Group 4 alkoxide complexes containing [NNO]-type scaffold: Synthesis, structural characterization and polymerization studies

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Group 4 Alkoxide Complexes containing [NNO]-type Scaffold:

Synthesis, Structural Characterization and Polymerization Studies

Received 00th January 20xx,

Accepted 00th January 20xx

Dipa Mandal, Debashis Chakraborty,* Venkatachalam Ramkumar and Dillip Kumar Chand*

A series of Ti(IV), Zr(IV) and Hf(IV) complexes are synthesized by complexationof [NNO]-type tridentate Schiff base ligands

{2-amino-3-((E)-(2-hydroxybenzylidene)amino)maleonitrile}, L1(H)₂; {2-amino-3-((E)-(2-hydroxy-3,5-dimethylbenzyl-

idene)amino)maleonitrile}, L2(H)₂and {2-amino-3-((E)-(3-(tert-butyl)-2-hydroxy-5-methylbenzylidene)amino)maleonitrile},

L3(H)₂with suitable group 4 metal alkoxides. The ligands are derived by the mono-condensation of diaminomaleonitrile

and salicylaldehyde derivatives. Allthe complexes are found to be dinuclear on the basis of their NMR and MS spectral

data. Three of the nine complexes are structurally characterized by single crystal X-ray diffraction studies. The crystal

structures confirmed the dinuclear nature of these complexes. The metal centers of the dinuclear core are connected

through two bridging alkoxy groups. The coordination environment of a metal center comprise of one unit of a di -

deprotonatedligand, one unitof terminal alkoxide and two units of the bridging alkoxide groups so as to form distorted

octahedral complexes. The catalytic activities of these complexes were investigated towards the ring-opening

polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA) under bulk. These complexes were found to be

highly active catalysts for the production of PCL [poly(caprolactone)] and PLA [poly(lactic acid)] with high rate of

conversion. The strong electron withdrawing cyano groups present in these complexes rationalized the high

polymerization activity. The polymers exhibited narrowmolecularweightdistributions (MWDs). Furthermore, the isolated

PLAs were remarkablyheterotactic-richwith maximum P_r=0.80, by using Hf catalyst with tert-butyl substituent on the

ortho- position of the phenolic moiety. These complexes are moderately active toward the ring-opening

homopolymerizationof rac-epoxides,such as rac-cyclohexene oxide (rac-CHO), rac-propylene oxide (rac-PO) and rac-

styrene oxide (rac-SO). These complexes exhibited good activity in ethylene polymerization upon activation with co-

catalystMAO and with a bulky substituenton the ortho- position of the phenolic moiety of the catalysts.

DOI: 10.1039/x0xx00000x

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caprolactone (-CL).^3-5Reasonably stereoselective PLA have

been prepared in the ROP of lactides using some of the above

mentioned group 4 complexes.^{3d-f,3j,3o-p,3r-s,3y,4e-f}Group 4 metals

Introduction

The development of homogeneous catalysts/initiators for

synthesizing ecofriendly biodegradable polymers, such as

poly(lactic acid) (PLA), poly(caprolactone) (PCL) and related

compounds, from renewable resources is an attractive area of

research.¹In the recent years, such polymers have been

extensively utilized as drug delivery excipients, adsorbable

sutures, medical implants and scaffolds for tissue engineering.²

Catalytic ring-opening polymerization (ROP) of cyclic esters

and lactides is considered as the most suitable route to

produce these polymers. Various catalytic systems based on

the complexes of Al, Zn, Mg, Ca, Sn, Nb, Ta, and selected

lanthanides (Nd, Sm, Yb) have been recently employed for the

ROP reactions.¹Several group 4 complexes prepared by using a

variety of polydentate chelating ligands have also been

employed for ROP, particularly that of lactides (LA) and -

are known to exhibit variable coordination number in their

complexes. The reactivity of these complexes as catalysts

could be tuned by varying the coordination environment

around the metal center. The substituents of the ligand

framework present in the catalyst play a crucial role in

determining the activity and stereoselectivity of ROP using

lactide. Kol et al., have reported that the group 4 complexes of

dithiodiolate ligands, where the ligand is equipped with

electron withdrawing CF₃groups showed highest catalytic

activity for the ROP of rac-LA under melt conditions.^3jThe

stereoselectivity of the catalyst is mainly influenced by the

central metal and the steric bulk present in the ligand

framework. The stereoselectivity in the ROP of rac-LA could be

increased by increasing the steric bulk at the ortho- and para-

positions of the phenolic ring in group 4 complexes of some

bis(phenolate) type ligands.^3o-p

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036,

Tamil Nadu, India

† Footnotes relating to the title and/or authors should appear here.

Electronic Supplementary Information (ESI) available: [details of any

supplementary information available should be included here]. See

DOI: 10.1039/x0xx00000x

Ring-opening homopolymerization of epoxides also have

attracted prominent attention for the synthesis of polyethers.

Such polyethers are important as they have been frequently

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used as thermoplastic or thermosetting materials, lubricants diamino-cis-2-butenedinitrile) with appropriate salicylaldehyde

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DOI: 10.1039/C5RA26721H

for textile fibers, adhesive on glass and metals, electrical derivatives in 1:1 ratio. The new ligands were identified by

insulator and foams.⁶ROP of epoxides such as cyclohexene recording their ¹H NMR, ¹³C NMR and EI-MS spectra (Figure S1

oxide (CHO), propylene oxide (PO) and styrene oxide (SO) have to S6 in ESI†); the ligands were also characterized by using

been accomplished using a variety of Zn, Al, Co, Sc and group 4 elemental analysis technique.

metals catalysts.^5i-j,7However, the group 4 catalysts were less

explored for ROP of the epoxides.^5i-j,7b-cIncorporation of

electron-withdrawing groups in the ligand moiety of early

transition metal complexes have been performed and such

complexes exhibited enhanced catalytic activity for the ROP of

CHO.^7c

Synthetic polymers obtained from olefin polymerization are

useful in daily life. High mechanical strength, flexibility,

superior toughness, chemical inertness and high tensile

Scheme 1. Synthesis of the ligands L1(H)₂-L3(H)₂

strength are the reasons for various applications of synthetic

polymers. A large number of catalysts based on transition and

Synthesis of the group 4 complexes, 1a-3a, 1b-3b and 1c-3c.

inner transition metals have been developed for olefin

polymerization, especially that of ethylene.^8-9Fujita et al.,

The synthetic route for the preparation of all the complexes is

shown in the Scheme 2. The complexes were prepared by

reacting the ligands L1(H)₂L3(H)₂respectively with Ti(OⁱPr)₄,

reported the use of group 4 complexes of phenoxy-imines as

catalysts

excellent

for

catalysts

ethylene

for

polymerization.^8a,10Subsequently,

ethylene polymerization were

.

Zr(OⁱPr)₄(HOⁱPr) and Hf(O^tBu)₄in a 1:1 molar ratio in dry

developed using fluorinated phenoxyimines^9a,9dand bulky

substituent bearing phenoxyimines^8a,9b,9gas catalysts. Group 4

complexes bearing coordinated alkoxide moieties, in addition

to the main ligand frame, have also been developed as active

catalysts for olefin polymerization, in presence of the co-

catalyst MAO.^5g-k,9e,9j

toluene at room temperature. After completion of the

reactions, toluene was evaporated to dryness.

products so obtained were recrystallized

The crude

from

dichloromethane or chloroform for further characterization.

All the complexes were identified by recording their ¹H NMR,

¹³C NMR and EI-MS spectra (Figure S7 to S33 in ESI†); the

complexes were also characterized by using elemental analysis

In this work, a series of new group 4 metal complexes of a

variety of salicylaldiminato ligands are prepared and used as

catalysts/initiators for the preparation of polymers. The Schiff

base ligands prepared by the mono-condensation of

diaminomaleonitrile with salicylaldehyde derivatives are used

for complexation with group 4 metal alkoxides. Each of the

ligands is crafted with electron withdrawing cyano group

whereas the steric bulk is varied on the ligands by varying the

substitution in the salicylaldehyde moiety. The complexes

formed are invariably dinuclear in nature where each metal

center contains one terminal and two bridging alkoxide

groups. The metal complexes are used as catalysts/initiators

for ROP of rac-LA, ε-CL, and rac-epoxides. The coordinated

alkoxide groups present in the catalysts helped to initiate the

polymerization reactions. The complexes are also used as pre-

catalysts for ethylene polymerization. The cyano group and

steric bulk introduced in the ligand moieties proved beneficial

and assisted to elevate the catalytic efficiency of the

complexes.

technique.

compounds exist in the dimeric state.

EI-MS

characterization

reveals

that

these

The appearances and pattern of the ¹H NMR spectra of all

the Ti and Zr complexes are comparable with each other.

