Synthesis of β-diketiminate derivatives of zinc alkoxides: Catalytic properties for ring opening polymerization

Article abstract of DOI:10.4067/S0717-97072011000400014

A series of zinc (II) complexes has been prepared based on a β-diketiminate ligand framework and exhibits the highest rate of polymerization with better stereoselectivity in the formation of polylactic from the rac-lactide with aggregation and narrow poly

Full text of DOI:10.4067/S0717-97072011000400014

J. Chil. Chem. Soc., 56, Nº 4 (2011)
SYNTHESIS OF β-DIKETIMINATE DERIVATIVES OF ZINC ALKOXIDES: CATALYTIC PROPERTIES FOR
RING OPENING POLYMERIZATION
TAIMUR ATHAR*, ABDUL HAKEEM, NEHA TOPNANI
Indian Institute of Chemical Technology, Hyderabad (A.P.). India
(Received: March 7, 2011 - Accepted: October 13, 2011)
ABSTRACT
A series of zinc (II) complexes has been prepared based on a β-diketiminate ligand framework and exhibits the highest rate of polymerization with better
stereoselectivity in the formation of polylactic from the rac-lactide with aggregation and narrow polydisperties. Degree of aggregation and their reactivity’s
depends on the nature of alkoxy groups, the role of metal ion, solvent and their structural properties. The nature of substituent group on β-diketiminate ligand
exerts a significant change affecting both the degree of selectivity and the rate of polymerization. The polymerizations are living, as evidenced by the narrow
polydisperties and with their linear nature and average molecular weight.
Keywords: β -diketiminato Zinc alkoxides, PLA and PCL, Ring opening polymerization
line. Solvents were distilled under nitrogen prior to use. Starting materials
were purchased from Aldrich and used as such. Ligands were synthesized as
reported in the literature. 14
1. INTRODUCTION
The remarkable advances have been reported for olefin polymerization by
using single – site catalysts. The stereochemical properties of Polylactic acid
(PLA) and polycaprolactone (PCL) depends on the efficiency of a catalyst. The
mechanical and physical properties, as well as their chemical and biological
degradation depend on the single site interaction of catalyst. Polylactic acid
(PLA) and polycaprolactone (PCL) are used as biopolymer due to their
biocompatibility, permeable and biodegradability properties since many
years.1-3 In last one decade, many efforts have been devoted for improving ROP
with the help of alkoxy based initiators. In alkoxy based catalyst the metal–
oxygen bonds play an important role for the ring-opening polymerizations-
copolymerizations and transesterification reactions. The rate of polymerization
depends on the following three parameters: nature of OR groups, the role of
central metal ion and the nature of solvent in their coordinative aggregates.
It is worth pointing that ROP of lactides can be initiated by alkoxy group in
two–step (1) an anionic-type insertion mechanism of the monomer at the rate
of propagation step without any transfer or termination reactions. Therefore the
knowledge of the catalyst behaviors along with their mechanistic approach are
essential at the molecular scale to better understand the conversion of monomer
into poly product with better controls in molecular weight with narrow
polydispersity. Ring-opening polymerization of lactides has been initiated by
various metal alkoxides (Al, Li, Mg, Fe, Sn, Zr Zn or Ti) since last one decade.
Rac –Lactide was sublimed under vacuum at 50oC. 1H and 13C NMR
1
spectra were recorded on a Varian Unity Inova-600 ( 600 MHz for H and
150 MHz for 13C ) or a Varian Mercury-400 ( 400 MHz for 1H and 100 MHz
1
for 13C ) spectrometer by using TMS as an internal standard for ( H ) or the
central line of CDCl ( 13C ). Microanalyses were performed by using a Heraeus
-RAPID instrument3 Results within± 0.4% with the theoretical value. GPC
measurements were performed on Postnova PN1122 by using THF as solvent
delivery system with TriSEC GPC software at 350C. Molecular weight and
polydispersity indexes were calculated by using polystyrene as a standard.
