Efficient ring-opening polymerization (ROP) of ?-caprolactone catalysed by isomeric pyridyl β-diketonate iron(III) complexes

Article abstract of DOI:10.1039/c7nj03571c

A series of Fe(iii) complexes of β-diketonate ligands, 1-(2-pyridyl)-3-(3-pyridyl)-1,3-propanedione (L1), 1-(2-pyridyl)-3-(2-pyridyl)-1,3-propanedione (L2), 1-(2-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L3), 1-(3-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L4), 1-(3-pyridyl)-3-(3-pyridyl)-1,3-propanedione (L5) and 1-(4-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L6), viz. [Fe(L1)₃] (1), [Fe(L2)₃] (2), [Fe(L3)₃] (3), [Fe(L4)₃] (4), [Fe(L5)₃] (5) and [Fe(L6)₃] (6) have been structurally characterized. All but one complex (1) catalyzed the ring-opening polymerization (ROP) of ?-caprolactone (?-CL) in near quantitative yield at 110 °C to give polymers with relatively narrow polydispersities (PDI). The comparison of in situ reaction and a reaction with preformed 1 indicated that the latter was a better catalyst, giving a higher molecular weight. Complex 2 catalyzed this reaction in a more modest yield reflecting its greater thermal stability, shorter Fe-O bonds and minimal distortion in fold angle among the isomeric complexes, suggesting that ligand dissociation is important for catalytic activity.

Full text of DOI:10.1039/c7nj03571c

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DOI: 10.1039/C7NJ03571C.
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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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ARTICLE
Efficient ring-opening polymerization (ROP) of ɛ-caprolactone
catalysed by isomeric pyridyl β-diketonate iron(lll) complexes†
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
b
a
c
Sze-Ling Lee, Fei-Long Hu, Xiu-Juan Shang, Yi-Xiang Shi, Ai Ling Tan, Jens Mizera, Jack K.
d
b
b
Clegg, Wen-Hua Zhang, David J. Young and Jian-Ping Lang∗
∗
e
DOI: 10.1039/x0xx00000x
A series of Fe(III) complexes of β-diketonate ligands, 1-(2-pyridyl)-3-(3-pyridyl)-1,3-propanedione (L1), 1-(2-pyridyl)-3-(2-
pyridyl)-1,3-propanedione (L2), 1-(2-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L3), 1-(3-pyridyl)-3-(4-pyridyl)-1,3-
propanedione (L4), 1-(3-pyridyl)-3-(3-pyridyl)-1,3-propanedione (L5) and 1-(4-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L6),
www.rsc.org/
3 3 3 3 3 3
viz. [Fe(L1) ] (1), [Fe(L2) ] (2), [Fe(L3) ] (3), [Fe(L4) ] (4), [Fe(L5) ] (5) and [Fe(L6) ] (6) have been structurally characterized.
All but one complex 1 catalyzed the ring-opening polymerization (ROP) of ɛ-caprolactone (ɛ-CL) in near quantitative yield
o
at 110 C to give polymers with relatively narrow polydispersities (PDI). The comparison of in situ reaction and a reaction
with performed 1 indicated that the latter was a better catalyst, giving a higher molecular weight. Complex 2 catalyzed this
reaction in a more modest yield reflecting its greater thermal stability, shorter Fe-O bonds and minimal distortion in fold
angle among the isomeric complexes suggesting that ligand dissociation is important for catalytic activity.
ions precludes their use as catalysts for the synthesis of materials
for biomedical applications because of potential contamination of
Introduction
Poly(ɛ-caprolactone) (PCL) is a versatile, inexpensive soft material
with the attractive properties of hydrolyzability, biodegradability,
permeability and bioresorbability. Potential biomedical applications
of this eco-friendly polymer range from sutures to resorbable
medical implants and prostheses, drug and contraceptive delivery
4
8-50
the resulting polymer.
residues has proved to be extremely difficult.
Iron catalysts have attracted interest because of the relatively
The elimination of these metallic
3
9, 51-54
4
9, 53-
benign toxicological footprint of this metal, and their low cost.
5
9
1
-7
A number of iron complexes display promising catalytic
systems as well as scaffolds for tissue engineering.
Non-
performance for the ring-opening of CL, yet, they remain
underexplored relative to main group and rare earth metal
biomedical applications of PCL include as components of
polyurethanes for packaging materials, in microelectronic devices
5
8, 60, 61
8
-11
catalysts.
and as components in adhesives and fibers.
PCL is made via the
One of the oldest and most reliable classes of bidentate ligands
ring-opening polymerization (ROP) of ɛ-caprolactone (CL) catalyzed
with a variety of organometallic complexes, most commonly
6
2-68
is the β-diketonates
have been employed for the polymerization of CL.
6-18 substituted β-diketonate ligands have not been investigated in this
and some catalysts bearing these ligands
3
0, 69-72
1
2-15
Dipyridyl-
organostannanes, in the presence of an alcohol initiator.