However, the extent of shift of the peaks as compared to the

corresponding ligands differs on a case to case basis. The ¹H

NMR spectrum of

a typical complex exhibited one singlet

corresponding to the secondary aldimine protons; however,

two sets of signals corresponding to two types of isopropoxide

groups at 1:1 ratio were observed. The peak corresponding to

the

phenolic

hydrogen

disappeared,

which

indicated

phenolate formation. Mono-deprotonation of the amine group

occurred during the complex formation as confirmed from the

appreciable downfield shift of the signal where the integration

ratio decreased to half of the original value (amido formation).

The integration ratio of the peaks confirmed only two units of

isopropoxide for each unit of the ligand. All these observation

supported the di-deprotonation of the ligand unit due to loss

of phenolic proton and one of the two amine protons. On the

basis of the NMR study presented in this work and related

literature, we assumed formation of binuclear complexes that

possess equal number of terminal and bridging isopropoxide

groups.^3f,3y,5e,5kThe EI-MS data (see Experimental Section and

in ESI†) indicated molecular formula corresponding to

[M(L)(OⁱPr)₂]₂for all the Ti and Zr complexes where L stands for

the di-deprotonated ligand.

Results and discussion

Synthesis of the ligands, L1(H)₂-L3(H)₂

The ligand L1(H)₂is reported whereas L2(H)₂and L3(H)₂are

new compounds.¹¹All the ligands were synthesized following

the reported procedure of the synthesis of L1(H)₂(see Scheme

1) by mono-condensation of diaminomaleonitrile (2,3-

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Scheme 2. Synthesis of the group 4 complexes, 1a-3a, 1b-3b and 1c-3c.

The ¹H NMR data of one of the Ti complexes (3a) is described The signal due to the coordinated amide group appeared at

here. The ¹H NMR spectrum of 3a in CDCl₃at room 6.26 ppm that is downfield shifted by 1.25 ppm as compared

temperature displayed signals assignable to two types of to the free ligand. The ¹H NMR spectra of 3a in CDCl₃at

isopropoxide groups, i.e. terminal and bridging isopropoxide variable temperature displayed no change in the signals

groups. The terminal isopropoxide groups exhibited one well- patterns as obtained at room temperature (Figure S35 in the

resolved doublet at δ=1.00–1.01 ppm [O-CH(CH₃)₂] and

a

ESI†). The NMR spectra of 3a in deuterated toluene displayed

multiplet at 4.14–4.16 ppm [O-CH(CH₃)₂]. The bridging only the solvent peaks, due to lack of solubility.

isopropoxide groups exhibited one doublet at 1.20–1.21 ppm

[O-CH(CH₃)₂] and a multiplet at 4.58–4.63 ppm [O-CH(CH₃)₂]. The ¹H and ¹³C NMR spectra of all the Ti and Zr complexes are

The two aromatic protons located at 7.15 and 7.37 ppm, and provided in ESI†. The NMR spectra support dimeric complex

the secondary aldimine proton [CH=N] at 8.63 ppm are slightly formation

as

understood

from

the

literature.^3f,3y,5e,5k

downfield shifted as compared to the signal for the Therefore, the molecular formula of all the Ti and Zr

corresponding protons in the free ligand (Figure S34 in the complexes could be generalized as [M(L)(OⁱPr)(-OⁱPr)]₂.

ESI†).

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The ¹H NMR spectra of all the Hf complexes are comparable The phenolate oxygen atoms are actually posit i_Vo_ien_wed_Artica_len_Oti_nlint_eo

DOI: 10.1039/C5RA26721H

with each other. The counter anion of the Hf salt that was used each other in their crystal structures. The geometry of N–H

for complexation is tert-butoxide where as in the case of Ti and hydrogen atoms in all complexes is ‘‘endo’’.^3fThe bond

Zr it is isopropoxide. The ¹H NMR of a typical Hf complex distances for M−O_terminal

bonds, M−O_phenoxidebonds, and

alkoxide

exhibited one singlet corresponding to the secondary aldimine M–N

bonds are comparable to the literature.^{3c,3f,3h,5e,5k}The

imine

protons. As per the data of Ti and Zr complexes, one may bond distances for the M−O_bridging

for Ti and Zr

alkoxide

presume two types of tert-butoxide groups in the Hf complex. complexes where the alkoxide stands for isopropoxide are

However, only one type of tert-butoxide group was observed comparable with literature data.^3f,3y,5e,5k

and the number of such group is one per ligand unit.

Interestingly, one more set of signals assignable to ethoxide

group was observed, that too one per ligand unit. It is

proposed, on the basis of literature, that the ethoxide group

was derived from the ethanol present in solvent of

crystallization i.e. chloroform.^5lThe integration ratio of the

peaks confirmed only one unit of tert-butoxide and one unit of

ethoxide for each unit of the ligands. Here also the di-

deprotonated ligands are present in the complexes. The EI-MS

data

indicated

molecular

formula

corresponding

to

[M(L)(O^tBu)(OEt)]₂for all the Hf complexes where L stands for

the di-deprotonated ligand. Assigning ethoxide for the

terminal or bridging position, so also for tert-butoxide is not

possible from the ¹H NMR spectrum. However, the bulky tert-

butoxide and relatively smaller ethoxide groups are proposed

to occupy terminal and bridging positions, respectively, on the

basis of steric consideration. It was confirmed by crystal

structure of one of the complexes i.e. 3c. The ¹H and ¹³C NMR

spectra of all Hf complexes are provided in the ESI. Therefore,

the molecular formula of all the Hf complexes could be

generalized as [M(L)(O^tBu)(-OEt)]₂.

Figure 1. Molecular structure of 3a with thermal ellipsoids

drawn at the 30% probability level. All hydrogen atoms except

the amido protons have been omitted for clarity. Selected

Bond Distances (Å) and Angles (°): Ti(1)−O(1) 1.8972(13,

Ti(1)−O(2)

1.7466(14),

Ti(1)−N(2)

Ti(1)−O(3)

2.1538(13),

Ti(1)−N(1)

3.2343(6);

160.97(6),

2.0912(16),

2.2047(15),

Ti(1)−Ti(1ⁱ)

O(2)−Ti(1)−O(3)

176.59(6),

O(3)−Ti(1)−N(2)

X-ray diffraction studies of complexes of [Ti(L3)(OⁱPr)₂]_2,3a;

O(1)−Ti(1)−N(1) 153.70(7).

[Zr(L3)(OⁱPr)₂]_2,3b and [Hf(L3)(O^tBu)( -OEt)] ₂, 3c.

The solid state structures of the binuclear complexes 3a, 3b

and 3c were determined by single crystal X-ray crystallography

analysis and presented in Figures 1-3, respectively. The

crystallographic parameters are collected in the Table S1 in

ESI†. The complexes 3a, 3b and 3c crystallized in monoclinic

(space group C2/c), triclinic (space group P1), and triclinic

(space group P1) systems, respectively. With regard to the

structural configuration, the molecular structures of all the

complexes are rather similar to one another. In a given

complex both the metal centers are hexa-coordinated with

distorted octahedral geometry. The coordination environment

of a metal center is defined by one [ONN]-type tridentate di-

deprotonated ligand, one terminal alkoxide group and two

bridged alkoxide groups. The ligand units are disposed in a

meridional fashion and the bridged alkoxides are located cis to

each other. The terminal alkoxides that are located in different

metal centers are disposed in an anti-fashion with respect to

each other. The relative positions of the ligand units should

also be described here to provide a complete picture of the

overall coordination environment. This could be defined in

more than one way to offer the same meaning, e.g. by

mentioning the relative positions of the phenolate oxygen

atoms coordinated to different metal centers of the complex.

Figure 2. Molecular structure of 3b with thermal ellipsoids

drawn at the 30% probability level. All hydrogen atoms except

the amido protons and solvent molecules have been omitted

for clarity. Selected Bond Distances (Å) and Angles (°):

Zr(1)−O(1) 2.003(3), Zr(1)−O(2) 2.090(3), Zr(1)−O(3) 1.893(3),

Zr(1)−N(1) 2.203(4), Zr(1)−N(2) 2.359(3), Zr(1)−Zr(2) 3.5257(9);

O(2)−Zr(1)−O(3)

167.75(12),

O(1)−Zr(1)−N(1)

144.85(14),

O(2)−Zr(1)−N(2) 164.79(12).

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However, in the case of Hf the bridging alkoxide is ethoxide were performed under solvent free conditions at the

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and such data is not available in the literature. The distances monomer to catalyst ratio of 200:1.

between two metal atoms in the isopropoxide bridged

dinuclear complexes 3a [3.2343(6) Å] and 3b [3.5257(9) Å] are

closely comparable with the metal-metal distances of some the ROP of ε-CL and rac-LA. The PCL and PLA formed exhibit

reported complexes of similar designs.^3f,3y,5e,5kThe dinuclear Hf narrow

molecular weight distributions [MWDs]. The

All the complexes exhibited high catalytic activities towards

complex 3c contains ethoxide bridges and the metal-metal percentage of monomer (ε-CL and rac-LA) conversion to

distance in the complex is 3.488(3) Å. There is no ethoxide product was determined from the integration ratio of relevant

bridged Hf complex in the literature, hence the present Hf-Hf protons in the corresponding ¹H NMR spectrum. In case of the

distance may be considered as a reference for future works.