Ligand Synthesis: The ligands with same N-aryl substituents in
b-diketiminate was prepared by using 2.0 equiv of ArNH2 in 2, 4-pentanedione
in toluene by using p-toluenesulfonic acid as catalyst and then refluxed for
2 days. The ligands with different N-aryl substituents in b-diketiminate was
prepared by using 1.0 equiv of ArNH with 2,4-pentanedione in toluene with
p-toluenesulfonic acid as catalyst and t2hen refluxed for 1day by using the same
equivalent aniline (Ar`NH2) was added into the solution and refluxed again
for 2 days
Synthesis of [{( BDI-2 )Zn( m-OBn )} Zn( m-OBn )2]
The
synthesis
of
2-(
2-M2ethoxyphenylimino
)-4-(
2,
6-diisopropylphenylamido )-pent-2-ene ( BDI-2 ) and [( BDI-2 )ZnEt] was
carried out as reported by Gibson[14] ( BDI-2 ) ZnEt ( 2 g , 4.37 mmol) was
dissolved in 20 ml hexane, followed by dropwise of addition of benzyl alcohol
( 0.55 mL , 5.24 mmol ) at 0oC with continuous stirring for 30 minutes .The
solution was warmed to room temperature and then stirred for 12 hours till
white solids precipitated out. The mixture was filtered through celite .The
filtrate was concentrated and white solid was crystallized from dried toluene
solids at 0oC. Yield: 2.2 g (1.62 mmol , 37% ).1H NMR ( CDCl3 , ppm ) at room
temperature: δ 7.10-6.45 ( 17H, m, ArH ), 4.69 ( 1H, s, 1H, s, HC{C(Me)NAr}
), 4.14 ( 4H, br s, OCH2Ar ), 3.44 ( 3H, s, OCH3 ), 2.87 ( 2H, br s, CHMe2 )2,
1.64 ( 3H, s, HC{(CH3)C=N(o-MeAr) }, 1.56 ( 3H, s, (CH3)C=N(2,6-DiIprAr) ), 0.94
( 6H, d, J = 6.8 Hz, ArCH(CH3) ), 0.77 ( 6H, s, ArCH(CH3)2 ). 13C NMR
( CDCl3 ,Hp–Hpm ) at room temperatu2re: δ 168.10 ( HC{C(CH3)NAr} ), 167.92
( HC{C(CH )NAr} ), 152.63 ( MeOCCN ), 144.41 ( NCCCMe2 ), 142.46 (
NCCCMe2 )3, 138.77 ( MeOCCN ), 127.74, 127.02, 125.73, 124.86, 124.64,
123.35, 120.74, 111.70 ( Ph ), 94.51 ( HC{C(CH3)NAr}2 ), 68.30 ( PhCH2O ),
55.207 ( CH3O ), 27.45 ( ArCH(CH3)2 ), 24.14 ( ArCH(CH )2 ), ( ArCH(CH3)2
), 23.71 ( HC{C(CH )NAr} ), 22.89 ( HC{C(CH )NAr}2 ).3Elemental analysis
calc. for C76H90N4O36Zn3.C62H5CH3: C: 69.04, H:3 6.84, N: 3.88%, Found: C:
68.82, H: 6.71, N: 3.81%.
4-10
So, it is important to analyze and discuss in detail the dependence of the
kinetic course of the reactions with catalyst in the solution state.
Monoanionic b-diketiminate ligand have been considered to be one of the
most versatile ligands in coordination chemistry and helps in fine tuning the
ligand properties with functional groups which leads to the formation of strong
metal –ligand bond which is capable to control stereo chemical properties via a
chain –end control mechanism . 11-12
Bulky substituents on the nitrogen atoms play an important role as a
catalyst for the activation of small molecule and polymerization. 13-16
The formations of β-diketiminate derivatives of Zinc alkoxides such as
dinuclear or trinuclear take place by changing the functional groups at the
molecular level. 15
The role of alkoxy group with different substituents has been investigated
for their acitivity towards the formation of dinuclear and trinuclear zinc
complexes.
Herein we report the synthesis, structure of b-diketiminate Zn alkoxide
complexes along with their solid-state molecular structures, their dynamic
behavior and their reactivity towards ROP based on b-diketiminate ligand by
using the concepts of molecular recognition within architectural properties of
macromolecules.