1
Various organometallic compounds, including Mg(ll),
1
9-21
22
23-25
26-28
29-32
33-35
36-38 respect, yet offer the opportunity for systematic exploration of the
importance of subtle differences in electron density and ligand
Ca(ll),
Ni(ll), Al(lll),
Ti(lV),
Zn(ll),
Sn(ll),
14, 44-46
In(lll),
3
9-41
42, 43
V(V),
Zr(lV),
and of some lanthanides
have been
stability associated with different positional isomers. We now
report the syntheses of a series of six neutral iron(III) complexes
investigated as catalysts for ring-opening polymerization (ROP) of
4
7
CL. However, the toxicity of catalysts derived from many of these
[
Fe(L) ] from symmetrical and asymmetrical dipyridyl-substituted β-
3
diketonate ligands with nitrogen atoms at positions 2, 3 or 4 (L1 –
62, 73-75
L6) (Scheme 1).
These isomeric complexes efficiently catalyze
the ring-opening polymerization of CL with high molecular weights
and differences in activity correlated with the positional influence
of the pyridine on the catalyst. Also, the efficiency of the Fe
complexes outperformed that from an in situ reaction with a
mixture of iron(III) salt and ligand.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
Journal Name
precipitate filtered, washed with ice water and dried under vacuum.
The resulting solid was recrystallized from CH Cl -Hexane (1:5).
L2: Off-white solid. Yield: 4.02 g (71.2%). MP: 103 − 104 °C. IR
2
2
-
1
(
(
KBr pellet) νmax/cm : 3134 (m), 3051(m), 2955 (m), 2924 (s), 2853
m), 1612 (m), 1580 (s), 1563 (s), 1462 (m), 1427 (m), 1285 (m),
1
213 (m), 1090 (m), 993 (m), 845(m), 787 (s), 739 (m), 704 (m), 613
m). Anal. Calcd. for C13 : C 69.03, H 4.42, N 12.19; found: C
8.94, H 4.32, N 12.35. H NMR (400 MHz, DMSO−d
) δ 8.81 (2H, d,
(
10 2 2
H N O
1
6
6
Scheme 1 Symmetrical and asymmetrical dipyridyl-substituted β-
J = 4.2 Hz), 8.15 (2H, d, J = 7.7 Hz), 8.07 (2H, m), 8.01 (1H, m), 7.68
diketonate ligands.
1
(
2H, m). The H NMR spectrum (DMSO–d
mixture of the keto and enol tautomers. UV-vis (CHCl
41 and 338.
L3: Off-white solid. Yield: 4.09 g (72.4%). MP: 109 − 110 °C. IR
6
) indicated a 12:88
3
, λmax (nm)):
2
Experimental Section
-1
General procedures
(KBr pellet) νmax/cm : 3439 (s), 3061 (m), 3037 (m), 2978 (m), 1625
(
(
s), 1579 (m), 1547 (s), 1463 (s), 1409 (m), 1384 (m), 1314 (s), 1213
s), 1066 (s), 993 (s), 782 (s), 740 (m), 706 (m), 612 (s), 465 (m).
ɛ-Caprolactone (CL, 99%) was obtained from J&K Scientific Ltd.
(
Beijing, China). Benzyl alcohol (BnOH) was obtained from Shanghai
Chemical Reagents Co. Ltd. (Shanghai, China). CL and toluene were
distilled over CaH under reduced pressure prior to use. All other
Anal. Calcd. for C H N O : C 69.03, H 4.42, N 12.19; found: C
1
3
10
2
2
1
6
4
9.19, H 4.66, N 12.01. H NMR (400 MHz, CDCl ) δ 8.83 (2H, d, J =
3
2
.6 Hz), 8.73 (1H, d, J = 4.0 Hz), 8.20 (3H, m), 7.92 (2H, m), 7.52 (1H,
chemicals and solvents were purchased from Sigma Aldrich without
further purification unless otherwise specified. IR spectra, using KBr
1
m). The H NMR spectrum (CDCl ) indicated a 2:98 mixture of the
3
keto and enol tautomers. UV-vis (CHCl , λ (nm)): 240.5 and 339.
-
1
3
max
pellets (400 – 4000 cm ), were collected on a Shimadzu IR Prestige-
L4: Yellow crystalline solid. Yield: 5.02 g (87.2%). MP: 179 − 180
2
1 Fourier Transform IR spectrophotometer. C, H and N elemental
-1
°
C. IR (KBr pellet) νmax/cm : 3107 (m), 3038 (m), 2919 (m), 2851 (m),
1
analyses were performed on a Carlo-Erba CHNO-S microanalyzer. H
NMR spectra were obtained from Agilent 400 MHz instruments; δ
values are given in ppm relative to tetramethylsilane. Thermal
1
614 (m), 1590 (s), 1545 (m), 1464 (m), 1413 (m), 1298 (m), 1247
(
(
m) , 1187 (m), 1114 (m), 1062 (m), 1021 (m), 936 (m), 844 (m), 791
s), 700 (m), 677 (m), 629 (m), 610 (m), 491 (m), 463 (m). Anal.