ROP of rac-LA, the monomer conversions of >97% were

obtained within short time of 8, 6, and 5 min using 3a, 3b and

3c, respectively. Thus, the present catalysts could be

considered at par with the most active reported group 4

complexes that are used for the ROP reactions.^3-5The presence

of strong electron withdrawing groups, such as cyano, in the

ligand backbone increased the Lewis acidity in the metal

center and resulted in higher activity. The experimental

number-average molecular weight [M_n(obs)] and molecular

weight distributions (MWDs or PDI) values of polymers were

measured by GPC analysis. Among the Ti, Zr and Hf complexes

of

a given ligand, the polymers obtained by using the Ti

complex resulted in the maximum correlation between the

M_n(theo) and M_n(obs). Also, relatively narrow PDI values were

observed in case of the polymers obtained by using the Ti

complex. An increasing tendency of catalytic activity with an

increase of the steric bulk in the form of the ortho- substituent

on the phenolate ring was observed. The steric bulk increases

from 1a to 2a to 3a so also the reactivity. As shown in Table 1,

the ROP of rac-LA under similar reaction conditions but in

presence different Ti complexes i.e. 1a, 2a and 3a, required 21,

Figure 3. Molecular structure of 3c with thermal ellipsoids

drawn at the 30% probability level. All hydrogen atoms except

the amido protons and solvent molecules have been omitted

for clarity. Selected Bond Distances (Å) and Angles (°):

Hf(1)−O(1)

1.919(11),

Hf(1)−Hf(2)

1.989(12),

Hf(1)−N(1)

Hf(1)−O(3)

2.310(14),

2.218(10),

Hf(1)−N(4)

Hf(1)−O(5)

2.206(14),

167.6(5),

3.4882(3);

O(3)−Hf(1)−O(5)

O(4)−Hf(1)−N(1) 160.9(4), O(1)−(Hf1)−N(4) 148.6(5).

15 and

8 min, respectively. The reactivity of Zr and Hf

The terminal and bridging alkoxide groups in 3a and 3b are

isopropoxide moieties. The M–O_alkoxidebond length related to

complexes also increased with the increase of steric bulk. Such

phenomenon of the influence of steric bulk was previously

the bridging alkoxides is relatively longer [Ti–O_bridging

alkoxide reported

for

the

group

4

complexes

of

imine-

alkoxide

thiobis(phenolate) ligands.^3pThe introduction of steric bulk at

the ortho-position of the phenoxide group is likely to protect

the metal center of the active catalyst from aggregation and

increase the catalytic activity.^1eA small discrepancy between

the M_n(theo) and the M_n(obs) were found with slightly broader

MWD values. This is attributed to the unavoidable

transesterification reactions and/or formation of cyclic

=2.1538(13) Å, Zr–O_bridging

=2.090(3) Å] than that of the

terminal alkoxides [Ti−O_terminal

=1.8972(13) Å, Zr−O_terminal

alkoxide

=1.893(3) Å] for a given metal complex. While the

alkoxide

terminal alkoxides are tert-butoxide and the bridging alkoxides

are ethoxide in 3c. It is emphasized here that ethoxide bridged

dinuclear Hf complexes are not reported in literature.

However, there are dinuclear Hf complexes where the bridging

units are groups such as oxide, hydroxide, isopropoxide, and

phenoxide.^5e,3i,3hThe Hf–O_ethoxidebond length related to the

bridging alkoxide is relatively longer [2.222(11) Å] than the Hf–

O_{tert-butoxide}bond length related to the terminal alkoxide

oligomers

during

propagation.

Overall

the

catalytic

performance of these complexes is comparable with reported

active group 4 catalysts for the ROP of ε-CL and rac-LA, in

terms of M_n(obs) and MWD values.^3-5

[1.926(11) Å]. Also, the M–N

bond length [Ti–N

imine

=2.2047(15), Zr–N

=2.359(3) Å, Hf–N

= 2.312(14) Å] is

imine

The microstructure of the PLAs obtained in the ROP of rac-

LA were investigated by analyzing the homonuclear decoupled

¹H NMR spectra (CDCl₃at 25 °C) of the PLA samples. The peaks

present in the methine region of the spectra were considered

for the analysis that indicated the formation of heterotactic-

enriched PLAs. The stereochemical structures could be

determined from the relative intensity of the tetrads (isi, iii,

iis/sii, sis) obtained in the homonuclear decoupled ¹H NMR,

where “i” and “s” indicate isotactic and syndiotactic diad,

respectively.¹²

also longer than the M–N_amidobond length [Ti–N_amido

=2.0912(16) Å, Zr–N_amido=2.203(4) Å, Hf–N_amido=2.233(14) Å]

for a given metal complex.

ROP of ε-CL and rac-LA

The performance of all these complexes (1a-1c, 2a-2c and 3a-

3c) towards the ROP of ε-CL and rac-LA were investigated

(Table 1) at 100 and 140 °C respectively. The polymerizations

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Table 1. Results for the ROP of ε-CL and rac-LA catalyzed by complexes 1a-1c, 2a-2c and 3a-3c^a

c

d

c

Entry

Catalyst

Monomer

Time

[min]

10

7

6

9

6

5

4

Conv.^b

[%]

93

91

90

96

95

97

96

98

92

88

89

96

95

93

98

97

M_n(obs)

M_n(theo)

M_w/M_n

P_r^e

[kg mol^-1]

20.45

18.36

16.65

20.98

18.05

17.89

21.18

19.45

17.34

25.42

21.98

22.33

25.06

24.76

22.90

28.15

25.03

23.43

[kg mol^-1]

22.90

28.90

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

1a

1b

1c

2a

2b

2c

3a

3b

3c

1a

1b

1c

2a

2b

2c

3a

3b

3c

ε-CL

rac-LA

1.19

1.20

1.23

1.15

1.19

1.21

1.12

1.16

1.18

1.17

1.20

1.21

1.15

1.19

1.20

1.14

1.16

1.17

3

21

17

15

12

10

8

0.62

0.67

0.70

0.69

0.74

0.75

0.68

0.77

0.80

6

5

^aReaction conditions: [M]_o/[C]_o= 200:1, 100 °C for ε-CL and 140 °C for rac-LA, solvent-free.

^bDetermined from ¹H NMR in CDCl₃at 25 °C. ^cMeasured by GPC at 27 °C in THF relative to polystyrene standards with Mark-

Houwink corrections for M_n. ^dM_n(theo) = [M]_o/[C]_o× mol wt (monomer) + mol wt (end group) at 100% conversion of monomer.

^eP_ris the probability for heterotactic enrichment calculated from homonuclear decoupled ¹H NMR spectrum in CDCl₃at 25 °C.

In particular, the PLAs obtained from the Hf catalyst (3c) with

tert-butyl groups as substituent on the phenol segment

exhibited maximum degree of heterotacticity (P_r

=

0.80)

[Figure S36 in ESI†]. The bulkier substituents led to higher

rigidity, especially if they were located at the ortho-position of

the phenol segment. Previously, Okuda et al. have reported

the increase of heterotacticity with increasing steric bulk at

ortho- and para-positions of the phenyl ring in (OSSO)-type

tetradentate bis(phenolate) ligand for the ROP of meso-LA.^3o

Kol et al. have demonstrated the gradual change of tacticity, of

the PLA obtained from ROP of rac-LA, from heterotactic-rich

through atactic to isotactic-rich depending on the substitution

pattern of the phenolate ring in the tetradentate-diainionic

imine-thiobis(phenolate) type ligand.^3pAlso, while moving

from Ti to Zr to Hf complexes of a given ligand, it was observed

that the PLAs produced from Hf and Zr catalysts featured

higher degrees of heterotacticity than their Ti counterparts

(e.g. P_r= 0.68 (3a), P_r= 0.77 (3b), P_r= 0.80 (3c).

Figure 4. Plots of M_nand M_w/M_nvs. [M]_o/[Cat]_oplots for ROP

of ε-CL and rac-LA at 100 °C and 140 °C respectively using 3a.