Synthesis of 2-(2-methoxyphenylimino)-4-(pentafluorophenylamido)-
2-pentene.(L -H). A toluene solution of pentafluoroaniline (21 g, 110 mmol)
with catalytic1 amount (0.1 g) of p-toluenesulfonic acid was added with 2, 4
-pentanedione (12 mL, 120 mmol). After being refluxed for 24 h with water
being continuously removed by using a Dean-Stark trap. Volatile materials
were removed under vacuum the resulting to the formation of slurry .Slurry was
2. EXPERIMENTAL PART
General Procedure
The reaction was performed in an inert atmosphere by using Schlenk
e-mail address: taimurathar2001@gmail.com
887

J. Chil. Chem. Soc., 56, Nº 4 (2011)
suspended in toluene (100 ml) followed by an addition of 2-methoxyaniline
(14 mL, 120 mmol). The solution was refluxed for 48 h with water being
removed by using a Dean-Stark trap again. The product was purified by column
chromatography. The filtrate was dried in vacuum to gives a yellow solid. Yield:
catalyst (0.0506 g, 0.05 mmol) and L-lactide (0.72 g, 20 mmol) in toluene
(20 mL) was stirred at room temperature for 2 min. Volatile materials were
removed and the residue was redissove in THF (10 mL). The mixture was
quenched with an aqueous acetic acid solution (0.35N, 10 mL) and the polymer
was precipitated out in n-hexane (40 mL) to give white crystalline solids.
Yield: 0.52 g (72%).
1
29 g (71%). H NMR (CDCl3, 400 MHz) at room temperature: d 12.38 (1H,
s, NH), 7.15–6.89 (4H, m, ArH), 5.00 (1H, s, b-CH), 3.81 (3H, s, ArOCH3),
2.12 (3H, s, CH CNArF ), 1.93 (3H, s, CH CNArOMe). 13C NMR (CDCl3, 400
MHz): at room 3tempera5ture d 171.45 (CH33CNArF5), 155.87 (CH3CNHOMe),
152.31 (MeOCCN), 140.53, 139.09, 138.92, 137.55, 136.74, 136.46, 128.95,
126.08, 125.36, 124.19, 120.28, 111.22 (Ph), 97.54 (b-C), 55.64 (OCH3), 22.23
(CH3CNArOMe), 20.60 (CH CNHF ). Anal. Calc. (Found) for C18H15F5N2O: C
58.38 (58.73), H 4.08 (4.12),3N 7.565(7.57) %.
Synthesis of [(L ) Zn (μ-OBn)2]. A L1-H (0.37 g, 1.00 mmol) was taken
in hexane followed b1y2the2addition of diethyl zinc (1.2 mL, 1.2 mmol). After
being stirred at 0oC for 8 hrs, a light yellow solution was obtained. Volatile
materials were removed in vacuum to yield yellow oil. Oil was taken in hexane
(20 mL) followed by the addition of BnOH (0.11 mL, 1.0 mmol), the mixture
was stirred for 8 hrs. Volatile materials were removed to yield a light yellow
powder. The solid was recrystallized from toluene at 0oC to yield colorless
crystals. Yield: 0.34 g (31%). 1H NMR (CDCl3, 400 MHz) at room temperature:
d 7.28–6.51 (9H, m, ArH), 4.75 (1H, s, b-CH), 4.57 (2H, br s, PhCH2O), 3.61
3. RESULTS AND DISCUSSION
Effect of functional group in N-aryl positions on the structure of
b-diketiminate Zn complexes:
The utility of above synthetic methodology for the preparation of zinc
complexes with a dinuclear and trinuclear complexes takes place with the
help of alkoxy ligands has been investigated. These complexes have wide
range potential applications. These studies show the effect of b-diketiminate
Zn alkoxide complexes with the help polar interactions. All dinuclear
complexes obtained by substitution with an electron donating groups at the
N-aryl positions. However the trinuclear complexes were obtained with the
help of electron withdrawing groups at the 1 or 3 positions in pentene along
with iso-propyl group at the 2,6 N –aryl positions. With the presence of an
electronegative substitution the polar interaction helps to overcomes the
p-resonance effect leads to formation of trinuclear complex with the help of
s-electron withdrawing effect. Spectroscopic and elemental analysis data are
consistent with the representative compound. The mechanism for the formation
of these complexes is shown in equation 1.