analyses were recorded on a Mettler Toledo Star System under a N2
Calcd. for C H N O : C 69.03, H 4.42, N 12.19; found: C 68.85, H
1
3 10 2 2
1
atmosphere with heating rate of 5 °C. Powder patterns were 4.43, N 12.21. H NMR (400 MHz, DMSO–d ) δ 9.39 (1H, s), 8.85 (3H,
6
obtained on a PANalyticalX’Pert PRO MPD system (PW3040/60) X- d, J = 5.0 Hz), 8.53 (1H, d, J = 8.0 Hz), 8.08 (2H, d, J = 5.5 Hz), 7.64
1
ray diffractometer. Molecular weight (M
polydispersity index (M /M
permeation chromatography (GPC) with tetrahydrofuran (THF) as (nm)): 234 and 336.
n
and
M
w
)
and (1H, m), 7.57 (1H, s). The H NMR spectrum (DMSO–d ) indicated an
6
, PDI) values were obtained by gel 4:96 mixture of the keto and enol tautomers. UV-vis (DMSO–d , λmax
n
6
w
-1
eluent at the flow rate of 0.35 mL∙min at 40 °C, using TOSOH-HLC-
320 gel permeation chromatograph (GPC) equipped with a
refractive index detector, TSK gel SuperMultiporeHZ-M columns
4.6 mm I.D. x 15 cm x 2, measurable molecular weights ranging
Syntheses of [Fe(Lx)
Ligand (1.12 g, 4.98 mmol) was dissolved in water (30 mL)
containing NaOH (0.24 g, 5.89 mmol) and Fe(NO ∙9H O (0.66 g,
.63 mmol) in 10 mL of water added slowly with rapid stirring for
h. The resulting red precipitate was collected by filtration, washed
Cl (100 mL) which was then dried
. The solutions were reduced in volume
using a rotary evaporator and hexane added to achieve a CH Cl to
3
] complexes
8
3
)
3
2
(
3
5
-1
1
3
from 10 to 10 g∙mol ). The sample solutions were filtered prior to
analysis and were injected using a TOSOH plus auto-sampler.
with water and dissolved in CH
over anhydrous Na SO
2
2
n
Calibration of M and PDI values was achieved using polystyrene
2
4
standards purchased from TOSOH.
2
2
hexane ratio of 1:3. Block red crystals formed by slow evaporation
of the solvent.
Preparation of ligands
The syntheses of ligands, Lx (x = 1 − 6) and Fe complexes, 1 − 6
were synthesized by modifications of literature procedures. Ligands
L1, L5 − 6 and complexes 5 − 6 were prepared as described
[
Fe(L1) ] (1). Yield: 0.80 g (67.3%). MP: 240 − 241 °C. IR (KBr
3
-1
pellet) vmax/cm : 3048 (m), 2961 (m), 2857 (m), 1587 (m), 1532 (s),
6
4, 74, 76
1461 (m), 1417 (m), 1380 (m), 1310 (m), 1285 (m), 1190 (m), 1072
previously.
(
m), 1018 (m), 991 (m), 787 (m), 742 (m), 616 (m), 550 (m). Anal.
Calcd. for C39 6.5Fe: C 63.26, H 3.81, N 11.35; found: C 62.77,
H 3.78, N 11.13.
Fe(L2) ] (2). Yield: 0.63 g (52.8%). MP: >300 °C. IR (KBr pellet)
max/cm : 3123 (m), 3049 (m), 3001 (m), 2918 (m), 1593 (m), 1539
28 6
H N O
Syntheses of β-diketonate ligands, Lx (x = 1 − 4)
To a suspension of NaH (60% dispersion in mineral oil, 2.00 g,
[
3
4
9.9 mmol) in anhydrous THF (10 mL) was added 2-acetyl-, 3-acetyl-
-1
v
or 4-acetylpyridine (3.03 g, 25.0 mmol), followed by slow addition
(
(
s), 1506 (s), 1460 (s), 1389 (s), 1362 (s), 1321 (m), 1285 (m), 1261
m), 1242 (m), 1076 (s), 991 (m), 945 (m), 800 (m), 743 (m), 714 (m),
of ethyl picolinate, ethyl nicotinate or ethyl iso-nicotinate (7.50 g,
4
9.7 mmol) in THF (10 mL). The yellow mixture was refluxed under
at 70 °C for 80 minutes and quenched with iced water (50 mL).
Glacial acetic acid was added to adjust the pH to 6 − 7 and the
6
1
15 (m), 556 (m). Anal. Calcd. for C39
27 6 6
H N O Fe: C 64.03, H 3.72, N
N
2
1.49; found: C 63.96, H 4.12, N 11.46.