Furthermore the ROP reactions were done using different

monomer-to-catalyst ratios ([M]_o/[Cat]_o) in the range of 100:1 In the plots of M_nand M_w/M_nvs. monomer conversion, the

to 800:1 using the catalyst 3a (Figure 4). In the plots of M_nand M_n(obs) values increased linearly with the monomer

M_w/M_n(MWD) vs. [M]_o/[Cat]_o, the M_n(obs) values of the conversion (Figure 5). This linear dependence of M_n(obs)

produced PCLs and PLAs increased proportionally to the values with the monomer to polymer conversion, supports

monomer-to-catalyst ratios with narrow MWDs, suggesting a that the propagating chains grew at nearly constant rate and

good control in the polymerization.

these polymerizations were well controlled. The plots of %

conversion of ε-CL and rac-LA against time showed a sigmoidal

6 | J. Name., 2012, 00, 1-3

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curve. This curve confirmed that the rate of polymer formation Kinetic Studies of the ROP of ε-CL and rac-LA.

ARTICLE

View Article Online

DOI: 10.1039/C5RA26721H

was very high at the initial stage, whereas at a later stage the

monomer conversion rate remained constant (Figure S37 in

ESI†).

Kinetic studies were conducted to find the order of the

reaction with respect to monomer and catalyst as well as the

rate constant. Polymerizations ε-CL and rac-LA were

monitored over a regular period of time by manual sampling to

determine the monomer conversion for [M]_o/[Cat]_o=200 at

100 and 140 °C respectively using 3a.

The ROP mechanism was found to be first order with respect

to ε-CL and rac-LA conversion using 3a, as clearly elucidated by

the

linear relationship between semilogarithmic plots

(ln([M]_t=0/[M]_t) vs. time (Figure 6). The apparent rate constant

(k_app) values [k_app= 0.875 min⁻¹for the ROP of ε-CL and k_app

=

0.484 min⁻¹for the ROP of rac-LA] were obtained from the

slope of these straight lines from the plot of (ln([M]_t=0/[M]_t) vs.

time.

Figure 5. Plots of M_nand M_w/M_nvs. monomer conversion for

ROP of ε-CL and rac-LA at [M]_o/[Cat]_o=200 at 100 °C and 140 °C

respectively using 3a.

Mechanistic aspects of the ROP of rac-LA.

To find the polymerization mechanism, low molecular weight

PLA (oligomer) were synthesized from rac-LA at monomer to

catalyst ratio 15:1 using catalyst 3a at 140 °C under solvent-

free condition. The end-group analysis of the isolated oligomer

was carried out using the techniques like ¹H NMR spectroscopy

and MALDI-TOF mass spectrometry. The ¹H NMR spectrum

clearly confirmed the incorporation of an iso-propyl end group

(δ=1.26–1.28 ppm signals for methyl protons and δ= 4.16–4.20

signals for methine protons), as shown in Figure S38 in the

ESI†.

Figure 6. Plots of ln([M]_o/[M]_t) vs. time (min) for the ROP of ε-

CL and rac-LA at 100 and 140 °C, respectively using 3a at

[M]_o/[Cat]_o=200:1.

It suggested that the initiation step occurs with insertion of the

coordinated monomer into the M–isopropoxide bond. In

addition, the MALDI-TOF mass spectrum of the oligomer

incorporates a series of signals with molecular weight interval

of 72.04 or 144.10, end-capped with isopropoxide group as

acetonitrile adduct, which indicates that isopropoxide moiety

acts as the initiating group for the controlled ROP of rac-LA

(Figure S39 in the ESI†). The expansion of the MALDI-TOF

spectrum shows a series of peaks with very small intensity,

that corresponds to the intramolecular transesterification

and/or cyclic intermediate formation [Figure S40 in ESI†]. The

1H NMR spectrum combined with MALDI-TOF mass spectrum

suggested that the ROP proceeds via the coordination-

insertion mechanism.^{5g -j}

Ring-Opening homopolymerization of rac-CHO, rac-PO and

rac-SO.

Next, the catalytic activities all these complexes (1a-1c, 2a-2c

and 3a-3c) toward the homopolymerization of rac-epoxides

(rac-CHO, rac-PO and rac-SO) were investigated. All

polymerizations were performed under solvent-free conditions

in monomer to catalyst ratio 1000:1 under argon atmosphere.

The temperature was maintained at 90 °C, 30 °C and 110 °C for

CHO, PO and SO respectively as chosen on the basis of

literature reports using same epoxide monomer.⁷The

polymerization results are summarized in Table 2.

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DOI: 10.1039/C5RA26721H

Table 2. Homopolymerization of rac-CHO, rac-PO and rac-SO catalyzed by 3a-3c^a

e

Entry

Catalyst

Epoxide

monomer

CHO

PO

Time

[h]

8

14

12

Conv.^b

[%]

82

78

79

68

63

62

72

Yield^c

[%]

80

75

77

65

61

59

71

TOF^d

[h^-1]

M_n(obs)

M_w/M_n

[kg mol^-1]

31.19

29.44

28.44

23.45

20.05

21.95

39.02

35.56

36.08

1

2

3

4

5

6

7

8

9

3a

3b

3c

3a

3b

3c

3a

3b

3c

102.0

97.5

98.7

48.5

45.0

44.2

60.0

56.6

55.8

1.29

1.33

1.34

1.31

1.35

1.37

1.32

1.35

1.36

PO

SO

68

67

64

65

SO

^aPolymerization conditions: [M]_o/[C]_o= 1000, under solvent-free. Polymerization data using catalysts 1a-1c and 2a-2c are given in

Table S2 in the ESI. ^bDetermined from ¹H NMR in CDCl₃at 25 °C. Based on gm of polymer obtained. ^dTurnover frequency (TOF)

c

=Mol of epoxide×(mol of catalyst)^-1h^-1. Measured by GPC at 27 °C in THF relative to polystyrene standards.

e

Herein all the complexes are moderately active as catalysts for For CHO polymerizations, the maximum M_n(obs)= 31.19 was

the ROP of epoxides to produce PCHO, PPO and PSO in good achieved and the MWDs are relatively broader [1.28-1.36].

yields. The rate of conversion of CHO is comparatively much However, the broader MWDs are comparable with the

faster than conversion PO and SO using any of the catalysts literature reports for the ROP of CHO using Zn and Al based

(Table 2 and Table S2). For example, in the case of the catalyst catalysts.^7e,7mThis is because of poor initiation and high

3a, 82% of CHO conversion (TOF= 102 h^-1) was observed after termination reactions during the polymerization. In case of PO

8 h, whereas only 68% PO conversion (TOF= 48.5 h^-1) and 72% and SO polymerizations, the maximum M_n(obs)= 23.45 and

SO conversion (TOF= 60.0 h^-1) happened even after 14 h and 39.02 were obtained respectively.

12

characterized by NMR spectroscopy and the experimental

number-average molecular weight [M_n(obs)] and molecular exhibited a sigmoidal curve for the polymerization of CHO, SO

h

respectively. All the polymers obtained were

Furthermore the plots between yield (%) of polymer vs. time

weight distributions (MWDs) values of polymers were and PO using 3a where [Epoxide]_o/[Cat]_o= 1000:1 (Figure S41

measured by GPC analysis. All the polymers (PCHO and PSO) in ESI†). This indicates that the yield (%) of polymer increased

were isolated as white solid, except for PPO, which was liquid. sharply with the time of polymerization at the initial stages

The ¹H NMR spectra obtained from the isolated PCHO, PPO and the yield became constant after a certain time.

and PSO (Figure 7) clearly assigned the characteristic peaks of

the respective polymer and signals are comparable with the Polymerization of Ethylene

literature.⁷

The catalysts 1a-1c, 2a-2c and 3a-3c were investigated toward

ethylene polymerization (Table 3). However, the catalysts were

activated using the co-catalyst MAO (methylaluminoxane). All

the polymerization reactions were performed in dry toluene at

60 °C under a constant pressure of ethylene (6 atm, 6.08 bar).

The ratio of [MAO]:[Cat] was maintained at 1000:1 in all the

reactions.