(3H, s, ArOCH3), 1.61 (3H, s, CH3CNArF ), 1.38 (3H, s, CH3CNArOMe). 13
C
NMR (CDCl3, 400 MHz) at room temperat5ure: d 171.93 (CH3CNArF5), 166.72
(CH CNHOMe), 152.02 (MeOCCN), 145.00, 143.29, 140.78, 139.09,138.50,
137.356, 136.65, 136.03, 129.02, 128.21, 127.91, 125.45, 125.29, 119.65 (Ph),
95.90 (b-C), 68.85 (PhCH O), 55.03 (OCH3), 23.16 (CH3CNArOMe), 20.56
(CH CNHF ). Anal. Calc. 2(Found) for C52H48F10N4O4Zn2 [(L6)2Zn2(μ-OBn)2 .
C2H63]: C 565.08 (56.76), H 4.34 (4.98), N 5.03 (5.48)%.
Synthesis of 2 - ( 2, 6 - diisopropylphenylimino ) - 4 - ( 4 -
chlorophenylimino ) - pent - 2 - ene ( BDI-DiiPr- pCl-H ).
2, 6-Diisopropylaniline (5.5 g, 31 mmole) was taken in toluene in the
presence of catalytic amount of para-toluenesulfonic acid, the 2, 4-pentadione
(3.1 mL, 30 mmol) was added. The mixture was refluxed for 36 hrs by using
Dean-Stark trap.
Volatile materials were removed in vacuum followed by the addition of
4-chloroaniline (3.95 g , 30.8 mmol ) in 20 ml toluene and then refluxed for
36 hrs to remove of water completely by using a Dean–Stark trap. A clear
solution was dried in vacuo after extracting with toluene to yield black solid.
Recrystallization of the complex takes place in 24 hours at –18oC in ethanol to
1
yield light yellow crystals. Yield: 8.52 g (23 mmol, 75%). H NMR ( CDCl3
, ppm ) at room temperature: δ 12.55 ( 1H, s, NH ), 7.23-7.14 ( 5H, m, ArH
), 6.82 ( 2H, d, ArH ), 4.88 ( 1H, s, HC{C(Me)NAr}2 ), 3.00 ( 2H, sept, JH-H
= 6 Hz, CH(Me)2 ), 2.01 ( 3H, s, H3CC=N(o-ClAr) ), 1.56 ( 3H, s, H CC=N(2,6-
DiIprAr) ), 1.21 ( 6H, d, JH–H = 6.8 Hz, ArCH(CH3)2 ), 1.12 ( 6H, d,3JH–H = 6.6
Hz, ArCH(CH3)2 ). 13C NMR ( CDCl3 , ppm ) at room temperature : δ 161.39
( HC{C(CH3)NAr} ), 159.31 ( HC{C(CH3)NAr} ), 144.80, 142.20, 140.44,
128.75, 127.88, 125.31, 123.40, 123.03 ( Ph ), 95.82 ( HC{C(CH3)NAr}2 ),
28.31 ( ArCH(CH3)2 ), 24.28 ( ArCH(CH3)2 ), 22.55 ( ArCH(CH3)2 ), 20.77
( H3CC=N(o-ClAr) ), 20.66 ( H3CC=N(2,6-DiIprAr). Mass spectrum ( EI, m/e ) :
368.4 [M]+. Elemental analysis calc. for C H29ClN2: C: 74.88, H: 7.92, N:
7.59%, Found: C: 74.50, H: 7.68, N: 7.24%.23
Ring Opening Polymerization of Lactides Initiated by b-diketiminate
Zn complex:
Alkoxides are considered as a potential precursor as a single-site catalyst
due to their high electropositive nature of elements which are stabilized
by the electronegative oxygen atoms, which will leads to a decrease in the
reactivity towards a acid-base side interaction which will leads to an anionic
polymerization in lactones. The slow activity in the alkoxide-bridged dimer
with different initiating groups leads to the difference in the catalytic action.