2
| J. Name., 2012, 00, 1-3
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NP lee wa s eJ od uo r nn oa tl aod fj uC s ht em ma ri sg it nr ys
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DOI: 10.1039/C7NJ03571C
Journal Name
ARTICLE
[
Fe(L3)
pellet) vmax/cm : 3417 (m), 3122 (m), 3037 (m), 2917 (m), 1590 (s), Polymerization reactions were performed under nitrogen
538 (m), 1464 (m), 1411 (m), 1363 (m), 1310 (m), 1286 (m), 1255 atmosphere using a dried Schlenk flask equipped with a magnetic
] (3). Yield: 0.76 g (63.4%). MP: 234 − 236 °C. IR (KBr ɛ-Caprolactone polymerization
3
-1
1
(
m), 1215 (m), 1091 (m), 1075 (s), 1064 (m), 1043 (m), 994 (m), 945 stirrer. A mixture of iron complex (14.68 mg, 0.2 mmol) and BnOH
m), 791 (s), 744 (s), 717 (s), 704 (s), 618 (s), 553 (m), 468 (m). Anal. (10.4 μL, 1.0 mmol) was added into each flask and degassed three
(
Calcd. for C39
H 3.89, N 11.30.
Fe(L4) ] (4). Yield: 0.97 g (81.5%). MP: 245 − 246 °C. IR (KBr
28 6
H N O6.5Fe: C 63.34, H 3.68, N 11.36; found: C 63.46,
times. Toluene (1 mL) and 1.11 mL of CL was added via a syringe
and the flask immersed into a preheated oil-bath. At the end of the
designated period, THF (3 mL) was added and the polymers were
precipitated with a mixture of methanol (215 mL) and 1M HClaq. (35
mL). The solid was filtered, washed with methanol and dried under
vacuum. Each experiment was replicated 3 − 5 times.
[
3
-1
pellet) vmax/cm : 3039 (m), 2923 (m), 2852 (m), 1589 (m), 1527 (s),
474 (m), 1425 (m), 1384 (m), 1363 (m), 1303 (m), 1237 (m), 1078
m), 1022 (m), 943 (m), 849 (m),778 (m), 704 (m), 683 (m), 623 (m),
43 (m). Anal. Calcd. for C42 Fe: C 65.12, H 4.42, N 10.85;
found: C 64.91, H 4.03, N 10.98.
1
(
5
34 6 6
H N O
Results and discussion
Synthesis
X-ray crystallographic determinations
The β-diketonate ligands, Lx (x = 1 − 6), were conveniently prepared
by a modified Claisen condensation between ethyl picolinate,
nicotinate or iso-nicotinate with 2-, 3- or 4-acetylpyridine. Higher
reaction temperatures produced better product yields with
Measurements for single crystals were carried out on either an
Agilent Xcalibur (1 - 2), Rigaku Mercury (3), Bruker APEX-II (fac-
2
3∙4H O), or Bruker D8-Quest (4) using graphite monochromated Mo
K⍺ (λ=0.71073 Å) radiation. The crystal structures were solved by
64, 73, 76
2
decreased reaction times.
Complexes 1 − 6 were obtained
direct methods and refined on F by full-matrix least squares
77
using 1 equiv. of Fe(NO ) ∙6H O to 3 equiv. of Lx (x = 1 − 6) in the
3
3
2
techniques using SHELXL-2014. In general non-hydrogen atoms
were refined anisotropically. Carbon-bound hydrogen atoms were
included in idealized positions and refined using a riding model.
Oxygen-bound hydrogen atoms that could not be located in the
difference Fourier map and were not modelled. In each structure
presence of sodium hydroxide. The symmetrical 5 and 6 have been
6
4, 76
reported previously.
2 2
Single crystals of 1∙0.5H O, 2, 3∙0.5H O,
4
∙0.5C H and 4∙0.5C H ∙0.5CH Cl , suitable for crystal structure
6
14
6
14
2
2
analysis were obtained by recrystallization from CH
2 2
Cl -hexane and
fac-3∙4H O were obtained from the slow evaporation of THF
2
2
except for fac-3∙4H O, there is positional disorder present in the
solution. Both ligands and coordination compounds were stable
under ambient conditions without any sign of decomposition after
pyridyl rings which was modelled using a variety of restraints and
constraints including rigid bodies in a number of instances, full
details are available in the CIFs. Key crystallographic parameters
are summarised in Table 1.
2 2
long periods. They were readily soluble in organic solvents CH Cl ,
CH OH, MeCN and acetone which is important for their application
3
as ɛ-ROP catalysts.