These complexes are found to be active pre-catalysts

towards ethylene polymerization, producing high molecular-

weight polyethylene. The high activity of the catalysts is due to

the presence of electron withdrawing group (i.e. cyano) in the

ligand back bone. Presence of electron withdrawing group is

known to reduce activation energy of the ethylene insertion

reaction and also facilitate the metal-carbon reactivity thereby

making the catalyst superior for ethylene polymerization.^8b

The catalysts 3a-3c exhibited higher activities, as compared to

2a-2c and 1a-1c, and the values fall in the range of 4.81 × 10⁵

1

Figure 7. H NMR spectra (500 MHz CDCl₃): a) PCHO; b) PPO; c) to 3.31 × 10⁵g mol^-1h^-1(Table 3). This is due to the presence of

PSO.

bulky tert-butyl groups at the ortho- and para-positions in the

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phenolic moiety of the catalysts 3a-3c. Some literature reports charge on the metal center. The presence of excess MAO

View Article Online

DOI: 10.1039/C5RA26721H

have shown that presence of steric bulk, on ortho-positions in resulted in a reversible deactivation of the active center,

the phenolate ring in the catalysts play a crucial role in therefore resulting in lower activity.^9k,14To evaluate the

improving

catalytic

activity

significantly

in

ethylene solvent

effect

on

have

the

been

catalytic

performance,

the

polymerization.^8a,9b,9gWe have also envisioned that the central polymerizations

performed

in toluene,

metal of the catalyst also has an influence on the catalytic dichloromethane and chloroform using [MAO]/[Cat] ratio=

activity. Remarkably, the Ti complexes exhibited maximum 1000:1 at 60 °C. Due to insolubility of these complexes in non

activity in contrast to their Zr and Hf analogues under the polar solvent, the solvents, such as hexane or pentane have

same catalytic conditions. For ethylene polymerization the not been used in this polymerization. The catalyst 3a was

tendency of the catalytic activity of the complexes of a given found to be most active in toluene, whereas in presence of

ligand is usually Ti>Zr>Hf.^9e,fThe higher electrophilicity of the dichloromethane and chloroform a detrimental effect on the

Ti center is the reason behind the higher activity of its catalytic activity was observed [Figure S44 in ESI†]. This is in

complexes in ethylene polymerization.^8bThus, Ti is the most line with the literature.^15-16

frequently used as metallic center in comparison to Zr and Hf

centers for ethylene polymerization.^8-10

Experimental Section

Table 3. Polymerization data for ethylene using complexes 1a-

Materials and Methods.

1c, 2a-2c and 3a-3c activated by MAO^a

Salicylaldehyde was purchased from Sigma-Aldrich, while 2-

d

Entry

Cat.

Yield^b

[gm]

1.06

0.99

0.97

1.51

1.24

1.18

2.41

1.85

1.66

Activity^c

M_n

M_w/M_n

hydroxy-3,5-dimethylbenzaldehyde

hydroxy-5-methylbenzaldehyde were prepared by following

literature procedures.¹⁷Diaminomaleonitrile, group

metal

and

3-(tert-butyl)-2-

[kg mol^-1]

87.89

78.92

75.45

109.11

99.35

94.26

122.75

119.50

115.29

1

2

3

4

5

6

7

8

9

1a

1b

1c

2a

2b

2c

3a

3b

3c

2.12

1.98

1.94

3.02

2.47

2.36

4.81

3.71

3.31

2.38

2.49

2.47

1.89

2.18

2.34

1.85

1.96

1.97

4

salts i.e. Ti(OⁱPr)₄, Zr(OⁱPr)₄·ⁱPrOH and Hf(O^tBu)₄; and the co-

catalyst MAO were purchased from Sigma-Aldrich and used

without further purification. Toluene was dried by refluxing

over sodium and benzophenone and distilled under argon

prior to use. The deuterated solvent for NMR studies i.e. CDCl₃

was acquired from Sigma-Aldrich and purified by distilling over

calcium hydride then stored in a glove box. The common

reagents and common solvents were acquired locally and

purified by usual methods. The monomers namely ε-CL, rac-

CHO, rac-PO and rac-SO were purchased from Sigma-Aldrich,

dried over calcium hydride overnight and distilled freshly prior

to use. Lactide monomer, rac-LA was purchased from Sigma-

Aldrich and purified by sublimation under argon atmosphere

and stored in glove box.

^aConditions: 10 μmol catalyst, [MAO]:[Cat] ratio = 1000, 50 mL

b

toluene, ethylene pressure = 6 atm, at 60 °C for 30 min. gm of

polymer obtained after 30 min. ^cActivity in (gm PE/mol cat ×h)

× 10⁵. ^dMeasured by GPC in 1,2,4-tri-chloro benzene (TCB) at

140 °C relative to polystyrene standards.

In order to find the effect of reaction parameters such as,

[MAO]/[C] ratio, temperature, and solvent, on the catalytic

performance, polymerizations were done systematically by

changing such parameters using the catalyst 3a. To investigate

the influence of reaction temperature on the activity, the

polymerizations were conducted at different temperatures in

the range of 25–80 °C, while the [MAO]/[Cat] molar ratio was

kept constant 1000:1. The activity was found to be slightly

increased with the elevation of reaction temperature from 25

°C to around 60 °C and then decreased with further increase of

temperature [Figure S42 in ESI†]. Therefore the reactions were

carried out at 60 °C. To find the optimum [MAO]/[Cat] ratio

required for maximum catalytic activity, the polymerizations

were performed with different [MAO]/[Cat] ratios at 60 °C.

The activity increased with increasing of [MAO]/[Cat] ratio and

then decreased with further enhancement of [MAO]/[Cat]

ratio [Figure S43 in ESI†]. The highest activity was obtained at

[MAO]/[Cat] ratio = 1000:1. It is proposed that the catalyst is

converted into a cationic species when reacted with MAO in

line with reported data.¹⁴The rate of chain propagation in the

polymerization increased due to the presence of positive

All manipulations for the preparation of complexes were

carried out using either standard Schlenk techniques or glove

box techniques under

a

dry argon atmosphere. The

polymerization reactions were also carried out under argon

atmosphere. All ¹H and ¹³C NMR spectra were recorded on a

Bruker Avance 400 MHz or 500 MHz spectrometers with

chemical shifts given in parts per million (ppm) using residual

solvent peak at 7.26 ppm as reference. EI spectra of the

compounds were recorded using JEOL GCMATE II instrument.

Elemental analyses were performed using

a Perkin Elmer

Series 11 analyzer. MALDI-TOF MS spectra were recorded on a

Bruker Daltonics instrument in dihydroxy benzoic acid matrix.

Molecular weights (M_nand M_w) and the MWDs (M_w/M_n) of

polymer samples were determined by GPC instrument with

Waters 510 pump and Waters 410 or 2414 differential

refractometer as the detector. Single crystal XRD data were

collected on a Bruker AXS (Kappa Apex 2) CCD diffractometer

equipped with graphite monochromated Mo (Kα) ( = 0.7107

Å) radiation source.

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Synthesis of the ligands

NMR (500 MHz, CDCl₃): δ = 1.15–1.16 (d, J= 5.5_ViH_ewz,_ArtC_icH_le(C_OH_nl₃_in)₂_e,

6H), 1.18–1.19 (d, J= 5.5 Hz, CH(CH₃)₂^D,^OI6^:H¹⁰),^.104³⁹.1^/C1–^5R4^A.1²7⁶⁷²(¹m^H,

The new ligands, L2(H)₂and L3(H)₂were prepared in a similar CH(CH₃)₂, 1H), 4.35–4.41 (m, CH(CH₃)₂, 1H), 6.23 (s, NH, 1H),

manner as reported for the known ligand L1(H)₂.¹¹

6.75–6.78 (d, Ar–H, 1H), 7.00–7.03 (d, Ar–H, 1H), 7.44–7.45 (m,

Ar–H, 1H), 7.51–7.53 (m, Ar–H, 1H), 8.37 (s, HC=N, 1H). ¹³C

Ligand L1(H)₂: To a solution of 2-hydroxy-benzaldehyde (1 g, NMR (125 MHz, CDCl₃): δ = 21.5 (CH(CH₃)₂), 24.9 (CH(CH₃)₂),

8.19 mmol) in absolute ethanol was added solution of 68.1 (O–CH(CH₃)₂), 71.7 (O–CH(CH₃)₂), 103.2 (CNH), 113.8

a

diaminomaleonitrile (0.874 g, 8.09 mmol) in absolute ethanol. (NCCNH), 117.1 (NCCN), 120.7 (Ar–C), 125.4 (Ar–C), 129.1

The ligand was formed via monocondensation and the reaction (NCCN), 133.4 (Ar–C), 136.3 (Ar–C), 138.0 Ar–C), 153.8 (Ar–O),

procedure was as per the report.¹¹

158.0 (HC=N). EI-MS m/z calculated for [M+]⁺. C₃₄H₄₀N₈O₆Ti₂:

752.46 found 752.1491. Anal. Calcd for C₃₄H₄₀N₈O₆Ti₂: C 54.27,

2-hydroxy-3,5- H 5.36, N 14.89. Found: C 54.25, H 5.34, N 14.86.