A preliminary experiment was conducted to investigate the utility of the
β-diketiminate ligands as a catalysts towards polymerization of rac-lactide by
using different functional groups at N-aryl position. These catalysts show the
good ROP to yield PLA with narrow polydispersties and good molecular weight
control and stereoselectivity. With the help of electrons donating group at the
N-aryl positions the polymerization reactions takes less than 10 mins. The rate
of polymerization differs with functional groups depending in the position at
the N-aryl substitution. It is concluded that Lewis acidity character increases
at the Zinc center due to the presence of an electron withdrawing groups and
lead to the formation of strong bond between Zinc and BnO- thereby affecting
the rate of ROP.
Ring opening polymerization (ROP) of L-lactide (LLA) with complexes
1 -3 was systematically examined in an inert atmosphere. These experimental
results show the complex 1 act as an efficient initiator for the polymerization of
LLA. More than 90% conversion takes place within 3 min in toluene at ambient
temperature and with narrow PDIs (1.09-1.18) as illustrated in Table 1. 15 All
four BnO- groups could be used as initiators on trinuclear Zn complex. The Mn
versus [LLA]0/[1] ratio plot as shown in Figure1.
Synthesis of [( BDI-DiiPr- pCl )Zn( m-OiPr )]2
(BDI-DiiPr-pCl-H) (5 g, 13.5 mmol) was dissolved in hexane, ZnEt2 (1M
in hexane, 16 ml, 16 mmol) was added drop wise at 0oC. The reaction mixture
was stirred for 12 hours at ambient temperature.
A volatile material was removed by vacuum followed by recrystallization
to takes place within 12 hours at 0oC in ethanol to yield white crystal. Yield:
3.52 g (3.575 mmole, 65%). 1H NMR ( CDCl , ppm ) at room temperature:δ
7.27-7.16 ( 3H, m, ArH ), 6.62( 2H, d, ArH )3, 6.46( 2H, t, ArH ), 4.81 ( 1H,
s, HC{C(Me)NAr}2 ), 3.73 ( 1H, sept, OCH(Me)2 ), 3.18 ( 2H, sept, CH(Me)2
),1.80 ( 3H, s, HC{C(CH3)NAr} ), 1.58 ( 3H, s, HC{C(CH3)NAr}2 ), 1.11 (
6H, d, JH–H = 6.4 Hz, ArCH(CH3)22 ), 1.07 ( 6H, d, JH–H = 7.2 Hz, ArCH(CH3)2 ),
0.74 ( 3H, d, JH–H = 7 Hz, OCH(CH3)2 ), 0.66 ( 3H, d, JH–H = 7 Hz, OCH(CH)2) ).
13C NMR ( CDCl3 , ppm ) at room temperature: δ 169.47 ( HC{C(CH3)NAr}),
165.82 ( HC{C(CH )NAr} ), 148.14,146.09, 142.47, 128.23, 127.90, 126.39,
125.64, 123.90 ( Ph3), 96.69 ( HC{C(CH3)NAr}2 ), 65.59 ( OCHMe ), 27.76 (
ArCH(CH3)2 ), 27.37 ( OC(CH3) ), 27.02 ( OC(CH )2 ), 24.62 ( ArC2H(CH )2 ),
24.47 ( ArCH(CH3) ), 24.37 ( H2C{C(CH3)NAr}2), 323.82 (HC{C(CH3)NA3r}2).
Elemental analysis 2calc. for C52H70Cl2N4O2Zn2: C: 63.19, H: 6.74, N: 5.76%,
Found:C: 63.42, H: 7.16,N: 5.69%.
Polymerization procedure: A typical polymerization procedure was
carried out by the synthesis of PLA at room temperature with the conversion
yield (94%) as supported by 1H NMR spectroscopic studies. A mixture of
888

J. Chil. Chem. Soc., 56, Nº 4 (2011)
Table 1 Polymerization of L-lactide using complex 1 as an initiator.
Entry
[M]/[Zn]
40
Temp.
r.t
Time(min)
Mn(GPC)a
Mn(cal.)b
Conv.(%)c
PDI
1.15
1.18
1.10
1.09
1
2
3
4
3
3
3
3
3299
1600
>99
>99
>99
>99
60
r.t
5100
2300
80
r.t
8000
3000
100
r.t.