Table 1 Crystal data and structure refinement parameters
1
∙0.5H
C H N O6.5Fe C H N O Fe
2
O
2
3∙0.5H
2
O
C H N O6.5Fe
fac-3∙4H
2
O
4∙0.5(C
6
H
14
)
6 2 2
4∙0.5(C H14)∙0.5(CH Cl )
Emperical formula
Formula weight
Crystal system
Space group
C H N O Fe C H N O Fe C42.5H N O FeCl
39 35 6 10 42 34 6 6 34 6 6
3
9
28
6
39 27
6
6
39 28
6
740.52
trigonal
R3ꢀ
731.51
monoclinic
C2/c
740.52
trigonal
R3ꢀ
803.58
trigonal
P3ꢀ
774.60
triclinic
817.06
triclinic
P1
P1
̅
̅
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å
ꢁcalcd/g cm
Z
14.2613(7)
14.2613(7)
30.489(2)
90
90
120
5370.2(6)
1.374
6
0.479
4761
9.0163(6)
22.1308(13)
17.6843(11)
90
91.643(6)
90
3527.2(4)
1.378
4
13.7398(16)
13.7398(16)
33.050(7)
90
90
120
5403.3(17)
1.365
6
16.0478(7)
16.0478(7)
8.2848(9)
90
90
120
1847.8(3)
1.444
2
0.477
13108
3056
10.600(3)
12.147(4)
16.590(5)
77.320(9)
73.287(9)
74.385(9)
1946.8(10)
1.321
2
10.5081(12)
12.0042(18)
16.4968(18)
77.517(3)
73.321(3)
75.069(3)
1903.2(4)
1.426
3
–3
2
–1
μ (Mo-Kα)/mm
Total reflections
Unique reflections
0.484
0.476
0.442
55304
6846
0.525
8275
3105
14450
2118
79809
8753
2104
R
int
0.0377
160
0.0361
326
0.0336
246
0.0374
169
0.0757
586
0.0459
623
No. parameters
F(000)
2292
0.0751
0.2468
1508
2292
0.0721
0.1807
834
0.0512
0.1506
804
846
a
R1
wR
0.0604
0.1610
1.046
0.0744
0.1914
1.027
0.0543
0.1233
1.038
b
2
c
GOF
1.099
1.117
1.037
a
b
2
o
2
c
2
2
2
1/2
c
2
o
2
c
2
1/2
R
1
= Σ||F
o
| – |F
c
||/Σ|F
o
|. wR
2
= {Σw(F
– F
) /Σw(F
o
) } . GOF = {Σw((F
– F
) )/(n – p)} , where n = number of reflections and p = total
numbers of parameters refined.
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Table 2 Selected bond lengths (Å) for complexes 1 – 4
Journal Name
1
2
3
∙0.5H
2
O
Fe(1)−O(2)
Fe(1)−O(2)
Fe(1)−O(2)
Fe(1)−O(1)
Fe(1)−O(1)
Fe(1)−O(2)
Fe(1)−O(1)
Fe(1)−O(2)
1.986(3)
1.982(3)
1.985(2)
1.9916(17)
1.983(3)
1.997(3)
Fe(1)−O(1)
Fe(1)−O(2)
Fe(1)−O(1)
Fe(1)−O(2)
Fe(1)−O(5)
Fe(1)−O(3)
Fe(1)−O(5)
Fe(1)−O(3)
1.999(3)
1.982(3)
1.998(2)
2.0019(17)
1.989(3)
1.997(3)
Fe(1)−O(3)
1.992(2)
∙0.5H
2
O
fac-3∙4H O
2
4
4
∙0.5(C
6
H
14
)
Fe(1)−O(4)
Fe(1)−O(6)
Fe(1)−O(4)
Fe(1)−O(6)
1.996(3)
2.000(3)
1.9987(17)
1.9782(17)
∙0.5(C H )∙0.5(CH Cl )
2
1.9992(17)
1.9952(17)
1.9916(18)
1.9898(18)
6
14
2
The β-diketonate ligands exhibited four intense bands in their IR face stacking (centroid-centroid distances of 3.99 Å) between
spectra (Table S1, ESI
) corresponding to aromatic (CꢂC), carbonyl adjacent molecules in the ab-plane form a two-dimensional
CꢂO), enol (CꢂC) and (C−O) stretching frequencies. These honeycomb-like lattice. Further π-interactions between these layers
†
7
8
(
absorptions for free ligand were absent in the spectra of the (centroid-centroid distances of 3.74 Å) result in the formation of
complexes with the enolate stretching bands all shifted to lower infinite channels that extend down the c-axis (Figure 3). These
frequencies. The experimental PXRD patterns for 1∙0.5H O, 2 and channels are filled with the waters of crystallisation which are
2
3
∙0.5H
single crystal X-ray data, indicating phase purity of each complex
Fig. S1, ESI ). There were two habits of crystal obtained from the
crystallization of complex 4 which differ by their solvation. Crystals
of 4∙0.5C 14 are block-shaped while those of 4∙0.5C 14∙0.5CH Cl
2
2
O matched the simulated PXRD patterns generated from stabilised by weak CpyridylH – hydrogen bonding.