Ligand L2(H)₂: To solution

a

of

dimethylbenzaldehyde (1 g, 6.66 mmol) in absolute ethanol

was added solution of diaminomaleonitrile (0.705 g, 6.52 Complex 1b: Ligand L1(H)₂(0.027 g, 0.129 mmol) and

mmol) in absolute ethanol. The mixture was refluxed for 24 h Zr(OⁱPr)₄·ⁱPrOH (0.050 g, 0.129 mmol) were reacted in 1:1

and reddish brown precipitate was observed. The resulting stoichiometric ratio and the reaction was carried out in similar

solution was cooled down to room temperature and the a manner as described for 1a. Yield 0.05 g, (88%). Mp: 211 ^oC

reddish brown precipitate were filtered off, washed with (decomp). ¹H NMR (500 MHz, CDCl₃): δ = 1.07–1.09 (d, J= 6 Hz,

ethanol and dried under vacuum to afford the pure product CH(CH₃)₂, 3H) 1.18–1.19 (d, J= 6 Hz, CH(CH₃)₂, 3H), 1.46–1.47

L2(H)₂. Yield 0.81 g, (52%). Mp: 208 ^oC. ¹H NMR (500 MHz, (d, J= 6 Hz, CH(CH₃)₂, 6H), 4.05–4.07 (m, CH(CH₃)₂, 1H), 4.66–

CDCl₃): δ = 2.24 (s, (CH₃), 3H), 2.28 (s, (CH₃), 3H), 4.95 (s, NH₂, 4.68 (m, CH(CH₃)₂, 1H), 6.10 (s, NH, 1H), 6.29–6.31 (d, Ar–H,

2H), 7.06 (s, Ar–H, 1H), 7.13 (s, Ar–H, 1H), 8.51 (s, HC=N, 1H), 1H), 6.56–6.59 (m, Ar–H, 1H), 6.93–6.94 (m, Ar–H, 1H), 7.31–

11.36 (s, Ar-OH, 1H). ¹³C NMR (125 MHz, CDCl₃): δ = 15.4 (Ar– 7.34 (m, Ar–H, 1H), 8.36 (s, HC=N, 1H). ¹³C NMR (125 MHz,

CH₃), 20.4 (Ar–CH₃), 108.0 (CNH₂), 112.0 (NCCNH₂), 113.8 CDCl₃): δ = 20.7 (CH(CH₃)₂), 21.7 (CH(CH₃)₂), 25.9 (CH(CH₃)₂),

(NCCN), 117.6 (Ar–C), 123.6 (NCCN), 126.0 (Ar–C), 129.2 (Ar– 69.6 (O–CH(CH₃)₂), 74.9 (O–CH(CH₃)₂), 104.0 (CNH), 113.1

C), 130.9 (Ar–C), 137.3 (Ar–C), 156.5 (Ar–O), 163.0 (HC=N). ESI- (NCCNH), 122.3 (Ar–C), 128.3 (NCCN), 128.9 (Ar–C), 129.1 (Ar–

MS m/z calculated for [M+H]⁺. C₁₃H₁₃N₄O: 241.26 found 241. C), 133.1 (Ar–C), 135.8 (Ar–C), 136.3 Ar–C), 155.8 (Ar–O), 158.2

Anal. Calcd for C₁₃H₁₂N₄O: C 64.99, H 5.03, N 23.32. Found: C (HC=N). EI-MS m/z calculated for [M]⁺. C₃₄H₄₀N₈O₆Zr₂: 839.18

64.97, H 5.01, N 23.31.

found 839.1240 Anal. Calcd for C₃₄H₄₀N₈O₆Zr₂: C 48.66, H 4.80,

N 13.35. Found: C 48.64, H 4.78, N 13.32.

Ligand L3(H)₂: The ligand L3(H)₂was prepared in a similar

manner as described for L2(H)₂by using 3-(tert-butyl)-2- Complex 1c: Ligand L1(H)₂(0.022 g, 0.106 mmol) and Hf(O^tBu)₄

hydroxy-5-methylbenzaldehyde (1 g, 5.20 mmol) instead of 2- (0.050 g, 0.106 mmol) were reacted in 1:1 stoichiometric ratio

hydroxy-3,5-dimethylbenzaldehyde

and

diaminomaleonitrile and the reaction was carried out in similar manner as

1

(0.551 g, 5.10 mmol). The resulting yellow precipitate were described for 1a. Yield 0.05 g, (87%). Mp: 217 ^oC (decomp). H

filtered off, washed with ethanol and dried under vacuum. NMR (500 MHz, CDCl₃): δ = 1.07 (s, C(CH₃)₃, 9H), 1.37–1.39 (t,

o

1

Yield 0.79 g, (55%). Mp: 185 C. H NMR (400 MHz, CDCl₃): δ = CH₂(CH₃), 3H), 4.28–4.33 (m, CH₂(CH₃), 2H), 6.10 (s, NH, 1H),

1.43 (s, C(CH₃)₃, 9H), 2.33 (s, (CH₃), 3H), 5.01 (s, NH₂, 2H), 7.09– 7.19–7.20 (m, Ar–H, 2H), ), 7.36–7.37 (d, Ar–H, 1H), 7.48–7.49

7.10 (d, Ar–H, 1H), 7.27–7.28 (d, Ar–H, 1H), 8.55 (s, HC=N, 1H), (d, Ar–H, 1H), 8.44 (s, CH=N, 1H). ¹³C NMR (125 MHz, CDCl₃): δ

11.79 (s, Ar-OH, 1H). ¹³C NMR (100 MHz, CDCl₃): δ = 20.7 (Ar– = 21.5 (CH(CH₃)₃), 26.4 (CH₂(CH₃), 68.0 (O–C(CH₃)₃), 74.0 (O–

CH₃), 29.4 (C(CH₃)₃),

34.9 (C(CH₃)₃), 107.4 (CNH₂), 112.0 CH₂(CH₃), 102.2 (CNH), 113.6 (NCCNH), 121.3 (NCCN), 124.8

(NCCNH₂), 113.5 (NCCN), 118.3 (Ar–C), 122.7 (NCCN), 128.7 (Ar–C), 126.3 (NCCN), 128.3 (Ar–C), 130.0 (Ar–C), 136.1 (Ar–C),

(Ar–C), 131.5 (Ar–C), 133.7 (Ar–C), 137.6 (Ar–C), 157.6 (Ar–O), 137.8 (Ar–C), 155.8 (Ar–O), 158.8 (HC=N). EI-MS m/z calculated

163.4 (HC=N). ESI-MS m/z calculated for [M+H]⁺. C₁₆H₁₉N₄O: for [M]⁺. C₃₄H₄₀N₈O₆Hf₂: 1013.71 found 1013.8488 Anal. Calcd

283.34 found 283. Anal. Calcd for C₁₆H₁₈N₄O: C 68.06, H 6.43, N for C₃₄H₄₀N₈O₆Hf₂: C 40.28, H 3.98, N 11.05. Found: C 40.26, H

19.84. Found: C 68.05, H 6.41, N 19.82.

3.96, N 11.02.

Synthesis of complexes 1a-1c, 2a-2c and 3a-3c

Complex 2a: Ligand L2(H)₂[2-amino-3-((E)-(2-hydroxy-3,5-

dimethylbenzylidene)amino)maleonitrile]

solution of L1(H)₂[2-amino-3-((E)-(2- mmol) and Ti(OⁱPr)₄(0.050 g, 0.175 mmol) were reacted in 1:1

hydroxybenzylidene)amino)maleonitrile] (0.037 g, 0.175 stoichiometric ratio and the reaction was carried out in similar

(0.042

g,

0.175

Complex 1a: To

a

o

1

mmol) in 5 mL of dry toluene was added a solution of Ti(OⁱPr)₄manner as described for 1a. Yield 0.07 g, (92%). Mp: 186 C. H

(0.050 g, 0.175 mmol) in 5 mL toluene, in 1:1 stoichiometric NMR (500 MHz, CDCl₃): δ = 1.07–1.08 (d, J= 6.5Hz, CH(CH₃)₂,

ratio. The reaction mixture was stirred for an additional period 6H), 1.30–1.32 (d, J= 6.5Hz, CH(CH₃)₂, 6H), 2.44 (s, (CH₃), 3H),

of 24 h at room temperature. The resulting brown precipitate 2.49 (s, (CH₃), 3H), 4.22–4.25 (m, CH(CH₃)₂, 1H), 4.54–4.57 (m,

was evaporated to dryness under vacuum and crystallized CH(CH₃)₂, 1H), 6.68 (s, NH, 1 H), 7.36 (s, Ar–H, 1H), 7.46 (s, Ar–

from chloroform solution. Yield 0.06 g, (90%). Mp: 205 ^oC. ¹H H, 1H), 8.46 (s, HC=N, 1H). ¹³C NMR (125 MHz, CDCl₃): δ = 18.2

10 | J. Name., 2012, 00, 1-3

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(CH(CH₃)₂), 20.4 (CH(CH₃)₂), 25.7 (Ar–CH₃), 31.0 (Ar–CH₃), 68.1 [M+H]⁺. C₄₄H₆₀N₈O₆Ti₂: 893.73 found 893.6053 Anal. Calcd for

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(O–CH(CH₃)₂), 73.7 (O–CH(CH₃)₃), 103.0 (CNH), 113.9 (NCCNH), C₄₄H₆₀N₈O₆Ti₂: C 59.20, H 6.77, N 12.5^D5.^OI:F¹o⁰u^.n¹⁰d³:^9/C^C5R5^A9².1⁶8⁷,^21HH

120.9 (NCCN), 125.4 (Ar–C), 129.7 (NCCN), 131.3 (Ar–C), 136.0 6.75, N 12.54.