10483
3700
aObtained from GPC analysis and calibrated by polystyrene standard. bCalculated from the molecular weight of L-lactide x [M]0/4[1]0 x conversion yield plus
Mw(BnOH). cObtained from 1H NMR analysis.
Figure 4. First-order kinetic plots for L-lactide polymerizations with time
in toluene with different concentration of complex 4 as an initiator.
Figure 1. Synthesis of polylactide 1H NMR spectrum of PLA-50 catalyzed
by compound 3 at 50 oC
Figure 5. Linear plot of kobs versus [1] for the polymerization of L-lactide
with [LA]0 = 0.5 M in toluene. k =17.24 M-2 s-1
Figure 2. Plot of Mn (GPC) vs. [M]/[Zn] with polydispersity indices
indicated by closed circles (GPC).
Figure 6. Plot of Mn (GPC) vs. [M]/[Zn] with polydispersity indices
indicated by closed circles (GPC).
Figure 3. Plot of Mn (GPC) vs. [M]/[Zn] with polydispersity indices
indicated by closed circles (GPC).
889

J. Chil. Chem. Soc., 56, Nº 4 (2011)
Table 2 Polymerization of L-lactide using complex 2 as an initiator.
Entry
[M]/[Zn]
50
Temp.
r.t.
Time(h)
Mn(GPC)a
6500
Mn(cal.)b
3700
Conv.(%)c
>99
PDI
1.07
1.06
1.05
1.03
1
2
3
4
2
2
2
2
100
r.t.
7700
7300
>99
150
r.t.
10700
18800
10900
14500
>99
200
r.t.
>99
aObtained from GPC analysis and calibrated by polystyrene standard. b Calculated
from the molecular weight of L-lactide x [M]0/4[1]0 x conversion yield plus Mw(BnOH). c Obtained from 1H NMR analysis.
Table 3 Polymerization of L-lactide using complex 3 as an initiator.
Entry
[M]/[Zn]
Temp.
r.t
Time(min)
Mn(GPC)a
5200
Mn(cal.)b
3650
Conv.(%)c
>99
PDI
1.07
1.05
1.03
1.03
1
2
3
4
25
50
60
75
20
20
20
20
r.t
10300
7250
>99
r.t
12050
8300
96
r.t
20700
10850
>99
aObtained from GPC analysis and calibrated by polystyrene standard. b Calculated
from the molecular weight of L-lactide x [M]0/4[1]0 x conversion yield plus Mw(BnOH). c Obtained from 1H NMR analysis.
The ROP reaction was carried out with complex 2; the results are given in
Table 2. When the [LLA]0/[2] ratio was increased, Mn of PLLA was increased
linearly, supporting the living property of polymerization with complex 2.
PDI (1.03-1.07) as illustrated in table 2 .The Mn versus [LLA]0/[2] ratio plot
shown as Figure 4. Electron withdrawing group attack the carbonyl group of
lactide by increasing Lewis acidic character of alkoxy group. More than 90%
conversion takes place within 2 hrs at room temperature in toluene, and their
kinetic studies13-14 reveals the catalytic process follows the first order reaction
and with a kinetic constant k =17.24 M-2 s-1 which are independent of type of
monomer used along its and complex concentration.( Figure 3 and 4) . More
than 90% conversion takes place within 20 min at room with complex 3 at a
desired [LLA]0/ [3] ratio in toluene and with PDIs in the range of (1.09-1.18) as
shown in Table 3. These results support the living property before the reaction
was quenched. The Mn versus [LLA]0/[3] ratio plot was shown as Figure [5,6]
These results show that stereoselectivities with Pr values are mostly
syndiotatic structure with high Pm value.15
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Binuclear and trinuclear β- diketiminate Zinc complexes were reported
and act as single, living initiators for the polymerization of rac-lactides to form
heterotactic PLA with new architectural properties. The ligand framework is
flexible with limited role to provide steric bulk for heterotactic chain –end
control.Micro structural analyses of the polymer exert a significant influence
to control the stereochemistry of monomer with the tacticity with growing
polymer chains and with good PDIs thereby reducing the rate of degradation.
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behavior in monomer. Much work needs to be done to fully understand the
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