(
†
(a)
6
H
6
H
2
are needle-shaped. Thermal stability for 1 − 6 was determined by
thermogravimetric analysis (TGA) over the range 25 °C to 800 °C.
Complexes 1 − 6 were stable to 187, 288, 241, 235, 220 and 240 °C,
respectively, followed by gradual weight decline until 455 °C (Fig.
S2, ESI†).
Crystal structures
As expected, the crystal structures of each of the complexes exhibit
distorted octahedral coordination geometries and bond lengths and
angles within the normal ranges (Fe–O = 1.987(3)−2.0019(17) Å; O-
Fe-O 86.33(7)−174.31 (14) °) (Table 2) which are similar to the
(
b)
6
4,76
structures of complex
6
previously reported.
Complexes
1
∙0.5H O and 3∙0.5H O are isostructural, crystallising in the trigonal
2
2
rd
ꢀ
space group R3 with 1/3 of the complex in the asymmetric unit. In
both cases, the pyridyl rings are disordered over two positions such
that both the meridional isomer and facial isomer are present
throughout the lattice. The disorder was modelled to reflect the
expected statistical distribution of the two isomers (ie 1:3 fac:mer).
The two isomers present in 1∙0.5H
isomerism is not possible in 2, 5 or 6 due to the symmetry of the (a) Facial and (b) Meridional. Water solvent molecules removed for clarity.
ligands. The crystal packing is of 1∙0.5H O and 3∙0.5H O is
dominated by offset face-to-face (centroid-centroid distances of
.76 Å - 3.94 Å) and edge-to-face (C(H) - centroid distances of 2.51
2
O are shown in Figure 1. Such Fig. 1 The two isomers present in the crystal structure of [Fe(L1)
3
]∙0.5H
2
O.
2
2
3
Å) π-interactions indicated by forming a comparatively tightly-
packed three-dimensional network.
Slow evaporation of a tetrahydrofuran solution of complex 3
produced red plate-like crystals which structural analysis revealed
2
to be the facial isomer fac-3∙4H O. This structure crystallised in the
rd
ꢀ
trigonal space group P3 again with 1/3 of the complex in the
asymmetric unit. A ORTEP representation of this complex is given in
Figure 2. The crystal packing of fac-3∙4H
2
O is again dominated by
π-
Fig. 2 ORTEP representation of the crystal structure of fac-3∙4H
2
O shown
π
interactions, however a more open lattice results. Offset face-to-
with 50 % probability ellipsoids. Water solvent molecules removed for
clarity.
4
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
Fig. 5 Crystal structure of mer-4∙0.5(C
solvents omitted for clarity.
6 2 2
H14)∙0.5(CH Cl ). Disorder and
Ring-opening polymerisation (ROP) of ɛ-CL
Optimization of conditions
2
Fig. 3 Part of the honeycomb-like crystal packing in fac-3∙4H O viewed
down the c-axis.
Complex 1 was used to determine the optimum conditions of
temperature, solvent, reaction time and concentration for the ROP
of ɛ-CL. Temperatures ranging from 90 − 120 °C were invesꢀgated
with lower temperature of 90 − 100 °C (Table 3, entries 1 and 2)
3
Complex 2 ([Fe(L2) ]) crystallises in the monoclinic space group C2/c
and the asymmetric unit contains one complete complex (Figure 4).
The 2-pyridyl groups are orientated mutually syn but anti with
respect to the oxygen groups benefitting from intramolecular
hydrogen bonding. π−π Interactions again dominate the crystal
packing forming a three-dimensional network.
resulting in a lower yield and molecular weight, M
narrow PDI (1.06 − 1.07). Elevaꢀng the temperature by 10 °C (Table
, entry 3) resulted in a doubling of yield (from 46% to 98%) and
n
, albeit with
3
essentially quantitative conversion after 24 hours. Increasing the
temperature further made little difference to the yield (Table 3,
entries 3 and 4) but a decrease in the observed molecular weight,
M
n
, and a broadening of polydispersity (PDI) was observed at 120
C. This is presumably due to an increase in the termination rate
and thermal depolymerization as well as an acceleration of
°
4
0, 53, 79, 80
transesterification.
Thus 110 °C producing higher
molecular weight and yield and narrow PDI was determined to be
the optimum reaction temperature.
Table 3 CL polymerization catalyzed by 1 in the presence of BnOH at
different temperatures
a
b
n
-
d
calc
-1
b
c
Entry
T(°C)
M
M
PDI
Yield (%)
1
(
gmol )
4200
(gmol )
3540
Fig. 4. Xray crystal structure of 2. Disorder omitted for clarity.