(Ar–C), 138.0 (Ar–C), 138.4 (Ar–C), 154.3 (Ar–O), 160.7 (HC=N).

EI-MS m/z calculated for [M+H]⁺. C₃₈H₄₈N₈O₆Ti₂: 809.57 found Complex 3b: Ligand L3(H)₂(0.036 g, 0.129 mmol) and

809.6174. Anal. Calcd for C₃₈H₄₈N₈O₆Ti₂: C 56.45, H 5.98, N Zr(OⁱPr)₄·ⁱPrOH (0.050 g, 0.129 mmol) were reacted in 1:1

13.86. Found: C 56.43, H 5.96, N 13.84.

stoichiometric ratio and the reaction was carried out in similar

manner as described for 1a. The resulting brown precipitate

Complex 2b: Ligand L2(H)₂(0.031 g, 0.129 mmol) and after solvent evaporation was crystallized from concentrated

o

Zr(OⁱPr)₄·ⁱPrOH (0.050 g, 0.129 mmol) were reacted in 1:1 solution of 1,2-dichloroethane. Yield 0.06 g, (93%). Mp: 186 C.

stoichiometric ratio and the reaction was carried out in similar ¹H NMR (500 MHz, CDCl₃): δ = 0.96–0.97 (d, J= 7.5Hz, CH(CH₃)₂,

manner as described for 1a. Yield 0.05 g, (90%). Mp: 202 ^oC 3H), 1.11–1.12 (d, J= 7.5 Hz, CH(CH₃)₂, 3H), 1.27–1.29 (d, J= 7

(decopm). ¹H NMR (500 MHz, CDCl₃): δ = 1.07–1.09 (d, J= 6 Hz, Hz, CH(CH₃)₂, 6H), 1.65 (s, C(CH₃)₃, 9H), 2.46 (s, (CH₃), 3H),

CH(CH₃)₂, 3H), 1.20–1.21 (d, J= 6 Hz, CH(CH₃)₂, 3H), 1.44–1.45 4.17–4.22 (m, CH(CH₃)₂, 1H), 4.32–4.38 (m, CH(CH₃)₂, 1H), 6.15

(d, J= 6.5 Hz, CH(CH₃)₂, 6H), 2.48 (s, (CH₃), 3 H), 2.53 (s, (CH₃), (s, NH, 1H), 7.40 (s, Ar–H, 1H), 7.48 (s, Ar–H, 1H), 8.45 (s,

3H), 3.96–3.98 (m, CH(CH₃)₂, 1H), 4.37–4.45 (m, CH(CH₃)₂, 1H), HC=N, 1H), [S (1,2-dichloroethane) = 3.74]. ¹³C NMR (125 MHz,

6.21 (s, NH, 1H), 7.32 (s, Ar–H, 1H), 7.55 (s, Ar–H, 1H), 8.55 (s, CDCl₃): δ = 20.7 (Ar–CH₃), 25.6 (CH(CH₃)₂), 25.9 (CH(CH₃)₂), 26.9

HC=N, 1H). ¹³C NMR (125 MHz, CDCl₃): δ = 20.4 (Ar–CH₃), 24.9 (CH(CH₃)₂), 29.9 (C(CH₃)₃), 35.2 (C(CH₃)₃), 72.9 (O–CH(CH₃)₂),

(Ar–CH₃), 25.5 (CH(CH₃)₂), 25.6 (CH(CH₃)₂), 30.4 (CH(CH₃)₂), 75.1 (O–CH(CH₃)₂), 102.3 (CNH), 113.8 (NCCNH), 122.3 (NCCN),

72.9 (O–CH(CH₃)₂), 75.0 (O–CH(CH₃)₂), 102.5 (CNH), 113.9 125.4 (Ar–C), 128.3 (NCCN), 133.2 (Ar–C), 135.8 (Ar–C), 136.3

(NCCNH), 120.9 (NCCN), 125.4 (Ar–C), 126.6 (Ar–C), 129.1 (Ar–C), 138.0 (Ar–C), 158.2 (Ar–O), 159.7 (HC=N), [S = 43.5]. EI-

(NCCN), 132.3 (Ar–C), 136.3 (Ar–C), 138.0 (Ar–C), 157.6 (Ar–O), MS m/z calculated for [M+H]⁺. C₄₄H₆₀N₈O₆Zr₂: 980.44 found

158.8 (HC=N). EI-MS m/z calculated for [M]⁺. C₃₈H₄₈N₈O₆Zr₂: 980.1991 Anal. Calcd for C₄₄H₆₀N₈O₆Zr₂: C 53.96, H 6.17, N

895.28 found 895.7510 Anal. Calcd for C₃₈H₄₈N₈O₆Zr₂: C 50.98, 11.44. Found: C 53.94, H 6.15, N 11.41.

H 5.40, N 12.52. Found: C 50.96, H 5.38, N 12.50.

Complex 3c: Ligand L3(H)₂(0.030 g, 0.106 mmol) and Hf(O^tBu)₄

Complex 2c: Ligand L2(H)₂(0.025 g, 0.106 mmol) and Hf(O^tBu)₄(0.050 g, 0.106 mmol) were reacted in 1:1 stoichiometric ratio

(0.050 g, 0.106 mmol) were reacted in 1:1 stoichiometric ratio and the reaction was carried out in similar manner as

and the reaction was carried out in similar manner as described for 1a. Yield 0.05 g, (89%). Mp: 194 ^oC (decomp). H

1

described for 1a. Yield 0.05 g, (88%). Mp: 208 ^oC. H NMR (500 NMR (500 MHz, CDCl₃): δ = 1.13 (s, C(CH₃)₃, 9H), 1.43–1.45 (t,

1

MHz, CDCl₃): δ = 1.44 (s, C(CH₃)₃, 9H), 1.81–1.83 (t, CH₂(CH₃), CH₂(CH₃), 3H), 1.67 (s, C(CH₃)₃, 9H), 2.46 (s, (CH₃), 3H), 4.32–

3H), 2.57 (s, (CH₃), 9H), 2.75 (s, (CH₃), 3H), 4.20–4.24 (m, 4.36 (m, CH₂(CH₃), 2H), 6.14 (s, NH, 1H), 7.28 (s, Ar–H, 1H),

CH₂(CH₃), 2H), 6.82 (s, NH, 1H), 7.18 (s, Ar–H, 1H), 7.48 (s, Ar– 7.52 (s, Ar–H, 1H), 8.47 (s, CH=N, 1H). ¹³C NMR (125 MHz,

H, 1H), 8.27 (s, CH=N, 1H). ¹³C NMR (125 MHz, CDCl₃): δ = 20.3 CDCl₃): δ = 20.7 (Ar–CH₃), 29.8 (C(CH₃)₃), 31.3 (CH₂(CH₃), 32.0

(CH(CH₃)₃), 25.4 (CH₂(CH₃), 32.4 (CH₃), 33.0 (CH₃), 70.5 (O– (C(CH₃)₃), 35.1 (C(CH₃)₃), 67.0 (O–CH₂(CH₃), 72.0 (O–C(CH₃)₃),

C(CH₃)₃), 76.0 (O–CH₂(CH₃), 103.5 (CNH), 114.6 (NCCNH), 121.6 104.1 (CNH), 113.1 (NCCNH), 122.3 (NCCN), 125.4 (Ar–C),

(NCCN), 125.4 (Ar–C), 126.5 (NCCN), 129.1 (Ar–C), 131.6 (Ar– 129.2 (NCCN), 133.0 (Ar–C), 136.1 (Ar–C), 138.0 (Ar–C), 138.3

C), 137.9 (Ar–C), 137.9 (Ar–C), 157.5 (Ar–O), 159.3 (HC=N). EI- (Ar–C), 155.6 (Ar–O), 159.6 (HC=N). EI-MS m/z calculated for

MS m/z calculated for [M]⁺. C₃₈H₄₈N₂₈O₆Hf₂: 1069.82 found [M+H]⁺. C₄₄H₆₀N₈O₆Hf₂: 1154.98 found 1154.2328 Anal. Calcd

1069.8386 Anal. Calcd for C₃₈H₄₈N₂₈O₆Hf₂: C 42.66, H 4.52, N for C₄₄H₆₀N₈O₆Hf₂: C 45.80, H 5.24, N 9.71. Found: C 45.78, H

10.47. Found: C 42.64, H 4.50, N 10.44.

5.22, N 9.69.

