According to the concentra-
tions of 7 established by the aforementioned equilibrium
study (Table 3), the polymerization rates catalyzed by
precatalyst 1–6 should display the following ratio:
0.13:0.29:0.46:0.67:0.82:1. In reality, the ratio of the reci-
procal of the reaction half-life is 0.039:0.030:0.16:
0.31:0.69:1, and the ratio of the observed initial rate is
0.068:0.068:0.20:0.29:0.69:1. Therefore, regardless of
how the comparison is made, the partial existence of
the catalyst in the presumed inactive form is not enough
to completely account for the decrease of reaction rate,
particularly in the cases where the basicity of the phos-
phine ligand is relatively strong. Further contradicting
the explanation solely based on the inactivity of acyl-
Co(CO)3L is that 2 is a somewhat slower precatalyst than
1, but more 7 is at equilibrium with 2 than with 1 under
1000 psi CO. On the other hand, the alternative mecha-
nism that Darensbourg and we proposed for the phos-
phine effect on the polymerization can be easily applied
Fig. 2. (a) Change of the concentration of
2 depicted by the
absorbance profile at 1957 cmꢂ1. (b) Polymer growth depicted by the
absorbance profile of the amide carbonyl at 1644 cmꢂ1. (c) Lactam
formation depicted by the absorbance profile of the amide carbonyl at
1750 cmꢂ1
.
Fig. 3. (a) Change of the concentration of
absorbance profile at 1964 cmꢂ1. (b) Polymer growth depicted by the
absorbance profile of the amide carbonyl at 1644 cmꢂ1
5 depicted by the
.
Table 4
Comparison of the polymerization ratea
Precatalyst Reaction half-life (min) Initial rate (·10ꢂ6 mol Lꢂ1 sꢂ1
)
Table 3
Equilibria between 1–6 and 7 at 60 ꢁC under 1000 psi CO in THF
1
2
3
4
5
6
278
366
68
4.2
4.2
CH3C(O)Co(CO)3PR3
Conversion to 7 (%)
K (M barꢂ1
)
12.2
17.8
42.2
61.4
PCy3
PMe2Ph
PPh3
1
2
3
4
5
6
13
29
4.8 · 10ꢂ7
2.9 · 10ꢂ6
9.7 · 10ꢂ6
3.4 · 10ꢂ5
9.2 · 10ꢂ5
Large
35
16
11
46
67
P(p-C6H4F)
P(m-C6H4F)
P(o-Tol)
a
Each experiment was conduct in 88 mL THF with 15 mmol of
precatalyst, 1000 psi of CO pressure, internal temperature of reactor
82
100
was 60 ꢁC, reaction monitored by ATR-IR.

5156
H. Xu et al. / Journal of Organometallic Chemistry 690 (2005) 5150–5158
Table 5
Carbonylative polymerization of N-butylaziridine using 1–6 as precatalyst
Entry
Catalyst
Aziridine/cat molar ratio
Reaction time (h)
Yield (%)a
Polymer
Mn (·103)b
PDIb
Lactam
1c
2c
1
2
3
4
5
6
5
5
5
6
6
6
6
6
6
40
40
40
40
40
40
31
46
80
21
27
34
62
72
120
16
16
12
4
92
90
8
10
3
9.72
6.00
9.68
9.30
8.85
10.45
6.89
9.98
17.0
5.53
7.36
8.89
16.0
17.6
30.1
1.71
1.71
1.54
1.39
1.32
1.15
1.32
1.32
1.38
1.15
1.16
1.13
1.15
1.11
1.40
3c
97
98
4c
2
5c
4
99
1
6c
4
100
100
99
0
7c
24
24
24
24
24
24
24
24
24
0
8d
1
9d
95
5
10d
11d
12d
13d
14d
15d
a
100
100
100
100
100
97
0
0
0
0
0
3
The total conversion is essentially quantitative for all the experiments. Product ratio estimated by 1H NMR.
Determined by GPC with a refractive index detector using chloroform as eluent relative to polystyrene standards.
b
c
Reaction conditions: 15 mmol of complex; 1000 psi of CO pressure; 88 mol of THF; internal temperature at 60 ꢁC, reaction monitored by ATR-
IR.
d
[catalyst] = 1.70 mM, reactions were run in a regular autoclave placed in an oil bath at 60 ꢁC.
to explain the observed rate difference (Scheme 2). Partic-
ularly, it is conceivable that PMe2Ph is a better carbon
nucleophile than PCy3 because of the steric factors,
and therefore would suppress the aziridine enchainment
more strongly than the latter.
under the typical polymerization conditions in the ab-
sence of a monomer is negligible as monitored by the
ReactIR over prolonged time. At 120 ꢁC under
1000 psi CO, the decomposition of 2 occurred over a
period of 12 hours and afforded a cobaltate species.
The decomposed catalyst solution is active for the car-
bonylation of aziridine and converts 40 equivalents of
N-butylaziridine to the b-lactam and the polymer in
2:1 ratio. Although we cannot characterize the coun-
ter-cation in the decomposed catalyst, we speculate that
it is a possibly a Lewis acid in view of CoatesÕ formula-
tion of [Lewis acid]+[Co(CO)4]ꢂ as potent catalysts for
the ring-expanding carbonylation of aziridines.
Although the decomposition of 2 only occurs at much
higher temperature than the polymerization tempera-
ture, it is conceivable that upon the carbonylative
enchainment of aziridine, the chain end unit would un-
dergo similar decomposition more readily.
3.3. Catalyst decomposition and lactam byproduct
As summarized in Table 5, the amount of lactam
byproduct varied from negligible to 10% when different
precatalysts were used. A back-biting, depolymerization
mechanism was previously proposed to explain the for-
mation of the b-lactam [53]. Despite the unfavorable en-
thalpy for the depolymerization, this mechanism remains
a possibility if the entropy could compensate for the en-
thalpy. On the other hand, it appears always likely that
catalyst decomposition is responsible for the byproduct
formation in any catalytic processes. In fact, the forma-
tion of lactam does not immediately occur at the begin-
ning of the polymerization but slightly lags behind
(Fig. 2). This induction period for lactam formation is
consistent with the progression of catalyst decomposi-
tion. Under such a scenario, the lactam production does
not involve the prior formation of the polyamide. Also
consistent with the explanation involving catalyst
decomposition is that the molecular weight distribution
is broadened whenever the b-lactam byproduct is
produced (Table 5). For precatalysts 5 and 6 where the
b-lactam byproduct is absent or minimal, the number
average molecular weight increases linearly as the mono-
mer loading is increased (Figs. 4 and 5).
20
15
10
n
M
5
0
0
20
40
60
80
100
Monomer-to-Cat Molar Ratio
The above observations prompted us to study the cat-
alyst decomposition process. The decomposition of 2
Fig. 4. Linear increase of Mn vs monomer conversion with 5 as the
precatalyst.

H. Xu et al. / Journal of Organometallic Chemistry 690 (2005) 5150–5158
5157
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