1
2
3
4
90
1.06
1.07
1.23
1.38
31%
46%
98%
99%
100
110
120
4790
5250
3
Complex 4 ([Fe(L4) ]) crystallised as the meridional isomer. Two
14070
13250
11190
11300
crystal structures of this material were obtained, which differ by
solvation. Both structures crystallised in the triclinic space group P1
with a full complex in the asymmetric unit (Figure 5). One complex
̅
a
Reaction condition: time: 24 h; solvent: toluene; ratio = [CL]
o
: [BnOH]
o
:
b
[ = 100 : 1 : 0.2. Determined by GPC in THF, calibrated to a polystyrene
1]
standard. Yield: weight of resulting polymer/weight of monomer used.
calc = yield (%) × 100 × M (CL).
o
(
(
4∙0.5(C
4∙0.5(C
6
H
H
14)) had
14)∙0.5(CH
a
block-like habit while the second
c
d
6
2
2
Cl )) has a needle-like habit. Other than the
M
differences in solvation, the structures are essentially identical.
Again the predominant interactions between molecules in the solid
A trace amount of polymer was produced in DMF (Table 4,
state are π-π stacking.
entry 1). Notably, 1 could initiate ROP of ɛ-CL in the absence of
solvent (Table 4, entry 4) and these conditions produced higher
yields than employing dioxane and chloroform as solvents (Table 4,
entries 2 and 3). However, using toluene resulted in higher
conversion (>95%), a narrower PDI (<1.23) and higher observed
n
molecular weight, M , compared to bulk polymerization. Observed
molecular weight, M
n
, decreased with increasing solvent polarity
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ARTICLE
Figure 6 and Table 4, entries 1 − 5). This inverse relationship molecular weight, M
Journal Name
(
n
(Figure 8). As discussed above, higher
suggests solvation of the catalyst impedes fast monomer insertion molecular weights with a broader distribution were observed at
40
resulting in lower molecular weight.
long reaction times (Table 6). These results are consistent with
rapid formation of the active monomer/initiator – catalyst complex,
Table 4 CL polymerization catalysed by 1 in the presence of BnOH in
followed by concentration dependent polymerization in
controlled manner until the late stages of the reaction.
a
different solvents
a
b
d
b
c
Entry
Solvent
M
n
M
calc
PDI
Yield (%)
-
1
-1
(
gmol ) (gmol )
Table 6 Time dependence of CL polymerization by 1
a
b
d
b
c
1
2
3
4
5
DMF
Dioxane
7090
7600
13000
13730
14070
460
7530
9250
10390
11190
1.12
1.17
1.27
1.27
1.23
4%
Entry
t(h)
Mn
(gmol )
3840
5120
7810
9490
9610
14070
Mcalc
(gmol )
3200
5020
7300
7880
8680
11190
PDI
Yield
(%)
-1
-1
66%
81%
91%
98%
CHCl
Nil
3
1
2
3
4
5
6
4
8
12
16
20
24
1.07
1.07
1.08
1.09
1.10
1.23
28%
44%
64%
69%
76%
98%
Toluene
a
Reaction condition: temperature: 110°C; time: 24 h; solvent: toluene; ratio
b
=
o o o
[CL] : [BnOH] : [1] = 100 : 1 : 0.2. Determined by GPC in THF, calibrated
c
to a polystyrene standard. Yield: weight of resulting polymer/weight of
d
a
monomer used. Mcalc = yield (%) × 100 × M (CL).
o o o
Reaction condition: temperature: 110 °C; ratio = [CL] : [BnOH] : [1] = 100
b
:
1 : 0.2; solvent: toluene. Determined by GPC in THF, calibrated to a
c
polystyrene standard.
Yield: weight of resulting polymer/weight of
d
monomer used. Mcalc = yield (%) × 100 × M (CL).
n
Fig. 6 Effect of solvent on the observed (M ) and calculated molecular
weight (Mcalc).
Unsurprisingly, higher amounts of catalyst 1 produced higher
conversions with 20 mol% resulting in an almost quantitative yield
of polymer after 24 h. This higher conversion was accompanied by
an increase in molecular weight and a modest broadening of the
molecular weight range (Table 5, entry 4).
Fig. 7 First-order kinetic plots for ROP of ɛ-CL versus time (h).
Table 5 CL polymerization using different amounts of catalyst 1
a
b
d
b
c
Entry
Ratio
M
n
M
calc
-1
PDI
Yield
(%)
-1
(
gmol ) (gmol )
1
2
3
4
100 : 1 : 0.01
100 : 1 : 0.05
100 : 1 : 0.10
100 : 1 : 0.20
8000
4170
7980
14070
4110
4910
7990
11190
1.04
1.07
1.08
1.23
36%
43%
70%
98%
a
Reaction conditions: temperature: 110 °C; time: 24 h; solvent: toluene;
b
ratio of [CL]
o
: [BnOH]
o
: [1]
c
o
.
Determined by GPC in THF, calibrated to a
polystyrene standard.
Yield: weight of resulting polymer/weight of
d
monomer used. Mcalc = yield (%) × 100 × M (CL).