Complex 3a: Ligand L3(H)₂[2-amino-3-((E)-(3-(tert-butyl)-2- General procedure for the ROP of ε-CL and rac-LA

hydroxy-5-methylbenzylidene)amino)maleonitrile]

(0.049

g, A dry reaction vessel of 100 mL capacity kept inside a glove

0.175 mmol) and Ti(OⁱPr)₄(0.050 g, 0.175 mmol) were reacted box was equipped with a magnetic bar. A mixture of ε-CL (1

in 1:1 stoichiometric ratio and the reaction was carried out in gm, 8.76 mmol) or rac-LA (1 gm, 6.94 mmol) and the desired

similar manner as described for 1a. Yield 0.07 g, (93%). Mp: amount of catalyst were taken in this vessel and then it was

o

180 C. ¹H NMR (500 MHz, CDCl₃): δ = 1.00–1.01 (d, J= 6 Hz, closed with a stopper and made air-tight using Teflon tape.

CH(CH₃)₂, 6H), 1.20–1.21 (d, J= 6 Hz, CH(CH₃)₂, 6H), 1.48 (s, The reaction vessel was then taken out side and immersed in

C(CH₃)₃, 9H), 2.37 (s, (CH₃), 3H), 4.14–4.16 (m, CH(CH₃)₂, 1H), an oil bath maintained at 100 °C for ε-CL and 140 °C in case of

4.58–4.63 (m, CH(CH₃)₂, 1H), 6.23 (s, NH, 1H), 7.15 (s, Ar–H, rac-LA. As the reaction mixture was heated, initially the lactide

1H), 7.37 s, Ar–H, 1H), 8.63 (s, HC=N, 1H). ¹³C NMR (125 MHz, melted and then became viscous over a period of time ranging

CDCl₃): δ = 20.9 (Ar–CH₃), 25.9 (CH(CH₃)₂), 26.0 (CH(CH₃)₂), 29.6 from 3 to 21 minutes depending on the catalyst (See Table 1).

(C(CH₃)₃), 35.2 (C(CH₃)₃), 68.2 (O–CH(CH₃)₂), 81.5 (O–CH(CH₃)₂), This indicated the completion of reaction. At this stage the

103.9 (CNH), 113.3 (NCCNH), 122.2 (NCCN), 125.4 (Ar–C), reaction mixture was cooled down to room temperature and

129.1 (NCCN), 131.1 (Ar–C), 134.2 (Ar–C), 135.6 (Ar–C), 138.0 dissolved in minimum amount of dichloromethane. The

(Ar–C), 155.9 (Ar–O), 163.6 (HC=N). EI-MS m/z calculated for polymer was precipitated by adding excess of cold methanol.

J . Name., 2013, 00, 1-3 | 11

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The precipitate was filtered and dried in vacuum to a constant samples were determined by GPC instrument with Waters 510

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1

DOI: 10.1039/C5RA26721H

weight. The conversion of ε-CL and rac-LA were analyzed by H pump and Waters 2414 differential refractometer as the

NMR spectroscopic studies of the isolated polymer. Data detector. The used column namely WATERS STRYGEL-HR4 of

concerning molecular weights (M_n) and the MWDs (M_w/M_n) of dimensions (4.6

×

300 mm) was connected during the

the polymer samples obtained by the ROP of LA were experiment. Measurements were done in 1,2,4-tri-chloro

determined by using a GPC instrument with Waters 510 pump benzene (TCB) at 140 °C. Number average molecular weights

and Waters 410 differential refractometer as the detector. The (M_n) and molecular weight distributions (MWDs) of poly

THF columns namely WATERS STRYGEL-HR5, STRYGEL-HR4 and olefins were measured relative to polystyrene standards.

STRYGEL-HR3 each of dimensions (7.8 × 300 mm) were used.

Number average molecular weights (M_n) and MWDs (M_w/M_n) X-ray crystallography

of polymers were measured relative to polystyrene standards Suitable single crystals of complexes 3a, 3b and 3c were

using THF as solvent at 27 °C.

obtained by crystallization from saturated solution of 1,2-

dichloroethane or chloroform at room temperature for X-ray

structural determinations. Single crystals were mounted on

General procedure for kinetic studies of ε-CL and rac-LA

The polymerizations of ε-CL and rac-LA were carried out at Bruker AXS (Kappa Apex 2) CCD diffractometer equipped with

[M]_o/[Cat]_o=200:1 ratio using 3a at 100 and 140 °C respectively graphite monochromated Mo (Kα) ( = 0.7107 Å) radiation

under an argon atmosphere in a sealed tube. Based on the source. The data were collected with 100% completeness for 

polymerization time for the maximum monomer conversion up to 25°.  and  scans was employed to collect the data. The

different time laps of regular intervals were assumed. Then frame width for  for was fixed to 0.5° for data collection. The

different polymerization reactions were carried out in a sealed frames were subjected to integration and data were reduced

tube using 0.5 g of monomer with the required amount of for Lorentz and polarization corrections using SAINT-NT. The

catalyst. In all cases the contents were immersed in a bath at multi-scan absorption correction was applied to the data set.

required temperature with constant stirring for the stipulated All structures were solved using SIR-92 and refinement was

time as assumed earlier. The each polymer obtained from done using SHELXL-97.¹⁸Location of all the hydrogen atoms

these polymerization reactions were separately analyzed by ¹H could be found in the difference Fourier map. The hydrogen

NMR spectroscopy. The [M]_t=0/[M]_tratio was calculated by atoms attached to carbon atoms were fixed at chemically

integration of the peak corresponding to the methine protons meaningful positions and were allowed to ride with the parent

for the monomer and polymer. The apparent rate constant atom during refinement.

These data were deposited with

(k_app) values were obtained from the slopes of the best-fit CCDC with the numbers: CCDC 1438097–1438099.

lines.

Conclusions

In conclusion, dinuclear group

General procedure for the homopolymerization of epoxides

The epoxides used are rac-cyclohexene oxide (CHO), rac-

4 complexes of tridentate

propylene oxide (PO) and rac-styrene oxide (SO). The

procedure of polymerization and method of characterization

of the products is very similar to that described for the ROP of

rac-LA. Here a mixture of rac-CHO (1 gm, 10.19 mmol) or rac-

PO (1 gm, 17.22 mmol) or rac-SO (1 gm, 8.32 mmol) with the

desired amount of catalyst have been used. However, the

reaction temperature and time depended upon the choice of

the epoxide (Table 2).

[NNO]-type of Schiff-base ligands have been synthesized in

good yields and completely characterized by NMR and Mass

techniques and the solid state structures of Ti, Zr and Hf

complexes of a given ligand have been elucidated by single

crystal X-ray diffraction studies. All these complexes

performed as highly active in ROP of ε-CL and rac-LA under

industrially preferred melt conditions. The narrow PDI values

of PCL and PLA suggested the occurrences of living

polymerization. All complexes acted as stereoselective

initiators, as the PLA obtained from rac-LA featured good

degree of heterotacticity. Mechanistic studies performed in

the ROP of rac-LA suggested the coordination-insertion

mechanism. Kinetic studies revealed the first order rate

constant with respect to ε-CL and rac-LA conversions. All

complexes showed moderate activity for ROP of rac-epoxides,

such as, CHO, PO and SO. Finally, application of these

complexes directed towards ethylene polymerization, where

these complexes exhibited good catalytic activity for producing

PE upon activation with MAO. Particularly the complexes,

bearing bulky tert-butyl ortho-phenolate substituent, exhibited

high catalytic activity.

General Procedure for ethylene polymerization

Polymerizations were performed in a 500 ml stainless steel

autoclave reactor equipped with a mechanical stirrer and a

temperature controller. First 10 μmol of the catalyst was

dissolved in 50 ml of toluene along with the required amount

of MAO (10 wt % in toluene) was injected into the reactor

under

argon

atmosphere.

Once

the

polymerization

temperature (60 °C) was reached, ethylene was purged into

the reactor. Then, the reaction mixture was stirred for 30 min

under 6.08 bar (6 atm) pressure of ethylene. After 30 min, the

polymerization was stopped and subsequently the produced

polymer was quenched with acidic methanol (5 mL HCl, 50 mL

methanol). The polymer was collected by filtration and dried

till under vacuum constant weight was achieved. Molecular

weights (M_nand M_w) and the MWDs (M_w/M_n) of polyethylene

12 | J. Name., 2012, 00, 1-3

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Acknowledgements

This work was supported by the Department of Science and

Technology, New Delhi. We thank IIT Madras for financial

support.

Instrumentation facilities

were

availed from

Department of Chemistry and SAIF, IIT Madras.

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DOI: 10.1039/C5RA26721H

Graphical abstract

A series of new dinuclear group 4 complexes containing [NNO]-type ligands are utilized as

catalysts for polymerization reactions.

rac-LA

Heterotactic-rich PLA

[P_r(max) = 0.80]

Polyether

Polyethylene

Article Doi

Group 4 alkoxide complexes containing [NNO]-type scaffold: Synthesis, structural characterization and polymerization studies

DOI: 10.1039/c5ra26721h

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c5ra26721h

Full text of DOI:10.1039/c5ra26721h

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