Kinetics of polymerization
A plot of In[CL] /[CL] versus time was linear (Figure 7), consistent
0
t
with
a
n
pseudo-first-order kinetic dependency on monomer Fig. 8 Graph of observed molecular weight (M ) and PDI versus conversion
-5 -1
(%).
concentration with a rate constant (k) of 1.94×10 s . Furthermore,
there was a linear correlation between percent conversion and
6
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ARTICLE
Effect of different initiators
signal at 3.65 ppm (triplet) is assigned to the end hydroxyl group.
Various hydroxylated initiators were investigated (Table 7), and all Small signals are also found for the initiating benzyl alcohol.
promoted the polymerization of ɛ-CL to varying degrees. Methanol,
ethanol and isopropanol (Table 7, entries 1, 2 and 4) facilitated the
2
reaction in lower yields (29% − 35%). Both H O and BnOH were
efficient initiators of ɛ-CL ROP with BnOH facilitating the reaction
almost quantitatively, presumably because of its slightly higher pKa.
Table 7 CL polymerization with different initiators
a
b
d
b
c
Entry
Initiator
M
n
M
calc
-1
PDI
Yield
(%)
-1
(
gmol ) (gmol )
1
2
3
4
5
6
MeOH
EtOH
Nil
9670
10110
15210
10430
9860
3310
3420
3200
4000
9930
11190
1.10
1.09
1.08
1.11
1.28
29%
30%
32%
35%
87%
98%
Scheme 2 Proposed mechanism of polymerization for complex 1.
Isopropanol
Conclusions
H
2
O
BnOH
14070
1.23
: [BnOH]
In summary, we have systematically prepared a series of
a
Reaction condition: temperature: 110 °C; ratio = [CL]
o
o
: [1]
o
= 100
3+
symmetrical and non-symmetrical Fe complexes of isomeric
pyridyl β-diketonate ligands. Six single crystal XRD structures were
obtained including both facial and meridional isomers of the
unsymmetrical ligands. All complexes catalysed the ring opening
polymerization of ɛ-CL, with 2 proving less active than the others.
Complex 2 decomposed at a slightly higher temperature in the TGA.
b
:
1 : 0.2; solvent: toluene. Determined by GPC in THF, calibrated to a
c
polystyrene standard.
Yield: weight of resulting polymer/weight of
d
monomer used. Mcalc = yield (%) × 100 × M (CL)
Effect of isomeric catalysts
Isomeric catalysts 1 – 6 were compared using the optimum
conditions discussed above. All complexes catalyzed the ROP of ɛ-CL This higher stability may be relevant to its decreased activity,
almost quantitatively with the exception of complex 2 which proved suggesting initial ligand dissociation may be slower and thus
to be less active (Table 8). The reason for this difference is not
immediately obvious, but does correlate with stability as
polymerization would be commensurately retarded. An
investigation of in situ catalyst formation versus performed 1
determined by delayed decomposition in the TGA (Fig. S2†). It
indicated that the performed catalyst gave higher molecular
weights.
would be expected that activation of the catalyst involves the
dissociation of at least one β-diketonate ligand and perhaps this is
slower for the 2,2 isomer due to electronic effects, leading to
slightly stronger coordinate bonds (Scheme 2). The ROP of ɛ-CL was
conducted by in situ mixing of the Fe(III) salts with L1, however, the Acknowledgements
observed and calculated molecular weights were lower, although
with a narrower PDI of 1.13.
The authors gratefully acknowledge financial support from
Universiti Brunei Darussalam for a Graduate Research Scholarship
GRS) to S-L L. The authors also gratefully acknowledge the National
(
Table 8 CL polymerization with different isomeric catalysts
a
b
d
b
c
Entry
Complex
M
n
M
calc
-1
PDI
Yield
(%)
Natural Science Foundation of China (Grant Nos. 21373142,
21401134 and 21531006), Science and Technology Department of
Jiangsu Province (BK20140307), Department of Education of Jiangsu
Province (14KJB150023), and the State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences (2015kf-07). J. P. Lang also expresses
his gratitude to the “333” Project of Jiangsu Province, the Priority
Academic Program Development of Jiangsu Higher Education
-1
(gmol ) (gmol )
1
2
3
4
5
6
7
1
1*
2
3
4
14070
6100
7890
12170
10900
12130
10380
11190
10730
7310
10840
13730
11410
11180
1.23
1.13
1.11
1.14
1.26
1.21
1.21
98%
94%
64%
95%
100%
100%
98%
5
6
a
Reaction condition: temperature: 110 °C; ratio = [CL]
o
: [BnOH]
o o
: [1] = 100 Institutions, and the “SooChow Scholar” Program of Soochow
b
:
1 : 0.2; solvent: toluene. Determined by GPC in THF, calibrated to a
University. J.K.C acknowledges the support of the Australian
Research Council.
c
polystyrene standard.
Yield: weight of resulting polymer/weight of
d
monomer used.
M
calc
=
yield (%) × 100 × M (CL). 1* prepare in situ by
mixing the Fe(III) salts with L1.
Notes and references
1
H NMR of products
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ARTICLE
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